Flexible direct current transmission line protection method based on current accumulation amount

Abstract

The invention relates to a flexible direct current transmission line protection method based on current accumulation, and belongs to the technical field of power system relay protection. The method comprises the following steps: reading current data measured at protection installation positions at two ends of a line; calculating the current change rate of a protection installation position according to the current data, judging whether a fault occurs, if not, repeating detection, and if the fault occurs, calculating the current accumulation amount at the two ends of the line to judge a fault area; and calculating a pole selection coefficient, and performing fault pole selection of the fault area according to the pole selection coefficient. The objective of the invention is to solve the technical problem of incorrect protection action caused by the influence of transition resistance in the prior art.

Description

Technical Field

[0001] This invention relates to a protection method for flexible DC transmission lines based on current accumulation, belonging to the field of power system relay protection technology. Background Technology

[0002] Flexible DC transmission is a novel DC transmission technology based on the voltage source converter (VSC) principle. It features significant advantages such as no commutation failures, active and reactive power decoupling, and flexible operation, effectively mitigating the interference between AC and DC power and improving grid stability. Its basic definition is a DC transmission method implemented through a voltage source converter, enabling flexible control of power output and making it suitable for various scenarios such as long-distance, high-capacity power transmission, isolated power supply, and offshore wind power integration.

[0003] The multi-terminal and complex nature of flexible DC transmission lines makes fault identification and location difficult. For example, single-ended protection methods have low sensitivity when there is a high-resistance ground fault at a remote end, while double-ended protection methods have a slow operating speed, making it difficult to meet the requirements of rapid protection. In addition, traditional protection methods are susceptible to the influence of transition resistance when facing single-phase ground faults, leading to incorrect operation. Summary of the Invention

[0004] The purpose of this invention is to provide a flexible DC transmission line protection method based on current accumulation, which aims to solve the technical problem that the existing technology is susceptible to the influence of transition resistance, leading to incorrect protection operation.

[0005] To achieve the above objectives, the technical solution of this invention is: a flexible DC transmission line protection method based on current accumulation. This method uses the current accumulation at both ends to represent the fluctuation characteristics of the fault current, thereby identifying the fault area and effectively solving the problem of low reliability of current protection against high-resistance faults. The method includes the following steps: Step 1: Read the current data measured at the protection installation points at both ends of the line; Step 2: Calculate the current change rate at the protection installation point based on the current data to determine if a fault has occurred. If no fault has occurred, repeat the test. If a fault has occurred, proceed to Step 3. Step 3: Calculate the current accumulation at both ends of the line to determine the fault area; Step 4: Calculate the polarity selection coefficient and perform fault polarity selection in the fault area based on the polarity selection coefficient.

[0006] Optionally, Step 2 specifically includes: When a fault occurs, the voltage drops and the current increases. The rate of change of current is used to construct the protection activation criterion, and the activation coefficient is defined as follows:

[0007] In the formula, Δ i It is the fault component current of a DC line, defined as the current after the fault minus the current before the fault. is the rated current of the line, and n is the sampling sequence number. When the starting coefficient s of the three fault points all meet the preset setting value, the protection is activated.

[0008] Optionally, the current accumulation amount is specifically:

[0009] In the formula, To protect the startup time, for time The differential current, by Current at time minus The current is obtained at time t, where dt is the sampling time interval and t is the current calculation time of the protection device. After the protection is started, as time increases, The time will automatically be pushed forward. for The accumulation of the rate of change of current at time t is due to the current The sum is obtained by adding the differential current amplitude at time t and the current rate of change amplitudes at all previous times.

[0010] Optionally, Step 3 specifically includes: The fault area identification coefficient is defined as follows:

[0011] In the formula, To protect the startup time, T is the window length. It is the change in positive electrode current. It is the sampling time interval. It is the change in negative current. When the fault area identification coefficient d is greater than the preset fault setting value, it is a fault within the occurrence area. When the fault area identification coefficient d is less than the preset fault setting value, it is a fault outside the occurrence area.

