A bidirectional series hybrid DC circuit breaker and a control method thereof

Abstract

This invention provides a bidirectional series-type hybrid DC circuit breaker and its control method, relating to the field of power equipment technology. The bidirectional series-type hybrid DC circuit breaker includes a main branch, a charging circuit, and a voltage injection unit. The main branch includes a mechanical switch, a current-limiting inductor, and a secondary winding of a first coupling inductor. The voltage injection unit includes a capacitor, a first H-bridge unit, a second H-bridge unit, a primary winding of the first coupling inductor, and a second coupling inductor. One end of the charging circuit is connected to the main branch, and the other end is connected to the voltage injection unit for charging the capacitor. Furthermore, a control method for the bidirectional series-type hybrid DC circuit breaker is also provided. The bidirectional series-type hybrid DC circuit breaker provided by this invention charges the capacitor by recovering the residual excitation current in the coupling inductor, allowing the capacitor voltage and the coupling inductor excitation current to return to their initial states, thereby achieving self-restart and enabling bidirectional interruption of fault current.

Description

Technical Field

[0001] This invention belongs to the field of power equipment technology, specifically relating to a bidirectional series hybrid DC circuit breaker and its control method. Background Technology

[0002] With the rapid development of renewable energy, medium- and low-voltage DC distribution networks have attracted widespread attention due to their ease of access, high transmission efficiency, and good stability. However, the absence of a natural current zero-crossing point and low line inductance in DC distribution networks make interrupting DC fault currents extremely challenging. DC circuit breakers (DCCBs), capable of quickly interrupting DC fault currents and selectively isolating faulty sections, have become indispensable equipment for ensuring the safe and stable operation of DC distribution networks.

[0003] The concept of a series-type hybrid DC circuit breaker (SHCB) was proposed. During fault interruption by the SHCB, the energy storage capacitor C in the voltage injection unit continuously charges the transformer's magnetizing inductance Lm, while the DC system current also charges Lm. Therefore, after fault interruption, the transformer retains a significant magnetizing current, causing a voltage drop in the energy storage capacitor. A key step in the SHCB is to quickly restore the transformer's magnetizing current and the energy storage capacitor's voltage to their initial state; this process is called the restart operation.

[0004] Numerous studies have explored restarting methods for SHCBs. First, restoring the voltage of the energy storage capacitor typically requires the addition of a charging device or circuit. Wired and wireless charging devices have been developed to enable rapid pre-charging of the energy storage capacitor. A simple charging circuit for a single-capacitor SHCB, consisting of power devices and a current-limiting resistor, has been proposed in the prior art. However, this design can only pre-charge the capacitor to the DC system voltage, limiting its applicability; moreover, the aforementioned simple charging circuit requires activating additional devices or circuits after each interruption fault, complicating the restarting process. Second, restoring the transformer's magnetizing current typically requires dissipating it through a resistor. In some SHCBs, the residual magnetizing current is dissipated through the power devices and the internal resistance of the windings. This process follows an LR first-order circuit, but due to the low resistance, the speed can be slow, limiting the SHCB's ability to reclose quickly. To address this issue, an independent dissipation branch with adjustable resistance has been introduced to accelerate the process. However, all these devices and methods result in energy waste.

[0005] Furthermore, the bidirectional operation of SHCBs relies on the ability of the voltage injection unit to generate a controllable polarity reverse voltage. Most existing SHCBs are unidirectional and require additional circuitry to achieve bidirectional functionality. For example, a full-bridge inverting circuit is placed on the primary side of the transformer to reverse the reverse voltage polarity.

[0006] Therefore, there is an urgent need to develop a series hybrid DC circuit breaker with bidirectional function and energy recovery capability. Summary of the Invention

[0007] This invention addresses the problems existing in the prior art by providing a bidirectional series hybrid DC circuit breaker. The bidirectional series hybrid DC circuit breaker includes a main branch, a charging circuit, and a voltage injection unit. It charges the capacitor by recovering the residual excitation current in the coupled inductor, and the capacitor voltage and the coupled inductor excitation current can be restored to their initial state, thereby achieving self-restart. Furthermore, the voltage injection unit based on the double H-bridge can generate a reverse voltage with controllable polarity, enabling the proposed series hybrid DC circuit breaker to interrupt bidirectional fault currents.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] A bidirectional series hybrid DC circuit breaker includes a main branch, a charging circuit, and a voltage injection unit;

[0010] The main branch circuit includes a mechanical switch, a current-limiting inductor, and a secondary winding of a first coupling inductor connected in series.

[0011] The voltage injection unit includes a capacitor, a first H-bridge unit, a second H-bridge unit, a primary winding of a first coupled inductor, and a second coupled inductor. The primary winding and secondary winding of the first coupled inductor constitute the first coupled inductor, and the second coupled inductor includes a primary winding and a secondary winding. The capacitor is connected to the first H-bridge unit and the second H-bridge unit, the first H-bridge unit and the second H-bridge unit are connected in parallel, the primary winding of the first coupled inductor and the secondary winding of the second coupled inductor are connected in series and then connected to the first H-bridge unit, and the primary winding of the second coupled inductor is connected to the second H-bridge unit.

[0012] One end of the charging circuit is connected to the main branch, and the other end is connected to the voltage injection unit, for charging the capacitor.

[0013] Furthermore, the first H-bridge unit includes a first fully controlled power device, a second fully controlled power device, a third fully controlled power device, and a fourth fully controlled power device; the second H-bridge unit includes a fifth fully controlled power device, a sixth fully controlled power device, a seventh fully controlled power device, and an eighth fully controlled power device.

[0014] The positive terminal of the capacitor is connected to the positive terminals of the first fully controlled power device, the second fully controlled power device, the fifth fully controlled power device, and the sixth fully controlled power device. The negative terminal of the capacitor is connected to the negative terminals of the third fully controlled power device, the fourth fully controlled power device, the seventh fully controlled power device, and the eighth fully controlled power device. The negative terminal of the capacitor is connected to the main branch.

