Huawei SUN2000 Inverter Technical Review and Comparative Study · Huawei SUN2000 Inverter Technical Review and Comparative Study November 2017 │ 01627 - v1.2 ... (PRs) and energy

Huawei SUN2000 Inverter Technical Review and Comparative Study

Phase 2: Case Study Comparison

Huawei Technologies UK Co., Ltd.

November 2017 01627 - v1.2


November 2017 │ 01627 - v1.2

Issue and Revision Record

Revision Date Originator Checker Approver Narrative

1.0 13/09/2017 JKH SG JH Draft

1.1 20/10/2017 JKH SG RSJA Update following client

comments

1.2 13/11/2017 MMS ## ## Final. Cover photo update

Disclaimer

This document has been prepared for the titled project or named part thereof and should not be

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or liability for this document to any party other than the person by whom it was commissioned.

As provided for in RINA Consulting’s proposal, to the extent that this report is based on

information supplied by other parties, RINA Consulting accepts no liability for any loss or damage

suffered by the client, whether contractual or tortious, stemming from any conclusions based on

data supplied by parties other than RINA Consulting and used by RINA Consulting in preparing

this report.

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Contents

Executive Summary .......................................................................................................................... 4

1 Introduction ................................................................................................................................ 7

2 Plant Design Comparison ......................................................................................................... 8

2.1 Key plant details .............................................................................................................. 8

2.2 System configuration ....................................................................................................... 9

2.3 Plant layout .................................................................................................................... 10

2.3.1 Huawei SUN2000 string inverter variant: low voltage design .......................... 10

2.3.2 Central inverter variant: low voltage design ...................................................... 11

3 Yield Study .............................................................................................................................. 13

3.1 Irradiation ....................................................................................................................... 13

3.2 Plant Availability ........................................................................................................... 14

3.3 PR calculation................................................................................................................ 14

3.4 PR losses ...................................................................................................................... 16

3.5 Yield estimations ........................................................................................................... 24

4 CAPEX Comparison ................................................................................................................ 26

4.1 CAPEX results ............................................................................................................... 26

5 OPEX Comparison .................................................................................................................. 28

5.1 Modelling Inputs and Assumptions ............................................................................... 28

5.1.1 O&M coverage .................................................................................................. 28

5.1.2 Modelling approach ........................................................................................... 29

5.2 Qualitative and quantitative OPEX results .................................................................... 29

5.2.1 Inverter repair expectations .............................................................................. 31


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Executive Summary

Huawei Technologies (UK) Co., Ltd. has appointed RINA Consulting Ltd. (‘RINA’) to perform a

technical review of Huawei SUN2000 33KTL inverters and a case study comparison of one PV

plant using the Huawei inverters with the same configuration using central inverters. This report

covers the case study comparison, with the technical review in a separate report. This report,

containing the second phase of work, should be considered in its entirety and RINA does not

endorse specific statements being extracted and used for marketing purposes.

This study compares the energy yield, capital expenditure (CAPEX) and operational expenditure

(OPEX) for two variants of one PV plant design: one using Huawei SUN2000 33KTL inverters

and the other, a comparable inverter from a market-leading brand of central inverters. The PV

plant which forms the basis of comparison is the Cowdown PV plant in the UK, which has a

capacity of 39.9 MWp and was chosen by Huawei to be representative of their UK PV plants. The

first variant is the as-built design, which uses Huawei SUN2000 33KTL string inverters and a mix

of Canadian Solar and Neo Solar Power PV modules. The one modification RINA has made to

the Huawei variant is to assume Power Line Communications (PLC) were used, rather than the

standard RS-485 communications system that was actually installed. The second variant is an

equivalent configuration that uses a market-leading brand of central inverters, which was

designed by RINA. Specifically, RINA undertook a preliminary design exercise to determine a

module string configuration for the central inverter systems; assumed cable lengths and cable

cross sections for both variants that would create equivalent cable voltage drops between the two

variants; and assumed DC combiner box specifications, numbers and locations in keeping with

experience of similar projects and in order to create an equivalent design to the Huawei string

inverter variant. We note that the central inverter variant is not necessarily fully optimised.

Yield Study

Plant Performance Ratios (PRs) and energy yields have been calculated for both system design

variants.

The Huawei string inverter variant has a PR of 86.4% for the Cowdown PV plant, which is higher

than that of the central inverter variant, which has a PR of 85.6%. PV plant losses change based

on various factors, including site specific, technology and component specific characteristics and

therefore and cannot necessarily be directly extrapolated to another location or project. Loss

factors which contribute to the difference in this PR calculation are: mismatch, inverter efficiency

and MPPT performance. We note that on a PV plant with a more homogeneous design, the

mismatch loss difference between the two variants might reduce slightly. We further note that the

inverter efficiency and MPPT performance losses are manufacturer, rather than technology

specific, although the Huawei performance is at the high-end of the market.

The yield calculations give a year one specific yield of 1,026 kWh/kWp for the Huawei SUN2000-

33KTL string inverter variant and 1,017 kWh/kWp for the central inverter variant, for this specific

site.

CAPEX

The Capital Expenditure (CAPEX) costs identified in this comparison exclude costs that we would

consider to be directly comparable between the two configurations. Also excluded from the


November 2017 │ 01627 - v1.2 5

CAPEX comparison are inverter and transformer costs. Inverter costs are excluded because they

are commercially sensitive and do not necessarily have a fixed price per unit. As the central

inverter comes within a power station that includes the transformer, transformer prices are also

excluded from both scenarios. Main components considered for each configuration are

summarised as:

AC and DC combiner box requirements, including monitoring equipment

Foundations for inverter and combiner boxes

AC and DC low voltage cabling requirements

The total CAPEX costs, for the components considered are:

£64.41 per kWp for the Huawei string inverter variant

£64.67 per kWp for the central inverter variant

The difference in CAPEX costs is almost negligible between the two systems. The difference

between the two systems could be greater if inverter and transformer costs were to be included

and both prices are sensitive to commercial negotiations and plant design factors. The string

inverter solution offers savings in some areas, namely: combiner boxes and by use of the PLC

system rather than a standard communications system.

OPEX

RINA’s proprietary lifecycle cost model was used to evaluate DC and AC component

repair/replacement costs which are categorised as corrective maintenance tasks and therefore

typically billed to project owners. Preventative maintenance requirements and corrective

maintenance labour costs are assumed to be included in a fixed price O&M agreement and not

considered in this OPEX calculation.

