CN115995828A - Subsynchronous oscillation suppression method and system for virtual synchronous doubly-fed fan through series compensation grid-connected system - Google Patents
Subsynchronous oscillation suppression method and system for virtual synchronous doubly-fed fan through series compensation grid-connected system Download PDFInfo
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Abstract
The invention discloses a subsynchronous oscillation suppression method and a subsynchronous oscillation suppression system of a virtual synchronous double-fed fan through a series compensation grid-connected system, wherein the method comprises the following steps: acquiring a VSG-based network side converter output impedance expression; revealing positive damping characteristics of the grid-side converter based on VSG on subsynchronous oscillation from the equivalent impedance angle of the doubly-fed wind turbine grid-connected system; analyzing a subsynchronous oscillation suppression mechanism of the virtual synchronous double-fed fan grid-connected system by combining active disturbance rejection control; and constructing an improved active disturbance rejection controller to perform subsynchronous oscillation suppression. The subsynchronous oscillation analysis method adopted by the invention has certain universality and can provide a theoretical analysis basis for subsynchronous oscillation problems caused by participation of high-proportion power electronic devices in a new energy power generation system. Meanwhile, the suppression method provided by the invention is not only suitable for the doubly-fed generator set, but also suitable for suppressing subsynchronous oscillation in a direct-drive fan and a weak alternating-current system.
Description
Technical Field
The invention relates to a subsynchronous oscillation suppression method and a subsynchronous oscillation suppression system for a virtual synchronous double-fed fan through a series compensation grid-connected system, and belongs to the field of renewable energy power generation systems.
Background
The Virtual Synchronous Generator (VSG) simulates the inertia and frequency response characteristics of the synchronous generator, and can overcome the defects of low inertia and weak damping of a high-proportion power electronic wind power generation system. The virtual synchronous double-fed fan has wide application prospect due to dynamic change of active support inertia frequency. However, the virtual synchronous control access causes the interaction characteristic of the doubly-fed fan and the power grid to be more complex, aggravates the dynamic characteristic of the system subsynchronous oscillation, and influences the stability level of the system subsynchronous oscillation. Therefore, it is needed to construct a subsynchronous oscillation suppression strategy suitable for the virtual synchronous double-fed fan grid-connected system, and improve the safe and stable operation capacity of the wind power system.
In the prior art, a great deal of literature expands analysis on parameter optimization, oscillation filtering, additional damping and other aspects aiming at research on a subsynchronous oscillation suppression strategy of a wind power system. The parameter optimization is mainly realized by optimizing and setting the parameters of the power electronic converter, so that the damping of the system at the resonance frequency can be effectively increased, and the system oscillation instability is avoided. But is limited by the requirements of stable operation boundary of the unit, and the adjustable range of the control parameters of the fan converter is limited. The oscillation filtering mainly designs a band-pass filter or a control structure with a filtering effect in the control part, filters out subsynchronous oscillation components contained in the control link, and further eliminates the interaction coupling of the machine network under the subsynchronous oscillation frequency band. However, such filter structure parameters are designed for a certain fixed resonance frequency point, and are only suitable for a single oscillation scenario, and if the operation condition of the power grid changes, the suppression strategy is difficult to effectively maintain the stable operation of the system. The additional damping is mainly introduced into an additional damping controller in the converter control link so as to improve the system damping and achieve the purpose of suppressing oscillation. The disadvantage of this type of damping strategy is that the design of the additional damping process is complicated and the additional damping controller parameters need to be reset once the system operating conditions change. Therefore, the multi-working-condition fan oscillation suppression strategy has important practical significance for wind power output. In addition, the existing control strategies are mainly focused on the design of the traditional double-fed fans, and the control structure of the virtual synchronous double-fed fans is greatly different from that of the traditional fans, so that the design ideas of the control strategies are different. Therefore, the problem of subsynchronous oscillation suppression of the virtual synchronous double-fed fans under multiple working conditions needs to be solved.
At present, many scholars develop small-interference stability analysis on the equipment level mainly around control model simplification, control strategy reconstruction and virtual synchronous control after control parameter optimization on the dynamic characteristic research of VSG. The scholars design a synchronous generator simplified equivalent model based on the VSG converter, and the dynamic characteristics and the operation standard of the synchronous generator are considered. The scholars propose a low voltage ride through control strategy based on VSG, which is easy to analyze the stability of the multi-terminal high voltage direct current system of the voltage source converter, so that the network side voltage source converter can provide frequency support and oscillation damping. The frequency variation parameter is introduced into the active loop control by a learner, the VSG grid-connected parameter is set by means of the self-adaptive moment of inertia control, the dynamic frequency adjustment capacity of the power grid is improved, and the parameter design process is complex. In summary, the existing literature lacks reasonable elucidation of the occurrence cause of the subsynchronous oscillation of the virtual synchronous double-fed fan, and is difficult to effectively inhibit the system oscillation from the source.
Disclosure of Invention
The invention provides a method and a system for suppressing subsynchronous oscillation of a virtual synchronous doubly-fed fan through a series compensation grid-connected system, which are used for analyzing the subsynchronous oscillation generation mechanism of the system under the control of VSG and combining with the self-immunity control to analyze the subsynchronous oscillation suppression mechanism of the virtual synchronous doubly-fed fan grid-connected system, so as to construct an improved self-immunity controller conforming to the mechanism; further, the stability of the designed improved active disturbance rejection controller is discussed; still further, different operating conditions are designed to verify the robustness of the proposed suppression strategy of the present invention.
The technical scheme of the invention is as follows: a subsynchronous oscillation suppression method of a virtual synchronous doubly-fed fan through a series compensation grid-connected system comprises the following steps: acquiring a VSG-based network side converter output impedance expression; revealing positive damping characteristics of the grid-side converter based on VSG on subsynchronous oscillation from the equivalent impedance angle of the doubly-fed wind turbine grid-connected system; analyzing a subsynchronous oscillation suppression mechanism of the virtual synchronous double-fed fan grid-connected system by combining active disturbance rejection control; and constructing an improved active disturbance rejection controller to perform subsynchronous oscillation suppression.
The obtaining the VSG-based network side inverter output impedance expression includes:
s1.1, adopting a power system small interference stability analysis method, realizing linearization of a VSG dynamic equation based on a control topological structure of the VSG, wherein a mathematical expression of a linearization result is as follows:
wherein F is PQ 、F PI 、
F 1 、F 2 、F L Are all 2 multiplied by 2 transfer function matrices; t (T) J And D p The moment of inertia and the damping coefficient are respectively; k (K) p And K i The excitation control proportion coefficient and the integral coefficient are respectively; k (K) v Controlling droop coefficients for reactive power-voltage; s is the Laplace operator; k (K) Vp1 And K Vi1 The d-axis proportion coefficient and the integral coefficient are respectively controlled for VSG output voltage; k (K) Vp2 And K Vi2 The q-axis proportion coefficient and the integral coefficient are respectively controlled for VSG output voltage; i sd And I sq D-axis component and q-axis component of steady-state quantity of stator current under synchronous rotation coordinate system; u (U) sd And U sq D-axis component and q-axis component of steady-state working point of stator voltage under synchronous rotation coordinate system; e (E) m Controlling the output voltage for excitation; delta is the difference between the phase-locked loop output phase angle and the grid-connected system voltage phase; l is a filter inductance; u (U) g Is the stable value of the network side voltage; omega N Is the synchronous angular velocity;
s1.2, acquiring VSG control parameters and system network operation parameters in a stable state, and taking a dq rotating coordinate system as a reference system to obtain a network side converter output impedance expression based on VSG, wherein the output impedance expression is as follows:
wherein [ Deltau ] sd ,Δu sq ] T The voltage disturbance vector is a grid-connected system stator side voltage disturbance vector; [ delta ] i sd ,Δi sq ] T The current response vector is the stator side current response vector of the grid-connected system; z is Z out Is a VSG-based grid-side inverter output impedance; z is Z dd 、Z dq 、Z qd And Z qq Respectively represent the output impedance Z out D-d, d-q, q-d, and q-q axis components of (a); i is denoted as identity matrix.
