What are High Voltage Transformers? Explanation About It's Testing and Voltage Regulation.

 What is a High Voltage Transformer?

A high voltage transformer is a type of transformer that operates at a high voltage level. For testing purposes, high voltage transformers are used in high-voltage labs. When the insulation is damaged, these high voltage transformers can be exposed to surges and transient voltages. 

These impulse voltages can be withstood by carefully designing the insulation. These transformers are typically single-phase core-type.

This type of transformer is generally oil-immersed. For separating high tension windings from low tension windings, bakelite sheets can be used. High voltage transformers that are used in HV cable testing must also supply enough electric current.

These transformers can produce a lot of heat so the cooling system is carefully designed. Special care also needs to be taken to ensure proper transformer voltage regulation.

For insulator testing purposes, the required current is very less but, while the insulator breaks down during testing, there would flow huge current through the transformer. To limit this current, a high transformer is earthed.

The earthed tank is connected to the secondary end of this transformer. A high voltage bushing connects the other end. This bushing has been specially manufactured and designed to withstand the full secondary high voltage in relation to the transformer tank's earthed potential.

This bushing also houses another tapping terminal. The tapping terminal and high voltage ends are connected to the primary transformer.

The tank is connected at one end by the secondary winding. The second transformer's tank is not grounded like the first. The transformer's secondary voltage is achieved by isolation and insulation.

The earth is connected at one end of the secondary or high voltage winding of the second transformer. The other end comes out of the high-voltage bushing to supply high voltage to the equipment or insulators being tested.

Voltage Regulation

Avoid surges at the high voltage side. The voltage regulation of the transformer must be precise enough to ensure accurate voltage measurements.

Also, it is important to avoid sudden voltage variations during testing. During testing, a voltage regulator shouldn't distort voltage waveforms.

A high voltage transformer's output voltage can be controlled by changing its input voltage on the primary side.

Variation of alternator field current.

• Inserting resistance ">resistance or
• Use an induction regulator
• Use tapped transformer.

Variation in Alternator Field Current

The variation of the alternator field current method can be used if only one alternator is being used to supply power to the high-voltage transformer.

An alternator produces a sinusoidal voltage waveform at zero loads. It is desirable that this voltage waveform does not become distorted by load conditions.

This is achieved by making a larger air gap between the stator and rotor, or by using a specially designed armature winding of the alternator.

In this instance, there is no need to connect an impedance in series with the primary transformer in order to regulate voltage. Voltage regulation can be achieved by varying the alternator field current to avoid distortion of the voltage waveforms due to inserted resistance.

The field current of the alternator is varied by a voltage divider, connected across DC supply to the field. This method can achieve zero voltage by neutralizing the residual magnetism in the field and severing its required field current.

Voltage Regulation through Inserting Resistance or Inductance

This method is used when there is no option to use a separate alternator in the lab for testing high voltage.

When testing small equipment, the high voltage transformer is fed by the AC supply mains. The variation of supply voltage to the H. V. the transformer is obtained by inserting resistance is in series with AC supply.

Smooth regulation of the voltage supplied to the primary transformer is possible with a sliding resistance.

Sometimes, the resistance can be connected to the main supply to act as a voltage divider to supply variable voltage to the transformer.

Although this method is simple, it has a problem with power loss. The resistance's power loss is too great to be used in high-power tests.

This method is expensive because of the high resistance needed for high power applications. This method is not suitable for low-medium power applications due to these drawbacks. This is generally limited to equipment with a rating between 2 kVA and 3 kVA.

Instead of resistance, voltage regulation can be achieved by connecting a choke coil (inductor) connected in series with transformer's primary.

You can alter the position of the choke coil's iron core to create voltage variation. This is done by inserting or withdrawing the iron core from the coil to achieve voltage variation.

• This method is more effective than resistance because it has a lower power loss.
• This method has its advantages, but it also has some disadvantages.
• This choke coil must be very large to produce higher power.
• The coil's iron core can cause voltage distortion.