Basic maintenance of the Solar PV system

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Introduction

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Performance of solar rooftop PV system with regular & basic maintenance improves. For best system performance, regular maintenance is necessary. This leads to increased system life and decrease in disruptions due to component failures. 

Also a data log for fault diagnosis during system maintenance should be maintained. 

[mk_blockquote font_family=”none”]“Solar power systems operate safely with regular maintenance”[/mk_blockquote]

Basic maintenance in a solar system that an owner has to care for :

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Solar Panels:

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One of the most important job is to clean the solar panels regularly. During the cleaning, panels need to be washed with the help of a hose pipe to remove the dust and dirt on the surface of the panel. This enhances power generation. 

Incase of dense or stubborn layers of dirt, modules need cleaning with a sponge or soft cloth or glass wiper after spraying some water. Glass layer needs to be wiped gently to avoid cracks due to small stone particles in the dust.  Even the bird droppings need a similar treatment. It helps decrease the effort to clean and at the same time removes the bird droppings completely. 

More importantly, the ideal time to clean panels is before 8 AM or after 5 PM. It avoids the microcracks on the module glass surface.

It is also suggested to remove the dry leaves if any to avoid long term problems like water clogging, short circuit due to moisture and fungus etc.PS: Since, modules are placed on the rooftop, it is very important to have safe walkways for the cleaning of the modules. Incase of a complicated rooftop, it is highly recommended to use a safety harness before considering cleaning the modules.

Modules with and without dust
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Inverter:

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Inverters convert DC into AC power. They generally require very little maintenance. 

  • Keep the inverter clean by wiping it with a dry cloth to avoid dust blowing inside. Necessary care needs to be taken while cleaning. 
  • Check the inverter LED color for the status of the system. If it displays GREEN, the system is working fine, if it displays BLUE, then there is a  problem with INTERNET connection which needs to sorted with help of internet provider, if it displays RED, there’s an ERROR in the system – do contact installer/customer care immediately. 
Inverter
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Balance of the system:

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Balance of the system includes the protection devices like Fuses, Surge Protection Devices (SPD), Circuit breakers, Monitoring interface and miscellaneous items.

All these needs to be checked once a while as they are first ones that are prone to failure due to grid instability, lightning etc. 

 

  • Check all the circuit breakers and switches if they are in working condition
  • Find if the Surge Protection Device (SPD) is working fine. If the SPD display color is dark, then it needs to be addressed immediately. Contact the installer immediately.
  • Check the earthing (grounding) cable to see if there are any disconnections in between. 
  • Lightning arrester connections and clamps should be checked if they in place without any disruptions
  • Monitoring. SolarEdge power optimizer systems come with module level monitoring. It helps in monitoring the system remotely. Regular look at it will help understanding drop in generation giving an alarm that modules need cleaning. In case of any other issues like failure of optimizer or panel will be known immediately decreasing the maintenance time and money and increasing productivity of system. Following image is the screen shot of one of the sites where there is shadow on two panels. This shows how shadow can effect the generation of each panel.
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Selection of SolarEdge power optimizer and inverter

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Introduction

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SolarEdge offers its latest cuttingedge technology to wide variety of customers from residential to commercial building owners. Offering ranges from 1kW till the maximum required or possible to install.

With wide range of selection, it’s very essential to choose the right power optimizer and inverter combination. It is decided based on the building requirement and size of the system needed.

There are typically two types of inverters we look at in rooftop segment. Single phase inverter and Three phase inverter. In single phase, we have Compact technology and HD Wave technology. Whereas, in three phase we have normal three phase inverters and Synergy technology. Single phase inverters range between 1-5kW where as three phase inverters are greater than or equal to 5kW.

Below chart gives overview details about the selection of optimizers possible for respective inverters.

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Selection of Power Optimizer based on Inverter size

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Inverter and Optimizer combination

Each of the inverter and the optimizers listed above have certain limitations in terms of string length. These factors are important to be considered as cables are designed for certain wattages based on the system size.

Following chart explains the minimum number of optimizers needed and maximum wattage each string is capable of.

