A flexible low-frequency power transmission system fault current analysis method

Abstract

The application discloses a flexible low-frequency power transmission system fault current analysis method and relates to the technical field of power system fault protection, which comprises the following steps: firstly, collecting physical parameters of flexible low-frequency power transmission system components and electrical parameters during system steady operation; according to the time sequence of M3C converter action, dividing the fault current in the flexible low-frequency power transmission system into the M3C converter topology response stage and the M3C converter control response stage; collecting electrical parameters during low-frequency power transmission system operation fault; during the M3C converter control response stage, establishing an equivalent source theoretical model of the M3C converter and a fault loop equation of the flexible low-frequency power transmission system; simultaneously solving the fault current in the M3C converter control response stage; and finally obtaining the flexible low-frequency power transmission system fault current. The influence of the flexible low-frequency power transmission system fault on the converter itself is considered, so that the analysis of the flexible low-frequency power transmission system fault current is more accurate.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power system fault protection, and in particular to a flexible low-frequency power transmission system fault current analysis method. BACKGROUND

[0002] General power transmission systems are divided into flexible low-frequency power transmission systems, power frequency power transmission systems, and direct current power transmission systems. The flexible low-frequency power transmission system reduces the influence of the skin effect by reducing the power transmission frequency, reduces the line reactance and charging power, and thus improves the power transmission efficiency. In the field of offshore wind power, compared with the traditional power frequency power transmission system, the cable charging current of the flexible low-frequency power transmission system is smaller, and the power transmission radius is larger; compared with the direct current power transmission system, there is no need to build an offshore converter station, and there is no problem of breaking the circuit breaker without a zero point, which has significant technical and economic advantages in the long-distance offshore wind power transmission scene.

[0003] However, compared with the power frequency power transmission system, the flexible low-frequency power transmission system has power electronic control links such as the alternating converter, and the fault response presents a multi-time scale characteristic; compared with the direct current power transmission system, the topology and control structure of the alternating converter are different from those of the rectifier / inverter in the direct current system, and the dynamic characteristics are quite different. Therefore, the fault characteristic analysis conclusions of the power frequency power transmission system and the direct current power transmission system cannot be applied to the flexible low-frequency power transmission system. At the same time, the fault current characteristics of the flexible low-frequency power transmission system can guide the design, testing and manufacturing of primary devices such as low-frequency circuit breakers and fault current limiters, avoiding the problems of large manufacturing difficulty and insufficient economy caused by the design of the primary devices still using the design ideas of the power frequency system. Therefore, the analysis of the fault current of the flexible low-frequency power transmission system is crucial.

[0004] The Chinese invention patent CN116148589A, which was published on May 23, 2023, discloses a "low-frequency power transmission system fault current simplified analysis method and system", which equivalent the M3C converter to a capacitor, divides the current evolution process after the fault into three stages, establishes the equivalent RLC circuit of each stage, and synthesizes the currents of each stage to obtain the fault current analysis formula.

[0005] The current short-circuit current calculation method equivalent the bridge arm of the converter to an RLC discharge power supply, and calculates the short-circuit current based on different discharge circuits after the fault. Although this method can reflect the current characteristics after the fault to some extent, it ignores the influence of the fault on the converter itself. SUMMARY

[0006] In order to overcome the defects in the prior art that ignore the influence of the flexible low-frequency power transmission system fault on the converter itself, the present application proposes a flexible low-frequency power transmission system fault current analysis method.

[0007] To achieve the above objectives, the present invention adopts the following technical solution, including:

[0008] A method for analyzing fault current in a flexible low-frequency transmission system, wherein the fault current in the flexible low-frequency transmission system is a three-phase short-circuit fault current, comprising:

[0009] S1: Obtain the equivalent inductance L of the low-frequency transformer in each phase circuit of the flexible low-frequency transmission system. T and equivalent resistance R T The equivalent inductance L of each phase in the three-phase circuit between the converter grid connection point and the fault point. Σ1 and resistance R Σ1 Bridge arm inductor L arm Bridge arm resistance R arm and bridge arm capacitor C n ;

[0010] S2: Based on the timing of the M3C converter's operation, the fault current in the flexible low-frequency transmission system is divided into two stages for calculation: the M3C converter topology response stage and the M3C converter control response stage.

