CN116345413A - Method and system for determining overvoltage and overcurrent based on phase-to-phase ground fault - Google Patents

Abstract

The present application proposes a method and system for determining overvoltage and overcurrent based on phase-to-ground faults, wherein the method includes: according to the corresponding total impedance value, positive-sequence voltage, and AC positive-sequence Current determines the corresponding negative-sequence fault current maximum value and the voltage maximum value of the non-fault phase when the phase-to-ground fault occurs; the negative-sequence fault current maximum value is used as the negative-sequence overcurrent when the phase-to-ground fault occurs, The maximum value of the voltage of the non-faulty phase is used as the overvoltage when a phase-to-ground fault occurs in the wind power transmission system through the flexible system. The technical solution proposed in this application can accurately determine the negative sequence when a phase-to-ground fault occurs based on the corresponding total impedance value, positive-sequence voltage and AC positive-sequence current of the system when a phase-to-ground fault occurs in the wind power direct transmission system. The overcurrent and overvoltage values are based on the determination of the negative sequence overcurrent and overvoltage values to control the stable operation of the wind power transmission system.

Description

Method and system for determining overvoltage and overcurrent based on phase-to-ground fault

technical field

The present application relates to the field of overcurrent and overvoltage, and in particular to a method and system for determining overvoltage and overcurrent based on phase-to-ground faults.

Background technique

The high voltage dc transmission system based on voltage source converter (VSC-HVDC) provides a cost-effective grid connection method for wind farms. The grid-connected technology of wind power through the flexible DC transmission system has also become one of the key directions. Wind farms usually use direct-drive wind turbines, and their grid side converters (GSCs) usually use two-level voltage source converters (2L-VSCs). Modular multilevel converter (MMC) has gradually become the mainstream topology of VSC-HVDC due to its unique advantages. The phase-to-phase fault is one of the most common faults in the power system. When a phase-to-phase fault occurs in the AC system between the GSC and the MMC at the sending end, the system will generate large transient overcurrent and overvoltage due to the influence of negative sequence current. Due to the low overcurrent and low overvoltage characteristics of power electronic devices, it is difficult for the system to achieve fault ride through, which affects the safe and reliable operation of the system. Therefore, it is of great significance to study the characteristics of overcurrent and overvoltage under phase-to-phase fault.

Contents of the invention

The method and system for determining overvoltage and overcurrent based on phase-to-ground faults provided by the present application at least solve the problem that the system is difficult to achieve fault ride-through due to the low overcurrent and low overvoltage characteristics of power electronic devices in the prior art, thus Technical issues that affect the safe and reliable operation of the system.

The embodiment of the first aspect of the present application proposes a method for determining overvoltage and overcurrent based on a phase-to-ground fault, the method comprising:

Obtain the corresponding total impedance value of the system, the positive-sequence voltage output by the multilevel converter at the sending end and the AC positive-sequence current output by the fan when a phase-to-ground fault occurs in the wind power transmission system through the flexible direct transmission system;

According to the total impedance value, the positive-sequence voltage and the AC positive-sequence current, respectively determine the maximum value of the negative-sequence fault current and the maximum voltage of the non-faulted phase when the wind power transmission system through the flexible direct transmission system has a phase-to-ground fault. value;

The maximum value of the negative-sequence fault current is taken as the negative-sequence overcurrent when a phase-to-ground fault occurs in the wind power transmission system through the flexible system, and the maximum voltage value of the non-faulted phase is taken as the occurrence of the wind power transmission system through the flexible system. Overvoltage at phase-to-earth fault.

Preferably, the method also includes:

According to the total impedance value, the positive sequence voltage and the AC positive sequence current, determine the positive sequence overvoltage corresponding to the common connection point of the fan in the wind power transmission system through the flexible direct transmission system when a phase-to-phase fault occurs;

Obtain the reactance value corresponding to the wind turbine commutation reactor in the system, and determine the occurrence phase according to the reactance value corresponding to the wind turbine commutation reactor, the total impedance value, the positive sequence voltage and the AC positive sequence current In the case of an indirect ground failure, the wind power is directly sent out to the negative sequence overvoltage corresponding to the common connection point of the wind turbines in the system.

Preferably, the formula for calculating the maximum value of the negative sequence fault current is as follows:

In the formula, I _fanmax is the maximum value of the negative sequence fault current, U _mp is the positive sequence voltage output by the multilevel converter at the sending end in the wind power direct transmission system through the flexible system, and X _all is the corresponding total reactance of the wind power direct transmission system through the flexible system value, α is the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system, and I _wp is the AC positive sequence current output by the wind turbine.

Preferably, the formula for calculating the maximum voltage of the non-fault phase is as follows:

In the formula, U _famax is the maximum value of the voltage of the non-fault phase, U _mp is the positive sequence voltage output by the multi-level converter at the sending end of the wind power direct transmission system through the flexible system, and X _all is the corresponding total voltage of the wind power direct transmission system through the flexible system. Reactance value, α is the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system through flexible direct transmission, and I _wp is the AC positive sequence current output by the wind turbine.

