1 Principle and effect of current transformer

The operation principle of current transformer is similar to that of transformer. The difference is that in the iron core of the transformer, the alternating main flux is the current of the communication voltage applied at both ends of the primary winding. The alternating main flux in the iron core induces the corresponding secondary electromotive force and secondary current in the secondary winding of the current transformer. Since the primary winding and secondary winding are wound on the same iron core, they are connected by the same alternating main flux. Therefore, numerically, the current and turn product of the primary winding and secondary winding should be equal, that is, i1n1 = i2n2, so I1 / I2 = N2 / N1 = k, K is called the transformation ratio of the current transformer.

Current transformer is mainly used to change the high voltage and large current in the power system into low voltage and small current according to a certain share, that is, no matter how large the primary side current is, the secondary side (usually) is 5A. Supply appearance, instrument, relay, etc. for measurement and maintenance.2 Precautions for current transformer in operation:

1) The secondary side of the current transformer during operation shall not be open circuit, and the equipment fuse and knife switch shall not be installed on the secondary side. This is because the number of turns of the secondary winding of the current transformer is far greater than that of the primary winding. Under the condition of open circuit, the current transformer is suitable for a step-up transformer. According to the flash of relevant materials, the voltage value can reach 100 v. Endanger the safety of operators. Therefore, when there is current at the primary side, the secondary side shall not be constructed except for selecting the corresponding short-circuit method (the current transformer is approximately in short-circuit condition during operation, so K1 and K2 can be directly short circuited and grounded). Remember.

2) For current transformer equipment, one end (usually K2), iron core and shell of the secondary side of the current transformer shall be firmly grounded. In order to prevent the primary side voltage from cascading to the secondary side due to insulation damage of primary and secondary side windings, endangering the safety of operators.3) When installing current transformer equipment, the accuracy level shall be considered. High precision measurement surface, low precision for maintenance. Be careful when choosing.

4) When using current transformer equipment, pay attention to the polarity (same name end). The terminals at the primary side are L1 and L2 (or P1 and P2). The current at the primary side flows in from L1 and out from L2. The terminals on the secondary side are K1 and K2 (or S1 and S2), that is, the terminals on the secondary side flow out from K1 and in from K2. L1 and K1, L2 and K2 are of the same polarity (same name), and no mistake is allowed. Otherwise, if the watt hour meter is connected, the watt hour meter will turn.

5) The primary winding of current transformer can be divided into single turn and multi turn, and LQG type is single turn. When using LMZ type (through core type), pay attention to whether there is through core data on the nameplate. If so, wear the required number of turns as required. Note: the number of turns through the center is subject to the number of turns through the hollow, not the number of turns around the periphery (otherwise, one turn will be wrong).6) The secondary winding of current transformer is divided into one winding and two windings. If there are two windings, one winding with high precision (small error value) is usually used for measurement, and the other with low precision (large error value) is usually used for maintenance.7) It is necessary to select 2.5mm2 copper core insulated wire for the connection line of current transformer. It is necessary to select 4mm2 copper core insulated wire according to some rules of some power industries, but my concept is not necessary.

3 Application of current transformer1) Measurement of communication currentIf the equipment is not operating at additional current value, setting current measuring equipment is a necessary skill method. For equipment above 40kW, it is necessary to install ammeter for monitoring.

The measurement of communication current has the methods of direct measurement and widening measurement through current transformer. Direct measurement is to connect the appropriate ammeter in series to the current circuit. The circuits shown in the figure below are connected through current transformer.

(a) Select a current transformer to measure the current in the three-phase balance line. The selection of current transformer is based on the range of ammeter, and the range of ammeter is based on the practical value of load current, which should account for 2 / 3 of the total range of ammeter to the full scale value. For example, if the additional current of a device is 100A, select an ammeter with a full-scale value of 150A. At this moment, the mutual inductance ratio of current transformer shall be 150 / 5A, and the ammeter with the ratio of 150 / 5A shall also be selected accordingly. When reading, read directly with the indicated value of the meter.

