ABB Relion SMU615 Technical Manual page 150

Section 5
Supervision functions
144
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.
1MRS758407 B
SMU615
Technical Manual