* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Fig.1. Schematic of the circuit model designed to simulate PV system
Stepper motor wikipedia , lookup
Ground (electricity) wikipedia , lookup
Audio power wikipedia , lookup
Electrical ballast wikipedia , lookup
Mercury-arc valve wikipedia , lookup
Electric power system wikipedia , lookup
Power factor wikipedia , lookup
Electrification wikipedia , lookup
Power MOSFET wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Pulse-width modulation wikipedia , lookup
Electrical substation wikipedia , lookup
Amtrak's 25 Hz traction power system wikipedia , lookup
Solar micro-inverter wikipedia , lookup
Current source wikipedia , lookup
Voltage regulator wikipedia , lookup
Stray voltage wikipedia , lookup
Opto-isolator wikipedia , lookup
History of electric power transmission wikipedia , lookup
Power engineering wikipedia , lookup
Surge protector wikipedia , lookup
Three-phase electric power wikipedia , lookup
Buck converter wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Voltage optimisation wikipedia , lookup
Power inverter wikipedia , lookup
Mains electricity wikipedia , lookup
Harmonic Analysis in A Selected Distributed Generator Devices PRADIPTA KUMAR TRIPATHY DURGESH MANJURE ELHAM B. MAKRAM Clemson University Clemson University Clemson University 303 Riggs Hall, Clemson University, Clemson, SC 29634, USA Abstract: In recent days photovoltaics (PVs) have come up as viable alternatives to conventional sources of energy production. Depending upon their size, PVs can operate as grid-connected or as stand-alone devices. PVs invariably involve a power-conditioning system, which consists of power electronic devices like converters/inverters. These devices are highly nonlinear and distort the current/voltage waveforms to a significant extent. Power quality issues are of prime concern to the customer and to the utility. Thus under this scenario, it has become imperative to study the characteristics of the PV system in detail. This paper presents results that seek to quantify the impact of the PV system, as one of the selected DG devices, on distribution system harmonics. Many residential loads are inherently nonlinear, and results of the interaction of the harmonics injected by these loads and those introduced by the PV system are also presented. Circuit models for various typical residential (nonlinear) loads and for the PV system have been created, and simulations have been carried out in time domain. Results are presented for different cases, and are succeeded by a brief analysis. 1 Introduction Interest in PV energy dates back to 1954, when the first PV system was designed. PV energy is often referred to as clean energy, because it is environment-friendly and doesn’t cause pollution. The recent developments in new low cost PV material, and the low manufacturing cost of PV cells have increased the usage of PV source for energy production throughout the world [1]. Deregulation of power system has allowed individual power producers to produce power autonomously for their own usage and for commercial sale in the energy market. For this purpose many power producers are installing PV power stations. These PV systems use a number of PV cells in series and parallel combinations to produce DC power from the absorbed solar power. Inverters are used to convert the DC power into usable AC power. PV systems can produce power ranging from small generating units of 5 KW to large installations of 0.1 MW. These PV systems can operate as stand-alone units supplying power to the residential loads. If the power generated is in excess, then it can be fed back to the AC grid. Due to the use of an inverter, the PV system draws nonlinear current thus resulting in distortion of the current and voltage waveforms and injection of harmonics into the system. directly and indirectly in various electrical equipments, usually used in residences. Household loads such as refrigerators, television sets, computers and heat pumps use power supply units with rectifiers as a part of the circuit. Another common household load, the compact fluorescent light (CFL) uses electronic ballasts which use rectifiers to input power to an inverter, which supplies the necessary AC power to the CFL [4,5]. Thus CFLs also create harmonic distortion. Recently more and more heat pumps with adjustable speed drives (ASD) are being installed as they have the advantage of better efficiency and better performance over the conventional ones [6]. These ASDs use single-phase bridge rectifiers to convert the AC power to DC. A three-phase six-pulse inverter converts this DC to AC and supplies to a three-phase induction motor that drives the fan load. All these loads draw nonlinear current and thus are sources of harmonic distortion. Harmonics injected into the grid cause severe problems like damaging the electrical equipments, interference with power