The Effects of Filter Capacitors on Cable Ripple at Different Sections of the Wind Farm Based Multi-Terminal DC System

Abstract

In most DC power systems, power electronic devices can introduce ripple content into the DC grid, where large input ripple currents on the DC link can have a negative influence. Under these circumstances, DC side filters play an important role in the reduction of ripple content. In this paper, based on a full detailed closed loop model of the entire offshore wind farm multi-terminal DC network, the effect of filter capacitors connected at different sections of the system on the limitation of the AC ripple content, particularly in the DC cables, is studied. In contrast to other work on HVDC for offshore wind, where simplified or equivalent circuits are mainly used while concentrating on the power and control system, this study can be regarded as the specific study of filter capacitors operating within a detailed system model. Utilising the advantages of PLECS, which is a highly effective tool in the simulation of power electronic circuits, no small-signal or simplified equivalent models are used in the system, and the entire study is based on detailed and accurate models of the semiconductor elements and transformers, which help to provide more realistic simulation results and a better understanding of the system. Another novel point in this paper is a new concept, relative losses, which is proposed to simplify extensively the calculation of losses in this research. Finally, the size of the filter capacitors at different sections of the system under different situations is suggested.

The Study of Multi-Terminal DC Systems in an Offshore Wind Environment: A Focus on Cable Ripple Analysis

“…Figure 1 shows a two-branch, multi-terminal DC system, which is the circuit of the simulation model in this study. This figure displays a more detailed circuit diagram of the generating side in ( [8], Figure 1), and the system specification is described as follows: Wind Model: This study focuses on wind power as the sole energy input source. Irrespective of wind direction, the wind model is characterized by various parameters, including the natural wind speed (S Wt ), the fundamental wind speed (S Wb ), the gust wind speed (S Wg ), the ramp wind speed (S Wr ), and the noise wind speed (S Wn ), as shown in Equation (1).…”

“…The detailed structure and circuit diagram of the Input Parallel Output Series (IPOS) DC-DC converter were provided in ( [8], Figure 4), resulting in three different unidirectional constructions within the simulation model [32]; this paper solely presents the system performance with the Identical Three-Phase Bridge IPOS Converter in ( [8], Figure 4b) and the Identical SAB IPOS Converter in ( [8], Figure 4c), as the performance of the system with the SAB + Three-Phase Bridge IPOS Converter in ( [8], Figure 4a) was expected to fall between them. All parameter labels in the simulation results were derived from Figure 1, with a comprehensive study conducted on the impact of the cable ripple content and AC losses from various perspectives.…”

“…As mentioned in [8], it can be challenging to synchronize the IGBT switching of all the multi-connected 5 MW DC-DC converters due to the long distance between them, and the extreme condition study method was applied in the simulation model, while the average value of the results obtained from the following two conditions can be regarded as the normal performance of the system.…”

“…In this simulation work, the system operated under different natural wind conditions, including gust and ramp wind, which means that results from a much longer simulation time needed to be monitored compared to the switching periods of the power electronic devices in the system. The steady or constant wind speed received at each turbine was set to around 10.3 m/s, which is the 10-year average wind speed of the area where the Hywind Scotland Pilot Park project was built [8,33]. The transient states are involved since the power derived from the wind is not constant, and the simulation results of the system with both Identical Three-Phase IPOS and Identical SAB IPOS structures are presented and compared.…”

“…For the Identical SAB IPOS converter used in the simulation model, there should be a 120 degree phase shift between the control signals of the switches of the three IPOS connected SAB converters in ( [8], Figure 4c) in order to reduce the ripple content of cables [8].…”