Discussion on the selection of accuracy level of current transformer for protection
current transformer for protection (TA) mainly cooperates with relay protection device. In case of short-circuit overload and other faults, it provides signals to relay device to cut off the fault circuit and protect the safety of power system. Its working condition is completely different from that of the transformer used for measurement. The normal primary current working range of the latter can have appropriate accuracy. When passing the fault short-circuit current, it is hoped that the transformer will be saturated as soon as possible, so as to protect the measuring instrument from short-circuit current damage. The former works only when the current is several times or dozens of times larger than the normal current, and its error (current and phase error) is required to be within the range of error curve, while composite error is used when checking current error and phase difference at the same time
when the primary current I1 of TA for protection is small, the secondary current I2 changes linearly; When I1 increases to a certain value, the magnetic density in the transformer core is very high. Due to the nonlinearity of ferromagnetic materials, the high-order harmonic content in the excitation current I0 is very large, and the waveform is sharp, which is far from the sine wave. Even if the friction experiment set by IL is an ideal sine wave, I2 is not sinusoidal
non sinusoidal wavelets cannot be analyzed by phasor diagram, and composite error (conceptual analysis) needs to be used, which makes I0 increase rapidly, which is equivalent to that part of I1 cannot be converted into I2, and I2 and I1 are no longer in direct proportion, thus increasing TA error
when the relay protection action is caused by the short-circuit fault of the power system, the short-circuit current I is large, which is generally more than 10 times the rated current, which increases the error and endangers the sensitivity and selectivity of the protection device
in addition, in principle, Ta itself is a special transformer, and all transformers have the requirements to operate under rated load. Therefore, if the TA secondary side load exceeds its rated secondary load value, it will also increase its error
2 solution ideas
as mentioned above, Ta error is inevitable, and its size is related to the excitation characteristics of TA core and secondary side load
to control this error, it is necessary to deal with the relationship between the maximum fault I at the location of TA, the ratio of this current to rated I1, the ratio of rated current and rated secondary load. Therefore, it is necessary to accurately understand the concept of accuracy level and its related accuracy level limit, rated current ratio and rated load. To solve this problem, it is necessary to select the appropriate accuracy level according to the actual situation of the substation
for TA used for protection, at the same time, the accuracy level is nominal as the percentage of the maximum allowable composite error of this level under the rated accuracy limit I1, The letter "P" after it indicates the protection [1, which is actually an artificially specified error level requirement made by TA. The accuracy limit coefficient refers to the ratio of i1max to rated I1 that can meet the composite error requirements. The rated current ratio refers to the ratio of rated I1 to I2. The rated load is the secondary load value  on which the accuracy level of the mutual inductor is determined.
in the early national standard current transformer (GBL) It is specified in that the accuracy level of TA protection is level B and D. When selecting the TA accuracy level for protection, the 10% error curve should be verified, To ensure that the current error does not exceed the specified value in case of short circuit [2. The current version stipulates that the accuracy level is 5p and 10p, which means that the composite error limit at the rated accuracy limit I1 is 5% and 10% respectively; the standard value series of accuracy limit coefficient are 5, 10, 15, 20, 30, etc., which means that under short-circuit fault, if the multiple of short-circuit current I compared with rated I1 is less than this value, the error is controlled within the accuracy level range.
therefore, two conclusions are obtained: 1) the accuracy level of protection TA must be selected at the same time The coefficient of definite limit value, such as 5p20 and 1200/5A, means I ≯ 20 times the rated I1, that is ≯ 20 × When 1200 = within the range of 24 Ka, the composite error is ≯ 5%; 2) By selecting two directions, the rated current ratio can be selected according to the given ilmax, secondary load value and 10% error curve and the accurate limit coefficient; According to the given i1max, rated current ratio and 10% error curve, you can also select the accurate limit value coefficient as of July 2017, and select the rated secondary load and secondary cable section according to the 10% error curve
in steady-state operation, TA secondary load should meet the requirements of 10% error curve. As long as the load allowed by the 10% error curve of TA secondary actual load, its measurement error is within 10%. The larger the secondary load is, the two or more raw materials of the core will collide with each other in the mixing chamber through these nozzles, and the easier it is to saturate
3 example description
110kV bus i1max25 Ka, TA secondary side load 40 VA in a 110kV substation
first, according to the requirements of relay protection devices, the steady-state error of TA for protection is ≯ 10%[3, and the accuracy level is selected as 10p according to the cost of engineering construction control equipment. According to the example, the curve in Figure 2, the accuracy limit coefficient is 24. According to ilmax, 25kA is divided by the accuracy limit coefficient to get 1042a. Considering the accuracy limit coefficient, we should try our best to