CN114421496B - Direct-current power distribution network low-frequency oscillation suppression method based on bidirectional power converter - Google Patents

Direct-current power distribution network low-frequency oscillation suppression method based on bidirectional power converter Download PDF

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CN114421496B
CN114421496B CN202210071990.2A CN202210071990A CN114421496B CN 114421496 B CN114421496 B CN 114421496B CN 202210071990 A CN202210071990 A CN 202210071990A CN 114421496 B CN114421496 B CN 114421496B
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current
distribution network
direct
low
power converter
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CN114421496A (en
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吴昌龙
罗华伟
李云丰
毛丽林
李彦昭
谢宇峥
许珊
肖劲鹏
陈小华
陈星�
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State Power Investment Group Hunan Loudi New Energy Co ltd
Hunan Jingyan Electric Power Design Co ltd
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State Power Investment Group Hunan Loudi New Energy Co ltd
Hunan Jingyan Electric Power Design Co ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
    • H02J3/241—The oscillation concerning frequency
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J1/00—Circuit arrangements for dc mains or dc distribution networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
    • H02J3/48—Controlling the sharing of the in-phase component
    • 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
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/70—Wind energy
    • Y02E10/76—Power conversion electric or electronic aspects
    • 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
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Rectifiers (AREA)
  • Dc-Dc Converters (AREA)

Abstract

The invention discloses a low-frequency oscillation suppression method for a direct-current power distribution network based on a bidirectional power converter, which comprises the steps of obtaining operation parameters of the direct-current power distribution network; judging whether the system is stable; constructing a low-frequency oscillation suppression module and setting module parameters; acquiring output current of a direct current side of the bidirectional power converter, inputting the output current into the set low-frequency oscillation suppression module to obtain original compensation current of the direct current distribution network, and obtaining compensation current of the direct current distribution network after amplitude limiting; and superposing the compensation current of the direct-current distribution network to a current inner ring of the bidirectional power converter to complete the low-frequency oscillation suppression of the direct-current distribution network based on the bidirectional power converter. The method stabilizes the direct-current voltage of the direct-current power distribution network and reduces the risk of oscillation instability of the direct-current power distribution network by adjusting the instantaneous active power injected into the direct-current power distribution network by the bidirectional power converter in time under the conditions of not increasing an external hardware circuit and not changing the steady-state operation characteristic of the direct-current power distribution network, and is high in reliability, good in practicability and easy to implement.

Description

Direct-current power distribution network low-frequency oscillation suppression method based on bidirectional power converter
Technical Field
The invention belongs to the field of electrical automation, and particularly relates to a low-frequency oscillation suppression method for a direct-current power distribution network based on a bidirectional power converter.
Background
With the development of economic technology and the improvement of living standard of people, electric energy has been widely developed to become essential secondary energy in production and life of people, and bring endless convenience to production and life of people. Therefore, ensuring stable and reliable supply of electric energy is one of the most important tasks of the power system.
Dc distribution networks have found widespread use in recent years. A dc power distribution network often uses a three-phase voltage source converter (VSC, also called a bidirectional power converter) to connect to an ac power distribution network, and the topology structure of the dc power distribution network is shown in fig. 1. A schematic diagram of a control architecture of a conventional bidirectional power converter is shown in fig. 2. Compared with the traditional alternating current transformer, the VSC has the characteristics of high flexibility, strong controllability and the like as a fully-controlled power electronic device. In addition, after the bidirectional power converter is coordinated and matched with the battery energy storage device, the wind power, the photovoltaic and other renewable energy sources can be more conveniently accessed and sent out.
The voltage and current of a direct current distribution network are theoretically constant physical quantities in a steady state situation, and their angular frequencies are zero. Therefore, compared with an alternating current distribution network, the direct current distribution network has no problems of frequency and power angle stability and the like. However, since the angular frequency is zero, when a short-circuit fault occurs in the dc power distribution network, a large short-circuit current is generated in a short time, which threatens the safe operation of the power electronic device. In order to suppress the short-circuit fault current and strive for more time for opening and closing the short-circuit fault current of the direct-current circuit breaker, a direct-current reactor is generally configured at a port of a direct-current power distribution network access device in engineering and used for suppressing the increase of the short-circuit fault current. However, the configuration of the dc reactors also has adverse effects, and the dc reactors introduce negative resistance effects by interacting with the output current of the access device, which may cause low frequency oscillations in the dc distribution network; and the larger the dc reactor, the greater the risk of oscillation instability. When a low-frequency oscillation occurs in a dc power distribution network, the oscillation of dc voltage, dc current, and power is inevitably induced, but the oscillation can be suppressed by suppressing any one of them. The essence of the oscillation of the direct-current power distribution network is a power oscillation phenomenon caused by the incoordination and mismatching of injected instantaneous active power and consumed instantaneous power, and the phenomenon is shown that certain devices have negative resistance effect in a medium-low frequency range due to output impedance of power electronic devices.
