Local information comprehensive protection method and system suitable for MMC-MTDC system

Abstract

The invention relates to a local information comprehensive protection method and system suitable for an MMC-MTDC system. The method comprises the following steps: S1, extracting real-time current and voltage information; s2, based on the current and voltage information, calculating a direct current voltage change rate of a direct current side of the current-limiting inductor, judging whether the direct current voltage change rate is greater than a first threshold value or not, if so, executing a step S3, and if not, executing a step S4; s3, calculating the sum of local differential currents, and judging whether a converter grounding fault exists or not based on the state of the sum of the local differential currents; s4, calculating the sum of single-side differential currents and a direct-current transient voltage root-mean-square value of the high-frequency part of the direct-current voltage; and S5, determining a fault type based on the sum of the single-side differential currents and the state of the root-mean-square value of the direct-current transient voltage. Compared with the prior art, the method has the advantages of quick response, high reliability and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of multi-terminal flexible DC transmission protection, and in particular to a local information comprehensive protection method and system applicable to an MMC-MTDC system. Background Art

[0002] With the continuous growth of global power demand and the widespread application of renewable energy, the traditional AC power transmission system is facing increasing challenges. As an effective solution, high-voltage DC (HVDC) technology can significantly reduce power transmission losses and improve system stability in long-distance power transmission. In HVDC technology, the modular multilevel converter (MMC), as a new type of power electronic converter, plays an important role in the multi-terminal flexible DC transmission system (MTDC). The MMC-MTDC system involves multiple converter stations and DC lines, and its fault characteristics and fault propagation mechanisms are complex. For example, a fault on the DC side may cause current and voltage changes at multiple ports in the system, posing challenges to traditional protection strategies. Due to the high dynamic characteristics of the MTDC system, the relay protection needs to respond within an extremely short time. Traditional protection devices may not be able to meet this requirement, especially when dealing with large-scale faults and system transition states. In a multi-terminal system, fault location and isolation are more complex. Accurately identifying the fault type, fault location, and quickly isolating the faulty part are the keys to the design of the protection system.

[0003] After a search of the prior art, it is found that Chinese Patent Application CN117895443A discloses a full-line quick-acting single-terminal quantity protection method for a flexible DC line based on voltage gradient energy ratio. This is a method that uses the single-terminal voltage measurement value of a flexible DC transmission line to improve the voltage gradient calculation and energy value calculation, and constructs multiple protection criteria. This method constructs an improved voltage gradient calculation by analyzing the voltage fault characteristics inside and outside the flexible DC line area to eliminate the influence of interference and constructs a protection startup criterion; calculates the sum of the squares of the voltage gradients at each sampling point to construct a voltage gradient energy value, and focuses on the difference in the fault voltage gradient energy value to construct a full-line fault protection criterion for longitudinal comparison of energy values; uses the transverse comparison of the positive and negative pole energy ratios to realize the fault pole selection criterion. However, this prior art still has deficiencies such as being unable to identify faults only using local information, being unable to immediately distinguish between faults on the DC side and AC side of the converter in the system, and not fully integrating the fault characteristic information of current and voltage. Summary of the Invention

[0004] The object of the present invention is to overcome the defects of the above-mentioned existing technologies and provide a local information integrated protection method and system applicable to the MMC-MTDC system (Modular Multilevel Converter-Multi-Terminal Flexible DC Transmission System), which can ensure stable and reliable operation when encountering various faults, at different positions and impedance conditions.

[0005] The object of the present invention can be achieved by the following technical solutions:

[0006] A local information integrated protection method applicable to the MMC-MTDC system includes the following steps:

[0007] S1. Extract real-time current and voltage information;

[0008] S2. Based on the current and voltage information, calculate the rate of change of the DC voltage on the DC side of the current-limiting inductor, and determine whether the rate of change of the DC voltage is greater than a first threshold. If so, execute step S3; if not, execute step S4;

[0009] S3. Calculate the sum of local differential currents, and based on the state of the sum of local differential currents, determine whether there is a converter grounding fault;

[0010] S4. Calculate the sum of unilateral differential currents and the root mean square value of the DC transient voltage of the high-frequency part of the DC voltage;

[0011] S5. Based on the state of the sum of unilateral differential currents and the root mean square value of the DC transient voltage, determine the fault type.

