Distribution Transformer Protection: Devices and Settings
Protection exists to disconnect a transformer before a fault destroys it, and to avoid disconnecting it when there is no real fault. Getting that balance right is a coordination exercise between the transformer, the MV switchgear and the LV board.
This article covers the standard devices and what each one detects.
Protection exists to disconnect a transformer before a fault destroys it, and to avoid disconnecting it when there is no real fault. Getting that balance right is a coordination exercise between the transformer, the MV switchgear and the LV board.
This article covers the standard devices and what each one detects.
Protection on the medium voltage side
MV protection is provided by the switchgear: fuses, a circuit breaker with a protection relay, or a combination. Common functions include overcurrent, earth fault and, for larger units, differential protection.
The transformer’s impedance sets the fault current, so protection settings must be based on the actual nameplate data.
Internal fault protection for oil immersed units
Buchholz relay. Detects gas accumulation and oil surges caused by internal arcing, alarms then trips
Pressure relief device. Vents sudden internal overpressure to prevent tank rupture
Oil level indicator. Alarms on low oil level
Oil and winding temperature indicators. Alarm and trip on high temperature
Protection for dry type units
PTC thermistors or Pt100 sensors embedded in the windings feed a temperature relay with alarm and trip outputs and, where fitted, staged forced-air cooling control.
Protection on the low voltage side
The LV board provides overcurrent and earth fault protection for outgoing circuits. Its incoming device must coordinate with the transformer’s upstream protection so that a downstream fault clears selectively.
Surge protection
Lightning arresters or surge arresters at the HV terminals protect the insulation from lightning and switching surges, particularly for outdoor and overhead-connected units.
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