Bidirectional series connection type hybrid direct current circuit breaker and control method thereof

Abstract

The invention provides a bidirectional series hybrid DC circuit breaker and a control method thereof, and relates to the technical field of power equipment. The bidirectional series hybrid DC circuit breaker comprises a main branch, a charging circuit and a voltage injection unit. The main branch comprises a mechanical switch, a current-limiting inductor, a first coupling inductor and a secondary winding; the voltage injection unit comprises a capacitor, a first H-bridge unit, a second H-bridge unit, a first coupling inductor primary winding and a second coupling inductor; and one end of the charging circuit is connected with the main branch, and the other end is connected with the voltage injection unit for charging the capacitor. In addition, the invention also provides a control method of the bidirectional series hybrid DC circuit breaker. According to the bidirectional series connection type hybrid direct current circuit breaker provided by the invention, the capacitor is charged by recovering the residual exciting current in the coupling inductor, and the voltage of the capacitor and the exciting current of the coupling inductor can be recovered to the initial state, so that self-weight starting is realized, and fault current can be bidirectionally broken.

Description

Technical Field

[0001] The present invention belongs to the technical field of electric power equipment, and in particular relates to a bidirectional series hybrid DC circuit breaker and a control method thereof. Background Art

[0002] With the rapid development of renewable energy, medium and low voltage DC distribution networks have attracted widespread attention due to their easy access, high transmission efficiency, and good stability. However, the DC distribution network does not have a natural current zero crossing point and the line inductance is low, making the interruption of DC fault current extremely challenging. DC circuit breakers (DCCBs) can quickly interrupt DC fault currents and selectively isolate fault sections, and have become an indispensable device to ensure the safe and stable operation of DC distribution networks.

[0003] The concept of series hybrid DC circuit breaker (SHCB) was proposed. During the fault breaking process of SHCB, the energy storage capacitor C in the voltage injection unit will continue to charge the transformer's excitation inductance Lm, and the DC system current will also charge Lm. Therefore, after the fault is broken, the transformer will retain a significant excitation current, causing the voltage of the energy storage capacitor to drop. The key step of SHCB is to quickly restore the transformer's excitation current and the voltage of the energy storage capacitor to the initial state. This process is called restart operation.

[0004] Many literatures have explored the restart operation methods of SHCB. First, restoring the voltage of the energy storage capacitor usually requires adding a charging device or circuit. Both wired and wireless charging devices have been developed to achieve fast pre-charging of the energy storage capacitor. A simple charging circuit for a single-capacitor SHCB is proposed in the prior art, which consists of a power device and a current-limiting resistor. However, this design can only pre-charge the capacitor to the DC system voltage, which limits its applicability; moreover, the above-mentioned simple charging circuit requires the activation of additional devices or circuits after each fault interruption, which complicates the restart operation process; secondly, restoring the excitation current of the transformer usually requires dissipating it through a resistor. There are also some SHCBs where the residual excitation current is dissipated through the internal resistance of the power device and the winding. The process follows the LR first-order circuit, but due to the low resistance, the speed may be slow, which limits the ability of the SHCB to quickly reclose. To solve this problem, an independent dissipation branch with an adjustable resistor is introduced to accelerate the process. However, all these devices and methods will lead to energy waste.

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

[0006] Therefore, it is urgent to develop a series hybrid DC circuit breaker with bidirectional function and energy recovery. Summary of the invention

[0007] In view of the problems existing in the prior art, the present invention provides a bidirectional series hybrid DC circuit breaker, which includes a main branch, a charging circuit and a voltage injection unit. The capacitor is charged 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 states, thereby achieving self-restart; and the voltage injection unit based on the double H-bridge can generate a reverse voltage with controllable polarity, so that the proposed series hybrid DC circuit breaker can interrupt the bidirectional fault current.

