Abstract: This paper introduces the working principle, structure, control method, operation mode, and frequency conversion schemes and application effects of high-voltage frequency converters in different application scenarios. It also focuses on some application issues of high-voltage frequency converters.
1. Introduction
Electric motors are major power-consuming equipment in industrial production, with high-voltage, high-power motors being particularly prominent. Most of these devices possess significant energy-saving potential. Therefore, vigorously developing high-voltage, high-power variable frequency speed control technology is both necessary and urgent. A high-voltage frequency converter is a series-superimposed high-voltage frequency converter, which uses multiple single-phase three-level inverters connected in series to output high-voltage AC power with variable frequency and voltage. According to the basic principles of motor theory, the motor speed satisfies the following relationship: n = (1 - s)60f/p = n₀ × (1 - s) (P: number of pole pairs; f: motor operating frequency; s: slip). From the formula, it can be seen that the synchronous speed n₀ of the motor is proportional to the motor's operating frequency (n₀ = 60fp). Since the slip s is generally small (0-0.05), the actual speed n of the motor is approximately equal to the synchronous speed n₀. Therefore, adjusting the power supply frequency f of the motor can change the actual speed of the motor. The slip s of the motor is related to the load; the larger the load, the greater the slip. Therefore, the actual speed of the motor will decrease slightly with the increase of the load.
The frequency converter itself consists of three parts: a transformer cabinet, a power unit cabinet, and a control cabinet. Three-phase high-voltage electricity enters through the high-voltage switch cabinet, is stepped down and phase-shifted to supply power to the power units within the power unit cabinet. The power units are divided into three groups, each group consisting of one phase, with the outputs of each phase power unit connected in series. The control unit in the main control cabinet performs rectification, inversion control, and monitoring of each power unit in the power unit cabinet via fiber optic cable. Based on actual needs, the frequency is set through the operating interface, and the control unit sends the control information to the power units for corresponding rectification, inversion, and adjustment, outputting a voltage level that meets the load requirements.
Currently, with the rapid development of modern power electronics and microelectronics technologies, high-voltage, high-power variable frequency speed control devices are becoming increasingly sophisticated. Previously difficult-to-solve high-voltage problems have been effectively addressed in recent years through device series connection or unit series connection. Their application areas and scope are also expanding, providing the technological prerequisites for industrial and mining enterprises to efficiently and rationally utilize energy (especially electrical energy).
2. Analysis of the main circuits of several commonly used high-voltage frequency converters
(1) Unit series multiplexed voltage source type high voltage frequency converter
The multi-phase voltage source inverter uses low-voltage single-phase inverters connected in series to compensate for the insufficient voltage withstand capability of IGBT power devices. Multi-phase inverters consist of several low-voltage power units connected in series in each phase. Each power unit is powered by a multi-winding phase-shifting isolation transformer, and control is achieved using a high-speed microprocessor, with fiber optic isolation for drive. However, it has the following drawbacks:
a) The number of power units and power devices used is too large. The 6kV system requires 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, which increases the internal resistance of the system and the number of fault points increases accordingly.
c) When a unit is damaged, the unit can be bypassed, but at this time the voltage at the center point of the output voltage imbalance is floating, causing voltage and current imbalance, which in turn increases the harmonics accordingly. If it is forced to run, it will eventually lead to the damage of the motor.
d) The output voltage waveform is good under rated load, but distortion is prominent below 25Hz;
d) The output voltage waveform is good under rated load, but distortion is prominent below 25Hz;
e) Due to the presence of a transformer in the system, further improvements in system efficiency are difficult to achieve. In a phase-shifting transformer, a 6kV three-phase 6-winding × 3 (10kV requires 12-winding × 3) delta connection will inevitably lead to internal circulating currents when the three-phase voltage is unbalanced (in reality, three-phase voltages cannot be perfectly balanced). This will increase internal resistance and current loss, consequently increasing the transformer's copper losses. Combined with the inherent losses of the transformer core, the transformer's efficiency will decrease, affecting the overall efficiency of the high-voltage frequency converter. This situation is more pronounced when operating below the rated load. At 10kV, the transformer has nearly 400 joints and nearly 100 cables. While the efficiency can reach 96% at rated load, it drops below 90% under light load.
(2) Neutral point clamped three-level PWM inverter
This series of frequency converters adopts a traditional voltage-type frequency converter structure. The inverter section of the neutral-point clamped three-level PWM frequency converter uses a traditional three-level method, so the output waveform will inevitably generate relatively large harmonic components, which is inherent to the three-level inverter method. Therefore, an output LC filter must be configured on the output side of the frequency converter for use with ordinary squirrel-cage motors. Similarly, due to harmonics, the motor's power factor, efficiency, and even lifespan will be affected to some extent. It can only reach its optimal operating state at the rated operating point, but as the speed decreases, the power factor and efficiency will decrease accordingly.
