A fault location method based on pre-charging type DC circuit breaker topology reuse

Abstract

The present application relates to the field of direct current distribution network, and aims to provide a fault positioning method based on pre-charging type direct current circuit breaker topology reuse. The method comprises using an improved pre-charging type circuit breaker topology in a direct current distribution network system, adding three switch circuits; when a fault current causes breaking, a charging device is used to charge the pre-charging capacitor; the opening and closing operation of the switch circuit is performed according to different short-circuit fault types, so as to construct different injection loops between the pre-charging capacitor and the fault point to realize the injection of the ranging current; the least square method is used to solve the oscillation frequency and the attenuation coefficient in the ranging current, and the fault distance of the ranging point is obtained. The present application only needs to make a simple modification to the existing circuit breaker topology, and the modification cost is very low; the problems such as the questionable feasibility of using a converter to construct a direct current voltage injection can be effectively overcome; the operation is simple, easy to design, and can be positioned repeatedly.

Description

Technical Field

[0001] This invention belongs to the field of DC distribution networks, specifically a fault location method based on the topology reuse of pre-charged DC circuit breakers. Background Technology

[0002] With the continuous depletion of fossil fuels globally, carbon emissions are constantly rising. In order to effectively mitigate the global greenhouse effect, distributed generation technology has emerged and has received widespread attention and importance from academia and industry.

[0003] DC distribution networks, as a platform for flexible access to distributed energy resources, offer advantages such as high energy conversion efficiency, low transmission line losses, and the ability to meet the needs of sensitive DC loads. With technological advancements, DC distribution networks will become the primary network connection method for urban power consumption. However, when a short-circuit fault occurs in a DC distribution network, the fault characteristics are characterized by a short duration, rapid current rise, and high overcurrent peaks. If not suppressed, these faults can damage the power electronic equipment within the distribution network. Therefore, the protection system must promptly trigger DC circuit breakers to interrupt the fault current to ensure the safe operation of the power grid. To avoid excessive current surges that could damage converters and other equipment, the overall fault clearing time must be within 3–5 ms. However, due to the short duration of the fault transient characteristics, the effective information provided during the fault period is limited, making fault location in DC distribution networks difficult.

[0004] Existing fault location schemes for DC distribution networks are derived from AC distribution networks, and the mainstream schemes can be divided into three categories: traveling wave reflection method, mechanism analysis method, and harmonic injection method. The traveling wave ranging method is relatively mature, but its application is limited by the characteristics of DC distribution networks, such as the interlacing of cable branches and the short length of cables. The mechanism analysis method is reliable in principle and requires little computation; however, it provides limited information under transient fault conditions and is easily affected by grid topology coupling and system operating noise, resulting in limited location accuracy. The harmonic injection method is simple to operate, easy to design, and can be repeatedly used for location, effectively overcoming the ranging error interference caused by interlacing cables, topology coupling, and system operating noise in DC distribution networks. Compared to the other methods, it is more suitable for DC distribution network systems, but it has drawbacks such as the need for additional equipment or questionable feasibility of the injection strategy.

[0005] Therefore, proposing a fault location method suitable for DC distribution networks that is fast, accurate, highly reliable, and reusable is of great significance for supplementing the current fault location schemes in DC distribution networks and promoting the rapid development and safety assurance of DC distribution networks. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a fault location method based on the topology multiplexing of pre-charged DC circuit breakers.

[0007] To solve the technical problem, the solution of the present invention is:

[0008] A fault location method based on pre-charged DC circuit breaker topology multiplexing is provided, including the following steps:

[0009] (1) Use of an improved pre-charge circuit breaker topology in DC distribution network systems:

[0010] The pre-charged circuit breaker topology includes a current-carrying branch, a transfer branch, and a power-dissipating branch arranged in parallel. In the current-carrying branch, the freewheeling diode D and the third switching circuit SW3 are connected in series and then in parallel with the first thyristor T1. The transfer branch includes a pre-charged capacitor C connected in series. in Inductor L in Resistance R in The second thyristor T2, one end of the first switching circuit SW1 is grounded and the other end is connected to the pre-charge capacitor C. in The energy-consuming branch includes a metal oxide varistor (MOV), one end of the second switching circuit SW2 is grounded and the other end is connected to the other pole of the DC bus; a current-limiting reactor is provided on the DC bus, and during normal operation, the first switching circuit SW1 and the second switching circuit SW2 remain off and the third switching circuit SW3 remains closed.

