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LARGE PHOTOVOLTAIC POWER PLANT DESIGN

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 SEGA 07
th
The 7 International Conference on Sustainable Energy and Green Architecture
Smart City and Urban Resiliency
Bangkok, May 21-22, 2018

LARGE PHOTOVOLTAIC POWER PLANT DESIGN

Snidvongs Suravut1*
1
Department of Engineering, Electrical Engineer, Bangkokthonburi University,
16/10 Leab Klong Thawewattana Rd., Thawewattana Bangkok, Thailand, 10170.
* airscan_t@yahoo.com

ABSTRACT panel, with a nominal rating, say, of about 300-600 VDC,

match to inverter size. Use large inverter, 1 MW, is
The recent increase in demand for solar power expensive, hard to maintenance, not easy for stocking.
systems is due to enhancements in manufacturing Use very small inverter, 300 W, is to complicate, still
crystalline panels, which reduces overall costs in expensive, so many wiring, easy to damage with
manufacturing and increases the efficiency of the lightening and grounding.
PV panels. Additional reasons for the demand in
solar power are: PV technology is proven and STRING INVERTER
reliable, PV modules have warranties exceeding
In a solar panel installation with a conventional,
30 years and government incentives.
central inverter, solar panels are connected in
There are two main requirements for solar series and parallel to form an array, which may
inverter systems: harvest available energy from be considered as a large PV panel, with a
the PV panel and inject a sinusoidal current into nominal rating, say, of about 300-600 VDC. The
the grid in phase with the grid voltage. In order to solar panels array is connected to a single central
harvest the energy out of the PV panel, a inverter in order to convert the generated solar
Maximum Power Point Tracking (MPPT) electricity to alternating current (AC). The usual
algorithm is required. This algorithm determines voltage power 120-180VAC at 60Hz (North
the maximum amount of power available from America) whereas 220-240VAC at 50Hz (Europe
the PV module at any given time. Interfacing to and Thailand).
the grid requires solar inverter systems to abide
With a central (string) inverter arrangement, solar
by certain standards given by utility companies.
panels are connected in a series (string connection)
Photovoltaic type, Field arrangement, voltage with a big disadvantage being that the overall
selection, inverter type selection, electrical system performance is defined by the performance
protection system, lightening protection system, of the system’s bottleneck. That is, the maximum
and grounding system must be designed output performance of the string is defined by the
appropriate to get optimum power and profit. minimum (poorest) performing solar panel depend
on solar panel efficiency. This is a main
To get appropriate optimum performance Solar
disadvantage of solar panel installations may face
cell and inverter size match to distribution
several sources of performance reduction. For
transformer, transmission line, bus bar, and
example, if a solar panel has a manufacturing defect
protection system is critical. This paper shows
reducing its performance level, in string connection
the design of photovoltaic power plant design
with a central inverter, it will affect negatively the
with safe ring.
performance of the whole string, even if the other
INTRODUCTION solar panels are performing to their maximum
power output. The same applies in situations of
Solar panels use sunlight to produce direct electricity partial shading onto a solar panel, from debris, or
(DC). To be able to use solar electricity, in both on-grid snow, or bird droppings etc., affecting the output of
and off-grid solar panel installations, we need to convert the whole string connection. That is why solar panel
direct current (DC) to alternating current (AC); solar cleaning is of ultimate importance, especially if
inverters, Cluster or Micro, are used to make this DC-AC central inverters are used. This is extremely
conversion, thus transform solar electricity in a usable important since solar panels have dramatically
form. They are safer to install, good for solar systems that lower outputs when shaded. Reduced performance
encounter shade, and allow for future system expansion. in solar electricity generation results in reduced
The solar panels are connected in series and parallel to earnings for on-grid installations, especially with
form an array, which may be considered as a large PV FIT’s, leading to longer payback periods and poor

