A DC system adaptive restart method based on converter control coordination

Abstract

The present invention relates to a DC system adaptive restart method based on converter control coordination. The method is aimed at the LCC-MMC hybrid DC transmission system, and uses LCC to inject a voltage forward wave F into the fault line. tw , if the fault disappears, F tw It will propagate along the line to the MMC and be reflected. At this time, the reverse wave B is detected at the LCC. tw1 ; If the fault exists, F w It will propagate along the line to the fault point and be reflected. At this time, the reverse traveling wave B is detected at the LCC. tw2 When F tw When propagating on a sound line, the reverse traveling wave B reflected to the LCC can be estimated by combining the analysis of the propagation process of the traveling wave on the overhead line and the boundary reflection coefficient of the DC circuit breaker. twe . Estimation of the reverse wave B by comparison twe and the measured reverse traveling wave B tw The waveform difference between the two can be used to proactively determine whether the fault has disappeared before the system is restarted, thus avoiding the DC system restarting due to a fault.

Description

Technical Field

[0001] The present invention relates to the technical field of power system protection and control, and in particular to a DC system adaptive restart method based on converter control coordination. Background Art

[0002] Hybrid DC combines the technical advantages of conventional DC based on line commutated converter (LCC) and flexible DC based on modular multilevel converter (MMC). It can not only reduce the risk of commutation failure, but also realize large-capacity, low-cost, long-distance power transmission and networking operation. my country has taken the lead in conducting research and construction of hybrid DC transmission systems.

[0003] As an emerging technology, the related theory and application research of hybrid DC is still relatively weak. In order to ensure the reliable operation of hybrid DC transmission system, domestic and foreign scholars have carried out some research on it, mainly involving topology design, control strategy optimization, fault current analysis and line protection distance measurement, while the research on system recovery technology after line fault is relatively lacking.

[0004] Overhead lines are the main transmission channels for high-voltage and large-capacity hybrid DC transmission systems, and have a high probability of transient faults. In order to ensure the safe and stable operation of the system and rapid power recovery, it is necessary to deeply analyze the characteristic differences between transient faults and permanent faults in hybrid DC lines, optimize the system recovery strategy, and study the adaptive restart technology suitable for hybrid DC systems, which has theoretical and practical significance.

[0005] Hybrid DC includes both LCC and MMC converters. In terms of fault clearing strategy, LCC clears faults by phase shifting, blocking trigger pulses, and throwing bypass pairs to achieve rapid restart of the system; MMC generally achieves fault isolation and clearing by tripping DC circuit breakers (DCCB), throwing controllable lightning arresters, full-bridge (hybrid bridge) MMC voltage reduction, and disconnecting AC circuit breakers. The new structure and new features of hybrid DC put forward new requirements for system recovery technology after fault clearing.

[0006] However, the hybrid DC projects that have been put into operation still follow the existing fault clearing and system recovery schemes of LCC-HVDC and MMC-HVDC systems. Among them, the LCC-HVDC system mainly uses the control characteristics of LCC to restore power transmission, which is called DC-line fault recovery sequences (DFRS). DFRS mainly includes three stages: phase shifting, which is to clear the fault current by adjusting the trigger angle of the rectifier side to 160°; deionization, when the DC current drops to zero, after a long fixed deionization time, the fault point is restored to insulation; restart, after a fixed deionization time (300-500ms), the DFRS controller reduces the trigger angle of the rectifier and restarts at the rated voltage or 75% of the rated voltage. If the fault disappears and the deionization time is long enough, the DC voltage can be successfully restored; if DFRS is repeated 2-3 times and the DC voltage is still lower than 0.1pu, the LCC is locked and judged as a permanent fault.

[0007] For flexible DC transmission systems equipped with DC circuit breakers, an automatic reclosing strategy with a fixed de-ionization time is often used to restore system power transmission. The automatic reclosing strategy proposed by ABB can identify permanent faults by detecting whether the DC side voltage can be successfully established after the circuit breaker is closed. If the DC voltage can be established, it means that the fault has disappeared and it is a transient fault. The circuit breaker can be closed to restore power transmission; if the DC voltage cannot be established, it is a permanent fault and the DC circuit breaker should be immediately reopened. For flexible DC systems that use full-bridge submodules to lock and clear faults, the fault restart method is to lock the converter to clear the fault current and go through a period of de-ionization, then unlock the converter again, and determine whether the fault has disappeared by detecting whether the DC voltage can be established normally.

