Clean Techn Environ Policy (2012) 14:521–533
DOI 10.1007/s10098-011-0411-z
REVIEW
Survey of grid-connected photovoltaic inverters
and related systems
N. A. Rahim • R. Saidur • K. H. Solangi
M. Othman • N. Amin
•
Received: 23 February 2011 / Accepted: 4 August 2011 / Published online: 19 August 2011
Springer-Verlag 2011
Abstract Grid-connected photovoltaic (PV) inverter
technology has advanced since it first attracted the attention
of policy makers. The objective of this article is to present
a survey of grid-connected PV inverters and their present
technology in Malaysia. Surveyed here are 186 PV inverter
products from 22 manufacturers, their power factors, system THDs, efficiencies, power inputs (from PV array),
costs, sizes, and protections investigated, and their datasheets collected from various manufacturers in Europe,
North America, and Asia. The data are compared within
regions and subject headings. Discussed are the lowering of
cost, the raising of efficiency levels, and reliability
improvement.
Keywords
PV policy
Inverter PV system PV technology Introduction
Solar energy has become the most popular source of
renewable energy, expected to produce by 2050 about 30%
of global energy (Ruoss et al. 2009; Bugutekin 2011). It is
clean, easy to harvest, environment friendly, and free. Also,
unstable fossil-fuel price and concern about global warming have drawn attention to alternative-energy electricity
N. A. Rahim M. Othman N. Amin
University of Malaya Power Energy Dedicated Advanced Centre
(UMPEDAC), Level 4 of Engineering Tower, Kuala Lumpur,
Malaysia
R. Saidur K. H. Solangi (&)
Department of Mechanical Engineering, Faculty of Engineering,
University of Malaya, 50603 Kuala Lumpur, Malaysia
e-mail: solangi.quest@gmail.com
(Saidur et al. 2007a, b),Taufiq et al. 2007 making renewable energy a solution to be considered (Saidur et al.
2010a; Ralegaonkar and Gupta 2010; Taleghani et al.
2010) as, e.g., about 89 Petawatts (89 9 1015 W) of sunlight strikes the earth but only about 15 Terawatts
(15 9 1012 W) (which is only 16%) of it are used (Smil
2006). PV cells can capture solar energy and convert it into
electricity, thus solar energy technology (known also as
solar PV technology) is essential to every country. A PV
system is connected to the grid by an inverter, which
converts the DC power generated from PV modules to the
AC power used in ordinary power supply of electrical
equipment. The inverter is therefore an important element
in grid-connected PV systems.
PV inverter technology has grown rapidly over the past
few decades, in line with PV development in Europe, US,
and Japan; Malaysia has a considerable number of gridconnected PV installations such as small-scale buildingintegrated systems (MBIPV 2009; Camdali 2010
Photovoltaic prices have dropped 50% over the decade,
and efficiency and reliability improved; in future, new,
reduced cost-to-efficiency inverter designs can be expected
in the market (Abella and Chenlo 2004). In Malaysia, on 11
May 2001, in a bid to encourage and intensify the use of
renewable energy in power generation, the government
launched the Small Renewable Energy Power Program
(SREP) (Saidur et al. 2010a; Newton 2007; Demirbas
2010). Small, renewable energy-generating power plants
can apply to sell electricity to utilities through the grid
distribution system, as per renewable energy agreement
between renewable energy producers and power utility
companies (Saidur et al. 2010b; Zgorena 1741; Saidur et al.
2009) included are all types of renewable energy, including
biomass, biogas, municipal waste, solar, mini-hydro, and
wind (Saidur et al. 2010b; KTAK 2009; Demirbas 2007;
123
522
N. A. Rahim et al.
Jayed et al. 2009. To maximize implementation effectiveness, this survey was designed so that useful and sufficient
information on PV design specification efforts could be
provided (Knier et al. 2009).
Over-load current/voltage
Polarity-reversal protection
•
International standards
Safety, equipment, and grid
•
Further information
Methodology
The survey was done among 186 PV inverter products
from 22 manufacturers. The inverters surveyed were divided into two power ranges: \10 kW and [10 kW. Data on
their performance and prices were collected through a
website questionnaire in a survey done among companies
and suppliers, and from the ‘‘Photon’’ international magazine. The data were then analyzed according to their geographical locations (North America, Europe, and Asia
Pacific); see Fig. 1.
