A Resonant Hybrid DC Circuit Breaker for Multi-Terminal HVDC Systems

: High-voltage direct current (DC) transmission systems and multi-terminal direct current transmission systems are attracting attention for expanding the grid to promote introduction of renewable energy. Fault clearing in DC systems is di ﬃ cult because there is no zero point of current. Hybrid circuit breakers are suitable for fault clearing in DC systems. Conventional hybrid circuit breakers have a hard-switching path that damages the switch. Hard switching damages the device and produces emissions due to harmonic noise. A novel resonant hybrid DC circuit breaker is proposed in this paper. The proposed circuit breaker reduces the damage to the switching device using soft switching due to the current zero point. The proposed circuit breaker is compared with conventional hybrid circuit breakers using numerical simulations. Interruption times and switching types of circuit breakers were compared. The simulation results of the fault clearing characteristics of the proposed breakers show that the proposed breakers have su ﬃ cient performance and are capable of stable reconnections in multi-terminal direct current transmission systems.


Introduction
The introduction of renewable energy sources (RESs), such as wind generators and photovoltaic generators, has been advancing in recent years [1][2][3]. However, power quality is reduced by the unstable output of RESs [4]. This problem is solved by a smoothing effect through the power system's expansion. The generation fluctuations of many RESs cancel each other out to yield the total system generation in a large area that fluctuates less [5,6]. Therefore, power system expansion is required. High-voltage direct current (HVDC) transmission and multi-terminal direct current (MTDC) transmission have attracted attention for system expansions [7][8][9][10]. Direct current (DC) systems are more efficient than alternating current (AC) systems in long-distance transmission [11]. Furthermore, there are no problems with inductance, capacitance, and phase displacement, which are common in AC transmission.
Modular multilevel converter (MMCs) attracted attention as grid-connected converters [12,13]. An MMC is composed of a number of cascaded cells and outputs voltages that are close to sine waveforms through multi-level conversion [14,15]. MMC cells are classified into half-bridge (HB) cells and full-bridge (FB) cells [16]. FB cells have four semiconductor switches per cell. HB cells have two semiconductor switches per cell. The advantage of HB cells is that HB cells cost less than FB cells and have lower steady-state conduction losses because of the smaller number of switching devices. two semiconductor switches per cell. The advantage of HB cells is that HB cells cost less than FB cells and have lower steady-state conduction losses because of the smaller number of switching devices. The advantage of FB cells is that they have a fault interruption capability [17]. However, because it blocks all converters connected to the grid, it is not possible to identify the point of failure. In addition, this clearing method requires a long preparation time for restoration [17]. HB cells do not have a fault interruption capability. A DC circuit breaker solves this problem.
DC fault interruption is difficult because there are no current zero points [18][19][20][21]. DC circuit breakers are the most reliable in fault interruption in DC systems [22]. There are three main types of DC circuit breakers: mechanical circuit breakers, semiconductor circuit breakers, and hybrid circuit breakers [23]. The response of mechanical circuit breakers is slow. Semiconductor circuit breakers have a steady-state loss problem. In 2012, a hybrid circuit breaker with high speed and low steadystate loss was developed by Asea Brown Boveri (ABB) ltd [24]. A hybrid circuit breaker is shown in Figure 1. A hybrid circuit breaker has a main path consisting of an ultrafast disconnector (UFD) and a load commutation switch (LCS), as well as a breaker path consisting of many semiconductor switches and a metal oxide varistor (MOV). The operation of a hybrid circuit breaker is shown in Figure 2 [25]. The main path carries the current during normal operation (a). When a fault is detected, the LCS is switched off immediately; hence, the fault current starts to be commutated to the breaker path (b). The UFD begins to open after the transition. The fault current flowing through the semiconductor switches in the breaker path increases until the UFD opens. When the UFD is fully open, the breaker path's semiconductor switches are turned off, and the current is commutated to the MOV (c). The MOV reduces the current and eliminates the fault. In the process of (b)-(c), the semiconductor switch in the breaker path rapidly turns off the high current and uses hard switching. Hard switching can exceed the reverse bias safety operation area (RBSOA) and cause significant damage to semiconductor devices [26]. In addition, hard switching causes electromagnetic interference (EMI) due to high-frequency noise [27].
A novel resonant hybrid DC circuit breaker is proposed in this paper. The proposed circuit breaker creates a current zero point. Zero-current switching (ZCS) avoids the negative effects of hard switching. A fault in an MTDC system was simulated to verify the practicality of the proposed circuit breaker. The proposed circuit breaker's interruption time is sufficiently short and provides stable power transmission during faults. The effectiveness of the proposed method was verified by MATLAB/Simulink simulations.
The remaining sections of the paper discuss the following: Section 2 describes a fault in the HB-MMC system. Section 3 explains the configuration of the MTDC system and the proposed hybrid circuit breaker. Section 4 discusses the principle of operation of the proposed circuit breaker. Section 5 presents the simulation results. Finally, Section 6 presents the inferences and conclusions concerning the proposed idea.

