Abstract The influence of adjacent phases on the electric field signal of the three-phase system of a 500kV optical current and voltage sensor (OMU) without a voltage divider capacitor is discussed through simulation calculation. The calculation shows that the adjacent phase influence in the system without an equalizing ring is less than 0.1%. When an equalizing ring is required for insulation, the error is larger, and compensation measures are needed. Peak value, base value, and waveform compensation were also calculated. Keywords 500kV, OMU signal, adjacent phase compensation 0 Introduction Combined electric field type optical current and voltage sensor (OMU) is a promising high-voltage testing device [1]. Previous research results [2] show that the setting of an equalizing ring has a good effect on improving insulation performance, but reduces the sensitivity of the electric field signal at the sensing point, which is not conducive to overcoming the test error caused by changes in the environmental electric field. In order to overcome the influence of thermal expansion and contraction of the insulating cylinder, it is better to fix the voltage sensor on the upper plate (the upper plate is then fixed on the insulating cylinder) than to fix the sensor directly on the insulating cylinder or the lower base, and the closer to the upper plate, the higher the sensitivity. This paper studies the electric field signal problem of a three-phase system (near the upper electrode), discussing the influence of the electric field signal of adjacent two-phase voltage sensor heads and its compensation measures. 1. Influence of the Three-Phase System Figure 1 shows the site layout. The upper electrode is a cylinder (with built-in current sensor) with radius r1 = 0.2m and height d2 = 0.4m, and a potential of 500kV; the insulating cylinder (with built-in voltage sensor) has a height d1 = 4m, and the support base is a grounded cylinder with a height d3 = 4m and a potential of 0V; the equipotential ring has radii r4 = 0.4m and r3 = 0.04m, with d4 = 0 when set horizontally and d = 0.2m when moved downwards. The calculation method is the simulated charge method. We selected point charges and ring-shaped line charges as simulated charges. The model does not consider the influence of the insulating medium (i.e., the insulating cylinder medium). Although this affects the calculation results, it does not affect the underlying laws and facilitates the calculation. The calculation error is based on the relative error of the potential at the check point on the boundary, and is controlled below 0.3%. The results of the single-phase axis electric field (RMS) distribution (see Figure 2) show that the field distribution is the most non-uniform when there is no equalizing ring. [IMG=Site Layout Diagram]/uploadpic/THESIS/2008/1/2008010710293533792G.jpg[/IMG] Figure 1 Site Layout Diagram [IMG=Electric Field Strength (RMS) Distribution]/uploadpic/THESIS/2008/1/20080107102941741626.jpg[/IMG] Figure 2 Electric Field Strength (RMS) Distribution Diagram 3 shows a three-phase system (phase distance 7.5 m), with phase A having an initial phase of 0°, phase B 120°, and phase C -120°. The sensor head can be installed on any one phase or all three phases; here, the three-phase structure is assumed to be identical. Under the first-order approximation condition (i.e., the influence of the three-phase system is the superposition of the single-phase case), it is assumed that the sensing head's induction of electric field vectors in different directions is based on the magnitude of the modulus. The influence of the instantaneous electric field signal (within one period) of two adjacent phases relative to the phase where the sensing head is located when the equalizing ring moves downward was calculated. The absolute error is set as the modulus of the instantaneous electric field intensity of the single-phase system minus the modulus of the synthesized instantaneous electric field intensity vector formed by the three-phase system. The results are shown in Figure 4 (sensor head on the side) and Figure 5 (sensor head in the middle). The figures show that the error is larger when the sensing head is in the middle than when it is on the side, but the waveform is completely symmetrical when the sensing head is in the middle, while it is asymmetrical when it is on the side. [IMG=Setup of Three-Phase System]/uploadpic/THESIS/2008/1/20080107102950875819.jpg[/IMG] Figure 3: Setup of Three-Phase System [IMG=Absolute Error of Instantaneous Electric Field Intensity (Modulus) of Side Phase]/uploadpic/THESIS/2008/1/2008010710295672047E.jpg[/IMG] Figure 4: Absolute Error of