Abstract. This paper deals with the proficient reappraisal of power quality jobs associated with the renewable based distributed coevals systems and how the usage power devices like STATCOM, DVR and UPQC play of import function in power quality betterment. IEEE and IEC criterion for the grid connected renewable energy systems are one of the critical point of involvement for the choice of usage power devices. Particular attending has been given on the compensation of reactive power, harmonic current and voltage fluctuation during the interconnected every bit good as islanding manner of operation. As the solar and air current are the most abundant resources hence our point of involvement are limited to PV and Wind energy systems merely.
Renewable Energy Systems, Grid integrating, Power Quality, Custom Power Devices, Distributed Generation
Centralized power coevals systems are facing deficit of fossil fuel and besides fouling the environment. Long transmittal lines are one of the chief causes for electrical power losingss. Therefore attending has been grown on distributed coevals ( DG ) web systems by the integrating of renewable energy system into the grid. With the addition of the renewable energy incursion to the grid, power quality ( PQ ) of the medium to low electromotive force power transmittal system is going a major country of involvement. Now a twenty-four hours, several surveies are accomplished for the integrating of renewable energy systems to the Grid utilizing power electronics convertors. The chief intent of the power electronic convertors is to change over the energy from one phase into another and integrate to the grid with the highest possible efficiency, the lowest cost and to maintain a superior public presentation. But as the inverters ( District of Columbia to ac convertor ) are in general really sensitive to voltage sags / harmonic currents which have a negative consequence on the life-time of the terminal user devices and on the web, hence Refering DG, a electromotive force / current perturbation can do the disjunction of a important sum of energy that could hold a major impact on the web operation.
On the other manus, Custom Power Devicess like STATCOM ( Shunt Active Power Filter ) , DVR ( Series Active Power Filter ) and UPQC ( Combination of series and shunt Active Power Filter ) are the latest development of interfacing devices between distribution supply ( grid ) and consumer contraptions to get the better of the any sort of electromotive force / current perturbations and better the power quality by counterbalancing the reactive and harmonic power generated or absorbed by the burden.
Now a twenty-four hours, Renewable Energy like solar and wind are the most conspicuously utilizing as DG beginnings and twenty-four hours by twenty-four hours perforating degree into the grid is besides increasing. And it has a important impact on the flow of power and electromotive force conditions at the clients and public-service corporation equipment. Though DG impacts include voltage support, variegation of power beginnings, decrease in transmittal and distribution losingss, better dependability [ 1 ] , but on the contrary, awareness on power quality jobs is besides turning up. Therefore this paper deals here with a proficient study on the research and development in the renewable energy based power system that are connected to the grid, sort the PQ jobs related to solar and weave energy integrated to the grid, impact of hapless PQ, indentify the likely connexion topologies of usage power devices into the system to get the better of the PQ jobs and suggest a usage power park for the hereafter grid connected distributed coevals system.
2. Power Quality Issues ( DG )
Though one of the most common jobs related to power quality is wiring and anchoring, about 70 to 80 % of all power quality related jobs can be attributed to faulty connexions and/or wiring [ 2 ] , power frequence perturbations, electro magnetic intervention, transient, harmonics, power factor are the other classs ( shown in table 1 ) that are related with the beginning of supply and types of burden [ 3 ] .
Table 1 – Classs of PQ jobs
Power Freq Disturbance
Electro Magnetic Interventions
Power System Transient
Power System Harmonics
Low Freq phenomena
Produce Voltage sag / crestless wave
High freq phenomena
interaction between electric and magnetic field
Fast, short-duration event
Produce deformation like notch, impulse
Low frequence phenomena
Produce wave form deformation
Current flow with different potencies
Caused by direct current or induced electrostatic field
Low power factor causes equipment harm
Within these phenomena, harmonics are the most ascendant one. Regardless of the burden class, the same cardinal theory can be used to analyze the power quality jobs associated with harmonics. And the effects of Harmonicss on power quality specially described in [ 4 ] . Harmonizing to the IEEE criterion, harmonics in the power system should be limited by two different methods ; one is the bound of harmonic current that a user can shoot into the public-service corporation system at the point of common yoke ( PCC ) and the other is the bound of harmonic electromotive force that the public-service corporation can provide to any client at the PCC. Detailss of these bounds can be found in [ 5 ] . Again, when the DG will be connected to the grid so for the stableness of the connexion some criterion on electromotive force, current every bit good as frequence bound should be followed. These interconnection criterion inside informations are given in [ 6 ] .
