CN115825539A - Voltage and current determination method and system for wind power grid-connected two-phase ground fault - Google Patents
Voltage and current determination method and system for wind power grid-connected two-phase ground fault Download PDFInfo
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Abstract
The application provides a method and a system for determining voltage and current of a wind power grid-connected two-phase ground fault, wherein the method comprises the following steps: acquiring corresponding fault current and non-fault phase voltage when the wind power is transmitted out of the system through the flexible direct transmission system to generate two-phase ground fault; determining negative sequence overcurrent of the wind power when the wind power is sent out through the flexible direct-transmission system in the two-phase grounding fault according to the fault current; and determining the overvoltage of the wind power direct-output system during the two-phase earth fault according to the voltage of the non-fault phase. According to the technical scheme, the accuracy of the maximum overvoltage value and the maximum negative sequence overcurrent value of the wind power soft direct-transmission system is improved, and the accuracy of a control strategy for two-phase ground fault ride-through is further improved.
Description
Technical Field
The application relates to the field of overcurrent and overvoltage, in particular to a method and a system for determining voltage and current of a wind power grid-connected two-phase ground fault.
Background
At present, wind power is one of the key attention directions through the grid connection technology of a flexible direct current sending system. While a two-phase ground fault is one of the most common faults in power systems. When a two-phase ground fault occurs in an alternating current system between a Grid Side Converter (GSC) and a sending end multilevel converter (MMC), the system may generate a large transient over-current and over-voltage under the influence of a negative sequence current. Due to the low overcurrent and low overvoltage characteristics of power electronic devices, fault ride-through of a system is difficult to achieve, and therefore safe and reliable operation of the system is affected. Therefore, the negative sequence current control strategy under the two-phase ground fault has great significance.
In the prior art, a control strategy for realizing two-phase earth fault ride-through of offshore wind power through a flexible direct-sending system exists, in the control strategy, an MMC is equivalent to a voltage source, a fan GSC restrains negative sequence current of the MMC to be zero, and at the moment, the negative sequence current in the system completely flows into the MMC. Meanwhile, in order to prevent the MMC from overcurrent, the MMC adjusts the amplitude of the negative sequence modulation voltage to make the sum of the positive sequence current and the negative sequence current flowing into the MMC smaller than a set safety value, thereby realizing fault ride-through. The control method effectively solves the problems of overvoltage and overcurrent when the MMC presents voltage source characteristics, thereby realizing fault ride-through. However, the calculation of the overvoltage and the overcurrent is not accurate, which further causes the control strategy of the two-phase ground fault ride-through to be inaccurate, and therefore, a scheme for quantifying the overcurrent and the overvoltage under the two-phase ground fault according to the fault position is urgently needed.
Disclosure of Invention
The method and the system for determining the voltage and the current of the wind power grid-connected two-phase ground fault at least solve the technical problem that the calculation of overvoltage and overcurrent in the prior art is inaccurate.
An embodiment of the first aspect of the application provides a method for determining voltage and current of a two-phase ground fault of wind power grid connection, and the method includes the following steps:
acquiring corresponding fault current and non-fault phase voltage when the wind power is transmitted out of the system through the flexible direct transmission system to generate two-phase ground fault;
determining negative sequence overcurrent of the wind power when the wind power is sent out through the flexible direct-transmission system in the two-phase grounding fault according to the fault current;
and determining the overvoltage of the wind power direct-output system during the two-phase earth fault according to the voltage of the non-fault phase.
Preferably, the obtained fault current is a negative-sequence fault current.
Further, the determining of the negative sequence overcurrent of the wind power through the flexible direct-current transmission system during the two-phase ground fault according to the fault current includes:
acquiring the distance from a fault point when the wind power is subjected to two-phase ground fault through the flexible direct transmission system to a multi-level converter at a transmission end of the wind power transmission system;
and determining the maximum value of the fault current according to the distance, and taking the maximum value of the fault current as the negative sequence overcurrent.
Further, the maximum value of the fault current is calculated as follows:
in the formula I fanmax Maximum value of negative sequence fault current, U mp Is the positive sequence voltage, X, output by the multi-level converter at the delivery end in the system for delivering the wind power through the flexible direct delivery all The total reactance value corresponding to the wind power soft direct-transmission system is defined as alpha, the distance from a fault point to a multi-level converter at a transmission end in the wind power soft-direct-transmission system is defined as I wp Is the AC positive sequence current output by the fan.
