An accurate and fast method for suppressing sub- / supersynchronous oscillations

Abstract

The present invention belongs to the technical field of wind farm monitoring, and relates to an accurate and fast method for suppressing sub / super synchronous oscillations. Additional damping control is added to the double-fed control to achieve suppression of subsynchronous oscillations. By extracting the currents of each phase on the double-fed stator and rotor side where subsynchronous oscillations occur in the wind power output system, the stator flux is calculated by a related formula, and then the oscillation component is extracted to obtain the oscillation information of the stator flux, thereby achieving simultaneous suppression of oscillations on the stator and rotor sides. A quasi-resonant controller is used to perform closed-loop control on the oscillation information, and then the subsynchronous component voltage component is obtained through gain and limiting, and this voltage is added to the reference voltage to achieve suppression of subsynchronous oscillations.

Description

Technical Field

[0001] The present invention belongs to the technical field of wind farm monitoring, and relates to an accurate and rapid method for suppressing sub / super synchronous oscillations, and in particular to a rapid and accurate method for suppressing system oscillations (subsynchronous oscillations, SSO) induced by fixed series compensation in a doubly-fed wind farm. Background Art

[0002] New energy power generation technology is developing rapidly in my country, and the trend of power electronics in the power system is becoming increasingly obvious. my country's power system will gradually enter an era of high proportion of renewable energy. In particular, the construction of the "Ubiquitous Power Internet of Things" focuses on the end of the "source-grid-load-storage" link of the system, supports data collection and specific business development, and provides sufficient and effective information and data support for the frequency adaptive active damping control method.

[0003] There is a risk of sub-synchronous oscillation (SSO) in large-scale doubly fed wind farms when they are transmitted via series compensation. The sub-synchronous oscillation problem of series compensation transmission is mainly caused by the mutual coupling between the DFIG converter control and the series compensation line, which is independent of the shaft system's natural modal frequency, so it is called sub-synchronous oscillation, or sub-synchronous control interaction (SSCI). In addition, wind resources have the characteristics of randomness, volatility, and intermittency, which leads to the corresponding volatility and randomness of power injection into the power system, resulting in its oscillation frequency having significant time-varying characteristics. Therefore, in order to ensure the stability of wind farms, it is of great engineering significance to study the SSCI suppression method under multiple operating conditions.

[0004] In response to the subsynchronous oscillation problem caused by wind power series compensation transmission, relevant scholars and research institutions have carried out a lot of research on suppression measures, which can be mainly divided into the following two categories: First, with the help of external equipment, the oscillation suppression is achieved by improving the system damping at the oscillation frequency on the grid side, such as the static synchronous compensator (STATCOM) and static VAR compensator (SVC) connected to the line. Second, through the DFIG's own converter control, the oscillation suppression is achieved by adding additional control, such as additional damping control, virtual resistance and other suppression strategies. From the actual application effect, the suppression method with the help of external equipment has limited overall suppression effect, and the cost is high compared with the suppression method through the wind turbine's own control, which restricts its application in engineering. The control strategy of additional damping is relatively complex in designing and optimizing the parameters of the damping link. If the parameters are not designed properly, the damping level of the system may be reduced. The suppression strategy of virtual resistance is to improve the oscillation damping of the system by introducing the rotor current into the DFIG converter. The operation is simple and easy to implement, but its control performance is poor. When the oscillation frequency is offset, the damping level of the system may be reduced. In addition, there are some other suppression methods, such as anti-disturbance control technology, sliding mode control, etc. It is reported in the literature that the controller based on anti-disturbance control can improve the anti-disturbance ability of the doubly fed wind turbine, thereby reducing torque fluctuations. However, the parameter design of the anti-disturbance control technology needs to be based on the actual power plant design. When the oscillation frequency drifts or the grid operation changes significantly, it is difficult to guarantee the control effect. Summary of the invention

[0005] Aiming at the problem of subsynchronous oscillation existing in traditional large-scale double-fed wind farms when transmitting power through series compensation, the present invention proposes a novel method for accurately and quickly suppressing subsynchronous / supersynchronous oscillation.

[0006] In order to achieve the above object, the present invention is implemented by adopting the following technical solutions:

[0007] The present invention discloses a method for suppressing subsynchronous oscillation of a double-fed transmission line via a series compensation. The method adds additional damping control to the double-fed control to suppress subsynchronous oscillation. By extracting the currents of each phase on the double-fed stator and rotor side where subsynchronous oscillation occurs in the wind power output system, the stator flux is calculated by a related formula, and then the oscillation component is extracted to obtain the oscillation information of the stator flux, thereby suppressing the oscillation on the stator and rotor sides at the same time. A quasi-resonant controller is used to perform closed-loop control on the oscillation information, and then the subsynchronous component voltage component is obtained through gain and limiting, and this voltage is added to the reference voltage to suppress the subsynchronous oscillation.