In order to solve the problem of low-frequency oscillation of a direct-current power distribution network, a common method in engineering is to change the control system parameters originally designed by the bidirectional power converter or add an additional passive damping filter device. Although it is a feasible way to change the originally designed control system parameters of the bidirectional power converter, the method changes the response speed, especially the transient response speed, of the bidirectional power converter. The additional installation of the passive damping filter device not only increases the occupied area and investment cost of the direct-current power distribution network, but also increases the operation maintenance cost and operation loss of the direct-current power distribution network system, so that the overall economy of the direct-current power distribution network system is sacrificed to a certain extent.
Disclosure of Invention
The invention aims to provide a method for suppressing low-frequency oscillation of a direct-current power distribution network based on a bidirectional power converter, which has high reliability, good practicability and easy implementation.
The invention provides a low-frequency oscillation suppression method for a direct-current power distribution network based on a bidirectional power converter, which comprises the following steps of:
s1, acquiring the operation parameters of the direct current distribution network;
s2, judging whether the system is stable according to the operation parameters acquired in the step S1:
if the system is judged to be unstable, continuing the subsequent steps;
if the system is judged to be stable, the algorithm is ended;
s3, constructing a low-frequency oscillation suppression module;
s4, setting module parameters of the low-frequency oscillation suppression module constructed in the step S3;
s5, acquiring output current of the direct current side of the bidirectional power converter, and inputting the output current to the low-frequency oscillation suppression module after setting is completed to obtain original compensation current of the direct current distribution network;
s6, limiting the original compensation current of the direct-current distribution network obtained in the step S5 to obtain the compensation current of the direct-current distribution network;
and S7, superposing the compensation current of the direct current distribution network obtained in the step S6 to a current inner ring of the bidirectional power converter, thereby finishing the suppression of the low-frequency oscillation of the direct current distribution network based on the bidirectional power converter.
Step S1 illustrates the operating parameters of the dc power distribution network, including the impedance of the bidirectional power converter
Figure 295010DEST_PATH_IMAGE001
And impedance of DC distribution network side
Figure 743309DEST_PATH_IMAGE002
(ii) a Wherein the impedance of the bidirectional power converter
Figure 54204DEST_PATH_IMAGE001
Is expressed as
Figure 232769DEST_PATH_IMAGE003
，U dcN Is a nominal value for the direct voltage and,C dc vsc_ for the dc-side support capacitance value,U sN for the amplitude rating of the ac side phase voltage,
Figure 312721DEST_PATH_IMAGE004
for the transfer function of the constant dc voltage controller,
Figure 666342DEST_PATH_IMAGE005
for the current inner loop transfer function of a bi-directional power converter,
Figure 12003DEST_PATH_IMAGE006
a low pass filter transfer function for the dc voltage,I dc0 is a steady state dc current value.
In step S2, the step of determining whether the system is stable according to the operation parameters obtained in step S1 specifically includes the following steps:
A. will be provided with
Figure 742062DEST_PATH_IMAGE007
Impedance substituting into a bidirectional power converter
Figure 942099DEST_PATH_IMAGE001
And impedance of DC distribution network side
Figure 466622DEST_PATH_IMAGE002
In the expression (c), the corresponding frequency domain expression is obtained as:
Figure 955372DEST_PATH_IMAGE008
Figure 269547DEST_PATH_IMAGE009
in the formula
Figure 324091DEST_PATH_IMAGE010
A modulo function for the complex number; agr () is a complex phase angle function;
B. under the most serious working condition of the system, drawing
Figure 285094DEST_PATH_IMAGE011
And
Figure 808610DEST_PATH_IMAGE012
followed by
Figure 614892DEST_PATH_IMAGE013
The amplitude and phase curves of the changes are recorded
Figure 789521DEST_PATH_IMAGE014
Amplitude curve of (1) and
Figure 655846DEST_PATH_IMAGE015
frequency corresponding to the intersection point of the amplitude curve
Figure 914263DEST_PATH_IMAGE016
；
C. Calculating the phase difference corresponding to the intersection point of the amplitudes of the two curvespha_diffIs composed of
Figure 320974DEST_PATH_IMAGE017
；
D. For the phase difference obtained in the step Cpha_diffAnd (4) judging:
if it is
Figure 350110DEST_PATH_IMAGE018
If yes, judging that the system is stable, and ending the algorithm;
otherwise, judging that the system is unstable and continuing the subsequent steps.
The constructing of the low-frequency oscillation suppression module in the step S3 specifically includes the following steps:
the input of the low-frequency oscillation module is the output current of the direct current side of the bidirectional power converter;
the low-frequency oscillation module comprises a low-pass filter, a high-pass filter and a proportional amplifier which are sequentially connected in series.
The low-pass filter is a first-order low-pass filter; the high pass filter is a first order high pass filter.
