A Differential Protection Method and System for M3C Frequency Converter

Abstract

The present invention belongs to the technical field of low-frequency power transmission, and specifically relates to a differential protection method and system for an M3C frequency converter. In the M3C frequency converter area, a low-frequency side differential protection and a power-frequency side differential protection are respectively configured. Based on the characteristic that the arm current itself contains both low-frequency components and power-frequency components, differential protection criteria based on the sampled values of the phase-separated fault components are respectively constructed. Specifically, the arm current, the valve side current on the power-frequency side, and the valve side current on the low-frequency side are collected; a fast differential protection starting criterion based on the change in differential current is constructed; a differential protection with the differential current fault component as the action quantity and the maximum value of the absolute value of the sum of the valve side current and the arm current as the braking value is constructed as the main criterion; when the differential protection starting criterion and the differential main criterion are both satisfied, the differential protection is executed. Thus, the present invention solves the problems of poor quick-acting performance and low reliability of the criterion for the differential protection of the M3C frequency converter for offshore low-frequency power transmission.

Description

Technical Field

[0001] The present invention belongs to the technical field of low-frequency power transmission, and particularly relates to a differential protection method and system for an M3C frequency converter. Background Art

[0002] In recent years, the development of offshore wind power has been rapid, showing a development trend from nearshore wind power to long-distance offshore wind power. The existing power transmission solutions mainly include: high-voltage power-frequency AC power transmission and flexible DC transmission solutions. However, these two power transmission solutions have the disadvantages of insufficient transmission distance (economical within 100 km) and high construction cost respectively.

[0003] The offshore low-frequency power transmission technology can make up for the disadvantages of insufficient transmission distance of high-voltage power-frequency AC power transmission and high construction cost of DC power transmission. Compared with the high-voltage power-frequency AC power transmission technology, since the transmission distance of submarine cable power transmission is inversely proportional to the frequency used, by reducing the power transmission frequency, the transmission distance can be increased. For example, when the power transmission frequency is reduced to about 15 Hz, the economically reasonable transmission distance of the submarine cable can reach 300 km, and the transmission distance is significantly increased; compared with high-voltage DC power transmission, by only building one converter station on land without building a station at sea, the construction and maintenance costs will be greatly reduced. Therefore, the low-frequency power transmission technology can completely solve the problem of transmitting power from far-offshore wind farms.

[0004] The modular multilevel matrix converter (M3C) adopts a cascaded H-bridge architecture and has advantages such as high modularity and strong scalability. It is one of the most critical devices in the low-frequency power transmission system. At the same time, since the M3C also has the disadvantage of weak current tolerance of sub-modules, a short-circuit fault in the frequency converter area will quickly cause overcurrent in the sub-module devices. Although the device-level overcurrent protection can quickly detect the fault, it cannot locate the fault area, which is not convenient for maintenance. Therefore, designing a differential protection for the frequency converter area with high reliability and fast speed is crucial for improving the safe and stable operation of the offshore low-frequency power transmission system.

[0005] At present, the application of offshore low-frequency power transmission projects is relatively few and is in the research stage. There is no mature converter protection strategy. The general idea is to configure two types of protection: the device-level body overcurrent protection and the system-level differential protection based on the effective value method. However, the device-level body overcurrent protection has good quick-acting performance but has the following problems: 1) It has no selectivity and cannot locate the fault area. Faults outside the converter area in the system will also cause the body overcurrent protection to act; 2) It has poor anti-interference ability. To ensure quick-acting performance, usually only two sampling points are judged to meet the setting value, that is, the action outlet. The system-level differential protection method based on the effective value can locate faults, but has poor quick-acting performance. Especially in the low-frequency region, compared with the power frequency region, the time of one cycle is longer, and the calculation time of the effective value will increase significantly, resulting in even worse quick-acting performance. It usually takes dozens of milliseconds to act and is difficult to cooperate with the device-level body overcurrent protection. Summary of the Invention

[0006] The purpose of the present invention is to provide a differential protection method for the M3C converter, which is used to solve the problems of poor quick-acting performance and low reliability of the criterion for the differential protection of the M3C converter for offshore low-frequency power transmission; a differential protection system for the M3C converter with good quick-acting performance and high reliability of the criterion is also provided.

[0007] To solve the above technical problems, the technical solutions provided by the present invention and the corresponding beneficial effects of the technical solutions are as follows:

[0008] A differential protection method for the M3C converter of the present invention includes the following steps:

[0009] When the power transmission system is working, for any one phase of the valve side three-phase on one side of the M3C converter and the valve side three-phase on the other side, denoted as phase x, the following processing is performed:

[0010] 1) Obtain current data, where the current data includes the valve side current i x , of phase x, and the three-phase bridge arm currents of the M3C converter connected to phase x;

[0011] 2) According to the current data, judge whether the number of times that meets the main criterion of the differential protection corresponding to phase x in R consecutive sampling operations is greater than or equal to S times, and whether it meets the differential protection start criterion. If both are satisfied, perform the corresponding differential protection on the valve side phase x; R≥S>0;

[0012] Among them, the differential protection start criterion formula for the valve side phase x is as follows:

[0013] ΔIdif_x(s)=Idif_x(s)-Idif_x(s-N)

