Offshore wind power DC line differential protection method, device and system

Abstract

The invention relates to the technical field of direct-current power transmission systems, in particular to an offshore wind power direct-current line differential protection method, device and system. The method comprises the following steps: respectively acquiring a first target current and a second target current at two ends of a direct-current submarine cable, and acquiring a direct-current voltage absolute value and a target voltage setting value at one end of the direct-current submarine cable; determining a current threshold value according to the first target current, the second target current and a preset protection threshold value; calculating a difference value between the first target current and the second target current; when the absolute value of the difference value is larger than the current threshold value, first delay time is calculated; when the absolute value of the direct-current voltage is larger than the target voltage setting value, second delay time is calculated; when the first delay time reaches the first preset time and the second delay time reaches the second preset time, differential protection is started, and the problem that an existing direct-current submarine cable differential protection method is low in reliability is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current transmission systems, and in particular to a differential protection method, device and system for offshore wind power direct current lines. Background Art

[0002] Wind power generation can be divided into onshore wind power and offshore wind power. Compared with onshore wind power, offshore wind power has the advantages of abundant wind energy resources, stable unit operation, high annual utilization hours, no occupation of land resources, and little negative impact on the environment. Therefore, offshore wind power has become an important direction for the development of renewable energy. As the layout of offshore wind power gradually moves from offshore to offshore, traditional AC submarine cable transmission technology has gradually encountered bottlenecks and is difficult to meet the transmission needs of offshore wind power. Therefore, DC submarine cable transmission based on flexible DC transmission technology (Voltage source converter based high voltage direct current transmission, VSC-HVDC) has become the preferred solution for offshore wind power transmission.

[0003] Compared with the traditional overhead lines on land, the inspection and maintenance of DC submarine cables are more difficult, so timely detection of DC submarine cable faults is more important, especially metallic grounding faults and transition resistance grounding faults. The existing technology generally configures DC voltage mutation protection as the main protection for DC submarine cable metallic grounding faults, but this protection cannot reliably identify faults grounded through transition resistance. Existing DC lines usually configure DC line longitudinal differential protection as the main protection for transition resistance grounding faults.

[0004] However, the existing DC line longitudinal differential protection is generally used to detect DC line grounding faults through transition resistance. However, if it is directly used in offshore flexible DC transmission systems, especially in systems with long DC submarine cables, the DC submarine cable has a large equivalent capacitance to the ground. When a single-pole grounding fault occurs, the line differential protection of the non-fault pole will malfunction, and the fault pole selection function of the differential protection will be lost, making it difficult to reliably locate the faulty submarine cable, which brings difficulties to the operation and maintenance of the system. Therefore, the existing DC submarine cable differential protection method has the problem of low reliability. Summary of the invention

[0005] The present invention provides a differential protection method, device and system for an offshore wind power DC line, which are used to solve the problem of low reliability of the existing DC submarine cable differential protection method.

[0006] On one hand, the present invention provides a differential protection method for an offshore wind power DC line, comprising:

[0007] Respectively obtaining a first target current and a second target current at both ends of the DC submarine cable, and obtaining an absolute value of a DC voltage and a target voltage setting value at one end of the DC submarine cable;

[0008] Determining a current threshold value according to the first target current, the second target current and a preset protection threshold value;

[0009] calculating a difference between the first target current and the second target current;

[0010] When the absolute value of the difference is greater than the current threshold, calculating a first delay time;

[0011] When the absolute value of the DC voltage is greater than the target voltage setting value, calculating a second delay time;

[0012] When the first delay time reaches a first preset time, and the second delay time reaches a second preset time, the differential protection is started.

[0013] Optionally, determining the current threshold according to the first target current, the second target current and a preset protection threshold value includes:

[0014] calculating a sum of the first target current and the second target current;

[0015] Multiplying the absolute value of the sum by a preset first coefficient to obtain a target current value;

[0016] The maximum value between the target current value and the protection threshold value is used as the current threshold.

[0017] Optionally, obtaining the first target current and the second target current at both ends of the DC submarine cable includes:

[0018] Obtaining a first current and a second current at two ends of the DC submarine cable;

[0019] Performing low-pass filtering on the first current to obtain a first target current;

[0020] The second current is low-pass filtered to obtain a second target current.

[0021] Optionally, the step of acquiring the target voltage setting value includes:

[0022] Obtaining the first voltage setting value under a single-pole grounding fault through a transition resistor at zero power;

[0023] Obtain the second voltage setting value under the condition of grounding fault between positive and negative poles through transition resistance at zero power;

[0024] The minimum value of the first voltage setting value and the second voltage setting value is multiplied by a preset sensitivity coefficient to obtain the target voltage setting value.

