Power distribution network area protection method and system based on carrier communication

Abstract

According to the power distribution network area protection method and system based on carrier communication, a power line carrier communication mode is applied, data transmission among nodes of a power distribution network is achieved by means of an existing power line, and communication channels are intelligently selected between a PLC host and a sub-machine according to the length and type of the line in the aspect of communication; a low-frequency channel is adopted for a short circuit to guarantee the communication stability; a long line starts a broadband channel and is combined with a relay transmission mode, and only logic quantity data is transmitted between devices, so that the data volume of a communication link is greatly reduced, and a fault area can be accurately positioned without data synchronization. In terms of protection area division and fault processing, protection areas are dynamically divided according to a line topological structure, the boundary of each area is accurately defined, and double-criterion directional element protection is configured. The centralized protection device can quickly position a fault area, quickly identify a fault type, and timely send a tripping or closing instruction to the power distribution terminal, thereby realizing quick fault isolation and power supply recovery.

Description

Technical Field

[0001] The present invention relates to the field of distribution network protection, and particularly to a method and system for distribution network regional protection based on carrier communication. Background Art

[0002] With the large-scale access of new energy to the distribution network, the protection problems of the distribution network have become increasingly prominent. Traditional protection schemes highly rely on optical fiber or wireless communication networks, and expose many defects in the complex application scenarios of the distribution network. On the one hand, the wiring difficulty is great. In places with complex geographical conditions such as islands and remote areas, the cost of laying optical fiber is high and the construction difficulty is extremely large. On the other hand, it is seriously affected by environmental interference. Wireless communication is vulnerable to weather and terrain blockage, and the signal is unstable. Although the overcurrent protection scheme is simple, it lacks selectivity. Once a fault occurs, the entire line will be cut off, resulting in a large area power outage, seriously affecting the power supply reliability. Therefore, there is an urgent need for an innovative protection scheme that can use the existing power line communication, accurately locate and isolate faults, realize the efficient and reliable operation of the distribution network, and ensure the stability of power supply. Summary of the Invention

[0003] The present invention aims at the deficiencies in the prior art and provides a method and system for distribution network regional protection based on carrier communication.

[0004] To achieve the above object, the present invention adopts the following technical solutions:

[0005] A method for distribution network regional protection based on carrier communication includes the following steps:

[0006] Configure a centralized protection device and a PLC communication host at the grid-connected switch of the substation, and configure distribution terminals and PLC communication slave machines at the main lines, branch lines, distributed power sources and circuit breakers of the distribution network; the centralized protection device, the distribution terminal, the PLC communication host and the PLC communication slave machine are communicatively connected, and the PLC communication host and the PLC communication slave machine build a communication network through the power line carrier channel to form a regional PLC network including at least 1 PLC communication host and multiple PLC communication slave machines; the centralized protection device divides the distribution network into several protection areas based on the distribution network topology according to the boundary point distribution principle; after the centralized protection device and the distribution terminals in each protection area collect the analog quantities of three-phase voltage and three-phase current, they judge the directions of the transient voltage element and the negative sequence current element, and send the logical quantities calculated by the distribution terminals to the centralized protection device through the regional PLC network. The logical quantities include the directions of the transient voltage element, the negative sequence current element, the switch position, the protection startup state, the PT disconnection state and the CT disconnection state. The centralized protection device discriminates the fault area according to the logical quantities sent by the distribution terminals in each area. After the fault area is located, a tripping instruction is sent to the distribution terminal in the fault area to isolate the fault, and a closing instruction is sent to the distribution terminal to restore power supply after the system returns to normal.

[0007] To optimize the above technical solution, the specific measures taken also include:

[0008] Further, the specific way that the PLC communication host and the PLC communication slave machine build a communication network through the power line carrier channel is as follows:

[0009] The PLC communication host and the PLC communication slave machine select different working broadband rates according to the line type and line length; based on the orthogonal frequency division multiplexing modulation method, the available channel bandwidth of 0.7 MHz - 1.6 MHz is adaptively divided into multiple orthogonal sub-channels. Among them, 0.7 - 1.1 MHz is the low-frequency channel group, and each sub-channel bandwidth of the low-frequency channel group is 125 kHz ± 5% and the adjacent interval is 50 kHz; 1.15 - 1.6 MHz is the wide-frequency channel group, and each sub-channel bandwidth of the wide-frequency channel group is 100 kHz ± 5% and the adjacent interval is 25 kHz. The low-frequency channel group is dedicated to short-distance communication with Ln ≤ 5 km, and the wide-frequency channel group is dedicated to relay communication with Ln > 5 km. Ln represents the line length between the PLC communication host and the PLC communication slave machine; when the relay communication transmission mode is enabled, select the adjacent PLC communication slave machine closest to the target PLC communication slave machine as the relay point, and establish a two-hop communication link to exchange data.

