A bidirectional current limiter based on continuous current commutation

Abstract

The present application discloses a bidirectional current limiter based on continuous current commutation, including: a current limiting branch including a DC voltage source and a current limiting reactance, the DC voltage source and the current limiting reactance are connected in series, and are used to adjust the current limiting threshold; an inner commutation branch includes two parallel IGBT groups, each IGBT group is composed of two series-connected IGBTs, and is used to convert the fault DC current into a high-frequency AC current when the line current reaches the current limiting threshold; an outer commutation branch includes two parallel diode groups, each diode group is composed of two series-connected diodes, and is used to achieve bidirectional current limiting of the circuit. When a DC fault occurs, the current limiting reactance can be instantaneously put into current limiting, and the commutation branch can convert the DC current into a high-frequency AC current, and can suppress inductance saturation through current limiting, and maintain the fault current within a certain stable range. Therefore, the present application solves the technical problem that the current limiting capability fails due to inductance saturation in the existing current limiting technology.

Description

Technical Field

[0001] The present application relates to the technical field of current limiters, and in particular to a bidirectional current limiter based on continuous current commutation. Background Art

[0002] With the development of power electronic devices such as VSC converters and DAB DC transformers, DC distribution networks can integrate the distribution characteristics of various new energy sources and loads to a greater extent and carry out multi-dimensional complementarity. Gradually, the DC distribution network has become a hot topic due to its advantages such as large power supply capacity, good economy, high reliability, low loss, and high power quality. However, compared with the traditional AC system, the DC power grid composed of power electronic equipment is a "low inertia, low impedance" system. After a serious DC short circuit fault occurs, the converter and the energy storage elements on the DC side discharge rapidly, resulting in a fast fault development speed, a wide impact range, a large amount of energy released, and a rapid increase in fault current.

[0003] In order to solve the above problems, certain measures can be taken to limit the amplitude and current rise rate of the short-circuit current in the DC power distribution system, namely, current limiting technology. However, there are still some problems in the existing current limiting technology. For example, the current limiting inductor is prone to inductance saturation as the fault current increases during the current limiting process, thus losing its current limiting ability; or the design cost of the circuit system is high, etc., which cannot meet the current current limiting needs. Summary of the invention

[0004] The present application provides a bidirectional current limiter based on continuous current commutation, which is used to solve the technical problem of current limiting capability failure caused by inductor saturation in existing current limiting technologies.

[0005] In view of this, the first aspect of the present application provides a bidirectional current limiter based on continuous current commutation, comprising: a current limiting branch, an inner commutation branch and an outer commutation branch;

[0006] The current limiting branch includes a DC voltage source and a current limiting reactance, wherein the DC voltage source is connected in series with the current limiting reactance to adjust the current limiting threshold;

[0007] The inner commutation branch comprises two parallel IGBT groups, each of which is composed of two series-connected IGBTs, and is used to convert the fault DC current into a high-frequency AC current when the line current reaches the current limiting threshold;

[0008] The external commutation branch includes two parallel diode groups, each of which is composed of two series-connected diodes, and is used to achieve bidirectional current limiting of the circuit.

[0009] Optionally, the current limiting branch and the two IGBT groups in the inner commutation branch are connected in series.

[0010] Optionally, the two diode groups in the inner commutation branch and the outer commutation branch are connected in series.

[0011] Optionally, it also includes: lightning arrester;

[0012] The lightning arrester is connected to the current-limiting reactance in the current-limiting branch, and is used to perform overvoltage protection on the current-limiting reactance.

[0013] It can be seen from the above technical solutions that the embodiments of the present application have the following advantages:

[0014] In the present application, a bidirectional current limiter based on continuous current commutation is provided, comprising: a current limiting branch, an inner commutation branch and an outer commutation branch; the current limiting branch comprises a DC voltage source and a current limiting reactance, the DC voltage source and the current limiting reactance are connected in series, and are used to adjust the current limiting threshold; the inner commutation branch comprises two parallel IGBT groups, each IGBT group is composed of two series-connected IGBTs, and is used to convert the fault DC current into a high-frequency AC current when the line current reaches the current limiting threshold; the outer commutation branch comprises two parallel diode groups, each diode group is composed of two series-connected diodes, and is used to achieve bidirectional current limiting of the circuit.