[0014] |ΔIdif_x(s)|>Δ1

[0015] Among them, Idif_x(s) is the calculated value of the differential current at the s-th sampling point; ΔIdif_x(s) is the calculated value of the change in differential current at the s-th sampling point, and Idif_x(s-N) is the calculated value of the differential current at the s-th sampling point; Δ1 is the starting threshold value;

[0016] The main criterion formula for differential protection corresponding to the valve side x is as follows:

[0017] |ΔIdif_x(s)|>MAX[k*MAX(|Δi x (s)|,|Δi xsum (s)|),Δ2]

[0018] Δi x (s)=i x (s)-i x (s-N)

[0019] Δi xsum (s)=i xsum (s)-i xsum (s-N)

[0020] Among them, Δi x (s) is the change in the current of phase x on the valve side; i x (s) is the sampling value of the current of phase x on the valve side at the s-th sampling point; i x (s-N) is the sampling value of the current of phase x on the valve side at the (s-N)-th sampling point; Δi xsum (s) is the change in the sum of the currents of the three-phase bridge arms corresponding to phase x on the valve side; i xsum (s) is the sum of the currents of the three-phase bridge arms corresponding to phase x on the valve side at the s-th sampling point; i xsum (s-N) is the sum of the currents of the three-phase bridge arms corresponding to phase x on the valve side at the (s-N)-th sampling point; ΔIdif_x(s) is the calculated value of the change in differential current at the s-th sampling point; k is the ratio coefficient; Δ2 is the threshold value of the main criterion for differential protection.

[0021] The beneficial effects of the above technical solution are as follows: Based on the arm current characteristics of the M3C frequency converter, the present invention proposes a differential protection method for the M3C frequency converter area. This differential protection method adopts the ratio restraint type, uses the arm current and the valve-side current on the low-frequency side or the valve-side current on the power-frequency side to construct a differential protection startup criterion based on the change amount of the differential current, so as to be able to achieve a rapid response to faults. It also proposes a main criterion for differential protection based on the ratio restraint method. The main criterion uses the absolute value of the change amount of the differential current as the action quantity, and uses the maximum value of the change amount of the valve-side current and the absolute value of the change amount of the sum of the arm currents as the restraint quantity. By counting the satisfaction of the main criterion at each sampling point, after the differential protection main criterion formula is satisfied S times in R consecutive sampling operations, the differential protection main criterion condition can be satisfied, improving the accuracy and reliability of the main criterion. In addition, the method of the present invention is simple, the setting of the fixed value is easy, the action speed is fast, and the sampling frequency requirement for the current measurement point is not high (10 kHz is sufficient), which is convenient for engineering implementation.

[0022] Further, for all phases of the valve side three-phase on one side of the M3C frequency converter and the valve side three-phase on the other side, the methods of steps 1) and 2) are used for differential protection.

[0023] The beneficial effects of the above technical solution are as follows: The present invention also proposes to configure differential protection on the low-frequency side and differential protection on the power-frequency side in the M3C frequency converter area. By using the selectivity of differential protection, reliable detection of different fault areas of the M3C frequency converter is achieved, and both the low-frequency side differential protection and the power-frequency side differential protection are configured by phase, that is, when any one of the three phases (u, v, w) on the low-frequency side simultaneously satisfies the differential protection startup criterion and the main criterion for the corresponding phase, the differential protection can act and trip, and the specific location of the fault can be determined.

[0024] Further, in order to improve the quick operation performance on the basis of ensuring accuracy, R = 10, 6 ≤ S ≤ 8.

[0025] Further, the following formula is used to obtain the differential current Idif_x of the x phase on the valve side:

[0026] Idif_x(s) = i x (s) - i xsum (s)

[0027] i xsum (s) = i 1 x (s) + i 2 x (s) + i 3 x (s)

[0028] where, i x (s) is the sampling value of the valve-side x-phase current at the s-th sampling point; i 1x (s), i 2 x (s), i 3 x (s) are the sampled values of the three-phase bridge arm currents corresponding to x; i xsum (s) is the sum of the sampled values of the three-phase bridge arm currents corresponding to the valve side x.

[0029] Furthermore, the starting threshold value Δ1 is the product of the maximum value of the calculated value ΔIdif_x(s) of the differential current change amount at the sampling moment during an external fault and a sensitivity coefficient greater than 1.

[0030] Furthermore, the value range of the sensitivity coefficient is from 1.2 to 1.5; the threshold value Δ2 of the main criterion of the differential protection is 0.05 pu.

[0031] Furthermore, the starting threshold value Δ1 is 0.1 pu.

[0032] Furthermore, the ratio coefficient k is 0.2.

[0033] The present invention also provides a differential protection system for an M3C frequency converter, which system includes: a processing module and a data acquisition module; the data acquisition module is used to acquire the three-phase currents on the power frequency side valve side, the bridge arm currents of the M3C frequency converter, and the valve side currents on the low frequency side; the processing module is sampled and connected to the data acquisition module; the processing module is used to execute computer instructions to implement a differential protection method for an M3C frequency converter according to the present invention.

