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Overview of voltage and current transformers A typical transformer uses the principle of electromagnetic induction to convert high voltage into low voltage, or convert large current into small current, to provide suitable voltage or current signals for measuring devices, protection devices, and control devices. The voltage transformer commonly used in power systems has a primary side voltage related to the system voltage, which is usually hundreds of volts to hundreds of kilovolts, and the standard secondary voltage is usually 100V and 100V/; while the current transformer commonly used in power systems has a primary side current of several amperes to tens of thousands of amperes, and the standard secondary current is usually 5A, 1A, 0.5A, etc.   1. Principle of voltage transformer The principle of voltage transformer is similar to that of transformer, as shown in Figure 1.1. The primary winding (high voltage winding) and the secondary winding (low voltage winding) are wound on the same iron core, and the magnetic flux in the iron core is Ф. According to the law of electromagnetic induction, the relationship between the voltage U of the winding and the voltage frequency f, the number of turns of the winding W, and the magnetic flux Φ is: 2. The principle of the current transformer It is also similar to the transformer in principle, as shown in Figure 1.2. The main difference from the voltage transformer is that under normal working conditions, the voltage drop on the primary and secondary windings is very small (note that it does not refer to the voltage to the ground), which is equivalent to a transformer in a short-circuit state, so the magnetic flux Φ in the iron core is also very small. At this time, the magnetic potential F (F=IW) of the primary and secondary windings is equal in magnitude and opposite in direction. That is, the current ratio between the primary and secondary of the current transformer is inversely proportional to the number of turns of the primary and secondary windings.   3. Terminals and polarity of transformer windings The voltage transformer winding is divided into the head end and the tail end. For a fully insulated voltage transformer, the voltage to the ground that the head end and the tail end of the primary winding can withstand is the same, while for a semi-insulated voltage transformer, the voltage that the tail end can withstand is generally only about a few kV. A and X are commonly used to represent the beginning and end of the primary winding of the voltage transformer, and a, x or P1, P2 are used to represent the beginning or end of the secondary winding of the voltage transformer; L1 and L2 are commonly used to represent the beginning and end of the primary winding of the current transformer, and K1, K2 or S1, S2 are used to represent the beginning or end of the secondary winding. Different manufacturers may have different numbers, usually with subscript 1 for the beginning and subscript 2 for the end.   When the induced potential of the terminals is in the same direction, they are called the same-name terminals; conversely, if the same-name terminals are connected to the same-name terminals, the magnetic flux they generate in the iron core is also in the same direction. The terminals with the same number as the beginning or the same end and the same induced potential direction are called subtractive polarity windings, and the voltage of the terminals is the result of subtracting the induced potential of the two windings. The correct number in the transformer is defined as subtractive polarity.   4. The main structural differences between voltage transformers and current transformers (1) Both voltage transformers and current transformers can have multiple secondary windings, but voltage transformers can have multiple secondary windings sharing one core, while current transformers must have independent cores for each secondary winding. There are as many cores as there are secondary windings.   (2) The primary winding of a voltage transformer has many turns and thin wires, while the secondary winding has fewer turns and slightly thicker wires. The primary winding of a high-voltage current transformer used in a substation has only 1 to 2 turns and thick wires, while the secondary winding has more turns. The thickness of the wire is related to the rated value of the secondary current.   (3) When the voltage transformer is operating normally, it is strictly forbidden to open the low-voltage terminal of the primary winding and short-circuit the secondary winding. When the current transformer is operating normally, it is strictly forbidden to open the secondary winding.   Hemei Electronics is committed to the research, development, production and sales of amorphous, nanocrystalline and Permalloy core products. Its main products include: Permalloy, silicon steel strip, silicon steel core, nanocrystalline core, Permalloy core, high-power transformer core, nanocrystalline magnetic ring inductor, electromagnetic ring coil, split-type current transformer, common mode inductor coil, precision current transformer and other products with good stability and high electrical parameters.