[0011] (2) When a fault current causes disconnection, the charging device charges the pre-charge capacitor C. in Charging is performed; switching circuits are opened and closed according to different short-circuit fault types to construct a pre-charge capacitor C. in Different injection loops between the fault point and the fault point are used to inject ranging current;

[0012] (3) The current is sampled during the current injection process, and the fault location circuit equation is established based on the constructed injection circuit. The sampling current is fitted by the nonlinear least squares method, and the attenuation coefficient and oscillation frequency of the injection circuit are calculated.

[0013] (4) Based on the calculated attenuation coefficient and oscillation frequency, further calculate the magnitude of the inductive component of the fault point distance from the precharge circuit breaker; combine the parameters of the current limiting reactor and the impedance per unit length of the cable to calculate the fault distance;

[0014] (5) Repeat steps (2)-(4) multiple times to calculate the average value of the obtained fault distance data in order to reduce the ranging error.

[0015] Compared with the prior art, the beneficial effects of the present invention are:

[0016] (1) This invention achieves active injection of ranging current in DC distribution network scenario by reusing the topology forced converter branch in the pre-charged DC circuit breaker, and uses the least squares method to solve for the oscillation frequency and attenuation coefficient in the ranging current to obtain the fault distance of the ranging point.

[0017] Compared to existing active injection methods that require additional equipment or converter devices for active injection, the solution of this invention only requires a simple modification to the existing pre-charge DC circuit breaker topology, adding only three switching circuits in hardware, resulting in extremely low modification costs. Therefore, this invention is applicable not only to newly constructed DC distribution network systems but also to the retrofitting and utilization of existing DC distribution network systems.

[0018] (2) Compared with the active injection method in the prior art, the solution of the present invention can effectively overcome the problem that the feasibility of using converter to construct DC voltage injection is questionable.

[0019] (3) The present invention is simple to operate, easy to design, and can be repeatedly positioned.

[0020] (4) The present invention can effectively overcome problems such as insufficient positioning information, intertwined power distribution network cables, topological coupling, and ranging error interference caused by system operating noise. Attached Figure Description

[0021] Figure 1 This is for the existing pre-charged DC circuit breaker topology.

[0022] Figure 2 This is the topology of the modified pre-charged DC circuit breaker.

[0023] Figure 3 A flowchart for fault location in a pre-charged DC circuit breaker topology multiplexing.

[0024] Figure 4 This is a single-pole grounding fault location circuit.

[0025] Figure 5 This is an inter-pole fault ranging circuit.

[0026] Figure 6 This is for comparing the ranging current with the fitted current. Detailed Implementation

[0027] The present invention will now be described in further detail with reference to the accompanying drawings.

[0028] I. Modifying the device topology

[0029] Figure 1It is a pre-charged DC circuit breaker topology in the prior art, which includes parallel-arranged current-carrying branches, transfer branches and energy-dissipating branches. The current-carrying branches are composed of mechanical switches or power electronic devices, the transfer branches are composed of pre-charged capacitors and power electronic switches, and the energy-dissipating branches are composed of ZnO surge arresters or other metallic oxide resistors.

[0030] To realize a fault location method for circuit breaker topology reuse in DC distribution network systems, this invention makes the following improvements to the existing pre-charged circuit breaker topology (e.g.) Figure 2 As shown):

[0031] The pre-charged circuit breaker topology includes a current-carrying branch, a transfer branch, and a power-dissipating branch arranged in parallel. In the current-carrying branch, the freewheeling diode D and the third switching circuit SW3 are connected in series and then in parallel with the first thyristor T1. The transfer branch includes a pre-charged capacitor C connected in series. in Inductor L in Resistance R in The second thyristor T2, one end of the first switching circuit SW1 is grounded and the other end is connected to the pre-charge capacitor C. in The energy-consuming branch includes a metal oxide varistor (MOV), one end of the second switching circuit SW2 is grounded and the other end is connected to the other pole of the DC bus; a current-limiting reactor is provided on the DC bus, and during normal operation, the first switching circuit SW1 and the second switching circuit SW2 remain off and the third switching circuit SW3 remains closed.

[0032] Existing pre-charged DC circuit breaker topologies can only interrupt fault current. This invention, by modifying them, enables active injection-type fault location using an added switching circuit (optionally a relay).

