Showing papers on "Grid-connected photovoltaic power system published in 1989"

Combined low-cost, high-efficient inverter, peak power tracker and regulator for PV applications

Abstract: A novel compound power converter that serves as a DC-to-AC inverter, maximum power point tracker (MPPT), and battery charger for stand-alone photovoltaic (PV) power systems is introduced. A theoretical analysis of the proposed converter is performed, and the results are compared with experimental results obtained from a 1.5 kW prototype. The overall cost of PV systems can thus be reduced by using load management control and efficiency-optimization techniques. Power flow through the converter is controlled by means of a combination of duty cycle and output frequency control. With load management, large domestic loads, such as single phase induction motors for water pumping, hold-over refrigerators, and freezers, can be driven by day at a much higher energy efficiency. This is due to the high efficiency of the inverter with high insolation, and because the inverter uses the energy directly from the solar array. The battery loss component is thus reduced. >

Modeling and Dynamic Performance of a Line-Commutated Photovoltaic Inverter System

Abstract: Interest in utility-interactive photovoltaic (PV) inverter systems has increased over the past decade and numerous central-station PV systems have been installed. It is anticipated that as PV system costs decrease, residential systems will be installed in increased numbers. Although a substantial amount of literature is available concerning the design, protection, safety, economics, and operating experience of residential and central-station PV systems, little information is available regarding their dynamic electrical characteristics and the computer modeling of these systems. Moreover, most of the available literature concerning modeling and/or dynamic performance focuses either upon the long-term dynamic behavior as it affects power system scheduling or upon the steady-state harmonic characteristics. In recent work, highly detailed computer models of a representative set of PV systems have been developed and several of these models have been verified by comparison with system measurements [1, 2]. However, the models described in [1, 2] are more complex than necessary for large-scale power system studies in which the fast switching transients associated with the dc-to-ac inverter are of little concern and only the slower cycle-to-cycle behavior of the PV system is of interest. In fact, it is not possible to incorporate these detailed models into conventional transient stability programs due, in part, to the very small time-step requirements associated with these models. In this paper, a three-phase self-commutated utility-interactive photovoltaic inverter system is described including its associated control system. A schematic diagram of the selected PV inverter system is depicted in Fig. 1.

Simulation studies of islanded behavior of grid-connected photovoltaic systems

Abstract: Simplified computer models are presented for predicting the run-on time of self-commutated inverters operating in a utility interactive mode. The inverter receives DC power from a photovoltaic (PV) array and delivers AC power to a local load or the utility lines. The models represent the dynamics of a phase-locked loop control circuit that is designed to destabilize the inverter operation and shut down the power-conditioning subsystem when a phase discrepancy between the line and some reference signal is detected. The sustained isolated operation (or islanding) of the PV system poses a possible safety concern to utility personnel and potential damage to utility-connected equipment. Two computer models, the TESLACO model and the APCC model, were implemented on an IBM PC using Pascal. They provide results compatible with experimental evidence and more elaborate computer modeling techniques. >

Enhancing the Utilization of Photovoltaic Power Generation by Superconductive Magnetic Energy Storage

Abstract: The authors demonstrate that a superconductive magnetic energy storage (SMES) system can enhance large-scale utilization of photovoltaic (PV) generation. Results show that power output from a SMES system can be used to smooth out PV power fluctuations so that the combined PV/SMES output is dispatchable and free from fluctuations. Power generated from PV arrays is shown to be fully utilized under different weather conditions, and PV penetration is increased to significant levels without adversely affecting the power system. Coupled with PV generation, a SMES system is even more effective in performing diurnal load leveling. A coordinated PV/SMES operation scheme is proposed, and its demonstration under different weather conditions is discussed. >

A Study of Islanding in Utility-Connected Residential Photovoltaic Systems Part I-Models and Analytical Methods

Abstract: Residential, utility-interactive, photovoltaic systems consist of a photovoltaic (PV) array, and a Power Conditioning System (PCS), or inverter, which converts the dc array energy to an ac voltage or current at the proper frequency for connection to an electric utility distribution feeder. One of the concerns wth such an interconnection is the potential for "islanding." This is a phenomenon in which one or more PCSs continue to operate, and keep the system energized even after the utility supply is disconnected. The loss of utility may be related to a scheduled outage, or the operation of circuit breakers, sectionalizers, or fuses in response to a system disturbance. Since the electric utility company cannot exert direct control on residential photovoltaic systems, islanding is generally considered to be undesirable. Concern is primarily related to the safety of personnel attempting to restore service in a system energized by islanded PCSs. Additionally, there is cause for concern as to the effect of transients and off-nominal frequency and/or voltage on connected load and on the PCS itself. Therefore, an understanding of the islanding problem is extremely important to the commercialization of utility-interactive PV systems. The objective of the work described in this paper and its companion [1] is to develop simple analytical methods to determine if islanding is possible in a system consisting of one or more PCSs connected to a single-phase distribution lateral. Attention is focused on three commercially available PCSs, the 'Gemini' manufactured by Windworks, Inc., the 'Teslaco' manufactured by Teslaco, Inc.

