Dear All,

I have few questions about generator as below.

1. I have heard people saying that generator should not be operated with very low load on it. Why? What will happen if the load, say 20% of the rated load of the generator?

2. What is the maximum current a generator can take above the rated current? For example, I have a generator of 500 KVA and the loads on it is two numbers of 200 KW motors and assume one of them is running with full load and the other one is starting. So the generator will take the starting current of the motor without any voltage drop or do I need to select an over sized generator?

3. When I keep a generator as standby for a building, can I connect both the neutral cable (generator and main) together or do I need to disconnect main neutral when generator power is ON?

Appreciate your advice.

Thanks & Regards,
KRA.

 

KRA,

1. The lower limit on the generator is normally associated with its prime mover, and not the generator itself. The size of generators you are quoting are normally driven by diesel engines. The problem with diesel engines is that when they run at light loads they tend to deposit carbon, from unburnt fuel, on the pistons, cylinder heads, etc. This is due to the fact that diesel engines do not atomise fuel very well at light loads, especially if the traditional fuel injection systems (not the newer common rail injection systems) are employed on the engine. At the higher loads, normally above 80% the carbon built up is burnt due to the higher temperatures available. In fact diesel engine manufacturers normally recommend that after a relatively long period of operation at light loads, the diesel engine should be run at full load for a number of hours to burn up the carbon deposits.

2. Assuming that your generator is rated at 0.8pf, then your 500kVA generator is capable of delivering 400kW, and if you include the 10% margin for governing purposes this increases to 440kW. With one motor running at 200kW, in trying to start the other you will be exposing the generator to two things:

a. your motor on starting, assuming it has direct on line starting, will absorb roughly 1200kVA (assuming 6 times nominal current is drawn on motor switch on). Most of it will be reactive power when the motor starts rotating. Presenting this reactive power to a 500kVA generator which already has 235kVA on it (assuming the running motor is operating at 0.85pg), will create severe voltage drops as I doubt it your generator can handle such a load.

b. In addition due to the motor and driven load inertia, then when the motor reaches the point of maximum torque, which is usually below the nominal speed of the motor+load (the load torque should never exceed the motor torque in any point of the acceleration curve), the real power drawn by the motor will exceed the 200kW. This coupled with the 200kW of the other motor already running will send the generator's prime mover in overload.

In actual fact I would think that the following will happen; when the second motor is switched on, this will create a severe voltage drop on the generator. This voltage drop will cause the other motor to slow down. If they both manage to remain running, as the generator voltage picks up, you will reach a point where both motors are trying to accelerate their loads, one as it is starting and the other as it was temporarily slowed down, and now it is recovering. Thus the real power drawn by the two motors will be higher than nominal, and thus higher than the capability of the generator prime mover (remember this is rated at max 440kW. Thus most probably your system will either collapse, and everything shuts down, or reach an equilibrium at a lower frequency, as the diesel engine will not manage to accelerate the loads.

3. Before answering you question re neutral, is your generator neutral grounded or floating, when NOT connected to the main supply?

 

KRA,

in answer to the 2nd part of your query... see Control List topic:

http://control.com/thread/1232621754

Regards, Phil Corso (cepsicon [at] aol [dot] com)

 

Dear Sir,

Thank you very much for the explanation.

2. Regarding the motor starting current, if I use a star/delta or soft starter to start the motors, in your opinion what size generator is required?

I have seen on a 1000kVA generator leaflet that the "Motor starting capability @ 30% Voltage dip is 1982 sKVA". Does this mean that the generator can deliver 1982kVA at 280V (70% of 400V)? 30% voltage drop while starting a motor is acceptable?

3. The generator neutral is not earthed but the neutral from the utility supply is earthed in the main panel and the transfer switch is only three pole not four pole (both neutrals are solidly liked).

Regards,
KRA