Siemens 7SR242 Duobias, Technical Manual

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©2015 Siemens Protection Devices Limited Chapter 7 Page 40 of 64
Worked Example
Y d
400/1
1600/1
90MVA, Z = 14%, Yd11, 132/33kV +10% to -20%
CT Specification
VK =
250V
T = 1/400
RCT = 3.5 Ohms
CT Specification
VK =
750V
T = 1/1600
RCT = 12 Ohms
RL = C = 1 Ohm RL = C = 4 Ohm
Figure 3-1 CT Requirements
Suitability of HV Current Transformers:
126.5V 1 3.5
0.14 400 10 132 3
10 90
4 V
3
6
K
i.e. less than 250V
Suitability of LV Current Transformers:
V 12
0.14 00 10 33 3
10 90
4 V
3
6
K
9 . 449 4
16
i.e. less than 750V
An indication of the suitability of a protection class CT e.g. class 5P to IEC60044 classification can be obtained.
The product of its rated burden expressed in ohms and the secondary current equivalent of its accuracy limit
primary current will give an approximation of the secondary voltage it can produce while operating within the limit
of its stated composite error.
3.2 CT Requirements for High Impedance Restricted Earth
Fault (64H)
For high impedance REF protection:
It is recommended that low reactance CTs conforming to IEC61869-2 Class PX are used. Class PX CTs
have a specified excitation characteristic and secondary winding resistance and these parameters are
used to calculate the performance of the protection in terms of primary operate current for internal faults
and stability during through fault conditions.
All current transformers should have an equal turns ratio.
The knee-point voltage of the CTs must be greater than 2 x 64H Setting Voltage (Vs) – see section 2.4.
A full explanation of how to specify CTs for use with REF protection, and set REF relays is available on our
Website:
www.siemens.com/energy .