Aleksandar Pregelj

Development of a Methodology for Improving Photovoltaic Inverter Reliability

In evaluating the energy-generation potential of a photovoltaic (PV) energy system, the system is usually assumed to work without interruptions over its entire life. PV energy systems are fairly reliable, but as any complex system, they may fail. In PV systems, the inverter is responsible for the majority of failures, and most inverter failures are blamed on the aluminum electrolytic capacitors typically used in the dc bus. This paper investigates the effects of common failure modes on the reliability of PV inverters and suggests a model framework for decomposing the inverter into subsystems for more detailed study. The challenges of statistical analysis based on small data sets are discussed, and simulations are performed to illustrate the proposed model using a simple decomposition into subsystems of the inverter used in the 342-kW PV system at the Georgia Tech Aquatic Center.

Recloser allocation for improved reliability of DG-enhanced distribution networks

The radial distribution feeder protection strategy is first presented in this paper without consideration for distributed generation (DG). Then, the addition of DG across the feeder (constrained in terms of power and/or energy capacity) is introduced in the model. If islanded operation of these DG sources is allowed on a feeder subjected to a disturbance, DG may reduce the number of interruptions and/or durations for customers residing within their protection zones, thus increasing the reliability of service. To that end, a procedure for finding optimal positions for DG and protection devices is presented for a feeder equipped with capacity-constrained distributed generators, using a custom-tailored genetic algorithm, and the improvement in reliability is demonstrated on a test feeder. Tuning of the genetic algorithm parameters and an adaptive algorithm that eliminates the need for parameter tuning are the subject of a separate paper by the same authors

Impact of renewable distributed generation on power systems

The traditional approach in electric power generation is to have centralized plants distributing electricity through an extensive transmission & distribution network. Distributed generation (DG) provides electric power at a site closer to the customer, eliminating the unnecessary transmission and distribution costs. In addition, it can reduce fossil fuel emissions, defer capital cost, reduce maintenance investments and improve the distribution feeder voltage conditions. In the case of small residential photovoltaic (PV) and wind systems, the actual generator locations and DG penetration level are usually not apriori known. The following study attempts to calculate the boundaries of the impact of randomly placed distributed generators on a distribution feeder. Monte Carlo simulations are performed, and boundaries for overall improvements are determined. The study shows that the knowledge of total penetration of small PV systems is sufficient to estimate the effects of DG on the feeder.

On optimization for security and reliability of power systems with distributed generation

Electricity market restructuring and supra-national agreements on the reduction of global greenhouse gas emissions have paved the way for an increase in the use of distributed generation - the connection of generation to the lower voltage power system. This paper formulates and discusses a methodology for the optimal siting and sizing of distributed generation a security constrained system can accept. Optimal siting is determined by sensitivity analysis of the power flow equations. The sizing method for a set of loading conditions, generation penetration level and power factor is formulated as a security constrained optimization problem. The information on optimal generation sites is used further to optimize system reliability assessed via reliability indices calculation. A genetic algorithm is designed to solve for optimal recloser positions when distributed generators are deployed in a securely optimal manner.

Impact of inverter configuration on PV system reliability and energy production

The loss of potential revenues due to PV system failures should be taken into consideration when the systems life cycle cost predictions are calculated. We demonstrate a procedure for quantifying the effects of inverter failures (as most dominant) on total lifetime PV system energy production, and investigate the suitability of several inverter configurations based on criteria of total lifetime energy output and life cycle costs. The overall PV system performance penalty due to inverter failures depends on several factors, such as the reliability characteristics of the inverter, inverter configuration and repair time. Using the Monte Carlo analysis, a performance-adjusting coefficient that accounts for these factors is proposed, and a straightforward analysis for determining the optimal inverter configuration is described.

Quantitative techniques for analysis of large data sets in renewable distributed generation

Distributed generation (DG) reduces losses and eliminates some of the transmission and distribution costs. It may also reduce fossil fuel emissions, defer capital costs, and improve the distribution feeder voltage conditions. The calculation of the effects of small residential photovoltaic and wind DG systems on various feeder operating variables is complicated by both the probabilistic nature of their output and the variety of their possible spatial allocations. A method based on a combination of clustering techniques and a convex hull algorithm is proposed that may reduce the computational burden by an order of magnitude, while still allowing accurate estimation of DG-enhanced feeder operation.

Green power: status and perspectives

The main objective of the so-called green power marketing is to provide selective customers of electric energy with choices to purchase electric energy from sustainable, environmentally friendly sources. This paper presents a rationale for marketing of green power provides a brief overview of some of the existing efforts in the United States and abroad, and attempts to identify the conditions and future trends for survival of renewable energy in the energy marketplace. Our goal is primarily to provide a representative sample of the current technology status, rather than a comprehensive coverage of worldwide initiatives in this area.

On optimization of reliability of distributed generation-enhanced feeders

Placement of protection devices in a conventional feeder (without distributed generation) is often performed so as to minimize traditional reliability indices (SAIDI, SAIFI, MAIFIe...), assuming the sole source(s) of energy at substation(s). Distributed generation (DG) and storage units located on the feeder may be constrained in terms of power and/or energy capacity, and may include renewable DG units whose output is dependent on the meteorological conditions. Those sources may reduce the number of faults and/or fault durations for customers residing within their protection zones, thus increasing the reliability of service. We propose a procedure for finding optimal recloser positions on the feeder equipped with power constrained distributed generators, using a specially tailored genetic algorithm.

Four-year performance assessment of the 342 kW PV system at Georgia Tech

The Georgia Tech Aquatic Center photovoltaic (PV) array, one of the worlds largest roof-mounted PV arrays, has been in operation since June 1996. This paper describes the system performance during the first four years of operation. The performance is compared to the simulation results, and the modeling and forecasting accuracy improved as a result. The system failure mechanisms are identified and reliability indices, such as mean time between failures, are calculated. The overall system performance is good, and experience in operation and maintenance useful in both modeling and forecasting of the performance of PV systems of similar size.

Recloser Allocation for Improved Reliability of DG-Enhanced Distribution Networks

The radial distribution feeder protection strategy is first presented in this paper without consideration for distributed generation (DG). Then, the addition of DG across the feeder (constrained in terms of power and/or energy capacity) is introduced in the model. If islanded operation of these DG sources is allowed on a feeder subjected to a disturbance, DG may reduce the number of interruptions and/or durations for customers residing within their protection zones, thus increasing the reliability of service. To that end, a procedure for finding optimal positions for DG and protection devices is presented for a feeder equipped with capacity-constrained distributed generators, using a custom-tailored genetic algorithm, and the improvement in reliability is demonstrated on a test feeder. Tuning of the genetic algorithm parameters and an adaptive algorithm that eliminates the need for parameter tuning are the subject of a separate paper by the same authors