Robert S. Balog

Control and Circuit Techniques to Mitigate Partial Shading Effects in Photovoltaic Arrays

Partial shading in photovoltaic (PV) arrays renders conventional maximum power point tracking (MPPT) techniques ineffective. The reduced efficiency of shaded PV arrays is a significant obstacle in the rapid growth of the solar power systems. Thus, addressing the output power mismatch and partial shading effects is of paramount value. Extracting the maximum power of partially shaded PV arrays has been widely investigated in the literature. The proposed solutions can be categorized into four main groups. The first group includes modified MPPT techniques that properly detect the global MPP. They include power curve slope, load-line MPPT, dividing rectangles techniques, the power increment technique, instantaneous operating power optimization, Fibonacci search, neural networks, and particle swarm optimization. The second category includes different array configurations for interconnecting PV modules, namely series-parallel, total-cross-tie, and bridge-link configurations. The third category includes different PV system architectures, namely centralized architecture, series-connected microconverters, parallel-connected microconverters, and microinverters. The fourth category includes different converter topologies, namely multilevel converters, voltage injection circuits, generation control circuits, module-integrated converters, and multiple-input converters. This paper surveys the proposed approaches in each category and provides a brief discussion of their characteristics.

Minimum Energy and Capacitance Requirements for Single-Phase Inverters and Rectifiers Using a Ripple Port

Converters with a dc port and a single-phase ac port must store energy to buffer the inherent double-frequency power flow at the ac port. The minimum energy storage required to isolate the power ripple from the dc port is presented, and leads to the minimum capacitance required for converters that use capacitive energy storage. This paper presents a ripple power port to manage energy storage and decouple capacitor ripple from power ripple. A ripple power port allows the designer to make a choice of capacitor voltage independent of other system voltages. A combination of an ac link converter and a ripple power port leads to a dramatic increase in reliability: it is shown that converters with nominal ratings up to 200 W can be designed with expected mean-time-between-failure ratings on the order of 1.4 × 106 h-sufficient for hundred-year operation in long-life applications such as photovoltaic converters and LED lamps. This large increase in life is achieved with minimal extra cost.

Cost-Effective Hundred-Year Life for Single-Phase Inverters and Rectifiers in Solar and LED Lighting Applications Based on Minimum Capacitance Requirements and a Ripple Power Port

Energy storage requirements for converters with a dc port and a single-phase grid-connected port are evaluated, based on the unavoidable double-frequency power requirement. The minimum energy storage requirement is linked to a minimum capacitance requirement for converters that use capacitance energy storage. It is shown how to employ a ripple power port to manage energy storage and decouple capacitor ripple from power ripple. A ripple power port allows a designer to make a choice of capacitor voltage independent of system voltages. This in turn allows film capacitors of modest size to be used for energy storage in these applications. A combination of ac link converter and ripple power port leads to a dramatic increase in reliability: it is shown that converters with nominal ratings up to 200 W can be designed with expected mean-time-between-failure ratings on the order of 1.4 million hours - sufficient for hundred-year operation in long-life applications such as photovoltaic converters and LED lamps. This large increase in life is achieved with minimal extra cost.

Reliability of Candidate Photovoltaic Module-Integrated-Inverter (PV-MII) Topologies—A Usage Model Approach

This paper proposes a new methodology for calculating the mean time between failure (MTBF) of a photovoltaic module-integrated inverter (PV-MII). Based on a stress-factor reliability methodology, the proposed technique applies a usage model for the inverter to determine the statistical distribution of thermal and electrical stresses for the electrical components. The salient feature of the proposed methodology is taking into account the operating environment volatility of the module-integrated electronics to calculate the MTBF of the MII. This leads to more realistic assessment of reliability than if a single worst case or typical operating point was used. Measured data (module temperature and insolation level) are used to experimentally verify the efficacy of the methodology. The proposed methodology is used to examine the reliability of six different candidate inverter topologies for a PV-MII. This study shows the impact of each component on the inverter reliability, in particular, the power decoupling capacitors. The results confirm that the electrolytic capacitor is the most vulnerable component with the lowest MTBF, but more importantly provide a quantified assessment of realistic MTBF under expected operating conditions rather than a single worst case operating point, which may have a low probability of occurrence.

High-frequency link inverter for fuel cells based on multiple-carrier PWM

Fuel-cell inverter applications typically have a relatively low voltage input, and require a battery bus for energy buffering. Circuit topology issues are examined based on these needs. The need for high step-up ratios, current control, low ripple, and battery storage leads to a current-sourced link converter as perhaps the best choice of conversion topology. High-frequency ac link conversion offers a possible way to reduce the number of power stages, in the form of a cycloconverter, known from previous work. It is shown that the control complexity in this converter can be addressed by adapting pulse-width modulation (PWM) techniques. Here, a multicarrier PWM approach is introduced as a convenient way to implement a high-frequency link inverter. The approach is a direct extension of conventional PWM, and supports square-wave cycloconversion methods that have appeared in prior literature. Simulation and experimental results are developed for a low-voltage ac link inverter, leading to a 48-V fuel cell input design.

