Design of a Capacitor Voltage Divider with Integrated Peaking Capacitor

Author： GOZ Electric Time：2024-11-03 09:35:06 Read：12

Structural design of capacitor voltage divider

Due to the existence of high-voltage leads, low-frequency, slow-decaying oscillation waves appear at the top of the voltage divider, causing serious distortion of the measured waveform. Therefore, a damping resistor needs to be connected in series in the lead to reduce the impact of the oscillation wave on the measurement, while the resistance and lead inductance will increase the response time of the measurement system, making the measurement of nanosecond pulse voltage inaccurate." At the same time, if it is a discrete capacitor voltage divider, the high-voltage arm and the low-voltage arm need to be connected with a cable. The wave process on this cable will superimpose high-frequency oscillations on the voltage of the low-voltage arm. Therefore, it is necessary to reduce the cable length as much as possible or change the structure of the capacitor voltage divider. In order to solve the above problems, this paper designs a coupled capacitor voltage divider. The voltage divider is mainly composed of an 8-layer peaking capacitor, a copper-clad film and a compensation resistor. The peaking capacitor is the basis of the capacitor voltage divider, combined with the copper-clad film to form an integrated structure, and the resistor compensation method is used to achieve The purpose of self-integration. As shown in the figure. Among them, the copper-clad film is composed of a metal film and an insulating film, and it is the key component of the capacitive voltage divider. In order to prevent the insulating film from flashing over along the surface, the area of the metal film should be slightly smaller than that of the insulating film! In addition, the insulating film should be made of materials with high relative dielectric constant, good toughness and good voltage resistance. Therefore, the length and width of the metal film in this article are 10mm smaller than the insulating film, and the insulating film is a polyimide film. The outermost metal ring of the peaking capacitor is the grounding terminal, and the copper-clad film is attached between the outermost insulating layer of the peaking capacitor and the ground electrode of the outermost metal ring, and the other structures of the peaking capacitor remain unchanged. The capacitance formed by the metal part of the copper-clad film dielectric and the multilayer film dielectric near the inner diameter side of the peaking capacitor is the high-voltage arm of the capacitive voltage divider, and the copper-clad film dielectric capacitor is used as the capacitor The low-voltage arm of the voltage divider. In order to avoid the influence of stray inductance on the high-frequency response of the voltage divider, the compensation resistor uses a non-inductive carbon core resistor, one end of which is connected to the copper film signal lead-out end of the low-voltage arm, and the other end is connected to the coaxial cable connector (BNC) to lead out the measurement signal.

Analysis and discussion of the calibration results of the capacitor voltage divider

From the high-voltage online calibration results, it can be seen that the voltage division ratio of the two probes has a certain error with the low-voltage square wave calibration and the theoretical voltage division ratio. In addition to the reading error caused by the low-frequency response, the main reason is the design structure of the peaking capacitor itself. In this article, the MV-level peaking capacitor is composed of a series of concentric capacitors in series. The entire capacitor is composed of 8 layers of insulating medium and 8 equally spaced metal rings. In order to make the voltage of the peaking capacitor evenly distributed, the capacitance value of each capacitor is required to be consistent. At the same time, in order to keep the capacitance value and field strength of each part consistent, each metal ring is designed to have different lengths and diameters. In order to optimize the electric field distribution at both ends of the metal ring, fillets are used to reduce the local field strength. The electric field distribution under a voltage of 1MV is calculated using finite element simulation software. It can be seen that the average field strength of each layer of capacitor does not exceed 85kVmm, but the field strength of the air gap between the metal ring and the insulating medium will be relatively high. Therefore, when the field strength reaches a certain value under a higher voltage, the air gap will have local discharge or even breakdown and form a discharge non-equilibrium plasma", which will increase the capacitance value of the peaking capacitor to a certain extent. In order to consider the influence of the plasma formed under high voltage on the structural capacitance, the plasma air gap between the electrodes is extended and the capacitance simulation calculation is performed using finite element simulation software, and the capacitance of each layer of the 8-layer capacitor can be obtained. The capacitance of the peaking capacitor will increase accordingly under the influence of high-voltage plasma. Therefore, the original simulation circuit is re-assigned for simulation. The capacitance of each part is also calculated according to the formula, where C is 115.9pF, C and C are 137.8pF and 735.0pF respectively, and other parameters remain unchanged. From the simulation, it can be obtained that the voltage divider ratio of the 1# probe is 10495, and the voltage divider ratio of the 2# probe is 15150, which is 5.20% and 0.02% different from the voltage divider ratio calibrated in the experiment. Therefore, from the above analysis, it can be seen that the change of the peaking capacitor capacitance under high voltage is one of the main factors affecting its voltage divider ratio. Since the working voltage of the peaking capacitor usually reaches hundreds of kV to MV, the voltage divider ratio calibrated online at high voltage is usually used as the voltage divider ratio of the voltage divider in actual measurement. In order to explore the changing trend of the probe voltage divider ratio under the action of higher voltage levels, the calibration experiment was carried out when the Marx charging voltage was higher. Considering the insulation tolerance performance of the peaking capacitor at a higher voltage level, a gas output switch and a 400Ω load resistor need to be connected after the peaking capacitor to achieve the truncation of the pulse signal at the first wave peak. Schematic diagram of the experimental platform. From the calibration results, it can be seen that the difference in the peaking capacitor voltage measured by the two probes of the peaking capacitor integrated capacitor divider does not change much, and does not change linearly with the increase of voltage. This may be due to the small air gap between the metal ring and the insulating medium. Under a small tolerance voltage, the air gap has broken down to form plasma, that is, the voltage divider ratio of the peaking capacitor integrated capacitor divider is relatively stable under a low tolerance voltage and will not cause a large error under high voltage.

