The continuously increasing demand for electric power and the economic access to remote renewable energy sources such as off-shore wind power or solar thermal generation in deserts have revived the interest in high-voltage direct current (HVDC) multiterminal systems (networks). A lot of work was done in this area, especially in the 1980s, but only two three-terminal systems were realized. Since then, HVDC technology has advanced considerably and, despite numerous technical challenges, the realization of large-scale HVDC networks is now seriously discussed and considered. For the acceptance and reliability of these networks, the availability of HVDC circuit breakers (CBs) will be critical, making them one of the key enabling technologies. Numerous ideas for HVDC breaker schemes have been published and patented, but no acceptable solution has been found to interrupt HVDC short-circuit currents. This paper aims to summarize the literature, especially that of the last two decades, on technology areas that are relevant to HVDC breakers. By comparing the mainly 20+ years old, state-of-the art HVDC CBs to the new HVDC technology, existing discrepancies become evident. Areas where additional research and development are needed are identified and proposed.

HVDC Circuit Breakers: A Review Identifying Future Research Needs

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.

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

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.

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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 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.

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

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.

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

A preferred mode of operating a parallel connected HVDC transmission system would be with at least one of the major rectifier stations in voltage control. Such a station could be realized using diode rectifier units.

J. P. Bowles

Papers

Multiterminal HVDC Transmission Systems Incorporating Diode Rectifier Stations