1, an overview of the
Protective current transformer is installed in the power system used in a loop when fault occurs in the fault current transformation of the electrical current to relay protection system equipment, so the protective current transformer is the most important work or its significance lies in the power system short circuit fault occurs can run normally, so as to guarantee the relay protection system can detect the fault information and promptly cut off the fault current, to protect the safety of the power system, the main equipment. At the same time, the protection of current transformer should ensure that the current transformer should be reliably run in the non-fault condition, which can not output the fault current signal and lead to the fault of the relay protection system.
At present our country electric power system for the main test project contains protective current transformer excitation characteristic test (volt-ampere characteristic curve), check change ratio, polarity, and map error curve, but no more in-depth analysis of these data, without considering the effects of parameters of transformer transformer secondary loop load, a lot of field test and don't understand our workers are responsible for maintenance of equipment designed for the protection of the meaning of these trials using current transformer.
2。 Protection of current transformer working principle
2.1 working principle of electromagnetic current transformer
Standard system in our country are basically identical with the IEC standard system, the most standard in the electric power system and the corresponding standard number in the IEC standards are equivalent, mutual - 1 is in the IEC standards for electromagnetic current transformer constraints and requirements, corresponding to our national standards GB1208, defined in mutual - 1 level of protective current transformer accuracy have 5 p, 10 p, 5 pr, pr and 10 PX five, installed in the power system protection in our country with a current transformer, the most common type of accurate level is 5 p, 10 p and PX。
Protection with current transformer and measuring principle of the current transformer used in exactly the same, the difference lies in the protection with emphasis on the performance of the current transformer is its ability to withstand short-circuit current, and measuring with emphasis on the performance of the current transformer is the current transmission error size. All the structure of the electromagnetic current transformer is shown in figure 1, secondary coil and a coil around the same closed on the core of a general 1 turn or number of turns, a number of turns is far less than the number of turns of the secondary, the number of turns ratio generally is equal to the ratio of its current.
The magnetic induction strength formula of the current transformer core is
The H is the magnetic field intensity, determined by the current I and the number of turns in the coil
B is the intensity of magnetic induction field in the core (different from magnetic field strength H),
U is the permeability of core, and the non-linear characteristic is shown in FIG. 2
And since u a non-linear when the iron core of the magnetic induction intensity B reaches a certain value, no longer increases with the increase of the H (saturated permeability after similar to vacuum magnetic permeability, magnetic effect almost can be ignored), transformer core into saturated state at this time.
3. Parameter evaluation
The two main parameters for the parameter evaluation of 5PR/10PR are ALF and the residual magnetic coefficient Kr, in which ALF is defined and evaluated in the same way as 5P/ 10P. 5 pr / 10 pr remanence coefficient Kr refers to the current transformer excitation saturation, excitation current in zero magnetic flux ratio of saturated magnetic flux in the core, figure 3 is HYVA - 405 is an exciting current measurement by the cycle of current transformer core flux curve along with the change of current hysteresis loop. In GB1208 and iec 60044-1, the residual magnetic coefficient of 5PR/10PR protective current transformer shall not exceed 10% to ensure that these transformers have low residual magnetic flux.
Protection with the main purpose of the current transformer test is to confirm that current transformer parameters meet the requirements of the protection system in the system short-circuit fault current transformer is not failure, in the normal operation of system does not lead to misoperation relay protection system, and the error of the transformer delivery to meet the standard requirements。 In the various parameters test of transformer, the secondary load is crucial impact protection with current transformer performance parameters, so the secondary circuit of current transformer load parameters must be considered and relay protection system, the influence of the secondary circuit have to confirm the actual link load less than the rating of the brand identity。