The Load as an Energy Asset in a Distributed DC SmartGrid Architecture

DC power systems can be made more reliable by considering the load as an important energy asset. Currently the ability to manage the total system is available only through a centralized controller, which limits flexibility, reconfigurability, and reliability. These limitations can be avoided while still providing system level coordination through the use of distributed controls based on local information. All elements of the power system including source, loads, and the network itself have influence, interaction, and coupling to all other elements. Therefore, it is necessary to model and control a microgrid as a system of systems that share some common aspects, such as voltage levels, but can operate independently. Using local information in the form of the bus voltage, these techniques do not rely on a centralized controller, which improves system reliability. However, it is important to design the microgrid in such a manner as to take advantage of the energy not just from the generation sources, but also the energy stored in the individual points-of-load as well.

Multi-Objective Optimization and Design of Photovoltaic-Wind Hybrid System for Community Smart DC Microgrid

Renewable energy sources continues to gain popularity. However, two major limitations exist that prevent widespread adoption: availability of the electricity generated and the cost of the equipment. Distributed generation, (DG) grid-tied photovoltaic-wind hybrid systems with centralized battery back-up, can help mitigate the variability of the renewable energy resource. The downside, however, is the cost of the equipment needed to create such a system. Thus, optimization of generation and storage in light of capital cost and variability mitigation is imperative to the financial feasibility of DC microgrid systems. PV and wind generation are both time dependent and variable but are highly correlated, which make them ideal for a dual-sourced hybrid system. This paper presents an optimization technique base on a Multi-Objective Genetic Algorithm (MOGA) which uses high temporal resolution insolation data taken at 10 seconds data rate instead of more commonly used hourly data rate. The proposed methodology employs a techno-economic approach to determine the system design optimized by considering multiple criteria including size, cost, and availability. The result is the baseline system cost necessary to meet the load requirements and which can also be used to monetize ancillary services that the smart DC microgrid can provide to the utility at the point of common coupling (PCC) such as voltage regulation. The hybrid smart DC microgrid community system optimized using high-temporal resolution data is compared to a system optimized using lower-rate temporal data to examine the effect of the temporal sampling of the renewable energy resource.

Model Predictive Control of PV Sources in a Smart DC Distribution System: Maximum Power Point Tracking and Droop Control

In a dc distribution system, where multiple power sources supply a common bus, current sharing is an important issue. When renewable energy resources are considered, such as photovoltaic (PV), dc/dc converters are needed to decouple the source voltage, which can vary due to operating conditions and maximum power point tracking (MPPT), from the dc bus voltage. Since different sources may have different power delivery capacities that may vary with time, coordination of the interface to the bus is of paramount importance to ensure reliable system operation. Further, since these sources are most likely distributed throughout the system, distributed controls are needed to ensure a robust and fault tolerant control system. This paper presents a model predictive control-based MPPT and model predictive control-based droop current regulator to interface PV in smart dc distribution systems. Back-to-back dc/dc converters control both the input current from the PV module and the droop characteristic of the output current injected into the distribution bus. The predictive controller speeds up both of the control loops, since it predicts and corrects error before the switching signal is applied to the respective converter.

Modern laboratory-based education for power electronics and electric machines

The study of modern energy conversion draws upon a broad range of knowledge and often requires a fair amount of experience. This suggests that laboratory instruction should be an integral component of a power electronics and electric machines curriculum. However, before a single watt can be processed in a realistic way, the student must understand not only the operation of conversion systems but also more advanced concepts such as control theory, speed and position sensing, switching signal generation, gate drive isolation, circuit layout, and other critical issues. Our approach is to use a blue-box module where these details are pre-built for convenience, but not hidden from the students inside a black box. Recent improvements to our blue-box modules are described in this paper and include a dual-MOSFET control box with independently isolated FET devices, a triple silicon controlled rectifier control box, a discretely built, high quality pulse-width modulation inverter, a small discrete brushless dc drive system, and a high-performance computer-controlled brushless dc dynamometer motor drive system. Complete details, sufficient to allow the reader to duplicate these designs, are publicly available.

Multilevel Medium-Frequency Link Inverter for Utility Scale Photovoltaic Integration

A multilevel topology with medium-frequency ac link for medium-voltage grid integration of utility photovoltaic (PV) plants is discussed in this paper. A megawatt-scale PV plant is divided into many zones, each comprising of two series-connected arrays. Each zone employs a medium-frequency transformer with three secondaries, which interface with the three phases of the medium voltage grid. An insulated-gate bipolar transistor full-bridge inverter feeds the MF transformer. The voltages at the transformer secondaries are then converted to three-phase line frequency ac by three full-bridge ac-ac converters. Second line frequency harmonic power does not appear in the dc bus, thereby reducing the dc capacitor size. Cascading several such cells, a high-quality multilevel medium-voltage output is generated. A new control method is proposed for the cascaded multilevel converter during partial shading while minimizing the switch ratings. The proposed topology eliminates the need for line frequency transformer isolation and reduces the dc bus capacitor size, while improving the power factor and energy yield. This paper presents the analysis, design example, and operation of a 10-MW utility PV system with experimental results on a scaled-down laboratory prototype.