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Solar Energy 97 (2013) 405–413

Power distribution architectures to improve system efficiencyof centralized medium scale PV street lighting system

RakeshBabu Panguloori a, PriyaRanjan Mishra a,⇑, Subrat Kumar b

a Philips Research India, Bangalore, Indiab Indian Institute of Technology Delhi, Delhi, India

Received 1 December 2012; received in revised form 29 July 2013; accepted 24 August 2013

Communicated by: Associate Editor Bibek Bandyopadhyay

Abstract

Typically, stand-alone/decentralized off-grid solar street lighting solutions are designed for autonomy of 3–5 days to meet 3–5 days oflow or no-insolation period. These decentralized systems meet the lighting load requirements even under bad weather condition but morethan two–third of the year, the surplus solar energy remains unutilized. The small amount of unutilized surplus energy from a singlestreet pole may not be sufficient to meet local energy needs. On the other hand centralized system will have enough energy capacityto meet many of the energy needs of rural community, such as by establishing solar charging stations (energy kiosks), or electric vehiclecharging stations or even feeding to grid . This paper investigates various centralized power distribution architectures with the objectiveof improving system efficiency and simultaneously reducing system complexity. Among various distribution architectures for centralizedstreet lighting system, narrow DC voltage bus architecture is most efficient and has higher reliability. Analytical work using MATHCADhas been undertaken to identify the boundaries of efficient distribution network; and is presented in this paper. During analysis, factorslike load at each point, distribution range, type of distribution network, extension to grid and safety are taken into consideration. Theanalytical work though has been undertaken for centralized street lighting system, it is equally relevant to cluster of homes in off-grid orgrid connected systems. One such 220 V DC centralized street lighting system has been designed and tested in lab as well as in field.Experimental results of 220 V DC centralized street lighting system are presented to illustrate system efficiency improvements over con-ventional 230 V AC system.� 2013 Elsevier Ltd. All rights reserved.

Keywords: Distributed generation; Solar street lighting system; Renewable energy; Centralized solar power system; Power autonomy; Distributionefficiency

1. Introduction

Solar energy is the cost-effective option for India toreduce energy scarcity without having to extend nationalgrid services to provide a reliable and secure energy supplyto remote areas (Gavanidou and Bakirtzis, 1992). Solarstreet lighting and solar home lighting solutions are widely

0038-092X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.solener.2013.08.034

⇑ Corresponding author. Tel.: +91 9980496004.E-mail addresses: rakeshbabu.panguloori@philips.com (R. Pangu-

loori), mishra.priyaranjan@philips.com (P. Mishra), kumar.subrat21@g-mail.com (S. Kumar).

promoted by the Indian government for rural communitydevelopment (MNRE, 2009). These solutions are well pop-ular as decentralized systems. LED lighting technology hasrapidly developed in recent years to the point that LEDscan be seriously considered for replacing conventional hal-ogen and incandescent lamps in general illumination, e.g.,street lighting (BEE, 2010). Providing street lighting isone of the most important responsibilities of a municipal-ity/village administration to improve safety and comfortfor both vehicular traffic and pedestrians. It also providesenhanced sense of security to village community. Energy

406 R. Panguloori et al. / Solar Energy 97 (2013) 405–413

efficient technologies and design can reduce street lightingcosts substantially, which will help municipalities toexpand their services by providing lighting in low-incomeand other underserved areas. TERI has reported that bythe introduction of LEDs, cost reduction of 25–30% is pos-sible because of reduced panel size, freight and storage cost(Debajit and Gopal, 2011).

Fig. 1 shows the power architecture of a decentralizedsolar LED street lighting system. It functions independentof the utility grid. The solar energy is used to charge a self-contained battery during day time and the charged batterypowers the LED light loads during night time. Batterycapacity is usually designed for power autonomy of 3–5 days to meet lighting loads under varying environmentalconditions, and are often overdesigned (Chih-Chiang andPi-Kuang, 2005; Nuttall et al., 2008; Marco and Guilher-me, 2009; Notton et al., 1996). Many times such systemneeds customized solar panels resulting in further increasein cost of solar system.

