Grid Pv System

Power Electronics and Control in Grid-Connected PV Systems

ECEN 2060

Grid-Connected PV System One possible grid-connected PV system architecture IPV

DC input VPV , I PV

PPV = VPV I PV

iac

+ PV array

VPV −

AC output +

Power electronics converter

vac −

AC utility grid

v ac (t ) = 2VRMS sin (ωt ) iac (t ) = 2 I RMS sin (ωt )

Pac = VRMS I RMS

pac (t ) = vac iac = VRMS I RMS (1 − cos(2ωt ))

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% P V I η converter = ac = RMS RMS PPV VPV I PV • Provide energy storage to balance the difference between PPV and pac(t) Desirable features • Minimum weight, size, cost • High reliability ECEN2060

2

Power Electronics for Grid-Connected PV System One possible realization: Energy-storage capacitor

IPV + PV array

VPV

iac +

+ Boost DC-DC converter

C

VDC

−

Single-phase DC-AC inverter

−

−

DC-DC control

vac

AC utility grid

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 ECEN2060

3

DC-AC Inverter Control One possible realization: Energy-storage capacitor

IPV + PV array

VPV

iac +

+ Boost DC-DC converter

C

−

VDC

Single-phase DC-AC inverter

−

−

DC-DC control

vac

AC utility grid

DC-AC control

= IRMSref

• 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:

IRMS = IRMSref One possible current control approach, based on a comparator with hysteresis, has been discussed in class, see Intro to Power Electronics notes

ECEN2060

4

Simulation model: pv_boost_dcac_averaged.mdl ECEN2060 6-module PV Array Ipv

1000

PV module (I)

Insolation

Vout (boost) = VDC

Ppv

Ipv

PV module (I)

Insolation

Boost

Vpv

PV1

Insolation

Vg

Vpv Ppv

Vout

DC-DC (averaged, C) current control D

Set Boost Iref to operate PV array at MPP

Iout

4.95

Iref

ef f iciency

PV module (I)

Ipv = Iref

PV current

Vpv

Duty

Boost scope

ef f iciency

Vpv Vpv

Ppv

PV module (I)

Insolation

(averaged)

D pin

PV module (I)

Insolation

3.94

Boost efficiency

iin Duty pin pin, pout pout

Iref

Set DC-AC Iref to balance the power, i,e to keep VDC constant

IRMSref

Average output AC power 1 s

Vpv

510.8

Ppv

Integrator(pout)

103.2 Vpv Vpv

Integrator(pin)

Pout

fac_out

DC-AC average power and efficiency

1 s

472.8

60

Ppv

PV5

PV module (I)

iac

DC-AC Inverter

Pout boost

Boost DC-DC

Ppv

PV output power

Ipv

iin

v ac

Vpv

PV4

Ipv

inverter

pout

Product Ipv

iac

492.6

Pout

Ppv

PV3

DC-AC

Iref

0.9643 Insolation

v ac Vdc

Vout

PV2

Ipv

DC-AC scope

199.8 Vpv

60 fac_in

0.9586 Compute efficiency

DC-AC Efficiency 493.2 Pin Average input AC power

Insolation

Ppv

PV6

Add

ECEN2060 PV + Boost DC-DC + DC-AC inverter averaged model

Ipv = Iref

ECEN2060

5

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 = 3.4 A

IRMSref is too low Pac < Ppv VDC increases

IRMSref = 4.4 A

IRMSref is too high Pac > Ppv VDC decreases

IRMSref = 3.94 A

IRMSref is just right Pac ≈ Ppv VDC starts at 200 V and returns to 200 V

ECEN2060

Tac = AC line period (1/60 seconds)

6

Average Power Balance by Automatic Feedback Control IPV

iac +

+ PV array

VPV

Boost DC-DC converter

+ Single-phase DC-AC inverter

VDC −

−

DC-DC control

vac

AC utility grid

−

+

IRMSref

− VDCref compensator

• 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 ECEN2060

7

Energy storage Energy-storage capacitor

IPV + PV array

VPV

iac +

+

Boost DC-DC converter

Pac pac(t) C

−

VDC

Single-phase DC-AC inverter

−

−

DC-DC control

vac

AC utility grid

DC-AC control

Pac − p ac (t ) = Pac − Pac (1 − cos 2ωt ) = Pac cos 2ωt Pac > pac(t), capacitor C is charged up

∆vDC Pac < pac(t), capacitor C is discarged • 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 ECEN2060

8

Energy storage capacitor C Pac − pac (t ) = Pac − Pac (1 − cos 2ωt ) = Pac cos 2ωt Pac > pac(t), capacitor C is charged up

∆vDC Pac < pac(t), capacitor C is discarged

• Energy supplied to the capacitor during the time when Pac > pac(t), i.e. when the capacitor is charged from VDCmin to VDCmax Tac / 8

π /2

ac

−π / 2

P ∆EC = ∫ Pac cos 2ωt dt = ac 2ω −T / 8

∫ cosθ dθ =

Pac

ω

• This energy must match the change in energy stored on the capacitor:

∆E C =

VDC max + VDC min 1 1 2 2 CV DC − CV = C ( V − V ) ≈ CVDC ∆VDC max DC min DC max DC min 2 2 2

• Solve for the ripple voltage:

CV DC ∆VDC = ECEN2060

Pac

ω

∆VDC =

Pac CV DC ω 9

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:

