Power Electronics and Control in Grid-Connected PV Systems

21
Power Electronics and Control in Grid-Connected PV Systems ECEN 2060

Transcript of Power Electronics and Control in Grid-Connected PV Systems

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