CN108933540A - A kind of quick recovery control method of flexible HVDC transmission system failure and device - Google Patents
A kind of quick recovery control method of flexible HVDC transmission system failure and device Download PDFInfo
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- CN108933540A CN108933540A CN201710390261.2A CN201710390261A CN108933540A CN 108933540 A CN108933540 A CN 108933540A CN 201710390261 A CN201710390261 A CN 201710390261A CN 108933540 A CN108933540 A CN 108933540A
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
- H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
- H02M7/12—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/21—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/217—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
- H02M7/219—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only in a bridge configuration
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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Abstract
The present invention provides a kind of quick recovery control method of flexible HVDC transmission system failure and device, average value including each mutually upper and lower bridge arm capacitance voltage of computing module multilevel converter obtains the DC component of each mutually upper and lower bridge arm capacitance voltage of modularization multi-level converter by the fundametal compoment in filtering average value；Compare the size between the difference of the DC component of upper and lower bridge arm capacitance voltage and dead zone link preset threshold, determines the output of dead zone link；When the difference for the DC component that the output of dead zone link is upper and lower bridge arm capacitance voltage, it is exported using the proportional controller based on proportional plus integral control and is adjusted, obtains offset voltage；Offset voltage is superimposed with bridge arm reference voltage, to realize the fast quick-recovery control of flexible HVDC transmission system failure.To accelerate to quickly recover to original operating status after flexible HVDC transmission system failure occurs, influence of the failure to whole system operation characteristic is reduced, the operation stability of system is improved.
Description
Technical Field
The invention relates to the field of High Voltage Direct Current (HVDC) transmission based on a Voltage Source Converter (VSC), which comprises a two-end DC transmission technology, a multi-end DC transmission technology, a DC power grid technology and the like, in particular to a fault quick recovery control method and device for a flexible DC transmission system.
Background
The world's concern about energy and environmental pollution has been raised in various countries, and reducing the consumption of fossil energy and improving the use efficiency of renewable energy are the directions in which various countries are making active efforts. With the vigorous development of renewable energy sources such as wind power, hydropower, photovoltaic and the like, the traditional alternating current transmission or conventional direct current transmission is difficult to meet the transmission requirement of electric energy, however, the electric energy generated by the renewable energy sources is delivered in a flexible and controllable manner by using VSC-HVDC, which is beneficial to improving the use efficiency of the renewable energy sources. As a Modular Multilevel Converter (MMC) is shown in fig. 2; the method has the advantages of low output voltage distortion rate, low device switching frequency, no device voltage sharing and the like, becomes the first topological structure of the current flexible direct current transmission technology, and creates favorable conditions for constructing a high-voltage large-capacity flexible direct current transmission system.
The overhead line power transmission is a preferred selection scheme of the current MMC applied to high-voltage flexible direct-current power transmission occasions due to the restriction of the voltage grade of a direct-current cable. When a direct current transmission system or a direct current power grid adopts an overhead wire as a transmission line, the overhead wire is not surrounded by an insulating layer, so that the direct current transmission system or the direct current power grid is often subjected to lightning stroke, flashover, short-circuit to ground fault and the like in practical engineering. When a short-circuit fault occurs, a current path is formed between a converter station in the direct-current transmission system and an alternating-current side grounding device through a fault point, so that direct-current fault current is fed in, and rapid discharge of capacitance and voltage of an MMC bridge arm in the converter station in the direct-current transmission system is caused, as shown in figure 3; there is a large difference in the difference between the capacitor voltage of the failed pole after the rapid discharge and the capacitor voltage of the non-failed pole. After the fault is ended, the average values of the capacitance and the voltage between the upper bridge arm and the lower bridge arm of each phase of the MMC are not equal any more, and because the equivalent resistance of the bridge arm of the MMC is small, the direct current component in the difference between the average values of the capacitance and the voltage of the upper bridge arm and the lower bridge arm can be gradually rebalanced after a long time. During this period of time, fundamental circulating currents will exist inside the MMC, increasing the fluctuation degree of the capacitor voltage and the loss of the system.
In order to improve the operation efficiency and performance of the whole direct current transmission system, it is necessary to perform quick recovery control on the flexible direct current transmission system after a fault occurs, so that the capacitance and voltage of the bridge arm of the MMC reach balance.
