CN112271912B - Active damping method for inhibiting dead zone harmonic waves at low-voltage side of power electronic transformer - Google Patents

Active damping method for inhibiting dead zone harmonic waves at low-voltage side of power electronic transformer Download PDF

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CN112271912B
CN112271912B CN202011246866.2A CN202011246866A CN112271912B CN 112271912 B CN112271912 B CN 112271912B CN 202011246866 A CN202011246866 A CN 202011246866A CN 112271912 B CN112271912 B CN 112271912B
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low
power electronic
lcl filter
dead zone
voltage side
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CN112271912A (en
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范建华
徐鹏飞
李健勋
李广琛
赵新举
李鸿儒
康磊
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Qingdao Topscomm Communication Co Ltd
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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
    • H02M1/00—Details of apparatus for conversion
    • H02M1/12—Arrangements for reducing harmonics from ac input or output
    • 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
    • H02M7/53871—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 with automatic control of output voltage or current
    • H02M7/53875—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 with automatic control of output voltage or current with analogue control of three-phase output
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/40—Arrangements for reducing harmonics

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  • Power Engineering (AREA)
  • Supply And Distribution Of Alternating Current (AREA)
  • Inverter Devices (AREA)

Abstract

The invention discloses an active damping for restraining harmonic waves of a dead zone at a low-voltage side of a power electronic transformerThe method simplifies the LCL filter into a first-order system by simplifying the LCL filter model, and then introduces the active resistor R AD And selecting an active resistance value R AD After current proportion feedforward of an active damping technology is realized, current control is corrected, a correction link is connected in series behind a controller, and suppression of harmonic waves of a dead zone on the low-voltage side of the power electronic transformer is completed. The method of the invention uses the active damping technology to improve the low-frequency impedance of the LCL filter, has simple realization principle and improves the effect of inhibiting the low-frequency harmonic component introduced by the dead zone in the DC/AC converter of the low-voltage side of the power electronic transformer based on the LCL filter.

