Influence of Current Transformer Overload on Metering
A 35KV wholesale user of our bureau has a metering current transformer model of lcz-35-100 / 5 and an energy meter model of dss331 and 3 100V3 1.5(6)A Due to continuous drought, the user's own hydropower output decreased. The current reached 162a at peak load in December 2001, which seriously exceeded the rated current of current transformer, resulting in the loss rate of 100m line as high as 19.6%. Therefore, we test and analyze the measurement error of current transformer when it is overloaded.
1. Influence of serious overload of current transformer on current transformer body
(1) Test object: current transformer, model: lcz-35-100 / 5; Secondary rated load: 50VA, accuracy level: 0.2; Manufacturer: Chongqing high voltage electrical appliance factory, No. 069.
Power factor during test: 0.8.
(2) When the load current is 5% to 165% of the rated current of the current transformer (current transformer regulation stipulates that 5%, 20%, 100% and 120% of the rated current are selected for the verification load point), the error (ratio difference and angle difference) is shown in Table 1.
Table 1
(3) It can be seen from the test data that when the load current is within 120% of the rated current, the greater the load, the smaller the error of current transformer; When the load current is between 120% 165% of the rated current, the error of current transformer increases with the increase of load current, but gradually tends to balance within the allowable range. This is because in the design of current transformer, the minimum saturation multiple determining the overload capacity of current transformer is generally 2 3 times of rated current; With the increasingly mature and advanced manufacturing technology of current transformers, some current transformers can withstand dozens of times of rated current in a short time.
2 Influence of current transformer overload on electric energy meter
When the current transformer is overloaded, it will not be saturated in a certain range, the secondary current will increase proportionally, and the supporting electric energy meter will also be overloaded.
Power factor during test: 1.0.
(2) In order to match the current transformer test, the load current is selected to be 10% 533% of the calibration current IB of the electric energy meter (jjg596-1999 regulation stipulates that when the power factor is 1.0, the verification points are 10%, 50%, 100% and 400% of the calibration current), and the error is shown in Table 2.
Table 2
The error r% curve when the above test error exceeds the rated maximum current Imax (400% IB) of the electric energy meter is shown in Figure 1.
Fig. 1 error curve of electric energy meter under overload
It can be seen that when the actual current is greater than the maximum rated current Imax of the electric energy meter, the electric energy meter will not burn due to certain overload capacity, but the negative error of the electric energy meter is large, and with the increase of overload current, the negative error becomes larger and larger, that is, the electric energy meter becomes slower and slower.
3 conclusion
When the current transformer is overloaded for a short time, the current transformer can operate normally, and the secondary current exceeds the maximum rated current of the electric energy meter, resulting in large negative error of the electric energy meter and slow measurement.
Because the overload of current transformer has certain concealment, it is impossible to verify a correction coefficient when making up the power. Therefore, it is necessary to reasonably configure current transformer, strengthen the supervision and inspection of power load and reduce power loss.