High Voltage Inverter - Power Control Device

The high voltage inverter is a speed control device that changes the output frequency and output voltage to control the speed of the AC high-voltage motor.

The frequency converter is a power control device that uses the on-off function of power semiconductor devices to convert the power frequency power supply to another frequency. With the rapid development of modern power electronics technology and microelectronics technology, high-voltage and high-power frequency conversion speed control devices continue to mature. The high-voltage problem that has been difficult to solve has been well solved in recent years by connecting devices or units in series.

1. Measuring instruments for high-voltage inverters

In the face of the inverter containing a large amount of harmonic, distortion or non-power frequency electricity, the accurate measurement method is to use an instrument with FFT function.

For the test of high-voltage and large-capacity inverters, due to the large voltage and current values, general instruments cannot meet the requirements, so voltage or current sensors should be used, and then connected to instruments for measurement. WP4000 frequency conversion power analyzer can realize the input, output and efficiency testing of high-voltage inverters with a voltage of 10kV and a current of 7000A according to the highest test of different frequency conversion power sensors.

include:

1.1 Input value: rated input voltage, rated input current, rated capacity, active power, power factor, input harmonics, input total harmonic distortion.

1.2 Output value: maximum rated output voltage, rated continuous current, rated power, frequency range, overload capacity, output harmonics, output total harmonic distortion.

1.3 Efficiency: within the designed frequency range, the efficiency at each frequency.

2. Basic principle of high voltage inverter

High-voltage and high-power frequency conversion speed control devices are widely used in various fans, water pumps, compressors, rolling mills, etc.

Pump loads widely used in industries such as metallurgy, chemical industry, electric power, municipal water supply, and mining account for about 40% of the energy consumption of the entire electrical equipment, and electricity bills even account for 50% of the cost of water production in waterworks. This is because: on the one hand, there is usually a certain margin in the design of the equipment; on the other hand, due to the change of working conditions, the pump needs to output different flow rates. With the development of the market economy and the improvement of automation and intelligence, the use of high voltage inverters to control the speed of pump loads is not only good for improving the process and product quality, but also meets the requirements for energy saving and economical operation of equipment. The inevitable trend of sustainable development. The benefits of variable speed control of pumping loads are numerous. From the application examples, most of them have achieved good results (some energy saving is as high as 30%-40%), which greatly reduces the water production cost of waterworks, improves the degree of automation, and is conducive to the integration of pumps and pipe networks. Reduced pressure operation reduces leakage and pipe burst and prolongs the service life of the equipment.

2.1 Adjustment method

Flow adjustment method and principle of pump load

The pump load is usually controlled by the flow of the liquid being delivered. For this reason, two methods are often used: valve control and speed control.

2.2 Valve control

This method adjusts the flow by changing the opening of the outlet valve. It is a mechanical method that has been around for a long time. The essence of valve control is to change the flow resistance by changing the fluid resistance in the pipeline. Because the speed of the pump remains unchanged, its head characteristic curve H-Q remains unchanged.

When the valve is fully open, the pipe resistance characteristic curve R1-Q and the head characteristic curve H-Q intersect at point A, the flow rate is Qa, and the pump outlet pressure head is Ha. If the valve is closed, the pipe resistance characteristic curve becomes R2-Q, and the intersection point of it and the head characteristic curve H-Q moves to point B, at this time the flow rate is Qb, and the pump outlet pressure head rises to Hb. Then the rise of the pressure head is: ΔHb=Hb-Ha. Then the energy loss shown in the shaded part is produced: ΔPb=ΔHb×Qb.

2.3 Speed control

It is an advanced electronic control method to adjust the flow rate by changing the speed of the pump. The essence of speed control is to change the flow rate by changing the energy of the delivered liquid. Because only the rotation speed changes, the opening of the valve remains unchanged, and the pipe resistance characteristic curve R1-Q also remains unchanged. The head characteristic curve Ha-Q at the rated speed intersects the pipe resistance characteristic curve at point A, the flow rate is Qa, and the outlet head is Ha.

When the speed decreases, the head characteristic curve becomes Hc-Q, and the intersection point between it and the pipe resistance characteristic curve R1-Q will move down to C, and the flow becomes Qc. At this time, assuming that the flow Qc is controlled to the flow Qb under the valve control mode, the outlet pressure head of the pump will be reduced to Hc. Therefore, the pressure head is reduced compared with the valve control method: ΔHc=Ha-Hc. According to this, the energy saved is: ΔPc=ΔHc×Qb. Compared with the valve control method, the energy saved is: P=ΔPb+ΔPc=(ΔHb－ΔHc)×Qb.