Disclosure of Invention
In order to meet the needs of the prior art, the invention provides a method and a device for controlling the quick recovery of the fault of the flexible direct current transmission system, and solves the problem that the flexible direct current transmission system after the fault occurs is difficult to quickly recover. The method has the advantages that the flexible direct current transmission system can be quickly restored to the original operation state after the fault occurs, the influence of the fault on the operation characteristic of the whole system is reduced, and the operation stability of the system is improved.
The technical scheme of the invention is as follows:
a fault quick recovery control method for a flexible direct current transmission system comprises the following steps:
calculating the average value of the capacitance voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel converter, and obtaining the direct current component of the capacitance voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel converter by filtering the fundamental component in the average value;
comparing the difference of the direct current components of the upper bridge arm capacitor voltage and the lower bridge arm capacitor voltage with a preset threshold value of a dead zone link, and determining the output of the dead zone link;
when the output of the dead zone link is the difference between the direct current components of the capacitor voltages of the upper bridge arm and the lower bridge arm, the output of the dead zone link is adjusted by using a proportional controller based on proportional-integral control to obtain compensation voltage;
and superposing the compensation voltage and the bridge arm reference voltage of the modular multilevel converter to realize the quick fault recovery control of the flexible direct current transmission system.
Preferably, the obtaining of the dc component of the capacitor voltage of the upper and lower arms of each phase of the modular multilevel converter includes:
determining average value u of capacitance and voltage of upper bridge arm of each phase of modular multilevel converter through the following formulacpj_aver：
Wherein j represents the j-th phase of the modular multi-level converter, and j is equal toa. b, c; p is physical quantity of an upper bridge arm, i represents the ith submodule, i is 1,2, …, N is the number of submodules with bridge arms connected in series, and u ispj_iThe voltage of the ith sub-module of the jth phase upper bridge arm is obtained;
determining average value u of capacitance and voltage of lower bridge arm of each phase of modular multilevel converter through the following formulacnj_aver：
Wherein n is a physical quantity of the lower arm, unj_iThe voltage of the ith sub-module of the jth phase lower bridge arm is obtained;
by filtering the average value u of the capacitance and voltage of the upper and lower bridge armscpj_averAnd ucnj_averThe fundamental component in the modular multilevel converter obtains the direct current component u of the capacitor voltage of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel convertercpj_aver' and ucnj_aver'。
Preferably, the difference between the dc components of the upper and lower arm capacitor voltages of each phase is determined by the following formula:
ucpnj_err＝ucpj_aver'-ucnj_aver' (3)
wherein j represents the j-th phase of the modular multilevel converter, and j is a, b and c which respectively represent the three phases of the modular multilevel converter; u. ofcpj_aver' and ucnj_aver' are direct current components of capacitance and voltage of upper and lower bridge arms of each phase of the modular multilevel converter respectively.
Preferably, the comparing the difference between the dc components of the upper and lower bridge arm capacitor voltages of each phase and the preset threshold of the dead zone link specifically includes:
when the direct current components of the capacitor voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel converter are inconsistent and the difference value is larger than the preset threshold value of the dead zone link, the dead zone link outputs the difference of the direct current components of the capacitor voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel converter, otherwise, zero is output.
Preferably, the compensation voltageAnd the voltage is obtained by multiplying the difference of the direct current components of the capacitor voltages of the upper and lower bridge arms of each phase of the modular multilevel converter by a proportionality coefficient K.
Preferably, the bridge arm reference voltage of the modular multilevel converter is used for adjusting the number of sub-modules input by a bridge arm of the modular multilevel converter;
if the output current value of the fault pole bridge arm is larger than zero, increasing the number of the submodules connected with the bridge arm in series; if the number of the submodules is less than zero, reducing the number of the submodules connected with the bridge arms in series;
if the output current value of the non-fault pole bridge arm is larger than zero, reducing the number of sub-modules connected in series with the bridge arm; and if the number of the submodules is less than zero, increasing the number of the submodules in series connection with the bridge arms.
Further, the j phase upper bridge arm reference voltage of the modular multilevel converter is respectively determined through the following formulaAnd lower bridge arm reference voltage
In the formula udcIs a reference value of the voltage on the direct current side,is a reference value of the voltage on the ac side,sign (x) for a circulating current suppression voltage reference valueIs a sign function; i.e. ipjAnd injRespectively showing the output current of the j-th phase upper bridge arm.
