ABB RER620 Technical Manual page 384

Section 6
Condition monitoring functions
378
• Change of voltage dU/dt
• Change of current dI/dt
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 ∆U exceeds the corresponding value of the Min Op voltage delta
setting and the magnitude of ∆I is below the value of the Min Op current delta setting
in any phase at the same time due to the closure of the circuit breaker, that is,
CB_CLOSED = TRUE.
• The magnitude of ∆U exceeds the value of the Min Op voltage delta setting and the
magnitude of ∆I is below the Min Op current delta setting in any phase at the same
time since the magnitude of the phase current in the same phase exceeds the Current
level 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 SEQRFUF with
an open breaker. If this is considered to be an important disadvantage, the CB_CLOSED
input is to be connected to FALSE.
The second condition requires the delta criterion to be fulfilled in one phase together with
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
The fuse-failure detection outputs FUSEF_U and FUSEF_3PH are controlled according
to the detection criteria or external signals.
1MAC309294-MB F
RER620
Technical Manual