Electrical Equipment Used in Power Plants

The following points highlight the seven important electrical equipment used in power plants. The equipment are: 1. Excitation Systems 2. Excitation Control 3. Automatic Voltage Regulators 4. Control Room 5. Plant Instrumentation  6. Plant Layout 7. Auxiliary Switchgear in Power Stations.

1. Excitation Systems:

The first step in the sophistication of the primitive excitation system was the introduction of an amplifier in the feedback path which amplified the error signal and made the system fast acting. With the increase in size of the unit and growth in the interconnection of the system, the excitation systems have become more and more complex.

The excitation system is required to provide the necessary field current to the rotor winding of a synchronous machine. The availability of excitation at all times is of paramount importance. Loss of excitation of a unit on the bus results in a more serious disturbance than that resulting from outage of the generator unit from the bus, as the remaining units must not only pick up the load dropped but also supply the large reactive current drawn by the unexcited alternator. In view of this an excitation system with better reliability is preferable, even if the initial cost is more.

The main requirements of an excitation system are reliability under all conditions of service, simplicity of control, ease of maintenance, stability and fast transient response.

The amount of excitation required depends on the load current, load power factor and speed of the machine. Larger the load currents, lower the speeds and lagging power factors, more the excitation required.

An excitation system may be individual one, in which each alternator is provided with its own exciter in the form of small generator on an extension of the main shaft, or centralised excitation system having two or more exciters feeding a bus­-bar to which field systems of all the alternators in the power plant are connected.

Though centralised excitation system is a cheaper arrangement but a fault in this system adversely affects all the alternators in the power plant. As such individual excitation system is widely used.

2. Excitation Control:

With the change in load on the supply system the terminal voltage of the alternator also changes due to variation in voltage drop in the synchronous impedance of the alternator. Since the alternators have to be run at a constant speed, the induced emfs, therefore, cannot be controlled by adjustment of speed. So to maintain constant voltage use of excitation regulation is made. The alternator excitation can be regulated by use of automatic or hand regulator acting in the field circuit of alternator exciter.

Automatic method of excitation control is preferred in modern practice.

The excitation control system does not differ much for dc and ac excitation systems as in an automatic voltage regulators, the generator, AVR and excitation control circuit form a closed loop system. The AVR senses the terminal voltage and if required varies the excitation so as to keep the terminal voltage within the specified limits.

A reactive volt ampere limiting device is built into the AVR. At a predetermined value of generator leading MVAR, the limiter overrides the basic AVR function and keeps the excitation current at a value to ensure that there is no further increase in rotor load angle and in the risk of instability.

A manual control system is invariably a part of the excitation control system and is employed whenever automatic control is not in action. When the AVR is in service, a circuitry is provided that makes the manual control to follow up the variations in excitation, so that in the event of failure of AVR, the generator excitation control is immediately transferred to manual control without any change in excitation current.

3. Automatic Voltage Regulators:

Synchronous generators have inherently large internal reactance and, therefore, high voltage regulation. So, in order to maintain terminal voltage of the generator under various operating conditions, provision of automatic voltage regulation equipment is essential.

An automatic voltage regulator operates on the principle of detection of error. The output voltage of an ac generator obtained through a PT is rectified, filtered and compared with a reference. The difference of the actual voltage and the reference voltage, known as error voltage, is amplified through an amplifier (rotary, magnetic or static) and supplied to the field circuit of the main exciter or pilot exciter. Thus, the amplified error signal controls the excitation of the main or pilot exciter through a buck or boost action. Exciter output control leads to the control of the main alternator terminal voltage.

The main functions of an AVR are as follows:

1. Control of system voltage within prescribed limits and have the operation of the machine nearer to the steady state stability limit.

2. Proper division of reactive load between the alternators operating in parallel.

3. Prevention of dangerous over-voltages on the occurrence of sudden loss of load on the system.

4. Increase of excitation under system fault conditions so that maximum synchronising power exists at the time of clearance of fault, to prevent loss of synchronism.

Modern alternators have high inherent reactance (more than 1 pu). The magnitudes of excitation for different load current and power factor conditions are very different. Hence automatic voltage regulators for modern alternators need stringent design characteristics.

On the occurrence of sudden change in load on the alternator, there should be a change in the excitation to provide the same terminal voltage under the new load conditions. The automatic voltage regulating equipment must operate on the exciter field which changes the exciter output voltage and then the alternator field current.

The response of a voltage that controls the alternator terminal voltage is thus a function of the responses of the voltage regulator, the exciter, and the main alternator excitation current (or flux). The response time of the regulator is’ the time required for the regulator to take action following the change in voltage.

Due to high inductance of alternator either a variation of field circuit resistance or change in the exciter voltage will not be able to give quick response. To achieve quick response, which is required due to violent fluctuations in industrial load, quick acting voltage regulators based on the ‘over-shooting the mark’ principle are used.

With the increase in load, the regulator of this type produces an increase in excitation more than that ultimately necessary. Before the voltage has the time to increase to the value corresponding to the increased excitation, the regulator reduces the excitation to the proper value.

For turbo-alternators a response time of 0.5 second is adequate, i.e., a mean rate of rise of 110 V/second for nominal 220 V. The rate of rise of voltage per second required for hydroelectric generators is considerable larger, because of the over speed to which these machines may be subjected.

4. Control Room:

The control room (or the operating room) is the nerve centre of a power station. The various controls performed from here are voltage adjustment, load control, emergency tripping of turbines etc. and the equipment and instruments housed in a control room are synchronising equipment, voltage regulators, relays, ammeters, voltmeters, wattmeters, kWh meters, kVARh meters, temperature gauges, water level indicators and other appliances, as well as a mimic diagram and suitable indicating equipment to show the opened or closed position of circuit breakers, isolators etc.