[0012] Further, the sum of local differential currents is the sum of local differential currents at each moment within a sampling period, and the local differential current is calculated based on the positive-pole current, negative-pole current, and neutral current within the region. The calculation formula is:

[0013] i LDZC = i p + i n + i g

[0014] wherein, i LDZC is the local differential current, and i p , i n , i g are the positive-pole current, negative-pole current, and neutral current respectively.

[0015] Further, in step S3, if the sum of local differential currents exceeds a second threshold, there is a converter grounding fault; otherwise, return to step S2.

[0016] Further, the sum of the unilateral differential currents is the sum of all unilateral differential currents within a sampling period. The unilateral differential current is calculated based on the in-phase positive and negative currents of the converter, and the calculation formula is:

[0017] i SEDC =i p +i n

[0018] In the formula, i SEDC is the unilateral differential current, and i p , i n are the positive current and the negative current respectively.

[0019] Further, the calculation formula for the root mean square value of the DC transient voltage is:

[0020]

[0021] In the formula, U R is the root mean square value of the DC transient voltage, n is the number of sampling points, and u t (i) is the DC voltage within the high-frequency band of sampling point i.

[0022] Further, in step S5, if the sum of the unilateral differential currents exceeds the third threshold and the root mean square value of the DC transient voltage exceeds the fourth threshold, a positive pole-to-ground fault occurs; if the absolute value of the sum of the unilateral differential currents is less than or equal to the third threshold and the root mean square value of the DC transient voltage exceeds the fifth threshold, a pole-to-pole fault occurs; if the sum of the unilateral differential currents is less than the negative value of the third threshold and the root mean square value of the DC transient voltage exceeds the sixth threshold, a negative pole-to-ground fault occurs.

[0023] Further, the third threshold is 0.02 kA to 0.40 kA.

[0024] Further, the first threshold is -886.75 to -1295 kV / ms.

[0025] The present invention also provides a computer-readable storage medium, including one or more programs for execution by one or more processors of an electronic device. The one or more programs include instructions for executing the local information comprehensive protection method applicable to the MMC-MTDC system as described above.

[0026] The present invention also provides a local information comprehensive protection system applicable to the MMC-MTDC system, including one or more processors, a memory, and one or more programs stored in the memory. The one or more programs include instructions for executing the local information comprehensive protection method applicable to the MMC-MTDC system as described above.

[0027] Compared with the prior art, the protection scheme of the present invention integrates multiple local information such as single-sided differential current, local differential current, DC voltage change rate, and root mean square value of DC transient voltage, ensuring that the system can maintain a stable and reliable operating state when encountering various faults, at different positions, and under different impedance conditions. The present invention has the following beneficial effects:

[0028] 1. Fast response ability: By real-time monitoring of key electrical parameters such as current and voltage, the present invention can activate the protection mechanism within a very short time after a fault occurs, significantly shortening the fault detection and isolation time, thereby effectively preventing the expansion of the fault and ensuring the safe and stable operation of the system.

[0029] 2. Flexible adaptability: For various complex fault scenarios in a multi-terminal flexible DC transmission system, such as internal pole-to-ground faults, external faults, converter faults, etc., the present invention integrates multiple local information such as single-sided differential current, local differential current, DC voltage change rate, and root mean square value of DC transient voltage, can accurately identify and take targeted protection measures, with strong adaptability, not affected by factors such as changes in system topology structure and operating conditions, and always maintaining an efficient protection effect.

[0030] 3. Reliability and stability: The present invention adopts a method of combining multiple criteria, comprehensively judges faults through multi-dimensional information such as voltage change rate, single-sided differential current, root mean square value of DC transient voltage, and local differential current, effectively avoiding misoperation of protection caused by misjudgment of a single index. At the same time, it has strong anti-interference ability and can stably and accurately execute the protection function in a high-noise environment, significantly improving the overall reliability of the system.

