Analysis of the most popular application of high v

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Analysis of high-voltage multilevel inverter applied in industrial system

1. Introduction

AC motor variable frequency speed regulation has been a major project of common concern in all walks of life at present. People are looking forward to using high efficiency, high reliability and economically acceptable frequency conversion technology to adjust the speed of AC motor for hundreds of years. In the second half of the 1960s, the progress of power semiconductor devices and their application in frequency converters has achieved the great development of high efficiency and energy conservation in developed countries during the first world energy crisis in the early 1970s. For AC motors, speed regulation can be achieved by changing the frequency. With the development of power electronic components such as silicon controlled rectifier, GTO, IGCT and IGBT, the development of corresponding control technology and the high integration of these power electronic components, frequency converters have been widely used in industry. Limited by the development and application of power electronic components, in the past ten years, low-voltage variable-frequency speed regulation devices have been mainly used, that is, the voltage is 380V ~ 690V. Large capacity high-voltage AC motors widely used in industry only use other speed regulation methods or non speed regulation forms to operate in industrial systems, thus consuming a lot of energy

according to the different research and development of the current main frequency converter manufacturers, the composition of the existing high-voltage frequency converter is also different. According to different voltages, it can be divided into direct high-voltage type and high low high type through step-up transformer (actually low-voltage frequency converter); According to the coupling form in the middle, it is divided into AC-AC frequency converter and AC-DC-AC frequency converter; According to the different combinations of intermediate DC coupling links, it is divided into voltage source inverter and current source inverter. We know that the topology of low-voltage inverter is a unified two-level structure. Due to the limited withstand voltage of power electronic components and the different switching frequencies of different power electronic components, the topology forms of high-voltage and high-capacity frequency converters developed in recent years are also different. However, considering the simplicity, reliability and economy of the whole system, the topology of the current high-voltage inverter is mainly concentrated in the form of three-level and four-level

in recent two years, China has successively introduced several sets of high-voltage converters with four-level voltage source topology from ALSTOM electric company in Europe in the new industrial projects. They have been applied in Taiyuan Iron and steel company, Baoxin stainless steel plant (subordinate to Baosteel), Qingdao Iron and Steel Co., Ltd. and Tianjin Seamless Steel Corporation in China. Its transmission system adopts the most advanced multi-level topology of Alstom company and the AC high-voltage variable-frequency speed regulation device of IGBT components. The biggest feature of this system is that the system is an AC high-voltage variable-frequency speed regulation device; The main circuit adopts four level IGBT structure; Three 4MW AC synchronous main motors share a common DC bus, which meets the requirements of high-performance process speed regulation of the system. At the same time, the system scheme is economical, reliable, energy-saving and optimized configuration. The rectifier adopts the most advanced converter called clean energy converter [1]. This kind of frequency converter can be used in the main rotation of fans, pumps and compressors, as well as in the production of rolling mills and large ship drives with high process performance requirements

2. System structure composition

in recent years, with the development of power electronic components and control system structure, the development of GTO, IGCT and IGBT and the development of frequency conversion technology structure, high-voltage and large capacity frequency converters have been rapidly applied in industrial systems. The variable-frequency drive first experiences multiple components in series on the basis of the original two-level control structure. The two-level output waveform is shown in Figure 1 (a). For the parallel connection of components, the output voltage should meet the requirements of components to withstand voltage. The problems caused by this connection mode are intertwined with complex current sharing devices, and the complexity of the circuit is often easy to cause damage to components; For the connection form of series components, the output current should also meet the requirements of the bearing capacity of the components, and ensure that the voltage distributed on the components should be balanced in any case, so it is also prone to system failures. Therefore, from the perspective of system reliability, they are difficult to ensure the reliable operation of the system, and the output waveform is also very poor

Figure 1 different level structure diagram and output waveform diagram

in recent years, with the development of power electronic components, the topology of frequency converter is also being developed. With the withstand voltage and current limitation of power electronic components, the topology of frequency converter has successively appeared in the form of three-level, four-level and multi-level structures. For three-level, four-level and multi-level inverters, it provides very small harmonic current to the motor, and the current waveform is closer to the sine wave current waveform required by the AC motor, as shown in Figure 1. Through this topology, we can see that with the increase of multi-level, the voltage amplitude decreases correspondingly, which reduces the voltage borne by power components and is more conducive to reducing dv/dt produced by the device. The current high-capacity, high-voltage frequency converter should not only ensure the output of high power, but also ensure the reliable operation of the system, but also ensure that the output waveform is closer to the sine wave. At present, the multi-level structure and output waveform often used in high-voltage and high-capacity converters when the safety protection device is damaged are shown in Figure 1. In recent years, the three-level structure scheme has used both GTO (and recently IGCT) components and IGBT solutions (at present, several famous large companies such as Siemens, abb and ALSTOM have such products). However, its disadvantage is that the conduction or blocking of components is guaranteed by clamping diodes, which have high voltage withstand requirements and a large number; The conduction load of switching devices is inconsistent; When the converter transmits active power, the impulse and discharge time of each capacitor on the DC side is different, which is easy to cause the imbalance of capacitor voltage and increase the difficulty of system dynamic control; At the same time, the expansion ability of this structure is also very limited

