CN114421494A - High-frequency oscillation suppression method and system for enhanced flexible direct current transmission system - Google Patents
High-frequency oscillation suppression method and system for enhanced flexible direct current transmission system Download PDFInfo
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Abstract
The invention discloses a high-frequency oscillation suppression method and a high-frequency oscillation suppression system for an enhanced flexible direct current transmission system, wherein the method is in a voltage feedforward cascade dead zone link or further in a current feedback cascade damping correction link; by using dead-zone element H in voltage feedforwardnThe voltage feedforward compensation circuit is used for eliminating the negative damping characteristic introduced by voltage feedforward under small disturbance and has a quick response characteristic under large disturbance; damping correction link H in current feedbackcFor eliminating the negative damping characteristic introduced by the control delay. The negative resistance area of the frequency band flexible direct current converter above 200Hz can be effectively eliminated, and the high-frequency oscillation of the frequency band flexible direct current transmission system above 200Hz can be effectively inhibited. The invention can realize the soft-direct current converter without negative damping area in the frequency band above 200Hz, and can not deteriorate the dynamic response characteristic and the fault ride-through characteristic of the original system.
Description
Technical Field
The invention relates to the field of flexible direct current transmission, in particular to a high-frequency oscillation suppression method and a high-frequency oscillation suppression system for an enhanced flexible direct current transmission system.
Background
The flexible direct current transmission technology has become one of the mainstream schemes of large-scale new energy grid connection, asynchronous grid connection and the like. The Modular Multilevel Converter (MMC) becomes a preferred converter topology of a flexible direct current transmission system due to the advantages of high voltage-resistant level, low output harmonic content, high modular integration level and the like. However, as the capacity and voltage class of the limp-to-dc system are continuously increased, the dynamic characteristics thereof have a greater and greater influence on the safe and stable operation of the power system. In the Luxi flexible straight project (350 kV/1000MW) which was put into operation in 2017, a high-frequency oscillation phenomenon of 1270Hz occurs, and in the Yubei flexible straight project which was put into operation in 2018, a high-frequency oscillation phenomenon of 1810Hz also occurs at the jaw side.
At present, the suppression of the high-frequency oscillation of a flexible direct system can be divided into an active strategy and a passive strategy, and the passive suppression strategy has the problems of large device loss, poor economy, poor suppression effect after the working condition of a power grid changes and the like. In the aspect of active suppression strategies, the method of cascading a voltage feedforward channel with a low-pass filter is a currently recognized simple and effective active suppression strategy, and in addition, the following active suppression strategies are proposed at home and abroad: voltage feedforward nonlinear filtering, active damping control and control link optimization, a self-adaptive strategy based on resonance monitoring and the like. However, except for the self-adaptive method, the current active suppression strategy is directed at specific power grid working conditions, the negative resistance characteristic of the MMC high-frequency section cannot be completely eliminated, and when the power grid working conditions change, the system still has a high-frequency oscillation risk. The reason for this problem is that the actual resistance characteristic of the MMC high-frequency simplified impedance model without considering PLL is smaller than the actual one, and the design of the suppression strategy is deficient. Although the adaptive control strategy has strong adaptability, the structure is complicated, and a plurality of groups of wave traps with different central frequencies and resonant frequency monitoring devices need to be designed. And the existing suppression strategy deteriorates the fault ride-through characteristics of the system, causing fault ride-through failure.
In order to solve the above problems, there is no high-frequency oscillation suppression strategy that can completely eliminate the high-frequency negative resistance characteristic and does not affect the fault ride-through characteristic of the system.
Disclosure of Invention
The present invention is directed to the problems in the prior art, and an object of the present invention is to provide a method and a system for suppressing high-frequency oscillation in an enhanced flexible dc power transmission system. The method does not need oscillation frequency monitoring, can completely eliminate the negative resistance characteristic of the MMC at the high frequency band, and can ensure the good fault ride-through characteristic of the MMC.
The invention provides a high-frequency oscillation suppression method for an enhanced flexible direct current transmission system, which is in a voltage feedforward cascade dead zone link.
Optionally, the voltage feedforward is cascaded with dead zone links, where a positive sequence d-axis voltage feedforward dead zone of the dead zone link is around 1pu, and positive sequence q-axis voltage feedforward and negative sequence dq-axis voltage feedforward dead zones are around 0pu, and the nonlinear transfer function is as follows:
wherein the non-linear voltage transfer function HnThe superscripts p and n respectively represent positive and negative sequence current loops, the superscripts dq respectively represent dq-axis channels, Up dIs the per unit value of the forward-sequence d-axis feedforward voltage, Up qIs the per unit value of the forward-sequence q-axis voltagen dIs the per unit value of the negative sequence d-axis feedforward voltage, Un qIs the per unit value of the negative sequence q-axis feedforward voltage, and d is the size of the dead zone.
Optionally, the dead zone link design step is as follows:
s1, setting the positive sequence d-axis voltage feedforward dead zone to be 0.75pu-1.25pu according to the determined grid-connected rated voltage of the flexible-direct system;
and S2, setting the positive sequence q-axis voltage feedforward dead zone and the negative sequence dq-axis voltage feedforward dead zone to be 0pu-0.25pu according to the determined grid-connected rated voltage of the flexible direct system.
Further, the method further comprises: and a current feedback cascade damping correction link.
Optionally, the current feedback cascade damping correction link is formed by cascading a third-order low-pass filter and a damping branch, and a transfer function is as follows:
where K is the gain, H1 h、H1 lAnd H3 lRespectively a 1 st order high pass filter, a 1 st order low pass filter and a 3 rd order low pass filter.
Optionally, the feedback current damping correction unit includes the following steps:
s1, establishing a high-frequency simplified impedance model of the MMC current converter, which contains PLL, according to the determined electrical and control parameters of the flexible-straight MMC current converter;
s2, according to the established MMC converter high-frequency simplified impedance model, firstly designing the cut-off frequency f of a third-order low-pass filter of a damping correction link3 lThe high-frequency impedance has a narrow negative resistance frequency band and a small highest negative resistance frequency as a design target to obtain a good high-frequency damping characteristic;
s3: on the basis of a third-order low-pass filter, an active damping link is designed to ensure that the frequency band soft-direct current converter above 200Hz has no negative resistance area.