(b) Two current transformers are selected and connected into an incomplete star to measure the three-phase current in the three-phase balanced or unbalanced line.(c) Three current transformers are selected and connected into a complete star connection method to measure the three-phase current in the three-phase balanced or unbalanced line.2) Measurement of communication electricity

In order to broaden the range of communication watt hour meter, the most commonly used method in the factory is to use current transformer to expand many ranges.(1) The top of the list is the measurement method of single-phase watt hour meter and current transformer. It can be used to measure three-phase balance or single-phase electricity metering.(2) The second is the measurement method of selecting two element three-phase three wire active watt hour meter and adding two current transformers. It can be used to measure the electricity in three-phase balanced or unbalanced lines.

(3) The third is the measurement method of selecting three-phase four wire active watt hour meter and adding three current transformers. It can measure the electricity metering in three-phase balanced or unbalanced lines.3. Maintenance in electric drive lineFor overload maintenance of high-power motor, it is usually impossible to buy corresponding thermal relay due to high current. Under such circumstances, it is usually handled by adding current transformer. The essence is to replace large current with small current, and the thermal relay within 5A can meet the requirements of overload maintenance.

(1) Current transformer is used for high-power self coupling step-down protection and overload maintenance in motor.(2) Current transformer is used for high-power frequency sensitive rheostat and overload maintenance in motor.(3) Current transformer is used for high-power star angle step-down protection and overload maintenance in motor.

To study the working characteristics of current transformer and confirm whether it will saturate when the external fault of protection passes through large current and affect the correctness of protection action, some test methods can be used for detection. Obviously, the most direct test method is to carry the actual load on the secondary side, apply the current from the primary side, observe the secondary current and find out the saturation point of the current transformer. However, for the protection level current transformer, its saturation point may exceed 15 20 times the rated current. When the current transformer becomes large, it will be difficult to carry out this test on site.

In addition, the saturation point of current transformer can also be measured through volt ampere characteristic test. As mentioned earlier, the saturation of the current transformer is caused by the excessive magnetic flux density of the core, and the magnetic flux density of the core can be reflected by the induced electromotive force of the current transformer. Therefore, the saturation current of the current transformer can be calculated from the saturation voltage value on the volt ampere characteristic curve. The test method of volt ampere characteristics is as follows: open the primary side, apply current from the secondary side, and measure the voltage drop on the secondary side winding. Due to the open circuit of the original square of the current transformer, without the demagnetization of the original square current, the iron core is easy to be saturated under the action of small current. Therefore, the volt ampere characteristic test does not need to add a large current, which is easy to realize in the field.

Under normal conditions, the magnetic flux of the iron core in the current transformer is in an unsaturated state. At this time, the load impedance and excitation current are small, while the value of excitation impedance is large, and the magnetic potential of primary winding and secondary winding is in balance. However, if the magnetic flux density of the iron core in the transformer increases and reaches saturation, ZM will decrease rapidly with the increase of saturation, and the linear proportional relationship between different excitation currents will be broken. The main factors causing the current transformer to reach saturation include: excessive current; Excessive load. When the load connected to the current transformer is too large, it will cause the increase of secondary voltage, resulting in the increase of magnetic flux density of iron core and saturation.

When the current transformer reaches saturation, the characteristics are as follows: the secondary current decreases, and the current waveform has large distortion of high-order harmonic component; The internal resistance decreases, even close to zero; If a fault occurs and the current waveform is near the zero point, the current transformer will cause linear relationship transmission; At the moment of failure, the transformer will lag about 5 seconds before reaching saturation. In general, it is forbidden to open the secondary circuit of current transformer. Because in the operation process of current transformer, once the secondary open circuit occurs, the primary current will be converted into excitation current, resulting in the increase of magnetic flux density of iron core and the rapid saturation of current transformer. Saturated magnetic flux will produce high voltage, which will cause great damage to the insulation facilities of primary and secondary windings, and is easy to pose a threat to personal safety.

