High-frequency harmonic protection method and system for flexible direct current system

Abstract

The invention discloses a high-frequency harmonic protection method and system for a flexible DC system, and the method comprises the steps: obtaining the AC voltage of a grid-connected point of the flexible DC system, and calculating the high-frequency harmonic component of the grid-connected point according to the AC voltage; obtaining the harmonic current of the valve side of the connection transformer of the flexible direct current system, and calculating the high-frequency harmonic component of the connection transformer according to the harmonic current; acquiring a bridge arm current of a bridge arm reactor of the flexible direct current system, and calculating a high-frequency harmonic component of the reactor according to the bridge arm current; and executing a protection instruction according to the grid connection point high-frequency harmonic component, the connection transformer high-frequency harmonic component and the reactor high-frequency harmonic component. By adopting the embodiment of the invention, high-frequency harmonic equivalence can be carried out on the grid connection point of the flexible direct current system, the valve side of the connection transformer and the key position of the bridge arm so as to realize rapid and reliable protection of key equipment, prevent harmonic waves of each frequency from exceeding the endurance capability of the equipment, ensure the safety of core equipment and the flexible direct current system and improve the reliability of the equipment. And the overall operation reliability of the flexible direct current system is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible direct current (DC) in power systems, and in particular to a high-frequency harmonic protection method and system for a flexible DC system. Background Art

[0002] Flexible DC technology is a key technology for building a new power system and supporting the "dual carbon" strategy. Flexible DC transmission uses a voltage source converter based on fully controlled devices, with technical advantages such as no commutation failure, independent decoupling control of active power and reactive power, flexible operation mode, strong support for the power grid, and easy construction of multi-terminal DC transmission systems. It has broad application prospects in the transmission of large-scale new energy bases such as Shagohuang and offshore wind power.

[0003] There is a complex coupling relationship between renewable energy and flexible DC systems, and between flexible DC systems and AC power grids. The overall interconnected system of renewable energy, flexible DC and AC power grids may have multiple resonance points in a wide frequency band, and the system is at risk of high-frequency oscillations.

[0004] The broadband oscillation mechanism of flexible DC systems is complex, with many influencing factors and strong uncertainty. The existing flexible DC systems are not mature in their ability to identify broadband oscillations and their protection measures, making it difficult to achieve oscillation suppression in the entire frequency band and under all system operating conditions, which seriously affects the safe and stable operation of the flexible DC systems. Summary of the invention

[0005] The technical problem to be solved by the present invention is how to suppress the high-frequency oscillation risk of a flexible DC system and provide a high-frequency harmonic protection method and system for a flexible DC system. By utilizing the high-frequency harmonic components at key positions such as the grid connection point, the connecting transformer side, and the bridge arm, and comprehensively considering the requirements of protection speed and reliability and the computing power of the hardware device, the protection system can be quickly and reliably operated to ensure the safety of core equipment and the flexible DC system.

[0006] In order to solve the above technical problems, an embodiment of the present invention provides a high-frequency harmonic protection method for a flexible DC system, comprising:

[0007] Acquire an AC voltage at a grid-connected point of the flexible DC system, and calculate a high-frequency harmonic component at the grid-connected point based on the AC voltage;

[0008] Acquire the harmonic current of the flexible DC system connecting the transformer valve side, and calculate the high-frequency harmonic component of the connecting transformer according to the harmonic current;

[0009] Acquire a bridge arm current of a bridge arm reactor of the flexible DC system, and calculate a high-frequency harmonic component of the reactor according to the bridge arm current;

[0010] A protection instruction is executed according to the high-frequency harmonic component of the grid connection point, the high-frequency harmonic component of the connection transformer and the high-frequency harmonic component of the reactor.

[0011] As an improvement of the above solution, the method of obtaining the AC voltage of the grid connection point of the flexible DC system and calculating the high-frequency harmonic component of the grid connection point according to the AC voltage includes:

[0012] Obtain the AC voltage at the grid connection point of the flexible DC system;

[0013] According to the AC voltage, a fundamental voltage and 2-n1 harmonic voltages of the grid connection point are obtained;

[0014] The total harmonic distortion rate is calculated according to the fundamental voltage and the harmonic voltage as the high-frequency harmonic component of the grid connection point.

