Introduction to Introduction to Power Electronics i PV S t n PV Systems
Transcript of Introduction to Introduction to Power Electronics i PV S t n PV Systems
Introduction toIntroduction to Power Electronics
i PV S tin PV Systems
ECEN 2060
References:• ECEN4797/5797 Intro to Power Electronics
ece.colorado.edu/~ecen5797• Textbook: R.W.Erickson, D.Maksimovic, Fundamentals of Power
Electronics, 2nd ed., Springer 2000, http://ece.colorado.edu/~pwrelect/book/SecEd.html
Example: Grid-Connected PV System O ibl id t d PV t hit t
AC
iac
++
IPV
PVPower
DC input
IV
One possible grid-connected PV system architecture
AC output tVtv RMSac sin2)( AC
utilitygrid
vac
VPVPV
arrayelectronicsconverter
PVPVPV IVP
PVPV IV , RMSac )( tIti RMSac sin2)(
RMSRMSac IVP
tIVivtp RMSRMS 2cos1)(
Functions of the power electronics converter• Operate PV array at the maximum power point (MPP) under all conditions
tIVivtp RMSRMSacacac 2cos1)(
• Generate AC output current in phase with the AC utility grid voltage• Achieve power conversion efficiency close to 100%
RMSRMSac IVP
PVPVPVconverter IVP
• Provide energy storage to balance the difference between PPV and pac(t)Desirable features
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Desirable features• Minimum weight, size, cost • High reliability
Power electronics converter
ACutilitygrid
iac
+
vac
+
VPV
IPV
PVarray
Powerelectronicsconverter
grid
“Inverter”
One possible realization:
AC
iac
++
IPV
DC DC Single-phase+
One possible realization:
ACutilitygrid
vac
VPVPV
arrayDC-DC
converterSingle-phase
DC-ACinverter
Energy-storage
VDC
gy gcapacitor
Class objectives: introduction to circuits and control of a DC-DC converter and a single phase DC AC inverter
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and a single-phase DC-AC inverter
Introduction to electronic power conversion
Four types of powerFour types of power electronics converters
• Control is invariably required• In the PV system, for example:
• Control input voltage of the DC-DCControl input voltage of the DC DC input voltage to operate PV at MPP
• Control shape of the DC-AC output current to follow a sinusoidal reference
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reference• Control current amplitude to balance
the input and output power
High efficiency is essential
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Circuit components for efficient electronic power conversion ?efficient electronic power conversion ?
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Ideal switch
Power semiconductor devices (e.g. MOSFETs, diodes) operate as near-ideal power switches:
• When a power switch is ON, the voltage drop across it is relatively small
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p g p y• When a power switch is OFF, the switch current is very close to zero
Capacitor
dtdv
Ci CC iC+
)()()( titvtp CCC CvC
For periodic vC(t), iC(t):
No losses (average capacitor power = 0)
0)0()(2
)()(1 22)(
)0(0
CC
Tv
CC
T
CC vTvT
CdvtvTCdttp
TP
C
)0(0 vC
1)(
TvT
CC C
Capacitor charge balance (average capacitor current = 0)
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0)0()()(1
)0(0
CCv
CCC vTvTCdv
TCdtti
TI
C
Inductor
iL+
LdtdiLv L
L
vLL
)()()( titvtp LLL
For periodic vL(t), iL(t):
No losses (average inductor power = 0)
0)0()(2
)()(1 22)(
)0(0
LL
Ti
iLL
T
LL iTiTLditi
TLdttp
TP
L
)0(0 iL
1)(
TiT
LL L
Inductor volt-second balance (average inductor voltage = 0)
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0)0()()(1
)0(0
LLi
LLL iTiTLdi
TLdttv
TV
L
Circuit components for efficient electronic power conversion
Power electronics converters are circuits consisting of semiconductor devices operated as (near-ideal) switches,
capacitors and magnetic components (inductors, transformers)
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p g p ( , )
Boost (step-up) DC-DC converter
switch control
Position 1 Position 2
Ts
DTs
fs = 1/Ts = switching frequency
Ts = switching period
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fs s g q y
D = switch duty ratio (or duty cycle), 0 D 1
Boost converter circuit
Power MOSFET and diode operate as near-ideal switches
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Power MOSFETs and diodesCharacteristics of several commercial power MOSFETsCharacteristics of several commercial power MOSFETs
+–
DTs Ts
Low on-resistance implies low pconduction losses
Fast switching enables high switching frequencies, e.g. 100’s of kHz to MHz g g g q g
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Voltage, current and frequency ratings of power semiconductor devices
(SCR)
Voltage rating
Current rating
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MOSFET: Metal Oxide Semiconductor Field Effect TransistorIGBT: Insulated Gate Bipolar TransistorSCR (or Thyristor): Silicon Controlled RectifierGTO: Gate Turn Off thyristor
Boost converter analysis