Though, the decentralized system meets the lightingloads even under worst cases but in most part of the year,the system remains underutilized. Since, the initial invest-ment for an off-grid solar LED Street lighting system ishigher compared to conventional grid powered streetlights, it is more important to utilize the solar energy muchmore effectively. In any typical year, the surplus energy var-ies from 5% to 30% of total watt hour load per day. Thesmall amount of surplus energy from a single street lightingsystem is not sufficient to meet local energy needs. Howeverin a centralized street lighting system, cumulative surplusenergy will be high enough to meet some of the local energyneeds. The surplus energy can be used for charging electricvehicle battery or for providing charging outlets for mobilelighting units / mobile-phones etc. The centralized stationcan also work as micro power distribution center. Thisleads to lower pay-back period for the entire system, lightweight and low cost pole structures along with severalother benefits. There is also possibility for grid interconnec-tivity in future, which will further lead to reduction in bat-tery capacity and hence further lower payback period.There is secondary benefit also; as the centralized systemcan be guarded within the fenced boundary to minimizethe risks of theft and sabotage (James, 2012; Pepermanset al., 2005; Chaurey and Kandpal, 2010). It is also easyto operate and maintain centralized system. Finally, thereis no need of design change in pole’s mechanical structure.

Overall system efficiency is most important parameter ina power distribution system and which in turn dependsupon power distribution architecture of centralized

Fig. 1. Power architecture of decentralized PV street lighting system.

systems. This paper investigates various centralized powerdistribution architectures with the objective of improvingsystem efficiency and simultaneously reducing system com-plexity. Analytical work undertaken to identify the bound-aries of efficient distribution network is presented. Thepresent application considers secondary roads (B-1, B-2classification as per BIS, 1981 (BEE, 2010)) with low trafficsuch as shopping street in town and approach roads in vil-lages. The average level of illumination on such road sur-face is 4–8 lux. During analysis of village electrification,factors like load at each dwelling, distribution range, typeof distribution network, extension to grid and safety aretaken into consideration. Lab test bed of 220 V DC central-ized system powering 15 street lights is presented and per-formance results are discussed in this paper.

The paper is organized as follows. Existing centralizedpower distribution architectures and the proposed narrowDC voltage bus architecture are presented in Section 2.The mathematical analysis to find various distributionpower losses and then the range of efficient distributionnetwork is given in Section 3. Comparison of system effi-ciency under fixed load condition for various cases is donein Section 4. Lab experimental results are discussed in Sec-tion 5. Finally conclusions are given in Section 6.

2. Centralized PV distribution architectures

Realizing the benefits of centralized system, many of thestreet lighting installers are executing AC-centralized PVstreet lighting system (Shay and Dugan, 2008; Avi, 2012)as shown in Fig. 2. The solar energy is stored in batteriesusing MPPT charge controller. The stored energy is thenconverted into 230 V AC using efficient inverter and sup-plied to street lights just as in conventional grid based sys-tem. It is typically designed for a group of 15 street poleson a 48 V battery bank. Since the distribution voltage is230 V AC, the distribution network can be wide enoughto cater 15 street poles spread across 400–500 meters.One of the advantages of this system is the output of theinverter is well regulated, so the LED driver does notrequire wide input voltage range unlike drivers for weakgrids (180 V�270 V input range). As the power level of

Fig. 2. Power architecture of N-pole conventional AC-centralized PVstreet lighting system.

48 V

PVModule

LEDDriver

PLEDDC bus

LEDDriver

PLED

LEDDriver

PLED

1st pole

Nth pole

(N-1)th pole

MPPT Charge

Controller

PVModule

MPPT Charge

Controller

PVModule

MPPT Charge

Controller

Fig. 4. Power architecture of PV centralized system proposed by Dakkaket al. (2003).

R. Panguloori et al. / Solar Energy 97 (2013) 405–413 407

street lighting is relatively higher than domestic, the LEDdriver involves two-stage power conversion. Hence, thesystem undergoes three stages of conversion in the powerpath. This reduces the system efficiency and limits thelength of street light distribution i.e. number of street poles.In addition to that, the reliability of inverter coupled PVsystem depends heavily on the reliability of PV inverters(Mohamed and Zhao, 2010; Masheleni and Carelse, 1997).

