relays
relays
(OP)
Could some one please explain the purpose and operation of a negative phase sequence relay WRT main generator output
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
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RE: relays
RE: relays
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.
RE: relays
RE: relays
RE: relays
RE: relays
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.
RE: relays
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!
RE: relays
RE: relays
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.