Simulation of Dc-Link Power Converter for Integrating Offshore Wind Turbine Generator to Grid

Transcription

1 Simulation of Dc-Link Power Converter for Integrating Offshore Wind Turbine Generator to Grid Chaitanya Krishna Jambotkar #1, Prof. Uttam S Satpute #2 #1Department of Electronics and Communication Engineering, KLS s VDRIT, Haliyal , India. #2Department of Electrical and Electronics Engineering, KLS s VDRIT, Haliyal , India. Abstract In the recent years Off-shore wind power plants have gained interest due to the fact that, there is an ample space available for its installation. Moreover it offers other advantages such as; high wind energy potential, turbine blades can be larger (no transportation and construction problems), also the blades can be allowed to rotate at higher speed (no noise constrain) thereby increasing the rated power. The most important requirements of Off-shore wind power plants are less weight, less maintenance and less cost; and these requirements can be met by eliminating or reducing gear size. So, Direct-drive Permanent Magnet Synchronous generator concept is found most attractive. In such scheme, the frequency of power generation depends upon the number of generator poles and the rotor speed. The rotor speed depends upon the wind speed, which is fixed for a site. So the frequency of power generation depends upon the number of poles of generator. In this project the concept of Fractional frequency Transmission system (FTTS) is introduced. This project deals with optimal selection of frequency for power converter such that the total cost of generation-transmission is minimum with maximum converter efficiency. This study requires the analysis on power quality of power converter which is integrated to grid. SVPWM based three phase controlled DC- Link power converter integrated to grid is implemented on MATLAB/Simulink platform (7.11 R2011b version) where in the power converter is simulated for lower frequencies such as 50/2, 50/3, 50/4, 50/5, 50/6, 50/7, 50/8 further all the results such as %, harmonic analysis, efficiency of converter etc are noted based on which we conclude the optimal frequency for power converter. Based on the obtained results we can conclude that f/6 i.e Hz is the optimal frequency for the power converter which is integrated to grid via FFTS. The generator and transmission lines are also expected to present the similar benefits. But the cost of power generation increases with the decrease in frequency. So systematic methodology has to be adopted in optimal selection of frequency for complete system design. Keywords- AC-DC-AC power converter, FFTS, PLL, SVPWM. I. INTRODUCTION In recent years, the electrical power generation from renewable energy sources, such as wind is increasingly attraction interest because of environmental problem and shortage of traditional energy source in the near future. Nowadays, the extraction of power from the wind on large scale became a recognized industry. It holds great potential showing that future will become undisputed number one choice form of renewable source of energy. The force that pushes this technology is the simple economics and clean energy. As a consequence of rising fossil fuel price and advanced technology, more and more homes business have been installing small wind turbines for the purposes of cutting energy bills and carbon dioxide emissions and are even selling extra electricity back to the national grid. The kinetic energy in the wind is converted into mechanical energy by the turbine by way of shaft and gear box arrangement because of the different operating speed ranges of the wind turbine rotor and generator. The generator converts this mechanical energy into electrical energy. However, as wind is an intermittent renewable source, the wind source extracted by a wind turbine is therefore not constant. For this reason, the fluctuation of wind power results in fluctuated power output from wind turbine generator. From the point of view of utilities, due to the fluctuation of generator output, it s not appropriate for the generator to be directly connected to power grid. In order to achieve the condition that the generator output power is suitable for grid connection, it is necessary to use a controller to manage the output produced by the wind turbine generator. At present the transmission technologies such as HVAC, MVAC, HVDC and FACTS are giving their level best. Each technology has got its own importance when evaluated on different parameters. Basically the fundamental issue that made the rise for various technologies was the power quality. The power quality is given by P max = V 2 X = V2 2пfL (1) % V = QX 2пfLQ V2 100 = (2) V 2 From equation (1) we can say that power transmission capacity increases by the factor of decrease in frequency and equation (2) says that voltage fluctuation will decrease by the factor of increase in transmission efficiency. Power converter is required at the grid side to convert the fractional frequency of transmission into a standard frequency 50Hz before integrating to grid. We have implemented ISSN: All Rights Reserved 2014 IJSETR 1940