1. Influence of transformer protection and Countermeasures

Generally, transformers have small capacity and high reliability. Most of them are installed on 10kV and 35kV buses. The high-voltage short-circuit current is the same as the short-circuit current of the system, while the short-circuit current on the low-voltage side is relatively large. If the protection of the transformer is not in place, it will seriously affect the safe operation of the transformer or the whole system. Traditional transformers have fuse protection devices, which have the advantages of safety and reliability. However, with the improvement of system automation requirements and the increase of short-circuit capacity, the traditional methods can not meet the needs. Some newly-built and reconstructed substations are often equipped with transformer switchgear, and the protection device of the system is similar to that of 10kV line, but the disadvantage is that the saturation of current transformer is often ignored. At the same time, due to the small capacity and primary current of the transformer, the common transformer is adopted. In order to ensure the accuracy of measurement, the transformation ratio of current transformer will be reduced. Once the transformer fails, it will cause the saturation of the current transformer and the decrease of the secondary current speed, resulting in the failure of the transformer protection. If a fault occurs at the high-voltage side of the transformer, the generated short-circuit current will automatically cut off the backup protection action. If a fault occurs at the low-voltage side, the short-circuit current generated cannot reach the starting value of backup protection, the fault will not be removed, and even the transformer will be burned, which will have a serious impact on the safe operation of the system.

To solve the protection failure of transformer, we need to start with the reasonable configuration of transformer, and consider the saturation problem caused by transformer fault when selecting current transformer. Current transformers with different functions shall be different from each other. For example, the transformer for measurement shall be set at the low-voltage side of the transformer to ensure the measurement accuracy requirements; The transformer for protection is generally set at the high-voltage side of the transformer to ensure the protection of the transformer.2. Influence of current protection and Countermeasures

After the current transformer is saturated, the secondary equivalent current will be reduced and the protection will refuse to operate. When it is far away from the power supply or the impedance coefficient is large, the short-circuit current at the line outlet will be small. However, if the scale of the system is expanded, the short-circuit current will increase, even hundreds of times of the primary current of the transformer, resulting in the saturation of the transformer that can normally operate in the system. At the same time, short-circuit current fault belongs to transient process, and there are a large number of different period components in the current, which will accelerate the saturation of current transformer. If a short-circuit fault occurs in a 10kV line, the saturation of the current transformer will reduce the current at the secondary side, resulting in the failure of the protection device. The removal of the switch on the low-voltage side of the bus and the main transformer will increase the scope and time of the fault, affect the reliability of power supply, and threaten the safe operation of the equipment in serious cases.

According to the above analysis, when the current transformer is saturated, the primary current will be transformed into excitation current. At the same time, the secondary current is zero and the current through the relay is zero, and the protection device in the equipment refuses to operate. In view of the above problems, the load impedance of the transformer should be reduced as much as possible, the sharing of current transformers should be avoided, and the cable cross-sectional area and cable length should be increased at the same time; The transformation ratio of current transformer shall not be too small, and attention shall be paid to the saturation problem caused by line short circuit.

Switching power transformer is a power transformer with switch tube. In the circuit, in addition to the voltage conversion function of ordinary transformer, it also has the functions of insulation isolation and power transmission. It is generally used in switching power supply and other occasions involving high-frequency circuits.Function and classification of switching power transformer

The switching power supply transformer and the switch tube form a self-excited (or other excited) intermittent oscillator to modulate the input DC voltage into a high-frequency pulse voltage. It plays the role of energy transfer and conversion. In the flyback circuit, when the switch is on, the transformer converts the electric energy into magnetic energy for storage, and releases it when the switch is off. In the forward circuit, when the switch is on, the input voltage is directly supplied to the load and the energy is stored in the energy storage inductor. When the switch is cut off, the energy storage inductor is used for continuous flow to the load.

Switching power transformer is divided into single excitation switching power transformer and double excitation switching power transformer. The working principle and structure of the two switching power transformers are not the same. The input voltage of the single excitation switching power supply transformer is a unipolar pulse, and it is also divided into positive flyback voltage output; The input voltage of double excitation switching power supply transformer is bipolar pulse, which is generally bipolar pulse voltage output.Principle of switching power transformer

For switching power supply, the working principle of switching transformer is different from that of ordinary transformer. The positive and negative half cycle waveforms of the input AC voltage or current of the ordinary transformer are symmetrical, and the input voltage and current waveforms are generally continuous. Within a cycle, the average value of the input voltage and current is equal to 0, which is the basic feature of the working principle of the ordinary transformer; Switching transformers generally work in the switching state, and their input voltage or current is generally not continuous, but intermittent. Most of the average value of input voltage or current in a cycle is not equal to 0. Therefore, switching transformers are also called pulse transformers, which is the biggest difference between switching transformers and ordinary transformers in working principle.

