# Voltage transformers

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Purpose and principle of operation of the voltage transformer.
The voltage transformer is used to reduce the high voltage supplied in AC installations to measuring instruments, protection and automation relays.

For direct connection to high voltage of the measuring and relay equipment, very bulky devices and relays would be required due to the need for their implementation with high-voltage insulation. The manufacture and use of such equipment is practically impracticable, especially at a voltage of 35 kV and above.
The use of voltage transformers allows the use of standard measuring instruments for high voltage measurements, expanding the measurement limits. In addition, the voltage transformer galvanically isolates the devices from high voltage, making them safe to use.
Voltage transformers are widely used in high-voltage equipment, the accuracy of measurements, electricity metering, as well as the reliability of protection in relay circuits and emergency automation depend on them.

Voltage transformer made in the form  toroidal transformerBasically he is no different from a power step-down transformer. It consists of an annular magnetic core wound from coiled electrical steel, a primary winding and one or more secondary windings. In fig. 1, a shows a diagram of a voltage transformer with one secondary winding. A high voltage U is applied to the primary winding1, and the voltage of the secondary winding U2 connected to the measuring device. The beginnings of the primary and secondary windings are marked with letters and aand the ends are X and NS
The ratio of the primary rated voltage to the secondary rated voltage is called the rated transformation ratio of the voltage transformer:
nNo.= U1nom./ U2nom.
The primary and secondary voltages of the voltage transformer coincide (when the transformer is operating without errors) in phase, as shown in Fig. 1b and the ratio of their absolute values is nnom At transformation ratio nNo.=1 voltage U2= U1.
Voltage transformers with two secondary windings are intended not only for power supply of measuring instruments and relays, but also for operation in an earth fault signaling device in a network with an isolated neutral or protection against earth faults in a network with a grounded neutral.

A diagram of a voltage transformer with two secondary windings is shown in fig. 2 a... The terminals of the second winding, used for signaling or protection in case of short-circuit to earth, are marked aD and xD ... In fig. shows a diagram of the inclusion of three such voltage transformers in a three-phase network.
Fig. 1 Scheme (a) and vector diagrams of the voltage transformer at nnom> 1 (b) nNo.= 1 (c)

The primary and first (main) secondary windings are star-connected. The neutral of the primary winding is grounded. Three phases and zero can be applied to measuring instruments and relays from the main secondary windings. Additional secondary windings are connected in an open delta circuit. From them, the sum of the phase voltage vectors of all three phases is fed to the signaling or protection devices.
During normal operation of the network, with a voltage transformer, this amount is zero. These can be seen from the vector diagrams in Fig. 2 c, where UA, Uand UC- vectors of phase voltages, on primary windings, a Uhell, Uvd, and Usd- vectors of voltages on secondary additional windings (triangle in Fig. 1 c). The sum of vectors Uhell, UbD and Usd obtained by combining them according to the connection scheme add. windings, while it was assumed that the directions of the vectors of both primary and secondary voltages correspond to the beginnings of the transformer windings. Final voltage ZU0 between the end of the phase winding with and the beginning of the phase winding a on the diagram is zero.

Fig 2 Voltage transformer with two secondary windings.

General information on the design of voltage transformers. The most commonly used single-phase transformers, with an operating voltage of 380 V to 50 kV. (3-phase voltage up to 18 kV). Single-phase and three-phase voltage transformers can have one or two secondary windings. Phase voltage is applied to the primary winding of the transformer with two secondary windings, connected to the phase-to-earth voltage in nominal mode. In the event of a ground fault, in a network with an isolated neutral, the phase-to-ground voltage may rise to line-to-line. Therefore, voltage transformers with two secondary windings, intended for use in a network with an isolated neutral and having a rated voltage equal to the phase voltage of the network, are designed for long-term operation under line voltage.
Voltage transformers with two secondary windings, manufactured for operation in a network with a grounded neutral, must withstand an increase in the primary phase voltage to 1.5Uf.nom

