Interference of return conductor on composite error of current transformer and countermeasures to reduce interference
May 30, 2024
The studied return conductor refers to the "return conductor" actually existing in the structure of the current transformer. The composite error average error of the secondary winding of the current transformer often exceeds the design value or even exceeds the standard limit value. The main reason is is the interference from the return conductor. In order to reduce the interference from the return conductor of the voltage transformer structure itself, the secondary turns can be unevenly distributed, and additional shielding of the winding can be used if necessary.
The generation of the return conductor and the interference of the electromagnetic field caused by the current in the return conductor to the secondary winding composite error (calculation omitted)
When there is a return conductor in the current transformer, the primary electromagnetic field is no longer a uniform inscribed circle family, and local electromagnetic fields may be concentrated, destroying the uniformity of the core electromagnetic field, thereby causing interference to the composite error.
The U-shaped primary winding has two conductors. If the ring-shaped secondary winding is placed on one conductor, the other conductor and the ring conductor are the return conductors. Because the magnetic flux generated by the current in the return conductor passes through the core, the magnetic density of the core near the return conductor increases, preventing the magnetic density of the core on the return conductor side from decreasing, causing the magnetic density of each section of the core to be different.
Since most of the magnetic flux paths caused by this external electromagnetic field are through gas or other non-magnetic substances, the equivalent circuit of the core is only a small section, so its magnetic induction intensity is directly related to the magnetic permeability of gas or other non-magnetic substances, and the magnetic induction The wire is linked to the secondary winding, which increases the secondary leakage reactance under normal operating conditions. When the current transformer operates at or above the rated voltage, the magnetization curve is close to a linear shape. In this case, the error of the measurement level and the average error of the rated voltage error of the maintenance level are basically consistent with the calculated values.
It can be seen from the above figure and calculation that under the conditions of the secondary load of the current transformer and its main parameters, when the current transformer operates at or above the rated voltage, the core flux density is in the parallel segment of the magnetized curve. , the magnetic permeability and magnetic flux density increase linearly, so the average error of the secondary winding rated voltage error is basically consistent with the measured value.
But when the primary current increases to a certain value, that is, under the condition of large overcurrent, as the magnetic flux density enters the saturation state, the magnetic flux density increases, the magnetic permeability decreases rapidly, and the error expands rapidly, which is very important. It may cause the error to exceed the limit value. In this case, as the magnetic flux density of the core near the return conductor side increases, that is, the local electromagnetic field is very strong, so there will be a significant difference between the average error and the measured value. When the specific primary current is less than the rated current, the magnetic flux density and magnetic permeability decrease, but the magnetic permeability decreases faster at this time, so there will also be a significant difference between the average error and the measured value.
Countermeasures to reduce return conductor interference
1. Use a method in which the secondary line turns are not evenly distributed.
Affected by the return conductor, if the turns of the secondary winding are evenly distributed, the core magnetic flux will be uneven, the composite error will be very large, and the difference between the average error and the basic measured value is very large. The use of non-uniformly distributed secondary windings can improve the magnetic flux distribution in the core, making it almost uniform, with small composite errors, and the average error is close to the measured value, thus reducing the interference from the return conductor.
The relative position of the return conductor and the core inside the voltage transformer is fixed. If a magnetic potential opposite to the return conductor's magnetic potential can be created close to the core, the return conductor's magnetic potential can be offset. According to the basic Biot-Savart law, the level of the electromagnetic field caused by the current at a certain point in the indoor space is inversely proportional to the distance between the two. It is known that the closer to the core, the smaller the required diamagnetic potential is.
The method of using non-uniform distribution of secondary wire turns can achieve this effect. The originally evenly distributed secondary wire turns are wound more on the core segment close to the return conductor side and less on the core segment away from the return conductor side. At this time, the secondary magnetic potential of the core section returning to the conductor side exceeds the magnetic potential when the normal distribution is uniform, and the resulting magnetic flux reverses the direction of the interference flux in this section of the core and returns to the conductor, thus canceling part of its effect. .
2. When installing, make sure that the middle turns of the secondary winding are close and sparse.
The secondary windings will also interfere with each other, which is likely to increase the error. The direction of the magnetic flux lines of the electromagnetic field between the windings is the same, which can cause accumulation. If the electromagnetic field distribution between the two windings is uneven, the accumulation result is likely to deviate from the uniform distribution of the secondary winding core magnetic flux, resulting in an increase in the average error and the measured value error. big.
In order to avoid this situation, the secondary winding should be installed so that the side with dense turns matches the dense side of the other secondary winding, and the thinner side matches the thinner side. There is a large gap between the transmission lines near two adjacent windings, which will cause large magnetic leakage. Paper circles can be used between the windings to reduce magnetic leakage interference.
3. When there is a TPY winding in the secondary winding, the method of lifting the paper coil can be used to reduce the interference of magnetic leakage.
Because the TPY winding has magnetic density, the magnetic flux leakage caused here is very large. In order to better prevent the interference of magnetic flux leakage, a paper ring with low magnetic permeability can be used to reduce this kind of interference. The paper ring can be raised in the middle of the secondary winding to reduce leakage. Magnetic interference. In addition, the magnetic potential at many magnetic density points of the TPY winding is poor and causes magnetic flux leakage. The installation should be at 90° with the center line of the U-shaped primary winding to reduce and balance the interference of magnetic flux leakage.
4. Use additional shielded windings
The main function of shielding the winding is to protect the ring core in the current transformer from interference from external stray magnetic flux. Shielded windings are multiple paired windings of other coil inductors on the voltage transformer core. Generally, two pairs of shielded windings with the same number of coil turns and opposite coil inductance positions are used. Each pair of windings is reversely rotated. Sex is tied together.
Their coil turns, winding direction and interface mode cause a leakage electromagnetic field in the high current transformer to be the same size as the stray electromagnetic field (the electromagnetic field generated in the core from the external current) but in the opposite direction to offset the stray electromagnetic field. Interference from electromagnetic fields. The higher section of the stray magnetic flux shields the winding. The potential difference of the magnetic induction is higher than that of the other section, shielding the equilibrium current flowing through the winding. This current demagnetizes the higher section of stray magnetic flux, and demagnetizes the stray magnetic flux in the lower section. The magnetization is increased so that the distance is not too close to the balanced level, so it is also called balanced winding.
The design of the shielded winding must comply with the following principles: the maximum magnetic induction intensity in the ring core is the least, and the shielding effect is best; the current in the shielded winding must be kept at a small level; the temperature of the surface winding part should be minimized ; The current distribution in the entire shielded winding must be kept uniform; the number of turns of the shielded winding coil should be kept as small as possible.
Result
The interference of the return conductor of the current transformer is the direct cause of the composite error of the measurement level and the maintenance level. The existence of the return conductor partially concentrates the primary electromagnetic field, so that the core magnetic flux distribution of the basic structure winding is uneven, and the composite type The error characteristics have greatly deteriorated. In order to reduce the interference of the return conductor on the composite error, it is necessary to find a way to make the core magnetic flux of the winding evenly distributed.