Step S4, which is to set the module parameters of the low-frequency oscillation suppression module constructed in step S3, specifically includes the following steps:
a. outputting current from DC side of bidirectional power converteri dc Obtaining a first current component by a first-order low-pass filteri dcf1 ；
The following formula is adopted as the transfer function of the first-order low-pass filter
Figure 138068DEST_PATH_IMAGE019
：
Figure 416603DEST_PATH_IMAGE020
In the formula
Figure 830266DEST_PATH_IMAGE021
Is the bandwidth of a first-order low-pass filter, and
Figure 917171DEST_PATH_IMAGE022
；f lpf is the cut-off frequency of a set first-order low-pass filter;
corresponding first current componenti dcf1 In the first placekExpression of individual control periods
Figure 640145DEST_PATH_IMAGE023
Comprises the following steps:
Figure 609238DEST_PATH_IMAGE025
in the formulaT s Is a sampling period;
b. b, mixing the first current component obtained in the step ai dcf1 Obtaining the second current component by a first-order high-pass filteri dcf2 ；
The following formula is adopted as the transfer function of the first-order high-pass filter
Figure 357752DEST_PATH_IMAGE026
：
Figure 112212DEST_PATH_IMAGE027
In the formula
Figure 756820DEST_PATH_IMAGE028
Is the bandwidth of a first-order high-pass filter, and
Figure 213209DEST_PATH_IMAGE029
；f hpf the cut-off frequency of a first-order high-pass filter needing to be set is obtained;
corresponding second current componenti dcf2 In the first placekExpression of individual control periods
Figure 968676DEST_PATH_IMAGE030
Comprises the following steps:
Figure 764593DEST_PATH_IMAGE032
in the formulaT s Is a sampling period;
c. b, the second current component obtained in the step b isi dcf2 Obtaining a third current component by a proportional amplifieri dcf3 (ii) a Wherein parameters of the proportional amplifierk damp Setting is needed;
d. adding the constructed low-frequency oscillation suppression module into a current inner ring of the bidirectional power converter; before setting is started, the cut-off frequency of a first-order high-pass filter is setf hpf Has an initial value of 2f cro Parameters of proportional amplifiersk damp Is 0;
e. fixed first step heightCut-off frequency of a pass filterf hpf Is an initial value; parameters of proportional amplifierk damp Increasing the preset proportional gain adjustment step length from the initial value to the proportional gain upper limit value;
in the process of increasing the parameters of the proportional amplifier, after the low-frequency oscillation suppression module is added, the impedance of the bidirectional power converter is obtained
Figure 832300DEST_PATH_IMAGE033
Corresponding amplitude curve and impedance of DC distribution network side
Figure 572723DEST_PATH_IMAGE002
Frequency corresponding to intersection point of corresponding amplitude curvef cro1 Phase difference value ofpha_diff1；
f. For the phase difference value obtained in the step epha_diff1, judging:
if it is
Figure 131880DEST_PATH_IMAGE034
If yes, judging that the parameter setting is finished;
otherwise, the cut-off frequency of the first-order high-pass filter is adjustedf hpf Subtracting the set cut-off frequency adjustment step length and obtaining the cut-off frequency of a new first-order high-pass filter
Figure 329774DEST_PATH_IMAGE035
As an initial value of the cut-off frequency of the new first order high pass filter;
g. repeating the step e to the step f to set the parameters, and judging as follows:
if the parameter setting is finished, the cut-off frequency of the current high-pass filter is usedf hpf And parameters of proportional amplifierk damp As a parameter of the final low frequency oscillation suppression module; ending the parameter setting process;
if the initial value of the cut-off frequency of the first-order high-pass filter is greater than 0, continuing parameter setting;
and if the initial value of the cut-off frequency of the first-order high-pass filter is equal to 0 and the parameter setting is not finished, determining that the parameter setting process fails, finishing the algorithm and giving an alarm.
E, calculating the upper limit value of the proportional gain by adopting the following formula
Figure 50606DEST_PATH_IMAGE036
：
Figure 481587DEST_PATH_IMAGE037
In the formula
Figure 578856DEST_PATH_IMAGE038
The ratio of the permitted fluctuations of the direct current oscillation relative to the rated value of the direct current;I dcN is a DC rated value;
Figure 660950DEST_PATH_IMAGE039
for built-up low-frequency oscillation-suppressing module outputdThe ratio allowed by the rated value of the shaft compensation current relative to the amplitude of the alternating current;I sN is a nominal value for the amplitude of the ac phase current.
Step S6, limiting the original compensation current of the dc distribution network obtained in step S5 to obtain the compensation current of the dc distribution network, specifically including the following steps:
the original compensation current of the direct current distribution network obtained in the step S5
Figure 162470DEST_PATH_IMAGE040
Amplitude limiting is carried out by adopting the following formula, so that the compensation current of the direct-current power distribution network is obtained
Figure 80747DEST_PATH_IMAGE041
：
Figure 184970DEST_PATH_IMAGE042
In the formula
Figure 606724DEST_PATH_IMAGE043
To setdThe axis compensates for the limited value of the current.
