CN112928938B - Virtual reactance-based MMC direct current fault current limiting method - Google Patents

Virtual reactance-based MMC direct current fault current limiting method Download PDF

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CN112928938B
CN112928938B CN202110319896.XA CN202110319896A CN112928938B CN 112928938 B CN112928938 B CN 112928938B CN 202110319896 A CN202110319896 A CN 202110319896A CN 112928938 B CN112928938 B CN 112928938B
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current
bridge arm
mmc
voltage
fault
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CN112928938A (en
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公铮
赵思涵
亓俊鹏
周娟
戴鹏
原亚雷
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China University of Mining and Technology CUMT
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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
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • 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
    • H02M1/00—Details of apparatus for conversion
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

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Abstract

The invention discloses a virtual reactance-based MMC direct-current fault current limiting method, and belongs to the technical field of direct-current power transmission and distribution. After a bipolar short-circuit fault occurs on the direct-current side of the MMC, the fault current rises rapidly, and the safety of a power electronic device is damaged. The method is based on the existing MMC basic control strategy, the virtual reactance in the bridge arm is mapped into the controller through a feedback function, the bridge arm reactance is equivalently increased, and after the fault occurs, the output direct current voltage of the MMC is reduced to realize the suppression of direct current fault current by correcting the bridge arm voltage reference value of the MMC. The method comprises the following specific steps: firstly, measuring bridge arm current of each phase and calculating the average current change rate of each phase. And then, calculating the DC voltage deviation generated on the virtual reactance of the bridge arm by the DC fault current. And finally, superposing the related deviation correction quantity to the bridge arm reference voltage of the MMC for correction, and achieving the purposes of fault current suppression and reduction of the on-off current of the direct current breaker by reducing the reference value of the bridge arm voltage after the fault.

