Neutral Voltage Displacement (NVD) is a form of protection used by Ausgrid that uses voltage transformers to detect when the fault-current contribution is interrupted by the opening of circuit-breakers. When this contribution from Ausgrid is interrupted, it is possible that the distribution network is still being energized by a generator without fault current flowing. The generator must be disconnected from the system when a situation as such arises for safety and legal reasons. Unfortunately, due to the use of delta-star transformer windings and the earthing arrangements in Ausgrid’s distribution network, it can be difficult to detect the earth-fault. This is why the NVD is a necessary protection. It is very important to make sure that the voltage transformers are the correct type and that they are properly configured.
Installations of NVD will be negotiated with Ausgrid based on the merits of the proposed generator connection point. Some generators may not have an 11kV connection point available for the customer to provide NVD protection, particularly for generators with a low-voltage connection point. In these cases, installation and location of the voltage transformers for the NVD should be discussed in the assessment process. Ausgrid may provide a voltage transformer to the customer so that NVD may be implemented. The settings for NVD should be negotiated with Ausgrid for each individual case.