Preferably, the determining the overvoltage of the wind power direct-current transmission system in the two-phase ground fault according to the voltage of the non-fault phase includes:
acquiring the distance from a fault point when the wind power is subjected to two-phase ground fault through the flexible direct transmission system to a multi-level converter at a transmission end of the wind power transmission system;
and determining the maximum voltage value of the non-fault phase according to the distance, and taking the maximum voltage value as the overvoltage.
Further, the maximum voltage value of the non-fault phase is calculated as follows:
in the formula of U famax Voltage maximum of non-faulted phase, U mp Is the positive sequence voltage, X, output by the multi-level converter at the delivery end in the system for delivering the wind power through the flexible direct delivery all The total reactance value corresponding to the wind power soft direct-transmission system is defined as alpha, the distance from a fault point to a multi-level converter at a transmission end in the wind power soft-direct-transmission system is defined as I wp Is the AC positive sequence current output by the fan.
Preferably, the method further comprises:
determining overvoltage corresponding to a common connection point of a motor in a wind power flexible-direct transmission system when two-phase grounding faults occur according to the distance between the fault point and a multi-level converter at the transmission end of the wind power flexible-direct transmission system;
wherein the corresponding overvoltage of fan common junction point includes: and positive sequence overvoltage and negative sequence overvoltage corresponding to the common connection point of the fan.
Further, the positive sequence overvoltage U corresponding to the common connection point of the fan when the two-phase ground fault occurs is determined according to the following formula wpmax ：
Determining negative sequence overvoltage U corresponding to the common connection point of the fan when two-phase ground fault occurs according to the following formula wnmax ：
In the formula of U mp Is the positive sequence voltage output by a multi-level converter at the delivery end in a system for delivering wind power through flexible direct delivery, I wp AC positive-sequence current, X, output by fan all The total reactance value corresponding to the wind power soft direct-sending system is alpha from the fault point to the wind power soft direct-sending systemDistance, X, of a middle-transfer-end multi-level converter weq And alpha is the distance from a fault point to the multi-level converter at the transmission end of the wind power in the system through flexible direct transmission.
An embodiment of a second aspect of the present application provides a voltage and current determination system for a two-phase ground fault of a wind power grid connection, including:
the acquisition module is used for acquiring corresponding fault current and non-fault phase voltage when the wind power is subjected to two-phase ground fault through the flexible direct-transmission system;
the first determining module is used for determining negative sequence overcurrent of the wind power when the wind power is subjected to the two-phase earth fault through the flexible direct-transmission system according to the fault current;
and the second determination module is used for determining the overvoltage of the wind power direct-output system during the two-phase ground fault according to the voltage of the non-fault phase.
Preferably, the obtained fault current is a negative sequence fault current.
The technical scheme provided by the embodiment of the application at least has the following beneficial effects:
the application provides a method and a system for determining voltage and current of a wind power grid-connected two-phase ground fault, wherein the method comprises the following steps: acquiring corresponding fault current and non-fault phase voltage when the wind power is transmitted out of the system through the flexible direct transmission system to generate two-phase ground fault; determining negative sequence overcurrent of the wind power when the wind power is sent out through the flexible direct-transmission system in the two-phase grounding fault according to the fault current; and determining the overvoltage of the wind power direct-output system during the two-phase earth fault according to the voltage of the non-fault phase. According to the technical scheme, the negative sequence over-current and the negative sequence over-voltage during the two-phase ground fault are determined according to the corresponding fault current during the two-phase ground fault and the voltage of the non-fault phase, the accuracy of the negative sequence over-current and the negative sequence over-voltage during the two-phase ground fault is improved, and the accuracy of a control strategy for the two-phase ground fault ride-through is further improved.
Additional aspects and advantages of the present application will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the present application.
