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INDIA SOLAREXCELLENCE

Inverterdesign andselection

March 2018

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© 2018, BRIDGE TO INDIA Energy Private Limited

Research/Authors:Mudit Jain, BRIDGE TO INDIAVinay Rustagi, BRIDGE TO INDIA

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Acknowledgements

Following the release of our earlier report “DC cable design and performance issues”, we are pleased to issue this report highlighting various technical and commercial aspects related to inverter design, selection and operations.

Many industry experts have contributed with their valuable time and support in preparation of this report. We acknowledge the support extended by the following companies:

a) PSU’s – NTPC and Solar Energy Corporation of India (SECI)

b) Inverter manufacturers – Huawei, Sungrow, ABB, SMA, Delta, Ingeteam, Gamesa, GE, TMEIC, Delta and Hitachi

c) Project developers – ReNew Power, Aditya Birla, Mytrah Energy, Hero Future Energies, Amp Solar, Acme, Fortum and Avaada Energy

d) EPC contractors – Mahindra Susten, Juwi India, Tata Power Solar, Sterling & Wilson, Bosch, Gensol, Oriano Solar, Rays Power Experts, Ujaas Energy, Enrich Energy, Emmvee Solar and Fourth Partner

e) Module manufacturers – Trina and First Solar

f) Consultancy firms – 3E and ReSolve Energy

We are especially grateful to NTPC, Huawei, 3E, Aditya Birla, ReNew Power, SMA, Gamesa, GE and Gensol for reviewing the final report and providing detailed valuable feedback.

http://www.bridgetoindia.com/reports/india-solar-excellence-2016-edition-dc-cable-design-performance-issues/


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Contents

1. Introduction 1

2. Reasons for inverter failure 2

3. Working of an inverter 43.1 MPPT controller 43.2 PWM 53.3 Reactive power support 63.4 Auxiliary power consumption 63.5 Other technical add-ons 7

4. Inverter design and selection 84.1 Central and string inverters 84.2 1,000 V vs 1,500 V systems 114.3 DC to AC inverter overloading 134.4 Master-slave combination 144.5 Paralleling 15

5. Regulations and standards for inverters 175.1 Harmonics 175.2 Frequency range 175.3 Voltage range 185.4 Other requirements 19

6. Commercial aspects 216.1 Cost 216.2 Up-time guarantee and warranty support 22

7. Inverter supply landscape 237.1 Utility scale market 237.2 Rooftop market 257.3 Domestic manufacturing 25

8. Conclusion 27

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List of figuresFigure 1: Solar plant layout 1Figure 2: System failures 2Figure 3: Functioning of inverter 4Figure 4: Layout with one or more MPPTs 5Figure 5: Effect of topologies on smoothening of sinusoidal wave 6Figure 6: Schematic diagram of central and string inverters 8Figure 7: Use of central and string inverters in India up to 2016 9Figure 8: Use of central and string inverters in India, 2017 9Figure 9: Market share for string and central inverters in the USA 10Figure 10: Cost savings for 1,500 V system in comparison to 1,000 V

systems, USD/W 12Figure 11: Will 1,500 V systems achieve 50% market share by 2019? 12Figure 12: Estimated global market share of 1,500 V inverters 13Figure 13: Benefits and losses of higher DC overloading 13Figure 14: DC overloading in India 14Figure 15: HVRT functionality of inverter proposed by CEA 19Figure 16: Price trend of central inverters in India, INR/W 21Figure 17: Global solar inverter average sales prices by technology

type, 2010-2022, USD/W 21Figure 18: Market share of top-10 inverter suppliers in India 23Figure 19: Inverter suppliers ranking trend globally 23Figure 20: Inverter suppliers ranking trend in India 24Figure 21: Share of top players in Indian utility scale solar

inverter market 24Figure 22: Share of top players in Indian rooftop solar inverter market 25

List of tablesTable 1: Comparison between string and central inverters 8Table 2: Standards followed in India 17Table 3: Inverter components and sourcing 25

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1. IntroductionAs solar sector continues to grow worldwide, technology has been continuously evolving to drive down cost, improve performance and lower the levelized cost of energy. Photovoltaic (PV) modules have played a key role in this process but an almost equally important role is played by inverters – the heart of any solar power system.

In most basic terms, an inverter can be defined as an electronic device that converts DC electricity into AC electricity. It acts as the gateway between the solar plant and the energy consumer or the grid.

Figure 1: Solar plant layout

Inverter

Inverter

Transformer

DC input

DC input

AC output

AC output

AC switchgear

Combiner box

Layout with string inverter

Layout with central inverter

OR

The inverter is the only component in a solar plant with interactive capabilities. Alongside inverting DC power to AC, it performs multiple other functions to produce high quality power:

i. System monitoring and controlling;

ii. Fault detection and protection for the solar plant;

iii. Grid management including power output variation, frequency and voltage management and dynamic grid support.


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2. Reasons for inverter failureMaintaining high system up-time is critical for ensuring financial viability of solar projects. Electric Power Research Institute (EPRI) found in its research that inverter failures account for 61% of total energy loss in solar plants. Out of all the components used in solar plants, inverters had the highest failure rate and resulted in the highest energy loss for all operational plants of SunEdison in 20121.