Comparing the two methods, it can be seen that under the same flow rate, the speed control avoids the energy loss caused by the increase of the pressure head and the increase of the pipe resistance under the control of the valve. When the flow rate is reduced, the speed control makes the pressure head drop significantly, so it only needs a much smaller power loss than the valve control, which can be fully utilized.

2.4 Efficiency Analysis

Efficiency Analysis of Pumps at Variable Speeds

As the speed decreases, the high efficiency section of the pump will move to the left. This shows that the speed control method can still make the pump run with high efficiency when the speed is low and the flow rate is low.

2.5 Research on water supply mode under variable frequency condition

In a water supply system consisting of multiple points and multiple pumping stations, it is necessary to control the pressure head at the outlet of the pumping station in order to adapt to the pipe network system and achieve better system performance indicators. This can be divided into constant pressure water supply, variable pressure Pressure water supply and variable pressure water supply in time intervals.

2.6 Constant pressure water supply

Keeping the outlet pressure head of the pumping station constant is the goal of the system control. The given outlet pressure head is Hg.

When the flow Q changes, the head characteristic H1-Q moves up and down due to the change of the speed, and the working point of the pump will move horizontally on the H=Hg line (A, B, C, D). Although this satisfies the requirement of the flow rate, energy is wasted because the pipe resistance characteristic R becomes steeper.

The constant pressure water supply system is relatively convenient to implement, and it is easy to coordinate with the medium and large pipe network systems of multi-pump station water supply. In the constant pressure control mode, because the pressure head at the outlet of the pump station remains unchanged, there is a certain gap between the parallel characteristics of the pump and the actual characteristics of the load, and the energy saving effect is not as good as that of the variable pressure water supply system.

2.7 Variable pressure water supply mode

In order to save energy, the outlet pressure head should be reduced (at least not increased) as the flow rate decreases. At this time, the "variable pressure water supply" method at the outlet end of the pump station can be used. Because the head characteristic moves down when the speed decreases, it intersects with the pipe resistance characteristic R1-Q at point C, and the flow rate decreases from Qa to Qc (the flow rate Qc is equal to QB in constant pressure control). The variable pressure control creates a larger pressure difference H=Hac, thus saving the energy shown in the shaded line. The variable pressure water supply reduces the outlet pressure head, which suppresses the loss caused by the change of pipe resistance characteristics and the additional loss of the water pump, and the energy saving effect is remarkable.

Through the analysis, the frequency converter can optimize the water supply mode during the speed regulation process of the pump load, and has achieved a better power saving effect.

3. Basic classification of high-voltage inverters

There are many types of high-voltage inverters, and their classification methods are also various. According to whether there is a DC part in the intermediate link, it can be divided into AC-AC inverter and AC-DC-AC inverter; according to the nature of the DC part, it can be divided into current type and voltage type inverter; according to whether there is an intermediate low-voltage circuit, it can be divided into High-high frequency converter and high-low-high frequency converter; according to the number of output levels, it can be divided into two-level, three-level, five-level and multi-level frequency converter; according to the voltage level and application, it can be divided into general frequency conversion Inverters and high-voltage inverters; according to the embedding method, it can be divided into diode embedding type and capacitor embedding type inverter, etc.

3.1 Current type

It is named because the inductance element is used in the DC link of the frequency converter. Its advantage is that it has four-quadrant operation capability and can easily realize the braking function of the motor. The disadvantage is that the inverter bridge needs to be forced to commutate, the structure of the device is complicated, and the adjustment is difficult. In addition, since the grid side adopts thyristor phase-shift rectification, the input current harmonics are relatively large, which will have a certain impact on the grid when the capacity is large.

3.2 High pressure type

It is named after the capacitor element is used in the DC link of the frequency converter. With the advancement of technology, the high voltage inverter can realize four-quadrant operation and vector control, and has become the mainstream product of the current transmission system speed regulation.

3.3 High Low High Type

It adopts the method of buck-boost and applies low-voltage or general-purpose frequency converters to medium and high-voltage environments, so it gets its name. The principle is to reduce the grid voltage to the rated or allowable voltage input range of the low-voltage inverter through the step-down transformer, and then transform the frequency and amplitude of alternating current through the conversion of the inverter, and then transform it into the AC current required by the motor through the step-up transformer. Voltage level.

