Post on 11-Jan-2022
Technical Guide for
String Configurations of
Huawei FusionSolar PV Solution
Version 1.0
Release Date Mar 9, 2020
Huawei Technologies Co., Ltd.
Version 01 (2020-03-09)2017-07-25 Copyright © Huawei Technologies Co., Ltd. i
Copyright Huawei Technologies Co., Ltd. 2020. All rights reserved.
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Version 01 (2020-03-09)2017-07-25 Copyright © Huawei Technologies Co., Ltd. ii
History Date Version Description. Author Review
2020-03-09 1.00 First version 420735/ Gu Xin 316493/ Wang Cong
526139/Zheng Yue 370202/Wang Guojun 313221/Peng Te 367395/Wang Tao 393232/Xie Wuyou 346773/Yuan Hongwen
Guide for String Configuration Content
Version 01 (2020-03-15) Copyright © Huawei Technologies Co., Ltd. iii
Content
1 Overview ......................................................................................................................................... 1
2 PV Modules in Series per String ................................................................................................ 2
2.1 IEC Standards ............................................................................................................................................................... 2
2.2 Cell Temperature Derived from Irradiance ................................................................................................................... 3
2.3 VOC Based on Cell Temperature and Irradiance ............................................................................................................ 5
2.4 Benefits from the Design with More Modules in One String ....................................................................................... 8
2.4.1 Lower CAPEX ........................................................................................................................................................... 8
2.4.2 Higher Yield ............................................................................................................................................................... 8
3 Module Orientation and Wiring .............................................................................................. 10
3.1 Module Orientation ..................................................................................................................................................... 10
3.2 Module Wiring ............................................................................................................................................................ 11
4 Y Connectors ................................................................................................................................ 12
4.1 Flexible DC/AC Ratio Design .................................................................................................................................... 13
4.2 Power Clipping Due to Input Current Limatation....................................................................................................... 13
4.3 System Reliability Design .......................................................................................................................................... 15
4.4 Case Study .................................................................................................................................................................. 16
5 Appendix ........................................................................................................................................ 17
5.1 Reference Documents ................................................................................................................................................. 17
Guide for String Configuration 1 Overview
Version 01 (2020-03-09) Copyright © Huawei Technologies Co., Ltd. 1
1 Overview
In the year of 2014, Huawei solar business released its revolutionary string solution for utility-
scale application with the capability of digitalization. Huawei FusionSolar Smart PV Solution
is dedicated to offering a leading PV solution towards optimal LCOE with higher reliability,
higher yields and easier maintenance. In the past six year, the overall shipment of Huawei
FusionSolar smart PV inverters has exceeded 118GW.
Despite of such achievement, Huawei string solution is still thought to be more expensive than
conventional central inverter solution since some plant designers believe that the layout of the
two are the same and balance of system cost is not taken into account. However, the design and
specifications of the string solution and balance of system equipment are different from those
of central inverter solution. For example, efficiencies at different DC voltage levels,
overloading capability, high temperature performance, etc. Any of these factors have strong
influences on the system design and thus affect the balance of system cost.
In 2020, a series of handbooks will be released to elaborate how to make best use of Huawei
string solution for utility-scale PV plants, covering the scope of string configuration, best
DC/AC ratio, block optimization and some special issues. In this document, string configuration
methodology is demonstrated under a given DC/AC ratio, including number of modules in
series per string, number of strings in parallel per inverter, module orientation, wiring and
approaches that helps achieve high DC/AC ratio. It is believed that these descriptions can be
useful for a more cost-performance system design.
Guide for String Configuration 2 PV Modules in Series per String
Version 01 (2020-03-09) Copyright © Huawei Technologies Co., Ltd. 2
2 PV Modules in Series per String
There are two typical voltage levels on DC side of utility-scale PV plant: 1000V and 1500V.