Drawings
The above and/or additional aspects and advantages of the present application will become apparent and readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:
fig. 1 is a flowchart of a voltage and current determination method for a two-phase ground fault of a wind power grid connection according to an embodiment of the present application;
FIG. 2 is a schematic diagram of a control process of an MMC provided according to an embodiment of the present application;
fig. 3 is a schematic diagram of a control process of a GSC according to an embodiment of the present application;
FIG. 4 is a diagram of an equivalent circuit of the system in the full mode according to an embodiment of the present application;
FIG. 5 is a diagram of an equivalent circuit of the system in the current limiting mode according to an embodiment of the present application;
FIG. 6 is a diagram of an equivalent circuit of a system in the event of a phase b and phase c ground fault, provided in accordance with one embodiment of the present application;
FIG. 7 is a schematic diagram illustrating a two-phase ground fault at α according to an embodiment of the present application;
FIG. 8 is a graph of a grounding coefficient versus a fault point provided in accordance with one embodiment of the present application;
FIG. 9 is a graph of negative sequence current coefficient versus fault point provided in accordance with an embodiment of the present application;
fig. 10 is a graph of a PCC positive sequence voltage versus a failure point of the GSC according to an embodiment of the present application;
fig. 11 is a diagram illustrating an equivalent circuit of a two-phase ground fault in a current limiting mode according to an embodiment of the present application;
fig. 12 is a first structural diagram of a voltage and current determination system for a two-phase ground fault of a wind power grid according to an embodiment of the present application;
fig. 13 is a second structural diagram of a voltage and current determination system for a two-phase ground fault of a wind power grid according to an embodiment of the present application.
Detailed Description
Reference will now be made in detail to the embodiments of the present application, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are illustrative and intended to explain the present application and should not be construed as limiting the present application.
The application provides a method and a system for determining voltage and current of a wind power grid-connected two-phase ground fault, wherein the method comprises the following steps: acquiring corresponding fault current and non-fault phase voltage when the wind power is transmitted out of the system through the flexible direct transmission system to generate two-phase ground fault; determining negative sequence overcurrent of the wind power when the wind power is sent out through the flexible direct-transmission system in the two-phase grounding fault according to the fault current; and determining the overvoltage of the wind power direct-output system during the two-phase earth fault according to the voltage of the non-fault phase. According to the technical scheme, the negative sequence overcurrent and overvoltage during the two-phase ground fault are determined according to the corresponding fault current and the voltage of the non-fault phase during the two-phase ground fault, the accuracy of the negative sequence overcurrent and overvoltage during the two-phase ground fault is improved, and the accuracy of a control strategy for the two-phase ground fault ride-through is further improved.
The method and the system for determining the voltage and the current of the two-phase ground fault of the wind power grid connection in the embodiment of the application are described below with reference to the attached drawings.
Example one
Fig. 1 is a flowchart of a method for determining voltage and current of a two-phase ground fault of a wind power grid according to an embodiment of the present application, and as shown in fig. 1, the method includes:
step 1: acquiring corresponding fault current and non-fault phase voltage when the wind power is transmitted out of the system through the flexible direct transmission system to generate two-phase ground fault;
it should be noted that the obtained fault current is a negative-sequence fault current.
Step 2: determining negative sequence overcurrent of the wind power through a flexible direct-transmission system during two-phase ground fault according to the fault current;
in an embodiment of the present disclosure, the step 2 specifically includes:
step 2-1: acquiring the distance from a fault point when the wind power is subjected to two-phase ground fault through the flexible direct transmission system to a multi-level converter at a transmission end of the wind power transmission system;
step 2-2: and determining the maximum value of the fault current according to the distance, and taking the maximum value of the fault current as the negative sequence overcurrent.
Wherein the maximum value of the fault current is calculated as follows:
in the formula I fan max Is the maximum value of negative sequence fault current, U mp Is the positive sequence voltage, X, output by the multi-level converter at the delivery end in the system for delivering the wind power through the flexible direct delivery all The total reactance value corresponding to the wind power soft direct-transmission system is defined as alpha, the distance from a fault point to a multi-level converter at a transmission end in the wind power soft-direct-transmission system is defined as I wp Is the AC positive sequence current output by the fan.
And step 3: and determining the overvoltage of the wind power direct-output system during the two-phase earth fault according to the voltage of the non-fault phase.
In an embodiment of the present disclosure, the step 3 specifically includes:
step 3-1: acquiring the distance from a fault point when the wind power is subjected to two-phase ground fault through the flexible direct transmission system to a multi-level converter at a transmission end of the wind power transmission system;
step 3-2: and determining the maximum voltage value of the non-fault phase according to the distance, and taking the maximum voltage value as the overvoltage.
