dc link voltage regulation of a standalone hybrid power system › hub › 2018-118-24 › 2 ›...

DC Link Voltage Regulation of aStandalone Hybrid Power System

C. Subba Rami Reddy,K. Ranjith Kumar,M. Surya Kalavathi,

EEE department,B.V. Raju Institute of Technology,

Narsapur, Medak, Telangana-502313JNTU Hyderabad,

Hyderabad, Telangana,

[email protected],[email protected],[email protected]

May 2, 2018

Abstract

A Standalone Hybrid Power System (SHPS) comprisesof photovoltaic (PV) as the main source of power generationand Hybrid Energy Storage System (HESS) is considered inthis paper. The PV power generation is intermittent in na-ture and may contain unacceptable fluctuations. High fluc-tuations in power generation and/or variations in load maycause power imbalance, which leads to variation in DC linkvoltage. This problem can be alleviated using HESS, whichis the combination of Lead Acid Battery and Ultra Capac-itor. The proposed HESS improves the life of the batteryby increases the state of charging (SOC) and reduces stresson the battery due to its characteristic features of high en-ergy and power densities. The performance of the proposedsystem is compared using PI control and fuzzy logic controland it is validated through MATLAB/Simulink.

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International Journal of Pure and Applied MathematicsVolume 118 No. 24 2018ISSN: 1314-3395 (on-line version)url: http://www.acadpubl.eu/hub/Special Issue http://www.acadpubl.eu/hub/

Key Words:Standalone hybrid power system, Photo-voltaic, Ultra-capacitor, Hybrid energy storage system, PIcontrol and Fuzzy logic control.

1 Introduction

World Energy Outlook report of 2016 paints a gloomy picture of theenergy needs to be met by the current scope of power generationin the world. The most effected countries are those that of devel-oping and underdeveloped countries. The existing infrastructure isincapable of meeting the ever increasing demands for power; result-ing in the lowest per capita consumption. Alternate methods suchas the use of renewable energy sources to augment the needs areincreasingly adopted. The disadvantages of such sources are beingovercome by adapting new technologies; HESS being one of them[1].Hybrid Power System (HPS) consists of both renewable and con-ventional energy sources to act as a stand-alone system, with theflexibility of grid connection in case of power export or emergencypower import conditions. In isolated sites, for stand-alone powergeneration HES is preferred due to the advancements in power elec-tronic technologies, power converter configurations etc. These tech-nologies, enabled system designers to present more reliable systems;which addresses the concern of use of alternate fuels, flexibility, effi-ciency, pollutant emissions and overall economy[2].India is bestowedwith many natural resources and hence the population is dispersedover the length and breadth of the country. There are scores oflocations with a sizable population far away from the load centers,for the mainstream power to be made available to them; in view ofeconomical and accessibility reasons. In such a scenario the HPSwith HESS is the need of the hour [3]. Lead Acid batteries havebeen the workhorses for standby power supply for many decades.They suffer from the inherent disadvantage of low energy density,in spite of having high power density. They have a life span of1000 full cycles and have large response times, for frequent loadfluctuations. Compared to this, the Ultra Capacitors have a verylong lifespan of 500,000 life cycles. Consequently, ultra capacitorcan be used to match the rapid load fluctuations [4-5]. The blendof these two types (i.e lead acid batteries and ultra-capacitors)iscrucial for various energy storage needs of both fast and slow fluc-

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International Journal of Pure and Applied Mathematics Special Issue

tuating power and it has turn out to be a research hotspot. Thehybrid energy storage system lowers the battery cost and improvesthe overall system efficiency [6]. A comparison between the battery,conventional capacitor and the ultra capacitor is presented in TableI [7].

TABLE I Comparison between Lead acid battery, Ultra-capacitorand Conventional Capacitor

In the existing literature [8-18], many researchers proposed thedifferent control strategies includes ANN, FL, MPC etc. for con-trolling power distribution between battery and ultra-capacitor.Branislav et al. [8] proposes a MPC for hybrid battery ultra-capacitor to improve the battery life time. Glavin et al. [9] proposesenergy control unit, is responsible for charging the battery/SC hy-brid system and supplying power to the load according to speci-fied conditions. Mendis et al. [10] proposes an energy managementstrategy for a HESS of DFIG based RAPS. Anthony et al. [11] pro-poses a control algorithm for improving battery life time in hybridenergy based standalone power system. Amine et al. [12] investi-gated the study of battery/SC combination of UPS. Haihua et al.[13] proposes dynamic energy management for renewable based mi-cro gird. Microgrid operated in two modes namely: islanded modeand grid connected mode. Energy balance is achieved in islandedmode and in grid connected mode prevention of intermittent natureof renewable sources and load fluctuations is achieved for stable op-eration. Manfredi et al. [14] uses UC alone to overcome the powerimbalance. Dougal et al. [15] proposes a hybrid energy system forpulsed load conditions. Fengyan et al. [4] proposes a power man-agement strategy for DC micro grid to achieve control constant DCvoltage for stable operation. Sercan et al. [16] proposes a rule basedcontrol of BESS for dispatching intermittent renewable sources foroptimal operation. In [6], authors used PI controller to improve theperformance of the hybrid energy storage system, whereas in this

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work, fuzzy logic control strategy is proposed for battery and SCESS to enhance the performance of the standalone hybrid powersystem and it shows the better performance when compared to PIcontrol.

