Multi-terminal hybrid DC line protection method based on measuring wave impedance phase characteristics

Abstract

The present invention relates to a multi-terminal hybrid DC line protection method based on measuring wave impedance phase characteristics. By measuring the line voltage and line current at each measurement point, phase-mode transformation and S-transformation are performed to extract the phase-frequency characteristics of the measured wave impedance at both ends of the line. Within the high-frequency range, the measured wave impedance phase at both ends of the line differs significantly under in-zone and out-of-zone fault conditions. Based on this relationship, a voltage traveling wave amplitude criterion is constructed to achieve fault identification. The present invention has been verified through PSCAD simulation, demonstrating that the proposed protection can distinguish T-zone busbar faults, while meeting rapidity and possessing excellent resistance to transition resistance and noise, meeting the line main protection requirements of multi-terminal hybrid DC systems.

Description

Technical Field

[0001] The present invention belongs to the technical field of power grid line protection, in particular to a multi-terminal hybrid DC line protection method based on measuring wave impedance phase characteristics. Background Art

[0002] In recent years, high-voltage direct current (HVDC) transmission systems have gained widespread adoption due to their outstanding advantages, including low losses, large capacity, and suitability for long-distance power transmission. Line commutated converter (LCC-HVDC) transmission systems based on thyristors (SCRs) are relatively mature and offer numerous advantages, including low cost, low losses, and high capacity. However, their converters rely on the AC system, requiring significant reactive power and carrying the risk of commutation failure. Modular multilevel converter (MMC-HVDC) technology, based on IGBTs (Insulated Gate Bipolar Transistor) technology, has rapidly developed due to its flexible control and lack of commutation failure risk. However, it also suffers from high construction costs, low capacity, and high losses. To address these challenges, the LCC-MMC hybrid HVDC system, which leverages the complementary strengths of the two converters, is an ideal solution for long-distance, high-capacity power transmission.

[0003] The protection of DC lines is one of the key issues for the safe operation of hybrid multi-terminal DC systems. In current research, HVDC transmission protection can be mainly divided into time domain protection and frequency domain protection. Time domain protection schemes mainly use the amplitude, integral, or differential of the signal as a criterion. For example, traveling wave protection based on time domain mutation is often used as the main protection for DC systems, but this type of protection has poor tolerance to transition resistance. Current differential protection has a high tolerance to transition resistance, but it is affected by distributed capacitance and has a long action delay, so it is often only used for backup protection. There is an existing technology that uses the ratio of time domain transient voltages under different sampling periods to construct protection criteria for inside and outside the zone. The protection speed is fast and has a strong tolerance to transition resistance. Frequency domain protection schemes mainly use the frequency characteristics of the fault signal to construct protection criteria. Existing technologies have discovered that DC lines and line boundaries have different attenuation and amplification effects on high- and low-frequency components, and utilize multi-band energy differences to distinguish between faults inside and outside the zone. Existing technologies have proposed a protection scheme based on the energy ratio of specific frequency bands on both sides of the line boundary by studying the impedance-frequency characteristics of DC filters. Existing technologies have constructed a fault voltage traveling wave energy spectrum matrix based on the attenuation of high-frequency energy by the current-limiting inductance of the flexible DC grid, amplifying the time-frequency differences between the fault traveling waves inside and outside the zone. These boundary element-based frequency domain protection schemes offer excellent reliability and speed, but for multi-terminal hybrid DC systems, the different boundary elements of different converters increase the complexity of fault information, and there is no boundary between the DC busbar in the T zone and the adjacent lines, making existing protection schemes difficult to directly apply to multi-terminal hybrid DC systems.

[0004] At present, domestic and foreign scholars have not yet fully grasped the research on line protection of hybrid DC systems. The existing technology constructs protection criteria by judging the flow direction through transient voltage information and active injection current, which does not rely on the bus boundary of the T zone, but has the problem of speed. The existing technology proposes a protection method based on the correlation coefficient by analyzing the similarity of the fault current of the lines at both ends of the hybrid DC system, but it is a two-terminal protection with insufficient speed. The existing technology analyzes the wave impedance amplitude-frequency characteristics of the three-terminal hybrid DC system and proposes a single-terminal protection based on measuring the wave impedance amplitude. However, when the DC filter of the LCC station stops operating due to a fault, the measured wave impedance amplitude of the out-of-zone fault will be greatly reduced, which may lead to false protection operation. Summary of the Invention

[0005] The purpose of the present invention is to overcome the shortcomings of the existing technology and propose a multi-terminal hybrid DC line protection method based on measuring the wave impedance phase characteristics. The method can distinguish T-zone busbar faults, has good resistance to transition resistance and noise while meeting the requirements of rapidity, and meets the main protection requirements of the multi-terminal hybrid DC system line.