Description

Active damping method for inhibiting dead zone harmonic waves on low-voltage side of power electronic transformer
Technical Field
The invention relates to the technical field of power electronic transformers, in particular to an active damping method for suppressing dead zone harmonics on the low-voltage side of a power electronic transformer.
Background
Compared with the traditional power transformer, the Power Electronic Transformer (PET) can not only realize the functions of voltage grade conversion, electrical isolation, energy transfer and the like, but also realize additional functions of power flow control, electric energy quality control and the like, and is a trend of the development of a future power system. When the DC/AC converter outputs fundamental wave (power frequency 50Hz) component voltage, a switch working mode of the DC/AC converter can introduce switch sub-high frequency harmonic in the output voltage of the three-phase inverter bridge, and the large high frequency impedance of the LCL filter can obviously filter the high frequency harmonic; the dead zone added for preventing the inverter bridge from being directly connected generates a large amount of low-frequency harmonic components, and the low-frequency impedance of the LCL filter is small, so that the suppression capability of the LCL filter is limited.
The traditional dead zone harmonic suppression method mainly comprises two types, namely dead zone compensation and optimization control. The dead zone compensation method starts from the modulation process of the switch, analyzes the magnitude of harmonic voltage caused by the dead zone and adjusts the magnitude of the modulation wave according to the harmonic voltage, so that the suppression of the dead zone harmonic is realized. However, harmonic voltage and current direction caused by dead zone are all related to factors such as nonlinearity of switching devices and fluctuation of direct current bus voltage, and the implementation difficulty of accurate dead zone compensation is high. The method for optimizing control starts from the design and optimization of a current controller, such as a resonance controller, a repetitive controller and the like which are reasonably designed, and essentially improves the equivalent impedance of an inverter, so that smaller harmonic current is generated under the same dead zone harmonic voltage. The optimization control methods can obtain better effects, but the principle is complex and troublesome to realize, and the research and development period is long.
Disclosure of Invention
Aiming at the defects and shortcomings of the prior art, the invention provides an active damping method for inhibiting dead zone harmonics at the low-voltage side of a power electronic transformer.
The purpose of the invention can be realized by the following technical scheme:
an active damping method for suppressing dead zone harmonics on the low-voltage side of a power electronic transformer comprises the following steps:
step 1: the model of the LCL filter is simplified, the low-voltage side DC/AC converter of the power electronic transformer is provided with the LCL filter, and the three-phase inverter bridge outputs the voltage V to the grid-connected current I g The transfer function of (a) is:
Figure BDA0002770312810000011
the LCL filter is simplified into a first-order system, and the transfer function of the LCL filter is as follows:
Figure BDA0002770312810000012
step 2: selecting an active resistance value R AD In the introduction of an active resistor R AD Then, transfer the letterThe numerical approximation is equivalent to:
Figure BDA0002770312810000013
wherein R is AD The value of (a) significantly affects the frequency range of harmonic suppression, R AD The values of (A) are as follows:
R AD ≥2πnf 1_max (L 1 +L 2 )
wherein f is 1_max The maximum value is taken when the voltage frequency fluctuation of the power grid is considered; n is the highest order of the low frequency harmonics to be suppressed.
And 3, step 3: realizing current proportion feedforward of active damping technology, detecting current I of inversion side in control system L1 And a given V is output by the inverter bridge ref Minus I L1 And R AD The product of (a) and (b).
And 4, step 4: correcting current control, and after adopting an active damping technology, approximating and equating a transfer function as:
Figure BDA0002770312810000021
wherein,
Figure BDA0002770312810000022
for time-delay links of digital control systems, T s Is the sampling period of the control system. Comparative G AD_d (s) and G(s), it is known that the transfer function of the controlled object changes from before after the active damping method is adopted.
In order to realize the same current control effect, a correction link is connected in series behind a controller, and the suppression of the dead zone harmonic wave at the low-voltage side of the power electronic transformer is completed at the same time, wherein the correction link process comprises the following steps:
Figure BDA0002770312810000023
further, step 2 utilizes an active damping technique to increase the low-frequency impedance of the LCL filter, thereby suppressing the low-frequency harmonic component introduced by the dead zone in the DC/AC converter based on the LCL filter on the low-voltage side of the power electronic transformer.
Furthermore, in the step 2, the requirement of the power quality related standard on 50 fundamental wave frequencies can be considered according to the required value, and the value of n is recommended to meet the condition that n is more than or equal to 50.
The invention has the beneficial technical effects that:
1. the low-frequency impedance of the LCL filter is increased by connecting resistors in series, so that the suppression capability of the LCL filter on dead zone harmonics is obviously improved.
2. The control effect achieved by using the active damping technology in the control is equivalent to that of a series resistor in an actual circuit, and no additional power loss is introduced.
3. Introducing an active damping technology and selecting a proper active resistance value R AD The low-frequency impedance of the LCL filter can be obviously improved, so that low-frequency harmonic components introduced by dead zones in the DC/AC converter based on the LCL filter on the low-voltage side of the power electronic transformer are suppressed.
Drawings
Fig. 1 is a power electronic transformer topology diagram of the active damping method for suppressing dead zone harmonics on the low voltage side of the power electronic transformer according to the present invention.
FIG. 2 is a schematic diagram of an LCL filter-based DC/AC converter on the low-voltage side of a power electronic transformer for the active damping method for suppressing dead zone harmonics on the low-voltage side of the power electronic transformer according to the present invention
Fig. 3 is a flow chart of the active damping method for suppressing dead zone harmonics on the low-voltage side of the power electronic transformer according to the present invention.
FIG. 4 is a LCL filter transfer function G of the active damping method for suppressing the dead zone harmonic on the low voltage side of the power electronic transformer according to the present invention LCL (s) bode diagram of its simplified form g(s).
FIG. 5 shows that the active resistance R is added in the active damping method for suppressing the dead zone harmonic at the low-voltage side of the power electronic transformer AD Simplified form G of post-LCL filter AD Bode diagram of(s).
Fig. 6 is a grid-tied current control topology without the active resistance technique.
Fig. 7 is a grid-connected current control topological diagram adopting an active resistance technology of the active damping method for suppressing the dead zone harmonic on the low-voltage side of the power electronic transformer.
Fig. 8 is a grid-connected current simulation result of grid-connected current control without using the active resistance technique.
Fig. 9 is a grid-connected current simulation result of the grid-connected current control using the active resistance technique according to the active damping method for suppressing the dead zone harmonic on the low-voltage side of the power electronic transformer of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not limit the invention.
