Multi-port DC circuit breaker based on dual capacitor current limiting and energy sink

Introduction

With the high degree of power electronics of the DC power grid, the low inertia nature of the DC system has caused many adverse effects on the development of the fault current (Mokhberdoran et al., 2017a; Choi et al., 2018; Mohan et al., 2021). If the fault is not removed in time, the electrical equipment will be burned and the fault scope will be expanded. The DC Circuit Breaker (DCCB) can quickly and selectively isolate the faulty line, which can effectively solve the above problems and ensure the stable operation of the DC power grid (Meyer and Rufer, 2006; Arman et al., 2014; Martinez-Velasco and Magnussonet al., 2017). However, when the DC circuit breaker operates, the fault current has risen sharply, and the energy storage elements in the network have also accumulated more energy. Therefore, the DC circuit breaker needs to interrupt a larger current in a short period of time, while dissipating more energy (Novello et al., 2011; Wen et al., 2016; Qu et al., 2019). By analyzing the research status of HVDC circuit breakers, this paper studies and proposes innovations on the existing current limiters and multi-port DC circuit breakers. The main research results are as follows.

To meet the future demand of high voltage and large capacity of DC power network, there is great room for the development of multi-port hybrid DCCB with current limiting capability (Zhao et al., 2019; Xu et al., 2019; Han et al., 2019). Reference (Mokhberdoran et al., 2017b) proposes a capacitive commutation type current-limited DCCB in which the FCL and DCCB share the main flow branch, the main current limiter is used to limit the current during the fault removal process, and the main circuit breaker is used to open and close the current-limited DCCB. In reference Wang et al., 2018, although the DCCB has current limiting capability, it may generate overvoltage problems at both ends of the inductor at the early stage of current conversion, threatening the safety of the equipment. Reference Wen et al., 2016 proposes a valve-segment modular design of current-limited DCCB, where the inductor branches are connected in parallel during normal operation to reduce the equivalent impedance and the impact on the dynamic performance of the system, and the inductor branches are connected in series instead of in parallel during fault conditions.

However, it is difficult to expand the main circuit breaker into multi-port form because of the large number of devices used in the main circuit breaker section and the complex structure. Reference Liu et al., 2017 proposes a multi-port current-limiting DCCB that shares the main current limiter and main circuit breaker, but the main circuit breaker needs to open the fault current of all branches, and if some energy can be transferred to other energy storage elements and then consumed separately, the main circuit breaker can be reduced. Electrical requirements. Reference Liu et al., 2019 proposes a typical half-bridge multi-port DC circuit breaker. The main breaking switch adopts an anti-parallel sub-module, and the selection switch adopts a series branch of a thyristor and an IGBT module, so the manufacturing cost is high. Reference A. (Mokhberdoran et al., 2018a) considers the effect of MP-DCCB and the inherent leveling resistance in the line on suppressing the development of fault current, but the excessively large current limiting inductance will affect the steady-state characteristics of the system during normal operation. At the same time, the energy storage in the current limiting inductor increases the energy consumption demand of the arrester and reduces the decay rate of the fault current. Reference A. (Mokhberdoran et al., 2018b) takes four DC lines sharing one MPCB as an example, and proves that using one unidirectional current conversion path and one EAP for multiple DC lines can optimize the number of components, simplify the EAP structure, and effectively improve economy and flexibility.

Although the above research can achieve rapid fault removal and isolation, there is still room for further improvement in terms of equipment integration, steady-state loss, and manufacturing costs. This paper proposes a multi-port DC circuit breaker topology based on dual-capacitor current limiting and energy sinking by improving the topology of the multi-port DC circuit breaker. The decay process of the fault current is decoupled from the energy discharge process of the energy storage element in the fault line, thereby shortening the time for the DC circuit breaker to isolate the fault line. At the same time, the arrester is no longer used for energy dissipation, but the energy of the fault current is temporarily stored in the capacitor. After the fault is removed, the excess energy is released through the discharge circuit, which further shortens the fault isolation time and equipment withstand voltage requirements of the DC circuit breaker. Finally, the function of the proposed circuit breaker is verified by building a DC grid model in the simulation software. The simulation results prove the effectiveness of decoupling the decay process of the fault current and the energy discharge process of the energy storage element in the fault line to shorten the fault isolation time, which can provide a new idea for the research of DC circuit breakers.

