Under fast varying solar irradiation and load resistance, a fast-converging maximum power point tracking system is required to ensure the photovoltaic system response rapidly with minimum power losses. Traditionally, maximum power point locus was used to provide such a fast response. However, the algorithm requires extra control loop or intermittent disconnection of the PV module. Hence, this paper proposes a simpler fast-converging maximum power point tracking technique, which excludes the extra control loop and intermittent disconnection. In the proposed algorithm, the relationship between the load line and the I-V curve is used with trigonometry rule to obtain the fast response. Results of the simulation and experiment using single-ended primary-inductor converter showed that the response of the proposed algorithm is four times faster than the conventional incremental conductance algorithm during the load and solar irradiation variation. Consequently, the proposed algorithm has higher efficiency.

A Fast-Converging MPPT Technique for Photovoltaic System Under Fast-Varying Solar Irradiation and Load Resistance

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

The perturb and observe (P&O) algorithm is one of the most commonly utilized maximum power point tracking (MPPT) control schemes for photovoltaic (PV) generators. However, the operation of this algorithm at high perturbation frequencies, when the system response to MPPT perturbations is never allowed to settle, has not been given adequate attention in the literature. This paper characterizes system behavior in this mode of operation for standalone PV systems feeding resistive loads and motor-pump loads. Simulation and experimental results show that the P&O algorithm operating at a high perturbation frequency may offer higher energy utilization efficiency and better system performance, despite the resulting nonperiodic waveforms of the system.

Operating Characteristics of the P&O Algorithm at High Perturbation Frequencies for Standalone PV Systems

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

This paper presents an active Anti-Islanding Detection (AID) method using Negative Sequence Voltage (NSV) injection to electrical grid through a three-phase photovoltaic (PV) inverters. Because islanding operation mode can cause a variety of problems, the islanding detection of grid-connected PV inverter is the mandatory feature. The islanding mode is detected by measuring the magnitude of Negative Sequence Impedance (NSI) calculated by the negative sequence voltage and current at the point of common coupling (PCC). Simulation and experiment 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.

Anti-islanding detection method using Negative Sequence Voltage

An adaptive maximum power point tracking (MPPT) scheme employing a variable scaling factor is presented. A MPPT control loop was constructed analytically and the magnitude variation in the MPPT loop gain according to the operating point of the PV array was identified due to the nonlinear characteristics of the PV array output. To make the crossover frequency of the MPPT loop gain consistent, the variable scaling factor was determined using an approximate curve-fitted polynomial equation about linear expression of the error. Therefore, a desirable dynamic response and the stability of the MPPT scheme were maintained across the entire MPPT voltage range. The simulation and experimental results obtained from a 3 KW rated prototype demonstrated the effectiveness of the proposed MPPT scheme.

http://www.ndsl.kr/soc_img/society/kipe/JRJJC3/2012/v17n5/JRJJC3_2012_v17n5_423.pdf

An Adaptive Maximum Power Point Tracking Scheme Based on a Variable Scaling Factor for Photovoltaic Systems

A three-level inverter having two direct current (DC)-links for converting DC voltages generated in a plurality of photovoltaic (PV) modules into alternating current (AC) voltages, wherein one end of a first DC-link of the two DC-links is connected to a first potential point at which a first potential is generated in the plurality of PV modules, and one end of a second DC-link of the two DC-links is connected to a second potential point at which a second potential that is lower than the first potential is generated in the plurality of PV modules, and both the other end of the first DC-link and the other end of the second DC-link are connected to a third potential point at which a third potential having a medium level between the first potential and the second potential is generated in the plurality of PV modules.

Three-level photovoltaic inverter configured for asymmetric control of DC-link voltages for separate MPPT driving

A three-level inverter having two direct current (DC)-links, and an apparatus and method for controlling the three-level inverter are disclosed. In the three-level inverter having two direct current (DC)-links for converting DC voltages generated in a plurality of photovoltaic (PV) modules into alternating current (AC) voltages, one end of a first DC-link of the two DC-links is connected to a first potential point at which a first potential is generated in the plurality of PV modules, and one end of a second DC-link of the two DC-links is connected to a second potential point at which a second potential that is lower than the first potential is generated in the plurality of PV modules, and both the other end of the first DC-link and the other end of the second DC-link are connected to a third potential point at which a third potential having a medium level between the first potential and the second potential is generated in the plurality of PV module.

Chun-Ho Lim

Papers

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

Anti-islanding detection method using Negative Sequence Voltage

An Adaptive Maximum Power Point Tracking Scheme Based on a Variable Scaling Factor for Photovoltaic Systems

Three-level photovoltaic inverter configured for asymmetric control of DC-link voltages for separate MPPT driving

Three-level inverter having two direct current (DC)-links, and apparatus and method for controlling three-level inverter