[0006] The present invention solves the technical problem by adopting the following technical solutions:

[0007] The multi-terminal hybrid DC line protection method based on measuring the wave impedance phase characteristic includes the following steps:

[0008] Step 1: Measure the line voltage and line current at the line outlets of the LCC station and the MMCⅠ station;

[0009] Step 2: Perform phase mode transformation on the line voltage and limiting current measured in step 1 to obtain the line mode voltage variation;

[0010] Step 3: Construct a startup criterion and determine whether the line mode voltage change satisfies the startup criterion. If so, proceed to step 4; otherwise, return to step 1.

[0011] Step 4: After the startup criterion is met, the initial line-mode voltage traveling wave and the initial line-mode current traveling wave are extracted and S-transformed to obtain the amplitude curve and phase curve of the voltage traveling wave and the current traveling wave;

[0012] Step 5: Calculate the initial traveling wave amplitude and wave impedance phase according to the amplitude curve and phase curve;

[0013] Step 6: Construct fault judgment criteria and pole selection criteria to determine whether the fault is within the zone or outside the zone. If it is determined to be an within-zone fault, the corresponding pole protection is activated and the pole is selected according to the pole selection criteria. Otherwise, it is determined to be an outside-zone fault and returns to step 1.

[0014] Moreover, the starting criterion in step 3 is:

[0015]

[0016] Among them, ΔU1(x) is the line mode voltage change, U1(x) is the line mode voltage signal of the x-th sampling point, i is a positive integer number, n is the number of sampling times, U N is the rated line voltage, k u1 is the line mode voltage variation coefficient.

[0017] Furthermore, the calculation method of the wave impedance phase in step 5 is:

[0018]

[0019] in, is the wave impedance phase, is the phase angle corresponding to t1, and is also the initial voltage traveling wave phase, is the phase angle corresponding to t1, and is also the initial current traveling wave phase.

[0020] Furthermore, the fault criterion in step 6 is: constructing a fault criterion according to whether the measurement point belongs to an LCC station or an MMC I station.

[0021] Furthermore, the fault criterion for the measurement point belonging to the LCC station is:

[0022]

[0023] in, is the measured wave impedance phase of the measuring point M1, where the measuring point M1 is located on line l AB Head end, protection range covers line l AB and l BC Full length, A u-max is the maximum amplitude of the initial voltage traveling wave, A u-set is the initial voltage traveling wave threshold.

[0024] Moreover, the fault criterion for the measurement point belonging to the MMC I station is:

[0025]

[0026] in, is the measured wave impedance phase at the measuring point M2, is the measured wave impedance phase of the measuring point M2, where the measuring points M2 and M3 are located on line l AB End and l BC Head end, protection range includes l AB Full length and l BC Full length and T-zone busbars.

[0027] Moreover, the pole selection criterion in step 6 is:

[0028]

[0029]

[0030] Where ΔU0 is the ground mode voltage difference, k u0 is the ground mode voltage variation coefficient, U N is the rated line voltage, n is the number of sampling times, and U0(x) is the ground mode voltage signal at the x-th sampling point.

[0031] The advantages and positive effects of the present invention are:

[0032] This method measures the line voltage and line current at each measurement point, performs phase-mode transformation and S-transformation, and extracts the phase-frequency characteristics of the measured wave impedance at both ends of the line. Within the high-frequency range, the measured wave impedance phase at both ends of the line differs significantly under in-zone and out-of-zone fault conditions. Based on this relationship, a voltage traveling wave amplitude criterion is constructed to identify faults. PSCAD simulations have verified that the proposed protection can distinguish T-zone busbar faults, maintains rapidity, and exhibits excellent resistance to transition resistance and noise, meeting the requirements for primary line protection in multi-terminal hybrid DC systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is a flow chart of the present invention;

[0034] Figure 2 A structural diagram of a PSCAD simulation model constructed for an embodiment of the present invention;

[0035] Figure 3 This is circuit 1 of the embodiment of the present invention. AB Simulation result diagram of M1 measuring point when positive pole fails;

[0036] Figure 4 This is circuit 1 of the embodiment of the present invention. AB Simulation result diagram of M2 measuring point when positive pole fails;

[0037] Figure 5 This is circuit 1 of the embodiment of the present invention. AB Simulation result diagram of M3 measuring point when positive pole fails;

[0038] Figure 6 This is outside the scope of the embodiment of the present invention. out1 Simulation results of the M1 measuring point during metallic bipolar fault. DETAILED DESCRIPTION

[0039] The present invention is further described below in conjunction with the accompanying drawings.

