Protective Current Transformers are designed to measure the actual currents in power systems and to produce proportional currents in their secondary windings which are isolated from the main power circuit. These replica currents are used as inputs to protective relays which will automatically isolate part of a power circuit.
Satisfactory operation of protective relays can depend on accurate representation of currents ranging from small leakage currents to very high overcurrent's, requiring the protective current transformer to be linear, and therefore below magnetic saturation at values up to perhaps 30 times full load current.
This wide operating range means that protective current transformers require to be constructed with larger cross-sections resulting in heavier cores than equivalent current transformers used for measuring duties. For space and economy reasons, equipment designers should, however, avoid over-specifying protective current transformers ITL technical staff are always prepared to assist in specifying protective CT's but require some or all of the following information.
- Protected equipment and type of protection.
- Maximum fault level for stability.
- Sensitivity required.
- Type of relay and likely setting.
- Pilot wire resistance, or length of run and pilot wire used.
- Primary conductor diameter or busbar dimensions.
- System voltage level.
A WORD OF CAUTION: RELAY MANUFACTURER'S RECOMMENDATIONS SHOULD ALWAYS BE FOLLOWED
Characterisation of a protective current transformer class is as follows:
Class P & PR (A current transformer to meet the composite error requirements of a short-circuit current under symmetrical steady-state conditions).
Class PX & PXR (A current transformer by specifying its magnetising characteristic).
Class TPX, TPY & TPZ (A current transformer to meet the transient error requirements under the conditions of an asymmetrical short-circuit current).
Rated Output: The burden including relay and pilot wires generally follow standard burdens 2.5, 5, 7.5, 10, 15 and 30VA.
The accuracy designation "P" (which stands for protection) uses the highest percentage composite error followed by the Accuracy Limit Factor (ALF).
Standard Protection Accuracy Classes are shown as 5P & 10P with Accuracy Limit Factors 5, 10, 15, 20, 30
Therefore the electrical requirement of a protection current transformer can be defined as:
RATIO - VA BURDEN - ACCURACY CLASS - ACCURACY LIMIT FACTOR
For example, 1600/5A, 15VA 5P10.
Accuracy limit Factor is defined as the multiple of rated primary current up to which the transformer will comply with the requirements of 'Composite Error'. Composite Error is the deviation from an ideal CT (as in Current Error) but takes account of harmonics in the secondary current caused by non-linear magnetic conditions through the cycle at higher flux densities.
Selection of Accuracy Class & Limit Factor.
Class 5P and 10P protective current transformers are generally used in overcurrent and unrestricted earth leakage protection. With the exception of simple trip relays, the protective device usually has an intentional time delay, thereby ensuring that the severe effect of transients has passed before the relay is called to operate. Protection Current Transformers used for such
In some systems, it may be sufficient to simply detect a fault and isolate that circuit. However, in more discriminating schemes, it is necessary to ensure that a phase-to-phase fault does not operate the earth fault relay.