Li-Chiun Lin

Power Loss Comparison of Single- and Two-Stage Grid-Connected Photovoltaic Systems

This paper presents power loss comparison of single- and two-stage grid-connected photovoltaic (PV) systems based on the loss factors of double line-frequency voltage ripple (DLFVR), fast irradiance variation + DLFVR, fast dc load variation + DLFVR, limited operating voltage range + DLFVR, and overall loss factor combination. These loss factors will result in power deviation from the maximum power points. In this paper, both single-stage and two-stage grid-connected PV systems are considered. All of the effects on a two-stage system are insignificant due to an additional maximum power point tracker, but the tracker will reduce the system efficiency typically about 2.5%. The power loss caused by these loss factors in a single-stage grid-connected PV system is also around 2.5%; that is, a single-stage system has the merits of saving components and reducing cost, and does not penalize overall system efficiency under certain operating voltage ranges. Simulation results with the MATLAB software package and experimental results have confirmed the analysis.

DC-Bus Voltage Control With a Three-Phase Bidirectional Inverter for DC Distribution Systems

This paper presents dc-bus voltage control with a three-phase bidirectional inverter for dc distribution systems. The bidirectional inverter can fulfill both grid connection and rectification modes with power factor correction. The proposed control includes two approaches, one line-cycle regulation approach (OLCRA) and one-sixth line-cycle regulation approach (OSLCRA), which take into account dc-bus capacitance and control dc-bus voltage to track a linear relationship between the dc-bus voltage and inverter inductor current. Since both of the approaches require the parameter of dc-bus capacitance, this paper first presents determination of dc-bus capacitor size and an online capacitance estimation method. With the OLCRA, the inverter tunes the dc-bus voltage every line cycle, which can reduce the frequency of operation-mode change and current distortion. The OSLCRA adjusts current command every one-sixth line cycle to adapt to abrupt dc-bus voltage variation. The two approaches together can prevent dc-bus voltage from wide variation and improve the availability of the dc distribution systems without increasing dc-bus capacitance. Experimental results measured from a three-phase bidirectional inverter have verified the feasibility of the discussed control approaches.

Two-Phase Modulated Digital Control for Three-Phase Bidirectional Inverter With Wide Inductance Variation

This paper presents two-phase modulated digital control for a three-phase bidirectional inverter with wide inductance variation in dc distribution systems. The bidirectional inverter can fulfill both grid connection and rectification with power factor correction. With the proposed control, the inverter can track its sinusoidal reference currents, and it is allowed to have wide inductance variation, reducing core size significantly. The control laws based on space-vector pulsewidth modulation are first derived with either an accurate approach or an approximated one, and the gate signals were then derived based on two-phase modulation for lower switching loss and switching noise spectra. Determination of control parameters and stability analysis are also presented. In the design and implementation, the inductances corresponding to various inductor currents were measured and tabulated into a single-chip microcontroller for tuning loop gain cycle by cycle, ensuring system stability. Measured results from a 10 kVA 3φ bidirectional inverter have been presented to confirm the feasibility of the discussed control approaches.

A D-

This paper presents a division-summation (D- Σ) digital control for a three-phase inverter to achieve active and reactive power injection to the ac grid. The proposed D- Σ approach summarizes the inductor-current variations over one switching cycle to derive control laws directly, which can overcome the limitation of d- q transformation. The inverter with this control can achieve various power factors (PFs) leading or lagging, by taking into account wide filter-inductance variation and grid-voltage distortion, reducing core size significantly. The control laws for achieving the desired features are derived in detail, and they are expressed in general forms for readily software programming. With the enhancement work of previous research, active and reactive power injection with PF from 0 to 1 can be controlled effectively, so that these control laws can be extended to wide current-tracking applications, such as static synchronous compensator and active power filter. In the design and implementation, the inductance values corresponding to various inductor currents were measured at the startup and stored in the controller for scheduling loop gain cycle by cycle. Measured results from a 10-kVA 3 φ inverter have confirmed the analysis and discussion of the proposed control approaches.

\Sigma

Division-summation (D-Σ) digital control has been successfully applied to the single-phase bidirectional inverter with an LC filter, which can cover wide inductance variation and achieve precise inverter current tracking. However, high frequency ripple current injection to the grid cannot be avoided, and an LCL filter is therefore required. Since there typically exist grid voltage harmonics, the injected grid current will contain harmonic components due to the effect of the LCL-filter capacitor. This paper presents an extended application of the D-Σ digital control associated with a filter-capacitor-current compensation to reduce the injected grid-current harmonics. The control laws of the inverter with the D-Σ digital control and compensation approach are derived in detail, and the reduction of grid-current harmonics is analyzed. With the proposed approaches, the phase margin between the output impedance of the inverter and grid impedance can be higher than 80° from low to high frequencies, and the inverter can achieve high harmonic voltage rejection ratio up to 39th harmonic, which is relatively suitable for weak grid connection. Experimental results measured from a 5-kW single-phase bidirectional inverter have verified the feasible application of the D-Σ digital control and proposed compensation.

