Solar Inverter; Central Inverter; String Inverter; Protection of Solar Inverter; Calculation of capacity of solar inverter, solar panels & batteries, Hybrid Solar Inverter;

What is Solar Inverter – Definition of solar inverter is given below:

Definition –

Inverter is an device that converts DC voltage (output from PV solar panel) into AC voltage is called solar inverter. The inverter output voltage should be in sign wave. It consists of PV (or Solar) cells, controller, inverter & battery (optional).

Details of each part is given below-

A. SOLAR PV CELLS –

Solar Cells work on the principle of Photovoltaic effect. Solar cells are made up of semiconductor material. The most commonly semiconductor material used is Silicon Crystal (other semiconductor material is selenium). Silicon Crystals are laminated into n-type & p-type layers, stacked on top of each other. Light falling on the crystals induces Photovoltaic effect which generates electricity.

Working principle–

N- type semiconductor material is having extra electron in it while p-type is having extra photon. A p-n junction is formed by placing p-type and n-type semiconductors next to one another. N-type semiconductor acts as a negative electrode & faces the sunlight & p-type acts as a positive electrode. The p-type, with one less electron, attracts the surplus electron from the n-type to stabilize itself. When sunlight (photon) hits n-type semiconductor, an electron gets excited & is attracted towards surface& this causes more negatives in the n-type semiconductor &a hole is created which carries positive charge so more positives in p-type which means flow of current with one positive terminal & one negative terminal. This is called photovoltaic effect.

     IMPORTANT POINT TO REMEMBER ABOUT SOLAR PV CELLS–

• A Solar Cell produces voltage equal to – 0.6 Volt,
• Many Solar panels are wired to get 12V.
• In order to generate 12 V we need to connect 20 cells in series (0.6 V x 20 no’s = 12 V).
• But practically more than 20 cells are wired to generate 14V or 16V to ensure to charge a battery properly. Secondly if the sun is low in the sky or cloudy we will still get same voltage from solar panels.
• Current from cell depends on the intensity of light, surface area of the cell & reflectivity of the device.
• Typically a small cell of 50mmx50mm might generate 0.6V at 100mA in full Sunlight.
• The efficiency of Solar cells varies with temperature. So it is not advisable to install Solar System in hot area even though there may be very good quality of Sunlight.
• To get maximum output from Solar Panels, these should be installed facing direction where sunlight falls on it for a longer period so south facing installation of solar panels is the best option in India.

B. FUNCTION OF CHARGE CONTROLLER/REGULATOR – 

It serves two functions –

• It regulates the voltage from the solar cell so that even if the panel produces more voltage say 16V, the unit will pass 12V.
• it continually adjusts the power going to the battery to stop over-loading or over-charging.

C. INVETER –

Inverters are used for the conversion of DC voltage to AC voltage. According to output voltage, they could be rectangle, trapezoid or sine shaped. But, the best quality, inverters have output voltage in sine wave so that they could be easily used for AC load or integrated with power grid.

Inverter is the heart of a solar power project. It is also known as Power Conditioning Unit (PCU).  A PCU consists of an electronic Inverter along with associated control, protection and data logging devices. Typically, the utility scale inverters are unidirectional and supply the power to the load/ grid in the form of AC power. The inverter has a feature of automatically adjusting with the grid conditions, such as the voltage & frequency levels.

Type of Inverter for Solar System –

There are mainly two category of solar inverters that are available: central and string. A central inverter is generally adopted for MW scale plant, since string inverter can handle comparatively less power, while adding to costs.

Comparison between Central Inverter and String Inverter
S. No	Central Inverter	String Inverter
1	Low Redundancy	High Redundancy
2	Less capital cost per watt	Marginally higher capital cost per watt compared to central inverter
3	Optimistic for large systems where production is consistent across arrays.	Modularity of string inverters is better for systems with different array angles and/or orientations.
4	In case of any downtime, the loss in production is high (e.g. if a 500kW central inverter fails. It may cause loss of 10% energy in a typical 5MW plant).	If a String inverter fails, the loss in energy generation and revenue is low due to lower inverter capacity.
5	Installation of central inverters requires a separate concrete foundation and an inverter room.	String inverters do not require any foundation can be mounted wherever possible to the array structure itself which is great advantage.
6	Inverter operation is sensitive to module mismatch, shading and modules operation at different MPP.	Inverter operation is less sensitive to module mismatch, shading and modules operation at different MPP.

DC & AC Cables –

Both DC & AC cables are used with solar inverter. DC cables are used between the PV/ solar panels & inverter because DC voltage is fed to inverter by solar panel while AC cables are used between inverter and load.

The specification of DC & AC cables are given below –

Specifications for DC Cables
Type	XLPE or XLPO insulated and sheathed, UV stabilized, Solar grade, single core multi-stranded flexible copper cables shall be used
Temp Range	-40oC to +90oC
Voltage rating	1500 V for DC side
Properties	Excellent resistance to heat, cold, water, oil, abrasion, UV radiation
Standards	As per EN 50618
Jacket/ Outer sheath	Cable Jacket should also be electron beam cross-linked XLPO, flame retardant, UV resistant
Color coding	Black color for Positive & Red stripe for negative

Standards for AC Cables
Type for CU cable	XLPE insulated and PVC sheathed, UV  stabilized four/five core multi-stranded flexible (Unarmored) copper cables shall be used
Type for AL cable	XLPE insulated and PVC sheathed, UV stabilized four core Aluminum Armored cables shall be used
Operating temperature range under normal condition	Upto 90oC
Temperature of the cable under Short circuit condition	Upto 250oC
Voltage rating	1100 V for AC side
Properties	Excellent resistance to heat, cold, water, oil, abrasion, UV radiation
Standards	As per IS 7098 Part-1, IS 5831-1984, IS:8130-1984

(Note- Cable size & number cores will depend on the capacity of the inverter. So, these cables should be selected carefully.)