The solar photovoltaic electricity generation and itsstorage in battery-bank are processed in DC system with-out any conversion from AC to DC or vice versa and atthe same time the end utilization in street lighting system(LED lighting) is also in DC. Therefore, it is more appro-priate to distribute electricity through DC grid withoutinvolving any inversions. Such distribution architecturewill result in higher overall energy efficiency (Hammer-strom, 2007; Panguloori et al., 2011; EPRI, 2006). In addi-tion, DC grid results in lower cable losses due to zeroreactive power loss and absence of skin effect unlike inAC grid.

Similar is the case in solar home applications. In stand-alone solar home solutions, the power generated in a sys-tem is supplied to individual home. For cost reasons, thesesystems are designed for average power profile. As a result,such systems are exposed to excessive transient burdens,and the battery is subjected to deep discharge to meethigher & extended energy requirements of the individualuser. In the centralized system designed for a group ofhomes, the load diversity factor helps in reducing the bur-den on the battery. The centralized PV system with powerdistribution on DC bus is named as DC micro grid. Thepower distribution is done on 48 V/96 V bus with fixed lineconnection to each individual home but limited to 10–20households (Debajit and Gopal, 2011) within the vicinityof the system. The distribution network is generally limitedto few hundred meters to limit distribution power losses. Ifthe same system is considered for street lighting as shownin Fig. 3, the power distribution is limited to few streetpoles. Another challenge with this architecture is theLED drivers need to be designed for wide input voltagerange (1:1.4) to meet varying battery voltage.

MPPTCharge

Controller

48 V/ 96VDC bus

PVArray

LEDDriver

PLED

LEDDriver

LEDDriver

LEDDriver

1st pole

2nd pole

Nth pole

(N-1)th pole

PLED

PLED

PLED

Fig. 3. Power architecture of N-pole conventional DC-centralized PVstreet lighting system.

In another example shown in Fig. 4; centralized PV sys-tem (Dakkak et al., 2003) employs plural PV subsystemswith centralized battery bank common to all loads. Com-pared to a conventional PV micro grid (common PV sys-tem instead of individual PV subsystems), this system haslow distribution loss when the bulk consumption happensduring day. In this subsystem, each PV subsystem powerscorresponding individual home and its adjacent homeswith its surplus power (Armenta, 1989). In case, if the loaddemand is less than the sum of all PV subsystems genera-tion, the system charges the common battery bank. Thispower distribution architecture does not offer any advan-tage if the generation and consumption happens at non-overlapping intervals such as street lighting applications.In such system, the power distribution architecture incursadditional distribution loss during day than the decentral-ized system.

Fig. 5 shows the proposed narrow DC voltage bus archi-tecture, where power module called DC bus regulator isused to step-up 48 V battery voltage to higher DC voltagetypically 220 V or 380 V for power distribution. The DCbus regulator can be a simple DC-DC converter, whichcan be controlled to regulate the DC bus voltage within anarrow band. Power efficiency of DC bus regulator plays

MPPTCharge

Controller

48 V

PVArray

LEDDriver

PLED

LEDDriver

PLED

LEDDriver

PLED

LEDDriver

PLED

V1

V2

VN-1

VNIN

IN-1

I2

I1

DC bus Regulator

Narrow voltage DC bus

1st pole

2nd pole

Nth pole

(N-1)th poleI'N-1

I'N-2

Fig. 5. Proposed narrow DC voltage bus architecture for street lightingsystem.

408 R. Panguloori et al. / Solar Energy 97 (2013) 405–413

major role in determining the overall system efficiency.Power efficiency depends upon power topology, so singleswitch boost converter or LLC resonant converter maybe suitable candidates for DC bus regulator. DC grid beingthe local grid offers the flexibility to control the DC buscharacteristics in a more appropriate way to suit the endapplication. If the “narrow DC bus” voltage is set closeto LED string voltage, then constant current linear regula-tor based LED driver can be used. This will result in cost-effective, compact, power efficient, luminaire integratedLED drivers. Even if switching regulator based LED driveris used, the system efficiency can be improved by modulat-ing the DC bus voltage to an appropriate value.

