تأثیر نوسانات سریع PV بر کیفیت توان در گرید ولتاژ پایین
Abstract
1- Introduction
2- System design
3- Case study and simulation data inputs
4- Simulation results
5- Discussion
6- Conclusion
References
Abstract
Cloud transients cause rapid fluctuations in the output of photovoltaic (PV) systems, which can significantly affect the voltage levels in a low-voltage (LV) grid with high penetration of PV systems. These voltage fluctuations may lead to violation of the existing power quality standards. This study estimates the impact of rapid PV output fluctuations on the power quality in an existing LV grid by performing load flow analyses for scenarios in the years 2017, 2030 and 2050 using PV data with 20-second resolution. In this study, we propose a system for the mitigation of PV output fluctuations by altering the charging processes of electric vehicles (EVs) and we assess the effectiveness of the proposed system. Results indicate that PV output fluctuations have minor impact on the voltage levels in the year 2030, but PV output fluctuations induce considerable voltage fluctuations in the year 2050. The magnitude of the voltage fluctuations is dependent on the location in the grid, the installed PV capacity and the grid configuration. These voltage fluctuations can induce visible and annoying light flicker for a significant part of the day in the year 2050. Implementing the proposed system shows that EV technology can contribute in reducing the amount of visible and annoying light flicker considerably, however at the expense of increased charging costs for EV owners.
Introduction
The ongoing surge in photovoltaic (PV) generation capacity in low voltage (LV) grids poses unprecedented challenges to distribution system operators (DSOs). Passing clouds induce short-term variability in the output of PV systems. Fluctuations of 45–90% of the rated PV capacity per minute induced by passing clouds have been reported for a large PV plant in Portugal [1] and fluctuations of 63% of the rated PV capacity per minute have been measured in a Hawaiian PV plant [2]. Cloud transients also result in considerable PV output fluctuations on a shorter time-scale; PV output fluctuations of 80% of the installed PV capacity per 2 seconds have been observed in Spain [3]. The intermittent nature of PV generation is the source of power quality issues. The main power quality problems associated with rapid PV output fluctuations are voltage fluctuations and light flicker, which is induced by voltage fluctuations [4]. Voltage fluctuations and flicker can cause damage to electrical appliances connected to the grid [5] and light flicker can cause annoyance and health problems to people exposed to it [6,7]. For this reason, European DSOs must comply to the EN-50160 power quality standards [8]. The relationship between PV power output fluctuations and light flicker has been addressed in multiple studies, with inconclusive results. A Malaysian case study demonstrated a positive relationship between the installed PV capacity and flicker values, and reported flicker values induced by PV power output fluctuations that violate the local flicker limits [9]. In addition, a study using a measurement setup for 69 PV modules concluded that fluctuations in PV generation can lead to considerable light flicker values, depending on the metric used to measure flicker values [10]. However, an empirical analysis of a 1.41 MW PV plant in Florida reported a low correlation between PV power output and measured flicker values [11], but this study did not look into the correlation between fluctuations in PV generation and flicker values. Similarly, Stewart et al. [12] concluded based on Hawaiian solar irradiance data that PV generation does not cause violation of the light flicker standards. The flicker values in this study were determined by using the flicker threshold values for 1-second and 2-second voltage fluctuations and by using 1-second and 2-second PV generation data, but this study did not consider the fact that PV output fluctuations on a longer timescale can also induce light flicker [6].