Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre [email protected] Overview of Distributed Power

Marco Liserre [email protected]

Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES)

Overview of Distributed Power Generation Systems (DPGS) and

Renewable Energy Systems (RES)

Marco Liserre

[email protected]



Outline Introduction to distributed power generation and renewable energy

systems

World energy scenario (including renewable energy)

Outlook on wind and photovoltaic energy

Integrating renewable energy sources with the future power system

Wind systems

Photovoltaic systems



Distributed power generation Relatively small generating units and storage

technologies

Provide electric capacity and/or energy at or near consumer sites to meet specific customer needs

Either be interconnected with the electric grid or isolated from the grid in "stand- alone"

The location value is important to the economics and operation



Renewable energy systems100

80

60

40

20

Cos

t of

elec

tric

ity

(¢/k

Wh)

0

1980 1985 1990 1995

Photovoltaics

4

3

2

1

0Cos

t of

etha

nol (

$/ga

l)

1980 1985 1990 1995

Biomass

40

30

20

10

0Cos

t of

elec

tric

ity (

¢/kW

h)

1980 1985 1990 1995

Wind

Cos

t of

elec

tric

ity (

¢/kW

h) 10

8

6

4

2

0

1980 1985 1990 1995

Geothermal

10

30

40

20

Cos

t of

elec

tric

ity (

¢/kW

h)

0

1980 1985 1990 1995

Solar Thermal

0

5

10

15

20

Biomass Electric

Cos

t of

elec

tric

ity (

¢/kW

h)

1980 1985 1990 1995

Source: Billman, Advances in Solar Energy submission, 1/8/99



World energy consumption The growth of energy demand in 2007 remained high despite high

energy prices China has surpassed the EU



World energy production The relative market share of oil is decreasing respect coal and gas



Renewable Energy scenario In 2007 the world renewable energy production share has been calculated as

19 %.

However 16 % is due to hydraulic energy production, hence wind and photovoltaic (the most promising renewable sources) energy production is still very modest.

The goal of the European Community is to reach 20 % in 2020, however the EU-27 energy is only 17% of world energy.

USA with 22% of energy share may adopt similar goals under the pressure of public opinion concerned by environmental problems (in California the goal is 20 % in 2010).



Renewable Energy scenario However the policies of Asia and Pacific countries, with 35% of energy share,

will be probably more important in the future energy scenario.

In fact countries like China and India require continuously more energy (China energy share increases 1 point every year from 2000).

The need for more energy of the emerging countries and the environmental concerns of USA and EU will drive the increase of the renewable energy production: the importance of renewable energy sources in the future energy scenario is not anymore under discussion !

The needed technology is available and it benefits of continuous improvement due to academic and industrial research activity

Knowledge transfer to industry on the basis of international conferences and workshops and educational programs.



Renewable Energy scenario

Wind energy – highest development

Solar energy – next highest development

Wave energy – largely unexplored

Tidal energy – largely unexplored

Small hydro (<10MW), 47GW used, 180 GW untapped (70% in developing countries). Oldest technology (not covered)

Biomass 18GW used (2000), largely unexplored. Used in CHP



Wind energy22

19891985 1992 1993 1996

15 m

30 m

46 m

37 m 600 kW

500 kW

300 kW

50 kW

46 m

112 m

4.500 kW

1.500 kW

70 m

200x

Growth of WTG‘s

Bigger and more efficient ! 3.6-6 MW prototypes running (Vestas, GE, Siemens Wind, Enercon)Danish Vestas and Siemens Wind stand for over 40% of the worldwide

market2 MW WT are still the "best seller" on the market!



Wind energy Wind energy can benefit of huge investments in research and education. Some of the most relevant goals of the research can be briefly summarized

as: to increase the power production of each wind turbine (over 5 MW), to increase the penetration of small wind turbine systems (under 50 kW) to create wind plants (preferably off-shore) that can behave similarly to standard oil & gas power plants respect to the grid (due to wind forecast and proper control strategies).

