05-11-2010 06:09 AM
We always choose only one neutral of the generators to be switched on to the system in multiple generator set parallel operation applications to avoid neutral circulation currents.
If this particular generator set , that the neutral is switched on to the system, shuts down or trips for some reason then other generators are operating without an earth reference and would create a floating system. Risk of high voltage or unbalance Voltage could result?.........not good for loads.
How we should design our control & protection system fast enough to activate other neutral switches of the running generators, in order to prevent tripping of the entire system due to activation of high voltage protection, and to maintain reliability of the operation.
What will happen to AVR, and generator set controller as both are sensing the generator output voltage to control the unit.
Do we need to add surge diverters / filters in series in to their sensing circuits.
We can introduce electrical interlocking between generator circuit breakers and neutral switches to avoid tripping of breakers before closing the neutral switch of the connected healthy unit. But what will happen if engine fails?
Are fibre optic circuits fast enough to send close commands to the neutral switches than copper based wiring,,,,,,, can we apply this.? Could be comparatively expensive.
Please add your thoughts to share the experience in this regard
05-14-2010 03:45 AM
My opinons as follows:
1. By all means, neutral earthing is needed in LV system, but in differnet countries, different codes and regulation.
2. whether adopting one neutral earthing depends on the design of whole earthing system.
3. The generator model and quantity decide the amount of circulation currents. The circulation currents are small for the same gensets.
4. The accepteance degree of the genset to circulation currents is different as well as the relay-protection system.
5. For steady status, the loads are shared in same, the circulation currets is small; For transient status, the circulation currents is large because the adjusting of AVR, but the time is short.
Some books say there is no problem with circulation current in 8 same units paralleling system.
Our experience is 15 units G3520,2000KW,10KV, 3 phase without neutral earthing in parallleling system as Power plant, but the system is pralleling with mains. So it can not explain the application of island mode.
Cat do write some words about one neutral earthing in paralleing application in OMM.
My suspense: Is the circulation currents really fearful for parallleling system?
That depends on the cause of circulation currents. I think there are two kinds of circulation currents.
One is harmonic currents from generator self and load;
the other results from the adjusting of AVR, exciting system.
Hope be helpful!
05-14-2010 06:47 AM
According to my experience ,Neutral Earthing is necessary for the mediuem voltage generator and you must ground the body of the generator because high voltage .In low voltage system ,Several time synchronize the genset but every time i must gorund the neutral of every generator and seperatly ground the body of the genset but why i do.Reason is that when you run the machine without load circulating current remain zero.if you putt the load on the genset gradually you will observed that neutarl current is normal and you did not feel any shock from the generator body .if you donot ground the neutarl of the generator and suddenly putt the load on the generator or ur system is variable load mean surage load .when will have come surage load on the generator you feel electric shock at the generator body and during this time you check the voltage across the neutral and body .you observed the voltage 220 or 110 and this time you try to connecting the ET sorfware with ecm it will not be connect and these voltage are not good for the engine ecm and emcp control panel.