L.-C. Lin

DC-bus voltage control for three-phase bi-directional inverter in DC microgrid applications

This paper presents dc-bus voltage control for a three-phase bi-directional inverter in dc-microgrid applications. The bi-directional 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 regulate dc-bus voltage to track a linear relationship between dc-bus voltage and inverter inductor current. With the OLCRA, the inverter can tune the dc-bus voltage to the desired voltage accurately 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 dramatic change and improve the availability of the dc-microgrid without increasing dc-bus capacitance. Determination of dc-bus capacitance is also presented in this paper. Experimental results measured from a 10 kVA three-phase bidirectional inverter have verified the feasibility of the discussed control approaches.

Design and implementation of multiple maximum power point trackers for multi-string PV panels in DC distribution systems

This paper presents design and implementation of multiple maximum power point trackers (MPPTs) with wide input voltage variation for dc distribution systems. The topology of the proposed MPPT combines buck and boost converters to accommodate PV input voltage from 0 V to 850 V. The control algorithm for tracking maximum power points is based on a “perturbation and observation” method. The proposed MPPT can on-line check the input configuration of PV-panel strings to determine the current for maximum power calculation. In the paper, operational principle, analysis and design of the proposed MPPT configuration and mode transition will be first presented. Flowcharts of the control algorithm include perturbation and observation method and on-line PV-panel string configuration check will be then explained. Experimental results obtained from three-string MPPTs with the maximum power of 10 kW have verified its performance and feasibility.

Iterative learning control with filter-capacitor current compensation for a three-phase four-wire inverter

This paper presents an iterative learning control with filter-capacitor current compensation for a three-phase four-wire inverter, which is adopted to uninterruptible power supply (UPS) applications. It can supply unbalanced, linear and rectified loads. With the proposed iterative learning control, the steady-state error can be reduced significantly cycle by cycle, and it is allowed to have wide inductance variation, reducing core size significantly. Additionally, the compensation for the filter-capacitor current is also considered in the control, which can help to shape output voltage waveform more accurately. In the design and implementation, the inverter inductances corresponding to various inductor currents are measured off-line and tabulated into a single-chip microcontroller for tuning loop gain every switching cycle, ensuring system stability. Experimental results measured from a 10 kVA inverter have verified the analysis and discussion.

Design of D-Σ digital controlled hybrid-frequency inverters with lcl filter for grid-connected applications

This study presents design of D-Σ digital controlled hybrid-frequency inverters with LCL filter for grid-connected applications. Hybrid-frequency inverters consist of inverters with different frequencies and power ratings connected in parallel. The lower frequency inverter has higher power rating, while the higher frequency inverter has lower one. The inverters are controlled to cancel out low-frequency switching-current ripples, and their equivalent switching frequency and dynamic response are dominated by the high switching frequency, reducing filter size. With different frequencies and power ratings, the inverters have extra power scheduling and control freedom. Moreover, with hybrid-frequency-inverter systems, it is no need to use high power high frequency switching devices. Features of smaller current ripples, passive filter size and better dynamic performance comparing to single frequency and conventional interleaved inverters are verified by simulation results.

A modified division-summation digital control for grid-connected inverter with wide inductance variation of LCL filter

The natural swinging filter inductance due to cores magnetic permeability can affect control stability. Conventional control methods generally adopt larger cores to avoid this issue. Division-Summation (D-Σ) digital control, on the other hand deals with the problem by including this filter inductance variation in the control law. It allows the use of much smaller core than conventional control methods for the same power rating. Non-linear model of filter inductor is used to estimate the inductance in each switching cycle. This estimate is used to achieve optimal control and no further proportionality constant parameter tuning is required. However, wide variation in both inverter-side and grid-side inductance needs to be considered with LCL filters. A modified D-Σ digital control method is presented in this paper to address this issue. The compensation term for active damping of resonance is also logically derived. Experimental results measured from a 5 kW single-phase grid-connected inverter have verified the feasibility of the proposed method.

An improved resonant frequency based LCL filter design method for grid-connected inverters

Resonant frequency of an LCL filter in grid-connected inverters decreases with increase in inductive grid impedance. It is also affected by drop in magnetic permeability of filter inductor with increase in current. This resonant frequency variation affects grid-voltage harmonic rejection and compromises stability. Hence, an improved resonant frequency based LCL filter design method is presented in this paper. A resonant frequency band is identified by considering four factors: grid-voltage harmonics, grid strength, filter inductance non-linearity and control stability boundary. The LCL parameters are then derived so that the resonant frequency remains within this identified band for a wide operating range. Division-Summation digital control method is used to elaborate the design method. Experimental results have demonstrated stability and much improved harmonic rejection under wide parameter variations.

Design and implementation of D-Σ digital controlled multi-function inverter to achieve APF, active power injection and rectification

There has been a growing demand of using multi-function inverters for grid-connected systems applied to nonconventional energy sources, such as solar, wind and so on. In addition to power quality conditioning, the inverter can also be used for bidirectional active power exchange with a three-phase four-wire grid. Therefore, the inverter acts as a multi-function compensator. The functions of the proposed inverter system include active power injection, rectification and active power filtering (APF) (including phase power balancing). This paper presents design and implementation of a three-leg split-capacitor shunt multi-function inverter with division-summation (D-Σ) digital control. The adopted D-Σ digital control can accommodate filter inductance variation, reducing core size significantly, and its control laws can be derived directly to cancel the variation effects of dc-bus voltage, switching period and filter inductance. An average power method is adopted in this paper for determining fundamental currents at the source side. In the design and implementation, the inductances corresponding to various inductor currents were estimated at the startup and stored in the microcontroller for scheduling loop gain cycle by cycle, which can insure system stability. Measured results from a three-phase four-wire inverter have confirmed the analysis and discussion.

Filter-capacitor current compensation for D-Σ digital controlled single-phase bi-directional inverter with LCL filter to reduce grid-current distortion

This paper presents filter-capacitor current compensation for Division-Summation (D-Σ) digital controlled singlephase bi-directional inverter with LCL filter to reduce grid-current distortion. The single-phase bi-directional inverter allows wide filter-inductance variation and it can operate in grid-connection mode and rectification mode with power factor correction. With the D-Σ digital control, the inverter can cover wide inductance variation and can track sinusoidal inductor current reference precisely. However since there typically exist harmonic voltages, the injected grid current will contain harmonic components. With estimated filter-capacitor current compensation, the grid-current distortion can be improved significantly. Experimental results from a 5 kW single-phase bi-directional inverter have verified the feasibility of the proposed compensation approach.

Power compensation based on digital predictive current controlled three-phase bi-directional inverter with wide inductance variation

This paper presents power compensation based on digital predictive current controlled three-phase bi-directional inverter with wide inductance variation. The three-phase bidirectional inverter can fulfill both real power and reactive power compensation for ac grid. With the proposed control, the inverter can track sinusoidal reference currents precisely with unity power factor or power factors -0.5 ~ +0.5, and it is allowed to have wide inductance variation, reducing core size significantly. In the design and implementation, the inductances corresponding to various inductor currents are measured and tabulated into a single-chip microcontroller for tuning loop gain cycle by cycle, ensuring system stability. Moreover, a one-phase shift detection method for anti-islanding operation based on the proposed control is also presented. Measured results from a 10 kVA 3ø bi-directional inverter have confirmed the feasibility of the discussed control approach and detection method.