In this way, due to the use of standard low-voltage inverters and step-down and step-up transformers, the voltage levels of the grid and motors can be matched arbitrarily. When the capacity is small (<500KW), the transformation cost is lower than that of direct high-voltage inverters. The disadvantage is that the step-up and step-down transformers are bulky and heavy, the frequency range is easily affected by the transformer, and the system efficiency is relatively low due to the introduction of the transformer.

Generally, high-low-high frequency converters can be divided into two types: current type and voltage type.

3.4 High current type

The circuit topology is named for the use of inductive components in the DC link of the low-voltage inverter. The input side adopts thyristor phase-shift control rectification to control the current of the motor, and the output side adopts forced commutation mode to control the frequency and phase of the motor. It can realize the four-quadrant operation of the motor.

3.5 High voltage type

The step-down transformer is introduced in the front stage to step down the grid, and then the low-voltage inverter is connected. The input side of the low-voltage inverter can be rectified by phase-shift control of silicon controlled rectifiers, or directly rectified by a diode three-phase bridge, and the intermediate DC part uses capacitor smoothing and energy storage. The inverter or converter circuit often uses IGBT components. Through SPWM conversion, alternating current with variable frequency and amplitude can be obtained, and then transformed into the voltage level required by the motor through a step-up transformer. It should be pointed out that a sine wave filter (F) needs to be placed between the converter circuit and the step-up transformer, otherwise the step-up transformer will generate heat due to input harmonics or excessive dv/dt, or damage the insulation of the winding . The cost of the sine wave filter is very high, generally equivalent to 1/3 to 1/2 of the price of the low-voltage inverter.

3.6 High and high frequency conversion

Gaogao inverter does not need a step-down transformer, and the power device directly builds a converter between the grid and the motor. Since the withstand voltage of power devices is difficult to solve, the most direct method at present is to use the method of connecting devices in series to increase the voltage level. The disadvantage is that it needs to solve the problem of device voltage equalization and buffering, and the technology is complicated and difficult. However, since this kind of frequency converter does not have a step-down transformer, its efficiency is higher than that of the low-high mode, and its structure is relatively compact.

3.7 High High Current

It uses GTO, SCR or IGCT components in series to realize direct high-voltage frequency conversion, and the voltage can reach 10KV. Since the inductance element is used in the DC link, it is not sensitive enough to the current, so it is not easy to cause an overcurrent fault, and the inverter is also very reliable and has good protection performance. Its input side adopts thyristor phase-controlled rectification, and the input current harmonics are relatively large. When the frequency conversion device has a large capacity, the pollution to the power grid and the interference to communication electronic equipment should be considered. Voltage equalizing and buffering circuits are complex in technology and high in cost. Due to the large number of components and the large volume of the device, it is difficult to adjust and maintain. The inverter bridge adopts forced commutation, and the heat generation is relatively large, so it is necessary to solve the heat dissipation problem of the device. Its advantage is that it has four-quadrant operation capability and can brake.

What needs to be specially explained is that due to the low input power factor and high input and output harmonics of this type of inverter, it is necessary to install high-voltage self-healing capacitors on its input and output sides.

3.8 High and high voltage

The circuit structure adopts IGBT direct series technology, also called direct device series high-voltage inverter. It uses high-voltage capacitors for filtering and energy storage in the DC link, and the output voltage can reach 13.8KV. Its advantage is that power devices with lower withstand voltage can be used, and all the IGBTs on the series bridge arms have the same function, which can realize mutual backup, or Design for redundancy. The disadvantage is that the number of levels is low, only two levels, and the output voltage dV/dt is also relatively large. It is necessary to use a special motor or install a common-mode voltage filter and a high-voltage sine wave filter, and the cost will increase a lot. Because it has the same topology as the low-voltage inverter, it has the same four-quadrant operation function as the low-voltage inverter, and can also realize vector control.

This kind of inverter also needs to solve the voltage equalization problem of the device, and generally requires a special design of the drive circuit and buffer circuit. There are also extremely strict requirements on the time delay of the IGBT drive circuit. Once the turn-on and turn-off times of the IGBT are inconsistent, or the slopes of the rising and falling edges are too different, it will cause damage to the power device.