[0034] The beneficial effects of the above technical solution are as follows: Based on the arm current characteristics of the M3C frequency converter, the system includes a processing module and a data acquisition module. The data acquisition module is used to collect the three-phase currents on the power frequency side valve side, the arm currents of the M3C frequency converter, and the valve side current on the low-frequency side. The processing module processes the data collected by the data acquisition module to implement the differential protection of the M3C frequency converter circuit. The differential protection method adopts the ratio restraint type, uses the arm current and the valve side current on the low-frequency side or the valve side current on the power frequency side to construct a differential protection starting criterion based on the change in differential current, so as to be able to achieve a rapid response to faults. A main criterion for differential protection based on the ratio restraint method is also proposed. The main criterion uses the absolute value of the change in differential current as the action quantity, uses the maximum value of the change in valve side current and the absolute value of the change in the sum of arm currents as the braking quantity, and by counting the satisfaction of the main criterion at each sampling point, after the differential protection main criterion formula is satisfied S times in R consecutive sampling operations, the differential protection main criterion condition can be satisfied, improving the accuracy and reliability of the main criterion. In addition, the method of the present invention is simple, the setting of fixed values is easy, the action speed is fast, and the sampling frequency requirement for current measurement points is not high (10 kHz is sufficient), which is convenient for engineering implementation. Therefore, the present invention provides an M3C frequency converter differential protection system with accurate and rapid fault location. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 is a flowchart of a differential protection method for an M3C frequency converter according to the present invention;

[0036] Figure 2 is a schematic diagram of a submarine power transmission system in an embodiment of an M3C frequency converter differential protection system according to the present invention;

[0037] Figure 3 is a schematic diagram of the differential protection area and measurement point configuration of an M3C frequency converter in an embodiment of an M3C frequency converter differential protection system according to the present invention;

[0038] Figure 4 is the waveform of the arm current i au in an embodiment of an M3C frequency converter differential protection system according to the present invention;

[0039] Figure 5 is a flowchart of a differential protection method for the low-frequency side in an embodiment of an M3C frequency converter differential protection system according to the present invention;

[0040] Figure 6 is a block diagram of the action output of differential protection for the low-frequency side in an embodiment of an M3C frequency converter differential protection system according to the present invention;

[0041] Figure 7 is a block diagram of the action output of differential protection for the power frequency side in an embodiment of an M3C frequency converter differential protection system according to the present invention;

[0042] Figure 8 It is a structural block diagram of an implementation manner of the M3C frequency converter differential protection system in an embodiment of the M3C frequency converter differential protection system of the present invention. Specific implementation manner

[0043] The present invention provides an M3C frequency converter differential protection method. The main purpose is to solve the technical problems that only the body overcurrent protection is configured for the M3C frequency converter, and the fault location cannot be determined, or the differential protection based on the effective value criterion has poor quick-acting performance. In this method, a low-frequency side differential protection and a power-frequency side differential protection are respectively configured in the M3C frequency converter area, covering the entire M3C frequency converter area. Based on the characteristics that the arm current itself contains both low-frequency components and power-frequency components, differential protection criteria based on the sampled values of the phase-fault components are respectively constructed. The specific steps mainly include: collecting the arm current, the valve side current on the power-frequency side, and the valve side current on the low-frequency side; constructing a fast differential protection startup criterion based on the change amount of the differential current; constructing a differential protection with the differential current fault component as the action quantity and the maximum value of the valve side current and the absolute value of the sum of the arm currents as the braking value as the main criterion; when the differential protection startup criterion is satisfied, the differential protection main criterion is put into operation, and in combination with a counter, the satisfaction situation of the main criterion is judged point by point at each sampling point. After repeated judgments at multiple sampling points, the differential protection main criterion can be satisfied; when both the differential protection startup criterion and the differential main criterion are satisfied, the differential protection can finally act and issue an instruction to block the M3C frequency converter and trip the AC circuit breaker. Therefore, this method has high reliability, good quick-acting performance, and is simple. It can be used as the main protection for the system-level protection in the frequency converter area, and cooperate with the body protection at the device level of the M3C frequency converter to achieve rapid detection and positioning of the fault area.

[0044] In order to make the purpose, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments.

[0045] An embodiment of an M3C frequency converter differential protection system:

