On-site distributed power supply network-related protection method, device and system satisfying frequency voltage ride-through capability improvement, and medium

Abstract

The invention provides an in-situ distributed power supply network-related protection method, device, system and medium satisfying frequency voltage ride-through capability improvement, and the method comprises the steps: determining the differentiated action requirements of a DER during system fault, island operation and system disturbance according to the low voltage ride-through and anti-island protection requirements of the DER; according to the differentiated action requirements, analyzing the electrical quantity characteristics of the DER grid-connected part in the system fault, island operation and system disturbance states, and obtaining a key characteristic boundary value; and starting a multi-criterion protection mechanism based on the voltage change rate, and realizing selective control of the DER in combination with the frequency change rate, the frequency and the differentiated action time limit. According to the method, the balance of disturbance ride-through, anti-islanding protection and fault protection capabilities of the DER system is realized, and the method has an important value for constructing a high-toughness, high-reliability and sustainable future power system.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power system relay protection, and particularly relates to a method, device and system for in-situ distributed power supply grid-connected protection meeting the improvement of frequency voltage ride-through capability and a medium. BACKGROUND

[0002] Under the guidance of the "double carbon" target, with the rapid growth of installed capacity of wind power, photovoltaic and other new energy in China, the demand for power grid transformation and upgrading from conventional power source dominance to new energy dominance promotes the continuous improvement of DER penetration rate in distribution network. The larger the proportion of DER in the distribution network, the more critical the support of voltage frequency when the system experiences local faults or disturbances, and the role of ensuring power supply reliability and power quality in normal operation. Therefore, the DER grid connection regulation clearly requires it to have low voltage ride-through capability to ensure that it can maintain grid-connected state and provide strong support for the grid in the event of system faults or disturbances causing voltage sag. However, the access of DER also brings the risk of unplanned islanding. When islanding, voltage and frequency out-of-limit may occur in the region, and DER may send reverse power to damage equipment, which not only endangers personnel safety, but also affects the reclosing success rate. Therefore, when the grid fails or operates in islanding, part of the DER is required to be disconnected from the grid to avoid affecting the reclosing and safe operation of the system. Therefore, in view of the above differentiated action requirements of the DER connected to the grid in different operating states of the system, it is necessary to configure corresponding grid-connected protection devices at the point of connection (POC) or point of common coupling (PCC) of the DER.

[0003] According to the Q / GDW11198-2014 Distributed Generation Grid-Connected Protection Technical Specification, grid-connected protection is defined as the part of the protection and control devices of the distributed generation that is related to the operation mode of the public grid or needs to be coordinated with the protection and security automatic devices in the public grid. The content includes DER side fault protection, voltage frequency protection, anti-islanding protection, fault splitting, etc. At present, there are more research results on anti-islanding protection, including remote island detection technology based on communication support, passive method based on local information, and active method based on active disturbance or impedance insertion. Passive protection identifies island operation by monitoring parameters such as voltage, frequency, frequency change rate, voltage phase offset, and harmonic distortion at the grid-connected point. Passive method is widely used because it is easy to implement and has less impact on power quality. However, when the load and DER output power are nearly matched, there is a blind area in island detection, which can be reduced by combining active method. For system faults, voltage frequency is often used as a characteristic quantity in actual engineering applications to prevent islanding protection from serving as fault protection, and DER is quickly removed within 2 seconds. Patent CN 117691738A discloses a method for adaptive configuration of anti-islanding protection setting based on fault characteristic quantity, which solves the problem that the current distributed power anti-islanding protection cannot consider both the rapid action of distributed power protection when islanding occurs and the support of voltage and frequency of distributed power during system disturbance; ensures that the distributed power exits in time when islanding is caused by fault, and continues to operate in grid-connected mode when the grid voltage and frequency are disturbed, providing power support for the system. The fault characteristic quantity detection includes zero sequence component, positive sequence sudden variable, negative sequence component, A-phase voltage sudden variable, B-phase voltage sudden variable, and C-phase voltage sudden variable. If one of the above characteristic quantities exceeds the threshold value, the islanding protection setting value is set to high sensitivity voltage and frequency anti-islanding protection setting value, otherwise the low sensitivity voltage and frequency anti-islanding protection setting value is started

[0004] The patent CN 117691738A discloses a method for adaptive configuration of islanding protection setting value based on fault characteristic quantities. The method mainly uses fault characteristic quantities such as zero sequence component, positive sequence sudden variable, negative sequence component, A-phase voltage sudden variable, B-phase voltage sudden variable, and C-phase voltage sudden variable to distinguish between normal operation, disturbance, and island operation of the system. When the system is in normal operation or disturbance, low-sensitivity voltage and frequency islanding protection setting value is started, DER is off the grid for more than 2s, and the voltage and frequency of the power grid are supported. When the system is in island operation, high-sensitivity voltage and frequency islanding protection setting value is started, and DER is quickly off the grid within 2s after the voltage and frequency exceed the limit. The patent realizes the compromise between the fast action of distributed power protection when islanding occurs and the voltage and frequency ride-through capability of distributed power when the system is disturbed. However, the characteristic quantities and threshold values of fault characteristic quantities when the system is disturbed and islanded, as well as the action time limit, are not analyzed and explained in detail, and the protection scheme does not fully consider the action requirements of DER protection and its voltage and frequency ride-through capability when the system fails. In actual engineering applications, voltage and frequency are often used as characteristic quantities for islanding protection, and DER is quickly removed within 2 seconds. However, the problem is that only using the current voltage and frequency characteristics cannot distinguish between system failure and island operation, resulting in the protection quickly removing DER regardless of the system failure, which causes non-selective DER off-grid, possibly leading to large-area DER off-grid when facing local system failure, and further exacerbating the unstable operation of the system. Therefore, the characteristics of some DERs that need to respond quickly when the system fails often conflict with the low-voltage ride-through capability of other DERs, and the DER grid protection action scheme must be proposed first to distinguish between different fault states and island operation, and then to fully utilize the voltage and frequency enhancement capability of DER.

[0005] For distribution networks with a high proportion of DERs, it is expected that the voltage and frequency support of DERs will be fully utilized during system disturbance, and that DERs in the island region will be quickly removed within 2s during unplanned island operation. At the same time, during system failure, only DERs that affect system safety and reclosing should be removed in time, and the remaining DERs should continue to operate to enhance the voltage and frequency support capability. This patent defines distributed power grid protection as: in medium-voltage distribution networks, comprehensive protection installed at POC and PCC, integrating fault protection and islanding protection functions.