[0010] Further, the specific way that the distribution network is divided into several protection areas according to the boundary point distribution principle is as follows:

[0011] Build a node relationship matrix, define the grid-connected switch as the topological root node, and starting from the root node, extend step by step along the direction of decreasing impedance amplitude. The line segments between adjacent distribution terminals and the branch lines directly connected to them form independent protection areas, and the boundaries of each area satisfy:

[0012]

[0013] Among them, |DU min | is the minimum fault fixed voltage mutation value, Iset is the setting current value flowing through the previous protection area, Z line_base is the line reference impedance. If a distribution terminal is installed on the distributed power source side, there is 1 protection area downstream of the distributed power source node, and dual-criterion direction elements are configured at the boundaries of each area.

[0014] Furthermore, the area PLC network executes a hierarchical data transmission mechanism, specifically:

[0015] In the steady-state operation stage, the distribution terminal and the centralized protection device exchange heartbeat data every set time period to monitor the normality of the communication link and perform time synchronization management; the content of the heartbeat data includes a data type code, a device status code, a 16-bit time synchronization counter, and a parity check code; in the fault handling stage, when a voltage or current mutation is detected, a fast transmission mechanism is triggered, and the distribution terminal sends 4 frames of key logical quantity data within 10 ms. The content of the logical quantity data includes a data type code, a direction element code, a protection logic code, a device status code, and a parity check code; the centralized protection device uses a sliding window mechanism to process continuous data frames, starts to perform fault area location and fault handling after receiving a frame of change quantity data within 10 ms, and sends an instruction to the distribution terminal. The content of the instruction includes a data type code, a device identifier, a protection area location code, a trip instruction, a closing instruction, and a parity check code; when any channel continuously receives 3 frames of data with failed checks, the channel switching mechanism is enabled.

[0016] Furthermore, the centralized protection device and the distribution terminal perform time synchronization, and the specific method is:

[0017] The centralized protection device generates heartbeat messages with time-stamped information, performs time synchronization through GPS, Beidou or SNTP, and splits the complete time-stamped information of the centralized protection device into multiple frames of logical quantity data for transmission. Specifically, each frame of heartbeat message includes a frame identifier and parity check bits. The frame identifier is used to distinguish the type of this frame of message. The distribution terminal performs data frame verification after receiving the heartbeat message and only processes the data that passes the verification. When 5 consecutive valid frames are received cumulatively, the time-stamped information is recombined and compared with its own clock. When the time difference between the two is greater than 1S, the distribution terminal subtracts the line transmission delay from the time stamp of the centralized protection device to make the time stamps of all distribution terminals consistent with that of the centralized protection device; the calculation method of the line transmission delay is shown in the following formula:

[0018]

[0019] Where, Dt represents the time difference of the PLC communication line transmission delay, Ln represents the line length between the distribution terminal and the centralized protection, Veff is the signal transmission speed, Kset is the delay coefficient, and Veff and Kset take different values according to the types of overhead lines, cable lines or hybrid lines, and ∈PlC is the jitter noise.

[0020] Further, the specific process of the fault area discrimination is as follows:

[0021] The centralized protection device executes different discrimination logics according to the opening and closing states of the grid-connected switches. When the grid-connected switch is in the closed position, the distribution network system is in the grid-connected state. It is stipulated that the positive direction is the direction of the fault current flowing to the bus, and the negative direction is the direction of the fault current flowing to the distribution network system. If the direction of the transient voltage element or the negative sequence current element of any distribution terminal is in the negative direction, it is considered that the discrimination result of this distribution terminal is the fault negative direction, otherwise it is the fault positive direction; for the protection area where the centralized protection device is located, if the discrimination result of the centralized protection device is the fault positive direction and the discrimination result of the adjacent node distribution terminal is the fault negative direction, then the fault is located in the protection area where the centralized protection device is located. For any other protection area, it is required that all nodes of the distribution terminal in the protection area adjacent to the near substation side of this protection area have completed the discrimination. When the discrimination results of the protection terminals of the upstream nodes of this protection area are all in the positive direction and the last node of this protection area is in the negative direction, then it is determined that this protection area is the fault area, otherwise it is determined as the non-fault area;

[0022] When the grid-connected switch is in the off position, the distribution network system is in the off-grid state. The distribution network system is powered by distributed power sources. It is stipulated that the reverse direction is the direction of the fault current flowing towards the bus, and the positive direction is the direction of the fault current flowing towards the distribution network system. For the protection area where the centralized protection device is located, when the discrimination result of the centralized protection device is the positive direction of the fault, it is determined that the protection area where the centralized protection device is located is the fault area. For any other protection area, when the first node in the protection area is not in the reverse direction and other nodes are in the reverse direction, it is determined that the protection area is the fault area.