[0015] As an improvement of the above solution, the step of obtaining the harmonic current on the connecting transformer valve side of the flexible DC system and calculating the connecting transformer high-frequency harmonic component according to the harmonic current includes:

[0016] Obtain the harmonic current on the side of the flexible DC system connected to the transformer;

[0017] According to the harmonic current, obtaining the 2nd to n2th order first harmonic current of the harmonic current;

[0018] According to the first harmonic current and the thermal characteristics of the connection variable high frequency harmonic, the equivalent thermal current is calculated as the connection variable high frequency harmonic component.

[0019] As an improvement of the above solution, the step of obtaining the arm current of the arm reactor of the flexible DC system and calculating the high-frequency harmonic component of the reactor according to the arm current includes:

[0020] Obtain the bridge arm current and skin effect coefficient of the bridge arm reactor of the flexible DC system;

[0021] According to the bridge arm current, a 2 to n3 second harmonic current of the upper bridge arm and a 2 to n3 third harmonic current of the lower bridge arm are obtained;

[0022] According to the skin effect coefficient, the second harmonic current and the third harmonic current, the bridge arm reactor equivalent current is calculated as the reactor high-frequency harmonic component.

[0023] As an improvement of the above solution, the calculation of the bridge arm reactor equivalent current as the high-frequency harmonic component of the reactor according to the skin effect coefficient, the second harmonic current and the third harmonic current includes:

[0024] According to the second harmonic current and the third harmonic current, a 2-n3th order fourth harmonic current flowing through the bridge arm reactor is obtained;

[0025] pass Calculate the equivalent current of the bridge arm reactor; where I n is the nth fourth harmonic current, k is the skin effect coefficient;

[0026] The bridge arm reactor equivalent current is taken as the reactor high frequency harmonic component.

[0027] As an improvement of the above solution, the protection instruction is executed according to the high-frequency harmonic component of the grid connection point, the high-frequency harmonic component of the connection transformer and the high-frequency harmonic component of the reactor, including:

[0028] Get the harmonic distortion rate protection setting value and harmonic distortion rate protection delay;

[0029] When the total harmonic distortion rate is greater than the harmonic distortion rate protection setting value and the duration is greater than the harmonic distortion rate protection delay, a first protection instruction is executed; the first protection instruction includes an alarm and a trip.

[0030] As an improvement of the above solution, the protection instruction is executed according to the high-frequency harmonic component of the grid connection point, the high-frequency harmonic component of the connection transformer and the high-frequency harmonic component of the reactor, including:

[0031] Obtain the inverse time protection setting value and inverse time protection time limit of high-frequency harmonic current;

[0032] When the equivalent thermal current is greater than the inverse time protection setting value and the duration is greater than the inverse time protection time limit, a second protection instruction is executed; the second protection instruction includes tripping.

[0033] As an improvement of the above solution, the protection instruction is executed according to the high-frequency harmonic component of the grid connection point, the high-frequency harmonic component of the connection transformer and the high-frequency harmonic component of the reactor, including:

[0034] Obtain the reactor overload protection setting, reactor thermal time constant and reactor continuous thermal current;

[0035] Calculating the reactor overload protection time limit according to the reactor thermal time constant, the reactor continuous thermal current and the bridge arm reactor equivalent current;

[0036] When the equivalent current of the bridge arm reactor is greater than the reactor overload protection setting value and the duration is greater than the reactor overload protection time limit, the third protection instruction is executed; the second protection instruction includes tripping.

[0037] As an improvement of the above solution, the calculation of the reactor overload protection time limit according to the reactor thermal time constant, the reactor continuous thermal current and the bridge arm reactor equivalent current includes:

[0038] pass Calculate the overload protection time limit of the reactor; where τ is the thermal time constant of the reactor, I eq is the equivalent current of the bridge arm reactor, and I∞ is the continuous thermal current of the reactor.

[0039] The embodiment of the present invention further provides a high-frequency harmonic protection system for a flexible DC system, comprising:

[0040] A grid-connected point high-frequency harmonic component calculation module, used to obtain the AC voltage of the flexible DC system grid-connected point, and calculate the grid-connected point high-frequency harmonic component according to the AC voltage;

[0041] A connection variable high frequency harmonic component calculation module is used to obtain the harmonic current on the connection variable valve side of the flexible DC system, and calculate the connection variable high frequency harmonic component according to the harmonic current;

[0042] A reactor high-frequency harmonic component calculation module is used to obtain the bridge arm current of the bridge arm reactor of the flexible DC system, and calculate the reactor high-frequency harmonic component according to the bridge arm current;

[0043] The protection instruction execution module is used to execute the protection instruction according to the high-frequency harmonic component of the grid connection point, the high-frequency harmonic component of the connection transformer and the high-frequency harmonic component of the reactor.