[0040] Multi-terminal hybrid DC line protection method based on measuring wave impedance phase characteristics, such as Figure 1 As shown, the following steps are included:

[0041] Step 1: Measure the pole-line voltage and pole-line current at the line outlets between the LCC station and the MMCⅠ station.

[0042] Step 2: Perform phase-mode transformation on the line-to-line voltage and limiting current measured in step 1 to obtain the line-to-line voltage variation.

[0043] Step 3: Construct a startup criterion and determine whether the line mode voltage change satisfies the startup criterion. If so, proceed to step 4; otherwise, return to step 1.

[0044] When a DC line fault occurs, a sudden voltage change is detected at the measuring point where the protection is installed. Because the line-mode voltage component has a higher traveling wave velocity than the ground-mode component and is less affected by line attenuation and distortion, the line-mode voltage change, ΔU1(x), is used as the triggering criterion.

[0045] The starting criteria are:

[0046]

[0047] Among them, ΔU1(x) is the line mode voltage change, U1(x) is the line mode voltage signal of the x-th sampling point, i is a positive integer number, n is the number of sampling times, U N is the rated line voltage, k u1 is the line mode voltage variation coefficient. The startup criterion is set according to the maximum fluctuation of the line mode voltage during normal system operation, so k u1 Set to 0.05. In order to detect the sudden change of voltage signal, the difference between three consecutive sampling points is used to effectively detect faults.

[0048] Step 4: After the startup criterion is met, the initial line-mode voltage traveling wave and the initial line-mode current traveling wave are extracted and S-transformed to obtain the amplitude curve and phase curve of the voltage traveling wave and the current traveling wave.

[0049] The calculation method of wave impedance phase is:

[0050]

[0051] in, is the wave impedance phase, is the phase angle corresponding to t1, and is also the initial voltage traveling wave phase, is the phase angle corresponding to t1, and is also the initial current traveling wave phase.

[0052] Step 5: Calculate the initial traveling wave amplitude and wave impedance phase according to the amplitude curve and phase curve.

[0053] Step 6: Construct fault judgment criteria and pole selection criteria to determine whether the fault is within the zone or outside the zone. If it is determined to be an within-zone fault, the corresponding pole protection is activated and the pole is selected according to the pole selection criteria. Otherwise, it is determined to be an outside-zone fault and returns to step 1.

[0054] By measuring the wave impedance phase characteristics, forward and reverse faults can be distinguished at the protection device. To distinguish between faults within the forward zone and those outside the forward zone, an initial voltage traveling wave amplitude criterion can be set. In this step, the fault judgment criteria are constructed based on whether the measurement point belongs to an LCC station or an MMC I station.

[0055] The fault criterion for the measurement point belonging to the LCC station is:

[0056]

[0057] in, is the measured wave impedance phase of the measuring point M1, where the measuring point M1 is located on line l AB Head end, protection range covers line l AB and l BC Full length, A u-max is the maximum amplitude of the initial voltage traveling wave, A u-set is the initial voltage traveling wave threshold.

[0058] The fault criterion for the measurement point belonging to MMCⅠ station is:

[0059]

[0060] in, is the measured wave impedance phase at the measuring point M2, is the measured wave impedance phase of the measuring point M2, where the measuring points M2 and M3 are located on line l AB End and l BC Head end, protection range includes l AB Full length and l BC Full length and T-zone busbars.

[0061] Determining the fault type is an important component of DC protection. This invention uses the ground-mode voltage mutation as a characteristic for fault selection: when an inter-pole fault occurs, the ground-mode voltage is 0; when a positive-pole ground fault occurs, the ground-mode voltage is negative and has a large amplitude; when a negative-pole ground fault occurs, the ground-mode voltage is positive and has a large amplitude. Therefore, fault selection can be identified using the ground-mode voltage. The ground-mode voltage difference is defined as:

[0062]

[0063] The extreme selection criterion is:

[0064]

[0065] Where ΔU0 is the ground mode voltage difference, k u0 is the ground mode voltage variation coefficient, U N is the rated line voltage, n is the number of sampling times, and U0(x) is the ground mode voltage signal at the x-th sampling point.

