Configuration method of low-voltage direct-current distribution network circuit breaker

Abstract

The invention provides a low-voltage direct-current distribution network circuit breaker configuration method, which comprises the following steps of: calculating a reliability index and / or an economic index of a system when different circuit breakers or different circuit breaker combinations are adopted under a segmented bus interconnection network topology; the circuit breaker or the combination of the circuit breakers is selected according to the requirements of one or more reliability indexes and / or economic indexes; the reliability index comprises a system average fault power failure frequency and a fault power failure average insufficient power supply quantity; the economic indexes comprise power failure loss caused by faults, operation loss of the circuit breaker and fixed cost.

Description

Technical Field

[0001] The present invention relates to the technical fields of distributed photovoltaic, energy storage system, regional autonomy, etc., and in particular to a method for configuring a low-voltage direct current distribution network circuit breaker. Background Art

[0002] Unlike medium-voltage DC transmission and distribution systems, low-voltage DC systems have more diverse types and characteristics of load equipment. This requires low-voltage DC systems to provide more flexible voltage level selection to meet the access needs of equipment with different power and voltage levels. At present, the topology types of low-voltage DC systems can be divided into radial type, "hand-in-hand" type and ring network type, and on this basis, hierarchical structure and redundant structure are derived.

[0003] The fault ride-through capability of the DC side is similar to the low voltage ride-through in the AC system. When a fault occurs on the DC side, each node does not disconnect from the DC distribution network before the DC voltage drops to the threshold, and injects power into the grid to promote the recovery of the DC voltage, thereby reducing the number of restarts of the DC distribution network after the fault, so as to improve the power supply reliability. With fault removal as the dividing point, low voltage ride-through is divided into two stages: in the first stage, the voltage drops under the action of the fault. The main design goal of this stage is to reduce the magnitude of the DC voltage drop through hardware designs such as current limiters, circuit breakers and converter topology structures to prevent it from falling below the decoupling threshold; the control goal of the second stage is to restore the DC voltage to a steady-state value.

[0004] For converters that do not have the ability to ride through short-circuit faults, since they contain many controllable switching devices, when the output current is greater than the rated current, the overcurrent protection inside the converter will operate to isolate the converter from the fault. The overcurrent protection of the converter is similar to the tripping characteristics of the DC circuit breaker, and both conform to the inverse time characteristics. However, since the response speed of the switch tube is faster than the tripping speed of the DC circuit breaker, when a branch that is electrically closer to the converter fails, the overcurrent protection will be activated first, and the selectivity of the relay protection cannot be met. The fault cannot be selectively removed, and manual line selection is required later, which increases the duration of the fault. Therefore, the short-circuit fault protection of the DC system needs to comprehensively consider the coordination between the fault ride-through capability of the converter and the circuit breaker.

[0005] Low voltage DC circuit breakers are the main equipment of low voltage DC power supply and distribution systems. Existing low voltage DC circuit breakers can also be divided into the following three categories: mechanical, solid-state and hybrid.

[0006] Mechanical DC circuit breakers use artificial zero crossing, magnetic arc blowing to increase arc voltage, arc induction and other methods to extinguish arcs. The artificial zero crossing method uses different principles such as self-excited oscillation, reverse pre-charging, coupled inductance, etc. to create a reverse current so that the current crosses the zero point and extinguishes the arc. The magnetic arc blowing method uses the combined effect of the magnetic field and the intermediate medium to introduce the arc into the arc extinguishing chamber composed of metal grids, increase the arc length, and finally make the arc voltage greater than the power supply voltage to achieve arc extinguishing. The arc induction method uses the change in the air gap length before and after the action to bypass the arc and achieve arc extinguishing. Mechanical DC circuit breakers have the advantages of low cost, low operating loss, and high reliability, but the breaking time is relatively long.

[0007] All-solid-state DC circuit breakers are power electronic device switches. The devices currently being studied are mainly emitter turn-off thyristors (ETO), gate turn-off thyristors (GTO), and insulated gate bipolar transistors (IGBT). Solid-state circuit breakers usually use power electronic devices to break the main circuit and bypass the current to the energy-consuming branch. Existing research mainly focuses on topological structure design to make the circuit breaker have better performance. Due to the conduction voltage drop of power electronic devices themselves, the operating loss and heat generation of solid-state circuit breakers are relatively large. Also because of the excellent control performance of power electronic devices, their operating speed is much higher than that of mechanical DC circuit breakers. Solid-state circuit breakers are characterized by strong controllability and rapidity, and can break current at the microsecond level, but they have high power consumption and are expensive.

[0008] The hybrid DC circuit breaker is a combination of a mechanical DC circuit breaker and a solid-state DC circuit breaker, combining the static characteristics of a mechanical circuit breaker (low conduction loss) and the dynamic characteristics of a fully solid-state circuit breaker (fast and controllable breaking). It uses a mechanical switch as the conducting part, and uses power electronic branches and energy-consuming branches to break the current. Hybrid DC circuit breakers usually use zero voltage switching (the voltage across the mechanical switch is zero) or zero current switching technology to achieve the switching between the mechanical switch and the solid-state circuit breaker from normal operation to fault breaking. Whether in terms of breaking time or operating loss, hybrid DC circuit breakers have excellent performance, but are relatively immature, complex in structure, and technically difficult, and do not have advantages in cost, volume, and reliability.

