In the power circuits of 380 Volts with high currents, according to the PUE, a converter of a special design, called a current transformer, is used. With its help, it is possible to reduce the value of the current indicator by the number of times specified by the technical characteristics. To understand the principle of operation of such converters, you will need to familiarize yourself with their design.
Electric current transformers contain the following structural elements:
- closed core (magnetic circuit);
- primary power winding;
- secondary (step-down) coil.
The primary winding is connected in series with the monitored circuit, so that the entire phase current flows through it. The secondary coil is loaded onto a device connected to the network - a protective relay or a measuring device. Due to the difference in the number of turns in each of the coils, the current component in the secondary winding is reduced to a value determined by the transformation ratio.
Since the resistance of the load circuits is insignificant, it is believed that these devices operate in a mode very close to short-circuit.
They usually have several groups of secondary windings, each of which is used for its own purposes. They can be connected to:
- protective devices (voltage relays, for example);
- accounting and diagnostic equipment;
- control equipment.
The resistance of the output windings is strictly standardized, since even a slight deviation from the value specified in the TU leads to an increase in the measurement error or a deterioration in the response characteristics.
The essential difference between a TT and its related voltage transformers lies in the functions performed by these devices and the principle of operation. Current transformers primarily provide protection for the connected load and the specified accuracy of the measurements. The second type is characterized by a purely converting mode of operation, which is related only to operation in power circuits.
Classification of current transformers
To understand what the TT is intended for, familiarization with the generally accepted classification of these devices will help. Known samples of converting devices differ in the following main features:
- Purpose - the function performed by each specific device.
- On-site installation method.
- Design features including the total number of turns in the primary winding.
- Operating voltage and type of conductor insulation.
- The number of transformation steps.
According to the purpose, the well-known TT samples are divided into laboratory, protective, measuring and so-called "intermediate" devices.
The last category is intended either for connecting measuring instruments or for equalizing current values in differential protection systems.
According to the method of installation, the following types are distinguished:
- only for outdoor installation (in switchgear cabinets);
- for internal installation schemes (in indoor switchgear);
- converters built into electrical units and switching devices, which include generators and power transformers;
- overhead devices mounted on top of the structure (on bushings).
Portable specimens are used for laboratory research as well as for inspection and measurement.
According to the design of the primary winding, current devices are divided into multi-turn, single-turn and bus models. In accordance with the operating voltage of the circuits in which these devices are installed, they are divided into transformers installed in networks up to and more than 1000 Volts.
According to the type of insulating materials used in them, these products are divided into the following types:
- with "dry" insulation based on porcelain or epoxy resin;
- with paper-oil or condenser protection;
- with compound filling.
According to the number of available transformation stages, all known devices installed in the power supply circuit are single-stage and two-stage (their other name is "cascade").
Various circuits for connecting current transformers mainly differ in the order of commutation of the primary and secondary windings. The first of them is characterized by the simplest sequential connection (the so-called "tie-in") into the rupture of the monitored phase bus. Another thing is secondary circuits, consisting of several windings, which can be disconnected according to the following schemes:
- “Full star, used when it is necessary to monitor the current parameters in each of the phases.
- "Incomplete star", used when there is no need to control all linear measuring circuits.
- A circuit for fixing currents of "zero sequence", which includes a control relay.
On outgoing feeders 6-10 kV, in order to save money, not three, but only two measuring transformers (without one phase) are often installed.
In this case, the secondary windings are switched on in an incomplete star scheme. A common circuit called "zero sequence current test" is formed by connecting the secondary windings to a full star. In this case, the control relay used in it is included in the break of the common wire ("zero"). With this type of disconnection, the current passing through the winding is made up of all three phase vectors. If the loads are balanced, in the event of single-phase or two-phase short circuits, the component resulting from the imbalance is released in the relay.
Main parameters and characteristics of current transformers
The technical parameters of any current transformer are described by the following main indicators:
- device class;
- Rated voltage;
- currents in the primary and secondary coils;
- AC transformation ratio (as a ratio);
- permissible measurement error when connecting an electricity meter;
- permeability and cross-section of the magnetic circuit (core);
- the magnitude of the magnetic path.
The voltage rating in kilovolts is usually given in the passport attached to each specific device. Its operating value ranges from 0.66 to 1150 kV. For more complete information about this and other indicators, you should read the reference literature on connecting transformers to electric meters.
The value of the rated current in the primary coil is also found out from the accompanying technical documentation. Depending on the specific model of the transducer, this parameter can range from 1.0 to 40 thousand Amperes. The values of the current indicator in the secondary coil are usually 1.0 or 5.0 Amperes (depending on the parameters of the primary circuit).
Sometimes, under the order, the manufacturer manufactures devices with secondary currents of 2.0 or 2.5 Amperes.
The transformation ratio (multiplicity) is an indicator of the proportion or ratio of the currents of the primary and secondary coils. The limiting multiplicity is understood as the ratio of the maximum primary current to its nominal value, provided that the total error at a fixed secondary load does not exceed 10%. The nominal limiting ratio means the same indicator at optimal load.This parameter characterizes the possibility of normal functioning of protective devices in emergency modes.
According to GOST 7746-89, there are three types of errors for CTs - current, angular and total. They are quantitative indicators of the deviation of the secondary current values, multiplied by the nominal factor, from the primary indicator.
The standard prescribes to calculate such errors only in the steady-state (with constant parameters) operating mode of the system and only if the form of the primary current does not differ from the sinusoidal one.
The current error mentioned in the description of the multiplicity characterizes the relative difference in the effective values of the currents, expressed as a percentage. Its angular equivalent is defined as the error between the vectors of two effective current components: the fundamental for the primary circuit and the first harmonic for the secondary. Based on these two values, the total error is calculated by summing them according to the formula given in the instructions.
The main purpose of measuring current transformers is to connect energy meters used to service three-phase power lines.