Network construction type new energy delivery line pilot protection method and system

Abstract

The invention belongs to the field of power system stability control, and discloses a network construction type new energy delivery line pilot protection method and system.The method includes the steps that for asymmetric faults, it is considered that under the influence of a negative sequence suppression strategy, the difference between the negative sequence current of a field station side and the negative sequence current of a system side is large, and the negative sequence current is adopted to construct a criterion; for a symmetric fault, the positive sequence current is adopted to construct a criterion by considering that the positive sequence current difference between a field station side and a system side is large under the influence of a current limiting strategy, and the criterion is constructed by considering that the positive sequence currents on the two sides are close due to the increase of the number of network construction type units, utilizing the reverse directions of the currents on the two sides and adopting the abrupt change of the positive sequence current. According to the method, three criteria are combined, the positive sequence current, the positive sequence current break variable and the negative sequence current are fused to form the composite sequence current, then the transmission line pilot protection criterion suitable for the network construction type new energy large-scale access system is constructed, and a good protection effect is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of protection of centralized transmission lines of network-type new energy units, and in particular to a longitudinal protection method and system for network-type new energy transmission lines. Background Art

[0002] Under the guidance of the dual carbon goals, a high proportion of renewable energy and a high proportion of power electronic equipment have gradually become typical features of the new power system. However, the poor stability of new energy access and the different fault characteristics also bring risks to the operation of the power system. In particular, the large-scale access of grid-connected new energy units leads to a decrease in the inertia of the system. When a system fails, it is very susceptible to voltage drops and instability, affecting the transient stability of the power system. The grid-connected new energy units adopt a control strategy similar to that of synchronous generators, which can actively provide support for the power system and provide a new solution for building a highly reliable new energy transmission system.

[0003] The converters currently used in new energy units can be divided into two types according to the control method: grid-following type and grid-building type. The principle of grid-following control technology is simple. It measures the phase information of the grid connection point through a phase-locked loop to achieve synchronization with the grid. However, its essence is a current source. It cannot provide voltage and frequency support by itself and must rely on the voltage and frequency of the grid. With the rapid increase in the installed capacity of new energy, the rotational inertia of the power grid has decreased and the stability has deteriorated. To solve this problem, scholars at home and abroad have proposed a grid-building control technology, which adopts a power synchronization strategy similar to that of a synchronous generator and can achieve synchronization without the help of a phase-locked loop. It is essentially a voltage source. It outputs stable voltage and frequency by setting voltage parameters, and provides inertia and damping for the power system to play a supporting role. Since the grid-building control technology is conducive to improving the short-circuit capacity and rotational inertia problems of the power system, it is often used in large-scale new energy transmission systems.

[0004] Therefore, for the centralized transmission lines of grid-type new energy units, the adaptability of traditional longitudinal protection is affected by the grid-type current limiting control strategy and negative sequence suppression strategy. When a fault occurs in the transmission line, the fault current provided by the inverter is limited and the value is small, while the phase varies depending on factors such as the control strategy, fault type, and fault location. Therefore, after the fault occurs, the phase of the inverter output current is uncertain, resulting in reduced protection sensitivity. At the same time, although the fault current provided by the grid-type new energy units is small during the fault, due to the centralized transmission of new energy units, the short-circuit current provided by the station side may be the same as that on the system side, resulting in a further decrease in the sensitivity of the protection. Summary of the invention

[0005] Purpose of the invention: In order to solve the problems arising from the above-mentioned prior art, the present invention provides a method for longitudinal protection of a grid-type new energy transmission line, which improves the sensitivity of the longitudinal protection of the centralized transmission line of a grid-type new energy unit. The present invention also provides a longitudinal protection system for a grid-type new energy transmission line.

[0006] Technical solution: According to a first aspect of the present invention, a longitudinal protection method for a network-type new energy transmission line is provided, the method comprising:

[0007] Confirm the faults and corresponding fault currents at the system side and the station side of the grid-type new energy transmission system, and determine the types of the faults and fault currents. The faults include asymmetric faults and symmetric faults, and the fault currents include positive sequence currents and negative sequence currents.

[0008] like Figure 2 As shown in the figure, taking the grid-following control of photovoltaic power generation as an example, the topological structure of the large-scale new energy transmission system is demonstrated. The station side consists of multiple photovoltaic generators under grid-following control, and the system side is the grid side. The power generated by the photovoltaic generator is converted into AC power by the converter and transmitted to the grid via the transmission line. PCC is the grid connection point of the photovoltaic generator.

[0009] If it is an asymmetric fault, the first criterion is constructed based on the negative sequence current on the station side and the system side: K3I w2 <I s2 , where K3 represents the negative sequence current proportionality coefficient, I w2 is the negative sequence current value provided by the station side when an asymmetric fault occurs, I s2 It is the negative sequence current value provided by the system side when an asymmetric fault occurs;

[0010] If it is a symmetrical fault, the second criterion is constructed based on the positive sequence current on the station side and the system side: K1I w1 <I s1 , where K1 represents the positive sequence current proportionality coefficient, I w1 is the positive sequence current value provided by the station side when a symmetrical fault occurs, I s1 It is the positive sequence current value provided by the system side when a symmetrical fault occurs;

[0011] If the number of grid-connected units increases and the positive sequence currents on both sides are close, the third criterion is constructed based on the positive sequence current mutation amount by taking advantage of the reverse current characteristics on both sides: K2ΔI w1 <ΔI s1 , where ΔI s1 is the sudden change of positive sequence current on the system side, ΔI w1 is the positive sequence current mutation amount at the station side, K2 represents the positive sequence current mutation amount proportional coefficient;

[0012] According to the first judgment, the second criterion and the third criterion, a composite sequence current is proposed which integrates the positive sequence current, the positive sequence current mutation and the comprehensive properties of the negative sequence current.