The studied return conductor refers to the "return conductor" actually existing in the structure of the current transformer. The composite error average error of the secondary winding of the current transformer often exceeds the design value or even exceeds the standard limit value. The main reason is is the interference from the return conductor. In order to reduce the interference from the return conductor of the voltage transformer structure itself, the secondary turns can be unevenly distributed, and additional shielding of the winding can be used if necessary.   The generation of the return conductor and the interference of the electromagnetic field caused by the current in the return conductor to the secondary winding composite error (calculation omitted)   When there is a return conductor in the current transformer, the primary electromagnetic field is no longer a uniform inscribed circle family, and local electromagnetic fields may be concentrated, destroying the uniformity of the core electromagnetic field, thereby causing interference to the composite error.   The U-shaped primary winding has two conductors. If the ring-shaped secondary winding is placed on one conductor, the other conductor and the ring conductor are the return conductors. Because the magnetic flux generated by the current in the return conductor passes through the core, the magnetic density of the core near the return conductor increases, preventing the magnetic density of the core on the return conductor side from decreasing, causing the magnetic density of each section of the core to be different.   Since most of the magnetic flux paths caused by this external electromagnetic field are through gas or other non-magnetic substances, the equivalent circuit of the core is only a small section, so its magnetic induction intensity is directly related to the magnetic permeability of gas or other non-magnetic substances, and the magnetic induction The wire is linked to the secondary winding, which increases the secondary leakage reactance under normal operating conditions. When the current transformer operates at or above the rated voltage, the magnetization curve is close to a linear shape. In this case, the error of the measurement level and the average error of the rated voltage error of the maintenance level are basically consistent with the calculated values. It can be seen from the above figure and calculation that under the conditions of the secondary load of the current transformer and its main parameters, when the current transformer operates at or above the rated voltage, the core flux density is in the parallel segment of the magnetized curve. , the magnetic permeability and magnetic flux density increase linearly, so the average error of the secondary winding rated voltage error is basically consistent with the measured value.   But when the primary current increases to a certain value, that is, under the condition of large overcurrent, as the magnetic flux density enters the saturation state, the magnetic flux density increases, the magnetic permeability decreases rapidly, and the error expands rapidly, which is very important. It may cause the error to exceed the limit value. In this case, as the magnetic flux density of the core near the return conductor side increases, that is, the local electromagnetic field is very strong, so there will be a significant difference between the average error and the measured value. When the specific primary current is less than the rated current, the magnetic flux density and magnetic permeability decrease, but the magnetic permeability decreases faster at this time, so there will also be a significant difference between the average error and the measured value.   Countermeasures to reduce return conductor interference   1. Use a method in which the secondary line turns are not evenly distributed.   Affected by the return conductor, if the turns of the secondary winding are evenly distributed, the core magnetic flux will be uneven, the composite error will be very large, and the difference between the average error and the basic measured value is very large. The use of non-uniformly distributed secondary windings can improve the magnetic flux distribution in the core, making it almost uniform, with small composite errors, and the average error is close to the measured value, thus reducing the interference from the return conductor.   The relative position of the return conductor and the core inside the voltage transformer is fixed. If a magnetic potential opposite to the return conductor's magnetic potential can be created close to the core, the return conductor's magnetic potential can be offset. According to the basic Biot-Savart law, the level of the electromagnetic field caused by the current at a certain point in the indoor space is inversely proportional to the distance between the two. It is known that the closer to the core, the smaller the required diamagnetic potential is.   The method of using non-uniform distribution of secondary wire turns can achieve this effect. The originally evenly distributed secondary wire turns are wound more on the core segment close to the return conductor side and less on the core segment away from the return conductor side. At this time, the secondary magnetic potential of the core section returning to the conductor side exceeds the magnetic potential when the normal distribution is uniform, and the resulting magnetic flux reverses the direction of the interference flux in this section of the core and returns to the conductor, thus canceling part of its effect. .   2. When installing, make sure that the middle turns of the secondary winding are close and sparse.   The secondary windings will also interfere with each other, which is likely to increase the error. The direction of the magnetic flux lines of the electromagnetic field between the windings is the same, which can cause accumulation. If the electromagnetic field distribution between the two windings is uneven, the accumulation result is likely to deviate from the uniform distribution of the secondary winding core magnetic flux, resulting in an increase in the average error and the measured value error. big.   In order to avoid this situation, the secondary winding should be installed so that the side with dense turns matches the dense side of the other secondary winding, and the thinner side matches the thinner side. There is a large gap between the transmission lines near two adjacent windings, which will cause large magnetic leakage. Paper circles can be used between the windings to reduce magnetic leakage interference.   3. When there is a TPY winding in the secondary winding, the method of lifting the paper coil can be used to reduce the interference of magnetic leakage.   Because the TPY winding has magnetic density, the magnetic flux leakage caused here is very large. In order to better prevent the interference of magnetic flux leakage, a paper ring with low magnetic permeability can be used to reduce this kind of interference. The paper ring can be raised in the middle of the secondary winding to reduce leakage. Magnetic interference. In addition, the magnetic potential at many magnetic density points of the TPY winding is poor and causes magnetic flux leakage. The installation should be at 90° with the center line of the U-shaped primary winding to reduce and balance the interference of magnetic flux leakage.   4. Use additional shielded windings   The main function of shielding the winding is to protect the ring core in the current transformer from interference from external stray magnetic flux. Shielded windings are multiple paired windings of other coil inductors on the voltage transformer core. Generally, two pairs of shielded windings with the same number of coil turns and opposite coil inductance positions are used. Each pair of windings is reversely rotated. Sex is tied together.   Their coil turns, winding direction and interface mode cause a leakage electromagnetic field in the high current transformer to be the same size as the stray electromagnetic field (the electromagnetic field generated in the core from the external current) but in the opposite direction to offset the stray electromagnetic field. Interference from electromagnetic fields. The higher section of the stray magnetic flux shields the winding. The potential difference of the magnetic induction is higher than that of the other section, shielding the equilibrium current flowing through the winding. This current demagnetizes the higher section of stray magnetic flux, and demagnetizes the stray magnetic flux in the lower section. The magnetization is increased so that the distance is not too close to the balanced level, so it is also called balanced winding.   The design of the shielded winding must comply with the following principles: the maximum magnetic induction intensity in the ring core is the least, and the shielding effect is best; the current in the shielded winding must be kept at a small level; the temperature of the surface winding part should be minimized ; The current distribution in the entire shielded winding must be kept uniform; the number of turns of the shielded winding coil should be kept as small as possible.   Result The interference of the return conductor of the current transformer is the direct cause of the composite error of the measurement level and the maintenance level. The existence of the return conductor partially concentrates the primary electromagnetic field, so that the core magnetic flux distribution of the basic structure winding is uneven, and the composite type The error characteristics have greatly deteriorated. In order to reduce the interference of the return conductor on the composite error, it is necessary to find a way to make the core magnetic flux of the winding evenly distributed.

