A DC distribution network fault location method based on MMC submodule reuse

Abstract

The present invention discloses a DC distribution network fault ranging method based on MMC submodule reuse, which belongs to the field of DC distribution network relay protection. The fault ranging method includes the following steps: cutting off the DC line power transmission after the fault occurs; controlling the A-phase bridge arm submodule to be removed, the B-phase and C-phase bridge arm submodules to be locked, and constructing a ranging discharge circuit suitable for single-pole grounding and bipolar short-circuit faults; controlling the A-phase discharge submodule to inject current into the fault ranging circuit in the system power-off state, and removing the submodule after a short-time discharge; changing the loop parameters by switching on and off the current-limiting inductor, and making the ranging circuit equivalent to an RL series circuit with variable inductance; identifying the time constants of different discharge currents, and calculating the fault distance and fault resistance. Compared with the traditional injection method, the present invention requires fewer additional components, less amount to be sampled, and lower sampling accuracy requirements. The accuracy of the ranging result can be improved by repeated measurements, and has good economy and practicality.

Description

Technical Field

[0001] The invention belongs to the field of DC distribution network relay protection, and in particular relates to a DC distribution network fault distance measurement method based on MMC submodule multiplexing. Background Art

[0002] With the continuous deepening of research in the field of DC power distribution, modular multi-level converter (MMC) has been widely used and studied in existing demonstration projects due to its advantages such as good waveform characteristics, large number of levels, modular structure, and low switching loss. However, with the continuous expansion of the scale of DC power distribution network, the line fault characteristics are becoming increasingly complex. At present, research on fault protection and control of DC distribution network is still in the theoretical stage. Compared with the perfect and mature AC distribution network, DC distribution network still lacks sufficient practical engineering support. In addition, the short line length of DC distribution network also greatly reduces the applicability of some fault location and distance measurement methods suitable for DC transmission system, and more accurate fault distance measurement is difficult to achieve.

[0003] At present, some research has been carried out on the fault location method of DC distribution network. According to the principle of its implementation, it can be divided into injection fault location, fault location based on multi-point information and local passive fault location. However, the existing methods have problems such as high sampling accuracy requirements of measurement devices, many additional components, high cost, large amount of samples to be sampled and complex principles. Therefore, it is necessary to carry out in-depth research on accurate and practical fault location methods suitable for MMC DC distribution network. Summary of the invention

[0004] In view of the deficiencies in the prior art, the purpose of the present invention is to provide a DC distribution network fault location method based on MMC sub-module multiplexing, which solves the problems existing in the existing fault location method, such as numerous additional components, high sampling accuracy requirements, and a large amount of samples to be sampled.

[0005] The purpose of the present invention can be achieved through the following technical solutions:

[0006] A DC distribution network fault location method based on MMC submodule multiplexing, the method comprising the following steps:

[0007] Step 1: Cut off the power transmission circuit after the fault occurs; when a line fault occurs, quickly cut off the DC circuit breakers CB5 and CB6 to terminate the line power transmission, and then disconnect the AC and DC side disconnector CB1 after reclosing and arc extinguishing;

[0008] Step 2: Adjust system components to construct a distance measurement discharge circuit; control the A-phase bridge arm submodule to be in a cut-off state, and the B-phase and C-phase bridge arm submodules to be in a locked state, and then close the A-phase bridge arm neutral point grounding switch CB2 and the distance measurement side DC circuit breaker CB5;

[0009] Step 3, control the discharge submodule to be switched on and off; control the A-phase bridge arm discharge submodule SM1 or SM2 to be switched on, connect the submodule capacitor C to the circuit for discharge, and switch off after 5ms;

[0010] Step 4: Change the loop parameters to continue the current flow; after the discharge continues for 5ms, close the current limiting inductor switching switch CB3 or CB4 to cut off the current limiting inductor L cl , thereby changing the circuit parameters and continuing to discharge until the end;

[0011] Step 5: Current signal acquisition and fault distance calculation: respectively collect the discharge current I before and after the current limiting inductor is removed in step 4 F1 with I F2 , the current discharge time constant τ is obtained through signal processing, and the fault point distance x and the fault resistance R are further calculated. f At the same time, in order to improve the ranging accuracy, steps 3 to 5 can be repeated to perform multiple measurements to obtain the average value.

