CN110061644B - Suppression strategy for unbalanced current of isolated MMC bridge arm - Google Patents
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
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- 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
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Abstract
The invention discloses a suppression strategy for unbalanced current of an isolated MMC bridge arm, which comprises the following steps of: for any bridge arm, collecting bridge arm voltage, and respectively obtaining bridge arm output voltage and bridge arm current according to a mathematical model of the isolated MMC circuit for collecting alternating current output current and direct current upper and lower bridge arm current; distributing initial values to each submodule and a driving signal of a bridge arm; comparing the power frequency alternating current component relation of the upper and lower bridge arm currents and the direct current component relation of the voltage by difference to obtain the magnitude of unbalanced current and voltage contained in the phase; and analyzing the current working state of the sub-modules, and correcting the voltage predicted value of each sub-module by using the output voltage of the bridge arm to obtain a driving control signal of the sub-module at the next moment. The invention has the advantages that: the problem that the direct current side of the isolated modular system contains alternating current components is solved, power loss is reduced, efficient utilization of device energy is achieved on the premise that a control system is stable, and production and manufacturing costs are saved.
Description
Technical Field
The invention relates to a suppression strategy for unbalanced current of an isolated MMC bridge arm.
Background
With the great improvement of the Voltage grade and the power grade of the power electronic switch device, High Voltage Direct Current (HVDC) applied to High-power occasions has received wide attention from the industry and academia, and is rapidly developed at home and abroad. Compared with the conventional Multilevel voltage source type Converter, the Isolated Modular Multilevel Converter (I-MMC) has the advantages of high modularization, easy expansion and low output waveform harmonic content, structurally replaces a multi-winding transformer, reduces the volume of the device, saves the cost, simultaneously brings convenience for the redundancy design of the system due to the modularized system structure, avoids the direct series-parallel connection of a switching device due to the multi-module series-parallel connection design scheme, solves the contradiction between the power device grade and the high-voltage power grid system grade, and greatly promotes the development of the flexible direct-current transmission technology. However, the problem of a multi-voltage current sensor is also brought by the serial-parallel connection structure of a large number of sub-modules, the voltage and precision grade of the sensor is met in a high-voltage system, and the design difficulty is extremely high. An isolated MMC topological structure capable of simultaneously realizing direct current output at a high-voltage direct current end and alternating current output at a high-voltage alternating current end through one-time conversion meets the requirements. Compared with the traditional MMC, the novel topological structure has the advantages that the high-voltage side has no isolation capacitor, the multi-loop control is not needed to be assisted, but the high-frequency transformer is arranged in the middle of the topological submodule, the leakage inductance of the high-frequency transformer can cause the loss of the voltage of the secondary side, and further the effective voltage value in the output voltage of each submodule can be different, and further the voltage of the high-voltage direct-current side is not pure and contains direct-current components, but contains a small amount of asymmetric alternating-current components, the alternating-current components can cause a series of severe influences on the load of a direct-current end, the selection requirement of the voltage-stabilizing capacitor is further enlarged, namely the quality of electric energy is reduced, and the economical efficiency is also reduced. It is therefore very necessary to solve this problem.
Disclosure of Invention
The invention aims to provide a suppression strategy for unbalanced current of an isolated MMC bridge arm, solves the problem that a direct current side of an isolated modular system contains alternating current components, reduces power loss, realizes efficient utilization of device energy and saves production and manufacturing cost on the premise of stable control system.
In order to solve the technical problems, the invention is realized by the following technical scheme: a suppression strategy for an unbalanced current of an isolated MMC bridge arm comprises the following steps:
1) for any bridge arm of the isolated MMC, bridge arm voltage is collected, alternating current output current and direct current upper and lower bridge arm current are collected for the isolated MMC, and the bridge arm voltage and the bridge arm current are respectively obtained according to a mathematical model of the isolated MMC;
2) distributing initial values to each submodule and a driving signal of a bridge arm;
3) comparing the alternating current component relationship of the upper bridge arm current and the lower bridge arm current by difference to obtain the magnitude of the contained unbalanced current; comparing the direct-current component relation of the upper bridge arm voltage and the lower bridge arm voltage by difference to obtain the magnitude of the contained unbalanced direct-current voltage;
4) and analyzing the current working state of the sub-modules according to the port value of each sub-module at the current moment, and correcting the voltage predicted value of each sub-module by using the bridge arm voltage to obtain a driving control signal of the sub-module at the next moment.
