Alarms
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5.4 OVER AVERAGE VOLTAGE
Indicates that the average of the monitored voltage phases, is outside the settings of the Over
Average Voltage Alarm.
The Average is calculated using voltage measurements taken over the proceeding ten seconds (10s
rolling average).
5.5 LOSS OF MAINS (R.O.C.O.F. & VECTOR SHIFT)
Active if either the R.O.C.O.F. or Vector Shift alarm has been activated.
5.5.1 WHAT IS LOSS OF MAINS DETECTION (LOM)?
LOM is the automatic detection of a mains failure when in parallel with another source of power.
Mains failure when in parallel leads to a section of the distribution network that is no longer connected
to the rest of the National Grid. That section is known as an ‘island’ and may be anything from a few
low voltage (LV) customers up to an area covering a substantial part of the country.
5.5.2 WHY DO WE NEED LOSS OF MAINS DETECTION (LOM)?
LOM is required in the UK by ‘G59’ and ‘G83’ which, although not legally binding in themselves, are
required by the Distribution Code (DC). Any generator connected to the National Grid is required to
comply with this.
5.5.3 WHY DO G59 AND G83 REQUIRE LOM DETECTION?
It is considered extremely dangerous for an island to be powered by one or more embedded
generators. Therefore LOM is required to be fitted to each point of connection to detect the formation
of an island and disconnect that generator from it, thus leaving the island without power.
Some of the risks generated by the ‘island’ are:
1. Linesmen may be attempting to repair a fault in the island and would be put in danger by it
being kept live by embedded generator.
2. Embedded generators may not maintain the voltage and frequency within legally required
limits, thus exposing other customers on the island to risk. This is due to the fact that they
usually operate in power and power factor control modes when grid connected rather than
voltage and frequency control modes that are used when running stand-alone.
3. The island may not be earthed appropriately and as a result protective devices may not
operate in the event of a fault, or the protective devices may not be in an appropriate location
to provide protection at all. Such a fault may expose people to danger e.g. a HV line laying on
the ground or a fallen tree lying across a HV line. There is a very significant chance of such a
fault since the operation of a protective device is one of the most likely causes of an island
forming.
4. Reconnecting the island to the grid while out of phase could cause damage to both the
generator and distribution network. Auto-recloser devices in the distribution network are very
common place as a result of the drive to maintain continuity of supply. These devices attempt
to reclose protective devices within typically 1-5 seconds of them tripping and rarely include
check-sync facilities. The need to disconnect a generator before an auto-recloser operates is
the drive for tripping times of less than 0.5s.
