grid converters for photovoltaic and wind power · pdf file• buck converter plus...

Grid Converters for Photovoltaic and Wind Power Systems

by R. Teodorescu, M. Liserre and P. RodriguezISBN: 978‐0‐470‐05751‐3Copyright Wiley 2011

Chapter 2Chapter 2

Photovoltaic Inverter Structures

Outline

• Structures

• TopologiesTopologies

• Modulation

• ControlControl

2

Structures

• PV DC voltage typically low for string inverters boost needed for low power

• For high power (>100 kW) central PV inverters w/o boost, typical three‐phase FBtopologies with LV MV transformer)topologies with LV‐MV transformer)

• Galvanic isolation necessary in some countries

• LF/HF transformer (cost‐volume issue)

• A large variety of topologies

• The optimal topology is not matured yet as for drives

T f l t l i h i hi h ffi i i d th id• Transformerless topologies having higher efficiency are emerging and the gridregulations are changing in order to allow them

3

StructuresPV Strings

Multi-string inverter

Central inverters

• 10‐250kW, three‐phase,

Module inverters

• 50‐180W, each panel has

String (Multi)inverters

• 1.5‐5 kW, typical residential

AC bus

, p ,several strings in parallel

• High efficiency, low cost, low reliability, not optimal MPPT

its own inverter enabling optimal MPPT

• Lower efficiency, difficult maintenance

application

• Each string has its own inverter enabling better MPPT

Th t i h diff tMPPT

• Used for power plants

maintenance

• Higher cost/kWp• The strings can have different

orientations

• Three‐phase inverters for power<5kW

4

High efficiency Mini‐central PV inverters (8‐15 kW) are also emerging for modular configuration in medium and high power PV systems

Structures: PV Modules

The advantages of PV modules compared totraditional PV systems are:

• Each module works independently

• High modularity allowing easy systemexpansion

• Use of standard AC installation material,which reduces costs of installation materialand system design.

• No mismatch losses at system level as eachAC module operates in its own MaximumPo er Point (MPP)Power Point (MPP).

• No need for string diodes.

• Low lightning induced surge voltages,b f h DC lbecause of the compact DC system layout.

Module Inverters manufacturers:

Enphase Petra Solar with “SunWave” Greenray Exeltech SolarBridge Enecsys Xslent Energy

5

Enphase, Petra Solar with SunWave , Greenray, Exeltech, SolarBridge, Enecsys, Xslent Energy Technologies, Sunsil, Xantrex, Mastervolt (Soladin 120), Tigo Energy (Module Maximizer) , National Semiconductor (Solar Magic), SolarEdge (Power Box)

Structures: PV ModulesPV‐DC Modules Double Stage

• Buck Converter plus Push‐Pull

PV‐DC Modules Single Stage

• Interleaved Boost with/without Charge pump System or Flybackp p y y

PV‐AC Module Double Stage

• Flyback plus Full‐Bridge or Flyback (Current COntrol) plus Full Bridge (Low Switching Frequancy operation)

PV DC M d l D bl St PV‐AC Modules Double StagePV‐DC Modules Double Stage PV AC Modules Double Stage

6

Structures: Transformer‐Based

DCPV DC DCPV DC ACDC

ACGridPV

Array

DC

DC

On low frequency (LF) side

ACGridPV

Array AC DC

On high frequency (HF) side

Boosting inverter with HF traformer based on FB boost t

Boosting inverter with LF transformer based on b t t

Both technologies are on the market! Efficiency 93 95%

converterboost converter

7

Both technologies are on the market! Efficiency 93‐95%

Structures: Transformer‐LessDC

DC

DC

ACGridPV

Array

• Time sharing configuration*

• FB inverter + boost

• Typical configuration

• High efficiency (>95%)

• Leakage current problem• Efficiency > 96%

• Extra diode to bypass boost when Vpv > Vg• Safety issue • Boost with rectified sinus reference

8*Source [Ogura, K.; Nishida, T.; Hiraki, E.; Nakaoka, M.; Nagai, S., "Time‐sharing boost chopper cascaded dual mode single‐phase sinewaveinverter for solar photovoltaic power generation system," Power Electronics Specialists Conference, 2004. PESC 04. 2004 IEEE 35thAnnual , vol.6, no., pp. 4763‐4767 Vol.6, 20‐25 June 2004]

