Raja Ayyanar

Active input-voltage and load-current sharing in input-series and output-parallel connected modular DC-DC converters using dynamic input-voltage reference scheme

This paper explores a new configuration for modular DC/DC converters, namely, series connection at the input, and parallel connection at the output, such that the converters share the input voltage and load current equally. This is an important step toward realizing a truly modular power system architecture, where low-power, low-voltage, building block modules can be connected in any series/parallel combination at input or at output, to realize any given system specifications. A three-loop control scheme, consisting of a common output voltage loop, individual inner current loops, and individual input voltage loops, is proposed to achieve input voltage and load current sharing. The output voltage loop provides the basic reference for inner current loops, which is modified by the respective input voltage loops. The average of converter input voltages, which is dynamically varying, is chosen as the reference for input voltage loops. This choice of reference eliminates interaction among different control loops. The input-series and output-parallel (ISOP) configuration is analyzed using the incremental negative resistance model of DC/DC converters. Based on the analysis, design methods for input voltage controller are developed. Analysis and proposed design methods are verified through simulation, and experimentally, on an ISOP system consisting of two forward converters.

Common-duty-ratio control of input-series connected modular DC-DC converters with active input voltage and load-current sharing

This paper proposes a simple control method to achieve active sharing of input voltage and load current among modular converters that are connected in series at the input and in parallel at the output. The input-series connection enables a fully modular power-system architecture, where low voltage and low power modules can be connected in any combination at the input and/or at the output, to realize any given specifications. Further, the input-series connection enables the use of low-voltage MOSFETs that are optimized for very low RDSON , thus, resulting in lower conduction losses. In the proposed scheme, the duty ratio to all the converter modules connected in input-series and output-parallel (ISOP) configuration is made common. This scheme does not require a dedicated input-voltage or load-current-share controller. It relies on the inherent self-correcting characteristic of the ISOP connection when the duty ratio of all the converters is the same. The proposed scheme is analyzed using the average model of a forward converter. The stability and performance of the scheme are verified through numerical simulation, both in frequency domain and in time domain. The proposed control method is also validated on an experimental prototype ISOP system comprising of two forward converters

Control Strategy to Mitigate the Impact of Reduced Inertia Due to Doubly Fed Induction Generators on Large Power Systems

The present work is based on developing a control strategy to mitigate the impact of reduced inertia due to significant DFIG penetration in a large power system. The paper aims to design a supplementary control for the DFIG power converters such that the effective inertia contributed by these wind generators to the system is increased. The paper also proposes the idea of adjusting pitch compensation and maximum active power order to the converter in order to improve inertial response during the transient with response to drop in grid frequency. Results obtained on a large realistic power system indicate that the frequency nadir following a large power impact in the form of generators dropping out is effectively improved with the proposed control strategy. The proposed control is also validated against the sudden wind speed change in the form of wind gust downs and wind ramp downs occurring in conjunction with the generators dropping out. A beneficial impact in terms of damping power system oscillations is also observed, which is validated by eigenvalue analysis. The affected mode is then excited with a large disturbance in time domain. The damping improvement observed in time domain and subsequent Prony analysis support the result obtained from eigenvalue analysis.

Topology comparison for Solid State Transformer implementation

In this paper, a comparison of six representative topologies for the implementation of Solid State Transformers (SST) is performed. The objective is to help identify the most suitable topology capable of supporting additional functionalities as compared to a regular transformer, e.g. on-demand reactive power support to grid, voltage regulation, and current limiting. The comparison is based on switch loss, switch count, control characteristics and supported functionalities. It has been concluded that a three-stage configuration comprising distinct AC-DC, DC-DC and DC-AC stages results in the most suitable implementation. A Simulink model corresponding to this three-stage configuration is developed to demonstrate the desired characteristics and functionalities of the SST.

