GLBS against GCB 1

[Schwatzernov]

What is the difference between GLBS(generator load break switch) and GCB(generator circuit breaker)applied to power plant porject.

I got caught with contractor that they proposed GLBS to be used for 1000MW coal fired power plant instead of GCB. The plant is connected to 500kV switchyard and has no start up transformer from power grid but back energizing from grid through generator step-up transformer and Unit Auxiliary Transformer while, at the time of back start, the GLBS is opened. They explained that if fault happens at generator system voltage, they will trip 500kV CB, 11kV CBs at the incomer of 11kV switchgear and trip 41E field CB.

Is there any problem if this is to be GLBS instead of GCB. How about if the fault happens at Unit Aux Transformer, even though field CB is tripped, the fault current still feeds from generator to the UAT. This increases damages to UAT ?

If the GCB is not necessary, why does the combined cycle power plant still need to use GCB instead of GLBS in every projects ? why don't they use GLBS which is cheaper ?

 

[ScottyUK]

Circuit breaker will clear fault current. A load-break switch will do what its name suggests, and under fault conditions it will have a very bad day if it tries to clear a fault. If you use a LBS then you will need to trip further back on the system to protect the equipment that should be covered by a GCB. Have a look at what happens to the plant if the generator protection operates - do you lose all your auxiliary power? Recovering from a plant from a blackout situation can take a long time, and cost a lot of money through lost revenue, penalties due to missed commercial commitments, etc.

Remember that the only reason the contractor has to use a LBS is to save money. He doesn't have to operate the plant.

 

[Schwatzernov]

Thanks ScottyUK,

Yes, you are right. However apart from view point on the plant operation and cost, in terms of safety/damage to the equipment,it seems that there is no issue ?

In order to justify to contractor reasonably, I have the following issues to be considered, kinldy advise me if I am wrong or I miss out some scenarios which is prone to safety/damage to equipment.[For information, generator excitation is static excitation system]

- Fault at unit aux transformer.
when fault happens at the UAT, generator protection will trip 500kV CB, 11kV incomer CB of switchgear and 41E generator field CB and the reactive energy stored in the field winding would be discharged to discharge resistor. Is my understanding correct that there is no current or just only negligible fault current (not higher than rated current of generator) still flows in a very short period(since the current is discharged to discharged resistor of field winding)for a few millisecond or a few seconds. If this my understanding is correct, so it seems that there is no issue for the equipment damages from the uninterrupted decrement fault current from generator ?

-Fault at generator or generator terminal(part between generator and GLBS)
Generator will issue the trip command to 500kV CB, field CB and 11kV incomer switchgear CB. So there is no fault current flowing from grid and back feed current from the process motors. So this case it seems that there is no issue for the view point of safety ?

-Fault at 500kV point between GSUT and 500kV CB.
I understand that this case also has no safety issue since HV relay e.g. 87B and 87GSUT and overall generator and GSUT differential relay will issue trip command to 500kV CB, 11kV CB and field CB. So there is no fault current flowing through GSUT. Even though there is small decrement through fault current, it is still in the through fault current withstand capability of GSUT. Is this correct ?

-Is there anything regarding to field winding discharge resistor related to type of excitation system or not ?
I understand that every types of excitation systems always have discharge resistor installed with field winding.

-what is the practice in Europe in application of GLBS and GCB ? This project, the contractor is Japanese company who has their own GLBS. They also insist that the GLBS is accepted world wide including nuclear power plant. They also made reference to the design of thermal power plant which there is even no any either GLBS or GCB between generator and GSUT. What is your opinion on this ? From the view point of safety/equipment damage, it seems to be no issue since the plant without any switching equipment is still has no issue regarding to safety.

If there is no issue of safety/damage, so what I can say is only the issue of plant operation and cost of re-starting the plant.

Thanks for any opinions or advices.

 

[ScottyUK]

Post a single-line diagram. It would help immensely.

 

[Schwatzernov]

Herewith enclosed is the single line.

