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SEC598F16
Photovoltaic Systems Engineering
Session 11
PV System Components
Charge Controllers
Inverters
Balance of Systems (BOS)
September 29, 2016
Session 11 content
Charge Controllers
Inverters
Balance of Systems
o
o
Purpose, utility
Operation, reliability, failure mechanisms
2
Learning Outcomes
• Introduction to the power electronics used
in PV systems
• Recognition of the importance of
controllers and inverters to the operation of
certain PV systems
• Understanding of BOS components and
their value to PV systems
3
PV Systems - Batteries
Lead Acid Battery
Lead acid batteries are less prone to electrolysis and sulfation if the
charging protocol does not employ a constant rate. The optimized
charging profile looks like this:
This current profile
is produced by a
“charge controller”
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PV Systems – Charge Controllers
A simplified version of a “stand-alone” system
that transfers DC electrical energy to a storage device
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PV Systems – Charge Controllers
Charge Controller
• The charge controller is an essential
electronic component in any PV system that
employs battery storage
• The charge controller carries out some
important functions:
o
o
o
It accepts the DC power from the PV array
It directs the DC power to the battery array
matching the optimal charging procedure, as
needed
It directs the DC power to the inverter for the AC
standby loads or grid connection
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PV Systems – Charge Controllers
• These features must be met in the choice
of charge controller:
o
o
o
Its DC output must match the battery array
voltage
Its DC output must normally supply enough
current to recharge the battery array in one day
Its DC current directed to the inverter must not
exceed the allowable inverter input
• There are quite a few Charge Controller
manufacturers with very high quality
products
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PV Systems – Charge Controllers
Pulse Width Modulation (PWM)
•An electronic technique that is used to:
•
•
•
switch from an input dc voltage at one level to an
output level at another level, as in DC power supplies
carry out the ‘perturb and observe’ process to find the
maximum power point
generate time dependent voltages that closely
approximate sinusoidal or other waveforms
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PV Systems – Charge Controllers
Pulse Width Modulation (PWM)
ac
line
voltage
AC-DC
converter
DC-DC
switching
power
supply
rectifier
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PV Systems – Charge Controllers
Pulse Width Modulation (PWM) – Duty Cycle
Vm
v(t)
Vavg
ton
toff
time
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PV Systems – Charge Controllers
Pulse Width Modulation (PWM) – Duty Cycle
Vavg = = 1
T
1
T
T
ò v(t)dt
0
ton
= VM
òV
M
dt
0
ton
ton
= VM
= VM D
T
( ton + toff )
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PV Systems – Charge Controllers
Pulse Width Modulation (PWM) – Duty Cycle
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PV Systems – DC-DC converters
Boost converter
•
The transfer characteristic is:
Vout
1
= Vin
1- D
en.wikipedia.org/wiki/Boost_converter
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PV Systems – DC-DC converters
Boost converter
ON
OFF
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PV Systems – DC-DC converters
Buck converter
•
The transfer characteristic is:
Vout
= D
Vin
en.wikipedia.org/wiki/Buck_converter
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PV Systems – DC-DC converters
Buck converter
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PV Systems – DC-DC converters
Buck-Boost converter
o The transfer characteristic is:
Vout
D
= Vin
D -1
en.wikipedia.org/wiki/Buckboost_converter
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PV Systems – DC-DC converters
Buck-Boost converter
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PV Systems – DC-DC converters
Summary
• Boost Converter
Vin £ Vout < ¥
• Buck Converter
0 < Vout < Vin
• Buck-boost Converter
0 < Vout < ¥
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PV Systems – DC-DC converters
Summary
• Boost Converter
•
•
Toyota Prius
LED Lamps
• Buck Converter
•
•
Vin £ Vout < ¥
0 < Vout < Vin
Impedance matching
Charge controllers
• Buck-boost Converter
0 < Vout < ¥
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PV Systems – Charge Controllers
12V, 24V, 48V
45A, 70A, 100A
1600W, 3200W
www.morningstar.com
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PV Systems – Charge Controllers
Charge controller block diagram
Isolation of PV array
and battery
Protection from
overcharging
Protection from deep
discharging
www.morningstar.com
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PV Systems – Charge Controllers
Charge controller operation
www.morningstar.com
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PV Systems – Inverters
A simplified version of a grid-tied utilityinteractive PV system
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PV Systems – Charge Controllers
A simplified version of a grid-tied PV system