[0023] Further, the discrimination method for the direction of the transient voltage element is as follows:

[0024] Both the centralized protection device and the distribution terminal adopt high-frequency sampling. When the system voltage changes, if |du / dt| > Δu or |di / dt| > Δi, the transient voltage element starts. Here, u is the voltage change amount, i is the current change amount, t is the time, Δu is the voltage change threshold setting value, and Δi is the current change threshold setting value. The phase voltage and phase current data of 20 sampling points before and after the start time of the transient voltage element are extracted as the transient data window. After the extraction of each phase data, the corresponding phase voltage array S1 and phase current array S2 are obtained. The mallat wavelet transform is performed on S1 and S2 respectively to obtain the wavelet transform detail coefficient d1_1 of the phase voltage and the wavelet transform detail coefficient d1_2 of the phase current. The first data point in the data window is removed, and the subsequent data points are moved forward by one position. After the cross-complementation of the wavelet transform detail coefficient d1_1 of the phase voltage and the wavelet transform detail coefficient d1_2 of the phase current, a complete wavelet transform is performed again to obtain the final wavelet transform detail coefficient. The final detail coefficients corresponding to the three-phase voltages are named d 1_U =[d 1_ua d 1_ub d 1_uc , and the final detail coefficients corresponding to the three-phase currents are named d 1_I =[d 1_ia d 1_ib d 1_ic , where d 1_ua、 d 1_ub、 d 1_uc are the final detail coefficients of the A-phase voltage, B-phase voltage, and C-phase voltage respectively; d 1_ia、 d 1_ib、 d 1_ic are the final detail coefficients of the A-phase current, B-phase current, and C-phase current respectively.

[0025] Calculate the sign function value of the phase voltage corresponding to the phase current. The formula is as follows:

[0026]

[0027] Among them, f = 1 indicates that the transient voltage element is in the positive direction, and f = 0 indicates that the transient voltage element is in the reverse direction, d 1_U is the final detail coefficient corresponding to the three-phase voltage, where d 1_I is the final detail coefficient corresponding to the three-phase current, ΔZ set is the threshold value characterized by the detail coefficient; Sgn(·) represents the sign function.

[0028] Furthermore, the discrimination method for the direction of the negative-sequence current element is as follows:

[0029] When the negative-sequence voltage is greater than the system negative-sequence voltage threshold value, it is detected whether the negative-sequence current is greater than the negative-sequence current threshold value. If both are satisfied, it is determined that the negative-sequence current element of this node is in the positive direction; otherwise, the negative-sequence current element of this node is in the reverse direction. The calculation formula for the negative-sequence voltage threshold value is:

[0030] U set = K′ rel U 2_min

[0031] Among them, Uset is the negative-sequence voltage threshold value, K’rel is the reliability coefficient for negative-sequence voltage discrimination, and U2_min is the minimum value of the negative-sequence voltage at the fault moment.

[0032] The beneficial effects of the present invention are as follows: The method and system for regional protection of a distribution network based on carrier communication of the present invention do not rely on optical fibers, 4G or 5G networks, are convenient to deploy, have the advantages of low investment and low operation and maintenance costs. At the same time, only logical quantity information is transmitted between the master and slave machines, the data volume is small and data synchronization is required, further improving the stability and reliability of the system; The centralized protection can effectively distinguish whether the fault type is a two-phase fault or a three-phase fault, can complete the discrimination of the fault area within hundreds of milliseconds, and quickly trip the circuit breaker to cut off the fault, is not affected by factors such as the access capacity of new energy, and has high action reliability, and is applicable to the distribution network scenarios of islands and remote areas. Description of the Drawings

[0033] Figure 1 is an example of the distribution network regional protection framework based on the carrier communication method in this embodiment;

[0034] Figure 2 is an example of the distribution network protection area division in this embodiment;

[0035] Figure 3 is an example of the protection logic of the centralized protection device in this embodiment;

[0036] Figure 4 is an example of the protection logic of the transient voltage direction element in this embodiment;

[0037] Figure 5 is an example of the protection logic of the negative-sequence current direction element in this embodiment. Specific Embodiment

[0038] The following will clearly and completely describe the technical solutions in the embodiments of the present application with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative efforts shall fall within the protection scope of the present application.

[0039] Embodiment 1

[0040] The present invention proposes a method for regional protection of a distribution network based on carrier communication, including the following steps:

[0041] As Figure 1 shown, a centralized protection device and a PLC communication host are configured at the grid-connected switch of the substation, and distribution terminals and PLC communication slave machines are configured at the main lines, branch lines, distributed power sources, and circuit breakers of the distribution network; the centralized protection device, the distribution terminals, the PLC communication host, and the PLC communication slave machines are communicatively connected, and the PLC communication host and the PLC communication slave machines construct a communication network through a power line carrier channel to form a regional PLC network including at least 1 PLC communication host and multiple PLC communication slave machines; the centralized protection device divides the distribution network into several protection regions based on the distribution network topology structure according to the boundary point distribution principle; the centralized protection device and the distribution terminals in each protection region collect three-phase voltage and three-phase current analog quantities and then judge the directions of transient voltage elements and negative sequence current elements, and send the logical quantities calculated by the distribution terminals to the centralized protection device through the regional PLC network. The logical quantities include the directions of transient voltage elements, the directions of negative sequence current elements, switch positions, protection startup states, PT disconnection states, and CT disconnection states. The centralized protection device discriminates the fault region according to the logical quantities sent by the distribution terminals in each region. After the fault region is located, a tripping instruction is sent to the distribution terminal in the fault region to isolate the fault, and a closing instruction is sent to the distribution terminal to restore power supply after the system returns to normal.

