A Review of Single-Pha Grid-Connected Inverters
for Photovoltaic Modules
Soeren Baekhoej Kjaer ,Member,IEEE ,John K.Pedern ,Senior Member,IEEE ,and Frede Blaabjerg ,Fellow,IEEE
试用报告Abstract—This review focus on inverter technologies for connecting photovoltaic (PV)modules to a single-pha grid.The inverters are categorized into four classifications:1)the number of power processing stages in cascade;2)the type of power de-coupling between the PV module(s)and the single-pha grid;3)whether they utilizes a transformer (either line or high frequency)or not;and 4)the type of grid-connected power stage.Various in-verter topologies are prented,compared,and evaluated against demands,lifetime,component ratings,and cost.Finally,some of the topologies are pointed out as the best candidates for either single PV module or multiple PV module applications.
Index Terms—AC module,photovoltaic (PV)power systems,single-pha grid-connected inverters.
I.I NTRODUCTION
P
HOTOVOLTAIC (PV)power supplied to the utility grid is gaining more and more visibility,while the world’s power demand is increasing [1].Not many PV systems have so far been placed into the grid due to the relatively high cost,compared with more traditional energy sources such as oil,gas,coal,nuclear,hydro,and wind.Solid-state inverters have been shown to be the enabling technology for putting PV systems into the grid.
The price of the PV modules were in the past the major contribution to the cost of the systems.A downward tendency is now en in the price for the PV modules due to a massive increa in the production capacity of PV modules.For example,
the price per watt for a PV module was between
4.4
7.9USD in 1992and has now decread to
2.6
3.5USD [2].The cost of the grid-connected inverter is,therefore,becoming more visible in the total system price.A cost reduction per inverter watt is,therefore,important to make PV-generated power more attractive [4].Focus has,therefore,been placed on new,cheap,and innovative inverter solutions,which has resulted in a high diversity within the inverters,and new system configurations.
This paper starts with an examination of the demands for the inverters,t up by utility grid companies,the PV modules,and the operators.This is followed by a historical review to e how
Paper IPCSD-05-002,prented at the 2002Industry Applications Society Annual Meeting,Pittsburgh,PA,October 13–18,and approved for publication in the IEEE T RANSACTIONS ON I NDUSTRY A PPLICATIONS by the Industrial Power Converter Committee of the IEEE Industry Applications Society.Manuscript submitted for review March 1,2004and relead for publication June 16,2005.S.B.Kjaer is with PowerLynx A/S,DK-6400Sønderborg,Denmark (e-mail:;sbkjaer@ieee).
J.K.Pedern and F.Blaabjerg are with the Institute of Energy Technology,Aalborg University,DK-9220Aalborg East,Denmark (e-mail:jkp@iet.aau.dk;fbl@iet.aau.dk).
Digital Object Identifier 10.1109/TIA.2005.853371
the demands were achieved in the past,how they are reached today,and perhaps how they will be realized in the future.Next follows an overview of some existing power inverter topologies for interfacing PV modules to the grid.The approaches are fur-ther discusd and evaluated in order to recognize the most suit-able topologies for future PV inverters,and,finally,a conclusion is given.
II.S PECIFICATIONS ,D EMANDS ,AND S TANDARDS
Inverter interfacing PV module(s)with the grid involves two major tasks.One is to ensure that the PV module(s)is operated at the maximum power point (MPP).The other is to inject a sinusoidal current into the grid.The tasks are further reviewed in this ction.
A.Demands Defined by the Grid
Since the inverter is connected to the grid,the standards given by the utility companies must be obeyed.In particular,the fu-ture international standard (still a Committee Draft for V ote-CDV)IEC61727[3]and the prent standards EN61000-3-2[4],IEEE1547[5]and the U.S.National Electrical Code (NEC)690[6]are worth considering.The standards deal with issues like power quality,detection of islanding operation,grounding,etc.Summaries are listed in Table I.
As en in Table I,the prent EN standard (applied in Eu-rope)is easier to cope with,regarding current harmonics,than the corresponding IEEE and IEC standards.This is also reflected in the chon inverter topologies,which have changed from large thyristor-equipped grid-connected inverters to smaller insulated-gate-bipolar-transistor (IGBT)/MOSFET-equipped ones.