Current and voltage harmonic measurements and modeling at the Gardner photovoltaic project

Abstract: Measurements on harmonics taken at the Gardner photovoltaic (PV) project during the summer of 1987 are reported. The project includes 28 PV homes, each with a roof-mounted 2 kW PV system, connected to one phase of a 13.8 kV three-phase feeder. The measurements discussed pertain to voltage and current distortion and power flow. Theoretical calculations were carried out in order to study the effects of PV inverters other than the type installed for this study. Computer modeling of the feeder in Gardner indicates, and measurements confirm, that small increases (0.2%) in voltage total harmonic distortion will occur on this feeder with the indicated penetration of PV systems using high-quality, forced-commutation inverters. >

Central-station photovoltaic plant with energy storage for utility peak load leveling

Abstract: The impact of photovoltaic (PV) power generation with energy storage on the electric utility's load shape for load leveling purposes is explored. Results show that utilities employing battery storage for peak load shaving might benefit from use of photovoltaic power, the extent of its usefulness being dependent on the specific load shapes as well as the photovoltaic array orientations. Typical utility load shapes both in the eastern and in the western parts of the US are examined for this purpose. Although photovoltaic power generation seems to have a bigger impact on the load of the western utility, utilities in both areas will experience considerable savings on the size of the battery system required to shave the peak loads and also in the off-peak base capacity required to charge the battery. >

A flexible control strategy to interface solar system with privileged load and utility line

Abstract: An intermediate photovoltaic power system (160 kVA, 380 V three-phase) that could be used as a stand-alone energy power supply and as an AC back up system has been designed. For this purpose it was necessary to use self-commuted inverters; a suitable power control logic was also required in order to compensate for reactive power and optimize power flow from the photovoltaic array to the AC power line. Good agreement between theoretical studies and simulation results confirms the validity of the proposed power control logic. >

Inverters for utility interactive photovoltaic power plants

Abstract: An introduction to the more important inverter circuits for photovoltaic (PV) plants is presented. Their relative merits and drawbacks are examined, with particular emphasis on power quality, reliability, and efficiency. It is concluded that transistor circuits will probably conquer the small power field, whereas high-power converters will use thyristors and GTOs. >

Battery power storage system

Abstract: PURPOSE: To improve power supply, safety and system efficiency during power interruption or faults, by providing a power interruption countermeasure for a main power system and a reverse flow countermeasure upon occurrence of a fault to an inverter type power storage system which stores midnight power and discharges power at a peak demand. CONSTITUTION: A system shown by a solid line 3 is a main system for feeding power directly from a substation 2 to a load 7 and the system shown by a dot and dash line 8 is a power storage system. Midnight power is charged through inverter 13, 14 into a battery 15. When base power of the main system is insufficient in day time, battery power is fed through the inverters 13, 14 to the loads 7, 20. Upon occurrence of fault in the power supply system, circuit breakers 18, 19 for an uninterruptible power system 17 are turned ON, OFF respectively and power stored in the battery 15 is fed to the uninterruptible load 20. Reverse power flow from the battery 15 to the main system 1 is prevented through operation of a reverse power flow detector 5 which causes interruption of a main system inlet circuit breaker 4 and instantaneous interruption of the main system. COPYRIGHT: (C)1991,JPO&Japio

Space Station Freedom photovoltaic power module design status

Abstract: Electric power for the Space Station Freedom will be provided by four photovoltaic (PV) power modules using silicon solar cells during phase I operation. Each PV power module requires two solar arrays with 32800 solar cells generating 18.75 kW of DC power for a total of 75 kW. The design incorporates an optimized thermal control system, pointing and tracking provision with the application of gimbals, and the use of orbital replacement units to achieve modularization. The design status of the PV power module, as derived from major tradeoff studies, is discussed at hardware levels ranging from component to system. Details of the design are presented where appropriate. >

Off-design modeling and performance of the Space Station solar dynamic power system

Abstract: A solar dynamic closed Brayton cycle system being developed for the Space Station Freedom is discussed. The aim of the work described here was to select a solar dynamic power module (SDPM) design and establish its behavior over the required range of operating conditions. The design point sizing and major considerations involved in the development of a mathematical computer model that predicts the behavior of the system based on different orbits are presented. Some of the orbital results for use in designing components are given. The overall results indicate that the SDPM design as operated satisfies the system requirements with achievable components and for moderate operating environments. >

Recommendations for improvements in the design and operation of future solar central receiver power plants based on experience gained from the Solar One Pilot Plant

Abstract: The Solar One Pilot Plant successfully demonstrated the feasibility of solar central receiver power plants. During its operating years many data were collected regarding the efficiency and availability of the various plant systems. The author summarizes these statistics and compares them with goals developed by the Department of Energy. On the basis of this comparison, design and operation improvements are recommended. >

Refitting a major microwave radio system with photovoltaic power supplies

Abstract: The authors report on the planning and implementation of a solar refit program on part of the Panaftel microwave system in West Africa, linking Senegal, Mali, Burkina Faso, Niger, and Benin. It is concluded that in spite of the high initial capital cost, conversion from fuel-dependent power systems to photovoltaic (PV) systems on major microwave routes can be economically sound. The main saving is in fuel cost. Although routine and emergency maintenance will in all probability be reduced, overall maintenance expenditures may not be very different between fuel-dependent systems and PV systems. This is due to the high cost of periodic replacement of aging batteries that is necessary. The effect of conversion on telecommunication system availability is positive, although it was not possible to quantify this factor due to other improvements being introduced during the same time frame. A simplification of maintenance power requirements is foreseeable with the introduction of PV systems. >

A rule based intelligent control for photovoltaic systems

Abstract: A rule-based intelligent control system is proposed for a photovoltaic (PV) system which consists of a PV generator, batteries, and variable load. The control system uses prediction to program the operation of the PV system. Specifically, the PV system is simulated on the basis of qualitative process theory. The intelligent system then allows for maximization of the life duration of the batteries by predicting and avoiding undesirable conditions as well as maximum possible coverage of the electrical demand based on the predicted available energy. An advantage of the control program is that it can enlarge its knowledge base (with the use of certain additional rules) by incorporating real data. Changing the contents of the knowledge base makes it possible to modify the performance of the controller so that it can be used in different systems with different needs. >