The function can be enabled and disabled with the Operation setting. The corresponding parameter values are "On" and "Off".
The operation of SEQSPVC can be described with a module diagram. All the modules in the diagram are explained in the next sections.
Negative phase-sequence criterion
A fuse failure based on the negative-sequence criterion is detected if the measured negative-sequence voltage exceeds the set Neg Seq voltage Lev value and the measured negative-sequence current is below the set Neg Seq current Lev value. The detected fuse failure is reported to the decision logic module.
Voltage check
The phase voltage magnitude is checked when deciding whether the fuse failure is a three, two or a single-phase fault.
The module makes a phase-specific comparison between each voltage input and the Seal in voltage setting. If the input voltage is lower than the setting, the corresponding phase is reported to the decision logic module.
Current and voltage delta criterion
- 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 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
| Fuse failure detection criterion | Conditions and function response |
|---|---|
| Negative-sequence criterion | If a fuse failure is detected based on the negative sequence criterion, the FUSEF_U output is activated. |
| If the fuse failure detection is active for more than five seconds and at the same time all the phase voltage values are below the set value of the Seal in voltage setting with Enable seal in turned to "True", the function activates the FUSE_3PH output signal. | |
| The FUSEF_U output signal is also activated if all the phase voltages are above the Seal in voltage setting for more than 60 seconds and at the same time the negative sequence voltage is above Neg Seq voltage Lev for more than 5 seconds, all the phase currents are below the Current dead Lin Val setting and the circuit breaker is closed, that is, CB_CLOSED is TRUE. | |
| Current and voltage delta function criterion | If the current and voltage delta criterion detects a fuse failure condition, but all the voltages are not below the Seal in voltage setting, only the FUSEF_U output is activated. |
| If the fuse failure detection is active for more than five seconds and at the same time all the phase voltage values are below the set value of the Seal in voltage setting with Enable seal in turned to "True", the function activates the FUSE_3PH output signal. | |
| External fuse failure detection | The MINCB_OPEN input signal is supposed to be connected through a protection relay binary input to the N.C. auxiliary contact of the miniature circuit breaker protecting the VT secondary circuit. The MINCB_OPEN signal sets the FUSEF_U output signal to block all the voltage-related functions when MCB is in the open state. |
| The DISCON_OPEN input signal is supposed to be connected through a protection relay binary input to the N.C. auxiliary contact of the line disconnector. The DISCON_OPEN signal sets the FUSEF_U output signal to block the voltage-related functions when the line disconnector is in the open state. |
The activation of the BLOCK input deactivates both FUSEF_U and FUSEF_3PH outputs.