The positive damping characteristic of the grid-side converter based on VSG to subsynchronous oscillation is revealed from the equivalent impedance angle of the doubly-fed fan grid-connected system, and the method comprises the following steps:
s2.1, equivalent output impedance Z of doubly-fed wind turbine G The mathematical expression of (2) is:
wherein R is G And X G The equivalent resistance and reactance of the doubly-fed wind turbine are respectively; r is R s Is the sum of the doubly-fed fan stator winding and the box-type variable resistor; r is R r Resistance of the rotor winding; r is R RSC Representing the rotor-side converter equivalent resistance; l (L) s Is the sum of the doubly-fed fan stator winding and the leakage inductance of the box transformer; l (L) lr Is rotor leakage inductance; l (L) m The excitation inductance is adopted; slip ratio
f r And j are expressed in rotor frequency and imaginary units, respectively; resonance frequency of inductance and capacitance series resonance circuit>
ω ss And f 0 Respectively the system equivalent series loop resonant frequency and the system reference frequency under the subsynchronous oscillation; x is X C 、X L The series compensation capacitance capacitive reactance and the system equivalent reactance are respectively;
s2.2, after the VSG is connected into the network side converter control of the doubly-fed fan, obtaining the equivalent output impedance Z of the doubly-fed fan based on the VSG G1 The expression is:
wherein R is VSG And X VSG The equivalent resistance and reactance of VSG are respectively; r is R G1 And X G1 And the equivalent resistance and the reactance of the doubly-fed wind turbine after being connected with the VSG are respectively.
The sub-synchronous oscillation suppression mechanism of the virtual synchronous double-fed fan grid-connected system is analyzed by combining active disturbance rejection control, and the sub-synchronous oscillation suppression mechanism comprises the following components:
taking d-axis analysis as an example, the mathematical expression of the output voltage of the current inner loop of the VSG connected to the lower network side converter is as follows:
wherein u is gd Outputting a voltage d-axis component for the grid-side converter; r is R g And L g The resistor and the inductor of the network side line reactor are respectively; i.e gd And i gq D-axis and q-axis components of the input current of the power grid are respectively; omega N Is the synchronous angular velocity; e (E) VSGd Outputting a voltage d-axis component for the VSG;
when the grid-connected system generates subsynchronous oscillation, the current inner loop output voltage expression of the grid-side converter under disturbance is rewritten as follows:
wherein Deltau sub_gd And delta E sub_VSGd The d-axis components of the output subsynchronous voltage and the VSG output subsynchronous voltage of the disturbance lower network side converter are respectively obtained; Δi sub_gd And Δi sub_gq Respectively the d and q axis components of the subsynchronous current;
analyzing the influence rule of the current inner loop PI control parameter of the grid-side converter on the subsynchronous oscillation to obtain the mathematical expression of the current inner loop output voltage of the grid-side converter based on PI control, wherein the mathematical expression is as follows:
wherein K is p2 And K i2 D-axis proportion coefficient and integral coefficient of the PI controller in the current inner loop control of the grid-side converter are respectively; s is the Laplace operator;
when the grid-connected system stably operates, the d-axis current expression of the grid-side converter under VSG control is obtained as follows:
when the system has a subsynchronous oscillation component, subsynchronous oscillation flows in a network side converter control link in the form of subsynchronous current and voltage components, so that the d-axis current of the network side converter becomes variable:
the equation transforms as follows:
wherein f 1 (Δi sub_gd ,Δi sub_gq ) The d-axis component of the subsynchronous current in the grid-connected system;
The d-axis current mathematical expression of the network side converter is obtained by adopting the front and back of the active disturbance rejection controller:
the q-axis is the same.
The construction of the improved active disturbance rejection controller comprises the following steps:
s4.1, replacing a PI controller of a current inner loop of the grid-side converter with an active disturbance rejection controller in order to inhibit subsynchronous oscillation of the doubly-fed wind turbine grid-connected system under VSG control, and rewriting a mathematical expression of the current of the grid-side converter into:
wherein i is gd And i gq D-axis and q-axis components of the input current of the power grid are respectively; u (u) gd And u gq The d-axis component and the q-axis component of the output voltage of the grid-side converter are respectively; b d And b q D-axis and q-axis components of the dynamic compensation factor, respectively; f (f) 1 (Δi sub_gd ,Δi sub_gq ) The d-axis component of the subsynchronous current in the grid-connected system; f (f) 2 (Δi sub_gd ,Δi sub_gq ) Represented as a subsynchronous current q-axis component in a grid-connected system; r is R g And L g The resistor and the inductor of the network side line reactor are respectively; omega N Is the synchronous angular velocity; e (E) VSGd Outputting a voltage d-axis component for the VSG;
it is known that the current inner loop of the grid-side converter controls the d axis to be coupled with the q axis; in order to realize accurate decoupling control of the output current of the network-side converter, the coupling term is included in the disturbance, and then the method is changed into the following steps:
wherein F is 1 (Δi sub_gd ,Δi sub_gq ) And F 2 (Δi sub_gd ,Δi sub_gq ) D-axis component and q-axis component of the summation of subsynchronous disturbance quantity and coupling quantity are respectively controlled for the network-side converter;
S4.2, constructing an improved active disturbance rejection controller, which comprises the following steps: an extended state observer, a linear error feedback LSEF;
s4.3, improving the performance analysis of the active disturbance rejection controller.
The extended state observer ESO is formed by placing a Fal function filter after the virtual synchronous doubly-fed fan is output by the series compensation grid-connected system and combining the extended state observer ESO with the traditional extended state observer ESO.
The method also comprises the verification step of: the verification proves that the improved active disturbance rejection controller strategy has subsynchronous oscillation suppression effects under different disturbance working conditions.
The different disturbance conditions include: different fan numbers, wind field wind speeds, series compensation degrees and grounding fault types.
According to another aspect of the present invention, there is also provided a subsynchronous oscillation suppression system of a virtual synchronous doubly-fed wind turbine via a series compensation grid-connected system, including: the first construction module is used for placing the Fal function filter after the virtual synchronous doubly-fed fan is output by the series compensation grid-connected system and constructing an extended state observer together with the ESO of the traditional extended state observer; the second construction module is used for constructing an improved active disturbance rejection controller according to the extended state observer and the linear error feedback LSEF; and the suppression module is used for suppressing subsynchronous oscillation of the virtual synchronous doubly-fed fan through the series compensation grid-connected system by adopting the improved active disturbance rejection controller.
Further comprises: and the verification module is used for verifying the subsynchronous oscillation suppression effect of the improved active disturbance rejection controller strategy under different disturbance working conditions.
The beneficial effects of the invention are as follows: the method disclosed by the invention reveals the influence mechanism of VSG on subsynchronous oscillation from the equivalent impedance of the doubly-fed fan grid-connected system, and on the basis of the analysis of the traditional subsynchronous oscillation mechanism, the method is used for mainly analyzing the action rule of the output impedance of the VSG on the equivalent impedance of the system, so that the method has stronger reference analysis and reference significance on the subsynchronous oscillation problem of the VSG applied to the direct-driven fan and the flexible direct-current weak power grid, and has stronger expansibility and extensibility. Aiming at the problem that VSG can not provide sufficient positive damping for the system, the invention deduces the subsynchronous oscillation suppression mechanism of the virtual synchronous double-fed fan grid-connected system combined with the active disturbance rejection control, and the analysis method has the advantages of clear physical meaning, clear deducing process and the like; in addition, the subsynchronous oscillation suppression strategy provided by the invention has stronger robustness to disturbance working conditions such as different serial supplements, wind speeds, fan numbers, grounding short circuit faults and the like, and can provide a certain theoretical analysis basis for the positive application of VSG to a doubly-fed fan grid-connected system. In conclusion, the subsynchronous oscillation analysis method adopted by the invention has certain universality and can provide a theoretical analysis basis for subsynchronous oscillation problems caused by participation of high-proportion power electronic devices in a new energy power generation system. Meanwhile, the suppression method provided by the invention is not only suitable for the doubly-fed generator set, but also suitable for suppressing subsynchronous oscillation in a direct-drive fan and a weak alternating-current system.