[0008] The above restart strategies are all automatic restart methods after a fixed de-ionization time. They lack pre-judgment of the nature of the fault and will be forced to restart regardless of the nature of the fault. For permanent faults, automatic restart will cause secondary impact on the converter valve. In addition, the automatic reclosing based on DC circuit breakers also has high requirements for the breaking capacity of the circuit breaker. To this end, scholars at home and abroad are committed to proposing adaptive restart technology that can actively identify the nature of the fault before the system restarts. However, the current research on adaptive restart technology for DC systems is mainly focused on LCC-HVDC systems and MMC-HVDC systems, and there has been no relevant research on hybrid DC systems. Summary of the invention

[0009] In view of the defects existing in the prior art, the purpose of the present invention is to provide a method for adaptive restart of a DC system based on converter control coordination. Based on the difference in the propagation paths of the voltage traveling wave injected by LCC on the healthy line and the faulty line, the method of the present invention can actively identify the nature of the fault before the system is restarted, avoiding the secondary impact problem caused by the system restarting when the fault exists.

[0010] In order to achieve the above purpose, the technical solution adopted by the present invention is:

[0011] A method for adaptive restarting of a DC system based on converter control coordination, characterized in that it comprises the following steps:

[0012] Step 1: After a line fault occurs, the sending-end LCC clears the fault current and the receiving-end MMC isolates the faulty line;

[0013] Step 2, injecting a voltage signal into the fault line;

[0014] Step 3, based on the voltage signal injected into the fault line in step 2, estimate the reverse traveling wave propagating to the LCC side; calculate the Pearson correlation coefficient PCC between the estimated reverse traveling wave and the actual reverse traveling wave;

[0015] Step 4: Determine whether the fault disappears based on the Pearson correlation coefficient PCC obtained in step 3.

[0016] Furthermore, in step 1, the sending-end LCC clears the fault current by adjusting the trigger angle, and the receiving-end MMC clears the fault line through the DC circuit breaker.

[0017] Further, the specific steps of step 2 are:

[0018] Step 2-1, after the fault point is freed, adjust the trigger angle;

[0019] Step 2-2, inject a voltage signal into the fault circuit and adjust the trigger angle.

[0020] Further, the fault described in step 1 includes a single-stage grounding fault and a double-stage short circuit fault; for a single-stage grounding fault, step 2-2 is to inject a voltage signal into the fault stage; for a double-stage short circuit fault, step 2-2 is to inject a voltage signal into any stage.

[0021] Furthermore, the specific steps of estimating the reverse traveling wave propagating to the LCC side in step 3 are:

[0022] Step 3-1, establish an overhead route model;

[0023] Step 3-2, obtaining a frequency domain expression of the voltage at one end of the overhead line in step 3-1;

[0024] Step 3-3, obtaining a time domain expression of the voltage at one end of the overhead line in step 3-1;

[0025] Step 3-4, estimate the reverse traveling wave transmitted to the LCC side, and calculate the Pearson correlation coefficient PCC between the estimated reverse traveling wave and the actual reverse traveling wave.

[0026] Further, the specific steps of step 4 are:

[0027] Step 4-1, if the Pearson correlation coefficient PCC is greater than 0.85, the fault is determined to be a transient fault;

[0028] Step 4-2: If the Pearson correlation coefficient PCC is less than or equal to 0.85, repeat steps 2 to 4 until PCC is greater than 0.85 and the fault disappears.

[0029] Step 4-3: If the number of restarts is greater than 3 times and the PCC is less than 0.85, the fault is determined to be a permanent fault.

