Treatment and Cause Analysis of Secondary Open Circuit of Current Transformer

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.

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).

Residual current transformer:Residual current transformer is the detection element of leakage protector. Its main function is to detect the residual current (grounding fault current such as electric shock and leakage) of the main circuit passing through the transformer core, and convert the residual current of the primary circuit into the output voltage of the secondary circuit. Residual current transformer is one of the most critical components of leakage protector.The excitation current of the primary circuit of residual current transformer is very small and works under the condition of weak magnetic field. For the residual current protection appliance with rated residual action current of 0.006a, the minimum working magnetic field strength is only about 0.080a/m. During operation, the main circuit is often impacted by hundreds of amps to thousands of AMPS starting current or tens of thousands of AMPS short-circuit current, and the residual current transformer is in extreme saturation state. under these circumstances. The transformer shall not affect the action characteristics due to the residual magnetism generated by impulse current. Therefore, iron nickel soft magnetic alloy with high initial permeability, high maximum permeability and low coercivity is generally used as the core material of residual current transformer. Fe Ni soft magnetic alloy has very high initial permeability and maximum permeability, very low coercivity, low saturation flux density and good temperature stability.:Both zero sequence current transformer and residual current transformer use the principle of electromagnetic induction to detect current; The vector operation of current signal can be realized by the superposition of magnetic field; Zero sequence current transformer is used to detect the vector sum of three-phase current, excluding zero line. If the three-phase is unbalanced, the zero sequence current transformer will have current. If the three-phase imbalance reaches a certain value, it can be protected; Residual current transformer mainly detects leakage current.In the three-phase system, the vector sum of three-phase and zero line is detected. In the single-phase system, the vector sum of live line and zero line is detected. Whether it needs to pass through the zero line depends on what current is detected by the transformer. Whether the detected current needs vector operation by a / B / C / N line.If necessary, the magnetic field generated by the current shall be superimposed to realize the operation,If you don't need it, just go through the order.For example:If only phase a current is detected, only phase a conductor needs to pass through the transformer.If the leakage current is detected in single phase, the leakage current is defined as live line current zero line current. Here, a vector superposition operation is done. Both live line and zero line need to pass through the transformer, and the entry direction needs to be correct.If the zero sequence current of three phases is detected, according to the definition, the zero sequence current is = A-phase current B-phase current c-phase current. Therefore, the conductor of a / B / C three phases needs to pass through the transformer, and the entry direction needs to be correct.If the leakage current of three phases is detected, according to the definition, the leakage current = A-phase current B-phase current c-phase current n-line current. Therefore, the wires of a / B / C / N need to pass through the transformer, and the entry direction needs to be correct.

Precautions for use of current transformerPay attention to the following points when using current transformer(1) The configuration of current transformer shall meet the requirements of measuring meter, relay protection and automatic device, and shall be powered by separate secondary winding respectively. Only in this way can it meet the requirements of different equipment without mutual influence.(2) When selecting current transformer, the rated voltage shall be equal to the voltage of the tested circuit, its primary rated current shall be greater than the maximum continuous working current of the tested circuit, and its structure, model and capacity shall meet the accuracy requirements.(3) Open circuit is not allowed for the secondary winding of current transformer in operation.(4) The secondary winding, iron core and shell of current transformer shall be reliably grounded, and only one grounding point is allowed.(5) According to the development of the power grid, check whether the dynamic thermal stability current and rated continuous thermal current of the current transformer meet the requirements under the actual operating current ratio. If the short-circuit current of the substation exceeds the specified value on the nameplate of the current transformer, change the current ratio or replace the current transformer in time.(6) If there is doubt about the rated continuous thermal current, the secondary winding shall be short circuited for temperature rise test. If it does not meet the requirements, the current transformer shall be replaced.What is the harm of open circuit at the secondary side of current transformerIn normal operation, the impedance of the current transformer is very small and close to the short-circuit state. Most of the magnetic potential generated by the primary current is offset by the magnetic potential generated by the secondary current, so that the total magnetic flux in the iron core and the induced potential of the secondary winding are small. However, when the secondary side of the current transformer is open circuit, it is equivalent that the load impedance Z is infinite and the secondary current is zero. The secondary current cannot generate magnetic potential to offset the magnetic potential on the primary side. The total magnetic flux is equal to the magnetic flux potential of the primary winding, that is, the primary current completely becomes the excitation current. As a result, the magnetic flux density of the iron core increases, resulting in iron core saturation and iron loss increase, resulting in serious heating of TA iron core, It may even burn out the insulation; At the same time, the increase of the magnetic flux density of the iron core will also distort the magnetic flux waveform into a flat top wave, which will induce a high voltage on the secondary side when the magnetic flux passes through zero, and its peak value can reach thousands of volts, which not only endangers the insulation of the current transformer itself, but also is extremely unsafe for the insulation of human body and secondary equipment. Therefore, the open circuit on the secondary side of the current transformer is never allowed in operation. If the secondary side of the current transformer in operation is open circuit, relevant protection and automatic devices can be disabled during treatment to reduce the secondary load to zero.

