Current differential protection method and system based on low-voltage current limiting control

Abstract

The invention provides a current differential protection method and system based on low-voltage current limiting control, and relates to the technical field of new energy relay protection. The method comprises the following steps: acquiring a voltage value of an alternating-current collection line and a phase difference value of current on two sides of the alternating-current collection line; when the voltage value is smaller than a preset voltage threshold value, a control mode of the converter station is switched to a low-voltage current-limiting control mode, and a voltage reference value of the converter station is set as an initial voltage reference value; when the phase difference value of the currents on the two sides is larger than a preset phase difference threshold value, a target voltage reference value of the converter station is determined based on the current positive sequence components on the two sides of the alternating current collection line, and the initial voltage reference value is switched into the target voltage reference value; and based on the relation between the ratio braking coefficient of the converter station under the target voltage reference value and a preset coefficient threshold value, current differential protection action or protection non-action of the converter station is determined. The problem that traditional differential protection cannot reliably act in the low-voltage current limiting period is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of new energy relay protection technology, and particularly relates to a current differential protection method and system based on low-voltage current limiting control. BACKGROUND

[0002] With the increasing demand for clean energy worldwide, the proportion of new energy power generation in the power system is increasing. Among them, new energy through flexible HVDC transmission system is widely used due to its strong controllability, low loss, and flexible grid connection. The AC collection line between the new energy station and the flexible converter is on both sides of the power electronic device. When the line is faulty, the short-circuit current is affected by different converter control modes, showing waveform distortion, controlled phase angle, and limited amplitude. Therefore, higher requirements are put forward for the reliability of current differential protection.

[0003] The existing technology generally adopts low-voltage current limiting control mode, that is, the voltage reference value of the converter station side is reduced after the fault, so as to suppress the short-circuit current within a safe range. However, in this mode, the phase difference between the current of the wind turbine side and the current of the converter station side is usually greater than 90° or even close to 180° when the inter-phase fault occurs. The traditional current differential protection is greatly offset by the subtraction of the differential current vector, the braking coefficient is increased, and the sensitivity is suddenly reduced, so it cannot reliably act. SUMMARY

[0004] The present application provides a current differential protection method based on low-voltage current limiting control to solve the problem that the traditional current differential protection cannot reliably act during low-voltage current limiting.

[0005] To achieve the above-mentioned purpose, the technical solution adopted by the present application is as follows: In a first aspect, the present application provides a current differential protection method based on low-voltage current limiting control, the method comprising: obtaining a voltage value of an AC collection line and a phase difference value of currents on both sides of the AC collection line; when the voltage value is less than a preset voltage threshold, switching the control mode of the converter station to a low-voltage current limiting control mode and setting the voltage reference value of the converter station to an initial voltage reference value; when the phase difference value of the currents on both sides is greater than a preset phase difference threshold, determining a target voltage reference value of the converter station based on the positive sequence components of the currents on both sides of the AC collection line, and switching the voltage reference value of the converter station from the initial voltage reference value to the target voltage reference value; and determining the current differential protection action or protection inaction of the converter station based on the relationship between the ratio braking coefficient of the converter station under the target voltage reference value and the preset coefficient threshold.

[0006] In some embodiments, determining the target voltage reference value of the converter station based on the positive sequence components of the currents on both sides of the AC collection line comprises: obtaining the positive sequence components of the currents on both sides of the AC collection line; determining the positive sequence voltage reference value of the converter station based on the positive sequence components of the currents; and performing dq transformation on the positive sequence voltage reference value to obtain the target voltage reference value.

[0007] In some embodiments, the positive sequence components of the currents on both sides of the AC collection line comprise the positive sequence components of the currents on the fan side and the positive sequence components of the currents on the converter station side; and determining the positive sequence voltage reference value of the converter station based on the positive sequence components of the currents on both sides of the AC collection line comprises: obtaining the transition resistance of the AC collection line and the negative sequence equivalent impedance of the converter station; determining the difference between the positive sequence components of the currents on both sides of the AC collection line based on the positive sequence components of the currents on the fan side and the positive sequence components of the currents on the converter station side; and determining the positive sequence voltage reference value based on the difference between the positive sequence components of the currents on both sides of the AC collection line, the transition resistance and the negative sequence equivalent impedance.

[0008] In some embodiments, the positive sequence voltage reference value is determined by the following formula: wherein, is the positive sequence voltage reference value, is the positive sequence components of the currents on the fan side, is the positive sequence components of the currents on the converter station side, is the transition resistance of the AC collection line, is the negative sequence equivalent impedance of the converter station.

[0009] In some embodiments, the transition resistance of the AC collection line is determined by the following formula: wherein, is the transition resistance of the AC collection line, and are the measured impedances of the impedance relays at the protection on both ends of the AC collection line, is the impedance value of the AC collection line, is the current on the converter station side, is the current on the fan side.

[0010] In some embodiments, determining the action or non-action of the current differential protection of the converter station based on the relationship between the ratio restraint coefficient of the converter station under the target voltage reference value and the preset coefficient threshold value comprises: determining the action of the current differential protection of the converter station when the ratio restraint coefficient is greater than or equal to the preset coefficient threshold value; and determining the non-action of the current differential protection of the converter station when the ratio restraint coefficient is less than the preset coefficient threshold value.

[0011] In some embodiments, the ratio restraint coefficient is determined by the following formula: wherein, ​is a ratio braking coefficient, is a current on the converter station side, is a current on the fan side.

[0012] In some embodiments, the method further comprises: when the two-side current phase difference value is less than or equal to a preset phase difference threshold value, and the ratio braking coefficient of the converter station under the initial voltage reference value is greater than or equal to a preset coefficient threshold value, determining a current differential protection action of the converter station.

