An Active Interphase Transformer for 12-Pulse Rectifier System to Get the Performance Like 24- Pulse Rectifier System

Transcription

1 An Active Interphase Transformer for 12-Pulse Rectifier System to Get the Performance Like 24- Pulse Rectifier System Milan Anandpara Tejas Panchal Vinod Patel Deaprtment of Electrical Engineering Deaprtment of Electrical Engineering AGM, R&D Department Institute of Technology Institute of Technology Amtech Electronics (I) Ltd. Nirma University Nirma University Gandhinagar, India Ahmedabad, India Ahmedabad, India Abstract- in this paper, an active interphase transformer is implemented in 12-pulse diode bridge rectifier system. This system draws near sinusoidal supply from the utility. In this paper, simulation results are provided for two schemes. Scheme I consists of an autotransformer (0.2 P 0 (PU)), two active interphase transformer, two diode bridge rectifiers & PWM VSI. A PWM VSI injects triangular shaped current into the auxiliary winding of active interphase transformer connected between two diode bridge rectifiers. This modification result in reduction of harmonics in utility line current. In scheme II boost converter is incorporated across the auxiliary winding of active interphase transformer. Both schemes draws clean power from the utility & is suitable for powering high power ac motor drive system, power supplies, UPS system to meet IEEE-519 harmonic current limit. A PWM VSI & boost converter connected across auxiliary winding of active IPT is of low kva & it is not directly exposed to line transients. Simulation results are provided for 415 V, 50 kva 12-pulse rectifier system for both schemes. Index Terms- autotransformer, actie interphase transformer, PWM VSI, harmonic reduction, boost PFC circuit. I. INTRODUCTION In most power electronics applications, diode rectifiers are commonly used in the front end of power converter as an interface with the electrical utility. The nonlinear operation of the diode bridge rectifiers causes highly distorted input current. The non-sinusoidal shape of the input current drawn by the rectifiers causes a number of problems in the sensitive electronic equipment. The distorted input current flowing through the system produces distorted voltages at the point of common coupling (PCC). Due to nonlinear nature of the diode bridge rectifier input line current have significant harmonics. For ac motor drive systems with no dc link smoothing inductor, the discontinuous conduction of the diode bridge rectifier result in a high THD in utility line current.[5] A number of methods have been proposed for harmonics reduction in utility line current. A conventional 12-pulse diode bridge rectifier results in 5 th & 7 th harmonics cancellation in utility line current. Many multipulse converters have been introduced to achieve clean power such as 12-pulse & 24- pulse systems. These multipulse converters are formed by combination of 6-pulse converter & isolation transformer (Y-Y & Y- ), which increase cost & complexity. In this scheme an active interphase transformer is incorporated in 12- Pulse diode bridge rectifier system with autotransformer arrangement. An interphase transformer is required to ensure the independent operation of two parallel connected diode bridge rectifiers.[5] In this paper two possible ways for implementation are shown and are named scheme I & II. In both schemes an autotransformer of low kva (0.2 P 0 ) is employed to generate 30 phase shift between two diode bridge rectifier which drastically reduces the cost, weight & volume over the conventional system (Y-Y & Y- connected 12-pulse rectifier system). Primary winding of Active IPT works as ZSBT. This winding will offer high impedance to cross conduction paths between two diode bridge rectifiers & offer low impedance and give independent operation of rectifier bridges in 12-pulse rectifier system. In scheme I low kva PWM VSI (Full Bridge) is connected across the auxiliary winding of active IPT. It will inject triangular shaped current I x into auxiliary winding of active IPT, near sinusoidal line current flow from the utility. In scheme II boost converter is connected across the auxiliary winding of Active Interphase Transformer. It will draw triangular shaped current I x, this will reduce harmonics in the utility line current to near sinusoidal current shape. Both the schemes I & II are rugged & give good performance with reduced kva components. II. SCHEMEI ACTIVE IPT WITH PWM VSI (FULL BRIDGE) Fig.1 shows the circuit diagram of this system. This system employs a polyphase autotransformer to provide ±15 phase shift between two Rectifier Bridges I & II & active interphase transformer. In this scheme inductance of interphase reactor (primary winding) & line impedances L s1 & L s2 are designed properly to get 12-pulse operation. A PWM VSI (full bridge inverter) injects triangular shape current (I x ) into the auxiliary winding (N x ) of active interphase transformer, by doing this near sinusoidal currents flow from the utility with less than 5% THD. This 12-pulse rectifier system has lower kva magnetic & fewer component counts. If the PWM VSI were to fail, this circuit will give 12-pulse operation with 5 th & 7 th harmonics cancellation in utility line current /14/$ IEEE

