Antonio Gomez-Exposito

Frequency Regulation Contribution Through Variable-Speed Wind Energy Conversion Systems

This paper presents a new method to enhance the participation of variable-speed wind energy conversion systems (WECS) in existing frequency regulation mechanisms. The proposed approach, based on a modified inertial control scheme, takes advantage of the fast response capability associated with electronically-controlled WECS, allowing the kinetic energy stored by rotational masses to be partly and transiently released in order to provide earlier frequency support. An additional improvement is achieved by communicating the WECS response to conventional generators so that these can eventually take care of the full load imbalance. Several simulations using a two-area test system are performed to demonstrate the benefits of the proposed scheme.

A Multilevel State Estimation Paradigm for Smart Grids

The main objective of this paper is to describe a multilevel framework that facilitates seamless integration of existing state estimators (SEs) that are designed to function at different levels of modeling hierarchy in order to accomplish very large-scale monitoring of interconnected power systems. This has been a major challenge for decades as power systems grew pretty much independently in different areas, which had to operate in an interconnected and synchronized fashion. The paper initially provides a brief historical perspective which also explains the existing state estimation paradigm. This is followed by a review of the recent technological and regulatory drivers that are responsible for the new developments in the energy management functions. The paper then shows that a common theoretical framework can be used to implement a hierarchical scheme by which even very large-scale power systems can be efficiently and accurately monitored. This is illustrated for substation level, transmission system level as well as for a level between different transmission system operators in a given power system. Finally, the paper describes the use and benefits of phasor measurements when incorporated at these different levels of the proposed infrastructure. Numerical examples are included to illustrate performance of the proposed multilevel schemes.

An Adaptive Nonlinear Controller for DFIM-Based Wind Energy Conversion Systems

An adaptive nonlinear controller for wind energy doubly fed induction machines is introduced in this paper. The proposed controller is based on the feedback linearization technique and includes a disturbance observer for estimation of parameter uncertainties. Estimated uncertainties values are injected in order to construct the control law, improving in this way the systems performance. The controller behavior, when tracking power references, is tested with realistic electromagnetic transients for DC /power systems computer-aided design simulations. In addition, the controller performance is checked in the presence of parameter uncertainties and nearby faults..

Two-Level State Estimation With Local Measurement Pre-Processing

This paper presents a two-level state estimation methodology intended to integrate in a simple and efficient manner the results provided by local estimators, handling measurements at the substation level, within the framework of existing EMS. This leads to a paradigm in which raw measurements are locally pre-processed by a linear estimator, in such a way that only a manageable subset of measurements is handled at the second level, significantly reducing the communication bandwidth. Simulation results show that, even when the two steps are centrally carried out, the proposed approach is computationally more efficient than the conventional one, particularly with highly redundant measurement sets.

Adaptive Control Strategy for VSC-Based Systems Under Unbalanced Network Conditions

A new adaptive control strategy, intended to improve the ride-through capability of high-voltage direct current (HVDC) systems under unbalanced network conditions and parameter uncertainties, is introduced. The proposed strategy resorts to a model reference adaptive control plus a resonant filter. The resonant filter scheme is based on a unique synchronous reference frame that prevents the use of the customary sequence component detector, increasing the controller bandwidth accordingly. Several tests are conducted to compare the proposed scheme against existing HVDC controllers, showing an improved performance regarding: 1) elimination of the 2ω ripple on the dc voltage arising during ac-side imbalances; 2) accurate and decoupled active and reactive power tracking when converter parameters are not perfectly known.

Hierarchical Wide-Area Control of Power Systems Including Wind Farms and FACTS for Short-Term Frequency Regulation

In this work a hierarchical scheme is proposed for the coordinated control of conventional synchronous generators, wind farm converters and flexible ac transmission systems. It comprises two levels, namely a fully decentralized set of controllers associated with the involved devices and a centralized coordinating controller based on synchronized wide-area signals provided by phasor measurement units. The proposed hierarchy of controllers is mainly focused on two objectives: transient frequency support and inter-area oscillations damping. Several works have recently proved that the fast-acting power converters of variable-speed generators can greatly improve the short-term frequency regulation capability in scenarios with a large penetration of wind energy. A case study is included showing that the dynamic performance and stability of power systems can indeed be enhanced when windmill converters are properly coordinated via a wide-area centralized controller.

An Improved Control Strategy for Hybrid Wind Farms

This paper addresses the control requirements of hybrid wind farms, comprising a relatively large number of conventional induction machines (IMs) along with one or very few permanent magnet synchronous machines (PMSMs), capable of compensating the reactive power demanded by the IMs during faulty conditions as well as attenuating the active power variations due to wind gusts. Based on the superposition theorem and the feedback linearization technique, a controller is designed to independently regulate the positive and negative sequence currents of the PMSM voltage source converters (VSCs), overcoming several drawbacks of existing approaches in the presence of unbalanced voltages. In the proposed scheme, the grid-side VSC currents are controlled in order to improve the ride-through capability of IMs, so that the whole wind farm can fulfill demanding grid codes in the absence of extra equipment, such as static compensators. As shown by the test results, combining IM-based wind farms with PMSMs accomplishes several relevant goals: delivering the reactive power consumption of the IMs, increasing the rated active power of the installation, and smoothing mechanical power oscillations.

Implicitly constrained substation model for state estimation

This paper addresses the problem of including detailed substation models in state estimation at the least possible cost. After reviewing existing alternatives, it is shown how Lagrange multipliers associated with topological constraints can be easily computed from an implicit model the size of which is slightly larger than that of the conventional bus-branch approach. This allows bad data and topological error processing to be carried out simultaneously.

Software Sensor-Based STATCOM Control Under Unbalanced Conditions

A new control strategy for static compensators (STATCOMs) operating under unbalanced voltages and currents is presented in this paper. The proposed strategy adopts a state observer (software sensor) to estimate ac voltages at the STATCOM connection point. This way, physical voltage sensors are not needed and the hardware gets simplified, among other advantages. Using the superposition principle, a controller is designed to independently control both positive and negative sequence currents, eliminating the risk of overcurrents during unbalanced conditions and improving the power quality at the STATCOM connection bus. Two operating modes are proposed for the computation of the reference currents, depending on whether the objective is to compensate unbalanced load currents or regulate bus voltages. The proposed observer allows positive and negative sequences to be estimated in a fraction of the fundamental cycle, avoiding the delay often introduced by filter-based methods. Overall, the STATCOM performance is improved under unbalanced conditions, according to simulation results presented in the paper.

State Estimation in Two Time Scales for Smart Distribution Systems

The monitoring of distribution systems relies on a critical set of pseudomeasurements and a varying but low number of redundant measurements. In the light of the different refreshing rates of both types of information, this paper considers a state estimation model structured in two time scales. Possibilities and limitations of the proposed model are discussed, and illustrated on a real distribution system comprising a diversity of load patterns.