[0030] The invention discloses a method for adaptively restarting a DC system based on converter control coordination, which has the following beneficial effects:

[0031] By actively judging the nature of the fault, the system is prevented from blindly restarting due to the fault, the success rate of the system restart is improved, the reliability of the system restart is improved, and the technical defects of blind restart of the DC system in the prior art are overcome. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The present invention has the following accompanying drawings:

[0033] Figure 1 A processing flow chart of a DC system adaptive restart method based on converter control coordination of the present invention;

[0034] Figure 2 A schematic diagram of an LCC-MMC ±800kV hybrid DC power transmission system provided by an embodiment of the present invention;

[0035] Figure 3 An overhead line parameter provided by an embodiment of the present invention: (a) is a frequency-dependent model parameter; (b) is a tower structure parameter;

[0036] Figure 4 Schematic diagram of calculation results when the present invention is applied to a single-pole ground short circuit fault: (a) calculation results corresponding to a permanent fault in Example 1; (b) calculation results corresponding to a transient fault in Example 1;

[0037] Figure 5 Schematic diagram of calculation results when the present invention is applied to a bipolar short circuit fault: (a) the calculation result corresponding to the permanent fault of Example 2; (b) the calculation result corresponding to the transient fault of Example 2; DETAILED DESCRIPTION

[0038] The present invention is further described in detail below in conjunction with the accompanying drawings.

[0039] The processing flow of a DC system adaptive restart method based on converter control coordination provided by the present invention is as follows: Figure 1 As shown, the specific steps include:

[0040] Step 1: After a line fault occurs, the LCC uses phase shift control (trigger angle adjusted to 160°) to clear the fault current, and the MMC uses a DC circuit breaker to isolate the faulty line. The tripped DC circuit breaker can be regarded as an open circuit point, and the traveling wave reflection coefficient is +1.

[0041] Step 2: Wait for 300ms of deionization time, adjust the LCC trigger angle from 160° to 87°, and then inject a voltage signal F into the fault line. tw Wait for 1ms, adjust the LCC trigger angle to 150° again, and record the restart number n as 1. For a single-pole grounding fault, it is necessary to inject a voltage signal into the faulty pole; for a bipolar short-circuit fault, it is sufficient to inject a voltage signal into any one pole.

[0042] Step 3: Based on the injected voltage signal F tw , estimate the reverse traveling wave B propagating to the LCC side twe . Calculate the actual reverse wave B tw With B twe The Pearson correlation coefficient PCC between them. The signal injection time is used as the data recording starting point for the two sets of reverse traveling waves, and the data length is 2.5τ. τ is the time required for the voltage traveling wave to propagate from one side of the line to the other side.

[0043] To accurately estimate the back-traveling wave B twe First, the overhead line model should be established in the frequency domain. Then, based on the recursive convolution theorem, the time domain expression of the overhead line model is obtained. In the frequency domain, the forward wave F(ω), reverse wave B(ω), terminal voltage U(ω) and terminal current I(ω) at one end of the overhead line satisfy the following equation:

[0044]

[0045] In the formula, n = k, m, representing the two ends of the line; Z c (ω) is the line characteristic impedance; R(ω), L(ω), G(ω) and C(ω) refer to the line resistance, inductance, conductance and capacitance respectively.

[0046] The forward and reverse waves on both sides of the line satisfy the following equation:

[0047]

[0048] In the formula, A(ω) is the attenuation function of the line; r(ω) is the propagation coefficient of the traveling wave; l is the line length. Combining the above two formulas, the frequency domain expression of the voltage at terminal k can be obtained:

[0049] U k (ω)=Z c (ω)I k (ω)+A(ω)F m (ω)

[0050] =Z c (ω)I k (ω)+A(ω)U m (ω)+Z c (ω)A(ω)I m (ω)

[0051] In order to obtain the time domain expression of the voltage at terminal k, it is necessary to perform recursive convolution calculation on the above formula. Using the vector matching method, Z is converted to c (ω) and A(ω) are expanded:

[0052]

[0053] Where, s = jω; N 1 、N 2 、N 3 Z c (ω), A(ω), and Z c (ω) is the number of poles of A(ω); τ is the time it takes for a traveling wave to propagate 1 km in an overhead line; b 0 is a free component; b i ,d i 、f i and a i 、c i 、e i are the corresponding residues and poles respectively. For actual overhead lines, b i ,d i 、f i and a i 、c i 、e i The solution can be obtained based on line parameters and voltage / current data fitting.

