When the transformer runs in the linear region of the core, the excitation impedance zm is very large, the excitation current is very small, and the recombination error is also small. If a short circuit occurs, the primary current is very large, the core is saturated, the zm drops sharply, the excitation current increases sharply, and the secondary current is distorted. In addition, this process is also closely related to the secondary load, under the same primary current, the larger the secondary load, the easier it is to saturate, the deeper the saturation, the more serious the waveform distortion, and the greater the recombination error. The purpose of the CT characteristic test is to calculate the composite error under various operating conditions through the excitation characteristics and related parameters, to ensure that the composite error meets the requirements, and finally to ensure the correct operation of relay protection and the safe operation of the secondary instrument.
Method:Calculate the secondary limit induced potential, the nameplate stipulates that the current transformer can adapt to the limit value of the operating state, the value of the induced potential E2 of the secondary winding in this state is equal to the secondary limit induced potential, check the excitation characteristics data chart, find the excitation current under the corresponding quadratic limit induced potential, and judge whether the composite error meets the requirements according to the excitation current. CT for general protection, and the composite error is less than 10, which means that the composite error meets the requirements.
Step 1(1) Measure the DC resistance, and the DC resistance of the transformer winding can be directly measured through the CTP series transformer tester (the CT winding impedance is approximately taken as the straight resistance value when calculating).
2) Calculate the rated load value of the CT nameplate capacity, rated load = rated capacity (rated current * rated current).
3) Calculate the quadratic limit induced potential, the quadratic limit electromotive force = CT accurate limit coefficient (ALF) * rated secondary current * (rated load + secondary winding impedance).
4) Find the excitation current value under the corresponding quadratic limit induced potential on the excitation curve.
5) According to the excitation current, judge whether the composite error meets the requirements. Composite error = excitation current (rated current * accurate limit factor).
For example:The parameters of a CT are: rated current ratio of 200A 5A, 10P10 level, rated load 20VA. The measurement of the CT winding secondary direct resistance is 02. The excitation test results are shown in the following table to determine whether the CT composite error meets the requirements.
A:
1) Calculate the vector sum of the secondary direct resistance and the rated secondary impedance:
2) Calculate the quadratic limit induced potential:
3) Check the table to obtain the excitation current under the secondary limit induced potential:
4) Calculate CT composite error:
The composite error of the CT is calculated to be 062% < 10%, from which the CT composite error can be obtained to meet the requirements.