Regional power grid broadband oscillation risk assessment method based on electromagnetic transient simulation

Abstract

The invention discloses a regional power grid broadband oscillation risk assessment method based on electromagnetic transient equivalent simulation, which innovatively provides a stability assessment method for solving the problem of regional power grid broadband oscillation risk assessment, and is characterized in that a voltage ratio is directly used as an equivalent open-loop transfer function of a system to carry out stability assessment. According to the method, the complexity of stability evaluation is simplified, the evaluation process is more direct and efficient, and meanwhile, the actual dynamic characteristics of the system can be reflected more accurately, so that the accuracy of stability evaluation is improved.

Description

Technical Field

[0001] The present invention relates to the field of stability analysis of power systems with a high proportion of renewable energy, and in particular to a method for assessing the risk of broadband oscillations in a regional power grid suitable for stability analysis of grid-connected power systems with a high proportion of renewable energy. Background Art

[0002] With the transformation of the global energy structure, the penetration rate of new energy such as wind power and photovoltaics in regional power grids continues to rise. Many regions are actively developing new energy and building a diversified and complementary clean energy system, and offshore wind power is developing rapidly. With the large-scale access of new energy, the structure and operation mode of regional power grids have undergone profound changes, and the degree of power electronics is increasing.

[0003] The impact of large-scale access to renewable energy on the power grid is becoming increasingly prominent. First, the randomness and volatility of renewable energy generation make it more difficult to balance the power grid and the operation mode more complex and changeable. Second, large-scale access to renewable energy has led to changes in the system stability mechanism, making the dynamic behavior of the power grid more complex, and traditional power system analysis methods and control strategies are difficult to fully apply. In addition, the high proportion of renewable energy access has increased the harmonic content in the power grid, and the risk of broadband oscillation has become more prominent, threatening the safe and stable operation of the power grid and restricting the efficient absorption of renewable energy.

[0004] Electromagnetic transient simulation can accurately simulate the electromagnetic transient process of power system components in the time domain, and describe in detail the changes in electrical quantities such as system voltage and current. By establishing power equipment models such as generators, transformers, transmission lines and new energy converters, electromagnetic transient simulation can characterize the dynamic response of the power system under various working conditions and support the safety and stability analysis of the power system.

[0005] However, there are still certain challenges when applying electromagnetic transient simulation technology to the risk assessment of broadband oscillation in regional power grids. On the one hand, wind turbine manufacturers regard the control logic as confidential and do not disclose it to the public due to commercial competition and other factors. It is impossible to directly obtain the control structure and parameters of the wind turbine, making it impossible to directly establish an accurate model of the unit during electromagnetic transient simulation modeling. On the other hand, wind turbines have small capacity and large number. Compared with conventional units, the number of computing nodes and simulation resources occupied by a single unit in the simulation has greatly increased, which is prone to the "curse of dimensionality" problem, resulting in low efficiency of simulation and safety and stability assessment, and it is difficult to meet the actual power grid needs. Summary of the invention

[0006] In order to overcome the difficulties of confidentiality of manufacturer control logic, lack of detailed parameter information of new energy units, and high dimensionality of accurate simulation modeling of actual systems, and to evaluate the risk of power grid oscillation under different operating modes, the present invention proposes a regional power grid broadband oscillation risk assessment method based on electromagnetic transient equivalent simulation, which is applied to large-scale offshore wind power systems and expands the application scope of the power grid broadband oscillation risk assessment method, specifically including: Step 1: On-site wave recording and oscillation feature extraction; When an accident occurs at an offshore wind farm in a certain area, the fault recording equipment is used to record the voltage, current, and power information of the grid connection point of the new energy station, and the oscillation frequency, oscillation amplitude, and divergence rate are mainly extracted as key information. By performing spectrum analysis on the recorded data and extracting oscillation characteristics, a real and reliable data foundation is provided for the simulation modeling of the present invention, ensuring that the modeling work of the present invention is based on actual accident scenarios and has a high degree of authenticity and accuracy.

