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Transcript of Grid Pv System
Power Electronics and Control in Grid-Connected PV Systems
ECEN 2060
2ECEN2060
Grid-Connected PV System
ACutilitygrid
iac
+
−
vac
+
−
VPV
IPV
PVarray
Powerelectronicsconverter
DC input
PVPVPV IVP =
PVPV IV ,
One possible grid-connected PV system architecture
AC output
( )tVtv RMSac ωsin2)( =
( )tIti RMSac ωsin2)( =
RMSRMSac IVP =
Functions of the power electronics converter
• Operate PV array at the maximum power point (MPP) under all conditions
• Generate AC output current in phase with the AC utility grid voltage
• Achieve power conversion efficiency close to 100%
PVPV
RMSRMS
PV
acconverter
IV
IV
P
P==η
• Provide energy storage to balance the difference between PPV and pac(t)
Desirable features
• Minimum weight, size, cost
• High reliability
( )( )tIVivtp RMSRMSacacac ω2cos1)( −==
3ECEN2060
Power Electronics for Grid-Connected PV System
One possible realization:
ACutilitygrid
iac
+
−
vac
+
−
VPV
IPV
PVarray
BoostDC-DC
converter
Single-phaseDC-ACinverter
Energy-storagecapacitor
+
−
VDCC
DC-DC control DC-AC control
Boost DC-DC converter
• Set the PV operating point (VPV, IPV) to MPP
• Efficiently step up VPV to a higher DC voltage VDC
DC-AC inverter
• Efficiently generate AC output current iac in phase with the AC grid voltage vac
• Balance the average power delivery from the PV array to the grid, Pac = Ppv * ηDC-DC * ηDC-AC
Energy storage capacitor C
• Balance the difference between the instantaneous power pac(t) and the average power
The system must be disconnected from the grid if the utility loses power
4ECEN2060
DC-AC Inverter Control
One possible realization:
ACutilitygrid
iac
+
−
vac
+
−
VPV
IPV
PVarray
BoostDC-DC
converter
Single-phaseDC-ACinverter
Energy-storagecapacitor
+
−
VDCC
DC-DC control DC-AC control
• The control variable for the DC-AC inverter is the RMS current reference IRMSref
• The inverter output current iac(t) is controlled so that it is in phase with the grid voltage vac(t)
and so that it’s RMS value equals the reference:
= IRMSref
IRMS = IRMSref
One possible current control approach, based on a comparator with hysteresis, has been
discussed in class, see Intro to Power Electronics notes
5ECEN2060
Simulation model: pv_boost_dcac_averaged.mdl
ECEN2060
PV + Boost DC-DC + DC-AC inverter averaged model
ECEN2060
6-module PV Array
Average output AC power
Average input AC power
DC-AC average power
and efficiency
Set Boost Iref to
operate PV array
at MPP
Set DC-AC Iref
to balance
the power, i,e
to keep VDC
constant
PV output power
60
fac_out
60
fac_in
103.2
Vpv
199.8
Vout (boost) = VDC
Product
510.8
Ppv
492.6
Pout boost
472.8
Pout
493.2
PinIpv
Insolation
Vpv
Ppv
PV module (I)
PV6
Ipv
Insolation
Vpv
Ppv
PV module (I)
PV5
Ipv
Insolation
Vpv
Ppv
PV module (I)
PV4
Ipv
Insolation
Vpv
Ppv
PV module (I)
PV3
Ipv
Insolation
Vpv
Ppv
PV module (I)
PV2
Ipv
Insolation
Vpv
Ppv
PV module (I)
PV1
4.95
PV current
3.94
Iref
1
s
Integrator(pout)
1
s
Integrator(pin)
1000
Insolation Vdc
Iref
v ac
iac
iin
D
pin
pout
DC-AC
inverter
(averaged)
DC-AC Inverter
0.9586
DC-AC Efficiency
DC-AC
scope
Compute
efficiency
Boost scope
0.9643
Boost efficiency
Vg
Iout
Iref
Vout
D
ef f iciency
Pout
Boost
DC-DC
(averaged, C)
current control
Boost DC-DC
Add
v ac
iac
iin
Duty
pin
pout
VoutVpv
Vpv
VpvPpv
Ipv = Iref
Ipv = Iref
Duty
ef f iciency
pin, pout
IRMSref
6ECEN2060
How to achieve average power balance?