Analysis was done through a summary of all the manufacturer data collected, and according to the following
criteria:
•
AC/DC analysis
Applicable grid conditions (voltage, frequency, and
phase)
Inverter power ratings
Harmonic current
Power factor
Conversion efficiency
Inverter control
•
Research instrument
A series of questions on PV inverter was emailed to
manufacturers of inverters. Those who responded supplied
all the requested data.
Statistical method
Design concept
Switching concept
•
Operating environment
Cost of inverter systems
Size and weight of inverter systems
Displays
Warranty
Cooling method
Protective functions
Isolation between AC and DC
AC/DC protective functions
Transient over-voltage protection
Islanding protection (Wikipedia et al. 2009)
Fig. 1 Flow chart of survey
methodology
Finding
Data
Website,
Questionnaire,
Photon
International
Analyze data
Statistical analysis was used to extract from the manufacturer datasheets, quantitative information such as population. The data were then grouped according to parameters
such as AC/DC analysis, inverter design, price, and protection. The AC/DC analysis included power rating, efficiency, power factor, etc. If the quantity of the same
subject varied, average values were used. To indicate price,
mean values were used, calculated through the following
equation (Table 1 gives an example of the calculation):
n
1 X
x ¼ xi
ð1Þ
n i¼1
where n is the number of sample and xi is the sample value
Result findings and discussion
The data gathered were summarized according to the
information provided by manufacturers of the PV inverters.
Technical data were grouped into DC/AC part, efficiency,
design concept, and ambience. Analysis of the main segments was based on power ranges: up to 10 kW (\10 kW),
and above 10 kW ([10 kW). Analysis of the data was
based on geographical sub-segments including North
America (including Canada), Europe, and Asia Pacific, as
follows:
(i)
Summarize
Graph, chart,
Table
123
North America
•
•
Kaco (USA)
Xantrex (Canada)
Survey of grid-connected photovoltaic inverters
Table 1 Average price of PV
inverters
523
Power rating (kW)
Price (USD)
Ave price (USD)
Price/kW
3
2153
1951.39
650.46
Average price/kW
1075.43
1332.16
2369.00
1
555.08
555.08
555.08
2
1750
1305.70
652.85
2951.65
894.44
3778.16
1079.47
2446.21
978.49
3606.67
901.67
1178.5
1094.28
1200
3.3
2543.31
2650.82
2543.31
2650.82
4370
3.5
5320
2585.95
4637.68
2569.00
2.5
4070
2050.95
1294.90
2369
4
2835
2665
5320
4.1
5320.00
5320.00
1297.56
4.2
5001.00
5001.00
1190.71
4.5
2955.00
2955.00
656.67
4.6
3530.95
3968.27
862.67
3275.89
3412.58
3275.89
3412.58
5480
5390
4.8
3485.95
3485.95
726.24
5.1
3085.00
3085.00
604.90
5.2
3590.95
3590.95
690.57
6
3717
3189.252
531.54
1878.30
1138.37
1075.43
2661.50
1.65
1836.84
1919.77
1836.84
1919.77
1.60
2587.78
2587.78
1617.36
1.80
3370
3370
1872.22
0.50
0.70
800.44
1151.95
800.44
1151.95
1600.88
1645.64
1.30
3074
3074
2364.62
1.50
1200
1720.67
1147.12
123
524
N. A. Rahim et al.
•
•
(ii)
PV Powered (USA)
Outback (USA)
Europe
•
•
•
•
•
•
(iii)
SMA (Germany)
Sunways (Germany)
Fronius (Austria)
Mastervolt (Netherlands)
Danfoss (Denmark)
Sunset (Germany)
Asia Pacific
•
•
•
•
•
•
•
•
Sharp (Japan)
Mitsubishi (Japan)
Sanyo (Japan)
Sungrow (China)
Seashine Forest (China)
Green Giant Tech (China)
Carspa (China)
Leonics (Thailand)
Figure 2 is a breakdown of the manufacturers of the
PV inverters surveyed. European countries constitute the
major PV producers. The dominant brands are SMA
(Sunny Boy) in Europe, Xantrex in North America,
Mitsubishi in Japan, and Sungrow in China. Figure 3
shows the distribution of AC (output) power rating for
the inverters surveyed; the two highest numbers of
inverters registering in the 2.5–3.5 kW and the
4.5–5.5 kW ranges.
Switching devices and control circuit
High-frequency switching by semiconductor switching
device is an essential element in PWM inverter control.
