The accuracy of the current transformer in measuring the current

Measurement accuracy levels are divided into (for special purposes, each accuracy level specifies the corresponding maximum permissible error limit (ratio difference and phase difference The words represent current errors. The standard accuracy level of the current transformer for protection is 5P and represents the percentage of the maximum allowable composite error of the accuracy level at the rated accuracy limit primary current (GB1208-1997). The rated accuracy limit primary current refers to the maximum primary current that the transformer can meet the composite error requirement.

Composite error refers to the root-mean-squared value of the instantaneous value of the primary current and the instantaneous value of the secondary current multiplied by the ratio at steady state when the positive signs of the primary and secondary currents are consistent with the provisions of the terminal marking. The accurate limit coefficient refers to the ratio of the rated accurate limit primary current to the rated primary current, and the standard accurate limit coefficient is , the smaller the general coefficient, the greater the secondary load. When the primary current is large enough, the core will be saturated and will not accurately reflect the primary current, and the accurate limit factor is the representative of this characteristic.

Application of DaniSense zero flux current sensors for high-precision current measurement.

The state divides the electric energy metering device into five categories according to the amount of measured electric energy and the importance of the measurement object

For all kinds of electric energy metering devices, the accuracy level of the current transformer is not less than:

class not less than or; class not less than or; class not less than; class not less than; class not less than;

If the ammeter is connected to the secondary side of the transformer, the secondary impedance of the transformer will increase, and the measurement accuracy will decrease, that is, the meter will "go" slowly.

Whether there is an impact mainly depends on the following two situations: 1. If the primary rated current of the current transformer is too large, the actual current flowing through the watt-hour meter is small, as long as the actual circuit is not lower than the "start" current value of the watt-hour meter, the measurement accuracy will not be affected. Roll the sail for example:

The actual rated current is about 45 A, and the commonly used 150 5 current transformer is selected, and the rate is 30 times. When fully loaded (45 A), the secondary current is 45 A 30 times A, and the measurement is still accurate. 2. If the primary rated current of the current transformer is too small, it is easy to cause the magnetic saturation of the iron core of the current transformer when the current is large, and the measurement error is increased, and it is also easy to generate large heat.

For example, the actual rated current is about 200 A Choose the commonly used 150 5 current transformer, which belongs to overload operation, and it is easy to cause the magnetic saturation of the iron core of the current transformer when the hail is fully loaded, and the measurement error increases, and it is easy to generate large heat.

Summary. Hello, too large or too small current transformer has an impact on the measurement accuracy.

Hello, too large or too small current transformer has an impact on the measurement accuracy.

The relevant information is as follows, the impact of the current transformer on the measurement accuracy is as follows, 1. The primary rated current of the current transformer is too large, and the actual current flowing through the watt-hour meter is small, as long as the actual circuit is not lower than the "forest key start" current value of the watt-hour meter, the measurement accuracy will not be affected.

2. If the primary rated current of the current transformer is selected to be too small, it is easy to cause the magnetic saturation of the core of the current transformer when the current is large, and the error of Zheng Qiaozhou is increased, and it is also easy to generate large heat.

There is a parameter on the nameplate of the current transformer, as shown in the icon, it is 400 5, as an example, when the main line current is 400, the output current of the secondary winding of the transformer is 5 amps, and the transformer transformation ratio is 400 5 80, when the measured current of the secondary winding of the transformer is 1 ampere, the primary current is 80, and the transformer conversion ratio is 80 to 1. This is a provision that if there are 2 wires in the middle, the ratio is 40 to 1

There is actually a formula for this: number of primary turns x primary current = number of secondary turns x secondary current.

Answer: If the current transformer ratio is determined purely from the rated current, the current transformer ratio can be selected as a multiple of the rated current; For the current transformer used for electric energy metering, the change ratio can be selected a little smaller, such as times, or even a little smaller, and the current transformer with "S" level can be selected for places where the load change is relatively large and there is often a small load; For the current transformer used for protection, the change ratio can be selected to be a little larger, such as 2 times, or even larger.

For the analog instrument, it is required that the pointer is about 75% of the instrument panel during normal operation current.

The commonly used current transformer secondary rated current is 5A, but 1A is also a national standard. When the secondary rated current of the selected current transformer is 5A, the ammeter is also selected as the rated current of 5A.

The selection of the current transformer is more complicated, not only to see the rated current, but also to verify its dynamic and thermal stability after being connected to the system, and analyze its capacity, accuracy and secondary load and other factors.

The current transformer current ratio is inversely proportional to the number of turns of the coil!

Does too large or too small the current transformer affect the metering accuracy? If the primary rated current of no current transformer is too large, the actual current flowing through the watt-hour meter will be small, and as long as the actual circuit is not lower than the "start" current value of the watt-hour meter, the metering accuracy will not be affected. If the primary rated dust spike bypass current of the current transformer is too small, it is easy to cause the magnetic saturation of the core of the current transformer when the current is large, and the measurement error is increased and the heat is easy to produce a large number of liters.

1. The influence of primary current.

When the primary current of the precision 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, in general, the one-time measurement current should be greater than 25% of the rated current of the transformer and less than 120%. Therefore, the primary side current of the transformer should be selected according to the rated range of the product.

Second, the impact of secondary load.

When the secondary load of the precision current transformer increases, the error (ratio difference and angular difference) also increases. Therefore, secondary loads should not exceed their rated value (volt-ampere or ohms) in use. The secondary load capacity of a precision current transformer depends on the rated capacity of the current transformer.

The rated capacity of the precision current transformer is the apparent power consumed by the rated secondary current I2E through the rated load Z2E, that is, S2E=I2E2XZ2E. In the production process of the transformer, the core and cross-sectional area of the current transformer will affect the rated capacity of the current transformer, which is not discussed here. The linear relationship between the input current of the current transformer and 1:

In the use of current transformers, the accuracy can only be guaranteed if the total impedance of the secondary wiring and the instrument current coil does not exceed the rated capacity (volt-ampere or ohm) specified on the nameplate. The rated load capacity of a current transformer with 10A 10mA and 1:1000 turns is generally not more than 200 ohms.

When the load is greater than the load capacity of the current transformer, the input current may be saturated before reaching the full scale, resulting in distortion of the transformer conversion.

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<> solution: Z=R+JXL=40+J40=40 Chaozheng2 45°( KVL:I(phasor) Z+us(phasor)=UAB(phasor).

i(phasor) = [50 45°-50 -45°] 40 2 45°=(25 2+J25 2-25 2+J25 2) 40 2 45°=50 2 90° 40 2 45°=.

So, the ammeter reading collapsed.

U (phasor) = i (phasor) z = .

u-φi=90°-45°=45°。u=50√2。

The power meter reads p=uicos=50 2.