CN117526393B - Method for determining oscillation risk of flexible direct current transmission system under different operation modes - Google Patents
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/002—Flicker reduction, e.g. compensation of flicker introduced by non-linear load
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
- H02J2003/365—Reducing harmonics or oscillations in HVDC
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- H—ELECTRICITY
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- H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
- H02J2203/10—Power transmission or distribution systems management focussing at grid-level, e.g. load flow analysis, node profile computation, meshed network optimisation, active network management or spinning reserve management
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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Abstract
The invention discloses a method for determining oscillation risk of a flexible direct current transmission system under different operation modes, which comprises the following steps: step 1: measuring to obtain the impedance of the alternating-current side power grid containing the near-area multi-layer topology of the flexible direct-current converter station; step 2: determining an impedance similarity calculation formula of the alternating-current side power grid in different operation modes; step 3: combining the operation modes to obtain an operation mode cluster and typical impedance; step 4: and obtaining MMC impedance according to the MMC small signal impedance model. Step 5: and (3) determining the oscillation risk under different operation modes by using an impedance method according to the typical impedance of the AC side power grid operation mode cluster in the step (3) and the MMC impedance in the step (4). The method can efficiently determine the oscillation risk of the flexible direct current transmission system under different operation modes; and the safe and stable operation level of the flexible direct current transmission project under the working condition of the complex power grid is improved.
Description
Technical Field
The invention belongs to the technical field of electric power, and particularly relates to a method for determining oscillation risk of a flexible direct current transmission system under different operation modes.
Background
Along with the improvement of the voltage and capacity grade of a single converter station in the flexible direct current transmission system, the access of the converter station is changed from 35kV distribution network access to 500kV main network access, and the influence of safe and stable operation on an alternating current large power grid is gradually increased. The Modular Multilevel Converter (MMC) is a mainstream choice of a high-power flexible direct-current transmission system with the advantages of modular design, easy expansion, low switching loss, small output voltage distortion and the like. With the use of a plurality of flexible direct current transmission systems, stability problems associated with current transformers are also increasingly pronounced, and power oscillations occurring in flexible direct current transmission systems relate to already from sub-synchronous frequency intervals to medium-high frequency intervals. These oscillation events, when occurring, cause the converter station to lock out, and the resulting surplus/deficit of power will have a severe impact on the ac grid that is connected. Numerous studies have shown that under certain control strategies or modes of operation, dynamic interactions of the power electronics of the flexible dc power transmission system converter station with the ac grid may occur at certain specific frequencies, resulting in system instability, in particular in voltage/current oscillations.
In order to analyze the oscillation instability risk of the flexible direct current transmission system, the prior literature establishes a detailed small signal impedance model of the MMC, and adopts a characteristic value analysis method, a Bode diagram method, a generalized Nyquist method or a polymerization impedance method and other stability analysis methods to explore the reason of oscillation. From the accident survey, it is clear that a change in the ac-side operating mode (in particular, a line shutdown) has a considerable influence on the generation of high-frequency resonances. However, in the conventional literature, when modeling the equivalent impedance of the alternating current side of the flexible direct current transmission system, the equivalent impedance of the alternating current side cannot be effectively simulated by approximately equivalent of a simple resistor, an inductor and a capacitor which are connected in series and parallel or a single cascading pi-shaped circuit. The traditional analysis model containing the complex communication network mainly depends on a eigenvalue analysis method, so that a state space equation of the system needs to be re-established after the operation mode of the system is changed, and the calculated amount is too large, therefore, only one operation mode is generally considered, and the oscillation risk caused by the change of the operation mode of the system is inconvenient to analyze.
Disclosure of Invention
The invention aims to provide a method for identifying the oscillation risk operation mode of a flexible direct current transmission system, which aims at solving the problem that the conventional oscillation analysis method of the flexible direct current transmission system cannot effectively analyze the influence of the change of the operation mode of the system on the oscillation risk, so as to effectively identify the oscillation risk of the system in different operation modes.
