Generator

I need some clarification on a problem we are facing in our plant. Ours is a power generation plant. In this plant we have a black start DG, which is used for startup or when power is not available from the grid. The specification of the DG is:
KVA=750
CURRENT=1040
Voltage=415

The make of the Generator is stamford and the engine is a Cummins engine. The out put of the Generator is connected to the main LT busbar for feeding Auxillary loads of the Gas turbine and steam turbine.

It has been observed that when we close this breaker during start ups the breaker trips immediately. Hence the protections have been bypassed and the breaker is closed. We are also not able to get the protection on which the breaker trips.

We are troubleshooting this problem so that the protections can be in line when the DG is started.
The DG has a generator protection relay IM3GV also a reverse power relay MRP11. Also there is a MC61A digital relay.

Can someone suggest how to tackle this problem? Thanks.

john,
I think that you are facing sincronyzation problems, I mean frecuency between the generator DG and the main LT busbar.

this trip is present with the main LT energized or not?

Regards
J.P
Instrument Techniccian

Fire the plant manager for permitting the protections to be bypassed in order to close the breaker. Talk about "asking for trouble"; this is pretty much a textbook definition of inviting it into the plant and allowing it to eat up profits for repairs when something gets damaged, or worse, when someone gets injured.

There are many methods of monitoring trip relays to determine which one is actuating. If a proper coordination study was not done when the panel was built, it's possible that the lockout relay can open so quickly that the seal-in flag of some electromechanical relays can not be actuated. That may not be the case with these particular relays, but if they're not indicating the cause of the problem, then they're not working correctly.

Get a knowledgeable technician in to evaluate the relays and the PT (Potential Transformer) and CT (Current Transformers) circuits feeding the relays to be sure they were properly "phased" (this should have been done during commissioning, but quite frequently on smaller units gets "overlooked"). If the CT polarity is wrong into the reverse power relay it will actuate when the unit is loaded shortly after synchronization--but there should still be some kind of indication that the relay has actuated.

You didn't say if it had been working previously then suddenly stopped working, or if this is a new plant and it's never worked before. Has there been any maintenance activity in the protective relaying and metering cabinets and associated wiring? The generator breaker circuit? In other words, if it was working previously, what has happened between the time it was working and now? If it's never worked and this is a new plant, there should be some kind of warranty or this should have been on some kind of punch list for troubleshooting and repair.

But definitely don't be running that generator with all the protections bypassed, or if someone does, stand across the street while it's being done.

You need to start by identifying what protective function is responsible for tripping the breaker and work from there. Without a more detailed power diagram to go by, it could be any number of things, including synchronization as suggested above (if there is power available on the bus your generator is connected to). It could also be the case that your load condition exceeds the rating of the generator and you're tripping out on an overcurrent condition (or the overcurrent protection is dialed down to low and tripping early).

Either way, once you arrive at what is tripping it, you can then evaluate what your protective settings should be across the board, for the manner in which you operate the genset.

Hey CSA,

Although I have not asked for additional information, I suspect the DG has not been set up for dead line isoc operation.

Everything else from everyone else, I agree with.

Does your black start dead line isoc generator have sync capability?

Yes, the generator has sync function and when it is synchronised with the bus then the breaker does not trip when the protections are in line. Only when closing on dead bus and the protections are in line, then the generator breaker trips immediately.

CTTech,

I would submit that if the diesel generator is to be used for (and I'm quoting the originator) "...startup or when power is not available from the grid...." that it should be capable of synchronization to the grid and/or isochronous operation on a deadbus.

The originator has not responded to questions about whether or not this had been previously working or if it's been a problem since commissioning. I also suspect the originator meant to say the DG was to be used when power was not available from the grid *and* also for starting when power was not available from the grid (not "or"), but that's just a presumption.

In reviewing my initial response, I guess I'm not really clear what breaker is being closed nor when it's being closed. Are we talking about the DG's breaker or the tie breaker between the DG breaker and the "LT" (LV???) busbar? Is there a DG breaker, or is the output of the DG connected to the "LT" busbar directly through a single breaker (though that doesn't seem possible)?

I'm wondering if the breaker that connects the output of the DG (downstream of the DG breaker) is only supposed to be closed when there's a loss of power from the grid. And, then once the DG is started, it's output breaker is closed to power the "LT" bus through the "tie" breaker.

But, I don't think we know enough about the configuration, really. If there's protection that's being bypassed, then it would seem that the breaker who's protection is being bypassed shouldn't be closed under the conditions that require the protections to be bypassed in order to close the breaker. In other words, the breaker isn't supposed to be closed during start-ups. It's possible that some voltage-sensing relay (other than the ones being referenced) is sensing that voltage is present when it shouldn't be and is tripping the breaker. We also don't know if the breaker is being tripped through a lock-out or what.

We really don't have enough to go on to respond. Except to say that the protective relays shouldn't be bypassed and that if it's felt that there is something wrong with the relays that's causing them to operate erroneously they should be repaired.

It would seem the PTs and CTs are functioning correctly and phase angle can be measured if sync is possible. Dead line operation requires additional setup.

What does your operational procedure require for dead line operation? For instance, I am required to operate two switches on the control panel and lock out a 13.8KV disconnect to operate the black start in dead line isoc mode. If your procedure is similar, it could be something as simple as a bad switch.

All of the questions that CSA posted need to be evaluated fully. The disabling of generator protection relaying even on a confirmable dead line in NOT the correct procedure. If a problem exists and is not corrected, the next sync could result in an uncontrolled energy release.

Hi,
I am now preparing a similar project, so I have chance to look deeper for black start.
The way I see your case, the protection of the breaker is sized or set well to avoid the irrush current for the aux transformer and excitation transformer. There are two way to solve this problem: one is to set your reverse overcurrent curve to around 20 times of FLA for instantanuous overcurrent if you have LSI setting, or you can just put some time delay but you need to be very careful. Try to get LSI protective device if I were you.

Best regards,
Jack

CSA,
Set LSI overcurrent protection for Instantanuous overcurrent at around 20 times FLA can solve this problem.

Jack