CN112953273B - Parameter design and control method of hybrid modular multilevel converter - Google Patents
Parameter design and control method of hybrid modular multilevel converter Download PDFInfo
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- CN112953273B CN112953273B CN202110326155.4A CN202110326155A CN112953273B CN 112953273 B CN112953273 B CN 112953273B CN 202110326155 A CN202110326155 A CN 202110326155A CN 112953273 B CN112953273 B CN 112953273B
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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
- 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
Abstract
The embodiment of the invention discloses a parameter design and control method of a hybrid modular multilevel converter, which improves the modulation ratio of the hybrid modular multilevel converter to an optimal operating point through the coordination and coordination of a third harmonic voltage injection method and a full-bridge submodule negative level utilization method. Firstly, the modulation ratio of the hybrid modular multilevel converter is improved to the maximum extent by a third harmonic voltage injection method, and then the modulation ratio is improved to the optimal point by the configuration of the cascade number of full-bridge sub-modules in a bridge arm. When the hybrid modular multilevel converter outputs rated active power, the modulation ratio operates at an optimal operating point, so that the fundamental frequency component of the bridge arm power fluctuation becomes zero, the capacitance and voltage fluctuation of the sub-modules is greatly reduced, the capacitance consumption of the hybrid modular multilevel converter is greatly reduced, the number of required full-bridge sub-modules is reduced to the greatest extent, and the total cost and the volume of the device are reduced.
Description
Technical Field
The invention relates to the technical field of voltage source converters and flexible direct current transmission, in particular to a parameter design and control method of a hybrid modular multilevel converter.
Background
The modular multilevel converter (MMC for short) has the advantages of easy realization of large level number, good harmonic performance, modularized design and the like, and has become the inevitable choice of the flexible direct-current transmission converter. However, there is a large low-frequency power fluctuation on the MMC bridge arm, which causes the sub-module capacitor voltage fluctuation, and therefore, a large sub-module capacitor value needs to be designed to limit the voltage fluctuation within an allowable range. In the existing engineering, the volume proportion of the direct current capacitor in the submodule can reach more than 60%, and the cost proportion can reach more than 40%. The huge cost and the volume problem that will bring of MMC electric capacity quantity have greatly restricted MMC's further development and application. Therefore, optimizing the parameters and operating characteristics of MMCs by various means has been a research hotspot.
Conventional MMC usually adopts the half-bridge submodule piece, also can constitute full-bridge MMC based on the full-bridge submodule piece, perhaps can constitute mixed modularization multilevel converter based on full-bridge submodule piece and half-bridge submodule piece. The full-bridge sub-module can output three levels of 1, 0 and-1, compared with the half-bridge sub-module which can only output two levels of 1 and 0. If the negative level output of the full-bridge submodule is utilized in normal operation, the conversion relation of the alternating current voltage and the direct current voltage of the hybrid modular multilevel converter can be changed to a large extent, and the method can also be regarded as an overmodulation technology. In recent years, research on the utilization of the negative level of the hybrid modular multilevel converter also finds that the negative level output of the full-bridge submodule can also have great influence on the power fluctuation of a bridge arm, and even the fundamental frequency power fluctuation component of the bridge arm can reach an extremely low point when the negative level of the full-bridge submodule is utilized to a certain degree.
Third harmonic injection is a commonly used overmodulation approach. By injecting third harmonic voltage into the reference wave in the modulation link, the amplitude of the reference wave can be reduced, the maximum modulation ratio which can be realized is improved, the AC output voltage capability of the MMC is improved, and other operation characteristics are improved. However, the improvement degree of the third harmonic injection method on the maximum modulation ratio of the MMC is relatively limited, and the influence on the optimization of the operational characteristics of the MMC is also relatively limited.
Although the relationship between alternating current and direct current voltage transformation of the hybrid modular multilevel converter can be changed by utilizing the third harmonic wave injection and the negative level, in the existing research, two methods are respectively processed, the advantages of the two methods cannot be synthesized to optimize the parameters and the operating characteristics of the hybrid modular multilevel converter, and the overall cost and volume are optimized.
Disclosure of Invention
The embodiment of the invention provides a parameter design and control method of a hybrid modular multilevel converter, which can ensure that the modulation ratio operation of the hybrid modular multilevel converter reaches the optimal point, simultaneously ensure that the number of required added full-bridge submodules is minimum, and further ensure that the volume and the cost of the hybrid modular multilevel converter are optimal.
