Setting Bus Undervoltage Relay

[Gwhiz]

After reading the other threads on the subject, I still have not come to a definitive answer on how to solve this particular problem.

I am working on some settings for a relay upgrade at one of our coal fired power plants. This requires the replacement of the undervoltage relay on the auxiliary buses. The previous relay was an IAC54J with an inverse time curve. The new relay is a Beckwith M-3310 27 definite time element that is part of the UAT protection package.

The 27 relay is not used to trip the entire bus. When a low voltage condition is sensed, the relay instead trips only some of the motors connected to the bus.

The settings of the IAV54J were a tap of 70 V and a time dial of 2.5. This provides for a time delay in the range of 1.5-7 s for tap values from 0-90%. These original settings were specified by a consulting firm when the unit was put in service in 1960, and all of the settings information has since been lost.

After reading the standards on the subject, it looks like a standard setting should be 80% of nominal voltage with a delay of 2-3 s for the instantaneous element.

My biggest concern is that the largest motor on this bus,a 2400 V, 1500 HP, 320 FLC, 1920 LRC boiler feed pump motor, may pull down the voltage significantly when starting and cause nuisance tripping of the undervoltage element. This motor is protected by IAC66B overcurrent relays with a tap setting of 500 A Primary (100:1 CT ratio) and a time dial of 3. These settings allow the motor about 13 s of LRC before tripping.
How can I determine the value that the supply voltage will be pulled down to and how long it will last?

Thanks in advance
Gwhiz

 

[peterb]

Gwhiz -
Calculate the voltage drop through the transformer (I presume that this would be the Station Auxiliary Transformer, rather than the UAT, as the BFP must be started before the unit auxiliaries are transferred to the UAT after synchronizing the unit). To do this manually, determine the transformer impedance, the system impedance (from the avaialable HV short circuit level), the pre-start operating voltage and the running load. The bus voltage level can be estimated by modelling the starting motor impedance in series with the transformer and the HV system, with the running load in parallel with the BFP (all on the same base). The calculation gets a little complex, but it is feasible to do.
Or there are many analysis programs which include motor starting modules - these can vary from the relatively simple approach above to a full model including the motor transient characteristics.
I think that the ballpark approach can be used to highlight whether there are any potential problems. The voltage will be depressed to the calculated value for the duration of the motor start, which could give you an indication of a reasonable relay setting.
80% sounds like a reasonable value - that is 80% of the motor rated voltage, not the bus rated voltage. Motors generally won't start successfully below this level, so this is a good starting point.
Is there a bus transfer scheme associated with this unit bus? That raises another consideration, if the motors need to ride through an open transfer and then re-accelerate.