R. Blasco-Gimenez

Control strategies for power smoothing using a flywheel driven by a sensorless vector-controlled induction machine operating in a wide speed range

This paper presents a novel control strategy for power smoothing. The system is based on a sensorless vector-controlled induction machine driving a flywheel. The problem of regulating the DC-link voltage against input power surges or sudden changes in load demand is addressed. The induction machine is controlled to operate in a wide speed range by using flux weakening above rated speed. A model reference adaptive system observer is used to obtain the rotational speed in the whole speed range. The observer parameters are adapted during flux weakening in order to obtain close tracking of the flywheel speed. Experimental results for the operation of the induction machine between zero to more than twice base speed are presented and discussed.

Distributed Voltage and Frequency Control of Offshore Wind Farms Connected With a Diode-Based HVdc Link

This paper introduces a new technique for the distributed voltage and frequency control of the local ac-grid in offshore wind farms based on synchronous generators. The proposed control technique allows the connection of the offshore wind farm using a diode based HVdc rectifier. The use of microgrid control techniques allowed the system comprising the wind farm and the diode HVdc rectifier to be operated in current or voltage control mode. Fault response to on-shore voltage sags of up to 80% has been shown to be comparable to that of thyristor rectifiers. The proposed control technique has been shown to be robust against load changes in islanded operation, capacitor bank switching, diode-rectifier ac-breaker tripping and wind turbine power limitation due to slow wind speeds. PSCAD simulations are used to prove the technical feasibility of the proposed control techniques both in steady state and during transients.

A topology for multiple generation system with doubly fed induction machines and indirect matrix converter.

In this paper, a new topology for a grid connected generation system based on two double fed induction generators is presented The scheme uses an indirect matrix converter consisting on an input rectifier, a three-to-two matrix converter which provides a DC voltage, and two voltage fed output converters. The input converter is connected to the grid and each of the output converters is connected to the rotor of the corresponding machine. The rotor currents of each generator are vector controlled and unity power factor operation at the indirect matrix converter input is considered. The duty cycles for the input converter are chosen in order to obtain maximum DC link voltage. The duty cycles for each of the output converter can be arranged in order to minimize the commutation losses of the input converter and/or reduce the ripple of the input current. The speed of the generators is used to decide the switching pattern for the output converters. The entire system is modelled and results are presented to show the feasibility of the proposed scheme.

Efficiency and Fault Ride-Through Performance of a Diode-Rectifier- and VSC-Inverter-Based HVDC Link for Offshore Wind Farms

This paper includes a technical feasibility study on the use of an HVDC diode-based rectifier together with an onshore voltage-source converter (VSC) for the connection of large offshore wind farms. A distributed control algorithm for the wind farm is used, where all the wind turbines contribute to the offshore-grid voltage and frequency control while allowing wind turbine optimal power tracking. Moreover, the proposed system shows good fault ride-through performance to solid faults at the onshore connection point, wind-farm ac grid, and HVDC line. The technical feasibility of the proposed solution has been validated by means of detailed PSCAD/EMTDC simulations. The efficiency of the complete system has also been studied and found to compare favorably with that of a VSC-HVDC-inverter station.

Diode-Based HVdc Link for the Connection of Large Offshore Wind Farms

This paper includes a technical feasibility study on the use of diode-based HVdc links for the connection of large offshore wind farms based on synchronous generators. A technique for the voltage and frequency control of the offshore ac grid is presented. The proposed control technique allows the operation of the rectifier end of the HVdc link in current or voltage control mode. Fault response to onshore voltage sags of up to 80% has been shown to be comparable to that of thyristor-based rectifiers. Moreover, the complete system shows an adequate fault-ride-through operation to solid short circuits at onshore inverter terminals. PSCAD® simulations are used to prove the technical feasibility of the proposed control techniques both in steady state and during transients.

LCC-HVDC Connection of Offshore Wind Farms With Reduced Filter Banks

Despite being more efficient, line commutated converter-HVDC links for the connection of large offshore wind farms have ac-filter bank size as one of their main drawbacks. This paper shows how the HVDC rectifier filter banks can be substantially reduced by taking advantage of the additional control possibilities offered by the use of wind turbines with fully rated converters. PSCAD simulations validate wind farm and diode rectifier HVDC link operation with a capacitor and filter bank five times smaller than its usual value. The proposed control algorithm allows for good harmonic and reactive power sharing between the different wind turbines. As the reduced capacitor bank operation leads to a redistribution of harmonic and reactive currents, an efficiency study has been carried out to evaluate the new power loss distribution with the reduced filter banks.

Centralized modular diagnosis and the phenomenon of coupling

This paper studies modular decomposition as an approach for failure diagnosis based on discrete event systems. This paper also analyses the problem of coupling produced in the implementation of centralized modular diagnosers, as coupled diagnosers cannot carry out their own diagnosis task when there is a failure in another subsystem sharing a common energy or material flow. In addition, we propose a method to avoid diagnoser coupling, by means of decoupling functions using nonlocal information with respect to the coupled diagnoser and generated in the diagnoser where the failure has been isolated.

Intermittent failure diagnosis in industrial processes

Intermittent failure diagnosis based on discrete event system (DES) in industrial processes is studied. Intermittent failures are defined as failures that can automatically recover once they have occurred. In this paper this sort of failures are modeled and some characteristic parameters are defined. The original approach to diagnose permanent failures is revisited and extended in order to include new diagnosis skills in the diagnosers. The developed methodology is applied to a classical industrial subprocess system composed by a pump and a valve. The developed diagnoser and this example are simulated in Matlab, employing Simulink for the continuous model and stateflow for the DES diagnoser.

STATCOM-controlled HVDC Power Transmission for Large Offshore Wind Farms: Engineering Issues

The paper considers a solution for integration of large offshore DFIG-based wind farms with a common collection bus controlled by a STATCOM into the main onshore grid using line-commutated HVDC connection. A design procedure is described and the controlled system is validated using PSCAD/EMTDC simulations confirming high performance of the proposed control strategy in both normal operation conditions and faults. Engineering issues related to STATCOM capacitor sizing and reduction of STATCOM rating are considered and their effectiveness is confirmed

Off-shore wind farm grid connection using a novel diode-rectifier and VSC-inverter based HVDC transmission link

This paper carries out a technical feasibility study on the use of HVDC diode based rectifiers for the connection of large off-shore wind farms, together with an on-shore voltage source converter. An integrated control technique for the overall system is developed and validated by means of PSCAD simulation of the distributed wind farm together with the complete HVDC link. The presented results show an adequate distributed control of the off-shore ac-grid voltage and frequency. Moreover, the proposed system shows good behavior during solid three phase faults at the on-shore connection point. It is concluded that the proposed topology is a technically feasible solution for the connection of large off-shore wind farms.