We have Two Frame-5, Mark-IV GTG system. One is running and another is Ready to Start Condition. We have no power from Grid. For this we usally run the GTG in Isochronous mode.

On 16/06/08 at 8.42 pm a big Flashing occured on Main Incomer VCB (Vacuum Circuit Breaker). Which Connect the 11 KVA Bus Bar. A larm came on Mark-IV Pannel "Circuit Braker Open" and GTG in FSNL mode. What is the cause of this flashing ?
After removing the VCB from the pannel, we found that the upper Contact area was burnt severly. We checked the earthig which is found OK. So , please help us for finding the Cause. Now the GTG in Shutdown Condition.

 

Responding to Rajkumar Mandal's 21Jun-08 (16:12)query... can you determine if only one or were two contacts damaged?

Regard, Phil Corso

 

Usually when I've encountered problems with vacuum circuit breakers, it's been because of a loss of vacuum or a problem with contact gap.

The contact gap, which is much smaller than the contact gap for a similarly-rated air-break circuit breaker, must be checked periodically and adjusted if necessary.

When was the last time maintenance was performed on the vacuum circuit breaker? Do you have the records from the activities of the maintenance? Do they indicate any problem with the bottle in question (usually, each phase has its own "bottle")?

 

Several things can cause a circuit breaker to flash internally. I will try to list a couple of causes:

1. Is the VCB of the plug-in type. If yes I would check the tension of the springs in the rose (multifingered contact) at the circuit breaker end. Obviously if it is destroyed, check the other side of the circuit breaker or that of the other gas turbine.

2. Check for the presence of dust and dampness in the circuit breaker structure. These will cause the insulation to break down.

3. Was the VCB plugged in properly? Don't rely on indication contacts. Check physically that the VCD contacts plug in properly.

4. When was the VCB checked last for tightness of the current carrying components, especially where bolted connections are concerned?

Normally circuit breakers fail due to:

1. overheating in components which (assuming the circuit breaker is properly rated for the environment it is working in) is caused by loose connections. This will result in insulation breakdown and flashover.

2. Circuit breakers that are in damp environments and operate relatively cold i.e. either because of failed anticondensation heaters or because their natural dissipation is not enough, they do not manage to displace the humidity that is accumulating inside.

3. Circuit breakers exposed to harsh chemical environments, eg. as in chemical processing plants. The corrosive environment will eat through components and thus the flash overs.

Try to analyse cm by cm where the flashing occured. You will most probably find the place where the flash was the most violent, and see which component of the circuit breaker is involved. Then see if the above applies.

 

Rajkumar, how old is the breaker?

 

The middle contact got severely damaged and some metal portion is melted. It was taken in for maintenance on May 6th 2006. My questions are:

How to check the Vacuum Interrupter and Spring Contact because after flashing it got damaged.

Why did the protection circuit not protect the VCB from this failure?

How to find out what is the main cause?

 

Responding to CSA's 22-Jun-08 (15:19) comment...

Can you provide some details about the kinds of problems you have encountered?

Regards, Phil C

 

Pretty basic, really; flashing/vaporizing of the contacts inside the vacuum bottle upon opening is the usual result of not being able to "open" the contacts because of loss of vacuum. I've also seen contact gaps out of adjustment which wouldn't "open" which caused operation of differential relays which opened other breakers in the circuit to interrupt the current flow. Most of the vacuum circuit breakers I've seen have individual assemblies for each phase with contacts inside a "bottle".

It's really not clear to me if the "vacuum bottle" for one of the phases of the circuit breaker lost its vacuum or if the primary disconnect (the "stab" or the "fingers" which connect the breaker to the high voltage bus or generator lead) is what was damaged. Usually when I've seen a vaccum bottle lose it's vacuum, the damage is such that one can't examine the contacts and the assembly must be replaced, and the originator of this tread says the contacts are damaged indicating that he's been able to examine the contacts. He also seems to want to return the breaker to service, which would seem to indicate that the vacuum bottle is intact, and that would also indicate that the damage is elsewhere on the breaker.

And I've never seen a vacuum bottle that can be "opened" to examine the contacts and then "closed" and had a vacuum pulled on it so that it can be restored to service. Usually, there are special tests that can determine or attempt to determine the integrity of the vacuum (leakage tests) to indicate when a vacuum bottle should be replaced. I'm not even aware of any facilities that can disassemble vacuum bottles and repair and replace contacts (though I'm sure there are companies who can claim to do so; there are people who'll claim to do anything).

So, this whole thread is kind of odd, and the other recent one related to VCBs, as well. I wouldn't know how one would determine if a vacuum bottle had lost its vacuum when installed and in service so that one could know it had failed. Perhaps some manufacturers have gauges indicating vacuum? I've always been told that installing a gauge and the the tubing to connect it to the vacuum bottle introduces even more possibilities for loss of vacuum so it's not commonly done.

So, again, this thread is confusing and puzzling on many fronts for me. I wish we had more information and/or could see pictures of the damage and could know who the manufacturer of the breaker was and the model number/size. But, even then, this whole thing sounds like people attributing some kind of failure to something which isn't well understood, simply because it's not well understood and so that must be the cause. (Ever notice how most problems are always the "control system"? Why? Because it's so complicated and not well understood, and complication and lack of understanding implies more possibilities for "problems"; besides, they have all those wires connected to them!)

And that leads to another thing I've seen about breakers which sustain failure/damage: Usually, its the result of improper "racking-in" or "racking-out" of the breaker. People just think they can apply a little extra force if the breaker doesn't seem to go into the cubicle properly or doesn't want to come out of the breaker cubicle very easily. Extra force, while it may be necessary to install or remove the breaker, is indicative of some kind of alignment problem which can lead to improper stresses and poor contact of the main, high voltage disconnects. It can also cause damage to the high voltage disconnects. And yet most people, when they encounter a situation with a high voltage breaker that requires a "little extra force", never take the time to try to determine why that extra force was required. They never look at the high voltage disconnects or the operating mechanism to see why the "little extra force" is required which could be the early indicator of more serious problems to come.

Alas, it's like Diagnostic Alarms on a Speedtronic turbine control panel. They're just nuisances; heck, the turbine doesn't trip because of Diagnostic Alarms, so they can just be ignored. (Sorry; I'll get off my soap box now.)

 

You would run a Hi-Pot Test of 31KV across the open contacts of the breaker with the applied voltage across one side of the breaker (line or Load) and ground the other side. If the vacum bottle has lost it's integrity it will fail.

Perfrom a DLRO on the contacts while closed, to see if they are damaged.