Pilot protection method for connected line of optical storage power station considering energy storage charging and discharging characteristics

Abstract

The invention discloses an optical storage power station connected line pilot protection method considering energy storage charge and discharge characteristics. The method comprises the following steps: measuring instantaneous current and voltage at two sides of a protected line through a current transformer and a voltage transformer; calculating power frequency quantity components of current and voltage at two sides of the line by using a discrete Fourier algorithm; calculating equivalent impedance on two sides of the line according to the power frequency components of the current and the voltage; and calculating equivalent impedance amplitudes of the two sides of the line according to the equivalent impedance of the two sides of the line, carrying out differential amplification, carrying out comparison based on the equivalent impedance amplitudes after differential amplification, and determining whether the fault is an internal fault according to a result. The characteristic that the short-circuit current amplitude of the optical storage power station is limited is considered, the concepts such as the equivalent impedance amplitude ratio are introduced, the difference of the equivalent impedance on the two sides of the line under the internal fault is enlarged through differential amplification, the error influence is reduced, and therefore the method is not influenced by operation of the optical storage power station as a power source or a load; and the method is not influenced by the transient control and protection strategy change of the grid-connected converter and a high-resistance grounding fault.

Description

Technical Field

[0001] The present invention relates to the field of data processing, and in particular to a longitudinal protection method for lines connected to a photovoltaic power station taking into account energy storage charging and discharging characteristics. Background Art

[0002] Energy storage has been gradually and widely used as a "power regulator" for the regional power grid to which photovoltaic power stations are connected. At present, most of the energy storage power stations in actual application are connected to the power grid at the same time as photovoltaic power stations using AC coupling. Their capacity accounts for a relatively large proportion, and their impact on the fault characteristics of the connected power grid is more significant. Although both photovoltaic and energy storage systems are connected to the grid through power electronic converters, the short-circuit current characteristics of both will be affected by the transient control and protection strategies of the converters, but the unique charging and discharging mode of energy storage gives it the characteristics of source-load duality, which results in more complex and diverse fault forms of photovoltaic power stations compared to single photovoltaic power stations. Therefore, the traditional relay protection principle based on the characteristics of synchronous machines has a higher risk of incorrect operation.

[0003] In the existing technology, most of the longitudinal protection criteria are proposed based on the short-circuit current variation characteristics provided by photovoltaic power stations, such as the correlation of instantaneous current waveforms, positive-sequence current components, and the phase difference between positive-sequence and negative-sequence current components. However, these criteria are mainly applicable to the situation where photovoltaic and other new energy sites always serve as power sources to feed short-circuit current to the fault point under power grid faults, and the impact of energy storage charging and discharging modes is not fully considered. Therefore, unlike a single photovoltaic power station, a photovoltaic power station or an energy storage power station may also absorb short-circuit current from the power grid similar to a load. In this scenario, the above technology is not applicable.

[0004] To this end, some studies have proposed new criteria for longitudinal protection based on the consideration of the impact of energy storage charging and discharging modes, combined with the differences in the trajectory curves of the instantaneous differential current / braking current on both sides of the line connected to the photovoltaic power station under normal operating conditions and internal and external faults. However, the above-mentioned new protection principles mainly rely on the short-circuit current variation characteristics of photovoltaic power stations or photovoltaic power stations, and the short-circuit current variation characteristics are affected by the transient control and protection strategies of the grid-connected converter. When the converter control and protection strategies change, this judgment basis will no longer be reliable.

[0005] In addition, in scenarios where a high proportion of photovoltaic and energy-storage power stations are connected, especially when a high-resistance grounding fault occurs, the existing protection principles may be difficult to accurately identify due to the unclear characteristics of the short-circuit current.

[0006] In summary, the existing technologies have at least the following problems: 1. Most solutions are only applicable to individual photovoltaic power stations but not to photovoltaic power stations; 2. Some solutions are affected by the converter control and protection strategies and are not universal; 3. When a high-resistance grounding fault occurs, the existing protection principle may be difficult to accurately identify due to the unclear short-circuit current characteristics. Currently, there is no solution that can solve the above problems at the same time. Summary of the invention

[0007] In view of the problems of narrow application scope, poor universality and difficulty in identifying some faults in the prior art, the present invention provides a longitudinal protection method for the lines connected to the photovoltaic power station taking into account the charging and discharging characteristics of energy storage, taking into account the characteristic of limited short-circuit current amplitude of the photovoltaic power station, introducing concepts such as equivalent impedance amplitude ratio, and amplifying the difference in equivalent impedance on both sides of the line under intra-regional faults through differential amplification to reduce the influence of errors. Therefore, the present invention is not affected by the operation of the photovoltaic power station as a power source or load, nor is it affected by changes in transient control and protection strategies of the grid-connected converter and high-resistance grounding faults. It can effectively distinguish between intra-regional and extra-regional faults of the power grid to which the photovoltaic power source is connected, and is suitable for scenarios where a high proportion of new energy stations with energy storage are connected to the power grid.

