Mukul C. Chandorkar

Advanced Control Architectures for Intelligent Microgrids—Part I: Decentralized and Hierarchical Control

This paper presents a review of advanced control techniques for microgrids. This paper covers decentralized, distributed, and hierarchical control of grid-connected and islanded microgrids. At first, decentralized control techniques for microgrids are reviewed. Then, the recent developments in the stability analysis of decentralized controlled microgrids are discussed. Finally, hierarchical control for microgrids that mimic the behavior of the mains grid is reviewed.

A Generalized Computational Method to Determine Stability of a Multi-inverter Microgrid

Microgrid-containing parallel-connected inverters, where each inverter is controlled by decentralized active power/voltage frequency and reactive power/voltage magnitude droop control laws results in flexible and expandable systems. These systems have been known to have stability problems for large values of active power/voltage frequency droop control gain. However, so far the stability analysis of multi-inverter systems has always been performed in a computationally intensive manner by considering the entire microgrid. In a practical microgrid, where the number of inverters may be large or the capacity of the units may differ, it becomes essential to develop a method by which stability can be examined without much computational burden. The system of differential algebraic equations has been simplified using justifiable assumptions to result in a final expression that allows the stability of the microgrid to be examined separately with respect to the droop control laws of each inverter transformed into an equivalent network. Moreover, the procedure allows taking into consideration the R/X ratio of the interconnecting cables. Analysis of final expressions validate the stability results reported in literature. Experimental results on hardware show the stable operation of the microgrid.

Improvement of Transient Response in Microgrids Using Virtual Inertia

Generation is shifting from a centralized power generating facility having large synchronous generators to distributed generation involving sources of smaller capacity. Most of these sources require inverters on the front end while being connected to the grid. Lower available kinetic energy, coupled with less short-circuit current ratio compared to large synchronous generators, compromises the transient stability of the microgrid when isolated from the main grid. Sources in the microgrid use droop control to share power according to their capacity without any form of communication. This paper proposes a novel controller for inverters to improve the frequency response of microgrid under disturbances involving large frequency deviations. It also discusses design of various parameters defined for the proposed control. The microgrid, which has two inverters and two synchronous generators, is simulated using Simulink/MATLAB software to test the proposed control strategy.

Novel architectures and control for distributed UPS systems

This paper is concerned with the circuit configuration and control of parallel distributed uninterruptible power supply (UPS) systems. These systems are expected to provide high quality continuous power to critical loads in the face of mains disturbances or failure. This paper discusses the system configuration issues of distributed UPS. These can be classified as online or line-interactive systems. Decentralized control without signal communication is a major requirement for distributed systems. This paper discusses the control issues which need to be addressed before distributed UPS systems can become practical.<<ETX>>

Real-Time Electrical Load Emulator Using Optimal Feedback Control Technique

This paper presents a method of emulating electrical loads using power electronic converters. The loads include machines such as induction motors and their associated mechanical load and also more complex machine systems such as wind-driven generators. The load emulator is, effectively, a dynamically controllable source or sink which is capable of bidirectional power exchange with either a grid or another power electronic converter system. Using load emulation, the feasibility of connecting a particular machine to a grid under various load conditions can be examined without the need for any electromechanical machinery. This paper considers the case of a power electronic voltage source inverter (VSI) emulating a three-phase induction motor connected to a three-phase ac grid. The VSI is operated in a mode where the current drawn from the ac grid is controlled by closed-loop control. The consistency of the experimental results with the simulation results proves the ability of the emulator and the proposed testing approach.

A Multifunctional Four-Leg Grid-Connected Compensator

There has been a growing demand for using active power filters (APFs) for grid-connected converter systems based on nonconventional energy sources such as solar, wind, and fuel cells. In addition to power quality conditioning, the APF can also be used for bidirectional active power exchange with the three-phase four-wire grid and, therefore, acts as a multifunctional compensator. This paper presents a four-leg shunt multifunctional grid-connected power quality compensator. The compensator reference currents are generated using the instantaneous p-q-r theory. This paper proposes a simple modification to the conventional theory to eliminate the source neutral current from the three-phase four-wire power system. A digital controller is used to implement dead-beat current control. The outputs of the digital current controller are used as reference voltages for a 3-D space vector modulator (3D-SVM). A new approach for pulse width modulation waveform generation in 3D-SVM is proposed. All the intermediate steps are discussed from the viewpoint of implementation on a digital signal processor field-programmable gate array platform. The multifunctional compensator is analyzed, simulated, and tested experimentally for the most practical conditions in the power system, and the results are presented.

Adaptive Current Differential Protection Schemes for Transmission-Line Protection

Throughout the history of power system protection, researchers have strived to increase sensitivity and speed of apparatus protection systems without compromising security. With the significant technological advances in wide-area measurement systems, for transmission system protection, current differential protection scheme outscores alternatives like overcurrent and distance protection schemes. Therefore, in this paper, we address this challenge by proposing a methodology for adaptive control of the restraining region in a current differential plane. First an error analysis of conventional phasor approach for current differential protection is provided using the concept of dynamic phasor. Subsequently, we extend the methodology for protection of series compensated transmission lines. Finally, we also evaluate the speed versus accuracy conflict using phasorlets. Electromagnetic Transient Program simulations are used to substantiate the claims. The results demonstrate the utility of the proposed approach.

New techniques for inverter flux control

Inverter flux control methods directly control the integral of the inverter output voltage vector, by means of the inverter switching. These methods have found applications in low- and medium-voltage industrial drives, and in inverters with outputs connected to the utility mains. This paper describes a new method for the direct control of the inverter flux vector, with the viewpoint of using this vector as the main forcing quantity in a closed-loop control system. It illustrates the use of the inverter flux in the control of a utility-connected inverter.

Decentralized operation of distributed UPS systems

This paper presents the decentralized operation of an interconnected network of multiple units of uninterruptible power supplies (UPS). The main aspect of decentralized operation is that the control of individual UPS units is based on local measurements only, and there is no data communication across the interconnected network. Also, special attention is given to achieving a single operating mode of controller operation, regardless of changes in the network configuration. The main emphasis of this paper is on the implementation of a decentralized control system for an experimental distributed UPS system. Operating results from the distributed system are presented and discussed.

Three-Dimensional Flux Vector Modulation of Four-Leg Sine-Wave Output Inverters

The time integral of the output voltage vector of a three-phase inverter is often termed the inverter flux vector. This paper addresses the control of a three-phase four-leg sine-wave output inverter having an LC filter at its output, by controlling the flux vector in three dimensions. Flux vector control has the property that an output filter resonance is actively damped by an output voltage control loop alone. Furthermore, an inverter switching action inherently regulates the output voltage rapidly against dc-bus voltage variations. The flux vector control of sine-wave output inverters finds several applications in three-phase four-wire systems. This paper presents a flux modulation method for three-phase four-leg inverters feeding unbalanced and nonlinear loads. All the necessary steps for the digital implementation of the flux modulator are presented. The switching behavior of the modulator has been evaluated, which is useful for the variable fundamental frequency applications of the inverters. To provide experimental validation, the modulator is implemented as a part of the control system for a stand-alone three-phase four-leg inverter with an LC filter at its output. Control system details are also provided. Experimental results indicate the effectiveness of the modulator and control system in providing balanced voltages at the output of the LC filter even under highly unbalanced conditions with nonlinear loads. The resonance damping and voltage regulation properties of the modulator are also apparent from the experimental results.