[0009] Among DC line faults, line break faults are less destructive to the system and are easy to identify. Ground faults are related to the system grounding method and are realized through insulation detection and residual current protection. Among them, inter-pole short-circuit faults have the largest current and are the most difficult to protect.

[0010] From the perspective of action speed, solid-state circuit breakers and hybrid circuit breakers can quickly cut off faults during the capacitor discharge phase after an inter-pole short circuit under a reasonable protection algorithm due to their excellent action performance. Therefore, if a low-voltage DC system is equipped with solid-state or hybrid DC circuit breakers, the impact on non-fault branch equipment can be minimized; if all mechanical DC circuit breakers are configured, even if the fault is correctly cut off, some non-fault branches may be temporarily powered off and quickly restarted. Although this short-term power outage has little impact on the reliability index of power shortage, it has a greater impact on the reliability index of power outage frequency.

[0011] Although solid-state circuit breakers and hybrid circuit breakers have absolute advantages in terms of speed, this is not the case in terms of the reliability and economy of the circuit breakers themselves. Since solid-state circuit breakers and hybrid circuit breakers have complex structures and contain power electronic devices, they have problems such as high cost and relatively poor reliability. In addition, the protection scheme using solid-state circuit breakers or hybrid circuit breakers has higher requirements for each link in order to quickly detect and eliminate faults, which will also increase the subsequent maintenance burden. Summary of the invention

[0012] In view of this, in view of the problem and gap that the prior art cannot accurately evaluate the configuration and selection of circuit breakers in a low-voltage DC distribution network through reliability and economy, the present invention provides a low-voltage DC distribution network circuit breaker configuration method taking reliability and economy into consideration.

[0013] The present invention specifically adopts the following technical solutions:

[0014] A method for configuring circuit breakers in a low-voltage DC distribution network, characterized in that: under a segmented busbar interconnection network topology, system reliability indicators and / or economic indicators when different circuit breakers or different combinations of circuit breakers are used are calculated, so as to select a circuit breaker or a combination of circuit breakers according to one or more reliability indicator requirements and / or economic indicators;

[0015] The reliability index includes the average frequency of power outage due to system failure and the average power shortage due to power outage due to system failure;

[0016] The economic indicators include power outage losses caused by faults, operating losses of circuit breakers and fixed costs.

[0017] Furthermore, the calculation of the reliability index takes into account the coordination between the circuit breakers used in each branch, the fault ride-through capability of the converter and the breaking characteristics of various low-voltage DC distribution network circuit breakers.

[0018] Furthermore, the calculation basis of the average fault power outage frequency of the system is: line fault rate, circuit breaker failure rate and the number of users and / or the distribution of users in each branch;

[0019] The calculation basis of the average power shortage due to power outage caused by fault is: line failure rate, circuit breaker failure rate, average repair time of line fault, power supply power of this branch, temporary system pressure loss due to long time for circuit breaker to clear fault and short power outage time suffered by other non-fault users, total system power, average repair time of circuit breaker fault and power supply power of all branches on the DC bus where this branch is located and / or power supply power of all branches on the DC bus that is not directly connected to this branch.

[0020] Furthermore, the power outage loss includes the average power outage electricity loss and the average power outage event loss; the economic loss caused by the average power outage power shortage is calculated based on the expected power shortage of the line failure multiplied by the electricity price of the corresponding power outage time; the average power outage event loss is calculated based on the number of power outage users and the single event power outage loss that distinguishes between long and short power outages.

[0021] Furthermore, the expected power shortage due to the line fault is calculated based on: line failure rate, circuit breaker failure rate, average repair time of line fault, power supply of this branch, temporary system pressure loss due to long time for circuit breaker to clear fault and short power outage time suffered by other non-faulty users, total system power, average repair time of circuit breaker fault, and power supply of all branches on the DC bus where this branch is located and / or power supply of all branches on the DC bus that is not directly connected to this branch.

[0022] Furthermore, the calculation of the power outage loss of a single event that distinguishes between long and short power outages is based on the coordination between the circuit breakers used in each branch, the fault ride-through capability of the converter and the breaking characteristics of various types of low-voltage DC distribution network circuit breakers and is based on: line failure rate, circuit breaker failure rate, power outage losses unrelated to the amount of electricity caused to users by a single long power outage, power outage losses unrelated to the amount of electricity caused to users by a single short power outage, the number of users, the number of users of all branches on the DC bus where this branch is located and / or the number of users of all branches on the DC bus that is not directly connected to this branch.

[0023] Furthermore, the operating loss of the circuit breaker is calculated by the annual average power corresponding to each primary branch, the annual average operating time corresponding to each primary branch, the average operating electricity price, the loss rate of the circuit breaker, and the annual average transmission power between segmented buses and the annual average operating time of the bus tie circuit breaker.

[0024] Furthermore, the fixed cost includes the cost of the fault isolation device itself and the corresponding protection device cost, which is calculated by converting it to the first year.