Another alternate design of DC bus regulator i.e. ‘DC-DC bidirectional converter’ is shown in Fig. 6. Since thesolar power generation and the load consumption (streetlighting) occurs at non-overlapping intervals, a singlepower module can replace MPPT charge controller andDC bus regulator units of Fig. 5. During day, the load isdisconnected and the bidirectional converter is operatedin buck mode as MPPT charge controller. During night,the bidirectional converter is operated in boost mode asDC bus regulator. In this interval, PV blocking diode pre-vents battery discharge and also protects PV array fromdamage. This approach improves the electronics utilizationfactor and results in lower footprint and lower cost.

Street lighting is a serially distributed load with equaldistance between consecutive poles. The current in the dis-tribution network decreases i.e. highest between centralizedstation and the first street pole and lowest between the lasttwo poles (provided all poles are connected in series). Insuch situation it is difficult to calculate distribution powerlosses. The next section deals with the analysis to calculatedistribution power losses at various distribution voltagesand range of distribution network.

3. Analysis on distribution power losses

In this section, the distribution power losses at variousdistribution voltages are calculated analytically by develop-ing a MATCAD routine. The street lighting is a serial dis-tributed load, so due to voltage drops in the cable the input

48 V

PVArray

LEDDriver

PLED

LEDDriver

PLED

LEDDriver

PLED

LEDDriver

PLEDNarrow voltage DC bus

DC-DCbidirectional

converter

1st pole

2nd pole

Nth pole

(N-1)th pole

Blocking diode

Load ON/OFF control

Fig. 6. Integrated MPPT and DC bus regulator in a single power module.

voltage and current of LED driver at each street pole willbe different. However, the lighting load at each poleremains constant. Referring to Fig. 5 the input current ofthe LED driver at any street pole is given as:

IN ¼P LED

gD � V Nð1Þ

where PLED is the power output of each LED lamp, gD isLED driver efficiency, IN, VN are the input voltage and cur-rent of LED driver of the Nth street pole.

Using Kirchhoff current law, the branch currents can bewritten as:

I 0N�1 ¼ IN ð2ÞI 0N�2 ¼ IN � 1þ I 0N�1 ð3Þ

The input voltage at various street poles can be found using(1)–(3) and is given as:

V N�1 ¼ V N þ 2� RL � L� I 0N�1 ð4Þ

where RL is the cable resistance per km and L is the dis-tance between two street light poles. MathCAD tool is usedto solve the above equations and the current values in var-ious branches are obtained. Finally, the distribution powerlosses are calculated using (5).

P LOSS ¼ 2� RL � L�XK

i¼1

ðI 0KÞ2 ð5Þ

For comparison among various centralized power distribu-tion architectures, 21 W decentralized solar street lightingsystem is considered as a reference. The technical specifica-tions of 21 W decentralized system are given below:

a. Solar module capacity: 100 Wp.b. Battery: 12 V, 100 AH lead-acid battery.c. Cables: 2.5 mm2 (resistance of 7 X/km).

For calculation of distribution power losses at variousstages, efficiency of AC LED driver and DC LED driverare assumed as 90% and 93% respectively. As per BIS-1981 classification of roads, the distance between two con-secutive poles for secondary roads is around 30 m. Thecomparative study is done for standard voltages of 12 V,24 V, 48 V, 96 V, 220 V, and 380 V respectively. To evalu-ate the optimum number of poles (or distribution length),the distribution losses is limited to 5% of the total systempower. Fig. 7 shows the distribution power losses whilefeeding the system at one end with lighting load of 21 Wat each pole. Similarly, the results for the case of 42 W ateach pole are shown in Fig. 8. It is seen from Figs. 7 and8 that the cable power losses raise exponentially after a cer-tain length. The intersection points on constant distribu-tion power loss line in Figs. 7 and 8 give the optimumnumber of poles for a particular distribution voltage. Ifthe distributed system is fed at the center of the system,then double the number of poles can be fed for the sameamount of distribution power losses. The results for

Fig. 7. Distribution power losses at various DC distribution voltageswhile feeding power at one end of the system (case1: 1 lamp per pole).

Fig. 8. Distribution power losses at various DC distribution voltageswhile feeding power at one end of the system (case2: 2 lamps per pole).

R. Panguloori et al. / Solar Energy 97 (2013) 405–413 409

various cases are tabulated in Table 1. The effective distri-bution lengths are 1.56 km, 1.74 km, 2.88 km for 220 VAC, 220 V DC and 380 V DC respectively.