[0042] The construction of the communication network between the PLC communication host and the PLC communication slave machine through the power line carrier channel is specifically as follows:

[0043] The PLC communication host and the PLC communication slave select different working broadband rates according to the line type and line length; based on the orthogonal frequency division multiplexing (OFDM) modulation method, the available channel bandwidth of 0.7 MHz - 1.6 MHz is adaptively divided into multiple orthogonal sub-channels. Among them, 0.7 - 1.1 MHz is the low-frequency channel group, and each sub-channel bandwidth of the low-frequency channel group is 125 kHz ± 5% with an adjacent interval of 50 kHz; 1.15 - 1.6 MHz is the wide-frequency channel group, and each sub-channel bandwidth of the wide-frequency channel group is 100 kHz ± 5% with an adjacent interval of 25 kHz. The low-frequency channel group is dedicated to short-distance communication with Ln ≤ 5 km, and the low-frequency band can ensure the stability of communication; the wide-frequency channel group is dedicated to relay communication with Ln > 5 km, where Ln represents the line length between the PLC communication host and the PLC communication slave; when enabling the relay communication transmission mode, select the adjacent PLC communication slave closest to the target PLC communication slave as the relay point, establish a two-hop communication link and then exchange data to ensure the efficient transmission of long-distance communication data.

[0044] As Figure 2 shown, the centralized protection device can adapt to the network topology structure, divide the protection area according to the boundary point distribution principle, and the network architecture of the distribution network mainly includes a tree-shaped network, a scattered network or a ring network architecture with open-loop operation. Dividing the distribution network into several protection areas according to the boundary point distribution principle is specifically as follows:

[0045] Establish a node relationship matrix, define the grid-connected switch as the topological root node, start from the root node, and extend step by step along the direction of decreasing impedance amplitude. The line segment between adjacent distribution terminals and the branch lines directly connected to it form independent protection areas, and the boundaries of each area satisfy:

[0046]

[0047] Among them, |DU min | is the minimum fault fixed voltage mutation value, Iset is the setting current value flowing through the previous protection area, and Z line_base is the line reference impedance. If a distribution terminal is installed on the distributed power source side, there is 1 protection area downstream of the distributed power source node, and dual-criterion direction elements are configured at the boundaries of each area to improve the accuracy of fault judgment.

[0048] The regional PLC network implements a hierarchical data transmission mechanism, specifically as follows:

[0049] During the steady-state operation phase, the distribution terminal and the centralized protection device exchange heartbeat data every set time interval (2S in this embodiment) to monitor the normality of the communication link and perform time synchronization management. The content of the heartbeat data includes a data type code, a device status code, a 16-bit time synchronization counter, and a parity check code. During the fault handling phase, when a voltage or current mutation is detected, a fast transmission mechanism is triggered. The distribution terminal sends 4 frames of key logical quantity data within 10 ms. The content of the logical quantity data includes a data type code, a direction element code, a protection logic code, a device status code, and a parity check code. The centralized protection device uses a sliding window mechanism to process continuous data frames. After receiving a frame of change quantity data within 10 ms, it starts to locate the fault area and handle the fault, and sends an instruction to the distribution terminal. The content of the instruction includes a data type code, a device identifier, a protection area location code, a trip instruction, a closing instruction, and a parity check code. When any channel continuously receives 3 frames of data with failed checks, the channel switching mechanism is enabled.

[0050] The centralized protection device and the distribution terminal perform time synchronization. The specific method is as follows:

[0051] The centralized protection device generates a heartbeat message with time stamp information and performs time synchronization through GPS, Beidou, or SNTP (Simple Network Time Protocol). The complete time stamp information of the centralized protection device is split into multiple frames of logical quantity data for transmission. Specifically, each frame of the heartbeat message includes a frame identifier and a parity check bit. The frame identifier is used to distinguish the type of this frame of message (year / month / day / hour / minute / second / millisecond). The distribution terminal performs data frame verification after receiving the heartbeat message and only processes the data that passes the verification. When 5 consecutive valid frames are received cumulatively, the time stamp information is recombined and compared with its own clock. When the time difference between the two is greater than 1S, the distribution terminal subtracts the line transmission delay from the time stamp of the centralized protection device so that the time stamps of all distribution terminals and the centralized protection device are consistent. The calculation method of the line transmission delay is shown in the following formula:

[0052]

[0053] Among them, Dt represents the time difference of the PLC communication line transmission delay, Ln represents the line length between the distribution terminal and the centralized protection, Veff is the signal transmission speed, Kset is the delay coefficient, and Veff and Kset take different values according to the type of overhead line, cable line, or hybrid line. ∈PlC is the jitter noise. Based on the PLC communication, the link transmission delay is within 50 ms. Using this time synchronization method, the time stamps of all distribution terminals and the centralized protection device in the area are basically the same, with an error within 10 ms.