Line-commutated inverters, based on thyristors, were at
first developed for the first grid-connected applications,
later replaced by self-commutated inverters that use insulated gate bipolar transistors (IGBTs), or field-effect
Asia
22%
Europe
50%
North America
28%
Fig. 2 Regional distribution of the PV inverters surveyed
123
transistors (FETs), in low-power applications. Currently,
popular switching devices in performing high-frequency
switching are IGBT, Metal Oxide Semiconductor Field
Effect Transistor (MOSFET), and HERIC (Highly Efficient & Reliable Inverter Concept), a new inverter topology with innovative switching, implemented by Sunways,
its efficiency greater than that of other PWM controls
(Sunways 2009).
Survey of the 186 products showed 102 products having
information about the switching concept. For instance,
about 52% used IGBT, 9% used MOSFET, 33% used both
IGBT and MOSFET, and 6% used HERIC. For power
rating[10 kW, 88% used IGBT, 10% used both IGBT and
MOSFET, and 2% used HERIC; see Fig. 4.
The findings show the IGBT switching frequency to
be around 20 kHz, and therefore IGBT can be used for
medium-to-high-power inverters exceeding 100 kW. In
contrast, the switching frequency of MOSFET is
30–32 kHz and up to 800 kHz at higher frequencies, but
its power capacity is low. The survey also shows that
IGBTs with a low switching frequency (e.g., 6, 15, and
16 kHz) are usually used for inverters of [10 kW rating.
For the output power range 1 to 10 kW, the switching
frequency is 20 kHz. Both IGBT and MOSFET can thus
be used. High-frequency switching can thus be said to
reduce an inverter’s output-current harmonics, size,
and weight (Ralegaonkar and Gupta 2010; Ishikawa
2002)
Operational AC/DC analysis
Operational AC voltage and frequency range
Operational AC voltage is the output of a PV inverter, fed
into the utility grid. Voltage and frequency generated
from the inverter are to operate smoothly, matching the
required voltage and frequency of the utility grid. Standard voltages and frequencies of a single-phase circuit
vary with region: 230 V and 50 Hz in Europe, 101/202 V
and 50/60 Hz in Japan, and 120/240 V and 60 Hz in
USA. Also, the standard voltage and frequency for a
three-phase circuit are 380/400 V and 50 Hz in Europe,
202 V and 50/60 Hz in Japan, and 480 V and 60 Hz in
the USA. At these values, the inverter can be operated
substantially without problem and within tolerances of
?10% and -15% for the voltage, and ±0.4 to 1% for the
frequency (Taufiq et al. 2007; Ishikawa 2002) Manufacturers must provide the information as there are differences in voltage and frequency of utility grids depending
on country. The survey shows the three frequencies (50,
50/60, and 60 Hz) determining which country can use
which type of PV inverter.
Survey of grid-connected photovoltaic inverters
Fig. 3 Distribution of power
ratings (AC Output) of the
products surveyed
525
28
30
26
Number
25
21
20
13
15
10
7
4
5
4
1
0
2
9.5
-1 0
.5
8.5
-9 .
5
7.5
-8 .
5
6.5
-7 .
5
5.5
-6 .
5
4.5
-5 .
5
3.5
-4 .
5
2.5
-3 .
5
1.5
-2 .
5
0.5
-1 .
5
0
Power Rating (AC output) in kW
Fig. 4 Switching concepts
of the products surveyed
Power rating < 10kW
IGBT & FET
33%
Power rating > 10 kW
HERIC
6%
IGBT
88%
IGBT
52%
FET
9%
HERIC
2%
IGBT & FET
10%
Operational DC voltage range
Applicable PV array power
Operational DC voltage is the input voltage at the DC side,
fed from the PV array. Inverters thus differ in their range of
operational DC voltage, according to their rated power and
the grid’s rated voltage. Table 2 shows the DC voltages for
a range of rated power capacities. Clearly, the wider the
voltage range, the more voltage the inverter can accept
from a PV array/string, allowing various ranges of voltages
from the PV array/string to be connected to the inverter.
Figure 5 is the survey result for ratio of applicable rated
power of PV arrays to rated output power of inverters.
