Harmonic injection-based double-end weak feed alternating current system model identification phase selection method and system

Abstract

The invention provides a double-end weak feedback AC system model identification phase selection method and system based on harmonic injection, and the method comprises the steps: collecting the amplitude information of negative sequence current and positive sequence voltage at a protection installation part of a new energy side, and achieving the switching of harmonic injection modes through employing the amplitude information as a starting criterion; on the basis of harmonic injection, an equivalent model of a fault phase and a non-fault phase of the double-end weak feed alternating current system under specific harmonic waves is constructed, and a criterion is constructed by using the difference of the model so as to judge the fault phase; and after a fault occurs, judging harmonic current and voltage amplitude signals of each phase collected at the protection installation position of the new energy side of the line by utilizing a protection criterion, and judging whether the phase is a fault phase or not.

Description

Technical Field

[0001] The present invention belongs to the field of protection for outgoing lines of MMC connected to new energy, and particularly relates to a phase selection method and system for model identification of a double-ended weakly-fed AC system based on harmonic injection. Background Art

[0002] Driven by the energy transition and the increasing maturity of power electronics technology, the large-scale access of new energy to the power system is an irresistible trend. Its fault characteristics are different from those of conventional power grids, making the power frequency protection have adaptation problems. Traditional protection is affected by the access of power electronic devices at both ends of the line, and there are risks of refusal to operate and misoperation. Existing research mainly focuses on the inadaptability of traditional phase selection elements in single-ended weakly-fed systems, studies the sequence component phase selection elements under power frequency quantities, and improves the control strategy and protection criterion. However, when constructing the criterion, the system at the opposite end of the new energy is considered as a synchronous machine system, which is a stable strong power source. When the opposite end is also a controlled weak power source, the constructed principle also has adaptability problems. Summary of the Invention

[0003] The purpose of the present invention is to provide a phase selection method and system for model identification of a double-ended weakly-fed AC system based on harmonic injection to solve the problem of unreliable traditional protection discrimination methods caused by characteristics such as frequency deviation and harmonics under the access of new energy.

[0004] To achieve the above purpose, the present invention adopts the following technical solutions:

[0005] In the first aspect, the present invention provides a phase selection method for model identification of a double-ended weakly-fed AC system based on harmonic injection, including:

[0006] Collect the amplitude information of the negative sequence current and positive sequence voltage at the protection installation location on the new energy side, and use the amplitude information as the starting criterion to realize the switching of the harmonic injection mode;

[0007] On the basis of harmonic injection, construct an equivalent model of the fault phase and non-fault phase of the double-ended weakly-fed AC system under specific harmonics, and construct a criterion using the difference of the models to further discriminate the fault phase;

[0008] After a fault, use the protection criterion to discriminate the amplitude signals of the harmonic currents and voltages of each phase collected at the protection installation location on the new energy side of the line to determine whether it is the fault phase.

[0009] Further, the collection of the amplitude information of the negative sequence current and positive sequence voltage at the protection installation location on the new energy side and the use of the amplitude information as the starting criterion include:

[0010] When a fault occurs in the system, for the new energy side, negative sequence current suppression is adopted, and whether the negative sequence current content and the positive sequence voltage drop at the grid connection point on the new energy side exceed the limit are used as the enabling criterion DN for limit control switching:

[0011]

[0012] In the formula, DN is the enable signal for controlling the switching, and "∪" represents logical "OR"; S n and S p are the positive and negative sequence discrimination signals respectively; U + and I - are the amplitudes of the positive and negative sequence components of the voltage at the installation location of the new energy side protection respectively; ε = 0.15, and the above variable U + is the per-unit value standardized with the rated voltage U N of the system as the reference value, and I - is the actual value.

[0013] Furthermore, the harmonic injection of the new energy side limiting module includes:

[0014] The new energy side limits the amplitude of the output current of the new energy side by setting the corresponding maximum values i dmax and i qmax and minimum values i dmin and i qmin at the input end of the command value. The specific output current value is:

[0015]

[0016] In the formula, i d and i q are the d q axis currents in the power frequency synchronous rotating coordinate system, and their maximum values are i dmax and i qmax respectively, and the minimum values are i dmin and i qmin respectively. i pv is the actual output current phasor of the new energy side;

[0017] Furthermore, in Case 1: When the input value is greater than or equal to the fundamental current maximum value or less than or equal to the fundamental current minimum value, the output current amplitude at this time is:

[0018]

[0019] Where: M and h are the amplitudes of the fundamental current and the harmonic current respectively; ω ex takes a value related to the frequency of the injected harmonic, and M, h, and ω ex are all parameters to be determined;

[0020] In Case 2: When the input value is less than the fundamental current maximum value and greater than the fundamental current minimum value, the output current amplitude at this time is:

[0021]

[0022] Where: I _in is the input current.