Preferably, the step 1 comprises the following steps: the following equation shows u for each submoduledcL、uuiOr uliAnd u for each legsuOr uslThe relationship between them, as shown in the equation:
udcLis an isolated MMC low-voltage direct-current port voltage uuiAnd uliSub-module voltages u in upper and lower bridge arms of isolated MMCsuAnd uslThe total voltage of an upper bridge arm and a lower bridge arm of the isolated MMC respectively and the voltage u of all modules in the upper bridge armuiSum of usuAll module voltages u in the lower bridge armliSum of usl，dui，dli(i is 1, …, n) is the equivalent modulation ratio of the upper and lower bridge arm submodules, k isuiAnd kli(i-1, …, n) is the transformer ratio of the upper and lower arm submodules, respectively, assuming d isu＝dui,dl＝dli,k＝kui＝kli(i＝1,…,n)；
The upper and lower bridge arm voltages meet the following conditions:
VdcHis isolated MMC high voltage direct current port voltage; v. ofacIs isolated MMC high voltage alternating current port voltage; duAnd dlThe following conditions need to be satisfied:
wherein the DC modulation index D is set to 0.5 and the AC modulation index D is set to 0.5aIs 0.5 to ensure duAnd dlBetween 0 and 1 to satisfy the operating conditions of the submodule;
based on the above equation:
at the same time, collecting the DC bus current IdcHAnd an alternating output current iacAnd obtaining upper and lower bridge arm currents of the bridge arms, wherein the upper and lower bridge arm currents meet the following conditions:
wherein IsuIs an isolated MMC upper bridge arm current, IslIs an isolated MMC lower bridge arm current, IdcHContaining a current AC component Ix。
Preferably, step 3) comprises the steps of:
1) through bridge arm current IsuAnd IslObtaining actual unbalanced current of the bridge arm, wherein the actual unbalanced current comprises a current alternating current component and a current direct current component;
2) carrying out closed-loop control on the unbalanced current of the bridge arm to obtain additional unbalanced current modulation voltage so as to obtain corresponding duAnd dlThe adjustment value of (2). Thereby realizing suppression of the alternating current component of the current on the direct current side.
Compared with the prior art, the invention has the advantages that:
1. the invention aims at solving the problem of alternating current component caused by unbalance contained on the direct current side of the isolated MMC under the unbalanced voltage of the alternating current voltage, and inhibits the fluctuation of the alternating current component at the direct current end.
2. The invention provides and designs an unbalance controller based on direct suppression of unbalance current based on instantaneous power theory analysis.
3. The principle of the designed isolated MMC controller is not complex, the controller is suitable for the conditions of balanced and unbalanced voltages, and the stability of the system is greatly improved.
Drawings
FIG. 1 is a schematic structural diagram of a single-phase high-frequency chain-based isolated modular cascaded converter in the invention;
FIG. 2 is a waveform of the output of the single stage isolated module of the present invention;
FIG. 3 is a block diagram illustrating the complete control of an isolated modular cascaded converter according to the present invention;
FIG. 4 is a simulation waveform without balance control in the present invention;
fig. 5 is a simulation waveform using the balance control in the present invention.
Detailed Description
Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The embodiments described below with reference to the drawings are illustrative and intended to be illustrative of the invention and are not to be construed as limiting the invention.
The suppression strategy for suppressing the unbalanced current between the bridge arms of the isolated modular cascaded converter is characterized in that the voltage and the current of each bridge arm are detected, the voltage and the current of each bridge arm are derived according to a mathematical model of a single-phase bridge arm of the isolated modular multilevel converter, and the voltage of each bridge arm is subjected to balance control, so that the unbalanced current component between the bridge arms is suppressed, and the dual effects of controlling the voltage of a balance module and the unbalanced current of each bridge arm are achieved.
The technical scheme for solving the technical problems comprises the following steps:
1) the values of the upper bridge arm voltage and the lower bridge arm voltage are controlled by adjusting the duty ratio to make the upper bridge arm voltage and the lower bridge arm voltage tend to target reference values;
2) controlling the alternating current component of the unbalanced current through an isolated MMC unbalanced current controller;
3) comparing the alternating current component relationship of the upper bridge arm current and the lower bridge arm current by difference to obtain the magnitude of the contained unbalanced current; and comparing the direct current component relation of the upper bridge arm voltage and the lower bridge arm voltage by difference to obtain the magnitude of the contained unbalanced direct current voltage.