[0015] The negative terminal of the first fully controlled power device is connected to the positive terminal of the third fully controlled power device and one end of the primary winding of the first coupled inductor;

[0016] The negative terminal of the second fully controlled power device is connected to the positive terminal of the fourth fully controlled power device and one end of the secondary winding of the second coupled inductor, and the other end of the secondary winding of the second coupled inductor is connected to the other end of the primary winding of the first coupled inductor.

[0017] The negative terminal of the fifth fully controlled power device is connected to the positive terminal of the seventh fully controlled power device and one end of the primary winding of the second coupled inductor.

[0018] The negative terminal of the sixth fully controlled power device is connected to the positive terminal of the eighth fully controlled power device and the other end of the primary winding of the second coupled inductor.

[0019] Furthermore, the charging circuit includes a first diode, a second diode, a first semi-controlled power device, a first resistor, and a second resistor;

[0020] The positive terminal of the first diode serves as the input terminal of the charging circuit and is connected to the first terminal of the mechanical switch in the main branch. The negative terminal of the first diode is connected to the positive terminal of the first semi-controlled power device and the negative terminal of the second diode.

[0021] The positive terminal of the second diode is connected to the tail end of the mechanical switch and one end of the current-limiting inductor, and the negative terminal of the second diode is connected to one end of the first resistor;

[0022] The negative terminal of the first semi-controlled power device is connected to one end of the second resistor, and the other ends of the second resistor and the first resistor are connected to the positive terminal of the capacitor.

[0023] Furthermore, the bidirectional series-type hybrid DC circuit breaker has the following operating stages when a short-circuit fault occurs:

[0024] Phase 1: The DC system is operating normally, the mechanical switch is closed, the voltage injection unit is in standby mode, and the main branch charges the capacitor through the charging circuit;

[0025] Stage 2: A short circuit fault occurs, and the current in the main branch increases;

[0026] Stage 3: When the main branch current reaches the protection threshold, the voltage injection unit is activated and generates a counter-voltage with the opposite polarity to the DC system voltage, modulating the main branch current into an AC ripple close to zero.

[0027] Phase 4: The mechanical switch is activated, and the short-circuit fault is disconnected without arcing through the mechanical switch;

[0028] Phase 5: After the mechanical switch is turned on, the energy stored in the first coupling inductor and the second coupling inductor is recovered and used to charge the capacitor.

[0029] Furthermore, when a forward short-circuit fault occurs, stage 3 of the bidirectional series hybrid DC circuit breaker includes the following operating states:

[0030] 3.1: When the first fully controlled power device and the fourth fully controlled power device are turned on, and the fifth fully controlled power device and the eighth fully controlled power device are turned on, a first counter-voltage with an amplitude higher than the DC system voltage is induced in the main branch through the action of the first coupling inductor and the second coupling inductor, causing the current in the main branch to decrease.

[0031] 3.2: After the main branch current drops to zero, the fifth fully controlled power device is turned off, the first fully controlled power device and the fourth fully controlled power device are turned on, and the seventh fully controlled power device and the eighth fully controlled power device are turned on. Through the action of the first coupling inductor and the second coupling inductor, a second counter-voltage with an amplitude lower than the DC system voltage is induced in the main branch, causing the main branch current to rise again.

[0032] 3.3: The voltage injection unit alternately injects the first counter-voltage and the second counter-voltage to modulate the main branch current into an AC ripple close to zero;

[0033] When a forward short-circuit fault occurs, stage 5 of the bidirectional series hybrid DC circuit breaker includes the following operating states:

[0034] After the mechanical switch is turned on, all fully controlled power devices are turned off; subsequently, the anti-parallel diodes of the second and third fully controlled power devices are turned on, and freewheeling current flows through the primary winding of the first coupled inductor and the secondary winding of the second coupled inductor; the anti-parallel diodes of the sixth and seventh fully controlled power devices are turned on, and freewheeling current flows through the primary winding of the second coupled inductor.

[0035] Furthermore, when a reverse short-circuit fault occurs, stage 3 of the bidirectional series hybrid DC circuit breaker includes the following operating states:

[0036] 3-1: The second fully controlled power device and the third fully controlled power device are turned on, and the sixth fully controlled power device and the seventh fully controlled power device are turned on. Through the action of the first coupling inductor and the second coupling inductor, a third counter-voltage with an amplitude higher than the DC system voltage is induced in the main branch, causing the main branch current to decrease.

[0037] 3-2: After the main branch current drops to zero, the sixth fully controlled power device is turned off, the second and third fully controlled power devices are turned on, and the anti-parallel diodes of the seventh and eighth fully controlled power devices are turned on. Through the action of the first coupling inductor and the second coupling inductor, a fourth counter-voltage with an amplitude lower than the DC system voltage is induced in the main branch, causing the main branch current to rise again.

[0038] 3-3: The voltage injection circuit alternately injects the third counter-voltage and the fourth counter-voltage to modulate the main branch current into an AC ripple close to zero;

[0039] When a reverse short-circuit fault occurs, stage 5 of the bidirectional series hybrid DC circuit breaker includes the following operating states:

[0040] After the mechanical switch is turned on, all fully controlled power devices are turned off; then the anti-parallel diodes of the first fully controlled power device and the fourth fully controlled power device are turned on, and freewheeling current flows through the primary winding of the first coupled inductor and the secondary winding of the second coupled inductor; the anti-parallel diodes of the fifth fully controlled power device and the eighth fully controlled power device are turned on, and freewheeling current flows through the primary winding of the second coupled inductor.

[0041] Furthermore, the condition for the bidirectional series hybrid DC circuit breaker to achieve self-restart is: E loss <E absorb ,

[0042] E loss E represents the total loss of the circuit breaker during the entire fault interruption process. absorb This represents the total energy absorbed by the circuit breaker from the DC system during the entire fault interruption process.