Drawings
FIG. 1 is a topology diagram of a main circuit of a virtual synchronous doubly-fed wind turbine grid-connected system according to one embodiment of the present invention;
FIG. 2 is a diagram illustrating a small signal output impedance model of an inverter under VSG control according to an embodiment of the present invention;
FIG. 3 is a diagram of an equivalent circuit of a small signal of an inverter in a dq rotation coordinate system according to an embodiment of the present invention;
fig. 4 is a diagram illustrating an output impedance bode of a VSG-controlled down-side converter according to an embodiment of the present invention;
FIG. 5 is a block diagram of a first order ADRC of an embodiment of the present invention;
FIG. 6 is a topology diagram of a function filter according to an embodiment Fal of the invention;
fig. 7 is a schematic diagram of a doubly fed fan grid-side converter rejection strategy based on active disturbance rejection control according to an embodiment of the present invention;
FIG. 8 is a test model of a series compensated grid-connected system of a doubly-fed wind turbine based on VSG control in accordance with an embodiment of the present invention;
FIG. 9 is a graph showing comparison between active output and current FFT analysis of a system before and after VSG access according to an embodiment of the present invention; the method comprises the steps of (a) outputting active power for a wind power plant under different serial supplements before and after the VSG is connected, and (b) analyzing a graph for current FFT in the wind power plant under different serial supplements before and after the VSG is connected;
FIG. 10 is a graph showing the comparison of response curves of a PI controller and an auto-disturbance-rejection controller under 0.4 series compensation according to an embodiment of the present invention; wherein, (a) is an active output waveform diagram of the wind power plant under different control, (b) is a reactive output waveform diagram of the wind power plant under different control, and (c) is a 690V-side A-phase current waveform diagram of the wind power plant under different control;
FIG. 11 is a graph showing the comparison of response curves of a PI controller and an auto-disturbance-rejection controller under 0.6 series compensation according to an embodiment of the present invention; wherein, (a) is an active output waveform diagram of the wind power plant under different control, (b) is a reactive output waveform diagram of the wind power plant under different control, and (c) is a 690V-side A-phase current waveform diagram of the wind power plant under different control;
FIG. 12 is a graph showing the response of active power under different measures when the number of fans is changed according to an embodiment of the present invention; wherein, (a) is the active output of the wind power plant under different control when the fan is 80, (b) is the active output of the wind power plant under different control when the fan is 67, and (c) is the active output of the wind power plant under different control when the fan is 50;
FIG. 13 is a graph showing the response of active power under different measures of suppression when the wind speed changes according to an embodiment of the present invention; wherein, (a) is the active output of the wind power plant under different control at the wind speed of 11m/s, (b) is the active output of the wind power plant under different control at the wind speed of 10m/s, and (c) is the active output of the wind power plant under different control at the wind speed of 9 m/s;
FIG. 14 is a graph showing the response of active power under different suppression measures for different ground short faults according to an embodiment of the present invention; the method comprises the steps of (a) outputting wind power plant active power under different control when a single-phase grounding short circuit fault exists, (b) outputting wind power plant active power under different control when a two-phase grounding short circuit fault exists, and (c) outputting wind power plant active power under different control when a three-phase grounding short circuit fault exists.
Detailed Description
The invention will be further described with reference to the drawings and examples, but the invention is not limited to the scope.
Example 1: as shown in fig. 1-14, a method for suppressing subsynchronous oscillation of a virtual synchronous doubly-fed fan through a series compensation grid-connected system includes: acquiring a VSG-based network side converter output impedance expression; revealing positive damping characteristics of the grid-side converter based on VSG on subsynchronous oscillation from the equivalent impedance angle of the doubly-fed wind turbine grid-connected system; analyzing a subsynchronous oscillation suppression mechanism of the virtual synchronous double-fed fan grid-connected system by combining active disturbance rejection control; and constructing an improved active disturbance rejection controller to perform subsynchronous oscillation suppression.
Further, an alternative implementation of the present invention is described below:
the invention selects the virtual synchronous double-fed fan grid-connected system shown in figure 1 as a topological structure for case implementation. The embodiment of the invention adopts a double-fed wind generating set, a rotor side converter (rotor side converter), a net side converter (net side converter), a direct current filter capacitor, a VSG controller, a phase-locked loop and a series compensation network. And voltage feedforward control is added to the grid-side converter on the basis of a conventional double-loop control strategy so as to realize virtual synchronous control of the double-fed fan. The specific process is that VSG control output voltage is connected to the current inner loop control of the network side converter of the doubly-fed fan and used as a feedforward signal for feedforward control. The direct current filter capacitor between the rotor side converter and the grid side converter forms a direct current link, and the LCL filter of the grid side converter is used as an output end to be connected with a power grid.
The derivation and analysis of the present invention will be described in further detail with reference to the accompanying drawings.
Specifically, the operation parameters and control parameters in this example can be expressed as follows:
table 1 single double-fed fan parameters
| Parameters (parameters) | Numerical value and unit |
| Rated power | 1.5MW |
| Rated frequency | 50H z |
| Rated voltage of stator | 690V |
| Stator resistor R s | 0.023p.u. |
| Stator leakage inductance L ls | 0.018p.u. |
| Rotor resistance R r | 0.016p.u. |
| Rotor leakage inductance L lr | 0.16p.u. |
| Excitation inductance L m | 2.9p.u. |
| DC voltage U dc | 1.15kV |
| 0.69/35kV field transformer X T1 | 0.06 |
| 0.69/35kV field transformer R T1 | 0.015 |
Table 2 doubly-fed wind turbine controller parameters
Table 3 transformer and line parameters
| Parameters (parameters) | Numerical value and unit | Parameters (parameters) | Numerical value and unit |
| 220kV line resistor R L1 | 0.01p.u. | 35/200kV transformer reactance X T2 | 0.06p.u. |
| 220kV line reactance X L1 | 0.1p.u. | 220/500kV transformer reactance X T3 | 0.14.p.u |
| 500kV line resistor R L2 | 0.005p.u. | Series compensation capacitor C under 0.4 series compensation degree L2 | (0.0962e-03)F |
| 500kV line reactance X L2 | 0.033 p .u. |
Note that the base capacity is 100MW
Referring to fig. 2-14, the method for suppressing subsynchronous oscillation of a virtual synchronous doubly-fed fan through a series compensation grid-connected system and a control system provided by the invention comprise the following specific steps:
step S1, deducing a network side converter output impedance expression based on a virtual synchronous generator VSG based on a small signal model of the VSG;
step S2, revealing the positive damping characteristic of VSG on subsynchronous oscillation from the equivalent impedance angle of the doubly-fed fan grid-connected system;
Step S3, deducing a mechanism for inhibiting the subsynchronous oscillation by combining the active disturbance rejection control in view of the fact that the effective inhibition of the subsynchronous oscillation cannot be realized due to the small output impedance of the VSG;
step S4, designing Fal function filters to improve and optimize output voltage waveforms, and constructing all parts of the active disturbance rejection control in detail so as to discuss the stability of the designed controller;
and S5, comparing and analyzing the traditional PI control, and verifying the adaptability of the proposed strategy to the next synchronous oscillation under different disturbance working conditions.
The step S1 specifically comprises the following steps:
s1.1, adopting a small interference stability analysis method of a power system, and realizing linearization of a dynamic equation of the VSG based on a control topology mechanism of the VSG. The control principle of VSG can be expressed as: VSG simulates the moment of inertia, primary frequency modulation and voltage regulation characteristics in synchronous generators, providing the necessary voltage and frequency support for the grid. Unlike synchronous generator, VSG can design active power and reactive power input independently according to grid frequency and voltage, has friendly grid connection, mainly comprises three parts of active loop control, excitation control and output voltage control, wherein the mathematical expression of active loop control and excitation control can be expressed as:
Wherein T is J And D p The moment of inertia and the damping coefficient are respectively; k (K) f And K v Active power-frequency control droop coefficients and reactive power-voltage control droop coefficients, respectively; p (P) ref And P set The droop control active power reference value and the droop control active power set value are respectively; f (f) N And f v Respectively a reference frequency and a synchronization frequency of a grid-connected system; u (u) rms And u N Mean square of grid-side voltages of grid-connected systemsValues and voltage ratings; omega N And omega v The reference angular frequency and the VSG output angular frequency of the grid-connected system are respectively; k (K) p And K i The method is divided into an excitation control proportion coefficient and an integral coefficient; q (Q) set And u ref The reactive power set value and the reactive power-voltage control voltage reference value of the grid-connected system are respectively; lambda (lambda) 1 Outputting a phase angle scaling factor for the VSG; θ m And E is m The output phase angle of the active control of VSG and the output voltage of the excitation control are respectively.
VSG controlled input active Power P m And reactive power Q m Can be expressed by a power equation as:
wherein u is d 、u q Respectively the dq axis components of the power grid voltage; i.e d 、i q The net side current dq axis components, respectively.
VSG outputs three-phase voltage e mi (i=a, b, c) is the output θ controlled by the active loop m And excitation control output E m The common decision can be realized by a synchronous equation, and the mathematical expression is as follows:
and when the VSG is stable, small interference is connected and local linearization processing is carried out, so that a small signal output impedance model of the converter at the lower network side under the control of the VSG can be obtained, as shown in fig. 2.
