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Conference programme 

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Poster session

Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Slavomir Seman Siemens AG, Germany
Co-authors:
Slavomir Seman (1) F P Rainer Zurowski (1) Timo Christ (1)
(1) Siemens AG, Erlangen, Germany

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Poster
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Presenter's biography

Biographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited

Mr. Seman has been working in wind industry for almost 10 years. He is currently Project Manager responsible for offshore wind park grid access - system architecture and simulations at Siemens AG, Erlangen, Germany. Mr. Seman has received his PhD degree from Helsinki University of Technology, Finland in 2006. After his studies he spent more than 5 years with ABB as Grid Access Specialist and Design Manager responsible for grid interface of WT frequency converters as well as for development of wind turbine simulation models.

Abstract

Investigation of dc converter nonlinear interaction with offshore wind power park system

Introduction

The offshore wind power plant system stability is mainly assessed by evaluation of voltage and frequency stability.

This is typically performed by evaluation of steady state power system characteristics (correlation between P, Q,
freq, Voltage magnitude) reflecting influence of the key components, such as Cables, Transformers, Filters.
Presented HVDC grid access solution utilizes diode rectifier units (DRU) replacing self-commutated HVDC converter
far end offshore.


Approach

The investigation will be focused on nonlinear interaction of DRU with offshore power system, influence
of filter circuits rating and active power operating points. Modeling of the array (wind park string) cables
using EMT type of simulation software will be described. The grid forming control capabilities of WTG line
side converters will also be included in the study. The concept stable operation will be demonstrated by
obtaining of stable power system characteristic intersection points with suitably chosen system control
droop under given constraints.

Main body of abstract

Voltage stability as seen from small signal theory is achieved by reactive power balance. It is well known from ideal
theory of line commutated converters that the commutation reactive power demand increases with increasing
commutation reactance. Realistic offshore power systems clearly deviate strongly from a purely inductive power
system characteristic due to the presence of many AC cables and even some AC filters. The constraints in this
context are imposed by active power control of WTG and given, i.e. fixed, DC voltage of DRU.
The two remaining offshore power system quantities, namely reactive power and frequency, are dependent on
each other due to operational characteristic of DRU under given constraints. Therefore steady-state stability can
be achieved by implementing a reactive power/frequency droop in the WTG line-side converter controller.
At first an idealized purely inductive offshore system is investigated and compared with theory regarding reactive
power when frequency is varied in a sensible frequency range. Then the impact of filter circuits and AC cabling
on reactive power is studied as well separately as combined under frequency variation. DRU-topologies under
consideration are either six-pulse or twelve-pulse configuration.


Conclusion

The reactive power demand of a DRU operated with constant DC back voltage and active power defined by WTG
operating points depends on the offshore power system AC cable network and AC filters. This effect has to be
understood and considered not only during system design phase but also in the defining phase for any kind of
offshore grid requirements. Stable operating points are established by the intersection of a suitably chosen reactive
power/frequency droop in the WTG line side control with the system reactive power/frequency characteristic of the
offshore power system under consideration.


Learning objectives
Line commutated converters impose additional constraints on a power system. Hence one degree of freedom is
lost in comparison to systems operated with self-commutated converters. System design and system requirements
should reflect this fact.