Digital Control for Three-Phase Inverter to Achieve Active and Reactive Power Injection

Unlike a dc distribution system with a three-phase inverter, the one with a single-phase inverter to regulate the dc-bus voltage will result in high voltage ripple. This paper presents dc-bus voltage regulation for a dc distribution system integrated with a single-phase bidirectional inverter. In a dc distribution system, a bidirectional inverter controls its inductor current to balance power flow and to regulate the dc-bus voltage. To reduce the line current distortion and the influence of dc-bus ripple voltage on regulation control, one line-cycle regulation approach (OLCRA) and quarter line-cycle regulation approach (QLCRA) are proposed. The OLCRA can regulate the dc-bus voltage with low ac current distortion, and the QLCRA can regulate the voltage under a fast load variation and with voltage ripple. Moreover, for enhancing the operational reliability and availability of the dc distribution system, the bidirectional inverter shifts the dc-bus voltage to different levels according to the load conditions. This shift mechanism can also reduce the possibility of entering under or over voltage protection without increasing dc-bus capacitance. In addition, the design of dc-bus capacitance for the system and the online estimation of the capacitance at the system startup and under the aging effect are proposed, which can help regulate the dc-bus voltage tightly in the long-term operation. The stability analysis of the dc-bus voltage regulation under capacitance variation is also addressed. Experimental results measured from a 5-kW system with a single-phase bidirectional inverter have verified the analysis and discussion.

Extended Application of D-

This paper presents power loss analysis for PV dc-distributed systems based on the loss factors of double line-frequency voltage ripple (DLFVR), fast irradiance variation + DLFVR, fast dc load variation + DLFVR, limited operating voltage range + DLFVR, and overall loss factor combination. These loss factors will result in power deviation from the maximum power points. In the paper, both single-stage and two-stage grid-connected PV systems are considered. All of the effects on a two-stage system are insignificant due to an additional maximum power point tracker, but the tracker will reduce the system efficiency typically about 2.5 %. The power loss caused by these loss factors in a single-stage grid-connected PV system is also around 2.5 %; that is, a single-stage system has the merits of saving components and reducing cost, while does not penalize overall system efficiency in dc-distribution applications. Simulation results with the MATLAB software package and experimental results have confirmed the analysis.

{\bf Σ}

Resonant frequency of an LCL filter in a grid-connected inverter is an important factor for control stability and grid-injected current quality. However, the constraints on resonant frequency are usually considered as an afterthought in conventional filter design methods. The fact that resonant frequency is affected by both internal and external factors during operation makes it even more critical. Hence, an improved resonant frequency based systematic LCL filter design method is presented in this paper. A resonant frequency band is identified by considering grid-voltage harmonics and control stability boundary. The variation in resonant frequency due to inductive line impedance as well as magnetic permeability of inductor core is taken into account to define the constraints on LCL parameters. The LCL parameters are then systematically derived from the identified constraints with almost no iteration. A division–summation (D-Σ) digital control method is used to validate the design method. Experimental results measured from a 5-kW single-phase grid-connected inverter have verified the effectiveness of the LCL filter design method over conventional methods.

Digital Control to a Single-Phase Bidirectional Inverter With an LCL Filter

This paper presents a load impedance estimation and iterative-learning control for a single-phase three-wire inverter acting as an uninterruptible power supply (UPS), which can supply unbalanced load, linear load and rectified load. The proposed load impedance estimation scheme is based on an RLC equivalent circuit concept to determine load parameters, RLC, insuring that the inverter can generate sinusoidal voltage. With an iterative learning control, the steady-state error of the output voltage can be reduced significantly. Additionally, the proposed control can accommodate wide inductance variation of the inverter, reducing core size significantly. In the design and implementation, the inverter inductances corresponding to various inductor currents are measured at the start-up and stored in the memory of the controller for scheduling loop gain every switching cycle, and the RLC load parameters are estimated online to determine control (duty ratio). Experimental results measured from a 5 kVA inverter have verified the analysis and discussion.

DC-Bus Voltage Regulation for a DC Distribution System With a Single-Phase Bidirectional Inverter

This paper presents current improvement for a 3⊘ bi-directional inverter with wide inductance variation. A bi-directional inverter fulfilling grid connection and rectification with power factor correction has been designed and implemented in the laboratory. With a digital predictive current control, the inverter can accommodate wide inductance variation, improving current distortion in high power applications significantly. However, under low current levels, inductor currents have serious distortion. To improve current distortion, this paper presents four attempts, including mid-point current sampling, smooth region transition, current interleaving, and duty splitting. Theoretical analysis and experimental results are presented to verify the discussion.