Further, the calculation formula of the positive sequence overvoltage corresponding to the common connection point of the fan when the phase-to-ground fault occurs is as follows: U _wpmax :

The formula for calculating the negative-sequence overvoltage corresponding to the common connection point of the fan when a phase-to-ground fault occurs is as follows:

In the formula, U _wpmax is the positive-sequence overvoltage corresponding to the common connection point of the fan when a phase-to-ground fault occurs, U _wnmax is the negative-sequence overvoltage corresponding to the common connection point of the fan when a phase-to-ground fault occurs, and U _mp is The positive-sequence voltage output by the multi-level converter at the sending end in the wind power transmission system through the flexible direct transmission system, I _wp is the AC positive sequence current output by the fan, X _all is the corresponding total reactance value of the wind power transmission system through the flexible direct transmission system, and α is the fault The distance from the point to the multi-level converter at the sending end of the wind power direct transmission system through the flexible system, X _weq is the corresponding reactance value of the wind turbine converter reactor, and α is the multi-level converter at the sending end of the fault point to the direct transmission system of the wind power through the flexible system Converter distance.

The embodiment of the second aspect of the present application proposes a system for determining overvoltage and overcurrent based on phase-to-ground faults, including:

The acquisition module is used to acquire the corresponding total impedance value of the system, the positive-sequence voltage output by the multilevel converter at the sending end, and the AC positive-sequence current output by the fan when a phase-to-ground fault occurs in the wind power direct transmission system;

The first determination module is used to respectively determine the maximum value of the negative sequence fault current corresponding to the phase-to-ground fault in the wind power direct transmission system through the flexible system according to the total impedance value, the positive sequence voltage and the AC positive sequence current and the maximum voltage of the non-fault phase;

The second determining module is used to use the maximum value of the negative sequence fault current as the negative sequence overcurrent when a phase-to-ground fault occurs in the wind power transmission system through the flexible direct transmission system, and use the maximum value of the voltage of the non-faulted phase as the The wind power is sent directly to the overvoltage when the phase-to-ground fault occurs in the system.

Preferably, the determination system also includes:

The third determination module is used to determine the positive sequence corresponding to the common connection point of the wind power in the wind power direct transmission system through the flexible direct transmission system when a phase-to-ground fault occurs according to the total impedance value, the positive sequence voltage and the AC positive sequence current Overvoltage;

The fourth determination module is used to obtain the reactance value corresponding to the wind turbine commutation reactor in the system, and according to the reactance value corresponding to the wind turbine commutation reactor, the total impedance value, the positive sequence voltage and the The AC positive-sequence current determines the negative-sequence overvoltage corresponding to the common connection point of the wind turbines in the wind power transmission system through the flexible direct transmission system when a phase-to-ground fault occurs.

Preferably, the formula for calculating the maximum value of the negative sequence fault current is as follows:

In the formula, I _fanmax is the maximum value of the negative sequence fault current, U _mp is the positive sequence voltage output by the multilevel converter at the sending end in the wind power direct transmission system through the flexible system, and X _all is the corresponding total reactance of the wind power direct transmission system through the flexible system value, α is the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system, and I _wp is the AC positive sequence current output by the wind turbine.

The embodiment of the third aspect of the present application proposes an electronic device, including: a memory, a processor, and a computer program stored in the memory and operable on the processor. When the processor executes the program, the first aspect is implemented. The method described in the examples.

The embodiment of the fourth aspect of the present application provides a computer-readable storage medium on which a computer program is stored. When the program is executed by a processor, the method as described in the embodiment of the first aspect is implemented.

The technical solutions provided by the embodiments of the present application bring at least the following beneficial effects:

This application proposes a method and system for determining overvoltage and overcurrent based on phase-to-ground faults, wherein the method includes: obtaining the corresponding total impedance value of the system when a phase-to-ground fault occurs in the wind power direct transmission system through the flexible system, The positive-sequence voltage output by the multilevel converter at the sending end and the AC positive-sequence current output by the wind turbine; according to the total impedance value, the positive-sequence voltage and the AC positive-sequence current, it is determined that the wind power is sent directly through the flexible When a phase-to-ground fault occurs in the system, the corresponding maximum value of the negative sequence fault current and the maximum value of the voltage of the non-fault phase; the maximum value of the negative sequence fault current is taken as the negative Sequence overcurrent, the maximum value of the voltage of the non-faulty phase is used as the overvoltage when the wind power transmission system through the flexible direct transmission system has a phase-to-ground fault. The technical solution proposed by this application can accurately determine the negative sequence when a phase-to-ground fault occurs based on the corresponding total impedance value, positive-sequence voltage and AC positive-sequence current of the system when a phase-to-ground fault occurs in the wind power direct transmission system. Over-current and over-voltage values, based on the determination of negative-sequence over-current and over-voltage values, control the stable operation of the wind power transmission system through the flexible system.

Additional aspects and advantages of the application will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the application.