system protection, causing load unbalance and increasing losses [7]. It has been seen that harmonic distortion in current doesn’t affect the load as much as harmonic distortion in voltage. Harmonics in the voltage impressed at the terminal of the equipment affect the equipment adversely [2, 3]. The problem of harmonics contributes significantly to the power quality issues and it needs to be given careful attention. In order to do so, it has become imperative to investigate the exact cause and nature of harmonics, their quantity and their interaction with the power system. This necessitates accurate modeling [8-10] of the power system components and study of power system behavior with respect to harmonics. This paper presents the study and analysis of harmonics in the presence of a PV system. At the outset, modeling of the PV system and a residential system has been discussed. The residential load also being a source of harmonics, studying the interaction of harmonics generated by the residential load and those injected by the PV system is one of the major aspects. Subsequently, the study results and the analysis of the harmonic distortion and the problems caused by the connection of PV system with the residential system have been presented. 2. PV system description and modeling The major contribution to the harmonic injection in the distribution system is due to the increased use of power electronics devices [2, 3]. These devices are being used A circuit model of PV system is simulated using PSCAD software as shown in Fig. 1. The PV system consists of a set of solar cells in series and parallel combinations, which produce DC power. While performing the computer simulation a DC voltage source (Vpv) was used for simulating the PV cell. The DC power is fed to a single-phase, full wave, line-commutated inverter to produce alternating sinusoidal voltage and current. The houses are connected through a common bus to the distribution system. Each house includes six compact fluorescent lights, each rated at 100 W, a 100 W TV load and a 150 W incandescent lighting load. The residential load also comprises of a 2 HP heat pump load. The main components of the circuit are: 1. Thyristors - T1, T2, T3, T4 2. Feedback diodes - D1, D2, D3, D4 T1 D1 T3 House1 L C D4 T2 D2 Isolation Transformer Filter Fig.1. Schematic of the circuit model designed to simulate PV system A control circuit is designed to periodically trigger the thyristors into conduction by a pulse train. The control circuit consists of a Voltage Controlled Oscillator, which provides the reference ramp for generating the firing pulse train. Frequency of the output wave is thus decided by the frequency of the firing pulses. The feedback diodes conduct when the voltage and current are of opposite polarities. The output voltage is alternating pulses of controlled width. The pulse width is modulated here by adding two square wave voltages, which are shifted in phase with respect to each other. Thus the inverter output voltage is smoothly adjusted from zero to a maximum by either phase advancing or phase retarding the control signals of one pair of diodes with respect to the other. This method of pulse control significantly reduces the harmonic contents in the output voltage and current [11, 12]. A transformer is used at the inverter output which isolates the potentials of the PV array and the grid and prevents supplying DC current to the grid. The harmonic distortion is further reduced by connecting a single-tuned filter [2, 13] at the output of the inverter. As per IEEE standard 519, THD, in voltage should not exceed 5 % in primary distribution lines or 8% on secondary lines [3]. 3. Residential modeling system House4 House3 Fig.2. Schematic of the residential system D3 Vpv T4 House2 description and A residential system [14] is developed using the standard library elements available in PSCAD software [15]. Fig. 2 shows a residential system with four houses. The CFLs use electronic ballast, which contains a diode bridge converter circuit, which in turn supplies power to an inverter. This inverter supplies the usable AC to the CFL [5]. Converters are also used in power supply units in electronic equipment like TV. The waveform for CFL is assumed to be the same as the waveform of TV load [14]. To model the TV/CFL load a single-phase rectifier is designed. Resistor R1, inductor L1 and capacitor C are connected to the rectifier circuit as shown in Fig. 3. The load is connected to the output of the rectifier circuit. By adjusting the parameters R1, L1 and C of the circuit to match the measured harmonic contents of a CFL and TV [14], the model for CFL and TV is designed. The typical parameter values for the CFL and TV are: R1 = 3.5 ohm L1 = 0.003 H C = 250 microfarad R1 L1 C 1-¢ AC Input Load 1-¢ Rectifier Fig.3. Schematic of the circuit used for simulating CFL load and TV load The heat pump model is designed as shown in Fig. 4. The single-phase rectifier is fed from the AC source and the output DC voltage of the rectifier is converted to AC voltage by the three-phase inverter. The inverter feeds to the three-phase induction motor. 