A flexible dc power transmission system fault fast recovery control apparatus, the apparatus comprising:
the calculation module is used for calculating the average value of the capacitance voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel converter and obtaining the direct current component of the capacitance voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel converter by filtering the fundamental component in the average value;
the comparison module is used for comparing the difference between the direct current components of the upper bridge arm capacitor voltage and the lower bridge arm capacitor voltage with a preset threshold of the dead zone link, and determining the output of the dead zone link;
the acquisition module is used for adjusting the output of the dead zone link by utilizing a proportional controller based on proportional-integral control to acquire compensation voltage if the output of the dead zone link is the difference of direct current components of capacitance voltages of an upper bridge arm and a lower bridge arm;
and the control module is used for superposing the compensation voltage and the bridge arm reference voltage of the modular multilevel converter so as to realize the quick fault recovery control of the flexible direct current transmission system.
Preferably, the computing module comprises
The first determining unit is used for determining the average value of the capacitor voltage of the upper bridge arm of each phase of the modular multilevel converter according to the following formula:
wherein j represents the j-th phase of the modular multi-electric converter, and j is a, b and c; p is an upper bridge arm physical quantity, i represents the ith submodule, i is 1,2, …, N and N is the number of submodules in series connection with bridge arms; u. ofpj_iThe voltage of the ith sub-module of the jth phase upper bridge arm is obtained;
the second determining unit is used for determining the average value of the capacitance and the voltage of the lower bridge arm of each phase of the MMC according to the following formula:
in the formula, n is the physical quantity of a lower bridge arm; u. ofnj_iThe voltage of the ith sub-module of the jth phase lower bridge arm is obtained;
a third determining unit for filtering the average value u of the upper and lower bridge arm capacitance voltagescpj_averAnd ucnj_averThe fundamental component in the modular multilevel converter obtains the direct current component u of the capacitor voltage of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel convertercpj_aver' and ucnj_aver'。
Preferably, the comparison module includes a difference obtaining unit and a determining unit; wherein,
the difference value obtaining unit is used for determining the difference of the direct current components of the capacitor voltages of the upper and lower bridge arms of each phase according to the following formula:
ucpnj_err＝ucpj_aver'-ucnj_aver' (7)；
and the judging unit is used for outputting the difference of the direct current components of the capacitor voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multi-level converter when the direct current components of the capacitor voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multi-level converter are inconsistent and the difference is larger than the preset threshold of the dead zone link, otherwise, outputting zero.
Preferably, the obtaining module includes a compensation determining unit, configured to obtain a compensation voltage according to a multiplication of a ratio coefficient K and a difference between dc components of upper and lower bridge arm capacitor voltages of each phase of the modular multilevel converter
Preferably, the control module includes:
the adjusting unit is used for adjusting the number of sub-modules input by a bridge arm of the modular multilevel converter;
an upper bridge arm reference voltage determining unit for determining the j-th phase upper bridge arm reference voltage of the modular multilevel converter according to the following formula
A lower bridge arm reference voltage determining unit for determining the j-th phase lower bridge arm reference voltage of the modular multilevel converter according to the following formula
Further, the adjusting unit includes: the first regulating subunit is used for increasing the number of the submodules connected in series with the bridge arms if the output current value of the fault pole bridge arm is greater than zero; if the number of the submodules is less than zero, reducing the number of the submodules connected with the bridge arms in series;
the second regulating subunit is used for reducing the number of the submodules connected in series with the bridge arms if the output current value of the non-fault pole bridge arm is greater than zero; and if the number of the submodules is less than zero, increasing the number of the submodules in series connection with the bridge arms.
Compared with the closest prior art, the invention has the beneficial effects that:
1. the invention relates to a fault quick recovery control method for a flexible direct current transmission system, which comprises the steps of calculating an average value of capacitance voltages of upper and lower bridge arms of each phase of a modular multilevel converter by defining the number of serially connected sub-modules of bridge arms of the modular multilevel converter, and obtaining direct current components of the capacitance voltages of the upper and lower bridge arms of each phase of the modular multilevel converter by filtering fundamental wave components in the average value; comparing the difference of the direct current components of the upper bridge arm capacitor voltage and the lower bridge arm capacitor voltage with a preset threshold value of a dead zone link, and determining the output of the dead zone link; the quick recovery control method only works after the difference between the direct current components of the capacitance voltages of the upper bridge arm and the lower bridge arm of each phase exceeds a preset value, and the operation performance of an original control system is not changed under the steady state condition.