[0033] II. Distance to locate the fault point

[0034] Based on the above improvements, this invention proposes a fault location method for pre-charged DC circuit breakers with reusable topologies for single-pole grounding faults or inter-pole short-circuit faults. The processing flow is as follows: Figure 3 As shown. Depending on the fault type (single-pole grounding fault or inter-pole short-circuit fault), the following two handling methods can be used:

[0035] (a) Cases of single-pole grounding faults

[0036] 1. When a fault current causes a disconnection, the charging device charges the pre-charge capacitor C. in Charging is performed; switching circuits are opened and closed according to different short-circuit fault types to construct a pre-charge capacitor C. in Different injection loops between the fault point and the fault point are used to inject ranging current;

[0037] (1) If no fault occurs, the circuit breaker will operate normally;

[0038] (2) If a short-circuit fault is confirmed, interrupt the fault current and wait for the transient energy absorbed by the circuit breaker topology to be completely dissipated; then disconnect the third switching circuit SW3; and use the charging device to charge the pre-charge capacitor C. in Charging begins and stops once the voltage reaches the set value.

[0039] (3) Close the first switch circuit SW1 and turn on the second thyristor T2 to form a single-pole ranging circuit to inject harmonic current into the fault point.

[0040] 2. During the current injection process, the current is sampled, and the fault location circuit equation is established based on the constructed injection circuit. The sampled current is fitted using the nonlinear least squares method to calculate the attenuation coefficient and oscillation frequency of the injection circuit.

[0041] (1) Single-pole grounding fault location injection circuit as follows Figure 4 As shown, by modeling the loop using Kirchhoff's laws, the corresponding ranging loop equation can be constructed:

[0042]

[0043] In the formula: L0 and R0 are the inductance and resistance per unit length of the cable, respectively. fcl L is the inductance value of the current-limiting reactor. in Let C be the internal reactance of the current-limiting branch of the circuit breaker, x be the distance from the fault point to the circuit breaker, and C be the internal reactance of the current-limiting branch of the circuit breaker. in i is the capacitance value of the pre-charge capacitor. L R is the ranging circuit current, t is the ranging time (the time of ranging current injection), and R is the current in the ranging circuit. f R is the transition resistance. in This refers to the internal reactance of the current-limiting branch of the circuit breaker.

[0044] Solving equation (1-1) yields:

[0045]

[0046]

[0047]

[0048]

[0049] In the above formulas, V c (0) represents the pre-charge capacitor C. in The charging voltage value, ω0 is the damped oscillation frequency, α is the injection circuit attenuation coefficient, x is the distance from the fault point to the circuit breaker, L0 and R0 are the inductance and resistance per unit length of the DC bus cable, and Lfcl L is the inductance value of the current-limiting reactor, t is the distance-measuring current injection time, and L is the distance-limiting current injection time. fcl R is the inductance value of the current-limiting reactor. f For the transition resistance; e is the natural constant, L in R is the internal reactance of the current-limiting branch of the circuit breaker. in C is the internal reactance of the current-limiting branch of the circuit breaker. in This is the capacitance value of the pre-charge capacitor.

[0050] (2) To ensure the accuracy of the subsequent nonlinear least squares fitting results, a minimum fitting error constraint is set:

[0051]

[0052] In the above formulas, i d For the sampling current, t i is the discrete sampling time, and n is the number of sampling current points.

[0053] (3) The attenuation coefficient α and damping oscillation frequency ω0 of the RLC circuit corresponding to the injection circuit are obtained by using the nonlinear least squares method.

[0054] 3. Based on the calculated attenuation coefficient and oscillation frequency, further calculate the magnitude of the inductive component of the fault point distance from the pre-charged circuit breaker; combine the parameters of the current-limiting reactor and the impedance per unit length of the cable to calculate the fault distance;

[0055] (1) Calculate the overall inductance L in the injection circuit using formula (4). all :

[0056]

[0057] (2) Inject the overall inductance L into the circuit all Subtract the inductance value L of the current-limiting reactor fcl and the stray inductance L inside the circuit breaker in This yields the inductance difference between the fault point and the injection point; the fault location is then calculated using the following formula:

[0058]

[0059] In the above formula, d xi This refers to the measured distance between the circuit breaker and the fault point obtained in the i-th measurement.

[0060] 4. To avoid the influence of random errors on the distance measurement accuracy, repeat steps (2) to (4) multiple times, and average the distance values ​​obtained from each measurement according to the following formula to obtain the final average measured distance:

[0061]

[0062] Where d xi Let d be the distance measured in the i-th measurement. x To measure the average fault distance, p represents the number of measurements.