[0012] Beneficial effects of the present invention:

[0013] 1. The DC distribution network fault distance measurement method based on MMC submodule reuse proposed in the present invention forms a discharge and distance measurement loop through the control submodule after the system loses power, and the loop is equivalent to an RL series circuit with variable inductance. The fault distance is obtained by collecting current signals and parameter identification calculation. The principle is relatively simple, the amount to be sampled is small, and the sampling accuracy requirement is low;

[0014] 2. The DC distribution network fault distance measurement method based on MMC submodule multiplexing proposed in the present invention adopts the submodule to provide a discharge signal source in a short time, making full use of the inherent components of the system. Compared with the distance measurement method of the additional injection type, it reduces the component configuration and further reduces the cost;

[0015] 3. The DC distribution network fault distance measurement method based on MMC submodule multiplexing proposed in the present invention can achieve repeated measurement of fault distance by repeatedly controlling the switching discharge and signal collection of submodules, which is conducive to further processing and improving the accuracy of distance measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0017] Figure 1 It is a schematic diagram of a DC power distribution system and a DC line fault distance measurement discharge path of the present invention;

[0018] Figure 2 It is a schematic diagram of a single-pole ground fault ranging discharge equivalent circuit of the present invention;

[0019] Figure 3 It is a schematic diagram of a bipolar short-circuit fault ranging discharge equivalent circuit of the present invention;

[0020] Figure 4 It is a flow chart of fault distance measurement of the present invention. DETAILED DESCRIPTION

[0021] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] The present invention proposes a DC distribution network fault location method based on MMC submodule multiplexing, which is applicable to a DC distribution network using a half-bridge MMC. Figure 1 As shown, a double-terminal DC distribution network with a rated voltage level of ±10kV is used. The distance measurement scheme needs to be supplemented with the bridge arm neutral point grounding switch CB2 and the line current limiting inductor switching switches CB3 and CB4 in the original system. CB2 is used to construct a distance measurement discharge circuit under a single-pole grounding fault, and has no effect on the bipolar short-circuit fault distance measurement circuit. It can be closed and grounded after the system is isolated and powered off. CB3 and CB4 are used to change the inductance parameters of the discharge circuit, thereby changing the current discharge time constant τ, and performing subsequent measurements and calculations. Its fault distance measurement method includes the following steps:

[0023] Step 1: Cut off the power transmission circuit after a fault occurs

[0024] When the system is operating normally, switches CB1, CB5, and CB6 remain closed, and CB2, CB3, and CB4 remain open. When a line fault occurs, the DC circuit breakers CB5 and CB6 are quickly disconnected to terminate the line power transmission. After reclosing and arc extinguishing, the AC and DC side disconnector CB1 is then disconnected.

[0025] Step 2: Adjust system components to build a distance measurement discharge circuit

[0026] The A-phase bridge arm submodule is controlled to be in the cut-off state, and the B-phase and C-phase bridge arm submodules are controlled to be in the locked state, and then the A-phase bridge arm neutral point grounding switch CB2 and the distance measurement side DC circuit breaker CB5 are closed;

[0027] At this time, the A-phase bridge arm is in a bidirectional conduction state with only the on-state resistance of each component; for the B and C-phase bridge arms, Figure 1 As shown, when the corresponding submodule injects current i SM When the direction is positive, each submodule is charged. SM When the direction is negative, it is a unidirectional conduction state with only the on-state resistance of each component. In the ranging process, since the capacitor voltage of a single discharge submodule is always smaller than the sum of the voltages of the single-phase bridge arm submodules, and the discharge current is always emitted from the high-voltage side of phase A, it is impossible to SM The negative direction passes through the B and C phase bridge arms, so the B and C phase bridge arm submodule capacitors can never be charged, nor can they be connected along i SM The negative direction conduction can be regarded as an open circuit state and has almost no effect on fault distance measurement;