[0013] According to different situations of symmetrical faults and asymmetrical faults on the line, by comparing the corresponding composite sequence currents on the system side and the station side, it is determined whether the comprehensive judgment criteria are met. If so, the protection trips; otherwise, the protection exits and continues to monitor the fault.

[0014] Further, including:

[0015] The sudden change of the positive sequence current on the system side ΔI s1 It is expressed as:

[0016]

[0017] in, It represents the positive sequence component of the fault current provided by the system side when a symmetrical fault occurs. It represents the positive sequence component of the through current provided by the station side during normal operation, I l express The amplitude, I s1 express The amplitude of

[0018] The station side positive sequence current mutation amount ΔI w1 It is expressed as:

[0019]

[0020] in, It represents the positive sequence component of the fault current provided by the station side when a symmetrical fault occurs, I w1 express The amplitude of .

[0021] Further, including:

[0022] The composite sequence current corresponding to the system side and the station side is obtained according to the first criterion, the second criterion and the third criterion, specifically:

[0023] The compound sequence current at the station side is expressed as:

[0024] I w-comp =I w1 +m1ΔI w1 +m2I w2 ;

[0025] Among them, m1 and m2 represent weight coefficients;

[0026] The system side compound sequence current is expressed as:

[0027] I s-comp=I s1 +m1ΔI s1 +m2I s2 .

[0028] Further, including:

[0029] The specific construction of the comprehensive criterion includes:

[0030] The composite sequence current is I w1 +m1ΔI w1 Less than I s1 +m1ΔI s1 , under asymmetric fault, I w2 +m2ΔI w2 Less than I s2 +m2ΔI s2 , therefore, the comprehensive criterion is expressed as:

[0031] K rel I w-comp <I s-comp ;

[0032] Among them, K rel is the reliability coefficient.

[0033] In a second aspect, the present invention further provides a grid-type new energy transmission line longitudinal protection system, the system comprising:

[0034] The system confirmation module is used to confirm the faults and corresponding fault currents at the system side and the station side of the grid-type new energy transmission system, and determine the types of faults and fault currents. The faults include asymmetric faults and symmetric faults, and the fault currents include positive sequence currents and negative sequence currents. The station side includes multiple photovoltaic power generation groups under grid-type control, and the system side is the grid side. The electric energy generated by the photovoltaic power generation group is converted into alternating current by the converter and transmitted to the grid side using the transmission line.

[0035] The criterion building module is used to build different criteria according to the faults in different situations. Specifically, if it is an asymmetric fault, the first criterion K3I is built based on the negative sequence current on the station side and the system side. w2 <I s2 , where K3 represents the negative sequence current proportionality coefficient, I w2 is the negative sequence current value provided by the station side when an asymmetric fault occurs, I s2 It is the negative sequence current value provided by the system side when an asymmetric fault occurs;

[0036] If it is a symmetrical fault, the second criterion is constructed based on the positive sequence current on the station side and the system side: K1I w1 <I s1, where K1 represents the positive sequence current proportionality coefficient, I w1 is the positive sequence current value provided by the station side when a symmetrical fault occurs, I s1 It is the positive sequence current value provided by the system side when a symmetrical fault occurs;

[0037] If the number of grid-connected units increases and the positive sequence currents on both sides are close, the third criterion is constructed based on the positive sequence current mutation amount by taking advantage of the reverse current characteristics on both sides: K2ΔI w1 <ΔI s1 , where ΔI s1 is the sudden change of positive sequence current on the system side, ΔI w1 is the positive sequence current mutation amount at the station side, K2 represents the positive sequence current mutation amount proportional coefficient;

[0038] A composite sequence current calculation module is used to propose a composite sequence current that integrates the positive sequence current, the positive sequence current mutation amount and the negative sequence current comprehensive properties according to the first judgment, the second criterion and the third criterion;

[0039] The comprehensive criterion judgment module is used to judge whether the comprehensive criterion is met by comparing the corresponding composite sequence currents on the system side and the station side according to different situations when symmetrical faults and asymmetrical faults occur on the line. If met, the circuit breaker protection trips; otherwise, the protection exits and continues to monitor the fault.

[0040] Further, including:

[0041] The sudden change of the positive sequence current on the system side ΔI s1 It is expressed as:

[0042]

[0043] in, It represents the positive sequence component of the fault current provided by the system side when a symmetrical fault occurs. It represents the positive sequence component of the through current provided by the station side during normal operation, I l express The amplitude, I s1 express The amplitude of

[0044] The station side positive sequence current mutation amount ΔI w1 It is expressed as:

[0045]

[0046] in, It represents the positive sequence component of the fault current provided by the station side when a symmetrical fault occurs, I w1 express The amplitude of .

[0047] Further, including:

[0048] The composite sequence current corresponding to the system side and the station side is obtained according to the first criterion, the second criterion and the third criterion, specifically:

[0049] The compound sequence current at the station side is expressed as:

[0050] I w-comp =I w1 +m1ΔI w1 +m2I w2 ;

[0051] Among them, m1 and m2 represent weight coefficients;

[0052] The system side compound sequence current is expressed as:

[0053] I s-comp =I s1 +m1ΔI s1 +m2I s2 .

[0054] Further, including:

[0055] The specific construction of the comprehensive criterion includes:

[0056] The composite sequence current is I w1 +m1ΔI w1 Less than I s1 +m1ΔI s1 , under asymmetric fault, I w2 +m2ΔI w2 Less than I s2 +m2ΔI s2 , therefore, the comprehensive criterion is expressed as:

[0057] K rel I w-comp <I s-comp ;

[0058] Among them, K rel is the reliability coefficient.