with the development of modern topology technology, the structural scheme of multilevel frequency conversion technology can be applied in industrial systems. Figure 2 is the topological structure diagram of the high-voltage, high-capacity, four-level converter recently introduced from ALSTOM in Europe for rolling mill control. Its structural characteristics, namely modular structure, can be clearly seen from Figure 2. The characteristic of this frequency converter is to ensure the series parallel connection of components. At the same time, it is not a simple series parallel connection of components, but a series connection from the structure, which ensures the safety and natural distribution of voltage. Its most obvious feature is:

at present, we know that the high-voltage standards used in industry are 3.3kV, 4.2kv, 5.5kv. 6 it usually takes several weeks to collect each time 6kV, according to these standards, through the series parallel topology technology of the overall unit device to meet the voltage requirements of different levels

due to this structural feature, it is very easy to realize the scheme that multiple frequency converters share a DC bus, which is widely used in today's system, so as to achieve the mutual exchange of energy within the system

this structure eliminates many partial voltage and shunt protection devices on components at all levels in our traditional structure, which can isolate each unit of the circuit from each other, making the system simple, reliable and easy to maintain. Thus, the factor of poor system reliability caused by multiple semiconductor components in series and parallel is eliminated

because this structure adopts IGBT components, its switching frequency is high, the trigger current is small, and IGBT is very easy to find in the market, which brings great opportunities for our development and application

Figure 2 Schematic diagram of four-level structure

it can be seen from Figure 2 that the output waveform of this structure is very close to the sinusoidal waveform. What are the development trends of large capacity AC variable frequency drive system for plastic packaging materials used in the food industry? Let's take a few cases to see that the transmission cable and motor and transformer windings are the most harmful. For the multi-level structure system, it is its advantage in this respect. It should be said that the more the level series, the closer the output waveform is to the sine wave

3. The control principle of the four-level transmission structure

the four-level control structure is shown in Figure 2. The distribution of high-power components in the main circuit is formed in pairs, and each pair is controlled based on the traditional two-level control idea. Figure 3 shows the operation principle diagram of this four-level, the voltage borne by each high-power component and the voltage distributed on each capacitor. It can be seen from the circuit structure that the voltage borne by the whole circuit is: V, 2/3v, 1/3v, but the voltage of each power component in the blocking state is always 1/3v. This structure technology successfully solves the dynamic and static problems of the voltage borne by each power component. At the same time, the control of a different pair of components also limits the problem of dv/dt in different time periods. In fact, the floating voltage borne by each component is provided by each capacitor. The circuit charges and discharges each capacitor during the commutation process, and its capacitor voltage complies with the following rule [2]:

here n is the number of pairs of high-power components in each phase, for example, there are 3 pairs of high-power components in each phase of the four-level structure, that is, here n=3. From the circuit structure, we know that the voltage passing through each power component depends on the voltage CK and CK-1 on the capacitor and is given by the following formula:

now we know that the voltage borne by each blocking high-power component is v/n, and the voltage of the conducting component is 0. This proves the four-level output voltage waveform in figure (3), namely: 0, v/n, 2 · v/n, V

The commutation control of the flat structure should meet the following requirements at the same time:

the capacitor voltage should be constant, that is,

in order to determine the control type of each pair of high-power components, we assume that the initial voltage value VCK is given by, and study the conditions to keep these voltages constant

each capacitor CK is connected with the power components, and depends on the switching state of the pair of components. The current on this capacitor is +i, 0, -i, which can be expressed as: here SK and sk+1 are 0 or 1 (this will depend on the switching state of the power components). This equation gives the stability condition of the stable state of the following voltage VCK k = 1... N:

when the current I is integral constant in a switching section, the stability condition of the stable state of the voltage VCK = 1... N can be written as:

Figure 3 operation control schematic diagram of the four-level structure

we know that the high-power components used for this four-level structure are IGBT, and the PWM modulation technology is used in the control loop. Its control loop adopts the high-performance controller (industrial computer), which is widely used in industrial systems at present, and uses it to distribute the working cycle and send the control cycle of the system. In a control cycle, it is divided into several stages, and in each stage, the switching action of IGBT power components is strictly controlled according to the law of on and off. From Figure 3, we can intuitively see the process of on and off of switch components in each stage. For example, in Section A: the switch of 1#, 2# and 3#, C1 is charged with forward current; and

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