[0013] In a second aspect, the present application provides a current differential protection system based on low-voltage current limiting control, comprising an AC collection line and a converter station; wherein the AC collection line is used to transmit electrical energy to the converter station, and the converter station is used to perform the steps of the current differential protection method based on low-voltage current limiting control provided in the first aspect or any one of the embodiments of the first aspect.

[0014] In a third aspect, the present application provides a current differential protection device based on low-voltage current limiting control, comprising: an acquisition module, configured to acquire a voltage value of an AC collection line and a two-side current phase difference value of the AC collection line; a control mode switching module, configured to switch a control mode of a converter station to a low-voltage current limiting control mode when the voltage value is less than a preset voltage threshold value, and set a voltage reference value of the converter station as an initial voltage reference value; a voltage reference value switching module, configured to determine a target voltage reference value of the converter station based on positive sequence components of two-side currents of the AC collection line when the two-side current phase difference is greater than a preset phase difference threshold value, and switch the voltage reference value of the converter station from the initial voltage reference value to the target voltage reference value; and a current differential protection module, configured to determine a current differential protection action or protection inaction of the converter station based on a relationship between a ratio braking coefficient of the converter station under the target voltage reference value and a preset coefficient threshold value.

[0015] Compared with the prior art, the present application has the following beneficial effects: Based on the low-voltage current limiting control mode of the converter station, the present application can adjust the dq-axis voltage reference value according to the two-side fault currents of the AC collection line, reduce the two-side fault current phase difference of the AC collection line, solve the problem that the traditional current differential protection cannot reliably act during low-voltage current limiting, and at the same time, when an external area fault occurs, the current differential protection will not malfunction, thereby ensuring the selectivity and reliability of the current differential protection. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is a topological structure diagram of a new energy through a flexible island sending-out system provided by an embodiment of the present application; Figure 2 is a topological structure diagram of a wind power AC collection line provided by an embodiment of the present application; Figure 3is a flowchart of a current differential protection method based on low-voltage current limiting control provided by an embodiment of the present application; Figure 4 is a low-voltage current limiting control mode block diagram provided by an embodiment of the present application; Figure 5 is another low-voltage current limiting control mode block diagram provided by an embodiment of the present application; Figure 6 is a flowchart of a method for determining a target voltage reference value provided by an embodiment of the present application; Figure 7 is a flowchart of a method for determining whether a current differential protection of a converter station acts or does not act provided by an embodiment of the present application; Figure 8 is a flowchart of another current differential protection method based on low-voltage current limiting control provided by an embodiment of the present application; Figure 9 is a flowchart of still another current differential protection method based on low-voltage current limiting control provided by an embodiment of the present application; Figure 10 is a K value diagram when a metallic inter-phase fault occurs in a line under an existing low-voltage current limiting control mode provided by an embodiment of the present application; Figure 11 is a K value diagram when a metallic inter-phase fault occurs in a line under the low-voltage current limiting control mode of the present application provided by an embodiment of the present application; Figure 12 is a converter station bridge arm current simulation diagram when a metallic inter-phase fault occurs in a line under the low-voltage current limiting control mode of the present application provided by an embodiment of the present application; Figure 13 is a K value diagram when a 30Ω transition resistance inter-phase fault occurs in a line under the low-voltage current limiting control mode of the present application provided by an embodiment of the present application; Figure 14 is a converter station bridge arm current simulation diagram when a 30Ω transition resistance inter-phase fault occurs in a line under the low-voltage current limiting control mode of the present application provided by an embodiment of the present application; Figure 15 is a structure block diagram of a current differential protection device based on low-voltage current limiting control provided by an embodiment of the present application. DETAILED DESCRIPTION

[0017] The present application will be further described below with reference to the drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present application, and cannot be used to limit the protection scope of the present application. It should be pointed out that the following detailed description is exemplary and is intended to provide further description of the present application.

[0018] The acquisition, transmission, storage, use, processing and the like of the data in the technical scheme of the present application comply with relevant provisions of national laws and regulations.

[0019] It should be noted that in the embodiments of the present application, some industry existing schemes such as certain software, components, models and the like may be mentioned, which should be considered as exemplary, and the purpose is only to illustrate the feasibility in the implementation of the technical scheme of the present application, but it does not mean that the applicant has or will necessarily use the scheme.

[0020] It should also be noted that the terms "include", "contain" or any other variants thereof are intended to cover non-exclusive inclusion, so that the process, method, product or equipment including a series of elements not only includes those elements, but also includes other elements not explicitly listed, or includes elements inherent to such process, method, product or equipment. Without more limitation, the element defined by the sentence "including one" does not exclude the presence of another identical element in the process, method, product or equipment including the element.

[0021] With the increasing demand for clean energy worldwide, the proportion of new energy power generation in the power system is increasing. Large-scale new energy through flexible DC transmission system has become a typical scenario in new-type power system. Flexible DC technology is particularly suitable for long-distance and large-capacity transmission of new energy such as offshore wind power, because it can be connected to passive network and reduce transmission loss.

[0022] As the main power transmission method of new energy at present, compared with traditional DC transmission technology, flexible DC transmission has no reactive power compensation problem. The MMC converter (Modular Multilevel Converter) of flexible DC transmission not only does not need to provide reactive power from the AC grid, but also can play the role of static var compensator, compensate the reactive power of the AC grid, and stabilize the voltage of the AC grid. Flexible DC transmission also does not have the problem of commutation failure. The MMC converter is a turn-off power electronic device, and there is no commutation failure problem. When the AC bus is faulty, as long as there is voltage on the AC bus, power can be transmitted. Moreover, it can also power the passive system, work in inverter passive form, provide synchronous power supply for new energy stations such as offshore wind farms, and has fault low voltage ride-through capability, which can realize the transmission of electric energy between new energy stations and land grid.

