A Novel Topology of Hybrid HVDC Circuit Breaker for VSC-HVDC Application

Van-Vinh Nguyen 1, Ho-Ik Son 1, Thai-Thanh Nguyen 1 ID , Hak-Man Kim 1,2,* and Chan-Ki Kim 3 1 Department of Electrical Engineering, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon 22012, Korea; nvvinh@inu.ac.kr (V.-V.N.); shi0617@inu.ac.kr (H.-I.S.); ntthanh@inu.ac.kr (T.-T.N.) 2 Research Institute for Northeast Asian Super Grid, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon 22012, Korea 3 Transmission & Distribution Lab., The R&D Center of Korea Electric Power Co., 105 Munji-ro, Yuseong-gu, Daejeon 34056, Korea; chankikim@kepco.co.kr * Correspondence: hmkim@inu.ac.kr; Tel.: +82-32-835-8769; Fax: +82-32-835-0773


Introduction
The reliable and safe operation of the high voltage direct current (HVDC) grid can be improved with the use of the DC circuit breakers (CBs) [1][2][3][4][5]. The requirements for the design of CBs can be different according to the type of HVDC technologies [6]. Typically, there are two types of HVDC, traditional line-commutated current source converter (LCC-HVDC) and voltage source converter (VSC-HVDC) [7]. In the case of LCC-HVDC, the rate of rise and magnitude of the fault current is not high due to the use of the DC side reactor [8]. As a result, the time interruption of the fault current could be up to few hundred milliseconds [9]. The main focus for the design of CB in the LCC-HVDC was on the mechanism of zero current crossing. However, in the VSC-HVDC technology, the fault current increases rapidly due to the capacitive behavior of the HVDC cable and low DC • A topology of HCB based on the thyristor technique is proposed to reduce the realization cost and on-state loss. The bidirectional HCB is designed to interrupt the fault current in both directions by using silicon-controlled rectifier (SCR) devices. The polarity of pre-charged capacitor in the proposed topology is independent on the direction of the fault, which results in the reduction of the interruption time in the case of bidirectional fault current interruption. As a result, it can be used in the VSC-HVDC grid due to lower fault current interruption time. Besides, the proposed topology is capable of the reclosing and rebreaking operations without an auxiliary power supply, which results in the cost reduction. • A switching control algorithm for the proposed HCB model to operate in different modes, such as normal, breaking, discharging, and reversing modes, is proposed. The algorithm represents sufficiently detailed protection sequence among the operation modes. The closing and opening operations of thyristors and switches detect and control accurately in case of normal and fault conditions.
The paper is organized as follows: Section 2 describes the proposed topology of HCB based on the inverse current injection method. The operation principles and design parameters of the proposed topology are explained in this section. In Section 3, a switching control algorithm for the proposed topology is presented. Simulation results are discussed in Section 4. Finally, the main conclusions are summarized in Section 5.

Configuration of Proposed Topology
The structure of the proposed HCB with bidirectional current interruption is shown in Figure 1, which is composed of a main branch, an energy absorbing branch, and auxiliary branches. The main branch has a fast mechanical switch S 2 . The auxiliary branches include a capacitor C, a resistor R, a fast mechanical switch S 3 , two inductors (L 1 and L 2 ), and five thyristors (T 1a , T 1b , T 2a , T 2b , and T 3 ). The T 1a , T 1b , T 2a , and T 2b are used for breaking the bidirectional fault current, whereas the T 3 is employed to reverse the polarity of capacitor voltage. The absorbing branch is composed of series-parallel metal oxide varistors MOVs to protect the overvoltage of the capacitor. In addition, a residual DC current disconnector S 1 is used to completely isolate the DC circuit. The mechanical switches S 1 , S 2 , and S 3 consist of the vacuum interrupter to extinguish the electrical arc. directions by using silicon-controlled rectifier (SCR) devices. The polarity of pre-charged capacitor in the proposed topology is independent on the direction of the fault, which results in the reduction of the interruption time in the case of bidirectional fault current interruption. As a result, it can be used in the VSC-HVDC grid due to lower fault current interruption time.
Besides, the proposed topology is capable of the reclosing and rebreaking operations without an auxiliary power supply, which results in the cost reduction.