Further, in the small signal model shown in FIG. 2, F PQ 、F PI 、F 1 、F L 、F P u Q 、F P i Q 、F 2 Represented as a 2 x 2 order transfer function matrix. Wherein:
wherein F is PQ 、F PI 、F 1 、F L 、
F 2 A transfer function matrix of order 2 x 2; f (F) PQ For power vector to vector [ delta theta ] m ,ΔE m ] T Is a transfer function matrix of (a); f (F) PI A PI control transfer function matrix; f (F) 1 Is the vector [ delta theta ] m ,ΔE m ] T To vector [ delta E ] d ,ΔE q ] T Is a transfer function matrix of (a); f (F) L Is the vector [ delta E ] VSGd ,ΔE VSGq ] T To the current response vector [ delta ] i sd ,Δi sq ] T Is a transfer function matrix of (a); />
And->
A small signal transfer function matrix for power calculation; f (F) 2 Is the voltage disturbance vector [ delta ] u sd ,Δu sq ] T To [ delta E ] d ,ΔE q ] T Is used for the transfer function matrix of the (a). T (T) J And D p The moment of inertia and the damping coefficient are respectively; k (K) p And K i The method is divided into an excitation control proportion coefficient and an integral coefficient; q (Q) set And u ref The reactive power set value and the reactive power-voltage control voltage reference value of the grid-connected system are respectively; θ m And E is m Respectively VSG active control output phase angle sumExcitation control output voltage; k (K) Vp1 And K Vi1 The d-axis proportion coefficient and the integral coefficient are respectively controlled for VSG output voltage; k (K) Vp2 And K Vi2 The q-axis proportion coefficient and the integral coefficient are respectively controlled for VSG output voltage; u (U) sd And U sq Respectively the steady-state working points of stator voltage under a synchronous rotation coordinate system; i sd And I sq Respectively the steady-state working points of stator current under a synchronous rotation coordinate system; l is a filter inductance; delta is the difference between the phase-locked loop output phase angle and the grid-connected system voltage phase; u (U) g Is the stable value of the network side voltage; s is the Laplace operator; omega N For synchronizing angular velocity.
S1.2 due to active power P in the whole control process ref Reactive power Q ref The command signal is unchanged, and the power command disturbance term can be considered as 0, and the direct current bus voltage is assumed to be constant. The control and operation parameters during VSG stabilization are obtained, as shown in tables 1-2, by taking dq rotation coordinates as a reference system, and combining with the converter small signal circuit diagram 3, the network side converter output impedance expression is obtained as follows:
wherein [ Deltau ] sd ,Δu sq ] T The method comprises the steps of defining the voltage disturbance quantity of a grid-connected system stator side; [ delta ] i sd ,Δi sq ] T The current response vector is the stator side current response vector of the grid-connected system; z is Z out Outputting impedance for the grid-side converter; z is Z dd 、Z dq 、Z qd And Z qq Respectively represent the output impedance Z out D-d, d-q, q-d, and q-q axis components of (c).
Further, the output impedance expression of the VSG-based grid-side inverter is derived as:
wherein I is expressed as an identity matrix.
Taking a VSG with a rated power of 1500kW as an example, the frequency characteristic of the output impedance of the VSG is analyzed, wherein the rated active power p=1500 kW, the rated reactive power q=0 kvar, the switching frequency=10 kHz, the filter inductance l=4.5 mH, and the rest control parameters are specifically shown in tables 1-2. The output impedance bode diagram of the VSG controlled lower net side converter can be obtained as shown in fig. 4.
As can be seen from fig. 4, the output impedance of the network inverter based on the VSG control at the reference frequency is positive, and exhibits a weak impedance characteristic. Analysis of the relationship between the output impedance of the VSG-controlled down-grid-side converter in the dq-axis direction can be regarded as Z dd Equivalent impedance Z to q-axis versus d-axis qd Amplitude is approximately equal, Z qq Equivalent impedance Z to d-axis versus q-axis dq The magnitudes are approximately equal. And corresponding impedance values are respectively overlapped on the dq axes, so that dq approximate decoupling can be realized. Further from the perspective of synchronous resistance and reactance characteristics of the synchronous generator, the output impedance of the VSG-controlled lower network side converter can be expressed as Z VSG ＝R VSG +jX VSG 。
The step S2 specifically comprises the following steps:
s2.1, deducing equivalent impedance of the doubly-fed fan grid-connected system. The capacitive reactance of the series capacitor in the doubly-fed wind turbine grid-connected system and the inductive reactance of the system are connected in series to form an inductance and capacitance series resonant circuit. Based on the prior literature, the subsynchronous oscillation generation and the equivalent impedance of the doubly-fed fan have a direct corresponding relation. Wherein, the equivalent impedance Z of the doubly-fed fan under subsynchronous oscillation G The mathematical expression of (c) can be expressed as:
wherein R is G And X G The equivalent resistance and reactance of the doubly-fed wind turbine are respectively; r is R s Is the sum of the doubly-fed fan stator winding and the box-type variable resistor; r is R r Resistance of the rotor winding; r is R RSC Representing the rotor-side converter equivalent resistance; x is X ls Dividing the doubly-fed wind machine into a stator winding and a box-type variable resistance leakage inductance; x is X lr Is rotor leakage reactance; x is X m The excitation inductance is adopted; slip ratio
f r Is the rotor frequency; resonance frequency of inductance and capacitance series resonance circuit>
j represents an imaginary unit; z is Z G The equivalent output impedance expression of the doubly-fed wind turbine is shown; omega ss And f 0 Respectively the system equivalent series loop resonant frequency and the system reference frequency under the subsynchronous oscillation; x is X C And X L The series compensation capacitance capacitive reactance and the system equivalent reactance are respectively.
It can be seen that the output impedance of the doubly-fed wind turbine is affected by the slip and the equivalent resistance of the rotor-side converter. Because the slip is negative, when the rotor resistance and the equivalent resistance of the rotor-side converter are greater than the vector addition sum of the rest resistances, the equivalent resistance of the system is represented as negative resistance, so that the inductance and capacitance resonant circuits continuously diverge and oscillate, and subsynchronous oscillation occurs.
S2.2, after the VSG is connected into the network side converter control of the doubly-fed fan, obtaining an equivalent output impedance expression of the doubly-fed fan based on the VSG, wherein the equivalent output impedance expression is as follows:
wherein R is VSG And X VSG The equivalent resistance and reactance of VSG are respectively; r is R G1 And X G1 The equivalent resistance and reactance of the doubly-fed wind turbine after being connected with the VSG are respectively; z is Z G1 Is equivalent output impedance of the doubly-fed fan based on VSG control.
Therefore, the VSG access can reduce the negative impedance degree of the system, so that the virtual synchronous control mode of the doubly-fed fan based on the grid-side converter VSG has an inhibiting effect on subsynchronous oscillation. However, because the output impedance value of the grid-side converter under the control of the VSG is smaller, obvious phenomenon of insufficient inhibition capability exists, and the positive and negative of the equivalent resistance of the system depend on the equivalent resistance of the rotor-side converter and the line resistance. Therefore, the VSG cannot completely suppress the occurrence of the subsynchronous oscillation, and attenuates the intensity of the subsynchronous oscillation only to some extent.
The step S3 specifically comprises the following steps:
because the VSG output impedance is smaller, subsynchronous oscillation risks exist in the doubly-fed fan grid-connected system based on the grid-side converter VSG control mode. For this reason, the next synchronous oscillation suppression mechanism of VSG control is explored in combination with active disturbance rejection control. The active disturbance rejection control can expand the disturbance and uncertain factors inside and outside the system into system state variables so as to feed back and compensate the system controller in real time, thereby realizing effective disturbance inhibition. The active disturbance rejection control structure is shown in fig. 5. Wherein v is 0 Is a controlled variable reference value; y is the output value of the controlled variable; z 1 A tracking signal for outputting y; z 2 Is the observed value of the total disturbance; f (f) 0 Is a known part of the system; u is the final control quantity of the system; b 0 Is a dynamic compensation factor.
To simplify the analysis process, d-axis analysis is taken as an example. The VSG control access lower grid side converter current inner loop output voltage mathematical expression can be expressed as:
wherein u is gd Outputting a voltage d-axis component for the grid-side converter; r is R g And L g The resistor and the inductor of the network side line reactor are respectively; i.e gd And i gq D-axis and q-axis components of the input current of the power grid are respectively; omega N Is the synchronous angular velocity; e (E) VSGd The d-axis component of the voltage is output for VSG.
When the grid-connected system generates subsynchronous oscillation, the current inner loop output voltage expression of the grid-side converter under disturbance can be rewritten as follows:
wherein Deltau sub_gd And delta E sub_VSGd The output subsynchronous voltage of the disturbance lower network side converter and the VSG output subsynchronous electricity are respectivelyPressing d-axis component; Δi sub_gd And Δi sub_gq The d and q axis components of the subsynchronous current, respectively.
Further analyzing the influence rule of the current inner loop PI control parameter of the network side converter on the subsynchronous oscillation, and because the differential link is replaced by the PI controller in the engineering practice, the better control effect can be obtained. Based on the above, the output voltage of the current inner loop of the network side converter based on PI control is obtained by mathematical expression:
it can be seen that the subsynchronous oscillation component passes through the current inner loop PI control link of the network side converter in the form of current, and further superimposes the subsynchronous component on the network side output voltage, so that the network side converter control part including VSG has subsynchronous components. Meanwhile, the subsynchronous oscillation quantity is further amplified under the action of the PI controller due to the proportion link, so that the oscillation is aggravated.
