Method for suppressing subsynchronous oscillation of new energy through direct current delivery system by using energy storage

Abstract

The invention discloses a method for suppressing subsynchronous oscillation of new energy through a direct current delivery system by applying energy storage, which comprises the following steps of: establishing a state space model of the system, setting a step response for the system, and verifying the correctness of the model; the method comprises the following steps: performing characteristic value analysis on a system to obtain each subsynchronous oscillation mode of the system, performing participation factor analysis on the system to obtain dominant factors of the subsynchronous oscillation modes, and analyzing the influence of energy storage access on system damping based on a characteristic root locus; according to an analysis result, additional damping control is added in an energy storage control link, an output current signal is directly added to double-loop control output of an energy storage inverter through extraction of a voltage component of a grid-connected point, filtering, phase shifting, amplification and amplitude limiting processing, then the current signal is transmitted to the grid-connected point of a new energy station, and system oscillation is counteracted through phase deviation. The method can effectively counteract system oscillation, restrain the problem of subsynchronous oscillation of the system, and is favorable for improving the stable operation level of the system.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy systems, and in particular to a method for suppressing subsynchronous oscillation of a new energy system via a direct current transmission system by using energy storage. Background Art

[0002] my country's renewable energy power generation and load centers are distributed inversely, so it is necessary to transmit electricity to the load center in large capacity and over long distances. Line-commutated-converter based high voltage direct current (LCC-HVDC) has solved the above problems to a certain extent with its mature technology, large capacity and low cost, and has become the main form of renewable energy power generation transmission.

[0003] As the proportion of renewable energy in the power grid system increases, the intermittent and unstable nature of renewable energy generation leads to inconsistency between its power generation characteristics and load demand. Therefore, it is necessary to introduce energy storage technology, which can store and release the electricity generated by photovoltaics, further solving the problem of deviation between power generation time and power consumption time. Therefore, energy storage is a necessary technology for large-scale access of renewable energy generation to the grid. With the rapid development of power electronic conversion technology and the continuous reduction of battery costs, energy storage technology will be developed on a large scale in new power systems. The access of a large number of power electronic equipment has led to increasingly complex system stability problems, and the new type of subsynchronous oscillation problem caused by this has also become increasingly complex. Since the new energy units are essentially different from traditional thermal power units in terms of composition structure, control strategy and grid connection mode, their subsynchronous oscillation (SSO) presents new characteristics, the oscillation mechanism is more complex, and the oscillation suppression method is also different.

[0004] In essence, the oscillation phenomenon of a high proportion of renewable energy through a DC transmission system is a manifestation of the interaction between renewable energy units, reactive compensation equipment, synchronous machines, LCC-HVDC and other diverse equipment through a weak power grid. However, existing literature has little research on the interaction between renewable energy units and energy storage, and even less research on methods to suppress subsynchronous oscillations of renewable energy combined with energy storage through a DC transmission system. Summary of the invention

[0005] The purpose of the present invention is to provide a method for suppressing subsynchronous oscillation of a new energy system via direct current transmission using energy storage, which can effectively offset system oscillation, suppress system subsynchronous oscillation problems, and help improve the level of stable operation of the system.

[0006] The objective of the present invention is achieved through the following technical solutions:

[0007] A method for suppressing subsynchronous oscillation of a new energy source through a direct current transmission system by using energy storage, the method comprising:

[0008] Step 1: Establish a state space model of the system, set a step response for the system, and verify the correctness of the model;

[0009] Step 2: By performing eigenvalue analysis on the system, the various subsynchronous oscillation modes of the system are obtained, and the participating factor analysis of the system is performed to obtain the dominant factors of the subsynchronous oscillation modes. The influence of energy storage access on system damping is analyzed based on the characteristic root loci;

[0010] Step 3: According to the analysis results, additional damping control is added to the energy storage control link. By extracting the voltage component of the grid connection point, the output current signal is directly added to the dual-loop control output of the energy storage inverter after filtering, phase shifting, amplification and limiting, and then transmitted to the grid connection point of the new energy station. The system oscillation is offset by phase shift, thereby suppressing the system subsynchronous oscillation.

[0011] It can be seen from the technical solution provided by the present invention that the above method can effectively offset system oscillations and suppress subsynchronous oscillation problems of the system, which is beneficial to improving the stable operation level of the system. It is also applicable to the oscillation suppression problems of other new energy grid-connected systems and has certain academic research and engineering application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0013] Figure 1 A schematic flow chart of a method for applying energy storage to suppress subsynchronous oscillation of a new energy source through a direct current transmission system provided in an embodiment of the present invention;

[0014] Figure 2 It is a structural schematic diagram of the additional damping control link according to an embodiment of the present invention;

[0015] Figure 3 The examples of the present invention include those with and without SSDC capacitor voltage U DC Schematic diagram of time domain simulation results;

[0016] Figure 4 The examples of the present invention include those with and without SSDC capacitor voltage U DC Schematic diagram of FFT analysis results. DETAILED DESCRIPTION

[0017] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments, which does not constitute a limitation of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0018] like Figure 1 The figure is a flow chart of a method for applying energy storage to suppress subsynchronous oscillation of a new energy source through a direct current transmission system provided by an embodiment of the present invention, the method comprising:

[0019] Step 1: Establish a state space model of the system, set a step response for the system, and verify the correctness of the model;

[0020] In this step, because the structure of the new energy DC transmission system is relatively complex, a modular block modeling method is adopted. The first step is to establish a state space model of each submodule in the system. Each submodule is connected through the input and output signal interface to obtain the state space model of the entire system.