Structures: String Inverters Transformer‐Less

G-PVC

Frame

Glass

l h l f

Parasitic capacitance of the PV array

G-PVC

Substrate

PV-cell

G-PVC

G-PVI• PV panel array has large surface

• Parasitic capacitance formed between grounded frame and PV cells

• Its value depends on the:Substrate

Surface of the PV array and grounded frame

Distance of PV cell to the module

Atmospheric conditions and dust which can increase the electrical conductivity of the panel’s

• Charging and discharging this capacitance leads to ground leakage currents (unsafe for human

surface

Leakage currentC a g g a d d sc a g g t s capac ta ce eads to g ou d ea age cu e ts (u sa e o u ainteraction; damage PV panels)

• Amplitude of leakage current depends on

Value of parasitic capacitance r

G-PVIG-PVI

Value of parasitic capacitance

Amplitude and frequency of imposed voltage

• RCM (Residual Current Monitoring) unit for monitoring

leakage ground currents

PV

Arr

ay

Filte

r

Cleakage ground currents

9

G-PVCG-PVI

Structures: ComparisonWith transformer

PV

Arr

ay

Filte

rLF transformer

r*

PV

Arr

ay

with HF

Filte

r

transformer

PV

Arr

ay

Filte

r

10

Structures: Central InvertersPV INVERTERPV Generetor

Central Power Flow Grid Side

Data Flow

MEDIUMVOLTAGE

InverterPower Flow

DC Connection

Grid SideAC

Connection

Data Flow

3-phaseTransformer

• High Performance for Large PV Plant High Level Monitoring High Level

Monitoring Center

High Performance for Large PV Plant, High Level Monitoring , High Level Intelligence, Reliability

• High Efficiency (up 98+%), Competitive prize/performance ratio.

• Typical structure – String inverter, 3‐phase FB proven technology (more parallel) with transformer to MV

• Manufacturers:

Aero Sharp, Aros (Sirio), Conergy, Control Techniques, EEI ‐ EquipaggiamentiElettronici Industriali Srl Eurener Green Power Helios System Hyunday HeavyElettronici Industriali Srl, Eurener, Green Power, Helios System, Hyunday Heavy, Integral Drive Systems AG, Ingeteam, Jema, Kaco, Layer, Leonics, Lti, Padcon, Pairan, Power One, PV Powered, Refu, Elettronica Santerno, SatCon, Schneider Electric, Siel, Siemens, Siliken, SMA, Sputnik, Voltwerk, Zigor

Structures: Central Inverters

ABB – PVS800*ABB PVS800• Multi‐level achieved by dual inverter

configuration• 100‐500 kW• 450‐750Vdc input • 400Vac out• Efficiency > 97.5%• Modular design, Long life‐time,PF Comp

12*Source [www.abb.com]

Structures: Central InvertersSUNNY CENTRAL up to 1250MV (2x Sunny Central 630HE)*

Efficient

• Without low voltage transformer Higher system efficiency due to direct connection to the mediumefficiency due to direct connection to the medium voltage Grid

Turnkey Delivery

• With medium‐voltage transformer and concrete substation for outdoor installation

OptionalOptional

• Grid management

• Control of reactive power

• Medium‐voltage switching stations for a flexible g gstructure of large solar parks

• AC transfer station with measurement

• Medium‐voltage transformers for other grid voltages (deviating from 20 kV)

13

voltages (deviating from 20 kV)

*Source [www.sma.de]

Structures: Central Inverters ‐Learn from Wind Power*

• Nominal power: 333kW@• Nominal power: 333kW @ 3AC690V+N

• Max. efficiency:>98,5% with UltraEta®topologyUltraEta topology

• MPP voltage range: 600‐1200VDC

• Max. DC Voltage: 1400V

/• Weight: approx. 400kg (1,20kg/kW)

• Outdoor‐qualified housing

• 690VAC is standard in the wind industry

• Standard‐components for up to 3MW + are cost‐effective and reliable

• Inverter cost (approx.) proportional to AC current

• Up to 1500VDC is enabled for PV components by DIN VDE0100

• Reduces installation time

14

Reduces installation time

• Reduces copper costs and cabling losses

*Source [www.refusol.de]