In order to solve the problems in the prior art, the invention provides a method for determining the oscillation risk of a flexible direct current transmission system under different operation modes, which comprises the following steps:
Step 1: measuring to obtain the impedance of the alternating-current side power grid containing the near-area multi-layer topology of the flexible direct-current converter station;
Using a harmonic test power supply in a specified frequency range to measure, and measuring an alternating current side power grid at the same frequency interval in the frequency range to obtain a group of alternating current side power grid impedance sequences, which are defined as Z grid＝{Zg_f1,Zg_f2,…,Zg_fi,…,Zg_fN }, wherein Z g_fi represents the alternating current side power grid impedance when the frequency is fi;
step 2: determining an impedance similarity calculation formula of the alternating-current side power grid in different operation modes;
N devices are shared in the AC side power grid, N is a positive integer, the layer of each device is determined, the sequence number of the N-1 fault operation mode of the device is defined according to the sequence number of the device, the initial operation mode is defined as the (n+1) th operation mode, the impedance similarity calculation formula of the AC side power grid in different operation modes is determined, and the impedance similarity of the AC side power grid in the (m) th and the (N) th operation modes comprises impedance amplitude similarity And impedance phase angle similarity/>m，n＝1,2…,N+1；
Setting an impedance amplitude similarity minimum value C |z|_min and an impedance phase angle similarity minimum value
Step 3: combining the operation modes to obtain an operation mode cluster and typical impedance;
Step 3.1: calculating the impedance similarity of the AC side power grid impedance in the AC side power grid initial operation mode and the AC side power grid impedance in other operation modes, and combining the operation modes with all the impedance amplitude similarity and the impedance phase angle similarity larger than the set value with the initial operation modes into a set;
Step 3.2: for the operation modes which cannot be combined with the initial operation mode, calculating the power grid impedance similarity under the two operation modes after two devices connected to the same node in the same layer are respectively cut off, and combining the two operation modes of which the impedance amplitude similarity and the impedance phase angle similarity are both larger than a set value into a set;
Step 3.3: determining a typical impedance for each mode of operation cluster;
Step 4: obtaining MMC impedance according to the MMC small signal impedance model;
an MMC small signal impedance modeling technology is adopted, an MMC small signal impedance model is established, and MMC impedance under different frequencies is obtained through the MMC small signal impedance model;
Step 5: and (3) determining the oscillation risk under different operation modes by using an impedance method according to the typical impedance of the AC side power grid operation mode cluster in the step (3) and the MMC impedance in the step (4).
Preferably, the multi-layer topology in the step 1 is a four-layer topology.
Preferably, in the step 2, the layer where each device is determined is specifically:
The method comprises the steps of taking a flexible direct current converter station bus as a center, defining the flexible direct current converter station bus as a layer 1 station, taking all 500kV transformer substation buses directly connected with the flexible direct current converter station bus as a layer 2 station in a scanning operation mode, and taking equipment between the flexible direct current converter station bus and the layer 2 station as layer 1 equipment; and continuously defining all 500kV transformer substation buses directly connected with the layer 2 station as layer 3 station, wherein equipment between the layer 2 station and the layer 3 station is layer 2 equipment, and so on, all 500kV transformer substation buses directly connected with the layer L station are used as layer L+1 station, equipment between the layer L station and the layer L+1 station is layer L equipment, and after the last layer station is determined, the equipment connected with the last layer station is directly defined as the last layer of equipment, so that each equipment in an alternating current side power grid has a corresponding layer.
Preferably, in the step2, the impedance similarity calculation formula of the ac side power grid under different operation modes specifically includes:
the impedance of the alternating-current side power grid in the nth and the mth operation modes is respectively Represents the ac side grid impedance in the nth mode of operation of the ac side grid at a frequency fi,/>Representing the impedance of the AC side power grid in the m-th operation mode of the AC side power grid when the frequency is fi, wherein the impedance similarity comprises impedance amplitude similarity/>And impedance phase angle similarity/>The impedance similarity calculation formula of the alternating-current side power grid in the m-th and n-th operation modes is as follows:
wherein, And/>Respectively/>And/>Amplitude of/(v)And/>Respectively/>And/>Is a phase angle of (c).
Preferably, the step 3.3: the typical impedance of each run mode cluster is determined as follows:
Selecting the operation mode with the largest number of devices contained in the operation mode cluster as a typical operation mode of the operation mode cluster; if two or more typical operation mode alternatives exist, selecting any operation mode meeting the conditions as the typical operation mode; the impedance of the alternating current side power grid in the selected typical operation mode is taken as the typical impedance of the operation mode cluster.
Preferably, the MMC small-signal impedance model in step 4 includes an electrical loop and a control system dynamic under an MMC average model, where the control system dynamic includes a phase-locked loop, a dc voltage control loop, an ac current control loop, a loop current suppression control loop, a voltage feedforward, and a control system delay dynamic.