The model performs statistical failure calculations for key components and takes defect

warranties into account. The system design is a key factor in determining project-specific failure

expectations and provides a more accurate approach to cost modelling than a more simplistic

forecast on a ‘cost-per-MW’ basis.

For the purpose of the OPEX comparison, RINA consider that some plant components will vary

between the two system variants and others will be common to both. Similarly to the CAPEX

assessment, those components that are common between the two systems are excluded from

the comparative OPEX analysis. At the request of the Client we are excluding inverter and

transformer costs from the OPEX comparison, instead presenting an analysis of the qualitative

differences between the two systems.

For the Cowdown PV plant, the central inverter system has higher DC and AC component

corrective maintenance costs, with the costs per kilowatt peak over the 25-year plant lifetime

being:

£2.80 per kWp for the DC and AC component corrective maintenance costs for the Huawei

string inverter variant, excluding inverter and transformer maintenance costs

£3.77 per kWp for the DC and AC component corrective maintenance costs for the central

inverter variant, excluding inverter and transformer maintenance costs

These differences stem mainly from the assumed relative price increase of DC combiner boxes

used on the central inverter plant due to more internal components than the AC combiner boxes

used on the string inverter plant. There are also slightly more DC combiner boxes required than


November 2017 │ 01627 - v1.2 6

AC combiner boxes leading to increased amount of repairs. The communications system costs

are expected to be a little lower for the Huawei PLC Smartlogger solution than the standard RS-

485 communications system, due to the additional monitoring equipment included in the DC

combiner boxes. These prices are specific to the plant design, component costs available at the

time of repair and component lifetime.

The failure profiles of both inverter technologies are presented in the body of the report, which

are indicative only and will depend on project specifics, such as the duration and specifications of

manufacturer warranty, the sub-component replacement schedules (in the case of central

inverters) and the O&M contract specifications. The lifetime of the inverters will have a large

impact on plant running costs, as it will influence how often you need to replace or repair the fleet

of inverters. As an example, the replacement ratio (number of times an inverter must be replaced

in the 25-year plant lifetime) is 1.76 when the Huawei string inverter lifetime is 12.5 years

compared with 1.20 when the inverter lifetime is 17.5 years.


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1 Introduction

RINA Consulting Ltd. (‘RINA’) has been appointed by Huawei Technologies (UK) Co., Ltd. as

Technical Advisor (‘TA’) to perform a two-part study of Huawei string inverters, of which the

second part is a case study comparison of Huawei SUN2000 33KTL string inverters against

central inverters.

RINA Consulting is the engineering consultancy division of the RINA Group, the result of the

integration of a number of internationally respected RINA companies including OST Energy,

D’Appolonia, Edif ERA (ERA Technology), Centro Sviluppo Materiali, G.E.T., Logmarin Advisors,

Polaris, SC Sembenelli Consulting and Seatech. RINA Consulting brings together a rich heritage

of engineering consultancy expertise into one unique organisation.

Huawei is a global Information and Communications Technology (ICT) company who also

manufacture solar PV inverter products.

Huawei manufactures a range of solar PV inverters and this review focusses on the SUN2000

33KTL model, part of the second series, “V2.1” of the SUN20000 string inverter product series.

This study will compare the design of an existing PV plant (Cowdown PV plant) in the UK using

Huawei SUN2000 33KTL inverters with an equivalent configuration that uses a market-leading

brand of central inverters, which shall not be named. The study is split into two parts. The first

report (Phase 1) consists of a technical review of the Huawei inverters used on the Cowdown PV

plant. The findings of the Phase 1 technical inverter review feed into the yield study and

CAPEX/OPEX comparison, which is the subject of this second report (Phase 2). The two reports

should be considered together.

The main scope of the case study comparison is as follows:

Yield Study

– A comparison of yield analyses of each variant. Each yield analysis will be carried out

according to OST’s standard methodology with focus in the losses related with the inverter

option

Qualitative and quantitative plant comparison

– Including comment on procurement, installation, maintenance, reliability, after sales

services, replacement schedule and CAPEX/OPEX evaluation.

Our opinions are contained throughout the report and issues of most significance are discussed

in the executive summary

This report should be considered in its entirety. RINA does not endorse specific statements being

used for marketing purposes.


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2 Plant Design Comparison

2.1 Key plant details

The Cowdown PV plant is a 39.9 MWdc photovoltaic power plant in the UK, which has been

constructed with 1,068 x Huawei SUN2000 33KTL string inverters. At the initial design stage, we

understand that following a design assessment the EPC contractor selected the Huawei string

inverter over a specific model of central inverter. The central inverter designs were not made

available to RINA and therefore, for the purposes of this comparison exercise, RINA has

designed an equivalent PV plant with the brand of central inverter we understand was originally

considered and rejected. The chosen inverter has a nominal AC power output of 1000 kVA,

housed in pairs within a 2000 kVA power station. RINA consider this central inverter brand to be

market-leading, but for reasons of confidentiality we have not disclosed which inverter this was

within this report.

Huawei provided various details of the as-built specifications of the plant at Cowdown. RINA used

this information to define a high level design for the central inverter plant variant. RINA designed

the central inverter variant to achieve a similar AC capacity to the Huawei string inverter variant.

The general description of the two plant variants is shown in Table 1. It should be noted that

RINA has provided a high level design for the purpose of comparison, however the central

inverter design should not be considered fully optimised, nevertheless we consider the design

adequate to highlight the differences between the using the Huawei string inverter and a central

inverter at this plant.

For both plant variants, nominal inverter capacity has been considered, although at 25°C, both

inverters can output a maximum AC power of 110% of nominal power. The nominal and

maximum power outputs are considered within the yield study. The overall DC:AC ratios for the

two plants only show minor variation, and both are within the range permitted by the

manufacturers.

Table 1: General plant details

Variant Huawei String Central

Overview

Location Cowdown, UK (51.196° N, 1.459° W)

DC capacity (Wp) 39,860,040

AC capacity (VA) (nominal) 32,040,000 32,000,000

DC:AC ratio 1.24 1.24

Site details

Site inclination Several slopes present, as per topographical map.

Near shading objects Small trees and bushes (4m to 6m) to the south of the plant

Overhead lines, with a tower height of 24m running in NE-SW

direction dividing the site in two main subarrays.