Analysis of an HB-MMC System Fault
An equivalent circuit of an HB-MMC during a fault is shown in Figure 3 [28]. The MMC operates as a converter during the fault. The equivalent circuit at fault consists of a circuit of resistance , inductance and capacitor . The conditions for the current to create a zero point are represented by Equation (1).
Let the initial current be ; the fault current is represented by Equation (2) [2].

Analysis of an HB-MMC System Fault
An equivalent circuit of an HB-MMC during a fault is shown in Figure 3 [28]. The MMC operates as a converter during the fault. The equivalent circuit at fault consists of a circuit of resistance R, inductance L and capacitor C. The conditions for the current to create a zero point are represented by Equation (1).
Let the initial current be i 0 ; the fault current i(t) is represented by Equation (2) [2].

Configuration of the MTDC System
In this paper, a modular multi-level converter is adopted as the AC/DC converter for the MTDC system. The configuration of the MMC is shown in Figure 4 [17]. The number of submodule stages is four. The configuration of the HB cell is shown in Figure 5 [17]. The HB-MMC-MTDC system assumed in this paper is shown in Figure 6 [29]. Power flow control between the four regions of the MTDC system is achieved by the MMC. The MTDC system's parameters are listed in Table 1. The transmission line model is shown in Figure 7 [30].

Configuration of the MTDC System
In this paper, a modular multi-level converter is adopted as the AC/DC converter for the MTDC system. The configuration of the MMC is shown in Figure 4 [17]. The number of submodule stages is four. The configuration of the HB cell is shown in Figure 5 [17]. The HB-MMC-MTDC system assumed in this paper is shown in Figure 6 [29]. Power flow control between the four regions of the MTDC system is achieved by the MMC. The MTDC system's parameters are listed in Table 1. The transmission line model is shown in Figure 7 [30].

Configuration of the MTDC System
In this paper, a modular multi-level converter is adopted as the AC/DC converter for the MTDC system. The configuration of the MMC is shown in Figure 4 [17]. The number of submodule stages is four. The configuration of the HB cell is shown in Figure 5 [17]. The HB-MMC-MTDC system assumed in this paper is shown in Figure 6 [29]. Power flow control between the four regions of the MTDC system is achieved by the MMC. The MTDC system's parameters are listed in Table 1. The transmission line model is shown in Figure 7 [30].

Configuration of the MTDC System
In this paper, a modular multi-level converter is adopted as the AC/DC converter for the MTDC system. The configuration of the MMC is shown in Figure 4 [17]. The number of submodule stages is four. The configuration of the HB cell is shown in Figure 5 [17]. The HB-MMC-MTDC system assumed in this paper is shown in Figure 6 [29]. Power flow control between the four regions of the MTDC system is achieved by the MMC. The MTDC system's parameters are listed in Table 1. The transmission line model is shown in Figure 7 [30].

Configuration of Circuit Breaker
The proposed resonant hybrid DC circuit breaker is shown in Figure 8. The hybrid circuit breaker consists of a mechanical switch, a semiconductor switch, a resonant circuit, and a demagnetization circuit. The mechanical switch conducts during normal operation. The semiconductor switch operates the resonant circuit to generate a current zero point on the transmission line at the time of the fault. ZCS avoids the negative effects of hard switching. Moreover, the residual inductance is low and the energy to demagnetize is low. The proposed circuit breaker is installed on the DC side of each converter, as shown in Figure 6. The circuit breaker limits the fault section and reduces the impact on other systems. In addition, the circuit breakers are connected in close proximity between the converters, and the AC system is largely unaffected by the DC faults. The parameters of the proposed breaker are shown in Table 2.

Configuration of Circuit Breaker
The proposed resonant hybrid DC circuit breaker is shown in Figure 8. The hybrid circuit breaker consists of a mechanical switch, a semiconductor switch, a resonant circuit, and a demagnetization circuit. The mechanical switch conducts during normal operation. The semiconductor switch operates the resonant circuit to generate a current zero point on the transmission line at the time of the fault. ZCS avoids the negative effects of hard switching. Moreover, the residual inductance is low and the energy to demagnetize is low. The proposed circuit breaker is installed on the DC side of each converter, as shown in Figure 6. The circuit breaker limits the fault section and reduces the impact on other systems. In addition, the circuit breakers are connected in close proximity between the converters, and the AC system is largely unaffected by the DC faults. The parameters of the proposed breaker are shown in Table 2.