[0023] Expand i' in Equation (2) or (3) IIDG to obtain the phasor form of the current output on the new energy side as:

[0024]

[0025] When the injection frequency is ω ex the harmonic can generate harmonics of two different frequencies, ω ex + ω p and ω ex - ω p respectively.

[0026] Furthermore, on the basis of harmonic injection, an equivalent model of the fault phase and non-fault phase of a double-ended weakly-fed AC system under specific harmonics is constructed, and a criterion is constructed using the difference of the model to distinguish the fault phase, including:

[0027] During normal operation, the impedance model of the line is expressed as a resistance R in series with an inductance X L and then in parallel with a capacitive reactance X C . At this time, the magnitude of the resistance R is not affected by the system frequency, and the capacitive reactance X C and the inductive reactance X L are obtained from the following formula:

[0028]

[0029] The angular frequency ω = 2πf. The larger f is, the larger the inductive reactance X L is, while the capacitive reactance X C is smaller; in the distributed parameter model under the 7th harmonic, the line resistance, inductance, and capacitance are in parallel. According to the parallel formula, the line impedance Z l is equivalent to the following formula:

[0030]

[0031] At this time, the line impedance is equivalent to the capacitive reactance of the distributed capacitance, and the system is capacitive, with the distributed capacitance current as the dominant. During normal operation, the load current shows a traversing characteristic, and the line current is all distributed capacitance current:

[0032]

[0033] where i N represents the harmonic current measured by the protection device on the new energy side;

[0034] When a fault occurs on the line, the fault phase line provides a short-circuit current from the harmonic source on the new energy side to the fault point. At this time, the line current is:

[0035] i N = i F1 + i NC + i F2 = i F + i C (9)

[0036] When a fault occurs in the line, the current of the faulty-phase line mainly consists of two parts: one is the fault current i F generated by the power source and the converter, and the other is the current i C generated by the distributed capacitance, and we get:

[0037] i N = i C + i F >> i C (10)

[0038] Based on this, a criterion is constructed by using the differences of the models.

[0039] Furthermore, after the fault, the protection criterion is used to discriminate the harmonic current and voltage amplitude signals of each phase collected at the protection installation location on the new energy side of the line to determine whether it is the faulty phase, including:

[0040] The discrimination of the faulty phase is realized by using the full-wave characteristics. By calculating the ratio of the energy of the phase current to the energy of the distributed capacitance current calculated by using the distributed capacitance parameters, an action criterion based on the line current energy ratio is constructed as follows:

[0041] E N / E C > k(11)

[0042] In the formula: EN represents the energy of the line current signal, EC represents the energy of the distributed capacitance current; k is the action coefficient, and k is selected as 6;

[0043] Among them, the energy calculation method of the signal is as follows:

[0044] E = ∫i 2 (t)dt (12)

[0045] Combining the above two formulas, the action criterion for the faulty phase is as follows:

[0046]

[0047] When k is greater than 6, this phase is the faulty phase.

[0048] In the second aspect, the present invention provides a model identification and phase selection system for a double-ended weakly-fed AC system based on harmonic injection, including:

[0049] A data acquisition module is used to collect the amplitude information of negative sequence current and positive sequence voltage at the installation location of the new energy side protection. The amplitude information is used as a starting criterion to realize the switching of the harmonic injection mode.

[0050] A fault phase discrimination module is used to construct an equivalent model of the fault phase and non-fault phase of a double-ended weakly-fed AC system under specific harmonics based on harmonic injection, and uses the difference of the model to construct a criterion to discriminate the fault phase.

[0051] A discrimination module is used to, after a fault, use protection criteria to discriminate the amplitude signals of harmonic currents and voltages of each phase collected at the installation location of the new energy side protection of the line to determine whether it is a fault phase.

