CURRENT TRANSFORMERS POLARITY AND CONNECTIONS FOR RELAYING


Polarity
Polarity marks designate the relative instantaneous directions of currents. At the same instant that the primary current is entering the marked primary terminal, the corresponding secondary current is leaving the similarly marked secondary terminal, having undergone a magnitude change within the transformer (see Figure 1).  

Figure 1 - Current Transformer Polarity

The primary H1 and secondary X1 terminals are marked with white dots, or with a ± symbol, or with H1 and X 1. As illustrated in Figure 1, the marked secondary conductor can be considered a continuation of the marked primary line as far as instantaneous current flow is concerned.

Connections
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 timeovercurrent 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 the Device 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 (see Figure 2).

Figure 2 - Wye connected CT

b)Vee connection.
A vee connection is basically a wye with one leg omitted, using only two CTs. Applied as shown in Figure 3, 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.

Figure 3 - Vee or open delta connected 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 4 is typically used for power transformer differential relay protection schemes where the power transformer has delta-wye-connected windings.

Figure 4 - Delta connected CT

The Cts on the delta side are connected in wye, and the CTs on the wye side are connected in delta. The delta connection is also used for overcurrent protection of grounding transformers where filtering 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

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