The parameter design and control method of the hybrid modular multilevel converter provided by the embodiment of the invention comprises the following steps:
according to the rated voltage U of the set DC port of the hybrid modular multilevel converterdcAnd the set capacitance voltage rated value U of the sub-module of the hybrid modular multilevel convertercCalculating and generating the number N of cascade half-bridge sub-modules contained in each bridge arm of the hybrid modular multilevel converter0And is and
according to the number N of cascade of half-bridge sub-modules contained in each bridge arm0Calculating the number K of cascade full-bridge submodules contained in each bridge arm of the hybrid modular multilevel converter, and
according to the rated voltage U of the set DC port of the hybrid modular multilevel converterdcDetermining the effective value U of the AC voltage of the hybrid modular multilevel converteracAnd is and
according to the effective value U of the alternating voltage of the hybrid modular multilevel converteracDetermining three-phase voltage reference value u after injection of third harmonic voltagera,urb,urc；
The three-phase voltage reference value ura,urb,urcInput to the hybrid modular multilevel converter to generate a pulsed control signal for the hybrid modular multilevel converter.
Preferably, the effective value U of the alternating voltage of the hybrid modular multilevel converteracDetermining three-phase voltage reference value u after injection of third harmonic voltagera,urb,urcThe method specifically comprises the following steps:
according to the set synchronous angular frequency omega of an alternating current system connected with the hybrid modular multilevel converter, the set bridge arm inductance value L of the hybrid modular multilevel converter and the set rated active current effective value I output by the hybrid modular multilevel converterNAnd an effective value of the alternating voltage U of the hybrid modular multilevel converteracCalculating and generating rated phase voltage U of an alternating current system connected with the hybrid modular multilevel converterSN；
According to the effective value U of the alternating voltage of the hybrid modular multilevel converteracCalculating and generating a three-phase fundamental voltage reference value u of the hybrid modular multilevel converter according to the set reference voltage phase angle delta of the hybrid modular multilevel converter and the synchronous angular frequency omega of an alternating current system connected with the hybrid modular multilevel converterra1,urb1,urc1；
According to the three-phase fundamental voltage reference value ura1,urb1,urc1Calculating the third harmonic voltage injection value u0；
According to the three-phase fundamental voltage reference value ura1,urb1,urc1And the third harmonic voltage injection value u0Determining three-phase voltage reference value u after injection of third harmonic voltagera,urb,urc。
Preferably, the cross-phase to which the hybrid modular multilevel converter is connected according to the set configurationThe synchronous angular frequency omega of a flow system, the set bridge arm inductance value L of the hybrid modular multilevel converter, and the set rated active current effective value I output by the hybrid modular multilevel converterNAnd an effective value U of the alternating voltage of the hybrid modular multilevel converteracCalculating and generating rated phase voltage U of an alternating current system connected with the hybrid modular multilevel converterSNSpecifically, it is calculated by the following formula:
preferably, the reference voltage effective value U according to the hybrid modular multilevel converteracCalculating and generating a three-phase fundamental wave voltage reference value u of the hybrid modular multilevel converter according to the set reference voltage phase angle delta of the hybrid modular multilevel converter and the synchronous angular frequency omega of an alternating current system connected with the hybrid modular multilevel converterra1,urb1,urc1Specifically, it is calculated by the following formula:
preferably, the reference voltage u is based on the three-phase fundamental wave voltagera1,urb1,urc1Calculating the third harmonic voltage injection value u0Specifically, it is calculated by the following formula:
wherein max is a function for taking the maximum value, and min is a function for taking the minimum value.
Preferably, the reference voltage u is based on the three-phase fundamental wave voltagera1,urb1,urc1And the third harmonic voltage injection value u0Determining three-phase voltage reference value u after injection of third harmonic voltagera,urb,urcSpecifically, the three-phase voltage reference value u is calculated by the following formulara,urb,urc：
ura＝ura1+u0，
urb＝urb1+u0，
urc＝urc1+u0。
Compared with the prior art, the parameter design and control method of the hybrid modular multilevel converter provided by the embodiment of the invention improves the maximum modulation ratio which can be realized by the hybrid modular multilevel converter through the coordination and coordination of the third harmonic voltage injection method and the full-bridge submodule negative level utilization method. Firstly, the modulation ratio of the hybrid modular multilevel converter can be improved by a third harmonic voltage injection method
And further increasing a proper number of full-bridge submodules in a bridge arm of the hybrid modular multilevel converter, and increasing the modulation ratio of the hybrid modular multilevel converter to be higher than that of the hybrid modular multilevel converter by using a negative level utilization method of the full-bridge submodules
The optimum point of (2). When the hybrid modular multilevel converter outputs rated active power, if the modulation ratio is operated at
In the process, the fundamental frequency component of the bridge arm power fluctuation of the hybrid modular multilevel converter can be changed into zero, so that the capacitor voltage fluctuation of the sub-module is greatly reducedAnd the capacitance of the sub-module can be smaller under the same limitation of the fluctuation rate of the capacitor voltage, so that the capacitance consumption of the hybrid modular multilevel converter is greatly reduced. Through the coordination of a third harmonic voltage injection method and a full-bridge submodule negative level utilization method, the modulation ratio can be improved to
The number of required full-bridge submodules is reduced to the maximum degree, and the overall cost and the volume of the device are reduced.