Description of drawings

The above and/or additional aspects and advantages of the present application will become apparent and easy to understand from the following description of the embodiments in conjunction with the accompanying drawings, wherein:

Fig. 1 is a flowchart of a method for determining an overvoltage and an overcurrent based on a phase-to-ground fault according to an embodiment of the present application;

Fig. 2 is a schematic diagram of the control process of the MMC provided according to one embodiment of the present application;

FIG. 3 is a schematic diagram of a control process of a GSC provided according to an embodiment of the present application;

FIG. 4 is an equivalent circuit diagram of the system in full regulation mode according to an embodiment of the present application;

FIG. 5 is an equivalent circuit diagram of a system in a current-limiting mode according to an embodiment of the present application;

FIG. 6 is an equivalent circuit diagram of the system when a phase-to-ground fault is provided according to an embodiment of the present application;

FIG. 7 is a schematic diagram of a phase-to-ground fault occurring at α according to an embodiment of the present application;

FIG. 8 is a graph showing the relationship between grounding factor and fault point according to an embodiment of the present application;

FIG. 9 is a graph showing the relationship between the negative sequence current coefficient and the fault point according to an embodiment of the present application;

FIG. 10 is a graph showing the relationship between the positive sequence voltage of the PCC point of the GSC and the fault point according to one embodiment of the present application;

FIG. 11 is an equivalent circuit diagram of a phase-to-ground fault in a current-limiting mode according to an embodiment of the present application;

FIG. 12 is a first structural diagram of a system for determining overvoltage and overcurrent based on a phase-to-ground fault according to an embodiment of the present application;

Fig. 13 is a second structure diagram of a system for determining overvoltage and overcurrent based on a phase-to-ground fault according to an embodiment of the present application.

Detailed ways

Embodiments of the present application are described in detail below, examples of which are shown in the drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the figures are exemplary, and are intended to explain the present application, and should not be construed as limiting the present application.

The method and system for determining overvoltage and overcurrent based on phase-to-ground faults proposed by this application, wherein the method includes: obtaining the corresponding total impedance value of the system when a phase-to-ground fault occurs in the wind power direct transmission system through the flexible system, The positive-sequence voltage output by the multilevel converter at the sending end and the AC positive-sequence current output by the wind turbine; according to the total impedance value, the positive-sequence voltage and the AC positive-sequence current, it is determined that the wind power is sent directly through the flexible When a phase-to-ground fault occurs in the system, the corresponding maximum value of the negative sequence fault current and the maximum value of the voltage of the non-fault phase; the maximum value of the negative sequence fault current is taken as the negative Sequence overcurrent, the maximum value of the voltage of the non-faulty phase is used as the overvoltage when the wind power transmission system through the flexible direct transmission system has a phase-to-ground fault. The technical solution proposed by this application can accurately determine the negative sequence when a phase-to-ground fault occurs based on the corresponding total impedance value, positive-sequence voltage and AC positive-sequence current of the system when a phase-to-ground fault occurs in the wind power direct transmission system. Over-current and over-voltage values, based on the determination of negative-sequence over-current and over-voltage values, control the stable operation of the wind power transmission system through the flexible system.

The method and system for determining overvoltage and overcurrent based on a phase-to-ground fault according to embodiments of the present application will be described below with reference to the accompanying drawings.

Embodiment one

Fig. 1 is a flow chart of a method for determining an overvoltage and overcurrent based on a phase-to-ground fault according to an embodiment of the present application. As shown in Fig. 1 , the method includes:

Step 1: Obtain the corresponding total impedance value of the system, the positive sequence voltage output by the multilevel converter at the sending end, and the AC positive sequence current output by the fan when a phase-to-ground fault occurs in the wind power transmission system through the flexible direct transmission system;

Step 2: According to the total impedance value, the positive-sequence voltage and the AC positive-sequence current, respectively determine the maximum value of the negative-sequence fault current and the non-fault phase the maximum voltage;

In the embodiment of the present disclosure, the formula for calculating the maximum value of the negative sequence fault current is as follows:

In the formula, I _fanmax is the maximum value of the negative sequence fault current, U _mp is the positive sequence voltage output by the multilevel converter at the sending end in the wind power direct transmission system through the flexible system, and X _all is the corresponding total reactance of the wind power direct transmission system through the flexible system value, α is the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system, and I _wp is the AC positive sequence current output by the wind turbine.

The formula for calculating the maximum voltage of the non-fault phase is as follows:

In the formula, U _famax is the maximum value of the voltage of the non-fault phase, U _mp is the positive sequence voltage output by the multi-level converter at the sending end of the wind power direct transmission system through the flexible system, and X _all is the corresponding total voltage of the wind power direct transmission system through the flexible system. Reactance value, α is the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system through flexible direct transmission, and I _wp is the AC positive sequence current output by the wind turbine.

Step 3: Use the maximum value of the negative sequence fault current as the negative sequence overcurrent when a phase-to-ground fault occurs in the wind power direct transmission system through the flexible system, and use the maximum voltage value of the non-faulted phase as the wind power direct transmission through the flexible system Overvoltage when a phase-to-ground fault occurs in the system.