Structures: Conclusions

• PV Modules are emerging on the market with more manufacturers Both dc‐dc and dc‐ac

• Transformerless multistring PV inverters feature in comparison withT f B d PV i tTransformer Based PV inverters: Higher efficiency

Lower volume and weightg

Today more than 70% of the PV inverters sold on the market aretransformerless achieving 98% max conversion efficiency and 97.7%“european” (weighted) efficiency [Photon Magazine]european (weighted) efficiency [Photon Magazine]

Further improvements in the efficiency can be achieved by using SiCMosFets. ISE Fraunhofer‐Freiburg reported recently* 99% efficiency (25%reduction in switching + conduction losses)reduction in switching + conduction losses)

• Central inverters are preferred in Large (multi MW) PV power plants 98+% efficiency is achieved with classical 2 level inverters

*Source [Burger, B., Schmidt,H. “25 YEARS TRANFORMLESS INVERTERS” – Proceedings of PVSEC 2007] 15

Topologies: FB Modulation

Modulation Basics (recapitulation)

ˆ; ;

2control d control

Ao Ao control tri atri tri

v V vV v v V mV V

2tri triV V

VSI connected to the grid using an H‐Bridge topologyGridFilterFilter Basic FB inverter

1S 1D 3S 3D

1L

LA

VSI connected to the grid using an H Bridge topology

PVC

2S 2D 4S 4D

2LgV

NB

+-

16

Topologies: FB ModulationHybrid PWM

Leg A is switched with high frequency andLeg B is switched with grid frequency

Bipolar PWMS1 + S4 and S2 + S3 are switched complementary at high frequency

Unipolar PWMLeg A and B are switched with high

frequency with mirrored sinusoidal ref.

Bipolar PWM Unipolar PWM

1

Reference and Carrier Signals

1


1


Hybrid PWM

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

-1

0

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

-1

0

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

-1

0

-1

0

1

Output voltage

-1

0

1

Output voltage

-1

0

1

Output voltage

17

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

1kHz triangular wave and 50Hz sinusoidal reference

Topologies: FB Bipolar PWM

• S1 + S4 and S2 + S3 are switched complementary at high frequency (PWM)

• No 0 output voltage possible

500

p g p

• The switching ripple in the current equals 1x switching frequency large filtering

• Voltage across filter is bipolar high core losses

• No common mode voltage VPE free for high frequency low leakage current0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02

-500

0

VIN

V

400

• Max efficiency 96.5% due to reactive power exchange L1(2)<‐> Cpv during freewheeling and that 2 switched are simultaneously switched every switching

• This topology is not suited to transformerless PV inverter due to low efficiency!0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02

0

100

200

300

Time [sec]

VD

C-G

ND

18

Topologies: FB Unipolar PWM

GridFilterFilter Basic FB inverterPV Array

S1 S3 D3

L1VPV

A

D1

Vg

N

L

S2 S4D2 D4

L2

CPV VAB = 0

A

B

• Leg A and B are switched with high frequency with mirrored sinusoidal reference

VPE Vg < 0, Ig <0. S1, S3 and D1 = ON

500

• Two 0 output voltage states possible: S1 and S2 = ON and S3 and S4 = ON

• The switching ripple in the current equals 2x switching frequency lower filtering

• Voltage across filter is unipolar low core losses

• V has switching frequency components high leakage current and EMI

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02-500

0

VIN

V

200

400

600

C-G

ND

19

• VPE has switching frequency components high leakage current and EMI

• Max efficiency 98% due to no reactive power exchange L1(2)<‐> Cpv during freewheeling

• This topology is not suited for TL PV inverter due to high leakage!

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02-200

0

Time [sec]

VD

C

Topologies: FB Hybrid PWM

• Leg A is switched with grid low frequency and Leg B is switched with high PWM freq

• Two 0 output voltage states possible: S1 and S2 = ON and S3 and S4 = ON

Th i hi i l i h l 1 i hi f hi h fil i

0

500

VIN

V

• The switching ripple in the current equals 1x switching frequency high filtering


• VPE has square wave variation at grid frequency high leakage current and EMI

• High efficiency 98% due to no reactive power exchange L1(2)<‐> Cpv during freewheeling and due to lower frequency switching in one leg

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02-500

0

500

C-G

ND

20

lower frequency switching in one leg.

