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A 35KV wholesale user of our bureau has a metering current transformer model of lcz-35-100 / 5 and an energy meter model of dss331 and 3 100V3 1.5(6)A Due to continuous drought, the user's own hydropower output decreased. The current reached 162a at peak load in December 2001, which seriously exceeded the rated current of current transformer, resulting in the loss rate of 100m line as high as 19.6%. Therefore, we test and analyze the measurement error of current transformer when it is overloaded.
1. Influence of serious overload of current transformer on current transformer body
(1) Test object: current transformer, model: lcz-35-100 / 5; Secondary rated load: 50VA, accuracy level: 0.2; Manufacturer: Chongqing high voltage electrical appliance factory, No. 069.
Power factor during test: 0.8.
(2) When the load current is 5% to 165% of the rated current of the current transformer (current transformer regulation stipulates that 5%, 20%, 100% and 120% of the rated current are selected for the verification load point), the error (ratio difference and angle difference) is shown in Table 1.
Table 1
(3) It can be seen from the test data that when the load current is within 120% of the rated current, the greater the load, the smaller the error of current transformer; When the load current is between 120% 165% of the rated current, the error of current transformer increases with the increase of load current, but gradually tends to balance within the allowable range. This is because in the design of current transformer, the minimum saturation multiple determining the overload capacity of current transformer is generally 2 3 times of rated current; With the increasingly mature and advanced manufacturing technology of current transformers, some current transformers can withstand dozens of times of rated current in a short time.
2 Influence of current transformer overload on electric energy meter
When the current transformer is overloaded, it will not be saturated in a certain range, the secondary current will increase proportionally, and the supporting electric energy meter will also be overloaded.
Power factor during test: 1.0.
(2) In order to match the current transformer test, the load current is selected to be 10% 533% of the calibration current IB of the electric energy meter (jjg596-1999 regulation stipulates that when the power factor is 1.0, the verification points are 10%, 50%, 100% and 400% of the calibration current), and the error is shown in Table 2.
Table 2
The error r% curve when the above test error exceeds the rated maximum current Imax (400% IB) of the electric energy meter is shown in Figure 1.
Fig. 1 error curve of electric energy meter under overload
It can be seen that when the actual current is greater than the maximum rated current Imax of the electric energy meter, the electric energy meter will not burn due to certain overload capacity, but the negative error of the electric energy meter is large, and with the increase of overload current, the negative error becomes larger and larger, that is, the electric energy meter becomes slower and slower.
3 conclusion
When the current transformer is overloaded for a short time, the current transformer can operate normally, and the secondary current exceeds the maximum rated current of the electric energy meter, resulting in large negative error of the electric energy meter and slow measurement.
Because the overload of current transformer has certain concealment, it is impossible to verify a correction coefficient when making up the power. Therefore, it is necessary to reasonably configure current transformer, strengthen the supervision and inspection of power load and reduce power loss.
The current sensor can detect both AC and DC. Measuring AC and DC flows out of AC and DC. External power supply is required.
The current transformer can only measure AC. when the output signal is AC 5A or ac1a, there is no need for external power supply.
Ordinary current transformer can only measure AC current. If the AC current transformer is open circuit, it will produce high voltage. In serious cases, it will break through the line insulation and produce electric shock, threatening personnel safety. Therefore, the secondary side cannot be open circuit during use, and the secondary side must be short circuited during disassembly.
Hall current sensor, DC and AC current can be measured. The secondary side does not have to be short circuited when the Hall current sensor is removed.
Ordinary current transformer can be measured directly without power supply. The Hall current sensor needs plus or minus 15V or plus or minus 12V working power supply.
Current transformers generally output AC 5 A or AC 1 A. The Hall current sensor generally outputs a small signal of 50mA or 100mA or 5V, and can also output 4-20mA, which is convenient for automatic control.
In terms of measurement accuracy, Hall current sensor is also better than current transformer. The measured current distortion, multiple harmonics and non sine wave have a great impact on the accuracy of current transformer measurement results. The influence on Hall current sensor is much smaller.