The inverters must also be able to detect an islanding situ-ation,and take appropriate measures in order to protect per-sons and equipment [7].Islanding is the continued operation of the inverter when the grid has been removed on purpo,by accident,or by damage.In other words,the grid has been re-moved from the inverter,which then only supplies local loads.The available detection schemes are normally divided into two groups:active and passive.The passive methods do not have any influence on the power quality,since they just monitor grid pa-rameters.The active schemes introduce a disturbance into the grid and monitor the effect.This may affect the power quality,and problems with multiple inverters in parallel with the grid are also known to exist [7],[8].
The IEEE [5]and the IEC [3]standards put limitations on the maximum allowable amount of injected dc current into the grid.The purpo of limiting the injection is to avoid saturation of the distribution transformers [7].However,the
0093-9994/$20.00©2005IEEE
TABLE I
S UMMARY OF THE M OST I NTERESTING S TANDARDS D EALING W ITH I NTERCONNECTIONS OF PV S YSTEMS TO THE G
RID
limits are rather small (0.5%and 1.0%of rated output current),and such small values can be dif ficult to measure precily with the exciting circuits inside the inverters.This can be mitigated with improved measuring circuits or by including a line-frequency transformer between the inverter and the grid.Some inverters u a transformer embedded in a high-frequency dc –dc converter for galvanic isolation between the PV modules and the grid.This does not,however,solve the problem with dc injection,but makes the grounding of the PV modules easier.
The NEC 690standard [6]demands that the PV modules shall be system grounded and monitored for ground faults,when the maximum output voltage of the PV modules reaches a certain ,50V [6],[7],[26].System ground involves the negative (positive)terminal of the PV array(s)being con-nected to ground.This can be troublesome for many high-power transformerless systems,since a sin
gle-pha inverter with neu-tral-to-line grid connection already is system grounded on the grid side.Other Electricity Boards only demand equipment ground of the PV modules in the ca of abnt galvanic iso-lation [7],[9].Equipment ground is the ca when frames and other metallic parts are connected to ground.
Assuming that both the grid voltage and grid current only contain the fundamental component and that they are in pha,the instantaneous power injected into the grid becomes equal
to
(1)
where is the average injected
肺炎有什么症状
power,is the angular
frequency,and is time.
B.Demands De fined by the Photovoltaic Module(s)
A model of a PV cell is sketched in Fig.1(a),and its electrical characteristic is illustrated in Fig.1(b).The most common PV technologies nowadays are the monocrystalline-and the multicrystalline-silicon modules,which are bad on traditional,and expensive,microelectronic manufacturing process [1].The MPP voltage range for the PV modules is normally de fined in the range from 23to 38V at a power gen-eration of approximate 160W,and their open-circuit voltage is below 45V .However,new technologies like thin-layer silicon,amorphous-silicon,and hoto Electro Chemical (PEC)are in development [1],[10].The types of PV modules can be made arbitrarily large by an inexpensive “roll-on –roll-off ”process.
Fig.1.Model and characteristics of a PV cell.(a)Electrical model with current and voltages de fined.(b)Electrical characteristic of the PV cell,expod to a given amount of (sun)light at a given temperature.As indicated,ripple at the PV module ’s terminals results in a somewhat lower power generation,compared with the ca where no ripple is prent at the terminals.
This means that new modules with only one cell may e the light in the future.The voltage range for the cells/modules is located around
0.5 1.0V at veral hundred amperes per square meter cell [11]–[13].