[0046] An embodiment of the M3C frequency converter differential protection system of the present invention. This system is optimally applied to the offshore low-frequency power grid. The offshore low-frequency power grid includes one side of an offshore platform, such as Figure 2As shown in the figure, the system includes: a wind turbine group, a low-frequency step-up transformer, and a low-frequency power transmission line, which are located on one side of the onshore platform. The offshore low-frequency power grid further includes an M3C frequency converter, a starting circuit, and a coupling transformer; the low-voltage side of the low-frequency step-up transformer is connected to the wind turbine group, and the high-voltage side is connected to the low-frequency side of the M3C frequency converter through a low-frequency line; the industrial-frequency side of the M3C frequency converter is connected to the coupling transformer, and the other side of the coupling transformer is used to connect to the receiving-end industrial-frequency power grid, that is, the coupling transformer is located between the M3C frequency converter and the receiving-end industrial-frequency power grid and is used to realize the exchange of energy and voltage. Specifically, the wind turbine group includes multiple wind turbines, all of which are used for wind power generation. The low-frequency step-up transformer boosts the electricity transmitted by the wind turbine group and then transmits it to the M3C frequency converter through a low-frequency line. After being frequency-converted by the M3C frequency converter, it is then transmitted to the coupling transformer through the starting circuit, and the coupling transformer exchanges the voltage to the receiving-end industrial-frequency power grid. In this embodiment, the system includes a processing module and a data acquisition module; the data acquisition module uses current sensors to collect the three-phase currents on the valve side of the industrial-frequency side, the arm currents of the M3C frequency converter, and the currents on the valve side of the low-frequency side; the processing module is sampled and connected to each current sensor, and multiple current sensors are arranged at each measuring point of the M3C frequency converter. Among them, the protection scope of the system and the required measuring point configuration are as Figure 3 shown. In this embodiment, each sensor uses an optical CT (current transformer), and the sensors at each measuring point ( Figure 3 the sensors are not drawn in the figure) are respectively used to collect the three-phase currents on the valve side of the low-frequency side (i u , i v , i w ), the three-phase currents on the industrial-frequency side (i a , i b , i c ), and the nine-phase arm currents (i au , i av , i aw , i bu , i bv , i bw , i cu , i cv , i cw)。The processing module is used to execute computer instructions to implement a differential protection method for the M3C frequency converter of the present invention, so as to determine the corresponding protection action, and send a command to lock the M3C frequency converter and a command to trip the AC circuit breaker to the control system of the M3C frequency converter. The main idea of a differential protection method for the M3C frequency converter of the present invention is as follows: The differential protection method is based on the characteristic that the arm current itself contains both low-frequency components and power-frequency components. For any one of the low-frequency side valve side u-phase, v-phase, and w-phase and the power-frequency side valve side a-phase, b-phase, and c-phase, denoted as the x-phase, a differential protection criterion based on the sampled values of the phase-separated fault components is constructed respectively, and then the differential protection is executed according to the main differential protection criterion. Among them, when executing the differential protection according to the main differential protection criterion, it is judged whether the number of times that the main differential protection criterion corresponding to the x-phase is satisfied in R consecutive sampling operations is greater than or equal to S times, and whether the differential protection startup criterion is satisfied. If both are satisfied, the corresponding differential protection is executed for the valve side x-phase; R≥S>0.

[0047] First, it is necessary to construct a fast differential protection startup criterion based on the change in differential current. The formula for this differential protection startup criterion is as follows:

[0048] The formula for the differential protection startup criterion corresponding to the valve side x-phase is as follows:

[0049] ΔIdif_x(s)=Idif_x(s)-Idif_x(s-N)

[0050] |ΔIdif_x(s)|>Δ1

[0051] Among them, Idif_x(s) is the calculated value of the differential current at the s-th sampling point; ΔIdif_x(s) is the calculated value of the change in differential current at the s-th sampling point, and Idif_x(s-N) is the calculated value of the differential current at the s-N-th sampling point; Δ1 is the startup threshold value;

[0052] Then, a differential protection with the fault component of the differential current as the action quantity and the maximum value of the valve side current and the absolute value of the sum of the arm currents as the braking value is constructed as the main criterion. The formula for this differential protection main criterion is as follows:

[0053] The differential protection main criterion corresponding to the valve side x-phase is expressed by the following formula:

[0054] |ΔIdif_x(s)|>MAX[k*MAX(|Δi x (s)|,|Δi xsum (s)|),Δ2]

[0055] Δi x (s)=i x (s)-i x (s-N)

[0056] Δi xsum (s) = i xsum (s) - i xsum (s - N)

[0057] Among them, Δi x (s) is the change in the valve-side x-phase current; i x (s) is the sampled value of the valve-side x-phase current at the s-th sampling point; i x (s - N) is the sampled value of the valve-side x-phase current at the (s - N)-th sampling point; Δi xsum (s) is the change in the sum of the three-phase bridge-arm currents corresponding to the valve-side x-phase; i xsum (s) is the sum of the three-phase bridge-arm currents corresponding to the valve-side x-phase at the s-th sampling point; i xsum (s - N) is the sum of the three-phase bridge-arm currents corresponding to the valve-side x-phase at the (s - N)-th sampling point; ΔIdif_x(s) is the calculated value of the differential current change at the s-th sampling point; k is the ratio coefficient; Δ2 is the threshold value of the main criterion for differential protection.

[0058] Among them, the valve-side x-phase differential current Idif_x is obtained by the following formula:

[0059] Idif_x(s) = i x (s) - i xsum (s)

[0060] i xsum (s) = i 1 x (s) + i 2 x (s) + i 3 x (s)

[0061] Among them, i x (s) is the sampled value of the x-phase current at the s-th sampling point; i 1 x (s), i 2 x (s), i 3 x (s) are respectively the sampled values of the three-phase bridge-arm currents corresponding to x-phase; i xsum (s) is the sum of the sampled values of the three-phase bridge-arm currents corresponding to x-phase. When the low-frequency side valve side is selected, x-phases are the three phases of the low-frequency side valve side, i 1 x (s), i 2 x (s), i 3 x(s) are also the arm currents connected to the valve side of the low-frequency side respectively. Taking the u-phase of the x-phase as the low-frequency side u-phase, the above differential current Idif_x can also be written as the formula corresponding to the differential current Idif_u of the u-phase of the low-frequency side in step (2) below. When the valve side of the power frequency side is selected, then the x-phase is the three phases of the valve side of the power frequency side, i 1 x (s), i 2 x (s), i 3 x (s) are also the arm currents connected to the valve side of the power frequency side respectively. Taking the a-phase of the x-phase as the power frequency side a-phase, the above differential current Idif_x can also be written as the formula corresponding to the differential current Idif_a of the a-phase of the power frequency side in step (2) below.

