The four common types of CTs are as follows:a) A wound CTA bar CTA window CTA bushing CT

CTs are usually connected on three-phase circuits in one of three ways, as follows:a)Wye connection . In the wye connection, a CT is placed in each phase with time-overcurrent relays (Device 51) placed in either two or three CT secondaries to detect phase faults. On grounded four-wire systems, a time-overcurrent relay (Device 51N) in the CT common wire known as a residually connected relay detects any ground fault or neutral load currents. If neutral load currents are not to be detected by theDevice 51N relay as ground-fault currents, a fourth CT is placed in the neutral conductor to cancel the neutral load currents. Secondary currents are in phase with primary currents.

b)Vee connection A vee connection is basically a wye with one leg omitted, using only two CTs. Applied as shown in Figure 3-8, this connection detects three-phase and phase-to-phase faults. A zero-sequence CT (window or bushing) and a ground overcurrent relay (Device 50GS or Device 51GS) are required to detect ground-fault currents. All three-phase conductors and the neutral (if present) shall pass through the CT.c) Delta connection . A delta connection uses three CTs with the secondaries connected in delta before the connections are made to the relays. The delta connection shown in Figure 3-9 is typically used for power transformer differential relay protection schemes where the power transformer has delta-wye-connected windings. The CTs on the delta side are connected in wye, and the CTs on the wye side are connected in delta. This connection is described in detail in Chapter 4. The delta connection is also used for overcurrent protection of grounding transformers where ltering out the third-harmonic currents is desirable. When connected in delta, the current in the relays is equal to times the CT secondary current. This fact should be considered when selecting the primary ratings of CTs and the secondary device ratings of delta- connected CTs.