[0006] The DER on-grid protection method based on local electrical quantity collects local electrical quantity, does not need to communicate with other protection devices, has fast action speed and high reliability. Through reasonable configuration of the DER on-site on-grid protection, balance is achieved between system disturbance ride-through capability, anti-islanding protection capability and fault protection capability, distributed power sources can be selectively and quickly removed in system fault and island operation, and can ensure not to be removed from the grid in power system disturbance, which can be effectively applied to active distribution network fault protection and power supply recovery scene, improve power supply guarantee capability and high-quality service level, and ensure safe and stable operation of the distribution network. SUMMARY

[0007] The application provides a just-in-place distributed power source on-grid protection method, device, system and medium that meet the improvement of frequency voltage ride-through capability, to solve the problem of non-selective large-area off-grid of distributed power sources (DER) in system fault, island operation and system disturbance of traditional on-grid protection, and aggravate the unstable operation of the system.

[0008] A just-in-place distributed power source on-grid protection method that meets the improvement of frequency voltage ride-through capability, comprising the following steps:

[0009] According to the low voltage ride-through and anti-islanding protection requirements of the distributed power source DER, the differentiated action requirements of the DER in system fault, island operation and system disturbance are determined;

[0010] According to the differentiated action requirements, the electrical quantity characteristics of the DER at the on-grid place in the system fault, island operation and system disturbance state are analyzed, and key characteristic boundary values are obtained, the electrical quantity characteristics include voltage rate of change, frequency rate of change and frequency, and the key characteristic boundary values include voltage rate of change threshold, frequency rate of change threshold, frequency threshold range and differentiated action time limit;

[0011] Based on the voltage rate of change, a multi-criteria protection mechanism is started, and the selective control of the DER is realized in combination with the frequency rate of change, the frequency and the differentiated action time limit.

[0012] Further, the system fault includes high-voltage incoming line fault, main transformer fault, adjacent line fault and this line fault, and the differentiated action requirements include:

[0013] When the system is disturbed, all DERs do not remove the grid;

[0014] When the high-voltage incoming line or the main transformer fails, all DERs exit operation;

[0015] When the adjacent line fails, the DER of this line does not remove the grid;

[0016] When the line is faulted, the fault section and downstream DERs are disconnected, and the upstream non-fault DERs remain connected.

[0017] When the island is operated, the DERs in the island region are disconnected, and the rest of the DERs are not disconnected.

[0018] Further, the distinction between the adjacent line fault and the line fault is achieved by a frequency rate of change threshold value: when the adjacent line is faulted, the RoCoF at the DER connected to the line is less than the frequency rate of change threshold value, and the protection does not act; when the line is faulted, the RoCoF at the DER connected downstream of the fault point is greater than the frequency rate of change threshold value, and the protection acts to disconnect the DER in the region.

[0019] Further, the multi-criteria protection mechanism is started based on the voltage rate of change, and the selective control of the DER is achieved in combination with the frequency rate of change, the frequency, and the differentiated action time limit, including:

[0020] When the voltage rate of change is less than the voltage rate of change threshold value, it is determined that the system is disturbed, the slow voltage-frequency protection is started, and the DER is disconnected according to the voltage / frequency overrun degree with a preset delay;

[0021] When the voltage rate of change is greater than or equal to the voltage rate of change threshold value, it is determined that the system is faulted or operated in island mode, the fast protection is started, and if the frequency rate of change is greater than the frequency rate of change threshold value or the frequency is out of the frequency threshold value range, and the duration exceeds the differentiated action time limit, the corresponding DER is immediately disconnected.

[0022] A just-in-place type distributed power grid protection device that meets the improvement of frequency and voltage ride-through capability, including the following steps:

[0023] A differentiated action requirement determination module is configured to determine the differentiated action requirements of the DER in system fault, island operation, and system disturbance according to the low voltage ride-through and anti-islanding protection requirements of the distributed power source DER;

[0024] An electrical quantity characteristic and electrical quantity characteristic acquisition module is configured to analyze the electrical quantity characteristics of the DER connected to the grid in the system fault, island operation, and system disturbance state according to the differentiated action requirements, and acquire key characteristic boundary values, wherein the electrical quantity characteristics include the voltage rate of change, the frequency rate of change, and the frequency, and the key characteristic boundary values include the voltage rate of change threshold value, the frequency rate of change threshold value, the frequency threshold value range, and the differentiated action time limit.

[0025] A distributed power source control module is configured to start a multi-criteria protection mechanism based on the voltage rate of change, and achieve the selective control of the DER in combination with the frequency rate of change, the frequency, and the differentiated action time limit.

[0026] Further, the system faults include high-voltage incoming line fault, main transformer fault, adjacent line fault and the line fault, and the differentiated actions include:

[0027] All DERs are not disconnected during system disturbance;

[0028] All DERs are disconnected during high-voltage incoming line fault or main transformer fault;

[0029] DERs on the line are not disconnected during adjacent line fault;

[0030] DERs in the fault section and downstream are disconnected, and DERs in the non-fault section upstream remain connected during the line fault;

[0031] DERs in the island region are disconnected, and the rest of DERs are not disconnected during island operation.

[0032] Further, the adjacent line fault and the line fault are distinguished by a frequency rate of change threshold value: DERs on the line are connected during the adjacent line fault, and the RoCoF at the DER connection point is less than the frequency rate of change threshold value, and the protection is not operated; DERs downstream of the fault point are disconnected during the line fault, and the RoCoF at the DER connection point is greater than the frequency rate of change threshold value, and the protection is operated to disconnect the DER in the region.

[0033] Further, the distributed power supply control module starts a multi-criteria protection mechanism based on the voltage rate of change, and realizes selective control of DERs in combination with the frequency rate of change, frequency and differentiated action time limit, including:

[0034] When the voltage rate of change is less than the voltage rate of change threshold value, it is determined that the system is disturbed, the slow voltage-frequency protection is started, and the DER is disconnected according to the voltage / frequency overrun degree with a preset delay;

[0035] When the voltage rate of change is greater than or equal to the voltage rate of change threshold value, it is determined that the fault or island operation is started, the fast protection is started, and if the frequency rate of change is greater than the frequency rate of change threshold value or the frequency exceeds the frequency threshold value range, and the duration exceeds the differentiated action time limit, the corresponding DER is immediately disconnected.