Fault Current Contribution From Synchronous Machine and Inverter Based Distributed Generators

There are advantages of installing distributed generation (DG) in distribution systems: for example, improving reliability, mitigating voltage sags, unloading subtransmission and transmission system, and sometimes utilizing renewables. All of these factors have resulted in an increase in the use of DGs. However, the increase of fault currents in power systems is a consequence of the appearance of new generation sources. Some operating and planning limitations may be imposed by the resulting fault currents. This paper discusses a model of inverter based DGs which can be used to analyze the dynamic performance of power systems in the presence of DGs. In a style similar to protective relaying analysis, three-dimensional plots are used to depict the behavior of system reactance (X) and resistance (R) versus time. These plots depict operating parameters in relation to zones of protection, and this information is useful for the coordination of protection systems in the presence of DG

Probabilistic Power Flow Studies for Transmission Systems With Photovoltaic Generation Using Cumulants

This paper applies a probabilistic power flow (PPF) algorithm to evaluate the influence of photovoltaic (PV) generation uncertainty on transmission system performance. PV generation has the potential to cause a significant impact on power system reliability in the near future. A cumulant-based PPF algorithm suitable for large systems is used to avoid convolution calculations. Correlation among input random variables is considered. Specifically correlation between adjacent PV resources are considered. Three types of approximation expansions based on cumulants, namely the Gram-Charlier expansion, the Edgeworth expansion, and the Cornish-Fisher expansion, are compared, and their properties, advantages, and deficiencies are discussed. Additionally, a novel probabilistic model of PV generation is developed to obtain the probability density function (PDF) of the PV generation production based on the environmental conditions. The proposed approaches with the three expansions are compared with Monte Carlo simulations (MCS) with results for a 2497-bus representation of the Arizona area of the Western Electricity Coordinating Council (WECC) system.

Fault Tolerant Circuit Topology and Control Method for Input-Series and Output-Parallel Modular DC-DC Converters

This paper presents a modular, fault tolerant dc-dc converter topology that utilizes common duty ratio control to ensure equal sharing of input voltage and output current in input-series output-parallel configuration. The input-series connection allows the use of low voltage MOSFETs optimized for very low RDS,ON resulting in lower conduction losses. The common-duty-ratio scheme does not require a dedicated control loop for input voltage or output current sharing. The fault tolerant protection and control scheme accommodates failure of one or more modules, and ensures input voltage and load current sharing among the remaining healthy modules. The design of a new sensing scheme for detection of fault is presented. The analysis of the topology and the underlying principles are presented. The dependence of peak current from the source and in the protection switch in case of failure of a single converter has been analyzed and the various design tradeoff issues are discussed. The theoretical predictions are validated with simulation and experimental results. The proposed method is simple and gives good dynamic response to changes in input, load, and during fault. This topology is especially suited for space applications where a high level of fault tolerance can be achieved through designed redundancy.

Input-series and output-series connected modular DC-DC converters with active input voltage and output voltage sharing

This paper proposes a new modular configuration, namely the input-series and output-series connection for DC-DC converters. A three-loop control scheme, including a dedicated input voltage controller is proposed to achieve equal sharing of the input as well output voltages by the series connected modules. The reference to the input voltage loop is chosen as the average of all converter input voltages. Such a reference minimizes the interaction among the various control loops. The proposed scheme is validated experimentally on a 400 W prototype system comprising of two forward converters.

Series-parallel connection of DC-DC converter modules with active sharing of input voltage and load current

This paper proposes a novel control strategy, such that equal rating DC-DC power converter modules, can be connected in series at the input side for higher input voltages and in parallel at the output side for higher output currents. Closed-loop control ensures that each module equally shares the total input voltage and the output current. Analytical discussion is confirmed by a laboratory hardware prototype. Such modular converters may find immediate application in working with various utility voltages around the world.

Building Block Converter Module for Universal (AC-DC, DC-AC, DC-DC) Fully Modular Power Conversion Architecture

This paper presents the configuration and control methods for a building block module suitable for application in a fully modular, AC-DC, DC-AC and DC-DC power conversion architecture, where the modules can be connected in any combination of series and parallel connections and support bidirectional power flow. The advantages of the proposed configuration, which is a combination of AC PWM full bridge stage and a dual active bridge based isolated DC-DC stage, are explained. The control implementations and average models of the building block module are discussed in detail. Analytical, simulation and experimental results are presented that confirm the feasibility of the fully modular power conversion approach with the proposed building block module.