udcLIs an isolated MMC low-voltage direct-current port voltage uuiAnd uliSub-module voltages u in upper and lower bridge arms of isolated MMCsuAnd uslThe total voltage of an upper bridge arm and a lower bridge arm of the isolated MMC respectively and the voltage u of all modules in the upper bridge armuiSum of usuAll module voltages u in the lower bridge armliSum of usl，dui，dli(i is 1, …, n) is the equivalent modulation ratio of each module, kuiAnd kli(i is 1, …, n) is the transformer ratio of each module, and d can be assumed to beu＝dui,dl＝dli,k＝kui＝kli(i＝1,…,n)。
The upper and lower bridge arm voltages meet the following conditions:
VdcHis isolated MMC high voltage direct current port voltage; v. ofacIs isolated MMC high voltage alternating current port voltage;
duand dlThe following conditions need to be satisfied
Wherein the DC modulation index D is set to 0.5 and the AC modulation index D is set to 0.5aIs 0.5 to ensure duAnd dlIs between 0 and 1 to satisfy the operating conditions of the submoduleWherein, the upper omega is the reference electrical angular frequency of the alternating current component, t is time, and theta is the phase corresponding to the alternating current component;
based on the above equation, it follows:
at the same time, collecting the DC bus current IdcHAnd an alternating output current IacAnd obtaining the current of the upper bridge arm and the lower bridge arm of each bridge arm, wherein the current of the upper bridge arm and the current of the lower bridge arm of each bridge arm meet the following conditions:
wherein IsuIs an isolated MMC upper bridge arm current, IslIs an isolated MMC lower bridge arm current, IdcHContaining a current AC component Ix。
The step 3) comprises the following steps:
1) through bridge arm current IsuAnd IslObtaining actual unbalanced current of the bridge arm, wherein the actual unbalanced current comprises a current alternating current component and a current direct current component;
2) carrying out closed-loop control on the unbalanced current of the bridge arm to obtain additional unbalanced current modulation voltage so as to obtain corresponding duAnd dlThe adjustment value of (2). Thereby realizing suppression of the alternating current component of the current on the direct current side.
And 4, step 4: and analyzing the current working state of the sub-modules according to the port value of each sub-module at the current moment, and correcting the voltage predicted value of each sub-module by using the bridge arm voltage to obtain a driving control signal of the sub-module at the next moment so as to realize the suppression of the unbalanced current at the direct current side.
FIG. 1 and FIG. 2 show a single-stage high-frequency isolated modular multilevel converter (HVDC) subject to the unbalanced current suppression strategy, in which a sub-module terminal on the high-voltage side is controlled by a controlled voltage source vuiOr vli(i ═ 1, …, n), and the established hybrid AC and DC modulation ratio is shown by the following equation:
the instantaneous output power at the high-voltage side of the submodule consists of two parts: pHVACu(l)iAnd PHVDCu(l)i。
In a conventional MMC is zero, which means that the power on the dc side of the submodule to the HV ac side is exactly equal to the power on the HV ac side to the ac side of the submodule. The total power fluctuations of the sub-modules in the single-phase branch are transferred to the common dc side and buffered by a common capacitor, unlike the MMC buffered by the individual capacitors of the sub-modules. The expressions for the bridge arm voltage and bridge arm current are as follows:
the alternating current component of the unbalanced current, namely the unbalanced current of the bridge arm, needs to be suppressed to zero, and the unbalanced current component is directly controlled; the controller is designed as follows:
parameter errors of high frequency isolation transformers between different sub-modules can cause stability problems such as voltage imbalance between the arms and circulating currents. However, in contrast to the conventional MMC structure, the secondary side voltages of all sub-modules are based on the same VdcLThe current manufacturing process can ensure that the voltage error of the sub-modules in the same arm is in a reasonable range, and does not need sub-module voltage balance control. Although voltage errors between the two arms may cause an imbalance in the arm voltages, balancing of the arm voltages may be achieved by additional auxiliary controls, which are simpler than conventional MMC structures.
As shown in fig. 3, the entire control strategy includes two parts: a balanced control strategy and a hybrid modulation strategy. Will bridge arm current ialAnd iauAs an input quantity of the PR controller. Ac modulation ratio d of upper armauEqual to the AC modulation dacSubtract PR controlDevice dmaAnd the ac modulation ratio d of the lower armalIs equal to dacPlus dma. When the variable exceeds 0, dauDecrease, otherwise dauIncrease, dalThe opposite is true. VdcuAnd VdclIs an input term of the PI controller. When V isdcuHas a direct current component greater than VdclTime, PI control DmdIs positive, otherwise DmdIs negative. DDUIs equal to D minus Dmd，DDLIs equal to D plus Dmd. The mixed modulation variable of the upper arm and the lower arm is duAnd dl。
To verify the effectiveness of the auxiliary voltage balance control, a simulation model of the structure was built. In the upper arm, the transformer turns ratio of one submodule is different from the others. Fig. 4 and 5 show the results of the unassisted balance control and the assisted balance control, respectively. Obviously, as shown in fig. 4, there are unbalanced voltages and circulating currents between the upper and lower arms, while VdcHIs fluctuating. Fig. 5 shows waveforms in the unbalanced structure under the balance control. The auxiliary control strategy can effectively balance the arm voltage and reduce the unbalanced current. VDCHThe fluctuation of (a) is significantly reduced.
