A hybrid cascade multi-terminal DC transmission line protection method based on current correlation coefficient

Abstract

The present invention relates to a hybrid cascade multi-terminal DC transmission line protection method based on current correlation coefficient, including S1, determining the fault identification criteria inside and outside the zone, using the correlation coefficient of high-frequency current to construct the identification criteria, and identifying the faults inside and outside the zone; S2, determining the protection start-up criteria; S3, determining the fault pole selection criteria; S4, determining the protection scheme. The present invention uses the DC voltage change to form the protection start-up criteria to determine whether the fault occurs; uses the cosine similarity to calculate the correlation coefficient of the high-frequency current to identify the faults inside and outside the zone; uses the voltage change of the two poles to select the fault pole to form an overall protection method. The present invention has a simple principle and a small amount of calculation. It only needs to calculate the correlation coefficient of the DC line fault current, and can accurately identify the DC line fault. The method has a high sensitivity to high-resistance grounding faults and does not require real-time synchronous transmission of data at both ends of the line.

Description

Technical Field

[0001] The invention belongs to the technical field of power system protection and control, and in particular relates to a hybrid cascade multi-terminal direct current transmission line protection method based on current correlation coefficient. Background Art

[0002] Line commutated converter-based high voltage direct current (LCC-HVDC) has been widely used in practical projects due to its large transmission capacity and low power consumption. However, it is highly dependent on the AC system. AC failure may cause DC commutation failure and even threaten the safety and stability of the power grid. VSC-HVDC (voltage source converter-based high voltage direct current) can avoid DC commutation failure, supply power to passive networks, and realize independent control of active power and reactive power, but it has higher economic investment and power loss.

[0003] As a new type of VSC converter, MMC (Modular multilevel converter) has become a hot topic of research due to its strong scalability and low harmonics. Hybrid cascade DC transmission is a typical hybrid multi-terminal DC transmission, which combines the advantages of LCC-HVDC and MMC-HVDC. Its sending end uses LCC converter station, and the receiving end is composed of high-voltage valve group LCC converter station and low-voltage valve group MMC converter station in series. This topology provides a more flexible transmission mode, which not only improves the stability of AC system voltage, but also reduces the probability of DC commutation failure. At the same time, it can also supply power to multiple load centers and promote energy consumption.

[0004] The reliability of DC line protection is a prerequisite for the safe operation of the power system. However, since the topology and operation mode of the hybrid cascade multi-terminal high-voltage DC transmission system are different from those of the traditional LCC-HVDC and VSC-HVDC systems, there are significant differences in line fault characteristics. Therefore, the line protection of the traditional LCC-HVDC system or VSC-HVDC system may not be suitable for the hybrid DC transmission system, and it is urgent to study new DC line protection.

[0005] Existing DC line protection can be divided into single-terminal protection and double-terminal protection. Among them, traveling wave protection is the main protection of the line. It runs fast, but has poor sensitivity to high-resistance grounding faults. As a backup protection, current differential protection needs to transmit the current waveform from one end of the line to the other end, which has high communication requirements. Some scholars have proposed double-terminal protection based on line boundary elements (DC filters and smoothing reactors). For example, longitudinal protection is formed based on the additional inductance voltage ratio, and longitudinal protection is formed based on the specific frequency current component of the DC filter. However, in the hybrid DC transmission system, there is no need to configure a DC filter on the MMC converter station side, and the current limiting inductance value is small. At this time, it may not be possible to form line protection based on boundary elements. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a hybrid cascade multi-terminal DC transmission line protection method based on current correlation coefficient. The present invention can overcome the shortcomings of traditional high-voltage DC transmission line protection, analyze the DC line fault to obtain a fault component network, and find that the high-frequency current of the fault line and the non-fault line is negatively correlated, and the high-frequency current of the non-fault line is strongly positively correlated, and this correlation is not affected by the fault position and transition resistance; then, cosine similarity is used to describe the current correlation and construct a protection identification criterion. The present invention can correctly identify the fault line, has strong tolerance to transition resistance, and has a simple algorithm and a small amount of calculation.

