Reliability analysis method of phase selection based on phase current difference sudden change variable considering new energy penetration and fault ride-through behavior

Abstract

A reliability analysis method of phase selection based on phase current difference sudden variable considering the penetration of new energy and fault ride-through behavior is proposed, including the following steps: based on the phase current difference sudden variable selection criterion, the phase selection failure index based on the amplitude ratio of phase current difference sudden variable is constructed; the fault equivalent impedance of different types of grid-connected inverters is determined according to the classification of new energy types; the equivalent impedance model of new energy station coupled with multiple types of energy is established; the phase selection failure index is converted into the relationship of positive and negative sequence current distribution coefficients, the equivalent impedance of new energy station is brought into sequence network analysis, the ratio of positive and negative sequence current distribution coefficients is calculated, and the relationship between the ratio and the critical failure condition is compared to complete the reliability analysis of phase selection method. The method comprehensively considers the influencing factors, has clear physical meaning, can effectively and accurately judge the risk of phase selection failure due to high proportion of new energy access to the line, and can provide certain guidance for the improvement of phase selection method.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of new energy power failure phase selection analysis, and particularly relates to a phase current difference mutation variable phase selection reliability analysis method considering new energy penetration rate and fault ride-through behavior. BACKGROUND

[0002] Large-scale development and long-distance transmission of new energy are effective means to promote efficient consumption of new energy. In actual operation, to ensure the stability of the system, new energy is usually bundled with synchronous power sources in the near area for transmission, forming a high-proportion wind, light, and fire bundled transmission system. Due to the special fault characteristics of new energy power sources, coupled with the coupling effect between different types of power sources, the electrical quantities measured by the single-ended measurement of the transmission line are quite different from those of the pure synchronous power system, and the difference is different due to the influence of penetration rate and fault conditions. The change of fault characteristics leads to the risk of reduced reliability of conventional fault phase selection methods, therefore, it is of great significance to develop a conventional phase selection method reliability analysis method considering the fault behavior and fault conditions of new energy, for risk analysis and method improvement.

[0003] In view of the special role of new energy, some analysis or improvement methods for phase selection have been reported, mainly based on different inverter grid-connected structures. For example, the document (Li, Y. Q., Tu, Q. R., Chen, Q. P., et al. Influence of large-scale photovoltaic power station on phase selection elements of transmission line protection[J]. Power System Technology, 2018, 42(09): 2976-2982.) analyzes the possible influence of photovoltaic power station on conventional phase selection methods, but the power source side type is single, only the inverter type photovoltaic power source is considered, and the scene of high-proportion wind, light, and fire bundled access on the power source side is not considered. In the process of constructing a new phase selection method, the document (Liang, Y. Y., Lu, Z. J. Photovoltaic grid-connected inverter sequence impedance angle reconstruction scheme for enhancing the adaptability of sequence component phase selection elements[J]. Power Automation Equipment, 2022, 42(01): 133-139.) analyzes the influence of photovoltaic power source on phase selection elements, but does not analyze the penetration rate and coupling of multiple types of energy. For double-fed power sources, the document (Zhang, J. F., Gao, L., Shen, Y. F., et al. Fault voltage sequence component phase selection element suitable for double-fed wind farm[J]. Power System Protection and Control, 2018, 46(10): 136-143.) points out that double-fed power sources may have adverse effects on conventional fault phase selection methods, but does not analyze the special influence of different fault conditions (such as voltage drop depth), and the analysis object also only contains single type of double-fed power source, and does not consider the coupling effect of multiple types of new energy.

[0004] In summary, the existing research reports basically consider that the access of new energy to the power grid has an adverse effect on fault phase selection, and some targeted analysis and improvement are carried out. However, the above analysis methods all have strong power dependence characteristics and are only aimed at a single type of energy, which may be insufficient for the scene of high-proportion wind, light, fire and bundled delivery. SUMMARY

[0005] In order to solve the problem of the lack of fault phase selection reliability analysis method in the high-proportion wind, light and fire bundled delivery system, the application proposes a phase current difference mutation variable phase selection reliability analysis method considering the new energy penetration rate and fault ride-through behavior. The method aims at the problem of failure or misjudgment of the fault phase selection method that the high-proportion wind, light and fire bundled delivery system may face, and realizes the quantitative analysis of the phase current difference mutation variable phase selection reliability considering the new energy penetration rate and voltage drop depth on the basis of constructing the equivalent impedance model of different types of new energy stations. The method comprehensively considers the influencing factors and has clear physical meaning, and can effectively and accurately judge the phase selection failure risk existing in the line due to high-proportion access of new energy, and can provide certain guidance for the improvement of the phase selection method.