2 Modeling of Standalone Hybrid Power

System

A stand alone hybrid power system consists of PV panels, batter-ies and Ultra Capacitors as shown in Fig.1. Varying load demandis not catered by battery system alone and hence a hybrid systemwith adequate storage capacity, capable of responding fast to meetthe load demands is required [19]. Hence battery and Ultra Capac-itors are chosen according to Ragone Plots theory [17]. These areinterfaced to the DC bus through bidirectional buck boost dc-dcconverter.

Fig.1: Standalone Hybrid Power System

Photovoltaic module The photovoltaic module has the equiv-alent circuit as in Fig 2 with the governing equations as given below[18]

I = Iph − Id − Is (1)

I = Iph − Is[e((V + IR)q

αKTNs

)

− 1] − V + IRs

Rsh

(2)

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Fig.2: Equivalent circuit of a PV cell

Where Iphis the current generated by the photovoltaic effect,Isis the reverse saturation current, q is the electric charge whichis equal to 1.6×10−19 C, KB is the Boltzmanns constant which isequal to 1.38×10−23 J/K, α is the ideality factor, T is the absolutetemperature, Ns is the number of cells in series, Rs is the seriesresistance and Rsh is the parallel resistance.

Lead acid batteryThe battery equivalent circuit represented one cell of the battery

[19] is as shown in fig.3.The output voltage was suitably enhancedto get a 12-volt battery, by adding six series connected cells. Withthe assumption that all the cells will behave identically, the mainbranch voltage is derived as:

Em = Em0 −KE(273 + θ)(1 − SOC) (3)

WhereEm- Open circuit voltage in voltsEm0- Open circuit voltage at full charge in voltsKE-Constant in Volts/◦CSOC-Battery state of chargeθ- Electrolyte temperature in ◦C

Fig. 3: Equivalent circuit of Lead-Acid Battery

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Ultra-capacitorUltra-capacitors are of characteristically high energy density

and high power density apparatus, which is constructed to take ad-vantage of the nano-distance of separation of ions in the electrolytewhich result in a massive ratio of charge storage to charge sepa-ration distance. The porous structure of the electrode can achievesurface areas higher than 2000m2/gm, which gives a massive chargestorage area. Charge transport is achieved by the ions through theporus separator. The potential difference between the electrodescreates the charge separation. The cycles of charge / dischargeeffects the performance of the ultra-capacitor and hence many an-alytical models are used to describe its performance [20]

Fig. 4: Internal cell construction

Fig.5: Ultracapacitor model

The ultra-capacitor is modeled as a three branch equivalent cir-cuit [21] as shown in Fig.5. The losses during charging and discharg-

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ing are represented by equivalent series resistance (ESR) Ra. Ultra-capacitor pore impedance is represented by other parallel branchesand Ca. R1C1 parallel branch responsible for the fast dynamicbehavior of ultra-capacitor. R2 accounts for self-discharging effectand C2 is the main capacitance responsible for energy storage andcharge handling.

3 Control strategy of hybrid power sys-

tem

The main objective of this work is to increase the SOC and reducethe stress on the battery thereby increase the lifetime of the bat-tery and DC-link voltage control in the case of input and outputpower variations. The fundamental design of this control approachis that the UC supports fast transients whereas battery supportsslow transients. The battery and ultra-capacitor are controlled bythe bidirectional buck-boost converter.

A. Using PI controller

Fig.6: Control strategy using PI control

Fig.6 [6] shows the control system related with the bi-directionalconverter for battery and UC. The reference current is generateddue to the difference in the dc grid voltage because of power im-balance. The dc component of the reference current after low passfilter (LPF) is the battery reference current. The net result oftotal reference current and dc component is the reference currentfor ultra-capacitor, is responsible for the decrease in SOC and in-creased stress on the battery. Therefore, the battery draws steadydc current and the rapid load fluctuations are bypassed to an ultra-capacitor.

B. Using fuzzy logic controller

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Fig. 7 Control strategy using FLC

Fig.7 shows the block diagram of fuzzy logic control strategy.This control algorithm further reduces the charging/dischargingstresses on battery and increases the life of the battery when com-pared to PI control.