The dimensions and weight of voltage transformers are not set by the power (as in power transformers), but by the volume of insulation of the primary winding and the dimensions of the high voltage terminals. With a low power of a voltage transformer operating in a mode close to no-load, the volume of high voltage insulation significantly exceeds the volume of copper in the primary winding required in terms of power. To ensure the necessary mechanical strength of the primary winding, it is necessary to overestimate the cross-section of its wire. An increase in the volume of the high voltage winding against the required power, naturally, also causes an increase in the size of the magnetic circuit. As a result, the dimensions and weight of a voltage transformer designed for a higher voltage is always larger than a transformer of the same design and power with a lower rated voltage of the primary winding. To reduce the size and weight of voltage transformers, their cascade (step) design is used. In this case, the operating voltage is distributed between the stages and the isolation of each of them is performed at a lower voltage. For the same purpose, at high voltage, voltage transformers of 10-15 kV are used, which are switched on through a capacitive voltage divider.

Voltage transformers with rated voltages from 380 V to 6 kV are available with dry insulation (windings are made with PET and PETD wires). 10-50 kV voltage transformers have oil insulation (the magnetic circuit is immersed in transformer oil).
To reduce the effect of atmospheric overvoltages on the turns of the upper (input) layers of the primary winding, they are protected in all voltage transformers of 3 kV and above by electrostatic screens, which are connected to the line inputs.
Isolation of the primary winding of a single-phase voltage transformer and its both terminals is performed at full operating voltage only for transformers with one secondary winding, which can be switched on to phase-to-phase voltage. Voltage transformers with two secondary windings, switched on for phase-to-ground voltage, have only one primary winding terminal, designed for full operating voltage; the other end is led out through the low voltage input. The section of the primary winding close to the grounded terminal is usually made with reduced insulation relative to the ground and the secondary winding.

Cascade voltage transformers. The principle of implementation is based on a transformer circuit consisting of two stages. Each stage is a transformer with a rated voltage equal to half the operating voltage, which is applied to the terminals of the high voltage winding. The transformer of each of the cascades is housed in a separate well insulated casing, with the casing of the first cascade being mounted directly on the casing of the second, as a result of which the high voltage input is double insulated with respect to earth.

• By Alexander Added on January 18, 2019 at 13:37

1.3v + 6.3v - 3A
2.50v - 60ma
3.250v 200ma
4.600v 200ma
5.12v + 12v - 2A

• By Oleg Added on February 22, 2019 at 01:03 am

how much will the transformer cost and what are the dimensions:
torus.
1.230v
screen
2.8v - 2A
3.8v - 2A
4.15v -0.5A
5.350V - 0.15A
6.350V - 0.15A

• By Nikolay Added on March 12, 2019 at 17:16

I need a 200W transformer with a primary winding for 220V, 230V, 240V, AND 260V with secondary separate windings for 4V, 5V, 6V and 7V. These are four separate windings. Each winding must withstand a load of 9 A.The output voltages correspond to 220 V when applied to the primary winding.

• By Sergey Added on April 9, 2019 at 14:14

I need transformers to increase the voltage with a factor of 1x2.5 with a primary 3-phase winding, the input of which will be supplied with energy from a wind generator U = 0-50 V, I = 0-40A.

• By Alexander Added on July 22, 2019 at 21:47

Hello! I need a transformer - primary winding - 220 Volts, Secondary 1 - 33 V. 1.5 A, Secondary 2 - 33V. 1.5A. How much does such a transformer cost? And how long does it take to get ready?

• By Victor Added on February 23, 2020 at 13:48

Toroid.
Power amplifier for sound power.
Primary 220 (V);
Secondary: 33-0-33 (B), 2 (A); 15-0-15 (B), 0.5 (A); 9 (B), 0.25 (A).
Qty. 1 PC.
Approximately —— dimensions, price, lead time?
From SW. Victor.

• By Michael Added on September 29, 2020 at 17:29

Hello, I wanted to know the approximate price of a step-up transformer.
Primary 12 V, 2 A
Secondary 6-10 kV, 0.2 A

• By Oleg Added on December 21, 2020 at 17:52

Hello.
Interested in a 220V transformer with an output voltage of 12V and a current of 25-30A.
What is the cost, dimensions and production time?
Thanks.