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Max Cable Length with Generator

Max Cable Length with Generator

Designing a 10kV generator installation with about 300 meters between alternator and breaker. Concerns raised that cable length may be  aproblem. Apart from burden on CT's which is OK are there any factors e.g. capacitances that limits cable length.


RE: Max Cable Length with Generator

What kinds of problems?

RE: Max Cable Length with Generator

That is what I am trying to find out. We did a project a number of years ago and this same issue came up, but with shorter cable. The generator supplier advised that the setup was just satisfactory. Nobody can now remember why this point was raised.

The only thing I can speculate is that capacitance on the cable may excite the machine and cause over-voltage or for example sustain over-voltage when the breaker is opened on load. Possible cable side VT failure?? I think any problem could only be with the breaker open.

RE: Max Cable Length with Generator

Some gas turbine generators, like the larger GE and Siemens machines, use the generator as a synchronous motor to start the turbine.  A large load-commutated-inverter VFD  (GE calls it an LCI) is connected to the generator terminals.  It spins the unit through a purge cycle, a fuel ignition sequence and then runs it up to full speed.  

Additional capacitance on the generator terminals can
affect the LCI operation.

Otherwise, I'm not aware of any generator issues with long cables.  I would be uneasy about the exposure to faults and would make sure the protection system included the cables in a differential zone.

RE: Max Cable Length with Generator

I would suspect that the capacitance of the generator windings will be much greater than the capacitance of even 300 meters of feeders. Why not measure or otherwise determine the capacitance of the machine windings and compare this value to the capacitance of the feeders? How much percentage increase in total capacitance will there be with the feeder capacitance added?
Given that surge capacitors are sometimes installed on generator terminals, I don't anticipate a capacitive problem with long feeders.
Protection may be an issue though.

"Why not the best?"
Jimmy Carter

RE: Max Cable Length with Generator

waross is right:
Just measure the capacitance of the stator winding. Its a simple procedure and can be done with 120 Volts/60 Hz (or similar) by using a divider transformer (Trenntransformator in German). Connect all three phases of the stator winding (usually they are allready star-point connected). Apply voltage to one of the motor terminals and connect the other leg to the stator core (stator core to be grounded), measure voltage and current flow and determine total winding capacitance (all three phases) as follows:

C = I / 2 * 3.14 * f * U

    C = capacitance
    I = current
    f = frequency
    U = voltage

Good luck and best regards


RE: Max Cable Length with Generator


The proper term is "isolating transformer", not divider transformer. Sorry for that.



RE: Max Cable Length with Generator

Is there an electrical code requirement that mandates a maximum cable length between the generator and its unit breaker?

RE: Max Cable Length with Generator

Depends, what is your governing code?

RE: Max Cable Length with Generator

In North America there is a limit for conductors without overcurrent protection. In Canada the limit is 3 meters from the source of supply or size reduction to the breaker.
If you go to 300% ampacity with your cables, you may stretch out to 7.5 meters before the breaker.
Check your local codes.

"Why not the best?"
Jimmy Carter

RE: Max Cable Length with Generator


There are two issues here,

1. one is the earthing method of generator. If the generator is a large one and high impedance earthing is proposed, then long cable is a concern. If the generator is medium resistance earthed (say 100A), then 300m cable may not be a concern.

2. Secondly, requirement for isolation close to the generator. Generally, this is a safety requirement and governed by national codes. To my knowledge, some codes stipulate that there should be an isolation facility within fifteen meters from the equipment.

RE: Max Cable Length with Generator

Hi raghun;
Under Canadian codes, the supply TO equipment must be isolated within 15 meters. (With exceptions)
Protection for conductors must be within 3 meters of the source. (With exceptions) In Canada the 3 meter rule would apply rather than the 15 meter rule.

"Why not the best?"
Jimmy Carter

RE: Max Cable Length with Generator

Installation of an overcurrent device within 3 m of a medium voltage generator is not usually practical. In my experience in Canada this is a gray area within the Canadian Electrical Code and local inspectors typically desire the generator unit circuit breaker to be "as close as practical" to the generator terminals. The protection for the phase conductors leaving a synchronous generator terminal box comes down to tripping of the field contactor (as well as the unit breaker) upon detection of a fault by the generator protective relay and phase CT's. The differential zone of the generator should at least overlap to the line side of the breaker.  

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