Disturbance rejection method and system for lcl type unified power flow controller

Abstract

The application discloses a disturbance suppression method and system applied to an LCL type unified power flow controller, relates to the technical field of power flow converter power control, and comprises the following steps: constructing an input and output mathematical model of a series side converter system in an s domain according to an LCL type filter, and reducing the order of the input and output mathematical model; performing linear space transformation on the reduced input and output mathematical model, and then compensating for the disturbance generated by the linear space transformation through a state observer LESO; designing a phase lead link based on the frequency characteristics of time delay; calculating current instruction values in a stationary coordinate system according to a power flow equation of the unified power flow controller, and designing a current tracking controller to realize real-time tracking of the current instruction values. The application can solve the technical problem that harmonics affect the output current quality of the series side converter in the related art.

Description

Technical Field

[0001] This application relates to the technical field of power control for power flow converters, and in particular to a disturbance suppression method and system applied to an LCL-type unified power flow controller. Background Technology

[0002] In active distribution networks, the use of power electronic control technology to improve power quality and supply reliability is becoming increasingly widespread. Taking the Unified Power Flow Controller (UPFC) as an example, it is a new type of control device among power electronic equipment. It can flexibly and quickly adjust the transmission power of lines, provide adjustable reactive power compensation, and simultaneously control node voltage, line overload, and distribution network losses. Its integration into active distribution networks is beneficial to improving the system's economy and reliability. Moreover, compared with traditional mechanical switching operations, UPFC offers faster response and more accurate adjustment, providing a more effective control method for solving operational problems in active distribution networks.

[0003] However, the parallel-side converter of a UPFC is mainly used to stabilize the intermediate DC bus capacitor voltage and support the sending-end voltage, while the series-side converter is mainly used to control the line power flow. Regarding the control of the parallel-side converter, its control is simple and mature, while the series-side converter commonly uses an L-type filter. However, this L-type filter is bulky and has poor suppression capability for switching harmonics; the voltage generated by the power grid or renewable energy sources is not an ideal three-phase sinusoidal voltage, often containing a large number of harmonics. These harmonics affect the output current quality of the series-side converter, further causing harmonic pollution to the electrical equipment in the line. Summary of the Invention

[0004] This application provides a disturbance suppression method and system for LCL-type unified power flow controllers to solve the technical problem that harmonics can affect the output current quality of series-side converters in related technologies.

[0005] Firstly, a disturbance suppression method for LCL-type unified power flow controllers is provided, comprising the following steps:

[0006] Based on the LCL-type filter, construct the input-output mathematical model of its series-side converter system in the s-domain, and reduce the order of the input-output mathematical model;

[0007] The reduced-order input-output mathematical model is subjected to a linear space transformation, and then the disturbance generated by the linear space transformation is compensated by the state observer LESO.

[0008] Design a phase lead element based on the frequency characteristics of delay;

[0009] The current command value in the stationary coordinate system is calculated based on the power flow equation of the unified power flow controller, and a current tracking controller is designed to track the current command value in real time.

[0010] In some embodiments, the input-output mathematical model includes:

[0011]

[0012]

[0013] In equations (1) and (2), L1 represents the converter-side inductance, and L′2 = L2 - (L r +L s ) represents the transformer-side inductance L2 and the line inductance (L r ,L s The equivalent inductance due to the combined effect, C f Indicates the filter capacitor, δ u =u s -u r Indicates the voltage source u at the sending end of the line. s With the receiving end voltage source u r The voltage difference between them, u inv i1 represents the converter output voltage, i1 represents the converter output current, and ω represents the converter output voltage. r and ω ir These represent the system's resonant frequency and anti-resonant frequency, respectively.

[0014] In some embodiments, the reduced-order mathematical model obtained after reducing the order of the input-output mathematical model includes:

[0015]

[0016]

[0017] The step of reducing the order of the input-output mathematical model further includes:

[0018] Time-domain transformation of equations (3) and (4) yields:

[0019]

[0020] In equation (5), i1 represents the vector [i 1α i 1β ] T u inv Represents vector [u invα ,u invβ ] T δ u [δ] uα ,δ uβ ] T;

[0021] Based on the parameter uncertainties of the series-side converter system, the time-domain expression of the system with parameter uncertainties is obtained as follows:

[0022]

[0023]

[0024] In equations (6) and (7), ΔL represents the fluctuation of the line inductance and the converter inductance, f d This indicates the possible presence of unknown disturbances in the system.

