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Power System Studies/Power System Analysis/Engineering
Power System Studies refers to the study of power evacuation from generation to loads, under control, protection and supervision; and under normal or contingency conditions; under various expected operation scenarios;capturing the behaviourof the electrical network, its elements, its control, protection and their responseunder different time frames, spanning few a micro seconds to several hours or even years of time.These studies may also be classified asStatic, Dynamic or Transient,depending on the mathematical models used in analysis and the time frame of examination of the behavior of the power system under consideration.The nature of the studies and their objectives may vary for different types of electrical network [or power systems] and the problems being analyzed, with possible different criteria. Thus a study for a transmission system , a distribution system or an Industrial network may not all have identical perspective, even though the type of analysis modules used in analyzing them are same.These studies may fulfil the objectives ofsystem planning, system design, system protection and control, developing system operation strategies, commercial and technical evaluation and feasibility studies, solutions for problems faced during system operation.These studies are pre-requisites forany new system, for any new renovation, modernization and expansion plans, and also for existing systems for arriving at solution for problems faced in the operation.As a rule, it is mandatory to perform the power system studies, where interconnection of two different systems are proposed. For example, interconnection of a industrial load to a distribution company, requires that standard set of studies to be performed, typically covering, the load flow, short circuit, relay coordination, harmonic analysis , motor startig studies and stability studies as applicable.List of Power System Studies/AnalysisThe power system study group have performed widest possible range of power system studies as follows. The studies cover planning, engineering, economic aspects of power system, design, operation, control and protection and uses appropriate static, dynamic or transient study models for power systems.Power Flow Studies.
Short Circuit Studies (Conventional/IEC909/ANSI/IEEE/G74).
Contingency Studies [Ranking and Evaluation].
Optimal Power Flow.
Reactive Power Optimization.
Capacitor Locations and Sizing.
Static/Dynamic Voltage Stability Analysis.
Transient Stability Analysis (Large Signal Performance).
Dynamic Stability Analysis (Small Signal Performance).
Power System Stabilizer Applications.
Protection System Studies (Overcurrent phase and earth fault, High set, Differential, Distance, Frequency, Voltage).
Equipment Protection Applications (Transformers, Transmission lines, Motors, Generators, Bus Protection).
Harmonic Measurements, Analysis and Filter Design.
Switching Transient Studies.
Insulation Coordination.
Motor Starting Studies.
Evaluation of energy transactions in de-regulated market. Energy pricing, Wheeling and banking charges, Transmission and Distribution Pricing, Grid Support charges.
Ground Mat Design.
Energy Audit Services.
Reliability Evaluation.
Long term energy and demand forecast along with associated system/finance/commercial planning.
EMTP, Line Constants, Parameter Evaluation, Insulation Coordination.
Power quality related studies
Power evacuation studies
Switch Yard and Substation Design
1) Load Flow/Power Flow Studies and Analysis
Power flow analysis/studies is the preliminary step used in anyPower Evacuation Studies.Power flow calculations provide active and reactive power flows and bus voltage magnitude and their phase angle at all the buses for a specified power system configuration and operating condition subject to the generationand/orregulating capabilities of generators, synchronous condensers, static var compensators, HVDC controls, FACTS controllers, tap changing under load transformers and specified net interchange between individual operating systems (utilities). This information is essential for the continuous evaluation of the current performance of a power system and for analyzing the effectiveness of alternative plans for system expansion to meet increased load demand. These analyses require the calculation of numerous power flow cases for both normal, and emergency (contingency) operating conditions. The output from power flow studies often provide the initial conditions needed for other analysis, such as short circuit studies, transient stability, economic dispatch, dynamic stability studies.
Applications of Power Flow Study and Analysis
Power flow study has the following applicationsTransmission expansion planning , operation planning
Distribution expansion planning , operation planning
Industrial/Commercial distribution system planning, operational planning
Network interconnection, Grid interconnection studies
Evaluation of energy transactions between various stake holders
Energy audit to accurately determine network losses and estimate billing losses if any
Sizing of transformers, cables, overhead lines, transformer tap ranges, shunt capacitors, shunt reactors, reactive power management, FACTS devices, HVDC operation
System security assessment via static contingency studies
Decision making tool in operation planning and operation of the system in load dispatch center
Motor starting studies using load flow type analysis, where the starting impedance of the Induction motor is modeled as constant impedance model with starting impedance.
Evaluation of static voltage stability using load flow technique
The following general criteria of acceptability of design is used in power flow studies
Voltage Drop at all buses should be within +/- 5% of the nominal rating for all operating conditions considered
No over load conditions of any electrical circuits for all operating conditions considered
Reactive power generation/import/export to be within specified limits for all operating conditions considered
Ensuring qualitypower supply to all loads, under specified contingency conditions, as per design philosophy adopted.
The following study cases/ power flow outputs are generally considered in power flow studiesExtreme operating conditions of maximum and minimum loading conditions will be considered to check the adequacy of the network, even though some of these conditions may not exist during normal operation
Contingency conditions such as outage of lines, transformers, generators will be considered and network adequacy for power evacuation will be assessed
Operating solutions suchas transformer taps, Generator Excitation, shunt reactive power compensations will be provided as needed.
Recommendations for strengthening and equipment upgradations will be provided to meet specific operating requirements.
