Sewan Choi

Polyphase transformer arrangements with reduced kVA capacities for harmonic current reduction in rectifier type utility interface

In this paper, polyphase transformer arrangements with reduced kVA capacities are presented for harmonic current reduction in high power rectifier utility interface systems. Based on the concept of an autotransformer, a proposed twelve-pulse rectifier system is realized with a resultant kVA rating of 0.18P/sub 0/ (pu). In this arrangement, the rectifier diodes each conduct for 120 degrees per cycle and the fifth and seventh harmonics are absent from the utility input line current. In the second scheme, an eighteen-pulse rectifier is realized with a kVA rating of 0.16P/sub 0/ (pu), rectifier diode conduction of 120 degrees per cycle and fifth, seventh, eleventh and thirteenth harmonic cancellation in the utility line currents. Additional applications for the proposed polyphase transformer arrangements include twelve and eighteen-pulse systems feeding multiple six-pulse nonlinear loads, such as AC motor drives, with an appropriate phase shift, and this arrangement substantially reduces the utility line current harmonics. For example, it is shown that the fifth and seventh harmonics generated by two separate six-pulse nonlinear loads are subtracted and this contributes to reduced harmonic distortion. Simulation results verifying the proposed concept are presented. >

Indirect Current Control Algorithm for Utility Interactive Inverters in Distributed Generation Systems

Distributed generation (DG) systems should go into intentional islanding operation to back up private emergency loads when the main grid is out of electric power. Conventional utility interactive inverters which are normally operated in current mode must change their operation into voltage mode to ensure stable supply voltage for emergency loads when intentional islanding operation occurs in DG systems. During the transition between current mode and voltage mode, a serious transient problem may occur on the output terminal voltage of utility interactive inverters. This paper proposes a new inverter system and its control algorithm for seamless transfer during intentional islanding operation in DG systems. Filter design guidelines and data for LCL filters that is appropriate for the proposed control algorithm are also presented.

New 24-pulse diode rectifier systems for utility interface of high-power AC motor drives

This paper proposes two new passive 24-pulse diode rectifier systems for utility interface of pulsewidth modulated (PWM) AC motor drives. The first approach employs an extended delta transformer arrangement, which results in near equal leakage inductance in series with each diode rectifier bridge. This promotes equal current sharing and improved performance. A specially tapped interphase transformer is then introduced with two additional diodes to extend the conventional 12-pulse operation to 24-pulse operation from the input current point of view. The proposed system exhibits clean power characteristics with fifth, seventh, eleventh, thirteenth, seventeenth, and nineteenth harmonics eliminated from the utility line currents. The second scheme is a reduced voltampere approach employing autotransformers to obtain 24-pulse operation. The voltampere rating of the polyphase transformer in the second scheme is 0.23P/sub 0/ (PU). Detailed analysis and simulations verify the proposed concept, and experimental results from a 208-V 10-kVA rectifier system are provided.

A new active interphase reactor for 12-pulse rectifiers provides clean power utility interface

In this paper, a new active interphase reactor for twelve-pulse diode rectifiers is proposed. The proposed system draws near sinusoidal currents from the utility. In this scheme, a low kVA [0.02 P/sub o/ (PU)] active current source injects a triangular current into an interphase reactor of a twelve-pulse diode rectifier. This modification results in near sinusoidal utility line currents with less than 1% THD. It is further shown that a low kVA, 12-pulse system with an autotransformer arrangement [kVA rating of 0.18 P/sub o/ (PU)] can be implemented with the proposed active interphase reactor. The resulting system draws clean power from the utility and is suitable for powering larger kVA AC motor drive systems. Detailed analysis of the proposed scheme along with design equations is illustrated. Simulation results verify the concept. Experimental results are provided from a 208 V, 10 kVA rectifier system.

A Low Cost Utility Interactive Inverter for Residential Fuel Cell Generation

This paper presents the development of a single-phase utility-interactive inverter for residential power generation to meet the specifications laid down for the 2005 Future Energy Challenge Competition sponsored by U.S. DOE and IEEE. The proposed inverter system is capable of working in both stand-alone and grid-connected mode. A control scheme for implementing both modes of operation is presented, which has simple structure with smaller number of sensors. The proposed control algorithm including the whole system control is implemented on a low cost, fixed-point DSP TMS320F2812. The experimental results from a 1 kW prototype show that the proposed inverter system exhibits not only low THD grid current during the grid-connected mode and well regulated inverter voltage during the stand-alone mode, but also smooth and automatic transfer between the two modes of operation.

Multiphase DC–DC Converters Using a Boost-Half-Bridge Cell for High-Voltage and High-Power Applications

In this paper, multiphase dc-dc converters are proposed for high-voltage and high-power applications. A generalized converter is configured such that the boost-half-bridge (BHB) cells and voltage doublers are connected in parallel or in series to increase the output voltage and/or the output power. In addition to reduced device voltage and current ratings by the connection, the proposed converter has the following features: high-step-up voltage gain with significantly reduced transformer turn ratio, low-input current ripple due to interleaving effect, zero-voltage switching turn-ON of switches and zero-current switching turn-OFF of diodes, no additional clamping and start-up circuits required, high-component availability and easy thermal distribution due to the use of multiple small components, and flexibility in device selection resulting in optimized design. A design guideline of determining the optimum circuit configuration for given output voltage and power level is presented. Experimental results are also provided to validate the proposed concept.

An approach to realize higher power PWM AC controller

PWM controlled AC controllers exhibit difficulties in the commutation of inductive load current from one bidirectional switch to another due to finite switch on/off times. A four step switching strategy for repeated on/off operation of bidirectional switches in an AC controller is proposed. The approach permits safe transition of inductive load current at all power factors from one bidirectional switch to another, even in the presence of source side stray inductance. Results from an experimental prototype AC controller coupled to an inductive load are discussed.<<ETX>>

Indirect Current Control for Seamless Transfer of Three-Phase Utility Interactive Inverters

This paper proposes an indirect current control algorithm for seamless transfer of three-phase utility-interactive voltage source inverters. With the proposed method the inverter is able to provide critical loads with a stable and seamless voltage during the whole transition period including both clearing time and control mode change. The LCL filter design which is suitable for the indirect current control is also proposed to meet the harmonic limits. The proposed control method is validated through simulation and experiment.

High Step-Up Soft-Switched Converters Using Voltage Multiplier Cells

In this paper, a soft-switched dc-dc converter using voltage multiplier cells is proposed for high-step up application. The proposed converter has the following advantages: 1) doubled voltage conversion ratio of the basic configuration (N = 1) compared to the conventional boost converter; 2) zero voltage switching turn ON of switches and zero current switching turn OFF of diodes; 3) low input current ripple, reduced size of the passive component and current stresses of switches due to interleaved structure, and 4) increased flexibility in device selection by adjusting the number of voltage multiplier cells. The proposed converter is compared to some high step-up converters. Experimental results from a 1-kW prototype are provided to validate the proposed concept.

Soft-Switched Interleaved Boost Converters for High Step-Up and High-Power Applications

This paper proposes a generalized scheme of new soft-switching interleaved boost converters that is suitable for high step-up and high-power applications. The proposed converter is configured with proper numbers of series and parallel connected basic cells in order to fulfill the required output voltage and power levels, respectively. This leads to flexibility in device selection resulting in high component availability and easy thermal distribution. Design examples of determining the optimum circuit configuration for given output voltage and power level are presented. Experimental results from a 1.5-kW prototype are provided to validate the proposed concept.