[0012] Optionally, the expression for the polarity selection coefficient is:

[0013] When a positive electrode fault occurs, the selection coefficient k is greater than the preset threshold A. When a negative electrode fault occurs, the selection coefficient k is less than the preset threshold B. When a bipolar fault occurs, the selection coefficient k satisfies A > k > B.

[0014] The beneficial effects of this invention are: This invention provides a flexible DC transmission line protection method based on current accumulation, which effectively solves the problem of low reliability of current protection against high resistance faults, has low requirements for data sampling rate and low algorithm computation, and has strong engineering applicability. Attached Figure Description

[0015] Figure 1 This is a topology diagram of the flexible DC transmission system of the present invention; Figure 2 This is a flowchart of the steps of the present invention. Detailed Implementation

[0016] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0017] Example 1: This example uses a bipolar fault on the rectifier side of a DC transmission line as an example. Figure 1 The diagram shows the topology of the flexible DC transmission system of this invention. The fault location is at... Figure 1 of f 1 location, Figure 2 The flowchart of the present invention illustrates a method for protecting flexible DC transmission lines based on current accumulation, comprising the following steps: Step 1: Read the current data measured at the protection installation points at both ends of the line; It is important to understand that during the initial traveling wave phase after a ground fault in a DC transmission line, the control systems of the converters on both sides are too slow to respond, therefore the equivalent voltage source and equivalent impedance of the converters remain approximately unchanged. According to the superposition theorem, the DC system after the fault can be equivalently divided into two parts: the network before the fault and the network added due to the fault. Since the DC line current before the fault is basically constant, this embodiment uses the fault component current to analyze the fluctuation characteristics of the DC line current, and then identifies the fault area based on the fluctuation characteristics of the current.

[0018] Step 2: Calculate the current change rate at the protection installation point based on the current data to determine if a fault has occurred. If no fault has occurred, repeat the test. If a fault has occurred, proceed to Step 3. Optionally, Step 2 specifically includes: When a fault occurs, the voltage drops and the current increases. The rate of change of current is used to construct the protection activation criterion, and the activation coefficient is defined as follows:

[0019] In the formula, Δ i It is the fault component current of a DC line, defined as the current after the fault minus the current before the fault. is the rated current of the line, and n is the sampling sequence number. When the starting coefficient s of the three fault points all meet the preset setting value, the protection is activated.

[0020] Specifically, the preset setting value is a threshold for judging the occurrence of a fault, which satisfies that the current fluctuation during normal operation of the line does not exceed the threshold. In this embodiment, the rated current of the line in the model is 5000A, so the setting value is 500A.

[0021] Step 3: Calculate the current accumulation at both ends of the line to determine the fault area; When a fault occurs within the DC line region, each quantity is expressed in the complex frequency domain. r Z is the equivalent commutation impedance on the rectifier side. p and Z z The equivalent impedances R of the smoothing reactor and the DC filter are respectively. f For ground fault transition resistance, U dc This represents the line-to-ground voltage before the fault. and These are the first forward and reverse current traveling waves propagating from the fault point to the rectifier side, respectively. This represents the fault component current on the rectifier side of the line. Before the reverse traveling wave of the second current reflected from the fault point reaches the rectifier side, The expression is:

[0022] In the formula, Let Z be the reflection coefficient on the rectifier side. In this embodiment, to simplify the analysis, the effect of traveling wave attenuation is ignored, and the DC line wave impedance Z is... c It can be considered to be purely resistive, then The expression is:

[0023] In the formula, This is the line wave impedance, which is taken as 300Ω in this embodiment; Furthermore, The expression is:

[0024] In the formula, This is the first positive voltage traveling wave propagating from the fault point to the rectifier side.

[0025] Furthermore, to obtain The expression is:

[0026] When a fault occurs outside the reverse region, each quantity is given its expression in the complex frequency domain.

