Steam Turbine Failure - Loss of control voltage 1

[TheElv]

Hello,

I'm looking into a steam turbine failure (150 MVA)which occured when the station site wide 110V D.C. supply was lost. The turbine ended up being driven backwards (motoring) as the shunt trip circuit breakers protecting the turbine / generator downstream were energise-to-open and there was no d.c. back-up so they couldn't open with no control voltage. I am interested in whether anyone has come across similar failure modes and anyone's thoughts on undervoltage versus shunt trip breakers and provision of local UPS for such installations.

Thanks

Jon

 

[Pikoplatt]

I suspect that the electronic governor for your turbine was powered by 110VDC, and the loss of 110VDC caused your turbine steam valve to go closed, resulting in the flow of reverse power into your turbine.

Normally, I have seen DC undervoltage alarms (27DC) on DC control voltage systems. Was there any pre-alarm before the incident, or was the loss of 110VDC due to a fault on the DC system, resulting in a trip of the DC main breaker? This can also happen if, you lost AC power to the 110VDC battery charger and one of your backup batteries had an open circuit condition. A UPS powering your battery charger would help if you had good batteries in your UPS. You can also have the battery Undervoltage alarm (27DC) wired to trip your Generator main breaker, with a pre-alarm at a higher voltage to warn you of the impending trip. Your 27DC trip setpoint must be selected to be above the value of the Electronic Governor minimum DC supply voltage rating.

 

[TheElv]

Thanks Pikoplatt,

The problem here was not the alarms as these were all being generated as normal. The issue here was the breakers unable to open on the trip signal because 110VDC has been lost. The circumstances resulting in the failure of the site wide DC system are still being investigated so can't comment on that at present.

Jon

 

[ScottyUK]

All big breakers are energise-to-trip so you need a high integrity tripping supply. My opinion is that the tripping & control battery should be independent of the turbine / generator lube & seal oil battery, but many EPC contractors save a few grand by combining the functions. Poor economy to say the least, and something which the owner's engineer should make sure is considered in the spec. Do you have a breaker on the HV side of the GSU transformer or on the generator side? If you have access to the HV side breaker, which would normally be on a separate battery serving the substation, then a hardwired tripping signal independent of the generator protection would be worth considering.


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If we learn from our mistakes I'm getting a great education!

 

[GTstartup]

The larger generator breakers I am familiar with have one AC trip coil and one DC trip coil. You might consider retrofiting yours like this. Slight downside with two different coils like this, is it can happen that one coil goes bad and it is not apparent until it's really needed. It's important to disable one periodically to make sure the other is working.

 

[slavag]

As saied above.
For this size generator ( actually for small too) YOU MUST have two trip coils with two separated sources, dosen't importnat DC1 and DC2 or DC, AC.
YOU MUST have two redandancy protection systems and.. possible HV side with separated power supply, in additional BFP systems, direct transfer trip system, etc, etc.

Best Regards.
Slava

 

[TheElv]

Thanks for the responses, folks. I can see the benefits of separate trip coils with dedicated supplies but have picked up that this doesn't seem to be standard practice amongst the major manufacturers. To answer ScottyUK's question, the breaker is on the HV side of the GSU. The hardwired trip independent of the generator protection seems sensible to me.

 

[GTstartup]

How have you "picked up" that it is not standard practice to have two dedicated trip coils? In my experience this is pretty much across the board standard.

 

[slavag]

GTstartp, Im fully agree with you. Only two trip coils.

 

[dpc]

Station Batteries and energize-to-trip dc systems are the norm, at least in the US and have been for a very long time. Obviously, the dc power source is the weak link in this system and must be reliable.

The older steam plants I have worked with often had only one dc trip coil on the generator breaker. Two trip coils is fairly common and is a good idea. But unless you have two separate battery systems, there is still a possible common mode failure.

Undervoltage releases are NOT common in the US and I have found them to be a big PITA.

BTW, I really doubt the turbine was driven backwards. It was motored by the generator, but was turning in its normal direction of rotation.

 

[rcw retired EE]

Based on my experience –
Generators < 50 MVA: all breakers have single trip coils and a single DC power supply.
Units > 100 MVA have redundant protection and trip coils and sometimes two separate battery systems. One battery is dedicated to protection and tripping. The second may run the DC Lube Oil pumps, emergency lighting and other critical loads, sometimes it runs the UPS also.