D. BATTERY –

Battery is an optional item in solar inverter. The purpose of battery is to store the power from solar panels when power is not required by the load & use this power at night or any other time when solar power is not available.

DIFFERENCE BETWEEN SOLAR CELL & PHOTO CELL – 

Solar cells convert light energy into electrical energy through photovoltaic effect (details are given in above section).

While photo cell is a type of resistor when light strikes the cell it allows more current to flow. It is better known as Light Dependent Resistor (LDR). Its resistance changes with light intensity, it means, on more light it offers less resistance & vice versa. It can be used as a light sensor which works in darkness. In darkness its resistance increases dramatically. Strictly speaking it is not a cell as it doesn’t generate electricity.

Frequently Asked Questions (FAQs) on Solar Inverter- 

Q1) How to protect solar inverter?

Ans) The solar inverter system should be provided with all necessary protections like earthing, Lightning, and grid islanding as follows:

LIGHTNING PROTECTION

The SPV power plants shall be provided with lightning & overvoltage protection. The main aim in this protection shall be to reduce the over voltage to a tolerable value before it reaches the PV or other sub system components. The source of over voltage can be lightning, atmosphere disturbances etc.

This can be achieved by deploying required number of Lightning Arrestors. The protection against induced high-voltages shall be provided by the use of metal oxide varistors (MOVs) and suitable earthing such that induced transients find an alternate route to earth.

SURGE PROTECTION

Internal surge protection shall consist of three MOV type surge-arrestors connected from +ve and –ve terminals to earth (via Y arrangement)

EARTHING PROTECTION

Each array structure of the PV yard should be grounded / earthed properly. In addition, the lightning arrester/masts should also be earthed inside the array field.

Earth resistance shall not be more than 5 ohms. It shall be ensured that all the earthing points are bonded together to make them at the same potential.

GRID ISLANDING:

In the event of a power failure on the electric grid (if solar PV system is connected with grid) , it is required that independent power-producing inverters attached to the grid turn off in a short period of time. This prevents the DC-to-AC inverters from continuing to feed power into small sections of the grid, known as “islands.” Powered islands present a risk to workers who may expect the area to be unpowered, and they may also damage grid-tied equipment. Therefore, following precautions needs to be taken –

1. The Rooftop PV system shall be equipped with islanding protection. In addition to disconnection from the grid (due to islanding protection) disconnection due to under and over voltage conditions shall also be provided.
2. A manual disconnect 4 pole isolation switch beside automatic disconnection to grid would have to be provided at utility end to isolate the grid connection by the utility personnel to carry out any maintenance. This switch shall be locked by the utility personnel. Tools & Tackles and Spares.

Q2) Why do solar cells need an inverter? Or What is an inverter for solar panel? 

Ans) A solar cell is a small generator & produce DC voltage. When these solar cells are connected in series & parallel combination, they work like a DC power generator. But, DC voltage is not used commercially for domestic and transmission & distribution of power. That is why inverters are used which converts DC voltage of solar cell/panel into AC voltage.

To get more details about inverter. Pls click on INVETER.

Q3) How to calculate capacity of solar panels, battery & inverter?

Ans) The calculation solar panels, battery & inverter depends on the load of the house/office. First of all let’s calculate the load /current/ wattage of the house/office which you wanted to run through solar panel.

• Load calculation –

Check the full load current of the building. This can be checked by switching on all the load (which are to be run through solar panel) of the building. Measure current with the help of clamp meter/Ampere meter. Let’s say 10A is the full load current.

• Check supply voltage. Consider single phase supply voltage 220V for calculation purpose.
• Calculate load by multiplying voltage & current –

P = 220*10 = 2200W   = 2.2 KW

• Solar panel capacity selection –

Capacity of solar panel would be according to load. Lets’ say max capacity of each panel is 500W available in market.

So, no. of solar panels required = 2200/500 = 4.4 or say 5 panels are required for the load of 2200W. (Note- keep value on higher side as solar panels efficiency deteriorate with time.)

In other words, solar plant capacity= 500*5 =2500W= 2.5 Kw.

(Note- No. of solar panels may vary according to the capacity of individual solar panel. As wattage of solar panel will increase, no. of solar panels will decrease & vice versa.)

• Battery capacity selection–

Check the power back up required from the battery. Let’s say 1 hour power back up is required. Consider 100Ah battery is available, power back up calculation is given below –

P = VI  where V=12V, Current I = 100A * 1 Hr = 100 A

P = 12 X 100 = 1200W.

Therefore, 2 nos batteries of 1200W are required for 2200W Load for 1 hr power back up.

Overall battery capacity is 2*1200W = 2400W= 2.4 Kw

(Note- If more hour power back up is required, then more batteries will be added. Also, batteries are available in different Ah ratings so battery quantity could be calculated accordingly.)

• Inverter capacity selection– Inverter capacity would be calculated based on load of the building.

Inverter capacity = Load + 20% safety margin

= 2200 + 2200*20%

= 2200 + 440 = 2640 W = 2.64Kw

So, inverter capacity would be around = 2.64 Kw

Q4) What is hybrid solar inverter?

Ans) A hybrid solar inverter is a combination of battery inverter & solar inverter in one unit when connected with hybrid solar system where all three sources grid, solar & battery are feeding power connected with single load. The function of hybrid inverter is to manage the power smartly/intelligently from all three sources at the same time.