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SYNCHRONOUS COMPENSATORS FOR MINI-GRIDS AND ISLANDING
Final Report
CONTRACT NUMBER: K/EL/00267/00/00
URN NUMBER: 04/1444
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SYNCHRONOUS COMPENSATORS FOR MINI-GRIDS AND ISLANDING
K/EL/00267/00/00
URN04/1444
Contractor
Econnect Ltd
The work described in this report was carried out under contract as part of the DTI Technology Programme: New and Renewable Energy, which is managed by Future Energy Solutions. The views and judgements expressed in this report are those of the contractor and do not necessarily reflect those of the DTI or Future Energy Solutions.
First published 2004 Econnect 2004
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EXECUTIVE SUMMARY Objective
The objective of the project was to demonstrate the viability of operating a stand-alone power system with 100% wind power using standard (grid-connected design) induction generator wind turbines, with synchronous compensators and Distributed Intelligent Load Controllers providing voltage control and frequency regulation respectively.
Background
There is a large global requirement for rural electrification as 2 billion people are without electricity. To supply these by extending existing grid systems would require massive investments in transmission and distribution plant in addition to the power stations to feed them. An alternative is to use local mini-grids to provide electricity using local indigenous renewable resources. The use of non-dispatchable renewable resources requires management of the balance between the available power and the system loads to ensure that the system's voltage and frequency remain within acceptable limits.
Econnect has developed a new generation of Distributed Intelligent Load Controllers to enable mini-grids (island grids, wind-diesel systems or autonomous systems) to use one or many standard (grid-connected design) wind turbines. The controllers use highly innovative software algorithms to control system frequency within the statutory limits, based on measurements of the frequency and rate of change of frequency of the supply. This information is processed using a fuzzy control algorithm, embedded in each controller, which informs load-switching decisions. There is no central controller and no communications link between the controllers. The global effect of the independent devices is to maintain power system stability for autonomous power systems powered solely from wind power. The controllers have undergone extensive modelling as well as successful site and laboratory tests on wind turbines.
In conjunction with the use of robust and well-established synchronous compensator technology to manage system voltage, the controllers offer a flexible, low-cost and efficient means of incorporating large amounts of renewable energy in electrical power systems.
The main purpose of the project was to develop equipment suitable for use in remote locations which would permit islanded operation of wind turbine systems equipped with induction generators. The project aimed to demonstrate methods by which induction wind turbine generators can operate in islanded mode, receiving voltage control from synchronous compensators and frequency control from distributed intelligent load controllers. This was to be achieved by developing a design methodology and applying it to the construction of two test systems, incorporating a 20kW stall-regulated wind turbine and a 300kW pitch-regulated wind turbine respectively. Each wind turbine was to be islanded from the electricity grid, fitted with the synchronous compensator and distributed load control technology, and tested.
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Work carried out
The project incorporated the following activities:
• Selection of appropriate test sites and agreement with wind turbine operators
The 20kW, stall-regulated Gazelle, and the 300kW, pitch-regulated Windmaster were identified as suitable test vehicles. The Gazelle had a simple control system. The Windmaster was more complex than the Gazelle and provided a natural progression in the development of the system.
• Data collection for wind turbines, and discussion with manufacturers / operators
• System design
Suitable synchronous compensators were selected in conjunction with Newage AVK-SEG, and appropriate load control configurations were specified.
• Dynamic computer modelling of the system
A computer model of each intended autonomous power system was developed using MATLAB / Simulink. Variable wind speed inputs and consumer loads were applied to investigate power quality and system stability.
• Specification and procurement of load control hardware, load banks, protection equipment, pony motors, variable speed drives, cabling, and datalogging equipment
• Load control software development
• System assembly and installation
• Development of a modular, PC-based, instrumentation and datalogging system
• Production of risk assessment and method statement documents
• Transport of equipment to the wind turbine site
• Synchronous compensator equipment commissioning
• Wind turbine modification
The objective during testing was to avoid any modification of the wind turbine system. However, in order to incorporate the synchronous compensator equipment, some changes had to be made. These were minor in the case of the Gazelle but more complex in the case of the Windmaster.
• Wind turbine testing
Following satisfactory commissioning tests, the wind turbines were disconnected from the grid and connected into the mini-grid system. A variety of tests were carried out, including optimisation of the wind turbine start procedures, simulated wind-diesel operation, and wind-only operation under a range of wind conditions.
• Decommissioning of test equipment
• Results analysis
• Dissemination of the results
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Results from computer simulation
The results from the computer models predicted successful operation of the stand-alone systems. The simulated control loads were seen to contribute the difference between the varying wind turbine input power and the varying base load power requirement. The random nature of the control load switching was clearly demonstrated. Predicted system frequency was maintained between approximately 48 and 52Hz, slightly outside the BS EN 50160 recommendation. Predicted voltage control was good, well within the required +/-10% of 230V stated in BS EN 50160. The models achieved this with different amounts of base load applied, different ratings of power factor correction capacitors, and different wind speeds. Some voltage imbalance between the phases was predicted and would be expected, since the loads were single-phase loads. On the Gazelle model this imbalance was well within limits; the Windmaster model indicated a potential area of concern.
The Windmaster model was employed to investigate the observed starting performance of the wind turbine in more detail, and was used to estimate the effects of modifying the starting strategy. It indicated that most of the power drain on the system during the start was due to the starting resistors rather than the motoring requirement of the wind turbine.
Results from wind turbine testing
Effective wind-only operation was achieved with both wind turbines. Under these conditions, the only power input into the system was from the wind turbine, and the only frequency (speed) control was provided by the load controllers. No adjustment to the load controller firmware was necessary to achieve stable frequency control in wind-only mode, and no additional adjustment was necessary to the AVR to achieve stable voltage control. Wind-only mode could be maintained stably for as long as there was enough wind to overcome the losses in the system, mainly the synchronous compensator's rotating losses.
Periods of wind-only operation were analysed to evaluate each system's performance.
Frequency control was good, within the BS EN 50160 requirement of +/-1Hz. With the Gazelle, there was a definite offset, however, with a low mean value of 48.6Hz, which put the system as configured outside acceptable limits. The load controller frequency setpoints were modified for the Windmaster tests and successfully removed this offset. On both systems, a slow oscillation in frequency was observed. The frequency of this oscillation, around 0.4Hz, was well outside the region where flicker would be a problem (8-10Hz).
Voltage control was good. The voltages on all three phases remained well within the BS EN 50160 voltage requirements during wind-only operation on both wind turbines. As the load control employed single-phase loads, with no restriction on the relative loading on each phase, this result was particularly encouraging. The values of negative sequence voltage achieved were well within the nominal 2% limit in BS EN 50160.
It was an aim of the work to be able to start the wind turbine without adversely affecting the power quality of the system. Starting the Gazelle was achieved with the
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frequency and voltages remaining within required limits. This was not the case with the Windmaster and would be an area for further development of the system.
The rotating losses within each system were evaluated. Observed system losses of 10% of the Gazelle's rated power and 5% of the Windmaster's rated power were in line with the Newage synchronous compensator design data.
Conclusions
Successful operation of the stand-alone system was achieved with both wind turbines. In areas where operability was limited, straightforward improvements have been identified. Neither machine was felt to be fundamentally unsuitable for stand-alone operation. Adaptation to stand-alone operation would ideally be carried out in conjunction with the manufacturer, which makes the Windmaster less suitable for further development.
The experience gained within this project has allowed a practical design methodology for stand-alone wind turbine systems to be developed.
The use of Newage's generators as a synchronous compensator with the stand-alone wind turbine system was relatively novel use of the equipment in comparison with Newage's standard diesel genset experience. Discussion with Newage's technical support department proved invaluable in assessing the technical implications of the proposed application and selecting appropriate machines. In both test situations the synchronous compensators and AVRs selected performed well with minimal adjustment to AVR settings, justifying the selection of well-established and robust technology. These generators compare well with power electronics alternatives in terms of cost.
The load control system demonstrated its ability to regulate system frequency within statutory limits for stand-alone wind turbine systems. No reprogramming of load controller firmware was necessary during testing. Frequency-sensitive controllable load up to the full rating of the wind turbine was felt to be essential, even if only to accommodate the large power swings during starting.
As intended during the design stage, minimal modifications were made to the wind turbine systems to allow them to operate in stand-alone mode. For both wind turbines it was necessary to widen out the frequency and voltage limits imposed by their protection systems.
The experience with starting the wind turbines suggested that a power electronic soft starter to limit the inrush current is essential in a stand-alone system if the back-up power system and synchronous compensator are not to be grossly oversized to cope with the starting requirements. Most modern induction generator wind turbines have soft starters, but older machines would need one installed in place of the starting resistors.
The experience gained with the Gazelle and the Windmaster provides confidence that most wind turbines with induction generators could be adapted to use in a stand-alone mode, using a synchronous compensator and distributed load control. The concept is applicable for any size of wind turbine. Wind turbines with power electronic grid interfaces might not require synchronous compensator technology to maintain stand-alone grid voltages (depending on their design), but could employ
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distributed load control to manage system frequency and make most efficient use of any surplus wind power.
The system developed here could also be used to allow existing diesel-generator-based systems to incorporate one or more wind turbines with ratings comparable to, or even significantly larger than, the diesel set's nominal kVA rating.
