1. Grid Connect Inverters NUER 19
Basic Principles

2. What is an Inverter?
A Solid State Device which main job is to convert a Direct Current Supply to an Alternating Current Supply
Inverter Symbol

3. An Inverter converts this Direct Current Input Wave Shape
To this Alternating Current Output wave shape

4. By using many very high speed switching devices an inverter is able to convert a series of precise D.C. Pulses into a synthesised A.C. Wave shape

5. Inverter Principle of Operation
Basically a 4 way switching bridge is used to re-route the direct current to alternate it at a set frequency through the load.

6. The Roles of a Grid Connect Inverter
To convert D.C. from the generating source Photovoltaic Array or Wind Turbine etc into an A.C. Wave that is then transformed and synchronised with the electricity grid
May Provide galvanic (voltage) isolation between the D.C. side and A.C. sides
To extract the maximum amount of energy from the generating source
Called Maximum Power Point Tracking (MPPT)
To log and display electrical data from the array such as Energy, Power Voltage Current, $ Income & CO2 offset
System monitoring and protection over: current, temperature & voltage
Earth fault & arc fault detection etc

7. System Diagram

8. Basic Portable Inverter Circuit with MOSFET’s and Centre Tap Transformer

9. Block Diagram for Typical Grid connect Inverter

10. Internal Inverter Components
Switching Devices Transistors MOSFET’s/IGBT’s
Double Wound Transformer
Diodes
Thermistor’s
Surge diverter MOV’s
Fuses
Filtering Inductors and Capacitors

11. PN Junction Diodes
Cathode
Anode
Diodes are semi conductor devices consisting of a PN Junction the P is the anode and the N is the cathode.
When a positive charge is placed on the anode & negative is place on the cathode the diode conducts and is said to be forward biased. If a negative is placed on the anode and positive on cathode the diode is said to be reverse biases and the diode will not conduct

12. Bridge Rectifier Converts A.C. Supply in to Basic D.C. Supply
Symbol
Input wave shape
Bridge Rectifier
Output wave shape
Bridge Rectifier

13. MOSFET Transistor Metal Oxide Semi Conductor Field Effect Transistor
Two N type semi Conductors within a P type base
When the gate is pulsed positive a electron bridge is created and current can flow through the source and drain.
MOSFET symbol

14. MOSFET’s N channel & P channel

15. IGBT Insulated Gate Bipolar Transistor
The insulated-gate bipolar transistor or IGBT is a three-terminal power semiconductor device, noted for high efficiency and fast switching. It switches electric power in many modern appliances: electric cars, variable speed refrigerators, air-conditioners, and even stereo systems with digital amplifiers. Since it is designed to rapidly turn on and off, amplifiers that use it often synthesize complex waveforms with pulse width modulation and low-pass filters.

16. Double Wound Transformer
Used for stepping the input voltage up or down providing galvanic isolation and helps with smoothing out the sin wave

17. Types of Wave Shapes of Inverters
Square Wave
No voltage regulation, cheap and nasty, limit to type of appliances that can be run off it
Modified Square Wave voltage
Still a square wave except the voltage is Regulated by Pulse Width Modulation (PWM)
True Sin wave (PWM with filters) less than 5% total Harmonic Distortion

18. Types of Waves (Continued)

19. Pulse Width Modulation to create a True Sine Wave
Pulse width modulation is a technique used to create sinusoidal wave from a series of coded D.C. pulses called a pulse train. For the pulse train to become
Smooth like a true sin wave a capacitive inductive filter must be used.

20. Classifying Grid Connect Inverters
Method of Commutation
Line Commutated
Self Commutated
Transformer/ Transformerless
How P.V. Array and Inverter Interface
Modular (at the modules)
String Inverter
Multi String
Mini Central
Central

21. Line Commutated Inverters
Using a thyristor Bridge arrangement the thyristor’s are switched (commutated)
By alternated pulses from the grid.
These are known as grid controlled inverters and when the grid falls these inverter will automatically stop

22. Self Commutated Inverters
Same Principle as Stand Alone Inverters
Using Pulse width modulation switching devices are MOSFET’s or IGBT
Must be used with a Grid Protection Device
To ensure safety of the line workers when the grid goes down.

23. Transformer Classifications Based on Transformers
Low Frequency Transformer (Safe, reliable, lowest efficiency very heavy) Galvanic Isolation

24. HF Inverters
High Frequency Transformer (Galvanic Isolation, no iron Core, Tx very light and more efficient than Low Frequency Tx more complicated electronics)

25. Transformerless Grid Connect Inverters
No Tranformer (transformerless) (No galvanic isolation cheap to build most efficient has some serious safety concerns)

26. Safety Concerns with Transformerless Inverters

27. Specification Sheet SMA Sunny Boy

28. Efficiency Curve

29. Classification by interface String Inverters Most common type

30. Multi String Inverter
Dual MPPT so that two completely separate arrays can be inputed into the inverter with no efficiency consequences .
Can have East West Facing Arrays

31. Central Inverters
Used for Large Solar Farms/Parks where the solar access is uniform across the park

32. List the Advantages and Disadvantages of Central Inverter over Multiple string Inverters