2.1

Principle of energy extraction by pulsed power electric field induction

The main content of pulse power technology research is how to store energy economically and reliably, and transmit large energy and large power to the load effectively. The ever-increasing requirements of energy, power, rise time, flatness, repetition rate, stability and life have put forward a series of scientific and technical problems for pulse power technology. Under the high-voltage AC line, there will be four different kinds of sensed electricity for the stopped conductor, namely electromagnetic induced voltage, induced current, electrostatic induced voltage and current. The types of induced electricity generated are different with different states of outage lines ⁵. In the actual measurement of lightning induced overvoltage phenomenon in distribution lines, due to the limitation of technical conditions, only the return lightning current waveform at the bottom of lightning channel can be measured.

At present, the commonly used power supply mode of monitoring devices is mainly to convert intermittent energy such as solar energy and wind-solar complementary energy into continuous power supply through energy storage components such as batteries and supercapacitors. In the engineering calculation model of lightning current waveform, the calculation of time and space distribution of return lightning current is based on the lightning current at the bottom of lightning channel ^6-7. The energy extraction by electric field induction of pulsed power needs to go through four processes: inductive energy storage from electric field, pulse shaping, high-frequency oscillation and mutual inductance output, as shown in Figure 1, in which pulse shaping is the process of development and suppression of air gap discharge.

Figure 1.

Working process of pulse power electric field induction energy extraction

Airborne conductor, as a suspended electrode, induces energy storage in the high-voltage electric field between the power line and the overhead ground wire, and maintains good insulation with them. At the same time, it is connected with a gas switch that remains disconnected and the high-voltage side of the pulse transformer, and forms an energy extraction circuit for suppressing discharge with the overhead ground wire. The mutual induction voltage is output from the low-voltage side of the pulse transformer after bridge rectification ⁸. Realize the energy transfer of the first level, the lowest level is directly grounded, and no transformer is used for isolation, which is beneficial to improve the circuit efficiency. The other levels are the same. This design improves the total voltage of the energy level and improves the circuit power.

2.2

Suppression discharge principle

The air gap discharge forms a spark discharge plasma channel to connect the circuit, and the turn-on time is generally in the order of 10-8 s. During the discharge process, the air gap conductivity increases rapidly, and the discharge pulse current increases rapidly, and the rising speed is extremely fast. At present, the towers used in power system can be divided into five types according to their shapes: goblet type, upper type, dry type, barrel type and cat head type, among which the top part of the cat head type tower is convenient for fixing and installing metal plates, and the top of the tower has a closed electric field environment ⁹. No matter what kind of induced electricity, the higher the voltage level, the greater the induced voltage and the greater the harm to the maintenance personnel. The simplest calculation model of lightning current waveform model at the bottom of lightning current channel is triangle model, which mainly includes three parameters, namely wavefront time, wave tail time and current amplitude ¹⁰. Therefore, relevant equipment should be installed on the distribution line below the EHV line and corresponding measures should be taken to eliminate the induced voltage in the line to ensure the safety of maintenance personnel. Because of the special structure of this plate, adding a metal plate with such an area to the original transmission line electric field environment will inevitably have a certain impact on the original electric field. In order to verify its safety, the electric field near the transmission line before and after installing the plate is analyzed in this paper ¹¹.

In the process of suppressing discharge, the gas switch plays the dual roles of pulse switch and oscillation at the same time, so the switch trigger and excitation control circuit are omitted. Because the pulse transformer is used to discharge, the main circuit bears a large impulse current, which will affect the service life of the equipment to some extent. At the same time, the electrical load is located at the low-voltage side of the pulse transformer and works in the flyback state, which has good electrical isolation between input and output and is suitable for discharging pulse power extraction ¹². In this paper, the way of electric field induction is used to obtain energy and supply power, and the design of the energy-obtaining polar plate is optimized. Through modular treatment, it is more conducive to wind protection and easier to install and maintain. At the same time, the charging circuit is developed based on inductance mode, which further improves the energy-obtaining power and overcomes the problem of impulse current.

2.3

Analysis of energy equivalent circuit

The reason for this change is that the electrostatic induction in the low-voltage line is generated by the capacitive coupling effect between the wires, and has no direct relationship with the transmission power in the high-voltage line. It is usually only used for qualitative analysis, and the results obtained by using the model in the extensive calculation often differ greatly from the actual situation. In the early calculation of lightning current waveform, the most widely used model is the double-exponential model.

The electromagnetic induction in the low-voltage line is caused by the alternating current in the high-voltage line, which produces the transformed magnetic field and thus the electromagnetic effect. The model is relatively simple in expression, easy to calculate and derive, and highly consistent with the measured natural lightning waveform, but its derivative at zero is not zero, which is not applicable to the field of simulation calculation ¹³. It is quite difficult to directly solve the pulse rise time by using the non-linear resistance of spark gap changing with current and time proposed by Wezel, and the process of spark gap resistance changing from high resistance value to close to 0 can be characterized by the characteristic time θ with the increase of spark gap conductivity:

Where p is the air gap pressure; d is the air gap distance, and a is the arc resistance coefficient; U is the breakdown voltage of the switch.

