0. DC to AC Inverter for Solar Panel
Stanislav Arendárik, (Aug 2010)

1. Main Topics Main topics: Solar panel description and characteristics
Main parameters of the inverter
Structure and block schematic of the inverter
Blocks description

2. Solar cell simple description
Photons in sunlight hit the solar panel and are absorbed by semiconductor material – silicon.
Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity. Due to special composition of solar cell, the electrons are only allowed to move in a single direction.
An array of the solar cells converts solar energy into a usable amount of direct current (DC) electricity.
source: wikipedia

3. Solar cell characteristics
A solar cell may be modeled by a current source in parallel with a diode, shunt and series resistances.
IL represents the max current of the solar panel (short current)
Diode forms the I-V characteristic
Shunt resistor represents the leakage currents (very small)
Series resistance represents the wiring losses

4. Solar cell characteristics - variations
For most crystalline silicon solar cells the change in VOC with temperature is about -0.50%/°C
Impact of the product of the series resistor and short-circuit current (ISCRS) is about 25mV/cell.
Impact of the parallel resistance is small.

5. Solar cell characteristics example
The maximum power point (MPP) on the I-V curve varies with real working conditions – high dependency on the irradiance and lower dependency on the temperature.

6. Maximum Power Point
In the technical parameters of the solar cell panel are defined:
VMP – voltage at MPP
IMP – current at MPP
The Inverter for the solar cell panel must achieve the operation on the MPP. This method is called as MPPT – maximum power point tracking.
The presented inverter has implemented the P&O (perturb & observe) algorithm for MPPT.
To get maximum power from a PV panel required operating at the optimum voltage.

7. Selected range of the solar panels
The amount of the solar cells is used to build one solar panel. Usually 72, 36, or other.
For this design was selected panel with 72 cells, this implies MPP at 36V, which fairly corresponds to 3 x 12V lead acid batteries in series as the back-up for the power source for the inverter. This battery is used as the energy accumulator for the off-grid inverter usage.
The characteristic of the selected solar panel:
PMAX = 180W
VSC = 44.4V
VMP = 36V
ISC = 5.25A
IMP = 5A

8. Inverter structure Option DEMO Inverter Charger Battery PV String
Boost stage
Output filter
MPPT
Inverter
GRID
PV Panel
Isolation
Isolation
Main HW/SW
Selected design way
Possible design way

9. Main Parameters of DC to AC Inverter
Input voltage source
Voltage level from solar panel VMP = +36V
Solar energy can be stored in Lead-Acid batteries
Battery charger can be implemented (phase 2)
Selected 3 pcs Lead-Acid batteries in series
Output voltage
This design aims 1-phase 230V 50Hz (115V 60Hz optional);
Single design, just different power components;
Tolerance +5% to -10%;
Output power
400VA;
Harmonic distortion of the output voltage
Lower than 3%;
Fault Protection
Output over-current, over-voltage, short-circuit;
Input under-voltage;
Signal output
Serial link RS-485 to main system

10. Block Diagram of DC to AC Inverter
Block Diagram of Inverter
The whole DC to AC inverter consists of main power parts:
MPP Tracking for solar panel output – software implemented
DC low voltage to DC high voltage converter
DC high voltage to AC sine output voltage inverter
Output filter
Isolated RS-485 line
Associated control and fault detection circuits
Both DC-DC converter and DC-AC inverter is controlled by one DSC MC56F8023.

11. DC to DC Converter
Push-Pull type with bridge rectifier on the isolated secondary side
Advantage:
Simple transformer windings
Simple control
Good efficiency
MPPT algorithm integrated in the software control loop

12. DC to DC Converter Push-Pull type control signals:
The duty-cycle for the PWM1 and PWM2 has the same value
The PWM2 control signal is shifted of T/2 later against the PWM1 signal
The actual duty-cycle value D depends on the actual power transferred through the transformer

13. DC to AC Inverter Full-bridge type inverter Advantages:
Simple +400V DCBus used to generate 230V AC
Simple PWM control
Simple reconstruction filter for AC line output
Can be controlled by half-bridge drivers or isolated gate transformers
Power MOSFETs or IGBTs can be used

14. DC to AC Inverter Full-bridge type inverter control signals:
The PWM3 and PWM4 are complementary
The duty-cycle varies ideal from zero to full period in sine amplitude and power line frequency
Real duty-cycle varies from 5% to 95% of the period
When the duty-cycle is <50%, the negative part of the output sine voltage is generated
When the duty-cycle is >50%, the positive part of the output sine voltage is generated

15. Output filter Integrated output EMI filter used Advantages:
Compact standard design
Good EMI properties
Sufficient load properties

16. Auxiliary Power Supplies
Auxiliary power supplies power the on-board circuitry
The HW comparator enables the first DC/DC converter if the input voltage is > 18V DC from PV panel
If the battery is connected, the inverter runs from battery and battery is charged by the solar panel
The main +12V DC power supply acts as the source for all other DC/DC converters
Two isolated power supplies provide the power for the isolated secondary side of the inverter
The control DSC is placed on the primary low voltage side
Isolated circuitry

17. Control circuits DSC MC56F8023 used
Controls the DC/DC converter and DC/AC inverter
MPPT software algorithm for the solar panel implemented
Battery charger control – as option
RS-485 line interface for supervisor’s line communication
Output over-current, over-voltage, short-circuit and input under-voltage fuses implemented
ON-GRID or OFF-GRID mode

18. Inverter view Battery connector Low power DC-DC converters
For control circuitry
Solar Panel
Connectors
Battery
present
relay
Full-Bridge Inverter
+ L-C Filter
Push-Pull
Primary side
DSC MC56F8023
Control Board
DCBus +400V
Power
Output
Output Filter
RS-485 Line
Battery Charger HW

19. Conclusion One standard DSC MC56F8023 used for whole inverter control
Possibility of use as the ON-GRID or OFF-GRID connected inverter
Possibility of use of wide range of the solar panels:
One piece of the 36V type with the output power up to 450W
Two pieces of the 18V types in series with the total output power up to 450W
Inverters can be connected in parallel at the output to boost the output power in the OFF-GRID usage
The inverter starts run when the sufficient amount of the power is available from the solar panel (if battery not connected)
The 3 x 12V lead-acid batteries in series are used as the energy accumulator

20. DEMO Connection DEMO Start-up sequence:
Switch OFF both switches (down position)
Connect load bulb (60W to 100W) at output
Connect the solar panel
The green LED shines, red LED not
Put OFF-GRID switch “ON” (up position)
Switch ON the main switch
The “Fault” LED indicates the over-current – when shines, switch OFF the main switch, wait about sec and switch ON the main switch.

21. Literature
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