[0054] When a fault occurs within a region, the centralized protection device and distribution terminals enable a fast message sending mechanism. The distribution terminals quickly send logical quantities such as the calculated transient voltage element direction, negative sequence current element direction, switch position, protection startup status, PT disconnection status, CT disconnection status, etc. After receiving the logical quantity information sent from the distribution terminals in each region, the centralized protection device first performs parity check on the data. The data that passes the check is used for protection logic calculation. Then, according to the regional protection fault location algorithm, the fault region is located, and a tripping instruction is sent to the distribution terminals within the located fault region, thereby isolating the fault region.

[0055] As Figure 3 shown, when a fault occurs in the distribution network, the centralized protection device can locate the fault region based on the transient voltage direction and negative sequence current direction sent from the distribution terminals in the system. The specific process of fault region discrimination is as follows:

[0056] The centralized protection device executes different discrimination logics according to the on-grid switch opening and closing states. When the on-grid switch is in the closed position, the distribution network system is in the on-grid state. It is stipulated that the positive direction is the direction of fault current flowing towards the bus, and the reverse direction is the direction of fault current flowing towards the distribution network system. If the transient voltage element direction or negative sequence current element direction of any distribution terminal is in the reverse direction, then the discrimination result of this distribution terminal is considered to be the reverse direction of the fault, otherwise it is the positive direction of the fault; for the protection region where the centralized protection device is located, if the discrimination result of the centralized protection device is the positive direction of the fault and the discrimination result of the adjacent node distribution terminal is the reverse direction of the fault, then the fault is located in the protection region where the centralized protection device is located. For any other protection region, it is required that all the distribution terminals of all nodes in the protection region adjacent to the near substation side of this protection region have completed discrimination. When the discrimination results of the protection terminals of the upstream nodes in this protection region are all in the positive direction and the last node of this protection region is in the reverse direction, then this protection region is determined to be the fault region, otherwise it is determined to be a non-fault region;

[0057] When the on-grid switch is in the open position, the distribution network system is in the off-grid state, and the distribution network system is powered by distributed power sources. It is stipulated that the reverse direction is the direction of fault current flowing towards the bus, and the positive direction is the direction of fault current flowing towards the distribution network system. For the protection region where the centralized protection device is located, when the discrimination result of the centralized protection device is the positive direction of the fault, then the protection region where the centralized protection device is located is determined to be the fault region. For any other protection region, when the first node in the protection region is not in the reverse direction and the other nodes are in the reverse direction, then this protection region is determined to be the fault region.

[0058] As Figure 4As shown, when a three-phase fault occurs in the system, the first extreme values of the transient quantities of voltage and current after the fault in the positive direction of protection exhibit an anti-polarity characteristic, while the first extreme values of the transient quantities of voltage and current after the fault in the reverse direction exhibit a co-polarity characteristic. The wavelet transform detail coefficients are used to represent the high-frequency transient quantities. The discrimination method for the direction of the transient voltage element is as follows:

[0059] Both the centralized protection device and the distribution terminal adopt high-frequency sampling. When the system voltage changes, if |du / dt| > Δu or |di / dt| > Δi is satisfied, the transient voltage element starts. Here, u is the voltage change, i is the current change, t is the time, Δu is the voltage change threshold setting value, and Δi is the current change threshold setting value. The phase voltage and phase current data of 20 sampling points before and after the starting moment of the transient voltage element are extracted as the transient data window. After extracting the data of each phase, the corresponding phase voltage array S1 and phase current array S2 are obtained. The mallat wavelet transform is performed on S1 and S2 respectively to obtain the wavelet transform detail coefficient d1_1 of the phase voltage and the wavelet transform detail coefficient d1_2 of the phase current. The first data point in the data window is removed, and the subsequent data points are shifted forward by one position. After cross-complementing the wavelet transform detail coefficient d1_1 of the phase voltage and the wavelet transform detail coefficient d1_2 of the phase current, a complete wavelet transform is performed again to obtain the final wavelet transform detail coefficient. The final detail coefficients corresponding to the three-phase voltages are named d 1_U =[d 1_ua d 1_ub d 1_uc , and the final detail coefficients corresponding to the three-phase currents are named d 1_I =[d 1_ia d 1_ib d 1_ic , where d 1_ua、 d 1_ub、 d 1_uc are the final detail coefficients of the A-phase voltage, B-phase voltage, and C-phase voltage respectively; d 1_ia、 d 1_ib、 d 1_ic are the final detail coefficients of the A-phase current, B-phase current, and C-phase current respectively.

[0060] Calculate the sign function value of the phase voltage corresponding to the phase current. The formula is as follows:

[0061]

[0062] where f = 1 indicates that the transient voltage element is in the positive direction, f = 0 indicates that the transient voltage element is in the reverse direction, d 1_U is the final detail coefficient corresponding to the three-phase voltages, where d 1_I is the final detail coefficient corresponding to the three-phase currents, and ΔZ set is the detail coefficient representation threshold setting value; Sgn(·) represents the sign function.

