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.

IRON CORE TYPES OF POWER TRANSFORMERS BASIC INFORMATION

Oriented (anisotropic) silicon-steel laminations.

The iron cores of conventional transformers consist of anisotropic silicon-steel laminations with lamination thickness ranging from 0.1 mm to 0.4 mm. In a transformer, the flux travels mostly within the limbs in the with-grain direction, and in the cross-grain direction only near the corners and lamination joints of transformer cores; thus oriented steel sheets are used.

The with- and cross-grain structure of oriented steel is determined by the rolling direction of the sheets during manufacture. Each side of a lamination is coated with insulating material so that no eddy currents can flow between laminations.

The coating does not significantly interfere with the passage of flux. The magnetic resistance, or reluctance, is only slightly increased and is taken into account via the iron-core stacking factor ϕFe = #(iron cross section of all laminations of core)/(cross section of entire core including insulation between laminations).

The stacking factor is in the range of 0.93 ≤ ϕFe ≤ 0.97 for 60 Hz units. For anisotropic electrical silicon steel the relative permeability is larger (and thus the magnetization required is smaller) in the with-grain direction (direction of rolling) than in the cross-grain direction. Similarly, the core losses are small in the with-grain direction and relatively large in the cross-grain direction.

Amorphous (glass-type) cores.

Amorphous magnetic materials either are obtained by quenching the molten material at high cooling rates or are manufactured by deposition techniques in a vacuum. The quenching process does not permit the forming of a crystalline structure, and therefore amorphous magnetic materials have a structure similar to glass.

The cores of transformers with amorphous alloy (AMTs) can be fabricated in the same manner as those made of oriented-silicon-steel. METGLAS (trademark of the Allied Signal Co) cores are 30% heavier than comparable oriented silicon-steel cores, but the no-load losses in amorphous alloy wound cores are only 30% of those in comparable oriented-siliconsteel wound cores.

However, the rated power efficiencies of present-day designs of AMTs and silicon-steel pole transformers with wound cores are about the same. For example, the rated power efficiencies of 20 kVA and 50 kVA wound-core AMTs at unity power factor are ηpower = 98.26% and 98.59%, respectively, while that of a 25 kVA oriented-silicon-steel wound core (10) at unity power factor is ηpower = 98.31%. The fabrication cost for AMTs with wound cores is higher than that for oriented silicon-steel wound cores.