[0015] The bidirectional current limiter based on continuous current commutation provided by the present application does not affect the flow movement of the DC system when the system is operating normally; when a DC fault occurs, the current limiting reactance can be instantly put into current limiting, and the commutation branch can convert the DC current into a high-frequency AC current, and the inductor saturation can be suppressed by current limiting to maintain the fault current within a certain stable range. Therefore, the present application can solve the technical problem that the current limiting capability fails due to inductor saturation in the existing current limiting technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A circuit structure topology diagram of a bidirectional current limiter based on continuous current commutation provided in an embodiment of the present application;

[0017] Figure 2 A schematic diagram of current flow during normal operation of the system provided in an embodiment of the present application;

[0018] Figure 3 A schematic diagram of instantaneous current flow when a fault occurs provided in an embodiment of the present application;

[0019] Figure 4 A schematic diagram of the fault current limiting state provided in an embodiment of the present application;

[0020] Figure 5 A schematic diagram of a fault current clear state provided in an embodiment of the present application;

[0021] Figure 6 A schematic diagram of the current limiter recovery state provided in an embodiment of the present application;

[0022] Figure 7 The equivalent circuit of the working process of the current limiter provided in the embodiment of the present application Figure 1 ;

[0023] Figure 8 The equivalent circuit of the working process of the current limiter provided in the embodiment of the present application Figure 2 ;

[0024] Fig. 9 The equivalent circuit of the working process of the current limiter provided in the embodiment of the present application Figure 3 ;

[0025] Fig.10 A comparison diagram of the theoretical analysis and simulation waveform of the fault current in the time domain provided in the embodiment of the present application;

[0026] Fig.11 A schematic diagram of the inductance selection principle provided in an embodiment of the present application. DETAILED DESCRIPTION

[0027] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0028] For easier understanding, see Figure 1 , an embodiment of a bidirectional current limiter based on continuous current commutation provided in the present application includes: a current limiting branch, an inner commutation branch and an outer commutation branch;

[0029] The current limiting branch includes a DC voltage source U b and current limiting reactance L fcl , a DC voltage source is connected in series with a current-limiting reactance to adjust the current-limiting threshold; when the line current reaches the current-limiting threshold, it automatically and instantly transfers to the current-limiting branch for flow movement.

[0030] The inner commutation branch includes two parallel IGBT groups, each of which consists of two series-connected IGBTs, namely T1-T4, which are used to convert fault DC current into high-frequency AC current when the line current reaches the current limiting threshold. The outer commutation branch includes two parallel diode groups, each of which consists of two series-connected diodes, namely D1-D4, which are used to achieve bidirectional current limiting of the circuit.

[0031] Furthermore, the current limiting branch and the two IGBT groups in the inner commutation branch are connected in series.

[0032] Furthermore, the two diode groups in the inner commutation branch and the outer commutation branch are connected in series.

[0033] DC fault current limiters are of great significance to the safe and stable operation of DC distribution networks. They can suppress the rise of DC fault current and reduce the current stress of other equipment. The development of fault line current can be divided into two situations: the direction of the original line current is the same as the direction of the feed-in current and the direction of the feed-in current is different. Figure 1 Taking the wiring method as an example, assume that before the fault occurs, the current direction of the line is from left to right. Case 1: The fault point is on its right side, and the direction of the current fed into the converter station is also from left to right. The current flowing through it will increase rapidly and the direction remains unchanged; Case 2: The fault point is on its left side, and the direction of the current fed into the converter station is from right to left. The current flowing through it will first decrease rapidly to zero, and then increase in the reverse direction. The action mode of the current limiter in Case 1 and Case 2 is basically the same. Taking Case 1 as an example, the working conditions of the current limiter in this embodiment under different conditions are analyzed:

[0034] First, in normal operation, T1 and T4 are turned on, T2 and T3 are turned off, and the DC voltage source U b The steady-state current generated is i b . when i dc b According to Kirchhoff's law, since the resistance of the four IGBTs is equal when they are turned on, the voltage source U b The current flowing through T1 and T3 is equal to the current flowing through T2 and T4. dc When it flows into the internal circuit commutation branch through the diode, it is also divided equally, with the line current flow direction as the positive direction. Therefore, the current flowing through the four commutation IGBTs is:

[0035]

[0036] Therefore, when the system is operating normally, T2 and T3 are turned on through the anti-parallel diodes, and the line current is as follows Figure 2 Direction of flow shown.