[0025] Optionally, the DC submarine cable is a positive DC submarine cable or a negative DC submarine cable.

[0026] The present invention also provides an offshore wind power DC line differential protection device, the device comprising:

[0027] An acquisition module, used to respectively acquire a first target current and a second target current at both ends of the DC submarine cable, and acquire an absolute value of a DC voltage and a target voltage setting value at one end of the DC submarine cable;

[0028] A determination module, configured to determine a current threshold value according to the first target current, the second target current and a preset protection threshold value;

[0029] A first calculation module, used for calculating the difference between the first target current and the second target current;

[0030] A second calculation module, configured to calculate a first delay time when the absolute value of the difference is greater than the current threshold;

[0031] A third calculation module, used for calculating a second delay time when the absolute value of the DC voltage is greater than the target voltage setting value;

[0032] A starting module is used to start the differential protection when the first delay time reaches a first preset time and the second delay time reaches a second preset time.

[0033] Optionally, the second calculation module includes:

[0034] a calculation submodule, configured to calculate a sum of the first target current and the second target current;

[0035] An operation submodule, used for multiplying the absolute value of the sum by a preset first coefficient to obtain a target current value;

[0036] The determination submodule is used to use the maximum value between the target current value and the protection threshold value as the current threshold.

[0037] Optionally, the acquisition module is specifically used to acquire a first current and a second current at both ends of the DC submarine cable; perform low-pass filtering on the first current to obtain a first target current; and perform low-pass filtering on the second current to obtain a second target current.

[0038] Optionally, the acquisition module is further specifically used to acquire a first voltage setting value under a single-pole grounding fault through a transition resistor at zero power; acquire a second voltage setting value under a grounding fault between a positive and negative poles through a transition resistor at zero power; and multiply the minimum value of the first voltage setting value and the second voltage setting value by a preset sensitivity coefficient to obtain the target voltage setting value.

[0039] On the other hand, the present invention provides an offshore wind power flexible direct current transmission system, comprising: an offshore wind farm, a positive direct current submarine cable, a negative direct current submarine cable, an offshore converter station, a first connection transformer, an onshore converter station, a second connection transformer, a first incoming line circuit breaker, a second incoming line circuit breaker, a first bridge arm reactor, a second bridge arm reactor and a starting circuit;

[0040] The offshore wind farm is connected to the first connection transformer via the first incoming line circuit breaker;

[0041] The first connection transformer is connected to one end of the positive DC submarine cable and one end of the negative DC submarine cable respectively through the offshore converter station and the first bridge arm reactor;

[0042] The positive DC submarine cable and the negative DC submarine cable are respectively provided with a grounding transition resistor;

[0043] The starting circuit is connected to the other end of the positive DC submarine cable and the other end of the negative DC submarine cable respectively through the onshore converter station and the second bridge arm reactor;

[0044] The second connection transformer is connected to the starting circuit and the second incoming circuit breaker respectively;

[0045] The second incoming line circuit breaker is connected to the onshore AC system;

[0046] The offshore converter station and the onshore converter station are both provided with a differential protection device, and the differential protection device is used to execute the method as described above.

[0047] It can be seen from the above technical solutions that the present invention has the following advantages:

[0048] The present invention provides a differential protection method for an offshore wind power DC line, the method comprising: respectively obtaining a first target current and a second target current at both ends of a DC submarine cable, and obtaining an absolute value of a DC voltage and a target voltage setting value at one end of the DC submarine cable; determining a current threshold value according to the first target current, the second target current and a preset protection threshold value; calculating a difference between the first target current and the second target current; when the absolute value of the difference is greater than the current threshold value, calculating a first delay time; when the absolute value of the DC voltage is greater than the target voltage setting value, calculating a second delay time; when the first delay time reaches a first preset time, and the second delay time reaches a second preset time, starting differential protection.

[0049] In the present invention, the first target current and the second target current at both ends of the DC submarine cable are respectively obtained, and the absolute value of the DC voltage and the target voltage setting value at one end of the DC submarine cable are obtained, so as to obtain the current data and voltage data of the DC submarine cable; and the current threshold is determined according to the first target current, the second target current and the preset protection threshold value, so as to provide a judgment basis for starting the differential protection; and by calculating the difference between the first target current and the second target current, when the absolute value of the difference is greater than the current threshold, the first delay time is calculated, and when the absolute value of the DC voltage is greater than the target voltage setting value, the second delay time is calculated, and when the first delay time reaches the first preset time and the second delay time reaches the second preset time, the differential protection is started, so as to realize the detection of the grounding fault of the DC submarine cable, and the present invention reliably identifies the faulty DC submarine cable by using the first target current, the second target current and the absolute value of the DC voltage to judge whether to start the differential protection, thereby improving the reliability of the detection of the grounding fault of the DC submarine cable. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0051] Figure 1 A topological schematic diagram of an offshore wind power flexible direct current transmission system provided by an embodiment of the present invention;