Module tables will mutually shade each other at certain times

of the day/year.

Metering point location At 33kV connection point


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Variant Huawei String Central

DC specifications

PV modules 153,792 in total:

42,912 x Neo Solar Power, NSP 260 W

13,536 x Neo Solar Power, NSP 255 W

85,824 x Canadian Solar, CS 260 W

11,520 x Canadian Solar, CS 255 W

Mounting Structure System Ground mount fixed tilt

Module tilt 18 degrees

Module azimuth South

Table configuration 1 table = 24 x 6 modules in landscape

Row spacing 10 m

AC specifications

Inverters 1068 x

Huawei SUN2000 30KTL

32 x

1000 kW central inverters

Power factor Unity

2.2 System configuration

The PV modules are connected in strings (a number of panels connected in series). A number of

strings may then be connected in parallel to an inverter. String and panel arrangements are

determined by the following factors:

The maximum power point (MPP) voltage range of the inverter

The highest MPP current capacity of the inverter and

The maximum system voltage of the panels.

The string and panel arrangement in the Huawei string inverter system variant was provided by

Huawei. RINA designed an arrangement for the central inverter variant. To ensure that the plant

variants were suitably designed, the electric characteristics of the strings and array are calculated

for 55°C and -5°C module temperature, as appropriate for the climatic conditions at Cowdown,

and compared against the above parameters.

A summary of the string configuration and power ratio of the string arrangements for the string

and central inverter configurations are detailed in Table 2 and Table 3 respectively. It should be

noted that 24 PV modules per module string was found to be suitable for both string and central

inverter configurations.

Table 2: System configuration for the Huawei string inverter variant

Array size (Wp) No. of strings of 24

modules per

inverter

No. of

inverters

Power ratio

(DC:AC)

Module

11,157,120 6 298 1.248 NSP 260

3,451,680 6 94 1.224 NSP 255

22,314,240 6 596 1.248 CS 260


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modules per

inverter

No. of

inverters

Power ratio

(DC:AC)

Module

2,937,600 6 80 1.224 CS 255

Table 3: System configuration for the central inverter variant


modules per

inverter

No. of

inverters

Power ratio

(DC:AC)

Module

1,422,720 114 2 1.225 NSP 260

1,028,160 84 NSP 255

2,432,520 198 2 1.212 NSP 255

9,734,400 195 8 1.217 NSP 260

12,604,800 202 10 1.260 CS 260

7,600,320 203 7 1.280 CS 260

848,640 135 1 1.278 CS 260

416,160 68 CS 255

2,521,440 206 2 1.273 CS 255

The DC:AC ratio is based on the nominal inverter power, which is in line with market standard

practices.

For both system variants, the maximum voltage of the inverters were found to be compatible with

the system designs for the climatic conditions at this site.

In the central inverter variant we note that the maximum current of the strings was higher than

that of the central inverter; however we have confirmation from the central inverter manufacturer

that this is acceptable and does not breach the warranty, provided that the DC:AC ratio is not

higher than manufacturer specifications.

Due to the different brands and power classes of modules on site, two of the central inverters

must be fed by PV modules of different power classes, which creates increased electrical

mismatch in the array feeding a central inverter. The Neo Solar Power (NSP) and Canadian Solar

(CS) PV modules have very different electrical properties, so we chose to combine modules of

the same brand but different nominal power into one inverter rather than combine NSP 260 with

CS 260 modules and NSP 255 with CS 255 modules.

2.3 Plant layout

2.3.1 Huawei SUN2000 string inverter variant: low voltage design

Details of the as-built layout for the Cowdown PV plant were provided by Huawei, a section of this

being shown in Figure 1. There are six parallel strings of modules on each table, which together

feed into one Huawei SUN2000 string inverter. The string inverters are positioned such that four

inverters are clustered at the end of each group of 24 strings of modules, as shown in the red

boxes in Figure 1, and a cluster of four inverters feeds into one AC combiner box as shown in the

blue circles in Figure 1. 17 AC combiner boxes feed into each 2 MVA substation. Cable lengths


November 2017 │ 01627 - v1.2 11

were assumed by RINA based upon the information provided in the Plant layout diagrams. Cable

dimensions are assumed by RINA based upon similar plants and keeping the overall cable

voltage drop similar between the two variants.

The string inverter design is summarised below:

4 Huawei SUN2000 string inverters per AC combiner box

17 AC combiner boxes per substation

DC string cable, Cu 1 x 6mm (modules to inverters): 571,200 m (= 1,142,400m for both poles)

AC combiner box cable, Al 3 x 35mm (inverters to AC combiner box): 16,320m of 35mm

cable (= 48,960m for all three phases)

AC substation cable, Al 2 conductors x 3 x 185mm (AC combiner box to substation): 13,600m

(= 81,600m for two conductors per three phases)

We are also aware that there are communications cables on the Cowdown plant. For the

purposes of this comparison, we are considering the plant as if Power Line Communications

(PLC) were employed. In this configuration, communications would be via the AC power cables,

for further details on PLC refer to Phase 1 Section 3.

Figure 1: Cowdown PV Plant Layout (section of) – Huawei string inverter variant, as built

2.3.2 Central inverter variant: low voltage design

For the central inverter variant, we assume that one 24-input DC combiner boxes will be

positioned at the same locations as an AC combiner box in the string inverter variant. The DC

combiner boxes include string-level monitoring hardware. To ensure that different module types

are not feeding into the same combiner box, this design variant needs more DC combiner boxes:

18 DC combiner boxes per 2 MVA substation rather than 17 AC combiner boxes per substation in

the string inverter variant. We assume that optimum combiner box positioning will result in the

overall impact of the extra combiner boxes on DC cable lengths being negligible, resulting in

identical DC cable lengths to the DC string cable lengths in the string inverter variant. Cable

dimensions are assumed by RINA based upon similar plants and keeping the overall cable

voltage drop similar between the two variants.

Design assumptions are summarised below:

21-24 PV module strings per DC combiner box

9 DC combiner boxes per 1 MVA central inverter

DC cable, Cu 1 x 6mm (modules to combiner box): 571,200m (= 1,142,400m for both poles)

DC inverter cable, Al 1 x 150mm (combiner box to inverter): 13,600m (= 27,200m for both

poles)

AC substation cable, Cu 2 conductors x 3 x 300mm (inverter to transformer): 960m of 300mm

cable (= 2,800m for two conductors per three phases)

6 strings


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RS-485 communications cables: 13,600m of cable


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3 Yield Study

Energy yields and plant Performance Ratios (PRs) have been calculated for both system design

variants.