[0007] The present invention solves the technical problem by the following technical solutions:

[0008] A hybrid cascade multi-terminal DC transmission line protection method based on current correlation coefficient, characterized in that the method comprises the following steps:

[0009] S1. Determine the fault identification criteria inside and outside the area

[0010] The fault identification criterion is constructed as follows:

[0011]

[0012] Where: r set is the set value, r cos12 , r cos13 and r cos23 They are the correlation coefficients of currents i1 and i2, i1 and i3, and i2 and i3, respectively. The correlation coefficient calculation formula is as follows:

[0013]

[0014]

[0015]

[0016] Where: i1, i2 and i3 are the high-frequency currents of the measuring points m1, m2 and m3 respectively;

[0017] For faults occurring in DC lines L1, L2 or L3, the high-frequency currents of the non-fault lines are completely positively correlated, and their correlation coefficient is theoretically 1; while the high-frequency currents of the fault line and the non-fault line are strongly negatively correlated, and their correlation coefficient is theoretically -1; when a fault occurs in the DC bus, the high-frequency currents of the three DC lines are all positively correlated, and their correlation coefficient is theoretically 1;

[0018] S2. Establishing fault start criteria

[0019] When the hybrid DC transmission system is operating normally, the DC voltage is approximately equal to a certain value. After a fault occurs, the DC voltage suddenly changes. Based on the change in voltage, it is determined whether the DC system has a fault. The fault start judgment criterion is as follows:

[0020]

[0021] Where: Δu x is the fault component voltage at the measuring point x, x is the measuring point m1~m6, U set represents the threshold, U set =k set U e0 , where k set is the margin coefficient, set to 2; U e0 is the maximum voltage change when the system is operating normally. According to the simulation test, U e0 is 8kV, then U set =k set U e0 =2×8kV=16kV;

[0022] S3, establish fault selection criteria;

[0023] In a bipolar HVDC transmission system, for a unipolar grounding fault, the voltage change of the faulty pole is more obvious than that of the non-faulty pole. For a bipolar short circuit fault, the voltage changes of the two poles are similar but not obvious. The faulty pole is selected by using the voltage changes of the two poles, as described in the following formula:

[0024]

[0025] Where: Δu x is the positive fault component voltage at the measuring point x, x is the measuring points m1, m2 and m3, Δu x' is the negative fault component voltage at the measuring point x', x' can be the measuring point m4, m5 and m6;

[0026] The fault selection criterion is constructed as follows:

[0027]

[0028] Where Q set1 and Q set2 is the threshold value. For bipolar DC transmission projects, the coupling coefficient of the DC lines on the same pole is usually less than 0.5, and the fault component voltage amplitude ratio between the fault pole and the non-fault pole exceeds 2.

[0029] Therefore, for a positive ground fault, Q>2, so Q set1 Can be set within the range of 1 to 2;

[0030] For negative ground fault, Q<0.5, so Q set2 Can be set in the range of 0.5 to 1;

[0031] Q set1 and Q set2 Set to 1.5 and 0.6 respectively;

[0032] S4. Determine the protection plan;

[0033] The DC voltage change is used to form the protection start-up criterion to determine whether a fault occurs. The cosine similarity is used to calculate the correlation coefficient of the high-frequency current to identify faults inside and outside the zone. The voltage change at both poles is used to select the fault pole to form an overall protection scheme.

[0034] Moreover, considering that the high-frequency signal is relatively weak and decays quickly, a high sampling rate is required, and the high-frequency current adopts 1 to 4 kHz to construct the fault identification criterion.

[0035] Furthermore, the high frequency current is extracted using a Chebyshev filter, and the data window is set to 2 ms.

[0036] Moreover, the setting value r in the fault identification criterion inside and outside the zone set Set to 0.

[0037] The advantages and beneficial effects of the present invention are:

[0038] 1. The protection method of the present invention has a small amount of calculation and only needs to calculate the correlation coefficient of the DC line fault current. It can accurately identify DC line faults with high identification efficiency, and ensure timely protection of multi-terminal DC transmission lines and ensure safe use.

