Measuring transformers are used in relay protection and automation circuits for analysis of current and voltage. This is necessary so that the relay protection can work on time in accordance with the settings set by the engineers when setting up electrical equipment. As a rule, current and voltage measuring transformers are used in relay protection and automation circuits. Consider each of the species separately.
Voltage transformers are used to guide the protection, this is a kind of support, relative to which the phase angle is measured, and also protection against earth faults using windings connected according to an open triangle scheme.
Measuring voltage transformers are performed with several cores: one is connected according to the "star" scheme, the second - according to the "open triangle" scheme. Today, manufacturing plants produce transformers with a third core, which is used for accounting.
For RU-35-110 kV, three single-phase voltage transformers are installed. For each connection group, separate machines are mounted. For RU-6-10 kV, a three-phase voltage transformer in one housing is most often used.
Measuring current transformers are used in all switchgears and are installed in phases.
These transformers are made of monolithic bus, which is the primary. The secondary windings are fixed on the bus, there can be several of them, and each winding (hereinafter the core) is designed for a certain accuracy class and a specific power.
Cores are displayed for protection, accounting and measurement circuits. It is important that there are never any loose core on transformers, otherwise the winding will burn out, and maintenance personnel are at risk of electric shock.
During installation, measuring current transformers must be installed according to the voltage input-output scheme (L1 - input, L2 - output). The direction of the current vectors of each phase depends on this. According to the physical sense, between the vectors of currents, the windings of which are connected according to the "Star" scheme, there should be 120 degrees. If, for some reason, the vectors are located differently, it is necessary to change the connection of CT cores, reversing the beginning with the end.
Current transformers are used for protection against short circuits and for the organization of differential protection. Measuring current transformers are an important attribute of modern electrification, it is a kind of sensor with which the current value and its direction are determined.
In electrical installations, a zero sequence transformer is used. This element is relevant for RU-0.4-6-10 kV, since the task of the transformer (in the bagel household) is to protect high-voltage circuits from earth faults. Its primary is the feeder itself, which is passed through the bagel. A value is removed from the secondary that changes according to a change in the magnetic field of the feeder.
Thus, transformers for measuring current and voltage are the basis of protection. Thanks to them, modern microprocessors can independently calculate power, resistance of a circuit section, angles between current and voltage. The results obtained do not need to be verified, since modern equipment is carried out according to the latest technologies.
Today, devices that are a comprehensive solution and include a switch, a disconnector, as well as built-in CTs and VTs are quite popular - this is convenient and quite profitable.