In addition, because the wind power plant is variable in system operation due to the factors of the number of fans, wind speed, ground faults and the like, subsynchronous oscillation suppression is difficult to effectively achieve in a PI control parameter adjustment mode. The analysis shows that if the output current of the network side converter is kept stable under the subsynchronous oscillation, the subsynchronous oscillation component in the output voltage of the network side converter can be eliminated, so that the action path of the subsynchronous oscillation is destroyed, and the purpose of inhibiting the subsynchronous oscillation is achieved.
Therefore, the active disturbance rejection control is adopted to replace PI control so as to estimate and compensate the influence of subsynchronous current on the output current of the fan in real time, and the stability of the output voltage of the network side can be realized, namely subsynchronous oscillation inhibition, and the inhibition principle is specifically expressed as follows:
when the grid-connected system stably operates, the d-axis current expression of the grid-side converter under VSG control can be obtained as follows:
when the system has a subsynchronous oscillation component, subsynchronous oscillation flows in a network side converter control link in the form of subsynchronous current and voltage components, so that the d-axis current of the network side converter becomes variable:
in order to clearly analyze the integral influence of the subsynchronous oscillation component on the voltage and the current of the grid-side converter, the following equation conversion is made:
Wherein f 1 (Δi sub_gd ,Δi sub_gq ) May be represented as a subsynchronous current d-axis component in a grid-tie system.
It can be seen that the disturbance of the subsynchronous oscillation to the network-side converter can be influenced by f 1 (Δi sub_gd ,Δi sub_gq ) It means that if this disturbance term is eliminated, subsynchronous oscillations will be suppressed. For this purpose, since the active-disturbance-rejection controller can be controlled to f 1 (Δi sub_d ,Δi sub_q ) And carrying out real-time estimation and real-time compensation on the output voltage of the network-side converter. The d-axis current mathematical expression of the network side converter is obtained by adopting the front and back of the active disturbance rejection controller as follows:
in the formula, the upper formula is before the active disturbance rejection controller is adopted, and the lower formula is after the active disturbance rejection controller is adopted; the comparison analysis can find that the output current i of the network side converter under the subsynchronous oscillation gd Consistent with the system steady run time expression. Under the active disturbance rejection controller, i gd Not subject to subsynchronous current component Δi sub_d Influence, block the output voltage u of the grid-side converter gd Medium-secondary synchronous voltage component Deltau sub_d The transmission process can further inhibit the subsynchronous vibrationThe purpose of swinging.
The step S4 specifically includes:
s4.1, in order to restrain the subsynchronous oscillation of the system under the control of VSG, the active disturbance rejection control is replaced by a network side converter current inner loop PI controller. The grid-side inverter current can be expressed as:
wherein i is gd And i gq D-axis and q-axis components of the input current of the power grid are respectively; u (u) gd And u gq The d-axis component and the q-axis component of the output voltage of the grid-side converter are respectively; b d And b q D-axis and q-axis components of the dynamic compensation factor, respectively; f (f) 1 (Δi sub_gd ,Δi sub_gq ) The d-axis component of the subsynchronous current in the grid-connected system; f (f) 2 (Δi sub_gd ,Δi sub_gq ) Represented as a subsynchronous current q-axis component in a grid-connected system; r is R g And L g The resistor and the inductor of the network side line reactor are respectively; omega N Is the synchronous angular velocity; e (E) VSGd The d-axis component of the voltage is output for VSG.
It is known that there is a coupling between the d-axis and the q-axis of the current inner loop control of the grid-side inverter. To achieve accurate decoupling control of the current, including the coupling term in the disturbance can then be changed to:
wherein F is 1 (Δi sub_gd ,Δi sub_gq )、F 2 (Δi sub_gd ,Δi sub_gq ) D and q axis sub-synchronous disturbance and coupling quantity are controlled by the network side converter respectively.
S4.2, construction of improved active disturbance rejection control, which comprises the following steps: a nonlinear differential Tracker (TD), an Extended State Observer (ESO), and a linear error feedback (LSEF), which are specifically as follows:
1) The tracking differential link of the first-order active disturbance rejection controller only outputs the tracking signal and does not output the differential signal, so that the nonlinear tracking differential link can be omitted.
2) Extended state observer link with filtering effect
An Extended State Observer (ESO) can track state information and estimate system disturbances based on system outputs and controlled object inputs to generate disturbance compensation quantities. Wherein, fal function is ESO core constituent unit, shows the correction characteristic of "big error little gain, little error big gain".
In addition, at present, noise interference existing in the system output measurement link is hardly considered, and the noise interference is generally existing in actual control. The larger gain coefficient in ESO can amplify the measurement noise, which has larger influence on the performance of the observer, and the filter is generally adopted in the actual control loop to process the output quantity of the system so as to filter noise interference, but the amplitude and the phase of the filtered signal have larger difference with the actual output of the system. If the observer is constructed using this as the system output, a large observation error tends to be caused. For this purpose, an improved extended state observer architecture is used, with a Fal function filter placed after the system output to improve the output waveform quality and to form an extended ESO with the ESO. Not only retains the original advantages and maintains the state order, but also can effectively process the noise introduced in the measurement link.
Fal the function filter control structure is shown in fig. 6. Wherein, the liquid crystal display device comprises a liquid crystal display device,
k Fal is a proportionality coefficient; alpha f A constant of 0 to 1; delta f A constant that affects the filtering effect; u is an input signal; y is U and passes through the output of Fal function filter control structureA signal. When the error e is smaller than delta f When in use, let->
The transfer function expression of the output Fal function feedback structure is then:
It can be seen that the transfer function expression of the output Fal function feedback structure is actually a low-pass filter, when δ f Increase or k Fal When decreasing, i.e. k 1 The bandwidth of the filter is narrowed, the tracking speed is reduced, and a better filtering effect can be obtained.
In summary, the disturbance term of the observed system is expanded into a new state variable, so that an extended state observer can be obtained, and the mathematical expression is as follows:
wherein beta is 01 、β 02 Is a gain coefficient; f (F) d Is the total disturbance quantity of the d axis of the system (compared with F 1 Multiple additions of system unmodeled parts); i.e gd And
respectively a d-axis component instantaneous value and an estimated value of the grid side power grid; e, e d The difference value between the d-axis component estimated value and the instantaneous value of the grid side power grid is obtained; />
The d-axis disturbance quantity observation value; />
And (5) outputting a value after the d-axis component of the network side voltage passes through a fal function filtering structure.
3) Linear State Error Feedback (LSEF) link
Because nonlinear state error feedback link control parameters are difficult to select, and linear state error feedback is adopted to replace the nonlinear state error feedback in order to simplify the design process of the controller, the disturbance can be well restrained. And by combining ESO estimation comprehensive disturbance, the feedback control quantity is subjected to real-time disturbance compensation, so that the purpose of suppressing oscillation is achieved. Therefore, on the basis of constructing the extended state observer, according to the linear state error feedback control design principle, an error feedback form can be obtained as follows:
Wherein e Nd Reference value i for d-axis current of grid-side converter gdref I with ESO output gd Signal difference of the estimated value; u (u) 0d The output quantity of the improved active disturbance rejection controller is not subjected to ESO dynamic compensation; k is an adjustable coefficient.
Further, the integrated disturbance combined with ESO estimation can perform real-time disturbance compensation on the feedback control quantity, thereby achieving the purpose of suppressing oscillation, and the disturbance estimation compensates the final control quantity formed
The method comprises the following steps:
it should be noted that, because the self-immunity control closed loop has a large dependence on the control parameters, the control parameters can achieve better robustness and adaptability through simple setting. Wherein the simulation sampling step h is taken as a basic parameter, and the gain parameter is beta 01 And beta 02 The selection is directly related to h, and beta is taken here 01 ＝1/h、β 02 ＝(β 01 ) 3 The method comprises the steps of carrying out a first treatment on the surface of the Dynamic participation factor b x The control quantity acts on the estimated value of the amplification factor of the system, and only slight adjustment is needed; the linear state feedback link parameter k is an adjustable coefficient, and the smaller the value is, the better the control effect is. Thus, the simulation effect can be properly selectedk value.
In conclusion, based on ESO estimation comprehensive disturbance, real-time disturbance compensation is carried out on feedback control quantity, and an active disturbance rejection controller is designed to replace a PI control link of a current inner loop of a network side converter, so that subsynchronous current and voltage components can be eliminated as targets on the basis of guaranteeing VSG control performance stability, and subsynchronous oscillation is effectively restrained. The control strategy proposed by the present invention is shown in fig. 7.
