The calculated delta quantities are compared to the respective set values of the
Current change rate
and
Voltage change rate
settings.
The delta current and delta voltage algorithms detect a fuse failure if there is a
sufficient negative change in the voltage amplitude without a sufficient change in
the current amplitude in each phase separately. This is performed when the circuit
breaker is closed. Information about the circuit breaker position is connected to the
CB_CLOSED input.
There are two conditions for activating the current and voltage delta function.
• The magnitude of dU/dt exceeds the corresponding value of the
Voltage change
rate
setting and magnitude of dI/dt is below the value of the
Current change
rate
setting in any phase at the same time due to the closure of the circuit
breaker ( CB_CLOSED = TRUE).
• The magnitude of dU/dt exceeds the value of the
Voltage change rate
setting
and the magnitude of dI/dt is below the
Current change rate
setting in any
phase at the same time and the magnitude of the phase current in the same
phase exceeds the
Min Op current delta
setting.
The first condition requires the delta criterion to be fulfilled in any phase at the
same time as the circuit breaker is closed. Opening the circuit breaker at one
end and energizing the line from the other end onto a fault could lead to an
improper operation of SEQSPVC with an open breaker. If this is considered to be
an important disadvantage, the CB_CLOSED input is to be connected to FALSE. In
this way only the second criterion can activate the delta function.
The second condition requires the delta criterion to be fulfilled in one phase
together with a high current for the same phase. The measured phase current
is used to reduce the risk of a false fuse failure detection. If the current on the
protected line is low, a voltage drop in the system (not caused by the fuse failure) is
not followed by a current change and a false fuse failure can occur. To prevent this,
the minimum phase current criterion is checked.
The fuse failure detection is active until the voltages return above the
Min Op
voltage delta
setting. If a voltage in a phase is below the
Min Op voltage delta
setting, a new fuse failure detection for that phase is not possible until the voltage
returns above the setting value.
Decision logic
If voltages are Wye-connected, it is recommended to scale the default
values of voltage-based settings with 1/sqrt(3) because the default
setting values apply for Delta-connected settings.
The fuse failure detection outputs FUSEF_U and FUSEF_3PH are controlled
according to the detection criteria or external signals.
Supervision functions
1MRS757644 H
900 620 series
Technical Manual
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