In the measurement frequency range, the Hall current sensor is much wider than the current transformer.
In addition, the response time of Hall current sensor is faster than that of current transformer.
The common current sensor / transformer converts a large current into a small current with the same frequency and phase for measurement or isolation. According to different transformation principles, there are current sensors / Transformers Based on electromagnetic induction principle, Hall effect and fluxgate.
Current transformer is similar to a transformer with few primary turns and more secondary turns. Ideally, the current ratio of the primary stage is inversely proportional to the turns ratio, and the current conversion ratio is marked with the rated current of the primary stage, such as "300A / 5A", indicating that the output current is 5A when the measured current is rated at 300A. Due to the leakage inductance and resistance of the primary coil, as well as the nonlinear excitation current and core magnetization curve, the ratio error and phase error of the transformer will be caused. The accuracy of transformer used for metering and billing is generally level 0.1 1. According to the principle of transformer, it can not measure DC current. It is usually designed for power frequency measurement. The accuracy is the parameter under power frequency. The bandwidth is narrow, so it is not suitable for harmonic analysis and non sinusoidal measurement. When using current transformer, be sure not to open the secondary circuit, otherwise high voltage will be generated, endangering personal and equipment safety.
Current transformer
The iron core in the current clamp is divided into two parts to avoid disconnecting the measured circuit. It is very easy to measure and widely used. There are two types based on the principle of electromagnetic induction and Hall effect.
The current clamp based on the principle of electromagnetic induction is the same as the transformer. The iron core is divided into two parts. When closed, the two parts of the iron core need to be closely combined. Some current clamps are connected to the secondary resistance, and the output is voltage signal, and the output without internal resistance is current signal. Affected by the closure degree of two iron cores, the accuracy of current clamp is usually worse than that of transformer. Similarly, the current clamp based on electromagnetic induction can only measure AC.
The current clamp based on Hall effect processes an air gap in the iron core to place the hall element. Hall element is used to measure the magnetic induction intensity in the air gap. According to different control modes, there are two types: open-loop and closed-loop. The open-loop hall type uses a Hall element with good linearity, and the output voltage of the hall element is directly proportional to the measured current. The closed-loop hall type uses zero flux technology, and there is a compensation coil on the iron core. When the primary measured current generates magnetic flux in the iron core, the hall element detects the magnetic induction intensity in the iron core and converts the error voltage into current to drive the compensation coil through negative feedback to offset the magnetic flux in the iron core. Finally, the measured current is consistent with the magnetic flux generated by the compensation coil in the opposite direction, By measuring the current of the compensation coil, the measured current can be converted according to the turns ratio.
Both open-loop and closed-loop Hall current clamp can measure DC and AC. The open-loop hall is affected by the nonlinearity of the iron core and the temperature characteristics of the hall element, the accuracy and linearity are poor, but the cost is low. The closed-loop hall has less dependence on the linearity of the hall element, and the iron core works under zero flux, so the accuracy is higher than that of the open-loop. However, the current clamp has the problem that the closing degree of the movable iron core is not ideal. There is almost no one equal to or better than 0.1%, and being able to achieve 1% is a very high index. The hall element needs to provide working voltage, so both current clamps need to supply power, and the closed-loop hall needs to drive the compensation coil, which consumes more power.
Current clamp
Generally, like Hall current clamp, there are two types of open-loop and closed-loop hall, and the output is current or voltage signal. Due to the closed form, the accuracy is higher than that of the same type of current clamp.
In addition, there are current sensors using fluxgate technology, whose accuracy is better than 0.05% or even 12ppm, but this type of sensor is very expensive and fragile. Once the sensor is not powered in use, the measured current will cause damage to the sensor.
Closed mouth current sensor
4 Connection between current sensor / transformer and power analyzer
The current measurement of pa5000 power analyzer has two groups of ports: direct input and sensor input. When the output of the used sensor / transformer is current signal, it is connected to the current direct input port of pa5000. Correctly set the "CT" proportion coefficient of PA according to the used sensor / transformer, such as 300A / 5A, then CT = 300A / 5A = 60.