[0039] 2. The protection method of the present invention has high sensitivity to high-resistance grounding faults, strong reliability, and a wide range of applications.

[0040] 3. The protection method of the present invention does not require real-time synchronization of data at both ends of the line and has low requirements for communication transmission. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is a topological diagram of a hybrid cascaded multi-terminal DC transmission system;

[0042] Figure 2 This is the fault component circuit diagram when the DC line L1 fails;

[0043] Figure 3 This is the fault component circuit diagram when the DC bus fails;

[0044] Figure 4 is a flow chart of the present invention;

[0045] Figure 5 This is a simulation experiment diagram of a metallic ground fault at the end of DC line L1;

[0046] Figure 6 The DC line L2 midpoint occurs R F =300Ω ground fault simulation experiment diagram;

[0047] Figure 7 This is a simulation experiment diagram of a DC bus metallic grounding fault. DETAILED DESCRIPTION

[0048] The present invention is further described in detail below through specific examples. The following examples are only illustrative and not restrictive, and the protection scope of the present invention cannot be limited thereto.

[0049] The present invention provides a hybrid cascade multi-terminal DC transmission line protection method based on current correlation coefficient. The typical topological structure of the hybrid cascade multi-terminal DC transmission line system is as follows: Figure 1 As shown in the figure, the sending end uses an LCC converter station, and the receiving end is composed of a high-pressure valve LCC converter station and a low-pressure valve MMC converter station in series. Due to land restrictions, at the receiving end of the hybrid DC system, the LCC converter station and the MMC converter station are built as substations, and the two are connected by a DC line, as shown in the figure. Figure 1 As shown. Specifically, at the receiving end of the hybrid DC system, an LCC converter station is configured at the edge of the developed area to convert part of the DC power into AC power. The LCC converter station is then connected to three MMC converter stations through overhead lines or DC cables to transmit the remaining power deep into every corner of the developed area. The use of three MMC converter stations in parallel can disperse the transmission power and reduce the requirements for the MMC transmission capacity. The advantages of this topology are as follows: the receiving-end MMC converter station is connected to different AC systems to provide power to multiple load centers. In addition, this topology makes full use of the advantages of high utilization and large transmission power of the DC corridor. As shown Figure 1As shown, the sending-end LCC station is connected to the receiving-end LCC station through the DC line L0. The receiving-end high-pressure valve LCC and the low-pressure valve MMC are connected by three DC lines L1 to L3. In order to improve the flexibility and reliability of the hybrid DC transmission system, when any of the DC lines L1 to L3 fails, the other non-fault lines of the DC system should be put into operation normally. Therefore, DC circuit breakers should be configured at both ends of the DC lines L1 to L3. The current limiting reactor can suppress the rise of the fault current and cooperate with the DC circuit breaker to cut off the fault line. Therefore, it is recommended to configure the current limiting reactor at both ends of the DC line. L dc represents the current limiting reactor, CB represents the DC circuit breaker. m1~m6 are the measuring points. f0~f5 represent the fault points. f0 represents the DC line L0 fault. f1~f3 represent the DC line L1~L3 fault. f4 and f5 represent the positive and negative DC bus faults respectively.

[0050] The line faults of the hybrid DC transmission system can be divided into unipolar grounding faults and bipolar short circuit faults. Since the fault characteristics of the bipolar short circuit fault are similar to those of the unipolar grounding fault, and due to the symmetry of the positive and negative poles, the present invention takes the positive pole fault as an example for detailed fault analysis. When a unipolar grounding fault occurs in the DC line L0 ( Figure 1 f0 in the figure), the discharge circuit and discharge mechanism at the sending end are similar to those of the traditional LCC-HVDC. Due to the unidirectional conductivity of the thyristor, the energy stored in the MMC submodule of the low-voltage valve group at the receiving end cannot feed current to the fault point through the high-voltage valve group LCC. Conventional line protection is applicable to the DC line L0, and the DC line L0 does not need to be equipped with a DC circuit breaker.