• This topology is not suited for transformerless PV inverter due to high leakage!

Source [Ray‐Shyang Lai; Ngo, K.D.T., "A PWMmethod for reduction of switching loss in a full‐bridge inverter," Power Electronics,IEEE Transactions on , vol.10, no.3, pp.326‐332, May 1995]

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02-500

VD

C

T ime [sec]

Topologies: H5 (SMA) Modulation

1S1D 3D

3S

5S

5DPVV

V

1L500

2S2D

4S4D

PVC

V

gV

2L

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02-500

0

VIN

V

PEV

S5 and S4 (S3) are switched with high frequencyS1 (S2) are switched with low grid freq

+-

S2

S1

S3

S4

V f V

+- S5

21Source [Victor M.et al.‐ US Patent Application, Pub.No.: US 2005/0286281 A1, Pub. Date: 29.Dec.2005]

Vref Vtri

Topologies: H5 (SMA)

• Extra switch in the dc link to decouple the PV generator from grid during 0 voltage

• Two 0 output voltage states possible: S5 = OFF S1 = ON and S5 = OFF S3 = ON

500

VTwo 0 output voltage states possible: S5 OFF, S1 ON and S5 OFF, S3 ON



• VPE is sinusoidal with grid frequency component low leakage current and EMI

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02-500

0

VIN

400

500

22Source [Victor M.et al.‐ US Patent Application, Pub.No.: US 2005/0286281 A1, Pub. Date: 29.Dec.2005]

• High max. efficiency 98% due to no reactive power exchange as reported by Photon Magazine for SMA SunnyBoy 4000/5000 TL single‐phase

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.020

100

200

300

VD

C-G

ND

T ime [sec]

Topologies: HERIC Modulation

1S1D

3S3D

PVV1L

500

2S2D

4S4D

PVC

S D

S D 2L

gV

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02-500

0

VIN

V

PEV

S+ and S‐ are switched with grid freqS1&S4 (S2&S3) are switched with high freq

23Source [Schmid H. et al. US Patent No: 7046534, issued 16 May 2006]

Topologies: HERIC

• Two 0 output voltage states possible: S+ and D‐ = ON and S‐ and D+ = ONTwo 0 output voltage states possible: S+ and D ON and S and D+ ON



• VPE is sinusoidal has grid frequency component low leakage current and EMI

24Source [Schmid H. et al. US Patent No: 7046534, issued 16 May 2006]

• High efficiency 98% due to no reactive power exchange as reported by Photon Magazine for Sunways AT series 2.7 – 5 kW single‐phase

Topologies: FBDC Bypass (H6)

1S1D

3S3D

5S

5D

1LPVV1PVC

2S2D

4S4D

D

D

6D

2L

gV

1PVC

2PVC

A

B

6SPEV

S5 and S6 are switched at high frequency, S1&S4(S2&S3) at line frequency( ) q y

25Source [Gonzalez S R, et.al.‐ International Patent Application, Pub No. WO2008015298, Pub. Date: 2 July 2007]

Topologies: FBDC Bypass (H6)

5D

5S

1D1S 3D3S

D A

1L1PVC

PVV

5D

5S

1S 3D

D A1L1PVC

PVV

3S1D

D

2S 4S 4D

B

6D

0ABV

2L

gV

2PVC

2D

0ABV

D

4S 4D

B

6D

2L

gV

2PVC

2D2S

• Two extra switches switching with high frequency and 2 diodes bypassing the dc bus. The 4 switches in FB switch at low fsw

• Two 0 output voltage states possible by “natural clamping# of D+ and D

6SPEV 6SPEV

• Two 0 output voltage states possible by “natural clamping# of D+ and D‐



• VPE is sinusoidal and has grid frequency component low leakage and EMI

26Source [Gonzalez S R, et.al.‐ International Patent Application, Pub No. WO2008015298, Pub. Date: 2 July 2007]