SaiddThe limited value of the shaft compensation current is specifically 0.05 times the rated value of the alternating current of the bidirectional power converter.
Step S7 is to superimpose the dc distribution network compensation current obtained in step S6 on the current inner loop of the bidirectional power converter, specifically, the dc distribution network compensation current obtained in step S6 and the dc distribution network compensation current obtained in step S6dThe shaft current reference values are inversely superimposed and then input to the current inner loop of the bidirectional power converter.
According to the low-frequency oscillation suppression method for the direct-current power distribution network based on the bidirectional power converter, the bidirectional power converter of the direct-current power distribution network is utilized, under the condition that an external hardware circuit is not added and the steady-state operation characteristic of the direct-current power distribution network is not changed, the direct-current of the bidirectional power converter is fed back to the current inner loop control equation, the instantaneous active power injected into the direct-current power distribution network by the bidirectional power converter is adjusted in time, the direct-current voltage of the direct-current power distribution network is stabilized, the risk of oscillation instability of the direct-current power distribution network is reduced, and the method is high in reliability, good in practicability and easy to implement.
Drawings
Fig. 1 is a schematic diagram of a connection topology of a dc distribution network and an ac system.
Fig. 2 is a schematic diagram of a control architecture of a conventional bidirectional power converter.
FIG. 3 is a schematic flow chart of the method of the present invention.
Fig. 4 is a schematic diagram of a control architecture of the bidirectional power converter of the present invention.
Fig. 5 is a schematic diagram illustrating an effect of the embodiment of the present invention.
Detailed Description
FIG. 3 is a schematic flow chart of the method of the present invention: the invention provides a low-frequency oscillation suppression method for a direct-current power distribution network based on a bidirectional power converter, which comprises the following steps of:
s1, acquiring the operation parameters of the direct current distribution network; the resonance oscillation of the direct-current power distribution network can be represented as mismatching of the output impedance of the power converter and the impedance of the direct-current network, and specifically, at least one output impedance has a negative damping effect; thus, the pre-derived parameters include the impedance of the bi-directional power converter
Figure 934937DEST_PATH_IMAGE033
And impedance of DC distribution network side
Figure 123866DEST_PATH_IMAGE002
(ii) a Wherein the impedance of the bidirectional power converter
Figure 359675DEST_PATH_IMAGE033
Is expressed as
Figure 652248DEST_PATH_IMAGE003
，U dcN For the purpose of a rated value of the direct-current voltage,C dc vsc_ for the dc-side support capacitance value,U sN for the amplitude rating of the ac side phase voltage,
Figure 354624DEST_PATH_IMAGE004
for the transfer function of the constant dc voltage controller,
Figure 981915DEST_PATH_IMAGE005
for the current inner loop transfer function of a bi-directional power converter,
Figure 224677DEST_PATH_IMAGE006
a low pass filter transfer function for the dc voltage,I dc0 is a steady state DC current value;
s2, judging whether the system is stable according to the operation parameters acquired in the step S1:
if the system is judged to be unstable, continuing the subsequent steps;
if the system is judged to be stable, the algorithm is ended;
in specific implementation, the following steps are adopted to judge whether the system is stable:
A. will be provided with
Figure 621024DEST_PATH_IMAGE044
Impedance substituting into a bidirectional power converter
Figure 540307DEST_PATH_IMAGE033
And impedance of DC distribution network side
Figure 654893DEST_PATH_IMAGE002
In the expression (c), the corresponding frequency domain expression is obtained as:
Figure 435768DEST_PATH_IMAGE045
Figure 952200DEST_PATH_IMAGE046
in the formula
Figure 543849DEST_PATH_IMAGE010
A modulo function for the complex number; agr () is a complex phase angle function;
Figure 411311DEST_PATH_IMAGE047
theoretically, infWhich may range from 0 to plus infinity, but in terms of low frequency oscillations in the dc distribution network,fto 500 Hz;
B. under the most severe working condition of the system, drawing
Figure 995876DEST_PATH_IMAGE011
And
Figure 366815DEST_PATH_IMAGE012
followed by
Figure 581895DEST_PATH_IMAGE013
The amplitude and phase curves of the changes are recorded
Figure 657692DEST_PATH_IMAGE014
Amplitude curve of (2) and
Figure 45948DEST_PATH_IMAGE015
frequency corresponding to the intersection point of the amplitude curve
Figure 271393DEST_PATH_IMAGE016
；
C. Calculating the phase difference corresponding to the intersection point of the amplitudes of the two curvespha_diffIs composed of
Figure 719692DEST_PATH_IMAGE017
；
D. For the phase difference obtained in the step Cpha_diffAnd (4) judging:
if it is
Figure 46899DEST_PATH_IMAGE018
If so, judging that the system is stable and does not generate resonant oscillation, and ending the algorithm without oscillation suppression;
otherwise, judging that the system is unstable, and continuing the subsequent steps; in practice, if satisfied