Drawings
Fig. 1 is a schematic diagram of a hybrid modular multilevel converter;
fig. 2 is a schematic diagram of the structure of the legs of a hybrid modular multilevel converter;
FIG. 3 is a view showing a method of specifying
Numerical lower bridge arm fundamental frequency power fluctuation amplitude following MacA varying two-dimensional curve;
fig. 4 is a schematic flowchart of a parameter design and control method of a hybrid modular multilevel converter according to an embodiment of the present invention;
fig. 5 is a waveform of a key operating characteristic of the hybrid modular multilevel converter after the method for designing and controlling parameters of the hybrid modular multilevel converter provided by the present invention is used.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Referring to fig. 1, fig. 1 is a schematic structural diagram of a hybrid modular multilevel converter, as shown in fig. 1, in six three-phase arms of the hybrid modular multilevel converterEach bridge arm of (1) contains N sub-modules, and each sub-module in the bridge arm contains N0The structure of the half-bridge sub-modules and the K full-bridge sub-modules can refer to the schematic structural diagram of the bridge arm of the hybrid modular multilevel converter shown in fig. 2.
In some existing projects, a certain proportion of full-bridge submodules are configured to reduce fundamental frequency power fluctuation components of bridge arms by utilizing negative level outputs of the full-bridge submodules. However, adding too many full-bridge sub-modules increases the overall cost and volume of the device. According to the existing research, the modulation ratio of the hybrid modular multilevel converter can be improved by both the third harmonic injection and the negative level utilization, but in the existing research, two types of methods are respectively processed, so that how to comprehensively utilize the advantages of the third harmonic injection and the negative level utilization to optimize the modulation ratio needs to be further researched on the basis.
Therefore, the influence of the coordination and the coordination of the third harmonic injection and the negative level utilization on the modulation ratio is researched, the influence of the modulation ratio on the fundamental frequency power fluctuation component of the bridge arm is further researched, the influence of the coordination and the coordination of the third harmonic injection and the negative level utilization on the fundamental frequency power fluctuation component of the bridge arm is further researched, the parameters and the operation characteristics of the hybrid modular multilevel converter are optimized, and the parameter design and control method of the hybrid modular multilevel converter is obtained.
Taking an a-phase bridge arm as an example, the influence of third harmonic injection and negative level utilization on the modulation ratio is firstly researched:
the equivalent voltage of the alternating current port of the hybrid modular multilevel converter obtained according to the output working condition is set as follows:
wherein U isacThe effective value of the fundamental frequency phase voltage of the alternating current port of the hybrid modular multilevel converter is shown, omega is the angular frequency of the fundamental frequency, k3And theta3Representing the amplitude factor and phase of the injected third harmonic voltage, respectively.
After the required output voltage of the alternating current port is determined, according to the existing research conclusion, the voltage of the A-phase bridge arm injected with the third harmonic voltage can be determined as follows:
wherein subscripts "ap" and "an" represent the a-phase upper and lower bridge arms, respectively.
The modulation ratio of the hybrid modular multilevel converter refers to the ratio of the amplitude of the fundamental frequency component of the alternating-current phase voltage to the voltage of the direct-current side, namely:
similarly, also with UdcThe third harmonic voltage injected is normalized by the waveform function f3(3 ω t) represents:
by substituting equations (1), (3) and (4) into equation (2), the hybrid modular multilevel converter bridge arm voltage can be expressed as:
setting the capacitor voltage of each submodule to UcAccording to the bridge arm voltage of the formula (5), the number of bridge arm levels (i.e. the sum of output levels of all sub-modules of the bridge arm) can be calculated as:
on the other hand, when viewed from the dc side, the dc voltage of the hybrid modular multilevel converter can be regarded as the sum of the upper and lower bridge arm voltages, and needs to be maintained at a constant value, that is:
wherein N is0Indicating that the sub-module capacitor voltage is UcDC voltage of UdcThe number of cascaded half-bridge sub-modules required in the time leg. Substituting equation (7) into equation (6) can yield:
the linear modulation region of the hybrid modular multilevel converter is that the bridge arm level number calculated by the formula (8) is within the output range of the hybrid modular multilevel converter.