[0023] However, there are many complex technical problems in the new energy through flexible DC transmission system. The AC transmission line between the new energy station and the flexible DC converter is composed of power electronic devices on both sides. When the AC collection line fails, the short-circuit current is affected by different converter control modes, and problems such as waveform distortion, controlled phase angle and limited amplitude occur.

[0024] When the AC collection line of the HVDC islanded transmission system fails, a series of key problems need to be solved. The specific problems are as follows: 1. Unlike the new energy station on land, the new energy station on the sea or in a harsh environment has high difficulty in operation and maintenance, the probability of failure of the HVDC transmission line is higher, and the failure is generally permanent, so the performance requirements of the current differential protection device are high; 2. The new energy power is connected to the power grid through power electronic devices, the system fault characteristics are affected by the control mode of the power electronic devices, the transient process is complex, the harmonic content is high, the fault characteristics are not obvious, the phase angle is controlled, and the traditional current differential protection faces the problems of reduced sensitivity and even serious failure; 3. In the new energy HVDC islanded transmission system, the two sides of the AC collection line are both power electronic converters, which are characterized by power electronic controlled power sources. When the AC collection line fails, the fault current flows from the new energy side to the converter station. In the case that the voltage reference value of the converter station is small, the current injected from the new energy side may cross, resulting in the reversal of the size relationship between the differential quantity and the braking quantity. Among them, when the phase-to-phase fault occurs, the transition resistance is not grounded, so whether the fault resistance exists or not will not affect the current injected from the new energy side to produce the crossing phenomenon.

[0025] In view of the above problems, in the prior art, when the new energy HVDC islanded transmission system fails, the new energy station and the HVDC converter adopt a negative sequence current control mode and a low voltage current limiting control mode respectively, which can avoid overvoltage of the AC collection line and reduce the fault current, but will weaken the fault characteristics and produce a phase difference, and the correct action of the differential protection is related to the amplitude and phase of the short-circuit current of the MMC converter on both sides. At present, the negative sequence current is injected to increase the amplitude of the fault current, so that the differential protection can act reliably, but there is a lack of research on the differential protection scheme based on the phase difference of the short-circuit current of the MMC converter on both sides. Since the phase difference of the fault currents on both sides is the most serious when the phase-to-phase fault occurs, the present application takes the phase-to-phase fault as an example.

[0026] In summary, a current differential protection scheme based on low voltage current limiting control is needed, which can adaptively provide dq-axis voltage reference values (in the dq synchronous rotating coordinate system, the d-axis is used for active power or DC voltage control, and the q-axis is used for reactive power or AC voltage value control) to the converter station according to the fault currents on both sides of the AC collection line, so as to reduce the phase difference of the fault currents on both sides of the line, make the current differential protection act correctly, and ensure the safe and stable operation of the whole system.

[0027] For the convenience of understanding, the new energy HVDC islanded transmission system in the offshore wind farm transmission scenario is taken as an example, and the principle of the current differential protection scheme based on low voltage current limiting control is combined with the principle of the low voltage current limiting control mode of the MMC converter. Figure 1 andFigure 2 This paper describes the new energy transmission system via a flexible DC island and its wind power AC collection line. For example... Figure 1 As shown, the transmission system includes a point of common coupling (PCC) 1, a sending-end MMC converter 2, a receiving-end MMC converter 3, and an AC collection line 6. The converter station uses a modular multilevel converter (MMC) as the sending-end MMC converter 2. In this transmission system, the AC power generated by the wind turbines in the wind farm is collected at the PCC (Power Control Center) 1 and then transferred to the AC collection line 6 to the sending-end MMC converter 2. The sending-end MMC converter 2 converts the AC power into high-voltage direct current (VDC) and remotely transmits it to the receiving-end MMC converter 3, where it is then inverted and converted into the AC power required by the power grid. Specifically, the wind power AC collection line is as follows: Figure 2 As shown, it includes an AC collection line 6, a wind farm grid-side converter 4, and a converter station 5. The AC power generated by the wind turbine generators in the wind farm is adjusted by the wind farm generator-side converter and the wind farm grid-side converter 4 and then collected in the AC collection line 6 before being sent to the converter station 5.

[0028] Figure 3 This is a schematic flowchart of a current differential protection method based on low-voltage current limiting control provided by an embodiment of the present invention. Figure 3 As shown, the current differential protection method based on low-voltage current limiting control includes steps S11 to S14.

[0029] In step S11, the voltage value of the AC collection line and the phase difference value of the current on both sides of the AC collection line are obtained.

[0030] In this embodiment of the invention, the voltage value refers to the effective value of the voltage at the beginning of the AC collection line, which can be collected in real time by an electromagnetic voltage transformer or a capacitive voltage transformer installed on the line side of the circuit breaker at the outlet of the collection bus.

[0031] In this embodiment of the invention, the phase difference between the two currents refers to the phase angle difference between the short-circuit current on the wind turbine side and the short-circuit current on the converter station side, which is obtained by existing synchronous sampling and positive sequence extraction methods. This invention does not limit the specific implementation method.

[0032] In step S12, when the voltage value is less than the preset voltage threshold, the control mode of the converter station is switched to the low-voltage current limiting control mode, and the voltage reference value of the converter station is set to the initial voltage reference value.

[0033] During normal operation, the converter station is controlled in normal operation mode, such as constant AC voltage-AC frequency control mode.