Ratios of the PV array’s average output power to the
inverter’s average rated output power are estimated to be
1.06 (\10 kW power rating) and 1.04 ([10 kW power
rating). These, however, are unconditional, as the array
output powers differed according to conditions (latitude,
angle of inclination of module, etc.) at the area where the
Table 2 Ranges of the PV
inverter DC voltages versus
their power ratings
Inverter rated power (AC)
Operational DC voltage range
\1 kW
1–5 kW
5–10 kW
[10 kW
40–130
100–500
180–480
150–400
150–700
330–550
125–200
100–600
195–550
180–380
165–300
330–600
110–350
200–380
240–450
200–780
333–500
125–400
140–450
200–400
200–450
240–450
350–600
210–420
220–380
350–600
400–1150
150–380
200–780
80–350
230–500
400–780
150–400
205–450
250–480
250–600
400–800
150–450
212–380
300–650
250–800
420–800
150–450
250–480
250–480
270–600
420–850
150–600
300–650
350–600
300–480
430–800
160–650
350–600
220–380
300–600
450–800
165–300
80–350
300–650
450–820
180–380
123
Normalised PV rated power kWp/kW
526
N. A. Rahim et al.
because avoiding disturbances (such as voltage fluctuation)
to the power system is important. The power factor was
seen to increase if THD decreased (see Fig. 6), achievable
via current-control scheme in the inverter circuit (a simple
control circuit and transient current suppression allow high
power factors) (Kissell 2006).
1.40
<10kW
>10kW
1.20
1.00
Inverter conversion efficiency
0.80
0.1
1
10
100
Inverter Power Rating (kW)
1000
Fig. 5 Distribution of rated powers of the PVs
PV power systems were installed (Knier et al. 2009; Rahim
et al. 2010).
AC harmonic current from inverter
In terms of AC harmonic current from inverter, the findings
of this survey show the Total Harmonic Distortion (THD)
range to be less than 5%, which translates to low harmonics
and good performance (Haeberlin 2001; Mekhilef et al.
2007). The values that remain low resulting from present
inverter technology which uses PWM control scheme to
control output waveform can reduce (with fewer problems)
an inverter’s harmonic current, because even with a few
harmonics, a grid inverter can be operated without any
problems, avoiding disturbance to the grid line (Kissell
2006; Mahlia et al. 2005). Note that THD of current generated by a PV inverter is regulated by the International
IEC 61000-3-2 Standard.
Power factor
The survey looked at power factors, which ranged from
0.98 to 1; the former is considered good as long as the
inverter’s AC output power factor is not let to drop,
Fig. 6 Current THD versus power factor
123
Inverter efficiency is defined as the relationship between
PDC and PAC, used to measure a PV inverter’s performance. According to the maximum/peak efficiencies and
the European and Californian (CEC) efficiencies that the
manufacturers provided in their product datasheets, overall
maximum efficiencies of the inverters surveyed showed
their distribution ranging from 90 to 97%; none was less
than 90%, or higher than 98%. Inverters with efficiencies
ranging from 94 to 96% were found to be the highest in
number for both the maximum and the European efficiencies. Figure 7 breaks down the efficiencies of the products
surveyed, showing the highest number of inverters to be in
the 95% to 96% efficiency range, followed by the 94 to
95% and the 96 to 97% ranges.
Figure 8 shows inverters from European countries having the highest average efficiency (95.25%), with North
American inverters coming in a close second (95.11%) and
Asian inverters next (94.43%). Interestingly, the highest
average efficiency of the Chinese inverter Sungrow is
98.5%, as is the highest average efficiency of European
inverters manufactured by SMS (also 98.5%), and as is the
highest average efficiency of North American inverters
manufactured by Solaron (almost the same as that of EU
countries) (Saidur et al. 2009).
Figure 9 further breaks down efficiency according to the
inverters’ isolation design. The maximum efficiency appropriated with the isolation type; among the high-frequency
transformer, low-frequency transformer, and transformerless
types, the transformerless has the highest efficiency.
Figures 10a and b are breakdowns of the inverter efficiencies of two power ratings. The findings show that
inverters from European countries have the highest efficiency (96.3%, low-frequency transformer), whereas the
ones from Asian countries have the lowest efficiency
(91.17%, high-frequency transformer). For transformerless
type, the highest efficiency is 98.37% (power rating [ 10 kW). There are no low-frequency transformer
data from North America for each of the power ratings
because low-frequency-transformer isolation is no longer
popular in North American market (it enlarges an inverter
system) (Mahlia et al. 2011). Power loss in a transformer
affects an inverter system’s efficiency entirely; transformerless inverter systems can thus be said to be the most
efficient.