[0027] at this time, The expression is:

[0028] In the formula, This is the equivalent impedance of the smoothing reactor;

[0029] In the formula, It is the equivalent impedance during a metallic fault within the zone. It is the equivalent impedance during metallic faults outside the zone; Specifically, for a typical DC transmission project, the smoothing reactor inductance is 0.2H, Z c =300Ω, the inductors L1, L2 and L3 of the DC filter on the LCC side are 17.4mH, 15.7mH and 3.2mH respectively, and the capacitors C1, C2 and C3 are 1μF, 3.047μF and 3.675μF respectively. The DC filter is a 12 / 24 / 36 tri-tuned filter.

[0030] When a nonmetallic fault occurs within the zone and a metallic fault occurs outside the zone, we get:

[0031] In the formula, It is the equivalent impedance during nonmetallic faults within the region. It is the equivalent impedance when there is a nonmetallic fault outside the zone.

[0032] It's important to understand that the amplitude of the high-frequency component of the fault current is greater during faults within the fault zone than during faults outside the fault zone. The larger the amplitude of the high-frequency component of the fault current, the more severe the fluctuations in the fault current. Therefore, the fluctuations in the fault current within the fault zone are more severe than those during faults outside the fault zone. (Transition resistance R) f The larger the resistance, the smaller the amplitude of the high-frequency component of the fault current during a fault within the zone. This results in a higher frequency at which the fault current on the rectifier side of the line within the zone is greater than that on the rectifier side of the line outside the zone, leading to a narrower frequency range for the high-frequency component and smaller fluctuations in the fault current waveform. In summary, the fluctuations in the line current waveform are always more severe during faults within the zone than during faults outside the zone, and the larger the transition resistance during a fault within the zone, the smaller the fluctuations in the line current waveform.

[0033] Furthermore, when the current waveform exhibits significant fluctuations, the differential current amplitude at each sampling moment is larger; conversely, when the current waveform fluctuates slightly, the differential current amplitude is smaller. Therefore, the current change rate amplitude can be used as the calculated value of the single-ended current fluctuation characteristic. The data from both ends can then be added together to identify faults inside and outside the zone. The current accumulation is defined as follows:

[0034] In the formula, To protect the startup time, for time The differential current, by Current at time minus The current is obtained at time t, where dt is the sampling time interval and t is the current calculation time of the protection device. After the protection is started, as time increases, The time will automatically be pushed forward. for The accumulation of the rate of change of current at time t is due to the current The sum of the differential current amplitude at time t is obtained by adding the current rate of change amplitudes at all previous times; that is, the starting time for the cumulative calculation is t. The time is, and the end time is At any given time, the accumulated calculation result is used as... Moment .over time, Keep increasing, calculate window length And it keeps increasing.

[0035] When the fluctuations in the current waveform vary significantly, the current amplitude at a single sampling point also changes considerably. The difference will be further amplified by accumulating the magnitude of the rate of change of current.

[0036] Optionally, Step 3 specifically includes: When a fault occurs within the fault zone, the sum of the currents on both sides drops sharply at the moment of the fault; when a fault occurs outside the fault zone, the sum of the currents on both sides rises sharply at the moment of the fault. This embodiment constructs a fault identification criterion for the protection system based on the above differences, defining a fault zone identification coefficient, expressed as:

[0037] In the formula, To protect the startup time, T is the window length. It is the change in positive electrode current. It is the sampling time interval. It is the change in negative current. When the fault area identification coefficient d is greater than the preset fault setting value, it is a fault within the occurrence area. When the fault area identification coefficient d is less than the preset fault setting value, it is a fault outside the occurrence area.

[0038] Specifically, in this embodiment, the PSCAD / EMTDC electromagnetic transient simulation software is used to simulate the entire protected line under fault conditions, and the fault identification setting value is set to 13000. If the current accumulation is less than the setting value, it is judged as an external fault; if the current accumulation is greater than the setting value, it is judged as an internal fault.

[0039] Step 4: Calculate the polarity selection coefficient and perform fault polarity selection in the fault area based on the polarity selection coefficient.