[0007] Step 2: Grid-side equivalent modeling; The regional scope of the modeled power grid is clarified, and based on the results of the regional power grid topology, operation mode, and line / transformer parameter collection, a power grid side model is constructed in the electromagnetic simulation software of the real-time digital simulator. The model includes power grids within the detailed modeling scope and power grids outside the detailed modeling scope. The power grid within the detailed modeling scope is kept consistent with the actual power grid topology parameters for simulation modeling, and the power grid outside the detailed modeling scope is modeled equivalently. Equivalent modeling is based on the following principles: the total capacity and total active power of the generators in the system are constant before and after the equivalent value, the power flow in the system is basically constant, the short-circuit current of each node in the system is approximately unchanged, and the dynamic change process of the fault voltage at the wind power grid connection point is basically consistent. In the scenario of wind power aggregation and grid connection, priority is given to the consistency of the short-circuit current of key nodes under different grid connection methods before and after the equivalent value.

[0008] Step 3: Equivalent modeling of wind power side; In order to construct the electromagnetic transient simulation model of the wind power side, the total capacity of the wind turbine set is determined according to the actual installed capacity of the station, and the station electromagnetic simulation model is constructed in the form of typical unit power multiplication. The grid-side converter of the unit adopts constant DC voltage control, and the machine-side converter of the unit adopts constant power control. All key control parameters are open and adjustable.

[0009] Step 4: Optimizing and adjusting wind turbine control parameters; For the above-mentioned offshore wind turbine model, the present invention extracts 5 types of control parameters, namely, DC bus voltage control parameters, inverter AC voltage control parameters, positive sequence current d axis control parameters, positive sequence current q axis control parameters and phase-locked loop control parameters. Set the same wind power grid-connected operating power as the on-site fault recording in step 1, adjust the 5 groups of wind power control parameters, and complete the simulation reproduction of the voltage oscillation frequency, amplitude and other characteristics of the wind power grid-connected point at the project site.

[0010] Step 5: Frequency domain stability evaluation; Figure 1 The wind power and grid-connected inverter are equivalent to the parallel connection of the ideal current source Is(s) and the output impedance Zw(s). The grid is equivalent to the series connection of the ideal voltage source Ug(s) and the grid impedance Zg(s). Among them, Zw(s) combines the frequency characteristics of the phase-locked loop, current control loop, filter and other factors, and has no effect on Zg(s). The expression of the grid-connected current Ig(s) is as follows: (1) In the formula, the stability of Ig(s) depends on 1 / [1+Zg(s) / Zw(s)], which can be equivalent to the closed-loop transfer function of a typical negative feedback control system, whose forward channel gain is 1 and the negative feedback channel gain is Zg(s) / Zw(s).

[0011] In conventional stability assessment methods, the ratio of grid-side impedance to wind farm-side impedance is usually used as the equivalent open-loop transfer function of the system, and the grid-connected system stability is analyzed based on the Nyquist stability criterion. However, the traditional stability assessment method has certain limitations, namely, the grid-side impedance and the wind farm-side impedance need to be tested separately. The present invention innovatively proposes a stability assessment method, which can reduce the number of impedance tests. The core of the method is to directly use the voltage ratio as the equivalent open-loop transfer function of the system to carry out stability assessment. The specific formula is as follows: (2) Where, Ig(s)= - Iw(s).

[0012] For a power grid under a certain operation mode, a certain section in the network is selected, and a broadband disturbance voltage is injected in series in the interconnection line between two nodes. The node voltage on the adjacent wind power side is taken as Uw, and the node voltage on the other side is taken as Ug. The ratio of the two voltage record data obtained by a set of broadband disturbance injection under a wide frequency band is calculated, and the stability of the node can be analyzed based on the Nyquist stability criterion. By injecting broadband disturbances in series on different interconnection lines of the regional power grid, the stability of each node in the regional power grid can be evaluated from a frequency perspective.