[0006] The technical scheme adopted by the application is as follows:

[0007] The phase current difference mutation variable phase selection reliability analysis method considering the new energy penetration rate and fault ride-through behavior comprises the following steps:

[0008] Step 1: based on the phase current difference mutation variable phase selection criterion, a phase current difference mutation variable amplitude ratio-based phase selection failure index is constructed; step 2: the new energy types are divided according to the inverter type and the doubly-fed type, and the fault equivalent impedance of different types of grid-connected inverters is determined;

[0009] Step 3: an equivalent impedance model of a new energy station coupled with multiple types of energy is established;

[0010] Step 4: the phase selection failure index in step 1 is converted into the relationship of the positive and negative sequence current distribution coefficients, the equivalent impedance of the new energy station in step 3 is brought into the sequence network analysis, the ratio of the positive and negative sequence current distribution coefficients is calculated, and the relationship between the ratio and the critical failure condition is compared to complete the reliability analysis of the phase selection method.

[0011] In the step 1, the A-phase grounding fault is taken as an example:

[0012] The phase current difference mutation variable phase selection criterion is shown in formula (1):

[0013]

[0014] In formula (1), m represents a reliability coefficient, and the commonly used setting value in engineering is 4-8; And respectively represent the amplitude of AB, BC, CA phase current difference variable; && is the logical "and" operation.

[0015] Based on the phase current difference variable selection criterion of formula (1), the phase current difference variable selection failure index R is constructed AG , as shown in formula (2):

[0016]

[0017] In the system of high proportion of wind, light, fire and package, only R AG The calculation value is greater than m, which can realize correct phase selection; otherwise, the phase current difference variable selection fails.

[0018] In step 2, the impedance behavior of single type new energy is analyzed as follows:

[0019] (1) Inverter type power supply:

[0020] Considering the inverter type power supply during fault, according to the national standard, the inverter type power supply will send reactive power to support system voltage, so as to implement low voltage ride through strategy; At the same time, in order to ensure power quality and power supply safety, the inverter type power supply will inhibit the negative sequence current output in the control link, and execute negative sequence suppression strategy.

[0021] At this time, the inverter type power supply station has no negative sequence current output, and does not participate in the composition of the system negative sequence network, and the negative sequence impedance is equivalent to infinity; The positive sequence equivalent impedance is equal to the ratio of the grid connection point voltage to the current, and the grid connection point voltage is affected by the fault condition, and the grid connection point current is the current output by the inverter type power supply under the low voltage ride through strategy, so the positive sequence equivalent impedance is essentially related to its low voltage ride through strategy.

[0022] (2) Doubly-fed type power supply:

[0023] Considering the two types of control methods during the fault of doubly-fed type power supply: when serious fault occurs, the crowbar current limiting strategy is put into operation, at this time the converter is locked, and the rotor is short-circuited through the crowbar resistance, to avoid the damage of large current to the rotor during serious fault; When non-serious fault occurs, the low voltage ride through strategy is executed by the converter, at this time the converter is controlled to output reactive power to support system voltage.

[0024] If the crowbar is put into current limiting and the converter is locked, the doubly-fed type power supply is equivalent to an asynchronous motor, at this time the equivalent positive and negative sequence impedance of the doubly-fed type power supply station is related to the slip. After the crowbar resistance is put into current limiting, the equivalent impedance network of the doubly-fed type station is shown in figure 7(a) and figure 7(b), and there is always a large difference between the positive and negative sequence impedance;

[0025] In figure 7(a) and figure 7(b), X rσ , R r and Rc X represents rotor leakage reactance, rotor resistance, crowbar resistance, respectively; X m X represents rotor leakage reactance, rotor resistance, crowbar resistance, respectively; X s X represents rotor leakage reactance, rotor resistance, crowbar resistance, respectively; X sσ X represents rotor leakage reactance, rotor resistance, crowbar resistance, respectively; X T X represents rotor leakage reactance, rotor resistance, crowbar resistance, respectively; X b X represents rotor leakage reactance, rotor resistance, crowbar resistance, respectively; X PV X represents rotor leakage reactance, rotor resistance, crowbar resistance, respectively; X DFIG X represents rotor leakage reactance, rotor resistance, crowbar resistance, respectively; X S X represents rotor leakage reactance, rotor resistance, crowbar resistance, respectively; X