[0052] Figure 2 A flow chart of a differential protection method for an offshore wind power DC line provided by an embodiment of the present invention;

[0053] Figure 3 A flow chart of a differential protection method for an offshore wind power DC line provided by another embodiment of the present invention;

[0054] Figure 4 A schematic diagram of a differential protection method for an offshore wind power DC line provided by an embodiment of the present invention;

[0055] Figure 5 A program flow chart of a differential protection method for an offshore wind power DC line provided by an embodiment of the present invention;

[0056] Figure 6 A structural block diagram of a differential protection device for an offshore wind power DC line provided in an embodiment of the present invention.

[0057] Figure 7Another structural block diagram of an offshore wind power DC line differential protection device provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0058] The present invention provides a method, device and system for differential protection of offshore wind power DC lines, which are used to solve the problem of low reliability of existing DC submarine cable differential protection methods.

[0059] In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, the technical solutions in the embodiments of the present invention will be clearly and completely described below in combination with the drawings in the embodiments of the present invention. Obviously, the embodiments described below are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0060] See also Figure 1 , which shows a bipolar offshore wind power flexible direct current transmission system provided by an embodiment of the present invention, the system comprising: an offshore wind farm, a positive direct current submarine cable, a negative direct current submarine cable, an offshore converter station, a first connection transformer 1, an onshore converter station, a second connection transformer 2, a first incoming line circuit breaker Q1, a second incoming line circuit breaker Q2, a first bridge arm reactor L1, a second bridge arm reactor L2 and a starting circuit 3; the offshore wind farm is connected to the first connection transformer 1 through the first incoming line circuit breaker Q1; the first connection transformer 1, through the offshore converter station and the first bridge arm reactor L1, respectively It is connected to one end of the positive DC submarine cable and one end of the negative DC submarine cable; the positive DC submarine cable and the negative DC submarine cable are respectively provided with grounding transition resistors; the starting circuit is respectively connected to the other end of the positive DC submarine cable and the other end of the negative DC submarine cable through the onshore converter station and the second bridge arm reactor L2; the second connecting transformer 2 is respectively connected to the starting circuit and the second incoming circuit breaker Q2; the second incoming circuit breaker Q2 is connected to the onshore AC system; the offshore converter station and the onshore converter station are both provided with differential protection devices, which are used to execute the differential protection method of the offshore wind power DC line.

[0061] It should be noted that the offshore converter station is provided with a flexible direct current converter VSC, which is used to convert alternating current into direct current, and transmit it to the onshore converter station through a direct current submarine cable. The onshore converter station is provided with a flexible direct current converter VSC, which is used to convert the received direct current into alternating current, and transmit it to the onshore alternating current system after being processed by the second connection transformer 2. In this embodiment, a reactor and a flexible direct current converter VSC are connected to each end of each direct current submarine cable. , They are respectively the current of the positive DC submarine cable and the current of the negative DC submarine cable on the offshore converter station side, and the locations of the two are the current collection points; , They are respectively the voltage of the positive DC submarine cable and the voltage of the negative DC submarine cable on the offshore converter station side, and the locations of the two are voltage collection points. , They are respectively the current of the positive DC submarine cable and the current of the negative DC submarine cable on the land converter station side, and the locations of the two are current collection points; , They are respectively the voltage of the positive DC submarine cable and the voltage of the negative DC submarine cable on the land converter station side, and the locations of the two are voltage collection points.

[0062] Among them, differential protection devices are installed in both the offshore converter station and the onshore converter station. The differential protection device in the offshore converter station collects the current of the positive DC submarine cable on the offshore converter station side. , the current of the negative DC submarine cable , voltage of positive DC submarine cable , voltage of negative DC submarine cable ; The differential protection device in the onshore converter station collects the current of the positive DC submarine cable on the onshore converter station side , the current of the negative DC submarine cable , voltage of positive DC submarine cable , voltage of negative DC submarine cable .

[0063] The differential protection device located in the offshore converter station and the differential protection device located in the onshore converter station are connected by inter-station communication, and the collected voltage and current are transmitted to each other. Based on the differential protection method of offshore wind power DC lines, the voltage, current and pre-configured differential protection criteria are used to realize the fault detection of the positive DC submarine cable and the negative DC submarine cable.

