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The rapid spread of distributed energy resources (DERs), the phasing out of large generating plants with the consequent reduction in system inertia, and the growth of the smart grid are combining to create new challenges for implementing and testing relay protection systems.
Two of the main concerns are maintaining network frequency stability and the provision of cost-effective relay protection; relay testing and test equipment are associated issues.
With network stability as its highest priority, the International Electrotechnical Commission (IEC) has released a new standard, IEC 60255-181:2019, which focuses on frequency protection functions, including Rate of Change of Frequency (ROCOF). This standard has important implications for test methodologies and, by extension, for test equipment.
Another important consideration is the growing use of self-powered relays (SPRs). These are being widely adopted in smaller DERs, and it is important to understand that they present specific challenges for testing even though they generally have the same basic protection functions as their separately powered counterparts.
Two commonly encountered issues
When testing frequency relays, there a two commonly encountered issues. The first is that relays from different manufacturers often respond differently to frequency variations, even though the settings are identical. The second is that not all relay test sets are able to accurately simulate the required waveform for frequency changes.
The Megger SVERKER900, however, simulates a voltage frequency ramp of the form prescribed by IEC 60255-181:2019,“Functional requirements for frequency protection”. The importance of using the correct waveform has not always been appreciated by manufacturers of relay test equipment. It is essential that the waveform used for testing corresponds to that produced by a rotating generator which changes speed while maintaining a constant net torque. IEC 60255 provides a formula for this.
In the past, attempts have been made to simulate the required frequency ramp by changing the frequency at each period or zero crossing. This produces a step-by-step change of frequency which does not correspond with reality.As a result, this method does not comply with IEC 60255 and should not be used.
Turning now to self-powered relays, these 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, eliminated. This simplifies the protection system and substantially reduces costs.
Soon, these considerations are likely to become even more important, as 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, creating a huge need for cost-effective protection.
Despite their benefits, self-powered relays present several challenges, particularly in relation to testing. Because of their integrated switch-mode power supplies, they present 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 much lower current.
Another issue is that of pre-fault conditions. As already mentioned, 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.
Finally, the binary input of the rel ay test set must be able to trigger on the trip voltage produced by the relay.
