Self-powered relay testing challenges

Electrical Tester - 6 January 2023

Authors: Stefan Larsson, Andrea Bonetti, and Lennart Schottenius

When testing self-powered relays, many technicians ask why a current of 1 A injected by the relay test set is not registered as 1 A by the relay. Stefan Larson, Power Protection Product Manager at Megger Sweden, provides the answer and discusses other challenges associated with testing self-powered relays.

For the last 40 years, self-powered relays have been used in MV/LV substations in the secondary distribution network. Traditionally, MV/LV transformers larger than 800 kVA were protected by one of these devices, while protection for smaller transformers was provided by an MV fuse. In the last 15 years, however, power utilities have moved toward protecting transformers as small as 100 kVA with self-powered relays, which means they are now common in substations and secondary distribution network kiosks.

Self-powered relays take the energy they need to operate from the current delivered to the relay by the current transformer. This means that the load current – and, when present, the fault current – in the circuit being monitored provides the energy needed to power the relay. This arrangement has the big benefit that the need for an external power supply, which typically takes the form of a battery with its related DC network infrastructure, is minimised or, in many cases, completely eliminated. This simplifies the protection system and substantially reduces costs.

In the near future, these considerations are likely to become even more important, as the concept of the ‘Smart Grid’ becomes ever more pervasive. Solar panels are increasingly being installed on the roofs of ordinary domestic properties, electric vehicles are being charged at home and at some point, they will hopefully be able to deliver energy to the grid (V2G). In other words, the Smart Grid will penetrate electrical systems at all voltage levels.

A key factor that will influence the speed of this penetration is cost, and in particular the cost of providing adequate protection for the Smart Grid. In principle, there would be little problem in protecting the Smart Grid using the proven solutions that have been developed for protecting high-voltage power networks. In relation to the Smart Grid, however, these solutions are too complex and too expensive. Self-powered relays make an important contribution toward addressing these issues and it is therefore expected that their usage will increase significantly as more and more Smart Grid systems are implemented.

Despite their benefits, self-powered relays also present a number of challenges, particularly in relation to testing. Because of their integrated switch-mode power supplies, they present a very non-linear load to the test set. This means that a nominally sinusoidal 1 A current injected by the test set may be heavily distorted by the relay which, as a result, might measure a much higher or a much lower current.

Another issue is that of pre-fault conditions. As we have already discussed, the energy needed for the operation of a self-powered relay is derived from the current transformers. This means that if there is no load current in the protected feeder, there is no energy to power the relay and, consequently, the relay is not active. If, under these conditions, a fault occurs, the fault current delivers energy to the relay which then starts up, detects the fault, and issues a trip command. The effective operate time, however, is the normal operate time of the relay plus the time that the relay takes to start up.

This situation is related to switching onto a fault condition: if the circuit breaker is closed onto a fault, there cannot be any pre-load into the protection relay before the breaker is closed. A similar situation can arise if the breaker is closed, but until a fault occurs, the load current is below the level necessary to provide enough energy to power the relay.

The issues associated with testing self-powered relays can be successfully addressed by using a test set such as Megger’s SVERKER 900, which has been developed from the outset with self-powered relays in mind. The on-board current generators in the SVERKER 900, together with sophisticated adaptive real-time current generation algorithms, allow the reliable testing of protection relays of all kinds, including self-powered types.

Uniquely, the SVERKER 900 is compatible with the many different kinds of burden associated with various types of protection relays. It easily copes with electromechanical relays, static relays, sophisticated numerical relays, self-powered relays, and relays with current transformer operated trip release units. The pre-fault instrument can perform multiple timing tests, which is particularly useful when testing self-powered relays, as the pre-fault provides the load necessary to keep the relay turned on.

The SVERKER 900 is designed to manage current generation for self-powered relays, taking into account:

  • The harmonics generated by self-powered relays, which can disturb the control circuits in a relay test instrument
  • The non-linear load presented by self-powered relays, which requires high-performance real-time control loops to ensure that the test instrument generates the correct waveforms
  • The need for the test instrument to generate a relatively large amount of power in relation to the injected current to allow for the power needed to provide a supply for the relay

The spread of Smart Grids means that self-powered protection relays are likely to be widely used in future, even in smaller power systems. Testing these relays may at first seem challenging, but in reality the challenges can be readily overcome. The key is to use a test set, such as the SVERKER 900, which has been specifically designed for use with self-powered relays and to cater for their special requirements.