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Design scheme of pure sine wave on-line uninterrupted inverter power supply

Mar 17, 2021

Design scheme of pure sine wave on-line uninterrupted inverter power supply

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1 Introduction

Today, with the increasing development of science and technology, modern industrial control equipment, communication devices, and scientific research implementation will all take electronic computers and monitoring equipment with microprocessors as the core. These devices are a kind of precision electronic equipment, most of which require AC power supply, and they have strict requirements on the power supply quality, reliability and continuity (uninterrupted) of the AC power supply, and the power supply interruption time shall not be greater than 25% The power frequency cycle. The voltage fluctuation, pulse interference and sudden interruption of the mains power will cause the random memory data loss and program destruction, resulting in extremely bad consequences. Especially in the financial, communications and power sectors, the requirements for municipal power are more stringent.


In order to solve the above problems, various uninterruptible inverter power supplies (UPS for short) have appeared one after another. And become an indispensable supporting equipment for each system. Because it has the functions of voltage stabilization, frequency stabilization, filtering and anti-interference, and more importantly, it can continue to output AC sine waves uninterrupted when the AC power is off, so the role of UPS in each system is conceivable. According to the function, it can be divided into on-line and standby. According to the output voltage, it can be divided into square wave, quasi-square wave and sine wave. This article introduces the design of an on-line uninterruptible inverter power supply whose output is a pure sine wave.


2 System structure and working principle

Design scheme of pure sine wave on-line uninterrupted inverter power supply


The system structure of the online uninterruptible inverter power supply device is shown in Figure 1. It consists of electronic bypass, AC/DC rectifier circuit, DC/AC inverter circuit controlled by SPWM, output switching circuit, DSP control and protection circuit, It is composed of a single-chip computer monitoring and a man-machine interface composed of LED and keyboard. The AC/DC rectifier circuit is a full-bridge diode rectifier, which rectifies the mains power into the DC voltage required for the inverter; the DC/AC part mainly converts the DC into AC, and the control part such as DSP generates a variable SPWM wave to control the inversion. Change the size of the output voltage, etc.; the output switch is the use of a single-chip microcomputer to switch the relay according to different situations to make it uninterrupted output.


Under normal online working conditions, the city input is rectified by the input isolation transformer and then converted into direct current. The direct current is converted into high-quality alternating current by inverter to supply power to the load. If the mains power fails, the input backup battery is automatically inverted. When the mains power is restored, it will automatically switch to the mains rectification and inverter power supply. If the mains normal inverter fails, the load will be automatically switched to the electronic bypass power supply through the output switch, and the inverter will be restored after the fault is eliminated.


3 Control part

Design scheme of pure sine wave on-line uninterrupted inverter power supply


Figure 2 is a schematic diagram of the main circuit of the inverter part. At present, there are two main ways to control the inverter part and generate SPWM waves. One is realized by traditional analog integrated circuits. This method is relatively mature and robust. Good, but the circuit is more complicated, the debugging is extremely inconvenient and extremely inflexible, and it cannot adapt to the requirements of modern digital technology (such as parallel machines, etc.). The other is to use all digital implementations, which are controlled by a high-speed processor (such as DSP) to directly generate SPWM waves, saving more analog integrated circuits. This method is simple and flexible, but it is more sensitive to system parameters. , It has high requirements for real-time performance and poor robustness. In view of the above reasons, this article introduces a control method that combines analog and digital.


The setting of the sine wave is given by the number, and the SPWM adjustment control is realized by the simulation. There are also many kinds of SPWM regulation and control parts. The traditional analog control method is a single voltage effective value control method. This method can only ensure that the steady-state output voltage is constant, while the voltage harmonic content is large, and the voltage waveform The quality cannot be guaranteed, and the dynamic response characteristics are not good. Therefore, in order to overcome the shortcomings of traditional control schemes, there are currently many "instantaneous" feedback adjustment methods. Mainly include: single voltage instantaneous value feedback, voltage instantaneous value feedback with filter inductor current inner loop, and voltage instantaneous value feedback with filter current inner loop. What this text chooses is the adjustment method of the voltage instantaneous value feedback of the inductor current inner loop.


Although the adjustment method of the voltage instantaneous value feedback of the inductor current inner loop has better dynamic response characteristics, because the instantaneous inductor current inner loop and the voltage instantaneous value feedback are used, the amplitude of the output voltage changes relatively during sudden load and sudden load shedding. Large, poor voltage stabilization effect, so this article adds a first-level output voltage rms outer loop on the original basis. That is, the high-speed processor of the digital part continuously collects the effective value of the output voltage. If the output voltage changes when the load is suddenly loaded or unloaded, the given sine wave is multiplied by a coefficient to adjust the output voltage.