In an embodiment of the present disclosure, the method further includes:

Step 4: According to the total impedance value, the positive sequence voltage and the AC positive sequence current, determine the positive sequence overvoltage corresponding to the common connection point of the fan in the wind power transmission system through the flexible direct transmission system when a phase-to-ground fault occurs;

Step 5: Obtain the reactance value corresponding to the fan commutation reactor in the system, and according to the reactance value corresponding to the fan commutation reactor, the total impedance value, the positive sequence voltage and the AC positive sequence current Determine the negative-sequence overvoltage corresponding to the common connection point of the fan in the wind power transmission system through the flexible direct transmission system when a phase-to-ground fault occurs.

Further, the calculation formula of the positive sequence overvoltage corresponding to the common connection point of the fan when the phase-to-ground fault occurs is as follows: U _wpmax :

The formula for calculating the negative-sequence overvoltage corresponding to the common connection point of the fan when a phase-to-ground fault occurs is as follows:

In the formula, U _wpmax is the positive-sequence overvoltage corresponding to the common connection point of the fan when a phase-to-ground fault occurs, U _wnmax is the negative-sequence overvoltage corresponding to the common connection point of the fan when a phase-to-ground fault occurs, and U _mp is The positive-sequence voltage output by the multi-level converter at the sending end in the wind power transmission system through the flexible direct transmission system, I _wp is the AC positive sequence current output by the fan, X _all is the corresponding total reactance value of the wind power transmission system through the flexible direct transmission system, and α is the fault The distance from the point to the multi-level converter at the sending end of the wind power direct transmission system through the flexible system, X _weq is the corresponding reactance value of the wind turbine converter reactor, and α is the multi-level converter at the sending end of the fault point to the direct transmission system of the wind power through the flexible system Converter distance.

In the embodiment of the present disclosure, to simplify the analysis, the wind farm can be equivalent to a GSC; the MMC and the GSC adopt a double closed-loop control strategy, including the voltage outer loop and the current inner loop. Negative sequence control is added to GSC.

Wherein, as shown in Figure 2 is the control process of MMC:

MMC adopts the traditional VF control strategy. First, the controller directly integrates the time with 50Hz frequency to generate the phase angle θ _MMC of the MMC modulation voltage, and sends this phase angle to the Park transform (T _abc/dq ) and inverse Park transform (T In _{the dq/abc} ) module, the current I _m of the MMC and the voltage U _ms of the PCC point are collected, and the positive and negative sequence components and the d and q axis components are separated, which are respectively I _mdqp , I _mdqn , U _msdqp , U _msdqn , Compare the positive sequence component U _msdqp of the PCC point voltage with its reference value, input the error amount into the PI controller, so as to obtain the command value of the positive sequence current, and then compare the positive sequence current I _mdqp with its command value, and calculate the error amount Input to the PI controller, the generated modulation voltage plus/minus the cross-coupling term (ωL _eq I _mdqp ) and the voltage feed-forward term (U _msdqp ), thereby generating a positive sequence modulation voltage, and then the modulation voltage is passed through the positive sequence modulation The voltage limiter generates the actual positive sequence modulation voltage U _mcdqp *. Similarly, negative-sequence control only retains the inner-loop control strategy, compares the negative-sequence current I _mdqn with its command value, inputs the error amount to the PI controller, and adds/subtracts the cross-coupling term (ωL _eq I _mdqn ) and a voltage feedforward term (U _msdqn ), thereby generating a negative sequence modulation voltage U _mcdqn *. After the positive and negative sequence modulation voltages are transformed by inverse Park (T _dq/abc ), a three-phase modulation voltage is generated. In the figure, U _mc is the voltage at the AC outlet of the MMC, U _ms is the voltage at the PCC point, I _m is the AC voltage at the MMC, U _wc is the voltage at the AC outlet of the GSC, U _ws is the voltage at the PCC point, and I _w is the AC voltage of the GSC , I _dc , u _mdc are the GSC DC side current and MMC DC side voltage respectively, θ _MMC , θ _VSC are the phase angles of the PCC point on the MMC side and the GSC side respectively, X _line is the AC transmission line impedance, X _w and X _m are respectively The impedance from the fault point to the GSC outlet and the fault point to the MMC outlet, X _weq and X _meq are the commutation reactors of GSC and MMC respectively.

Among them, the control process of GSC is shown in Figure 3. GSC adopts the network-following control strategy. Firstly, the positive sequence voltage phase θ _VSC of the PCC point of the fan is collected through the phase-locked loop (PLL), and the phase angle is sent to the Park transform ( T _abc/dq ) and inverse Park transformation (T _dq/abc ) modules, and collect the GSC current and PCC point voltage, and separate them into positive and negative sequence components and d, q axis components, which are respectively I _wdqp , I _wdqn , U _wsdqp , U _wsdqn , and then compare the DC voltage U _wdc of the GSC with its reference value, input the error amount into the PI controller to obtain the command value of the positive sequence current, and then compare the positive sequence current I _wdqp with its command value Values are compared, the error amount is input to the PI controller, and the generated modulation voltage is added/subtracted to the cross-coupling term (ωLI _wdqp ) and the voltage feed-forward term (U _wsdqp ), thereby generating a positive-sequence modulation voltage. Similarly, negative sequence control only retains the inner loop control strategy, compares the negative sequence current I _wdqn with its command value, inputs the error amount to the PI controller, and adds/subtracts the cross-coupling term (ωLI _wdqn ) and the voltage feedforward term (U _wsdqn ), thus generating a negative sequence modulation voltage. Three-phase modulation voltages U _wcp * and U _wcn * are generated by inverse Park transformation (T _dq/abc ) of the positive and negative sequence modulation voltages.

