Technical Guide
Testing self-powered relays with SVERKER 900
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So this is also a “secondary injection”, but through a current transformer. The test through the secondary test
current is comfortably done by converting the relay characteristics/settings into the values of the secondary test
current.
3.2. Pre-fault quantities
For overcurrent relays, the necessary energy for the relay operation is delivered by the current transformers.
This mean that in general, if there is no load current in the protected feeder, there is no energy to power the
relay and the relay is not active
3
. If the power system fault occurs, the fault current delivers energy to the relay,
the relay starts-up, it detects the fault and issues the operate (trip) command. In this case. The operate time (trip
time) of the relay is the normal relay operate time plus the time that the relay needs to start-
up (to “wake-up”).
This situation is associated to a
“switch onto fault condition” [7]: if the circuit breaker is closed on a permanent
fault, there cannot be any pre-load into the protection relay before the circuit breaker is closed.
A similar situation can also happen if the circuit breaker is closed and the load level is below the necessary level
to provide enough energy to the relay to be in operation, before the fault occurs.
Typically, we can suppose that the load current is available so that the relay is powered and ready to do its job
when the power system fault occurs. To test this condition it is necessary to inject with the relay test set a
certain level of pre-fault current for a certain period, before the fault current is really injected.
In SVERKER 900 this means that the “pre-fault and fault” instrument shall be used, where the pre-fault currents
are set at a certain current level for a reasonable time which is long enough to let the protection relay
be “ready”
when the fault is simulated. One second is long enough to reach this status
4
.
If the pre-fault current is set to zero, or the pre-fault time is set to zero, the switch onto fault (or low load)
condition is tested.
3.3. SVERKER 900 and self-powered relays
SVERKER 900 is designed to manage the current generation into self-powered relays. The major technical
challenges for succeeding in this are:
1) Harmonics generated from the self-powered relay that can disturb the control circuits in SVERKER 900;
2) Non-linear load presented by the self-power relay, that requires high real time control loops to make sure
the correct waveforms are generated by the SVERKER 900;
3) The power associated to the injected currents is relatively high, as the relays draw the energy from the
current signal for their functionality. This power must be delivered by the current generators of SVERKER
900.
Figure 3. The SVERKER 900 with
indication “Current generator MkII” in
the serial number is able to generate
current signals for testing self-powered
relays.
3 Unless the eventual presence of some small batteries, that can keep the relay alive in absence of the load, and that are
recharged as soon as the load appears; this is not a representative situation.
4 Self-powered relays have usually a very fast start-up time, less than 100 milliseconds. It has not been observed a self-
powered relay that needs more than 200 ms to start-up. One second of pre-fault is a reasonable time.