When the output of the dead zone link is the difference between the direct current components of the capacitor voltages of the upper bridge arm and the lower bridge arm, the output of the dead zone link is adjusted by using a proportional controller based on proportional-integral control to obtain compensation voltage; and superposing the compensation voltage and the bridge arm reference voltage of the modular multilevel converter to realize the quick fault recovery control of the flexible direct current transmission system. The method can quickly realize the rebalance of the capacitance voltage after the flexible direct current transmission system breaks down, improve the recovery speed of the system, and reduce the fundamental wave circulating current introduced by the unbalance of the bridge arm capacitance voltage in the MMC.
2. The rapid recovery control method and the conversion device are simple and easy to implement, do not need additional peripheral hardware circuits, only need to obtain the average value of the capacitance and voltage of the upper bridge arm and the lower bridge arm of each phase of the MMC, do not have excessive algorithm operation flows, and have the advantage of easy implementation.
3. The quick recovery control method has universality, and is not only suitable for the grounding short-circuit fault of the positive bus of the direct-current power grid, but also suitable for the grounding short-circuit fault of the negative bus and the condition that the capacitance and the voltage of the bridge arm of the MMC are unbalanced due to other faults.
Drawings
FIG. 1: the embodiment of the invention provides a flow chart of a direct current transmission system fault rapid recovery control method;
FIG. 2: the MMC structure schematic diagram of the grounding device containing the alternating current side provided by the embodiment of the invention;
FIG. 3: the embodiment of the invention provides a schematic diagram of a positive bus grounding short circuit fault current path of a direct current transmission system;
FIG. 4: the embodiment of the invention provides a direct current transmission system fault rapid recovery control method and a block diagram.
Detailed Description
The following describes embodiments of the present invention in further detail with reference to the accompanying drawings.
This patent proposes a method for controlling rapid recovery of a fault of a flexible direct current transmission system, as shown in fig. 1, including:
s1, calculating the average value of the capacitance voltage of the upper and lower bridge arms of each phase of the modular multilevel converter, and obtaining the direct current component of the capacitance voltage of the upper and lower bridge arms of each phase of the modular multilevel converter by filtering the fundamental component in the average value;
there are many methods for extracting the dc component in the average value of the capacitor voltage, such as a low pass filter and a trap filter, and the present invention is not limited to this, and all of them are specified as methods for obtaining the dc component.
The MMC consists of 6 bridge arms which are divided into A, B, C three phases, and each phase consists of an upper bridge arm and a lower bridge arm. In the method for controlling the quick fault recovery of the direct current transmission system, the average value of the capacitance and voltage of the upper bridge arm and the lower bridge arm of each phase is required to be used as the input of the quick recovery control method, so that the average value of the capacitance and voltage of the upper bridge arm and the lower bridge arm of each phase is required to be obtained. In the fault recovery process of the MMC, because the phase difference of a fundamental component of a capacitor voltage is 180 degrees under the action of fundamental current of each bridge arm submodule capacitor, the fundamental component needs to be filtered, only a direct current component needs to be reserved, and therefore the difference of the direct current components in the capacitor voltages of an upper bridge arm and a lower bridge arm needs to be extracted and used as the input of a dead zone link.
Assuming that the number of the submodules connected in series with each bridge arm is N, obtaining the direct current components of the capacitor voltages of the upper and lower bridge arms of each phase of the modular multilevel converter comprises the following steps:
determining average value u of capacitance and voltage of upper bridge arm of each phase of modular multilevel converter through the following formulacpj_aver：
Wherein j represents the j-th phase of the modular multi-electric converter, and j is a, b and c; p is physical quantity of an upper bridge arm, i represents the ith submodule, i is 1,2, …, N is the number of submodules with bridge arms connected in series, and u ispj_iThe voltage of the ith sub-module of the jth phase upper bridge arm is obtained;
determining average value u of capacitance and voltage of lower bridge arm of each phase of modular multilevel converter through the following formulacnj_aver：
Wherein n is a physical quantity of the lower arm, unj_iThe voltage of the ith sub-module of the jth phase lower bridge arm is obtained;
by filtering the average value u of the capacitance and voltage of the upper and lower bridge armscpj_averAnd ucnj_averThe fundamental component in the modular multilevel converter obtains the direct current component u of the capacitor voltage of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel convertercpj_aver' and ucnj_aver'。
S2, comparing the difference of the direct current components of the upper and lower bridge arm capacitor voltages with a preset threshold of the dead zone link, and determining the output of the dead zone link;
and determining the difference of the direct current components of the capacitor voltages of the upper and lower bridge arms of each phase according to the following formula:
ucpnj_err＝ucpj_aver'-ucnj_aver' (3)
wherein j represents j phase of the modular multilevel converter, j is a, b and cThree phases of the modular multilevel converter are represented; u. ofcpj_aver' and ucnj_aver' are direct current components of capacitance and voltage of upper and lower bridge arms of each phase of the modular multilevel converter respectively.
