Electromagnetic leakage protection characteristics robust design

 

 Analysis of factors affecting the robustness of leakage protection characteristics

Influence of compensation capacitance dispersion on the robustness of protection characteristics

In order to analyze the influence of the dispersion of compensation capacitor C1 on the leakage action value, equation (2) can be obtained by transforming

(4)

Then the absolute sensitivity of C1 is

(5)

The relative sensitivity of C1 is

(6)

From equations (5) and (6), sensitivity curves with C1 under different inductance values can be obtained, as shown in Figure 12.

As can be seen from Figure 12, when the compensation capacitor is a resonant capacitor, the sensitivity is the lowest, that is, the action value is the most stable, but the action time is the longest. As can be seen from FIG. 12a, the absolute sensitivity is axisymmetrically C1=C0, and the change of excitation inductance L0 does not affect the change trend of absolute sensitivity. As can be seen from FIG. 12b, when C1 < C0, its relative sensitivity is generally smaller than when C1 > C0. The larger the excitation inductance, the smaller the relative sensitivity of C1. When L0≥250 H, the relative sensitivity S < 1.0 when C1 < C0, then the capacitance tolerance has little influence on the action value. Therefore, when the excitation induction is as large as possible and the compensation capacitor is smaller than the resonant capacitor, the relative sensitivity of the compensation capacitor can be reduced, that is, the influence of the dispersion of the compensation capacitor on the protection characteristics can be reduced.

Fig.12 Relationship between sensitivity and C1 under different L0

    Since the action time changes in a ladder-like manner with the increase of compensation capacitance, the action time changes little on the same ladder. As can be seen from Figure 10, the influence of C1 tolerance on the action time can be reduced when C1 value is set in the middle of the ladder.

3.1.2 Influence of dispersion of magnetic parameters on robustness of protection characteristics

As can be seen from FIG. 11, when Br > 0.2T and Bs > 0.7T, the change curves of R1, C1 relationship and action time are relatively dense, and the action value and action time are less affected by the dispersion of Br. When Hc changes, the distance between R1 and C1 curves is large. Only when the value of compensating capacitor C1 is close to the value of resonant capacitor C0, the change of Hc has little effect on the action value. Under different magnetic parameters Br, Hc and Bs, the leakage action value and action time are simulated and calculated, and the results are shown in Figure 13.

The intersection points in FIG. 13a, 13c and 13e are the design points of protection characteristics under the parameters in Table 2. It can be seen that the change of Bs has the least impact on the action value and action time, while the change of Hc has the most impact. As can be seen from FIG. 13b, 13d and 13f, the change trend of action time is basically consistent with the action value, and the action time is relatively short when the capacitance is small. If the action value is close to IDn, the action time changes greatly, even exceeding the standard requirement of 300 ms. Therefore, in parameter design, it should be ensured that the action value is less affected by the dispersion of magnetic parameters, and there is a certain margin between the action value of rated residual current IDn.

Fig.13 Influence of magnetic parameters of residual current transformer on IDd and tC

    In order to further clarify the influence of Bs on action value and action time, the changes of action value and action time with Br and Hc under different Bs were simulated and analyzed, as shown in FIG. 14.

It can be seen from FIG. 14 that under different Bs, the change curves of action value and action time with Br and Hc are very close, which proves that the change of Bs has the least influence on action value and action time. Therefore, in the design of magnetic properties parameters, Bs can be determined first, and then the influence of Br and Hc dispersion on the action value and action time can be analyzed. In this paper, Bs is determined to be 1.0T.

Compared with FIG. 13c and FIG. 14c, the influence of Hc change on action value and action time decreases after Br is increased. Under the conditions of Bs= 1.0T and Br= 0.2T, the R1 and C1 matching curves and action time curves of simulation analysis when Hc is 0.6, 0.7 and 0.8A /m, respectively, are shown in Figure 15. As can be seen from FIG. 15, the resonant capacitance values of the three groups are relatively close. When the compensation capacitor C1=18 nF, its values are close to the resonant capacitance values and are located in the gentle interval of the curve.

Figure 14 Influence of magnetic parameters on action value and action time under different Bs

 

Fig.15 Relationship curves of R1 and C1 and tC under different Hc

    In summary, when the value of compensating capacitor C1 is determined, the design of Br and Hc makes the compensating capacitor C1 close to the resonant capacitor, which can reduce the influence of magnetic parameters on the action value and action time, and thus improve the robustness of leakage protection characteristics.

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