[0066] like Figure 2 As shown in Figure 2, based on the multi-terminal hybrid DC line protection method based on measuring the wave impedance phase characteristics, a PSCAD simulation model was constructed to verify the correctness of the proposed protection scheme. The sampling frequency was 100 kHz and the data window was 3 ms. The voltage and current components at a frequency of 10 kHz were extracted for analysis.

[0067] 1. Typical in-area fault simulation analysis:

[0068] Set at t = 0.75s, line l AB A ground fault with a positive electrode and a transition resistance of 100Ω occurs at the midpoint, and the fault duration is 0.1s. The simulation results of the M1, M2, and M3 measurement points are as follows: Figure 3 、 Figure 4 and Figure 5 As shown in Figure 1, sub-figures (a), (b), (c), and (d) are the line mode voltage variation (startup criterion), the initial voltage traveling wave amplitude at 10 kHz after S-transformation, the ground mode voltage variation (fault pole selection), and the wave impedance phase at 10 kHz after S-transformation, respectively.

[0069] according to Figure 3 、 Figure 4 and Figure 5 As shown, when line l AB When a positive ground fault occurs, all four measuring points can be activated and determine the positive fault. For M1, the maximum amplitude of the high-frequency voltage traveling wave is A u-max =25.71kV, measuring wave impedance phase Meet the fault judgment criteria within the area; for the measuring points M2 and M3 in the MMCⅠ station, the maximum amplitude of the high-frequency voltage traveling wave A u-max =14.98kV, measuring wave impedance phase Judged as l AB A malfunction occurred.

[0070] When the triggering element meets the judgment criteria, it immediately intercepts the 3ms data window signal. After calculating the initial voltage traveling wave amplitude and wave impedance phase, it outputs the action result if it is determined to be an internal fault. For LCC stations, only the output result of the M1 measuring point is required to determine the action status. For MMCⅠ stations, the protection judgment only requires data from two measuring points within the station, M2 and M3. No inter-station communication is required, which provides high speed.

[0071] 2. Typical out-of-area fault simulation analysis:

[0072] Assume that at t = 0.75s, the valve side of the current limiting reactor of the LCC station f out1 A metallic bipolar fault occurs, which lasts for 0.1s. The simulation results of the M1 measurement point are as follows: Figure 6 shown.

[0073] 3. Analysis of protection effectiveness under different fault conditions:

[0074] Depend on Figure 6 As shown in the figure, when a two-pole fault occurs outside the current-limiting reactor of the LCC station, the M1 measuring point meets the startup requirements and determines that the two-pole fault occurs. The maximum amplitude of the initial voltage traveling wave is A u-max =2.193kV, measuring wave impedance phase The reverse zone fault condition is met.

[0075] To further verify the adaptability of the protection principle, the performance tests of the proposed protection were carried out under different fault locations, transition resistances and fault types. The simulation results are shown in Table 1.

[0076] Table 1 Test results of protection algorithms under different fault conditions

[0077]

[0078] It can be seen from Table 1 that when an internal fault occurs, the maximum value of the initial voltage traveling wave amplitude at each measuring point is A max are significantly greater than the setting value 5, and are significantly different from the fault outside the zone; when a forward fault occurs, At around 90°, it is obviously larger than the set value of 45°, and and Around 180°, it is also much larger than the set value of 90°; when a reverse fault occurs, and The values are all around 0°, which is consistent with the theoretical analysis results mentioned above. In summary, the protection principle is not affected by the fault location, transition resistance and fault type, and can accurately identify faults inside and outside the zone. The fault type can be correctly identified by using the pole selection criterion. For the M2 and M3 measuring points of the MMCⅠ station, it can clearly determine the fault type. AB 、l BC And T zone busbar failure.

[0079] 4. Analysis of the impact of noise interference on protection:

[0080] In order to analyze the impact of noise interference on protection performance, the line l AB and l BC At the midpoint (f in12 、f in22 ) and current limiting reactor L1 valve side (f out1White noise with a signal-to-noise ratio of 20-40 dB was added to the simulation data for metallic faults. The lower the signal-to-noise ratio, the stronger the noise signal. The simulation results are shown in Table 2. The addition of noise causes irregular fluctuations in the amplitude of the high-frequency voltage traveling wave and the phase of the wave impedance. Under 20-40 dB noise, the protection circuit breaker operates correctly for all in-zone faults; under 30-40 dB noise, it does not malfunction for out-of-zone faults. In summary, the proposed protection circuit breaker has a certain degree of noise tolerance.