In general, the time for the spark gap resistance to drop far below the total impedance of the rest of the discharge circuit, and much shorter than the rising time of the discharge current. The details of the change of spark resistance are not very important. Considering that the energy recovery circuit has a large inductance element in series, the development time of the spark gap is ignored, and the gas switch is regarded as an ideal switch with no delay at the turn-on moment.

3.1

Introduction to video device

When the distribution line is in a shutdown state, both ends of the line must be grounded immediately. Generally, the maintenance personnel of low-voltage distribution lines will ignore this operation, or in the grounding process, the operation is not scientific, which makes the grounding incomplete, causing serious safety hazards to the maintenance personnel and nearby residents. It is composed of USB camera, raspberry pie, 4G module and power module. The USB camera uses the common drive-free USB camera on the market. Raspberry pie is based on the Raspbian version of the operating system to drive the camera and 4G module. The USB camera captures video and pictures. Raspberry pie uses the 4G network to upload video and pictures to a specific server. When you need to view the device status, you can only download the corresponding information. The structure block diagram of the inductive energy source is shown in Figure 2. It includes the energy taking unit, the main control unit and the follow-up circuit. The energy taking unit includes two energy taking cores of different materials. The measuring coil measures the current on the line in real time as a control signal. The main control unit selects different magnetic cores by judging the current value.

Figure 2.

Structural block diagram of inductive energy harvesting power supply

According to the elimination method of temporary grounding of low-voltage distribution line, and then combined with relay and contactor control, the corresponding safety device can be designed. Its basic principle is that when the distribution line is in the state of power failure and maintenance, AC contactors and other devices will automatically ground both ends of the distribution line to eliminate the induced voltage in the line. After the distribution line is struck by lightning, the current will form a certain field line distribution excitation on the power transmission line, that is to say, the field line distribution excitation can establish a function in time and space, thus generating lightning-induced overvoltage. As shown in Table 1, the energy extraction power supply starts to work, and the dead zone is small. By switching the tap or the magnetic core, the magnetic core is prevented from being over-saturated, and stable energy supply can be realized within the current variation range.

Table 1.

Test data at 550 turns

The induced voltage elimination device is applied to the distribution line under 500kV. In the actual calculation process, the electric field perpendicular to the ground is less affected. In the design of energy extraction, more attention is paid to the suppression of energy extraction power in the process of discharge. In the process of suppressing discharge, the energy stored in the inductor is expressed as:

Substitute t_s into the formula (2):

Then the average power of energy extraction is obtained:

It can be seen from formula (3) that the average power P will change with the change of power line voltage U_ac.

When the observation point is close to the lightning current, the horizontal electric field Ez is also less affected. This electric field can be calculated by the above formula. Through the analysis of simulation software, it can be concluded that due to the influence of inductance in the line, the voltage on the grounding resistance will rapidly drop to zero after a slight oscillation, thus effectively eliminating the induced voltage in the low-voltage line.

3.2

Comprehensive test

The distribution line is a high-voltage overhead line, and the occurrence of lightning induced overvoltage is a relatively complex natural phenomenon. In this paper, the simulation calculation model widely used by the current academic community can be more close to the calculation results under actual working conditions to a certain extent. Based on the power consumption of the video equipment and the time required to complete a video and image acquisition task, calculate the required energy and set the threshold voltage for the video equipment to start work. The measured value is 9.2V. Install automatic grounding device at both ends of low-voltage distribution line. Once the line is disconnected and needs maintenance, the grounding device can act quickly to ground both ends of the line, so as to eliminate the induced voltage and ensure the safety of line maintenance personnel.

When measuring the space electric field of a lightning stroke, the function waveform in the calculation results of the pilot stage is relatively slow, and the decline speed is closely related to the development speed of the pilot. When the waveform drops to the pole and rises upward, the speed is mainly related to the development speed of the lightning flow channel, and the waveform of the final stage is also dominated by the lightning flow. Carry out the experiment according to the size and installation position of the plate given in the paper, record the indication of the DC microammeter under different plate size and voltage, and simulate the parasitic capacitance under the corresponding plate size, calculate the corresponding induced current and record.

The plate size is 0.50m × The curves of experimental current and simulated current with voltage at 0.25m are shown in Figure 3. It can be seen that the simulation results are basically consistent with the experimental values, and the induced current increases with the increase of voltage. When the line voltage is fixed at 50kV, the induced current generated under different plate sizes is measured and simulated. The change curve is shown in Figure 4. It can be seen that the simulation results are basically consistent with the experimental values, and the induced current increases with the increase of the plate, which is consistent with the theory that the capacitance value is proportional to the area of the plate in the principle of parallel plate capacitor.

Figure 3.

Induced currents corresponding to different voltages

Figure 4.

Induced currents corresponding to plates of different sizes.

The induced current distribution and the lightning withstand level of the line prove that the overhead ground wire can effectively improve the lightning withstand level of the line. No matter whether the plate size or voltage changes, the simulation results of finite element software are basically consistent with the experimental values, so we can know that the simulation results of the method proposed in this paper are desirable. However, the initial permeability is low, which is suitable for energy extraction at high current. Permalloy has a high initial permeability, but it is easy to saturate when the current is slightly larger. Microcrystalline alloy has high initial magnetic permeability, and it is easy to obtain large output at low current, which is suitable for energy extraction at low current and reduces the flashover probability of distribution lines during induction lightning.