Circuit breaker topology and operating principle IGBT devices

Circuit breaker topology design

The topology of the proposed multi-port DC circuit breaker based on double capacitor current limiting and energy collection (DC CL&EC MP-DCCB) is shown in Figure 1. The topology consists of five main branches, which are low loss branch, voltage regulator and sink branch, current transfer and current limiting branch, energy dissipation branch and state recovery branch.

FIGURE 1

FIGURE 1. DC CL and EC MP-DCCB topology structure.

In Figure 1, the low loss branch (LLB) is a series connection of an ultra-fast mechanical switch UFD and a double-controlled load transfer switch LCS, which is responsible for the delivery of DC operating current during normal system operation and the transfer of auxiliary fault current to the current transfer and current limiting branch when a line short circuit fault occurs on the DC side. Voltage stabilizing and current-collection branch (VS and CCB) consists of a mechanical switch T_v1 and a small number of fully controlled valves T_v2 and voltage stabilizing and current-collection capacitors C_v that can withstand large voltages and are inexpensive. The role of this branch is twofold: first, it is responsible for stabilizing the output voltage on the DC side during steady-state operation of the system; second, it temporarily stores the fault current energy when clearing the DC side fault, and then releases the excess energy after the fault current is cleared. The current-transferring and current-limiting branch (CT and CLB) is composed of a fully controlled valve group T_c and a current-limiting capacitor C_c in series, which is used to provide a current path during fault removal and to limit the growth rate of the fault current by using the current-limiting capacitor C_c. The energy consuming branch (ECB) consists of a diode and an energy dissipation resistor R_e, which is used to consume the energy stored in the fault line after the ultra-fast mechanical switch connected to the fault line can withstand the high voltage; the state recovery branch consists of a mechanical switch T_s1, a semi-controlled thyristor switch T_s2, a diode D_s3 and a recovery resistor R_s. This branch is used to consume the energy stored in the current-limiting capacitor C_c after the capacitor C_v is restored to about the system-level voltage, so that the capacitor C_c returns to the zero state.

Analysis of the working principle of circuit breaker

Take the three-port form of DC CL&EC MP-DCCB shown in Figure 1 as an example to illustrate the working principle of the proposed circuit breaker, whose installation position in the four-terminal DC network and the location of short-circuit fault occurrence are also shown in Figure 2.

FIGURE 2

FIGURE 2. Four-terminal DC network.

Suppose t₀ moment line L₂₃ near the converter station MMC₂ end of a single-pole ground short-circuit fault, multi-port DC circuit breaker P₁ port connected to the converter station MMC₂, P₂ port connected to line L₁₂, P₃ port connected to line L₂₃, the specific workflow of the proposed circuit breaker isolation fault line is shown in Figure 3, the action timing sequence is shown in Figure 4, the specific working principle is described below.

FIGURE 3

FIGURE 3. Multi-port DC circuit breaker action flow.

FIGURE 4

FIGURE 4. Fault Isolation Action Timing.

Normal working condition

When the system is in normal operation (t < t₀), the DC circuit breaker carries out power transmission through the internal low-loss branches, at this time, each ultra-fast mechanical switch UFD and load transfer switch LCS is on, switches T_v1 and T_v2 are on, and the voltage stabilization and current convergence branches are connected to the DC system, at this time, capacitor C_v is used as the voltage stabilization capacitor. When the DC output voltage of the converter station fluctuates in a small range, the regulator and sink branch can play the role of stabilizing the DC output voltage and improving the quality of the DC power supply. The transfer and current-limiting branch are in the open-circuit state, and no current flows through the energy-consuming branch. The current path in normal operation is shown in Figure 4A.

DC line short circuit

1) t₀≤t < t₁

After a 0.3 ms delay, the relay protection device detects a suspicious fault in the DC system at the moment t₁. t₀≤t < t₁, the DC fault current grows freely, and the fault current path is the same as the normal working state of the DC system, and the current path is shown in Figure 4A.

2) t₁≤t < t₂

At the moment of t₁, the fully controlled valve group T_c of the transfer and current limiting branch is given a signal, while the fully controlled valve group Tv₂ of the voltage regulator and current sink branch is blocked and the load transfer switch LCS₃ is turned off, at which time the current limiting capacitor C_c starts charging and suppresses the growth rate of the fault current. The purpose of blocking valve T_v2 is to prevent the voltage regulator and current sink capacitor C_v of the same voltage level as the system from discharging to the fault point. Again, due to the extremely short opening time of the power electronic switch, the above actions are considered to occur simultaneously during the analysis. If the fault is accurately detected at the moment t₂, the subsequent actions are carried out, otherwise LCS₃ is re-conducted, and the transfer and current limiting branches are withdrawn, and the system returns to normal operation. The current path at this stage is shown in Figure 4B.

