Generator Testing - Current Balance

[Raptor1963]

Have something really strange happening w/ a medium voltage generator, 12.47kv @ 1690kW 0.8pf., run through a step-down transformer 12.47kV to 480V, we are floating the neutral at the generator, bonded in the transformer, so delta-to-delta, thru a Cutler-Hammer Magnum 3000a breaker, to a Crestchic 3.3MVA resistive/reactive load bank. We started a full load test yesterday and after the first hour we burnt up one of the 7/phase 4/0 DLO type cable connected to the load bank. We assumed it was a bad cable, replaced, and re-started the test. After about 30 minutes we smoked another cable. Now we start looking at the current in the cables, now take into account monitoring the current/voltage at 3 different places, genset control panel, secondary side of the transformer, and load bank PC program, these all read even currents, equal voltages, and even kW/kVAR distribution across all phases. We have one single copper buss connection connection point on each end, the load bank/breaker/transformer, cable all came off the same roll/same lengths. For some odd reason the current chooses to split un-equally through the cables, to the point of overload. One cable in 'C' phase has double the current of the other cables in that phase, 'B' phase is pretty equal, 'A' phase has two cables that are running 1.5x the others in that phase. We tried taking off the reactive part of the load, same results. Thought maybe a load bank problem, so I brought in another loadbank, check current through the cables, same problem. Checked the cables on both sides of the transformer, unbalanced both sides. Any HELP?????

 

[ramsy]

Re: Generator Testing - Current Balance

..cable all came off the same roll/same lengths.

Cables must be the same material (impedance), using the same conduit type (reactance), as well as same lengths, and termination methods. Make shure your not mixing any AL with CU or copper cladding, or sheilded cables with non-shielded (reactance can be significant).

 

[stormywyo]

All the terminations should be the same, as stated, and torqued to the same value. I would look at a difference in the torque values on the terminations. You can change the current in the various paralleled conductors by tightening or loosening the connections.

 

[mhulbert]

How close are your lengths for each cable? It doesn't take much to have an imbalance. Sometimes with temp setups like this...a connector burns up on one end and takes a little cable insulation with it...so the tech cuts off a little cable and installs a new connector. The shorter the cable the more current it has, it sounds like you have 2-3 short cables. Do you KNOW that they are all the exact same length, down to the inch?

Mike

 

[dave_asdf]

I don't know much about load banks, but is it possible they can be wired up for different voltages by changing the series/parallel configuration of the resistors? if so it might be possible that the load bank has less resistance on one lug than others. If all the cables connect to one bar i'm way off, but if there's multiple stabs that cables are landed to check the resistance of the load bank. maybe some are connected in parallel and should be changed to series.

 

[catchtwentytwo]

Dave may have a good point.

We have a 2 MW 480 volt load bank. The larger load steps have multiple fused elements and are controlled by multiple contactors.

Here is a photo from Simplex that I hope is viewable:

 

[Pierre C Belarge]

Raptor wrote:
"we are floating the neutral at the generator, bonded in the transformer"

What do you really mean by this? Is the generator controlled by a 3 or 4 pole transfer switch? After you answer these questions, maybe the track these guys are following (of which I agree with) may not be the problem.

 

[LoadBanks]

Parallel Cable Amperage imbalance

Parallel Cable Amperage imbalance

From our experience running up to 33 cables in parallel on large loads, there are several contributing factor for cable hogging the load. 1. Connection quality at both ends and in the middle if camlocs are used. 2. Mutual inductance ( up to 20%). 3. Triplexing of runs.

If one runs all phases flat on the ground, no tray or conduit, let's say four A conductors then four B conductors then four C conductors, the cables that are next to the adjoining phase will hog the load privided all else is equal. The solution is to separate or tri-plex and de-rate the ampacity. We typically will derate the free air rating by 5 to 10% and round up to next nearest cable quantity. When the cables are all piled up together, this does not constitue free air and can be worse than conduit rating because the conduit can transfer heat and a pile of rubber cannot expel heat as good.

We support companies doing these tests safely as our primary function of our business. Email me if you have any questions. ctaylor @ comrent .com