Phase-Lock technology of full digital UPS based on DSP*

Ya-jing ZHANG，Guo-xu ZHANG

Department of Information Engineering，Tangshan College，Tangshan 063000, China

Phase-Lock technology of full digital UPS based on DSP*

Ya-jing ZHANG?，Guo-xu ZHANG

DepartmentofInformationEngineering，TangshanCollege，Tangshan063000,China

In order to avoid a big impact on load when the UPS inverter is switching, the output voltage of UPS inverter must be consistent with the grid voltage in frequency and phase domain. Fast and reliable software phase-locked tracking technology can accurately provide the standard sinusoidal voltage for the digital inverter with the same frequency and phase as those of the grid voltage. This paper mainly discusses the phase-locked technology of full digital UPS based on TMS320LF2407.

DSP, UPS, Phase-lock

1.Introduction

UPS (uninterrupted power supply) can provide reliable and stable power for load no matter whatever the power grid is normal or abnormal. UPS is used for key loads such as computer rooms, hospital, and so on[1-3]. These workplaces need UPS to provide the important devices when the power break down. The traditional on-line UPS is very complex and expensive since, because of many power parts and analog controllers are in the UPS. Therefore, people have paid more and more attentions on the study of full digital UPS, which is more reliable and more suitable for the development of modern science and technology[4]. Digital control technology is simpler and more stable than analog control technology. With the development of microelectronic technology, the full digital UPS including advanced control strategies became a reality since, more and more solutions about power electronics technology have arisen[5-7].

In this paper, we will mainly discuss the phase-locked technology, which is one of key technologies in a full digital UPS based on TMS320LF2407.

The phase-locked technology plays a very important role in many fields such as communication, radar, navigation, space technology and computer. There are two parts in phase-locked technology, i.e., analog and digital. There are some defects for in analog phase-locked technology in many respects that includes reliability, stability and integration; digital phase-locked technology includes two parts, one is digital logic phase-locked whichthat is made of digital logic device, the other is software phase-locked which isthat based on DSP[8]. Digital logic phase-locked circuit has ve only two statues: turn-on and cut-off. As compared with analog phase-locked technology, digital logic phase-locked technology has ve lots of advantages in terms of noise immunity, integrated level and reliability. It also can eliminate adverse effects in circuit that is caused by loop filter and phase detector’s DC shift. But in digital logic phase-locked loop, we need to transfer analogy electric level to square pulse or discrete data, so it will be much difficult than before for the system analysis and design. Software phase-locked technology can be used for the design of phase-locked loop, which has all the advantages in digital logic phase-locked technology. In addition, it can increase integration and intelligence of system and simplify the hardware circuit, and it is easily to modify or extend as well.

2.The importance of phase-locked

When UPS runs, there are two switches in working process: one is at the moment when the power starts, bypass supplies power to the load, the inverter operates no-load. At the same time, phase-locked function gets started to adjust the inverter’s output to trace the power frequency and phase. When they are the same, the inverter will begin to supply power to the load. The other is at the moment when the inverter circuit goes wrong or the load arises fluctuation. The control system will block PWM output to stop the inverter, at the same time the switch of bypass will get connected, the load will be supplied directly by grid voltage[5].

In order to avoid too much impact on the load when the switch is working, the power frequency and phase of output voltage from UPS inverter must be consistent with grid voltage. Therefore, phase-locked technology is very necessary in UPS system. Software phase-locked technology is one of the key parts in digital UPS system because the fast and reliable software phase locking technology can supply normal voltage sinusoidal wave to inverter which has the same frequency and phase with grid voltage.

3.The fundamental principle of phase-locked loop

PLL (phase-locked loop) is a closed-loop phase-controlled system, which can follow the frequency and phase of input signal automatically. PLL consists of phase comparator, low pass filter (LPF) and voltage controlled oscillator (VCO). The block diagram of control is shown in Figure 1.

Figure 1. The block diagram of phase-locked control process

Following is the working principle of PLL. Two signals with different frequency and phase will be sent into the phase comparator, one is the output of VCOuo(t), the other is the sampled signal of grid voltageui(t). Therefore, an error signalue(t) will be produced. The amplitude of error signal is directly proportional to the phase difference betweenuo(t) andui(t).ue(t)will be processed by LPF, then it will send out a control voltage signaluc(t), which is the average value ofue(t). VCO will adjust the frequency and phase of output voltage signaluo(t) under the control ofuc(t), so that the difference of frequency and phase betweenuo(t) andui(t) will get gradually decreased.