Instantaneous Electric Field Intensity (Modulus) of Side Phase with Equalizing Ring Downward Movement d4＝0.2 m, H＝3.98 m 2. Comparison of Errors of Different Setting Methods of Equalizing Ring in Three-Phase System Due to the insulation requirements of the 500kV system, the equalizing ring has different setting methods, and the sensor head has different setting positions. To compare the error distribution and propose a compensation basis, we calculated the relative error distribution (E_single - E_tri)/E_single for the peak values ​​(modulus at 90° and 270°) and the absolute error distribution (E_single - E_tri) for the base values ​​(modulus at 0°, 180°, and 360°) under different conditions, as shown in Figures 6, 7, 8, and 9 (the three-phase equalization ring is set up in the same way for each case). [IMG=Absolute error of instantaneous electric field strength (modulus) of the middle phase]/uploadpic/THESIS/2008/1/20080107103001192259.jpg[/IMG] Figure 5 Absolute error of instantaneous electric field strength (modulus) of the middle phase when the equalization ring is moved down d4 = 0.2 m, H = 3.98 m. Obviously, regardless of whether the sensor head is set on the side or in the middle, the error is the smallest (below 0.1%) when there is no equalization ring, which meets the engineering requirements and no compensation measures are needed. This is because without an equalizing ring, the electric field distribution is more concentrated, resulting in a lower field strength at distances and thus less interference to adjacent phases. With an equalizing ring, the electric field distribution is relatively uniform, leading to a higher field strength at distances and stronger interference. Comparing with Figure 2, it can be seen that interference is greater where the field strength is lower along the axis (both peak relative error and base absolute error are larger), thus requiring compensation measures. The compensation method depends on the data processing section and can employ peak value compensation or base value compensation separately. Waveform compensation can also be used according to engineering requirements. [IMG=Relative Error Distribution of Side-Phase Peak Values ​​(Modes)]/uploadpic/THESIS/2008/1/2008010710301074678M.jpg[/IMG] Figure 6: Relative Error Distribution of Side-Phase Peak Values ​​(Modes) [IMG=Absolute Error Distribution of Side-Phase Base Values ​​(Modes)]/uploadpic/THESIS/2008/1/2008010710302753749T.jpg[/IMG] Figure 7: Absolute Error Distribution of Side-Phase Base Values ​​(Modes) [IMG=Relative Error Distribution of Intermediate Phase Peak Values ​​(Modes)]/uploadpic/THESIS/2008/1/20080107103039442025.jpg[/IMG] Figure 8 Relative error distribution of intermediate phase peak (mode) [IMG=Absolute error distribution of intermediate phase base value (mode)]/uploadpic/THESIS/2008/1/2008010710315285061N.jpg[/IMG] Figure 9 Absolute error distribution of intermediate phase base value (mode) Figure 10 shows a typical sensor head arrangement, without a voltage equalization ring but with the upper electrode base plate recessed, and the sensor head is set inside. Calculations show that when the sensor head is set on the edge phase, the single-phase peak electric field strength is 350.557 kV/m, the three-phase composite peak (mode) electric field strength is 350.047 kV/m, the relative error is 0.14%, the single-phase base electric field strength is 0 kV/m, and the three-phase composite base (mode) electric field strength is 2.363 kV/m. When the sensor head is set on the middle phase, the single-phase peak electric field strength is 350.557 kV/m, the three-phase composite peak (mode) electric field strength is 349.853 kV/m, the relative error is 0.2%, the single-phase base electric field strength is 0 kV/m, and the three-phase composite base (mode) electric field strength is 5.738 kV/m. [IMG=A sensor head arrangement]/uploadpic/THESIS/2008/1/2008010710315833168S.jpg[/IMG] Figure 10 A sensor head arrangement 3 Conclusion The adjacent phase influence of the system without equalizing ring is less than 0.1%; when equalizing ring is set for insulation, the error is large and compensation measures (peak value, base value, waveform compensation) need to be taken. The actual compensation measures need to be determined in combination with the response curve of the sensor head to the electric field signal in different directions. References 1. Rogers A J. Optical measurement of current and voltage on power system. IEEE Electr Power Applications, 1979, 2(4):120 2. Ye Qizheng et al. Research on sampling method of electric field signal of 500 kV capacitor OMU without voltage divider. High Voltage Engineering, 1999, 25(1):46