Survey of grid-connected photovoltaic inverters
Fig. 7 Survey distribution of
the maximum and the European
efficiencies
527
Max and Europen Efficiency distribution survey
35
Max efficiency
Number
30
EU efficiency
25
20
15
10
5
0
<90
90-91
91-92
92-93
93-94
94-95
95-96
96-97
97-98
>98
Maximum efficiency %
European Efficiency
Max Efficiency
95.5
95.25
95.11
Efficiency (%)
95
94.81
94.43
94.5
94.2
94
93.74
93.5
93
Europe
Asia
N. America
Region
Fig. 8 Average maximum and European efficiencies of the products
surveyed
Transformerless
Low Frequency
High Frequency
100
99
97.48
Efficiency (%)
98
IEC 60529 (of the International Organization for Standardization). Figure 11 shows that waterproofing and dust
proofing are required in outdoor installations. The survey
found most of the \10 kW inverters to be suitable for
outdoor installation, subject to 71% dust proofing and
waterproofing. Only 46% installations of inverters
[10 kW required dust proofing and waterproofing. PV
power systems with high power rating for grid interconnection and generally installed in houses are assumed to
be often installed indoors, where dust proofing and
waterproofing are not required. Dust proofing and waterproofing are still not required in some outdoor installation
specifications, so, in such situations, the inverters are
installed in external boxes.
Table 3, listing the maximum and the minimum temperatures of indoor and outdoor inverter installations, show
solar inverters as generally installed outdoors; they can
withstand extreme temperature. Still, inverter system
operation are unsuitable at temperatures above operating
conditions, as overheating, fire, or damaged insulation can
result (Tuladhar et al. 2008; Sanjay Kumar et al. 2009).
97
96.07
95.94
96
Protection function
95.25
94.63
95
Isolation AC/DC
94
93.35
93
< 10 kW
> 10 kW
Power Rating
Fig. 9 Average maximum efficiencies according to isolation type
Operating conditions
Operating conditions such as indoor and outdoor installation specifications, ambient temperature, waterproofing,
and dust proofing, are important flexibility issues. Waterproofing and dust proofing requirements for power ratings
\10 kW and [10 kW are summarized next.
The coding systems for environmental rating of enclosures are NEMA and the IP rating system according to
Figures 12a and b show the distribution of various AC/DC
transformer categories. Figure 12a shows the high-frequency transformers (44%) and the low-frequency transformers (36%) constituting most of the \10 kW PV
inverters. Constitution of the [10 kW inverters (Fig. 12b)
are 54% inverters with low-frequency transformers, 27%
transformerless inverters, and 19% inverters with highfrequency transformers (Wong 2004).
Figures 13a and b segregate the inverters by region.
European inverters with high-frequency transformers in the
\10 kW range are found to have the most transformer
isolation. European inverters with low-frequency transformer in the [10 kW range again have the most transformer isolation.
123
528
N. A. Rahim et al.
Fig. 10 Averages of maximum
efficiencies according to
isolation type, for power rating
a \10 kW, b [10 kW power
rating
(a)
97
Transformerless
96.25 96.3
95.65
96
Efficiency (%)
Low Frequency
94.93
95
High Frequency
95.93
95.13
94.4
94
93
92
91.17
91
90
Europe
N.America
Asia
Region
(b)
Transformerless
Low Frequency
High Frequency
100
99
Efficiency (%)
98
98.37
97.44
97.15
97
96.07
96
95.18
95
94.64
95
94
93
92
92
91
Europe
N.America
Asia
Region
Fig. 11 Breakdown of
installation environments,
power rating a \10 kW,
b power rating [10 kW
(a)
(b)
No Dust
proof, Water
proof
15%
Dust Proof,
Waterproof
46%
Indoor Installation
Temperature
minimum (C)
Outdoor Installation
Temperature
minimum (C)
Minimum
temperature (C)
Maximum
temperature (C)
10
40
-25
50
-20
-25
45
55
-20
-15
55
60
0
60
-10
65
Anti-islanding protection
The survey’s findings suggest most (73%) of the inverters
use the ENS (impedance measurement) anti-islanding
detection method; the remaining used the active or the
123
Dust
Proof,
Waterpro
of
71%
No Dust
proof, No
water
proof
4%
No Dust
proof, No
water proof
39%
Table 3 Maximum and
minimum temperatures of
indoor and outdoor installations
of inverters
No Dust
proof,
Water
proof
25%
passive method. In the passive method, the detection of
islanding depends on grid voltage and grid frequency; rate
of change of voltage and of frequency are detected, and
three-phase voltage drop is monitored. Passive method
could be insufficient in preventing islanding under perfect
Survey of grid-connected photovoltaic inverters
529
High Freq
Transformer
44%
(a)
Low Freq
Transformer
36%
Transformerless
20%
High Freq
Transformer
19%
(b)
Low Freq
Transformer
54%
Transformerless
27%
Fig. 12 Distribution of inverter DC/AC isolations, power rating
a \10 kW, b [10 kW
(a)
Transformerless
High freq transformer
Low freq transformer
16
14
Number
12
10
8
6
4
2
0
Europe
North America
injection, monitoring of phase jumps, harmonics, positive
feedback methods or unstable current, and phase controllers, is suggested (Abella and Chenlo 2004; Ozbalta and
Ozbalta 2010). All the anti-islanding protections surveyed
comply with UL1741, IEEE1547, IEEE-929:2000, AS
4777, DIN VDE 0126.
