GE P743 User Manual

P74
x/EN AP/Na7
Application Notes
(AP) 6
-8 MiCOM P74
1, P742, P743
AP
• the maximum load is on the 2 buses (biggest bias current)
• The internal short-circuit value is minimum.
During the internal fault:
• the CZ bias current is: Icc min (1 bar) + 4 x IloadMax (1 bar)
• the CZ differential current is: Icc min (1 bar)
Thus the biggest slope for the Check Zone to detect the fault is:
Icc min (1 bar)
(4 x IloadMax (1 bar)) + Icc min (1 bar)
If for example:
Icc min = IloadMax, the slope must be below 20%
b) Recommendation is 25% for China
In china, the requirement is to be able to trip for a resistive fault that is counted
twice by the Check Zone (for example one and half circuit breaker substation).
− ID>1 Alarm Timer to not operate for an external fault shall be greater than the
longest protection time (such as line, overcurrent, etc…)
2.1.1.2 87BB CU Settings (Compensated Earthed Network Schemes)
2.1.1.2.1 Sub-station features
Only 4 values have to be known:
1. Maximum load current in a feeder
2. Minimum phase to phase fault current (Ph-Ph min.) in a bus
3. Maximum single phase steady state faulty current (Ph-N Max.) in a bus
4. Number of independent bars
2.1.1.2.2 Differential Busbar Protection
9 values have to be chosen:
1. ID>1 (from 5 A to 500 A (primary value)), recommendation equal to 1,2 x (Ph-N Max.)
2. Slope k1 (ID>1) (from 0% to 50%), recommendation is 10%.
3. ID>1 Alarm Timer (from 0 to 100 s) shall be greater than the longest Busbar protection
time
4. Slope k2 (from 20% to 90%) but recommendation 65%.
5. ID>2 (from 50 A to 50000 A (primary value)), recommendation is:
6. Lower than 0,8 x (Ph-Ph min) and Higher than 1,2 x Iload Max and if possible equal to
6 x (ID>1).
7. Slope kCZ (from 0% to 90%) but recommendation 30%.
8. IDCZ>2 (from 50 A to 50000 A (primary value)), recommendation is:
9. Lower than 0,8 x (Ph-Ph min) and Higher than 1,2 x Iload Max and if possible equal to
6 x (ID>1).
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

GE P743 Specifications

General

Brand	GE
Model	P743
Category	Relays
Language	English