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1. INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 3, October – December (2012), © IAEME & TECHNOLOGY (IJEET)ISSN 0976 – 6545(Print)ISSN 0976 – 6553(Online)Volume 3, Issue 3, October - December (2012), pp. 164-174 IJEET© IAEME: www.iaeme.com/ijeet.aspJournal Impact Factor (2012): 3.2031 (Calculated by GISI) ©IAEMEwww.jifactor.com A MODERN TWO DOF CONTROLLER FOR GRID INTEGRATION WITH SOLAR POWER GENERATOR Sweeka Meshram1, Ganga Agnihotri2, Sushma Gupta3 1 (Deptt. Of Electrical Engg., MANIT Bhopal, 462051, India, email@example.com) 2 (Deptt. Of Electrical Engg., MANIT Bhopal,462051, India, firstname.lastname@example.org) 3 (Deptt. Of Electrical Engg., MANIT Bhopal,462051, India, email@example.com) ABSTRACT This paper presents the design, analysis and implementation of the power grid integration with the solar power generator using the two Degree Of Freedom (DOF) controller. Micro grid distribution generation (DG) systems usually have inverters and these interfacing inverter are directly connected to the power grid through passive filter. In this paper, a two DOF controller is employed for controlling the DG inverter, which behaves as an uninterruptible power supply (UPS) for its local community loads. The two DOF controller is capable of connecting/disconnecting the power grid with the variation in the solar power generation and load. The proposed system is a reliable because it is capable of providing continuous smooth power flow with control. The interfacing inverter must have the capability to regulate the AC bus voltage at the occurrence of nonlinearities and irregular natures of the loads. To meet this requirement a fast two DOF controller is applied to the proposed system. The effectiveness of the grid connected solar power generator system and adopted control technique is verified. Keywords: Power grid, Solar System, Two DOF Controller, PLL. I. INTRODUCTION With the increasing electricity demand, it is very necessary to promote the new form of power generation using the renewable energy such as wind, solar and fuel cell. The power generation systems generating the electricity from many small energy sources are known as the Distributed Generation (DG) system. Most countries generate electricity in large centralized facilities, such as fossil fuel (coal, gas powered), nuclear, large solar power plants or hydropower plants. These plants have excellent economies of scale and allow collection of energy from many sources and may give lower environmental impacts and improved security of supply. Among the DG systems, the photovoltaic (PV) technology based DG is gaining approval as an approach of maintaining and civilizing living standards without harming the environment. Fig. 1 shows the PV installed capacity in the world. The PV installation has an 164
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN0976 – 6553(Online) Volume 3, Issue 3, October – December (2012), © IAEMEexponential growth because the many grid connected PV system is supported by thegovernment and private companies. Fig. 1 Installed PV Power Capacity The power generation from the solar energy has two major tribulations. One problemis the PV cell has very limited power generation capability. The series and parallelcombination of the PV cell/module may not generate the sufficient power required for theapplication. Another problem is power generation is not possible when solar irradiation is notavailable such as in the night and in the rainy days. Thus continuous power flow is notpossible when using the PV system in standalone mode. This problem can be solved byconnecting utility grid in parallel with the PV generator. For the parallel operation of the gridand PV generator the frequency and amplitude of voltage of both the system should be sameand the generated voltage of the PV generator should be in phase with the grid voltage. Butwith the variation in the load the amplitude/frequency of grid voltage may be disturbed. Tovary the PV generator voltage according to the grid voltage variation, an interfacing device isrequired. In the grid connected DG system an inverter works as an interfacing device. Forgenerating the switching pulses for the DG inverter, a controller is required. This controllertracks the phase angle of the three phase grid voltage and generates the switching pulses suchthat the inverter will output the voltage waveform which in phase with the grid voltage. Aphase tracking system has been developed for tracking the phase angle of the grid voltage .A new green power inverter for interfacing the fuel cell with the grid has been developed .As the advancement is being continuously in power electronics, it is suggested that theinverters based on the power electronics devices are efficient for interfacing purpose . Anovel vector control system using deadbeat-controlled PWM inverter with LC filter has beendeveloped . Using the LCL filter, a modified direct power control strategy has beendeveloped for connecting the inverter to the grid . A two