[0025] And, an electronic device, comprising a memory, a processor and a computer program stored in the memory and executable on the processor, characterized in that when the processor executes the program, the steps of the low-voltage DC distribution network circuit breaker configuration method as described above are implemented.

[0026] A non-transitory computer-readable storage medium having a computer program stored thereon, characterized in that when the computer program is executed by a processor, the steps of the low-voltage DC distribution network circuit breaker configuration method as described above are implemented.

[0027] Compared with the prior art, the present invention and its preferred solution realize quantitative calculation of reliability and economy of low-voltage DC distribution network circuit breaker configuration selection, provide a scientific basis for low-voltage DC distribution network circuit breaker configuration, ensure that low-voltage DC distribution network circuit breaker configuration meets safety and cost requirements to the maximum extent, and greatly improve the efficiency of related work. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments:

[0029] Figure 1 A schematic diagram of different circuit breaker configurations for the primary branch circuit according to an embodiment of the present invention;

[0030] Figure 2 It is a schematic diagram of the configuration of upper and lower branch circuit breakers according to an embodiment of the present invention;

[0031] Figure 3 It is a schematic diagram of the configuration of a sectional bus circuit breaker according to an embodiment of the present invention;

[0032] Figure 4 A schematic diagram of a branch circuit considering a circuit breaker type under a typical topology of segmented busbar interconnection according to an embodiment of the present invention;

[0033] Figure 5 This is a schematic diagram of the DC distribution network topology of a local power building. DETAILED DESCRIPTION

[0034] Hereinafter, the specific embodiments of the present application will be described in detail with reference to the accompanying drawings. According to these detailed descriptions, those skilled in the art can clearly understand the present application and can implement the present application. Without violating the principles of the present application, the features in the various embodiments can be combined to obtain new implementations, or certain features in certain embodiments can be replaced to obtain other preferred implementations.

[0035] In order to make the features and advantages of the present invention more clearly understood, the following embodiments are specifically described in detail with reference to the accompanying drawings:

[0036] Reliability calculation of the system under different configurations

[0037] (1) Reliability indicators of low voltage DC distribution network

[0038] With reference to DL / T 836.12016 Power Supply Reliability Evaluation Procedure for Power Supply System Part 1: General Requirements, when designing a circuit breaker configuration scheme, the embodiment of the present invention first selects two reliability indicators related to faults and independent of each other: the system average fault power outage frequency SAIFI-F and the average power shortage due to fault power outage AENT-F.

[0039] The system average fault outage frequency SAIFI-F (times / household) refers to the average number of fault outages for power supply system users during the statistical period. The calculation method is:

[0040]

[0041] The average power shortage AENT-F (kWh / time) refers to the average amount of power shortage per power outage during the statistical period. The calculation method is:

[0042]

[0043] (2) Analysis of the coordination of breaking characteristics of different types of circuit breakers

[0044] 1) Coordination of different types of circuit breakers between adjacent branches

[0045] When two primary branches use mechanical circuit breakers and solid-state circuit breakers as fault isolation devices respectively, the fault branch diagram is as follows: Figure 1 As shown in FIG. 1 , due to the difference in action removal time, a line fault will not only trigger the protection of the faulty branch itself, but also affect the protection of other branches.

[0046] Assumptions Figure 1 Both circuit breakers are configured with protection algorithms that match themselves. For the fault in this branch, the solid-state circuit breaker or hybrid circuit breaker can quickly cut off the fault during the capacitor discharge process, while the mechanical circuit breaker often cuts off the fault after the impact current process. Specifically, if an inter-pole fault occurs at the F1 position, even if the molded case circuit breaker operates correctly, the entire system will still experience a complete capacitor discharge shock and trigger the protection of the solid-state circuit breaker of the adjacent branch. The non-fault branch must be restarted after reclosing. If an inter-pole fault occurs at the F2 position, the solid-state circuit breaker or hybrid circuit breaker can quickly cut off the fault during the rising stage of the capacitor discharge impact current, and the protection of the non-fault branch will not operate.

[0047] 2) Cooperation between the upper and lower levels on the first and second level branches

[0048] Taking mechanical circuit breakers and solid-state circuit breakers as two types of fault isolation devices, there are two configuration options for each of the upper and lower branches, and there are four situations in total, namely: (a) both the upper and lower branches are equipped with mechanical circuit breakers, (b) the upper branch is equipped with mechanical circuit breakers and the lower branch is equipped with solid-state circuit breakers, (c) the upper branch is equipped with solid-state circuit breakers and the lower branch is equipped with mechanical circuit breakers, (d) both the upper and lower branches are equipped with solid-state circuit breakers. The fault diagram is as follows Figure 2 shown.

[0049] (a) When mechanical circuit breakers are configured at both the upper and lower levels, the time difference between the upper and lower levels can make the upper level protection serve as the backup protection of the lower level, and ensure that the lower level protection does not cross-level action when the lower level protection is correctly operated. However, due to the inherent slow action speed of mechanical circuit breakers, whether the inter-pole short-circuit fault of the upper branch or the lower branch will inevitably cause the system voltage to drop temporarily or even cause some equipment to be disconnected from the grid while being correctly removed, so the user side will experience a short-term fault power outage. If there is a small probability that the circuit breaker refuses to operate, the number of users who experience long-term power outages will be further increased from the users of the faulty branch.

