Grid-connected photovoltaic power systems:Technical and potential problems—A review
Mohamed A.Eltawil a ,b ,*,Zhengming Zhao a
a The State Key Laboratory of Power System,Department of Electrical Engineering,Tsinghua University,Beijing 100084,China b
Agricultural Engineering Department,Kafrelsheikh University,Box 33516,Egypt
Contents 1.Introduction .....................................................................................................1132.Glossary of terms 1143.Global PV module and its electrical performance .......................................................................1144.
Grid-connected 1174.1.Power value ...............................................................................................1204.2.Ratio between load and 1205.Potential problems associated with high penetration levels of grid-tied PV ..................................................1216.
Grid-connected inverters—control types and 1226.1236.2.Inverters’operational analysis .................................................................................1247.
Islanding
(124)
Renewable and Sustainable Energy Reviews 14(2010)112–129
A R T I C L E I N F O Article history:
Received 11May 2009Accepted 14July 2009
Keywords:
Grid-connected photovoltaic
Penetration levels of grid tied PV Inverter technology
Islanding detection methods
A B S T R A C T
Traditional electric power systems are designed in large part to utilize large baload power plants,with limited ability to rapidly ramp output or reduce output below a certain level.The increa in demand variability created by intermittent sources such as photovoltaic (PV)prents new challenges to increa system flexibility.This paper aims to investigate and emphasize the importance of the grid-connected PV system regarding the intermittent nature of renewable generation,and the characterization of PV generation with regard to grid code compliance.The invest
igation was conducted to critically review the literature on expected potential problems associated with high penetration levels and islanding prevention methods of grid tied PV.According to the survey,PV grid connection inverters have fairly good performance.They have high conversion efficiency and power factor exceeding 90%for wide operating range,while maintaining current harmonics THD less than 5%.Numerous large-scale projects are currently being commissioned,with more planned for the near future.Prices of both PV and balance of system components (BOS)are decreasing which will lead to further increa in u.The technical requirements from the utility power system side need to be satisfied to ensure the safety of the PV installer and the reliability of the utility grid.Identifying the technical requirements for grid interconnection and solving the interconnect problems such as islanding detection,harmonic distortion requirements and electromagnetic interference are therefore very important issues for widespread application of PV systems.The control circuit also provides sufficient control and protection functions like maximum power tracking,inverter current control and power factor control.Reliability,life span and maintenance needs should be certified through the long-term operation of PV system.Further reduction of cost,size and weight is required for more utilization of PV systems.Using PV inverters with a variable power factor at high penetration levels may increa the number of balanced conditions and subquently increa the probability of islanding.It is strongly recommended that PV inverters should be operated at unity power factor.
ß2009Elvier Ltd.All rights rerved.
*Corresponding author at:Agricultural Engineering Department,Kafrelsheikh University,Box 33516,Egypt.Tel.:+20473232896(2113);fax:+20473232032.E-mail address:in (M.A.Eltawil).Contents lists available at ScienceDirect
Renewable and Sustainable Energy Reviews
j o u r n a l ho m e p a g e :w w w.e l s e v i e r.c o m /l o ca t e /r s e r
1364-0321/$–e front matter ß2009Elvier Ltd.All rights rerved.doi:10.2009.07.015
8.Performance and reliability of inverter hardware .......................................................................1269.
The overall conclusion 126Acknowledgements ...............................................................................................127References (127)
1.Introduction
Grid interconnection of PV power generation system has the advantage of more effective utilization of generated power.However,the technical requirements from both the utility power system grid side and the PV system side need to be satisfied to ensure the safety of the PV installer and the reliability of the utility grid.Clarifying the technical requirements for grid interconnection and solving the problems such as islanding detection,harmonic distortion requirements and electromagnetic interference are therefore very important issues for widespread application of PV systems [1].Grid interconnection of PV systems is accomplished through the inverter,which convert dc power generated from PV modules to ac power ud for ordinary power supply to electric equipments.Inverter system is therefore very important for grid-connected PV systems.
Grid connection and extension costs are significant factors for integrating renewable energy sources-electricity (RES-E)genera-tion technologies into an existing electricity network.Prices of both PV and BOS are decreasing following a trend of incread production and improved technology.This explains the high amount of subsidies for R&D and application of PVs in indus-trialized countries.The solar PV electric power generation will play an important role in the future energy supply in China.