DOF controller is also developedfor controlling the DG inverter for the parallel operation of the fuel cell with grid . Theperformance analysis of these systems shows that these systems have harmonics andtherefore an adjustable harmonic mitigation technique for harmonic reduction of thephotovoltaic system utilizing the surplus capacity of the interactive inverter has beendeveloped . The modeling, analysis and testing of the inverter based micro grid system hasbeen done . The two DOF controller is the best controller because it can control the powerflow bi-directionally to/from the grid and enables the smooth parallel operation of the gridwith the DG [9-12]. In this paper, analysis of the grid connected PV generator based DG system using thetwo DOF controller been done. In this paper, a DG inverter is designed, which can performthe task of the Uninterruptible Power Supply (UPS). The solar power based DG system is 165
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN0976 – 6553(Online) Volume 3, Issue 3, October – December (2012), © IAEMEdesigned for feeding the particular local load. In the rainy days/night, the solar powergenerator is not capable to supply the required power demand. In that case, the power grid isconnected for meeting up the power demand. When the power demand is greater than thesolar power generation, then the grid is connected automatically using the two DOFcontroller. Thus continuous power supply is possible for supplying to the local load.II. STRUCTURE OF PV SYSTEM BASED DPGS The structure of the Distributed Power Generation System (DPGS) based on the PVsystem is shown in the fig. 2. The maximum power is extracted using the MPPT and stored inthe battery. The MPPT uses the Incremental Inductance technique. The task of the powertransformation from the PV system to the utility grid is carried out using the powerconversion unit. The power conversion unit is the combination of DC-DC converter andPWM AC inverter. The stored DC power (generated by the PV array) in the battery energystorage system is not able feed directly to the load/grid. The DC/DC boost converter is usedfor increasing the level of the DC voltage up to the level reliable for synchronizing the grid.The three level AC inverter works as a grid interfacing device and able to generate the ACvoltage matches the grid voltage level with reduced harmonics. The generated AC electricitynow can be fed to the local load or to the grid. Fig. 2 Structure of the PV System based DPGS The most important parts of the proposed system are the controllers. There are two typesof the controller is adopted for the purpose of controlling the DPGS i.e. PV generator sidecontroller and grid side controller. a) PV side Controller: The task of this controller is to extract the maximum generated power from the solar renewable energy sources. The protection of the PV side converter is also handled by this controller. b) Grid side Controller: This controller performs the many functions such as controls the PV generated active power which is fed to the grid, controls the reactive power transfer between DPGS and grid, controls the DC link voltage of the PV array, enhances the power quality and the main function is to synchronize the grid with the PV generator .III. TWO DEGREE OF FREEDOM CONTROLLER The grid connected Solar Power Generator must have the high control bandwidth formaintaining its AC voltage undistorted during load changeover. The two DOF controllerprovides the high control bandwidth with reliable operation. The grid voltage is taken asreference frame for phase synchronization of the PV generator with the grid. The controlling 166
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN0976 – 6553(Online) Volume 3, Issue 3, October – December (2012), © IAEMEof the DG inverter is a necessary part of the system and it has been done using the PLL(Phase Locked Loop). With the variation in the load and due to the harmonic component, thegrid voltage is usually distorted. If this distorted grid voltage is considered as a reference forthe synchronization, then the power quality of the local AC bus of the PV generator may bepoor. The quality may be improved using the PLL. The PLL calculates the phase angle (θ) ofthe grid voltage (VG_abc). The PLL is generally a frequency controller. The three phase gridvoltage and capacitor voltage (VC_abc) across the filter capacitor is converted into two phasequantity. The d-axis component of the capacitor voltage (VCd) is compared with the d-axiscomponent of the grid voltage (VGd) and the error signal is processed with the gain (Gm)which is equal to 1 for the designed system. The output is then processed with the inversedynamic [G-1(s) = Lf.Cf + 1] and it is also compared to VCd. The error is processed with the PIcontroller. The output of the PI controller and inverse dynamic gain is then added. The VCd isalso processed with the ω block and the output is added to ∆Id. Fig. 3 Block Diagram of Two DOF Controller for DG Inverter Similar process is adopted for obtaining the ∆Iq. These two quantities i.e. ∆Id and ∆Iq isthen converted into three phase and considered as a control signal for generating the PWMswitching pulses for DG inverter. In the tracking performance, the two DOF controller isbetter than the conventional PI controller. Table.1 shows the comparison between theconventional PI controller and the new Two DOF Controller. TABLE.1 COMPARISON BETWEEN PI CONTROLLER AND 2 DOF CONTROLLER Sr. Two DOF Controller Conventional PI Controller No. 1. It Implies fast controlling of power. It has slow power controlling capacity. 