[0034] When the AC collection line fails, the converter station and each line switch start according to the low voltage criterion, that is, whether the voltage value of the AC collection line is less than the preset voltage threshold. If it is detected that the voltage value of the AC collection line is less than the preset voltage threshold, the control mode of the converter station is switched from the normal operation mode to the low-voltage current limiting control mode, and the voltage reference value of the converter station is set to the initial voltage reference value.

[0035] For example, in the normal operation of the new energy station through the flexible transmission system, the converter station adopts the fixed AC voltage-AC frequency control mode; when it is detected that the AC collection line of the new energy station through the flexible transmission system fails, the converter station and each line switch start according to the low voltage criterion, and the control mode of the converter station is switched to the low-voltage current limiting control mode. It should be noted that in the existing low-voltage current limiting control mode, the converter station mainly reduces the d-axis voltage reference value component to reduce the voltage and limit the fault current.

[0036] In step S13, when the phase difference value of the two sides of the current is greater than the preset phase difference threshold, the target voltage reference value of the converter station is determined based on the positive sequence component of the current on both sides of the AC collection line, and the voltage reference value of the converter station is switched from the initial voltage reference value to the target voltage reference value.

[0037] When the phase difference of the fault currents on both sides of the AC collection line is greater than the preset phase difference threshold, it will affect the performance of the current differential protection, and may cause the differential protection to refuse to act, so that the line short-circuit fault cannot be removed in time, which endangers the safe and stable operation of the entire system.

[0038] Therefore, the embodiment of the present application improves the existing low-voltage current limiting control mode based on the phase difference of the fault currents on both sides of the AC collection line. In the improved low-voltage current limiting control mode, when it is detected that the phase difference value of the two sides of the current is greater than the preset phase difference threshold, the target voltage reference value is further determined, and the voltage reference value of the converter station is switched from the initial voltage reference value to the target voltage reference value (that is, the voltage reference value is recalculated and updated).

[0039] In the embodiment of the present application, the phase difference of the fault currents on both sides is directly used to quantify the severity of the fault, and once the phase difference exceeds the limit, the voltage reference value is automatically updated, so that the "action amount" after the subtraction of the differential protection vector quickly recovers, the braking coefficient synchronously falls back, the protection sensitivity is enhanced, and the refusal dead zone during the traditional low-voltage current limiting is eliminated.

[0040] In step S14, the current differential protection action or protection inaction of the converter station is determined based on the relationship between the ratio braking coefficient of the converter station under the target voltage reference value and the preset coefficient threshold.

[0041] It should be noted that the greater the ratio braking coefficient, the greater the differential current is greater than the braking current, corresponding to the internal fault; the smaller the ratio braking coefficient, the smaller the differential current is less than the braking current, corresponding to the external fault or disturbance. Therefore, taking the ratio braking coefficient exceeding the preset coefficient threshold as the action condition can automatically highlight the internal fault and suppress the external unbalance, thereby ensuring the reliable and non-malfunction of the differential protection during the low-voltage current limiting period.

[0042] For example, assuming that the ratio braking coefficient is K and the preset coefficient threshold is 0.8, if the K value is greater than or equal to 0.8, it is an internal fault and the current differential protection action condition is met, and the current differential protection correctly acts; if the K value is less than the set value 0.8, it is an external fault and the current differential protection action condition is not met, and the current differential protection does not act.

[0043] For the convenience of understanding, the above-mentioned current differential protection method based on low-voltage current limiting control is exemplarily illustrated below.

[0044] In some specific embodiments, the midpoint of the 1.4s line is set to be faulty, and after the new energy is sent out through the flexible transmission system, the converter and each line switch are started according to the low-voltage criterion, the low-voltage starting criterion coefficient is 0.8, the converter station is switched to the low-voltage current limiting control, and the low-voltage current limiting control mode block diagram at this time is as shown in Figure 4 As shown in Figure 4 u s is a preset voltage value, i s is a preset current value, and the subscripts ref, d, q and abc represent reference value, d-axis, q-axis and three-phase respectively, and f is a preset reference frequency. u sn is a voltage reference value in normal operation, u sl is a voltage reference value in low-voltage current limiting, u sabc is a three-phase voltage reference value, u sdref is a d-axis voltage reference value, i sdref is a d-axis current reference value, i sqref is a q-axis current reference value, u sd is a d-axis voltage value, u sq is a q-axis voltage value, u diffd is a d-axis differential mode voltage component, u diffq is a q-axis differential mode voltage component. abc / dq represents the coordinate transformation (Park transformation) from the three-phase stationary coordinate system abc to the two-phase synchronous rotating coordinate system dq, PI is a proportional integral controller, and the product of ωL represents the equivalent inductance of the MMC converter mathematical model, which is used to calculate the dq-axis cross-coupling voltage and decoupling compensation. Assuming that the preset phase difference threshold is 90°, when the phase difference of the currents on both sides of the line is less than or equal to the set value 90°, the protection acts.

[0045] When the phase difference of the currents on both sides of the line is greater than the set value 90°, another low-voltage current limiting control mode (i.e., improved low-voltage current limiting control mode) is started, and the low-voltage current limiting control mode block diagram is as shown in Figure 5 As shown in Figure 5 u s is a preset voltage value, i s is a preset current value, and the subscripts ref, d, q, and abc represent reference value, d-axis, q-axis, and three-phase, respectively. sn u sl is a voltage reference value during normal operation, u sabc is a three-phase voltage reference value, u sdref is a d-axis voltage reference value, i sdref is a d-axis current reference value, i sqref is a q-axis current reference value, u sd is a d-axis voltage value, u sq is a q-axis voltage value, u diffd is a d-axis differential mode voltage component, and u diffq is a q-axis differential mode voltage component. is a d-axis voltage reference value during the improved low-voltage current limiting control mode, is a q-axis voltage reference value during the improved low-voltage current limiting control mode, and the obtained dq-axis voltage reference values are transmitted to the converter station.