[0064] Based on this, this embodiment configures positive line differential protection for the positive DC submarine cable to detect positive line faults, and configures negative line differential protection for the negative DC submarine cable to detect negative line faults; and when a bipolar short circuit fault occurs, both the positive and negative DC line differential protections are activated, thereby accurately locating the faulty submarine cable and improving the reliability of fault detection.

[0065] See also Figure 2 , which shows a flow chart of a differential protection method for an offshore wind power DC line provided by an embodiment of the present invention. This embodiment uses the method for Figure 1 Taking the differential protection device in the system shown in FIG. 1 as an example, the method includes:

[0066] Step 101: respectively obtain a first target current and a second target current at both ends of the DC submarine cable, and obtain an absolute value of a DC voltage and a target voltage setting value at one end of the DC submarine cable.

[0067] It should be noted that the first target current and the second target current refer to the currents at both ends of the DC submarine cable after low-pass filtering. According to the aforementioned system, the two ends of the DC submarine cable are connected to the offshore converter station and the onshore converter station respectively. Therefore, the first target current can be the current on the offshore converter station side, and the second target current can be the current on the onshore converter station side. The absolute value of the DC voltage refers to the absolute value of the DC voltage at one end of the DC submarine cable. The target voltage setting value serves as a criterion for determining whether the low voltage auxiliary criterion is met.

[0068] In one example, the DC submarine cable can be a positive DC submarine cable or a negative DC submarine cable. When the differential protection device is located at the offshore converter station, the acquired DC voltage absolute value is the DC voltage absolute value of the DC submarine cable on the offshore converter station side. For example: when the DC submarine cable is a positive DC submarine cable, and the execution subject is the differential protection device located in the offshore converter station, the acquired first target current and second target current are respectively the current of the positive DC submarine cable on the offshore converter station side after low-pass filtering. , the current of the positive DC submarine cable after low-pass filtering on the land converter station side The absolute value of the DC voltage obtained is the absolute value of the voltage of the positive DC submarine cable on the offshore converter station side. .

[0069] Step 102: Determine a current threshold value according to the first target current, the second target current and a preset protection threshold value.

[0070] It should be noted that the protection threshold value can be determined based on the differential current value generated when a single-pole grounding fault occurs in a DC submarine cable through a transition resistor. For example, the resistance range of the transition resistor can be 300 ohms-500 ohms, and 0.1 times the rated differential current value is used as the protection threshold value.

[0071] The current threshold serves as a criterion for determining whether the current differential protection criterion is met.

[0072] Step 103: Calculate the difference between the first target current and the second target current.

[0073] Step 104: When the absolute value of the difference is greater than the current threshold, calculate a first delay time.

[0074] It should be noted that after receiving the first target current and the second target current, this embodiment calculates the difference between the first target current and the second target current, and when the absolute value of the difference between the first target current and the second target current is greater than the current threshold, it means that the current differential protection criterion is met, and a ground fault may have occurred in the cable. At this time, the delay is started from zero and the delay time is accumulated.

[0075] It is understandable that since the differential protection devices of both the offshore converter station and the onshore converter station will perform the judgment of whether the current differential protection criterion is met, and since the two stations exchange the collected current data and voltage data through inter-station communication, in order to ensure the synchronization of the differential current calculation of the two stations, a delay period is first performed before entering the start of the differential protection. Therefore, this step starts to accumulate the delay time (i.e., the first delay time) after determining that the absolute value of the difference between the first target current and the second target current is greater than the current threshold. Therefore, the accumulated delay time in this embodiment can improve the accuracy of the differential protection action.

[0076] Step 105: When the absolute value of the DC voltage is greater than the target voltage setting value, calculate the second delay time.

[0077] It should be noted that when a ground fault occurs in a DC cable, in order to avoid the problem of differential protection malfunction caused by the discharge of the equivalent capacitance of the DC cable to the ground, which is similar to the characteristics of the positive and negative DC cables short circuit, this embodiment uses the DC voltage of the DC cable to construct a low voltage auxiliary criterion to reliably identify the faulty pole. The principle is: when a ground fault occurs in the positive DC cable, the DC voltage of the positive DC cable is significantly reduced; when a ground fault occurs in the negative DC cable, the DC voltage of the negative DC cable is significantly reduced. If a bipolar short circuit fault occurs, the DC voltage of the positive DC cable and the DC voltage of the negative DC cable are both significantly reduced.

[0078] Therefore, this step determines whether the absolute value of the DC voltage is greater than the target setting value. If so, it means that the voltage differential protection criterion is met. At this time, the delay is started from zero and the delay time is accumulated.