In a second aspect of the present invention, there is provided an enhanced high-frequency oscillation suppression system for a flexible direct current transmission system, for implementing the above high-frequency oscillation suppression method, including:
the voltage feedforward module is used in a voltage feedforward cascade dead zone link; or, further comprising:
the current feedback module is used for cascading a damping correction link in current feedback;
the electricityA voltage feedforward module using a dead-zone link H in voltage feedforwardnThe voltage feedforward compensation circuit is used for eliminating the negative damping characteristic introduced by voltage feedforward under small disturbance and has a quick response characteristic under large disturbance;
the current feedback module adopts a damping correction link H in current feedbackcFor eliminating the negative damping characteristic introduced by the control delay.
Further, the high-frequency oscillation suppression system of the enhanced flexible direct current transmission system specifically includes:
the DDSRF-PLL module realizes the phase locking of the positive sequence power frequency voltage phase theta of the power grid voltage through a double-decoupling synchronous rotating coordinate system phase-locked loop;
a power loop module for feeding back the deviation between the reference power and the actual power via a PI controller HpqControlling to realize the error-free tracking of power reference and generating the dq-axis current reference of the positive and negative sequence current control loop module;
the power measurement module is used for calculating the actual output power of the MMC equipment module by collecting the dq axis component of the positive sequence voltage and current and feeding the actual output power back to the power loop module;
a positive and negative sequence current control loop module including a voltage feedforward module and a current feedback module for passing the deviation between the dq-axis current reference and the feedback current generated by the power loop module through a current loop PI controller HiPerforming a homodyne control and generating a positive-negative sequence dq-axis modulation voltage, the dq-axis current passing through omega0L feedforward link is used for decoupling, and dead zone link H in voltage feedforward modulenThe damping correction link H in the current feedback module is used for eliminating the negative damping characteristic introduced by voltage feedforward under small disturbance and has the quick response characteristic under large disturbancecFor eliminating negative damping characteristics introduced by control delays;
1/4 power frequency period delay module, which realizes the separation and extraction of positive and negative sequence voltage and current through the set delay, and realizes the transformation of the positive and negative sequence voltage and current from the static coordinate system to the rotating coordinate system through the park transformation module of the positive and negative theta;
CCSC module, through PI control link HCCRealize the double frequency circulation current inside the MMCRestraining dq axis components and generating three-phase double-frequency circulating current restraining modulation voltage;
and the modulation module is used for carrying out inverse park conversion on the dq-axis modulation voltage generated by the dq-axis current inner ring control link to generate an alpha beta component under a static coordinate system and superposing the alpha beta component, further generating three-phase fundamental frequency modulation voltage through Clark conversion, and superposing the three-phase fundamental frequency modulation voltage and the circulating current suppression voltage to generate the final MMC three-phase modulation voltage.
Compared with the prior art, the embodiment of the invention has at least one of the following advantages:
the method and the system can eliminate the negative resistance characteristic of the high-frequency impedance of the MMC, ensure the interaction stability of the high-frequency-band flexible impedance and the power grid, and ensure the fault ride-through characteristic similar to that of a non-high-frequency oscillation suppression strategy.
The method and the system have strong capability of adapting to the working condition of the power grid, completely eliminate the negative resistance characteristic of the MMC in the height frequency band (more than 200Hz), and realize the inhibition of high-frequency oscillation when the working condition of the power grid changes without adopting a complex adaptive method.
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Embodiments of the invention are further described below with reference to the accompanying drawings:
FIG. 1 is a detailed functional block diagram of a preferred embodiment of the method and system of the present invention;
FIG. 2 is a flow chart of a dead band link design in accordance with a preferred embodiment of the present invention;
FIG. 3 is a schematic diagram illustrating the impedance characteristics of an MMC in accordance with a preferred embodiment of the present invention;
FIG. 4 is a comparison of the delay settings of the damping correction element in the current feedback of a preferred embodiment of the present invention;
FIG. 5 is a diagram illustrating the effect of fault-ride-through characteristics according to a preferred embodiment of the present invention.
Detailed Description
The present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way. It should be noted that variations and modifications can be made by persons skilled in the art without departing from the spirit of the invention. All falling within the scope of the present invention. Portions not described in detail below may be implemented using conventional techniques.
Existing suppression strategies degrade the fault ride-through characteristics of the system, causing fault ride-through failures. The embodiment of the invention provides a high-frequency oscillation suppression strategy which can completely eliminate the high-frequency negative resistance characteristic and does not influence the fault ride-through characteristic of a system.
In an embodiment of the invention, a method for suppressing high-frequency oscillation of an enhanced flexible direct current transmission system is provided, and the method comprises a voltage feedforward cascade dead zone link, a dead zone link H in voltage feedforwardnThe negative damping characteristic introduced by voltage feedforward under small disturbance can be eliminated, and the fast response characteristic under large disturbance is achieved; through this setting, need not the oscillation frequency monitoring, and can eliminate the negative resistance characteristic of MMC high-frequency channel completely, and can guarantee the good fault ride through characteristic of MMC.
Specifically, in a preferred embodiment, in a dead-zone link cascaded with voltage feedforward, a positive sequence d-axis voltage feedforward dead zone of the dead-zone link is around 1pu, and positive sequence q-axis voltage feedforward and negative sequence dq-axis voltage feedforward dead zones are around 0pu, the nonlinear transfer function is as follows:
wherein the non-linear voltage transfer function HnThe superscripts p and n respectively represent positive and negative sequence current loops, the superscripts dq respectively represent dq-axis channels, Up dIs the per unit value of the forward-sequence d-axis feedforward voltage, Up qIs the per unit value of the forward-sequence q-axis voltagen dIs the per unit value of the negative sequence d-axis feedforward voltage, Un qIs the per unit value of the negative sequence q-axis feedforward voltage, and d is the size of the dead zone.