[0043] Furthermore, E absorb The calculation formula includes the induced energy stored in the DC system and the energy released by the DC system during the entire fault interruption process:

[0044]

[0045] Where n1 is the turns ratio of the primary to the secondary side of the first coupled inductor, L m1 L is the magnetizing inductance of the first coupled inductor. loop For a current-limiting inductor, I T Main branch current protection threshold, U dc The voltage of the DC bus, i M The current in the main branch is t5-t1, which is the duration of the fault current interruption process.

[0046] The total loss E of the circuit breaker during the entire fault interruption process loss Including the conduction loss E of the component con and switching loss E sw ,Right now:

[0047] E loss =E con +E sw ;

[0048] Conduction loss E con The calculation formula is:

[0049]

[0050] R H1 i is the loop resistance of the first H-bridge unit. H1 R is the current of the first H-bridge unit. H2 i is the loop resistance of the second H-bridge unit. H2 This represents the current in the second H-bridge unit;

[0051] Switching loss E sw The calculation formula is:

[0052] E sw =(E on +E off +E rec )[f S [(t5-t2)+4],

[0053] E on For the conduction loss of fully controlled power devices, E off For the turn-off loss of fully controlled power devices, E rec For the reverse recovery loss of the anti-parallel diode of a fully controlled power device, f s t5-t2 is the switching frequency of the fifth or sixth fully controlled power device during the current modulation period, and t5-t2 is the duration of the current modulation process.

[0054] Furthermore, to facilitate the self-restarting of the bidirectional hybrid DC circuit breaker, when a short-circuit fault occurs, the main branch current i is first reduced.M The current limit is set to a value greater than zero, I. lim Then, the main branch current is modulated to a zero-current AC ripple to achieve arc-free interruption of the mechanical switch.

[0055] This invention also provides a control method for a bidirectional series hybrid DC circuit breaker, applied to the bidirectional series hybrid DC circuit breaker described above, comprising the following steps:

[0056] S1. The main branch is operating normally and has completed charging of the capacitor;

[0057] S2. Monitor the current signal of the main branch. When the current of the main branch exceeds the preset main branch current threshold, determine the direction of the short-circuit fault current.

[0058] S3. Based on the determined current direction of the short-circuit fault, enter the corresponding working state;

[0059] S4. Determine whether there is excess induced energy in the first coupled inductor and the second coupled inductor;

[0060] S5. If there is excess induced energy, the third fully controlled power device, the fourth fully controlled power device, the seventh fully controlled power device, and the eighth fully controlled power device are turned on to deplete the excess induced energy.

[0061] S6. If there is no excess induced energy, perform a reclosing operation to restore the main branch to normal operation.

[0062] Compared with the prior art, the present invention has the following beneficial effects:

[0063] (1) By recovering the residual excitation current in the first and second coupling inductors to charge the capacitor, both the capacitor voltage and the excitation current can be restored to their initial state, thus completing the self-restart. This simplifies and accelerates the restart operation while realizing energy regeneration;

[0064] (2) The voltage injection unit based on the double H-bridge can generate a polarity controllable reverse voltage, enabling the proposed series hybrid DC circuit breaker to interrupt bidirectional fault current.

[0065] (3) Through theoretical analysis, this invention also proposes the conditions for the self-restart of the bidirectional series hybrid DC circuit breaker, which can provide theoretical design guidance for the industrial application of the bidirectional series hybrid DC circuit breaker.

[0066] (4) The present invention also proposes an improved interruption strategy to facilitate the self-restart of the ER-SHCB. When a fault occurs, the current is first limited to a current limiting value I greater than zero. limThen, the ripple is modulated to near zero to achieve arc-free breaking of the mechanical switch. This allows the DC system to transfer more energy to the first coupled inductor and the second coupled inductor, that is, to make E absorb The total energy absorbed by the circuit breaker from the DC system during the entire fault interruption process is greater, thus satisfying the self-restart condition. Attached Figure Description

[0067] Figure 1 This is a topology diagram of the bidirectional series hybrid DC circuit breaker in this invention;

[0068] Figure 2 This is a key waveform diagram of a short-circuit fault occurring in this invention;

[0069] Figure 3a This is a schematic diagram of the DC system in this invention when no fault has occurred (where the un-thickened parts represent non-conducting circuits);

[0070] Figure 3b This is a schematic diagram of a forward short-circuit fault that has just occurred in this invention (where the un-thickened part represents the non-conducting circuit);

[0071] Figure 3c This is stage 3.1 in this invention when a forward short-circuit fault occurs (where the un-bolded part represents the non-conducting circuit);

[0072] Figure 3d This is stage 3.2 in this invention when a forward short-circuit fault occurs (where the un-bolded part represents the non-conducting circuit);

[0073] Figure 3e This is stage 4 when a forward short-circuit fault occurs in this invention (where the un-thickened part is the non-conducting circuit);

[0074] Figure 3f This is stage 5 when a forward short-circuit fault occurs in this invention (where the un-thickened part is the non-conducting circuit);

[0075] Figure 4 This is a schematic diagram of the improved disconnection strategy for achieving reliable self-restart operation in this invention;

[0076] Figure 5 This is a control flowchart for a short-circuit fault in a bidirectional series hybrid DC circuit breaker according to the present invention.

[0077] In the attached diagram, S represents a mechanical switch; L represents... loop1. Current-limiting inductor; Q1, First fully controlled power device; Q2, Second fully controlled power device; Q3, Third fully controlled power device; Q4, Fourth fully controlled power device; Q5, Fifth fully controlled power device; Q6, Sixth fully controlled power device; Q7, Seventh fully controlled power device; Q8, Eighth fully controlled power device; C, Capacitor; T1, First coupling inductor; T2, Second coupling inductor; D1, First diode; D2, Second diode; Q C 1. First half-power device; R1, first resistor; R2, second resistor; U dc DC system voltage; U c Capacitor voltage; U s The voltage of the mechanical switch; u VI The reverse voltage generated by the voltage injection unit; i M Main branch current; i H1 The current of the first H-bridge; i H2 The current in the second H-bridge; I T , Main branch current protection threshold; I lim Traffic limit value. Detailed Implementation

[0078] This invention addresses the problems existing in the prior art by proposing a bidirectional series hybrid DC circuit breaker. Because the proposed bidirectional series hybrid DC circuit breaker has an energy recovery function, it will be referred to as ER-SHCB below.

