DSTATCOM BASED POWER QUALITY IMPROVEMENT OF MICROGRID

Transcription

1 DSTATCOM BASED POWER QUALITY IMPROVEMENT OF MICROGRID VIJAY KUMAR K PG scholar,balaji institute of Technology & Science, JNTUH, Warangal, Telangana, India MD ERSHAD ALI M.Tech,Asst. Professor,Balaji Institute of Technolgy & Science JNTUH, Warangal, Telangana, India ABSTRACT-Renewable energy system applications have become very popular. One of the areas for increasing the penetration of wind lies within power electronics which can add a great deal of flexibility and control to existing systems both at the transmission and distribution levels. This paper explains a new kind of inverter that can replace the existing inverter of a small to medium sized permanent-magnet wind machine (10kW to 20kW) that can offer VAR control and power factor correction in a dynamic manner. This inverter is called a D-STATCOM Inverter. The D- STATCOM Inverter and focus of this research, is a voltage- source converter that can provide the grid with VAR control and power factor correction independent of the wind turbines production. The voltage of the DC link along with the individual voltages across each of the link and auxiliary capacitors. In this paper D-STATCOM inverter is used for large amount of harmonics. The main advantages of proposed controller are to eliminate steady state error and harmonic content and improve the system response. The proposed inverter utilizes the hybrid-clamped topology. All simulations were done in MATLAB/Simulink environment. Key points D-STATCOM, Hybrid-Clamped Topology, MMC Topology, Multi-Level Inverter, OHSW Harmonic Elimination Technique. INTRODUCTION Wind energy conversion systems are usually passive generators. The generated power does not depend on the grid requirement but entirely on the fluctuant wind condition. Since most utilities do not track the end points of their distribution lines carefully, where most of the wind turbines are connected to the grid, increasing the application of renewable energies in utilities can result in problems for the whole system dynamics. Active power is controlled by shifting the phase angle while reactive power control is achieved by modulation index control. It is clear that in order to achieve the penetrations of wind and other renewable our administration is asking for, the electric grid will have to be greatly expanded and new concepts for control will have to be exploited [3]. One of the areas for increasing the penetration of wind lies within power electronics which can add a great deal of flexibility and control to existing systems both at the transmission and distribution levels. To this date, much of the focus of power electronic devices including the vast array of FACTS devices lies within the transmission sector, for the sole reason that these technologies are expensive and need to be placed in locations where they can realize the greatest return on investment. This approach would leave out the vast majority of the electric grid which is the distribution system on the low voltage side of distribution transformers. At this voltage and power level it is almost impossible to justify the immense cost of implementing any kind of power electronics that can regulate line voltages, power flows, and VAR support. To overcome the large cost associated with FACTS type devices it is advantageous to turn towards a smaller and more distributed scale. Rather than having one large FACTS device on a distribution line, such as a unified power flow controller. This paper explains a new kind of inverter that can replace the existing inverter of a small to medium sized permanent-magnet wind machine (10kW to 20kW) that can offer VAR control and power factor correction in a dynamic manner. This inverter is called a D- STATCOM Inverter. The distribution static compensator (DSTATCOM) is used for load compensation in power distribution network. The distribution static compensator (DSTATCOM) is a shunt active filter, which injects currents into the point of common coupling (PCC) (the common point where load, source, and DSTATCOM are connected) such that the harmonic filtering, power factor correction, and load balancing can be achieved. DESCRIPTION OF THE D-STATCOM DESIGN: The proposed inverter is able to correct the power factor of the line, especially at the end points of the distribution lines where there is not enough attention to the line behavior. Multiple D-STATCOM inverters on the feeder lines would help utilities increase their knowledge of the distribution system leading to greater efficiency, reliability, and control. The unique work in this paper is the bringing together and combination of several relatively new concepts. The design objectives of the project include: minimize the overall switching frequency of the inverter; minimize the total