Other protection functions
All the inverters surveyed had their own AC/DC protection
functions on their DC and AC sides. Protection for the DC
side includes against reverse DC voltage, DC over-power,
over-current, and DC grounding fault. Protection for the
AC side includes against AC over/under-voltages, AC
over-current, frequency increase/drop, and detection of AC
grounding, also detection of any of the direct current
occurring in some transformerless systems (Rajamohan
et al. 2010).
Another function is the detection of temperature rise,
where protection accompanies operation of the inverter
system, and voltage and current are detected and used in
the control circuit, as are information from various types of
sensors. Protection functions are integral with the inverter
system. In an overload, this includes Operating Point
Change (OPC), Power Limit (PL), Power Reduction (PR),
Switch Off (SO), and Current Limit (CL). Figure 14 breaks
down the inverters’ protection behavior when overloaded;
for both power ratings, OPC and PL seem to dominate.
Reverse DC-voltage protection is done by an internal diode
in the inverter circuit. Protection against lightning and
surge voltage are performed by a surge arrester and a
varistor on both the DC and the AC sides.
Asia
Region
(b)
Transformerless
High freq Transformer
Low freq Transformer
25
Number
20
15
10
5
0
Europe
North America
Asia
Region
Fig. 13 Distribution of types of inverter isolation by region, power
rating a \10 kW, b [10 kW
balanced load conditions in both active and reactive powers, so its combination with active method, which is based
on e.g., frequency shift, impedance monitoring by current
Cost of inverter systems
The findings clearly show that an exact average price per
kW for PV inverters is difficult to determine, because an
inverter’s rated power is not the only price determiner, but
factors influencing the cost of inverter production (manufacturing technology, efficiency, performance, size, weight,
reliability, type of isolation design, etc.) are, too. Visual
display, data monitoring, data storage, and communication
capabilities also contribute to price. Figure 15a shows the
average price of inverters as concordant with their power
ratings.
Figure 15b shows the average price per kW of
0.5–6 kW inverters as $1075.43 (i.e., $1.075/W). Inverter
size affects cost, so the price does not fully reflect the
inverter’s improvements. 4.2 kW inverter, for instance, is
about 54% cheaper than a 1.3 kW inverter. Prices of
inverters with high power ratings can thus be stated as
cheaper than those with low power ratings.
123
530
N. A. Rahim et al.
Fig. 14 Breakdown of
protection types
40
OPC,PL
Number
35
30
OPC
25
SO,PL
20
PL
15
PL,PR
10
5
0
<10kW
>10kW
Power rating
Normalised Inverter cost (USD/kW)
35.00
6000
5000
4000
3000
2000
1000
0
0
1
2
3
4
5
6
Rated power (kW)
(b)
2500
Normalize weight (kg / kW)
Average Inverter cost (USD)
(a)
Transformerless
High Freq Transformer
Low Freq Transformer
30.00
25.00
20.00
15.00
10.00
5.00
0.00
0
2000
2
4
6
8
10
12
Rated Power (kW)
1500
Fig. 16 Weights of the inverter systems
1000
Display
500
0
0
1
2
3
4
5
6
Rated power (kW)
Fig. 15 a Average cost of PV inverters. b Average price per kW of
PV Inverters
Size and weight of inverter systems
In PV system installations, an inverter’s size and weight
matter. As stated in isolation protection, the presence or
absence of an isolation transformer in an inverter system is
the main contributor to system size and weight. Manufacturers, however, mostly use transformerless or high-frequency transformer for isolation to reduce the weight and
size of their inverter systems. For now, size and weight are
reduced via the use of a high-frequency transformer,
because high frequency minimizes the size of the cable
used in the transformer, reducing size of both the transformer and the inverter system. Figure 16 is the distribution of the inverter systems, based on their weight (Radzi
and Rahim 2009), the average weight for transformerless
shown as 6.22 kg/kW, for high-frequency transformer as
5.64 kg/kW, and for low-frequency transformer as
14.69 kg/kW.