Wherein the maximum voltage value of the non-fault phase is calculated by the following formula:
in the formula of U famax Voltage maximum of non-faulted phase, U mp For wind power channel flexibilityDirect-out of positive-sequence voltage, X, output from a transmit-side multilevel converter in a system all The total reactance value corresponding to the wind power soft direct-transmission system is defined as alpha, the distance from a fault point to a multi-level converter at a transmission end in the wind power soft-direct-transmission system is defined as I wp Is the AC positive sequence current output by the fan.
In the disclosed embodiment, the method provided by the invention further comprises:
and 4, step 4: determining overvoltage corresponding to a common connection point of a fan in the wind power flexible direct transmission system when two-phase grounding faults occur according to the distance from the fault point to the multi-level converter at the transmission end of the wind power flexible direct transmission system;
wherein the corresponding overvoltage of fan common junction point includes: and positive sequence overvoltage and negative sequence overvoltage corresponding to the common connection point of the fan.
Determining positive sequence overvoltage U corresponding to the common connection point of the fan when two-phase ground fault occurs according to the following formula wpmax ：
Determining negative sequence overvoltage U corresponding to the common connection point of the fan when two-phase ground fault occurs according to the following formula wnmax ：
In the formula of U mp Is the positive sequence voltage output by a multi-level converter at the delivery end in a system for delivering wind power through flexible direct delivery, I wp AC positive-sequence current, X, output by fan all The total reactance value corresponding to the system for transmitting the wind power out through the flexible direct transmission system, alpha is the distance from a fault point to a multi-level converter at the transmitting end of the system for transmitting the wind power out through the flexible direct transmission system, X weq And alpha is the distance from a fault point to the multi-level converter at the transmission end of the wind power in the system through flexible direct transmission.
It should be noted that when the fan outputs an ac positive sequence current I wp Positive sequence voltage U output by multi-level converter at transmission end in system for transmitting lagging wind power out through flexible direct transmission mp At 90 deg., i.e. when the fan only emits reactive power, the non-fault phase overvoltage level of the fault point reaches its maximum value U famax The negative sequence current level of the system reaches the maximum value I fanmax The positive and negative sequence voltages of the PCC point of the fan respectively reach the maximum value U wpmax And U wnmax 。
In the embodiment of the disclosure, to simplify the analysis, a wind farm may be equivalent to one GSC; the MMC and the GSC adopt a double closed-loop control strategy, the double closed-loop control strategy comprises a voltage outer loop and a current inner loop, and meanwhile, in order to control negative sequence current, negative sequence control is added into the MMC and the GSC respectively.
Wherein, as shown in fig. 2, the control process of the MMC is as follows:
the MMC adopts a traditional VF control strategy, firstly, a controller directly integrates time by using 50Hz frequency to generate a phase angle theta of MMC modulation voltage MMC This phase angle is sent to Park transformation (T) respectively abc/dq ) And inverse Park transform (T) dq/abc ) In the module, collecting the current I of MMC m And PCC point voltage U ms Separating positive and negative sequence components and d and q axis components, I mdqp 、I mdqn 、U msdqp 、U msdqn The positive sequence component U of the PCC point voltage msdqp Comparing with its reference value, inputting the error amount into PI controller to obtain the command value of positive sequence current, and then inputting the positive sequence current I mdqp Comparing with its command value, inputting error amount to PI controller, adding/subtracting cross coupling term (ω L) from the generated modulation voltage eq I mdqp ) And the voltage feedforward term (U) msdqp ) Thereby generating a positive sequence modulation voltage which is then passed through a positive sequence modulation voltage limiter to generate the actual positive sequence modulation voltage U mcdqp * . Similarly, the negative sequence control retains only the inner loop control strategy, with the negative sequence current I mdqn Comparing with its command value, inputting error amount to PI controller, adding/subtracting cross coupling term (ω L) from the generated modulation voltage eq I mdqn ) And the voltage feedforward term (U) msdqn ) Thereby generating a negative-sequence modulation voltage U mcdqn * . Converting the positive and negative sequence modulation voltage by inverse Park (T) dq/abc ) Then, three-phase modulation voltage is generated. In the figure, U mc For the voltage, U, at the MMC AC outlet ms Is the PCC point voltage, I m For MMC alternating voltage, U wc For the voltage at the GSC AC outlet, U ws Is the PCC point voltage, I w Is GSC AC voltage, I dc 、u mdc Respectively GSC direct side current and MMC direct side voltage, theta MMC 、θ VSC Phase angle, X of PCC points on MMC and GSC sides respectively line For the impedance of the AC transmission line, X w And X m Impedances, X, of fault point to GSC outlet and fault point to MMC outlet, respectively weq And X meq Converter reactors which are GSC and MMC respectively. f. of (1) Indicating a single phase ground fault.
