5) Revenue loss: While a dc substation is out of service, the power of the wind farm it connects to would be blocked or curtailed. Historically, operations and maintenance considerations have led to different onshore and offshore system designs. Figure 6 shows a simplified single-line diagram of a two-level, point-to-point link. The onshore substation design optimizes the system's electrical performance. The connection transformer has a tap changer to best match the network ac voltage to the converter. Since physically large transformers are essential components of a plant and including a tap changer decreases a plant's reliability, offshore connection transformers do not have a tap changer. A decrease in the offshore converter's efficiency is accepted to obtain lower costs in overall operations and maintenance. figure 5. The HelWin 2 foundation next to the HelWin 1 platform. (Used with permission of Siemens, www.siemens .com/press.) ac Network +Vdc ac Breaker and Preinsertion Resistance Phase Reactor Wind Farm ac Filter ac Filter Onshore -Vdc Offshore figure 6. A point-to-point, representative, single-line diagram of an HVdc two-level substation (based on published data from the BorWin 1 HVdc offshore platform). july/august 2019 ieee power & energy magazine 65http://www.siemens.com/press http://www.siemens.com/press