2. It has larger bandwidth. It has smaller bandwidth. 3. It has smaller phase lag. It has larger phase lag. 4. Managing of power to/from grid is It is less efficient as compared to the 2 efficient using this controller. DOF Controller. 5. Able to prevent from an excessive With this controller, a large circulating circulating current. current flows from grid. 6. It has fast disturbance rejection capability. It has no disturbance rejection capability. 167
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN0976 – 6553(Online) Volume 3, Issue 3, October – December (2012), © IAEMEIV. CONTROLLING OF POWER FLOW The two DOF controller is able for grid synchronization as well as helps DG inverter towork as a uninterruptible power supply (UPS). Fig. 4 shows the power circuit diagram inwhich PV generator is connected with grid through the inductor LG. The grid voltage (VG)and grid current (IG) is considered as a reference for controlling the DG inverter. It isconsidered as the grid has unity power factor i.e. VG ∠0. Fig. 4 Power Circuit of Grid Connected PV Generator The phasor diagram of the DG system circuit is shown in the fig. 5. When the real poweris supplied to the grid from the PV generator the VC is leading VG by an angle δ which isgreater than zero. The voltage across the grid inductor VLG is having 900 angle to gridvoltage. The real power flow from PV generator to the grid can be mathematically expressedas: ܸீ . ܸ ܸீ . ܸ ܲ= sin ߜ = ߱ீܮ ߱ீܮ The power transfer depends on the angle δ. If δ is greater than 0, power flows from PVgenerator to the grid. If δ< 0, PV generator receives power from the grid and if δ = 0, thenthere will be dynamic isolation between PV generator and grid. Fig. 5 (a) Fig. 5(b) Fig. 5 (a) Vectors when DG Supplies Real Power to Grid, (b) Vectors when DG supplies Reactive Power to Grid For transmitting the reactive power VG and VC should be in phase and the grid current IGwill have 900 to VG. The transmitting VAR is given as: ଶ ܸ ܸ . ܸீ ܳ= − ߱ீܮ ߱ீܮV. RESULTS AND DISCUSSION A 25 kV, 2500 MVA power grid is connected to 100 kW generating PV System. Thearray consists of 66 strings of 5 series connected PV modules connected in parallel. One PVmodule has 96 PV cells. The PV array is delivering the 100 kW at 1000 W/m2 at maximumpower. The open circuit voltage (VOC) of the one module is 64.2 V and voltage at themaximum power point (Vmp) is 54.7 V. The short circuit current (ISC) of the one module is 168
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN0976 – 6553(Online) Volume 3, Issue 3, October – December (2012), © IAEME5.96 A and current at the maximum power point (Imp)is 5.58 A. At 1000 W/m2, solar array isgenerating 400 V DC voltage and 321 A DC current. The generated DC voltage is convertedusing the 3 level DC/AC inverter. The switching frequency of the inverter is 3 kHz. Fig. 6 (a)shows the output voltage of the solar inverter before filtering and fig. 6 (b) shows theenlarged waveform of the solar inverter output voltage before filtering. Fig. 6 (a) Solar Inverter Output Voltage before filtering Fig. 6 (b) Solar Inverter Output Voltage before filtering For making the inverter output voltage into pure sinusoidal AC voltage a LC filter is used.The numeric values of the filter inductance (LF) and capacitance (CF) are 20 mH and 350 µF[12,13]. The resonance problem is avoided using the damping resistance RD , connected inseries with the filter capacitor and valued as 8 Ω. The damping resistor is able to absorb theswitching frequency ripple and the LC filter gives the pure sinusoidal AC voltage with fewerharmonic. Fig. 7 (a) shows the solar inverter output voltage after filtering using the LC filterand fig. 7 (b) shows the enlarged waveform of solar inverter output voltage after filtering. Fig. 7 (a) Solar Inverter Output Voltage after filtering Fig. 7 (b) Solar Inverter Output Voltage after filtering 169