Wherein, as shown in fig. 3, the GSC adopts a network-following type control strategy, and firstly acquires the positive sequence voltage phase θ of the PCC point of the wind turbine through a phase-locked loop (PLL) VSC The phase angle is fed to the Park transformation (T) abc/dq ) And inverse Park transform (T) dq/abc ) In the module, the current of the GSC and the voltage of a PCC point are collected and separated into a positive sequence component, a negative sequence component, a d-axis component and a q-axis component which are I respectively wdqp 、I wdqn 、U wsdqp 、U wsdqn Secondly, the DC voltage U of GSC wdc Comparing with its reference value, inputting the error amount into PI controller to obtain the command value of positive sequence current, and then inputting the positive sequence current I wdqp Comparing with its command value, inputting error amount into PI controller, adding/subtracting cross coupling term (ω LI) from the generated modulation voltage wdqp ) And the voltage feedforward term (U) wsdqp ) Thereby generating a positive sequence modulation voltage. Similarly, the negative sequence control retains only the inner loop control strategy, with the negative sequence current I wdqn Comparing with its command value, inputting error amount into PI controller, adding/subtracting cross coupling term (ω LI) from the generated modulation voltage wdqn ) And a voltage feedforward term (U) wsdqn ) Thereby generating a negative-sequence modulation voltage. Converting the positive and negative sequence modulation voltage by inverse Park (T) dq/abc ) Then, three-phase modulation voltage U is generated wcp * And U wcn *。
It should be noted that, when a shallow fault occurs, the positive sequence current output by the MMC is smaller than the current limit value thereof, and at this time, the MMC is in a full modulation mode and can be equivalent to a voltage source; when a deep fault occurs, the MMC outputs positive sequence current to be limited to a limiting value, the MMC is in a current limiting mode and can be equivalent to a current source, and the GSC can be equivalent to the current source. Wherein, the system equivalent circuit without considering negative sequence control and in the full-regulation mode is shown in FIG. 4, and the system equivalent circuit without considering negative sequence control and in the current-limiting mode is shown in FIG. 5, in which U mcplim And I mlim Respectively MMC positive sequence voltage amplitude limit value and MMC total current amplitude limit value, U wc For the voltage at the MMC AC outlet, X w Impedance of the fault point to the GSC outlet, I w Is GSC alternating current, I m For MMC alternating current, X m Impedance of the fault point to the MMC outlet, f (n) Is the point of failure.
The following is described with respect to the full modulation mode:
the key to realize two-phase ground fault ride-through is to obtain the key influencing factor of system overvoltage and overcurrent. Therefore, the invention adopts an analysis method for decoupling the circuit characteristic and the control characteristic, ignores the negative sequence control of the MMC and the GSC to obtain the expressions of the over-current and over-voltage of the system, and extracts the key influence factors in the expressions.
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 fig. 6, wherein the calculation formula of the fault current may be
In the formula I fap The positive sequence current of the system when the phase b and the phase c are in ground fault, I fan The negative sequence current of the system when the phase b and the phase c are in ground fault, I fa0 Zero sequence current of the system when the phase b and the phase c are grounded, U mp For positive sequence voltage, X, output from a transmitting-end multi-level converter in the system n0 ＝X n '+X 0 '，
X mn Is the negative sequence component of the impedance from the fault point to the MMC outlet, X wn Is the negative sequence component of the impedance from the fault point to the GSC outlet, X m0 Is the zero sequence component of the impedance from the fault point to the MMC outlet, X w0 Is the zero sequence component of the impedance from the fault point to the GSC outlet, X mp Is the positive sequence component of the impedance from the fault point to the MMC outlet, X n0 The sum of the negative and zero sequence impedances, in the system the transformer is usually directly grounded, so the invention assumes that the positive, negative and zero sequence impedances in the system are the same. When a two-phase ground fault occurs at α, X as shown in FIG. 7 mp 、X n ' and X 0 ' can be expressed as
In the formula, X all All reactances from the GSC ac outlet to the MMC ac outlet are indicated, including the MMC converter reactor, the line reactance, the GSC filter and its converter reactors. Will be a formula
Substitution into
In the middle, it can be found that when the GSC output current lags the MMC output voltage by 90 °, i.e. the fan only emits reactive power, the negative sequence current in the system reaches the maximum, which is
Then the formula X n0 ＝X n '+X 0 '，
Substitution into
In (b) finishing to obtain
Wherein ki is negative sequence current coefficient expressed as
Further obtain
Meanwhile, according to the operation characteristics of the power grid, transient overvoltage may occur in a non-fault phase, the overvoltage level of a fault point is highest, and the problem that the fan is disconnected due to the overvoltage is also considered in the PCC of the GSC.