Further consider the case of adding a full-bridge submodule, that is, a bridge arm of the hybrid modular multilevel converter may include a half-bridge submodule and a full-bridge submodule, wherein the half-bridge submodule may output two levels of 1 and 0, and the full-bridge submodule may output three levels of 1, 0 and-1. If a bridge arm contains N submodules (N can be more than or equal to the reference number N)0) And K full-bridge submodules in the N submodules can output negative levels, and at any time, the level range output by the bridge arm must be between-K and N, which is the linear modulation region of the hybrid modular multilevel converter, namely:
by substituting formula (8) for formula (9), the following relationship can be obtained:
the expression (10) is the condition that the hybrid modular multilevel converter needs to meet when operating in the linear modulation region, and comprehensively reflects the influence of third harmonic injection and negative level utilization on the linear modulation region.
Will Mac(max)Defined as the maximum modulation ratio achievable, i.e. M when the constraint of equation (10) is satisfiedacThe maximum value that can be obtained. Maximum modulation ratio M when using conventional half-bridge MMC and without third harmonic injectionac(max)Is 1.0.
The utilization of the negative level of the full-bridge submodule is not considered (i.e. K is temporarily set to 0, and N is temporarily set to N)0) In the presence of third harmonic injection, equation (10) can be expressed as:
-1≤Macsin(ωt)+f3(3ωt)≤1(11)
if proper third harmonic voltage is injected, the sine reference wave can be cut to a certain extent, and the voltage at M isacThe condition of equation (11) can still be satisfied when the voltage exceeds 1, which is the basic principle of overmodulation by third harmonic voltage injection. Maximum modulation ratio Mac(max)Is defined as the third harmonic injection overmodulation coefficient kzsqIn practice this is also equivalent to a reduction of the peak of the reference voltage waveform, i.e.:
according to the existing theory, the third harmonic alone is adopted to inject the overmodulation coefficient kzsqCan achieve the purpose of
On the other hand, if the number N of the sub-modules of the actual bridge arm cascade is larger than the reference number N0I.e. N>N0The right-hand positive boundary value in equation (10) will obviously increase. However, the requirement of positive-negative symmetry of the linear modulation region requires that the negative boundary on the left side in equation (10) be enlarged to the same extent to satisfy the requirement of enlargement of the linear modulation region. Thus in N>N0It is also necessary to configure a certain number of full-bridge power modules to output a certain number of negative levels (i.e., K is larger)At a certain value). After the third harmonic injection is adopted, the positive and negative peak values of the reference wave become +/-Mac/kzsqThe positive and negative boundary conditions of equation (10) may be expressed as follows:
for equation (13), if the positive and negative boundaries need to be extended to the same extent, the following condition should be satisfied:
from equation (14) we can obtain:
N＝N0+K(15)
the above analysis shows that when the modulation ratio is increased by the negative level utilization of the full-bridge module, it is at N0On the basis of a plurality of half-bridge submodules, K full-bridge submodules capable of outputting negative levels are added to form a hybrid modular multilevel converter, so that the cascading number of the bridge arm submodules reaches N-N0+ K. The negative level utilization coefficient is defined as the number K of additional full-bridge sub-modules and the reference number N of sub-modules0The ratio of (a) to (b), namely:
in this case, the linear modulation region constraint shown in equation (10) can be rewritten as:
according to equation (17), when the third harmonic injection and negative level utilization methods are combined, the maximum modulation ratio achievable by the hybrid modular multilevel converter can be expressed as:
Mac(max)＝kzsq(1+2kNVSF)(18)
the equation (18) comprehensively reflects the influence of the third harmonic injection and the negative level utilization on the maximum modulation ratio of the hybrid modular multilevel converter.
As can be seen from equation (18), the maximum modulation ratio M is obtained by the third harmonic injection method aloneac(max)Can be increased to kzsqAnd k iszsqCan be maximally reached
Although the improvement degree of the third harmonic injection method on the maximum modulation ratio is limited, the third harmonic injection method does not need to add an additional full-bridge sub-module, and is a scheme with better economical efficiency.