[0036] A just-in-place type distributed power supply grid protection system meeting the improvement of frequency and voltage ride-through capability, comprising a computer readable storage medium and a processor;

[0037] The computer readable storage medium is used to store executable instructions;

[0038] The processor is used to read the executable instructions stored in the computer readable storage medium, and execute the just-in-place type distributed power supply grid protection method meeting the improvement of frequency and voltage ride-through capability.

[0039] A non-transitory computer-readable storage medium having stored thereon a computer program, which, when executed by a processor, implements the grid-connected protection method for a distributed power supply that satisfies the improved frequency voltage ride-through capability.

[0040] The application proposes differentiated action requirements of DER grid-connected protection of DER in system fault (including high-voltage incoming line fault, main transformer fault and line fault), island operation and system disturbance to improve the frequency voltage ride-through capability of DER according to the DER low voltage ride-through and anti-islanding protection requirements; analyzes the electrical quantity difference of DER grid-connected place in different operating states to obtain the key characteristic boundary value; and proposes a multi-criteria grid-connected protection started by the voltage change rate, which is composed of the frequency change rate (RoCoF), frequency and voltage characteristic criteria and differentiated action time limit. The scheme fully utilizes the frequency voltage support capability of DER in system disturbance, and more importantly, ensures that in different system fault conditions or island operation, the DER affecting safety and reclosing can be quickly and selectively removed within 0.35s, and other DERs of multiple point distributed grid connection are not caused to be off-grid in a large area, affecting the safe and stable operation of the power grid. This grid-connected protection integrating multiple functions realizes the balance of DER system disturbance ride-through, anti-islanding protection and fault protection capability, and has important value for building a high-resilience, high-reliability and sustainable future power system. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 is a typical medium-voltage distribution network system diagram;

[0042] Figure 2 is a grid-connected protection logic diagram;

[0043] Figure 3 is the RoCoF and frequency change curve when the main transformer is three-phase short-circuit;

[0044] Figure 4 is the RoCoF and frequency change curve when the main transformer is two-phase short-circuit;

[0045] Figure 5 is the RoCoF and frequency change curve when the adjacent line is three-phase short-circuit;

[0046] Figure 6 is the RoCoF and frequency change curve when the adjacent line is two-phase short-circuit;

[0047] Figure 7 is the RoCoF and frequency change curve when the line is three-phase short-circuit;

[0048] Figure 8 is the RoCoF and frequency change curve when the line is two-phase short-circuit;

[0049] Figure 9is the RoCoF and frequency change curve when operating in island mode;

[0050] Figure 10 is the RoCoF and frequency change curve when the system is disturbed;

[0051] Figure 11 is a flowchart of the method for grid-connected protection of the on-site distributed power supply according to the embodiment of the application. DETAILED DESCRIPTION

[0052] To make the objectives, technical solutions, and advantages of the embodiments of the application clearer, the technical solutions in the embodiments of the application will be described below in connection with the drawings in the embodiments of the application. Obviously, the described embodiments are only a part of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the protection scope of the application.

[0053] Referring to Figure 11 The embodiment of the application provides a method for grid-connected protection of on-site distributed power supply meeting the improvement of frequency and voltage ride-through capability, and comprises the following steps:

[0054] Step 1: According to the low-voltage ride-through and anti-islanding protection requirements of the distributed power supply DER, the differentiated action requirements of the DER in system fault, island operation, and system disturbance are determined.

[0055] The embodiment of the application studies the new form of power grid scenario shown in Figure 1 The typical new form of power grid scenario shown in the figure is taken as the research object, and the distribution network containing a high proportion of DER is taken as the research object. A plurality of distributed power supplies are connected to the public power grid through special switch intervals. DER2 and DER3 are directly connected to the user-side power grid through the point of common coupling POC. The user power grid is interconnected with the public power grid through the point of common coupling PCC. DER1, DER4, and DER5 are directly connected to the public power grid through the PCC point. PD1-PD6 are the grid-connected protections configured at each POC and PCC. In the subsequent discussion, line II is referred to as the current line, and line I is referred to as the adjacent line.

[0056] Based on the low-voltage ride-through standards, island protection, and fault protection requirements involved in different operating scenarios, specific requirements for the action of the DER grid-connected protection are proposed.

[0057] (1) When the system power is subjected to a large disturbance, a large number of DERs will be disconnected from the grid, which will further aggravate the power imbalance of the system and seriously threaten the stability of the power system. Therefore, during the disturbance, it is necessary to ensure that all DERs are not disconnected from the grid, and all grid-connected protections are not actuated.

[0058] (2) If a fault occurs in the high-voltage incoming line or the main transformer, in order to prevent the distribution network side DER from continuing to operate and causing asynchronous reclosing problems, or even affecting the normal operation of the standby automatic transfer device, all DERs should be taken out of operation and put back into operation after the power grid returns to stable operation.

[0059] (3) When a fault occurs on an adjacent line, the short-circuit current provided by the DER of this line will not affect its reclosing, and the power supply reliability of the non-faulty part must still be guaranteed after the faulty line is disconnected. Therefore, it should be ensured that the DER in this line does not disconnect from the grid.

[0060] (4) If a fault occurs on this line, the line protection device closest to the fault point will operate. To avoid affecting reclosing, the DER in the fault section and downstream should trip as soon as possible; at the same time, to give full play to the frequency and voltage support function of the DER in the non-fault section, the DER in the upstream non-fault section should remain in the network state.

[0061] (5) If the circuit breaker trips due to protection malfunction or human factors, and this leads to an islanded operation, the DERs in the islanded area should be disconnected from the network as soon as possible, while the other DERs should not be disconnected.

[0062] The key to ensuring the reliable operation of grid-connected protection systems lies in accurately distinguishing between system disturbances, faults, and islanded operation. This requires precisely identifying different fault states, with the challenge being the ability to differentiate between multiple fault states. Therefore, it is essential to extract the electrical quantity characteristics under various operating conditions to accurately identify the power system's operating status and fault location. This is crucial for resolving issues related to grid-connected protection of distributed generation systems.

[0063] Step 2: Based on the differentiated action requirements, analyze the electrical quantity characteristics at the DER grid connection under the system fault, islanded operation, and system disturbance states, and obtain key characteristic boundary values. The electrical quantity characteristics include voltage change rate, frequency change rate, and frequency. The key characteristic boundary values ​​include voltage change rate threshold, frequency change rate threshold, frequency threshold range, and differentiated action time limit.

