Protective relays fundamental

Power system protection plays a crucial role in the reliable and safe operation of electrical energy supply systems. It is an indispensable component for preventing or at least minimising faults and failures in the power grid.

The occurrence of faults and malfunctions in electrical power transmission and distribution systems is unavoidable. These faults can have various causes, such as overvoltages caused by lightning strikes, short circuits caused by mechanical damage during construction work or bridging of the insulation by animals or branches. In addition, faults can occur due to ageing of the insulation or operating errors such as switching earth fault and short-circuit devices or opening disconnectors under load.

In power supply systems, the most common faults are short-circuit and overload faults. Due to their low prices, low-voltage systems are usually protected by miniature circuit breakers. They are able to recognise an overload and switch off automatically. At higher voltage levels, it is no longer possible to interrupt high currents so easily. More complex protective devices are used there. These recognise a fault and can open a corresponding power circuit breaker to switch off the fault.

The protective devices installed in the systems cannot prevent faults from occurring, but usually only intervene after they have occurred. Their main task is to quickly and reliably detect faults and issue a control command to switch off the affected network element, e.g. a cable or a transformer. This prevents major damage, minimises hazards and ensures the continued operation of unaffected grid elements. The latter is particularly important for the stability and reliability of the power supply. For example, severe short circuits can lead to voltage dips in the entire grid.

Various grid protection devices are presented, parameterised and investigated in the experiments:

• Reverse power relay
• Time Over/undervoltage relay
• Time overcurrent relay (I)DMT

Learning objectives

• Operation of protective relays
• Parameterisation of protection devices and hands-on exercises
• Configuration of the protective circuit; combination with circuit breakers
• Tripping in the event of a fault
• Testing further important functions
• Requirements for protective devices

Target Audience

The experiments also provide practical experience with real industrial devices. They are thus suitable for both trainees and students.