[0031] 4. Utilization of local information: By making full use of locally collected data, the present invention proves that various types of faults can be effectively detected without relying on remote current communication. This not only significantly improves the speed of fault detection but also enhances the reliability of the system. Due to reducing the dependence on remote communication, the system can still maintain efficient and stable operation in the face of communication interruption or delay, thereby improving the response ability and fault handling efficiency of the overall protection system.

[0032] 5. Easy integration and upgrade: The protection method and system design of the present invention conform to the development trend of digitalization and intelligentization of modern power systems, are easy to be seamlessly docked with other intelligent devices, control systems, and communication networks, facilitate remote monitoring, fault diagnosis, and optimization and upgrade, and are conducive to realizing efficient operation and maintenance management of power systems.

[0033] 6. Having strong anti-noise and anti-high impedance capabilities: Although in practical applications, noise may cause fluctuations in measurement data and the transition resistance will also have a certain impact on fault measurement, the protection method of the present invention can still maintain accurate identification of faults. This indicates that the solution demonstrates excellent adaptability and robustness in the face of complex noise environments and high impedance conditions. Through advanced signal processing and data analysis techniques, the present invention can effectively filter out noise interference and accurately determine the fault type, ensuring that the protection system still operates reliably and efficiently under various adverse conditions. Description of the Drawings

[0034] Figure 1 It is a flow chart of the method of the present invention;

[0035] Figure 2 It is a schematic diagram of the structure of a four-terminal modular multilevel converter - multi-terminal flexible DC transmission system applied in an embodiment of the present invention;

[0036] Figure 3 It is a topology diagram of a modular multilevel converter;

[0037] Figure 4 It is a diagram of positive pole metallic grounding fault information, where (4a) is the change of DC voltage rate of change over time, (4b) is the change of unilateral differential current over time, and (4c) is the measured value of DC voltage;

[0038] Figure 5 It is a diagram of negative pole with 200-ohm transition fault grounding fault information, where (5a) is the change of DC voltage rate of change over time, (5b) is the change of unilateral differential current over time, and (5c) is the measured value of DC voltage;

[0039] Figure 6 It is a diagram of pole-to-pole metallic fault information, where (6a) is the change of DC voltage rate of change over time, (6b) is the change of unilateral differential current over time, and (6c) is the measured value of DC voltage;

[0040] Figure 7 It is a diagram of external DC bus single-pole metallic grounding fault information, where (7a) is the change of DC voltage rate of change over time, (7b) is the change of unilateral differential current over time, and (7c) is the measured value of DC voltage;

[0041] Figure 8 It is the response of the AC system single-phase-to-ground metallic fault at station T4 (fT4), where (8a) is the change of DC voltage rate of change over time and (8b) is the change of the local differential current of converters T3 and T4 over time;

[0042] Figure 9It is a diagram of a positive - pole metallic grounding fault with 35 dB noise information. Among them, (9a) is the variation of the DC voltage change rate over time, (9b) is the variation of the unilateral differential current over time, and (9c) is the measured value of the DC voltage. Detailed implementation manners

[0043] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented on the premise of the technical solution of the present invention, and gives detailed implementation manners and specific operation processes. However, the protection scope of the present invention is not limited to the following embodiments.

[0044] Embodiment 1

[0045] In this embodiment, the unilateral differential current is defined as the difference between the positive - terminal current and the negative - terminal current, while the local differential current refers to the difference between the positive - pole, negative - pole, and neutral currents within the local differential region (LDZ region). When a multi - terminal flexible DC transmission system encounters a fault, the values of the unilateral differential current and the local differential current will change significantly, and different fault types will result in different current change patterns. At the same time, the DC voltage change rate on both sides of the current - limiting inductor in the fault state will show obvious fluctuations. Based on the above, this embodiment provides a local information comprehensive protection method applicable to the MMC - MTDC system (modular multilevel converter - multi - terminal flexible DC transmission system). This method integrates a protection scheme of multiple local information such as the unilateral differential current, local differential current, DC voltage change rate, and DC transient voltage root - mean - square value, aiming to ensure that the system can still operate stably and reliably when encountering various faults, at different positions, and under different impedance conditions.

