VOLTAGE REGULATION TECHNIQUES OF POWER TRANSFORMER BASIC INFORMATION

It is a general practice to have some means of adjustment to maintain constant voltage at the output terminals by compensating for the variations of the input voltage. This is done by tapping out or adding turns to the primary or input winding and maintaining the volts per turn, and thus the output voltage.

This operation is usually performed when the transformer is de-energized; this is called off-circuit tap changing. In dry type transformers, the usual method is to bring out the tap terminals on the outer surface of the coil or on a terminal board, where the linking to obtain the required turns is done manually with the unit de-energized.

It is possible, though not usual, to have tap switches similar to those used in liquid- filled units. Until recently, dry-type transformers were never supplied with under-load-tap-changing equipment. This was due to the fact that under-load tap changing involves breaking of load current at full voltage, thereby requiring switching equipment with capabilities comparable to those of circuit breakers.

To do this in air was cumbersome, bulky, and extremely expensive. But with the increased capacities and voltages of dry- type transformers, the demand for such equipment has increased, and recently voltage regulators became commercially available.

Two different approaches are used to provide underload voltage regulation. One takes the traditional approach of the liquid-filled units by providing motor-driven selector switches combined with a spring activated vacuum diverter switch.

The other approach uses a separate regulator winding feeding a buck/boost transformer connected in series with the primary winding. Voltage regulation is achieved by means of low-voltage vacuum contactors that modify the tap settings of the regulating winding of the buck/boost transformer, circumventing high-voltage switching equipment.

The contactors are usually controlled by programmable logic controllers (PLC). In cases where high speed response is required, the second approach has successfully used thyristors in place of vacuum contactors, thereby achieving a cycle switching.

POWER TRANSFORMER SERIES IMPEDANCE AND REGULATION BASIC AND TUTORIALS

The series impedance of a transformer consists of a resistance that accounts for the load losses and a reactance that represents the leakage reactance. This impedance has a very low power factor, consisting almost entirely of leakage reactance with only a small resistance.

As discussed earlier, the transformer design engineer can control the leakage reactance by varying the spacing between the windings. Increasing the spacing ‘‘decouples’’ the windings and allows more leakage flux to circulate between the windings, increasing the leakage reactance.

While leakage reactance can be considered a transformer loss because it consumes reactive power, some leakage reactance is necessary to limit fault currents. On the other hand, excessive leakage reactance can cause problems with regulation.

Regulation is often defined as the drop in secondary voltage when a load is applied, but regulation is more correctly defined as the increase in secondary output voltage when the load is removed. The reason that regulation is defined this way is that transformers are considered to be ‘‘fully loaded’’ when the secondary output voltage is at the rated secondary voltage.

This requires the primary voltage to be greater than the rated primary voltage at full load.

Let Ep equal the primary voltage and let Es equal the secondary voltage when the transformer is fully loaded. Using per-unit values instead of primary and secondary voltage values, the per-unit secondary voltage will equal Ep with the load removed. Therefore, the definition of regulation can be expressed by the following equations.


Regulation = (Ep - Es)/ Es


Since Es = 1 by definition,
Regulation Ep - 1 (3.8.2)
Regulation depends on the power factor of the load. For a near-unity power
factor, the regulation is much smaller than the regulation for an inductive load
with a small lagging power factor.

Example 3.4
A three-phase 1500 KVA 12470Y-208Y transformer has a 4.7% impedance. Calculate the three-phase fault current at the secondary output with the primary connected to a 12,470 V infinite bus. Calculate the regulation for a power factor of 90% at full load.

The three-phase fault is a balanced fault, so the positive-sequence equivalent circuit applies. The full-load secondary current is calculated as follows:

I 1.732 500,000 VA per phase/208 V 4167 A per phase
The per-unit fault current is the primary voltage divided by the series impedance:
1/0.047 = 21.27 per unit

The secondary fault current is equal to the per-unit fault current times the fullload current:
If 21.27 per unit 4167 A per phase 88,632 A per phase To calculate regulation, the secondary voltage is 1∠0° per unit by definition.

Applying a 1 per unit load at a 90% lagging power factor, I 1.0∠ 25.8°. Since the series impedance is mainly inductive, the primary voltage at full load Ep can be calculated as follows:

Ep 1∠0° + 1.0∠ 25.8° X 0.047∠90°
1.02 + j0.042 = 1.021 per unit
Regulation = Ep - 1 = 0.021 = 2.1%

TRANSFORMER PERCENT (%) REGULATION BASIC AND TUTORIALS

When a transformer is energized with no load, the secondary voltage will be exactly the primary voltage divided by the turns ratio (NP/NS). When the transformer is loaded, the secondary voltage will be diminished by an amount determined by the transformer impedance and the power factor of the load.

This change in voltage is called regulation and is actually defined as the rise in voltage when the load is removed. One result of the definition of regulation is that it is always a positive number.

The primary voltage is assumed to be held constant at the rated value during this process. The exact calculation of percent regulation is given in Equation

where cos 􀁕 is the power factor of the load and L is per unit load on the transformer.

The most significant portion of this equation is the cross products, and since %X predominates over %R in the transformer impedance and cos 􀁕 predominates over sin 􀁕 for most loads, the percent regulation is usually less than the impedance (at L = 1).

When the power factor of the load is unity, then sin 􀁕 is zero and regulation is much less than the transformer impedance.

A much simpler form of the regulation calculation is given in Equation

For typical values, the result is the same as the exact calculation out to the fourth significant digit or so.

POWER TRANSFORMERS VOLTAGE REGULATION BASICS AND TUTORIALS

VOLTAGE REGULATION OF POWER TRANSFORMERS BASICS
How To Compute The Voltage Regulation of Power Transformers?

The regulation that occurs at the secondary terminals of a transformer when a load is supplied consists, as previously mentioned, of voltage drops due to the resistance of the windings and voltage drops due to the leakage reactance between the windings. 

These two voltage drops are in quadrature with one another, the resistance drop being in phase with the load current. The percentage regulation at unity power factor load may be calculated by means of the following expression: 

(copper loss x 100/output) + [(percentage reactance) ²/200]

This value is always positive and indicates a voltage drop with load. The approximate percentage regulation for a current loading of a times rated full-load current and a power factor of cosΦ₂ is given by the following

expression: 

percent regulation = a(Vr cosΦ₂ + Vx SinΦ₂) + (a²/200) (Vx cosΦ₂ - Vr SinΦ₂)²

where VR = percentage resistance voltage at full load

                = copper loss x 100 / rated kva

At loads of low power factor the regulation becomes of serious consequence if the reactance is at all high on account of its quadrature phase relationship.