[0079] like Figure 1 As shown, the proposed ER-SHCB consists of three parts: a main branch, a charging circuit, and a voltage injection unit; the main branch includes a mechanical switch S and a current-limiting inductor L connected in series. loop The voltage injection unit includes a capacitor C, a first H-bridge unit, a second H-bridge unit, a primary winding of the first coupled inductor T1, and a second coupled inductor T2.

[0080] The primary winding and secondary winding of the first coupled inductor constitute the first coupled inductor T1, and the second coupled inductor T2 includes the primary winding and secondary winding of the second coupled inductor; wherein, the capacitor C is connected to the first H-bridge unit and the second H-bridge unit, the first H-bridge unit and the second H-bridge unit are connected in parallel, the primary winding of the first coupled inductor and the secondary winding of the second coupled inductor are connected in series and then connected to the first H-bridge unit, and the primary winding of the second coupled inductor is connected to the second H-bridge unit.

[0081] One end of the charging circuit is connected to the main branch, and the other end is connected to the voltage injection unit. The charging circuit charges the capacitor C. The charging circuit includes a first diode D1, a second diode D2, and a first semi-controlled power device Q. C First resistor R1 and second resistor R2, first semi-controlled power device Q C A thyristor can be used. The positive terminal of the first diode D1 serves as the input terminal of the charging circuit and is connected to the starting terminal of the mechanical switch S in the main branch. The negative terminal of the first diode D1 is connected to the first semi-controlled power device Q. C The positive terminal of the first diode is connected to the negative terminal of the second diode D2; the positive terminal of the second diode D2 is connected to the other end of the mechanical switch S and the current-limiting inductor L. loop One end of the first diode D2 is connected to the first resistor R1, and the cathode of the second diode D2 is connected to one end of the first resistor R1; the first semi-controlled power device Q C The negative terminal of the capacitor is connected to one end of the second resistor R2, and the other end of the second resistor R2 and the first resistor R1 is connected to the positive terminal of the capacitor C.

[0082] Specifically, the first H-bridge unit includes a first fully controlled power device Q1, a second fully controlled power device Q2, a third fully controlled power device Q3, and a fourth fully controlled power device Q4; the second H-bridge unit includes a fifth fully controlled power device Q5, a sixth fully controlled power device Q6, a seventh fully controlled power device Q7, and an eighth fully controlled power device Q8, and the first fully controlled power device Q1 to the eighth fully controlled power device Q8 can all be IGBTs.

[0083] The positive terminal of capacitor C is connected to the positive terminals of the first fully controlled power device Q1, the second fully controlled power device Q2, the fifth fully controlled power device Q5, and the sixth fully controlled power device Q6. The negative terminal of capacitor C is connected to the negative terminals of the third fully controlled power device Q3, the fourth fully controlled power device Q4, the seventh fully controlled power device Q7, and the eighth fully controlled power device Q8. The negative terminal of capacitor C is connected to the main branch.

[0084] The negative terminal of the first fully controlled power device Q1 is connected to the positive terminal of the third fully controlled power device Q3 and one end of the primary winding of the first coupled inductor T1.

[0085] The negative terminal of the second fully controlled power device Q2 is connected to the positive terminal of the fourth fully controlled power device Q4 and one end of the secondary winding of the second coupled inductor T2, and the other end of the secondary winding of the second coupled inductor T2 is connected to the other end of the primary winding of the first coupled inductor T1.

[0086] The negative terminal of the fifth fully controlled power device Q5 is connected to the positive terminal of the seventh fully controlled power device Q7 and one end of the primary winding of the second coupled inductor.

[0087] The negative terminal of the sixth fully controlled power device is connected to the positive terminal of the eighth fully controlled power device and the other end of the primary winding of the second coupled inductor T2.

[0088] In the proposed ER-SHCB, the voltages and currents in the main branch and voltage injection unit can be derived as follows:

[0089]

[0090] In the formula, u H1 u H2 The output voltages of the first H-bridge and the second H-bridge are determined by the switching states of the first fully controlled power device Q1 through the eighth fully controlled power device Q8. M i H1 i H2 The currents in the main branch, the first H-bridge, and the second H-bridge are represented respectively; n1 and n2 are the turns ratios of the primary and secondary sides of the first coupling inductor T1 and the second coupling inductor T2, respectively; L m1 L m2 These are the magnetizing inductors of the first coupling inductor T1 and the second coupling inductor T2, respectively.

[0091] Using formula (1), under the assumption that mechanical switch S is on, the reverse voltage u generated by the voltage injection unit is derived. VI and the corresponding slope of the main branch current As shown in Table 1. For simplicity, Table 1 only lists the 5 key modes used in ER-SHCB, U c This is the voltage across capacitor C. The voltage injection unit can generate reverse voltages of different levels by appropriately selecting design parameters (n1, n2, and U). c ), It can be adjusted to be positive or negative. This allows the voltage injection unit to actively regulate the fault current, a process known as active current modulation. Furthermore, the voltage injection unit can generate a polarity-controllable reverse voltage, enabling the ER-SHCB to disconnect bidirectional fault current.

[0092] Table 1 Countervoltage generated by the voltage injection unit (u) VI and the corresponding slope of the main branch current.