[0046] The current differential protection method based on low-voltage current limiting control provided by the embodiments of the present application uses a two-stage strategy of “low-voltage current limiting + adaptive adjustment of phase difference driving voltage reference value”, which not only suppresses fault currents but also ensures that the current differential protection still has sufficient sensitivity during low-voltage current limiting, thereby achieving reliable and rapid fault identification of the AC collection line of the new energy flexible DC transmission system.

[0047] In a specific embodiment, for determining the target voltage reference value of the converter station based on the positive sequence components of the currents on both sides of the AC collection line in the step S13, the step shown in Figure 4 may be used.

[0048] Figure 6 is a flowchart of a method for determining a target voltage reference value provided by the embodiments of the present application, as shown in Figure 6As shown, the method comprises steps S21 to S23.

[0049] In step S21, the positive sequence components of the currents on both sides of the AC collection line are obtained.

[0050] In some embodiments, the positive sequence components of the currents on both sides include the positive sequence component of the current on the fan side and the positive sequence component of the current on the converter station side.

[0051] For example, the positive sequence component of the current on the fan side is obtained , and the positive sequence component of the current on the converter station side is obtained .

[0052] In step S22, based on the positive sequence components of the currents on both sides, the positive sequence voltage reference value of the converter station is determined.

[0053] The above step S22 can further include: obtaining the transition resistance of the AC collection line and the negative sequence equivalent impedance of the converter station; determining the difference between the positive sequence components of the currents on both sides of the AC collection line based on the positive sequence component of the current on the fan side and the positive sequence component of the current on the converter station side; and determining the positive sequence voltage reference value based on the difference between the positive sequence components of the currents on both sides, the transition resistance, and the negative sequence equivalent impedance.

[0054] In some embodiments, the positive sequence voltage reference value can be determined by the following formula 1.

[0055] Formula 1: , wherein is the positive sequence voltage reference value, is the positive sequence component of the current on the fan side, is the positive sequence component of the current on the converter station side, is the transition resistance of the AC collection line, is the negative sequence equivalent impedance of the converter station.

[0056] wherein, for the negative sequence equivalent impedance of the converter station The calculation method simplifies the converter station into a capacitance-inductance series-parallel model, calculates the sum of the sub-module and bridge arm inductance impedance values in the upper and lower bridge arms, and then takes the parallel result of the upper and lower bridge arm impedance values to obtain the negative sequence equivalent impedance of the converter station Specific numerical values.

[0057] wherein, for the transition resistance The calculation method is that, since the transition resistance cannot be measured by the device during line fault, impedance relays are installed at the protections on both ends of the line, and the measured impedance values obtained by the two impedance relays are and , from which the transition resistance value can be approximately calculated. Specifically, the transition resistance can be determined by the following formula 2.

[0058] Formula 2: , wherein, is the transition resistance of the AC collection circuit, and are the measured impedance of the impedance relays at both ends of the AC collection circuit, respectively, is the impedance value of the AC collection circuit, is the current at the converter station side, is the current at the fan side.

[0059] In step S23, the positive sequence voltage reference value is dq-transformed to obtain a target voltage reference value.

[0060] The positive sequence voltage reference value is dq-transformed, and the angle required for the dq-transformation is still the angle used for the dq-transformation when the sending-out system is in normal operation , after obtaining the d-axis voltage reference value and the q-axis voltage reference value, the voltage reference value in the original low-voltage current-limiting control strategy is replaced by the dq-axis voltage reference values obtained by the above method.

[0061] It should be noted that, in order to prevent overcurrent of the converter, the d-axis voltage reference value is still processed in the same way as in the original low-voltage current-limiting control mode, that is, multiplied by a proportional coefficient for voltage reduction processing.

[0062] In a specific embodiment, for the relationship between the ratio braking coefficient of the converter station at the target voltage reference value and the preset coefficient threshold value in the above step S14, determining the current differential protection action or protection inaction of the converter station can adopt the steps as shown in Figure 7 .

[0063] After the voltage reference value of the converter station is replaced by the target reference value, the detection ratio braking coefficient K is calculated in real time.

[0064] In some embodiments, the ratio braking coefficient is determined by the following formula 3: Formula 3: , wherein, is the ratio braking coefficient, is the current at the converter station side, is the current at the fan side.

[0065] Figure 7 is a flowchart of a method for determining the current differential protection action or protection inaction of a converter station provided by an embodiment of the present application, as shown in Figure 7 , step S14 can further include step S141 and step S142.

[0066] In step S141, when the ratio braking coefficient is greater than or equal to the preset coefficient threshold value, it is determined that the current differential protection of the converter station acts.

[0067] Suppose the preset coefficient threshold is 0.8, if the current differential protection ratio braking coefficient K value is greater than or equal to 0.8, it is an internal fault and the current differential protection action condition is met, and the current differential protection correctly acts.

[0068] In step S142, when the ratio braking coefficient is less than the preset coefficient threshold, it is determined that the current differential protection of the converter station does not act.

[0069] Suppose the preset coefficient threshold is 0.8, if the current differential protection ratio braking coefficient K value is less than 0.8, it is an external fault and the current differential protection action condition is not met, and the current differential protection does not act.

[0070] The embodiment of the present application takes the numerical comparison result of the ratio braking coefficient and the preset coefficient threshold as the criterion under the target reference value, so that the current differential protection accurately identifies the internal fault and immediately outputs an action signal in the low-voltage current limiting control mode, and reliably maintains the non-action state in the external fault, thereby eliminating the protection dead zone caused by the fault current phase difference, and improving the fault identification accuracy, action speed and operation safety of the new energy flexible direct transmission system AC collection line.