•
A switching control algorithm for the proposed HCB model to operate in different modes, such as normal, breaking, discharging, and reversing modes, is proposed. The algorithm represents sufficiently detailed protection sequence among the operation modes. The closing and opening operations of thyristors and switches detect and control accurately in case of normal and fault conditions.
The paper is organized as follows: Section 2 describes the proposed topology of HCB based on the inverse current injection method. The operation principles and design parameters of the proposed topology are explained in this section. In Section 3, a switching control algorithm for the proposed topology is presented. Simulation results are discussed in Section 4. Finally, the main conclusions are summarized in Section 5.

Configuration of Proposed Topology
The structure of the proposed HCB with bidirectional current interruption is shown in Figure 1, which is composed of a main branch, an energy absorbing branch, and auxiliary branches. The main branch has a fast mechanical switch S2. The auxiliary branches include a capacitor C, a resistor R, a fast mechanical switch S3, two inductors (L1 and L2), and five thyristors (T1a, T1b, T2a, T2b, and T3). The T1a, T1b, T2a, and T2b are used for breaking the bidirectional fault current, whereas the T3 is employed to reverse the polarity of capacitor voltage. The absorbing branch is composed of series-parallel metal oxide varistors MOVs to protect the overvoltage of the capacitor. In addition, a residual DC current disconnector S1 is used to completely isolate the DC circuit. The mechanical switches S1, S2, and S3 consist of the vacuum interrupter to extinguish the electrical arc.

Operation Principle
The operation modes of the proposed HCB are based on predefined the states of the switches in the proposed HCB. The switches can be turned-on or turned-off depending on the operation modes. The operation of proposed HCB can be divided into four modes such as normal mode, breaking mode, discharging mode, and reversing mode. The states of the switches are different according to the direction of the current, as shown in Table 1. The direction of the fault current is positive when it flows from A-side to B-side and negative for the flow from B-side to A-side. The delay time for operating switches in the proposed HCB is neglected for the case of interpretation in the description. Table 1. Switching states for positive and negative current scenarios.

Positive Currents
Modes represents for the closed or turned-on state; b 0 represents for the opened or turned-off state.
In order to easily understand the operation principle of the proposed HCB, the detailed current waveforms for the positive current interruption are illustrated in Figure 2. The operation of the proposed HCB is divided into eight intervals corresponding to four operation modes. It contains a normal mode (t 0~t2 ) to transfer the power between the sides without any interruption, a breaking mode (t 2~t5 ) to break the fault current, a discharging mode (t 5~t6 ) to reduce the capacitor voltage to the rated voltage, and a reverse mode (t 6~t7 ) to change the capacitor polarity. The detailed operation characteristics of each mode are explained as follows.

Operation Principle
The operation modes of the proposed HCB are based on predefined the states of the switches in the proposed HCB. The switches can be turned-on or turned-off depending on the operation modes. The operation of proposed HCB can be divided into four modes such as normal mode, breaking mode, discharging mode, and reversing mode. The states of the switches are different according to the direction of the current, as shown in Table 1. The direction of the fault current is positive when it flows from A-side to B-side and negative for the flow from B-side to A-side. The delay time for operating switches in the proposed HCB is neglected for the case of interpretation in the description.  In order to easily understand the operation principle of the proposed HCB, the detailed current waveforms for the positive current interruption are illustrated in Figure 2. The operation of the proposed HCB is divided into eight intervals corresponding to four operation modes. It contains a normal mode (t0~t2) to transfer the power between the sides without any interruption, a breaking mode (t2~t5) to break the fault current, a discharging mode (t5~t6) to reduce the capacitor voltage to the rated voltage, and a reverse mode (t6~t7) to change the capacitor polarity. The detailed operation characteristics of each mode are explained as follows. Under normal conditions, the switches S1 and S2 are closed while the switch S3 is opened. Therefore, the DC current between the terminals can stably flow through the main branches on the CB as shown in Figure 3a. The DC current start to increase when a fault occurs at t = t1. At this moment, the normal mode is kept until the DC current is larger than a predefined threshold Itrip to detect a short-circuit fault. Under normal conditions, the switches S 1 and S 2 are closed while the switch S 3 is opened. Therefore, the DC current between the terminals can stably flow through the main branches on the CB as shown in Figure 3a. The DC current start to increase when a fault occurs at t = t 1 . At this moment, the normal mode is kept until the DC current is larger than a predefined threshold I trip to detect a short-circuit fault.

