A network for the transmission of electric energy basically consists of (long-distance) connections. These connections are normally comprised of overhead lines, but underground cables are also used in densely populated areas and in special cases.

All components in the high-voltage network must serve to transmit electricity along the connections: 

• Switchgear to energize and de-energize the overhead lines and underground circuits;
• Transformers to reach the most economical voltage level for transmission;
• Capacitor banks and shunt reactors to control the voltage;
• Control centers and telecommunication systems to operate the grid;
• Protection systems to prevent the collapse of the network's functionality in the event of equipment failure.    

One of the most important elements in the protection system to assure a functional and reliable network is the circuit-breaker. The circuit­breaker will energize and de-energize a section of the high-voltage network in normal applications. The current may be lagging or leading, but will normally not exceed the rated nominal current of the system. For merely switching these currents, the circuit-breaker would not have to be as complicated as it is. However, in case of a short circuit somewhere in the network, the circuit-breaker is the only means of protecting the network. Therefore, it must fulfil more difficult requirements. 

If a short circuit is not interrupted immediately, the backup protection systems will trip a larger part of the transmission grid, leading to the outage of more overhead lines, busbars and substations, and consequently the interruption of the power supply for a larger area. The power supply and voltage level for the remaining part of the grid may be endangered.

During a short circuit, the synchronizing power of the network is (partially) lost. Longer duration of a short circuit may result in dynamic stability problems for the generating plants. Therefore, fast elimination of the fault is necessary.

Rapid, selective, and reliable clearing of the short-circuit currents is required. In this context, KEMA's involvement is to reassure, as an independent and impartial testing authority, the circuit-breakers' performance and reliability.

For more information, please contact the KEMA laboratories.