[0071] Figure 8 is another flowchart of the current differential protection method based on low-voltage current limiting control provided by the embodiment of the present application, as shown in Figure 8 The current differential protection method based on low-voltage current limiting control includes steps S11, S12 and S15.

[0072] In step S11, the voltage value of the AC collection line and the two-side current phase difference value of the AC collection line are obtained.

[0073] In step S12, when the voltage value is less than the preset voltage threshold, the control mode of the converter station is switched to the low-voltage current limiting control mode, and the voltage reference value of the converter station is set to the initial voltage reference value.

[0074] The specific details of steps S11 and S12 can be referred to the description of steps S11 and S12 in the above Figure 3 , which will not be described here.

[0075] In step S15, when the two-side current phase difference value is less than or equal to the preset phase difference threshold, and the ratio braking coefficient of the converter station under the initial voltage reference value is greater than or equal to the preset coefficient threshold, it is determined that the current differential protection of the converter station acts.

[0076] It should be noted that when the two-side current phase difference value is less than or equal to the preset phase difference threshold, the ratio braking coefficient of the converter station under the initial voltage reference value is generally greater than or equal to the preset coefficient threshold, so the current differential protection of the converter station can be directly determined to act.

[0077] The current differential protection method based on low-voltage current limiting control provided by the embodiment of the application can quickly trigger the differential protection action to realize fast fault removal, and can avoid control fluctuation caused by frequent switching of the voltage reference value, and improve system operation stability.

[0078] For the convenience of understanding, the current differential protection method based on low-voltage current limiting control provided by the embodiment of the application is specifically described below. Figure 9 For the convenience of understanding, the current differential protection method based on low-voltage current limiting control provided by the embodiment of the application is specifically described below.

[0079] Figure 9 is another flowchart of the current differential protection method based on low-voltage current limiting control provided by the embodiment of the application, as shown in Figure 9 The overall flow of the current differential protection method based on low-voltage current limiting control is as follows: When it is detected that the new energy station through the AC collection line of the flexible transmission sending-out system fails, the converter station and each line switch start according to the low-voltage criterion, wherein the low-voltage criterion starts, and the criterion formula is: , is a safety voltage, is a real-time voltage, and the low-voltage starting criterion coefficient is 0.8.

[0080] If is not established, the real-time voltage is reacquired for judgment; if is established, the converter station fault control starts, that is, the control mode of the converter station is switched to the low-voltage current limiting control mode.

[0081] After the converter station is switched to the low-voltage current limiting control mode, the current phases of the two sides of the line are acquired, and it is judged whether the fault current phase difference of the two sides of the line is greater than a preset phase difference threshold value, wherein the preset phase difference threshold value is 90°: , wherein, is a B-phase short-circuit current phase provided by the fan side (the same principle for A-phase and C-phase, and the formula is taken as an example for B-phase), is a B-phase short-circuit current phase provided by the converter station side.

[0082] If is established, that is, when the fault current phase difference of the two sides of the line is less than or equal to 90°, the current differential protection ratio braking coefficient K value is greater than or equal to 0.8 at this time, the current differential protection action condition is met, and therefore the current differential protection correctly acts; if If not, the improved low-voltage current limiting protection mode is started, including: obtaining the positive sequence components of the currents on both sides of the line, that is, extracting the positive sequence components of the fault currents on both sides of the AC collection line and wherein is the positive sequence component of the short-circuit current provided by the fan side, is the positive sequence component of the short-circuit current provided by the converter station side; the extracted positive sequence current components are substituted into the above formula 1 to calculate the positive sequence voltage reference value of the converter station; the dq transformation is performed on the positive sequence voltage reference value to obtain the d-axis voltage reference value and the q-axis voltage reference value and transmit them to the converter station.

[0083] Further, under the action of the d-axis voltage reference value and the q-axis voltage reference value, the K value is calculated in real time, wherein the set value of the current differential protection ratio braking coefficient K value is 0.8, and the K value is calculated according to the above formula 3 based on the fan side current and the converter station side current after the action of the d-axis voltage reference value and the q-axis voltage reference value.

[0084] If the current differential protection ratio braking coefficient K value is greater than or equal to 0.8, it is an internal fault and the current differential protection action condition is met, and the current differential protection correctly acts; if the current differential protection ratio braking coefficient K value is less than 0.8, it is an external fault and the current differential protection action condition is not met, and the current differential protection does not act.

[0085] The embodiment of the application, when the converter station is in a low-voltage current limiting control mode, calculates and updates the dq-axis voltage reference value in real time according to the phase characteristics of the fault currents on both sides of the AC collection line, thereby reducing the phase angle difference between the currents on both sides, restoring the sensitivity of the differential protection, ensuring the correct action of the protection, and maintaining the safe and stable operation of the system.

[0086] In order to further verify the advantages of the current differential protection method based on low-voltage current limiting control provided by the embodiment of the application compared with the traditional current differential protection method, the action time, the ratio braking coefficient K curve and the bridge arm current instantaneous value curve of the two methods are compared under the conditions of metallic interphase faults and interphase faults through 30 Ω transition resistance, so as to quantitatively evaluate the technical effects of the embodiment in improving sensitivity and eliminating the dead zone of the protection during low-voltage current limiting.