Preferably, in the step 5, the risk of oscillation under different operation modes is determined by using an impedance method, specifically:
When the phase difference between the typical impedance of the operation mode cluster and the MMC impedance at a frequency point with equal amplitude exceeds 180 degrees, the flexible direct current transmission system has an oscillation risk at the frequency point; at this time, all operation modes in the operation mode set are system oscillation risk operation modes, and the corresponding frequency is an oscillation risk frequency point of the operation mode cluster.
Compared with the prior art, the invention has the beneficial effects that:
1. According to the method, the characteristics that the impedance of the alternating current power grid of the port of the flexible direct current converter station is influenced by the change of the running mode and weakened along with the fact that the input or cut equipment is far away from the converter station are utilized, different running modes of the alternating current power grid are classified by utilizing the impedance similarity, and the oscillation risk of the flexible direct current power transmission system under the different running modes is determined efficiently.
2. According to the invention, the impedance calculation is utilized to avoid repeatedly establishing a state space model for the power grid model under different alternating current power grid operation modes, so that the calculated amount is reduced, and the oscillation risk analysis speed of the system flexible direct current power transmission system is improved.
3. According to the invention, corresponding oscillation risk frequency points are provided for oscillation risks under different operation modes, so that the safe and stable operation level of the flexible direct-current transmission project under the working condition of a complex power grid can be improved.
Drawings
Fig. 1 is a schematic diagram of an MMC-based flexible dc power transmission system according to an embodiment of the invention;
FIG. 2 is an electrical topology of a three-phase MMC of an embodiment of the present invention;
FIG. 3 is a schematic diagram of an MMC control system according to an embodiment of the present invention;
FIG. 4 (a) is a graph of the amplitude of the grid impedance as a function of frequency for an initial mode of operation of the grid according to an embodiment of the present invention;
FIG. 4 (b) is a graph of the phase of the grid impedance as a function of frequency for an initial mode of operation of the grid according to an embodiment of the invention;
FIG. 5 (a) is a graph of MMC small signal impedance magnitude as a function of frequency for an embodiment of the present invention;
FIG. 5 (b) is a graph of MMC small signal impedance phase versus frequency for an embodiment of the present invention;
Fig. 6 is a flow chart of a method of determining risk of oscillation of a flexible dc power transmission system in different modes of operation according to the present invention.
Detailed Description
For a better understanding of the technical solution of the present invention, the following detailed description of the specific embodiments of the present invention refers to the accompanying drawings and examples.
According to the invention, the characteristics that the impedance of the alternating current power grid of the port of the flexible direct current converter station is influenced by the change of the operation mode and weakened along with the fact that the input or cut equipment is far away from the converter station are utilized to classify the impedance of the alternating current power grid in different operation modes, so that the oscillation risk operation mode of the flexible direct current power transmission system of the system is identified efficiently.
The invention improves the oscillation analysis method of the current flexible direct current transmission system mainly as follows: the accuracy of vibration risk identification is improved by establishing the alternating current power grid with the multilayer topology, meanwhile, the state space model is prevented from being repeatedly established for the power grid model under different alternating current power grid operation modes by utilizing impedance calculation, different operation modes are classified by utilizing impedance similarity, vibration risks under different classifications are obtained, corresponding vibration risk frequency points are further provided for the operation mode classification with the vibration risks, and the safe and stable operation level of the flexible direct current power transmission project under the complex power grid working condition is improved.