[0026] If the crowbar is not put in, the low voltage ride through control is carried out by the converter, the doubly-fed power station does not output negative sequence current due to the input of negative sequence suppression strategy, and does not participate in the formation of the system negative sequence network; therefore, the negative sequence impedance equivalent is infinite; the positive sequence equivalent impedance is related to the low voltage ride through strategy, and the positive sequence equivalent impedance is equal to the ratio of the grid connection point voltage to the current, the grid connection point voltage is related to the fault condition, and the grid connection point current is equal to the current output by the doubly-fed power station under the low voltage ride through strategy. In step 3, the equivalent impedance modeling of the new energy station coupled with multiple types of energy is as follows:

[0027] The power side new energy penetration rate p is defined as shown in formula (3):

[0028]

[0029] In formula (3), p represents the power side new energy penetration rate, p belongs to 0-1; k1 and k2 represent the superposition coefficients of the inverter type and the doubly-fed type generating units respectively; P PV , P DFIG represent the rated power of the inverter type and the doubly-fed type generating units respectively, P S is the rated power of the synchronous generator.

[0030] The decoupling method of the superposition coefficients k1 and k2 is as follows:

[0031]

[0032] In formula (5), A and B are the rated power ratios of the synchronous power and the inverter type power and the doubly-fed type power respectively; a is the wind-solar ratio.

[0033] 1) For the inverter type power: under the low voltage ride through target, the positive sequence equivalent impedance Z PV1 of the inverter type station is:

[0034]

[0035] In formula (8), U represents the positive sequence voltage of the grid connection point; represents the positive sequence current output by the inverter type station; U PV1 and φ are the effective value and phase of the positive sequence voltage respectively; Id represents d-axis current; δ represents positive sequence current phase; Z represents the value of impedance of single generating unit in inverter type field station.

[0036] For inverter type power supply, the negative sequence equivalent impedance of inverter type field station is equivalent to infinity to the outside due to the existence of negative sequence suppression strategy.

[0037] 2) For double-fed type power supply, if crowbar is not put into current limiting, the behavior characteristics of positive and negative sequence equivalent impedance of field station are the same as those of inverter type power supply shown in formula (8);

[0038] If crowbar is put into current limiting, the positive and negative sequence impedance of double-fed type power supply field station is irrelevant to fault condition, and the calculation method is as follows:

[0039]

[0040] In formula (9), Z DFIG1 and Z DFIG2 are positive and negative sequence impedance of double-fed type field station; X rσ , R r and R c respectively represent rotor leakage reactance, rotor resistance and crowbar resistance; X m is excitation reactance; R s and X sσ respectively represent stator resistance and leakage reactance; X T and X b respectively represent transformer and main transformer reactance; s represents slip, usually between-0.3 and 0.3.

[0041] In step 4, the phase selection failure index shown in formula (1) is converted into impedance related index, and m=4 is taken to obtain:

[0042]

[0043] In formula (13), C1 and C2 are positive and negative sequence current distribution coefficients.

[0044] According to sequence network analysis, the impedance calculation method of C1 and C2 is:

[0045]

[0046] In formula (14), Z S is power supply side impedance; Z L is line impedance; Z SN is receiving end system impedance. Subscript 1 represents positive sequence impedance; subscript 2 represents negative sequence impedance;

[0047] The equivalent impedance of the inverter type and double-fed type power supply considering the penetration rate and voltage drop depth shown in formula (9), formula (8) is brought into formula (14), and the ratio is calculated, if the ratio is less than 0.64, the phase current difference mutation selection under the current penetration rate and voltage depth will fail to judge; otherwise, the phase selection method is not affected.

[0048] The phase current difference mutation selection reliability analysis method considering the new energy penetration rate and fault ride-through behavior has the following technical effects:

[0049] 1) The present application takes the high-proportion wind, light and fire bundled external transmission system as the object, takes the phase current difference mutation selection as an example, establishes the critical risk index of phase selection failure, constructs the equivalent impedance model of the new energy station considering the penetration rate and fault ride-through behavior, calculates the fault phase selection risk coefficient of the power supply side of the coupled multi-type energy, determines the reliability of the conventional phase selection method, and can depict the potential risk of fault phase selection.

[0050] 2) The method aims to establish the model of the equivalent impedance of the new energy station about the penetration rate and the voltage drop depth, apply the sequence network analysis method, calculate the relationship between the ratio of the positive and negative sequence current distribution coefficients and the critical failure value, and analyze the reliability of the phase current difference mutation selection method of the high-proportion new energy bundled external transmission line.

