High Voltage Power Converter for Large Wind Turbine

Research output: Book/ReportPh.D. thesis

Abstract

The increasing penetration of the wind energy has resulted in newly planned installations of offshore wind turbines. In order to minimize installation, material and transportation costs of the offshore wind power plants, large multi-MW wind turbine systems are being preferably employed and developed, which allow high power generation of each single unit. Nevertheless, further increase in the power ratings of the newly emerging turbines becomes a major concern related to the operating voltage level.

In order to accommodate larger powers, presently employed low voltage (690 V)
systems already require multi-parallel converter and filter modules, which increase the overall complexity.

In this thesis, a concept for the medium voltage wind turbine is examined and
evaluated, where voltage increase is dictated by the removal of the step-up transformer. As a result, an entire wind turbine electrical system operates at 20 kV level - identical as for the collector distribution network. Medium voltage operation allows the converter unit along with the filter to be installed on the base platform inside the tower. In this manner, more space in the nacelle can be flexibly accommodated by the mechanical parts.

Due to limited voltage level of the generator insulation system (15 kV) along with
the increasing grid integration requirements, special care has been made over the search for optimal full-scale power converter circuitry, which additionally has to compensate voltage differences between the generator-side and a grid-side. Three converter topologies with different conversion philosophies have been introduced (A, B and C), their performance examined and eventually compared with the conventional low voltage system. System A is a back-to-back MMC converter, which is commonly used in HVDC application. System B consists of the generator-side 2-level converter, DC/DC boost unit and a grid-side NPC-3L converter. System C is made of a seriesconnected full-bridge cells on the generator-side, and a grid-side NPC-5L converter.

The performance of the proposed topologies is analyzed both under the normal and fault operation. In normal operation, medium and low voltage converter topologies are compared with regard to the efficiency and the required amount of silicon material in the semiconductor switches. In fault operation, maximum temporary ratings of the collector feeder components are compared also for different grounding schemes, which impact is the result of the removed step-up transformer.

Finally, the ground fault detection scheme for feeder cable system is proposed -
with the usage of current differential relay. Due to lack of the galvanic separation
between the wind turbines and the feeder cable sections, careful investigation for the relay selective operation has been made, which distinguishes ground faults located at the wind turbine terminals from faults within the protected cables.

The obtained results from the computer simulations in EMTDC/PSCAD software
show, that the best performance has been achieved by the transformer-less turbine with a back-to-back modular multilevel converter (MMC) topology, which is single grounded only through its DC link common-mode point. It has also occurred that the results derived from losses and short circuit analyses have become advantageous over the equivalent conventional system consisting of low voltage wind turbines equipped with the step up transformer.
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Details

The increasing penetration of the wind energy has resulted in newly planned installations of offshore wind turbines. In order to minimize installation, material and transportation costs of the offshore wind power plants, large multi-MW wind turbine systems are being preferably employed and developed, which allow high power generation of each single unit. Nevertheless, further increase in the power ratings of the newly emerging turbines becomes a major concern related to the operating voltage level.

In order to accommodate larger powers, presently employed low voltage (690 V)
systems already require multi-parallel converter and filter modules, which increase the overall complexity.

In this thesis, a concept for the medium voltage wind turbine is examined and
evaluated, where voltage increase is dictated by the removal of the step-up transformer. As a result, an entire wind turbine electrical system operates at 20 kV level - identical as for the collector distribution network. Medium voltage operation allows the converter unit along with the filter to be installed on the base platform inside the tower. In this manner, more space in the nacelle can be flexibly accommodated by the mechanical parts.

Due to limited voltage level of the generator insulation system (15 kV) along with
the increasing grid integration requirements, special care has been made over the search for optimal full-scale power converter circuitry, which additionally has to compensate voltage differences between the generator-side and a grid-side. Three converter topologies with different conversion philosophies have been introduced (A, B and C), their performance examined and eventually compared with the conventional low voltage system. System A is a back-to-back MMC converter, which is commonly used in HVDC application. System B consists of the generator-side 2-level converter, DC/DC boost unit and a grid-side NPC-3L converter. System C is made of a seriesconnected full-bridge cells on the generator-side, and a grid-side NPC-5L converter.

The performance of the proposed topologies is analyzed both under the normal and fault operation. In normal operation, medium and low voltage converter topologies are compared with regard to the efficiency and the required amount of silicon material in the semiconductor switches. In fault operation, maximum temporary ratings of the collector feeder components are compared also for different grounding schemes, which impact is the result of the removed step-up transformer.

Finally, the ground fault detection scheme for feeder cable system is proposed -
with the usage of current differential relay. Due to lack of the galvanic separation
between the wind turbines and the feeder cable sections, careful investigation for the relay selective operation has been made, which distinguishes ground faults located at the wind turbine terminals from faults within the protected cables.

The obtained results from the computer simulations in EMTDC/PSCAD software
show, that the best performance has been achieved by the transformer-less turbine with a back-to-back modular multilevel converter (MMC) topology, which is single grounded only through its DC link common-mode point. It has also occurred that the results derived from losses and short circuit analyses have become advantageous over the equivalent conventional system consisting of low voltage wind turbines equipped with the step up transformer.
Original languageEnglish
PublisherDepartment of Energy Technology, Aalborg University
Number of pages178
StatePublished - Jun 2014
Publication categoryResearch

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