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Selection of Power Optimizer for Inverter

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String length

**With P730 – For SE27.6k, SE55k, SE82.8k: It is allowed to install upto 13,500W per string when there are 3 strings connected to the inverter. The maximum power difference between the strings is upto 2000W

**For SE27.6k the maximum inverter DC power is 37,250W

**No optimizer listed in the above table can be mixed in same string.

For data sheets of the inverter and optimizer

With the help of SolarEdge designer tool, it is possible to do optimal design and create a generation report for the sytem to be proposed. Use of this tool helps in reducing the installation cost, time and at the same time increases the reliability.

[mk_title_box font_family=”none”]About Sunlit Future[/mk_title_box]

Sunlit Future with an experience of more than two decades also are distributors of SolarEdge power optimizer technology products. We offer technical support in terms of design, installation and commission of the system for the partners. Our aim is to provide quality installation to gain optimal generation of the system.

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Safe DC option with SolarEdge power optimizers

Safe DC option is an integrated feature in SolarEdge power optimizers and inverters. Safety of installers, maintenance personnel and firefighters is ensured with this option. This option shuts down DC current and voltage in strings when inverter is off or disconnected or in safety mode.

High string voltages are de-energized automatically to 1 Vdc per Power Optimizer and max. 50 Vdc per string in any of the above cases. Electrocution and unsafe high voltage risks are eliminated with this Trade Mark option.

Safe DC option can be triggered from SolarEdge fire fighter gateway aswell, providing centralised safety management.

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Working of Safe DC

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The SolarEdge Solution consists of a Inverter which communicates to Power Optimizers over Power line communication (PLC).
Safe DC is a mechanism which is build into the software of the Inverter and the Power Optimizer.

The Power Optimizers only harvest full power from the panels if the PLC signal is constantly renewed. If it fails the Power Optimizers initiates the Safe DC mechanism, which lowers the DC voltage to 1 Vdc per Optimizer. This feature works even when more than one panel is connected to it.

The 1 Vdc is necessary during installation and maintenance stage to check the proper working of the Power Optimizers.

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Advantages of SolarEdge Safe DC mechanism

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Installation

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During installation, panels are connected with Power Optimizers in strings. Safe DC is activated by default, lowering the operational voltage of the Power Optimizer to safe 1 Vdc, if the sun shines.

The maximum number of Power Optimizers in one String of panels is 50, therefore max. string voltage is 50 Vdc if all Power Optimizers are working properly.

Full power harvest can be safely activated when the Power Optimizers are connected and communicate with an SolarEdge inverter.

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[mk_button url=”https://www.solaredge.com/sites/default/files/solaredge_case_study_fire_safety_tasmasia_australia.pdf”]Case study on Safe DC[/mk_button]
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Emergency

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In case of a fire firefighters, mostly the AC wires of the house were cut to terminate the grid connection. Safe DC is automatically activated by the inverter if the grid connection is lost. This reduces the per power optimizers voltage to 1 Vdc. Eventually, max. 50 Vdc per String, lowering the risk for firefighters tremendously.

If properly educated, firefighters can also manually activate Safe DC by switching off the Inverter, the AC breaker or the AC/DC safety switch.

The Power Optimizers will also activate Safe DC by themselves if their thermal sensor senses a temperature above 85 °C, because such high temperatures typically indicate a fire near the PV System.

To activate Safe DC manually, inverter should be turned off so that it stops sending PLC signals to the Power Optimizers. Safe DC can also be activated if the connection to the AC side is interrupted by switching off the main AC breaker or AC/DC safety switch.

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Maintenance

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Maintaining PV system should be done regularly. Safe DC should be activated in order to reduce the high DC string voltages to a safe level. Maintenance like cleaning the panels, checking for defects, replacing panels or power optimizers should be considered only after activating the Safe DC mechanism.

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Modbus Meter – For Zero Export from SolarEdge

Modbus Meter

SolarEdge Modbus Meter primarily helps in limiting the export of power to the grid generated using solar power plant enabling it for zero export. It also helps in finding the generation of the solar power plant, export to the utility grid, import from the utility grid and full time consumption inside the house 

It particularly is useful for large scale installations like schools, hospitals, industries and commercial buildings where the major loads are during the day time.

