The increasing interest in integrating intermittent renewable energy sources into microgrids presents major challenges from the viewpoints of reliable operation and control. In this paper, the major issues and challenges in microgrid control are discussed, and a review of state-of-the-art control strategies and trends is presented; a general overview of the main control principles (e.g., droop control, model predictive control, multi-agent systems) is also included. The paper classifies microgrid control strategies into three levels: primary, secondary, and tertiary, where primary and secondary levels are associated with the operation of the microgrid itself, and tertiary level pertains to the coordinated operation of the microgrid and the host grid. Each control level is discussed in detail in view of the relevant existing technical literature.

Trends in Microgrid Control

The placement of a minimal set of phasor measurement units (PMUs) so as to make the system measurement model observable, and thereby linear, is investigated. A PMU placed at a bus measures the voltage as well as all the current phasors at that bus, requiring the extension of the topological observability theory. In particular, the concept of spanning tree is extended to that of spanning measurement subgraph with an actual or a pseudomeasurement assigned to each of its branches. The minimal PMU set is found through a dual search algorithm which uses both a modified bisecting search and a simulated-annealing-based method. The former fixes the number of PMUs while the latter looks for a placement set that leads to an observable network for a fixed number of PMUs. In order to accelerate the procedure, an initial PMU placement is provided by a graph-theoretic procedure which builds a spanning measurement subgraph according to a depth-first search. From computer simulation results for various test systems it appears that only one fourth to one third of the system buses need to be provided with PMUs in order to make the system observable. >

Power system observability with minimal phasor measurement placement

The objective of this paper is to provide a review of distributed control and management strategies for the next generation power system in the context of microgrids. This paper also identifies future research directions. The next generation power system, also referred to as the smart grid, is distinct from the existing power system due to its extensive use of integrated communication, advanced components such as power electronics, sensing, and measurement, and advanced control technologies. At the same time, the need for increased number of small distributed and renewable energy resources can exceed the capabilities of an available computational resource. Therefore, the recent literature has seen a significant research effort on dividing the control task among different units, which gives rise to the development of several distributed techniques. This paper discusses features and characteristics of these techniques, and identifies challenges and opportunities ahead. The paper also discusses the relationship between distributed control and hierarchical control.

Distributed Control Techniques in Microgrids

Motivated by technological advances in power electronics and signal processing, and by the interest in using direct drives for robot manipulators, we investigate the control problem of high-performance drives for switched reluctance motors (SRM's). SRM's are quite simple, low cost, and reliable motors as compared to the widely used dc motors. However, the SRM presents a coupled nonlinear multivariable control structure which calls for complex nonlinear control design in order to achieve high dynamic performances. We first develop a detailed nonlinear model which matches experimental data and establish an electronic commutation strategy. Then, on the basis of recent nonlinear control techniques, we design a state feedback control algorithm which compensates for all the nonlinearities and decouples the effect of stator phase currents in the torque production. The position dependent logic of the electronic commutator assigns control authority to one phase, which controls the motion, while the remaining phase currents are forced to decay to zero. Simulations for a direct drive, single link manipulator with the SRM are reported, which show the control performance of the algorithm we propose in nominal conditions and test its robustness versus the most critical parameter uncertainties of payload mass and stator resistance.

Feedback linearizing control of switched reluctance motors

Towards a theory of voltage collapse in electric power systems

In the control of adjustable speed drives, the performance of inexpensive digital integrated circuits is approaching the stage where traditional control algorithms may be displaced by new algorithms that better exploit their speed and the functional capabilities of their software. The concept of ``instantaneous torque control'' is introduced as an objective worth pursuing in the application of such digital IC's to drive systems. Instantaneous torque control would in principle permit the fastest possible response and the elimination of torque ripple, along with many other advantages not possible with conventional control algorithms, most of which are set up to control a time-averaged torque. Some of the fundamental principles of instantaneous torque control are developed for the switched reluctance motor, which is used as an example because, like the brushless dc permanent-magnet motor with concentrated windings, it has the potential for rapid response, but it can have appreciable torque ripple with unfavorable firing angles. A reference frame transformation that would eliminate the rotor position from the voltage and torque equations is not known for either of these machines. This opens up a number of interesting questions as to the generality of instantaneous torque control algorithms, and whether they can be incorporated into the general or unified theory of electrical machines.

Instantaneous Torque Control of Electric Motor Drives

The authors study the minimal information structure needed for monitoring and control of the voltage profile and the reactive power flow of a power system. Emphasis is on the minimum amount of information (data) required for reliable decision-making. An effort is made to work with fewer voltage data and therefore make the real-time monitoring and control more manageable. An annealing algorithm for selecting pilot points is presented. These are the load buses at which the voltage is to be measured in real time. Simulation results obtained with the algorithm for the Central Illinois Light Company system are presented. >

Secondary voltage control using pilot point information

In this paper, a new methodology for power system dynamic response calculations is presented. The technique known as the waveform relaxation has been extensively used in transient analysis of VLSI circuits and it can take advantage of new architectures in computer systems such as parallel processors. The application in this paper is limited to swing equations of a large power system. Computational results are presented.

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Transient Stability Simulation by Waveform Relaxation Methods

In this paper, an approach to modeling, analysis, and design of slow distributed voltage control schemes is proposed. In particular, a dynamical voltage model governed by the under load tap-changing transformers as control tools is studied. Rigorous conditions are derived to predict when the LTC based scheme may be poorly coordinated and not able to maintain voltages within the limits. The proposition is that nonconvergence of LTC control scheme is one of the causes of a systemwide voltage collapse.

Discrete Models of Slow Voltage Dynamics for under Load Tap-Changing Transformer Coordination

In the control of adjustable speed drives, the performance of inexpensive digital integrated circuits is approaching the stage where traditional control algorithms may be displaced by newer algorithms that better exploit their speed and the functional capabilities of their software. This paper introduces the concept of “instantaneous torque control” as an objective worth pursuing in the application of such digital IC's to drive systems. Instantaneous torque control would in principle permit the fastest possible response and the elimination of torque ripple, along with many other advantages not possible with conventional control algorithms, most of which are set up to control a time-averaged torque. This paper develops some of the fundamental principles of instantaneous torque control for the switched reluctance motor, which is used as an example because, like the brushless dc permanent-magnet motor with concentrated windings, it has the potential for rapid response but it can have appreciable torque ripple with unfavorable firing angles. Neither of these machines satisfies the conditions for the existence of a reference-frame transformation that will eliminate the rotor position from the voltage and torque equations, and this opens up a number of interesting questions as to the generality of instantaneous torque control algorithms, and whether they can be incorporated into the general or unified theory of electrical machines.

Marija Ilic-Spong

Papers

Instantaneous Torque Control of Electric Motor Drives

Secondary voltage control using pilot point information

Transient Stability Simulation by Waveform Relaxation Methods

Discrete Models of Slow Voltage Dynamics for under Load Tap-Changing Transformer Coordination

Instantaneous torque control of electric motor drives