Voltage transformer

Antiresonant voltage transformer

Voltage transformers for high voltage networks

A voltage transformer is one of the types of transformer designed not to convert electrical power to power various devices, but to galvanically decouple high voltage circuits (6 kV and higher) from low (usually 100 V) secondary voltage .

It is used in measuring circuits, converting the high voltage of generator power lines to a low-voltage voltage convenient for measurement.

In addition, the use of a voltage transformer allows isolating low-voltage measuring circuits for protection, measurement and control from high voltage, which, in turn, allows the use of cheaper equipment in low-voltage networks and reduces the cost of their isolation.

Since the voltage transformer is not designed to transmit power through it, the main mode of operation of the voltage transformer is the idle mode.

Content

Principle of Operation

The measuring voltage transformer by the principle of execution differs little from the power step-down transformer. It consists of a steel core recruited from plates of sheet electrical steel, a primary winding and one or two secondary windings. As a result of manufacturing, the required accuracy class must be achieved: in amplitude and angle. Three-phase voltage transformers with zero leads are carried out on a five-core magnetic circuit, so that when a short circuit occurs on the high voltage side, the total magnetic flux closes along the core steel (when a short circuit occurs through air, a large current occurs that leads to overheating of the transformer). Based on the above reasons, three-phase transformers with a three-core magnetic circuit do not have external zero conclusions and are not used for registering "earth faults". The less the secondary winding of the voltage transformer is loaded (that is, the closer the mode is to idle mode, or, in other words, the greater the resistance of the secondary winding circuit), the actual transformation coefficient Kt is closer to the nominal value. This is especially important when connecting measuring instruments to the secondary circuit, since the transformation coefficient affects the accuracy of the measurements. Depending on the load, the same voltage transformer can operate in different accuracy classes: 0.5; one; 3.

Types of voltage transformers

Grounding voltage transformer - a single-phase voltage transformer, one end of the primary winding of which must be tightly grounded , or a three-phase voltage transformer, the neutral of the primary winding of which must be tightly grounded (a transformer with loose insulation of one of the terminals is a single-phase ZNOM type or three-phase NTMI type and US).
Non-grounded voltage transformer is a voltage transformer in which all parts of the primary winding, including the terminals, are isolated from earth to a level corresponding to the voltage class.
Cascade voltage transformer - voltage transformer, the primary winding of which is divided into several series-connected sections, the transmission of power from which to the secondary windings is carried out using tie and equalizing windings.
Capacitive voltage transformer - voltage transformer containing a capacitive divider .
Double-winding transformer - a voltage transformer having one secondary voltage winding.
Three-winding voltage transformer - a voltage transformer having two secondary windings: main and additional.

Application

If there are several secondary windings in a three-phase system, the main windings are connected “into a star”, forming phase voltage outputs a , b , c and a common zero point o , which must necessarily be grounded to prevent the effects of insulation breakdown on the side of the primary winding (in practice, the phase is most often grounded “ B ” windings of the LV voltage transformer). Additional windings are usually connected according to the "open triangle" circuit in order to control the voltage of the zero sequence. In normal mode, this voltage is in the range of 1-3 V due to the error of the windings, sharply increasing in emergency situations in high voltage circuits, which makes it possible to easily connect high-speed relay protection devices and automation (for circuits with isolated neutral, usually a signal). To register the earth in the network, grounding of the zero output of the HV winding of the voltage transformer is necessary (for the passage of harmonics of the zero sequence).

Features of the operation of voltage transformers are regulated by Chapter 1.5 of the Rules for the installation of electrical installations . So, the load of the secondary windings of the measuring transformers, to which the meters are connected, should not exceed the nominal values. The cross-section and length of wires and cables in the voltage circuits of the meter must be selected so that the voltage loss in these circuits is not more than 0.25% of the nominal voltage when powered by voltage transformers of accuracy class 0.5 and not more than 0.5% when powered by voltage transformers accuracy class 1.0. To meet this requirement, the use of individual cables from voltage transformers to meters is allowed. Voltage losses from voltage transformers to technical meters should not exceed 1.5% of the rated voltage.

Features of the operation of VT in networks with isolated and grounded neutrals

In networks with a grounded neutral, when an earth fault occurs, the voltage of the damaged phase near the fault location decreases to zero, the vector $3U_{0}$ ${\ displaystyle 3U_ {0}}$ obtained by adding the phase voltage vectors (the addition of phase vectors located 120 ° relative to each other), and therefore the voltage $3U_{0}$ ${\ displaystyle 3U_ {0}}$ increases to phase voltage.

In networks with insulated neutral, when phase to ground, all phase voltages (with respect to the zero point) remain unchanged, but with respect to earth the phase voltages increase to linear, while being transformed into the secondary winding (with mandatory grounding of the zero point of the primary winding of the VT) they are geometrically summed. Moreover, the vectors of these stresses are 60 ° relative to each other, then $3U_{0}={\sqrt {3}}U_{b}={\sqrt {3}}U_{c}$ ${\ displaystyle 3U_ {0} = {\ sqrt {3}} U_ {b} = {\ sqrt {3}} U_ {c}}$ where $U_{b}$ ${\ displaystyle U_ {b}}$ , $U_{c}$ ${\ displaystyle U_ {c}}$ - voltages of undamaged phases relative to earth. As the voltage of the undamaged phases relative to earth increased to ${\sqrt {3}}$ ${\ displaystyle {\ sqrt {3}}}$ then $3U_{0}=3U_{f}$ ${\ displaystyle 3U_ {0} = 3U_ {f}}$ , i.e $3U_{0}$ ${\ displaystyle 3U_ {0}}$ increases to triple the value of the phase voltage relative to zero.

Based on the above features, VTs for operation in networks with a grounded neutral additional winding is performed at 100 V, and for networks with an insulated neutral 100/3 V.

The phenomenon of ferroresonance

Voltage transformers in networks with isolated neutral can enter ferroresonance with spurious capacitances of distribution networks (this is especially an undesirable phenomenon characteristic of cable networks), which can lead to their failure. To prevent damage to voltage transformers as a result of ferroresonance, NAMI type antiresonant voltage transformers have been developed.