Purpose and principle of operation of voltage transformers

A classic voltage transformer (VT) is a device that converts one value to another. The process is accompanied by a partial loss of power, but it is justified in situations where it is necessary to change the parameters of the input signal. In the design of such a transformer, winding elements are provided, with the correct calculation of which it is possible to obtain the required output voltage.

Purpose and principle of operation

The voltage transformer converts the operating potential through the principle of electromagnetic induction

The main purpose of voltage transformers is to convert the input signal to the level specified by the user's tasks - when the operating potential needs to be reduced or increased. This can be achieved due to the principle of electromagnetic induction, formulated as a law by scientists Faraday and Maxwell. According to him, in any loop located close to another similar turn of the wire, an EMF is induced with a current, proportional to the flux of magnetic induction penetrating them. The magnitude of this induction in the secondary winding of the transformer (consisting of many such turns) depends on the current in the primary circuit and on the number of turns in both coils.

The current in the secondary winding of the transformer and the voltage at the load connected to it are determined only by the ratio of the number of turns in both coils. The law of electromagnetic induction allows you to correctly calculate the parameters of a device that transfers power from input to output with the desired ratio of current and voltage.

What is the difference between a current transformer and a voltage transformer

The main difference between current transformers (CTs) and voltage converters is their different functional purpose. The former are used only in measuring circuits, allowing the level of the controlled parameter to be reduced to an acceptable value. The latter are installed in AC electric lines and output voltages used to operate the connected household equipment.

Their differences in design are as follows:

  • as the primary winding in current transformers, the power supply bus is used, on which it is mounted;
  • the parameters of the secondary winding are designed for connection to a measuring device (an electric meter in a house, for example);
  • in comparison with VT, the current transformer is more compact and has a simplified switching circuit.

Current and voltage transformers meet different requirements in terms of the accuracy of the converted values. If this indicator is very important for a measuring device, then for a voltage transformer it is of secondary importance.

Classification of voltage transformers

According to the generally accepted classification, these devices, according to their purpose, are divided into the following main types:

  • power transformers with and without grounding;
  • measuring devices;
  • autotransformers;
  • special matching devices;
  • isolation and peak transformers.

The first of these varieties are used to deliver uninterruptible power supply to the consumer in a form acceptable to him (with the required amplitude). The essence of their action is to transform one level of potential into another with the aim of subsequent transfer to the load.Three-phase devices installed at a transformer substation, for example, allow reducing high voltages from 6.3 and 10 kV to a household value of 0.4 kV.

Autotransformers are the simplest inductive designs that have one winding with taps to adjust the output voltage. Matching products are installed in low-current circuits, ensuring the transfer of power from one stage to another with minimal losses (with maximum efficiency). With the help of so-called "isolation" transformers, it is possible to organize the electrical isolation of circuits with high and low voltage. Thus, the protection of the owner of the house or summer house from high-potential electric shock is guaranteed. In addition, this kind of converters allows:

  • transfer electricity from source to consumer in the desired and safe form;
  • protect load circuits with sensitive devices included in them from electromagnetic interference;
  • block the entry of a constant current component into the working circuits.

Peak transformers are another type of devices that convert electrical energy. They are used to determine the polarity of pulse signals and match it with the output parameters. This type of converters is installed in signal circuits of computer systems and radio communication channels.

Instrument voltage and current transformers

Special instrument transformers are a special type of converters that allow monitoring devices to be included in power circuits. Their main purpose is to convert current or voltage into a value that is convenient for measuring network parameters. The need for this arises in the following situations:

  • when taking readings with electric meters;
  • if voltage and current protection relays are installed in power supply circuits;
  • if it contains other automation devices.

Gauges are classified by design, installation type, transformation ratio and number of stages. According to the first feature, they are built-in, through passage and support, and at the location - external or intended for installation in closed-type switchgear cells. According to the number of conversion steps, they are divided into single-stage and cascade, and according to the conversion ratio - into products that have one or more values.

Features of VT operation in networks with isolated and grounded zero point

Electric high-voltage networks are available in two versions: with an insulated neutral bus, or with a compensated and grounded neutral. The first mode of connecting the zero point allows you not to disconnect the network in case of single-phase (OZ) or arc faults (DZ). PUE allow the operation of lines with an isolated neutral for up to eight hours with a single-phase closure, but with the proviso that at this time work is underway to eliminate the malfunction.

Damage to electrical equipment is possible due to an increase in the phase voltage to linear and the subsequent appearance of an alternating arc. Regardless of the cause and mode of operation, this is the most dangerous type of short circuit with a high overvoltage factor. It is in this case that the probability of the appearance of ferroresonance in the network is high.

The ferroresonant circuit in power networks with an isolated neutral is a zero sequence chain with nonlinear magnetization. A three-phase non-grounded VT is essentially three single-phase transformers connected in a star-star manner. With overvoltage in the zones where it is installed, the induction in its core increases by about 1.73 times, causing the appearance of ferroresonance.

To protect against this phenomenon, special methods have been developed:

  • manufacture of VTs and TTs with low self-induction;
  • the inclusion of additional damping elements in their circuit;
  • manufacturing of 3-phase transformers with a single magnetic system in 5-rod design;
  • grounding the neutral wire through a current-limiting reactor;
  • use of compensation windings, etc .;
  • the use of relay circuits that protect the VT windings from overcurrents.

These measures protect the measuring VTs, but do not completely solve the safety problem. Grounded devices installed in networks with an isolated neutral bus can help in this.

The nature of the operation of low voltage transformers in modes with a grounded neutral is characterized by increased safety and a significant reduction in ferroresonance phenomena. In addition, their use increases the sensitivity and selectivity of protection in a single-phase circuit. This rise becomes possible due to the fact that the inductive winding of the transformer is included in the ground circuit and briefly increases the current through the protective device installed in it.

The PUE provides justification for the admissibility of short-term grounding of the neutral with a small inductance of the VT winding. For this, automation is used in the network, which, with power contacts, when an OZ occurs, after 0.5 seconds, briefly connects the transformer to the busbars. Due to the effect of a solidly grounded neutral, a current limited by the inductance of the VT begins to flow in the protective circuit in the event of a single-phase earth fault. At the same time, its value is sufficient to trigger the protection against OZ and create conditions for extinguishing a dangerous arc discharge.

home.techinfus.com/en/
Add a comment

Foundation

Ventilation

Heating