External Faults On C-1 - GE B90 Instruction Manual

CHAPTER 8: APPLICATION OF SETTINGS

8.4.2 External faults on C-1

The following table presents the results of analysis of an external fault on circuit C-1 (C-1 is connected to the North bus; C-
3, C-4, and C-5 are connected to the South bus).
For security reasons, it has been assumed that the fault current being a sum of several contributors (C-3, C-4, and C-5 in
this case) has a time constant of the DC component of the maximum among the time constants of the contributors. The
fault current is supplied from circuits C-3, C-4, and C-5 connected to the South bus, thus through CT-3, CT-4, and CT-6. The
current passes through the tie breaker threatening saturation of CT-7 and CT-8.
By comparing the secondary currents (column 3 in the table below) with the limits of linear operation for the CTs (column 4
in the Limits of Linear Operations of the CTs table earlier), it is concluded that CT-1 will saturate during this fault, producing
a spurious differential signal for the North bus zone differential protection. All other CTs do not saturate due to the AC
components. The amount of the spurious differential current (magnetizing current of CT-1) can be calculated using the
burden, magnetizing characteristic and primary current of the noted CT by solving the following equations:
For I = 116.67 A, R = 1.61 Ω and the characteristic shown earlier in the Approximate CT Magnetizing Characteristics
s s
figure, the solution is I
magnetizing
The magnetizing current of the saturated CT-1 appears to the differential element protecting the North bus as a
differential signal of 29.73 A, while the restraint signal is the maximum of the bus currents (112.8 A in this case).
Consequently, the higher slope of the characteristic should not be lower than 29.73 A / 112.8 A, or 26%, and the pickup of
the high set differential elements should not be lower than 29.73 A, or 2.97 pu.
The CTs identified as operating in the linear mode as far as the AC components are considered can, however, saturate due
to the DC components. Saturation does not occur if V
If the above condition is violated, CT time-to-saturate for a full DC saturation can be estimated as follows. The CT
saturation factor K capability curve is defined as
s
where
T is a primary system time constant
1
T is the secondary CT time constant, which can be estimated by the following equation:
2
where
N is the CT ratio
V is the CT voltage at 10 A exciting current obtained from the CT excitation curve
s
CT limiting factor K is defined by the following equation:
S_LIM
where
I is the maximum CT primary fault current
p
The figure illustrates the K CT saturation capability curve and K
s
B90 LOW IMPEDANCE BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
= 29.73 A, I = 112.8 A.
relay
> I x R
sat s
SLOPES AND HIGH SET THRESHOLD
x (1 + ω x T ), where ω is radian system frequency (2πf).
s dc
limiting factor.
S_LIM
Eq. 8-4
Eq. 8-5
Eq. 8-6
8
Eq. 8-7
8-7