Breaking Mode (t2~t5)
When the DC current reaches a predefined threshold at t = t2, the interruption processes start with opening S2 on the main branch. An electric arc occurs inside the vacuum interrupter of the mechanical switch S2 and the fault current rises continuously while the arc is extinct. Simultaneously, the thyristors (T1a and T1b) are turned-on for the positive current, whereas thyristors (T2a and T2b) are turned-on for negative current, as shown in Figures 3c and 4c. Therefore, the inverse current generated by the resonance circuit including the inductor L1 and capacitor C is injected to the main branch. Due to this, the arc current on the main branch is rapidly decreased and naturally dissipated with the zero crossing of the current at t = t3. In addition, the fault current is completely commutated

Breaking Mode (t 2~t5 )
When the DC current reaches a predefined threshold at t = t 2 , the interruption processes start with opening S 2 on the main branch. An electric arc occurs inside the vacuum interrupter of the mechanical switch S 2 and the fault current rises continuously while the arc is extinct. Simultaneously, the thyristors (T 1a and T 1b ) are turned-on for the positive current, whereas thyristors (T 2a and T 2b ) are turned-on for negative current, as shown in Figures 3c and 4c. Therefore, the inverse current generated by the resonance circuit including the inductor L 1 and capacitor C is injected to the main branch. Due to this, the arc current on the main branch is rapidly decreased and naturally dissipated with the zero crossing of the current at t = t 3 . In addition, the fault current is completely commutated from the main branch to the auxiliary branch with capacitor C, as shown in Figures 3d and 4d. Subsequently, the fault current oscillates by the capacitor C and inductor L 1 . The MOV starts to absorb the fault current at t = t 4 when the voltage of the capacitor C is over the protection level, as shown in Figures 3e and 4e. At t = t 5 , when the fault current reaches to zero, the thyristors (T 1a and T 1b ) are turned-off for the positive fault current, whereas thyristors (T 2a and T 2b ) are turned-off for negative fault current, as shown in Figures 3f and 4f. Finally, the DC circuit is isolated by turning-on the residual DC current disconnector S 1 .  Figures 3f and 4f. Finally, the DC circuit is isolated by turning-on the residual DC current disconnector S1.

Discharging Mode (t 5~t6 )
Although the voltage of the capacitor C is protected by the MOV, the overcharged voltage of capacitor C is not suitable for the reclosing operation. Therefore, the loop circuit with the resistor R is used to recover to magnitude of the rated voltage by turning-on the fast mechanical switch S 3 . During the period of t = t 5~t6 , the capacitor voltage V C is discharged to magnitude of the rated voltage.

Reversing Mode (t 6~t7 )
For the reclosing operation, the voltage polarity of the capacitor C should also be changed to the initial condition. Therefore, the reverse capacitor mode is done for the period of t = t 6~t7 by turning-on thyristor T 3 . After this time, the proposed HCB can be reclosed on the DC circuit and have the rebreaking capability by repeating the above operation modes.

Design Parameters
The component parameters in the proposed topology should be properly designed to successful interrupt the fault current. The operation time and interruption capability of the proposed HCB can affect the circuit parameters. In the following sub-sections, the process of designing parameters is explained in detail below.

Design of Auxiliary Branch with L 1 -C
During the breaking operation, the injected current from the auxiliary branch with L 1 -C should be higher than the main branch. The injected current can be obtained as follows: where V pre−C is the pre-charging voltage of the capacitor. From Equation (1), the maximum magnitude of the injected current is V pre−C √ C/L 1 . Assuming the minimum injected current for successful interruption of the fault current, the time taken by the injected current to reach the fault current is equal to π √ L 1 C /2 corresponding to a quarter of the period the injected current. Therefore, the relationship between the inductance L 1 and the capacitance C can be represented as follow: where I trip is the predefined threshold to detect the fault, M is the margin of the breaking current, and T i is the time that the fault current is equal to the injected current. The T i should be decided within 1 milliseconds by considering the rate of the fault current rise. Rearranging the Equations (2) and (3), the inductance L 1 and the capacitance C are obtained as follows:

Design of Auxiliary Branches with R and L 2
The auxiliary branches with R and L 2 are used to recover the capacitor voltage to the initial voltage. The parameters can be obtained as follow: where T d is the time taken by the discharging mode and T r is the time taken by the reversing mode.

Design of Energy Absorbing Branch
The MOV is used to protect the capacitor overvoltage by absorbing the energy on the DC circuit. The energy absorbed by the MOV can be represented as follow [24]: where V MOV is the protection voltage level, T E is the time for absorbing the energy, and i f is the fault current.

