The current transformer is a device whose primary winding is connected in series with the working circuit and the secondary winding is used for measuring. Such devices are used not only in laboratories to estimate quantities. The true place of current transformers near power plants, where they help control the modes, making adjustments to the process of equipment operation.
Protection and Measurement with Current Transformers
Once it was necessary to transfer energy over a distance. This happened at the time of the development of history, when generators began to be located near rivers. The factories were located at ordinary places: at the place of occurrence of resources, near large cities - sources of labor. It turned out that the voltage 220, especially 110 V, is inefficient to transmit over a distance - losses grow. Explanation - with a constant power consumption current increases, which directly leads to an increase in the heat generated in the wires.
The option to increase the cross-section of the wire was quickly discarded as too expensive. Then they began to use step-up transformers. As a result, it was found that with acceptable efficiency it is possible to transmit electricity over long distances only at a voltage of tens of kilovolts. It is clear that so much power is required to control. Part of the consequences of breaking the phase wires of power lines:
- The death of people who are supposed to fix the problem and accidentally find themselves in place.
- Failure of three-phase power motors.
- Explosive and flammable situations.
In a year, a section of 100 km of the 380 V transmission line accounts for 40 to 50 accidents, 40% for a break in the phase wire. In the course of eliminating abnormal situations 4-5 people die. Overhead lines are unreliable, but this is the best method today for transmitting electrical energy over a distance that requires control and protection measures. In addition, current transformers are used in measuring equipment. For example, in tandem with three-phase voltage meters.
Classification of current transformers
Current transformers are usually classified:
- By type of current. The measured voltage varies by type. For measurements in the DC circuit, cutting the signal into pulses is used. Direct transformation is not possible:
- for alternating current;
- for DC.
- To destination. We have already said that current transformers are often used for measurements( for example, kWh).Call systems where you need to protect personnel to improve safety. Of course, the techniques are applied to localize and eliminate emergency situations:
- By type of conversion. Controllers or meters work with current or voltage. Accordingly, the following transformers are manufactured:
- By way of presenting information:
- By type of installation:
- for indoor use;
- for work in the open air( according to GOST 15150 placement category 1);
- By way of installation:
- reference( installation on the plane);
- checkpoints( mainly input devices to the building);
- embedded( sometimes without primary winding, represents the magnetic core, worn on the insulation of the current-carrying core): bus( put on the power bus);detachable( magnetic core consists of two parts, bolted together).
- By the number of transformation ratios. According to GOST, a number of stresses are distinguished, differing from each other in order. To interface with the same control devices, the transformation ratio has to be changed:
- with one transformation ratio;
- with multiple transformation ratios;
- By the number of stages of transformation. It is not always possible to obtain an acceptable signal level using a single transform. Then it is necessary to increase and relieve the number of windings repeatedly, lowering or increasing:
- According to the design of the primary winding:
- single-turn: with its own primary winding( primary winding is rectangular or round rod or U-shaped);without own primary winding;
- By type of insulation between the primary and secondary windings:
- with viscous( in the form of compounds);
- with hard( composite materials, porcelain);
- with gaseous( air);
- with a combination( oil and paper).
- According to the principle of current conversion:
The design, in other cases and the principle of operation, is determined by the voltage for which the device is intended. Current transformers are divided into two families: for low voltage( up to 1 kV) and high( other).Devices are very specific. Instruments familiar to the school course of physics resemble only current transformers with a multi-turn winding, which approximately resembles a coil.
Parameters of current transformers
When choosing to work in tandem with a three-phase meter, first of all, pay attention to the transformation ratio. A number of values is standardized, and you need to choose devices that can work in pairs. It was said above that in other cases the transformation ratio may be changed, and it is necessary to use it.
In addition to the operating voltage, the current in the primary winding( of the network under study) plays a role. It is clear that with increasing heat increases, and once the current-carrying part can burn. This requirement is not so relevant for transformers without primary winding. The nominal secondary current is usually 1 or 5 A, which serves as a criterion for coordination with the mating devices.
Relates to pay attention to the load resistance in the measurement circuit. There is hardly a counter out of the general row, but it is necessary to control the moment. Otherwise, the accuracy of the readings is not guaranteed. The load factor is usually not lower than 0.8.This applies to measuring instruments with inductances in the composition. GOST normalizes the value in volt-amperes. To obtain resistance in ohms, you must divide the number by the square of the secondary current.