[0093] <![CDATA[Q1]]> <![CDATA[Q2]]> <![CDATA[Q3]]> <![CDATA[Q4]]> <![CDATA[Q5]]> <![CDATA[Q6]]> <![CDATA[Q7]]> <![CDATA[Q8]]> <![CDATA[Counter voltage u VI > <![CDATA[Current slope di of the main branch M / dt]]> Mode 1 1 0 0 1 1 0 0 1 <![CDATA[n1U C (1+n2)]]> <![CDATA[(U dc -n1U C (1+n2)) / L loop <0]]> Mode 2 1 0 0 1 0 0 0 1 <![CDATA[n1U C ]]> <![CDATA[(U dc -n1U C ) / L loop >0]]> Mode 3 0 0 1 1 0 0 1 1 0 <![CDATA[U dc / L loop ]]> Mode 4 0 1 1 0 0 1 1 0 <![CDATA[-n1U C (1+n2)]]> <![CDATA[-(U dc -n1U C (1+n2)) / L loop >0]]> Mode 5 0 1 1 0 0 0 1 0 <![CDATA[-n1U C ]]> <![CDATA[-(U dc -n1U C ) / L loop <0]]>

[0094] Taking the interruption of forward fault current as an example, the working principle of ER-SHCB is explained in detail. Figure 2The conceptual waveform for ER-SHCB interrupting fault current. Figures 3a-3f This is the operating mode for ER-SHCB to interrupt fault current.

[0095] The detailed fault diagnosis process is as follows:

[0096] Phase 1 (before t0): The DC system operates normally, mechanical switch S is closed, and the voltage injection unit is in standby mode. During this phase, the main branch charges capacitor C through the first diode D1 and the first resistor R1 to offset the self-discharge of capacitor C. Since the first resistor R1 is sufficiently large, it can be considered an open circuit during fault interruption. Furthermore, the first half-power device Q... C The second resistor R2 is used to quickly charge the capacitor C during the first startup, and then keep it disconnected.

[0097] Stage 2 (t0-t1): A short-circuit fault occurs, and the main branch current i M Rapid rise.

[0098] Stage 3 (t1-t4): When the main branch current i M Reaching protection threshold I T At this time, the voltage injection unit is activated and generates a counter-voltage with the opposite polarity to the DC system voltage, modulating the main branch current into a near-zero AC ripple. This process involves three sub-stages:

[0099] 3.1(t1-t2): The first fully controlled power device Q1 and the fourth fully controlled power device Q4 are turned on, and the fifth fully controlled power device Q5 and the eighth fully controlled power device Q8 are turned on (mode 1 in Table 1). The voltage injection unit induces a high first counter-voltage n1U. c (1+n²). Because this voltage exceeds the DC system voltage U... dc This causes the main branch current i M Rapid decline;

[0100] 3.2(t2-t3): Main branch current i M After the voltage drops to zero, the fifth fully controlled power device Q5 turns off, while the first fully controlled power device Q1 and the fourth fully controlled power device Q4 turn on. The body diodes of the eighth fully controlled power device Q8 and the seventh fully controlled power device Q7 turn on (mode 2 in Table 1), inducing a lower second counter-voltage n1U. C Because this voltage is slightly lower than the DC system voltage U dc Therefore, the main branch current i M It began to rise again;

[0101] 3.3(t3-t4): The voltage injection unit alternately injects the first counter-voltage and the second counter-voltage, surrounding the DC system voltage U.dc Fluctuations will affect the main branch current i M Modulated to near-zero AC ripple.

[0102] Phase 4 (t4-t5): When the mechanical switch S is turned on, the first fully controlled power device Q1 and the fourth fully controlled power device Q4 are turned on, and the fifth fully controlled power device Q5 and the eighth fully controlled power device Q8 are turned on (mode 1 in Table 1). The voltage injection unit induces a high first counter-voltage n1U. c (1+n²). At this stage, the mechanical switch will experience a small negative voltage U. s U S =U dc -n1U c (n2+1), as capacitor C discharges, the voltage will further approach zero, which allows the mechanical switch to open in an environment of near-zero current and zero voltage, thus achieving arc-free interruption.

[0103] During phases 3-4, the capacitor C in the voltage injection unit is the magnetizing inductance L of the first coupling inductor. m1 Second coupling inductor L m2 The magnetizing inductor is charged, and the main branch current i M This also charges the magnetizing inductor. Therefore, a significant first coupled inductor magnetizing current i is generated. H1 The second coupled inductor excitation current i H2 And the voltage U of the capacitor c It will decrease.

[0104] Phase 5 (t5-t6): Mechanical switch S completes insulation restoration; that is, after mechanical switch S is opened, all fully controlled power devices are turned off. Subsequently, the anti-parallel diodes of the second fully controlled power device Q2, the third fully controlled power device Q3, the sixth fully controlled power device Q6, and the seventh fully controlled power device Q7 begin freewheeling. The remaining magnetizing current of the first coupling inductor T1 and the second coupling inductor T2 is used to charge capacitor C. With appropriate parameter design, this energy feedback process can fully charge capacitor C, as shown in the theoretical analysis below. Therefore, the magnetizing currents of the first coupling inductor T1 and the second coupling inductor T2, and the capacitor voltage U... c All return to their initial state, and ER-SHCB achieves self-restart.

[0105] When the bidirectional series hybrid DC circuit breaker experiences a reverse short-circuit fault, stages 1, 2, and 4 are the same as those for a forward short-circuit fault. Only stages 3 and 5, which differ from the forward short-circuit fault, are described below:

[0106] 3-1: The second fully controlled power device Q2 and the third fully controlled power device Q3 are turned on, and the sixth fully controlled power device Q6 and the seventh fully controlled power device Q7 are turned on. Through the action of the first coupling inductor T1 and the second coupling inductor T2, an amplitude higher than the DC system voltage U is induced in the main branch. dc The third counter-voltage causes the main branch current to drop rapidly;

[0107] 3-2: After the main branch current drops to zero, the sixth fully controlled power device Q6 turns off, the second fully controlled power device Q2 and the third fully controlled power device Q3 turn on, and the anti-parallel diodes of the seventh fully controlled power device Q7 and the eighth fully controlled power device Q8 turn on. Through the action of the first coupling inductor T1 and the second coupling inductor T2, a fourth counter-voltage with an amplitude lower than the DC system voltage is induced in the main branch, causing the main branch current i M It rose again;

[0108] 3-3: The voltage injection circuit alternately injects the third counter-voltage and the fourth counter-voltage to reduce the main branch current i M Modulated to near-zero AC ripple;

[0109] When a reverse short-circuit fault occurs, stage 5 of the bidirectional series hybrid DC circuit breaker includes the following operating states:

[0110] After the mechanical switch is turned on, all fully controlled power devices are turned off; then the anti-parallel diodes of the first fully controlled power device Q1 and the fourth fully controlled power device Q4 are turned on, and freewheeling current flows through the primary winding of the first coupling inductor T1 and the secondary winding of the second coupling inductor T2; the anti-parallel diodes of the fifth fully controlled power device Q5 and the eighth fully controlled power device Q8 are turned on, and freewheeling current flows through the primary winding of the second coupling inductor T2.