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Recommendations
This project has clearly demonstrated the suitability of distributed load control for controlling system frequency in stand-alone wind turbine systems. It is recommended that the technologies employed here are developed for full commercial application.
The methodology developed from this work will produce robust system designs with a high chance of successful operation with minimal development work or modifications on site. The next step would be to demonstrate long-term operation of such a system, successfully integrated into an existing stand-alone diesel-based mini-grid. This will require further development of the back-up power supply and the accompanying supervisory system. A diesel generator with a clutch to allow its alternator to act as the synchronous compensator or a pony motor powered by an existing mini-grid are both viable alternatives.
Although the fundamental governing operation of the load controllers does not require any communication between the load controllers, integration into a system which uses more than one type of generation source might require some communications ability for supervisory control purposes. This aspect of the load controllers is currently under development and has been successfully demonstrated with hydropower and photovoltaic systems. A full system demonstration would consolidate this work.
The system concept has potential for use on offshore wind turbines in the event of an outage due to a fault on the electrical network connecting the wind farm to the shore. A system which enables one or more offshore wind turbines to operate without a mains grid connection could maintain a wind-powered supply to an array of wind turbines, and therefore safeguard them from damage (for example, due to loss of auxiliary equipment, or collisions with vessels or aircraft). This solution would also permit charging of any back-up batteries, reduce the size of any diesel genset required for back-up and minimise the amount of diesel fuel required to be stored at the turbine.
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CONTENTS
Executive Summary......................................................................................................... iv
1 INTRODUCTION......................................................................................................... 1
1.1 Background ......................................................................................................... 1
1.2 Aims and objectives of the project.................................................................... 1
1.3 Benefits................................................................................................................ 2
1.4 Previous work ..................................................................................................... 3
1.5 Collaborators....................................................................................................... 3
1.6 Project overview ................................................................................................. 3
2 DESIGN CONSIDERATIONS...................................................................................... 5
2.1 Requirements...................................................................................................... 5
2.2 System design philosophy................................................................................. 5
2.3 Frequency and voltage control .......................................................................... 5
2.4 Load management.............................................................................................. 6
2.5 Reactive power compensation and voltage control......................................... 6
3 SYSTEM SIMULATION.............................................................................................. 9
3.1 Wind Turbine ...................................................................................................... 9
3.2 Synchronous Compensator and Automatic Voltage Regulator (AVR) ........... 9
3.3 Control Loads and Distributed Intelligent Load Controllers............................ 9
3.4 Simulations carried out.................................................................................... 10
3.5 Gazelle simulation results................................................................................ 10
3.6 Windmaster simulation results........................................................................ 10
4 DATALOGGING SYSTEM........................................................................................ 13
4.1 Requirements and specification ...................................................................... 13
4.2 Instrumentation and data recording ............................................................... 13
5 20KW DEMONSTRATION USING GAZELLE WIND TURBINE AT SUNDERLAND, TYNE AND WEAR ........................................................................................................... 15
5.1 Gazelle wind turbine......................................................................................... 15
Wind turbine and generator........................................................................................... 15
Existing Gazelle control, sequencing and protection................................................... 15
Soft starter....................................................................................................................... 16
Auxiliary equipment ....................................................................................................... 16
5.2 System design and development.................................................................... 16
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Synchronous generator selection.................................................................................. 16
AVR selection .................................................................................................................. 16
Pony motor...................................................................................................................... 17
Auxiliary equipment ....................................................................................................... 17
5.3 Test results........................................................................................................ 17
Wind-only operation....................................................................................................... 17
Simulated wind-diesel mode ......................................................................................... 17
Starting ............................................................................................................................ 17
5.4 Discussion of Gazelle results ........................................................................... 18
Power quality .................................................................................................................. 18
Wind turbine starting...................................................................................................... 19
Reactive power compensation and rotating losses ..................................................... 19
6 300KW DEMONSTRATION USING WINDMASTER WIND TURBINE AT BLYTH, NORTHUMBERLAND...................................................................................................... 21
6.1 Windmaster wind turbine ................................................................................ 21
Wind turbine details ....................................................................................................... 21
Starting resistors............................................................................................................. 21
Blade pitch control .......................................................................................................... 21
Existing Windmaster control and protection ................................................................ 22
Windmaster auxiliary equipment .................................................................................. 22
6.2 System design and development.................................................................... 22
Synchronous generator selection.................................................................................. 22
AVR .................................................................................................................................. 22
Governing load controllers and control loads .............................................................. 23
Pony motor...................................................................................................................... 23
6.3 Test results........................................................................................................ 23
Wind-only operation....................................................................................................... 23
Simulated wind-diesel mode ......................................................................................... 23
Wind turbine starting...................................................................................................... 23
Rotating losses................................................................................................................ 24
6.4 Discussion of Windmaster results................................................................... 24
Power quality .................................................................................................................. 24
Analysis of starting performance - updated modelling results ................................... 25
Potential for improving system performance during wind turbine starting .............. 26
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Windmaster protection settings .................................................................................... 27
Load control .................................................................................................................... 27
Design assumptions and requirements ........................................................................ 27
7 FINAL DESIGN METHODOLOGY ............................................................................ 28
8 DISCUSSION AND CONCLUSIONS........................................................................ 30
8.1 Synchronous compensator technology.......................................................... 31
8.2 Load controller technology.............................................................................. 31
8.3 Suitability of the Gazelle and Windmaster wind turbines ............................. 31
8.4 Applicability to other wind turbines................................................................ 32
8.5 Recommendations and Further Work ............................................................. 32
9 REFERENCES............................................................................................................ 34
10 APPENDIX A - FIGURES .......................................................................................... 35
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LIST OF FIGURES 1. System configuration
2. Predicted frequency and active power flows from Gazelle computer simulation
3. Predicted voltages and reactive power flows from Gazelle computer simulation
4. Predicted frequency and active power flows from Windmaster computer simulation
5. Predicted voltages and reactive power flows from Windmaster computer simulation
6. Instrumentation requirements
7. Transducers installed
8. Measured frequency and active power flows from wind-only Gazelle operation
9. Examples of control load switching from wind-only Gazelle operation
10. Measured voltages and reactive power flows from wind-only Gazelle operation
11. Measured frequency and active power flows from wind-only Windmaster operation
12. Measured voltage and reactive power flows from wind-only Windmaster operation
13. Measured frequency and active power flows during Windmaster start
14. Measured voltage and reactive power flows during Windmaster start
LIST OF TABLES 1. Frequency and voltage results - wind-only Gazelle operation
2. Frequency and voltage results - wind-only Windmaster operation
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1 INTRODUCTION 1.1 Background
There is a large global requirement for rural electrification as 2 billion people are without electricity. To supply these by extending existing grid systems would require massive investments in transmission and distribution plant in addition to the power stations to feed them. An alternative is to use mini-grids to provide electricity using local indigenous renewable resources.
The UK has pioneered island renewable energy systems, eg Fair Isle and Foula, with distributed load management to enable these grids to operate with 100% wind energy penetration. These projects have been individually successful, but have not been replicated in the global market. It is believed that there are two key reasons why this is so:
• the systems have employed purpose-built wind turbines with synchronous generators rather than standard (grid-connected design) induction generator machines
• the grids cannot successfully accommodate more than one or two turbines.
Recognising this opportunity, Econnect has developed a new generation of Distributed Intelligent Load Controllers to enable mini-grids (island grids, wind diesel systems or autonomous systems) to use one or many standard (grid connected design) wind turbines. Econnect received a SMART award in 1995 and SPUR award in 1997 to undertake this development. The development was supported by two Econnect Eng. D. Studentships at UMIST. The controllers used highly innovative software algorithms to control system frequency within the statutory limits. They underwent extensive modelling as well as successful preliminary site and laboratory tests on wind turbines with induction and synchronous generators.
In order to develop Econnect's role as system integrators for a number of demonstration systems, and hence expand the market for the load controllers, the need to demonstrate the technology with in-service wind turbines was identified.
When developed, this load controller technology also has the potential to provide safe and stable operation of grid-connected embedded generation in islanded mode under network fault conditions. This could offer significant benefits in improving security of supply to customers, particularly in remote rural areas where islanding may be more beneficial.