[0050] (b) When the upper branch is equipped with a mechanical circuit breaker and the lower branch is equipped with a solid-state circuit breaker, the difference in the operating speed of the circuit breakers themselves can also ensure the selectivity of the upper and lower protections under a reasonable protection algorithm. Different from situation a, when the lower branch is short-circuited, if the lower solid-state circuit breaker operates correctly, the system voltage drops slightly, the non-fault branch equipment can ensure continuous operation, and only the fault branch itself will be out of power for a long time in the entire system; if the lower solid-state circuit breaker refuses to operate and the upper mechanical circuit breaker operates correctly as a backup protection, the users who will experience a long power outage will be expanded to all users in the same upper branch as the fault branch, and users other than this part will experience a short fault power outage due to the long fault removal time; if both the upper and lower protections refuse to operate, all users in the system will experience a long fault power outage.

[0051] (c) When the upper branch is equipped with a solid-state circuit breaker and the lower branch is equipped with a mechanical circuit breaker, if a short circuit occurs in the lower branch and both the upper and lower protections do not refuse to operate, the upper solid-state circuit breaker will operate before the lower mechanical circuit breaker and fail to meet the protection selectivity. Manual line selection is required to complete local fault finding. However, at this time, for the part other than the upper branch to which the low-voltage DC system fault belongs, the equipment can operate continuously without experiencing a short power outage. If a short circuit occurs in the lower branch and the lower protection refuses to operate but the upper protection does not, the users who experience a long power outage are also all users in the same upper branch as the faulty branch, and users other than this part will experience a short fault power outage due to the long fault removal time.

[0052] (d) When both the upper and lower levels are equipped with solid-state circuit breakers, due to the excellent operating speed of solid-state circuit breakers, considering the three situations of correct operation of the lower-level protection, failure of the lower-level protection to operate and correct operation of the upper-level backup protection, and failure of both the upper and lower-level protections, the fault scope and users with long-term power outages will be expanded from the faulty lower-level branch itself to all users on the same upper-level branch as the faulty branch and all users in the entire system. However, in each case, the remaining branch equipment can maintain continuous operation and these users will not experience short-term power outages.

[0053] 3) Coordination of different types of circuit breakers for segmented busbars and branches

[0054] Assuming that the busbars are divided into mechanical circuit breakers and solid-state circuit breakers, consider the following situation: Figure 3 shown.

[0055] When a line fault on a DC bus or its downstream branches is not cleared correctly or takes a long time to be cleared, configuring a solid-state circuit breaker on the segmented bus can limit the scope of the fault. If a mechanical circuit breaker is configured, the fault range will be further expanded.

[0056] Based on the above analysis, the configuration and coordination of circuit breakers can be summarized into eight situations in Table 1.

[0057] Table 1 Circuit breaker configuration and coordination

[0058]

[0059] (3) Impact of circuit breakers on system reliability under typical topologies

[0060] When there are many incoming and outgoing line circuits on the bus in a low-voltage DC system, in order to control the impact of faults and maintenance to the smallest possible range, disconnectors or DC circuit breakers are often used to segment the bus. Take the system topology of the DC distribution network of a local power building as an example. Figure 4 Considering that the system structure is not complicated, the failure consequence analysis method is used to analyze the Figure 4 The system shown in the figure traverses all branch faults, whether the circuit breaker refuses to operate, and whether the system has the ability to ride through short-circuit faults, and uses this to calculate the reliability indicators of equations (1) and (2).

[0061] like Figure 4 As shown in the figure, suppose there are N users in the low voltage DC system and the total system capacity is P total, among which n1 households are connected to DC bus 1 through the primary branch of mechanical circuit breakers, n2 households are connected to DC bus 1 through the primary branch of solid-state circuit breakers, m1 households are connected to DC bus 2 through the primary branch of mechanical circuit breakers, and m2 households are connected to DC bus 2 through the primary branch of solid-state circuit breakers. Assuming that the segmented bus is equipped with a mechanical circuit breaker, k = 0; if the segmented bus is equipped with a solid-state circuit breaker, k = 1.

[0062] Table 2 Eight branch fault conditions considering different circuit breaker configurations under the segmented busbar interconnection topology

[0063]

[0064]

[0065] Considering the probability of line failure and the probability of circuit breaker failure, according to Figure 4 The circuit breaker types and branch conditions shown in Table 2 are used to calculate the power outage time, number of power outage users and expected power shortage for eight types of branch faults, and then calculate the system average fault power outage frequency SAIFI-F and the average power shortage AENT-F:

[0066] 1) When both converters have fault ride-through function

[0067] a. Primary branch with mechanical circuit breaker

[0068] Assume that the faulty branch is connected to DC bus 1. When the line fault in the above case occurs, assume that the busbar is equipped with a mechanical circuit breaker. If the faulty branch circuit breaker operates correctly, only the users of the faulty branch in the entire system will experience a long power outage, and its repair time is the repair time of the line fault. The remaining N-1 users will experience a short power outage. If the faulty branch circuit breaker fails, users on all branches (n1+n2) of DC bus 1 will experience a long power outage, and the repair time is the circuit breaker repair time (it can be seen from Table 3 that the circuit breaker repair time is greater than the line repair time), and the remaining users (m1+m2) will experience a short power outage. When the above line fault occurs, assuming that the busbar is equipped with a solid-state circuit breaker, if the faulty branch circuit breaker operates correctly, only the users of the faulty branch will experience a long power outage, and its repair time is the repair time of the line fault, and the users on other branches (n1+n2-1) of DC bus 1 will experience a short power outage; if the faulty branch circuit breaker fails, the users on all branches (n1+n2) of DC bus 1 will experience a long power outage, and because the busbar circuit breaker operates quickly, the system at the other end will not be affected by the fault. The above analysis is also applicable to DC bus 2.