[0008] The above-mentioned method for accurate and rapid suppression of sub- / super-synchronous oscillations has the following specific steps:

[0009] (1) Pre-filtering of the sampled current / voltage signal, the formula is as follows:

[0010]

[0011] Among them, ω L is the low-pass filter G LP (s) cutoff angular frequency, ω L The value is selected according to the oscillation range of the SSCI of the wind farm, and the high-frequency component is filtered out without affecting the amplitude-frequency and phase-frequency characteristics of the subsynchronous oscillation component; ξ is the damping coefficient, and its value is 0.707; s corresponds to t in the real number domain, represents a complex number, and refers to a variable in space.

[0012] (2) Data calculation

[0013] The rotor current is transformed into the stator coordinate system, and then the stator flux component is calculated using the following formula:

[0014]

[0015]

[0016] Among them, U s represents the stator voltage vector, U r represents the rotor voltage vector, ψ s represents the stator flux, ψ r Represents the rotor flux, R s Represents the stator resistance, R r Indicates rotor resistance, L s Represents stator inductance, L m represents the mutual inductance of the stator and rotor, L r Represents the rotor inductance, I s represents the stator current vector, I r represents the rotor current vector, ω represents the synchronous angular velocity, ω slip Indicates the slip angular velocity.

[0017] (3) Extraction of sub- / supersynchronous oscillation components

[0018] The fundamental frequency component is filtered out from the stator flux component by a second-order generalized integrator SOGI to obtain the subsynchronous component. The formula is as follows:

[0019]

[0020] Where k is the damping coefficient and ω is the estimated value of the subsynchronous angular frequency. Output ε v is the error signal of frequency ω in the input subsynchronous signal v.

[0021] (4) Closed-loop control and gain and limiting

[0022] The quasi-resonant controller is used for rotor flux control, and the center frequency of the quasi-resonant controller is obtained by FFT. The frequency band of the stator current is monitored in real time. The formula is:

[0023]

[0024] Among them, K r is the resonance coefficient, ω1 is the center frequency, ω c is the cut-off frequency;

[0025] The magnetic flux oscillation component of the control system is 0, and then the stator and rotor oscillation components in the control system are 0, cutting off the oscillation path of the system and achieving the oscillation suppression of the system.

[0026] Compared with the prior art, the advantages and positive effects of the present invention are:

[0027] 1. Obtain real-time rotor flux through real-time information such as wind turbine stator and rotor current, so as to avoid data errors caused by time delay, resulting in incomplete system suppression and greater losses.

[0028] 2. Before extracting the sub / super synchronous rotor flux signal, a second-order generalized integrator is used to filter out other signals to avoid interference from other signals and improve the reliability of the system in suppressing oscillation.

[0029] 3. The system oscillation can be suppressed by the real-time oscillation frequency to avoid the situation where the system cannot be suppressed due to frequency drift.

[0030] 4. The damping of the system at the oscillation frequency is increased without changing the original structure of the system. It only acts on the oscillation point, indicating the effectiveness of the control. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Schematic diagram of the oscillation suppression control structure.

[0032] Figure 2 This is the SOGI schematic diagram.

[0033] Figure 3 Schematic diagram of stator flux closed-loop control.

[0034] Figure 4 Schematic diagram of extracting rotor flux oscillation components.

[0035] Figure 5 Schematic diagram of the stator flux suppression oscillation simulation verification.

[0036] Figure 6 Schematic diagram of the stator flux suppression oscillation experimental verification. DETAILED DESCRIPTION

[0037] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described below in conjunction with specific embodiments. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0038] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways than those described herein. Therefore, the present invention is not limited to the specific embodiments of the following disclosure.

[0039] Example 1

[0040] Since series-compensated transmission lines have the risk of subsynchronous oscillation, subsynchronous oscillation may occur when the system operating conditions change or the system is disturbed. Figure 1 As shown in the figure, this oscillation is mainly caused by the interaction between the transmission line part of the system and the control link. This oscillation is called subsynchronous oscillation SSO, also known as subsynchronous oscillation interaction (SSCI), which causes the system to oscillate. If the damping of the system is negative, the system oscillation diverges, causing losses.