with battery backup
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PV Systems - Maximum Power Point Tracking
The PV system produces electrical power and is best
utilized when the maximum power produced can be
fully delivered to the electrical “load” – this can only
happen when the power source and the power load
“match”
C.S.Solanki, Solar Photovoltaic Technology and Systems
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PV Systems - MPPT
Other representative electrical loads
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PV Systems - MPPT
An approach to assuring a better match is the use of
Maximum Power Point Tracking (MPPT) – an electronic technique
that moves the operating point along the maximum power
hyperbola (I*V = constant) associated with the PV array until it
intersects the electronic load IV characteristic
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PV Systems - MPPT
 Perturb and Observe
PV operating points from P&O algorithm
N.Fermia et al., Power Electronics and Control Techniques for Maximum
Harvesting in PV Systems
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PV Systems - MPPT
 Perturb and Observe
Time domain behavior
N.Fermia et al., Power Electronics and Control Techniques for Maximum
Harvesting in PV Systems
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PV Systems - MPPT
 Perturb and Observe
P&O flowchart
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PV Systems - MPPT
 Perturb and Observe
N.Fermia et al., Power Electronics and Control Techniques for Maximum
Harvesting in PV Systems
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PV Systems - Inverters
The inverter is the essential electronic system that
converts the DC electrical output from the PV array into
the AC electrical input for the residence, national
electrical grid, and so on
DC input
INVERTER
AC output
33
PV Systems - Inverters
Heart of the inverter – the “H-bridge”
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PV Systems - Inverters
The H-bridge in operation
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PV Systems - Inverters
The output of the inverter is controlled
by pulse width modulation (PWM)
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PV Systems - Inverters
• State of the Art Inverters:
o
High efficiency – 98% or higher
o
Dual independent MPPT systems
o
Integrated DC disconnect and combiner inputs
o
No fans or electrolytic capacitors
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PV Systems - Inverters
J.M.Jacob, Power Electronics: Principles and Applications
38
PV Systems – Balance of Systems (BOS)
Components
• The Balance of System components are the smaller and less
expensive items needed to complete the assembly of a PV
system
• Many of the BOS components must meet certain codes and
standards. Some of the codes are building codes, others are
environmental in nature, others still are electrical codes. Some
are specified by national regulatory bodies, others by local
authorities
• In the United States, the National Electrical Code (NEC)
specifies the requirements for many BOS components
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PV Systems – BOS
• The Balance of System components are the smaller
and less expensive items needed to complete the
assembly of a PV system
o
o
o
o
o
o
o
o
o
o
o
Disconnects
Surge protectors
Overcurrent protection devices
Ground fault detection and interruption devices
Grounding connections
Wiring
Connectors
Receptacles
Enclosures
Combiner boxes
Array mounts
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PV Systems – BOS
• Switches, Circuit Breakers, Fuses, Receptacles
o
o
All of these electrical components used in the DC sections of
the PV system must be rated for DC electricity. Similarly, all
of the components used in the AC sections of the PV system
must be rated for AC electricity.
The NEC specifies circuit breaker sizes matched to wire
sizes:
 #10 THHN wire can carry a maximum current of 40A, but the
maximum circuit breaker size allowed for use with this wire is 30A
o
Different voltages require different receptacles:
 12 VDC will not damage a receptacle designed for 120 VAC, but 120
VAC will surely damage a 12 VDC receptacle
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PV Systems – BOS
• Ground Fault Protection
o
o
o
o
The NEC requires that metallic frames and other metal parts
of PV systems be connected to ground – this is done with
grounding connectors.
The current leaves the PV array through the positive
conductor and the same amount returns in the negative
conductor. One of these is also connected to ground at one
point in the system, and is then known as the grounded
connector.
If the ungrounded connector were to become connected to
ground, then current could also flow in the grounding
connectors.
This situation is known as a ground fault, and the NEC
specifies that if a ground fault occurs, the PV array must
then be disconnected from the inverter and electrical loads
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PV Systems – Inverters
A simplified version of a grid-tied utilityinteractive PV system
43
PV Systems – BOS
A more detailed version of the grid-tied utilityinteractive PV system
44