It can also be seen from equation (18) that for the negative level utilization method, as long as the number K of full-bridge submodules is increased, (1+ 2K) in equation (18)NVSF) The term can be increased accordingly, theoretically, the maximum modulation ratio M can be increased infinitelyac(max). However, the larger the value K is, the more additional full-bridge sub-modules need to be added, so that the selection of the negative level utilization coefficient needs to be considered by combining various factors.
The direct effect of overmodulation is to change the proportional relation of the alternating current and direct current voltages of the hybrid modular multilevel converter, and improve the alternating current side output voltage capability of the hybrid modular multilevel converter under the condition of constant direct current voltage. However, overmodulation also has a significant effect on bridge arm power fluctuations, which in turn affects sub-module capacitance-voltage fluctuations and capacitance usage.
In order to study the influence of the third harmonic injection and the negative level fluctuation on the bridge arm power fluctuation, on the basis of determining the influence of the third harmonic injection and the negative level utilization on the modulation ratio of the hybrid modular multilevel converter, the influence of the modulation ratio on the bridge arm power fluctuation needs to be further determined so as to obtain the influence of the third harmonic injection and the negative level utilization on the bridge arm power fluctuation.
Similarly, taking the a-phase upper arm as an example for analysis, the a-phase upper arm voltage can be expressed as follows:
wherein k is3And theta3Representing the amplitude factor and phase of the injected third harmonic voltage, respectively.
The influence of double frequency circulation is not considered for the moment, and the bridge arm current on the phase A can be expressed as:
wherein, IacIs the AC output fundamental current amplitude of the hybrid modular multilevel converter,
is a mixed modular multilevel converter AC side power factor angle, IdcIs the DC side current of the hybrid modular multilevel converter.
The bridge arm power fluctuation can be calculated according to the bridge arm voltage and the bridge arm current as follows:
parm(t)＝uap(t)iap(t)(21)
according to the definition of the modulation ratio, the relation between the AC/DC port voltage and the DC port voltage of the hybrid modular multilevel converter is as follows:
neglecting the loss of the hybrid modular multilevel converter, according to the principle of power conservation, the power of the alternating current port and the power of the direct current port of the hybrid modular multilevel converter should satisfy the following relationship:
wherein, IacIs an alternating port current, IdcIs the dc port current.
By substituting equation (22) for equation (23), the relationship between the ac port current and the dc port current can be obtained as follows:
substituting the formula (22) into the formula (19), substituting the formula (24) into the formula (20), making the expressions of the bridge arm voltage and current only contain the alternating-current side voltage and current quantity, then substituting the expressions into the formula (21) to calculate and expand, and obtaining the analytic expressions of the fundamental frequency component, the frequency doubling component, the frequency tripling component and the frequency quadrupling component of the bridge arm power fluctuation as follows respectively:
wherein, S is the apparent power that mixed modularization multilevel converter exported, promptly:
S＝3UacIac (29)
obviously, the lower the frequency of the fluctuation component of the bridge arm power is, the more significant the influence on the fluctuation of the capacitor voltage is, and therefore, the analysis and optimization of the fundamental frequency component of the fluctuation of the bridge arm power are also the most important. From the equation (25), it can be seen that, under the condition that S is constant under the output apparent power of the hybrid modular multilevel converter, the amplitude of the fundamental frequency fluctuation of the bridge arm power mainly consists of the modulation ratio MacAngle of power factor
And (4) determining. Modulation ratio MacHas a direct influence on the fundamental frequency power fluctuations. Both negative level utilization and third harmonic injection affect the modulation ratio MacAnd through MacThe influence on the power fluctuation of a bridge arm is reflected, and the main reason for the influence of overmodulation on the capacitor voltage fluctuation and the capacitor consumption of the submodule of the hybrid modular multilevel converter is also shown.
Referring to FIG. 3, FIG. 3 is a view of a specific embodiment of the present invention
Numerical lower bridge arm fundamental frequency power fluctuation amplitude following MacA two-dimensional curve of change. As in fig. 3
Time curve shows the power factor of the hybrid modular multilevel converter operation
That is, the pure active power is output, and the analytic expression of the fundamental frequency power fluctuation of the bridge arm can be rewritten as follows:
this means that when M isacWhen the following conditions are satisfied, the control unit,
namely, it is
When it is, p will bearm(1)And (5) 0, namely, the fluctuation of the fundamental frequency power of the bridge arm is reduced to zero. This is a very important property: in that
Under the working condition of (1), the basic frequency work of the bridge armThe amplitude of the rate fluctuation will follow MacThe increase of (2) is a change from fall to rise, and
it reaches a very low point of zero. The fundamental frequency power fluctuation of the bridge arm is the most obvious component for influencing the voltage fluctuation of the sub-module capacitor, so that the capacity consumption of the hybrid modular multilevel converter can be greatly reduced, and the fundamental frequency power fluctuation is also an important optimization target for overmodulation utilization of the hybrid modular multilevel converter.