[0021] Calculate the initial power flow of the system, linearize the state space model of the system at the stable operating point corresponding to the power flow result, and build the state space model of each submodule on the MATLAB / Simulink platform based on the obtained small signal differential equations;

[0022] 5s after the start of the simulation, the DC voltage command value Udcref of the photovoltaic power station is set to step, and the step amplitude is 0.05kV. Other reference values ​​remain unchanged. The step response curves of the PSCAD / EMTDC time domain simulation model and the small signal model are obtained. The two step response curves are compared. If the two curves are consistent, the correctness of the model is verified.

[0023] Step 2: By performing eigenvalue analysis on the system, the various subsynchronous oscillation modes of the system are obtained, and the participating factor analysis of the system is performed to obtain the dominant factors of the subsynchronous oscillation modes. The influence of energy storage access on system damping is analyzed based on the characteristic root loci;

[0024] In this step, the state space model of the system obtained in step 1 is used to solve the eigenvalue of the state space model of the system to obtain each mode of subsynchronous oscillation and the frequency of each mode pair;

[0025] Perform participation factor analysis on each mode of subsynchronous oscillation, and obtain the dominant factors corresponding to the subsynchronous oscillation of the system from the results of participation factor analysis;

[0026] By changing the presence or absence of energy storage and the size of the pure energy storage capacity, the position changes of the characteristic root loci are observed and analyzed to obtain the influence of pure energy access on the system damping.

[0027] For example, taking a photovoltaic power station based on new energy as an example, through the characteristic value analysis of the photovoltaic storage through the LCC-HVDC transmission system, it can be known that there is a sub-synchronous SSO mode with an oscillation frequency of 15.56Hz, and its dominant factors are the DC capacitor voltage and the energy storage control link parameters. Therefore, additional damping control can be selected to be installed in the energy storage control link.

[0028] Step 3: According to the analysis results, a supplementary subsynchronous damping controller (SSDC) is installed in the energy storage control link. By extracting the voltage component of the grid connection point, the output current signal is directly added to the dual-loop control output of the energy storage inverter after filtering, phase shifting, amplification and limiting, and then transmitted to the grid connection point of the new energy station. Through phase shift, the system oscillation is offset, thereby effectively suppressing the system subsynchronous oscillation.

[0029] In this step, if Figure 2 FIG. 1 is a schematic diagram of the structure of the additional damping control link of the embodiment of the present invention. The additional damping control includes four links: filtering, phase shifting, amplification and limiting, wherein:

[0030] The filtering link is used to filter out the high-frequency components introduced by the switching devices in the DC capacitor voltage uDC;

[0031] The phase shift link is used to adjust the phase of the signal to ensure that the signal maintains the correct phase relationship during transmission;

[0032] The amplification link is used to enhance the amplitude of the signal so that the signal will not be attenuated during the transmission process, ensuring that the system can correctly receive and process the signal;

[0033] The limiter link is used to limit the amplitude of the signal to prevent the signal from being too large or too small and protect the system from damage;

[0034] The specific expression can be seen from the attached figure, where: N(s) and D(s) are the numerator and denominator of the filter transfer function; T1 and T2 are time constants; m is the number of phase shift links; K is the gain.

[0035] In the specific implementation, since different oscillation frequencies require different control parameters, the parameters of the additional damping control lack adaptability. This application uses a particle swarm algorithm to set the parameters of the phase shift link, making the parameter setting more reasonable and conducive to a more stable system. Specifically:

[0036] When the particle swarm optimization algorithm is used to set the parameters of the phase shift link, the objective function is the damping ratio corresponding to the characteristic root of the system characteristic equation, the particle swarm is initialized, the position and velocity parameter restrictions are set, the initial population number, the spatial dimension, and the maximum number of iterations are set;

[0037] In this chapter, the spatial dimension of the particles is set to 2, and the number of particles is set to 50. Substitute the values ​​of the parameters T1 and T2 of the phase shift link into the eigenvalue analysis method to obtain the state matrix of the system; calculate the eigenvalue of the system state matrix and the corresponding damping ratio; substitute the damping ratio into the objective function to calculate the fitness value of each particle; and obtain the optimal values ​​of the parameters T1 and T2 of the damping controller through iteration.

[0038] It is worth noting that the contents not described in detail in the embodiments of the present invention belong to the prior art known to professional and technical personnel in the field.

