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Chapter 7) 7SR158 Applications Guide
Section 1: Introduction
The 7SR158 relay can be applied for all voltage and frequency functions. The relay is used to provide under-
frequency load shedding. The 'Rate of Change of Frequency' or 'Vector Shift' functions can be used to detect loss
of connection between a distributed generator and the main utility supply ('Loss of Mains' or Islanding').
1.1
Loss of Mains (LoM)
Where the electrical connection between Distributed generation (DG), such as a Wind Farm, combined heat &
power system, etc. and the electricity supply system is broken (Loss-of-Mains), it is usual for the embedded
generation to be physically disconnected from the system by means of tripping interconnecting Circuit-Breakers
(CB's). The DG must not be allowed to supply a part of the network once it has become electrically isolated from
the main grid supply.
In some cases the disconnected generation may be allowed to continue in operation to feed local loads – as an
'islanded' system. However the islanding may have adverse safety and operational implications for both the
Generator Operator and Utility Provider and it is then a requirement to disconnect from both the electricity system
and local loads when LoM occurs. Following restoration of the electricity system, the embedded generation must
then be safely re-synchronised with the system before the connection is restored.
Restoring the electrical network with the distributed generation still connected can lead to CB damage and voltage
disturbances. The LoM detection must therefore be fast enough to ensure that the electricity supply is not
restored – for example by auto-reclosure – before the embedded generation is disconnected.
At the same time, the embedded generation must remain connected – the LoM protection must remain stable -
during normal operating conditions, such as voltage depression.
Vector Shift (78) and 'Rate-of-Change –of-Frequency' (81R) monitoring is used to detect loss of mains. The
method applicable to a particular system depends largely on the generator's inertia (ability to resist change). For
smaller generators, perhaps up to 250kV, vector shift will be most applicable. These small machines are not
usually able to tolerate sudden changes in load and so react in an abnormal manner by phase 'jumping'. Larger
generators are more able to resist sudden changes in load, their slower reaction resulting in a gradual rolling-off
of the frequency. This makes ROCOF more applicable. These are just general guidelines, however, and a
complete LOM protection provides both elements.
©2014 Siemens Protection Devices Limited
Chapter 7) Page 5 of 24
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