Summary of load flow studies and concise reports in tabular formats and single line diagram formats will be provided, along with the summary of recommendations
Contingency Ranking and Evaluation - System Security and Adequacy Evaluation
Contingency evaluation studies typically refers to evaluation of network adequacy and security under credible network element outage conditions. Typically outage of important transmission lines , transformers, generating units are usually considered in the evaluation. The evaluation is carried out by using static as well as dynamic analytical tools such as load flow analysis and transient stability analysis. Real time control and monitoring solutions in enercy control centers or energy management systems or load dispatch centers usually use an algorithm called contingency ranking algorithm to shortlist credible contingencies for real time evaluation and control of power systems. Often contingency ranking algorithm will use some approximate and fast load flow type algorithms from a list of contingencies and rank them in the decreasing order of severity. This ordered or ranked list will be considered for a detailed contingency evaluation to assess system security.
General Features of PowerApps Power Flow Software
Power Flow is the analysis module of PowerApps dedicated to power flow analysis in three-phase electric power networks. It is equipped with powerful analytical options and alternative solution techniques.Gauss-Seidel.
Newton-Raphson.
Fast-Decoupled.
AC/DC Load Flow, FACTS devices.
Load flow solution of multiple-islanded systems. The solution is available for each of the islands having a reference (slack) node. The reference node is automatically identified by the algorithm as the largest generator node in each island.
Limit violation reports, summary reports.
Unbalalanced 3 phase load flow, including 1 and 2 phase load flow for lines drawn separately from a 3 phase supply point.
Choice of objectives for the OPF/RPO (Transmission loss minimization, Voltage Stability improvement, Removal of operating violations, Economic dispatch).
Optimal Load Flow.
OPF/RPO control options are active power injections, reactive power injections, shunt compensations, series compensations, phase shifters, transformer taps.
OPF/RPO sensitivity calculations with respect to the performance objective provides information for suitable location of shunt reactive power compensation and also identifies most effective controllers for optimization.
No limits on the number of study cases and related reports in a single execution of the program
2) Short Circuit Studies, Fault Calculations
Short circuit calculations provide currents and voltages on a power system during fault conditions. This information is required to design an adequate protective relaying system and to determine interrupting requirements for circuit breakers at each switching location. Fault conditions can be balanced or un-balanced shunt faults or series (open conductor) faults. Often information about contributions to a fault from rotating machines such synchronous machines, large motors would be required as a function of time to determine making and breaking requirements. Fault calculations may consider or ignore pre-fault power flow conditions. Short Circuit is the PowerApps analysis module dedicated to simulating fault conditions in three-phase electric power systems. User friendly data entry, a multitude of reports and flexibility in applying all industry-accepted standards are features that make it an Indispensable tool for these very common and important system studies. PowerApps Short Circuit Module adheres to North American ANSI C37.5, ANSI C37.010, ANSI C37.13 and International IEC-60909 guidelines. It also supports conventional short-circuit studies without reference to any particular standards.Short circuit studies provide post fault bus voltages at different busbars in the network for a fault at any one of the location in the network. These results are typically given as fault MVAs, fault currents in kA at different bus barsand fault contributions from adjacent bus sections to the fault, on a single line diagram for various operating conditions. Short circuit studies for minimum fault level condition at the main switch board may be of interest in relay coordination to check, whether relays can distinguish between the maximum load currents and minimum fault currents. In the event, the minimum fault currents in the relays are very close to the maximum load currents, it may be necessary to suggest voltage restraint for relays to ensure that the relays will operate only for fault conditions and not for healthy full load conditions.The deliverables from the short circuit studies will include the following-Tabular report of conventional short circuit levels at all buses-Tabular reports of Making/Breaking Current levels at all the buses-Report on single line diagrams showing fault levels, fault kA for both conventional and IEC 60909 type calculations-Recommendations with respect to operating strategy, to limit short circuit levels where needed
General Features of Short Circuit Study/Calculation Software
Fault levels for asymmetrical and symmetrical faults including bolted faults.
ANSI/IEEE standards.
IEC standards including 363 and 909.
G74 British standard, a computer algorithm based standard for IEC 909 standard. IEC 909 standard specified multiplication factors based on hand calculation procedures and simplifying assumptions.
Short circuit analysis of multiple-islanded systems with solution for each of the islands.
Default flat 1.0 pu positive sequence bus voltage based calculations.
Option to consider pre-fault bus voltages from load flow along with the sequence impedances for loads.
Automatic one line diagram creation.
Multiple case studies in single execution of the program for different network configurations and/or different source impedances or ratings.
Automatic generation of reports for all the specified study cases on the single line diagram.
Induction motor models.
Fault calculations for network with multiple islands with sources in each island.
Detailed system wide post fault bus voltages and flows for specified bus faults along with impedance seen at each relay locations.
Output contains, detailed phase quantities, sequence quantities of voltages, currents, driving point impedances, transfer impedances, contribution from sources, and contribution from adjacent buses.
Results of fault calculations with mutual coupling matches perfectly with published examples.
