New development of the hottest three-phase uninter

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The new development of three-phase uninterruptible power supply

Abstract: This paper makes a general evaluation of the circuit topology of each module, the circuit structure of the whole machine and various popular control strategies of the three-phase uninterruptible power supply system, points out the problems existing in the design and application of uninterruptible power supply and the new hot spots of current research, and finally predicts the development trend of ups

key words: three-phase uninterruptible power supply; Inverter in parallel; Digital control

o introduction

for a long time in the future, the situation of insufficient power supply, large voltage fluctuation and serious interference will still exist in China. The rapid development of various industries and fields puts forward higher and higher requirements for power supply quality, especially the contradiction between the requirements of power supply quality of important systems, important departments and important electrical equipment with strong real-time performance and the actual situation of electricity in our country is becoming increasingly acute. Therefore, uninterruptible power supply (UPS), as a green power supply with voltage stabilization, frequency stabilization and pure purification, has increasingly become the focus of people's attention supported by the Chunlei talent project of Ningbo Institute of materials. In order to continuously improve the performance of UPS, researchers have done a lot of research on UPS system, and put forward many circuit topologies and control strategies

1 circuit topology of UPS

the reliable operation of UPS is inseparable from the coordination of various modules. The following is a brief analysis of the circuit topology of the main functional modules of ups

1.1 rectifier and power factor correction circuit

rectifier circuit constitutes a DC power supply device in application. It is the interface circuit between public power and power electronic devices, and its performance will affect the operation and power quality of public power. High performance UPS, such as 3-point anti break or 4-point anti break, requires high input power factor and minimize the harmonic component of input current. Traditional single-phase UPS mostly adopts analog method, while three-phase UPS mostly adopts phase controlled rectifier circuit and voltage type single tube rectifier circuit

1.1.1 the traditional three-phase phase phase controlled rectifier circuit and voltage type single tube rectifier circuit adopt the semi controlled power device as the switch, which has the following problems:

1) the existence of side harmonic current will reduce the power factor at the equipment side and increase reactive power

2) the phase controlled rectifier commutation mode leads to the distortion of electric voltage during the commutation period, which not only affects the performance of its own circuit, but also interferes with the electricity and brings adverse effects on other equipment in the same grounding point

3) the phase controlled rectifier is a time-delay link, which cannot realize the rapid adjustment of output voltage

voltage type single transistor rectifier circuit is the abbreviation of three-phase uncontrolled rectifier bridge plus boost circuit. Its disadvantages are: the high peak current not only hinders the improvement of system power, but also increases the conduction loss and switching loss; In order to maintain the improvement of side power factor, the boost circuit must have a certain boost ratio, which will lead to high DC output voltage for three-phase circuits

1.1.2 current type three-phase bridge rectifier circuit

the current type three-phase bridge rectifier circuit is shown in Figure 1. Its advantage is that the feedback control is simple, and there is no need to add current feedback to the control circuit. It can realize the sinusoidal input current only by adjusting the duty cycle of each switch tube; The voltage on the DC side is low. The disadvantage is that the sinusoidal degree of the input current is not very good, and a shunt capacitor must be added at the input side to realize phase shift. This kind of circuit is now becoming one of the research hotspots. This kind of circuit is suitable for high-power rectifier circuit and does not require high power factor

1.1.3 voltage type three-phase bridge rectifier circuit

voltage type three-phase bridge rectifier circuit is shown in Figure 2, which is characterized by the use of high-frequency PWM rectification technology, and the device is in high-frequency switching state. Because the on and off states of the device can be controlled, the current waveform of the rectifier is controllable. The advantage of this circuit is that the input current in the same phase with the input voltage can be obtained, that is, the input power factor is 1, and the harmonic content of the input current can be close to zero; Energy can flow in both directions. Normally, energy flows from the AC side to the DC side. When the DC output voltage is higher than the given value, energy flows from the DC side to the AC side, with high conversion efficiency. The disadvantage is that it belongs to boost rectifier circuit, and the DC side voltage requirements are high. This kind of circuit is also a research hotspot in recent years

1.2 battery pack and charge discharge circuit

battery pack is the energy storage unit of ups. When the mains power is normal, it absorbs the energy from the mains power and stores it in the form of chemical energy. Once the mains power is interrupted, it converts the stored chemical energy into electrical energy to supply power to the inverter and maintain the continuity of load power supply. In the UPS system with medium and small power, the voltage of the battery pack is usually relatively low, so the charging and discharging circuit with bidirectional energy flow is usually used [4]. In order to improve efficiency in high-power system, the battery pack is usually directly connected to the DC bus in parallel to simplify the circuit

1.3 inverter circuit

inverter is the core of UPS, which converts DC electric energy into AC energy with stable voltage and frequency required by users. The following still takes the three-phase inverter as the object to analyze the research hotspots of inverter in recent years

1.3.1 three phase half bridge inverter circuit

three phase half bridge inverter circuit is the most widely used in three-phase inverter circuit. This circuit is characterized by the use of fully controlled devices to form the inverter, which has the advantages of high power density, good performance, small size and lightweight. This circuit is convenient to use new control strategy to improve the quality of inverter. However, it is difficult to achieve 100% independent load

