Post on 03-Feb-2018
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 trans- former
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
Reference and Carrier Signals
1
Reference and Carrier Signals
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
• 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
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
• The switching ripple in the current equals 1x switching frequency high filtering
• Voltage across filter is unipolar low core losses
• 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
• The switching ripple in the current equals 1x switching frequency high filtering
• Voltage across filter is unipolar low core losses
• 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‐
• The switching ripple in the current equals 1x switching frequency high filtering
• Voltage across filter is unipolar low core losses
• 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
• The switching ripple in the current equals 1x switching frequency high filtering
• 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
• The switching ripple in the current equals 1x switching frequency high filtering
• Voltage across filter is unipolar low core losses
• 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
• The switching ripple in the current equals 1x switching frequency high filtering
• Voltage across filter is unipolar low core losses
• 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-
GridFilterFilter HB inverterPV Array
L1
VPV
CPV1
AB
S1 D1VPV/2
Clamping Switch
VAB = 0
S+D+
GridFilterFilter HB inverterPV Array
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
• Voltage across filter is unipolar low core losses
• 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]