A current transformer is a current conversion device. It transforms high voltage and low voltage currents into lower voltage currents to supply appearance and maintenance equipment and to isolate appearance and maintenance equipment from high voltage circuits. The current transformer's secondary current is 5A, which makes it safe and convenient to measure the appearance and relay maintenance equipment, and also enables it to be standardised in production.   The structure of the current transformer is composed of iron core, primary winding, secondary winding, terminal and insulation support. The primary winding of the current transformer turns less, connected in series in the demand for measuring current in the line, the flow of large measured current, the secondary winding turns more, connected in series in the measurement of the appearance or relay maintenance circuit.   　　The secondary circuit of the current transformer is not allowed to open circuit. Current transformer in operation, its secondary circuit has always been closed, but because of the measurement of the appearance and maintenance equipment series winding impedance is very small, the current transformer operation is close to the short-circuit situation, the primary current magnetising force is greatly offset by the secondary current, the total flux density is not large, the secondary winding potential is not large. When the current transformer open circuit, the secondary circuit impedance is infinite, the current is equal to zero, the primary current has completely become the excitation current, in the secondary winding seizure of a very high potential, hostage to personal safety, constitute the appearance of the maintenance equipment, transformer secondary insulation damage.   　　The secondary circuit of the current transformer must be grounded to avoid a breakdown of insulation, the secondary string of high pressure, hostage to personal safety, damage to equipment.  

The current transformer is a special transformer. Its function is to convert large current into a small standard current, and it works in conjunction with equipment such as measuring instruments, calibration instruments, and relays. This can have the effect of expanding the detection range of the instrument and improving the reliability and safety of the power circuit. The schematic diagram of the wiring circuit of the current transformer is as shown in the figure. The primary electromagnetic coil of the current transformer is connected in series in the primary main power circuit, and the electromagnetic coils of its secondary connected instruments and automobile relays are also connected in series.   Usually when people use current transformers, they mainly consider the two performance parameters of transformation ratio and accuracy.   1. Transformation ratio   The rated voltage value of the secondary side of the current transformer is 5A or 1A. Under normal circumstances, 5A is selected. Common transformation ratios for accurate measurement of 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 appropriately choose the transformation ratio of the current transformer?   The "Design Specifications for Electrical Measuring Instruments in Electrical Power Installations" requires that "the selection of the detection range of pointer measuring instruments should ensure that the rated current of the power project is marked at 2/3 of the instrument scale." According to this standard, people can use the following The formula is used to calculate the transformation ratio N of the selected current transformer.   In this formula calculation, I is the maximum load current of the control loop, 0.7 means that the maximum load current is indicated at 70% of the instrument panel, and 5 is the secondary rating of the current transformer.   current. Then select the transformation ratio of the relative current transformer based on the measured transformation ratio value. For example, if I is 50A, N=14.29 can be obtained according to the calculation method, so a current transformer with a 75/5 ratio is selected.   2. Accuracy   The accuracy of the current transformer is also called precision, because the ratio deviation and angular deviation of the current transformer are unavoidable. The accuracy of a current transformer is calibrated as a percentage of the deviation limit of this ratio. For example, the maximum ratio deviation of a class 0.5 current transformer is ±0.5%. There are 0.1, 0.2, 0.5, 1, 3, and 5 levels of current transformers for accurate measurement, and 5P and 10P level two current transformers for maintenance. Under normal circumstances, choose level 0.2 for metrological verification, choose level 0.5 for precision measurement, and choose level 1 for general monitoring instruments.   In addition, people must pay attention when using current transformers: the secondary side of the current transformer must be grounded to prevent high voltage from the primary side from flowing into the secondary side, which may endanger the safety of equipment and life; pay attention to the first and second sides of the current transformer Optical rotation, optical rotation will ablate the instrument in serious cases; the secondary side of the current transformer cannot be leaded, and it is forbidden to install a circuit breaker on the secondary side to prevent the secondary side lead from magnetic induction of high voltage, endangering life and road safety.