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

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

[0010] The main branch includes a mechanical switch, a current limiting inductor, and a first coupled inductor secondary winding connected in series in sequence;

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

[0013] Further, 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 electrode of the capacitor is connected to the positive electrode of the first fully-controlled power device, the positive electrode of the second fully-controlled power device, the positive electrode of the fifth fully-controlled power device, and the positive electrode of the sixth fully-controlled power device, and the negative electrode of the capacitor is connected to the negative electrode of the third fully-controlled power device, the negative electrode of the fourth fully-controlled power device, the negative electrode of the seventh fully-controlled power device, and the negative electrode of the eighth fully-controlled power device, and the negative electrode of the capacitor is connected to the main branch;

[0015] The cathode of the first fully-controlled power device is connected to the anode of the third fully-controlled power device and one end of the primary winding of the first coupled inductor;

[0016] The negative electrode of the second fully-controlled power device is connected to the positive electrode 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 cathode of the fifth fully-controlled power device is connected to the anode of the seventh fully-controlled power device and one end of the primary winding of the second coupled inductor;

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

[0019] Further, the charging circuit is provided with a first diode, a second diode, a first half-controlled power device, a first resistor, and a second resistor;

[0020] The positive electrode of the first diode is used as the input end of the charging circuit and is connected to the first end of the mechanical switch in the main branch, and the negative electrode of the first diode is connected to the positive electrode of the first semi-controlled power device and the negative electrode of the second diode;

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

[0022] The cathode of the first half-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 anode of the capacitor.

[0023] Furthermore, when a short circuit occurs in the bidirectional series hybrid DC circuit breaker, there are the following working stages:

[0024] Phase 1: the DC system operates 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 main branch current increases;

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

[0027] Stage 4: The mechanical switch is turned on, and the short-circuit fault part is disconnected without arcing by the mechanical switch;

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

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

[0030] 3.1: 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. Through the action of the first coupled inductor and the second coupled inductor, a first counter voltage having an amplitude higher than the DC system voltage is induced in the main branch, causing the current of 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 coupled inductor and the second coupled 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 injects the first counter voltage and the second counter voltage alternately to modulate the main branch current into an AC ripple close to zero;

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

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

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

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

[0037] 3-2: After the main branch current drops to zero, the sixth fully-controlled power device is turned off, the second fully-controlled power device and the third fully-controlled power device are turned on, and the anti-parallel diodes of the seventh fully-controlled power device and the eighth fully-controlled power device are turned on, and through the action of the first coupled inductor and the second coupled 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 loop alternately injects a third counter voltage and a fourth counter voltage to modulate the main branch current into an AC ripple close to zero;

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

[0040] After the mechanical switch is turned on, all fully-controlled power devices are turned off; then the anti-parallel diode of the first fully-controlled power device and the anti-parallel diode of the fourth fully-controlled power device are turned on, and the current is continued through the primary winding of the first coupled inductor and the secondary winding of the second coupled inductor; the anti-parallel diode of the fifth fully-controlled power device and the anti-parallel diode of the eighth fully-controlled power device are turned on, and the current is continued 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: loss <E absorb ,

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

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

[0044]

[0045] Where n1 is the turns ratio between the primary and secondary sides of the first coupled inductor, L m1 is the magnetizing inductance of the first coupled inductor, L loop is the inductance of the current limiting inductor, I T is the main branch current protection threshold, U dc is the voltage of the DC bus, i M is the current of the main branch, t5-t1 is the duration of the fault current breaking process;

[0046] The total loss of the circuit breaker during the entire fault breaking process is E loss Including the conduction loss E of the components 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 is the loop resistance of the first H-bridge unit, i H1 is the current of the first H-bridge unit, R H2 is the loop resistance of the second H-bridge unit, i H2 is the current of 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 is the conduction loss of the fully controlled power device, E off is the turn-off loss of the fully controlled power device, E rec is the reverse recovery loss of the anti-parallel diode of the fully controlled power device, f s is the switching frequency of the fifth fully-controlled power device or the sixth fully-controlled power device during the current modulation period, and t5-t2 is the duration of the current modulation process.

[0054] Furthermore, in order to promote the self-restart of the bidirectional hybrid DC circuit breaker, when a short circuit occurs, the main branch current iM Limit to a current limit value I greater than zero lim , and then modulate the main branch current to zero current AC ripple to achieve arc-free breaking of the mechanical switch.

