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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 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 offshore wind power plants connected to symmetric monopole interconnectors without requiring dc-circuit breakers.

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