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Impact of Solar Panels on Power Quality of Distribution Networks and
Transformers
Article in Canadian Journal of Electrical and Computer Engineering · January 2015
DOI: 10.1109/CJECE.2014.2359111
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CANADIAN JOURNAL OF ELECTRICAL AND COMPUTER ENGINEERING, VOL. 38, NO. 1, WINTER 2015
45
Impact of Solar Panels on Power Quality of
Distribution Networks and Transformers
Impact de panneaux solaires sur la qualité
des réseaux de distribution et de
transformateurs de puissance
Mohamed A. Awadallah, Bala Venkatesh, Senior Member, IEEE, and Birendra N. Singh
Abstract— This paper presents an investigation on the impact of solar panels (SPs) on the power quality of
distribution networks and transformers. Both solar farms and residential rooftop SP are modeled with the
distribution network according to Canadian Utility data. Total harmonic distortion of voltages and currents
on both sides of the distribution transformer are monitored under different operation conditions. A laboratory
setup employing a single-phase inverter and three-phase transformer is used to test system performance in the
presence of phase unbalance and harmonics. Core and winding temperatures are measured under various loads.
Simulation and experimentation results show that the performance of distribution networks and transformers
under the impact of SPs is within standard limits.
Résumé— Cet article présente une enquête sur l’impact des panneaux solaires (PS) sur la qualité des réseaux de
distribution et de transformateurs de puissance. Les fermes solaires ainsi que les toits avec des PS résidentiels sont
modélisés avec le réseau de distribution basé sur les données canadiennes de services publics. Les distorsions
harmoniques totales des tensions et des courants sur les deux côtés du transformateur de distribution sont
contrôlées sous différentes conditions de fonctionnement. Une installation de laboratoire utilisant un onduleur
monophasé et un transformateur triphasé est utilisée pour mesurer la performance du système de test avec
un déséquilibre de phase et d’harmoniques. Les températures du noyau et d’enroulement sont mesurées avec
différentes charges. Les résultats de simulation et expérimentaux montrent que la performance des réseaux de
distribution et des transformateurs sous l’impact des PS respecte les standards.
Index Terms— Distribution networks, distribution transformers, harmonics, solar panels (SPs).
I. I NTRODUCTION
OLAR photovoltaic (PV) energy is one of the most rapidly
developing renewable sources. Solar cells are made of
semiconductor materials which convert light energy of the sun
into dc electricity. Therefore, the usage of inverters with solar
panels (SP) becomes inevitable before solar power can be used
by local loads or transmitted into the grid. SPs are normally
installed in distribution networks, rather than the generation
or transmission levels of power systems. Both small rooftop
SP installations and large solar farms create voltage harmonics
and inject current harmonics into the distribution network by
the associated inverters. On the other hand, distribution trans-
S
Manuscript received February 27, 2014; revised May 29, 2014; accepted
September 15, 2014. Date of current version March 31, 2015. This work was
supported by Hydro One Networks Inc., Toronto, ON, Canada, through the
Centre for Urban Energy, Ryerson University, Toronto.
M. A. Awadallah was with the University of Zagazig, Zagazig 44516, Egypt.
He is now with the Centre for Urban Energy, Ryerson University, Toronto,
ON M5B 2K3, Canada (e-mail: awadalla@ryerson.ca).
B. Venkatesh is with the Centre for Urban Energy, Department of Electrical
and Computer Engineering, Ryerson University, Toronto, ON M5B 2K3,
Canada (e-mail: bala@ryerson.ca).
B. N. Singh is with Hydro One Networks Inc., Toronto, ON M5G 2P5,
Canada (e-mail: bob.singh@hydroone.com).
Associate Editor managing this paper’s review: Davood Yazdani.
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/CJECE.2014.2359111
formers are subject to second-quadrant operation (i.e., when
the active power flow is reversed) under light load conditions
when SP operate at or close to full capacity.