In Ontario, Canada, the local code does not allow any non-protection loads on the tripping batteries for any breaker. Too many times the emergency lights or a DC motor depleted a single battery and protection was lost in a long outage.

Redundant tripping can be done by hard wiring a failure relay to trip backup breakers in the HV switchyard that usually has a separate battery system. The normal generator breaker failure relaying might not work because it is usually fed by the same battery.

Using a UPS to run a battery charger is asking for problems. The UPS is a lot more complicated than a straight charger and battery and it has a higher failure rate. The UPS runs off a battery. Sometimes it's the same battery as the protection system. Using a battery with a 30-60 minute run time to charge a battery system with a 48-72 hour run time doesn't make sense. A battery failure will still loose the DC power.

Just put in a second battery with dual chargers. The backup battery can be much smaller since it only powers the redundant relay and trip coils.

I investigated one steam turbine failure where the station battery supplied all protection, emergency lighting and the UPS power. The battery charger ran off the UPS panel, so the battery discharged itself through the UPS until all DC power was lost.

Under voltage releases are very problematic. They will cause several nuisance trips.

Best solution is two battery systems with redundant chargers supplying separate protection and breaker trip coils.

 

[Pikoplatt]

rcwilson, I like the two independent charger & battery and trip coil systems you recommended. It is a simpler and more secure trip system, with no common point of failure, besides the ac supply to battery chargers, assuming separate sources not available. I would keep the DC undervoltage alarms, agree the trips can create more problems, especially as I have seen breakers still trip at 60VDC on a 125VDC nominal trip system. Also some protection relays can opearte over a wide rage of DC voltage 60-280VDC. Thanks for your post.

 

[TheElv]

Thanks everyone,

Some good advice here and the benefits of the redunduncy from independent chargers, batteries and coils is clear. Of course, one has to consider the best value solutions taking into account the probability of various failure modes, as well as the consequences (which are likely to be severe) which is why I was also interested in how common such failures as I described in the initial post really are. Btw dpc was correct - the turbine wasn't actually driven backwards -it was motored but still in normal direction of rotation.

 

[rmw]

You said steam turbine failure. How badly did it trash the turbine? I know that is an ME question, but this is still a electrical power forum.

rmw

 

[RRaghunath]

Lots of good points.
One thing I like to add is about the Breaker fail protection.
Is the HV breaker on GSU provided with BF protection. It is likely that this also did not help because the DC failure is plant wide. Nevertheless worth looking at.

 

[Hoxton]

I think that the point behind all this is that if a feeder / distribution breaker fails to open on a relay operation (trying to chose my words carefully!)then something else should operate to deal with this. (breaker fail; trip circuit supervision; an upstream breaker opening etc, etc.)

With a generator breaker, there is often no backup, since the fuel / steam / water source is removed by the action of the trip command. The generator will then motor drive the prime mover. Reverse power relay operation has no effect on a failed breaker....There is also no fault current to operate another breaker, as in the case of a distribution system.

Remember that the actual beaker could fail, not just the trip circuit, so you may have to disconnect the entire power station!

I have found:
Turbines steam amd gas do not like being run in motor.

Reciprocating gas engines, being low compression, will run overnight and longer.

Diesels - never tried it, but the high compression could cause them to stall?

It just needs engineering....

 

[ScottyUK]

There's almost always backup, it just may take out a fairly significant amount of plant in the process. At the plant I worked at until recently the generator breaker was on the high side of the GSU transformer: the breaker fail scheme would have cleared half the substation with 500-odd MW of generation, a cable feeder and one half of a double-circuit line. It was never called to operate in anger that I recall.


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If we learn from our mistakes I'm getting a great education!

 

[AGibson]

Non-redunant MV/HV breaker systems are very common and the failure mode described is also fairly standard, as most power systems have historically been designed to fail energised.
A lot of people have cremated power stations (and high power consumers)precisely because of this design fault, which is standard practice unless otherwise specified.
If you want a safe system, you need fully redundant power sources (separate and independant battery banks, chargers & cabling) and a back up means of isolation (a separate upstram breaker which trips if the primary means of isolation fails (i.e. due to a burnt out trip coil).