4.Phase-locked technology of on line UPS

Because the control circuit of single phase UPS inverter will produce the SPWM wave in different ways, so there exist significant differences in phase-locked control method.

4.1.Analog phase-locked technology of on line UPS

As shown in Figure 1, the phase-locked control diagram of traditional online UPS is presented, its diagram of phase-locked control is shown in Figure 1. When the power supply is at normal situation, the detection circuit of grid voltage will send a high level signal and, 50 Hz grid voltage will isbe converted into a single-polarity invertible full wave rectification signal by the waveform transformation circuit. Then the signal will be sent to the input of analog switch 1, after it passeding the analog switch 2, a bunch of grid voltage synchronization signal will occur. The output of frequency transformer will be is synchronizeding by following the signal with 20 ms periods, so the grid voltage synchronization signal which is addeding to the control port of multivibrator can adjust the phase of high frequency pulse from high frequency oscillator and, to ensure that the sine wave generator could an output 50 Hz reference sinusoidal wave. The frequency and phase will be always synchronized with the grid voltage due to the phase-locked synchronization circuit (PLL). When the power supply is at abnormal situation, multivibrator will send out a signal whose local frequency is 20 kHz and output 500 Hz pulse train by frequency divider. Finally a normal sine wave will occur with the stable frequency at 50 Hz by sine wave generator.

4.2.Digital phase-locked technology of on line UPS

Digital phase-locked technology of on line UPS uses the microprocessor as the core control chip and applies the software method to lock phase. The circuit consists of AC voltage transformer, precision rectifier circuit, zero-crossing comparator, low-pass filter, inverter, analog switch and microprocessor. The block diagram of this circuit is shown in Figure 3.

Figure 2. Phase-locked control diagram of online UPS with output transformer

Figure 3. The diagram of digital phase-locked control circuit in online UPS

Following is the working principles is as follows. Grid AC voltage is insulated and reduced to be a low voltage AC signal whosewhich frequency and phase areis the same as grid voltage by using potential transformer and, it will be is sent to the precision rectifier circuit. The output is a positive polarity half wave rectifier signal, to measure amplitude of voltage by using A/D converter. The voltage will be is also sent to zero comparator. The output is the polarity of AC voltage, and it will be sent to microprocessor by using I/O. Therefore, we can get the real-time wave data about external AC voltage. By using the data and D/A converter, we can get sine wave. Because there are lots of harmonic waves in grid voltage, the output of potential transformer is not a normal sine wave, so the output of microprocessor is not normal sine wave as well.

Therefore, we can use digital low-pass filter to filter the higher frequency harmonic waves after the PWM output to ensure that the output is stable and clear enough.

Following is the working process. PWM (pulse-width modulation) circuit in microprocessor is modulated by digit sequence to produce a square wave pulse that is proportional to signal amplitude. If microprocessor uses crystal oscillator whose frequency is of 20 MHz, and the resolution of output from PWM circuit will be is 8 bits and then the highest frequency of output square wave will be 78 kHz. We can use a RC low pass filter with minor integration constant addeding to the output of PWM circuit’s output to produce a smooth half wave output. This signal it will be sent to analog switch directly, at the same time it will be sent to inverter circuit as well. The output is negative polarity half wave voltage and, this voltage will be sent to the same analog switch. The microprocessor will control the two signals with opposite polarity output in turn by switching the working status of analog switch. Thus a sine wave with the same frequency and phase at grid voltage will be produced. When the commercial power is failed, the microprocessor can use the normal 50 Hz sinusoidal sequence stored in memory to control the output of inverter.

4.3.Software phase-locked technology of on line UPS

With the development of microelectronic technology, there have been many special microprocessors that is used for motor control with high cost-efficiency, which integrates PWM generation circuit in its own chip and can adjust the output frequency of PWM wave by software programming. Software phase-locked control technology in UPS is based on this kind of microprocessors, which can change the frequency by programming.

There are two kinds of ways by software. One is that both the grid voltage and inverter output voltage are needed to be filtered and shaped, and then they will be changed to square wave with the same frequency. Capture pins on microprocessor will catch the rising edge of this square wave, which will be used to calculate the frequency and phase difference and the result could be used to adjust the frequency of SPWM to ensure that the frequency and phase of grid voltage and inverter output voltage will be kept as the same.