[0055] The present invention also provides a control method for a bidirectional series hybrid DC circuit breaker, which is applied to the bidirectional series hybrid DC circuit breaker described above, and comprises the following steps:

[0056] S1, the main branch operates normally and has completed charging of the capacitor;

[0057] S2, monitoring the current signal of the main branch, and when it is detected that the main branch current exceeds a preset main branch current threshold, determining the direction of the short-circuit fault current;

[0058] S3, entering a corresponding working state according to the determined current direction of the short-circuit fault;

[0059] S4, determining whether there is excess inductive energy in the first coupled inductor and the second coupled inductor;

[0060] S5. If there is excess inductive 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 exhaust the excess inductive 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 coupled inductor and the second coupled inductor to charge the capacitor, both the capacitor voltage and the excitation current can be restored to the initial state, thereby completing the self-restart. This simplifies and speeds up the restart operation while achieving energy regeneration;

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

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

[0066] (4) The present invention also specifically proposes an improved breaking strategy to facilitate the self-restart of the ER-SHCB. When a fault occurs, the current is first limited to a current limit value I greater than zero. lim, and then modulate to a ripple close to zero to achieve arc-free breaking of the mechanical switch, so that the DC system can transfer more energy to the first coupled inductor and the second coupled inductor, that is, make E absorb (The total energy absorbed by the circuit breaker from the DC system during the entire fault interrupting process) is greater, thus meeting the self-restart condition. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Figure 1 It is a topological structure diagram of the bidirectional series hybrid DC circuit breaker in the present invention;

[0068] Figure 2 It is the key waveform diagram when a short circuit fault occurs in the present invention;

[0069] Figure 3a is a schematic diagram of a DC system in the present invention when no fault occurs (the non-bold part is a non-conducting circuit);

[0070] Figure 3b 1 is a schematic diagram of a forward short circuit fault just occurring in the present invention (the non-bold part is a non-conducting circuit);

[0071] Figure 3c This is stage 3.1 when a forward short circuit fault occurs in the present invention (wherein the non-bold part is a non-conducting circuit);

[0072] Figure 3d This is stage 3.2 when a forward short circuit fault occurs in the present invention (wherein the non-bold part is a non-conducting circuit);

[0073] Figure 3e This is stage 4 when a forward short circuit fault occurs in the present invention (wherein the non-bold part is a non-conducting circuit);

[0074] Figure 3f This is stage 5 when a forward short circuit fault occurs in the present invention (wherein the non-bold part is a non-conducting circuit);

[0075] Figure 4 is a schematic diagram of an improved breaking strategy for achieving reliable self-restart operation in the present invention;

[0076] Figure 5 It is a control flow chart for a short circuit fault of a bidirectional series hybrid DC circuit breaker in the present invention.

[0077] In the attached figure, S, mechanical switch; L loop, 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 coupled inductor; T2, second coupled inductor; D1, first diode; D2, second diode; Q C , first half-power device; R1, first resistor; R2, second resistor; U dc , DC system voltage; U c , capacitor voltage; U s , voltage of 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 of the second H bridge; I T , main branch current protection threshold; I lim , current limiting value. DETAILED DESCRIPTION

[0078] In view of the problems existing in the prior art, the present invention proposes a bidirectional series hybrid DC circuit breaker. Since the proposed bidirectional series hybrid DC circuit breaker has an energy recovery function, it is hereinafter referred to as ER-SHCB.

[0079] like Figure 1 As shown in FIG. 1 , 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, a current limiting inductor L connected in series in sequence. loop and the secondary winding of the first coupled inductor T1; the voltage injection unit is provided with a capacitor C, a first H-bridge unit, a second H-bridge unit, the primary winding of the first coupled inductor T1, and a second coupled inductor T2.

[0080] The first coupled inductor primary winding and the first coupled inductor secondary winding constitute the first coupled inductor T1, and the second coupled inductor T2 includes the second coupled inductor primary winding and the second coupled inductor secondary winding; wherein the capacitor C is connected to the first H-bridge unit and the second H-bridge unit, the first H-bridge unit is connected in parallel with the second H-bridge unit, the first coupled inductor primary winding and the second coupled inductor secondary winding are connected in series and then connected to the first H-bridge unit, and the second coupled inductor primary winding 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, and the capacitor C is charged by the charging circuit. The charging circuit is provided with a first diode D1, a second diode D2, a first half-controlled power device Q C , a first resistor R1 and a second resistor R2, a first half-controlled power device Q C The positive electrode of the first diode D1 is used as the input end of the charging circuit and is connected to the first end of the mechanical switch S in the main branch. The negative electrode of the first diode D1 is connected to the first half-controlled power device Q C The anode of the second diode D2 is connected to the other end of the mechanical switch S and the current limiting inductor L loop The cathode of the second diode D2 is connected to one end of the first resistor R1; the first half-controlled power device Q C The negative electrode of is connected to one end of the second resistor R2, and the other ends of the second resistor R2 and the first resistor R1 are connected to the positive electrode 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. The first fully-controlled power device Q1 to the eighth fully-controlled power device Q8 can all use IGBTs.

