Relay Protection And Substation Automation Of Modern Ehv Power Systems (Moscow – Cheboksary, September 10-12, 2007)

Relay Protection and Substation Automation of Modern EHV Power Systems (Moscow – Cheboksary, September 10-12, 2007)

New and re-discovered theories and practices serving the modern power system protection and control

Adaptive 100% Stator Earth Fault Protection Based on Third Harmonic Voltage Measurement I. Brnčić, Z. Gajić, S. Roxenborg, ABB AB, Substation Automation Products Swedenzoran.gajic@se.abb.com T. Bengtsson ABB AB, Corporate Research Sweden

KEYWORDS Generator protection, Stator earth fault protection, Protective relaying.

Introduction
Short circuit between the stator winding in the slots and stator core is the most common type of electrical fault in generators. According to a big US utility, 70 to 80 % of all electrical faults in their generators have been – orstarted as - earth-faults of the stator winding. The protection described in the paper is capable of detecting earth faults occurring at any point of the stator winding, and is sufficiently sensitive to indicate also the deterioration of insulation, leakages, in the winding sections situated close to the neutral point of the stator winding. The 100 % stator earth-fault protection incorporates twoseparate protections which complement each other and together cover the whole stator winding. The sub-protections are: 1. 2. The 3rd harmonic-voltage-based differential protection (ANSI designation 59 THD), Fundamental frequency (zero-sequence) over-voltage protection (ANSI designation 59 N)

The paper describes in more detail only the 3rd harmonic-voltage-based differential protection..Generator earth fault protection
The stator earth-fault protection of large generators connected to their step-up power transformers should preferably be able of detecting small earth current leakages occurring even in the vicinity of the generator neutral. A high resistance earth-fault close to the neutral is not deadly itself, but must anyway be detected in order to prevent a possibility for a doubleearth-fault when another earth-fault can for example occurs near the generator terminals. A typical example is a water-cooled generator, where deteriorations of winding insulation near the neutral should also be detected, because any incipient leakage of cooling water in the vicinity of the neutral point would cause a gradual deterioration of insulation without giving rise to immediate breakdown(the voltage stress imposed on the insulation near the neutral being a few hundred volts only). If such deteriorations remain undetected, a sustained leakage of water may damage the insulation of another conductor in the same slot or, possibly, that of a conductor in an adjacent slot. If this conductor is situated close to the phase terminal, its normal service voltage with respect to the earthedstator frame lies in the order of magnitude of 10 kV. This voltage will of course sooner or later cause the breakdown of the deteriorated insulation, and the first earth-fault of the stator winding will
SA2007-000670

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Relay Protection and Substation Automation of Modern EHV Power Systems (Moscow – Cheboksary, September 10-12, 2007)

develop near the phase terminal. Under the effect ofthis first earth-fault, voltage of the generator neutral will rise to the level of up to 10 kV with respect to the earthed stator frame. This sudden voltage rise will cause most likely a flashover at the weakened spot near the neutral. Thus a most hazardous type of fault, a double earth-fault will result. The 100 % stator winding protection described in this paper is capable of detecting not only any(low) resistance earth-faults anywhere along the stator winding of the protected generator, but is also sufficiently sensitive to indicate the deterioration of insulation in the winding sections situated close to the neutral point of the stator winding, which is not immediately critical, but must anyway be detected in order to prevent a double earth-faults.

The 100 % stator earth fault...