[0062] A differential protection method for an M3C frequency converter of the present invention specifically includes the following steps, as Figure 5 shown:

[0063] (1) Analog quantity acquisition. Acquire the currents i u , i v , i w of the three phases of the valve side of the low-frequency side (defined as the u, v, and w phases), each phase only contains low-frequency components; acquire the currents i a , i b , i c of the three phases of the valve side of the power frequency side (defined as the a, b, and c phases), each phase only contains power frequency components; acquire the 9-phase arm currents i au , i av , i aw , i bu , i bv , i bw , i cu , i cv , i cw of the M3C frequency converter. The arm current of each phase is the superposition of power frequency components and low-frequency components. As Figure 4 shown is the waveform of the arm current i au , which contains both power frequency components and low-frequency components. The sum i au , i bu , i cu of the three-phase arm currents of the low-frequency side only contains low-frequency components (the frequency in the waveform is 20 Hz); the sum i usum of the three-phase arm currents of the power frequency side only contains power frequency components. au , i av , i aw ; the sum i asum of these currents only contains power frequency components.

[0064] (2) Differential current calculation. Calculate the differential current on the low-frequency side and the differential currents of each phase on the power-frequency side respectively. For the low-frequency side, taking phase u as an example (the same applies to phases v and w), calculate the arm current i au 、i bu 、i cu of the three phases, and the sum current i usum . This sum current only contains low-frequency components. Subtract it from the valve-side current i u on the low-frequency side to obtain the differential current Idif_u of phase u on the low-frequency side. The calculation formula is:

[0065] i usum (s) = i au (s) + i bu (s) + i cu (s)

[0066] Idif_u(s) = i u (s) - i usum (s)

[0067] For the power-frequency side, taking phase a as an example (the same applies to phases b and c), calculate the arm current i au 、i av 、i aw of the three phases, and the sum current i asum . This sum current only contains power-frequency components. Subtract it from the valve-side current i a on the power-frequency side to obtain the differential current Idif_a of phase a on the power-frequency side. The calculation formula is:

[0068] i asum (s) = i au (s) + i av (s) + i aw (s)

[0069] Idif_a(s) = i a (s) - i asum (s)

[0070] (3) Construction of differential protection startup criterion. Construct the differential protection startup criterion on the low-frequency side and the fast startup criterion of differential protection on the power-frequency side respectively. The construction methods of the differential protection startup criteria on the low-frequency side and the power-frequency side are the same. Taking the differential current of phase u on the low-frequency side as an example for illustration (the same applies to phases v and w on the low-frequency side and all phases on the power-frequency side), calculate the change amount of the s-Nth sampling point of the differential current, and construct a fast startup criterion based on the change amount of the differential current. The calculation formula is:

[0071] i usum (s) = i au (s) + i bu (s) + i cu (s)

[0072] Idif_u(s) = iu (s)-i usum (s)

[0073] ΔIdif_u(s) = Idif_u(s) - Idif_u(s - N)

[0074] |ΔIdif_u(s)| > Δ1

[0075] where i au (s), i bu (s), i cu (s) are the sampled values of the arm current connected to phase u respectively; i usum (s) is the sum of the sampled values of the arm current connected to phase u; Idif_u(s) is the calculated value of the differential current at the s-th sampling point; i u (s) is the sampled value of the phase u current of the valve side current on the low-frequency side at the s-th sampling point; ΔIdif_u(s) is the calculated value of the change in differential current at the s-th sampling point, and Idif_u(s - N) is the calculated value of the differential current at the (s - N)-th sampling point; Δ1 is the starting threshold value, which can be determined by multiplying the maximum value of ΔIdif_u(s) during external faults by a sensitivity factor greater than 1. The sensitivity factor is usually taken as 1.2 to 1.5, and a typical starting threshold value Δ1 can be taken as 0.1 pu.

[0076] (4) Construction of the main criterion for differential protection. After the starting criterion for differential protection is satisfied, the main criterion for differential protection based on the ratio restraint method will be put into operation. The main criterion uses the absolute value of the change in differential current as the operating quantity, and the maximum value of the change in valve side current and the absolute value of the change in the sum of arm currents as the restraint quantity. Taking the phase u of the differential current on the low-frequency side as an example for illustration (the same applies to phases v and w on the low-frequency side and the power frequency side), the calculation formula is:

[0077] Δi u (s) = i u (s) - i u (s - N)

[0078] Δi usum (s) = i usum (s) - i usum (s - N)

[0079] |ΔIdif_u(s)| > MAX[k * MAX(|Δi u (s)|, |Δi usum (s))|, Δ2]

