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Photovoltaic-grid connection in the UAE: Technical perspective

Ammar M. Al-Sabounchi*, Esmaeel Al-Hammadi, Saeed Yalyali, Hamda A. Al-Thani

National Energy and Water Research Center, Abu Dhabi Water & Electricity Authority, Abu Dhabi, United Arab Emirates

a r t i c l e i n f o

Article history:

Available online 10 February 2012

Keywords:

Utility-interactive PV System

Distributed generation

Distribution feeder

Dust deposition

a b s t r a c t

Connection of utility-interactive PV generators at the distribution level, namely PV distributed generation

(PVDG), could bring many benets to the distribution network. However, deployment of PVDG systems,

in any country, requires actual data on the performance of these systems under actual weather condi-tions. Additionally, it needs compliance with the electrical structure and regulations of the power

distribution network in that country. Hence, applying PVDG technology in the UAE brings forth many

considerations and this work aims at tackling potential technical ones. Among these is the role of daily

load curve and PV production curve in determining feasible locations and capacities of PVDG systems.

The analyses are based on existing case study feeders at the 11 kV level of Abu Dhabi distribution

network. Accordingly, the work results in suitable recommendations on feasible locations of PVDG

systems. Also it denes rational objectives and constraints for optimal sizing and location of such

systems.

The other consideration tackled in this work is the performance of PVDG systems in actual UAE

weather conditions. Actual data from two pilot PVDG systems installed in Abu Dhabi are collected and

analyzed. The production of PV array, consistency of voltage and frequency and the conversion efciency

of PV modules and inverters along with the impact of ambient temperature are considered. In the same

connection, the inuence of accumulated dust deposition on the production of PV array in UAE is also

taken over in this work.

2012 Elsevier Ltd. All rights reserved.

1. Introduction

Knowing that the daily average solar insolation in the UAE is

topped to around 6 kWh/m2 makes the sense for the country to go

for solar energy systems over other renewables. Regarding solar

energy systems, interfacing of photovoltaic (PV) systems with the

utility grid at the distribution level, so called PV distribution

generation (PVDG), has been drawing considerable attention in the

UAE. As in other distribution generation (DG) systems, the PVDG

can bring many benets to the network. These may include

reduction of line losses, improvementof voltage prole, postponing

network upgrade, improvement of system reliability, and reductionof global warming concerns[1].

However, connection of PVDG systems (and any other DG) may

bring some concerns to power distribution companies. One of the

main concerns is that the PVDG systems deviate from the tradi-

tional concept of hierarchical power ow from the substation to

consumers. This mayresult in power ow in the upstream direction

that may disturb the automatic voltage regulators[2]. As for power

quality concern, the PVDG system shall inject reliable electricity of

voltage and frequency within the acceptable limits [3].

This work aims at brieng potential outcomes of research

activities conducted, by the National Energy and Water Research

Center (NEWRC), to investigate the performance of PVDG systems

in the UAE. It is worth mentioning that the UAE power grid is

mainly supplied by thermal/gas power plants generating three

phase electricity at 50 Hz frequency. The generated electricity is

connected to the transmission lines at 132 kV, 220 kV and 400 kV

voltage levels via step up transformers. The transmission lines, in

turn, provide the distribution network at 33 kV and 11 kV voltage

levels via step down transformers. At this stage, the electricity isdistributed through 11 kV underground and overhead feeders

supplying three phase 11/0.4 kV distribution transformers.

Based on above, the conguration of distribution feeders along

with the trends of load and solar irradiance curves are investi-

gated fundamentally in this work. The performance of PVDG

systems under actual UAE weather conditions is also tackled. Two

pilot systems, implemented by NEWRC in Abu Dhabi at the 0.4 kV

level, are monitored and evaluated for this purpose. The outcomes

of these systems are believed valuable in generating solid

recommendations for feasible installation of PVDG systems in

the UAE.* Corresponding author.

E-mail address: ammar.munir@adwea.ae(A.M. Al-Sabounchi).

Contents lists available atSciVerse ScienceDirect

Renewable Energy

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m/ l o c a t e / r e n e n e

0960-1481/$ e see front matter 2012 Elsevier Ltd. All rights reserved.

doi:10.1016/j.renene.2012.01.070

Renewable Energy 49 (2013) 39e43

mailto:ammar.munir@adwea.aehttp://www.sciencedirect.com/science/journal/09601481http://www.elsevier.com/locate/renenehttp://dx.doi.org/10.1016/j.renene.2012.01.070http://dx.doi.org/10.1016/j.renene.2012.01.070http://dx.doi.org/10.1016/j.renene.2012.01.070http://dx.doi.org/10.1016/j.renene.2012.01.070http://dx.doi.org/10.1016/j.renene.2012.01.070http://dx.doi.org/10.1016/j.renene.2012.01.070http://www.elsevier.com/locate/renenehttp://www.sciencedirect.com/science/journal/09601481mailto:ammar.munir@adwea.ae