Comparing the difference between the direct current components of the upper and lower bridge arm capacitor voltages of each phase with a preset threshold of a dead zone link specifically comprises: when the direct current components of the capacitor voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel converter are inconsistent and the difference value is larger than the preset threshold value of the dead zone link, the dead zone link outputs the difference of the direct current components of the capacitor voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel converter, otherwise, zero is output.
S3, when the output of the dead zone link is the difference between the direct current components of the upper and lower bridge arm capacitance voltages, the output is adjusted by using a proportional controller based on proportional-integral control to obtain a compensation voltage;
and the dead zone link is used for judging whether the fault quick recovery control method is enabled. When the MMC is in normal operation, the direct current components in the average value of the capacitance voltages of the upper bridge arm and the lower bridge arm of each phase cannot be completely equal, so that certain slight difference exists. In order to ensure that the fault fast recovery controller is not repeatedly adjusted under the normal operation condition, a dead zone link is required to be arranged. When the difference of the direct current components in the average values of the capacitance voltages of the upper bridge arm and the lower bridge arm of each phase of the MMC exceeds a threshold value set by the dead zone link, the dead zone link outputs the difference of the direct current components in the average values of the capacitance voltages of the upper bridge arm and the lower bridge arm, and otherwise, zero is output, namely the quick recovery control method is not started. Therefore, the setting of the dead zone link can ensure that the fault quick recovery control method is possible to be enabled only after the fault occurs and does not work under the normal operation condition.
The proportional link is used as a controller for generating compensation voltage by a fault quick recovery method. The difference between the direct current components in the average value of the capacitance voltages of the upper bridge arm and the lower bridge arm of each phase of the MMC cannot be too large even after a fault occurs, in order to realize the rapid recovery of the capacitance voltages, the difference needs to be amplified, the response speed of the rapid recovery control method is improved, and in addition, the difference is required to be amplifiedIn steady state situations where it is not functioning, a proportional controller is the best approach. Compensating voltageAnd the voltage is obtained by multiplying the difference of the direct current components of the capacitor voltages of the upper and lower bridge arms of each phase of the modular multilevel converter by a proportionality coefficient K.
Meanwhile, in order to prevent each phase voltage at the alternating current side of the MMC from generating a large direct current component, the compensation voltage output by the proportional controller needs to be limited, and the specific size of the limit needs to be determined according to the actual engineering voltage grade.
And S4, superposing the compensation voltage and the bridge arm reference voltage of the modular multilevel converter to realize the quick fault recovery control of the flexible direct current transmission system.
The bridge arm reference voltage of the modular multilevel converter is used for adjusting the number of sub-modules input by a bridge arm of the modular multilevel converter;
if the output current value of the fault pole bridge arm is larger than zero, increasing the number of the submodules connected with the bridge arm in series; if the number of the submodules is less than zero, reducing the number of the submodules connected with the bridge arms in series;
if the output current value of the non-fault pole bridge arm is larger than zero, reducing the number of sub-modules connected in series with the bridge arm; and if the number of the submodules is less than zero, increasing the number of the submodules in series connection with the bridge arms.
In order to realize the rapid recovery of the capacitor voltage, the essence of the flexible direct current power transmission system after the fault occurs is to change the energy absorbed or released by the MMC bridge arm. For a fault pole, the capacitance voltage of a fault pole bridge arm is low, and in order to realize quick recovery, when bridge arm current is used for charging a submodule inputted by the fault pole, the submodule inputted by the fault pole bridge arm needs to be increased; on the contrary, when the bridge arm current is discharged for the sub-module in which the fault pole is put, the input number of the fault pole sub-module needs to be reduced. For a non-fault pole, the capacitance voltage of a bridge arm of the non-fault pole is higher, and when the current of the bridge arm charges sub-modules which are input by the non-fault pole, the energy absorbed by the bridge arm needs to be reduced, so that the number of input sub-modules is reduced; on the contrary, when the bridge arm current is discharged from the sub-modules which are not input with the fault poles, the energy released by the bridge arm needs to be increased, so that the number of input sub-modules is increased. The MMC bridge arm reference voltage is controlled to increase or decrease the number of the sub-modules, so that the compensation voltage generated in the step of quickly recovering the proportion of the controller needs to be displayed in a mode of being superposed into the bridge arm reference voltage.