Description

Virtual reactance-based MMC direct current fault current limiting method
Technical Field
The invention belongs to the field of direct current transmission and distribution, and particularly relates to an MMC direct current fault current limiting method based on virtual reactance.
Background
The flexible direct current transmission and distribution technology based on Modular Multilevel Converters (MMC) has the advantages of no commutation failure, independent active and reactive control and the like, and is an effective means for solving the problem of long-distance collection and sending of large-scale renewable energy sources. Because the damping coefficient of the direct current power grid is low, once a short-circuit fault occurs on the direct current side, the capacitance of the MMC sub-module is rapidly discharged, the fault current is rapidly increased, and the safety of power electronic devices in the system is damaged.
At present, most direct current power grids adopt direct current circuit breakers to isolate faults, and high-speed large-capacity direct current circuit breakers with large on-off currents are difficult to manufacture and high in cost. Therefore, reducing dc fault currents is key to achieving large-scale use of flexible dc distribution networks. Most of the current methods for reducing fault current are additional current limiting devices such as a current limiting reactor, a superconducting current limiter or a solid-state current limiter, the installation of the current limiting devices greatly increases the investment cost of a direct current power grid, and the dynamic performance of the system can be seriously influenced by the overlarge current limiting reactor. Therefore, a simple and reliable MMC current limiting method needs to be designed, direct current fault current is restrained, and the on-off requirement of the direct current breaker is reduced.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides an MMC direct current fault current limiting method based on a virtual reactance, based on the existing MMC basic control strategy, the virtual reactance in a bridge arm is mapped into a controller through a feedback function, the bridge arm reactance is equivalently increased, the reference value of the bridge arm voltage of the MMC is corrected, and the direct current fault current is restrained by reducing the output direct current voltage of the MMC.
In order to solve the technical problems, the technical scheme of the invention is as follows:
step 1: the current sensors are utilized to measure the current of the upper and lower bridge arms of each phase of the MMC in real time and are recorded as i pj 、i nj And calculating the average current change rate of the phase according to the upper bridge arm current and the lower bridge arm current of the phase, wherein the specific calculation method comprises the following steps: and obtaining the bridge arm current change rate after the measured bridge arm currents are subjected to a differential link, adding the bridge arm current change rates, and multiplying the added bridge arm current change rates by 1/2 to obtain the average current change rate of each phase.
And 2, step: and (3) passing the average current change rate obtained by the calculation through a hysteresis comparator to obtain an action signal, wherein the output value of the comparator is 1 when the average current change rate is greater than the action value, and the output value of the comparator is 0 when the average current change rate is less than the return value.
And 3, step 3: setting differential coefficient K according to MMC system circuit parameters D Size of (2) with K D The value of (b) reflects the magnitude of the virtual reactance; then di is mixed j The ratio of/dt to the differential coefficient K D And multiplying the action signals of the hysteresis comparator to obtain the deviation correction quantity delta u of the bridge arm voltage comj * I.e. by
Figure BDA0002992376670000011
And 4, step 4: correcting the deviation of the bridge arm voltage by delta u comj * Superimposed to a bridgeThe arm reference voltage is corrected to obtain the final bridge arm reference voltage for modulation, including
Figure BDA0002992376670000012
Wherein u is pj 、u nj Reference voltages of an upper bridge arm and a lower bridge arm of each phase under the MMC basic control are respectively.
Through the 4 steps, the fault current can be effectively inhibited when the direct current side of the MMC system has a short-circuit fault. Compared with the prior art, the invention has the beneficial effects that: (1) When the direct current system normally operates, the change rate of the average current of each phase is zero, the current limiting control link does not act, the current rises rapidly after a fault occurs, the current limiting control link is automatically started, and the reference voltages of the bridge arms are respectively corrected to realize the active suppression of the fault current; (2) The virtual reactance is introduced, the virtual reactance is not a physical reactance, the effect of the reactor can be played by only adding one control link, the effective inhibition of fault current is realized, and the investment cost of the MMC is reduced.
Drawings
FIG. 1 shows a basic MMC control method in a flexible DC power grid; FIG. 2 is a block diagram of MMC bridge arm virtual reactance current-limiting control.
Detailed Description
The MMC dc fault current-limiting method based on virtual reactance according to the present invention will be described in further detail with reference to the accompanying drawings. It is to be understood that the following description is intended only by way of example, and is not intended to limit the scope of the invention.
The technical problem to be solved by the invention is to adopt a virtual reactance control method, map the newly added virtual reactance in the bridge arm into a controller through a feedback function based on a basic control strategy of the MMC, equivalently increase the bridge arm reactance, and reduce the output direct-current voltage of the MMC to realize the suppression of direct-current fault current by correcting a bridge arm voltage reference value of the MMC. The technical scheme adopted by the invention is as follows:
step 1: firstly, a current sensor is utilized to measure the current of an upper bridge arm and a lower bridge arm of each phase of the MMC in real time, and the current is recorded as i pj 、i nj (j = a, b, c), depending on the phaseThe upper and lower bridge arm currents of (2) are used to calculate the average current change rate di of the phase j As shown in fig. 2, firstly, the measured bridge arm current is subjected to a differentiation link to obtain the bridge arm current change rate, and then the bridge arm current change rates are added and multiplied by 1/2 to obtain the average current change rate of each phase, which includes:
Figure BDA0002992376670000021
when the direct current system normally operates, the current phases of the upper bridge arm and the lower bridge arm of each phase are opposite, the average current change rate is zero, and when the direct current system breaks down, the average current change rate is rapidly increased.
Step 2: in order to prevent the false start of the virtual reactance current-limiting link from influencing the normal operation of the system when the system is in normal operation, the average current change rate di of each phase is calculated j And/dt obtains an action signal after passing through the hysteresis comparator, wherein the output value of the comparator is 1 when the average current change rate is greater than the action value, and the output value of the comparator is 0 when the average current change rate is less than the return value.
And step 3: setting differential coefficient K according to MMC system circuit parameters D Size of (1), with K D The value of (c) reflects the magnitude of the virtual reactance; then di is added j The ratio of/dt to the differential coefficient K D Multiplying action signals of the hysteresis comparator to obtain the voltage drop of the virtual reactance under the fault current, and taking the voltage drop as the correction quantity delta u of the voltage deviation of the bridge arm comj * Is provided with
Figure BDA0002992376670000022
And 4, step 4: as shown in FIG. 2, the bridge arm voltage deviation correction amount Δ u is subtracted from the bridge arm reference voltage comj * Then obtaining the corrected bridge arm voltage reference value, namely the bridge arm reference voltage finally used for modulation, wherein
Figure BDA0002992376670000023
Wherein u is pj 、u nj Respectively on each phase,The reference voltage of the lower bridge arm is generated by MMC basic control method, the concrete principle is shown in figure 1, wherein, U dc For the DC side voltage of MMC j The reference voltage is an alternating current reference voltage of MMC, the bridge arm current rapidly rises after the fault, and the deviation correction quantity delta u of the bridge arm voltage comj * And increasing the reference voltage of the bridge arm for modulation, so that the direct-current voltage output by the MMC is reduced, and the direct-current fault current is inhibited.
It should be noted that the above 4 steps are taken as the summary of the invention, and the 4 steps are an integral whole.
The invention provides an MMC direct current fault current limiting method based on virtual reactance, which has the following advantages:
1. the virtual reactance is introduced, when the direct current system normally operates, the phases of current of an upper bridge arm and a lower bridge arm of the same phase of the MMC are opposite, the average current change rate of each phase is zero, and the current-limiting control cannot be acted; after the direct current system breaks down, the current of the bridge arm of the MMC rises rapidly, the current-limiting control acts automatically, the reference voltage of each phase of the bridge arm is corrected respectively, the direct current voltage output by the MMC is reduced, active suppression of fault current can be achieved, independent control can be achieved for each phase of the bridge arm voltage, and switching control of a controller is not needed.
2. The virtual reactance provided by the invention is not a physical reactance, and the effective suppression of the fault current can be realized only by adding a current-limiting control link in the MMC basic control, so that the volume and the investment cost of the MMC device cannot be increased, and the current-limiting control cannot influence the internal electrical characteristics of the MMC.
The foregoing is illustrative of one embodiment and advantages of the present invention, but the scope of the present invention is not limited thereto. It will be apparent to those skilled in the art that variations and modifications may be made in the above-described embodiments without departing substantially from the technical spirit and principles of the invention described herein, and that such variations and modifications are to be regarded as the scope of the invention.

