Grid-connected photovoltaic power systems: Technical and potential problems—A review

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Abstract

Traditional electric power systems are designed in large part to utilize large baseload power plants, with limited ability to rapidly ramp output or reduce output below a certain level. The increase in demand variability created by intermittent sources such as photovoltaic (PV) presents new challenges to increase system flexibility. This paper aims to investigate and emphasize the importance of the grid-connected PV system regarding the intermittent nature of renewable generation, and the characterization of PV generation with regard to grid code compliance. The investigation was conducted to critically review the literature on expected potential problems associated with high penetration levels and islanding prevention methods of grid tied PV. According to the survey, PV grid connection inverters have fairly good performance. They have high conversion efficiency and power factor exceeding 90% for wide operating range, while maintaining current harmonics THD less than 5%. Numerous large-scale projects are currently being commissioned, with more planned for the near future. Prices of both PV and balance of system components (BOS) are decreasing which will lead to further increase in use. The technical requirements from the utility power system side need to be satisfied to ensure the safety of the PV installer and the reliability of the utility grid. Identifying the technical requirements for grid interconnection and solving the interconnect problems such as islanding detection, harmonic distortion requirements and electromagnetic interference are therefore very important issues for widespread application of PV systems. The control circuit also provides sufficient control and protection functions like maximum power tracking, inverter current control and power factor control. Reliability, life span and maintenance needs should be certified through the long-term operation of PV system. Further reduction of cost, size and weight is required for more utilization of PV systems. Using PV inverters with a variable power factor at high penetration levels may increase the number of balanced conditions and subsequently increase the probability of islanding. It is strongly recommended that PV inverters should be operated at unity power factor.

Introduction

Grid interconnection of PV power generation system has the advantage of more effective utilization of generated power. However, the technical requirements from both the utility power system grid side and the PV system side need to be satisfied to ensure the safety of the PV installer and the reliability of the utility grid. Clarifying the technical requirements for grid interconnection and solving the problems such as islanding detection, harmonic distortion requirements and electromagnetic interference are therefore very important issues for widespread application of PV systems [1]. Grid interconnection of PV systems is accomplished through the inverter, which convert dc power generated from PV modules to ac power used for ordinary power supply to electric equipments. Inverter system is therefore very important for grid-connected PV systems.

Grid connection and extension costs are significant factors for integrating renewable energy sources-electricity (RES-E) generation technologies into an existing electricity network. Prices of both PV and BOS are decreasing following a trend of increased production and improved technology. This explains the high amount of subsidies for R&D and application of PVs in industrialized countries. The solar PV electric power generation will play an important role in the future energy supply in China.

According to the present plan, total PV power installations will reach 350 MW by 2010, 1.8 GW by 2020 and 600 GW by 2050. According to forecasts made by the Chinese Electric Power Research Institute, renewable energy installations will account for 30% of the total electric power installations in China by 2050, of which PV installations will account for 5% [2].

In fact, growing of PV for electricity generation is one of the highest in the field of the renewable energies and this tendency is expected to continue in the next years [3]. As an obvious consequence, an increasing number of new PV components and devices, mainly arrays and inverters, are coming on to the PV market [4]. The energy production of a grid-connected PV system depends on various factors. Among these we distinguish the rated characteristics of the components of the PV system, the installation configuration, the geographical siting of the PV system, its surrounding objects, and defects that occur during its operation. The need for PV arrays and inverters to be characterized has then become a more and more important aspect [5], [6], [7], [8], [9]. Due to the variable nature of the operating conditions in PV systems, the complete characterization of these elements is quite a difficult issue.

The performance of grid-connected PV systems can be evaluated by investigating the performance ratio (PR) [10], which is defined by the ratio of the system efficiency and the nominal efficiency of PV modules under STC [11]. The average values of PR were found to be 66% for one hundred rooftop mounted PV in Germany [12], [13], [14], 55–70% for eight grid-connected PV systems in Europe [15], while it was 63–76% in the Netherlands [16]. These values apply to systems using solar cells made of poly- and mono-crystalline silicon.

From the performance analysis of 260 PV plants in the IEA-PVPS Task 2 database the annual performance ratios for the different types of systems [17], could be 0.6–0.8, 0.1–0.6 and 0.3–0.6 for grid-connected PV systems, stand-alone systems without back-up and stand-alone systems with back-up, respectively. The well maintained PV systems showed an average PR value of typically 0.72 at an availability of 98%. Despite good results, which have been obtained in many of the grid-connected systems, the investigation of the operational behavior of the reported PV systems has identified further potential for optimization.

It is often assumed, in the analysis of grid-connected generators, that the grid supply exhibits a perfect voltage waveform and that the embedded generators themselves are unaffected by perturbations of the grid, i.e. that any disturbance produced is due solely to the embedded sources. In reality, however, the operation of these power electronic generators, and hence the current waveform they source into the network, can be significantly affected by minor distortion of the voltage waveform at the point of connection [18].

With the proliferation of production and improved technologies, the system requires to be standardized, and thus ensuring, issues of safety and quality in manufacture, application, and use. Standards will serve to build consumer confidence, reduce costs and further expand PV development [19].