[0064] Analysis is required based on internet protection requirements. Figure 1 The electrical quantity changes under different operating conditions are categorized as system disturbance, system fault, and islanded operation. System faults include high-voltage incoming line faults, main transformer faults, adjacent line faults, and faults on the local line. The characteristics of the electrical quantities under different operating conditions are described below.

[0065] 2.1 Electrical Quantity Characteristics During System Disturbance

[0066] At the initial moment of system disturbance, power shortage occurs in the system, voltage and frequency change, and then each unit redistributes the power shortage in the swing process, so that the voltage and frequency reach a new steady state. If there is a power shortage, the low-voltage load shedding and low-frequency load shedding safety automatic devices will further ensure the safe and stable operation of the power grid. At present, the setting value of the voltage frequency protection and the frequency change rate protection in the islanding protection is to distinguish between system disturbance and island operation, so the electrical quantity boundary value during system disturbance can be determined by referring to the setting value, the voltage is 85%U N and 110%U N , and the frequency is 49.5 Hz and 50.5 Hz. For RoCoF, due to the high penetration of new energy forming a low-inertia power system, the frequency change rate increases during system disturbance, and some countries increase the trigger value to ±1 Hz / s to distinguish between system disturbance. In addition, the blocking condition of the low-voltage load shedding device is to prevent the device from malfunctioning during a short circuit, and the voltage change rate is set to 20%~30%V / s, and the voltage change rate boundary value during system failure can be determined in turn.

[0067] 2.2, electrical quantity characteristics during system failure

[0068] 2.2.1, during high-voltage line or main transformer failure

[0069] When the high-voltage line or main transformer fails, it will cause the voltage of the distribution network to drop, and the voltage of the DER grid-connected point to drop differently. If the fault is a three-phase short circuit, the distribution network and the large power grid will be isolated due to the fault, and the support of the main grid will be lost, at this time the voltage and frequency at POC and PCC will be greatly offset, and the voltage change rate and RoCoF will exceed the boundary value during system disturbance. If the fault is a two-phase short circuit, the distribution network remains connected to the large power grid through the non-fault phase, and the frequency will hardly change, but the voltage will decrease to different degrees, and the voltage change rate will exceed the boundary value. When the high-voltage line protection or the main transformer protection acts immediately or after a delay, the distribution network side and the large power grid are completely disconnected, and island operation occurs, causing voltage and frequency changes.

[0070] 2.2.2, during adjacent line failure

[0071] When the distribution line fails, it will inevitably cause the bus voltage to drop, and the bus voltage drop amplitude is related to the fault location and fault type. If the adjacent line has a three-phase short circuit, the closer the fault point is to the bus, the lower the bus voltage, and it may even drop to zero. The mutation of the bus voltage will cause the voltage along the line of all outgoing lines of the substation to have a corresponding mutation.

[0072] When the fault occurs in the adjacent line, the frequency in the line is also related to the fault point position and the fault type, so it is necessary to further analyze the fault position boundary value of the adjacent line when the frequency change rate of the line is 1 Hz / s. When the fault occurs in the downstream area of the boundary position, the frequency of the line remains unchanged, which is different from the frequency characteristics when the fault occurs in the line, and the characteristics can be used to effectively distinguish the faults of the line and the adjacent line.

[0073] The system frequency change is caused by the mismatch between the input mechanical power and the output electromagnetic power of the synchronous motor. According to the rotor motion equation before and after the fault, the following equation is obtained:

[0074] (1)

[0075] where H is the inertia time constant of the unit, is the unit value of the difference between the mechanical input power and the electromagnetic output power, is the rated frequency of the system, is the steady-state equivalent reactance of the generator, is the transient reactance of the generator, is the line impedance from the generator outlet bus to the fault point, is the steady-state operating power angle of the generator, is the power angle of the generator after the fault.

[0076] In general, the value range of is 1.5~2.4Ω, the value range of is 0.15~0.31Ω, the value range of H is 4~8s, and the operating power angle is usually 30°. As can be seen from equation (1), when the distance from the fault point to the bus increases, the system frequency change rate will decrease. If the system frequency change rate is 1 Hz / s, equation (1) can be obtained as follows:

[0077] (2)

[0078] Under different , and H, the maximum fault impedance when RoCoF is 1 Hz / s , the line reactance is about 0.4Ω / km, and the maximum fault distance is about 92.5m, i.e. when the distance from the fault point to the bus in the adjacent line is greater than 92.5m, the frequency change rate in the line will be less than 1 Hz / s. If the first section protection is configured in the adjacent line, the protection range is usually greater than 92.5m, which can also be understood as, when the fault occurs outside the first section protection range of the adjacent line, the frequency change rate in the line will be less than 1 Hz / s.

[0079] 2.2.3, the fault of the line

[0080] When the line is three-phase short-circuited, the voltage at the grid-connected point of each DER changes suddenly, and the frequency changes to a different extent. The frequency upstream of the fault point is almost unchanged because of the support of the large power grid, while the frequency downstream of the fault point changes greatly because the downstream area is isolated from the main grid. Subsequently, the protection of the line closest to the fault point acts, and the downstream area of the protection forms a fault island. The island area loses the support of the large power grid and is accompanied by a fault, causing a serious imbalance of power, which will inevitably lead to a sharp drop in voltage and frequency. That is, after the line is three-phase short-circuited, the frequency change rate of the DER grid-connected point downstream of the line closest to the fault point is greater than 1 Hz / s, while the frequency change rate of the DER grid-connected point in the non-fault area upstream of the fault point and in the adjacent line is less than 1 Hz / s.

[0081] When the line is two-phase short-circuited, the distribution network is still electrically connected to the main grid, and the system frequency does not change. Subsequently, the protection of the line closest to the fault point acts, forming a fault island. After that, the situation is the same as that when the line is three-phase short-circuited.

[0082] 2.2.4, Operating island

[0083] The analysis of the electrical quantity variation characteristics and the blind area of the local passive protection caused by non-planned island operation due to switch operation, protection misoperation, etc. is relatively complete. In the island operation state, the higher the degree of source-load power mismatch, the more significant the changes in voltage and frequency, and the changes are characterized by sudden changes. When the source-load power is basically balanced, the changes in voltage and frequency are very small. Based on this, relying only on local voltage frequency protection and frequency change rate protection will inevitably have a detection blind area. In actual engineering applications, active protection measures must be combined to eliminate these blind areas.

