CN113872218A - Method for analyzing running characteristic of series capacitance coupling type dynamic voltage restorer - Google Patents

Method for analyzing running characteristic of series capacitance coupling type dynamic voltage restorer Download PDF

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CN113872218A
CN113872218A CN202111178467.1A CN202111178467A CN113872218A CN 113872218 A CN113872218 A CN 113872218A CN 202111178467 A CN202111178467 A CN 202111178467A CN 113872218 A CN113872218 A CN 113872218A
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dvr
voltage
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李稳良
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Feilai Electric Shanghai 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
    • H02J3/12—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load
    • H02J3/16—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load by adjustment of reactive power
    • 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
    • H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
    • H02J3/1807—Arrangements for adjusting, eliminating or compensating reactive power in networks using series compensators
    • H02J3/1814—Arrangements for adjusting, eliminating or compensating reactive power in networks using series compensators wherein al least one reactive element is actively controlled by a bridge converter, e.g. unified power flow controllers [UPFC]
    • 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
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
    • 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
    • Y02E40/30—Reactive power compensation

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Abstract

The invention discloses a method for analyzing the running characteristics of a series capacitance coupling type dynamic voltage restorer, which belongs to the technical field and comprises the following steps: s1: the basic circuit design of the series capacitance coupling DVR; s2: adjusting the running state of the series capacitance coupling DVR under the condition of load fluctuation; s3: determining an optimal structure of a series capacitance coupling DVR; s4: the operation boundary of the inverter is analyzed through simulation and verification, an operation state adjustment strategy suitable for the series capacitance coupling type DVR is provided, and normal and stable operation of the device is guaranteed under the condition of load fluctuation. And then, the influence of the series capacitor on the stability and the dynamic performance of the control system is analyzed by combining a voltage-current double closed-loop control strategy, and the final selection of the series capacitor coupling type DVR structure is completed. Finally, the effectiveness and the feasibility of the series capacitance coupling DVR are verified through simulation and experiments.

Description

Method for analyzing running characteristic of series capacitance coupling type dynamic voltage restorer
Technical Field
The invention relates to the technical field of capacitive coupling type dynamic voltage restorers, in particular to a method for analyzing the running characteristics of a series capacitive coupling type dynamic voltage restorer.
Background
A Dynamic Voltage Restorer (DVR) is the most economical Dynamic compensation device for solving the Voltage drop problem in the power system at present by virtue of its advantages of high operation efficiency, high reliability, good rapidity, etc.
Generally, when the DVR adopts the minimum energy compensation strategy, although the active demand can be reduced to a certain extent, the compensation time can be prolonged, and the compensation capability can be increased, the problem of the amplitude increase of the output voltage of the DVR is inevitably brought.
Disclosure of Invention
The present invention is directed to a method for analyzing the operating characteristics of a series capacitive coupling dynamic voltage restorer, so as to solve the problems in the background art.
In order to achieve the purpose, the invention provides the following technical scheme: the method for analyzing the operation characteristics of the series capacitance coupling type dynamic voltage restorer comprises the following steps:
s1: the basic circuit design of the series capacitance coupling DVR;
s2: adjusting the running state of the series capacitance coupling DVR under the condition of load fluctuation;
s3: determining an optimal structure of a series capacitance coupling DVR;
s4: and (5) simulating and verifying.
Further, regarding the PWM inverter whose basic circuit topology is composed of full-controlled devices and the dc-side energy storage capacitor C in S1dcThe circuit comprises an LC output filter, a series transformer T, a bypass switch K and a series capacitor C.
Further, for S1, the basic circuit adds a series capacitor C to the LC filter, and the series capacitor C can bear a voltage of 90 ° of large hysteresis load current when the DVR adopts the minimum energy compensation strategy.
Further, for S1, the magnitude of the voltage sustained by the LC filter is closely related to the magnitude of the series capacitor C, and the uncertainty of the dropping amplitude of the grid voltage and the random fluctuation of the load also affect the LC filter, and the magnitude of the voltage sustained by the structural filter branch is
ULCC＝Il(1/ωC-ωLf)＝k·Udvm (1)
In the formula, k is a bearing coefficient, the uncertainty of the grid voltage drop amplitude and the random fluctuation of the load need to be comprehensively considered for the value of k, and when the DVR is applied to the occasion with smaller grid voltage drop amplitude, the value of k can be properly reduced; on the contrary, the k value can be properly increased, and the formula (1) is solved and simplified to obtain
Figure BDA0003296351280000021
Wherein Sn is the rated capacity m of DVR, and in order to make the fundamental wave impedance of the filtering branch circuit be capacitive under power frequency and inductive under characteristic harmonic frequency, it is necessary to meet the requirements
Figure BDA0003296351280000022
Where s is the angular frequency of the switching frequency, the series capacitance C can be determined by equations (2) and (3), and furthermore, for the filter inductance Lf and the filter capacitance CfThe determination is made according to the selection principle of the conventional DVR.