Outline of the Switching Control Algorithm
The operation mode of proposed HCB is shown in Figure 5. The main components of the control method are divided into two parts. First is the checking mode part and second is the mode selector part. In mode checking, the present operation mode of the HCB is recognized by measuring the DC line current and capacitor voltage. In addition, the function to detect any fault is also included to activate the HCB. From the data of the checking mode, a control unit is required to select the operation mode and control signals. Hence, the real signal of switching state is transferred to each switch by considering the time delay of the real devices. The time delay was considered as 2 ms for the fast mechanical switch and 20 ms for the residual switch [26]. In case of the switch based on the semiconductor, the time delay can be neglected because the switching time is short (0-40 µs) [22]. Therefore, the time delay of the mechanical switch is only considered in the simulation model to reflect the characteristics of real devices.
where is the time taken by the discharging mode and is the time taken by the reversing mode.

Design of Energy Absorbing Branch
The MOV is used to protect the capacitor overvoltage by absorbing the energy on the DC circuit. The energy absorbed by the MOV can be represented as follow [24]: where is the protection voltage level, is the time for absorbing the energy, and is the fault current.

Outline of the Switching Control Algorithm
The operation mode of proposed HCB is shown in Figure 5. The main components of the control method are divided into two parts. First is the checking mode part and second is the mode selector part. In mode checking, the present operation mode of the HCB is recognized by measuring the DC line current and capacitor voltage. In addition, the function to detect any fault is also included to activate the HCB. From the data of the checking mode, a control unit is required to select the operation mode and control signals. Hence, the real signal of switching state is transferred to each switch by considering the time delay of the real devices. The time delay was considered as 2 ms for the fast mechanical switch and 20 ms for the residual switch [26]. In case of the switch based on the semiconductor, the time delay can be neglected because the switching time is short (0-40 μs) [22]. Therefore, the time delay of the mechanical switch is only considered in the simulation model to reflect the characteristics of real devices.

Flowchart of the Switching Control Algorithm
It is important for the controller to accurately detect the operation status of the HCB as well as the fault depending on the direction of the current. Figure 6 shows the switching control algorithm of the HCB that includes the checking mode and the selecting mode. The variable k is used for limiting the number of the operation of the HCB (N limit = 2). The fault can be detected by the predefined threshold I trip to detect the fault. The two measurement signals, DC line current and the capacitor voltage, are required for the algorithm. The detailed procedures of the switching control algorithm are shown as follows.

•
Initially, the number of the operation of HCB and the threshold for detecting the fault are determined. After closing HCB in the normal mode, the control system checks the operation mode of the HCB by measuring the DC line current and capacitor voltage.

•
If the magnitude of the DC line current is higher than the predefined threshold, the fault is detected and the direction of the fault current is determined (positive or negative). At the same time, the proper breaking mode is selected according to the direction of the fault current. For example, the breaking mode is active by turning on the thyristors (T 1a and T 1b ) for the forward direction and the thyristors (T 2a and T 2b ) for the reverse direction. The breaking mode is kept until the DC line current is dropped to zero.

•
The DC line can be isolated by the HCB after the breaking mode. Considering the reclosing operation of the HCB, the capacitor voltage is checked whether the magnitude and polarity is same with the pre-charged voltage. If capacitor voltage is not equal to the pre-charged value, the capacitor voltage is controlled by the discharging mode and reversing mode.

Flowchart of the Switching Control Algorithm
It is important for the controller to accurately detect the operation status of the HCB as well as the fault depending on the direction of the current. Figure 6 shows the switching control algorithm of the HCB that includes the checking mode and the selecting mode. The variable k is used for limiting the number of the operation of the HCB (Nlimit = 2). The fault can be detected by the predefined threshold Itrip to detect the fault. The two measurement signals, DC line current and the capacitor voltage, are required for the algorithm. The detailed procedures of the switching control algorithm are shown as follows. • Initially, the number of the operation of HCB and the threshold for detecting the fault are determined. After closing HCB in the normal mode, the control system checks the operation mode of the HCB by measuring the DC line current and capacitor voltage.

•
If the magnitude of the DC line current is higher than the predefined threshold, the fault is detected and the direction of the fault current is determined (positive or negative). At the same time, the proper breaking mode is selected according to the direction of the fault current. For example, the breaking mode is active by turning on the thyristors (T1a and T1b) for the forward direction and the thyristors (T2a and T2b) for the reverse direction. The breaking mode is kept until the DC line current is dropped to zero.