The inverters must guarantee that the PV module(s)is oper-ated at the MPP,which is the operating condition where the most energy is captured.This is accomplished with an MPP tracker (MPPT).It also involves the ripple at the terminals of the PV module(s)being suf ficiently small,in order to operate around the MPP without too much fluctuation.Analys of the circuit in Fig.1(a)show that there is a relationship between the ampli-tude of the voltage ripple and the utilization
ratio
,
given as
[14]
面膜美即
吴耀邦(2)
where is the amplitude of the voltage
ripple,
and are the power and voltage at the
MPP,
and are the coef-ficients describing a cond-order Taylor approximation of the current,and the utilization ratio is given as the average generated power divided by the theoretical MPP power.The coef ficients are computed
as
(3)
(4)
酥炸带鱼
(5)
西柚英文
(6)(7)
Calculations show that the amplitude of the ripple voltage should be below 8.5%of the MPP voltage in order to reach a utilization ratio of 98%.For example,a PV module with an MPP voltage of 35V should not be expod to a voltage ripple of more than 3.0V (amplitude),in order to have a utilization ratio of 98%.As en in the previous ction,the power injected into the grid follows a sinusoidal wave,raid to the cond
power,
,for which reason the inverter must contain a power
decoupling device.
C.Demands De fined by the Operator
The operator (the owner)also has a few words to say.First of all,the inverter must be cost effective,which is easily achieved with similar circuits as the ud in today ’s single-pha power-factor-correction (PFC)circuits and variable-speed drives (VSDs).However,the ur also demands a high ef fi-ciency over a wide range of input voltage and input power since the variables are de fined in very wide ranges as functions of solar irradiation and ambient temperature.Fig.2shows the average irradiation during a normal year in Denmark (Northwestern Europe)[15].The figure shows that most of the potential energy is available in the range from 50to 1000W/m of irradiation.
Further,the inverter must be highly reliable (long operational lifetime)since most PV module manufacturer offer a warranty of 25years on 80%of initial ef ficiency,and a materials and workmanship warranty of five years [27].
The main limiting components inside the inverters are the electrolytic capacitors ud for power decoupling between the PV module and the single-pha grid [16]–[19].The operational lifetime for electrolytic capacitors is given by
[20]
(8)
where
is the operational
lifetime,is the lifetime at a hotspot temperature
of
,is the hotspot temperature,
and
is the temperature increa which reduces the lifetime by a factor of two.However,the equation assumes a constant tem-perature,which can be approximated when the inverter is placed indoors and neglecting the power loss inside the capacitor,but certainly not when the inverter is integrated with the PV module,as for the ac module.In the ca of a varying temperature a mean value of (8)must be applied to determine the lifetime [20].
III.E VOLUTION OF PV I NVERTERS
A.The Past —Centralized Inverters
The past technology,illustrated in Fig.3(a),was bad on centralized inverters that interfaced a large number of PV mod-ules to the grid [25].The PV modules were divided into ries connections (called a string),each generating a suf ficiently high voltage to avoid further ampli fication.The ries connections were then connected in parallel,through string diodes,in order to reach high power levels.This centralized inverter includes
Fig.2.Meteorological data.(a)Irradiation distribution for a Danish reference year.(b)Solar energy distribution for a Danish reference year.Total time of irradiation equals 4686h per year.Total potential energy is equal to 1150kWh =(m 1y ) 130W/m
[15].
Fig.3.Historical overview of PV inverters.(a)Past centralized technology.(b)Prent string technology.(c)Prent and future multi-string technology.(d)Prent and future ac-module and ac cell technologies.
some vere limitations,such as high-voltage dc cables between the PV modules and the inverter,power loss due to a central-ized MPPT,mismatch loss between the PV modules,loss in the string diodes,and a non flexible design where the ben-e fits of mass production could not be reached.The grid-con-nected stage was usually line commutated by means of thyris-tors,involving many current harmonics and poor power quality.The large amount of harmonics was the occasion of new in-verter topologies and system layouts,in order to cope with the emerging standards which also covered power quality.
Fig.4.Three types of PV inverters.Plea note that the sign for the PV module shall be interpreted as either a single PV module,or as multiple PV modules in ries/parallel connections.(a)A single power processing stage that handles the MPPT,voltage amplification,and grid current control.(b)Dual power processing inverter where the dc–dc converter is responsible for the MPPT and the dc–ac inverter controls the grid current.V oltage amplification can be included in both stages.(c)Dual-stage inverter,where each PV module or string is connected to a dedicated dc–dc converter that is connect
ed to a common dc–ac inverter.