[0052] Furthermore, the collection of the amplitude information of negative sequence current and positive sequence voltage at the installation location of the new energy side protection and the use of the amplitude information as a starting criterion include:

[0053] When a fault occurs in the system, for the new energy side, negative sequence current suppression is adopted, and whether the negative sequence current content and the positive sequence voltage drop at the grid connection point of the new energy side exceed the limit are used as the enabling criterion DN for amplitude limiting control switching:

[0054]

[0055] In the formula, DN is the enabling signal for control switching, and "∪" represents logical "or"; S n 、S p are the positive and negative sequence discrimination signals respectively; U + 、I - are the amplitudes of the positive and negative sequence components of the voltage at the installation location of the new energy side protection respectively; ε = 0.15, and the above variable U + is the per-unit value standardized with the system rated voltage U N as the reference value, and I - is the actual value.

[0056] In a third aspect, the present invention provides a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, the steps of the method for identifying and selecting a phase of a double-ended weakly-fed AC system model based on harmonic injection are implemented.

[0057] In a fourth aspect, the present invention provides a computer-readable storage medium, which stores a computer program. When the computer program is executed by a processor, the steps of the method for identifying and selecting a phase of a double-ended weakly-fed AC system model based on harmonic injection are implemented.

[0058] Compared with the prior art, the present invention has the following technical effects:

[0059] The present invention adopts the idea of active detection protection, injects a harmonic source, and utilizes the characteristic that the harmonic source is not affected by the system control strategy to amplify the fault electrical quantity. When constructing a criterion, the influence of the control of the far-end system can be ignored and it can be equivalent to a constant impedance, while the injection end can be equivalent to a constant power source at the injected harmonic frequency. Based on this, a protection criterion is constructed by using the difference between the fault-phase and non-fault-phase models, which can adapt to different system characteristics, ensure the correct operation of the phase selection element, have a strong ability to withstand the transition resistance, and have high sensitivity.

[0060] Under the access of new energy, due to the influence of characteristics such as frequency deviation and harmonics, the traditional protection discrimination method may have unreliable problems. The present invention injects a harmonic source and uses the harmonic characteristics to discriminate the fault phase, effectively improving the reliability and accuracy of the protection.

[0061] The present invention constructs an equivalent model of the fault phase and non-fault phase of a double-ended weakly-fed AC system under specific harmonics, and constructs a criterion by using the difference of the model, which can well adapt to the system characteristics after the access of new energy.

[0062] In an actual power system, there may be a transition resistance at the fault point, which will affect the accuracy of the traditional protection discrimination. The present invention injects harmonics and constructs an action criterion based on the line current energy ratio, enhancing the protection's ability to withstand the transition resistance and improving the sensitivity of the protection.

[0063] The present invention adopts the idea of active detection protection, and amplifies the fault electrical quantity by injecting a harmonic source, so as to more easily construct a reliable criterion. This method is not affected by the system control strategy and can more accurately identify the fault phase.

[0064] The traditional protection method may require a complex configuration and setting process. The present invention injects harmonics and constructs a criterion by using the difference of the model, simplifying the protection configuration and setting process, and improving the practicability and operability of the protection.

[0065] The method of the present invention can adapt to different system characteristics and operation modes, without the need for frequent adjustment or modification of the protection. This improves the adaptability and flexibility of the protection and reduces the operation and maintenance costs.

[0066] In summary, the phase selection method and system for model identification of a double-ended weakly-fed AC system based on harmonic injection proposed by the present invention have remarkable technical effects such as improving the reliability of protection discrimination, adapting to the system characteristics of new energy access, enhancing the ability to withstand the transition resistance, realizing active detection protection, simplifying the protection configuration and setting, and improving the protection adaptability. These technical effects make the present invention have a wide application prospect and practical value in the field of power system protection. Description of the Drawings

[0067] Figure 1 Flow chart of the present invention

[0068] Figure 2 Topological structure of large-scale photovoltaic direct connection to the power system in the embodiment of the present invention

[0069] Figure 3 New energy measurement control loop and its topological diagram

[0070] Figure 4 Line equivalent model of non-fault phase

[0071] Figure 5 Line equivalent model of fault phase

[0072] Figure 6 Energy ratio of each phase of the present invention when different types of faults occur in the double-ended weakly-fed transmission line Specific implementation manner

[0073] The present invention is further described below with reference to the accompanying drawings:

[0074] Example 1. Please refer to Figure 1 , the present invention provides a method for identifying and selecting phases of a double-ended weakly-fed AC system model based on harmonic injection, including:

[0075] Collect the amplitude information of the negative-sequence current and positive-sequence voltage at the new energy side protection installation location, and use the amplitude information as the starting criterion to realize the switching of the harmonic injection mode;

[0076] On the basis of harmonic injection, construct the equivalent models of the fault phase and non-fault phase of the double-ended weakly-fed AC system under specific harmonics, and use the differences of the models to construct criteria to further distinguish the fault phase;

[0077] After a fault, use the protection criterion to discriminate the amplitude signals of the harmonic currents and voltages of each phase collected at the new energy side protection installation location of the line to determine whether it is the fault phase.