Further, for S1, the inverter output power of DVR that can effectively reduce the output voltage of the inverter after the minimum energy compensation strategy is adopted for the large hysteresis load current 90 ° that can be borne by the series capacitor C can be expressed as
Figure BDA0003296351280000023
In the formula, Pinv and Qinv are respectively the active power output by the inverter and the active power and the reactive power which are not respectively output by the inverter;
Pinv＝UinvIl cosβ (5)
Qinv＝UinvIl sinβ (6)
under the same working condition, the output active power of the DVR with the traditional structure is the same as that of the inverter of the series capacitance coupling DVR, as shown in formula (7):
Pinv＝Uinv1Il cosβ1＝Uinv2Il cosβ2 (7)
similarly, the output reactive power of the inverters of the DVR with two structures can be obtained as
Figure BDA0003296351280000031
Figure BDA0003296351280000032
Due to the existence of the series capacitor C, the output reactive power of the inverter can be reduced to a certain extent, the active demand of the inverter is unchanged under the condition that the overall DVR control mode is not changed, but the overall capacity of the inverter can be correspondingly reduced due to the reduction of the reactive demand.
Further, aiming at the adjustment strategy analysis under the load current fluctuation in S2, the voltage U borne by the filter branch circuitLCCIs directly subjected to load current IlWhen the load current is reduced due to the fluctuation of the load, ULCCAnd also decreases as the load current decreases, resulting in ULCCWhen the voltage is too small, the output voltage of the inverter exceeds the limit voltage value of the inverter, and corresponding operation state adjustment is needed at the moment to ensure that the output voltage of the inverter is within the limit range of the inverter and ensure that the DVR works in the minimum energy compensation state within the output capacity range;
when the load current is reduced, the broken line circular arc with O as the center represents the limit circle of the output voltage of the inverter, because of the load current IlReduction of (1), ULCCWill also decrease accordingly, resulting in the output voltage U of the inverterinvWith a consequent increase when IlWhen reduced to a certain extent, UinvWill exceed its limit voltage value Uinv-maxFurther, the normal operation of the DVR may be affected.
Further, in S3, the DVR and the series capacitive coupling DVR are analyzed and compared in terms of control stability, filter effect, and the like, and U issAnd UlRespectively the grid voltage and the load voltage,
Figure BDA0003296351280000033
Uinvand UdvrRespectively representing a reference compensation voltage given value of the DVR, an inverter voltage command value and an output voltage; i isl、ILfRespectively load current and filter inductance current; u shapeCThe voltage borne by the series capacitor; KPWM is inverseTaking KPWM as 1 for simplifying analysis, and respectively forming a scheme a and a scheme b;
scheme b, a transfer function G from the given value of the reference compensation voltage to the actual value of the output voltage can be obtainedLfCCfAnd the transfer function Z of the load current to the actual value of the output voltageLfCCfAre respectively as
Figure BDA0003296351280000041
Figure BDA0003296351280000042
In formulae (10) and (11)
Figure BDA0003296351280000043
For scheme a, the same reasoning can be followed
Figure BDA0003296351280000044
Figure BDA0003296351280000045
In the formulae (12) and (13)
Figure BDA0003296351280000046
And the frequency of the actual value of the output voltage to the given value of the reference compensation voltage can be obtained by changing the size of the series capacitor C.
Further, aiming at the step S4, in order to verify the feasibility of the series capacitive coupling type DVR, a single-phase DVR simulation model is built in an MATLAB/Simulink simulation platform for simulation analysis.