B.The Prent—String Inverters and AC Modules
The prent technology consists of the string inverters and the
ac module[25].The string inverter,shown in Fig.3(b),is a re-
duced version of the centralized inverter,where a single string
of PV modules is connected to the inverter[7].The input voltage
may be high enough to avoid voltage amplification.This re-
quires roughly16PV modules in ries for European systems.
The total open-circuit voltage for16PV modules may reach as
much as720V,which calls for a1000-V MOSFET/IGBT in
order to allow for a75%voltage de-rating of the miconduc-
tors.The normal operation voltage is,however,as low as
450
510V.The possibility of using fewer PV modules in ries also exists,if a dc–dc converter or line-frequency transformer is ud for voltage amplification.There are no loss associated with string diodes and parate MPPTs can be applied to each string.This increas the overall efficiency compared to the cen-tralized inverter,and reduces the price,due to mass production. The ac module depicted in Fig.3(d)is the integration of the inverter and PV module into one electrical device[7].It removes the mismatch loss between PV modules since there is only one PV module,as well as supports optimal adjustment between the PV module and the inverter and,hence,the individual MPPT.It includes the possibility of an easy enlarging of the system,due to the modular structure.The opportunity to become a“plug-and-play”device,which can be ud by persons without any knowledge of electrical installations,is also an inherent feature. On the other hand,the necessary high voltage-amplification may reduce the overall efficiency and increa the price per watt, becau of more complex circuit topologies.On the other hand, the ac module is intended to be mass produced,which leads to low manufacturing cost and low retail prices.
The prent solutions u lf-commutated dc–ac inverters, by means of IGBTs or MOSFETs,involving high power quality in compliance with the standards.
C.The Future—Multi-String Inverters,AC Modules,and AC Cells股票机构
The multi-string inverter depicted in Fig.3(c)is the further development of the string inverter,where veral strings are in-terfaced with their own dc–dc converter to a common dc–ac in-verter[7],[28].This is beneficial,compared with the centralized system,since every string can be controlled individually.Thus, the operator may start his/her own PV power plant with a few modules.Further enlargements are easily achieved since a new string with dc–dc converter can be plugged into the existing plat-form.Aflexible design with high efficiency is hereby achieved. Finally,the ac cell inverter system is the ca where one large PV cell is connected to a dc–ac inverter[11]–[13].The main challenge for the designers is to develop an inverter that can amplify the very low voltage,
0.5 1.0V and100W per square meter,up to an appropriate level for the grid,and at the same time reach a high efficiency.For the same reason,entirely new converter concepts are required.
IV.Classifications of Inverter Topologies
Next follows a classification of different inverter technolo-gies.The topologies are categorized on the basis of number of power processing stages,location of power decoupling capaci-tors,if they employ transformers or not,and types of grid inter-face.
A.Number of Power Processing Stages
The number of power processing stages,in cascade,is thefirst grouping here.Fig.4shows three cas of single-and multiple-stage inverters.
The inverter of Fig.4(a)is a single-stage inverter,which must handle all tasks ,MPPT,grid current control and,per-haps,voltage amplification.This is the typical configuration for a centralized inverter,with all the drawbacks associated with it. The inverter must be designed to handle a peak power of twice the nominal power,according to(1).
Fig.4(b)depicts a dual-stage inverter.The dc–dc converter is now performing the MPPT(and perhaps voltage amplification). Dependent on the control of the dc–ac inverter,the output from the dc–dc converters is either a pure dc voltage(and the dc–dc converter is only designed to handle the nominal power),or the output current of the dc–dc converter is modulated to follow a rectified sine wave(the dc–dc converter should now handle a peak power of twice the nominal power).The dc–ac i歌曲《父亲》原唱
nverter is in the former solution controlling the grid current by means of pulwidth modulation(PWM)or bang-bang operation.In the latter,the dc–ac inverter is switching at line frequency,“un-folding”the rectified current to a full-wave sine,and the dc–dc converter takes care of the current control.A high efficiency can be reached for the latter solution if the nominal power is low.On the other hand,it is advisable to operate the grid-connected in-verter in PWM mode if the nominal power is high.