[0025] In some embodiments, a linear space transformation is performed on the reduced-order input-output mathematical model to obtain the following state-space equations:

[0026]

[0027]

[0028] In equations (8) and (9), L1 represents the converter-side inductance, and L′2 = L2 - (L r +L s ) represents the transformer-side inductance L2 and the line inductance (L r ,L s The equivalent inductance resulting from the combined action of the two.

[0029] In some embodiments, the mathematical model for compensating for disturbances generated by linear space transformations using the state observer LESO includes:

[0030]

[0031] In equation (10), x2 This represents the total disturbance. express x2 The observed values.

[0032] In some embodiments, the phase lead element in the phase lead element designed based on the frequency characteristics of the delay includes:

[0033]

[0034] Where, if a>b, G a The phase characteristics are:

[0035]

[0036] In equations (11) and (12), the values ​​of the adjustment parameters a and T are based on G. a The choice of operating frequency band and size.

[0037] In some embodiments, the step of calculating the current command value in the stationary coordinate system based on the power flow equations of the unified power flow controller includes:

[0038] Based on the circuit topology of the unified power flow controller, its power flow equations are determined as follows:

[0039]

[0040] The desired active power P on the transmission line * line and reactive power Q * line If the series transformer turns ratio is 1, the desired current command value in the synchronous rotating coordinate system can be obtained.

[0041] Transform the desired current command value into a current command value in a stationary coordinate system.

[0042] In some embodiments, the mathematical expression for the current command value in the stationary coordinate system includes:

[0043]

[0044] In equation (14), θ represents the output angle of the phase-locked loop.

[0045] In some embodiments, the tracking equations of the designed current tracking controller include:

[0046]

[0047] In equation (15), k p Represents the proportional gain, k r ω1 represents the generalized integral gain, and ω1 represents the angular frequency of the tracked reference current.

[0048] Secondly, embodiments of this application also provide a disturbance suppression system applied to an LCL-type unified power flow controller, including a memory and a processor. The memory stores a computer program that runs on the processor. When the processor executes the computer program, it implements the steps of the disturbance suppression method applied to an LCL-type unified power flow controller as described above.

[0049] The beneficial effects of the technical solution provided in this application include: compared with the L-type unified power flow controller, the LCL-type unified power flow controller is smaller in size, controls the current of the series-side converter, suppresses the harmonic current output of the series-side converter, and can also solve the delay in the digital controller, improve the system robustness and stability of the unified power flow controller, avoid the overshoot and oscillation problems caused by coupling phenomenon, and improve the quality of the series-side output current of the power flow controller. Attached Figure Description

[0050] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0051] Figure 1 A flowchart illustrating a disturbance suppression method applied to an LCL-type unified power flow controller, as provided in an embodiment of this application;

[0052] Figure 2 A schematic diagram of the system topology of the LCL-type unified power flow converter provided in the embodiments of this application;

[0053] Figure 3 This is a frequency response diagram of the reduced-order model and the series-connected mathematical model of the LCL-type unified power flow controller in the embodiments of this application;

[0054] Figure 4 This is a diagram of the internal structure of the LESO state observer in the embodiments of this application;

[0055] Figure 5 This is a block diagram of the equivalent transfer function of the LCL-type unified power flow converter system in the embodiments of this application;

[0056] Figure 6 This is a comparison chart of the inner-loop open-loop frequency characteristics with and without delay compensation in the embodiments of this application;

[0057] Figure 7 This is a comparison chart showing whether there is an improved PR control in the embodiments of this application;

[0058] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0059] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0060] The flowchart shown in the attached diagram is for illustrative purposes only and does not necessarily include all content and operations / steps, nor does it necessarily have to be performed in the order described. For example, some operations / steps can be broken down, combined, or partially merged, so the actual execution order may change depending on the actual situation.

[0061] This application provides a disturbance suppression method for an LCL-type unified power flow controller. Compared with the L-type unified power flow controller, the LCL-type unified power flow controller is smaller in size, controls the current of the series-side converter, suppresses the harmonic current output by the series-side converter, and can also solve the delay in the digital controller, improve the system robustness and stability of the unified power flow controller, avoid the overshoot and oscillation problems caused by coupling phenomenon, and improve the quality of the series-side output current of the power flow controller.