In summary, the linearized equivalent model of the uninterruptible inverter power supply system is shown in Figure 3, where Vo, IL, IC, and Io are the parameters shown in Figure 2. Kp is the coefficient multiplied by the digital processor according to the change of the effective value of the collected output. ΔUd adjusts the bias of the output transformer, which is mainly caused by the existence of a DC component in the transformer due to various non-ideal factors. Vf is the output voltage feedback, and Ilf is the output inductor current feedback.


Design scheme of pure sine wave on-line uninterrupted inverter power supply


4 output switch

Most UPS output switching generally adopts three methods: the first is to use non-contact solid state relays; the second is to use conventional relays with contacts; the third is a combination of solid state relays and conventional relays. Generally speaking, solid state relays are suitable for high current and high switching speed occasions. The disadvantage is that they have on-state voltage drop and LC circuit, so the on-state voltage drop loss will be converted into heat energy, and a radiator must be added for heat dissipation; and LC The leakage current caused by the circuit makes the solid-state relays unable to completely turn off, especially in the case of small current; in addition, most solid-state relays require zero voltage or zero current to turn off, so the switching time is also negatively affected. Conventional relays with contacts are the opposite of solid state relays. The contact resistance is small and no radiator is needed; there is no leakage current and the off time is not affected by the nature of the load; but the switching time is longer and there is arcing, which affects its life. Nowadays, many UPSs use two kinds of relays together, solid state relays are used for switching moments, and then conventional contact relays are used to increase the switching speed without requiring heat dissipation. But for low-power UPS, the switching speed of conventional relays can still meet the output switching requirements of UPS.


5 Phase lock function

In order for UPS to achieve uninterrupted output, in addition to the short switching time, the inverter output voltage and the electronic bypass voltage must be consistent in frequency and phase, so that the switching waveform is perfect. In the past, people used analog phase-locked loops to track the phase and frequency of the electronic bypass to achieve the purpose of phase-locking. This article uses the DSP external capture port to collect the phase and frequency of the electronic bypass at high speed, with fast tracking speed and high accuracy.


6 Alarm and protection function

In the actual operation of the UPS, various unexpected problems may sometimes occur, whether it is itself or outside, so the alarm and protection functions in the UPS device are indispensable. Alarms can remind operation and maintenance personnel to deal with problems in time to prevent accidents, while protection can prevent permanent damage to equipment and continuous expansion of accidents. For example, when the AC mains power fails, the battery is powered by inverter, but the capacity of the battery is limited, and deep discharge will cause permanent damage to the battery. In order to avoid the occurrence of deep discharge faults, during the battery inverter power supply process, the battery voltage is too low alarm and protection function must be provided, as long as the battery voltage reaches the lower discharge limit voltage, the inverter will stop, and the operator will be notified to deal with it in time.


In addition, during the use of UPS, there will sometimes be transient output short-circuits. Therefore, output short-circuit protection must be provided to prevent the expansion of accidents. Others such as over temperature, AC input, etc. must have alarm and protection functions.


7 Experimental results

Using the main circuit structure and control methods introduced above, this article has developed a 1KVA UPS. AC input and output are both isolated by a power frequency transformer. The input transformer ratio is 1:1, the input filter inductance is 5mH, and the output The transformer ratio is 1:2, the output AC filter inductance is 2 mH, the output filter capacitor is 25uF, the output switching adopts contact and non-contact relays, and the digital sine wave setting and control part are produced by Texas Instruments The TMS320F240 chip and the power device of the inverter main circuit adopt the 6MBP50RA060 module produced by Fuji (with drive and protection directly inside).


The test results of part of the device are:


1) The switching time is <5ms.


2)  Harmonic distortion <3%.


3)  Stabilization accuracy <±1%.


4)  Frequency stabilization accuracy <±0.5% (DC power supply).


8  Conclusion

This article uses input and output power frequency transformer isolation, a combination of digital and analog control technology, and directly uses the 6MBP50RA060 module produced by Fuji to design a 1kVA online uninterruptible inverter power device device. This device has the following advantages:


1) IPM full-bridge inverter, high frequency SPWM modulation, the output is a perfect sine wave.


2) Reliable input and output power frequency transformer isolation.


3) High-precision voltage stabilization and frequency stabilization.


4) Smaller harmonic distortion.


5) High-speed dual-CPU microprocessor, fast control, perfect monitoring, and friendly man-machine interface.


6) Intuitive LED work flow chart instructions, clear LED digital tube display.


7) High-precision digital phase lock.


8) Accurate sound and light alarm.


9) Timely and reliable protection function.


10) Standard RS485 communication interface