[0008] The following is the technical solution of the present invention.

[0009] The longitudinal protection method of the lines connected to the photovoltaic power station taking into account the energy storage charging and discharging characteristics includes the following steps: S1: For the protected line, the instantaneous current and voltage on both sides of the protected line are measured through current and voltage transformers; S2: Calculate the power frequency components of the current and voltage on both sides of the line using the discrete Fourier algorithm; S3: Calculate the equivalent impedance on both sides of the line based on the power frequency components of current and voltage; S4: Calculate the equivalent impedance amplitude on both sides of the line according to the equivalent impedance on both sides of the line and amplify it differentially. Calculate the ratio based on the equivalent impedance amplitude after differential amplification. Determine whether it is an in-zone fault according to the result. If so, send an action signal to the protection device and the circuit breakers on both sides of the line will trip. Otherwise, the protection will not operate.

[0010] Since the energy storage system has the dual characteristics of source and load in the photovoltaic power station, that is, it can be used as both a power source and a load, its charging and discharging mode has a significant impact on the short-circuit current. When calculating the equivalent impedance, the present invention indirectly considers the charging and discharging mode of the energy storage system, thereby improving the fault identification accuracy under different operating modes (power source or load) of the photovoltaic power station. In addition, by calculating the equivalent impedance amplitude on both sides of the line and performing differential amplification, the difference in the equivalent impedance on both sides of the line under in-zone faults and out-of-zone faults can be significantly enlarged, reducing false operations while increasing the reliability and stability of protection, and based on the current shunting phenomenon caused by the fault, it can more accurately determine whether the fault occurs in the area.

[0011] At the same time, the present invention does not rely on the specific short-circuit current variation characteristics, but determines the fault location by comparing the equivalent impedance amplitude ratio on both sides of the line. Therefore, it is not affected by the transient control and protection strategy of the converter and is not affected by high-resistance grounding faults.

[0012] Preferably, the step S2: calculating the power frequency components of the current and voltage on both sides of the line using a discrete Fourier algorithm comprises: ; in, is the sampling signal, is the number of sampling points, is the frequency index, is the representation in the frequency domain; extract The real and imaginary parts of the current and voltage on the photovoltaic storage side are obtained by , , and the power frequency components of the current and voltage on the system side , .

[0013] Preferably, the step S3: calculating the equivalent impedances on both sides of the line according to the power frequency components of the current and voltage, comprises: ; in , are the equivalent impedances of the photovoltaic storage side and the system side, , are the power frequency components of the current and voltage on the photovoltaic storage side, , They are the power frequency components of current and voltage on the system side respectively.

[0014] Preferably, the method further includes calculating the change in the power frequency current amplitude according to the power frequency components of the current and voltage, including: ; In the formula, , are the changes in the power frequency current amplitude on the photovoltaic storage side and the system side, and is the current power frequency current amplitude on the photovoltaic storage side and the system side; and is the power frequency current amplitude at the previous moment.

[0015] Preferably, in S4, the equivalent impedance amplitudes on both sides of the line are calculated according to the equivalent impedances on both sides of the line and differentially amplified, including: Calculate the equivalent impedance amplitude of the line on the photovoltaic storage side and the system side based on the equivalent impedance on both sides of the line , ; Based on the equivalent impedance amplitudes on the photovoltaic storage side and the system side and the changes in the power frequency current amplitude, the equivalent impedance amplitudes are differentially amplified to obtain differentiated maximum impedance and differentiated minimum impedance.

[0016] Preferably, the differential amplification of the equivalent impedance amplitude based on the equivalent impedance amplitude on the photovoltaic storage side and the system side and the change in the power frequency current amplitude to obtain the differentiated maximum impedance and the differentiated minimum impedance includes: ; In the formula, and are the differentiated maximum impedance and the differentiated minimum impedance, respectively. , are the equivalent impedance amplitudes on the photovoltaic storage side and the system side, is a natural constant, , They are the changes in the power frequency current amplitude on the photovoltaic storage side and the system side respectively.

