Comparative Study of Voltage Equalizing Circuits for Photovoltaic String

Partial shading is one of the inevitable condition that photovoltaic (PV) strings encounter. When photovoltaic strings are partially shaded, numerous peaks appear on their power versus voltage (P-V) characteristic curve, significantly reducing their ability to deliver power. This paper presents, three power electronics topologies that permit voltage equalization by charge redistribution between the PV modules wired in series. Three popular topologies, namely; switched-capacitor (SC), Buck-boost switched capacitor (BBSC) and PWM with switched capacitor (SC-PWM) converters have been examined. Complete experimentation have been done using MATLAB/Simulink environment. A comparative analysis have been done under various shading conditions. The findings demonstrate that the converters under consideration can improve the performance of string under partial shading.


I. INTRODUCTION
Globally, a sizable amount of power is produced from coal each year.Yet, the use of coal for power generation causes environmental problems.Thus, it is crucial to replace coal with some additional green and clean energy sources (such as solar, wind, tidal, etc.).Over the past few decades, solar and wind energy production have significantly increased due to their favorable environmental effects and financial benefits.[1][2] [3].The solar photovoltaic (PV) technology has grown the fastest in comparison to other renewable energy sources.The installed capacity is increasing at an increasing rate.The majority of the world is moving towards solar PV as the most affordable alternative for generation of electricity, which is expected to boost investment in the subsequent years.2021 year saw an all-time high 179 Twh of solar PV energy produced, a 22% increase over 2020.Solar photovoltaics (PV) ranked third behind wind and hydropower with 3.6% of the world's total electricity produced coming from renewable sources.However, experts claim that the biggest obstacles to installing PV systems are their reliance on irradiance, limited efficiency in converting energy, large space needed, and effects of partial shading [4] .
In order to reduce the hotspot issue, the bypass diodes are coupled across each module on a serially connected PV string [4].Each module operates at a different voltage level, resulting in multiple peaks with a single global maximum power point (GMPP) and numerous local maximum power points (LMPP) [5].Additionally, the power that is extracted from PV has been reduced significantly [5] as a result of the activation of bypass diodes.PV arrays can be connected using a variety of interconnection methods, including Bridge Linked (BL), Total Cross Tied (TCT), Series-Parallel (SP) and Honey Comb (HC) [6] to increase maximum power that is to be extracted, but multiple peaks still exists.Traditional technique to track maximum power point (MPPT) controllers, such as InC and P&O, do not have the capability to track the GMPP.Numerous methods have been reported to deal with these issues have been proposed [7].As a result, power losses will occur if it tracks the LMPP.The GMPP can, however, be precisely tracked by MPPT algorithms based on artificial neural network (ANN) ,artificial intelligence and controller such as fuzzy logic controller [8], [9], and bio-inspired optimization algorithms including the particle swarm optimization (PSO) [10].However, multiple peaks can still be produced by PSCs in the I-V and P-V curves.

Figure.1. P-V curve with voltage equalizer as compared to that with bypass diode
In order to solve the partial shading issue, DC-DC Voltage Equalizers (which include switching devices and storing elements) are suggested.It equalizes the voltages across all of the modules in addition to preventing multiple peaks from forming on the P-V and I-V curves during PSCs.VE increased PV's ability to extract power when compared to the activation of bypass diodes, as shown in Fig. 1 for modules connected in series [12].As compared to bypass diodes, which result in multiple peaks being produced, in case of voltage equalizer each module will have the same operating voltage, which prevents the formation of many peaks and allows each module to operate at different current levels during PSCs.
Three voltage equalizer topologies, SC-PWM, SC, and BBSC, are compared in this paper for a PV module Integrated application under partial shading.The structure of this study is as follows.Simulation study with three VE topologies are discussed in Section II along with how they operate and various functionalities is discussed in Section II.It displays the simulation results for various shading scenarios in Section III.It compares three VE-based topologies overall using a variety of performance metrics, including GMPP, mismatch loss, tracking loss.Section IV draws conclusion Switches with even and odd numbers are alternately driven.Similar to conventional SCCs, the SCC automatically unifies the voltages of the PV modules.This topology integrates a PWM buck converter and Switched capacitor ladder converter.To balance the voltage between modules, a switched capacitor ladder converter is used.The inductor (L) receives the square wave generated between the two switches.As a result, the output can be controlled using D, just like in a conventional buck converter.The size of the capacitor and inductor is correlated with the amount of energy stored.The capacitors C1 to C3 are 90 F, C4 to C5 are 30 μF, Cout is 60 μF, Lout is 15 mH, switching frequency of 150 kHz with a 50% duty cycle and in addition the SC with PWM converter being set up in a ladder architecture with the sub-modules.[11].

S
Circuit diagram illustrating the connections of BBSC voltage equalizer with PV string are shown in Fig. 2(c).In this topology, buck-boost converters are used for voltage equalisation along a downward path, while SCs are used for power transfer in an upward direction.[15].BBSC can therefore operate in either direction.The following specifications apply to the BBSC converter, which is set up as a ladder architecture with sub-modules: a capacitor of 220 µF, an inductor of 0.5 mH, and a switching frequency of 50 kHz with a 50% duty cycle.