Procedure to avail a net meter takes anywhere between 4 weeks to 30 weeks. Having modbus meter helps in consuming the power generated from solar rooftop system from the minute of commissioning it. This takes care of all the day time loads till the bidirectional meter is availed.

This data is available in the remote monitoring app of SolarEdge giving us an understanding of generation from the system and consumption of the building as shown in the figure below. 

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modbus

Red colored graph in the above figure shows the loads of the building during the 24 hours. Blue graph indicates the generation and consumption in the building based on the total connected loads in the building.

It is in align with the total consumption. Total electrical load connected in the building is greater than the installed capacity of solar, that is why the generation most of the time is less than the consumption.

Advantages of having Modbus Meter

In case of the system not having a bidirectional meter

  • Limits the export of solar power by generating what is required for building’s self consumption
  • Continues to generate power based on the maximum and minimum energy needs of the building
  • Can study the connected loads in the building all the times
  • Can be used from the day of commissioning of the system till the bidirectional meter is installed
  • This feature is highly useful for the buildings with high day time loads – Institutes, Hospitals, Commerical and Industrial buildings
  • Remotely available monitoring data

In case of the system having a bidirectional meter,

  • Electrical loads can be seen on a real time basis through out the day and night
  • Peak time non-essential loads could be shifted to non-peak hours
  • Remotely available monitoring data within the SolarEdge app
  • Avoids loss of power generation
  • It also can be used during the export limitations
  • This meter holds the data of import, export and the connected loads which can also be seen in the monitoring portal

Modbus connection

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Working of the SolarEdge Modbus Meter

Export and Import data is read by a current transformer which is connected parallelly with the Electricity Board power supply to the SolarEdge Modbus meter through the communication cable, Modbus meter is connected to a SolarEdge Inverter. 

Power optimizers are connected to Inverter through Power Line Communication (PLC) ensuring the monitoring data to be available in Modbus meter and the monitoring portal.

This could be used in single and three-phase systems as well. 

Current Transformer

When ordering a Modbus meter it is also necessary to consider ordering the Current Transformer.

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Current Transformer ER Model Rated RMS Current (A) Internal (A X B)

External (C X D)

SE-ACT-0750-250 250 20 x 20 mm/61 x 60.4 mm
SE-CTS-2000-1000 1000 50.8 x 50.8 mm/121 x 127 mm
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ctdimensions

More details on Installation of the Modbus meter:  SolarEdge

SolarEdge Modbus Meter

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Silicon Hetrojunction (HJT) Modules from Waaree

Bifacial modules are gaining the traction due to its ability to generate more kWh in limited conditions, Waaree group had come up with their new Super Silicon HJT modules. These HJT modules generate higher kWh and long-lasting.

PV module market around the world has been cost-driven. It is no wonder that the crystalline technology has been the chalk horse of the industry. Further, with the Levelized cost of electricity (LCOE) from solar plants have started breaching grid parity. The focus now is to extract maximum kWh from the plant in order to ensure the investors have maximized returns. The technology up-gradation in conjunction with module design up-gradation has played a significant role in such a shift.

Utilization of bifacial module which is capable of generating power from both sides has been gaining traction. Given the fact that enhanced energy output could be expected under the right designing considerations. However, the currently available bifacial module utilizes PERC/PERT crystalline solar cells have limited bifaciality. They are coupled with other inherent issues like LID, PID, LeTID, etc. It was hence necessary to bank upon a pragmatic solution.

Silicon Hetrojunction (HJT)

Silicon heterojunction (HJT) solar cell which has been introduced in the mid-19th century, utilizes both crystalline and thin-film technology. The cell has crystalline silicon (n-type) sandwiched between amorphous (thin film) layers of silicon on both sides. It gives the cell its bifacial nature. The monocrystalline technology in the cell has better absorption of light. Whereas, the amorphous has superior passivation (which enables high electron collection) characteristics.

The cells are thinner (~120 μm) compared to crystalline silicon cell making it slightly flexible and further also making them highly resistant to micro-cracks. The n-type silicon present in HJT does not have Boron, which makes these cells LID free. Further, the mitigation of electron from the cell which is responsible for PID is reduced to zero in HJT cells. Thanks to the presence of highly conductive oxide layer (ITO) which protects the cell electrically.