Further, in the above embodiment, the dead zone link design may adopt the following steps:
s1, setting the positive sequence d-axis voltage feedforward dead zone to be 0.75pu-1.25pu according to the determined grid-connected rated voltage of the flexible-direct system;
and S2, setting the positive sequence q-axis voltage feedforward dead zone and the negative sequence dq-axis voltage feedforward dead zone to be 0pu-0.25pu according to the determined grid-connected rated voltage of the flexible direct system.
Of course, the above parameters are the settings of the above preferred embodiment of the present invention, and in other embodiments, other parameter ranges may be selected as needed, and the present invention is not limited to the above parameter ranges.
On the basis of the above embodiment, in order to better implement a high-frequency oscillation suppression strategy that completely eliminates the high-frequency negative resistance characteristic and does not affect the system fault ride-through characteristic, the method may further include: in the current feedback cascade damping correction link, a damping correction link H in current feedback is adoptedcThe negative damping characteristic introduced by the control delay can be eliminated.
Specifically, in a preferred embodiment, in the current feedback cascade damping correction link, the damping correction link is formed by cascading a third-order low-pass filter and a damping branch, and a transfer function is as follows:
where K is the gain, H1 h、H1 lAnd H3 lAre respectively provided withThe filter includes a 1 st order high pass filter, a 1 st order low pass filter and a 3 rd order low pass filter.
Specifically, in a preferred embodiment, the feedback current damping correction procedure includes the following steps:
s1, establishing a high-frequency simplified impedance model of the MMC current converter, which contains PLL, according to the determined electrical and control parameters of the flexible-straight MMC current converter;
s2, according to the established MMC converter high-frequency simplified impedance model, firstly designing the cut-off frequency f of a third-order low-pass filter of a damping correction link3 lThe high-frequency impedance has a narrow negative resistance frequency band and a small highest negative resistance frequency as a design target to obtain a good high-frequency damping characteristic;
s3: on the basis of a third-order low-pass filter, an active damping link is designed to ensure that a soft direct current converter of a required frequency band (such as a frequency band above 200Hz) has no negative resistance region.
In order to better realize the technical scheme, a new high-frequency impedance simplified model considering a phase-locked loop (PLL) is established, the improved high-frequency impedance simplified model can more accurately represent the actual resistance characteristic of the flexible-straight system, and the conservatism of high-frequency stability analysis of the flexible-straight system is effectively reduced. The modeling of the simplified model comprises two parts of a positive-negative sequence current loop and a phase-locked loop, and particularly, the final impedance expression is shown as (8.a), the modeling part of the current loop is shown as (8.b), and the modeling of the phase-locked loop part is shown as (7.a) - (7. e). And further, the high-frequency oscillation suppression method of the enhanced flexible direct current transmission system with more excellent effect is realized on the basis of the model.
Specifically, the high frequency impedance simplified model in S2, wherein the PLL correlation modeling is as follows:
the variables in (7.a) to (7.f) are explained below:
z in (7.a)αβIs a 2 nd order impedance matrix in a MMC equipment static alpha beta coordinate system, wherein Tu1p、Tu2p、Tu1n、Tu2nIs a sub-type of the alpha beta impedance of the MMC equipment, which is generated by a phase-locked control link, and the specific expansion expression of the alpha beta impedance is shown as (7. b). T isip、TinIs another molecular formula for forming the alpha beta impedance of the MMC equipment, which is generated by a current control link, Tip、TinThe specific expansion expression of (2) is shown as (7. c). s (0.5L)arm+LT)I、(0.5Rarm+RT) I is the remainder of the sub-formula forming the MMC equipment alpha beta impedance, which is generated by the AC port circuit structure of the MMC, s is the complex frequency, LarmAnd RarmBridge arm inductance and bridge arm resistance and R being MMCTAnd LTThe leakage reactance and the resistance of the MMC alternating current outlet transformer converted to the MMC side are shown, and I is a 2-order unit array.
In (7.b)
Representing the inverse transformation of the positive sequence park, the superscript theta represents the positive sequence, the superscript-represents that the frequency domain component after the inverse transformation is reduced by 1 time of fundamental frequency, and the superscript + represents that the frequency domain component after the inverse transformation is increased by 1 time of fundamental frequency;
representing negative sequence park transformation, superscript-theta representing negative sequence, superscript-representing a 1-fold decrease in fundamental frequency of the frequency domain component after inverse transformation, and superscript + representing a 1-fold increase in fundamental frequency of the frequency domain component after inverse transformation. In a stationary coordinate system, a variable with an arbitrary frequency f, which is passed through a fundamental frequency f1After the park transformation or the park inverse transformation, the frequency deviation of plus and minus one harmonic is generated, namely f +/-f1For LTI systems, the frequency is f + f1The transfer function complex frequency independent variable corresponding to the variable is s + j omega1Frequency of f-f1The transfer function complex frequency independent variable corresponding to the variable is s-j omega1。Gd(s+jω1) Corresponding to a frequency of f + f1Time-delayed transfer function of variable, Gd(s-jω1) Corresponding to a frequency f-f1Delay transfer function of variable, s in parentheses is complex frequency, j is complex number, ω1Is the fundamental angular frequency. Hi(s+jω1) Corresponding to a frequency of f + f1Current loop PI transfer function of variable, Hi(s-jω1) Corresponding to a frequency f-f1Current loop PI transfer function of the variable. Hc(s+jω1) Corresponding to a frequency of f + f1Damping of variables correcting the link transfer function, Hc(s-jω1) Corresponding to a frequency f-f1Current loop PI transfer function of the variable.
The direct current components of positive sequence d-axis, positive sequence q-axis, negative sequence d-axis and negative sequence q-axis output by the MMC equipment respectively, the superscript p represents a positive sequence, n represents a negative sequence and 0 represents a direct current component.