[0011] S3: Collect the dq-axis voltage E at the output point of the M3C converter during a low-frequency power transmission system malfunction. d and E q M3C converter grid connection point dq axis voltage u d and u q The phase-locked loop's phase-locked angle θ and its angular frequency ω;

[0012] S4: The discharge scenarios of each sub-converter during the topology response stage of the M3C converter are divided into two categories: Scenario 1: discharge from one phase arm to two phase arms; Scenario 2: discharge from two phase arms to one phase arm; the fault current i during the topology response stage of the M3C converter... ft This includes the sum of the discharge currents of each sub-converter during the topology response phase;

[0013] S5: Establish the equivalent source theoretical model of the M3C converter and the fault loop equation of the flexible low-frequency transmission system, and simultaneously solve the fault current i during the control response stage of the M3C converter. fc ;

[0014] S6: Obtain the fault current i of the flexible low-frequency transmission system f The expression is:

[0015]

[0016] Where t0 is the initial time of the topology response, t1 is the initial time of the control response, t2 is the converter lockout time, and t is the fault occurrence time.

[0017] Preferably, in step S4, the M3C converter topology responds to the phase fault current i ft is the sum of the currents of each sub-converter; the discharge condition of each sub-converter is divided into two, which are the discharge condition of one-phase bridge arm to two-phase bridge arm or the discharge condition of two-phase bridge arm to one-phase bridge arm; correspondingly, the current of the sub-converter is the current in the equivalent circuit of the discharge condition of one-phase bridge arm to two-phase bridge arm or the current in the equivalent circuit of the discharge condition of two-phase bridge arm to one-phase bridge arm.

[0018] Preferably, in step S4, the calculation process of the current in the equivalent circuit of the discharge condition of one-phase bridge arm to two-phase bridge arm is as follows:

[0019] For any sub-converter, the discharge condition of one-phase bridge arm to two-phase bridge arm is equivalent to a loop composed of an equivalent capacitor C1, an equivalent inductor L1 and an equivalent resistor R1 connected in series.

[0020] The current i ft1 in the equivalent circuit of the discharge condition of one-phase bridge arm to two-phase bridge arm is:

[0021]

[0022] wherein A1 and A2 are constant coefficients of the discharge condition of one-phase bridge arm to two-phase bridge arm; ω1 is the angular frequency; the expressions of the equivalent capacitor C1, the equivalent inductor L1 and the equivalent resistor R1 are as follows:

[0023] C1=(C na +C nb )*C nc / (C na +C nb +C nc )

[0024]

[0025] wherein C na , C nb and C nc are capacitors in the three-phase bridge arm.

[0026] Preferably, step S5 comprises:

[0027] S51: establishing an equivalent source theoretical model based on the control characteristics of the converter according to the characteristics of the converter grid-connected point voltage controlling the converter outlet point voltage;

[0028] S52: respectively establishing fault loop equations from the converter grid-connected point and the converter outlet point to the fault point;

[0029] S53: obtain the dq-axis components of the fault current of the flexible low-frequency power transmission system by simultaneously using the equivalent source theoretical model of the M3C converter and the fault loop equation of the flexible low-frequency power transmission system;

[0030] S54: obtain the fault current i fc .

[0031] Preferably, the equivalent source theoretical model is:

[0032]

[0033] wherein u d , u q are the dq-axis components of the outlet point voltage of the M3C converter; k p1 , k p2 are the inner and outer loop proportional regulation coefficients of the voltage-frequency control; k i1 , k i2 are the inner and outer loop integral regulation coefficients of the voltage-frequency control; E * d , E * q are the dq-axis outer loop command values; E d , E q are the dq-axis components of the grid-connected point voltage of the M3C converter, which is the voltage feedforward term; i d , i q are the dq-axis components of the system fault current; w is the phase-locked loop angular frequency. is the integral of the difference between the grid-connected point voltage command value and the actual value of the converter.

