A NOVEL APPROACH TO ENHANCE THE POWER QUALITY USING CMLI BASED CUSTOM POWER DEVICES

Transcription

1 A NOVEL APPROACH TO ENHANCE THE POWER QUALITY USING CMLI BASED CUSTOM POWER DEVICES 1 M. KAVITHA, 2 A. SREEKANTH REDDY & 3 D. MOHAN REDDY Department of Computational Engineering, RGUKT, RK Valley, Kadapa (DT), A.P, India, mrkus.1780@gmail.com, sreekanth04eee@gmail.com & usmohanus@gmail.com Abstract:- In recent years electrical power quality had obtained more attention in power engineering. Now-a-days, there has been an increased use of non-linear loads which has resulted in an increased fraction of non-sinusoidal currents and voltages in electric network. And the power distribution system is suffering from severe power quality problems. These power quality problems include high reactive power burden, harmonics currents, load unbalance, voltage sags, voltage swells and excessive neutral current etc. To overcome such problems numerous topologies have been introduced and studied extensively for utility and drive applications in the recent literature. Among them multilevel converters and custom power devices are suitable in high voltage and high power applications due to their ability to synthesize waveforms with better harmonic spectrum and attain higher voltage with a limited maximum device rating. Hence, this paper provides MATLAB/SIMULINK files of a novel device called CMLI based Custom Power Device (DSTATCOM). This combination reaches the IEEE standards with 3.85% of THD after mitigation of power quality problems. Keywords: Power Quality, Power quality problems, CMLI, DSTATCOM, PWM Technique. I. INTRODUCTION Modern power systems are highly complex and are designed as such to fulfil the growing demands of power with better power quality. This power quality is effecting by many power quality problems like voltage collapse, reactive power burden, harmonics currents, load unbalance, voltage sags, voltage swells and excessive neutral current etc. Voltage collapse and some other problems are occurs when power systems are heavily loaded, faulted, or have reactive power shortages. Voltage collapse is system instability and it occurs due to many power system components. Voltage instability is the inability of the power system to meet the demand for reactive power in the heavily loaded system. One of the most important causes of voltage instability in a system is the occurrence of reactive power imbalance in the system. Reactive power imbalance occurs when system is faulted, heavily loaded and voltage fluctuation is there. This problem can be effectively tackled by the introduction of high power electronic controllers which can inject or absorb reactive power as per system requirement. One of the most important reactive power sources is FACTS (Flexible A.C transmission system) device [1]. These devices allow Flexible operation of AC transmission system without stressing the system. The multilevel inverter has gained much attention in recent years due to its advantages in lower switching loss better electromagnetic compatibility, higher voltage capability, and lower harmonics. Multilevel cascaded inverters have been also proposed for such applications as static Var generation, an interface with renewable energy sources, and for battery-based applications. The inverter could be controlled to either regulate the power factor of the current drawn from the source or the bus voltage of the electrical 142 system where the inverter was connected. Several topologies for multilevel inverters have been proposed, the most popular being the diode-clamped, flying capacitor, and cascade H-bridge structures. The pulse width modulation (PWM) cascaded multilevel inverter strategy reduces the total harmonic distortion and enhances the fundamental output voltage. According to IEEE-519 standards, the power quality is good if the THD is lesser than 5%.This paper presents 3.68% of THD which reaches the IEEE standards by increasing the number of level of CMLI based DSTATCOM using PWM [2]. II. MULTILEVEL INVERTER TOPOLOGIES The multilevel converter has drawn tremendous interest in the power industry. The general structure of the multilevel converter is to synthesize a sinusoidal voltage from several levels of voltages, multilevel voltage source converters are emerging as a new breed of power converter options for high power applications. These converter topologies can generate high-quality voltage waveforms with power semiconductor switches operating at a frequency near the fundamental. Multilevel topologies are able to generate better output quality, while operating at lower switching frequency [3]. This implies lower switching dissipation and higher efficiency. Moreover, this topology utilizes switches with lower breakdown voltage; therefore, it can be used in higher power applications at lower cost. Compared to the multi pulse converter, multilevel converters are more flexible and have a wide application. They can be used as active power filters and to handle unbalanced loads. No phase shift transformer is required in this configuration, so a

