GE DS200TCDAG1B Vector network analyzer

Restricted Earth Fault Restricted earth fault protection (also known as “Restricted Earth Fault”, acronym REF) is applied in order to detect earth faults occurring both on the secondary winding of the distribution transformers as well as on the connection cable between the transformer and the LV circuit-breaker, faults which could be unlikely detected by the protections placed on the primary winding (e.g. by the transformer residual current protections against internal faults), because of their modest intensity. A possible solution to detect and eliminate this type of fault, is using an external toroid connected to the star centre of the transformer and using the signal coming from this device to open the MV circuit-breaker and disconnect the fault.

shows a principle diagram 

comprising a MV/LV earthed delta-star transformer, where the symbol Id,G> identifies the protection REF (restricted earth fault) whereas Id> indicates the residual current protection of the transformer. The distribution system on the LV side is type TN-S. Taking into consideration an earth fault on the secondary windings of the transformer, it is evident that the single-phase earth fault on the LV side is seen from the MV side as a two-phase fault. By means of some simple calculations, it is possible to demonstrate how, while the REF protection is able to detect the whole fault current, the residual current protection of the transformer sees only a percentage (even if very small) of the fault current.

For example

for a 20 kV / 400 V earthed delta-star transformer, if the fault occurs at halfway of a winding, the residual current protection of the transformer shall detect a current which is 0.6% of the effective earth fault current. This could make it impossible to set the residual current protection of the transformer so that it can trip with such a low threshold. It is necessary to point out that the toroid connected on the star centre detects also every current deriving from an earth fault on the supply side (non-restricted earth fault): therefore it is necessary to provide a protection able to discriminate (time selectivity) between the two fault typologies. The solution offered by the protection trip units by ABB is defined “Double G”. For further information on the “Double G” function, it is possible to consult the technical documentation given by ABB.

 Company product range

----------------------Ruichang Mingsheng Automation Equipment Co., Ltd----------------------

PLC module, programmable controller, CPU module, IO module, DO module, AI module, DI module

Network communication module,

Ethernet module, motion control module, analog input module, analog output module, digital input module, digital output

Module, redundancy module, power module, relay output module, relay input module, processor module