[0034] Preferably, the fault loop equations of the grid-connected point and the outlet point of the converter to the fault point are:

[0035]

[0036] wherein E d , E q are the dq-axis components of the grid-connected point voltage of the M3C converter; u d , u q are the dq-axis components of the outlet point voltage of the M3C converter; i d , i q are the dq-axis components of the system fault current.

[0037] Preferably, in step S53, the expressions of the dq-axis components i d , i q of the fault current of the flexible low-frequency power transmission system are:

[0038]

[0039] wherein, c1, c2, c3, c4, c5, c6, c7 and c8 are different proportional coefficients; α1, α2, α3, α4, α5 and α6 represent different attenuation components respectively; β1, β2, β3, β4, β5 and β6 represent different angular frequencies of periodic components respectively; μ1, μ2, μ3, μ4, μ5 and μ6 represent different phase angles of periodic components respectively; t is the fault occurrence time.

[0040] Preferably, in step S54, the M3C converter control responds to the fault current i fc , and the calculation formula is:

[0041]

[0042] wherein, i d and i q are dq-axis components of the system fault current respectively.

[0043] Preferably, in step S4, the calculation process of the current in the equivalent circuit of the two-phase bridge arm discharging to the one-phase bridge arm is as follows:

[0044] For any sub-converter, the two-phase bridge arm discharging to the one-phase bridge arm is equivalent to a loop composed of an equivalent capacitor C2, an equivalent inductor L2 and an equivalent resistor R2 connected in series.

[0045] The analytical expression of the current i ft2 in the equivalent circuit of the two-phase bridge arm discharging to the one-phase bridge arm is as follows:

[0046]

[0047] wherein, A3 and A4 are constant coefficients of the two-phase bridge arm discharging to the one-phase bridge arm; w2 is the angular frequency, and the calculation formula is as follows:

[0048]

[0049] wherein, the calculation formulas of the equivalent capacitor C2, the equivalent inductor L2 and the equivalent resistor R2 are as follows respectively:

[0050] C2=C na *(C nb +C nc ) / (C na +C nb +C nc )

[0051]

[0052] wherein, C na , C nbC nc are capacitors in the three-phase bridge arm.

[0053] A flexible low-frequency power transmission system suitable for the fault current analysis method of the flexible low-frequency power transmission system, comprising: an M3C converter, a low-frequency transformer, a low-frequency power transmission line, a fan and an alternating current power supply; the power frequency side of the M3C converter is connected with the alternating current power supply, the low-frequency side is connected with the valve side of the low-frequency transformer, one end of the low-frequency line is connected with the network side of the low-frequency transformer, and the other end is connected with the fan;

[0054] The M3C converter comprises three sub-converters, each of which comprises the same three-phase bridge arm, each phase bridge arm is composed of a bridge arm inductor, a bridge arm resistor and N sub-modules in series, and each sub-module comprises an energy storage capacitor and four insulated gate bipolar diodes.

[0055] One end of the three sub-converters is connected with three-phase lines of the alternating current power supply respectively, and each phase line comprises an inductor and a resistor; the other end of the three sub-converters is connected with three-phase lines of the valve side of the low-frequency transformer.

[0056] The advantages of the present application are:

[0057] (1) The present application firstly collects the physical parameters of the components of the flexible low-frequency power transmission system and the electrical parameters during the stable operation of the system, divides the fault current in the flexible low-frequency power transmission system into the M3C converter topology response stage and the M3C converter control response stage according to the time sequence of the action of the M3C converter, collects the electrical parameters during the operation fault of the low-frequency power transmission system, establishes the equivalent source theoretical model of the M3C converter and the fault loop equation of the flexible low-frequency power transmission system in the M3C converter control response stage, solves the fault current in the M3C converter control response stage, and finally obtains the fault current of the flexible low-frequency power transmission system; the influence of the fault of the flexible low-frequency power transmission system on the converter itself is considered, so that the analysis of the fault current of the flexible low-frequency power transmission system is more accurate.

[0058] (2) The present application derives the full-stage fault current analysis of the flexible low-frequency power transmission system under three-phase short-circuit fault from the equivalent circuit of the topology response stage and the equivalent source theoretical model of the control response stage, so that the fault current parameters can be obtained without a large amount of modeling simulation, and the present application has strong engineering application value.