[0062] The following detailed description of some embodiments of this application is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0063] like Figure 1 As shown in the figure, this application provides a disturbance suppression method applied to an LCL-type unified power flow controller, including the following steps:

[0064] S1: Construct the input-output mathematical model of the series-side converter system of the LCL filter in the s-domain, and reduce the order of the input-output mathematical model;

[0065] S2: Perform a linear space transformation on the reduced-order input-output mathematical model, and then use the state observer LESO to compensate for the disturbances generated by the linear space transformation;

[0066] S3: Design of phase lead element based on frequency characteristics of delay;

[0067] S4: Calculate the current command value in the stationary coordinate system based on the power flow equation of the unified power flow controller, and design a current tracking controller to track the current command value in real time.

[0068] In this embodiment, an LCL filter is used to construct the filter. Compared with an L-type filter, the LCL filter has a smaller size and stronger suppression capability of PWM switching harmonics. Therefore, the use of an LCL filter can realize the miniaturization of the power flow controller.

[0069] In this embodiment, the low-order harmonic voltage in the voltage source causes harmonic pollution to the user-side equipment. Therefore, a disturbance suppression method is used to compensate and control the current of the series-side converter to suppress the harmonic current output by the series-side converter. Furthermore, considering that the delay in the implementation of the unified power flow controller will cause the system to be unstable, this embodiment also designs a phase lead element for delay compensation, which expands the allowable parameter fluctuation range of the LCL filter, thereby improving the robust stability of the system.

[0070] In short, the embodiments of this application suppress harmonic interference generated by the voltage source on the power flow controller, expand the stability domain of the system, and also achieve miniaturization.

[0071] Furthermore, the input-output mathematical model includes:

[0072]

[0073]

[0074] In equations (1) and (2), L1 represents the converter-side inductance, and L′2 = L2 - (L r +L s ) represents the transformer-side inductance L2 and the line inductance (L r ,L s The equivalent inductance due to the combined effect, C f Indicates the filter capacitor, δ u =u s -u r Indicates the voltage source u at the sending end of the line. s With the receiving end voltage source u r The voltage difference between them, u inv i1 represents the converter output voltage, i1 represents the converter output current, and ω represents the converter output voltage. r and ω ir These represent the system's resonant frequency and anti-resonant frequency, respectively.

[0075] In step S1, as Figure 2 As shown, the input-output relationship of the series-side converter system of the LCL filter in the s-domain is determined by the voltage-current relationship:

[0076]

[0077]

[0078] in,

[0079] Furthermore, in step S1, according to Figure 3 It can be determined that u in formulas (1) and (2)inv and δ u The frequency domain characteristics of the transfer function are similar at low frequencies, thus a low-frequency approximate model of the system can be obtained. In other words, the reduced-order mathematical model obtained after reducing the order of the input-output mathematical model includes:

[0080]

[0081]

[0082] The step of reducing the order of the input-output mathematical model further includes:

[0083] Time-domain transformation of equations (3) and (4) yields:

[0084]

[0085] In equation (5), i1 represents the vector [i 1α i 1β ] T u inv Represents vector [u invα ,u invβ ] T δ u [δ] uα ,δ uβ ] T ;

[0086] Based on the parameter uncertainties of the series-side converter system, the time-domain expression of the system with parameter uncertainties is obtained as follows:

[0087]

[0088]

[0089] In equations (6) and (7), ΔL represents the fluctuation of the line inductance and the converter inductance, f d This indicates the possible presence of unknown disturbances in the system.

[0090] Preferably, a linear space transformation is performed on the reduced-order input-output mathematical model to obtain the following state-space equations:

[0091]

[0092]

[0093] In equations (8) and (9), L1 represents the converter-side inductance, and L′2 = L2 - (L r +L s ) represents the transformer-side inductance L2 and the line inductance (L r ,Ls The equivalent inductance resulting from the combined action of the two.

[0094] In this embodiment, after performing a linear space transformation on the reduced-order input-output mathematical model, let state variable x2 represent the total perturbation and state variable x1 represent i1, then we obtain...

[0095]

[0096] The state-space equations are further obtained as follows:

[0097]

[0098] in, C = [1 0].

[0099] Preferably, such as Figure 4 As shown, it is

[0100] The state observer when C = [1 0] has the following specific form:

[0101]

[0102] in, express L represents the observer gain as [β1, β2]. T .