Educational investments are mainly done by universities to prepare a future category of engineers for the wind industry but also by leader wind companies that want to form highly specialized engineers through specific PhD programs



Photovoltaic energy

The cost of PV electricity will reach the break-even point soon in many countries

Optimistic ! Silicon shortage has slowed the price reduction



Photovoltaic energy Despite the silicium shortage in the last years the PV industry is growing at more than 30% PV Module technology is also developing fast toward higher efficiency and lower cost of 4-5

€/Wp, expected 3€/Wp in 5 years. From experience 7%/year fallString technology is dominating. Multi-string for residential applications Mini-central three-phase inverters 8-15 kW are emerging for modular configuration in medium

and high power systems (commercial roof-tops) Central inverters are available for plants up to MW range (1MW – SMA) Reliability is increased now 5 years but extended 20 years (not free!) Increase functionality available (built-in logger, communication, grid support, etc) Cost is still high (400- 500€/kWp) and high efforts are done in order to reduce it to 250-300

€/kWp in the next 5 years by: mass production better topologies with fewer components design-to-cost

PV electricity cost is expected to reach the break-even cost around 2015 where mass PV penetration is expected



Photovoltaic energy The most relevant goals of photovoltaic energy are 40% cost reduction of

photovoltaic panels and of the power converter stage in 5 years and the increase of the efficiency of both and the reliability of the latter considerably.

These goals are driving the research towards several directions such as: maximum power extraction algorithms, advanced anti-islanding algorithms for higher safety levels

higher efficiency of the power converter (98 % efficiency is the goal

for transformerless topologies)



Power system evolutionActive distribution grids with a significant

amount of medium-scale and small-scale generators (ranging from hundreds of kW to tens of MW), involving both conventional and renewable technologies, together with storage systems and flexible high-voltage transportation systems connecting those grids with lower cost and ROW (Right Of Way)

restrictions.

The importance of storage in the overall scenario is crucial



Smart micro-grids (SMG)

The safe operation in any condition (grid-connected or stand-alone) relies also on good simulation tools to predict the behavior of the overall system considering the specific operation of the renewable energy sources.

Within active grids, generators and loads can both play a role as operators in electricity markets

Distribution grids have to be equipped with protection systems and real-time control systems leading to smart micro-grids (SMG) usually operated in connection to distribution grids but with the capability of automatically switching to a stand-alone operation if faults occur in the main distribution grid, and then re-connected to the grid.



Information Technology Networking

The operation of a SMG can result in higher availability and quality compared with strictly hierarchical management of power generation and distribution. The security of the system can be improved by the ability of feeding final users, reacting to demand variations in a short time by redispatching energy thanks to smart systems. This allows to reduce risks and consequences of black-outs, avoiding the increase of the global production.

Hydrogen distribution network

Photovoltaic systems highly integrated in the

buildings




problems . . .




possible solutions . . .

Automated Demand Response

Color-based indication of grid status

from Dr. Peter Palensky’s contribution to IEEE – IECON 2008 Panel Discussion Session On Industrial Electronics for Renewable Energy



Wind systems



Limited speed range (-30% to +20%, typical)Small-scale power converter (Less power losses, price)Complete control of active Pref and reactive power Qref

Need for slip-ringsNeed for gear

Doubly-fed induction generator - wounded rotor

Producers: Vestas, Gamesa, NEG Micon, GE Wind, Nordex, REpower Systems,

DEWind

Power range: 0.85 MW to 4.2 MW

Wind turbine systems

Gear

Doubly-fedinduction generator

Pitch

Grid

DC

AC

AC

DC

Pref Qref



Full speed range No brushes on the generator Complete control of active and reactive power Proven technology Full-scale power converter Need for a gear

Induction generator - Squirrel cage rotor

Mainly for low power stand-alone

Producers: Verteco (converter rated for 50% power), Neg Micon, Siemens



Gear

Inductiongenerator

Pitch

DC

AC

AC

DC

Pref Qref



Full speed range Possible to avoid gear (multi-pole generator) Complete control of active and reactive power Small converter for field Need of slip-rings Full scale power converter Multi-pole generator may be big and heavy

Synchronous generator - External magnetized

Gear

SynchronousGenerator

Pitch

GridDC

AC

AC

DC

Pref Qref

VII

DC

AC

inverter

or

diode-bridge + chopper

Producers: Enercon, Largey,





Full speed range Possible to avoid gear (multi-pole generator) Complete control of active and reactive power Brushless (reduced maintenance) No power converter for field (higher efficiency) Full scale power converter Multi-pole generator big and heavy Permanent magnets needed

