Black start follow up

[PowerfulStuff]

Hi,
I've just been through a 'black start' on a 100MW islanded power station after boiler feed water was contaminated with mud! Anyway the power is back on but I had a few quick ones to see of anyone else has seen them before-

1/ During energisation of a new generator step up transformer the MWTU reverse power relay tripped repeatedly when excitation was applied to the generator (i.e. before the secondary circuit breaker was closed). I swapped the relay out but it has since tested OK. Is there any testing on a new transformer that would lead to excessive core magnetisation and hence an inrush current high enough to cause a false trip on the relay. The manual shows insensitivity up to 20 times rated current.

2/ One of the diesel generators has a 'series boost unit' in the controller which we diagnosed as having failed, replaced it and all OK. It's an inductor/capacitor unit which appears to provide field boost depending on load current based on how it is wired up -AC field supply goes through and one phase CT connected. It is wired before the field controller. It is 30 years old and (funnily enough) there are no manuals. Anyone got any thoughts about what it might do beyond my guesses?

Thanks,
Martin

 

[rcw retired EE]

#1 - Is the 32R relay onthe GSU transformer or the generator?

#2- Series Boost unit - Could be a PPT or a reactive compensation circuit.

A Power Potential Transformer (PPT) supplies power to the excitation system. Since a fault on the system drops the generator terminal voltage, the voltage supply to the exciter/field system is also reduced. The PPT uses current transformers to get power for the excitation from the fault current. But a PPT would have CT's on all phases insead of just one.

A reactive compensation circuit uses only one CT on "B" phase. The voltage regulator sensing circuit is on phase A-C, so B phase current is 90 degrees out of phase with Voltage A-C. The CT secondary current goes through a resistor, which creates a voltage drop proportional to the reactive current. This signal is used to provide reactive droop control on the voltage regulator.

The resistor can be adjusted to add or subtract from the voltage regulator's reference. As the generator picks up reactive load, the compensation signal increases the feedback voltage to the regulator causing it to lower the output voltage, reducing reactive load. (reactive droop compensation). Setting the resistor to subtract from the reference makes the regulator compensate for some downstream voltage drop, such as a transformer or transmission line. In effect, the VR is regulating voltage at a point downstream of the generator terminals.

 

[dpc]

I believe the Series Boost (SBO) unit was(is?) a Basler option for supply additional excitation boost during short circuits to aid in clearing faults and motor starting. It is a single-phase device to boost power to the voltage regulator (not the exciter directly) when output voltage dips. It can use either one or two CTs. Basler website might have more information if it is a Basler unit.

Any chance your 32 relay has ended up with CT and/or PT inputs swapped such that it is looking in the wrong direction?

You could try increasing the time delay on the 32 a little bit.

 

[ScottyUK]

A generator raising excitation is about as gentle a means of energising a transformer as you'll find. Inrush should be minimal, so I don't think you'll find much joy down that road.

Did the generator successfully close onto the bus with the new 32 relay, or is the problem still unresolved?


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[PowerfulStuff]

Thanks for the replies. As a bit more information-

#1 - the 32R relay has VT's and CT's between the generator and GSU transformer. It is set very low (0.015) to protect the steam turbine, but with a suprisingly long time (10s).

My reason for picking inrush as a possible cause is that it tripped three times in a row at the time excitation was applied. Low speed trending shows 'instantaneous' rise to rated volts but with a 2s sampling time is a bit difficult to know for sure. The AVR is an older Op-amp controller which may not have much in the way of ramping.

After the relay was swapped out the turbine stayed running with excitation on and closed onto the bus OK. I'm unsure about the relay swap as the same relay didn't trip on another turbine and has been tested OK since. After losing many millions of production dollars no-one is keen to do anything that will risk another trip so I'm sitting on my hands until the third turbine is back in service (around 1 month).

#2 - The aim of the diesel generators is to run large MV boiler pumps and fans to then provide steam to the larger turbines. The idea proposed by DPC is pretty good. As the excitation supply comes from the generator output, any voltage dip would also pass on to the voltage regulator. Given it's age I don't think that it would be quick enough to respond directly, so the idea of boosting voltage supply seems like a very good idea.


Thanks for your help - I've learnt something (and hope to not have to use it again for a while!)

 

[GTstartup]

This is just a possibility for you to look at. I have seen reverse power relays trip incorrectly with just current applied and no voltage. This is because the relay doesn't have a voltage reference and therefore doesn't know "where it is". Most installations have a voltage balance relay monitoring PT circuits to preven a misoperation like this. By building up the voltage slowly and already connected to the transformer, you may be getting some current but not enogh secondary voltage to allow the relay to operate correctly. Test the relay again but try to simulate the actual operating conditions. i.e. enough tripping current and slowly bring up the test voltage. See what happens.