CN108448919B - Modular multilevel converter circulating current suppression method based on virtual reactance - Google Patents

Modular multilevel converter circulating current suppression method based on virtual reactance Download PDF

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CN108448919B
CN108448919B CN201711399082.1A CN201711399082A CN108448919B CN 108448919 B CN108448919 B CN 108448919B CN 201711399082 A CN201711399082 A CN 201711399082A CN 108448919 B CN108448919 B CN 108448919B
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voltage
bridge arm
circulating current
virtual reactance
reactance
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张亮
朱泱
孙玉坤
孟高军
水恒华
袁宇波
花婷
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Nanjing Institute of Technology
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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

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Abstract

The invention belongs to the technical field of direct current transmission, and particularly relates to a modular multilevel converter circulating current inhibition method based on virtual reactance. The method virtualizes a hardware equivalent reactor through a software algorithm to change the equivalent output reactance of each phase of bridge arm, thereby effectively inhibiting the circulating current problem of the modular multilevel converter.

Description

Modular multilevel converter circulating current suppression method based on virtual reactance
Technical Field
The invention belongs to the technical field of direct current transmission, and particularly relates to a modular multilevel converter circulating current suppression method based on virtual reactance.
Background
The Modular Multilevel Converter (MMC) has attracted wide attention of various worldwide scholars since the MMC is proposed for the first time in 2002, compared with the traditional multilevel converter, the modular multilevel converter has good voltage expansion capability and power expansion capability and good output voltage waveform quality due to the highly modular design, but the modular multilevel converter has the greatest advantage of having a common direct-current bus, so that energy exchange can be performed between three-phase bridge arms, and three-phase balance control is facilitated.
Energy of the modular multilevel converter is dispersedly stored in capacitors of each sub-module, theoretically, parameters of each sub-module are completely the same, but actually, due to the problems of small difference of capacitance parameters, different input time of the sub-modules, loss of devices and the like, energy of the converter cannot be evenly distributed, and due to the structural characteristic that three-phase bridge arms are connected in parallel at two ends of direct-current voltage, circulation current is generated between the bridge arms.
The form of a first-order differential equation satisfied by the circulation current and each electrical quantity of the MMC is as follows:
Figure GDA0001616836680000011
it can be seen from the first-order differential equation that the voltage drop of the internal circulation of the x-phase on the bridge arm reactance and the bridge arm equivalent resistance is just caused by the fact that the sum of the voltages of the upper and lower bridge arms is unequal to the voltage of the direct current side, and the circulation only exists in the MMC internal circuit, although the circulation has no influence on a power supply or a load connected to the outer side of the MMC, the existence of the circulation can distort the bridge arm current, cause the loss of a switching device, and even influence the safe operation of a compensation system.
The existing method for inhibiting the circulating current is to reduce the circulating current among three-phase bridge arms by increasing reactance values of bridge arm reactance on hardware, but the tracking capability of an MMC system is influenced by selecting an excessively large reactance value of the bridge arm, and the problems of volume increase, cost increase and the like are caused by excessively large inductance. Therefore, in order to solve the existing problems, a circulating current suppression method of a modular multilevel converter based on virtual reactance is proposed, which aims to improve the equivalent output bridge arm reactance of the MMC and not only increase the bridge arm reactance on hardware to suppress circulating current.
Disclosure of Invention
The invention aims to provide a circulating current suppression method of a modular multilevel converter based on a virtual reactance, which is characterized in that a suppression model corresponding to a bridge arm circulating current is established by analyzing a mechanism generated by the MMC circulating current, and the virtual reactance is provided for correcting the voltages of an upper bridge arm and a lower bridge arm of the MMC so as to achieve the aim of circulating current suppression. The method virtualizes a hardware equivalent reactor through a software algorithm to change the equivalent output reactance of each phase of bridge arm, thereby effectively inhibiting the circulating current problem of the modular multilevel converter.
The invention adopts the following specific technical scheme:
a design scheme of a circulating current suppression method of a modular multilevel converter based on virtual reactance specifically comprises the following steps:
establishing a corresponding circulation current suppression model according to a circulation current generation mechanism of the MMC, wherein the circulation current suppression model is formed by a voltage source VzVoltage source 2VdiffAnd bridge arm reactance LaBridge arm resistor RaAre connected in series. Using voltage sources of opposite polarity 2VdiffCorrecting the upper and lower bridge arm voltages to reduce the voltage source VzTo achieve suppression of circulating current izThe purpose of (1).