2 AC-DC-AC converter i.e. a pair of rectifier and inverter as power converter in our project. SVPWM technique is used to produce an appropriate firing angle to the switching device so that the output of the inverter circuit is always maintained at 50Hz at appropriate voltage level. SVPWM is used ahead of other PWM techniques as it provides less, switching losses, and a bit boost in voltage level compared to other PWM techniques. II. SYSTEM DESCRIPTION The power converter system is analyzed by giving various lower frequencies as input to it such as f/2, f/3, f/4, f/5, f/6, f/7, f/8 at standard transmission voltage level. The block diagram implementation of power converter integrated to grid is shown in the figure 1. III. MATLAB/SIMULINK IMPLEMENTATION A. SVPWM Modeling The proposed approach is based on the instantaneous values of the reference voltages of a, b and c phases only and the actual switching times for each inverter leg are deduced directly. The obtained load current is converted from three phase Iabc to two phase components Id and Iq respectively. The two phase currents are then compared with the reference values of the two phase components and the obtained output is again converted back to three phase components. The obtained three phase components are used to obtain the Uα and Uβ by using the transformation as stated below. U α = 2 3 I a + I b cos 2π 3 + I c cos 2π (3) U β = 2 3 I b sin 2π 3 I c cos 2π (4) Fig 1 Block diagram of the power converter integrated to grid It is assumed that the power is transmitted at the fractional frequency and given as input to the controlled rectifier which converts a fractional frequency AC voltage to a DC voltage which acts as input to the controlled inverter the output of the controlled inverter is produced with reference to the grid voltage and frequency. Then the output of controlled inverter is compared with the grid code as defind by IEC61727 [3] and conditions for synchronization such as voltage magnitude, phase and frequency. If the incoming line (output of inverter) satisfies all these conditions then the control signal will be provided to the circuit breaker to integrate the incoming line to the grid. The following figure 2 shows 2 Electrical scheme of a wind turbine equipped with a direct-drive PMSG. Fig. 3 The sinusoidal wave of three phases The Figure 3 illustrates the characteristic of sinusoidal waves of a 3-phase system. The period of sinusoidal waves of 3- phase is considered as 6 sectors, which are divided at the critical points of (π/3), (2π/3), (π), (4π/3), (5π/3) and (2π) radians. The blue, green and red lines indicate A neutral, B neutral and C neutral voltages in a 3-phase balanced system. Analysis of the figure reveals the following table (the digit 1 is representative of the corresponding phase amplitude being greater than or equal to 0, the digit 0 expressing the amplitude is less than 0.). Fig 2 Electrical scheme of a wind turbine equipped with a direct-drive PMSG. ISSN: All Rights Reserved 2014 IJSETR 1941

3 For the neutral voltage of Phase A, B and C, in this interpretation of the logical relationship an alternative method of expressing the relationship in algebra is given by, N = A + 2B + 4C (4) Where N represents the sector mapping Based on the above equation the above table can be modified as follows M = 2 3 I ref I dc (7) The generated times are the pulse width modulated based on the sector and the time periods obtained based on the following table. B. Rectifier Fig 4 Vector synthesis schematic in sector 1 The Figure 4 illustrates the vectors located in sector, assuming Tz is the unit of time. According to the Figure 4 the time T1 and T2 are obtained as follows: The bridge configuration is used to implement controlled rectifier unit in MATLAB/Simulink platform. The switching device used is IGBT (Insulated gate bipolar transistor) because of its low on state voltage and less switching time. The below figure 5 shows the Simulink model of SVPWM based three phase controlled rectifier. I ref sin 2π 3 = I 1 T 1 sin π 3 α (5) Simplifying the above equations we get the equations for T1, T2 and T0 as follows [2], T 1 = I ref I 1 sin π 3 α sin 2π 3 = M sin π 3 α T 2 = I ref I 1 sin α sin 2π 3 = M sin α T 0 = T z T 1 T (6) Where M is the ratio of modulation, T0 is the dead time of the inverter. Fig 5 SVPWM based controlled Rectifier ISSN: All Rights Reserved 2014 IJSETR 1942