3.9 Embedded type

Clamp type inverters can generally be divided into diode clamp type and capacitor clamp type.

3.10 Diode type

It can not only realize the clamping of the diode midpoint, but also realize the output of three levels or more, and its technical difficulty is lower than that of the direct device series inverter. Since the DC link uses capacitive components, it still belongs to the voltage type inverter. This kind of inverter needs to be equipped with an input transformer, which is used for isolation and star-delta transformation, can realize 12-pulse rectification, and provides intermediate clamping zero level. Power devices such as IGBT are forcibly embedded in the middle zero level through auxiliary diodes, so that both ends of IGBT will not be burned due to overvoltage, and multi-level output is realized.

With this frequency converter structure, the sine wave filter may not be installed in the output. However, due to the use of transformers, the cost has increased.

3.11 Capacitive type

It adopts the method of adding floating capacitors on the same bridge arm to realize the embedding of power devices, and this kind of frequency converter is rarely used.

4. The units of the high-voltage inverter are connected in series

4.1 Basic information

This is a circuit topology that has only been developed in recent years. It is mainly composed of three parts: input transformer, power unit and control unit. Adopting modular design, it got its name because it solves the problem of high voltage by connecting power units in series. It can directly drive AC motors without output transformers, let alone any form of filters.

The 6KV inverter can be composed of 15 or 18 power units, and each phase is composed of 5 or 6 power units in series, and form a Y-shaped connection to directly drive the motor. The circuits and structures of each power unit are exactly the same, and they can be interchanged or used as spares for each other.

The input part of the frequency converter is a phase-shifting transformer, the primary side is connected in Y shape, and the secondary side is connected in a delta-shaped extension. There are 15 to 18 pairs of three-phase windings in total, which supply power to each power unit respectively. They are evenly divided into three parts I, II and III, and each part has 5 to 6 pairs of three-phase small windings with an average phase offset of 8.5 or 10 degrees.

4.2 Features

The characteristics of the inverter are as follows:

4.2.1 Using multiple PWM control, the output voltage waveform is close to a sine wave.

4.2.2 Multiple rectification circuits, the number of pulses is as high as 30 or 36, the power factor is high, and the input harmonics are small.

4.2.3 Modular design, compact structure, easy maintenance, enhanced product interchangeability.

4.2.4 Direct high voltage output without output transformer.

4.2.5 Very low dv/dt output without any form of filter.

4.2.6 The use of optical fiber communication technology improves the anti-interference ability and reliability of the product.

4.2.7 The automatic bypass circuit of the power unit can realize the function of non-stop in failure.

4.3 Disadvantages

4.3.1 Since the transformer adopts the extension delta connection method to achieve 8.5 degrees or 10 degrees of phase shift, the corresponding error caused by the process causes the internal circulation of the transformer to be large, the heat generation is high, and the efficiency of the transformer is low, thereby reducing the efficiency of the entire system.

4.3.2 Due to the different load ratios, not all power units output power, resulting in harmonics that cannot cancel each other out. Therefore, when the load is lower than the rated value, the harmonics increase rapidly. For the same reason, the starting torque is small, the motor jitters and generates heat, and the noise is also high.

4.3.3 Since the motor needs to be grounded to protect the motor from the influence of the common-mode voltage, the common-mode voltage is introduced to the transformer, which makes the transformer bear greater electrical stress, reduces the reliability and life of the transformer.

4.3.4 Due to the introduction of complex phase-shifting isolation transformers, the cost increases.

5. Production status of high-voltage inverters

5.1 Development background

With the rapid development of modern power electronics technology and computer control technology, the technological revolution of electric transmission has been promoted. It has become a development trend that AC speed regulation replaces DC speed regulation, and computer digital control replaces analog control. AC motor frequency conversion speed regulation is a major means to save electric energy, improve production process, improve product quality, and improve the operating environment. Frequency conversion speed regulation is recognized as the most promising speed regulation method at home and abroad because of its high efficiency, high power factor, and excellent speed regulation and braking performance.

The previous high-voltage inverters were composed of thyristor rectifiers, thyristor inverters and other devices, which had many disadvantages, such as large harmonics, which had an impact on the power grid and motors. Some new devices developed will change this situation, such as IGBT, IGCT, SGCT and so on. The high-voltage inverter composed of them has excellent performance and can realize PWM inverter and even PWM rectification. Not only does it have small harmonics, but the power factor is also greatly improved.

