Power Plant Electrical System

Electrical System in Power Plant:

Major components of electrical power plant are Generator and its auxiliaries, Extra High Voltage (EHV) Substation, Power Transformers, Circuit Breakers, Cables, UPS Systems, Battery banks. The Diagram shows basic single line diagram of power plant electrical system.

Power Plant Electrical system Single line diagram

EHV Substation:

Extra High Voltage (EHV) switch-yard acts as interface between the power plant electrical system and electrical grid. Power plant switch-yard can be AIS (Air Insulated Switch-yard) type which is conventional switch-yard or can be GIS (Gas Insulated Switch-yard) type. GIS substation is employed where the space requirement for the switch-yard is less, switch-yard located near the coastal region or in highly polluted areas.In GIS substation all the switchgear components are enclosed in SF6 gas modules. 

• Main and Transfer switching scheme or Double main bus bar scheme is generally employed for 220kV substation
•  Double main bus bar scheme or One-and -half breaker schemes are employed for 400kV substation.

Adequate number of transmission lines are provided for evacuating the power from generating station. The number of transmission lines required is conformed by carrying the power system studies

• Typical 220kV line can transmit power upto 250MW for a distance of about 300 kms
• Typical 400kV line can transmit power upto 450MW for a distance of about 300 to 400 kms

 Some of the components or switch-yard equipment are:

Busbars, Circuit Breakers, Isolators, Earth Switches, Current Transformers, Potential Transformers, Surge arresters, wave trap, CVTs, Lightning arresters etc.

 Generator – Its Auxiliaries: 

Electrical Generator converts mechanical energy input to electrical energy and delivers power to power plant electrical system and to grid. Voltage level at which generator generates power will be between 11kV and 33kkV. Some of the auxiliary systems associated with generators above 220MVA are: Stator water system (cooling the stator winding), hydrogen cooling system (rotor cooling of generator), seal oil system (provides seal for hydrogen gas not to escape), and lube oil system (provide lubrication to the generator)

 Generator Circuit Breaker (GCB):

Generator Circuit Breaker is placed in between the generator terminals and Generator Step-up transformer. In addition to providing protection to the generator, GCB provides electrical supply path from electrical grid to power plant electrical system when generator is not in operation by opening Generator Circuit Breaker. Connections between power plant generator, generator transformer and unit auxiliary transformers are through Isolated Phase Bus Duct (IPBD)

Power Transformers:

Generator Transformer, Start-up transformer, and Unit Auxiliary Transformers are the power transformers employed in the power plant.

 Generator Transformer: 

This transformer is used for step-up the voltage for transmission of power to the grid when power plant generator is in operation. It provides one of the paths to electrical supply from grid to power plant electrical system by opening the Generator Circuit Breaker

 Start-up Transformer:

This transformer connects distribution of power plant electrical system to electrical grid. HV winding of start-up transformer is connected to switchyard bus and LV winding is connected to power plant 6.6V bus. This transformer provides  electrical supply path to power plant electrical system from grid supply during starting of the plant. This transformer act as one of the offsite power supply source

Types of Boilers

Fire Tube Boilers

Water Tube Boilers

 Unit Auxiliary Transformers:

Unit auxiliary transformers connects the generator terminals and steps down the voltage to supply power to power plant electrical system. 

All the power transformers are resistive grounded. Forced water and forced air cooling is provided for all the power transformers

 6.6kV Switchgear:

Distribution system arrangement for the  power plant electrical system consists of 6.6kV buses, 415V buses and DC buses. 6.6kV buses (4 in numbers) are connected from secondary windings of unit auxiliary transformers and secondary windings of stat-up transformers. Major loads such as large motors, distribution auxiliary transformers are connected to power plant high voltage buses. Motor ratings above 200kW are connected to 6.6kV buses. Motors driving large pumps such as Boiler Feed Pumps, Condenser cooling water pumps etc whose rating above 200kW are connected to 6.6kV buses. Distribution transformers connected 415V buses are connected to these high voltage buses. All the loads are connected to high voltage buses (6.6kV buses) through cables and circuit breakers. SF6 or Vacuum circuit breakers are generally employed for switching purpose

 415V Switchgear:

Power plant Low Voltage (LV) system (415 V) is connected from power plant High voltage (6.6kV) buses through distribution auxiliary transformers. Small motor loads, Power Control Centers and other static loads are connected to 415 V buses. From power control centers feeders are connected to motor control centers and other distribution loads

 Battery and UPS system:

Power plant electrical system consists of some emergency loads such as generator and turbine lubrication pumps, power supply to instrumentation and controls in main control room and in substation control room, emergency lighting and computer systems. In order to provide supply to these emergency loads UPS and battery banks are used to supply the power. During normal operation of the plant, UPS systems and battery banks are charged with normal power supply deriving from 415V buses of the plant.