Generator temperature 1

[sngpl]

We have five horizontal, 3.3 MW each, air-cooled hydroelectric generators. Generator 2 has started to trip on high stator winding temperature, while running at full load. Its worth mentioning that this generator was rewound (not by the original manufacturer) a couple of years because its was damaged due to a fire incident. My questions are:

1- would the generator capability curves change after rewinding the generator?
2- is it possible that the unit got derated after rewinding?
3- I want to measure the resistance of the stator RTDs for calculating the temperature of the winding and also at the same measuring the actual stator winding temperature with an infra red heat sensor. This is to ascertain that the RTDs are functioning properly. If I disconnect the RTDs would it trip the generator? Because I think the protection system would see a very high resistance i.e. the open terminals of the RTD.
4- any further suggestions?

 

[jbartos]

Suggestions to ScottyUK (Electrical) Aug 18, 2003 marked ///\\My post did state "For a generator at constant power output...", which means that the MW value is a fixed value. The reactive power may be varied without affecting the real power output. Your example went from 230MW to 275MW, which suggests you are looking at the constant MVA curve, not the constant MW line. Normally the prime mover - engine or turbine - isn't able to deliver the full MVA rating of the generator. At our site, the 365MVA generator is only able to deliver 310MW at unity power factor because that is the limit of the turbine. The same generator will deliver 365MVA by supplying 310MW and about 200MVAr. Thus my constant MW assumption is valid for normal operation of the machine.

Define MVA as total volt-amps of the generator
Define MW as real power output of generator
Define MVAr as reactive power output of generator

If we agree that MVA = sqrt(MW^2 + MVAr^2), then we surely must also agree that when MVA = MW, MVAr must equal zero.

When MVA = MW, the power factor is unity. For any condition other than MVAr = 0, MVA must be greater MW. The stator heating is largely governed by I^2R in the stator, and the minimum value for stator I occurs when MVA=MW, i.e. when the power factor is unity. For any other power factor, the stator heating is greater.
///Please, revisit my posted link and notice:
""Past limiter designs have been simplistic with fixed level and timers, and were not generally designed for maximum utilization of the capability region. Present designs, as shown, are much more flexible and can be tailored towards maximum utilization if the generator capability. This may be a reason to consider upgrading an older excitation system.""
This statement is in agreement with my posting where I indicated:
""~20% over the rated generator MW can trip the generator unless the manufacturer provided that much of design margin for MW.""

ScottyUK (Electrical) I am not sure that I follow your logic that is based on elementary or textbook relationships and not noticing the manufacturer implemented generator hardware constraints in terms of limiters, for example.\\

 

[electricpete]

ScottyUK - you are right on the money. Thanks for setting in and correcting the misinformation provided by others on this thread.

 

[jbartos]

Suggestion to ScottyUK (Electrical) Aug 8, 2003 marked ///\\It would be unusual for a generator to trip at unity power factor.
///Not quite, see

http://www.transmission.bpa.gov/orgs/opi/Power_Stability/FldCurPanelGE.pdf

for:
""Past limiter designs have been simplistic with fixed level and timers, and were not generally designed for maximum utilization of the capability region."" This implies that they might have tripped at PF=1.0.
\\ Most units can accept a certain amount of leading power factor loading, as illustrated on the capability curve, but are normally designed to run at about 0.85 lagging.
///Yes, true; or are designed at about 0.8 power factor lagging.\\\