A SCHEME FOR HARMONIC REDUCTION WITH MULTILEVEL …pep.ijieee.org.in/journal_pdf/1-463-153001491425-31.pdf · 2018-06-26 · Abstract - Many electrical rotating machines are operated

International Journal of Electrical, Electronics and Data Communication, ISSN(p): 2320-2084, ISSN(e): 2321-2950 Volume-6, Issue-4, Apr.-2018, http://iraj.in

A Scheme for Harmonic Reduction with Multilevel Inverter in Large Synchronous Machine Powered by Static Drive

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A SCHEME FOR HARMONIC REDUCTION WITH MULTILEVEL INVERTER IN LARGE SYNCHRONOUS MACHINE POWERED BY

STATIC DRIVE

1ARUN KUMAR DATTA, 2N. R. MONDAL, 3J. SANTHOSH

Central Power Research Institute, Bhopal, India E-mail: [email protected], [email protected], [email protected]

Abstract - Many electrical rotating machines are operated nowadays with static drives. Inverters used in theses drives inject harmonics in the system if they are not properly designed. To study this phenomenon, one large machine with static drive (known as SFC) has been taken in this paper. Simulation technique is used to create the virtual model of the system by acquiring parameters from the real system. Built model is validated and further analysis is carried out extracting the system waveforms and frequency spectrums. The level of harmonics is found to be high in the present design of static frequency converter (SFC). To reduce the harmonics level, multilevel inverter has been envisaged as a replacement of the current source inverter used in SFC. Simulation results support this proposed scheme. Though the study is carried out on a specific system but the findings are applicable to all such similar machines. Index terms - Alternators, Inverter, Simulation, Static Frequency Converter, Synchronous Machines. I. INTRODUCTION Synchronous machines have many excellent features. Starting problem has limited its use by conventional method. By virtue of modern static drives it is being used in a widespread manner. After the invention of semiconductor devices various static starting devices for synchronous motors are developed. These devices named adjustable speed drive (ASD) or variable speed drive (VFD) are boon to the motor technology. Among them the thyristor converter has firmly established as the static electrical power conversion equipment of many types ranging from a few hundred watts to tens megawatts. Though the concept of frequency converter is very old but for the large electric machines this soft starting technology is thought of at a later stage. Under the static drive technology Static Frequency Converter (SFC) is envisaged, designed and applied on many large machines [1] - [3]. Uses of SFC has increased in the recent years in the field of aviation industry, computer installations, communications, military installations, motor speed control, ships and power transmission. One large rating synchronous machine driven with SFC is taken as study object of this paper. This machine is installed at a laboratory of Central Power Research Institute, Bhopal. The harmonic profile of this machine and its supply source have been analysed with MATLAB/Simulation techniques. II. SPECIAL PURPOSE ALTERNATOR Short circuit test Healthy power system is need of the day. For uninterrupted power supply, system must employ good quality equipment. Hence all power system equipment must pass through some stringent quality tests before putting into service. Short circuit is one of the mandatory type tests conducted under the most

severe conditions. It is performed to prove the equipment’s ability to withstand the severities under the condition of actual fault. Short circuit alternator Energy requirement during short circuit test is enormously high and equals to equipment’s fault level. Normally these tests require a stable source with high fault power level. A specially designed high power three phase alternator is best suited as a source of this energy.

Figure 1: Alternator with short circuit test circuit.

Fig. 1 shows the single line diagram of short circuit test conducted in the laboratory. Conventional alternators are normally coupled with a prime mover for the mechanical energy input. But the machine taken up here is without any prime mover. Rather the said machine is connected with Static Frequency Converter (SFC) as a static drive. Starting, running and stopping of this machine are done with this SFC. Another significant benefit with SFC is that the single



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machine is operable in dual mode, either as a motor or as a alternator. That’s why it is also popularly known as motor-less alternator. By virtue of this static drive the alternator is run as a synchronous motor to speed up to its rated rpm. Alternator mode is switched only prior to short circuit tests to supply electric energy to test object. III. STATIC FREQUENCY CONVERTER Description SFC works on the principle of LCI (Load Commutated Inverter) and is very much popular in the field of gas turbine base power plant, pump storage power plant and railways [4] - [8]. LCI operation is simple and reliable. It uses load-commutated, phase-controlled thyristor technology to supply power to the stator windings of a high efficiency synchronous motor. The power circuit has a source rectifier connected to the input power supply, a DC link reactor and an inverter connected to the synchronous motor (Fig. 2). The source rectifier connected with the reactor acts as a DC current source. Its output current is impressed at the DC input of the machine side inverter [9]. The LCI controls the motor torque to regulate motor speed. Motor torque is controlled through the DC link current. The only problem faced in the LCI is for the load side thyristor commutation at low speed where natural commutation doesn’t take place hence forced commutation is used [10]. This is happened due to low induced back emf generation at machine terminals. Normally this problem persists up to 10% of the rated speed. Though SFC is used to run a synchronous motor, however implementation of this technology in a short circuit testing plant is actually eliminated a high power motor used as a prime mover for a large synchronous generator [11] – [13].

