CN110112947B - Method for designing number of units of cascaded H-bridge converter capable of enduring asymmetric voltage sag - Google Patents
Method for designing number of units of cascaded H-bridge converter capable of enduring asymmetric voltage sag Download PDFInfo
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- CN110112947B CN110112947B CN201910414609.6A CN201910414609A CN110112947B CN 110112947 B CN110112947 B CN 110112947B CN 201910414609 A CN201910414609 A CN 201910414609A CN 110112947 B CN110112947 B CN 110112947B
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- 238000000034 method Methods 0.000 title claims abstract description 21
- 230000010363 phase shift Effects 0.000 claims description 4
- 238000010586 diagram Methods 0.000 description 3
- 230000016507 interphase Effects 0.000 description 3
- 238000004804 winding Methods 0.000 description 2
- 230000009286 beneficial effect Effects 0.000 description 1
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- 230000037111 immune power Effects 0.000 description 1
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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
- H02M7/53—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 using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/537—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 using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
- H02M7/5387—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 using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
Abstract
The invention provides a method for designing the number of units of a cascaded H-bridge converter which can tolerate asymmetric voltage sag, which comprises the steps of firstly obtaining an average voltage expression of the direct current side of each power unit under the asymmetric voltage sag according to the bridge arm conduction characteristic of each power unit uncontrolled rectifier under the asymmetric voltage sag; and determining the number of cascade units required by the cascade H-bridge converter which can tolerate asymmetric voltage sag according to the output rated voltage, the direct-current voltage and the modulation ratio constraint condition of the cascade H-bridge converter by using the direct-current side average voltage expression. The invention takes the improvement of the voltage sag tolerance of the cascaded H-bridge converter as a starting point, so that the cascaded H-bridge converter has the immune asymmetric voltage sag capacity, the voltage sag treatment cost is greatly reduced, and the implementation is easy.
Description
Technical Field
The invention relates to the technical field of power electronics, in particular to a method for designing the number of units of a cascaded H-bridge converter, which can tolerate asymmetric voltage sag.
Background
The cascaded H-bridge converter has the obvious advantages of high voltage level, high power level, high waveform quality, high modularization and the like, and is gradually widely applied to various fields in recent decades. Research related to cascaded H-bridge converters over the past decades has focused mainly on the power balancing and energy exchange issues inside the cascaded H-bridge converters and the modulation methods and control strategies of the cascaded H-bridge converters. The research is generally based on the premise that the power grid voltage is an ideal condition, so that the research difficulty of the cascaded H-bridge converter is simplified. However, the power quality problems such as voltage sag and the like exist in the power grid, which affect the normal operation of the power electronic equipment, so that the cascaded H-bridge converter, which is one of the power electronic equipment, is affected by the voltage sag.
In order to protect power electronic devices and the like from voltage sag, current research mainly focuses on how to design voltage sag compensation devices such as SVC, D-STATCOM, DVR, and UPQC, but neglects the sag tolerance capability of the power electronic devices themselves, and increases the voltage sag control cost.
Disclosure of Invention
The invention aims to provide a design method for the number of cascade units of a cascade H-bridge converter for resisting asymmetric voltage sag (voltage sag when two phases of an electric network are in interphase short circuit and two phases of an electric network are in ground short circuit).
In order to achieve the purpose, the invention adopts the technical scheme that:
a design method for the number of units of a cascaded H-bridge converter enduring asymmetric voltage sag aims at providing the number N of cascade and the rated phase voltage U of the secondary side of a phase-shifting transformer by taking the sum of the direct-current side voltages of all power units in each phase of the cascaded H-bridge converter enduring the asymmetric voltage sag as the target and still being larger than the amplitude of the output voltage of the cascaded H-bridge converter2NThe method for obtaining the number of the power units of the cascaded H-bridge converter for enduring the asymmetric voltage sag comprises the following steps:
step one, obtaining rated voltage U of output side of cascaded H-bridge converteroN；
Step two, determining the voltage grade of the IGBT, comprehensively considering the safety according to the voltage-resistant grade of the IGBT, and further determining the rated voltage U of the direct-current side of each power unitdcN；
Step three, determining the average voltage variation k of each power unit of the cascaded H-type converter at the direct current side when the asymmetric voltage sag occurs at the power grid side;
step four, combining UoN、UdcNAnd k, outputting a rated voltage modulation ratio constraint condition by the cascaded H-bridge converter, and determining the number of power units required by the cascaded H-bridge converter to endure asymmetric voltage sag.