Synchronous generator - Permanent magnets

inverter

or

diode-bridge + chopper

Producers: Largey, Mitsubishi, Pfleiderer Wind Energy



PM-synchronousGeneratorMulti-pole

Pitch

GridDC

AC

AC

DC

Pref Qref



SG Example 1

20 kW mini-WT multipolar permanent magnet synchronous generator with axial

flux produced by JONICA IMPIANTI (JIMP)



SG Example 2

from “WindBlatt 02/03”

WT Enercon 300 kW multipolar synchronous generator installed in Antartica



SG Example 3Multibrid WT

5 MW multipolar synchronous generator (Multi) with ibrid gear (brid) for offshore applications

Prokon Nord

synchronous generator with permanet magnets surface mounted and radial flux

3 kV NPC converter from Alstom or ABB



Trends2002

- Power electronics is now in wind turbines- Direct-driven genertaor market share is growing

no

gear-box



Basic power conversion and control:

Wind turbine systems control

Rotor

Power conversion & power control

Power transmission

Gearbox (obtional) Generator

Power conversion

Power converter(obtional)

Power conversion & power control

Supply grid

Power transmission

Wind power

Mechanical power Electrical power

Electrical control

Power control

Pref Qref

Consumer



Basic demands:

Electrical:

Mechanical:

• Interconnection (conversion, synchronization)

• Overload protection

• Active and reactive power control

• Power limitation (pitch)

• Maximum energy capture

• Speed limitation/control

• Reduce acoustical noise

Control loops with different bandwidth




- Controllers (internal)- Modulation- Overall system control


Gear

Inductiongenerator

Pitch

GridDC

AC

AC

DC

Power control Grid control

v ,v ,vra rb rci ,i ,ira rb rc

s ,s ,sra rb rcs ,s ,sga gb gc

v ,v ,vga gb gci ,i ,iga gb gc



Control of permanent magnet synchronous generator system

0 20 40 60 80 100 120 140 160 180 2000

1000

2000

3000

4000

5000

6000CARATTERISTICA DI CONTROLLO

Velocità rotore [rpm]

Co

pp

ia [N

*m]

12 m/s

14 m/s

9 m/s

8 m/s

7 m/s6 m/s5 m/s4 m/s

10 m/s

1

T *

60/(2*pi)1

wm




Control of synchronous generator system

- Control of active and reactive power




Control of doubly-fed induction generator system


DFIG

AC

DC

Rotor controlGrid control

DC

AC

vDC

Transformer

Grid converter

Rotor converter

Pref

Qref

v ,v ,vra rb rc

s ,s ,sra rb rc

s ,s ,sga gb gcv ,v ,vga gb gc

i ,i ,iga gb gc

ra rb rci ,i ,i



Detailed example

Operating range



Control of doubly-fed induction generator system (generator-side)

k

k

k

k

k+

k+

k+

k+

T ·s

T ·s

T ·s

T ·s

P I-contro ller

P I-contro ller

P I-contro ller

P I-contro ller

P r e f

Q re f

Q m e a s

P m e a s

i rq

v rq

v rd

i r a s ra

i r b s rb

i rc s rc

i rd

3

2

- Complete control of active and reactive power

3

2

3

2 2

ms s rq

s

m ss s rd

s m

LP v i

L

L vQ v i

L fL




Detailed exampleBasic power flow



Photovoltaic systems



PV Inverter Topologies

PV Inverters

with boost

without boost (central inverters)

with isolation

transformerless

on the LF side

on the HF side

transformerless

• PV dc voltage typical low for string inverters boost needed for low power• For high power (>100 kW) central PV inverters w/o boost, typical three-

phase FB topologies with LV-MV trafo• 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 • Transformerless topologies having higher efficiency are emerging and the

grid regulations are changing in order to allow them



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

DC

ACGridPV

Array

DC

DC

On low frequency (LF) side

DC

ACGridPV

Array

DC

AC

AC

DC

On high frequency (HF) side

PV inverters with boost converter and isolation

Grid

N

L

FilterFilterFB boost with HF trafo FB inverterFilterPV Array

S5 S7

S6 S8

S1 S3

S2 S4

D1 D3

D2 D4

D5 D7

D6 D8

VPE

Boosting inverter with HF trafo based on FB boost converter [2]

Grid

N

L

FilterFilterBoost without trafo FB inverterFilterPV Array

S5

S1 S3

S2 S4

D1 D3

D2 D4

D5

LF Trafo

VPE

Boosting inverter with LF trafo based on boost converter



DC

DC

DC

ACGridPV

Array

Transformerless PV inverters with boost

• Efficiency >95%•Leakage current problem•Safety issue

•Efficiency > 96%•Extra diode to bypass boost when Vpv > Vg•Boost with rectified sinus reference

