Section 4: Current Transformer Requirements; Current Transformer Ratio Selection; Current Transformer Class/Rating - Siemens 7SR18 Solkor Manual

7SR18 Applications Guide

Section 4: Current Transformer Requirements

The two primary criteria to be met when specifying current transformers (CT) for use with the relay are CT ratio
and knee-point voltage. The CT connections and polarity are shown at the end of this section.
4.1

Current Transformer Ratio Selection

The first criterion is to select a CT ratio to step the primary rated current of the protected circuit down to
approximately a relay nominal current of 1 A or 5 A.
1 A is preferable as there is only 1/5th of the current but 5 times voltage output compared to 5 A CT secondary
(see Example. below). This has the advantage of higher driving voltage for relays which may have high
impedance, but the 7SR18 has a low impedance i.e. 0.05 Ω for the 1 A CT input therefore, the voltage is not a
great consideration. The majority of the CT burden is the cables from the CT to the relay.
Ratios should be chosen to provide the relay with about rated current at full feeder rating. As a general comment,
1 A secondary rated CT's are superior to 5 A, for all types of protection relays, as they are less prone to
saturation. Where possible 1 A rated CT's are recommended, however the relay does have 1 A and 5 A rated CT
terminals.
The relay settings can then be chosen to allow the use of sensitive settings. The relay can be connected with 1 A
rated CT's at one end and 5 A rated CT's at the other end. CT ratio correction is provided in the range of 0.25 to
3.0 to cater for retrofit applications whereby the ratio at one end may be different to that of the other. This setting
operates on the secondary level of current from the line CT's. The setting range of the CT ratio correction factor of
0.25 to 3.0 must be taken into account when considering protection of a circuit with different CT ratios.
Example of Applying CT Ratio Correction
A feeder circuit rated at 600 A with maximum anticipated load of 600 A, has a line CT ratio of 600/1 at one end
and 800/5 at the other end. The ratio correction would be set to 600/800 = 0.75 on the relay connected to the
600/1 CT, and 800/800 = 1.0 on the other relay. Each relay has 1 A and 5 A inputs for connection to the CT's
allowing for example, 600/1 A at one end of the feeder circuit and 800/5 A at the other.
4.2

Current Transformer Class/Rating

The second criterion is the specification of the CT class/rating. The relay is a biased current differential relay and
therefore, to ensure stability for high values of through-fault current (i.e. high multiples of the rated current) a CT
class e.g. 5P20 to IEC61869-2:2012 classification, can be obtained.
The CT requirements may be altered by selection of any one of three relay Bias Break Point settings. A lower
Bias Break Point setting will lower the CT requirements.
The following page contains formulae that may be used to select appropriate CT knee-point voltages. The CT
e.m.f. is chosen to allow the protection to be stable for the worst case through-fault.
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