There are a number of losses associated with the harvesting of sunlight for the generation of DC

power and there are further losses in the conversion of DC power from the modules to the useful

AC power that feeds to the grid, the cumulative loss of which defines the plant PR.

These specific losses are dependent on the system design and key plant components, which

were detailed in Section 2. Each individual loss has been modelled taking into account

information provided (and considered sufficient) and RINA proprietary models or experience.

Explanations for each of the losses are provided.

RINA has taken the following steps to establish the energy yield for the Project:

Acquired and analysed Global Horizontal Irradiation (GHI) data from a range of databases

Uplifted the calculated GHI profile to establish the GII as appropriate for the plant location and

module orientation

Modelled the system configuration performance ratio and resultant first year specific yield

estimate

Calculated combined uncertainties to give specific yield estimates at various probabilities of

exceedance.

Each step is described in the following sections.

3.1 Irradiation

We have taken Global Horizontal Irradiation (GHI) values from the following databases for

comparison:

Meteonorm 7.1

SolarGIS

PV GIS (CM SAF)

Helioclim 3v5

NREL CSR

A Weighted Mean (WM) value from the above data sources has been used, weighted accounting

for the number of years of data available and inversely weighted based on the spatial resolution

of each source. The weighted mean value of 1050 kWh/m2/year has been selected for the

continuation of our irradiation assessment.

GHI values have been uplifted to Global Inclined Irradiation (GII) values using the industry

standard Perez model. For the site in question, we understand module orientation to be due

south with modules tilted at 18°. We would consider that an inclined irradiation of

1,118 kWh/m2/year can legitimately be applied to the site and this figure has been carried

forward in the analysis.


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3.2 Plant Availability

The calculated PR does not include any allowance for plant or grid availability losses. Plant

availability is often linked to, and guaranteed within, the EPC and O&M contracts for the lifetime

of the project.

A central inverter failure generally results in a higher proportion of the plant being unproductive

and the failures can be more complex to repair and may require specialist labour. The increased

complexity and generation impact from a single unit outage can lead to central inverters

contributing towards a lower overall plant availability. Conversely string inverters have an

increased component count and therefore more frequent failures may be expected on-site;

typically string inverter issues can be resolved by the O&M team by replacing the string inverter

with spares stock, assuming number of failures are within expectations and spares are available.

Plant unavailability can be mitigated through the availability guarantee provided by the O&M

contractor or separate guarantees direct with the inverter supplier. RINA notes that typical

financial model assumptions for plant availability do not typically differentiate between string and

central inverter technologies.

For the Huawei string inverters, we note that string-level I-V testing can be undertaken via the

inverter equipment, which would reduce any downtime resulting from annual I-V curve checking

as part of the O&M activities. For the central inverter system, string-level I-V curve checking

would require the string’s combiner box to be disconnected. For one 24-string combiner box, as

per the Cowdown example, approximately 30 minutes would be required to test all 24 strings,

during which time the capacity of the 24 strings – ~6.2 kWp – would be lost. Assuming that

typically 5% of strings would be I-V tested annually, the Cowdown plant would experience around

7.5 hours per year in which 6.2 kWp of capacity is lost. Actual production of the combiner box

during 30 minutes might not reach 6.2 kW; however, as I-V curve checking should be undertaken

only when irradiance levels are above 600 kW/m2, a minimum of 3.7 kW of the 6.2 kW capacity

would be lost, which results in approximately 28kWh per year. Furthermore, undertaking the

testing for the central inverter solution results in additional time and materials costs for the O&M

contractor, which may be reflected in the contract price.

3.3 PR calculation

The PR calculated in this section is modelled at the export meter. The PR calculation is

undertaken based on the design and technical specifications of the plant, PVsyst modelling and

RINA proprietary tools. Where relevant, PVsyst software (v6.52) is used, calculations are

performed using hour-by-hour irradiance and ambient temperature values generated for the site.

The shading has been estimated according to the 3D model shown in Figure 2.


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Figure 2: PVsyst near shading model

The breakdown of the losses for both system variants are reported below in Table 4.


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3.4 PR losses

Table 4: Losses breakdown

Loss String Central Description Comment

Near shadings:

irradiance loss

2.5% 2.5% Loss of irradiance affecting the

modules due to obstruction of

direct sunlight from surrounding

objects. E.g. external shading from

nearby trees or large buildings, and

mutual shading from neighbouring

modules.

Modelled using PVsyst near shading engine, according to the model in

Figure 2. The following near shading objects were noted:

Small trees and bushes along the southwestern subarrays ranging

from 4m to 6m.

Overhead lines running in NE-SW direction dividing the site in two

main subarrays, with towers of height of 24m.

Row spacing of modules is 10m.

Each PV module table comprises 6 rows of modules in landscape

orientation, tilted at 18 degrees from horizontal.

Spectral losses 0.0% 0.0% This takes into account the effect of

operating at a different air mass

and solar spectrum than those at

STC.

These losses are considered to be effectively zero for crystalline

silicon modules.

Angular / IAM 2.0% 2.0% The loss due to times when the sun

is not 90° to the module. This

causes an increase in the reflection

of light from the front glass.

Modelled through the modules’ PAN files which have been modified to

match the information reported on the datasheets and laboratory

testing data where available.

Soiling 1.0% 1.0% Loss of light reaching the cells.

Over its working life the module will

collect dust, dirt, bird droppings and

vegetation on its surface.

Potential soiling loss has been estimated based on local precipitation

records. We would recommend that soiling is monitored and limited

under O&M.

Low irradiance

performance

2.2% 2.2% This loss considers the relative

efficiency of the module when

Modelled through the modules’ PAN files which have been modified to

match the information reported on the datasheets and laboratory


November 2017 │ 01627 - v1.2 17


operating at an irradiance level

other than STC.

testing data where available.

Light Induced

Degradation

(LID)

0.7% 0.7% This degradation causes

performance loss and occurs

during a module’s first operating hours in outdoor conditions and will

reduce the module’s performance when compared to the standard

values measured at STC.

