The evaluation of the role of the shield winding in the current transformer for generator protection during short-circuit fault (Shenyang Transformer Co., Ltd., Shenyang 110025, Liaoning).

China Classification: TM452: A 1 Lulu Due to the large rated current of large generators, the phase-to-phase distance is small, the magnetic field of each phase has a strong effect on the toroidal current transformer on the adjacent phase casing, so magnetic shielding measures must be taken. One of the measures is to add a magnetic shield winding to the transformer. For the shielded winding design of the current transformer, the shielding effect and heating problem of the winding under the rated current are usually considered, and the shielding effect at the time of combining current is rarely considered. In this paper, the shielding effect of the shield winding on the protection current transformer is analyzed in the case of short-circuit fault.

2 Shielding principle of shielded windings The influence of the adjacent phase current on the core of the toroidal transformer can be expressed. A circle whose center is the center of the adjacent phase conductor and whose radius is 0 is regarded as the main path of the stray flux of the adjacent phase current. The magnetic flux enters the iron core from the point, and the two passes through the semicircle on both sides of the iron core to meet the i point.

The adjacent phase current stray flux and the shield winding shield winding are composed of two pairs of shield windings arranged orthogonally on the circumference of the core. Each pair of windings is connected in reverse polarity, and each winding occupies 1/4 of the circumference of the core. . Only a pair of windings that shield the adjacent phase currents in the horizontal position are drawn, which are equivalent to two open magnetic low precision current transformers that are shorted to each other. According to the principle of transformer, the first ampere is the magnetic phase difference and the adjacent phase current A is in phase. The secondary ampere is basically equal to the opposite direction, which just balances the external magnetic potential and plays the role of shielding the core.

The current is called the balancing current, and the phase difference from the adjacent phase current A is TT, which can be calculated by the following formula.

3 The residual stray flux balancing current generated by the shield winding must have a certain amount of residual stray flux present, which depends on the impedance of the shield winding. If the small leakage reactance is neglected, the magnetic flux is proportional to the winding resistance, and the phase difference between the magnetic flux and the balancing current is /2 (hysteresis). The residual stray flux density at rated current is as follows. Calculated: balance current shield winding phase subjacent rated current, the AS transformer core cross-sectional area, m2 / * rated frequency, the main magnetic flux Hz 4 rated current of the secondary winding of the secondary winding of the main magnetic flux density is calculated using the formula Ba : Total balance current and residual stray flux current transformer total balance current residual stray flux residual stray flux and main flux minimum phase difference aA mutual inductance 0.5VT'0.5VTtt/6B mutual sensor VT' The V3"B*tt/2C mutual inductance 0.5VTl, 0.5VTBtt/6 is visible. Under normal conditions, the total balance current and residual stray flux of the B mutual inductance are the largest.

N2n secondary winding turns Z, secondary load impedance (resistance, reactance Zb), D due to generator primary current transformer primary current of several thousand amps, secondary winding turns, winding resistance than secondary load impedance The value is relatively large. Regardless of the load power factor, the impedance angle of the secondary circuit of the transformer is small. The phase difference between the secondary current of the transformer main flux 52 and the secondary current /2 (lag primary current /,) is close to TT/. 2, the following assumes that the phase difference is TT/2. The main magnetic flux of the local maximum magnetic flux density ring current transformer in the core is closed by the circumference of the core, and the remaining stray magnetic flux is closed by the partial core and the air circuit, and the two kinds of magnetic fluxes The phasor sum on a portion of the core having the smallest phase difference from each other forms a local maximum magnetic flux of the core.

The three-phase current transformer installed on the generator bushing has stray flux in the core which is related to the arrangement of the bushing. In this paper, only the three casings of A, B and C are arranged in a single arrangement, as shown. For the double arrangement of the six sleeves at the line end and the neutral point, the main difference is that the return conductor current is increased, and the analysis can be analogized.

The three-phase conductor arrangement and the three-phase currents of the A and C-phase stray flux generators are symmetrical, and the structural parameters of the three-phase group current transformers are identical. In the three-phase current (/A, /B, /c), since the A and C phases are on the left and right sides of the B phase, the current/A sense is opposite to the stray flux generated in the B mutual inductor core ( ), the balance current / / phasor difference generated by the B mutual inductance, constitutes the total balance current / ' of the transformer, which leads to /B, the phase difference is tt / 2, see. Because the B and C phases are on the same side of the A phase, the current/B, /c are in the same direction as the stray flux generated in the A mutual inductor core, and they are balanced current generated in the A mutual inductance /, / China Internet The basic situation is to publish the “Statistical Report on the Development of China's Internet Network”. The statistics of the report show that in recent years, China's Internet has developed rapidly. By January 2002, the total number of Internet computers in China had reached 12.54 million, including 2.34 million dedicated computers and 10.2 million dial-up computers. The number of Internet users reached 33.7 million, an increase of 50% over the same period of last year. Among the users who are online, according to the industry distribution, users belonging to scientific research and education accounted for 10.2%, and manufacturing accounted for 8.2%; according to occupational distribution, professional and technical personnel accounted for 19.9. %, clerks and related personnel accounted for 17.2%; according to gender, males accounted for 60%, females accounted for 40%; by geographical distribution, the top ten were Guangdong aged and below accounted for 79.9%, and over 35 years old accounted for 20.1%; according to academic distribution Bachelor's degree or above accounted for 58.9%, and junior college and below accounted for 41.1%; the impact on the work, it is considered that the impact is very large, accounting for 20.6%, and the impact is relatively large, accounting for 36.4%.

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