1. Principle of power acquisition by collection unit The fault indicator acquisition unit uses the electromagnetic induction principle of "moving electricity generates magnetism, and moving magnet generates electricity" to generate electricity on the secondary coil through the electromagnetic coupling of the permalloy core. Voltage and current realize the transmission of electric power and achieve the purpose of drawing electricity from the conductor. As shown in the figure, when a primary current flows through the primary winding N1, an alternating magnetic flux Φ1 is generated in the core magnet. This alternating magnetic flux Φ1 passes through the core magnetic circuit and generates an induced voltage value U on the secondary winding. Compared with other electromagnetic mutual inductance transformations, the primary winding here is a cable or wire, and the number of turns is 1 turn. When the alternating magnetic flux Φ1 forms a closed loop in the magnetic core, it will be hindered by magnetic resistance Φr, magnetic space radiation (magnetic leakage ΦL), etc. Part of the magnetic flux will be lost, and only this part of the magnetic flux that passes through the secondary winding Φ2 can effectively transfer electric energy to the secondary side, that is, Φ1=ΦL+Φr+Φ2. It can be seen that under the same working conditions, the smaller the reluctance Φr and magnetic leakage ΦL, the greater the power output by the secondary side will be. Since the permalloy magnetic core acquisition unit needs to meet the requirements of live installation and disassembly, the magnetic core needs to adopt the cutting and snapping mode, that is, the magnetic core needs to be cut into two halves during production and then closed to form a ring magnetic circuit during installation. When the magnetic circuit is closed, an air gap will be generated due to the cutting gap. Compared with the original annular core, its magnetic resistance Φr and magnetic leakage ΦL are greatly increased. Under the same working conditions, the better the cutting process, the smaller the air gap, and the smaller the magnetic resistance. The smaller the resistance Φr and magnetic leakage ΦL will be, the greater the secondary side output power will be. In order to reduce the magnetic circuit loss and increase the secondary side output power level under the same wire current, it is necessary to use magnetic core materials with high magnetic permeability and high saturation magnetic flux density Bs value, making the selection of magnetic core materials tend to be high-end. . After our many experiments, the materials that meet the power extraction requirements of the fault indicator collection unit are: permalloy 1J85, iron-based nanocrystal 1K107.   Physical Properties/Magnetic Properties of Magnetic Materials: 2. The relationship between magnetic flux and power/correlation calculation between theoretical formulas: (1)Φ1=B*Ae (2)B=μ*H (3)μ=AL*Le/0.4*π* N²*Ae(N²=1) (4) H=0.40π*N*I/Le in: μ: magnetic permeability B: Magnetic flux density Ae: core cross-sectional area H: magnetic field strength AL: inductance coefficient Le: equivalent magnetic circuit length It can be seen that at the same primary side current, the higher the magnetic permeability μ, the shorter the equivalent magnetic path length Le of the magnetic core, the larger the core cross-sectional area Ae, and the larger the primary side magnetic flux Φ1.   In order to minimize the losses of Φr and ΦL after the magnetic flux on the primary side is transferred to the secondary side, it is necessary to reduce the reluctance loss and magnetic leakage loss, that is, to increase the magnetic permeability, optimize the effective magnetic path length of the magnetic core, and adopt excellent technology to ensure The cut surfaces of the core fit snugly. In our actual processing practice, the magnetic permeability of 1K107 iron-based nanocrystalline magnetic core after cutting can reach more than 15,000, and the magnetic permeability of 1J85 permalloy magnetic core after cutting can reach more than 12,000. What is the magnetic permeability of permalloy core after cutting? To be broken down in the next episode.   3. Analysis of the reasons for burnt power coil Since the power coil works under power frequency conditions, if the magnetic core is properly designed, the iron loss caused by the eddy current in the magnetic core will not be too large, so the permalloy magnetic core will not burn out. The situation is mainly caused by improper design of the secondary winding. When the operating current on the primary side of the fault indicator rises, the output current on the secondary side will also rise accordingly, that is, I1:I2 =N1:N2 is satisfied. Therefore, it is necessary to consider the winding copper wire according to the possible continuous maximum current on the primary side conductor. The wire diameter and current density on the winding should not be too large, otherwise high heat will be generated on the winding, causing the coil to burn out.   4. Example application 1J85 (0.2mm permalloy) and 1K107 (0.025mm iron-based nanocrystal) are both ultra-high permeability soft magnetic materials. Under power frequency conditions, the eddy current loss is almost negligible. In the current fault indicator acquisition unit In power applications, its excellent magnetic properties are unmatched by other permalloy core magnetic materials. The price of magnetic cores between 1J85 and 1K107 is about 2:1. In large-volume applications, 1K107 has outstanding magnetic performance advantages and price advantages.

Regarding the relationship between the number of core turns and the transformation ratio of a current transformer, this question mainly involves the core-type transformer. According to relevant experiments, it is known that the ratio of its primary current and secondary current is equivalent to the number of primary turns. Inversely proportional to the secondary turns ratio. In other words, if the number of core turns of this transformer is 1 turn, its transformation ratio is 500/5; if the number of core turns is 2 turns, the transformation ratio becomes 250/5. The transformation ratio changes with the change in the number of core turns.   Based on this relationship, we can also calculate the current of the current transformer, which can be used to control the size of the current transformer based on the current to prevent the equipment from being burned due to excessive current.   The relationship and calculation of current transformer ratio and number of turns   Calculation of current transformer ratio and number of turns   Some current transformers have lost their factory nameplates during use. When the user's load changes and the current transformer ratio must be changed, the transformer must first be tested to determine the maximum primary rated current of the transformer, and then the ratio change must be carried out as needed. and calculation of turns.   For example, a current transformer with a maximum primary rated current of 150A needs to be used in a 50/5 transformer application. The formula is converted into the number of primary core turns = the maximum primary rated current of the current current transformer/the primary current of the transformer to be converted. =150/50=3 turns, which is converted into a 50/5 current transformer, and the number of core turns at one time is 3 turns.   