The Belgian site being studied in this paper is connected to the public transmission grid (380 kV) by a High Voltage (HV) station. This HV station incorporates a 25 kV substation linked to a second substation via 25 kV underground cables. The second substation supplies the electric installations on the site.
The substations contain 25 kV circuit breakers in interior type HV cells with a metallic casing (see pictures below). The insulation is realised by SF6 gas. The safety relays and control devices are located in cabinets situated on the front panel of the cells. Their LV supply (110 VDC and 24 VDC) is provided by cabinets situated in an adjacent room.
Description of the problem
When operating the new circuit breakers, the control circuits received over-voltages and broke down. The same thing happened during other operations and events, including in-line short circuits, section switch closures, and above all the connection of the 25 kV cables to the earth to discharge them.
Thousands of Euros were going up in smoke every time one of these events occurred. The poor EMC of the installation was generating voltage surges between the LV equipment and the earth, causing the breakdowns.
The company called in an EMC consultant to study the phenomenon and propose a solution.
Since the earthing concepts of the HV station with the incorporated substation and the second substation are different, part of the consultant’s study had to be executed separately. The first substation contained an earth loop connected to the mesh of the 380 kV public grid earthing system, while the second substation earthing system consisted of earth rods.
The EMC consultant provided the company with a solution to bring the surges below the 500 V limit tolerated by the LV equipment. He recommended, among other things, building a meshed ground grid and improving the continuity of the cable shielding. In the solution, all connections to the earth were kept as short as possible and the installation of data cables and power cables next to each other was avoided.
After implementing the proposed measures, a thorough inspection and test of the substation was carried out. The measures reached their objective and no more breakdowns have occurred.Log in to post comments