The voltage of the fault point healthy phase voltage (phase a) is
The sound phase amplitude of the fault point is U famax ＝k u (U mp +αI wp X all ) In the formula, ku is a two-phase short circuit grounding coefficient,
when the output current of the GSC lags behind the voltage of the MMC by 90 degrees, namely the GSC only sends out reactive power, the healthy and complete phase voltage of a fault point is the maximum, and the positive sequence voltage and the negative sequence voltage of the PCC point of the GSC are
U wn ＝αjI fan X weq It can be seen that when the GSC output current lags behind the MMC voltage by 90 °, i.e. the GSC only sends out reactive power, the positive-negative sequence voltage amplitude is maximum
Further, based on the relationship between the grounding coefficient and the fault point α as shown in fig. 8, the relationship between the negative sequence current coefficient and the fault point α as shown in fig. 9, and the relationship between the PCC positive sequence voltage of the GSC and the fault point α as shown in fig. 10, it can be obtained that:when the GSC positive sequence current lags behind the MMC positive sequence voltage by 90 degrees, the overvoltage and overcurrent levels of the system are the highest; the closer the fault point is to the MMC, the higher the system overvoltage and overcurrent levels are; system negative sequence current level and overvoltage water average and GSC positive sequence current I wp It is related.
The following is described with respect to the current limited mode:
when the alternating current system has a deep fault, the fault current reaches the current amplitude limiting value of the MMC, and at the moment, the MMC has the characteristic of a current source and is in a current limiting mode. The system overvoltage is significantly lower in the current limiting mode than in the full modulation mode, and therefore the system overvoltage level is no longer analyzed in the current limiting mode. However, in the current limiting mode, the output current of the MMC is equal to the amplitude limiting value of the current limiter, and reaches the upper limit of system protection, and is larger than the current in the full modulation mode, the negative-sequence current is further increased, and the problem of the overcurrent of the bridge arm of the MMC is further aggravated.
The equivalent circuit of the two-phase ground fault in the current limiting mode is shown in fig. 11, where the negative-sequence current of the two-phase ground fault is I fan ＝-(I wp +I mp ) It can be found that the magnitude of the system negative sequence current is independent of the fault location, and is only dependent on the magnitude and phase of the MMC and GSC output currents. When the GSC output current is in phase with the MMC output current, namely the GSC only sends out reactive power, the negative sequence current of the system is the maximum. Like the full modulation mode, in the current limiting mode, the negative sequence current in the two-phase ground short circuit fault can be reduced to a certain extent by reducing the positive sequence current in the system.
In conclusion, the voltage and current determining method for the two-phase ground fault of the wind power grid-connected system provided by the application determines the negative sequence overcurrent and overvoltage during the two-phase ground fault according to the corresponding fault current during the two-phase ground fault and the voltage of the non-fault phase, so that the accuracy of the negative sequence overcurrent and overvoltage during the two-phase ground fault is improved, and the accuracy of the control strategy for the two-phase ground fault ride-through is further improved.
Example two
Fig. 12 is a voltage and current determining system for a two-phase ground fault of a wind power grid connection according to an embodiment of the present application, as shown in fig. 12, including:
the acquiring module 100 is used for acquiring fault current and non-fault phase voltage corresponding to the two-phase ground fault of the wind power when the wind power is sent out from the system through the flexible direct transmission line;
the first determining module 200 is configured to determine a negative sequence overcurrent of the wind power through the flexible direct-current transmission system when the two-phase ground fault occurs according to the fault current;
and a second determining module 300, configured to determine, according to the voltage of the non-fault phase, an overvoltage of the wind power when the two-phase ground fault occurs through the flexible direct-current wind power output system.
In an embodiment of the present disclosure, the obtained fault current is a negative sequence fault current.