•
The DC line can be isolated by the HCB after the breaking mode. Considering the reclosing operation of the HCB, the capacitor voltage is checked whether the magnitude and polarity is same with the pre-charged voltage. If capacitor voltage is not equal to the pre-charged value, the capacitor voltage is controlled by the discharging mode and reversing mode.

Test System for Case Studies
In order to test the performance of the proposed HCB, a symmetrical monopole VSC-HVDC was modeled in MATLAB/Simulink, as illustrated in Figure 7. The AC network next to the HVDC link is substituted by an equivalent resistor (R)-inductor (L) impedance, which enabled the X/R ratio of the power system to be determined. The converter transformer is a wye-delta connection. A phase reactor is added between the converter and transformer to filter the harmonics during conversion. Each HCB is located at the output of both the rectifier and inverter sides. The detail specifications of the HVDC link are as follows [27]

Test System for Case Studies
In order to test the performance of the proposed HCB, a symmetrical monopole VSC-HVDC was modeled in MATLAB/Simulink, as illustrated in Figure 7. The AC network next to the HVDC link is substituted by an equivalent resistor (R)-inductor (L) impedance, which enabled the X/R ratio of the power system to be determined. The converter transformer is a wye-delta connection. A phase reactor is added between the converter and transformer to filter the harmonics during conversion. Each HCB is located at the output of both the rectifier and inverter sides. The detail specifications of the HVDC link are as follows [27]: The basic parameters of the designed HCB topology are summarized in Table 2. Four HCBs are applied to the positive and negative pole at each side. The mechanical switches in the HCB are considered as the breaker blocks provided by MATLAB/Simulink. The arc extinction process is modeled by opening the mechanical switches when its current passes through 0 at the first current zero crossing.
The simulations are carried out under two different scenarios to verify the performances of the proposed HCB. The first scenario is under the temporary fault while the second scenario is under permanent fault on the middle of the DC transmission line at 0.5 s.

Temporary Fault on DC Line
The current and capacitor voltage waveforms of the proposed HCBs during a temporary fault are shown in Figures 8-12. Under normal conditions, the current direction on the positive pole is from rectifier to inverter while the current direction on the negative pole is opposite. It is assumed that the fault occurs at 0.5 s and it is cleared from the HVDC system after 5 ms. The DC current is increased due to the short circuit. The fault currents flow from both sides to the fault location. At this moment, the fault is detected when the fault current reaches the positive threshold for HCB 1 and HCB 3 and the negative threshold for HCB 2 and HCB 4. After detecting the fault, the turn-off signal is transferred to switch S2 in each HCB. At the same time, the thyristors (T1a and T1b) in HCB 1 and  Table 2. Four HCBs are applied to the positive and negative pole at each side. The mechanical switches in the HCB are considered as the breaker blocks provided by MATLAB/Simulink. The arc extinction process is modeled by opening the mechanical switches when its current passes through 0 at the first current zero crossing. The simulations are carried out under two different scenarios to verify the performances of the proposed HCB. The first scenario is under the temporary fault while the second scenario is under permanent fault on the middle of the DC transmission line at 0.5 s.