[0028] For a positive grounding fault, the discharge current path is from the neutral grounding point G2 of the A-phase bridge arm to the positive fault grounding point G3 via the A-phase upper bridge arm. Figure 1 Path f1 As shown; for a negative grounding fault, the discharge current path is from the neutral grounding point G2 of the A-phase bridge arm to the negative fault grounding point G4 via the lower bridge arm of the A-phase, as shown Figure 1 Path f2 As shown; for a bipolar short-circuit fault, the discharge current path passes through the upper and lower bridge arms of phase A, the positive and negative lines, and the short-circuit fault resistor R f2 ,like Figure 1 Path f3 As shown;

[0029] Submodule removal refers to the corresponding level S up Set to 0, S down Set to 1, the submodule locks the corresponding level S up Set to 0, S down Set to 0.

[0030] Step 3: Control the input and removal of discharge submodules

[0031] Control the A-phase bridge arm discharge submodule SM1 or SM2 to be in the on state, and the submodule capacitor C is connected to the circuit for discharge and is removed after 5ms;

[0032] Among them, the submodule input refers to the corresponding level S up Set to 1, S downSet to 0;

[0033] For positive grounding and negative grounding faults, the submodules are SM1 and SM2 respectively. For bipolar short circuit faults, the submodule can be SM1 or SM2. When the submodule is put into operation, the internal path is Path1, and when the submodule is removed, it is Path2. Figure 1 shown.

[0034] Step 4: Change the loop parameters to continue current

[0035] After the discharge continues for 5ms, close the current limiting inductor switching switch CB3 or CB4 to cut off the current limiting inductor L cl , thereby changing the circuit parameters and continuing to discharge until the end;

[0036] Among them, for the positive pole grounding fault and the negative pole grounding fault, the current limiting inductor switch is CB3 and CB4 respectively, and for the bipolar short circuit fault, the current limiting inductor switch can be selected as CB3 or CB4.

[0037] Step 5: Current signal acquisition and fault distance calculation

[0038] Collect the discharge current I before and after the current limiting inductor is removed in step 4 respectively F1 with I F2 The signal window length is 5ms before and after the inductance removal, for a total of 10ms. The current discharge time constant τ is obtained through signal processing, and the fault point distance x and the fault resistance R are further calculated. f .

[0039] For a single-pole grounding fault, the equivalent circuit of the distance measurement discharge is as follows: Figure 2 As shown, C represents the submodule capacitance, CB equal It represents the equivalent switch of the control submodule. That is, the control submodule is equivalent to the control of the corresponding capacitor C. The discharge current I F1 with I F2 It can be expressed as:

[0040]

[0041]

[0042] In formulas (1) and (2), I1 and I2 represent the corresponding discharge current amplitudes, t1 represents the discharge submodule removal time, t2 represents the current limiting inductor removal time, τ1 and τ2 represent the corresponding discharge time constants, which can be expressed as:

[0043]

[0044]

[0045] In formulas (3) and (4), L arm is the bridge arm inductance, L cl is the line current limiting inductor, R arm is the half-bridge arm on-state equivalent resistance, that is, the sum of the on-state resistances of the bridge arm path components, which are known quantities; R f1 is the fault grounding resistance, L line is the fault section line inductance, R line is the fault section line resistance, the above are unknown quantities; the corresponding components are as follows Figure 2 As shown; L line With R line It can be expressed as:

[0046] L line =l0x(5)

[0047] R line =r0x(6)

[0048] In formulas (5) and (6), l0 and r0 represent the inductance and resistance per unit length of the line, respectively, which are known quantities, and x represents the distance to the fault point, which is an unknown quantity;

[0049] The Prony algorithm uses a linear combination of exponential functions with different amplitudes, phases, frequencies, and attenuation factors to fit the data to be measured. Assuming that there are k1 attenuated DC components, k2 attenuated high-frequency components, and 1 power frequency component in the corresponding signal, the signal can be expressed as:

[0050]

