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Protection Maloperation Investigations

Protection maloperations happen and the effects can be costly.

AEE has investigated the following protection maloperation at a 300 MW gas turbine power station.

Each time one of the two generators was nearing its rated output, the generator step up transformer protection would trip on overcurrent. On first inspection, all of the relay setting parameters looked to be in order. There was anecdotal evidence that during the protection commissioning, the transformer HV bushing CTs had been reconnected from star to delta configuration.

This had been necessary because of an unforeseen limitation in the transformer differential protection relay. The relay could not cope with the comparatively large transformation ratio of 330/15 kV. When the transformer HV CTs were altered from star to delta, the ratio of HV to LV CT currents seen by the relay was reduced a value that lay within the range of available settings values. The relay’s differential setting parameters were duly altered to reflect the new lower HV CT current ratio.

In the relay architecture, however, there was no automatic link between the relay’s differential function and the relay’s overcurrent functions. Whilst the differential elements were adjusted for the HV CT delta connection, the overcurrent elements were not. The unintended consequence of the reconnection of the CTs was that the overcurrent elements saw 173% of the current that they had previously seen. Hence the relay tripped on healthy load current. Simply multiplying the overcurrent pick up current setting by a factor of 1.73 put things right. The other gas turbine generator’s step up transformer was rated at 132/15 kV and, because of the lower turns ratio, did not need to have the HV CT connections altered when it was commissioned.

There were also random protection operations on both units which could not be attributed to any specific relays. AEE audited the protection systems on both units and recommended the following remedial measures:

  • The removal of accidentally crossed wiring between AC protection & DC control circuits,
  • The earthing of the CT secondary wiring where it had been installed floating free from earth,
  • Improved harmonic current blocking settings in the transformer differential relays.

Reliability is now level with industry best practice.