Eddy Aeloiza

Hybrid distribution transformer: Concept development and field demonstration

Todays distribution system is expected to supply power to loads for which it was not designed. Moreover, high penetration of distributed generation units is redefining the requirements for the design, control and operation of the electric distribution system. A Hybrid Distribution Transformer is a potential cost-effective alternative solution to various distribution grid control devices. The Hybrid Distribution Transformer is realized by augmenting a regular transformer with a fractionally rated power electronic converter, which provides the transformer with additional control capabilities. The Hybrid Distribution Transformer concept can provide dynamic ac voltage regulation, reactive power compensation and, in future designs, form an interface with energy storage devices. Other potential functionalities that can be realized from the Hybrid Distribution Transformer include voltage phase angle control, harmonic compensation and voltage sag compensation. This paper presents the concept of a Hybrid Distribution Transformer and the status of our efforts towards a 500 kVA, 12.47 kV/480 V field demonstrator.

Control of three-phase buck-type rectifier in discontinuous current mode

In the three-phase buck-type rectifier, the current on the dc-link inductor becomes discontinuous under light load condition, at which point the current ripple is larger than the dc current value. Traditional control algorithms and modulation schemes do not work consistently well in discontinuous current mode (DCM), causing input current distortion and output voltage ripple. In this paper, the three-phase buck-type rectifier is modeled and analyzed in DCM. The DCM transfer function is derived and compared with the one for continuous current mode (CCM). It is shown that the pole and gain of the DCM transfer function changes significantly compared to that of CCM. A new modulation scheme for DCM is then proposed, which places the space vectors in such way to keep the dc-link current continuous during the active states. A digital controller is then used to eliminate the sampling error caused by the large current ripple, successfully controlling the rectifier in DCM. Simulation and experimental results are used to verify that the input current distortion and the output voltage ripple are dramatically reduced under the proposed DCM modulation and control strategy.

All-SiC power module for delta-type current source rectifier

To reduce the conduction loss, a novel three-phase current source rectifier, named Delta-type Current Source Rectifier (DCSR), has been proposed in previous paper. This rectifier has delta-type connection on its input side, and its dc-link current can be shared by multiple devices at a time to reduce up to 20% conduction loss. The SiC devices are expected to be the next-generation power devices due to their low conduction and switching losses. In this paper, an all-SiC power module is built to realize a high-density DCSR. The switching performance of the power module is characterized under different operation conditions. Then DCSR is compared with the traditional CSR on both switching speed and switching loss. It is shown that the turn-on speed is accelerated and the switching energy is lower in DCSR. The equivalent parasitic inductance is also lower in DCSR with two paralleled minor commutation loops. The switches can operate at higher switching speed without serious resonance in DCSR.

Multilevel multichannel interleaved AC-DC converter for high current applications

In this paper an analysis and implementation of a voltage source multi-level multi-channel interleaved ac/dc converter structure is presented. The primary application is on grid-tie converters to interface renewable energy sources, energy storage devices and VAR Compensators. The multilevel structure ensures reaching higher voltage levels and the parallel interleaved approach allows for reaching higher currents and higher effective carrier frequency. Significant reduction on the size and weight of the passive components, such as the ac interface reactor and the grid side ac filter can be attained, as well as reduction of the ripple and RMS value of the dc bus capacitors current. The converter structure has a modular architecture where each converter sub-module will be able to switch at switching frequencies that would comfortably ensure low total switching losses and increased effective carrier frequency. The reduction of the input inductance will also contribute to reduction on copper losses and therefore higher converter efficiency. The reduction on dc bus capacitors current will result in lower heat dissipation, thereby extending its lifetime. The paper presents an example with three 3-Level interleaved channels where the control of the circulating currents among the sub-channels is attained by the addition of a closed-loop zero-component controller to the traditional d-q synchronous reference frame. Simulations and experimental results for a down-scale prototype 230 V/15 kVA are presented and discussed.

A Novel Three-Phase Current Source Rectifier With Delta-Type Input Connection to Reduce the Device Conduction Loss

The conduction loss of semiconductor devices is large in traditional three-phase current source rectifiers (CSR). In this paper, a new CSR topology is proposed to reduce the conduction loss. This rectifier, named Delta-type Current Source Rectifier (DCSR), has delta-type connection on its input side and its dc-link current can be shared by multiple devices at a time. Its principle of operation, modulation scheme and design method are discussed in detail in this paper. Based on the analysis, the conduction loss can be reduced by up to 20% with the proposed topology. A 7.5 kW prototype is then built to experimentally verify the performance of DCSR.

Modulation scheme for delta-type current source rectifier to reduce input current distortion

To reduce the conduction loss, a novel three-phase current source rectifier, named Delta-type Current Source Rectifier (DCSR), has been proposed in previous research. This rectifier has delta-type connection on its input side, and its dc-link current can be shared by multiple devices at a time to reduce up to 20% conduction loss. A high-efficiency modulation scheme for DCSR has been proposed, where the conduction states involve more switches to share the dc-link current. However, it causes current distortion when the input voltages have intersections. In this paper, the phenomenon is analyzed in detail. The clamped voltage on the diode bridge will fluctuate at the voltage intersections, resulting in false current pulse and distortion. An improved modulation scheme is then proposed for DCSR to reduce the input current distortion without sacrificing much efficiency. Through experiment in a 7.5 kW prototype, its effectiveness is verified and the total harmonic distortion (THD) of the input current is reduced dramatically.

A high current bidirectional DC-DC converter for concept demonstration of grid-scale SMES systems

This paper presents the development and testing results of the high current bidirectional DC-DC converter system for concept demonstration of the next generation high current superconducting magnetic energy storage (SMES) system under a program funded by ARPA-E. The grid-scale SMES system could be enabled by new superconductor materials and manufacturing technologies and targets MWh level energy storage applications. In the SMES power conversion system, interleaving of three-level NPC DC-DC converters with voltage reversal for bidirectional power flow is proposed to meet the requirements of new SMES coil systems, such as high current and voltage ratings, low current and voltage ripples, wide operation range, and modularity. To demonstrate the concept, a 700A/12V SMES lab coil prototype and a 6-channel interleaved bidirectional DC-DC converter with three-level NPC modules have been developed and tested. The paper focuses on the detail design and implementation of the high current DC-DC converter system including the power stage, passive components and control electronics, and the testing results for the proposed converter solution for SMES applications.

Modulation Scheme Analysis for High-Efficiency Three-Phase Buck-Type Rectifier Considering Different Device Combinations

The three-phase buck-type rectifier features a step-down ac-dc conversion function, smaller filter size, inrush current limiting capability, and potential for high efficiency, where its switching loss is dependent on the modulation scheme and the specific semiconductors used. In this paper, three different device combinations are compared through experiments on their switching characteristics for the buck rectifier application. It is shown that the switching performance of two series-connected devices becomes worse than a single device due to the superposition of the nonideal semiconductor characteristics. Moreover, the switching loss in the commutation between two switches is usually higher than the one in the commutation between a switch and the freewheeling diode. Taking into consideration both types of commutations, the switching loss of the buck rectifier is then modeled and the analytical equations are derived for four space vector modulation schemes. According to the analysis, each modulation scheme has its own field for high-efficiency application. The most advantageous modulation scheme is identified in this paper for each of the device combinations investigated.