The MMC-based flexible direct current transmission system shown in fig. 1 includes an ac side power grid and a flexible direct current converter station (MMC). The alternating-current side power grid is a radial network, the flexible direct-current converter station is connected with the first 500kV transformer substation bus B1 through a flexible direct-current converter station bus A0 and a first overhead line L1 of the alternating-current side power grid, and is further connected to second to fourth 500kV transformer substation buses C1-C3 which are farther through second to fourth overhead lines L2-L4 respectively, and further the second to fourth 500kV transformer substation buses C1-C3 are further connected to fifth to seventh 500kV transformer substation buses D1-D3 which are farther through fifth to seventh overhead lines L5-L7 respectively. The other ends of the fifth to seventh 500kV transformer substation buses D1-D3 are respectively connected with first to third Thevenin equivalent power supplies S1-S3 to represent the rest equivalent area alternating current power grids, and in addition, first to fifth parallel reactors XL1, XL2, XL3, XL4 and XL5 are respectively connected to five nodes of the third to seventh 500kV transformer substation buses C2, C3, D1, D2 and D3. The flexible direct current converter station bus and all substation buses are only used as ideal electrical nodes, and no additional electrical parameters exist, so that the flexible direct current converter station bus and all substation buses are not considered as equipment. The running mode of the alternating current system comprises an initial running mode and a running mode after any one device in the alternating current system is cut off. If N devices are shared in the ac system, the operation mode after any one device in the ac system is cut off is called an N-1 fault operation mode, where N represents the remaining devices, so that the devices represented by N are different in different N-1 fault operation modes. The running mode of the alternating-current side power grid specifically comprises an initial running mode of the alternating-current side power grid and an N-1 fault running mode of equipment participating in power transmission, such as a power transmission line, a parallel compensation reactor and a power supply, after one of the equipment fails, the equipment is cut off.
The MMC-based flexible dc power transmission system shown in fig. 1 includes 15 devices and 8 electrical nodes in the ac side power grid, and each device is cut off to define a new N-1 fault operation mode, and the ac side power grid of this embodiment includes 16 operation modes in total, in which any device is not cut off and one device is cut off.
The flexible direct current converter station comprises an inverter and a step-up transformer. The converter adopts the electrical topology of a three-phase MMC, as shown in fig. 2. The control system of the inverter comprises a phase-locked loop, direct current voltage control, alternating current control, voltage feedforward and control system delay, as shown in fig. 3. The model of the step-up transformer is an ideal transformation ratio transformer considering only leakage inductance. The MMC-based flexible direct current power transmission system further comprises a direct current side model, and the direct current side simulates a passive load through resistors.
The method for determining the oscillation risk of the flexible direct current transmission system in different operation modes according to the embodiment is shown in fig. 6, and includes the following steps:
Step 1: and measuring to obtain the impedance of the alternating-current side power grid containing the flexible direct-current converter station near-area multi-layer topology.
The multi-layer topology is preferably a four-layer topology, and the alternating-current side power grid only considers the impedance of the alternating-current side power grid containing the four-layer topology of the near zone of the flexible direct-current converter station, because the influence of the topology structure of the alternating-current side power grid after exceeding four layers on the impedance of the interface is negligible according to the existing empirical data. The ac side power grid impedance is the port where the MMC accesses the ac side power grid, i.e., at A0 of fig. 1. The method comprises the steps of measuring by using a harmonic test power supply in a certain frequency range, and measuring an alternating-current side power grid at the same frequency interval in the frequency range to obtain a group of alternating-current side power grid impedance sequences, which are defined as Z grid＝{Zg_f1,Zg_f2,…,Zg_fi,…,Zg_fN, wherein Z g_fi represents the alternating-current side power grid impedance at the frequency fi. The specific frequency range and frequency interval of the harmonic test power supply are determined according to the actual analysis precision requirement. The alternating-current side power grid comprises a power transmission line taking distributed parameters into consideration, a parallel compensation reactor, a 500kV transformer substation bus and a Thevenin equivalent power supply model. In this embodiment, graphs of the impedance amplitude and phase of the ac side power grid according to the frequency change in the ac side power grid initial operation mode are shown in fig. 4 (a) and fig. 4 (b).
Step 2: and defining an impedance similarity calculation method of the alternating-current side power grid under different operation modes.
The method comprises the steps of taking a flexible direct current converter station bus (A0) as a center, defining the flexible direct current converter station bus as a layer 1 station, taking all 500kV transformer substation buses directly connected with the flexible direct current converter station bus as a layer 2 station (B1) of an operation mode scanning, and taking equipment between the flexible direct current converter station bus and the layer 2 station (L1) as layer 1 equipment; and continuously defining all 500kV transformer substation buses directly connected with a layer 2 station as a layer 3 station (C1-C3), wherein equipment between the layer 2 station and the layer 3 station is layer 2 equipment (L2-L4), and so on, all 500kV transformer substation buses directly connected with the layer L station are layer 1 station, equipment between the layer L station and the layer L+1 station is layer L equipment, when the last layer station is determined, the equipment connected with the last layer station is directly defined as the last layer equipment, each equipment in an alternating current side power grid belongs to a corresponding layer, corresponding N-1 fault operation modes are also provided for each equipment, N pieces of equipment are contained in the alternating current side power grid for simplifying the subsequent description process, N is a positive integer, N is numbered for each equipment, corresponding operation mode numbers are corresponding to the equipment, and are also numbered N, so that N-1 fault operation modes corresponding to the equipment are shared, and the initial operation mode numbers N are set to be n+1. In this embodiment, the ac side power grid includes 15 devices in total, so that 15N-1 fault operation modes are total, the initial operation mode number is 16, and the number of layers and operation mode numbers of the specific devices are shown in table 1.
