FEEDING A DUAL LOAD
Some power companies use a delta–delta-connected transformer bank to feed two types of loads. These loads consist of a 240-V, three-phase industrial load and a 120/240-V, single-phase, three-wire lighting load.
One single-phase transformer supplies the single-phase, three-wire lighting load. This transformer usually is larger than the other two transformers. The 120/240-V, single-phase, three-wire service is obtained from this transformer by bringing out a tap from the mid- point of the 240-V, low-voltage secondary winding. Many transformers have two 120-V windings. As explained in Unit 13, these windings can be connected in series, with a tap brought out at the midpoint to give 120/240-V service.
Three single-phase transformers are connected as a delta–delta bank in Figure 14–5. Each transformer has two 120-V windings. When these windings are connected in series, each transformer has a total output of 240 V. The high-voltage primary windings are con- nected in closed delta. The low-voltage output windings or secondary windings are also connected in closed delta to give three-phase, 240-V service to the industrial power load. Because the middle transformer also feeds the single-phase, three-wire, 120/240-V light- ing load, a tap is made at the midpoint on the secondary output side of the transformer to give 120/240-V service. This tap feeds to the neutral wire of the single-phase, three-wire system and is grounded.
A check of the connections in Figure 14–5 shows that there is 120 V to ground on both lines A and C of the three-phase, 240-V secondary system. However, line B will have 208 V to ground. The condition can be a serious hazard and cannot be used for lighting service.
THE WYE CONNECTION
The wye connection is another standard method of connecting single-phase trans- formers to obtain three-phase voltage transformation. The wye connections must be made systematically to avoid errors. The student must be familiar with and able to use the basic voltage and current relationships for this type of connection. In Unit 10, the following information was given for the three-phase wye connection:
• The line current and the coil winding current are equal.
• The line voltage is equal to M3 times the coil winding voltage.
Figure 14–6 shows three single-phase transformers connected in wye–wye. The H2 leads of the high-voltage windings are considered to be the ends of each of the high-volt- age windings and are connected together. The H1 (beginning) lead of each transformer is
connected to one of the three line leads. When this connection is shown in a schematic diagram, it looks like the letter Y (which is written as wye). The connection may also be called a star connection.
In general, the low-voltage winding leads are marked and the polarity is shown on the transformer nameplate. However, the following procedure should be used to make the low- voltage secondary connections:
1. Energize the three-phase, wye-connected, high-voltage side of the transformer bank.
The voltage output of each of the three transformers must be the same as the name- plate rating.
2. Deenergize the primary. Connect the X2 ends of two low-voltage secondary windings, as shown in Figure 14–7A. With all three X1s open and clear, energize the primary. If the connections are made correctly, the voltage across the open ends is M3 times the secondary winding voltage. In this case, the voltage across the open ends is 208 V, as indicated in the vector diagram.
The secondaries of the two transformers are shown connected incorrectly in Figure 14–7B. In this case, the voltage across the open ends is the same as the secondary voltage of each transformer. According to the vector diagram, the resultant voltage is only 120 V. The connections can be corrected by reversing the leads of transformer 2. As a result, the voltage across the open ends will be 208 V.
Secondaries Properly Connected
The correct wye connections are shown in Figure 14–8A for the secondary windings of the three transformers. The voltage across each pair of line leads is equal to M3 times the secondary coil voltage or 208 V. The vector diagram in Figure 14–8B shows the relationship between the coil voltages and the line voltages in a three-phase, wye-connected system.
The wye–wye connection can be used in those applications where the load on the secondary side is balanced. If the load consists of three-phase motor loads only, and the load currents are balanced, then the wye–wye connection can be used. This connection cannot be used if the secondary load becomes unbalanced. An unbalanced load causes a serious imbalance in the three voltages of the transformer bank.
A fourth wire known as the neutral wire is added to eliminate unbalanced voltages. The neutral wire is connected between the source and the common point on the primary side of the transformer bank.
The diagram in Figure 14–9 shows a wye–wye-connected transformer bank having a three-phase, four-wire, 2400/4160-V input and a three-phase, four-wire, 120/208-V output. On the high-voltage input side, the neutral wire is connected to the common point. This is the point where all three high-voltage primary winding (H2 ) leads terminate. The voltage from the neutral to any one of the three line leads is 2400 V. Each high-voltage winding is connected between the neutral and one of the three line leads. This means that each high- voltage winding is connected across 2400 V. The voltage across the three line leads is M3 X 2400 V, or 4160 V. The neutral wire maintains a nearly constant voltage across each of the high-voltage windings, even when the load is unbalanced. The neutral conducts any unbalance of current between the source and the neutral point on the input side of the transformer bank. The neutral wire is grounded and helps protect the three high-voltage windings from lightning surges.
For the transformer bank shown in Figure 14–9, the three-phase, four-wire sys- tem feeds from the low-voltage side of the bank to the load. Each low-voltage wind- ing is connected between the secondary neutral and one of the three line leads. The voltage output of each secondary winding is 120 V. Thus, there is 120 V between the neutral and any one of the three secondary line leads. The voltage across the line wires is M3 X 120, or 208 V. The use of a three-phase, four-wire secondary provides two voltages that can be used for different load types:
2. The use of a neutral wire means that 120 V is available for lighting loads.
Neutral Wire Used on Primary and Secondary
Figure 14–10 shows the connections for a wye–wye-connected transformer bank. Note that both the primary and secondary sides contain a neutral wire. Each transformer has two 120-V, low-voltage windings connected in parallel. The voltage output for each single-phase transformer is 120 V. Because this is a three-phase, four-wire system, the
following voltages are available: (1) a three-phase, 208-V service for motor loads and (2) a 120-V, single-phase service for the lighting loads. The lighting load should be distributed evenly between the three transformers. Thus, an attempt is always made to balance the lighting circuits from the line wire to the neutral.
Nearly all wye–wye-connected transformer banks use three single-phase transformers having the same kilovolt-ampere capacity. The individual kVA ratings are added to find the capacity of the transformer bank. For example, if a bank consists of three transformers, each rated at 25 kVA, the total rating is 25 + 25 + 25 = 75 kVA.
In a wye–wye connection, a defective transformer must be replaced before the wye– wye transformer bank can be reenergized. Unlike a delta–delta bank, a wye–wye trans- former bank cannot be temporarily reconnected in an emergency using two single-phase transformers only.