Figure 2: SFC block diagram.

Operation Frequency converters are of many types depending on the technology and application. The general configuration (Fig. 3) of SFC here is a combination of two 6-pulse thyristor bridges with an intermediate dc link reactor [14], [15]. Hence it is a three stage process. In the first stage, AC power frequency is converted to DC power with a six pulse phase-controlled rectifier known as network bridge (NB). It is controlled in such a way as to produce a unidirectional current of controllable magnitude in

smoothing inductor. Smoothing reactor filters out the ripples from the DC current in the second stage. In the third stage this current is then switched in a thyristor inverter called machine bridge (MB) to provide variable frequency three phase current to a synchronous motor. MB produces three-phase alternating current, the frequency of which is varied from a very low value up to the nominal value [16]. The current waveform of the inverter is of the six-step type. Voltage waveform is dependent on the properties and loading of the synchronous motor. Precise speed control can be achieved through inverter frequency control. Reversal of rotation can also be achieved by reversal of the phase sequence of the thyristor switching as the frequency goes through zero value. Reversal of power flow is achieved by reversing the polarity of the voltage in the direct-current link between the rectifier and the inverter. Both these bridges can also be made to operate in vice versa mode depending upon the machine requirements e.g. motoring, braking etc. Thus, all four-quadrant operation of the drive is possible. With these features of SFC, the machine can be operated in dual mode i.e. motor and generator. Thyristor firing angle in the NB and MB are set by SFC controller with various feedback loops (Fig. 3). This controller is popularly known as power electronic controller (PEC) which acts very fast in µsec range during short circuit test sequence [17], [18].

Figure 3: Schematic of static drive for short circuit alternator. Waveforms & FFT Analysis Waveforms are recorded at the various points of SFC while it is in operation. Being a current source inverter its current waveforms are much more distorted than the voltage waveforms. The input and output voltage & current waveforms and their FFTs of SFC are depicted in Fig. 4a to Fig. 7b.

Figure 4a: SFC input voltage.



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Figure 4b: FFT of SFC input voltage.

Figure 5a: SFC input current.

Figure 5b: FFT of SFC input current.

Figure 6a: SFC output voltage.

Figure 6b: FFT of SFC output voltage.

Figure 7a: SFC output current.



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Figure 7b: FFT of SFC output current.

IV. SYSTEM MODELLING To understand the intricacies of any system a thorough analysis is necessary, which can be done in a better way through modelling. MATLAB/Simulink [19] software is found to be appropriate to make the system model. Model of the existing system is created (Fig. 8) with the data (Table 1) drawn from the working system. In Simulink model, one three phase source, one step down transformer, two thyristor bridges are taken. A reactor is placed in between the two bridges. Step down transformer is inside the rectifier block. Rectifier and inverter both are in six pulse configuration. After running the model various waveforms are plotted. For validation of the model, simulated waveforms are compared with the waveforms from the actual system recorded with a high speed recorder. Machine voltage and current from the simulated and actual system are depicted in Fig. 9 & Fig. 10, which are found to be similar in nature. After the validation of SFC Simulink model, processing of these waveforms is done through Fast Fourier Transformation (FFT), the feature available in the Simulink platform. The frequency spectrums of these waveforms are drawn and %age THD levels at various points are tabulated in Table 2. Parameter Value Input Source 33kV, 3-ph Transformer Nominal Power 3.5MVA Transformer Primary Input, Delta 33kV, 50Hz Transformer Secondary Output, Star 1.72kV Thyristor Bridge 3 arms

Snubber Resistance 2000 Ω Snubber Capacitance 0.1µF Link Reactor 4.8mH Link Reactor 12kV Half Adder 20 Motor Voltage 1.72kV 3-ph S.C. level at base voltage 1500MVA

Table 1 SFC model parameters

Figure 8: Simulink model of SFC used in short circuit

alternator.

Figure 9: Machine voltage.

Figure 10: Machine current.