[0037] Secondly, in the fault current limiting state, that is, a fault occurs in the DC line, the line current increases rapidly, and when the line current exceeds the threshold, that is, i dc >i b , then T2 is quickly turned off by the reverse voltage drop, the line current is instantly transferred to the current limiting branch, and the current limiting inductor is immediately put into current limiting. Figure 3 When the system determines that there is a line fault (when the current flowing through T2 is detected to be 0), it immediately gives T2 and T3 a trigger pulse, and gives T1 and T4 a shutdown pulse. The current path is changed from the previous D1-T1-L fcl -T4-D4 to D1-T3-L fcl ​-T2-D4, where the current in the current-limiting inductor commutates, and the inductor discharges outward in the opposite direction to suppress the increase of the fault current. When the current in the inductor becomes zero, the inductor itself starts to limit the increase of the DC current. After T / 2 time, trigger pulses are given to T1 and T4, and turn-off pulses are given to T2 and T3. The current path is changed from D1-T3-L fcl -T2-D4 turns into D1-T1-L fcl -T4-D4, similarly, the current in the current-limiting inductor is commutated, and the inductor discharge and the inductor's own action suppress the current increase in a short time.

[0038] See also Figure 4 , through the trigger control of IGBT, it is turned on alternately according to a certain period, so that the DC current is D1-T1-L fcl -T4-D4 (State1) and D1-T3-L fcl -T2-D4 (State2) switches between the two circuits to convert the DC current flowing through the current limiting branch into a high-frequency AC current, and then utilizes the inductance characteristics to suppress the increase of fault current and prevent inductance saturation.

[0039] See also Figure 5 and Figure 6 After a period of time, when the system detects the closing signal of the DC circuit breaker, it gives T1 and T4 trigger pulses, gives T2 and T3 shut-off pulses, and the current limiter returns to the current-passing state. By controlling the on-off of the IGBT, the DC current is converted into a high-frequency AC current inside the current limiter. Therefore, after the current limiting continues for a period of time, when the detector detects that the line current is close to zero, the main circuit breaker of the DC circuit breaker is disconnected and the lightning arrester is put into use. The current limiter in this embodiment can achieve safe and low-loss shutdown of small currents.

[0040] The current limiter provided in this embodiment converts DC current into high-frequency AC current by controlling the alternate conduction of IGBTs. Therefore, the DC circuit breaker can be controlled to disconnect when the current is close to 0, which greatly improves the shutdown reliability of the DC circuit breaker and reduces the disconnection capacity and design cost of the DC circuit breaker.

[0041] According to the working conditions of the current limiter under the above different states, the current limiting process can be analyzed in detail, and then the core parameters in the current limiting process can be calculated. b Replacement. Considering the extreme case, assuming that the fault occurs near the converter station, the line impedance is zero. When the fault current exceeds the current limiting setting value, the fault current is instantly transferred to the current limiting branch. The current limiting reactor can limit the fault current without any time delay, effectively limiting the increase of the fault current. Then the system controls the on and off of the IGBT to limit the fault current within a certain range. The following is an analysis of the change of the fault current within a commutation cycle T.

[0042] 0-T / 2: T2 and T3 are given a trigger signal, and T1 and T4 are given a shutdown signal. The equivalent circuit is as follows: Figure 7 As shown. In half a commutation cycle, the current development is divided into two stages, the current limiting inductor discharge stage and the capacitor discharge stage. At the instant of commutation, the current in the current limiting inductor is forced to commutate, resulting in a transient spike overvoltage at both ends of the inductor. The magnitude of this voltage is related to the magnitude of the current when the current in the current limiting inductor commutates. Due to the presence of the spike overvoltage, the diodes D1 and D4 are turned off, so the line current i in this stage dc is 0. When the transient peak voltage gradually decreases and is less than the terminal capacitor voltage v dc When , D1 and D4 are turned on, the current development enters the terminal capacitor discharge stage.