[0017] Preferably, in S4, based on the ratio of the equivalent impedance amplitudes after differential amplification, it is determined whether it is an intra-zone fault according to the result, and if so, an action signal is sent to the protection device to trip the circuit breakers on both sides of the line, otherwise, the protection does not act, including: ; In the formula, is the impedance adjustment factor; If the inequality holds, an action signal is sent to the protection device and the circuit breakers on both sides of the line trip. Otherwise, the protection does not operate.

[0018] Preferably, the impedance adjustment coefficient Take 1.2.

[0019] The present invention also provides an electronic device, including a memory and a processor, wherein the memory stores a computer program, and when the processor calls the computer program in the memory, the steps of the method for longitudinal protection of lines connected to a photovoltaic power station taking into account the charging and discharging characteristics of energy storage are implemented.

[0020] The present invention also provides a storage medium, in which computer executable instructions are stored. When the computer executable instructions are loaded and executed by a processor, the steps of the method for longitudinal protection of lines connected to a photovoltaic power station taking into account the charging and discharging characteristics of energy storage are implemented.

[0021] The substantial effects of the present invention include: Since the energy storage system has the dual characteristics of source and load in the photovoltaic power station, that is, it can provide electricity to the power grid as a power source, and can also absorb electricity from the power grid as a load when needed, this difference in charging and discharging mode has a significant impact on the short-circuit current phase angle and amplitude of the photovoltaic power station. When calculating the equivalent impedance, the present invention fully considers the impact of the charging and discharging mode of the energy storage system on the short-circuit current, and effectively amplifies the difference between the equivalent impedances on both sides of the line under the fault in the area by introducing the impedance adjustment coefficient related to the current change on both sides of the line, thereby improving the accuracy of fault identification.

[0022] In addition, in normal operation or out-of-zone faults, the equivalent impedance amplitudes on both sides of the line are basically equal; in the case of an in-zone fault, due to the shunting effect of the fault point current, the equivalent impedance amplitudes on both sides of the line will have significant differences. Therefore, the present invention determines whether the fault occurs in the zone by calculating the equivalent impedance amplitude ratio on both sides of the line and comparing it with the set impedance adjustment coefficient. This method is not affected by the operation of the photovoltaic power station as a power source or load, and can accurately distinguish between faults inside and outside the zone.

[0023] In addition, the traditional relay protection principle based on the characteristics of synchronous machines is difficult to adapt to changes in the control strategy of power electronic converters. The photovoltaic and energy storage systems in the photovoltaic power station are connected to the grid through power electronic converters, and their short-circuit current characteristics are affected by the transient control and protection strategies of the converters. The present invention does not rely on the specific short-circuit current change characteristics, but judges the fault location by comparing the equivalent impedance amplitude ratio on both sides of the line. Therefore, even if the control and protection strategies of the power electronic converter change, as long as the current shunting phenomenon caused by the fault still exists, the present invention can accurately identify the fault.

[0024] In summary, the present invention effectively solves the problems existing in the prior art by considering the influence of energy storage charging and discharging characteristics and using the equivalent impedance amplitude ratio for fault identification principles or logic, thereby improving the reliability and accuracy of the longitudinal protection of the lines connected to the photovoltaic power station. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a data flow diagram of an embodiment of the present invention; Figure 2 This is a data diagram of the equivalent impedance amplitude ratio and protection action status on both sides of the fault line according to an embodiment of the present invention; wherein, "√" indicates protection action, and "×" indicates protection inaction; Figure 3 The equivalent impedance and measured impedance curve of the photovoltaic power station side on the line when the transition resistance on the line is 30Ω, 50Ω, and 100Ω and the A phase grounding fault occurs under the embodiment of the present invention; wherein, a) and b) are respectively the equivalent impedance and measured impedance curves when used as a power source; c) and d) are respectively the equivalent impedance and measured impedance curves when used as a load; Figure 4 The protection action characteristic curve diagram of the embodiment of the present invention is when the transition resistance on the line is 30Ω, 50Ω, and 100Ω and the phase A grounding fault occurs; Among them, a) and b) are the protection action characteristic curves when the A-phase grounding fault occurs when the transition resistance on the lower line is 30Ω when it is used as a power supply or load, c) and d) are the protection action characteristic curves when the A-phase grounding fault occurs when the transition resistance on the lower line is 50Ω when it is used as a power supply or load, e) and f) are the protection action characteristic curves when the A-phase grounding fault occurs when the transition resistance on the lower line is 100Ω when it is used as a power supply or load. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution will be clearly and completely described below in combination with the embodiments. 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.