When the output of the current sensor / current transformer used is a voltage signal, connect it to the external sensor port (BNC interface) of pa5000, and correctly set the "sensor ratio" of PA according to the sensor / transformer used, with the unit of MV / A. if the current clamp parameter is converted from 1000A to 1V, the sensor ratio is set to 1mV / A.
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.
The selection of voltage transformer shall be based on the primary wiring mode (including Y-type connection and V-type connection), the power consumption grade of primary voltage, the capacity requirements of secondary line and the requirements for transformation accuracy.Current transformers are mainly assembled in different switchgear. Due to different models of current transformers, current transformers often have great differences in structure (including copper bar lapping form, iron core, shape, etc. and dynamic and thermal stability tolerance). For example, the current transformers equipped with the central handcart cabinet are mostly lzzbj9 or AS12, but there are many models equipped with fixed cabinets.There are many differences between voltage transformers of the same model and specification. Generally, there are many combinations mainly due to different transformation ratio, capacity of secondary coil, accuracy of protection coil and metering coil. When selecting the transformation ratio of the current transformer, the actual load rated current should be obtained first. This current is preferably 65% - 85% of the measurement range of the current transformer.
Key points of current transformer selection1. The rated current (primary side) shall be 1.0 1.3 times of the load current during normal operation of the line;2. The rated voltage shall be 0.5kV or 0.66kv;
3. Pay attention to the accuracy level. If it is used for measurement, the accuracy level shall be 0.5 or 0.2; If the load current changes greatly, or the load current is 30% lower than the rated current at the primary side of the current transformer during normal operation, class 0.5 shall be selected;4. Determine the transformation ratio and turns as required;5. Model and specification selection: determine the transformation ratio according to the primary load current of the power supply line, and then determine the model according to the actual installation situation;
6. For the selection of rated capacity, the secondary rated capacity of current transformer shall be greater than the actual secondary load, and the actual secondary load shall be 25 100% of the secondary rated capacity. The capacity determines the load impedance at the secondary side, and the load impedance affects the measurement or control accuracy. The load impedance is mainly affected by the coil resistance, reactance and wiring contact resistance of measuring instrument and relay Influence of secondary connection conductor resistance.The key points of current transformer selection are as follows: Standard value of rated primary and secondary current
The standard value of rated primary current of current transformer for power system is shown in the following table, and the value in brackets is only limited to old products:The rated secondary current standard is 1a and 5A.In addition to the numbers listed in the above table, the standard values of rated primary current of instrument current transformer include: 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.5, 0.75, 1.5, 2, 2.5, 3, 4, 7.5, 8, 25, 80, 120 and 250. The standard value of rated secondary current is 0.5A in addition to 1a and 5A.
Accuracy levelThe accuracy levels of domestic current transformers include: 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 3 and 5. Near the rated current, the error of current transformers at all levels can also be roughly considered as corresponding to level 0.01... Level 5, and the allowable error is 0.01%... 5%.The accuracy level of instrument current transformer is generally higher than 0.5. Current transformer for power system: used for measuring
The amount of electric energy shall not be lower than grade 0.5. For large electric energy, grade 0.2 or 0.1 is required; For measuring current and power, level 1 is generally used; Levels 3 and 5 are only used to monitor the current, that is, roughly observe the current. Level 5 (there are 10 for old products) is only used on the bushing of high voltage circuit breaker.The accuracy level of current transformer for protection is represented by 5p and 10p, which is also equivalent to its allowable error of 5% and10%
Rated capacity / rated loadThe load of current transformer refers to the total impedance of electrical instruments and connecting wires connected to the secondary of current transformer,It includes the impedance of these instruments or relays, as well as the resistance of connecting wires and the contact resistance of connecting points.
