Grid Connect Inverters
NUER 19
Basic Principles
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
An Inverter converts this
Direct Current Input Wave
Shape
To this Alternating Current
Output wave shape
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
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.
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
System Diagram
Basic Portable Inverter Circuit
with MOSFET’s and Centre Tap Transformer
Block Diagram for Typical Grid connect
Inverter
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
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
Bridge Rectifier
Converts A.C. Supply in to Basic D.C. Supply
Symbol
Input wave shape
Bridge Rectifier
Output wave
shape
Bridge Rectifier
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
MOSFET’s N channel & P channel
IGBT
Insulated Gate Bipolar Transistor
The insulated-gate bipolar
transistor or IGBT is a threeterminal 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.
Double Wound Transformer
Used for stepping the
input voltage up or
down providing
galvanic isolation and
helps with smoothing
out the sin wave
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
Types of Waves (Continued)
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.
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
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
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.
Transformer Classifications Based on
Transformers
• Low Frequency Transformer (Safe, reliable, lowest
efficiency very heavy) Galvanic Isolation
HF Inverters
• High Frequency Transformer (Galvanic Isolation, no
iron Core, Tx very light and more efficient than Low
Frequency Tx more complicated electronics)
Transformerless Grid Connect Inverters
• No Tranformer (transformerless) (No galvanic
isolation cheap to build most efficient has some
serious safety concerns)
Safety Concerns with Transformerless
Inverters
Specification Sheet SMA Sunny Boy
Efficiency Curve
Classification by interface
String Inverters
Most common type
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
Central Inverters
• Used for Large Solar Farms/Parks where the
solar access is uniform across the park
List the Advantages and Disadvantages of
Central Inverter over Multiple string Inverters