2 A. Implelentation of PWM gatingsignal generator for full bridge inverter Fig.1 Circuit diagram of 12-pulse rectifier system with active interphase transformer& PWM VSI. Fig.3 Block diagram of PWM gating signal generator for PWM VSI. Fig. 3 shows the block diagram of gating signal generator for PWM inverter. The reference for the injected current is synchronized with the input voltages & is configured with standard digital logic circuits & phase locked loop electronics. The reference current and the feedback current are compared and the current error is then compared to a triangular carrier wave to generate PWM gating signals for inverter. The close loop operation of this scheme ensures to follow for clean power requirement. B. Analysis of scheme I In this section, analysis of scheme I is presented in detail. Fig. 2 & 2 show the winding configuration & vector diagram of the autotransformer., sin (1), sin 15 (2),, , (3) Fig.2 Delta connected autotransformer vector configuration on a three limb core. diagram winding tan (4), (5) Where is the rms of the line to line voltage. kva rating of delta connected autotransformer is found out by, 6 3 (6) 0.2 (7) This 12-pulse rectifier requires a transformer kva of only 20% of the system kva. Thus transformer size is reduced by 80% as compared to conventional (Y-Y & Y- ) 12-pulse scheme. C. kva rating of Active Interphase Transformer & PWM VSI The Voltage (V m ) across primary winding is found out by, (8)

3 The voltage (V x ) across the auxiliary winding in the active interphase transformer is given by, The rms value of injected current Ix is given by, (9) operation with 5 th & 7 th harmonics cancellation in utility line current. A. Implementation of the single phase boost PFC circuit (10) The kva rating of active interphase transformer is, (11) (12) The kva rating of PWM VSI is given by, (13) Fig.5 Circuit diagram of Boost PFC Circuit (14) Magnetizing inductance L 1 for primary winding of active interphase transformer is given by,., Where K 1 is the desired percentage ripple of load current I 0. The active interphase transformer employed designed for turn ratio N P: N x = 1: 2. (15) in simulation was The MC34262 are active power factor controllers specifically designed for use as a preconverter in electronic ballast & in off-line power converter application. The load current I 0 is sensed and used in the control of I x magnitude, such that near sinusoidal current flow from the utility under varying load condition. The boost converter is of low kva (0.05 (pu)) of system power. The kva rating of autotransformer & active interphase transformer is same as that of describe in scheme I.[5] Table I. Specification for both schemes of 50 kva, 415 V 12-pulse rectifier systems III. SCHEME II ACTIVE IPT WITH BOOST PFC CIRCUIT Component Rating System kva Active Interphase Transformer Autotransformer Magnetizing Inductance of Interphase reactor L 1 50 Kva 4.7 kva 10 kva 25 mh AC Line Reactance L s1, L s2 kva rating of PWM VSI Switching Frequency of Inverter Output Voltage Output Current kva rating of boost converter Inductance of boost converter 770 μh 5.5 kva 5000 Hz 580 V 77.5 A 2.25 kva 2 mh Fig.4 Circuit diagram of 12-pulse rectifier system with active interphase transformer& Boost PFC Circuit. In this scheme boost converter is connected across auxiliary winding of active interphase transformer. The boost converter output is fed back to the dc link. The boost converter is controlled by ON SEMICONDUCTORS MC Current I x in the auxiliary winding of Active IPT reduce harmonics in the utility line current to a near-sinusoidal current shape. When current I x become zero, this circuit will give 12-pulse IV. SIMULATION RESULTS System shown in Fig. 1 is simulated in PSIM. Specification of 415 V, 50 kva 12-pulse rectifier systems with PWM VSI & boost converter is given in Table I. A. Simulation results at full load (scheme I) These are the simulation results when PWM VSI injects triangular shaped current I x into the auxiliary winding of the active interphase transformer.