Figure 2: System failures

Despite best efforts, malfunctioning of inverters occurs regularly. Most common reasons include:

i. Failure of fans: Fans play a crucial role in maintaining rated ambient temperature of the inverter. Failure of fans is a common problem.

ii. Malfunctioning of IGBT: IGBT is one of the most crucial components of inverter. Malfunctioning of IGBT can lead to distortion of output waveform or complete shutdown of the system.

iii. Fire: This is the most hazardous risk caused by DC arcs, short circuiting or overheating.

iv. High temperature: The operating temperature of the inverter reaches over 50 degree C. In some cases, inverter trips at such high temperatures.

v. Grounding faults: Grounding faults can arise with water logging or broken sheath and insulation of cables and can cause tripping of inverter.

vi. Grid fluctuation: Inverter trips if fluctuations in grid are beyond the operating levels of inverter. However, if the protection system of inverter doesn’t work, it can also cause burning of circuits.

Inverter selection is a very important attribute of a solar plant with critical impact on its performance. Although inverters are standard products, there are many variants available with different features and design parameters which must be considered in detail while finalizing inverter selection. There is no "best" product for all purposes.

1 GTM Research

Number of failure events Energy loss from failure

Source: GTM white paper 2014, “Ensuring Success In Global Utility Solar PV Projects”

36% Inverter

Planned outage 8%

DC subsystem

4%

Support structure 3%

External factors 20%

21% AC subsystem

Others 8%

43% Inverter

Others 13%

Planned outage 5%

DC subsystem 6%

Support structure 7%

External factors 14%

14% AC subsystem


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3. Working of an inverter

An inverter encompasses multiple functional components, two of which are described in more detail below:

i Maximum Power Point Tracker (MPPT)

ii Pulse Width Modulator (PWM)

Figure 3: Functioning of inverter

3.1 MPPT controllerIn a large solar plant, power from multiple PV strings is fed into a single inverter. Typically, 100 – 200 strings may feed power into a single 1 MW central inverter (for detailed discussion on central vs string inverters, please refer to Section 4.1). An MPPT controller extracts maximum power output from the PV array by optimizing output voltage. It is highly useful at times of array underperformance because of, for example, lower irradiation or higher ambient temperature. The MPPT controller dynamically optimizes power output even with varying irradiation, temperature and other ambient conditions.

MPPT can be segregated in two parts – hardware MPPT controller and a software algorithm. Hardware MPPT convertor is basically a DC to DC convertor, which can isolate the strings in blocks and optimize the performance of each block. It is useful in installations in uneven terrains with much higher variation in individual string performances. Not all PV inverters have a hardware MPPT controller or DC to DC converter, but all PV inverters have a software MPPT algorithm to optimize power output function. In modern central inverters, only the software algorithm is incorporated whereas, most string inverters have hardware MPPT controller in addition to software algorithm.

With increasing inverter block sizes, MPPT plays an increasingly important role. If there are multiple strings connected to a single MPPT, the performance of some strings is enhanced but other strings may be forced to underperform and required to operate at a lower voltage than they are capable of. Increasing the number of MPPTs in an inverter enhances overall system performance by reducing the number of strings connected with one MPPT.

Inverter

DC power

Voltage optimization

PWMHardware MPPT controller

AC power

AC side

ACDC

MPPT algorithm

SET

OK

MMPTController

MPPT controller is useful at times of array

underperformance because of lower irradiation or higher

ambient temperature


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Figure 4: Layout with one or more MPPTs

Inverter Inverter

Inverter with single MPPT Inverter with 4 MPPTs

MPPT

PWMPWM

The cost of a MPPT controller is negligible for large size inverters. Total performance advantage of using MPPT could be around 30% to 40% over an inverter without any MPPT. This explains why all commercially used PV inverters come with MPPT controllers. The only exception where MPPT controllers are not used are very small inverters, usually up to 1 kW size, where cost outweighs performance advantage.

3.2 PWMDC to AC conversion of electricity essentially means converting a direct (or constant) wave form to a sinusoidal waveform. In practice, it is impossible to replicate the exact sinusoidal wave. Solar inverters use PWM to transform DC power into a granular step function to replicate sinusoidal waveform. Most efficient method of carrying PWM switching is through Insulated Gate Bipolar Transistor (IGBT), which is used in all modern day solar inverters.

The most basic conversion technique is referred to as the basic topology or a 2-level topology. Higher topologies of inverter effectively create intermediary steps in the wave form to improve smoothness. Circuit designs for PWM have been evolving to achieve higher level of resemblance with sine wave by increasing the number of topologies. Around 75% of inverters used in India follow 2-level topology. Other inverters use 3- or 4-level topologies. Lot of research is being undertaken to increase the number of topologies and commercial production of 5-level topology inverters is about to begin.

PWM operation inherently generates harmonics. Increasing the number of topologies can also reduce total harmonic distortion (THD), but at an additional cost. Increasing topologies requires use of more IGBTs - a 3-level topology typically requires double the number of IGBTs than that required for a 2-level topology.

