# Development of A Loss Minimization Based Operation Strategy for Embedded BTB VSC HVDC
## 1 Introduction  
In recent years, the electric-power industry has changed from a conventional alternating current (AC) system to a hybrid system, in which special facilities such as high voltage direct current (HVDC) and flexible AC transmission system (FACTS) work together. In particular, HVDC has been used for over 50 years, mainly for asynchronous interconnection, long-distance transmission, submarine, and underground cable transmission, etc. The most commonly used system is the line-commutatedconverter (LCC) HVDC, which uses thyristor valves, but the development of voltage-sourced converter (VSC) HVDC technology is also on the rise with the advances in power electronics technology [1; 2; 3]. In general, HVDC is mainly used with point-to-point (PTP) type which is used for large-capacity, long-distance transmission. These HVDCs are intended to transmit power unidirectionally between the two regions, so no special operation strategies are required. Recently, however, small-and medium-scale VSC HVDCs were installed in the grid to provide additional grid services in addition to unidirectional power transmission, because of the independent control of active/reactive power and converter switching operation of the VSC HVDC, which can provide grid services for an AC system, as shown in Table 1 [2; 4]. In particular, it is easy to provide grid services for the AC grid when embedded HVDC is installed in the grid, where embedded HVDC is defined as \"a DC link with multiple ends that are physically connected from a single synchronous AC network. With such a connection, these connections allow DC to perform basic functions of bulk power transmission as well as some additional control functions within the AC network such as power flow control, voltage control, stability improvement\" [5]. In other words, embedded HVDC is mainly used for AC power flow control by active power control at both ends of the converter, improvement of voltage stability by reactive power control, suppression of failure propagation in the AC system, and system stability through frequency control. With the introduction of embedded VSC HVDC, there have been various studies of embedded VSC HVDC operation strategies as shown in [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. References [6; 7; 8] are early research on the operation strategy of the HVDC, which improves the stability of power facilities by Remedial Action Scheme (RAS). As defined [9; 10], an RAS is designed to take corrective action to maintain system reliability and provide acceptable system performance. This operating strategy has been applied to many HVDC projects such as Manitoba HVDC, Eastern/Western Alberta Transmission Line, and the RAS is used as the basis of emergency control in this paper [11; 12; 13; 14; 15; 16]. The operating point of HVDC should be determined prior to these emergency controls, and most HVDCs are intended for long-distance transmission, so the operating point is determined according to the rated capacity or N-1 failure without any special strategy [17]. However, by changing the HVDC operating point, system total loss can be reduced, and such studies have also been conducted. This method mainly focuses on reducing transmission loss with Multi-Terminal DC (MTDC) [18; 19], and studies on finding operating points based on objective functions such as system loss, voltage deviation, and reliability criteria have been variously conducted on small scale systems [20; 21]. These conventional studies have the drawback such as they were conducted in a small-scale radial system, not a large-scale meshed system; the studies manly consider only AC loss; and there is no integrated control scheme for HVDC operation considering system total loss.