[0044] Compared with the prior art, the present invention discloses a high-frequency harmonic protection method and system for a flexible DC system, which obtains the AC voltage of the grid-connected point of the flexible DC system, and calculates the high-frequency harmonic component of the grid-connected point according to the AC voltage; obtains the harmonic current of the valve side of the flexible DC system connecting transformer, and calculates the high-frequency harmonic component of the connecting transformer according to the harmonic current; obtains the bridge arm current of the bridge arm reactor of the flexible DC system, and calculates the high-frequency harmonic component of the reactor according to the bridge arm current; and executes the protection instruction according to the high-frequency harmonic component of the grid-connected point, the high-frequency harmonic component of the connecting transformer and the high-frequency harmonic component of the reactor. By adopting the embodiment of the present invention, the high-frequency harmonic equivalence can be performed on the grid-connected point, the valve side of the connecting transformer and the key positions of the bridge arm of the flexible DC system based on the equivalent harmonic calculation principle, so as to realize the rapid and reliable protection of key equipment, avoid the harmonics of each frequency exceeding the tolerance capacity of the equipment, ensure the safety of the core equipment and the flexible DC system, and improve the overall operation reliability of the flexible DC system. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 It is a schematic flow chart of the steps of a high-frequency harmonic protection method for a flexible DC system provided by an embodiment of the present invention;

[0046] Figure 2 is a structural schematic diagram of a flexible DC system provided by an embodiment of the present invention;

[0047] Figure 3 is a schematic flow chart of steps for generating a first protection instruction provided by an embodiment of the present invention;

[0048] Figure 4 is a schematic flow chart of steps for generating a second protection instruction provided by an embodiment of the present invention;

[0049] Figure 5 is a schematic diagram of an inverse time current characteristic curve of a bridge arm reactor provided by an embodiment of the present invention;

[0050] Figure 6 This is a schematic diagram of a step flow of generating a third protection instruction provided by an embodiment of the present invention.

[0051] Figure 7 It is a structural schematic diagram of a high-frequency harmonic protection system for a flexible DC system provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0052] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0053] In the description of the specification and claims, it is to be understood that the terms first, second, etc. in the specification and claims are only used for the purpose of describing the same technical features, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated, nor necessarily describing the order or time sequence. The terms are interchangeable where appropriate. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include at least one of the features.

[0054] The embodiment of the present invention provides a high-frequency harmonic protection method for a flexible DC system. Figure 1 In this embodiment, the high-frequency harmonic protection method for the flexible DC system is specifically performed through steps S1 to S4:

[0055] S1. Acquire an AC voltage at a grid-connected point of a flexible DC system, and calculate a high-frequency harmonic component at the grid-connected point based on the AC voltage.

[0056] S2. Obtain the harmonic current on the connecting transformer valve side of the flexible DC system, and calculate the connecting transformer high-frequency harmonic component according to the harmonic current.

[0057] S3. Obtain a bridge arm current of a bridge arm reactor of the flexible DC system, and calculate a high-frequency harmonic component of the reactor according to the bridge arm current.

[0058] S4. Execute a protection instruction according to the high-frequency harmonic component of the grid connection point, the high-frequency harmonic component of the connection transformer and the high-frequency harmonic component of the reactor.

[0059] See also Figure 2 , Figure 2 It is a flexible DC system in the embodiment of the present invention. The AC voltage of the grid connection point can be obtained through the grid-side AC voltage transformer, and the harmonic current connected to the transformer side can be obtained through the current measuring device connected to the transformer side; the bridge arm current can be obtained through the upper bridge arm current measuring device and the lower bridge arm current measuring device in the bridge arm reactor. It should be noted that in the embodiment of the present invention, the bridge arm current is finally obtained by measuring the upper and lower bridge arm currents respectively. In some embodiments, the bridge arm reactor can also be equivalent to obtain an equivalent reactor, and the current of the equivalent reactor is directly measured and used as the bridge arm current.

[0060] It should be noted that the applicant has found that in the actual application of traditional flexible DC systems, the continuous effect of high-frequency harmonics will lead to heat accumulation in key equipment such as connecting transformers and bridge arm reactors, reduce the temperature rise margin, and even cause insulation breakdown or equipment burning, seriously affecting the safe and stable operation of the flexible DC system.