SET

OK

MMPTController

SET

OK

MMPTController

SET

OK

MMPTController

SET

OK

MMPTController

SET

OK

MMPTController


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Figure 5: Effect of topologies on smoothening of sinusoidal wave

2 level topology

V(t) V(t)

t

3 level topology

3.3 Reactive power support It is well understood now that increasing penetration of variable solar power has ramifications for grid stability, if not properly managed. Inverters can help in improving grid stability and management by providing reactive power support.

Reactive power arises due to phase differences between voltage and current. This phase difference is created by electric and magnetic fields in inductive and capacitive loads such as motors or capacitor banks. DISCOMs mandate customers with high power load to maintain a power factor close to one. Residual reactive power demand is managed by the DISCOMs themselves.

An inverter is capable of running on non-unity factor on demand. It can help in managing reactive power in two ways – it can adjust power factor very quickly (not usually possible for conventional AC power rotating generators) and some PV inverters can provide reactive power even when not generating real power.

Germany was the first country to mandate reactive power support from inverters. Most tenders or regulations in India do not mandate such support. Karnataka is the only state to mandate solar plants to supply around 20% of capacity as reactive power support. Supplying reactive power support reduces active power output, but the impact is not significant. Even at full load, the impact of 20% reactive power support could be only around 2% loss of active power. Although the impact is small, retrospective change in technical requirements for reactive power support is a potential risk for project developers.

String inverters are usually capable of operating from 0.8 lagging to 0.8 leading power factor and can provide only static reactive power support i.e. the parameters need to be fixed upfront. String inverters can offer dynamic reactive power support only by retrofitting capacitor banks at plant level. On the other hand, central inverters offer power factor over a much wider range and can provide dynamic reactive power support.

3.4 Auxiliary power consumptionThe operating temperature inside the inverters is much higher than ambient temperature. Usually, inverters come with attached fans to reduce the temperature. However, these fans require auxiliary power to operate. Further, inverters also need auxiliary power for displays and control systems.

Until recently, most inverters derived this auxiliary power by taking connection from an auxiliary transformer installed at the plant. But this required power

Retrospective change in technical requirements for

reactive power support is a potential risk for project

developers

Inverters can provide reactive power support even when they are not generating real power


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to be stepped down once again and with so many conversions at multiple stages, losses used to increase reducing the actual inverter efficiency during operations at high temperature. Most modern inverters offer DC to DC convertor inside the main unit to derive power at the required voltage right from the DC source reducing losses considerably.

3.5 Other technical add-onsNegative grounding

A major factor impacting performance of solar PV modules is the loss due to potential induced degradation (PID). This loss arises due to ion migration between module and its grounding cover under high voltage, which creates leakage current through frames creating potential difference between PV cells and ground. PID is usually caused because of humidity and module surface conductivity. Most solar PV modules available in the market today are PID resistant, but they do not eliminate PID entirely. This risk can be mitigated by attaching a negative grounding kit, commonly known as Ground Fault Detector Interrupter (GFDI) with the inverter to negate the potential difference between modules and ground by dragging the ions back from the frame.

For string inverters, installation of GFDI kit would require isolation transformer with each inverter which is deemed to be very expensive. Instead of using GFDI kit (which is a preventive measure), a few string inverters supply anti PID kit. Usually, this anti-PID kit connects PV modules to positive offset battery and reverse charge the module by inputting positive voltage to modules at night, dragging the migrated ions back. This is only a corrective step and may not reverse the PID effect entirely. Moreover, its difficult to control reverse charging level and module manufacturer may not approve this as it could damage the modules. A more improved version of anti-PID kit in string inverters works during the day time by injecting DC voltage and adjusting module and ground voltage levels eliminating PID effect (which is a preventive measure).

DC optimizers

DC optimizer is a DC to DC converter which has an inbuilt MPPT functionality and can also be used for string monitoring. It is a step ahead of string inverters and can maximize the power of each module individually instead of whole string. DC optimizers are attached to the selected number of strings, thus providing better redundancy, improved MPPT performance and more granular monitoring. Their use is especially beneficial at uneven sites and rooftop projects, where risk of shading or other non-confirm power output variation is relatively high. Depending upon the extent of use, DC optimizers can cost between 50%-100% of the string inverter cost

Inverters can help correct PID effect in modules and improve

performance


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4. Inverter design and selectionInverter selection for a solar plant depends on many factors – size of the plant, site layout and levelling, technical features required etc. and is a compromise between cost, output and performance. It also dictates design considerations for the entire balance of system – cabling, combiner boxes, connectors, switchgears etc.

4.1 Central and string inverters There are primarily two types of inverters:

a) String inverters – These inverters are usually available in the range of 1 kW to 150 kW. They typically provide higher optimization of generation in case of variation within the strings.

b) Central inverters – These inverters are available in the range of 250 kW to 2,000 kW.

Figure 6: Schematic diagram of central and string inverters

Table 1: Comparison between string and central inverters

String inverters Central inverters

Size Typically, between 1 kW to 150 kW Typically, between 250 kW to 2.5 MW

Monitoring String level At central inverter level. For string level monitoring, string monitoring level combiner boxes are required.

Number of MPPTs Multiple MPPTs for same power output:• Better redundancy, breakdown results in

only partial loss of power in the system;• Improved output.

Usually single MPPT per inverter, but a few inverters may have up to 4 MPPTs.