Configuration of Circuit Breaker
The proposed resonant hybrid DC circuit breaker is shown in Figure 8. The hybrid circuit breaker consists of a mechanical switch, a semiconductor switch, a resonant circuit, and a demagnetization circuit. The mechanical switch conducts during normal operation. The semiconductor switch operates the resonant circuit to generate a current zero point on the transmission line at the time of the fault. ZCS avoids the negative effects of hard switching. Moreover, the residual inductance is low and the energy to demagnetize is low. The proposed circuit breaker is installed on the DC side of each converter, as shown in Figure 6. The circuit breaker limits the fault section and reduces the impact on other systems. In addition, the circuit breakers are connected in close proximity between the converters, and the AC system is largely unaffected by the DC faults. The parameters of the proposed breaker are shown in Table 2.

Parameter Design
The MMC during the failure is represented in Figure 3. Considering = 4, , , and are represented by Equations (3) The condition for having a current zero point is given by Equation (2). Assuming the inductor to be 1 mH, the condition of the resonant capacitor is represented by Equation (6).
Considering the calculation error,

Parameter Design
The MMC during the failure is represented in Figure 3. Considering n = 4, R, L, and C are represented by Equations (3)-(5).
The condition for having a current zero point is given by Equation (2). Assuming the inductor to be 1 mH, the condition of the resonant capacitor is represented by Equation (6).

Operation Principle of the Circuit Breaker
The operation of the proposed breaker is shown in Figure 9. In the case of a fault, the current is increased towards the fault point (a). The semiconductor switch and the resonant circuit are turned on after detecting a drop in DC transmission voltage or a rise in DC current due to the fault (b). The impedance at the semiconductor branch approaches zero due to the resonance phenomenon. Therefore, the rectification of the semiconductor switch creates a current zero point at the mechanical Sustainability 2020, 12, 7771 7 of 14 switch. The mechanical switch turns off at the current zero point (c). After the mechanical switch is turned off, the semiconductor switch turns off at the current zero point. After the current interruption, the thyristor Tha is turned on and the residual inductance is demagnetized (d). The operation of the proposed breaker is shown in Figure 9. In the case of a fault, the current is increased towards the fault point (a). The semiconductor switch and the resonant circuit are turned on after detecting a drop in DC transmission voltage or a rise in DC current due to the fault (b). The impedance at the semiconductor branch approaches zero due to the resonance phenomenon. Therefore, the rectification of the semiconductor switch creates a current zero point at the mechanical switch. The mechanical switch turns off at the current zero point (c). After the mechanical switch is turned off, the semiconductor switch turns off at the current zero point. After the current interruption, the thyristor ℎ is turned on and the residual inductance is demagnetized (d).

Simulation Conditions
In this paper, the fault clearing characteristics and a system restart are simulated in the case of a short-circuit fault. The system was simulated in MATLAB/Simulink and was modeled as follows.

•
The AC system assumed to have a 230 kV, three-phase AC power supply.

•
The fault detection method is shown in Figure 10. A fault is determined when the voltage is less than 100 kV and the current exceeds 300 A.
The DC fault current interruption characteristics of the ABB's hybrid circuit breaker are simulated for comparison with the proposed circuit breaker. The ABB's hybrid circuit breaker is shown in Figure 1. Assuming that the time to open the UFD takes 1.5 ms, hard switching of the circuit breaker is observed. The circuit breaker's interruption time should be within 5 ms [24]. If the proposed circuit breaker has an interrupting capability within 5 ms and achieves ZCS, the effectiveness of the proposed method is demonstrated.
The proposed circuit breaker simulated the fault interruption of the MTDC system. The sequence of the fault interruption simulation is as follows. A short-circuit failure occurs in the MTDC system at t = 3. After the fault clearing, the circuit breaker is re-closed and power transmission

Simulation Conditions
In this paper, the fault clearing characteristics and a system restart are simulated in the case of a short-circuit fault. The system was simulated in MATLAB/Simulink and was modeled as follows.

•
The AC system assumed to have a 230 kV, three-phase AC power supply.

