No abstract

AC overvoltage emerges on dc buses of a magnetically controlled shunt reactor when a turn-to-turn fault occurs within the control winding (CtrW). Overvoltage protections are thus employed. However, this paper addresses a problem in the application of overvoltage protection against CtrW turn-to-turn faults where such a protective scheme may operate in an unacceptable time delay or even malfunction in some fault cases. It is shown that the ac overvoltage on dc buses during a turn-to-turn fault tends to be intermittent voltage spikes rather than standard sinusoidal waveform, and the fault feature may even not exist in some cases. The cause of the problem is investigated in this paper by dividing the operating state for the excitation system into three stages. It is proved that the appearance of overvoltage on dc buses is dependent on zero-crossing of the total control current. Simulation results and physical model tests have demonstrated the theoretical analysis in this paper. Moreover, a new protective scheme that is capable of detecting a 2% CtrW turn-to-turn fault by using the fundamental component in the total control current is proposed.Index Terms-AC overvoltage, magnetically controlled shunt reactor (MCSR), protective scheme, the control winding (CtrW), total control current, turn-to-turn faults.

Studies of volcanoes are essential for understanding the internal structure of the Earth and its evolution. Holocene intraplate volcanism is widespread across northeast (NE) China, including Changbaishan volcano (CBV), Longgang volcano (LGV), and Jingpohu volcano (JPHV) as shown in Figure 1 (Liu et al., 2001). Mt. Changbaishan (also known as Mt. Paektu in Korean), straddling the border between China and North Korea, is located more than 1,000 km northwest of the Japan Trench along which the Pacific Plate is subducting beneath the Eurasian Plate (Figure 1). As the largest and most active intraplate volcanic center in NE Asia, CBV has exhibited a complex history of eruptions spanning from the Early Miocene (∼20 Ma) to the Holocene (Wei et al., 2013). Notably, a destructive eruption in 946 CE known as the "Millennium Eruption" with a global significance (e.g., Oppenheimer et al., 2017;Wei et al., 2013) shaped a massive caldera that today is partially filled with a crater lake, covering an area of nearly 20 km 2 (see the top-middle inset in Figure 1). During July 2002 and July 2005, an episode of unrest beneath CBV signified by earthquake swarms, ground inflation, and geochemical anomalies in the gas emissions has led to growing concerns over its potential eruption in the near future (Xu et al., 2012) and has drawn much public attention from nearby countries and volcanologists worldwide (Stone, 2011;Witze, 2016).

The Tengchong volcano (TCV), situated in the west of the southeastern Tibetan Plateau, is one of the largest active volcanoes in China (Figure 1a). The TCV is characterized by large-volume magmatic gases (CO 2 and sulfide) emission (C.