Therefore, in the method, the modulation ratio of the hybrid modular multilevel converter is enabled to be equal to the modulation ratio when the rated active current is output by the hybrid modular multilevel converter through the coordination of the third harmonic injection method and the full-bridge submodule negative level utilization method
The fundamental frequency power fluctuation of the bridge arm at the moment is minimized.
To achieve the purpose, rated voltage of a DC port of the hybrid modular multilevel converter is set to be U firstlydcSetting the sub-module capacitor voltage rating of the hybrid modular multilevel converter to be UcCalculating and generating half-bridge submodule cascade number N of each bridge arm of hybrid modular multilevel converter0The method comprises the following steps:
by the third harmonic injection method, the third harmonic injection can be over-modulated by a modulation coefficient kzsqTo achieve
To make the modulation ratio be
According to equation (18), the negative level utilization factor should be:
according to the definition of the negative level utilization coefficient as shown in equation (16), i.e.
And according to the number N of the half-bridge sub-modules of each bridge arm in cascade connection0Calculating and generating the number K of cascade full-bridge sub-modules of each bridge arm of the hybrid modular multilevel converter as follows:
thus, in each bridge arm of the hybrid modular multilevel converter, at least N is required0Half-bridge sub-modules and K full-bridge sub-modules.
In order to ensure that the modulation ratio of the mixed modular multilevel converter when outputting rated active power is just operated
It is necessary to design the rated voltage of the ac system to which the hybrid modular multilevel converter is connected at a suitable value. When the DC voltage of the hybrid modular multilevel converter is UdcModulation ratio of
The effective value of the voltage of the alternating current port of the hybrid modular multilevel converter is as follows:
setting synchronous angular frequency omega of an alternating current system connected with the hybrid modular multilevel converter, setting bridge arm inductance of the hybrid modular multilevel converter to be L, and setting an effective value of rated active current output by the hybrid modular multilevel converter to be INAt the modulation ratio
Rated phase voltage U of alternating current system connected with hybrid modular multilevel converterSNIt can be calculated as follows:
setting a reference voltage effective value U of a hybrid modular multilevel converteracSetting the phase angle delta of the reference voltage of the hybrid modular multilevel converter, and calculating and generating the three-phase fundamental voltage reference value u of the hybrid modular multilevel converterra1,urb1,urc1The expression is as follows:
for implementing the third harmonic voltage injection method, the reference value u is obtained according to the three-phase fundamental voltagera1,urb1,urc1The generated third harmonic voltage injection value can be calculated in the following way:
wherein max is a function for taking the maximum value, and min is a function for taking the minimum value.
At three-phase fundamental reactive current instantaneous reference value ura1,urb1,urc1In-phase superposition of third harmonic voltage injection value u0And the three-phase voltage reference value u after the third harmonic voltage is injected can be calculatedra,urb,urcThe expression is as follows:
reference value u of three-phase voltagera,urb,urcObtaining the mixed modular multilevel converter as the input of the pulse modulation controller of the mixed modular multilevel converterPulse control signal of level converter.
According to the analysis, the invention can design and control the parameters of the hybrid modular multilevel converter, thereby achieving the maximum modulation ratio which can be realized, simultaneously, the required full-bridge sub-modules are minimum, and the fundamental frequency fluctuation of a bridge arm is minimum.
Therefore, referring to fig. 4, a method for designing and controlling parameters of a hybrid modular multilevel converter according to an embodiment of the present invention includes steps S1 to S3:
step S1, according to the rated voltage U of the DC port of the mixed modular multilevel converterdcAnd the set capacitance voltage rated value U of the sub-module of the hybrid modular multilevel convertercCalculating and generating the number N of cascade half-bridge sub-modules contained in each bridge arm of the hybrid modular multilevel converter0And is and
step S2, according to the number N of the half-bridge sub-modules contained in each bridge arm in cascade connection0Calculating the number K of cascade full-bridge submodules contained in each bridge arm of the hybrid modular multilevel converter, and
step S3, according to the rated voltage U of the DC port of the mixed modular multilevel converterdcDetermining the effective value U of the AC voltage of the set hybrid modular multilevel converteracAnd is and
step S4, according to the effective value U of the alternating voltage of the hybrid modular multilevel converteracDetermining three-phase voltage reference value u after injection of third harmonic voltagera,urb,urc；
Step S5, the three-phase voltage reference value ura,urb,urcInput to the hybrid modular multilevel converter to generate a pulsed control signal for the hybrid modular multilevel converter.