[0051] When a single-pole ground fault occurs on the DC line L1, L2 or L3 ( Figure 1 f1, f2 or f3), the fault discharge mechanism analysis is similar. Here, the fault of DC line L1 is used as an example for explanation. After the f1 fault, not only the MMC1 submodule capacitor discharges to the fault point, but the high-voltage valve group LCC and the other two parallel converter valves MMC2 and MMC3 will also inject fault current into the fault point. At this time, the rising speed of the fault current is much greater than that of the traditional LCC-HVDC or VSC-HVDC. In addition, for faults occurring in the DC lines L1, L2 or L3, because one side of the line is the LCC converter and the other side is the MMC converter, the fault analysis is more complicated, and there are large differences in the line fault characteristics compared with the traditional LCC-HVDC and VSC-HVDC. Traditional LCC-HVDC or VSC-HVDC line protection may not be suitable for DC lines L1, L2 and L3. Therefore, the present invention mainly studies the line protection of DC lines L1, L2 and L3.

[0052] DC line fault characteristics analysis:

[0053] In order to avoid the influence of the DC control system after the fault (the control system usually takes 10ms to respond), the current and voltage used in the present invention are both within 2ms after the fault occurs, at which time the superposition theorem is applicable. Figure 1 Take f1) in the figure as an example to illustrate. At this time, the corresponding fault component circuit is as follows: Figure 2 shown.

[0054] Figure 2 middle, and Represent the currents at measuring points m1, m2 and m3 respectively. is the current flowing through the LCC. Indicates the DC bus voltage. is the fault point voltage. F is the transition resistance. dc is the equivalent impedance of the current limiting reactor, Z dc =j2πfL dc , f represents the frequency. p1 is the equivalent impedance of DC line L1 from the fault point to the measuring point m1, Z p2 is the equivalent impedance of the remaining part of the DC line L1. line2 , Z line3 are the equivalent impedances of DC lines L2 and L3 respectively. LCC is the equivalent impedance on the LCC side. MMC1 , Z MMC2 and Z MMC3 They are the equivalent impedances of MMC1, MMC2 and MMC3 respectively. In the fault analysis process, the MMC converter can be equivalent to the RLC series impedance.

[0055] Figure 2 In the example, after the DC line L1 fails, the currents measured at the non-fault lines m2 and m3 are shown in equations (1) and (2), respectively.

[0056]

[0057]

[0058] In the formula, Z eq2 , Z eq3 are the equivalent impedances of branches L2 and L3, respectively, as shown in equations (3) and (4).

[0059] Z eq2 =3Z dc +Z line2 +Z MMC2 (3)

[0060] Z eq3 =3Z dc +Zline3 +Z MMC3 (4)

[0061] The current measured at the fault line m1 can be expressed as formula (5).

[0062]

[0063] In the formula, Z eq Z eq2 , Z eq3 and Z LCC The parallel impedance is shown in (6).

[0064] Z eq =Z eq2 / / Z eq3 / / Z LCC (6)

[0065] When a fault occurs in the DC line, the fault current rises rapidly. To avoid damage to the MMC equipment caused by overcurrent, the protection is required to detect the fault in a very short time (generally within 3ms). Therefore, in the transient fault analysis and related protection research, only the stage before MMC blocking needs to be considered. This stage is mainly based on the discharge of submodule capacitors. In this stage, the MMC converter can be equivalent to an RLC series impedance, such as Figure 2 Take MMC2 as an example, its equivalent impedance Z MMC2 It can be expressed as formula (7).

[0066] Z MMC2 =jωL MMC2 +R MMC2 +1 / jωC MMC2 (7)

[0067] Where, L MMC2 , R MMC2 and C MMC2 are the equivalent inductance, resistance and capacitance of MMC2 respectively, as shown in formula (8), and ω is the angular frequency.

[0068]

[0069] Where, L arm is the bridge arm inductance, R arm is the bridge arm resistance, C sub is the submodule capacitance, and N is the number of submodules put into operation in each phase unit.