[0051] 3) The present application fully considers the particularity of the multi-type new energy bundled access, establishes the failure risk index of the phase current difference mutation selection through the sequence network analysis, considers the fault ride-through behavior of the new energy under different fault conditions and different grid-connected structures, establishes the equivalent impedance model of the new energy station under the change of the penetration rate, and further realizes the quantitative analysis of the reliability of the conventional phase selection method. The advantages are:

[0052] ① The considered factors are comprehensive, including different types of new energy access, new energy penetration rate, fault conditions and fault ride-through strategy of new energy, which is in line with actual engineering application;

[0053] ② Strong adaptability, suitable for high-proportion wind, light and fire bundled external transmission lines, and can also be extended to wind and fire, or fire and light bundled lines;

[0054] ③ The physical meaning is clear, the calculation is simple, and the reliability analysis result is accurate. BRIEF DESCRIPTION OF DRAWINGS

[0055] The present application will be further described below in combination with the drawings and examples;

[0056] Figure 1 For the high-proportion new energy bundled external transmission scene.

[0057] Fig. 2(a) is a schematic diagram of the positive sequence network of the system;

[0058] Fig. 2(b) is a schematic diagram of the negative sequence network of the system.

[0059] Fig. 3(a) is a schematic diagram of positive and negative sequence impedance under negative sequence suppression of inverter type power supply;

[0060] Fig. 3(b) is a schematic diagram of positive and negative sequence impedance under negative sequence suppression of double-fed type power supply.

[0061] Figure 4 is a schematic diagram of equivalent impedance of inverter type station.

[0062] Fig. 5(a) is a schematic diagram of phase selection risk of different power supply types (crowbar resistance is not in action);

[0063] Fig. 5(b) is a schematic diagram of phase selection risk of different power supply types (crowbar resistance is in action).

[0064] Fig. 6(a) is a phase current difference mutation variable phase selection case (fault current);

[0065] Fig. 6(b) is a phase current difference mutation variable phase selection case (phase current difference mutation variable amplitude).

[0066] Fig. 7(a) is a positive sequence equivalent impedance network when the crowbar of double-fed type power supply is put in;

[0067] Fig. 7(b) is a negative sequence equivalent impedance network when the crowbar of double-fed type power supply is put in. DETAILED DESCRIPTION

[0068] A phase current difference mutation variable phase selection reliability analysis method considering new energy penetration rate and fault ride-through behavior is provided. The method first constructs a failure index of phase current difference mutation variable phase selection, takes single type new energy station impedance as the basis, considers actual fault conditions and new energy access conditions, establishes a station equivalent model containing multiple types of new energy coupling, and finally realizes reliability analysis of phase current difference mutation variable phase selection based on sequence network calculation of positive and negative sequence current distribution coefficient ratio relationship.

[0069] Specifically includes the following steps:

[0070] Step 1: Construction of phase current difference mutation variable phase selection failure index:

[0071] Taking A-phase grounding fault as an example, the phase current difference mutation variable phase selection criterion is shown in formula (1):

[0072]

[0073] Wherein, m represents the reliability coefficient, and the commonly used setting value in engineering is 4-8; and respectively represent the amplitudes of AB, BC and CA phase current difference mutation variables; && is the logical "and" operation.

[0074] Based on the phase selection criterion of phase current difference mutation amount (1), a phase selection failure index R based on phase current difference mutation amount is constructed. AG As shown in equation (2):

[0075]

[0076] Among the systems that focus on high-proportion wind, solar, and thermal power bundling, only R... AG The calculated value must be greater than m for correct phase selection to be achieved; otherwise, the phase selection will fail due to the sudden change in phase current difference.

[0077] Step Two: Impedance Behavior Analysis of Single-Type New Energy Sources

[0078] Based on the structure of grid-connected inverters, new energy sources are classified into two categories: inverter-type and doubly-fed type. The impedance of different types of new energy sources under fault ride-through conditions is primarily considered.

[0079] (1) Inverter type power supply:

[0080] Considering the low-voltage ride-through strategy and negative-sequence suppression strategy during inverter power supply faults, the inverter power supply station has no negative-sequence current output and does not participate in the formation of the system's negative-sequence network, so the negative-sequence impedance is equivalent to infinity. The positive-sequence equivalent impedance is related to its low-voltage ride-through strategy and mainly involves the voltage drop depth. Different fault conditions (voltage drop depths) have different positive-sequence current command values, and the corresponding positive-sequence impedances are different.