PE g q y p g

• High max efficiency 96.5% due to no reactive power exchange as reported by Photon Magazine for Ingeteam Ingecon Sun TL series (2.5/3.3/6 kW, single‐phase)

Modulation: ZVFBR(AAU+UPC)

A

B

PV

S

S5

+500

+-

S2

S4

S1

S3

-

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02-500

0

VIN

V

500

Vref Vtri

S4

27Source [T. Kerekes, R. Teodorescu, P. Rodriquez, G. Vazquez and E. Aldabas, A new high‐efficiency single‐phase transformerless PV

inverter topology; IEEE Transactions on Industrial Electronics]

Topologies: ZVFBR (AAU+UPC) A

PV

A

PVB

B

A94%

96%

98%

100%

B

PV

84%

86%

88%

90%

92%

HB-BipHERICHB-ZVR

• Derived from HERIC but the bidirectional grid‐short‐circuiting switch is implemented using a diode bridge and 1 switch

• Zero voltage is achieved by turning the FB off and turning T5 on ‐> higher losses as HERIC due to hi h i hi f f T5

500W 1000W 1500W 2000W 2500W 2800W84%

high switching frequency of T5


• Voltage across filter is bipolar due to deadtime clamping higher core losses

• VPE is constant low leakage current and EMI

28Source [T. Kerekes, R. Teodorescu, P. Rodriquez, G. Vazquez and E. Aldabas, A new high‐efficiency single‐phase transformerless PVinverter topology; IEEE Transactions on Industrial Electronics]

VPE is constant low leakage current and EMI

• High efficiency due to no reactive power exchange during zero voltage

Topologies: UltraEta ‐ REFU

• Three‐level output. Requires double PV voltage input in comparison with FB but it include time‐shared boost

• Zero voltage is achieved by short‐circuiting the grid using the bidirectional switchZero voltage is achieved by short circuiting the grid using the bidirectional switch



• VPE w/o high freq component low leakage current and EMI . No L in neutral!

29Source [Hantschel J. German Patent Application, Pub. No. DE102006010694 A11, Pub Date: 20 Sep 2007]

• High max efficiency 98% due to no reactive power exchange, as reported by Photon Magazine for Refu Solar RefuSol (11/15 kW, three‐phase)

Topologies: NPC (Danfoss Solar)

500• Three‐level output. Requires double PV voltage input in comparison with FB. Typically needs boost.

• Two 0 output voltage states possible: S2 and D+ = ON and S3 and D‐ = ON. For zero voltage during Vg>0, Ig<0, S1 and S3 switch in opposition and S2 and S4 for Vg<0, Ig>0

-500

0

500

VIN

V



• VPE is equal –Vpv/2 w/o high freq comp low leakage and EMI . No L in N!

• High max efficiency 98% due to no reactive power exchange as reported by Danfoss

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02500

100

200

300

400

500

VD

C-G

ND

30Source [A. Nabae, I. Takahashi, and H. Akagi; "A New Neutral‐Point‐Clamped PWM Inverter"; IEEE Transactions on Industry

Applications, vol. IA‐17, no. 5, 1981, pp. 518‐523]

• High max efficiency 98% due to no reactive power exchange, as reported by DanfossSolar TripleLynx series (10/12.5/15 kW)

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.020

T ime [sec]

Topologies: CONERGYGridFilterFilter HB inverterPV Array

L1

VPV

CPV1

AB

S1 D1VPV/2

Clamping Switch

VAB = VPV/2

S+D+

GridFilterFilter HB inverterPV Array

L1

VPV

CPV1

AB

S1 D1VPV/2

Clamping Switch

VAB = -VPV/2

S+D+

VPE

Vg

N

CPV2

S2 D2

VPV/2

Vg > 0, Ig > 0. S1 =ON, S+, S- and S2 = OFF

S-D-

VPE

Vg

N

CPV2

S2 D2

VPV/2

Vg < 0, Ig > 0. S2 =ON, S+, S- and S2 = OFF

S-D-


L1

VPV

CPV1

AB

S1 D1VPV/2

Clamping Switch

VAB = 0

S+D+


L1

VPV

CPV1

AB

S1 D1VPV/2

Clamping Switch

VAB = 0

S+D+

VPE

Vg

N

CPV2

S2 D2

VPV/2

Vg > 0, Ig > 0. S+ =ON, S-, S1 and S2 = OFF

S-D-

VPE

Vg

N

CPV2

S2 D2

VPV/2

Vg < 0, Ig < 0. S- =ON, S+, S1 and S2 = OFF

S-D-

• Only 4 switches needed with 2 of them (S+ and S‐) rated only Vpv/4

• Three‐level output. Requires double PV voltage input in comparison with FB. Typically needs boost.