According to the prent plan,total PV power installations will reach 350MW by 2010,1.8GW by 2020
and 600GW by 2050.According to forecasts made by the Chine Electric Power Rearch Institute,renewable energy installations will account for 30%of the total electric power installations in China by 2050,of which PV installations will account for 5%[2].
In fact,growing of PV for electricity generation is one of the highest in the field of the renewable energies and this tendency is expected to continue in the next years [3].As an obvious conquence,an increasing number of new PV components and devices,mainly arrays and inverters,are coming on to the PV market [4].The energy production of a grid-connected PV system depends on various factors.Among the we distinguish the rated characteristics of the components of the PV system,the installation configuration,the geographical siting of the PV system,its surrounding objects,and defects that occur during its operation.The need for PV arrays and inverters to be characterized has then become a more and more important aspect [5–9].Due to the variable nature of the operating conditions in PV systems,the complete characterization of the elements is quite a difficult issue.
Nomenclature C T empirical constant relating to the impact of cell temperature on output (dimensionless)
DF diode factor,dimensionless e electric charge on an electron,1.60Â10À19C E A array output energy (kWh)E PV energy to grid (kWh)
G STC reference irradiance at STC (1kW/m 2)
h c convective heat transfer coefficient (W/(m 2K))h r radiative heat transfer coefficient (W/(m 2K))H T mean daily irradiance in array plane (kWh/m 2d)H (t ,b )is the incident irradiance in the plane of the PV generator
I L light generated current (A)I o diode current (A)
I sc short circuit current at reference values (A)I mp current at maximum power point (A)
k Boltzmann constant,1.38Â10À23J/(K mol)K a thermal conductivity of air (W/(m K))L distance from entry point (m)
m number of parallel connected cells (dimensionless)n number of ries connected cells (dimensionless)N number of panels in surface (dimensionless)P panel power output (W)
P mp is the dc power supplied by the PV generator when operating in the maxim power point P O peak power (W P )
P 0PV is the maximum power of the PV generator Q solar insolation (W/m 2)
Q ref reference insolation (usually 1000W/m 2)(W/m 2)T c temperature in the rear part of the cell or PV module (K)
T ref reference temperature (usually at 298K)(K)
V mp voltage at maximum power point at reference values (V)
V mpp voltage at maximum power point (V)
V oc
open circuit voltage at reference values (V)h e electricity generating efficiency,dimensionless l mp
is temperature coefficients of maximum power voltage of the PV modules
AFD active frequency drift (frequency bias)AFDPF AFD with positive feedback (aka SFS)BOS balance of system components DSP digital signal processor
FCC federal communications commission GTO gate turn off device
JFET junction field-effect transistor OFR over frequency relay
PCS power conditioning system (aka inverter)PJD pha jump detection PLL
pha locked loop
POCC point of common coupling SFS sandia frequency shift
SMS slide-mode frequency shift
SOV silicon oxide varistor,a transient surge suppression device
STC standard test conditions UFR under frequency relay
UIPV utility-interactive photovoltaic (system)
M.A.Eltawil,Z.Zhao /Renewable and Sustainable Energy Reviews 14(2010)112–129113
The performance of grid-connected PV systems can be evaluated by investigating the performance ratio(PR)[10],which is defined by the ratio of the system efficiency and the nominal efficiency of PV modules under STC[11].The average values of PR were found to be66%for one hundred rooftop mo
unted PV in Germany[12–14],55–70%for eight grid-connected PV systems in Europe[15],while it was63–76%in the Netherlands[16].The values apply to systems using solar cells made of poly-and mono-crystalline silicon.
From the performance analysis of260PV plants in the IEA-PVPS Task2databa the annual performance ratios for the different types of systems[17],could be0.6–0.8,0.1–0.6and0.3–0.6for grid-connected PV systems,stand-alone systems without back-up and stand-alone systems with back-up,respectively.The well maintained PV systems showed an average PR value of typically 0.72at an availability of98%.Despite good results,which have been obtained in many of the grid-connected systems,the investigation of the operational behavior of the reported PV systems has identified further potential for optimization.
It is often assumed,in the analysis of grid-connected generators,that the grid supply exhibits a perfect voltage waveform and that the embedded generators themlves are unaffected by perturbations of the hat any disturbance produced is due solely to the embedded sources.In reality, however,the operation of the power electronic generators,and hence the current waveform they source into the network,can be significantly affected by minor distortion of the voltage waveform at the point of connection[18].