Figure 973267DEST_PATH_IMAGE048
However, if the stability margin does not reach the set value, the oscillation suppression can be performed in the subsequent steps;
s3, constructing a low-frequency oscillation suppression module; the method specifically comprises the following steps:
the input of the low-frequency oscillation module is the output current of the direct current side of the bidirectional power converter;
the low-frequency oscillation module comprises a low-pass filter, a high-pass filter and a proportional amplifier which are sequentially connected in series;
in specific implementation, the low-pass filter is a first-order low-pass filter; the high-pass filter is a first-order high-pass filter;
s4, setting module parameters of the low-frequency oscillation suppression module constructed in the step S3; the method specifically comprises the following steps:
a. since the actual dc current contains a certain amount of high-frequency glitch component, the dc side output current of the bidirectional power converter is required to suppress its adverse effecti dc Obtaining a first current component by a first-order low-pass filteri dcf1 ；
The following formula is adopted as the transfer function of the first-order low-pass filter
Figure 53218DEST_PATH_IMAGE019
：
Figure 875681DEST_PATH_IMAGE020
In the formula
Figure 673873DEST_PATH_IMAGE021
Is the bandwidth of a first-order low-pass filter, and
Figure 653199DEST_PATH_IMAGE022
；f lpf the specific implementation value of the cutoff frequency of the first-order low-pass filter is about 500 Hz;
corresponding first current componenti dcf1 In the first placekExpression of individual control periods
Figure 587657DEST_PATH_IMAGE023
Comprises the following steps:
Figure 643337DEST_PATH_IMAGE049
in the formulaT s Is a sampling period;
b. when the bidirectional power conversion transmits power, a direct current component exists in the direct current, and in order to prevent the direct current component from entering the oscillation suppression algorithm, the compensated current has the direct current component, so that the current inner loop controller is saturated, and the first current component obtained in the step a is usedi dcf1 By a first-order high-passThe filter obtains the second current componenti dcf2 ；
The following formula is adopted as the transfer function of the first-order high-pass filter
Figure 928825DEST_PATH_IMAGE050
：
Figure 947728DEST_PATH_IMAGE027
In the formula
Figure 2272DEST_PATH_IMAGE028
Is the bandwidth of a first-order high-pass filter, and
Figure 166537DEST_PATH_IMAGE029
；f hpf the cut-off frequency of a first-order high-pass filter needing to be set is obtained;
corresponding second current componenti dcf2 In the first placekExpression of individual control periods
Figure 673741DEST_PATH_IMAGE030
Comprises the following steps:
Figure 745603DEST_PATH_IMAGE051
in the formulaT s Is a sampling period;
c. b, the second current component obtained in the step b isi dcf2 Obtaining a third current component by a proportional amplifieri dcf3 (ii) a Wherein parameters of the proportional amplifierk damp Setting is needed;
d. adding the constructed low-frequency oscillation suppression module into a current inner ring of the bidirectional power converter; before setting is started, the cut-off frequency of a first-order high-pass filter is setf hpf Has an initial value of 2f cro Parameters of proportional amplifiersk damp Is 0;
e. fixed cut-off frequency of first order high pass filterf hpf Is an initial value; parameters of proportional amplifierk damp The step size is adjusted by a set proportional gain (preferably 0.05 times)
Figure 160711DEST_PATH_IMAGE052
) Increasing from an initial value to a proportional gain upper limit value;
in the process of increasing the parameters of the proportional amplifier, after the low-frequency oscillation suppression module is added, the impedance of the bidirectional power converter is obtained
Figure 292615DEST_PATH_IMAGE033
Corresponding amplitude curve and impedance of DC distribution network side
Figure 552695DEST_PATH_IMAGE002
Frequency corresponding to intersection point of corresponding amplitude curvef cro1 Phase difference value ofpha_diff1；
At this time, after the low-frequency oscillation suppression module is added, the impedance of the bidirectional power converter
Figure 162668DEST_PATH_IMAGE033
Is expressed as
Figure 942536DEST_PATH_IMAGE053
Wherein
Figure 183024DEST_PATH_IMAGE054
Is a transfer function of the low frequency oscillation suppression module, and
Figure 930400DEST_PATH_IMAGE055
；
meanwhile, the proportional gain upper limit value may be calculated by the following equation
Figure 344064DEST_PATH_IMAGE056
：
Figure 227707DEST_PATH_IMAGE057
In the formula
Figure 950681DEST_PATH_IMAGE038
The ratio of the permitted fluctuations of the direct current oscillation relative to the rated value of the direct current;I dcN is a DC rated value;
Figure 185353DEST_PATH_IMAGE039
for built-up low-frequency oscillation-suppressing module outputdThe ratio allowed by the rated value of the shaft compensation current relative to the amplitude of the alternating current;I sN a nominal value for the amplitude of the AC phase current;
f. for the phase difference value obtained in step e
Figure 137129DEST_PATH_IMAGE058
And (4) judging:
if it is
Figure DEST_PATH_IMAGE059
If yes, judging that the parameter setting is finished;
otherwise, the cut-off frequency of the first-order high-pass filter is adjustedf hpf Minus a set cut-off frequency adjustment step (preferably 0.1 times)f cro ) And the cut-off frequency of the obtained new first-order high-pass filter