S5.3, analyzing the performance of the controller. Since the d and q axes have the same expression form, only the d axis is taken as an example to analyze three parts of ESO stability, ESO observation error and closed loop system stability.
1) ESO stability analysis
To simplify the calculation process, let:
selecting alpha 1 ＝α 2 ，δ 1 ＝δ 2 The following steps are:
ρ 1 (e d )＝ρ 2 (e d )＝ρ(e d )
then, expanding the disturbance term of the observed system into a new state variable, a simplified expanded state observer can be obtained, whose expression becomes:
the above expression can be equivalent to a coefficient-variable linear system, and is restated to be in a matrix form, and the expression is:
further, the ESO transfer function expression can be calculated by Laplace transformation as follows:
for the above equation, it can be considered as a variable coefficient linear system of perturbation within a limited range of parameters, and its stability can be analyzed by the root track method. It is known that the characteristic equation of the system can be expressed as: s is(s) 2 +β 01 ρ(e d )s+β 02 ρ(e d ) =0, listing the us table as shown in table 4:
TABLE 4 Table 4
The system stability conditions available from the us criteria are:
due to the control parameter beta of the invention 01 、β 02 Are all larger than 0, and select alpha 1 ＝α 2 ＝0.5、δ 1 ＝δ 2 =0.2, it can be seen that ρ (e d )>0(e d Not equal to 0), and the designed ESO is obtained to meet the stability requirement.
2) ESO observation error analysis
Based on the analysis, the total disturbance term is used as a system expansion state variable to obtain a network-side converter control output current state space model expression as follows:
Based on the analysis, ESO observation error analysis can be realized, and the mathematical expression of the error equation is as follows:
when the error system enters a steady state, the right end converges to zero, i.e.:
since Fal (e) d1 ,0.5,0.2)＝|e d1 |0.5*sign(e d1 ) Therefore, the steady state error is:
obviously if beta 02 Is sufficiently greater than w d The ESO estimation error will be small enough to meet the requirement of observation accuracy.
3) Stability analysis of network-side converter closed-loop control system
The d-axis voltage due to the filtering characteristic of the Fal function feedback structure
The waveform quality is optimized to obtain the voltage u after filtering gd The expression is:
based on the control topological structure relation shown in fig. 5, the mathematical expression of the d-axis closed-loop control system of the network-side converter can be obtained as follows:
combining the above formulas, the d-axis closed loop transfer function of the network side converter can be obtained as follows:
wherein: ρ=ρ (e d )，Δ＝k 1 s 3 +(kb d β 01 ρ+β 02 ρ+k 1 β 01 ρ)s 2 +(kk 1 b d β 01 ρ+kb d β 02 ρ+2k 1 β 02 ρ)s+kk 1 b d β 02 ρ。
From this analysis, the closed loop system characteristic equation with the network side converter as the analysis can be expressed as:
k 1 s 3 +(kb d β 01 ρ+β 02 ρ+k 1 β 01 ρ)s 2 +(kk 1 b d β 01 ρ+kb d β 02 ρ+2k 1 β 02 ρ)s+kk 1 b d β 02 ρ=0, listing the us table as shown in table 5:
TABLE 5
| s 3 | k 1 | kk 1 b d β 01 ρ+kb d β 02 ρ+2k 1 β 02 ρ |
| s 2 | kb d β 01 ρ+β 02 ρ+k 1 β 01 ρ | kk 1 b d β 02 ρ |
| s 1 | b 1 | |
| s 0 | kk 1 b d β 02 ρ |
Wherein b 1 ＝kk 1 b d β 01 ρ+kb d β 02 ρ+2k 1 β 02 ρ-kk 1 2 b d β 02 ρ/(kb d β 01 ρ+β 02 ρ+k 1 β 01 ρ)。
The system stability conditions available from the us criteria are:
the invention requires the control parameter beta 01 、β 02 、k、k 1 、b d And ρ is greater than 0, k 1 >0、kb d β 01 ρ+β 02 ρ+k 1 β 01 ρ>0 and kk 1 b d β 02 ρ>0. Next, prove b 1 Greater than 0.
Satisfying beta for control parameters 02 ＝(β 01 ) 3 ，β 02 Far greater than k, k 1 、b d . By combining the above analysis, |e can be obtained d |<1. Then ρ is known from Fal functional expression>1(|e d |≠1,|e d |<1). Furthermore, (kb) d β 01 ρ+β 02 ρ+k 1 β 01 ρ)>β 02 ρ, obtainable by the shrinkage method, (kk) 1 2 b d β 02 ρ/β 02 ρ)>[kk 1 2 b d β 02 ρ/(kb d β 01 ρ+β 02 ρ+k 1 β 01 ρ)]。
Thus, the following relationship can be obtained:
kk 1 b d β 01 ρ+kb d β 02 ρ+2k 1 β 02 ρ-(kk 1 2 b d β 02 ρ/β 02 ρ)＝kk 1 b d β 01 ρ+kb d β 02 ρ+2k 1 β 02 ρ-kk 1 2 b d >0
based on the analysis, b can be obtained 1 >0. In summary, it can be seen that the d-axis closed-loop control system of the grid-side converter meets the stability requirement. The q-axis closed-loop control system also meets the stability requirement.
The step S5 specifically comprises the following steps:
a simulation model is built based on a certain wind farm in North China, the accuracy of analysis of the influence mechanism of VSG on subsynchronous oscillation and the applicability of active disturbance rejection control on subsynchronous oscillation are verified, and the simulation test model is shown in FIG. 8. The outlet capacity of the doubly-fed wind field under the model is 100MW, and the doubly-fed wind field consists of 67 identical 1.5MW doubly-fed fans, and each doubly-fed fan passes through a 0.69/35kV in-field transformer T 1 The double-fed wind power plant is connected to the same bus, and an equivalent model of a polymerization fan is adopted to simulate the whole double-fed wind power plant. The whole doubly-fed wind field is connected with a 35/220kV transformer T 2 Is connected to a 220kV line and finally is subjected to 220/500kV voltage boosting to change into T 3 And the power transmission device is connected to a 500kV line for long-distance power transmission, and a series capacitor is arranged in the 500kV line for compensation. Z in the figure L1 、Z L2 220kV and 500kV line impedance, C L2 The capacitive reactance of the capacitor is series compensation. The effectiveness of the proposed subsynchronous oscillation analysis mechanism and the proposed suppression strategy of the present invention will be verified from three aspects. The relevant control parameters can be seen in tables 1-3.
1) Analysis and verification of subsynchronous oscillation mechanism of grid-connected system before and after VSG access
In order to verify the accuracy of the mechanism of the influence of the VSG on the subsynchronous oscillation of the doubly-fed wind power grid-connected system, the wind speed of the wind power plant is kept to be 11m/s, and two series compensation operation working conditions of 0.4 and 0.6 are set to obtain an FFT analysis chart of the output active power and the current in the doubly-fed wind power plant before and after the VSG is connected, as shown in figure 9.
It can be seen that VSG reduces active power P output by wind farm after being accessed w The oscillation amplitude has a suppression effect on subsynchronous oscillation. The method has the advantages that the frequency of subsynchronous oscillation is reduced after the VSG is connected under the serial compensation degree of 0.4 and 0.6, the waveform distortion rate is reduced, the subsynchronous oscillation characteristic is weakened, the effect of subsynchronous oscillation inhibition is achieved through virtual synchronous control of the doubly-fed fans based on the grid-side converter VSG, the equivalent negative impedance amplitude of the system under the high serial compensation degree is larger when the output impedance of the VSG is fixed, and the effect of inhibiting the subsynchronous oscillation by the VSG under the low serial compensation degree is better. However, because the VSG output impedance is smaller in the control mode, sufficient positive damping cannot be provided for the system, and particularly, in fig. 9, the degree of secondary synchronous oscillation after the VSG is accessed is weakened to a certain extent, and the experimental analysis result is consistent with the theoretical analysis conclusion.