[0059] In the third aspect, the present invention also provides a meshed new energy transmission line longitudinal connection protection device, the meshed new energy transmission line longitudinal connection protection device comprising: a memory, a processor, and a meshed new energy transmission line longitudinal connection protection program stored in the memory and executable on the processor, the meshed new energy transmission line longitudinal connection protection program implementing the steps of the above-mentioned meshed new energy transmission line longitudinal connection protection method when executed by the processor.

[0060] In a fourth aspect, the present invention provides a computer-readable storage medium, on which is stored a meshed-type new energy transmission line longitudinal connection protection program, which, when executed by a processor, implements the steps of the meshed-type new energy transmission line longitudinal connection protection method as described above.

[0061] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0062] (1) The present invention is aimed at the grid-type new energy centralized transmission system, comprehensively considers the fault current characteristics of the station side and the system side under various scenarios, integrates the positive sequence current, the positive sequence current mutation, and the negative sequence current to form a composite current, and then proposes a new criterion for longitudinal protection based on the composite current, which overcomes the influence of the control strategy of the grid-type new energy unit on the traditional longitudinal protection;

[0063] (2) The present invention integrates positive sequence current, positive sequence current mutation and negative sequence current to form a comprehensive criterion, avoiding the protection misoperation or refusal to operate caused by the influence of the control strategy on the phase angle criterion. The protection principle is simple and easy to implement. The simulation results show that the criterion has strong adaptability and can operate correctly under different fault types, different numbers of grid-type new energy units and different control strategies, which significantly improves the adaptability of the longitudinal protection.

[0064] (3) The grid-connected new energy units of the present invention are affected by the negative-sequence current suppression strategy, resulting in a smaller output negative-sequence current. The protection criterion is constructed using the fault current, thereby simplifying the identification of asymmetric faults, which is conducive to rapid protection action and the correct action of the longitudinal protection, thereby improving the sensitivity of the longitudinal protection of the centralized transmission lines of the grid-connected new energy units. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 This is a flow chart of the longitudinal protection method according to an embodiment of the present invention;

[0066] Figure 2 A schematic diagram of the topological structure of a network-based new energy delivery system according to an embodiment of the present invention;

[0067] Figure 3 A schematic diagram of positive sequence current on the system side and the station side in a symmetrical fault scenario according to an embodiment of the present invention;

[0068] Figure 4 It is a schematic diagram of positive sequence current on the system side and the station side in the centralized transmission scenario of the grid-connected new energy units according to an embodiment of the present invention;

[0069] Figure 5 It is a schematic diagram of the sudden change of positive sequence current on the system side and the station side in the centralized transmission scenario of the grid-connected new energy units according to an embodiment of the present invention;

[0070] Figure 6 A schematic diagram of negative sequence currents on the system side and the station side in an asymmetric fault scenario according to an embodiment of the present invention;

[0071] Figure 7 This is a schematic diagram of the action time of the longitudinal protection under a single-phase grounding fault according to an embodiment of the present invention;

[0072] Figure 8 This is a schematic diagram of the action time of the longitudinal protection under a two-phase fault according to an embodiment of the present invention;

[0073] Fig. 9 This is a schematic diagram of the action time of the longitudinal protection under a three-phase fault according to an embodiment of the present invention;

[0074] Fig.10 It is a schematic diagram of the action time of the longitudinal protection under the three-phase grounding fault according to the embodiment of the present invention;

[0075] Fig.11 This is a flow chart of the longitudinal protection method based on comprehensive criteria described in an embodiment of the present invention. DETAILED DESCRIPTION

[0076] 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.

[0077] The present invention proposes a new principle of longitudinal protection for grid-connected renewable energy transmission lines based on composite sequence current characteristics, comprehensively considering the influence of current limiting measures, negative sequence suppression strategy and fault type, integrating positive sequence current, positive sequence current mutation and negative sequence current to form a comprehensive criterion, discarding the phase criterion, and avoiding the uncertainty risk of phase angle caused by control strategies.

[0078] The simulation results show that the criterion has strong adaptability and can operate correctly under different fault types, different numbers of grid-connected new energy units and different control strategies, which significantly improves the adaptability of the longitudinal protection.

[0079] The technical solution adopted by the present invention is: a new principle of longitudinal protection for grid-type new energy transmission lines based on composite sequence current characteristics, the method includes symmetrical fault identification criteria based on positive sequence current ratio, station proximal symmetrical fault identification criteria based on positive sequence current mutation ratio, asymmetrical fault identification criteria based on negative sequence current ratio, composite sequence current construction, and comprehensive longitudinal protection criteria based on composite sequence current ratio.

[0080] According to a first aspect of the present invention, a method for longitudinal protection of a network-type new energy transmission line is provided, the method comprising:

[0081] Confirm the faults and corresponding fault currents at the system side and the station side of the grid-type new energy transmission system, and determine the types of the faults and fault currents. The faults include asymmetric faults and symmetric faults, and the fault currents include positive sequence currents and negative sequence currents.