And modulating voltage direct current components of a positive sequence d axis, a positive sequence q axis, a negative sequence d axis and a negative sequence q axis which are output by the MMC equipment respectively, wherein the superscript p represents a positive sequence, n represents a negative sequence and 0 represents a direct current component. D1Is a coupling term controlled by a positive sequence current loop, as shown in (7.D), DpIs a sub-formula such as (7.e) associated with the phase locked loop.
In (7.c)
Represents the positive sequence park transformation and the positive sequence,
representing negative park transformation, superscript theta representing positive sequence fundamental frequency park transformation, -theta representing negative sequence fundamental frequency park transformation, superscript + representing that the frequency is increased by one time of fundamental frequency after park transformation, and superscript-representing that the frequency is decreased by one time of fundamental frequency after park transformation. The other variables have the same meanings as in (7. b).
(7.d) wherein LT、LarmIs the same as (7.a), ω1Is the fundamental angular frequency, and j is a complex number.
(7.e) A inp、Bp、CpAnd An、Bn、CnAre each the component Dp、DnA sub-formula of (1). A. thepAnd AnSub-formula Y contained in5p、Y6p、Y7pAnd Y5n、Y6n、Y7nThe detailed form is shown as (7. f). B ispIs composed of
BnIs composed of
Same as (7. c). CpAnd CnIn (1)
And
the direct current component of the dq axis grid-connected voltage is obtained by positive sequence fundamental frequency park conversion marked by DDSRF-PLL in figure 1, wherein the superscript p represents positive sequence park conversion, and 0 represents the direct current component;
and
is a dq axis grid-connected voltage direct current component obtained by negative sequence fundamental frequency park conversion marked by DDSRF-PLL in figure 1, wherein the superscript n represents negative sequence park conversion, and 0 represents a direct current component;
and
the negative sequence double fundamental frequency park marked by DDSRF-PLL in figure 1 is converted to obtain q-axis grid-connected voltage direct-current component;
and
a dq axis grid-connected voltage direct-current component is obtained by converting a positive sequence double fundamental frequency park marked by DDSRF-PLL in the figure 1; further, constitution CpAnd CnThe sub-formula of (1): y is1p、Y2p、Y3p、Y4p、Y7p、Y1n、Y2n、Y3n、Y4n、Y7nThe detailed form is shown in (7. f). DpAnd DnSub-formula Gpllp、GpllnIs the open loop transfer function of the phase locked loop, corresponding to a frequency of f + f1And f-f1The variable of (2). At GpllpAnd GpllpIn, Kp_pll、Ki_pllThe proportional parameter and the integral parameter of the phase-locked loop PI link are respectively, and s is complex frequency.
Y in (7.f)1pTo Y7pIs the constitution DpSub-formula of (Y)1nTo Y7nIs the constitution DnA sub-formula of (1). Wherein
Is the park transformation of positive sequence double fundamental frequency, the transformed variable frequencies are respectivelyThe base frequency is reduced and increased by two times,
and
the park transformation of negative sequence double fundamental frequency is adopted, and the transformed variable frequency is respectively reduced and increased by double fundamental frequency; hlp(s+3jω)、Hlp(s-3jω)、Hlp(s+jω)、Hlp(s-j ω) is the transfer function of the low-pass filter, corresponding to a frequency of f +3f, respectively1、f-3f1、f+f1、f-f1The variable of (2).
And
the negative sequence park transformation in figure 1 results in the positive second harmonic component of the dq axis grid-connected voltage,
and
the negative sequence park in figure 1 transforms to get the negative second harmonic component of the dq axis grid-connected voltage,
and
the positive second harmonic component of the dq axis grid-connected voltage is obtained by the negative sequence double frequency park conversion in figure 1,
and
the negative sequence double frequency park in figure 1 is transformed to obtain the negative second harmonic component of the dq axis grid-connected voltage,
same as in (7. e);
same as in (7. c).
In addition, the high-frequency impedance simplified model in S2 above, in which the positive-negative sequence current loop portion is modeled as follows:
z in (8.a)αβIs a 2 nd order impedance matrix in a MMC equipment static alpha beta coordinate system, wherein Tu1p、Tu2p、Tu1n、Tu2nIs a sub-type of the alpha beta impedance of the MMC equipment, which is generated by a phase-locked control link, and the specific expansion expression of the alpha beta impedance is shown as (7. b). T isip、TinIs another molecular formula for forming the alpha beta impedance of the MMC equipment, which is generated by a current control link, Tip、TinThe specific expansion expression of (c) is shown as (8. b). s (0.5L)arm+LT)I、(0.5Rarm+RT) I is the remainder of the sub-formula forming the MMC equipment alpha beta impedance, which is generated by the AC port circuit structure of the MMC, s is the complex frequency, LarmAnd RarmBridge arm inductance and bridge arm resistance and R being MMCTAnd LTIs leakage reactance and resistance converted from MMC AC outlet transformer to MMC side, I is 2-order unit array, j is complex number, ZpnIs a modified sequence impedance of an MMC installation, which is a 2 x 2 square matrix of impedances, ZppIs that the MMC equipment ignores the positive sequence impedance of the positive and negative sequence coupling, which is equal to the first row, first column element Z in the modified sequence impedance matrixpn(1，1)。
In (8.b)
Representing the inverse transformation of the positive sequence park, the superscript theta represents the positive sequence, the superscript-represents that the frequency domain component after the inverse transformation is reduced by 1 time of fundamental frequency, and the superscript + represents that the frequency domain component after the inverse transformation is increased by 1 time of fundamental frequency;
representing negative sequence park transformation, superscript-theta representing negative sequence, superscript-representing a 1-fold decrease in fundamental frequency of the frequency domain component after inverse transformation, and superscript + representing a 1-fold increase in fundamental frequency of the frequency domain component after inverse transformation. In a stationary coordinate system, a variable with an arbitrary frequency f, which is passed through a fundamental frequency f1After the park transformation or the park inverse transformation, the frequency deviation of plus and minus one harmonic is generated, namely f +/-f1For LTI systems, the frequency is f + f1The transfer function complex frequency independent variable corresponding to the variable is s + j omega1(s is the complex frequency, j is the complex number, ω1Is the fundamental angular frequency) with a frequency f-f1The transfer function complex frequency independent variable corresponding to the variable is s-j omega1。Gd(s+jω1) Corresponding to a frequency of f + f1Time-delayed transfer function of variable, Gd(s-jω1) Corresponding to a frequency f-f1Delay transfer function of variable, s in parentheses is complex frequency, j is complex number, ω1Is the fundamental angular frequency. Hi(s+jω1) Corresponding to a frequency of f + f1Current loop PI transfer function of variable, Hi(s-jω1) Corresponding to a frequency f-f1Current loop PI transfer function of the variable. Hc(s+jω1) Corresponding to a frequency of f + f1Damping of variables correcting the link transfer function, Hc(s-jω1) Corresponding to a frequency f-f1Current loop PI transfer function of the variable. L isarm、Rarm、RTAnd LTHas the same meaning as (8. a).