2) PI controller and improved active disturbance rejection controller inhibition effect contrast
To verify the effectiveness of the active disturbance rejection control in suppressing the subsynchronous oscillations, the ability of the ADRC to suppress the subsynchronous oscillations with the PI control and the conventional ADRC is compared and analyzed under different serial supplements (wherein, in FIGS. 10 and 11, the ADRC control is improved, namely, the active disturbance rejection controller is improved). Wherein, the controller parameter setting is respectively: k (k) Fal ＝0.5，α f ＝0.5，b d ＝0.01，β 01 ＝6，β 02 =108. Simulation results are shown in fig. 10, with a series compensation of 0.4 added at 8 s. In addition, in order to analyze the suppression capability of the control strategy presented herein to subsynchronous oscillation under different serial supplements, a simulation experiment at the time of 0.6 serial supplements was set, and the experimental results are shown in fig. 11. Based on simulation results, as the 500KV power transmission line side of the doubly-fed wind field grid-connected system is connected with the series compensation capacitor under PI control, the fan output active power contains a subsynchronous frequency component with larger amplitude, the waveform of the current at 690V side in the field is severely distorted, and continuous subsynchronous oscillation occurs. When the improved active disturbance rejection controller is adopted, with the connection of the series compensation capacitor, the wind power plant outputs active power, reactive power and current only in a slight transient process when the wind power plant is just put into operation, and then the subsynchronous oscillation disturbance quantity is effectively and rapidly eliminated, so that the system tends to be stable. Compared with the traditional active disturbance rejection controller, the improved active disturbance rejection controller combined with the Fal function filter can maintain stable output current And the influence of the interaction of the control part of the doubly-fed fan and the series compensation capacitor is effectively weakened, and the inhibition effect is better.
3) Improved adaptive analysis of active disturbance rejection control
In order to verify the robustness of active disturbance rejection control in suppressing subsynchronous oscillation, characteristics such as output randomness, fluctuation and the like of the doubly-fed wind power plant are considered, serial compensation degree is set to be 0.4, and control parameter k is maintained Fal ＝0.5，α f ＝0.5，b d ＝0.01，β 01 ＝6，β 02 =108, the adaptability of the active disturbance rejection control at different fan counts and wind speeds was analyzed.
(1) Fan number change
The number of wind turbines of the wind farm is changed to study the adaptability of the active disturbance rejection control to the change of the number of the wind turbines, the wind farm is set according to the operation conditions of 67 wind turbines, two operation states of 80 wind turbines and 50 wind turbines are increased to carry out simulation analysis, and simulation results are shown in figure 12 (wherein ADRC control is the improved active disturbance rejection controller in figure 12). It can be known that under the condition of three grid-connected fans, if the system does not take inhibition measures, the active output of the wind power plant is unstable after the failure. With the reduction of the number of the grid-connected fans, the output power of the wind power plant is reduced, the subsynchronous oscillation degree is increased, the system instability is enhanced, the equivalent impedance of the system is changed due to the fact that the number of the fans connected in parallel is changed, and then the damping characteristic of the system is affected. However, after the active disturbance rejection control is added, the oscillation degree is rapidly reduced, the active output amplitude of the wind power plant is weakened at the initial stage of oscillation, then the oscillation is effectively inhibited during the stabilization, and the active output is stable.
(2) Wind field wind speed variation
In view of the variable wind speed of the wind farm, the wind farm output fluctuation is shown, and the adaptability of the active disturbance rejection control to the wind speed needs to be researched. Three different wind speeds are set, other parameters and operation conditions are kept unchanged, subsynchronous oscillation suppression capability of the active disturbance rejection controller to the VSG access grid-connected system is studied, and an active output response diagram of a wind power plant is shown as a diagram in fig. 13 (wherein ADRC control is the improved active disturbance rejection controller in fig. 13). The analysis can be considered that when the wind speed is changed from 11m/s to 9m/s, subsynchronous oscillation phenomenon is generated in the wind power plant under PI control, and the oscillation intensity is reduced along with the increase of the wind speed; the wind speed change during the active disturbance rejection control hardly affects the active disturbance rejection control inhibition effect, and can effectively weaken the oscillation jitter amplitude at the initial stage of oscillation, and then the oscillation is rapidly inhibited, so that the stable and safe operation of the whole system is ensured.
(3) Ground short fault variation
The ground short fault acts as a disturbance and has the capability of inducing subsynchronous oscillation of the system. Therefore, the line serial compensation is set to be in an input state, other parameters are kept constant, and the adaptability of the active disturbance rejection controller to different grounding short circuit faults is verified. When the system enters a steady state, different grounding short-circuit faults are set on the 690V/35kV high-voltage side at 8s, and the duration of the faults is close to 0.2s. The wind farm active output response curves for different ground short faults are shown in fig. 14 (wherein, in fig. 14, ADRC control is the improved active disturbance rejection controller of the present invention). It can be seen that under the condition of a ground short circuit fault, the wind power plant active output P based on PI control w Rapidly diverging oscillations occur, subsequently P when the controller enters the clipping region w The system presents an unstable state and subsynchronous oscillation occurs; with improved ADRC control, P after failure w After a short-time transient process, normal operation can be restored, and subsynchronous oscillation is effectively inhibited. It follows that ADRC control has good suppression capability for subsynchronous oscillations in the event of a ground short fault.
Example 2: a subsynchronous oscillation suppression system of a virtual synchronous double-fed fan through a series compensation grid-connected system comprises: the first construction module is used for placing the Fal function filter after the virtual synchronous doubly-fed fan is output by the series compensation grid-connected system and constructing an extended state observer together with the ESO of the traditional extended state observer; the second construction module is used for constructing an improved active disturbance rejection controller according to the extended state observer and the linear error feedback LSEF; and the suppression module is used for suppressing subsynchronous oscillation of the virtual synchronous doubly-fed fan through the series compensation grid-connected system by adopting the improved active disturbance rejection controller.
Further, the method further comprises the following steps: and the verification module is used for verifying the subsynchronous oscillation suppression effect of the improved active disturbance rejection controller strategy under different disturbance working conditions.
The foregoing embodiment numbers of the present invention are merely for the purpose of description, and do not represent the advantages or disadvantages of the embodiments.
In the foregoing embodiments of the present invention, the descriptions of the embodiments are emphasized, and for a portion of this disclosure that is not described in detail in this embodiment, reference is made to the related descriptions of other embodiments.
While the present invention has been described in detail with reference to the drawings, the present invention is not limited to the above embodiments, and various changes can be made without departing from the spirit of the present invention within the knowledge of those skilled in the art.
Claims (10)
1. A subsynchronous oscillation suppression method of a virtual synchronous double-fed fan through a series compensation grid-connected system is characterized by comprising the following steps:
acquiring a VSG-based network side converter output impedance expression;
revealing positive damping characteristics of the grid-side converter based on VSG on subsynchronous oscillation from the equivalent impedance angle of the doubly-fed wind turbine grid-connected system;
analyzing a subsynchronous oscillation suppression mechanism of the virtual synchronous double-fed fan grid-connected system by combining active disturbance rejection control;
and constructing an improved active disturbance rejection controller to perform subsynchronous oscillation suppression.
2. The method for suppressing subsynchronous oscillation of a virtual synchronous doubly-fed wind turbine system through series compensation grid connection according to claim 1, which is characterized by comprising the following steps: the obtaining the VSG-based network side inverter output impedance expression includes:
S1.1, adopting a power system small interference stability analysis method, realizing linearization of a VSG dynamic equation based on a control topological structure of the VSG, wherein a mathematical expression of a linearization result is as follows:
wherein F is PQ 、F PI 、
F 1 、F 2 、F L Are all 2 multiplied by 2 transfer function matrices; t (T) J And D p The moment of inertia and the damping coefficient are respectively; k (K) p And K i The excitation control proportion coefficient and the integral coefficient are respectively; k (K) v Controlling droop coefficients for reactive power-voltage; s is the Laplace operator; k (K) Vp1 And K Vi1 The d-axis proportion coefficient and the integral coefficient are respectively controlled for VSG output voltage; k (K) Vp2 And K Vi2 The q-axis proportion coefficient and the integral coefficient are respectively controlled for VSG output voltage; i sd And I sq D-axis component and q-axis component of steady-state quantity of stator current under synchronous rotation coordinate system; u (U) sd And U sq D-axis component and q-axis component of steady-state working point of stator voltage under synchronous rotation coordinate system; e (E) m Controlling the output voltage for excitation; delta is the difference between the phase-locked loop output phase angle and the grid-connected system voltage phase; l is a filter inductance; u (U) g Is the stable value of the network side voltage; omega N Is the synchronous angular velocity;
s1.2, acquiring VSG control parameters and system network operation parameters in a stable state, and taking a dq rotating coordinate system as a reference system to obtain a network side converter output impedance expression based on VSG, wherein the output impedance expression is as follows:
Wherein [ Deltau ] sd ,Δu sq ] T The voltage disturbance vector is a grid-connected system stator side voltage disturbance vector; [ delta ] i sd ,Δi sq ] T The current response vector is the stator side current response vector of the grid-connected system; z is Z out Is a VSG-based grid-side inverter output impedance; z is Z dd 、Z dq 、Z qd And Z qq Respectively represent the output impedance Z out D-d, d-q, q-d, and q-q axis components of (a); i is denoted as identity matrix.