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Position 1
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Position 2
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Inductor voltage and capacitor current waveforms
D’ 1 DD’ = 1-D
Periodic steady-state operation• Inductor volt-second balance: average inductor voltage = 0
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• Capacitor charge balance: average capacitor current = 0
Inductor volt-second balance
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Boost DC voltage conversion ratio M = Vout/Vg
Boost DC-DC converter steps-up a DC input voltage by a ratio M
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p p p g ywhich is electronically adjustable by changing the switch duty ratio D
Simulink modelECEN2060ECEN2060
Switched-mode BoostDC-DC converter
200 4
1/100
1/Rload
iout
boost_switching.mdl
200.4
Vout
100
Vin
0.5
Duty cycle
Vg
Iout
D
Vout
IL
switch control
BoostDC-DC
(switching)
Boost DC-DCD
Vg
Vout
Vout
iL
switch control
Scope
Input voltage Vg = 100 VI d t L 200 HInductance L = 200 HCapacitance C = 10 FLoad resistance R = 100 Switch duty cycle D = 0.5Switch duty cycle D 0.5Output voltage Vout = 200 VInput current Ig = IL = 4 APower P = 400 W
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Switching frequency fs = 100 kHzSwitching period Ts = 10 s
Averaged (DC) model
No losses:gout V
DV
11
outg ID
I
1
1D1
outoutgg IVIV
Ideal boost DC-DC converter works as an ideal DC transformer with an electronically adjustable step-up ratio
1+ +
1:nIg Iout
DDMn
11)(Vg Vout
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Modeling of lossesL i i h d d• Losses in switched-mode power converters: Conduction losses, due to voltage drops across inductor
winding resistance and across power semiconductorwinding resistance, and across power semiconductor switches when ON
• Conduction losses depend strongly on the output power
Switching losses, due to energy lost during ON/OFF transitions
• Switching losses are not strongly dependent on output power; aSwitching losses are not strongly dependent on output power; a portion of switching loss remains even at zero output power
• Switching losses are proportional to the switching frequency
Other losses incl dingOther losses, including:• Losses in magnetic cores• Power needed to operate control circuitry
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Switching waveforms and switching losses MOSFET turn on transition
vt
MOSFET turn-on transition
Drain voltage
zoom-in
t
itDrain current
vtDTs Ts
it +
L
+
IoutiL
Ig
id
vd+ _+ _vL
t
it
+–
DTs Ts
+–
_
C R
_
voutvtvgateVg pt = vt it
Switching power loss = Transition energy loss * Switching frequency
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Switching waveforms and switching losses MOSFET turn off transition
vt
MOSFET turn-off transition
Drain voltage
zoom-in
t
itDrain current
it
DTs Ts
it +
L
+
IoutiL
Ig
id
vd+ _+ _vL
vt
+–
DTs Ts
+–
_
C R
_
voutvtvgateVg
Switching power loss = Transition energy loss * Switching frequency
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Averaged (DC) model with losses
+ +1D : 1Ig Iout
RL
Vg
VoutIsw
• Small RL models conduction losses due to inductor winding resistance and power switch resistancesp
• Small Isw models switching and other load-independent losses• Efficiency with losses, when the load current Iout is known:
swswoutL IIIR
2
2)(1
1
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outoutout IIVD 2)1(
Example: efficiency for various RL
Assume:• Resistive load
R = Vout/Iout
• Isw = 0
RDRL 1
)1(1
1
2
)(
Note that it is more difficult to achieve high efficiency if a large
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Note that it is more difficult to achieve high efficiency if a large step-up ratio is required (i.e. if duty-ratio D is close to 1)
Single-phase DC-AC grid-connected inverter
vacLi
iin
1 2+ acLiL
iac
+– VDC
1
1
2
2
• Switches in position 1 during DTs, in position 2 during (1D)Ts
• Switching frequency fs is much greater than the AC line frequency (60 Hz or 50 Hz)• By controlling the switch duty ratio D it is possible to generate a sinusoidal ACBy controlling the switch duty ratio D, it is possible to generate a sinusoidal AC
current iac (+ small switching ripple) in phase with the AC line voltage, as long as the input DC voltage VDC is sufficiently high, i.e. as long as VDC is greater than the peak AC line voltage
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Position 1
vacLi
iin
1 2+ acLiL
iac
+– VDC
1
1
2
2 + vL
acDCL vVv ii acL ii
Li ii
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Lin ii
Position 2
vacLi
iin
1 2+ acLiL
iac
+– VDC
1
1
2
2 + vL
acDCL vVv
ii acL ii
Li ii
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Lin ii
Inductor volt-second balanceN t th t it hi f f li f• Note that switching frequency fs >> ac line frequency
• Over a switching period, vac(t) const.
