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Application Notes P746/EN AP/G31
MiCOM P746 (AP) 6-
13
AP
Explanations of the values:
1. ID>1 shall be higher than 120% of the highest phase to neutral fault to not operate in
case of phase to neutral fault.
2. Slope k1 recommendation is 10% to meet 10Pxx current transformers
3. Slope k2 (ID>2) recommendation is 50 / 65%
To be always stable in the worth CT ratio conditions (between the biggest CT and the
smallest CT). 60% is OK as long as the CT ratio is less than 5.
1. ID>2 shall be lower than 80% of the minimum phase to phase fault current to operate
sub-cycle for the minimum fault and higher than 120% Iload Max (120% to allow 20%
margin) and if possible equal to 6 × (ID>1) to be insensitive to the worth CT saturation.
2. IDCZ>2 same as ID>2
3. Slope kCZ (IDCZ>2) recommendation is 30%
The requirement is to be able to trip for a fault that is counted twice by the Check Zone (for
example one and half circuit breaker substation).
2.3 Additional protection settings
2.3.1 Dead Zone protection (DZ)
On a feeder, if the isolators or the breaker is open, a dead zone (or end zone) is said to exist
between the open element and the CT. The P746 can protect this zone with the Dead Zone
protection. This is a simple time delayed overcurrent element which is only active when a
dead zone is identified in the local topology.
2.3.1.1 Setting guidelines
For each CT connected to a Feeder Circuit Breaker (not on bus couplers or bus sections)
For the phase:
•
I>DZ must be below 80% of the minimum Dead Zone fault level (and if possible bigger
than the maximum load).
•
I>DZ Time delay must be at least 50ms if the CB status positions are used (any value
otherwise)
IMPORTANT NOTE:
When a dead zone fault occurs and the bias current flowing through the bus is small,
there could be a maloperation of the 87BB.
To prevent that, it is recommended to enable an additional criterion such as voltage
(voltage criteria can be used for single busbar only in one box mode).
2.3.2 Circuit Breaker Fail (CBF)
2.3.2.1 Setting guidelines
Typical timer settings to use are as follows:
CB fail reset mechanism tBF time delay
Typical delay for 2 cycle
circuit breaker
CB open
CB auxiliary contacts
opening/ closing time (max.) +
error in tBF timer + safety margin
50 + 10 + 50 = 110 ms
Undercurrent elements
CB interrupting time +
undercurrent element (max.) +
safety margin operating time
50 + 15 + 20 = 85 ms