Grid

N

L


S5

S1 S3

S2 S4

D1 D3

D2 D4

D5

VPE Leakage circulating current

•Time sharing configuration

•FB inverter + boost

Grid

N

L


S5

S1 S3

S2 S4

D1 D3

D2 D4

D5

VPE Leakage circulating current

• Typical configuration



Frequency analysis of voltage to earth Vpe for FB with UP and BP PWM switching

Spectrum of voltage to earth Spectrum of leakage current

Based on ICp and VCp and different frequencies the leakage capacitance was calculated at: Cp=13.6nF (7.06nF/kWp). Cp is useful in high-frequency analysis and in damping resonances

VAB, VPE and IPE for FB-UP



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

No 0 output voltage possible

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

Voltage across filter is bipolar high core losses

No common mode voltage VPE free for high frequency low leakage current

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

This topology is not suited to transformerless PV inverter due to low efficiency!

High efficiency topologies derived from H-bridge FB with Bipolar PWM Switching

Vg

Grid

N

L

FilterFilter Basic FB inverterPV Array

S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

VPV

CPV

VPE

VAB = VPV

A

B

S1 + S4 = ONS1 + S4 and S2 + S3 are switched complementary at high frequency.

Vg

Grid

N

L


S1

S3

S2S4

D1 D3

D2 D4

L1

L2

VPV

CPV

VPE

VAB = - VPV

A

B

S2 + S3 = ONS1 + S4 and S2 + S3 are switched complementary at high frequency.



Leg A and Leg B are switched with high frequency with mirrored sinusoidal reference 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 needed Voltage across filter is unipolar low core losses 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 to transformerless PV inverter due to high leakage!

High efficiency topologies derived from H-bridge FB with Unipolar PWM Switching

Vg

Grid

N

L


S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

VPV

CPV

VPE

VAB = VPV

A

B

Vg > 0, Ig > 0. S1 and S4 = ON

Vg

Grid

N

L


S1

S3

S2S4

D1 D3

D2 D4

L1

L2

VPV

CPV

VPE

VAB = - VPV

A

B

Vg < 0, Ig < 0. S2 and S3 = ON

Vg

Grid

N

L


S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

VPV

CPV

VPE

VAB = 0

A

B

Vg > 0, Ig >0. S1, S3 and D3 = ON

Vg

Grid

N

L


S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

VPV

CPV

VPE

VAB = 0

A

B

Vg < 0, Ig < 0. S2, S4 and D4 = ON

Vg

Grid

N

L


S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

VPV

CPV

VPE

VAB = 0

A

B

Vg > 0, Ig >0. S2, S4 and D2 = ON

Vg

Grid

N

L


S1 S3

S2 S4

D3

D2 D4

L1

L2

VPV

CPV

VPE

VAB = 0

A

B

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

D1



Leg A is switched with grid low frequency and Leg B is switched with high PWM frequency Two 0 output voltage states possible: S1 and S2 = ON and S3 and S4 = ON The switching ripple in the current equals 1x switching frequency high filtering needed Voltage across filter is unipolar low core losses 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. This topology is not suited to transformerless PV inverter due to high leakage!

High efficiency topologies derived from H-bridge FB with Hybrid PWM Switching

Vg

Grid

N

L


S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

VPV

CPV

VPE

VAB = VPV

A

B

Vg > 0, Ig > 0. S1 and S4 = ON. Leg A switched at 50 Hz, Leg B at 16 kHz

Vg

Grid

N

L


S1

S3

S2S4

D1 D3

D2 D4

L1

L2

VPV

CPV

VPE

VAB = - VPV

A

B

Vg < 0, Ig < 0. S2 and S3 = ON. Leg A switched at 50 Hz, Leg B at 16 kHz

Vg

Grid

N

L


S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

VPV

CPV

VPE

VAB = 0

A

B

Vg > 0, Ig >0. S1, S3 and D3 = ON.Leg A switched at 50 Hz, Leg B at 16 kHz

Vg

Grid

N

L


S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

VPV

CPV

VPE

VAB = 0

A

B

Vg < 0, Ig < 0. S2, S4 and D4 = ON. Leg A switched at 50 Hz, Leg B at 16 kHz



High efficiency topologies derived from H-bridge H5 (SMA)– ηmax=98%

Vg

Grid

N

L

FilterFilter H5 FB inverterPV Array

S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

VPV

CPV

VPE

S5

D5

Vg > 0. S5, S1 and S4 =ONS5 and S4 are switched at high frequency. S1 is switched at line frequency