2 lower investment cost, plus a lower power loss, can be expected [4]. The multilevel converter configuration can be further classified into three different configurations: 1. Diode-clamped converter 2. Flying capacitor converter 3. Cascaded converter. Compared to the multi pulse converter, multilevel converters has many advantages. They can be used as active power filters and to handle unbalanced loads. No phase shift transformer is required in this configuration, so a lower investment cost, plus a lower power loss, can be expected. The multilevel converter configuration can be further classified into three different configurations. Among the available multilevel converter topologies, the cascaded multilevel inverter constitutes a promising alternative, providing a modular design that can be extended to allow a transformer less connection. The cascaded H- bridge multilevel Inverter uses separate dc sources (SDCSs). The multilevel inverter using cascadedinverter with SDCSs synthesizes a desired voltage from several independent sources of dc voltages, which may be obtained from batteries, fuel cells, or solar cells. In Cascaded multilevel inverter inverters are connected in series [5]. Each H-bridge converter unit provides three voltage levels (-V, 0, V). As number of levels increases the output waveform becomes perfect. The cascaded multilevel inverter is as shown in figure1. Figure2. Typical output waveform of cascaded five level inverter III. CALCULATION OF NUMBER OF LEVELS Depending on the number of levels, the bridges are cascaded. In this topology, the number of output phase voltage levels is defined by m = 2s+1, where s is the number of dc sources. The following is the generalized structure of Cascaded H-Bridge multilevel inverter. All levels of CMLI can be calculated based on the following formulae. The number of levels in the line-to-line voltage waveform will be k = 2N 1... (1) While the number of levels in the line to load neutral of a star (Y) load will be p = 2k 1... (2) The number of capacitors or isolated supplies required per phase is N cap = 1 2 (N 1)... (3) The number of possible switch states is n states = N phases... (4) Figure1. Cascaded multilevel inverter Compared with the other two multilevel configurations and the multi pulse converter, the cascaded converter eliminates clamping diodes, flying capacitors, bulky zigzag transformer and requires least component mountings. The modularity of this configuration makes it much easier to implement converters with a large number of levels. Larger dc-side capacitors are required compared to the diode clamped and flying capacitor converter under balanced condition but it provides separate phase control to support significant voltage unbalance [6]. The output waveform of cascaded five level inverter is shown in figure2. And the number of switches in each leg is S n = 2(N 1)... (5) IV. FACTS A Flexible AC Transmission System (FACTS) is an ac transmission system incorporating power electronic-based or other static controllers which provide better power flow control and enhanced dynamic stability by control of one or more ac transmission system parameters (voltage, phase angle. and impedance). Two of the most important FACTS devices, which have broad application in electric utility industry, are SVC (Static Var Compensator) and DSTATCOM (Distributed Static synchronous Compensator) [7]. DSTATCOM, also named ASVG (Advanced Static Var Generator) is one of the new-generation FACTS devices, and 143

3 recognized to be one of the key technologies in future power system. DSTATCOM has played an important role in power industry since 1980s. The DSTATCOM is basically a DC-AC voltage source converter with an energy storage unit, usually a DC capacitor [8]. It operates as a controlled Synchronous Voltage Source (SVS) connected to the line through a coupling transformer. Figure3 shows the schematic configuration of DSTATCOM. The controlled output voltage is maintained in phase with the line voltage, and can he control to draw either capacitive or inductive current from the line in a similar manner of a synchronous condenser, but much more rapidly. Figure4. Pulse width modulation Figure3. Schematic configuration of DSTATCOM Compared to SVC and other conventional reactive power compensators, DSTATCOM has several advantages listed below. DSTATCOM has a dynamic performance far exceeding the other Var compensators. The overall system response time of DSTATCOM can reach l0ms or less. DSTATCOM has the ability to maintain full capacitive output current at low system voltage, which also makes it more effective than SVC in improving the transient stability. Compared with SVC, DSTATCOM can easily realize redundancy design, which brings a higher reliability. IGCT, IGBT, used in DSTATCOM, require simpler gate drives and snubber circuits, and also make DSTATCOM more reliable [10]. STATCOM has a smaller installation space, about 50% of that for SVC. V. PWM TECHNIQUE This modulation technique can be used to encode information for transmission. The below notches can be form based on the following equations. If the notches are more then the reduction process of harmonics is easy. This pule width modulation provides that facility. When V control >V tri, V A0= V dc /2... (6) When V control <V tri, V A0 = -V dc /2... (7) Figure4 shows the pulse width modulation. In general, a multilevel inverter with m voltage levels requires (m 1) triangular carriers. In the PWM, all the triangular carriers have the same frequency and the same peak-to-peak amplitude, but there is a phase shift between any two adjacent carrier waves, given by φ cr =360 0 /(m 1). The modulating signal is usually a three-phase sinusoidal wave with adjustable amplitude and frequency. The gate signals are generated by comparing the modulating wave with the carrier waves [10]. It means for five-level inverter, four triangular carriers are needed with a 90 phase displacement between any two adjacent carriers. In this case the phase displacement of V cr1 = 0, V cr2 = 90, V cr1- = 180 and V cr2- = 270. VI. MATLAB/SIMULINK CIRCUITS AND RESULTS The power quality is increasing by decreasing the THD value, this can be done by increasing the number of levels of CMLI. If the number of levels of CMLI increases the desired output waveform will have good sinusoidal shape. As we know that if number of levels increases harmonics effect reduces which means that we can get better sinusoidal waveform. By comparing five, seven, nine and eleven level cascaded multilevel inverters, ninth level inverter is effective and efficient to get expected smoother sinusoidal waveform which helps us to enhance the power quality. Below table1 shows the practical values of five, seven and nine levels THD. Table1. THD (%) values of different levels of CMLI No. of levels THD (%) value VI.i. Matlab/Simulink Circuits The matlab/simulink circuit of five level cascaded multilevel inverter is shown in figure5. 144