[0059] (3) The present application divides the fault current into the topology response stage (M3C converter hardware discharge characteristics) and the control response stage (converter control strategy), overcomes the difficulty of traditional methods in considering the transient response and control characteristics of power electronic equipment, and significantly improves the accuracy of fault current calculation.

[0060] (4) The application is directed to the complex structure of the M3C converter, and the sub-converter discharge condition is divided into two categories of "one-phase-to-two-phase discharge" and "two-phase-to-one-phase discharge", and equivalent circuit models (including capacitance, inductance and resistance parameters) are established respectively, which more truly reflect the physical process in the initial stage of fault.

[0061] (5) The equivalent source theory model is introduced in the fault analysis of the application, and the control parameters such as the inner and outer loop proportional / integral coefficients of the voltage-frequency control, the phase-locked loop dynamic (angular frequency w) are solved together with the fault loop equation, so as to realize the accurate analysis of the current in the control response stage.

[0062] (6) The method of the application collects key variables such as dq-axis voltages (Ed, Eq, ud, uq) of the converter outlet point / grid-connected point, phase-locked angle, and bridge arm parameters (Larm, Rarm, Cn), covering all elements from hardware to control system, and is suitable for fault reproduction and protection design in actual engineering. BRIEF DESCRIPTION OF DRAWINGS

[0063] Figure 1 It is a topology diagram of the flexible low-frequency power transmission system in the application;

[0064] Figure 2 It is a topology structure diagram of the M3C converter;

[0065] Figure 3 It is a structure diagram of the SM sub-module of the M3C converter;

[0066] Figure 4 It is an equivalent circuit diagram of the discharge condition one in the topology response stage;

[0067] Figure 5 It is an equivalent circuit diagram of the discharge condition two in the topology response stage;

[0068] Figure 6 It is a structure diagram of the control system of the M3C converter;

[0069] Figure 7 It is a result diagram of the fault current analytical calculation verification of the embodiment of the application;

[0070] Figure 8 It is a flowchart of the calculation method of the application. DETAILED DESCRIPTION

[0071] The technical solutions in the embodiments of the application will be described clearly and completely in combination with the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the application.

[0072] As shown in Figure 1 A flexible low-frequency power transmission system, comprising: an M3C converter (modular multilevel matrix converter), a low-frequency transformer, a low-frequency power transmission line, a fan and an AC power supply; the power frequency side of the M3C converter is connected with the AC power supply, the low-frequency side is connected with the valve side of the low-frequency transformer, one end of the low-frequency line is connected with the network side of the low-frequency transformer, and the other end is connected with the fan;

[0073] The low-frequency side of the M3C converter is in a constant voltage / frequency control mode;

[0074] The M3C converter comprises three sub-converters, each of which comprises the same three-phase bridge arm, each phase bridge arm is composed of a bridge arm inductance L arm , a bridge arm resistance R arm and N sub-modules in series, each sub-module contains an energy storage capacitor C and four insulated gate bipolar transistors, has positive, negative and zero three-level output capability, adjusts the switching state of the sub-module, accurately matches the AC voltage difference on both sides, and realizes the direct decoupling of the low-frequency side and the power frequency side of the M3C converter;

[0075] One end of each of the three sub-converters is connected with a three-phase line of the AC power supply, each phase line comprises an inductance L and a resistance Rs, and the voltages in the three-phase line are U sa , U sb and U sc ; the other end of each of the three sub-converters is connected with a three-phase line of the valve side of the low-frequency transformer, and the equivalent inductance and the equivalent resistance of the low-frequency transformer in each phase line are L T and R T ;

[0076] As shown in Figure 8 The present application provides a flexible low-frequency power transmission system fault current analysis method, the flexible low-frequency power transmission system fault current is a three-phase short-circuit fault current, the equivalent inductance and the resistance of each phase circuit in the three-phase circuit connected with the grid connection point and the fault point of the converter are L Σ1 and R Σ1 , and the method comprises:

[0077] S1: obtaining the equivalent inductance L T and the equivalent resistance R T of the low-frequency transformer in each phase line of the flexible low-frequency power transmission system, the equivalent inductance L Σ1 and the resistance R Σ1 of each phase line in the three-phase line connected with the grid connection point and the fault point of the converter, the bridge arm inductance L arm , the bridge arm resistance R arm and the bridge arm capacitor;

[0078] S2: According to the time sequence of the M3C converter action, the fault current in the flexible low-frequency power transmission system is divided into two stages for calculation, which are the M3C converter topology response stage and the M3C converter control response stage;

[0079] S3: Collect the dq-axis voltage E d and E q of the M3C converter outlet point when the low-frequency power transmission system operates in fault; d and u q of the dq-axis voltage of the M3C converter grid connection point;

[0080] S4: The discharge situation of each sub-converter in the M3C converter topology response stage is divided into two categories, which are one-phase bridge arm discharging to two-phase bridge arms and two-phase bridge arms discharging to one-phase bridge arm, which are respectively recorded as situation one and situation two; the fault current i ft in the M3C converter topology response stage is the sum of the discharge currents of each sub-converter in the topology response stage, that is, i ft =i ft x+i ft y+i ft z, wherein x, y, and z are the discharge situations in different sub-converters, and can be valued as 1 or 2; i ft1 and i ft2 are the currents in the equivalent circuit of the one-phase bridge arm discharging to two-phase bridge arms situation and the two-phase bridge arms discharging to one-phase bridge arm situation, respectively;

[0081] The calculation process of the current in the equivalent circuit of the one-phase bridge arm discharging to two-phase bridge arms situation is as follows:

[0082] For any sub-converter, the equivalent circuit diagram of the one-phase bridge arm discharging to two-phase bridge arms situation is as shown in Figure 4 , which can be equivalent to a loop composed of an equivalent capacitor C1, an equivalent inductor L1, and an equivalent resistor R1 connected in series, the positive electrode of the equivalent capacitor C1 is connected with the equivalent inductor L1, and the negative electrode of the equivalent capacitor C1 is connected with the equivalent resistor R1;

[0083] The analytical expression of the current i ft1 in the equivalent circuit of the one-phase bridge arm discharging to two-phase bridge arms situation is as follows:

[0084]

[0085] wherein A1 and A2 are constant coefficients determined by the initial values of the capacitor voltage and the inductor current at the fault time, ω1 is the angular frequency, and the calculation formula is as follows:

[0086]

[0087] Wherein, the equivalent capacitance C1 is the capacitance C of the discharging phase bridge arm na in parallel with the capacitances C of the other two phase bridge arms nb nc in series; the equivalent inductance L1 is the inductance L of the discharging phase bridge arm arm in parallel with the inductances L of the corresponding phase low-frequency line Σ1 + L T in series, and then in parallel with the inductances L of the other two phase bridge arms arm in parallel with the inductances L of the corresponding phase low-frequency line Σ1 + L T in series; the equivalent resistance R1 is the resistance R of the discharging phase bridge arm arm in parallel with the resistance R of the corresponding phase low-frequency line Σ1 + R T in series, and then in parallel with the resistance R of the other two phase bridge arms arm in parallel with the resistance R of the corresponding phase low-frequency line Σ1 + R T in series, namely:

[0088] C1= (C na + C nb ) * C nc / (C na + C nb + C nc )

[0089]

[0090] C na , C nb , C nc are the capacitances in the three phase bridge arms respectively.

[0091] The calculation process of the current in the equivalent circuit of the discharging situation of the two phase bridge arms to the one phase bridge arm is as follows:

[0092] For any sub-converter, the equivalent circuit diagram of the discharging situation of the two phase bridge arms to the one phase bridge arm is as shown in Figure 5 , the discharging situation of the two phase bridge arms to the one phase bridge arm can be equivalent to a loop composed of an equivalent capacitance C2, an equivalent inductance L2 and an equivalent resistance R2 in series, the positive electrode of the equivalent capacitance C2 is connected with the equivalent inductance L2, and the negative electrode of the equivalent capacitance C2 is connected with the equivalent resistance R2;