VAB = + VPV

A

B

Vg

Grid

N

L


S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

VPV

CPV

VPE

S5

D5

VAB = - VPV

A

B

Vg < 0. S5, S2 and S3 = ONS5 and S2 are switched at high frequency. S3 is switched at line frequency

Vg

Grid

N

L


S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

VPV

CPV

VPE

S5

D5

VAB = 0

A

B

Vg > 0. S5 and S4 = OFF, S1 and D3 =ONS5 and S4 are switched at high frequency. S1 is switched at line frequency

Vg

Grid

N

L


S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

VPV

CPV

VPE

S5

D5

VAB = 0

A

B

Vg < 0. S5 and S2 = OFF, D1 and S3 = ONS5 and S2 are switched at high frequency. S3 is switched at line frequency

Extra switch in the dc link to decouple the PV generator from grid during zero voltage Two 0 output voltage states possible: S5 = OFF, S1 = ON and S5 = OFF, S3 = ON The switching ripple in the current equals 1x switching frequency high filtering needed Voltage across filter is unipolar low core losses VPE is sinusoidal with grid frequency component low leakage current and EMI High max. efficiency 98% due to no reactive power exchange as reported by Photon Magazine for SMA SunnyBoy 4000/5000 TL single-phase



Vg

Grid

N

L

FilterFilter HERIC FB inverterPV Array

S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

S+ D+

S-D-

VPV

CPV

VPE Vg > 0. S1 and S4 =ON, S+ = ONS1 and S4 are switched at high frequency. S+ is switched at line frequency

VAB = + VPV

A

B

Vg

Grid

N

L


S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

S+ D+

S-D-

VPV

CPV

VPE Vg < 0. S2 and S3 =ON. S- = ONS2 and S3 are switched at high frequency. S- is switched at line frequency

VAB = - VPV

A

B

Vg

Grid

N

L


S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

S+ D+

S-D-

VPV

CPV

VPEVg > 0. S1 and S4 =OFF. S+ and D- = ON

S5 and S4 are switched at high frequency. S+ is switched at line frequency

VAB = 0

A

B

Vg

Grid

N

L


S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

S+ D+

S-D-

VPV

CPV

VPE Vg < 0. S2 and S3 =OFF. S- and D+ = ONS2 and S3 are switched at high frequency. S- is switched at line frequency

VAB = 0

A

B

High efficiency topologies derived from H-bridge HERIC (Sunways)-ηmax=98%

Two 0 output voltage states possible: S+ and D- = ON and S- and D+ = ON The switching ripple in the current equals 1x switching frequency high filtering needed Voltage across filter is unipolar low core losses VPE is sinusoidal has grid frequency component low leakage current and EMI High efficiency 98% due to no reactive power exchange as reported by Photon Magazine for Sunways AT series 2.7 – 5 kW single-phase



High efficiency topologies derived from H-bridge FB – DC Bypass (Ingeteam)-ηmax=96.5%

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- The switching ripple in the current equals 1x switching frequency high filtering needed Voltage across filter is unipolar low core losses VPE is sinusoidal and has grid frequency component low leakage current and EMI 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)

Vg

Grid

N

L

FilterFilter FB inverterPV Array

S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

VPV

VPE

S5

D5

S6

D6

CPV1

CPV2 B

A

DC Bypass

VAB = VPV

Vg > 0. S5, S6,S1 and S4 = ONS5 and S6 are switched at high frequency, S1 and S4 at line frequency

D+

D-

Vg

Grid

N

L


S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

VPV

VPE

S5

D5

S6

D6

CPV1

CPV2 B

A

DC Bypass

VAB = - VPV

Vg < 0. S5, S6,S1, S2 and S3 = ONS5 and S6 are switched at high frequency, S2 and S3 at line frequency

D+

D-

Vg

Grid

N

L


S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

VPV

VPE

S5

S6

CPV1

CPV2 B

A

DC Bypass

VAB = 0

Vg > 0. S1 and S4 = ONS5 and S6 are switched at high frequency, S1 and S4 at line frequency

