Influence of Current Transformer Overload on Metering

1 What are the advantages and disadvantages of machine winding and manual winding in the manufacture of transformer?The advantages of machine wound transformer are high efficiency and beautiful appearance, but winding the ring transformer with tall and small holes is more troublesome, and it is not as reliable as manual winding in terms of insulation treatment process. Manual winding can make the magnetic leakage of the transformer very small. During the winding process, it can be adjusted at any time according to the layout of coil turns. Therefore, the real hi-end transformer must be wound purely manually. The only disadvantage of pure manual winding is low efficiency and slow speed.2 Which of the ring type, EI type, R type and C type power transformers is the best?They have their own advantages and disadvantages, and there is no saying who is the best, so strictly speaking, any kind of transformer can be done best. In terms of structure, the ring type can minimize the magnetic flux leakage, but the EI type can do better in the sense of intermediate frequency density. In terms of magnetic saturation alone, the EI type is stronger than the ring type, but the ring type is better than the EI type in efficiency. Nevertheless, the key to the problem is whether you can give full play to their respective advantages, which is the most fundamental to do a good job of transformer.In the current imported amplifiers, the application of ring transformer is still the mainstream, which basically explains a problem. Enthusiasts should be objective and fair about the * price of transformers. You can't refer to something that isn't done well and say it's bad. Some people say that the ring transformer is easy to magnetic saturation, so why don't you find a way to make it not easy to magnetic saturation? This could have been achieved by technical means. Without enough effort or blindly in order to save cost, it is of course prone to magnetic saturation. Similarly, as long as you make it carefully, the efficiency of EI transformer can be very high.The quality of the transformer has a great impact on the sound, because the transmission energy of the transformer is closely related to the iron core and coil, and its transmission rate plays a decisive role in the influence of the sound. Like EI transformer, people usually think its medium frequency is thicker and high frequency is thinner. Why? Because its transmission speed is relatively slow. And the ring? The low frequency is relatively fierce, while the medium and high frequency are slightly weaker. Why? Because its transmission speed is relatively fast, but if you can make the ring type and EI type perfect through effective structural changes, the key depends on what you do.However, at least one thing is certain that R-type transformer is not easy to do well. It can be used as the power supply of low current front-stage power amplifier and CD player. If it is used as the power supply of rear-stage power amplifier, it has serious defects. Because the structural form of R-type transformer itself is not easy to change, while ring type and EI type are relatively easy to achieve the purpose of beautiful sound by changing the structure. The power amplifier power supply made of R-type transformer usually has a hard sound and lacks aura, and the low-frequency power amplifier is often hard without bouncing force.III. is the greater the silicon content of silicon steel sheet of transformer core, the better?It has not been found that the silicon content of silicon steel sheet has little effect on the quality of transformer, while the oriented and non oriented are related to the model of iron core. Secondly, even if the iron core of the same model is not handled well, the quality difference is also very large, and sometimes the difference is as high as 40% or 50%.For a good iron core and the same material, the heat treatment and wire winding process are very key. A good heat treatment only needs a small 10mA excitation current to reach 15000 Gauss, while a bad heat treatment may need 50mA excitation current to reach the corresponding 15000 Gauss. There is a great difference between the two. Judging whether the iron core is good or not from a professional point of view is mainly through several indicators such as excitation current, iron loss and saturation parameters.IV. if the splicing process is adopted for the belt silicon steel sheet of the ring transformer, does it mean that the quality is certainly not good?It can not be generalized, but it is not easy to splice too many broken heads, because one more broken position will have one more magnetic flux leakage point, so it is best not to have more than 2-3 joint points. In terms of manufacturing process, all broken ends and splices shall be pickled first, but for the ring transformer manufacturing high-grade audio equipment, strictly speaking, it is best to use silicon steel without splicing, and its process quality will be more guaranteed.V. what is particular about the silicon steel sheet material in the transformer?Because the loss of silicon steel in alternating magnetic field is very small, silicon steel sheet is mainly used as magnetic material in transformers. Silicon steel sheet can be divided into hot rolling and cold rolling. Cold rolled silicon steel strip has the advantages of small volume, light weight and high efficiency because of its high magnetic conductivity and low loss. The performance of hot rolled silicon steel strip is slightly inferior to that of cold rolled silicon steel strip.Common EI type transformers are made of silicon steel plate into E-type and I-type sheets with a thickness of 0.35-0.5mm, which are inserted into the winding package after heat treatment. Most of these iron cores use hot-rolled silicon steel sheets (the models of high-quality silicon steel sheets with high silicon content are d41, d42, d43 and D301). The iron core of ring and C-type transformers is formed by winding cold-rolled silicon steel strip. The C-type transformer is cut after heat treatment and paint dipping.The leakage inductance of the transformer is generated by the magnetic flux that does not pass through the primary and secondary coils. These magnetic fluxes pass through the air and form a closed magnetic circuit. The leakage inductance can be reduced by enhancing the coupling density between the primary and secondary of the transformer. The leakage inductance of a good transformer shall not exceed 1 / 100 of the primary coil inductance, and that of a high fidelity hi fi output transformer shall not exceed 1 / 500.