[0078] First, sample the positive-sequence voltage and negative-sequence current at the new energy side protection installation location, start harmonic injection according to the amplitudes of the positive-sequence voltage and negative-sequence current, secondly, inject harmonics into the new energy side amplitude-limiting module, sample the harmonic currents and voltages at the new energy side protection installation location, adopt the energy integration algorithm, and use the ratio of the energy of the line harmonic current to the energy of the line distributed capacitance current to compare with the setting value to discriminate the fault phase.

[0079] Example 2. The present invention provides a method for identifying and selecting phases of a double-ended weakly-fed AC system model based on harmonic injection, including:

[0080] Step 1. Analyze the working principle of the amplitude-limiting module and the harmonic injection method of the new energy side amplitude-limiting module;

[0081] Step 2. At the initial stage of the fault, by using the characteristics under different fault types and combining with the control strategy of the system, negative-sequence current suppression is adopted on the new energy side, but it cannot be completely suppressed to zero. Therefore, the degree of positive-sequence voltage dip and the magnitude of negative-sequence current are used as the starting criteria for harmonic injection. In this paper, when the positive-sequence voltage drops to 0.9 p.u. and the negative-sequence current takes the actual value of 0.15, by collecting the magnitude information of the negative-sequence current and positive-sequence voltage at the protection installation location on the new energy side, and using this information as the starting criterion, the switching of the harmonic injection mode is realized;

[0082] Step 3. On the basis of harmonic injection, analyze the equivalent models of the fault phase and non-fault phase of the double-ended weakly-fed AC system under specific harmonics, and construct a criterion by using the differences of the models to further identify the fault phase;

[0083] Step 4. After the fault, use the protection criterion to discriminate the harmonic current and voltage amplitude signals of each phase collected at the protection installation location on the new energy side of the line, and adopt the energy integration algorithm to improve the anti-interference ability of the protection algorithm to determine whether it is the fault phase.

[0084] Furthermore, in Step 1, different from synchronous generators, power electronic converters still have high controllability under fault conditions. To ensure that the inverter does not trip out of the network within a certain period of time and at the same time provides voltage support to the system, when the drop depth exceeds the action dead zone specified by the national standard, the inverter starts low-voltage ride-through. At this time, the power outer loop is disconnected, and the command values \(i\) dref 、\(i\) qref of the current inner loop are given by the low-voltage ride-through strategy according to the voltage dip situation at the grid connection point. At the same time, in order to protect power electronic devices, a limiting link is generally set in the control of the new energy side, such as attached Figure 1 . Taking the new energy side as an example, by setting the corresponding maximum values \(i\) dmax 、\(i\) qmax and minimum values \(i\) dmin 、\(i\) qmin at the input end of the command value to limit the output current amplitude of the new energy side. The specific output current value is:

[0085]

[0086] where \(i\) d 、\(i\) q are the d q -axis currents in the power-frequency synchronous rotating coordinate system, and their maximum values are \(i\) dmax 、\(i\) qmax respectively, and the minimum values are \(i\) dmin 、\(i\) qmin respectively, and \(i\) pv is the actual output current phasor of the new energy side.

[0087] It can be seen that harmonic injection can be achieved by changing the dq-axis output value. On this basis, in order to realize harmonic current injection, the limit value is modified, which is collectively divided into two cases.

[0088] Case 1: When the input value is greater than or equal to the fundamental current maximum value or less than or equal to the fundamental current minimum value, the current amplitude output at this time is:

[0089]

[0090] where: M and h are the amplitudes of the fundamental current and the harmonic current respectively; ω ex is related to the frequency of the injected harmonic, and M, h, and ω ex are all parameters to be tuned.

[0091] Case 2: When the input value is less than the fundamental current maximum value and greater than the fundamental current minimum value, the current amplitude output at this time is:

[0092]

[0093] where: I_in is the input current.