[0080] where Δi u (s) is the change in phase u of the valve side current on the low-frequency side; i u (s) is the sampled value of the phase u of the valve side current on the low-frequency side at the s-th sampling point; iu (s-N) is the sampling value of the s-Nth sampling point of the valve-side current u-phase on the low-frequency side; Δi usum (s) is the change in the sum of the u-phase currents of the bridge arm; i usum (s) is the sum of the u-phase currents of the bridge arm at the s-th sampling point; i usum (s-N) is the sum of the u-phase currents of the bridge arm at the s-Nth sampling point; ΔIdif_x(s) is the calculated value of the change in differential current at the s-th sampling point, which is used as the operating quantity of the u-phase differential protection; k is the ratio coefficient, and its value range is 0.1 - 0.5. In this embodiment, k takes 0.2; MAX(|Δi u (s)|,|Δi usum (s)|) is the braking value, taking the maximum value between Δi u (s) and i usum (s); Δ2 is the threshold value of the main criterion for differential protection. This threshold value is used to prevent the misoperation of this criterion when the system is disturbed and can take a relatively small value, such as 0.05 pu.

[0081] (5) Determination of the satisfaction condition of the main criterion for differential protection. To improve the anti-interference ability of the main criterion, a counter is used to count the satisfaction situation of the main criterion at each sampling moment: if the criterion is satisfied at the current moment, the count value is incremented by 1; otherwise, the count value remains unchanged. The upper limit value of the counter is the set value R. In this embodiment, R = 10, and the lower limit value is denoted as 0. If the criterion is satisfied S times in R consecutive operations, then the main criterion for differential protection is satisfied, where R > S. R can take 10, and S can take 6 to 8. Under the condition that the starting criterion is satisfied, by repeating the satisfaction times S in R samplings within the set period, it can be considered that both the starting criterion and the main criterion are satisfied, thereby improving the rapidity of differential protection.

[0082] Specifically, in this embodiment, after the starting criterion is satisfied, a time window will be opened, for example, 1 ms. The main criterion must complete the judgment within 1 ms. Then the sampling frequency is 10 kHz, the operating period is 0.1 ms, and there are 10 sampling points corresponding to 1 ms, that is, R = 10. After 1 ms, the main criterion will exit until the next starting criterion is satisfied and a new round of discrimination is carried out. Judge the number of times the main criterion is satisfied in 10 samplings. If the number of times the main criterion is satisfied is greater than or equal to S times, it is considered that both the main criterion and the starting criterion are satisfied. S times means that there are a total of S times satisfied in R times. S times itself does not have to be continuous, as long as the total number reaches S.

[0083] (6) Differential protection action output. Such as Figure 6 、 Figure 7As shown, when the differential protection startup criterion and the differential main criterion are both satisfied, the differential protection (if the fault occurs in the low-frequency side area, the low-frequency side differential protection operates; if the fault occurs on the power frequency side, the power frequency side differential protection operates) finally operates and trips, sending a command to lock the M3C frequency converter and a command to trip the AC circuit breaker.

[0084] Based on the arm current characteristics of the M3C frequency converter, the present invention proposes a differential protection method for the M3C frequency converter. This differential protection method adopts a ratio restraint type, uses the valve side current on the low-frequency side, the valve side current on the power frequency side, and the arm current to construct a startup criterion based on the change in differential current and a differential protection main criterion of the ratio restraint method, and uses a counter to count the satisfaction of the main criterion at each sampling point. After repeated judgments at multiple points, the main criterion can be satisfied, improving the reliability of the differential protection method. At the same time, the method uses sampling value calculation. By repeating the satisfaction times S in R samplings within a set period, it can be considered that both the startup criterion and the main criterion are satisfied simultaneously, which can improve the quick-acting performance of the differential protection. In addition, the method of the present invention is simple, easy to set the fixed value, has a fast action speed, and has a low requirement for the sampling frequency of the current measurement point (10 kHz is sufficient), which is convenient for engineering implementation.

[0085] Specifically, the present invention has the following advantages:

[0086] 1) It proposes to configure differential protection on the low-frequency side and the power frequency side in the M3C frequency converter area, covering the entire M3C frequency converter area, and uses the selectivity of differential protection to realize reliable detection of different fault areas in the M3C frequency converter.

[0087] 2) Based on the arm current characteristics, it proposes startup criteria for the low-frequency side and the power frequency side based on the change in sampled value differential current, realizing a fast response to faults; there is no problem of long calculation time and poor quick-acting performance caused by the fact that one cycle in the low-frequency area is longer than that in the power frequency area.

[0088] 3) It proposes a differential protection main criterion based on the ratio restraint method. The main criterion uses the absolute value of the change in differential current as the action quantity, uses the maximum value of the change in valve side current and the absolute value of the change in the arm sum current as the restraint quantity, and uses a counter to count the satisfaction of the main criterion at each sampling point. After repeated judgments at multiple points, the conditions of the differential protection main criterion can be satisfied, improving the reliability and accuracy of the main criterion.

[0089] 4) The differential protection on the low-frequency side and the differential protection on the power frequency side are both configured by phase. That is, when any one of the three phases (u, v, w) on the low-frequency side simultaneously satisfies the startup criterion and the main criterion of the corresponding phase, this differential protection can operate and trip, sending a command to lock the M3C frequency converter and a command to trip the AC circuit breaker, and can accurately judge the specific area of the fault and which phase it is located in, realizing the function of accurately positioning the fault location.