According to the correction quantity V of voltagediffConsidering the voltage drop of a virtual reactance, the circulating current suppression method of the modular multilevel converter based on the virtual reactance is provided. The virtual reactor is composed of a current comparison unit, an operation processing unit and a voltage generation unit. And comparing the current signal of the ring current of the bridge arm with a reference signal by using a current comparison unit of the virtual reactor, processing the current signal by using the virtual reactor to generate a corresponding voltage correction signal, and correcting the voltages of the upper and lower bridge arms of the MMC by using a voltage generation unit. The arithmetic processing unit is a virtual reactance controller designed based on an on-line analysis of the circulating current characteristics.
The invention has the beneficial effects that: compared with the prior art, the invention has the following beneficial effects: (1) aiming at the harm of bridge arm current distortion and capacitance voltage fluctuation caused by circulation, a circulation suppression model is provided, and the voltage drop of circulation at the resistance and reactance of a bridge arm is effectively reduced by correcting the voltages of an upper bridge arm and a lower bridge arm so as to achieve the purpose of circulation suppression; (2) the invention provides a circulating current restraining method, wherein a virtual reactance is provided for solving the problems of size and cost increase of an MMC (modular multilevel converter) caused by overlarge bridge arm reactance, and a negative feedback is added into a voltage ring for circulating current control to virtualize a hardware equivalent reactor so as to greatly reduce the cost and the size of the MMC device; (3) the virtual reactance controller designed by the invention greatly increases the gain of the current error of the virtual reactance transfer function at the frequency doubling position, thereby increasing the equivalent output impedance and achieving the expectation of reducing the frequency doubling circulating current.
Drawings
Fig. 1 is a general block diagram of a circulating current suppression method of a modular multilevel converter based on virtual reactance according to the present invention.
Fig. 2 is a circulating current equivalent circuit and a circulating current suppression model of a circulating current suppression method of a modular multilevel converter based on virtual reactance.
Fig. 3 is a virtual reactor of the circulating current suppression method of the modular multilevel converter based on the virtual reactance.
Detailed Description
For the purpose of enhancing the understanding of the present invention, the present invention will be described in further detail with reference to the accompanying drawings and examples, which are provided for the purpose of illustration only and are not intended to limit the scope of the present invention.
Referring to fig. 1 to 3, a specific embodiment of a modular multilevel converter circulating current suppression method based on virtual reactance according to the present invention is shown, and the present invention is further described with reference to the drawings and the specific embodiment.
Fig. 1 is a general structure diagram of a modular multilevel converter circulating current suppression method based on virtual reactance: the three-phase modular multilevel converter is connected with an alternating current power grid through an alternating current bus, three-phase bridge arms are connected in parallel at two ends of direct current voltage, each phase of bridge arm is divided into an upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm are respectively formed by connecting N identical sub-modules 1, a bridge arm resistor 2, a bridge arm reactor 3 and a virtual reactor 4 in series. The submodule 1 comprises a half bridge formed by connecting two IGBT switching tubes in series and a direct-current energy storage capacitor in parallel, and the equivalent reactance of each phase of bridge arm is increased by controlling the virtual reactance 4, so that the bridge arm circulation of the MMC is restrained.
The mechanism of the circulation generation is that bridge arm current is coupled into a submodule energy storage capacitor through the switching action of an upper IGBT tube and a lower IGBT tube of the submodule, the submodule capacitor outputs fluctuation voltages with different frequencies due to the charging and discharging principle of the submodule capacitor, the voltage with different frequency fluctuations is superposed in the bridge arm voltage, so that the bridge arm voltage is unequal to the direct current side voltage, and circulation with corresponding frequency is caused. The fluctuating components of odd-numbered multiples cancel each other out when summing, so the MMC circulating current is mainly composed of even-numbered multiples and is mainly doubled. Although the double-frequency circulating current only flows between the three-phase bridge arms, the existence of the double-frequency circulating current still can cause damage to bridge arm current and sub-module capacitance, and is not beneficial to stable operation of the MMC. It is therefore necessary to suppress double frequency circulating currents.
Fig. 2 shows a circulating current equivalent circuit and a suppression model of a modular multilevel converter circulating current suppression method based on virtual reactance. FIG. 2 is a simplified equivalent circuit of the circulating current, voltage source VzThe voltage difference caused by the unequal sum of the voltages of the upper bridge arm and the lower bridge arm of the MMC and the voltage of the direct current side, namely the voltage drop of the circulating current on the reactance and the resistance of the bridge arm, and the circulating current suppression model shown in the figure 2 is that a voltage source V with opposite polarity is added on the basis of an equivalent circuitdiff，VdiffUsed for correcting the voltages of the upper and lower bridge arms so as to reduce the circulating current voltage drop VzEnsuring that the sum of the upper bridge arm voltage and the lower bridge arm voltage of the MMC is always equal to the direct-current side voltage so as to achieve the aim of restraining the circulating current izThe purpose of (1). Expressed as follows by the mathematical formula:
Figure GDA0001616836680000031
