Comparison of two typical control methods in inverter controller

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Comparison of two typical control methods in inverter controller [Copy link]

Abstract: This paper compares and analyzes the two control methods of capacitor current feedback and inductor current feedback in inverter control. The control model of the controller is derived, the design method of the controller is analyzed, the experimental results under various sudden loads and constant load conditions are given, and the characteristics, advantages and disadvantages of the two control methods are pointed out. The experiment is carried out in an 800VA inverter, the switching frequency of the inverter is 30kHz, and the output voltage is a sine wave of 400Hz and 115V effective value.

Keywords: capacitor current; inductor current; controller

　

0 Introduction

There are many control methods that can be used for inverters, and different control methods have their own unique advantages and applicable occasions [1][2]. From the perspective of the control loop, it can be divided into open-loop control, single-loop control, dual-loop control and multi-loop control.

Open-loop control cannot meet the requirements of UPS inverters in terms of static or dynamic characteristics. In order to obtain good static and dynamic characteristics of the inverter output voltage, single-loop instantaneous value feedback control of the output voltage can be used. This control method can adjust the waveform of the output voltage in real time, better suppress the nonlinear characteristics of components and the influence of DC bus voltage fluctuations, and improve the static and dynamic characteristics of the inverter to a certain extent. However, since this control method only has single voltage loop control, when the load undergoes a relatively large dynamic change (such as a sudden increase in the current of the load), the output voltage of the inverter will have a relatively large distortion, and the dynamic adjustment is relatively slow. Since this system is a second-order oscillation link, the lighter the load, the longer the dynamic adjustment time, and the root trajectory of the closed-loop system is close to the imaginary axis when the load is light, the system stability is poor. In order to further improve the control characteristics of the inverter, dual-loop and multi-loop control can be used. Since multi-loop control is relatively complex, it is rarely used in actual applications. Dual-loop control has been widely used due to its good control performance and convenient control. This paper compares and analyzes two typical dual-loop control methods of output voltage and filter capacitor current feedback and output voltage and filter inductor current feedback.

1 Inverter control model with two feedback loops

_{Figure 1 is the main circuit diagram of the full- bridge} inverter. Vd is a DC voltage source, S1 _~ S4 _are four IGBT switches, L and C are filter inductors and filter capacitors, which are used to filter out high-order harmonics in the inverter system. rL and rC are the equivalent series impedances of the filter inductors and filter capacitors. _{ZL is the load, which} can be purely resistive or nonlinear. The inverter main circuit diagram shown in Figure 1 is a nonlinear system due to the presence of switching devices. However, when the switching frequency of the device is much greater than the fundamental frequency of the inverter output voltage, it can be analyzed using state space averaging and linearization techniques. As shown in Figure 1, the following dynamic equations of the inverter model can be obtained:

(1)

(2)

v0 ₌ ( 3)

i _L =i _C + i _Z (4)

Where: i _C , i _L , i _Z are the currents of inductance, capacitance and load respectively.

Figure 1 Main circuit of full-bridge inverter

The above dynamic equations show the interrelationships between the various quantities in the inverter. In the process of establishing the above equations, the inverter can be regarded as an amplifier with a constant gain. Based on the previous dynamic equations, a controller model can be designed as shown in Figures 2 and 3. The coefficients are defined as listed in Table 1.

Figure 2 Schematic diagram of the inductor current feedback inverter control model

Figure 3 Capacitor current feedback inverter control principle block diagram

Table 1 Coefficient definition

coefficient	definition
k	Proportional feedback factor of the current loop
k V	Proportional feedback factor of the voltage loop
GPI ( s )	PI regulation function
τ 1	Current feedback filter constant
τ 2	Voltage feedback filter constant
C	Current regulation ratio
k PWM	SPWM amplification factor

　

From the control block diagram above, it can be seen that the difference between the control of inductor current feedback and capacitor current feedback is only the different paths of current feedback. The structure of the control loop is the same, so the same design method can be used when designing closed-loop parameters. Therefore, from the perspective of feedback parameter design, the design methods of the two control methods can be exactly the same, except that the values of the parameters vary.

