Comparative analysis of common high voltage inverter technology

As we all know, the frequency conversion speed regulation scheme of high-power fans and pumps can receive significant energy-saving effects, and its direct economic benefits are great, and macroeconomic and social benefits are even greater. It can be expected that the development of new high-frequency AC motor frequency conversion speed regulation technology is one of the leading directions of energy conservation in China. At present, there are two main problems that hinder the application of variable frequency speed control technology in high-voltage and high-power AC drive: First, China's large-capacity (200kW or more) motors have high supply voltage (6kV, 10kV), and the power devices that make up the inverter. The low pressure level is low, which causes difficulties in voltage matching. Secondly, the high-voltage high-power variable frequency speed control system has high technical content, high difficulty and high cost, while the general energy-saving transformation of fans and pumps requires low input and high return. , causing economic difficulties

These two world-wide problems have hindered the popularization and application of high-voltage large-capacity variable-frequency speed control technology. Therefore, how to solve high-voltage power supply and use high-tech to produce low-cost and high-reliability frequency conversion speed control devices is the current concern of related industries in the world. Hot spot. Generally speaking, in the case of high voltage power supply and limited power capability of the power device, the power device can be used in series to solve the problem. However, when the device is used in series, there is a problem of static and dynamic voltage equalization due to the difference in dynamic resistance and pole capacitance of each device. If the voltage equalization measures of R and RC in parallel with the device are adopted, the circuit will be complicated and the loss will increase. At the same time, the requirements of the series connection circuit of the device are greatly improved, and the series device should be turned on and off at the same time, otherwise Each device has different breaking times and is subject to uneven voltage, which may cause damage to the device or even the entire device may collapse.

Harmonic problems are common problems of all frequency converters, especially in high-power variable frequency speed regulation. Harmonics can pollute the power grid, and other electrical equipment on the same power grid, and even affect the normal operation of the power system; harmonics can also interfere with communication and control systems, and in severe cases, communication will be interrupted, system 瘫痪; harmonic current will also The motor loss is increased, so that the heat is increased, the efficiency and the power factor are lowered, and the "derating" is used.

There is also efficiency problem, the greater the capacity of the variable frequency speed control, the more important the efficiency of the system. Using different main circuit topologies, the type and amount of power devices used, and the use of transformers, filters, etc., all affect the efficiency of the system. In order to improve system efficiency, we must try to minimize the loss of power switching devices and frequency converters.

Reliability and redundancy design problems, general high-voltage and high-power drag systems require high system reliability, especially in important sectors of the national economy such as power, energy, metallurgy, mining and petrochemical industries. It will cause huge loss of people's life and property. Therefore, it is also crucial to facilitate the use of redundant design and bypass control functions in the design of high-voltage inverters.

At present, high-voltage inverters in the world do not have mature and consistent topologies like low-voltage inverters, but are limited to power devices with current voltage tolerance. How to deal with the requirements of high-voltage operating conditions, domestic and foreign inverter manufacturers Eight Immortals crossing the sea, each has its own tricks, so its main circuit structure is not consistent, but they have successfully solved the problem of high voltage and large capacity. Of course, there are also differences in performance indicators and prices. Such as the perfect harmonic-free inverter produced by ROBICON; the BulleTIn1557 and PowerFlex7000 series inverters produced by Rockwell (AB), the SIMOVERTMV medium-voltage inverter produced by Siemens of Germany, and the ABB company of Sweden. ACS1000 series inverter; SILCOVERT?TH inverter produced by Italian ANSALDO company, perfect harmonicless inverter produced by Mitsubishi and Fujitsu of Japan, and Cage, Xianxing, Leadworth, and Chengdu Jialing Co., Ltd. High-voltage inverters, etc.

In this paper, several common main circuit topologies of medium and high voltage inverters are analyzed and compared, and the reliability, redundancy design, harmonic content and dv/dt indicators of medium and high voltage inverters with different circuit structures are discussed in depth. And put forward his own views on the development direction of medium and high voltage inverters.