[0043] See also Figure 8 and Fig. 9 , the Laplace equation of the current limiting inductor discharge stage can be expressed as:

[0044]

[0045] Among them, s is the Laplace operator, R s_eq is the IGBT equivalent snubber resistance, C s_eq is the IGBT equivalent snubber capacitance, I l0 is the initial current when current commutation occurs in the current limiting inductor, U cs_eq0 is the initial voltage of the equivalent buffer capacitor, which is generally the IGBT turn-on voltage. The current at any time when current commutation occurs in the current limiting inductor is expressed as:

[0046]

[0047]

[0048] According to the loop current law, the Laplace equation of the system end capacitor discharge stage (in this embodiment, the DC voltage source is equivalent) can be expressed as:

[0049]

[0050]

[0051] i dcf (s) = i dcf1 (s)+i dcf2 (s)

[0052] Among them, R eq is the system equivalent impedance, I dc_lD When the diode is turned on, the residual current in the current limiting inductor is U cs_eqD When the diode is conducting, it is the voltage across the equivalent buffer capacitor, Udc is the system end capacitor voltage, and the fault current can be expressed in the time domain:

[0053]

[0054] According to the above formula, the fault current analysis curve can be drawn. Fig.10 , the current is limited to below the rated current, and there is always a reverse current in the current limiting reactor every time the topology switches, which also verifies from a theoretical level that the topology has a strong current limiting capability and square inductor saturation capability. In addition, U cs_eqD ,I dc_lD The solution process is:

[0055]

[0056] According to the conduction principle of the diode, when the transient voltage across the current-limiting inductor gradually decreases and is less than the terminal capacitor voltage U dc When , the diode is conducting, so:

[0057]

[0058] From the above formula, we can solve t D , and then find U cs_eqD and I dc_lD :

[0059]

[0060]

[0061] T / 2-T: T1 and T4 are given a trigger signal, and T2 and T3 are given a shutdown signal. The only difference from 0-T / 2 is that the DC voltage source U b is reversed.

[0062] The parameters that need to be set in the current limiter include the current limiter commutation period and the current limiting inductance value. Other parameters such as the current limiting peak value are given by the system. In general, the system overcurrent threshold is given in advance. From the perspective of system safety and system stability, the steady-state current limiting peak value is set to 1.5 times the steady-state value. Due to the limitation of the rated switching frequency of the IGBT, from the perspective of equipment safety and switching losses, the current limiter commutation frequency is only 90% of the rated IGBT switching frequency. Taking into account extreme cases, assuming that the ground fault occurs near the converter station, the line reactance is zero. According to Fig. 9 Analysis shows that when the commutation period and current limiting peak value are determined, the value of the current limiting reactance, that is, the current limiting inductance, is determined. Fig. 9 Solving the linear equation can give the current limiting inductor L fcl However, the solution process is complicated, so this example uses a graphical method to solve it. Please refer to Fig.11 :

[0063]

[0064] Among them, I yz is the system overcurrent threshold, t=t / 2-t D .

[0065] Further, it also includes: a lightning arrester;

[0066] The lightning arrester is connected to the current-limiting reactance in the current-limiting branch to provide overvoltage protection for the current-limiting reactance.

[0067] It is understandable that the arrester plays a vital role when overvoltage occurs in power electronic devices. In this embodiment, the arrester mainly protects against overvoltage occurring at both ends of the power electronic devices at the moment of fault, and also dissipates the residual current in the system during the current limiter fault recovery stage.

[0068] In the current limiting process of the current limiter, the current in the inductor is commutated by controlling the on and off of the IGBT to limit the fault current. However, this also leads to a large instantaneous current when a fault occurs, which causes a transient spike overvoltage at both ends of the current limiting inductor when commutation occurs. When the current limiting enters a steady state, the overvoltage gradually decreases to a stable state. Therefore, the operating voltage U ac In each half commutation cycle, the current development in the current limiter can be divided into two stages, the inductor freewheeling stage and the capacitor discharge stage. In the inductor freewheeling stage, the voltage across the inductor is much greater than the voltage U of the capacitor (VSC capacitor or DAB capacitor). dc , causing D1 and D4 to turn off, resulting in overvoltage at both ends of the inductor. The capacitor discharge stage is small and is not taken into consideration when designing the arrester action voltage.

[0069]

[0070] Among them, κ is the reliability coefficient, U ac It is the arrester action voltage.