[0081] (2) Doubly fed power supply:

[0082] Considering the crowbar current limiting strategy and low voltage ride-through strategy during a doubly-fed power supply fault, if the crowbar is engaged for current limiting and the converter is locked, the doubly-fed power supply is equivalent to an asynchronous motor. In this case, the equivalent positive and negative sequence impedances of the power station are related to the slip rate, and there is a significant difference between the two. If the crowbar is not engaged, the converter performs low voltage ride-through control. Due to the existence of the negative sequence suppression strategy, the doubly-fed power station has no negative sequence current output, and the equivalent negative sequence impedance is infinite. The equivalent positive sequence impedance is related to its low voltage ride-through strategy, similar to that of an inverter power supply.

[0083] Step 3: Equivalent impedance modeling of new energy power plants with multiple energy types coupled:

[0084] Since the power supply side includes the coupling of multiple types of new energy sources, the new energy penetration rate p on the power supply side is first defined as shown in equation (3):

[0085]

[0086] Where p represents the penetration rate of new energy sources on the power supply side, and p belongs to 0 to 1; k1 and k2 represent the superposition coefficients of inverter-type and doubly-fed induction generator units, respectively; P PV P DFIGP S P

[0087] The formula (3) is arranged to obtain the relationship of superposition coefficient about permeability:

[0088]

[0089] It can be seen that the superposition coefficients k1 and k2 are not completely decoupled, which is inconvenient for analysis. Further, the wind-solar ratio k2P DFIG / k1P PV =a, at this time, the formula (4) can be simplified as:

[0090]

[0091] Wherein A, B are the rated power ratio of synchronous power supply and inverter type power supply, doubly-fed power supply respectively; a is the wind-solar ratio.

[0092] (1) Inverter type station:

[0093] In order to facilitate analysis, it is assumed that the wind power access is 0 (a=0), and the equivalent impedance behavior of the inverter under the negative sequence suppression and low voltage ride through control is analyzed by taking the photovoltaic station as an example. The greater the permeability, the greater the positive sequence current output of the photovoltaic station, and the positive sequence current of the inverter type station can be expressed as:

[0094]

[0095] Wherein: represents the positive sequence current output of a single generating unit.

[0096] Combined with the low voltage ride through requirement of new energy in China, can be expressed as:

[0097]

[0098] In the formula, I sig1 represents the positive sequence current amplitude of the inverter type station; φ, δ respectively represent the voltage and current phase after the fault; and are d, q axis current command values respectively; i d(0) represents the d-axis current command value in steady state operation; I max is the overcurrent limit value allowed by the inverter, generally taking 1.2 times the rated current; U PV1 represents the positive sequence voltage effective value of the grid-connected point.

[0099] The formula (7) is brought into the formula (6), and it is assumed that the voltage after the fault is The expression of the invertor station positive sequence impedance about the penetration and the voltage drop depth is as follows:

[0100]

[0101] (2) Doubly-fed station:

[0102] For the doubly-fed power supply, if the crowbar is not put into current limiting, the station equivalent impedance is the same as the invertor power supply, which is calculated according to formula (6)-(8);

[0103] If the crowbar is put into current limiting, the positive and negative sequence impedances of the doubly-fed station are independent of the fault condition, and the calculation method is as follows:

[0104]

[0105] Wherein, Z DFIG1 and Z DFIG2 are the positive and negative sequence impedances of the doubly-fed station; X rσ , R r and R c represent the rotor leakage reactance, rotor resistance and crowbar resistance respectively; X m is the excitation reactance; R s and X sσ represent the stator resistance and leakage reactance respectively; X T and X b represent the transformer and main transformer reactance respectively; s represents the slip.

[0106] Step four: reliability analysis of the phase selection method considering the penetration and fault ride-through behavior:

[0107] According to the sequence network, the calculation method of the phase selection failure index, and is as follows:

[0108]

[0109] The formula (10) is brought into the failure index shown in formula (2), and the following is obtained:

[0110]

[0111] Taking the maximum margin R AG =4 as the critical condition of phase selection failure, formula (10) can be solved as follows: or