Reactive Power Optimization[RPO], Optimal Power Flow [OPF], Economic Dispatch[ED], Available Transfer Capability [ATC] Calculations
The power flow solution calculates power flows and determines
bus voltages at an operating point for a given network
configuration and generation and load specifications. However, it
is left to the engineering judgement of the system planner to
determine optimum way of system operation considering- Operating
objectives- Operating constraints [Commercial and Security
Constraints] and- Equipment capability constraints.Such an exercise
using load flow tool is very tedious and time consuming for a
practical power system with large number of operating controls and
constraints.A properly designed optimal power flow [OPF] solution
provides the best and most optimum practical solution to achieve
improvement in a single or multiple hierarchical objectives while
respecting various constraints on the system operation. An OPF can
determine the most effective subset of controls and their solution
for a given operating condition to improve the specified
objectives. OPF can consider different objectives for improvement
such as transmission loss minimization, voltage stability
improvement and minimization of system operating cost.
OPF/RPO analysis module of PowerApps is based on the Primal-dual LP
programming approach and has the following features:Newton-Raphson
load flow for solution at an operating point.
OPF/RPO solution of multiple-islanded systems. The solution is available for each of the islands having a reference (slack) node. The reference node is automatically identified by the algorithm as the largest generator node in each island.
Choice of objectives for the OPF/RPO (Transmission loss minimization, Voltage Stability improvement, Removal of operating violations, Economic dispatch, ATC calculations).
Optimal load flow as per selected objectives and specified constraints
OPF/RPO control options are active power injections, reactive power injections, shunt compensations, series compensations, phase shifters, transformer taps.
OPF/RPO sensitivity calculations with respect to the performance objective provides information for suitable location of shunt reactive power compensation and also identifies most effective controllers for optimization.
No limits on the number of study cases and related reports in a single execution of the program.
3) Static Voltage Stability
This is a stability phenomenon, where the power system looses its ability to control load bus voltage due to various reasons. This phenomenon can lead to failure of the total or partial power system due to interventions of various control and protection actions.The reasons for voltage instability could be- Failure to provide necessary power support to the loads as a consequence of power transfer limit. The power transfer limit is determined not only by the bus voltage phase angle, but also by bus voltage magnitude- Failure to meet power requirements due to equipments reaching their control and operating limits. Examples are transformer tap limits, generator reactive power supply capabilities.- Inconsistency in the load power requirements as function of bus voltage and power supply characteristics.PowerApps provides various analytical tools for assessment of static voltage stability using load flow solution or output from static state estimation. Further the reactive power optimization algorithm provides a method of improving static voltage stability. The analytical tools areV-P (nose) curves or PV curves
Sensitivity Indices. Sensitivity of bus voltage magnitude for active (P) and reactive (Q) injection at a bus.
Sensitivity of net reactive power generation for a given bus reactive power injection.
Minimum Singular Value Decomposition of the complete load flow Jacobian as well us reduced Jacobian formulations. [ P.A.Lof, T.Smed, G.Andersson, D.J.Hill Two IEEE Transaction publications, 1992, 1993]. Further, identification of critical buses based on left and right singular vectors are also implemented in PowerApps.
Voltage Stability Index L proposed by P.Kessel and H.Glavitsch. [ IEEE Transactions on Power Delivery, 1986].
Static Voltage Stability Evaluation using relative bus voltage phasors at an operating point given by load flow solution or static state estimation.. [A New and Fast Technique for Voltage Stability Analysis of a Grid Network Using System Voltage Space", Published in International Journal of Electrical Power & Energy Systems, Elsevier Science Ltd.UK.]
Improvements in static voltage stability using a reactive power optimization tool. [Optimal Static Voltage Stability Improvement Using a Numerically Stable SLP Algorithm, for Real Time Applications", Published in International Journal of Electrical Power & Energy Systems, Elsevier Science Ltd.UK]
4) Transient Stability Analysis (Large Signal Performance)
The recovery of a power system subjected to a severe large disturbance is of interest to system planners and operators. Typically the system must be designed and operated in such a way that a specified number of credible contingencies do not result in failure of quality and continuity of power supply to the loads. This calls for accurate calculation of the system dynamic behavior, which includes the electro-mechanical dynamic characteristics of the rotating machines, generator controls, static var compensators, loads, protective systems and other controls. Transient stability analysis can be used for dynamic analysis over time periods from few seconds to few minutes depending on the time constants of the dynamic phenomenon modeled. Transient Stability Analysis is the PowerApps simulation module dedicated to simulating electromechanical transients in three phase electric power systems. It features an extensive library of equipment and controller models, the capability to include user-defined controls, a very flexible user-interface and powerful graphics. Transient Stability Analysis module utilizes the simultaneous implicit trapezoidal integration solution technique for network, machine and controller equations. The program supports the capability to test the step response of controllers and User Defined Modeling for system equipment and controllers.
General Features of Transient Stability Analysis
Transient models of excitation systems, turbine governors, static-var compensators, power system stabilizers and HVDC controllers.
Load shedding / islanded operation.
Transient stability analysis of multiple-islanded systems with solution for each of the islands.
Choice of generator models. From simple classical generators with constant voltage behind transient reactance to modelling detailed synchronous machines with variable voltages behind sub-transient reactances.
Standard IEEE excitation system models and turbine and governor models.
Commercial excitation models and governor models.
Models for power system stabilizers and different stabilizing signals.
Modelling load characteristics similar to that in the load flow analysis.
Modelling load characteristics as function of frequency.
Dynamic models of Induction motor and its load.
Motor starting studies. Motor modelling by their equivalent circuits or by the measured response during starting along with mathematical model for load torque as function of speed.
Under frequency/Under Voltage relay operation simulation.