[0082] If it is an asymmetric fault, the first criterion is constructed based on the negative sequence current on the station side and the system side: K3I w2 <I s2 , where K3 represents the negative sequence current proportionality coefficient, I w2 is the negative sequence current value provided by the station side when an asymmetric fault occurs, I s2 It is the negative sequence current value provided by the system side when an asymmetric fault occurs;

[0083] If it is a symmetrical fault, the second criterion is constructed based on the positive sequence current on the station side and the system side: K1I w1 <I s1 , where K1 represents the positive sequence current proportionality coefficient, I w1 is the positive sequence current value provided by the station side when a symmetrical fault occurs, I s1 It is the positive sequence current value provided by the system side when a symmetrical fault occurs;

[0084] If the number of grid-connected units increases and the positive sequence currents on both sides are close, the third criterion is constructed based on the positive sequence current mutation amount by taking advantage of the reverse current characteristics on both sides: K2ΔI w1 <ΔI s1 , where ΔI s1 is the sudden change of positive sequence current on the system side, ΔI w1 is the positive sequence current mutation amount at the station side, K2 represents the positive sequence current mutation amount proportional coefficient;

[0085] According to the first judgment, the second criterion and the third criterion, a composite sequence current is proposed which integrates the positive sequence current, the positive sequence current mutation and the comprehensive properties of the negative sequence current.

[0086] According to different situations of symmetrical faults and asymmetrical faults on the line, by comparing the corresponding composite sequence currents on the system side and the station side, it is determined whether the comprehensive judgment criteria are met. If so, the protection trips; otherwise, the protection exits and continues to monitor the fault.

[0087] like Fig.11 As shown, the specific steps of the method are as follows:

[0088] Step 1: Considering that the current limiting strategy of grid-connected new energy units leads to a large difference in the positive sequence current amplitude between the station side and the system side, a criterion based on the positive sequence current amplitude ratio is proposed.

[0089] Step 2: Considering the scenario of centralized transmission of new energy, the increase in the number of grid-connected new energy units makes the positive sequence current on the station side and the system side close during faults. It is proposed to use the through current under normal working conditions as a benchmark to measure the sudden change in the positive sequence current on both sides, and then construct a judgment criterion based on the sudden change in the positive sequence current.

[0090] Step 3: Considering the asymmetric fault conditions, the grid-connected new energy units basically do not provide negative-sequence current under the action of the negative-sequence suppression strategy. The negative-sequence current on the station side and the system side is quite different. A criterion based on the negative-sequence current ratio is proposed.

[0091] Step 4: Comprehensively consider the impact of fault conditions and grid-type unit control strategies, assign different weights to the positive sequence current, positive sequence current mutation and negative sequence current to form the composite sequence current on the station side and the system side.

[0092] Step 5: Based on the composite sequence current, the protection criteria proposed in steps 1 to 3 are integrated to construct a comprehensive longitudinal protection criterion.

[0093] According to the above method, the method specifically includes:

[0094] Step 1: If Figure 1 As shown in the figure, for the grid-type new energy transmission system, if a symmetrical fault occurs in the transmission line, the short-circuit current provided by the system-side power supply is:

[0095]

[0096] Where: Indicates the equivalent power supply on the system side; Z s Represents the equivalent impedance on the system side; Z l represents the total impedance of the transmission line; α represents the ratio of the length of the line from the fault location to the system side to the total length.

[0097] Although grid-connected new energy units use relevant control strategies to simulate the operating characteristics of synchronous generators to enable them to have support capabilities, they are limited by internal power electronic devices and have poor tolerance for fault currents. Therefore, grid-connected new energy units are generally equipped with current limiting strategies.

[0098] When a fault occurs on the system side, the fault current of the grid-connected new energy unit generally does not exceed the rated current. k times, k is generally 1.2-1.5. For the grid-type new energy centralized transmission system, the fault current provided by the station side can be expressed as:

[0099]

[0100] In the formula, N represents the number of grid-connected new energy units.

[0101] If the number of new energy units connected is small, then Much greater than Since the positive sequence currents on both sides of the line also meet the fault characteristics, the positive sequence current amplitude ratio can be selected to construct the criterion:

[0102] K1I w1 <I s1 (3)

[0103] Where K1 represents the positive sequence current proportional coefficient.

[0104] Step 2: From formula (2), it can be seen that the maximum fault current provided by the station side is only related to the number of new energy units N. Therefore, if the number of grid-type new energy units installed is large, I w1 May increase to I s1 This situation becomes more obvious as the fault location approaches the station side, mainly because the increase in line impedance weakens the fault current provided by the system side. This situation will cause the sensitivity of the criterion constructed by the positive sequence current amplitude ratio to decrease, and in severe cases, the protection will refuse to operate.

[0105] Taking into account the reverse direction of the fault current on the system side, the positive sequence current mutation is introduced to construct the protection criterion based on the through current provided by the station side during normal operation. At this time, the positive sequence current mutation on the station side can be expressed as:

[0106]

[0107] In the formula, Indicates the positive sequence component of the through current provided by the station side during normal operation; It indicates the positive sequence component of the fault current provided by the station side when a symmetrical fault occurs; I l express The amplitude of I w1 express The amplitude of .

[0108] The sudden change of positive sequence current on the system side can be expressed as:

[0109]

[0110] In the formula, It represents the positive sequence component of the fault current provided by the system side when a symmetrical fault occurs; I s1 express The amplitude of .

[0111] At this time, although the positive sequence currents on both sides of the line are almost the same, Under the influence of , there is a large difference in the positive sequence current mutation on both sides of the line. Therefore, a criterion for improving the protection sensitivity can be constructed:

[0112] K2ΔI w1 <ΔI s1 (6)

[0113] Where: K2 represents the proportional coefficient of the positive sequence current mutation.

[0114] Step 3: For the grid-type renewable energy transmission system, if an asymmetric fault occurs in the transmission line, the fault current changes on the station side and the system side are more complex, and it is difficult to identify the fault only by the positive sequence component. The renewable energy on the station side has a negative sequence suppression strategy. When an asymmetric fault occurs, the negative sequence current I w2 Almost 0, while the system side negative sequence current I s2 Relatively large, even in high resistance grounding, I s2 Also much larger than I w2 Therefore, the negative sequence current can be used to construct the protection criterion for asymmetric faults:

[0115] K3I w2 <I s2 (7)

[0116] Where: K3 represents the negative sequence current proportional coefficient.

