IEEE C57.105-2019 pdf free.IEEE Guide for Application of Transformer Connections in Three-Phase Electrical Systems.
The zigzag winding is also combined with either wye or delta connections in two-winding transformers to provide a desired phase relation and also a grounded neutral.
In this guide, the symbol ZZ indicates the zigzag connection, and the symbol ZiG indicates that the neutral is grounded. Table I shows application characteristics of transformer banks with zigzag-grounded secondaries.
4.4 T-T connections
Some three-phase distribution transformers consist of two single-phase transformers in a common tank with the interconnection of the two primaries and two secondaries in the manner shown in Figure 3(a) and Figure 3(b). the end ofone winding connecting to the midpoint of the other to form a configLlralion similar to the letter T. This connection is also known as a Scott connection. The crossbar of the T is called the main winding, and the stem is called the teaser winding. Voltages of the main and teaser coils are such that balanced three-phase voltages are obtained. The neutral point ofthe secondary winding could be at one-third of the teaser coil from the main to simulate a grounded Y. four-wire connection as shown in Figure 3(b) or it could be at the midpoint of the main winding to simulate a delta with a grounded mid-tap. four-wire connection as shown in F igurc 3(b). Although not actually a symmetrical connection, the three-phase T-connected transformer simulates the symmetrical Y and delta connections.
The possible application characteristics of T-connected transformers are shown in Table 1 and simulate standard delta-Y, Y-delta. delta-delta, and Y-Y connections. T-connected transformers may be paralleled with delta andY combinations provided the usual requirements for paralleling are met (see 4.5,3).
The T-connection has three very interesting properties. First. if the neutral is grounded. a T-connected winding presents a low-impedance to zero-sequence current and permits the flow of such currents independently of the other winding. The implication of this fact is that the primary should not be grounded since the transformer will then act as a grounding transformer (see 4,6). T-connected windings are capable of providing a four-wire grounded service, even with an ungrounded primary, such as four-wire three-phase 120/208 V or 277/480 V service as shown in Figure 3(aL or four-wire three-phase 120/240 V service as shown in Figure 3(b) and may do so regardless of relative orientation of the windings. The symbol TG will be used in this guide to represent a T-connected winding with neutral grounded.IEEE C57.105 pdf free download.