Additionally, the cell has a lower temperature coefficient (almost half of the crystalline technology-based cells). All the above factors when coupled together enables HJT based module to have lower degradation rates throughout its lifetime.

Since these cells have high bifaciality, micro-crack resistance, no LID, PID & LeTID and lower temperature coefficient it is a perfect technology for Indian conditions. Waaree Energies recently in REI-2019 launched their SUPER HJT module which utilizes HJT solar cells. Available next year, this module has already started gaining popularity among the local and international markets.

Further, with the detailed scrutiny happening for each and every module we manufacture, the end customer can rest assured that their plant would be up and running for 25+ years. It ensures more than the desired return.

India has set an ambitious target of meeting 175GW of its power demand by renewable energy in 2022. While there has been a lot of improvement from its state of cumulative capacity in the early 21st century. From that state, the sector has seen a lot of government intervention and private partnerships in order to establish and develop a solar power plant in the country.

A new plan by the government (yet in draft version) enables Public Sector Undertaking (PSU) companies to utilize and/or source green energy. The plant shall likely be 1.8 GW of capacity (similar to coal-based ultra-mega power projects (UMPPs)) and may be divided into 3 parts. The country already has a plan to develop 12GW of the power plant through the PSU route. While there have been cases of tender cancellation in various states citing various reasons, we believe that the role of government in fuelling the solar PV market growth is crucial.

The role of PSU's in enabling India to meet its renewable energy targets

Cumulative & Expected capacity addition of solar energy in India by 2022

Health benefits from Renewable Energy sources implementation estimated

Implementing renewable energy sources for meeting the enhancement in demand of the world is a no brainer now. However, it is also a well-known fact that renewable energy comes with environmental advantages too! A recent study titled “Health co-benefits of sub-national renewable energy policy in the US” reviews the health benefits of implementing renewable source in Rust Belt, US.

The study compares the impacts of Renewable Portfolio Standards (RPS) on air quality and human health. The study compares Business As Usual (BAU) RPS case along with enhanced RPS cases of +50% & +100% and finally the case of pricing CO2 emissions to no renewable energy implementation by 2030.

The study finds that the co-benefits of the BAU, RPS + 50%, and RPS + 100% scenarios correspond to co-benefits of 8¢, 12¢, and 13¢ per kWh of new renewable generation (or $94, $120, $119 per ton of CO2 reduced respectively). It further suggests that CO2 emission pricing shall further have a positive impact with a health benefit realized up to $211 per ton of CO2 reduced. The study concludes by finding that health co-benefits in the region were 35%–79% higher depending on the RPS scenario. We believe that for a country like India which has ambitious missions like 24×7 Power for all and Health for all, studies like such should be taken into consideration by the policymakers to enable implementation of right sources.

Cost benefits from implementing renewable energy sources under different scenarios in 2030 comparing to no renewables implementation (Source: Emil G Dimanchev et al 2019 Environ. Res. Lett. 14 085012)

The state of solar grid parity in China and takeaways for India

Solar PV has gained tremendous popularity around the world and China is amongst the top leader in the market. Even though the country had both supporting government policies and attractive upfront subsidies for PV which is making it a competent source of electricity in the past few years. The country (and many countries around the world) breaching the grid parity, the dynamics are poised to be changed.

A new study titled “City-level analysis of subsidy-free solar photovoltaic electricity price, profits and grid parity in China” and published in “Nature Energy” reviews the situation of China. The study analysed more than 344 prefecture-level Chinese cities and finds that all these cities without any subsidy in solar PV, can meet grid parity.

Further with the country also being a coal-powered giant and utilizing desulfurized coal (which emits lower emission) for power production; the study finds that around 22% of the cities can produce electricity which can match prices of such sources.

The subsidy cuts in recent years was a clear note that the PV industry now needs to be focused on quality products rather than on the scale. But, economies of scale has been instrumental in the case of India. Very often it is forgotten that the solar PV plant is to last for 25 years which makes quality a crucial factor of review. We believe that the utility/customers which are only utilizing quality products would be able to reap the long term benefits of solar PV.