2, power device series two-level current type high-voltage inverter

The BulleTIn1557 series of medium voltage inverters from Rockwell Company of the United States has a circuit structure of AC/DC current source type and a two-level inverter with power device GTO series. The control method adopts the speed sensorless direct vector control, the motor torque can be changed rapidly without affecting the magnetic flux, and the advantages of the pulse width modulation and the current source structure are integrated, and the operation effect is approximate to the DC transmission device. The company offers several options to meet harmonic rejection requirements such as standard 12- and 18-pulse and PWM rectifiers, standard harmonic filters and power factor compensators to make their harmonics compliant with the IEEE519?1992 standard. Provisions. Figure 1 shows the main circuit topology of the BulleTIn1557 inverter with an 18-pulse rectifier.

AB recently introduced the new generation of medium voltage inverter PowerFlex7000 series, replacing the original GTO with a new power device, symmetrical gate commutated thyristor (SGCT), which simplifies the drive and absorption circuits and improves system efficiency. Each 6kV system The bridge arm is connected in series with three SGCTs with a withstand voltage of 6500V.

The advantage of the current source inverter is that it is easy to control the current, which is convenient for energy feedback and four-quadrant operation. The disadvantage is that the performance of the inverter is related to the parameters of the motor. It is difficult to achieve multi-motor linkage, poor versatility, large harmonic components of current, pollution and The loss is large and the common mode voltage is high, which has an effect on the insulation of the motor.

AB's inverter adopts a two-level inverter scheme in which the power devices are connected in series. The structure is simple and the power devices used are few, but the series connection brings the voltage equalization problem, and the dv/dt of the two-level output will cause the insulation of the motor. Hazard, it is required to improve the insulation level of the motor; and the harmonic component is large, and the output filter needs to be specially designed to be used by the power supply machine. Even so, the total harmonic distortion THD can only reach about 4%.

The input terminal adopts a controllable device to realize PWM rectification, which is convenient for energy feedback and four-quadrant operation, but at the same time, the harmonics on the grid side are increased, and the line reactor filter is added to meet the requirements of the power grid, which also increases the volume and cost.

Because it is a direct high-voltage frequency conversion, the grid voltage and the motor voltage are the same, it is easy to implement the bypass control function, so that the motor can be put into the grid operation in the event of a device failure.

3, unit series multiplexed voltage source inverter

American Robincon Company uses the unit series multiplexing technology to produce a perfect harmonic-free (PERFECTHARMONY) high-voltage inverter with a power of 315kW～10MW. It can realize direct 3.3kV or 6kV high-voltage output without output transformer; the first high-voltage inverter The advanced IGBT power switching device is adopted to achieve the perfect harmonic-free output waveform, which can meet the strict requirements of harmonics of power supply departments of various countries without additional filters; the input power factor can reach above 0.95, THD<1%, The overall efficiency (including the input isolation transformer) is as high as 97%. The reason for achieving such a high indicator is the adoption of three new

High-voltage frequency conversion technology: First, the direct in-line superposition of single-phase bridge SPWM inverters with independent power supply is adopted in the output inverter part; secondly, multi-phase multiple superposition rectification technology is adopted in the input rectification part; third is on the structure Power unit modular technology is used.

The so-called multiplex technology is that each phase consists of several low-voltage PWM power units connected in series. Each power unit is powered by a multi-winding isolation transformer, controlled by a high-speed microprocessor and driven by optical fiber. The multiplex technology fundamentally solves the harmonic problems generated by the general 6-pulse and 12-pulse inverters, and achieves perfect harmonic-free frequency conversion. Figure 2 shows the main circuit topology of the 6kV inverter. Each group consists of five power units with a rated voltage of 690V connected in series. Therefore, the phase voltage is 690V×5=3450V, and the corresponding line voltage is 6000V. Each power unit is powered by 15 secondary windings of the input isolation transformer, and 15 secondary windings are divided into 5 groups with a phase difference of 12° between each group. In Fig. 3, the middle Δ connection method is used as a reference (0°), and there are two sets of four sets of windings which are advanced (+12°, +24°) and hysteresis (-12°, -24°) ​​respectively. The required phase difference angle can be achieved by different connection groups of the transformer.