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

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

[0085] The cathode of the second fully-controlled power device Q2 is connected to the anode 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 cathode of the fifth fully-controlled power device Q5 is connected to the anode of the seventh fully-controlled power device Q7 and one end of the primary winding of the second coupled inductor.

[0087] The cathode of the sixth fully-controlled power device is connected to the anode 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 voltage and current in the main branch and the 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 to the eighth fully-controlled power device Q8. M 、i H1 、i H2 They represent the current of the main branch, the first H-bridge, and the second H-bridge respectively; n1 and n2 are the turns ratios of the primary and secondary sides of the first coupled inductor T1 and the second coupled inductor T2 respectively, L m1 , L m2 They are the excitation inductances of the first coupled inductor T1 and the second coupled inductor T2 respectively.

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

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

[0093] <![CDATA[Q1]]> <![CDATA[Q2]]> <![CDATA[Q3]]> <![CDATA[Q4]]> <![CDATA[Q5]]> <![CDATA[Q6]]> <![CDATA[Q7]]> <![CDATA[Q8]]> <![CDATA[Counter-voltage u VI > <![CDATA[The current slope di 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]]> Modal 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 2This is the conceptual waveform of the fault current broken by ER-SHCB. Figure 3a-3f This is the working mode of ER-SHCB for breaking fault current.

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

[0096] Phase 1 (before t0): The DC system operates normally, the mechanical switch S is closed, and the voltage injection unit is in standby mode. In this phase, the main branch charges the capacitor C through the first diode D1 and the first resistor R1 to offset the self-discharge of the capacitor C. Since the first resistor R1 is large enough, it can be regarded as an open circuit during the disconnection fault. In addition, 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 remain disconnected.

[0097] Phase 2 (t0-t1): A short circuit occurs, and the main branch current i M Rise quickly.

[0098] Phase 3 (t1-t4): When the main branch current i M Reaching protection threshold I T When the voltage injection unit is activated, it 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. 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, the fifth fully controlled power device Q5 and the eighth fully controlled power device Q8 are turned on (mode 1 in Table 1), and the voltage injection unit senses a high first counter voltage n1U c (1+n2). Since this voltage exceeds the DC system voltage U dc , causing the main branch current i M Rapid descent;

[0100] 3.2(t2-t3): Main branch current i M After the voltage drops to zero, the fifth fully-controlled power device Q5 is turned off, while the first fully-controlled power device Q1 and the fourth fully-controlled power device Q4 are turned on, and the body diodes of the eighth fully-controlled power device Q8 and the seventh fully-controlled power device Q7 are turned on (mode 2 in Table 1), inducing a lower second reverse voltage n1U C , since this voltage is slightly lower than the DC system voltage U dc , and then 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 around the DC system voltage Udc Fluctuation, the main branch current i M Modulated to near zero AC ripple.

[0102] Phase 4 (t4-t5): When the mechanical switch S starts to turn on, the first fully controlled power device Q1 and the fourth fully controlled power device Q4 are turned on, the fifth fully controlled power device Q5 and the eighth fully controlled power device Q8 are turned on (mode 1 in Table 1), and the voltage injection unit senses a high first counter voltage n1U c (1+n2). At this stage, the mechanical switch will be subjected to a small negative voltage U s , U S =U dc -n1U c (n2+1), as the capacitor C discharges, the voltage will further tend to zero, which allows the mechanical switch to be opened in an environment of approximately zero current and zero voltage, thereby achieving arc-free breaking.

