Power quality issues due to high penetration of rooftop solar pv in low voltage distribution networks a case study

Increased penetration of rooftop solar PV is causing undesirable technical impacts on the distribution networks. Several urban distribution transformers in Sri Lanka are exceeding fifty percent of the solar PV over the transformer capacity, which shed a green light to assess the cumulative effect of the rooftop solar PVs. In this study, a distribution feeder with high solar PV penetration has been selected and power quality issues such as harmonics, over voltage and DC injection are analyzed under various conditions. Effects of nonlinear loads were also assessed to create an accurate representation of the existing network. The simulation results identify whether the selected system satisfies the statutory limits imposed by various global regulations concerned with power quality.31931 00 ©2019 IEEE Power Quality Issues Due to High Penetration o.

2019 IEEE 14th International Conference on Industrial and Information Systems (ICIIS), 18-20 Dec., Peradeniya, Sri Lanka Power Quality Issues Due to High Penetration of Rooftop Solar PV in Low Voltage Distribution Networks: A Case Study H H H De Silva Department of Electrical Engineering, University of Moratuwa, Sri Lanka ORCID: https://orcid.org/0000-00024359-1643 D K J S Jayamaha Department of Electrical Engineering, University of Moratuwa, Sri Lanka ORCID: https://orcid.org/0000-00026803-8107 sufficient for analyzing the voltage profile [5-7] In [8], detailed modelling of the PV system is used to analyze the voltage levels and losses along the feeder Futuristic analysis based on a stochastic approach to increase the solar penetration levels is also covered in [8-10] However, the studies reveal that voltage levels have not exceeded the statutory limits due to the existing solar PV penetration levels [6,8,9,11,12] In harmonic analysis related to distribution networks, total voltage harmonic distortion (VTHD) and total current harmonic distortion (ITHD) are observed under different conditions in [13,14] The aggregated effect of multiple single phase inverters and three phase inverters has been analyzed separately in [15] However, according to [11] and [14], with the considered solar PV penetration levels, recorded VTHD levels were within the statutory limits while ITHD levels were violated in many cases Several studies have considered solar PV as the main or only means of harmonics injection, while the effects of non-linear loads being omitted [9-14] In studies, which assess the harmonic injection by multiple solar PV inverters, a finite pattern on harmonics is not quite observable due to the primary and secondary emissions of the solar PV inverters [13] The resultant harmonics on the system may either attenuate or enhance the individual harmonic content Abstract—Increased penetration of rooftop solar PV is causing undesirable technical impacts on the distribution networks Several urban distribution transformers in Sri Lanka are exceeding fifty percent of the solar PV over the transformer capacity, which shed a green light to assess the cumulative effect of the rooftop solar PVs In this study, a distribution feeder with high solar PV penetration has been selected and power quality issues such as harmonics, over voltage and DC injection are analyzed under various conditions Effects of non-linear loads were also assessed to create an accurate representation of the existing network The simulation results identify whether the selected system satisfies the statutory limits imposed by various global regulations concerned with power quality Keywords— DC Injection, harmonics, low voltage network, over voltage, power quality, rooftop solar PV I N W A Lidula Department of Electrical Engineering, University of Moratuwa, Sri Lanka ORCID: https://orcid.org/0000-00033556-4693 INTRODUCTION With the popularity of the national promotional projects for roof-top solar PV in Sri Lanka, electricity consumers showed a tendency of increased installation of roof-top solar PVs Ceylon Electricity Board (CEB), the electrical power generation, transmission and distribution authority of Sri Lanka has published that 70 MW of rooftop solar were installed by the end of 2017 [1] However, unplanned interconnection of distributed generators (DGs) could lead to technical impacts on power system reliability, power quality and stability In this paper, both solar PV and non-linear loads are modeled in detail to replicate the accurate picture of the distribution feeder In analyzing current harmonics at the system level, total demand distortion (TDD) is preferred over ITHD [2] and used in this paper in contrast to other studies [5-8,10-14] Total demand provides a common reference for measuring current distortion compared to the fundamental current component DC injection is equally important due to its impact on the transformers [16,17] However, DC injection due to solar PV has not been recognized in the reported power quality studies, despite its position in regulatory standards [4, 16] The paper is organized as follows In Section II, power quality issues due to increased rooftop solar PV and the limitations are presented In Section III, the details of the modeled network are presented Section IV presents the results of the study and a qualitative analysis on the results The effects of solar irradiance and loading condition are the main variables and are presented under six different cases Finally, section V draws the conclusion Particularly, power quality issues arising from increased integration of solar PVs to the utility grid have gained significant research attention The power electronic converter that interfaces the solar PV to the system is the main factor that affects the power quality of the utility network Thyristor-based, line-commutated inverters are considered undesirable on the power system due to the generated harmonic currents [2] To achieve better control and to address the harmonic issue, the inverter technology evolved to pulse width modulation (PWM) technology, which resulted in a better interface system to the solar PV [2] Once a roof-top solar PV system is installed, it will be commissioned if and only if all the system parameters are within statutory limits imposed by the standards such as [3] and [4] However, cumulative effects are not particularly evaluated at the commissioning level and therefore, the impact of increased integration of rooftop solar PV to the local network is found a timely necessity and has been evaluated by several research studies II POWER QUALITY ISSUES DUE TO INCREASED SOLAR PV INTEGRATION The power quality issues associated with solar PV corelates with the intermittent nature of the PV generation and the effects from the power electronics interface Possible In distribution level, power quality issues in literature mainly focus on voltage issue In many such studies, PV inverters are modeled as constant PQ sources, which is 978-1-7281-3706-3/19/$31.00 ©2019 IEEE 395 Authorized licensed use limited to: University of Exeter Downloaded on June 09,2020 at 16:58:48 UTC from IEEE Xplore Restrictions apply 2019 IEEE 14th International Conference on Industrial and Information Systems (ICIIS), 18-20 Dec., Peradeniya, Sri Lanka power quality issues and their causes related to solar PV are listed in Table I [10] For the harmonic measurements, standards not specify whether line-line or line-neutral currents/voltages to be used Based on vectorial analysis, line-line harmonic voltages could range between to 1.15 times measured between lineneutral harmonic voltages at a given frequency These two extreme cases correspond to an angle between two vectors of 0º and 180º respectively [19] However, field measurements at typical LV distribution sites show that these extreme cases are not observed and the harmonic currents/voltages measured in line-line are very similar to those measured in line-ground Hence in this study, line - ground voltages/ currents are chosen for the harmonic analysis The harmonic current limits indicate the minimum quality of current waveform the customer can inject into the grid at the PCC At the same time, the utility is responsible for providing a clean low distorted voltage to the customer In addition to harmonic distortion it is worthwhile to analyze the other power quality requirements related to solar PV integration A Harmonic Limitation Requirements and Standards THD The THD definition in (1) is derived from IEEE 519 standard [3] where, Mh is the RMS value of the hth harmonic component of the quantity M, which can be either voltage or current [2,3], and M1 is the fundamental value of the quantity M concerned In a system-level study, TDD is more appropriate to evaluate the effect of current harmonics at varying load conditions [2] It is a general practice to check the current harmonic levels with the base as the inverter rated current, which is known as total rated distortion (TRD) [18] However, this study employs maximum demand for the feeder at considered cases as the base to calculate the TDD In the TDD definition given in equation (2), Ih is the hth harmonic current and IL is the maximum demand current at the PCC TDD = B Other Power Quality Requirements and Standards Over Voltage: The LV standards specify a ± 6% of the nominal voltage to be maintained [20,21] Specifically, for DG, temporary over voltage (TOV) limits are specified in [4] to be maintained at the PCC However, this study will not focus on the TOV limits, but will consider the 6% threshold × As defined in IEEE 519 standard, the recommended limits for LV VTHD and TDD are given in Table II As per the guidelines in [3], since the PV inverter falls into the power generation equipment category, the TDD limits are considered from the category Isc/IL