Given the DC voltage level, one important step is to decide how many modules can be
connected in series per string. The general principle is to connect as more modules per string as
possible while the maximum open circuit voltage (VOC) of the string at the lowest ambient
temperature should be no higher than the defined voltage level. Meanwhile, the number of
modules in series is advised to keep the string operating voltage always within the range of the
operating voltage of MPPT since inverter output power derates quickly when the string voltage
is out of the range. In particular for the most central solutions, the largest number of modules
in series is determined by the upper limit of the operating voltage of MPPT which is not as high
as the voltage level of the entire system. But for string solution, designers still follow the
philosophy of designing central solution, which in fact does not make the best use of all merits
from string solution.
In this section, we will describe the methodology of deciding number of modules in series based
on the corresponding IEC standards and practices.
2.1 IEC Standards
Section 7.2 of IEC 62548 is considered as the norm of the calculation method for the number
of modules in series per string, which defines the correction of voltage by the extremely low
ambient temperature VOC ARRAY as follows.
Correction of the voltage for the lowest expected operating temperature shall be calculated according to manufacturer’s instructions. Where manufacturer’s instructions are not available
for crystalline and multi-crystalline silicon modules VOC ARRAY shall be multiplied by a
correction factor according to Table 5 using the lowest expected operating temperature as a reference.
Most string designs are based on this description with Eq. (2.1) and will adopt record low
temperature for the voltage correction, no matter it is the temperature at night or in daytime.
DC,MAX
OC T,V Lowest
VN
V (1+C (T -25)) (2.1)
where CT,V temperature coefficient of VOC.
However, recently the industry has found that some deviations between the assumptions in the
standard IEC 52548 and the fact, which typically results in a higher investment on CAPEX.
(1) Overestimation on the temperature when the modules are operating. Photovoltaic voltage
Guide for String Configuration 2 PV Modules in Series per String
Version 01 (2020-03-09) Copyright © Huawei Technologies Co., Ltd. 3
of one string only occurs when the irradiance is not zero but the historically lowest temperature
is unlike to occur in the night when there is no light.
(2) Overestimation on the voltage of modules under irradiance. The voltage of module is
determined by the temperature of pn junction of encapsulated solar cells. Under irradiance, solar
cells have an increase in temperature when generate voltage and such thermal energy can spread
to everywhere of the solar cells within milliseconds since silicon-based solar cells are good
thermal conductors. Therefore, when the modules are under irradiance, the cell temperature will
increase and then a further correction of module voltage should be taken based on the irradiance.
As a result, Section 7.2 of IEC TS 62738 released in 2018 suggests considering the mean of
annual extreme low dry bulb temperatures during sunlight hours and a voltage correction
based on the irradiance for calculating VOC ARRAY, as is shown in Figure 2.1.
Figure 2.1 Key information from IEC 62738: 2018
2.2 Cell Temperature Derived from Irradiance
Several scholars have studied the temperature behavior of PV modules under irradiance. Two
approaches will be introduced and compared to confirm the accuracy of each other.
(1) NOCT method. Typically, the module manufacturers denote the NOCT parameters on the
datasheets which provide the possibility of deriving the module temperature under different
irradiances and ambient temperatures. Under 1m/s wind speed, module temperature and solar
irradiation are in linear correlation, according to the research results from Ross RG (1,2):
CELL amb inc
NOCT-20T =T + G
800 (2.2)
where Tamb is the ambient temperature, TCELL is the cell temperature, Ginc is solar irradiation on
module surface (W/m2) and NOCT is the nominal operating cell temperature.
When modules are open-circuit, Eq. (2.2) should be written as follows according to energy
conservation law.
Guide for String Configuration 2 PV Modules in Series per String
Version 01 (2020-03-09) Copyright © Huawei Technologies Co., Ltd. 4
CELL amb inc
NOCT-20 1T =T + G *
800 1-Eff (2.3)
Where Eff is the PV module efficiency (related to the module area).
Take the high-efficiency module for example, NOCT is 41±3 ℃ and module efficiency is 20%.