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 SEGA 07
th
The 7 International Conference on Sustainable Energy and Green Architecture
Smart City and Urban Resiliency
Bangkok, May 21-22, 2018

financial performance of a solar panel investment. number of central (string) inverters, in a modular
From conducted research it has been found that a structure, is preferable than using micro-
shade of as little as 9% on the solar panel surface in inverters.
a module can lead to power reduction of the whole
system as much as 54%.[1] MICRO INVERTER
Another disadvantage of string inverters is that Indeed, micro-inverters have brought about a
they normally come at a limited selection of new concept in solar photovoltaic system design,
power ratings; this means that the power rating of with manufacturers claiming an output
the solar panels has to be matched to the power performance increase of around 5-20%. This
rating of the string inverter. For example, an improved system performance can have
array of 20 solar panels with a total power of significant financial benefits in solar PV systems.
4900Wp will need an inverter rated at 4900W+, Since their development, micro inverters have
say 5000W or 5300 W. This leads to system brought about alternative design solutions on
unutilized inverter capacity that you actually pay solar photovoltaic arrays, introducing new
for but you never use. In addition, it places competition to established manufacturers of the
limitations on the option of expansion of the conventional central (string) inverters. A good
solar panel system. For example, in order to add example is Enphase micro inverters that have
more panels, one need to consider keeping the forced established invert manufacturers, such as
match between the power rating of solar panels SMA, to introduce their own micro-inverter in
and the central inverter used. Some additional order to maintain their position in the market.
disadvantages of string inverters include their Following the establishment of both micro and
increased need for cooling and their need of large central (string) inverters as alternative solutions
diameter wire to handle the low voltage of DC in to solar photovoltaic systems, a number of
a string connection. Due to larger power ratings, inverter companies have launched their own
central inverters have integrated cooling fans solutions in both products. The answer to the
making them more bulky and noisy. question of which inverter is best, micro inverter
or central inverter, primarily depends on
parameters which are custom specific to each
case of a solar photovoltaic system.
Unlike central (string) inverters which are used to
connect on an array of many solar panels, micro-
inverters, as their name ‘micro’ suggests, are
designed to connect on to a single or a pair of
solar panels dual micro-inverters. Their power
rating is designed for the output of a single panel,
e.g. 245w, or between 400-500w for duel micro-
Figure 1, In the traditional photovoltaic string inverters.
converter architecture, all of the solar modules in
Micro-inverters’ smaller power rating also results
an array feed energy into a single string inverter.
in smaller size than of central inverters which has
Source: Renewable Green Energy Power, April
several technical advantages. For example,
1, 2018.
micro-inverters are less prone to failure from heat
So why should anyone undergo the as they do not have the same need for cooling as,
disadvantages of string inverters if there is not bigger, central inverters do.
benefit? The main reason for anyone using a
string inverter is leveraged cost. Provided a Apart from these technical improvements, the
system’s power rating is optimized between solar main advantage of micro-inverters, compared to
panels and inverter, string inverters may have the central inverters, is that they manage to isolate
cost advantage. The other main issue is location each solar panel, or pair of 2 solar panels, from
and size of the solar photovoltaic system. When the whole system without affecting the overall
dealing with large scale photovoltaic power performance of the photovoltaic system. For
plants, especially in rural areas with no example, in a solar panel installation with 20
surrounding buildings, string inverters are a solar panels and a total power of 4,900w, if one
preferable solution. In PV power plants, using a of the solar panels is shaded from snow, or bird