[0027] It should be understood that in various embodiments of the present invention, the size of the sequence number of each process does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0028] It should be understood that in the present invention, "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus comprising a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products or apparatuses.

[0029] It should be understood that in the present invention, "plurality" refers to two or more than two. "And / or" is merely a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. The character " / " generally indicates that the objects associated before and after are in an "or" relationship. "Contains A, B and C", "Contains A, B, C" means that A, B, and C are all included, "Contains A, B or C" means that one of A, B, and C is included, and "Contains A, B and / or C" means that any one, any two, or any three of A, B, and C are included.

[0030] The technical solution of the present invention is described in detail with specific embodiments below. The embodiments may be combined with each other, and the same or similar concepts or processes may not be described in detail in some embodiments.

[0031] Example: The longitudinal protection method of the lines connected to the photovoltaic power station taking into account the energy storage charging and discharging characteristics includes the following steps: S1: For the protected line, the instantaneous current and voltage on both sides of the protected line are measured through current and voltage transformers; S2: Calculate the power frequency components of the current and voltage on both sides of the line using the discrete Fourier algorithm; S3: Calculate the equivalent impedance on both sides of the line based on the power frequency components of current and voltage; S4: Calculate the equivalent impedance amplitude on both sides of the line according to the equivalent impedance on both sides of the line and amplify it differentially. Calculate the ratio based on the equivalent impedance amplitude after differential amplification. Determine whether it is an in-zone fault according to the result. If so, send an action signal to the protection device and the circuit breakers on both sides of the line will trip. Otherwise, the protection will not operate.

[0032] The data flow diagram of this embodiment is shown in Figure 1 Since the energy storage system has the dual characteristics of source and load in the photovoltaic power station, that is, it can be used as both a power source and a load, its charging and discharging mode has a significant impact on the short-circuit current. This embodiment indirectly considers the charging and discharging mode of the energy storage system when calculating the equivalent impedance, thereby improving the fault identification accuracy under different operating modes (power source or load) of the photovoltaic power station. In addition, by calculating the equivalent impedance amplitude on both sides of the line and performing differentiated amplification, the difference in the equivalent impedance on both sides of the line under in-zone faults and out-of-zone faults can be significantly enlarged, reducing false operations while increasing the reliability and stability of protection, and based on the current shunting phenomenon caused by the fault, it can more accurately determine whether the fault occurs in the area.

[0033] At the same time, this embodiment does not rely on the specific short-circuit current variation characteristics, but determines the fault location by comparing the equivalent impedance amplitude ratio on both sides of the line. Therefore, it is not affected by the transient control and protection strategy of the converter and is not affected by high-resistance grounding faults.

[0034] Specifically, S2: using the discrete Fourier algorithm to calculate the power frequency components of the current and voltage on both sides of the line, including: ; in, is the sampling signal, is the number of sampling points, is the frequency index, is the representation in the frequency domain; extract The real and imaginary parts of the current and voltage on the photovoltaic storage side are obtained by , , and the power frequency components of the current and voltage on the system side , .

[0035] Specifically, the step S3: calculating the equivalent impedances on both sides of the line according to the power frequency components of the current and voltage, includes: ; in , are the equivalent impedances of the photovoltaic storage side and the system side, , are the power frequency components of the current and voltage on the photovoltaic storage side, , They are the power frequency components of current and voltage on the system side respectively.

[0036] In this embodiment, the change in the power frequency current amplitude is calculated according to the power frequency component of the current and voltage, including: ; In the formula, , are the changes in the power frequency current amplitude on the photovoltaic storage side and the system side, and is the current power frequency current amplitude on the photovoltaic storage side and the system side; and is the power frequency current amplitude at the previous moment.

[0037] Among them, before the failure and Basically close to 0, once a fault occurs and It will become noticeably larger.

[0038] Next, we analyze the change characteristics of the equivalent impedance measured on both sides of the line under normal operating conditions, out-of-area faults, and in-area faults. When the photovoltaic power station is operated as a power source or load in different scenarios, ignoring the influence of the distributed capacitance current on the line, the fundamental frequency current measured at the protection installation on both sides of the line under normal operating conditions is equal in magnitude and opposite in direction. In this case, the equivalent impedance measured on the photovoltaic storage side and the system side is basically equal, that is, the equivalent impedance ratio on both sides is close to 1, as shown in the following formula: ; When the line optical storage side area Outside the system side area When a fault occurs, the relationship between the equivalent impedances measured at the protection installations on both sides of the line is basically similar to that under normal operating conditions, that is, the amplitudes of the equivalent impedances measured on the photovoltaic storage side and the system side are equal, and the above relationship still holds.