How to install current transformer1. The construction shall be carried out according to the drawing, the wiring shall be correct, the number marks at both ends of the conductor shall be clear, and the label range shall meet the requirements of the specification.2. The secondary circuit conductor or cable shall be copper wire. The section of current transformer circuit conductor shall not be less than 2.5mm2, and the section of voltage transformer circuit conductor shall not be less than 1.5mm23. Special current terminals shall be selected for the first terminal block at the outlet of the current transformer, and the secondary winding not used by the current transformer shall be short circuited and grounded at the terminal block.4. There shall be no joint for the secondary circuit conductor in the panel and cabinet, and there shall be no joint in the middle of the control cable or conductor. If there must be a joint, the long terminal box shall be used for transition connection.5. The polarity of current transformer shall not be reversed, and the phase sequence and phase shall meet the requirements of design and regulations. For the wiring of transformer for differential protection, the current phasor diagram of two arms must be measured before putting into operation to check the correctness of wiring.6. The arrangement of secondary circuit conductors shall be neat and beautiful, the connecting screws between conductors and electrical components and terminal strip must be free of virtual connection and looseness, and the distance between conductor binding and clamping points shall meet the requirements of the regulations.7. The secondary circuit shall be well insulated from the ground, and there shall be no mixed line between the voltage circuit and the current circuit.8. The current and voltage circuits shall be grounded at one point at the outlet of the secondary side of the transformer.Current transformer installation specification1. The current transformer must be installed firmly, and the exposed metal part of the transformer shell shall be reliably grounded.2. The same group of current transformers shall be installed in the same direction to ensure that the positive directions of the primary and secondary circuit currents of this group of current transformers are consistent, and it is easy to observe the nameplate as much as possible.3. Open circuit is not allowed at the secondary side of current transformer. When only one secondary circuit is used for secondary double turn transformer, the other secondary circuit shall be reliably short circuited.4. The secondary side of low-voltage current transformer can not be grounded. Because the insulation grade of conductor, watt hour meter and transformer used by low-voltage metering device is the same, the maximum voltage that can be borne is basically the same; In addition, after the secondary winding is grounded, the insulation level of the primary circuit of the whole device to the ground is reduced, which is easy to damage the watt hour meter or transformer with insulation weakness under the action of high voltage (such as induced lightning stroke). In order to reduce lightning damage, it is also better to have the secondary side ungrounded.Precautions for use of current transformer1. When measuring CT with magnetic shunt compensation, there may be a large difference between the error value measured by the instrument and the error value measured by the traditional method, which is determined by the test method of the tester. Because the compensation amount of magnetic shunt compensation cannot be measured on the basis of this test method; However, under 100% and 120% current percentage, the verification conditions of the instrument are met.2. There is no display after startup. Please check whether the power supply is connected properly and whether the fuse is blown.3. In the storage record, the storage time is garbled. Currently, empty data is stored, and the stored data in the storage record is checked again under continuous power down.4. This interface is a general platform based on Huatian power development, and the display part is separated from the measurement part. When measuring an item, although the "Cancel" key exits the page of the current measurement item, the instrument is still measuring the item. Do not touch the tested object and test clip!5. Please press the "reset" button or shut down the instrument in case of any crash, splash screen or white screen of the instrument, or interrupt the measurement of the instrument.6. Since the display part of the instrument is separated from the measurement part, when any interface displays all asterisks, the instrument cannot communicate. If repeated "reset" and shutdown cannot be repaired, the instrument has been damaged.