In order to better understand the technical solution of the present invention, the following provides a design description of a preferred embodiment, wherein the design steps are as follows:
s101, according to the determined grid-connected rated voltage of the flexible direct system, setting a positive sequence d-axis voltage feedforward dead zone to be 0.75pu-1.25pu, and setting a positive sequence q-axis voltage feedforward dead zone and a negative sequence dq-axis voltage feedforward dead zone to be-0.25 pu-0.25 pu;
s201, establishing the MMC improved high-frequency simplified impedance model and verifying according to the determined electrical and control parameters of the MMC converter;
s301, designing the cut-off frequency of a third-order low-pass filter of a damping correction link according to the established high-frequency simplified impedance model of the converter to obtain better high-frequency damping characteristics, so that the high-frequency impedance has a narrower negative resistance frequency band and a lower highest negative resistance frequency;
s401: on the basis of a third-order low-pass filter, an active damping control link in a damping correction link is designed to modify the non-ideality of the damping characteristic of a specific frequency band so as to ensure that no negative resistance characteristic exists above 200 Hz.
In this embodiment, by the control strategy of S101 to S401, the negative resistance characteristic of the MMC high-frequency impedance can be eliminated, the interaction between the high-frequency band flexible circuit and the power grid is ensured to be stable, and the fault ride-through characteristic similar to that of the non-high-frequency oscillation suppression strategy is ensured.
Based on the same technical concept, in another embodiment of the present invention, there is further provided an enhanced flexible direct current transmission system high-frequency oscillation suppression system, for implementing the high-frequency oscillation suppression method in any one of the embodiments, specifically including: a voltage feedforward module, or a voltage feedforward module and a current feedback module. The voltage feedforward module is in a voltage feedforward cascade dead zone link; the current feedback module is in a current feedback cascade damping correction link. Specifically, the voltage feedforward module adopts a dead zone link H in voltage feedforwardnThe voltage feedforward compensation circuit is used for eliminating the negative damping characteristic introduced by voltage feedforward under small disturbance and has a quick response characteristic under large disturbance; the current feedback module adopts a damping correction link H in current feedbackcFor eliminating the negative damping characteristic introduced by the control delay. The system of the embodiment does not need oscillation frequency monitoring, can completely eliminate the negative resistance characteristic of the MMC high-frequency band, and can ensure the good fault ride-through characteristic of the MMC. The voltage feedforward module and the current feedback module in this embodiment may be implemented by using the corresponding technical features in the above method embodiments.
FIG. 1 is a detailed functional block diagram of a preferred embodiment of the method and system of the present invention. In some embodiments, to better perform the high-frequency oscillation suppression of the enhanced flexible direct current transmission system, the high-frequency oscillation suppression system may specifically include:
MMC and power grid equipment equivalent modules;
the DDSRF-PLL module directly collects grid-connected voltage as input, realizes phase locking on the power frequency voltage phase theta of the positive sequence of the power grid voltage through a double-decoupling synchronous rotating coordinate system phase-locked loop, and provides the angles of park transformation and park inverse transformation for the power loop module, the power measurement module, the positive and negative sequence current control loop module, the CCSC module, the 1/4 power frequency period delay module and the modulation module.
A power loop module for feeding back the deviation between the reference power and the actual power via a PI controller HpqControlling to realize the error-free tracking of power reference and generating the dq-axis current reference of the positive and negative sequence current control loop module;
the power measurement module is used for calculating the actual output power of the MMC equipment module by collecting the dq axis component of the positive sequence voltage and current and feeding the actual output power back to the power loop module;
a positive and negative sequence current control loop module including a voltage feedforward module and a current feedback module for subtracting the feedback current from the dq-axis current reference generated by the power loop module and passing through a current loop PI controller HiPerforming a homodyne control and generating a positive-negative sequence dq-axis modulation voltage, the dq-axis current passing through a value corresponding to omega1(0.5Larm+LT)(ω1Is the angular frequency of the fundamental frequency, LarmIs bridge arm reactance, LTTransformer leakage reactance) is equal, and a dead zone link H in a voltage feedforward module is used for decouplingnThe damping correction link H in the current feedback module is used for eliminating the negative damping characteristic introduced by voltage feedforward under small disturbance and has the quick response characteristic under large disturbancecFor eliminating negative damping characteristics introduced by control delays;
1/4 power frequency cycle delay module, which realizes the positive and negative sequence separation and extraction of voltage and current collection signal under normal operation state through the set delay, and realizes the transformation of positive and negative sequence voltage and current from static coordinate system to rotating coordinate system through the park transformation module of positive and negative theta, and provides the feedback of the positive and negative sequence current of dq axis and feedforward of the positive and negative sequence voltage of dq axis for the positive and negative sequence current control loop module;
CCSC module, through PI control link HCCThe suppression of double frequency circulation dq axis components inside the MMC is realized, and three-phase double frequency circulation suppression modulation voltage is generated;
and the modulation module is used for carrying out inverse park conversion on the dq-axis modulation voltage generated by the positive-negative sequence current control loop module to generate an alpha beta component under a static coordinate system and superposing the alpha beta component, further generating a three-phase fundamental frequency modulation voltage through Clark conversion, and superposing the three-phase fundamental frequency modulation voltage and a three-phase double-frequency circulating current restraining modulation voltage generated by the CCSC module to generate the final MMC three-phase modulation voltage.