The solar cell temperature for open-circuit condition could be simplified to:
CELL amb incT =T +0.033G (2.4)
(2) Thermal behavior characteristic method. In PVsyst Help document (Navigation: Project
design > Array and system losses >Array Thermal losses), there’s detailed description about
how to calculate TCELL, as is shown in Figure 2.2.
Figure 2.2 How to calculate TCELL in PVsyst
CELL amb inc
1T =T + (Alpha*G *(1-Eff))
U (2.5)
Where Alpha is the absorption coefficient of solar irradiation. The usual value is 0.94. U: the
thermal behavior is characterized by a thermal loss factor
C vU=U +U *V (2.6)
For free-standing systems (with air circulation all around the collectors) that is usually used in
utility-scale plants, with the absence of reliable measured data, PVsyst proposes default values
without wind dependency:
2
CU =29W/m K , 2
VU =0W/m K/m/s
According to PVsyst own measurements and the values from some PVsyst users, when using
standard meteorological data such as the US TMY2 data (usually around 4-5 m/s on an average
in continental - non-coastal areas), and free-standing system, the following U-values are
proposed:
2
CU =25W/m K , 2
VU =1.2W/m K/m/s
With an average wind velocity of 3 m/s, this corresponds to U = 28.6 W/m²·k, close to the
PVsyst standard value. Take the same module used for Method (1) as example (Eff=0 when
modules are open-circuit), the Eq. (2.5) could be simplified as below:
CELL amb incT =T +0.033G (2.7)
which is the same as the result of Eq. (2.4). Details can be referred to PVsyst Help documents.
Therefore, according to Eqs. (2.4) and (2.7), when the irradiation received by open-circuit
modules is 1000 W/m2, the cell temperature will increase by ~33℃, which is in good agreement
with what we can observe in a real PV plant.
Guide for String Configuration 2 PV Modules in Series per String
Version 01 (2020-03-09) Copyright © Huawei Technologies Co., Ltd. 5
2.3 VOC Based on Cell Temperature and Irradiance
In the past, VOC is assumed as a constant even if irradiance varies a lot. However this is not the
fact. In the following (Figure 2.3) is the picture from datasheet of a module at 25 oC but with
different irradiances. It is obvious that VOC is increasing with the increase of irradiance although
the increase is not very large.
Figure 2.3 I-V curves of a module at 25 oC with different irradiances
The following equation shows the relationship between VOC and irradiance is based on the
single diode model (3,4):
inc SC_STCLOC
0 0
G IInkT nkTV = ln +1 = ln +1
q I q 1000I
(2.8)
Where T is Kelvin temperature, K is the Boltzmann constant 1.38×1023J/K, n is the number of
cells per module; q is the elementary charge 1.6×10-19 C, IL is short circuit current, VOC is the
open-circuit voltage, I0 is the constant for the diode reverse bias saturation current, which can
be calculated based on the STC values provided by the manufacturers as is shown by Eq. (2.9).
OC
SC0 qV @STC
-1nkT
I @STCI =
e
(2.9)
Therefore, VOC of module under given temperature and irradiance can be written as:
inc SCOC inc T,V amb inc
0
G I @nkT 1V (@T,G )= ln +1 *[1+C (T + (Alpha*G *(1-Eff))-25)]
q 1000I U
STC
(2.10)
Where CT,V is the temperature coefficient of VOC, the value of Tamb is equal to the mean of
annual extreme low dry bulb temperatures during sunlight hours and Eff is 0 for open circuit.
For the number of modules per string N, it should meet the requirements showed by the
following equations (5,6):
DC,MAX
OC inc
VN
V (@T,G ) (2.11)
where VDC,MAX is the Min{Vmax_module, Vmax_inverter}.