[0063] As Figure 5 shown, when a two-phase short circuit occurs in the distribution network, according to the negative sequence suppression control strategy, negative sequence voltages exist at the nodes in the fault area. The discrimination method for the direction of the negative sequence current element is as follows:

[0064] When the negative sequence voltage is greater than the system negative sequence voltage threshold, detect whether the negative sequence current is greater than the negative sequence current threshold. If both are satisfied, it is determined that the negative sequence current element of this node is in the positive direction; otherwise, the negative sequence current element of this node is in the reverse direction. The calculation formula for the negative sequence voltage threshold is:

[0065] U set = K′ rel U 2_min

[0066] where Uset is the negative sequence voltage threshold, K’rel is the reliability coefficient for negative sequence voltage discrimination, and U2_min is the minimum value of the negative sequence voltage at the fault moment.

[0067] The distribution network area protection method and system based on carrier communication of the present invention do not rely on optical fibers, 4G or 5G networks, are convenient to deploy, have the advantages of less investment and low operation and maintenance costs. At the same time, only logical quantity information is transmitted between the master and slave machines, the data volume is small and data synchronization is required, further improving the stability and reliability of the system; the centralized protection can effectively distinguish whether the fault type is a two-phase fault or a three-phase fault, can complete the discrimination of the fault area within hundreds of milliseconds, and quickly trip the circuit breaker to cut off the fault, is not affected by factors such as the access capacity of new energy, and has high action reliability, and is applicable to the distribution network scenarios of islands and remote areas.

[0068] Embodiment 2

[0069] The present invention proposes a distribution network area protection system based on carrier communication corresponding to the method of Embodiment 1, including:

[0070] A centralized protection device and a PLC communication host installed at the grid-connected switch, and distribution terminals and PLC communication slave machines at each node of the distribution network system;

[0071] The centralized protection device, the distribution terminal, the PLC communication host and the PLC communication slave machine are communicatively connected. The PLC communication host and the PLC communication slave machine construct a communication network through the power line carrier channel to form a regional PLC network including at least 1 PLC communication host and multiple PLC communication slave machines;

[0072] The regional PLC network transmits logical quantities for time synchronization and fault location;

[0073] The centralized protection device divides the distribution network into several protection areas according to the boundary point distribution principle based on the distribution network topology structure;

[0074] The centralized protection device and the distribution terminal judge the directions of the transient voltage element and the negative sequence current element, and the centralized protection device locates the fault area according to the directions of the transient voltage element and the negative sequence current element, and sends a tripping command to the distribution terminal in the fault area;

[0075] The distribution terminal executes the switching command to isolate the fault area and restore power supply.

[0076] The implementation manners of each module and the module functions in the system are completely consistent with the steps of the method in the first embodiment, so they will not be described in detail here.

[0077] Those of ordinary skill in the art can realize that the units and algorithm steps of each example described in combination with the embodiments disclosed in the present application can be implemented by electronic hardware or a combination of computer software and electronic hardware. Whether these functions are executed in a hardware or software manner depends on the specific application and design constraints of the technical solution. Professional technicians can use different methods to implement the described functions for each specific application, but such implementation should not be considered to exceed the scope of the present application.

[0078] The above is only the preferred implementation manner of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions falling within the idea of the present invention belong to the protection scope of the present invention. It should be noted that for those of ordinary skill in the art, several improvements and refinements made without departing from the principle of the present invention should be regarded as the protection scope of the present invention.

Claims

1. A method for regional protection of a distribution network based on carrier communication, characterized in that, Including the following steps: A centralized protection device and a PLC communication host are configured at the grid-connected switch of the substation, and distribution terminals and PLC communication slave machines are configured at the main lines, branch lines, distributed power sources and circuit breakers of the distribution network; the centralized protection device, the distribution terminal, the PLC communication host and the PLC communication slave machine are communicatively connected, and the PLC communication host and the PLC communication slave machine construct a communication network through a power line carrier channel to form a regional PLC network including at least 1 PLC communication host and multiple PLC communication slave machines; the centralized protection device divides the distribution network into several protection areas based on the distribution network topology structure according to the boundary point distribution principle; after the centralized protection device and the distribution terminals in each protection area collect the three-phase voltage and three-phase current analog quantities, they judge the transient voltage element direction and the negative sequence current element direction, and send the logic quantities calculated by the distribution terminals to the centralized protection device through the regional PLC network, and the logic quantities include the transient voltage element direction, the negative sequence current element direction, the switch position, the protection start state, the PT disconnection state and the CT disconnection state. The centralized protection device discriminates the fault area according to the logic quantities sent by the distribution terminals in each area. After the fault area is located, a tripping command is sent to the distribution terminal in the fault area to isolate the fault, and a closing command is sent to the distribution terminal to restore power supply after the system returns to normal.