• Two 0 voltage states using the bidirectional clamping switch (S+ and S‐)g g p g

• The switching ripple in the current equals 1x switching freq high filtering


• VPE is equal –Vpv/2 w/o high freq comp low leakage and EMI . No L in N!

h ff d h d b (

31

• High max efficiency 96.1% due to no reactive power exchange, as reported by Conergy IPG series (2‐5 kW single‐phase)

Source [Knaup P – International Patent Application, Pub No. WO 2007/048420 A1, Pub. Date: 3 May 2007]

Topologies: TL ComparisonTopologies derived from FB

• Actually both HERIC, H5, REFU and FB‐DCBP topologies are converting the 2 level FB (or HB) inverter in a 3 levelone

• This increases the efficiency as both the switches and the output inductor are subject to half of the input voltagestress

• The zero voltage state is achieved by shorting the grid using higher or lower switches of the bridge (H5) or by usingadditional ac bypass (HERIC or REFU) or dc bypass (FB‐DCBP)additional ac bypass (HERIC or REFU) or dc bypass (FB DCBP)

• H5 and HERIC are isolating the PV panels from the grid during zero voltage while REFU and FB‐DCBP is clamping theneutral to the mid‐point of the dc link

• Both REFU and HERIC use ac by‐pass but REFU uses 2 switches in anti‐ parallel and HERIC uses 2 switches in series(back to back). Thus the conduction losses in the ac‐bypass are lower for the REFU topology

• REFU and H5 have slightly higher efficiencies as they have only one switch switching with high‐frequency whileHERIC and FB_DCBP have two

FB ZVR d i f HERIC b diff i l i f h bidi i l i h i di d b id d• FB‐ZVR derives from HERIC but uses a different implementation of the bidirectional switch using a diode bridge andone switch. Constant Vpe but moderate high efficiency (lower than HERIC but higher than FB‐UP). Can also workwith non‐unitary PF

Topologies derived from NPC

• The classical NPC and its “variant” Conergy‐NPC are both three‐level topologies featuring the advantages ofunipolar voltage across the filter, high efficiency due to disconnection of PV panels during zero‐voltage state and

i l l k d d d C li k id i

32

practical no leakage due to grounded DC link mid‐point

• Due to higher complexity in comparison with FB‐derived topology, these structures are typically used in three‐phase PV inverters with ratings over 10 kW (mini‐central)

Typical control structure for dual-stage PV inverterControl Structure OverviewTypical control structure for dual stage PV inverter

Basic functions – common for all grid‐connected i

PV specific functions – common for PV inverters• Maximum Power Point Tracking – MPPT

Very high MPPT efficiency in steady state (typical > inverters

• Grid current control THD limits imposed by standards Stability in case of grid impedance

variations

99%) Fast tracking during rapid irradiation changes

(dynamical MPPT efficiency) Stable operation at very low irradiation levels

• Anti‐Islanding – AI as required by standards (VDE0126, IEEE1574 etc )

Ancillary Support – (future?)• Voltage Control• Fault Ride‐through • Power Quality

Harmonics Ride‐through grid voltage disturbances (not required yet!)