[0069] for Figure 4 For the primary branch that directly supplies power to users, the line failure rate is:

[0070] λ i =λ0·l+λ other (3)

[0071] In the formula, λ i represents the failure rate of the i-th branch, λ0 represents the failure rate of the cable per unit length, l represents the line length, λ other Indicates the comprehensive failure rate of other parts of the circuit such as terminals.

[0072] The expected number of users with power outages caused by line failures in this area (EOU1) is:

[0073]

[0074] In the formula, λ i is the line failure rate, λ MCB is the mechanical circuit breaker failure rate, MTTR l is the average repair time of the line fault, T0 is the time it takes for the mechanical circuit breaker to clear the fault, causing the system to temporarily lose pressure and causing short power outages for other non-fault users, and MTTR MCB is the mean time to repair a mechanical circuit breaker fault.

[0075] The expected power shortage (EENS1) is:

[0076]

[0077] Where P i is the power supply power of the i-th branch, P total is the total system power, is the power supply power of all branches on the DC bus where the i-th branch is located, is the power supply of all branches on the DC bus that are not directly connected to the i-th branch.

[0078] b. Configure the primary branch of the solid-state circuit breaker

[0079] The expected number of users with power outages (EOU2) caused by line failures in this area is:

[0080]

[0081] In the formula, λ SSCB is the mechanical circuit breaker failure rate, MTTR SSCB is the mean time to repair a mechanical circuit breaker fault.

[0082] The expected power shortage (EENS2) is:

[0083]

[0084] 2) One end of the converter has fault ride-through, and the other end does not

[0085] Different from situation 1), if the converter does not have a fault ride-through capability, it cannot provide a continuous ride-through current to the mechanical circuit breaker when a fault occurs, and the circuit breaker cannot operate even if it does not fail.

[0086] a. Primary branch with mechanical circuit breaker on the fault-through side

[0087] The expected number of users with power outages caused by line failures in this area (EOU3) is:

[0088]

[0089] The expected power shortage (EENS3) is:

[0090]

[0091] b. Primary branch with solid-state circuit breaker on the fault-through side

[0092] The expected number of users with power outages caused by line failures in this area (EOU4) is:

[0093]

[0094] The expected power shortage (EENS4) is:

[0095]

[0096] c. Primary branch without mechanical circuit breaker on the fault-through side

[0097] The expected number of users with power outages caused by line failures in this area (EOU5) is:

[0098]

[0099] The expected power shortage (EENS5) is:

[0100]

[0101] d. The expected number of users without power outages caused by line faults in this area of ​​the primary branch without solid-state circuit breakers on the fault-through side (EOU6) is:

[0102]

[0103] The expected power shortage (EENS6) is:

[0104]

[0105] 3) Both converters have no fault ride-through

[0106] a. Primary branch with mechanical circuit breaker

[0107] The expected number of users with power outages caused by line failures in this area (EOU7) is:

[0108]

[0109] The expected power shortage (EENS7) is:

[0110]

[0111] b. Configure the primary branch of the solid-state circuit breaker

[0112] The expected number of users with power outages caused by line failures in this area (EOU8) is:

[0113]

[0114] The expected power shortage (EENS8) is:

[0115]

[0116] The above eight situations discuss Figure 4 The possible situations of fault ride-through with different converters and all line faults in the topology shown, the expected power outage time, the expected number of power outage users and the expected power shortage can further obtain the reliability index of the low-voltage DC system. When both ends have the fault ride-through capability, a=1, b=2; when only one end has the fault ride-through capability, a=3, b=6; when both ends do not have the fault ride-through capability, a=7, b=8.

[0117] System average fault outage frequency SAIFI-F:

[0118]

[0119] Average power shortage due to power outage AENT-F:

[0120]

[0121] There are currently three types of DC circuit breakers. Solid-state circuit breakers can quickly cut off faults during the current rise process of capacitor discharge after a short circuit. Hybrid circuit breakers operate slightly slower than solid-state circuit breakers, but can still cut off faults during capacitor discharge after a short circuit. Mechanical circuit breakers take a long time to isolate faults, and can generally only cut off faults when the fault enters a steady state.