Several common detection wiring modes of transformer:The transformer winding deformation tester is mainly composed of main measuring unit and notebook computer, parallel three special measuring cables, measuring clips and grounding wires.The main measuring unit system and the test object are connected by 50W high-frequency coaxial cable. The frequency sweeping signal is injected into the test object through the signal clip (yellow) through the connecting cable through the output port (excitation output); The signal is obtained from the test object by the signal measuring clip (green) and transmitted to the (response input) via cable; The synchronous reference signal is obtained from the injection point of the tested object by signal measurement and transmitted to the input (reference input) through cable. The shell of the tested object and the shielding layer of the test cable must be reliably connected and grounded. Large transformers generally use the connection point between the outgoing line of the iron core grounding sleeve and the oil tank as the public grounding point, and the shell point of the transformer is grounded.Three phase yn shaped measuring wiring1. The Yellow clamp (input) is clamped on the o point of the transformer, the green clamp (measurement) is clamped on the a phase of the transformer, and the measurement of representative and three-phase yn type a phase.2. The Yellow clamp (input) is clamped on the o point of the transformer, the green clamp (measurement) is clamped on the B of the transformer, and represents the measurement of three-phase yn type B.3. The clamp (input) is clamped on the o point of the transformer, the green clamp (measurement) is clamped on the C of the transformer, and represents the measurement of three-phase yn type C.Three phase Y-shaped measuring wiring1. The Yellow clamp (input) is clamped on phase a of the transformer, the green clamp (measurement) is clamped on phase B of the transformer, and represents the measurement of three-phase Y-type ab.2. The color clamp (input) is clamped on phase B of the transformer, the green clamp (measurement) is clamped on phase C of the transformer, and represents the measurement of three-phase Y-type BC.3. The Yellow clamp (input) is clamped on phase C of the transformer, the green clamp (measurement) is clamped on phase a of the transformer, and represents the measurement of three-phase Y-type ca.Three phase delta measuring wiring1. The Yellow clamp (input) is clamped on phase a of the transformer, the green clamp (measurement) is clamped on phase B of the transformer, and represents the measurement of three-phase type ab.2. The Yellow clamp (input) is clamped on phase B of the transformer, the green clamp (measurement) is clamped on phase C of the transformer, and represents the measurement of three-phase BC.3. The Yellow clamp (input) is clamped on phase C of the transformer, the green clamp (measurement) is clamped on phase a of the transformer, and represents the measurement of three-phase ca.Single phase x, y, Z measuring wiring1. The Yellow clamp (input) is clamped on the X point of the transformer, the green clamp (measurement) is clamped on the a of the transformer, and represents the measurement of single-phase x, y and Z ax.2. The Yellow clamp (input) is clamped on the Y-point of the transformer, the green clamp (measurement) is clamped on the B-point of the transformer, and represents the measurement of single-phase x, y and Z-type by.3. The Yellow clamp (input) is clamped on the Z point of the transformer, the green clamp (measurement) is clamped on the C of the transformer, and represents the measurement of single-phase x, y and Z CZ.matters needing attention:The instrument shall be preheated for 15 minutes before measurement. If the temperature is low in winter, the preheating time shall be appropriately extended to ensure the normal measurement of the instrument.Pay attention to the grounding wire in strict accordance with the schematic diagram. In particular, the grounding clip of the response signal shall be connected with the grounding clip of the excitation signal through the connecting wire, and then the grounding wire of the excitation signal shall be grounded with the iron core to ensure the correct flow direction of the signal current.Author: Dingsheng power www.kv-kva.com reprint please indicate

The working principle of current transformer is similar to that of transformer. Its basic working principle is to work according to the electric magnetic electric conversion process. To a certain extent, it can be considered that current transformer is a special transformer. The function of making current transformer is mainly to measure the large current in the main circuit. During measurement, the large current in the main circuit is changed into small current according to the proportional relationship, and then sent to the meter to measure the current or power.Working process of current transformerCurrent transformer is actually a special converter that changes a large current into a small current. This converter can only be used in AC circuit and can measure the large current in AC circuit. The following is mainly about its measurement process. Friends familiar with the transformer structure know how the transformer works, so the current transformer is also similar to the double winding transformer in structure. Different from the transformer, the number of turns of the primary winding of the current transformer around the iron core is very small, generally only one to two turns, and the enamelled wire is very thick (mainly because the primary winding needs to be connected in high current), it