[0094] Expanding i' in Equation (2) or (3) gives the current phasor form output on the new energy side as: IIDG It can be seen that when injecting a harmonic with frequency ω

[0095]

[0096] two different frequencies of harmonics, ω ex + ω ex and ω p - ω ex can be generated. p

[0097] Furthermore, in step 2, when a system fault occurs, for the new energy side, negative sequence current suppression is adopted. According to the fact that the voltage drops deeply during a symmetrical fault and negative sequence current will be generated during an asymmetrical fault, the negative sequence current content at the connection point of the new energy side and whether the positive sequence voltage drop exceeds the limit are used as the enabling criterion D for the limit control switch N :

[0098]

[0099] In the formula, D N is the enabling signal for the control switch, and "∪" represents the logical "or"; S n , S p are the positive and negative sequence discrimination signals respectively; U + , I - ​They are the positive and negative sequence component amplitudes of the voltage at the installation location of the new energy side protection. Considering that the negative sequence component only appears when the system has an asymmetric fault, the value of ε can be relatively small. To ensure sensitivity, in this paper, ε = 0.15 is taken. The above variable U + is the per-unit value standardized with the system rated voltage U N as the reference value, and I - is the actual value. When the starting criterion is met, harmonic injection is started.

[0100] Furthermore, in step 3, based on the harmonic injection method in step 1 and the starting method of harmonic injection in step 2, an analysis is carried out on the equivalent models of the fault phase and non-fault phase of the double-ended weakly-fed AC system under specific harmonics with harmonic injection. Here, a specific analysis of the system under the 7th harmonic is carried out:

[0101] During normal operation, the impedance model of the line can be expressed as a resistance R in series with an inductance X L and then in parallel with a capacitive reactance X C . At this time, the magnitude of the resistance R is not affected by the system frequency, and the capacitive reactance X C and the inductive reactance X L are obtained from the following formula:

[0102]

[0103] It can be seen that the magnitudes of the capacitive reactance and inductive reactance are related to the angular frequency ω, and ω = 2πf, so their values are related to the frequency f. The larger f is, the larger the inductive reactance X L is, and the smaller the capacitive reactance X C . In the distributed parameter model under the 7th harmonic, since the capacitive reactance X C of the line distributed capacitance is much smaller than the line resistance R, the line inductance X L , as well as the system impedances Z pv and Z MMC on both sides, and the system impedances on both sides can be approximated as short circuits because they are much larger than the line impedance. And because the line resistance, inductance, and capacitance are in parallel, according to the parallel formula, the line impedance Zl at this time can be equivalent to the following formula:

[0104]

[0105] At this time, the line impedance can be equivalent to the capacitive reactance of the distributed capacitance, the system is capacitive, and the distributed capacitance current is dominant. During normal operation, the load currents all show a traversing characteristic, and the line currents are all distributed capacitance currents:

[0106]

[0107] In the formula, iN represents the harmonic current measured by the protection device on the new energy side.

[0108] When a fault occurs in the line, the faulty-phase line provides short-circuit current from the harmonic source on the new energy side to the fault point. At this time, the line current is:

[0109] i N =i F1 +i NC +i F2 =i F +i C (9)

[0110] It can be seen from the above analysis that during normal operation, the line current of each phase only contains the line distributed capacitance current, which conforms to the capacitance model. When a fault occurs in the line, the current of the faulty-phase line mainly consists of two parts: one is the fault current i F , generated by the power supply and the converter, and the other is the current i C generated by the distributed capacitance. Moreover, the amplitude of the fault current is significantly different from the distributed capacitance current, and the line current does not conform to the capacitance model. It can be obtained that:

[0111] i N =i C +i F >>i C (10)

[0112] Based on this, a criterion is constructed by using the difference of the models.

[0113] Furthermore, in step 4, since the method of comparing currents through current sampling values in step 3 has very poor anti-interference ability and is extremely prone to misoperation or refusal of protection due to fluctuations at individual points or sampling errors, an energy integration algorithm is used to improve the anti-interference ability of the protection algorithm, and the full-wave characteristics are fully utilized to realize the discrimination of the faulty phase. By calculating the ratio of the energy of the phase current to the energy of the distributed capacitance current calculated using the distributed capacitance parameters, an operating criterion based on the line current energy ratio is constructed as follows:

[0114] E N / E C >k(11)

[0115] In the formula: E N represents the energy of the line current signal, and E C represents the energy of the distributed capacitance current; k is the operating coefficient. During normal operation, since the differential current signal conforms to the capacitance model, k is approximately 1 under normal circumstances. In practice, considering many factors such as the reliability of protection and harmonic interference caused by system faults, k can be appropriately increased to 5 - 8. In this paper, k is selected as 6. Values greater than this range may affect the sensitivity of the protection.