[0013] Step 6: Time domain typical working condition voltage recording analysis; Based on different grid connections and renewable energy output conditions, typical operation modes are set, and the stability margin of each typical operation mode is evaluated from the frequency domain perspective. The operation mode with the lowest stability margin is selected for further time domain simulation and recording analysis, which is used to evaluate the impact range and oscillation propagation law of the most serious broadband oscillation event in the regional power grid. The specific time domain simulation and recording method is: simulate the adjustment of the operation mode (system disturbance occurs), record the voltage oscillation waveform of each node in the regional power grid, and analyze its oscillation amplitude and amplitude convergence / divergence characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Small signal equivalent circuit diagram of wind power grid-connected system Figure 2 This is a schematic diagram of an equivalent power grid in a certain area of ​​an embodiment of the invention Figure 3 This is a schematic diagram of the steps of the broadband oscillation risk assessment method proposed in the invention Figure 4 Schematic diagram of wind turbine circuit and control structure of wind farm equivalent of a new energy inverter grid-connected system of the invention embodiment Figure 5 This is the frequency domain stability evaluation result of line 2 maintenance in the embodiment of the invention Figure 6 The frequency domain stability evaluation result of the invention embodiment line 2 under maintenance and wind farm 3 with a small step Figure 7 The time domain recording analysis result of the line 1 maintenance of the embodiment of the invention DETAILED DESCRIPTION

[0015] The embodiment of the present invention (a certain offshore wind power collection access 220 area power grid) is as follows Figure 2 As shown, with the 500kV busbars of a certain substation 1 and a certain substation 2 as the boundary, an equivalent power grid system with a maximum voltage level of 500kV is constructed. The external network of the system is equivalent to an ideal power supply and an equivalent reactance. The 220-area power grid includes 27 220kV nodes and 4 wind farms, with capacities of 604MW, 495MW, 215MW and 400MW respectively. For long-distance lines, considering the influence of the distribution of parameters such as inductance, capacitance, resistance and conductivity along the line on the electrical characteristics of the line, the present invention adopts a distributed parameter transmission line model to build. For short-distance lines, the distribution characteristics of the parameters such as the inductance and capacitance of the line have a relatively small influence on the line operation characteristics, so a Π-type equivalent line is used when constructing the network.

[0016] The process of the method for assessing the risk of broadband oscillation in a regional power grid according to the present invention is shown in the attached figure. Figure 3 As shown in the figure, the specific oscillation risk assessment process is as follows: Step 1: Extraction of oscillation characteristics from on-site recording; According to actual engineering cases, in a certain offshore wind power collection and access area, the grid-connected power of offshore wind farm 3 increased suddenly due to wind speed changes, and the grid-connected power of the wind farm jumped from 0.3pu to 0.56pu. Subsequently, the busbars on each side of wind farm 3 experienced voltage fluctuations for about one minute, which eventually led to the failure and shutdown of 56 wind turbines in the farm. According to the fault recording data of the offshore wind farm centralized control station, the frequency of the oscillation component in the voltage RMS curve is 15Hz, the oscillation range is about 60kV, and it quickly diverges and presents nearly equal amplitude oscillations.

[0017] Step 2: Grid-side equivalent modeling According to the collection results of the grid topology, operation mode, and line / transformer parameters of the 220-area offshore wind power collection and access, the grid side model is constructed in the electromagnetic simulation software of the real-time digital simulator. The grid outside the detailed modeling range is equivalent to a voltage source with impedance, and the BPA short-circuit current calculation program is used to compare the short-circuit currents of substation 1 and substation 2 before and after the equivalent value. The short-circuit currents before and after the equivalent value are basically the same, among which the equivalent reactance of the external voltage source of substation 1 is 0.0254H, and the equivalent reactance of the external voltage source of substation 2 is 0.0538H.