[0041] The present embodiment provides a method for suppressing subsynchronous oscillations of a doubly-fed transmission line via a series compensation. The method adds additional damping control to the doubly-fed control to suppress subsynchronous oscillations. By extracting the currents of each phase on the doubly-fed stator and rotor side where subsynchronous oscillations occur in the wind power output system, the stator flux is calculated by a related formula, and then the oscillation component is extracted to obtain the oscillation information of the stator flux, thereby suppressing the oscillations on the stator and rotor sides at the same time. A quasi-resonant controller is used to perform closed-loop control on the oscillation information, and then the subsynchronous component voltage component is obtained through gain and limiting, and this voltage is added to the reference voltage to suppress subsynchronous oscillations.

[0042] The schematic diagram of oscillation suppression is as follows Figure 1 As shown in the figure, I is the stator and rotor current signals of the wind farm collected by the system; II is the calculation module, which is used to obtain the stator flux; III is the rotor flux obtained by the system, which contains oscillation components when oscillating; IV is the improved structure of the second-order generalized integrator (SOGI), which can extract fixed-frequency signals, such as Figure 3 As shown; V is the oscillating flux of the extraction system, which is 0 in steady state; VI is the resonant regulator (R regulator), which can achieve error-free tracking of fixed frequency signals, such as Figure 4 As shown; VII is the traditional control block diagram of the wind farm, adding the above control links to the traditional control, such as Figure 5 shown.

[0043] The whole system mainly includes data calculation module II, steady-state signal filtering unit IV, oscillation component tracking link VI and converter traditional control link VII.

[0044] The method for suppressing double-fed subsynchronous oscillation of a series-compensated transmission line comprises the following steps:

[0045] (1) Pre-filtering of sampled current / voltage signals.

[0046] The second-order Butterworth GLP(s) is designed to filter out high-order harmonics, and the attenuation coefficient is set to be greater than 40dB. The current / voltage signal in the high-proportion renewable energy system is filtered using a low-pass filter, and its transfer function is:

[0047]

[0048] In the formula, ω L is the low-pass filter G LP The cut-off angular frequency (s) is selected based on the oscillation range of the wind farm SSCI, and the high-frequency component is filtered out without affecting the amplitude-frequency and phase-frequency characteristics of the subsynchronous oscillation component; ξ is the damping coefficient, which is 0.707 in this embodiment. This part reduces the impact of high-frequency noise.

[0049] (2) Data calculation part:

[0050] According to the analysis requirements, the acquired wind farm current signal is calculated. Since the stator current and the rotor current are not in the same coordinate system and are difficult to calculate, the rotor current is transformed into the stator coordinate system.

[0051] Coordinate transformation calculation formula:

[0052]

[0053] The voltage and flux are calculated as:

[0054]

[0055]

[0056] In this link, the stator flux vector is calculated based on the voltage and current signals obtained in the previous link.

[0057] (3) Extract sub / supersynchronous oscillation components;

[0058] Since the acquired stator flux contains not only oscillating components but also fundamental frequency components, the second-order generalized integrator (SOGI) can track the sinusoidal signal of a specific frequency without static error. Its error output signal can realize the difference signal of its input. When ω is a certain angular velocity, the signal at ω is attenuated by -80db, while the signal at other frequencies will not be attenuated. Therefore, the fundamental frequency signal can be extracted through SOGI, such as Figure 2 shown.

[0059] The transfer function of SOGI is:

[0060]

[0061]

[0062] Where k is the damping factor and ω is the estimated value of the subsynchronous angular frequency. The output εv′ is the error signal of the frequency ω in the input subsynchronous signal v. In other words, when the input v = V m cos(ωt)+V m1 cos(ω1t), the output signal is v=V m1 cos(ω1t), this method can be used to filter out the baseband signal and reduce its impact.

[0063] When ω is designed as the base frequency, SOGI is used. The SSCI frequency of the double-fed series-compensated grid-connected system is generally not very large. By reasonably selecting the value of k, the power frequency component can be extracted, reducing the influence of the power frequency component on the extraction of the subsynchronous oscillation component, accurately removing the base frequency component, and obtaining the subsynchronous component, thus preparing for the subsequent suppression of the subsynchronous component.

[0064] (4) Closed-loop control link and gain and limiting;

[0065] The rotor flux control part adopts a quasi-resonant controller, which does not require complex shift link parameter design and has good adaptability to grid disturbances. Its transfer function is:

[0066]

[0067] Among them, K r is the resonance coefficient, ω1 is the center frequency, ω c is the cut-off frequency. The design of this resonator reduces the gain at the resonant frequency, but increases the gain of the frequency band near it to reduce the adverse effects of frequency offset. The center frequency of the quasi-resonant controller is obtained by FFT, and the frequency band of the stator current is monitored in real time. The coefficient K in the additional control loop can be adjusted according to different control systems, and a limiting link is designed to prevent the suppression control from affecting the original control system.

