4.122
SEL-751 Relay Instruction Manual Date Code 20170927
Protection and Logic Functions
Group Settings (SET Command)
The count calculator receives angle shift detection information from all three
phases and records the number of times that the angle shift threshold of all
phases has been exceeded in two consecutive quarter-cycles. If the angle shift
count exceeds four and no blocking conditions exist, 78VSO is asserted indi-
cating an islanding or loss of mains condition.
Power system short circuit conditions can also cause the voltage angle change
to exceed the angle shift setting threshold. To prevent possible false tripping,
the vector shift element is blocked for undervoltage conditions. If any of the
phase voltages fall below the voltage supervision threshold setting 78VS59,
the output of the vector shift element is blocked. You can program the
78VSBL SEL
OGIC control equation to provide additional blocking conditions
as required in your application.
Depending on the DG loading conditions, a vector shift occurs once when an
islanding event happens, causing a change in two consecutive period measure-
ments after which the voltage stabilizes. For this reason, a delayed operation
of the element is not applicable. Although the vector shift element allows for
fast and reliable detection of DG islanding conditions, the limitation of this
element needs to be realized. This element is based on the sudden phase
change in the voltage waveform. If there is no load current change between the
DG and the utility at the point of common coupling, there is no vector shift
and this element does not detect the islanding condition. For this element to
operate properly, the load change must be at least 20 percent of the rated
power of the DG.
The vector shift element (78VS) and the fast rate-of-change-of-frequency ele-
ment (81RF) can be used to detect islanding conditions. The vector shift ele-
ment is designed to detect islanding conditions at the moment when the
islanding condition happens and typically responds within 1.5–3 cycles after
the islanding condition occurs. On the other hand, 81RF is designed to detect
islanding conditions during and after the voltage shift occurs; the 81RF
element complements the 78VS element by providing more dependable
protection.
Vector Shift Element Settings
See Table 4.47 for the range and default settings for the vector shift element.
Set E78VS := Y to enable the vector shift element. Set the vector shift angle
pickup threshold setting 78VSAPU to the desired angle to detect a vector shift
condition. The factory-default value of this setting is 10 degrees. Determine
this setting based on the generator impedance and your studies.
The vector shift voltage supervision threshold setting 78VS59 defines the
minimum voltage magnitude required for this element in percentage of the
nominal voltage setting VNOM. The factory-default threshold is 80 percent of
the VNOM setting. If VNOM setting is OFF, then the vector shift element is
turned off.
The vector shift SEL
OGIC control equation, 78VSBL, allows you to define
additional blocking conditions of the vector shift element. For instance, block
the element for a few cycles when the DG breaker is closed to keep the ele-
ment secure from operating under this condition.
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