The realization of the fault quick recovery control method of the flexible direct current transmission system finally needs to be implemented on changing the output voltage reference value of the bridge arm of the MMC, namely changing the energy absorbed or released by each bridge arm of the MMC. When the bridge arm current is larger than zero, more sub-modules are required to be put into the fault pole to increase the energy absorbed by the bridge arm, and when the bridge arm current is smaller than zero (the reference direction of the bridge arm current in the attached figure 2 is taken as a positive direction), the put-in number of the sub-modules is required to be reduced to reduce the energy released by the bridge arm.
When the bridge arm current is larger than zero, the number of input sub-modules of the non-fault pole is required to be reduced to reduce the energy absorbed by the bridge arm; on the contrary, when the bridge arm current is less than zero, the input quantity of the sub-modules needs to be increased to increase the energy released by the bridge arm. The MMC corrects the number of the added sub-modules by each phase of the upper bridge arm and the lower bridge arm, thereby being beneficial to quickly realizing fault recovery, namely realizing the rebalancing of the capacitor voltage of the sub-modules.
In summary, the method for fast recovery control in consideration of the fault can be obtained, and therefore, the j-th phase upper bridge arm reference voltage of the modular multilevel converter is determined by the following formulaAnd lower bridge arm reference voltage
In the formula udcIs a reference value of the voltage on the direct current side,is a reference value of the voltage on the ac side,sign (x) is a sign function for the circulating current suppression voltage reference value; i.e. ipjAnd injRespectively showing the output current of the j-th phase upper bridge arm.
Based on the above inventive concept, this embodiment further provides a flexible dc power transmission system fault fast recovery control device, as shown in fig. 4, including:
the calculation module is used for calculating the average value of the capacitance voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel converter and obtaining the direct current component of the capacitance voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel converter by filtering the fundamental component in the average value;
the comparison module is used for comparing the difference between the direct current components of the upper bridge arm capacitor voltage and the lower bridge arm capacitor voltage with a preset threshold of the dead zone link, and determining the output of the dead zone link;
the acquisition module is used for adjusting the output of the dead zone link by utilizing a proportional controller based on proportional-integral control to acquire compensation voltage if the output of the dead zone link is the difference of direct current components of capacitance voltages of an upper bridge arm and a lower bridge arm;
and the control module is used for superposing the compensation voltage and the bridge arm reference voltage of the modular multilevel converter so as to realize the quick fault recovery control of the flexible direct current transmission system.
The calculation module comprises a first determination unit and is used for determining the average value of the upper bridge arm capacitance voltage of each phase of the modular multilevel converter according to the following formula:
wherein j represents the j-th phase of the modular multi-electric converter, and j is a, b and c; p is an upper bridge arm physical quantity, i represents the ith submodule, i is 1,2, …, N and N is the number of submodules in series connection with bridge arms; u. ofpj_iThe voltage of the ith sub-module of the jth phase upper bridge arm is obtained;
the second determining unit is used for determining the average value of the capacitance and the voltage of the lower bridge arm of each phase of the MMC according to the following formula:
in the formula, n is the physical quantity of a lower bridge arm; u. ofnj_iThe voltage of the ith sub-module of the jth phase lower bridge arm is obtained;
a third determining unit for filtering the average value u of the upper and lower bridge arm capacitance voltagescpj_averAnd ucnj_averThe fundamental component in the modular multilevel converter obtains the direct current component u of the capacitor voltage of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel convertercpj_aver' and ucnj_aver'。
The comparison module comprises a difference value acquisition unit and a judgment unit; wherein,
a difference value obtaining unit, configured to determine a difference between dc components of the upper and lower bridge arm capacitance voltages of each phase according to the following formula:
ucpnj_err＝ucpj_aver'-ucnj_aver' (7)；
and the judging unit is used for outputting the difference of the direct current components of the capacitor voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multi-level converter when the direct current components of the capacitor voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multi-level converter are inconsistent and the difference is larger than the preset threshold of the dead zone link, otherwise, outputting zero.