D5

D6

D+

D-

Vg

Grid

N

L


S1 S3

S2 S4

D1 D3

D2 D4

L1

L2

VPV

VPE

S5

S6

CPV1

CPV2 B

A

DC Bypass

VAB = 0

Vg < 0. S2 and S3 = ONS5 and S6 are switched at high frequency, S2 and S3 at line frequency

D5

D6

D+

D-



High efficiency topologies derived from H-bridge REFU ηmax=98% -

Three-level output. Requires double PV voltage input in comparison with FB but it include time-shared boost Zero voltage is achieved by shortcircuiting the grid using the biderectional switch The switching ripple in the current equals 1x switching frequency high filtering needed Voltage across filter is unipolar low core losses VPE without high frequency component low leakage current and EMI . No L in neutral! High max efficiency 98% due to no reactive power exchange, as reported by Photon Magazine for Refu Solar RefuSol (11/15 kW, three-phase)

Vg

Grid

N

L

FilterBoost AC BypassPV Array

S1 S3

S2 S4

L

S+S-

VPV

VPE

A

DC Link H HB Boost BypassDC Link L

Vg > 0, VPV < |Vg|, Ig > 0. S1 and S+ =ONS1 is switched at high frequency. S+ is switched at line frequency

VAB = + VPV/2

VDC

B

Vg

Grid

N

L


S1 S3

S2 S4

L

S+S-

VPV

VPE

A


Vg > 0, VPV < |Vg|, Ig > 0. S3 and S+ =ONS3 is switched at high frequency. S+ is switched at line frequency

VAB = + VDC/2

VDC

B

L

Vg

Grid

N

L


S1 S3

S2 S4

S-

VPV

VPE

A


Vg > 0, Ig > 0. S+ =ONS+ is switched at line frequency

VAB = 0

VDC

S+B

Vg

Grid

N

L


S1 S3

S2 S4

L

S+S-

VPV

VPE

A


Vg < 0,VPV > |Vg| Ig < 0. S2 and S- =ONS2 is switched at high frequency. S- is switched at line frequency

VAB = - VPV/2

VDC

B

Vg

Grid

N

L


S1 S3

S2 S4

L

S+S-

VPV

VPE

A


Vg < 0, VPV < |Vg| Ig < 0. S4 and S- =ONS2 is switched at high frequency. S- is switched at line frequency

VAB = - VDC/2

VDC

B

L

Vg

Grid

N

L


S1 S3

S2 S4

S-

VPV

VPE

A


Vg < 0, Ig < 0. S- =ONS- is switched at line frequency

VAB = 0

VDC

S+B



High efficiency topologies derived from H-bridge Summary

• Actually both HERIC, H5, REFU and FB-DCBP topologies are converting the 2 level FB (or

HB) inverter in a 3 level one.

• This increases the efficiency as both the switches and the output inductor are subject to

half of the input voltage stress.

• The zero voltage state is achieved by shorting the grid using higher or lower switches of

the bridge (H5) or by using 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 the neutral 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 while HERIC and FB_DCBP have two.



High efficiency topologies derived from NPCHalf Bridge Neutral Point Clamped (HB-NPC)-ηmax=98% -

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 opsition and S2 and S4 for Vg<0, Ig>0 The switching ripple in the current equals 1x switching frequency high filtering needed Voltage across filter is unipolar low core losses VPE is equal –Vpv/2 without high frequency component low leakage current and EMI . No L in N! High max efficiency 98% due to no reactive power exchange, as reported by Danfoss Solar TripleLynx series (10/12.5/15 kW)

GridFilterFilter NPC inverterPV Array

L1

VPV

CPV1

VPE

Vg

N

A

CPV2

BL

S1

S2

S3

S4

D1

D2

D3

D4

D+

D-

Vg > 0, Ig > 0. S1 and S2 =ON, S3 and S4 = OFFS1 is switched at high frequency. S2 is switched at line frequency

VAB = + VPV/2VPV/2

VPV/2


L1

VPV

CPV1

VPE

Vg

N

A

CPV2

BL

S1

S2

S3

S4

D1

D2

D3

D4

D+

D-

Vg > 0, Ig > 0. S2 =ON, D+ = ON, S1, S3 and S4 = OFFS1 is switched at high frequency. S2 is switched at line frequency