Figure DEST_PATH_IMAGE060
As an initial value of the cut-off frequency of the new first order high pass filter;
g. repeating the step e to the step f to set the parameters, and judging as follows:
if the parameter setting is finished, the cut-off frequency of the current high-pass filter is usedf hpf And parameters of proportional amplifiersk damp As a parameter of the final low frequency oscillation suppression module; ending the parameter setting process;
if the initial value of the cut-off frequency of the first-order high-pass filter is greater than 0, continuing parameter setting;
if the initial value of the cut-off frequency of the first-order high-pass filter is equal to 0 and the parameter setting is not completed, determining that the parameter setting process fails, finishing the algorithm and giving an alarm; at this time, the design of three-phase VSC and other controller parameters of the direct-current distribution network system is determined to have problems, the direct-current distribution network is difficult to keep stable even after the oscillation suppression module is added, at this time, the control system parameters need to be redesigned, and the step S1 is returned to perform all the steps again;
in specific implementation, after parameter setting is completed, one-time manual fine adjustment can be performed on the parameter setting, so that the damping characteristic of the system is further improved;
s5, acquiring output current of the direct current side of the bidirectional power converter, and inputting the output current to the low-frequency oscillation suppression module after setting is completed to obtain original compensation current of the direct current distribution network;
s6, the actual direct current distribution network has large direct current impact in the rapid dynamic and transient processes, particularly in the initial period of fault occurrence and recovery, which can cause the existence of transient impact; in order to suppress the influence of the direct current distribution network, amplitude limiting is performed on the original compensation current of the direct current distribution network obtained in the step S5, and compensation current of the direct current distribution network is obtained; the method specifically comprises the following steps:
the original compensation current of the direct current distribution network obtained in the step S5
Figure DEST_PATH_IMAGE061
Amplitude limiting is carried out by adopting the following formula, so that the compensation current of the direct-current distribution network is obtained
Figure DEST_PATH_IMAGE062
：
Figure 485064DEST_PATH_IMAGE042
In the formula
Figure 67356DEST_PATH_IMAGE043
To setdA limited value of the shaft compensation current, preferably 0.05 times the ac rating of the bi-directional power converter;
s7, superposing the compensation current of the direct-current distribution network obtained in the step S6 to a current inner ring of the bidirectional power converter, and accordingly finishing the suppression of the low-frequency oscillation of the direct-current distribution network based on the bidirectional power converter; specifically, the compensation current sum of the dc distribution network obtained in step S6dAfter the shaft current reference values are reversely superposed, the shaft current reference values are input into a current inner ring of the bidirectional power converter; as shown in particular in fig. 4.
Fig. 5 is a schematic diagram showing the simulation effect of the method of the present invention applied to an actual system. As can be seen from fig. 5, after the oscillation suppression method provided by the present invention is adopted, the low-frequency oscillation suppression effect of the dc distribution network is very obvious.
The method improves the positive damping characteristic of the output impedance of the bidirectional power converter in a low frequency band, has the capability of reducing the oscillation instability risk of the direct current power distribution network, and can effectively inhibit oscillation after the direct current power distribution network system generates a low frequency oscillation phenomenon; according to the method, only the direct current of the bidirectional power converter is fed back to the current inner loop control equation, and the compensation value of the d-axis reference current is generated after the direct current passes through the oscillation suppression algorithm, so that excessive algorithm operation processes are avoided, and the method has the advantages of clear concept and easiness in implementation; the method does not need to set a starting trigger signal outside, can be put into use along with the operation of the bidirectional power converter all the time, and does not change the steady-state operation performance of the bidirectional power converter and the direct-current power distribution network on the premise of inhibiting the low-frequency oscillation of the direct-current power distribution network; the method is not only suitable for occasions where the bidirectional power converter provides the positive damping characteristic for the direct-current power distribution network, but also suitable for occasions where other full-control direct-current power electronic devices provide the positive damping characteristic for the direct-current power distribution network.

