Description - GE B90 Instruction Manual

10.4 SLOPES AND HIGH SET THRESHOLD
10.4SLOPES AND HIGH SET THRESHOLD
To set the higher slope and threshold of the high set (unbiased) differential operation, external faults must be analyzed.
Consider an external fault for the North bus relay. It is justified to assume bus configurations that give maximum stress to
the maximum number of CTs. For this purpose we will assume the tie breaker, B-7 closed; all the circuitry capable of sup-
plying the fault current to be in service; moreover, they are connected to the South bus in order to analyze the CT-7 and CT-
8 carrying the fault current.
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 com-
ponents. 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
s
ure, the solution is I
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 differ-
ential 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
(2πf).
If the above condition is violated, CT time-to-saturate for a full DC saturation can be estimated as follows. The CT satura-
tion 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
S_LIM
10
10-6
I
relay
I
relay
= 1.61 Ω and the characteristic shown earlier in the Approximate CT Magnetizing Characteristics fig-
s
= 29.73 A, I = 112.8 A.
magnetizing relay
ω T × ×
1
K = ------------------------ - e
S
T T
–
1
T
is defined by the following equation:
K
B90 Low Impedance Bus Differential System
2 2
I I
= –
s magnetizing
×
R V
=
s magnetizing
> × × ( ω T ×
V I R 1
+
sat s s
T
2
( ωt )
sin
t – T1 ⁄ t – T2 ⁄
–
e
–
2
V × N
s
= ------------------------------
2
× ( ω R × )
20
s
V × N
s
= ------------- -
S...LIM
I × R
p s
10 APPLICATION OF SETTINGS

10.4.1 DESCRIPTION

10.4.2 EXTERNAL FAULTS ON C-1
) , where ω is radian system frequency
dc
(EQ 10.4)
(EQ 10.5)
(EQ 10.6)
(EQ 10.7)
GE Multilin