Load shedding.
Islanded operation.
Element opening/closing.
Loss of generators.
Multiple transient stability disturbance scenarios for each base case load flow study. Note that, multiple load flow case studies can be performed followed by multiple transient stability simulations for each load flow study case.
Plots of selected bus frequencies and bus voltages. Note bus frequencies are different from generator frequencies.
Reports and Recommendations from Transient Stability Studies
Plots of Dynamic response of Generator rotor angles, frequencies, power outputs, voltages, excitation system outputs, governor-prime mover outputs
Plots of Line Flows, transformer flows, bus voltages, bus frequencies
Plots of Motor dynamic variables where required
Plots of the system variables that are of interest from protection point of view [example frequencies, distances seen from distance relays, fault currents seen from overcurrent relays etc]
Recommendation related to protection and control, operating strategy, Control settings of equipments [for example power system stabilizer, relay settings, load shedding schmes etc], based on various study cases considered
5) Dynamic Stability Analysis (Small Signal Performance)
The dynamic behaviour of power systems subjected normal power impacts is influenced by the following factors:The system load level.
The network characteristics.
The Generator and its controller characteristics.
The load characteristics.
The system is dynamically stable if the oscillatory response following a perturbation quickly settles down to a new stable operating point without sustained oscillations. These studies are typically carried out using linearized model of the system.
General Features of Dynamic Stability Analysis
Component modelling similar to Transient Stability Studies.
Linearized model of network algebraic equations and first order differential equations used at an operating point.
Eigen values analysis used for the evaluation of the system stability.
Option for time domain simulation with the linearized model and with specified perturbation.
Transfer function approach with single machine, infinite bus models.
Can be executed for multiple islanded systems and for multiple load flow study cases.
Options for root locus plots, Bode plots for simple single machine infinite bus models.
6) Power System Stabilizer Applications
The dynamic stability of a system can be improved by providing suitably tuned power system stabilizers on selected generators to provide damping to critical oscillatory modes. Suitably tuned Power System Stabilizers (PSS), will introduce a component of electrical torque in phase with generator rotor speed deviations resulting in damping of low frequency power oscillations in which the generators are participating. The input to stabilizer signal may be one of the locally available signal such as changes in rotor speed, rotor frequency, accelerating power or any other suitable signal. This stabilizing signal is compensated for phase and gain to result in adequate component of electrical torque that results in damping of rotor oscillations and thereby enhance power transmission and generation capabilities. State-space techniques described under Dynamic Stability Studies or classical control theory such as Bode plots, root locus techniques can be used to determine suitable parameters for power system stabilizers.The design can then be verified with a transient stability analysis for practical system disturbances.A Typical Control Schematic Diagram of Power System Stabilizer
7) Power System Protection Studies and Relay CoordinationCASE STUDY : Protection Co-ordination Study
In any power system netowrk, protection should be designed such that protective relays isolate the faulted portion of the network at the earliest, to prevent equipment damage, injury to operators and to ensure minimum system disruption enabling continuity of service to healthy portion of the network.When relays meant to protect specific equipments, transmission/distribution lines/feeders or primary zone protective relays, do not operate and clear the fault in their primary protection zone,backuprelays located in the backup zone, must operate to isolate the fault, after providingsufficient time discriminationfor the operation of the primary zone relays.The protective relays must also be able todiscriminate between faulted conditions, normal operating conditions and abnormal operating conditionsand function only for the specific protection for which they are designed, without operating for any normal and short term acceptable abnormal events for which they are not intended to act and provide protection.The term or phraserelay coordinationtherefore covers the concept ofdiscrimination, Selectivityandbackupprotection as explained in the foregoing discussion. Further the coordination is not confined only to relays and equipment operating characteristics, but also includes other protective device characteristics such as Fuse, MCB's, Circuit Breakers as applicable.Relay coordination calculation module must consider the operating characteristics of the relays, normal operating and thermal or mechanicalwithstand characteristicsof the equipments and must determine the optimum relay settings to achieve the objectives stated to protect the equipments and to ensure continuity of power supply to healthy part of network.Apart from the fault or short circuit conditions, protection system must also be designed to provide protection against thermal-withstand limits, motor stalling, negative sequence current with-stand limits, protection against abnormal frequencies, and protection against unbalance operating conditions as applicable to various equipments and operating situations.Frequency relay settings can be determined by using a dynamic simulation tool, such as transient stability analysis.Frequency Control Engineering;Transient Stability Analysis
Overcurrent Phase/Earth Fault Relays
Overcurrent phase fault relays.
Overcurrent earth fault relays.
High set relay settings to ensure protection against primary zone faults.
Coordination with maximum load current.
Coordination with fuse characteristics.
Coordination with maximum motor starting current and time.
Coordination with transformer inrush current.
Coordination with primary-back up pairs.
Coordination with thermal withstand capabilities ([I-square]t = K characteristics).
Coordination with safe stall limits for Motors.
Automatic generation of TCCs showing all relevant coordination.
Automatic identification of primary and back up relay pairs.
Provision for user defined back up relays for specific primary relays.
Solution for multiple island networks.
Multiple study cases for different network and source configurations in a single execution of the program.
Built in libraries of commercial relays, IEEE and IEC characteristics.
Distance Relay Settings
Zone setting calculations for zone 1 and 2.
Zone 3 setting calculations based on inbuilt short circuit calculations.