[0071] The capacity design of the arrester includes two aspects: the energy consumed by the overvoltage of the device during the fault instantaneous suppression and the residual energy dissipated in the system during the fault recovery phase of the current limiter. The fault instantaneously causes the current limiting inductor to commutate, resulting in an overvoltage at both ends of the current limiting inductor. When the transient overvoltage gradually becomes smaller than the arrester action voltage, the arrester stops working. The arrester action time is expressed as t ac express:

[0072]

[0073]

[0074] Solving the above equation, we can get t ac , therefore, the energy consumed to suppress device overvoltage is:

[0075]

[0076] According to the working principle of the current limiter, the circuit breaker is disconnected when the line current is close to zero. Therefore, the design capacity of the lightning arrester in the DCCB can be greatly reduced, but the current limiting inductor still stores some energy, such as Figure 5 As shown, when the IGBT is given a turn-off pulse, the current is dissipated through the arrester connected in parallel with the IGBT. This part of the energy can be calculated as:

[0077]

[0078] The total arrester design capacity is:

[0079] W=W1+W2

[0080] The obtained capacity W is the model of the lightning arrester designed in this embodiment.

[0081] Regarding the device selection of IGBT and diode, the rated current and rated voltage are the main parameters to be considered. According to the working principle of the current limiter, the rated current of the IGBT and diode should be greater than the current or overcurrent setting threshold when the current limiter is commutated.

[0082]

[0083] Among them, I r is the rated current of IGBT and diode, I set is the system overcurrent threshold. The difference is that the rated voltage of the IGBT and diode should be greater than the action voltage of the lightning arrester installed at both ends:

[0084]

[0085] U r_G ≥k RVR_G U ac

[0086] Among them, κ RVR_D U is the ratio of the residual voltage to the operating voltage of the arrester installed at both ends of the diode. dc is the voltage at the output capacitor of the DC transformer. The rated voltage of the diode is divided by 2 because the two sets of diode arms bear the voltage across the current limiting inductor in the current limiting state.

[0087] The bidirectional current limiter based on continuous current commutation provided by the embodiment of the present application does not affect the flow movement of the DC system when the system is operating normally; when a DC fault occurs, the current limiting reactance can be instantly put into current limiting, and the commutation branch can convert the DC current into a high-frequency AC current, and the inductor saturation can be suppressed by current limiting to maintain the fault current within a certain stable range. Therefore, the embodiment of the present application can solve the technical problem that the current limiting capability fails due to inductor saturation in the existing current limiting technology.

[0088] In the several embodiments provided in the present application, it should be understood that the disclosed devices and methods can be implemented in other ways. For example, the device embodiments described above are only schematic. For example, the division of the units is only a logical function division. There may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or units, which can be electrical, mechanical or other forms.

[0089] The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0090] In addition, each functional unit in each embodiment of the present application may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above-mentioned integrated unit may be implemented in the form of hardware or in the form of software functional units.

[0091] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present application is essentially or the part that contributes to the prior art or all or part of the technical solution can be embodied in the form of a software product, and the computer software product is stored in a storage medium, including a number of instructions for executing all or part of the steps of the method described in each embodiment of the present application through a computer device (which can be a personal computer, a server, or a network device, etc.). The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (full name in English: Read-Only Memory, English abbreviation: ROM), random access memory (full name in English: Random Access Memory, English abbreviation: RAM), disk or optical disk and other media that can store program codes.

[0092] As described above, the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. A person skilled in the art should understand that the technical solutions described in the aforementioned embodiments can still be modified, or some of the technical features can be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A bidirectional current limiter based on continuous current commutation, characterized in that: include: Current limiting branch, inner commutation branch and outer commutation branch; The current limiting branch includes a DC voltage source and a current limiting reactance, wherein the DC voltage source is connected in series with the current limiting reactance to adjust the current limiting threshold; The inner commutation branch comprises two parallel IGBT groups, each of which is composed of two series-connected IGBTs, and is used to convert the fault DC current into a high-frequency AC current when the line current reaches the current limiting threshold; The current limiting branch and the two IGBT groups in the inner commutation branch are connected in series; The external commutation branch includes two parallel diode groups, each of which is composed of two series-connected diodes, for realizing bidirectional current limiting of the circuit; The two diode groups in the inner commutation branch and the outer commutation branch are connected in series.

2. The bidirectional current limiter based on continuous current commutation according to claim 1, characterized in that: Also includes: lightning arrester; The lightning arrester is connected to the current-limiting reactance in the current-limiting branch, and is used to perform overvoltage protection on the current-limiting reactance.

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