Estimated based on the review of the available literature on LID

effects in p-type polycrystalline modules.

Module quality /

power tolerance

-1.0% -1.0% The loss / gain due to the module

power tolerance is a result of the

deviation in the average effective

module efficiency with respect to

manufacturer specifications.

Estimated based on the datasheet module tolerance information.

Module

temperature

losses

1.0% 1.0% This takes into account of the loss

when operating at a temperature

other than STC.

Modelled within PVsyst using module datasheet information and

based on ambient temperature information taken from Meteonorm 7.

Near shadings:

Electrical effect

0.5% 0.5% Loss due to the current of a string

of modules / cells being reduced to

the current in the most shaded

module / cell, this loss identifies the

electrical effect due to the near

shadings described above in the

first row of this table.

Modelled using PVsyst near shading engine, according to Figure 2

As both variants have the same PV module configuration with the

same number of modules per string, the loss arising from shading-

induced variations in current along the string is similar in both cases.

Scenarios such as passing clouds over the area of a large array or

part of an array being shaded by external objects can cause partial

shading effects, whereby some module strings are shaded and others

unshaded. If unshaded and shaded module strings are feeding into

the same MPPT input of an inverter there will be a temporary

mismatch between the parallel strings. The loss will depend on the

location, shading objects and the number of strings per MPPT input. In


November 2017 │ 01627 - v1.2 18


the comparison considered here, there are 2 strings per MPPT for the

Huawei string inverter and approximately 200 strings per MPPT for the

central inverter. The maximum difference between the two Cowdown

plant variants was estimated by using PVsyst to model the section of

the plant with most external shading objects around it. The electrical

shading loss was higher for the central inverter system with the

maximum loss difference between the two variants being less than

0.05%.

Mismatch 0.3% 0.6% Similar to the previous loss, this

effect takes place when combining

modules with varied characteristics,

such as variations between

allowable manufacturing

tolerances, which will again limit

the current and ultimately the

power for all the modules linked in

the string.

Mismatch arises from any source

which will cause an imbalance in

the current or voltage across the

modules. Current mismatch is more

impacting than voltage mismatch.

Sources of mismatch include

thermal gradients across modules,

uneven soiling, uneven module

degradation, mismatched modules

on a string, mismatched strings into

This loss is based on:

inverter type

DC system configuration, informed by a review of relevant literature

PV module power tolerance, informed by our experience of module

flash test data.

Current mismatch: in the two plant variants here, both inverters have

the same number of modules per string so the current mismatch along

module strings is similar in both cases.

Voltage mismatch: shading of part of an array can induce mismatch

across array sections.

In the comparison considered here, the terrain contains slopes,

resulting in different inter-row shading for different parts of the plant,

which can increase mismatch.

Partial shading events can be caused by, for example, passing clouds,

which is dependent on site location, external shading objects and

differences in inter-row shading between parts of the array, with the


November 2017 │ 01627 - v1.2 19


an MPPT input and partial shading.

Specifically, current mismatch

manifests as the mismatch

between modules connected in

series in a string. We note that

uneven degradation of modules

could lead to mismatch losses

increasing over time. Voltage

mismatch is the difference in

voltages across parallel strings

running into one MPPT input.

overall electrical loss dependent on the number of strings per MPPT

input.

There are 2 strings per MPPT for the Huawei string inverter and

approximately 200 strings per MPPT for the central inverter, which will

slightly increase the mismatch loss for the central inverter variant, as

taken into account in the Near shadings: Electrical effect loss, as

described in the table row, above.

Voltage drops across different lengths of cables to the combiner

box/inverter input can be a source of voltage mismatch also. We have

modelled both system variants to have the same voltage drop

between PV panels and transformers; however, due to the nature of

the string and central inverter configurations, the cable length between

the PV modules and the string inverter are shorter than the total cable

length between the PV modules and the central inverter. For this

reason, the central inverter variant has more potential for mismatch

due to voltage drops.

In addition, due to the different brands and power classes of modules

on site, two of the central inverters must be fed by strings of different

power classes. This has been taken into account and results in a

slight rise in mismatch loss for the central inverter variant.

Ohmics, DC

wiring

0.5% 1.2% Electrical loss due to the Joule

Effect proportional to the voltage

drop along the wiring between the

modules and the inverters.

A max. voltage drop of 2.5% at STC has been estimated based on

RINA’s experience for this type of project design and site conditions. As the central inverter variant has a larger volume of DC wiring, there

is more power DC wiring loss for this variant.

More detailed analysis can be applied if cable loss calculations are


November 2017 │ 01627 - v1.2 20


provided for review.

Inverter

efficiency

1.5% 1.7% Loss due to inverter inefficiencies

in converting DC power from the

modules to an AC power the grid

can accept.

Modelled in PVsyst based on datasheet inverter efficiency information.

We have simulated as close as possible the efficiency values reported

on the datasheets provided. The Huawei string inverter values were

validated by third party laboratory reports, with relatively high

efficiencies presented.

It is important to remember that these values are manufacturer- and

product-specific and is not purely driven by differences in string or

central inverter technology.

Under sizing of

the inverter

0.2% 0.1% This power loss is caused when

inverter power is determined by the

AC output power rating of the

inverter components rather than

available DC power at its input from

the PV system. This loss typically

occurs due to the DC field being

oversized with respect to the AC

rated output of the inverter.

Modelled in PVsyst based on module and inverter datasheet

information as well as the electrical design for the site.

The values of this loss depend on the array design and

temperature/power characteristics of the inverters, as such, it is

possible through plant design to achieve the same value for both the

cases, and is therefore not purely driven by differences in string or

central inverter technology.

MPPT

performance

0.2% 0.8% Loss due to accuracy of the

maximum power point tracking

algorithm of the inverter. As

operating conditions change, the

inverter must determine the

maximum power available from the

modules and adjusts the operating

point as required. The accuracy of

We have developed a methodology to estimate the MPPT loss, based

on climatic information for the site region, and detailed static and

dynamic MPPT inverter efficiency information provided. The Huawei

string inverter has been tested by a third party laboratory and found to

have MPPT system efficiency at the high end of the market.

For the central inverter, we have not been provided with detailed

information on the dynamic and static MPPT efficiencies. Therefore,


November 2017 │ 01627 - v1.2 21


this control algorithm incurs a loss.