Compared with the prior art, the invention has the beneficial effects that:
the method for analyzing the running characteristic of the series capacitance coupling type dynamic voltage restorer analyzes the running boundary of the inverter, provides a running state adjusting strategy suitable for a series capacitance coupling type DVR, and ensures the normal and stable running of the device under the condition of load fluctuation. And then, the influence of the series capacitor on the stability and the dynamic performance of the control system is analyzed by combining a voltage-current double closed-loop control strategy, and the final selection of the series capacitor coupling type DVR structure is completed. Finally, the effectiveness and the feasibility of the series capacitance coupling DVR are verified through simulation and experiments.
Drawings
FIG. 1 is a block diagram illustrating the basic circuit structure and control of a series capacitive coupling DVR according to the present invention;
FIG. 2 is an equivalent circuit of the system of the present invention;
FIG. 3 is a diagram of DVR operating phasors of the invention;
FIG. 4 is a graph of the operating phasors of the DVR with varying load current according to the invention;
FIG. 5 is a graph of the operating phasors of a DVR with varying power factor for the present invention;
FIG. 6 is a block diagram of the system control of the series capacitive coupling DVR of the present invention;
FIG. 7 is a Bode plot of output voltage versus reference voltage for the present invention;
FIG. 8 is a Bode plot of output voltage versus load current for the present invention;
FIG. 9 is a waveform of a simulation of series capacitive coupling DVR compensation verification of the invention;
fig. 10 shows simulation waveforms for adjusting the state of the series capacitive coupling DVR according to the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The method for analyzing the operation characteristics of the series capacitance coupling type dynamic voltage restorer comprises the following steps:
the method comprises the following steps: the basic circuit design of the series capacitance coupling DVR;
the basic circuit structure of series capacitance coupling DVR is shown in figure 1, the topological structure of the basic circuit comprises a PWM inverter formed by full-control devices, and a direct current side energy storage capacitor CdcThe LC output filter, the series transformer T, the bypass switch K, and the series capacitor C may be divided into two different structures according to the installation location of the series capacitor C, i.e., installed on the right side (scheme a) or the left side (scheme b) of the filter capacitor Cf, as shown in fig. 1.
The equivalent circuit of the series capacitive coupling DVR in the two schemes a and b is shown in fig. 2. In order to simplify the analysis, the series transformer is considered as an ideal transformer, and the influence of transformer coupling is neglected in the theoretical analysis. L in the figuref、CfRespectively a filter inductor and a filter capacitor; c1、C2Is a series capacitor; u shapeinvFor the inverter output voltage, ULFor filtering the voltage across the inductor, UCIs the voltage across the series capacitor; u shapes、UlUdvr is the network side voltage, the load side voltage and the compensation voltage injected by DVR respectively; zlIs the load equivalent impedance.
The basic circuit is additionally provided with a series capacitor C on the basis of the LC filter, and when the DVR adopts a minimum energy compensation strategy, the series capacitor C can bear the voltage of large hysteresis load current of 90 degrees;
when the DVR adopts a minimum energy compensation strategy, the series capacitor C can bear a voltage of a larger hysteresis load current of 90 degrees, so that the output voltage of the inverter can be remarkably reduced on the basis of not changing the filtering effect, and the capacity of the inverter and the voltage level of a direct current side are further effectively reduced;
the DVR has three different operation modes including pure reactive compensation according to different voltage drop depths
Figure BDA0003296351280000061
Critical compensation
Figure BDA0003296351280000062
And minimum active power compensation
Figure BDA0003296351280000063
The phasor diagrams of the conventional DVR and the series capacitive coupling DVR when they operate in these three modes and when the conventional DVR employs the in-phase compensation strategy are shown in fig. 3.
The voltage borne by the LC filter is closely related to the size of the series capacitor C, and the uncertainty of the dropping amplitude of the power grid voltage and the random fluctuation of the load also influence the LC filter, and the voltage borne by the structural filter branch is
ULCC＝Il(1/ωC-ωLf)＝k·Udvm (1)
In the formula, k is a bearing coefficient, the uncertainty of the grid voltage drop amplitude and the random fluctuation of the load need to be comprehensively considered for the value of k, and when the DVR is applied to the occasion with smaller grid voltage drop amplitude, the value of k can be properly reduced; on the contrary, the k value can be properly increased, and the formula (1) is solved and simplified to obtain
Figure BDA0003296351280000071
Wherein Sn is the rated capacity m of DVR, and in order to make the fundamental wave impedance of the filtering branch circuit be capacitive under power frequency and inductive under characteristic harmonic frequency, it is necessary to meet the requirements
Figure BDA0003296351280000072
Where s is the angular frequency of the switching frequency, the series capacitance C can be determined by equations (2) and (3), and furthermore, for the filter inductance Lf and the filter capacitance CfThe determination is made according to the selection principle of the conventional DVR.
