Transformers in the infrastructure of energy supply systems can have different meanings. Classical designs are used to convert individual current parameters to values that are optimal for making measurements. There are other varieties, the list of tasks of which includes the correction of voltage characteristics to the level optimal from the point of view of further transmission and distribution of energy resource. Moreover, the purpose of the current transformer is determined not only by its construction device, but also by the list of additional functions, not to mention the principle of operation.
Device transformers
Almost all modifications of this type of transformer are equipped with magnetic circuits, which are equipped with a secondary winding. The latter is loaded during operation in accordance with regulatory values in terms of resistance. Compliance with certain load indicators is important for subsequent measurement accuracy. An open winding cannot create compensation for magnetic fluxes in the core, which contributes to overheating of the magnetic circuit, and in some cases to its combustion.
At the same time, the magnetic flux generated by the primary row winding is characterized by higher performance, which can also contribute to overheating of the magnetic wire and its core. I must say that the conductive infrastructure forms a common system on which current and voltage transformers are based. The purpose of the electrical unit in this case is not of fundamental importance - the features of the operation are determined more likely by the materials used. In the case of current converters, for example, the core of the magnetic circuit is made of amorphous nanocrystalline alloys. This choice is due to the fact that the design is able to work with a wider range of technical and operational values, depending on the accuracy class.
Purpose of current transformer
The main task of a traditional current transformer is conversion. Hardware electrotechnical filling adjusts the characteristics of the served current, using the primary winding connected in series to this end. In turn, the secondary winding performs the function of directly measuring the converted current. To do this, this part provides relays with measuring instruments, as well as protection devices and automatic regulation. In particular, the purpose of a measuring current transformer may be to measure and record using low voltage devices. In this case, the condition is met under which a high-voltage current is recorded with the access of personnel to direct monitoring of the process. The fixation of operating quantities is required for a more rational use of energy during transmission in subsequent lines. Perhaps this is one of the few common sub-functions that transforming and power models of transformers have. In more detail it is worth considering the differences between these units.
Differences from voltage transformer
Most often, experts indicate a method of performing insulation between the windings. In current transformers, the primary winding is isolated from the secondary in accordance with the total received voltage. In this case, the secondary winding will be grounded, therefore, its potential corresponds to a similar indicator. In addition, measuring transformers operate in conditions close to short circuit situations, since they have a very modest level of resistance on the secondary line. In this nuance, the specific purpose of the measuring current and voltage transformers is manifested, as well as the difference in the requirements for operating conditions.
So, if operation under the threat of a short circuit for a power voltage transformer is unacceptable due to the risk of an accident, then for a conventional current converter this mode of operation is considered normal and safe. Although, of course, such transformers also have their own threats, for the prevention of which special means of protection are provided.
Principle of operation
Electromagnetic induction is the basic principle on which the working process of such transformers is based. As already noted, the main functional elements are the magnetic conductor and two levels of windings. The first one is supplied with electric charge from alternating current, and the second level implements directly the working function in the form of measurement. As current flows through the turns of the winding, induction occurs.
Further, according to the law of electromagnetic induction, which just determines the purpose and principle of operation of current transformers, the operating quantities on the line are fixed. The user using special equipment can determine the characteristics of the magnetic flux - therefore, the frequency and voltage of the current source are recorded. The technical parameter for examining the characteristics of the circuit will be the speed of measurement - this value is not the target, but it is important to evaluate it to understand the efficiency of the transformer itself.
Varieties of current transformers
There are three main categories of current converters. The most common are the so-called dry transformers, in which the first level of the winding is not at all isolated from the first. Accordingly, the parameters of the secondary current directly depend on the indicator of the conversion coefficient.
Toroidal models are also popular, the design of which provides for the possibility of their installation on a cable or bus. For this reason, the need for a primary winding, which is equipped with typical current and voltage transformers, is completely lost. The purpose and arrangement of such models is determined by their special principle of operation - in this case, the primary current will flow through the central conductor in the housing, allowing the secondary winding to directly record operating indicators. But for various reasons, including those associated with low accuracy of measurements and unreliability of the design, such models are rarely used to evaluate the current characteristics. Most often they are used for auxiliary protective link in case of short circuit.
Also used are high voltage transformers - gas and oil. They are usually involved in specialized projects in industry.
Transformation ratio
To evaluate the efficiency of the transformer itself, a conversion coefficient was introduced. Its nominal value is usually indicated in the official documentation for the transformer. This coefficient denotes the ratio of the primary rated current to that of the second winding. For example, this can be a value of 100/5 A. It can change dramatically depending on the number of sections with turns.
It should also be borne in mind that the nominal coefficient does not always correspond to the actual one. The deviation is determined by the conditions in which the current transformers are operated. The purpose and principle of operation in many respects determine the error indicators, but this nuance is not a reason for refusing to take into account the nominal transformation ratio. Knowing the magnitude of the same error, the user can level it using special electrical equipment.
Installation of a current transformer
The simplest busbar models of transformers practically do not require the use of special equipment or even tools. Such a device can be installed by one master using special clamping fittings. Standard designs require the creation of a foundation on which support racks are mounted. Further, a frame is fastened by electric welding, which will act as a kind of electrical box for concluding the necessary equipment. At the final stage, equipment is installed. What will be the set of technical equipment determines the purpose of the current transformer and the features of its future operation. At a minimum, the infrastructure required to measure the characteristics of the serviced circuit is being integrated.
Ways to connect transformers
To simplify the procedure for connecting wiring to equipment, component manufacturers mark them - for example, current relays and transformers can be designated TAa, TA1, KA1, etc. Thanks to this marking, service personnel can quickly and accurately pair the elements that the current transformer is equipped with . The device, purpose and principle of operation of the installation in this case are closely interrelated and affect the connection method, but at the same time, the served network as such has a significant impact on the nature of the technical implementation of the conversion system. For example, three-phase lines with insulated neutrals allow transformers to be installed in only two phases. This feature is due to the fact that networks with a range of 6 -35 kV do not have a neutral wire.
Transformer verification
The complex of verification measures consists of several operations. First of all, it is a visual inspection of the object, during which the integrity of the structure, the correctness of the same markings, compliance with the passport data, etc. are evaluated. Then, the equipment is demagnetized - for example, by gradually increasing the current on the first level winding. After that, the current value gradually decreases to zero.
Next, the main calibration steps are prepared, to which the measuring current transformers will be exposed. The purpose and principle of action is important to take into account in such preparation, since the load level and other operational factors cause different values of errors in the registration of the characteristics of the working environment. The verification itself provides for assessing the compliance of the polarity of the terminals of the windings with the normative parameters, as well as fixing errors with their subsequent verification with the values indicated in the passport of the unit.
Transformer Safety
The main hazards in the operation of current transformers are related to the quality of the windings. It is important to consider that under the layers of turns a metal base works, which in a bare form can pose a considerable threat to personnel. Therefore, a maintenance schedule is drawn up, in accordance with which current transformers are regularly checked. The purpose and principle of operation in this case can be oriented both to voltage conversion and current measurement. In both cases, maintenance personnel must carefully monitor the condition of the windings. As protection measures, shunt shorts are introduced into the working structure, and the grounding of the winding leads is also supported.
Conclusion
As the operational loads on the wiring line increase, the service life of the service stations decreases markedly. Although the purpose of the current transformer is not related to high voltage conversion, such equipment is also subject to severe wear. In order to increase the operational life of such plants, manufacturers use more technologically advanced materials both for electromagnetic equipment and for performing the same winding. Along with this, equipment on measuring relays is being improved, as a result of which the measurement error coefficient is also minimized.