Technical Guide
Testing self-powered relays with SVERKER 900
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1. Introduction to this Technical Guide.
Self-powered relays will be an important component for the protection of the smart grid. While they allow
reducing the cost of the protection system, they are definitely a challenge for relay test sets, that are required to
provide the voltage and current signals to simulate the power system fault, but also the generated signals need
to have necessary electric power to supply the protection relay.
SVERKER 900 is designed to manage this task and this Technical Guide describes how two self-powered
relays can be tested.
This document guides through the testing of two self-powered overcurrent relays from SEG GmbH (formerly
Woodward), WIC-1 and WIP-1, with the relay test set SVERKER 900, for commissioning/maintenance
purposes. The Technical Guide details on the principles adopted in the testing with reference to relevant IEC
standards and to the relay manuals. In addition, basic instructions on the correct usage of the testing
possibilities offered by SVERKER 900 are described.
2. About self-powered relays: past and future.
Traditionally the self-powered relays have been used in secondary distribution network, in MV/LV substations
for the last 40 years. Normally if the MV/LV power transformer is greater than 800 kVA the transformer is
protected with a self-powered relay, and if the transformer has lower rated power, it is normally protected with a
MV- Fuse.
In order to be operational, self-powered relays drain the necessary energy from the current signal delivered by
the main CTs (some other applications may drain this energy from voltage transformers instead). Therefore, the
load currents, and eventually the fault currents, deliver the energy to the relay for its operation.
The need to have an external power supply (typically a battery system with all the related DC network structure)
for the relay functionality and for the tripping of the circuit breaker is then minimized, if not completely removed,
bringing to a clear cost reduction and more simple protection system.
Looking at the near future, it can be said that the concept of smart grid
1
is penetrating our society more and
more: solar panels are installed on the roofs of common people houses, electrical vehicles are charged at our
homes and hopefully one day will be able to deliver energy to the grid (V2G [2])
2
. More technically, smart grids
penetrates all the “voltage levels”.
One of the important factors that will affect the speed of this penetration is the “cost” to do it. Also for the
protection of the smart grid power system, the cost is important. Technically the solutions to protect the smart
grid are in principle available from the competence in protecting the high voltage power system networks
(transmission networks), but the smart grid cannot tolerate the costs of the high voltage system protection, in
terms of complexity and price of the equipment.
Self-powered relays provide an important contribution to reduce the cost of the protection of the smart grid [3],
and it is then foreseen that their usage will grow in the near future, the more the smart grid is implemented.
3. General topics related to testing self-powered relays.
3.1. Direct secondary injection or injection through test circuits
There is in principle no practical difference between these two types of injection.
The “direct secondary injection” is the usual secondary injection for a usual protection relay. The test set applies
the current waveforms to the analog inputs of the protection relay. The self-powered relay will draw the energy
for its functionality from the injected currents [4].
The injection through test terminals on board of the relay is foreseen for some self-powered relays (Figure 1, [5]
and [6]). This simplifies the practical maintenance operations in the field because there is no need to short
circuit the secondary side of the main CTs and to create a connection from the relay test set to the relay analog
inputs, as usually done for the conventional direct secondary injection.
1 The definition of smart grid is not easy. According to the International Electrotechnical Vocabulary (IEV, IEC 60050) [1],
smart grid is one electric power system that utilizes information exchange and control technologies, distributed computing
and associated sensors and actuators, for purposes such as:
– To integrate the behaviour and actions of the network users and other stakeholders,
– To efficiently deliver sustainable, economic and secure electricity supplies
While this definition is very generic, in practice smart grid is in these years associated to distributed power generation
(photovoltaic, wind), to energy storage, to standardised communication protocols and methods (IEC 61850).
2 V2G means Vehicle to Grid. Many tests are done already today in 2020 for achieving this goal.