Considering the MPPT voltage range, N should meanwhile following Eq. (2.12)
Guide for String Configuration 2 PV Modules in Series per String
Version 01 (2020-03-09) Copyright © Huawei Technologies Co., Ltd. 6
MPPT, MIN MPPT,MAX
m inc m inc
V VN
V (@T1,G ) V (@T2,G ) (2.12)
where VMPPT,MIN and VMPPT,MAX the lower and upper limits of MPPT range, respectively.
m incV (@T1,G ) can be referred to Eq. (2.10) with VOC replaced by Vm where T1 is the mean of
annual extreme high dry bulb temperatures during sunlight hours. m incV (@T2,G ) is similar to
that of m incV (@T1,G ) where T2 is the mean of annual extreme low dry bulb temperatures during
sunlight hours.
In the following, one example is showed for better understanding the application of
methodology for calculating module number per string.
Assuming that the mean of historical annual lowest temperatures is 10 ℃, LR4-72HPH-440M
is the given module (ISC= 11.46A, VOC=48.90V, CT,V=-0.28%/oC) and SUN2000-185KTL is the
given inverter (SUN2000-185KTL is certified to have maximum DC voltage of 1500 V and it
will not be damaged if DC input voltage is no higher than 1550V), the values of VOC at different
irradiances can be calculated according to Eqs. (2.10) - (2.12). The final number Nfinal can be
determined by the smallest number under different irradiances.
Table 2.1 Calculating maximum module number based on constant Tamb
Irradiance
(W/m2)
Tamb
(oC)
Tmodule
(oC)
Corr. Module VOC
(V) N Nfinal
Corr. String VOC
(V)
100 10.0 13 46.10 32 31 1429.07
150 10.0 15 46.66 32 31 1446.60
200 10.0 17 47.00 31 31 1457.00
250 10.0 18 47.21 31 31 1463.48
300 10.0 20 47.34 31 31 1467.46
350 10.0 22 47.41 31 31 1469.71
400 10.0 23 47.44 31 31 1470.68
450 10.0 25 47.44 31 31 1470.69
500 10.0 27 47.42 31 31 1469.92
550 10.0 28 47.37 31 31 1468.52
600 10.0 30 47.31 31 31 1466.60
650 10.0 31 47.23 31 31 1464.24
700 10.0 33 47.15 31 31 1461.51
750 10.0 35 47.05 31 31 1458.45
800 10.0 36 46.94 31 31 1455.11
850 10.0 38 46.82 32 31 1451.52
900 10.0 40 46.70 32 31 1447.71
950 10.0 41 46.57 32 31 1443.69
1000 10.0 43 46.44 32 31 1439.50
1050 10.0 45 46.29 32 31 1435.14
1100 10.0 46 46.15 32 31 1430.64
Table 2.1 shows the detailed calculation process of Eq. (2.11). It can be seen that the maximum
number of modules in series can be up to 31 and the largest string VOC, 1471V, occurs when the
irradiance received by modules is 400 W/m2. Without correction, string VOC would be 1580V
when 31 modules are connected in series, which is 109 V difference to that of the corrected
value.
In addition, it can be prove that
Guide for String Configuration 2 PV Modules in Series per String
Version 01 (2020-03-09) Copyright © Huawei Technologies Co., Ltd. 7
MPPT, MIN MPPT,MAX
m inc m inc
V V<31
V (@T1,G ) V (@T2,G )
Therefore, the module number per string should be 31.
Another approach for more accurate calculation is to summarize the lowest temperatures at the
corresponding irradiances through hourly meteorological data of the past 20-30 years and then
calculate the module number per string.
