Winding and Lead Resistance Measurements
The internal resistance of CT windings and external impedance (including lead resistance) are required for relay setting calculations, and for calculating ratio correction error when applying CTs. The internal winding resistance and the lead resistance can be calculated or measured with a resistance bridge. If it is desired to separate the winding resistance and lead resistance, the resistance of a full winding and of a tap then should be measured. All measurements should be made at the CT short-circuiting terminal block. After completion of this test, the CT should be demagnitized to remove any residual magnetism as discussed under Section 9.2.7.
For relay applications and for calculating CT ratio correction factors, burden measurements may be necessary. In such cases, the total burden of the circuit, which is the sum of the internal CT burden and the external connected burden must be determined. The internal burden is the resistance of secondary wind- ing and the lead resistance from the winding to the short-circuiting terminal block as discussed in Section 9.2.5. The internal burden can be converted to volt-amperes at rated secondary current. The external burden can be mea- sured in volt-amperes by measuring the voltage required to drive the rated current through the connected burden (load).
The residual magnetism of a CT is known as remanence. The performance of both class C and T CTs is influenced by remanence. The core of the CT is subject to hysteresis, i.e., when current is interrupted the flux density in the core does not become zero when current does. When flux in the core is high due to high current or because the current contains a high DC component and when this current is interrupted, the residual magnetism in the core will be high, possibly being above the flux equivalent of the knee point on the excitation curve as shown in Figure 9.5. When the CT is next energized, the flux changes will begin from the remanence value and therefore may lead to the saturation of the CT. When this occurs, much of the primary current is used for exciting the core, and thereby significantly reducing and distorting the secondary output. This condition can be corrected by demagnitizing the core of the CT. This can be accomplished by applying a suitable variable alternating voltage to the secondary, with initial magnitude sufficient to force the flux density above the saturation point, and then decreasing the applied voltage slowly and continuously to zero. Test connections are identical to those as shown in Figure 9.4 for performing this test.
Grounding of CT
It is common practice to connect the secondary of the instrument (current and voltage) transformers to the station or substation ground system. The primary purpose of grounding is for personnel safety and correct perfor- mance of the instrument transformers, relays, meters, and other equipment connected to the instrument transformer secondaries. The grounding point in the instrument transformer secondary circuit should be located electrically at one end of the secondary winding of each instrument transformer and physically at the first point of switchboard or relay panel of the instrument transformer secondary circuit. The following are some examples for ground- ing instrument transformers.
1. A single instrument transformer secondary winding should be con- nected to a single ground.
2. Where more than one instrument transformer is used, such as three single-phase transformers wye-connected to form a three-phase connection, the common point of the secondary windings of all instrument transformers should be connected to a single ground.
3. The secondary circuit of multi-instrument transformers where no common point of connection is available for all of the transformer secondaries, such as delta-connected transformers, should have the common point between the greatest number of the secondary wind- ings connected to ground.
4. When secondary windings of more than one instrument transformers are interconnected and do not have a common neutral connection, then the common secondary neutral connection for the greatest number of these transformers should be the point of connection to ground.
5. The grounding conductor for instrument transformers should be as large, or larger than the secondary phase conductors. The grounding conductor should never be smaller than number 12 AWG copper, or equal, for grounding of instrument transformers.
6. CTs that are not used should have the full winding shorted at the CT location and grounded.