Each of the power cells in Figure 3 is a three-phase input, low-voltage insulated gate bipolar transistor (IGBT), single-phase output low-voltage PWM voltage inverter. The power unit circuit is shown in Figure 4. Each power unit output voltage is 1, 0, -1 three state levels, 5 units per phase are superimposed, and 11 different level levels can be generated, which are ±5, ±4, ±3, ±2 , ±1 and 0. Figure 5 is a phase-synthesized positive wave output voltage waveform. The high-voltage inverter composed of this multiplex technology, also called the unit series multi-level PWM voltage type inverter, uses power units in series instead of using conventional devices in series to achieve high-voltage output, so there is no device voltage equalization. problem. Each power unit is subjected to full output current, but only withstands 1/5 of the output phase voltage and 1/15 of the output power. Due to the multiplexed PWM technology, the frequency converter modulates the fundamental voltage by five pairs of triangular carriers that are sequentially phase-shifted by 12°. For the five signals obtained by the fundamental wave modulation of the A phase, five power units of A1 to A5 are respectively controlled, and the phase voltage waveform having the step level of 11 steps shown in FIG. 5 is superimposed, and the line voltage waveform has 21 steps. Ping, it is equivalent to 30-pulse frequency conversion, theoretically 19 times less harmonics can be offset, the total voltage and current distortion rate can be less than 1.2% and 0.8%, respectively, called the perfect harmonic-free inverter. Its input power factor can reach more than 0.95, no need to set input filter and power factor compensation device. The power unit of the same phase of the inverter outputs the same fundamental voltage, and the carrier between the units in series is shifted by a certain phase. If the IGBT switching frequency of each power unit is 600 Hz, when five power units are connected in series, the equivalent The output phase voltage switching frequency is 6 kHz. The power unit uses a low switching frequency to reduce switching losses, while the high equivalent output switching frequency and multi-level can greatly improve the output waveform. Improvements in waveforms, in addition to reducing output harmonics, can also reduce noise, dv/dt values, and motor torque ripple. Therefore, this kind of frequency converter has no special requirements for the motor, and can be used for a universal cage type motor, and does not need to be derated, and there is no special limitation on the length of the output cable. Since the power unit has enough filter capacitors, the inverter can withstand a -30% supply voltage drop and 5 cycles of power loss. Although the main circuit topology increases the number of devices, the efficiency of the inverter can be as high as 96% or more because the IGBT driving power is very low, and it is not necessary to use a voltage equalizing circuit, an absorbing circuit, and an output filter.

The advantages of the unit series multiplexed inverter are:

1) Since the power unit is connected in series, the low-voltage IGBT with mature technology and low price can be used to form an inverter unit, and the number of series units can be adapted to different output voltage requirements;

2) Perfect input and output waveforms, making it suitable for any occasion and motor use;

3) Since the multi-power unit has the same structure and parameters, it is convenient to modularize the power unit and realize redundant design. Even in the case of individual unit failure, the unit can be short-circuited by the unit bypass function, and the system can still be normal or Derating run.

The disadvantages are:

1) The number of power units and power devices used is too large. The 6kV system uses 150 power devices (90 diodes, 60 IGBTs). The device is too large, heavy, and the installation position is a problem.

2) Energy feedback and four-quadrant operation cannot be realized, and braking cannot be achieved;

3) Bypass switching control cannot be realized when the grid voltage and the motor voltage are different.

