Arghya Department of Electrical Department of Electrical Department

                                                                        Arghya Mitra                                                      Ghanasyam P                                     Assistant Professor                                               Astha Verma    Department of Electrical                               Department of Electrical                                 Department of Electrical             Engineering                                                    Engineering                                                        Engineering     Visvesvaraya National                                 Visvesvaraya National                                         Visvesvaraya National    Institute of Technology                                  Institute of Technology                                      Institute of Technology          Nagpur, India                                                  Nagpur, India                                                       Nagpur, India     [email protected]

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com         Fault Analysis of Grid Integrated Solar PV system with Adaptive Controller                                                                                                                                                                                                                                                      Abstract- This paper concentrates on faultanalysis of grid integrated photovoltaic (PV) system with varying insolation. Anew topology is proposed in order to reduce the effect of fault in grid andconnected loads. A 50 KW solar plant is designed with maximum power pointtracking (MPPT), DC-DC converter and a 3 phase Voltage source inverter (VSI)with decoupled control. Switching of VSI is designed in such a way that it willnot inject any reactive power during normal condition and inject maximumreactive power during fault condition. Reactive power injection improves RMSvoltage in grid and thereby improves system stability. Simulation is carriedout in PSCAD/EMTDC software and the behavior of proposed system is studied.

 Index Terms—Grid Integrated, Photovoltaic,Fault analysis, solar PV, Power Control, Maximum Power Point. I.     INTRODUCTION  W orldenergy demand is rapidly increasing day by day due to increase in population,industrialization, urbanization etc. The International Energy outlook 2017(IEO2017) states that the world energy consumption will increase up to 28% in2040. Major energy demand is fulfilled by non-renewable sources. Energy fromthese are not clean as they causes pollution and global warming also they aredepleting.  All these reasons made usmore interest in renewable sources like wind, solar, hydro etc. Out of thesesolar energy provides clean and inexhaustible source of energy with no movementor vibration during its operation.

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It will not emit any type of greenhousegases 1 or waste products.                         Nowadaysgrid integrated solar system 2-4 is gaining popularity. Two differentconfiguration of grid connected PV system are single or two stage. Single stageconfiguration use only one power converter so it can provide maximum efficiencyand low cost as compared to two stage configuration 5. In two stageconfiguration, two power converters are used. MPPT technology have beenimplemented in DC side to extract maximum power from PV module 6-7. Perturb& Observe method and Incremental Conductance are more popular 8. Gridconnected system requires an efficient inverter.

Power injected to the gridmust be clean with less harmonics and unity power factor.  The decoupled controller in inverter can improvethe injection of required real and reactive power to the grid 9-10.    Stability analysis isvery important here because the injected solar power can cause a large impactin grid side 11. Analysis of stability of grid integrated PV system isstudied under various fault conditions. During fault condition, the RMS voltagewill fall. This can cause many problems in load side. Here the inverter isconfigured in such a way that it will not inject any reactive power to gridduring normal condition and will inject some reactive power during faultperiod.

Reactive power injection will improves the grid side voltage profileand thereby improving the stability. The study is carried out in a 50KW PVsystem. Simulation is done in PSCAD/EMTDC software.II.     MODELLINGA.     PV Array Solar energy can be converted to electricity using photovoltaiccell or solar cell.

Solar cells are the basic unit of a PV module. It iscomposed of semiconductor material. Voltage and current rating of a PV moduledepends on the number of cells connected in series and parallel respectively.PV array consist of series and parallel connected modules. Required systemvoltage and current depends on number of modules connected.        PV cell can be modelledfrom the equivalent circuit of PV. Figure (1) shows the equivalent circuit ofPV.

An ideal solar cell comes with a current source in parallel with a diode.But in practice a small series resistor and a parallel resistor is present. Figure1: Equivalent circuit of PV cell The equation to describe Voltage-current characteristics is givenas:         (1)     Where IL is the photocurrent and ID is the reversesaturation current of diode.

