Neutral Earth connection

Yes looking into this further, it is not a good situation at all. The substation is as said a split substation - there is an automatic bus coupler between the two LV sections. The primary feed comes from an 11kv to 400v transformer, one transformer per side. Its been configured to run with both tansformers closed and the bus coupler open. should there be an outage on the HV side the couplers close and either side of the substation remains powered from the live transformer. 


If the mains is lost totally then all the output circuit breakers on the substation open, the remote generators start up and the HV from the generators is supplied to the substations through the existing network. However when the HV fails the main incomer circuit breakers also open and this removes the neutral earth link as this is located on the transformer, precisely for the reason quoted, should there be no earth reference there then there is a case that the transformer secondary voltages could trouble the insulation of the LV wiring. 

Proposal is to use neutral earth contactors on the substation bus to ensure that the neutral remains earthed when the incoming circuit breakers are open. Also in addition to this a neutral earth contactor is included on the UPS input switchboard so that if the supplies to the UPS input switchboard are opened then the neutral will remain referenced to earth. The supply is designed and operated as a TN-S system so that should be maintained. The site is a hospital and the UPS supply is used to support critical equipment so this needs to be kept as stable as it can be. There is further work required as some other issues have been identified with the system, The UPS supply although has two independent feeds these feeds pass through the UPS input switchboard which will then in effect be a single point of failure. Some way to automatically ensure that there can not be a supply loss will be required. There may be a way of doing this and also looking at use of a transformer to provide galvanic isolation between the UPS and supply so a local earth point could be utilised. Looking at data centers it may be worth adopting what they use for critical data and employ an additional UPS along with the relevant static switchers to ensure no break supplies to the super critical loads.


With UPS units and circuit breakers it becomes a mine field to try and ensure that the supply is maintained and remains within its design parameters, especially when the UPS system is used to fully support the Isolated Power Supply IPS systems on site.

Yes looking into this further, it is not a good situation at all. The substation is as said a split substation - there is an automatic bus coupler between the two LV sections. The primary feed comes from an 11kv to 400v transformer, one transformer per side. Its been configured to run with both tansformers closed and the bus coupler open. should there be an outage on the HV side the couplers close and either side of the substation remains powered from the live transformer. 


If the mains is lost totally then all the output circuit breakers on the substation open, the remote generators start up and the HV from the generators is supplied to the substations through the existing network. However when the HV fails the main incomer circuit breakers also open and this removes the neutral earth link as this is located on the transformer, precisely for the reason quoted, should there be no earth reference there then there is a case that the transformer secondary voltages could trouble the insulation of the LV wiring. 

Proposal is to use neutral earth contactors on the substation bus to ensure that the neutral remains earthed when the incoming circuit breakers are open. Also in addition to this a neutral earth contactor is included on the UPS input switchboard so that if the supplies to the UPS input switchboard are opened then the neutral will remain referenced to earth. The supply is designed and operated as a TN-S system so that should be maintained. The site is a hospital and the UPS supply is used to support critical equipment so this needs to be kept as stable as it can be. There is further work required as some other issues have been identified with the system, The UPS supply although has two independent feeds these feeds pass through the UPS input switchboard which will then in effect be a single point of failure. Some way to automatically ensure that there can not be a supply loss will be required. There may be a way of doing this and also looking at use of a transformer to provide galvanic isolation between the UPS and supply so a local earth point could be utilised. Looking at data centers it may be worth adopting what they use for critical data and employ an additional UPS along with the relevant static switchers to ensure no break supplies to the super critical loads.


With UPS units and circuit breakers it becomes a mine field to try and ensure that the supply is maintained and remains within its design parameters, especially when the UPS system is used to fully support the Isolated Power Supply IPS systems on site.