Relatively poor redundancy as number of inverters is small and each inverter has a large capacity.

Design flexibility High Low

Ambient protection Suited for outdoor use (IP65 or IP54 protection) Generally suited for indoor use only (most prevalent protection codes followed are IP 42 and IP 20). A few companies supply central inverters for use in outdoor applications with IP54 or IP65 protection.

Weight Typically, less than 100 kg Typically, much more than 1,000 kg

Upfront cost INR 3.50-4.00/ W• Lower DC wiring but higher AC wiring cost• Additional requirement of sub-LT panel• Additional requirement of main LT panel

with AC breaker

INR 1.80-2.40/ W• Additional requirement of string combiner boxes• Container or civil structures required for

housing• Higher DC wiring cost

Central inverter

String combiner boxes

Central inverter

String inverters

String inverter

AC

AC

AC

DC

DC

DCAC

DCAC

DCAC

DCAC


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Central inverters have been used globally for a long time with proven track record. In India, they account for 98% of total installed utility scale capacity. Primary reasons for such dominant historic market share include:

a) Cost advantage: Central inverters are cheaper than string inverters;

b) Simple site terrains: Utility scale projects in India have been built mostly on flat lands. As different strings perform similarly in such conditions, central inverters produce optimal output with little performance risk;

c) Low maintenance due to lesser terminations on AC side.

Figure 7: Use of central and string inverters in India up to 2016

99%

Utility scale – 9.3 GW Rooftop solar – 1.2 GW

Source: BRIDGE TO INDIA research

In contrast, string inverters are preferred for rooftop solar plants:

a) Light weight: Being much lighter and resistant to harsh atmospheric conditions, string inverters can be installed on rooftops or other suitable outdoor places;

b) Flexibility: String inverters allow optimization of system design consistent with roof size, orientation and shading profile.

However, utility scale projects are increasingly using string inverters. As per our research, 9% of new utility scale solar capacity commissioned in 2017 uses string inverters and this trend is expected to accelerate in the coming years.

Figure 8: Use of central and string inverters in India, 2017

9% of new utility scale solar capacity commissioned in 2017

uses string inverters

Central inverters 99%

String inverters 1%

Hybrid inverters 16%


String inverters 82%

Source: BRIDGE TO INDIA researchNote: The data is for utility scale solar capacity addition in 2017 – 8.3 GW


String inverters 9%


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Increased adoption of string inverters for utility scale projects is in line with experience in other international markets including USA and China. As much as 50% of utility scale capacity installed in China in 2016 used string inverters.

In the USA, only 5% of utility scale solar installations in 2015 were built with string inverters - mostly small projects with site-specific requirements favouring usage of string inverters. But the market has been gradually moving towards string inverters with string inverter share expected to grow up to 28% in 2020.

Figure 9: Market share for string and central inverters in the USA

String vs central inverter debate

Historically, there was a big cost gap between string and central inverters even after accounting for savings in BOS costs in the case of string inverters. That price advantage made central inverters the pre-dominant choice for utility-scale projects. But the gap has been narrowing slowly with faster reduction of prices in string inverters than that of central inverters, resulting in a heightened debate on relative technical merits of central and string inverters. The jury is still out to find the “best solution”:

i. Flexibility and yield: Traditionally, central inverters were used in large open spaces with flat terrain (minimum contour variation). This configuration worked well even with limited number of MPPTs (1-4) in central inverters as the voltage in all the strings is generally equal. String inverters were seen as beneficial for projects built on uneven sites due to their multi-MPPT string design and better power output.

Although efficiency of central inverters is usually higher, string level optimization can deliver better power output due to increased reliability with increase in redundancy from relatively flat terrains as well. Therefore, as project developers increasingly focus on reducing levelized cost of energy, many of them prefer multi MPPT design using string inverters.

Source: GTM Research

Central invertersString inverters

String inverter share is expected to grow from 5% in

2015 to 28% in 2020 USA

2015

100%

90%

80%

70%

60%

50%

40%

30%

20%

10%

0%2016E 2017E 2018E 2019E 2020E

5% 8%

15%22% 26%

72%74%78%85%92%95%

28%

String inverters can deliver better power output due

to increased reliability and better redundancy


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ii. Installation time: As string inverters do not need special containers or civil structures for housing, they can be installed much faster than central inverters, useful for projects with tight deadlines.

iii. System availability and maintenance: Because of their large size and weight, central inverters are difficult to replace and require dedicated technical support at all times. Any failure results in shut down of a large part of the plant. But better reliability means that extended warranty or replacement is required only for select components such as IGBT, fans and harmonic filter.

On the other hand, small, modular design of string inverters means that spares can be maintained and installed quickly in the event of any failures. String inverters are believed to simplify technical maintenance requirements, but their extended warranty costs are higher in comparison.

As per a National Renewable Energy Laboratory (NREL) study in 2016, “Best Practices in Photovoltaic System Operations and Maintenance”, cost of string inverter maintenance is higher in comparison to central inverters. However, falling costs of string inverters and improvements in technical features of both types of inverters means that this difference has shrunk over the years.

