Causes and compensation methods for affecting transformer error - News - Global IC Trade Starts Here.

inductance

First, the cause of the impact of TA error

(1) Influence of primary current on error: In the actual operation of TA, when the primary current increases from 5% to 120% Ib, the magnetic flux density in the core is also proportionally increased. The ratio difference and the angular difference decrease with the increase of the current, but the change of the ratio difference is small, and the variation of the angular difference is large.

When the primary current exceeds 120% Ib, the running TA will cause magnetic saturation and cause a serious negative overshoot. When the primary current is less than 5% Ib, the magnetic field strength h of the TA is small, and the magnetic flux density is smaller, thus causing a positive error.

(2) The effect of secondary load on error:

TA specifies the rated upper and lower limit load values ​​during the design and manufacture process, that is, the connected secondary load must be

The accurate operation of the TA is guaranteed within the allowable range. The error of TA is proportional to the magnitude of the secondary load. When the secondary load increases, the magnetic flux density of the transformer core also increases, causing the ratio and angular error to change to a negative value. When the secondary load connected by the TA is less than the lower limit value, the error changes to the positive overshoot.

(3) The effect of the number of turns on the error:

The error of TA is inversely proportional to the square of the number of turns of the secondary winding. When the number of turns of the secondary winding is increased, the error of the TA can be reduced, but as the number of turns of the secondary winding increases, the internal impedance of the secondary winding also gradually increases, which limits the error to some extent. decline. Therefore, under the premise of ensuring that the accuracy of the TA meets the requirements, the number of winding turns should be as small as possible.

(4) The effect of the average magnetic path length on the error:

The error of the transformer is inversely proportional to the average magnetic path length. The smaller the core length of the core, the smaller the relative error of the TA. As the core increases, the magnetic path lengthens, and the error increases. Therefore, the TA should select the appropriate core size and control the magnetic path length within the specified range.

(5) The influence of the core cross section on the error:

The error of TA is inversely proportional to the cross-sectional area of ​​the core. The TA error can be reduced when the core cross-sectional area is increased. In fact, as the cross-sectional area of ​​the core increases, the magnetic permeability of the core decreases, and the average magnetic path length of the core increases, which causes the internal impedance of the secondary coil to increase. Therefore, the height and width of the iron core should be selected when designing and manufacturing the TA. For the laminated core, the height h is generally slightly larger than the width b. For the ring type iron core, since the inner diameter is smaller than the outer diameter, the selection of the iron core is 1.5b ≤ h ≤ 2b.

(6) Influence of core material on error:

The error of TA is inversely proportional to the magnetic permeability of the core. The better the material selected for the core, the higher the magnetic permeability and the smaller the size of the core. Therefore, in order to reduce the overall size of the TA, the choice of high quality core material is the main way.

2 Reasons for affecting TV error

(1) The effect of a voltage on the error:

In the case where the secondary load is constant, the change in the primary voltage affects the ratio and angular difference of the TV. When the primary voltage is greater than the rated value, the TV error will change in the positive direction.

(2) The effect of secondary load on error:

The secondary load of the TV is proportional to the error. When the secondary load increases or decreases, the ratio and angular difference associated with it change, so the secondary negative wearing of the TV cannot exceed the rated amount of the given accurate level.

(3) Influence of winding impedance on error:

The accuracy of the TV is inversely proportional to the impedance of the primary and secondary windings. As the impedance of the winding increases, the accuracy of the TV decreases. In addition, the magnetizing current of the TV also has a certain influence on the accuracy.

3 Compensation of transformer error

(1) Compensation for TA error: 1 补偿 number compensation: We know from I1W1=I2W2= two equations, and the current of TA is inversely proportional to the number of turns. If the secondary winding is less than the rated number of turns W by WX匝, WX is the number of turns to be compensated. When the number of turns is compensated, the secondary side current will increase inversely to achieve the purpose of compensation error. 2 Magnetic shunt compensation: In the double core compensation, if only the compensation parameter is increased, the error of TA can be ideally compensated at 10% Ib. However, in order to compensate for the constant value, it is necessary to consider the compensation of the magnetic shunt, that is, to reduce the wearing area of ​​the auxiliary core. 3 Magnetic shunt short circuit compensation: The angular difference of TA is generally larger than 10% Ib. When using magnetic shunt compensation, it compensates the difference of TA and compensates for the angular difference. effect.

(2) Compensation of TV error: 1 In order to reduce the impedance and reactance of the winding, the cross-sectional area of ​​the primary and secondary coil wires of TV should be selected according to 0.4~0.8A/mm2. Since the resistance and impedance are inversely proportional to the length of each turn of the coil, it is better to use a circular core for minimizing the length of each turn. 2 In order to reduce the TV error, the magnetic flux density in the core should be relatively small. Choosing high-quality core materials, the connection between the cores must be compact, which can reduce TV error and no-load current and core loss. 3 In order to improve the accuracy of the TV difference, we can adjust the number of turns of the primary winding. To improve the accuracy of the angular difference, we can adjust the wire diameter of the primary winding. When the ratio difference of the TV is a negative overshoot, the number of turns of the primary winding can be appropriately reduced so that the secondary load is equal to half of the rated capacity at a given accurate level, and the difference ratio is close to zero. When the angular difference is positively out of tolerance, the wire diameter of the primary winding can be appropriately reduced (for example, 0.23 mm2 instead of 0.21 mm2 wire diameter).