•
The fault detection method is shown in Figure 10. A fault is determined when the voltage is less than 100 kV and the current exceeds 300 A.   Figure 11 shows the mechanical switch current and Figure 12 shows the semiconductor switch current. When a fault is detected, the LCS is turned off and the current in the mechanical switch generates a current zero point. The current in the semiconductor switch is rectified and flows into the MOV when the UFD opens. The MOV current is shown in Figure 13. Current is reduced and fault  The DC fault current interruption characteristics of the ABB's hybrid circuit breaker are simulated for comparison with the proposed circuit breaker. The ABB's hybrid circuit breaker is shown in Figure 1. Assuming that the time to open the UFD takes 1.5 ms, hard switching of the circuit breaker is observed. The circuit breaker's interruption time should be within 5 ms [24]. If the proposed circuit breaker has an interrupting capability within 5 ms and achieves ZCS, the effectiveness of the proposed method is demonstrated.

Simulation Results of the ABB's Breaker
The proposed circuit breaker simulated the fault interruption of the MTDC system. The sequence of the fault interruption simulation is as follows. A short-circuit failure occurs in the MTDC system at t = 3. After the fault clearing, the circuit breaker is re-closed and power transmission resumes from t = 3.3 s. The utility of the proposed circuit breaker is demonstrated when the effective power of each system is stable and restored. Figure 11 shows the mechanical switch current and Figure 12 shows the semiconductor switch current. When a fault is detected, the LCS is turned off and the current in the mechanical switch generates a current zero point. The current in the semiconductor switch is rectified and flows into the MOV when the UFD opens. The MOV current is shown in Figure 13. Current is reduced and fault clearing is achieved. The current interruption time is 1.5 ms. In Figure 12, the semiconductor switch is turned off rapidly from 2 kA. In these simulation results, the hard switching of conventional hybrid circuit breakers was observed.   Figure 11 shows the mechanical switch current and Figure 12 shows the semiconductor switch current. When a fault is detected, the LCS is turned off and the current in the mechanical switch generates a current zero point. The current in the semiconductor switch is rectified and flows into the MOV when the UFD opens. The MOV current is shown in Figure 13. Current is reduced and fault clearing is achieved. The current interruption time is 1.5 ms. In Figure 12, the semiconductor switch is turned off rapidly from 2 kA. In these simulation results, the hard switching of conventional hybrid circuit breakers was observed.

Simulation Results of the Proposed Breaker
The current of a mechanical switch is shown in Figure 14 and the current of a semiconductor switch is shown in Figure 15. The mechanical switch is found to pass through the zero point during rectification to complete the interruption. The semiconductor switch current is interrupted at the current zero point. The current interruption time is between 1 and 1.1 ms. ZCS can be observed in Figure 15.
The transmission line current is shown in Figure 16 and the transmission line voltage is shown in Figure 17. In the case of the fault, the transmission of power to other systems is stable and voltage fluctuations are controlled. The submodule capacitor voltage of the MMC at the point of fault is shown Figure 18. The submodule capacitor voltage outside the point of fault is shown in Figure 19. The voltage fluctuation is small and stable, and protection of the MMC is achieved. The active power of each converter is shown in Figure 20. The fault point's active power has zero power, and the other systems are making up for the lack of power. At t = 3.3 s, the system is restarted. When reconnected, the time required to recover the current is 1 s. In these simulation results, the proposed circuit breaker achieves soft-switching fault interruption. In addition, the reconnections are smooth. Therefore, the practicality of the proposed circuit breaker has been demonstrated.

Simulation Results of the Proposed Breaker
The current of a mechanical switch is shown in Figure 14 and the current of a semiconductor switch is shown in Figure 15. The mechanical switch is found to pass through the zero point during rectification to complete the interruption. The semiconductor switch current is interrupted at the current zero point. The current interruption time is between 1 and 1.1 ms. ZCS can be observed in Figure 15.
The transmission line current is shown in Figure 16 and the transmission line voltage is shown in Figure 17. In the case of the fault, the transmission of power to other systems is stable and voltage fluctuations are controlled. The submodule capacitor voltage of the MMC at the point of fault is shown Figure 18. The submodule capacitor voltage outside the point of fault is shown in Figure 19. The voltage fluctuation is small and stable, and protection of the MMC is achieved. The active power of each converter is shown in Figure 20. The fault point's active power has zero power, and the other systems are making up for the lack of power. At t = 3.3 s, the system is restarted. When reconnected, the time required to recover the current is 1 s. In these simulation results, the proposed circuit breaker achieves soft-switching fault interruption. In addition, the reconnections are smooth. Therefore, the practicality of the proposed circuit breaker has been demonstrated.

