Assessment of collection systems for HVDC connected offshore wind farms

Abstract

a b s t r a c tA technical and economic comparison is made between DC and AC collection systems of offshore wind farms. DC collection systems have the advantages of reduced weight and size of the DC cables and DC cables are free from reactive power compensation. The heavy 50/60 Hz transformers in the offshore transmission platform of AC collection systems can be replaced with smaller size medium frequency transformers in DC collection systems. However, the need for a high power DC-DC converter with high voltage transformation ratios and DC protection methods will remain a challenge for the DC collection systems. Also, DC collection systems do not necessarily reduce the power conversion stages compared to the AC collection systems even if HVDC (High Voltage DC) transmission is used to transfer the offshore wind power from the collection systems to the onshore grids. A cost assessment study verifies that the cost reductions achieved by the reduced size of the DC cables and offshore platform are outweighed by the cost of DC protective devices and DC-DC converters. This is because the length of the DC collection cables is relatively short compared to the long distance HVDC cables. The technical comparison supported by the simulation results shows that the total losses in the DC collection systems are higher than in AC collection systems. The effect of collection bus voltages on the losses is analysed for the DC collection systems.

Enhanced dielectric properties of polyethylene/hexagonal boron nitride nanocomposites

“…For example, the adoption of renewable generation involving offshore wind necessarily requires the transmission of large amounts of energy from the point of production to centres of demand. As transmission distances increase, a change from conventional alternating current (AC) to direct current (DC) becomes increasingly attractive [2]. Indeed, this approach also enables the interconnection of nonsynchronous, national power systems, thereby enabling the more efficient use of resources.…”

High voltage cascaded step-up DC-DC Marx converter for offshore wind energy systems

“…In this topology, the cluster is formed by two parallel lines of wind turbines, each containing 5 wind turbines in series. Hence, there is a total of 10 wind turbines; each rated at 5 MW, 1.2 kV [13], [14]. Therefore, the cluster bus voltage is 6 kV and the rated power is 50 MW.…”

Evaluation of electrical losses in MVAC collector systems in offshore wind farms

“…Although some researchers are considering DC collector types, results of investigations state that losses and costs are slightly higher than in AC collector systems [4,5,11]. Therefore, the methodology proposed in this document will apply to AC technology to be used also in the case study.…”

“…Therefore, taking into account the above conditions, the turbine models shown in Table II will be included in the case study. In Table II the amount of annual energy generated by the wind turbines (AEP) is calculated by multiplying the power generated as function of wind speed, probability of occurrence of the wind speed and the number of hours in a year as in [4]. Turbine power generation data are obtained by turbine manufacturers' datasheets [18]- [22].…”

“…Furthermore, it is evident that investment costs are higher and could explain why developers are taking into account new collector designs, especially with redundancy, efficiency/low losses and costs [3][4][5].…”