Research on Intelligent UPS Control Technology-EEWORLD

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1 Introduction

Driven by the rapid development of computer networks and related network communication technologies, UPS (uninterruptible power supply) is being used more and more widely in various fields of the national economy to ensure that the data, files, graphics and other materials obtained during information resource sharing have high authenticity, reliability, continuity and high fidelity. At the same time, with the development of information technology, new technologies such as intelligent information processing and network-based remote monitoring are gradually applied to UPS, forming a fully intelligent UPS system, which is convenient for users and improves reliability. This paper focuses on UPS control technology, analyzes the characteristics of UPS PID control technology, and deeply analyzes the composite control strategy based on PID control and repetitive control.

2 Overview of control strategy

The digital control technology of UPS inverter has become a hot spot in the current inverter research field. A variety of inverter digital control methods have emerged, including digital PID control, state feedback control, deadbeat control, repetitive control, fuzzy control, etc., which have strongly promoted the development of UPS technology.
Each control scheme has its own advantages and disadvantages. Although some control methods have good dynamic response speed, the steady-state output voltage harmonic distortion does not meet the requirements; although some control methods have high dynamic and steady-state accuracy at the same time, they are very sensitive to parameter changes and have poor robustness; some control methods have good steady-state accuracy, but the dynamic response effect is very poor; some control methods are limited by the hardware level and cannot be well applied at present. Therefore, an inevitable development trend is that various control schemes penetrate each other, learn from each other's strengths and weaknesses, and form a composite control scheme.

3 Digital PID control

In UPS inverter control, the most commonly used and simplest method is PID control. The specific implementation methods include voltage instantaneous value feedback control and voltage and current dual closed-loop feedback control. Figure 1 shows voltage instantaneous value feedback control.

Figure 1 UPS inverter digital PID control
The advantage of the control strategy of instantaneous voltage feedback is that only one voltage sensor is used. The disadvantage is that the system dynamic response characteristics are not good, the tracking characteristics are not very good, and the waveform quality is poor. Figure 2 shows the output voltage waveform when this control method is used on a 10KVA inverter with a capacitive load. It can be seen from the figure that the waveform distortion is large and it is difficult to meet the requirements of high-quality power supply.

Figure 2 Voltage waveforms with inductive load currents of 33A and 20A
One of the ways to improve the dynamic characteristics of the voltage source inverter is to add a current closed loop. In this control strategy, the current of the filter capacitor (that is, the differential of the output voltage) is introduced into the control system as a feedback variable to improve the quality of the output waveform. It must use a Hall sensor to detect the filter capacitor current, which increases the complexity and cost of the system.

[page]4 Composite control based on PID control and repetitive control

The inverter controller is a regulation system with a sinusoidal reference setting, rather than a constant value setting regulation system. At the same time, the system disturbance, that is, the load current, is not a constant value disturbance. When the load is linear, the load current changes sinusoidally; when the load is non-linear, the current changes according to the non-sinusoidal law. For the problem of zero-static tracking of sinusoidal instructions, a sinusoidal signal model with the same frequency as the reference setting can be implanted in the controller.

(1-1)
Where ω is the angular frequency of the sinusoidal command. It can be verified that when the command and the disturbance both change sinusoidally with the angular frequency ω, a stable control system containing the internal model shown in (1-1) is error-free. However, this is only a conclusion obtained under the assumption of a linear load. The actual load is much more complex than this, and most of them are rectifier loads. Such load current is non-sinusoidal, containing the fundamental wave and multiple harmonics of the fundamental frequency at integer multiples. Therefore, the actual disturbance frequency components are very rich. If all these frequency disturbances are to be error-free, the method of implanting a sinusoidal signal internal model is not suitable.
The disturbance signal has a common feature, that is, it repeats with the same waveform in each fundamental wave cycle. Therefore, the repetitive control based on the internal model principle can adopt the following "repetitive signal generator" internal model. Its transfer function is:

(1-2)
Where L is the inverter output fundamental wave period. Discretize it to get a positive feedback link with a delay of one cycle. This delay link is the fatal disadvantage of repetitive control, and its regulation effect on tracking error lags behind one power frequency cycle. Therefore, it is considered to combine the PID control method with the repetitive control method to form a new UPS inverter waveform control method based on PID and repetitive control. The steady-state output waveform quality of the system is improved by repetitive control, and the dynamic characteristics of the system are improved by digital PID control, so that the system has both good steady-state and dynamic characteristics.
The purpose of feedforward control is to improve the control effect of the digital PID controller, further reduce the fluctuation and waveform distortion of the output voltage in the dynamic process, and improve the stability of the digital PID control system. The discrete repetitive controller is used to eliminate the periodic tracking error of the system and reduce the output voltage waveform distortion when the UPS inverter is loaded with nonlinear rectification. The digital PID controller is used to adjust the output voltage tracking error in real time to reduce the output voltage fluctuation and distortion when the system is disturbed. The control block diagram is shown in Figure 3. The main links in the figure are introduced as follows:
1) z-N: cycle delay link, so that the error information of this cycle affects the correction amount from the next cycle.
2) Q(z): Set to overcome the inaccuracy of the object model and enhance the stability of the system. It can be a constant less than 1.
3) S(z): Compensation link, used to transform the object characteristics.
4) zk: Phase compensation, to meet the system frequency response requirements.
5) a: Proportional factor. Used to maintain the stability of the system.

Figure 3 Composite control block diagram based on PID control and repetitive control

[page]5 Experimental results

According to the above model, the controller parameters can be preliminarily determined by simulink simulation. The more conservative parameters are used to test the 10KVA inverter module. The parameters are adjusted to achieve better static and dynamic characteristics of the system. The system parameters are: input DC voltage is 200V, output frequency is 50HZ, switching frequency is 19.6HZ, filter inductor is 120uH, filter capacitor is 15uF. Figure 4 shows the voltage and current waveforms with resistive load and current of 36A on the left; the voltage and current waveforms with rectifying load and current of 20A on the right.

Figure 4 Voltage and current waveforms with resistive and rectifying loads
From the above two figures, we can see that the composite control strategy based on PID control and repetitive control has a good waveform control effect, especially for nonlinear rectifier loads, it has a good harmonic suppression effect, and the system also has good dynamic response characteristics. Therefore, the composite control strategy based on PID control and repetitive control introduced in this article has a high application value.

References

[1] Hou Zhenyi, Wang Yiming, “UPS circuit analysis and maintenance”, Science Press, 2001
[2] Li Chengzhang, “Modern UPS power supply and circuit diagram”, Electronic Industry Press, 2001
[3] S. Karve, “Three of a kind [UPS topologies, IEC standard],” IEE Review, vol.46, no.2, pp.27-31, March 2000
[4] Ma Xudong, UPS intelligent control technology based on DSP, Electrical Automation, No.1, 2003, pp.26-27
[5] H. Pinheiro, P. Jain, “Comparison of UPS topologies based on high frequency transformers for powering the emerging hybrid fiber-coaxial networks,” IEEE-INTELEC \'99, pp.9-12, 1999
[6] Lin Xinchun, Duan Shanxu, UPS full digital control system based on DSP, Power Electronics Technology, No.2, 2001, pp.51-53

Reference address：Research on Intelligent UPS Control Technology

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