[0063] (ii) Cases of inter-electrode short-circuit faults

[0064] 1. When a fault current causes a disconnection, the charging device charges the pre-charge capacitor C. in Charging is performed; the switching circuit is opened and closed to construct the pre-charge capacitor C. in An injection circuit is established between the fault point and the location to inject ranging current.

[0065] (1) If no fault occurs, the circuit breaker will operate normally;

[0066] (2) If a short-circuit fault is confirmed, interrupt the fault current and wait for the transient energy absorbed by the circuit breaker topology to be completely dissipated; then disconnect the third switching circuit SW3; and use the charging device to charge the pre-charge capacitor C. in Charging begins and stops once the voltage reaches the set value.

[0067] (3) Close the second switch circuit SW2 and turn on the second thyristor T2 to form an inter-electrode ranging circuit to inject harmonic current into the fault point.

[0068] 2. During the current injection process, the current is sampled, and the fault location circuit equation is established based on the constructed injection circuit. The sampled current is fitted using the nonlinear least squares method to calculate the attenuation coefficient and oscillation frequency of the injection circuit.

[0069] (1) Inter-electrode short-circuit fault location injection circuit as follows Figure 5 As shown, by modeling the loop using Kirchhoff's laws, the corresponding ranging loop equation can be constructed:

[0070]

[0071] Solving equation (1-2) yields the following injection current equation;

[0072]

[0073]

[0074]

[0075]

[0076] In the above formulas, V c (0) represents the pre-charge capacitor C. inThe charging voltage value, ω0 is the damped oscillation frequency, α is the injection circuit attenuation coefficient, x is the distance from the fault point to the circuit breaker, L0 and R0 are the inductance and resistance per unit length of the DC bus cable, and L fcl L is the inductance value of the current-limiting reactor, t is the distance-measuring current injection time, and L is the distance-limiting current injection time. fcl R is the inductance value of the current-limiting reactor. f For the transition resistance; e is the natural constant, L in R is the internal reactance of the current-limiting branch of the circuit breaker. in C is the internal reactance of the current-limiting branch of the circuit breaker. in This is the capacitance value of the pre-charge capacitor.

[0077] (2) To ensure the accuracy of the subsequent nonlinear least squares fitting results, a minimum fitting error constraint is set:

[0078]

[0079] In the above formulas, i d To sample and obtain the injected current, t i Where n is the discrete sampling time, and n is the number of sampling current points.

[0080] (3) The attenuation coefficient α and damping oscillation frequency ω0 of the RLC circuit corresponding to the injection circuit are obtained by using the nonlinear least squares method.

[0081] 3. Based on the calculated attenuation coefficient and oscillation frequency, further calculate the magnitude of the inductive component of the fault point distance from the pre-charged circuit breaker; combine the parameters of the current-limiting reactor and the impedance per unit length of the cable to calculate the fault distance;

[0082] (1) Calculate the overall inductance L in the injection circuit using formula (9). all :

[0083]

[0084] (2) Inject the overall inductance L into the circuit all Subtract the inductance value L of the current-limiting reactor fcl and the stray inductance L inside the circuit breaker in This yields the inductance difference between the fault point and the injection point; the fault location is then calculated using the following formula:

[0085]

[0086] In the above formula, d xi This refers to the measured distance between the circuit breaker and the fault point obtained in the i-th measurement.

[0087] 4. To avoid the influence of random errors on the distance measurement accuracy, repeat steps (2) to (4) multiple times, and average the distance values ​​obtained from each measurement according to the following formula to obtain the final average measured distance:

[0088]

[0089] Where d xi Let d be the distance measured in the i-th measurement. x To measure the average fault distance, p represents the number of measurements.

[0090] Figure 6 In a low-voltage DC distribution network at 375V, where a single-pole grounding fault is 100m from the circuit breaker, this method was used for distance measurement. The measured distance was found to be 94.4m, as shown in the figure. The measurement method of this invention exhibits good overall fitting performance and high accuracy.