3. Grid integrating of Renewable Energy Systems – Power Quality Issues
A Solar Photovoltaic Systems:
Though the end product of PV panel depends on the solar strength and cloud screen, the PQ jobs are non merely depends on irradiation but besides based on the overall public presentation of solar photovoltaic system including PV faculties, inverter, filters commanding mechanism etc. Studies, presented in [ 7 ] , shows that the short fluctuation of irradiance and cloud screen play an of import function for the low-tension distribution grids with high incursion of PV. Therefore particular attending should be paid to the electromotive force profile and the power flow on the line. It besides suggests that Voltage and power extenuation can be done utilizing super capacitance which increases around one fifth of the PV system cost. Voltage crestless wave may besides happen when heavy burden is removed from the connexion. Refering DG, any sort of electromotive force perturbation causes the disjunction of inverter from the grid and therefore it consequences losingss of energy. Besides long term public presentation of grid connected PV system shows a singular debasement of efficiency due to the fluctuation of beginning and public presentation of inverter [ 8 ] .
Fig 2: General construction of grid-connected PV system
The general block diagram of grid connected PV system is shown in fig 2 and the system can be a single-phase or three stage depending on the grid connexion demands. The PV array can be a individual or a twine of PV panels either in series or parallel manner connexion. Centralized or decentralized manner of PV systems can besides be used and the overview of these PV-Inverter-Grid connexion topologies along with their advantages and disadvantages are figured out in [ 9 ] .
These power electronics convertors along with the operation of non-linear contraptions inject harmonics to the grid. On the complete manus electromotive force fluctuation due to the irradiation, cloud screen or shadowing consequence could do the PV system instable in term of grid connexion. Therefore, particular control design is required in the commanding portion of the inverter [ 10-11 ] .
In general, grid-connected PV inverter is non able to command the reactive and harmonic currents drawn from the non-linear tonss. An interesting controlling mechanism has been presented in [ 12 ] where PV system is used as an active filter to counterbalance the reactive and harmonic current every bit good as shooting power to the grid. This system can besides run in base entirely manner. But the overall control circuit becomes slightly more complex. Present research [ 13 ] besides shows that a singular accomplishment has been done on betterment of Inverter control to supply the reactive power compensation and harmonic suppression as accessory services. A multifunctional PV Inverter for grid connected system ( fig 3 ) has been developed late and presented in [ 14 ] shows the dependability through UPS functionality, harmonic compensation, reactive power compensation capableness along with the connexion capableness during the electromotive force sag status. But consequence shows that PQ betterment is still out of IEEE scope.
Fig 3 – Concept of a Multifunctional PV-Inverter System integrated into an industrial grid
B. Wind Energy System:
A simplified diagram stand foring some of the common types of air current energy systems are shown in Fig 4. From the design perspective position it is found that some generators are straight connected to the grid through a dedicated transformer while others incorporate power electronics. Many designs, nevertheless, include some degree of power electronics to better controllability. Whatever the design or connexion constellation, each turbine itself has consequence on the power quality of the transmittal system. Recent analysis and survey [ 15 ] shows that Comparing the consequences of the tower shadow and perpendicular air current shear to the consequences of the horizontal air current shear and swerve mistake, it can be concluded that the impact of the swerve mistake and horizontal air current shear on the power ( torsion ) and voltage oscillations is more terrible than effects due to the tower shadow and perpendicular air current shear.
Fig 4: Different types of air current energy system
A literature study [ 16 ] of the new grid codifications adapted for air current power integrating has identified the jobs of incorporate big sums of air current energy to the electric grid. It suggests that new air current farms must be able to supply electromotive force and reactive control, frequence control and mistake ride-through capableness in order to keep the electric system stableness. Whereas, for the bing air current farms with variable velocity double fed initiation generators and synchronal generators, frequence response into the turbine control system can be incorporated by the package upgradation. Wind farms with fixed velocity generators have to be phased out because they can non offer electromotive force or frequence control. An overview of the developed accountants for the convertor of grid connected system has besides been discussed in [ 17 ] and showed that the double fed initiation generator ( DFIG ) has now the most efficient design for the ordinance of reactive power and the accommodation of angular speed to maximise the end product power efficiency. These generators can besides back up the system during electromotive force droops. However, the drawbacks of converter-based systems are harmonic deformations injected into the system. Bing a single-stage buck-boost inverter, late proposed Z-source inverter ( ZSI ) can be a good campaigner to extenuate the PQ jobs for future DG systems connected to the grid [ 18 ] ( fig 5 ) .
