Booster Transformer

BOOSTER TRANSFORMER

There are two types of booster transformers, transformers, namely i) ii)

In-phase Booster Quadrature Booster

i)

In-phase Boosters In this type, an additional voltage in-phase with or in-phase opposition to the existing line-to-earth voltage is injected into the system. This transformer is conjunction with the on-load tape-changing transformer compensates the voltage drop in the line and controls the reactive voltamperes (VAr) transferred in an interconnected system.

There are various common arrangements for connecting a booster transformer are: (a) A boosting auto-transformer T 2 proved with tapping is connected with the secondary of the main transformer transformer T1 (Fig. 13.3). This arrangement is costly due to the fact that the booster transformer is to be insulated to withstand the line surges. (b) The primary winding of the booster transformer T 2 is connected on the primary side of the main transformer transformer T1. The secondary of T 2 is tapped and is connected in series with the secondary of T 1 (Fig. 13.4). Figure 13.5 shows the connection of the booster for a solidly grounded system. This arrangement is cheap due to reduced insulation. (c) The tertiary winding W T of the main transformer T 1  energises the primary of the booster transformer transformer TB (Fig. 13.6). (d) A regulating transformer T R  is inserted in the circuit as shown in Fig 13.7. A reversing switch S is used to arrange for bucking or boosting of voltage. The primary of the boosting transformer is supplied by the regulating transformer. The drop in transformer is negligible and the boost voltage vb (Fig. 13.8) is added to the system voltage. The arrangements shows by Fig. 13.3 to 13.7 are drawn for one phase only. They can be used for three-phase system. For example, the equivalent three-phase diagram of fig. 13.4 is shown in Fig. 13.9.

ii)

Quadrature Booster Quadrature booster injects a voltage leading the system voltage by 90 o. The amount of real power (kW) flowing in controlled by such a booster. The reason is obvious. The line impedance is practically reactive. The injection of a series in-phase results in the flow of a quadrature current (reactive kVA), while if the injection voltage by nearly 90 o. Thus, the resulting current is in-phase with the phase with the phase voltage of the line or, in other words, real power (kW) flows.

As mentioned earlier, a quadrature booster is used to control the flow of real power and the phase angle. Fig 13.10 shows the method of obtaining a boost voltage at 90 o to the existing voltage for phase A. The  booster transformer is energized from a regulating transformer connected across phase B and C. The connections of booster for the other two phases can similarly be shown. Figure 13.11 shows the phasor diagram of the voltages. A boost voltage V BC, is injected in phase A to make its v oltage Va’ A phase displacement δ between the input voltage Va and output voltage Va’ takes place. Thus, it is possible to control phase shift δ by tap-changing on the regulating transformer. Voltage control by tap-changing is relatively slow. For large transformer it takes 15-20 seconds for tap-changing and, therefore, the method is not suitable for circuits with rapidly varying loads.