Date of Award

May 2017

Degree Type


Degree Name

Master of Science



First Advisor

Lingfeng Wang


HVDC, Operating Reliability, Reliability


Wind energy, especially offshore wind energy, is the focus of policy for countries that want to make significant use of renewable energy. With the development of semiconductor technology, high voltage direct current (HVDC) technology is being widely used for the transmission of wind power from offshore windfarms to onshore power grids. The application of HVDC technology can benefit the power system in many ways, such as operation security, reliability performance and economy. With the increasing number of applications of HVDC, the reliability performance of HVDC plays an important role in the overall power system reliability. Although the reliability of HVDC transmission system has been studied for some time, most of the research is only for the reliability evaluation of the planning stage, but the evaluation of operational reliability is rarely mentioned.

Converters and transformers are major components in VSC-HVDC transmission systems. Constant failure rate for components (transformers and converters) has been proven to be feasible and it is widely used in power system to calculate medium or long term average reliability indices [23]. Constant failure rate is an average value for long-time. However, the average failure rates cannot represent the components’ failure probability under different operating conditions and operating environment.

In this thesis, a converter real-time failure model and a transformer real-time failure model were built and tested. These two models were applied to two VSC-HVDC transmission systems (radial, regional) to calculate the operating reliability indices. And a set of sensitivity analyses was conducted to evaluate the influence of various factors.

The converter real-time failure model is based on the power loss of power electronics which is caused by the wind speed and its variation, and the influence of ambient temperature was considered. For the transformer real-time failure model, the aging failure caused by the mechanical strength loss, the random failure based on weather condition and the failure caused by overload protection was considered. To calculate the operating reliability for two systems, systems were simplified by using the minimum cut set method. Combined with the real-time availability of the transformer and the converter, and other components, the system hourly reliability was calculated. In the calculation, the difference between offshore and onshore was considered. The sensitivity analyses demonstrated the influence of season, DC cable capacity, and wind turbine parameters.

According to the calculation results, the reliability of the major components can be more accurately reflected by these condition-dependent models. The availability for the system varies evidently with different operating and environment conditions. The ambient temperature and the wind speed are the main affecting factors. The one-year simulation results demonstrate that the system reliability exhibits some degree of seasonal nature. Also, it was shown that the system topology, DC cable capacity, and wind turbine parameters could affect the system reliability.