3) t₂≤t < t₃

The fault detection phase ends at t₂ and the fault location is accurately detected, at which time the ultra-fast mechanical switch UFD3 directly connected to the faulty branch is tripped, the current limiting capacitor continues to charge, and the fault current limiting capability is further enhanced. By the time UFD3 reaches the rated opening distance at the moment t₃, it can provide sufficient electrical strength for the removal of the faulty line, and the current path in the time period t₂≤t < t₃ is also shown in Figure 4B.

4) t₃≤t < t₄

At the moment of t₃, UFD₃ opens to the rated breaking position, at which time a shutdown signal is applied to the transfer and current limiting branch switch T_c, and the fault current is passed through the diode of the voltage regulator and sink branch and capacitor C_c to form a circuit and store the fault current energy in capacitor C_c. At this point, capacitor C_c exists in the fault circuit as a sink capacitor, and the current path is shown as the red dashed line in Figure 4C. At the same time, the energy stored in the flat reactor on the fault line is released through the energy dissipation branch - flat reactor - fault grounding point circuit, and the current path is shown as the blue dashed line in Figure 4C. Thus, the decoupling of fault current decay and fault line energy release is achieved, which can significantly shorten the action time of DC circuit breaker to remove the DC fault line.

5) t ≥ t₄

The energy storage element on the faulty line ends at t₄, the faulty line current drops to 0, and the faulty line completes its isolation work. t₅, the fault current on the voltage stabilization and energy sink branch drops to 0, at which time the mechanical switch T_v1 is available for arc-free breaking, and the non-faulty line resumes normal operation. t₄≤t < t₅ time period and the current path after t₅ moment are shown in Figures 4D, E respectively.

Circuit breaker key parameters design

As the DC circuit breaker working principle research mainly focuses on the fault opening process, for the energy dissipation process of the internal energy storage elements of the circuit breaker after the completion of the fault opening, in order to restore the fault line to normal operation as soon as possible for transient faults, the recovery resistance R_s can be taken as a smaller value to reduce the current limiting capacitor C_c state recovery loop (current limiting capacitor C_c-switch T_s1, T_s2, Ds3, recovery resistance R_s - energy dissipation resistance Re) of the time constant. Therefore, the key components in the proposed multi-port DC circuit breaker topology include current-limiting capacitor C_c, voltage regulator and sink capacitor C_v, and energy dissipation resistor R_e. For the current-limiting capacitor C_c in this paper, the parameter design principle is the same as that of the common transfer capacitor C (Wang et al., 2019). Therefore, the value of the current-limiting capacitor C_c is set to 215μF, and the design of the other two key parameters is analysed below.

Voltage regulator and sink capacitor C_v design

During normal system operation, the voltage regulator and sink capacitor C_v can be equated to a large supply of system-level voltage, and there is not much significance in discussing its parameters in steady state. While during short circuit fault removal on the DC side of the line, the parameters of capacitor C_v are designed to directly affect the time required for the fault current to flow to capacitor C_v to eventually drop to 0 after the ultra-fast mechanical switch connected to the fault line is fully tripped and the peak voltage that capacitor C_v eventually reaches, i.e., the stage shown in Figure 4C. The equivalent circuit for this stage and its simplified circuit are shown in Figure 5, where U is the equivalent DC voltage source for the converter station and L_dc is the flat wave reactor.

FIGURE 5

FIGURE 5. Parametric analysis of capacitor C_v equivalent circuit and its simplified circuit.

The following equation can be written from the simplified circuit in Figure 5.