Other types of inverters

There is a third type of inverter – hybrid solar inverter – used primarily in rural and remote areas. Hybrid solar inverters can run even when grid power is unavailable. Such inverters are typically connected with both grid and batteries for supplying power throughout the day for use in locations with frequent power cuts. Improvement in grid power supply and implementation of net-metering policies is expected to make hybrid inverter usage scarce over time.

Another technology gaining traction in the inverter market is micro-inverters. These inverters are designed to operate with a single PV module. Combination of each PV module and microinverter produces AC power output which can be connected directly with home power supply. Such inverters are great for complex installations with shading issues or difficult roof layouts and can monitor performance of each individual module. Microinverters are suited for very small applications and are more efficient, but also significantly costlier, estimated to be around four times than string inverters limiting their use to very small installations.

4.2 1,000 V vs 1,500 V systemsThe solar industry moved from 600 V systems to 1,000 V a few years ago and is now looking to upgrade to 1,500 V to further bring down the levelized cost of energy. Increasing DC side voltage results in multiple benefits:

i. Reduced power losses – Shifting to 1,500 V systems reduces DC power losses by 33%2;

ii. Infrastructure cost saving – due to higher number of modules in the string, number of combiner boxes and length of DC cable is reduced;

iii. Lower installation time and cost – due to fewer equipment installation;

2 BRIDGE TO INDIA report, “India Solar Excellence | DC Cable Design And Performance Issues”, http://www.bridgetoindia.com/reports/india-solar-excellence-2016-edition-dc-cable-design-performance-issues/




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DC components such as switchgear, fuse and circuit breakers for 1,500 V systems are slightly costlier in comparison to 1,000 V systems but this cost difference is expected to be eliminated as industry eco-system adjusts over the next 2-3 years. A parallel can be drawn from the US market, where most project developers have already started adopting 1,500 V systems. Several component suppliers also have set up facilities to supply the equipment with higher rating.

Total expected savings in project cost from using 1,500 V inverter over 1,000 V inverter are estimated at about INR 3.20/ W, or about 5% of total capital cost, as shown in the chart below.

Figure 10: Cost savings for 1,500 V system in comparison to 1,000 V systems, USD/W

Indian solar market is in the early stages of this transition to 1,500 V systems. A total capacity of over 1 GW, expected to be commissioned in early 2018, is being planned with 1,500 V architecture. Leading industry players are optimistic that more than 50% of industry will migrate to 1,500 V systems by 2019.

Figure 11: Will 1,500 V systems achieve 50% market share by 2019?

Cables, conduits

and trenching

Direct labour

Combiner boxes

AC subsystem

PV modules

PV inverters

Netsaving

0.08

0.06

0.04

0.02

0


Source: BRIDGE TO INDIA survey of project developers, EPC players and inverter suppliers in India - a total of 18 responses were received

No 28%

Yes 72%

IHS has forecast international market share of 1,500 V systems to rise from 6% in 2016 to 50% by 2020.

Total expected savings in project cost from using 1,500 V

inverter over 1,000 V inverter are estimated at about 5% of

total capital cost


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Figure 12: Estimated global market share of 1,500 V inverters

4.3 DC to AC inverter overloadingIn an attempt to reduce levelized cost of power, it is common industry practice to pair inverters with over-sized DC module capacity. A 1 MW DC plant rarely produces 1 MW of power as solar modules operate at their maximum efficiency only during peak hours of noon and that too during select months of the year. DC overloading allows the plant to increase generation during non-peak hours. However, it creates a situation where peak-hour generation may cross capacity constraints of the inverter forcing the inverter to ‘clip off’ extra generation.

Optimal DC:AC overloading requires optimizing the trade-off between clipping losses and extra generation at different times of the day and year. This trade-off is complicated by the fact that solar modules degrade over time causing reduction in clipping losses in the later years.

Figure 13: Benefits and losses of higher DC overloading

Source: IHS, “PV inverter market tracker Q3 2016”

1,500 V 1,000 V

100%

80%

60%

40%

20%

0%2015 2016 2017E 2018E 2019E 2021E

3%

97% 94% 84% 68% 56% 50%

6% 16%

32% 44%

50%

''Clipping'' losses (energy not generated due to inverter power limitation)Additional energy generated by increasing DC-to-AC ratio

6am 6pmNoon

Time of day

Low DC-to-AC ratio

High DC-to-AC ratio

Syst

em p

ower

1 MWAC

Increasing DC capacity reduces effective cost on transmission lines, AC side equipment and soft costs, while increasing power output at the same time.

Optimal DC:AC overloading requires optimizing the

trade-off between clipping losses and extra generation at

different times of the day and year


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New project tenders in India in recent years do not prescribe any cap on DC:AC overloading.

Choosing optimal DC:AC ratio depends on multiple parameters:

i. Irradiation: Higher irradiation results in higher clipping losses and hence, lower DC overloading is suggested for such projects. For projects installed in areas with lower irradiation, higher DC overloading is recommended.

ii. Site temperature: Site temperatures affect power output from the modules. Most utility scale project sites in India have high temperatures (over 40 degree C) during peak-hours, resulting in underperformance of the system and lower clipping losses. Hence, higher overloading may be desirable for such sites.

iii. Land availability: Increasing DC overloading requires more land. Land availability constraints, in solar parks or other projects, may dictate the amount of overloading.

iv. Inverter capability: Inverters from different manufacturers come with varying capability to accommodate DC:AC overloading depending upon the strength of the MPPT algorithm and IGBT design.

