Research Of Critical Technology On Single-phase Back-to-back Green Recovery-Energy-Based System For Burn-in Test

AC power electronic products, such as Uninterruptible Power Supply (UPS), AC motor drives, etc., are widely used in industrial, commercial and residential applications for high power quality requirements. However, before these AC sources can be used in practice, they must be tested for reliability and stability. Traditionally, the method of consumption load is in common. In this way, not only the tested power is wasted but also the operation is inconvenient. To meet the requirements of energy-saving and environmentally friendly features for AC power source burn-in test, green recovery-energy-based system composed by power switches is proposed. This paper focuses on the single-phase back-to-back green recovery-energy-based system, and the in-depth work is organized as follows:According to the steady-state vector relationship, the principle on the exchange of active and reactive power between the single-phase PWM rectifier and AC source is analyzed, the reason that PWM rectifier can be operated as controllable current source is also point out and the average state space mathematical model is established for control. Based on the instantaneous energy balance, for single-phase system, the instantaneous ac input power fluctuates at the twice frequency of AC source, which leads to the fluctuation of DC bus voltage at the twice frequency of AC source, unfortunately, causing the input ac current distortion. The relationship between Total Harmonic Distortion (THD) of input ac current and voltage-loop bandwidth is revealed. That offers limits for voltage controller design. The low-pass filter and notch filter for improving the current quality are compared.The linearized small-signal mathematical of PWM rectifier is set up for discussing the effect of operation of PWM rectifier under consumer mode and the power feedback mode, and circuit parameters on the closed-loop poles. The stability problem under power feedback operation is also discussed. At the same time, the influence of all kinds of disturbance on the dynamics of DC bus voltage is analyzed for the improved control. The dynamics test is mainly for the load transient, and the concept of the equivalent out impedance for DC bus is given. Due to the limitation of voltage-loop bandwidth, the load current feed forward control is proposed. The difference between full load current feed forward control and direct load current feed forward control is investaged by the output impedance bode. To overcome the defects of digital realization of observer such as aliasing due to the limited sampling frequency and DC bus current PWM harmonics, the method to estimate DC bus current by instantaneous input power is presented. Also a novel filter is proposed for reducing the impact of twice harmonics of DC bus voltage and current.Due to better attenuating current harmonics around switching frequency injected to the grid, L-filter is substituted by LCL-filter in grid connection of PWM rectifier. The transfer functions models of LCL-filter, the pulse-width-modulation theory and the harmonics requirements for input current of grid-connect converter are described. According to the restrictions such as harmonics suppression, steady-state characteristics etc., the throughout LCL-filter parameters design is analyzed. Because of the resonance phenomenon of LCL-filter, it may bring stability problems in the current loop. In order to overcome them, a control strategy applying the filter-capacitor current feedback in the current loop is proposed to achieve the function of damping resistor, and stability conditions for controller parameters are also analyzed. Then, the input impedance integrated the controller with the LCL-filter parameters in high frequency region is modeled and analyzed by the frequency responses of the transfer functions models, including the consumer mode and the power feedback mode. Also, the influence of the disturbance of Point of Common Coupling (PCC) voltage is studied.Both the essence of DC bus voltage fluctuation and the harmonics relationship in steady state between the AC side and the DC side of AC/DC converter and DC/AC converter will be revealed by the integrated modeling. Especially, when the frequency of AC tested source is different from that of the grid, the harmonics and inter-modulation production in the DC bus is analyzed in detail, and the generation of the output current harmonics and inter-harmonics injected to the grid is revealed, by means of instantaneous power theory and inter-modulation theory. Considering the steady-state harmonics and transient DC bus voltage fluctuation, a whole design for the critical storage component-DC bus capacitor is presented. To suppress the output current harmonics and inter-harmonics injected into the grid, a novel filter is given. Also, the difficulty of a wide range of DC bus voltage fluctuation and the balance of transient energy during dynamic is overcame by a feed forward control of the input current reference RMS.Under real grid-connected conditions, the PCC voltage is distorted in the distribution network due to the resonant and other nonlinear loads connected to the same PCC. Applying the harmonic damping concept, a novel grid-connected current reference strategy is proposed. This strategy makes the grid-connected converter of back-to-back system behaves as a positive resistance for the specific harmonic (at the resonant frequency) of PCC voltage to damp the possible resonance and suppress the harmonics of the PCC voltage, independent of a negative resistance for fundamental component to realize power feedback. This study provides theoretical and experimental basis for promoting the process of green recovery-energy-based system to a practical application. Three current reference algorithm (the sinusoidal grid-connected current reference, grid-connected current reference produced by PCC voltage directly, and the proposed method) are compared.