[0122] Among the reliability indicators of the DC system, the average system fault outage time, the average system fault outage frequency, the average power shortage due to fault outages and other indicators are closely related to the selection of circuit breakers. This section proposes a calculation model for indicators such as the average system fault outage time, the average system fault outage frequency, and the average power shortage due to fault outages when different circuit breakers or different circuit breaker combinations are used under the segmented busbar interconnection network topology. The use of different circuit breakers or circuit breaker combinations will affect various reliability indicators of the system. There are different requirements for reliability indicators in different scenarios, and the type of circuit breaker can be selected according to a certain reliability indicator. However, for general scenarios, the selection of circuit breakers should consider economic indicators related to the comprehensive reliability indicator. Since the two reliability indicators exist in different dimensions, they cannot be directly compared and require a unified quantitative indicator. Therefore, the economic indicator is further considered in the following embodiments of the present invention.

[0123] Analysis of circuit breaker configuration method based on system economics

[0124] The relay protection scheme of the low-voltage DC system is closely integrated with the DC circuit breaker. The choice of the circuit breaker will directly affect the propagation and development of the branch fault in the system, and then affect the power supply reliability index of the system. On the one hand, the system voltage level and the cost proportion of the relay protection part are factors that need to be considered in the protection design of the low-voltage DC system. Compared with the traditional low-voltage AC system, the use of complex protection with communication in the low-voltage DC system is worth considering. The differences in protection schemes brought about by the differences in the development of DC circuit breakers at this stage need to be evaluated from an objective perspective. On the other hand, the application scenarios of the system and the importance of users are also conditions that cannot be ignored in protection design. The IEEE standard IEEEStd 493-2007IEEE Recommended Practice for the Design of Reliable Industrial and Commercial Power Systems divides user types into three categories: commercial users, industrial users, and organizational / institutional users, and divides the power outage loss into the average loss per power outage event and the average power outage loss. For industrial users, a single short-term power outage may produce a large number of defective products and cause high economic losses; while for general commercial and residential users, the loss of a short-term power outage is relatively small.

[0125] In view of the above reasons, the present invention takes economy as one of the important indicators to measure the protection configuration and circuit breaker selection, and calculates the comprehensive economic cost of circuit breaker configuration from three aspects: power outage loss caused by fault, circuit breaker operation loss and fixed cost:

[0126] C 综合经济成本 =C 停电损失 +C 运行损耗 +C固定成本 (twenty two)

[0127] Wherein, C comprehensive economic cost is the comprehensive economic cost index of the circuit breaker proposed in this embodiment, C power outage loss is the economic loss caused by fault power outage under the circuit breaker configuration, C operation loss is the economic loss caused by the normal operation loss of the circuit breaker, and C fixed cost is the fixed cost of the circuit breaker and the corresponding protection.

[0128] (1) Power outage losses

[0129] The power outage loss caused by a fault is divided into average power outage loss and average power outage event loss according to IEEE Std 493-2007.

[0130] C 停电损失 =C 电度损失 +C 停电事件损失 (twenty three)

[0131] Where C is the economic loss caused by the average power outage and power shortage, and C is the economic loss caused by the power outage event that is not related to the amount of electricity.

[0132] The average power outage loss can be calculated by multiplying the expected power shortage under the eight conditions of line failure shown in Table 3 by the electricity price of the corresponding power outage time:

[0133]

[0134] In the formula, p i The unit kilowatt-hour power outage price corresponds to the power outage duration.

[0135] The average power outage loss can be calculated based on the number of users experiencing power outages in the eight cases shown in Table 3 and the power outage loss of a single event that distinguishes between long and short power outages.

[0136] Table 3 Eight branch fault conditions considering different circuit breaker configurations under the segmented busbar interconnection topology

[0137]

[0138]

[0139] For example, in case 1, when both converters at both ends are fault-ridden, the power outage loss C1 caused by line fault in the n1 primary branch equipped with a mechanical circuit breaker is:

[0140]

[0141] Where C1 is the average power outage loss caused by the primary branch failure of this type of mechanical circuit breaker in the system to the users of the entire system, p longis the power outage loss caused to users by a single long power outage that is not related to the amount of electricity, p short It refers to the power outage loss caused to users by a single short-term power outage that is unrelated to the amount of electricity consumed.

[0142] Case 2: When both converters at both ends are fault-ridden, a line fault occurs in the primary branch n2 equipped with a solid-state circuit breaker, causing a power outage loss C2:

[0143]

[0144] Case 3: Only one end of the converter is fault-ridden. The power outage loss C3 caused by the line fault of the primary branch of the mechanical circuit breaker n1 at one end of the fault-ridden is:

[0145]

[0146] Case 4: Only one end of the converter is equipped with fault ride-through. The power outage loss C4 caused by line fault in the primary branch n2 with solid-state circuit breaker at one end of the fault ride-through is:

[0147]

[0148] Case 5: Only one end of the converter has fault ride-through, and one end is not equipped with a mechanical circuit breaker m1. The power outage caused by a line fault in the primary branch causes a power outage loss C5:

[0149]

[0150] Case 6: Power outage loss C6 caused by line fault in the primary branch m2 with solid-state circuit breaker configured at one end without fault ride-through:

[0151]

[0152] Case 7: When both converters at both ends do not have fault ride-through, a line fault occurs in the n1 primary branch equipped with a mechanical circuit breaker, resulting in a power outage loss C7:

[0153]

[0154] Case 8: When both converters at both ends do not have fault ride-through, a line fault occurs in the primary branch n2 equipped with a solid-state circuit breaker, resulting in a power outage loss C8:

[0155]

[0156] In summary, the sum of the calculations of C1 to C8 can be obtained as follows:

[0157]

[0158] (2) Operation loss

[0159] Since the solid-state circuit breaker has a power electronic device with a forward conduction voltage drop in series in the main current branch, the operating loss of the solid-state circuit breaker is higher than that of the mechanical circuit breaker and the hybrid circuit breaker. The average annual loss caused by the circuit breaker operating loss is:

[0160]

[0161] Where P i is the annual average power corresponding to each primary branch, P m is the annual average transmission power between segmented buses, T i is the annual average operating time corresponding to each primary branch, T m is the average annual operating time of the bus tie circuit breaker, φ1 is the loss rate of the mechanical circuit breaker, φ2 is the loss rate of the solid-state circuit breaker, and p is the average operating electricity price.

[0162] (3) Fixed costs

[0163] The fixed costs of different circuit breaker configurations include the cost of the fault isolation device itself and the cost of the corresponding protection device, which are converted to the first year and given directly in the form of a constant.

[0164] When the configuration of relay protection is not reasonable or the cost is too high based on reliability, selectivity, speed and sensitivity, this section proposes that circuit breaker selection can be considered from three aspects: power outage loss, operating loss and fixed cost. The economic-based circuit breaker selection model can be widely used in application scenarios such as commercial, industrial and public institutions, and can also be combined with the aforementioned reliability evaluation to form a complete circuit breaker configuration plan.

[0165] The above solution of the present invention is further demonstrated and introduced in combination with a specific application example below:

[0166] Configuration scheme of DC distribution network circuit breaker in a certain power building

[0167] The low-voltage DC system uses two 630kVA three-port AC / DC converters without low-voltage ride-through to connect to the AC grid. The converter stations are equipped with fault ride-through functions and are three-level VSC structures. Space vector pulse width modulation technology is used to control the IGBT switches on the converters. The system uses true bipolar wiring to build a ±750V DC bus. The distributed photovoltaic system, outdoor energy storage system, V2G charging pile, bidirectional inverter PCS, communication system and DC loads in the system are connected to the positive and negative bus in a symmetrical manner. Figure 5 shown.

[0168] according to Figure 5Based on the application scenarios and system topology, the circuit breaker selection needs to meet the reliability requirements. In addition, since the current hybrid DC circuit breakers require the use of a large number of fully controlled devices, are extremely expensive, and have poor applicability in DC power grids, the research team only focuses on two types of branch circuit breakers: mechanical circuit breakers and solid-state circuit breakers.

[0169] According to IEEE Std 493-2007 and other relevant literature, the reliability parameters of the lines and circuit breakers of the low voltage DC system and the power outage loss parameters of commercial users can be obtained as shown in Tables 4 and 5.

[0170] Table 4 Reliability parameters of low voltage DC system lines and circuit breakers

[0171] Device Type Failure rate Mean time to repair Cable (per 1000 feet) 0.009230 8.9 Mechanical circuit breaker 0.010000 16 Solid-state circuit breakers 0.112070 16

[0172] Table 5 Power outage losses for commercial users

[0173]

[0174]

[0175] According to the above reliability and economic parameters, Figure 5 The index calculation and comparison of each branch of the low-voltage DC system under different circuit breaker configurations are carried out, and the results are shown in Tables 6 and 7.

[0176] Table 6 Impact of line faults and circuit breaker failures on reliability parameters

[0177]

[0178]

[0179] Table 7 Power outage losses caused by line faults and circuit breaker failures

[0180]

[0181]

[0182] It can be seen from Table 6 and Table 7 that the advantages and disadvantages of the two configuration schemes of MCB and SSCB for branches are not absolute. They have different effects on different parameters under different circumstances. From the perspective of reliability, when the bus coupler is configured with MCB, configuring SSCB for any branch can reduce the average fault power outage frequency and the average power shortage. When the bus coupler is configured with SSCB, configuring SSCB for any branch can reduce the average fault power outage frequency, but increase the average power shortage, while the effect of configuring MCB is just the opposite. From the perspective of power outage loss, the loss of irrelevant electricity caused by a single power outage event is far greater than the loss caused by the power outage. When the bus coupler is configured with MCB, configuring SSCB for any branch can reduce the power loss and event loss. When the bus coupler is configured with SSCB, configuring SSCB for any branch can reduce the event loss, but increase the power loss, while the effect of configuring MCB is just the opposite. From the perspective of operating loss and fixed cost, configuring mechanical circuit breakers for all branches in the system is the solution with the lowest loss and cost. Taking into account reliability and economics including power outage losses, operating losses and fixed costs, mechanical circuit breakers are selected as fault isolation devices.

[0183] Based on the same inventive concept, the present invention also provides a computer device, which includes: one or more processors, and a memory for storing one or more computer programs; the program includes program instructions, and the processor is used to execute the program instructions stored in the memory. The processor may be a central processing unit (CPU), or other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), field-programmable gate arrays (FPGA) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components, etc. It is the computing core and control core of the terminal, which is used to implement one or more instructions, specifically for loading and executing one or more instructions in a computer storage medium to implement the above method.