[0117] Step 4: From the analysis of steps 1 to 3, it can be seen that when a fault occurs in the grid-type renewable energy transmission line, if it is an asymmetric fault, the negative sequence current on the station side and the system side is greatly different under the influence of the negative sequence suppression strategy. Therefore, criterion (7) is proposed; if it is a symmetrical fault, the positive sequence current on the station side and the system side is usually greatly different under the influence of the current limiting strategy. Therefore, criterion (3) is proposed. If the increase in the number of grid-type units leads to the close positive sequence current on both sides, the reverse current on both sides is used to construct criterion (6) with the positive sequence current mutation amount.

[0118] Criteria (3), (6) and (7) take into account various situations when a transmission line fails and do not conflict with each other. Therefore, a composite sequence current is proposed that integrates the positive sequence current, the positive sequence current mutation and the comprehensive properties of the negative sequence current.

[0119] The compound sequence current at the station side is:

[0120] I w-comp =I w1 +m1ΔI w1 +m2I w2 (8)

[0121] In the formula, I w1 Indicates the amplitude of the positive sequence component at the station side; ΔI w1Indicates the sudden change in amplitude of the positive sequence component at the station side; I w2 It represents the amplitude of the negative sequence component at the station side; m1 and m2 represent weight coefficients, which can generally be 4-6. In the present invention, m1=m2=4.

[0122] The system side compound sequence current is:

[0123] I s-comp =I s1 +m1ΔI s1 +m2I s2 (9)

[0124] In the formula, I s1 Indicates the amplitude of the positive sequence component on the system side; ΔI s1 Indicates the sudden change in the amplitude of the positive sequence component on the system side; I s2 Indicates the amplitude of the negative sequence component on the system side.

[0125] Step 5: It can be seen from the analysis that when a symmetrical fault occurs in the line, the composite sequence current I w1 +m1ΔI w1 Much smaller than I s1 +m1ΔI s1 , under asymmetric fault, I w2 +m2ΔI w2 Much smaller than I s2 +m2ΔI s2 , so the comprehensive protection criterion can be constructed:

[0126] K rel I w-comp <I s-comp (10)

[0127] In the formula, K rel is the reliability coefficient, which can generally be taken as 1.2-1.5.

[0128] If a fault occurs in the transmission line, the fault current on the system side and the station side is collected, and the positive sequence component and negative sequence component are solved by Fourier transform, and the composite sequence current is synthesized respectively. By comparing the composite sequence currents on both sides, it is determined whether the comprehensive criterion is met. If it is met, the protection trips, if not, the protection exits and continues to monitor the fault. The specific protection process is as follows Figure 1 As shown:

[0129] The fault is started and the current signals on both sides of the line are collected 5ms after the fault occurs. The positive sequence component I1, the positive sequence current mutation ΔI1 and the negative sequence component I2 of the fault current are solved by Fourier transform.

[0130] If I s1 / I w1 >K1, the circuit breaker will trip.

[0131] Otherwise, if ΔI s1 / ΔI w1 >K2, the circuit breaker will trip.

[0132] Otherwise, if I s2 / I w2 >K3, the circuit breaker will trip.

[0133] Otherwise, the composite sequence current I comp =I1+m1ΔI1+m2I2, and judge, if I s-comp / I w-comp >K rel , the circuit breaker protection trips, otherwise, the fault restart is performed again.

[0134] Example: The present invention verifies the new principle of differential protection for grid-type new energy transmission lines. A photovoltaic power generation model is built in the PSCAD power system simulation software to connect a grid-type converter as a grid-type power source. The adaptability of differential protection in three scenarios, namely, symmetrical fault, asymmetrical fault, and centralized access of grid-type new energy units, is verified in turn.

[0135] Scenario 1: The number of grid-connected new energy units is small, and a three-phase grounding fault occurs on the transmission line. The fault occurs 2 seconds later. The currents on the station side and the system side before and after the fault are extracted, and the positive sequence, negative sequence, and zero sequence components are extracted using Fourier transform. The positive sequence components of the currents on both sides are compared. Figure 3 It can be seen that after the fault occurs, under the action of the current limiting strategy, the positive sequence component amplitude of the fault current of the grid-connected new energy unit does not increase significantly, while the positive sequence component amplitude of the fault current on the system side is larger, and the positive sequence component amplitudes on both sides differ greatly. The fault can be identified by comparing the amplitudes, which is consistent with the theory.

[0136] Scenario 2: Increase the number of grid-connected new energy units to form a centralized access model for grid-connected new energy units, and set the fault occurrence time to 2s. Extract the current on the station side and the system side before and after the fault occurs, and use Fourier transform to extract the positive sequence, negative sequence, and zero sequence components. Compare the positive sequence components of the current on both sides. Figure 4 At this time, due to the increase in the number of grid-connected new energy units N, although the fault current is still restricted by the suppression strategy, the fault current sent from the station side is generally large, resulting in little difference in the positive sequence components of the fault current on the station side and the system side. At this time, the effect of using the positive sequence component to identify the fault is poor.

[0137] Since the fault currents on the station side and the system side are in opposite directions after a transmission line fault occurs, the positive sequence component mutation is introduced to construct the protection criterion based on the transmission line current during normal operation. Keeping the conditions unchanged, the fault positive sequence components on the station side and the system side are subtracted from the positive sequence components under normal operating conditions, such as Figure 5 It can be seen that the mutation amount of the positive sequence current on both sides is quite different, which can be used to construct the protection criterion.