With the proliferation of production and improved technolo-gies,the system requires to be standardized,and thus ensuring, issues of safety and quality in manufacture,application,and u. Standards will rve to build consumer confidence,reduce costs and further expand PV development[19].
PV simulation tools are uful to(i)perform detailed analysis of system performance under realfield operating conditions,(ii) investigate the impact of different load profiles,(iii)verify system sizing and determine the optimal size of PV components and(iv) asss the viability of a PV system in terms of energy production and life cycle cost of the system[20].Various simulation tools are currently available to perform PV simulation and can be found in [21–29].
Empirical relationships have also been developed using real field test data for different types of PV cells to characterize different PV parameters to predict PV performance[30].Different mathematical models have been developed for individual PV components to perform simulation of the overall PV system[31–33].A scenario ba PV software tool has been developed to determine the future progress of grid-connected PV systems[34]. Various long-term PV performance models have been developed to simplify the process of hour by hour simulation[35–39].The developed models are uful to design optimal configurations of PV systems.
At prent,the main PV-powered products include solar street, traffic signal,garden and lawn lamps,calculators and solar toys etc. China has become the largest producer of PV-powered products in the world.The annual usage of solar cells for the products has reached20MW p and there is a great deal of exportation[2].
With so many additional functions being allocated to the inverter,the inverter becomes ever more critical to the system function,and the reliability of current technology inverters becomes a significant issue of concern.This investigation aims to emphasize the importance of the grid-connected PV system regarding the intermittent nature of renewable generation,and the characterization of PV generation with regard to grid code compliance.Also,will focus on the technical requirements for grid interconnection and solving the interconnect problems such as islanding detection,harmonic distortion requirements and electromagnetic interference.
2.Glossary of terms and acronyms
Thefield of power electronics abounds with unfamiliar and ambiguous terminology.The glossary in Table1provides definitions in general u in the PV industry as related to inverters and should help establish a common language for the different types of inverters and the power components ud in
them.Some functions such as the inverter control methods or ties to standards and codes are also defined here[40,41].
The current commercially available inverter hardware ud for uninterruptible power supplies or for remote(short-term)power applications was found to be incompatible with the new requirements of a PV power system[42].Costs were too high and efficiencies were too low.For stand-alone applications,the existing hardware had been designed to interface with small power tools or lighting loads[43].Parameters such as voltage regulation,power quality,high overall efficiency,low tare loss when loads were turned off,and provisions for permanent connections were not available.For utility-interactive applications, it was found that the uninterruptible power supply(UPS)inverters were costly,inefficient,and could not work with the wide input voltage window prented by PV modules.They also lacked controls for MPPT,and they needed extensive modifications for the required wake-up and shut down functions for the diurnal cycles of the PV power source[42].
3.Global PV module and its electrical performance
The production of solar cells has grown at an average annual rate of37%in the past from77.6MWp in1995to 1817.7MWp in2005,and at an average annual rate of45%in the p
ast5years(from287.7MWp in2000to1817.7MWp in2005)[2]. Fig.1shows the production capacity for some countries and regions in the year of2005.
One feature of the global PV industry is that PV-generated electricity is replacing other forms of electricity at an increasingly high rate.This is most evident in the growth-rate for grid-connected electricity,which has become the dominant market for PV-generated electricity as shown in Fig.2.Other applications for PV-generated electricity include communication and signaling, special commercial and industrial applications,rural off-grid systems,consumer u and large-sized power plants not connected to the grid.
The electrical energy produced by a solar cell at any time instant depends on its intrinsic properties and the incoming solar radiation.Details of the solar cell physics can be found in standard texts[47].The algorithm adopted in ESP-r,as described below,was developed as a result of the JOULE project PV-HYBRID-PAS funded by the European Commission and is reported in[48].The diode factor(DF)of a PV array with m number of cells in parallel and n cells in ries is defined as
DF¼
e
kT ref
V mpÀV OC
n
ln
I ScÀI mp
I Sc
!À1
(1)
At a particular cell temperature T c,the un-illuminated currentflow in the p–n junction is then:
I o¼2T cÀT ref=C T
I Sc
1Àexp
eðV OC=nÞ
ref
!À1
(2)
I o is known as the diode current.In the equation,C T is an empirical constant depending on the impact of cell temperature output.This normally carries a value of10for crystalline silicon PV modules,
M.A.Eltawil,Z.Zhao/Renewable and Sustainable Energy Reviews14(2010)112–129 114
Table1
Definitions of some terminologies as they pertain to this paper.