[0084] Step 3, Start the multi-criteria protection mechanism based on the voltage change rate, combine the frequency change rate, frequency, and differential action time limit to achieve selective control of DER.

[0085] 3.1 DER grid protection principle design

[0086] First, use the voltage change rate (dV / dt) to distinguish between system disturbance and fault and operating island. When the voltage change rate is less than the set value, it indicates that the system is running normally or a disturbance has occurred. At this time, start the voltage frequency protection, and according to the DER grid-connected operation requirements in GB / T29319-2024, different time delays are set for DER to be disconnected from the grid according to the voltage frequency out-of-limit degree, so as to fully play the role of DER in voltage frequency support during system disturbance. When the voltage change rate is greater than the set value, it can be determined that the system has a fault, or a power mismatch operating island has occurred. Given the differential action requirements of DER in different fault and island scenarios, the electrical quantity characteristics can be used to further accurately distinguish the different states of the system.

[0087] Secondly, RoCoF can be used to distinguish the fault of the line from the fault of the adjacent line. When the RoCoF detected at the DER grid-connected point is less than 1 Hz / s, it is determined that the fault is in the adjacent line, and the DER grid-connected protection in the line does not act, and continues to run to support the frequency and voltage of the power grid. A time delay strategy can be used to solve the problem of DER misoperation when the fault is near the adjacent line. The grid-connected protection and the line I section protection cooperate through time limit, and a time difference of 0.2s is adopted. This scheme can meet the requirements that when the high-voltage incoming line or the main transformer fails, or after the corresponding protection acts, the DER on the distribution network side can quickly disconnect from the grid, and will not affect the reclosing and backup power supply; when the fault occurs in the line, the DER grid-connected protection in the non-fault area upstream of the fault point does not act, which improves the frequency / voltage ride-through capability and does not affect the reclosing, while the DER in the fault area and downstream will disconnect from the grid as soon as possible, which does not endanger the equipment and personal safety and reclosing.

[0088] Finally, for the island operation state, in addition to the frequency change rate protection, over / under frequency protection can also be added. However, it is difficult to completely eliminate the island detection blind area by relying on passive on-site protection alone, and active island protection must be used in cooperation.

[0089] 3.2 Setting calculation of grid-connected protection

[0090] (1) Voltage change rate

[0091] When the power grid fails, the voltage starts to decrease from the steady state, and then reaches the steady state value again after a short fluctuation. The voltage change rate (dV / dt) is defined as:

[0092] (3)

[0093] wherein, is the amplitude of the steady-state voltage after the fault, is the rated voltage amplitude, is the dynamic duration of the voltage, i.e. the time from the moment before the fault to the moment when the steady-state value is reached again. According to the analysis results in 2.1, to avoid DER protection misoperation when the system is disturbed, when the rated voltage is 1, the voltage change rate threshold is set to 20%~30% V / s. If 20ms is taken, according to the calculation of formula (3), the voltage change is , i.e. , which is equal to 0.4%~0.6% V. Based on this, the time window is set to 20ms, and when exceeds 0.6% V of the rated voltage, it is used as the starting threshold of the fast grid-connected protection.

[0094] (2) RoCoF and action time limit

[0095] According to the analysis results in section 2.1, the RoCoF threshold value is set to 1 Hz / s, and the time window is selected to be 10 ms. The sum of the inherent action time of the line protection I section and the breaker trip time is about 0.15 s, and on this basis, a time difference of 0.2 s is added. Therefore, the action time limit of the frequency change rate protection is set to 0.35 s.

[0096] (3) Fast over / under frequency setting value

[0097] The setting value of the fast over / under frequency is set to the over / under frequency setting value specified in the island protection of GB / T20046-2006 standard, that is, 49.5 Hz and 50.5 Hz, and the action time limit is also uniformly set to 0.35 s.

[0098] (4) Slow over / under frequency and slow voltage setting value

[0099] The setting value of the slow over / under frequency and slow voltage is set to the over / under frequency and voltage limit setting value specified in GB / T29319 standard, and the action time limit is taken to the grid connection time limit requirement under different voltages and frequencies specified in the standard.

[0100] 3.3 Grid-connected protection logic diagram

[0101] According to 3.1 and 3.2, the DER grid-connected protection logic diagram is shown in 2. When the voltage change rate of a time window is less than 0.6%V, the slow protection is started, the setting value and the action time limit are specified in GB / T29319, otherwise the fast protection is started, and the DER can be quickly removed if the frequency or frequency change rate continues to exceed the limit for 0.35 s.

[0102] 4 Simulation verification

[0103] According to The simulation model is built by Matlab / Simulink. In the model, the high-voltage side is a 110 kV system, which is connected to the main transformer through a high-voltage incoming line and is stepped down to 10 kV. There are two outgoing lines in the substation, and the length of line I is 4 km. DER1 is connected to PCC1, which is 2 km away from the outlet of line I. The main line of line II is 8 km long, which is divided into two sections by sectionalizing circuit breaker QF2, with the upstream section being 3 km long and the downstream section being 5 km long. DER2 and DER3 are connected to PCC2 through POC1 and POC2, and are connected to PCC2 through a 1-km grid-connection line. The point is 2 km away from the outlet of line II. DER4 and DER5 are connected to the grid through PCC3 and PCC4, respectively. PCC3 is located 1 km downstream of sectionalizing circuit breaker QF2, and PCC4 is located 3 km downstream of PCC3. The unit length impedance of the line is (0.17+j0.29) Ω, and the load is modeled as a constant impedance. The power of Load1~Load3 is (3.32+j1.56) MVA, (4.98+j2.34) MVA, and (1.66+j0.78) MVA, respectively. The total penetration rate of DER is 80%, and the capacity of DER1~5 is 2.6 MW, 0.7 MW, 0.7 MW, 2 MW, and 1.9 MW, respectively. At 0.3 s, faults are set at f2~f4, and island operation and system disturbance are simulated. The voltage effective value at each grid-connected point one cycle after the fault occurs is measured, and the frequency change rate (RoCoF) and frequency change curve before and after the fault are recorded to determine the action of the grid-related protection and to verify the effectiveness of the grid-related protection technology of the on-site distributed power supply that meets the frequency / voltage ride-through capability improvement.