Limit operation modes are usually characterized by an electrodynamic resistance current arising from a short circuit. In the passport they write the value at which the device will work for an arbitrarily long time without failure. Under short-circuit conditions, the current is so strong that it begins to have a mechanical effect. Sometimes, instead of the current of electrodynamic resistance, its multiplicity to nominal is indicated. It remains only to perform the multiplication operation. The specified parameter does not apply to devices without primary winding.
In addition, the thermal resistance current is determined, which the transformer withstands without critical overheating. This kind of stability can be expressed by multiplicity. But they share the currents of thermal stability in time until the device remains intact:
- Three seconds.
There are dependencies between the currents of electrodynamic and thermal resistance presented in the figure. The temperature of the primary winding of aluminum should not exceed 200 degrees Celsius, and of copper - from 250 to 300, depending on the type of insulation. For high-voltage transformers, mechanical resistance is standardized, determined by the action of wind at a speed of 40 m / s( hurricane):
- 500 N for products with a rated voltage up to 35 kV.
- 1000 N for products with a rated voltage from 110 to 220 kV.
- 1500 N for products with a rated voltage of 330 kV.
Inclusion of a current transformer in the circuit and principle of operation of
In general, the device consists of a magnetic circuit and two windings. But the current transformer, unlike the usual, is turned on in a special way. The primary winding sequentially enters the main circuit, where the consumers are located, the secondary is closed to the measuring device or the protective relay.
When a current flows in the primary winding inside the magnetic core, a field appears causing a response. Simultaneously, a current is induced in the secondary winding. Its field is opposite to the originator, and the resulting flow is equal to the difference between the original and the newly formed. It is only a few percent of the original and, in fact, is the transmission link of the system. The resulting magnetic field permeates along the path of the core the turns of the primary and secondary windings, suggesting in the first counter-emf, and in the second emf.
Electromotive force generates a secondary current, the ratio to the primary depends on the ratio of the number of turns. This is the transformation ratio. The secondary current will remain unchanged, and the primary current will grow until the resulting field becomes equal to the field at idle. As a result, the device will acquire a sufficiently low resistance.
Let us explain for a complete understanding of the behavior of a transformer in idle mode. In this case, the primary current induces a magnetic field in the magnetic core. The stream circulates in a closed loop of electrical steel with a small attenuation. Its action is such that the created EMF in the primary winding in the direction opposite to the voltage of the network. This happens because in the inductance the current lags 90 degrees, the induced emf lags 90 degrees behind the magnetic field.
Now imagine that the secondary winding was loaded. As a result, the field energy begins to be transmitted to the output, forming a current. From the secondary winding, a magnetic field is formed in antiphase from the source that generated it. Counter-EMF at the entrance falls, consumption begins to grow. The increased current increases the primary magnetic field. The process goes on until equilibrium is reached. This will happen when the resulting magnetic field equals the field at idle. The device will begin to consume more energy, now the system is working.
From what has been said it is clear:
- It is useless to turn on any type of transformer in the idle mode in the network. Energy will be spent only on losses due to the magnetic reversal of the core( eddy currents are almost not formed, due to the special design in the form of plates isolated from each other).
- A small number of turns in current transformers is required to reduce the consumption in the specified circuit segment to a minimum. Individual copies do not have a primary winding. What looks logical for large flowing currents.
We have seen that there is a magnetic coupling between the currents. The name of the transformers seems to be quite logical. Structures for overload protection( in the short circuit mode) and differential circuits that compare the magnitudes of the currents of the phase and neutral wires are developed. In the latter case, a certain insensitivity threshold is provided for the circuit to account for leakage currents of the system.
The considered class of devices has two types of errors that require mention:
- The current error is the difference between the actual transformation ratio and the nominal one.
- The angular error is the divergence of the vector of the output current from the ideal case( in antiphase relative to the input).
There are special methods for compensating for these disadvantages. For example, by means of a coil correction, the current error is eliminated. The angle of divergence is eliminated by the correct choice of the magnitude of the magnetic induction in the core.