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2. Methodology

Six 11 kV distribution feeders supplying residential, industrial

and commercial load demands in Abu Dhabi are taken as casestudies. Those consist ofve underground (UG) and one overhead

(OH) feeders, inside the city and at suburbs, representing the

prevalent types of 11 kVfeeders in Abu Dhabi. The data of feederare

collected and analyzed in terms of physical structure and load

demand. Additionally, the daily average rates of solar irradiance in

Abu Dhabi are provided. On performance evaluation aspect, the

two pilot PVDG systems mentioned above are installed in Abu

Dhabi at ground and rooftop levels.

2.1. Physical structure

The 11 kV feeders and laterals branching from them in Abu

Dhabi are extended in three phases. They are considered in

balanced mode operation by the distribution company. Conse-quently, the feeders are usually extended in three wires (Y or D)

with no return bath for the neutral. Based on the six case studies,

the capacity of the 11/0.4 kV distribution transformers along 11 kV

feeders is in the range of 0.2e1.5 MVA. At the same time the

installed capacity of the feeders is in the range of 10e20 MVA. The

11 kV feeders are usually extended in XLPE underground cables of

wire sizes in the range of 185e300 mm2. The available overhead

feeders are usually found in the suburbs with wire size around

95 mm2 carried on crossarm poles.

2.2. Load demand

The daily demand proles at the input of the feeders were

collected from the power distribution company for four seasonal

days reecting the power consumption trends of the four seasons.For each feeder the active power demand (MW) at each time

interval is represented in per unit of feeder installed capacity

(MVA). The process is repeated for the four seasonal days with the

results are depicted inFigs. 1e3.

The load curves ofFig. 1shows that the highest load demand in

Abu Dhabi is in summer, which is very normal due to the high

cooling load. At the same time, the difference between summerand

winter demands is considerably high. This is due to the absence of

cooling load in winter, also the moderate ambient temperature in

winter drops the need for real heating load.

As for peak load demand (PLD) and peak load time (PLT), Fig. 4

summarizes the results of the feeders over usual summer day.

According to thegure the average of the highest PLD -in summer-

is around25% of feeder installed capacity. This could be an indicatorfordistribution load relief in Abu Dhabi and consequently long time

before network upgrade is needed.

Additionally, Fig. 4 shows that PLT most likely occur at late

daylight hours in Abu Dhabi, or even after nightfall like in feeders

4-UG & 5-UG. Not to mention that the average PLT of the six feeders

is found to be at 5:30pm.

2.3. Solar irradiance

The daily solar irradiance curves, in per unit of 1000 W/m 2, are

illustrated in Fig. 5 over four usual seasonal days. The gure is

plotted based on actual measurements collected from the two pilot

PVDG systems. Since the PV power production is directly

Fig. 1. Per-unit daily summer load curves.

Fig. 2. Per-unit daily winter load curves.

Fig. 3. Per-unit daily spring & fall load curves.

Fig. 4. PLD and PLT of feeders in summer day.

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of PV surplus power will be produced in winter resulting in

unpreferably powerow in the upstream direction. In this course,

Table 1yields that the average PLD of feeders in winter is around

47% of that in summer.

To have an idea about the space area corresponding to PVDG

system sizing, the multi-crystalline silicon PV modules are taken asexample. According to datasheets, the approximate area required to

produce 1 kWp at standard conditions is around 7.5e8.0 m2. Asthis

area is not small, it is deemed rational that prospective PVDG

systems in Abu Dhabi are most likely t at no higher than the 11 kV

level.

3.1.3. Voltage constraint/benet

Connection of PVDG system on distribution feeder reduces the

current ow from the substation up to the point of common

coupling (PCC) resulting in improvement of voltage prole. This

benet is secured as long as the PVDG system capacity is sized in

compliance with the sizing constraint avoiding any surplus PVDG

power generation. Otherwise, the surplus power in the upstream

direction could be high enough that raises the voltage at some

downstream nodes to unacceptable values. This may affect the

performance of automatic voltage regulators that rely on the trend

of decreasing voltage prole along the feeder[2].

3.2. Performance evaluation

For credible performance evaluation of PVDG systems in the

UAE, two pilot systems rated at 36 kW and 9 kW were installed in

Abu Dhabi at the 0.4 kV level. Many parameters are being evaluated

through these systems including production of power, consistency

of voltage & frequency, efciency of system components, reduction

of CO2emission, impact of dust deposition, and others. This section

represents selective synthesized data, taken from the 36 kW

system, on some potential performance parameters. Out of the 13

multiple inverters of the system, inverter 9 and the PV strings

supplying it are taken as example.