4 (f) Fig.6 Simulation results at full load condition Input line current I a. Rectifier-I input current I a1. Rectifier-I output current I d1. (d) Triangular shaped injected current I x. (e) Output Power P o. (f) Frequency spectrum of input line current. B. Simulation results under varying load condition (scheme I) (d) Fig.7 Input line current waveforms I a at 75% load at 50% load at 25 % load. (e) C. Simulation results (scheme II) These are the simulation results when boost PFC converter draws triangular shaped current I x into the auxiliary winding of the active interphase transformer. Simulation results under varying load condition are shown in Fig. 8, 8, 8, 8(d) & 8(e).

5 Table II. % THD of supply current under varying load condition for both scheme I & II 12-Pulse Rectifier system with Active IPT at varying load %THD of Supply Current for Scheme I %THD of Supply Current for Scheme II 100% load 2.8% 4.9% 75% load 3% 4.95% 50% load 3.7% 5% 25% load 4.3% 5.1% D. Simulation results without current injection I x for scheme I & II (d) Fig.9 Input line current I a. Rectifier-I input current I a1. Frequency spectrum of input line current I a. If active PWM VSI or boost converter malfunctions, circuit showed in Fig. 1 & Fig. 4 gives 12-pulse operation with 5 th & 7 th harmonics cancellation as (% THD = 11%) shown in Fig. 9. (e) Fig.8 Input line current I a. Rectifier-I input current I a1. Rectifier-I output current I d1. (d) Triangular shaped injected current I x (e) Output Power P o. V. CONCLUSION By incorporating active interphase transformer in 12-pulse rectifier system, a suitable method of harmonic reduction in utility line currents can be achieved. This system provide clean input power, while non-linear loads such as ac drives,

6 UPS system, induction heating systems & power supplies are connected to electrical utility in both schemes I & II. A triangular shaped current of (300 Hz frequency) is injected into the auxiliary winding of active interphase transformer yields near sinusoidal utility line currents in both schemes I & II. The utility line current THD is measured around 2.8% in scheme I& 4.9 % in scheme II at full load. Both schemes I & II gives superior performance under varying load condition with reduced kva component. This system meets the IEEE- 519 harmonics current standard for the utility line current. ACKNOWLEDGEMENT I show gratitude toward Mr. Vinod Patel, AGM R&D Department, AMTECH Electronics (I) Ltd., who gave me a chance to do this project & their guidance during a project. I would like to thank Mr. Tejas Panchal, Assistant Professor, Nirma Institute of Technology for their positive attitude toward the project. Special Thanks to my parents & almighty for their encouragement during the project. REFERENCES [1] D.A. Paice, Power Electronics Converter Harmonics: Multipulse Method For Clean Power, New York: IEEE Press, [2] P. Enjeti and I. Pitel, An active interphase reactor for 12-pulse rectifier, U.S. Patent disclosure. [3] S. Choi, P. Enjeti and I. Pitel, New polyphase transformer arrangement with reduced kva capacities for harmonic current reduction in rectifier type utility interface, in IEEE PESC conf., 1995, pp [4] J. Schaefe, Rectifier Circuits, Theory and Design, New York: Wiley, [5] Bang Sup Lee, Jaehong Hahn, Prasad N. Enjeti and Ira J. Pitel A Robust Three-Phase Active Power-Factor-Correction and Harmonic Reduction Scheme for High Power, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 46, NO. 3, [6] S. Miyari, S. Lida, K. Nakata and S. Masukawa New method for reducing harmonics involved in input and output of rectifier with interphase reactor, IEEE Trans. Ind. Application, vol.ia-22,pp , September-Octomber 1986.