2. S.Gomathy M.E.,M.B.A
✓ Most of the appliances around us use AC power while the PV
modules produce DC power.
✓ Therefore, the conversion of DC power to AC power is required
before we can use it for running our appliances.
✓ Also, in standalone solar PV systems, the energy is stored in
batteries, in the form of DC power.
✓ In this case also, when we want to make use of stored energy in
batteries we need to convert the DC power stored in energy to AC
power used by appliances.
✓ This conversion of DC power into AC power can be obtained using
devices called DC to AC converters or DC/AC converters or
inverters.
3. S.Gomathy M.E.,M.B.A
Input power:
✓ Inverter converts DC input power into AC output power.
✓ Since input side is DC power side, therefore, the input power
fed to the inverter can be given in terms of input DC current
and input DC voltage.
Output power:
The output of the inverter is AC power. The AC power can be given in
terms of RMS value of current, RMS value of voltage and Power Factor
(PF).
4. S.Gomathy M.E.,M.B.A
Efficiency of inverter:
The efficiency of an inverter can be given in terms of output power and
input power, and can be written in the following way:
5. S.Gomathy M.E.,M.B.A
Problem
Input DC power of an inverter is 300 W. Output AC power is 250 W.
What is the efficiency of the inverter?
Solution Following parameters are given for inverter:
• Input power = 300 W
• Output power = 250 W
6. S.Gomathy M.E.,M.B.A
Types of Inverters
✓ The solar inverters are an important interface between the solar PV module
and the load.
✓ Depending on whether battery is used in the PV system or not, the solar
inverters can be classified in three broad categories:
Standalone inverters or Off-grid inverters
✓ These inverters are not connected to grid.
✓ They are normally used in standalone PV power systems.
✓ In standalone system, there is no back-up of power for energy storage,
therefore, this type of inverters has battery back-up to supply the power to
the load in case of non sunshine hours.
7. S.Gomathy M.E.,M.B.A
Grid-tie inverters or grid-interactive inverter
• These inverters are connected to grid and do not have battery back-up.
• They have special circuitry to match inverter output voltage and frequency
with that of grid.
• Grid is used as battery back-up when power generated by PV array is
insufficient.
• These inverters also have in-built MPPT to extract maximum power from
PV array.
• When the sun is shining and PV array is generating more power than our
usage, then the extra power after meeting our load is supplied to the grid.
• If PV array power is less than our load requirement then some power is
drawn from the grid to make up the shortage power.
8. S.Gomathy M.E.,M.B.A
Inverter Specifications
Battery voltage:
✓ Manufacturers specify the range of voltages, called DC input
window that can be applied to the input of inverter.
DC current:
✓ It is the maximum DC current that the inverter can handle.
Voltage range:
✓ Manufacturers specify the range of voltages, called DC input
window that can be applied to the solar panel input of inverter.
Maximum short circuit current:
✓ It is the maximum current that the inverter can handle from the
panel.
9. S.Gomathy M.E.,M.B.A
Output
✓ Inverter rated output power is the power level inverter which can maintain
for a long period and so it is sometimes termed continuous output power.
✓ Generally, continuous output is specified at particular temperature.
✓ In practice, the available power output depends on temperature.
✓ With increase in the temperature of the inverter than the rated temperature,
its power output decreases because of increase in internal losses in inverter.
✓ One must try to get the actual data sheet of inverter before purchasing, as
the design and performance can vary from one manufacturer to other
manufacturer.
10. S.Gomathy M.E.,M.B.A
✓This graph is only indicative to demonstrate that the power output
of inverter decreases with increase in temperature.
11. S.Gomathy M.E.,M.B.A
Output voltage:
• This is the AC output voltage that results at the output of inverter.
• It is either 230 V or 400 V since our loads also run at same voltage
levels.
Frequency:
• Frequency is 50 Hz in India and 60 Hz in USA.
• Inverters are designed for specific frequency.
Efficiency:
• This is the ratio of output power to input power of a inverter.
• The efficiency of a inverter varies with the load connected to it at
the output.
• Typical efficiency-vs-load curve of a inverter is presented as a
function of load.
• This graph is only indicative to demonstrate that the power output of
inverter depends on the load.
13. S.Gomathy M.E.,M.B.A
No load power consumption:
✓ This is the power consumed by the inverter under no-load condition,
i.e., the case when no load is working on an inverter.
✓ This no-load power consumption should be as small as possible to
avoid power loss in inverter.
Surge power (W):
✓ Many inductive loads draw 5 to 6 times normal running power for
few seconds during starting.
✓ This is called surge power.
✓ Inverter should be able to supply the surge power required by the
inductive loads.
14. S.Gomathy M.E.,M.B.A
Output waveform:
✓ The inverters are supposed to convert a DC power into AC power
output.
✓ A perfect AC power output should have sinusoidal behaviour,
meaning the AC current and AC voltage should be pure sine wave.
✓ In practice, converting DC power into square wave AC power
output is much simpler (and low cost) as compared to converting a
DC power into perfect sine wave AC power output which is more
expensive.
✓ On the other hand, the AC power with square wave is considered
low quality AC power while the AC power with sine wave is
considered high quality AC power.
✓ The quality of the AC power affects the performance and life of the
appliances which use this AC power.
15. S.Gomathy M.E.,M.B.A
Different output waveforms of an inverter
• Inverters are available with the following types of output
waveforms.
1. Square wave
2. Sinusoidal
3. Modified sinusoidal
16. S.Gomathy M.E.,M.B.A
Quality of power in inverters:
✓ Pure sine wave is ideal for almost all AC loads and is considered as the best
quality power.
✓ Any deviation from sinusoidal is said to introduce harmonics thereby
degrading the quality of AC power.
✓ The extent of degradation of AC power or the quality of AC power is
measured using the term Total Harmonic Distorsion (THD).
✓ In short, THD indicates the devation of waveform from pure sinusoid
✓ The more the THD, the more the deviation from sinusoid and power is
✓ said to be of lower quality.
✓ From this discussion, we can say the following:
✓ THD (sine wave) < THD (Modified sine wave) < THD (square wave)
✓ The THD is specified by the inverter. It should be as low as possible.
✓ The THD value for good quality sine waver inverters is always less
than 4%.
18. S.Gomathy M.E.,M.B.A
Problem
• A house has the following AC loads rated at 230 V.(i) Three 80 W lights
(ii) Two 100 W fans.(iii) A 60 W radio. All the loads are powered
simultaneously. A 12 V battery is available. Choose the appropriate
inverter?
Solution
Output power estimation: Total AC load to be powered simultaneously is as
follows:
19. S.Gomathy M.E.,M.B.A
• The inverter should be able to supply this current, the wire
should be chosen to withstand this amount of current.
Output voltage and frequency of inverter:
• These values should match with load rating values. So, 230 V
and 50 Hz are to be chosen.
Surge capability of inverter:
• For accurate surge estimation, we need to know the surge of all
individual loads.
• Normally, 3 to 4 times of output power is sufficient to meet the
surge of loads.
• So, inverter should be capable of providing 500 × 4 = 2000 W
surge power.