Fig.8: Membership function for error input

The fuzzy logic controller contains a Takagi-mamdani inferenceengine and two fuzzy inputs such as dc link voltage error Er andchange in dc link voltage error Er. Triangular and trapezoidal mem-bership functions are chosen for both of the fuzzy inputs as shownin figures 8,9 and 10. There are seven membership functions foreach input and output including NB (Negative Big), NM (NegativeMedium), NS (Negative Small), Z (Zero), PB (Positive Big), PM(Positive Medium) and PS (Positive Small). Fuzzy logic relatesoutputs to the inputs using a list of if-then statements called fuzzyrules. Fuzzy rules are in the form of, IfEr is NB and∆Er is NBthen O is NB

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Fig.9 Membership function for change in error input

Fig. 10: Membership function for output

Fig.11 shows the surface view of the fuzzy rules and Table IIshows the rule base matrix.

Table II Rule base matrix

Fig. 11: Surface view of fuzzy rules

4 Results& Discussion

The simulation of the proposed hybrid power system with PI con-trol and Fuzzy Logic Control is carried out in MATLAB/Simulink.Arbitrary signals of solar irradiance, temperature and operatingvoltage of the PV system as shown in figures 12, 13 &14 is consid-ered (for both modes i.e. battery alone mode and HESS mode) to

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account the real climatic conditions. PV array generates the maxi-mum power as shown in fig.15for the specified input conditions. PIcontroller guarantees that the current drawn from PV array tracksthe maximum PV current for the specified climatic conditions.

Fig.12: Solar Irradiance

Fig. 13: Operating Voltage

Fig. 14: Temperature

Fig.15: Maximum power generated by PV

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Battery alone as energy storage modeThe mean load requirement in the period 0 to 2 sec is 150W

as shown in fig.16 which is smaller than the available PV power of200W and here excess of 50W power is consumed by the battery.In the period 2 to 4 sec the mean load power is 230W which ismore than the maximum available PV power. The deficit power inthis period is supplied by the battery. Therefore DC bus voltagecan be maintained at the required level for the duration of powersurplus and deficit conditions with the existence of bidirectionalconverter. PV power generated constantly varies according to thesolar irradiance, temperature and load. Therefore, over a period oftime the power imbalance also severely changes.

At the point when this exceedingly changing and fluctuant irreg-ularity power is given to the battery, it experiences regular chargingand discharging operations. It builds stress on the battery and itmight have the adverse impact on the lifetime and execution of thebattery. To stay away from this, ultra-capacitors are associatedas an extra energy storage device to the grid, using bi-directionalbuck-boost converter. Since the ultra-capacitor can respond fasterto rapid changes, the stress of the battery can be decreased.

Fig. 16: Load power variation

Hybrid energy storage system modeAccording to the load variations, the UC and battery charges

and discharges to maintain the DC bus voltage. The performanceof the UC under HESS mode is shown in figures 17, 18 &19. Thehigh frequency variations in the reference current are absorbed bythe UC. The UC discharges more rapidly during the period of 2to 4 sec as the fluctuations are rigorous in this region. Therefore,current increases to 7 A rapidly at 2 sec to alleviate the stress onthe battery.

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Fig. 17: Ultracapacitor current in HESS mode

Fig.18: Ultracapacitor voltage in HESS mode

Fig. 19: Ultracapacitor SOC in HESS mode

Fig. 20: Comparison of DC bus voltage

Fig. 20 shows the dc grid voltage, which is maintained at thereference level even though there is imbalance power unlike previouscase. It is also observed from fig. 20, FLC maintains constant DCvoltage in HESS mode with lesser ripples compared to PI controller.

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Fig. 21 shows the comparison of battery current in both modeswith PIC and FLC and it is observed that FLC with HESS modegives better performance. In HESS mode, the battery current issmoothened and current rating is also reduced when compared tobattery alone mode as depicted in fig.21.

Fig. 22 shows the battery SOC in both modes and it is observedthat SOC increases in HESS mode. Due to the increase in SOC ofthe battery depth of discharge reduces which in turn increases lifetime of the battery.

Fig.21 Comparison of battery current

Ripple content can be calculated by using following formula

Ripple(%) =Xmax −Xmin

Xavg

× 100 (4)

where

Xavg =Xmax +Xmin

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Ripple comparison with PI control and Fuzzy Logic Control isshown in Table III.

Fig. 22Comparison of battery state of charge

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TABLE III Ripple comparison with PI and Fuzzy Logic Controller

5 Conclusion

Hybrid energy storage system which comprises of battery and ultra-capacitor for standalone hybrid power system is presented in thiswork. Two modes of operation such as battery alone mode andHESS mode is considered and the performance of these two modesare compared with PI and fuzzy logic controller. In the batteryalone mode, due to its low power density the power balance opera-tion imposes severe stresses on the battery and effected its life time.To improve battery life time and reduces the stresses ultra-capacitoris added to the battery. This combination has the advantages ofhigh power density and high energy density. Therefore, HESS modewith fuzzy logic controller shows the improved performance.

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dc link voltage regulation of a standalone hybrid power system › hub › 2018-118-24 › 2 ›...

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