[0046] As Figure 1 shown, this method includes the following steps:

[0047] Step 1: For a single converter, detect and read the relevant parameters of the system from current and voltage transformers in real time, and then calculate the DC voltage change rate du dc_L / dt of the DC side of the current - limiting inductor on the DC transmission line. This change rate serves as the basis for starting protection. During normal operation, the DC voltage remains highly stable. For external faults of the AC system and internal and external faults of the converter station, the change is also minimal. However, for internal single - phase grounding faults or internal pole - to - pole faults, this DC voltage shows significant changes. When du dc_L / dt is greater than the first threshold DV set , where DV set is a negative value, further sum the local differential currents I LDZC sum, that is n is the sampling point within the sampling time window τ. In this embodiment, τ is 1 ms (millisecond). If I LDZC exceeds the second threshold I set.2, in this embodiment, take I set.2 It can be taken as 0.1 kA (kiloampere). If there is a ground fault in the converter AC system, otherwise continue to detect the rate of change of DC voltage du dc_L / dt to detect potential fault phenomena.

[0048] Step 2: If the rate of change of DC voltage du dc_L / dt is less than the first threshold DV set , there may be a fault in the system. Further calculate the sum of the unilateral differential currents I SEDC , that is Compare the magnitude of I SEDC with the third threshold I set.1 . If I SEDC exceeds the threshold I set.1 (I set.1 >0), then there may be a positive pole to ground fault. On the contrary, if I SEDC is less than the negative value of the threshold I set.1 , it indicates a negative pole to ground (NPTG) fault. If the absolute value of I SEDC is less than or equal to I set.1 , then a pole to pole fault may occur.

[0049] Step 3: Calculate the root mean square value of the DC transient high-frequency voltage. The calculation formula is Measure n sampling points within the sampling time window τ. u t is the DC voltage in the high-frequency band. Considering that when a fault occurs in the DC system, high-frequency transient signals will be generated, and the current-limiting inductors installed at both ends of the DC line can be regarded as high resistance to high-frequency signals. Therefore, by analyzing the change of the root mean square value of the transient high-frequency voltage, different positions and types of faults can be effectively distinguished, including positive pole grounding fault, negative pole grounding fault, and pole-to-pole fault. Specifically:

[0050] If the sum of the unilateral differential currents exceeds the third threshold I set.1 , and the root mean square value of the DC transient voltage exceeds the fourth threshold U R.set.1 , then a positive pole to ground fault occurs; if the absolute value of the sum of the unilateral differential currents is less than or equal to I set.1 , and the root mean square value of the DC transient voltage exceeds the fifth threshold U R.set.2 , then a pole-to-pole fault (inter-pole fault) occurs; if the sum of the unilateral differential currents is less than the negative value of I set.1 , and the root mean square value of the DC transient voltage exceeds the sixth threshold U R.set.3 , then a negative pole to ground fault occurs.

[0051] In the simulation platform PSCAD / EMTDC, a four-terminal modular multilevel converter - multi-terminal flexible DC transmission system is constructed in this embodiment. The system consists of converter stations T1, T2, T3, T4 and DC transmission lines. Star-connected reactors (Lg) and neutral point grounding resistors (Rg) are installed on the AC side to ground the system, as Figure 2 shown. The most basic unit in the three-phase modular multilevel converter is the half-bridge sub-module (SM), as Figure 3 shown. It includes two insulated gate bipolar transistor (IGBT) switches (T1 and T2), two diodes (D1 and D2) and a capacitor (Csm). A large number of SMs are connected in series with an arm inductor (L) to form a converter arm, and the upper and lower converter arms of the same phase form a phase unit.

[0052] In this embodiment, the above-mentioned constructed model is systematically verified under various fault conditions, including DC faults and AC faults, to test and verify the effectiveness and reliability of the proposed protection method. Through these simulation experiments, we can evaluate the performance of the protection system under different fault scenarios and optimize the protection strategy to address the challenges in practical applications.

[0053] The system parameters set in the simulation system are shown in Table 1.

[0054] Table 1 System parameter settings

[0055]

[0056]

[0057] Take the protection unit u installed near L34 and the frequency converter T3 as an 34 example. First, the starting criterion DV set should be set. When various internal DC faults occur, the measured voltage rate should be less than DV set . Considering the most severe external fault (inter-pole short-circuit fault) and the least severe internal fault (positive pole grounding short-circuit fault, with a transition resistance of 200 Ω), Table 2 shows the measured data when different types of faults occur.

