A phase-locked loop (PLL) is a nonlinear negative-feedback control system that synchronizes its output in frequency as well as in phase with its input PLLs are now widely used for the synchronization of power-electronics-based converters and also for monitoring and control purposes in different engineering fields In recent years, there have been many attempts to design more advanced PLLs for three-phase applications The aim of this paper is to provide overviews of these attempts, which can be very useful for engineers and academic researchers

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Three-Phase PLLs: A Review of Recent Advances

In this paper, benchmark of Si IGBT, SiC MOSFET, and Gallium nitride (GaN) HEMT power switches at 600-V class is conducted in single-phase T-type inverter. Gate driver requirements, switching performance, inverter efficiency performance, heat sink volume, output filter volume, and dead-time effect for each technology is evaluated. Gate driver study shows that GaN has the lowest gate driver losses above 100 kHz and below 100 kHz, SiC has lowest gate losses. GaN has the best switching performance among three technologies that allows high efficiency at high-frequency applications. GaN-based inverter operated at 160-kHz switching frequency with 97.3% efficiency at 2.5-kW output power. Performance of three device technologies at different temperature, switching frequency, and load conditions shows that heat sink volume of the converter can be reduced by 2.5 times by switching from Si to GaN solution at 60  $^{\circ }$ C case temperature, and for SiC and GaN, heat sink volume can be reduced by 2.36 and 4.92 times, respectively, by increasing heat sink temperature to 100  $^{\circ }$ C. Output filter volume can be reduced by 43% with 24, 26, and 61 W increase in device power loss for GaN-, SiC-, and Si-based converters, respectively. WBG devices allow reduction of harmonic distortion at output current from 3.5% to 1.5% at 100 kHz.

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Single-Phase T-Type Inverter Performance Benchmark Using Si IGBTs, SiC MOSFETs, and GaN HEMTs

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.

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

Partially shaded conditions (PSCs) often occur in large photovoltaic generation systems (PGSs). PSCs cause losses in system output power, hot spot effects, and system safety and reliability problems. When PSC occur, the PGS power–voltage characteristic curve exhibits multiple peak values; that is, the curve comprises a global maximum power point and multiple local maximum power points. Current literature includes various studies of global maximum power point tracking (GMPPT) algorithms and hardware architectures suitable for PSC; because the substantial quantity of PSC literature, this subject must be comprehensively reviewed. To focus on GMPPT techniques used in PSC, traditional maximum power point tracking techniques and circuit architectures that cannot distinguish GMPP and LMPP were not discussed.

A review of maximum power point tracking techniques for use in partially shaded conditions

Unique pitfalls in parallel three-level T-type inverters (3LT
 2
 Is) are potential zero-sequence circulating currents (ZSCCs) which are more complex than parallel two-level inverters and can cause current discrepancy, current waveform distortion, power losses, etc. In this paper, the ZSCC paths in the parallel 3LT
 2
Is are first presented, and an equivalent model of the ZSCCs is developed. It is seen from this model that the ZSCCs consist of conduction, switching, and hybrid components. Based on the aforementioned analysis, an original sharing neutral bus structure is proposed to eliminate the conduction ZSCCs. With regard to the switching ZSCCs composed of high-frequency and low-frequency harmonics, modified LCL filters are proposed to eliminate the former, and zero-sequence control loops are put forward to suppress the latter. Furthermore, the proposed schemes are also proven to be effective to elimination of the hybrid ZSCCs. Experimental results validate the developed models and the proposed ZSCC elimination schemes.

Modeling and Elimination of Zero-Sequence Circulating Currents in Parallel Three-Level T-Type Grid-Connected Inverters

It is important to improve the overall efficiency of a photovoltaic (PV) inverter when it is connected to the grid. Fundamentally, the conversion efficiency from dc to ac power of an inverter is important. However, in the presence of partial shading, maximum power point tracking (MPPT) on PV modules is more important than the conversion efficiency. In this paper, a new control method for a three-level inverter is proposed. With the proposed method, each dc-link voltage of the three-level inverter can be asymmetrically regulated. When PV modules are split into two and each split module is connected to the respective dc-link capacitors of the inverter, the asymmetric control can be helpful because separate MPPTs are possible. The effectiveness of the proposed method was examined through experiments with a T-type three-level inverter, where each dc-link capacitor was supplied by a PV simulator emulating two separate PV modules under different shading conditions.

Asymmetric Control of DC-Link Voltages for Separate MPPTs in Three-Level Inverters

A grid synchronization technique is essential for grid-connected power converters. Phase-locked loop (PLL) has been widely exploited as an implementation technique, but additional efforts are required to deal with severely distorted grid voltages. In this paper, an observer is proposed to enhance the performance of PLL. A state equation is newly formulated to construct the observer, which extracts positive-sequence voltage from the distorted grid voltage. Additionally, guidelines are suggested to set the observer gains by considering its internal transfer functions. The PLL and proposed observer have been tested in simulations and experiments in contrast to dual second-order generalized integrator frequency-locked loop. The results have shown that the proposed method reveals a better phase-tracking performance for grid synchronization.

Phase-Locked Loop Based on an Observer for Grid Synchronization

Grid synchronization technique is essential for grid-connected power converters. Phase locked loop (PLL) has been exploited as an implementation technique. However, additional efforts are required for PLL to deal with severely distorted grid voltages. In this paper, an observer is proposed to enhance the performance of PLL. A state equation is formulated to extract positive-sequence voltage from the distorted grid voltage and is utilized to construct the observer. Guidelines are suggested to set the gains of the observer along with its internal transfer functions. The PLL combined with the proposed observer has been tested in simulations and experiments in contrast to dual second order generalized integrator frequency locked loop (DSOGI-FLL). The results have validated that the proposed method reveals better phase-tracking performance for grid synchronization.

Phase locked loop based on an observer for grid synchronization

This paper presents an active anti-islanding detection method using negative sequence voltage injection to the grid through a three-phase photovoltaic inverters. Because islanding operation mode can cause a variety of problems, the islanding detection of grid-connected photovoltaic inverter is the mandatory feature. The islanding mode is detected by measuring the magnitude of negative sequence impedance calculated by the negative sequence voltage and current at the point of common coupling. Simulation and experimental test are performed to verify the effectiveness of the proposed method which can detect the islanding mode in the specified time. The test has been done in accordance with the condition on IEEE Std 929-2000.

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Anti-islanding Detection of Photovoltaic Inverter Based on Negative Sequence Voltage Injection to Grid

This study aimed to assess the trends in the incidence of thyroid cancer in Incheon Province, South Korea, from 2004 to 2013 and to identify its association with thyroid cancer screening.

Trends in the incidence of thyroid cancer in Incheon Province, South Korea, from 2004 to 2013: A representative sample study from Incheon cancer registry

Woo-Chull Kim

Papers

Asymmetric Control of DC-Link Voltages for Separate MPPTs in Three-Level Inverters

Phase-Locked Loop Based on an Observer for Grid Synchronization

Phase locked loop based on an observer for grid synchronization

Anti-islanding Detection of Photovoltaic Inverter Based on Negative Sequence Voltage Injection to Grid

Trends in the incidence of thyroid cancer in Incheon Province, South Korea, from 2004 to 2013: A representative sample study from Incheon cancer registry