As shown in fig. 1, in a preferred embodiment of the present invention, a method and a system for suppressing high-frequency oscillation of an enhanced flexible dc power transmission system are provided, wherein a voltage and current high-frequency oscillation at an MMC grid-connected PCC point is suppressed by cascading a dead zone in a voltage feedforward link and a damping correction device in a current feedback link. The strategy needs to design parameters of a voltage feedforward dead zone, a current feedback high-order low-pass filter and an impedance correction device.
Fig. 2 is a flow chart of a dead zone link design according to a preferred embodiment of the present invention. Referring to fig. 2, the specific design steps are as follows:
1. the range design of the voltage feedforward dead zone link is to ensure that the link is not influenced by the normal small disturbance of the power grid voltage, so as to ensure that the converter is the same as the constant feedforward under the small disturbance of the power grid voltage, and because the normal voltage allowed by the power grid voltage is +/-0.1 pu, the dead zone d in the embodiment is 0.25pu, so as to ensure a sufficient voltage constant feedforward interval.
2. According to the electrical and control parameters of the converter, a high-frequency simplified model of the converter is established, a positive and negative sequence decoupling control structure of the converter applied to practical engineering shown in figure 1 is referred, and quarter power frequency period delay, positive and negative sequence current inner loop control and DDSFR-PLL are considered, such as (8. a). The parameters are shown in tables 1 and 2:
TABLE 1 MMC Electrical parameters and control parameters
TABLE 2 MMC control parameters
TABLE 3 Transmission line equivalent impedance parameter
3. The cut-off frequency of the third-order low-pass filter of the damping correction controller is designed to ensure that the high-frequency impedance has a narrower negative resistance frequency band and a lower highest negative resistance frequency, and the cut-off frequency is designed to be 150 Hz.
4. The design of an active damping link of a damping correction controller aims at ensuring no negative resistance characteristic above 200Hz, and K is 1.34, the low-pass cut-off frequency is 200Hz, and the high-pass cut-off frequency is 1100 Hz. Fig. 3 shows the designed impedance characteristic of the MMC, and fig. 3 is a schematic diagram of the impedance characteristic of the MMC according to a preferred embodiment of the present invention.
Fig. 4 is a three-phase voltage waveform of a point-of-connection equipped on an MMC to verify the effectiveness of the present invention in suppressing high-frequency oscillations. The system has no control delay before three seconds, the voltage waveform of the three phases of the grid-connected point is stable, 550us of control delay is input after three seconds, the system starts to oscillate, and the high-frequency oscillation is effectively inhibited after 3.25 seconds of input of the high-frequency oscillation inhibiting strategy.
5. The fault ride through characteristics of the strategy are tested, the three-phase voltage of the PCC point is reduced from 1pu to 0pu at 3s, and the voltage of the PCC point is recovered at 3.1s, so that the amplitude of the direct current, the direct current voltage and the alternating current of the flexible direct current system is similar to that of the non-oscillation suppression strategy during the fault ride through period, as shown in fig. 5, fig. 5 is a fault ride through characteristic effect diagram of a preferred embodiment of the invention, and as can be seen from the diagram, the high-frequency oscillation suppression method provided by the invention cannot deteriorate the fault ride through characteristics of the flexible direct current system.
In summary, in the embodiments of the present invention, by adding a dead zone link to the voltage feedforward signal, or further adding a high-order low-pass filter and a damping correction link to the current feedback signal, the negative resistance region of the flexible dc-to-ac converter in the frequency band above 200Hz can be effectively eliminated, and effective suppression of high-frequency oscillation of the flexible dc power transmission system in the frequency band above 200Hz is achieved. The invention can realize the soft-direct current converter without negative damping area in the frequency band above 200Hz, and can not deteriorate the dynamic response characteristic and the fault ride-through characteristic of the original system.
The embodiments were chosen and described in order to best explain the principles of the invention and the practical application, and not to limit the invention. Any modifications and variations within the scope of the description, which may occur to those skilled in the art, are intended to be within the scope of the invention.
Claims (10)
1. A high-frequency oscillation suppression method for an enhanced flexible direct current transmission system is characterized by being in a voltage feedforward cascade dead zone link.
2. The method for suppressing high-frequency oscillation of the enhanced flexible direct current transmission system according to claim 1, wherein the voltage feedforward is cascaded with a dead zone link, wherein a positive sequence d-axis voltage feedforward dead zone of the dead zone link is around 1pu, positive sequence q-axis voltage feedforward and a negative sequence dq-axis voltage feedforward dead zone of the dead zone link are around 0pu, and the nonlinear transfer function is as follows:
wherein the non-linear voltage transfer function HnThe superscripts p and n respectively represent positive and negative sequence current loops, the superscripts dq respectively represent dq-axis channels, Up dIs the per unit value of the forward-sequence d-axis feedforward voltage, Up qIs the per unit value of the forward-sequence q-axis voltagen dIs the per unit value of the negative sequence d-axis feedforward voltage, Un qIs the per unit value of the negative sequence q-axis feedforward voltage, and d is the size of the dead zone.
3. The method for suppressing high-frequency oscillation of the enhanced flexible direct current transmission system according to claim 1, wherein the dead zone link design step is as follows:
s1, setting the positive sequence d-axis voltage feedforward dead zone to be 0.75pu-1.25pu according to the determined grid-connected rated voltage of the flexible-direct system;
and S2, setting the positive sequence q-axis voltage feedforward dead zone and the negative sequence dq-axis voltage feedforward dead zone to be 0pu-0.25pu according to the determined grid-connected rated voltage of the flexible direct system.
4. An enhanced flexible direct current transmission system high frequency oscillation suppression method according to any one of claims 1-3, characterized by further comprising: and a current feedback cascade damping correction link.