[0093] The analytical expression of the current i ft2 in the equivalent circuit of the discharging situation of the two phase bridge arms to the one phase bridge arm is as follows:

[0094]

[0095] ​Wherein, A3 and A4 are constant coefficients determined by the initial value of the capacitor voltage, inductance current fault moment, w2 is the angular frequency, the calculation formula is:

[0096]

[0097] Wherein, the equivalent capacitor C2 is the capacitor C nb 、 nc Parallel with the third phase bridge arm capacitor C na Again in series with the capacitor value; the equivalent inductance L2 is the inductance L arm And the corresponding phase low-frequency line inductance L Σ1 +L T Parallel, and then with the third phase bridge arm inductance L arm And the corresponding phase low-frequency line inductance L Σ1 +L T In series again in series inductance value; the equivalent resistance R2 is the bridge arm resistance R arm And the corresponding phase low-frequency line resistance R Σ1 +R T Parallel, and then with the third phase bridge arm resistance R arm And the corresponding phase low-frequency line resistance R Σ1 +R T In series again in series resistance value; that is:

[0098] C2=C na *(C nb +C nc ) / (C na +C nb +C nc )

[0099]

[0100] S5: the establishment of M3C converter equivalent source theory model and flexible low frequency power transmission system fault loop equation, simultaneous solving M3C converter control response stage of fault current i fc ;

[0101] S51: according to the characteristics of the converter grid connected point voltage control converter outlet point voltage, the corresponding based on the equivalent source theory model of converter control characteristics is established to calculate the fault current of flexible low frequency power transmission system;

[0102] The equivalent source theory model of M3C converter is:

[0103]

[0104] Wherein, u d , u q The dq axis component of M3C converter outlet point voltage; kp1 , k p2 are the inner and outer loop proportional regulation coefficients of voltage-frequency control, respectively; i1 , k i2 are the inner and outer loop integral regulation coefficients of voltage-frequency control, respectively; * d , E * q are the dq-axis outer loop command values, respectively; d , E q are the dq-axis components of the M3C converter grid-connection point voltage, which are the voltage feedforward terms; d , i q are the dq-axis components of the system fault current; w is the phase-locked loop angular frequency;

[0105] S52: As can be seen from the above formula, due to the existence of coupling feedforward phase between the dq axes, the fault current cannot be solved by a single fault loop equation, so fault loop equations are established from the M3C converter grid-connection point and the converter outlet point to the fault point, respectively:

[0106]

[0107] wherein, L Σ1 and R Σ1 are the equivalent inductance and resistance from the M3C converter grid-connection point to the fault point, respectively; T , R T are the equivalent inductance and resistance of the low-frequency transformer in each phase line, respectively;

[0108] S53: The equivalent source theoretical model of the M3C converter and the fault loop equation of the flexible low-frequency power transmission system are solved simultaneously to obtain the dq-axis components i d , i q of the fault current of the flexible low-frequency power transmission system:

[0109]

[0110] wherein, c1, c2, c3, c4, c5, c6, c7 and c8 are all proportional coefficients; α1, α2, α3, α4, α5 and α6 represent different attenuation components, respectively; β1, β2, β3, β4, β5 and β6 represent different angular frequencies of periodic components, respectively; μ1, μ2, μ3, μ4, μ5 and μ6 represent different phase angles of periodic components, respectively; t is the fault occurrence time;

[0111] S54: After Park transformation, the fault current i fc of the M3C converter control response stage is expressed as:

[0112]

[0113] Wherein, theta is the phase-locked angle of the phase-locked loop;

[0114] S6: obtaining the fault current i of the flexible low-frequency power transmission system f , the expression is:

[0115]

[0116] Wherein, t0 is the initial time of the topology response, t1 is the initial time of the control response, t2 is the time when the converter is blocked, and t is the fault occurrence time.

[0117] In order to verify the effect of the application, a flexible low-frequency power transmission system is built on the PSCAD platform based on the parameters in Table 1, and a three-phase short-circuit fault is set at the midpoint of the low-frequency line to verify the effect of the application.