$\frac{L_{\mathrm{d}\mathrm{c}}}{2}\frac{di}{dt}+u_{cv}-U=0(1)$

$i=C\frac{d{u}_{cv}}{dt}(2)$

$u_{cv}=\frac{1}{C}\int idt(3)$

The analysis shows that the smaller the capacitor C_v parameter is, the faster the capacitor absorbs the fault current, the shorter the time for the fault current to drop to 0, but at the same time the higher the peak voltage that the capacitor eventually reaches. The model is simulated in PSCAD/EMTDC software, and the results are shown in Figure 6. In Figure 6A, it can also be seen that when the capacitor C_v parameter is taken as 50 μF, the fault current can be decayed to 0 in 5.8 ms after the fault occurs, which takes the shortest time, but the peak voltage of capacitor C_v reaches 373 kV, which seriously affects the insulation level and insulation cost of the equipment. The decay time of the fault current increases slightly as the value of the capacitance C_v parameter is gradually increased. When the capacitance C_v parameter reaches 200 μF, the fault current excision time remains basically unchanged, and only increases by 0.8 ms compared with the fault current excision time required for capacitance C_v parameter taken as 50 μF, but the peak voltage of capacitance C_v decreases from 373 kV to 259 kV. The influence of the final peak voltage change is significantly reduced. Through the above analysis, considering the fault current excision time, the peak voltage of capacitor C_v and the insulation effect of capacitance parameter on the equipment, the value of voltage regulator and sink capacitor C_v parameter is taken as 200 µF in this paper.

FIGURE 6

FIGURE 6. Fault current and peak capacitor voltage curves for different C_v parameters.

Energy-consuming resistor R_e design

The energy dissipation resistance R_e mainly affects the energy release time of the energy storage elements in the fault line after the fault current is transferred to the voltage regulator and sink branch, which means that it affects the isolation time of the fault line. To meet the rapidity requirements of the circuit breaker, the model is built in the PSCAD/EMTDC software for simulation analysis, and the fault line current curves under different energy dissipation resistance R_e parameters are shown in Figure 7. As can be seen from Figure 7, the decay time of the current on the faulty line is gradually shortened when the energy dissipation resistor R_e parameter is gradually increased. However, when the energy dissipation resistor R_e increases to 150 Ω, the fault line can be isolated 4.8 ms after the fault occurs. When the resistance value continues to increase, the isolation time of the fault line basically no longer changes. In summary, the energy dissipation resistor R_e parameter in this paper is taken as 150 Ω.

FIGURE 7

FIGURE 7. Fault line current curves for different energy dissipation resistance R_e parameters.

Simulation analysis

The proposed circuit breaker function is verified by building the four-terminal DC network model shown in Figure 2 in PSCAD/EMTDC simulation software. The parameters of each converter station are set as shown in Table 1, and the parameters of DC circuit breaker components are shown in Table 2: line parameters are r₀ = 0.01 Ω/km, l₀ = 0.82 mH/km, and DC line flat wave reactance is taken as 20 mH. It is assumed that a single-pole short circuit fault occurs at the first end of L₂₃ when the system is in stable operation for 1.5 s. The three-port DC circuit breaker connected to MMC₂ of the converter station is used as an example to verify the opening performance and fault opening time of the circuit breaker topology proposed in this paper.

TABLE 1

TABLE 1. Converter station parameters.

TABLE 2

TABLE 2. DC circuit breaker parameters.

Open capacity verification

Figure 8 shows that the current flow in each branch inside the proposed multi-port DC circuit breaker during the opening of the breaker for DC fault current.

FIGURE 8

FIGURE 8. DC fault current opening and closing curves.

Figure 8A shows the current situation of the low-loss branch, steady-state operation, the converter station MMC₂ and MMC₃ for the receiving end of the converter station, the operating current flow on line L₂₃ for the converter station MMC₃ to MMC₂, after the occurrence of a fault, the current on line L₂₃ first decays rapidly to 0 and then increases in the reverse direction to 0.68 kA, to 1.5003 s when the transfer and current-limiting branch is put in. The fault current is transferred to the transfer and current-limiting branch, and the current condition of this branch is represented by the current I_Cc flowing through the current-limiting capacitor C_c, and the I_Cc current curve is shown in Figure 8B. At 1.5003 s, the transfer and current limiting branch current increases rapidly to 0.68 kA. Due to the presence of current limiting capacitor C_c, the fault current increases to 0.68 kA and then decreases slightly after the fault current charges the current limiting capacitor continuously, and the current limiting capacity of the current limiting capacitor gradually increases. The rise rate of the fault current gradually decreases to 1.504 s fault line connected to the ultra-fast mechanical switch breaks to the rated open position, transfer, and current limiting branch out, I_Cc current value quickly reduced to zero. Thereafter the energy contained in the fault current is divided into two parts: one is that the feeder current of other converter stations cannot be changed instantaneously due to the role of inductive elements in the network. This part of the current energy is temporarily stored by the sink capacitor, and the feeder current of the converter station rapidly decreases to zero within a few milliseconds, after which the non-faulty part of the network gradually resumes normal operation to reduce the impact of the fault, as shown in Figure 8C. The current fluctuation at 1.5 s in this curve is caused by the short-time discharge and charging of the voltage regulator and sink capacitor at the moment of the fault, but the branch is removed in time 0.3 ms after the fault to prevent further discharge of the voltage regulator and sink capacitor. Second, the energy stored in the flat wave reactor in the faulty line, this part of energy is released through the loop formed by the energy dissipation branch and the faulty line. The current situation during this time is shown in Figure 8D for the part t > 1.504 s. As can be seen from Figure 8D, the fault branch current completes the energy release of the energy storage element at 1.5048 ms, and the time required to isolate the faulted line is significantly reduced.