This loss is purely related to the

ability of the inverters to track the

maximum power point as tested

according to standard EN50530,

refer to RINA Phase 1 report for

detailed Huawei test results.

Mismatch losses for the projects

are considered separately, see

above.

we have used a standard assumption for the central inverter.

It is important to remember that these values are manufacturer- and

product-specific rather than being purely driven by differences in string

or central inverter technology.

Ohmics, AC LV

wiring




inverters and the transformers.

The loss has been estimated based on RINA’s experience for this type of project design and site conditions. RINA’s has assumed a highly

efficient short run low load loss cable connection from the central

inverters to the LV:MV transformers. As the string inverter variant has

a larger volume of AC wiring, the AC wiring loss is greater for this

variant.



Ohmics, AC MV

wiring




transformers and point of

connection.

Estimated based on our experience for this type of project design and

site conditions.



LV-MV

transformer

1.2% 1.2% Loss due to how efficiently the

transformer is able to convert the

power from LV to MV for

compliance with the connection

Estimated based on our experience for this type of project design and

site conditions.


November 2017 │ 01627 - v1.2 22


characteristics of the grid.

Power factor loss 0.0% 0.0% Loss due to operating the plant at a

power factor other than unity.

For the purpose of this yield RINA has assumed a power factor of

unity at the inverter level. There is likely to be a small amount of

reactive power required to overcome the inductive losses of the

electrical transmission from the inverters to obtain unity power factor

at the connection point. We note that if a non-unity power factor were

chosen, both Huawei string inverters and the central inverter have

similar power factor capabilities.

Self-consumption 0.4% 0.4% Loss due to energy consumption

from the site deducted from the

generation, which can include fans,

heaters, air conditioning, CCTV,

lights, etc. This loss does not

account for the imported energy

when the plant is not producing

energy, i.e. self-consumption

during standby and night-time is

excluded.

Estimated based on our experience for this type of project.

This loss is also exclusive of any large on site loads.

In the two system variants examined here, we have assumed

equivalent features for the site. The central inverters used are outdoor

inverters so self-consumption is expected to be low, furthermore any

imported energy from the grid is not considered as a loss in

production.

Module

degradation

0.2% 0.2% Loss due to natural degradation of

the modules performance during its

operating life.

RINA has undertaken a literature review on module degradation, and

considers a linear annual degradation rate of 0.4% to be an

appropriate assumption for crystalline silicon modules.

For year one, we take the average degradation of 0% at the starting

point and 0.4% at the end of year 1. Therefore, for the purposes of

modelling first year PR numbers a value of 0.2% has been used.

We note that modules within a string may degrade at different rates


November 2017 │ 01627 - v1.2 23


around an average value so, over time, power mismatch of modules

along each module string may increase. Similarly, the overall output of

each string will change with time and might contribute to the voltage

mismatch.

PR 86.4% 85.6%

It can be seen that the string inverter variant has a higher PR than the central inverter variant for the Cowdown PV plant. PV plant losses change

based on various factors, including site specific, technology and component specific characteristics and therefore and cannot be directly extrapolated

to another location or project. For example, the mismatch loss difference between the two variants would reduce if the plant only comprised of one

type and power class of PV module, whereas in this case, the plant design favours string inverters with regards to mismatch loss. Furthermore, many

losses are manufacturer specific, for example inverter efficiency and MPPT performance. Comments on Huawei’s efficiency metrics can be found in Phase 1 Section 2.2


November 2017 │ 01627 - v1.2 24

3.5 Yield estimations

Specific yield is a measure of the output of a PV system per unit of installed capacity (kWh/kWp).

It is a function of the irradiance experienced by a system, and its PR. Year one specific yield

calculations for the Project are shown in Table 4 and Table 5.

Table 4: Year one energy yield for the Huawei string inverter system

PR Installed

Capacity (kWp)

Probability of

Exceedence

Spec. Yield

kWh/kWp

1st Year Production

(kWh)

86.4% 39,860.64 P50 1,026 40,892,170

P75 988 39,364,132

P90 953 37,988,850

P99 894 35,621,892

Table 5: Year one energy yield for the central inverter system variant

PR Installed

Capacity (kWp)

Probability of

Exceedence

Spec. Yield

kWh/kWp

1st Year Production

(kWh)

85.6% 39,860.64 P50 1,017 40,532,487

P75 979 39,015,424

P90 945 37,650,020

P99 886 35,300,063

The yield calculations give a year one specific yield of 1,026 kWh/kWp for the Huawei SUN2000-

33KTL string inverter and 1,017 kWh/kWp for the central inverter. It should be noted that the

energy yield calculation is site specific and cannot be directly extrapolated to another location.

Various factors will alter these yield values including choice of PV components and plant layout.

Plant and grid availability has been excluded from our year one specific yield figures in Table 4

and 5.

P75 and P90 numbers are based on the uncertainties feeding into our yield analysis, which can

be separated into three discrete parts:

Variation in year on year irradiation

Duration of forward modelling period irradiation variability

Uncertainties in the PR modelling assumptions.

The three uncertainties listed above are combined using the common standard error approach.

The overall combined uncertainty for a one year period for the two system variants are shown in

Table 6.


November 2017 │ 01627 - v1.2 25

Table 6: Combined uncertainties over a one year modelling time period

Uncertainty Uncertainties –

Huawei string case

study

Uncertainties –

central inverter case

study

Standard error historic irradiation uncertainty 0.8%

Standard error irradiation uncertainty over time 3.1%

Standard error PR uncertainty 4.5% 4.6%

Combined uncertainty 5.5% 5.5%


November 2017 │ 01627 - v1.2 26

4 CAPEX Comparison

The Capital Expenditure (CAPEX) costs identified in this report are considered to be the upfront

costs to procure, construct and commission the associated PV plant.

Many costs between the two plants will be identical, for example:

PV modules and string cabling

PV module mounting structure

High voltage components

For the purposes of this comparison, we also assume that trenching and conduits for cables will

be identical.

As the aim of the report is to compare the CAPEX costs of the two designs, RINA will only

compare costs that differ between the string and central PV plant designs and will thus exclude

the components listed above. Also excluded from the CAPEX comparison are inverter and

transformer costs. Inverter costs are excluded because they are commercially sensitive and each

piece of equipment does not necessarily have a fixed price per unit. The central inverter comes

within a power station that includes the transformer. For this reason, transformer prices are also

excluded from both scenarios.