[0087] Figure 10 It is shown that the K value during the metallic interphase fault in the traditional low-voltage current limiting mode is less than 0.8 throughout the process, and the protection is refused to act; Figure 11 and Figure 12It is shown that, after the method provided in the embodiment of the present application is used, when the converter station adopts the improved low-voltage current limiting control mode and the line has a metallic inter-phase fault, the K value is greater than the set value 0.8 within 1.404-1.414 s, the K value is greater than 0.8 within the interval of 1.404 s-1.414 s, the protection correctly acts, and the peak value of the bridge arm current (including the bridge arm current under the A phase, the B phase and the C phase) does not exceed 2 times the rated current, meeting the current limiting requirement. Figure 13 and Figure 14 It is further proved that, under the 30 Ω high-resistance inter-phase fault, the K value still exceeds the limit within 1.434 s-1.444 s, the protection reliably acts, and the bridge arm current (including the bridge arm current under the A phase, the B phase and the C phase) also does not exceed the limit. The K value is always lower than 0.8 under the external fault condition, and the protection does not misjudge. In summary, the method provided in the embodiment of the present application can reliably act the differential protection while maintaining the current limiting level during the low-voltage current limiting period.

[0088] The embodiment of the present application also provides a current differential protection system based on low-voltage current limiting control, which comprises an alternating current collection line and a converter station; wherein the alternating current collection line is used to transmit electric energy to the converter station, and the converter station is used to execute the steps of the current differential protection method based on low-voltage current limiting control provided in any of the above-mentioned embodiments.

[0089] In some embodiments, the converter station of the embodiment of the present application can specifically adopt a V / F control method (a motor speed regulation technology for maintaining a constant voltage-to-frequency ratio, V / f=C).

[0090] Under the condition that the converter station of the embodiment of the present application adopts the V / F control method, the control system of the converter station in the embodiment of the present application at least comprises a low-voltage criterion module, a converter station V / F control module, a fault current phase difference detection module, an improved low-voltage current limiting control mode module and a differential module. The low-voltage criterion module is connected with the converter station V / F control module and the fault current phase difference detection module; the fault current phase difference detection module is connected with the differential module; and the improved low-voltage current limiting control mode module is connected with the converter station V / F control module, the fault current phase difference detection module and the differential module.

[0091] The low-voltage criterion module is used to collect the line voltage and judge whether the alternating current collection line has a fault. Specifically, the low-voltage criterion module detects the system voltage level in real time during the system operation, judges whether the system has a fault, and transmits a fault signal to the converter station V / F control module and the fault current phase difference detection module.

[0092] The converter station V / F control module is configured to generate and send the control instruction of the converter station. Specifically, the new energy grid connection needs the converter station to provide stable voltage support. In normal operation, a constant AC voltage reference value is output. When receiving the fault signal sent by the low voltage criterion module, the converter station adopts a low voltage current limiting control mode to actively reduce the d-axis voltage reference value, so as to reduce the voltage and limit the fault current. When receiving the dq-axis voltage reference value sent by the improved low voltage current limiting control mode module, the voltage reference value in the original low voltage current limiting control mode is replaced by the dq-axis voltage reference value sent by the improved low voltage current limiting control mode module.

[0093] The fault current phase difference detection module is configured to collect the fault currents on both sides of the line and extract the phase, and then determine whether the phase difference of the fault currents on both sides is greater than a set value. Specifically, after receiving the fault signal sent by the low voltage criterion module, the fault current phase difference detection module collects the fault currents on both sides of the line and extracts the phase, and then determines whether the phase difference of the fault currents on both sides is greater than a set value. If it is greater than the set value, the signal is transmitted to the improved low voltage current limiting control module to start the improved low voltage current limiting control mode (i.e., the voltage reference value of the converter station is switched from the initial reference value to the target reference value); otherwise, the signal is transmitted to the differential module.

[0094] The improved low voltage current limiting control mode module is configured to generate the d-axis voltage reference value and the q-axis voltage reference value and transmit them to the converter station V / F control module. Further, the improved low voltage current limiting control mode module includes a fault current sequence component extraction module and a voltage reference value calculation module. The fault current sequence component extraction module is configured to extract the positive sequence component of the fault current on both sides of the line, and the voltage reference value calculation module is configured to give the d-axis voltage reference value and the q-axis voltage reference value. Specifically, after receiving the start signal sent by the fault current phase difference detection module, the improved low voltage current limiting control mode module extracts the positive sequence component of the fault current on both sides of the AC collection line, and then substitutes the extracted positive sequence current component into the corresponding formula to calculate the positive sequence voltage reference value of the converter station. Then, the positive sequence voltage reference value is dq-transformed to obtain the d-axis voltage reference value and the q-axis voltage reference value. The obtained dq-axis voltage reference value is transmitted to the converter station V / F control module, and the voltage reference value in the original low voltage current limiting control mode is replaced by the dq-axis voltage reference value obtained by the above method.

[0095] The differential module is configured to calculate the K value and determine whether the protection can act.

[0096] The specific details and benefits of the current differential protection system based on low voltage current limiting control provided by the embodiments of the present application can be referred to the description of the current differential protection method based on low voltage current limiting control described above, which will not be described here.

[0097] Figure 15is a structural block diagram of a current differential protection device based on low-voltage current limiting control provided by an embodiment of the present application, as shown in the figure, the device 100 comprises an acquisition module 110, a control mode switching module 120, a voltage reference value switching module 130, and a current differential protection module 140. Figure 15

[0098] The acquisition module 110 is configured to acquire a voltage value of the AC collection line and a phase difference value of the two sides of the AC collection line. The control mode switching module 120 is configured to switch the control mode of the converter station to a low-voltage current limiting control mode when the voltage value is less than a preset voltage threshold, and set the voltage reference value of the converter station as an initial voltage reference value. The voltage reference value switching module 130 is configured to determine a target voltage reference value of the converter station based on the positive sequence components of the two sides of the AC collection line when the phase difference of the two sides of the AC collection line is greater than a preset phase difference threshold, and switch the voltage reference value of the converter station from the initial voltage reference value to the target voltage reference value. The current differential protection module 140 is configured to determine the current differential protection action or protection inaction of the converter station based on the relationship between the ratio restraint coefficient of the converter station under the target voltage reference value and a preset coefficient threshold.