[0061] Exemplarily, the protection instructions include alarm and trip. The protection instruction of the trip can be to control the tripping of the corresponding device only, or to control the tripping of its front-end device or the whole system. For example, when the high-frequency harmonic component of the grid connection point exceeds the tolerance of the grid connection point, the flexible DC system is controlled to trip, or an alarm is issued; when the high-frequency harmonic component of the connecting transformer exceeds the tolerance of the connecting transformer, the connecting transformer is controlled to trip; when the high-frequency harmonic component of the reactor exceeds the tolerance of the bridge arm reactor, the bridge arm reactor or its front-end device is controlled to trip.

[0062] In the above scheme, high-frequency harmonic equivalence is performed on the grid-connected points, valve sides of the connecting transformers and key positions of the bridge arms of the flexible DC system based on the principle of equivalent harmonic calculation, so as to achieve rapid and reliable protection of key equipment and avoid harmonics of various frequencies exceeding the tolerance capacity of the equipment. This can ensure the safety of core equipment and the flexible DC system and improve the overall operational reliability of the flexible DC system.

[0063] As a preferred implementation, step S1, obtaining the AC voltage of the grid connection point of the flexible DC system, and calculating the high-frequency harmonic component of the grid connection point according to the AC voltage, includes:

[0064] Obtain the AC voltage at the grid connection point of the flexible DC system;

[0065] According to the AC voltage, a fundamental voltage and 2-n1 harmonic voltages of the grid connection point are obtained;

[0066] The total harmonic distortion rate is calculated according to the fundamental voltage and the harmonic voltage as the high-frequency harmonic component of the grid connection point.

[0067] For example, by Calculate the total harmonic distortion rate; where U sn is the nth harmonic voltage at the grid connection point, U s1 is the fundamental voltage at the grid connection point.

[0068] As a preferred implementation, step S2, obtaining the harmonic current of the flexible DC system connecting the transformer valve side, and calculating the connecting transformer high-frequency harmonic component according to the harmonic current, includes:

[0069] Obtain the harmonic current on the side of the flexible DC system connected to the transformer;

[0070] According to the harmonic current, obtaining the 2nd to n2th order first harmonic current of the harmonic current;

[0071] According to the first harmonic current and the thermal characteristics of the connection variable high frequency harmonic, the equivalent thermal current is calculated as the connection variable high frequency harmonic component.

[0072] For example, by Calculate the equivalent thermal current; where I vn is the nth first harmonic current of the harmonic current, K is a preset conversion coefficient, and the conversion coefficient is determined according to the thermal characteristics under the connection variable high frequency harmonic.

[0074] As a preferred implementation manner, step S3, obtaining the arm current of the arm reactor of the flexible DC system, and calculating the high-frequency harmonic component of the reactor according to the arm current, is specifically performed through steps S31-S33:

[0075] S31, obtaining the bridge arm current and skin effect coefficient of the bridge arm reactor of the flexible DC system;

[0076] S32, obtaining a 2nd to n3th second harmonic current of the upper bridge arm and a 2nd to n3th third harmonic current of the lower bridge arm according to the bridge arm current;

[0077] S33. Calculate the bridge arm reactor equivalent current as the high-frequency harmonic component of the reactor according to the skin effect coefficient, the second harmonic current and the third harmonic current.

[0078] It should be noted that the bridge arm current includes an upper bridge arm current and a lower bridge arm current. Since there is a corresponding relationship between the upper bridge arm and the lower bridge arm of the bridge arm inductor, in an embodiment of the present invention, the harmonic orders of the second harmonic current and the third harmonic current are consistent, and the time of acquisition thereof should also be consistent, which can be conducive to the subsequent calculation of the data unification of the high-frequency harmonic components of the inductor.

[0079] Furthermore, the step S33 includes:

[0080] According to the second harmonic current and the third harmonic current, a 2-n3th order fourth harmonic current flowing through the bridge arm reactor is obtained;

[0081] pass Calculate the equivalent current of the bridge arm reactor; where I n is the nth fourth harmonic current, k is the skin effect coefficient;

[0082] The bridge arm reactor equivalent current is taken as the reactor high frequency harmonic component.

[0083] It should be noted that the bridge arm inductor passes through alternating current, and the current distribution inside the wire is not uniform. By introducing the skin effect coefficient, the fourth harmonic current can be corrected, and the obtained bridge arm inductor equivalent current takes into account the skin effect of the wire.

[0084] Illustratively, in the embodiment of the present invention, n1=n2=n3=40.

