Application of Discrete Sliding Mode Control in UPS Inverter Design-EEWORLD

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1 Introduction

UPS (Uninterruptible power supply) is a power electronic device between the AC power grid and the load. Its basic function is to ensure uninterrupted power supply to the load and ensure continuous normal operation of the load when the AC power fails. Modern UPS should also have the characteristics of outputting high-quality electric energy, having little impact on the power grid, and being highly intelligent.

There are two types of control strategies based on linear control theory and nonlinear control theory in the control concept of UPS. Among them, nonlinear control theory mainly adopts sliding mode control, repetitive control, and deadbeat control. The sliding mode control system has strong robustness, that is, it has excellent insensitivity to the model error, parameter change and external disturbance of the controlled object. However, the traditional continuous sliding mode control has the problem of system jitter in practical applications, which will generate a large number of harmonics, so it cannot be widely used in UPS systems. With the development of discrete sliding mode control theory and digital control technology in recent years, the sampling time has been greatly shortened, the switching frequency of the system has been increased, and the jitter phenomenon has been effectively eliminated. The use of discrete sliding mode control in the design of UPS inverter can improve the reliability and availability of UPS and meet the higher performance requirements of modern UPS systems. [2][3]

2 UPS system composition

The UPS system is mainly composed of a rectifier, a battery pack and an inverter. The inverter main circuit of the UPS adopts a full-bridge inverter circuit with an LC filter. By controlling the static switch between the inverter output and the load, the battery pack and the inverter can continue to supply power to the load when the city power is cut off. The simplified schematic diagram of the system is shown in Figure 1.

In order to increase the switching frequency of the system and eliminate the jitter phenomenon, the core control device of the UPS uses TI's DSP TMS320LF2407, which has an instruction cycle of 33ns, and has peripheral modules such as analog-to-digital conversion, serial peripheral interface, serial communication interface module, event manager, etc., a 16-bit fixed-point core and instruction set. The instruction set source code is upwardly compatible with the TMS320C5x series and has good portability. The high-performance computing power of its core enables it to run complex control algorithms, improve the flexibility and logical judgment ability of the controller, and thus make the system performance more superior. [4]

Figure 1 Single-phase online UPS system
In the inverter and its output filter circuit, the inductor current

and capacitor voltage

is the controlled variable, the unknown load current

It can be regarded as external interference. Assuming that the switching frequency is much higher than the modulation frequency, the UPS inverter state space equation can be obtained:

Represents system control input.

Represent the current sensor and voltage sensor gains respectively.

Indicates the nominal value of the DC bus voltage.

In practical applications, UPS must have excellent performance, such as anti-interference, small voltage waveform distortion, good tracking performance, and the ability to work normally under non-linear load or overcurrent conditions. This paper designs a discrete sliding mode controller with a dual-loop structure to meet these requirements of UPS inverters.

[page]3 Inverter control design

The inverter adopts a dual-loop structure design, which consists of a current predictor and a current controller, and both design their own switching planes, and both adopt a free sequence control method. The current predictor should have a faster response than the current controller and be insensitive to load changes. The current controller will have a unilateral behavior characteristic to effectively eliminate the jitter phenomenon that occurred when continuous sliding mode control was previously used. The current predictor estimates the required inductor current by tracking the error of the output voltage, and the current controller is used to adjust the inductor current and generate a control signal for the PWM inverter. In this way, the inverter will have the characteristics of current limiting and fixed switching frequency. The block diagram of its control system is shown in Figure 2.

Figure 2 Control system block diagram

3.1 Current predictor design

In order to eliminate the static error of the output voltage, an error integral term is added to the system sliding mode plane design

, and the state equation is obtained

is the reference control quantity,

is the integral gain

After combining with (2), assuming the sampling period is T, the discrete system equation can be obtained

is the feedforward autocorrelation coefficient,

The determination of will ensure that the system state tends to be stable on the sliding mode hyperplane.

The current predictor control term is

Get the control item.

(11)

3.2 Current controller design

Discretize (1), where the sampling period is T, and we can get

(12)
The sliding mode function of the system can be selected as

(13)
in

is the tracking error
Then the current controller control term is

(14)

The proof of the existence of sliding mode motion is similar to the above, and it can be obtained that if the condition 01 is satisfied, the system can reach the sliding mode plane without jitter. And its control term can be obtained as

(15)

[page]4 Simulation results

Now design a 1KVA single-phase inverter with the following parameters:

Using the TMS320LF2407 evaluation board provided by TI and the simulation software VisSim/ECD designed by a third-party company specifically for TI's 2000 series DSP, the corresponding algorithm can be simulated online and the corresponding target file C language source code can be generated. After simulation, the voltage and current waveforms of the system when it is put into operation and unloaded at full load are shown in Figure 3. The results show that the controller shows good robustness under load changes and returns to normal in a short time. It has good tracking performance in steady-state operation. Under full load, the voltage is maintained at more than 99%. In addition, under non-linear load conditions, the controller also shows good voltage regulation performance.

Figure 3. Simulation results of the system when fully loaded and unloaded

5 Conclusion

This paper proposes a discrete sliding mode control algorithm for UPS inverter design. The design adopts a dual-loop structure and has the characteristics of current limiting and fixed switching frequency, which achieves the purpose of eliminating jitter. The simulation results show that the system can effectively suppress the output voltage waveform distortion, has good voltage regulation performance and strong load capacity.

References

[1] Gao Weibing, Theory and Design Method of Variable Structure Control [M]. Beijing: Science Press. 1996

[2] Tian Hongqi, Sliding Mode Control Theory and Its Application [M]. Wuhan: Wuhan Press. 1995

[3] Li Chengzhang, Modern UPS Power Supply and Circuit Diagram Collection [M]. Beijing: Electronic Industry Press. 2001

[4] Texas Instruments Inc. TMS320CLF240xA DSP Controllers Reference Guide [Z].
Texas Instruments Inc., 2001

[5] Tzuen-Lih Chern et al. Microprocessor-Based Modified Discrete Internal Variable-Structure Control for UPS [J].
IEEE Trans Ind. Electron, 1999, 46(2): 341-347

[6] Hu Qing et al. Application of Sliding Mode Variable Structure Control in DC-DC Converter [J]. Journal of Shenyang University of Technology, 2002, 24(2): 57-60

Reference address：Application of Discrete Sliding Mode Control in UPS Inverter Design

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