[0070] Referring to the existing MMC-HVDC transmission project, the bridge arm inductance L arm Generally, it is 100~300mH, so the equivalent inductance L of MMC is MMC Generally 66.67~200mH; bridge arm resistance R armGenerally, it is 0.3Ω, so the equivalent resistance of MMC is R MMC Generally, it is 0.2Ω; the number of submodules N is usually 100 to 300, and the submodule capacitance C sub The value is generally 10 to 20 mF, so the equivalent capacitance C of MMC MMC It is generally 0.1 to 0.6 mF. Combining formula (7), it can be deduced that when the frequency f satisfies formula (9), the inductive impedance of the MMC is greater than the capacitive impedance, that is, ωL MMC >1 / ωC MMC The equivalent resistance of MMC is generally small, so it can be obtained that at high frequencies, MMC can be equivalent to inductance L MMC .

[0071]

[0072] When the frequency is greater than 1kHz, the equivalent impedance Z on the LCC side LCC Can be regarded as inductance L LCC , DC lines can also be equivalent to inductors at high frequencies.

[0073] Combining the above analysis, it can be concluded that at high frequencies, the current of the non-fault line satisfies equation (10).

[0074]

[0075] Where, L eq2 and L eq3 Z eq2 and Z eq3 The equivalent inductance, L line2 and L line3 Z line2 and Z line3 The equivalent inductance, K 23 for and Current ratio.

[0076] It can be seen from formula (10) that for a fault occurring in the DC line L1, the high-frequency currents of the non-fault lines L2 and L3 are completely positively correlated.

[0077] The currents of the faulty line and the non-faulty line satisfy equations (11) and (12).

[0078]

[0079]

[0080] Where, L eq Z eq The equivalent inductance of K 12 and K 13 Respectively and and The current ratio.

[0081] It can be concluded that for the fault occurring in the DC line L1, the high-frequency currents of the faulty line and the non-faulty line are strongly negatively correlated, that is, and and are negatively correlated.

[0082] The fault analysis of the DC line L2 or L3 is similar to the fault analysis of the DC line L1, so it will not be described here.

[0083] In summary, it can be concluded that for faults occurring on DC lines L1, L2 or L3, the high-frequency current of the non-fault line is positively correlated, while the high-frequency current of the fault line is negatively correlated with the high-frequency current of the non-fault line. Therefore, the correlation coefficient of the current can be used to determine the fault line.

[0084] DC bus fault characteristics analysis:

[0085] When a fault occurs on the busbar of the DC lines L1 to L3, Figure 1 For the f4 fault in the figure, for the DC lines L1, L2 and L3, this fault is an out-of-zone fault. However, if the DC circuit breaker installed on the line is not disconnected quickly, the large fault current will impact the MMC valve, which may threaten its safe operation. Therefore, the busbar fault should also be quickly identified and the DC circuit breaker configured on the LCC side of the DC lines L1 to L3 should be tripped.

[0086] The fault component circuit after the DC bus fault is as follows Figure 3 As shown in the figure, Z line1 is the equivalent impedance of line L1. Figure 3 As shown, after a DC bus fault occurs, the currents measured by m1, m2 and m3 are shown in equations (13), (14) and (15) respectively.

[0087]

[0088]

[0089]

[0090] In the formula, Z eq1 is the equivalent impedance of the L1 branch, as shown in formula (16).

[0091] Z eq1 =3Z dc +Z line1 +Z MMC1 (16)

[0092] Combining the above analysis, it can be concluded that at high frequencies, the currents in the DC lines L1, L2, and L3 satisfy equations (17), (18), and (19).

[0093]

[0094]

[0095]

[0096] Where, L eq1 Z eq1 The equivalent inductance, L line1 It's Z line1 The equivalent inductance.

[0097] It can be concluded that when a DC bus fault occurs, the high-frequency currents flowing through the three parallel DC lines L1, L2 and L3 are completely positively correlated. and and and are positively correlated.

[0098] In summary, for faults occurring in DC lines L1, L2 or L3, the high-frequency currents of the non-fault lines are completely positively correlated, and the high-frequency currents of the fault lines and non-fault lines are strongly negatively correlated. For DC bus faults, the high-frequency currents flowing through the three parallel DC lines are all positively correlated. Therefore, the correlation coefficient of the current can be used to identify the fault.