[0104] 4.1 Main transformer fault

[0105] (1) Three-phase short circuit

[0106] A three-phase short circuit fault occurs in the main transformer at 0.3 s, and the transformer protection acts at 0.4 s. The voltage effective value measured at each grid-connected point one cycle after the fault occurs is shown in Table 1. The RoCoF and frequency change curve measured at different grid-connected points before and after the fault are shown in Figure 3 .

[0107] Table 1 Voltage measurement of grid-related protection after three-phase short circuit of main transformer

[0108]

[0109] After the main transformer fault, the voltage mutation of each DER grid-connected point exceeds 0.6%UN, and the fast protection starts immediately; After the fault, the frequency of each grid-connected point of the distribution network side drops sharply, and after the main transformer fault is cut off, the system enters the island operation state, and the frequency continues to decrease. When the duration of RoCoF or frequency out-of-limit exceeds 0.35s, the grid-related protection at this place will act immediately. Therefore, all DERs on the distribution network side act 0.25s after the main transformer protection acts. Reclosing must be coordinated with grid-related protection, and further time delay action can ensure that reclosing is not affected.

[0110] (2) Two-phase short circuit

[0111] The main transformer 0.3s occurs AB two-phase short circuit, and the transformer protection acts at 0.4s. The voltage effective value measured at each grid-connected point after one cycle of fault is shown in Table 2. The RoCoF and frequency change curves measured at different grid-connected points before and after the fault are shown in Figure 4 .

[0112] Table 2 Voltage measurement value of grid-related protection after two-phase short circuit of main transformer

[0113]

[0114] After the main transformer fault, the voltage mutation of each DER grid-connected point exceeds 0.6%UN, and the fast protection starts immediately; DER grid-connected point frequency does not change until the main transformer protection trips and island operation occurs, and the frequency and frequency change rate are out of limit, and after 0.35s, all DER grid-related protection on the distribution network side acts.

[0115] 4.2 Fault at f3 of adjacent line

[0116] (1) Three-phase short circuit

[0117] At 0.3s, a three-phase short circuit occurs at f3, which is 2km away from the outlet of adjacent line I, and the line outlet protection II segment acts for 0.55s. Therefore, the line protection acts at 0.85s. The voltage effective value measured at each grid-connected point after one cycle of fault is shown in Table 3. The RoCoF and frequency change curves measured at different grid-connected points before and after the fault are shown in Figure 5 .

[0118] Table 3 Voltage measurement value of grid-related protection after three-phase short circuit of adjacent line

[0119]

[0120] After the fault of adjacent line I, the voltage jump of each DER grid-connected point exceeds 0.6%UN, and the fast protection starts immediately. Since the fault position is not near the bus, the line still has the support of the large power grid, so the frequency of the DER grid-connected point in the line basically remains unchanged, and the RoCoF is much smaller than 1 Hz / s, and PD2~5 will not misoperate.

[0121] As for PCC1 in line I, before the export protection acts, it is connected with the large power grid, and the frequency remains stable. After the export protection acts, line I enters the fault island state, and the RoCoF and frequency measured by PD1 both exceed the limit value. After 0.35s, the protection acts, and DER1 is cut off.

[0122] (2) Two-phase-to-phase short circuit

[0123] At 0.3s, AB two-phase short circuit occurs at f3, which is 2km away from the export in adjacent line I, and the line protection acts at 0.85s. The voltage effective value measured by each grid-connected point after one cycle of fault is shown in Table 4. The RoCoF and frequency change curves measured by different grid-connected points before and after the fault are shown in Figure 6 .

[0124] Table 4 Voltage measurement value of grid-related protection after two-phase short circuit of adjacent line

[0125]

[0126] After the fault of adjacent line I, the voltage jump of each DER grid-connected point exceeds 0.6%UN, and the fast protection starts immediately. The frequency, RoCoF and grid-related protection action of each grid-connected point are basically the same as those in three-phase short circuit, and will not be described again.

[0127] 4.3 Fault of the line

[0128] (1) Three-phase short circuit

[0129] At 0.3s, three-phase short circuit fault occurs at f4, which is 2km downstream of the line II sectionalizer, and the line II sectional protection acts in 0.55s. Therefore, the sectional protection acts at 0.85s. The voltage effective value measured by each grid-connected point after one cycle of fault is shown in Table 5. The RoCoF and frequency change curves measured by different grid-connected points before and after the fault are shown in Figure 7 .

[0130] Table 5 Voltage measurement value of grid-related protection after three-phase short circuit of the line

[0131]

[0132] After the three-phase short-circuit of the line II, the voltage jump of each DER grid-connected point exceeds 0.6%UN, and the fast protection starts immediately; the upstream and downstream of the fault point are not disconnected from the main grid, and the frequency of each grid-connected point is basically unchanged; the downstream of the fault point appears a fault island, and the frequency measured by PD4 drops rapidly, and the RoCoF increases rapidly. After the sectionalization protection acts at 0.85s, PCC3 also enters the fault island state, and the frequency measured by PD3 begins to decrease, and the RoCoF rises rapidly. Therefore, when the frequency or RoCoF of PD3 and PD4 exceeds the limit value and the duration exceeds 0.35s, the protection acts immediately to cut off DER4 and DER5, while other distributed power continues to operate.

[0133] (2) Two-phase short-circuit

[0134] At 0.3s, the AB two-phase short-circuit occurs at f4, and the line II sectionalization protection acts in 0.55s, so the sectionalization protection acts at 0.85s. The voltage effective value measured by each grid-connected point after one cycle of the fault is shown in Table 6. The RoCoF and frequency change curves measured by different grid-connected points before and after the fault are shown in Figure 8 .

[0135] Table 6 Voltage measurement value of the grid-related protection after the two-phase short-circuit of the line

[0136]

[0137] After the two-phase short-circuit of the line II, the voltage jump of each DER grid-connected point exceeds 0.6%UN, and the fast protection starts immediately; since the upstream and downstream of the fault point are not disconnected after the fault, the frequency of each grid-connected point is basically unchanged. After the line sectionalization protection acts at 0.85s, PD3 and PD4 are in a fault island state, and the electrical quantity change characteristics and protection action are consistent with those in the three-phase short-circuit, which will not be described again.