The topological diagram of the power electronic transformer applied in the invention is shown in figure 1, and the power electronic transformer is provided with a medium-voltage alternating current port, a high-voltage alternating current port and a low-voltage alternating current port. The middle-voltage AC port side and the high-voltage AC port side are of modular structures, the A, B, C three-phase module is respectively composed of N modules, H bridges of the N modules of each phase are connected in series to form an input stage of a Cascade H Bridge (CHB) structure, and AC/DC conversion is realized; the direct current bus of each H bridge is connected with the low-voltage direct current bus through a series resonance type double-active H bridge isolation converter, and DC/DC conversion and isolation are realized. And on the side of the low-voltage alternating current port, the AC/DC converter is connected with a low-voltage direct current bus and a low-voltage alternating current power grid to realize DC/AC conversion.
Fig. 2 shows a schematic diagram of a DC/AC converter based on an LCL filter on the low-voltage side of a power electronic transformer, wherein the DC/AC converter outputs fundamental wave (power frequency 50Hz) component voltage, and the switching mode of the DC/AC converter introduces switching sub-high frequency harmonics into the output voltage of a three-phase inverter bridge, and the large high frequency impedance of the LCL filter can significantly filter the high frequency harmonics. In addition, the dead zone added for preventing the inverter bridge from being directly connected generates a large amount of low-frequency harmonic components, and the low-frequency impedance of the LCL filter is small, so that the suppression capability of the LCL filter is limited.
FIG. 3 shows a flow of steps for implementing the present invention, introducing active damping technique and selecting a suitable active resistance value R AD The low-frequency impedance of the LCL filter is obviously improved, so that low-frequency harmonic components introduced by dead zones in the DC/AC converter based on the LCL filter on the low-voltage side of the power electronic transformer are suppressed.
The invention discloses an active damping method for inhibiting dead zone harmonics at the low-voltage side of a power electronic transformer, which comprises the following steps of:
step 1: the LCL filter model is simplified. The low-voltage side DC/AC converter of the power electronic transformer is provided with an LCL filter, and a three-phase inverter bridge outputs a voltage V to a grid-connected current I g The transfer function of (a) is:
Figure BDA0002770312810000031
the above equation shows that the LCL filter is a third order system with a transfer function G LCL The bode diagram of(s) is shown in fig. 4. Fig. 4 shows that the LCL filter has a good high-frequency filtering effect because of its large high-frequency impedance; however, the low-frequency impedance is small, and therefore, the suppression capability of the low-frequency harmonic component is limited. Harmonic components introduced by dead zones in the DC/AC converter are mainly low frequencies, for which the LCL filter has a limited suppression capability.
To simplify the analysis of the low frequency band, the LCL filter is simplified to a first order system, whose transfer function is:
Figure BDA0002770312810000032
a bode diagram of a simplified form g(s) of the LCL filter transfer function is shown in fig. 4.
Step 2: selecting an active resistance value R AD . The low-frequency impedance of the LCL filter can be remarkably increased by connecting resistors in series in the LCL filter, and the suppression capability of the LCL filter on dead zone harmonics can be remarkably improved. In the introduction of an active resistor R AD The transfer function is then approximately equivalent to:
Figure BDA0002770312810000041
increasing the active resistance R AD Simplified form G of post-LCL filter AD The bode diagram of(s) is shown in FIG. 5, knowing that G AD (s) comprises two asymptotes, the intersection of which is called the turning point, and the angular frequency of which is:
Figure BDA0002770312810000042
on the left side of the turning point (direction of decreasing frequency), G AD The bode diagram of(s) is mainly determined by the active resistance R AD Indicates that R is AD The low-frequency impedance of the LCL filter can be obviously increased; to the right of the turning point, R AD The influence of (c) is small. This indicates that R AD The value of (a) significantly affects the frequency range of harmonic suppression, and a proper active resistance value R should be selected AD 。
R AD The values of (A) are as follows:
R AD ≥2πnf 1_max (L 1 +L 2 )
wherein f is 1_max The maximum value is taken when the voltage frequency fluctuation of the power grid is considered; n is the highest order of the low frequency harmonics to be suppressed. n can be an active resistance value R selected according to a demand value by considering the requirement on 50 fundamental wave frequencies in the related standard of the power quality AD At least 50 fundamental wave frequencies are restrained, and the value of n is recommended to satisfy n more than or equal to 50.
And step 3: and realizing the current proportion feedforward of the active damping technology. The current in the actual circuit will generate voltage drop at the two ends of the resistor after flowing through the resistor, and in order to realize the same effect, the current I on the inversion side is detected in the control system L1 And the output voltage of the inverter bridge is given by V ref Minus I L1 And R AD The product of (a).
The grid-connected current control topological graph without the active resistance technology and the grid-connected current control topological graph with the active resistance technology are respectively shown in fig. 6 and fig. 7, wherein fig. 7 shows the details of the current proportional feedforward implementation of the active damping technology. The control effect achieved by using the active damping technique in the control is equivalent to the series resistance in the actual circuit, and no additional power loss is introduced.
And 4, step 4: the current control is corrected. After the active damping technique is adopted, the transfer function is approximately equivalent to:
Figure BDA0002770312810000043
wherein,
Figure BDA0002770312810000044
a time-delay element of a digital control system, T s Is the sampling period of the control system. Comparative G AD_d (s) and G(s), it is known that the transfer function of the controlled object changes from before after the active damping method is adopted.
In order to realize the same current control effect, a correction link is connected in series behind a controller, and the suppression of the dead zone harmonic wave at the low-voltage side of the power electronic transformer is completed at the same time, wherein the correction link process comprises the following steps:
Figure BDA0002770312810000045
fig. 7 shows the position of the calibration segment.
The grid-connected current simulation result of grid-connected current control without adopting the active resistance technology and the grid-connected current simulation result of grid-connected current control adopting the active resistance technology are respectively shown in fig. 8 and 9, the current waveform of fig. 8 contains a large amount of low-frequency harmonics, while the current waveform of fig. 9 adopting the method of the invention is closer to sine, the low-frequency harmonic components are obviously reduced, and the effectiveness of the invention is shown.
The above-mentioned embodiments are illustrative of the specific embodiments of the present invention, and are not restrictive, and those skilled in the relevant art can make various changes and modifications to obtain corresponding equivalent technical solutions without departing from the spirit and scope of the present invention, so that all equivalent technical solutions should be included in the scope of the present invention.