Settings for different commercially available relays.
Different relay characteristics MHO , Polygonal , Lens , Circle , R/X Blinder, Offset characteristic.
R/X diagrams.
Solution for multiple islanded network in a single execution of the program.
Solution for multiple study cases with different network configurations in a single execution of program.
8) Harmonic Measurements, Analysis and Filter Design
Harmonics in power systems can result in undesirable influence such as Capacitor heating/failure, Telephone interference, Rotating equipment heating, Relay misoperation, Transformer heating, Switchgear failure, Fuse blowing. The main sources of harmonics in power system are static power converters, arc furnaces, discharge lighting and any other load that requires non-sinusoidal current. In order to limit the harmonic current propagation in to the network, harmonic filters are placed close to the source of the harmonic currents. Harmonic filters provide low impedance paths to harmonic currents and thus prevent them from flowing into the power network. Harmonic analysis program computes indices such as total voltage harmonic distortion factor at system buses to evaluate the effect of the harmonic sources and to evaluate the effectiveness of the harmonic filters. Also, driving point impedance plots of the buses of interest are generated to identify whether series or parallel resonance phenomenon occurs at any harmonic frequency of interest.
Our Approach to Harmonic Analysis
We use 4 step approach as described in this section.-In the first step for existing and functional networks harmonic current measurements is performed at selected points to identify the harmonic currents injected into the network by the harmonic sources. These measurements reflect harmonic currents for one operating configuration and the loads prevailing at the time of measurements only. These may not represent conservative estimates of harmonic currents available.-In the second step, the measurement information of the first step will be used along with design data of harmonics where available from non-linear loads, generating harmonic currents. A computer network model will be prepared as per IEEE standards and the effect of various harmonic sources at various harmonic orders will be examined. Various harmonic distortion factors will be computed as outlined in relevant IEEE standards. The advantage of computer model and simulation is that it can take care of large number of operating configurations and conservative estimates of harmonic currents, which cannot be covered by field measurements. Field measurements of the first step, can however be used to validate the computer model developed.-In the third step, harmonic driving point impedances of all buses of interest will be generated at various harmonic orders and plots of the driving point impedances will be generated with respect to a range of harmonic orders [orders 1 through 50]. These plots indicate series and parallel resonance conditions in network.-In the fourth step, analysis of results of the first 3 steps will be carried out and solutions needed to solve any harmonic related problems will be obtained. These solutions are verified by using the computer model developed. The problems that might arise could be excessive harmonic distortion factors beyond relevant IEEE specified standards, existence of resonance conditions close to an exciting harmonic frequency. Where these problems are encountered, solutions will be provided by introduction of harmonic filters and its design will be verified again by using the computer model developed. Recommendations include specifications on sizing of individual components of the harmonic filters.
Our Guide Lines for Harmonic Measurements
Case 1: In this case the power supply to individual loads are supplied by dedicated panels, with no other loads other than the specific non-linear load. The load size is significantly large enough to warrant a specic dedicated harmonic filter. The measurements will be taken for this load feeder
Case 2: A single supply switch board supplies several non-linear loads. All loads are sufficiently small and nearly similar to each other. In this case dedicated harmonic filters for individual loads may not be necessary. A common filter may be provided at the incomer, provided the outgoing feeder loads are reasonably constant. The measurement will be done on the incomer of the switch board only.
Case 3: A single switch board supplies several non-linear loads. The nature of the loads are significantly different from each other. The net switch board load is not constant or uniform making it difficult to arrive at a common filter at the incomer. In this case we take harmonic measurements at each outgoing feeder and design individual load filters.
Apart from the above cases for harmonic measurements for purpose of filter design, it may be necessary to carryout measurements at point of common power coupling at HV levels to ensure that statutory requirements are satisfied.From the guidelines provided, it is fairly straightforward to examine the electrical network and to determine the number of measurement points. Measurements may have to be performed at different short circuit levels at the point of grid coupling as the electrical network characteristics changes with fault levels.
General Features of Harmonic Measurements, Filter Design and Analysis
Distortion Factor Calculations as per IEEE 519 Standard.
Impedance Frequency Scans to identify parallel and series resonance points and bus locations.
Modelling of harmonic sources and filters.
Modelling of all electric circuits as function of frequency.
Analysis using design data or Field measurements.
Analysis for various network configurations, fault levels.
Simultaneous solution of multiple islanded network.
Single execution and report generation for multiple study cases.
Calculation of harmonic current flows in specified circuit elements.
Display of computed harmonic distortion factors, harmonic bus voltages, harmonic currents on single line diagram for all study cases.
Evaluation of adequacy of filter design.
Design of filters considering the harmonic currents to be filtered and reactive power compensation needed at fundamental frequency.
Field measurements of power flows, harmonics and reports on the same.
Harmonic Analysis Related
DocumentsValidationDocument_HarmonicAnalysis_IEEEStd399_1997
Typical Harmonic and Power Quality Measurements Report Extract
Sample Single Line Diagram and Driving Point Impedance Plot for Harmonic Analysis
9) EMTP and Line Constants (LC)
The features of EMTP and Line Constants Program given below are similar to a program developed by M/s Microtran Power System Analysis Corporation, with the exception that available software will not handle Power Electronics Circuits, ideal transformers and rotating machines. Details of EMTP and Line constant features, the technical documents, user documents and a free student limited edition of the Microtran software can be down loaded from the web site http://www.microtran.com.