[0138] Scenario 3: Set a single-phase grounding fault on the transmission line, and the fault occurs at 2s. Extract the current on the station side and the system side before and after the fault occurs, and use Fourier transform to extract the positive sequence, negative sequence, and zero sequence components. Compare the negative sequence components of the current on both sides. Figure 6 As shown. It can be seen that, affected by the negative sequence suppression strategy, the grid-connected new energy units basically do not provide negative sequence current in the case of asymmetric faults, so the difference between the negative sequence current on the station side and the system side is large, such as Figure 6 As shown, it can therefore be used to construct protection criteria.

[0139] The above three scenarios cover all fault conditions when the transmission line of the grid-forming new energy unit fails, so the positive sequence current, positive sequence mutation, and negative sequence current are integrated to form a composite current. From the above analysis and simulation phenomena, it can be seen that when the transmission line fails, the positive sequence current, positive sequence current mutation, and negative sequence current on the station side are all smaller than those on the system side, so the composite current ratio on both sides can be used to determine whether a fault occurs. In addition, due to the centralized transmission and asymmetric fault scenarios of the grid-forming new energy units, the protection criteria corresponding to the other two scenarios are relatively weak, so the weights are added to the protection criteria in these two scenarios to optimize the protection effect, and the weights of the present invention are both 4.

[0140] First, the influence of high-resistance grounding on the negative sequence current protection criterion is verified separately. A single-phase grounding fault is set, the fault position remains unchanged, and the grounding resistance is set to 100Ω, 1000Ω, 3000Ω and 5000Ω respectively. The comparison of the negative sequence current amplitude on the station side and the system side is shown in Table 1.

[0141] Table 1 Analysis of the influence of high-resistance grounding on the negative sequence current protection criterion

[0142]

[0143] It can be seen that although the increase in grounding resistance will weaken the negative sequence current on the system side, it is still much larger than the negative sequence current on the station side, and the ratio of the negative sequence current amplitudes on both sides is large. Therefore, the fault characteristics shown by this criterion are more obvious and are basically not affected by the grounding resistance.

[0144] Secondly, different fault conditions are set to determine whether the fault judgment criterion proposed in the present invention can correctly judge the fault. The results are shown in Table 2.

[0145] Table 2 Working conditions of protection criteria under different fault conditions

[0146]

[0147]

[0148] From the above data, it can be seen that the protection criteria under different fault types are all greater than the set threshold value of 1.5. Therefore, the faults of the grid-type new energy transmission lines can be identified and removed in time, thus having a good use effect.

[0149] Finally, verify the quickness of the protection and determine whether the criterion can quickly remove the fault. Set single-phase grounding fault, two-phase fault, three-phase fault and three-phase grounding fault respectively, and the fault occurrence time is 2s. By comparing the amplitude ratio of the composite current on the system side and the station side with the set action threshold, the protection action time is determined, such as Figure 7 , Figure 8 (a) and (b) in Fig. 9 (a) and (b) in Fig.10 As shown in (a) and (b) in Figure 7 In (a), the horizontal axis is time t / s, and the vertical axis is: I s-comp / I w-comp From the figure, we can see that the mutation time occurs in 2s. Figure 7 (b) in the figure shows the circuit breaker action time.

[0150] The protection action time under the four fault types is 2.003248s, 2.003421s, 2.004622s, and 2.003380s respectively. It can be seen that the longitudinal protection action time is within 5ms, which can meet the protection speed requirement.

[0151] In a second aspect, the present invention further provides a grid-type new energy transmission line longitudinal protection system, the system comprising:

[0152] A system confirmation module is used to confirm the faults and corresponding fault currents at the system side and the station side of the grid-type new energy transmission system, and determine the types of the faults and fault currents. The faults include asymmetric faults and symmetric faults, and the fault currents include positive sequence currents and negative sequence currents.

[0153] The criterion building module is used to build different criteria according to the faults in different situations. Specifically, if it is an asymmetric fault, the first criterion K3I is built based on the negative sequence current on the station side and the system side. w2 <I s2 , where K3 represents the negative sequence current proportionality coefficient, I w2 is the negative sequence current value provided by the station side when an asymmetric fault occurs, I s2 It is the negative sequence current value provided by the system side when an asymmetric fault occurs;

[0154] If it is a symmetrical fault, the second criterion is constructed based on the positive sequence current on the station side and the system side: K1I w1 <Is1 , where K1 represents the positive sequence current proportionality coefficient, I w1 is the positive sequence current value provided by the station side when a symmetrical fault occurs, I s1 It is the positive sequence current value provided by the system side when a symmetrical fault occurs;

[0155] If the number of grid-connected units increases and the positive sequence currents on both sides are close, the third criterion is constructed based on the positive sequence current mutation amount by taking advantage of the reverse current characteristics on both sides: K2ΔI w1 <ΔI s1 , where ΔI s1 is the sudden change of positive sequence current on the system side, ΔI w1 is the positive sequence current mutation amount at the station side, K2 represents the positive sequence current mutation amount proportional coefficient;

[0156] A composite sequence current calculation module is used to propose a composite sequence current that integrates the positive sequence current, the positive sequence current mutation amount and the negative sequence current comprehensive properties according to the first judgment, the second criterion and the third criterion;

[0157] The comprehensive criterion judgment module is used to judge whether the comprehensive criterion is met by comparing the corresponding composite sequence currents on the system side and the station side according to different situations when symmetrical faults and asymmetrical faults occur on the line. If met, the circuit breaker protection trips; otherwise, the protection exits and continues to monitor the fault.