The maximum primary rated current can be calculated for this purpose. For example, the transformation ratio of the original current transformer is 50/5 and the number of core turns is 3. If you want to convert it into a 75/5 transformer for use, you should calculate it first. Output the maximum primary rated current: Maximum primary rated current = primary current in the original application × original number of core turns = 50 × 3 = 150A, the number of core turns after conversion to 75/5 is 150/75 = 2 turns That is, when the original 50/5 current transformer with 3 core turns is converted into a 75/5 current transformer, the strain stress of the core turns is 2 turns. Another example is that the original 50/5 current transformer with 4 core turns needs to be used as a 75/5 current transformer. We first calculate the maximum primary rated current as 50×4=200A. After conversion, The number of turns for threading the core should be 200/75≈2.66 turns. When threading the core, the number of turns can only be an integer, either 2 turns or 3 turns.   When we pass through 2 turns, the primary current has become 200/2=100A, resulting in a 100/5 transformer, which causes a deviation. The deviation is (original transformation ratio - current transformation ratio) / current transformation ratio = (15-20)/20=--0.25, which is -25%. In other words, if you still calculate the electricity consumption according to the 75/5 ratio, you will undercount the electricity consumption by 25%. And when we use 3 turns, we will eventually overcount the customer's electricity consumption. Since its primary current becomes 200/3=66.66A, a 66.6/5 transformer is produced, and the deviation is (15-13.33)/13.33=0.125, that is, 12.5% is overcalculated when measuring the electrical energy based on the 75/5 transformation ratio. of electricity. Therefore, when we do not know the maximum primary rated current of the current transformer, we cannot casually replace the ratio, otherwise it may cause errors in measurement verification.   The relationship between the current transformer ratio and the number of core turns   The relationship between the current transformer ratio and the number of turns. For a core-type transformer, the ratio of its primary current to the secondary current is equivalent to the inverse ratio of the ratio of the primary turns to the secondary turns;   Transformer, with 1 turn through the core, the transformation ratio is 500/5; with 2 turns through the core, the transformation ratio is 250/5;   Primary current/secondary current=500/5=100/1=number of secondary turns/number of primary turns (number of secondary turns is 100 turns);   There are 2 turns in the core, the number of secondary turns/the number of primary turns = 100/2 = primary current/secondary current, the secondary current is 5A, and the primary current can be calculated to be 250A;   In other words, if the number of core turns changes, the applied transformation ratio changes, but the transformer itself does not change, and its secondary turns number does not change, it is still 100 turns;   Another optimization algorithm is:   Primary current × number of core turns = primary current when the core turns 1 (here 250A × 2 = 500A)   If the factory nameplate only says 150/5, it means that the primary side of this transformer (one wire across the transformer) can only allow a current pass of no more than 150 amps. If it exceeds, the transformer will be burned. However, in actual applications, the current on the primary side may not necessarily meet the current standard of 150 A, but the requirement of 150 A current magnetic induction can be obtained based on calculations, such as 75/5, 50/5, 30/5.150/5. The current on the primary side is 150 A, and the secondary output is 5 A. The transformation ratio is 150 divided by 5, which is equivalent to 30 times, 75/5, 50/5, 30/5 and so on. 75 requires 2 loops; 50 requires 3 loops; 30 requires 5 loops. In other words, to meet the requirement of outputting 5 amps on the secondary side, the primary side must have a current induction of 150 amps. If the primary side can only be 75 amps, then after two turns, 75*2 will satisfy the primary side. The 150 A current is magnetically induced, and so on for the others.   It can be seen that knowledge in these aspects is very important. If it can be mastered very well, it can have a certain maintenance effect on the current transformer. If you need to know more about this and obtain such products, please contact Hemei Electronics Company.

Why can't the current transformer be opened when it is running? What is the principle? What are the adverse effects of opening a circuit?   Most welders often hear this sentence: "The secondary side of the current transformer cannot be open-circuited, and the secondary side of the voltage transformer cannot be short-circuited." At work, everyone must regard this as the "Quran". When disassembling the secondary line of the current transformer, first add a short rotor or short wiring to connect the secondary side, and then perform surgical removal or wiring work. , Only in this way can the safety of the person be guaranteed, which is a very appropriate approach. When I removed the current wire of the electric meter before, I encountered a situation where the wiring was not firm and the terminals were crackling with charge and discharge. So why can't the secondary current transformer be open circuited? What is the principle? What adverse effects will it cause? The following is a detailed analysis for everyone, I hope everyone can understand it. When the current transformer is working normally, the load connected to the secondary coil is an ammeter or meter current coil and a smart transmitter, etc. The impedance of these coils is not large, and most of them operate in short circuit conditions. Fault conditions. In this case, the magnetic flux generated by the primary current of the current transformer and the secondary coil current cancel each other, keeping the magnetic flux density in the core at a moderate level, usually at a few tenths of a Tesla ( The unit of magnetic flux density: T), because the secondary coil resistor is not large, the operating voltage of the secondary coil is also very low. Under normal operating conditions, the magnetic fluxes of current transformers cancel each other, and the relative density of the magnetic fluxes is not large. When the secondary winding of the current transformer is open-circuited, if the primary current does not change, the secondary circuit is broken, or the resistor is very large, then the current on the secondary side is 0, or very small, and the secondary coil Or the magnetic flux of the iron core is not large and cannot offset the primary magnetic flux. At this time, all the primary current turns into self-induced electromotive force, saturating the iron core. This change is sudden, called a sudden change, and its magnetic flux density reaches several Teslas.   Adverse effects of secondary open circuit of current transformer   When this kind of situation occurs, it will cause the following adverse effects:   1. The second generation of thousands of volts of voltage (this has not been certified, it is a copied theory), the high voltage will penetrate the insulation layer of the current transformer, causing the entire substation equipment casing to be induced and electrified, and will also It will cause electric shock accidents to maintenance workers and threaten their lives.   2. A sudden change in the saturation state of the iron core will increase the core loss of the voltage transformer, and the iron core will become hot and damage the voltage transformer.   3. The saturated iron core of the voltage transformer is in a saturated state, the measurement calibration is inaccurate, and the CT ratio difference and angle difference increase. 

