TEXT 6. Generator Terminal Equipment

The generator output is connected to terminal equipment via cable, busbar, or isolated phase bus. The terminal equipment comprises current transformers (CTs), voltage transformers (VTs), and surge sup­pression devices. The CTs and VTs are used for unit protection, metering and synchronizing, and for governor and excitation system functions. The surge protection devices, consisting of surge arresters and capacitors, protect the generator and low-voltage windings of the step-up transformer from lightning and switching-induced surges.

The generator circuit breaker and associated isolating disconnect switches are used to connect and disconnect the generator to and from the power system. The generator circuit breaker may be located on either the low-voltage or high-voltage side of the generator step-up transformer. In some cases, the generator is connected to the system by means of circuit breakers located in the switchyard of the generating plant. The generator circuit breaker may be of the oil filled, air-magnetic, air blast, or compressed gas insulated type, depending on the specific application. The circuit breaker is closed as part of the generator synchronizing sequence and is opened (tripped) either by operator control, as part of the automatic unit stopping sequence, or by operation of protective relay devices in the event of unit fault conditions.

The generator transformer steps up the generator terminal voltage to the voltage of the power system or plant switchyard. Generator transformers are generally specified and operated in accordance with inter­national standards for power transformers, with the additional consideration that the transformer will be operated close to its maximum rating for the majority of its operating life. Various types of cooling systems are specified depending on the transformer rating and physical constraints of the specific appli­cation. In some applications, dual low-voltage windings are provided to connect two generating units to a single bank of step-up transformers. Also, transformer tertiary windings are sometimes provided to serve the AC station service requirements of the power plant.

The excitation system fulfills two main functions:

 It produces DC voltage (and power) to force current to flow in the field windings of the generator. There is a direct relationship between the generator terminal voltage and the quantity of current flowing in the field windings. It provides a means for regulating the terminal voltage of the generator to match a desired set point and to provide damping for power system oscillations.

Prior to the 1960s, generators were generally provided with rotating exciters that fed the generator field through a slip ring arrangement, a rotating pilot exciter feeding the main exciter field, and a regulator controlling the pilot exciter output. Since the 1960s, the most common arrangement is thyristor bridge rectifiers fed from a transformer connected to the generator terminals, referred to as a “potential source controlled rectifier high initial response exciter” or “bus-fed static exciter”. Another system used for smaller high-speed units is a brushless exciter with a rotating AC generator and rotating rectifiers.

Modern static exciters have the advantage of providing extremely fast response times and high field ceiling voltages for forcing rapid changes in the generator terminal voltage during system faults. This is necessary to overcome the inherent large time constant in the response between terminal voltage and field voltage (referred to as Td_o, typically in the range of 5 to 10 sec). Rapid terminal voltage forcing is necessary to maintain transient stability of the power system during and immediately after system faults. Power system stabilizers are also applied to static exciters to cause the generator terminal voltage to vary in phase with the speed deviations of the machine, for damping power system dynamic oscillations.

Various auxiliary devices are applied to the static exciter to allow remote setting of the generator voltage and to limit the field current within rotor thermal and under excited limits. Field flashing equipment is provided to build up generator terminal voltage during starting to the point at which the thyristors can begin gating. Power for field flashing is provided either from the station battery or alternating current station service.

The governor system is the key element of the unit speed and power control system. It consists of control and actuating equipment for regulating the flow of water through the turbine, for starting and stopping the unit, and for regulating the speed and power output of the turbine generator. The governor system includes set point and sensing equipment for speed, power and actuator position, compensation circuits, and hydraulic power actuators which convert governor control signals to mechanical movement of the wicket gates (Francis and Kaplan turbines), runner blades (Kaplan turbine), and nozzle jets (Pelton turbine). The hydraulic power actuator system includes high-pressure oil pumps, pressure tanks, oil sump, actuating valves, and servomotors.

Older governors are of the mechanical-hydraulic type, consisting of ballhead speed sensing, mechanical dashpot and compensation, gate limit, and speed droop adjustments. Modern governors are of the electro- hydraulic type where the majority of the sensing, compensation, and control functions are performed by electronic or microprocessor circuits.

1. Read and translate the text.

2. Read the text again and form the antonyms of the following words and expressions or find them in the dictionary:

disconnect

high-voltage

close

step-up transformer

cooling system

advantage

rotating

3. Put the following words in the correct order to form a sentence.

1) System, function, fulfills, produce, the, d.c.,to, excitation, voltage, main, the.

2) Exciters, advantages, several, modern, have, static.

3) Static, the, allow, voltage, auxiliary, remotely, the, devices, exciter, generator, to, set.

4) Power, system, key, the, control, governor, the, is, element, of, system.

4. Answer the questions to the text.

1) How is the generator output connected to terminal equipment?

2) Where is the governor circuit breaker located?

3) What is the function of a generator?

4) What functions of the excitation system do you know?

5) What generators were generally used prior to the 1960s?

6) What are the advantages of modern static exciters?

7) Why are various auxiliary devices applied to the static exciter?

8) What is the key element of the power control system?

5. Write the summary of the text.