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

[0104] Phase 5 (t5-t6): The mechanical switch S completes insulation recovery, that is, after the mechanical switch S is turned on, all fully-controlled power devices are turned off; then the anti-parallel diode of the second fully-controlled power device Q2, the anti-parallel diode of the third fully-controlled power device Q3, the anti-parallel diode of the sixth fully-controlled power device Q6, and the anti-parallel diode of the seventh fully-controlled power device Q7 begin to continue current, and the residual excitation current of the first coupled inductor T1 and the second coupled inductor T2 is used to charge the capacitor C. Through appropriate parameter design, this energy feedback process can fully charge the capacitor C, see the theoretical analysis below. Therefore, the excitation current of the first coupled inductor T1, the second coupled 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 has a reverse short circuit fault, stages 1, 2, and 4 are the same as those of a forward short circuit fault. Only stages 3 and 5 that are different 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, the sixth fully-controlled power device Q6 and the seventh fully-controlled power device Q7 are turned on, and through the action of the first coupled inductor T1 and the second coupled inductor T2, a voltage with an amplitude higher than the DC system voltage U is induced in the main branch. dc A third counter voltage is provided, so that the current of the main branch drops rapidly;

[0107] 3-2: After the main branch current drops to zero, the sixth fully-controlled power device Q6 is turned off, the second fully-controlled power device Q2 and the third fully-controlled power device Q3 are turned on, and the anti-parallel diodes of the seventh fully-controlled power device Q7 and the eighth fully-controlled power device Q8 are turned on, and through the action of the first coupled inductor T1 and the second coupled 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 rise again;

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

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

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

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

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

[0113]

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

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

[0116]

[0117] Where n1 is the turns ratio between the primary and secondary sides of the first coupled inductor, L m1 is the magnetizing inductance of the first coupled inductor, L loop is the inductance of the current limiting inductor, I T is the main branch current protection threshold, U dc is the voltage of the DC bus, i M is the current of the main branch, and t5-t1 is the duration of the fault current breaking process.

[0118] Since SHCB usually adopts an air core structure to prevent magnetic saturation, the magnetization loss of the first coupling inductor T1 and the second coupling inductor T2 can be ignored. loss The conduction loss E of the component con and switching loss E sw Composition, expressed as:

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

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

[0121]

[0122] In the formula, R H1 Represents the loop resistance of the first H-bridge unit, including the internal resistance of the power device, the primary winding of T1 and the secondary winding of T2; R H2 Represents the loop resistance of the second H-bridge unit, including the internal resistance of the power device and the primary winding of T2; i H1 is the current of the first H-bridge unit, i H2 is the current of the second H-bridge unit;

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

[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 They represent the conduction loss, turn-off loss and reverse recovery loss of the IGBT's anti-parallel diode, respectively. These parameters can be estimated from the device data sheet. s represents 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] In order to ensure that ER-SHCB can reliably realize self-restart, U c (t6)≥U c (t1), because the excess energy can be dissipated in the internal resistance of the device and winding. Combined with formula (2), the condition for self-restart operation is:

[0127] E loss <E absorb , (7)

[0128] In addition, formulas (3)-(6) show that the following methods can promote the establishment of self-restart conditions:

[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 disconnection strategies to increase E absorb .

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

[0133] The present invention also provides a control method for a bidirectional series hybrid DC circuit breaker, which is applied to the bidirectional series hybrid DC circuit breaker described above. Figure 5 As shown, the following steps are included:

[0134] S1, the main branch operates normally and has completed charging of the capacitor C;

[0135] S2, monitoring the current signal of the main branch, when the main branch current i M When the preset main branch current threshold is exceeded, the direction of the short-circuit fault current is determined;

[0136] S3. Enter the corresponding working state according to the direction of the short-circuit fault current determined. 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 Rapidly decrease; the voltage injection unit switches between mode 1 and mode 2, i M Modulate to an AC ripple close to zero; at the same time, 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 Rapidly decrease; the voltage injection unit switches between mode 4 and mode 5, i M Modulate to an AC ripple close to zero; at the same time, 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, determining whether there is excess inductive energy in the first coupled inductor T1 and the second coupled inductor T2;

[0140] S5. If there is excess inductive 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, which is used to exhaust the excess inductive 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, which inherits the low conduction loss and fast breaking characteristics of conventional series DC circuit breakers, and has the following unique advantages:

[0143] 1. By recovering the residual excitation current in the first coupling inductor and the second coupling inductor to charge the capacitor, both the capacitor voltage and the excitation current can be restored to the initial state, thereby completing the self-restart. This simplifies and speeds up 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, so that the proposed bidirectional series DC circuit breaker can 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, rather than to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions of the technical solution of the present invention by ordinary technicians in this field do not deviate from the essence and scope of the technical solution of the present invention.

