**Voltage Regulation of Transformer**

The voltage regulation of a transformer can be described as the change in the secondary voltage as the current varies from full load to no load while keeping the primary voltage constant.

Generally, a transformer is utilized to supply loads that operate at basically constant voltage. The amount of secondary (load) current which is drawn entirely depends on the load linked to the transformer secondary side terminals. As this current varies with changing load, with the same applied primary voltage, the load voltage varies accordingly. This change in the load voltage is primarily due to the voltage drop across the transformer internal impedance. Voltage regulation is actually a measure of how much the voltage varies as the load is varied.

It is clear from the transformer equivalent circuit in figure 1 that the secondary current I_{s} produces voltage drop I_{s}R_{s} and Is X_{s} across the resistive and reactive components respectively. Also, the primary current Ip causes primary circuit voltage drops I_{p}R_{p} and I_{p}X_{p}. Consequently, the effective primary voltage E_{p} is less than the input voltage V_{i}, and the output voltage V_{o} is less than the calculated value of E_{s}.

Fig.1: Complete Equivalent Circuit of Transformer

It appears that the transformer output voltage is greatest on no-load and that under loaded conditions the voltage drop across resistive and reactive components of the equivalent circuit cause V_{o} to drop below its no-load level. (Note that, depending on the power factor of the load, the output full-load voltage may actually be larger than the no-load voltage).

**Voltage Regulation of Transformer Formula**

The percentage change in output voltage from no-load to full-load is termed the voltage regulation of the transformer. Ideally, there should be no change in V_{o} from no-load to full-load (i.e., regulation = 100%). For the best possible performance, the transformer should have the lowest possible regulation. Mathematically, voltage regulation can be expressed as

\[\begin{matrix} Voltage\text{ }Regulation=\frac{{{V}_{o(NL)}}-{{V}_{o(FL)}}}{{{V}_{o(FL)}}} & {} & \left( 1 \right) \\\end{matrix}\]

Where V_{o(NL) }is the transformer no-load output voltage, and V_{o(FL) }is the full load output voltage. Voltage regulation for a transformer is illustrated in figure 2.

Fig.2: Transformer Voltage Regulation

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The three phasor diagrams of figure 3 illustrate how the voltage regulation depends on the power factor of the load. In diagram (a), the current is in phase with the voltage (the load is resistive and its power factor is unity), whereas in diagram (b), the current lags the voltage (the load is inductive, the power factor is called lagging), and in diagram (c), the current leads the voltage (the load is capacitive, the power factor is said to be leading).

Fig.3: Voltage Regulation at (a) Unity (b) Lagging (c) Leading Power Factor

**Voltage Regulation of Transformer at Unity Power Factor**

Figure 3 (a) shows a phasor diagram for the case of a resistive load (unity power factor) on the transformer (i.e., the load current is in phase with the secondary voltage). Since the current I_{2} is in phase with the secondary voltage V_{o (FL)}, the voltage drop across R_{e2} is also in phase with V_{o (FL)}. The drop across the leakage reactance X_{e2} leads the secondary voltage V_{o (FL)} by 90^{o}, as shown. The circular arc has a radius equal to V_{o (FL)} and therefore represents a line of constant secondary voltage. The referred value of primary voltage V_{o (NL)} is beyond the arc, so it is bigger than the secondary voltage V_{o (FL)}, which means the voltage regulation calculated by equation (1) is **positive**.

**Voltage Regulation of Transformer at Lagging Power Factor**

Figure 3 (b) shows a phasor diagram for the case of an inductive load (lagging power factor) on the transformer (i.e., the load current lags the secondary voltage by 90^{o}). The referred value of primary voltage V_{o (NL)} is beyond the arc, so it is bigger than the secondary voltage V_{o (FL)}, which means the voltage regulation calculated by equation (1) is positive for an inductive case. For any current that lags the secondary voltage by 0 to 90^{o}, the voltage regulation will be **positive**.

**Voltage Regulation of Transformer at Leading Power Factor**

Figure 3 (c) shows a phasor diagram for the case of a capacitive load (leading power factor) on the transformer i.e., the load current leads the secondary voltage by 90^{o}. As a result, the referred value of primary voltage V_{o (NL)} is actually smaller than the secondary voltage V_{o (FL)}, which means the voltage regulation calculated by equation (1) is **negative** for a capacitive case.

**Negative Voltage Regulation**

Negative voltage regulation can only occur with leading power factor loads; however, not all leading loads will cause negative voltage regulation.

Negative voltage regulation means the voltage increases with the load.

This is a very undesirable condition because it can lead to an unstable condition. Many loads use more power as the voltage increases. Thus, the voltage goes up, causing the power to go up, potentially causing the voltage to go up some more.

Many utilities impose a penalty on large customers if they operate at a leading power factor. Most loads are lagging, so leading power factor usually results from a failure to remove power factor correction capacitors when the load decreases.

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