[0099] A specific protection scheme of a hybrid cascade multi-terminal DC transmission line protection method based on current correlation coefficient is:

[0100] S1. Determine the fault identification criteria inside and outside the area

[0101] According to the previous analysis, the frequency used should be greater than 1kHz, and considering that the high-frequency signal is weak, the decay speed is fast, and the sampling rate requirement is high, the selected frequency should not be too high. The present invention uses 1-4kHz high-frequency current to construct the fault identification criterion. The Chebyshev filter is used to extract the high-frequency current. In order to avoid the adjustment influence after the control system fails, the data window is set to 2ms.

[0102] Cosine similarity is used to describe the similarity of fault current waveforms. This method is not affected by changes in signal amplitude. The cosine correlation coefficient is expressed as formula (20).

[0103]

[0104] In the formula, r cosis the cosine correlation coefficient, ranging from -1 to 1. Signals x and y are expressed as: x = {x1, x2, …, x n}, y={y1,y2,…,y n}. When two signals are completely positively correlated, the correlation coefficient r cos =1; when completely negatively correlated, r cos =-1.

[0105] For the currents at measuring points m1 to m3, their correlation can be described by equations (21), (22) and (23).

[0106]

[0107]

[0108]

[0109] Where i1, i2 and i3 are the high frequency currents at measuring points m1, m2 and m3 respectively. cos12 , r cos13 and r cos23 They are the correlation coefficients between currents i1 and i2, i1 and i3, and i2 and i3 respectively.

[0110] According to the above inference, for faults occurring in DC lines L1, L2 or L3, the high-frequency currents of the non-fault lines are completely positively correlated, and their correlation coefficient is theoretically 1; while the high-frequency currents of the fault line and the non-fault line are strongly negatively correlated, and their correlation coefficient is theoretically -1. When a fault occurs in the DC bus, the high-frequency currents of the three DC lines are all positively correlated, and their correlation coefficient is theoretically 1.

[0111] Based on this, the fault identification criterion is constructed as shown in formula (24).

[0112]

[0113] In the formula, r set is the setting value, set to 0.

[0114] S2. Establishing fault start criteria

[0115] When the hybrid DC transmission system operates normally, the DC voltage is approximately equal to a certain value. After a fault occurs, the DC voltage changes suddenly. Therefore, it is possible to judge whether a fault occurs in the DC system based on the change in voltage and construct the protection start criterion as shown in formula (25).

[0116]

[0117] In the formula, Δu xU is the fault component voltage at the measuring point x, where x is the measuring point m1 to m6. set represents the threshold, U set =k set U e0 , where k set is the margin coefficient, set to 2. e0 is the maximum voltage change when the system is operating normally. According to the simulation test, U e0 is 8kV. Therefore, U set =k set U e0 =2×8kV=16kV.

[0118] S3. Establishing the fault selection criteria

[0119] In a bipolar HVDC transmission system, for a single-pole ground fault, the voltage change of the fault pole is more obvious than that of the non-fault pole. For a bipolar short-circuit fault, the voltage changes of the two poles are similar. Therefore, the voltage changes of the two poles can be used to select the fault pole, as described in equation (26).

[0120]

[0121] In the formula, Δu x is the positive fault component voltage at the measuring point x, where x is the measuring points m1, m2 and m3. x' is the negative fault component voltage at the measuring point x', where x' can be the measuring points m4, m5 and m6.

[0122] The fault pole selection criterion is constructed as shown in formula (27).

[0123]

[0124] In the formula, Q set1 and Q set2 is the threshold. For bipolar DC projects, the coupling coefficient of the DC lines on the same pole is usually less than 0.5. Therefore, the voltage amplitude ratio of the fault component between the fault pole and the non-fault pole exceeds 2. Therefore, for a positive ground fault, Q>2, so Q set1 It can be set within the range of 1 to 2. For a negative ground fault, Q<0.5, so Q set2 It can be set in the range of 0.5 to 1. set1 and Q set2 Set them to 1.5 and 0.6 respectively.