at the moment, the inhibition of the circulation current between the three-phase bridge arms is converted into a voltage correction VdiffAnd correction amount of voltage VdiffCan be regarded as a reactance value LvirIs considered a virtual reactance, as opposed to a physical reactance. Defining the virtual reactance controller as Lvir(s), the voltage correction amount VdiffAnd virtual reactance and circulating current izThe relationship between satisfies:
Figure GDA0001616836680000045
wherein iz(s) is the circulating current between actual bridge arms of MMCz ref(s) is a reference value for the circulating current. Substituting the expression (2) into the inhibition model of the circulation to obtain an expression of the circulation, wherein the expression is as follows:
Figure GDA0001616836680000041
therefore, the frequency doubling component of the circulating current and the equivalent output impedance after the virtual reactance is introduced are respectively as follows:
Figure GDA0001616836680000042
Z(s)＝Lvir(s)+sLa+Ra(5)
fig. 3 shows a virtual reactor 4 of the method for suppressing the circulating current of the modular multilevel converter based on the virtual reactance. The virtual reactor is internally composed of a current comparison unit 41, an operation processing unit 42 and a voltage generation unit 43, wherein the current comparison unit 41 is used for circulating current reference value iz ref(s) and the actual detected circulation value izAnd(s) comparing, wherein the output end of the comparing unit is processed by the operation processing unit 42 to generate a corresponding voltage correction signal and send the voltage correction signal to the voltage generating unit 43, so that the correction of the MMC upper and lower bridge arm voltages is completed.
The arithmetic processing unit 42 is a virtual reactance controller designed based on an on-line analysis of the circulating current. The circulating current reference value is generally IdcThe/3 is a direct current component of the circulating current, so the error signal output by the current comparing unit 41 is an alternating current component of double frequency, and in order to realize accurate non-difference tracking of the double frequency alternating current signal, the arithmetic processing unit must implant a signal model with the same frequency as the double frequency, and according to the internal model principle, the 2 ω is required to be tracked0A pair of conjugate pole-dumps comprised by the transfer function of the virtual reactance
Figure GDA0001616836680000043
Provided, represented by formula (6):
Figure GDA0001616836680000044
wherein k isaThe design is that in order to reduce the steady-state error of the circulating current, the arithmetic processing unit can realize the control of the AC signal of the circulating current double frequency without the steady-state error through the processing of the formula (6), but the design is thatThe gain of the amplified circulating current error at the double frequency is also required to be multiplied by a certain proportionality coefficient, so that the virtual reactance generates a very high gain at the double frequency circulating current, and therefore, the calculation processing unit is designed as follows:
Figure GDA0001616836680000051
wherein k isbThe design of (1) is to increase the proportional gain of the virtual reactance and improve the dynamic response of the virtual reactance to the circulating current. The introduction of the virtual reactance not only needs to track and control the circulating current, but also aims to restrain the double-frequency circulating current of the bridge arm, so a low-pass filter is added on the basis of the formula (7), and the transfer function of the low-pass filter is as follows:
Figure GDA0001616836680000052
wherein ω iscAnd (3) replacing an integral element in the operation processing unit by the expression (8) for the cut-off frequency of the low-pass filter to obtain:
Figure GDA0001616836680000053
the voltage generating unit 43 receives the correction voltage generated by the arithmetic processing unit 42, and sends the correction to the modular multilevel converter, and generates modulation wave signals of the upper and lower bridge arm voltages to drive the IGBT tube, thereby correcting the upper and lower bridge arm voltages of the MMC.
The invention relates to a circulating current restraining method of a modular multilevel converter based on a virtual reactance, which has the advantages that after a virtual reactor is introduced, although the voltages of an upper bridge arm and a lower bridge arm of each phase of an MMC are changed, the output voltage of the bridge arm of each phase is still equal to half of the difference between the voltage of the lower bridge arm and the voltage of the upper bridge arm, the output characteristic of the whole modular multilevel converter is the same as that before the virtual reactance is introduced, and therefore the external characteristic of the MMC cannot be influenced by the access of the virtual reactance. Its two virtual reactances are not physical reactances but only in the circulating currentA negative feedback is added in the controlled voltage loop, so that the size of the MMC device system cannot be changed along with the increase of the virtual reactance, and the cost of the MMC is greatly reduced. Thirdly, the invention considers that the MMC has a complex internal electrical environment and is not beneficial to the bridge arm reactance L in practical engineeringaAnd bridge arm resistance RaThe expected reduction of double frequency circulating current is thus achieved by substantially increasing the value of the introduced virtual reactance, thereby increasing the equivalent output impedance.
The foregoing illustrates and describes the principles, general features, and advantages of the present invention. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are described in the specification and illustrated only to illustrate the principle of the present invention, but that various changes and modifications may be made therein without departing from the spirit and scope of the present invention, which fall within the scope of the invention as claimed. The scope of the invention is defined by the appended claims and equivalents thereof.