Claims

1. A bidirectional series hybrid DC circuit breaker, characterized in that: It 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 first coupled inductor secondary winding connected in series in sequence; The voltage injection unit is provided with a capacitor, a first H-bridge unit, a second H-bridge unit, a first coupled inductor primary winding, and a second coupled inductor; the first coupled inductor primary winding and the first coupled inductor secondary winding constitute a first coupled inductor, and the second coupled inductor includes a second coupled inductor primary winding and a second coupled inductor secondary winding; wherein the capacitor is connected to the first H-bridge unit and the second H-bridge unit, the first H-bridge unit is connected in parallel with the second H-bridge unit, the first coupled inductor primary winding and the second coupled inductor secondary winding are connected in series and then connected to the first H-bridge unit, and the second coupled inductor primary winding 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, and is used to charge the capacitor.

2. The bidirectional series hybrid DC circuit breaker according to claim 1, characterized in that: 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 electrode of the capacitor is connected to the positive electrode of the first fully-controlled power device, the positive electrode of the second fully-controlled power device, the positive electrode of the fifth fully-controlled power device, and the positive electrode of the sixth fully-controlled power device, and the negative electrode of the capacitor is connected to the negative electrode of the third fully-controlled power device, the negative electrode of the fourth fully-controlled power device, the negative electrode of the seventh fully-controlled power device, and the negative electrode of the eighth fully-controlled power device, and the negative electrode of the capacitor is connected to the main branch; The cathode of the first fully-controlled power device is connected to the anode of the third fully-controlled power device and one end of the primary winding of the first coupled inductor; The negative electrode of the second fully-controlled power device is connected to the positive electrode 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 cathode of the fifth fully-controlled power device is connected to the anode of the seventh fully-controlled power device and one end of the primary winding of the second coupled inductor; The cathode of the sixth fully-controlled power device is connected to the anode of the eighth fully-controlled power device and the other end of the primary winding of the second coupled inductor.

3. The bidirectional series hybrid DC circuit breaker according to claim 2, characterized in that: The charging circuit is provided with a first diode, a second diode, a first half-controlled power device, a first resistor, and a second resistor; The positive electrode of the first diode is used as the input end of the charging circuit and is connected to the first end of the mechanical switch in the main branch, and the negative electrode of the first diode is connected to the positive electrode of the first semi-controlled power device and the negative electrode of the second diode; The anode of the second diode is connected to the tail end of the mechanical switch and one end of the current limiting inductor, and the cathode of the second diode is connected to one end of the first resistor; The cathode of the first half-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 anode of the capacitor.

4. The bidirectional series hybrid DC circuit breaker according to claim 3, characterized in that: When a short circuit occurs in the bidirectional series hybrid DC circuit breaker, the following working stages are involved: Phase 1: the DC system operates 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 main branch current increases; Phase 3: When the main branch current reaches the protection threshold, the voltage injection unit is activated and generates a counter voltage with a polarity opposite to the DC system voltage, modulating the main branch current into an AC ripple close to zero; Stage 4: The mechanical switch is turned on, and the short-circuit fault part is disconnected without arcing by the mechanical switch; Phase 5: After the mechanical switch is turned on, the energy stored in the first coupled inductor and the second coupled inductor is recovered to charge the capacitor.

5. The bidirectional series hybrid DC circuit breaker according to claim 4, characterized in that: When a forward short circuit fault occurs in the bidirectional series hybrid DC circuit breaker, the stage 3 includes the following working states: 3.1: 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. Through the action of the first coupled inductor and the second coupled inductor, a first counter voltage having an amplitude higher than the DC system voltage is induced in the main branch, causing the current of 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 coupled inductor and the second coupled 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 injects the first counter voltage and the second counter voltage alternately to modulate the main branch current into an AC ripple close to zero; When a forward short circuit fault occurs in the bidirectional series hybrid DC circuit breaker, the stage 5 includes the following working states: After the mechanical switch is turned on, all fully-controlled power devices are turned off; then the anti-parallel diode of the second fully-controlled power device and the anti-parallel diode of the third fully-controlled power device are turned on, and the current is continued through the primary winding of the first coupled inductor and the secondary winding of the second coupled inductor; the anti-parallel diode of the sixth fully-controlled power device and the anti-parallel diode of the seventh fully-controlled power device are turned on, and the current is continued through the primary winding of the second coupled inductor.

