One of 30 practical technical questions and answer

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30 practical technical questions and answers of low voltage current transformer (one)

1 What is the meaning of the rated current ratio on the nameplate of the current transformer

answer: rated current ratio refers to the ratio of primary rated current to secondary rated current. Usually expressed as an irreducible fraction. The so-called rated current is that under this current, the transformer can operate for a long time without heat damage

2. What is current transformer? This experiment shows that the animal body has good biocompatibility with the stent made of stainless steel wire. After the stent is placed, there is no inflammatory and necrotic change in the blood vessel wall. The accuracy level

answer: there is a certain error in the transformation current of current transformer, and the accuracy level is specified according to the transformation ratio error generated by current transformer under rated working conditions. Current transformers above level 0.1 are mainly used for testing, precision measurement or as a standard to calibrate low-level transformers, and can also be used to calibrate instruments in conjunction with standard instruments, which are often called standard current transformers; Try to make clothing materials have new characteristics: it allows heat radiation, air and water vapor to connect electrical measuring instruments through level 0.2 and 0.5 Changchuan; Level 3 and below current transformers are mainly connected to some relay protection devices and control equipment

3. How is the polarity mark of current transformer regulated

answer: the polarity signs include plus polarity and minus polarity. The commonly used current transformers are generally minus polarity, that is, when the primary current flows from L1 to L2. When, the secondary current flows from K1 end to K2 through the external circuit. L1 and K1, L2 and K2 are homopolar ends respectively

4. What is the meaning of current transformer rated capacity

answer: the rated capacity of current transformer is the apparent power consumed when the rated secondary current I2E passes through the rated load z2e, that is, s2e=

generally, i2e=5a, so s2e=25z2e. In the use of current transformer, its accuracy can be guaranteed only when the total impedance of secondary connection and instrument current coil does not exceed the rated capacity (volt ampere or Ohm) specified on the nameplate

5. What is current transformer error

answer: due to the influence of the structure and material performance of the current transformer iron core, the current transformer has an exciting current of 0, which makes it produce errors

from the vector diagram of current transformer primary current 1 and converted secondary current 2 (as shown in Figure 2), the converted secondary current rotates 180 ˚ The latter 2, compared with the primary current 1, is not only unequal in size, but also does not coincide in phase, that is, there are two kinds of errors, which are called specific difference (ratio error) and angular difference (phase error)

6. What is the meaning of 10% multiple marked on the nameplate of current transformer

answer: according to the regulations, the numerical error of current transformer used for relay protection device shall not exceed 10%, and the two angle error shall not exceed 7 ˚。

10% multiple is the multiple of the primary current to its rated value when the current error of the current transformer is 10% under the specified secondary load and any power factor. The 10% multiple is generally only related to relay protection devices

7. What are the main factors affecting the error of current transformer

answer: (1) influence of primary current. When the primary current of current transformer is very small, the error will increase; When the primary current is greater than the rated current for a long time, the error will also increase. Therefore, generally, the primary measurement current should be greater than 25% of the rated current of the transformer and less than 120%

(2) influence of secondary load When the secondary load of current transformer increases, the error (, ratio difference and poor close cooperation between Jiaoshan Southeast University, Shandong University of science and technology and scientific research institutes) also increases Therefore, the secondary load should not exceed its rated value (volt ampere or Ohm) in use

in addition, the power frequency and the residual magnetism of the iron core also affect the transformer error

8. Why can't the secondary of current transformer be open circuit

answer: when the secondary coil of the current transformer is closed, the magnetic potential of the primary and secondary windings cancel each other, the magnetic flux in the iron core is very small, and the induced potential on both sides is very low, so it will not affect the work of the load. If the secondary winding is open circuit, the magnetic potential of the primary winding will sharply increase the magnetic flux of the iron core, and the number of turns of the secondary winding is more, so the induced potential of the secondary system group is very high, which will break down the insulation, damage the equipment and endanger the safety of the measuring personnel

9. How to judge the polarity of current transformer

answer: the polarity of the current transformer should be verified before it is put into operation. The commonly used DC verification wiring is shown in Figure 3

when the switch DK is closed, the pointer of the milliammeter or multimeter deflects towards the + terminal, which indicates that the secondary terminal K1 of the transformer connected to the milliammeter or multimeter + terminal and the primary terminal L1 of the transformer connected to the battery turn are of the same polarity. If the pointer of the milliammeter or multimeter deflects towards one terminal at the moment when the switch DK is closed, it indicates that the polarity of L1 and L2 is opposite

10. Why install current transformers for measurement

answer: when the current exceeds a certain value, the watt hour meter and other measuring instruments should be connected to the circuit through the current transformer, so that the measuring instruments and staff can avoid direct contact with the high current circuit, so as to ensure the safety of the instruments and personnel. In terms of instrument manufacturing, it is also easy to achieve standardization, and the instrument transformer can be used to arbitrarily expand the measurement range, and the accuracy of the instrument is also easy to improve

11. How to choose current transformer

answer: in addition to considering the place of use, the current transformer for measurement should also be selected according to the following parameters:

(1) selection of rated voltage. To make the line voltage UX of the measured line adapt to the rated voltage ue of the transformer, u UX is required

(2) selection of rated transformation ratio. The rated primary current ile should be selected according to the maximum working current iz passing through the current transformer for a long time. Ile should be 1. It is better to make the current transformer operate near the rated current, so that the measurement is more accurate

(3) selection of accuracy level: generally, current transformers with accuracy level not lower than 0.5 are selected for watt hour meters and all measuring instruments

(4) selection of rated capacity. In order to accurately calculate the secondary load of current transformer, it must be below the rated capacity (impedance), and its error will not exceed the given accuracy level

12 what are the commonly used low-voltage current transformers

answer: lqg-0.5, indoor device coil type, suitable for 500 volts and below AC

LMZ. 5、LMZJ. 5. Lmm1 0.5 is an indoor bus tree 6 Safety protection device: Grease casting insulation such as pattern breaking shutdown, overload protection, beam limit position protection, overcurrent, overvoltage, overspeed protection, etc. it is suitable for electric energy measurement and relay protection in AC circuits of 500 volts and below, especially for rural lines with large seasonal load changes, and has strong moisture resistance

l current transformer m bus type

q coil type G improved type

z cast insulation J increased capacity

13 What are the operating characteristics of current transformers

answer: (1) the primary current does not change with the change of secondary load

(2) the primary current changes depending on the change of system load

(3) the power consumed by the secondary line changes with the change of the secondary resistance

(4) the secondary measurement impedance is very small, which is approximate to the short-circuit state

(5) the secondary rated current is 5 amps

14. What kinds of wiring are commonly used for current transformers

answer: (L) a current transformer, as shown in Figure 4, is mainly used to measure the current of one phase in the three power devices of load balance

(2) star connection, as shown in Figure 5, is mainly used to measure the three-phase current of three-phase four wire device of three-phase power device with unbalanced load

(3) incomplete star wiring, as shown in Figure 6, is mainly used to measure and display the three-phase current in three-phase three wire power devices

(4) two phase current difference wiring, as shown in Figure 7, should be used in 6-10 kV neutral ungrounded systems to protect high-voltage motors with small capacity

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