is connected in series in the measured circuit when connected, and the wiring mode is shown in the figure belowThe number of turns of enamelled wire wound by the secondary of current transformer is many times more than that of the primary. Because the secondary is connected with the measuring instrument, the secondary is connected in series with the current coil of ammeter or watt hour meter to form a closed loop. The resistance value of these coils is very small, so the secondary can be similar to short circuit. We can deduce from the approximate short circuit of the secondary coil The voltage of the primary coil out of the transformer is almost zero. According to the electromagnetic principle, we can know that the primary input voltage determines the main magnetic flux. When the input side voltage is almost zero, the magnetic flux is zero and the excitation current is zero, so the total magnetic potential is zero. Finally, we will get such an expression: I1 = kxi2 (where I2 is the reading of the secondary connection ammeter of the current transformer).We can know from the expression I1 = kxi2 that the current transformer is equivalent to a step-up transformer. When the primary current is large, the current induced by the secondary is the current reduced by a certain proportion. In the formula, K is the reduced proportion, which we call the transformation ratio. For example, when k is 30 / 5, it means that the current flowing through the primary is 30a, and the current induced by the secondary side is 5A, The transformation ratio is 6. In other words, we reduce the current of the main circuit by 6 times through the current transformer, which can not only expand the measuring range of the instrument, but also ensure the safety of personnel and equipment.Precautions for use of current transformerThere are two points to pay attention to in the use of current transformer. One is that the shell and iron core of current transformer shall be reliably grounded for safety; the other is that its secondary cannot be disconnected when the current transformer is in use, otherwise it will induce high voltage and endanger personal safety.

Voltage transformer and current transformer have the same point. They all use the working principle of electricity magnetism electricity. As shown in Figure 1 below.Both voltage transformer and current transformer are components for measurement and protection in power supply system.Common voltage transformers are reduced to the rated voltage of the instrument to provide working voltage for the circuit; Its symbol is represented by TV; Its output voltage is connected in parallel with the instrument. See the figure below for its physical map.This configuration of high-voltage voltage transformer is mainly used for measurement and protection of voltage level of 10kV and above; The high-voltage winding of the voltage transformer is connected in parallel with the tested circuit, and the low-voltage winding is connected in parallel with the voltage coil of the measuring instrument. Due to the large internal impedance of the voltage coil, when the voltage transformer works, it is equivalent to a no-load transformer, so its secondary side cannot be short circuited, otherwise the winding coil will be burned.Its working principle is shown in the figure below.Current transformer provides rated 1 5A current with different ampere turn ratio for instruments and watt hour meters. It is represented by the symbol TA, and its physical diagram is shown in the figure below.Because the number of coils on the primary side of the current transformer is small and all are in series, its current depends on the line load current and has nothing to do with the secondary side load. Because the impedance of the current coil connected to the secondary side is very small, the current transformer is equivalent to a short-circuit transformer when it works normally. Therefore, the actual current value in the line is very large, so its secondary side cannot be open circuit during normal operation; Short circuit at secondary side of voltage transformer; One terminal on the secondary side of both of them is a protective grounding terminal, which must be protected and grounded according to the safety regulations during installation.The same point: they are all transformers. The primary side is connected with the measured circuit, and the secondary side is connected with instruments or control elements. They are generally used for the measurement and control of AC circuitsdifference:The primary side of the current transformer is connected in series in the measured circuit, and the voltage induced by the secondary side changes with the current of the primary side;The primary side of the voltage transformer is connected in parallel with the measured circuit, and the voltage induced by the secondary side changes with the terminal voltage of the primary sideThe key is the current transformer. The secondary side cannot open circuit, and the secondary side open circuit is equivalent to infinite impedance. According to the principle of mutual inductance, the resistance reflected to the primary side is infinite. When the current at the primary side is large, a large voltage drop will be generated at the primary side, which is very easy to burn the transformer. Therefore, when there is no measuring instrument at the secondary side of the current mutual inductor, the secondary side should also be short circuited with wires.In the same point, they all obtain monitoring data at the secondary side through electromagnetic induction. The difference is that the monitoring objects are different, one is voltage and the other is current.