From the above results, advantages of centralized dis-tributed generation with DC grid are evident. The sameconcept can be extended for rural electrification. This anal-ysis helps in identifying the proper place and type of distri-bution network in remote villages to minimize power

Table 1Optimum No. of street light poles fed at various distribution voltages in diffe

Distributionvoltage

Power fed at one end of the system Pow

No. of poles with 1lamp/pole

No. of poles with 2lamps/pole

No.lamp

380 V DC 48 33 96220 V DC 29 20 58220 V AC 26 19 5296 V DC 14 9 2848 V DC 7 5 1424 V DC 3 2 612 V DC 2 1 4

* Power fed at the center of the system and with 1 lamp of 21 W on each pol

distribution losses. Understanding lack of safety awarenessand scattered nature of village dwellings i.e. wide spreadacross a few kilometers, or dual bus voltage DC grid is pro-posed. In such situation small power distribution clusterswith bus voltage 48 V (less than SELV limit) can be formedto feed end consumers. These small power distributionclusters can be interlinked at 380 V DC to leverage surplusenergy and create reliable energy network. The illustrationof such distribution system is shown in Fig. 9.

The distribution power losses can also be obtainedapproximately by solving (1)–(5) with the assumption thateach LED driver or dwelling draws equal amount of cur-rent at its input. Eq. (6) gives power losses for a case wherepower is fed at one end of the system with 1 lamp per pole.Similarly, for the case of feeding power at the center of thesystem, the power losses can be obtained using (7).

P LOSS ¼ RL � L� P LED

V N

� �2

� N � ðN þ 1Þ � ð2N þ 1Þ6

� �ð6Þ

P LOSS ¼ 2� RL � L� P LED

V N

� �2

�N2� N

2þ 1

� �� ðN þ 1Þ

6

� �ð7Þ

From the above equations, it is observed that the powerlosses are directly proportional to square of the loadpower, inversely proportional to distribution voltage andhas exponential dependence on the No. of street poles‘N’. Hence, higher voltage allows wide distribution net-work for street lighting.

4. System efficiency comparison

In the present section overall system efficiency compari-sons are done for 220 V AC, 220 V DC, and 380 V DC.The comparative study is performed for various combina-tions, such as 10 poles or 20 poles with 1 lamp per pole or 2lamps per pole. For calculation of losses at various stages,efficiency of AC LED driver and DC LED driver are

rent cases.

er fed at the center of the system Equivalent distributionlength*

of poles with 1/pole

No. of poles with 2lamps/pole

66 2.88 km40 1.74 km38 1.56 km18 0.84 km10 0.42 km4 0.18 km2 0.12 km

e.

Fig. 9. View of distribution system with 48 V clusters interlinked with380 V DC.

Lighting load power

LED driver lossDistribution

power lossLoss in Inverter/ DC bus regulator

Input power from centralized

battery bank

Fig. 10. Segregation of various losses in the power flow path.

410 R. Panguloori et al. / Solar Energy 97 (2013) 405–413

assumed as 90% and 93% respectively. Efficiency for theinverter and the DC bus regulator are taken as 90% and95% respectively. Fig. 10 illustrates various componentlosses in the power flow path. The distribution power lossesfor various cases are based on empirical study undertakenin Section 3.

Accounting different losses in Fig. 10, it can be observedthat efficiency improvement of around 7% with 220 V DC

Fig. 11. System efficiencies at 220 V AC, 220 V DC a

and 8% with 380 V DC is possible in comparison with220 VAC. Variation of system efficiency with distributionvoltages are shown in Fig. 11. It is also observed inFig. 11 that for smaller load, the system efficiencies of220 V DC and 380 V DC systems are very close. However,as the load or the distribution length increases, 380 V DCsystem gives 2–3% higher efficiency than 220 V DC system.

Practically cost effective solutions are available for DC-DC converters with 93 plus efficiency whereas cost effectivesolutions are not available for inverter with 90 plus effi-ciency. So, it is worth studying the effect of inverter effi-ciency on overall system efficiency. Here, the study iscarried out for a 10 pole system with 21 W load on eachpole and comparison study has been undertaken between220 V AC and 220 V DC systems. The percentage ofimprovement in overall system efficiency for 220 V DC sys-tem over 220 V AC system is shown in Fig. 12. The systemefficiency improvement can be as high as 10% at 85% inver-ter efficiency. Further benefits like compact, low cost LEDdriver is possible with the narrow DC voltage architecture.To validate our empirical and theoretical studies, we devel-oped a centralized DC solar street lighting system. Thesame has been tested in lab as well as in the field. Theresults are given in the next section.