Application specific integrated circuit(ASIC)A highly integrated circuit package containing hundreds o
f logic functions that is modified by burning-away internal paths to produce application specific circuit functions.ASICs are ud to provide designflexibility and to reduce cost and parts count in the control ction of an inverter.
ac PV building block A complete,environmentally protected PV modular system consisting of a PV module,a complete integrated inverter
enclod with a housing eliminating exposure of any dangerous voltage and generally doubling as the module frame or
mounting structure that also enclos all of the necessary ac bus work,interconnects,communication,surge protection
and terminations[44].
ASTM American Standards for Testing Materials.
Azimuth An Azimuth different from1808(south)shifts the theoretical power peak toward east or west according with the
orientation of the PV generator.
Bi-directional inverter An inverter that can be operated in all four quadrants of the voltage/current regime hence may function as an inverter or
as a rectifier by applying the proper drive signals.Powerflow may be in either direction.
Burden The impedance(load)of the circuit connected to the condary winding of an instrumentation transformer.Note:for
voltage transformers it is convenient to express the burden in terms of the equivalent volt-amperes and power factor at a
specified voltage and frequency(from IEEE Std.100-1996)[43].
CHP or micro-CHP Combined heat and power or the micro-combined heat and power.
Converter A general term ud to describe a device for changing direct current power to alternating current power or vi versa or
from one frequency to another.
Current-controlled inverter An inverter designed to convert dc power to ac power where the output current is controlled and unaffected by output
voltagefluctuations.Typically ud in utility-interactive applications where voltage is controlled by the utility. Disconnect switch A switching device that breaks an electrical circuit.The devices may have ac or dc voltage and current ratings and
may or may not be rated for breaking under load.Disconnect switches usually provide a visible break,and may have a
locking feature to provide control over the status of the disconnect switch.
Energy efficiency The ratio of output energy to input energy during an identified period.
Electromagnetic interference or compatibility(EMI/EMC)Generally refers to electromagnetic interference(radio frequencies)produced by a device and electromagnetic compatibility(EMC)of the device.Inverters must not emanate excessive EMI or be susceptible to normal EMI.EMI may be radiated as a radio wave or conducted on the ac and dc lines.
ESL Equivalent ries inductance,a term associated with the inductance associated with the constru
ction and
leads of capacitors.
ESR Equivalent ries resistance,a term associated with the power loss of a capacitor.
ETO Emitter-turn-off thyristor:a new solid-state switch consisting of a thyristor device under development that is configured
to facilitate device turn-off via emitter signals and generally switches faster than the commercial GTOs and can handle
more power than IGBTs.
Field-effect transistor(FET)Field-effect transistor:a solid-state device that us a voltagefield to control the currentflow through it.Devices
ud in today’s inverters are usually metal-oxide-silicon FETs(MOSFETs)and are generally ud when the dc voltage is
less than100V.They can easily be wired in parallel with each other to increa the current/power rating of the inverter. HALT Highly accelerated life tests that are conducted in a manner to reveal component and package layout weakness that
have been related to premature failure mechanisms and mean-time-to-first-failure(MTBF).
IGBT Insulated gate bi-polar transistor:a solid-state switch that combines the advantages of the FET and a bi-polar transistor.
It requires low control power but has the advantages of low loss when in the‘‘on’’state.IGBTs are generally ud when
input voltages are greater than100V.IGBTs have a wide range of capabilities and are now being integrated with
built-in drivers and lf-protection.
Interconnection The equipment and procedures necessary to connect a power generator to the utility grid.IEEE Std.100-1996[43]Def:
the physical plant and equipment required to facilitate the transfer of electric energy between two or more entities.It can
consist of a substation and an associated transmission line and communications facilities or only a simple
electric power feeder.
Inverter A device designed to convert dc power to ac power.Inverters are also commonly referred to as power conditioning
systems and power conditioners in PV applications.Inverters are often referred to as static power converters(SPC)
in standards documents.