[0039] However, if the line When a fault occurs in the area, the current flow direction on the photovoltaic storage side will be directly related to the energy storage charging and discharging mode. When the energy storage works in the charging mode, if the photovoltaic storage power station is operated as a load, the current on the photovoltaic storage side of the line will no longer flow from the bus to the fault point, but to the outside of the area. The current flows in the same direction under fault conditions. However, in this case, the magnitude relationship of the equivalent impedance measured on both sides of the line is different from that outside the area. There is a big difference between the faults in the area, which is mainly because the current amplitudes on the photovoltaic storage side and the system side are quite different when the fault occurs in the area. In fact, whether the photovoltaic storage power station is operated as a power source or a load, the short-circuit current on the photovoltaic storage side is usually limited to times the rated current, while the short-circuit current on the system side is usually times the rated current Even in the scenario where the proportion of photovoltaic power stations connected is high, when the short-circuit current provided by the system during the fault is basically the same as that of the photovoltaic power station, the factors affecting the magnitude of the short-circuit current provided by the photovoltaic power station are different from those of the connected system, so the change amplitude of the short-circuit current on the photovoltaic power station side of the line is completely different from that on the system side. This will make the equivalent impedance value measured on the photovoltaic power station side of the line under the fault in the area significantly different from the equivalent impedance measured on the system side, that is, satisfying the following formula: ; Through the above analysis, it can be seen that this embodiment uses the proposed measured equivalent impedance ratio to identify faults inside and outside the area, and will not be affected by the current reversal problem at the protection installation caused by the photovoltaic power station operating as a load in the charging mode. Under faults inside the area, the equivalent impedance ratio on both sides of the line is always much greater than or much less than 1; while under faults outside the area or normal operating conditions, regardless of whether the photovoltaic power station operates as a power source or a load, the equivalent impedance ratio at the protection installation on both sides of the line is close to 1.

[0040] Specifically, in S4, the equivalent impedance amplitudes on both sides of the line are calculated according to the equivalent impedances on both sides of the line and differentially amplified, including: Calculate the equivalent impedance amplitude of the line on the photovoltaic storage side and the system side based on the equivalent impedance on both sides of the line , ; Based on the equivalent impedance amplitudes on the photovoltaic storage side and the system side and the changes in the power frequency current amplitude, the equivalent impedance amplitudes are differentially amplified to obtain the differentiated maximum impedance and the differentiated minimum impedance, including: ; In the formula, and are the differentiated maximum impedance and the differentiated minimum impedance, respectively. , are the equivalent impedance amplitudes on the photovoltaic storage side and the system side, is a natural constant, , They are the changes in the power frequency current amplitude on the photovoltaic storage side and the system side respectively.

[0041] Specifically, in S4, based on the ratio of the equivalent impedance amplitude after differential amplification, it is determined whether it is an internal fault according to the result. If it is, an action signal is sent to the protection device, and the circuit breakers on both sides of the line trip. Otherwise, the protection does not operate, including: ; In the formula, is the impedance adjustment factor; If the inequality holds, an action signal is sent to the protection device and the circuit breakers on both sides of the line trip. Otherwise, the protection does not operate.

[0042] In this embodiment, the impedance adjustment coefficient Take 1.2.

[0043] This embodiment also provides an electronic device, including a memory and a processor, wherein the memory stores a computer program, and when the processor calls the computer program in the memory, the steps of the method for longitudinal protection of lines connected to a photovoltaic power station taking into account the charging and discharging characteristics of energy storage are implemented.

[0044] This embodiment also provides a storage medium, in which computer executable instructions are stored. When the computer executable instructions are loaded and executed by a processor, the steps of the method for longitudinal protection of lines connected to a photovoltaic power station taking into account the charging and discharging characteristics of energy storage are implemented.

[0045] In order to verify the reliability and stability of the method described in this embodiment, an electromagnetic transient simulation model of a power grid containing a photovoltaic power station and a storage power station is built based on PSCAD / EMTDC. It is assumed that the total active power of the photovoltaic power station and the storage power station are 60MW and -60MW respectively, and the area outside the , and within the area Single-phase grounding, two-phase-to-phase grounding, two-phase-to-phase or three-phase short circuit occurs (the transition resistance is 15Ω).

