Introduction; Current Transformer Requirements - Siemens 7SG164 Ohmega 400 Series Manual

1 Introduction

This family of Digital Distance Relays give full scheme protection with independent measurements for every
zone and fault loop. Impedance starting elements are not required. Optional features provide a full range of
protection functions supplements by control, metering, data storage, fault locator, auto-reclose and fibre optic
data communication capabilities.
The relays can be applied to either overhead line or cable feeders and, depending on the availability and type of
teleprotection channels available, can be configured to provide unit protection in a number of different models.

2 Current Transformer Requirements

The current transformers used with the relay should be class TPS to IEC 46-6 (ie BS3938 class x). The CT's
should have a knee point voltage Vk as follows:-
V = K.I (1 + X ) (0.03 + R
k p p
N R
p
V = K.I (1 + X ) (0.06 + R
k e e
N R
e
Where:
I = Primary phase fault current calculated for X
p
Ie = Primary earth fault current calculated for X
N = C.T. ratio
X /R = power system resistance to reactance ratio for the total plant including the feeder line parameters
p p
calculated for a phase fault at the end of zone 1.
X /R = similar ratio to above but calculated for an earth fault at the end of zone 1.
e e
R = C.T. internal resistance
ct
R = lead burden, C.T. to relay terminals
I
K = factor chosen to ensure adequate operating speed and is <1. K is usually 0.5 for distribution systems, a
higher value is chosen for primary transmission systems.
Both V values should be calculated and the higher value chosen for the C.T. to be used.
k
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+ R )
ct I
+ R )
ct I
/R ratio at the end of zone 1.
p p
ratio at the end of zone 1
e
7SG164 Ohmega 400 Series Applications
Chapter 7 Page 3 of 8