Section 3: Current Transformer (Ct) Requirements; Ct Ratio; Thermal And Overcurrent Protection Cts; Earth Fault Protection Cts - Siemens 7SR105 Rho User Manual

Section 3: Current Transformer (CT) Requirements

3.1

CT ratio

The CT primary rating is usually chosen to be equal to or greater than the motor full load current.
Input power =
P.F.
Full load current =
The C.T. ratio should be chosen as equal to or the next standard above the motor full load rating here a 100 Amp
primary is chosen. The secondary could be either 1 or 5A, to reduce CT burden a 1A secondary is used, so a
current ratio of 100/1 is chosen.
The motor full load CT secondary current is:
MotorFLC 86 3 .
=
CT Ratio
100
3.2

Thermal and Overcurrent Protection CTs

Motors are typically located in industrial environments with relatively low fault current. The motor is the end load of
the system and therefore has no onerous grading requirements. The CT can typically be a class 10P10 with VA
rating to match the load.
Definite Time and Instantaneous Overcurrent
For industrial systems with requirements as for i.d.m.t.l. relays item (a) above, a class 10P10 (or 20).
Note that overcurrent factors do not need to be high for definite time protection because once the setting is
exceeded magnitude accuracy is not important.
3.3

Earth Fault Protection CTs

Considerations and requirements for earth fault protection are the same as for Phase fault. Usually the relay
employs the same CT's e.g. three phase CTs star connected to derive the residual earth fault current.
The accuracy class and overcurrent accuracy limit factors are therefore already determined and for both these
factors the earth fault protection requirements are normally less onerous than for overcurrent.
3.4

Sensitive Power (32S)

For sensitive reverse power applications e.g. Ps < 5%Pn class 1 metering CTs are recommended with a rated
burden to match the secondary load connected.
For higher settings a 5P10 CT can be used.
©2018 Siemens Protection Devices
Output 400
=
× efficiency 0.85 × 0.955
KVA 493
= = 86.3 Amps
3 kV 3 × 3 3 .
1
×
= . 0 86 A ( = . 0 86 × In )
7SR105 Rho and 7SR17 Rho Applications Guide
= 493 KVA
Chapter 7 Page 21 of 31