PV simulation tools are useful to (i) perform detailed analysis of system performance under real field operating conditions, (ii) investigate the impact of different load profiles, (iii) verify system sizing and determine the optimal size of PV components and (iv) assess the viability of a PV system in terms of energy production and life cycle cost of the system [20]. Various simulation tools are currently available to perform PV simulation and can be found in [21], [22], [23], [24], [25], [26], [27], [28], [29].

Empirical relationships have also been developed using real field test data for different types of PV cells to characterize different PV parameters to predict PV performance [30]. Different mathematical models have been developed for individual PV components to perform simulation of the overall PV system [31], [32], [33]. A scenario base PV software tool has been developed to determine the future progress of grid-connected PV systems [34]. Various long-term PV performance models have been developed to simplify the process of hour by hour simulation [35], [36], [37], [38], [39]. The developed models are useful to design optimal configurations of PV systems.

At present, the main PV-powered products include solar street, traffic signal, garden and lawn lamps, calculators and solar toys etc. China has become the largest producer of PV-powered products in the world. The annual usage of solar cells for these products has reached 20 MWp and there is a great deal of exportation [2].

With so many additional functions being allocated to the inverter, the inverter becomes ever more critical to the system function, and the reliability of current technology inverters becomes a significant issue of concern. This investigation aims to emphasize the importance of the grid-connected PV system regarding the intermittent nature of renewable generation, and the characterization of PV generation with regard to grid code compliance. Also, will focus on the technical requirements for grid interconnection and solving the interconnect problems such as islanding detection, harmonic distortion requirements and electromagnetic interference.

Section snippets

Glossary of terms and acronyms

The field of power electronics abounds with unfamiliar and ambiguous terminology. The glossary in Table 1 provides definitions in general use in the PV industry as related to inverters and should help establish a common language for the different types of inverters and the power components used in them. Some functions such as the inverter control methods or ties to standards and codes are also defined here [40], [41].

The current commercially available inverter hardware used for uninterruptible

Global PV module and its electrical performance

The production of solar cells has grown at an average annual rate of 37% in the past decade, i.e. from 77.6 MWp in 1995 to 1817.7 MWp in 2005, and at an average annual rate of 45% in the past 5 years (from 287.7 MWp in 2000 to 1817.7 MWp in 2005) [2]. Fig. 1 shows the production capacity for some countries and regions in the year of 2005.

One feature of the global PV industry is that PV-generated electricity is replacing other forms of electricity at an increasingly high rate. This is most evident

Grid-connected PV systems

Grid-connected PV systems include building integrated PV (BIPV) systems and terrestrial PV systems (including PV power plants in saline-alkali land, tideland and desert). At the scale of the entire interconnected electric power grid, generated electric power must be consumed within milliseconds of being generated. Excess power can be accumulated with energy storage systems such as pumped hydro, but conventional energy storage systems respond much more slowly than the load changes so throttling

Potential problems associated with high penetration levels of grid-tied PV

An extensive literature search was conducted to collect the available information on expected problems associated with high penetration levels of grid tied PV. The penetration level is defined as the ratio of nameplate PV power rating (Wp) to the maximum load seen on the distribution feeder (W). The results of that literature survey are presented below.

Ref. [69] examined cloud transient effects if the PV were deployed as a central-station plant, and it was found that the maximum tolerable

Grid-connected inverters—control types and harmonic performance

Inverter technology is the key technology to have reliable and safety grid interconnection operation of PV system. It is also required to generate high quality power to ac utility system with reasonable cost. To meet with these requirements, up to date technologies of power electronics are applied for PV inverters. By means of high frequency switching of semiconductor devices with pulse width modulation (PWM) technologies, high efficiency conversion with high power factor and low harmonic

Islanding detection methods

Islanding detection methods may be divided into four categories: passive inverter-resident methods, active inverter-resident methods, active methods not resident in the inverter, and the use of communications between the utility and PV inverter [85].

  • i.

    Passive inverter-resident methods rely on the detection of an abnormality in the voltage at the point of common coupling (PCC) between the PV inverter and the utility.

  • ii.

    Active inverter-resident methods use a variety of methods to attempt to cause an

Performance and reliability of inverter hardware

Performance and reliability of inverters, and most other power electronics, in PV systems has been perceived by many to be poor over the past 20 years. The word ‘perceived’ is used here because many other factors have contributed to reported failures other than simply inverter problems. Utility-interactive PV inverter islanding or problems may occur as a result of the following conditions [42], [85]:

  • i.

    A fault that is detected by the utility, and which results in opening a disconnecting device,

The overall conclusion and recommendation

  • The tendency of over-sizing excessively the PV generator in relation to the inverter still exists, and this procedure can affect the inverter's operational lifetime. The maximum PV power in a power network for which balanced conditions never occur is approximately two to three times the minimum night load of the relevant power network.

  • Balanced conditions occur very rarely for low, medium and high penetration levels of PV systems. The probability that balanced conditions are present in the power

Acknowledgements

The investigation presented in this paper have been done in the frame of research work at the Department of Electrical Engineering, Tsinghua University, Beijing for postdoctoral program, which has been funded by the Tsinghua University.

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