1.2 Aims and objectives of the project
The main aim of the project was to develop equipment suitable for use in remote locations which would permit islanded operation of wind turbine systems equipped with induction generators. A structured set of objectives was produced as follows:
• demonstrate methods by which induction wind turbine generators can operate in islanded mode, receiving voltage control from synchronous compensators and frequency control from distributed intelligent load controllers
• develop a design methodology for synchronous compensators and apply it to the construction of such units for two test systems
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• site test a 20kw stall regulated wind turbine, to be islanded from the electricity grid and fitted with the above technology
• site test a 300kw pitch regulated wind turbine, to be islanded from the electricity grid and fitted with the above technology
• publish the results and test data to inform relevant parties as to the benefits of mini-grid technology which incorporates these technologies
1.3 Benefits
The main beneficiaries of this project were identified as Econnect, mini-grid developers, distribution network operators, embedded generator owners and remote rural communities. It was envisaged that the project would:
• provide test results to demonstrate the capability of DILCs in providing safe, stable and sustained operation of mini-grids or islanded sections of the interconnected grid with 100% wind energy penetration with high power quality
• provide a design methodology for synchronous compensators, a key component in mini-grid systems using standard grid connected wind turbines and DILCs to achieve sustained and stable 100% wind penetration
• establish Distributed Intelligent Load Control as the preferred option for the developers and operators of mini-grids world-wide to integrate wind energy at high penetration
• raise awareness of the possibilities for operating embedded generation in islanded mode by offering proven technology and test results with wind turbines designed for grid connection
• offer a possible method of island operation, thereby improving security of supply, reducing customer minutes lost (CML) and customer interruptions (CI), improving voltage support and protection discrimination for customers and networks particularly in remote rural areas
• develop collaborations between Econnect and UK companies to bid successfully for development and operation of mini-grid systems using renewable energy
• open up new export markets for UK business
• provide opportunities for reducing lost generation time from embedded generation plant due to network faults
• establish and promote UK expertise in the world-wide mini-grid market and for islanded operation
The use of Distributed Intelligent Load Control technology to control system frequency:
• is cheaper than using wind turbines with inverters and cheaper than energy storage
• breaks the maximum wind penetration threshold considered acceptable by many utilities
• enables cheaper electricity supply in mini-grids due to saving of wear and tear on gensets
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1.4 Previous work
This project was a development of work carried out by Econnect in conjunction with UMIST in December 1999 [1,2]. The aim of these tests was to investigate whether an adequate level of control could be achieved using distributed intelligent load controllers to manipulate a series of resistive loads in order to function as an electronic governor for a wind-diesel system, while running in wind-only mode. The system was located at the Rutherford Appleton Laboratories Energy Research Unit (RAL) in Oxfordshire. The experimental wind-diesel system consisted of a 45kW stall-regulated 3-bladed wind turbine, with rotor diameter 17m, and a 48kW turbocharged diesel generator set coupled to a 68kW/85kVA synchronous alternator via an electromagnetic clutch. The performance was good although some stability problems were encountered. The site tests at RAL provided useful information and demonstrated that the system could operate satisfactorily in the wind-only mode of operation using the load controllers and a synchronous compensator.
The next steps were to demonstrate that this could be achieved on a commercially-available wind turbine, designed for grid-connected operation, and then to show that the system would also work successfully on a larger, pitch-regulated commercially-available wind turbine.
1.5 Collaborators
Support for, and interest in, the project was obtained from a synchronous generator manufacturer (Newage AVK-SEG) and a wind turbine manufacturer (Gazelle Wind Turbines Ltd). Two wind turbine operators kindly permitted their wind turbines to be disconnected from the grid and tested on stand-alone systems (the Business and Innovation Centre, Sunderland, and Blyth Harbour Wind Farm Company). Windcluster had originally agreed to make a wind turbine available for testing, but project timescales did not fit in with repowering of the wind farm in question. Significant technical and logistical assistance was provided by NaREC and by AMEC Wind.
1.6 Project overview
The project incorporated the following activities:
• Selection of appropriate test sites and agreement with wind turbine operators
• Data collection for wind turbines, and discussion with manufacturers
• System design, including
- selection of suitable synchronous compensators
- specification of load control configuration, protection equipment and back-up power equipment
• Dynamic computer modelling of the system
• Specification and procurement of
- load control hardware
- load banks
- protection equipment
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- pony motors and variable speed drives
- cabling
- instrumentation and datalogging equipment
• Load control software development
• System assembly and installation
• Development of datalogging systems
• Production of risk assessment and method statement documents
• Equipment commissioning
• Wind turbine modification
• Wind turbine testing
• Decommissioning of test equipment
• Results analysis
• Dissemination of the results [10,11,12]
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2 DESIGN CONSIDERATIONS 2.1 Requirements
The main technical operating requirements for a wind turbine-based stand-alone electrical power system are to:
• maintain a continuous supply with voltage and frequency within required limits [3]
• operate generating equipment safely and within its limitations, to maximise its operating life
• ensure adequate protection against failures within the system
• maximise use of the available wind power
The solution developed needed to be a low cost, robust and reliable design, with minimal, and straightforward, maintenance requirements.
2.2 System design philosophy
Research into electrical systems has often concentrated on "high-tech" solutions appropriate for use in well-funded, grid-connected applications, and much current development is focussed on static solutions and techniques based on power electronics, such as StatCom [4]. These solutions are less appropriate for a remote location where technical skills are limited, such high-tech equipment is unfamiliar, and technical support and spares from the manufacturer are difficult and time-consuming to access. In this type of environment, where it is envisaged stand-alone wind turbine technology would be of most benefit, experience has shown that straightforward, low-cost and "low-tech" solutions are often most appropriate. It was therefore decided within this project to aim to produce a solution with low-cost, robust, well-established, off-the-shelf equipment, and to preserve as much commonality with the grid-connected wind turbine system as possible, in order to:
• minimise bespoke engineering design and manufacturing work, and hence minimise cost and delivery times
• maximise system reliability
• allow straightforward maintenance and product support from equipment manufacturers
2.3 Frequency and voltage control
In any autonomous power system which employs a rotating generator, the system's electrical frequency is directly proportional to the generator's rotational speed. Constant frequency is maintained by matching the active power supply into the system with the active power drawn from the system by the loads. If this is not achieved, either surplus energy in the system is absorbed in accelerating all the rotating machines, increasing the frequency, or any excess load is supplied from the kinetic energy of the rotating machines, causing a drop in frequency.
A “conventional” autonomous power system, with a dispatchable energy source such as a diesel generator, achieves this balance by controlling the power output using the diesel governor [1]. In a system which relies purely on a non-dispatchable
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renewable energy source like a wind turbine, this balance is achieved by matching the load demand to the variable power input. Although frequency is fundamentally dependent on active power, variation in reactive power flows around the network will modify component losses and hence cause changes in active power.
Voltage stability is dependent on the active and reactive power flows within a system. In a large system, where the line inductive impedance is considerably greater than its resistance, reactive power tends to be the dominant component; in a smaller system like those under consideration here, the line resistance is more comparable with the inductive impedance and both active and reactive power flows affect voltage levels. Voltage, unlike frequency, can also vary at different points around the network, depending on the power flows at each point and the line characteristics between points.
The strategy employed in this project was to use the DILCs and control loads to balance the flow of active power in the system, and hence regulate the system frequency. Reactive power, and hence voltage control, was supplied by a synchronous compensator. The system configuration is shown in figure 1.
2.4 Load management
The load management strategy employed a number of small, individually-controlled deferrable loads, distributed around the mini-grid. This approach is scalable to any size of system. These loads would typically be water or space heaters, switched on and off by individual governing load controllers, developed by Econnect. Each controller sampled the supply voltage waveform and calculated the frequency and rate of change of frequency of the supply. This information was then processed using a “fuzzy logic” algorithm, to produce an appropriate switching decision. Load switching was achieved using a solid state TRIAC device that switched at zero crossing points, ensuring negligible supply distortion.
The global combination of the independent switching decisions from all the load controllers formed a decentralised, dynamic load management system. The fuzzy logic algorithm had an adjustable time delay plus a self-tuning facility, which enabled each individual controller to tailor its decision-making process to improve system stability. The use of multiple small loads produced a controllable load with good resolution. The resulting system was flexible and had high redundancy; it would also permit easy expansion to cope with increased supply capacity. In a typical stand-alone application, deferrable loads around the network would make effective use of the excess wind power.
2.5 Reactive power compensation and voltage control
The test systems retained the existing power factor correction capacitors, and used a synchronous compensator to meet the fluctuating system requirements above the base reactive power provided by the capacitors. Capacitors offered a low cost and efficient way of providing a proportion of the required reactive power, whilst the synchronous compensator exhibited great advantages, such as flexibility of operation under all load conditions, and an essentially inductive source impedance that could not cause harmonic resonances with the network.
It was decided to limit the capacitor rating within both demonstration systems to those already installed for power factor correction. Significant increases in capacitor
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rating would have introduced additional capacitor switching requirements, with additional control equipment requirements and potential reliability concerns. High levels of capacitance connected could also increase the risk of self-exciting the wind turbine and/or reaching the underexcitation limit on the synchronous compensator in the event of some system failures, with associated risks of resonance, overvoltage and component damage.
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3 SYSTEM SIMULATION The first step in the design process was to develop a computer model of each intended autonomous power system. The models were developed using MATLAB / Simulink, and incorporated all the components in the proposed test system by employing Simulink's Power Systems Blockset and Fuzzy Logic Toolbox.
For each system, a mini-grid containing a wind turbine, a Newage synchronous compensator with a backup power source, variable consumer loads, and control loads under the control of Distributed Intelligent Load Controllers (DILCs), was simulated. Variable wind speed inputs and variable consumer loads were applied to investigate the power quality and stability of the system.
3.1 Wind Turbine
The wind turbine was modelled in two sections, the aerodynamic performance and the generator. The windspeed was input from a data file in the form of windspeed v time data. Various sets of windspeed data were used, to investigate the effects of varying mean windspeed and turbulence levels.
A simplified approach to calculating the relationship between windspeed and aerodynamic torque was adopted, which significantly reduced the level of complexity of the model, and permitted valid results to be obtained provided the rotor speed of the wind turbine remained close to its design value.