[0079] Step 106: When the first delay time reaches the first preset time and the second delay time reaches the second preset time, the differential protection is started.

[0080] It should be noted that there is no order between step 103 and step 105, and there is no order between step 104 and step 105, and they can be performed simultaneously.

[0081] The first preset time can be determined based on the inter-station communication delay time between the offshore converter station and the onshore converter station or the DC submarine cable discharge time, wherein the first preset delay time needs to be greater than the discharge time of the faulty DC submarine cable. For example: when the inter-station communication delay time is greater than the discharge time of the faulty DC submarine cable, the first preset time is the inter-station communication delay time, and when the inter-station communication delay time is less than the discharge time of the faulty DC submarine cable, the first preset time is the discharge time of the faulty DC submarine cable. In one example, the numerical range of the discharge time of the faulty DC submarine cable is generally 100ms-300ms. The inter-station communication delay time can be obtained through testing.

[0082] The second preset time can be determined according to the inter-station communication time between the offshore converter station and the onshore converter station. The inter-station communication time is directly related to the length of the DC submarine cable. In one example, the inter-station communication delay time is generally within 10ms.

[0083] In this embodiment, when the difference between the first target current and the second target current is greater than the current threshold, and when the absolute value of the DC voltage is greater than the target voltage setting value, it can be accurately determined that the DC submarine cable has a ground fault. In order to enable the two stations to accurately start the differential protection, this embodiment accumulates the first delay time and the second delay time respectively, and after the first delay time reaches the first preset time and the second delay time reaches the second preset time, a protection action signal is output to start the differential protection, thereby accurately identifying and isolating the faulty DC submarine cable, and improving the reliability and accuracy of detecting the faulty DC submarine cable.

[0084] This embodiment obtains the first target current and the second target current at both ends of the DC submarine cable respectively, and obtains the absolute value of the DC voltage and the target voltage setting value at one end of the DC submarine cable, so as to obtain the current data and voltage data of the DC submarine cable; and determines the current threshold value according to the first target current, the second target current and the preset protection threshold value, so as to provide a judgment basis for starting the differential protection; and calculates the difference between the first target current and the second target current, and when the absolute value of the difference is greater than the current threshold value, calculates the first delay time, and when the absolute value of the DC voltage is greater than the target voltage setting value, calculates the second delay time, and when the first delay time reaches the first preset time, and when the second delay time reaches the second preset time, starts the differential protection, so as to realize the detection of the DC submarine cable grounding fault, and the present invention reliably identifies the faulty DC submarine cable by using the first target current, the second target current and the absolute value of the DC voltage to judge whether to start the differential protection, thereby improving the reliability of the DC submarine cable grounding fault detection.

[0085] Please refer to Figure 3 , which shows a flow chart of a differential protection method for an offshore wind power DC line provided by an embodiment of the present invention. This embodiment uses the method for Figure 1 Taking the differential protection device in the system shown in FIG. 1 as an example, the method includes:

[0086] Step 201: respectively obtain a first target current and a second target current at both ends of the DC submarine cable, and obtain an absolute value of a DC voltage and a target voltage setting value at one end of the DC submarine cable.

[0087] It should be noted that the steps of obtaining the first target current and the second target current include:

[0088] S1, obtaining a first current and a second current at two ends of a DC submarine cable;

[0089] S2. low-pass filter the first current to obtain a first target current;

[0090] S3. Perform low-pass filtering on the second current to obtain a second target current.

[0091] The first current is the original current at the offshore converter station side of the DC submarine cable, and the second current is the original current at the onshore converter station side of the DC submarine cable.

[0092] This embodiment performs low-pass filtering on the first current and the second current respectively to obtain the corresponding first target current and the second target current, thereby avoiding the situation where high-frequency disturbances in the DC system may cause false operation of the protection, and further improving the accuracy of DC submarine cable grounding fault detection.

[0093] In one example, the low pass filter cutoff frequency may be 250 Hz.

[0094] Step 202: Obtain a first voltage setting value under a single-pole grounding fault through a transition resistor at zero power.

[0095] It should be noted that, when the system zero power is obtained in this step, the voltage setting value of a single-pole (i.e., a single-pole DC submarine cable) grounding fault through a transition resistance is used as the first voltage setting value.

[0096] In one example, the transition resistance may range from 300 ohms to 500 ohms.

[0097] Step 203, obtaining a second voltage setting value when there is a grounding fault between the positive and negative electrodes via a transition resistor at zero power.

[0098] It should be noted that, when the system zero power is obtained in this step, the voltage setting value between the positive and negative poles (i.e., between the positive DC submarine cable and the negative DC submarine cable) under the grounding fault through the transition resistance is used as the first voltage setting value.