Another improvement to the high-voltage frequency converters in series with power cells is the use of high voltage IGBT devices to reduce the number of power cells in series. For example, with a 3300V withstand voltage IGBT device, a frequency converter with two power units connected in series can output 4.16kV medium voltage; for a 6kV output, only three units are connected in series. The reduction in the number of power cells and devices reduces losses and faults, which helps to increase the efficiency and reliability of the device and reduce the size of the device. However, due to the reduction of the level series, the output harmonics increase, and in order to obtain an excellent output waveform, an output filter must be added. In addition, since the high voltage IGBT is much more expensive than the ordinary low voltage IGBT, although the power device is reduced, the cost does not necessarily decrease.

4, neutral point clamp three-level PWM inverter

In the PWM voltage source type inverter, when the output voltage is high, in order to avoid the static and dynamic voltage equalization caused by the series connection of the device, and reduce the influence of output harmonics and dv/dt, the inverter part can adopt the neutral point. Clamped three-level mode (Neutralpointclamped: NPC). The power device of the inverter can be a high voltage IGBT or an IGCT. ABB's ACS1000 series inverters are three-level inverters with a new power device, integrated gate commutated thyristor (IGCT), with output voltage ratings of 2.2kV, 3.3kV and 4.16kV. Figure 6 shows the main circuit topology of the ACS100012 pulse-rectified three-level voltage source inverter. Siemens uses high-voltage IGBT devices to produce a similar SIMOVERTMV inverter.

The rectification part uses a 12-pulse diode rectifier, and the inverter part uses a three-level PWM inverter. It can be seen from Fig. 6 that the series of frequency converters adopt the traditional voltage type inverter structure, and the total number of devices is reduced to 12 by using a high withstand voltage IGCT power device. As the number of devices decreases, the cost is reduced, and the circuit structure is simple, thereby reducing the size and reliability.

Since the rectified portion of the frequency converter is non-linear, the resulting higher harmonics will cause pollution to the power grid. To this end, in the 12-pulse rectification wiring diagram of the ACS1000 series inverter shown in Figure 6, two sets of three-phase bridge rectifier circuits are connected by a rectifier transformer, the primary windings are connected in a triangle, and the secondary windings are connected in a group. In a triangle shape, the other group is connected in a star shape, and the line voltages of the two secondary windings of the rectifier transformer are the same, but the phases are different by 30°, so that the 5th and 7th harmonics will have a phase of 180° at the primary of the transformer. Move, and thus cancel each other out, the same 17th and 19th harmonics will cancel each other out. After the series connection of the two rectifier bridges, the rectified output waveform of 12 pulses can be obtained, which is smoother than the 6-pulse, and the diode withstand voltage of each rectifier bridge can be reduced by half. The 12-phase rectification circuit is used to reduce the characteristic harmonic content, because the characteristic harmonic order is N=KP±1 (P is the rectification phase number and K is the natural number). Therefore, the characteristic harmonics on the grid side are only 11, 13, 23, 25 times, and so on. If a 24-pulse rectifier circuit is used, the grid side harmonics will be further suppressed. Both options allow the input power factor to be guaranteed above 0.95 over the full power range, eliminating the need for power factor compensation capacitors.

The inverter's inverter part adopts the traditional three-level mode, so the output waveform will inevitably produce relatively large harmonic components (THD up to 12.8%), which is inherent in the three-level inverter mode. The voltage waveform is shown in Figure 7. Therefore, an output LC filter must be configured on the output side of the inverter for use in ordinary squirrel cage motors. After passing through the LC filter, it can make THD <1%. Also due to harmonics, the power factor and efficiency of the motor will be affected to some extent. Only at the rated operating point can the best working condition be achieved. As the speed decreases, the power factor and efficiency will decrease accordingly.