[0099] The specific details and benefits of the current differential protection device based on low-voltage current limiting control provided by the embodiment of the present application can be referred to the description of the current differential protection method based on low-voltage current limiting control above, and will not be repeated here.

[0100] Finally, it should be noted that the above content is only used to illustrate the technical solutions of the present application, and is not a limitation on the protection scope of the present application. Simple modifications or equivalent replacements of the technical solutions of the present application made by those skilled in the art do not deviate from the essence and scope of the technical solutions of the present application.​

Claims

1. A current differential protection method based on low-voltage current limiting control, characterized in that, The method includes: Obtain the voltage value of the AC collection line and the phase difference value of the current on both sides of the AC collection line; When the voltage value is less than the preset voltage threshold, the control mode of the converter station is switched to the low-voltage current limiting control mode, and the voltage reference value of the converter station is set to the initial voltage reference value. When the phase difference between the currents on both sides is greater than a preset phase difference threshold, the target voltage reference value of the converter station is determined based on the positive sequence components of the currents on both sides of the AC collection line, and the voltage reference value of the converter station is switched from the initial voltage reference value to the target voltage reference value; and Based on the relationship between the ratio braking coefficient of the converter station under the target voltage reference value and the preset coefficient threshold, the current differential protection of the converter station is determined to operate or not operate.

2. The current differential protection method based on low-voltage current limiting control according to claim 1, characterized in that, Determining the target voltage reference value of the converter station based on the positive sequence components of the currents on both sides of the AC collection line includes: Obtain the positive sequence components of the current on both sides of the AC collection line; Based on the positive sequence components of the currents on both sides, the positive sequence voltage reference value of the converter station is determined; The target voltage reference value is obtained by performing a dq transformation on the positive sequence voltage reference value.

3. The current differential protection method based on low-voltage current limiting control according to claim 2, characterized in that, The positive sequence components of the current on both sides include the positive sequence component of the current on the wind turbine side and the positive sequence component of the current on the converter station side. The determination of the positive-sequence voltage reference value of the converter station based on the positive-sequence components of the currents on both sides includes: Obtain the transition resistance of the AC collection line and the negative sequence equivalent impedance of the converter station; Based on the positive sequence component of the current on the wind turbine side and the positive sequence component of the current on the converter station side, the difference between the positive sequence components of the current on both sides of the AC collection line is determined. The positive sequence voltage reference value is determined based on the difference in the positive sequence components of the currents on both sides, the transition resistance, and the negative sequence equivalent impedance.

4. The current differential protection method based on low-voltage current limiting control according to claim 3, characterized in that, The positive sequence voltage reference value is determined using the following formula: ， in, This is the positive sequence voltage reference value. This represents the positive sequence component of the current on the fan side. This represents the positive sequence component of the converter station side current. Transition resistor for AC convergence lines, This is the negative sequence equivalent impedance of the converter station.

5. The current differential protection method based on low-voltage current limiting control according to claim 3, characterized in that, The transition resistance of the AC junction line is determined by the following formula. : ， in, Transition resistor for AC convergence lines, and These are the measured impedances of the impedance relays at both ends of the AC convergence line. The impedance value of the AC collection line. For converter station side current, This refers to the current on the fan side.

6. The current differential protection method based on low-voltage current limiting control according to any one of claims 1 to 5, characterized in that, The determination of whether the current differential protection of the converter station should operate or not operate based on the relationship between the ratio braking coefficient of the converter station under the target voltage reference value and the preset coefficient threshold includes: When the ratio braking coefficient is greater than or equal to the preset coefficient threshold, the current differential protection of the converter station is determined to operate. When the ratio braking coefficient is less than the preset coefficient threshold, the current differential protection of the converter station is determined not to operate.

7. The current differential protection method based on low-voltage current limiting control according to claim 6, characterized in that, The ratio braking coefficient is determined using the following formula: ， in, This is the ratio braking coefficient. For converter station side current, This refers to the current on the fan side.

8. The current differential protection method based on low-voltage current limiting control according to claim 1, characterized in that, The method further includes: When the phase difference between the currents on both sides is less than or equal to a preset phase difference threshold, and the ratio braking coefficient of the converter station under the initial voltage reference value is greater than or equal to a preset coefficient threshold, the current differential protection of the converter station is determined to operate.

9. A current differential protection system based on low-voltage current limiting control, characterized in that, It includes an AC collection line and a converter station; wherein the AC collection line is used to transmit electrical energy to the converter station, and the converter station is used to perform the steps of the current differential protection method based on low-voltage current limiting control as described in any one of claims 1 to 8.

10. A current differential protection device based on low-voltage current limiting control, characterized in that, The device includes: The acquisition module is used to acquire the voltage value of the AC collection line and the phase difference value of the current on both sides of the AC collection line; The control mode switching module is used to switch the control mode of the converter station to the low-voltage current limiting control mode when the voltage value is less than the preset voltage threshold, and to set the voltage reference value of the converter station to the initial voltage reference value. A voltage reference value switching module is used to determine the target voltage reference value of the converter station based on the positive sequence components of the currents on both sides of the AC collection line when the phase difference between the currents on both sides is greater than a preset phase difference threshold, and to switch the voltage reference value of the converter station from the initial voltage reference value to the target voltage reference value; and The current differential protection module is used to determine whether the current differential protection of the converter station should operate or not operate based on the relationship between the ratio braking coefficient of the converter station under the target voltage reference value and the preset coefficient threshold.

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