[0103] Furthermore, the total disturbance x2 was observed using a Luneburger observer, and the observed value was... To suppress the impact of the total disturbance, the estimated value of the total disturbance is compensated, and the control rate is:

[0104]

[0105] Where u f This indicates the output of the outer loop PR controller.

[0106] Optionally, the mathematical model for compensating for disturbances generated by linear space transformations using the state observer LESO includes:

[0107]

[0108] In equation (10), x2 This represents the total disturbance. express x2 The observed values.

[0109] In this embodiment, the observer gain is designed to make the observed values... Tracking x2 allows the outer-loop controller to control an ideal nominal object. For selecting the observer gain, first determine the maximum frequency ω of the desired disturbance suppression. d The observer's poles are positioned at ω, which is 5 to 10 times the imaginary axis. d At this point, a value of 5 is chosen to effectively suppress disturbances. The specific calculation formula is as follows:

[0110] |λI-(A-LC)|=[λ+5ω d ] 2 ,

[0111] If the coefficients of the characteristic polynomials with respect to λ on both sides of the calculation formula are the same, then the observer gain L can be directly obtained.

[0112] Preferably, the phase lead element in the phase lead element designed based on the frequency characteristics of the delay includes:

[0113]

[0114] Where, if a>b, G a The phase characteristics are:

[0115]

[0116] In equations (11) and (12), the values ​​of parameters a and b are adjusted according to G. a The choice of operating frequency band and size.

[0117] Furthermore, the steps for calculating the current command value in the stationary coordinate system based on the power flow equations of the unified power flow controller include:

[0118] Based on the circuit topology of the unified power flow controller, its power flow equations are determined as follows:

[0119]

[0120] The desired active power P on the transmission line * line and reactive power Q * line If the series transformer turns ratio is 1, the desired current command value in the synchronous rotating coordinate system can be obtained.

[0121] Transform the desired current command value into a current command value in a stationary coordinate system.

[0122] Furthermore, the mathematical expression for the current command value in the stationary coordinate system includes:

[0123]

[0124] In equation (14), θ represents the output angle of the phase-locked loop.

[0125] Preferably, the tracking equations of the designed current tracking controller include:

[0126]

[0127] In equation (15), k p Represents the proportional gain, k r ω1 represents the generalized integral gain, and ω1 represents the angular frequency of the tracked reference current.

[0128] In this embodiment, according to Figure 4 The input-output transfer function of the observer is determined as follows:

[0129]

[0130] in,

[0131] In the formula, b=1 / (L1+L′2), u(s)=u inv (s).

[0132] In the actual implementation of the controller, there is a computational delay of 1 clock cycle between sampling and control output. The delay element is expressed in the s-domain as follows:

[0133]

[0134] In the formula, T s Indicates the sampling period.

[0135] Furthermore, the delay produces a large phase lag at the mid and high frequencies of the system, which reduces the system's stability domain.

[0136] For a conventional system, the open-loop transfer function of the inner loop is:

[0137]

[0138] Where, ω o Indicates the bandwidth of LESO;

[0139] The phase of the open-loop transfer function at the resonant frequency is:

[0140]

[0141] As can be seen from the open-loop transfer function of a conventional system, the phase lag caused by the delay at the resonant frequency is known through the phase formula of the open-loop transfer function. This causes the open-loop phase to cross -180° at a lower frequency, reducing the allowable resonant frequency range of the system and thus worsening the system's robustness, which is not conducive to engineering practice.

[0142] Optionally, in order to suppress phase lag caused by delay in the desired frequency band, the following phase lead element is designed:

[0143]

[0144] Where a>b, G a The phase characteristics are:

[0145]

[0146] Adjusting the values ​​of parameters a and b can be done by selecting G. a The effective frequency band and magnitude. For G a Differentiating the phase characteristic formula and setting the derivative to zero, we can obtain G. a Maximum leading angular frequency ω m and maximum leading angle

[0147]

[0148] To expand the system's stability domain, the phase generated by the phase-leading element needs to be consistently greater than the phase lag generated by the delay element in the desired frequency band.

[0149] Obviously, The second derivative in (0, ω) m Since G is less than 0, G is not less than 0. a In (0,ω) m The curve above is convex. Let...

