Research on a phase-locked control technology for a fully digital UPS inverter

　　1. Introduction

　　UPS, uninterruptible power supply, refers to a form of power supply that can provide reliable and stable power supply to the load under normal or faulty conditions of the mains. It is mostly used in some critical loads such as computer rooms, hospitals and other occasions, providing the most power failure protection for the load. However, the traditional online UPS has multiple power sections and analog controllers, which is a very complex and expensive system. Therefore, the research on high-quality, high-reliability fully digital UPS (uninterruptible power supply) suitable for the development of modern science and technology has become a topic of great concern. Digital control has become a hot topic in the field of power supply research with many advantages such as simple and flexible control, more stable output performance, and the ability to achieve functions that are difficult to achieve with analog control. With the development of microelectronics technology, more and more solutions have been provided for power electronics, making the full digital system of UPS power supply and the introduction of various advanced control strategies gradually become a reality.

　　This paper mainly discusses one of the key technologies in UPS based on TMS320LF2407 digital control platform - phase-locked control technology.

　　2. Phase-locked meaning

　　There are two switching processes during the operation of the uninterruptible power supply:

　　First, when the power supply is started, the bypass supplies power to the load, and the inverter runs at no-load. At the same time, the phase-locking function is started to adjust the inverter output to track the grid frequency and phase. When the inverter output tracks the grid frequency, it switches to the inverter to supply power to the load. Second, when the inverter circuit fails, or when the load is impactful (such as when starting the load) or overloaded, the control system will block the PWM output to stop the inverter from supplying power to the load, and at the same time turn on the bypass switch, so that the grid directly supplies power to the load.

　　In order to effectively ensure that the inverter bypass switching process does not produce excessive impact on the load, the UPS inverter output voltage must be consistent with the frequency and phase of the grid voltage. Therefore, the UPS system introduces phase-locked control technology, and software phase-locked technology is one of the important links of digital UPS. Fast and reliable software phase-locked tracking technology can accurately provide a standard voltage reference sine wave with the same frequency and phase as the grid voltage for the inverter digital control.

　　3. Basic principles of phase-locked loop

　　The phase-locked loop is a closed-loop phase control system that can automatically track the frequency and phase of the input signal. It consists of three parts: a phase comparator, a low-pass filter, and a voltage-controlled oscillator. The control block diagram is shown in Figure 1.

　　Figure 1: Phase-locked control block diagram

　　Its working principle is: the output signal uo(t) of the voltage-controlled oscillator and the sampling signal ui(t) of the power grid, two signals with different frequencies and phases, are sent to the phase comparator, and the amplitude of the generated error signal ue(t) is proportional to the phase difference between the uo(t) and ui(t) signals. After being processed by the low-pass filter, ue(t) will send out a control voltage signal uc(t) equivalent to the average value of the ue(t) signal. Under the control of the signal uc(t), the voltage-controlled oscillator will adjust the frequency and phase of the output voltage signal uo(t), thereby gradually reducing the frequency and phase difference between the two signals uo(t) and ui(t).

　　4. Phase-locked control technology for online UPS

　　Depending on the different ways in which the control circuit of the single-phase UPS inverter generates SPWM waves, the methods for implementing UPS phase-locked control vary greatly, which are discussed below.

　　4.1 Analog phase-locked control technology for online UPS

　　The block diagram of the phase-locked control principle of the traditional online UPS power supply is shown in Figure 2. When the power supply is normal, the grid voltage detection circuit outputs a high level, and the 50Hz grid voltage is converted into a unipolar "inverted full-wave rectifier" signal with a period of 20ms through the waveform conversion circuit, and then sent to the input end of analog switch 1. After passing through analog switch 2, a series of grid voltage synchronization tracking signals are generated. Since the output signal of the frequency converter is a synchronous capture signal with a period of 20ms, the grid synchronization tracking signal added to the control end of the multivibrator can adjust the phase of the high-frequency output pulse of the high-frequency oscillator to ensure that the sine wave generator outputs a 50Hz reference sine wave. After the phase-locked synchronization circuit, that is, the "phase-locked loop", the sine wave is always in a synchronous tracking state with the same frequency and phase as the grid voltage. When the power supply is abnormal, the multivibrator generates a signal with a local oscillation frequency of 20kHz, outputs a 500Hz pulse sequence through the frequency divider, and then generates a stable 50Hz standard sine wave through the sine wave generator.

　　Figure 2: Schematic diagram of phase-locked control of online UPS power supply with output transformer

　　The traditional sine wave signal generator uses a feedback oscillation circuit to output a sine wave using the circuit's self-oscillation and frequency selection. However, the low-frequency analog oscillator has a disadvantage:

　　It is greatly affected by voltage and temperature, and the frequency and amplitude stability of the output signal are poor, making it difficult to meet the requirements of being used as an AC reference. Moreover, the use of analog devices makes the control circuit structure quite complex, which is inconvenient to produce and difficult to debug.