The aerodynamic torque was applied to a predefined asynchronous machine block, available from the Power Systems Blockset library. The parameters used in the model of the asynchronous machine were obtained from the generator data sheet for each wind turbine [5,6].
3.2 Synchronous Compensator and Automatic Voltage Regulator (AVR)
The synchronous compensator subsystem comprised two major components, the synchronous machine and the AVR.
The models initially employed a simplified synchronous machine block from the Power Systems Blockset library. The simplified synchronous machine block modelled both the electrical and mechanical characteristics of a synchronous machine [7], using parameters from the datasheet for each machine selected.
A simple model of the AVR was developed. The AVR model sensed all three voltage phases and used a mean root-mean-square (rms) value as its input voltage, outputting a field voltage which was applied to the synchronous machine model block.
3.3 Control Loads and Distributed Intelligent Load Controllers
Single-phase control loads were simulated in line with the system design. Each phase had a number of switchable resistive loads, each with its own Distributed Intelligent Load Controller (DILC). The controllers were implemented using the MATLAB Fuzzy Logic Toolbox library [8]. This permitted easy integration of the controllers into the SIMULINK simulations. A more detailed description of the fuzzy control algorithm can be found in [1].
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3.4 Simulations carried out
Simulations were carried out over a wide range of operating conditions. Results for the Gazelle and Windmaster wind turbine systems are described below. Overall the simulations results were encouraging, and gave confidence that the system design would perform satisfactorily when implemented. The results also highlighted areas for investigation during wind turbine testing.
3.5 Gazelle simulation results
Results for the base Gazelle computer model (post-start up) are shown in figures 2 and 3.
The graph of active power in figure 2 shows the predicted control load profile making up the difference between the varying wind turbine input power and the varying base load power requirement. The random nature of the control load switching is clearly demonstrated. This is due both to the variable time delays built into each DILC, and to the fuzzy logic and the random component within the algorithms they use. It is an essential aspect of the use of distributed load control that the controllers act independently and at different times to ensure stable and smooth operation.
Predicted system frequency was maintained between approximately 48 and 52Hz. This was slightly outside the BS EN 50160 recommendation. Previous experimental work [1] had demonstrated better frequency control, but at the expense of rapid controller switching which can result in flicker.
Predicted voltage control (figure 3) was good, well within the required +/-10% of 230V stated in BS EN 50160. The AVR model achieved this with variations in active power input between 7kW and 20kW, and total reactive power requirements fluctuating between 12kVAr and 18kVAr, of which 3-9kVAr was supplied by the synchronous machine. The worst predicted imbalance between two-phase voltages was 7V, or 3% of nominal. Some imbalance would be expected, since all the consumer loads and control loads were independent single-phase loads.
Figure 3 shows the approximately constant reactive power contribution from the capacitors at around 9kVAr, varying slightly as the voltage and frequency varied. The reactive power from the synchronous compensator made up the shortfall between the contribution from the power factor correction capacitors and the reactive power required by the wind turbine and the base loads.
Variations were modelled, with different amounts of base load applied, different ratings of power factor correction capacitors, and different wind speeds. None of the results obtained indicted any cause for concern.
A separate investigation of wind turbine starting, using a model which incorporated a simplified representation of the soft starter, was carried out. This provided some useful pointers regarding the processes that take place during starting, but the approximate nature of the soft starter model meant that its results were not directly comparable with the test results.
3.6 Windmaster simulation results
Results for the base Windmaster computer model (post-grid connection) are shown in figures 4 and 5.
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Predicted system frequency remained between approximately 48Hz and 52Hz. This was slightly outside the BS EN 50160 recommendation. The model showed a much slower response to windspeed fluctuations than the Gazelle model. This was due to the much greater system inertia, and resulted in less frequent controller switching operations.
Figure 4 shows the predicted control load profile making up the difference between the varying wind turbine input power and the varying base load power requirement.
Predicted voltage control was good, within the required ±10% of 230V stated in BS EN 50160. The AVR model achieved this with variations in active power input between 180kW and 300kW, and total reactive power requirements (from the wind turbine and the base load) fluctuating between 100kVAr and 350kVAr. The largest predicted imbalance between two-phase voltages was 20V, or 8.7% of nominal. Some imbalance would be expected, since the control loads were independent single-phase loads.
Figure 5 shows the predicted (approximately constant) reactive power contribution from the capacitors at around 100kVAr, varying slightly as the voltage and frequency varied. The reactive power from the synchronous compensator made up the shortfall between the contribution from the power factor correction capacitors and the reactive power required by the wind turbine and the base loads.
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4 DATALOGGING SYSTEM 4.1 Requirements and specification
A datalogging system was developed to allow the performance of the system to be evaluated. The parameters that were considered to be necessary were system frequency, system voltages, line currents, real power flows and reactive power flows, for all three phases for the synchronous compensator, the wind turbine and the loads, plus wind turbine rotor speed (shown in figure 6). Although proprietary devices existed to measure and log all these parameters, the cost of a sufficiently comprehensive off-the-shelf system was prohibitive. The same information could be obtained from a smaller set of measurements which then allowed the remaining parameters to be calculated. Consequently a modular PC-based system, measuring the minimum number of parameters possible, was specified, to give maximum flexibility at low cost.
4.2 Instrumentation and data recording
Transducers were installed around the system as shown in figure 7. These all produced a galvanically-isolated low voltage signal, suitable for connection to a PC-based data acquisition card, and proportional to instantaneous values of voltage or current as appropriate. It was assumed that a single voltage measurement on each phase would suffice for the entire system. As the voltages were measured close to the current measurements, this would give accurate calculated power flows.
The transducer output signals were fed into a National Instruments DAQ-6024E data acquisition card installed in a Dell Latitude laptop. A National Instruments Labview software application was developed to sample the inputs, carry out some signal analysis, and log frequency, amplitude and phase information for each signal. The data files were post-processed to calculate the required parameters.
On the Gazelle system, digital signals indicating the switching status of the load controllers were also optically isolated and recorded via a National Instruments DAQ-1200 data acquisition card installed in the same laptop.
For the Windmaster testing, an Areva M871 fast transient datalogger was also used to record currents, voltages and calculated parameters.
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5 20KW DEMONSTRATION USING GAZELLE WIND TURBINE AT SUNDERLAND, TYNE AND WEAR
5.1 Gazelle wind turbine
Wind turbine and generator
The Gazelle was a three-bladed, stall-regulated downwind wind turbine rated at 20kW. Its rotor ran at 106rpm, driving the generator via a 2-stage planetary 14:1 gearbox. It was usually connected to a three-phase 400V / 50Hz mains supply. The Gazelle's wind turbine generator was a 4-pole asynchronous Brook Crompton W-DA180LJ motor. Calculations predicted that the Gazelle would have a reactive power requirement of 14.6kVAR at its rated power output of 20kW. Of this, approximately 10kVAR (90% of its no-load requirement) was provided by the power factor correction capacitors after startup.
The Gazelle operated between average windspeeds of 4 m/s and 20 m/s. Estimates suggested that the stall regulation of the Gazelle would be successful in limiting instantaneous power to around 20kW at gust windspeeds below about 35m/s, and this value was used to size the system loads.
Existing Gazelle control, sequencing and protection
Sequencing control and protection for the Gazelle was carried out by a Mitsubishi FX2N-FX32MR PLC.
During its start up the grid-connected Gazelle drew power from the network to motor itself up to its operational speed. Due to its aerodynamic characteristics, it did not receive much aerodynamic assistance until near its operating speed and hence would not usually self-start. Thus in a stand-alone configuration a backup power supply was required to assist the Gazelle. The power factor correction capacitors were connected after reaching synchronous speed and therefore the inrush current of the induction generator was required to be supplied by the grid, or in this case, by the synchronous machine.
The Gazelle's PLC was set up to shut it down under the following conditions:
• low windspeed (identified by motoring of the generator)
• rotor overspeed above 10% (nominal 106rpm)
• supply voltage outside -10%/+6% limits
• excessive vibration
• winding overtemperature
• loss of hydraulic pressure (for the brake supply)
• over-voltage
• under-voltage
• over-frequency
• under-frequency
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Soft starter
The Gazelle was fitted with a thyristor soft-starter, which allowed the voltage applied to the stator to build up gradually when first connecting the turbine to the grid. Typical direct-on-line starting of the Brook Crompton motor would otherwise result in inrush currents of the order of 7.5 times the rated motor current. The voltage ramp applied by the soft starter reduced this stator inrush current and hence the local voltage dip. When the generator terminal voltage reached its full value, the soft starter was bypassed to minimise power dissipation in its power electronic switches. The soft starter generated a poor quality voltage waveform while it ramped up the voltage.
Auxiliary equipment
The Gazelle was equipped with a variety of auxiliary devices requiring power, including a hydraulic pump.
5.2 System design and development
Synchronous generator selection
Discussions were held with Newage about a suitable synchronous machine to use. They advised the use of a slightly oversized machine initially. Consequently a two-bearing 42.5kVA UCI224C machine was selected. The UCI224C weighed 281kg and had dimensions of 792 x 690 x 552 mm.
Efficiency data was provided by Newage, in the form of power dissipated by the machine for varying amounts of reactive power supplied. This suggested that there would be a standing loss of around 700W even when the machine was generating no reactive power at all. During normal operation the synchronous compensator would be providing an average of 4.6kVAr plus consumer reactive power requirements, which would result in the order of 1kW dissipated.
