Power Generation Plant Electrical System - Overview
Switchyard or Substation
Switchyard or Substation is one of the major source of interlink that present between the power plant distribution system and external grid. During plant starting condition electrical power that is required to start the unit is received from the grid through switchyard and stepdown transformer. Some of the major components of the switchyard are listed below
Major Components of Switchyard:
Some of the major components of the Switchyard are:
- Circuit Breakers
- Isolators and Earth Switches
- Current Transformers and Potential Transformers
- Wave Traps
- Surge Arrestors and many more
Types of Substations:
There are three types of substation based on physical configuration. Air Insulated Substation is the conventional substation where all the components are present in the open atmosphere.
Gas Insulated Substation (GIS) where all the electrical components are placed in the hermetically sealed SF6 compartments. The advantage of GIS substation is that it occupy about 1/10th of the area that a conventional Air Insulated Substation occupy. Other type of substation is Hybrid Insulated Substation. This type of substation is a mixture of GIS and AIS Substation.
Different busbar switching configurations are available. Based on the factors such as reliability, availability, simplicity, ability to expand in future, ease of maintenance and cost switching schemes that are applicable will be chosen. In power plants mostly double main bus bar scheme and one and half breaker scheme is employed for power plants that are generating more than 400 to 500 MW
Electrical Generator connected to Turbine generates electricity in power plant. In case of Hydro electric power plant, turbine is driven by water which in turn drives the Generator, In Thermal and Nuclear Power plant Turbine-Generator set is driven by steam that comes from boiler or stream generator. Synchronous Generators are employed for generating electrical power.
Electrical Generator Auxiliaries:
While generating electrical power heat is generated in windings of the generator and core of the generator. This steam has to be removed else it damages the insulation of the machine. Therefore for large generators hydrogen cooling is used to remove the heat from the core. Stator conductors are cooled by water passing inside the conductors. A seal is provided for escaping of the hydrogen gas. For this purpose Seal oil system is also used. In case of small generators forced air or natural air cooling is used to remove the heat generated.
In Power Plant electrical transformers are used for stepping down and stepping up the electrical power. Some of the major transformers that are available in generating station are:
This Transformer is used for stepping up the power from generating voltage to transmission voltage. This transformer also acts as step down transformer and supply the power to electrical system in the plant when main plant Generator is not working. This can be possible by opening the Generator Circuit Breaker (GCB) when plant Generator is not in operation. Generator Transformer can be a single three winding transformer or three single phase transformers either connected in star or delta.
This transformers are the stepdown transformers that are connected to the Generator Terminals. These transformers steps down the power from generating voltage to plant distribution bus voltage. Unit Auxiliary Transformers will be of 2-winding three phase transformer. These transformers are connected to plant High Voltage buses
Start Up Transformer:
This transformer is used for supplying the power to plant loads for starting of the plant when main plant generator is not in operation. This transformer is connected to switchyard bus bar and plant distribution High Voltage (HV) bus. This transformer can be 2-winding or 3-winding three phase transformer.
All power transformers mentioned cooling requirements depends on the rating of the machines.
In Power plant small rating distribution transformers serve the purpose of stepping the power from High Voltage (HV) or Medium Voltage (MV) to Low Voltage (LV) and supply power to low voltage loads in the plants. Generally these distribution transformers are natural air cooled.
Power Plant Switchgear consists of High Voltage or Medium Voltage Switchgear and Low Voltage Switchgear. For High Voltage applications SF6 circuit breakers are employed. In case of medium voltage breaking Vacuum Circuit Breakers (VCBs) are used. For Low voltage applications, air break circuit breakers are generally employed.
UPS and Battery Systems
UPS and Battery Systems are present in power plant for supplying the loads which are critical to the plant. Battery and UPS systems are employed for operation of the circuit breakers, emergency lightning operation, AC and DC instrumentation supply.
Electrical System in power generation plants includes a structure of major components from Extra High Voltage (EHV) systems to Low Voltage (LV) systems. Some of the major components includes Generators, Switchyard equipment, Transformers, Switchgear, UPS and Battery Systems