Table 2.2 Calculating maximum module number based on calibrated Tamb
Irradiance
(W/m2)
Tamb
(oC)
Tmodule
(oC)
Corr. Module VOC
(V) N Nfinal
Corr. String VOC
(V)
100 10.5 14 46.04 32 32 1473.17
150 10.0 15 46.66 32 32 1493.27
200 11.0 18 46.87 32 32 1499.89
250 13.0 21 46.82 32 32 1498.23
300 14.0 24 46.81 32 32 1498.07
350 15.0 27 46.75 32 32 1496.08
400 16.0 29 46.65 32 32 1492.75
450 16.5 31 46.58 32 32 1490.51
500 16.5 33 46.55 32 32 1489.60
550 17.0 35 46.43 32 32 1485.91
600 16.0 36 46.50 32 32 1488.13
650 17.5 39 46.22 32 32 1479.15
700 18.0 41 46.06 32 32 1474.08
750 21.0 46 45.56 32 32 1457.83
800 22.0 48 45.31 33 32 1449.92
850 22.5 51 45.12 33 32 1443.91
900 22.5 52 45.00 33 32 1439.86
950 28.0 59 44.11 34 32 1411.55
1000 30.0 63 43.70 34 32 1398.31
1050 32.0 67 43.28 34 32 1384.87
1100 33.0 69 42.99 34 32 1375.65
Table 2.2 shows the improved correction of string VOC when one applies different Tambs to the
corresponding irradiances. It can be seen that the maximum number of modules in series
increases to 32 and the largest string VOC occurs when the irradiance received by modules is
200 W/m2. Therefore, this methodology increases the modules in series, which in practice has
been adopted by Indian developers in their projects, who increased the number of modules in
series from 30 to 34.
Importantly, this methodology is suitable for both central and string solutions. However, the
string configuration with 32 modules per string in the above case is not suitable for central
solution with a narrower MPPT voltage range and a lower voltage for maximum inverter
efficiency. This is because the string voltage with 32 modules in series overcomes the upper
limit of MPPT range and the operating voltage range does not match the one for highest inverter
efficiency. Therefore inverter will derate and cause a yield loss.
One should also notice that PVsyst does not support this correction methodology of string
voltage now. When this methodology is discussed, the designers should increase Tamb value of
the project file using Eq. (2.7), typically 12-13 oC. Sometimes, the voltage levels for both PAN
file and OND file should be increased to avoid warnings of PVsyst.
Guide for String Configuration 2 PV Modules in Series per String
Version 01 (2020-03-09) Copyright © Huawei Technologies Co., Ltd. 8
2.4 Benefits from the Design with More Modules in One String
With given DC/AC ratio, configuring one string with more modules in series lead to the benefits
in the following two aspects:
(1) Lower CAPEX: Reducing strings at DC side and thus the cost for supporting structure and
DC cables as well as the corresponding labor cost
(2) Higher yield and thus more revenue
2.4.1 Lower CAPEX
Take a standard block with 36 units of SUN2000-185KTL. For example, detailed block design
is shown in Table 2.3.
Table 2.3 Comparison between different string configurations
Project Location Abu Dhabi
Design Temperature 50 ℃
PV Module LR4-72HPH-440M
Inverter Model SUN2000-185KTL
Inverter Power (kW) 160
Comparison
Modules per string 30 Module per string 27 Module per string
Number of strings 576 648
No. of Tracker Tables 576 648
No of Inputs per Inverter 16 18
DC input power per Inverter (W) 213840 211200
DC/AC Ratio 1.32 1.3365
DC Cables (m) 40,612 50,758
Cable Cost (Material + Labor $/Wp) 0.0179 0.0189
Tracker cost (Material + Labor $/Wp) 0.096 0.101
CAPEX Saving ($/WAC ) A- $0.006/WAC A
It can be seen that 30 modules in series can save 0.006 $/WAC compare with 27 modules in
series. The main differences are from the cable and tracker cost since the 30-module per string
reduce the string number and the supporting structure of trackers.
2.4.2 Higher Yield
As is shown in Figure 2.4, at a given load percentage, the efficiency of inverter increases from
880V to 1174V (1200V), with an efficiency gap of about 0.3-0.5%. This indicates that the
inverter operates at higher string voltage.
Figure 2.4 Efficiency Curve of SUN2000-185KTL-H1
Guide for String Configuration 2 PV Modules in Series per String
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By means of PVsyst simulation, string voltage of 30-module is around 100V higher than that
of 27-module, which shifts from 858-1116V to 954-1240V, as are shown in Figure 2.5 and
Table 2.4.