[0116] Among them, the calculation method of the energy of the signal is as follows:

[0117] E=∫i2 (t)dt (12)

[0118] Combining the above two equations, the operating criterion for the faulty phase is obtained as follows:

[0119]

[0120] When k is greater than 6, then this phase is the faulty phase.

[0121] Example 3: Establish a large-scale double-ended weakly-fed system as shown in the appendix Figure 2 as a simulation model. Set symmetrical and asymmetrical faults to occur at the system sending end.

[0122] When AG, ABCG, ABG, and AB faults occur, process the positive-sequence voltage and negative-sequence current signals of the outgoing line on the PV side. Use the amplitudes of the two to determine whether to inject. Then process the 7th harmonic current and voltage signals of the outgoing line on the PV side. Take a 40 ms data window length to calculate the ratio of the distributed capacitance current energy signal under the 7th harmonic of each phase to the magnitude of the 7th harmonic current energy flowing through the protection installation location to determine whether it is the faulty phase. The above simulation results are as shown in the following table.

[0123] Table 1 Discrimination situations for different types of faults in the outgoing line of the double-ended weakly-fed system

[0124]

[0125] Example 4: Establish a large-scale double-ended weakly-fed system as shown in the appendix Figure 2 as a simulation model. Set the fault point on the outgoing line of the system to have an AG ground fault through different transition resistances.

[0126] When an AG fault occurs, process the positive-sequence voltage and negative-sequence current signals of the outgoing line on the PV side. Use the amplitudes of the two to determine whether to inject. Then process the 7th harmonic current and voltage signals of the outgoing line on the PV side. Take a 40 ms data window length to calculate the ratio of the distributed capacitance current energy signal under the 7th harmonic of each phase to the magnitude of the 7th harmonic current energy flowing through the protection installation location to determine whether it is the faulty phase. The above simulation results are as shown in the following table.

[0127] Table 2 Discrimination situations when the outgoing line of the double-ended weakly-fed system has an AG ground fault through a transition resistance

[0128]

[0129] In another embodiment of the present invention, a phase selection system for identifying a double-ended weakly-fed AC system model based on harmonic injection is provided, which can be used to implement the above-mentioned method for identifying and selecting phases of a double-ended weakly-fed AC system model based on harmonic injection. Specifically, the system includes:

[0130] A data acquisition module is configured to acquire the amplitude information of negative sequence current and positive sequence voltage at the installation location of the new energy side protection, and use the amplitude information as a starting criterion to achieve the switching of the harmonic injection mode.

[0131] A fault phase discrimination module is configured to, based on harmonic injection, construct an equivalent model of the fault phase and non-fault phase of the double-ended weakly-fed AC system under specific harmonics, and use the difference of the model to construct a criterion to discriminate the fault phase.

[0132] A discrimination module is configured to, after a fault, use the protection criterion to discriminate the amplitude signals of harmonic currents and voltages of each phase collected at the installation location of the new energy side protection of the line to determine whether it is a fault phase.

[0133] In the embodiments of the present invention, the division of the modules is illustrative, only a logical function division. In actual implementation, there may be other division methods. In addition, in each embodiment of the present invention, the functional modules can be integrated in one processor, or can exist separately physically, or two or more modules can be integrated in one module. The above integrated modules can be implemented in the form of hardware or in the form of software functional modules.

[0134] In another embodiment of the present invention, a computer device is provided. The computer device includes a processor and a memory. The memory is used to store a computer program. The computer program includes program instructions. The processor is used to execute the program instructions stored in the computer storage medium. The processor may be a Central Processing Unit (CPU), or may also be other general-purpose processors, Digital Signal Processors (DSPs), Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. It is the computing core and control core of the terminal, and is suitable for implementing one or more instructions. Specifically, it is suitable for loading and executing one or more instructions in the computer storage medium to implement the corresponding method flow or corresponding function. The processor in the embodiments of the present invention can be used for the operation of the phase selection method based on the double-ended weakly-fed AC system model identification by harmonic injection.

[0135] In another embodiment of the present invention, the present invention further provides a storage medium, specifically a computer-readable storage medium (Memory). The computer-readable storage medium is a memory device in a computer device, used to store programs and data. It can be understood that the computer-readable storage medium here can include both the built-in storage medium in the computer device and, of course, the extended storage medium supported by the computer device. The computer-readable storage medium provides a storage space, which stores the operating system of the terminal. And, in this storage space, there is also stored one or more instructions suitable for being loaded and executed by the processor. These instructions can be one or more computer programs (including program codes). It should be noted that the computer-readable storage medium here can be a high-speed RAM memory or a non-volatile memory, such as at least one disk memory. One or more instructions stored in the computer-readable storage medium can be loaded and executed by the processor to implement the corresponding steps of the method for identifying and selecting a phase of a double-ended weakly-fed AC system model based on harmonic injection in the above embodiments.

