Analysis and application of main circuit diagram of high voltage inverter

2. Analysis of the main circuits of several commonly used high-voltage inverters

(1) Unit series multiple voltage source type high voltage inverter

Unit series multiplexing voltage source type High voltage inverter Use low voltage single-phase inverters in series to make up for the lack of voltage resistance of power device IGBT. The so-called multiplexing means that each phase is composed of several low voltage power units in series, and each power unit is powered by a multi-winding phase-shifting isolation transformer, controlled by a high-speed microprocessor and driven by optical fiber isolation. However, it has the following disadvantages:

a) Too many power units and power devices are used. A 6kV system needs 150 power devices (90 diodes and 60 IGBTs). The device is too large and heavy, and the installation location and infrastructure investment are problematic.

b) Too many high-voltage cables are required, the internal resistance of the system increases invisibly, too many connections are made, and the number of fault points increases accordingly;

c) When a unit is damaged, the unit can be bypassed, but at this time the output voltage is unbalanced and the voltage at the center point is floating, causing voltage and current imbalance, so that harmonics also increase accordingly. If it is barely running, it will eventually cause damage to the motor;

d) The output voltage waveform is good at rated load, and the distortion is prominent below 25Hz;

e) Due to the existence of transformers in the system, it is not easy to further improve the system efficiency; in the phase-shifting transformer, the 6kV three-phase 6 windings × 3 (10kV requires 12 windings × 3) extended triangle connection method, when the three-phase voltage is unbalanced (in fact, the three-phase voltage cannot be absolutely balanced), the internal circulation generated will inevitably cause an increase in internal resistance and current loss, and correspondingly increase the copper loss of the transformer. At this time, coupled with the inherent loss of the transformer core, the efficiency of the transformer will be reduced, which will affect the efficiency of the entire high-voltage inverter. This situation is more significant when the load is lower than the rated load. At 10kV, the transformer has nearly 400 joints and nearly 100 cables. The efficiency can reach 96% at rated load, but at light load, the efficiency is less than 90%.

(2) Neutral point clamped three-level PWM inverter

This series of inverters adopts the traditional voltage type inverter structure. The inverter part of the neutral point clamped three-level PWM inverter adopts the traditional three-level method, so it is inevitable that a relatively large harmonic component will be generated in the output waveform, which is inherent in the three-level inverter method. Therefore, an output LC filter must be configured on the output side of the inverter to be used for ordinary squirrel cage motors. Also due to harmonics, the power factor, efficiency, and even life of the motor will be affected to a certain extent. Only at the rated operating point can the best working state be achieved, but as the speed decreases, the power factor and efficiency will decrease accordingly.

Multi-level + multiplexed high-voltage inverter. The original intention of multi-level + multiplexed high-voltage inverter is to solve the problem of limited withstand voltage of high-voltage IGBT, but this method not only increases the complexity of the system, but also reduces the advantages of good multiplexed redundancy performance and simple three-level structure. Therefore, this type of inverter is actually not advisable.

The performance-price advantage of this type of inverter is not significant. Instead of using both multi-level and multiplexing technologies, it is better to use the multiplexing inverter or three-level inverter with high-voltage IGBT mentioned above.

(3) Current source high voltage inverter

The current source type high voltage inverter with power devices directly connected in series is constructed by connecting a large inductor in series in the circuit, and then directly connecting power devices with slower switching speeds such as SCR (or GTO, SGCT, etc.) in series.

Although this method uses fewer power devices and is easy to control current, it does not really solve the problem of series connection of high-voltage power devices. Because even if the power device fails, due to the current limiting effect of the large inductor, di/dt is limited. Although the power device is not easy to be damaged, the problem it brings is serious pollution to the power grid and low power factor. In addition, the current source type high-voltage inverter is sensitive to changes in the grid voltage and motor load, and cannot be made into a truly universal product.

The current source type high voltage inverter is the earliest product, but wherever the voltage type inverter arrives, it is forced to withdraw because it is obviously at a disadvantage economically and technically.

3. IGBT direct series high voltage inverter

3.1 Introduction to the main circuit

Figure 1. IGBT directly connected in series for high voltage frequency conversion

As shown in Figure 1, the system in the figure enters the inverter directly from the high voltage of the power grid through the high voltage circuit breaker, undergoes high voltage diode full-bridge rectification, DC smoothing reactor and capacitor filtering, and then is inverted by the inverter. With the addition of a sine wave filter, high voltage frequency conversion output is easily achieved and directly supplied to the high voltage motor.

The two-level voltage type high-voltage inverter with power device IGBT directly connected in series is a high-voltage speed control system with no input and output transformer, IGBT direct series inverter, and output efficiency of 98%, which is successfully designed by using the existing mature technology of inverter and applying unique and simple control technology. For occasions requiring rapid braking, a DC discharge braking device is used, as shown in Figure 2:

Figure 2. Main circuit diagram of IGBT direct series high-voltage inverter with DC discharge braking device

If four-quadrant operation is required, energy feedback is required, or the short-circuit capacity of the input power supply side is small, a PWM rectifier circuit as shown in Figure 3 can also be used to make the input current truly a perfect sine wave.