[0058] Table 2 Measured DV when different types of faults occur set

[0059]

[0060] Theoretically, DV set can be set between -886.75 and -1295 kV / ms. It can be triggered when internal faults occur and will not be triggered when external faults occur. However, this may reduce the sensitivity. Considering this, the DV setThe value is -800 kV / ms, which can ensure reliable triggering of the starting criterion when a fault occurs.

[0061] After the starting criterion is triggered, calculate I SEDC to identify the fault pole, I set.1 should be set. Theoretically, I set.1 is a value slightly greater than zero. Then calculate UR_L and determine whether it is an internal fault. Table 3 shows the calculation results of UR_L and I SEDC when different types of PTG faults occur. The second column data in Table 3 are the calculation results of U R_L and I SEDC under normal conditions, and the third row data represents that the transition resistance is 0 Ω or 200 Ω. For the setting of I set.1 , it can be seen from Table 3 that theoretically, the value of I set.1 can be set between 0.02 kA and 0.40 kA, and in this paper, it is set to 0.2 kA. For the setting of U R_L.34 , considering that the maximum value of U R_L.34 is 0.21 kV when an external fault occurs and the minimum value of U R_L.34 is 3.16 kV when an internal fault occurs, so U R.set.1 can be set to a value between the two. In the present invention, U is set to 1 kV. In addition, for positive pole grounding short - circuit faults and negative pole grounding short - circuit faults, so U R .set.3 is also set to 1 kV.

[0062] Table 3 Calculation results of U R_L and I SEDC for different types of positive pole grounding short - circuit faults

[0063]

[0064] Table 4 shows the calculation results of U R.set.2 and I R_L when U SEDC is set to 3 kV for different types of inter - pole short - circuit faults.

[0065] Table 4 Calculation results of U R_L and I SEDC for different types of inter - pole short - circuit faults

[0066]

[0067] Table 5 shows the performance of the protection scheme under different transition resistances. For internal faults, we divide L34 into 100 equally spaced parts, set 3 faults at different positions, and add different transition resistances; for external faults, more faults are set by changing the transition resistances at different positions such as the DC bus, AC bus, and converter. For internal faults, when the transition resistance is generally high, the detection rate of this protection scheme is 100%, and the average number of detections is short.

[0068] Performance of the protection scheme under different transition resistances in Table 5

[0069]

[0070]

[0071] The influence of white noise on the protection scheme of the present invention is shown in Table 6. Considering the most severe external fault (inter-pole short-circuit fault) and the least severe internal fault (single-pole ground short-circuit fault), simulation experiments are carried out to detect whether the protection action is correct under different white noise conditions. It can be clearly seen from Table 6 that as the signal-to-noise ratio (SNR) decreases (indicating a higher noise level), the influence of white noise becomes more obvious. In the case of low noise, this scheme maintains a high detection rate and a short operation time, highlighting its robustness in such an environment.

[0072] Performance of the protection scheme under different white noise conditions in Table 6

[0073]

[0074] Figure 4 Shows the response of the metal positive pole to ground fault of L43 (125 km from T3), and this fault occurs at 2 s. It can be seen from the system response curve in (4a) that the change rate of the DC voltage changes very quickly when the fault occurs. Since the fault location is near the converter T4, the change rate of the DC voltage at the position of u 43 (the position of voltage u 43 is as shown in Figure 3 ) changes first, and the startup criterion is triggered very quickly. Then, from the system response curve shown in (4b), calculate the I of T4 within 1 ms SEDC to identify the fault pole, and the result is 0.7775 kA. It is greater than the threshold I set.1 . Calculate the DE of u 43 to judge whether it is an internal fault. As shown in (4c), at 5 consecutive sampling points, its absolute value is greater than the threshold DE set.1 . According to the flowchart of the protection scheme proposed by Figure 1 , through the protection unit u 43, the fault discrimination process for detecting the internal PPTG fault T3 is similar: 0.4 ms after the fault occurs, the protection unit u located near L43 and T3 is triggered 43 The starting criterion. Then, calculate the I of T3 within 1 ms SEDC , and the result is 0.7796 kA, which is greater than the threshold I set.1 . Calculate the U of u 34 R_L , and the result is 12.42 kV. As shown in (4c), it is greater than the set threshold U R.set.1 , so U34 can detect the internal positive pole to ground short - circuit fault.

