String Configurations of Huawei FusionSolar PV Solution

Technical Guide for

String Configurations of

Huawei FusionSolar PV Solution

Version 1.0

Release Date Mar 9, 2020

Huawei Technologies Co., Ltd.

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History Date Version Description. Author Review

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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


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



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.



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.

mk:@MSITStore:D:/Program%20Files%20(x86)/PVsyst6.8.5/PVsyst6.chm::/project_design.htm

mk:@MSITStore:D:/Program%20Files%20(x86)/PVsyst6.8.5/PVsyst6.chm::/project_design.htm

mk:@MSITStore:D:/Program%20Files%20(x86)/PVsyst6.8.5/PVsyst6.chm::/array_losses.htm



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)



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



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.



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



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


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


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


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



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



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



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.



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


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.

https://www.intechopen.com/profiles/15889/arzu-sencan

String Configurations of Huawei FusionSolar PV Solution

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