5.2 Industry Characteristics

Inverter is a device that makes the motor run at variable speeds to achieve energy-saving effects. It is customary to call a motor with a rated voltage between 3kV and 10kV a high-voltage motor, so it is generally developed for motors that operate under high-voltage environments from 3kV to 10kV The frequency converter is called a high voltage inverter. Compared with low-voltage inverters, high-voltage inverters are suitable for frequency conversion and speed regulation of high-power wind power and water pumps, and can receive significant energy-saving effects.

With the increasing demand for energy saving and environmental protection and the acceleration of equipment upgrading and transformation, China's high-voltage inverter industry has shown a steady growth trend. The market size increased from 1.1 billion yuan in 2005 to 6.3 billion yuan in 2011, with a compound annual growth rate of 35.4%. ; The proportion in frequency converter also increased from 12.9% in 2006 to 22.8% in 2011. In 2012, with the improvement of the frequency conversion rate of downstream industries, the growth rate of the high-voltage inverter market is expected to reach 34.92%. China's high-voltage inverter industry mainly has the following operating characteristics.

5.3 Domestic Status

With the further deepening of technical research, domestic high-voltage inverters can already be compared with imported inverters in theory and function, but due to the limitation of process technology, the gap with imported products is still quite obvious. These situations are mainly manifested in the following aspects:

5.3.1 The products of major foreign brands are stepping up to occupy the domestic market and accelerating the pace of localization.

5.3.2 The research and development capabilities and industrialization scale are increasing year by year.

5.3.3 The power of domestic high voltage inverters is getting bigger and bigger. At present, the largest application in China has reached 20,000KW.

5.3.4 The technical standards of domestic high voltage inverters are yet to be regulated.

5.3.5 The supporting industries for high voltage inverters are underdeveloped.

5.3.6 The production process is average, which can meet the technical requirements of frequency converter products, and the price is relatively low.

5.3.7 The key power semiconductor components used in frequency converters are completely dependent on imports, and will continue to rely on imports for quite a long time.

5.3.8 The technological gap with developed countries is narrowing, and products with independent intellectual property rights are being applied in the national economy.

5.3.9 A frequency converter with the functions of instantaneous power failure and recovery, fault recovery and so on has been developed.

5.3.10 Some manufacturers have developed four-quadrant high-voltage inverters.

5.3.11 The high voltage inverter with vector control has also been applied.

5.4 Current situation abroad

The frequency converter manufacturers of major foreign brands have formed serialized products, and their control systems have also realized full digitalization. Almost all products have vector control function, and perfect craftsmanship is also a major feature of foreign brands. In developed countries, as long as there is a motor, there will be a frequency converter at the same time. Its current development is mainly manifested as follows:

5.4.1 Technology development started early and has a considerable scale of industrialization.

5.4.2 Inverters capable of providing extra-large power have exceeded 10,000KW.

5.4.3 The technical standards of frequency conversion and speed regulation products are relatively complete.

5.4.4 Supporting industries and industries related to frequency converters have begun to take shape.

5.4.5 Capable of producing power devices in inverters, such as IGBT, IGCT, SGCT, etc.

5.4.6 High voltage inverters are widely used in various industries and have achieved remarkable economic benefits.

5.4.7 Product internationalization and localization intensified.

5.4.8 New technologies and new processes emerge one after another, and are applied to products in large quantities and quickly.

6. Common measures for high-voltage inverters

Common measures for high-voltage inverter anti-interference:

6.1 The E terminal of the high-voltage inverter should be connected with the control cabinet and the shell of the motor, and should be connected to the safety ground. The grounding resistance should be less than 100Ω, which can absorb the surge interference.

6.2 Install an inductive magnetic ring filter at the input or output of the high-voltage inverter. Flatness and 3-4 laps help to suppress high-order harmonics (this method is simple and easy, and the price is low).

6.3 The above-mentioned magnetic ring filter can also be wound on the incoming line of the high-voltage inverter control signal terminal or the analog signal given terminal according to the site conditions.

6.4 In the electric control cabinet equipped with a high voltage inverter, the power line and signal line should be routed separately, and the metal hose should be well grounded.

6.5 The analog signal line should be shielded line, and the single end should be connected to the simulated ground at the high voltage inverter.