[0112] Taking the high proportion of new energy bundled sending scene shown by formula (11) as an example, the sequence network of the system is analyzed in detail. Figure 1 In the topology shown by formula (12), the power supply side contains three typical energy sources of wind, light and fire, which jointly constitute the energy base on the power generation side. Figure 1 In the topology shown by formula (12), the power supply side contains three typical energy sources of wind, light and fire, which jointly constitute the energy base on the power generation side. Figure 1The sequence network of the power system is shown in FIG. 2(a) and FIG. 2(b). The subscripts 1 and 2 in FIG. 2(a) and FIG. 2(b) represent positive and negative sequence parameters, respectively; and represent the current fed by the photovoltaic and wind turbine, respectively; is the voltage of the power source side synchronous generator; is the equivalent voltage of the receiving end system; SN is the equivalent impedance of the receiving end; LM and Z LN represent the impedance of the line on both sides of the fault point; T+L(P) , Z T+L(W) and Z T+L(S) represent the sum of the outgoing line impedance and the transformer impedance of the photovoltaic station, the wind power station and the synchronous generator group, respectively; PV , Z DFIG and Z SM are the internal resistances of the photovoltaic, wind power and synchronous generator, respectively. For the sending end system on the back side of the outgoing line, the wind, light and thermal power systems can be equivalent, and the equivalent internal resistance of the system is simplified as Z S , as shown in the blue part in the figure.

[0113] In the scenario of negative sequence suppression, the negative sequence impedance of the new energy is infinite (equivalent open circuit), and in the sequence network, the negative sequence impedance of the new energy station can be regarded as not being connected. The positive sequence impedance is a certain fixed value, and according to FIG. 2(a) and FIG. 2(b), the impedance calculation method of the positive and negative sequence current distribution coefficient is:

[0114]

[0115] In formula (12), the numerator is not affected by the connection of the new energy. The equivalent positive sequence impedance Z S1 on the power source side in the denominator is determined by the parallel relationship, and therefore Z S1 < Z SM1 , that is, the positive sequence impedance on the power source side is smaller than the positive sequence impedance of the synchronous generator. The equivalent negative sequence impedance Z S2 on the power source side in the denominator is affected by the negative sequence open circuit of the new energy, and therefore Z S2 = Z SM2 . It can be known from the above that in the wind, light and thermal power bundled outgoing scenario, the equivalent positive sequence impedance Z S1 on the power source side will develop in the direction of being smaller than the equivalent negative sequence impedance Z S2 , and it can be known that the positive and negative sequence current distribution coefficient should be taken as

[0116] The equivalent impedance of the inverter type and the doubly-fed type in step three is brought into formula (12), and the ratio of C2 and C1 is calculated, if less than or equal to 0.64, it is proved that the phase selection face failure risk under the current penetration and fault condition. If the ratio of C2 and C1 is greater than 0.64, it is proved that there is no risk of phase selection failure under the current fault condition and penetration.

[0117] Example verification:

[0118] Taking the typical high-proportion new energy bundling sending scene shown in Figure 1 as an example, a simulation model is built in PSCAD to verify the effectiveness of the method.

[0119] Among them, the installed capacity of the inverter type photovoltaic station is 250MW, the installed capacity of the doubly-fed wind power is 500MW, and the wind-light ratio is 2. The rated capacity of the synchronous generator is 500MW.

[0120] (1) Verification of the behavior of the equivalent impedance of a single type of new energy.

[0121] The backside equivalent impedance of the inverter type station and the doubly-fed type station is measured respectively, and the results are shown in FIG. 3(a) and FIG. 3(b). As can be seen, under the scene of negative sequence suppression, the negative sequence equivalent impedance of the inverter type station is much larger than the positive sequence impedance, which is consistent with the theoretical analysis, and in the sequence network analysis, the negative sequence impedance under negative sequence suppression can be regarded as an open circuit. After the crowbar resistance is put into the doubly-fed type power supply, due to the existence of the slip rate, the positive and negative sequence impedances are not equal, and there is a large difference between them, which lays a foundation for subsequent analysis of the risk of phase selection failure.

[0122] (2) Verification of the relationship between the equivalent impedance of the inverter type station and the penetration and voltage drop depth

[0123] Since the fault ride-through behavior of the inverter type station is only related to the voltage drop depth, the voltage drop depth is used to reflect the fault ride-through behavior of the inverter type power supply. Taking A=20 in formula (8), the impedance relationship shown in FIG. 8 can be drawn. Further, the equivalent impedance of the inverter type station is measured in PSCAD, as shown by the circles in FIG. 9. As can be seen, the measured impedance value changes in the same trend as the theoretical impedance plane when the penetration changes, which shows that the equivalent impedance modeling method of the inverter type station is reasonable and has high accuracy. Figure 4 Figure 4

[0124] ​​(3) The phase selection method reliability analysis considering the penetration and fault ride-through behavior. The voltage drop to 0.6 p.u. is set as the limit of the double-fed power source to put in the pry resistance, the impedance change law obtained above is brought into formula (14) and the ratio is calculated to obtain the phase selection risk failure, i.e. the phase selection reliability analysis conclusion. As can be seen from Fig. 5, when the penetration p is 0, it is equivalent to that there is no new energy access in the system, and the grid only contains conventional synchronous power source, and the ratio of positive and negative sequence current distribution coefficient returns to 1. After high proportion of new energy is bundled and sent out, with the increase of penetration and the deepening of voltage drop, the ratio of C1 and C2 decreases, and the trend is shown in Fig. 5(a), Fig. 5(b). The critical plane is drawn with C1 / C2=0.64, and it can be known that when the ratio of C1 and C2 is reduced to below 0.64, the conventional phase current difference mutation variable phase selection fails, and the reliability analysis process is completed.