The acquisition module comprises a compensation determination unit used for determining capacitance of upper and lower bridge arms of each phase of the modular multilevel converter according to the capacitanceThe difference of the DC components of the voltage is multiplied by a proportionality coefficient K to obtain a compensation voltage
The control module includes:
the adjusting unit is used for adjusting the number of sub-modules input by a bridge arm of the modular multilevel converter;
an upper bridge arm reference voltage determining unit for determining the j-th phase upper bridge arm reference voltage of the modular multilevel converter according to the following formula
A lower bridge arm reference voltage determining unit for determining the j-th phase lower bridge arm reference voltage of the modular multilevel converter according to the following formula
Wherein, the adjusting unit includes: the first regulating subunit is used for increasing the number of the submodules connected in series with the bridge arms if the output current value of the fault pole bridge arm is greater than zero; if the number of the submodules is less than zero, reducing the number of the submodules connected with the bridge arms in series;
the second regulating subunit is used for reducing the number of the submodules connected in series with the bridge arms if the output current value of the non-fault pole bridge arm is greater than zero; and if the number of the submodules is less than zero, increasing the number of the submodules in series connection with the bridge arms. As will be appreciated by one skilled in the art, embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein.
The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flow diagrams and/or block diagrams, and combinations of flows and/or blocks in the flow diagrams and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting the same, and although the present invention is described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications and equivalents may be made to the embodiments of the invention without departing from the spirit and scope of the invention, which is to be covered by the claims.
Claims (13)
1. A method for controlling fault quick recovery of a flexible direct current transmission system is characterized by comprising the following steps:
calculating the average value of the capacitance voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel converter, and obtaining the direct current component of the capacitance voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel converter by filtering the fundamental component in the average value;
comparing the difference of the direct current components of the upper bridge arm capacitor voltage and the lower bridge arm capacitor voltage with a preset threshold value of a dead zone link, and determining the output of the dead zone link;
when the output of the dead zone link is the difference between the direct current components of the capacitor voltages of the upper bridge arm and the lower bridge arm, the output of the dead zone link is adjusted by using a proportional controller based on proportional-integral control to obtain compensation voltage;
and superposing the compensation voltage and the bridge arm reference voltage of the modular multilevel converter to realize the quick fault recovery control of the flexible direct current transmission system.
2. The method of claim 1, wherein obtaining the DC component of the upper and lower leg capacitor voltages of each phase of the modular multilevel converter comprises:
determining average value u of capacitance and voltage of upper bridge arm of each phase of modular multilevel converter through the following formulacpj_aver：
Wherein j represents the j-th phase of the modular multi-electric converter, and j is a, b and c; p is physical quantity of an upper bridge arm, i represents the ith submodule, i is 1,2, …, N is the number of submodules with bridge arms connected in series, and u ispj_iThe voltage of the ith sub-module of the jth phase upper bridge arm is obtained;
determining average value u of capacitance and voltage of lower bridge arm of each phase of modular multilevel converter through the following formulacnj_aver：
Wherein n is a physical quantity of the lower arm, unj_iThe voltage of the ith sub-module of the jth phase lower bridge arm is obtained;
by filtering the average value u of the capacitance and voltage of the upper and lower bridge armscpj_averAnd ucnj_averThe fundamental component in the modular multilevel converter obtains the direct current component u of the capacitor voltage of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel convertercpj_aver' and ucnj_aver'。
3. The method of claim 1, wherein the difference between the dc components of the upper and lower leg capacitor voltages for each phase is determined by:
ucpnj_err＝ucpj_aver'-ucnj_aver' (3)
wherein j represents the j-th phase of the modular multilevel converter, and j is a, b and c which respectively represent the three phases of the modular multilevel converter; u. ofcpj_aver' and ucnj_aver' are direct current components of capacitance and voltage of upper and lower bridge arms of each phase of the modular multilevel converter respectively.
4. The method of claim 1, wherein the comparing the difference between the dc components of the upper and lower bridge arm capacitor voltages of each phase with a preset threshold of a dead band link comprises:
when the direct current components of the capacitor voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel converter are inconsistent and the difference value is larger than the preset threshold value of the dead zone link, the dead zone link outputs the difference of the direct current components of the capacitor voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel converter, otherwise, zero is output.
5. The method of claim 1, wherein the compensation voltageAnd the voltage is obtained by multiplying the difference of the direct current components of the capacitor voltages of the upper and lower bridge arms of each phase of the modular multilevel converter by a proportionality coefficient K.
6. The method of claim 1, wherein the bridge arm reference voltages of the modular multilevel converter are used for adjusting the number of sub-modules put into a bridge arm of the modular multilevel converter;
if the output current value of the fault pole bridge arm is larger than zero, increasing the number of the submodules connected with the bridge arm in series; if the number of the submodules is less than zero, reducing the number of the submodules connected with the bridge arms in series;
if the output current value of the non-fault pole bridge arm is larger than zero, reducing the number of sub-modules connected in series with the bridge arm; and if the number of the submodules is less than zero, increasing the number of the submodules in series connection with the bridge arms.