Multilevel + multiplexing high-voltage frequency converters. The original intention of multilevel + multiplexing high-voltage frequency converters was to solve the problem of limited voltage withstand capability of high-voltage IGBTs. However, this approach not only increases system complexity but also diminishes the advantages of good redundancy performance and simple three-level structure. Therefore, this type of frequency converter is actually not advisable.
The performance-price advantage of this type of frequency converter is not significant. Rather than using both multilevel and multiplexing technologies, it would be better to use the aforementioned high-voltage IGBT multiplexing frequency converter or three-level frequency converter.
(3) Current source type high voltage frequency converter
A current source type high-voltage frequency converter with power devices directly connected in series is constructed by connecting a large inductor in series in the line and then directly connecting power devices with slow switching speeds, such as SCRs (or GTOs, SGCTs, etc.), in series.
While this approach uses fewer power devices and is easier to control current, it doesn't truly solve the problem of series connection of high-voltage power devices. Even if a power device fails, the current-limiting effect of the large inductor restricts di/dt, making the device less prone to damage. However, this results in severe grid pollution and a low power factor. Furthermore, current-source high-voltage frequency converters are sensitive to changes in grid voltage and motor load, making them unsuitable for truly universal applications.
Current source type high voltage frequency converters were the earliest products, but wherever voltage source frequency converters went, they were forced to withdraw because they were clearly at a disadvantage economically and technically.
3. Direct high-voltage frequency converter with IGBTs in direct series connection
3.1 Introduction to the Main Circuit
Figure 1. IGBT direct series high voltage frequency converter
As shown in Figure 1, the system in the figure receives high voltage from the power grid directly through a high voltage circuit breaker into the frequency converter. After passing through a high voltage diode full-bridge rectifier, a DC smoothing reactor, and a capacitor filter, it is then 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 frequency converter with IGBTs directly connected in series is a high-voltage speed control system with no input/output transformer, IGBT direct series inverter, and output efficiency of 98%, which is successfully designed by using existing mature frequency converter technology and unique and simple control technology.
For applications 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 frequency converter with DC discharge braking device.
If four-quadrant operation is required, or if energy feedback is needed, or if the short-circuit capacity on 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 achieve a perfect sine wave.
Figure 3. Main circuit diagram of IGBT direct series high-voltage frequency converter with energy feedback and four-quadrant operation.
3.2 Voltage and current output waveforms and harmonic diagrams of IGBT direct series high-voltage frequency converter at 25Hz, 30Hz, 40Hz, and 50Hz:
3.3 Core and Key Technologies
(1) Series technology for high-speed power devices
According to novelty searches, no country in the world has produced a high-voltage frequency converter with IGBTs directly connected in series. The reason is as some authoritative figures have stated: "IGBTs cannot be connected in series. Because the switching time is short, on the order of microseconds, it is difficult to ensure that all the IGBTs connected in series switch simultaneously. Otherwise, if some turn on early, all the voltage would be applied to the IGBTs that turn on late. In that case, if a 1200V IGBT is subjected to 6000V, it will burn out. If one burns out, the entire series will follow. It is impossible to connect them in series."
(2) Sine wave technology
It is common knowledge in the industry that high-voltage motors have strict requirements on the output voltage waveform of frequency converters. Solving the problem of frequency converter output voltage waveform issues involves two approaches: first, optimizing the PWM waveform; and second, developing specialized filters.
In the past, some people believed that "three-level voltage waveforms are always superior to two-level waveforms, and even low-voltage frequency converters should adopt three-level waveforms in the future." This statement may not be entirely accurate. While the total harmonic content of three-level waveforms may be lower than that of two-level waveforms, the 11th and 13th harmonics are particularly high and difficult to handle. In contrast, with two-level waveforms, harmonics below the 60th order can be significantly reduced with proper waveform optimization. Filtering harmonics above the 60th order is naturally much easier. Three-level waveforms were used out of necessity to avoid the difficulties of connecting components in series.
(3) Common-mode voltage rejection technology
Simply resolving the IGBT series connection doesn't eliminate the need for an input transformer. This is due to the presence of common-mode voltage. In the low-voltage inverter field, common-mode voltage is one of the contributing factors to motor bearing damage discovered in recent years. In the high-voltage inverter field, common-mode voltage is an even more critical issue that must be addressed. Common-mode voltage (also called zero-sequence voltage) refers to the voltage between the center point of the motor stator winding and ground.
Common-mode voltage is also a cause of external interference, especially in long-distance transmission equipment. Whether it's a current-source or voltage-source frequency converter, the generation of common-mode voltage is inevitable. Based on the mechanism of common-mode voltage generation, technicians have adopted a "blocking and channeling" approach to eliminate common-mode voltage within the frequency converter.