[0111] To ensure that the bidirectional series hybrid DC circuit breaker can achieve self-restart, that is, to restore the voltage and excitation current of the capacitor C to their initial state, the following analysis process is performed:

[0112] For a bidirectional series-type hybrid DC circuit breaker during the entire fault breaking process (t1-t6), the following formula can be derived from the law of conservation of energy:

[0113]

[0114] In the formula, E loss E represents the total loss of the ER-SHCB during the entire fault interruption process. absorbThis represents the total energy absorbed by the ER-SHCB from the DC system during the entire fault interruption process.

[0115] Among them, E absorb This includes both the induced energy stored in the DC system and the energy released by the DC system during the entire fault interruption process, calculated as follows:

[0116]

[0117] Where n1 is the turns ratio of the primary to the secondary side of the first coupled inductor, L m1 L is the magnetizing inductance of the first coupled inductor. loop For a current-limiting inductor, I T Main branch current protection threshold, U dc The voltage of the DC bus, i M The current in the main branch is t5-t1, which is the duration of the fault current interruption process.

[0118] Since SHCBs typically employ a hollow structure to prevent magnetic saturation, the magnetization losses of the first coupling inductor T1 and the second coupling inductor T2 can be neglected. Therefore, E loss The conduction loss E of the component con and switching loss E sw Composition, represented as:

[0119] E loss =E con +E sw (4)

[0120] E con The calculation is as follows: The secondary winding of the first coupled inductor T1 is usually designed with low internal resistance, and the current i flowing through it is... M with i H1 and i H2 Compared to its relatively small value, its conduction loss can be ignored. Furthermore, the conduction loss during the energy feedback process (t5-t6) is assumed to be the same as the conduction loss during the fault interruption period (t1-t5). Based on these approximations, E con The following can be calculated:

[0121]

[0122] In the formula, R H1 This represents the loop resistance of the first H-bridge unit, including the internal resistance of the power devices, the primary winding of T1, and the secondary winding of T2; R H2 This represents the loop resistance of the second H-bridge unit, including the internal resistance of the power devices and the primary winding of T2; i H1 i represents the current of the first H-bridge unit. H2 This represents the current in the second H-bridge unit;

[0123] E sw The calculation is as follows: Since the conduction losses of the first diode D1 and the second diode D2 are very small, they can usually be ignored. Only the conduction and turn-off losses of the IGBT and the reverse recovery losses of the diodes are considered. During the entire fault-breaking process (t1-t6), Q1, Q4, Q5, and Q8 turn on and off with the activation and deactivation of the voltage injection unit. Q5 or Q6 operates at a high switching frequency for current modulation. Therefore, E sw The following can be calculated:

[0124] E sw =(E on +E off +E rec )[f S [(t5-t2)+4], (6)

[0125] In the formula, E on E off and E rec These represent the on-state loss, off-state loss, and reverse recovery loss of the IGBT's anti-parallel diode, respectively. These parameters can be estimated from the device datasheet. s This indicates the switching frequency of the fifth fully controlled power device Q5 or the sixth fully controlled power device Q6 during the current modulation period, and t5-t2 is the duration of the current modulation process.

[0126] To ensure that ER-SHCB reliably achieves self-rebooting operation, U must be satisfied. c (t6)≥U c (t1), because excess energy can be dissipated in the internal resistance of the device and windings. Combining equation (2), the condition for self-restart operation is:

[0127] E loss <E absorb (7)

[0128] Furthermore, equations (3)-(6) show that the following methods can promote the fulfillment of the self-restart condition:

[0129] 1. Select power devices with low switching losses to reduce E sw ;

[0130] 2. Use power devices with low on-resistance and coupled inductor windings with low internal resistance to minimize E. on ;

[0131] 3. Apply improved segmentation strategies to increase E absorb .

[0132] In one specific embodiment, by means of, Figure 4The operation shown facilitates the self-restart of the bidirectional hybrid DC circuit breaker. In the event of a fault, Phase I first performs current-limiting modulation to reduce the main branch current i M The current limit is set to a value greater than zero, I. lim Then, proceed to stage II, where the main branch current i M An AC ripple modulated to zero current is used to achieve arc-free breaking of the mechanical switch S. The current limiting value I is... lim The larger the value, the more energy the DC system transmits to the first coupling inductor T1 and the second coupling inductor T2, and the easier it is for the residual energy in the first coupling inductor T1 and the second coupling inductor T2 to fill the capacitor.

[0133] This invention also provides a control method for a bidirectional series hybrid DC circuit breaker, applicable to the bidirectional series hybrid DC circuit breaker described above, such as... Figure 5 As shown, it includes the following steps:

[0134] S1. The main branch is operating normally and has completed charging of the capacitor C;

[0135] S2. Monitor the current signal of the main branch. When the current i of the main branch is detected... M When the current exceeds the preset main branch current threshold, determine the direction of the short-circuit fault current;

[0136] S3. Based on the direction of the short-circuit fault current as determined, enter the corresponding working state. When a forward short-circuit fault occurs, enter S3-1; when a reverse short-circuit fault occurs, enter S3-2.