[0184] It needs to be further explained that, based on the same inventive concept, the present invention also provides a computer storage medium, on which a computer program is stored, and the computer program is executed by a processor to execute the above method. The storage medium can adopt any combination of one or more computer-readable media. The computer-readable medium can be a computer-readable signal medium or a computer-readable storage medium. The computer-readable storage medium can be, for example, but not limited to, an electrical, magnetic, optical, electrical, magnetic, infrared, or semiconductor system, device or device, or any combination of the above. More specific examples (non-exhaustive list) of computer-readable storage media include: an electrical connection with one or more wires, a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber, a portable compact disk read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the above. In the present invention, a computer-readable storage medium can be any tangible medium containing or storing a program, which can be used by an instruction execution system, device or device or used in combination with it.

[0185] In the description of this specification, the description with reference to the terms "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present disclosure. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0186] The above shows and describes the basic principles, main features and advantages of the present disclosure. Those skilled in the art should understand that the present disclosure is not limited by the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present disclosure. Without departing from the spirit and scope of the present disclosure, the present disclosure may have various changes and improvements, and these changes and improvements fall within the scope of the present disclosure to be protected.

[0187] The present invention is not limited to the above-mentioned optimal implementation mode. Anyone can derive other various forms of low-voltage DC distribution network circuit breaker configuration methods under the inspiration of the present invention. All equal changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A method for configuring a low voltage DC distribution network circuit breaker, characterized in that: Under the sectional bus interconnection network topology, the system reliability index and / or economic index when different circuit breakers or different combinations of circuit breakers are used are calculated, so as to select the circuit breaker or the combination of circuit breakers according to the requirements of one or more reliability indexes and / or economic indexes; The reliability index includes the average frequency of power outage due to system failure and the average power shortage due to power outage due to system failure; The economic indicators include power outage losses caused by faults, operating losses of circuit breakers and fixed costs.

2. The low voltage DC distribution network circuit breaker configuration method according to claim 1, characterized in that: The calculation of the reliability index takes into account the coordination between the circuit breakers used in each branch, the fault ride-through capability of the converter and the breaking characteristics of various low-voltage DC distribution network circuit breakers.

3. The low voltage DC distribution network circuit breaker configuration method according to claim 2, characterized in that: The calculation basis of the average fault outage frequency of the system is: line fault rate, circuit breaker failure rate and the number of users and / or the distribution of users in each branch; The calculation basis of the average power shortage due to power outage caused by fault is: line failure rate, circuit breaker failure rate, average repair time of line fault, power supply power of this branch, temporary system pressure loss due to long time for circuit breaker to clear fault and short power outage time suffered by other non-fault users, total system power, average repair time of circuit breaker fault and power supply power of all branches on the DC bus where this branch is located and / or power supply power of all branches on the DC bus that is not directly connected to this branch.

4. The low voltage DC distribution network circuit breaker configuration method according to claim 1, characterized in that: The power outage loss includes the average power outage electricity loss and the average power outage event loss; the economic loss caused by the average power outage power shortage is calculated based on the expected power shortage of the line failure multiplied by the electricity price during the corresponding power outage time; the average power outage event loss is calculated based on the number of power outage users and the single event power outage loss that distinguishes between long and short power outages.

5. The low voltage DC distribution network circuit breaker configuration method according to claim 4, characterized in that: The expected power shortage of the line fault is calculated based on: line failure rate, circuit breaker failure rate, average repair time of line fault, power supply power of this branch, temporary system pressure loss due to long time of circuit breaker clearing fault and short power outage time suffered by other non-fault users, total system power, average repair time of circuit breaker fault and power supply power of all branches on the DC bus where this branch is located and / or power supply power of all branches on the DC bus that is not directly connected to this branch.

6. The low voltage DC distribution network circuit breaker configuration method according to claim 4, characterized in that: The calculation of the single-event power outage loss that distinguishes between long and short power outages is based on the coordination between the circuit breakers used in each branch, the fault ride-through capability of the converter and the breaking characteristics of various types of low-voltage DC distribution network circuit breakers and is based on: line failure rate, circuit breaker failure rate, power outage losses to users that are not related to the amount of electricity caused by a single long power outage, power outage losses to users that are not related to the amount of electricity caused by a single short power outage, the number of users, the number of users of all branches on the DC bus where this branch is located, and / or the number of users of all branches on the DC bus that is not directly connected to this branch.

7. The low voltage DC distribution network circuit breaker configuration method according to claim 1, characterized in that: The operating loss of the circuit breaker is calculated by the annual average power corresponding to each primary branch, the annual average operating time corresponding to each primary branch, the average operating electricity price, the loss rate of the circuit breaker, the annual average transmission power between segmented buses and the annual average operating time of the bus tie circuit breaker.

8. The low voltage DC distribution network circuit breaker configuration method according to claim 1, characterized in that: The fixed costs include the cost of the fault isolation device itself and the corresponding protection device cost, which are calculated by converting to the first year.

9. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the program, the steps of the low-voltage DC distribution network circuit breaker configuration method according to any one of claims 1 to 8 are implemented.

10. A non-transitory computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the low-voltage DC distribution network circuit breaker configuration method according to any one of claims 1 to 8 are implemented.

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