[0085] As a preferred implementation, step S4, executing a protection instruction according to the high-frequency harmonic component of the grid connection point, the high-frequency harmonic component of the connection transformer and the high-frequency harmonic component of the reactor, is performed through steps S411-S412:

[0086] S411, obtaining harmonic distortion rate protection setting value and harmonic distortion rate protection delay;

[0087] S412. When the total harmonic distortion rate is greater than the harmonic distortion rate protection setting value and the duration is greater than the harmonic distortion rate protection delay, execute a first protection instruction; the first protection instruction includes an alarm and a trip.

[0088] When the total harmonic distortion rate is greater than the harmonic distortion rate protection setting, it is considered that the harmonic oscillation may be abnormal. Furthermore, by combining the duration of the harmonic oscillation abnormality, if the duration is greater than the harmonic distortion rate protection delay, it means that the high-frequency harmonic component of the grid connection point exceeds the tolerance capacity of the grid connection point. Continuing operation may cause irreversible failure of the flexible DC system. At this time, the first protection instruction is executed.

[0089] See also Figure 3 In the specific implementation, the total harmonic distortion rate is calculated according to the 2nd to n1th harmonic voltages of the flexible DC system grid-connected point, and the total harmonic distortion rate is compared with the harmonic distortion rate protection constant. The duration is also required to be compared with the harmonic distortion rate protection delay. When both requirements are met, the first protection instruction corresponding to the flexible DC system grid-connected point is executed.

[0090] As a preferred implementation, step S4, executing a protection instruction according to the high-frequency harmonic component of the grid connection point, the high-frequency harmonic component of the connection transformer and the high-frequency harmonic component of the reactor, is performed through steps S421-S422:

[0091] S421, obtaining the inverse time protection setting value and inverse time protection time limit of the high-frequency harmonic current;

[0092] S422. When the equivalent thermal current is greater than the inverse time protection setting value and the duration is greater than the inverse time protection time limit, execute a second protection instruction; the second protection instruction includes tripping.

[0093] It should be noted that the inverse time protection setting value and the inverse time protection time limit correspond to multiple sections of high-frequency harmonic current. For example, the inverse time protection setting value and the inverse time protection time limit of section I are I 1set ,I 1time , the inverse time protection setting value and inverse time protection time limit of section II are I 2set ,I 2time ,…, the X-segment inverse time protection setting value and inverse time protection time limit are I 10set ,I 10time , where I 1set >I 2set >…>I 10set , and I 1time 2time <… 10time The inverse time protection setting value and the inverse time protection time limit can be directly obtained from the connecting transformer manufacturer.

[0094] In the embodiment of the present invention, a total of 8 to 10 sections of inverse time protection constants and the inverse time protection time limits corresponding to high-frequency harmonic currents are adopted.

[0095] See also Figure 4 In a specific implementation, the equivalent thermal current is calculated based on the 2nd to 40th first harmonic current of the flexible DC system connecting the transformer valve side. Preferably, the equivalent thermal current is first compared with the thermal current protection constant, and then the constants and time limits of the multi-stage high-frequency harmonic protection are compared. If the equivalent thermal current does not meet the constant and time limit requirements corresponding to any harmonic level, the second protection instruction corresponding to the connecting transformer valve side is executed.

[0096] ​​As a preferred implementation, step S4, executing a protection instruction according to the high-frequency harmonic component of the grid connection point, the high-frequency harmonic component of the connection transformer and the high-frequency harmonic component of the reactor, is performed through steps S431-S432:

[0097] S431, obtaining the reactor overload protection setting value, the reactor thermal time constant and the reactor continuous thermal current;

[0098] S432, calculating the reactor overload protection time limit according to the reactor thermal time constant, the reactor continuous thermal current and the bridge arm reactor equivalent current;

[0099] S433. When the equivalent current of the bridge arm reactor is greater than the reactor overload protection setting value and the duration is greater than the reactor overload protection time limit, execute the third protection instruction; the second protection instruction includes tripping.

[0100] For example, see Figure 5 The reactor overload protection setting value and the reactor thermal time constant are determined by Figure 6 The inverse time characteristic curve equation shown is determined.

[0101] Furthermore, the step S432 includes:

[0102] pass Calculate the overload protection time limit of the reactor; where τ is the thermal time constant of the reactor, I eq is the equivalent current of the bridge arm reactor, and I∞ is the continuous thermal current of the reactor.