Transformer winding machine is a special equipment used to wind transformer coils. In the process of transformer coil production, winding equipment is divided according to transformer voltage level or capacity, including automatic winding machine for winding distribution transformer high and low voltage coils and foil coil winding machine; There are vertical winding machines and horizontal winding machines for winding large power transformer coils. Selecting appropriate transformer winding equipment is the key to the production of high-performance transformers. According to different needs, transformer winding machines can be divided into the following types. Automatic winding machine automatic winding machine is a special equipment for winding high-voltage coils of distribution transformers. The coil wound by this equipment has the advantages of compact structure, small volume and high strength. In order to further improve the production efficiency, the host speed of the new winding machine is increased from 300 rad / min to 500 rad / min.The wire arrangement mechanism is increased from single head to two to three heads, that is, three coils can be wound at one time. At the same time, multiple wires (up to 4) are used and wound; Add radial pressing mechanism; The tension of each pay off reel is controlled independently, so that the winding wires are arranged neatly, the turns are compact and uniform, and the current density is evenly distributed. At the same time, it is equipped with end insulation uncoiling mechanism and wire tensioning mechanism. The computer can preset a variety of parameters, which can complete the coil winding at one time, greatly improve the degree of automation and reduce the labor intensity of workers.Foil coil winding machineFoil coil winding machine is the key equipment for winding low-voltage foil coil. Tension control and foil strip deviation correction are the key technologies of foil coil winding machine. The tension of the foil strip of the existing domestic or imported foil coil winding machine is generated by mechanical friction, and its tension is not stable enough, which will cause defects such as uneven tightness of the wound coils, inconsistent DC resistance between coils, out of tolerance of coil outer diameter and so on. Domestic manufacturers have developed a new type of digital foil winding machine, which uses DC motor to generate foil belt tension. It is characterized by convenient, accurate and stable tension adjustment, and the tension remains unchanged in the whole winding process, so as to ensure the winding quality of the coil. In particular, it has a wide tension adjustment range and is more suitable for winding thinner foil strips without damaging the end edges of the foil strips. Moreover, the tension motor is in the power generation state in the tension state, and the generated electric energy can be fed back to the winding motor or power grid. The friction energy consumption of friction free braking tension mode is significant in energy saving. Moreover, the control system will automatically calculate the coil diameter of the uncoiler, so as to adjust the motor output torque at any time to ensure the constant tension in the whole winding process, which plays a key role in ensuring the coil quality. At the same time, the deviation adjustment system of the equipment adopts photoelectric analog sensor to non-contact detect the edge of the foil belt, and dynamically correct the deviation through the servo motor. The system has high precision, fast response, high reliability and high deviation adjustment accuracy. According to the existing data, the equipment technology has been at the world leading level. Vertical winding machine transformer winding machine transformer winding machine the larger the transformer capacity used now, the greater the impact on the power grid, and the higher the security is required. The manufacturing quality performance of transformer requires not only reliable electrical performance, but also more reliable mechanical stability and short-circuit resistance. The stability is mainly reflected in the manufacturing process control of the coil, that is, the electromagnetic balance of the high and low voltage coil is required, and the axial turns are close and evenly distributed. Therefore, it is required that the coil winding process should be compressed in the axial direction and tightened in the radial direction. The vertical winding machine just uses the self weight of the wire cake to compress naturally in the axial direction, and the tensioning device is used to control the tension in the radial direction. Therefore, the vertical winding machine is widely used for the winding of high-voltage coils in large transformers, and even the low-voltage spiral coils are produced by the vertical winding machine.

Comparison of power transformers:1 What are the advantages and disadvantages of machine winding and manual winding in the manufacture of transformer?The advantages of machine wound transformer are high efficiency and beautiful appearance, but winding the ring transformer with tall and small holes is more troublesome, and it is not as reliable as manual winding in terms of insulation treatment process. Manual winding can make the magnetic leakage of the transformer very small. During the winding process, it can be adjusted at any time according to the layout of coil turns. Therefore, the real hi-end transformer must be wound purely manually. The only disadvantage of pure manual winding is low efficiency and slow speed.2 Which of the ring type, EI type, R type and C type power transformers is the best?They have their own advantages and disadvantages, and there is no saying who is the best, so strictly speaking, any kind of transformer can be done best. In terms of structure, the ring type can minimize the magnetic flux leakage, but the EI type can do better in the sense of intermediate frequency density. In terms of magnetic saturation alone, the EI type is stronger than the ring type, but the ring type is better than the EI type in efficiency. Nevertheless, the key to the problem is whether you can give full play to their respective advantages, which is the most fundamental to do a good job of transformer.In