[0118] Table 1: Parameters of the flexible low-frequency power transmission system

[0119]

[0120] The simulation waveform is basically consistent with the theoretical calculation waveform, thereby verifying the accuracy of the method.

[0121] Of course, for those skilled in the art, the application is not limited to the details of the above exemplary embodiments, but also includes the same or similar structures that can be realized in other specific forms without departing from the spirit or essential characteristics of the application. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting, and the scope of the application is defined by the appended claims rather than the above description, and therefore all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the application. Any reference signs in the claims should not be regarded as limiting the claims involved.

[0122] In addition, it should be understood that although the present specification is described in terms of embodiments, not every embodiment contains only one independent technical solution, and the description of the specification is only for the sake of clarity, and those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can be combined appropriately to form other embodiments that those skilled in the art can understand.

[0123] The technical, shape, and structure parts not described in detail in the application are well-known technologies.

Claims

1. A method for analyzing fault current of a flexible low frequency power transmission system, the fault current being a three-phase short-circuit fault current, characterized in that, Comprise: S1: obtaining the equivalent inductance of the low-frequency transformer in each phase circuit in the flexible low-frequency power transmission system L T and the equivalent resistance R T , the equivalent inductance of each phase circuit in the three-phase circuit of the converter grid connection point and the fault point and the resistance , the bridge arm inductance L arm , the bridge arm resistance R arm , and the bridge arm capacitance C n ; S2: according to the timing of M3C converter action, the fault current in the flexible low frequency power transmission system is divided into two stages, which are M3C converter topology response stage and M3C converter control response stage; S3: Collecting the dq-axis voltage at the outlet point of the M3C converter during the operation fault of the low-frequency power transmission system E d and E q , the dq-axis voltage at the grid-connected point of the M3C converter u d and u q , the phase locking angle of the phase-locked loop , the angular frequency of the phase-locked loop w ; S4: dividing the discharge condition of each sub-converter of the M3C converter topology in response to the phase into two categories, including condition one: one-phase bridge arm discharges to two-phase bridge arm, and condition two: two-phase bridge arm discharges to one-phase bridge arm; the fault current of the M3C converter topology in response to the phase i ft including the sum of the discharge currents of each sub-converter of the topology in response to the phase; S5: Establish the equivalent source theoretical model of M3C converter and the fault loop equation of flexible low frequency power transmission system, and solve the fault current of M3C converter control response stage i fc ; S6: Obtain fault current of flexible low-frequency power transmission system i f The expression is: wherein, t 0 is the initial moment of the topological response, t 1 is the initial moment of the control response, t 2 is the moment of the converter blocking, t is the time of the fault occurrence.

2. The method of claim 1, wherein, In step S4, the M3C converter topology responds to the phase fault current i ft is the sum of the currents of each sub-converter; the discharge condition of each sub-converter is divided into two kinds, which are the discharge condition of one-phase bridge arm to two-phase bridge arms or the discharge condition of two-phase bridge arms to one-phase bridge arm; correspondingly, the current of the sub-converter is the current in the equivalent circuit of the discharge condition of one-phase bridge arm to two-phase bridge arms or the current in the equivalent circuit of the discharge condition of two-phase bridge arms to one-phase bridge arm.

3. The method of claim 1, wherein, In step S4, the calculation process of the current in the equivalent circuit of the situation that the one-phase bridge arm discharges to the two-phase bridge arm is: For any sub-converter, discharging from one phase bridge arm to two phase bridge arms is equivalent to a loop composed of equivalent inductance C 1, equivalent capacitance L 1, and equivalent resistance R 1 in series. Current in the equivalent circuit for the case of discharging from the one-phase bridge arm to the two-phase bridge arm i ft1 The analytical expression is: wherein, A 1 and A 2 is a constant for the case of discharging from one phase leg to two phase legs; is the angular frequency; equivalent capacitance C 1, equivalent inductance L 1, and equivalent resistance R The expressions for 1, 1, and 1, respectively, are: wherein C na , C nb , C nc are the capacitances in the three-phase bridge arms, respectively.