Our research shows that for projects commissioned in India in 2017, DC:AC overloading was over 10% for over 75% of total AC capacity.

Figure 14: DC overloading in India


Note: Total capacity considered for this data is 7.7 GW

4.4 Master-slave combinationWith increasing project sizes, block sizes are also increasing in utility scale projects to reduce the requirement of control rooms and AC evacuation systems. Within a block, instead of having a single large central inverter, several smaller inverters are stacked together to provide better redundancy. These stacks are operated in a master-slave mode. When PV modules start generating power in the morning, the master inverter switches on. As the power exceeds a certain threshold, a second inverter – first slave – is switched on and so on such that inverters always operate at their optimum efficiency level. When the generation reduces due to lower irradiation, the slave units are automatically disconnected.

30%

15%11%

13%

31%

Over 30%

Between 21% to 30%

Between 11% to 20%

Between 1% to 10%

No overloading


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For string inverter system, such kind of arrangement is not required as all the inverters operate as independent master units only.

4.5 ParallelingIn general, multiple inverters are installed in one control room in utility scale projects. Traditionally, only one inverter is connected to each isolated winding of transformer. Paralleling of the inverter means that multiple inverters can be connected to single winding of the inverter transformer. Usually, the output of inverters is clubbed together before connecting with a particular winding.

There could be two potential issues with paralleling – synchronization of output from multiple inverters and scope of reverse flow of power from inverter to another. There is no regulation to govern paralleling and the developer (or installer) has to demonstrate satisfactory control processes to mitigate the two issues for final safety approval from the Chief Electrical Inspector of Government (CEIG). For string inverters, lower quantum of output power makes it easier to control such issues and paralleling is used extensively globally in all rooftop projects. As a result, proof of concept is usually not required during CEIG inspection for string inverters.


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5. Regulations and standards for inverters

The Central Electricity Authority (CEA) has laid down national regulations/ guidelines covering grid connectivity, plant construction, power supply and metering standards. There is no specific regulation for solar PV. All key regulatory requirements for integrating solar plants with grid are met by inverters.

Despite India not having a standard grid code for inverters, IEC standards are commonly followed as in most European countries, but there could be differences in the way different state authorities certify project approvals.

Table 2: Standards followed in India

Typically, there are three key requirements mandated by most technical standards across the world and proposed revised guidelines are looking to adopt more stringent norms.

5.1 Harmonics Harmonic control is one of the most important requirements for inverters to manage grid safety. In solar plants, current harmonics are caused by deviation from the perfect sinusoidal wave while converting from DC to AC. India adopts IEEE 519 and caps THD at 5%. But most recent tenders restrict this to either 3% or 4%. Furthermore, draft changes suggested by CEA propose to cap THD at 2%3, whereas the USA restricts this to 3%.

Increasing topologies can help in reducing THD - an inverter working on 2 level topologies could generate THD of around 4% whereas an inverter working on 5 level topologies could effectively reduce THD to less than 1%. AC harmonic filter can also play an important role in reducing THD.

5.2 Frequency rangeFluctuation in frequency can damage electrical and power generating equipment. Traditionally, power flowed only in one direction – from power station to consumers – and grid managers were responsible for managing grid frequency by controlling power generation to effectively meet power demand. However, with increasing penetration of rooftop solar plants, managing the grid is becoming more complex as the number of distributed generating plants is increasing at a rapid pace.

3 CEA, Draft technical standards, http://www.cea.nic.in/reports/regulation/draft_tech_std_elec_plants&elec_lines.pdf

Requirements IEC codes

Efficiency measurement IEC-61683

Environmental testing IEC 60068-2/ IEC 62093

Electromagnetic capability IEC 61000-6-2, IEC 61000-6-4

Harmonic Control in Electrical Power Systems IEEE 519

Utility - interconnected photovoltaic inverters - Test procedure of islanding prevention measures and other electrical safety

IEEE1547/ IEC 62116/ UL 1741 or equivalent EN/BIS standards

Safety of power converters for use in photovoltaic power systems IEC 62109

Technical Guidelines for Generating plant connected to Medium voltage network

BDEW 2008

All key regulatory requirements for integrating

solar plants with grid are met by inverters


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Abrupt changes in power supply-demand at a grid substation can lead to voltage and frequency shocks and trip solar PV systems. This phenomenon is known as grid tripping or over voltage tripping. It could potentially have devastating effects as identified in Germany.

50.2 Hz problem in Germany and introduction of droop functionality

Standard grid frequency in Germany is stipulated as 50 Hz. A regulation introduced in Germany in 2005 mandated inverters connected at low voltage grids to switch off if frequency touches 50.2 Hz. This was done in order to bring back frequency to acceptable limits. At the time, solar PV played a negligible role in the German energy mix. But it grew exponentially and by 2011, installed capacity reached over 20 GW with more than half connected to the low voltage network. Shutting down all inverters simultaneously could drastically reduce grid frequency resulting inverters to turn back on. With inverters being turned on, frequency could potentially rise quickly again to 50.2 Hz, causing a ‘yo-yo’ effect.