PWM (pulse width modulation) controls the switch tube to turn on the rectified DC voltage, so that the high-frequency current flows into the primary side of the high-frequency transformer of the switching power supply, and the induced current is generated at the secondary side of the transformer. After rectification, the required voltage or multi-channel voltage can be obtained.Test details of switching power transformer1. Check whether there are obvious abnormalities by observing the appearance of the transformer: such as whether the coil lead is broken, desoldered, whether the insulating material has scorched traces, whether the iron core fastening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed, etc.

2. Insulation test: use multimeter R & times; 10K gear shall measure the resistance between iron core and primary, primary and secondary, iron core and secondary, electrostatic shielding layer and secondary and secondary windings respectively. The pointer of multimeter shall point to the infinity position. Otherwise, the insulation performance of the transformer is poor.3. Coil on-off detection: place the multimeter in R & times; Gear 1: in the test, if the resistance value of a winding is infinite, it indicates that the winding has open circuit fault.4. Discrimination of primary and secondary coils: the primary pin and secondary pin of power transformer are generally led out from both sides, and the primary winding is mostly marked with 220V, while the secondary winding is marked with rated voltage, such as 15V, 24V, 35V, etc. Then identify according to these marks.

5. Detection of no-load current: A. open all secondary windings by direct measurement method, and place the multimeter in AC current gear (500mA, connected in series into the primary winding. When the plug of the primary winding is plugged into 220V AC mains power, the multimeter indicates the no-load current value. This value shall not be greater than 10% 20% of the full load current of the transformer. Generally, the normal no-load current of the power transformer of common electronic equipment shall be about 100mA. If it exceeds too much, it indicates that the transformer has short-circuit fault. B. the indirect measurement method is used in voltage transformation A 10 / 5W resistor is connected in series in the primary winding of the power transformer, and the secondary is still no-load. Turn the multimeter to the AC voltage gear. After power on, use two probes to measure the voltage drop u at both ends of the resistor R, and then use Ohm's law to calculate the no-load current I empty, i.e. I empty = u / R. connect the primary stage of the power transformer to 220V mains power, and use the AC voltage of the multimeter to measure the no-load voltage of each winding in turn (U21, u22, U23 and u24) shall meet the required values, and the allowable error range is generally: high voltage winding 10%, low voltage winding 5%, and the voltage difference between two groups of symmetrical windings with center tap shall be 2%.

6. Test the temperature range of power transformer: generally, the allowable temperature rise of low-power power transformer is 40 50 . If the insulating material used is of good quality, the allowable temperature rise can be improved.7. Detect and judge the same name end of each winding: when using the power transformer, sometimes in order to obtain the required secondary voltage, two or more secondary windings can be used in series. When using the power transformer by series method, the same name end of each winding in series must be connected correctly without mistake. Otherwise, the transformer cannot work normally.

8. Comprehensive detection and discrimination of short-circuit fault of power transformer: the main symptoms of short-circuit fault of power transformer are serious heating and abnormal output voltage of secondary winding. Generally, the more inter turn short-circuit points in the coil, the greater the short-circuit current, and the more serious the heating of transformer. The simple method to detect and judge whether there is short-circuit fault of power transformer is to measure no-load power Current. For the transformer with short-circuit fault, its no-load current value will be much greater than 10% of the full load current. When the short-circuit is serious, the transformer will heat rapidly within tens of seconds after no-load power on, and it will feel hot to touch the iron core by hand. At this time, it can be concluded that there is a short-circuit point in the transformer without measuring the no-load current.

What is the difference between switching power supply and transformer in use?Switching power supply: the switching power supply can stably turn the voltage within a certain range into a very accurate low voltage or high voltage (for example, 110v-250 input, the output voltage can be stably controlled at the required voltage positive and negative difference of 0.5V).Transformer: the output voltage of the transformer changes with the input voltage, that is, the input voltage increases, the output voltage increases, the input voltage decreases, and the output voltage decreases.