Figure 2.5 Array voltage of 30 and 27 modules per string from PVsyst at Abu Dhabi
Table 2.4 Operating voltage range with different modules in series
Series Nos. Operating Voltage
27 858V to 1116V
30 954V to 1240V
Table 2.5 and Figure 2.6 show the simulation results for 30-module and 27-module string
solutions. The annual weighted average inverter efficiency gap is about 0.18% and the 30-
module solution gains 0.19% more energy yield.
Table 2.5 Inverter efficiency during simulation
Modules per string 30 modules per string 27 modules per string
Inverter efficiency 98.69%(a) 98.51%(b)
Specific Production 2102 kWh/kWp/year 2098 kWh/kWp/year
(a) (b)
Figure 2.6 PVsyst simulations with (a) 30 modules and (b) 27 modules per string
Assume PPA price is 0.02 $/kWh, and discount rate is 3%, then for a 100MWp project 30
module per string solution results in $ 118,000 or 0.16 US cents/WAC extra yield income in 20
years compared with 27 modules per string solution.
To sum up, the total benefits brought by the longer string solution is about 0.76 US cents/WAC
in the example project in Abu Dhabi. Therefore, it is recommended that the module number per
string of the string solution should be as large as possible for lower CAPEX and higher yield,
with reference to the methodology aforementioned in sections 2.1-2.3.
Guide for String Configuration 3 Module Orientation and Wiring
Version 01 (2020-03-09) Copyright © Huawei Technologies Co., Ltd. 10
3 Module Orientation and Wiring
3.1 Module Orientation
In a solar PV plant, no matter whether tracker or fixed racks are used, the modules can be
designed either portrait or landscape. Figures 3.1 and 3.2 show that portrait means the short
edge of module is parallel to the ground, while landscape means the long edge of module is
parallel to the ground.
Figure 3.1 Portrait module orientation
Figure 3.2 Landscape module orientation
Guide for String Configuration 3 Module Orientation and Wiring
Version 01 (2020-03-09) Copyright © Huawei Technologies Co., Ltd. 11
xP/xL means that modules are placed with x rows under Potrait/Landscape orientation. For
fixed structure, typical module orientations are 2P, 2L, 3L and 4L. For tracker + monofacial
module, typical module orientation is 1L, 2P, 2L and 3L. For tracker + bifacial module, typical
module orientation is 1L, 2P, and 2L. 2P is most used. Usually if one applies portrait design,
CAPEX will be lower compared with landscape due to the cable saving from both the part of
module and the part from module table to inverter, but with heavier shading the yield will
decrease by 0.2% to 0.5% due to the diode turn-on effect of shaded modules. For bifacial system,
additional mismatch loss occurs for portrait configuration, which can be up to 0.4% when the
background is of sand, according to our experience.
3.2 Module Wiring
There are two main types of module wirings: (a) C type; (b) spreading. C type can save cable
cost while spreading can increase yield due to better uniformity. The designer should balance
the cost and the yield to come up with a better design.
For bifacial systems, C-type is usually used for grass or lake background. Spreading is used for
concrete, desert or snow background.
Figure 3.3 C type module wiring
Figure 3.4 Spreading module wiring
Guide for String Configuration 4 Y Connectors
Version 01 (2020-03-09) Copyright © Huawei Technologies Co., Ltd. 12
4 Y Connectors
In order to achieve the optimal LCOE, high DC/AC ratio may be required. For Huawei
SUN2000-185KTL, there are 9 MPPTS and 18 inputs, which restrains the upper limit of DC/AC
ratio. Therefore, the 2 in 1 solution with the implementation of Y connectors (Figure 4.1) is
proposed to increase the maximum inputs to 27 inputs. The technical specifications are shown
in Table 4.1.
Figure 4.1 Y Connector
Table 4.1 Technical Specifications of Y Connector
Rated Voltage 1500 V Rated Current 2 x 16 A (Fuse Rating of 15 A or 20 A)
Color Black, TUV Certification Cable (EN50618) Working Environment Temperature range: -40℃ ~ +85℃, Humidity
range: 5% ~ 95% Min. Installation Temperature -10℃
Storage temperature range -40℃ ~ +70℃ Application Outdoor Use
Protection Rating IP67 DC Connector Type Staubli MC4 EVO2
Allowed Cable Diameter Range 4.7 mm - 6.4 mm Durability Insertion /Removal cycles 100 times
RoHS Satisfied
Note: Y-branch connector specifications must follow the rated maximum current of module.