2. The method for protecting a distribution network area based on carrier communication according to claim 1, wherein The specific method for the PLC communication host and the PLC communication slave machine to construct a communication network through a power line carrier channel is as follows: The PLC communication host and the PLC communication slave machine select different working broadband rates according to the line type and line length; based on the orthogonal frequency division multiplexing modulation method, the available channel bandwidth of 0.7 MHz - 1.6 MHz is adaptively divided into multiple orthogonal sub-channels. Among them, 0.7 - 1.1 MHz is the low-frequency channel group, and each sub-channel bandwidth of the low-frequency channel group is 125 kHz ± 5% and the adjacent interval is 50 kHz; 1.15 - 1.6 MHz is the broadband channel group, and each sub-channel bandwidth of the broadband channel group is 100 kHz ± 5% and the adjacent interval is 25 kHz. The low-frequency channel group is dedicated to short-distance communication with Ln ≤ 5 km, and the broadband channel group is dedicated to relay communication with Ln > 5 km, where Ln represents the line length between the PLC communication host and the PLC communication slave machine; when the relay communication transmission mode is enabled, the adjacent PLC communication slave machine closest to the target PLC communication slave machine is selected as the relay point, and after establishing a two-hop communication link, data is exchanged.

3. The method for protecting a distribution network area based on carrier communication according to claim 1, characterized in that, The specific method for dividing the distribution network into several protection areas according to the boundary point distribution principle is as follows: A node relationship matrix is established, and the grid-connected switch is defined as the topological root node. Starting from the root node, it extends step by step along the direction of decreasing impedance amplitude. The line segment between adjacent distribution terminals and the branch lines directly connected to it form independent protection areas, and the boundaries of each area satisfy: Among them, |DU min | is the minimum fault fixed voltage mutation value, Iset is the setting current value flowing through the previous protection area, and Z line_base is the line reference impedance. If a distribution terminal is installed on the distributed power source side, there is 1 protection area downstream of the distributed power source node, and double-criterion directional elements are configured at the boundaries of each area.

4. The method for protecting a distribution network area based on carrier communication according to claim 1, wherein, The regional PLC network executes a hierarchical data transmission mechanism, specifically as follows: During the steady-state operation phase, the distribution terminal and the centralized protection device exchange heartbeat data every set period of time to monitor the normality of the communication link and perform time synchronization management. The content of the heartbeat data includes a data type code, a device status code, a 16-bit time synchronization counter, and a parity check code. During the fault handling phase, when a voltage or current mutation is detected, a fast transmission mechanism is triggered. The distribution terminal sends 4 frames of key logical quantity data within 10 ms. The content of the logical quantity data includes a data type code, a direction element code, a protection logic code, a device status code, and a parity check code. The centralized protection device uses a sliding window mechanism to process continuous data frames. After receiving a frame of change quantity data within 10 ms, it starts to locate the fault area and handle the fault, and sends an instruction to the distribution terminal. The content of the instruction includes a data type code, a device identifier, a protection area location code, a tripping instruction, a closing instruction, and a parity check code. When any channel continuously receives 3 frames of data with failed checks, the channel switching mechanism is enabled.

5. The method for regional protection of a distribution network based on carrier communication according to claim 1, characterized in that The specific method for the centralized protection device to synchronize time with the distribution terminal is as follows: The centralized protection device generates a heartbeat message with time stamp information and performs time synchronization through GPS, Beidou, or SNTP. The complete time stamp information of the centralized protection device is split into multiple frames of logical quantity data for transmission. Specifically, each frame of the heartbeat message includes a frame identifier and a parity check bit. The frame identifier is used to distinguish the type of this frame of message. The distribution terminal performs data frame verification after receiving the heartbeat message and only processes the data that passes the verification. When 5 consecutive valid frames are received cumulatively, the time stamp information is recombined and compared with its own clock. When the time difference between the two is greater than 1 s, the distribution terminal subtracts the line transmission delay from the time stamp of the centralized protection device so that the time stamps of all distribution terminals and the centralized protection device are consistent. The calculation method of the line transmission delay is shown in the following formula: Where Dt represents the time difference of the PLC communication line transmission delay, Ln represents the line length between the distribution terminal and the centralized protection, Veff is the signal transmission speed, Kset is the delay coefficient, Veff and Kset take different values according to the type of overhead line, cable line, or hybrid line, and ∈PlC is the jitter noise.