Claims (2)

1. An active damping method for suppressing dead zone harmonics on the low-voltage side of a power electronic transformer is characterized by comprising the following steps:
step 1: the model of the LCL filter is simplified, the low-voltage side DC/AC converter of the power electronic transformer is provided with the LCL filter, and the three-phase inverter bridge outputs the voltage V to the grid-connected current I g The transfer function of (a) is:
Figure FDA0003754384330000011
the LCL filter is simplified into a first-order system, and the transfer function of the LCL filter is as follows:
Figure FDA0003754384330000012
step 2: selecting an active resistance value R AD Introducing an active resistor R AD The transfer function is then approximately equivalent to:
Figure FDA0003754384330000013
wherein R is AD The value of (a) significantly affects the frequency range of harmonic suppression, R AD The values of (A) are as follows:
R AD ≥2πnf 1_max (L 1 +L 2 )
wherein f is 1_max The maximum value is taken when the voltage frequency fluctuation of the power grid is considered; n is the highest number of times of low-frequency harmonic suppression, the low-frequency impedance of the LCL filter is improved by using an active damping technology, and the low-frequency harmonic component introduced by a dead zone in the DC/AC converter of the low-voltage side of the power electronic transformer based on the LCL filter is suppressed;
and step 3: implementing active damping techniquesCurrent proportional feedforward, detecting current I on inverter side in control system L1 And a given V is output by the inverter bridge ref Minus I L1 And R AD The product of (a) and (b),
and 4, step 4: correcting current control, and after adopting an active damping technology, approximating and equating a transfer function as:
Figure FDA0003754384330000014
wherein,
Figure FDA0003754384330000015
for time-delay links of digital control systems, T s To control the sampling period of the system, contrast G AD_d (s) and G(s), it is known that the transfer function of the controlled object is changed from before after the active damping method is adopted,
in order to realize the same current control effect, a correction link is connected in series behind a controller, and the suppression of the harmonic waves of the dead zone at the low-voltage side of the power electronic transformer is completed at the same time, wherein the correction link process is as follows: 0
Figure FDA0003754384330000016
2. The active damping method for suppressing the dead zone harmonic waves at the low-voltage side of the power electronic transformer according to claim 1, wherein n in the step 2 can consider the requirement on the frequency of 50 fundamental waves in the power quality related standard according to a required value, and the value of n is more than or equal to 50.
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CN201813171U (en) * 2010-10-08 2011-04-27 天津理工大学 Two-stage photovoltaic grid-connected control device based on pole assignment and repetitive control combination
CN101950983A (en) * 2010-10-08 2011-01-19 天津理工大学 Two-stage photovoltaic grid-connected control system based on combination of pole allocation and repetitive control
CN102290820B (en) * 2011-08-22 2013-07-03 中国矿业大学 LCL (Lower Control Unit) filtering controlled rectifying active damping control method of electric currents on feedback variable current side
CN103516248B (en) * 2013-09-26 2016-01-13 武汉大学 Based on the LLCL filtering combining inverter of single current loop control
CN105186573A (en) * 2015-10-27 2015-12-23 夏百战 Current control system based on LCL filter technology
JP6848622B2 (en) * 2017-04-05 2021-03-24 富士電機株式会社 Power converter and its control device

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