Further, in the second step, the voltage class of the IGBT is determined according to the voltage class of the IGBT power device commonly used by the H-bridge converter and by combining the comprehensive cost, the control complexity and the application scene of the H-bridge converter, and then the rated voltage U at the direct current side of each power unit is determined by comprehensively considering the safety of the voltage class of the IGBTdcN。
Furthermore, in the third step, when the unbalanced voltage sag occurs in the power grid, the three-phase uncontrolled rectification is converted into the single-phase uncontrolled rectification, and the input alternating voltage is
U2NIs the secondary side rated voltage, U, of the phase-shifting transformer2N＝UdcN/1.35, wherein
The phase angle is shifted for the phase-shifting transformer, at this time, the DC side voltage of each power unit
0.9≤KdLess than or equal to 1.414, and the variation k of the DC voltage average value of each power unit satisfies the following conditions:
the calculation process is as follows:
further, in general KdTaken as 1.2, when KdWhen 1.2 is taken, k ≈ 0.85.
Further, in the fourth step, U is combinedoN、UdcNAnd k, outputting a rated voltage modulation ratio constraint condition m which is less than or equal to 1 by the cascade H-bridge converter, and determining the work required by the cascade H-bridge converter to resist asymmetric voltage sagNumber of rate units N:
asymmetric voltage sag, modulation ratio
N can be represented as
The number of cascade units required for tolerating asymmetric voltage sag can be known from m being less than or equal to 1
And determining the value of N according to the unit redundancy requirement.
Furthermore, the design method for the number of the cascade units is suitable for a cascade H-bridge converter connected to a 10kV power distribution network, each cascade unit rectifier adopts a three-phase uncontrolled rectifier, and the modulation mode is carrier phase shift modulation.
Further, the asymmetric voltage dip is caused by a two-phase metallic short circuit occurring in the vicinity of the bus bar.
The invention has the beneficial effects that: the invention fully considers the influence of interphase voltage sag on the H-bridge converter, realizes the asymmetrical voltage sag capability of the cascaded H-bridge converter when the interphase metallic direct short circuit of the immune power grid occurs, ensures that the H-bridge converter and the load thereof can still normally operate when the asymmetrical voltage sag occurs in the power grid, improves the operation reliability of the system, simultaneously does not need to add other voltage sag treatment devices to the system, and reduces the cost of the system.
Drawings
FIG. 1 is a diagram of a cascaded H-bridge converter topology used in the present invention;
FIG. 2 is a topology diagram of each power cell of a cascaded H-bridge converter used in the present invention;
fig. 3 is a voltage waveform diagram of the power supply side, the secondary side of the phase-shifting transformer, the direct current side of each power unit and the cascade output side of the cascaded H-bridge converter under asymmetric voltage sag.
Detailed Description
The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings.
Fig. 1 is a topology of a cascaded H-bridge converter including a multi-winding phase-shifting transformer and a plurality of power cells in an embodiment of the invention. The power cell topology is shown in fig. 2, and includes a three-phase uncontrolled rectifier (D1-D6) and an H-bridge inverter (Q1-Q4). The output voltage of the multi-winding phase-shifting transformer passes through a three-phase uncontrolled rectifying circuit in each power unit to serve as a direct-current power supply of an H-bridge inverter of each power unit, and each power unit is connected in series to form a bridge arm and then supplies power to a load in a PS (packet switched) modulation mode.
The method for designing the cascade number of the cascaded H-bridge converter enduring the asymmetric voltage sag is further described by combining the examples, and the method comprises the following steps:
step one, acquiring performance parameters of the cascaded H-bridge converter.
The relevant electrical parameters when the invention is implemented are set as follows:
rated voltage U at output side of cascaded H-bridge converteroNAre all 10 kV.
Step two, in this case, the voltage classes of the medium-voltage IGBT are 1000V, 1700V and 3300V, the cost and the control complexity are comprehensively considered, the voltage class of the IGBT in this case is 1700V, a certain safety margin is considered, and the rated voltage U of the direct-current side is provideddcNWas selected to be 900V. And step three, when the power grid has asymmetric voltage sag, the direct voltage average sag degree k of the connected H-bridge converter under the asymmetric voltage sag is approximately equal to 0.85.
Step four, according to UoN＝10kV，UdcN900V, k 0.85, is represented by formula
N is more than or equal to 11 can be calculated, in consideration of primary redundancy, N in the case is finally 12, a Matlab/Simulink model is built, and the correctness of the design method of the number of the cascading units is verified.
Fig. 3 shows waveforms of voltages on the power supply side of the cascaded H-bridge converter, the secondary side of the phase-shifting transformer, the dc side of each power unit, and the cascaded output side in an asymmetric voltage sag.
Fig. 3(a) shows the power-side voltage waveform of the cascaded H-bridge converter with asymmetric voltage sag, which is seen to occur at 0.1s,
FIG. 3(b) shows the phase angle shift for asymmetric voltage sag
The secondary side line voltage waveform of the phase-shifting transformer is combined with the conduction characteristics of diodes of bridge arms of the three-phase uncontrolled rectifier, so that the three-phase uncontrolled rectifier of each power unit can be equivalent to a single-phase uncontrolled rectifier under the condition of asymmetric voltage sag.
