Hao Yi

A Source-Current-Detected Shunt Active Power Filter Control Scheme Based on Vector Resonant Controller

A shunt active power filter (APF) with current detection at the source side is considered as a closed-loop system from the view of the whole power distribution system, which is expected with better harmonics filtering performance compared with the one with load current detection. The conventional source current detection control scheme can be mainly grouped into two types: 1) current-source-based (CS) scheme and 2) power-balance-based (PB) scheme. As introduced in this paper, the CS scheme has good dc-voltage regulation, but it bears inevitable conflict between filtering performance and system stability, whereas the PB scheme has excellent stable filtering performance, but it bears great influence on the dc voltage in the transient state. In this paper, a novel source current detection control scheme is proposed, which reserves the advantages of conventional schemes and, meanwhile, avoids their limitations. The proposed scheme employs a vector resonant controller, with which the shunt APF performs as an equivalent multiband rejection filter that series between nonlinear loads and the grid source, blocking selected harmonics components from the load side flowing into the source side. The proposed scheme is simple in structure, is needless of a harmonics extraction algorithm, and shows good stable and transient performance. Finally, the conclusions obtained in this paper are validated through experiments on the laboratory prototype.

System Operation and Energy Management of a Renewable Energy-Based DC Micro-Grid for High Penetration Depth Application

A renewable energy-based dc micro-grid with hybrid energy storage, consisting of battery and ultracapacitor, is investigated. To achieve high penetration depth of renewable sources into the utility grid, a novel system operation strategy and the corresponding energy management method is proposed. In the operation strategy, the ultracapacitor unit works as the sole voltage source of the micro-grid to support the dc link in both connected and islanding mode. The micro-grid is controlled to deliver/absorb predefined amount of power to/from the utility grid during connected mode and zero during islanding mode. This design will certainly simplify the power dispatching algorithm of the power system and increase the possibility of including large quantities of micro-grids into the utility grid. The energy management method is dedicated to conducting the net power of the micro-grid effectively. The net power is separated into high- and low-frequency components. The high-frequency power is suppressed by the ultracapacitor automatically and the low frequency power is shared by the battery and an adjustment unit. A small-scale dc micro-grid structure with a single dc link is considered for investigation. MATLAB/Simulink simulation results are presented to validate the proposed system operation strategy and the energy management method.

A Three-Phase PLL Algorithm Based on Signal Reforming Under Distorted Grid Conditions

This paper proposes a novel three-phase phase-locked loop (PLL) algorithm, which focuses on the reforming of the primary signals before grid synchronization rather than improving the phase estimation methodologies. The unbalanced signals are reformed to balanced ones without damage to the phase angle, through which the negative sequence of the grid voltages is removed. This eliminates the estimation errors of conventional synchronous reference frame PLL and enhances its response speed with a higher bandwidth. The reforming process is supposed to be carried out at every zero-crossing point of the three-phase voltages and choose one phase as reference to balance the other two. Coefficients for the signal reforming are calculated at one zero-crossing point and updated until the next comes. In implementation, a certain phase is chosen as the reference all along and the reforming process will be suspended when it just crosses the zero line. This PLL algorithm has a fast and precise character to reform the three-phase grid voltages and is flexible for application. Under heavily distorted grid conditions, it can still perform effectively even with multiple zero-crossings. Comprehensive experimental results from a digital signal processor-based laboratory prototype are provided to validate the performance of this PLL algorithm.

Static Synchronous Generator Model: A New Perspective to Investigate Dynamic Characteristics and Stability Issues of Grid-Tied PWM Inverter

With increasing penetration of the renewable energy, the grid-tied PWM inverters need to take corresponding responsibilities for the security and stability of future grid, behaving like conventional rotational synchronous generator (RSG). Therefore, recognizing the inherent relationship and intrinsic differences between inverters and RSGs is essential for such target. By modeling the typical electromechanical transient of grid-tied PWM inverters, this paper first proves that PWM inverters and RSGs are similar in physical mechanism and equivalent in mathematical model, and the concept of static synchronous generator (SSG) is thereby developed. Furthermore, the comprehensive comparison between RSG and SSG is carried out in detail, and their inherent relation is built. Based on these findings, the rationality and feasibility of migrating the concepts, tools, and methods of RSG stability analysis to investigate the dynamic behaviors and stability issues of SSG is therefore confirmed. Taking stability issues as an example, the criteria of small signal and transient stability of a typical grid-tied PWM inverter is put forward to demonstrate the significance of the developed SSG model (including synchronizing coefficient, damping coefficient, inertia constant, and power-angle curve), providing clear physical interpretation on the dynamic characteristics and stability issues. The developed SSG model promotes grid-friendly integration of renewable energy to future grid and stimulates interdisciplinary research between power electronics and power system.

A Novel Real-Time Voltage and Frequency Compensation Strategy for Photovoltaic-Based Microgrid

Weather-dependent photovoltaic (PV) system power variation may become frequent and rapid, which could be regarded as perturbation for the microgrid (MG). This perturbation would lead to voltage and frequency fluctuations and dramatically degrade the MG performances. Conventional secondary control strategy could restore the voltage and frequency after a certain time delay, but it cannot satisfy the speed demands when frequent and rapid perturbations appear. To overcome the limitation, this paper proposes a novel real-time voltage and frequency compensation strategy to suppress the fluctuations. The proposed strategy realizes voltage regulation by an adaptive virtual impedance loop and realizes frequency regulation by a virtual frequency-impedance loop. The novel real-time strategy is simple to be implemented and needless of communication; therefore, it is much more suitable for the PV-based MG. Finally, a PV-based MG is established as laboratory prototype, and experimental results validate the effectiveness of the proposed method.

