PO198

On-Shore MML power converters for the connection of Diode Rectifier Units

Ricardo Vidal-Albalate2, Ruben Pena3, Salvador Aņķ-Villalba1, Enrique Belenguer2, Ramon Blasco-Gimenez 1
1Universitat Politecnica de Valencia, Valencia, Spain, 2Universitat Jaume I de Castello, Castello, Spain, 3Universidad de Concepcion, Concepcion, Chile, 4Universidad de Magallanes, Punta Arenas, Chile

Abstract

The connection of large off-shore wind farms using diode rectifier units presents important advantages, due to the simplicity of the diode rectifier converter, its robustness and weight and loss reduction.

Moreover, series connected diode rectifier units allow for increased reliability as the system is capable of reduced power operation in the case of the failure of one unit. Diode rectifier converters require the use of full bridge on-shore MML power converters. Such converters allow for reduced HVDC-link voltage operation when one diode rectifier unit is faulty and also might help to improve transient response durent faults.

However, the full bridge MML is more complex than its half bridge counterpart and has higher losses. This paper presents a comparative study of a full bridge MML and a mixed full half-bride cell MML power converter, which presents lower losses than the full bridge MML, while keeping the same functionality.

 

Method

The characteristics of both converters are compared by means of detailed PSCAD simulations, considering detailed models of the on-shore converters, HVDC cables, diode rectifier converters and off-shore wind farm including 150x8MW individual wind turbines.

EMT simulations are used to compare the technical characteristics of both converters during normal operation, faults and reduced power operation.

Also, a comparative loss study is carried out during both normal and reduced power operation.

Results

The results show that a hybrid MML power converter, consisting of one third of full bridge cells and two thirds of half bridge cells  can offer the same performance as a full-cell MML power converter at reduced cost and losses.

If only faults on one diode rectifier unit are to be considered, then, the number of full bridge cells can be optimised and further cost and loss reduction can be achieved at the expense of not guaranteing operation when two diode rectifier units are faulty.

The considered cost reduction is achieved at the expense of losing the fault blocking capability offered by the full bridge MMC.

Conclusions

Diode rectifier units have been proposed as an alternative of the use of full power MMC rectifier stations for the connection of large off-shore wind farms.

Although the use of large power diode rectifiers is common in other industries, it is important to further reduce risks and costs for this technology to be applied to the HVDC connection of off-shore wind farms.

One known disadvantage of the diode rectifier technology, as currently proposed, is the use of an on-shore full-bridge cell MML converter. This paper will include a thorough validation of a simpler alternative for on-shore MML converter, with lower initial cost and lower losses than a full-bridge MMC.

Objectives

Learn the operation of a off-shore wind farm connected via diode rectifiers, including the on-shore MML power converter.

Learn the limitations and characteristics of full-bridge MML converters when operated with diode rectifier units.

Learn the alternatives for the on-shore MML converters and the technical trade-offs of each solution.