[0046] Figure 2 This is a data chart of the equivalent impedance amplitude ratio and protection action on both sides of the fault line. It can be seen that no matter the photovoltaic power station is operated as a power source or a load, , When an out-of-zone fault occurs at , the amplitude ratio of the equivalent impedances on both sides of the fault phase and the non-fault phase is close to 1, and the proposed protection criterion is reliable and does not operate. When an internal fault occurs in the area, under any fault type, the equivalent impedance amplitude ratio on both sides of the fault phase is much larger than 1.2, while the equivalent impedance amplitude ratio on both sides of the non-fault phase is close to 1. The protection judgment criterion can realize the fault phase action and the non-fault phase no action.

[0047] At the same time, comparison shows that under the same fault conditions, when the photovoltaic power station operates as a load, the equivalent impedance amplitude ratio is relatively large, that is, the fault characteristics are more obvious, which helps the proposed protection to better identify faults and effectively avoids the influence of the complex phase angle change characteristics of the short-circuit current on both sides of the fault line on the accuracy of fault identification.

[0048] At the same time, for the single-phase ground short circuit fault that is easily affected by transition resistance, the operation mode and total output power of the photovoltaic power station here are the same as those in the previous article, with the output power of the transmission line Taking the A phase ground short circuit at the midpoint of the line as an example, when the transition resistance is 30Ω, 50Ω, and 100Ω respectively, the action characteristics of the proposed protection are compared and analyzed. Figure 3 and Figure 4 They are the measured equivalent impedance and measured impedance curve and the protection action characteristic curve at the photovoltaic power station side.

[0049] contrast Figure 3 In a) and b), it can be found that when the photovoltaic power station is used as a power source, the equivalent impedance measured at the protection installation on the line is not affected by the transition resistance and is smaller than the impedance amplitude of the actual fault line (calculated according to the parameters given in the previous article, the line impedance amplitude is 6.8Ω). However, the measured impedance at the protection installation gradually increases with the increase of the transition resistance. When the transition resistance is 30Ω, the measured impedance is about 80Ω, which is much larger than the fault line impedance.

[0050] In addition, if Figure 3 As shown in c) and d), when the photovoltaic power station operates as a load, the equivalent impedance and measured impedance measured on the line change in the same trend as the transition resistance changes, that is, the larger the transition resistance, the smaller the measured equivalent impedance and measured impedance. In fact, the measured equivalent impedance and measured impedance on the line will be affected by the current ratio on both sides of the line. Under a high-resistance grounding fault, the current ratio on both sides of the line is small, so the equivalent impedance and measured impedance are also relatively small. However, due to the different fault response characteristics of the photovoltaic power station and the connected system, the magnitude of the reduction in short-circuit current on the photovoltaic power station side is quite different from the magnitude of the reduction in short-circuit current on the system side. This will make the equivalent impedance amplitude ratio on both sides of the line always greater than the protection threshold value even if a high-resistance grounding fault occurs under intra-area faults. In addition, by comparison Figure 3 From c) and d), it can be found that when the photovoltaic power station operates as a load, the equivalent impedance on the photovoltaic power station side is always greater than the fault line impedance, while the system side is usually a conventional power supply, and its equivalent impedance is smaller than the fault line impedance. Therefore, the proposed protection criterion based on the equivalent impedance amplitude ratio can effectively identify high-resistance grounding faults.

[0051] like Figure 4 As shown in the figure, a) and b) are the protection action characteristic curves when the A-phase grounding fault occurs when the transition resistance is 30Ω on the lower line when it is used as a power source or load, c) and d) are the protection action characteristic curves when the A-phase grounding fault occurs when the transition resistance is 50Ω on the lower line when it is used as a power source or load, e) and f) are the protection action characteristic curves when the A-phase grounding fault occurs when the transition resistance is 100Ω on the lower line when it is used as a power source or load. It can be seen that under different transition resistances, even if 50Ω and 100Ω high-resistance grounding faults occur, the equivalent impedance amplitude ratio on both sides of the line when the photovoltaic power station is running as a load is It is always greater than the equivalent impedance amplitude ratio when the photovoltaic power station is operated as a power source, that is, the proposed protection has a higher sensitivity. At the same time, it can be found by comparison that when the photovoltaic power station is operated as a power source, the larger the transition resistance, the smaller the equivalent impedance amplitude ratio, but they are all greater than the protection action threshold value, and the protection can act reliably. It is worth pointing out that the decrease in the equivalent impedance amplitude ratio is mainly due to the small difference in current and current change on both sides of the line under high-resistance grounding faults.