The current transformer is an instrument that converts the large current in the primary side circuit into a small current that can be measured by the secondary side ammeter. It is mainly used for current monitoring and measurement or measurement of electrical energy.   Introduces the monitoring and measurement of current: Current monitoring and measurement is divided into two forms, one is ammeter detection of current; the other is thermal overload protection measurement.   The wiring method for ammeter to detect current is as shown below: Current transformer circuit diagram The P1 terminal of the current transformer in the figure is connected to the ammeter A1, and the A2 terminal is connected in series with the P2 terminal of the transformer and is grounded.   How to short-circuit other unused transformers? As we all know, the secondary side of the current transformer must not be opened. Because the current transformer is a step-up transformer, in an open circuit state, high voltage will be generated on the secondary side, which may easily cause electric shock to the human body. Therefore, the secondary side must be short-circuited when not in use. The simplest method is to connect the P1 and P2 terminals directly with a 2.5mm wire.   How to replace P1 and P2 when they are reversely connected? When the P1 terminal is connected to the P2 terminal, just connect the P1 and P2 terminals back to their original positions.   Hemei Electronics is committed to the research and development, production and sales of amorphous, nanocrystalline and permalloy magnetic core products. Its main products are: nanocrystalline strips, ultra-microcrystalline iron cores, ultra-microcrystalline magnetic cores, Permalloy cores, high-power transformer cores, nanocrystalline magnetic ring inductors, electromagnetic ring coils, switching transformers, common mode inductors, precision current transformers and other products have good stability and high electrical parameters. advantage.   Read more:  https://www.hemeielectricpower.com/

In power engineering, current transformers convert large currents into small currents and are used for the protection, measurement and measurement of power systems. Today I will explain to you how to choose a current transformer.   The technical conditions for current transformer selection and verification include: primary circuit voltage, primary circuit current, secondary load, secondary circuit current, accuracy level and transient characteristics, relay protection and measurement requirements, dynamic stability multiple, thermal Stability factor, mechanical load and temperature rise. In addition to the above technical conditions, it should also be calibrated according to the use environment: ambient temperature, maximum wind speed, relative humidity, pollution, altitude, earthquake intensity and system wiring method.   The rated primary voltage of the current transformer should not be less than the rated primary voltage of the circuit, which is generally the nominal voltage of the system instead of the highest working voltage.   The rated primary current of the current transformer is an important and difficult point in the selection. Generally, it can be selected according to the rated current or maximum operating current of the primary equipment to which it belongs. The standard values of the rated primary current are: 10A, 12.5A, 15A, 20A, 25A, 30A, 40A, 50A, 60A, 75A and their decimal multiples or decimals. The primary current of the current transformer winding for measurement and metering should be selected strictly according to the loop operating current. In actual use, the transformation ratio difference on each side of the current transformer connected to the main transformer and busbar differential protection should not be greater than 4 times.   As shown in Figure 1, for each circuit of the 35kV bus of a 50MVA booster station, the rated current of the main transformer circuit is 866A, the outlet circuit is 600A, the reactive power compensation circuit is 300A, and the rated current of the station transformer circuit is 10A. Because the rated current of the station transformer loop is very small, the primary current of the protection level winding of the current transformer of this loop is 1/4 times the current of the main transformer side; the primary current of the main transformer loop is selected as 1000A, and the primary current of the protection level of the station transformer loop is optional. It is 250A. Considering the consistency of the transformer ratio, the primary current of the station transformer circuit protection level can be selected as 300A, and the measurement and measurement is generally selected as 50A.   When the high-voltage side of the main transformer is a directly grounded system, the primary rated current of the neutral point current transformer should be 50% to 100% of the rated current of the high-voltage side of the transformer, and meet the specified error limit. The rated primary current of the discharge gap zero-sequence current transformer of the neutral point complete device should be selected according to 100A.   The cores of the current transformers on each side used for transformer differential protection and differential protection of the same busbar should have the same core form.   For P and PR class current transformers, they are only required to meet steady-state non-saturation, while TP class current transformers are required to be non-saturated throughout the entire working cycle.   The above is the current transformer selection compiled by the editor based on specifications and actual engineering experience. Please also pay attention to the reference in practical applications.   Read more:  https://www.hemeielectricpower.com/

Motor energy saving and core material development Today, it can be said that more than half of the total global electricity consumption comes from motors. High-efficiency motors are urgently needed to control carbon dioxide emissions and thereby curb global climate warming. The key to achieving this high-efficiency motor is the core material. The core material of the motor, as the name implies, is made of iron-containing materials. The core material is made of magnets and plays an important role in controlling the magnetic flux of the motor (forming a magnetic field), but it also becomes a source of heat due to iron losses (hysteresis loss and eddy current loss). So, will it be possible to achieve high efficiency of the motor by choosing a core material with small iron loss? Not so. Core materials have many properties. Moreover, even the same core material has different required properties when used in motors and when used in transformers. In recently developed high-efficiency motors, amorphous alloys are used instead of non-oriented electromagnetic steel sheets as core materials, and combined with ferrite magnets, ultra-high efficiencies previously thought to be impossible have been achieved. This article will explain the selection and use of motor core materials based on this case.   