[0054] We can further obtain the time domain magnitude u of the voltage at terminal k: k (t) is:

[0055]

[0056] Using recursive convolution to calculate the exponential function in the above formula, we can get:

[0057]

[0058] Where Δt is the sampling calculation step length; m i ,s ni , p ni ,q ni are all constants, and can be calculated based on a i 、b i 、c i ,d i 、e i 、f i and Δt are calculated; i k (t) is the current data corresponding to the previous sampling point. tw Based on the above formula, the reverse wave B transmitted to the LCC side can be estimated based on the two conditions that the reflection coefficient is +1 after the MMC trips. twe .

[0059] The Pearson correlation coefficient PCC measures the degree of correlation between two sets of data and has a high statistical significance. The PCC between two variables is defined as the quotient of the covariance and standard deviation between the two variables:

[0060]

[0061] In the formula, X and Y represent two sets of data variables; n represents the number of samples contained in each set of variables.

[0062] τ is the time required for the voltage traveling wave to propagate from one side of the line to the other side, satisfying the following formula:

[0063]

[0064] Where L is the line length; v is the speed of traveling wave propagation.

[0065] Step 4: Determine whether the fault has disappeared based on the calculated PCC:

[0066] 1) If PCC>0.85, it means B tw and B twe The injected voltage traveling wave is highly similar and is reflected back to the LCC side by the tripped DC circuit breaker on the opposite side, which determines that the nature of the fault is a transient fault.

[0067] 2) If PCC <= 0.85, it means B tw and B tweThe similarity is low, and the injected voltage traveling wave is reflected back to the LCC side by the fault point, and it is determined that the fault has not disappeared. However, in order to avoid misjudging a transient fault that exists for a long time as a permanent fault, after a short de-ionization time of 100ms, repeat steps 2-4 to determine whether the fault disappears again, and increase the number of restarts n by 1. If the calculated PCC>0.85 after the signal is injected twice, it indicates that the fault has disappeared. In order to avoid restart failure due to incomplete insulation recovery at the fault point, a de-ionization time of 100ms is required again at this time. If the calculated PCC<0.85 when the number of restarts n>3, the fault is determined to be a permanent fault.

[0068] Figure 2 This is a schematic diagram of an LCC-MMC ±800kV hybrid DC transmission system provided in an embodiment of the present invention. The system parameters are shown in Table 1.

[0069] Table 1 System simulation parameters

[0070]

[0071] Example 1: The fault type is a positive ground short circuit fault, and the fault distance is L fault =300kM, fault resistance R fault =100Ω, sampling frequency f s =10kHz, line sag H=19.5m, resistivity r=0.0322Ω / km. Fault occurrence time t 0 =0ms, MMC outlet circuit breaker at t 1 =3ms, the LCC trigger angle is at t 2 =10ms and adjust to 160°.

[0072] Step 1) After 300ms of deionization time, at t 3 =310ms, adjust the LCC trigger angle to 87° and maintain for 1ms, then adjust the LCC trigger angle to 160° again. Collect and record the injected voltage forward wave F tw .

[0073] Step 2) Based on the injected voltage forward wave F tw , estimate the reverse traveling wave B propagating to the LCC side twe . Collect and record the reverse traveling wave B that actually propagates to the LCC side tw . With the signal injection time t 3 As the starting point of data recording for the two sets of reverse traveling waves, the data length is 2.5τ. For a 1000km line, τ=3.341ms.

[0074] Step 3) Calculate B twe and B twThe Pearson correlation coefficient PCC between them is used to determine the nature of the fault based on the size of the PCC.

[0075] Example 2: Fault type is bipolar short circuit fault, fault distance L fault =300kM, fault resistance R fault =100Ω, sampling frequency f s =10kHz, line sag H=19.5m, resistivity r=0.0322Ω / km. Fault occurrence time t 0 =0ms, MMC outlet circuit breaker at t 1 =3ms, the LCC trigger angle is at t 2 =10ms and adjust to 160°.

[0076] Step 1) After 300ms of deionization time, at t 3 =310ms, adjust the LCC trigger angle to 87° and maintain for 1ms, then adjust the LCC trigger angle to 160° again. Collect and record the injected voltage forward wave F tw .