[0137] S3-1. If it is a forward short-circuit fault, the voltage injection unit enters mode 1, forcing the main branch current i M Rapid descent; the voltage injection unit switches between mode 1 and mode 2, causing i M The AC ripple is modulated to near zero; simultaneously, within the modulated AC ripple time window, when the voltage injection unit enters mode 1, the mechanical switch S is turned on; after the mechanical switch S is turned on, the voltage injection unit enters mode 4 to achieve energy recovery.

[0138] S3-2. If it is a reverse short-circuit fault, the voltage injection unit enters mode 4, forcing the main branch current i M Rapid descent; the voltage injection unit switches between mode 4 and mode 5, causing i M The AC ripple is modulated to near zero; simultaneously, within the modulated AC ripple time window, when the voltage injection unit enters mode 4, the mechanical switch S is turned on; after the mechanical switch S is turned on, the voltage injection unit enters mode 1 to achieve energy recovery.

[0139] S4. Determine whether there is excess induced energy in the first coupled inductor T1 and the second coupled inductor T2;

[0140] S5. If there is excess induced energy, the third fully controlled power device Q3, the fourth fully controlled power device Q4, the seventh fully controlled power device Q7, and the eighth fully controlled power device Q8 are turned on, that is, the voltage injection unit enters mode 3 to exhaust the excess induced energy.

[0141] S6. If there is no excess induced energy, perform a reclosing operation to restore the main branch to normal operation.

[0142] This paper proposes a bidirectional series DC circuit breaker with energy recovery function. It inherits the low conduction loss and fast breaking characteristics of conventional series DC circuit breakers, and at the same time has the following unique advantages:

[0143] 1. By recovering the residual magnetizing current in the first and second coupling inductors to charge the capacitor, both the capacitor voltage and magnetizing current can be restored to their initial states, thus completing the self-restart. This simplifies and accelerates the restart operation while achieving energy regeneration;

[0144] 2. The voltage injection unit based on the double H-bridge can generate a polarity-controllable reverse voltage, enabling the proposed bidirectional series DC circuit breaker to interrupt bidirectional fault current.

[0145] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, and is not intended to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions made by those skilled in the art to the technical solution of the present invention do not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A bidirectional series hybrid DC circuit breaker, characterized by: Includes the main branch, charging circuit, and voltage injection unit; The main branch circuit includes a mechanical switch, a current-limiting inductor, and a secondary winding of a first coupling inductor connected in series. The voltage injection unit includes a capacitor, a first H-bridge unit, a second H-bridge unit, a primary winding of a first coupled inductor, and a second coupled inductor. The primary winding and secondary winding of the first coupled inductor constitute the first coupled inductor, and the second coupled inductor includes a primary winding and a secondary winding. The capacitor is connected to the first H-bridge unit and the second H-bridge unit, the first H-bridge unit and the second H-bridge unit are connected in parallel, the primary winding of the first coupled inductor and the secondary winding of the second coupled inductor are connected in series and then connected to the first H-bridge unit, and the primary winding of the second coupled inductor is connected to the second H-bridge unit. One end of the charging circuit is connected to the main branch, and the other end is connected to the voltage injection unit, for charging the capacitor; The first H-bridge unit includes a first fully controlled power device, a second fully controlled power device, a third fully controlled power device, and a fourth fully controlled power device; the second H-bridge unit includes a fifth fully controlled power device, a sixth fully controlled power device, a seventh fully controlled power device, and an eighth fully controlled power device. The positive terminal of the capacitor is connected to the positive terminals of the first fully controlled power device, the second fully controlled power device, the fifth fully controlled power device, and the sixth fully controlled power device. The negative terminal of the capacitor is connected to the negative terminals of the third fully controlled power device, the fourth fully controlled power device, the seventh fully controlled power device, and the eighth fully controlled power device. The negative terminal of the capacitor is connected to the main branch. The negative terminal of the first fully controlled power device is connected to the positive terminal of the third fully controlled power device and one end of the primary winding of the first coupled inductor; The negative terminal of the second fully controlled power device is connected to the positive terminal of the fourth fully controlled power device and one end of the secondary winding of the second coupled inductor, and the other end of the secondary winding of the second coupled inductor is connected to the other end of the primary winding of the first coupled inductor. The negative terminal of the fifth fully controlled power device is connected to the positive terminal of the seventh fully controlled power device and one end of the primary winding of the second coupled inductor. The negative terminal of the sixth fully controlled power device is connected to the positive terminal of the eighth fully controlled power device and the other end of the primary winding of the second coupled inductor.

2. The bidirectional series hybrid DC circuit breaker according to claim 1, characterized in that: The charging circuit includes a first diode, a second diode, a first semi-controlled power device, a first resistor, and a second resistor. The positive terminal of the first diode serves as the input terminal of the charging circuit and is connected to the first terminal of the mechanical switch in the main branch. The negative terminal of the first diode is connected to the positive terminal of the first semi-controlled power device and the negative terminal of the second diode. The positive terminal of the second diode is connected to the tail end of the mechanical switch and one end of the current-limiting inductor, and the negative terminal of the second diode is connected to one end of the first resistor; The negative terminal of the first semi-controlled power device is connected to one end of the second resistor, and the other ends of the second resistor and the first resistor are connected to the positive terminal of the capacitor.

3. The bidirectional series hybrid DC circuit breaker according to claim 2, characterized in that: The bidirectional series hybrid DC circuit breaker operates in the following stages when a short-circuit fault occurs: Phase 1: The DC system is operating normally, the mechanical switch is closed, the voltage injection unit is in standby mode, and the main branch charges the capacitor through the charging circuit; Stage 2: A short circuit fault occurs, and the current in the main branch increases; Stage 3: When the main branch current reaches the protection threshold, the voltage injection unit is activated and generates a counter-voltage with the opposite polarity to the DC system voltage, modulating the main branch current into an AC ripple close to zero. Phase 4: The mechanical switch is activated, and the short-circuit fault is disconnected without arcing through the mechanical switch; Phase 5: After the mechanical switch is turned on, the energy stored in the first coupling inductor and the second coupling inductor is recovered and used to charge the capacitor.