[0138] 4.4 Operating island

[0139] At 0.3s, the line II sectionalization circuit breaker QF4 is tripped, and its downstream enters the operating island state. The voltage effective value measured by each grid-connected point after one cycle of the breaker tripping is shown in Table 7. The RoCoF and frequency change curves measured by different grid-connected points before and after the island are shown in Figure 9 .

[0140] Table 7 Voltage measurement value of the grid-related protection after the operating island appears

[0141]

[0142] After 0.3s, PCC3 and PCC4 enter island operation state, due to the mismatch between power supply power and load power, the voltage and frequency of the two places change suddenly, among which the voltage mutation exceeds 0.6%UN, and the fast protection starts immediately; after the frequency and the frequency change rate exceed the limit for 0.35s, DER4 and DER5 are disconnected from the grid. While the grid-related protection is not in the island operation area, the voltage and frequency at the installation location are basically unchanged, and the protection will not malfunction.

[0143] 4.5 System disturbance

[0144] Load3 in line II is tripped at 0.3s, and the voltage effective value measured at each grid-connected place after one cycle is shown in Table 8. The RoCoF and frequency change curve measured at different grid-connected places before and after the load tripping is shown in Figure 10 .

[0145] Table 8 Voltage measurement value of grid-related protection after system disturbance

[0146]

[0147] After the load tripping, the voltage mutation at each DER grid-connected place is less than 0.6%UN, and the slow protection starts; the voltage and frequency quickly recover to normal after a short time fluctuation, and neither the voltage nor the frequency exceeds the limit, so all grid-related protections will not act, and the DER continues to operate in grid-connected mode.

[0148] The present application has the following advantages:

[0149] (1) Under the premise of meeting the requirements of DER low voltage ride through and anti-islanding protection in China, the specific requirements of differentiated action of distribution network with high DER penetration in various operating states are clarified, and the target of proposing grid-related protection meeting the voltage and frequency improvement capability is clarified.

[0150] (2) The voltage change rate (dV / dt) can effectively distinguish system disturbance, fault and operating island. When the voltage change rate is less than the setting value, the voltage and frequency protection is started, and the DER plays a full role in voltage and frequency support during system disturbance according to the over-limit degree of characteristic quantity with different delay time. When the voltage change rate is greater than the setting value, the fast protection is started, and the frequency and frequency change rate over-limit and its duration are used to selectively remove the DER affecting the safe and stable operation of the system within 0.35s.

[0151] (3) When the high-voltage incoming line or the main transformer fails, after the line protection or transformer protection acts, the grid-related protection on the distribution network side can act according to the change of frequency or RoCoF, and will not affect the reclosing and standby automatic switching.

[0152] (3) By comprehensively using RoCoF and frequency change, and combining with the time delay measure, the line fault and the adjacent line fault can be effectively distinguished. In the adjacent line fault, the grid-connected protection of the line can be ensured not to be mis-operated, so that the DER can fully support the frequency and voltage before and after the system fault. Meanwhile, when the line fault occurs, the grid-connected protection of the non-fault area upstream of the fault point will not be mis-operated, and the DER of the fault section and the downstream thereof affecting the reclosing will be completely disconnected from the grid, so that the differentiated action requirement of the DER in the multi-point distributed grid in the system fault is realized.

[0153] (4) The protection scheme optimizes the grid-connected protection, balances the penetration capability, the anti-islanding protection capability and the fault protection capability of the DER in the system disturbance, realizes that the DER can be selectively and quickly removed in the system fault and the island operation, ensures that the reliable frequency and voltage support is provided in the system disturbance, can be effectively applied to the fault protection and power supply recovery scene of the active distribution network, improves the power supply guarantee capability, and guarantees the safe and stable operation of the distribution network.

[0154] Another embodiment of the present application provides a grid-connected protection system of a local distributed power supply meeting the frequency and voltage penetration capability improvement, comprising a computer readable storage medium and a processor.

[0155] The computer readable storage medium is used for storing executable instructions.

[0156] The processor is used for reading the executable instructions stored in the computer readable storage medium, and executing the grid-connected protection method of the local distributed power supply meeting the frequency and voltage penetration capability improvement.

[0157] Another embodiment of the present application provides a grid-connected protection device of a local distributed power supply meeting the frequency and voltage penetration capability improvement, comprising the following steps.

[0158] A differentiated action requirement determination module is used for determining the differentiated action requirements of the DER in the system fault, the island operation and the system disturbance according to the low voltage penetration and the anti-islanding protection requirements of the DER.

[0159] An electrical quantity characteristic and electrical quantity characteristic acquisition module is used for analyzing the electrical quantity characteristics of the DER at the grid-connected place in the system fault, the island operation and the system disturbance state according to the differentiated action requirements, and acquiring key characteristic boundary values, wherein the electrical quantity characteristics include the voltage change rate, the frequency change rate and the frequency, and the key characteristic boundary values include the voltage change rate threshold value, the frequency change rate threshold value, the frequency threshold value range and the differentiated action time limit.

[0160] A distributed power supply control module is used for starting the multi-criteria protection mechanism based on the voltage change rate, and realizing the selective control of the DER in combination with the frequency change rate, the frequency and the differentiated action time limit.

[0161] Another embodiment of the present application provides a non-transitory computer readable storage medium, having stored thereon a computer program, which, when executed by a processor, implements the method for in-situ distributed generation grid protection to meet frequency voltage ride through capability improvement.

[0162] Those skilled in the art will understand that embodiments of the present application can be provided as methods, systems, or computer program products. Accordingly, the present application can take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present application can take the form of a computer program product on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROMs, optical storage devices, etc.) embodying computer readable program code.

[0163] The present application is described with reference to the flowchart and / or block diagram illustrations of the methods, apparatus (systems) and computer program products according to embodiments of the present application. It will be understood that each block of the flowchart and / or block diagrams, and combinations of blocks in the flowchart and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general purpose computer, special purpose computer, embedded processing device or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart and / or block diagram block or blocks. Figure 1 one or more functions specified in the flowchart and / or block diagram block or blocks. Figure 1 means for carrying out each of the one or more functions specified in the flowchart and / or block diagram block or blocks.

[0164] These computer program instructions can also be stored in a computer- readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instructions which implement the flowchart and / or block diagram block or blocks. Figure 1 one or more functions specified in the flowchart and / or block diagram block or blocks. Figure 1 means for carrying out each of the one or more functions specified in the flowchart and / or block diagram block or blocks.