Figure 9 shows the use of ABB hybrid DC circuit breaker compared to the current limiting DC circuit breaker CC&CL-HDCCB based on capacitive commutation proposed in literature CUZNER R M et al., 2017 and the resistance-inductance current limiting hybrid DC circuit breaker proposed in literature Wang et al., 2019 fault current under each scheme of LR-HDCCB. Combining with Figure 8, it can be seen that the peak fault current of the circuit breaker proposed in this paper is 8.8 kA, which is 20.7% lower than that of the ABB solution and the breaking time is shorter. Compared with LR-HDCCB, the current limiting capability is slightly insufficient. However, it can be seen from Figure 12 that the voltage across the circuit breaker of the LR-HDCCB is extremely high during the commutation stage, even higher than the system-level voltage. This is caused by the parallel connection of resistance and inductance in the commutation stage, which may cause the ultra-fast mechanical switch to be difficult to break and even cause equipment damage. Compared with CC&CL-HDCCB, the current limiting capability is similar, but the fault clearing time advantage of the scheme proposed in this paper is obvious. From the above analysis, it can be concluded that the multi-port DC circuit breaker proposed in this paper has a good current limiting capability and has the shortest fault breaking time.

FIGURE 9

FIGURE 9. Fault current comparison.

Figure 10 shows that the voltage rise of the current limiting capacitor C_c and the sink capacitor C_v during the opening of the proposed circuit breaker for DC line faults.

FIGURE 10

FIGURE 10. Current limiting capacitor and sink capacitor voltage curves.

Among them, the voltage of current-limiting capacitor C_c reaches its peak voltage after the ultra-fast mechanical switch connected to the fault line breaks to the rated position, and its peak voltage is 100 kV, as shown in the red curve in Figure 10. The voltage rise of the sink capacitor C_v is shown in the blue curve in Figure 10. The voltage of the sink capacitor C_v at steady state is approximately the system voltage. The moment the fault occurs, its voltage fluctuates slightly, and then the circuit breaker acts in time, its voltage remains stable until the fault line connected to the ultra-fast mechanical switch breaks to the rated position sink branch re-entered, transfer and current-limiting branch out of operation. Sink capacitor C_v pooling converter station fault current, capacitor C_v voltage gradually increases until the converter station feeder current drops to zero, capacitor C_v voltage reaches its peak value of 260 kV.

Figure 11 shows that the voltage curves at both ends of the main breakout switch of the proposed multi-port circuit breaker. The proposed main breakout switch refers to the IGBT full control switch of the current transfer and current limiting branch and the IGBT full control switch of the voltage regulator and current sink branch.

FIGURE 11

FIGURE 11. Multi-port circuit breaker main breakout switch voltage curve at both ends.

Since the system is operating at steady state when the circuit breaker internal voltage regulator and sink branch is on, current transfer and current limiting branch is open, and capacitor C_v carries the system-level voltage, the current transfer and current limiting branch is first on and then off during DC fault opening and closing, the voltage regulator and sink branch is first off and then on, and the fully controlled switching states of the two branches are complementary, so they are jointly regarded as the main breaking switch of the circuit breaker to observe the voltage conditions at both ends. As can be seen from Figure 11, the main breakout switch (mainly the current transfer and current-limiting branch switch) bears the system-level voltage during steady state. After 0.3 ms after the fault, the transfer branch is triggered and the voltage at both ends of the main breakout switch drops to zero, and then the voltage stabilization and current convergence branch switch is blocked. The main breakout switch (mainly the voltage regulator and sink branch switch) bears the voltage, and then the current limiting capacitor C_c is continuously charged. The voltage across the main breakout switch gradually decreases until the 1.504 s sink capacitor C_v resumes conduction and pools the fault current energy. The voltage of capacitor C_v gradually increases to a peak voltage of 260 kV, and the voltage across the main breakout switch (mainly the current transfer and current limiting branch switch) increases to a final voltage of 156 kV. Accordingly, the number of devices for the IGBT full control switch of the current transfer and current limiting branch switch and the IGBT full control switch of the voltage regulator and current sink branch switch is calculated.