[0052] The above process fully proves that: compared with the existing technical solutions, the method described in this embodiment can achieve the situation where a high proportion of energy storage and new energy stations are connected. When a fault occurs, the protection can reliably, accurately and quickly distinguish between internal and external faults, and has good adaptability in various fault situations.

[0053] In summary, the substantial effects of this embodiment include: Since the energy storage system has the dual characteristics of source and load in the photovoltaic power station, that is, it can provide electricity to the power grid as a power source, and can also absorb electricity from the power grid as a load when needed, this difference in charging and discharging mode has a significant impact on the short-circuit current phase angle and amplitude of the photovoltaic power station. When calculating the equivalent impedance, this embodiment fully considers the impact of the charging and discharging mode of the energy storage system on the short-circuit current, and effectively amplifies the difference between the equivalent impedances on both sides of the line under the fault in the area by introducing the impedance adjustment coefficient related to the current change on both sides of the line, thereby improving the accuracy of fault identification.

[0054] In addition, in normal operation or out-of-zone faults, the equivalent impedance amplitudes on both sides of the line are basically equal; in the case of an in-zone fault, due to the shunting effect of the fault point current, the equivalent impedance amplitudes on both sides of the line will have significant differences. Therefore, this embodiment calculates the equivalent impedance amplitude ratio on both sides of the line and compares it with the set impedance adjustment coefficient to determine whether the fault occurs in the zone. This method is not affected by the operation of the photovoltaic power station as a power source or load, and can accurately distinguish between faults inside and outside the zone.

[0055] In addition, the traditional relay protection principle based on the characteristics of synchronous machines is difficult to adapt to changes in the control strategy of power electronic converters. The photovoltaic and energy storage systems in the photovoltaic power station are connected to the grid through power electronic converters, and their short-circuit current characteristics are affected by the transient control and protection strategies of the converters. However, this embodiment does not rely on the specific short-circuit current change characteristics, but judges the fault location by comparing the equivalent impedance amplitude ratio on both sides of the line. Therefore, even if the control and protection strategies of the power electronic converter change, as long as the current shunting phenomenon caused by the fault still exists, this embodiment can accurately identify the fault.

[0056] In summary, this embodiment effectively solves the problems existing in the prior art by considering the influence of energy storage charging and discharging characteristics and using the equivalent impedance amplitude ratio for fault identification and other principles or logics, thereby improving the reliability and accuracy of the longitudinal protection of the lines connected to the photovoltaic power station.

[0057] Through the description of the above implementation methods, technical personnel in the relevant field can understand that for the convenience and simplicity of description, only the division of the above-mentioned functional modules is used as an example. In actual applications, the above-mentioned functions can be assigned to different functional modules as needed, that is, the internal structure of the specific device can be divided into different functional modules to complete all or part of the functions described above.

[0058] In the embodiments provided in the present application, it should be understood that the disclosed structures and methods can be implemented in other ways. For example, the embodiments of the structure described above are only schematic. For example, the division of modules or units is only a logical function division. There may be other division methods in actual implementation. For example, multiple units or components can be combined or integrated into another structure, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be an indirect coupling or communication connection through some interfaces, structures or units, which can be electrical, mechanical or other forms.

[0059] The units described as separate components may or may not be physically separated, and the components shown as units may be one physical unit or multiple physical units, that is, they may be located in one place or distributed in multiple different places. Some or all of the units may be selected according to actual needs to achieve the purpose of the present embodiment.

[0060] In addition, each functional unit in the embodiment of the present application may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above-mentioned integrated unit may be implemented in the form of hardware or in the form of software functional units.

[0061] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a readable storage medium. Based on this understanding, the technical solution of the embodiment of the present application is essentially or the part that contributes to the prior art or all or part of the technical solution can be embodied in the form of a software product, which is stored in a storage medium, including several instructions to enable a device (which can be a single-chip microcomputer, chip, etc.) or a processor (processor) to perform all or part of the steps of the various embodiments of the present application. The aforementioned storage medium includes: U disk, mobile hard disk, read only memory (ROM), random access memory (RAM), disk or optical disk and other media that can store program code.

[0062] The above contents are only specific implementation methods of the present application, but the protection scope of the present application is not limited thereto. Any technician familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A longitudinal protection method for lines connected to a photovoltaic power station taking into account the energy storage charging and discharging characteristics, characterized in that: The following steps are involved: S1: For the protected line, the instantaneous current and voltage on both sides of the protected line are measured through current and voltage transformers; S2: Calculate the power frequency components of the current and voltage on both sides of the line using the discrete Fourier algorithm; S3: Calculate the equivalent impedance on both sides of the line based on the power frequency components of current and voltage; S4: Calculate the equivalent impedance amplitude on both sides of the line according to the equivalent impedance on both sides of the line and amplify it differentially. Calculate the ratio based on the equivalent impedance amplitude after differential amplification. Determine whether it is an in-zone fault according to the result. If so, send an action signal to the protection device and the circuit breakers on both sides of the line will trip. Otherwise, the protection will not operate.