It should be noted that when a shallow fault occurs, the positive sequence current output by the MMC is less than its current limit value. At this time, the MMC is in full modulation mode and can be equivalent to a voltage source; when a deep fault occurs, the MMC outputs a positive sequence current Is limited to the limiting value, MMC is in current limiting mode, can be equivalent to a current source, and GSC can be equivalent to a current source. Among them, the equivalent circuit of the system without considering the negative sequence control and in the full regulation mode is shown in Figure 4, and the system equivalent circuit is shown in Figure 5 when the negative sequence control is not considered and in the current limiting mode. In the figure, U _mcplim and I _mlim is the MMC positive sequence voltage limit value and MMC total current limit value respectively, U _wc is the voltage at the MMC AC outlet, X _w is the impedance from the fault point to the GSC outlet, I _w is the GSC AC current, and I _m is MMC AC current, X _m is the impedance from the fault point to the MMC outlet, f ⁽ⁿ⁾ is the fault point.

The following introduction is made about the full modulation mode:

The key to realizing phase-to-ground fault ride-through is to obtain the key influencing factors of system overvoltage and overcurrent. Therefore, the present invention adopts the decoupling analysis method of circuit characteristics and control characteristics, ignores the negative sequence control of MMC and GSC to obtain the expressions of system overcurrent and overvoltage, and extracts the key influencing factors.

式中，I_fap为b相和c相发生接地故障时系统正序电流，I_fan为b相和c发生接地故障时系统负序电流，U_mp为系统中送端多电平换流器输出的正序电压，/>

X_mn为故障点到MMC出口的阻抗负序分量，X_wn为故障点到GSC出口的阻抗负序分量，在系统中，变压器通常直接接地，所以本发明假设系统中正、负、零序阻抗相同。如图7所示当相间接地故障发生在α处，X_mp、X_n'可表示为/>

式中，X_mp为故障点到MMC出口的阻抗正序分量，X_all表示从GSC交流出口到MMC交流出口处所有电抗，包括MMC换流电抗器、线路电抗、GSC滤波器及其换流电抗器。将公式/>

代入/>

中，可以发现，当GSC输出电流滞后MMC输出电压90°，即风机仅发出无功功率时，系统中负序电流达到最大，为/>

将公式/>

代入

中，整理得到/>

式中，k_i为负序电流系数，其表达式为/>

When a two-phase ground fault occurs in the system, taking the b-phase and c-phase faults as an example, and ignoring the system resistance, the equivalent circuit of the system is shown in Figure 6, where the calculation formula of the fault current can be

In the formula, I _fap is the positive sequence current of the system when the ground fault occurs in phase b and c, I _fan is the negative sequence current of the system when the ground fault occurs in phase b and c, U _mp is the output of the multilevel converter in the system positive sequence voltage, />

X _mn is the negative-sequence component of the impedance from the fault point to the MMC outlet, and X _wn is the negative-sequence component of the impedance from the fault point to the GSC outlet. In the system, the transformer is usually directly grounded, so the present invention assumes that the positive, negative and zero-sequence impedances in the system are the same . As shown in Figure 7, when the phase-to-phase fault occurs at α, X _mp and X _n ' can be expressed as />

In the formula, X _mp is the positive sequence component of the impedance from the fault point to the MMC outlet, and X _all represents all reactances from the GSC AC outlet to the MMC AC outlet, including the MMC commutation reactor, line reactance, GSC filter and its commutation reactance device. will the formula />

substitute />

, it can be found that when the output current of the GSC lags the output voltage of the MMC by 90°, that is, when the fan only emits reactive power, the negative sequence current in the system reaches the maximum, which is />

will the formula />

substitute

In, arrange to get />

In the formula, _ki is the negative sequence current coefficient, and its expression is />

At the same time, according to the operating characteristics of the power grid, transient overvoltage may occur in non-fault phases, and the overvoltage level at the fault point is the highest. The PCC point of the GSC should also consider the problem of wind turbine off-grid due to overvoltage.

可以得到当GSC输出电流滞后MMC电压90°，即GSC仅发出无功功率时，故障点健全相电压最大，U_famax＝k_u(U_mp+αI_wpX_all)，GSC的PCC点正序电压和负序电压为U_wp＝-I_fnX_n'+I_wpX_wp，U_wn＝αjI_fnX_weq，可以看出，当GSC输出电流滞后MMC电压90°，即GSC仅发出无功功率时，正负序电压幅值最大为

The healthy phase voltage (phase a) at the fault point is U _fa = U _fap + U _fan + U _fa0 = 2jI _fap X _n ', and the healthy phase amplitude at the fault point is U _fa = k _u (U _mp +αI _wp X _all ) , where, ku is the phase-to-phase short-circuit grounding coefficient,