Thanks to the adoption of the three core technologies mentioned above, the efficiency of the IGBT direct high-voltage frequency converter reaches over 98%. The output voltage is sinusoidal, and the common-mode voltage is minimized. It is suitable for any asynchronous or synchronous motor, requiring no derating, and can handle long-distance transmissions of several kilometers without issue. For extremely long transmission distances, line voltage compensation should be considered, such as increasing the voltage or increasing the conductor cross-section.
4. System Features:
4.1 Intelligent Power Unit
All power modules are intelligently designed with powerful self-diagnostic capabilities. In the event of a fault, the power module quickly relays the fault information to the main control unit, which promptly shuts down the main power components (IGBTs) to protect the main circuit. Simultaneously, the fault location and type are accurately displayed on the Chinese human-machine interface. The design incorporates standardization considerations for unit modules within a certain power range, ensuring consistency in structure and function. When a module fails, an alarm notification is received, and a backup module with equivalent functionality can be installed within minutes, minimizing downtime.
The 6kV grid voltage is stepped down by a secondary-side multiplexed isolation transformer to power the power units. Each power unit is a three-phase input, single-phase output AC/DC PWM voltage source inverter. Adjacent power units are connected in series at their outputs, forming a Y-connection, enabling direct high-voltage output for high-voltage motors. At the 6kV voltage level, each phase consists of six power units with a rated voltage of 600V connected in series, resulting in a maximum output phase voltage of 3464V and a line voltage of approximately 6000V. Changing the number of power units connected in series in each phase or altering the output voltage level of the power units allows for high-voltage output at different voltage levels. Each power unit is powered by a separate set of secondary windings from the input transformer, and the power units and the transformer secondary windings are mutually insulated. The secondary windings employ an extended delta connection for multiplexing, reducing input harmonic currents. The 6kV voltage level inverter supplies power to 18 power units. The 18 secondary windings are divided into groups of three, forming 6 different phase groups with a 10-degree electrical angle difference between each other, forming a 36-pulse rectifier circuit structure. The input current waveform is close to a sine wave. This equal-value split-phase power supply method greatly reduces the total harmonic current distortion, and the power factor of the inverter input can reach above 0.95.
(1) Voltage level is 3kV-10kV;
(2) The system comes with a specially designed high-voltage switchgear, which is highly efficient and safe to match the high-voltage frequency converter itself, and includes a variable/power frequency switching device and an electronic vacuum circuit breaker;
(3) Fully Chinese operating interface, based on Windows operating platform, color LCD touch screen, facilitating local monitoring, parameter setting, function selection and debugging;
(3) Built-in PLC programmable controller, which makes it easy to change and expand the control logic;
(4) The high-voltage main circuit and the low-voltage control circuit use optical fiber transmission and are safely isolated, which makes the system highly resistant to interference;
(5) The control circuit uses fully digital communication.
(6) The rectifier and inverter units of the system are designed with a modular building block structure, which makes the whole machine small in size and light in weight, and easy to install and maintain;
(7) The device can be operated on the machine itself or remotely controlled, and has complete and convenient operation function selection;
(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 degree of automation control of the system, realizing full closed-loop monitoring of the entire industrial control system, and thus obtaining more complete and reliable automated operation;
(10) Possesses comprehensive fault monitoring and reliable fault alarm protection functions;
(11) High input power factor and low output voltage harmonic content, eliminating the need for power factor compensation and harmonic suppressors;
(12) The output voltage is a standard sine wave, which does not damage the insulation of the cable and motor, reduces the vibration and wear of mechanical parts such as bearings and blades of the motor, extends the service life of the motor, and the length of the cable output to the motor can reach 20km;
(13) Employing unique common-mode voltage rejection technology, the system's common-mode voltage is ≤1000V, eliminating the need to improve the motor's insulation class and requiring no dedicated motor;
(14) It is easy to achieve energy feedback and four-quadrant operation; and it can directly draw out DC for DC power transmission;
(15) No special requirements are placed on 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 Example: Application of IGBT Direct Series High Voltage Frequency Converter in Slag Flushing Pumps of Ironmaking Plants
5.1 Application Overview
Yongfeng Steel Plant is a major production plant of Chongqing Iron & Steel Group Corporation, responsible for smelting the company's required molten iron and iron blocks. During the blast furnace smelting process, a large amount of molten slag is generated. This slag is typically cooled and dispersed using a large flow of medium-pressure water, and then transported to a slag pool for recycling as a byproduct of ironmaking. Blast furnace production is uninterrupted, generally involving 15 tapping operations per day. Slag is discharged once before and once after tapping, with each discharge lasting approximately 30 minutes. During this time, the water pumps of the slag flushing system must operate at full capacity. At other times, the pumps only need to maintain approximately 30% water flow to prevent pipe blockage. Blast Furnace #4 uses a ZGB-300 slag flushing pump. Under normal system conditions, the inlet and outlet valves are closed before startup, and after startup, the valves are opened to approximately 90%, with the unit running 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 rate is adjusted by regulating the valve to 30% (at which time the motor current is 25A), consuming 214kW of power. This results in significant energy loss and frequent valve operation, which greatly reduces the service life of the pump and increases the time spent replacing the valve during production downtime. Therefore, the company decided to modify the slag flushing pump of blast furnace No. 4.