Figure 3. Main circuit diagram of IGBT direct series high-voltage inverter with energy feedback and four-quadrant operation

3.2 IGBT direct series high voltage inverter 25Hz, 30Hz, 40Hz, 50Hz voltage, current output waveform and harmonic diagram:

(1) Series connection technology of high-speed power devices

According to new research, no country in the world has produced a high-voltage inverter with IGBTs directly connected in series. The reason is as some authoritative sources said: "IGBTs cannot be connected in series. Because the switching time is short, in microseconds, it is difficult to ensure that all tubes connected in series switch on and off at the same time. Otherwise, some will open early, and all the voltage will be applied to the tubes that open late. Then, this 1200V tube with 6000V will burn out, and one tube will burn out. It is impossible to connect in series."

(2) Sine wave technology

It is common knowledge that high-voltage motors have strict requirements on the output voltage waveform of the inverter. To solve the output voltage waveform of the inverter, we start from two aspects: one is to optimize the PWM waveform; the other is to develop a special filter.

In the past, some people believed that: "The voltage waveform of three-level must be better than that of two-level. In the future, even low-voltage inverters should adopt three-level." This statement may not be comprehensive. The total harmonic content of three-level may be lower than that of two-level, but because the 11th and 13th harmonic content of three-level is particularly high, it is particularly difficult to handle. As long as the waveform of two-level is well optimized, the harmonics below 60 can be greatly reduced. It is naturally much easier to filter harmonics above 60. People use three-level to avoid the difficulty of connecting devices in series.

(3) Common mode voltage protection technology

Simply solving the series connection of IGBTs does not eliminate the input transformer. The reason lies in the existence of common-mode voltage. In the field of low-voltage inverters, common-mode voltage is one of the causes of motor bearing damage found in recent years. In the field of high-voltage inverters, common-mode voltage is one of the key issues that must be solved. Common-mode voltage (also called zero-sequence voltage) refers to the voltage between the center point of the motor stator winding and the ground.

Common mode voltage is also the cause of external interference, especially for long-distance transmission equipment. Whether it is a current source or a voltage source inverter, it is inevitable that it will generate common mode voltage. Based on the mechanism of common mode voltage generation, technicians have adopted a "blocking and unblocking" method to eliminate the common mode voltage inside the inverter.

Due to the adoption of the above three core key technologies, the efficiency of IGBT direct high-voltage inverter reaches more than 98%. The output voltage is sinusoidal and the common-mode voltage is minimized. It is suitable for any asynchronous motor and synchronous motor, and there is no need to reduce the capacity. Long-distance transmission of several kilometers is no problem. When the transmission distance is too long, line voltage compensation should be considered, such as increasing the voltage or increasing the wire cross-section.

4 System Features:

(1) The voltage level is 3kV-10kV;

(2) The system comes with a specially designed high-voltage switch cabinet, which is efficiently and safely matched with its own high-voltage inverter, and contains variable/power frequency switching devices and electronic vacuum circuit breakers;

(3) Fully Chinese operation interface, based on Windows operating platform, color LCD touch screen, convenient for on-site monitoring, parameter setting, function selection and debugging;

(3) Built-in PLC programmable controller, easy to change and expand control logic relationship;

(4) The high-voltage main circuit and the low-voltage control circuit are transmitted by optical fiber and safely isolated, making the system highly anti-interference;

(5) The control circuit communication method adopts fully digital communication;

(6) The system's rectifier unit and inverter unit are designed with a modular building block structure. The whole machine occupies a small area, is light in weight, and is easy to install and maintain;

(7) The device can be operated locally or remotely controlled, with complete and convenient operating function options;

(8) The system has standard computer communication interfaces RS232 or RS422, RS485, which can be easily configured with the user's DCS system or industrial control system to establish a workstation for the entire system, further improving the system's automation control level and achieving full closed-loop monitoring of the entire industrial control system, thereby achieving more complete and reliable automation operation;

(10) It has comprehensive fault monitoring and reliable fault alarm protection functions;

(11) The input power factor is high, the output voltage harmonic content is small, and no power factor compensation and harmonic suppressor are required;

(12) The output voltage is a standard sinusoidal waveform, which does not damage the insulation of the cable and the motor, reduces the vibration and wear of the mechanical parts such as the bearings and blades of the motor, and extends the service life of the motor. The cable length output to the motor can reach 20km;

(13) Using unique anti-common mode voltage technology, the system common mode voltage is ≤1000V, no need to increase the insulation level of the motor, no need for a dedicated motor;

(14) It is easy to realize energy feedback and four-quadrant operation; and can directly lead out DC for DC transmission;

(15) There are no special requirements for the user's high-voltage asynchronous motor. It is applicable not only to new and old asynchronous motors, but also to synchronous motors.