Fig 5 – PMSG-based WECS with District of Columbia encouragement chopper and ZSI
Anti-islanding is one of the of import issues for grid connected DG system. A major challenge for the islanding operation and control strategies is the protection coordination of distribution systems with bidirectional flows of mistake current. This is unlike the conventional over-current protection for radial systems with unidirectional flow of mistake current. Therefore extended research in being carried out and an overview of the bing protection techniques with islanding operation and control, for forestalling disjunction of DGs during loss of grid, has been discussed in [ 19 ] .
4. Impact of Power Quality Problems
The impacts of power quality are normally divided into three wide classs: direct, indirect and societal. A item of these impacts has been described in [ 20 ] .
A recent study based on interviews and web based entry, conducted over a 2-year period in 8 European states, has been reported in [ 21 ] . Survey reported PQ costs due to the consequence of Voltage dips and crestless waves, Short breaks, Long breaks, Harmonics, Surges and transients, Flicker, imbalance, earthing and electromagnetic compatibility ( EMC ) jobs. It is found that the cost of wastage caused by hapless PQ for EU-25 harmonizing to this analysis exceeds ˆ150bn where Industry accounts for over 90 % of this wastage. Dips and short breaks account for about 60 % of the overall cost to industry and 57 % for the entire sample. The survey besides shows that the economic impact of unequal PQ costs industry some 4 % of turnover and services some 0,15 % . Fig 6 shows the PQ costs for the EU-25 states in sector-wise. At the same clip it is necessary to see the impact of DG in footings of the cost of power quality. In [ 22 ] , a method to measure the dip and break costs due to DG into the grid has been proposed. Based on the operating hours, the frequences of PQ events occur and cost of PQ evens indicates the positive or negative impact of DG.
5. Extenuation of PQ jobs
There are two ways to extenuate the power quality jobs – either from the client side or from the public-service corporation side. First attack is called burden conditioning, which ensures that the equipment is less sensitive to power perturbations, leting the operation even under important electromotive force deformation. The other solution is to put in line conditioning systems that suppress or counteracts the power system perturbations. There are several devices like Flywheels, Supercapacitors, Energy storage systems, Changeless Voltage Transformers, Noise Filters, Isolation Transformers, Transient Voltage Surge Suppressors, Harmonic Filters etc are used for the extenuation of specific PQ jobs. Custom power devices ( CPD ) like DSTATCOM, DVR and UPQC are late added in the extenuation of any sort of PQ jobs associated with public-service corporation distribution and the type of terminal user contraptions. Here our particular concern is on CPD and how they play of import function in extenuating PQ jobs due to the renewable energy systems, particularly solar and air current, integrated with the grid.
Fig 6 – Extrapolation of PQ cost to EU economic system in LPQI surveyed sectors
Fig 7 – The cost per event identified by the Survey
6. Role of Custom Power Devicess
Custom Power ( CP ) construct was foremost introduced by N.G. Hingorani in 1995 [ 23 ] . Custom Power embraces a household of power electronic devices, or a tool chest, which is applicable to distribution systems to supply power quality solutions. This engineering has been made possible due to the widespread handiness of cost effectual high power semiconducting material devices such as GTOs and IGBTs, low cost microprocessors and techniques developed in the country of power electronics.
DSTATCOM is a shunt-connected usage power device which primary purposes are power factor rectification, current harmonics filtering, burden DC offset cancellation and burden reconciliation. It can besides be used for electromotive force ordinance at a distribution coach [ 24 ] . It is frequently referred as shunt or parallel active power filter. It can be consist of electromotive force or current beginning PWM convertor depending on the storage component as a District of Columbia capacitance or inductance, Fig. 8. It operates as a current beginning and compensates current harmonics by shooting the harmonic constituents generated by the burden but stage shifted by 180 grades. Furthermore, with an appropriate control strategy, the shunt active power filter can besides counterbalance the burden power factor.
Fig 8 – System constellation of DSTATCOM
DVR is a series-connected usage power device to protect sensitive tonss from supply side perturbations except outages. It can besides move as a series active filter, insulating the beginning from harmonics generated by tonss. It consists of a voltage-source PWM convertor equipped with a District of Columbia capacitance and connected in series with the public-service corporation supply electromotive force through a low base on balls filter ( LPF ) and a matching transformer [ 25 ] , fig 9. This device injects a set of governable Ac electromotive forces in series and synchrony with the distribution feeder electromotive forces such that the load-side electromotive force is restored to the desired amplitude and waveform even when the beginning electromotive force is imbalanced or distorted.