ssacDC
sacDCL TtDTvV
DTtvVv
,0 ,
ssacDC
sT
acDCacDCacDCLL vVDvVDvVDdttvT
V 0)12())(1()()(1
sT0
12)( DVv
DMDC
ac
1)(1 DM
V t b t th th k f
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VDC must be greater than the peak of vac
Control of AC line current
vacLi
iin
1 2+ acLiL
iac
+– VDC
1
1
2
2
Control objectives:Control objectives:• iac = IM sin (t), in phase with AC line voltage vac(t)• Amplitude IM (or RMS value) adjustable to control power delivered to the AC line
tVtv RMSac sin2)(
tIti RMSac sin2)(
tIVivtp RMSRMS 2cos1)(
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RMSRMSac IVP
tIVivtp RMSRMSacacac 2cos1)(
A simple current controller
+ vacLiL
+ V
iin
1 2
iac
+– VDC
12
sensediLsin(t)
I +
_+
iref
iref iL
switchcontrol
IMrefiref iL comparator
with hysteresismultiplier
)sin( tIi Mrefref switch controlcomparatorwith hysteresis)sin( tIi Mrefref
iL < iref – i/2: position 1iL > iref + i/2: position 2
position 1
with hysteresis
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iL is always within i/2 of irefposition 2 iref iL
i/2i/2
current ripple
Simulink modelECEN2060ECEN2060
Switched-Mode DC-AC Inverter
200
Vdc
Vdc
Iref
v ac
iac
iin
DC-ACinverter
(switching)
dcac_switching.mdl
W f (t) i (t) i (t) d it h t lStep
Scope
Irefswitch control
DC-AC
Waveforms vac(t), iac(t), iin(t), and switch control over one AC line period (1/60 s)
Input voltage VDC = 200 V
Inductance L = 2 mHAC: 120Vrms, 60HzIMref = 32 = 4.2 AiL = 1 AiL = 1 APac = 360 WWith this simple controller switching
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controller, switching frequency is variable
Averaged DC-AC inverter model with losses
+ +1 : D 1Iin iacRL
VDC
vacIsw
ideal transformer
• Small RL models inductor winding resistance and power switch resistancesswitch resistances
• Small Isw models switching and other losses
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DC-AC inverter efficiency exampleECEN2060
Switched-Mode DC-AC Inverter (averaged model)
200
Vdc
Vdc
v ac
iac
iin
D
DC-AC
inverter
(averaged)
v ac
iac
iin
Duty
Simulink modeldcac averaged.mdl
Input voltage VDC = 200 VAC: 120Vrms, 60HzR = 0 8
60
fac
Scope
566.2
Pin
1s
Integrator
4.5
IRMS
0.9537
Iref pin
pout
DC-AC
pin
pout
dcac_averaged.mdl RL = 0.8
Isw = 50 mAPac = 0 to 600 W
80%
90%
100%60
fac1
540
Pout
1s
Integrator1
0.9537
EfficiencyDivide
• Inverter efficiency of about
40%
50%
60%
70%
80%• Inverter efficiency of about 95% is typical
• At high power levels, conduction losses due to RL
10%
20%
30%
40%conduction losses due to RLdominate
• At low power levels, efficiency drops due to switching and
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0%0 100 200 300 400 500 600
Pac [W]
other fixed losses