VAB = 0VPV/2

VPV/2


L1

VPV

CPV1

VPE

Vg

N

A

CPV2

BL

S3

S4

D1

D2

D3

D4

D+

D-

Vg < 0, Ig < 0. S3 and S4 =ON, S1 and S2 = OFFS4 is switched at high frequency. S3 is switched at line frequency

VAB = - VPV/2VPV/2

VPV/2

S1

S2


L1

VPV

CPV1

VPE

Vg

N

A

CPV2

BL

S1

S2

S3

S4

D1

D2

D3

D4

D+

D-

Vg > 0, Ig > 0. S3 =ON, D- = ON, S1, S2 and S4 = OFFS4 is switched at high frequency. S3 is switched at line frequency

VAB = 0VPV/2

VPV/2



High efficiency topologies derived from NPCConergy NPC -ηmax=96% -

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 output voltage states possible using the bidirectional clamping switch (S+ and S-) The switching ripple in the current equals 1x switching frequency high filtering needed Voltage across filter is unipolar low core losses VPE is equal –Vpv/2 without high frequency component low leakage current and EMI . No L in N! High max efficiency 96.1% due to no reactive power exchange, as reported by Conergy IPG series (2-5 kW single-phase)

GridFilterFilter HB inverterPV Array

L1

VPV

CPV1

VPE

Vg

N

A

CPV2

B

S1

S2

D1

D2

VPV/2

VPV/2

Clamping Switch

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

VAB = VPV/2

S+

S-

D+

D-


L1

VPV

CPV1

VPE

Vg

N

A

CPV2

B

S1

S2

D1

D2

VPV/2

VPV/2

Clamping Switch

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

VAB = -VPV/2

S+

S-

D+

D-


L1

VPV

CPV1

VPE

Vg

N

A

CPV2

B

S1

S2

D1

D2

VPV/2

VPV/2

Clamping Switch

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

VAB = 0

S+

S-

D+

D-


L1

VPV

CPV1

VPE

Vg

N

A

CPV2

B

S1

S2

D1

D2

VPV/2

VPV/2

Clamping Switch

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

VAB = 0

S+

S-

D+

D-



High efficiency topologies derived from NPCSummary

• The classical NPC and its “variant” Conergy-NPC are both three-level topologies featuring

the advantages of unipolar voltage across the filter, high efficiency due to disconnection of

PV panels during zero-voltage state and 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).

• These topologies are also very attractive for high power in the range of hundreds of kW)

central inverters) where the advantages of multi-level inverters are even more important.



PV Inverter Topologies -Conclusions• The “race” for higher efficiency PV inverters has resulted in a large variety of “novel” transformerless topologies derived from H-Bridge with higher efficiency and lower CM/EMI (H5, HERIC)

• 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 transformerless achieving 98% max conversion efficiency and 97.7% “european” (weighted) efficiency

• Further improvements in the efficiency can be achieved by using SiC MosFets. ISE Fraunhofer-Freiburg reported recently 98.5% efficiency (25% reduction in switching + conduction losses)

• For 3-phase systems the trend is to use 3 independent controlled single-phase inverters like 3xH5 or 3xHERIC but 3FB-SC and 3NPC (not proprietary) are also present on the market. 3NPC achieve higher efficiency 98%

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



Control Structure Overview

PV Panels

String

dc-dc

boost

LCL

Low pass

filter

C

VPV

IPV

+

-

L

N

Trafo&

Grid

Anti-IslandingProtections

Grid /PV plant Monitoring

Ig

Vg

dc-ac

PWM-VSI

VdcPWM PWM

MPPT

Active filtercontrol

Grid support(V,f,Q)

Ancillary functions

PV specific functions

Basic functions (grid conencted converter)

CurrentControl

VdcControl

MicroGridControl

GridSynchronization

Basic functions – common for all grid-connected invertersGrid current control

THD limits imposed by standardsStability in case of grid impedance variationsRide-through grid voltage disturbances (not required yet!)

DC voltage controlAdaptation to grid voltage variationsRide-through grid voltage disturbances (optional yet)

Grid synchronization Required for grid connection or re- connection after trip.

PV specific functions – common for PV invertersMaximum Power Point Tracking – MPPT

Very high MPPT efficiency in steady state (typical > 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)Grid Monitoring

Operation at unity power factor as required by standardsFast Voltage/frequency detection

Plant MonitoringDiagnostic of PV panel arrayPartial shading detection

Ancillary Support – (future?)Voltage ControlFrequency controlFault Ride-through Q compensationDVR



Introduction to Maximum Power Point Tracking

- MPPTThe MPP is affected by temperature and irradiance.