TABLE 1
The impedance of the alternating-current side power grid in the nth and the mth operation modes is respectively Represents the ac side grid impedance in the nth mode of operation of the ac side grid at a frequency fi,/>Representing the ac side grid impedance in the mth mode of operation of the ac side grid at a frequency fi, N, m=1, 2 …, n+1; because the impedance is complex, wherein the real part represents the impedance amplitude and the imaginary part represents the impedance phase angle, the impedance similarity of the AC side power grid in the m and n operation modes comprises the impedance amplitude similarity/>And impedance phase angle similarity/>The specific calculation model is as follows:
wherein, And/>Respectively/>And/>Amplitude of/(v)And/>Respectively/>And/>Is a phase angle of (c).
Setting a minimum value C |z|_min and of the impedance similarityThe minimum value of the impedance similarity can be set according to an oscillation suppression method adopted by the flexible direct current system, for example, the oscillation suppression method of adding a first-order low-pass filter to a feed-forward voltage channel is adopted, and the minimum impedance amplitude similarity and the impedance phase angle similarity which cannot be invalid by the original oscillation suppression method after switching between two operation modes are obtained through testing, so that the minimum value of the impedance similarity is obtained. According to the formula, the smaller the impedance similarity is, the larger the influence on the impedance of the alternating current system is after the equipment fails and is cut off.
In this embodiment, C |z|_min and0.99 And 0.97 respectively.
Step 3: the run mode combinations result in a run mode cluster and a typical impedance.
Step 3.1: calculating the impedance similarity of the AC side power grid impedance in the AC side power grid initial operation mode and the AC side power grid impedance in other operation modes, and combining the operation modes with all the impedance amplitude similarity and the impedance phase angle similarity larger than the set value with the initial operation modes into a set.
And (3) calculating the impedance similarity between the AC side power grid impedance in the initial operation mode of the AC side power grid and the AC side power grid impedance in other operation modes by using the formulas (1) and (2). The impedance similarity results of the power grid impedance in the initial operation mode of the ac side power grid and the power grid impedance in other operation modes are shown in table 1.
In table 1, all the operation modes with the impedance amplitude similarity and the impedance phase angle similarity larger than the set values are combined with the initial operation mode to form a cluster, that is, the operation modes of the ablation device XL1, the ablation device S1, the ablation device XL3, the ablation device S2, the ablation device XL4 and the ablation device S3 are combined with the initial operation mode, and at this time, 8 operation modes can not be combined with the initial operation mode.
Step 3.2: and for the operation modes which cannot be combined with the initial operation mode, calculating the power grid impedance similarity corresponding to the two operation modes after two devices connected to the same node in the same layer are respectively cut off, and combining the two operation modes of which the impedance amplitude similarity and the impedance phase angle similarity are both larger than the set value into a set.
In this embodiment, the impedance amplitude similarity between the two operation modes of the ablation device L7 and the ablation device XL2 is 0.9972 and the impedance phase angle similarity is 0.9834. And as the impedance amplitude similarity and the phase angle similarity under the two operation modes are both larger than the set minimum value, combining the two operation modes into one operation mode cluster.
The cluster division of the power grid operation mode is completed. The 16 operation modes of the ac power grid of the embodiment are divided into 8 operation mode clusters by merging, and the division result of the final operation mode clusters is shown in table 2.
TABLE 2
Step 3.3: a typical impedance for each run mode cluster is determined.
A typical impedance determination method for each run-mode cluster is as follows: selecting the operation mode with the largest number of devices contained in the operation mode cluster as a typical operation mode of the operation mode cluster; if two or more typical operation mode alternatives exist, selecting any operation mode meeting the conditions as the typical operation mode; the impedance of the alternating current side power grid in the selected typical operation mode is taken as the typical impedance of the operation mode cluster.