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 SEGA 07
th
The 7 International Conference on Sustainable Energy and Green Architecture
Smart City and Urban Resiliency
Bangkok, May 21-22, 2018

droppings or other debris, the reduction in the smaller diameter connection cables which all
performance of the affected solar panel will not conclude to better reliability the overall system.
influence the performance of the other panels,
The main disadvantage of micro-inverters has, up
thus will isolate the ‘damage’ to a single panel.
until recently, been their increased initial cost.
Considering manufacturing defects in solar
Because a system with micro-inverters requires
panels and performance factors such as, shading,
one micro-inverter for every panel (or every two
temperature issues etc., manufacturers claim that
panels in cases of dual-micro-inverters) the
an array of solar panels with micro-inverters can
overall cost is marginally higher than that of a
produce as much as 5% to 25% more than the
system with central, string-inverters. However,
equivalent array with a string inverter, depending
with the introduction of dual micro-inverters and
on custom specific circumstances. [2]
technological advancement in manufacturing
Furthermore, Micro-inverters, unlike central integrated micro-inverters in solar panels the cost
(string) inverters are very flexible offering many difference is continually reducing.
system expansion options without significant
limitations. Because they tend to become an SOLAR FIELD DESIGN
integrated part of the solar panel itself, In Thailand with diffuse insolation average of
850 W/m2, hot and humid climate, monsoon area,
and near equator, should use Monocrystalline
solar panels.
Monocrystalline solar panels have the highest
efficiency rates since they are made out of the
highest-grade silicon. The efficiency rates of
monocrystalline solar panels are typically 15-
20%. SunPower produces the highest efficiency
solar panels on the U.S. market today. Their E20
series provide panel conversion efficiencies of up
Figure 2, Micro-inverters installation
to 20.1%. Update (April, 2013): SunPower has
Source: Renewable Green Energy Power, April 1, 2018.
now released the X-series at a record-breaking
or of two panels in dual-inverters, they offer the efficiency of 21.5%. [4]
option of adding as many solar panels with a
micro-inverter to an existing system as one Monocrystalline silicon solar panels are space-
wants. In addition, extended panels are not efficient. Since these solar panels yield the
limited to have the same power rating of existing highest power outputs, they also require the least
panels. amount of space compared to any other types.
Monocrystalline solar panels produce up to four
Another main advantage of micro-inverters is times the amount of electricity as thin-film solar
their ability to produce grid-matching power at panels.
the level of a single solar panel. Each solar panel
has its own micro-inverter; solar panel arrays are Monocrystalline solar panels live the longest.
connected in a parallel connection configuration Most solar panel manufacturers put a 25-year
and then connected to the power grid. The warranty on their monocrystalline solar panels.
parallel structure with micro-inverters, opposed Tend to perform better than similarly rated
to string connections with central string- polycrystalline solar panels at low-light
inverters, has the crucial advantages of isolating conditions.
each panel, thus isolating problems of low
performance from the entire system. The sources In solar field you have area larger than any other
for such problems may include shading on building, with low chances of shading on your
panels, malfunction of the solar panel or the solar panels and in an open area with cooling
micro-inverters themselves etc. Considering that breezes, the cheaper option of string inverters
micro-inverters operate at lower power levels was better solution.
than central (string) inverters, they offer Small String inverter 11-22kW with MPPT much
additional advantages such as lower heat-loads, better to use as one inverter can connect from 1
improved Mean Time Between Failures (MTBF), to 3 strings. Solar String inverter had input

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 SEGA 07
th
The 7 International Conference on Sustainable Energy and Green Architecture
Smart City and Urban Resiliency
Bangkok, May 21-22, 2018