We are aware that for the Cowdown PV plant being assessed, the communications system is a

standard RS-485 communications system. Despite this, for the purposes of this study, we will

consider the Huawei string inverter variant as having the power line communications (PLC)

system implemented as this is Huawei’s preferred solution. We are, however, aware that the PLC

might not be suitable for all designs as the length of the AC power cables must be limited

depending on the type of cable, and we are not aware of why PLC was not implemented on the

Cowdown project.

The CAPEX figures come from a variety of sources. The primary source was a price list for a

Huawei SUN2000 string inverter plant and an equivalent central inverter plant that Huawei have

stated is from an EPC contractor. RINA has not verified the provenance of the price list but did

check that the numbers used for the CAPEX comparison were in line with our general pricing

expectations. As the site details were different from the Cowdown site, e.g. the cable lengths and

size of combiner boxes, some of the prices were scaled accordingly. Prices from our sources,

were sometimes in euros and US dollars. In these cases we converted to pounds sterling using

an exchange rate correct for the time the price was quoted.

4.1 CAPEX results

The result of the CAPEX comparison for specific components is summarised in Table 7. The two

sets of CAPEX figures are based on the design assumptions stated in Section 2.3.


November 2017 │ 01627 - v1.2 27

Table 7: CAPEX summary for the two plant variants

Huawei string inverter variant Central inverter variant

Equipment Total cost (£) Equipment Total cost (£)

Major equipment Major equipment

AC Combiner Boxes 130,036 DC Combiner Boxes

inclusive of monitoring

hardware

429,660

Smart logger and

associated equipment.

32,000

Foundations Foundations

Inverter, combiner box and

substation foundations/piles

224,135 inverter/transformer station

and combiner box

foundations/piles

81,984

Electrical (Drop. Volt. 2.19% at STC) (cable

details found in Section 2.3)

Electrical (Drop. Volt. 2.16% at STC) (cable

details found in Section 2.3)

DC string cables (modules

inverters)

1,882,447 DC string cables (modules

combiner boxes)

1,882,447

AC LV cables (inverters

combiner boxes)

49,386 DC cables (combiner boxes

inverters)

96,761

AC LV cables (combiner

boxes substation

248,814 AC cables (inverters

transformers)

60,274

RS-485 cable for

communications

26,599

Total (£)£ £ 2,566,818 £ 2,577,725

Total (£/kWp) £64.40/kWp £64.67/kWp

Delta 0 +0.1%

Based on the assumptions made here, it can be seen that the string and central inverter variants

have negligible difference in CAPEX values, taking into account only the components listed in the

above table. It should be noted that inverter costs are excluded which form a high proportion of

project costs. We note that the key variables and cost differences between the two variants are

very sensitive to component price negotiated with suppliers, general design optimisation and

component quality and absolute prices for both variants are sensitive to fluctuations in material

prices and currency exchange rates.

The string inverter solution offers particular savings in some areas, namely:

combiner boxes: AC combiner boxes may be cheaper than DC combiner boxes as they have

fewer internal components

communications system: communication wiring is not required for PLC, resulting in costs and

installation time savings

The central inverter variant can offer savings in cabling as shorter lengths of more expensive AC

cabling are required than in the string inverter design.


November 2017 │ 01627 - v1.2 28

5 OPEX Comparison

A major part of the expenditure associated with the operation of a PV plant is in repair and

replacement of the failed equipment to ensure high performance and availability of plant during

operation.

Maintenance activities will depend upon several factors including the specifications of O&M

contract, any after-sales services available from component manufacturers, preventative

maintenance plans and the reliability, complexity and overall lifespan of plant components.

5.1 Modelling Inputs and Assumptions

For the purpose of predicting future operating expenditure associated with maintenance activities,

RINA has developed a comprehensive lifecycle costing model for PV plants (the ‘Model’), taking into account extensive and up-to-date failure analysis of the components used on a PV plant.

Using this Model, Corrective Maintenance (CM) costs are calculated.

The Model:

Performs statistical failure calculations for key components by considering:

– expected mean component lifetime (years)

– a standard deviation (in years) around that lifetime

– random failure rate (%)

Takes account of manufacturers defect warranties, EPC contractual warranties and any

known purchased extended warranty contracts for equipment

The OPEX Model is developed from a technical foundation applicable to PV systems worldwide

and regional financial assumptions have not been included in the Model, such as:

No inflation

No discount rates

Availability of spare parts at today’s prices

No import taxes and duties or non-typical shipping costs

5.1.1 O&M coverage

Maintenance activities can be divided in terms of preventative and corrective maintenance

components.

O&M costs are not included in the OPEX comparison however we expect the following to be

included in the O&M contract:

Preventative maintenance and consumables

Corrective maintenance:

– Equipment failure diagnosis

– Labour time required for equipment procurement and repair.

O&M costs can often be slightly higher for central inverter systems than string inverter systems,

mainly because central inverter maintenance activities can be more complex than those of string

inverters, sometimes require specific scheduled maintenance activities and require more highly

skilled technicians, who will have a higher cost of labour. Furthermore we note that the Huawei


November 2017 │ 01627 - v1.2 29

string inverters offer string-level I-V curve tracing and analysis, which can be done without the

need of the strings being disconnected. For the central inverter variant, the combiner box would

need to be disconnected before annual I-V curve tracing is carried out. Therefore savings may be

found with fixed price O&M contracts for string inverter sites compared with equivalent contracts

for central inverter sites. The O&M cost, and any differences to be found between string and

central inverter sites, will depend on project specifics.

We expect material costs for replacement components to be excluded from the O&M contract. As

such, RINA’S OPEX analysis considers only the replacement component costs, i.e. the

components are billed to the project owner by the O&M contractor.

5.1.2 Modelling approach

The system design is a key factor in determining project-specific failure expectations and

provides a more accurate approach to cost modelling than a more simplistic forecast on a ‘cost-per-MW’ basis.

Similarly to the CAPEX comparison above, for the purpose of the OPEX comparison, RINA

consider that some plant components will vary between the two system variants and others will

be common to both. Those components that are common between the two system variants are

excluded from the comparative OPEX analysis.

Plant components that are common to both plants are:

PV modules and string cabling

PV module mounting structure

High voltage system.

