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Dear sir,

I am working in CPP with 750 MW power generation capacity. There are 6 gas turbine generators. Generators have both overvoltage and overexcitation protection. If overexcitation occurs, then overvoltage protection can also sense this fault. So my question is why are both protections used? Is the function of both protections the same or different? Are both really needed?

Very simply, under certain conditions and depending on the settings of the relays, it is possible for the generator terminal voltage to exceed the over-voltage protection setting without the exciter output exceeding the over-excitation relay setting.

Responding to Girish's 13-Jan-09 query...

Let's start with the technical definitions, as related to power systems, given in IEEE Standard 100:

o Overvoltage (1) (general). A voltage above the normal rated nvoltage of a device or circuit.

o Overvoltage (2) (rotating machinery). An abnormal voltage higher than normal voltage, such as might be caused by lightning or switching surges.

Overvoltage at power line frequency can be caused by: a malfunctioning voltage regulator; human error; a sudden load change, especially the kVAr component; or, a sudden speed increase experienced in hydro plants.

o Overexcitation. While protective devices can be guard against voltage regulator failure, none are provided for overexcitation (nor for its converse, underexcitation)..

The terms are more often used to explain the changes a generator's air-gap flux, hence the internal generated voltage (emf) when describing paralleled generators or generator-grid operation.

o Field overcurrent. I have added this term because a generator's rotor can experience excessive damage for certain rotor-winding faults.

Regards, Phil Corso (cepsicon@aol.com)

Phil, most exciter regulators have at least the option of under-excitation limits or even tripping (to prevent exceeding some minimal power factor, leading,). Overexcitation limits or tripping are also options on most exciter regulators, for the same reasons, to limit exciter output to protect the exciter or the generator rotor or to try to prevent generator terminal "over-voltage".

CSA, when I said "none were provided for over excitation" I was alluding to the fact there is no ANSI Protection Device Designation Number. The limits imposed by the AVR essentially serves to maintain operation within the Generator's Capability Curve.

Too bad you hadn't read my paper, "The Physics of... Armature-Reaction." You would have been informed about ancillary functions provided by voltage regulators!

Phil,

Please refer to this World-Wide Web posting:

http://www.kilowattclassroom.com/Archive/DevNos.pdf, Sheet 4, Device Number 59.

I have seen under-excitation protection schemes enumerated as 27 devices. Sometimes, it's as simple as a Loss-of-field Relay, a 41 device.

CSA... my point(s) exactly.

The Website you suggested is a list of the ANSI Protection Device Designation Numbers, I mentioned.

While Device # 59, Overvoltage, is listed, there is none listed for Overexcitation.

Device #40 (not #41), Loss of Field, does not directly measure "under excitatation", but, instead measures armature current and terminal voltage, indicative of the importation of kVAr resulting from abnormally low excitation.

Phil

We use 61 or 24 as the ANSI number for Over excitation. As such ANSI have kept some numbers free which can be used for some of the protection not defined.