Further, the first determining module 200 includes:
the first obtaining unit 201 is configured to obtain a distance from a fault point when the two-phase ground fault occurs in the wind power soft-direct-transmission system to a multi-level converter at a transmission end of the wind power soft-direct-transmission system;
a first determining unit 202, configured to determine a maximum value of the fault current according to the distance, and use the maximum value of the fault current as the negative sequence overcurrent.
Wherein the maximum value of the fault current is calculated as follows:
in the formula I fanmax Maximum value of negative sequence fault current, U mp Is the positive sequence voltage, X, output by the multi-level converter at the delivery end in the system for delivering the wind power through the flexible direct delivery all The total reactance value corresponding to the wind power soft direct-transmission system is defined as alpha, the distance from a fault point to a multi-level converter at a transmission end in the wind power soft-direct-transmission system is defined as I wp Is the AC positive sequence current output by the fan.
In an embodiment of the present disclosure, the root second determining module 300 includes:
a second obtaining unit 301, configured to obtain a distance from a fault point when the two-phase ground fault occurs in the wind power flexible-direct transmission system to the multi-level converter at the transmission end of the wind power flexible-direct transmission system;
a second determining unit 302, configured to determine a voltage maximum value of the non-faulty phase according to the distance, and use the voltage maximum value as the overvoltage.
Wherein the maximum voltage value of the non-fault phase is calculated by the following formula:
in the formula of U famax Voltage maximum of non-faulted phase, U mp Positive sequence voltage, X, output by a multi-level converter at the delivery end of a system for delivering wind power through flexible direct delivery all The total reactance value corresponding to the wind power soft direct-transmission system is defined as alpha, the distance from a fault point to a multi-level converter at a transmission end in the wind power soft-direct-transmission system is defined as I wp Is the AC positive sequence current output by the fan.
In the embodiment of the present disclosure, as shown in fig. 13, the system further includes:
a third determining module 300, configured to determine, according to a distance between the fault point and a multilevel converter at a sending end of the wind power system passing through the flexible direct sending-out system, an overvoltage corresponding to a common connection point of a fan in the wind power system passing through the flexible direct sending-out system when a two-phase ground fault occurs;
wherein the corresponding overvoltage of fan common junction point includes: and positive sequence overvoltage and negative sequence overvoltage corresponding to the common connection point of the fan.
Determining positive sequence overvoltage U corresponding to the common connection point of the fan when two-phase ground fault occurs according to the following formula wpmax ：
Determining negative sequence overvoltage U corresponding to the common connection point of the fan when two-phase ground fault occurs according to the following formula wnmax ：
In the formula of U mp Is the positive sequence voltage output by a multi-level converter at the delivery end in a system for delivering wind power through flexible direct delivery, I wp AC positive-sequence current, X, output by fan all The total reactance value corresponding to the wind power soft direct-transmission system is defined as alpha, the distance from a fault point to a multi-level converter at a transmission end in the wind power soft-direct-transmission system is defined as X weq And alpha is the distance from a fault point to the multi-level converter at the transmission end of the wind power in the system through flexible direct transmission.
In conclusion, the voltage and current determining system for the two-phase ground fault of the wind power grid-connected system improves the accuracy of negative sequence overcurrent and overvoltage during the two-phase ground fault, and further improves the accuracy of a control strategy for the two-phase ground fault ride-through.
In the description herein, reference to the description of the term "one embodiment," "some embodiments," "an example," "a specific example," or "some examples," etc., means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the application. In this specification, the schematic representations of the terms used above are not necessarily intended to refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, various embodiments or examples and features of different embodiments or examples described in this specification can be combined and combined by one skilled in the art without contradiction.
Any process or method descriptions in flow charts or otherwise described herein may be understood as representing modules, segments, or portions of code which include one or more executable instructions for implementing steps of a custom logic function or process, and alternate implementations are included within the scope of the preferred embodiment of the present application in which functions may be executed out of order from that shown or discussed, including substantially concurrently or in reverse order, depending on the functionality involved, as would be understood by those reasonably skilled in the art of the present application.
While embodiments of the present application have been shown and described above, it will be understood that the above embodiments are exemplary and should not be construed as limiting the present application and that changes, modifications, substitutions and alterations in the above embodiments may be made by those of ordinary skill in the art within the scope of the present application.