AVR selection
The performance of the AVR was critical to the control, stability, and hence availability, of the entire network. The fact that it controlled the system voltages means that it had the potential to permit equipment damage through overvoltage, and also lose speed control of the wind turbine through undervoltage. Different types of AVR were available, and the AVR selection is outlined below.
A separately-excited permanent magnet (PMG) MX321 AVR was selected for the synchronous machine. The PMG provides a constant supply of excitation power which is not contaminated by the load or influenced by the magnetic field of the generator. This type of AVR is less vulnerable to non-linear loads on the system (in this case, the Gazelle's soft starter) and will maintain a short-circuit current in the event of a short circuit fault, presenting circuit breakers with enough current to effect a trip. The MX321 AVR chosen senses voltages from all three phases to determine the excitation level of the generator, which is advantageous on a system with unbalanced loads present. The quoted voltage control accuracy for the MX321, 0.5%, is well within the EN50160 requirements.
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Pony motor
As the motoring requirements of the Gazelle's generator during start-up were unknown, it was decided to size the pony motor slightly larger than the Gazelle's generator, at 30kW. The pony motor employed was a 30kW ABB induction motor, connected to the shaft of the synchronous machine using a flexible coupling. A 30kW variable speed drive powered from the mains supply was used to control the speed and torque of the pony motor.
Auxiliary equipment
All auxiliary devices on the Gazelle were powered from the synchronous compensator supply.
5.3 Test results
Wind-only operation
Wind-only operation was achieved as shown in figures 8 - 10. The variable speed drive powering the pony motor was shut down, and on some occasions, disconnected, while the system continued to operate powered only by the wind turbine. At this point, the only frequency (speed) control was provided by the load controllers.
No adjustment to the load controller firmware was necessary to achieve stable frequency control in wind-only mode, and no additional adjustment was necessary to the AVR to achieve stable voltage control.
Wind-only mode could be maintained stably for as long as there was enough wind to overcome the losses in the system, mainly the synchronous compensator's rotating losses, but with some losses due to the auxiliary wind turbine equipment (PLC, Pilz protection relay, G59 relay, etc).
Simulated wind-diesel mode
Before wind-only operation was achieved, the system was operated in simulated wind-diesel mode. The variable speed drive would automatically power the pony motor to maintain system frequency if the wind alone was insufficient to maintain the frequency. Once there was enough wind to accelerate the pony motor / synchronous compensator assembly above this setpoint, the drive stopped providing active power to the pony motor, and merely provided excitation current. The pony motor was therefore acting like an infinitely-variable diesel generator.
Starting
Some starting investigations were carried out on the grid-connected wind turbine, to optimise the soft starter settings before trying a stand-alone start. Settings were identified which reduced the current drawn by the wind turbine to reasonable levels.
A wind turbine start-up strategy was then developed using the pony motor. The aim was to ensure that starting of the Gazelle could be carried out without compromising system power quality, ie the synchronous compensator had to provide acceptable power quality before, during and after the start. This was important because the Gazelle needed to be motored up to synchronous speed due to its fixed-pitch rotor blades.
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Starting tests were first carried out with no wind. This was a worst case situation - in normal operation, if there was no wind, the wind turbine wouldn't be started. However it did provide a consistent set of conditions under which to investigate the power requirements of the wind turbine during starting.
In order to ensure that no loads were switched on during wind turbine starting, the motor speed setpoint was set to 1450rpm (48.3Hz).
Start failures due to undervoltage were remedied by increasing the no-load voltage setpoint on the AVR to 240V rather than 220V, still well within statutory requirements.
Start failures due to underfrequency most often occurred as the soft starter was bypassed - this presumably indicated that the switching generated a sudden step change, which caused a demand for too much power. This was alleviated by slowing down the rate at which the turbine was motored up to speed. There may be scope for further adjustments to the soft starter settings to reduce the power requirements further, which would allow a smaller pony motor to be incorporated in a future system design.
5.4 Discussion of Gazelle results
Power quality
An 800-second period of wind-only operation was analysed to evaluate the system's performance. During this period the mean wind turbine power generated was 6.3kW. Table 1 shows mean, maximum, minimum and standard deviation values for frequency and voltage, as these are the main parameters specified in BS EN 50160. The requirements stated in BS EN 50160 are also shown.
Frequency, Hz Phase-to-neutral voltage, V, rms
BS EN 50160 red yellow blue BS EN 50160
Mean 48.59 50 237.1 237.5 237.9 230
Maximum 49.71 51* 244.0 243.51 244.9 253
Minimum 46.98 49* 227.3 227.8 227.9 207
Standard deviation 0.41 - 2.8 2.7 2.8 -
* for 95% of week
Table 1 - Frequency and voltage results - wind-only Gazelle operation
Frequency control was good. Assuming a normal distribution, a standard deviation of 0.41Hz would suggest a variation of +/-0.82Hz for 95% of the time, within the BS EN 50160 requirement of +/-1Hz. There was a definite offset in the control, however, with a low mean value of 48.6Hz, which puts the system as configured outside acceptable limits. However it should be straightforward to modify the load controller firmware to remove this offset.
A slow oscillation in frequency was observed. This is a feature inherent in the use of the distributed load control technique. However the frequency of this oscillation, around 0.4Hz, was well outside the region where flicker would be a problem (8-10Hz).
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A lightbulb was connected to the red phase voltage, to evaluate visually whether there was any flicker generated by the load controller switching. None was evident.
Voltage control was good. The AVR was adjusted to give a phase voltage of 240Vrms with no load on the synchronous machine - this was to provide some margin above the G59 relay's minimum during starting. The BS EN 50160 voltage requirements are fairly wide and the voltage on all three phases remained well within them. As the load control employed single-phase loads, with no restriction on the relative loading on each phase, this was particularly encouraging.
Excessive negative sequence voltages could cause concern about machine winding overcurrent / overheating; however the values achieved, (mean 1.06% of positive sequence voltage, standard deviation 0.1%) were well within the nominal 2% limit in BS EN 50160.
Wind turbine starting
It was an aim of the work to be able to start the wind turbine without adversely affecting the power quality of the system. Although starting the wind turbine did have an effect on frequency and voltage, these effects were within reasonable limits, with a 6.3% voltage drop and a frequency variation between 47.6Hz and 50Hz during the start sequence.
Reactive power compensation and rotating losses
Tests were carried out with three levels of static reactive power compensation: the original 37uF capacitors, no capacitors, and 61.6uF capacitors. The capacitors were switched in after starting and hence provided none of the starting reactive power requirement. The following comments therefore refer only to the "steady-state", post-starting, reactive power requirements.
The original configuration, with 37uF capacitors, resulted in a reactive power requirement from the synchronous compensator between 6kVAr and 10kVAr. The uncompensated wind turbine required between 12kVAr and 16kVAr reactive power from the synchronous compensator. Introducing the larger 61.6uF capacitors reduced the additional reactive power component to between 0kVAr and 7kVAr.
In wind-only mode, with the original capacitors fitted, the synchronous compensator / pony motor assembly absorbed an average of 1.9kW from the system, with a wide variation around this mean. The Newage design data [9] suggested that real power losses, from the synchronous machine only, at around 8kVA would be approximately 1.3kW; the remaining 0.6kW could be accounted for by the additional rotating losses of the pony motor.
The measurements made with different power factor correction capacitors indicated a slight general increase in the synchronous compensator assembly's real power requirement as more apparent power was drawn from the synchronous compensator, in line with the design data provided by Newage.
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6 300KW DEMONSTRATION USING WINDMASTER WIND TURBINE AT BLYTH, NORTHUMBERLAND
6.1 Windmaster wind turbine
Wind turbine details
The Windmaster was a three-bladed, pitch-regulated upwind wind turbine rated at 300kW. Its rotor ran at 50.9rpm, driving the generator via a 3-stage parallel axis gearbox. The generator was connected to the local 11kV / 50Hz mains via a three-phase 11 / 0.380 transformer. The Windmaster's generator was a 4-pole asynchronous ABB machine (wQU 315 L4 DA). It was calculated that the generator would have a reactive power requirement of almost 200kVAr at its rated power output of 300kW - this could approach 230kVAr in gust conditions. Of this, approximately 100kVAr would be provided by the power factor correction capacitors at all times and hence the synchronous compensator would need to provide the remaining 100-130kVAr. Approximately 70% of the no-load reactive power was provided on the low voltage side by power factor correction capacitors.
The Windmaster operated when average windspeed was between 4 m/s and 25 m/s, achieving rated power of 300kW at 13m/s. Local turbulence could result in highly variable instantaneous windspeed and hence highly variable instantaneous power. Above rated windspeed, the wind turbine's power output could easily exceed the rated power for short periods without causing the wind turbine to cut out. This gust power was estimated at 370kW at the Blyth site, based on observations by site engineers, and this defined the total capacity required for the load banks.
Starting resistors
The Windmaster was not fitted with a power electronic soft-starter. Inrush currents were reduced by the use of starting resistors when the generator was first connected to the grid, when its speed reached approximately 1498rpm. At approximately 1500rpm, the resistors were bypassed to avoid overheating and fire, and to minimise power dissipation. In an off-grid system, this facility should reduce the current demand from the wind turbine generator, and hence assist the AVR and synchronous machine to maintain system voltage during wind turbine connection, although this approach is considerably less flexible than employing a power-electronics-based soft starter.