Temporary Fault on DC Line
The current and capacitor voltage waveforms of the proposed HCBs during a temporary fault are shown in Figures 8-12. Under normal conditions, the current direction on the positive pole is from rectifier to inverter while the current direction on the negative pole is opposite. It is assumed that the fault occurs at 0.5 s and it is cleared from the HVDC system after 5 ms. The DC current is increased due to the short circuit. The fault currents flow from both sides to the fault location. At this moment, the fault is detected when the fault current reaches the positive threshold for HCB 1 and HCB 3 and the negative threshold for HCB 2 and HCB 4. After detecting the fault, the turn-off signal is transferred to switch S 2 in each HCB. At the same time, the thyristors (T 1a and T 1b ) in HCB 1 and HCB 3 are turned-on while the thyristors (T 2a and T 2b ) in HCB 2 and HCB 4 are turned-on. The current on the auxiliary branch with thyristors (T 1a and T 1b or T 2a and T 2b ) is injected to the branch with L 1 -C. Therefore, the current on the switch S 2 is naturally dissipated because the current on the branch with the switch S 2 reaches to zero. After that, the fault current flows through the auxiliary and is fully dissipated at about 0.5074 s. During breaking operation, the capacitor voltages in each HCB are increased to the protection level of MOV and the polarity is opposite to the initial condition as shown in Figure 11. In order to reclose the DC line, the capacitor voltage through the auxiliary branches with R-C and L 2 -C is reversed after the time delay of S 1 . At t = 0.5284 s, the reverse voltage mode of the HCBs is conducted and HVDC system is recovered to the normal mode by reclosing the HCBs. HCB 3 are turned-on while the thyristors (T2a and T2b) in HCB 2 and HCB 4 are turned-on. The current on the auxiliary branch with thyristors (T1a and T1b or T2a and T2b) is injected to the branch with L1-C. Therefore, the current on the switch S2 is naturally dissipated because the current on the branch with the switch S2 reaches to zero. After that, the fault current flows through the auxiliary and is fully dissipated at about 0.5074 s. During breaking operation, the capacitor voltages in each HCB are increased to the protection level of MOV and the polarity is opposite to the initial condition as shown in Figure 11. In order to reclose the DC line, the capacitor voltage through the auxiliary branches with R-C and L2-C is reversed after the time delay of S1. At t = 0.5284 s, the reverse voltage mode of the HCBs is conducted and HVDC system is recovered to the normal mode by reclosing the HCBs.    Therefore, the current on the switch S2 is naturally dissipated because the current on the branch with the switch S2 reaches to zero. After that, the fault current flows through the auxiliary and is fully dissipated at about 0.5074 s. During breaking operation, the capacitor voltages in each HCB are increased to the protection level of MOV and the polarity is opposite to the initial condition as shown in Figure 11. In order to reclose the DC line, the capacitor voltage through the auxiliary branches with R-C and L2-C is reversed after the time delay of S1. At t = 0.5284 s, the reverse voltage mode of the HCBs is conducted and HVDC system is recovered to the normal mode by reclosing the HCBs.    Therefore, the current on the switch S2 is naturally dissipated because the current on the branch with the switch S2 reaches to zero. After that, the fault current flows through the auxiliary and is fully dissipated at about 0.5074 s. During breaking operation, the capacitor voltages in each HCB are increased to the protection level of MOV and the polarity is opposite to the initial condition as shown in Figure 11. In order to reclose the DC line, the capacitor voltage through the auxiliary branches with R-C and L2-C is reversed after the time delay of S1. At t = 0.5284 s, the reverse voltage mode of the HCBs is conducted and HVDC system is recovered to the normal mode by reclosing the HCBs.

Permanent Fault on DC Line
After the initial fault current is cleared with a typical temporary fault, the reclosing and rebreaking operations are carried out continuously with the proposed HCB. The current and capacitor voltage waveforms of fours the HCBs in a typical permanent fault condition are illustrated in Figures 13-17. The reclosing operation of the HCBs is started at 0.5274 s after the delay time by turned-off switch S1. For this operation, the discharging and reversing capacitor voltage modes are performed and are finished at 0.5284 s. After this time, the HCBs allow the rebreaking operation by reclosed the switch S1. The rebreaking operation of the HCBs begins at 0.5484 s after 20 ms the delay time of switch S1. The procedure to rebreak the fault current is same operation in case of the temporary fault. Hence, the fault current interruption repeatedly of the HCBs is done at 0.5558 s. In order to fulfil the operation of the HCBs, the voltage of the capacitor (C) is recovered to the normal condition by the implementation of the discharging and reversing modes again. This process takes 10 ms after a delay time of turned-off switch S1 at 0.5758 s. At 0.5768 s, the operation of HCBs for a typical permanent fault condition is completed. In addition, the peak fault current is limited at 7 (kA) and the operation time to break fault current is 5 ms in both the process of fault current interruption.

Permanent Fault on DC Line
After the initial fault current is cleared with a typical temporary fault, the reclosing and rebreaking operations are carried out continuously with the proposed HCB. The current and capacitor voltage waveforms of fours the HCBs in a typical permanent fault condition are illustrated in Figures 13-17. The reclosing operation of the HCBs is started at 0.5274 s after the delay time by turned-off switch S1. For this operation, the discharging and reversing capacitor voltage modes are performed and are finished at 0.5284 s. After this time, the HCBs allow the rebreaking operation by reclosed the switch S1. The rebreaking operation of the HCBs begins at 0.5484 s after 20 ms the delay time of switch S1. The procedure to rebreak the fault current is same operation in case of the temporary fault. Hence, the fault current interruption repeatedly of the HCBs is done at 0.5558 s. In order to fulfil the operation of the HCBs, the voltage of the capacitor (C) is recovered to the normal condition by the implementation of the discharging and reversing modes again. This process takes 10 ms after a delay time of turned-off switch S1 at 0.5758 s. At 0.5768 s, the operation of HCBs for a typical permanent fault condition is completed. In addition, the peak fault current is limited at 7 (kA) and the operation time to break fault current is 5 ms in both the process of fault current interruption.