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN0976 – 6553(Online) Volume 3, Issue 3, October – December (2012), © IAEME For analyzing the performance of the grid connected solar system with thenew fast two DOF controller variable load is connected. At t = 0 sec., the SolarSystem is able to supply the power to community load of about 18 kW, 5 kVARat 0.9 lagging PF. The grid connection to the solar system depends on powerdemand of the local load. At t = 0.5 sec. a step load of about 15 kW, 5 kVAR at0.9 lagging PF is connected. The solar system is able to supply power to thisextra load, hence grid is connected to the solar system and ensures thecontinuous power with the variation in the load. At t = 0.8 sec. again load isincreased by 12 kW, 4 kVAR at 0.9 lagging PF and results show that the systemis also able to supply this load also. At t = 1.2 sec. load of about 12 kW, 4kVAR is disconnected. At t = 1.5 sec. 15 kW, 5 kVAR load is alsodisconnected. Now the solar system is able to supply the power to the remainingload, therefore the grid will be disconnected automatically. Fig. 8 (a) and fig. 8 (b) shows the contribution of the active and reactivepower of the power grid respectively according to the variation in the load.From 0 to 0.5 sec. and 1.5 to 2 sec. the solar system operates as a standalonesystem and during those periods the grid does not supply power to the solarsystem. Fig. 8 (a) Active Power Variation of Grid Fig. 8 (b) Reactive Power Variation of Grid 170
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN0976 – 6553(Online) Volume 3, Issue 3, October – December (2012), © IAEME Fig. 9 (a) and fig. 9 (b) shows the involvement of the active and reactive power of theSolar System with the variation in the load. Fig. 9 (a) Active Power Variation of Solar Inverter Fig. 9 (b) Reactive Power Variation of Solar Inverter Fig. 10 shows the waveform of load voltage. Fig. 10 Load Voltage Fig. 11(a) shows the waveform of the current through the load. From 0 sec. to 0.5 sec., solarsystem supplies power to the load of about 18 kW, 5 kVAR. At this load the peak value ofthe load current is 31.2 A. At 0.5 sec. load is increased and grid is connected. With thevariation in the load the peak value of the load current reached to 62.1 A. Fig. 11 (a) Load Current due to step change in load and grid connection 171
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN0976 – 6553(Online) Volume 3, Issue 3, October – December (2012), © IAEME Fig. 11 (b) shows the load current, when the load is again increased at t = 0.8 sec. by astep of about 12 kW, 4 kVAR. The peak load current at t = 0.8 sec has been reached to 82.7A. Fig. 11 (b) Load Current due to again step change in load From t = 1.2 sec the disconnection of the load is started. At t = 1.2 sec. 12 kW, 4 kVAR isdisconnected and the load current regain the peak value of load current of about 62.1 A. Fig.12 (a) shows the variation in the load current due to step change in the load. Fig. 12 (a) Load Current due to step load disconnection Fig. 12 (b) shows the load current when the load is of about 18 kW, 5 kVAR and the gridis disconnected. During this period the Solar System is again operating in standalone mode. Fig. 12 (b) Load Current due to step load and grid disconnection 172
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN0976 – 6553(Online) Volume 3, Issue 3, October – December (2012), © IAEME Fig. 15 (a) shows the phase angle of the grid voltage. According to this phase angle PLLgenerates the PLL output as shown in the fig. 15 (b). Using this output, the two DOFcontroller generates the switching pulses for the solar inverter such that the solar inverteroutput the voltage waveform which is in phase with the power grid voltage. Fig. 15 (a) Phase Angle of the Grid Voltage Fig. 15 (b) PLL Output The variation of the load is taken into consideration to prove that the two DOF controller isefficient and can control very fast with the variation in the load. With this controller connection anddisconnection of the power grid with the Solar System is possible with good performance.VI. CONCLUSION In this paper, performance analysis of the grid connected PV generator based DG system usingTwo DOF controller has been done. The Two DOF controller enabled the control of power flow bi-directionally to and from the grid and smooth parallel operation between the PV generator based DGsystem and grid. This controller also enables the regulated inverter output voltage and trying to keepconstant amplitude and frequency of the grid voltage with the variation in the linear and nonlinearload. The system is analyzed under the grid connected and stand alone mode. Initially the PVgenerator is considered as a grid connected. When the PV generator voltage is equal to the gridvoltage i.e. (VC =VG), the grid will automatically disconnect from the DG and PV generator operatesin the isolation mode. Depending upon the VC, grid will receive/supply the power from the PVgenerator. The results verify the operation of the adopted system for the grid connected/disconnectedmode and it is apparent that the system is able to put into the service feeding the load.REFERENCES S.-K. Chung, A phase tracking system for three phase utility interface inverters, IEEETransaction on Power Electronics, 15(3), 2000, 431–438, May 2000. G. K. Andersen, C. Klumpner, S. B. Kjaer, and F. Blaabjerg, A new green powerinverter for fuel cells, Proceeding on IEEE PESC, 2, 2002, 727–733. F. Blaabjerg, Z. Chen, and S. Kjaer, Power electronics as efficient interface indispersed power generation systems, IEEE Transaction on Power Electronics, 19(5), 2004,1184–1194. 173
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