Fig. 6depicts the dc power production and efciency of the PVstrings at different irradiances. Also Fig. 7 shows the impact of

ambient temperature on PV modules efciency. Mind that the

measurements were taken in June 6th, 2009 to show the worst

possible status in summer. Thegure manifests the inverse relation

between ambient temperature and efciency that raises the

maximum efciency to around 13% at 7am. Based on the

same gure, the daily average efciency during daytime hours is

around 11%.

Similar evaluation is conducted to inverter 9 at the same day,

with the results shown in Fig. 8. The gure shows that the daily

average operation efciency of the inverter is topped to 93%. Also it

manifests a sort of consistent efciency, even with the variations of

dc input power.

As for power quality items, Fig. 9 shows high consistency involtage and frequency of the ac power delivered by inverter 9.

According to the gure, the average operating voltage and

frequency are 237.19 V and 49.97 Hz. This means only 1.17% and

0.04%, respectively, below standard nominal values.

Respecting daily operating time of the inverters, Fig.10 indicates

the daily monthly average hours of inverter 9 measured over the

year of 2009. It shows relatively long operating time extend to 13 h

daily in June but drop to around 8.5 h in December; means 10.75 h

daily annual average of around.

Coming to the inuence of accumulated dust deposition on the

performance PV modules, Fig. 11 shows serious inuence on the

production of the PV system. The gure shows the drop in power

production of PV modules after they had been left without cleaning

Fig. 8. Conversion efciency of inverter.

Fig. 9. Operating ac voltage & frequency of inverter.

Fig. 10. Daily monthly average of invertersoperating hours.

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for six consecutive months. To be more specic, the powerproduction of inverter 9 was measured on daily basis at irradiance

800 W/m2 with the PV modules been cleaned once at the beginning

of the month. The procedure was repeated for three months with

the results illustrated inFig.12. According to the gure, the average

monthly drop in PV power production, due to accumulated dust

deposition, is around 27%. This can provide suitable guidance

compromising between imposed cleaning frequency, on one side,

and the extra PV modules to compensate the impact of dust

deposition, on the other side.

4. Conclusions

The PV utility-interactive, especially PVDG systems, is promising

and highly recommended technology in UAE. In this course, it is

recommended that prospective PVDG systems are installed at no

higher than the 11 kV level. This is according to the sizing and

spacing constraints of PV arrays along with the load demand trends

of six case study feeders representing the most likely 11 kV feeders

in Abu Dhabi.

In the same connection, if the size of the PVDG system is less

than the PLD delivered by the 11/0.4 kV transformer at the PCC,

then it is recommended to connect it at the low tension side of

0.4 kV. Additional loss saving will be gained this way across the

windings of the transformer. The PVDG system could be even

connected after the circuit breakers of the low tension side if its size

is within the ampacity of those breakers. On the contrary, if the

capacity is higher than the average maximum limit recommended

in Table 1, then it is suitable to distribute it in separate PVDG

systems at different 11 kV feeders. Alternatively, it is recommended

to be installed on next higher voltage level of the grid, considering

availability of space area.

In the same connection, it seems rational to rate the size of

PVDG systems based on winter load demand. This practice is

deemed suitable for hot summer regions like in UAE and the GCC

countries. As a matter of fact it may avoid, to great extent, the

possibility of surplus PV power production and consequent reverse

power ow during the whole year. To this end, it is recommended

to rate the PVDG system capacity based on the average maximum

limit of winter inTable 1.

As for space area, it seems suitable interfacing the PVDG systems

with feeders at the suburbs where the space concern is less than

inside the city. Also suburb feeders are usually extended over

longer distances facing more voltage drop problems. Connection of

DG systems, including PVDG, at feasible sizing and location is ideal

solution for improving the voltage prole of such feeders.

On different note, the accumulated dust deposition seems to be

serious concern affecting the production of PVDG systems. The

impact of dust deposition on the power production of PV moduleswas illustrated inFigs. 11 and 12. This may provide suitable guid-

ance for PV designers in dealing with the matter. This may impose

suitable cleaning frequency of PV modules and adding extra

modules to compensate the reduction in PV production between

cleaning times.

Last but not least, the PVDG systems are expected to show good

performance in terms of operating voltage and frequency along

with conversion efciency of inverters. Additionally, the high

ambient temperature in Abu Dhabi seems to be of moderate effect

on the efciency of PV modules.

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Fig. 12. Monthly power prole with dust deposition.

Fig. 11. Impact of six months dust deposition.

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