CAT XQ400-3456 w/SR4B & VR6

[zzzztt302]

Experienced equipment failure within a load connected to a distribution system powered by the subject M/G.Point of L-L/L-G fault was transformer isolated from M/G 480 VAC output.During the failure, the M/G output went to O volts but did not trip off-line (output CB remain closed). After the fault was cleared/isolated, the M/G set was shutdown and restarted. M/G output came up normally. Is this an indication of regulator over-excitation shutdown? If so what would be potential causes? M/G is SE PMG.
Thanks

 

[catserveng]

Sounds like that is the most likely cause, the VR6 (is the same as a Basler AVC63-12)has a fixed over excitation shutdown that resets after unit stops, best explanation of it's function is in the Basler manual, the CAT manual doesn't do a very good job of describing that function.

As I remember the OEL is fixed at 90 VDC for 10 seconds, so if you go full field and the PMG has enough power to keep up, the OEL will stop excitation.

What kind of electrical protections are you using? Relying on the CAT control panel? What panel? If you had an event that required enough field to trip the OEL function in the AVR, the AVR stopped exciting and the breaker still didn't open, sounds like you have an improperly protected system.

Hope that helps,

Mike L.

 

[zzzztt302]

Thanks for the input Mike.I've been reviewing the Basler info based upon reading some of your earlier posts on related topics. Initially it was thought that the M/G went out on OV (which prompted the question"why didn't the M/G output breaker trip",assuming that it is suppose to on OV/OC. A review of M/G fault codes indicated none present. A review of regulator operation made it apparent that OEL trip was a potential cause. I'm assuming this (OEL) does not shunt trip the m/g output CB, based upon observation, correct? Reportedly, none of the down stream distribution ahead of a MV transformer cleared during the event. The load is non-linear, and can exhibit high THD. Is it possible that if the fault were restriking in nature (vs. a hard fault)that this di/dt , dv/dt, reflected back to the M/G regulator could have caused a problem from an EMI perspective? Can the OEL respond in an integral fashion?

Thanks
Phil

 

[catserveng]

The VR6 doesn't have a fault output, so it can't drive a trip or external alarm signal.

The standard CAT protection packages tend to have some holes in them, the EMCP3 is better than the EMCP II, but not easy to properly setup, and frankly, not as good as an off the shelf protective relay like a Basler GPS100. Since most packages this size go into standby systems, minimum protection is usually a good thing, that may not be the case in your situation.

The OEL as far as I know needs to see a full field condition for the time period before it shuts off, if the field output drops below the threshold and then goes back again, I don't think it is cumlative. I had a jaw crusher a couple of years ago the operators were overfilling and the motor would nearly lock, had a data logger on it and saw the field go full, voltage start to fall off, then the rock would break and the field would drop back down, this went on for about a week until they popped a couple diodes.

I never seen this AVR have OEL related problems on high harmonic loads, usually if the load is distorted enough you'll have issues with maintaining rated voltage, the AVR will either drive low or high depending on how it interpets the waveform and voltage value. Most common mode of failure where I've seen the OEL on these do it's job is on paralleled units and the rotating rectifier lost a diode or two (or three or more), and the field went full, then went out altogether, resulting in a loss of excitation or reverse VAR trip. Did have a rental project a few years back with 10 units feeding a large single step up transformer, first unit on line on Monday usually couldn't heat it up, and it would hit the OEL on really bad days.

Hope that helps,

 

[zzzztt302]

Thanks for you help Mike.That is valuable information.Have a good weekend.

Phil.

 

[zzzztt302]

Mike L,

On your last post you mentioned reverse VAR trip. Is this a function that the regulator acts on alone or takes an input from an auxiliary sensor (option)? Would this condition set a fault code and shunt trip the M/G output circuit breaker?

Thanks
Phil

 

[catserveng]

The VR6 only has the OEL and no external indication of a regulator shutdown. The newer CDVR has some additional protections and an alarm and shutdown output to use for monitorinh/control.

I worked in the CAT system for a lot of years, I never was a fan of the CAT electrical protections, EMCPII was lousy, EMCP3.X is supposed to be better but problematic, hard to program and impossible to properly (in my opinion) test and verify. In most applications I tried to install a generator protective relay, used Basler GPR's, which are pretty basic but at least better than the package protections, and everything else up to an SEL300G. IF your system is important, protect it. The generator is supplied with enough stuff to barely protect itself in a stand alone condition, and not always even that. After looking at hundreds of broken, burnt, toasted and abused generator ends over the years, I like to see at least a simple generator protective relay in addition to whatever the manufacturer supplies, not just CAT, but even some of supposed better products, like Cummins Power Command, I have found lacking at times.

I like to see at least over and under voltage and frequency, over current and over load, even on standby packages (surprises me to this day how I still run into overloaded standby generators even with all the sizing softwares and supposed better engineering practises). If a generator is supporting a large UPS or drive system I also like to add a reverse VAR protection, since so many use PF correction caps and leave them connected when operating on the generator.

my 2.5 cents worth