# Main Topics Main topics: Solar panel description and characteristics

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

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

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

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

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.

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.

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.

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

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

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

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.

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

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

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

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

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

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

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

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

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

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.

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