Core materials can be divided into two categories   Core materials are also called soft magnetic materials. Figure 1 shows the main soft magnetic materials.   What kind of motor core material is better? If there is no heating effect of iron loss, using soft magnetic materials with large Bs in the core can obtain the maximum magnetic flux density Bm of the core. In addition, the volume of the core is inversely proportional to the square of Bm. From the perspective of miniaturization, materials with large Bs also have advantages. On the other hand, from the perspective of improving efficiency, soft magnetic material cores with small iron losses are preferred. Therefore, high-quality soft magnetic materials with high Bs and low iron loss located in the lower right area of Figure 2 are needed. However, as shown in Figure 2, there is a compromise between high Bs and low iron loss, and the ideal soft magnetic material that can satisfy both does not exist. It can also be seen from Figure 2 that when the frequency is 1kHz, in terms of high Bs, Perminde alloy has advantages; in terms of low iron loss, nanocrystalline soft magnetic alloy and PC-type permalloy (high magnetic permeability material) are more advantageous. Advantage.   Comparing iron-based amorphous materials and non-oriented electrical steel sheets Comparison of the initial magnetization curves of iron-based amorphous alloy SA1 and non-oriented electromagnetic steel plate 35H300 is shown in Figure 3. For SA1, in order to relax the stress exerted on the alloy during casting, the magnetic properties can be improved through magnetic field-free heat treatment in a nitrogen atmosphere. However, since SA1 will become embrittled under heat treatment, heat treatment is mostly not performed when used in motors. When it exceeds B-0.8T, the magnetizing field strength of SA1 becomes greater than that of 35H300 (the magnetic permeability becomes smaller).   Evaluate core processing Figure 4 shows the iron loss comparison between SA1 and 35H300 slotted and cut cores without heat treatment. In the case of 400Hz and B.=1.0T, the iron loss of the SA1 core is 2Wkg and the iron loss of the 35H300 core is 20Wkg. The former is 1/10 of the latter. Based on this, it is extremely effective in improving the efficiency of motors, such as in battery-driven motors, and has the effect of extending battery life. In addition, the higher the driving frequency, the greater the iron loss difference between the two. This situation also applies to high-speed motors. Figure 5 shows the structure of an 11kW double-rotor axial gap motor, which uses an SA1 stator core and a ferrite sintered magnet rotor. The stator core is a simple structure in which amorphous alloy SA1 is cut into long strips, laminated, and fixed with resin. It is a slit-cut core. The stator core is made of non-oriented electromagnetic steel plates of the same shape, which are cut into long strips and then laminated and fixed with resin to obtain another motor. The loss comparison between it and the motor shown in Figure 5 is shown in Figure 6. Compared with the 35H300 stator core, the SA1 stator core can reduce the iron loss by 1/5. In addition, by reducing the excitation current, the copper loss can be reduced by about 15%, reaching an efficiency of 93.2% in line with the IE4 standard. Furthermore, on the basis of this motor, the characteristics of amorphous alloy are fully utilized, which is smaller than the traditional three-phase induction motor. A high-efficiency motor that meets the IE5 standard is being developed.   Read more:  https://www.hemeielectricpower.com/

Background: Nowadays, China has made great progress in terms of life, economy and science and technology, among which the electric power industry is the most obvious. The domestic electric power industry has now set a benchmark in the international industry.Under the strategic goals of "power Internet of Things" and "digital grid", comprehensive monitoring of the status of the power grid has become an important step in realizing the digitalization and intelligence of the power grid. In this process, the application scenarios of power detection and monitoring equipment have become increasingly complex, the scale has gradually increased, and the demand for safe and stable power supply has also continued to increase. However, current transmission line monitoring equipment mainly supplies energy through wiring power supply or solar + energy storage power supply. It faces severe challenges such as high cost, frequent manual maintenance, and poor reliability, which seriously restricts the promotion, application and development of online transmission line monitoring equipment. .   In order to solve the problem of power supply for power detection and monitoring equipment, we will take you to learn about one of the safer energy supply solutions for equipment in the current power industry - passive source of electromagnetic induction energy!   When it comes to passive source of power, I believe the first thing that pops up in everyone’s mind must be the wireless charging function used on smartphones! In principle, the understanding is correct, but compared with the technology of passive source of electricity, the fields of application are essentially different. The former is currently suitable for medium, high and low voltage transmission lines. Passive source power is referred to as CT power extration, which mainly consists of three components: magnet core, power module and electrical equipment.   CT power extraction is based on the principle of electromagnetic induction, which induces energy from power lines in a non-electrical connection. The magnetic core material is processed into the required shape (usually a ring shape) through a special process, and the AC wire is passed through the inner hole of the magnetic core material. The magnetic core will sense the magnetic field around the wire and pass through the magnetic ring coil. Electric energy is induced, and after the electric energy is converted into AC/DC power through the coil end, a DC regulated power supply is output. (The magnetic core can be processed into a closed-loop or open-close structure according to requirements)   Features: 1. Stable power supply - The CT power supply device can be externally attached to the wire to provide stable power output, and can realize self-protection under short circuit and surge current, and achieve long-term stable operation with low heat consumption. It is a safe and reliable power supply method. 