Fig. 3(c) shows waveforms of voltages on the dc sides of 12 power units in a single phase of the cascaded H-bridge converter under the asymmetric voltage sag, where the voltages on the dc sides of the power units under the asymmetric voltage sag change in cosine with the phase shift angle of the phase-shifting transformer supplying power to the power units, and the change rule of the voltages on the dc sides of the power units under the asymmetric voltage sag is consistent with the voltage expression of the dc sides of the power units under the asymmetric voltage sag given in step three.
FIG. 3(d) shows the phase shift angle for asymmetric voltage sag
The voltage waveform of the direct current side of the power unit supplied with power by the phase-shifting transformer is consistent with a theoretical value obtained by the expression of the average voltage of the direct current side of the asymmetrical voltage sag lower-link H-bridge converter given in the step three, and the calculation formula of the direct current side of each power unit of the asymmetrical voltage sag lower-link H-bridge converter provided by the invention is verified to be applicable by combining the graph shown in the figure 3(a) and the graph shown in the figure 3 (b).
Fig. 3(e) shows the output side line voltage of the cascaded H-bridge converter with asymmetric voltage sag, and it can be seen from fig. 3(e) that the amplitude of the output side voltage of the converter is slightly decreased when 0.1s sag occurs, but the output side voltage is rapidly restored to the rated value under the action of the control device, so the asymmetric voltage sag does not greatly affect the cascaded H-bridge converter.
Therefore, the cascaded H-bridge converter has the capability of immune asymmetric voltage sag under the design scheme of the number of the cascade.
The above description is only an embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention.
Claims (6)
1. A method for designing the number of units of a cascaded H-bridge converter, which can tolerate asymmetric voltage sag, is characterized by comprising the following steps:
step one, obtaining rated voltage U of output side of cascaded H-bridge converteroN；
Step two, determining the voltage grade of the IGBT, comprehensively considering the safety according to the voltage-resistant grade of the IGBT, and further determining the rated voltage U of the direct-current side of each power unitdcN；
Step three, determining the average voltage variation k of each power unit of the cascaded H-type converter at the direct current side when the asymmetric voltage sag occurs at the power grid side;
step four, combining UoN、UdcNK, outputting a rated voltage modulation ratio constraint condition by the cascaded H-bridge converter, and determining the number of power units required by the cascaded H-bridge converter to tolerate asymmetric voltage sag;
in the third step, when the power grid has asymmetric voltage sag, the three-phase uncontrolled rectification is changed into the single-phase uncontrolled rectification, and the input alternating voltage is
U2NIs the secondary side rated voltage, U, of the phase-shifting transformer2N＝UdcN/1.35, wherein
The phase angle is shifted for the phase-shifting transformer, at this time, the DC side voltage of each power unit
0.9≤KdThe variation k of the DC voltage average value of each power unit is less than or equal to 1.414, and satisfies the following conditions:
the calculation process is as follows:
2. the method for designing the number of units of the cascaded H-bridge converter, which can tolerate asymmetric voltage sag, according to claim 1, is characterized in that: in the second step, according to the voltage grade of the IGBT power device commonly used by the H-bridge converter, the comprehensive cost, the control complexity and the application scene of the H-bridge converter are combined to determine the voltage-withstanding grade of the IGBT, and then the safety is comprehensively considered by the voltage-withstanding grade of the IGBT to determine the rated voltage U of the direct current side of each power unitdcN。
3. The method for designing the number of units of the cascaded H-bridge converter, which can tolerate asymmetric voltage sag, according to claim 1, is characterized in that: general KdTaken as 1.2, when KdWhen 1.2 is taken, k ≈ 0.85.
4. The method for designing the number of units of the cascaded H-bridge converter, which can tolerate asymmetric voltage sag, according to claim 1, is characterized in that: in the fourth step, the U is combinedoN、UdcNAnd k, outputting a rated voltage modulation ratio constraint condition m by the cascaded H-bridge converter to be less than or equal to 1, and determining the number N of power units required by the cascaded H-bridge converter to endure asymmetric voltage sag:
asymmetric voltage sag, modulation ratio
N is represented by
The number of cascade units required for tolerating asymmetric voltage sag can be known from m being less than or equal to 1
And determining the value of N according to the unit redundancy requirement.
5. The method for designing the number of units of the cascaded H-bridge converter, which can tolerate asymmetric voltage sag, according to claim 1, is characterized in that: the design method for the number of the cascade units is suitable for a cascade H-bridge converter connected to a 10kV power distribution network, each cascade unit rectifier adopts a three-phase uncontrolled rectifier, and the modulation mode is carrier phase shift modulation.
6. The method for designing the number of units of the cascaded H-bridge converter, which can tolerate asymmetric voltage sag, according to claim 1, is characterized in that: the asymmetric voltage dip is caused by a two-phase metallic short circuit occurring in the vicinity of the bus bar.
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