5. Experimental results and discussion

In this section, performance results of field installed ACcentralized solar street lighting system by a state govern-ment agency (Tamilnadu Energy Development Agency,TEDA) are presented and compared with the lab/test bedresults of 220 V DC centralized solar lighting solution.TEDA has executed AC centralized solar street lighting

nd 380 V DC under different loading conditions.

Fig. 12. Effect of inverter efficiency on system efficiency [DC busregulator = 95%, DC LED driver efficiency = 93%, and AC LED driverefficiency = 90%].

Table 2Measured field data of two AC centralized systems.

Siteno.

Inverter efficiency

At full load (full brightness)(%)

At 1/3rd load (one thirdbrightness) (%)

1 84 772 83 78

Fig. 13. Prototype of 400 W DC bus regulator.

Fig. 14. Efficiency profile of DC bus regulator at various loading andbattery voltage conditions.

Fig. 15. Block schematic of DC centralized solar street lighting controller.

Fig. 16. Close front view of DC centralized solar street lighting controller.

Table 3Test results of LED driver.

LED driver Efficiency on 220 V AC(%)

Efficiency on 220 VDC (%)

At full load (20 W) 83 86At 1/3rd of full load

(6 W)80 80

R. Panguloori et al. / Solar Energy 97 (2013) 405–413 411

system to power a cluster of LED lamps through inverter.The specifications of the system are given below:

� PV array of 600 Wp capacity of Crystalline SPVmodules.� Low maintenance tubular lead acid / VRLA Capacity of

capacity 48 V, 100 AH.� 15 Number of white LED luminaires of 20 W each.

The LED luminaires are programmed with an operationcycle of full brightness (20 W) for 5 h and one third bright-ness (6 W) for 7 h. Table 2 shows the measured field data ofAC centralized systems at two sites.

From Table 2, it is clear that the efficiency of inverterfalls significantly with the load whereas this type of loadconsumption profile demands flat load efficiency profilefrom the inverter. So, we undertook the development ofDC bus regulator with flat load efficiency profile. To suitthis, 400 W DC bus regulator to convert 48 V DC to220 V DC has been developed using interleaved two phase

Table 4Overall system efficiency comparison in discharge path (from battery to LED load).

System characteristics ginv or gconv(%) gD (%) gT (%) go (%) PCF Hrs. of operation/day Energy consumption factor/day

220 V AC at full load 84 83 97 67.6 1.48 5 7.4220 V AC at 1/3rd of full load 77 80 98 60.3 1.65 7 11.6220 V DC at full load 96 86 98 81 1.23 5 6.17220 V DC at 1/3rd of full load 97 80 99 76.8 1.3 7 9.11

Total energy consumption per day for AC street lighting system for unit load = 11.27.Total energy consumption per day for DC street lighting system for unit load = 9.2.Energy reduction per day = 11.27–9.2 = 2.07 (in percentage = 2.07/11.27�100 = 18%).

Here ginv, efficiency of inverter in AC system; gconv, efficiency of converter in DC system; gD, efficiency of LED driver; gT, transmission efficiency; go = (ginv

or gconv)�gD�gT = overall system efficiency; PCF, power consumption factor per unit load = (1/go).

Fig. 17. Photograph of 7 W linear LED driver.

412 R. Panguloori et al. / Solar Energy 97 (2013) 405–413

boost converter topology. The most widely used boundaryconduction mode (BCM) of operation (Chris and Balogh,2008) is chosen for the power module and realized withFairchild controller IC FAN9612 (Fairchild, 2010). Eventhough the BCM operation has higher RMS current inthe inductor and switching devices, it provides zero currentturn ON for the MOSFET, which reduces the switchingloss. BCM operation also eliminates diode reverse recoveryand hence avoids the need of fast recovery diodes.FAN9612 has phase shedding function, which shuts OFFone phase of DC bus regulator at light load. Since switch-ing losses also decreases along with decrease in copperlosses due to shedding of one-half of the converter, higherefficiency at reduced load power can be achieved. The labprototype of 400 W DC bus regulator is shown in Fig. 13.