The second method is that only grid voltage will be filtered and shaped and then it will change the square wave with the same frequency. Capture pins on microprocessor will catch the rising edge of this square wave to cause an interrupt, then it will judge whether the phase of SPWM output is earlier or later than the phase of grid voltage. The result will be used to adjust the frequency of the next SPWM output by changing the time of timer. Therefore, the frequency will be exactly tracked.

The above two methods have many things in common. We assume that the frequency of grid voltage is 50 Hz, then we divide each voltage cycle intoNequal parts according to the carrier frequency of SPWM (for example, if the carrier frequency is 78 kHz, thenNis 150). The corresponding sinusoidal quantity are stored in memory of microprocessor as the list,Nremains constant in each SPWM time, we can adjust the output frequency by adjusting the frequency of carrier. The implementation depends on two interrupts: one is the interrupt of SPWM carrier timer and the other is the interrupt of capture.

Since there exist two methods that the output of inverter could lock the phase of grid voltage by adjusting the frequency of SPWM, the steady accuracy of track is better than the dynamic accuracy of track. When PLL starts working, the tracing velocity will be very low, so the speed of synchronous response still remains to rise and their anti-jamming and fault-tolerance remain to rise too.

4.4.Program flow of phase-locked loop

The program flow of phase-locked is shown in Figure 4. Notice that when the frequency and phase of power is tracked, the speed can not be too fast because it could effect the output waveform and the stability will be destroyed. In this paper, the method we used is similar with fuzzy control. When we detect that the frequency of input is higher (or lower) than the provided value, we will increase (or decrease) the value of PWM time-meter according to the step until the frequency of output fall into the normal range. We can use the same method to adjust the phase. According to the testing result, the method has been proved that it is simple and convenient, furthermore, it is easy to control and the error is smaller in output between the PPL and the power.

According to the above method, we can calculate the speed of tracking: whenfis 50 Hz and it remains constant, we only need to adjust the phase, so the speedVpwill be:

Vp=360°/192=1.875°/20ms=93.75°/s

And the timetof tracking is:

t=Ph/1.875°*20ms=

1/93.75*ph=0.010 67*ph

Where,Phis the error of phase between the output voltage and grid voltage.

When the frequencyfis out of the tracking range, i.e., from 47 Hz to 53 Hz, the speed of adjustment frequencyVfis:

Vf=(1 040-980)*20ms/3Hz=0.4s/Hz

And the timetof tracking is:

t=Vf*0.02

Figure 4. The program flow of phase-locked loop

5.Conclusion

The phase-locked technology of full digital UPS based on TMS320LF2407 has been thoroughly discussed in this paper. It has been shown that the precision of full digital phase-locked technology is much higher and easier to implement and it could meet all the requirements about phase-locked technology of UPS.

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摘要：介紹了一種電磁比例泄壓閥的結(jié)構(gòu)設(shè)計與性能測試方法。該電磁比例泄壓閥采用比例電磁鐵作為驅(qū)動元件，壓力可以適時調(diào)節(jié)，滿足實驗室內(nèi)分析儀器的不同壓力需求；閥體采用內(nèi)嵌傾斜彈簧圈加強的往復式密封件，增強了密封能力，應(yīng)用在實驗室內(nèi)有高壓液體的分析儀器上，起著安全保護的作用。此外，與壓力傳感器、流量傳感器配合，還可以構(gòu)成閉環(huán)控制。

關(guān)鍵詞： 電磁比例泄壓閥；高壓密封；結(jié)構(gòu)設(shè)計；性能測試

中圖分類號：TH137.52+1

基于DSP的全數(shù)字UPS逆變器鎖相控制技術(shù)*

張雅靜?，張國旭

唐山學院 信息工程系, 河北 唐山 063000

為保證不間斷電源逆變器交直流切換過程不對負載產(chǎn)生過大的沖擊，UPS逆變輸出電壓必須與電網(wǎng)電壓的頻率及相位保持一致。快速可靠的軟件鎖相跟蹤技術(shù)可以準確地為逆變器數(shù)字化控制提供與電網(wǎng)電壓同頻同相的標準正弦電壓。主要討論基于TMS320LF2407數(shù)字化控制平臺的全數(shù)字不間斷電源中的鎖相控制技術(shù)。

DSP;UPS;鎖相

TM571

電磁比例泄壓閥的結(jié)構(gòu)設(shè)計與性能測試

韓鳳霞?

北京信息科技大學 機電實習中心，北京 100192

2014-02-05

10.3969/j.issn.1001-3881.2014.12.016

*Project supported by Science and Technology Bureau of Tangshan (No.12110215a)

? Ya-jing ZHANG, E-mail: 50513835@qq.com