PBV transformer: anzapfa device, principle of operation, operation and repair

Electricity consumers work more efficiently at rated voltage. However, this condition is rather difficult for everyone. Acceptable for consumers is its deviation up to + 5%. To achieve a voltage value close to the nominal value, the number of turns of the transformer windings is changed. This can be done in two ways:

• using a transformer tap-changer;
• adjusting the voltage under load.

Connection of voltage regulation with changing the number of turns

There are several methods of maintaining the voltage value of consumers within the required limits. Among them, a special place is occupied by the method of its regulation. The advantages of this method are:

• improvement of the voltage regime for consumers;
• an increase in the permissible voltage loss;
• improving the quality of electricity supplied to consumers.

When designing electrical networks, the means, boundaries and degrees of regulation, the place of installation of the regulators, as well as their automation system are chosen.

The value of the primary and secondary voltage is directly proportional to the number of turns of the windings in which it flows:

U 1 / U 2 ≈W 1 / W 2,

where U 1, U 2 - respectively primary and secondary voltage;

W 1 / W 2 - respectively, the number of turns of the primary and secondary winding

It follows from this that in order to change the voltage at the output of the transformer, it is necessary to change the number of turns of one of the windings. Due to this, the winding that will be involved in the switching is produced with taps.

Despite the simplicity of the process, there are some difficulties. When switching from one branch to another, in no case should the current circuit be interrupted. Simultaneously with this requirement, it is forbidden for the switch contacts to close two adjacent branches, otherwise a short circuit of this part of the winding cannot be avoided. And this, in turn, will lead to its damage due to the large currents that have arisen.

There are two ways to meet these conditions: switching the taps of the windings after disconnecting all its windings from the network and during operation, with a load.

Basic concepts about pbv transformer

The transformer tap-changer has a very simple decoding, which consists in the first letters of the words - "switching without excitation". This means that all switching operations must be carried out at the transformer, which is disconnected from the power supply.

But another name for the transformer pbv device is also widely known - anzapfa. Anzapfa (switch) is a device with which the number of winding turns can be changed to regulate the output voltage.

The switch is designed to change the transformation ratio within 5%, changing the number of turns of the high voltage winding involved in the work.

Place of installation of anzapfa

For transformers that have a multilayer cylindrical winding with a power of up to 560 kVA, the location of the anzapf is near the zero point.

If the transformer is manufactured with a capacity of up to 1000 kVA, voltage up to 10 kV and has continuous windings, a reverse circuit with a branch near the zero point is used.

In transformers over 1000 kVA and 35 kV, a circuit with an adjusting branch in the middle of the winding is used. In this case, the anzapf consists of three elements. They are placed on a common axis, one on top of the other. The switch closes simultaneously a pair of contacts in any phase. This type of switch design is the cheapest and the smallest.

To keep the switching currents low, the unzapfu must always be installed in the high voltage winding. This achieves the manufacture of taps and the switch of the device more compact dimensions. At the same time, much more turns are wound at the high voltage winding, due to which a higher regulation accuracy is achieved.

When switching the anzapf from one stage to another, turn the switch handle. It is located on the roof of the tank.

When adjusting in a way without excitation, disconnecting the transformer first from the low and then high voltage side is a prerequisite..

The switch handle drive is covered by a cap. The designations + 5%, "Nom", -5% are marked near the handle indicator. When the handle indicator is turned to the indication + 5%, all winding turns are activated. When reading "Nom" - 5% less. When set to -5%, the winding turns are 10% less in operation.

In some types of transformer, instead of the designations + 5%, "Nom", -5%, the numbers I, II, III are indicated. In this case, the reading of I corresponds to + 5%, II - "Nom", III - 5%.

If the power of the transformers is in the range from 25 to 6300 kVA, then they are made with taps with manual switching to adjust the voltage within ± 5% in steps of 2.5%.

Methods of off-circuit transformer

Switching a transformer without excitation can be done in two ways:

1. Changing the voltage using the primary winding.
2. Regulation by installing anzapfa in the secondary winding.

If the voltage is changed using the primary winding, then the anzapfu is installed in it. This method only finds application in step-down transformers. This method is also called voltage regulation by changing the magnetic flux.

Regardless of the voltage loss in the winding, you can take U 1 ≈ E 1. The electromotive force in the primary winding will not change due to constant parameters: frequency and voltage of the network:

E 1 = 4.44 f W 1 F m

Considering that changes in frequency during operation are not expected, the product W 1 Ф m will not change. Therefore, the magnetic flux can be reduced by connecting more turns of the primary winding. For example, in order to achieve a voltage drop at the terminals of the secondary winding of 2.5%, it is necessary to increase the number of turns of the primary winding by 2.5%.

The branch terminals of step-down transformers can provide + 10% surcharge. To do this, you need to connect -5% turns to them.

For example, depending on the terminal to which the switching device is connected, the surcharge percentage for a 10 kV step-down transformer will vary.

Clamp	Mains voltage, V	Surcharge,%
+ 5% or I	10500	0
"Nom" or II	10000	-5
-5% or III	9500	+10

The second method is used in step-up transformers. The low voltage winding (primary) is connected to the mains.

If the frequency and voltage are unchanged, the magnetic flux will be stable, and the electromotive force E 2 will change in accordance with the change in the turns of the secondary winding, depending on the formula:

E 2 = 4.44 f W 2 F m

The formula indicates that if the number of turns on the terminals of the secondary winding decreases, then the voltage will also decrease. Analysis of the formula confirms that the number of turns and the voltage value are directly proportional.

Very often in step-up transformers to obtain the highest voltage, the required number of turns is already connected and taken into account. Therefore, when idling, the step-up transformer will be without surcharge.

Operation and repair of devices

In transformers, 10% of their breakdowns are faults associated with damage to the contact system of the anzapfa:

• Loose fit of movable and immovable parts of the contacts. This is due to a decrease in the contact pressure, as a result of which an oxide film forms on the contact surface.
• Over time, the junction of the control branches with the parts of the switching device weakens.
• During a long service life, the strength of the connection between the control tap and the winding decreases. The main reason is poor quality soldering.

All these factors lead to heating of the damaged site, which can subsequently cause an emergency breakdown of the entire transformer. Therefore, the maintenance and repair of anzapfa equipment occupy a worthy place among the rest of the equipment.

The first step in repairing the switch device is inspection. Assessment of the state of fixed and moving contacts is necessary, since they are in transformer oil for a long time during operation. Because of this, they become covered with an oxide film. To remove it, it is necessary to thoroughly clean the contacts with a rag that was previously moistened with purified gasoline. If the contacts are burnt and melted, they are replaced with new ones, which can be purchased, or you can make yourself. In case of self-production, an important condition is the selection of materials for contacts, similar in quality to the factory ones.

After replacing damaged parts, tighten the fasteners, check for jamming, correct contact of movable and fixed contacts, update the inscriptions near the cover switch.

After completing all the operations of setting up the anzapfa, it is necessary to test the quality of its performance. For this, switchings are made to all stages within ten cycles. Interference in the operation of the device should not be traced.

The imperfection of all these methods of regulation without excitation is that in order to switch the branches, the transformer must be disconnected from the power source. This creates interruptions in the supply of electricity to consumers.

A common method is voltage regulation under load.