Although power system harmonics are known to be consequences of nonlinear loads, accurate measurement of voltage
and current harmonics is quite tricky [1]. Tracking down
harmonic sources is also challenging as well as effective
filtering and mitigation techniques [2]. A few publications in
the literature have considered the effects of SP on distribution
networks. Impact of the SP at the Sydney Olympic Village
on accommodating network is addressed in [3], where voltage
and current total harmonic distortion (THD) remain within
standard limits even if all SP operate simultaneously. In a
weak network supplied by SP, the replacement of incandescent
lamps by compact fluorescent lamps—for energy saving—
increases voltage THD [4]. The initial THD of 3.14% can
reach 10.15%, 22.2%, and 34% if 30%, 60%, and 90% of the
lighting load is replaced, respectively. In [5], the effects of
SP on power grids of two small Greek islands are studied and
compared with the case of Diesel generator supplies. Although
voltage THD with SP operation is higher than the Diesel
generator case, yet, it remains under standard limits.
With the combination of linear and nonlinear loads on a
transformer, an expansion of the standard K-factor evaluates
the composite harmonic current [6]. Rad et al. [7] report
0840-8688 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
46
CANADIAN JOURNAL OF ELECTRICAL AND COMPUTER ENGINEERING, VOL. 38, NO. 1, WINTER 2015
TABLE I
C ASES OF U NBALANCED O PERATION
harmonic measurements on six distribution transformers along
with other performance indices. Harmonic effect on winding
eddy current loss is found much more significant than other
stray loss. A laboratory setup is proposed to measure losses of
high switching frequency converter transformers, and compare
with those computed through finite elements [8]. In [9],
voltage and current harmonics caused by various nonlinear
lighting loads are presented, and derating of the distribution
transformer is accordingly proposed. A calculation routine of
the reduced per unit load, at which distribution transformers
maintain full lifetime under current harmonics, is presented
in [10]. Harmonics generated by different nonlinear loads are
measured in the laboratory, and their impact on transformer
losses, temperature rise, and loss of life is studied [11]. The
finite element method is used to calculate the hottest spot temperature through field strength solution at different parts of the
transformer [12]. The change in the temperature rise because
of harmonics is used to estimate the lifetime expectancy and
propose a new loading profile to keep lifetime unaffected.
This paper introduces a study on the impact of SP
and their associated inverters on the distribution network and
transformer. A MATLAB/Simulink model is built for SP and
distribution network according to Canadian Utility data; both
solar farms and residential rooftop SP are considered. Voltage
and current THD are monitored under different conditions.
In a laboratory setup, a single-phase grid-tied commercially
available inverter for solar power applications is used to
feed a three-phase transformer connected to the grid. Losses,
efficiency, and voltage and current THD are measured as well
as core and winding temperatures. Results show that SP do
not have significant harmful impact on distribution networks
or transformer, as long as they keep low relative rating with
respect to the power carrying capacity of the system.
II. S IMULATION R ESULTS
A. Analysis of Solar Farm
A simulation model is built in MATLAB/Simulink for
a solar farm on the basis of Canadian Utility data for the
system shown in Fig. 1. The transmission system is modeled
as 115 kV, 170 MVA, three-phase source. Two 83 MVA,
115/27.6 kV transformers exist within the distribution system
shown as one block in Fig. 1. Bus 1 is the point of common
coupling (PCC), where the solar farm is connected to the
system at this point. The solar farm includes 17 SP of 500 kW
each. One SP is separately connected to a 27.6 kV/265 V,
500 kVA transformer, whereas the remaining 16 SPs are
connected in pairs to eight transformers of 1 MVA each. The
SP module employs maximum power point tracking (MPPT)
Fig. 1.
Solar farm system model.
Fig. 2.
SP model.
via a dc converter, whereas the associated inverter applies
sinusoidal pulse-width modulation (PWM) and has an LCL
low-pass filter (Fig. 2).