The series capacitor C can bear large hysteresis load current of 90 degrees, the output voltage of the inverter can be effectively reduced after a minimum energy compensation strategy is adopted, and the output power of the inverter of the DVR can be expressed as
Figure BDA0003296351280000073
In the formula, Pinv and Qinv are respectively the active power output by the inverter and the active power and the reactive power which are not respectively output by the inverter;
Pinv＝UinvIl cosβ (5)
Qinv＝UinvIl sinβ (6)
under the same working condition, the output active power of the DVR with the traditional structure is the same as that of the inverter of the series capacitance coupling DVR, as shown in formula (7):
Pinv＝Uinv1Il cosβ1＝Uinv2Il cosβ2 (7)
similarly, the output reactive power of the inverters of the DVR with two structures can be obtained as
Figure BDA0003296351280000074
Figure BDA0003296351280000075
Due to the existence of the series capacitor C, the output reactive power of the inverter can be reduced to a certain extent, the active demand of the inverter is unchanged under the condition that the integral DVR control mode is not changed, but the overall capacity of the inverter can be correspondingly reduced due to the reduction of the reactive demand;
step two: adjusting the running state of the series capacitance coupling DVR under the condition of load fluctuation;
by reasonably designing the size of the capacitor C, when the minimum energy compensation method is adopted to compensate the voltage of the power grid, the series capacitor C can bear a voltage UC with a larger hysteresis load current of 90 degrees, and therefore the output voltage and the capacity of the inverter can be effectively reduced. However, the value of UC changes under the influence of load fluctuations, and further, the voltage of the output required by the inverter changes accordingly. In consideration of the maximum output capacity of the inverter, in the case of load fluctuation, in order to enable the output voltage of the inverter not to exceed the limit value, a corresponding adjustment strategy needs to be made if necessary so as to ensure the normal and stable operation of the device.
Analysis of regulation strategy under load current fluctuation, and voltage U borne by filtering branchLCCIs directly subjected to load current IlWhen the load current is reduced due to the fluctuation of the load, ULCCAnd also decreases as the load current decreases, resulting in ULCCWhen the voltage is too small, the output voltage of the inverter exceeds the limit voltage value of the inverter, and corresponding operation state adjustment is needed at the moment to ensure that the output voltage of the inverter is within the limit range of the inverter and ensure that the DVR works in the minimum energy compensation state within the output capacity range;
when the load current is reduced, the voltage phasor relationship representing the DVR operation state is shown in FIG. 4a, wherein the dotted arc with O as the center represents the limit circle of the inverter output voltage, due to the load current IlReduction of (1), ULCCWill also decrease accordingly, resulting in the output voltage U of the inverterinvWith a consequent increase when IlWhen reduced to a certain extent, UinvWill exceed its limit voltage value Uinv-maxFurther, the normal operation of the DVR can be influenced;
in order to avoid the adverse effect caused by the reduction of the load current, the operating state of the DVR needs to be adjusted, specifically, as shown in fig. 4b, in order to make the output voltage of the inverter meet the limit output requirement, the load voltage Ul needs to be adjusted by a certain angle, which can be expressed as the angle where the output voltage Uinv of the inverter meets the condition Uinv≤M·Udc
Wherein M is a modulation ratio; u shapedcIs the DC side voltage of the inverter, can obtainThe limit voltage value that the inverter can output is Uinv-max＝M·UdcWhich can be deduced from the relationship between Us and Ul,Ulcc＝(1/ωC-ωLf)·Il，
Figure BDA0003296351280000091
after the adjustment angle is determined, the adjusted compensation voltage amplitude value can be obtained as
Figure BDA0003296351280000092
Can obtain Ul' and UdvrThe included angle of
Figure BDA0003296351280000093
Therefore, the phase of the adjusted compensation voltage is ═ +, and the amplitude and the phase of the compensation voltage can be changed by adjusting the angle, so that the output voltage of the inverter finally meets the requirement of the limit voltage.
