Steam Turbine Failure - Loss of control voltage
Steam Turbine Failure - Loss of control voltage
(OP)
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
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






RE: Steam Turbine Failure - Loss of control voltage
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.
RE: Steam Turbine Failure - Loss of control voltage
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
RE: Steam Turbine Failure - Loss of control voltage
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If we learn from our mistakes I'm getting a great education!
RE: Steam Turbine Failure - Loss of control voltage
RE: Steam Turbine Failure - Loss of control voltage
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
RE: Steam Turbine Failure - Loss of control voltage
RE: Steam Turbine Failure - Loss of control voltage
RE: Steam Turbine Failure - Loss of control voltage
RE: Steam Turbine Failure - Loss of control voltage
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.
RE: Steam Turbine Failure - Loss of control voltage
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.
RE: Steam Turbine Failure - Loss of control voltage
RE: Steam Turbine Failure - Loss of control voltage
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.
RE: Steam Turbine Failure - Loss of control voltage
rmw
RE: Steam Turbine Failure - Loss of control voltage
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.
RE: Steam Turbine Failure - Loss of control voltage
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....
RE: Steam Turbine Failure - Loss of control voltage
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If we learn from our mistakes I'm getting a great education!
RE: Steam Turbine Failure - Loss of control voltage
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).