The “50.2 Hz problem” identified in Germany forced regulators and grid operators to mandate inverters to alter their output and not shut down even if frequency touches 50.2 Hz. By 2012, regulations mandated inverters to continue functioning even above 50.2 Hz albeit with lower power output. Subsequently, systems were retrofitted with a ‘droop functionality’ such that even in case of frequency fluctuations, inverters were required to stay connected and gradually reduce power in steps until desired frequency level was reached. Inverters are now required to shut down only at a relatively higher level of 51.5 Hz.

In India, conventional power plants are required to support the grid through frequency response. However, there is no such regulation for solar PV plants. The proposed amendments by CEA, “Technical Standards for Connectivity to the Grid” require solar projects of over 10 MW capacity to provide immediate real power primary frequency response proportional to frequency deviations from 50 Hz. The rate of real power response would be required to have droop characteristic of 3-6% - the same as that for conventional plants.4

The USA doesn’t mandate droop functionality. A 2014 study by Smart Inverter Working Group, set up by the California Public Utilities Commission, came with following recommendations for solving this problem5.

a) Systems ramp up over time instead of connecting at full capacity after the requisite time break;

b) The system should connect randomly within a time window of a few minutes.

5.3 Voltage rangeVoltage fluctuations can result from solar PV plant operations or because of a fault in the distribution grid. In solar plants, the voltage fluctuation usually results from sudden change in power generation because of moving cloud cover or any other fault. A safe operating voltage range is specified for the inverter to ensure safe and reliable operation of the grid and the solar plant.

CEA mandates that inverters should be disconnected from the grid when grid voltage exceeds 110% of nominal voltage or falls below 80% of nominal voltage.

4 CEA guideline of technical standards for connectivity to the grid regulations http://www.cea.nic.in/reports/regulation/draft_technical_std_grid_regulations.pdf5 “Recommendations for Updating the Technical Requirements for Inverters in Distributed Energy Resources”, Smart Inverter Working Group, http://www.energy.ca.gov/electricity_analysis/rule21/documents/recommendations_and_test_plan_documents/Recommendations_for_updating_Technical_Requirements_for_Inverters_in_DER_2014-02-07-CPUC.pdf

http://www.energy.ca.gov/electricity_analysis/rule21/documents/recommendations_and_test_plan_documents/Recommendations_for_updating_Technical_Requirements_for_Inverters_in_DER_2014-02-07-CPUC.pdf




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This is similar to requirements in Germany for medium voltage levels. On the other hand, the USA allows for a much wider operating range of 50 - 120%. However, in case of short circuit in the grid, when there is momentary flow of a high current, current regulations require inverter to be tripped. But this could cause the line voltage to decrease further leading to a cascading effect. CEA has therefore proposed changes to mandate a high voltage ride through (HVRT) feature. According to the proposed amendment, solar plants must remain connected to the grid even when the voltage at the interconnection point rises above the specified value as per the figure below:

Figure 15: HVRT functionality of inverter proposed by CEA

5.4 Other requirementsThere are three other technical requirements for inverters as prescribed by CEA and other industry standards:

a) Flicker: It is defined as the fluctuation of voltage due to fluctuations in electric supply. Large penetration of solar in the grid can produce significantly bothersome flicker. India follows IEC 61000-3-3 and IEC 61000-3-11 standards to control flicker as followed in Germany and the USA.

b) Anti-islanding function: When the grid is unavailable, the inverter must stop supplying power to ensure grid safety. This requirement of anti-islanding is mandated for grid connected systems only.

c) DC injection into AC grid: DC current within the AC network could cause significant disturbance within distribution and measurement transformers. CEA restricts maximum permissible level of DC injection at 0.5% of the full rated output at the interconnection point. This function is specifically required for inverters connected at low or medium voltage.

Source: CEA guideline of technical standards for connectivity to the grid

145%

140%

135%

130%

125%

120%

115%

110%

105%

100%0 1 2

Time (s)3.0 4

Volt

age

as p

erce

ntag

e of

no

min

al v

olta

ge


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6. Commercial aspects

6.1 Cost As a result of technological innovation, improving volumes and enhanced competition, inverter prices have been coming down rapidly. Central inverter prices have come down by 42% in the last three years and reached INR 1.8/W.

Figure 16: Price trend of central inverters in India, INR/W



As mentioned earlier, string inverter usage has also recently started becoming mainstream. There is no reliable long-term price data for string inverters in the Indian market, but international experience shows that cost difference with central inverters has been coming down over time.

Figure 17: Global solar inverter average sales prices by technology type, 2010-2022, USD/W

0.7

0.6

0.5

0.4

0.3

0.2

0.1

02013 2014

Micro inverter String inverter Central inverter

2015 2016 2017 2018 2019 2020 2021 2022

Q1 2015

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0Q2 2015 Q3 2015 Q4 2015 Q1 2016 Q2 2016 Q3 2016 Q4 2016 Q1 2017 Q2 2017 Q3 2017 Q4 2017

3.1 3 2.82.6

2.4 2.3 2.2 21.9 1.8 1.8 1.8


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6.2 Up-time guarantee and warranty supportTypically, the inverter suppliers offer around 99% up-time guarantee. They have to maintain strong service networks for urgent maintenance and rectification. Most inverter suppliers have opened multiple service centres in states with high concentration of solar projects such as Rajasthan, Gujarat, Andhra Pradesh, Telangana, Karnataka and Tamil Nadu.