Analyzing an adjusting strategy under the fluctuation of the load power factor:
when the load fluctuates to cause the change of the load power factor angle, the U is usedLCCMay also cause the inverter output voltage to exceed its limit voltage range, as shown in fig. 5 a. As can be seen from the figure, due to the increasing power factor angle, the inverter output voltage Uinv increases accordingly, and finally exceeds the limit circle of the inverter output voltage. Of course, the inverter output voltage may exceed its limit range when the power factor angle is reduced, and therefore, when the inverter output voltage exceeds its limit range due to the change of the load power factor angle, the corresponding operation state adjustment is also needed, as shown in fig. 5 b.
Step three: determining an optimal structure of a series capacitance coupling DVR;
reference is made to the figure6, the DVR and the series capacitance coupling type DVR are analyzed and compared in the aspects of control stability, filtering effect and the like, and U is obtainedsAnd UlRespectively the grid voltage and the load voltage,
Figure BDA0003296351280000101
Uinvand UdvrRespectively representing a reference compensation voltage given value of the DVR, an inverter voltage command value and an output voltage; i isl、ILfRespectively load current and filter inductance current; u shapeCThe voltage borne by the series capacitor; KPWM is equivalent gain of the inverter, KPWM is taken as 1 for simplifying analysis, and a scheme a and a scheme b are respectively formed;
scheme b, a transfer function G from the given value of the reference compensation voltage to the actual value of the output voltage can be obtainedLfCCfAnd the transfer function Z of the load current to the actual value of the output voltageLfCCfAre respectively as
Figure BDA0003296351280000102
Figure BDA0003296351280000103
In formulae (10) and (11)
Figure BDA0003296351280000104
For scheme a, the same reasoning can be followed
Figure BDA0003296351280000105
Figure BDA0003296351280000106
In the formulae (12) and (13)
Figure BDA0003296351280000107
The frequency of the actual value of the output voltage to the given value of the reference compensation voltage can be obtained by changing the size of the series capacitor C; and by changing the size of the series capacitor C, a frequency response bode plot of the actual value of the output voltage to the load current can be obtained, as shown in fig. 7.
As can be seen from fig. 7: when the scheme b is adopted, a resonance peak exists in the amplitude-frequency characteristic curve and the phase-frequency characteristic curve of the scheme b near 100Hz, and the resonance peak moves leftwards along with the gradual increase of C, and the existence of the resonance peak may cause resonance to influence the stability of the whole system; when the scheme a is adopted, a resonance peak cannot occur, namely, after the series capacitor C is added on the right side of the filter capacitor, the stability of the system cannot be influenced. Meanwhile, no matter the scheme a or the scheme b is adopted, the amplitude-frequency characteristic curve and the phase-frequency characteristic curve of the filter are basically superposed with those of the traditional DVR, namely, compared with the traditional DVR, the frequency response characteristic of the filter in high and low frequency bands is basically unchanged, so that the filter effect of the filter is slightly influenced after the series capacitor C is added.
As can be seen from fig. 8: when the scheme b is adopted, the amplitude-frequency characteristic curve and the phase-frequency characteristic curve of the scheme b also have a resonance peak in the vicinity of 100 Hz. In the low frequency band, for the series capacitor coupling type DVR, when the value of the series capacitor C is the same, the amplitude-frequency characteristic curve and the phase-frequency characteristic curve of the series capacitor are basically overlapped no matter the series capacitor is on the left side or the right side of the filter capacitor, which shows that the system disturbs the load current under the two schemes
The inhibition effect of the dynamic low-frequency band is basically the same; in the high frequency band, the amplitude-frequency characteristic curve and the phase-frequency characteristic curve of the traditional DVR are basically superposed when the scheme b is adopted, namely the high frequency attenuation capacities of the amplitude-frequency characteristic curve and the phase-frequency characteristic curve are basically the same, but when the scheme a is adopted, the scheme a has better high frequency attenuation capacity in the high frequency band, the influence of harmonic components in load current on output voltage can be better inhibited, the increase of C can enable the scheme b to have better high frequency attenuation capacity in the high frequency band, the influence of the harmonic components in the load current on the output voltage can be better inhibited, and the structure of the scheme a is finally selected by combining the analysis.
Step four: simulation and verification;
in order to verify the feasibility of the series capacitance coupling type DVR, a single-phase DVR simulation model is set up in an MATLAB/Simulink simulation platform for simulation analysis.
