EE394V_DG_Week6

8/7/2019 EE394V_DG_Week6

1/12

1 Alexis Kwasinski, 2010

Power electronic interfaces

Power electronic converters provide the necessary adaptation functions tointegrate all different microgrid components into a common system.

8/7/2019 EE394V_DG_Week6

2/12


Power electronic interfaces

Integration needs: Component with different characteristics:

dc or ac architecture. Sources, loads, and energy storage devices output.

Control issues: Stabilization

Operational issues: Optimization based on some goal

Efficiency (e.g. MPPT) Flexibility Reliability

Safety Other issues:

Interaction with other systems (e.g. the main grid)

8/7/2019 EE394V_DG_Week6

3/12


Power electronics basics

Types of interfaces: dc-dc: dc-dc converter ac-dc: rectifier dc-ac: inverter ac-ac: cycloconverter (used less often)

Power electronic converters components: Semiconductor switches:

Diodes MOSFETs IGBTs SCRs

Energy storage elements Inductors Capacitors

Other components: Transformer Control circuit

8/7/2019 EE394V_DG_Week6

4/12



Types of interfaces: dc-dc: dc-dc converter ac-dc: rectifier dc-ac: inverter ac-ac: cycloconverter (used less often)

Power electronic converters components: Semiconductor switches:

Diodes MOSFETs IGBTs SCRs

Energy storage elements Inductors Capacitors

Other components: Transformer Control circuit

Diode MOSFET

IGBT SCR

8/7/2019 EE394V_DG_Week6

5/12


Power electronics basics dc-dc converters

oV !Buck converter

1oV !

Boost converter

1oV !

Buck-boost converter

8/7/2019 EE394V_DG_Week6

6/12



Rectifiers

R ectifier Filter

t t t

vv v

8/7/2019 EE394V_DG_Week6

7/12



Inverters dc to ac conversion Several control techniques. The simplest technique is square wavemodulation (seen below).The most widespread control technique is Pulse-Width-Modulation (PWM).

8/7/2019 EE394V_DG_Week6

8/12


Power electronics basic concepts

Energy storage When analyzing the circuit, the state of each energy storage elementcontributes to the overall systems state. Hence, there is one state variableassociated to each energy storage element.

In an electric circuit, energy is stored in two fields: Electric fields (created by charges or variable magnetic fields andrelated with a voltage difference between two points in the space) Magnetic fields (created by magnetic dipoles or electric currents)

Energy storage elements: Capacitors: Inductors:

C L

8/7/2019 EE394V_DG_Week6

9/12


Power electronics basic concepts

Capacitors: state variable: voltage Fundamental circuit equation:

The capacitance gives an indication of electric inertia. Compare theabove equation with Newtons

Capacitors will tend to hold its voltage fixed. For a finite current with an infinite capacitance, the voltage must beconstant. Hence, capacitors tend to behave like voltage sources (thelarger the capacitance, the closer they resemble a voltage source) A capacitors energy is

C C

dvi C

dt !

dvm

dt !

r r

212

C W C v!

8/7/2019 EE394V_DG_Week6

10/12


Power electronics basic concepts Inductors

state variable: current Fundamental circuit equation:

The inductance gives an indication of electric inertia. Inductors willtend to hold its current fixed.Any attempt to change the current in an inductor will be answered withan opposing voltage by the inductor. If the current tends to drop, thevoltage generated will tend to act as an electromotive force. If thecurrent tends to increase, the voltage across the inductor will drop, like

a resistance. For a finite voltage with an infinite inductance, the current must beconstant. Hence, inductors tend to behave like current sources (thelarger the inductance, the closer they resemble a current source) An inductors energy is

LL

d iv L

d t !

21

2

LW L i!

8/7/2019 EE394V_DG_Week6

11/12



Harmonics Concept: periodic functions can be represented by combiningsinusoidal functions

Underlying assumption: the system is linear (superposition principleis valid.) e.g. square-wave generation.

01

( ) cos( )n nn

f t c c n t [ Ug

!

!

8/7/2019 EE394V_DG_Week6

12/12



Additional definitions related with Fourier analysis

01

( ) ( cos( ) sin( ))n nn

f t a a n t b n t [ [ g

!

!

2( ) cos( )

T

na f t n t d t T X

X

[ !

1tan nnn

ba

U !

0

1( )

T

a f t d t T

X

X

!

2( )sin( )

T

nb f t n t d t T X

X

[ !

2 2n n nc a b! 0 0 (dc components)a c!

Documents

EE394V_DG_Week6