Regulating the Exciter Voltage During Start of High Inrush Motor


Thread Starter


We have a GE frame 6B, brushless excitation and air cooled generator.
Question is concerning exciter voltage regulation in island mode.
MkVI and EX2100 control systems.

plant is a sweetening refinery with multiple electric motor driven gas compressors. The inrush for start of one of these electric motors is significant, effectively an 8MW motor load.

GE philosophy for the software is that island mode simply breaks the generator control mode, disables VAR or PF control mode on generator. In island, for plant black start we require excitation regulation to counter the inrush of the compressor electric motors on start. Should control mode "OFF" already provide regulation to a manual set-point? Or is it normal for this sort of island dynamic large load to have a voltage regulator code section purely for island mode?

System is MkVI and EX2100 excitation. The voltage or exciter regulation is actually made in the MkVI and simply sends raise and lower pulses to EX2100. even though EX2100 has code sections that seem it could control itself the error from set-point and control response.

Is it possible, if the load of island was significant enough, that in island PF or VAR control is a possible operation mode?

It seems the code is based upon an export power producer with an expected island auxiliary load of no more than accessories to run turbine. What we actually have is a power station to provide all plant power, though connected to grid, without any positive export power.

Anyone have experience of large plant island start and associated regulation of exciter voltage during start of high inrush large motor load?

inrush usually relates to current and MVA, never MW.

What are the actual nameplate parameters of:

1) Generators; MW, kV, A, and Pf?

2) Motors; MW, kV, A, and LRA/FLA ratio (Locked-Rotor-Amperes to Full-Load-Amperes)?

Phil Corso
I am not an expert but in my basic understanding the purpose of the AVR is to maintain a fixed voltage on the generator terminals by increasing or decreasing the DC current flowing through the generator field. So, when starting a large inductive load you will see an increase in VAR flow, a drop in voltage at the generator terminal voltage, and the AVR will respond by increasing field current until the generator terminal voltage reaches setpoint (voltage prior to motor start), or go as far as it can upto the AVR programmed Over Excitation Limit.

The PF and VAR control modes will not help in this situation since they typically are slow acting loops, the AVR is designed to respond very fast to voltage changes. Turning the AVR OFF will worsen the situation since the Exciter will not change the Field Current and you will see voltage drop and stay low until manually raised.

What are you seeing happen when you start the gas compressor that you think is a problem? Are there limiter alarms coming in on the EX2100? Is the generator able to pick up the load?