Blade pitch control
The Windmaster, like most wind turbines at this rating and above, had variable-pitch rotor blades, which were adjusted to allow the rotor to self-start, and to regulate the power output above the rated wind speed. This is in contrast with the stall-regulated Gazelle. The pitch-control mechanism introduced a new control mechanism, with potential for interaction with the frequency management strategy of the load controllers. The pitch angles of the three rotor blades were controlled hydraulically. The linear motion of a two-directional hydraulic cylinder, with a stroke of 350mm, was transformed into a rotational motion of up to 90 degrees, by the pitch mechanism.
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Existing Windmaster control and protection
Sequencing control and protection for the Windmaster was carried out by an HMZ Data communications system.
During its start up the Windmaster's generator was not connected to the grid. Its variable pitch blades were angled to provide aerodynamic torque to accelerate the rotor up to its operational speed. When the generator reached approximately 1455rpm, the power factor correction capacitors were connected to the grid. When the generator speed reached approximately 1498rpm, the generator was connected to the grid. The initial connection was made through starting resistors in series with the generator, to reduce the inrush current, and then the resistors were bypassed and the generator was connected directly to the grid.
This sequence meant that the synchronous machine was not required to motor the generator up to its operational speed, and was assisted in exciting the generator because the power factor correction capacitors were connected before the wind turbine generator was connected.
The Windmaster's controller was set up to carry out a shut down on any of the following events (among others):
• rotor overspeed / rotor underspeed
• excessive vibration
• winding overtemperature
• over-voltage / under-voltage
• over-frequency / under-frequency
Windmaster auxiliary equipment
The Windmaster was equipped with a variety of auxiliary devices with various voltage supply requirements. This included a 250bar hydraulic pump supplying the motor for the blade pitch control and the yaw motor.
6.2 System design and development
Synchronous generator selection
In relation to the Gazelle system, a smaller synchronous machine was specified for the Windmaster system (250kVA in comparison with a wind turbine of 300kW), in agreement with Newage. A two-bearing 250kVA HCI434C was selected, with a weight of 885kg and a moment of inertia of 3.35kgm2. Typical efficiency was quoted by the manufacturer as 93% at rated output. This gave a predicted full load loss of 17.5kW, providing an order of magnitude estimate of system losses at around 5% of wind turbine rating.
AVR
As a result of the successful experience gained with the Gazelle system, the same MX321 AVR was selected for the Windmaster testing.
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Governing load controllers and control loads
The increased rating of the Windmaster compared with the Gazelle introduced some extra considerations to be taken into account when designing the control load configuration. It was not possible simply to scale up the Gazelle configuration, due to the limit on the size of load that could be switched by each load controller.
A 250kW variable base load was specified, in addition to a total control load sized at 150kW, which employed seventeen load controllers per phase, switching a range of 1kW, 2kW and 4kW single-phase loads. The 250kW base load fulfilled the role of a consumer load. In addition, two three-phase inductor banks, of 100kVAr and 25kVAr respectively, were employed.
Pony motor
The motoring requirements of the Windmaster's generator during the grid connection process were unknown, but not expected to be significantly larger than the rating of the wind turbine's generator. Therefore the pony motor was sized at 200kW, smaller than the generator but with the ability to deliver higher power for short timescales. The pony motor employed was an ABB induction motor, connected to the shaft of the synchronous machine using a flexible coupling. A 400kW variable speed drive powered from the mains supply was used to control the speed and torque of the pony motor. This high rating was used for availability reasons rather than technical ones.
6.3 Test results
Wind-only operation
Effective wind-only operation was achieved as shown in figures 11 and 12. At this point, the only power input into the system was from the wind turbine, and the only frequency (speed) control was provided by the load controllers.
No adjustment to the load controller firmware was necessary to achieve stable frequency control in wind-only mode, and no additional adjustment was necessary to the AVR to achieve stable voltage control.
Wind-only mode could be maintained stably for as long as there was enough wind to overcome the losses in the system, mainly the synchronous compensator's rotating losses.
Simulated wind-diesel mode
The fact that the frequency setpoint on the variable-speed drive was below the DILC frequency target meant that the motor drive would only power the pony motor if the wind was insufficient to maintain this lower frequency. Once there was enough wind to accelerate the pony motor / synchronous compensator assembly above this speed, the drive stopped providing active power to the pony motor, and merely provided excitation current. If the wind dropped again, the drive provided active power to maintain its speed setpoint. It was therefore acting like an infinitely-variable diesel generator.
Wind turbine starting
Starting the wind turbine on the stand-alone system proved difficult. A successful start required all the components of the system (ie variable speed drive, AVR and
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load controllers) to operate successfully immediately to prevent the wind turbine exceeding any of its protection limits.
A successful start is shown in figures 13 and 14. In order to ensure that no loads were switched on during wind turbine starting, the pony motor speed setpoint was set below the load controller setpoint. The PFC capacitors were switched in at 249.5 seconds; the starting resistors were switched in at 269.5 seconds, and the bypass contactor K1 was closed at 262 seconds.
The events that occurred during the start had almost contradictory requirements. For example, the moment at which the wind turbine was first connected required a large flow of real and reactive power into the wind turbine system to avoid under-frequency and under-voltage. As soon as that first inrush was overcome and the turbine's rotor became supersynchronous, the turbine started to generate. The power input from the variable speed drive then had to be removed, and large amounts of load had to be applied instantly to prevent an overspeed.
Initially the G59 frequency and voltage protection prevented any progress being made past the initial connection of the starting resistors. Temporarily bypassing these limits by employing an alternative G59 relay allowed the bypass contactor K1 to be closed and some successful starts to be achieved. Figures 13 and 14 show the momentary frequency and voltage dips that occurred when connecting the generator. Many starts were terminated just after K1 was closed, due to excessive rotor speed excursions, the limits for which effectively mirrored the frequency settings which had been successfully bypassed. Unfortunately it was not possible to access the control software to modify these limits, which would have been originally set up to accommodate grid-connected operation.
Rotating losses
In wind-only mode, the synchronous compensator / pony motor assembly absorbed an average of 15.6 kW from the system.
6.4 Discussion of Windmaster results
Power quality
Only a short period of wind-only operation was achieved with the Windmaster due to the restrictive rotor speed limits within the controller. This period of operation has been analysed to evaluate the system's performance. During this period the mean wind turbine power generated was 41.3 kW. Table 2 shows mean, maximum, minimum and standard deviation values for frequency and voltage, as these are the main parameters specified in BS EN 50160. The requirements stated in BS EN 50160 are also shown.
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Frequency, Hz Phase-to-neutral voltage, V, rms
measured BS EN 50160
red yellow blue BS EN 50160
Mean 50.32 50 219.0 217.9 217.1 230
Maximum 50.82 51* 219.9 218.8 218.2 253
Minimum 49.58 49* 218.0 216.7 215.5 207
Standard deviation 0.23 - 0.397 0.295 0.517 -
* for 95% of week
Table 2 - Frequency and voltage results - wind-only Windmaster operation
Frequency control was good. Assuming a normal distribution, a standard deviation of 0.23Hz would suggest a variation of +/-0.46Hz for 95% of the time, within the BS EN 50160 requirement of +/- 1Hz. The mean value of 50.32Hz means that the overall frequency level was within statutory limits.
A slow oscillation in frequency was observed. This is a feature inherent in the use of the distributed load control technique. However the frequency of this oscillation, around 0.4Hz, was well outside the region where flicker would be a problem (8-10Hz).
Voltage control was good. The AVR was adjusted to give a phase voltage of 220Vrms with no load on the synchronous machine, to meet the requirements of the wind turbine. The BS EN 50160 voltage requirements are fairly wide and the voltages on all three phases remained well within them. As the load control employed single-phase loads, with no restriction on the relative loading on each phase, this was particularly encouraging, particularly in view of the predictions regarding voltage imbalance obtained from the computer model. Negative sequence voltages were well within the nominal 2% limit in BS EN 50160 (mean 1.1% of positive sequence voltage, standard deviation 0.32%).
Analysis of starting performance - updated modelling results
The Simulink model was updated so that its behaviour reflected the observed results more closely, in order to investigate the observed starting performance of the wind turbine in more detail.
• It was found that the original version of the model predicted a system voltage collapse at the moment of connection of the wind turbine. This was not reflected in the observed performance of the system, and so the simplified synchronous machine model was replaced with a full synchronous machine model, and the AVR model was replaced with the Power Systems Blockset Excitation block. Where specific machine parameters were not available for the HCI434C, the default per-unit values were employed.
• A delay was added into the diesel model to allow for its response time in reacting to a frequency dip.
• Additional measurements were incorporated to allow the power consumption of the starting resistors to be evaluated.
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The updated model's behaviour during starting was qualitatively correct, however it did not replicate quantitatively the exact frequency and voltage response of the system, most likely due to the simplifications in the AVR and pony motor controller models. The frequency dip and the overall voltage drop during the start were underpredicted; in contrast, the model suggested that application of out-of-balance single-phase loads would result in larger voltage imbalances between the phases than were seen in practice. The updated model was therefore used to provide an overall indication of the processes involved during wind turbine starting, and to estimate the effects of modifying the starting strategy.
