IEEE C57.105-2019 pdf free download.IEEE Guide for Application of Transformer Connections in Three-Phase Electrical Systems.
4.1.2 Types of sources The primary supply may be either an ungrounded, uni-grounded, effectively grounded, or impedance- grounded source. An ungrounded or uni-grounded supply may have a grounded shield wire or cable sheath, but such conductor should not be mistaken for a neutral conductor if it is not intended for such service. Both ungrounded and uni-grounded supplies are frequently called three-wire or delta source because such systems consist of only the three-phase conductors beyond the source transformer. An ungrounded system has only the three-phase wires emanating from the source transformer winding and no intentional connection to ground. A uni-grounded system has the three-phase wires emanating from the source transformer winding and also an intentional low-impedance connection from the source transformer winding to ground. The effectively grounded distribution system has a solidly grounded neutral conductor derived from the source. Usually the neutral is multi-grounded along the feeder. Such systems are referred to as “four-wire” or “grounded wye” systems. The impedance-grounded systems are usually found in industrial plant circuits operating at distribution voltage levels of 4160 V and higher. Such systems may be wye or delta. If wye, the neutral is derived from the source, but is insulated and grounded through a resistor or reactor. If delta, the grounding is through a grounding transformer to which an external impedance may or may not be connected. Such systems allow the fow of suffcient ground fault current to afford positive high-speed protective relaying but also limit the ground fault current to minimize damage to faulted equipment. Although the impedance-grounded wye systems are sometimes said to have a “foating neutral,” this terminology is misleading. The neutral is stabilized at the source and with well-balanced loads the neutral voltage remains near ground voltage except during fault conditions.
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 of one winding connecting to the midpoint of the other to form a confguration 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 of the 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 Figure 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 and Y 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 fow 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(a), or four-wire three-phase 120/240 V service as shown in Figure 3