A Digital Hysteresis Current Controller for Three-Level Neural-Point-Clamped Inverter With Mixed-Levels and Prediction-Based Sampling

For a grid-tied three-level neural-point-clamped (NPC) inverter, fully digital hysteresis controller with simple variable hysteresis band is insufficient to stabilize switching frequency (fs). The challenge mainly comes from the restriction of analogue-to-digital converter (ADC) sampling frequency, which leads to inaccurate current-boundary-touching capture. As a result, fs jitter appears. This phenomena becomes much more serious around grid voltage zero-crossing points due to zero-approaching hysteresis band, where obvious fs variation happens. To stabilize fs in a fully digital manner, two techniques are proposed in this paper: the mixed-level scheme and the prediction-based sampling method. The former one is used to cope with the obvious fs variation around grid voltage zero-crossing points. It switches operating status from three-level state into two-level state around grid voltage zero-crossing points to enlarge the hysteresis band for digital implementation. The latter one is used to resolve fs jitter. It makes the sampling and switching to happen at the moment of current boundary-touching to achieve the most effective boundary-comparing with the lowest ADC sampling frequency. With these two techniques, fs of the grid-tied three-level NPC inverter could be well stabilized with a fully digital hysteresis controller, and therefore, high-quality grid-side could be achieved.

Comparison Analysis of Resonant Controllers for Current Regulation of Selective Active Power Filter with Mixed Current Reference

Instead of extracting every selected harmonic component, the current reference of selective active power filter (APF) can be also obtained by filtering out the fundamental component from distorted load current for computation efficiency. This type of mixed current reference contains kinds of harmonic components and easily involves noises. In this condition, selective harmonic compensation must be realized by the current controller. With regard that selectivity is the most significant feature of controller, this paper presents specific comparison analysis between two types of resonant controllers: proportional-resonant (PR) controller and vector-resonant (VR) controller. The comparison analysis covers the relations, performances, and stability of both controllers. Analysis results conclude that the poorer selectivity of the PR controller could be relatively improved, but limitations from system stability make the improvement hardly realized. By contrast, the VR controller exhibits excellent selectivity and is more suitable for selective APF with mixed current reference. Experimental results from laboratory prototype validate the reasonability of analysis. And the features of each resonant controller are concluded.

An enhanced load power sharing strategy for low-voltage microgrids based on inverse-droop control method

It is important for an autonomous microgrid to share the load demand properly among multiple distributed generation (DG) units. Normally, the traditional ω-P and E-Q droop control method is used for its “plug and play” feature. However, when employed in a low-voltage microgrid, the conventional droop control is subject to the power coupling and steady-state reactive power sharing errors. Furthermore, the complex microgrid configurations often make the reactive power sharing more challenging. In this paper, an enhanced power sharing strategy is proposed based on inverse-droop control, which can behave well in low-voltage microgrids but has real power sharing errors. So a synchronous regulation process for real power sharing is added to inverse-droop control. After the regulation starts, an integration term is added to keep the well shared reactive power, and the real power errors are eliminated through the frequency regulation, just like the work of traditional droop control. Finally, the Matlab simulation results validate the feasibility of the proposed strategy.

Analysis and Optimization of Flexible MCPT Strategy in Submodule PV Application

Distributed maximum power point tracking (DMPPT) can effectively increase the power generation ability of photovoltaic (PV) systems in mismatch conditions. To further recover mismatch power loss inside PV panels, submodule realization is developed as an improved penetration of DMPPT concept. This paper proposes an optimized submodule level DMPPT system with a flexible time-sharing maximum current point tracking algorithm which detects the output current of series-connected dc-dc converters. Comparing with current PV optimizer and other submodule DMPPT solutions, the proposal can always guarantee that each PV submodule outputs individual true maximum power regardless of irradiance cases while eliminating the need for several MPPT units and current sensors in the system. Enormous potential of single control IC can be fully developed with fewer components (voltage sensors, current sensors, and relevant AD/DA units) for further integration in PV application.

Analysis of Existence-Judging Criteria for Optimal Power Regions in DMPPT PV Systems

This paper quantitatively evaluates different distributed maximum power point tracking (DMPPT) photovoltaic (PV) systems in mismatch cases. To help designers obtain the full benefits of the DMPPT concept, a general analysis of the optimal power region (OPR) in different DMPPT system configurations is undertaken. Taken into account in the evaluation of OPRs are functional constraints derived from DMPPT converters, mismatch conditions, and system architectures and parameters of related PV units. To simplify the calculation process and enable rapid discovery of OPRs, judging criteria are proposed for 4 DMPPT structures. If the judging criteria are met, DMPPT PV systems can extract ideal maximum power regardless of mismatch issues. Finally, experimental results are provided to show the effectiveness of the proposed static models, formulas, and judging criteria.