[0099] It is understandable that step 201 to step 203 have no sequential order and they can be performed simultaneously.

[0100] Step 204: multiply the minimum value of the first voltage setting value and the second voltage setting value by a preset sensitivity coefficient to obtain a target voltage setting value.

[0101] It should be noted that the sensitivity coefficient can be set according to actual conditions.

[0102] In one example, the sensitivity coefficient may be about 1.2.

[0103] Step 205: Calculate the difference between the first target current and the second target current.

[0104] It is understandable that there is no sequence between step 204 and step 205, and they can be performed simultaneously.

[0105] Step 206: When the absolute value of the difference is greater than the current threshold, calculate a first delay time.

[0106] Step 207: When the absolute value of the DC voltage is greater than the target voltage setting value, calculate the second delay time.

[0107] Step 208: When the first delay time reaches the first preset time and the second delay time reaches the second preset time, the differential protection is started.

[0108] It should be noted that the differential protection device is equipped with a current differential protection criterion and a low voltage auxiliary criterion, wherein the current differential protection criterion can be used as the main criterion and the low voltage auxiliary criterion can be used as the auxiliary criterion, thereby realizing accurate determination of the DC submarine cable grounding fault.

[0109] Taking the differential protection device of the offshore converter station as an example, for the positive DC submarine cable, the configured current differential protection criterion is as follows:

[0110] , delay T1;

[0111] In the formula, , They are respectively the current at the offshore converter station side and the current at the onshore converter station side of the positive DC submarine cable after low-pass filtering. To protect the threshold value, the delay T1 should be able to avoid the DC submarine cable discharge time, where the submarine cable discharge time is generally 100ms-300ms.

[0112] The current differential protection criteria for the negative DC submarine cable are the same as those for the positive DC submarine cable and will not be elaborated here.

[0113] Taking the differential protection device of the offshore converter station as an example, for the positive DC submarine cable, the configured low voltage auxiliary criteria are as follows:

[0114] , delay T2;

[0115] For negative DC submarine cables, the configured low voltage auxiliary criteria are as follows:

[0116] , delay T2;

[0117] In the formula, is the target voltage setting value. The delay time T2 can be taken as the inter-station communication time, where the inter-station communication time is directly related to the length of the submarine cable and is generally within 10ms.

[0118] Target voltage setting value Should meet:

[0119]

[0120] In the formula, It can be set according to the voltage value under the grounding fault of a single pole through a transition resistor (generally considered to be 300 ohms-500 ohms) under zero power of the DC system; It can be set according to the voltage value under the short circuit fault between positive and negative poles through transition resistance under zero power of DC system; Take the minimum value of the two and multiply it by the sensitivity coefficient , Generally, it can be around 1.2.

[0121] Among them, the logic of the current differential protection criterion and the low voltage auxiliary criterion is "AND". Therefore, in this embodiment, when the differential protection device receives the first target current, the second target current and the absolute value of the DC voltage, the three are substituted into the current differential protection criterion and the low voltage auxiliary criterion. When the current differential protection criterion and the low voltage auxiliary criterion are met and the delay reaches the delay time requirement, the differential protection is started, which improves the accuracy of the DC submarine cable grounding fault detection.

[0122] It can be understood that the differential protection principle of the onshore converter station is consistent with the protection principle of the offshore converter station. For details, please refer to the offshore converter station and will not be repeated here.

[0123] In one embodiment, see Figure 4 , this embodiment provides a principle description of the differential protection method, such as Figure 4 As shown, the positive line represents the positive DC submarine cable, and the negative line represents the negative DC submarine cable. The differential protection device of this station collects the positive line current, receives the positive line current of the opposite station, and collects the positive line voltage. If the positive line currents of the two stations meet the positive line differential current criterion, and the positive circuit voltage meets the positive line low voltage auxiliary criterion, then after delaying T1 and T2 respectively, the export protection action signal is output, and the positive line differential protection is actuated. The negative line differential protection is similar to the positive line differential protection.

[0124] In one embodiment, see Figure 5 In actual application programs, the program flow of the differential protection method may include: first, collecting the DC current and DC voltage of the positive and negative lines, and receiving the DC line current of the positive and negative poles of the station through the inter-station communication between the protection devices of the two stations. After that, the positive and negative currents are low-pass filtered, and the differential current and the braking current are calculated; after that, the judgment of whether the low voltage auxiliary criterion is met and the judgment of whether the current protection criterion is met are respectively performed. If both criteria are met, the protection action signal is exported, otherwise, the collection action is re-executed.

[0125] In this embodiment, the differential current represents the absolute value of the difference between the first target current and the second target current, and the braking current represents the current threshold.