(From left) Historical LCOE of solar PV generation in China and (right) LCOE of various generating sources (Source: Yan J., et al. City-level analysis of subsidy-free solar photovoltaic electricity price, profits and grid parity in China. https://doi.org/10.1038/s41560-019-0441-z)

Source: Waaree Group

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Maximum Power Point Tracking – MPPT

What is MPPT

Maximum Power Point Tracking (MPPT), a simple digital electronic DC to DC converter. It optimizes to match between the Photovoltaic (PV) panels and utility grid (ESCOM) or battery. Converts higher DC output of modules according to the need of the grid or the battery. 

MPPT block diagram

Block Diagram of MPPT Photo Voltaic system

Image Source 

Why is it needed

Solar modules are cool devices, harnesses photons and converts into usable DC supply. Assuming we don’t have MPPT.

Let’s consider a 300 Watt solar module and 24-volt battery with a charge of 10 amps. It equals to 240 Watts. A loss of 60 Watts has occurred when compared with modules generation of 300 Watt. 60 Watt will be a loss with no generation due to mismatch with the battery. 

MPPT technique is used to avoid such losses and generate the maximum possible at any given time, in the optimizers and inverters.

Working of MPPT

This MPPT compares the output of the module and the voltage of the utility grid or the battery. MPPT looks for the best voltage for the utility grid or battery and brings out the maximum current from the power. Power (W) = Voltage (V) X Current (I). There are various MPPT techniques. Most of these techniques are 95-97% efficient.

Some of the MPPT techniques used are:

  1. Perturbation and observation (P&O) technique 
  2. The incremental conductance (Inc Cond) technique 
  3. Ripple correlation technique 
  4. Short circuit current (SCC) technique 
  5. Open circuit voltage (OCV) technique

In a case, the solar panel is producing 300 Watts and connected to 24 Volt battery, MPPT takes in the voltage and Current panel is producing say 40 Volt, 9.5 Amps and converts it into 24 volts, 15.83 Amps not letting incur any loss. Happy all.

With respect to the usage of MPPT in DC optimizer technology and string inverter technology, even though the technology remains the same the way it is used varies. 

MPPT in String inverters

Based on the inverter capacity solar modules are connected in series. Each string is connected to the inverter. Here series of modules are connected to the inverter with inbuilt MPPT component in it. Each series is called a String. Here MPPT optimizes at the string level. In a panel of 72 cells, all the cells are connected in series. 

Series means connecting positive of the first cell to the negative of the second cell and so on. Negative terminal from the first cell and positive terminal of the last cell in the panel are pulled from the panel allowing to connect to another module. When the positive output of the first panel is connected to the negative of the second panel and so on it connects all the modules in series forming a string. 

Once the output is passed through the MPPT it is inverted into the AC output. Usable power in the building.

With all the modules in series, if there is an issue in a generation it affects the generation of complete string. Possible generation issue could be a bird dropping or shadow on the panel. Generation of all the modules will be completely dependent on least generating panel, compromising on the generation of the system.

MPPT in Power optimizers

In the case of power optimizers technology, each module is connected with a DC power optimizer. Here MPPT technique is provided at module level not letting the generation of the system go in vain. Possibility of external factors like bird dropping and shadow on one single panel are few of them. Each panel will be having independent MPPT technique generating power irrespective of the adjacent module. 

All the power optimizers after optimising the power from each module are connected in series and connected to inverter through a junction box. Each module is independent of each other.

This kind of technology helps better in roofs in various directions, shadows etc. One optimizer can be connected to two modules reducing the need to a number of optimizers but having the benefit of maximum generation. 

String Vs Power Optimizers (MPPT)

Technology remains the same in both cases. In string inverters, it is at a string level whereas in case of optimizers it is at the panel level. In optimizer technology, it is also possible to monitor the generation at the panel level. This helps in the reduction of the maintenance work saving time, money and enhancing the generation bringing down the diagnosis of the system to the module level. With module-level MPPT it prevents loss due to module mismatch.

Conclusion

Maximum Power Point Technique (MPPT) as the name says, helps in generating optimal power within the constraints of a string level of module level. This technique is used not just in photovoltaics but also in Wind power in harnessing maximum possible power at that instance.

References:

Waaree   Wikipedia  SolarEdge Wiki  Solar Electric  SolarEdge

 

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