5. Application Examples

Application of IGBT directly connected in series with high voltage inverter in slag flushing pump of iron making plant

5.1 Application Overview

Yongfeng Steel Plant is a major production plant of Laiwu Steel Group Co., Ltd., responsible for the smelting of molten iron and iron blocks required by the company. A large amount of slag is produced during the smelting of molten iron in the blast furnace. It is usually cooled and flushed with medium-pressure water with a large flow rate, and transported to the slag pool for recovery as a by-product of ironmaking production. Blast furnace production is uninterrupted. Under normal circumstances, iron is tapped 15 times a day. Slag is released once before and after the blast furnace taps iron. The two slag tapping times are about 30 minutes. During this time, the water pump of the water slag flushing system is required to work at full load. The rest of the time, the water pump only needs to maintain about 30% water flow to prevent pipeline blockage. 4#-Blast furnace uses ZGB-300 slag flushing pump. When the original system is running, the inlet and outlet valves of the pipeline are closed before starting, and the valve opening is about 90% after starting. The unit runs at full speed, the grid voltage is 6300V, the motor operating current is 33A, the power factor is 81.6%, and the power consumption is 294kW. When slag flushing water is not needed, the water flow is adjusted by adjusting the valve at 30% (the motor current is 25A at this time), and the power consumption is 214kW. On the one hand, it leads to a large amount of energy-saving losses. On the other hand, frequent operation of the valve greatly reduces its service life and increases the time for stopping production and replacing the valve. For this reason, the company decided to transform the slag flushing pump of No. 4 blast furnace.

5.2 Renovation plan

From the motor speed formula n=60f×(1-s)/p, we can know that as long as the motor frequency f is changed, the motor speed can be adjusted. The high-voltage and high-power inverter controls the on and off of the IGBT (insulated gate bipolar power field effect transistor) to make the output frequency continuously adjustable. Moreover, as the frequency changes, the output current, voltage, and power will change, that is, when the load is large, the speed is large and the output power is large, and when the load is small, the speed is small and the output power is also small.

From fluid mechanics: Q′=Q(n′/n), H′=H(n′/n)2, P′=P(n′/n)3, we can know that when the pump speed is lower than the rated speed, the power saving is: E=〔1-(n′/n)3〕×P×T(kWh)

It can be seen that through frequency conversion transformation, the flow Q, pressure H and shaft power P of the slag flushing pump will undergo significant changes, which not only saves energy but also greatly improves the equipment operation performance. According to the actual characteristics of the slag flushing pump, it has been specifically transformed. The slag flushing pump works at 49.5Hz when flushing slag and works at 25Hz when not flushing slag. Considering that the process does not require very high speed regulation accuracy, this system only uses open-loop control and is operated in the blast furnace duty room. When slag flushing is required, a "1" signal is given to the regulation system, and the motor runs at high speed. When slag flushing is not required, this signal is cancelled and the motor runs at low speed, achieving a good energy-saving effect.

5.3 Actual operation status of the modified system

According to 18 months of operation, after repeated tests, all operating parameters have been normal, the inverter has good quality and performance, is safe and reliable, and all indicators have met the design requirements.

(1) Good harmonic suppression effect. The voltage harmonic content is less than 3%, which complies with IEEE519-1992 and GB/T14549-93 standards.

(2) Various protection functions are complete. Overcurrent, overvoltage, undervoltage, fault protection and other functions are reliable, and multi-link protection functions such as lightning protection of the external power grid are taken into consideration.

(3) Various indication functions are complete. It has functions such as input and output current and voltage, operating frequency, fault display, and operating status indication.

(4) Easy to operate. Similar to the function operation mode of ordinary low-voltage inverters, function setting and adjustment are simple and convenient.

5.4 Transformation benefits

Compared with the operation without inverter, the unit can save power ΔP1=P50-P49.5=80kW when running at 49.5Hz;

Compared with the operation without inverter, the unit can save power ΔP2=214kW-P25=132kW when running at 25Hz;

Annual power saving: ΔW = (H1ΔP1+H2ΔP2) = 365(7.5×80+16.5×132) = 1013970kWh;

(Note: 365 days per year, H1: slag flushing time = 15 × 30/60 = 7.5 hours; H2: no slag flushing time = 24-7.5 = 16.5 hours);

Economic benefit: ΔW electricity price = 1013970 × 0.56 = 567823 yuan (Note: Laiwu Steel Plant industrial electricity price is 0.56 yuan/kWh);

Realize the motor soft start function, prolong the motor life, and greatly reduce the failure rate of the slag flushing pump;

Improved the level of automation and saved a lot of industrial water;

From the above, we can see that the comprehensive economic benefits can reach more than 600,000 yuan per year, and all costs can be recovered in one year.

5.5 Conclusion

Because the IGBT direct series high-voltage inverter has no input and output transformers, is small in size, has a high cost-effectiveness, and has good comprehensive performance, it surpasses other domestic and foreign products. It is the representative of the new generation of high-performance high-voltage frequency conversion products, and provides a feasible way for the technical transformation of high-voltage frequency conversion speed regulation technology in other processes in the factory. It has great promotion value in the field of high-voltage frequency conversion transformation.