The controller parameters should be designed so that the system has a large amplification factor in the low-frequency area to improve the steady-state performance of the system; the slope of the mid-frequency band should not be too large or too small. In addition, the mid-frequency band should have a certain width so that the system has a large phase margin. The higher the crossover frequency of the system, the faster the system responds, but too high a frequency will introduce high-frequency interference. The design of the high-frequency band requires that the amplitude-frequency characteristics of the controller decrease rapidly as the frequency increases to improve the system's anti-interference ability.

2 Experimental results of two feedback control methods

According to the control principle described above, an inverter with a rated output power of 800VA, a sine wave output voltage of 115V, and an output frequency of 400Hz is designed. In this inverter, the two control modes of inductor current feedback and capacitor current feedback are compared to illustrate the characteristics of the two control modes.

For comparison purposes, the parameters in all the inductor current feedback experiments in this paper are the same. Similarly, the control parameters in all the capacitor current feedback experiments are also the same. Figures 4 to 9 are the experimental waveforms under linear load, and Figures 10 to 13 are the experimental waveforms under rectifier bridge load. It can be seen from Figures 4 to 7 that the THD of the no-load output voltage and full-load output voltage waveforms under the two control modes are basically the same. Since the capacitor current is the differential of the output voltage, it has a predictive effect on the change of the output voltage, and the dynamic response speed should be very fast. The current of the inductor current feedback also includes the capacitor current, so it also has a faster response speed. It can be seen from Figures 8 and 9 that the switching time and overshoot of the inverter are basically the same when the capacitor current feedback and the inductor current feedback are basically the same. It can be seen from Figures 10 to 13 that for the rectifier bridge load, the output voltage under capacitor current feedback control is much better than the output voltage waveform quality under inductor current control. The THD of the output voltage in Figure 13 is 7%. By appropriately changing the control parameters, the THD in Figure 13 can be reduced. Since the capacitor current is the differential of the output voltage, it is very sensitive to the sudden change of the load current and can be corrected before the output voltage is distorted. Therefore, as shown in the experimental waveform above, the waveform quality of the inverter output voltage when the capacitor current is fed back is much better than that when the inductor current is fed back. The inductor current feedback can be used to achieve automatic current limiting protection according to the system requirements, using the same parameters as in steady state, as shown in Figure 14. Unlike the inductor current feedback, the capacitor current cannot reflect the load size in steady state. Therefore, the capacitor current feedback cannot directly rely on the same feedback parameters as in steady state to achieve current limiting protection. If current limiting protection is to be achieved, other methods need to be used [3].

Figure 4 Capacitor current feedback linear load no-load output voltage experimental waveform THD = 0.8%

Figure 5 Inductor current feedback linear load no-load output voltage experimental waveform THD = 0.9%

Figure 6 Capacitor current feedback linear load full load output voltage experimental waveform THD = 1.2%

Figure 7 Inductor current feedback linear load full load output voltage experimental waveform THD = 1%

Figure 8 Capacitor current feedback linear load full load to no load switching output voltage experimental waveform

Figure 9 Inductor current feedback linear load full load to no load switching output voltage experimental waveform

Figure 10 Capacitor current feedback rectifier bridge load no-load output voltage experimental waveform THD = 1.0%

Figure 11 Inductor current feedback rectifier bridge load no-load output voltage experimental waveform THD = 1.1%

Figure 12 Capacitor current feedback rectifier bridge full load output voltage experimental waveform THD = 1.3%

Figure 13 Inductor current feedback rectifier bridge load full load output voltage experimental waveform THD = 7%

Figure 14 Output voltage and inductor current experimental waveforms when inductor current feedback current limiting

3 Conclusion

The analysis shows that the design principles of capacitor current feedback and inductor current feedback controllers are the same. The output voltage waveform quality generated by the two control methods is relatively good when the linear load is in steady state, and the dynamic response is also relatively fast in transient state. However, for the rectifier bridge load, the control result of capacitor current feedback is much better than that of the inductor current. Therefore, the load adaptability of capacitor current feedback is much better than that of inductor current feedback, and it is suitable for linear loads, rectifier bridges and other loads. Inductor current feedback is suitable for linear loads and occasions that require current limiting protection. The design in this article is realized through analog control. Compared with the various popular complex digital control methods, the output waveform quality is basically the same, but it has the advantages of simple design and low cost.