Chapter 14 - Monitoring and Control
In some situations it is possible for both "bus" and "line" sides of a circuit breaker to be live when a circuit breaker is
open - for example at the ends of a feeder that has a power source at each end. Therefore, it is normally necessary
to check that the network conditions on both sides are suitable, before closing the circuit breaker. This applies to
both manual circuit breaker closing and autoreclosing. If a circuit breaker is closed when the line and bus voltages
are both live, with a large phase angle, frequency or magnitude difference between them, the system could be
subjected to an unacceptable shock, resulting in loss of stability, and possible damage to connected machines.
The System Checks functionality involves monitoring the voltages on both sides of a circuit breaker, and if both
sides are live, performing a synchronisation check to determine whether any differences in voltage magnitude,
phase angle or frequency are within permitted limits.
The pre-closing system conditions for a given circuit breaker depend on the system configuration, and for
autoreclosing, on the selected autoreclose program. For example, on a feeder with delayed autoreclosing, the
circuit breakers at the two line ends are normally arranged to close at different times. The first line end to close
usually has a live bus and a dead line immediately before reclosing. The second line end circuit breaker now sees a
live bus and a live line.
If there is a parallel connection between the ends of the tripped feeder the frequencies will be the same, but any
increased impedance could cause the phase angle between the two voltages to increase. Therefore just before
closing the second circuit breaker, it may be necessary to perform a synchronisation check, to ensure that the
phase angle between the two voltages has not increased to a level that would cause unacceptable shock to the
system when the circuit breaker closes.
If there are no parallel interconnections between the ends of the tripped feeder, the two systems could lose
synchronism altogether and the frequency at one end could "slip" relative to the other end. In this situation, the
second line end would require a synchronism check comprising both phase angle and slip frequency checks.
If the second line-end busbar has no power source other than the feeder that has tripped; the circuit breaker will
see a live line and dead bus assuming the first circuit breaker has re-closed. When the second line end circuit
breaker closes the bus will charge from the live line (dead bus charge).
SYSTEM CHECKS IMPLEMENTATION
The System Checks function provides Live/Dead Voltage Monitoring, two stages of Check Synchronisation and
System Split indication.
The System Checks function is enabled or disabled by the System Checks setting in the CONFIGURATION column. If
System Checks is disabled, the SYSTEM CHECKS menu becomes invisible, and a SysChks Inactive DDB signal is
The device provides inputs for a three-phase "Main VT" and at least one single-phase VT for check synchronisation.
Depending on the primary system arrangement, the Main VT may be located on either the line-side of the busbar-
side of the circuit breaker, with the Check Sync VT on the other. Normally, the Main VT is located on the line-side (as
per the default setting), but this is not always the case. For this reason, a setting is provided where you can define
this. This is the Main VT Location setting, which is found in the CT AND VT RATIOS column.
The Check Sync VT may be connected to one of the phase-to-phase voltages or phase-to-neutral voltages. This
needs to be defined using the CS Input setting in the CT AND VT RATIOS column. Options are, A-B, B-C, C-A, A-N, B-
N, or C-N.
The settings in the VOLTAGE MONITORS sub-heading in the SYSTEM CHECKS column allow you to define the
threshold at which a voltage is considered live, and a threshold at which the voltage is considered dead. These
thresholds apply to both line and bus sides. If the measured voltage falls below the Dead Voltage setting, a DDB