Claims (3)

1. A modular multilevel converter circulating current suppression method based on virtual reactance is characterized by comprising the following steps:
step one, establishing a corresponding circulation current suppression model according to a mechanism generated by MMC circulation current, wherein the circulation current suppression model is formed by a voltage source VzVoltage correction signal VdiffAnd bridge arm reactance LaBridge arm resistor RaAre connected in series;
step two, correcting the signal V according to the pair voltagediffThe analysis of the bridge arm voltage compensation circuit is regarded as the voltage drop of a virtual reactance, the virtual reactance is composed of a current comparison unit, an operation processing unit and a voltage generation unit, the current comparison unit of the virtual reactance is used for comparing a current signal of bridge arm circulation with a reference signal, a corresponding voltage correction signal is generated after the processing of the operation processing unit of the virtual reactance, the voltage generation unit is used for correcting the voltages of an upper bridge arm and a lower bridge arm of an MMC, and the operation processing unit is a virtual reactance controller designed according to the online analysis of circulation characteristics;
at the step of first classOn the basis of effective circuit, a voltage correction signal V with opposite polarity is addeddiff，VdiffUsed for correcting the voltages of the upper and lower bridge arms so as to reduce the voltage source VzEnsuring that the sum of the upper bridge arm voltage and the lower bridge arm voltage of the MMC is always equal to the direct-current side voltage so as to achieve the aim of restraining the circulating current izThe purpose of (1) is expressed by a mathematical formula as:
Figure FDA0002395562520000011
2. the method of claim 1, wherein the virtual reactance-based modular multilevel converter circulating current suppression method is characterized in that a virtual reactance controller is defined as Lvir(s), then the voltage correction signal VdiffAnd virtual reactance and circulating current izThe relationship between satisfies:
Figure FDA0002395562520000012
wherein iz(s) is the circulating current between actual bridge arms of MMCz ref(s) is a reference value of the circulation, and the expression (2) is substituted into the inhibition model of the circulation to obtain an expression of the circulation, wherein the expression is as follows:
Figure FDA0002395562520000013
therefore, the frequency doubling component of the circulating current and the equivalent output impedance after the virtual reactance is introduced are respectively as follows:
Figure FDA0002395562520000014
Z(s)＝Lvir(s)+sLa+Ra(5)。
3. the virtual reactance-based modular multilevel converter circulating current suppression method according to claim 2, wherein the arithmetic processing unit is embedded in the circuit boardEntering a signal model with the same frequency as the double frequency, and dividing 2 omega according to the internal model principle0A pair of conjugate pole-dumps comprised by the transfer function of the virtual reactance
Figure FDA0002395562520000021
Provided, represented by formula (6):
Figure FDA0002395562520000022
wherein k isaThe design of (1) is that in order to reduce the steady-state error of the circulating current, through the processing of the formula (6), the arithmetic processing unit can realize the control of the alternating current signal of the circulating current frequency doubling without the steady-state error, but in order to amplify the gain of the circulating current error at the frequency doubling position, the transfer function is multiplied by a certain proportionality coefficient, so that the virtual reactance generates a very high gain at the frequency doubling circulating current position, therefore, the arithmetic processing unit is designed as follows:
Figure FDA0002395562520000023
wherein k isbThe design of (2) is to increase the proportional gain of the virtual reactance and improve the dynamic response of the virtual reactance to the circulating current, and a low-pass filter is added on the basis of the formula (7) in the transfer function, wherein the transfer function of the low-pass filter is as follows:
Figure FDA0002395562520000024
wherein ω iscAnd (3) replacing an integral element in the operation processing unit by the expression (8) for the cut-off frequency of the low-pass filter to obtain:
Figure FDA0002395562520000025
the voltage generating unit receives the voltage correcting signal generated by the operation processing unit, sends the voltage correcting signal to the modular multilevel converter, and generates modulating wave signals of the upper bridge arm voltage and the lower bridge arm voltage so as to drive the IGBT tube to correct the upper bridge arm voltage and the lower bridge arm voltage of the MMC.
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CN111030487B (en) * 2019-12-12 2021-02-19 西安交通大学 Phase capacitance voltage control method applied to modular multilevel converter
CN111525561B (en) * 2020-05-14 2022-06-17 上海交通大学 Method for evaluating stability of modular multilevel converter under multi-frequency cross coupling
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