The typical impedance of the operation mode cluster containing the initial operation mode is necessarily the impedance of the ac side power grid of the initial operation mode, because the ac side power grid contains all devices and the number of the devices is the largest when the initial operation mode is adopted.
In the present embodiment, in the operation mode cluster including the operation mode of the cutting device XL2 and the operation mode of the cutting device L7, since the ac side power grid contains a larger number of devices when the device XL2 is cut, the power grid impedance in this operation mode is taken as a typical impedance of the operation mode cluster. Typical impedances for the various run mode clusters in this embodiment are shown in table 2.
Step 4: and obtaining MMC impedance according to the MMC small signal impedance model.
The method comprises the steps of establishing an MMC small-signal impedance model by adopting an existing MMC small-signal impedance modeling technology, wherein the MMC small-signal impedance model generally comprises an electric loop and control system dynamics under an MMC average value model, and the control system dynamics comprises a phase-locked loop, a direct-current voltage control loop, an alternating-current control loop, a circulation suppression control loop, a voltage feedforward and a control system delay dynamics.
And calculating to obtain MMC impedance under different frequencies through an MMC small signal impedance model. In this embodiment, the relationship between the amplitude and the phase of the MMC small signal impedance calculated according to the MMC small signal impedance model and the frequency is shown in fig. 5 (a) and fig. 5 (b).
Step 5: and determining the oscillation risk under different operation modes by using an impedance method according to the typical impedance of the AC side power grid operation mode cluster in the step 3 and the MMC impedance in the step 4.
The oscillation risk of each operation mode set is analyzed by using an impedance method, and the method specifically comprises the following steps: when the typical impedance of the operational mode cluster is more than 180 degrees out of phase with the MMC impedance at a frequency point of equal amplitude, the flexible dc transmission system is at risk of oscillation at that frequency point. At this time, all operation modes in the operation mode set are system oscillation risk operation modes, and the corresponding frequency is an oscillation risk frequency point of the operation mode cluster.
It should be noted that, for an operation mode in which a certain device of the ac side power grid is cut off, the whole MMC-based flexible dc power transmission system cannot normally operate, and the oscillation risk is not considered.
In this embodiment, since no power source is caused in the flexible dc power transmission system based on the MMC after the line L1 is cut, the flexible dc power transmission system based on the MMC cannot normally operate, so that the risk of oscillation is not considered.
The typical impedance of the operation mode cluster and the small-signal impedance of the MMC are combined in table 2, and the oscillation risk judgment results and the oscillation risk frequencies of the other 7 operation mode clusters are shown in table 3.
TABLE 3 Table 3
All oscillation risk operation modes and oscillation risk frequency points of the alternating-current side power grid are all determined, namely the oscillation risk of the flexible direct-current power transmission system under different operation modes is determined.
While particular embodiments of the present invention have been described above with reference to the accompanying drawings, it will be understood by those skilled in the art that these are by way of example only, and that various changes and modifications may be made to these embodiments without departing from the principles and spirit of the invention. The scope of the invention is limited only by the appended claims.
Claims (6)
1. A method for determining the risk of oscillation of a flexible direct current transmission system in different modes of operation, comprising the steps of:
Step 1: measuring to obtain the impedance of the alternating-current side power grid containing the near-area multi-layer topology of the flexible direct-current converter station;
Using a harmonic test power supply in a specified frequency range to measure, and measuring an alternating current side power grid at the same frequency interval in the frequency range to obtain a group of alternating current side power grid impedance sequences, which are defined as Z grid＝{Zg_f1,Zg_f2,…,Zg_fi,…,Zg_fN }, wherein Z g_fi represents the alternating current side power grid impedance when the frequency is fi;
step 2: determining an impedance similarity calculation formula of the alternating-current side power grid in different operation modes;
N devices are shared in the AC side power grid, N is a positive integer, the layer of each device is determined, the sequence number of the N-1 fault operation mode of the device is defined according to the sequence number of the device, the initial operation mode is defined as the (n+1) th operation mode, the impedance similarity calculation formula of the AC side power grid in different operation modes is determined, and the impedance similarity of the AC side power grid in the (m) th and the (N) th operation modes comprises impedance amplitude similarity And impedance phase angle similarity/>m，n＝1,2…,N+1；
Setting an impedance amplitude similarity minimum value C z_min and an impedance phase angle similarity minimum value
Step 3: combining the operation modes to obtain an operation mode cluster and typical impedance;