K represents Boltzmann’s constant, q is the chargeof electron. T is the temperature and ‘?’ is the p-n junction ideality factor.RS and RSH represents the series and shunt equivalentresistance of PV cell. Analysis is done by taking reference insolation at 1000W/m2 and temperature at 25oc. B.     DC-DC Chopper    Since the solar PV voltagedepends upon the incoming solar radiation which is varying, it is veryimportant to use a DC-DC converter to keep the voltage at a constant level.

Here we are using a buck converter to step down the PV voltage. Step downprocess is performed by the combination of mainly four components which are anelectronic switch, diode, inductor and an output capacitor. The circuit diagramof buck converter is shown in figure 2.  When the switch is ON theinput PV voltage comes across the load also inductor stores some energy. Whenthe switch is OFF, the energy stored in the inductor supplies the load. Theoutput equation of buck converter is given by:                                                                        (2)        Where D is the duty ratio.

Figure 2: Circuit Diagram of buck converter  C.     Voltage Source Inverter (VSI) Voltage source inverter is one of the main part of this system.It converts the input DC voltage to 3 phase AC voltage. The main purpose ofinverter is to supply the required active and reactive power to grid. In normalcondition it will inject zero reactive power. VSI consist of semiconductor switches which is triggered according tocontroller.

Here 6 IGBTs are used in three legs. Antiparallel diodes are usedto allow bidirectional current flow. The circuit diagram of VSI is given infigure 3.   Figure 3: 6 pulse converter   The technique used to generate gate pulse is Sinusoidal PulseWidth Modulation (SPWM). The sinusoidal modulating signal is compared with the carriersignal. The carrier is a high frequency triangular signal. If the magnitude ofmodulating signal is greater than carrier signal, the corresponding switch inthe upper leg is triggered.

If not, the corresponding switch in the lowerswitch is triggered. The RMS voltage in the AC side is given by the equation:                                   (3)     Where ma is the modulation index which is given by:                                                             (4) By changing modulation index, we can change the magnitude ofoutput voltage. D.     Grid Integration          Theinverter output is connected to grid through a transformer and LC filter isused for filtering. One important thing while integrating to grid is thesynchronization of voltage magnitude and frequency. It is done by thecontrollers in grid side converter. A step up transformer is used to step upthe voltage to 11 KV.

III.     CONTROLLER A.     MPPT Controller    MPPT controller is usedto extract the maximum power available at the PV module. Here perturb andobserve (P) algorithm is used. It calculates the voltage (VMPP)at which maximum power is obtained.

This voltage is continuously compared with PVvoltage to get perturbation. The MPPT controller governing equation is given by                                        (5) When { & { }          (6){ & { }          (7)                                        (8) When { & { }          (9){ & { }         (10)        Where  is thepresent voltage.  is thereference voltage which is obtained by adding/subtracting a small voltage ?Vbased on the conditions.

 B.     Decoupled Controller 1.     Conventional Controller      Decoupled controller is used in VSI to controlthe power flow between the inverter and grid.

This power can be active orreactive power. Our aim is to provide clean energy with unity power factor togrid. The best method to control P and Q is by decoupled controller. By usingPark’s transformation abc components can be converted to dq0 i.e. three constant DC components which are direct, quadratureand zero components. DC bus voltage (VDC)ensures the active power flow between VSI and grid.

It is maintained to aconstant value VDCref bycontrollers. Reactive power flow is controlled by q axis component of current (Iq). For operating at unity powerfactor, the reference q axis component (Iqref)is set to zero. The voltage equation of circuit between VSI and grid is givenby:                             (11)                     (12) Where Id and Iq are d axis and q axis current, Vd and Vq are d axis and q axis component of terminal voltage.

