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Lack of Reactive Current Load Sharing, CCC

Lack of Reactive Current Load Sharing, CCC

Lack of Reactive Current Load Sharing, CCC

(OP)
I am looking for a possible answer to an outcome i experienced with a CCC system. Utilizing 2 1750kw, 4160vac Gensets,in the system. If i had lack of CCC control, or lack of reactive current load sharing could a possible outcome be overheating of the Excitation field leading to catastrophic failure of these components (exciter rotor, stator that exploded due to over heating). The closest forum i have read on this was a answer of-
A lack of reactive current load sharing would mean that paralleled generators would 'fight' each other to supply the required reactive load. Thr results of this can be quite scarry, ammeters swing around and genereal geneset load instability ensues as each genset ships and then rejects load.  

RE: Lack of Reactive Current Load Sharing, CCC

When generators are operated singly the load determines the reactive current.
When generators are operated in parallel with similar generators or with a utility the voltage setting or excitation level determines the reactive current.
If generators are in Voltage control mode and one or both have inappropriate Voltage settings the generator with the higher voltage will supply a disproportionate amount of the reactive current.
I suspect that you may have had a failing component that caused the problem. A couple of shorted rotating diodes will overheat the exciter until something fails. A loose and possibly arcing connection in the quadrature CT circuit (CCC) could have interesting results.

CCC: Control Center Complex, Charge Current Controller, Crisis Coordination Center, Central Control Complex, Capacity Coordination Cell, Ah, there it is, Cross Current Compensator.  

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

RE: Lack of Reactive Current Load Sharing, CCC

(OP)
This was a system that was using Cross Current Compensation. The origional problem was the obsolete VR3 went out. We updated the system with VR6 regulators. The existing gear was Redco. It was ovelooked that the existing CTs were 5000:1 which i found later to be incompatible with the new VRs. All other components were tested, rectifier and all windings. The initial test resembled the units fighting eachother with the one ultimatly failing. 6 hours after failure the exciter rotor was still hot even after we removed it from the unit.

RE: Lack of Reactive Current Load Sharing, CCC

What exactly failed? The PMG or the brushless exciter?

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

RE: Lack of Reactive Current Load Sharing, CCC

(OP)
Only the exciter rotor.

RE: Lack of Reactive Current Load Sharing, CCC

How are the rotating diodes and the surge suppressor?  

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

RE: Lack of Reactive Current Load Sharing, CCC

Any half-decent AVR should have a over-excitation limiter (OEL) designed to prevent the rotor current exceeding the safe operating limit of the rotor. Some are a hard limit, others use an approximate I2t characteristic which allows a short overload before the limiter pulls the current back. Was the OEL set correctly?

If you have cooked the rotor did you have any significant imbalance on the load? Bigger machines almost always have a negative phase sequence relay to prevent damage due to negative sequence heating of the rotor forging; little machines hope for the best and usually get away with it because they are not so critically loaded.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Lack of Reactive Current Load Sharing, CCC

Hi Scotty: As I understand it, it was just the exciter that fried.

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

RE: Lack of Reactive Current Load Sharing, CCC

Based on your descriptions, these are two CAT 3516's (likely older mechanical engines) with CAT SR4 generators, correct?

I've seen this a couple of times before under the same circumstances.  I am no longer associated with CAT or it's dealers, so what I give you here is my opinion based on my experience.

When the VR6 was retofitted in, the old droop CT was left in, correct?  How was the droop adjusted on the new AVR?  Was the old droop CT properly landed on the 1 amp droop CT input of the AVR?  Even when hooked up in a cross current circuit, the unit needs some voltage droop, usually about 2-3%.  Now for the bad news, the VR3 equipped units and the VR6 equipped unit will likely not work worth a darn in cross current compensation.  You may get them to hang together for awhile, but the slightest change in the voltage adjust setting or the droop pot setting will cause the units to not share VAR's properly, usually resulting in one of the units going full field and the other going to little or no excitation output.

I got burned enough times retrofitting units that had to operate in islanded paralleled applications to where if I had to change to the newer AVR on a unit I would do everything I could to talk the customer into changing them all, and to install correctly sized droop CT's with 5 amp secondaries.

The main issues is that these are two different AVR's, and their droop characteristics are NOT the same, and I've tried pretty much everything to make them work reliably with not much luck, usually what ends up happening is that the two AVR's are effectively operating in droop and most of the time they will hang togethere for part of the load range.

As to your failure, if the exciter stator fried, it was likely on the unit that was supplying all the VAR's.  As noted above, the AVR's should have some form of OEL.  The VR3, depending on part number, uses fuses to protect itself, need to make sure correct fuses where installed.  Refernece CAT publication SENR3905 for VR3's on MV tailends.

On the VR6, it has a fixed OEL, at about 125 VDC after 10 seconds it is supposed to shut off the AVR.  This assumes the PM is not falling on it's rear end, unfortunately some CAT PM's had trouble maintaining output under high excitation level conditions.  So check your no load and full load PM output voltage when you get it back together, make sure it's above 90 VAC and doesn't change much between no load and full load. IF the PM output is falling off, the excitation can go to full and never get to the point of the OEL shuting it off, after a few minutes the exciter gets pretty hot and sometimes fails.  Worst case I've seen is that the main rotor also can fail on an older machine.

Also note above was issue of a loss of excitation relay, is one installed on each generator and are they properly installed?

On the machine with the fried exciter, make sure you do a fairly complete set of tests before putting it back into service, I would recommend doing a pole drop test on the rotor to make sure it's ok.  Also replace the diodes and surge suppressor, even if they test good they have been stressed so toss them.  Does this unit have the individual 6 diodes, three positive and three negative?  Or does it have the positive and negative modular diodes assemblies?

When you get it back on line record the no load and full load field volts and amps on each unit, even with different AVR's they should be pretty close for given condition.  If not something is wrong.

If you want to stay with a cross current compensation system, I would strongly recommend you replace both AVR's and droop CT's, get a resistive/reactive laod bank to properly set them up, and document all the information to use as reference at a later date.

The CAT VR6 is basically a Basler AVC63-12 (only offered in a 240 VAC sensing version thru CAT), and I find the Basler manual to be better than the CAT manual. The Basler instructions for a cross current circuit works quite well on VR6 equipped units of the same size and rating.

Personally I'm not a fan of cross current compensation systems on these generators, have had much better long term operation with an active VAR sharing control like the Woodward DSLC or EGCP.

Hope that helps.

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