[0125] S4. Determine the protection plan

[0126] The protection scheme flow chart is as follows: Figure 4As shown. The DC voltage change is used to form the protection start criterion to determine whether a fault occurs. The cosine similarity is used to calculate the correlation coefficient of the high-frequency current to identify the fault inside and outside the area. The voltage change of the two poles is used to select the fault pole to form an overall protection scheme.

[0127] Simulation Verification

[0128] Build topology in PSCAD Figure 1 The simulation model of the hybrid cascade multi-terminal DC transmission system shown in the figure. The sending end uses LCC converters, and the receiving end consists of an LCC converter station and an MMC converter station in series. The rated DC voltage and transmission power are 800kV and 8GW respectively. The length of the DC line L0 is 2000km, and the length of the DC lines L1 to L3 are both 200km. The DC lines all use frequency-dependent models. The sampling frequency is 10kHz, and the fault occurs at 2s.

[0129] DC line fault simulation results

[0130] Figure 5 Given a metallic ground fault at the end of DC line L1 ( Figure 1 The simulation results of f1) in . Figure 5 As shown in a, the voltage fault component Δu at the measuring points m1~m3 m1 , Δu m2 and Δu m3 After the fault, the value suddenly changes significantly, and its amplitude is greater than the threshold value U set (U set Assuming it is 16kV), the protection start-up components installed on the DC lines L1 to L3 can operate reliably. Figure 5 As shown in b~5c, the high-frequency currents i1 and i2, i1 and i3 are negatively correlated. The correlation coefficient r is calculated using the data 2ms after the fault. cos12 =-1.00, r cos13 =-1.00, both less than the threshold value (r set =0). Figure 5 As shown in Figure d, the high-frequency currents i2 and i3 are positively correlated. In addition, due to the topological symmetry of the three parallel branches, the waveforms of i2 and i3 are almost the same, and their correlation coefficient r is calculated. cos23 =1.00, greater than the threshold r set , it is judged that the fault occurs on the DC line L1. Figure 5 e can be seen that Δu m1 The amplitude is greater than Δu m4 , the fault selection function Q = 9.01 is calculated, which is greater than the threshold Q set1 Therefore, it is finally determined that a positive-pole ground fault has occurred in DC line L1.

[0131] Figure 6is the transition resistance R at the midpoint of the DC line L2 F =300Ω ground fault ( Figure 1 f2) simulation in. Figure 6 As shown in a, after the fault, Δu m1 , Δu m2 and Δu m3 The amplitude of changes significantly, and the starting components installed on the DC lines L1 to L3 can operate reliably. Figure 6 As shown in b~6d, the high-frequency currents i1 and i2, i2 and i3 are completely negatively correlated, while i1 and i3 are positively correlated, which verifies the previous inference that the high-frequency currents of the fault line and the non-fault line are strongly negatively correlated, while the high-frequency currents of the non-fault line are strongly positively correlated. The correlation coefficient r is calculated cos12 =-1.00,r cos13 =1.00,r cos23 =-1.00, it is judged that the fault occurred on the DC line L2. Figure 6 It can be seen that Δu m2 The amplitude is greater than Δu m5 , calculated Q = 7.13, greater than Q set1 , thus determining that a positive ground fault has occurred in the DC line L2.

[0132] DC bus fault simulation results

[0133] Figure 7 Simulation of a fault on the positive DC bus ( Figure 1 f4), such as Figure 7 As shown in a, after the fault occurs, Δu m1 ,Δu m2 and Δu m3 The amplitude of changes significantly, and the starting components installed on the DC lines L1 to L3 can operate reliably. Figure 7 As shown in Figure b, the high-frequency currents i1 and i2, i1 and i3, and i2 and i3 are all completely positively correlated. In addition, due to the topological symmetry of the three parallel branches, the waveforms of i1, i2, and i3 are almost the same. The correlation coefficient r is calculated cos12 =1.00,r cos13 =1.00,r cos23 =1.00, both greater than the threshold r set , it is judged as a DC bus fault.