The result shows that the control method provided by the invention overcomes the defect, the unbalanced current control suppression effect is obvious, and meanwhile, the fluctuation of the active power of the system is effectively suppressed.
The above description is only an embodiment of the present invention, but the technical features of the present invention are not limited thereto, and any changes or modifications within the technical field of the present invention by those skilled in the art are covered by the claims of the present invention.
Claims (2)
1. The utility model provides a restrain tactics to isolated form MMC bridge arm unbalance current which characterized in that: the method comprises the following steps:
1) for any bridge arm of the isolated MMC, bridge arm voltage is collected, alternating current output current and direct current upper and lower bridge arm current are collected for the isolated MMC, and the bridge arm is obtained according to a mathematical model of the isolated MMCThe voltage and bridge arm current specifically comprises the following steps: the following equation shows u for each submoduledcL、uuiOr uliAnd u for each legsuOr uslThe relationship between them, as shown in the equation:
udcLis an isolated MMC low-voltage direct-current port voltage uuiAnd uliRespectively is the sub-module voltage u in the upper and lower bridge arms of the isolated MMCsuAnd uslThe total voltage of an upper bridge arm and a lower bridge arm of the isolated MMC respectively and the voltage u of all modules in the upper bridge armuiSum of usuAll module voltages u in the lower bridge armliSum of usl，dui，dli(i is 1, …, n) is the equivalent modulation ratio of the upper and lower bridge arm submodules, k isuiAnd kli(i-1, …, n) is the transformer ratio of the upper and lower arm submodules, respectively, assuming d isu＝dui,dl＝dli,k＝kui＝kli(i＝1,…,n)；
The upper and lower bridge arm voltages meet the following conditions:
VdcHis isolated MMC high voltage direct current port voltage; v. ofacIs isolated MMC high voltage alternating current port voltage;
duand dlThe following conditions need to be satisfied:
wherein the DC modulation index D is set to 0.5 and the AC modulation index D is set to 0.5aIs 0.5 to ensure duAnd dlIs between 0 and 1 to satisfy the operating conditions of the submodule, where ω is the reference electrical angle of the alternating current componentThe frequency, t is time, and theta is a phase corresponding to the alternating current component;
based on the above equation:
at the same time, collecting the DC bus current IdcHAnd an alternating output current iacAnd obtaining upper and lower bridge arm currents of the bridge arms, wherein the upper and lower bridge arm currents meet the following conditions:
wherein IsuIs an isolated MMC upper bridge arm current, IslIs an isolated MMC lower bridge arm current, IdcHContaining a current AC component Ix；
2) Distributing initial values to each submodule and a driving signal of a bridge arm;
3) comparing the alternating current component relationship of the upper bridge arm current and the lower bridge arm current by difference to obtain the magnitude of the contained unbalanced current; comparing the direct-current component relation of the upper bridge arm voltage and the lower bridge arm voltage by difference to obtain the magnitude of the contained unbalanced direct-current voltage;
4) and analyzing the current working state of the sub-modules according to the port value of each sub-module at the current moment, and correcting the voltage predicted value of each sub-module by using the bridge arm voltage to obtain a driving control signal of the sub-module at the next moment.
2. The suppression strategy for the unbalanced current of the isolated MMC bridge arm of claim 1, wherein:
the step 3) comprises the following steps:
1) through bridge arm current IsuAnd IslObtaining actual unbalanced current of the bridge arm, wherein the actual unbalanced current comprises a current alternating current component and a current direct current component;
2) carrying out closed-loop control on the unbalanced current of the bridge arm to obtain additional unbalanced current modulation voltage so as to obtain corresponding duAnd dlIs adjusted to thereby achieveThe ac component of the current on the dc side is now suppressed.
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN104811067A (en) * | 2015-04-30 | 2015-07-29 | 山东大学 | PR (proportional resonant) controller-based NMC-HVDC (modular multilevel converter-high voltage direct current) circulating current suppression method |
| CN107863895A (en) * | 2017-09-29 | 2018-03-30 | 广东电网有限责任公司电力调度控制中心 | The balance method of upper and lower bridge arm capacitive energy during a kind of bridge arm asymmetry |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN104811067A (en) * | 2015-04-30 | 2015-07-29 | 山东大学 | PR (proportional resonant) controller-based NMC-HVDC (modular multilevel converter-high voltage direct current) circulating current suppression method |
| CN107863895A (en) * | 2017-09-29 | 2018-03-30 | 广东电网有限责任公司电力调度控制中心 | The balance method of upper and lower bridge arm capacitive energy during a kind of bridge arm asymmetry |
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