4 Figure5. 5 level CMLI The matlab/simulink circuit of phase shift pulse with modulation is shown in figure6. Figure8. Circuit of CMLI based DSTATCOM using PWM VI.ii. Matlab/Simulink Results VI.ii.i. Without DSTATCOM The matlab/simulink results of five level cascaded multilevel inverter without distributed static synchronous compensator is shown in figure9. Figure6. Circuit of PWM The matlab/simulink circuit of five level cascaded multilevel inverter without distributed static synchronous compensator is shown in figure7. Figure9. Waveform of source voltage Figure10. Waveform of source current Figure7. Circuit of CMLI without DSTATCOM The matlab/simulink circuit of cascaded multilevel inverter based distributed static synchronous compensator based phase shift pulse width modulation shown in figure8. Figure11. Waveform of load current 145

5 The cascaded multilevel inverter without distributed static synchronous compensator will act like level shift pulse width modulation technique. This circuit presents 28.28% of THD value, which gives the poor power quality than cascaded multilevel inverter with distributed static synchronous compensator. The harmonic spectrum of five level cascaded multilevel inverter based distributed using phase shift pulse width modulation is shown in figure16. Figure12. Harmonic spectrum of CMLI without DSTATCOM VI.ii.ii. With DSTATCOM The matlab/simulink results of five level cascaded multilevel inverter based distributed static synchronous compensator using phase shift pulse width modulation is shown in figure13. Figure13. Waveform of source voltage Figure16. Harmonic spectrum of CMLI based DSTATCOM using PWM VII. CONCLUSION This paper is clearly showing the difference in results of THD before and after using of CMLI based DSTATCOM. According to IEEE-519 standards, the power quality is good if the THD (%) is lesser than 5%. The proposed circuit can easily gives the 3.68% (nine level) of THD in small running time. This can be done by increasing the number of inverters. Therefore, this chapter presents MATLAB/SIMULINK circuit of CMLI based DSTATCOM using PWM which helps to mitigate all the power quality issues and increases the power system s power quality in shorter time. Figure14. Waveform of source current REFERNCES [1] P. Ferracci, Power Quality, Schneider Electric Cahier Technique no. 199, September [2] IEEE Recommended Practice For Monitoring Electric Power Quality. New York: IEEE, Inc.Leng, O.S. (2001). Simulating Power Quality Problems. [3] F. Z. Peng, al., "A multilevel voltage-source inverter with separate DC sources for static var generation," IEEE Trans. on Industry Applications, vol. 32, no. 5, pp.b , September/October, [4] Z. Du, L. M. Tolbert, J. N. Chiasson, and B. Ozpineci, Reduced switching-frequency active harmonic elimination for multilevel converters, IEEE Trans. Ind. Electron., vol. 55, no. 4, pp , Apr [5] O. Lopez, J. Alvarez, J. Doval-Gandoy, and F. D. Freijedo, Multilevel multiphase space vector PWM algorithm, IEEE Trans. Ind. Electron., vol. 55, no. 5, pp , May Figure15. Waveform of load current [6] M. Glinka and R.Marquardt, A new AC/AC multilevel converter family, IEEE Trans. Ind. Electron., vol. 52, no. 3, pp , Jun

6 [7] Madrigal, E.Acha., Modelling of Custom Power Equipment Using Harmonic Domain Techniques, IEEE [8] K. R. Padiyar, Facts Controllers in Power Transmission and Distribution, [9] Lin Kongxing, Li Jugen. Li Xiangmng, cl al. Preliminary SNdy on the Applications of STATCOM to Central China Power System in 2010, Automation of Eleclric Power Systems, (23) [10] C.J. Hatziadoniu, A 12-pulse static synchronous compensator for the distribution system employing the 3- level inverter, IEEE Trans. on Power Delivery, vol. 12, no. 4, pp , Oct [11] Mr. D.Mohan Reddy and Dr.T.Gowri Manohar Comparative Analysis Of A Cascaded Seven Level And Five Level Mli Based Distribution Statcom For Compensation Of Harmonics And Reactive Power Using Reference Frame Theory. International Journal of Electrical Engineering and Technology (IJEET), Volume 4, Issue 2, March - April pp *** 147