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.

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.

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.

This paper presents an aggregation technique based on the resolution of a multi-objective optimization problem applied to the admittance model of a wind power plant (WPP). The purpose of the presented aggregation technique is to reduce the order of the wind power plant model in order to accelerate WPP simulation while keeping a very similar control performance for both the simplified and the detailed models. The proposed aggregation technique, based on the admittance model order reduction, ensures the same DC gain, the same gain at the operating band frequency, and the same resonant peak frequency as the detailed admittance model. The proposed aggregation method is validated considering three 400-MW grid-forming Type-4 WPPs connected to a diode rectifier HVDC link. The proposed aggregation technique is compared to two existing aggregation techniques, both in terms of frequency and time response. The detailed and aggregated models have been tested using PSCAD-EMTsimulations, with the proposed aggregated model leading to a 350-fold reduction of the simulation time with respect to the detailed model. Moreover, for the considered scenario, the proposed aggregation technique offers simulation errors that are, at least, three-times smaller than previously-published aggregation techniques.

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.

The connection of diode rectifier (DR) based wind power plants to existing or planned High Voltage dc (HVdc) interconnectors can lead to important savings on cost and system robustness. Since the DR station usually operates in a bipolar configuration, its connection to symmetric monopoles is particularly challenging. However, there are no published detailed studies on the protection of DR connection wind power plants to symmetric monopole interconnectors or even to bipolar interconnectors. This paper includes the comparative study of five different protection strategies for such systems, including both solid and resistive DR station grounding and strategies with and without the use of dc-circuit breakers. An analytical study allows for the calculation of fault current during fault on-set for both half-bridge and hybrid Modular Multi-level Converter (MMC) stations. Using detailed Electromagnetic Transient (EMT) simulation studies, the different protection strategies are evaluated in terms of current, voltage and isolation requirements of each element, as well as the need for dc-circuit breakers, fast communication or larger surge arresters. Moreover, a distance fault detection algorithm is included for the wind turbine converters to distinguish between local ac-grid and dc-cable faults. From the simulation results it is possible to conclude that DR high impedance grounding, together with wind turbine distance protection can be used for the protection of DR based off-shore wind power plants connected to symmetric monopole interconnectors without requiring dc-circuit breakers.