[0051] Among them, A 1k , A 2k , A0 represents the corresponding component amplitude, α k , β k represents the corresponding component attenuation constant, ω k , ω0 represents the corresponding component angular frequency, θ k , θ0 represents the initial phase angle of the corresponding component. Therefore, the ranging discharge current is suitable for this signal model, and each modal parameter can be identified. Among them, the relationship between the current time constant and the corresponding attenuation constant can be expressed as:

[0052]

[0053] Therefore, the Prony algorithm can be used to identify the discharge current signal and obtain the discharge time constants τ1 and τ2. In summary, by combining formulas (3)-(6), the fault point distance x and the fault grounding resistance R can be obtained. f1 :

[0054]

[0055]

[0056] Similarly, for a bipolar short-circuit fault, the distance measurement discharge equivalent circuit is as follows: Figure 3 As shown, the corresponding time constant can be expressed as:

[0057]

[0058]

[0059] Therefore, by combining equations (5)-(6) and (11)-(12), we can obtain the fault point distance x and the fault resistance R in a similar way: f2 :

[0060]

[0061]

[0062] At the same time, in order to improve the ranging accuracy, steps 3 to 5 can be repeated to perform multiple measurements to obtain an average value.

[0063] The fault location flow chart of the present invention is as follows: Figure 4 As shown in the figure, when a fault occurs, the power transmission is first cut off, and the fault type is judged, and then the AC and DC side disconnectors are cut off; then, the A-phase submodule is controlled to be removed, the B and C-phase submodules are locked, and the neutral point grounding switch of the A-phase bridge arm and the DC circuit breaker on the ranging side are closed; then, according to the fault type, the corresponding submodule is controlled to be put into operation and removed after 5ms; after the discharge continues for 5ms, the current-limiting inductor is removed, the loop parameters are changed, and the discharge is completed; finally, the discharge current before and after the inductor is removed is collected, and the time constant is identified by fitting, and the fault distance and fault resistance are calculated.

[0064] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements all fall within the scope of the present invention to be protected.

Claims

1. A DC distribution network fault location method based on MMC submodule multiplexing, characterized in that: The DC distribution network includes A, B, and C phase bridge arm submodules; The AC / DC side isolating switch CB1 is connected between the DC side outlet of the transformer and the neutral points of the A, B, and C phase bridge arms; the neutral point grounding switch CB2 of the A phase bridge arm is connected between the grounding grid and the neutral point of the A phase bridge arm; the current limiting inductor switching switch CB3 is connected in parallel with the upper bridge arm current limiting inductor L cl On both sides; the current limiting inductor switching switch CB4 is connected in parallel with the lower bridge arm current limiting inductor L cl Both sides; upper arm current limiting inductor L cl Between the DC outlet side and the positive DC outlet line and the lower bridge arm current limiting inductor L cl A DC circuit breaker CB5 is connected between the DC outlet side and the negative DC outlet line; a DC circuit breaker CB6 is connected to the ends of the positive and negative DC lines, and together with CB5, plays the role of starting or terminating line power transmission; A, B, C phase upper and lower bridge arm outlets are connected to the DC circuit breaker CB5 with a limited current inductor L cl ; The A-phase bridge arm discharge submodules SM1 and SM2 are single submodules selected to release energy, which are the upper terminal modules of the upper bridge arm and the lower bridge arm of the A-phase, respectively; the submodule capacitor C is the charge and discharge capacitor in each bridge arm submodule; The method comprises the following steps: Step 1: Cut off the power transmission circuit after the fault occurs; when a line fault occurs, quickly cut off the DC circuit breakers CB5 and CB6 to terminate the line power transmission, and then disconnect the AC and DC side disconnector CB1 after reclosing and arc extinguishing; Step 2: Adjust system components to construct a distance measurement discharge circuit; control the A-phase bridge arm submodule to be in a cut-off state, and the B-phase and C-phase bridge arm submodules to be in a locked state, and then close the A-phase bridge arm neutral point grounding switch CB2 and the distance measurement side DC circuit breaker CB5; Step 3, control the discharge submodule to be switched on and off; control the A-phase bridge arm discharge submodule SM1 or SM2 to be switched on, connect the submodule capacitor C to the circuit for discharge, and switch off after 5ms; Step 4: Change the loop parameters to continue the current flow; after the discharge continues for 5ms, close the current limiting inductor switching switch CB3 or CB4 to cut off the current limiting inductor L cl , thereby changing the circuit parameters and continuing to discharge until the end; Step 5: Current signal acquisition and fault distance calculation: respectively collect the discharge current I before and after the current limiting inductor is removed in step 4 F1 with I F2 , the current discharge time constant τ is obtained through signal processing, and the fault point distance x and the fault resistance R are further calculated. f At the same time, in order to improve the ranging accuracy, steps 3 to 5 can be repeated to perform multiple measurements to obtain the average value; In step 5, for a single-pole grounding fault, the discharge current I F1 with I F2 It is expressed as: In formulas (1) and (2), I1 and I2 represent the corresponding discharge current amplitudes, t1 represents the discharge submodule removal time, t2 represents the current limiting inductor removal time, τ1 and τ2 represent the corresponding discharge time constants, which are expressed as: In formulas (3) and (4), L arm is the bridge arm inductance, L cl is the line current limiting inductor, R arm is the equivalent resistance of the half-bridge arm in the on-state state, the above are known quantities; R f1 is the fault grounding resistance, L line is the fault section line inductance, R line is the fault section line resistance, the above are unknown quantities; L line With R line It is expressed as: L line =l0x(5) R line =r0x(6) In formulas (5) and (6), l0 and r0 represent the inductance and resistance per unit length of the line, respectively, which are known quantities, and x represents the distance to the fault point, which is an unknown quantity; The Prony algorithm is used to identify the discharge current signal and obtain the discharge time constants τ1 and τ 2， Combine equations (3)-(6) to obtain the fault point distance x and the fault grounding resistance R f1 : Similarly, for a bipolar short-circuit fault, the fault point distance x and the fault resistance R can be similarly obtained. f2 :