[0081] Table 2 Test results of noise interference resistance

[0082]

[0083] 5. Analysis of the impact of sampling frequency on protection:

[0084] The protection method proposed in the present invention needs to extract the fault traveling wave, so there are certain requirements for the sampling frequency. In order to achieve better simulation effects, the 100kHz sampling frequency selected by the present invention has left a certain margin. Tables 3 and 4 respectively give the simulation results when the sampling frequencies are 50kHz and 25kHz. It has been verified that when the sampling frequency is 50kHz, the protection principle can still correctly identify faults and operate reliably; when the sampling frequency is 25kHz, the wave impedance phase results are still basically consistent with the above analysis conclusions, but the initial traveling wave amplitude is reduced, resulting in the protection may refuse to operate when a fault occurs with a transition resistance of 500Ω. In summary, considering the reliability of the protection, the sampling frequency should not be lower than 50kHz.

[0085] Table 3 Simulation results when the sampling frequency is 50kHz

[0086]

[0087]

[0088] Table 4 Simulation results when the sampling frequency is 25kHz

[0089]

[0090] It should be emphasized that the embodiments described in the present invention are illustrative rather than restrictive. Therefore, the present invention includes but is not limited to the embodiments described in the specific embodiments. Any other embodiments derived by those skilled in the art based on the technical solutions of the present invention also fall within the scope of protection of the present invention.

Claims

1. A multi-terminal hybrid DC line protection method based on measuring the wave impedance phase characteristics, characterized by: The following steps are involved: Step 1: Measure the line voltage and line current at the line outlets of the LCC station and the MMCⅠ station; Step 2: Perform phase mode transformation on the pole line voltage and pole line current measured in step 1 to obtain the line mode voltage change; Step 3: Construct a startup criterion and determine whether the line mode voltage change satisfies the startup criterion. If so, proceed to step 4; otherwise, return to step 1. Step 4: After the startup criterion is met, the initial line-mode voltage traveling wave and the initial line-mode current traveling wave are extracted and S-transformed to obtain the amplitude curve and phase curve of the voltage traveling wave and the current traveling wave; Step 5: Calculate the initial traveling wave amplitude and wave impedance phase according to the amplitude curve and phase curve; Step 6: Construct fault judgment criteria and pole selection criteria to determine whether the fault is internal or external. If it is internal, the corresponding pole protection is activated and the pole selection is performed according to the pole selection criteria. Otherwise, it is determined to be an external fault and the process returns to step 1. The fault criterion in step 6 is as follows: construct the fault criterion based on whether the measurement point belongs to the LCC station or the MMCⅠ station; The fault criterion for the measurement point belonging to the LCC station is: in, is the measured wave impedance phase of the measuring point M1, where the measuring point M1 is located on line l AB Head end, protection range covers line l AB and l BC Full length, A u-max is the maximum amplitude of the initial line mode voltage traveling wave, A u-set is the initial line mode voltage traveling wave threshold; The fault criterion for the measurement point belonging to MMCⅠ station is: in, is the measured wave impedance phase at the measuring point M2, is the measured wave impedance phase of the measuring point M3, where the measuring points M2 and M3 are located on line l AB End and l BC Head end, protection range includes l AB Full length and l BC Full length and T-zone busbars.

2. The multi-terminal hybrid DC line protection method based on measuring the wave impedance phase characteristics according to claim 1, characterized in that: The starting criterion in step 3 is: Among them, ΔU1(x) is the line mode voltage change, U1(x) is the line mode voltage signal of the x-th sampling point, i is a positive integer number, n is the number of sampling times, U N is the rated line voltage, k u1 is the line mode voltage variation coefficient.

3. The multi-terminal hybrid DC line protection method based on measuring the wave impedance phase characteristics according to claim 1, characterized in that: The calculation method of the wave impedance phase in step 5 is: in, is the wave impedance phase, is the phase angle corresponding to t1, and is also the initial line mode voltage traveling wave phase, is the phase angle corresponding to t1, and is also the initial line mode current traveling wave phase.

4. The multi-terminal hybrid DC line protection method based on measuring wave impedance phase characteristics according to claim 1, characterized in that: The pole selection criterion in step 6 is: Where ΔU0 is the ground mode voltage difference, k u0 is the ground mode voltage variation coefficient, U N is the rated line voltage, n is the number of sampling times, U0(x) is the ground mode voltage signal at the x-th sampling point, and i is a positive integer number.

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