Claims

1. A fault location method based on topology multiplexing of a pre-charged DC circuit breaker, characterized in that, Includes the following steps: (1) Use of an improved pre-charge circuit breaker topology in DC distribution network systems: The pre-charged circuit breaker topology includes a current-carrying branch, a transfer branch, and a power-dissipating branch arranged in parallel. In the current-carrying branch, the freewheeling diode D and the third switching circuit SW3 are connected in series and then in parallel with the first thyristor T1. The transfer branch includes a pre-charged capacitor C connected in series. in Inductor L in Resistance R in The second thyristor T2, one end of the first switching circuit SW1 is grounded and the other end is connected to the pre-charge capacitor C. in The energy-consuming branch includes a metal oxide varistor (MOV), one end of the second switching circuit SW2 is grounded and the other end is connected to the other pole of the DC bus; a current-limiting reactor is provided on the DC bus, and during normal operation, the first switching circuit SW1 and the second switching circuit SW2 remain off and the third switching circuit SW3 remains closed. (2) When a fault current causes disconnection, the charging device charges the pre-charge capacitor C. in Charging is performed; switching circuits are opened and closed according to different short-circuit fault types to construct a pre-charge capacitor C. in Different injection loops between the fault point and the fault point are used to inject ranging current; (3) During the current injection process, the current is sampled, and the fault location circuit equation is established based on the constructed injection circuit. The sampled current is fitted using the nonlinear least squares method to calculate the attenuation coefficient and oscillation frequency of the injection circuit. Specifically, this includes: (3.1) The injection current equations are obtained by modeling different ranging circuits using Kirchhoff's laws. For a single-pole ground fault, the injection current equation is: in, For inter-electrode short-circuit faults, the injection current equation is: in In the above formulas, V c (0) represents the pre-charge capacitor C. in The charging voltage value, ω0 is the damped oscillation frequency, α is the injection circuit attenuation coefficient, x is the distance from the fault point to the circuit breaker, L0 and R0 are the inductance and resistance per unit length of the DC bus cable, and L fcl R is the inductance value of the current-limiting reactor, t is the distance-measuring current injection time, and R is the distance-limiting current injection time. f For the transition resistance; e is the natural constant, L in R is the internal reactance of the current-limiting branch of the circuit breaker. in C is the internal reactance of the current-limiting branch of the circuit breaker. in This refers to the capacitance value of the pre-charge capacitor. (3.2) To ensure the accuracy of the subsequent nonlinear least squares fitting results, a minimum fitting error constraint is set: In the above formulas, i d For sampling and obtaining the injected current in step 2, t i The discrete sampling time in step 2 is n, where n is the number of sampling current points. (3.3) The attenuation coefficient α and damping oscillation frequency ω0 of the RLC circuit corresponding to the injection circuit are obtained by using the nonlinear least squares method. (4) Based on the calculated attenuation coefficient and oscillation frequency, further calculate the magnitude of the inductive component of the fault point distance from the precharge circuit breaker; combine the parameters of the current limiting reactor and the impedance per unit length of the cable to calculate the fault distance; (5) Repeat steps (2)-(4) multiple times to calculate the average value of the obtained fault distance data in order to reduce the ranging error.

2. The method according to claim 1, characterized in that, Step (2) specifically includes: (2.1) If no fault occurs, the circuit breaker will operate normally; if a short-circuit fault is determined to have occurred, the fault current will be interrupted, and the transient energy absorbed by the circuit breaker topology will be completely dissipated; then the third switching circuit SW3 will be disconnected; the pre-charge capacitor C will be charged by the charging device. in Charging begins and stops once the voltage reaches the set value. (2.2) For a single-pole grounding fault, close the first switch circuit SW1 and turn on the second thyristor T2 to form a single-pole ranging circuit to inject harmonic current into the fault point; for an inter-pole short-circuit fault, close the second switch circuit SW2 and turn on the second thyristor T2 to form an inter-pole ranging circuit to inject harmonic current into the fault point.

3. The method according to claim 1, characterized in that, Step (4) specifically includes: (4.1) Calculate the overall inductance L in the injection circuit all : For a single-pole ground fault, the following formula is used for calculation: For inter-electrode short-circuit faults, the following formula is used for calculation: (4.2) Inject the overall inductance L into the circuit all Subtract the inductance value L of the current-limiting reactor fcl and the stray inductance L inside the circuit breaker in This yields the inductance difference between the fault point and the injection point; the fault location is then calculated using the following formula: In the above formula, d xi This refers to the measured distance between the circuit breaker and the fault point obtained in the i-th measurement.

4. The method according to claim 1, characterized in that, To avoid the influence of random errors on the distance measurement accuracy, steps (2) to (4) are repeated multiple times, and the distance values ​​obtained from each measurement are averaged according to the following formula to obtain the final average measured distance: Where d xi Let d be the distance measured in the i-th measurement. x To measure the average fault distance, p represents the number of measurements.

5. The method according to claim 1, characterized in that, The first switching circuit SW1, the second switching circuit SW2, and the third switching circuit SW3 are all relays.

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