[0125] Further, based on the established PSCAD simulation model, the line midpoint AG fault is set, the voltage drop is to 0.83 p.u., the penetration is 60%, the negative sequence suppression and low voltage ride-through are started, and the pry protection does not act. The effectiveness of the reliability analysis method of the application is verified.

[0126] According to the actual fault working condition, combined with Fig. 5(a), it can be known that the actual ratio of C1 and C2 is below the failure risk plane of 0.64, and there is a risk of failure. The actual fault current and the phase current difference mutation variable amplitude measured on the power side are shown in Fig. 6(a), Fig. 6(b), and it can be seen that the effective value of the three-phase current difference mutation variable does not meet the criterion shown in formula (1), for example Less than 4, the conventional phase current difference mutation variable selection has failed, and the fault phase cannot be correctly selected, which is consistent with the conclusion of the risk analysis of the application, and illustrates the correctness and effectiveness of the phase current difference mutation variable phase selection reliability analysis method of the application.

Claims

1. A phase current difference sudden change variable selection reliability analysis method considering new energy penetration rate and fault ride-through behavior, characterized in that The method comprises the following steps: Step 1: constructing a phase selection failure index based on the phase current difference mutation variable amplitude ratio based on a phase current difference mutation variable phase selection criterion; Step 2: dividing the new energy type according to the inverter type and the doubly-fed type, and determining the fault equivalent impedance of the grid-connected inverter of different types; Step 3: establishing an equivalent impedance model of a new energy station coupled with multiple types of energy; In step 3, the equivalent impedance modeling of the new energy station coupled with multiple types of energy is as follows: Power supply side new energy penetration rate is defined as shown in equation (3): (3)； In formula (3), p represents the new energy penetration rate on the power supply side; k 1、 k 2 respectively represent the superposition coefficients of the inverter type and the doubly-fed type power generation units; 、 respectively represent the rated power of the inverter type and the doubly-fed type power generation units, is the rated power of the synchronous generator; wherein the superposition coefficients k 1、 k 2 the decoupling representation method is: (5)； In formula (5): A , B Pwind / Psolar and Pwind / Pinv are the rated power ratio of synchronous power supply and inverter power supply, double-fed power supply, respectively; a Pwind / Psolar is the wind-solar ratio. Step 4: converting the phase selection failure index in step 1 into a relationship of positive and negative sequence current distribution coefficients, bringing the equivalent impedance of the new energy station in step 3 into the sequence network analysis, calculating the ratio of the positive and negative sequence current distribution coefficients, comparing the relationship with the critical failure condition, and completing the reliability analysis of the phase selection method.

2. The phase current difference abrupt variable directional reliability analysis method considering new energy penetration rate and fault ride-through behavior according to claim 1, characterized in that: In step 1, for the A-phase ground fault, the phase current difference mutation variable phase selection criterion is as shown in formula (1): (1)； In formula (1): represents a reliability coefficient; , and respectively represent the amplitude of the phase current difference mutation variable of AB, BC, and CA; is a logical AND operation.

3. The phase current difference abrupt variable directional reliability analysis method considering new energy penetration rate and fault ride-through behavior according to claim 2, characterized in that: The phase selection failure index is constructed based on the phase current difference abrupt variable selection criterion of formula (1) As shown in formula (2): (2)； In the system of high proportion of wind, light, fire and package, only The calculated value is greater than m The correct phase selection can be realized; otherwise, the phase current difference mutation phase selection fails.

4. The phase current difference abrupt variable directional reliability analysis method considering new energy penetration rate and fault ride-through behavior according to claim 1, characterized in that: In step 2, for the inverter type power supply, during the fault of the inverter type power supply, the inverter type power supply will output reactive power to support the system voltage, so the low voltage ride through strategy is implemented; at the same time, the inverter type power supply will suppress the negative sequence current output in the control link, and the negative sequence suppression strategy is executed; At this time, the inverter type power station has no negative sequence current output, does not participate in the construction of the system negative sequence network, and the negative sequence impedance equivalent is infinite; the positive sequence equivalent impedance is equal to the ratio of the grid connection point voltage and current, the grid connection point voltage is affected by the fault condition, and the grid connection point current is the current output by the inverter type power supply under the low voltage ride through strategy, so the positive sequence equivalent impedance is essentially related to the low voltage ride through strategy.

