Summary of Medium Voltage Inverter and Its Application

1 Introduction

Frequency converters are developing towards low noise, high performance and high reliability. General frequency converters have achieved good results in saving energy, controlling industrial production processes and improving the level of automated production in enterprises with their advantages of wide speed regulation range, high speed regulation accuracy, fast dynamic response, high efficiency and convenient operation. With the widespread application of general frequency converters, medium voltage frequency converters are being promoted and applied. There are many solutions for medium voltage variable speed regulation, such as medium-low-medium solution, medium-low solution and medium-medium solution. Users should choose the solution with the best performance-price ratio based on the load of their own medium voltage motor. This article expresses some views on the latest progress and application of medium voltage frequency converters for reference.

2 Medium Voltage Inverter Products and Overview

(1) Siemens

Siemens has a complete range of transmission products, covering all application areas, including current source type, voltage source type and common DC bus type. In the application field of medium voltage inverter, Siemens adopts medium-low-medium solution to better solve the speed regulation problem of (300-630) kW/6kV motor, that is, adding a step-down transformer on the input side of the general inverter and a step-up transformer on the output side to form a medium voltage inverter drive system. Its main features are high reliability and low price. Considering that the inverter output contains high-order harmonics and DC components, the output step-up transformer needs special design. If the medium voltage 6kV motor is changed to 690V or 3300V motor, only the step-down transformer and inverter are used to form the inverter drive system, which is the so-called medium-low solution. Medium voltage inverters can be used for new engineering projects and technical transformation projects. In new engineering projects, reasonable choices can be made for motors and inverter drive systems according to process requirements. In technical transformation projects, 6kV motors can be changed to motors with 3300V or lower voltage levels. Although the cost and workload are increased, the motor and inverter drive system are more reasonable. The medium-low solution not only solves the speed regulation problem of variable torque loads such as fans and pumps, but also provides a better solution for loads with higher starting torque and acceleration torque (such as extruders, hoists and other torque loads). For the convenience of matching, Siemens has provided 3300V and 690V medium and low voltage motors in China. The following is an introduction to the direct high-voltage medium voltage inverter launched by Siemens using high-voltage IGBT and three-level technology.

SIMOVERTMV medium voltage inverter adopts the principle of vector conversion field oriented control with excellent performance. It has extremely high dynamic performance, excellent torque quality and perfect control characteristics. It adopts high-voltage IGBT with high reliability, simple drive, low trigger power and no need for buffer circuit. It adopts three-level technology to reduce the impact on the motor. Figure 1 is the schematic diagram of the MV system.

The MV series medium voltage inverter maintains the modular structure of Siemens low voltage inverter. Its input transformer is three-winding. The MV inverter with AFE active front end can be used in weak power grids and has kinetic energy reserve power supply, flying restart power supply and automatic restart function. The rated motor voltage is 2.3kV, 3.3kV, 4.16kV and 6kV.

(2) American A-B Company

The medium voltage inverter of AB Company is Bulletin1557 series. The control mode adopts speed sensorless direct vector control. The motor torque can change quickly without affecting the magnetic flux. The operation effect is similar to that of DC transmission device. The circuit structure is AC-DC-AC current source type. The power device used is GTO, which combines the advantages of pulse width modulation technology and current source power structure. Its harmonics comply with IEEE5191992. AB can provide several solutions to meet the requirements of harmonic suppression, such as standard 12-pulse or 18-pulse rectifiers, standard harmonic filters or power factor compensators and PWM rectifiers. Figure 2 shows the Bulletin1557 inverter with 18-pulse rectifier structure.

Figure 1 Schematic diagram of MV system

Figure 218 Pulse structure of Bulletin 1557 inverter

Figure 3 Schematic diagram of the structure of ACS1000

Figure 4 Structural schematic diagram of PMT-F500

AB Company recently launched a new generation of medium voltage inverter Power

Flex7000 has the characteristics of simple system structure, adopts SGCT-symmetrical gate-commutated thyristor, current source-PWM technology, direct vector control technology, plus its inherent energy regeneration capability, good braking and acceleration and deceleration performance, and is easy to use.

(3) ABB

ABB's medium voltage inverter is the ACS1000 series, which uses IGCT as the main switching device, direct torque control (DTC), and three-level technology. IGCT is a GTO with a turn-off gain of 1. In order to solve the non-uniform turn-off of many small cathode units and the cathode current shrinkage effect during the GTO turn-off process, the GTO gate driver and GTO are integrated into one component, so that the negative gate of the GTO rises to the amplitude of the anode current within 1μs. This is the basic principle of IGCT. Direct torque control is an AC speed control technology that has developed in parallel with vector control technology since 1985. In DTC, stator flux and torque are used as the main control variables. At present, the power range of ACS1000 is 315kW~5000kW, and the output voltage is 2.3kV, 3.3kV and 4.16kV, etc. IGCT has certain advantages in high-power medium voltage inverters. Figure 3 is the structural schematic diagram of ACS1000.