When the charging voltage of the Marx main circuit is 48kV, the difference between the two probe voltages increases significantly. This is mainly due to the limitation of the gas switch tolerance pulse voltage amplitude, and the switch breaks down at the pulse front edge. Since the response time of the 2# probe is longer than that of the 1# probe, its high-frequency response is relatively poor, resulting in a slow front edge of the final output waveform, and the rise time increases from 81.0ns to 83.4ns. When the output switch breaks down, the amplitude measured by the 2# probe is relatively small relative to the true value, which eventually leads to a small output voltage amplitude of the 2# probe. If the experimental parameters are adjusted so that the switch breaks down at the peak, the voltage difference measured by the two probes can be effectively reduced.

According to the voltage measured by the 1# probe, the 2# probe is calibrated online at high voltage. From the data analysis in Table 4, it can be seen that the voltage divider ratio of the 2# probe is maintained at around 15200 at different voltage levels. Except for the large voltage divider ratio caused by the switch front edge breakdown when the Marx charging voltage is 48kV, the others are relatively stable. The monthly average voltage divider ratio is 15283, which is less than 0.9% relative error compared to the voltage divider ratio (15128) at low voltage levels, which can be ignored. Therefore, the voltage divider has good voltage divider ratio stability at higher voltage levels.

Conclusion

This paper designs an integrated capacitor voltage divider for measuring the pulse voltage on the peaking capacitor. The voltage divider realizes the direct measurement of the peaking capacitor voltage and is successfully applied to the peaking capacitor voltage measurement of the Manx pulse source. The main conclusions are as follows: Based on the peaking capacitor, a reasonably designed polyimide copper-clad film is cleverly embedded in the grounding end of the outermost concentric capacitor of the peaking capacitor to form a new type of peaking capacitor integrated capacitor voltage divider: According to the actual structure of the peaking capacitor integrated capacitor voltage divider, the response characteristics of the voltage divider are analyzed, the theoretical calculation formula is given and simulation calculations are carried out, and the simulated voltage divider ratio is consistent with the theoretical voltage divider ratio. The high-voltage online calibration voltage divider ratio is quite different from the square wave calibration and theoretical voltage divider ratio. This is mainly because the peaking capacitor capacitance is affected by the plasma under high voltage. Therefore, considering the influence of this factor, the simulation calculation analysis shows that the theoretical and calibrated voltage divider ratios of the two probes are consistent, and the relative errors are less than 5.20%. Experiments have verified that at higher voltage levels, the voltage divider ratio of the integrated capacitor voltage divider changes less with voltage, does not change linearly with voltage, and the relative error of the voltage divider ratio is small, so the voltage divider ratio of the voltage divider has good stability.