Page 68 - Energize June 2022

TECHNICAL

In NOJA Power’s OSM Recloser If the race to disconnect load is faster than the frequency falling, a network can regain
system, the over-frequency feature control by reaching the point where remaining generation matches the load that is left
monitors the power frequency and can connected to the network.
be configured to either alarm or trip the At this point, network operators can progressively increase power, and reconnect parts
recloser if the frequency goes too high. of the network, avoiding a complete blackout event.
Under-frequency works by monitoring the system frequency to see if it reaches a
Under-frequency threshold below the operating frequency. If this occurs, the relay activates and can either
Under-frequency occurs when there is a automatically trip a circuit breaker or alert the network operator.
generation shortfall. That is, when there is
too much load for the available generation Rate of change of frequency
capacity. The last frequency element for consideration is rate of change of frequency, known as
Under-frequency protection is often ROCOF.
used in a “load shedding scheme”, where This protection deals with a fundamental shortfall of over-frequency and under-
the distribution network automatically frequency protection – the non-detection zone.
disconnects parts of the network when Both over- and under-frequency protection require the network conditions to become
there is insufficient generation. critical before they are detected.
This allows the network operator to If the network is normally at 50 Hz, and a generation shortfall occurs, a circuit breaker
partially reduce service, rather than allowing without ROCOF would only know about the problem when the frequency reaches a low
the entire system to experience a blackout. point. By this time, the frequency may have fallen too far to be saved by load shedding.
Using OSM Reclosers positioned Instead, ROCOF looks at the movement of frequency over time. While minor
around the distribution network, these movements in frequency are tolerable, a major shift of say 0,5 Hz/s, is more indicative of
devices can be automatically triggered serious issues.
to “shed load” by progressively powering By monitoring ROCOF, circuit breakers can operate faster, because the protection
down parts of the network when there is system can see the problem before the frequency has dropped.
insufficient generation. Using our car analogy again, ROCOF is like the eyes of the driver. ROCOF can see the

Figure 1: Multiple NOJA Power OSM reclosers installed in a distribution substation

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