Islanding A condition in which a portion of the utility system that contains both load and distributed resources remains energized
while isolated from the remainder of the utility system[41].
Islanding is the electrical phenomenon in a ction of a power network disconnected from the main supply,where the
loads in that disconnected ction are entirely powered by PV systems and where the voltage and frequency are
maintained around nominal values.
At the point of disconnection of an island it is esntial that the active power and reactive power at the point of
disconnection be very clo to zero.The disconnection of the islanding must also happen without introducing a short
circuit between the phas and/or between one pha and ground.Any fault forces the voltage to a very low value and
all PV systems will immediately switch off and islanding will not occur.
Islanding is a balanced condition in a disconnected part of a power network where the load is sustainable powered by
the connected PV systems.A balanced condition of only a few conds is not categorized as a sustainable power
balance.Within the IEA Task V working group a period of5or more conds is treated as a possible islanding.
Line-commutated inverter An inverter designed to be attached to the utility grid or other ac source that requires the switch current to pass
through zero in order to turn the switching devices‘‘off.’’Several versions of small,single-pha,line-commutated
inverters were ud early in the PV program.Line-commutated inverters are still ud for some three-pha
intermediate-sized and all large(>500kW)inverters.
Maximum power point tracker(MPPT)Circuitry associated with utility-interactive inverters(and some larger stand-alone)that continuously adjust the dc operating point to obtain the maximum power available from a PV array at any given time.
Modular inverter An inverter design that is compatible with the paralleling or summing with one or more inverters of the
same or similar design.
MOSFET Metal oxidefield-effect transistor.
MOV Metal oxide varistor,a commonly ud surge suppression device.
MSD Mains monitoring units with allocated all-pole switching devices connected in ries(ENS).
MTBF Mean-time before failure.
Multi-level inverter An inverter using a circuit topology that switches gments of the energy source in and out of the output circuit in
order to synthesize a current sourced low frequency(typically50or60Hz)sine waveform.
NEC National Electrical Code,a publication of the National Fire Protection Association.
M.A.Eltawil,Z.Zhao/Renewable and Sustainable Energy Reviews14(2010)112–129115
Table 1(Continued )NFPA
The National Fire Protection Association,the organization responsible for the National Electrical Code and numerous other installation related codes.
Non-islanding inverter
An inverter defined in IEEE 929as one that will cea to energize the utility line in 10cycles or less when subjected to islanded loads that are >Æ50%mismatch to inverter real-power output and power factor is less than 0.95[45].Alternatively,a disconnection from the line is required within 2s if the load to inverter match is <50%,the power factor is >0.95and the quality factor is 2.5or less.
Point of common coupling
The point at which the electric utility and the customer interface occurs.Typically,this is the customer side of the utility revenue meter.Note :in practice,for building-mounted PV systems (such as residential PV systems)the customer
distribution panel may be considered the PCC.This is for convenience in making measurements and performing testing.PCS
Power conditioning subsystem or power conditioning system (e SPC the IEEE definition associated with inverters)Performance test conditions (PTC)
A fixed t of ambient conditions that constitute the dry-bulb temperature (208C),the in-plane irradiance (1000W/m 2global for flat-plate modules,850W/m 2for concentrators),and wind speed (1m/s)at which electrical performance of the PV system is reported.
Power conditioning unit (PCU)
A device that converts the dc output of a PV array into utility-compatible ac power.The PCU (inverter)may include
(if so equipped)the array maximum power tracker,protection equipment,transformer,and switchgear.See also inverter,power conditioning subsystem (PCS),and static power converter (SPC).Note :the term ‘‘Inverter’’is most commonly ud.PVUSA PVs for utility scale applications.
PWM
Pul width modulated:a method ud in lf-commutated inverters to generate a synthesized waveform
(e.g.a 50-or 60-Hz sine wave)through a combination of varying the duration of time that the switches in a bridge are turned ‘‘on’’and ‘‘off.’’PWM switching frequencies may be constant or vary.PWM offers the advantages of using high-frequency transformers and much smaller filter components.PWM frequencies may range from 5to 100kHz for PV inverters.Many utility-interactive inverters u PWM.RCMU Residual current monitoring unit.
SAD Silicon Avalanche device,a transient surge suppression device.
SBIR
Small business innovative rearch program conducted by veral programs of the U.S.Government.
