Power Electronics & FACTS

(EE5519)

Dr Mohamed DarwishRoom: H207

Email: mohamed.darwish@brunel.ac.uk

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.