The relationship between secondary rated capacity Sn and rated load Zn can be expressed by the following formula: SN = i2n2 * ZnAccording to the standard, the standard value of rated secondary load of current transformer shall be 5, 10, 15, 20, 30, 40, 50, 60, 80 or 100VA. For current transformers with rated secondary current of 5A, the impedance of rated load is: 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.6, 2.0, 2.4, 3.2 or 4.0 . Rated voltage
The rated voltage of current transformer refers to the line voltage on the line connected to the primary winding. Current transformer primaryThe winding is connected in series on the line, so the rated voltage of the current transformer is not the voltage at both ends of the primary winding of the current transformer, but the insulation voltage of the primary winding of the current transformer to the secondary winding and ground. Therefore, the rated voltage of the current transformer only shows the insulation strength of the current transformer, and has no direct relationship with the capacity of the current transformer.
Current transformer is widely used in our work and life. Its function is mainly to convert the primary current with large value into the secondary current with small value through a certain transformation ratio. It has the functions of protection, measurement and so on. In daily life, it is easy to have open circuit fault of current transformer, so it is very important to know how to deal with it.
First, what is the secondary open circuit fault? If the secondary open circuit occurs, all the primary current will be used for excitation to seriously saturate the iron core. At the same time, the alternating magnetic flux will induce a very high voltage on the secondary coil, with a peak value of thousands of volts or even tens of thousands of volts. Obviously, such a high voltage acting on the secondary coil and secondary circuit will have disastrous consequences for personal safety and equipment safety, and the insulation of the coil will be burned out due to overheating. Therefore, the regulation stipulates that open circuit is strictly prohibited during the operation of current transformer. So in case of secondary open circuit fault, how should we check and deal with it?
Judge whether there is a secondary open circuit fault through the following points1. Whether the indication of loop instrument is abnormal, which is generally displayed as lower or zero. The current circuit of the measuring meter is open. In case of slow or no rotation of the meter. If there is no indication from the meter sometimes, it indicates that the current transformer may be in a semi open circuit state (poor contact). We should be vigilant.2. Whether the current transformer body has noise, whether the vibration is uniform, and whether there are serious heating, smoking and other phenomena. Of course, these phenomena are not obvious when the load is small.
3. Check whether the secondary circuit terminal and element wire head of current transformer are discharged and ignited.4. Misoperation or refusal of relay protection. This situation can be found and handled in case of false tripping or skip tripping. The watt hour meter, relay, etc. are burnt out by smoke. The secondary open circuit of current transformer will be caused by the burnout of reactive power meter and watt hour meter, transmitter of telecontrol device and relay of protection device.Next, how to deal with the secondary open circuit fault
1. If the secondary open circuit of current transformer is found, first distinguish which group of current circuit the fault belongs to, the phase of the open circuit and whether it has an impact on the protection, report to the dispatcher and remove the protection that may misoperate.2. Try to reduce the primary load current. If the current transformer is seriously damaged, transfer the load and cut off the power for inspection and treatment. If the state-owned bypass is used, the bypass power supply can be used to ensure the power supply.2. Try to short-circuit the current transformer for the second time on the nearest test terminal, and then check and deal with the open circuit point. During the short-circuit, the special short-circuit wiring for short-circuit shall be used. The short-circuit shall be proper and reliable, and the winding of fuse or general conductor is prohibited. According to the drawing.
3. Pay attention to the phenomenon of short-circuit. If there is spark during short-circuit, the short-circuit is effective. The fault point is in the circuit below the short-circuit contact, which can be further found. If there is no spark during short-circuit, the short-circuit may be invalid. The fault point may be in the circuit before the short contact. The short contact can be changed forward point by point to narrow the range to clarify the open point
5. Within the fault range, check the terminals and components prone to failure, and check that there are parts of the circuit that have been touched during operation. For the faults detected, those that can be handled by themselves can be handled immediately, and then the exited protection can be put into operation. If the open circuit point is on the wiring terminal of the transformer body, the power shall be cut off for treatment. In case of failure that cannot be handled or found out by yourself, report to the superior and send professional maintenance personnel to check and deal with it. At this time, the current transformer shall be short circuited for the second time, or the load shall be transferred and the power shall be cut off.
Obviously, how to deal with the open circuit problem of current transformer not only involves basic composition knowledge, but also needs to understand the causes of faults and have professional maintenance experience
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