4 C. Inverter IV. RESULTS The bridge configuration is used to implement controlled inverter unit in MATLAB/Simulink platform. The switching device used is IGBT (Insulated gate bipolar transistor) because of its low on state voltage and less switching time. The below figure 6 shows the Simulink model of SVPWM based three phase controlled inverter. The following table provides the values for voltage and current at lower fractional frequency simulated for controlled rectifier circuit. Table 4 values for voltage and current at fractional frequencies for controlled rectifier FREQUENCY (Hz) f (50) f/2 (25) f/3 (16.67) f/4 (12.5) f/5 (10) f/6 (8.33) f/7 (7.142) f/8 (6.25) Vdc Idc (A) (Vdc) (Idc) e+005 e Fig 6 SVPWM Based controlled Inverter Integrated to grid D. PLL Technique Phase Locked Loop (PLL) is a phase tracking algorithm widely applied in communication technology, being able to provide an output signal synchronized with its reference input in both frequency and phase. Here, the PLL technique is utilized to extract the phase angle of the grid voltages. The PLL is implemented in dq synchronous reference frame. This structure needs the coordinate transformation from abc to dq and the lock is realized by setting the reference to zero. A PI controller is used to control the variable. This structure can provide both the frequency of grid as well as the grid voltage angle. The following figure 7 shows output voltage waveform of the controlled rectifier circuit. Fig 7 Output voltage waveform of the controlled Rectifier circuit. ISSN: All Rights Reserved 2014 IJSETR 1943

5 The following figure 8 shows output current waveform of the controlled rectifier circuit. The following figure 9 shows the control signal to produce firing angle using SVPWM Technique. Fig 8 Output current waveform of the controlled rectifier circuit. The following table provides the values for voltage and current at lower fractional frequency simulated for controlled inverter circuit integrated to grid with synchronization frequency maintained at 50Hz constantly for all fractional frequency given as input to the controlled inverter circuit. Fig 9 Control signal to produce firing angle using SVPWM Technique. The following figure 10 shows the line voltages of the controlled inverter. Table 5 values for voltage and current at fractional frequencies for controlled inverter integrated to grid FRQUE NCY (Hz) f (50) f/2 (25) f/3 (16.67) f/4 (12.5) f/5 (10) f/6 (8.33) f/7 (7.142) f/8 (6.25) Vdc Vl Vph Fsy n (Hz) (Vout) (Iout) Fig 10 Line voltages of the controlled inverter. The following figure 11 shows the phase voltages of the controlled inverter. Fig 11 Phase voltages of the controlled inverter. ISSN: All Rights Reserved 2014 IJSETR 1944

6 The following figure 12 shows the output voltages of the controlled inverter after filter circuit. Fig 12 Output voltages of the controlled inverter after filter circuit. V. CONCLUSION The paper presents a novel idea of integrating Dc-Link power converter to grid via FFTS. Complete design of system requires frequency response characteristics of different components involved in. The paper has attempted to obtain the frequency response characteristics of power converter. From the results following different points are concluded; SVPWM method of producing PWM is beneficial as it produces boost in output voltage compared to other PWM techniques and uses input voltage to better extent. f/6 i.e Hz is optimal frequency for power converter as % and harmonic variations are less for this frequency value than other frequencies and power converter efficiency is also high at this frequency. Lower frequency operation of system is beneficial. The following figure 13 shows waveforms for Grid voltages and currents. REFERENCES [1] X. Wang, "The Fractional Frequency Transmission System" in Proc, Inst., Elect, Eng., Japan, Power System, Tokyo, Japan, Jul [2] Wang Xifan, Cao Chengjun, Zhou Zhichao, Experiment on Fractional Frequency Transmission System IEEE Trans on Power System, vol. 21, no.1, Feb [3] Uttam S Satpute, S H Jangamshetti, D R Joshi, Feasibility Study of Fractional Frequency Transmission System IEEE Trans on Power Electronics, Drives and Energy Systems (PEDES) & 2010 Power India, Fig 13 Waveforms for Grid voltages and currents. The below figure 14 shows the graph for efficiency of power converter. From graph, we can say that f/6 is the optimal frequency for power converter which is same as illustrated using power quality of power quality of power converter in the tabular column of both rectifier and inverter. [4] Sandhya Rani, A.Appaprao, A Space Vector PWM Scheme for Three level Inverters Based on Two-Level Space Vector PWM International Journal Of Power System Operation and Energy Management (IJPSOEM) Volume-1, Issue-1, 2011, pp [5] P.Tripura, Y.S.Kishore Babu, Y.R.Tagor, Space Vector Pulse Width Modulation Schemes for Two-Level Voltage Source Inverter ACEEE Int. J. on Control System and Instrumentation, Vol. 02, No. 03, October 2011 pp [6] M. H. Rashid, Power Electronics Handbook, New York [7] B. K. Bose, Modern Power Electronics and AC Drives, Englewood Cliffs, NJPrentice-Hall, [8] Ned Mohan, Power Electronics, Willey Publications. Fig 14 The efficiency plot of power converter. ISSN: All Rights Reserved 2014 IJSETR 1945