2. A DC distribution network fault location method based on MMC submodule multiplexing according to claim 1, characterized in that: The distance measurement scheme requires the configuration of a bridge arm neutral point grounding switch CB2 and line current limiting inductor switching switches CB3 and CB4 in the original system.

3. A DC distribution network fault location method based on MMC submodule multiplexing according to claim 1, characterized in that: In step 2, for a positive grounding fault, the discharge current path is from the neutral grounding point G2 of the A-phase bridge arm to the positive fault grounding point G3 via the A-phase upper bridge arm.

4. A DC distribution network fault location method based on MMC submodule multiplexing according to claim 1, characterized in that: In step 2, for a negative pole grounding fault, the discharge current path is from the neutral grounding point G2 of the A-phase bridge arm to the negative pole fault grounding point G4 via the A-phase lower bridge arm.

5. A DC distribution network fault location method based on MMC submodule multiplexing according to claim 1, characterized in that: In step 2, for a bipolar short circuit fault, the discharge current path passes through the upper and lower bridge arms of phase A, the positive and negative lines, and the short circuit fault resistor R f2 .

6. A DC distribution network fault location method based on MMC submodule multiplexing according to claim 1, characterized in that: In the steps 2 and 3, the submodule removal finger corresponds to the level S up Set to 0, S down Set to 1, the submodule locks the corresponding level S up Set to 0, S down Set to 0, the submodule input refers to the corresponding level S up Set to 1, S down Set to 0.

7. A DC distribution network fault location method based on MMC submodule multiplexing according to claim 1, characterized in that: In the step 3, for positive pole grounding fault and negative pole grounding fault, the submodules put into operation are SM1 and SM2 respectively, and for bipolar short circuit fault, the submodule put into operation can select SM1 or SM2.

8. A DC distribution network fault location method based on MMC submodule multiplexing according to claim 1, characterized in that: In the step 4, for the positive pole grounding fault and the negative pole grounding fault, the current limiting inductor switches are CB3 and CB4 respectively, and for the bipolar short circuit fault, the current limiting inductor switch can be CB3 or CB4.

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