Most power transformers permit you to tap into the windings at a variety of different positions. Some transformers are configured with voltage sensors and motors which permit them to change tap settings automatically in response to voltage variations on the power system. These transformers are referred to as tap-changing-under-Ioad (TCUL) devices. A TCUL device that controls voltage but does not transform large amounts of power is referred to as a regulating transformer. Regulating transformers that control voltage magnitude on distribution systems are usually called voltage regulators or simply regulators.
TCUL devices that regulate voltage magnitudes are categorized by a tap range and a tap increment. The tap range defines the limits of the device’s regulating ability. The tap increment defines the resolution of the device. The tap changing mechanism of many transformers is categorized by a regulation range and a step count, e.g. ±10 percent, 32 steps. The tap increment is computed from this data as follows.
| (12) |
where
Tinc is the transformer’s tap increment.
Tmax is maximum increase in voltage provided by the transformer (sometimes called boost). If the transformer can not boost the voltage, Tmax is zero.
Tmin is maximum decrease in voltage provided by the transformer (sometimes called buck). If the transformer can not decrease the voltage, Tmin is zero.
nsteps is the number of tap steps.
Table 3 catalogs the tap range and increment of common load tap-changers.
| Regulation | ||||
|---|---|---|---|---|
| Range | nsteps | Tmax | Tmin | Tinc |
| ±10% | 32 | 10.0 | -10.0 | 0.62500 |
| ±10% | 16 | 10.0 | -10.0 | 1.25000 |
| ±10% | 8 | 10.0 | -10.0 | 2.50000 |
| +10% | 32 | 10.0 | 0.0 | 0.31250 |
| +10% | 16 | 10.0 | 0.0 | 0.62500 |
| +10% | 8 | 10.0 | 0.0 | 1.25000 |
| ±7.5% | 32 | 7.5 | -7.5 | 0.46875 |
| ±7.5% | 16 | 7.5 | -7.5 | 0.93750 |
| ±7.5% | 8 | 7.5 | -7.5 | 1.87500 |
| +7.5% | 32 | 7.5 | 0.0 | 0.23437 |
| +7.5% | 16 | 7.5 | 0.0 | 0.46875 |
| +7.5% | 8 | 7.5 | 0.0 | 0.93750 |
| ±5% | 32 | 5.0 | -5.0 | 0.31250 |
| ±5% | 16 | 5.0 | -5.0 | 0.62500 |
| ±5% | 8 | 5.0 | -5.0 | 1.25000 |
| +5% | 32 | 10.0 | 0.0 | 0.15625 |
| +5% | 16 | 10.0 | 0.0 | 0.31250 |
| +5% | 8 | 10.0 | 0.0 | 0.62500 |