Figure 12 shows a comparison of the voltage across the circuit breaker during fault removal for each scheme. In the LR-HDCCB scheme, since the capacitor is charged first, then discharged, and then reversely charged to the final voltage, the voltage across the circuit breaker is determined by the arrester voltage and the current-limiting inductor voltage until the arrester consumes energy. In the CC&CL-HDCCB scheme, in the commutation stage, the voltage across the circuit breaker is extremely high due to the action of the resistance-inductance parallel part. With the gradual input of the inductance, the voltage across the circuit breaker gradually decreases until the voltage jumps again in the energy consumption stage of the arrester. The circuit breaker consumes energy through the arrester, and the voltage across both ends gradually drops until the fault is removed. It can be seen from the comparison results that the proposed circuit breaker shortens the operation time and reduces the voltage level during the breaking process of the circuit breaker, which is beneficial to the safe and stable operation of the equipment.

FIGURE 12

FIGURE 12. Voltage comparison across DC circuit breakers.

Figure 13 shows the comparison of the arrester energy consumption of each scheme. In the figure, the power consumption of the arrester under the ABB scheme is 5250 kJ. In this scheme, because part of the energy is stored in the commutation capacitor, the power consumption of the arrester is reduced to 3684 kJ. In the LR-HDCCB scheme, due to the continuous energy consumption of the resistance-inductive parallel branch in the commutation stage and later, the power consumption of the arrester in this scheme is slightly smaller than 2874 kJ, but there is still the problem of excessive voltage across the circuit breaker in the commutation stage. In the CC&CL-HDCCB scheme, part of the energy is stored in the capacitor during the operation of the circuit breaker, and the energy consumption of the arrester is slightly less than 3305 kJ. Because the final value voltage of the capacitor in this scheme is higher than the system level voltage and much higher than the final value voltage of the capacitor in this scheme, it affects the insulation cost of the equipment. From the above analysis, it can be seen that this solution not only reduces the energy consumption of the arrester, but also takes into account the insulation cost of the equipment, which is beneficial to prolong the service life of the arrester and reduce the cost.

FIGURE 13

FIGURE 13. Arrester energy consumption comparison.

Fault breaking time check

To calibrate the decoupling of the decay removal process of the fault current from the discharge process of the residual energy of the faulty line to achieve further reduction of the fault opening time of the DC circuit breaker. The simulation of the fault line opening current profile of the multi-port DC circuit breaker proposed in this paper is compared with that of the multi-port DC circuit breaker proposed in the literature Han et al., 2019, and the results are shown in Figure 14.

FIGURE 14

FIGURE 14. Fault line opening and closing time comparison.

It is obvious from Figure 14 that both schemes have the same current limiting effect on the fault current, with the peak fault current at 8.8 kA. However, the multi-port DC circuit breaker topology proposed in this paper decouples the fault current decay process from the fault line energy release process, and the fault line current drops to 0 at 1.5048 s. Compared with the circuit breaker topology proposed in the literature Han et al., 2019, the fault The isolation of the line is completed 2 ms earlier, reducing the time required to isolate the faulty line by about 29.4%. Thus, the effectiveness and necessity of decoupling the decay removal process of the fault current from the discharge process of the residual energy of the faulty line is verified.

Economic analysis

From the simulation results in Figure 8, the peak fault current of the low-loss branch is 2.06 kA and the rated current of IGBT is 2 kA. Considering the current margin, two branches of the low-loss branch need to be connected in parallel, and the number of IGBTs in series in each branch is 20. Because the peak fault current of the transfer and current-limiting branch is 8.6 kA, five branches need to be connected in parallel, and the number of IGBTs in series in this branch needs to be determined according to the voltage at both ends of the branch. As can be seen from Figure 11, the peak withstand voltage of this branch is 193 kV, and considering the engineering conditions and safety margin, the voltage withstand capability of IGBT is 2.25 kV, so the number of IGBTs to be connected in series is 193/2.25 = 86, so the number of IGBTs required for this branch is 86 × 5 = 430. The peak current of the voltage regulator and sink branch is 8.36kA, so this branch needs to be connected in parallel with five branches, and it can be seen from Figure 11 that the peak withstand voltage of this branch is 188kV, and the number of IGBTs need to be connected in series is 188/2.25 = 84, so the number of IGBTs needed for this branch is 84 × 5 = 420.