1-¢ AC Input 1-¢ Rectifier 3-¢ Inverter 3-¢ Induction motor Fig.4. Schematic of the heat pump circuit The use of inverter and converter circuits makes CFLs, TVs and heat pumps sources of harmonics. 4. Simulation Results and Discussion The system shown in Fig. 5 is a 12.47 kV radial distribution system. Two transformers step the voltage down to 4.16 kV and then to 208 V. The residential system is connected to one of the phases, as shown, at 120 V. The system is considered as balanced. This system is used for various case studies performed to analyze harmonic distortion. The analysis is done in time domain using PSCAD software. Primary current Primary voltage 12.47 kV/4.16 kV 4.16 kV/0.208 kV Single phase loads 12.47 kV Substation Fig. 6 shows the waveforms and harmonic spectra for voltage and current on the load side. It was seen that the odd harmonic components are dominant in voltage and current waveforms. Specifically the 3rd and 5th harmonic components are dominant in the load voltage and amount to 13.5 % and 8.1 % respectively. THD in voltage was found to be 27.1 % and that in current was found to be 24.1 %. One reason for the high value of THD in voltage and current could be the considerable amount of distortion created due to the nonlinear loads present in each individual house (voltage distortion was found to be 3.5 % due to a single house). Thus, the harmonics created by all the houses add up and the total harmonic distortion from the four houses exceeds the limit. PV system also supplies nonlinear current due to the internal inverter circuit and thus causes harmonic distortion. Load voltage Three phase loads Load current Load voltage House3 House4 Magnitude(amperes) House2 Magnitude(volts) Line reactor House1 100 50 0 -50 -100 Residential loads 40 20 0 -20 -40 -60 -150 4.82 PV System Load current 60 150 4.84 4.86 4.88 4.9 4.83 4.84 4.85 4.86 4.87 4.88 4.89 4.9 Time(seconds) Isolation transformer Time(seconds) Harmonic distortion in load voltage 30 25 25 20 Magnitude(%) Following case studies are done to investigate the harmonic distortion: Magnitude(%) Fig.5. Schematic of test system used for the simulations Harmonic distortion in load current 20 15 10 5 Cases 2 and 3 are compared to: (i) (ii) Investigate the harmonic injection into the AC distribution system by the connection of DG and Investigate the change in harmonic distortion in the residential system load current and load voltage. 0 5 10 15 20 25 30 10 5 0 0 5 10 Harmonic Order 15 20 25 30 Harmonic Order Fig.6. Load current and load voltage distortion for case 1 B) Analysis and comparison of distortion when PV system is connected to the AC grid in parallel with the residential system load Figs. 7 and 8 show the comparison of the load voltage distortion and load current distortion when the residential system is fed by the distribution system and PV system simultaneously. Load voltage Load voltage with PV 10 9 8 Magnitude(%) 1) In case 1, the PV system is used as a stand-alone unit supplying power to the residential system. A singletuned filter is used to reduce the harmonics at the inverter output, before it is fed to the residential system. 2) In case 2, the residential load is fed from the distribution system only. 3) In case 3, the PV system is connected to the AC grid through a line reactor and residential load is fed simultaneously by the two sources; the AC grid and the PV system. 0 15 7 6 5 4 A graphical comparison of the case studies has been presented here onwards. FFT is applied to a single cycle of the waveforms and the harmonic spectrum thus obtained has been plotted. 3 2 1 0 3 A) Analysis of distortion in the load voltage and current in the residential system due to PV system alone 5 7 9 11 13 15 17 Harmonic order 19 21 23 25 Fig.7. Harmonic spectrum of load voltage for cases 2 and 3 Load current Load current with PV 45 4 40 3.5 35 30 25 20 Primary Voltage Primary Voltage with PV 4.5 Magnitude(%) Magnitude (%) 50 3 2.5 2 1.5 15 1 10 0.5 5 0 0 3 5 7 9 11 13 15 17 Harmonic order 19 21 23 Fig.8. Harmonic spectrum of load current for cases 2 and 3 The plots show a decrease in the magnitude of the individual harmonic components in the presence of PV. THD for load current was found to be 49.3 %, when fed by the distribution system only. However, it decreased to 44.14 % when PV system is also connected to the system. In this case, the PV inverter acts as a harmonic sink rather than a source of harmonic generation. So, the current distortion decreased. However the current distortion didn’t decrease to a great extent because of the higher amount of distortion caused due to the nonlinear loads present in the houses in comparison to the harmonic