Simulation example:
• 6-module (85 W each) PV array with full sun (1,000 W/m2 insolation)
• PV array operates at MPP: Ppv = 6*85 W = 510 W
• AC grid RMS voltage: 120 V
• Run simulations for 3 different values of IRMSref and observe boost output voltage Vout(t) = VDC(t)
IRMSref = 3.94 A
IRMSref = 4.4 A
IRMSref = 3.4 A
Tac = AC line period (1/60 seconds)
IRMSref is too low
Pac < Ppv
VDC increases
IRMSref is too high
Pac > Ppv
VDC decreases
IRMSref is just right
Pac ≈ Ppv
VDC starts at 200 V
and returns to 200 V
7ECEN2060
Average Power Balance by Automatic Feedback Control
• Voltage VDC is sensed and compared to a reference value VDCref (e.g. VDCref = 200 V)
• The difference VDC – VDCref is the error signal for the feedback controller
• If the error is positive, i.e. if VDC is greater than VDCref, the compensator increses IRMSref
• If the error is negative, i.e. if VDC is less than VDCref, the compensator decreases IRMSref
• In steady-state, IRMSref adjusted by the automatic feedback controller is just right so that
VDC = VDCref, error signal is zero, and the average power Pac delivered to the AC grid
matches the power generated by the PV array
• Stability, dynamic responses and realizations of feedback controllers are topics beyond the
scope of this class. These topics are addressed in Circuits, and more advanced Control
and Power Electronics courses
ACutilitygrid
iac
+
−
vac
+
−
VPV
IPV
PVarray
BoostDC-DC
converter
Single-phaseDC-ACinverter
+
−
VDC
DC-DC control
VDCref
IRMSref+−
compensator
8ECEN2060
Energy storage
ACutilitygrid
iac
+
−
vac
+
−
VPV
IPV
PVarray
BoostDC-DC
converter
Single-phaseDC-ACinverter
Energy-storagecapacitor
+
−
VDCC
DC-DC control DC-AC control
• Capacitor C provides energy storage necessary to balance instantaneous power delivered to the grid
• Magnitude of the resulting voltage ripple ∆VDC at twice the line frequency (2 x 60 = 120 Hz) depends on the average power Pac and capacitance C
Pac pac(t)
tPtPPtpP acacacacac ωω 2cos)2cos1()( =−−=−
Pac > pac(t), capacitor C is charged up
Pac < pac(t), capacitor C is discarged
∆vDC
9ECEN2060
Energy storage capacitor C
tPtPPtpP acacacacac ωω 2cos)2cos1()( =−−=−
• Energy supplied to the capacitor during the time when Pac > pac(t), i.e. when the capacitor
is charged from VDCmin to VDCmax
Pac > pac(t), capacitor C is charged up
Pac < pac(t), capacitor C is discarged
∆vDC
ωθθ
ωω
π
π
acac
T
T
acC
Pd
PdttPE
ac
ac
∫∫−−
===∆
2/
2/
8/
8/
cos2
2cos
• This energy must match the change in energy stored on the capacitor:
( ) DCDCDCDC
DCDCDCDCC VCVVV
VVCCVCVE ∆≈+
−=−=∆22
1
2
1 minmaxminmax
2min
2max
• Solve for the ripple voltage:
ωac
DCDC
PVCV =∆
ωDC
acDC
CV
PV =∆
10ECEN2060
Energy storage analysis example
• DC-AC inverter input voltage: VDC = 200 V
• Average power delivered to the grid: Pac = 600 W
• Find C so that ∆VDC = 40 V (i.e. +/-10% of the DC voltage at the input of the DC-AC inverter)
• Solution:
J 6.1602
600===∆
πωac
C
PE
ωac
DCDC
PVCV =∆
F 200Hz 602*V 200*V 40
W600µ
πω==
∆=
DCDC
ac
VV
PC
• Note that the energy supplied (or absorbed) by the capacitor is relatively small:
• The total energy stored on the capacitor is also small
J 42
1 2 == DCC CVE
• This example illustrates the need for only relatively small energy storage in a grid-
connected system, easily accomplished by a capacitor, in sharp contrast to stand-alone
PV systems that require very significant energy storage (e.g. batteries)
11ECEN2060
Maximum Power Point (MPP) Tracking
ACutilitygrid
iac
+
−
vac
+
−
VPV
IPV
PVarray
BoostDC-DC
converter
Single-phaseDC-ACinverter
Energy-storagecapacitor
+
−
VDCC
DC-DC control DC-AC control
Choices for the Boost DC-DC control variable:• Duty cycle D• Input current reference Iref
• Input voltage reference Vref
• The objective of the MPP tracking algorithm is to adjust the DC-DC control
variable so that the PV array operates at the maximum power point
• In the example discussed here:
• It is assumed that the Boost output voltage Vout = VDC is constant
• Iref is used as the control variable for the Boost DC-DC converter
• PV array current ideally tracks the Boost input current reference: IPV = Iref
12ECEN2060
Reminder: PV array characteristic
• Example: six 85 W modules in series, full sun
0 20 40 60 80 100 1200
1
2
3
4
5
6
Vpv [V]
Ipv [A]
13ECEN2060
0 20 40 60 80 100 1200
50
100
150
200
250
300
350
400
450
500
Ppv as a function of Vpv
• Example: six 85 W modules in series, full sun
Vpv [V]
Ppv [W]
14ECEN2060
0 1 2 3 4 5 60
50
100
150
200
250
300
350
400
450
500
Ppv as a function of Ipv = Iref
• Example: six 85 W modules in series, full sun
Ipv = Iref [A]
Ppv [W]
Objective: adjust Ipv = Iref to operate at MPP
MPP
15ECEN2060
0 1 2 3 4 5 60
50
100
150
200
250
300
350
400
450
500
Simple “perturb and observe” MPP tracking algorithm
Ipv = Iref
Ppv
MPPInitialize Iref, ∆Iref, Pold
Measure Ppv
Ppv > Pold ?