[0126] See also Figure 6 , a structural block diagram of an offshore wind power DC line differential protection device provided by an embodiment of the present invention, the differential protection device comprises:

[0127] An acquisition module 301 is used to respectively acquire a first target current and a second target current at both ends of the DC submarine cable, and acquire an absolute value of a DC voltage and a target voltage setting value at one end of the DC submarine cable;

[0128] A determination module 302, configured to determine a current threshold value according to a first target current, a second target current and a preset protection threshold value;

[0129] A first calculation module 303, used to calculate the difference between the first target current and the second target current;

[0130] A second calculation module 304, configured to calculate a first delay time when the absolute value of the difference is greater than a current threshold;

[0131] A third calculation module 305, configured to calculate a second delay time when the absolute value of the DC voltage is greater than the target voltage setting value;

[0132] The starting module 306 is used to start the differential protection when the first delay time reaches the first preset time and the second delay time reaches the second preset time.

[0133] Further, the second calculation module 304 includes:

[0134] A calculation submodule, used for calculating the sum of the first target current and the second target current;

[0135] An operation submodule, used for multiplying the absolute value of the sum by a preset first coefficient to obtain a target current value;

[0136] The determination submodule is used to take the maximum value between the target current value and the protection threshold value as the current threshold.

[0137] Furthermore, the acquisition module 301 is specifically used to acquire a first current and a second current at both ends of the DC submarine cable; perform low-pass filtering on the first current to obtain a first target current; and perform low-pass filtering on the second current to obtain a second target current.

[0138] Furthermore, the acquisition module 301 is specifically used to obtain a first voltage setting value under a single-pole grounding fault through a transition resistor at zero power; obtain a second voltage setting value under a grounding fault between a positive and negative poles through a transition resistor at zero power; and multiply the minimum value of the first voltage setting value and the second voltage setting value by a preset sensitivity coefficient to obtain a target voltage setting value.

[0139] In one embodiment, Figure 7 As shown, in actual application, the functions of the differential protection device 400 in the system can be realized by setting a data communication unit 401, a data acquisition unit 404, a data processing unit 403, a logic judgment unit 404, and a protection export unit 405. Among them, the data communication unit 401 realizes the functions of receiving the positive and negative DC currents of the station through inter-station communication, and sending the current signal of the station to the station protection device. The data acquisition unit 404 realizes the positive and negative DC current and DC voltage collection functions of the offshore wind power flexible transmission system. The data processing unit 403 realizes the functions of low-pass filtering of the positive and negative DC currents and differential current calculation. The logic judgment unit 404 realizes the protection logic functions such as the main judgment criteria of the current differential protection and the low voltage auxiliary judgment criteria; the protection export unit 405 realizes the function of the protection action signal export.

[0140] The present invention provides a method, device and system for differential protection of an offshore wind power DC line. The method provided is based on collecting the DC current and DC voltage of the positive and negative poles of the DC submarine cable line, receiving the DC submarine cable current of the opposite station through the inter-station communication between the differential protection devices of the two stations, and constructing the main criterion of current differential protection, and constructing the auxiliary criterion by using the DC voltage of the positive and negative poles to realize fault pole selection, and can reliably detect the grounding fault on the DC submarine cable; and the fixed value setting steps are given, and the algorithm is simple, the fixed value setting is easy, the CPU resources are less occupied, and the engineering implementation is convenient, and the reliability and accuracy are strong.

[0141] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working processes of the systems, devices and units described above can refer to the corresponding processes in the aforementioned method embodiments and will not be repeated here.

[0142] In the several embodiments provided in the present application, it should be understood that the disclosed systems, devices and methods can be implemented in other ways. For example, the device embodiments described above are only schematic. For example, the division of units is only a logical function division. There may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be an indirect coupling or communication connection through some interfaces, devices or units, which can be electrical, mechanical or other forms.

[0143] The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0144] In addition, each functional unit in each embodiment of the present invention may be integrated into one processing unit, or each functional unit may be physically separate, or two or more functional units may be integrated into one processing unit. The above integrated unit may be implemented in the form of hardware or in the form of software functional units.

[0145] If the integrated unit 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 this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of the technical solution can be embodied in the form of a software product. The computer software product is stored in a storage medium, including several instructions for a computer device (which can be a personal computer, server, or network device, etc.) to perform all or part of the steps of the methods of each embodiment of the present invention. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), disk or optical disk, etc. Various media that can store program codes.