[0075] Figure 5 shows the negative pole to ground fault response of L43 (125 km away from T3). The fault occurs at 2 s, and the fault discrimination process with a transition resistance of 200 Ω is similar to the positive pole to ground fault. The main difference is that the value of I SEDC is negative, so the internal negative pole to ground short - circuit fault can be detected. Generally speaking, the proposed protection scheme can quickly and accurately identify internal faults.

[0076] Figure 6 shows the response of the metal pole - to - pole short - circuit of L43 (125 km away from T3). It can be seen from the system response curve in (6a) that the rate of change of the DC voltage changes rapidly when the fault occurs. Compared with (6a), since the pole - to - pole short - circuit fault is more serious than the positive pole to ground short - circuit, the rate of change of the DC voltage is greater. Due to the fault location being close to the T4 converter, the rate of change of the DC voltage of u 43 changes first, and the starting criterion is triggered very quickly. From the system response curve in (6b), calculate the I of T4 SEDC to determine the fault pole. The result is 4.75×10 -4 kA, and its absolute value is less than the threshold I set.1 . Calculate the U of U43 R_L to judge whether it is an internal fault, as shown in (6c).

[0077] From Figure 7 's system response curve in (7a), it can be seen that the rate of change of the DC voltage changes rapidly when the fault occurs. The changes of u 31 and u 34 are almost the same because they are connected to the same bus. Due to the fault location being close to the converter T3, the rate of change of the DC voltage of u 34 changes first, and the starting criterion is triggered very quickly. Then, from the system response curve in (7b), calculate the I of T4 within 1 ms SEDC to identify the fault pole. At the same time, calculate the U of u 34 R_L to determine whether it is an internal fault, as shown in (7c).

[0078] Figure 8 It is the single-phase metal grounding fault response diagram of the AC system at T4 station. The fault occurrence time is 2 s and the duration is 50 ms.

[0079] Figure 9 It shows the response of the metal positive pole to ground fault at L43 (125 km away from T3), and the fault occurs at 2 s. To test the noise response protection scheme, white noise with a signal-to-noise ratio of 35 db is added to the measured voltage and current, and the test results are as Figure 9 shown.

[0080] The above method uses the polarity of the unilateral differential current to select the faulty pole, uses the root mean square value of the DC transient voltage to select the faulty DC line, and uses the local differential current to detect the faults in the converter AC system. The comprehensive test results show that this protection scheme integrates local information and can identify different faults without the need for far-side current communication of long-distance transmission lines. Various test results prove the effectiveness of the proposed protection scheme.

[0081] If the above method is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on such an understanding, the technical solution of the present invention, in essence, or the part that makes a contribution to the prior art, or a part of this technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions for causing a computer device (which may be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the methods described in various embodiments of the present invention. And the aforementioned storage medium includes: various media such as USB flash drives, mobile hard disks, read-only memories (ROM, Read-Only Memory), random access memories (RAM, Random Access Memory), magnetic disks, or optical discs that can store program codes.

[0082] Embodiment 2

[0083] This embodiment provides a local information comprehensive protection method applicable to the MMC-MTDC system, including one or more processors, a memory, and one or more programs stored in the memory. The one or more programs include instructions for executing the protection method as described in Embodiment 1.

[0084] Those skilled in the art should understand that the embodiments of the present invention can be provided as a method, a system, or a computer program product. Therefore, the present invention can take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present invention can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk memory, CD-ROM, optical memory, etc.) that contain computer-usable program code. The solutions in the embodiments of the present invention can be implemented in various computer languages. For example, object-oriented programming languages such as Java and interpreted scripting languages such as JavaScript, etc.

[0085] The present invention is described with reference to the flowcharts and / or block diagrams of methods, apparatuses (systems), and computer program products according to embodiments of the present invention. It should be understood that each flow and / or block in the flowchart and / or block diagram, and the combination of flows and / or blocks in the flowchart and / or block diagram, can be realized by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, such that the instructions executed by the processor of the computer or other programmable data processing devices generate means for realizing the functions specified in Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks.