For a long time, as a standard device in instruments and equipment, current transformer has been used to measure precision current. Even in harsh environment and high temperature conditions, this device is very accurate, convenient and reliable.In applications such as switching power supply, motor current load detection, lighting and instruments, current transformer is generally used as control, circuit protection and monitoring devices. With the increasing stock of current transformer, how to select a suitable current transformer needs to consider many factors. This paper introduces a simple selection method, which is very helpful to select suitable cost-effective devices in many applications. Although spot devices are cheap and desirable, they have some functional limitations in use. Some applications may require special products or even complete customization.Many factors should be considered in the selection of current transformer, such as size, frequency, function and current range.Input currentFirstly, the selection of current transformer must clarify and verify many indicators, such as size, frequency, function and the range of sampling current. Its accuracy and efficiency actually depend on these parameters. In addition to the possible compromise on the accuracy of the current transformer, if the current of the current transformer exceeds the rated current specification specified by the manufacturer, its operating temperature will continue to rise and cannot be controlled, resulting in circuit failure.In addition, if the rating of a current transformer is much higher than its "sampling current", the size of this device will inevitably be large, which is too expensive for its application. Generally speaking, it is a wise choice that the rated value of the selected current transformer is about 30% higher than the maximum expected value of its "sampling current".Turns ratioThe turns ratio of common current transformer ranges from 1:10 to 1:1000. The higher the turn ratio (r = nsec / NPRI), the higher the resolution of current measurement.However, it should be noted that too high turn ratio will lead to the increase of distributed capacitance and leakage inductance, which will reduce the accuracy of current transformer and the working performance at high frequency (caused by self resonance). However, if the turn ratio is too low (low induction coefficient), the output signal may be distorted or "decreased" (the single-stage input signal must be skewed), resulting in unstable control circuit and inaccurate measurement results.Induction coefficient and excitation currentThe secondary inductance of current transformer determines the fidelity of output signal. The value of the induction coefficient is inversely proportional to the excitation current, which is commonly known as "induced current".In order to ensure the maximum fault-tolerant performance of current transformer, the excitation current should be several times smaller than the amplitude of sampling current. For most applications such as switching power supply, it is ideal to take 10% of the sampling current as the maximum excitation current. For example, if a circuit must guarantee a maximum loss of 10% for a sampling current of 1 20A at 100kHz, the maximum value of the excitation current must be set to 100mA (i.e. 10% of the minimum sampling current value).The sampling current of 1A will produce an error of 10%, and the sampling current of 20a will produce an error of 0.5%. If the excitation current is not indicated in the data book provided by the manufacturer, it can be calculated by the following formula:e=CLdI/dt|dI/dt|=e/LWhere e is the device output voltage (unit: V), l is the induction coefficient (unit: H), and|di / dt|is the ratio of excitation current to time (unit: A / s).Output voltage and load resistanceThe output voltage (VO) should be set as low as possible to reduce the intervention loss. Assuming that the optimal secondary output voltage of a circuit is 0.5V and the output current is 20a, the current transformer with a turn ratio of 1:100 will produce a secondary current of about 200mA. The load resistance should be: RO = VO / is = 0.5 / 0.2 = 2.5 .

The current sampling unit is a sampling circuit that provides the current sampling signal to the single chip microcomputer when the induction cooker works. The single chip microcomputer always detects the change of the input current, and automatically adjusts the PWM signal according to the detected current sampling signal to make the induction cooker do the constant processing of the output power. The single chip microcomputer will also detect the input current of the induction cooker according to the change of the detected current sampling signal, so as to automatically make various actions. When the single chip microcomputer detects the appropriate pulse number of the pot in the synchronous circuit, it will use 0.5 2S to detect the change of the current, and determine whether the material and heating area of the heating pot meet the heating requirements through the difference of the current change. When the current sampling signal changes too much, the single chip microcomputer will judge whether there is no pot. There are two common current sampling unit circuits in the mainstream induction cooker circuits in the market, namely, the current sampling unit using current transformer sampling and the current sampling unit using resistance voltage drop sampling. The working principles of these two sampling circuits will be introduced below.The current sampling unit is a sampling circuit that provides the current sampling signal to the single chip microcomputer when the induction cooker works. The single chip microcomputer always detects the change of the input current, and automatically adjusts the PWM signal according to the detected current sampling signal to make the induction cooker do the constant processing of the output power. The single chip microcomputer will also detect the input current of the induction cooker according to the change of the detected current sampling signal, so as to automatically make various actions. When the single chip microcomputer detects the appropriate pulse number of the pot in the synchronous circuit, it will use 0.5 2S to detect the change of the current, and determine whether the material and heating area of the heating pot meet the heating requirements through the difference of the current change. When the current sampling signal changes too much, the single chip microcomputer will judge whether there is no pot. There are two common current sampling unit circuits in the mainstream induction cooker circuits in the market, namely, the current sampling unit using current transformer sampling and the current sampling unit using resistance voltage drop sampling. The working principles of these two sampling circuits will be introduced below.(1) The current sampling unit using current transformer sampling is shown in figure (a) below. The AC voltage secondary measured by current transformer CT1 is rectified by bridge rectifier composed of d10 D13. The DC voltage smoothed by EC5 is sent to the I-A / D port of CPU, and CPU detects the input current of induction cooker according to the change of this voltage signal. The turn ratio of current transformer CT1 is 1:3000. If the turn ratio is large, the induced current is linear when working with high current. VR1 is an adjustable resistance of 0 10K , which is mainly used to adjust the power deviation caused by structural error. It can also change the reference of current detection by adjusting this resistance to achieve the purpose of adjusting the output power of induction cooker. When the VR1 resistance increases, the voltage of the corresponding current detection will increase. When the primary current of CT1 is constant, the voltage induced by the secondary of CT1 will increase accordingly. The program will control accordingly according to the change of analog signal of a / D port. According to the requirements of software constant power, the power will decrease relatively.Transfer files for [membrane switch] proofing

