Delta–wye transformer connections may also be used for voltage transformation. The delta–wye connection is used both to step up and step down voltage. As shown in Figure 14–11, the primary windings are connected in delta and the secondary windings are connected in wye.
The delta–wye transformer bank shown in Figure 14–11 is used to step up the volt- age at a generating station. The input voltage is stepped up by the transformer ratio. This voltage is further increased by the factor of 1.73. The high-voltage output is then connected directly to three-phase transmission lines. These transmission lines deliver the energy to users who may be miles away from the generating station. The use of the delta–wye connection means that the insulation requirements are reduced for the secondary windings. This fact is very important when the secondary side has a very high voltage. (Recall that the coil voltage is only 58% of the line voltage, where 1 -:- M3 = 1 -:- 1.73 = 0.58 = 58%.)
In Figure 14–11, the ac generators deliver energy to the generating station bus bars at a three-phase potential of 13,800 V. For the three single-phase transformers, the primary windings are each rated at 13,800 V. The windings are connected in delta to the bus bars of the generating station. The coil and line voltages are the same in a delta connection. Therefore, each primary winding has 13,800 V applied to it. The stepup ratio of the transformers shown is 1:5. This means that the voltage output of the secondary of each single-phase transformer is 5 X 13,800 = 69,000 V. The three secondary windings are connected in wye. Each high-voltage secondary winding is connected between the secondary neutral and one of the three line wires. The voltage between the neutral and any one of the three line wires is the same as the secondary coil voltage, or 69,000 V. The voltage across the three line wires is M3 X 69,000 = 119,370 V, or 120,000 V.
Three-phase output voltages can be balanced because of the neutral wire on the high- voltage secondary. This is true even when the load current is unbalanced. The neutral wire is grounded at the transformer bank. It is also grounded at intervals on the transmission line. The neutral wire helps protect the high-voltage secondary windings of the three single-phase transformers from damage due to lightning surges.
Stepping Down Voltages Using the Delta–Wye Connection
The delta–wye connection can also be used for applications where the voltage is stepped down. For example, assume that energy is to be transferred from a 13,800-V, three-phase, three-wire distribution system to a 277/480-V, three-phase, four-wire sys- tem. This voltage is then used to supply the power and lighting needs of a large office building.
Figure 14–12 shows the delta–wye-connected transformer bank used in this case. The primary side of the transformer bank is connected in delta to a three-phase, three- wire, 13,800-V distribution circuit. The line voltage and the voltage across each primary coil winding are all equal to 13,800 V. The transformer ratio is 50:1. As a result, the secondary coil winding voltage of each transformer is 13,800 -:- 50 = 276 V.
The secondary side of the transformer bank is connected in wye. The voltage from the grounded neutral to each of the three line wires is 277 V. The voltage across the line wires is M3 X 277, or 480 V.
Three-Phase, Four-Wire System of Power and Lighting
The standard voltage for lighting circuits is 120 V. Industrial power applications normally use either 208 or 240 V. However, power and lighting applications also can be served by a 277/480-V, three-phase, four-wire system.
Modern lighting applications generally require a high level of light intensity. Fluorescent lighting units furnish light having the required intensity. Standard 120-V fluorescent lighting fixtures are used with special ballasts for operation on 277-V circuits. Motors wound for 480 V, rather than 208 or 220 V, can be used for air- conditioning units, fans, pumps, and elevators. In office buildings, the lighting demand can be as much as 7 to 10 volt-amperes per square foot (VA/ft2). The motor load may aver- age as much as 4 VA/ft2. A 277/480-V, three-phase, four-wire system has the following advantages:
• The voltage drop is reduced in feeders and branch circuits, resulting in an increase in the operating efficiency.
• The load demands on the 277/480-V system can be increased with a minimum of changes and expense.
All office buildings have miscellaneous loads requiring 120 V. These loads include desk lamps, office machines, and communications equipment. Under normal conditions, such loads are only a fraction of the total load and can be supplied from small air-cooled transformers. These transformers are located on each floor of the building. They are connected to the 277/480-V system and step down the voltage from 480 V to 120 V.
The wye–delta transformer bank is used to step down relatively high transmission line voltages at the load center. A transformer bank of this type is commonly used to step down three-phase voltages of 60,000 V or more. There are two advantages in using the wye–delta connection. The first is that the three-phase voltage is reduced by the transformer ratio times 1.73. The second advantage is that there is a reduction in the insulation requirements for the high-voltage windings. Less insulation is required because the actual primary coil voltage is only 58% of the primary line voltage.
The diagram of a wye–delta transformer bank is shown in Figure 14–13. This bank is located at the end of a three-phase, four-wire transmission line. The primary three-phase voltage of 60,900 V is stepped down to 4400 V, three phase. Three single-phase trans- formers are used. The high-voltage side of each transformer is rated at 1000 kVA, 35,200
V. Each low-voltage side is rated at 4400 V. The voltage ratio of each transformer is 8:1. Assume that the three-phase primary line voltage is 60,900 V. As shown in Figure 14–13, the three single-phase transformers are connected in wye on the high-voltage side. The primary line voltage is 60,900 V. Therefore, the voltage impressed across the primary winding of each transformer is
Balanced three-phase voltages are obtained even when there are unbalanced load cur- rents because of the neutral wire on the high-voltage primary input. The neutral wire is grounded and gives lightning surge protection.
For both delta–wye and wye–delta connections, the three single-phase transform- ers generally have the same kVA capacity. The total capacity of the transformer bank in kVA is obtained by adding the kVA ratings of the three transformers. In Figure 14–13, for example, each single-phase transformer is rated at 1000 kVA and the total capacity of the transformer bank is 3000 kVA.
It is possible to achieve three-phase transformation of energy by using two trans- formers only. One connection that will do this is called the open-delta connection (V connection). On occasion, one of the three transformers in a delta–delta bank will become defective. To restore three-phase service to consumers as soon as possible, the defective transformer is cut out of the system and the configuration of the open-delta connection is used.
The following example describes a typical use of the open-delta connection. A delta– delta connection is made using three 50-kVA transformers. Each one is rated at 2400 V on the high-voltage side and 240 V on the low-voltage side. This closed-delta transformer bank steps down 2400 V, three phase to 240 V, three phase to supply an industrial consumer. One of the transformers is damaged by lightning, resulting in a power failure. The three-phase service must be restored at once.
By disconnecting all of the leads of the damaged transformer, the closed-delta bank becomes an open-delta bank, as shown in Figure 14–14.
The student may expect that the total kVA capacity of the open-delta bank will be two- thirds of the capacity of the closed-delta bank. Actually, the capacity of an open-delta bank is only 58% that of a closed-delta bank. For this example, the total capacity of the delta– delta bank is equal to the sum of the kVA capacities of the three transformers: 50 + 50 + 50 = 150 kVA. When one transformer is disconnected, an open-delta connection is formed. The total kVA capacity is now only 58% of the capacity of the closed-delta connection: 150 X 0.58 = 87 kVA (or 86.6% of the total capacity of the two remaining transformers).
Capacity of the Open-Delta Connection
In Figure 14–15, three 1-kVA transformers are connected to form a closed-delta connection. The secondary voltage of each transformer is 100 V. The maximum current for each winding is 10 A (1000 kVA/100 V = 10 A). The total power for this connection can be found by using the formula