Biased Characteristic Breakpoints; Description - GE b30 Instruction Manual

CHAPTER 8: APPLICATION OF SETTINGS
• The trip signal are routed directly to the B-6 and B-7 breakers while it is supervised by the status of S-2, S-4 and S-6 for
the B-2, B-3 and B-4 breakers, respectively

8.3 Biased characteristic breakpoints

8.3.1 Description

The limits of linear operation of the CTs need to be found in order to set the breakpoints of the biased differential
characteristic. The settings for the North and South bus relays are analyzed simultaneously from this point on as the two
differential zones share some CTs and the results of computations apply to both the relays.
For microprocessor-based relays it is justified to assume the burden of the CTs to be resistive. The limits of the linear
operation of a CT, neglecting the effects of the DC component and residual magnetism, can be approximated as follows:
where
I is the maximum secondary current transformed without saturation (AC component only, no residual magnetism),
max
R is the total burden resistance
s
V is the saturation voltage of the CT
sat
The total burden resistance depends on both the fault type and connection of the CTs. For single-line-to-ground faults and
CTs connected in Wye, the burden resistance is calculated as:
where
B30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
Figure 8-4: South bus zone
BIASED CHARACTERISTIC BREAKPOINTS
Eq. 8-1
8
Eq. 8-2
8-5