and
Reference:
1. IEEE Std 242-2001 (Buff Book)
Section: 12.5.5.4 Overexcitation protection beginning by page 485.
V/Hz = 1.05 per unit for a generator (i.e., generator base) and above causes the generator and associated transformer overexcitation
Also, when generator operates at lower speed V/Hz ratio is monitored to protect the generator and load.
Simply put it is the nominal voltage to the rated Hz.
For one of my generators;
13.8kv/60 = 230volts/Hz
What this does is provide a 'sag' to the AVR such that the motor or generator does not go into over saturation.
The reason that I mention motors is that it also applies to VFD's as well.
V/Hz relation is a quantity used to determine the degree of heating a generator or transformer can be subjected to as a consequence of overdervoltages and/or underfrecuencies. Protection of generator and step up transformer against this condition is accomplished through a temporized protection scheme implemented in the AVR (for obvious reasons).
At the moment i don't remenber typical curves and settings for this protection, but i will investigate.
We have a 15.5 KV rated generator at 50 Hz. The alarm on overvoltage is set at 16.2 KV. Tripping stage one is 17.02 KV for 10 sec and 20 KV instantaneously.
Actually i wanted to know that before the tripping voltages are acheived, what will be the role of VPFL, volts per freq limiter? What does it do as part of AVR. What are the alarms settings of VPFL typically and whats the ratioanle.
V/F is usually taken as ratio of per unit voltage to per unit frequency. E.g., frequency is at 50.5 Hz, when will V/F limiter operate?
Also as i dont have access to IEEE standards, i cannot get further information from the reference. Unless someone can send me excerpts or the standard to me at irfan.akhtar@elpaso.com
It was not possible to tell from the original post which standards were applicable to the subject generator.
Most IEEE standards are available online or emailed PDFs, but access is fee based. Given the followup information supplied, they may have limited value to you.
The volt-hertz limiter within the AVR is usually a clamp which prevents excitation current increasing once the V/Hz limit is reached, thus preventing over-fluxing of either the generator or, more probably, the generator transformer. An alarm is usually generated by the AVR when this clamp becomes active. A separate relay outside of the AVR provides the tripping function, providing independence of the protection function in the event that the volt-hertz limiter within the AVR fails or does not respond rapidly enough.