2. The longitudinal protection method for lines connected to a photovoltaic power station taking into account the energy storage charging and discharging characteristics according to claim 1 is characterized in that: S2: Calculating the power frequency components of the current and voltage on both sides of the line using a discrete Fourier algorithm, including: ； in, is the sampling signal, is the number of sampling points, is the frequency index, is the representation in the frequency domain; extract The real and imaginary parts of the current and voltage on the photovoltaic storage side are obtained by , , and the power frequency components of the current and voltage on the system side , .

3. The longitudinal protection method for lines connected to a photovoltaic power station taking into account the energy storage charging and discharging characteristics according to claim 1 is characterized in that: S3: calculating the equivalent impedance on both sides of the line according to the power frequency components of the current and voltage, including: ； in , are the equivalent impedances of the photovoltaic storage side and the system side, , are the power frequency components of the current and voltage on the photovoltaic storage side, , They are the power frequency components of current and voltage on the system side respectively.

4. The longitudinal protection method for lines connected to a photovoltaic power station taking into account the energy storage charging and discharging characteristics according to claim 1 is characterized in that: It also includes calculating the change in the power frequency current amplitude based on the power frequency components of the current and voltage, including: ； In the formula, , are the changes in the power frequency current amplitude on the photovoltaic storage side and the system side, and is the current power frequency current amplitude on the photovoltaic storage side and the system side; and is the power frequency current amplitude at the previous moment.

5. The longitudinal protection method for lines connected to a photovoltaic power station taking into account the energy storage charging and discharging characteristics according to claim 1 is characterized in that: In the above S4, the equivalent impedance amplitudes on both sides of the line are calculated according to the equivalent impedances on both sides of the line and differentially amplified, including: Calculate the equivalent impedance amplitude of the line on the photovoltaic storage side and the system side based on the equivalent impedance on both sides of the line , ; Based on the equivalent impedance amplitudes on the photovoltaic storage side and the system side and the changes in the power frequency current amplitude, the equivalent impedance amplitudes are differentially amplified to obtain differentiated maximum impedance and differentiated minimum impedance.

6. The longitudinal protection method for lines connected to a photovoltaic power station taking into account the energy storage charging and discharging characteristics according to claim 5 is characterized in that: The method of differentially amplifying the equivalent impedance amplitude based on the equivalent impedance amplitude on the photovoltaic storage side and the system side and the change in the power frequency current amplitude to obtain the differentiated maximum impedance and the differentiated minimum impedance includes: ； In the formula, and are the differentiated maximum impedance and the differentiated minimum impedance, respectively. , are the equivalent impedance amplitudes on the photovoltaic storage side and the system side, is a natural constant, , They are the changes in the power frequency current amplitude on the photovoltaic storage side and the system side respectively.

7. The longitudinal protection method for lines connected to a photovoltaic power station taking into account the energy storage charging and discharging characteristics according to claim 6 is characterized in that: In S4, based on the ratio of the equivalent impedance amplitude after differential amplification, it is determined whether it is an internal fault according to the result. If it is, an action signal is sent to the protection device, and the circuit breakers on both sides of the line are tripped. Otherwise, the protection does not act, including: ； In the formula, is the impedance adjustment factor; If the inequality holds, an action signal is sent to the protection device and the circuit breakers on both sides of the line trip. Otherwise, the protection does not operate.

8. The longitudinal protection method for lines connected to a photovoltaic power station taking into account the energy storage charging and discharging characteristics according to claim 7 is characterized in that: The impedance adjustment factor Take 1.

2.

9. An electronic device, characterized in that: It comprises a memory and a processor, wherein the memory stores a computer program, and when the processor calls the computer program in the memory, it implements the steps of the method for longitudinal protection of lines connected to a photovoltaic power station taking into account the charging and discharging characteristics of energy storage as described in any one of claims 1 to 8.

10. A storage medium, characterized in that: The storage medium stores computer executable instructions, which, when loaded and executed by the processor, implement the steps of the method for longitudinal protection of lines connected to a photovoltaic power station taking into account the energy storage charging and discharging characteristics as described in any one of claims 1 to 8.

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