How to select the transformation ratio and accuracy of main technical parameters of current transformerCurrent transformer is a special transformer. Its function is to change large current into standard small current and work with measuring instruments, measuring instruments, relays and other equipment. This can expand the measuring range of the instrument and improve the reliability and safety of the circuit. The wiring schematic diagram of current transformer is shown in the figure below. The primary coil of current transformer is connected in series in the primary main circuit, and the coils of secondary connected instruments and relays are also connected in series.The selection of current transformer parameters is mainly based on the following six points:1. Rated primary voltage - determined by the nominal voltage of the system. High voltage current transformer can be used in low voltage system, such as 10kV current transformer in 6kV system.2. Rated primary current - selected according to the rated current level specified in gb1208. If the primary current cannot be selected according to the specified levels, the following methods can be used: When the transformation ratio requirements of protection circuit and measurement circuit are different, the secondary winding current transformer with tap can be used. The current transformer with primary tap can also be changed. Generally, it can be connected in series and parallel, and the current transformer with multiple transformation ratio or half transformation ratio can be obtained. The current transformer for measuring circuit is of class s for special purposes, and it shall maintain the accuracy requirements within the range of 10 110% of the rated current.3. Rated secondary current - 1A and 5A.Selection principle: for all conditions of new power plant and substation, 1A should be selected. If the original TA is 5A in favor of transformer installation or expansion project, and in some cases to reduce the secondary open circuit voltage of TA, the rated secondary current can be 5A. For one plant, the rated secondary current in the station can be 1a and 5A at the same time.4. The accuracy level and transient characteristics are described in the following topics.5. Number of iron cores - there are two types of current transformer cores: one is the single core type in which a current transformer has only one primary winding and secondary winding, and most low-voltage current transformers are this type; One is the multi core type with one primary winding and two or more secondary windings. Each secondary winding is configured according to different purposes. On the premise of meeting the accuracy level, the electric energy metering instrument and measuring meter can share a secondary winding.6. According to the structure, it can be divided into oil immersed, resin cast and SF6 Current transformers.At ordinary times, we mainly consider the transformation ratio and accuracy when selecting current transformer.1. Transformation ratioThe rated current at the secondary side of the current transformer is 5A or 1a, and 5A is generally selected. The transformation ratios commonly used for measuring current transformers are 5 / 5, 10 / 5, 15 / 5, 20 / 5, 25 / 5, 30 / 5, 40 / 5, 50 / 5, 75 / 5, 100 / 5, 150 / 5, 200 / 5, 250 / 5, 300 / 5, 400 / 5, 500 / 5, 600 / 5, 750 / 5, 800 / 5, etc. so how to correctly select the transformation ratio of current transformers?Code for design of electrical measuring instruments for power installations stipulates that "the selection of measuring range of pointer measuring instruments should ensure that the rated value of power equipment is indicated at 2 / 3 of the instrument scale." according to this specification, we can use the following formula to select the transformation ratio n of current transformer.In this formula, I is the maximum load current of the circuit, 0.7 means that the indication at the maximum load current refers to 70% of the instrument panel, and 5 is the secondary rated current of the current transformerCurrent value. Then the transformation ratio of the corresponding current transformer is selected according to the calculated transformation ratio. For example, I is 50a, and N = 14.29 can be obtained according to the calculation formula, so the current transformer with 75 / 5 transformation ratio is selected.2. AccuracyThe accuracy of current transformer is also called accuracy. Due to the ratio error and angle error of current transformer, it is inevitable. The accuracy of current transformer is numerically equal to the percentage of the error limit of this ratio. For example, the maximum ratio error of current transformer of class 0.5 is 0.5%. The measuring current transformer has grades 0.1, 0.2, 0.5, 1, 3 and 5, and the current transformer for protection has grades 5p and 10p. Generally, level 0.2 is selected for metering, level 0.5 for measurement and level 1 for general monitoring instruments.At the same time, we need to pay attention when using current transformer: the secondary side of current transformer must be grounded to prevent high voltage from channeling from the primary side to the secondary side, which will endanger the safety of equipment and life; Pay attention to the polarity of the first and second sides of the current transformer, and burn the instrument in case of wrong polarity; The secondary of current transformer shall not be open circuit. It is strictly prohibited to install fuse on the secondary side to prevent high voltage induced by open circuit on the secondary side, which will endanger life safety.