(4) Mitsubishi Electric

Mitsubishi Electric's recently developed medium-voltage inverter PMT-F500HV series suitable for square torque loads adopts a unit series multi-level structure and a 24-phase rectification method on the input side to suppress the interference of high-order harmonics on the power grid. The power factor can reach 0.95 in the entire speed regulation range. Figure 4 is the structural principle diagram of PMT-F500HV.

(5) Robincon Corporation, USA

Robincon's medium-voltage inverter is composed of multiple low-voltage power units connected in series and superimposed to achieve high-voltage output. Each power unit is powered by the secondary coil of the integrated input isolation transformer. There is a small phase difference between the secondary coils when they are wound to eliminate most of the harmonic currents generated by each unit. Each power unit uses a three-phase input and single-phase output PWM inverter composed of low-voltage IGBTs. The inverter outputs 0-480V adjustable voltage and 0-120Hz adjustable frequency. It uses multi-level pulse width modulation technology, and the output voltage waveform is close to a sine wave.

3. Medium voltage inverter solution selection

The application of medium-voltage inverters is gradually being promoted. It plays an important role in energy saving and improving the automation level of enterprises. Some medium-voltage motors that need speed regulation are key equipment in the production process. Due to their large investment, it is necessary to make a comprehensive analysis and comparison of medium-voltage inverters and load conditions before making a choice. In recent years, some noteworthy problems have also appeared in the application of medium-voltage inverters: For example, in the medium-low-medium solution, the inverter output contains high-order harmonics and DC components, which leads to unsatisfactory equipment operation; some medium-voltage motors are constant torque loads, or have high starting torque and acceleration torque, which are not applicable to all medium-voltage inverters. Due to improper selection, the equipment is in a long-term fault state, which not only affects production but also causes investment waste. There are also lessons in this regard: whether in new engineering projects or technical transformation projects, when using medium-voltage inverters, the supplier's technical solutions, ability to solve engineering problems, after-sales service quality, etc. should be comprehensively considered. When choosing, a longer-term consideration should be made, and the development after a few years should be considered, because the operation of the equipment is long-term. From the application and practice of low-voltage inverters in the past twenty years, it can be concluded that companies with good sales performance attach great importance to after-sales service and have established a 24-hour service system.

With the development and application of power electronic devices such as high-voltage IGBT and IGCT, medium-voltage inverters have also developed rapidly. The use of new devices will make the structure of medium-voltage inverters simpler and more reliable. For 6kV medium-voltage motors, there are those that directly use high-voltage inverters, such as Siemens MV series medium-voltage inverters, which have achieved many results in China. Due to the differences between domestic power supply and distribution and foreign power supply, users should choose the best solution according to the specific load conditions to obtain the best economic benefits. In the process of inverter transformation of 6kV medium voltage motor, the 6kV motor with Y connection can be changed to Δ connection, and the voltage is reduced from 6kV to 3.47kV, which is close to the 3.3kV medium voltage inverter commonly used in the world. It combines the actual load situation and the requirements of the voltage source inverter for the original motor insulation level, and can obtain a reasonable performance-price ratio; if the 6kV motor is changed to a lower medium voltage such as 2.13kV, 3.3kV, 4.16kV or a low voltage such as 690V and the corresponding medium and low voltage inverter is selected, it is also a more economical solution, because foreign manufacturers have many years of application experience and performance in this regard and it is a standard product with low cost. In order to meet the requirements of domestic users, Siemens will produce medium and low voltage motors of various specifications in China to adapt to the matching of drive systems. These solutions will be suitable for various load conditions of synchronous motors and asynchronous motors.

4 Conclusion

The medium voltage inverter developed with high-voltage IGBT has been widely used for its good dynamic performance, high reliability and simple structure. The development and application of high-voltage IGBT and IGCT will enable the medium voltage inverter to achieve a better performance-price ratio. The SIMOVERTMV voltage source medium voltage inverter developed by Siemens using high-voltage IGBT and three-level technology can meet the requirements of various medium voltage motor loads. In order to reduce costs and obtain higher dynamic performance and reliability, there are many options for the transformation of 6kV medium voltage motor variable frequency drive system. The research, development and application of medium voltage inverters should be combined with the actual situation of power supply and distribution at home and abroad, and keep up with the development and application of power electronic devices such as high-voltage IGBT and IGCT, microelectronics technology and control theory, so as to obtain the best effect in both development and application.