2 harmonic distortion in order to maintain compliance with IEEE standards and maximize energy conversion efficiency; and finally, keep the cost of the inverter as low as possible. The D-STATCOM Inverter falls under the category of devices known as custom power electronics. Most of the research on custom power electronics has stemmed from the research done on STATCOM devices that integrate battery energy storage. Generally, the purpose of the D-STATCOM Inverter is to increase the value to the utility of a small wind turbine located on a single-phase feeder line by providing extra control and information. The D-STATCOM Inverter represents one stage of a three-stage power electronics block that makes up the entire converter structure. The first of these stages is the wind turbines maximum power point tracker (MPPT). This stage transfers the maximum power from the wind turbine while transforming the quasi DC output of the permanent generator into a uniform DC voltage. The second stage, the DC-DC boost, boosts the DC voltage provided by the MPPT to the desired DC link voltage required by the D-STATCOM inverter. The final stage, the D-STATCOM inverter and focus of this research, is a voltage source converter that can provide the gird with VAR control and power factor correction independent of the wind turbines production. This provides the utilities with both control of and information about the feeder line which they never had before, thus increasing the value and potential penetration of small wind. Figure 1 shows the structure of the system. P = sin δ (1) Q = (2) The two equations have four variables which govern the active and reactive power range. These variables are the inductance of the transformer X, the voltage of thed- STATCOM DC link Es, the voltage of the secondary side of the transformer El, the range of the voltage angle δ, and the range of the modulation index m. in defining these four variables a few assumptions are made. One is that the range of the modulation index is from 0.6 to 1. The second is the inductance of the transformer. For this design, the inductance of the transformer is 0.05H and X/R ratio of the transformer is assumed to be large so that eqations (1) and (2) will hold. The final variable is the angle δ which is assumed to have an operating range from -90 degrees to +90 degrees in order to maintain quarter wave symmetry. Figure 2 provides a graphic view of the designed operating range of the D-STATCOM inverter in accountance with the chosen values and equations (1) and (2). Figure 2: Overview of the D-STATCOM inverter design PROBLEM OPTIMIZATION: The optimization problem as defined by Barkati is to solve, for a given modulation index, a series of equations define by (3). Equation (3) represents the amplitudes of every harmonic. Figure 1: System structure The design of the D-STATCOM inverter incorporates a 5- level topology called the hybrid- clamped and uses the OSHW technique. In designing the inverter there were three basic criteria: 1) the inverter should be able to support turbines rated from 10 to 20KW; 2) the inverter should be able to provide up to 20 kvars of capacitive compensation regardless of the active power conversion; and 3)the inverter must be connected to a single- phase feeder line. The active and reactive power flow of the DSTATCOM is governed by Equations (1) and (2) which are listed below. H (α) = cos(nα ) (3) For a single phase inverter using quarter wave symmetry, containing m-levels, k switching angles, and j DC link capacitors, the system of equations to be solved contains k non- linear equations and is represented by the following: cos(α ) + cos(α ) + + cos(α ) = cos(3α ) + cos(3α ) + cos(3α ) = h = 0 cos(5α ) + cos(5α ) + + cos(5α ) = h = 0 cos(nα ) + cos(nα ) + + cos(nα ) = h = 0 (4)

3 Where M represents the modulation index that fundamental component should be solved for. The Fourier series of this waveform is written as: V(wt) = V H (α) sin(nwt) (5) WhereV represents the DC voltage across one DC link capacitor. A. 5-Level Two Angle OHSW: The two equations to be solved for a 5-level inverter are: cos(α ) + cos(α ) = M (6) cos(3α ) + cos(3α ) = 0 (7) Where M is the modulation index and ranges from 0.6 to 1. The objective function or cost function is given by, Cost Function(α, α ) = w 2M H + w H (8) SIMULATION RESULTS: The D-STATCOM Inverter was designed and simulated using MATLAB/SIMULINK. At the highest level, the model consists of five distinct parts. These are a Thevinin equivalent of the grid, a data acquisition block, the D-STATCOM controller, the power electronics circuit, and the wind turbine model. To confirm the operation of the D- STATCOM Inverter, different cases and simulations were carried out in Sim Power Systems toolbox. Figures 3(a) and 3(b) show the results of a 20-second simulation in which the load on the grid was initialized to 50 kw and kvars, giving a power factor of 0.82 (lagging). The voltages for both the DC link and auxiliary capacitors in the hybrid clamped topology are initialized to 1000 V. For the first 6 seconds of the simulation the output of the wind turbine is set to 0 W in order to give the D-STATCOM Inverter enough time to adjust to the required compensation demanded by a target power factor of 0.9 (lagging). The top graph of Figure 3(a) shows the power factor of the feeder line during the course of the 20-second simulation. Starting at the 0th second, the power factor of the line is 0.82 (lagging) as it is defined entirely by the load. As soon as the simulation starts, the D-STATCOM Inverter begins to provide compensation and the power factor is adjusted. The second graph shows the P and Q provided by the feeder line to the load. Initially, the feeder line is supplying the entire load of 50 kw and 34.8 kvars. When the DSTATCOM Inverter provides capacitive VAR compensation, the amount of VARS provided by the feeder line to the load is decreased to about 20 kvars. Additionally, as the output of the wind turbine, shown in Figure 3(b), is increased, the amount of active power provided by the feeder line to the load is decreased by the same amount. (b) Figure 3: (a) Feeder line power factor, feeder line P and Q, D- STATCOM power factor, and delivered P and Q of the D-STATCOM. (b) Modulation index, angle delta, and wind turbine output power In the above figures the maximum output power of the turbine is 11 KW, but the control system works properly for up to 20 KW wind turbines. Overall, the D-STATCOM Inverter is able to provide the feeder line with VAR compensation which is independent of the active power provided by the wind turbine. (a)