The location of control room in relation to other sections of the power station is also very significant. It should be located away from the sources of noise and it should be near the switch house so as to save multi-core cables used for interconnections. Of course, if there is any fire in the switch house, the control room should remain unaffected.

Also there should be access from the control room to the turbine house. The control room should be neat and clean, well ventilated, well lighted and free from draughts. There should be no glare and the colour scheme should be soothing to eyes. The instruments should have scales clearly marked and properly calibrated and all the apparatus and circuits should be labelled so that they are clearly visible.

The various locations of control room in relation to other sections of the power station are shown in Fig. 17.13.

5. Plant Instrumentation:

The modern power stations with small operating staff strength needs complete instrumentation and automatic controls for maximum effectiveness and economy. Instruments may be indicating, recording or integrating and are grouped in a central room, fully air-conditioned.

Instruments are installed in a power plant for a number of reasons. They help in efficient working and operation of a power plant by furnishing continuous information about the conditions of pressure, temperature, and flow throughout the plant. They provide those charged with the supervision of the plant a basis upon which to direct its operation so as to achieve the best performance possible and furnish data for the calculation of the performance of the plant or any part of the plant so that results may be compared from time-to-time.

Cost accounting systems will be based on adequate meter readings and correct cost allocations may point towards possible economics to be affected. Furthermore, they may be employed for checking the internal condition of the equipment and indicating when and where maintenance or repair is required.

The functions of the instruments are thus summarized as:

(i) Operating guidance,

(ii) Economical supervision,

(iii) Performance calculations,

(iv) Cost and cost allocation, and

(v) Maintenance/repair guidance.

Classification of Instruments:

All the plant instruments may be classified into two groups— mechanical instruments i.e., the instruments used for measuring mechanical quantities such as pressure, temperature, flow, speed and electrical instruments i.e., the instruments used for measuring electrical quantities such as current, voltage, power, energy, power factor.

This classification should not be confused with a division into mechanically operated and electrically operated instruments, for there are several instruments recording mechanical quantities that are electrically operated.

Mechanical instruments include temperature measuring devices, pressure measuring instruments, flow-meters, fuel measuring instruments, gas analysis instruments, speed measuring instruments, level recorders, gong alarms, steam calorimeters and fuel calorimeters atmospheric measuring instruments (barometers, hygrometers, thermometers).

Electrical instruments include ammeters, voltmeters, wattmeters, kWh meters, kVARh meters, synchroscope, power factor meters, reactive-volt-ampere meters, ground detectors.

6. Plant Layout:

The main items of the electrical power plant are alternators, exciters, transformers, regulating equipment, bus-bars reactors, switching equipment, protective equipment, batteries, and carrier-current equipment. The first step in the laying out of a new power plant is to draw the line diagram of the main connections.

The substations have the following distinct circuits:

1. Main Circuits:

Main circuits are the circuits through which power flows from generators to transmission lines. The components in series with the main circuit of power flow include bus-bars, power transformers, circuit breakers, isolators, CTs, line tap units, series capacitors, series reactors, diode or thyristor rectifiers. The components in shunt circuits connected phase to ground include shunt capacitors, shunt reactors, static VAR sources, harmonic filters, voltage or potential transformers, surge arresters.

2. Auxiliary Power Control Circuits:

Auxiliary power control circuits through which power flows to substation auxiliaries. The supply conductors are usually power cables.

3. LV Control Circuits:

LV Control Circuits – measurement, protection, control, monitoring, communication circuits. The supply conductors are usually control cables.

4. Auxiliary Low Voltage AC and Low Voltage DC Supply Circuits:

While laying out the main connections, the important factors need to be considered are given below:

1. The bus-bar arrangement should allow the alternators and transformers to be operated under the most efficient conditions.

2. Provision should be made to disconnect any part without interfering with the normal operation for the purpose of inspection, maintenance, repair and testing.

3. Reliable and adequate supply availability to the essential auxiliaries.

4. Protective gear of required performance, metering, synchronising and other control apparatus should be provided.

5. Short-circuit conditions should be maintained within limits by either reactors or way of grouping.

The next step is to decide the switchgear arrangement, which depends upon capacity of station, method of control, number of generating units and feeders, system of connections, reliability, safety, space, flexibility, simplicity and investment.

The main switchgear is usually located in separate building(s). The auxiliary switchgears are grouped into units or panels and are placed as nearly as possible to the auxiliaries which they serve.

7. Auxiliary Switchgear in Power Stations:

The auxiliaries in thermal power stations include boiler auxiliaries, condenser auxiliaries, generator and turbine auxiliaries, station auxiliaries etc.

Switching and bus arrangement for the supply to the station auxiliaries should be designed to provide reliability, simplicity and low cost. Major considerations in the layout of such a system are the size and nature of the plant, and its manner of use, the sources of electric power which may be available, and the amount of duplicate equipment which is provided for the essential and non-essential auxiliaries.

The auxiliary motors are of ratings, ranging from fractional kW to thousand kW. The total power required by auxiliaries is of the order of 6 to 8 per cent of the station output. The purpose of auxiliary switchgear is to facilitate switching, control and protect various auxiliaries.

In a 200 MW unit capacity class generating stations, the auxiliary system is generally at two voltage levels such as 11 kV and 415 V. However, with 500 MW unit class power stations, three voltage levels (such as 11 kV, 6.6/3.3 kV and 415 V) are necessary, so that the auxiliary switchgear of enough breaking capacity can be installed.

Single-line diagrams and auxiliary power connections should be prepared as in the case of the main electrical connections to indicate the source of power and the switching arrangement which will meet the operating and service reliability requirements.