the current imported amplifiers, the application of ring transformer is still the mainstream, which basically explains a problem. Enthusiasts should be objective and fair about the * price of transformers. You can't refer to something that isn't done well and say it's bad. Some people say that the ring transformer is easy to magnetic saturation, so why don't you find a way to make it not easy to magnetic saturation? This could have been achieved by technical means. Without enough effort or blindly in order to save cost, it is of course prone to magnetic saturation. Similarly, as long as you make it carefully, the efficiency of EI transformer can be very high.The quality of the transformer has a great impact on the sound, because the transmission energy of the transformer is closely related to the iron core and coil, and its transmission rate plays a decisive role in the influence of the sound. Like EI transformer, people usually think its medium frequency is thicker and high frequency is thinner. Why? Because its transmission speed is relatively slow. And the ring? The low frequency is relatively fierce, while the medium and high frequency are slightly weaker. Why? Because its transmission speed is relatively fast, but if you can make the ring type and EI type perfect through effective structural changes, the key depends on what you do.However, at least one thing is certain that the R-type power transformer is not easy to do well. It can be used as the power supply of low current front-stage power amplifier and CD player. If it is used as the power supply of rear-stage power amplifier, it has serious defects. Because the structural form of R-type transformer itself is not easy to change, while ring type and EI type are relatively easy to achieve the purpose of beautiful sound by changing the structure. The power amplifier power supply made of R-type transformer usually has a hard sound and lacks aura, and the low-frequency power amplifier is often hard without bouncing force.3 Does the silicon steel sheet of transformer core contain more silicon, the better?It has not been found that the silicon content of silicon steel sheet has little effect on the quality of transformer, while the oriented and non oriented are related to the model of iron core. Secondly, even if the iron core of the same model is not handled well, the quality difference is also very large, and sometimes the difference is as high as 40% or 50%.For a good iron core and the same material, the heat treatment and wire winding process are very key. A good heat treatment only needs a small 10mA exciting current to reach 15000 Gauss, while a bad heat treatment may need 50mA exciting current to reach the corresponding 15000 Gauss. There is a great difference between the two. Judging whether the iron core is good or not from a professional point of view is mainly through several indicators such as excitation current, iron loss and saturation parameters.4 If the splicing process is adopted for the belt silicon steel sheet of the ring transformer, does it mean that the quality is certainly not good?It can not be generalized, but it is not easy to splice too many broken heads, because one more broken position will have one more magnetic flux leakage point, so it is best not to have more than two to three joint points. In terms of manufacturing process, all broken ends and splices shall be pickled first, but for the ring transformer manufacturing high-grade audio equipment, strictly speaking, it is best to use silicon steel without splicing, and its process quality will be more guaranteed.5 What about the silicon steel sheet material in the transformer?Because the loss of silicon steel in alternating magnetic field is very small, silicon steel sheet is mainly used as magnetic material in transformers. Silicon steel sheet can be divided into hot rolling and cold rolling. Cold rolled silicon steel strip has the advantages of small volume, light weight and high efficiency because of its high magnetic conductivity and low loss. The performance of hot rolled silicon steel strip is slightly inferior to that of cold rolled silicon steel strip.Common EI type transformers are made of silicon steel plate into E-type and I-type sheets with a thickness of 0.35-0.5mm, which are inserted into the winding package after heat treatment. Most of these iron cores use hot-rolled silicon steel sheets (the models of high-quality silicon steel sheets with high silicon content are d41, d42, d43 and D301). The iron core of ring and C-type transformers is formed by winding cold-rolled silicon steel strip. The C-type transformer is cut after heat treatment and paint dipping.The leakage inductance of the power transformer is generated by the magnetic flux that does not pass through the primary and secondary coils. These magnetic fluxes pass through the air and form a closed magnetic circuit. The leakage inductance can be reduced by enhancing the coupling density between the primary and secondary of the transformer. The leakage inductance of a good transformer shall not exceed 1 / 100 of the primary coil inductance, and that of a high fidelity hi fi output transformer shall not exceed 1 / 500.fqj

Shunt capacitor, formerly known as phase-shifting capacitor. It is mainly used to compensate the reactive power of inductive load in power system, so as to improve power factor, improve voltage quality and reduce line loss. Single phase shunt capacitor is mainly composed of core, shell and outgoing line structure. A metal foil (as a pole plate) is stacked with insulating paper or plastic film and wound together. A capacitor core is formed by pressing several elements, insulating parts and fasteners, and impregnated with insulating oil. The lead of the capacitor plate is led to the outlet connecting piece at the lower end of the outlet porcelain bushing after being connected in series and parallel. The metal shell of the capacitor is filled with insulating medium oil.Introduction to shunt capacitorMost of the power loads in the power grid, such as motors and transformers, belong to inductive loads. In the process of operation, it is necessary to provide corresponding reactive power to these equipment. After the reactive power compensation equipment such