Iref = Iref +∆Iref
∆Iref = −∆Iref
Always step Iref in the direction of increasing Ppv
Pold = Ppv
Continue
in the
same direction
Change
direction
YES NO
16ECEN2060
MATLAB code: MPP tracking algorithm initialization
Initialize Iref, ∆Iref, Pold
Measure Ppv
Ppv > Pold ?
Iref = Iref +∆Iref
∆Iref = −∆Iref
Pold = Ppv
Continue in the same direction
Change direction
Initialize Iref, ∆Iref, Pold
Measure Ppv
Ppv > Pold ?
Iref = Iref +∆Iref
∆Iref = −∆Iref
Pold = Ppv
Continue in the same direction
Change direction
YES NO
17ECEN2060
MATLAB code: MPP tracking algorithm
Initialize Iref, ∆Iref, Pold
Measure Ppv
Ppv > Pold ?
Iref = Iref +∆Iref
∆Iref = −∆Iref
Pold = Ppv
Continue in the same direction
Change direction
Initialize Iref, ∆Iref, Pold
Measure Ppv
Ppv > Pold ?
Iref = Iref +∆Iref
∆Iref = −∆Iref
Pold = Ppv
Continue in the same direction
Change direction
YES NO
18ECEN2060
Simulation model: pv_boost_mpp_Iref.mdlECEN2060
6-module PV Array
85 x 6 = 510 W DC system
Insolation 1-5
Insolation 6
PV power
PV energy [kWh]
Ideal PV energy [kWh]
ECEN 2060 PV array with
MPP tracking
Boost DC-DC converter
1 time unit = 1 minute
PV voltage
Output energy [kWh]
-K-
kWh (pv)
-K-
kWh (out)
103.4
Vpv
200
Vout
Select
insolation
for modules
1-5
Select
insolation
for
module 6
Select
controller
1000
S6 (constant)
S6
(time varying)
1000
S1-5
(constant)
S1
(time varying)
510.8
Ppv
Ipv
Insolation
Vpv
Ppv
PV module (I)
PV6
Ipv
Insolation
Vpv
Ppv
PV module (I)
PV5
Ipv
Insolation
Vpv
Ppv
PV module (I)
PV4
Ipv
Insolation
Vpv
Ppv
PV module (I)
PV3
Ipv
Insolation
Vpv
Ppv
PV module (I)
PV2
Ipv
Insolation
Vpv
Ppv
PV module (I)
PV1
PV MPP
scope
P IrefMPPT
MPP tracking
controller
MPPtrackIref.m
4
Iref
(constant)
1
Ipv = Iref
4.94
Ipv
1
s
Integrate
Ppv
1
s
Integrate
Pout1
s
Integrate
Pideal
4.081
Epv
3.936
Eout
4.087
Eideal
-K-
Convert to
kWh
Compute
Ppv
0.9644
Boost efficiencyVout
Vg
Iref
Iout
Pout
ef f iciency
D
BoostDC-DC
(averaged)
Iref control
Boost DC-DC
Add
-K-
85/1000
5
5 modules
1
1 module
Ipv
Vpv
Vpv
Vpv
Vpv
Ppv
Ppv Iref 1
Iref
Iref
Iref
Pout
Ppv
ideal
Ppv
ideal
Pout, Ppv , PidealPout, Ppv , PidealPout, Ppv , Pideal
Duty
ef f iciencyef f iciency
19ECEN2060
MPP tracking operation
Boost DC-DC converter duty cycle D
PV array voltage Vpv
Boost DC-DC converter input current reference, Iref = Ipv
PV array output power Ppv compared to ideal Ppv @ MPP
20ECEN2060
The Future of
Grid-Connected PV Systems
Ipv, Vpv
ConverterPV
ControllerIpv, Vpv
Ipv, Vpv
ConverterPV
ControllerIpv, Vpv
Ipv, Vpv
ConverterPV
ControllerIpv, Vpv
Ipv, Vpv
ConverterPV
ControllerIpv, Vpv
Ipv, Vpv
ConverterPV
ControllerIpv, Vpv
Ipv, Vpv
ConverterPV
ControllerIpv, Vpv
Inverter 60 Hz ACUtility
• Scalable modular power electronics: distributed DC-DC conversion
• Much improved performance in the presence of module mismatches or partial shading
• Ongoing projects in the Colorado Power Electronics Lab (CoPEC) at CU ECE Dept led
by Prof. Erickson
Innovations in system architecture, control, and power electronics circuit design
21ECEN2060
Module-Integrated DC-DC Converter (MIC) for the Smart PV Roofs