1 Overview
PWM front-end controlled rectifier is widely used in three-phase AC/DC voltage system due to its advantages such as DC voltage change, input power factor correction (PFC) and input current harmonic control. The three-phase AC/DC voltage system composed of front-end rectifier, DC capacitor and inverter is widely used in online UPS. The structure diagram of online UPS based on DSP control is shown in Figure 1.
In Figure 1, the main circuit consists of an input transformer, an input filter circuit, a voltage and current detection circuit, a battery, a power circuit, an output filter circuit, and a static switch. The power circuit includes three parts, namely, the input PFC, a three-phase full-bridge inverter, and a DC/DC part. The circuit signal is controlled by TMS320C2812. The controller is developed by TI Software Company and can be easily assembled, executed, and error checked. A general PFC boost rectifier controller usually has two feedback loops, an outer voltage loop and an inner current loop. The voltage regulator generates a current-controlled d-axis current, while the q-axis current control is a unity power factor of zero, and its control is shown in Figure 2.
Under normal working conditions, the regulator outputs a stable DC bus voltage and d-axis current control, but the inverter load is unbalanced, which will produce a fluctuating DC voltage. Therefore, the rectifier will cause front-end total harmonic distortion (THD) input current under unbalanced load.
Related studies have shown that the cause of the DC voltage filtering problem is due to the unbalanced inverter load current and unbalanced input voltage. However, their control goal is not to improve the input power quality, but to minimize the DC link voltage.
Some researchers have used the switching function concept of the power converter to show the existence of harmonic DC bus voltage. This paper will use these quantified engineering to deal with the harmonic fluctuation problem, and the simulation and experimental results will effectively prove the new control technology proposed in this paper. 2 System Analysis A standard DSP-based online UPS system is shown in Figure 3. The system consists of a push-type front-end rectifier, a DC link, and a voltage source inverter. The two power converters use standard space vector PWM control to produce fast voltage regulation and minimize total harmonic distortion control inverters.
The impact on load balance is analyzed as follows. The input of the inverter:
Where SA, SB and SC are the switches of the three inverters at the top of the switch for the switching function, as follows:
Expanding on these functional swaps, assuming standard sinusoidal phase currents, is as follows:
Where AK is a component of order k. After all trigonometric transformations with AK≡0, we can get:
Where Iinv0 is the DC component of the inverter input current; Iinvn is the current of the n-order part. From formula (4), we can see that IoutA=IoutB=IoutC and ΦA=ΦB=ΦC, and Iinvn=0, if n>0 the three-phase load current is balanced. Otherwise, the existence of the AC component will cause a chain reaction.
From formula (4), it can be concluded that considering the fixed three-phase current , Iinv0 is only proportional to A1, Iinv2 is a linear combination of A1, A3Iinv4 and A3A5 are a linear combination, and so on. In the low frequency range, since Ak≡0, Iinv0=0. According to the standard space vector PWM, the algorithm for each harmonic is:
Where q=0,1,2,…∝; ωm is the modulation frequency; ωc is the carrier frequency; ωm≤ωc, a is the modulation index; Jv(z) is the first kind of Bessel function. Formula (5) is only applicable to the frequency range far below the carrier frequency, when the carrier frequency in the primary band can be ignored. In the system study of this paper, m/c=1/90 is applicable to the specified calculation, and the modulation index is assumed: A1=1, A3=1.142×10-4, A5=3.020×10-8.
The second harmonic will cause unbalanced front-end three-phase input current. Suppressing the second harmonic DC voltage will not solve the current unbalanced problem, because it is still unstable. The control strategy proposes to eliminate the loss of control, but there is a second harmonic component and feedback. 3 Control strategy In power supply applications, the standard voltage frequency of the basic inverter output is 50Hz, but the DC bus harmonics must be twice, which can be designed to prevent digital band-stop filters with known harmonic frequencies. In the digital filter, the lower and upper cutoff frequencies ω1 and ω2 of 2n are designed and simulated using MATALAB. Discrete time sliding mode controller (DSMC), which has been proven to be more effective, is used for the inner current loop. The description of DSMC simulation is as follows. The rectifier circuit including the input inductor as shown in Figure 3 can be used as a mode LTI system and represented in state space. In discrete time, the system can be described as follows:
In the formula, input current iin; rectifier control voltage vpwm; input power supply voltage vin all represent the reference coefficients for participating in the synchronization dq, and Ai, Bi and Ei are circuit parameters determined by the system. Given the current inverter command iref (k), the DSMC simulation is equivalent to the control formula as follows:
The DC bus voltage and PWM technology can be used to determine the rectifier control voltage limit speed, and the actual control voltage formula can be obtained:
4 Simulation results
In order to intuitively compare the traditional and proposed control techniques, different models were established under unbalanced loads. The undesirable negative sequence component of the input current is nearly eliminated, and the total harmonic distortion of the input current is also reduced. This result means that the decoupling between the inverter and the rectifier is achieved in the unbalanced load input current DC link. Figures 4 and 5 show the different dynamic performances between the controllers with harmonic compensation. By comparison, it can be seen that the traditional control technology has unbalanced three-phase input current and low distortion, and the control technology of this paper is stable.
5 Experimental results
Based on two digital controllers TMS320C2812DSP to control the rectifier and inverter, the simulation experiment was carried out using the same load in Figure 3. Figure 6 shows the DC measurement and screening values collected online in steady state. Obviously, the 100Hz component represented by the direct measurement Udc is significantly suppressed, which is proved by the fourth-order filter in the transient test.
The above simulation experiments were repeated, and the results are shown in Figures 7 and 8.
It can be seen that the control technology proposed in this paper improves the balanced three-phase input current , and similar waveforms are shown in the simulation results of Figure 4 and Figure 5. 6 Conclusion This paper proposes a new standard rectifier inverter control technology system with front-end PWM rectifier to achieve decoupling conversion and DC capacitor connection between three-phase inverters under unbalanced loads. Based on the analysis of the influence of the front-end controlled rectifier on the unbalanced load, a voltage current loop is designed and used to prevent the DC voltage feedback of the second harmonic component and filter the input current of the rectifier and inverter so that it does not destroy the dynamic response of the DC bus voltage. The new control technology proposed in this paper is effectively proved by simulation and experimental results.
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