5. The phase current difference abruptness variable directional reliability analysis method considering new energy penetration rate and fault ride-through behavior according to claim 4, characterized in that: In step 2, for the doubly-fed type power supply, consider two types of control methods during the fault of the doubly-fed type power supply: when a serious fault occurs, the crowbar current limiting strategy is put into operation, at this time the converter is locked, and the rotor is short-circuited through the crowbar resistance to avoid damage to the rotor caused by large current during a serious fault; when a non-serious fault occurs and the crowbar protection is not triggered, the low voltage ride through strategy is executed by the converter, at this time the converter is controlled to output reactive power to support the system voltage.

6. The phase current difference mutation variable phase selection reliability analysis method considering the new energy penetration rate and fault ride through behavior according to claim 5, characterized in that: If the crowbar is put into current limiting and the converter is locked, the doubly-fed type power supply is equivalent to an asynchronous motor, at this time the positive and negative sequence impedances of the doubly-fed type power station are related to the slip; after the crowbar resistance is put into current limiting, there is a large difference between the positive and negative sequence impedances of the equivalent impedance network of the doubly-fed type station; If the crowbar is not put into operation and the low voltage ride through control is performed by the converter, the doubly-fed type power station does not output negative sequence current due to the implementation of the negative sequence suppression strategy, does not participate in the construction of the system negative sequence network, and the negative sequence impedance equivalent is infinite; the positive sequence equivalent impedance is related to the low voltage ride through strategy, and the positive sequence equivalent impedance is equal to the ratio of the grid connection point voltage and current, the grid connection point voltage is related to the fault condition, and the grid connection point current is equal to the current output by the doubly-fed type power supply under the low voltage ride through strategy.

7. The phase current difference abruptness variable directional reliability analysis method considering new energy penetration rate and fault ride-through behavior according to claim 1, characterized in that: For inverter power supply: positive sequence equivalent impedance of inverter station under low voltage ride through target Is: (8)； In formula (8), represents a positive sequence voltage of a grid connection point; represents a positive sequence current output from an inverter-type substation; and are, respectively, an effective value and a phase of a positive sequence voltage; represents a d-axis current; represents a phase of a positive sequence current; represents a value impedance of a single power generation unit in an inverter-type substation; For the inverter type power supply, the negative sequence equivalent impedance of the inverter type station is infinite due to the existence of the negative sequence suppression strategy.

8. The phase current difference abruptness variable directional reliability analysis method considering new energy penetration rate and fault ride-through behavior according to claim 7, characterized in that: For the doubly-fed type power supply, if the crowbar is not put into current limiting, the behavior characteristics of the positive and negative sequence equivalent impedance of the station are the same as those shown in formula (8) for the inverter type power supply. If the crowbar is put into current limiting, the doubly-fed power station's Positive and negative sequence impedance Irrespective of the fault condition, the calculation method is as follows: (9)； In formula (9), and are positive and negative sequence impedances of the double-fed station, respectively; X rσ , R r and R c represent rotor leakage impedance, rotor resistance, and crowbar resistance, respectively; X m is the excitation reactance; R s and X sσ represent stator resistance and leakage impedance, respectively; X T , X b represent transformer and main transformer reactance, respectively; s represents the slip.

9. The phase current difference abruptness variable directional reliability analysis method considering new energy penetration rate and fault ride-through behavior according to claim 8, characterized in that: In step 4, the phase selection failure index shown in formula (1) is converted into an impedance-related index, taking m = 4, to obtain: (13)； In formula (13): C 1、 C 2 is a positive and negative sequence current distribution coefficient; According to the sequence network analysis, it is obtained C 1、 C The impedance calculation method of 2 is: (14)； In formula (14): Z S Zs is the impedance of the power supply side; Z L Zl is the impedance of the line; Z SN Zr is the impedance of the receiving system; subscript 1 represents the positive sequence impedance; subscript 2 represents the negative sequence impedance; The equivalent impedance of the inverter type and doubly-fed type power supply considering the penetration rate and voltage drop depth shown in formula (9) and formula (8) is brought into formula (14), and the ratio is calculated. If the ratio is less than 0.64, the phase current difference jump under the current penetration rate and voltage depth will fail to select the phase; otherwise, the phase selection method is not affected.

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