voltage range from 160-1,000VDC and start Scope of Design

voltage 350VDC. Series and parallel connection
Photo Voltaic modules layout, foundation, steel
for solar panel to get voltage, current, and power
support, protection system, cables, conduits,
as required input for Solar String inverter.
electrical equipment layout, AC/DC principle
diagram, lightening protection, grounding
system, power station, and monitoring system.
The system in detail
This PV power station, 22kV side uses the cable
way hand in hand, concentrated In the #5
interconnected of box type transformer, computer
protection, set #5 box transformer concentration,
telemetry, remote communication, remote
control, remote automation function, telemetry
part should contain the three phase current,
voltage, active power, reactive power,
environmental temperature (multifunction
meter), protection, remote communication,
Figure 3, Solar panels arrangement with string solar remote control performed by DTU remote
inverter communication part should contain the breaker
The composition of the entire system mainly points, closing breaker and isolation, the location,
includes solar nodules, cluster grid Inverter, grounding switch breaker position, main feed line
lightening protection, AC protection, DC circuit breaker tripping pressure loss and pressure
protection, Combination box, panels support, switch function.
concrete footing, cable, cable support, conduits, 1. The composition of the entire PV
outdoor, transformer and monitoring system etc. system mainly Include PV components, cluster
Solar Modules install above ground in order to grid Inverter, DC Medium voltage power switch
improve efficiency of panels, power generation box, AC Medium voltage power switch box,
and beautiful effect, installation angle of 20 22kV/400VAC transformer (oil-immersed),
degree, guarantee the inverter in each period are power cable, cable bridge, conduits, lightening,
in the best MPPT working state, this design uses grounding and monitoring system.
the set of string and cluster type Inverter, ensure 2. Solar Cell Module using high quality
that every squo.re loop has the best efficiency of Mono crystalline 567,000 panels of 300W.
Inverter. This project corresponding inverter, 3. Inverter using high efficiency and stable
photovoltaic phalanx access through the grid 20kW solar grid tie inverter, a total of 6,750 sets.
inverter, the 450VDC convert to 230/ 400VAC, 4. Outdoor step up distributed transformer
incorporated into the outdoor AC distribution 2,500kVA, 22+/-2 x 2.5% /0.4kV, Uk=6%Dyn11.
after access to the 22KV up pressure change, 5. Power cable use 0.6/kV XLPE insulated
Meet the requirement of local grid. cross linked polyolefin sheathed flame-retardant
Design standard power cable.
6. PV Modules connecting structure and
1. SJ/TU127-1977 body building lighting protection system, all
2. GB/T184 79-2001 other equipment chassis has to be reliable
3. GB50217-2007 grounding.
4. GB/T18802.12-2006 The main equipment parameters
5. GB50052-2009
6. GB50057-2010 1. PV modules, using Mono crystalline, at
7. GB/T19939-2005 the same time with high conversion rate
8. IEC60364-7-712-2002, IEC60076 performance, effectively absorb the solar energy
IEC60298, IEC60439, IEC60947 into electrical energy, power generation capacity
is high. According to the site condition, we adopt
The corresponding standards PV components, high efficiency power generation performance of
Inverter and other technical manuals and samples.

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 SEGA 07
th
The 7 International Conference on Sustainable Energy and Green Architecture
Smart City and Urban Resiliency
Bangkok, May 21-22, 2018

567,000 panels of 300W Monocrystalline

components, as shown in table 1 below.

Table 1. 300W Monocrystalline Specifications

Model
Pmax 300W
Vmpp 32.2VDC
Impp 9.32A
Ipvc 9.97A
Maximum system voltage 1,000VDC
Dimension (LxWxH mm) 2,000x1,000x40
Voltage temperature factor -0.42%/℃
Notes• Standard test conditions: AM 1.5, Insolation
1,000W/m2, 25℃.
The choice of Inverter
Figure 4, Cluster grid tie inverter GroWatt 20000UE
Choice of GroWatt 20000UE, 20kW, MPPT cluster Source: GroWatt, April 1, 2018.
grid tie inverter as shown in Figure 3. The selected
model has the advantages of high conversion
efficiency, stable and reliable performance, realize the
effective utilization of solar energy sources.
1. Full digital DSP28335 + CLPD control
technology.
2. MPPT control, timely track the maximum
output power of PV panels.
3. Sine wave output, automatic synchronization
grid current harmonic content is small, the grid no
pollution, no impact.
4. Advance disturbance detection technology,
realizing the island protection operation, reduce
disturbance to the grid.
5. Active power adjust function 0 to 100% Figure 5, Remote monitoring system with RS-485
6. Adjustable reactive power, power factor range
-0.8 to 0.8. Table 2. GroWatt 20000UE Specifications
7. RS485 data with computer remote Model 20000UE
monitoring running state. Max PV array power 20,800W
8. High accurate output measurement. Max PV input voltage 1,000VDC
9. Protection function, high reliability, MPPT voltage range 400-800VDC
over voltage protection, short circuit protection, Maximum PV input current 26ADC/string
island protection, overheat protection, over load Output voltage 3x230/400VAC 32A
protection, DC grounding protection, AC Number of MPPT 2
grounding protection, Phalanx insulation Protection grade IP65
Impedance protection and residual current Frequency 50/60 Hz
protection phalanx. Rated power output 20kW
10. Active and reactive power grid PV Efficiency 98%
scheduling. Dimension (LxWxH mm) 570x740x235
11. The flow control function (option). Weight 60kgs
Certificate: PEA, MEA, CE, IEC62109, EN50438
Lighting protection design
Lighting protection system prevent direct lighting
flesh with using component frame as a pick up. a. all stents using equipotential grounding.
Solar panels direct lighting prevention measures b. output, and lighting surge protector on
mainly include two aspects. the cable. (DC part, communication part is