 

[waross]

They also made reference to the design of thermal power plant which there is even no any either GLBS or GCB between generator and GSUT

If the generator protection zone is extended to adequately cover the Unit Step-up Transformer, Then transformer faults and generator faults may both be cleared by the generator shut-down trip.
No switch or breaker between the generator and the transformer is not as bad as a switch that does not clear a fault.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Schwatzernov]

They will design too have protection zone covers GSUT and UAT. Fault at any these equipment i.e. generator, GSUT, UAT, IPB will cause tripping of 500kV, 11kV incomer CB and generator field breaker.

The reason that contractor used is that without GCB, even there is no GLBS, there is no harm to any equipment as the field breaker is tripped so there is no fault current to harm any equipment.

However I think that even field breaker trip, the magnetic flux in the field winding cannot abruptly reduce to zero but it has time constant. Even though there is de-excitation which discharge current to discharge resistor but the short circuit current from generator cannot become zero immediately or even decrease to zero for a few millisecond and several seconds which such fault current contributed from generator would damage transformer(in fault happens at transformer winding e.g. UAT or GSUT). The point they argued is that the transformer can withstand the fault current for 2 seconds and there is no harm.

Fault at generator, the GCB cannot help since the fault is fed from grid through GSUT, however the 500kV is tripped, so GCB also doesn't give any benefit.

Another thing I wonder is that if the above reasons are true, apart from the reason of operation for back energizing the plant station service system, why is it necessary to use GCB in combined cycle power plant ?

 

[rcw retired EE]

"Why is it necessary to use GCB in combined cycle power plant ?"

It is unusual to have a start up transformer on a combined cycle plant. Most combined cycle plants backfeed power from high voltage through the GSU and UAT to the turbine starting system to start the generator. The GCB is needed to isolate and synchronize the generator.

On many CC plants, the steam turbine is synchronized at high voltage and does not have a GCB.

The GCB also provides a little better protection for a GSU or UAT fault by interrupting the decaying fault current from the generator to minimize transformer damage. That might mean the difference between a transformer rewind and a replacement. Search this forum for some good comments by others on this issue. For small generators the current decay is fast enough that the GCB does not add much benefit when the relay reaction time and breaker’s trip and clearing times are considered.

 

[ScottyUK]

"It is unusual to have a start up transformer on a combined cycle plant."

Plants designed on the cheap are a maintenance nightmare. The GSU transformer and auxiliary step-down transformer become virtually impossible to take out of service for maintenance, especially during an planned outage when staff numbers swell and the need for auxiliary power increases. It's one thing the designers of the older stations did right, before the accountants and their allies took over.

 

[rcw retired EE]

Scotty- You point is well taken. You're lucky you don't maintain some of the "cost effective" plants with only one HV breaker on a 2x1 combined cycle. Two gas turbines and one steam turbine on 3 GSU transformers and two UAT's on a single HV breaker.

Each generator does have its own circuit breaker but no disconnect switch. YOu have to shut down the whole plant to do anything. (Not my choice!)

 

[Schwatzernov]

Is it true that when the field CB has been tripped, the there is no decaying fault current from generator anymore. The fault current from generator is totally abruptly zero. This is the contractor clarified.

Between tripping field CB and not tripping field CB, what is the difference of decaying fault current contributed from generator ?

Do anyone have the short circuit decrement curve compared between the one when field CB trip and the other when field CB doesn't trip ?

 

[waross]

The DC offset of the fault current depends on the X:R ratio of the path of the fault current, the point on the wave when the fault occurs.
The decay of the DC component depends on the resistance of the path of the fault current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Schwatzernov]

waross, I understand that point but I am thinking that is it correct that after field CB trip, there is no decaying current anymore. The current becomes zero immediately. So no effect to equipment that the short circuit happened.

My understanding is that even though the field CB has tripped, since generator still has residual magnetic flux in the field winding, so it is incorrect that the fault current from generator becomes immediately zero but becoomes gradually decaying to zero. The time it comes to zero depends on its time constant which depends on its X/R.