Claims (10)
1. A voltage and current determination method for a wind power grid-connected two-phase ground fault is characterized by comprising the following steps:
acquiring corresponding fault current and non-fault phase voltage when the wind power is transmitted out of the system through the flexible direct transmission system to generate two-phase ground fault;
determining negative sequence overcurrent of the wind power when the wind power is sent out through the flexible direct-transmission system in the two-phase grounding fault according to the fault current;
and determining the overvoltage of the wind power direct-output system during the two-phase earth fault according to the voltage of the non-fault phase.
2. The method of claim 1, wherein the obtained fault current is a negative sequence fault current.
3. The method of claim 2, wherein the determining the negative sequence over-current of the wind power through a flexible direct-current delivery system at the time of the two-phase ground fault according to the fault current comprises:
acquiring the distance from a fault point when the wind power is subjected to two-phase ground fault through the flexible direct transmission system to a multi-level converter at a transmission end of the wind power transmission system;
and determining the maximum value of the fault current according to the distance, and taking the maximum value of the fault current as the negative sequence overcurrent.
4. The method of claim 3, wherein the maximum value of the fault current is calculated as follows:
in the formula I fanmax Is the maximum value of negative sequence fault current, U mp Positive sequence voltage, X, output by a multi-level converter at the delivery end of a system for delivering wind power through flexible direct delivery all The total reactance value corresponding to the wind power soft direct-transmission system is defined as alpha, the distance from a fault point to a multi-level converter at a transmission end in the wind power soft-direct-transmission system is defined as I wp Is the AC positive sequence current output by the fan.
5. The method of claim 1, wherein the determining the overvoltage of the wind power through the soft direct wind delivery system in the case of the two-phase ground fault according to the voltage of the non-fault phase comprises:
acquiring the distance from a fault point when the wind power is subjected to two-phase ground fault through the flexible direct-transmission system to a multi-level converter at a transmission end of the wind power through the flexible direct-transmission system;
and determining the maximum voltage value of the non-fault phase according to the distance, and taking the maximum voltage value as the overvoltage.
6. The method of claim 5, wherein the voltage maximum for the non-faulted phase is calculated as follows:
in the formula of U famax Voltage maximum of non-faulted phase, U mp Is the positive sequence voltage, X, output by the multi-level converter at the delivery end in the system for delivering the wind power through the flexible direct delivery all The total reactance value corresponding to the system for transmitting the wind power out through the flexible direct transmission system, alpha is the distance from a fault point to a multi-level converter at the transmitting end in the system for transmitting the wind power out through the flexible direct transmission system, I wp Is the AC positive sequence current output by the fan.
7. The method of claim 1, wherein the method further comprises:
determining overvoltage corresponding to a common connection point of a motor in a wind power flexible-direct transmission system when two-phase grounding faults occur according to the distance between the fault point and a multi-level converter at the transmission end of the wind power flexible-direct transmission system;
wherein the corresponding overvoltage of fan common connection point includes: and positive sequence overvoltage and negative sequence overvoltage corresponding to the common connection point of the fan.
8. The method of claim 7, wherein the positive sequence overvoltage U corresponding to the fan common connection point when a two-phase ground fault occurs is determined as follows wpmax ：
Determining negative sequence overvoltage U corresponding to the common connection point of the fan when two-phase ground fault occurs according to the following formula wnmax ：
In the formula of U mp Is the positive sequence voltage output by a multi-level converter at the delivery end in a system for delivering wind power through flexible direct delivery, I wp AC positive-sequence current, X, output by fan all The total reactance value corresponding to the wind power soft direct-transmission system is defined as alpha, the distance from a fault point to a multi-level converter at a transmission end in the wind power soft-direct-transmission system is defined as X weq And alpha is the distance from a fault point to the multi-level converter at the transmission end of the wind power in the system through flexible direct transmission.
9. A voltage and current determination system for a wind power grid-connected two-phase earth fault is characterized by comprising:
the acquisition module is used for acquiring corresponding fault current and non-fault phase voltage when the wind power is subjected to two-phase ground fault through the flexible direct-transmission system;
the first determining module is used for determining negative sequence overcurrent of the wind power when the wind power is subjected to the two-phase earth fault through the flexible direct-transmission system according to the fault current;
and the second determining module is used for determining the overvoltage of the wind power direct-transmission system during the two-phase ground fault according to the voltage of the non-fault phase.
10. The system of claim 9, wherein the captured fault current is a negative sequence fault current.
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