6.6 The interference can also be improved by adjusting the carrier frequency of the high-voltage inverter. The lower the frequency, the less interference, but the greater the electromagnetic noise.

6.7 The RS485 communication port must be connected with the upper computer to adopt the transmission mode of photoelectric isolation, so as to improve the anti-interference performance of the communication system.

6.8 The power supply of the external computer or instrument should be separated from the power supply of the power unit of the high-voltage inverter, and try to avoid sharing an internal transformer.

6.9 Independent shielding should also be carried out on the instrument equipment that is subject to interference. Instruments such as temperature controllers, PID regulators, PLCs, sensors or transmitters on the market must be equipped with metal shielding shells and connected to security grounds. If necessary, the above-mentioned inductive magnetic ring filter can be installed at the power inlet end of this type of instrument.

7. Maintenance of high-voltage inverter

General installation environment requirements for high-voltage inverters: the minimum ambient temperature is -5°C, and the maximum ambient temperature is 40°C. A large number of studies have shown that the failure rate of high-voltage inverters increases exponentially with the increase of temperature, and the service life decreases exponentially with the increase of temperature. If the ambient temperature rises by 10°C, the service life of high-voltage inverters will be halved. In addition, whether the high-voltage inverter is running well or not has a lot to do with the cleanliness of the environment. Summer is a period of high-voltage inverter failures. Only through good maintenance work can the occurrence of equipment failures be reduced. Users must pay attention.

During the maintenance of the high-voltage inverter in summer, attention should be paid to the temperature of the installation environment of the inverter, and the dust inside the inverter should be cleaned regularly to ensure the smoothness of the cooling air passage. Strengthen inspections and improve the surrounding environment of inverters, motors and lines. Check whether it is fastened, ensure the correct and reliable connection of each electrical circuit, and prevent unnecessary downtime accidents.

7.1 Precautions

7.1.1 Carefully monitor and record the display parameters on the inverter man-machine interface, and immediately report any abnormalities

7.1.2 Carefully monitor and record the ambient temperature of the inverter room. The ambient temperature should be between -5°C and 40°C. The temperature rise of the phase-shifting transformer cannot exceed 130°C

7.1.3 When the temperature is high in summer, the ventilation and heat dissipation of the inverter installation site should be strengthened. Ensure that the surrounding air does not contain excessive dust, acids, salts, corrosive and explosive gases

7.1.4 Summer is a rainy season, and rainwater should be prevented from entering the inverter (for example, rainwater enters along the air outlet of the air duct)

7.1.5 The filter screen on the inverter cabinet door should be cleaned once a week; if the working environment is dusty, the cleaning interval should be shortened according to the actual situation

7.1.6 During the normal operation of the inverter, a piece of A4 paper of standard thickness should be firmly adsorbed on the air inlet filter of the cabinet door

7.1.7 The frequency conversion room must be kept clean and tidy, and should be cleaned at any time according to the actual situation on site.

7.1.8 The ventilation and lighting of the frequency conversion room must be good, and the ventilation and heat dissipation equipment (air conditioner, ventilation fan, etc.) can operate normally.

7.2 Maintenance items

7.2.1 Use a vacuum cleaner with a plastic nozzle to thoroughly clean the inside and outside of the inverter cabinet to ensure that there is no excessive dust around the equipment.

7.2.2 Check the ventilation and lighting equipment of the frequency conversion room to ensure that the ventilation equipment can operate normally.

7.2.3 Check that the connection between the cables inside the inverter is correct and reliable

7.2.4 Check that all the grounding in the inverter cabinet should be reliable, and the grounding points should not be rusted

7.2.5 Every six months (within) each connecting nut of the internal cable of the inverter should be tightened again

7.2.6 When the frequency converter resumes operation after a long shutdown, the insulation of the frequency converter (including the phase-shifting transformer and the main circuit of the bypass cabinet) should be measured, and a 2500V megohmmeter should be used. The inverter can only be started after the insulation is qualified.

7.2.7 Check the tightness of all electrical connections, check whether there are abnormal discharge traces in each circuit, whether there are strange smells, discoloration, cracks, damage, etc.

7.2.8 After each maintenance of the inverter, carefully check whether there are any missing screws and wires, etc., to prevent short-circuit accidents caused by small metal objects. Especially after making major changes to the electrical circuit, ensure that the connection of the electrical connection line is correct and reliable to prevent the occurrence of "reverse power transmission" accidents.