Protection and Automation
The power system comprises many expensive components and so the complete power system represents a large capital investment. The system must therefore be utilised as much as possible in a secure and reliable way, but safety is at all times number one. No matter how well designed, faults will always occur and they are a risk to life and property. The protection has to detect a fault or an abnormal operation condition of the system. The most common parameters, which give information about the presence of a fault, are the voltages and currents at the terminals of the protected component or the protected zone. Also, information about the state of circuit breakers and switches, whether they are open or closed, can be used.
Protective relays detect faults in the network and should be selective when they trip a circuit breaker to isolate the faulty part in the network. Functional testing of protective relays began in the 1980-ties and made use of networks of lumped inductances, capacitances and resistances to simulate a part of the power system under faulted conditions. But the flexibility was rather poor and the relays could only be tested for a limited number of steady state situations. To overcome this, KEMA developed in 1985 world’s first computer based test facility for protective relays. The computer calculates the voltage and current wave-shapes that are converted in real-time to analogue signals by means of voltage- and current-amplifiers. With this technology, protective relays could be tested for a wide variety of settings and different system time scales, including transient phenomena.
The first digital protective relay came on the market around 1990 and many of these digital relays were tested at KEMA. Software became an important issue as the behavior of a relay could be excellent for one setting but inadequate for another setting. Digital relays in a substation are sensitive to influences from outside and this required EMC and environmental tests. KEMA’s experience with EMC tests for classical components was helpful to define the correct requirement levels. KEMA shared its knowledge with IEC and in 2005 a new series of standards for protective relays was being published.
Also, digital communication entered the transmission and distribution sector in the 90-ties, in the beginning the main problem was to merge different technologies, ranging from a traditional relay-controlled substation to manufacturer specific turnkey solutions. KEMA took the lead in the discussion on interoperability and defined the blue print for open protocols that resulted in the IEC 61850 protocol.
Till 2007 there was a strict separation between primary components, e.g. the circuit breaker and its digital control device. This changed with the introduction of the first digital auto-recloser, where the control circuit is integrated in the same housing as the breaker. The IEC standards do not cover such an integration. KEMA developed methods to test the control device whilst the breaker receives a trip signal to open its contacts when there is a short circuit. Digitalization of the T&D networks is just beginning.