Fig 9 ( a ) : Rectifier supported ( B ) DC capacitance supported DVR
UPQC is the integrating of series and shunt active filters, connected back-to-back on the dc side and sharing a common DC capacitance [ 26 ] , Fig 10. The series constituent of the UPQC is responsible for extenuation of the supply side perturbations: electromotive force sags/swells, spark, electromotive force imbalance and harmonics. It inserts electromotive forces so as to keep the burden voltages at a coveted degree ; balanced and distortion free. The shunt constituent is responsible for extenuating the power quality jobs caused by the consumer: hapless power factor, burden harmonic currents, burden imbalance, DC offset. It injects currents in the Ac system such that the beginning currents become balanced sinusoids and in stage with the beginning electromotive forces.
STATCOM is already reported for air current power applications such as islanding public presentation, stableness sweetening, transient, flicker extenuation of air current generator, etc. [ 27-28 ] . As the traditional STATCOM works merely in taking and dawdling operating manner, its application is hence limited to reactive power support merely. On the other manus, air current is intermittent and stochastic in nature. Therefore the fluctuating power can non be smoothed by utilizing STATCOM, because it has no active power control ability. To get the better of this job, Battery Energy Storage System ( BESS ) has been incorporated with STATCOM ( STATCOM/BESS ) [ 29 ] , which has both existent and reactive power control ability ( fig 11 ) .
Fig. 10: System constellation of UPQC
Fig 11 – STATCOM BES and BR to better power quality and stableness of air current farm
Similarly DVR can besides be used with BESS to command the reactive and active power flow with harmonic electromotive force extenuation during the grid connected to the distributed coevals system [ 30 ] , fig 12.
Fig 12 – Power quality control utilizing DVR and BESS
Very recent research studies that a important research and development has been done on UPQC execution to the grid connected PV or Wind energy systems [ 31-32 ] . As the UPQC has about all the extenuation belongingss of bing PQ jobs in the transmittal and distribution grid along with the counterbalancing harmonics and reactive power generated by the burden hence arrangement of UPQC in distributed coevals web is really of import.
A construction has been proposed in [ 31 ] , fig 13, where PV is connected to DC nexus in UPQC as an energy beginning. It works both in interconnected and islanded manner. UPQC has the ability to shoot power utilizing PV to sensitive burden during beginning electromotive force break. Advantage of this system is voltage break compensation and active power injection to grid in add-on to the other normal UPQC abilities. It has the higher efficiency and working ability in comparison with other common grid-connected PV systems and besides causes a decrease of entire system cost. But the system ‘s functionality may fall down if adequate Sun is non available during the electromotive force break status.
Fig 13: UPQC with grid connected PV
Similarly the application of a incorporate power quality conditioner ( UPQC ) to get the better of the grid integrating jobs of the FSIG is investigated in [ 32 ] , fig 14. As the ability of air current generator connected to the grid influenced by the in the event of system mistakes and dynamic reactive power compensation and the air current driven, fixed-speed initiation generator ( FSIG ) on its ain fails to carry through these demands therefore The function of the UPQC in heightening the mistake ride-through capableness of the generator has been investigated under both full and partial terminal electromotive force Restoration. Consequence shows UPQC as a premier contestant for the application of PQ extenuation in grid connected wind energy system.
Fig 14: Grid connected wind energy system with UPQC
Fig 15 – Assorted electromotive force, velocity and power responses to a three-phase mistake with UPQC
Another interesting thought was proposed in [ 33 – 34 ] to better the power quality in a power portion utilizing the usage power devices and it has been extended more by utilizing supervisory control technique to organize the usage power devices by turn outing the pre-specified quality of power [ 35 ] , fig 18.
Recent tendencies in the power coevals and distribution system shows that incursion degree of DG into the grid increasing quickly. End user contraptions are going more sophisticated to the power quality status. Extensive Research on commanding of the Custom power devices for the extenuation of PQ jobs are besides transporting out. Therefore now more concentration should be given on the arrangement of CPDs in between conventional grid, GD and user contraptions for the PQ betterment of overall distribution webs.
Fig 18: Single line diagram of CPP