• DC voltage control Adaptation to grid voltage variations Ride‐through grid voltage disturbances

(optional yet)

IEEE1574, etc.)• Grid Monitoring

Operation at unity power factor as required by standards

Fast Voltage/frequency detection• Plant Monitoring

Harmonics Unbalance

33

(optional yet)• Grid synchronization

Required for grid connection or re‐connection after trip

g Diagnostic of PV panel array Partial shading detection

• Sun Tracking (mechanical MPPT) 1‐2 axis motion controller tracking of Sun

Typical control structure for dual-stage PV inverterControl Structure for Dual‐StageTypical control structure for dual stage PV inverter

Typical control structure for dual‐stage PV inverter

pvI

dc voltage

PLLacV

sinr̂I ˆ

refIDDCVdc dc

refDCV ,

current dc acrefgI grid

dc-dc conv dc-ac inv

MPPTpvV dc voltagecontroller

2pvP pvP

acRMSV

*ˆrefI

refI

PVarray

PWMDCdc-dc

convcurrent

controller PWM dc-acinv

gI

refg , ~

• The MPPT is implemented in the dc‐dc boost converter.

• The output of the MPPT is the duty cycle function As the dc link voltage V is

acRMSVacRMS

• The output of the MPPT is the duty‐cycle function. As the dc‐link voltage VDC iscontrolled in the dc‐ac inverter the change of the duty‐cycle will change voltageat the output of the PV panels, VPV as:

V

• The dc‐ac inverter is a typical current controlled voltage source inverter (VSI)with PWM and dc‐voltage controller.

DVKV PV

DC

1

g

• The power feed‐forward requires communication between the two stages andimproves the dynamics of MPPT 34

Typical control structure for single-stage PV inverter [16]Control Structure for Single‐StageTypical control structure for single stage PV inverter [16]

PLLacV

MPPTPV

array

pvI

pvV

*pvV dc voltage

controller

sinˆrI refI

r̂efI PWM dc-ac

invcurrent

controller

I

2pv

acRMS

P

V

pvP

acRMSV*ˆrefI

• In these topologies ‐which are becoming more and more popular in countries withlow grid voltage (120V) like Japan and thus the voltage from the PV array is highenough the MPPT is implemented in the dc ac inverterenough ‐ the MPPT is implemented in the dc‐ac inverter

• Also in topologies with boost transformer on AC side

• The output of the MPPT is the dc‐voltage reference. The output of the dc‐voltagecontroller is the grid current reference amplitude. The power feed‐forwardimproves the dynamic response as MPPT runs at a slow sampling frequencies (typ. 1Hz)

• A PLL is used to synchronize the current reference with the grid voltage

35

Control Implementation

Ipv + U L

P VP anelsString

F ull-bridgeInverter

V SI-P W M

LC LLow pass

f ilterGridC

V pv

-V N

I g

V g Isolat ionTransform er

D C/D CC onverter

V dc

12

dc 12

dc MPPT

PW M

V p v

I p v

PW Mdc

Gdc

P LL

GC

V d c ref

r e fI

s in

V g

*acV dc

+ +- -I g V d c

V d c

+

Control structure

ˆrI

• The current controller GC can be of PI or PR (Proportional Resonant) type

• Other non‐linear controllers like hysteresis or predictive control can be used forcurrent control

• The DC voltage controller can be P type due to the integration effect of the typicallarge capacitor

36

Conclusion

• The “race” for higher efficiency PV inverters has resulted in a large variety of “novel”• The race for higher efficiency PV inverters has resulted in a large variety of noveltransformerless topologies derived from H‐Bridge with higher efficiency and lowerCM/EMI (H5, HERIC)

E i l t hi h ffi i b hi d ith 3 l l t l i ( NPC)• Equivalent high‐efficiency can be achieved with 3‐level topologies (ex NPC)

• Today more than 70% of the PV inverters sold on the market are transformerlessachieving 98% max conversion efficiency and 97.7% “european” (weighted) efficiency

• Further improvements in the efficiency can be achieved by using SiC MosFets. ISEFraunhofer‐Freiburg reported recently* 99% efficiency (25% reduction in switching +conduction losses)

• For 3‐phase systems the trend is to use 3 independent controlled single‐phaseinverters like 3xH5 or 3xHERIC but 3FB‐SC and 3NPC (not proprietary) are also presenton the market. 3NPC achieve higher efficiency 98%g y

• The general trend in PV topologies is “More Switches for Lower Losses”

37*Source [Burger, B., Schmidt,H. “25 YEARS TRANFORMLESS INVERTERS” – Proceedings of PVSEC 2007]

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