4. The bidirectional series hybrid DC circuit breaker according to claim 3, characterized in that: When a forward short-circuit fault occurs, the bidirectional series hybrid DC circuit breaker in stage 3 includes the following operating states: 3.1: When the first fully controlled power device and the fourth fully controlled power device are turned on, and the fifth fully controlled power device and the eighth fully controlled power device are turned on, a first counter-voltage with an amplitude higher than the DC system voltage is induced in the main branch through the action of the first coupling inductor and the second coupling inductor, causing the current in the main branch to decrease. 3.2: After the main branch current drops to zero, the fifth fully controlled power device is turned off, the first fully controlled power device and the fourth fully controlled power device are turned on, the anti-parallel diode of the seventh fully controlled power device and the eighth fully controlled power device are turned on, and through the action of the first coupling inductor and the second coupling inductor, a second counter-voltage with an amplitude lower than the DC system voltage is induced in the main branch, causing the main branch current to rise again; 3.3: The voltage injection unit alternately injects the first counter-voltage and the second counter-voltage to modulate the main branch current into an AC ripple close to zero; When a forward short-circuit fault occurs, stage 5 of the bidirectional series hybrid DC circuit breaker includes the following operating states: After the mechanical switch is turned on, all fully controlled power devices are turned off; subsequently, the anti-parallel diodes of the second and third fully controlled power devices are turned on, and freewheeling current flows through the primary winding of the first coupled inductor and the secondary winding of the second coupled inductor; the anti-parallel diodes of the sixth and seventh fully controlled power devices are turned on, and freewheeling current flows through the primary winding of the second coupled inductor.

5. The bidirectional series hybrid DC circuit breaker according to claim 3, characterized in that: When a reverse short-circuit fault occurs, stage 3 of the bidirectional series hybrid DC circuit breaker includes the following operating states: 3-1: The second fully controlled power device and the third fully controlled power device are turned on, and the sixth fully controlled power device and the seventh fully controlled power device are turned on. Through the action of the first coupling inductor and the second coupling inductor, a third counter-voltage with an amplitude higher than the DC system voltage is induced in the main branch, causing the main branch current to decrease. 3-2: After the main branch current drops to zero, the sixth fully controlled power device is turned off, the second and third fully controlled power devices are turned on, and the anti-parallel diodes of the seventh and eighth fully controlled power devices are turned on. Through the action of the first coupling inductor and the second coupling inductor, a fourth counter-voltage with an amplitude lower than the DC system voltage is induced in the main branch, causing the main branch current to rise again. 3-3: The voltage injection circuit alternately injects the third counter-voltage and the fourth counter-voltage to modulate the main branch current into an AC ripple close to zero; When a reverse short-circuit fault occurs in the bidirectional series hybrid DC circuit breaker, stage 5 includes the following operating states: After the mechanical switch is opened, all fully controlled power devices are turned off; subsequently, the anti-parallel diodes of the first fully controlled power device and the fourth fully controlled power device are turned on, freewheeling through the primary winding of the first coupled inductor and the secondary winding of the second coupled inductor; the anti-parallel diodes of the fifth fully controlled power device and the eighth fully controlled power device are turned on, freewheeling through the primary winding of the second coupled inductor.

6. The bidirectional series hybrid DC circuit breaker according to claim 4 or 5, characterized in that: The condition for the bidirectional series hybrid DC circuit breaker to achieve self-restart is: ， This represents the total loss of the circuit breaker during the entire fault interruption process. This represents the total energy absorbed by the circuit breaker from the DC system during the entire fault interruption process.

7. The bidirectional series hybrid DC circuit breaker according to claim 6, characterized in that: The calculation formula includes the induced energy stored in the DC system and the energy released by the DC system during the entire fault interruption process: ， in, The ratio of the number of turns on the primary side to the number of turns on the secondary side of the first coupled inductor. The magnetizing inductance of the first coupling inductor. An inductor that is a current-limiting inductor. The current protection threshold for the main branch circuit. The voltage of the DC bus. The current in the main branch, The duration of the fault current interruption process; The total loss of the circuit breaker during the entire fault breaking process Including the conduction losses of components and switching losses ,Right now: ； Conductive loss The calculation formula is: ， The loop resistance of the first H-bridge unit. The current of the first H-bridge unit, The loop resistance of the second H-bridge unit. This represents the current in the second H-bridge unit; Switching losses The calculation formula is: ， For the conduction loss of fully controlled power devices, For the turn-off loss of fully controlled power devices, The reverse recovery loss of the anti-parallel diode in a fully controlled power device. This refers to the switching frequency of the fifth or sixth fully controlled power device during current modulation. This represents the duration of the current modulation process.

8. The bidirectional series hybrid DC circuit breaker according to claim 7, characterized in that: To facilitate the self-restarting of the bidirectional hybrid DC circuit breaker, when a short-circuit fault occurs, the main branch current is first reduced. Limit the rate to a value greater than zero. Then, the main branch current is modulated to a zero-current AC ripple to achieve arc-free interruption of the mechanical switch.

9. A control method for a bidirectional series-type hybrid DC circuit breaker, characterized in that: The application of a bidirectional series-type hybrid DC circuit breaker as described in any one of claims 1-8 includes the following steps: S1. The main branch is operating normally and has completed charging of the capacitor; S2. Monitor the current signal of the main branch. When the current of the main branch exceeds the preset main branch current threshold, determine the direction of the short-circuit fault current. S3. Based on the determined direction of the short-circuit fault current, enter the corresponding working state; S4. Determine whether there is excess induced energy in the first coupled inductor and the second coupled inductor; S5. If there is excess induced energy, the third fully controlled power device, the fourth fully controlled power device, the seventh fully controlled power device, and the eighth fully controlled power device are turned on to deplete the excess induced energy. S6. If there is no excess induced energy, perform a reclosing operation to restore the main branch to normal operation.

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