It is still practically accepted to have such ratings by considering that solar farms do not usually operate at rated capacity
all the time owing to changes in environmental conditions.
SPs typically give rated output at standard test conditions
(1000 W/m2 solar irradiation and 25 °C cell temperature)
which may not be maintained all the time. On the other
hand, from the simulation-work viewpoint, the SP impact
on distribution transformers becomes more serious as the SP
rating increases. Therefore, simulation results would be more
conclusive in the present case.
The system shown in Fig. 1 is modeled under balanced
and unbalanced conditions with and without capacitors at
the PCC when the solar farm operates at rated power. The
THD of phase voltages and currents on both sides of the
transformer are monitored. In balanced case, system voltages
of the transmission system are equal to rated value and
shifted from each other by 120°. Three cases of voltage
unbalance are considered as shown in Table I. In practice, it
is unacceptable to have voltages more than 1.1 pu. However,
some phase voltage values in Table I are exaggerated to make
sure the impact of voltage unbalance on harmonic distortion is
insignificant. The THD of phase voltages and currents at both
sides of the transformer are given in Table II. Results imply
that phase voltage THD is always less than the standard
permissible limit of 5%, except for phase-B secondary voltage
under unbalanced case #3. However, current THD is mostly
under the standard limit, except for phase-A primary current
AWADALLAH et al.: IMPACT OF SPs ON POWER QUALITY
47
TABLE II
P ERCENTAGE THD OF P HASE V OLTAGES AND C URRENTS U NDER BALANCED AND U NBALANCED C ONDITIONS
TABLE III
P ERCENTAGE THD OF P HASE V OLTAGES AND C URRENTS U NDER D IFFERENT S HUNT C APACITOR R ATINGS AT PCC
under case #3 of unbalanced operation. None of the individual
voltage or current harmonic components exceeds the standard
limit of 3% under any case. The only exception is the fifth
secondary voltage harmonic on phase-B under unbalanced
case #2, which is 3.05%. Results show that waveform distortion is more sensitive to unbalance in voltage magnitudes than
phases. It should be noted that waveform distortion in cases
of unbalance may not be due only to SP. Some phase voltage
magnitudes significantly exceed rated value which could cause
transformer saturation leading to more distortion. Finally, it
could be concluded that voltage and current THD are mostly
within acceptable values with SP operation under balanced and
unbalanced conditions.
The next procedure to remedy the negative impact on
voltage and current waveform distortion is to install shunt
capacitors at PCC to help filter out harmonics. Capacitors
with different VAR ratings are tried; results of new THD
are shown in Table III. It can be noticed that there is no
significant changes in THD values for capacitor rating up
to 8 MVA. At 10 MVA, voltage THD on both sides and
secondary current THD are noticeably reduced. Capacitors at
such rating are likely to assist low-pass filters of SP by
reducing the cutoff frequency. However, for ratings above
14 MVA, all THD increase again, possibly due to transformer
saturation as a result of voltage increase at PCC.
B. Analysis of Residential Rooftop SP
Impact of residential rooftop SP on distribution network
and transformer is then considered. The difference between
Fig. 3.
Residential SP system model.
residential SP and solar farms is not only the output power,
but also the method of integration with the system. Solar
farms are usually located far outside residential areas, and are
normally connected to the grid at PCC using long feeders.
On the contrary, residential SP are located in urban areas, and
are connected to the grid at many points inside the distribution
network. Three-phase inverters are commonly used with solar
farms, and transformers are mostly employed to raise the
voltage up to the distribution level. However, because residential SP are usually less than 10 kW in rating, singlephase inverters are used to connect directly to the 240 V
feeders.
48
CANADIAN JOURNAL OF ELECTRICAL AND COMPUTER ENGINEERING, VOL. 38, NO. 1, WINTER 2015
TABLE IV
P ERCENTAGE THD OF P HASE V OLTAGES AND C URRENTS U NDER D IFFERENT N UMBERS OF R ESIDENTIAL SP
Fig. 4.