Such auxiliary can be purchased from Huawei or the manufacturers based on the following recommended models: If the rated current of the fuse of the Y-branch connector is 15 A, the
Guide for String Configuration 4 Y Connectors
Version 01 (2020-03-09) Copyright © Huawei Technologies Co., Ltd. 13
recommended model is 904095944 (Luxshare) or A040959443039 (Comlink); if the rated current of the fuse of the Y-branch connector is 20 A, the recommended model is 904095945
(Luxshare) or A040959453039 (Comlink).
4.1 Flexible DC/AC Ratio Design
Figure 4.2 Connection of Y Connector Note: For safety issues, Y connectors should be close to modules side and away from inverter.
As is shown in Figure 4.2, Y connector solution provides possibility to connect one more string
to one MPPT. For SUN2000-185KTL @ 40 ℃ as design temperature (175kW as nominated),
one can see in Table 4.2 the DC/AC ratio for 18 inputs is limited by 1.36 when LR4-72HPH-
440M 440W modules are applied. If Y connectors are adopted, the DC/AC ratio can be up to
1.81, which is much more flexible for different requirements.
Table 4.2 DC/AC Ratio with Y Connectors
No. of Strings 18 19 20 21 22 23 24
440W*26 1.177 1.242 1.307 1.373 1.438 1.504 1.569
440W*30 1.358 1.433 1.509 1.584 1.659 1.735 1.810
4.2 Power Clipping Due to Input Current Limatation
According to the datasheet (7), SUN2000-185KTL supports up to 26A as the max current per
MPPT. One may have the question that whether there is power loss due to current limitation
(so-called current clipping) when Y connectors are used. For this question, below explains why
power loss occurs when input current is larger than the limit.
As is shown in Figures 4.3-4.4, current clipping happens when the current sum of 3 inputs
exceeds 26A, which only occurs during the midday. It is noted that power clipping
simultaneously occurs when the current exceeds the limit. At this moment, V increases and thus
I decreases according to the inverter operation mechanism, which indicates that current clipping
may be hence minimized. In some cases, in order to match the DC/AC ratio, some inputs are
left unplugged and therefore current clipping will not occur for the MPPTs with empty inputs.
PV1+
PV1-
PV2+
PV2-
PV+
PV-
PV+
PV- MPPT1
Inverter MPPT
String1
String2
PV+
PV-
String3
Y Connector with fuse
Guide for String Configuration 4 Y Connectors
Version 01 (2020-03-09) Copyright © Huawei Technologies Co., Ltd. 14
Figure 4.3 Diagram for current clipping
Figure 4.4 Output variation after current clipping
Therefore, the power loss due to current clipping may happen but the loss should be very small.
To obtain an approximate value for the loss caused by the current clipping, the following
calculation methodology can be used. (PVsyst does not support the evaluation of power loss
due to current clipping ).
1. Export hourly data of irradiance and temperature from MetroNorm or SolarGIS;
2. Using Perez Model and view-factor method (for bifacial modules) to calculate irradiance
received by PV modules;
3. Calculate current, voltage and power of PV modules;
4. Estimate if total power of strings are higher than maximum power of inverters, if so, then
calculate input voltage and current.
5. Estimate if current is higher than threshold value, if so, to calculate the needed voltage
increase when reducing current to the limitation value, and the difference between post-
power clipping and after- power clipping.
