Three-phase systems are widely used in power generation, transmission, distribution, and industrial applications. CT selection for three-phase systems must consider phase balance, metering method, protection scheme, and wiring configuration. The goal is to obtain accurate current information for all phases and, where required, for earth-fault detection.
In a fully instrumented three-phase system, three CTs are usually installed, one on each phase. This provides complete information for metering, protection, and monitoring. Each CT should have the same ratio, accuracy class, and burden. Matching characteristics are important for differential protection and for accurate power measurement. If one CT differs from the others, the system may produce incorrect readings or false trips.
In some applications, only two CTs are used. This is common in balanced systems or where cost and space are limited. The two-CT method can still provide useful information for metering and overcurrent protection, but it may not detect all unbalanced conditions. For earth-fault protection, a residual connection or a separate zero-sequence CT may be required.
The ratio should be selected according to the phase current. The normal load should fall within the recommended range of the CT primary rating. The secondary output must match the meters and relays. Burden must be calculated for each phase, especially if the cable runs are long. Correct polarity is essential. Reversed polarity on one phase can cause incorrect power readings and protection misoperation.
For protection, the CTs must coordinate with the relays. Differential protection requires matched CTs with similar characteristics. Overcurrent protection requires adequate accuracy limit factor. Earth-fault protection may use a core-balance CT or a residual connection of the three phase CTs. The protection scheme should be studied carefully.
Installation should follow phase identification and grounding requirements. The CTs should be mounted securely and clearly labelled. Testing should verify ratio, polarity, phase sequence, and burden. After commissioning, the system should be checked for balanced readings and correct power measurement.
In conclusion, select three-phase CTs by choosing three matched units for full metering and protection, or two units for limited applications. Match ratio, accuracy, burden, and polarity. Consider earth-fault detection and relay coordination. A correctly designed three-phase CT system will provide accurate data and reliable protection.