In an alternative embodiment, said step S4 ″ is performed according to the effective value U of the ac voltage of said hybrid modular multilevel converteracDetermining three-phase voltage reference value u after injection of third harmonic voltagera,urb,urc", specifically includes:
according to the set synchronous angular frequency omega of an alternating current system connected with the hybrid modular multilevel converter, the set bridge arm inductance value L of the hybrid modular multilevel converter and the set rated active current effective value I output by the hybrid modular multilevel converterNAnd an effective value of the alternating voltage U of the hybrid modular multilevel converteracCalculating and generating rated phase voltage U of an alternating current system connected with the hybrid modular multilevel converterSN；
According to the effective value U of the alternating voltage of the hybrid modular multilevel converteracCalculating and generating a three-phase fundamental voltage reference value u of the hybrid modular multilevel converter according to the set reference voltage phase angle delta of the hybrid modular multilevel converter and the synchronous angular frequency omega of an alternating current system connected with the hybrid modular multilevel converterra1,urb1,urc1；
According to the three-phase fundamental voltage reference value ura1,urb1,urc1Calculating the third harmonic voltage injection value u0；
According to the three-phase fundamental voltage reference value ura1,urb1,urc1And the third harmonic voltage injection value u0Determining three-phase voltage reference value u after injection of third harmonic voltagera,urb,urc。
In an alternative embodiment, the set synchronous angular frequency ω of the ac system to which the hybrid modular multilevel converter is connected is set according to the set synchronous angular frequency ω of the ac system to which the hybrid modular multilevel converter is connectedBridge arm inductance value L of the flat converter and set rated active current effective value I output by the hybrid modular multilevel converterNAnd an effective value of the alternating voltage U of the hybrid modular multilevel converteracCalculating and generating rated phase voltage U of an alternating current system connected with the hybrid modular multilevel converterSNSpecifically, it is calculated by the following formula:
in an alternative embodiment, the reference voltage effective value U according to the hybrid modular multilevel converteracCalculating and generating a three-phase fundamental voltage reference value u of the hybrid modular multilevel converter according to the set reference voltage phase angle delta of the hybrid modular multilevel converter and the synchronous angular frequency omega of an alternating current system connected with the hybrid modular multilevel converterra1,urb1,urc1Specifically, it is calculated by the following formula:
in an alternative embodiment, the three-phase fundamental voltage reference value u is determined according to the three-phase fundamental voltage reference value ura1,urb1,urc1Calculating the third harmonic voltage injection value u0Specifically, it is calculated by the following formula:
wherein max is a function for taking the maximum value, and min is a function for taking the minimum value.
In an alternative embodiment, the three-phase fundamental voltage reference value u is determined according to the three-phase fundamental voltage reference value ura1,urb1,urc1And the third harmonic voltage injection value u0Determining three-phase voltage reference value u after injection of third harmonic voltagera,urb,urcSpecifically, the three-phase voltage reference value u is calculated by the following formulara,urb,urc：
ura＝ura1+u0，
urb＝urb1+u0，
urc＝urc1+u0。
In a specific embodiment of a mixed modular multilevel converter with 1250MW rated active power, the modulation ratio of the mixed modular multilevel converter can be improved to 1250MW active power by adopting the parameter design and control method of the mixed modular multilevel converter provided by the invention
Wherein the third harmonic injection method is adopted to make the overmodulation coefficient reach
Then, the number of the full-bridge submodules is selected to increase the modulation ratio when 1250MW active power is output to
The required increased full-bridge sub-modules are relatively reduced, and the method has more advantages in cost. Referring to fig. 5, fig. 5 is a waveform curve of a key operation characteristic of the hybrid modular multilevel converter after the parameter design and control method of the hybrid modular multilevel converter provided by the present invention is used, and it can be seen from the figure that, since the modulation ratio operates at an extreme point, the fundamental frequency components in the bridge arm power fluctuation and the sub-module capacitance voltage fluctuation are also basically in the same timeThe voltage fluctuation amplitude of the sub-module capacitor is greatly reduced, a relatively small capacitance value can be selected, the capacitor consumption of the hybrid modular multilevel converter is greatly reduced, and the volume and the cost of the hybrid modular multilevel converter are reduced.