6. Yield Loss = Clipping loss / Whole Year energy yield
Table 4.3 Annual yield loss caused by the current limitation
Region Spain
Sevilla
UAE
Abu Dhabi
Vietnam
Phu Thuy
Mexico
Chihuahua
Argentina
Cauchari
India
Bhadla
DC/AC 1.09 1.31 1.22 1.28 1.2 1.5
Current clipping 0.05% 0% 0.01% 0% 0.02% 0%
In Table 4.3, yield losses due to current clipping are calculated based on 330-340W modules,
typical locations for PV projects and typical DC/AC ratio. It can be seen that for all of typical
locations around the world, the yield loss is no larger than 0.05%, which is actually negligible for a real project. Moreover, it can be seen that there is no yield loss due to current clipping
Guide for String Configuration 4 Y Connectors
Version 01 (2020-03-09) Copyright © Huawei Technologies Co., Ltd. 15
when the DC/AC ratio is larger than 1.25, indicating that in most areas of the world, current
clipping never has a negative effect.
4.3 System Reliability Design
Figure 4.5 Short-circuit Schematic Diagram
Huawei Y connector solution has very high reliability with failure rate lower than 0.03%. To
fulfil 1.25*I protection level, there are 2 kinds of inside fuses: 15A and 20A (8, 9). According to
IEC 61730 and 62548, reverse current tolerance of module should fulfil 1.35 times of rated
current of protection device, so there should be one fuse inside one Y connector when 3 strings
are connected to one MPPT. See more details in Table 4.4:
Table 4.4 System Reliability Design by Using Y Connector Solution
Max Panel
operating
Current
Fuse inside Y-
Connector
Reverse
current
tolerance of
Panel
Minimum
Nominal
current of PC
cable
Max. Short-
Circuit Current
of SUN2000-
185KTL
Design Value I 1.25I 1.6875I 1.56I
Application <12A 15A 20.25A
>32A(4mm2) 40A 20A 27A
Figure 4.6 Y connector monitoring for protection
In addition, to enhance the reliability of entire system, Huawei string inverter can detect the
abnormal Y connectors and report to the FusionSolar management system. Moreover, Huawei smart I-V curve diagnosis supports the Y connector scenario.
Guide for String Configuration 4 Y Connectors
Version 01 (2020-03-09) Copyright © Huawei Technologies Co., Ltd. 16
4.4 Case Study
Huawei Y connector solution has been already widely used globally.
Figure 4.7 Huawei Y connector used in First Solar Thin-Film Panels
Thin film module usually has low operating current (1~2A), using Y connector instead of DC
combiner box can save CAPEX (Fig. 4.7). Huawei Y connector solution has been accepted by
large thin-film module manufacturers such as First Solar, and has already been used in many
projects.
Figure 4.8 Huawei Y connector used in India
Huawei Y connector solution is popular in high DC/AC ratio scenarios. For example, a
project in India (Fig. 4.8), commissioned in June 2017, does not have any Y connectors failure
till now.
Guide for String Configuration 5 Appendix
Version 01 (2020-03-09) Copyright © Huawei Technologies Co., Ltd. 17
5 Appendix
5.1 Reference Documents
1. Ross RG, Smokler MI. Flat-Plate Solar Array Project Final Report. 1986: 86-31.
2. Ross RG. Flat-Plate Photovoltaic Array Design Optimization. 14th IEEE Photovoltaic Specialists
Conference. 1980:1126-1132.
3. E.M.G. Rodrigues, R. Melício. Simulation of a Solar Cell considering Single-Diode Equivalent Circuit
Model. May 2011.
4. M.A. Green, Solar Cells : Operating Principles, Technology and System Applications, Springer.
5. IEC 62548: Photovoltaic (PV) arrays – Design requirements. Edition 1.0 2016-09.
6. IEC TS 62738: Design guidelines and recommendations for ground-mounted photovoltaic power plants.
ED1, 2018.
7. SUN2000-185KTL-H1 Datasheet - (20191122).
8. Huawei Y Branch Connector Technical Specifications - (20190506) 20A.
9. Huawei Y Branch Connector Technical Specifications - (20190506) 15A.
10. Arzu Şencan. Modeling and Optimization of Renewable Energy Systems. May 11th 2012.