2. Wide application - The magnetic core of the CT power taking device can be specially customized according to different working environments. It has excellent waterproof, moisture-proof, high temperature resistance and other characteristics, and can be stable even in complex and severe weather such as rain and snow. , effective power supply. 3. Compact size - CT power extraction device has high power density, small size and light weight, which can greatly reduce the air resistance on overhead lines and reduce the load-bearing capacity of the line.   Application Environment: 1. Suitable for conventional medium voltage or low voltage transmission lines 2. Suitable for underground mines or tunnels 3. Overhead cables, underground branch cables, branch boxes, etc. 4. Rail transit electric traction system Product related companies: CT power extraction is a necessity derived from the development of the power industry. Currently, the quality of products produced by domestic manufacturers is uneven. Among them, Hefei Hemei Electronics Technology Co., Ltd., a leading material manufacturer, is an example: Hemei Electronics is a A leading domestic designer and manufacturer specializing in the research and development of electromagnetic signal coupling and magnetoelectric power conversion technology, it has been committed to promoting the progress and development of magnetoelectric signal coupling measurement technology, passive self-power supply technology, and mid-range wireless power transmission technology.   Hemei Electronics mainly produces passive self-powered (CT power supply) devices and CT power supply modules. Ultra-high-precision AC measurement sensors, permalloy cores, magnetic shielding boxes, etc.   The products are mainly used in passive power supply of intelligent sensing equipment, passive power supply AC measurement of AC power supply systems such as power and railways and monitoring equipment, amplification and isolation of carrier communication signals in smart home systems, etc.

Power supply companies use digital technology to develop and apply a fully functional fault location system by comprehensively processing and integrating fault alarm data, real-time operational data, offline data, grid structure, equipment parameters and many other information on the distribution network.   1 Background The power supply company under the jurisdiction of 95 10kV lines, in 2017 due to fault tripping caused by the grid eight events 43, an average of 0.45 / article, while the average time to find the fault event 136 minutes, the line fault outage has seriously affected the reliability of power supply and power supply service capacity. 2018 Weinan Power Supply Company stipulates that the distribution network eight grid events are strictly controlled within 14 times, the production management of distribution networks Requirements are higher, and in this regard, the Xintong operation and inspection class combined with the demand for small current connection fault diagnosis, based on the first half-wave grounding line selection principle, using wireless communication technology, data acquisition technology, computer technology, the development and application of the distribution network grounding fault location device.   2 Innovation Points The device is innovative in terms of reliability, availability, advancement and maintainability: maintenance-free. The internal use of large-capacity, long-life disposable lithium batteries, with CT power extraction, power extraction current to the supercapacitor charging, supercapacitor to the RF wireless communication module power supply; fault detection, load collection function integration. When a line fault occurs, it detects the type of grounding and short-circuit faults to achieve fault location. During normal operation of the line, load and voltage information is uploaded at regular intervals; Ground fault detection with direction. On the basis of collecting line current and capacitance current, it collects relative ground voltage, increases the criterion of voltage and direction, eliminates the false action caused by excitation inrush current, and greatly improves the correct rate of grounding detection; two-way wireless communication. With local wireless frequency division, frequency hopping network and remote GPRS ‘four remote’ two-way interactive communication dual maintenance function; integrated multi-technology fault indicator. Unconventional technical means to solve a series of problems such as machine power consumption, communication energy, self-power supply, fault detection, fault recording, online monitoring, high current impact, high and low temperature, protection level, lightning surge, EMC, volume, weight and so on.   3 Achieve benefits Economic benefits. After the implementation of the project, each line trip or grounding can reduce the accident point in a wide range, reduce manual patrol 2 man-days, each man-day 200 yuan, 2 × 200 = 400 yuan, reduce vehicle costs 100 yuan, a total of 500 yuan, according to the 128 flat line road grounding and tripping 58 times in the whole year of 2014, the annual savings of 29,000 yuan; change the traditional fault handling mode, reduce the fault processing time, effectively controlling the scope and time of the fault. The average outage time within the application area of the pilot project construction is reduced from 20.84 hours to 6.85 hours, and the annual power supply is increased by 313,568,000 kWh, which can directly generate economic benefits of 171,500 yuan according to the average price of electricity of 0.5471 yuan/kWh. Social benefits. After the implementation of this project, significant economic benefits have been achieved in addition to effectively improving power supply reliability, and the project has been awarded the First Prize of Science and Technology by Weinan Power Supply Company.   4 Outlook The distribution network line ground fault location device has been fully applied in State Grid Pucheng County Power Supply Company, and 19 sets of related devices have been deployed in Pucheng County power grid, providing a good basis for the operation of Pucheng power grid line equipment. As a useful attempt of 10 kV distribution network line fault diagnosis, the distribution network ground fault location device has explored the principle of fault routing, communication channels, power solutions, etc., which provides a theoretical and practical basis for further improvement of the fault indication device, and the continuous improvement in the field of fault diagnosis and localisation is an important supplement to realise the high-reliability and low-cost of distribution automation at the level of the agricultural network. The development prospect is huge.   Read more:  https://www.hemeielectricpower.com/