There are various methods available for transforming 3-phase voltages to higher or lower 3-phase voltages i.e. for handling a considerable amount of power. Safety 3-phase Isolation transformers and autotransformers are the more popular three-phase transformers. The most common connections are:
- Y- Y
- △- △
- Y – △
- △- Y
- open-delta or V- V
33.3. Star/Star orY/Y Connection
This connection is most economical for small, high-voltage transformers because the number of turns/phases and the amount of insulation required is minimum. In Fig. 33.4 a bank of 3 transformers connected in Y on both the primary and the secondary sides is shown. The ratio of line voltages on the primary and secondary sides is the same as the transformation ratio of each transformer. However, there is a phase shift of 30° between the phase voltages and line voltages both on the primary and secondary sides. Of course, line voltages on both sides as well as primary voltages are respectively in phase with each other. This connection works satisfactorily only if the load is balanced. With the unbalanced load to the neutral, the neutral point shifts thereby making the three line-to-neutral (i. e. phase) voltages unequal. The effect of unbalanced loads can be illustrated by placing a single load between phase(or coil) a and the neutral on the secondary side. The power to the load has to be supplied by primary phase (or coil)A. This primary coil A cannot supply the required power because it is in series with primaries B and C whose secondaries are open. Under these conditions, the primary coils B and C act as very high impedances so that primary coil4 can obtain but the very little current through them from the line. Hence, the secondary coil cannot supply any appreciable power. In fact, a very low resistance approaching a short-circuit may be connected between point a and the neutral and only a very small amount of current will flow. This, as said above, is due to the reduction of voltage Ean because of the neutral shift. In other words, under short-circuit conditions, the neutral is pulled too much towards the coil. a. This reduces Eam but increases Eon and Ebn and Ecn (however line voltage EAB, EBC and ECA are unaffected). On the primary side, Ean will be practically reduced to zero whereas EBN and ECN will rise to nearly full primary line voltage. This difficulty of shifting (or floating) neutral can be obviated by connecting the primary neutral (shown dotted in the figure) back to the generator so that primary coil A can take its required power from between its line and the neutral. It should be noted that if a single-phase load is connected between lines a and b, there will be a similar but less pronounced neutral shift which results in an overvoltage on one or more step transformers.
Another advantage of stabilizing the primary neutral by connecting it to the neutral of the generator is that it eliminates distortion in the secondary phase voltages. This is explained as follows. For delivering a sine wave of voltage, it is necessary to have a sine wave of flux in the core, but on account of the characteristics of iron, a sine wave of flux requires a third harmonic component in the exciting current. As the frequency of this component is thrice the frequency of the circuit, at any given instant, it tends to flow either towards or away from the neutral point in all three transformers. If the primary neutral is isolated, the triple frequency current cannot flow. Hence, the flux in the core cannot be a sine wave and so the voltages are distorted. But if the primary neutral is earthed i.e. joined to the generator neutral, then this provides a path for the triple-frequency currents and e m. fs. and the difficulty is overcome. Another way of avoiding this trouble of oscillating neutral is to provide each of the transformers with a third or tertiary winding of relatively low kVA rating. This tertiary winding is connected in△and provides a circuit in which the triple-frequency component of the magnetizing current can flow (with an isolated neutral, it could not). In that case, a sine wave of the voltage applied to the primary will result in a sine wave of phase voltage in the secondary. As said above, the advantage of this connection is that insulation is stressed only to the extent of line to neutral voltage i.e.58% of the line voltage.
33.4. Delta-Delta or△-△Connection
This connection is economical for large, low-voltage transformers in which the insulation problem is not so urgent, because it increases the number of turns/phases. The transformer connections and voltage triangles are shown in Fig. 33.5. The ratio of transformation between primary and secondary line voltage is exactly the same as that of each transformer. Further, the secondary voltage triangle ABC occupies the same relative position as the primary voltage triangle ABC i.e. there is no angular displacement between the two. Moreover, there is no internal phase shift between phase and line voltages on either side as was the case in Y- Y connection. This connection has the following advantages:
- As explained above, in order that the output voltage is sinusoidal, it is necessary that the magnetizing current of the transformer must contain a third harmonic component. In this case, the third harmonic component of the magnetizing current can flow in the O-connected transformer primaries without flowing in the line wires. The three phases are 120° apart which is 3x 120= 360° with respect to the third harmonic, hence it merely circulates in the△. Therefore, the flux is sinusoidal which results in sinusoidal voltages.
- No difficulty is experienced from unbalanced loading as was the case in Y- Y connection. The three-phase voltages remain practically constant regardless of load imbalance.
- An added advantage of this connection is that if one transformer becomes disabled, the system can continue to operate in open-delta or in V- V although with reduced available capacity. The reduced capacity is 58% and not 66. 7% of the normal value, as explained in Art. 33.7.
33.5. Wye/Delta or Y/△ Connection
The main use of this connection is at the substation end of the transmission line where the voltage is to be stepped down. The primary winding is Y-connected with grounded neutral as shown in Fig.33.6. The ratio between the secondary and primary line voltage is 1/√3 times the transformation ratio of each transformer. There is a 30° shift between the primary and secondary line voltages which means that the Y-△ transformer bank cannot be paralleled with either a Y- Y or a△-△bank. Also, third harmonic currents flow in △to provide a sinusoidal flux.
33.6. Delta/Wye or△/ Y Connection
This connection is generally employed where it is necessary to step up the voltage for example, at the beginning of high tension transmission system. The connection is shown inFig. 33.7. The neutral of the secondary is grounded for providing 3-phase 4-wire service. In recent years, this connection has gained considerable popularity because it can be used to serve both the 3-phase power equipment and single-phase lighting circuits. This connection is not open to the objection of a floating neutral and voltage distortion because. the existence of a△-connection allows a path for the third-harmonic currents. It would be observed that the primary and secondary line voltages and line currents are out of phase with each other by 30°. Because of this 30° shift, it is impossible to parallel such a bank with a△-△or Y- Y bank of transformers even though the voltage ratios are correctly adjusted. The ratio of secondary to primary voltage is 3 times the transformation ratio of each transformer.
33.7. Open-Delta or V – V connection
If one of the transformers of a△-△is removed and 3-phase supply is connected to the primaries as shown in Fig.33.11, then three equal 3-phase voltages will be available at the secondary terminals on no load. This method of transforming 3-phase power by means of only two transformers is called the open-△ or V- V connection.
It is employed:
- When the three-phase load is too small to warrant the installation of a full three-phase transformer bank.
- When one of the transformers in a△-△bank is disabled so that service is continued although at reduced capacity, till the faulty transformer is repaired or a new one is substituted.
- When it is anticipated that in the future the load will increase necessitating the closing of the open delta. One important point to note is that the total load that can be carried by a V- V bank is not two-thirds of the capacity of a△-△bank but it is only 57.7%of it. That is a reduction of 15% (strictly, 15.5%) from its normal rating.