While the foregoing is directed to the preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention.
Claims (6)
1. A parameter design and control method of a hybrid modular multilevel converter is characterized by comprising the following steps:
according to the rated voltage U of the set DC port of the hybrid modular multilevel converterdcAnd the set capacitance voltage rated value U of the sub-module of the hybrid modular multilevel convertercCalculating and generating the number N of cascade half-bridge sub-modules contained in each bridge arm of the hybrid modular multilevel converter0And is and
according to the number N of cascade of half-bridge sub-modules contained in each bridge arm0Calculating the number K of cascade full-bridge submodules contained in each bridge arm of the hybrid modular multilevel converter, and
according to the rated voltage U of the set DC port of the hybrid modular multilevel converterdcDetermining the effective value U of the AC voltage of the hybrid modular multilevel converteracAnd is and
according to said hybrid modular multilevel converterAc voltage effective value UacDetermining three-phase voltage reference value u after injection of third harmonic voltagera,urb,urc；
The three-phase voltage reference value ura,urb,urcInput to the hybrid modular multilevel converter to generate a pulsed control signal for the hybrid modular multilevel converter.
2. Parameter design and control method of a hybrid modular multilevel converter according to claim 1, characterized in that the ac voltage rms U of the hybrid modular multilevel converter is a function ofacDetermining three-phase voltage reference value u after injection of third harmonic voltagera,urb,urcThe method specifically comprises the following steps:
according to the set synchronous angular frequency omega of an alternating current system connected with the hybrid modular multilevel converter, the set bridge arm inductance value L of the hybrid modular multilevel converter and the set rated active current effective value I output by the hybrid modular multilevel converterNAnd an effective value of the alternating voltage U of the hybrid modular multilevel converteracCalculating and generating rated phase voltage U of an alternating current system connected with the hybrid modular multilevel converterSN；
According to the effective value U of the alternating voltage of the hybrid modular multilevel converteracCalculating and generating a three-phase fundamental voltage reference value u of the hybrid modular multilevel converter according to the set reference voltage phase angle delta of the hybrid modular multilevel converter and the synchronous angular frequency omega of an alternating current system connected with the hybrid modular multilevel converterra1,urb1,urc1；
According to the three-phase fundamental voltage reference value ura1,urb1,urc1Calculating the third harmonic voltage injection value u0；
According to the three-phase fundamental voltage reference value ura1,urb1,urc1And the third harmonic voltage injection value u0Determining three-phase voltage reference value u after injection of third harmonic voltagera,urb,urc。
3. The method according to claim 2, wherein the parameters of the hybrid modular multilevel converter are designed and controlled according to the set synchronous angular frequency ω of the ac system to which the hybrid modular multilevel converter is connected, the set bridge arm inductance value L of the hybrid modular multilevel converter, and the set active current effective value I of the hybrid modular multilevel converterNAnd an effective value U of the alternating voltage of the hybrid modular multilevel converteracCalculating and generating rated phase voltage U of an alternating current system connected with the hybrid modular multilevel converterSNSpecifically, it is calculated by the following formula:
4. parameter design and control method of a hybrid modular multilevel converter according to claim 2, characterized in that the said reference voltage root-mean-square value U according to the hybrid modular multilevel converteracCalculating and generating a three-phase fundamental voltage reference value u of the hybrid modular multilevel converter according to the set reference voltage phase angle delta of the hybrid modular multilevel converter and the synchronous angular frequency omega of an alternating current system connected with the hybrid modular multilevel converterra1,urb1,urc1Specifically, it is calculated by the following formula:
5. the method according to claim 2, wherein the parameters of the hybrid modular multilevel converter are designed and controlled according to the three-phase fundamental voltage reference value ura1,urb1,urc1Calculating the third harmonic voltage injection value u0Specifically, it is calculated by the following formula:
wherein max is a function for taking the maximum value, and min is a function for taking the minimum value.
6. The method according to claim 2, wherein the parameters of the hybrid modular multilevel converter are designed and controlled according to the three-phase fundamental voltage reference value ura1,urb1,urc1And the third harmonic voltage injection value u0Determining three-phase voltage reference value u after injection of third harmonic voltagera,urb,urcSpecifically, the three-phase voltage reference value u is calculated by the following formulara,urb,urc：
ura＝ura1+u0，
urb＝urb1+u0，
urc＝urc1+u0。
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