123
Information about the display’s number of lines, characters, and pixels was found through analysis of the survey
data, which showed the display as being essential to conveyance of information on e.g., system parameters, and to
the monitoring of system functions. The displays are supposedly liquid crystal display (LCD) or LED displays,
some including features such as background illumination
and color display.
Warranty
The findings show that product warranty is important
proof of performance reliability. The manufacturer warranties range from 2 (minimum) to 5 years (maximum).
A few offer 10 years warranty out of the 20 years of a
PV panel’s life. The manufacturers, though, do not
expect a full 20-year lifetime, because an inverter must
be replaced every 10 years (; NREL 2006; Demirci
Sankir et al. 2011) Yuan and Zhang 2006 so its performance is maintained. Any inverter having lived the
20 years of its life would be promising to the future of
building-integrated PV applications (Ozbalta and Ozbalta
2010).
Survey of grid-connected photovoltaic inverters
531
Fig. 17 Sunny Webbox
Data logging, communication, and diagnostic
characteristics
The survey discovered the latest inverters’ features such as
data logging and communication capabilities. Inverter
manufacturers provided the software required to process
inverter-sent data, allowing quick diagnostics when the
system does not operate properly, so the user is able to
adjust to optimal operation, the system parameters.
For data logging, some manufacturers provide memory for
up to 1 GB, and for communication, some provide interfaces
such as RS232, RS485, USB, Bluetooth, GSM, GPRS, and
modem, allowing users to connect the inverter with other
devices such as notebook, controller, and mobile phone.
Figure 17 is an example of an inverter communication device;
it allows users to access their system performance data anytime, anywhere in the world, via a web server (SMA 2009).
Cooling method
Inverter cooling is an important consideration for an inverter’s reliability, long life, and ability to operate at full power
over a wide range of temperatures. The survey’s findings
show that a suitable cooling method is by either a fan or
natural air convection. Inverters with a wide range of
ambient temperatures (-25 to 65C) usually do not use a fan
as its cooling method, but inverters with ambient temperatures between -25 and 45C do. At this point, cooling can be
summarized as depending on the range of ambient temperatures. Of the inverters surveyed, about 56% of power rating
\10 kW and 52% of power rating[10 kW used fans. A fan
cooling system has been suggested as not necessarily having
to be continuously ON but can turn ON automatically when
temperature or power output reaches a particular level,
reducing noise, improving the inverter’s efficiency.
Conclusions
Technology for PV grid inverters has advanced with rapidly increasing world market demand for solar energy in
line with growth of PV. The rise of technologies of recent
PV inverters has brought down price, increased efficiency,
and improved reliability and availability. The PV grid
inverters surveyed generally performed rather well in terms
of conversion efficiency: 95% overall average efficiency, a
power-factor that is close to unity over a wide operating
range, and capable of keeping current harmonic THD at
less than 5%.
Transformer power loss makes transformerless designs
more effective than ones with transformers (whether low,
or high, frequency). Also, a transformer increases size and
123
532
N. A. Rahim et al.
weight of an inverter system, but a high-frequency transformer or a transformerless system can reduce transformer
size and weight, minimizing size and weight of entire
inverter systems.
High power rating has been claimed to lower price. A
warranty of 5 to 10 years seems to be sufficient convincing
of an inverter’s reliability and durability. A PV inverter
must have flexible waterproofing and dust proofing. Features such as data logging, system monitoring, and memory
storage capacity for data acquisition and communication
are preferred considerations. Suitable fan cooling matters
to assessment of inverter lifespan and reliability.
Future work could consider the following:
•
•
•
•
Improve survey ethics, which, when lacking, can cause
difficulties when collecting data.
Use of database system software to make survey more
systematic and user friendly.
Arrange costs of the PV inverters in present survey
against those obtained through previous surveys.
Increase the number of survey respondents.
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