III. RESULTS & DISCUSSION
Under shading, the generated power, multiple peak count, losses due to mismatch, losses that are tracked and power improvement in comparison to the bypass diode of PV arrays with various configurations are studied [12].
Where 'ML' is losses due to mismatch, 'Pwoutpv' is PV module's output power with no shading, 'Pwspv' is PV module's output power with shading.ܶ = ܲ ீெ − ܲ ெ (݅݅) Where 'TL' is tracking loss, PGMPP is power at global maximum peak and PLMPP is power at local maximum peak.
Two partial shading conditions have been taken to compare the output of three topologies: switched capacitor, buck-boost switched capacitor, and switched capacitor with PWM.Single peak is produced and the maximum power also increased drastically.Table 1 displays the downward/upward shading taken into account for the five series-connected PV arrays.Two partial shading conditions that have taken are:- Insolation level (W/m 2 ) Downward Shading 1000 900 800 Upward shading 800 900 1000 Table 1 Insolation levels on PV modules during different shading conditions

Case-1:-Downward shading
The modules in this shading scenario are exposed to irradiance in decreasing order of 1000W/m 2 , 900W/m 2 , 800W/m 2 at a temperature of 25 o C. As a result, the voltage between the rows that are shaded and those that are not is different.Fig. 3 displays the I-V and P-V characteristic curves of the PV arrays in a variety of configurations under the lower shading scenario.According to calculations, the array's total power is 558.5W,529W, and 504W respectively, with single peaks for BBSC, SC, and SC-PWM.On the other hand, the characteristic curve P-V generated by the bypass diode configuration had three peaks, the last of which was the global peak at 447.4W and the first MPP (local) at 180W.In lower shading condition, with a bypass diode, switched capacitor with PWM, switched capacitor, and buckboost switched capacitor, the array experienced mismatch losses of 140.5W, 83.9W, 161.19W, and 69.02W, respectively.With a bypass diode, tracking loss with lower shading equals 267.4W.With the array's reconfiguration using SC-PWM, SC, and BBSC, the power output increased by 11.23%, 15.42%, and 19.82% with respect to the bypass diode.The series-connected PV module with BBSC offers the best outcome among the three techniques in downward shading.

Case-2:-Upward shading
The modules in this shading scenario are exposed to irradiance in the following increments: 800W/m 2 , 900W/m 2 , and 1000W/m 2 at a temperature of 25 o C. As a result, the voltage between the rows that are shaded and those that are not is different.Fig. 4 displays the P-V characteristic curves of the arrays in various configurations for the upper shading condition.According to calculations, the arrays' total power is 530W, 503W, and 468.7W, respectively, with single peaks for BBSC, SC, and SC-PWM.In contrast, the bypass diode configuration's curve P-V produced 447.IV.CONCLUSION Three power electronics configurations that permit voltage equalization between the series-wired PV modules to balance PV modules have been analyzed.The switched-capacitor (SC), Buck-boost switched capacitor (BBSC), and PWM with switched capacitor (SC-PWM) converters for the purpose of power balancing are configured and verified by simulation in this study .These three techniques have been compared in this paper.A local MPP was successfully eliminated by voltage equalizers and the maximum power that could be extracted increased significantly as compared to that without equalization.Simulation model with proposed voltage equalizers has been presented in this paper and also characteristic curves of configuration with proposed equalizers and with bypass diodes have been presented and compared.
II. METHODOLOGY Simulation research has been conducted with MATLAB/Simulink environment.Simulink model consisting of three modules connected in series with each having the following parameters has been developed; maximum power of 200.376W,Voc (open circuit voltage) of 33.2 V, Isc (short circuit current) of 7.75 A, maximum power voltage of 26.4 V and maximum power current of 7.59 A, temperature coefficient of Voc & Isc ( in % / 0 C) is -0.395 & 0.117.Fig. 2 (a) illustrates the circuit diagram of the PWM with SC voltage equalizer.

Fig 2 (Figure. 2 .
Fig 2(b) illustrates the circuit diagram of SC equalizer connected to a PV string.There is no charge redistribution and, consequently, no loss when the sub-modules are irradiated uniformly and have perfectly matched characteristics.The SC converter divides the differential charge among the adjacent sub-modules based on the partial shading ratio when partial shading occurs[14].The following specifications apply to the SC converter, which is set up as a ladder architecture with sub-modules: 20 µF capacitor, and 250 kHz switching frequency with a 50% duty cycle.

Fig. 3 .
Fig.3.P-V curve of various PV string with different voltage equalizers under downward shading 4 W with three peaks, the final peak being the global peak and the first MPP (local) peaking at 178 W. The array experienced mismatch losses of 137.5W, 119.2W, 84.9W, and 57.9W with a bypass diode, switched capacitor with PWM, switched capacitor, and buck-boost switched capacitor in upper shading condition, respectively.Tracking loss with upper shading equals 272.4W with a bypass diode.The array's power output increased by 3.90%, 10.45%, and 15.01% when SC-PWM, SC, and BBSC are used to reconfigure the array when compared to bypass diode.The most effective of the three methods is the series-connected PV module with BBSC in upward shading.

Table 2 .
Performance of PV string with different voltage equalizer circuits