Staying grounded

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Orlando De Leon of Omicron Electronics outlines the main challenges specific to testing circuit breakers installed in gas insulated substations when both sides are grounded.

When it comes to testing circuit breakers installed in a gas insulated substation (GIS), the main challenge is the safe access to the high voltage conductors and/or terminals. In GIS systems, the high voltage terminals are normally fully enclosed within the housing, therefore it is not possible to connect test instruments directly to the high voltage conductors and/or terminals. The common approach is to use the connection path provided by the built-in earthing switches. The earthing switches will commonly have insulated terminations externally on the GIS with an external bolted linkage to the earth structure of the GIS called ground connections.

The circuit breaker is tested via an inductive current change measurement using the parallel ground connection of the circuit breaker while the GIS remains grounded on both sides. Figure 1 shows the electrical diagram for the set-up.

 

Figure 1: Electrical diagram for a current sensor arrangement on a GIS

The current sensor is a Rogowski coil that is laid around the ground connection of the earthing switch, as shown in Figures 2 and 3. This connection ensures that intrusive changes or additional connections are not needed to be made in the GIS, therefore there is no added risk taken by removing a ground connection or de-commissioning bay parts of the GIS.  

 

Figure 2: Rogowski coil connection on an earthing switch

 

Figure 3: Rogowski coil connection on an earthing switch

The test is performed by injecting a DC current (up to 200A) through the circuit breaker/housing of the GIS using ground connections of the earthing switches, as shown in Figures 4 and 5. The current clamp is for the current injection and the black Rogowski coil is the current sensor in both figures. 

 

Figure 4: Current injection and Rogowski coil connection on an earthing switch

 

Figure 5: Current injection and Rogowski coil connection on an earthing switch

If the circuit breaker is closed, this current will take two different paths, the main contacts of the circuit breaker and the housing of the GIS, once the circuit breaker is opened, the only possible path for the current will be the housing of the GIS. This condition is shown in Figure 6 where the circuit breaker is in a closed position and when the circuit breaker opens and the change of states is measured by the current sensor. 

 

Figure 6: Diagram opening operation

If the circuit breaker is open, the current will flow only through the housing of the GIS and when the circuit breaker closes the current will take two different paths, the housing of the GIS and the main contacts of the circuit breaker. This condition is shown in Figure 7 where the circuit breaker is in an open position and when the circuit breaker closes and the change of states is measured by the current sensor. 

Figure 7: Diagram closing operation

The current sensor will measure the changes in the current each time an operation is made. The current change that is measured through the ground connection of the circuit breaker is then used to determine the opening and closing times, while the circuit breaker remains grounded on both sides throughout the entire measurement.