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 SEGA 07
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The 7 International Conference on Sustainable Energy and Green Architecture
Smart City and Urban Resiliency
Bangkok, May 21-22, 2018

equipped with surge protection). In view of the

induction lighting measures are
1. For the DC input line into induction
lighting protection.
2. For the AC output line into induction
lighting protection, installation of over voltage
surge protector, and external grid systems also
have a lighting protection system to protect.
3. All of the Introduction of the
distribution through reliable processing metal
shell to reduce thunder electric wave invasion of
amplitude. Figure 6 SafeRing configuration
SAFE RING Source: ABB SF6 insulated compact switch gear,
SafeRing[3] is a ring main unit for the secondary type SafePlus, and SF6 insulated Ring Main
distribution network. SafeRing can be supplied in Unit, type SafeRing 12/24 kV, April 4, 2018.
15 different configurations suitable for most
switching applications in 12/24 kV distribution SYSTEM SETUP
networks. It is extendable and combined with the A system setup for 135 MW solar cell power
SafePlus concept, which is ABB’s flexible, plant shown in Figure 7.
modular compact switchgear, they represent a
complete solution for 12/24 kV secondary
distribution networks. SafeRing and SafePlus
have identical user interfaces. SafeRing is a
completely sealed system with a stainless-steel
tank containing all live parts and switching
functions. A sealed steel tank with constant
atmospheric conditions ensures a high level of
reliability as well as personnel safety and a
virtually maintenance-free system.
The SafeRing concept offers a choice of either
switch-fuse combination or circuit-breaker with
relay for protection of the transformer. SafeRing
can be supplied with an integrated remote control
and monitoring unit.
SafeRing is designed for use in the following
applications: - Compact secondary substations -
Small industries - Wind power plants - Hotels,
shopping centres, office buildings, business
centres etc. - Light mining applications, airports,
hospitals, tunnels and underground railways
C - Cable switch
De - Direct cable connection with earthing
D - Direct cable connection
F - Switch-fuse-disconnector
V - Vacuum circuit-breaker
Be - Busbar earthing
Sl - Busbar sectionalizer, load break switch
Sv - Busbar sectionalizer, vacuum circuit-breaker Figure7 Box type oil immersed transformer
single line diagram
CB - Circuit-breaker module
M - Metering module

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 SEGA 07
th
The 7 International Conference on Sustainable Energy and Green Architecture
Smart City and Urban Resiliency
Bangkok, May 21-22, 2018