General Features of EMTP and LC
Lumped R, L, C elements.
Multiphase pi-circuits.
Single and three-phase n-winding transformers.
Transposed and untransposed distributed parameter transmission lines with constant or frequency dependent parameters.
Nonlinear resistances and surge arrester models (including metal oxide arresters), as well as nonlinear inductances with user-defined residual flux.
Switches with any number of opening/closing sequences, and other switching control criteria to simulate circuit breakers with multiple closing- opening sequences, spark gaps, etc.
Diodes, thyristors, and anti-parallel thyristor models with either fixed, or user-defined firing controls.
Voltage and current sources. In addition to standard mathematical functions (sinusoids, surge functions, steps, ramps), the user may specify sources point by point as functions of time, or calculate them inside the user-supplied subroutine SOURCE.
Synchronous machines with armature, field, and damper windings. The model also includes a shaft- mass system representation for the simulation of torsional oscillations.
Initial conditions can be determined automatically by the program or they can be supplied by the user. The program can also be used to obtain steady-state phasor solutions at a given frequency or over a user-defined frequency range. The "frequency scan" capability is particularly useful for harmonics, resonance and subsynchronous resonance studies.
User-supplied linear or nonlinear models using the entry point routine CONNEC. The procedure is quite simple: the user compiles his version of CONNEC with any compiler capable of creating a DLL. A sample version of CONNEC and detailed interface information is available to would-be developers.
10) Switching Transient Studies
These studies are generally performed to assess the transients associated with:Energization of overhead transmission lines and study of associated transients, surge arrester ratings, transient mitigation methods.
Energization of capacitor banks / reactors in industrial or public utility facilities.
Transients associated with various switching actions such as fault application and clearance.
Insulation Coordination
Insulation Coordination is the process of determining the proper
insulation levels of various components in a power system as well
as their arrangements. It is the selection of an insulation
structure that will withstand voltage stresses to which the system
or equipment will be subjected to, together with the proper surge
arrester. The process is determined from the known characteristics
of voltage surges and the characteristics of surge arresters.
The following standards are used by the consultants, while
performing the insulation coordination:Insulation Co-ordination,
Part 1: Definitions, principles and rules IEC 71-1, standard.
Insulation Co-ordination, Part 2: Application guide IEC 71-2, standard.
IEEE Guide for the Application of Insulation Coordination. IEEE Std 1313-2-1999.
Summary of Application of EMTP Studies*EMTP is used world-wide for switching and lightning surge analysis, insulation coordination and shaft torsional oscillation studies, protective relay modeling, harmonic and power quality studies, HVDC and FACTS modeling. Typical EMTP studies are:Lightning overvoltage studies
Switching transients and faults
Statistical and systematic overvoltage studies
Very fast transients in GIS and groundings
Machine modeling
Transient stability, motor startup
Shaft torsional oscillations
Transformer and shunt reactor/capacitor switching
Ferroresonance
Power electronic applications
Circuit breaker duty (electric arc), current chopping
FACTS devices: STATCOM, SVC, UPFC, TCSC modeling
Harmonic analysis, network resonances
Protection device testing
Insulation Coordination Studies for a 132 kV Submarine Cable InterconnectionA Case Study Description Implemented in the Middle East RegionAbstractThe concept of insulation coordination is well known, however, the exact and detailed method performing these studies are not practiced to the same extent as regular power system analysis and studies. A study case is presented in this paper, where power frequency temporary over voltage, switching frequency and lightning over voltage studies [fast and very fast front over voltage studies] are performed strictly in accordance with the IEC standards 60071 parts 1 and 2. The power frequency over voltage studies were performed using standard power system analysis tools such as load flow, short circuit and transient stability studies. The statistical switching and lightning over voltage studies were performed using the EMTP software. The details of the studies are presented in this paper.Keywords-Insulation coordination, Power Frequency Overvoltage Studies, Switching Frequency Overvoltage studies, Lightning overvoltage studies, Selection of withstand levels, Surge arrester applications.A.IntroductionThe utility operating company in a middle east country region has been operating the offshore oilfield in island U for over 40 years. Over the years, various installations were upgraded / added to the existing complex consuming significant amount of spare power generation capacity. The facilities in the island U is now facing up-grades for new process installation as the utility envisages various business opportunities in and around its facilities in the island. Consequently, the electrical local load growth demands additional power generation. Therefore, upgrades of the existing power generating system are envisaged to meet these demands.In relation to the above, the operating utility in the island U intends to meet the additional load demand at the island, by means of providing a sub-sea cable link, of approximately 40 KMs, from D Island power system to the U island power system. In respect of this proposed tie-in various engineering studies, power system studies and insulation coordination studies were performed. This paper outlines the insulation coordination studies performed and presents the summary of the studies.B.Description of the system.The islands of U and D both have gas turbine generators with the island D having the surplus generation. The 132 kV sub-sea cable link is initially planned to operate at 33 kV level and later to be upgraded to 132 kV operating voltage level. The studies were performed considering that the initial operating voltage will be at 33 kV level. Though the proposed sub-sea cable is adequately sized, the initial proposed operating conditions envisaged a maximum export of about 8 MW power from the island D to U, which is lower than the cable capacity.The single line diagram of the system considered for the analysis is shown in the figure 1.C.Insulation Coordination StudyInsulation co-ordination procedure consists of the selection of the highest voltage for the equipment together with a corresponding set of standard rated withstand voltages which characterize the insulation of the equipment needed for the application. The optimization of the selected set of withstand voltagesUwmay require reconsideration of some input data and repetition of part of the procedure till satisfactory results are obtained.The first step in the insulation coordination is the determination of the representative over voltages in the system [Urp] to which the electrical circuit is subjected to , under various operating conditions and switching phenomena.IEC standard 60071 classifies these over voltages asa.Low frequency continuous over voltages. Power Frequency Load flow analysis used to determine these over voltages.b.Low frequency temporary over voltages. These are determined by transient stability analysis and unbalanced short circuit studies involving ground faults.c.Transient slow front overvoltages. These are determined by statistical switching , line energization studies with or without pre-insertion resistors or other means of over voltage control.d.
