In this post the Single Line Diagram of the typical Power plant is explained, it covers the Bus Transfer methods also. Single Line Diagram-SLD gives information about how the Electrical System is distributed throughout the plant.
Power Plant-Single Line Diagram:
The above Single Line Diagram consists of the following equipment. The Notation of the SLD components are as follows
Parts of Single Line Diagram:
- GT- Generator Transformer
Rating: 3 phase 315 MVA 16.5/400KV
- ST- Station Transformer
Rating: 400KV/11.5/11.5KV, 80/40/40 MVA
- UAT- Unit Auxiliary Transformer
Rating: 16.5/11.5KV, 25/20 MVA
- TR- Service Transformer
Rating: 11/3.5kV,6.3 MVA
- GEN- Generator
Rated Voltage: 16.5KV
- EHV Bus- Extra High Voltage Bus
Rated Voltage: 400KV
- Unit Buses- Buses 1 & 2
- Station Buses -Buses 3 to 6
- X- Circuit Breaker
- NO-Normally Open Status of Circuit Breaker
- NC-Normally Closed Status of Circuit Breaker
Single Line Diagram of Power Plant-Equipment Description:
1. Unit Bus:
All the auxiliaries which are required to run for the successful operation of the unit, are connected to unit buses
2. Station Bus:
All the auxiliaries which are common to the station are connected to station buses.
3. Tie Bus:
On interruption of supply to various buses, Changeover from one supply to other is resorted using Tie Buses.
4. GT- Generator Transformer
The GT derives power from the generator terminals and delivers it to the EHV buses.
5. UAT- Unit Auxiliary Transformer
Unit Auxiliary Transformer [UAT] will get the Supply from Generator Transformer which is charged only when the unit is running. If the unit is not running then UAT gets the supply from the station transformer.
UAT provides the supply to the Unit Auxiliary Loads like ID and FD fans, Mill motors, CW and CEP pump motors, ESP transformers, FOPH transformers, ACW, DMCW, compressor pump motors, etc.
The UAT derives power from the generator terminals and delivers it to the unit buses. (Buses 1 & 2)
6. ST- Station Transformer
In any power plant, the station transformer will get the supply from the grid to provide supply to the auxiliaries which are common to all the units. For Ex: BFP, AHP and CHP Aux Transformers, Lightning and welding Transformers, etc. The station transformer derive power from EHV buses and deliver it to the station buses [Buses 3 to 6]
7. Bus Transfer Methods-Single Line Diagram:
The following are the types of Bus transfer methods used in any typical power plant.
1.Station-to-Unit Transfer: [Manual Live Changeover]
During the start-up of a unit, there is no power at the Generator terminals. During this time the unit bus is fed from the station transformer through the station bus and the respective station to unit ties. Post synchronization, when the unit reaches approximately 30% load, the supply to the unit bus is switched over to the unit transformer without interruption.
The changeover is done manually with appropriate provisions to ensure synchronism between the station to unit tie-breaker (Bkr B) and the incoming supply (Bkr A). In this changeover, the two supplies are Momentarily paralleled and therefore the tie (Bkr B) is automatically tripped. It is essentially a “Make before Break” scheme. This changeover is called a manual live changeover.
2. Unit-to-Station Transfer: [Auto Transfer Changeover]
In the event of a generator trip, load throw-off, turbine trip, boiler trip, etc., it is required to automatically transfer the unit bus from the UAT Incomer (Bkr A) to the alternate feed available from the ST through the station to unit tie-breaker (Bkr B).
Once the generator trip, the unit bus loses its normal source of supply as the EHV breaker (Bkr E) also tripped. To maintain an uninterrupted power supply to the auxiliaries (mainly motor loads) it is required to switch over to the healthy station source. This switchover has to be very fast otherwise the decaying bus voltage would cause all the connected auxiliaries to trip.
There are several methods available for doing Auto changeover. The widely used method is Fast Transfer-Open Transition.
The transfer is supervised by a high-speed check synchronizing relay which permits the closing of the station unit tiebreaker. Check synchronizing relay permissive is issued after comparing UAT bus voltage and ST bus voltage. In the majority of cases, fast transfer within 8 cycles is successful.
Open Transition can be done in two ways.
- Simultaneous Transfer:
In this method, the tripping command for UAT breaker (A & C) and closing command of ST breaker (B & D) are initiated simultaneously. Typical tripping time is 3 to 4 cycles and closing time is 5 to 6 cycles. Thus the unit bus is without any external connection for a dead time of about 2 cycles.
2. Cascade Transfer:
In this method, the tripping command of the UAT breaker (A & C) is given and the closing command for the ST breaker (B & D) is initiated after ensuring that the UAT breaker is open. Here unit bus is without any external connection for a dead time of about 8 cycles.
8. Distribution System (Supplied by Service TRF#A, B, C and D)
The 3.3 kV distribution bus configuration is having two bus sections with a bus-coupler breaker normally open. Each incomer and the related transformer is connected to one bus section and rated to cater to a full load of both the bus sections.
Auto Changeover is provided, in these buses to changeover to the healthy bus by closing the bus coupler, should there be any interruption in any of the incoming feeders.
In case of any fault, the incomer breaker of the affected section has to be tripped first before the bus coupler is closed due to an auto changeover. Otherwise, the upstream fault may be fed by the changeover healthy source causing it also to trip. It is always a “Break before Make” transfer.