[0165] These computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the flowchart and / or block diagram block or blocks. Figure 1 one or more functions specified in the flowchart and / or block diagram block or blocks. Figure 1 means for carrying out each of the one or more functions specified in the flowchart and / or block diagram block or blocks.

[0166] It should be pointed out finally that the above embodiments are only used to illustrate the technical solutions of the present application but not to limit it. Although the present application has been described in detail with reference to the above embodiments, it should be understood by those skilled in the art that the specific embodiments of the present application can be modified or replaced equivalently without departing from the spirit and scope of the present application, and any modification or equivalent replacement should be covered in the protection scope of the claims of the present application.

Claims

1. A method for local distributed power source grid connection protection that improves frequency voltage ride-through capability, characterized in that, Includes the following steps: Based on the low voltage ride-through and anti-islanding protection requirements of distributed generation (DER), the differentiated action requirements of DER under system faults, islanding operation, and system disturbances are clarified. Based on the differentiated action requirements, analyze the electrical quantity characteristics at the DER grid connection under the system fault, islanded operation and system disturbance states, and obtain key characteristic boundary values. The electrical quantity characteristics include voltage change rate, frequency change rate and frequency. The key characteristic boundary values ​​include voltage change rate threshold value, frequency change rate threshold value, frequency threshold value range and differentiated action time limit. The protection mechanism is based on voltage change rate to initiate a multi-criteria protection mechanism, which combines frequency change rate, frequency, and differentiated action time limit to achieve selective control of DER.

2. The method as described in claim 1, characterized in that, The system faults include high-voltage incoming line faults, main transformer faults, adjacent line faults, and faults on this line. The differentiated action requirements include: During system disturbances, all DERs remain connected to the network; When the high-voltage incoming line or main transformer fails, all DERs will be taken out of operation. When an adjacent line fails, the DER of this line will not be disconnected from the network; When a fault occurs on this line, the faulty section and downstream DER are disconnected from the network, while the upstream non-faulty section DER remains connected to the network. When operating an island, the DERs within the island area will be disconnected from the network, while the other DERs will remain connected to the network.

3. The method as described in claim 2, characterized in that, The distinction between adjacent line faults and local line faults is achieved through a frequency change rate threshold: when an adjacent line fault occurs, the RoCoF at the local line's DER grid connection point is less than the frequency change rate threshold, and the protection does not operate; when the local line fault occurs, the RoCoF at the downstream DER grid connection point is greater than the frequency change rate threshold, and the protection operates to disconnect the DER in that area.

4. The method as described in claim 1, characterized in that, The multi-criteria protection mechanism based on voltage change rate, combined with frequency change rate, frequency, and differentiated action time limit, achieves selective control of the DER, including: When the voltage change rate is less than the voltage change rate threshold, it is determined to be a system disturbance, and slow voltage-frequency protection is activated. The DER disconnects the network after a preset delay based on the degree of voltage / frequency exceeding the limit. When the voltage change rate is greater than or equal to the voltage change rate threshold, it is determined to be a fault or islanded operation, and fast protection is activated. If the frequency change rate is greater than the frequency change rate threshold or the frequency exceeds the frequency threshold range and the duration exceeds the differentiated action time limit, the corresponding DER is immediately disconnected.

5. A local distributed power source grid connection protection device that improves frequency voltage ride-through capability, characterized in that, Includes the following steps: The differentiated action requirement determination module is used to define the differentiated action requirements of the distributed generation DER under system faults, islanding operation, and system disturbances, based on the low voltage ride-through and anti-islanding protection requirements of the DER. The electrical quantity characteristics and electrical quantity characteristic acquisition gate module are used to analyze the electrical quantity characteristics at the DER grid connection under the system fault, islanding operation and system disturbance states according to the differentiated action requirements, and to obtain key characteristic boundary values. The electrical quantity characteristics include voltage change rate, frequency change rate and frequency. The key characteristic boundary values ​​include voltage change rate threshold value, frequency change rate threshold value, frequency threshold value range and differentiated action time limit. The distributed power control module is used to initiate a multi-criteria protection mechanism based on the voltage change rate, and to achieve selective control of DER by combining the frequency change rate, frequency, and differentiated action time limit.

6. The apparatus as claimed in claim 5, characterized in that, The system faults include high-voltage incoming line faults, main transformer faults, adjacent line faults, and faults on this line. The differentiated action requirements include: During system disturbances, all DERs remain connected to the network; When the high-voltage incoming line or main transformer fails, all DERs will be taken out of operation. When an adjacent line fails, the DER of this line will not be disconnected from the network; When a fault occurs on this line, the faulty section and downstream DER are disconnected from the network, while the upstream non-faulty section DER remains connected to the network. When operating an island, the DERs within the island area will be disconnected from the network, while the other DERs will remain connected to the network.

7. The apparatus as claimed in claim 5, characterized in that, The distinction between adjacent line faults and local line faults is achieved through a frequency change rate threshold: when an adjacent line fault occurs, the RoCoF at the local line's DER grid connection point is less than the frequency change rate threshold, and the protection does not operate; when the local line fault occurs, the RoCoF at the downstream DER grid connection point is greater than the frequency change rate threshold, and the protection operates to disconnect the DER in that area.

8. The apparatus as claimed in claim 5, characterized in that, The distributed power supply control module initiates a multi-criteria protection mechanism based on voltage change rate, and combines frequency change rate, frequency, and differentiated action time limits to achieve selective control of the DER, including: When the voltage change rate is less than the voltage change rate threshold, it is determined to be a system disturbance, and slow voltage-frequency protection is activated. The DER disconnects the network after a preset delay based on the degree of voltage / frequency exceeding the limit. When the voltage change rate is greater than or equal to the voltage change rate threshold, it is determined to be a fault or islanded operation, and fast protection is activated. If the frequency change rate is greater than the frequency change rate threshold or the frequency exceeds the frequency threshold range and the duration exceeds the differentiated action time limit, the corresponding DER is immediately disconnected.

9. A local distributed power source grid connection protection system that improves frequency voltage ride-through capability, comprising: Computer-readable storage media and processors; The computer-readable storage medium is used to store executable instructions; The processor is used to read executable instructions stored in the computer-readable storage medium and execute the local distributed power grid protection method that satisfies the frequency voltage ride-through capability improvement as described in any one of claims 1-4.

10. A non-transitory computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the local distributed power grid protection method for improving frequency voltage ride-through capability as described in any one of claims 1-4.

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