FIG. 9 is a simulation waveform of series capacitive coupling DVR compensating grid voltage, in which the grid voltage drop amplitude is 20% at 0.10-0.20 s, 40% at 0.30-0.40 s, and the output reference value of DVR is U at 0.10-0.20 s* dvr71.8V; as can be seen from fig. 9, when the output reference value of the DVR is 159V at 0.30 to 0.40s, the series capacitive coupling DVR can still operate normally when the dc side voltage is only 200V. When the voltage of the power grid drops, the series capacitor coupling type DVR can output enough voltage to complete compensation, and the output voltage of the inverter is small. With the increase of the voltage drop amplitude, the series capacitance coupling DVR outputs the voltage U at the inverterinvIn the lower case, Udvr can still reach the reference value U* dvrAnd the compensation of the voltage of the dropping power grid can be realized. Even under the lower direct current side voltage level, the series capacitance coupling type DVR also has good compensation effect.
Fig. 10 is a simulation waveform of the series capacitance coupling type DVR compensating power grid voltage, when the load current is reduced to 1/4, the voltage ULCC on the DVR filter branch is also reduced to 1/4, which results in the output voltage of the inverter rising, the power grid voltage is set to drop within 0.30-0.50 s in the simulation, the dropping amplitude is 30%, wherein 0.30-0.40 s is not subjected to state adjustment, and 0.40-0.50 s is subjected to corresponding state adjustment.
During the period of not carrying out state adjustment for 0.30-0.40 s, the output voltage of the inverter is greatly increased and exceeds the set limit range Uinv-maxThe compensation voltage outputted by the DVR cannot reach the corresponding reference value, and the harmonic content is large, which affects the final compensation effect. Is carried out for 0.40 to 0.50sThe state adjustment is carried out, the compensation voltage required to be output is reduced after the adjustment is carried out, the output voltage of the inverter is limited within the limit range, and the series capacitor coupling DVR still has a good compensation effect.
The optimal topological structure of the series capacitance coupling DVR is obtained through analysis and comparison, so that the stability is ensured and the dynamic performance is good; considering that the output voltage of the inverter can exceed the limit range under the condition of load fluctuation, an operation state adjusting strategy suitable for the series capacitance coupling type DVR is provided so as to ensure the normal and stable operation of the device. Finally, the effectiveness and feasibility of the series capacitor coupling DVR are fully verified through simulation and experiments, and when a minimum energy compensation strategy is adopted, the structure not only can realize minimum active output, but also can remarkably reduce the output voltage of the inverter, so that the capacity and the direct current side voltage level of the inverter are effectively reduced.
For practical engineering application of the series capacitance coupling type DVR, some problems need to be noticed, for example, considering that a DVR system is coupled to a power grid side, the series capacitance may generate electrical oscillation with inductive devices such as line parasitic parameters, transformers, motors, and the like.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art should be able to cover the technical solutions and the inventive concepts of the present invention within the technical scope of the present invention.

Claims (8)

1. The method for analyzing the operation characteristics of the series capacitance coupling type dynamic voltage restorer is characterized by comprising the following steps of:
s1: the basic circuit design of the series capacitance coupling DVR;
s2: adjusting the running state of the series capacitance coupling DVR under the condition of load fluctuation;
s3: determining an optimal structure of a series capacitance coupling DVR;
s4: and (5) simulating and verifying.
2. The method for analyzing operational characteristics of a series capacitive coupling dynamic voltage restorer of claim 1, wherein the method is applied to a PWM inverter whose basic circuit topology is composed of fully controlled devices, a dc-side energy storage capacitor C in S1dcThe circuit comprises an LC output filter, a series transformer T, a bypass switch K and a series capacitor C.
3. The method for analyzing operational characteristics of a series capacitive coupling dynamic voltage restorer of claim 1, wherein a series capacitor C is added to the basic circuit based on the LC filter in S1, and when the DVR employs a minimum energy compensation strategy, the series capacitor C can bear a voltage of 90 ° of a large hysteresis load current.