The model indicated that the majority of the power drain on the system during the start was due to the starting resistors, which absorbed a peak of around 200kW, whereas the maximum motoring power required by the generator was only around 50kW. The peak current flow seen in the model was 480A. These values would vary depending on the assistance provided by aerodynamic torque at the moment of connection. Running the model with the starting resistors bypassed gave a predicted maximum current flow of 512A, a peak power drawn by the wind turbine generator of 80kW, and a 50% reduction in the predicted frequency excursion.
The immediate conclusion from these observations is that the resistors appeared to be generating as large a problem as they solved for the stand-alone system - they reduced the instantaneous current demand of the system, but greatly increased the real power demand placed on the backup power source.
The model suggested that complete removal of the starting resistors would be likely to result in a smaller underfrequency but a larger voltage drop (though this would depend on how much the observed voltage drop was a consequence of the under-frequency roll-off protection in the AVR). The risk associated with such a starting strategy might be that in high winds there would be insufficient voltage at the moment of wind turbine generator connection to "pull in" the wind turbine rotor and prevent an overspeed.
Potential for improving system performance during wind turbine starting
A number of options exist for improving starting performance.
• Replacement of the starting resistors with a soft-starter (as was used in the Gazelle testing and is employed in all modern induction-generator wind turbines). This would limit the voltage applied to the generator terminals without affecting the mini-grid system voltage, and hence reduce the inrush current requirement, without increasing the real power demand on the system. Sequencing of the power factor correction capacitors would probably need to be modified in order to avoid interaction with the soft starter.
• Increasing the inertia, and hence the amount of stored rotational energy, within the system. The simplest way to do this would be by connecting a flywheel onto the synchronous compensator's shaft.
• Improving the speed and magnitude of the response of the backup power source. This is likely to be a more expensive and risky method than the previous two suggestions, as it deals with the symptoms rather than the cause of the problem. It might necessitate an upgrade of the synchronous compensator.
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Windmaster protection settings
The Windmaster controller's speed, frequency and voltage limits proved restrictive in operating the stand-alone system. Adapting a grid-connected wind turbine to operate within a stand-alone system is likely to require some modification of protection limits. This is difficult in the case of the Windmaster, as the controller is old and the manufacturer does not exist in their original form.
Load control
The original design of the Windmaster system allowed for a combination of control load plus base load. It became apparent during testing that the transient events during starting, in conjunction with large and rapid variations in wind speed, required a larger proportion of controllable load to be available in order to regulate system frequency during connection of the wind turbine. In an improved design, some of the control load could be of larger magnitude than the load-controller-managed loads, and employ relays to carry out overfrequency load adding and underfrequency load shedding in a slower manner than the dynamic control provided by the load controllers.
Design assumptions and requirements
The satisfactory performance of the system vindicates the design assumptions made.
Assuming that the system modifications discussed above would be successful in reducing the starting power requirements, then it should be possible to derate the backup power source (diesel) to a suitable size for the essential system loads.
The configuration tested here had the backup power source directly coupled to the synchronous compensator. An alternative, probably preferable, solution is to operate a separate diesel genset (with its own synchronous generator) on the system, and use it to power a small electric pony motor to accelerate the synchronous compensator to synchronous speed when the wind turbine is to be started. This allows both synchronous machines to operate in parallel in wind-diesel mode if necessary, and the diesel to be shut down in wind-only mode, to minimise losses without using a clutch.
The synchronous compensator appeared to be sized adequately for steady-state wind turbine operation. The synchronous compensator maintained satisfactory system voltage when the wind turbine was running. A larger machine might have been preferable in view of the temporary reactive power absorption requirement, and the current inrush, observed on starting. However these requirements are both specific to the Windmaster turbine. They would probably be surmountable if developing the Windmaster design, and may not apply in the case of alternative wind turbines. It is not obvious that the HCI434C synchronous compensator would be undersized in a fully-optimised Windmaster system, but it can probably be concluded that it would not be advisable to select a smaller machine.
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7 FINAL DESIGN METHODOLOGY Design of a given synchronous compensator system will be very specific to the situation under consideration, depending heavily on whether a system is being designed from scratch, or a wind turbine is being retrofitted into an existing mini-grid. It is however possible to generalise about the processes involved. The following activities are likely to be required in all cases.
1. Obtain wind turbine information including:
• power output and method of regulation (stall or pitch)
• generator rating and mechanical and electrical machine parameters, including operating voltage
• power factor correction capacitor rating and sequencing
• type of soft-starter installed, and start sequencing
• shutdown sequencing and protection limits
(Close collaboration with the wind turbine manufacturer at all stages of the project will be extremely helpful in evaluating the implications of operating the proposed wind turbine in stand-alone mode and carrying out any modifications required.)
2. Evaluate wind turbine reactive power requirements and motoring requirements during starting (if possible, measure on a grid-connected machine). Specify suitable soft-starter if not already included in wind turbine package.
3. Obtain mini-grid system information including:
• details of existing and planned back-up power supplies (diesels / batteries etc) and control systems
• type and size of available control loads
• type, size and priority of existing and planned consumer loads
• operating voltage
4. Select appropriate synchronous compensator type(s) in line with active and reactive power requirements (both absorbing and generating) and complementary power supplies. A permanent magnet AVR with three-phase sensing was employed successfully here.
5. Identify a suitable control load configuration. A good starting point would be 50% of wind turbine rating on each phase, comprising between five and ten small resistive loads, each with its own load controller. The load sizes would be dictated by the load controllers' switching capacity and the wind turbine rating.
6. Carry out computer modelling to assess the performance of the proposed system, paying particular attention to system moments of inertia and frequency stability. Confirm synchronous compensator selection.
7. Integrate synchronous compensator equipment and wind turbine into mini-grid design.
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31
8 DISCUSSION AND CONCLUSIONS 8.1 Synchronous compensator technology
The use of Newage's generators as a synchronous compensator with the stand-alone wind turbine system was a relatively novel use of the equipment in comparison with Newage's standard diesel genset experience. Discussion with Newage's technical support department proved invaluable in assessing the technical implications of the proposed application and selecting appropriate machines. In both test situations the synchronous compensators and AVRs selected performed well with minimal adjustment to AVR settings, justifying the selection of this well-established and robust technology. These generators compare well with power electronics alternatives in terms of cost.
8.2 Load controller technology
The load control system demonstrated its ability to regulate system frequency within statutory limits for stand-alone wind turbine systems. No reprogramming of load controller firmware was necessary during testing, as the flexibility provided by the dip switches on each load controller board permitted easy adjustment of the necessary parameters.
Frequency-sensitive controllable load up to the full rating of the wind turbine was felt to be essential, even if only to accommodate the large power swings during starting.
8.3 Suitability of the Gazelle and Windmaster wind turbines
In both cases the system design was carried out and implemented with a large amount of assistance from people who were extremely familiar with the wind turbine in question. The detailed information provided was essential to achieving successful operation of the stand-alone system for both machines.
As intended during the design stage, minimal modifications were made to the wind turbine systems to allow them to operate in stand-alone mode. The Windmaster's requirement for a 24-hour power supply to the controller and protection systems, to ensure its safety when shut down, complicated its conversion into a stand-alone machine. The Gazelle did not have this requirement as its rotor was held stationary by its brakes when not connected to a voltage supply.
For both wind turbines it was necessary to widen out the frequency and voltage limits imposed by their protection systems. These limits are imposed to allow an islanded condition (or effectively, grid failure) to be identified so that the wind turbine can be disconnected. This protection is part of the connection requirements defined by the network operator, and therefore is not strictly part of the wind turbine's intrinsic protection. Widening these limits was straightforward with the Gazelle, as it employed a frequency and voltage measurement relay with adjustable settings, separate from its controller. For the Windmaster, the frequency, voltage, and rotor speed limits were hard-coded in its controller, and so a less temporary stand-alone system would require modifications to the controller software, agreed in conjunction with the manufacturer.
The experience with starting the wind turbines suggested that a power electronic soft starter to limit the inrush current is essential in a stand-alone system if the back-up power system and synchronous compensator are not to be grossly oversized to cope
32
with the starting requirements. Most modern induction generator wind turbines have soft starters, but older machines like the Windmaster would need one to be installed in place of the starting resistors.
Successful operation of the stand-alone system was achieved with both wind turbines. In areas where operability was limited, straightforward improvements have been identified. Neither machine was felt to be fundamentally unsuitable for stand-alone operation. Adaptation to stand-alone operation would ideally be carried out in conjunction with the manufacturer, which makes the Windmaster less suitable for further development.
8.4 Applicability to other wind turbines
The experience gained with the Gazelle and the Windmaster provides confidence that most wind turbines with induction generators could be adapted for use in a stand-alone mode, using a synchronous compensator and distributed load control. The concept is applicable for any size of wind turbine.
Wind turbines with power electronic grid interfaces might not require synchronous compensator technology to maintain stand-alone grid voltages (depending on their design), but could employ distributed load control to manage system frequency and make most efficient use of any surplus wind power.
The system developed here could also be used to allow existing diesel-generator-based systems to incorporate one or more wind turbines with ratings comparable to, or even significantly larger than, the diesel set's nominal kVA rating.