5. The method for suppressing high-frequency oscillation of the enhanced flexible direct current transmission system according to claim 4, wherein the current feedback cascade damping correction link is formed by cascade connection of a third-order low-pass filter and a damping branch, and a transfer function is as follows:
where K is the gain, H1 h、H1 lAnd H3 lRespectively a 1 st order high pass filter, a 1 st order low pass filter and a 3 rd order low pass filter.
6. The method for suppressing high-frequency oscillation of the enhanced flexible direct current transmission system according to claim 4, wherein the feedback current damping correction step comprises the following steps:
s1, establishing a high-frequency simplified impedance model of the MMC current converter, which contains PLL, according to the determined electrical and control parameters of the flexible-straight MMC current converter;
s2, according to the established MMC converter high-frequency simplified impedance model, firstly designing the cut-off frequency f of a third-order low-pass filter of a damping correction link3 lThe high-frequency impedance has a narrow negative resistance frequency band and a small highest negative resistance frequency as a design target to obtain a good high-frequency damping characteristic;
s3: on the basis of a third-order low-pass filter, an active damping link is designed to ensure that the frequency band soft-direct current converter above 200Hz has no negative resistance area.
7. The enhanced flexible direct current transmission system high frequency oscillation suppression method according to claim 6, wherein the high frequency impedance simplified model in S2, wherein the PLL correlation modeling is as follows:
wherein:
Zαβis a 2 nd order impedance matrix in a MMC equipment static alpha beta coordinate system, wherein Tu1p、Tu2p、Tu1n、Tu2nThe sub-type of the MMC equipment alpha beta impedance is formed and is generated by a phase-locked control link; t isip、TinThe other molecular formula of the MMC equipment alpha beta impedance is formed and is generated by a current control link; s (0.5L)arm+LT)I、(0.5Rarm+RT) I is the remainder of the sub-formula forming the MMC equipment alpha beta impedance, which is generated by the AC port circuit structure of the MMC, s is the complex frequency, LarmAnd RarmBridge arm inductance and bridge arm resistance and R being MMCTAnd LTThe leakage reactance and the resistance of the MMC alternating current outlet transformer converted to the MMC side are represented by I, and the I is a 2-order unit array;
(7.d) wherein LT、LarmIs the same as (7.a), ω1Is the fundamental angular frequency, j is a complex number;
(7.e) A inp、Bp、CpAnd An、Bn、CnAre each the component Dp、DnA sub-formula of (1); a. thepAnd AnSub-formula Y contained in5p、Y6p、Y7pAnd Y5n、Y6n、Y7nThe detailed form is shown as (7. f); b ispIs composed of
BnIs composed of
Same as (7. c); cpAnd CnIn (1)
And
the direct current component of the dq axis grid-connected voltage obtained by positive sequence fundamental frequency park conversion is obtained, wherein a superscript p represents the positive sequence park conversion, and 0 represents the direct current component;
and
the direct current component of the dq axis grid-connected voltage obtained by negative sequence fundamental frequency park conversion is obtained, wherein the superscript n represents the negative sequence park conversion, and 0 represents the direct current component;
and
converting the negative sequence double fundamental frequency park to obtain a q-axis grid-connected voltage direct-current component;
and
the direct current component of the dq axis grid-connected voltage is obtained by converting the positive sequence double fundamental frequency park; further, constitution CpAnd CnThe sub-formula of (1): y is1p、Y2p、Y3p、Y4p、Y7p、Y1n、Y2n、Y3n、Y4n、Y7nThe detailed form is shown as (7. f); dpAnd DnSub-formula Gpllp、GpllnIs the open loop transfer function of the phase locked loop, corresponding to a frequency of f + f1And f-f1A variable of (d); at GpllpAnd GpllpIn, Kp_pll、Ki_pllProportional and integral parameters of the PI link of the phase-locked loop, s is complex frequency, j is complex number, omega1Is the fundamental angular frequency;
y in (7.f)1pTo Y7pIs the constitution DpSub-formula of (Y)1nTo Y7nIs the constitution DnA sub-formula of (1); wherein
Is the park transformation of positive sequence double fundamental frequency, the variable frequency after transformation is respectively reduced and increased by double fundamental frequency,
and
the park transformation of negative sequence double fundamental frequency is adopted, and the transformed variable frequency is respectively reduced and increased by double fundamental frequency; hlp(s+3jω)、Hlp(s-3jω)、Hlp(s+jω)、Hlp(s-j ω) is the transfer function of the low-pass filter, corresponding to a frequency of f +3f, respectively1、f-3f1、f+f1、f-f1A variable of (d);
and
the positive second harmonic component of the dq axis grid-connected voltage is obtained by negative sequence park conversion,
and
negative second harmonic component of dq axis grid-connected voltage is obtained by negative sequence park conversion,
and
the positive second harmonic component of the dq axis grid-connected voltage is obtained by the negative sequence double frequency park conversion,
and
the negative sequence double frequency park conversion is carried out to obtain the negative second harmonic component of the dq axis grid-connected voltage,
same as in (7. e);
same as in (7. c).