[0103] See also Figure 6 In the specific implementation, the equivalent current of the bridge arm inductor is calculated according to the 2nd to 40th second harmonic current of the upper bridge arm of each phase and the 2nd to 40th third harmonic current of the lower bridge arm of each phase, and the skin effect of the upper and lower bridge arms of each phase is taken into account, so as to realize the inverse time protection of the bridge arm inductor.

[0104] It should be noted that, in the embodiment of the present invention, the bridge arm reactor equivalent current and the reactor continuous thermal current are both obtained based on the skin effect coefficient.

[0105] A high-frequency harmonic protection method for a flexible DC system provided by an embodiment of the present invention can perform high-frequency harmonic equivalence on key positions of the grid connection point, the valve side of the connecting transformer and the bridge arm of the flexible DC system based on the equivalent harmonic calculation principle, so as to achieve fast and reliable protection of key equipment, avoid harmonics of various frequencies exceeding the tolerance capacity of the equipment, ensure the safety of core equipment and the flexible DC system, and improve the overall operation reliability of the flexible DC system.

[0106] See also Figure 7The embodiment of the present invention provides a high-frequency harmonic protection system for a flexible DC system. The high-frequency harmonic protection system for a flexible DC system includes a grid-connected point high-frequency harmonic component calculation module 11, a connection variable high-frequency harmonic component calculation module 12, a reactor high-frequency harmonic component calculation module 13 and a protection instruction execution module 14, wherein:

[0107] A grid-connected point high-frequency harmonic component calculation module 11 is used to obtain an AC voltage at a grid-connected point of the flexible DC system, and calculate a grid-connected point high-frequency harmonic component according to the AC voltage;

[0108] A connection variable high frequency harmonic component calculation module 12 is used to obtain the harmonic current on the connection variable valve side of the flexible DC system, and calculate the connection variable high frequency harmonic component according to the harmonic current;

[0109] A reactor high-frequency harmonic component calculation module 13 is used to obtain the bridge arm current of the bridge arm reactor of the flexible DC system, and calculate the reactor high-frequency harmonic component according to the bridge arm current;

[0110] The protection instruction execution module 14 is used to execute the protection instruction according to the high-frequency harmonic component of the grid connection point, the high-frequency harmonic component of the connection transformer and the high-frequency harmonic component of the reactor.

[0111] As a preferred implementation, the grid connection point high frequency harmonic component calculation module 11 includes:

[0112] An AC voltage acquisition unit, used to acquire the AC voltage of a flexible DC system grid connection point;

[0113] A harmonic voltage extraction unit, used for obtaining the fundamental voltage and 2 to n1 harmonic voltages of the grid connection point according to the AC voltage;

[0114] The total harmonic distortion rate calculation unit is used to calculate the total harmonic distortion rate according to the fundamental voltage and the harmonic voltage as the high-frequency harmonic component of the grid connection point.

[0115] As a preferred implementation, the connection variable high frequency harmonic component calculation module 12 includes:

[0116] A harmonic current acquisition unit is used to acquire the harmonic current of the flexible DC system connected to the transformer valve side;

[0117] A first harmonic current extraction unit, used for obtaining a 2nd to n2th order first harmonic current of the harmonic current according to the harmonic current;

[0118] The equivalent thermal current calculation unit is used to calculate the equivalent thermal current as the connection variable high frequency harmonic component according to the first harmonic current and the thermal characteristics under the connection variable high frequency harmonic.

[0119] As a preferred implementation, the reactor high-frequency harmonic component calculation module 13 includes:

[0120] An initial data acquisition unit, used to acquire the bridge arm current and skin effect coefficient of the bridge arm reactor of the flexible DC system;

[0121] A bridge arm harmonic current extraction unit, used to obtain the 2nd to n3rd second harmonic current of the upper bridge arm and the 2nd to n3rd third harmonic current of the lower bridge arm according to the bridge arm current;

[0122] The reactor equivalent current calculation unit is used to calculate the bridge arm reactor equivalent current as the reactor high-frequency harmonic component according to the skin effect coefficient, the second harmonic current and the third harmonic current.

[0123] Further, preferably, the reactor equivalent current calculation unit is specifically used for:

[0124] According to the second harmonic current and the third harmonic current, a 2-n3th order fourth harmonic current flowing through the bridge arm reactor is obtained;

[0125] pass Calculate the equivalent current of the bridge arm reactor; where I n is the nth fourth harmonic current, k is the skin effect coefficient;

[0126] The bridge arm reactor equivalent current is taken as the reactor high frequency harmonic component.