[0158] Further, including:

[0159] The sudden change of the positive sequence current on the system side ΔI s1 It is expressed as:

[0160]

[0161] in, It represents the positive sequence component of the fault current provided by the system side when a symmetrical fault occurs. It represents the positive sequence component of the through current provided by the station side during normal operation, I l express The amplitude, I s1 express The amplitude of

[0162] The station side positive sequence current mutation amount ΔI w1 It is expressed as:

[0163]

[0164] in, It represents the positive sequence component of the fault current provided by the station side when a symmetrical fault occurs, I w1 express The amplitude of .

[0165] Further, including:

[0166] The composite sequence current corresponding to the system side and the station side is obtained according to the first criterion, the second criterion and the third criterion, specifically:

[0167] The compound sequence current at the station side is expressed as:

[0168] I w-comp =I w1 +m1ΔI w1 +m2I w2 ;

[0169] Among them, m1 and m2 represent weight coefficients;

[0170] The system side compound sequence current is expressed as:

[0171] I s-comp =I s1 +m1ΔI s1 +m2I s2 .

[0172] Further, including:

[0173] The specific construction of the comprehensive criterion includes:

[0174] The composite sequence current is I w1 +m1ΔI w1 Less than I s1 +m1ΔI s1 , under asymmetric fault, I w2 +m2ΔI w2 Less than I s2 +m2ΔI s2 , therefore, the comprehensive criterion is expressed as:

[0175] K rel I w-comp <I s-comp ;

[0176] Among them, K rel is the reliability coefficient.

[0177] The other technical features of the grid-type new energy transmission line longitudinal protection system described in the present invention are similar to the corresponding protection method and will not be repeated here.

[0178] Secondly, the present invention also provides a meshed new energy transmission line longitudinal protection device, which includes: a memory, a processor, and a meshed new energy transmission line longitudinal protection program stored in the memory and executable on the processor. When the meshed new energy transmission line longitudinal protection program is executed by the processor, the steps of the meshed new energy transmission line longitudinal protection method as described above are implemented.

[0179] Finally, the present invention provides a computer-readable storage medium, on which a meshed new energy transmission line longitudinal connection protection program is stored. When the meshed new energy transmission line longitudinal connection protection program is executed by a processor, the steps of the meshed new energy transmission line longitudinal connection protection method as described above are implemented.

[0180] Those skilled in the art will appreciate that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0181] Although the preferred embodiments of the present invention have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0182] Obviously, those skilled in the art can make various changes and modifications to the embodiments of the present invention without departing from the spirit and scope of the embodiments of the present invention. Thus, if these modifications and variations of the embodiments of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for longitudinal protection of a network-type new energy transmission line, characterized in that: The method includes: Confirm the faults and corresponding fault currents at the system side and the station side of the grid-type new energy transmission system, and determine the types of faults and fault currents. The faults include asymmetric faults and symmetric faults, and the fault currents include positive sequence currents and negative sequence currents. The station side includes multiple photovoltaic power generation groups under grid-type control, and the system side is the grid side. The electric energy generated by the photovoltaic power generation group is converted into alternating current by the converter and transmitted to the grid side using the transmission line. If it is an asymmetric fault, the first criterion is constructed based on the negative sequence current on the station side and the system side: K3I w2 <I s2 , where K3 represents the negative sequence current proportionality coefficient, I w2 is the negative sequence current value provided by the station side when an asymmetric fault occurs, I s2 It is the negative sequence current value provided by the system side when an asymmetric fault occurs; If it is a symmetrical fault, the second criterion is constructed based on the positive sequence current on the station side and the system side: K1I w1 <I s1 , where K1 represents the positive sequence current proportionality coefficient, I w1 is the positive sequence current value provided by the station side when a symmetrical fault occurs, I s1 It is the positive sequence current value provided by the system side when a symmetrical fault occurs; If the number of grid-connected units increases and the positive sequence currents on both sides are close, the third criterion is constructed based on the positive sequence current mutation amount by taking advantage of the reverse current characteristics on both sides: K2ΔI w1 <ΔI s1 , where ΔI s1 is the sudden change of positive sequence current on the system side, ΔI w1 is the positive sequence current mutation amount at the station side, K2 represents the positive sequence current mutation amount proportional coefficient; According to the first judgment, the second criterion and the third criterion, a composite sequence current is proposed which integrates the positive sequence current, the positive sequence current mutation and the comprehensive properties of the negative sequence current; According to different situations of symmetrical faults and asymmetrical faults on the line, by comparing the corresponding composite sequence currents on the system side and the station side, it is determined whether the comprehensive judgment criteria are met. If so, the protection trips; otherwise, the protection exits and continues to monitor the fault.

2. The method for longitudinal protection of a network-type new energy transmission line according to claim 1 is characterized in that: The sudden change of the positive sequence current on the system side ΔI s1 It is expressed as: in, Indicates that when a symmetrical fault occurs, the system side provides the positive sequence component of the fault current. In normal operation, the positive sequence component of the through current provided by the station side, I l express The amplitude, I s1 express The amplitude of The station side positive sequence current mutation amount ΔI w1 It is expressed as: in, Indicates that when a symmetrical fault occurs, the station side provides the positive sequence component of the fault current, I w1 express The amplitude of .

3. The method for longitudinal protection of a network-type new energy transmission line according to claim 2 is characterized in that: The composite sequence current corresponding to the system side and the station side is obtained according to the first criterion, the second criterion and the third criterion, specifically: The compound sequence current at the station side is expressed as: I w-comp =I w1 +m1ΔI w1 +m2I w2 ; Among them, m1 and m2 represent weight coefficients; The system side compound sequence current is expressed as: I s-comp =I s1 +m1ΔI s1 +m2I s2 。 4. The method for longitudinal protection of a network-type new energy transmission line according to claim 3 is characterized in that: The specific construction of the comprehensive criterion includes: The composite sequence current is I w1 +m1ΔI w1 Less than I s1 +m1ΔI s1 , under asymmetric fault, I w2 +m2ΔI w2 Less than I s2 +m2ΔI s2 , therefore, the comprehensive criterion is expressed as: K rel I w-comp <I s-comp ; Among them, K rel is the reliability coefficient.