[0090] In other embodiments, a differential protection system for an M3C frequency converter of the present invention is implemented in the following structural manner:

[0091] As Figure 8 shown, the system structure is divided into: a data acquisition unit, a data processing unit, a logic judgment unit, and a protection outlet unit. The data acquisition unit is used to collect the three-phase currents on the low-frequency side valve side, the M3C frequency converter arm currents, and the low-frequency side valve side currents on the power frequency side. The data processing unit is used to obtain the current data collected by the data acquisition unit, calculate the differential current on the low-frequency side and the differential currents of each phase on the power frequency side. If the corresponding protection threshold requirements are met, the startup criterion is satisfied; implement the main criterion algorithm for differential protection based on the ratio restraint method. After the startup criterion is satisfied, this main criterion for differential protection is put into operation. The logic judgment unit obtains the main criterion for differential protection in the data processing unit, and is used to judge the satisfaction of the main criterion for differential protection at each sampling point by using a counter, that is, if it is satisfied S times in R operations, the main criterion for differential protection is satisfied; finally, the logic judgment unit feeds back the judgment result to the data processing unit. The protection outlet unit acts based on the processing result of the data processing unit, that is, when both the startup criterion and the main criterion for differential protection are satisfied, the protection action exits.

[0092] An embodiment of a differential protection method for an M3C frequency converter:

[0093] An embodiment of a differential protection method for an M3C frequency converter of the present invention, as Figure 1 shown, first collect the three-phase currents (i u , i v , i w ) on the low-frequency side valve side, the three-phase currents (i a , i b , i c ) on the power frequency side, and the nine-phase arm currents (i au , i av , i aw , i bu , i bv , i bw , i cu , i cv , i cw ). Then, based on the characteristic that the arm current itself contains both low-frequency components and power-frequency components, for any one phase of the u-phase, v-phase, and w-phase on the low-frequency side valve side and the a-phase, b-phase, and c-phase on the power-frequency side valve side, denoted as the x-phase, construct a differential protection criterion based on the sampled values of the phase-separated fault components, and then perform differential protection according to the main criterion for differential protection. Among them, when performing differential protection according to the main criterion for differential protection, judge whether the number of times that the main criterion for differential protection corresponding to the x-phase is satisfied in R consecutive sampling operations is greater than or equal to S times, and whether the differential protection startup criterion is satisfied. If both are satisfied, perform the corresponding differential protection on the valve side x-phase; R≥S>0.

[0094] First, it is necessary to construct a fast differential protection starting criterion based on the change in differential current. The formula for this differential protection starting criterion is as follows:

[0095] ΔIdif_x(s) = Idif_x(s) - Idif_x(s - N)

[0096] |ΔIdif_x(s)| > Δ1

[0097] Where, Idif_x(s) is the calculated value of the differential current at the s-th sampling point; ΔIdif_x(s) is the calculated value of the change in differential current at the s-th sampling point, Idif_x(s - N) is the calculated value of the differential current at the (s - N)-th sampling point; Δ1 is the starting threshold value;

[0098] The main criterion for the differential protection corresponding to the valve side x phase is expressed by the following formula:

[0099] |ΔIdif_x(s)| > MAX[k * MAX(|Δi x (s)|, |Δi xsum (s)|), Δ2]

[0100] Δi x (s) = i x (s) - i x (s - N)

[0101] Δi xsum (s) = i xsum (s) - i xsum (s - N)

[0102] Where, Δi x (s) is the change in the valve side x phase current; i x (s) is the sampled value of the valve side x phase current at the s-th sampling point; i x (s - N) is the sampled value of the valve side x phase current at the (s - N)-th sampling point; Δi xsum (s) is the change in the sum of the three-phase bridge arm currents corresponding to the valve side x phase; i xsum (s) is the sum of the three-phase bridge arm currents corresponding to the valve side x phase at the s-th sampling point; i xsum (s - N) is the sum of the three-phase bridge arm currents corresponding to the valve side x phase at the (s - N)-th sampling point; ΔIdif_x(s) is the calculated value of the change in differential current at the s-th sampling point; k is the ratio coefficient; Δ2 is the threshold value of the main criterion for differential protection. Among them, the differential current Idif_x of the valve side x phase is obtained by the following formula:

[0103] Idif_x(s) = i x (s) - i xsum (s)

[0104] i xsum (s) = i 1 x (s) + i 2 x (s) + i 3 x (s)

[0105] where i x (s) is the sampled value of the x-phase current at the s-th sampling point; i 1 x (s), i 2 x (s), i 3 x (s) are respectively the sampled values of the three-phase bridge arm currents corresponding to the x-phase; i xsum (s) is the sum of the sampled values of the three-phase bridge arm currents corresponding to the x-phase. When the low-frequency side valve side is selected, the x-phase is respectively the three phases of the low-frequency side valve side, and i 1 x (s), i 2 x (s), i 3 x (s) are also respectively the bridge arm currents connected to the low-frequency side valve side. Taking the x-phase as the low-frequency side u-phase as an example, the above differential current Idif_x can also be written as the formula corresponding to the u-phase low-frequency side differential current Idif_u in step (2) of an embodiment of the M3C frequency converter differential protection system of the present invention. When the power-frequency side valve side is selected, the x-phase is the three phases of the power-frequency side valve side, and i 1 x (s), i 2 x (s), i 3 x (s) are also respectively the bridge arm currents connected to the power-frequency side valve side. Taking the x-phase as the power-frequency side a-phase as an example, the above differential current Idif_x can also be written as the formula corresponding to the power-frequency side a-phase differential current Idif_a in step (2) of an embodiment of the M3C frequency converter differential protection system of the present invention. This method is consistent with the M3C frequency converter differential protection method introduced in an embodiment of the M3C frequency converter differential protection system of the present invention and achieves the same effect, so it will not be elaborated here.

