# HVdc Circuit Breakers: Prospects and Challenges
## 2 High Voltage Direct Current Networks  
The first commercial installation of HVdc transmission technology is seen in 1954 [24]. HVdc technology is not only feasible and attractive from a technical point of view but also is economical. Typically, HVdc systems are employed for two reasons: (i) To transfer bulk power over large distances via long transmission lines; (ii) to connect unsynchronized ac networks and networks operating at different frequencies (back-back). The dc transmission has low losses than ac for long transmission lines. Mainly, HVdc converter technology can be divided into two: Classical technology--Thyristors-based line commutated converters (LCC) and recent technology--insulated gate bipolar transistors (IGBTs)-based (self-commutated converters) voltage source converters (VSCs) [25; 26; 27; 28; 29]. Power flow in LCC-HVdc is regulated by adjusting the firing and extinction time of gate signals before commutation to the next valve, as the thyristor is only a turn-on device. The flow of power is unidirectional; thereby, LCC is sometimes called a current source converter (CSC) [30]. Thus, the change in power flow direction would be problematic, as it requires a polarity change. Reactive power (Q) is consumed at both inverter and rectifier sides, which has to supply via filters and extra capacitors from the ac side. The LCC-HVdc substation produces less than 1% losses at rated current, of which 50% comes from the converter station transformer [31]. However, LCC technology is continuously advancing (e.g., capacitor commutated conversion consumes relatively less reactive power [32; 33]), and the filter size is also compromised by employing tuned active dc and ac filters [25; 34; 32; 33]. The recently installed LCC-HVdc transmission links are provided in Table 1. LCC has the highest power and voltage rating in HVdc converter topologies, blocking voltage of 10 kV with a current rating up to 6250 A [35; 36; 37]. Constant dc-link voltage is obtained from self-commutated voltage source converters, where large capacitors are used. Real power (P) is transmitted softly in both directions and can provide reactive power at both receiving (inverters) and sending ends, which reduces the filter size considerably. Filter size is also reduced as only high-frequency harmonics are present because of pulse width modulation (PWM) due to the availability of high power IGBTs. However, a VSC station contributes to losses by 1.6% [38; 39; 31], of which 70% comes from converter valves. Five terminal VSC-HVdc Zhoushanwith \(\pm\) 200 kV [38]. Each station's transmission capacity is 100, 400, 100, 300, and 100 MW, respectively. Details of some prominent VSC-HVdc transmission links are given in Table 2.  
In the event of dc fault, anti-parallel diodes to IGBTs start acting as an uncontrolled rectifier. A high rate of dc fault current is experienced due to the small dc inductance of the system. Furthermore, dc capacitors discharge and aid the fault current [40; 41]. VSCs are no longer well-regulated, and thus the fault current is only restricted by the ac breakers [28]. Here, the DCCBs should be doing the same, but HVdc CBs are not available. Table 3 summarizes the dissimilarities between LCC-HVdc and VSC-HVdc transmission technologies. Significant differences concerning the DCCB are; loss of VSC control in dc fault and different sizes of the inductances and capacitances (result in a different rate of rising dc fault current). VSC technology is rapidly advancing. New topologies are proposed to reduce converter losses (<1%), harmonics contents, and capability to extinguish the dc fault current [40; 41; 42; 43; 44; 45]. [PERSON] proposed a multi-modular converter (MMC) topology, an advanced form of VSC-HVdc circuits [46; 47]. This topology is based on the cascaded connections of several sub-modules of two IGBTs and a parallel capacitor. Sub-modules arrangements may use full-bridge (FB) or half-bridge (HB) cascaded connections. FB-MMC can override the overcurrent fault condition of HB-MMC. Thus, dc fault reverse blocking is