relays

Could some one please explain the purpose and operation of a negative phase sequence relay WRT main generator output

 

[mpparent]

I believe that a negative sequence relay is used for protection against unbalanced loads, open conductors, etc., where these conditions can cause overheating.

 

[peterb]

A negative sequence overcurrent relay (ANSI device # 46)responds to unbalanced load or fault currents in the generator circuit. The generator has a limited capability to supply negative sequence currents, as these induce double power frequency rotor current which leads to rotor overheating.

Typical numbers for large turbogenerators would be of the order of 10% continuous negative sequence current, with an (I2 squared x time) constant in the range 15-30 seconds, where I2 is in per unit of rated current.

The relays will include a negative sequence filter to determine the I2 level. Usually, a low set alarm setpoint is provided in addition to the trip setpoint. In today's world, the negative sequence relay will probably be one element in a multifunction relay, but the principle remains the same. Check the various manufacturer's websites for more details.

 

[busbar]

To add just a bit—there are negative-sequence voltage (device 47) and current (device 47) relays. Examples are GE ICR voltage and SGC current—ABB CVQ voltage and COQ current relays.

 

[MarkAthay]

Can anyone tell me how negative sequence relaying would be used on the relaying for a distribution or transmission circuit? Particularly interested for on a distribution circuit where coordination with a down-steam fuse is involved. Thanks for any help you may lend me.

 

[redtrumpet]

Mark - I'm not sure why you would want negative sequence relaying on a distribution circuit. As far as I know, the positive and negative sequence impedances of conductors and transformers are equal, so negative sequence relaying offers no advantage over conventional relaying of positive sequence voltage, current, and/or impedance quantities. I have asked for utility impedances at points of connection for many locations in the past, and the positive and negative sequence impedances have, without exception, been identical.

 

[dpc]

It is possible to use negative sequence current to for through-fault protection of delta-wye transformers. For a line-to-ground fault on the secondary (wye) side, there will be negative sequence current in the primary. This allows more sensitive protection from the primary side than standard overcurrent relaying.

But in general I agree that negative sequence has not been traditionally used much for line and distribution relaying. Now that the negative sequence overcurrent relaying is provided as a standard with digital relaying, maybe there are some additional applications we should be looking at.

 

[MarkAthay]

I just moved over from one utility to another and one of their consultants had talked them into having negative sequence settings on their distribution breakers. I'm going through a recoordination process between the main breakers and low-side breakers at one substation and I'm trying to figure out WHAT to do with the negative sequence settings. I want to have good coordination with the fuses so I have reliable fuse operation, and I'm worried that these negative sequence settings could gum up the works.

During a fault, what happens to the negative sequence currents? Something in the back of my mind says the GREATLY go up and can? exceed the positive sequence currents. True or false? Should I just deep-six the negative sequence settings and get on with life? I'm using Schweitzer relays that can do everything but bake bread....

Thanks again!

 

[stevenal]

I've only seen I2 exceed I1 when phases were mistakenly swapped on the CT wiring. SEL recommends coordinating with fuses for line to ground faults as worst case. I1=I2=I0 for line to ground faults. Check out their web site at

http://www.selinc.com

for tech papers and application guides. Coordination software is handy to have also.

 

[jbartos]

Suggestion: References:
1. ANSI/IEEE Std 242-1986, Section 11.4.5 Phase Balance Current Relay - Device 46.
2. ANSI C50.13-1977

Reference 1, industry standard, mentions Dev. 46 only. Unbalanced loads, unbalanced system faults, and open conductors, or other unsymmetrical operating conditions result in an unbalance of generator phase voltages. The resulting unbalanced (negative sequence) currents induce double system frequency currents in the rotor that quickly cause the rotor overheating. Serious damage to the generator will occur if the unbalanced condition is lasting. The ability of the generator to withstand these negative sequence currents is defined by Reference 2 as I2**2 x t =K. Generators under 100MVA are generally able to carry negative sequence currents up to 8-10% of full load current continuously within tolerable overheating.
Usually, a negative sequence relay, which consists of time overcurrent unit with extremely inverse characteristics matching the generator I2**2 x t curves, is recommended. Static types of the relay are more sensitive capable of detecting negative sequence currents down to the continuous capability of the generator.