Voltage vector measurement is blocked when all phase-neutral voltages fall below 20 V. To allow the voltage
to stabilize this under voltage inhibit will reset 300 ms after all phase voltages recover to levels above 20 V.
Tripping Function
These elements can be used to trip the circuit‑breaker and isolate the local network from the grid.
Triggering of waveform storage must be configured if required. If the element is a tripping function it must be
configured to a designated trip contact to trigger a fault record.
Settings Example
When a generator is connected to an electricity supply system, the generator will respond to any transitory
changes in load to keep both voltage and frequency levels as close to constant as possible. When a LoM event
occurs, however, the generator is unable to respond instantaneously to the sudden, large-scale change in load
and an instantaneous shift in the phase of the generator voltage can result (Voltage Vector Shift). The sudden
change in load causes a sudden change in cycle length. The cycle length becomes shorter or longer depending
on whether the newly formed island has a surplus or shortage of generation.
[dw_7SR5_function78SettingsExample, 1, en_US]
Figure 5-50 Vector Shift
This vector shift can be in either direction depending on whether load increases or decreases. Where the DG
capacity is insufficient to supply the local load and power is supplemented from the mains then a LoM event
will cause the DG load to increase.
With an increase in load, the voltage would tend to “jump back” as shown in Figure 5-50. This would occur
when a trip on the mains network has left the distributed generation to try and supply the whole of the down-
stream load. This would be the usual condition leading to a LOM tripping.
Alternatively, if the local generation is supplying both a local load and the mains network, as happens with
some industrial systems, disconnection from the mains network would create an islanding condition in which
there may be an excess of electrical supply and the voltage would tend to “jump forward”.
There is an obvious link between the 78 element and the 81R (Rate-of-Change-of-Frequency) element.
However the equivalent vector shift seen for expected ROCOF levels should not result in operation of the 78
element.
The highest 81R pickup setting is 10 Hz/s.
This equates to a ROCOF of 0.1 Hz over the 10 ms measuring interval.
Giving an equivalent vector shift of (360°/50 Hz) ⋅ 0.1 Hz = 0.72°.
At 60 Hz system frequency, the equivalent is 0.5° vector shift at maximum ROCOF setting.
Attempting to differentiate between the 2 elements at inordinately high levels of ROCOF would impose a
delay on the operation of the 78 element and so this is considered acceptable. Momentary changes in phase
can occur due to load switching operations. These can be as great as 5° and must be allowed for in the setting
Protection and Automation Functions
5.26 78VS Voltage Vector Shift
Reyrolle 7SR5, Overcurrent Protection Device, Device Manual 339
C53000-G7040-C014-1, Edition 11.2019
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