　　4.2 Digital phase-locked control technology for online UPS

　　The digital phase-locked control technology of online UPS uses a microprocessor as the core control chip and uses software to implement phase lock. Its circuit generally consists of the following parts: AC voltage transformer, precision rectifier circuit, zero-crossing comparator, low-pass filter, inverter, analog switch and microprocessor. The circuit block diagram is shown in Figure 3.

　　Figure 3: Block diagram of the digital phase-locked control circuit for online UPS

　　The working principle of this circuit is as follows: the AC voltage of the power grid is isolated and stepped down by the voltage transformer to become a low-voltage AC signal with the same frequency and phase as the power grid voltage. One path passes through the precision rectifier circuit to become a half-wave DC voltage signal with positive polarity. The voltage amplitude is measured by the A/D converter inside the microprocessor; the other path outputs the positive and negative polarity of the AC signal through the voltage zero-crossing comparator and enters the single-chip computer through the I/O port. In this way, the real-time waveform data of the external AC voltage can be measured. After the collected waveform digital sequence is converted by D/A, a sine wave can be output. Since the power grid voltage contains a large number of harmonic components, the AC voltage signal collected by the voltage transformer is not a pure sine wave, so the waveform generated by the direct output method is not a stable and pure sine wave.

　　Therefore, a digital low-pass filter is added after the PWM output to filter out high-frequency harmonic components, thereby ensuring the stability and purity of the output voltage.

　　The specific implementation process is: first, use the digital sequence to modulate the PWM pulse width modulation circuit inside the single-chip microcomputer so that the width of the pulse square wave generated is proportional to the signal amplitude. If the microprocessor uses a 20MHz crystal oscillator and the PWM output is 8-bit resolution, the maximum frequency of the output square wave is 78KHz, so adding an RC low-pass filter with a very small integral constant at the PWM output end can obtain a very smooth half-wave output waveform, and the phase delay caused by the low-pass filter can be ignored. The signal is sent directly to the analog switch in one way, and the other way is sent to the inverting circuit to become a negative half-wave voltage signal, and then sent to the analog switch. The positive and negative voltage signals are switched by the analog switch controlled by the single-chip microcomputer, and a sinusoidal wave signal with phase synchronization with the external power grid is output. When the power grid fails, the microprocessor will read the standard 50Hz sinusoidal wave sequence stored in its memory to control the inverter output.

　　4.3 Software phase-lock control technology for online UPS

　　With the development of microelectronics technology, many cost-effective dedicated microprocessors for motor control have emerged. The microprocessor integrates PWM wave generation circuits, and the PWM wave output frequency can be changed through software programming. The software phase-locked control in UPS is based on such microprocessors and is implemented in a program calculation manner. There are two ways to implement it in software. One is to filter and shape the two signals of the mains voltage and the inverter output voltage, and transform them into square wave signals with the same frequency. The jump of the rising edge of the square wave is captured by the capture pin of the microprocessor, and the frequency and phase difference are calculated by the captured value, so as to adjust the frequency of the output SPWM wave, so that the frequency and phase of the two signals are consistent. The principle block diagram of the phase-locked synchronous control is shown in Figure 4. Another method only needs to square-wave transform the mains voltage. In the capture interrupt of the mains conversion square wave, the phase of the SPWM output wave is judged to be ahead or behind the mains phase, and the output frequency of the next cycle of the SPWM wave is adjusted by changing the SPWM timer period value, thereby realizing frequency tracking.

　　Figure 4: Software phase-locked synchronization control

　　The common points of the two methods are: both set the grid voltage frequency to 50Hz in advance, divide each voltage cycle into N equal parts according to the SPWM carrier frequency (when the carrier frequency is 78KHz, N=150), make the sine value at the corresponding moment into a table and store it in the memory of the microprocessor, keep the number of output sequences per SPWM cycle N unchanged, and adjust the carrier frequency to adjust the output voltage frequency; their implementation process depends on two interrupts, one is the SPWM carrier cycle timer interrupt, and the other is the capture interrupt (the capture interrupt mode can be set to make the capture interrupt occur at the zero phase moment of the sine wave cycle).

　　Since both methods achieve phase locking of the inverter output to the AC power voltage by adjusting the SPWM frequency, the tracking accuracy is high in steady state, but the dynamic performance is poor, and the tracking adjustment speed is slow when the phase-locked loop starts. Therefore, the synchronization response speed needs to be improved, and the anti-interference and fault tolerance capabilities of the synchronization need to be further enhanced.

　　5. Conclusion

　　The software phase-locking technology of UPS based on TMS320LF2407 digital control platform studied in this paper has high phase-locking accuracy and is easy to implement, which can well meet the phase-locking technology requirements of uninterruptible power supply.