It can be obtained that when the GSC output current lags the MMC voltage by 90°, that is, when the GSC only emits reactive power, the healthy phase voltage at the fault point is the largest, U _famax = k _u (U _mp +αI _wp X _all ), and the positive sequence voltage of the PCC point of the GSC And the negative sequence voltage is U _wp ＝-I _fn X _n '+I _wp X _wp , U _wn ＝αjI _fn X _weq , it can be seen that when the GSC output current lags the MMC voltage by 90°, that is, when the GSC only emits reactive power , the maximum positive and negative sequence voltage amplitude is

Further, based on the relationship curve between the grounding coefficient and the fault point α is shown in Figure 8, the relationship curve between the negative sequence current coefficient and the fault point α is shown in Figure 9, and the relationship curve between the positive sequence voltage of the PCC point of GSC and the fault point α As shown in Figure 10, it can be obtained that: when the positive sequence current of the GSC lags the positive sequence voltage of the MMC by 90°, the overvoltage and overcurrent levels of the system are the highest; the closer the fault point is to the MMC, the higher the overvoltage and overcurrent levels of the system; Both the negative sequence current level and the overvoltage level are related to the positive sequence current _Iwp of the GSC.

The following introduction is made about the current limit mode:

When a deep fault occurs in the AC system, the fault current reaches the MMC current limit value. At this time, the MMC presents the characteristics of a current source and is in the current limit mode. The overvoltage of the system in the current limiting mode is obviously lower than that in the full modulation mode, therefore, the system overvoltage level is no longer analyzed in the current limiting mode. However, in the current limiting mode, the MMC output current is equal to the limit value of the current limiter, and reaches the upper limit of the system protection, which is greater than the current in the full modulation mode. The negative sequence current further increases, and the overcurrent problem of the MMC bridge arm is further aggravated.

可以发现，系统负序电流的大小与故障位置无关，仅与MMC和GSC输出电流大小和相位有关。当GSC输出电流与MMC输出电流同相，即GSC仅发出无功功率时，系统负序电流最大。与满调制模式相同，在限电流模式下，通过降低系统中的正序电流，可以在一定程度上降低相间接地短路故障时的负序电流。The phase-to-ground fault equivalent circuit in the current-limiting mode is shown in Figure 11, and the negative-sequence current of the phase-to-ground fault is

It can be found that the magnitude of the system negative sequence current has nothing to do with the fault location, but only with the magnitude and phase of the MMC and GSC output current. When the GSC output current is in phase with the MMC output current, that is, when the GSC only emits reactive power, the negative sequence current of the system is the largest. Same as the full modulation mode, in the current limiting mode, by reducing the positive sequence current in the system, the negative sequence current during the phase-to-ground short circuit fault can be reduced to a certain extent.

In summary, the method for determining the overvoltage and overcurrent based on phase-to-ground faults proposed in this application can be based on the corresponding total impedance value and positive-sequence voltage of the system when a phase-to-ground fault occurs in the wind power transmission system through the flexible direct transmission system. and AC positive sequence current to accurately determine the negative sequence overcurrent and overvoltage values when a phase-to-ground fault occurs, and based on the determination of negative sequence overcurrent and overvoltage values to control the stable operation of the wind power transmission system through the flexible system.

Embodiment two

Fig. 12 is a system for determining an overvoltage and overcurrent based on a phase-to-ground fault according to an embodiment of the present application, as shown in Fig. 12 , including:

The acquisition module 100 is used to acquire the corresponding total impedance value of the system, the positive sequence voltage output by the multilevel converter at the sending end, and the AC positive sequence current output by the wind turbine when the wind power is transmitted directly through the flexible system when a phase-to-ground fault occurs;

The first determination module 200 is used to respectively determine the maximum negative sequence fault current corresponding to the phase-to-ground fault in the wind power direct transmission system through the flexible system according to the total impedance value, the positive sequence voltage and the AC positive sequence current value and the maximum voltage of the non-fault phase;

The second determination module 300 is used to use the maximum value of the negative sequence fault current as the negative sequence overcurrent when a phase-to-ground fault occurs in the wind power direct transmission system through the flexible system, and use the maximum value of the voltage of the non-faulted phase as the specified value. The above-mentioned overvoltage occurs when the phase-to-ground fault occurs in the wind power direct transmission system.

In an embodiment of the present disclosure, as shown in FIG. 13 , the determination system further includes:

The third determination module 400 is used to determine the positive current corresponding to the common connection point of the wind power in the wind power transmission system through the flexible direct transmission system when a phase-to-ground fault occurs according to the total impedance value, the positive-sequence voltage and the AC positive-sequence current. Sequence overvoltage;

The fourth determination module 500 is used to acquire the reactance value corresponding to the wind turbine commutation reactor in the system, and according to the reactance value corresponding to the wind turbine commutation reactor, the total impedance value, the positive sequence voltage and the The above-mentioned AC positive-sequence current determines the negative-sequence overvoltage corresponding to the common connection point of the fan in the wind power transmission system through the flexible direct transmission system when a phase-to-ground fault occurs.

