S. Añó-Villalba

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.

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.

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.

Fault analysis of uncontrolled rectifier HVDC links for the connection of off-shore wind farms

The present paper introduces the use of an uncontrolled rectifier HVDC link for the connection of off-shore wind farms. The use of an uncontrolled rectifier instead of a line commutated thyristor (LCT) rectifier represents clear advantages in terms of installation cost and operation reliability. However, the rectifier can no longer be used to limit the HVDC link current during on-shore grid faults. The present paper introduces a method to control the HVDC link voltage and current by modifying the off-shore ac grid voltage, which, in turn, is controlled by the wind turbines. Simulation results show good performance in steady state and during on-shore ac grid f aults.

Connection of off-shore wind power plants to VSC-MTdc networks using HVdc diode-rectifiers

This paper introduces the use of a diode based HVdc rectifier for the connection of a large off-shore wind power plants (WPP) to an existing multiterminal VSC-HVdc grid. Wind turbines with fully rated converters are used to control the offshore ac-grid voltage and frequency and the power delivered by the WPP. Moreover, the proposed topology allows for black-start operation of the off-shore WPP and exhibits a good response against perturbations such as off-shore faults. The technical feasibility of the proposed topology has been validated by means of detailed PSCAD simulations, assuming a WPP consisting on five aggregated wind turbines of different ratings to evaluate the distributed nature of the control strategy.

Connection of Off-Shore Wind Farms Using Diode Based HVDC Links

The development of off-shore wind farms located at a large distance from the coastline imposes a series of technical challenges. At distances larger than 50–70 km and installed powers larger than 500 MW, the use of HVDC links based on line commutated converters (LCC-HVDC) is the most advantageous solution for the connection to the on-shore transmission grid. At the same time, the market share of wind turbines with fully rated converters is increasing. Currently, manufacturers are offering either direct-drive, single-stage or double-stage gearbox multi-megawatt wind turbines for off-shore applications. The additional fault-ride through and control capabilities of wind turbines with fully rated converters can be exploited to control off-shore ac-grid voltage and frequency. Moreover, the wind turbine front-end converters can also be used to control the power transmited through the HVDC link. At this stage, the use of a controlled rectifier is no longer needed, and a diode-based rectifier can be used at the off-shore side of the HVDC link. This chapter shows how an adequate control system on each wind turbine can be used to enable the diode-based HVDC rectifier to be operated in voltage or current control mode, in a similar way as standard HVDC links. Moreover, the proposed control strategy includes adequate protection and fault ride-through capability against most common grid disturbances.

Connection of off-shore wind farms using a diode based HVDC link with reduced filter banks

This paper shows that it is possible to substantially reduce the rectifier capacitor and filter banks when using diode based HVDC link for the connection of large off-shore wind farms. The considered wind turbines are based on full power power converters. The paper includes a control strategy that allows the wind turbine front-end converters to control the offshore ac-grid voltage and frequency and, at the same time, absorb a substantial amount of the harmonic currents produced by the LC-HVDC rectifier. The proposed control strategy for filter reduction is validated by means of PSCAD simulation, highlighting adequate active, reactive and harmonic current sharing between the wind turbines. Impact on overall efficiency is is also analysed and discussed.

Harmonic filter reduction of off-shore wind farms connected with a diode based HVDC link

The filter bank size is one of the main deterrents for the use of otherwise more efficient LCC-HVDC links for the connection of large off-shore wind farms. This paper shows how the HVDC rectifier filter banks can be substantially reduced by taking advantage of the additional control flexibility offered by the use of wind turbines with fully rated converters. PSCAD simulation results are shown where rectifier filter banks are substantially reduced. System wide implications and impact on overall efficiency is analyzed and discussed.