Alberto B. Diez

Scheduling of Droop Coefficients for Frequency and Voltage Regulation in Isolated Microgrids

This paper details a procedure based on bifurcation theory to evaluate the impact that droops and primary reserve scheduling have on the microgrid stability. The methodology is based on finding the worst primary reserve share-that is, the share closest to instability-that can be found after rescheduling the droops of selected generating units that support frequency (and voltage) regulation. The solution-which consists of a measure of the distance to instability in a given direction-is found in a multi-parameter space endowed with coordinates corresponding to the droop coefficients. Two stages are proposed to achieve the solution. First, an investigation of the distance to bifurcation is computed in a one-dimensional parameter space in a defined search direction. Then the direction of this search is updated by calculating the normal vector at the found bifurcation point. The procedure is iteratively repeated until the closest bifurcation is found. The proposed approach is analyzed in a 69-bus and 11-generation unit isolated microgrid. It is shown through the analysis of some scenarios how the distances and normal vectors provide valuable insight on the correct scheduling from the stability point of view, giving advice on how the primary reserve should be more reliably scheduled.

Measuring, modeling and decoupling of saturation-induced saliencies in carrier signal injection-based sensorless AC drives

The focus of this paper is the measuring, modeling, and decoupling of saturation-induced saliencies in carrier signal injection based sensorless control. First techniques for the measurement of saturation-induced saliencies are presented. The goal of these measurements is to provide useful information on the position and magnitude of the various saturation-induced saliencies. Using the results from several different experimental measurements, models are developed explaining the source and behavior of the saturation-induced saliencies. The paper concludes by presenting methods for decoupling the effects caused by the parasitic saturation-induced saliencies, eliminating the errors that they cause in rotor position or flux angle estimation.

Online diagnostics in inverter-fed induction machines using high-frequency signal injection

Fault diagnostics for induction machines using an injected high frequency carrier signal is presented and analyzed in this paper. Both stator winding fault and broken: rotor bar detection is covered. Measurement of the resulting high frequency negative sequence current is shown to be capable of detecting both types of faults at their incipient stage. Though sharing similar physical principles to techniques applied to line-connected machines, the use of a high frequency signal is shown to provide important advantages for inverter fed machines, such as providing the same performance and drastically reduced sensitivity to the working condition of the machine, i.e. torque and flux levels, and fundamental excitation frequency.

Static and dynamic behavior of saturation-induced saliencies and their effect on carrier-signal-based sensorless AC drives

This paper analyzes the origin and the behavior of saturation-induced saliencies in induction machines, and their influence on carrier signal injection based sensorless techniques. The modeling of saturation-induced saliencies is necessary for the estimation of flux position, while the minimization of their influence is desired for the estimation of rotor position. Specifically focusing on rotor position estimation, there are two ways to achieve this minimization, the first being the use of a machine design that reduces the magnitude of the undesired saturation-induced saliencies and the second being the compensation in the estimator of the undesired saturation-induced saliencies. The modeling of saturation-induced saliencies, not only statically, but also dynamically, i.e. when the operating point of the machine changes, is addressed by this paper.

Complex-Valued State Matrices for Simple Representation of Large Autonomous Microgrids Supplied by

This paper focuses on representing the state space model of a microgrid in which power regulated (PQ ) and voltage/frequency regulated (Vf) generation units share a distribution system. The generation units considered in this paper are inverter interfaced. This introduces some interesting modeling problems which are treated in the paper, such as the decoupled cascaded control schemes or the non-negligible grid dynamics. A modeling approach is proposed based on four defined complex vectors. These vectors allow for complex-valued system matrices to be formed in a quite automated way. Moreover, a convenient partition of the system matrices is proposed, which in turn allows fast and easy modifications. Additionally, a multivariable methodology is proposed to simultaneously find the control system gains in an optimal sense. A 69-bus radial system, supplied by 20 generation units, is used to demonstrate how the proposal is of easy implementation to conduct small-signal stability analyses.

PQ

The diagnosis of stator winding faults in inverter-fed AC machines using an injected high-frequency carrier-signal voltage is analyzed in this paper. Measurement of the resulting carrier signals (either the negative-sequence carrier-signal current or the zero-sequence components) is used to detect turn faults at an incipient stage. The carrier frequency and magnitude are shown to have significant impact on the performance of the method, with the criteria for their selection being presented. The proposed technique shows low sensitivity to the working condition of the machine, i.e., torque level, flux levels, and fundamental excitation frequency.

and

This paper details a modeling procedure that incorporates composite loads in stand-alone microgrids in which, because of the low system inertia provided by inverter-interfaced generation units, the grid dynamics is not neglected. The paper introduces a methodology based on 1) separately treating the plants (RL grid elements) from reference frames and control systems; and 2) establishing a vector valued function to methodologically describe all plants in a similar way. Induction motors equations are rearranged to be integrated within the model, giving as a result a highly structured, compact system model. Next, bifurcation theory is adapted to the problem to show that composite loads are a need in the microgrid modeling if more realistic results about oscillations and mainly about load margin are pursued. Thanks to the modeling procedure, this is proven by means of a series of analyses conducted in a microgrid of considerable larger dimensions than those presented to date in the literature.

Vf

Induction motor field-weakening techniques have been developed which provide maximum torque capability above rated speed. Unfortunately most of these techniques are valid only for steady-state operation and show significant sensitivity to DC bus voltage and machine parameters. This paper analyzes the requirements of dynamically providing maximum torque under field-weakening operation. Three major issues are addressed: current regulator design; saturation techniques for current regulators in order to ensure best performance under voltage constraints; and flux regulator design to minimize transient errors when varying flux. Maximum DC bus utilization through the proper use of saturation techniques, dynamic response and reduced sensitivity are the advantages of the proposed solution.

Generation

Diagnostics of induction machines using the zero sequence voltage is presented in this paper. Both stator-winding fault and broken rotor bar detection are covered. Faults in the stator windings or damaged rotor bar cause asymmetries in the phase impedances of the machine, which give rise to zero sequence voltage components. The measured zero sequence voltage is shown to contain reliable information on the condition of the machine, with little influence from the operating point or imbalances in the phase voltages. The method will be shown to be valid both for line-connected machines and inverter-fed machines.

High-Frequency Carrier-Signal Voltage Selection for Stator Winding Fault Diagnosis in Inverter-Fed AC Machines

Fault diagnostics for induction machines using an injected high frequency carrier signal is presented and analyzed in this paper. Both stator winding fault and broken: rotor bar detection is covered. Measurement of the resulting high frequency negative sequence current is shown to be capable of detecting both types of faults at their incipient stage. Though sharing similar physical principles to techniques applied to line-connected machines, the use of a high frequency signal is shown to provide important advantages for inverter fed machines, such as providing the same performance and drastically reduced sensitivity to the working condition of the machine, i.e. torque and flux levels, and fundamental excitation frequency.Fault diagnostics for induction machines using an injected high-frequency carrier signal is presented and analyzed in this paper. Both stator winding fault and broken rotor bar detection is covered. Measurement of the resulting high-frequency negative-sequence current is shown to be capable of detecting both types of faults at their incipient stage. Though sharing similar physical principles to techniques applied to line-connected machines, the use of a high-frequency signal is shown to provide important advantages for inverter-fed machines, such as providing the same performance and drastically reduced sensitivity to the working condition of the machine, i.e., torque and flux levels, and fundamental excitation frequency.