6. The method for protecting a distribution network area based on carrier communication according to claim 1, wherein The specific process of the fault area discrimination is as follows: The centralized protection device executes different discrimination logics according to the on-grid switch's open / close state. When the on-grid switch is in the closed position, the distribution network system is in the on-grid state. The positive direction is defined as the direction of the fault current flowing towards the bus, and the negative direction is defined as the direction of the fault current flowing towards the distribution network system. If the direction of the transient voltage element or the negative sequence current element of any distribution terminal is in the negative direction, then the discrimination result of that distribution terminal is considered as the fault negative direction; otherwise, it is the fault positive direction. For the protection area where the centralized protection device is located, if the discrimination result of the centralized protection device is the fault positive direction and the discrimination result of the distribution terminal of the adjacent node is the fault negative direction, then the fault is located in the protection area where the centralized protection device is located. For any other protection area, it is required that the discrimination of all nodes' distribution terminals in the protection area adjacent to the near substation side of this protection area is completed. When the discrimination results of the protection terminals of the upstream nodes in this protection area are all in the positive direction and the last node of this protection area is in the negative direction, then this protection area is determined as the fault area; otherwise, it is determined as a non-fault area. When the on-grid switch is in the open position, the distribution network system is in the off-grid state, and the distribution network system is powered by distributed power sources. The negative direction is defined as the direction of the fault current flowing towards the bus, and the positive direction is defined as the direction of the fault current flowing towards the distribution network system. For the protection area where the centralized protection device is located, when the discrimination result of the centralized protection device is the fault positive direction, then the protection area where the centralized protection device is located is determined as the fault area. For any other protection area, when the first node in the protection area is not in the negative direction and other nodes are in the negative direction, then this protection area is determined as the fault area.

7. The method for protecting a distribution network area based on carrier communication according to claim 6, characterized in that The discrimination method of the direction of the transient voltage element is as follows: Both the centralized protection device and the distribution terminal adopt high-frequency sampling. When the system voltage changes, if |du / dt| > Δu or |di / dt| > Δi is satisfied, the transient voltage element is activated, where u is the voltage change, i is the current change, t is the time, Δu is the voltage change threshold value, and Δi is the current change threshold value. The phase voltage and phase current data of 20 sampling points before and after the activation moment of the transient voltage element are extracted as the transient data window. After the extraction of each phase data, the corresponding phase voltage array S1 and phase current array S2 are obtained. The mallat wavelet transform is respectively performed on S1 and S2 to obtain the wavelet transform detail coefficient d1_1 of the phase voltage and the wavelet transform detail coefficient d1_2 of the phase current. The first data point in the data window is removed, and the subsequent data points are moved forward by one position. After the cross-complementation of the wavelet transform detail coefficient d1_1 of the phase voltage and the wavelet transform detail coefficient d1_2 of the phase current, a complete wavelet transform is performed again to obtain the final wavelet transform detail coefficient. The final detail coefficients corresponding to the three-phase voltages are named d 1_U =[d 1_ua d 1_ub d 1_uc , and the final detail coefficients corresponding to the three-phase currents are named d 1_I =[d 1_ia d 1_ib d 1_ic , where d 1_ua、 d 1_ub、 d 1_uc are the final detail coefficients of the phase A voltage, phase B voltage, and phase C voltage respectively; d 1_ia、 d 1_ib、 d 1_ic are the final detail coefficients of the phase A current, phase B current, and phase C current respectively. Calculate the sign function value of the phase voltage corresponding to the phase current. The formula is as follows: Among them, f = 1 indicates that the transient voltage element is in the positive direction, and f = 0 indicates that the transient voltage element is in the reverse direction, d 1_U is the final detail coefficient corresponding to the three-phase voltage, where d 1_I is the final detail coefficient corresponding to the three-phase current, ΔZ set is the threshold value characterized by the detail coefficient; Sgn(·) represents the sign function.

8. The method for protecting a distribution network area based on carrier communication according to claim 6, characterized in that, The discrimination method of the direction of the negative sequence current element is as follows: When the negative sequence voltage is greater than the system negative sequence voltage threshold value, detect whether the negative sequence current is greater than the negative sequence current threshold value. If both are satisfied, then it is determined that the negative sequence current element of this node is in the positive direction; otherwise, the negative sequence current element of this node is in the negative direction. The calculation formula for the negative sequence voltage threshold value is: U set＝ K′ rel U 2_min Where, Uset is the negative sequence voltage threshold value, K’rel is the reliability coefficient for negative sequence voltage discrimination, and U2_min is the minimum value of the negative sequence voltage at the fault moment.

9. A distribution network regional protection system based on carrier communication, characterized in that, It includes: A centralized protection device and a PLC communication host installed at the on-grid switch, distribution terminals and PLC communication slave machines at each node of the distribution network system; The centralized protection device, distribution terminals, PLC communication host and PLC communication slave machines are communicatively connected. The PLC communication host and PLC communication slave machines build a communication network through the power line carrier channel, forming a regional PLC network including at least 1 PLC communication host and multiple PLC communication slave machines; The regional PLC network transmits logical quantities for time synchronization and fault location; The centralized protection device divides the distribution network into several protection areas based on the distribution network topology structure according to the boundary point distribution principle; The centralized protection device and the distribution terminal judge the directions of the transient voltage element and the negative-sequence current element, and the centralized protection device locates the fault area according to the directions of the transient voltage element and the negative-sequence current element, and sends a tripping command to the distribution terminal in the fault area; The distribution terminal executes the switching command to isolate the fault area and restore power supply.

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