[0040] Optionally, the expression for the polarity selection coefficient is:

[0041] When an intra-zone fault occurs, the sum of the currents on both sides suddenly drops at the moment of the fault. When a fault occurs, the voltage drops and the current increases. The rate of change of current is used to construct the protection activation criterion. Therefore, when a positive fault occurs, the polarity selection coefficient k is greater than the preset threshold A. When a negative fault occurs, the polarity selection coefficient k is less than the preset threshold B. When a bipolar fault occurs, the polarity selection coefficient k satisfies A > k > B.

[0042] Optionally, in this embodiment, A is set to 1.1 and B is set to 0.9. When 1.1 > k > 0.9, it is identified as a bipolar fault within the region; when k > 1.1, it is identified as a positive fault; and when k < 0.9, it is identified as a negative fault.

[0043] In summary, this invention first reads the current data measured at the protection installation points at both ends of the line; then it calculates the current change rate at the protection installation points to determine if a fault has occurred. If the current change rate at three consecutive points is greater than the starting setting value, the current accumulation at both ends is calculated. If the current accumulation is less than the setting value, it is determined to be an external fault; if the current accumulation is greater than the setting value, it is determined to be an internal fault. Then, the fault polarity selection process begins. In this process, the polarity selection coefficient k is calculated. When 1.1 > k > 0.9, it is identified as an internal bipolar fault; when k > 1.1, it is identified as a positive fault; and when k < 0.9, it is identified as a negative fault. This invention provides an important theoretical foundation and technical support for the safe and stable operation of flexible DC transmission lines.

[0044] The specific embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A protection method for flexible DC transmission lines based on current accumulation, characterized in that, The method includes the following steps: Step 1: Read the current data measured at the protection installation points at both ends of the line; Step 2: Calculate the current change rate at the protection installation point based on the current data to determine if a fault has occurred. If no fault has occurred, repeat the test. If a fault has occurred, proceed to Step 3. Step 3: Calculate the current accumulation at both ends of the line to determine the fault area; Step 4: Calculate the polarity selection coefficient and perform fault polarity selection in the fault area based on the polarity selection coefficient.

2. The method for protecting flexible DC transmission lines based on current accumulation as described in claim 1, characterized in that, Step 2 specifically refers to: When a fault occurs, the voltage drops and the current increases. The rate of change of current is used to construct the protection activation criterion, and the activation coefficient is defined as follows: ； In the formula, Δ i It is the fault component current of a DC line, defined as the current after the fault minus the current before the fault. is the rated current of the line, and n is the sampling sequence number. When the starting coefficient s of the three fault points all meet the preset setting value, the protection is activated.

3. The method for protecting flexible DC transmission lines based on current accumulation as described in claim 1, characterized in that, The current accumulation amount is specifically: ； In the formula, To protect the startup time, for time The differential current, by Current at time minus The current is obtained at time t, where dt is the sampling time interval and t is the current calculation time of the protection device. After the protection is started, as time increases, The time will automatically be pushed forward. for The accumulation of the rate of change of current at time t is due to the current The sum is obtained by adding the differential current amplitude at time t and the current rate of change amplitudes at all previous times.

4. The method for protecting flexible DC transmission lines based on current accumulation as described in claim 1, characterized in that, Step 3 specifically refers to: The fault area identification coefficient is defined as follows: ； In the formula, To protect the startup time, T is the window length. It is the change in positive electrode current. It is the sampling time interval. It is the change in negative current. When the fault area identification coefficient d is greater than the preset fault setting value, it is a fault within the occurrence area. When the fault area identification coefficient d is less than the preset fault setting value, it is a fault outside the occurrence area.

5. The method for protecting flexible DC transmission lines based on current accumulation according to claim 4, characterized in that, The expression for the polarity selection coefficient is: ； When a positive electrode fault occurs, the selection coefficient k is greater than the preset threshold A. When a negative electrode fault occurs, the selection coefficient k is less than the preset threshold B. When a bipolar fault occurs, the selection coefficient k satisfies A > k > B.

Images