Claims

1. A differential protection method for an M3C frequency converter, characterized in that: It includes the following steps: When the power transmission system is working, for any one phase among the three valve sides on one side of the M3C frequency converter and the three valve sides on the other side, denoted as phase x, the following processing is carried out: 1) Obtain current data, where the current data includes the x-phase valve-side current i x , and the three-phase bridge-arm currents of the M3C frequency converter connected to the x-phase; 2) According to the current data, judge whether the number of times that satisfies the main differential protection criterion corresponding to phase x in R consecutive sampling operations is greater than or equal to S times, and whether it satisfies the differential protection starting criterion. If both are satisfied, execute the corresponding differential protection for valve side phase x; R≥S>0; Among them, the formula for the differential protection starting criterion corresponding to valve side phase x is as follows: ΔIdif_x(s)=Idif_x(s)-Idif_x(s-N) |ΔIdif_x(s)|>Δ1 Where, Idif_x(s) is the calculated value of the differential current at the s-th sampling point; ΔIdif_x(s) is the calculated value of the change in differential current at the s-th sampling point, Idif_x(s-N) is the calculated value of the differential current at the s-th sampling point; Δ1 is the starting threshold value; The formula for the main differential protection criterion corresponding to valve side phase x is as follows: |ΔIdif_x(s)| > MAX[k * MAX(|Δi x (s)|, |Δi xsum (s)|), Δ2] Δi x (s) = i x (s) - i x (s - N) Δi xsum (s) = i xsum (s) - i xsum (s - N) Among them, Δi x (s) is the change in the valve-side phase-x current; i x (s) is the sampled value of the valve-side phase-x current at the s-th sampling point; i x (s - N) is the sampled value of the valve-side phase-x current at the (s - N)-th sampling point; Δi xsum (s) is the change in the sum of the three-phase bridge-arm currents corresponding to the valve-side phase-x; i xsum (s) is the sum of the three-phase bridge-arm currents corresponding to the valve-side phase-x at the s-th sampling point; i xsum (s - N) is the sum of the three-phase bridge-arm currents corresponding to the valve-side phase-x at the (s - N)-th sampling point; ΔIdif_x(s) is the calculated value of the differential current change at the s-th sampling point; k is the ratio coefficient; Δ2 is the threshold value of the main criterion for differential protection.

2. The differential protection method for an M3C frequency converter according to claim 1, characterized in that: For all phases among the three valve sides on one side of the M3C frequency converter and the three valve sides on the other side, the methods in steps 1) and 2) are used for differential protection.

3. The differential protection method for an M3C frequency converter according to claim 1, characterized in that: R = 10, 6≤S≤8.

4. The differential protection method for an M3C frequency converter according to claim 1, characterized in that: The following formula is used to obtain the differential current Idif_x of valve side phase x: Idif_x(s) = i x (s) - i xsum (s) i xsum (s) = i 1 x (s) + i 2 x (s) + i 3 x (s) where, i x (s) is the sampled value of the valve-side x-phase current at the s-th sampling point; i 1 x (s), i 2 x (s), i 3 x (s) are the sampled values of the three-phase bridge arm currents corresponding to the x-phase respectively; i xsum (s) is the sum of the sampled values of the three-phase bridge arm currents corresponding to the valve-side x-phase.

5. The differential protection method for an M3C frequency converter according to claim 1, characterized in that: The starting threshold value Δ1 is the product of the maximum value of the calculated value ΔIdif_x(s) of the change in differential current at the sampling moment during an external fault and a sensitivity coefficient greater than 1.

6. The differential protection method for an M3C frequency converter according to claim 5, characterized in that: The value range of the sensitivity coefficient is 1.2 to 1.5; the threshold value Δ2 of the main differential protection criterion is 0.05 pu.

7. The differential protection method for an M3C frequency converter according to any one of claims 1 to 6, characterized in that: The starting threshold value Δ1 is 0.1 pu.

8. The differential protection method for an M3C frequency converter according to any one of claims 1 to 6, characterized in that: The value range of the ratio coefficient is: 0.1<k<0.

5.

9. An M3C frequency converter differential protection system, characterized in that: This system includes: a processing module and a data acquisition module; the data acquisition module is used to acquire the three-phase currents of the valve side on the power frequency side, the currents of the M3C frequency converter bridge arms, and the currents of the valve side on the low-frequency side; the processing module is sampled and connected to the data acquisition module; the processing module is used to execute computer instructions to implement the differential protection method for the M3C frequency converter according to any one of claims 1-8.

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