A Novel Approach to Offshore Wind Power Using Low-Frequency AC Transmission

Here we are proposing a Low-Frequency AC handling power up to 1 GW with high reliability. LCCs wind power plant collection system is dc based, and connects to order harmonics, which inevitably results in the requirement the LFAC transmission line with a 12-pulse thyristor converter. for auxiliary equipment, such as capacitor banks, ac filters, Simulation results. VSC-HVDC systems are able to independently regulate activeand reactive power exchanged with the onshore grid and Index Terms offshore power, 12 pulse converters, underwater power cables, wind energy. the converters can be identified as drawbacks of VSC-HVDC significant component of the future electric generation applied for distances greater than 100 km for offshore wind portfolio due to greater space availability and better wind power transmission. energy potential in offshore locations [1], [2]. The integration of offshore wind power plants with the main power grid is a High-voltage low-frequency ac (LFAC) transmission has been subject of ongoing research. Presently, high-voltage ac recently proposed. In LFAC systems, an intermediate(HVAC) and high-voltage dc (HVDC) are well-established frequency level is used, which is created using a technologies for transmission [3]. cycloconverter that lowers the grid frequency to a smaller value, typically to one-third its value. In general, the main HVAC transmission is advantageous because it is relatively advantage of the LFAC technology is the increase of power straightforward to design the protection system and to change capacity and transmission distance for a given submarine voltage levels using transformers. However, the high cable compared to 50-Hz or 60-Hz HVAC. This leads to capacitance of submarine ac power cables leads to substantial cost savings due to the reduction in cabling considerable charging current, which, in turn, reduces the requirements and the use of normal ac breakers for protection. active power transmission capacity and limits the transmission distance. HVAC is adopted for relatively short (up to 50 75 The proposed system differs from previous work in that the km) underwater transmission distances [4]. wind turbines are assumed to be interconnected with a medium-voltage (MV) dc grid, in contrast with current Two classes of HVDC systems exist, depending on the types practice, where the use of MV ac collection grids is standard. of power-electronic devices used: 1) line-commutated DC collection is becoming a feasible alternative with the converter HVDC (LCC-HVDC) using thyristors and 2) development of cost-effective and reliable dc circuit breakers, voltage-source converter HVDC (VSC-HVDC) using selfand studies have shown that it might be advantageous with commutated devices, for example, insulated-gate bipolar respect to ac collection in terms of efficiency and improved transistors (IGBTs). The main advantage of HVDC production costs. technology is that it imposes essentially no limit on transmission distance due to the absence of reactive current in the transmission line. LCC-HVDC systems are capable of Page 51 transmission system for offshore wind power. The LFAC system is interconnected with the power grid with a cycloconverter. The consume reactive power from the ac grid and introduce lowA new method to design the system illustrated. The system s performance is observed by using s components and controls is and static synchronous compensators. On the other hand, I. I Offshore wind power plants are expected to represent a NTRODUCTION systems. Power levels (typically on the order of 300 and reliability are lower than those of LCC-HVDC. HVDC is 400MW) Fig. 1. Configuration of the proposed LFAC transmission system. The main reason for using a dc collection system with LFAC transmission is that the wind turbines would not need to be redesigned to output low-frequency ac power, which would lead to larger, heavier, and costlier magnetic components (e.g., step-up transformers and generators). At the sending end of the proposed LFAC system, a dc/ac 12pulse thyristor-based inverter is used to generate lowfrequency (20or 16 2/3-Hz) ac power, as shown in Fig. 1. By eliminating grid-side inverters, a medium-voltage dc collection system can be formed by interconnecting the rectified output of the generators. synchronous generators with fully rated power converters (Type-4 turbines) are commonly used in offshore wind plants. The required dc voltage level can be built by using high-power dc dc converters and/or by the series connection of wind turbines. For example, multi-MW permanent-magnet At the onshore substation (the receiving end), a thyristor-based cycloconverter is used as an interface between the lowfrequency side and the 60or 50-Hz onshore power grid. Thyristor-based converters can transmit more power with increased reliability and lower cost compared to VSC-HVDC systems. However, large filters are necessary at both ends to suppress low-order harmonics and to supply reactive power. Furthermore, the system can be vulnerable to main power grid disturbances. The proposed LFAC system could be built with commercially available power system components, such as the receiving-end transformers and submarine ac cables designed for regular power frequency. The phase-shift transformer used at the sending end could be a 60-Hz transformer derated by a factor of three, with the same rated current but only one-third of the original rated voltage. Another advantage of the proposed LFAC scheme is its feasibility for multiterminal transmission, since the design of multiterminal HVDC is complicated. In summary, LFAC transmission could be an attractive technical solution for medium-distance transmission. The objective of this paper is to set forth as a design process for an LFAC system for point-to-point transmission. The system configuration and control strategies are outlined in Section II. The selection of the major system components is discussed in Section III-A, and filter design is discussed in Section III-B. Design example and time-domain simulation studies of the proposed LFAC system are provided in Sections IV and V, respectively. II. SYSTEM CONFIGURATION AND CONTROL The proposed LFAC transmission system is shown in Fig. 1, assuming a 60-Hz main grid. At the sending end, a mediumvoltage dc collection bus is formed by rectifying the ac output power of series-connected wind turbines. A dc current source represents the total power delivered from the wind turbines. A dc/ac 12-pulse thyristor-based inverter is used to convert dc power to low-frequency (20-Hz) ac power. It is connected to a three-winding transformer that raises the voltage to a higher level for transmission. AC filters are used to suppress the 11th, 13th, and higher-order (23rd) current harmonics, and to supply reactive power to the converter. A smoothing reactor is connected at the dc terminals of the inverter. At the receiving end, a three-phase bridge (6-pulse) cycloconverter is used to generate 20-Hz voltage. A filter is connected at the low-frequency side. At the grid side, ac filters are used to suppress odd current harmonics, and to supply reactive power to the cycloconverter. Simply put, the operation of the LFAC transmission system can be understood to proceed as follows. First, the cycloconverter at the receiving end is activated, and the submarine power cables are energized by a 20-Hz voltage. In the meantime, the dc collection bus at the sending end is charged using power from the wind turbines. After the 20-Hz voltage and the dc bus voltage are established, the 12-pulse inverter at the sending end can synchronize with the 20-Hz voltage, and starts the transmission of power.