Claims (7)

1. A low-frequency oscillation suppression method for a direct-current power distribution network based on a bidirectional power converter comprises the following steps:
s1, acquiring the operation parameters of the direct current distribution network;
s2, judging whether the system is stable according to the operation parameters acquired in the step S1:
if the system is judged to be unstable, continuing the subsequent steps;
if the system is judged to be stable, the algorithm is ended;
s3, constructing a low-frequency oscillation suppression module; the method specifically comprises the following steps:
the input of the low-frequency oscillation module is the output current of the direct current side of the bidirectional power converter;
the low-frequency oscillation module comprises a low-pass filter, a high-pass filter and a proportional amplifier which are sequentially connected in series;
the low-pass filter is a first-order low-pass filter; the high-pass filter is a first-order high-pass filter;
s4, setting module parameters of the low-frequency oscillation suppression module constructed in the step S3; the method specifically comprises the following steps:
a. outputting current from DC side of bidirectional power converteri dc Obtaining a first current component by a first-order low-pass filteri dcf1 ；
The following formula is adopted as the transfer function of the first-order low-pass filter
Figure DEST_PATH_IMAGE001
：
Figure 532940DEST_PATH_IMAGE002
In the formula
Figure DEST_PATH_IMAGE003
Is the bandwidth of a first-order low-pass filter, and
Figure 43556DEST_PATH_IMAGE004
；f lpf is the cut-off frequency of a set first-order low-pass filter;
corresponding first current componenti dcf1 In the first placekExpression of one control cycleFormula (II)
Figure DEST_PATH_IMAGE005
Comprises the following steps:
Figure DEST_PATH_IMAGE007
in the formulaT s Is a sampling period;
b. b, mixing the first current component obtained in the step ai dcf1 Obtaining the second current component by a first-order high-pass filteri dcf2 ；
The following formula is adopted as the transfer function of the first-order high-pass filter
Figure 735875DEST_PATH_IMAGE008
：
Figure DEST_PATH_IMAGE009
In the formula
Figure 193402DEST_PATH_IMAGE010
Is the bandwidth of a first-order high-pass filter, and
Figure DEST_PATH_IMAGE011
；f hpf the cut-off frequency of a first-order high-pass filter needing to be set is obtained;
corresponding second current componenti dcf2 In the first placekExpression of individual control periods
Figure 53779DEST_PATH_IMAGE012
Comprises the following steps:
Figure 407400DEST_PATH_IMAGE014
in the formulaT s Is a sampling period;
c. c, mixing the second current component obtained in the step bi dcf2 Obtaining a third current component by a proportional amplifieri dcf3 (ii) a Wherein parameters of the proportional amplifierk damp Setting is needed;
d. adding the constructed low-frequency oscillation suppression module into a current inner ring of the bidirectional power converter; before setting is started, the cut-off frequency of a first-order high-pass filter is setf hpf Has an initial value of 2f cro Parameters of proportional amplifiersk damp Is 0;
e. fixing the cut-off frequency of a first order high-pass filterf hpf Is an initial value; parameters of proportional amplifierk damp Increasing the preset proportional gain adjustment step length from the initial value to the proportional gain upper limit value;
in the process of increasing the parameters of the proportional amplifier, after the low-frequency oscillation suppression module is added, the impedance of the bidirectional power converter is obtained
Figure DEST_PATH_IMAGE015
Corresponding amplitude curve and impedance of DC distribution network side
Figure 471171DEST_PATH_IMAGE016
Frequency corresponding to the intersection point of the corresponding amplitude curvef cro1 Phase difference value ofpha_diff1；
f. For the phase difference value obtained in the step epha_diff1, judging:
if it is
Figure DEST_PATH_IMAGE017
If so, judging that the parameter setting is finished;
otherwise, the cut-off frequency of the first-order high-pass filter is adjustedf hpf Subtracting the set cut-off frequency adjustment step length and obtaining a new first-order high-pass filterCut-off frequency of
Figure 217541DEST_PATH_IMAGE018
As an initial value of the cut-off frequency of the new first order high pass filter;
g. repeating the step e to the step f to set the parameters, and judging as follows:
if the parameter setting is finished, the cut-off frequency of the current high-pass filter is usedf hpf And parameters of proportional amplifierk damp As a parameter of the final low frequency oscillation suppression module; ending the parameter setting process;
if the initial value of the cut-off frequency of the first-order high-pass filter is larger than 0, continuing parameter setting;
if the initial value of the cut-off frequency of the first-order high-pass filter is equal to 0 and the parameter setting is not completed, determining that the parameter setting process fails, finishing the algorithm and giving an alarm;
s5, acquiring output current of the direct current side of the bidirectional power converter, and inputting the output current to the low-frequency oscillation suppression module after setting is completed to obtain original compensation current of the direct current distribution network;
s6, limiting the original compensation current of the direct-current distribution network obtained in the step S5 to obtain the compensation current of the direct-current distribution network;
and S7, superposing the compensation current of the direct current distribution network obtained in the step S6 to a current inner ring of the bidirectional power converter, thereby finishing the suppression of the low-frequency oscillation of the direct current distribution network based on the bidirectional power converter.