as shunt capacitor is installed in the power grid, it can provide the reactive power consumed by the inductive load and reduce the reactive power provided by the power grid to the inductive load and transmitted by the line. Due to the reduction of the flow of reactive power in the power grid, it can reduce the power loss caused by the transmission of reactive power by the line and transformer.Reactive powerReactive power, many electrical equipment work according to the principle of electromagnetic induction, such as distribution transformer and motor. They all rely on the establishment of alternating magnetic field to convert and transfer energy. The electric power required to establish alternating magnetic field and induced magnetic flux is called reactive power. Therefore, the so-called "reactive power" is not "useless" electric power, but its power is not converted into mechanical energy and heat energy; Therefore, in the power supply and consumption system, in addition to active power supply, reactive power supply is also required, both of which are indispensable. The unit of reactive power is var.In the power grid, there are two kinds of electric power supplied by the power supply to the load: one is active power and the other is reactive power. Active power is the electrical power required to maintain the normal operation of electrical equipment, that is, the electrical power that converts electrical energy into other forms of energy (mechanical energy, optical energy, thermal energy). For example, a 5.5kW motor converts 5.5kW power into mechanical energy to drive the water pump to pump water or thresher to thresh;Various lighting devices convert electric energy into light energy for people's living and working lighting. Reactive power is more abstract. It is the electric power used for the electric field and magnetic field in the circuit, and used to establish and maintain the magnetic field in electrical equipment. All electrical equipment with electromagnetic coils must consume reactive power in order to establish a magnetic field. For example, a 40W fluorescent lamp not only needs 40W active power (the ballast also needs to consume part of the active power) to emit light, but also needs about 80var reactive power for the coil of the ballast to establish an alternating magnetic field. It is called "reactive power" because it does not work externally.Reactive power is by no means useless power, it is very useful. The motor needs to establish and maintain the rotating magnetic field to make the rotor rotate, so as to drive the mechanical movement. The rotor magnetic field of the motor is established by obtaining reactive power from the power supply. The transformer also needs reactive power to generate magnetic field in the primary coil of the transformer and induce voltage in the secondary coil. Therefore, without reactive power, the motor will not rotate, the transformer will not change voltage, and the AC contactor will not pull in.Under normal circumstances, electrical equipment should not only obtain active power from the power supply, but also obtain reactive power from the power supply. If the reactive power in the power grid is in short supply, the electrical equipment does not have enough reactive power to establish a normal electromagnetic field, then these electrical equipment can not work under rated conditions, and the terminal voltage of the electrical equipment will drop, thus affecting the normal operation of the electrical equipment.Reactive power also has certain adverse effects on power supply and consumption, mainly in the following aspects:(1) Reduce the output of generator active power.(2) When the apparent power is constant, increasing reactive power will reduce the power supply capacity of power transmission and transformation equipment.(3) The flow of reactive power in the power grid will increase the line voltage loss and power loss.(4) When the system lacks reactive power, it will cause low power factor operation and voltage drop, so that the capacity of electrical equipment can not be brought into full play.Function of shunt capacitor in compensating reactive powerThere are two methods for power capacitor as compensation device: series compensation and parallel compensation. Series compensation is to connect capacitors directly in series to high-voltage transmission lines to improve transmission line parameters, reduce voltage loss, improve its transmission capacity and reduce line loss. The capacitor of this compensation method is called series capacitor, which is applied to high-voltage long-distance transmission lines, and is rarely used by power users. Parallel compensation is to connect the capacitor directly with the compensated equipment to the same circuit to improve the power factor. The capacitor used in this compensation method is called shunt capacitor, which is adopted by power enterprises. There are usually three ways according to the installation position of the capacitor.1. The centralized compensation capacitor bank is centrally installed on the 6 10kV bus of the enterprise or local general step-down substation to improve the power factor of the whole substation and basically balance the reactive power within the power supply range of the substation. It can reduce the reactive power loss of high-voltage line and improve the power supply voltage quality of the substation.2. For group compensation, capacitor banks are installed on the high-voltage or low-voltage bus of workshop or village terminal substation with low power factor, also known as decentralized compensation. This method has the same advantages as centralized compensation, only the reactive power compensation capacity and range are relatively small. However, the effect of grouping compensation is obvious and widely used.3. Local compensation: the capacitor or capacitor assembly is located near the asynchronous motor or inductive electrical equipment for local reactive power compensation, also known as separate compensation or individual compensation. This method can not only improve the power factor of the power supply circuit for electric equipment, but also improve the voltage quality of electric equipment. It is very suitable for small and medium-sized equipment.