The measured efficiency results are depicted in Fig. 14.An average efficiency of 96% is achieved throughout thedesired load range and also at various battery voltage con-ditions. Fig. 15 shows block schematic of 220 V DC cen-tralized solar lighting solution. In addition to 220 V DCoutput, controller has 220 V AC output also. This featurehas been provided to bypass the controller if it fails duringoperation. The luminaires can be driven by both supply i.e.DC in normal operation and AC in fault condition. Thecomplete system is assembled in a metal sheet enclosurewith DC energy meters, the front view of the same is shownin Fig. 16.

Further to investigate overall system efficiency, we testedluminaires with AC driver at both 220 V DC and at 220 V

AC. The test results are given in Table 3. From Table 3, itis clear that though LED driver efficiency at rated power(full brightness) differs slightly but at reduced power level,LED drivers has shown comparatively higher efficiency inDC operation. This is due to losses in power factor sectionof LED driver in AC operation.

Based on efficiency figures of Table 2, Fig. 14 andTable 3, overall efficiency of AC and DC centralized streetlight systems have been calculated and tabulated in Table 4.In Table 4, power and energy consumption factors are nor-malized to unit load. From Table 4, it can be concludedthat for DC operation, overall system efficiency improvesin the range of 13% and 17% during rated load (full bright-ness) and dim load (1/3rd brightness) respectively. Further,based on TEDA load profile the reduction in energy con-sumption per day is as high as 18%.

As mentioned in Section 2, if the narrow DC bus voltagein Fig. 5 is set close to LED string voltage, then compact,power efficient, luminaire integrated linear regulator basedLED drivers can be used. A lab test prototype of 7 W LEDdriver is built to verify the concept. The photograph of theprototype is shown in Fig. 17. Supertex’s high voltage lin-ear regulator IC (LR8) has been used as constant currentsource to realize the circuit. Since, LR8 output is currentlimited, four such ICs has been used in parallel to meetthe requirement LED current. An efficiency of 96% isachieved. The whole circuit can be realized on silicon assingle chip and can be sandwiched between lamp and heatsink. It is also possible to design the drivers to suit DC grid

R. Panguloori et al. / Solar Energy 97 (2013) 405–413 413

voltage and to adopt for a particular application. Replac-ing efficiency figure of compact LED driver in Table 4,the overall system efficiency will be around 90% and simi-larly PCF will become 1.1. The energy reduction per dayat full load will be around 25%. Linear LED driver hasnot been included in the system as there is issue of efficiencyin dimming mode. And our load profile pattern needs bothi.e. dimming as well as full brightness. We are working onthis aspect also.

6. Conclusions

In this paper, various centralized power distributionarchitectures are compared in terms of their system effi-ciency and complexity for street lighting application. Anarrow DC voltage bus architecture is proposed to simplifythe LED driver topology and to improve system efficiencyand reliability. Mathematical treatment to identify the effi-cient distribution network range for various voltages is pre-sented. For the street light specification considered in thispaper, the effective distribution length is found as1.56 km/1.74 km/2.88 km at 220 V AC/220 V DC/380 VDC respectively.

A more practical comparison is given between 220 V ACand 220 V DC systems on overall system efficiency. WithDC operation, the overall system efficiency can beimproved by 13% and by 17% during rated load and dimload respectively. And with incorporation of linear driverin narrow bus voltage DC grid, the possibility of reductionin energy consumption per day is as high as 25%. The pres-ent work is also useful to system designer as MATHCADtreatment of analytical equations provide useful tool todetermine distribution losses more accurately. More suchinnovative ideas dealing with reliability, safety issues, faultanalysis etc. will be required in future to make solar systemaffordable to larger masses.

Acknowledgements

The authors would like to acknowledge and thankSudheer Kumar for his valuable input on technical specifi-cations of present decentralized solar street lighting system.We like to thank Joyson Patil for his support in develop-ment, installation and testing of centralized DC system.We also thank Prof. Bhim Singh and IIT Delhi for agreeingto academic collaboration. At last we also acknowledgePhilips Innovation Campus for providing valuable supportand Dr. Narendranath Udupa & Geetha Mahadevaiah fortheir encouragement.

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