Transient fast front switching overvoltages. These are determined
by statistical switching studies, with unbalanced faults, fault
removal, and switch reclose with or without fault removal on
energized lines.e.Transient very fast front lightning over
voltages. These are determined from the lightning over voltage
studies.D.Power Frequency Overvoltage StudiesThe objective of load
flow studies is to examine the power frequency over voltages in the
system under all possible operating conditions. These operating
conditions involved no load cases or complete loss of load
conditions as well, to assess the extent of over voltages under no
load conditions. A list of sample study cases considered for load
flow, short circuit and Transient stability studies is as
follows1.Two generators of island U in operation along with all D
island generators at Peak Load2.All generators of both islands in
operation at peak load, sharing loads3.Two U generators in
operation at peak load with loss connection between D island and U
island.4.Two U island generators in operation along with all D
island generators at No Load5.All D island generators along with
two U island generators in operation with reduced output at Peak
Load such that there is maximum flow in Sub-Sea cable from island D
to U6.All U island generators in operation along with all D island
generators except one gas turbine at peak Load such that there is
maximum flow in Sub-Sea cable from U to DFor each of the network
configurations, the short circuit studies were also performed to
compute the over voltages on healthy phases during ground fault
conditions.Further transient stability studies were performed for
the following disturbance scenarios and events-3 Phase faults and
fault removal by isolating the faulted circuit , to determine the
over voltages , their duration , upon fault removal.-Loss of load
conditions resulting in over voltages.E.Switching Frequency, Fast
Front, Very Fast Front OvervoltagesStatistical switching studies
were performed using the ATP-EMTP software for the following
switching scenarios with and without proposed surge arresters.
These studies involveda.Line Energization studies with receiving
end open endedb.Single phase fault, with single phase reclosing
after fault clearancec.Single phase fault, with single phase
reclosing after unsuccessful fault clearanced.Single phase fault,
with 3 phase reclosing after fault clearancee.Single phase fault,
with 3 phase reclosing after unsuccessful fault clearancef.3 phase
fault, with 3 phase reclosing after fault clearanceg.3 phase fault,
with 3 phase reclosing after unsuccessful fault clearanceApart from
the above lightning over voltage studies of a typical 33 kV
overhead line was also considered for the insulation coordination
calculations to arrive at the conservative values , as the
lightning was not applicable for the sub-sea cable
system.F.Modeling of Sub-sea cable for EMTP studiesIn
electromagnetic transient simulations there are basically two ways
to represent transmission lines/cables:i.Lumped parameter models:
Nominal and exact PI-modelsii.Distributed parameter
models/traveling wave models: Bergeron and frequency-dependent
modelsNominal PI-model: The nominal PI-model is one of the simplest
representations that can be done of a cable line. It includes the
cable's total inductance, capacitance, resistance and conductance
(usually not considered) modeled as lumped parameters.Exact
PI-model: The exact PI-model, sometimes also called the equivalent
pi-model, is a more advanced version of the nominal pi-model that
considers the distributed nature of the impedance and admittance.
This model is accurate when used in the frequency domain for a
single frequency and is normally used to validate other
models.Bergeron model: The Bergeron model is a constant-frequency
model based on traveling wave theory. The cable is considered to be
lossless and its distributed resistance is added as a series lumped
resistance. Typically, the model is divided into two sections, it
can be divided in more sections, but the differences in the results
are minor. This model is a constant-frequency model and its use is
only recommended for the cases when only one frequency is
considered.Frequency Dependent (FD) models: As the name indicates,
FD-models are models that have frequency-dependent cable
parameters. When compared with the previous models, the use of the
frequency domain increases the results accuracy. In FD-models all
calculations are performed in the frequency domain and the
solutions converted to time domain by the using transformations
such as Fourier-transform or Z-transform.For this study Exact PI
model was considered.G.Sample Simulation Results and PlotsText Box:
Figure 2: Voltage profile at one end of the Sub-Sea cable [without
Surge Arrester]Text Box: Figure 3: Voltage profile at one end of
the Sub-Sea cable [with Surge
Arrester]H.Conclusions/RecommendationsThe lightning, switching
overvoltage and insulation co-ordination studies were carried out
for the 33kV sub-sea cable system. The model representing the 33kV
system is carried out as recommended in IEC 60071-2, and in
accordance with the next extend as required by EMTP. All studies
were based on the relevant international standard, i.e. IEC
60071-2.