To sum up, it can be concluded that the switching power supply first changes the AC into DC, and then changes the DC into AC with higher frequency through the power switch tube. The voltage conversion through the high-frequency power transformer not only improves the efficiency, but also greatly reduces the volume after the frequency is high, and saves the copper and iron loss. Because it is controlled by the power switch tube, the switch tube is conductive at low current The power on time is short and the output voltage can be maintained. When the load is large, the switch tube works continuously to maintain the output voltage. Therefore, the output voltage of the switching power supply is stable and can be selected as high-precision instruments such as LED display screen.

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Inspection during operation of power transformer(1) The on-site meter indication of the upper oil temperature and high and low winding temperature of the transformer shall be the same as that of the meter or CRT of the control panel. Check whether each temperature is normal and whether it is close to or exceeds the maximum allowable limit.(2) Whether the oil level on the transformer conservator is normal, each oil level gauge shall be free of dirt or damage, and there shall be no condensation inside.

(3) Whether the indication of the transformer oil flow meter is normal, whether the color of the transformer oil becomes sharply darker, and there shall be no oil leakage and oil leakage at all parts of the body.(4) The electromagnetic noise of transformer shall have no abnormal change compared with the past. The body and accessories shall not vibrate, and the temperature of each component shall be normal.(5) Whether the cooling system operates normally; For forced oil circulation air-cooled transformer, whether individual fans stop running; Whether the running fan motor is overheated, abnormal sound and abnormal vibration; Whether the oil pump operates normally.

(6) Whether each switch in the transformer cooler control device is in the specified position for operation.(7) Whether the transformer shell grounding, iron core grounding and grounding devices at each point are intact.(8) Whether the insulating parts on the transformer tank cover, such as bushing and porcelain bottle, are damaged, cracked and discharged. Whether the oil level indication of the oil charging sleeve is normal.

(9) Whether the connectors of the primary circuit of the transformer are in good contact and whether there is heating.(10) Whether the indication of hydrogen monitoring device is abnormal.(11) Whether the fire water circuit of the transformer is intact and the pressure is normal.

(12) Whether the desiccant of the moisture absorber is invalid must be checked regularly, replaced and dried.Maintenance of power transformer in operation(1) The staff shall regularly check the chromatography of transformer insulating oil and check the indicated value of hydrogen monitoring device, so as to find the possible abnormalities in the transformer in time.

(2) During normal operation of transformer, the temperature of main transformer, auxiliary high voltage transformer and start / standby transformer shall be processed, output and printed by computer every hour. The temperature of auxiliary transformer shall be recorded during regular inspection.(3) According to the provisions of "equipment regular switching test system", the cooler of main transformer, auxiliary high voltage transformer and start / standby transformer shall be tested and switched for operation once every half a month.(4) According to the provisions of "equipment regular switching test system", the tap lifting remote control test shall be carried out for the on load voltage regulating devices of main transformer, auxiliary high voltage transformer and start / standby transformer once every half a month.

(5) Check the main transformer, auxiliary high voltage transformer and start / standby transformer according to the provisions of "equipment regular switching test system".Precautions in operation of power transformer(1) No overload operation: long-term overload operation will cause coil heating, gradually aging insulation and short circuit.

(2) Regular inspection of insulating oil quality: the oil quality shall be tested regularly, and the unqualified oil shall be replaced in time or other measures shall be taken.(3) Prevent insulation aging damage of transformer iron core and insulation aging caused by long-term heating of iron core.(4) Prevent the insulation from being damaged due to careless maintenance. If it is found that it is damaged, it shall be handled in time.

(5) Ensure good conductor contact and local overheating due to poor contact.(6) To prevent lightning stroke, the transformer will be burned due to insulation breakdown.(7) Short circuit protection: in case of short circuit of transformer coil or load, if the protection system fails or the protection setting value is too large, the transformer may be burned. To do this, install reliable short-circuit protection.

(8) Well protected grounding.(9) Ventilation and cooling: if the transformer coil conductor is class a insulation, its insulator is mainly paper and cotton yarn. When the temperature rises by 8 , the insulation life will be reduced by about half; The normal temperature of the transformer is below 90 , and the service life is about 20 years; If the temperature rises to 105 , the service life is 7 years. Keep good ventilation and cooling during transformer operation.