The three-level inverter has a simple structure, small volume, low cost, and the minimum number of power devices used (12), which avoids the series connection of the devices and improves the reliability index of the device. According to the current withstand voltage level of IGCT and high voltage IGBT, the maximum output voltage level of the three-level inverter is 4.16kV. When the output voltage requires 6kV, 12 power devices can not meet the requirements, and the device must be connected in series, except for the increase. In addition to cost, it will inevitably bring about a voltage equalization problem, lose the advantages of a three-level structure, and will greatly affect the reliability of the system. If 9kV withstand voltage IGCT is used in the future, the three-level inverter can directly output 6kV, but the harmonics and dv/dt also increase accordingly. The filtering function must be strengthened to meet the THD index. Or use the four-level inverter described below. Before the appearance of 9kV voltage-resistant devices, for the 6kV high-voltage motor, the Y/â–³ switching method can be used to change the Y-connected 6kV motor to the â–³ connection method, the line voltage is 3.47kV, and 3.3kV or 4.16kV is used. The output inverter can meet the requirements, and also meets the requirements of the IGCT voltage inverter to increase the insulation level of the motor by one level, so this solution may be the most economical and reasonable. However, after the Y/â–³ is changed, the motor voltage is inconsistent with the grid voltage, and the bypass function cannot be realized. When the inverter fails, the production must be guaranteed to be normal. The motor must first be changed back to the Y-connection. Invest in the 6kV grid. For this reason, the Y/â–³ change of the motor should be realized by the Y/â–³ switching cabinet to realize the bypass function. The bypass switching of the ACS1000 series itself is done when the motor voltage is consistent with the grid voltage. If the active input front end is used, energy feedback and four-quadrant operation can be realized, but the three-level structure is not easy to implement redundant design.

5, multi-level high-voltage inverter

With the development of modern topology technology, multi-level high-voltage variable frequency speed control technology has been applied in practice. The representative of this high-voltage inverter is the ASPAVDM6000 series high-voltage inverter produced by ALSTOM of France. The inverter structure is shown in Figure 8.

The power devices are not simply connected in series, but are connected in series, and the capacitors are clamped to ensure safe voltage distribution. Its main features are:

1) Through the series-parallel topology of the monolithic unit to meet the needs of different voltage levels (such as 3.3kV, 4.16kV, 6.6kV, 10kV).

2) This structure allows the system to adopt a DC bus scheme to achieve energy exchange between multiple high-voltage inverters.

3) This structure does not have many voltage dividing and shunting devices on the power devices of the conventional structure, which eliminates the low reliability of the system, so that the system structure is very simple, reliable and easy to maintain.

4) The output waveform is very close to the sine wave, which can be applied to the general induction motor and synchronous motor speed regulation without reducing the capacity. There is no influence of dv/dt on the motor insulation. The motor has no extra temperature rise. It is a technologically advanced. High voltage inverter. The output voltage and motor current waveform are shown in Figure 9.

5) ASPAVDM6000 series high-voltage inverter can use 12-pulse, 18-pulse diode rectification or thyristor rectification according to the different requirements of the grid for harmonics; if the electric energy is to be fed back to the grid, the thyristor rectifier bridge can be used; Wave, power factor, and four-quadrant operation, the active front end can be selected.

6, multi-level + multiple inverter

Japan's Fuji Corporation uses the high-voltage IGBT developed medium-voltage inverter FRENIC4600FM4 series, which brings together many advantages of multi-level and multi-inverter, it is realized by multiple serial connection of multiple medium-voltage three-level PWM inverter power units The direct high-voltage output thus constitutes a double perfect harmonic-free system: multiple superposition rectification for the grid, the harmonics meet the requirements of IEEE519?1992; the motor is a perfect harmonic-free sine wave output, which can directly drive any brand of AC mouse Cage motor.