1.3.2 H-bridge inverter

for the super large capacity inverter, due to the substantial improvement of power level, new requirements are put forward for the structure of the inverter, and the H-bridge arm inverter is one of the choices. The inverter output transformer adopts the multi winding connection method, the primary side of the output transformer adopts three independent windings, and the inverter output adopts three independent h bridges. This is convenient to control, but the cost is high

1.3.3 three phase four leg conversion technology

due to the inherent defects of the three-phase circuit, the three-phase four leg inverter itself began to seek a new circuit structure, so the three-phase four leg inverter appeared, as shown in Figure 3. The output of this circuit structure is three-phase four wire system, the three-phase voltage can be controlled independently, and the control method is flexible, but the algorithm of this topology is relatively complex. The PWM vector rotates in three-dimensional space, and the digital control method must be used to realize the generation of spatial PWM waveform. This circuit has become one of the research hotspots in recent years

1.4 three phase UPS complete circuit

1.4.1 traditional three-phase UPS circuit structure

traditional three-phase UPS structure, the input adopts thyristor rectification, the output adopts inverter, the battery is directly connected to the DC bus, and the rectifier acts as a charger at the same time. The output adopts transformer isolation, which can realize complete isolation of input and output, and ensure that the electrical disturbance will not interfere with the load. When the mains power is cut off, the battery outputs stable alternating current through the inverter; When the inverter fails, the bypass output voltage ensures the reliability of power supply. The main disadvantage of this structure is its large volume and weight

1.4.2 high frequency chain three-phase UPS

in order to reduce costs and reduce the volume and weight of UPS, high frequency chain three-phase UPS appeared, as shown in Figure 4. This circuit eliminates the huge power frequency transformer and adopts high-frequency rectification for input, which can obtain higher input power factor and lower input harmonic current. Its disadvantage is that the input and output are not isolated by transformers, and the electrical disturbance may cause disturbance to the output of UPS; The output three-phase voltage forms the neutral line by the midpoint of the battery and capacitor, so the amplitude of positive and negative DC voltage must be kept equal in the control, otherwise the output neutral line will have a large DC component, which is unfavorable to the load and the transformer in the load; The input adopts three-phase four wire system, and there is current flowing through the neutral line, which may cause the neutral line potential offset and interfere with the load; The input and output are not isolated, so the circulation problem in parallel is difficult to solve

1.4.3 New Interactive UPS

because the above two ups have to undergo two full power transformations, the efficiency of the system is low. From the perspective of improving the efficiency of the system, a series parallel compensation high-capacity structure has emerged, which is a new interactive structure, as shown in Figure 5. This topology also has no transformer isolation for input and output, so it will have the disadvantage of high-frequency chain ups. The output frequency of this UPS must be consistent with that of electricity, and the suppression ability of Electrical disturbance is not strong, so the power supply quality is worse than the traditional three-phase ups. Its characteristic is that the energy from input to output is not transformed by full power, but also composed of two high-frequency converters, but converter 1 can only bear 20% of the power at most. In terms of cost, the cost of this structure is lower. In terms of control method, converter 1 is a voltage compensator, which is used to compensate for the distortion of electric voltage; Converter 2 is a current compensator, which is used to compensate the harmonic current of the load and supply power to the load as a full power voltage source inverter when the mains power is cut off

1.4.5 ups with parallel input and output

in this circuit, the input end is formed by multiple rectifiers in parallel to supply power to the DC bus. At the same time, the DC bus provides DC voltage to multiple inverters, and the output ends of multiple inverters are directly connected to supply power to the load at the same time. This method can enhance the capacity of UPS, increase the reliability of the system, reduce the cost and enhance the maintainability. However, the more parallel modules, the more difficult it is to solve the current sharing problem between modules

2 control technology of uninterruptible power supply

with the rapid development of control theory and various microcontrollers with rich functions and excellent performance, a variety of discrete control methods have emerged. From the number of control feedback loops can be divided into single loop, double loop, multi loop control. The control effect can be improved by increasing the number of feedback loops as much as possible when the hardware allows. From the perspective of control principle, it includes digital PID control, state feedback control, deadbeat control, repetitive control, sliding mode variable structure control, fuzzy control, neural control, space vector control and other methods

digital PID control has good adaptability and strong robustness; The algorithm is simple and easy to implement with single chip microcomputer or DSP. However, there are two limitations: on the one hand, the sampling quantization error of the system reduces the control accuracy of the algorithm; On the other hand, the sampling and calculation delay make the controlled system a system with pure time delay, which reduces the stability domain of PID controller and increases the design difficulty

predictive control can achieve small output current distortion and strong anti noise ability. However, this algorithm requires accurate load model and circuit parameters, so its robustness is poor, and the delay caused by numerical calculation is also a problem in practical application. Hysteresis control has fast response speed and high stability, but the switching frequency of hysteresis control is not fixed, which reduces the reliability of the circuit and the spectrum of the output voltage, which is detrimental to the system performance

The basic idea of deadbeat control is to calculate the PWM pulse width of the next switching cycle according to the state equation of the inverter and the output feedback signal. Therefore, theoretically, the output voltage can be very close to the reference voltage in phase and amplitude, and the output voltage error caused by load change or non-linear load can be corrected in a switching cycle.

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