Permanent Fault on DC Line
After the initial fault current is cleared with a typical temporary fault, the reclosing and rebreaking operations are carried out continuously with the proposed HCB. The current and capacitor voltage waveforms of fours the HCBs in a typical permanent fault condition are illustrated in Figures 13-17. The reclosing operation of the HCBs is started at 0.5274 s after the delay time by turned-off switch S 1 . For this operation, the discharging and reversing capacitor voltage modes are performed and are finished at 0.5284 s. After this time, the HCBs allow the rebreaking operation by reclosed the switch S 1 . The rebreaking operation of the HCBs begins at 0.5484 s after 20 ms the delay time of switch S 1 . The procedure to rebreak the fault current is same operation in case of the temporary fault. Hence, the fault current interruption repeatedly of the HCBs is done at 0.5558 s. In order to fulfil the operation of the HCBs, the voltage of the capacitor (C) is recovered to the normal condition by the implementation of the discharging and reversing modes again. This process takes 10 ms after a delay time of turned-off switch S 1 at 0.5758 s. At 0.5768 s, the operation of HCBs for a typical permanent fault condition is completed. In addition, the peak fault current is limited at 7 (kA) and the operation time to break fault current is 5 ms in both the process of fault current interruption.

Permanent Fault on DC Line
After the initial fault current is cleared with a typical temporary fault, the reclosing and rebreaking operations are carried out continuously with the proposed HCB. The current and capacitor voltage waveforms of fours the HCBs in a typical permanent fault condition are illustrated in Figures 13-17. The reclosing operation of the HCBs is started at 0.5274 s after the delay time by turned-off switch S1. For this operation, the discharging and reversing capacitor voltage modes are performed and are finished at 0.5284 s. After this time, the HCBs allow the rebreaking operation by reclosed the switch S1. The rebreaking operation of the HCBs begins at 0.5484 s after 20 ms the delay time of switch S1. The procedure to rebreak the fault current is same operation in case of the temporary fault. Hence, the fault current interruption repeatedly of the HCBs is done at 0.5558 s. In order to fulfil the operation of the HCBs, the voltage of the capacitor (C) is recovered to the normal condition by the implementation of the discharging and reversing modes again. This process takes 10 ms after a delay time of turned-off switch S1 at 0.5758 s. At 0.5768 s, the operation of HCBs for a typical permanent fault condition is completed. In addition, the peak fault current is limited at 7 (kA) and the operation time to break fault current is 5 ms in both the process of fault current interruption.

Conclusions
This paper proposed a novel topology of HCB with the capabilities of bidirectional fault current interruption, reclosing, and rebreaking. In addition, a switching control algorithm for the HCBs was developed to operate the proposed HCB in different operation modes, such as the normal mode, breaking mode, discharging mode, and reversing the capacitor after clearing the fault. The simulation results showed that the HVDC system could be recovered by reclosing the HCB for the temporary fault. In the case of the permanent fault, the proposed HCB could interrupt the fault current continuously. The reclosing and rebreaking capabilities of the proposed HCB could supply the benefit of reliable and safe operation of the HVDC grid. The proposed HCB has lower on-state losses than the semiconductor-based HCB such as ABB HCB due to the use of thyristor instead of IGBT. The proposed HCB can reduce the fault interruption time in the case of bidirectional fault interruption because the polarity of the pre-charged capacitor does not change after identifying the direction of the fault current. Simulation results have shown that the fault interruption time of the proposed HCB was about 7 ms, which is about 30% lower than the interruption time of the HCB model in [25] (the interruption time is about 10 ms).
With the capabilities of bidirectional fault current interruption, reclosing, and rebreaking, the proposed HCB topology can be considered a potential solution for the protection of the multi-terminal VSC-HVDC (MTDC) system. Our future work will focus on the testing of the proposed HCB topology in the MTDC system.