CV DC ∆VDC = C=

Pac

ω

Pac 600 W = = 200 µF ∆VDCVDC ω 40 V * 200 V * 2π 60 Hz

• Note that the energy supplied (or absorbed) by the capacitor is relatively small:

∆EC =

Pac

ω

=

600 = 1.6 J 2π 60

• The total energy stored on the capacitor is also small

EC =

1 2 CVDC = 4J 2

• This example illustrates the need for only relatively small energy storage in a gridconnected system, easily accomplished by a capacitor, in sharp contrast to stand-alone PV systems that require very significant energy storage (e.g. batteries) ECEN2060

10

Maximum Power Point (MPP) Tracking Energy-storage capacitor

IPV + PV array

VPV

iac +

+ Boost DC-DC converter

−

C

Single-phase DC-AC inverter

VDC

−

−

DC-DC control

vac

AC utility grid

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 ECEN2060

11

Reminder: PV array characteristic • Example: six 85 W modules in series, full sun Ipv [A]

6

5

4

3

2

1

0

ECEN2060

0

20

40

60

80

100

120

Vpv [V]

12

Ppv as a function of Vpv • Example: six 85 W modules in series, full sun Ppv [W]

500 450 400 350 300 250 200 150 100 50 0 0

ECEN2060

20

40

60

80

100

120

Vpv [V]

13

Ppv as a function of Ipv = Iref • Example: six 85 W modules in series, full sun MPP Ppv [W]

500 450 400 350 300 250 200 150 100 50 0 0

1

2

3

4

5

6

Ipv = Iref [A]

Objective: adjust Ipv = Iref to operate at MPP ECEN2060

14

Simple “perturb and observe” MPP tracking algorithm MPP Initialize Iref, ∆Iref, Pold

500 450

Ppv

400 350

Measure Ppv

300 250 200

YES

150

Ppv > Pold ?

100 50 0 0

NO

1

2

3

4

5

6

Ipv = Iref

Always step Iref in the direction of increasing Ppv

Change direction

Continue in the same direction

∆Iref = −∆Iref

Iref = Iref +∆Iref Pold = Ppv

ECEN2060

15

MATLAB code: MPP tracking algorithm initialization Initialize Iref, ∆Iref, Pold

Measure Ppv

YES

NO

Ppv > Pold ? Change direction

Continue in the same direction

∆Iref = −∆Iref

Iref = Iref +∆Iref Pold = Ppv

ECEN2060

16

MATLAB code: MPP tracking algorithm Initialize Iref, ∆Iref, Pold

Measure Ppv

YES

NO

Ppv > Pold ? Change direction

Continue in the same direction

∆Iref = −∆Iref

Iref = Iref +∆Iref Pold = Ppv

ECEN2060

17

Simulation model: pv_boost_mpp_Iref.mdl Insolation 1-5

Ipv

S1 (time varying) 1000 S1-5 (constant)

ECEN2060 6-module PV Array 85 x 6 = 510 W DC system PV module (I)

Insolation

Select insolation for modules 1-5

103.4

Vpv

1 time unit = 1 minute

Vpv

Ppv

0.9644

4.94

PV1

Ipv

ECEN 2060 PV array with MPP tracking Boost DC-DC converter

PV voltage

PV module (I)

Insolation

200

Ipv

Vout

Vout Vpv

Vpv

Vpv

Iout Boost DC-DC (averaged) Pout Iref control

Boost efficiency Pout

Vg

ef f iciency

ef f iciency

Ppv Iref

PV2

D

Duty

Boost DC-DC Ipv

PV module (I)

Insolation

Ppv

PV3

Vpv Ppv

Ipv

PV module (I)

Insolation

P

MPPT Iref

Vpv

Iref 1

4 Select controller

Iref (constant)

Insolation

PV MPP scope

Iref

Ppv

PV module (I)

Vpv

PV power

Ppv

510.8

PV5

Insolation 6

Iref

Compute Ppv

PV4

Ipv

Vpv

MPP tracking controller MPPtrackIref.m

Vpv

Ppv PV energy [kWh]

Ipv

S6 (time varying) 1000 S6 (constant)

PV module (I)

Insolation

1 s

Vpv Ppv

PV6

Select insolation for module 6

Ipv

Integrate Ppv

Add 1

Ppv Pout, Ppv , Pideal Ppv ideal

5

1

Epv

kWh (pv)

Iref

Ipv = Iref

5 modules

4.081

-K-

Ideal PV energy [kWh] -K85/1000

Ppv ideal

1 s Integrate Pideal

-KConvert to kWh

4.087 Eideal

Integrate Pout 1 s

kWh (out) -K-

Output energy [kWh] 3.936 Eout

1 module

ECEN2060

18

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

ECEN2060

19

The Future of

Grid-Connected PV Systems Ipv, Vpv

Ipv, Vpv

PV

Converter

PV

Converter

Ipv, Vpv

Controller

Ipv, Vpv

Controller

Ipv, Vpv

Ipv, Vpv Converter

PV

Converter

PV

Inverter Ipv, Vpv

Controller

Ipv, Vpv

Ipv, Vpv

60 Hz AC Utility

Controller

Ipv, Vpv

PV

Converter

PV

Converter

Ipv, Vpv

Controller

Ipv, Vpv

Controller

Innovations in system architecture, control, and power electronics circuit design

• 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 ECEN2060

20

Module-Integrated DC-DC Converter (MIC) for the Smart PV Roofs

ECEN2060

21