Working principle of power transformerAs for the power transformer, it is a special transformer whose output and input share a set of coils. The step-up and step-down are realized with different taps. The tap voltage of less than the common coil is reduced. The tap voltage of more than the common coil increases. The working principle of power transformer is to use the principle of electromagnetic induction. The alternating current we use generates a magnetic field through coil n, and there is a coil m next to coil n. because the magnetic field generated by coil n is not constant, it changes accordingly according to the change of current. The change of magnetic field causes the corresponding current in coil m, and the frequency of coil m will change according to the different turns ratio of coil N and coil M. Of course, now many power transformers are becoming single coil (called autotransformer), and the principle is basically the same.Autotransformer is a transformer with only one winding. When it is used as a step-down transformer, some wire turns are extracted from the winding as the secondary winding; When used as a step-up transformer, the applied voltage is only applied to part of the wire turns of the winding. Generally, the part of the winding belonging to both primary and secondary is called common winding, and the rest of the autotransformer is called series winding. Compared with ordinary transformers, autotransformer with the same capacity has not only small size, but also high efficiency, and the larger the transformer capacity, the higher the voltage. This advantage is more prominent. Therefore, with the development of power system, the improvement of voltage level and the increase of transmission capacity, self coupled transformer is widely used because of its large capacity, low loss and low cost.Function of power transformerIn addition to the small volume of power transformer, there is no clear boundary between power transformer and electronic transformer. Generally, the power supply of 60Hz power network is very large, which may cover as large a capacity as half a continent. The power limitation of electronic devices is usually limited by the ability of rectification, amplification and other components of the system. Some of them belong to the power amplification, but compared with the power generation capacity of the power system, it still belongs to the scope of small power. Various electronic equipment are commonly used in transformers for the following reasons: providing various voltage levels to ensure the normal operation of the system; Provide electrical isolation of parts operating at different potentials in the system; Provide high impedance for AC current, but low impedance for DC; Maintain or modify the waveform and frequency response at different potentials.Loss of power transformerIn the power supply voltage regulator, when the current passes through the primary coil, a certain amount of heat will be generated. (the magnetic flux generated by the coil flows in the iron core, because the iron core itself is also a conductor, and the potential will be induced on the plane perpendicular to the magnetic line of force. This potential forms a closed loop on the broken surface of the iron core and generates current, like P a vortex, so it is called "eddy current". This "Eddy current" increases the loss of the transformer and heats the iron core of the transformer and increases the temperature rise of the power transformer.) more energy loss is generated in the "eddy current", which is the "iron loss" in the term. In addition, when a large number of copper wires are used in our power transformer, a large amount of heat will be generated when the current passes through, which is what we call "copper loss" The main heat sources of power transformer are "copper loss" and "iron loss" It is also these two phenomena that make the power transformer lose more power. Therefore, the temperature rise of the transformer is mainly caused by iron loss and copper loss. Because the power transformer has iron loss and copper loss, its output power is always less than the input power. Therefore, we introduce an efficiency parameter to describe it, = Output power / input power.

Introduction to working principle of power transformerThe high-voltage winding used in domestic transformers is generally connected into Y connection method, and the connection method of medium voltage winding and low-voltage winding shall be determined according to the system situation. The so-called system situation refers to the relationship between the voltage phasor of high voltage transmission system and the voltage phasor of medium voltage or low voltage transmission system. If it is a low-voltage distribution system, it can be determined according to the standard.High voltage windings are often connected into Y connection because the phase voltage can be equal to 57.7% of the line voltage and the voltage per turn can be lower.1). The voltage phasors of domestic 500, 330, 220 and 110kV transmission systems are in the same phase. Therefore, for three-phase three winding or three-phase autotransformer with the following voltage ratio, star connection method shall be used for high-voltage and medium voltage windings. When the three-phase three iron core column iron core structure is adopted, the low-voltage winding can also adopt star connection method or angle connection method, which depends on that the voltage phasor of the low-voltage transmission system is in phase with the voltage phasor of the medium voltage and high-voltage transmission system or lags by 30 electrical angle.500 / 220 / lvkv YN, yn0, yn0 or YN, yn0, D11220 / 110 / lvkv YN, yn0, yn0 or YN, yn0, D11330 / 220 / lvkv YN, yn0, yn0 or YN, yn0, D11330 / 110 / lvkv YN, yn0, yn0 or YN, yn0, D112). There are two different phase angles for domestic 60 and 35kV Transmission system voltages.For example, YNd11 connection method is adopted for 220 / 60kV transformer and YN, yn0 and D11 connection method is adopted for 220 / 69 / 10kV transformer. The difference between the two 60kV transmission systems is 30 electrical angle.When the 220 / 110 / 35kV transformer adopts YN, yn0 and D11 connection method and the 110 / 35 / 10kV transformer adopts YN, yn0 and D11 connection method, the voltage phasor of the