[0136] Those skilled in the art should understand that the embodiments of the present invention can be provided as a method, a system, or a computer program product. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present invention can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk memories, CD-ROMs, optical memories, etc.) containing computer-usable program codes.

[0137] The present invention is described with reference to the flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to the embodiments of the present invention. It should be understood that each flow and / or block in the flowchart and / or block diagram, as well as the combination of flows and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing devices generate a device for implementing the functions specified in Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks.

[0138] These computer program instructions can also be stored in a computer-readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer-readable memory generate a manufactured product including an instruction device, and the instruction device implements the functions in the flow Figure 1One or more processes and / or boxes Figure 1 The functions specified in one or more boxes.

[0139] These computer program instructions can also be loaded onto a computer or other programmable data processing device, so that a series of operation steps are executed on the computer or other programmable device to generate a computer-implemented process. Thus, the instructions executed on the computer or other programmable device provide steps for implementing the functions specified in Figure 1 One or more processes and / or boxes Figure 1 The steps of the functions specified in one or more boxes.

[0140] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: the specific implementation manners of the present invention can still be modified or equivalently replaced, and any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the protection scope of the claims of the present invention.

Claims

1. A two-terminal weakly-fed AC system model identification and phase selection method based on harmonic injection, characterized in that: include: Collect the amplitude information of negative sequence current and positive sequence voltage at the protection installation on the new energy side, use the amplitude information as the starting criterion, and realize the switching of harmonic injection mode; Based on harmonic injection, an equivalent model of the fault phase and non-fault phase of the double-terminal weak-feed AC system under specific harmonics is constructed, and the criterion is constructed by using the difference of the model to identify the fault phase. After a fault occurs, the protection criteria are used to identify the harmonic current and voltage amplitude signals of each phase collected at the protection installation on the new energy side of the line to determine whether it is a faulty phase.

2. The double-terminal weakly-fed AC system model identification and phase selection method based on harmonic injection according to claim 1 is characterized in that: The collecting of the amplitude information of the negative sequence current and the positive sequence voltage at the protection installation on the new energy side and using the amplitude information as a starting criterion includes: When a fault occurs in the system, negative sequence current suppression is used for the renewable energy side, and whether the negative sequence current content and positive sequence voltage drop at the renewable energy side grid connection point exceed the limit is used as the enabling criterion DN for the switching of the limit control: In the formula, DN is the enable signal for control switching, "∪" represents the logical "or"; S n , S p are positive and negative sequence discrimination signals respectively; U + ,I - are the positive and negative sequence component amplitudes of the voltage at the protection installation on the new energy side; ε=0.15, the above variable U + The system rated voltage U N is the reference value normalized value, I - is the actual value.

3. The double-terminal weakly-fed AC system model identification and phase selection method based on harmonic injection according to claim 2 is characterized in that: Harmonic injection of the limiting module on the new energy side, including: The new energy side sets the corresponding maximum value i at the input end of the command value. dmax 、i qmax and the minimum value i dmin 、i qmin To limit the output current amplitude of the new energy side, the specific output current value is: Where i d 、i q are d in the industrial frequency synchronous rotating coordinate system q The shaft current, its maximum value is i dmax 、i qmax , the minimum values ​​are i dmin 、i qmin ,i pv is the actual output current phasor of the new energy side.

4. The double-terminal weakly-fed AC system model identification and phase selection method based on harmonic injection according to claim 3 is characterized in that: Case 1: When the input value is greater than or equal to the maximum fundamental current value or less than or equal to the minimum fundamental current value, the output current amplitude is: Where: M and h are the amplitudes of fundamental current and harmonic current respectively; ω ex The value of is related to the frequency of the injected harmonic, M, h, ω ex These are parameters to be adjusted; Case 2: When the input value is less than the maximum fundamental current value and greater than the minimum fundamental current value, the output current amplitude is: Where: I _in is the input current; In formula (2) or (3), i′ IIDG The expanded current phasor form of the new energy side output is: or When the injection frequency is ω ex The harmonics of can be generated with a frequency of ω ex +ω p and ω ex -ω p Two harmonics of different frequencies.