11) Motor Starting Studies
The starting current of most ac motors is several times normal full load current when starting them directly on line at full rated voltage. The starting torque varies directly as the square of the applied voltage. Excessive starting current results in drop in terminal voltage and may result in the following:Failure of motor starting due to low starting torques.
Unnecessary operation of under voltage relays.
Stalling of other running motors connected to the network.
Voltage dips at the power sources and consequent flicker in the lighting system.
Motor starting studies can help in the selection of best method of starting, the proper motor design, and the proper system design for minimizing the impact of the motor starting.Analysis of motor starting methods can be performed by both static and dynamic simulation techniques as follows. These techniques have their own conveniences, advantages and drawbacks. We believe mainly in transient (dynamic) motor starting studies that reproduce observed (measured) motor starting conditions.- Load flow type solution with the perceived starting impedance of the motor modeled as part of network modeling- Short circuit method type of calculations considering pre-fault short circuit conditions and using voltage drop calculations considering motor starting currents. Alternatively- Where accurate dynamic model of the motor electric circuit and load torque characteristics are available, dynamic model of the motor can be used in traditional transient stability algorithm to assess the impact of the motor starting.- In the absence of accurate model information, transient stability studies can be used where the observed (measured) starting current can be used as nodal injection at the motor bus as a dynamic event and the system response to this dynamic event can be evaluated.The various methods described above can take into account all types of motor starting such as- Direct on line- With compensation- Auto transformers- Soft Starters- Start Delta Start- Performance curves supplied from manufacturer- Variable frequency drives
12) Power Quality
To us Power Quality is characterized byStable AC voltages at near nominal values and at near rated frequency subject to acceptable minor variations, free from annoying voltage flicker, voltage sags, frequency fluctuations.
Near sinusoidal current and voltage wave forms free from higher order harmonics
All electrical equipments are rated to operate at near rated voltage and rated frequency. Hence the first point is one of the criteria of for assessing the power quality.As indicated inhttp://www.powerapps.org/Harmonic Measurements, Analysis and Filter Design.aspx, harmonics in power supply can result in the following- Capacitor heating/failure- Telephone interference- Rotating equipment heating- Relay misoperation- Transformer heating- Switchgear failure- Fuse blowingTo address the issues of power quality - we undertake detailed field measurements, monitor electrical parameters at various PCCs, feeders to assess the operating conditions in terms of power quality. If problems are found, we perform detailed studies using a computer model. The accuracy of computer model is first built to the degree where the observed simulation values matches with those of the field measurement values. This provides us with a reliable computer model using which we workout remedial measures. For the purpose of the analysis we may use load flow studies, dynamic simulations, EMTP simulations, harmonic analysis depending on the objectives of the studies.We also evaluate the effectiveness of harmonic filters through the computer model built, paying due attention to any reactive power compensation that these filters may provide at fundamental frequency for normal system operating conditions. Additionally the equipment ratings will also be addressed to account for harmonic current flows and consequent over heating.
13) Power Evacuation Studies
The phrasePower Evacuation Studiesis a generic term associated with plans for evacuating power generated from a generating source to a load centre. In the simplest form, it may mean only load flow studies with proposed transmission and distribution plans. When complete engineering is involved, the entire spectrum of power system analysis/studies may have to be performed.Power Evacuation Studies may mean, studies related to new generation facility and its connectivity to the grid for evacuation of the power or may mean studies related to existing facilities to study alternative plans of power evacuation for operational purposes.The objective of the studies is usually the checking feasibility of the various technical and economic aspects and therefore may encompass various other studies as follows.Load Flow or Power Flow Studies using standard load flow analysis techniques.
Static and Dynamic Contingency studies, using load flow analysis, transient stability analysis, small signal stability analysis, voltage stability analysis techniques. This is done to check the adequacy of the evacuation design or plan to withstand credible contingencies and to assess the reliability aspects of power evacuation.
Reactive Power Compensation Studies for capacitor locations, sizing, optimum settings for generator excitations, transformer taps. These studies are carried out using reactive power optimization techniques based typically on linear programming methods. The objective is to ensure that power is supplied to load centers at acceptable voltage levels and with minimum transmission losses.
Optimal Power Flow Studies. For economic dispatch or other suitable objectives. The other suitable objective may contain, scheduled power exchange, removal of operational infeasibilities, improving stability margin.
Engineering studies, such as site survey, plant and equipment locations, various engineering plans and specifications [civil, structural, mechanical, instrumentation, piping, electrical etc], transmission routes, substation layout, circuit breaker sizing, ground mat design, insulation coordination, protection and coordination to complete the designed Power Evacuation arrangement.
14) Switch Yard and Substation Design
We cover the following aspects of electrical engineering with respect to switch yard and substation designDetailed single line diagrams
Electrical layout drawings
Busbar design
Breaker, isolator, switching arrangements, disconnector and earthing switches, sizing calculations, specifications
Substation automation design, PLCC system design and implementation
Instrument transformers. CTs and PTs selection and specifications
Lightning (surge) Arrester specifications
Neutral grounding resistors calculations and specifications
Shunt reactor, series reactor, shunt capacitor requirements depending on reactive power control needed, power transmission requirements, short circuit current limitation requirements
Power transformer, distribution transformer, sizing, tap range requirement calculations and specifications
Earthing and ground mat design
Lighting calculations and related specifications
Insulation coordination studies
Power cable selection, routing, schedules
Auxiliary standby power design