In this post the Single Line Diagram of typical Power plant is explained, it covers the Bus Transfer methods also. Single Line Diagram SLD gives the information about how the Electrical System is distributed through out the plant.
Single Line Diagram of Power Plant-System Description:
All the auxiliaries which are required to run for the successful operation of the unit, are connected to unit buses
All the auxiliaries which are common to the station are connected to station buses.
On interruption of supply to various buses, Changeover from one supply to other is resorted using Tie Buses.
GT- Generator Transformer
The GT derive power from the generator terminals and deliver it to the EHV buses.
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 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 and compressor pump motors etc.
The UAT derive power from the generator terminals and deliver it to the unit buses. (Buses 1 & 2)
ST- Station Transformer
In any power plant, 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 to the station buses [Buses 3 to 6]
The Notation of the SLD components are as follows
- 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
Bus Transfer Methods:
Station-to-Unit Transfer: [Manual Live Changeover]
During 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 unit bus is switched over to 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 manual live changeover.
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 station to unit tie breaker (Bkr B).
Once the generator trip, unit bus lose its normal source of supply as EHV breaker (Bkr E) also tripped. To maintain 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 high speed check synchronizing relay which permits closing of station unit tie breaker. Check synchronizing relay permissive is issued after comparing UAT bus voltage and ST bus voltage. In 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.
- Cascade Transfer:
In this method, the tripping command of UAT breaker (A & C) is given and closing command for ST breaker (B & D) is initiated after ensuring that UAT breaker is open. Here unit bus is without any external connection for a dead time of about 8 cycles.
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 the full load of both the bus sections.
Auto Changeover is provided, in these buses to changeover to the healthy bus by closing the buscoupler, should there be any interruption in any of the incoming feeder.
In case of any fault, the incomer breaker of the affected section has to be tripped first before the buscoupler 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 “Break before Make” transfer.