The system use Mono crystal Solar Cell 300W in To get appropriate optimum performance Solar
series to get 450VDC and parallel in 3 sets to get cell and inverter size match to distribution
20kW with Cluster inverter GROwatt 20000UE to transformer, transmission line, bus bar, and
make small cluster 1 MW system, connect as protection system is critical. The network system
network with SafeRing. Input 22kV transmission with SafeRing will reduce cost and help stable
line connected with ABB Gsec/T2F DS1, system with better protection.
Transformer protection SF6 type, Lightening
Figure 7 and 8 shown how to setup system for
protection, Grounding system, Kvar automatic
networking solar power plant with small cluster 1
control, AE4000SWE Air Circuit Breaker,
MW connect with SafeRing as network system to
SIRCO switch, and MCB 40A. Box type
scale up easily such as 135 MW. RS485
transformer was used in the system. Remote
monitoring system help to monitor and control
monitoring system with RS-485 as shown in
the power plant real time. SF6 disconnection and
Figure 5 was installed. Output transmission line
protection make the job much easier to control
22kV connected with ABB Gsec/T2 DS2 Bus bar
the system. The box type transformer with all
Sectionalizer SF6 type, it connected to the input
protection system in the box make the system
of next 1 MW cluster. With this type of Ring
simple, easy to increase the size, and low cost.
connection as network will make the system
stable and better protection. The size of CONCLUSION
transmission line will minimum with maximum
power handling. The single line diagram as String inverter was used for large system instead
shown in Figure 8. of micro grid inverter which is appropriate for
small system in order to avoid evident risks of
DISCUSSIONS underperformance. In residential areas the
chances of underperformance, either from
The answer to the question of which inverter is
shading or from technical faults, are usually high.
best; micro-invert or string inverter is rather
On the other hand, if you have a roof higher than
dependable on custom specific circumstances. If
any other building, with low chances of shading
you are dealing with a small system such as
on your solar panels and in an open area with
residential solar panels perhaps it would be
cooling breezes, string inverters is a better
justifiable to undergo the extra initial cost of
solution.
micro-inverters in order to avoid evident risks of
underperformance. In residential areas the Type of solar panel, solar configuration, type of
chances of underperformance, either from inverter is a matter of custom specific
shading or from technical faults, are usually high. circumstance of your solar photovoltaic system;
On the other hand, if you have a roof higher than size, location, temperature, risk of shading and of
any other building, with low chances of shading course, financial constraints.
on your solar panels and in an open area with Photovoltaic type, Field arrangement, voltage
cooling breezes, the cheaper option of string selection, inverter type selection, electrical
inverters might be a better solution. protection system, lightening protection system,
Once again, the type of solar panel, solar and grounding system must be designed
configuration, best type of inverter is a matter of appropriate to get optimum power and profit.
custom specific circumstance of your solar To get appropriate optimum performance Solar
photovoltaic system; size, location, temperature, cell and inverter size match to distribution
risk of shading and of course, financial transformer, transmission line, bus bar, and
constraints! If you are interested in installing protection system is critical. The network system
solar panels for your power plant you may be with SafeRing will reduce cost and help stable
faced with choosing the best solution for your system with better protection.
solar photovoltaic system.
ACKNOWLEDGEMENTS
Photovoltaic type, Field arrangement, voltage
selection, inverter type selection, electrical This research paper was supported by
protection system, lightening protection system, BangkokThonburi University. We thank
and grounding system must be designed sincerely our colleagues from the Electrical
appropriate to get optimum power and profit. Engineering Section, Faculty of Engineering,

7
 SEGA 07
th
The 7 International Conference on Sustainable Energy and Green Architecture
Smart City and Urban Resiliency
Bangkok, May 21-22, 2018

BangkokThonburi University who provided [2] “Enphase Microinverter M190″, Enphase

warm encouragement to conduct this research. Energy, 2017.
REFERENCES [3] ABB product catalogue, Application, ABB
SF6 insulated compact switch gear, type
[1] “Increase Power Production”, eIQ Energy, SafePlus, and SF6 insulated Ring Main Unit,
2018. type SafeRing 12/24 kV, April 4, 2018.
[4] SunPower, 300W Panel specification, 2017.

Figure 8 Box type oil immersed transformer with SafeRing.

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