7. The method of claim 6, wherein the j-th phase upper bridge arm reference voltage of the modular multilevel converter is determined by the following formulaAnd lower bridge arm reference voltage
In the formula udcIs a reference value of the voltage on the direct current side,is a reference value of the voltage on the ac side,sign (x) is a sign function for the circulating current suppression voltage reference value; i.e. ipjAnd injRespectively showing the output current of the j-th phase upper bridge arm.
8. A fault fast recovery control device for a flexible direct current transmission system, the device comprising: the calculation module is used for calculating the average value of the capacitance voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel converter and obtaining the direct current component of the capacitance voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel converter by filtering the fundamental component in the average value;
the comparison module is used for comparing the difference between the direct current components of the upper bridge arm capacitor voltage and the lower bridge arm capacitor voltage with a preset threshold of the dead zone link, and determining the output of the dead zone link;
the acquisition module is used for adjusting the output of the dead zone link by utilizing a proportional controller based on proportional-integral control to acquire compensation voltage if the output of the dead zone link is the difference of direct current components of capacitance voltages of an upper bridge arm and a lower bridge arm;
and the control module is used for superposing the compensation voltage and the bridge arm reference voltage of the modular multilevel converter so as to realize the quick fault recovery control of the flexible direct current transmission system.
9. The apparatus of claim 8, wherein the computing module comprises
The first determining unit is used for determining the average value of the capacitor voltage of the upper bridge arm of each phase of the modular multilevel converter according to the following formula:
wherein j represents the j-th phase of the modular multi-electric converter, and j is a, b and c; p is an upper bridge arm physical quantity, i represents the ith submodule, i is 1,2, …, N and N is the number of submodules in series connection with bridge arms; u. ofpj_iThe voltage of the ith sub-module of the jth phase upper bridge arm is obtained;
the second determining unit is used for determining the average value of the capacitance and the voltage of the lower bridge arm of each phase of the MMC according to the following formula:
in the formula, n is the physical quantity of a lower bridge arm; u. ofnj_iThe voltage of the ith sub-module of the jth phase lower bridge arm is obtained;
a third determining unit for filtering the average value u of the upper and lower bridge arm capacitance voltagescpj_averAnd ucnj_averThe fundamental component in the modular multilevel converter obtains the direct current component u of the capacitor voltage of the upper bridge arm and the lower bridge arm of each phase of the modular multilevel convertercpj_aver' and ucnj_aver'.
10. The apparatus of claim 8, wherein the comparing module comprises a difference obtaining unit and a determining unit; wherein,
the difference value obtaining unit is used for determining the difference of the direct current components of the capacitor voltages of the upper and lower bridge arms of each phase according to the following formula:
ucpnj_err＝ucpj_aver'-ucnj_aver' (7)；
and the judging unit is used for outputting the difference of the direct current components of the capacitor voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multi-level converter when the direct current components of the capacitor voltages of the upper bridge arm and the lower bridge arm of each phase of the modular multi-level converter are inconsistent and the difference is larger than the preset threshold of the dead zone link, otherwise, outputting zero.
11. The apparatus of claim 8, wherein the obtaining module comprises a compensation determining unit for obtaining the compensation voltage according to multiplication of a scaling coefficient K and a difference between DC components of the upper and lower bridge arm capacitor voltages of each phase of the modular multilevel converter
12. The apparatus of claim 8, wherein the control module comprises:
the adjusting unit is used for adjusting the number of sub-modules input by a bridge arm of the modular multilevel converter;
an upper bridge arm reference voltage determining unit for determining the j-th phase upper bridge arm reference voltage of the modular multilevel converter according to the following formula
A lower bridge arm reference voltage determining unit for determining the j-th phase lower bridge arm reference voltage of the modular multilevel converter according to the following formula
13. The apparatus of claim 12, wherein the adjustment unit comprises: the first regulating subunit is used for increasing the number of the submodules connected in series with the bridge arms if the output current value of the fault pole bridge arm is greater than zero; if the number of the submodules is less than zero, reducing the number of the submodules connected with the bridge arms in series;
the second regulating subunit is used for reducing the number of the submodules connected in series with the bridge arms if the output current value of the non-fault pole bridge arm is greater than zero; and if the number of the submodules is less than zero, increasing the number of the submodules in series connection with the bridge arms.
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