Jose Luis Rodriguez-Amenedo

Automatic generation control of a wind farm with variable speed wind turbines

Wind farms are considered as negative loads from the point of view of the utility manager. Modem variable speed wind turbines offer the possibility for controlling active and reactive power separately. This paper presents a new integrated control system of a wind farm according to the utility manager requirements. This control system is based on two control levels: a supervisory system controls active and reactive power of the whole wind farm by sending out set points to all wind turbines, and a machine control system ensures that set points at the wind turbine level are reached. The system has been validated by numerical simulation using data from a wind farm with 37 variable speed wind turbine sited in northern Spain. Automatic generation control of these characteristics promises an improved performance of the system and a better grid integration of the wind energy without significant extra costs.

Direct Power Control Applied to Doubly Fed Induction Generator Under Unbalanced Grid Voltage Conditions

This paper analyzes the effects of unbalanced voltage on doubly fed induction generators. It also presents a novel control strategy based on direct power control (DPC+) applied to this type of generators, predominant in wind energy applications, that enables them to work under perturbed conditions and achieve optimum results. Although the technique can be implemented to control both rotor converters and grid converters, we will hereby exemplify the former which regulates stator active and reactive power. The results obtained with DPC+ are then compared through experimental tests to indicate that the technique is suitable and achieves good dynamic responses while controlling current distortion, power or torque oscillations. The validation of results has been performed through experimental tests on a 20-kW generator.

Providing Ride-Through Capability to a Doubly Fed Induction Generator Under Unbalanced Voltage Dips

This paper analyzes the effect of unbalanced voltage over doubly fed induction generators (DFIGs) and presents a novel control strategy, named dynamic programming power control plus (DPPC+), based on dynamic programming control. The high penetration of wind energy in the electrical grids demands for new requirements for the operation of wind energy conversion systems (WECSs). DFIG is the most employed WECS, and the DPPC+ guarantees their operation under unbalance conditions achieving the required objectives. Although the technique can be implemented to control both rotor and grid converters, we hereby expound the former, which regulates stator active and reactive power. The validation of the results obtained with DPPC+ has been performed through the use of experimental tests on a 20-kW test bench, consisting of a DFIG and induction motor drive. The obtained results show that the DPPC+ is suitable for achieving a good dynamic response while controlling current distortion and power and/or torque oscillations for both steady-state conditions and unbalanced voltage dips, showing the low-voltage ride-through capability.

Dynamic Programming Power Control for Doubly Fed Induction Generators

This paper presents a novel control strategy, dynamic programming power control (DPPC), to be applied to doubly fed induction generators most commonly used in wind energy applications. Although the technique can be implemented to control both rotor and grid converters, it will hereby be expounded the former, which regulates stator active and reactive powers. The results obtained are compared with those from other techniques, such as direct torque/power control (DTC/DPC) through the use of experimental tests, and indicate that DPPC achieves gains in dynamic response and considerable improvements in terms of ripple reduction and frequency spectrum as a result of constant switching frequency operation. The validation of results has been performed through experimental tests on a 6-kW generator.

Extended direct power control of a three-level Neutral Point Clamped Voltage Source Inverter with unbalanced voltages

This paper presents the control of a three- level Neutral Point Clamped (NPC) Voltage Source Inverter under unbalanced grid voltages. The control method used is the Extended Direct Power Control (EDPC), which is a generic approach for Direct Power Control of multilevel inverters based on geometrical considerations. It defines, for each inverter vector, two geometrical loci of no power change (a circumference for active power and a straight line for reactive power). Each locus divides the plane into two regions. The relative position of grid voltage vector in these regions determines whether the application of a certain inverter voltage vector will increase or decrease power. Besides, dc link middle point voltage is controlled by taking into account the switching state of the inverter (those states which make the middle point voltage changing). Finally, a decision algorithm chooses the inverter vector to be applied among those which fulfil the three requirements. In case of imbalances in grid voltages, negative sequence components appear and system variables start oscillating at twice the fundamental frequency, affecting the performance of the inverter. In fact, if power is kept constant, output current becomes distorted, presenting mainly third and fifth harmonic components. In order to obtain sinusoidal balanced output currents and thus improve inverter performance under voltage imbalances, power reference is compensated with oscillating terms so that negative sequence current components disappear. The whole control system has been tested on a three-level NPC Voltage Source Inverter connected to the grid and results confirm the validity of the method.

Modeling and Control of LCC Rectifiers for Offshore Wind Farms Connected by HVDC Links

This paper presents a voltage and frequency control (VFC) and an average-value model (AVM) of a line-commutated converter for a rectifier station in an offshore wind farm (OWF) connected by a high-voltage direct current link. A capacitor bank is placed at the AC terminals of the rectifier station to perform VFC within the OWF. The proposed model uses the active and reactive power generated by the OWF as inputs, while the state variables are the voltage magnitude and phase angle at the capacitor bank bus. The proposed VFC is based on the orientation of the voltage vector at the capacitor bank bus toward a synchronous reference axis. It is then demonstrated that frequency control is achieved by regulating the reactive power balance at the capacitor bank bus, while voltage control is carried out by regulating the active power balance. Moreover, it is demonstrated that in a diode rectifier, although voltage cannot be controlled as in a thyristor rectifier, it is bounded within acceptable limits. In addition, small-signal study is performed to facilitate controller design and system stability analysis. VFC and the accuracy of the proposed AVM are validated by simulation, using both the proposed AVM and a detailed switching model.