4. The method for analyzing the operational characteristics of a dynamic voltage restorer of the series capacitive coupling type according to claim 1, wherein for step S1, the magnitude of the voltage sustained by the LC filter is closely related to the magnitude of the series capacitance C, and the uncertainty of the voltage drop amplitude of the power grid and the random fluctuation of the load also affect the LC filter, and the magnitude of the voltage sustained by the structural filter branch is
ULCC＝Il(1/ωC-ωLf)＝k·Udvm (1)
In the formula, k is a bearing coefficient, the uncertainty of the grid voltage drop amplitude and the random fluctuation of the load need to be comprehensively considered for the value of k, and when the DVR is applied to the occasion with smaller grid voltage drop amplitude, the value of k can be properly reduced; on the contrary, the k value can be properly increased, and the formula (1) is solved and simplified to obtain
Figure FDA0003296351270000011
Wherein Sn is the rated capacity m of DVR, and in order to make the fundamental wave impedance of the filtering branch circuit be capacitive under power frequency and inductive under characteristic harmonic frequency, it is necessary to meet the requirements
Figure FDA0003296351270000021
Where s is the angular frequency of the switching frequency, the series capacitance C can be determined by equations (2) and (3), for the filter inductance Lf and the filter capacitance CfThe determination is made according to the selection principle of the conventional DVR.
5. The method for analyzing operational characteristics of a series capacitive coupling dynamic voltage restorer of claim 1, wherein for S1, the maximum hysteretic load current 90 ° that can be sustained on the series capacitor C, the inverter output power of the DVR that can be effectively reduced by the output voltage of the inverter after the minimum energy compensation strategy is adopted can be represented as:
Figure FDA0003296351270000022
in the formula, Pinv and Qinv are respectively the active power output by the inverter and the active power and the reactive power which are not respectively output by the inverter;
Pinv＝UinvIlcosβ (5)
Qinv＝UinvIlsinβ (6)
under the same working condition, the output active power of the DVR with the traditional structure is the same as that of the inverter of the series capacitance coupling DVR, as shown in formula (7):
Pinv＝Uinv1Ilcosβ1＝Uinv2Ilcosβ2 (7)
similarly, the output reactive power of the inverters of the DVR with two structures can be obtained as
Figure FDA0003296351270000023
Figure FDA0003296351270000024
6. The method for analyzing operational characteristics of a dynamic voltage restorer of the series capacitive coupling type according to claim 1, wherein a voltage U applied to the filter branch is analyzed for the adjustment strategy under the fluctuation of the load current in S2LCCIs directly subjected to load current IlWhen the load current is reduced due to the fluctuation of the load, ULCCAnd also decreases as the load current decreases, resulting in ULCCWhen the voltage is too small, the output voltage of the inverter exceeds the limit voltage value of the inverter, and corresponding operation state adjustment is needed at the moment to ensure that the output voltage of the inverter is within the limit range of the inverter and ensure that the DVR works in the minimum energy compensation state within the output capacity range;
when the load current is reduced, the broken line circular arc with O as the center represents the limit circle of the output voltage of the inverter, because of the load current IlReduction of (1), ULCCWill also decrease accordingly, resulting in the output voltage U of the inverterinvWith a consequent increase when IlWhen reduced to a certain extent, UinvWill exceed its limit voltage value Uinv-maxFurther, the normal operation of the DVR may be affected.
7. The method for analyzing operational characteristics of a series capacitive coupling type dynamic voltage restorer of claim 1, wherein U is a value obtained by analyzing and comparing the DVR and the series capacitive coupling type DVR in S3 in terms of control stability and filtering effectsAnd UlRespectively the grid voltage and the load voltage,
Figure FDA0003296351270000034
Uinvand UdvrRespectively representing a reference compensation voltage given value of the DVR, an inverter voltage command value and an output voltage; i isl、ILfAre respectively loadsCurrent and filter inductor current; u shapeCThe voltage borne by the series capacitor; KPWM is equivalent gain of the inverter, KPWM is taken as 1 for simplifying analysis, and a scheme a and a scheme b are respectively formed;
scheme b, a transfer function G from the given value of the reference compensation voltage to the actual value of the output voltage can be obtainedLfCCfAnd the transfer function Z of the load current to the actual value of the output voltageLfCCfAre respectively as
Figure FDA0003296351270000031
Figure FDA0003296351270000032
In formulae (10) and (11)
Figure FDA0003296351270000033
For scheme a, the same reasoning can be followed
Figure FDA0003296351270000041
Figure FDA0003296351270000042
In the formulae (12) and (13)
Figure FDA0003296351270000043
And the frequency of the actual value of the output voltage to the given value of the reference compensation voltage can be obtained by changing the size of the series capacitor C.
8. An assembling method of an operation characteristic analysis method of a series capacitive coupling type dynamic voltage restorer according to claim 1, wherein aiming at the S4, in order to verify the feasibility of the series capacitive coupling type DVR, a single-phase DVR simulation model is built in an MATLAB/Simulink simulation platform for simulation analysis.
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