3. The method for suppressing subsynchronous oscillation of a virtual synchronous doubly-fed wind turbine system through series compensation grid connection according to claim 1, which is characterized by comprising the following steps: the positive damping characteristic of the grid-side converter based on VSG to subsynchronous oscillation is revealed from the equivalent impedance angle of the doubly-fed fan grid-connected system, and the method comprises the following steps:
s2.1, equivalent output impedance Z of doubly-fed wind turbine G The mathematical expression of (2) is:
wherein R is G And X G The equivalent resistance and reactance of the doubly-fed wind turbine are respectively; r is R s Is the sum of the doubly-fed fan stator winding and the box-type variable resistor; r is R r Resistance of the rotor winding; r is R RSC Representing the rotor-side converter equivalent resistance; l (L) s Is the sum of the doubly-fed fan stator winding and the leakage inductance of the box transformer; l (L) lr Is rotor leakage inductance; l (L) m The excitation inductance is adopted; slip ratio
f r And j are expressed in rotor frequency and imaginary units, respectively; resonance frequency of inductance and capacitance series resonance circuit>
ω ss And f 0 Respectively the system equivalent series loop resonant frequency and the system reference frequency under the subsynchronous oscillation; x is X C 、X L The series compensation capacitance capacitive reactance and the system equivalent reactance are respectively;
s2.2, after the VSG is connected into the network side converter control of the doubly-fed fan, obtaining the equivalent output impedance Z of the doubly-fed fan based on the VSG G1 The expression is:
wherein R is VSG And X VSG The equivalent resistance and reactance of VSG are respectively; r is R G1 And X G1 And the equivalent resistance and the reactance of the doubly-fed wind turbine after being connected with the VSG are respectively.
4. The method for suppressing subsynchronous oscillation of a virtual synchronous doubly-fed wind turbine system through series compensation grid connection according to claim 1, which is characterized by comprising the following steps: the sub-synchronous oscillation suppression mechanism of the virtual synchronous double-fed fan grid-connected system is analyzed by combining active disturbance rejection control, and the sub-synchronous oscillation suppression mechanism comprises the following components:
taking d-axis analysis as an example, the mathematical expression of the output voltage of the current inner loop of the VSG connected to the lower network side converter is as follows:
wherein u is gd Outputting a voltage d-axis component for the grid-side converter; r is R g And L g The resistor and the inductor of the network side line reactor are respectively; i.e gd And i gq D-axis and q-axis components of the input current of the power grid are respectively; omega N Is the synchronous angular velocity; e (E) VSGd Outputting a voltage d-axis component for the VSG;
when the grid-connected system generates subsynchronous oscillation, the current inner loop output voltage expression of the grid-side converter under disturbance is rewritten as follows:
wherein Deltau sub_gd And delta E sub_VSGd The d-axis components of the output subsynchronous voltage and the VSG output subsynchronous voltage of the disturbance lower network side converter are respectively obtained; Δi sub_gd And Δi sub_gq Respectively the d and q axis components of the subsynchronous current;
analyzing the influence rule of the current inner loop PI control parameter of the grid-side converter on the subsynchronous oscillation to obtain the mathematical expression of the current inner loop output voltage of the grid-side converter based on PI control, wherein the mathematical expression is as follows:
wherein K is p2 And K i2 D-axis proportion coefficient and integral coefficient of the PI controller in the current inner loop control of the grid-side converter are respectively; s is the Laplace operator;
when the grid-connected system stably operates, the d-axis current expression of the grid-side converter under VSG control is obtained as follows:
when the system has a subsynchronous oscillation component, subsynchronous oscillation flows in a network side converter control link in the form of subsynchronous current and voltage components, so that the d-axis current of the network side converter becomes variable:
the equation transforms as follows:
wherein f 1 (Δi sub_gd ,Δi sub_gq ) The d-axis component of the subsynchronous current in the grid-connected system;
the d-axis current mathematical expression of the network side converter is obtained by adopting the front and back of the active disturbance rejection controller:
the q-axis is the same.
5. The method for suppressing subsynchronous oscillation of a virtual synchronous doubly-fed wind turbine system through series compensation grid connection according to claim 1, which is characterized by comprising the following steps: the construction of the improved active disturbance rejection controller comprises the following steps:
s4.1, replacing a PI controller of a current inner loop of the grid-side converter with an active disturbance rejection controller in order to inhibit subsynchronous oscillation of the doubly-fed wind turbine grid-connected system under VSG control, and rewriting a mathematical expression of the current of the grid-side converter into:
Wherein i is gd And i gq D-axis and q-axis components of the input current of the power grid are respectively; u (u) gd And u gq The d-axis component and the q-axis component of the output voltage of the grid-side converter are respectively; b d And b q D-axis and q-axis components of the dynamic compensation factor, respectively; f (f) 1 (Δi sub_gd ,Δi sub_gq ) The d-axis component of the subsynchronous current in the grid-connected system; f (f) 2 (Δi sub_gd ,Δi sub_gq ) Represented as a subsynchronous current q-axis component in a grid-connected system; r is R g And L g The resistor and the inductor of the network side line reactor are respectively; omega N Is the synchronous angular velocity; e (E) VSGd Outputting a voltage d-axis component for the VSG;
it is known that the current inner loop of the grid-side converter controls the d axis to be coupled with the q axis; in order to realize accurate decoupling control of the output current of the network-side converter, the coupling term is included in the disturbance, and then the method is changed into the following steps:
wherein F is 1 (Δi sub_gd ,Δi sub_gq ) And F 2 (Δi sub_gd ,Δi sub_gq ) D-axis component and q-axis component of the summation of subsynchronous disturbance quantity and coupling quantity are respectively controlled for the network-side converter;
s4.2, constructing an improved active disturbance rejection controller, which comprises the following steps: an extended state observer, a linear error feedback LSEF;
s4.3, improving the performance analysis of the active disturbance rejection controller.
6. The method for suppressing subsynchronous oscillation of a virtual synchronous doubly-fed wind turbine via a series-fed grid system according to claim 5, wherein the extended state observer ESO is an extended state observer formed by placing a Fal function filter after the virtual synchronous doubly-fed wind turbine is output via the series-fed grid system and combining the filter with a conventional extended state observer ESO.
7. The method for suppressing subsynchronous oscillation of a virtual synchronous doubly-fed wind turbine system via a series compensation grid connection according to claim 1, further comprising the step of verifying: the verification proves that the improved active disturbance rejection controller strategy has subsynchronous oscillation suppression effects under different disturbance working conditions.
8. The method for suppressing subsynchronous oscillation of a virtual synchronous doubly-fed wind turbine system via a series compensation grid system according to claim 7, wherein the different disturbance conditions comprise: different fan numbers, wind field wind speeds, series compensation degrees and grounding fault types.
9. The subsynchronous oscillation suppression system of the virtual synchronous double-fed fan through the series compensation grid-connected system is characterized by comprising the following components:
the first construction module is used for placing the Fal function filter after the virtual synchronous doubly-fed fan is output by the series compensation grid-connected system and constructing an extended state observer together with the ESO of the traditional extended state observer;
the second construction module is used for constructing an improved active disturbance rejection controller according to the extended state observer and the linear error feedback LSEF;
and the suppression module is used for suppressing subsynchronous oscillation of the virtual synchronous doubly-fed fan through the series compensation grid-connected system by adopting the improved active disturbance rejection controller.
10. The virtual synchronous doubly-fed wind turbine generator system subsynchronous oscillation suppression system of claim 9, further comprising:
and the verification module is used for verifying the subsynchronous oscillation suppression effect of the improved active disturbance rejection controller strategy under different disturbance working conditions.
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| CN116382123A (en) * | 2023-05-26 | 2023-07-04 | 南方电网数字电网研究院有限公司 | Offshore wind turbine grid-connected characteristic testing method for main control and converter combined hardware in loop |
| CN116544969A (en) * | 2023-06-28 | 2023-08-04 | 哈尔滨理工大学 | Control method and device for restraining subsynchronous oscillation of direct-drive wind power plant under weak current network |
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| CN116382123A (en) * | 2023-05-26 | 2023-07-04 | 南方电网数字电网研究院有限公司 | Offshore wind turbine grid-connected characteristic testing method for main control and converter combined hardware in loop |
| CN116382123B (en) * | 2023-05-26 | 2023-09-01 | 南方电网数字电网研究院有限公司 | Offshore wind turbine grid-connected characteristic testing method for main control and converter combined hardware in loop |
| CN116544969A (en) * | 2023-06-28 | 2023-08-04 | 哈尔滨理工大学 | Control method and device for restraining subsynchronous oscillation of direct-drive wind power plant under weak current network |
| CN116544969B (en) * | 2023-06-28 | 2023-12-29 | 哈尔滨理工大学 | Control method and device for restraining subsynchronous oscillation of direct-drive wind power plant under weak current network |
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