5. The method for longitudinal protection of a network-type new energy transmission line according to claim 4 is characterized in that: According to different situations of symmetrical faults and asymmetrical faults occurring in the line, by comparing the corresponding composite sequence currents on the system side and the station side, it is judged whether the comprehensive criterion is met, which specifically includes: The current signals on both sides of the line are collected several seconds after the fault occurs, and the positive sequence component I1, the positive sequence current mutation amount ΔI1 and the negative sequence component I2 of the fault current are solved by Fourier transform; wherein the positive sequence component I1 includes the positive sequence component on the system side and the positive sequence component on the station side, the positive sequence current mutation amount ΔI1 includes the positive sequence current mutation amount on the system side and the positive sequence current mutation amount on the station side, and I2 includes the negative sequence component on the system side and the negative sequence component on the station side; If I s1 I w1 >K1, the circuit breaker will trip. Otherwise, if ΔI s1 / ΔI w1 >K2, the circuit breaker will trip. Otherwise, if I s2 I w2 >K3, the circuit breaker will trip. Otherwise, the composite sequence current I w-comp =I w1 +m1ΔI w1 +m2I w2 and I s-comp =I s1 +m1ΔI s1 +m2I s2 , and judge, if I s-comp / I w-comp >K rel , the protection trips, otherwise, the protection exits and continues to monitor the fault.

6. A grid-type new energy transmission line longitudinal protection system, characterized in that: The system includes: The system confirmation module is used to confirm the faults and corresponding fault currents at the system side and the station side of the grid-type new energy transmission system, and determine the types of faults and fault currents. The faults include asymmetric faults and symmetric faults, and the fault currents include positive sequence currents and negative sequence currents. The station side includes multiple photovoltaic power generation groups under grid-type control, and the system side is the grid side. The electric energy generated by the photovoltaic power generation group is converted into alternating current by the converter and transmitted to the grid side using the transmission line. The criterion building module is used to build different criteria according to the faults in different situations. Specifically, if it is an asymmetric fault, the first criterion K3I is built based on the negative sequence current on the station side and the system side. w2 <I s2 , where K3 represents the negative sequence current proportionality coefficient, I w2 is the negative sequence current value provided by the station side when an asymmetric fault occurs, I s2 It is the negative sequence current value provided by the system side when an asymmetric fault occurs; If it is a symmetrical fault, the second criterion is constructed based on the positive sequence current on the station side and the system side: K1I w1 <I s1 , where K1 represents the positive sequence current proportionality coefficient, I w1 is the positive sequence current value provided by the station side when a symmetrical fault occurs, I s1 It is the positive sequence current value provided by the system side when a symmetrical fault occurs; If the number of grid-connected units increases and the positive sequence currents on both sides are close, the third criterion is constructed based on the positive sequence current mutation amount by taking advantage of the reverse current characteristics on both sides: K2ΔI w1 <ΔI s1 , where ΔI s1 is the sudden change of positive sequence current on the system side, ΔI w1 is the positive sequence current mutation amount at the station side, K2 represents the positive sequence current mutation amount proportional coefficient; A composite sequence current calculation module is used to propose a composite sequence current that integrates the positive sequence current, the positive sequence current mutation amount and the negative sequence current comprehensive properties according to the first judgment, the second criterion and the third criterion; The comprehensive criterion judgment module is used to judge whether the comprehensive criterion is met according to different situations of symmetrical faults and asymmetrical faults on the line by comparing the corresponding composite sequence currents on the system side and the station side. If it is met, the protection trips; otherwise, the protection exits and continues to monitor the fault.

7. The grid-type new energy transmission line longitudinal protection system according to claim 6 is characterized in that: The sudden change of the positive sequence current on the system side ΔI s1 It is expressed as: in, It represents the positive sequence component of the fault current provided by the system side when a symmetrical fault occurs. It represents the positive sequence component of the through current provided by the station side during normal operation, I l express The amplitude, I s1 express The amplitude of The station side positive sequence current mutation amount ×I w1 It is expressed as: in, It represents the positive sequence component of the fault current provided by the station side when a symmetrical fault occurs, I w1 express The amplitude of .

8. The grid-type new energy transmission line longitudinal protection system according to claim 7 is characterized in that: The composite sequence current corresponding to the system side and the station side is obtained according to the first criterion, the second criterion and the third criterion, specifically: The compound sequence current at the station side is expressed as: I w-comp =I w1 +m1ΔI w1 +m2I w2 ; Among them, m1 and m2 represent weight coefficients; The system side compound sequence current is expressed as: I s-comp =I s1 +m1ΔI s1 +m2I s2 。 9. The grid-type new energy transmission line longitudinal protection system according to claim 8 is characterized in that: The specific construction of the comprehensive criterion includes: The composite sequence current is I w1 +m1ΔI w1 Less than I s1 +m1ΔI s1 , under asymmetric fault, I w2 +m2ΔI w2 Less than I s2 +m2ΔI s2 , therefore, the comprehensive criterion is expressed as: K rel I w-comp <I s-comp ; Among them, K rel is the reliability coefficient.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores a meshed new energy transmission line longitudinal protection program, which, when executed by a processor, implements the steps of the meshed new energy transmission line longitudinal protection method as described in any one of claims 1 to 5.

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