2. The method according to claim 1, wherein the operating parameters of the dc power distribution network in step S1, specifically including the impedance of the bidirectional power converter
Figure DEST_PATH_IMAGE019
And impedance of DC distribution network side
Figure 948737DEST_PATH_IMAGE016
(ii) a Wherein the impedance of the bidirectional power converter
Figure 207680DEST_PATH_IMAGE019
Is expressed as
Figure 493168DEST_PATH_IMAGE020
，U dcN For the purpose of a rated value of the direct-current voltage,C dc vsc_ for the dc-side support capacitance value,U sN for the amplitude rating of the ac side phase voltage,
Figure DEST_PATH_IMAGE021
for the transfer function of the constant dc voltage controller,
Figure 279114DEST_PATH_IMAGE022
for the current inner loop transfer function of a bi-directional power converter,
Figure DEST_PATH_IMAGE023
a low pass filter transfer function for the dc voltage,I dc0 is a steady state dc current value.
3. The method for suppressing the low-frequency oscillation of the direct-current distribution network based on the bidirectional power converter as recited in claim 2, wherein the step S2 of determining whether the system is stable according to the operation parameters obtained in the step S1 specifically comprises the following steps:
A. will be provided with
Figure 864817DEST_PATH_IMAGE024
Impedance substituting into a bidirectional power converter
Figure 560240DEST_PATH_IMAGE019
And impedance of DC distribution network side
Figure 83757DEST_PATH_IMAGE016
In the expression ofThe expression to the corresponding frequency domain is:
Figure DEST_PATH_IMAGE025
Figure 421197DEST_PATH_IMAGE026
in the formula
Figure DEST_PATH_IMAGE027
A modulo function for the complex number; agr () is a complex phase angle function;
B. under the most serious working condition of the system, drawing
Figure 110673DEST_PATH_IMAGE028
And
Figure DEST_PATH_IMAGE029
followed by
Figure 242577DEST_PATH_IMAGE030
The amplitude and phase curves of the changes are recorded
Figure DEST_PATH_IMAGE031
Amplitude curve of (1) and
Figure 768236DEST_PATH_IMAGE032
frequency corresponding to the intersection point of the amplitude curve
Figure 128942DEST_PATH_IMAGE033
；
C. Calculating the phase difference corresponding to the intersection point of the amplitudes of the two curvespha_diffIs composed of
Figure DEST_PATH_IMAGE034
；
D. Step by stepPhase difference obtained in step Cpha_diffAnd (4) judging:
if it is
Figure 423657DEST_PATH_IMAGE035
If yes, judging that the system is stable, and ending the algorithm;
otherwise, judging that the system is unstable and continuing the subsequent steps.
4. The method according to claim 3, wherein the proportional gain upper limit value in step e is calculated by the following formula
Figure DEST_PATH_IMAGE036
：
Figure 992041DEST_PATH_IMAGE037
In the formula
Figure DEST_PATH_IMAGE038
The ratio of the permitted fluctuations of the direct current oscillation relative to the rated value of the direct current;I dcN is a DC rated value;
Figure 257194DEST_PATH_IMAGE039
for built-up low-frequency oscillation-suppressing module outputdThe ratio allowed by the rated value of the shaft compensation current relative to the amplitude of the alternating current;I sN is a nominal value for the amplitude of the ac phase current.
5. The method for suppressing the low-frequency oscillation of the direct-current distribution network based on the bidirectional power converter as claimed in any one of claims 1 to 4, wherein the step S6 is performed by limiting the original compensation current of the direct-current distribution network obtained in the step S5 to obtain the compensation current of the direct-current distribution network, and specifically comprises the following steps:
the original compensation current of the direct current distribution network obtained in the step S5
Figure DEST_PATH_IMAGE040
Amplitude limiting is carried out by adopting the following formula, so that the compensation current of the direct-current power distribution network is obtained
Figure 936437DEST_PATH_IMAGE041
：
Figure DEST_PATH_IMAGE042
In the formula
Figure 836391DEST_PATH_IMAGE043
To setdThe axis compensates for the limited value of the current.
6. The method according to claim 5, wherein said method for suppressing low frequency oscillations in a DC power distribution network based on bidirectional power convertersdThe limited value of the shaft compensation current is specifically 0.05 times the rated value of the alternating current of the bidirectional power converter.
7. The method for suppressing low-frequency oscillation of a DC distribution network based on a bidirectional power converter as claimed in claim 6, wherein the step S7 is performed by superimposing the compensation current of the DC distribution network obtained in the step S6 on the current inner loop of the bidirectional power converter, specifically the compensation current of the DC distribution network obtained in the step S6 and the sum of the compensation current of the DC distribution network obtained in the step S6dAnd after the shaft current reference values are reversely superposed, the shaft current reference values are input into a current inner ring of the bidirectional power converter.
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