4. The method of claim 1, wherein, Step S5 comprises: S51: according to the characteristics of the converter grid point voltage controlling the converter outlet point voltage, an equivalent source theoretical model based on the converter control characteristics is established; S52: the fault loop equations from the converter grid point and the converter outlet point to the fault point are respectively established; S53: the equivalent source theoretical model of the M3C converter and the fault loop equations of the flexible low frequency power transmission system are solved to obtain the dq axis components of the fault current of the flexible low frequency power transmission system; S54: Obtain the fault current of the M3C converter control response stage based on the Park transformation i fc .

5. The method of claim 4, wherein, The equivalent source theoretical model is: wherein, u d 、u q are the dq-axis components of the M3C converter outlet point voltage, respectively; k p1 , k p2 are the inner and outer loop proportional regulation coefficients of the voltage-frequency control, respectively; k i1 、k i2 are the inner and outer loop integral regulation coefficients of the voltage-frequency control, respectively; d 、 q are the dq-axis outer loop command values, respectively; E d 、E q are the dq-axis components of the M3C converter grid point voltage, and is the voltage feedforward term; i d 、i q are the dq-axis components of the system fault current; w is the phase-locked loop angular frequency; is the integral of the difference between the converter grid point voltage command value and the actual value.

6. The method of claim 4, wherein, The fault loop equations from the converter grid point and the converter outlet point to the fault point are: wherein, E d 、E q are the dq-axis components of the M3C converter point of common coupling voltage, respectively; u d 、u q are the dq-axis components of the M3C converter point of common coupling voltage, respectively; i d 、i q are the dq-axis components of the system fault current, respectively.

7. The method of claim 4, wherein the fault current is calculated by the following equation: ###0002### where, I is the fault current, V is the voltage, R is the resistance, L is the inductance, and C is the capacitance. In step S53, the dq-axis components of the fault current of the flexible low-frequency power transmission system i d 、i q are respectively expressed as: wherein c1, c2, c3, c4, c5, c6, c7 and c8 are all different proportional coefficients; , , , , and represent different attenuation components, respectively; , , , , and represent different angular frequencies of periodic components, respectively; , , , , and represent different phase angles of periodic components, respectively; and t is the time of fault occurrence.

8. The method of claim 4, wherein, In step S54, the M3C converter control responds to the fault current of the phase i fc The calculation formula is: wherein i d 、i q are the dq-axis components of the system fault current, respectively.

9. The method of claim 1, wherein, In step S4, the calculation process of the current in the equivalent circuit of the situation that the two-phase bridge arm discharges to the one-phase bridge arm is: For any one sub-converter, the discharge situation of two-phase bridge arms to one-phase bridge arm is equivalent to a loop composed of equivalent capacitance C 2, equivalent inductance L 2, and equivalent resistance R 2 in series. Current in the equivalent circuit for the case of discharging from a two-phase bridge arm to a one-phase bridge arm i ft2 The analytical expression is: wherein A 3 and A 4 is the constant factor of the situation that two-phase bridge arms discharge to one-phase bridge arm; 2 is the angular frequency, and the calculation formula is: Wherein, the equivalent capacitance C2, the equivalent inductance L 2, the calculation formula of equivalent resistance R2 is respectively; wherein C na , C nb , C nc are the capacitances in the three-phase bridge arms, respectively.

10. A flexible low frequency power transmission system adapted to be used in a method of fault current analysis for a flexible low frequency power transmission system according to any one of claims 1-9, characterized in that, Comprise: M3C converter, low frequency transformer, low frequency transmission line, fan and AC power supply; the power frequency side of the M3C converter is connected with the AC power supply, the low frequency side is connected with the valve side of the low frequency transformer, one end of the low frequency transmission line is connected with the network side of the low frequency transformer, and the other end is connected with the fan; The M3C converter comprises three sub-converters, each of which comprises the same three-phase bridge arm, each phase bridge arm is composed of a bridge arm inductance, a bridge arm resistance and N sub-modules in series, and each sub-module comprises an energy storage capacitor and four insulated gate bipolar diodes; One end of the three sub-converters is respectively connected with the three-phase line of the AC power supply, and each phase line comprises an inductance and a resistance; the other end of the three sub-converters is connected with the three-phase line of the valve side of the low frequency transformer.

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