In the ABB solution: the current peak value of the low-loss branch is 6.26 kA, the LCS needs to be connected in parallel with four branches, and the required number of IGBTs is 80; the peak fault current is 11.1 kA, the main circuit breaker needs to be connected in parallel with six branches, and the main circuit breaker needs to be connected in parallel. The number of IGBTs required by the branch is 6 × 2×300/2.25 = 1,600. The total number of IGBTs required is 1,680. In the LR-HDCCB scheme: the current peak value of the low-loss branch is 6.26 kA, the LCS needs to be connected in parallel with four branches, and the number of IGBTs required is 80; the peak fault current is 6.60 kA, and the main breaking branch needs to be connected in parallel with four branches. The number of IGBTs required for the main breaking branch is 4 × 300/2.25 = 534. The total number of IGBTs required is 614. In the CC&CL-HDCCB scheme: the peak value of the current flowing through the main breaking branch is 8.31 kA, five branches need to be connected in parallel, and the number of IGBTs required is 2 × 5×150/2.25 = 667. The number of thyristors required is 364. If the two-port DC circuit breaker has the same breaking capacity as the circuit breaker in this article, three devices are required, so the number of components of the above circuit breaker needs to be multiplied by three times.

To sum up, the circuit breaker topology proposed in this paper requires a total of 920 IGBTs, which is slightly less economical than the topology proposed in the literature Han et al., 2019, but still better than ABB hybrid DC circuit breakers and capacitor commutation-based circuit breakers, current-limiting DC circuit breakers and resistance-inductance current-limiting hybrid DC circuit breakers. First of all, this paper takes advantage of the advantages of hybrid equipment and adopts the idea that multiple current limiters or circuit breakers at the same DC bus share the main current limiting circuit or breaking circuit, and design a current branch corresponding to n ports and low on-state loss. Secondly, designing the main current limiter by utilizing the advantages of the transfer capacitor in terms of voltage and current resistance level, cost, blocking ability and reliability. Finally, using the full control of IGBT to design the main circuit breaker; adopts the action sequence of current limiting and then breaking to reduce the energy consumption demand of the arrester.

Table 3 shows the economic comparison of each scheme. It can be seen that this scheme not only has a good current limiting effect and improves equipment integration, but also greatly improves the economy.

TABLE 3

TABLE 3. Economic comparison.

Conclusion

This paper proposes a multi-port DC circuit breaker based on dual-capacitor current limiting and sink energy, which decouples the decay and removal process of the fault current from the discharge process of the residual energy of the fault line. The transfer branch capacitor is used to achieve the current limiting function, and the voltage regulator and sink branch capacitor provide a path for the source-side fault current after decoupling the circuit and make it decay quickly. The transfer branch capacitor is used to limit the current, the voltage regulator and the sink branch capacitor to provide a path for the source-side fault current to decay quickly. The proposed circuit breaker’s breaking capacity and fault breaking time were verified by building a four-terminal DC network model in the PSCAD/EMTDC simulation software. The analysis of the simulation results shows that the proposed circuit breaker is economically superior. At the same time, the operation time has been improved compared with the capacitor-based multi-port DC circuit breaker, which is conducive to the rapid isolation of DC faults and the recovery of the grid as soon as possible and can provide ideas for the development of DC circuit breaker research. Cuzner and Singh, 2017; Ahmad and Wang, 2019.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Author contributions

WG and XL designed the topology of multi-port DC circuit breaker, WY and QH calculated the charging capacitance parameters of circuit breaker, SL and JB verified the topology results by simulation, and carried out a literature review.

Funding

This paper is supported by the Research Project on the Theory and Technology of Dispatching Operation Control of Zhejiang New Power System (SGTYHT/21-JS-226).

Conflict of interest

WG, XL, WY, and QH were employed by the company Electric Power Dispatching and Control Center of State Grid Zhejiang Electric Power Co. Ltd.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

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