Experimental setups. (a) Delta. (b) Star. (c) Star grounded.
A MATLAB/Simulink model is built for the system (Fig. 3)
on the basis of Canadian Utility data. Saturable transformer
modules at 27.6 kV/416 V, 100 kVA are used in the model,
where a maximum of 10 SP are connected to phase A of
each transformer. The model is run for different numbers of
SP and THD of voltages and currents are recorded. Results are
shown in Table IV. Results indicate that harmonics caused by
SP on phase A can intrude into other phases having no SP.
With the increase in SP numbers, voltage and current THD
boost up, but remain within standard limits.
III. E XPERIMENTAL R ESULTS
The experimental work aims at testing the SP impact on
distribution transformers in a laboratory setup. A three-phase
dry-type, /Y, 240/240 V, 10 kVA, 220 °C temperature class
transformer is tested using a single-phase grid-tied, 208 V,
3 kW, commercially available inverter for solar power applications. Three different configurations are used for testing
as shown in Fig. 4. The primary side connected to the grid
could be delta, star, or star grounded, where two terminals of
the secondary winding are connected to the inverter. The dc
source used to feed the inverter can run in current mode to
best represent SP characteristics. The inverter employs MPPT
over two channels that could be separately connected to two
SP or paralleled together to the same one. The active power
flow from the inverter to the grid through the transformer
Fig. 5.
Voltage THD under different configurations. (a) Primary.
(b) Secondary.
is controlled via the dc input. Voltage and current THD are
measured across the transformer as well as active and reactive
powers. Core and winding temperatures are also measured by
two independent thermocouples.
It should be noted that such experimental setup represents
what exists in reality when the load profile on the distribution
network is very low and SP are connected to one phase at
the secondary side of the distribution transformer. It also
represents the case of only one house in a neighborhood having
a rooftop SP connected to the distribution transformer through
a single-phase inverter. Nevertheless, three-phase energization
of the transformer is essential for the balance of the flux in
the core.
As inverter rated power is 3 kW, active power is varied
between 500 W and 2500 W in steps of 500 W; measurements
AWADALLAH et al.: IMPACT OF SPs ON POWER QUALITY
Fig. 6.
Current THD under different configurations. (a) Primary.
(b) Secondary.
are taken at each loading point. Owing to the large thermal
time constant of the transformer, the highest load of 2500 W
is set for about 4 h to reach steady-state temperature. Then,
active power is reduced through the inverter dc input, and
every load power is set for about 40 min. Both core and
winding temperatures are recorded every 10 min. The voltage
and current THD values are plotted against active power as
shown in Figs. 5 and 6 at the grid and inverter sides of the
transformer. The voltage THD is always within the permissible
5% limit. Current THD has a high value close to 30% at light
load; it decreases with loading approaching the standard limit.
Transformer losses and efficiency are plotted for different
winding configurations as shown in Fig. 7. It should be noted
that losses are still much less than rated, as transformer is
lightly loaded. It is evident from the plots that efficiencies are
almost comparable for different configurations, while the delta
connection yields the lowest losses. Temperature variations
are shown in Figs. 8–10 for delta, star, and star grounded
connections, respectively. From such temperature plots, the
heating thermal time constant for core and winding average
out to 104.575 and 67.214 min, respectively. By considering
that the temperature class of the transformer is 220 °C, it is
evident that both core and winding temperatures are well
below the rated limit at all loads. It is true even in cases the
current THD becomes close to 30%. It is believed that current
harmonics add to the winding loss, and hence to the winding
temperature rise. However, because the transformer is lightly
49
Fig. 7. Transformer losses and efficiency under different configurations.
(a) Losses. (b) Efficiency.
Fig. 8.