5.2 Renovation Plan
From the motor speed formula n=60f×(1-s)/p, it can be seen that the motor speed can be adjusted by changing the motor frequency f. High-voltage, high-power frequency converters control the conduction and cutoff of IGBTs (Insulated Gate Bipolar Transistors) to make the output frequency continuously adjustable. Moreover, as the frequency changes, the output current, voltage, and power will also change; that is, the speed and output power are high when the load is high, and the speed and output power are low when the load is low.
From fluid mechanics: Q′=Q(n′/n), H′=H(n′/n)², P′=P(n′/n)³, we know that when the pump speed is below the rated speed, the energy saving is: E=〔1-(n′/n)³〕×P×T(kWh)
It is evident that through frequency conversion modification, the flow rate Q, pressure H, and shaft power P of the slag flushing pump will undergo significant changes, resulting in not only energy savings but also greatly improved equipment operating performance. Specific modifications were made to the slag flushing pump based on its actual characteristics. The pump operates at 49.5Hz during slag flushing and at 25Hz when not flushing. Considering that the process does not require very high speed regulation accuracy, this system only employs open-loop control and is operated from the blast furnace control room. When slag flushing is needed, a "1" signal is sent to the regulating system, causing the motor to run at high speed. When slag flushing is not needed, this signal is canceled, and the motor runs at low speed, achieving excellent energy-saving results.
5.3 Actual operating status of the modified system
After 18 months of operation, and after repeated testing, all operating parameters remained normal, indicating that the frequency converter 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) Comprehensive protection functions. The overcurrent, overvoltage, undervoltage, and fault protection functions are reliable, and multi-stage protection functions such as lightning protection against external power grids are also taken into account.
(3) It has complete indication functions. It has functions such as input and output current and voltage, operating frequency, fault display, and operating status indication.
(4) Easy to operate. The operation mode is similar to that of ordinary low-voltage frequency converters, and the function settings and adjustments are simple and convenient.
5.4 Benefits of the renovation
Compared to operating without a frequency converter, the unit can save 80kW of power when operating at 49.5Hz.
Compared to operating without a frequency converter, the unit can save ΔP2=214kW-P25=132kW of power when operating at 25Hz.
Annual electricity savings: ΔW = (H1ΔP1 + H2ΔP2) = 365(7.5 × 80 + 16.5 × 132) = 1013970 kWh;
(Note: Based on 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 benefits: ΔW electricity price = 1013970 × 0.56 = 567823 yuan (Note: Laiwu Steel Plant industrial electricity price is 0.56 yuan/kWh);
The soft-start function of the motor extends the motor life and greatly reduces the failure rate of the slag flushing pump.
It improved the level of automation and saved a significant amount of industrial water.
As can be seen from the above, the comprehensive economic benefits can reach more than 600,000 yuan per year, and the entire cost can be recovered in one year.
6. Conclusion
Through technical transformation of the slag pump system using variable frequency speed control, and after a long period of operation and testing, the product has proven to be reliable, fully functional, and technologically advanced. This demonstrates that domestically developed high-voltage frequency converters have reached a world-class level in technology. Because the IGBT direct-series high-voltage frequency converter has no input/output transformers, is small in size, has a high cost-performance ratio, and excellent overall performance, it surpasses other domestic and international products. It represents a new generation of high-performance high-voltage frequency converters and provides a feasible path for the technical transformation of other processes within the plant, possessing significant promotional value in the field of high-voltage frequency conversion retrofitting. AC variable frequency speed control transmission devices have been widely used in various industries in my country and have achieved excellent economic and energy-saving benefits. With the introduction of new high-power semiconductor devices and continuous updates and developments in control theory, frequency converters are now developing towards higher power and higher voltage, with increasingly sophisticated control accuracy and dynamic characteristics. Vigorously developing variable frequency speed control technology necessitates raising my country's variable frequency speed control technology to a new level, narrowing the gap with world-class levels, improving independent development capabilities, and meeting the needs of key national economic projects and the market. Standardize my country's variable frequency speed control technology, improve product reliability and process level, and achieve large-scale and standardized production.
