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Power Electronics and FACTS (EE5519)
WednesdayTuesday
Modern Power Semiconductors
(MD)
Monday Thursday Friday
9:00 – 10:00
10:00 – 11:00
11:00 – 12:00
12:00 – 13:30
13:30 – 14:30
14:30 – 15:30
15:30 – 16:30
Dissipative & Energy Efficient Snubbers
(MD)
Converter Topologies
(MD)
Electrical Generators for Wind, Wave and
Tidal Power
(ES)
Introduction to Switching Functions
(CM)
Introduction to FACTS (MI)
Modelling of FACTS Controller
(MI)
FACTS Development &
Applications (MI)
SEP Workshop &
Specification of module
assignments
IndividualStudyTime Laboratory exercise:
Phase shifters & voltage controllers
(MD)
Applications of the switching function in
Power Electronics(CM)
Laboratory exercise: PSPICE
Simulations(MD)
Tutorial(MD)
V.S. & C.S. Inverters
(MD)
Rotary & Static UPS
systems (MD)
High frequency
UPS systems (MD)
LUNCH
Voltage Regulators
(MD)
Power Quality & Harmonics
(MD)
Passive & Active Power Filters
(MD)
MD → Mohamed DarwishMI → Malcolm Irving
CM → Christos MarouchosES → Ed Spooner
• Buck Converter
• Boost Converter
• Buck/Boost Converters
DC-DC Converter Topologies
DC DC
Applications:
• Power Supplies
Properties:• High efficiency (ideally 100%)
• Robust and withstand extreme conditions (short-circuiting & open circuiting)
• Allow energy to flow in either direction (in some cases)
• To provide electrical isolation (in some cases)
The Buck Converter
The buck converter is a step-down converter, in the sense that the output voltage can never be greater than the input voltage.
Vin
t
Vout
t
Vin Vout
Insert low-pass filter to remove switching harmonics and pass only DC component.
VinVout
Vin
t
VD
t
Vout
t
Practical Buck ConverteriLS
LoadD
L
Vin VoutVD C
iout
iL
t
VD
Vout
Vin
Iout
t
Δ I
ton toff
Applying vL = L di/dtΔ Ion = (Vin – Vout ) DT / L
‘S’ is Closed:
A voltage appears across L for a time ton . If L is finite, the result will be a small increase in the current through it, Δ I(on) .
D = ton / (ton + toff) & T = ton + toff
ton = DTtoff = (1 – D) T
‘S’ is Open:
When S is open and diode conducts, a voltage – Vout appears across L for time toff . Again applying: vL = L di/dt we find that:Δ Ioff = – Vout (1 – D) T / L
In the steady state:ΔIon + Δ Ioff = 0
(Vin – Vout ) DT / L = Vout (1 – D) T / L
Vout = D Vin
The value of the capacitor is determined by the amount of voltage ripple acceptable at the output.
The inductance value is not critical, but there are several factors that influence its choice:
• Imax depends on Δ I, so to minimise the switch and diode current ratings L should be large.
• The losses in the practical switch and diode increase somewhat as Imax increases, so L should be large.
• The cutoff frequency of the output ‘LC’ filter is ω = 1/√(LC). For a given filtering effect there is a trade-off between L and C.
• To minimise the ripple current rating of C, L should be large.
• To ensure continuous mode of operation, L should be large.
• From size, weight and cost considerations we would like L to be as small as possible.
The Boost Converter
The buck converter is a step-up converter. It is also called fly-back converter.
Vin LoadVoutC
DL
S
iin iout
Vs
Vin
t
Vs
t
Vout
t
iL
t
VS
Vin
Iavg
Δ I
tton toff
VoutV
The Boost Converter
VSVin Vout
Δ Ion = Vin DT / L
‘S’ is Closed:
A voltage, v in appears across ‘L’ during ton . If L is finite, the result will be a small increase in the current through it, Δ I(on) .
D = ton / (ton + toff) & T = ton + toff
ton = DTtoff = (1 – D) T
Δ Ioff = (Vin - Vout ) (1 – D) T / L
‘S’ is Open:
A voltage, v in – vout (negative voltage) appears across ‘L’during toff . The result is a small decrease in the current:
In the steady state:Δ Ion + Δ Ioff = 0
Vin D T / L + (Vin – Vout ) (1 – D) T / L = 0
Vout = Vin / (1 – D)
Since ‘D’ lies between 0 and 1 Vout is greater than Vin and of the same polarity.
Vout can be varied by PWM (changing ‘D’)
The choice of the capacitor is theoretically determined by the amount of voltage ripple acceptable at the output.
The inductance value is not critical, but there are several factors that influence its choice:
• Imax depends on Δ I, so to minimise the switch and diode current ratings L should be large.
• The losses in the practical switch and diode increase somewhat as Imax increases, so L should be large.
• The cutoff frequency of the output ‘LC’ filter is ω = 1/√(LC). For a given filtering effect there is a trade-off between L and C.
• To minimise the ripple current rating of C, L should be large.
• To ensure continuous mode of operation, L should be large.
• From size, weight and cost considerations we would like L to be as small as possible.
The Buck / Boost Converter
Buck / Boost converter is a dc-to-dc converter that has the capability of stepping up or stepping down the output voltage.
It is also labeled as indirect converter because thesource is never directly connected to the load.
It is the inductor in the circuit that controls the flow of energy from the input side to the output side.
S D
Vin Vout LoadCVLL
i L+ +
- -
- +
-+
-
+
i outi in
iL
t
VL
Iin + Iout
Δ I
tton toff
The Buck / Boost Converter
Vin
-Vout
S D
Vin Vout LoadCVLL
i L+ +
- -
+
-
-
+
-
+
i outi in
‘S’ is Closed:
A voltage, v in appears across ‘L’ during ton and Δ Ion = Vin DT / L
‘S’ is Open:
A voltage,– vout (negative voltage) appears across ‘L’ during toff and Δ Ioff = - Vout (1 – D) T / L
In the steady state:
Δ Ion + Δ Ioff = 0
Vin DT / L - Vout (1 – D) T / L = 0 Vout = Vin D / (1 – D)
When 0 < D < 0.5 Vout < Vin
When D = 0.5 Vout < Vin
When 0.5 < D < 1 Vout < Vin
Practical Consideration:
• The switch must be a semiconductor device (BJT, MOSFET, etc.). It will need some form of drive circuit.Its conduction and switching loss may be significant, so it willneed a snubber circuit and heatsinking to ensure reliable operation.
• Diode must be a fast type; low stored charge. It will also need heatsink.
• Inductor design – already discussed.
• The ripple current capability and equivalent series impedance ofreservoir capacitors must be considered.