[0150] Considering The open-loop phase of the system can be obtained to satisfy the following relationship:

[0151]

[0152] according to It can be seen that in (0,ω) m The open-loop phase of the system is above -180°, meaning the stability region of the system is (0, ω). m Choose appropriate parameters to make ω m Greater than ω s / 6 can expand the stability region.

[0153] In other words, the parameter adjustment sequence for the lead element is as follows: 1) Determine the desired maximum stable frequency ω m ;2) According to Calculate the maximum lead angle required 3) Based on the maximum leading angle Calculate parameter a using the formula; 4) Calculate the maximum lead angular frequency ω mThe formula is used to calculate parameter b.

[0154] Specifically, according to Figure 5 The open-loop transfer function of the LESO loop can be obtained as follows:

[0155] G o (s)=G a (s)G d (s)G p (s)G y (s)

[0156] In the LESO loop, Ga(s) is used to compensate for the phase lag caused by the delay element Gd(s), thereby preventing the system from crossing -180° in phase at the resonant frequency.

[0157] according to Figure 6 Bode plots of the open-loop system show that the system cannot be stable without a delay compensation stage. In the embodiments of this application, by employing a lead compensation stage, the system satisfies the Nyquist phase condition at the resonant frequency, and the system is stable.

[0158] It should be noted that G a The parameters a and b not only relate to the stability region but also affect the closed-loop performance. An excessively large a requires a very small proportional gain in the outer-loop PR controller to stabilize the system, which reduces the system bandwidth and impacts tracking accuracy and response speed. Therefore, the final parameters need to be fine-tuned based on the outer-loop performance.

[0159] Furthermore, according to Figure 2 The formula for determining the power flow of a circuit based on its circuit topology is as follows:

[0160]

[0161] Let P be the desired active and reactive power on the transmission line. * line and Q * line Sampling the receiving-end node voltage U2, and considering the series transformer (YY) with a turns ratio of 1, the desired current command value is:

[0162]

[0163] To avoid coupling between the dq axes in the synchronous rotating coordinate system, the current command value is transformed to the stationary coordinate system αβ:

[0164]

[0165] In the formula, θ represents the output angle of the phase-locked loop.

[0166] In this context, the current command value in the stationary coordinate system is a sinusoidal signal. To reduce the steady-state error of current tracking, a proportional resonant (PR) controller is used.

[0167] Optionally, based on Figure 5 The transfer function of the system after LESO closed-loop is:

[0168]

[0169] The LCL-type filter has an infinitely large resonant peak at the resonant frequency. After passing through the LESO closed loop, the resonant peak is weakened, but it still limits the bandwidth of the current tracking loop. Therefore, the following improved current tracking controller tracking equation is adopted:

[0170]

[0171] In the formula, k p Represents the proportional gain, k r ω1 represents the generalized integral gain, and ω1 represents the angular frequency of the tracked reference current.

[0172] Actual power grids or voltage sources exhibit certain frequency fluctuations, ω i This is used to extend the operating range of the PR controller. Based on the transfer function of the system after the LESO closed loop and the tracking equation of the improved current tracking controller, the open-loop transfer function of the outer-loop controller can be obtained as follows:

[0173] G opo (s)=G c (s)G cp (s),

[0174] Among them, controller G c The latter half is a first-order low-pass filter, composed of... Figure 7 It can be seen that the low-pass filter further weakens the resonance peak of the inner closed-loop system and increases k. p The upper limit of the system bandwidth is increased, enabling the series converter of the power flow controller to quickly track the current command value and improve the dynamic response of power flow control.

[0175] In this embodiment of the invention, a mathematical model of the series-side converter of a unified power flow converter based on an LCL filter is established and approximated at low frequencies. Then, a linear space transformation is performed on the reduced-order model, and a LESO observer is designed to estimate the disturbance. Next, the frequency characteristics of the controller delay are analyzed, and a phase lead element is designed according to the desired stability domain. Finally, the reference value of the converter output current is calculated according to the power command, and an outer-loop tracking controller combining a PR controller and a first-order low-pass filter is designed. This can promote the miniaturization design of the power flow controller, improve system stability, improve the quality of the series-side output current of the power flow controller, and also has practicality.

[0176] This application also provides a disturbance suppression system for an LCL-type unified power flow controller, including a memory and a processor. The memory stores a computer program that runs on the processor. When the processor executes the computer program, it implements the steps of the disturbance suppression method for an LCL-type unified power flow controller as described above.