Simulation Results of the Proposed Breaker
The current of a mechanical switch is shown in Figure 14 and the current of a semiconductor switch is shown in Figure 15. The mechanical switch is found to pass through the zero point during rectification to complete the interruption. The semiconductor switch current is interrupted at the current zero point. The current interruption time is between 1 and 1.1 ms. ZCS can be observed in Figure 15.
The transmission line current is shown in Figure 16 and the transmission line voltage is shown in Figure 17. In the case of the fault, the transmission of power to other systems is stable and voltage fluctuations are controlled. The submodule capacitor voltage of the MMC at the point of fault is shown Figure 18. The submodule capacitor voltage outside the point of fault is shown in Figure 19. The voltage fluctuation is small and stable, and protection of the MMC is achieved. The active power of each converter is shown in Figure 20. The fault point's active power P 1 has zero power, and the other systems are making up for the lack of power. At t = 3.3 s, the system is restarted. When reconnected, the time required to recover the current is 1 s. In these simulation results, the proposed circuit breaker achieves soft-switching fault interruption. In addition, the reconnections are smooth. Therefore, the practicality of the proposed circuit breaker has been demonstrated.

Discussion
A comparison table is shown in Table 3. The ABB's hybrid circuit breakers can eliminate HVDC system failures within 5 ms. However, hard switching in fault interruption was observed. The proposed hybrid circuit breaker can eliminate the fault of the HVDC system within 5 ms. ZCS was observed in fault interruption. In the fault simulation in the MTDC system, the circuit breaker showed sufficient fault rejection performance, and a stable system restart was observed.

Discussion
A comparison table is shown in Table 3. The ABB's hybrid circuit breakers can eliminate HVDC system failures within 5 ms. However, hard switching in fault interruption was observed. The proposed hybrid circuit breaker can eliminate the fault of the HVDC system within 5 ms. ZCS was observed in fault interruption. In the fault simulation in the MTDC system, the circuit breaker showed sufficient fault rejection performance, and a stable system restart was observed.

Discussion
A comparison table is shown in Table 3. The ABB's hybrid circuit breakers can eliminate HVDC system failures within 5 ms. However, hard switching in fault interruption was observed. The proposed hybrid circuit breaker can eliminate the fault of the HVDC system within 5 ms. ZCS was observed in fault interruption. In the fault simulation in the MTDC system, the circuit breaker showed sufficient fault rejection performance, and a stable system restart was observed.

Discussion
A comparison table is shown in Table 3. The ABB's hybrid circuit breakers can eliminate HVDC system failures within 5 ms. However, hard switching in fault interruption was observed. The proposed hybrid circuit breaker can eliminate the fault of the HVDC system within 5 ms. ZCS was observed in fault interruption. In the fault simulation in the MTDC system, the circuit breaker showed sufficient fault rejection performance, and a stable system restart was observed.

Discussion
A comparison table is shown in Table 3. The ABB's hybrid circuit breakers can eliminate HVDC system failures within 5 ms. However, hard switching in fault interruption was observed. The proposed hybrid circuit breaker can eliminate the fault of the HVDC system within 5 ms. ZCS was observed in fault interruption. In the fault simulation in the MTDC system, the circuit breaker showed sufficient fault rejection performance, and a stable system restart was observed.

Conclusions
In this paper, a resonant hybrid DC circuit breaker for multi-terminal HVDC systems was proposed. The hard switching with conventional hybrid circuit breakers upon interruption has an adverse effect on the semiconductor switch. The resonant hybrid circuit breaker proposed in this paper provides soft-switching interruption and avoids the adverse effects of hard switching. The ABB's circuit breaker and the proposed circuit breaker were compared in a simulation. The ABB's circuit breaker has an interruption time of 1.5 ms and is suitable for the protection of HVDC systems. However, the current of 2 kA was rapidly interrupted, and hard switching was observed upon the fault current interruption. The proposed circuit breaker interrupts the current in 1 to 1.1 ms. The fault current is interrupted at the current zero point when the current is interrupted. Smooth reconnections of the system were confirmed to be about 1 s after fault clearing. Therefore, the proposed circuit breaker avoids the adverse effects of hard switching and can be applied to MTDC systems.
The proposed circuit breaker can be applied to the fault interruption of equipment handling high power in addition to current interruption of power lines. In the future, it is necessary to conduct simulations on transmission lines with distributed power sources, as well as experiments on actual equipment and simulations on high-power devices.

Conflicts of Interest:
The authors declare no conflict of interest.

Nomenclature
The following notations are used in this manuscript. The voltage of the upper-arm cell capacitor in phase a [V] v cAb1 , v cAb2 The voltage of the upper-arm cell capacitor in phase The voltage of the upper-arm cell capacitor in phase c The voltage of the lower-arm cell capacitor in phase a [V] v cBb1 , v cBb2 The voltage of the lower-arm cell capacitor in phase The voltage of the lower-arm cell capacitor in phase c