The Engineer's Place for News and Discussion®

 Previous in Forum: Converting 3 Phase to 1 Phase in Reverse? Next in Forum: Class 0.3 CT for Revenue Metering

### Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.

### Rating Vote:

Member

Join Date: Jan 2012
Posts: 9

### Functioning of AVR on Heavily Unbalanced System?

08/11/2012 10:05 AM

How do AVRs treat heavily unbalanced system? Say for a small 400V 15KVA generator, two of the phases are fully loaded and one phase is open, what will AVR do?

I guess, it works by calculating V_avg = (Vab_rms + Vbc_rms + Vca_rms)/3, and tries to maintain V_avg = 400V by varying the excitation.

Interested in this topic? By joining CR4 you can "subscribe" to

Guru

Join Date: May 2007
Location: NYC metropolitan area.
Posts: 811
#1

### Re: Functioning of AVR on heavily unbalanced system?

08/11/2012 3:20 PM

Generally speaking AVRs know nothing about the degree of imbalance so your assumption is correct only if the AVR senses all three phases, but not all do.

The governor won't know either, all it sees is the speed dropping as the load increases (isolated mode), leading to the possibility of two phases carrying the full load that the governor can supply, at the expense of overloading those phases.

Unbalanced operation is undesirable on any machine, larger machines have relaying to alarm and prevent it while smaller sets lack any real protection against it.

__________________
Curious minds want to know, engineering minds get answers....
Member

Join Date: Jan 2012
Posts: 9
#2

### Re: Functioning of AVR on heavily unbalanced system?

08/11/2012 10:11 PM

Thanks.

So some AVRs sense only one set of phase voltage, say Vab, and adjusts excitation based on it?

However, I don't see why such unbalance is damaging except for terminal voltages being unbalanced. As long as current in each phase don't exceed the rated current, why would the generator care?

In my above example, I wasn't telling that full machine power would be used by only 2-phases, but rather, only their shares. If the loads are placed Phase-to-neutral on two of the phases, such that the line current don't exceed rated current, whats bad here? Is the neutral current that will flow now, bad for generator? if yes why?

Guru

Join Date: May 2009
Location: Ketchikan, AK, USA
Posts: 12929
#3

### Re: Functioning of AVR on Heavily Unbalanced System?

08/12/2012 12:43 AM

Under the assumptions in post 2, I don't see a problem for the generator, either. However, any 3-phase motors running in this condition may overload or stall.

In general, different AVRs may respond differently.

__________________
In vino veritas; in cervisia carmen; in aqua E. coli.
2
Guru

Join Date: May 2007
Location: NYC metropolitan area.
Posts: 811
#4

### Re: Functioning of AVR on Heavily Unbalanced System?

08/13/2012 6:49 PM

Operation of a generator with a phase open is not acceptable and utility grade generators have protective relaying to trip the unit within a relatively short period.

Basically the fact that a line conductor is open doesn't mean that no current is flowing in the windings, in fact there are very high negative sequence currents in all three windings, and these double frequency currents induce huge amounts of eddy currents in the stator laminations, rotor surface, and surrounding metal parts. These undesirable currents cause intense heating that rapidly breaks down insulating materials shortening the life of the generator.

A thorough discussion of the reasons require an understanding of the internals of a generator and symmetrical component analysis, so here's the short version from a NERC (North American Electric Reliability Council) slide on the subject, from slide 6:

http://www.nerc.com/docs/pc/spctf/Phoenix%20Prot%20Coord%20Workshop_Session%207_Device%2032%2046%20et%20al_20100317.pdf

"...The Need for Negative Phase Sequence or
Unbalanced Overcurrent Protection - Function 46

"There are a number of system conditions that may cause unbalanced three-phase currents in a generator. The most common causes are:

•system asymmetries (untransposed lines),
•unbalanced system faults, and
•open phases.

These system conditions produce negative-phase-sequence components of current that induce a double-frequency current in the surface of the rotor, the retaining rings, the slot wedges, and to a smaller degree, in the field winding.
These rotor currents may cause high and possibly dangerous temperatures in a very short time.

The ability of a generator to accommodate unbalanced currents is specified by IEEE Std C50.12, IEEE Std C50.13, and IEC 60034-1 in terms of negative- sequence current (I2 ). This guide specifies the continuous I2 capability of a generator and the short time capability of a generator, specified in terms I22t=K, as shown in Figure 4-39 (curve drawn using data from IEEE Std C50.13)."

IEEE C37.102-2006 - Guide for AC Generator Protection, Section 4.5.2 (emphasis added)..."

Sorry for the jargon but that's how utility engineers talk, so whenever you come across a phrase you've never seen before just Google until it starts to make sense. Enjoy the trip!

__________________
Curious minds want to know, engineering minds get answers....