Causes and precautions of secondary electrification of current transformerWhen the current transformer (hereinafter referred to as CT) is working, the secondary circuit must not be open circuit, otherwise it will cause high voltage and electrification in the secondary circuit, which may damage the electrical equipment or endanger personal safety. Therefore, in case of electrification once but twice, the power shall be cut off immediately for inspection. The following is a preliminary analysis of the causes of CT secondary electrification, and some problems that should be paid attention to.1. Cause of failure(1) Due to negligence of work or unclear use principle of CT, the secondary circuit is disconnected for operation or CT secondary line is not installed, so that the secondary circuit is in open circuit state and charged.(2) Since the secondary pile heads (i.e. K1 and K2 pile heads) of CT are not connected properly or in a loose state, the secondary circuit is charged.(3) The secondary circuit is electrified because the pile head of the electrical equipment connected in the secondary circuit of CT is not connected properly or in a loose state (such as electrical instruments, protective relays, electric energy meters, etc.).(4) The CT secondary is charged due to the disconnection of the secondary conductor. The reasons for disconnection are generally bitten by small animals, aging fracture, mechanical strength tension fracture, etc.2. Cause of wrong wiringThe secondary wrong wiring of CT will cause secondary electrification. The author has encountered two cases:(1) Two ammeters are connected in series from KL of two-phase CT, as shown in Figure 1. Although there is no secondary open circuit in this connection, when the CT secondary connection flows from phase a of CT to phase C of CT and passes through two CT secondary coils with large impedance, the CT secondary tends to open circuit, so the secondary is charged. And with the increase of load, the secondary voltage appears higher.(2) An ammeter is connected in series from KL of two-phase CT, as shown in Figure 2. There is no open circuit in t secondary in this wrong wiring; However, the principle of electrification is the same as above, and one ammeter has no indication.3. Causes of irregular wiringThe informal wiring in Figure 3 is to introduce the voltage from the CT secondary KL pile head into the voltage and current coil of the electric energy meter to make the current secondary part of the electric energy meter charged. The essence of this informal wiring is to connect the I of CT "L" is connected with KL, and the voltage pile heads L, 4 and 7 of the electric energy meter are respectively connected with the current pile heads 2, 5 and 8. This connection method omits the three voltage secondary lines a, B and C, and uses the CT secondary connection line to introduce the electric energy meter, so the CT secondary is charged with 380 / 220V voltage. Although this connection method has little impact on the measurement, it is extremely nonstandard and unsafe, so it should be eliminated.4. Cause of insulation damage(1) The insulation is damaged due to overload heating, and the primary voltage jumps into the secondary voltage to make it live.(2) The secondary may also be charged due to poor product quality, insulation aging and other reasons.(3) Some CT's are located in wet places for a long time, and their insulation is damaged by moisture all the year round, which makes the primary voltage jump into the secondary voltage and make it charged.5 safety problems needing attention in the use of current transformer1 During the operation of CT, the secondary measurement shall not be open circuit. During the normal operation of CT, because the secondary impedance is very small and close to the short-circuit state, the excitation current of its iron core tends to zero, so the secondary circuit does not carry voltage. However, when the secondary open circuit or the secondary impedance is very large, the excitation current increases, the CT iron core heats due to the significant increase of magnetic flux, and the induction of CT secondary coil is high Voltage endangers personal safety and equipment safety. Therefore, open circuit is never allowed during CT secondary operation. During installation, attention must be paid to reliable wiring, and each connecting pile head shall be tight and firm.(2) One end of CT secondary measurement must be grounded (generally KZ grounding) to prevent the voltage of primary measurement from channeling to secondary measurement in case of CT primary and secondary coil breakdown.(3) In case of abnormal vibration, noise and heating of CT during operation, it shall be stopped for inspection and treatment.(4) For CT in operation, the secondary line shall not be disconnected at will and work between the secondary lines, otherwise it will cause open circuit danger.