6. The bidirectional series hybrid DC circuit breaker according to claim 4, characterized in that: When a reverse short circuit fault occurs in the bidirectional series hybrid DC circuit breaker, the stage 3 includes the following working states: 3-1: the second fully-controlled power device and the third fully-controlled power device are turned on, the sixth fully-controlled power device and the seventh fully-controlled power device are turned on, and through the action of the first coupled inductor and the second coupled inductor, a third counter voltage with an amplitude higher than the DC system voltage is induced in the main branch, causing the current of the main branch to decrease; 3-2: After the main branch current drops to zero, the sixth fully-controlled power device is turned off, the second fully-controlled power device and the third fully-controlled power device are turned on, and the anti-parallel diodes of the seventh fully-controlled power device and the eighth fully-controlled power device are turned on, and through the action of the first coupled inductor and the second coupled 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 loop alternately injects a third counter voltage and a 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, the stage 5 includes the following working states: after the mechanical switch is turned on, all fully-controlled power devices are turned off; then the anti-parallel diode of the first fully-controlled power device and the anti-parallel diode of the fourth fully-controlled power device are turned on, and the current is continued through the first coupled inductor primary winding and the second coupled inductor secondary winding; the anti-parallel diode of the fifth fully-controlled power device and the anti-parallel diode of the eighth fully-controlled power device are turned on, and the current is continued through the second coupled inductor primary winding.

7. The bidirectional series hybrid DC circuit breaker according to claim 5 or 6, characterized in that: The conditions for the bidirectional series hybrid DC circuit breaker to achieve self-restart are: AND loss <E absorb , E loss It represents the total loss of the circuit breaker during the entire fault interruption process, E absorb It represents the total energy absorbed by the circuit breaker from the DC system during the entire fault interrupting process.

8. The bidirectional series hybrid DC circuit breaker according to claim 7, characterized in that: E absorb It includes the inductive energy stored in the DC system and the energy released by the DC system during the entire fault interruption process. The calculation formula is: Where n1 is the turns ratio between the primary and secondary sides of the first coupled inductor, L m1 is the magnetizing inductance of the first coupled inductor, L loop is the inductance of the current limiting inductor, I T is the main branch current protection threshold, U dc is the voltage of the DC bus, i M is the current of the main branch, t5-t1 is the duration of the fault current breaking process; The total loss of the circuit breaker during the entire fault breaking process is E loss Including the conduction loss E of the components con and switching loss E sw ,Right now: AND loss =And con +E sw ; Conduction loss E con The calculation formula is: R H1 is the loop resistance of the first H-bridge unit, i H1 is the current of the first H-bridge unit, R H2 is the loop resistance of the second H-bridge unit, i H2 is the current of the second H-bridge unit; Switching loss E sw The calculation formula is: AND sw =(And on +E off +E rec )[f S (t5-t2)+4], E on is the conduction loss of the fully controlled power device, E off is the turn-off loss of the fully controlled power device, E rec is the reverse recovery loss of the anti-parallel diode of the fully controlled power device, f s is the switching frequency of the fifth fully-controlled power device or the sixth fully-controlled power device during the current modulation period, and t5-t2 is the duration of the current modulation process.

9. The bidirectional series hybrid DC circuit breaker according to claim 8, characterized in that: In order to promote the self-restart of the bidirectional hybrid DC circuit breaker, when a short circuit occurs, the main branch current i M Limit to a current limit value I greater than zero lim , and then modulate the main branch current to zero current AC ripple to achieve arc-free breaking of the mechanical switch.

10. A control method for a bidirectional series hybrid DC circuit breaker, characterized in that: The method is applied to a bidirectional series hybrid DC circuit breaker according to any one of claims 1 to 9, comprising the following steps: S1, the main branch operates normally and has completed charging of the capacitor; S2, monitoring the current signal of the main branch, and when it is detected that the current of the main branch exceeds a preset main branch current threshold, determining the direction of the short-circuit fault current; S3, entering a corresponding working state according to the determined direction of the short-circuit fault current; S4, determining whether there is excess inductive energy in the first coupled inductor and the second coupled inductor; S5. If there is excess inductive 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 inductive 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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