5. The double-terminal weakly-fed AC system model identification and phase selection method based on harmonic injection according to claim 1 is characterized in that: Based on the harmonic injection, an equivalent model of the fault phase and the non-fault phase of the double-terminal weak-feed AC system under specific harmonics is constructed, and a criterion is constructed by using the difference of the model to identify the fault phase, including: In normal operation, the line impedance model is represented by a resistor R in series with an inductor X. L Then connect the capacitor X in parallel C At this time, the value of the resistor R is not affected by the system frequency, and the capacitive reactance X C With inductive reactance X L The size is obtained by the following formula: Angular frequency ω=2πf, the larger f is, the greater the inductive reactance X L The larger the capacitive reactance X C The smaller the line impedance is, the smaller the line impedance is. In the distributed parameter model under the 7th harmonic, the line resistance, inductance and capacitance are connected in parallel. According to the parallel formula, the line impedance Z is l The equivalent is the following formula: At this time, the line impedance is equivalent to the capacitive reactance of distributed capacitors, the system is capacitive, and the distributed capacitor current is dominant. In normal operation, the load current shows a ride-through characteristic, and the line current is distributed capacitor current: Where i N Indicates the harmonic current measured by the protection device on the new energy side; When a line fault occurs, the fault phase line provides short-circuit current to the fault point from the harmonic source on the renewable energy side. The line current at this time is: i N =i F1 +i NC +i F2 =i F +i C (9) When a line fault occurs, the current of the fault phase line mainly consists of two parts: one is the fault current i generated by the power supply and converter. F , and the second is the current i generated by the distributed capacitance C ,get: i N =i C +i F >>i C (10) This is used as a basis for constructing the difference in the model.

6. The double-terminal weakly-fed AC system model identification and phase selection method based on harmonic injection according to claim 1 is characterized in that: After a fault occurs, the protection criteria are used to identify the harmonic current and voltage amplitude signals of each phase collected at the protection installation on the new energy side of the line to determine whether it is a faulty phase, including: The full-wave characteristics are used to identify the fault phase. By calculating the ratio of the phase current energy to the distributed capacitor current energy calculated using the distributed capacitor parameters, an action criterion based on the line current energy ratio is constructed, as shown in the following formula: E N / E C >k (11) Where: EN represents the energy of the line current signal, EC represents the energy of the distributed capacitor current; k is the action coefficient, and k is selected as 6; The energy of the signal is calculated as follows: E=∫i 2 (t)dt (12) Combining the above two equations, the action criterion of the fault phase can be obtained as follows: When k is greater than 6, the phase is a fault phase.

7. A double-terminal weakly-fed AC system model identification and phase selection system based on harmonic injection, characterized in that: include: The data acquisition module is used to collect the amplitude information of the negative sequence current and positive sequence voltage at the protection installation on the new energy side, and use the amplitude information as the starting criterion to realize the switching of the harmonic injection mode; The fault phase identification module is used to construct an equivalent model of the fault phase and the non-fault phase of the double-terminal weak-feed AC system under specific harmonics based on harmonic injection, and to construct a criterion based on the difference of the model to identify the fault phase; The identification module is used to identify the harmonic current and voltage amplitude signals of each phase collected at the protection installation on the new energy side of the line after a fault occurs, using the protection criteria to determine whether it is a faulty phase.

8. The double-terminal weak-fed AC system model identification and phase selection system based on harmonic injection according to claim 7 is characterized in that: The collecting of the amplitude information of the negative sequence current and the positive sequence voltage at the protection installation on the new energy side and using the amplitude information as a starting criterion includes: When a fault occurs in the system, negative sequence current suppression is used for the renewable energy side, and whether the negative sequence current content and positive sequence voltage drop at the renewable energy side grid connection point exceed the limit is used as the enabling criterion DN for the switching of the limit control: In the formula, DN is the enable signal for control switching, "∪" represents the logical "or"; S n , S p are positive and negative sequence discrimination signals respectively; U + ,I - are the positive and negative sequence component amplitudes of the voltage at the protection installation on the new energy side; ε=0.15, the above variable U + The system rated voltage U N is the reference value normalized value, I - is the actual value.

9. A computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the computer program, the steps of the double-terminal weak-fed AC system model identification and phase selection method based on harmonic injection as described in any one of claims 1 to 6 are implemented.

10. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, the steps of the double-terminal weak-fed AC system model identification and phase selection method based on harmonic injection as claimed in any one of claims 1 to 6 are implemented.

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