4 with the use of a small filter. The simulated THD is actually lower than the predicted THD. This is due to some of the harmonics being suppressed by the inductances and capacitances in the circuit. Table 1: Simulated vs. Predicted Results for THD for the OHSW Method up to the 100th harmonic Figure 4: Variation of capacitor voltages using OHSW Figure 4 depicts the voltage of the DC link along with the individual voltages across each of the link and auxiliary capacitors. Figures 5 depicts the power factor of the feeder line, the P and Q on the feeder line, the output power factor of the inverter, the output P and Q of the inverter, the modulation index, the angle delta, and the power produced by the wind turbine/solar array. The output waveform of the 5-level OHSW should contain no even harmonics, because of quarter-wave symmetry, and the 3rd order harmonic should be eliminated due to the OHSW technique. Results show that while the 3rd and even harmonics have not been eliminated they have been suppressed to a low level. Figure 5: Feeder line power factor, feeder line P and Q, D- STATCOM power factor, and delivered P and Q of the D- STATCOM Table 1 summarizes the differences between the simulated and predicted values. The presence of even harmonics is the result of non-optimal switching times in the simulation, inductances and capacitances in the inverter, and variations in the current and voltage waveforms. The contribution to the THD by the even harmonics is of little concern as all of the even harmonics are relatively the same size and the higher order ones can be further suppressed CONCLUSION This paper has presented a novel method to improve the power quality at point of common coupling (PCC) for small to mid-sized wind turbine applications. Controller is simple, and is based on sensing the line currents only. The proposed 5-level inverter is capable of regulating the power factor of the line using hybrid-clamped multilevel topology. It is also able to eliminate a great number of harmonics using OHSW technique. The THD of the source current using the proposed controller is well below 5%, the harmonics are eliminated.the results show that the OHSW technique is a feasible modulation scheme for the D- STATCOM Inverter and that the hybrid-clamped topology is capable of operating under the dynamic conditions presented by a wind turbine. Also, simulation results show that the THD is actually lower than the predicted THD because some of the harmonics are suppressed by the inductances and capacitances in the circuit. REFERENCES [1] AWEA, AWEA U.S. wind industry annual market report year ending 2010, American Wind Energy Association, Washington DC, [2] AWEA, AWEA U.S. wind industry annual market report year ending 2009, American Wind Energy Association, Washington DC, [3] S. Fink, C. Mudd, K. Porter, and B. Morgenstern. Wind energy curtailment case studies May National Renewable Energy Laboratory: Golden, CO. NREL/SR-550-

5 4671, [4] A. Chen and X. He, Research on hybrid-clamped multilevel-inverter topologies, IEEE Trans. Industrial Electronics, vol. 53, no. 6, pp , [5] A. Chen and X. He, A hybrid multilevel inverter topology with neutral point voltage balancing ability, IEEE Annual Power Electronics Specialist Conference, Aachen, Germany, pp , [6] J. Zhao, X. He, R, Zhao, A novel PWM control method for hybrid clamped multilevel inverters, IEEE Trans. Industrial Electronics, vol. 37, pp , [7] S. Barkati et al, "Harmonic elimination in diode-clamped multilevel inverter using evolutionary algorithms," Electric Power Systems Research, pp , Elsevier, VIJAY KUMAR K currently pursuing his M.Tech in Electrical Power Systems in Balaji institute of Technology and Sciences, Warangal, Telangana, India affiliated to JNTU University, Hyderabad. He has done his B.Tech degree from Kakatiya Institute of Technology and Science, affiliated to Kakatiya University, Warangal, Telangana, India in 2008 and his fields of interest include Industrial Drives, Power Systems and Control Systems. MD ERSHAD ALI has completed his M.Tech in Electrical Power Systems from Jayamukhi Institute of Science & Technology JNTUH, Telangana, India in 2013 and B.tech from Vagdevi College of Engineering in Presently working as Asst. Professor in Balaji Institute of Technolgy & Science from 2009 to till date. His fields of interest include Renewable Energy Sources and Industrial Drives. [8] S. Barkati, E.M. Berkouk, M.S. Boucherit, Partical swarm optimization for harmonic elimination in multi-level inverters, Electrical Engineering 91, Springer, pp , [9] R. N. Ray, D. Chatterjee, S. K. Goswami, An application of PSO technique for harmonic elimination in a PWM inverter, Applied Soft Computing 9, Elsevier, pp , [10] S. Sirisukprasert, Optimized harmonic steppedwaveform for multilevel inverter, M.S. thesis, Department of Electrical Engineering, Virginia Polytechnic Institute State University, Blacksburg, VA, 1999.