Claims (1)

1. A virtual reactance-based MMC direct-current fault current limiting method is characterized by comprising the following steps:
step 1: the current sensors are utilized to measure the current of the upper and lower bridge arms of each phase of the MMC in real time and are recorded as i pj 、i nj The measured bridge arm current is subjected to a differential link to obtain the bridge arm current change rate, the bridge arm current change rates are added and multiplied by 1/2 to obtain the average current change rate di of the j phase j The specific calculation formula is as follows:
Figure FDA0003712436650000011
and 2, step: the average current change rate di of the j phase j The/dt passes through a hysteresis comparator to obtain an action signal;
and step 3: setting a differential coefficient K according to MMC system circuit parameters D Size of (2) with K D The value of (b) reflects the magnitude of the virtual reactance; then di is mixed j Dt and the differential coefficient K D Multiplying action signals of the hysteresis comparator to obtain the voltage drop of the virtual reactance under the fault current, and using the voltage drop as the correction quantity of the voltage deviation of the bridge arm
Figure FDA0003712436650000012
And 4, step 4: correcting the deviation of bridge arm voltage
Figure FDA0003712436650000013
Superimposed to the bridge arm reference voltage u pj 、u nj To obtain the final bridge arm reference voltage for modulation, including
Figure FDA0003712436650000014
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CN105633941B (en) * 2015-01-20 2018-04-17 华北电力大学 A kind of commutation failure suppressing method based on virtual current limiter
CN104600997B (en) * 2015-02-04 2017-02-22 国家电网公司 Self coupled modular multilevel high-voltage DC-DC transformer and control method thereof
CN106655237B (en) * 2016-12-31 2019-05-14 华中科技大学 The fault ride-through method of multiterminal flexible high pressure DC transmission system DC monopole ground connection
CN106998075A (en) * 2017-05-26 2017-08-01 云南电网有限责任公司电力科学研究院 Suppress the method and system of the 12 pulsation follow-up commutation failures of inverter of IGCT

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