Usually, inverters come with a warranty of five years. Manufacturers offer extended warranties up to 20 years even though most developers are reluctant to buy these because of additional cost. Industry practice is to factor in some additional maintenance and/or replacement cost during the course of plant life of 25 years. But finding compatible spare parts may become a challenge due to rapid evolution in technology.


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7. Inverter supply landscape

7.1 Utility scale marketInverter market globally is becoming increasingly consolidated with market share of top-10 inverter suppliers increasing over the years, from 55% in 2013 to 79% in 2016. Indian inverter market is much more consolidated with top-10 players always accounting for over 90% of total market.

Figure 18: Market share of top-10 inverter suppliers in India

100%

80%

60%

40%

20%

0%2013

55%

92% 94% 96%

Global

India

79%72%

66%

92%

2014 2015 2016

Source: GTM research, BRIDGE TO INDIA research. Data for India is based on projects installed and international data is for capacity shipped.

Source: GTM Research. Data is for capacity shipped.

Historically, SMA maintained a leadership position globally for many years but it recently lost the number 1 position to Huawei. Sungrow has remained in second position for last four years. Incidentally, Huawei and Sungrow are both biggest proponents of usage string inverters in utility scale projects (Sungrow also sells central inverters).

Figure 19: Inverter suppliers ranking trend globally

2013 2014 2015 2016

1

2

3

4

5

6

7

8

9

10

Ran

k

Huawei

Sungrow

SMA

Sineng

TMEIC

TBEA

ABB

Kstar

GE

Solaredge


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Source: BRIDGE TO INDIA research. Data is based on projects installed.


SMA also dominated the Indian market in early years before losing its leadership position to ABB, which has remained at the top for last five years. TMEIC and Sungrow have been improving their ranking consistently and reached second and fourth positions respectively in 2017. Huawei ranking is relatively lower in India because of its late entry in the market.

Figure 20: Inverter suppliers ranking trend in India

Cumulative market shares of leading industry players are shown in the chart below. The increasing popularity of string inverters suggests that both Huawei and Sungrow will climb up the ranking in the coming years.

Figure 21: Share of top players in Indian utility scale solar inverter market

Cumulative market share until 2017 – 17.4 GW

A

ABB

10

9

8

7

6

5

4

3

2

1

2013 2014 2015 2016 2017

ABB

Hitachi

TMEIC

SMA

Schneider

Sungrow

TBEA

Delta

Huawei

GE

Others12%

GE 1%Delta1%

TBEA 2%

Huawei 4%

Schneider 5%

Sungrow 6%

Hitachi 10%

14% TMEIC

14% SMA

27% ABB


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7.2 Rooftop marketRooftop market in India is still very small and accounts for barely 10% of total solar market. Delta and SMA are the two undisputed leaders in this market and together account for almost half of the cumulative market share.

Figure 22: Share of top players in Indian rooftop solar inverter market

Cumulative market share until September 2017 – 1.9 GW


7.3 Domestic manufacturing

We understand that ABB, TMEIC, Schneider Electric, Delta and Hitachi have local assembly plants in India. They import critical components such as IGBT, controller and harmonic filter, accounting for about 40% of the total cost of an inverter. Other components such as cabinet, meters and other accessories are usually procured locally. Other leading companies rely on imports of completely built up units.

Table 3: Inverter components and sourcing

Component Sourcing

IGBT Imported

Controllers Imported

Harmonic filter Imported

Cooling unit Can be sourced locally

Cabinet Can be sourced locally

Meters Can be sourced locally

Other accessories Can be sourced locally

Others 18%

Schneider 2%

28% Delta

19% SMA (incl zever)

6% Kaco Su-kam 5%

ABB 4%

Fronius 4%Sungrow 4%

Statcon energiaa 3%Enertech 3%

Consul neowatt 2%

Vispra 2%


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8. Conclusion

Inverters are the heart of a solar power generating plant. They account for only about 5% of total system cost but have a disproportionate effect on system performance and functionality. Inverters perform many critical functions in addition to converting power from DC to AC and are undergoing rapid evolution in technology and standards. They also account for bulk of total energy loss in solar plants and have a relatively high failure rate in comparison to other components in solar plants. Due consideration is therefore needed for design, installation and operation of inverters:

1. DC:AC overloading: Most tenders allow repowering of solar plants in future and inverter selection must consider plant design requirement not only for today but also for future.

2. Maintenance requirements: Developers are reluctant to take out extended warranty covers because of high cost. But maintenance over 25-year period can be challenging particularly as changing industry standards and improving technology make it difficult to procure compatible spare parts in future.

3. Add-on features: CEA has already proposed draft regulations for voltage ride through and frequency response requirements. As seen in Germany, India might also impose mandatory droop functionality for retrofitting of inverters. Selecting an inverter with such advanced features may save from problems at a later date.


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Photograph: Huawei


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