[0039] The effect of the method described in the embodiment of the present invention is described below with a specific example. In this example, a time domain simulation model of the photovoltaic energy storage via LCC-HVDC transmission system is built in the PSCAD / EMTDC time domain simulation platform, and a time domain simulation is performed to compare the two working conditions of SSDC with and without additional damping control of energy storage. The total simulation time is set to 7.0s. An inductor is connected in series in the line within 4.0s, and other system parameters remain unchanged. The comparison results of the photovoltaic energy storage via LCC-HVDC transmission system under the two working conditions of SSDC with and without energy storage are obtained, as shown in FIG. Figure 3 The example of the present invention is shown with and without SSDC DC capacitor voltage U DC Schematic diagram of time domain simulation results, such as Figure 4 The example of the present invention is shown with and without SSDC DC capacitor voltage U DC Schematic diagram of FFT analysis results.

[0040] from Figure 3 It can be seen that after adding SSDC to the energy storage control link, the DC capacitor voltage U dc The oscillation weakens; Figure 4 It can be seen that when there is no additional control, the DC capacitor voltage U dc There is an obvious subsynchronous component in the PV power station. After adding SSDC, the DC capacitor voltage u DC The subsynchronous component in the system is reduced, SSO is suppressed, and the system runs stably. Figure 3 , Figure 4 It shows that the added additional damping control SSDC provides positive damping for the oscillation mode, verifying the effectiveness of the additional control.

[0041] In addition, a person skilled in the art can understand that all or part of the steps in the above-mentioned embodiment method can be implemented by instructing related hardware through a program, and the corresponding program can be stored in a computer-readable storage medium. The above-mentioned storage medium can be a read-only memory, a disk or an optical disk, etc.

[0042] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with the technical field within the technical scope disclosed in the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims. The information disclosed in the background technology section of this article is only intended to deepen the understanding of the overall background technology of the present invention, and should not be regarded as an admission or in any form that the information constitutes prior art known to those skilled in the art.

Claims

1. A method for suppressing subsynchronous oscillation of a new energy source via a direct current transmission system by using energy storage, characterized in that: The method comprises: Step 1: Establish a state space model of the system, set a step response for the system, and verify the correctness of the model; Step 2: By performing eigenvalue analysis on the system, the various subsynchronous oscillation modes of the system are obtained, and the participating factor analysis of the system is performed to obtain the dominant factors of the subsynchronous oscillation modes. The influence of energy storage access on system damping is analyzed based on the characteristic root loci; Step 3: According to the analysis results, additional damping control is added to the energy storage control link. By extracting the voltage component of the grid connection point, the output current signal is directly added to the dual-loop control output of the energy storage inverter after filtering, phase shifting, amplification and limiting, and then transmitted to the grid connection point of the new energy station. The system oscillation is offset by phase shift, thereby suppressing the system subsynchronous oscillation.

2. According to the method of claim 1, the method of using energy storage to suppress subsynchronous oscillation of a new energy source through a direct current transmission system is characterized in that: In step 1, First, the state space model of each submodule in the system is established. Each submodule is connected through the input and output signal interface to obtain the state space model of the entire system. Calculate the initial power flow of the system, linearize the state space model of the system at the stable operating point corresponding to the power flow result, and build the state space model of each submodule on the MATLAB / Simulink platform based on the obtained small signal differential equations; 5s after the start of the simulation, the DC voltage command value Udcref of the photovoltaic power station is set to step, and the step amplitude is 0.05kV. Other reference values ​​remain unchanged. The step response curves of the PSCAD / EMTDC time domain simulation model and the small signal model are obtained. The two step response curves are compared. If the two curves are consistent, the correctness of the model is verified.

3. According to claim 1, the method for suppressing subsynchronous oscillation of a new energy source through a DC transmission system by using energy storage is characterized in that: In step 2, the state space model of the system is obtained through step 1, and the eigenvalue of the state space model of the system is solved to obtain each mode of subsynchronous oscillation and the frequency of each mode pair; Perform participation factor analysis on each mode of subsynchronous oscillation, and obtain the dominant factors corresponding to the subsynchronous oscillation of the system from the results of participation factor analysis; By changing the presence or absence of energy storage and the size of the pure energy storage capacity, the position changes of the characteristic root loci are observed and analyzed to obtain the influence of pure energy access on the system damping.

4. The method for suppressing subsynchronous oscillation of a new energy source through a direct current transmission system by using energy storage according to claim 1 is characterized in that: In step 3, the additional damping control installed includes four links: filtering, phase shifting, amplification and limiting, among which: The filtering link is used to filter out the high-frequency components introduced by the switching devices in the DC capacitor voltage uDC; The phase shift link is used to adjust the phase of the signal to ensure that the signal maintains the correct phase relationship during transmission; The amplification link is used to enhance the amplitude of the signal so that the signal will not be attenuated during the transmission process, ensuring that the system can correctly receive and process the signal; The limiting link is used to limit the amplitude of the signal to prevent the signal from being too large or too small and protect the system from damage.

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