[0177] The system embodiments provided in this application and the method embodiments described above have been described in detail, and will not be repeated here.

[0178] In the description of this application, it should be noted that the terms "upper," "lower," etc., indicating the orientation or positional relationship are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. Unless otherwise expressly specified and limited, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication between two elements. For those skilled in the art, the specific meaning of the above terms in this application can be understood according to the specific circumstances.

[0179] It should be noted that in this application, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0180] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A disturbance suppression method applied to an LCL-type unified power flow controller, characterized in that, Includes the following steps: Based on the LCL-type filter, construct the input-output mathematical model of its series-side converter system in the s-domain, and reduce the order of the input-output mathematical model; The reduced-order input-output mathematical model is subjected to a linear space transformation, and then the disturbance generated by the linear space transformation is compensated by the state observer LESO. Design a phase lead element based on the frequency characteristics of delay; The current command value in the stationary coordinate system is calculated based on the power flow equation of the unified power flow controller, and a current tracking controller is designed to track the current command value in real time. The phase lead element in the phase lead element designed based on the frequency characteristics of the delay includes: （11） Among them, if a >b, G a The phase characteristics are: （12） In equations (11) and (12), the adjustment parameters a and b Size according to G a The selection of the operating frequency band and size; The tracking equations of the designed current tracking controller include: （15） In equation (15), k p Indicates proportional gain. k r Represents the generalized integral gain. This indicates the angular frequency of the reference current being tracked.

2. The disturbance suppression method applied to an LCL-type unified power flow controller as described in claim 1, characterized in that, The input-output mathematical model includes: （1） （2） In equations (1) and (2), Indicates the converter-side inductance. Indicates the transformer-side inductance and line inductance The equivalent inductance due to the combined effect Indicates the filter capacitor. Indicates the voltage source at the sending end of the line. With the receiving end voltage source The voltage difference between them Indicates the converter output voltage. Indicates the converter output current. and These represent the system's resonant frequency and anti-resonant frequency, respectively.

3. The disturbance suppression method applied to an LCL-type unified power flow controller as described in claim 1, characterized in that, The reduced-order mathematical model obtained by reducing the order of the input-output mathematical model includes: （3） （4） The step of reducing the order of the input-output mathematical model further includes: Time-domain transformation of equations (3) and (4) yields: （5） In equation (5), Representing vectors , Representing vectors , express ; Based on the parameter uncertainties of the series-side converter system, the time-domain expression of the system with parameter uncertainties is obtained as follows: （6） （7） In equations (6) and (7), ΔL represents the fluctuation of the line inductance and the converter inductance. f d This indicates the possible presence of unknown disturbances in the system.

4. The disturbance suppression method applied to an LCL-type unified power flow controller as described in claim 1, characterized in that, A linear space transformation is performed on the reduced-order input-output mathematical model, resulting in the following state-space equations: （8） （9） In equations (8) and (9), Indicates the converter-side inductance. Indicates the transformer-side inductance and line inductance The equivalent inductance due to the combined effect.

5. The disturbance suppression method applied to an LCL-type unified power flow controller as described in claim 1, characterized in that, The mathematical model for compensating for disturbances caused by linear space transformations using the LESO state observer includes: （10） In equation (10), x 2 represents the total disturbance. express x The observed value of 2.

6. The disturbance suppression method applied to an LCL-type unified power flow controller as described in claim 1, characterized in that, The steps for calculating the current command value in the stationary coordinate system based on the power flow equations of the unified power flow controller include: Based on the circuit topology of the unified power flow controller, its power flow equations are determined as follows: （13） Desired active power on the transmission line and reactive power If the series transformer turns ratio is 1, the desired current command value in the synchronous rotating coordinate system is obtained. Transform the desired current command value into a current command value in a stationary coordinate system.

7. The disturbance suppression method applied to an LCL-type unified power flow controller as described in claim 6, characterized in that, The mathematical expressions for the current command value in the stationary coordinate system include: （14） In equation (14), θ represents the output angle of the phase-locked loop.

8. A disturbance suppression system applied to an LCL-type unified power flow controller, characterized in that, The system includes a memory and a processor. The memory stores a computer program that runs on the processor. When the processor executes the computer program, it implements the steps of the disturbance suppression method applied to an LCL-type unified power flow controller as described in any one of claims 1 to 7.

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