[0077] Step 2) Based on the injected voltage forward wave F tw , estimate the reverse traveling wave B propagating to the LCC side twe . Collect and record the reverse traveling wave B that actually propagates to the LCC side tw . With the signal injection time t 3 As the starting point of data recording for the two sets of reverse traveling waves, the data length is 2.5τ. For a 1000km line, τ=3.341ms.

[0078] Step 3) Calculate B twe and B tw The Pearson correlation coefficient PCC between them is used to determine the nature of the fault based on the size of the PCC.

[0079] Figure 4 The calculation results of Example 1 are given. Figure 4 (a) Calculation results corresponding to the transient fault in Example 1, Figure 4 (b) Calculation results corresponding to permanent fault in Example 1. Figure 5 The calculation results of Example 2 are given. Figure 5 (a) Calculation results corresponding to the transient fault in Example 2, Figure 5(b) Calculation results corresponding to the permanent fault in Example 2. The waves in the dotted box in the figure are the data used to compare the waveform similarity. For a single-pole grounding fault, the calculated PCCs are 0.9463 (transient fault) and 0.2596 (permanent fault); for a double-pole short-circuit fault, the calculated PCCs are 0.999 (transient fault) and 0.217 (permanent fault). Therefore, by comparing the PCCs, the proposed method can accurately identify the nature of the fault.

[0080] Further comparison Figure 4 and Figure 5 It can be seen that for a bipolar short-circuit fault, the measured reverse traveling wave B tw and estimate the back-traveling wave B twe The waveform similarity under transient fault is as high as 0.999. For single-pole grounding fault, affected by the coupling of sound lines, the measured reverse wave B tw and estimate the back-traveling wave B twe It is not completely consistent. The waveform similarity under transient fault is 0.9463.

[0081] In addition, when LCC is used to inject characteristic voltage signals into the fault line, the initial voltage traveling wave F tw The polarity of F is positive. For transient faults, tw Total reflection with positive polarity occurs at the tripped DC circuit breaker, such as Figure 4 As shown in (a) and 5(a), the polarity of the actually detected reverse wave (red line) is positive. For permanent faults, F tw A reflection with negative polarity occurs at the fault point, such as Figure 4 As shown in (b) and 5(b), the polarity of the actually detected back-travelling wave (red line) is negative. This is also the main reason for the difference between the estimated back-travelling wave and the actual back-travelling wave. Considering that PCC mainly reflects the difference between the waveforms by comparing the amplitude and change trend of the two sets of data, if the polarity of the wave head of the measured back-travelling wave and the estimated back-travelling wave is inconsistent, the calculated PCC will be much less than 1, which also ensures the feasibility of the proposed method.

[0082] In summary, the present invention avoids the DC system restarting due to a fault by actively judging the nature of the fault before the system restarts, realizes adaptive restart, improves the success rate of the DC system restart, improves the reliability of the DC system restart, and overcomes the technical defects of blind restart of the DC system in the prior art.

[0083] The embodiment of the present invention has extremely high reliability, can withstand 800Ω fault resistance and is not affected by the fault distance, and has low requirements on system sampling frequency (10kHz). For a 1000km line, the fault nature can be accurately identified after 7ms of signal injection.

[0084] Those skilled in the art can understand that the accompanying drawings are only schematic diagrams of an embodiment, and the modules or processes in the accompanying drawings are not necessarily required to implement the present invention.

[0085] Each embodiment in this specification is described in a progressive manner, and the same or similar parts between the embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments. In particular, for the device or system embodiment, since it is basically similar to the method embodiment, the description is relatively simple, and the relevant parts can be referred to the partial description of the method embodiment. The device and system embodiments described above are merely schematic, wherein the units described as separate components may or may not be physically separated, and the components displayed as units may or may not be physical units, that is, they may be located in one place, or they may be distributed on multiple network units. Some or all of the modules may be selected according to actual needs to achieve the purpose of the scheme of this embodiment. Ordinary technicians in this field can understand and implement it without paying creative labor.

[0086] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

[0087] The contents not described in detail in this specification belong to the prior art known to professional and technical personnel in this field.