Plant components that differ between the two plants:

DC and AC cabling (excluding string cabling)

Communications systems

Inverter type

Transformer ratings are the same between the two plants; however the central inverter

solution contains the transformer within the inverter.

At the request of the Client we are excluding inverter and transformer costs from the OPEX

comparison, instead presenting an analysis of the qualitative differences between the two

systems.

5.2 Qualitative and quantitative OPEX results

The DC and AC component repair/replacement costs are shown in Figure 3. These figures are

excluding the inverter and transformer. The main reactive maintenance tasks that contribute

towards these costs are summarised below;

Central inverter:

– DC main distribution cables – insulation faults

– DC Combiner box replacement:

DC Combiner box component replacement, such as; Combiner box surge protection

DC isolators

Monitoring hardware

Fuse holders


November 2017 │ 01627 - v1.2 30

String inverter:

– AC LV distribution cables – insulation faults

– AC Combiner box replacements and repairs

– AC Combiner box component replacement, such as;

Combiner box AC surge protection

Circuit breakers

Smartlogger communications box replacements

We assumed a 2 year warranty for all of the above components.

Figure 3: DC and AC component corrective maintenance costs

The resulting costs per kilowatt peak over the 25-year plant lifetime, for the components analysed

are:

£2.80 per kWp for the Huawei string variant

£3.77 per kWp for the central variant.

The higher costs for the DC subsystem in the central inverter configuration are primarily the

following:

the DC combiner boxes having more internal components than the AC combiner boxes used

on the string inverter plant

the slightly greater number of DC combiner boxes than AC combiner boxes

The standard RS-485 communications system costs being a little higher than for the Huawei

PLC Smartlogger solution, which does not require an additional RS-485 cable present which

might require repair. We note, however that there is a limited track record for the Smartlogger

solution so reliability cannot be vouched for as easily as for a standard communications

system.

The costs above are specific to Cowdown PV plant and are sensitive to the following

assumptions:

Plant design: specific number of components and dimensions of cables

Cost of each component

Lifetime of components.

If the plant design is optimised, component numbers can be reduced, which may decrease plant

maintenance activities over the plant lifetime (depending on other component specifications,

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

20,000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Co

st (

£)

Year

Central inverter String Inverter


November 2017 │ 01627 - v1.2 31

failure rates etc.). RINA has used standard cost and lifetime assumptions for the DC and AC

component OPEX analysis, with the exception of the Smartlogger, for which we have considered

the price as given by Huawei. Lifetime of components will change with each different component

brand and model and costs may be dependent on commercial factors rather than being a

standard fixed item.

5.2.1 Inverter repair expectations

Figure 4 shows the modelled failure profile for string inverters across the plant’s 25-year lifetime.

Figure 5 shows the modelled failure profile for the central inverters. These profiles were

calculated based on expected mean inverter lifetimes (or those of their components), the

standard deviation around that lifetime and a random failure rate. Whilst the products are under

manufacturer warranty, no costs are expected to be incurred by the Client. The warranty period

for both inverters is considered to be 5 years, which can be extended in both systems, typically in

5 year intervals up to 20 years. The failure profiles shown are qualitative only, with actual OPEX

costs for the inverters dependent on several commercial factors.

Figure 4: String inverter corrective maintenance cost profile at different inverter mean

lifetimes

For the string inverter, we expect some minor preventative maintenance tasks might be required,

e.g. cleaning of heat sinks, however the full unit is swapped out upon failure which falls under

corrective maintenance. Failure rate data provided by Huawei for the Phase 1 report (see Phase

1 Section 4) indicated relatively low failure rates, averaging at around 0.3% per annum. From the

various pieces of failure and reliability evidence presented by Huawei, we consider that a lifetime

in the region of 15 to 20 years could be reasonably assumed, which is greater than our standard

assumption of 12.5 years. Huawei claim that the lifetime is 25 years, however we have not seen

the calculation that demonstrates this lifetime.

As can be seen from Figure 4, the lifetime of the Huawei string inverter is a key consideration

when it comes to plant running costs. The replacement ratio (number of times an inverter must be

replaced in the 25-year plant lifetime) 1.76 when the inverter lifetime is 12.5 years compared with

1.20 when the inverter lifetime is 17.5 years. The difference between these two replacement

ratios stresses the importance of obtaining failure rate and lifetime testing data when considering

OPEX costs.

0%

2%

4%

6%

8%

10%

12%

14%

16%

18%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25An

nu

al

cost

of

inve

rte

r re

pla

cem

en

ts

(% o

f to

tal co

st o

ve

r p

lan

t li

feti

me

)

Year

12.5 yr mean lifetime, 5 yr warranty 17.5 yr mean lifetime, 5 yr warranty


November 2017 │ 01627 - v1.2 32

Figure 5: Central inverter corrective maintenance cost profile

The OPEX costs for the central inverter system are based on corrective maintenance, i.e.

replacement of central inverter sub-components at the time those sub-components fail. We are

aware that another common approach to central inverter maintenance is to have a refurbishment

schedule, whereby the inverter manufacturer recommends an overhaul and service including the

replacement of key components at certain intervals during the plant lifetime. Different central

inverter components have different lifetimes and thus different replacement frequencies. A

corrective maintenance cost profile of an inverter with a planned refurbishment schedule would

look different from Figure 5 and there would be more concentrated expenditure during the

planned refurbishment time.

In both cases, the total maintenance costs are sensitive to the assumed lifetime of components:

lifetime assumptions of string inverters and central inverter component replacement frequencies

have a considerable impact on the corrective maintenance expenditure for a project.

Furthermore, the duration and specifications of manufacturer warranty and after-sales packages,

the preventative maintenance plans and the O&M contract specifications will have a large impact

on the OPEX costs.

0%

2%

4%

6%

8%

10%

12%

14%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25An

nu

al

cost

of

inve

rte

r re

pa

irs

(% o

f to

tal co

st o

ve

r p

lan

t li

feti

me

)

Year

5 yr warranty


November 2017 │ 01627 - v1.2

RINA Consulting Ltd.

2nd Floor Offices

Nile House, Nile Street

Brighton, BN1 1HW, UK

+44 (0)1273 819 429

www.rinaconsulting.org

http://www.rinaconsulting.org/

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Huawei SUN2000 Inverter Technical Review and Comparative Study · Huawei SUN2000 Inverter Technical Review and Comparative Study November 2017 │ 01627 - v1.2 ... (PRs) and energy