Step 3.1: calculating the impedance similarity of the AC side power grid impedance in the AC side power grid initial operation mode and the AC side power grid impedance in other operation modes, and combining the operation modes with all the impedance amplitude similarity and the impedance phase angle similarity larger than the set value with the initial operation modes into a set;
Step 3.2: for the operation modes which cannot be combined with the initial operation mode, calculating the power grid impedance similarity under the two operation modes after two devices connected to the same node in the same layer are respectively cut off, and combining the two operation modes of which the impedance amplitude similarity and the impedance phase angle similarity are both larger than a set value into a set;
Step 3.3: determining a typical impedance for each mode of operation cluster;
the typical impedance of each run mode cluster is determined as follows:
Selecting the operation mode with the largest number of devices contained in the operation mode cluster as a typical operation mode of the operation mode cluster; if two or more typical operation mode alternatives exist, selecting any operation mode meeting the conditions as the typical operation mode; taking the impedance of the alternating-current side power grid in the selected typical operation mode as the typical impedance of the operation mode cluster;
Step 4: obtaining MMC impedance according to the MMC small signal impedance model;
an MMC small signal impedance modeling technology is adopted, an MMC small signal impedance model is established, and MMC impedance under different frequencies is obtained through the MMC small signal impedance model;
Step 5: and (3) determining the oscillation risk under different operation modes by using an impedance method according to the typical impedance of the AC side power grid operation mode cluster in the step (3) and the MMC impedance in the step (4).
2. The method for determining the oscillation risk of the flexible direct current transmission system under different operation modes according to claim 1, wherein the multi-layer topology in the step 1 is a four-layer topology.
3. The method for determining the oscillation risk of the flexible direct current transmission system under different operation modes according to claim 1, wherein the determining in the step 2 is specifically performed by the layer where each device is located:
The method comprises the steps of taking a flexible direct current converter station bus as a center, defining the flexible direct current converter station bus as a layer 1 station, taking all 500kV transformer substation buses directly connected with the flexible direct current converter station bus as a layer 2 station in a scanning operation mode, and taking equipment between the flexible direct current converter station bus and the layer 2 station as layer 1 equipment; and continuously defining all 500kV transformer substation buses directly connected with the layer 2 station as layer 3 station, wherein equipment between the layer 2 station and the layer 3 station is layer 2 equipment, and so on, all 500kV transformer substation buses directly connected with the layer L station are used as layer L+1 station, equipment between the layer L station and the layer L+1 station is layer L equipment, and after the last layer station is determined, the equipment connected with the last layer station is directly defined as the last layer of equipment, so that each equipment in an alternating current side power grid has a corresponding layer.
4. The method for determining the oscillation risk of the flexible direct current transmission system in different operation modes according to claim 1, wherein the impedance similarity calculation formula of the alternating current side power grid in the step 2 in different operation modes is specifically as follows:
the impedance of the alternating-current side power grid in the nth and the mth operation modes is respectively Represents the ac side grid impedance in the nth mode of operation of the ac side grid at a frequency fi,/>Representing the impedance of the AC side power grid in the m-th operation mode of the AC side power grid when the frequency is fi, wherein the impedance similarity comprises impedance amplitude similarity/>And impedance phase angle similarityThe impedance similarity calculation formula of the alternating-current side power grid in the m-th and n-th operation modes is as follows:
wherein, And/>Respectively/>And/>Amplitude of/(v)And/>Respectively/>And/>Is a phase angle of (c).
5. The method for determining the oscillation risk of a flexible dc power transmission system according to claim 1, wherein the MMC small signal impedance model in step 4 includes an electrical loop and a control system dynamic in an MMC average model, and the control system dynamic includes a phase-locked loop, a dc voltage control loop, an ac current control loop, a loop current suppression control loop, a voltage feedforward, and a control system delay dynamic.
6. The method for determining the risk of oscillation of the flexible direct current transmission system in different operation modes according to claim 1, wherein the determining the risk of oscillation in different operation modes by using an impedance method in step5 is specifically:
When the phase difference between the typical impedance of the operation mode cluster and the MMC impedance at a frequency point with equal amplitude exceeds 180 degrees, the flexible direct current transmission system has an oscillation risk at the frequency point; at this time, all operation modes in the operation mode set are system oscillation risk operation modes, and the corresponding frequency is an oscillation risk frequency point of the operation mode cluster.
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