[0127] As a preferred implementation, the protection instruction execution module 14 includes:

[0128] A grid connection point limit acquisition unit is used to obtain the harmonic distortion rate protection setting value and the harmonic distortion rate protection delay;

[0129] The first protection instruction execution unit is used to execute the first protection instruction when the total harmonic distortion rate is greater than the harmonic distortion rate protection set value and the duration is greater than the harmonic distortion rate protection delay; the first protection instruction includes alarm and tripping.

[0130] As a preferred implementation, the protection instruction execution module 14 includes:

[0131] A connection variable limit acquisition unit is used to obtain the inverse time protection setting value and inverse time protection time limit of high-frequency harmonic current;

[0132] The second protection instruction execution unit is used to execute the second protection instruction when the equivalent thermal current is greater than the inverse time protection set value and the duration is greater than the inverse time protection time limit; the second protection instruction includes tripping.

[0133] As a preferred implementation, the protection instruction execution module 14 includes:

[0134] A reactor limit acquisition unit is used to obtain the reactor overload protection setting value, the reactor thermal time constant and the reactor continuous thermal current;

[0135] An overload protection time limit calculation unit, used to calculate the reactor overload protection time limit according to the reactor thermal time constant, the reactor continuous thermal current and the bridge arm reactor equivalent current;

[0136] The third protection instruction execution unit is used to execute the third protection instruction when the equivalent current of the bridge arm reactor is greater than the reactor overload protection set value and the duration is greater than the reactor overload protection time limit; the second protection instruction includes tripping.

[0137] Further, preferably, the third protection instruction execution unit is specifically used to:

[0138] pass Calculate the overload protection time limit of the reactor; where τ is the thermal time constant of the reactor, I eq is the equivalent current of the bridge arm reactor, and I∞ is the continuous thermal current of the reactor.

[0139] A high-frequency harmonic protection system for a flexible DC system provided by an embodiment of the present invention can perform high-frequency harmonic equivalence on key positions of the grid connection point, the valve side of the connecting transformer and the bridge arm of the flexible DC system based on the equivalent harmonic calculation principle, so as to achieve fast and reliable protection of key equipment, avoid harmonics of various frequencies exceeding the tolerance capacity of the equipment, ensure the safety of core equipment and the flexible DC system, and improve the overall operation reliability of the flexible DC system.

[0140] A person skilled in the art can understand that all or part of the processes in the above-mentioned embodiments can be implemented by instructing the relevant hardware through a computer program, and the program can be stored in a computer-readable storage medium, and when the program is executed, it can include the processes of the embodiments of the above-mentioned methods. The storage medium can be a disk, an optical disk, a read-only memory (ROM) or a random access memory (RAM), etc.

[0141] The above is a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several improvements and modifications without departing from the principle of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A high-frequency harmonic protection method for a flexible DC system, characterized in that: include: Acquire an AC voltage at a grid-connected point of the flexible DC system, and calculate a high-frequency harmonic component at the grid-connected point based on the AC voltage; Acquire the harmonic current of the flexible DC system connecting the transformer valve side, and calculate the high-frequency harmonic component of the connecting transformer according to the harmonic current; Acquire a bridge arm current of a bridge arm reactor of the flexible DC system, and calculate a high-frequency harmonic component of the reactor according to the bridge arm current; A protection instruction is executed according to the high-frequency harmonic component of the grid connection point, the high-frequency harmonic component of the connection transformer and the high-frequency harmonic component of the reactor.

2. A high-frequency harmonic protection method for a flexible DC system according to claim 1, characterized in that: The step of obtaining the AC voltage of the flexible DC system grid connection point and calculating the high-frequency harmonic component of the grid connection point according to the AC voltage includes: Obtain the AC voltage at the grid connection point of the flexible DC system; According to the AC voltage, a fundamental voltage and 2-n1 harmonic voltages of the grid connection point are obtained; The total harmonic distortion rate is calculated according to the fundamental voltage and the harmonic voltage as the high-frequency harmonic component of the grid connection point.

3. A high-frequency harmonic protection method for a flexible DC system according to claim 1, characterized in that: The step of obtaining the harmonic current of the flexible DC system connected to the variable valve side and calculating the connected variable high-frequency harmonic component according to the harmonic current includes: Obtain the harmonic current on the flexible DC system connected to the transformer valve side; According to the harmonic current, obtaining the 2nd to n2th order first harmonic current of the harmonic current; According to the first harmonic current and the thermal characteristics of the connection variable high frequency harmonic, the equivalent thermal current is calculated as the connection variable high frequency harmonic component.