In the embodiment of the present disclosure, the formula for calculating the maximum value of the negative sequence fault current is as follows:

In the formula, I _fanmax is the maximum value of the negative sequence fault current, U _mp is the positive sequence voltage output by the multilevel converter at the sending end in the wind power direct transmission system through the flexible system, and X _all is the corresponding total reactance of the wind power direct transmission system through the flexible system value, α is the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system, and I _wp is the AC positive sequence current output by the wind turbine.

In the embodiment of the present disclosure, the formula for calculating the maximum voltage of the non-fault phase is as follows:

In the formula, U _famax is the maximum value of the voltage of the non-fault phase, U _mp is the positive sequence voltage output by the multi-level converter at the sending end of the wind power direct transmission system through the flexible system, and X _all is the corresponding total voltage of the wind power direct transmission system through the flexible system. Reactance value, α is the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system through flexible direct transmission, and I _wp is the AC positive sequence current output by the wind turbine.

Further, the calculation formula of the positive sequence overvoltage corresponding to the common connection point of the fan when the phase-to-ground fault occurs is as follows: U _wpmax :

The formula for calculating the negative-sequence overvoltage corresponding to the common connection point of the fan when a phase-to-ground fault occurs is as follows:

In the formula, U _wpmax is the positive-sequence overvoltage corresponding to the common connection point of the fan when a phase-to-ground fault occurs, U _wnmax is the negative-sequence overvoltage corresponding to the common connection point of the fan when a phase-to-ground fault occurs, and U _mp is The positive-sequence voltage output by the multi-level converter at the sending end in the wind power transmission system through the flexible direct transmission system, I _wp is the AC positive sequence current output by the fan, X _all is the corresponding total reactance value of the wind power transmission system through the flexible direct transmission system, and α is the fault The distance from the point to the multi-level converter at the sending end of the wind power direct transmission system through the flexible system, X _weq is the corresponding reactance value of the wind turbine converter reactor, and α is the multi-level converter at the sending end of the fault point to the direct transmission system of the wind power through the flexible system Converter distance.

To sum up, the overvoltage and overcurrent determination system based on phase-to-phase ground fault proposed by this application and the technical solution proposed by this application can be based on the corresponding phase-to-phase ground fault of the wind power transmission system through the soft direct transmission system. The total impedance value, positive sequence voltage and AC positive sequence current can accurately determine the negative sequence overcurrent and overvoltage value when a phase-to-ground fault occurs, and control the stability of the wind power transmission system through the flexible system based on the determination of the negative sequence overcurrent and overvoltage value run.

Embodiment three

In order to realize the above-mentioned embodiments, the present disclosure also proposes an electronic device, including: a memory, a processor, and a computer program stored in the memory and operable on the processor. When the processor executes the program, the implementation of The method described in Example 1.

Embodiment Four

In order to realize the above-mentioned embodiments, the present disclosure also proposes a computer-readable storage medium, on which a computer program is stored, wherein, when the program is executed by a processor, the method as described in the first embodiment is implemented.

In the description of this specification, descriptions referring to the terms "one embodiment", "some embodiments", "example", "specific examples", or "some examples" mean that specific features described in connection with the embodiment or example , structure, material or characteristic is included in at least one embodiment or example of the present application. In this specification, the schematic representations of the above terms are not necessarily directed to the same embodiment or example. Furthermore, the described specific features, structures, materials or characteristics may be combined in any suitable manner in any one or more embodiments or examples. In addition, those skilled in the art can combine and combine different embodiments or examples and features of different embodiments or examples described in this specification without conflicting with each other.

Any process or method descriptions in flowcharts or otherwise described herein may be understood to represent a module, segment or portion of code comprising one or more executable instructions for implementing custom logical functions or steps of a process , and the scope of preferred embodiments of the present application includes additional implementations in which functions may be performed out of the order shown or discussed, including in substantially simultaneous fashion or in reverse order depending on the functions involved, which shall It should be understood by those skilled in the art to which the embodiments of the present application belong.

Although the embodiments of the present application have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limitations on the present application, and those skilled in the art can make the above-mentioned The embodiments are subject to changes, modifications, substitutions and variations.

Claims (10)

1. A method for determining overvoltage and overcurrent based on phase-to-ground fault, characterized in that, the method comprises: Obtain the corresponding total impedance value of the system, the positive-sequence voltage output by the multilevel converter at the sending end and the AC positive-sequence current output by the fan when a phase-to-ground fault occurs in the wind power transmission system through the flexible direct transmission system; According to the total impedance value, the positive-sequence voltage and the AC positive-sequence current, respectively determine the maximum value of the negative-sequence fault current and the maximum voltage of the non-faulted phase when the wind power transmission system through the flexible direct transmission system has a phase-to-ground fault. value; The maximum value of the negative-sequence fault current is taken as the negative-sequence overcurrent when a phase-to-ground fault occurs in the wind power transmission system through the flexible system, and the maximum voltage value of the non-faulted phase is taken as the occurrence of the wind power transmission system through the flexible system. Overvoltage at phase-to-earth fault. 2. The method of claim 1, further comprising: According to the total impedance value, the positive sequence voltage and the AC positive sequence current, determine the positive sequence overvoltage corresponding to the common connection point of the fan in the wind power transmission system through the flexible direct transmission system when a phase-to-phase fault occurs; Obtain the reactance value corresponding to the wind turbine commutation reactor in the system, and determine the occurrence phase according to the reactance value corresponding to the wind turbine commutation reactor, the total impedance value, the positive sequence voltage and the AC positive sequence current In the case of an indirect ground failure, the wind power is directly sent out to the negative sequence overvoltage corresponding to the common connection point of the wind turbines in the system. 3. The method according to claim 1, wherein the formula for calculating the maximum value of the negative sequence fault current is as follows:

In the formula, I _fanmax is the maximum value of the negative sequence fault current, U _mp is the positive sequence voltage output by the multilevel converter at the sending end in the wind power direct transmission system through the flexible system, and X _all is the corresponding total reactance of the wind power direct transmission system through the flexible system value, α is the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system, and I _wp is the AC positive sequence current output by the wind turbine. 4. The method according to claim 1, wherein the calculation formula of the maximum voltage of the non-fault phase is as follows:

In the formula, U _famax is the maximum value of the voltage of the non-fault phase, U _mp is the positive sequence voltage output by the multi-level converter at the sending end of the wind power direct transmission system through the flexible system, and X _all is the corresponding total voltage of the wind power direct transmission system through the flexible system. Reactance value, α is the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system through flexible direct transmission, and I _wp is the AC positive sequence current output by the wind turbine. 5. The method according to claim 2, wherein the calculation formula of the positive-sequence overvoltage corresponding to the common connection point of the fan when the phase-to-ground fault occurs is as follows: U _wpmax :

The formula for calculating the negative-sequence overvoltage corresponding to the common connection point of the fan when a phase-to-ground fault occurs is as follows:

In the formula, U _wpmax is the positive-sequence overvoltage corresponding to the common connection point of the fan when a phase-to-ground fault occurs, U _wnmax is the negative-sequence overvoltage corresponding to the common connection point of the fan when a phase-to-ground fault occurs, and U _mp is The positive-sequence voltage output by the multi-level converter at the sending end in the wind power transmission system through the flexible direct transmission system, I _wp is the AC positive sequence current output by the fan, X _all is the corresponding total reactance value of the wind power transmission system through the flexible direct transmission system, and α is the fault The distance from the point to the multi-level converter at the sending end of the wind power direct transmission system through the flexible system, X _weq is the corresponding reactance value of the wind turbine converter reactor, and α is the multi-level converter at the sending end of the fault point to the direct transmission system of the wind power through the flexible system Converter distance. 6. A system for determining overvoltage and overcurrent based on phase-to-ground faults, characterized in that it includes: The acquisition module is used to acquire the corresponding total impedance value of the system, the positive-sequence voltage output by the multilevel converter at the sending end, and the AC positive-sequence current output by the fan when a phase-to-ground fault occurs in the wind power direct transmission system; The first determination module is used to respectively determine the maximum value of the negative sequence fault current corresponding to the phase-to-ground fault in the wind power direct transmission system through the flexible system according to the total impedance value, the positive sequence voltage and the AC positive sequence current and the maximum voltage of the non-fault phase; The second determination module is used to use the maximum value of the negative sequence fault current as the negative sequence overcurrent when a phase-to-ground fault occurs in the wind power transmission system through the flexible direct transmission system, and use the maximum value of the voltage of the non-faulted phase as the The wind power is sent directly to the overvoltage when the phase-to-ground fault occurs in the system. 7. The determination system according to claim 6, wherein the determination system further comprises: The third determination module is used to determine the positive sequence corresponding to the common connection point of the wind power in the wind power direct transmission system through the soft direct transmission system when a phase-to-ground fault occurs according to the total impedance value, the positive sequence voltage and the AC positive sequence current Overvoltage; The fourth determination module is used to obtain the reactance value corresponding to the wind turbine commutation reactor in the system, and according to the reactance value corresponding to the wind turbine commutation reactor, the total impedance value, the positive sequence voltage and the The AC positive-sequence current determines the negative-sequence overvoltage corresponding to the common connection point of the wind turbines in the wind power transmission system through the flexible direct transmission system when a phase-to-ground fault occurs. 8. The determining system as claimed in claim 6, wherein the calculation formula of the maximum value of the negative sequence fault current is as follows:

In the formula, I _fanmax is the maximum value of the negative sequence fault current, U _mp is the positive sequence voltage output by the multilevel converter at the sending end in the wind power direct transmission system through the flexible system, and X _all is the corresponding total reactance of the wind power direct transmission system through the flexible system value, α is the distance from the fault point to the multilevel converter at the sending end of the wind power transmission system, and I _wp is the AC positive sequence current output by the wind turbine. 9. An electronic device, characterized in that it comprises: a memory, a processor, and a computer program stored on the memory and operable on the processor, when the processor executes the program, it realizes the requirements as claimed in claims 1 to 5. any one of the methods described. 10. A computer-readable storage medium, on which a computer program is stored, wherein, when the program is executed by a processor, the method according to any one of claims 1 to 5 is realized.

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