The secondary load of a current transformer directly affects its correct operation. Generally speaking, the larger the secondary load, the greater the error of the transformer. As long as the secondary load does not exceed the manufacturer's set value, the manufacturer should guarantee that the error produced by the transformer is within its accuracy class or 10% error curve range. Therefore, during the use of a current transformer, it is necessary to understand its rated secondary load and actual secondary load. Only when the actual secondary load is less than the rated secondary load can the error meet the requirements. However, in actual use, sometimes due to changes in secondary equipment or inadequate design considerations, the secondary load temporarily increases, causing the actual secondary load to exceed the rated secondary load of the transformer; or the measured secondary load is greater than the rated secondary load, at which point the error value of the current transformer will exceed the manufacturer's specified value. When the error of the current transformer exceeds the manufacturer's specified value, it will bring adverse effects to secondary equipment such as relay protection and metering devices. Below, we will discuss remedial measures to be taken when the error of the current transformer exceeds the manufacturer's specified value.
1. Increase the cross-sectional area of the secondary cable or reduce the length of the cable
Increasing the cross-sectional area of the secondary cable in the current circuit or reducing the length of the cable actually reduces the impedance of the secondary circuit conductors and reduces the secondary load. When the impedance and installation position of the relay protection device or metering device cannot be changed, it is usually possible to choose to increase the cross-section of the secondary cable to reduce the secondary load; when the installation position of the relay protection device or metering device can be changed, it is usually chosen to reduce the length of the secondary cable to reduce the secondary load. For example, for the energy metering meter on a 10kV outgoing line in a substation, if the meter was originally installed in the control room, when the error of the current transformer exceeds the manufacturer's specified value, the meter can be directly installed on the high-voltage cabinet from the control room, thereby reducing the impedance of the circuit.
2. Connect the secondary coils of spare current transformers in series to double the load
Connect the secondary coils of two current transformers of the same phase with the same transformation ratio and identical characteristics in series. When the secondary coils of current transformers are connected in series, the secondary terminal voltage of each current transformer is:
U2=（ZJ+2ZL）I2/ZI2/2
Where ZJ——relay impedance
ZJ——secondary connection wire impedance
Z——total impedance of the circuit
The load of each current transformer is:
Z′=U2sub/I2=（ZJ+2ZJ）/2=Z/2
The value of Z′ is half of that when using one current transformer. Therefore, in order to meet the needs of the secondary load, the secondary coils of two current transformers can be connected in series. After series connection, the transformation ratio remains unchanged, and the capacity doubles. This wiring method is often used in the wiring of the bushing current transformer for transformer differential protection.
3. Increase the transformation ratio of the current transformer or use a current transformer with a secondary rated current of 1A
Increasing the transformation ratio of the current transformer, that is, increasing the multiple of m10, to reduce the saturation of the iron core. According to the principle that the loss of the line is proportional to the square of the current, it can be known that the loss of the line becomes smaller, and the output impedance becomes larger, so the load capacity is strengthened. Especially when using a current transformer with a secondary current of IA, under the condition of unchanged line length, the load capacity is 25 times the original, and the effect is very obvious.
4. Reduce the secondary load
Try to select relays with a large setting current, because the coil of a relay with a large setting current has a thicker wire diameter and fewer turns, so the impedance is correspondingly smaller; or change the series connection of the relay coil to a parallel connection, because the impedance of the series connection is greater than that of the parallel connection; or use a microcomputer protection device to replace the electromagnetic relay. For the metering circuit, an all-electronic multifunctional energy meter can be used to replace the induction energy meter, and at the same time, taking advantage of the multifunctional characteristics of the all-electronic multifunctional energy meter, one meter can be used for multiple purposes, so that the original one outgoing line with four energy meters (forward and reverse) is changed to one energy meter, greatly reducing the secondary load.
5. Select a current transformer with high volt-ampere characteristics
Select a current transformer with a higher volt-ampere characteristic curve. Because the iron core cross-section of this type of current transformer is larger, it can increase the saturation multiple of the iron core, thereby reducing the error.
In summary, when the error of the current transformer exceeds the manufacturer's specified value, different handling methods can be adopted according to different situations and combined with the actual site conditions, to ensure that the error of the current transformer is within the specified range, so as to ensure the correct operation of the relay protection device and the accuracy of the metering device.