above two 35kV Transmission systems is also different by 30 electrical angle.Therefore, caution should be taken when determining the connection method of 60 and 35kV windings, which must meet the requirements of voltage phasor of transmission system. The connection method of 60 and 35kV winding is determined according to the relative relationship of voltage phasor. Otherwise, even if the capacity and voltage ratio are right, the transformer cannot be used, the connection method is wrong, and the transformer cannot be connected to the transmission system.3). The phasors of domestic 10, 6, 3 and 0.4kV transmission and distribution systems also have two phases. In Shanghai, there is an electrical angle with a voltage phasor difference of 60 between 10kV and 110kV transmission systems. At this time, the three-phase three winding power transformer with 110 / 35 / 10KV voltage ratio and YN, yn0 and Y10 connection method can be used, but the three-phase three core column core is limited.4). However, it should be noted that when the single-phase transformer is connected into three-phase group connection method, the three-phase group of yny0 connection method cannot be used. Yny0 connection method cannot be adopted for three-phase shell transformer.When YN, yn0 and yn0 connection method must be adopted for three-phase five column iron core transformer, there shall be a fourth winding connected into angular connection method in the transformer, and its outlet shall not be led out (the outlet led out during electrical test in structure is not in this example).5). When transformers of different connection groups operate in parallel, the general rule is that the connection group label must be the same.6). When the distribution transformer is used in thunder prone areas, yzn11 connection method can be adopted. When Z connection method is adopted, the impedance voltage algorithm is different from Yyn0 connection method, and the copper consumption of Z connection winding is more. Yzn11 connected distribution transformer has good lightning protection performance.7). Yny0 connection method cannot be adopted when four wound core frames are adopted for three-phase transformer.8). The above connection methods are used for domestic transformers. For export, appropriate connection methods and connection group labels shall be provided as required.9). Generally, there are taps in the high-voltage winding connected with the tap changer. Therefore, when selecting tap changer (including on load voltage regulating tap changer and non excitation voltage regulating tap changer), attention must be paid to the coordination of transformer connection method and tap changer connection method (including connection method, test voltage, rated current, voltage of each stage, voltage regulation range, etc.). For on load tap changer used in on load tap changer of YN connected on load tap changer, it should be noted that the midpoint must be able to be led out

The secondary parameters of current transformer include transformation ratio and accuracy level.For the current transformer for protection, the accuracy level shall be nominal as the percentage of the maximum allowable composite error of the accuracy level under the rated accuracy limit primary current, followed by "P" for protection.Rated accuracy limit primary current refers to the maximum current that can ensure that the composite error does not exceed the allowable value of the accuracy level indicated by the current transformer when leaving the factory. It is generally indicated by the accuracy limit coefficient, and the rated accuracy limit coefficient is generally indicated after its accuracy level; The so-called rated accuracy limit coefficient refers to the ratio of rated accuracy limit primary current to rated primary current.Transformation ratio: it refers to the ratio of primary current to secondary side current. It is an important parameter for relay protection setting calculation and measurement.The selection of transformation ratio shall first consider the indication accuracy of the measuring instrument under rated working conditions and meet the requirements of rated input current and working accuracy of the protection device. For example, when the rated input current of the protection device is 5a, under normal working conditions, the secondary output current of the current transformer of the measurement stage should be between 1 4.5a. If it is too small, (such as less than 0.5A), it is unreasonable. For the protection level current transformer, the transformation ratio is generally greater than that of the measurement level current transformer because it is necessary to ensure that it is unsaturated in case of system fault.Note that the capacity of the primary winding of the current transformer with constant series ratio is doubled; The parallel transformation ratio is doubled and the capacity remains unchanged. For secondary winding, the series transformation ratio remains unchanged and the capacity is doubled; The parallel transformation ratio is reduced by half and the capacity remains unchanged.Accuracy level: at present, there are six accuracy levels of current transformers used in China: 0.1, 0.2, 0.5, 1, 3 and 5. According to the discussion of measurement, measurement and protection, the measurement needs to be measured accurately during operation to meet the measurement requirements under normal load, and the protection needs to be protected in fault state to meet the requirements under limit conditions.Measurement accuracy level: 0.1, 0.2, 0.5, etc. For example, level 0.5 means that the transmission error of current transformer shall not be greater than 0.5% under rated working conditions.Protection accuracy level: generally, P level is adopted, for example, 5p20, which means that the error is 5% under 20 times of rated current. Therefore, although the accuracy of protection level is lower than that of measurement level, it has strong anti saturation ability.Therefore, CT winding cannot be used incorrectly, otherwise it is prone to saturation, and there will be misoperation or refusal of operation for relay protection (longitudinal differential protection is easy to misoperate because the detection difference flow is large. Backup protection will refuse to operate because the collected value is too small).