Three Phase Transformer Connections
All large blocks of AC power are transmitted and distributed at high voltage levels by three phase transformer connection systems. Single phase transformers can be electrically connected to form three-phase transformer banks. However, the norm is to combine the individual phase coils into one three-phase transformer unit on a common core as illustrated in Figure 1. The advantages to using a single unit 3-phase transformer, as opposed to three single-phase transformers include:
Fig. 1. Typical three phase transformer connections unit
The main advantage to using a bank of single-phase transformer units occurs in the case of a fault. With a single three-phase transformer, if one of the phase windings becomes defective, the entire three-phase unit must be taken out of service. If a single-phase transformer in a three-phase bank becomes defective, it can usually be disconnected and partial service restored until a replacement is obtained. However, the reliability of modem transformers is so good that most large transformers are 3-phase.
Fig. 2. Three phase transformer windings
Three-phase transformers are generally designed and constructed for specific voltages. For example, a transformer may have a 4800-volt Delta primary and a 120/208-volt Wye secondary. The three-phase transformer has one core with three sets of windings. A primary and secondary winding are placed one on top of the other on each of the three legs of the corn-on core as illustrated in Figure 2.
Fig. 3. Single phase transformers used to form a three phase transformer bank
Single-phase transformers can also be used to form a 3-phase transformer bank. It is important that single-phase transformers are identical when connecting them to form a three-phase system. They should be identical in voltage, kVA, impedance, manufacturer and model number (Figure 3).
Three-phase transformer connections are named after the way the windings are connected inside the transformer. A 3-phase "Wye" (also known as a Star system) configuration is a connection in which one end of each of the windings of a polyphase transformer is connected to a common point (the neutral point) and the other end is connected to its appropriate line terminal. The polarity markings are fixed on any transformer. In the Wye connection of the high-voltage transformer coils, the coil ends with the polarity H2 are connected together. The H2 terminals are then connected to the neutral (N). A line wire (A, B, and C) is then connected to each of the remaining Hl coil ends.