Juan Manuel Mauricio

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.

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..

Fault Ride-Through Enhancement of DFIG-Based Wind Generation Considering Unbalanced and Distorted Conditions

A control strategy is proposed to enhance the operation, under network disturbances, of a wind turbine driven doubly fed induction generator (DFIG). The scheme allows us to overcome low voltages, imbalances, and harmonic distortions at the point of common coupling. The control law is designed using a feedback linearization approach plus resonant filters; this law directly controls the DFIG stator powers from the rotor voltages, unlike the most used nested two-loop approaches. An accurate control of the active and reactive powers delivered to the grid permits us to fulfill severe grid code requirements and to improve the fault ride-through capability. Under unbalanced conditions, the reference currents of both grid- and rotor-side converters are coordinately chosen to simultaneously eliminate the double-frequency pulsations in the total active power and electromagnetic torque. Several tests and disturbances to provide a realistic assessment and validation have been performed, showing the adequacy of the proposed controller. Comparisons with other control approaches with different objectives are also presented to illustrate the advantages regarding elimination of the double-frequency ripples and harmonic rejection capability.

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.

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.

VSC-Based MVDC Railway Electrification System

This paper proposes a new railway electrification system in which the voltage-source converter (VSC) becomes the basic building block. This will allow existing railways, comprising several ac and dc subsystems, to be transformed into simpler medium-voltage dc (MVDC) multiterminal power systems feeding mobile loads. Moreover, the VSC-based unified scheme will substantially facilitate the connectivity among otherwise heterogeneous railway systems, while the integration of distributed generation and storage is achieved in a straightforward fashion. In addition to the general MVDC architecture, details are provided about the dc catenary layout, dual-voltage locomotive configurations, and dc-dc links between urban and long-distance railways. The need for a supervisory control system, and its role in coordinating local VSC controllers, so that the resulting power flows are optimized while the catenary voltage is kept within limits, are discussed. The proposed railway electrification paradigm is compared with the standard 25-kV, ac electrification system by means of a real case study.

Modeling and Control of an HVDC-VSC Transmission System

A voltage source converter based HVDC (HVDC-VSC) transmission system is considered in this paper. This technology is one of the most promising FACTS in future power systems. A 7-th order non-linear averaged model is chosen for the HVDC-VSC, suitable for transient stability analysis and control design. Its behavior is validated against a more realistic EMTDC-PSCAD model in an open-loop test. After that, an LgV controller is developed based on the model obtained. The controller is applied to the EMTDC model and its capability to track set-points related with active power, reactive power and other suitable variables is tested. The performance of the proposed controller shows better results than PI controllers proposed in earlier papers, having better stability margin and shorter time response

Flywheel Energy Storage Model, Control and Location for Improving Stability: The Chilean Case

A flywheel energy storage (FES) plant model based on permanent magnet machines is proposed for electro-mechanical analysis. The model considers parallel arrays of FES units and describes the dynamics of flywheel motion, dc-link capacitor, and controllers. Both unit and plant-level controllers are considered. A 50-MW FES plant model is tested in the Northern Chile Interconnected System (NCIS) when connected to the Argentinian Interconnected System (AIS). The FES plant provides transient support for primary frequency regulation and its impact on stability is studied using small-signal analysis and time domain simulations. To identify the best location to install the FES plant, eigenvalue sensitivity for the interarea mode is analyzed. The results are validated for different operation scenarios of wind and solar power. By installing the FES plant in the NCIS best location, the damping ratio of the interarea mode is increased from 0.53% to 13.1%. Moreover, the power transfer from the NCIS to the AIS can be augmented from 90 to 180 MW while still keeping the damping ratio above 9%. These findings are promising and may lead to useful planning criteria for energy storage deployment.