Due to the high-voltage rectifier diode and high-voltage IGBT, the inverter used in the main circuit of the system is greatly reduced, the reliability is improved, the loss is reduced, and the volume is reduced. The integrated efficiency of the inverter can reach 98%, the power factor is up to 0.95, no need to add phase-in capacitor or AC/DC reactor, and no output filter is needed, which greatly simplifies the system structure. Rate unit, which is not dominant in the number of devices, is more than double the device of the three-level three-phase inverter of the same voltage and power level, and also has exactly one more than the ordinary single-phase inverter power unit. Times of the device. For example, with a 3300V withstand voltage IGBT device, a 6kV system with a unit series multiple circuit requires three units in series for each phase, a total of 9 units, a total of 54 rectifier diodes, 36 IGBTs, and a three-level power unit. Each phase requires two units in series, a total of 6 units, a total of 72 rectifier diodes, 48 ​​IGBTs, a full 1/3 of the device and double the redundancy cost of the power unit, reducing the multiplex The inverter has good redundancy performance and increases the cost of the device. Therefore, this type of frequency converter is actually not desirable.

7 transformer coupled output high voltage inverter

In the main circuit topology of medium and high voltage inverters, in addition to the two-level, multi-level and unit series multiplexing scheme mentioned above, in 1999, a new type of transformer-coupled unit series high-voltage inverter main circuit topology was proposed. structure. The main idea is to use a transformer to superimpose the outputs of three conventional two-level three-phase inverter units consisting of high-voltage IGBTs or IGCTs to achieve higher voltage output, and these three conventional inverters can be used with ordinary low-voltage inverters. The control method of the device greatly simplifies the circuit structure and control method of the inverter.

The program consists of the following components:

——An 18-pulse input transformer can basically realize the input current without harmonics;

- three conventional two-level three-phase DC/AC inverters;

- three output transformers with a change of 1:1;

- High voltage motor.

The following is an analysis of how it works in several ways.

1) Voltage relationship

Considering the line voltage of the motor, you can get:

UKL=Ua1b1+Ub1a2+Ua2b2

ULM=Ub2c2+Uc2b3+Ub3c3(1)

UMK=Uc3a3+Ua3c1+Uc1a1

Since the output transformer has a ratio of 1:1, that is, Ub1a2=Ua3b3, Uc2b3=Uc1b1, Uc1a3=Ua2b2, then it is available.
UKL=Ua1b1+Ua2b2+Ua3b3 ULM=Ub1c1+Ub2c2+Ub3c3(2)

This relationship between the voltages of UMK=Uc1a1+Uc2a2+Uc3a3 is shown in FIG. Each inverter adopts SPWM or space voltage vector PWM (SVPWM) control method, and the effective value of each inverter output line voltage is [] aE, where E is the inverter input DC voltage and a is the modulation depth. In harmonic injection SPWM and SVPWM, a can be up to 1.15. The effective value of the motor line voltage obtained by equation (2) is [] aE.

For a high-voltage motor with a line voltage of 2300V, E=1090V, an IGBT with a rated voltage of 1700V can be used to form the system; for a high-voltage motor with a line voltage of 4160V, E=1970V, an IGBT with a rated voltage of 3300V can be used; When the line voltage of the high-voltage motor is 6600V and E=3130V, the IGCT with the rated voltage of 4500V should be used; therefore, the scheme has strong adaptability.

2) Current relationship

Set the motor three-phase current balance, the effective value of the current is I, without considering the current harmonics ia1=Isin(ωt)ib2=Isin(ωt-120°)(3)ic3=Isin(ωt+120°)

In Fig. 12, ia1=i4-i6, ib2=i6-i2, i2+i4+i6=0, so that ia1=Isin(ωt+90°)ib2=Isin(ωt-30°)(4)ic3=Isin(ωt-150° )

Considering that the primary and secondary currents of the output transformer are equal, the three output currents of the first inverter can be calculated as ia1=Isin(ωt)ib1=Isin(ωt-120°)(5)ic1=Isin( Ωt+120°)

The three output currents of the other two inverters also satisfy the above relationship, namely: ia1=ia2=ia3=Isin(ωt)ib1=ib2=ib3=Isin(ωt-120°)(6)ic1=ic2=ic3= Isin(ωt+120°)

This means that the three inverter output currents are fully balanced.