[0086] The preferred specific embodiments of the present invention have been described in detail above. It should be understood that those of ordinary skill in the art can make many modifications and variations based on the concept of the present invention without creative efforts. Therefore, all technical solutions that can be obtained by those skilled in the art in the technical field of the present invention through logical analysis, reasoning, or limited experiments based on the concept of the present invention on the basis of the prior art should be within the protection scope determined by the claims.

Claims

1. A local information comprehensive protection method applicable to MMC-MTDC system, characterized in that: The following steps are involved: S1, extracting real-time current and voltage information; S2, based on the current and voltage information, calculating the DC voltage change rate on the DC side of the current limiting inductor, and determining whether the DC voltage change rate is greater than a first threshold value, if so, executing step S3, if not, executing step S4; S3, calculating the local differential current sum, and judging whether there is a converter grounding fault based on the state of the local differential current sum; S4, calculating the total of the single-side differential current and the DC transient voltage RMS value of the high-frequency part of the DC voltage; S5. Determine a fault type based on the state of the single-side differential current sum and the DC transient voltage RMS value.

2. The local information comprehensive protection method applicable to the MMC-MTDC system according to claim 1 is characterized in that: The local differential current sum is the sum of the local differential currents at each moment in the sampling period. The local differential current is calculated based on the positive current, negative current and neutral current in the region. The calculation formula is: i LDZC =i p +i n +i g In the formula, i LDZC is the local differential current, i p 、i n 、i g They are positive current, negative current and neutral current respectively.

3. The local information comprehensive protection method applicable to the MMC-MTDC system according to claim 1 is characterized in that: In step S3, if the sum of the local differential currents exceeds the second threshold, there is a converter ground fault, otherwise the process returns to step S2.

4. The local information comprehensive protection method applicable to the MMC-MTDC system according to claim 1 is characterized in that: The sum of the single-sided differential currents is the sum of all single-sided differential currents in the sampling period. The single-sided differential current is calculated based on the positive current and the negative current of the same phase of the converter. The calculation formula is: i SEDC =i p +i n In the formula, i SEDC is the unilateral differential current, i p 、i n are the positive current and the negative current respectively.

5. The local information comprehensive protection method applicable to the MMC-MTDC system according to claim 1 is characterized in that: The calculation formula of the DC transient voltage root mean square value is: Where U R is the RMS value of the DC transient voltage, n is the number of sampling points, u t (i) is the DC voltage in the high frequency band at sampling point i.

6. The local information comprehensive protection method applicable to the MMC-MTDC system according to claim 1 is characterized in that: In step S5, if the sum of the single-sided differential current exceeds the third threshold value and the root mean square value of the DC transient voltage exceeds the fourth threshold value, a positive pole-to-ground fault occurs; if the absolute value of the sum of the single-sided differential current is less than or equal to the third threshold value and the root mean square value of the DC transient voltage exceeds the fifth threshold value, a pole-to-pole fault occurs; if the sum of the single-sided differential current is less than the negative value of the third threshold value and the root mean square value of the DC transient voltage exceeds the sixth threshold value, a negative pole-to-ground fault occurs.

7. The local information comprehensive protection method applicable to the MMC-MTDC system according to claim 6 is characterized in that: The third threshold is 0.02kA to 0.40kA.

8. The local information comprehensive protection method applicable to the MMC-MTDC system according to claim 1 is characterized in that: The first threshold is -886.75 to -1295 kV / ms.

9. A computer-readable storage medium, characterized in that: The method comprises one or more programs executed by one or more processors of an electronic device, wherein the one or more programs comprise instructions for executing the local information integrated protection method applicable to an MMC-MTDC system as claimed in any one of claims 1 to 8.

10. A local information integrated protection system suitable for MMC-MTDC system, characterized in that: The system comprises one or more processors, a memory and one or more programs stored in the memory, wherein the one or more programs include instructions for executing the local information integrated protection method applicable to the MMC-MTDC system as claimed in any one of claims 1 to 8.

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