[0068] In order to achieve zero net difference tracking of the rotor flux, the flux of the control system is 0, and a resonant regulator is used such as Figure 3 shown.

[0069] Figure 3 The transfer function of the mid-resonant regulator is:

[0070] When the magnetic flux oscillation component of the control system is 0, that is, the stator and rotor oscillation components in the control system are 0, the oscillation path of the system is cut off, thereby suppressing the oscillation of the system. This control method is simple and easy to implement, and it will increase the damping of the system at the oscillation component, move the natural oscillation frequency of the system to a higher damping point, and improve the reliability of the system in suppressing oscillation. Its control is an additional control, such as Figure 4 As shown, it will not affect the normal operation of the system. Only when the system oscillates, this control strategy will work to suppress the oscillation of the system.

[0071] The control effect is equivalent to increasing the equivalent resistance of the system, which is equivalent to increasing the damping of the system and improving the stability of the system.

[0072] The subsynchronous oscillation of the simulation system is used to verify the feasibility of the suppression strategy. The simulation results are shown in Figure 5 As shown in Figure 1, the system oscillation is triggered at 0.1s, and this suppression strategy is put into use. The simulation results of suppressing system oscillation show that this suppression strategy can quickly and effectively suppress the subsynchronous oscillation of the system. The corresponding experimental verification of the double-fed wind power generation system with series compensation capacitor transmission is carried out, and the results are shown in Figure 1. Figure 6 As shown in the figure, after the system oscillates, the proposed suppression method is put into use to suppress the oscillation of the system. The oscillation of the system decays rapidly and the system becomes stable. The experimental results show that this suppression strategy is very effective.

[0073] The above description is only a preferred embodiment of the present invention and does not limit the present invention in other forms. Any technician familiar with the profession may use the technical content disclosed above to change or modify it into an equivalent embodiment with equivalent changes and apply it to other fields. However, any simple modification, equivalent change and modification made to the above embodiment based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. An accurate and fast method for suppressing sub- / supersynchronous oscillations, It is characterized in that Extract the current of each phase on the doubly fed stator and rotor side of the wind power output system where subsynchronous oscillation occurs, calculate the stator flux, and extract the oscillation component to obtain the oscillation information of the stator flux, so as to suppress the oscillation on the stator and rotor sides at the same time; use the quasi-resonant controller to perform closed-loop control on the oscillation information, and then obtain the subsynchronous component voltage through gain and limiting, add the subsynchronous component voltage to the reference voltage, perform additional damping control, and suppress the subsynchronous oscillation; Here are the steps: (1) Pre-filtering of the sampled current / voltage signal, the formula is as follows: Among them, ω L is the low-pass filter G LP (s) cutoff angular frequency, ω L The value is selected according to the oscillation range of the SSCI of the wind farm, and the high-frequency component is filtered out without affecting the amplitude-frequency and phase-frequency characteristics of the subsynchronous oscillation component; ξ is the damping coefficient, and its value is 0.707; s corresponds to t in the real number domain, represents a complex number, and refers to a variable in space; (2) Data calculation The rotor current is transformed into the stator coordinate system, and then the stator flux component is calculated using the following formula: Among them, U s represents the stator voltage vector, U r represents the rotor voltage vector, ψ s represents the stator flux, ψ r Represents the rotor flux, R s Represents the stator resistance, R r Indicates rotor resistance, L s Represents stator inductance, L m represents the mutual inductance of the stator and rotor, L r Represents the rotor inductance, I s represents the stator current vector, I r represents the rotor current vector, ω represents the synchronous angular velocity, ω slip Indicates the slip angular velocity; (3) Extraction of sub- / supersynchronous oscillation components The fundamental frequency component is filtered out from the stator flux component by a second-order generalized integrator SOGI to obtain the subsynchronous component. The formula is as follows: Where k is the damping coefficient, ω is the estimated value of the subsynchronous angular frequency, and the output ε v is the error signal of frequency ω in the input subsynchronous signal v; (4) Closed-loop control and gain and limiting The quasi-resonant controller is used for rotor flux control, and the center frequency of the quasi-resonant controller is obtained by FFT. The frequency band of the stator current is monitored in real time. The formula is: Among them, K r is the resonant term coefficient, ω 1 is the center frequency, ω c is the cut-off frequency; The magnetic flux oscillation component of the control system is 0, and then the stator and rotor oscillation components in the control system are 0, cutting off the oscillation path of the system and achieving the oscillation suppression of the system.

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