Core and winding temperature under delta connection.
loaded, overall winding temperature is still within very safe
limits. Although such high current distortion does not have
serious effect on the distribution transformer, its impact on
other network components could be critical. In addition, it is
believed that the reversal of active power flow, which results in
the second quadrant operation, has insignificant impact on the
transformer performance. The temperature plots also show that
both winding and core temperatures are consistently increasing
exponentially as far as the load is kept constant. However, as
50
CANADIAN JOURNAL OF ELECTRICAL AND COMPUTER ENGINEERING, VOL. 38, NO. 1, WINTER 2015
Such voltage is zero under all load levels as the primary
voltage is more influenced by the balanced grid voltage.
In addition, the current flowing from the star point to
ground, in case of star-grounded connection, is given in
Table V, as well.
IV. C ONCLUSION
Fig. 9.
Core and winding temperature under star connection.
This paper presents a study on the impact of SPs on the
power quality of distribution networks and transformers. Solar
farms and rooftop residential SPs are independently simulated
when incorporated into distribution systems. Results show
that voltage and current are mostly within permissible limits
under different conditions. An optimum value of capacitors
connected at the PCC can help the system reduce voltage
and current THD. Results also show that voltage and current
distortion increases as the number of SP inverters connected
to the system increases.
An experimental setup is built in the laboratory to test
a three-phase dry-type transformer when fed by a singlephase grid-tied inverter, and connected to the grid. Voltage
and current THD, losses and efficiency of the transformer,
and core and winding temperatures are all measured under
different SP output powers. Results show that all THD values
are within standard limits, losses and efficiency increase with
loading, and temperature is always well below the rated load
value.
A PPENDIX
Fig. 10.
Core and winding temperature under star-grounded connection.
TABLE V
A SSESSMENT OF E XPERIMENTAL S YSTEM U NBALANCE
load is reduced, winding temperature decreases exponentially.
Whereas, core temperature maintains the same rising behavior
because the supply voltage and frequency are unchanged.
To assess the unbalance of transformer operation, the
standard deviations of line current magnitudes at the
primary (grid) side are computed for different configurations
in Table V. It is clear from such table that the star connection
gives less unbalance than both delta and star-grounded, which
are comparable. It is also obvious that unbalance in line
currents increases with loading for all configurations. The
unbalance is also assessed by measuring the neutral point
voltage with respect to ground in case of star connection.
AWADALLAH et al.: IMPACT OF SPs ON POWER QUALITY
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51
Mohamed A. Awadallah was born in Zagazig,
Egypt, in 1971. He received the B.S. (Hons.) and
M.S. degrees from Zagazig University, Zagazig, in
1993 and 1997, respectively, and the Ph.D. degree
from Kansas State University, Manhattan, KS, USA,
in 2004, all in electrical engineering.
He is currently a Visiting Research Fellow with
the Centre for Urban Energy, Ryerson University,
Toronto, ON, Canada. His current research interests
include motor drives, smart grids, and renewable
energy.
Dr. Awadallah is a member of the Eta Kappa Nu, Tau Beta Pi, and Phi
Kappa Phi.
Bala Venkatesh (SM’08) received the Ph.D. degree
from Anna University, Chennai, India, in 2000.
He is currently a Professor and the Academic
Director of the Centre for Urban Energy, Ryerson
University, Toronto, ON, Canada. He is also a Registered Professional Engineer in the provinces of
Ontario and New Brunswick, Canada. His current
research interests include power system analysis and
optimization.
Birendra N. Singh received the M.Eng. degree
from the Memorial University of Newfoundland,
St John’s, NL, Canada.
He taught Electrical Engineering Courses with
Ryerson University, Toronto, ON, Canada. He is
currently the Manager of the Technology Development with Hydro One Networks Inc., Toronto.
He has over 30 years of diversified experience in
the electric utility industry with Newfoundland and
Labrador Hydro, St John’s, Toronto Hydro, Toronto,
and Hydro One, Toronto. He is also a Registered
Professional Engineer in the province of Ontario, Canada.