GE P645

To calculate the stability voltage the maximum through fault level should be considered. The maximum through
fault lev
el, ignoring the source impedance, I
F
, is:
I
F
= I
FLC
/ X
TX
= 394 / 0.05 = 7873 A
The required stability voltage, VS, and assuming one CT saturated is:
V
s
= KI
F
(R
CT
+ 2R
L
)
The following figure can be used to determine the K factor and the operating time. The K factor is valid when:
● 5 ≤ X/R ≤ 120
and
● 0.5In ≤ I f ≤ 40In
We recommend a value of VK/VS = 4.
With the transformer at full load current and percentage impedance voltage of 394A and 5% respectively, the
prospective fault current is 7873 A and the required stability voltage Vs (assuming that one CT is saturated) is:
V
s
= 0.9 x 7873 x (0.5 + 2 x 0.98) / 400 = 45.5 V
The CTs knee point voltage should be at least 4 times Vs so that an average operating time of 40 ms is achieved.
5.3.2.2 PRIMARY CURRENT CALCULATION
The primary operating current should be between 10 and 60 % of the winding rated current. Assuming that the
r
elay effective setting or primary operating current is approximately 30% of the full load current, the calculation
below shows that a setting of less than 0.3 A is required.
Eective setting = 0.3I
FL
C
/ CT Ratio = 30.3 x 394 / 400 = approximately 0.3 A
5.3.2.3 STABILISING RESISTOR CALCULATION
Assuming that a setting of 0.1A is selected the value of the stabilizing resistor, R
ST
, r
equired is
R
ST
= V
s
/ (IREF> Is1 (HV)) = 45.5 / 0.1 = 455 ohms
To achieve an average operating time of 40 ms, Vk/Vs should be 3.5.
The Kneepoint voltage is:
V
K
= 4V
s
= 4 x 45.5 = 182 V.
If the actual V
K
is greater than 4 times V
s
, then the K factor increases. In this case, V
s
should be recalculated.
Note:
K can reach a maximum value of approximately 1.
5.3.2.4 CURRENT TRANSFORMER CALCULATION
The effective primary operating current setting is:
I
P
= N(I
s
+ nI
e
)
By re-arranging this equation, you can calculate the excitation current for each of the current transformers at the
stability voltage. This turns out to be:
I
e
= (0.3 - 0.1) / 4 = 0.05 A
Chapter 8 - Restricted Earth Fault Protection P64x
186 P64x-TM-EN-1.3