COORDINATION WORKING MODE OF BOILER AND TURBINE At power plants with fossil fuel coordination working mode consist several hierarchical organized regulation levels. Main purpose of coordination control is to secure that system boiler-turbine-generator in automatic pressure control behave like one entity. For that purpose is necessary to integrate multitude different regulation circuit into one structure called main controller. This regulator must control two main control signals: steam pressure at boiler output and electrical power at generator output.

TURBINE CONTROL &AVRDONE BY:Marthad abdolmonem 044065Mohammed ahmed osman 054054Suhail mohammed khairi 044035Rughem osman karfis 014026PART ONE:TURBINE CONTROL

COORDINATION WORKING MODE OF BOILER AND TURBINE

At power plants with fossil fuel coordination working mode consist several hierarchical organized regulation levels. Main purpose of coordination control is to secure that system boiler-turbine-generator in automatic pressure control behave like one entity. For that purpose is necessary to integrate multitude different regulation circuit into one structure called main controller. This regulator must control two main control signals:

steam pressure at boiler output (boiler following mode) electrical power at generator output (turbine following mode)

Main regulator is containing two regulation contour

Dependency which regulation contour (boiler or turbine load) control substantial control signals (steam pressure or electrical power) we determine two coordinated working mode:

boiler load control control boiler thermal load, that is coal quantity which have to distribute over mills and convey to boiler burning place.

turbine load control controlled value is turbine valve position, placed between super heaters and turbine height pressure chamber and determine steam flow to turbine moving blade. Which of these two mode will be in use depend of many parameters, but in basic starts from idea: for control value (pressure or power) which have to change quickly and frequently usually using turbine load controller. Boiler load regulation is more required than turbine load controller by reason of complicated combustion process in boiler.

Combustion process in burning place have big time constants and many disturbances: coal or air shortage in burning place, failure equipment in plant, inferior water supply at boiler,...

From that reasons boiler load control usually used for values which needed stay constant for a long time without frequent set point changing. Turbine is relative fast executive object related to boiler and that characteristic determine turbine usage possibility this objects in coordinated working mode.Main controller in boiler coordination mode (figure 1) contain pressure regulator, which generate thermal load as output and power regulator which control position of turbine valves.

Pressure regulator have to preserve reference pressure during disturbance such as changing coal calorific value or changing position of turbine valves. Coal calorific value representing quantity of thermal energy released during complete combusting a unit mass of fuel. This value is variable and depend of fuel grade

sliding pressure VS constant pressure ModesOperation mode of steam turbine depends on the control concept that is implemented on steam power plant. Unit's power is generally controlled by manipulating of steam flow through turbine. Turbine's steam flow depends on steam pressure and size of input area through which steam enters turbine. This area is manipulated (control) by position of turbine's control valves. Therefore, there are two operational modes (in general):

- mode with constant pressure in front of turbine where turbine's pressure is controlled by changing control valves position and

mode with sliding pressure in front of turbine, where control valves are maintained at some constant position (for example 80 or 100% opened). In such case steam pressure is controlled by boiler control loops (by manipulating coal feeders).

Very closely related to this operational modes are so called "control concepts"

Concept of "boiler leading" means that steam pressure in front of turbine is controlled by control valves (constant) while power output setpoint is associated with boiler controller that manipulate coal firing.

On the other side, there is "turbine leading" concept in which, power output setpoint is associated by control valves and therefore every demand in power is immediately followed by changing of control valves position. Then, steam pressure is controlled by boiler.

*note that this concept is not fully "sliding mode" because, control valve position is not constant, but this concept is usually called sliding pressure mode, because steam pressure is changing and boiler pressure controller needs to adapt to these changes. Constant pressure implies stable pressure of the steam generator and main steam line over the units load range. Meanwhile, the basic nature of a simple, rotating turbine is to require less pressure as load and flow rate are reduced, and if the main steam pressure is limited to only that required for each load, this mode is referred to as pure sliding pressure.

However, when we speak generally of sliding pressure, we often mean modified sliding pressure, as shown in Figure 2. This mode has a limited amount of pressure throttling to provide a modest amount of fast-response load reserve. A unit under constant pressure will have significant load reserve at any reduced load, due to its significant pressure throttling or the availability of admission valve(s).

By opening the throttle valve or an admission valve, the pressure in the turbine and steam generator move toward equalization. The sudden reduction of pressure in the steam generator prompts an instantaneous expulsion of steam mass due to the increase in a specific volume of steam within the confines of the system, and it provides a temporary load increase even before the fuel-handling and -firing system can be loaded to support any sustained higher load. Pure sliding-pressure operation does not offer this kind of load or frequency response and is therefore generally not practiced.

Fig 2.PART TWO:AUTOMATIC VOLTAGE REGULATOR (AVR)A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level.It may use an electromechanical mechanism, or passive or active electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.AVRWith the exception of passive shunt regulators, all modern electronic voltage regulators operate by comparing the actual output voltage to some internal fixed reference voltage.

Any difference is amplified and used to control the regulation element in such a way as to reduce the voltage error. This forms a negative feedback control loop; increasing the open-loop gain tends to increase regulation accuracy but reduce stability (avoidance of oscillation, or ringing during step changes). There will also be a trade-off between stability and the speed of the response to changes. If the output voltage is too low (perhaps due to input voltage reducing or load current increasing), the regulation element is commanded, up to a point, to produce a higher output voltage - by dropping less of the input voltage, if the output voltage is too high, the regulation element will normally be commanded to produce a lower voltage. However, many regulators have over-current protection, so that they will entirely stop sourcing current (or limit the current in some way) if the output current is too high, and some regulators may also shut down if the input voltage is outside a given range.

Operation of AVR linked to Generators:

AVR is linked with the main stator windings and the exciter field windings to provide closed loop control of the output voltage with load regulation. In addition to being powered from the main stator, the AVR also derives a sample voltage from the output windings for voltage control purposes. In response to this sample voltage, the AVR controls the power fed to the exciter field, and hence the main field, to maintain the machine output voltage within the specified limits, compensating for load, speed, temperature and power factor of the generator.

A frequency measuring circuit continually monitors the generator output and provides output under-speed protection of the excitation system, by reducing the output voltage proportionally with speed below a pre-settable threshold.

Potential Divider and Rectifier takes a proportion of the generator output voltage and attenuates it. This input chain of resistors includes the range potentiometer and hand trimmer which adjust the generator voltage. A rectifier converts the a.c. into d.c. for further processing.

The Amplifier (Amp) compares the sensing voltage to the Reference Voltage and amplifies the difference (error) to provide a controlling signal for the power devices. The Ramp Generator and Level Detector and Driver infinitely control the conduction period of the Power Control Devices and hence provides the excitation system with the required power to maintain the generator voltage within specified limits.

The Stability Circuit provides adjustable negative ac feedback to ensure good steady state and transient performance of the control system.

The Low Hz Detector measures the period of each electrical cycle and causes the reference voltage to be reduced approximately linearly with speed below a presettable threshold. The Synchronising circuit is used to keep the Ramp Generator and Low Hz Detector locked to the generator waveform period.

The Low Pass Filter prevents distorted waveforms affecting the operation of the AVR. Power Control Devices vary the amount of exciter field current in response to the error signal produced by the Amplifier. Suppression components are included to prevent sub cycle voltage spikes damaging the AVR components and also to reduce the amount of conducted noise on the generator terminals. The Power Supply provides the required voltages for the AVR circuitry.

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