Voltage Collapse Explained

[saladhawks]

I have read several articles attempting to explain voltage collapse in an electrical system, but most are at the graduate academic level with too much math for my taste. I know all about VARS (reactive power) being required to support Voltage, but I am still missing something.

I have attempted to look at voltage collapse as a similar event to connecting more Watt (real power) load than a generator can sufficiently support. In this case, the prime mover would be inadequate and the rotor would slow down. This would lead to a loss of rotor speed, which would coincidentally lead to a loss of generator frequency, with the generator eventually tripping out via an under frequency relay.

In the case when more VAR (reactive power) load is connected than a generator can sufficiently support, the generator exciter will continue to increase its output until it exceeds its thermal limit, with a over-excitation relay eventually tripping out the generator. Is this the point when the voltage actually collapses????

 

[ccjersey]

Well when it collapses depends on what the source of excitation power is. If it's the stator, there's only so much voltage that can be lost before it becomes unable to support the field. That's why permanent magnet exciters are fitted, to enhance field current capability. In that case, with a separate exciter generator, the revs of the prime mover would be the main impact on field current capability.

 

[waross]

Many Automatic Voltage regulators include Under Frequency Roll Off. As the prime mover is over loaded and slows down, the voltage is reduced in proportion to the reduction in frequency. This is common on small standby sets. The opposite philosophy applies to larger networked sets. On larger sets, field boosting is sometimes used to supply enough fault current to ensure breaker tripping. Current transformers are used to supply the excitation under conditions of high current.
Remember that a fault will pull down the terminal voltage of a generator. When the voltage drops below the minimum needed by an AVR powered by line voltage the excitation will be reduced also. Under these conditions the current will be less than would be indicated by simple calculations based on rated terminal voltage and full excitation.
respectfully

 

[ePOWEReng]

An important aspect of voltage collapse is how a tranmission system responds to the loss of critical transmission lines. If a parallel line trips out between an area with excess generation and an area with more load than generation then that tranmission path becomes weaker, the inductive reactance increases and the path "sucks" more VARs from the system. Take a look at what Surge Impedance loading is (when you increase loading further past the SIL then the path becomes more inductive and sucks more VARs):

http://www.o-t-s.com/sil.htm

If the transmission system VAR support is a high % of capacitor banks, it can compound the problem, since you just dropped the voltage do to losing a line, now your reactive support from your cap banks is smaller too (Q=V^2/Xc). This is part of the reason why generators are set on voltage control instead of pf control, it allows them to provide VARs to support the system, temporarily until more Cap banks are switched in or transmission lines come back in service. See the Aug 1996 blackout reports.

 

[Tinh123]

hold6448,

I have been looking for the 1996 blackout report for quite some time, but can't get a hold of it. Can you tell me where I may be able to obtain this report? Thanks.

 

[ePOWEReng]

http://www.nerc.com/~filez/reports.html