3) Power relationship After the voltage-current relationship is obtained, we can easily obtain the power relationship between the various parts of the high-voltage inverter. Obviously, the apparent power of three inverters VA1, VA2, VA3 is VA1=VA2=VA3=[]aEI, and the apparent power VA of the whole high-voltage inverter is VA=[]aEI, that is, three inverses. The transformer divides the output of the entire inverter.

4) PWM strategy

Since the voltage, current and power of the three inverters are completely symmetrical, the three inverters can adopt the same control law. The line voltage applied to the motor is equal to three times the voltage of one inverter output line, which is equivalent to A two-level PWM high-voltage inverter, although this method is simple, it is not suitable because dv/dt is too large.

A better method is to shift the PWM signals of the three inverters by 1/3 switching cycles. For the SPWM, the three inverters each use a triangular wave, and the three triangular waves have phase differences. 120°. Figure 13 is a graph of the motor line voltage obtained by this method, in which the voltage frequency is 40 Hz, and a third harmonic of 15% is injected. It can be seen that this is a high-voltage inverter with a line voltage of 7 levels, which is equivalent to the line voltage waveform of a four-level inverter.

5) Output transformer The output transformer plays a very important role in this solution, and may also be the weak link of this solution, because a transformer with too large capacity will limit its application. In general, the transformer can adopt the structure shown in FIG. From the previous analysis, it is known that the effective value of the voltage between the windings of the output transformer is [] aE, and the current flowing through each winding is equal, the effective value is, so the capacity of the transformer can be obtained as [] aE, that is, the output The capacity of the transformer is 1/3 of the total capacity of the inverter, which is much smaller than the capacity of the output transformer in the high-low-high scheme.

This high-voltage inverter solution has the following outstanding advantages:

1) Three high-voltage inverters can be constructed with three conventional inverters as the core;

2) Three conventional frequency converters operate in balanced symmetrical operation, each sharing 1/3 of the total output power;

3) The output of the whole inverter can be equivalent to the 7-level PWM output waveform is better than the ordinary three-level inverter, the same as the four-level inverter. The total harmonic distortion THD<0.3%, dv/dt is also low;

4) The output transformer capacity is only 1/3 of the total capacity, which can be built-in or externally mounted;

5) 18-pulse input diode rectifier with small harmonics on the grid side and high power factor.

8, the conclusion

Power devices in series with two-level current-type inverters are increasingly limited in their use due to their own shortcomings.

The unit serial multi-frequency converter is due to the low voltage of the power device at that time, the system is complex, the number of components is large, the volume is large, and the failure rate is high; but it is beaten up and won the unparalleled input and output waveform, which is called “perfect Harmonic"; The improved method is to use high-voltage IGBT or IGCT to form the power unit to reduce the number of cells and reduce the volume, but at the expense of the waveform, the output filter is added to make the harmonics reach the standard.

The three-level inverter using high-voltage IGBT and IGCT has the advantages of simple structure, high reliability, small number of components and high efficiency. In the face of high-voltage power supply, multi-level can be used. Who will use multiple multiplexes? However, the waveform is slightly worse, and an LC output filter is required. Even so, the cost is lower than that of the multiple inverter. At present, due to the limitation of the withstand voltage of the device, the output voltage can only reach 4.16kV. If the output is 6kV, the motor Y/â–³ can be used to change the connection. It seems that this is the most economical and reasonable solution for 6kV motor energy-saving retrofit.

Transformer coupled output high-voltage inverter, is expected to achieve 6kV, 10kV high-voltage output with current voltage level devices, is a promising new high-voltage frequency conversion program.

With the continuous development of power devices, GTO is about to exit the stage in medium-power high-voltage inverters. High-voltage IGBTs and IGCTs are promising devices and are the hope of solving medium- and high-voltage inverters. IGCTs have lower conduction voltages. The loss is small and has certain advantages, and will become the main power device of the high-voltage inverter.