Self-commutated inverter
An inverter that us switches and controls that may be turned ‘‘on’’or ‘‘off’’at any time.Generally this inverter us a PWM method to generate a synthesized waveform.Self-commutated inverters may be utility-interactive or stand-alone.They may be voltage controlled or current controlled.
Silicon controlled rectifier (SCR)
A miconductor that is a member of the thyristor family.It cannot be switched from ‘‘on’’to ‘‘off’’with gate controls unless current through it pass below a holding threshold (typically through zero).The devices are typically ud in line-commutated inverters.
Supervisory control and data acquisition (SCADA)
Equipment ud to monitor and control power generation,transmission,and distribution equipment (IEEE Std.100).Def.:A system operating with coded signals over communication channels so as to provide control of remote equipment
(using typically one communication channel per remote station).The supervisory system may be combined with a data acquisition system,by adding the u of coded signals over communication channels to acquire information about the status of the remote equipment for display or for recording functions.
Static power converter (SPC)
Terminology ud in some standards for any static power converter with control,protection and filtering functions ud to interface an electric energy source with an electric utility system.Sometime
s referred to as power conditioning subsystem (PCS)or power conditioning units.Typically sold as inverters for PV applications.
Stand-alone inverter (S-A)
An inverter designed to operate with the loads connected directly to its output and independent of any other ac power source.This inverter requires a battery at the input to provide dc voltage regulation and surge currents.
The stand-alone inverter provides frequency and voltage regulation,over current protection and surge capabilities for the loads.The S-A inverter must be a lf-commutated,voltage-controlled inverter so that loads can be operated within their specified voltages.
Stand-by loss For a utility-interactive power conditioner,this is the active and reactive power drawn from the utility grid when the power conditioner is in stand-by mode.
String inverter
An inverter designed to u a single PV string of modules for its input.The ac output of many inverters can be combined and fed to a common transformer.String inverters can be ud to reduce
dc wiring and protection costs and to improve redundancy of a large system.
Standard reporting conditions (SRC)
For PV performance measurements,a fixed t of conditions that constitute the device temperature,the total irradiance,and the reference spectral irradiance distribution to which electrical performance data are translated (e ASTM Std.E 1328).
Standard test conditions (STC)A particular t of SRC defined as 1000W/m 2irradiance,258C cell temperature,and Air Mass 1.5spectrum (e ASTM Std.E 1328).
TEAM-UP Technology experience to accelerate markets in utility PVs.
Thyristor
A term ud for a family of miconductor switching devices characterized by bi-stable switching (either ‘‘on’’or ‘‘off’’)through internal regenerative feedback.Some thyristors can be forced to turn ‘‘off’’but many will turn ‘‘off’’only when current through it falls below a holding current threshold.
Tilt
Higher values of tilt angle usually increa the power production in winter and decrea it in summer.Furthermore,when the sun covers a large path (summer period)a high tilt angle restricts the production curve.When tilt is equal to 908the maximum theoretical visibility of the sun path is limited to 1808.
Transistor (bipolar transistor)
A miconductor device characterized by output current that is dependent upon an input current.They exhibit low forward loss but require more drive power than FETs or IGBTs.Several early inverters ud bi-polar power transistors as switching devices.
TSD Transient surge device sometimes referred to as TSSD or transient surge suppression device.
Utility
For this document,the organization having jurisdiction over the interconnection of the PV system and with whom the owner would enter into an interconnection agreement.This may be a traditional electric utility,a distribution company,or some other organization.IEEE Std.100-1996.Def:An organization responsible for the installation,operation,or maintenance of electric supply or communications systems.
Utility-interactive inverter (U-I)
An inverter designed to be connected to the utility grid or other stable ac source.This inverter does not require dc energy storage and usually incorporates a MPPT to maximize power delivered to the grid.It may
be lf-or line-commutated and may be voltage-or current-controlled.Non-islanding requirements now apply to U-I inverters in the United States,some European countries and in Japan.
VJFET
Vertical-junction field-effect transistor:Generally referring to the physical construction of a field-effect (SiC)device as referred to in this report.
Voltage-controlled inverter
An inverter designed to convert dc power to ac power where the output voltage is controlled.Typically ud in
stand-alone applications since the output voltage must be regulated within the inverter.Voltage contr
olled inverters are also ud as utility-interactive where they employ a line-tie impedance to limit current flow between the inverter and the utility.
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