V. APPLICATION OF MULTILEVEL INVERTER Table 2 reveals that the existing CSI based SFC generates high level of harmonics both in source side as well as in load side. Hence prolonged use of CSI



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based SFC is not advisable for large machine because of generation of shaft voltages which are vulnerable to the bearing of the machine [20]. To suggest a solution, multilevel inverter (MLI) can be a better edge over the conventional CSI based. MLI has numerous benefits. The important one; its output voltage waveforms are near to sinusoidal thus improving its harmonic profile, which is the main objective of this study. Hence a static drive with MLI technology has been proposed for this large machine. To justify this theme some data are necessary to substantiate it. Practical testing on this working system is not advisable. Therefore virtual experimentation is found to be the best way to give any concrete suggestion. With this concept a 3-level CSI inverter is put in place of conventional CSI in the model shown earlier in Fig. 8, keeping all other blocks unchanged. VI. RESULTS & DISCUSSION Simulation study on the 3-level inverter based SFC is carried out. Simulated waveforms and frequency spectrums of source and load for voltage and current are drawn. Outputs of 3-level inverter are compared with the existing inverter and are depicted in Fig. 11 to Fig. 18. All the results are tabulated in Table II.

Figure 11: Simulated source voltage & frequency spectrum

(existing).

Figure 12: Simulated source voltage & frequency spectrum (3-

level inverter).

Figure 13: Simulated source current & frequency spectrum

(existing).

Figure 14: Simulated source current & frequency spectrum (3-

level inverter).

Figure 15: Simulated motor voltage (existing).



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Figure 16: Simulated motor voltage (3-level inverter).

Figure 17: Simulated motor current (existing).

Figure 18: Simulated motor current (3-level inverter).

Parameter Existing CSI Proposed 3-L Inverter Inverter %THD %THD Source Voltage 2.46 3.39 Source Current 18.12 4.88 Motor Voltage 20.74 2.25 Motor Current 30.11 2.67

Table 2 Simulation Results

CONCLUSION In spite of many advantages every static drive has few drawbacks. Harmonics generation is the major one which not only affects the machine but also travels through the connected supply network. Shaft voltage is the outcome of high THD which must be arrested by some suitable measure for safe operation of any large machine. Elimination of these harmonics can be done by proper selection of converter. A system of large machine is taken to study these phenomena. Simulation techniques are applied to find out a suitable solution for the problem of harmonics. Modification of the source is one of the best solutions. With simulation technique multilevel PWM inverter is replaced the conventional current source inverter. Further analysis shows that the proposed inverter reduces harmonics level radically which can be implemented physically. This work is conducted on a particular machine however outcome of this study can be employed on other machines connected with static sources. REFERENCES [1] Oliver Drubel, “Converter Dependent Design of Induction

Machines in the Power range below 10MW”, 1-4244-0743-5/07 2007 IEEE.

[2] P. Langhorst, C. Hancock, "Simple Truth about Motor Drive Compatibility", MagneTek Publication, pp. 24-28, July 1996.

[3] Hisanori Taguchi, Shinzo Tamai, Yasuhiko Hosokawa and Akinobu Ando, “APS Control Method for Gas Turbine Startup by SFC”, International Power Electronics Conference, pp. 264-269, IEEE, 2010.

[4] Shin-Hyun Park, Seon-Hwan Hwang, Jang-Mok Kim, Ho-Seon Ryu, Joo-Hyun Lee, “A Starting-up control algorithm of large synchronous generation motor for Gas Turbosets”, IEEE International Symposium on Industrial Electronics ISIE, pp. 502-508, 2008.

[5] Zhang Yu-Zhi, “Study of Process of Starting Pumped Storage Machines by Static Frequency Converter with Field Current Controlled”, IEEE International Conference on Signal Processing Systems, pp. V1-224 –V1-227, IEEE2010.

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[8] John Rosa, “Utilization and Rating of Machine Commutated Inverter-Synchronous Motor Drives”, IEEE Transactions on Industrial Applications, Vol. IA-15, pp.155-164, Mar./Apr. 1979.

[9] Robert L. Steigerwald and Thomas A. Lipo, “Analysis of a Novel Forced- Commutation Starting Scheme for a Load-Commutated Synchronous Motor Drive”, IEEE Trans. on Industry Applications, Vol. IA-15, No. 1, Jan/Feb 1979.

[10] Datta Arun Kumar, Manisha Dubey, N. R. Mondal, B. V. Raghavaiah, “Motor-less operation of Short Circuit generator – A CPRI Perspective”, International Conference on Electrical Power and Energy Systems (ICEPES-2010), pp.439-445, 26-28 August 2010 MANIT, Bhopal.

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[12] Datta Arun Kumar, Ansari M. A., Mondal N. R., Raghavaiah B. V. “A Novel Use of Power Electronics: Prime Mover-less Alternator with Static Drive & Excitation System”, International Journal of Electronics & Communication Technology (IJECT), Vol. 3, Issue 1, pp. 472-475, January - March 2012.

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A SCHEME FOR HARMONIC REDUCTION WITH MULTILEVEL …pep.ijieee.org.in/journal_pdf/1-463-153001491425-31.pdf · 2018-06-26 · Abstract - Many electrical rotating machines are operated