8.5 Recommendations and Further Work
This project has clearly demonstrated the suitability of distributed load control for controlling system frequency in stand-alone wind turbine systems. It is recommended that the technologies employed here are developed for full commercial application.
The methodology developed from this work will produce robust system designs with a high probability of successful operation with minimal development work or modifications on site. The next step would be to demonstrate long-term operation of such a system, successfully integrated into an existing stand-alone diesel-based mini-grid. This will require further development of the back-up power supply and the accompanying supervisory system. A diesel generator with a clutch to allow its alternator to act as the synchronous compensator or a pony motor powered by an existing mini-grid are both viable alternatives.
Although the fundamental governing operation of the load controllers does not require any communication between the load controllers, integration into a system which uses more than one type of generation source might require some communications ability for supervisory control purposes. This aspect of the load controllers is currently under development and has been successfully demonstrated with hydropower and photovoltaic systems. A full system demonstration would consolidate this work.
The system concept has potential for use on offshore wind turbines in the event of an outage due to a fault on the electrical network connecting the wind farm to the shore. A system which could allow one or more offshore wind turbines to operate without a
33
mains grid connection could maintain a wind-powered supply to an array of wind turbines, and therefore safeguard them from damage (for example, due to loss of auxiliary equipment, or collisions with vessels or aircraft). This solution would enable the charging of any back-up batteries, reduce the size of any diesel genset required for back up and minimise the amount of diesel fuel required to be stored at the turbine.
34
9 REFERENCES 1 P.Taylor, N Jenkins, “The Development and Application of Distributed Fuzzy
Load Control to an Autonomous Wind Diesel System”. Proceedings of EWEA Special Topic Conference “Wind Power for the 21st Century”, Kassel, Germany, September 2000, pp290-293
2 P. Taylor, "Distributed Intelligent Load Control of Autonomous Renewable Energy Systems", EngD thesis, UMIST / Econnect
3 BS EN 50160 : European Standard, EN50160, “Voltage Characteristics of electricity supplied by public distribution systems.” CENELEC, November 1994
4 A. Collinson, F. Dai, A. Beddoes, J Crabtree (EA Technology Ltd), "Solutions for the connection and operation of distributed generation", DTI New & Renewable Energy Programme, July 2003
5 Data sheet provided by Brooke Crompton, 4 July 2002
6 Asea Brown Boveri ABB Elektromotoren GmbH, Asynchronous Generator Specification, Type wQU 315 L4DA, EMO/ME 14th January 1992.
7 Mathworks Matlab SimPowerSystems User Guide
8 Mathworks Matlab Fuzzy Logic Toolbox User Guide
9 Newage International “Technical Reference Manual”. Edition 4 / 2000.
10 R. Kemsley, "Voltage and Frequency Control for High-Penetration Stand-Alone Wind Power Systems" (poster), European Wind Energy Conference and Exhibition, Madrid, Spain, 16-19 June 2003
11 R. Kemsley, "Control solutions for stand-alone wind systems", BWEA25 - Wind Connections, Glasgow, UK, 28-30 October 2003
12 R. Kemsley, A. Maloyd, P. Taylor, "Islanded operation as a means of safeguarding offshore wind turbines under fault conditions", MAREC 2004: 3rd International Conference on Marine Renewable Energy, Blyth, UK, 7–9 July 2004
35
10 APPENDIX A - FIGURES
~
~
wind turbine
bus bar
induction generator
synchronous compensator
load
distributed intelligentload controllers
load
load
load
load
AVRexcitation
Key : active power
reactive power
measuredvoltage
Power factor correction capacitors
load
load
control loadsdistributed
between phases
consumerloads
Figure 1 - System configuration
36
0 2 4 6 8 10 12 14 16 18 200
10
20
30
40
50
60
Frequency, Hz
0 2 4 6 8 10 12 14 16 18 20-20
-10
0
10
20
kW
time, seconds
Wind turbine powerSynch. comp. powerControl load powerBase load power
Figure 2 - Predicted frequency and active power flows from Gazelle computer simulation
37
0 2 4 6 8 10 12 14 16 18 200
50
100
150
200
250
300
V
0 2 4 6 8 10 12 14 16 18 20-10
-5
0
5
10
15
time, seconds
kVAr
VblueVyellowVred
Wind turbine reactive powerSynch. comp. reactive powerControl load reactive powerBase load reactive powerPFC caps. reactive power
Figure 3 - Predicted voltages and reactive power flows from Gazelle computer simulation
38
0 2 4 6 8 10 12 14 16 18 200
10
20
30
40
50
60
Freq
uenc
y, H
z
0 2 4 6 8 10 12 14 16 18 20-300
-200
-100
0
100
200
kW
time, seconds
Wind turbine powerSynch. comp. powerControl load powerBase load power
Figure 4- Predicted frequency and active power flows from Windmaster computer simulation
39
0 2 4 6 8 10 12 14 16 18 200
50
100
150
200
250
300
V
VblueVyellowVred
0 2 4 6 8 10 12 14 16 18 20-150
-100
-50
0
50
100
150
time, seconds
kVAr
Wind turbine reactive powerSynch. comp. reactive powerControl load reactive powerBase load reactive powerPFC caps. reactive power
Figure 5 - Predicted voltages and reactive power flows from Windmaster computer simulation
40
E
variablespeeddrive
fused mains supply from transformer
synchronous compensator
pony motor
stand alonemini-grid
AVR
Manualswitch
N3ph
VSD fuse switch
Generator circuit breaker
to wind turbine
Load 1circuit breaker
Load 2circuit breaker
Load 3circuit breaker
to load banks
Auxiliary supplyfuse switch
Wind turbinecircuit breaker
WIND TURBINECURRENTSVOLTAGES
POWERREACTIVE POWER
WIND TURBINEROTOR RPM
LOADCURRENTSVOLTAGES
POWERREACTIVE POWER
SYNCHRONOUS COMPENSATORCURRENTSVOLTAGES
POWERREACTIVE POWER
Figure 6 - Instrumentation requirements
41
E
variablespeeddrive
fused mains supply from transformer
synchronous compensator
pony motor
stand alonemini-grid
AVR
Manualswitch
N3ph
VSD fuse switch
Generator circuit breaker
to wind turbine
Load 1circuit breaker
Load 2circuit breaker
Load 3circuit breaker
to load banks
Auxiliary supplyfuse switch
Wind turbinecircuit breaker
INSTANTANEOUSCURRENTS
INSTANTANEOUSVOLTAGES
INSTANTANEOUS CURRENTS
ROTOR RPM
INSTANTANEOUSCURRENTS
Figure 7 - Transducers installed
42
-20
-15
-10
-5
0
5
10
15
20
440 450 460 470 480 490 500time, seconds
pow
er, k
W
0
5
10
15
20
25
30
35
40
45
50
frequ
ency
, Hz
wind turbine power load power synch. comp. power system frequency
frequency
Figure 8 - Measured frequency and active power flows from wind-only Gazelle operation
43
300 310 320 330 340 350 360time, seconds
load 1, red phase
load 5, red phase
load 1, yellow phase
load 5, yellow phase
load 1, blue phase
load 5, blue phase
Figure 9 - Examples of control load switching from wind-only Gazelle operation
44
-15
-10
-5
0
5
10
15
440 450 460 470 480 490 500time, seconds
reac
tive
pow
er, k
VAr
0
50
100
150
200
250
phas
e-ne
utra
l vol
tage
, Vrm
s
wind turbine reactive power load reactive power synch. comp. reactive powerVred Vyellow Vblue
Figure 10 - Measured voltages and reactive power flows from wind-only Gazelle operation
45
-100
-50
0
50
100
150
560 570 580 590 600 610 620time, seconds
Pow
er, k
W
5
10
15
20
25
30
35
40
45
50
55
freq
uenc
y, H
z
wind turbine power total load power synch. comp. power system frequency
mot
orin
gge
nera
ting
Figure 11 - Measured frequency and active power flows from wind-only Windmaster operation
46
-150
-100
-50
0
50
100
150
200
250
300
550 560 570 580 590 600 610 620
time, seconds
volta
ge, V
rms
0
30
60
90
120
150
180
210
240
270
phas
e-ne
utra
l vol
tage
, V
wind turbine system reactive power load reactive power synch. comp. reactive powerV d V ll Vbl
gene
ratin
gab
sorb
ing
voltages
Figure 12 - Measured voltage and reactive power flows from wind-only Windmaster operation
47
-350-300-250-200-150-100
-500
50100150200250300350400450500550600650700
249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269time, seconds
kW /
kVA
r
10121416182022242628303234363840424446485052
freq
uenc
y, H
z
wind turbine system power load power synch. comp. power frequency
mot
orin
gge
nera
ting
Figure 13 - Measured frequency and active power flows during Windmaster start
48
-350-300-250-200-150-100-50
050
100150200250300350400450500550600650700
249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269
time, seconds
kVA
r
-800-750-700-650-600-550-500-450-400-350-300-250-200-150-100-50050100150200250
phas
e-ne
utra
l vol
tage
, Vrm
s
wind turbine system reactive power load reactive power synch. comp. reactive powerVred LEM rms Vyellow LEM rms Vblue LEM rms
gene
ratin
gab
sorb
ing
Figure 14 - Measured voltage and reactive power flows during Windmaster start
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