Claims

1. A method for adaptive restart of a DC system based on converter control coordination, characterized in that: The steps include: Step 1: After a line fault occurs, the sending-end LCC clears the fault current and the receiving-end MMC isolates the faulty line; Step 2, injecting a voltage signal into the fault line; Step 3, based on the voltage signal injected into the fault line in step 2, estimating the reverse traveling wave propagating to the LCC side; Calculate the Pearson correlation coefficient PCC between the estimated anti-traveling wave and the actual anti-traveling wave; Step 4: judging whether the fault disappears based on the Pearson correlation coefficient PCC obtained in step 3; The specific steps of estimating the reverse traveling wave propagating to the LCC side in step 3 are: Step 3-1, establishing an overhead line model; Step 3-2, obtaining the frequency domain expression of the voltage at one end of the overhead line described in step 3-1, specifically: In the frequency domain, the forward wave F(ω), reverse wave B(ω), terminal voltage U(ω) and terminal current I(ω) at one end of the overhead line satisfy the following equations: In the formula, n = k, m, representing the two ends of the line; Z c (ω) is the line characteristic impedance; R(ω), L(ω), G(ω) and C(ω) refer to the line resistance, inductance, conductance and capacitance respectively; The forward and reverse waves on both sides of the line satisfy the following equation: Where A(ω) is the attenuation function of the line; r(ω) is the propagation coefficient of the traveling wave; l is the line length; the above two equations can be combined to obtain the frequency domain expression of the voltage at terminal k: U k (ω)=Z c (oh)I k (ω)+A(ω)F m (oh) =Z c (oh)I k (ω)+A(ω)U m (ω)+Z c (ω)A(ω)I m (oh) Step 3-3, obtaining the time domain expression of the voltage at one end of the overhead line in step 3-1, specifically: Using the vector matching method, Z c (ω) and A(ω) are expanded: Where, s = jω; N1, N2, N3 are Z c (ω), A(ω), and Z c (ω) the number of poles of A(ω); t is the time it takes for a traveling wave to propagate l km in an overhead line; b0 is a free component; b i ,d i 、f i and a i 、c i 、e i are the corresponding residues and poles respectively; We can further obtain the time domain magnitude u of the voltage at terminal k: k (t) is: Step 3-4, estimating the reverse traveling wave transmitted to the LCC side, and calculating the Pearson correlation coefficient PCC between the estimated reverse traveling wave and the actual reverse traveling wave; The above estimation of the reverse traveling wave transmitted to the LCC side is: Use recursive convolution to apply the above u k (t) The exponential function in the expression can be calculated as follows: Where Δt is the sampling calculation step length; m i ,s ni , p ni ,q ni are all constants, according to a i 、b i 、c i d i 、e i 、f i and Δt are calculated; i k (t) is the current data corresponding to the previous sampling point; further combined with the injected voltage traveling wave F tw Based on the above formula, the reverse wave B transmitted to the LCC side can be estimated based on the two conditions that the reflection coefficient is +1 after the MMC trips. twe .

2. A method for adaptive restarting of a DC system based on converter control coordination according to claim 1, characterized in that: In step 1, the sending-end LCC clears the fault current by adjusting the trigger angle, and the receiving-end MMC clears the fault line through the DC circuit breaker.

3. The method for adaptive restarting of a DC system based on converter control coordination according to claim 1, characterized in that: The specific steps of step 2 are: Step 2-1, after the fault point is freed, adjust the trigger angle; Step 2-2, inject a voltage signal into the fault circuit and adjust the trigger angle.

4. A method for adaptive restarting of a DC system based on converter control coordination according to claim 3, characterized in that: The faults described in step 1 include unipolar grounding faults and bipolar short circuit faults; for unipolar grounding faults, step 2-2 is to inject a voltage signal into the faulty pole; for bipolar short circuit faults, step 2-2 is to inject a voltage signal into any pole.

5. The method for adaptive restarting of a DC system based on converter control coordination according to claim 3, characterized in that: The specific steps of step 4 are: Step 4-1, if the Pearson correlation coefficient PCC is greater than 0.85, the fault is determined to be a transient fault; Step 4-2: If the Pearson correlation coefficient PCC is less than or equal to 0.85, repeat steps 2 to 4 until PCC is greater than 0.85 and the fault disappears. Step 4-3: If the number of restarts is greater than 3 times and the PCC is less than 0.85, the fault is determined to be a permanent fault.

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