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Very often a neutral is used with delta-delta connections or open-delta connections. For instance, both three-phase 240 volts and single-phase 120/240 volts may be obtained from the same transformer bank. One transformer is often larger than the other two to serve the single-phase load portion, and the other transformer or transformers are smaller, to serve only their portion of the three-phase load with the larger transformer.
Figure 27-4 illustrates a four-wire open-delta connection and the resultant voltages. Note that Y to N gives 208 volts, which is 120 volts X 1.73, and results from phase relationships or angular displacements. Leg Y is often called the wild leg, and the NEC requires that be identified wherever it appears with the neutral.
Figure 27-4 Open-delta four-wire connection and vectors.
Also, one will sometimes find one phase of a closed-delta or open-delta transformer connection grounded. This won’t change any voltage from those shown in Figures 27-2 and 27-3. There is no NEC violation; this is merely one method of grounding the secondary of a delta bank. This method of grounding is not found too often, but it is used at times.
So far this discussion has dealt with single-phase transformers connected for three-phase service. There are three-phase delta-delta
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Figure 27-5 Three-phase delta-delta connected transformer.
transformers in one enclosure. Figure 27-5 shows such a transformer, and the connections and the vectors are shown.
Delta-delta connections are very satisfactory. There are no problems with third harmonics, and there is no angular phase shift between primary and secondary.
Wye-wye connected transformers are not so popular. See Figure
27-6. This is because they have an inherent instability of the neutral. The three-phase, three-legged core types are a little more stable than the five-legged core types or three single-phase units. If the H0 and X0 terminals are grounded together and the load unbalance doesn’t exceed 10%, they give fairly satisfactory results, but with unbalance of more than 10%, they are subject to excessive heating.
Figure 27-6 Three-phase wye-wye connected transformer.
Transforming Polyphase Power 299
The most popular three-phase transformers are delta-wye. These have no third harmonics or other problems. Figure 27-7 shows a delta input and a wye output with three single-phase transformers. This will work with inputs such as 480 volts, with the output connected for 120 volts on each transformer, thus giving 208 volts three-phase and 120 volts single-phase.
Figure 27-7 Three single-phase transformers in a delta-wye connection.
To obtain full output, each transformer winding must carry full current at the rated voltage. Figure 27-7A shows such a connection. To clarify, Figure 27-7B is shown. If a single three-phase transformer is to be replaced by three single-phase transformers and the connection is delta-wye, the phase relationships of both current and voltage must be taken into account, due to the 120° displacements. Note from the vectors shown in Figure 27-7B that there is a 30° angular displacement between the windings, which was mentioned in the last chapter under paralleling of transformers.
Figure 27-8 shows a three-phase delta-wye transformer connection and the phase relationships.
Two-Phase to Three-Phase Conversion
The Scott connection for changing two-phase to three-phase, while not much used, is illustrated in Figure 27-9. Note that special transformers are required.
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Figure 27-8 Three-phase delta-wye transformer.
Figure 27-9 Voltage relationships in Scott-connected transformers for two phases to three phases.
The high-voltage windings usually have taps for adjusting the low-voltage output to the desired voltage if the incoming line voltage is either too high or too low. For instance, if it is desired to get 120/208 V output with a normal input of 480 V, this is one thing, but if the input is higher or lower than 480 volts, these taps will adjust for the desired voltage output.
For example, such transformers usually have taps in the vicinity of 2x/2% ranges, which may be internally changed, such as 456/467/480/504 volts. A diagram is shown on the nameplate of the transformer and the necessary changes are made to meet the voltage conditions.
Large transformers usually have an external dial for tap changing. Others may even have motor-operated tap changers, which change under load as the voltages change. These, of course, would be automatic.
Transforming Polyphase Power 301
Delta- and Wye-Connection Voltages and Currents
A comparison of winding and line voltages and current is shown in Figure 27-10 for delta connections and in Figure 27-11 for wye connections. Note that in Figure 27-10, for the delta connection, that the winding voltage and the line voltage are the same, while the line amperes are 1.73 times the winding current.
Figure 27-10 Voltage and current in a delta transformer.