4. A high-frequency harmonic protection method for a flexible DC system according to claim 1, characterized in that: The method of obtaining the arm current of the arm reactor of the flexible DC system and calculating the high-frequency harmonic component of the reactor according to the arm current includes: Obtain the bridge arm current and skin effect coefficient of the bridge arm reactor of the flexible DC system; According to the bridge arm current, a 2 to n3 second harmonic current of the upper bridge arm and a 2 to n3 third harmonic current of the lower bridge arm are obtained; According to the skin effect coefficient, the second harmonic current and the third harmonic current, the bridge arm reactor equivalent current is calculated as the reactor high-frequency harmonic component.

5. A high-frequency harmonic protection method for a flexible DC system as claimed in claim 4, characterized in that: The calculating, according to the skin effect coefficient, the second harmonic current and the third harmonic current, the equivalent current of the bridge arm reactor as the high-frequency harmonic component of the reactor comprises: According to the second harmonic current and the third harmonic current, a 2-n3th order fourth harmonic current flowing through the bridge arm reactor is obtained; pass Calculate the equivalent current of the bridge arm reactor; where I n is the nth fourth harmonic current, k is the skin effect coefficient; The bridge arm reactor equivalent current is taken as the reactor high frequency harmonic component.

6. A high-frequency harmonic protection method for a flexible DC system according to claim 2, characterized in that: The executing of the protection instruction according to the high-frequency harmonic component of the grid connection point, the high-frequency harmonic component of the connection transformer and the high-frequency harmonic component of the reactor comprises: Get the harmonic distortion rate protection setting value and harmonic distortion rate protection delay; When the total harmonic distortion rate is greater than the harmonic distortion rate protection setting value and the duration is greater than the harmonic distortion rate protection delay, a first protection instruction is executed; the first protection instruction includes an alarm and a trip.

7. A high-frequency harmonic protection method for a flexible DC system as claimed in claim 3, characterized in that: The executing of the protection instruction according to the high-frequency harmonic component of the grid connection point, the high-frequency harmonic component of the connection transformer and the high-frequency harmonic component of the reactor comprises: Obtain the inverse time protection setting value and inverse time protection time limit of high-frequency harmonic current; When the equivalent thermal current is greater than the inverse time protection setting value and the duration is greater than the inverse time protection time limit, a second protection instruction is executed; the second protection instruction includes tripping.

8. A high-frequency harmonic protection method for a flexible DC system according to claim 4, characterized in that: The executing of the protection instruction according to the high-frequency harmonic component of the grid connection point, the high-frequency harmonic component of the connection transformer and the high-frequency harmonic component of the reactor comprises: Obtain the reactor overload protection setting, reactor thermal time constant and reactor continuous thermal current; Calculating the reactor overload protection time limit according to the reactor thermal time constant, the reactor continuous thermal current and the bridge arm reactor equivalent current; When the equivalent current of the bridge arm reactor is greater than the reactor overload protection setting value and the duration is greater than the reactor overload protection time limit, the third protection instruction is executed; the second protection instruction includes tripping.

9. A high-frequency harmonic protection method for a flexible DC system according to claim 8, characterized in that: The calculating the reactor overload protection time limit according to the reactor thermal time constant, the reactor continuous thermal current and the bridge arm reactor equivalent current comprises: pass Calculate the overload protection time limit of the reactor; where τ is the thermal time constant of the reactor, I eq is the equivalent current of the bridge arm reactor, and I∞ is the continuous thermal current of the reactor.

10. A high-frequency harmonic protection system for a flexible DC system, characterized in that: include: A grid-connected point high-frequency harmonic component calculation module, used to obtain the AC voltage of the flexible DC system grid-connected point, and calculate the grid-connected point high-frequency harmonic component according to the AC voltage; A connection variable high frequency harmonic component calculation module is used to obtain the harmonic current on the connection variable valve side of the flexible DC system, and calculate the connection variable high frequency harmonic component according to the harmonic current; A reactor high-frequency harmonic component calculation module is used to obtain the bridge arm current of the bridge arm reactor of the flexible DC system, and calculate the reactor high-frequency harmonic component according to the bridge arm current; The protection instruction execution module is used to execute the protection instruction according to the high-frequency harmonic component of the grid connection point, the high-frequency harmonic component of the connection transformer and the high-frequency harmonic component of the reactor.

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