8. The enhanced flexible direct current transmission system high frequency oscillation suppression method according to claim 6, wherein the high frequency impedance in S2 is a simplified model, wherein the positive and negative sequence current loop and related modeling are as follows:
z in (8.a)αβIs a 2 nd order impedance matrix in a MMC equipment static alpha beta coordinate system, wherein Tu1p、Tu2p、Tu1n、Tu2nThe sub-type of the MMC equipment alpha beta impedance is formed, the sub-type is generated by a phase-locked control link, and a specific expansion expression of the sub-type is shown as (7. b); t isip、TinIs another molecular formula for forming the alpha beta impedance of the MMC equipment, which is generated by a current control link, Tip、TinThe specific expansion expression of (2) is shown as (8. b); s (0.5L)arm+LT)I、(0.5Rarm+RT) I is the remainder of the sub-formula forming the MMC equipment alpha beta impedance, which is generated by the AC port circuit structure of the MMC, s is the complex frequency, LarmAnd RarmBridge arm inductance and bridge arm resistance and R being MMCTAnd LTIs leakage reactance and resistance converted from MMC AC outlet transformer to MMC side, I is 2-order unit array, j is complex number, ZpnIs a modified sequence impedance of an MMC installation, which is a 2 x 2 square matrix of impedances, ZppIs that the MMC equipment ignores the positive sequence impedance of the positive and negative sequence coupling, which is equal to the first row, first column element Z in the modified sequence impedance matrixpn(1，1)；
In (8.b)
Representing the inverse transformation of the positive sequence park, the superscript theta represents the positive sequence, the superscript-represents that the frequency domain component after the inverse transformation is reduced by 1 time of fundamental frequency, and the superscript + represents that the frequency domain component after the inverse transformation is increased by 1 time of fundamental frequency;
representing negative sequence park transformation, superscript-theta representing negative sequence, superscript-representing a 1-fold decrease in fundamental frequency of the frequency domain component after inverse transformation, and superscript + representing a 1-fold increase in fundamental frequency of the frequency domain component after inverse transformation. In a stationary coordinate system, a variable with an arbitrary frequency f, which is passed through a fundamental frequency f1After the park transformation or the park inverse transformation, the frequency deviation of plus and minus one harmonic is generated, namely f +/-f1For LTI systems, the frequency is f + f1The transfer function complex frequency independent variable corresponding to the variable is s + j omega1S is the complex frequency, j is the complex number, ω1Is the fundamental angular frequency with a frequency f-f1The transfer function complex frequency independent variable corresponding to the variable is s-j omega1；Gd(s+jω1) Corresponding to a frequency of f + f1Time-delayed transfer function of variable, Gd(s-jω1) Corresponding to a frequency f-f1A time-delayed transfer function of the variable; hi(s+jω1) Corresponding to a frequency of f + f1Current loop PI transfer function of variable, Hi(s-jω1) Corresponding to a frequency f-f1A current loop PI transfer function of the variable; hc(s+jω1) Corresponding to a frequency of f + f1Damping of variables correcting the link transfer function, Hc(s-jω1) Corresponding to a frequency f-f1Current loop PI transfer function of the variable.
9. An enhanced flexible direct current transmission system high-frequency oscillation suppression system for realizing the high-frequency oscillation suppression method according to any one of claims 1 to 8, characterized by comprising the following steps:
the voltage feedforward module is used in a voltage feedforward cascade dead zone link; or, further comprising:
the current feedback module is used for cascading a damping correction link in current feedback;
the voltage feedforward module adopts a dead zone link H in voltage feedforwardnThe voltage feedforward compensation circuit is used for eliminating the negative damping characteristic introduced by voltage feedforward under small disturbance and has a quick response characteristic under large disturbance;
the current feedback module adopts a damping correction link H in current feedbackcFor eliminating the negative damping characteristic introduced by the control delay.
10. An enhanced flexible direct current transmission system high frequency oscillation suppression system according to claim 9, comprising:
the DDSRF-PLL module is used for directly collecting grid-connected voltage as input, realizing phase locking on the power frequency voltage phase theta of the positive sequence of the power grid voltage through a double-decoupling synchronous rotating coordinate system phase-locked loop and providing angles of park transformation and park inverse transformation for the power loop module, the power measurement module, the positive and negative sequence current control loop module, the CCSC module, the 1/4 power frequency period delay module and the modulation module;
a power loop module for feeding back the deviation between the reference power and the actual power via a PI controller HpqControlling to realize the error-free tracking of power reference and generating the dq-axis current reference of the positive and negative sequence current control loop module;
the power measurement module is used for calculating the actual output power of the MMC equipment module by collecting the dq axis component of the positive sequence voltage and current and feeding the actual output power back to the power loop module;
a positive and negative sequence current control loop module including a voltage feedforward module and a current feedback module for subtracting the feedback current from the dq-axis current reference generated by the power loop module and passing through a current loop PI controller HiPerforming a homodyne control and generating a positive-negative sequence dq-axis modulation voltage, the dq-axis current passing through a value corresponding to omega1(0.5Larm+LT) Decoupling by equal feedforward gain link, and adopting dead zone link H in voltage feedforward modulenEliminating negative damping characteristic introduced by voltage feedforward under small disturbance, and having quick response characteristic under large disturbance, and adopting damping correction link H in current feedback modulecEliminating negative damping characteristics introduced by control delay; omega1Is the angular frequency of the fundamental frequency, LarmIs bridge arm reactance, LTIs the leakage reactance of the transformer;
1/4 power frequency period delay module, which realizes the positive and negative sequence separation and extraction of voltage and current collection signal through the set delay, and realizes the transformation of positive and negative sequence voltage and current from the static coordinate system to the rotating coordinate system through the park transformation module of positive and negative theta, and provides the feedback of the positive and negative sequence current of dq axis and the feedforward of the positive and negative sequence voltage of dq axis for the positive and negative sequence current control loop module;
CCSC module, through PI control link HCCThe suppression of double frequency circulation dq axis components inside the MMC is realized, and three-phase double frequency circulation suppression modulation voltage is generated;
and the modulation module is used for carrying out inverse park conversion on the dq-axis modulation voltage generated by the positive-negative sequence current control loop module to generate an alpha beta component under a static coordinate system and superposing the alpha beta component, further generating a three-phase fundamental frequency modulation voltage through Clark conversion, and superposing the three-phase fundamental frequency modulation voltage and a three-phase double-frequency circulating current restraining modulation voltage generated by the CCSC module to generate the final MMC three-phase modulation voltage.
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| CN116436042A (en) * | 2023-06-13 | 2023-07-14 | 昆明理工大学 | Wind-water-fire system stability analysis method considering wind turbine frequency modulation dead zone |
| CN116436042B (en) * | 2023-06-13 | 2023-08-15 | 昆明理工大学 | Wind-water-fire system stability analysis method considering wind turbine frequency modulation dead zone |
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