[0146] The terms "first", "second", "third", "fourth", etc. (if any) in the specification of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than those illustrated or described herein, for example. In addition, the terms "including" and "having" and any of their variations are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0147] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A differential protection method for an offshore wind power DC line, characterized in that: The method comprises: Respectively obtaining a first target current and a second target current at both ends of the DC submarine cable, and obtaining an absolute value of a DC voltage and a target voltage setting value at one end of the DC submarine cable; Determining a current threshold value according to the first target current, the second target current and a preset protection threshold value; calculating a difference between the first target current and the second target current; When the absolute value of the difference is greater than the current threshold, calculating a first delay time; When the absolute value of the DC voltage is greater than the target voltage setting value, calculating a second delay time; When the first delay time reaches a first preset time, and the second delay time reaches a second preset time, the differential protection is started.

2. The method according to claim 1, characterized in that The determining of the current threshold according to the first target current, the second target current and a preset protection threshold value includes: calculating a sum of the first target current and the second target current; Multiplying the absolute value of the sum by a preset first coefficient to obtain a target current value; The maximum value between the target current value and the protection threshold value is used as the current threshold.

3. The method according to claim 1, characterized in that The obtaining of the first target current and the second target current at both ends of the DC submarine cable comprises: Obtaining a first current and a second current at two ends of the DC submarine cable; Performing low-pass filtering on the first current to obtain a first target current; The second current is low-pass filtered to obtain a second target current.

4. The method according to claim 1, characterized in that: The step of obtaining the target voltage setting value comprises: Obtaining the first voltage setting value under a single-pole grounding fault through a transition resistor at zero power; Obtain the second voltage setting value under the condition of grounding fault between positive and negative poles through transition resistance at zero power; The minimum value of the first voltage setting value and the second voltage setting value is multiplied by a preset sensitivity coefficient to obtain the target voltage setting value.

5. The method according to claim 1, characterized in that The DC submarine cable is a positive DC submarine cable or a negative DC submarine cable.

6. A differential protection device for an offshore wind power DC line, characterized in that: The device comprises: An acquisition module, used to respectively acquire a first target current and a second target current at both ends of the DC submarine cable, and acquire an absolute value of a DC voltage and a target voltage setting value at one end of the DC submarine cable; A determination module, configured to determine a current threshold value according to the first target current, the second target current and a preset protection threshold value; A first calculation module, used for calculating the difference between the first target current and the second target current; A second calculation module, configured to calculate a first delay time when the absolute value of the difference is greater than the current threshold; A third calculation module, used for calculating a second delay time when the absolute value of the DC voltage is greater than the target voltage setting value; A starting module is used to start the differential protection when the first delay time reaches a first preset time and the second delay time reaches a second preset time.

7. The device according to claim 6, characterized in that The second calculation module includes: a calculation submodule, configured to calculate a sum of the first target current and the second target current; An operation submodule, used for multiplying the absolute value of the sum by a preset first coefficient to obtain a target current value; The determination submodule is used to use the maximum value between the target current value and the protection threshold value as the current threshold.

8. The device according to claim 6, characterized in that The acquisition module is specifically used to acquire the first current and the second current at both ends of the DC submarine cable; perform low-pass filtering on the first current to obtain a first target current; and perform low-pass filtering on the second current to obtain a second target current.

9. The device according to claim 6, characterized in that The acquisition module is specifically used to acquire a first voltage setting value under a single-pole grounding fault through a transition resistor at zero power; acquire a second voltage setting value under a grounding fault between a positive and negative poles through a transition resistor at zero power; and multiply the minimum value of the first voltage setting value and the second voltage setting value by a preset sensitivity coefficient to obtain the target voltage setting value.

10. An offshore wind power flexible direct current transmission system, characterized in that: include: Offshore wind farm, positive DC submarine cable, negative DC submarine cable, offshore converter station, first connection transformer, onshore converter station, second connection transformer, first incoming line circuit breaker, second incoming line circuit breaker, first bridge arm reactor, second bridge arm reactor and starting circuit; The offshore wind farm is connected to the first connection transformer via the first incoming line circuit breaker; The first connection transformer is connected to one end of the positive DC submarine cable and one end of the negative DC submarine cable respectively through the offshore converter station and the first bridge arm reactor; The positive DC submarine cable and the negative DC submarine cable are respectively provided with a grounding transition resistor; The starting circuit is connected to the other end of the positive DC submarine cable and the other end of the negative DC submarine cable respectively through the onshore converter station and the second bridge arm reactor; The second connection transformer is connected to the starting circuit and the second incoming circuit breaker respectively; The second incoming line circuit breaker is connected to the onshore AC system; The offshore converter station and the onshore converter station are both provided with a differential protection device, and the differential protection device is used to execute the method according to any one of claims 1 to 5.

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