distortion caused due to the PV inverter. It was found that voltage distortion at the load went down from 11.9 % to 7.2 %. Figs. 9 and 10 show the harmonic spectra for current and voltage on the high voltage side of the transformer when PV system is connected with the distribution grid. It was seen that THD in the primary injected current is 38.4 % Primary current Primary current with DG 40 35 Magnitude(%) 30 25 20 15 10 5 0 3 5 7 9 11 13 15 17 Harmonic order 19 21 23 3 5 7 25 25 Fig.9. Harmonic spectrum of primary current for cases 2 and 3 9 11 13 15 17 Harmonic order 19 21 23 25 Fig. 10 Harmonic spectrum of primary voltage for cases 2 and 3 when the residential load is supplied by distribution system only. However, it goes down to 21.9 %, when the PV system is also connected to the system. The voltage distortion on the high voltage side drops from 5.1 % to 3.1 %. When PV system was not connected to the system, only the distribution system was feeding to the residential load, so the harmonics injected into the distribution system were entirely due to the nonlinear residential load. However, after PV system is connected to the system, harmonics due to the PV inverter also gets injected to the distribution system. Though the distortions from both the sources look quite high in magnitude, resultant distortion is less. One reason for the decrease in distortion could be due to phase-angle cancellation. 5. Conclusion The PV system and the residential system have been modeled to investigate the harmonic interaction by performing various case studies. It was found that the PV system did not add any distortion to the high amount of distortion on the load side already present due to the nonlinear load. Thus the harmonics produced on the load side by the PV system were not significant compared to the high current distortion caused due to the household nonlinear loads. The distortion injected to the distribution system decreased after connecting the PV system due to harmonic phasor cancellation. However, an accurate estimate of how the THD is going to change and how the individual harmonic components will change for increased size of PV system or expansion of the number of PV systems needs further investigation and analysis. References [1] US Department of Energy, “Photovoltaic energy program overview”, Fiscal year 2000 [2] Stevens John, “The issue of Harmonic Injection from Utility Integrated Photovoltaic Systems Part 1: The Harmonic Source,” IEEE Transaction on Energy Conversion, 3(3), Sept. 1988,pp 507 –510 [3] Stevens John, “The issue of Harmonic Injection from Utility Integrated Photovoltaic Systems Part 2: The Harmonic Source,” IEEE Transaction on Energy Conversion, 3(3), Sept. 1988, pp 511 –515 [4] Cyganski D, Orr J. A. et al., “Current and Voltage harmonic Measurements and Modeling at the Gardner Photovoltaic project,” IEEE Transactions on Power Delivery, 4(1), Jan. 1989, pp 800 -809 [5] Johnson Karl and Zavadil Robert, “Assessing the Impact of Nonlinear Load on Power Quality in Commercial Buildings- An Overview,” IEEE Industry Application Society Annual Meeting, Dearborn, MI, USA, Sept.28-Oct 4 1991, pp 18631869 [6] Domijan A., Jr., Hancock O. and Maytrott C., “A Study and Evaluation of Power Electronic Based Adjustable Speed Motor Drives for Air Conditioners and Heat Pumps with an Example Utility Case Study of the Florida Power and Light Company,” IEEE Transactions on Energy Conversion, 7(3), Sept. 1992, pp 396 –404 [7] Manjure Durgesh, “Effect of Nonlinear Loads on Power system Harmonics,” M. S. Thesis, Clemson University, Clemson, Dec. 1999 [8] Wasynesuk O. and Anwah N. A., “Modeling and Dynamic Performance of a Self-commutated Photovoltaic Inverter System,” IEEE Transaction on Energy Conversion, 4(3), Sept. 1989 pp 322 -328 [9] Wasynesuk O., “Modeling and Dynamic Performance of a Line-commutated Photovoltaic Inverter System,” IEEE Transaction on Energy Conversion, 4(3), Sept. 1989, pp 337 -343 [10] Mansoor A., Grady W. M., Staats P.T., Thallman R. S., Doyle M. T., Samotyj M. J., “Predicting the Net Harmonic Currents Produced by Large Numbers of distributed Single-Phase Computer Loads”, IEEE Transactions on Power Delivery, 12(3), Jul. 1997, pp 1258 -1266 [11] Mohan N., Undeland Tore M., Robbins William P., “Power Electronics: Converters, Applications, and Design,” New York, Wiley 1995, Second Edition [12] Vokas G. A. and Machias A. V., “Harmonic voltages and currents on two greek islands with photovoltaic stations: study and field measurements,” IEEE Transaction on Energy Conversion, 10(2), Jun.1995 pp 302 –306 [13] Bor-Ren Lin, “Power Converter Control based on Neutral and Fuzzy methods” EPRI Research 35(1995), pp 193-206 [14] Dwyer Rory, Khan Afroz K, McCluskey Robert K, Sung Roger, “Evaluation of harmonic Impacts from compact Fluorescent lights on distribution system,” IEEE Transactions on Power Systems, 10(4), Nov. 1995, pp 1772 -1779