Sandeep Bala

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.

A high power medium-voltage dc amplifier system

In this paper the concept of a medium-voltage dc amplifier system is proposed. The dc amplifier system must provide a medium-voltage dc bus with the possibility of superposing a high bandwidth time-varying signal and should be capable of producing voltage excursions at a high slew-rate. This is intended to facilitate the development of new technologies, i.e. new high power non-linear loads based on power electronics, in all electric ships as part of the newly proposed MVDC ship power system. This paper sets forth the set of parameters and specifications of the medium-voltage dc amplifier system and proposes a specific circuit topology to achieve the required characteristics. Simulation results verify the feasibility and demonstrate highly dynamic performance of the proposed system configuration.

Autonomous power electronic interfaces between microgrids

The use of back-to-back (BTB) voltage source converters (VSCs) to interface large scale asynchronous ac power systems is well established. BTB VSCs also provide a convenient means to interface small scale microgrids and the utility power grid. The relatively small size of such interfaces and broad scale of their application would mandate autonomous fast operation based on the health of the microgrids, although their preferred operating points (set-points) may be dispatched from relatively slow supervisory systems. This paper presents a method to control such interfaces so as to preserve microgrid-like operation of each ac system and to autonomously transfer power across the interfaces. The paper describes the control architecture, analytical model, and design method that ensure robust operation. We demonstrate the design of a 2-microgrid interface using a dynamic phasor model, which we show to be more accurate than the reduced order VSC model commonly used in microgrid analyses. The interface can also alter the steady state power flow between microgrids to suit energy management strategies in a smart grid. Finally, we present simulation and experimental test results of the interface operation.

On the choice of voltage regulators for droop-controlled voltage source converters in microgrids to ensure stability

Microgrids are emerging to be an attractive means to cluster various distributed generation systems with local loads while operating either in a grid-connected mode or in an islanded mode in a semi-autonomous fashion. Voltage source converter (VSC) based distributed generation devices are often an essential component in such microgrids, particularly in interfacing dc energy storage devices. While system level control of VSCs that incorporate frequency and voltage droop have been shown to be essential in ensuring autonomous system operation in a distributed manner, the impact of VSC internal voltage regulator dynamics on system stability has not been definitively established. This paper presents a systematic comparison of voltage regulators using a dynamic phasor based model of an inverter system interfaced to an infinite grid; the model is used to examine system stability boundaries. The impact of various options for realizing VSC voltage regulators in conjuction with varying interconnection system impedance parameters are studied to establish stability boundaries for each case. A dynamic phasor model is used to evaluate operating point stability and the results are verified using computer simulation results.

Considerations for the design of power electronic modules for hybrid distribution transformers

This paper discusses various factors to be considered in the design and implementation of the power electronics for a hybrid distribution transformer. The hybrid distribution transformer is a voltage and/or VAr compensation device that provides enhanced control in distribution systems. The device comprises a fractionally rated power electronics (PE) module combined with a distribution transformer. The design and selection methodology for the PE module is detailed using qualitative and quantitative methods. Practical considerations as relates to factory acceptance tests are considered in determining the impact of the PE module connected to the transformer for testing and certification.

Lowering failure rates and improving serviceability in offshore wind conversion-collection systems

Reliability and maintenance are key concerns for offshore wind farms. Electrical systems are responsible for more failures than mechanical or structural systems, but they are also comparatively easier to repair. Nevertheless, it is important to lower the rate of failure and improve the ease of maintenance of electrical systems. This paper presents approaches to achieve these goals at the component level as well as at the system level. ABBs PCS6000 is used as the example to discuss component level issues. This paper also compares five possible conversion-collection system architectures considering their reliability and amenability to maintenance. The failure rates can be lowered and the serviceability can be improved by incorporating these considerations properly at the design stage.

Design issues in a medium-voltage DC amplifier with a multi-pulse thyristor bridge front-end

In this paper the design issues of a medium-voltage DC amplifier with a multi-pulse thyristor bridge front-end are presented. A medium voltage dc amplifier is needed in de-risking new technologies coming onboard future electric ships. Based on the required system dynamic specifications, an initial design reveals the problems with a multi-pulse thyristor front-end, and then provides a novel solution to meet the requirements, are presented. The proposed system solution is validated through both simulation and experimental results.

A multi-loop control system for series DC active filter in a medium-voltage DC amplifier

In order to cope with increasing power demand onboard future combatant ships, US navy has embarked on medium-voltage dc as the next generation integrated power system. A viable dc power supply system solution for such ships is based on the mature technology of multi-pulse thyristor bridge front-end rectifiers. Due to the intrinsic ripple characteristic associated with thyristor commutations in such front-ends, a high bandwidth series dc active filter can be used to smooth out the dc-bus. In this paper, we propose a new multi-loop feedback and feed-forward control system for the series dc active filter in order to meet the stringent dc-bus ripple attenuation and disturbance rejection requirements. Analysis and simulations results show promise of multi-loop control scheme. Experimental results of the 12kVA/400Vdc laboratory test-bed with feed-forward control are provided.

Resource Aggregation Using Microgrids

Microgrids are emerging as a flexible approach to aggregate diverse distributed and renewable energy sources with the electric grid. This chapter presents the salient features of the microgrid, including operating modes, example configurations, control, and dynamic properties. A detailed outline of stability properties of the microgrid in DC and AC configurations is presented. A brief summary of their potential and challenges in the future is discussed in the concluding section.