The task of MPPT is to track this MPP regardless of weather or load conditions so that the PV system draws maximum power from the solar array.

The MPPT is a nonlinear and time-varying system that has to be solved.

All algorithms are based on the fact that, looking at the power characteristic, at the left of the MPP the dP/dV > 0, at the right dP/dV < 0 and at MPP dP/dV = 0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.70

6

Cell voltage [V]

0 0.1 0.2 0.3 0.4 0.5 0.6 0.70

0.5

1

1.5

2

2.5

Cell voltage [V]

1000 W/m2

600 W/m2

200 W/m2

4

2I ce

ll[A

]P

cel

l[W

]

15o C

40o C

75o C

15o C

40o C

75o C

MPP

dP/dV = 0, MPP

P

V

dP/dV = 0

dP/dV < 0

dP/dV > 0

MPP



MPPT Comparison Most common methods:

Perturb&Observe – PO Incremental Conductance – IC Constant Voltage

Preliminary results indicate that IC method compares favorably with PO and CV methods

Still PO is preferred due to implementation simplicity Combined PO+CV is best!



Typical control structure for dual-stage PV

inverter

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 VDC is controlled in the dc-ac inverter the change of the duty-cycle will change voltage at the output of the PV panels, VPV as:

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

The power feedforward requires communication between the two stages and improves the dynamics of MPPT

MPPT

pvI

pvVdc voltagecontroller

PLLacV

sinˆrI

2pv

acRMS

P

V

pvP

acRMSV

*ˆrefI

ˆrefI

PV

array

DPWM

DCVdc-dcconv

refDCV ,

currentcontroller

PWM dc-acinv

gI

refgI ,

~grid

dc-dc conv dc-ac inv

D

VKV PV

DC

1



Typical control structure for single-stage PV inverter

In these topologies -which are becoming more and more popular in countries with low grid voltage (120V) like Japan and thus the voltage from the PV array is high enough- the MPPT is implemented in the dc-ac inverter

Also in topologies with boost trafo on ac side (SMA)

The output of the MPPT is the dc-voltage reference. The output of the dc-voltage controller is the grid current reference amplitude. The power feedforward improves the dynamic response as MPPT runs at a slow sampling frequencies (typ. 1 Hz).

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

M P P TP V

a r r a y

p vI

p vV

*p vV d c v o l t a g e

c o n t r o l le r

P L La cV

s i n ˆ

rI r e fI

2p v

a c R M S

P

V

p vP

a c R M SV

*ˆr e fI

ˆr e fI

P W M d c - a ci n v

c u r r e n tc o n t r o l l e r

I



Practical PV inverter control implementation

Dual-stage full-bridge PWM inverter with LCL filter and grid trafo

12

dc 12

dc

PV

Panels

String

Full-bridge

Inverter

VSI-PWM

LCL

Low pass

filter

GridCVpv

Ipv +

-

U

V

L

N

Ig

Vg

MPPT

Gdc

PLL

GC

PWM

Vdc ref

Vpv

Ipv

refI

sin

Vg

*acV dc

+ +- -Ig Vdc

Vdc

IsolationTransformer

+

Control structure

ˆrI

DC/DC

Converter

PWMdc

Vdc

•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 for current control

•The dc voltage controller can be P type due to the integration effect of the typical large capacitor



PV Inverter Control Structures - Conclusions

The most typical control structure is the current controlled voltage source inverter with PWM

Typically boost dc-dc converter is required The MPPT is a necessary feature in order to extract the maximum power

from a panel array at any conditions of irradiation and temperature. PO and INC are the most used ones. PO+CV is also possible According to the topology (dual- or single-stage) the MPPT is implemented

in the dc-dc converter or in the dc-ac inverter PR current controller better than PI control for sinusoidal references PLL is typically required for synchronization



Acknowledgment

Part of the material is or was included in the present and/or past editions of the

“Industrial/Ph.D. Course in Power Electronics for Renewable Energy Systems – in theory and practice”

Speakers: R. Teodorescu, P. Rodriguez, M. Liserre, J. M. Guerrero,

Place: Aalborg University, Denmark

The course is held twice (May and November) every year

Documents

Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Marco Liserre [email protected] Overview of Distributed Power