[0134] The method proposed in the present invention has a simple principle and a small amount of calculation. It only needs to calculate the correlation coefficient of the DC line fault current. Compared with the traveling wave protection, this method has a higher sensitivity to high-resistance grounding faults. The current correlation is not affected by the fault resistance. The cosine correlation coefficient is used to measure the current correlation, which further reduces the influence of the transition resistance on the method.

[0135] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art will appreciate that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A hybrid cascade multi-terminal DC transmission line protection method based on current correlation coefficient, characterized in that: The method comprises the following steps: S1. Determine the fault identification criteria inside and outside the area The fault identification criterion is constructed as follows: Where: r set is the set value, r cos12 , r cos13 and r cos23 They are the correlation coefficients of currents i1 and i2, i1 and i3, and i2 and i3, respectively. The correlation coefficient calculation formula is as follows: Where: i1, i2 and i3 are the high-frequency currents of the measuring points m1, m2 and m3 respectively; For faults occurring in DC lines L1, L2 or L3, the high-frequency currents of the non-fault lines are completely positively correlated, and their correlation coefficient is theoretically 1; while the high-frequency currents of the fault line and the non-fault line are strongly negatively correlated, and their correlation coefficient is theoretically -1; when a fault occurs in the DC bus, the high-frequency currents of the three DC lines are all positively correlated, and their correlation coefficient is theoretically 1; S2. Establishing fault start criteria When the hybrid DC transmission system is operating normally, the DC voltage is approximately equal to a certain value. After a fault occurs, the DC voltage suddenly changes. Based on the change in voltage, it is determined whether the DC system has a fault. The fault start judgment criterion is as follows: Where: Δu x is the fault component voltage at the measuring point x, x is the measuring point m1~m6, U set represents the threshold, U set =k set U e0 , where k set is the margin coefficient, set to 2; U e0 is the maximum voltage change when the system is operating normally. According to the simulation test, U e0 is 8kV, then U set =k set U e0 =2×8kV=16kV; S3, establish fault pole selection criteria; In a bipolar HVDC transmission system, for a unipolar grounding fault, the voltage change of the faulty pole is more obvious than that of the non-faulty pole. For a bipolar short circuit fault, the voltage changes of the two poles are similar but not obvious. The faulty pole is selected by using the voltage changes of the two poles, as described in the following formula: Where: Δu x is the positive fault component voltage at the measuring point x, x is the measuring point m1, m2 and m3, Δu x' is the negative fault component voltage at the measuring point x', where x' is the measuring points m4, m5 and m6; The fault selection criterion is constructed as follows: Where Q set1 and Q set2 is the threshold value. For bipolar DC transmission projects, the coupling coefficient of the DC lines on the same pole is less than 0.5, and the fault component voltage amplitude ratio between the fault pole and the non-fault pole exceeds 2. Therefore, for a positive ground fault, Q>2, so Q set1 Set it in the range of 1 to 2; For negative ground fault, Q<0.5, so Q set2 Set it in the range of 0.5 to 1; Q set1 and Q set2 Set to 1.5 and 0.6 respectively; S4. Determine the protection plan; The DC voltage change is used to form the protection start-up criterion to determine whether a fault occurs. The cosine similarity is used to calculate the correlation coefficient of the high-frequency current to identify faults inside and outside the zone. The voltage change at both poles is used to select the fault pole to form an overall protection scheme.

2. The hybrid cascade multi-terminal DC transmission line protection method based on current correlation coefficient according to claim 1 is characterized in that: Considering that high-frequency signals are weak, decay quickly, and require a high sampling rate, the high-frequency current uses 1 to 4 kHz to construct the fault identification criterion.

3. The hybrid cascade multi-terminal DC transmission line protection method based on current correlation coefficient according to claim 1 is characterized in that: The high frequency current is extracted by using a Chebyshev filter, and the data window is set to 2 ms.

4. The hybrid cascade multi-terminal DC transmission line protection method based on current correlation coefficient according to claim 1 is characterized in that: The setting value r in the fault identification criterion inside and outside the zone set Set to 0.

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