Rg and Lg are resistance and inductance respectively of the line. Thecontrol structure is given in figure 4:    Figure 4  a: d axis control block                 b: q axis control block         The expression for FF1 and FF2 in figure 4 is given by                                                      (13)                                                                (14) 2.     Proposed Controller                  The mainaim of decoupled controller is to control the flow of required amount of realand reactive power. In conventional controller we set the reactive power flowto zero.

But sometime reactive power can be used to improve the voltageprofile. During fault condition, the terminal voltage in the grid side drops.This can cause major problem in grid side connected loads. These may beself-clearing faults which lasts not more than few milliseconds.In this controller, the grid voltage is sensed and it is comparedwith some predefined values to detect fault. And if any fault is occurred,reactive power is injected to the grid. The amount of reactive power injecteddepends upon the type of fault occurred.

The control structure of proposedcontroller is given in figure 5.    Figure 5: proposed qaxis control block  IV.     SYSTEM UNDER STUDYA 50 KW solar plant is designed in PSCAD/EMTDC and various studieswere conducted. The Solar PV parameters are listed in Table 1. The blockdiagram of system is given in figure 6.

It is observed that the system isfollowing maximum power point tracking (MPPT) technology. A DC-DC buckconverter is used to step down PV voltage from 1000 V to 500 V. The MOSFET isswitching in such a way that it is providing a constant DC voltage in theoutput.   TABLE 1: Solar PV parameters Parameters Values Module SC current 16.1A Module OC voltage 85V Voltage rating of module 50.61V Current rating of module 10A Module power rating 506.1W No of modules in series 20 No of modules in parallel 5 Voltage rating of Array 1012V Current rating of Array 50A Power rating 50.

6 KW Reference Irradiation 1000 W/m2 Reference Temperature 250C Series resistance per cell 0.2? Shunt resistance per cell 1000?     Figure 6: Block diagram of system                       The inverter outputis connected to the grid through a 50 KVA 230V/11KV transformer. A load of50KW+30KVAR is connected with bus. It is observed that the active powerrequired by the load is supplied by PV system and the reactive power requiredis supplied from the grid during constant insolation. Different types of faultswere made in the bus and studies were conducted. Here we are considering singleline to ground (LG) fault during positive and negative slope of insolation. Aself-clearing fault of duration of 50 millisecond is considered.

V.     RESULTS AND DISCUSSIONSSimulation results of the above model is described in thissection. Different cases of fault were analyzed with constant as well asvarying irradiation.  A.       CASE I    Here LG fault isconsidered with constant insolation. Fault is applied after 10 seconds.

It isclear from figure 7(a) that the RMS voltage is improving during fault period.Figure 7(b) shows the amount of reactive power injected during fault. Figure7(c) shows the improvement in the load side.   Figure 7(a): RMS voltage Figure 7(b): P load    Figure 7(a):Q injected                                                 B.       CASE IIHere LG fault is considered during positive slope of insolation.Fault applied after 5 seconds. Figure 8(a), 8(b), 8(c) shows the simulation resultsfor conventional and proposed model.

Here also the improvement is observed.     Figure 7(a):RMS voltage         Figure 7(b): P load             Figure 7(a): Q injected   C.      CASE IIIHere LG fault is considered during negative slope of insolation.Figure 9(a), 9(b), 9(c) shows the simulation results for conventional andproposed model. Fault applied after 21 seconds.  Figure 7(a): RMS voltage                              Figure 7(b): P load             Figure 7(a): Q injected VI.     Conclusion This paper presents theanalysis of various faults that can be occurred in a grid connected PV system.

Controllers like MPPT, Decoupled controllers were modelled successfully. DC-DCchopper and Voltage source inverter were modelled.  Studies were concentrated in LG faults withdifferent conditions of irradiation and a new topology is implemented.

The simulationresults shows that the proposed controller is giving more improved results ascompared to conventional controllers. The modelling and studies were carriedout in PSCAD/EMTDC software. References1    M.

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