[0018] The power grid within the detailed modeling scope is consistent with the actual power grid topology parameters for simulation modeling. After completing the power equipment model, the equivalent error evaluation is carried out to calculate the three-phase short-circuit current of each node in the entire network under three operating conditions: line 1 maintenance (circuit breaker 1 is disconnected), line 2 maintenance (circuit breaker 2 is disconnected), and full wiring. The results are as follows: the three-phase short-circuit current of node 1, node 2, and node 3 under the full wiring operation state is 35.03kA, 29.73kA, and 6.12kA, with errors of 0.15%, 0.06%, and 3.9%; the three-phase short-circuit current of line 1 under maintenance operation is 35.03kA, 29.73kA, and 6.12kA, with errors of 0.25%, 0.14%, and 2.52%; the three-phase short-circuit current of line 2 under maintenance operation is 35.03kA, 29.73kA, and 7.63kA, with errors of 0.07%, 0.29%, and 2.55%. It can be seen that the short-circuit currents before and after the equivalent value at different key nodes and in different typical modes are basically the same, and the obtained equivalent parameters can be used to construct a grid-side simulation model.

[0019] Step 3: Building a wind farm equivalent model The 220 area power grid contains 27 220kV nodes and 4 wind farms, with capacities of 604MW, 495MW, 215MW and 400MW respectively. The electromagnetic transient simulation model of 4 typical 2.5MW permanent magnet direct-drive offshore wind turbines was constructed, and the electromagnetic simulation model of the station was constructed in the form of typical unit power multiplication. According to the total capacity of the wind farm and the series-parallel relationship of the on-site units, the multiplication coefficient of wind farm 1 is 86, wind farm 2 is 198, wind farm 3 is 241.6, and wind farm 4 is 160. Figure 4 The grid-side converter of the unit adopts constant DC voltage control, the machine-side converter of the unit adopts constant power control, and all key control parameters are open and adjustable.

[0020] Step 4: Optimize and adjust wind turbine control parameters When the fault condition of area 220 is line 2 maintenance, the grid-connected power of wind farm 3 steps from 0.3pu to 0.56pu. In order to complete the working condition reproduction, the left side of the grid-connected point of wind farm 3 is regarded as the grid side, and the right side is regarded as the wind farm side. Combined with wind speed control, the grid-connected power of wind farm 3 is realized from 0.3pu to 0.56pu. By continuously adjusting the 5 groups of control parameters of wind farms 1, 2, and 4, the impedance characteristics of the grid side can meet the voltage fluctuation caused by the step of wind farm 3. At this time, the DC bus voltage control parameters, inverter AC voltage control parameters, positive sequence current decoupling d-axis control parameters, positive sequence current decoupling q-axis control parameters, and phase-locked loop control parameters of wind farms 1, 2, and 4 are 1.3, 0.7, 1.2915, 1.3, and 40, respectively. Therefore, the above parameters and wind speed switching can complete the simulation reproduction of the voltage oscillation frequency, amplitude and other characteristics of the wind power grid-connected point on the project site.

[0021] Step 5: Frequency domain stability assessment; According to the stability assessment method proposed in the present invention, for a power grid under a certain operation mode, stability assessment can be carried out by measuring the voltages on both sides of the grid connection point. Based on the simulation model after the above parameters are adjusted, the connecting line between wind farm 4 and wind farm 3 is selected, and a 1% broadband disturbance voltage is injected in series. The voltage of wind farm 3 node is taken as Uw, and the voltage of wind farm 4 node is taken as Ug. An impedance frequency scan of 1~99Hz and 2Hz interval is carried out, and the ratio of the two voltage record data obtained by broadband disturbance injection under broadband bandwidth is calculated. The characteristic root trajectory is as follows Figure 5 As shown in Figure 1, according to the Nyquist stability criterion, the stability margin of line 2 during maintenance is 0.007322, which is consistent with the stability when there is no step. At this time, the wind speed is adjusted to make the grid-connected power of wind farm 3 step from 0.3pu to 0.56pu. Repeating the above operation, the characteristic root locus can be obtained as follows: Figure 6 As shown, it can be seen that the stability margin is -0.06511 when the field fault occurs in area 220, which is consistent with the instability that actually causes voltage fluctuations.

[0022] Keep the above parameters unchanged, disconnect circuit breaker 1, and put line 1 into maintenance state. Repeat the above operations to evaluate the grid oscillation risk in the maintenance operation state of line 1 in the frequency domain. At this time, the stability margin of line 1 during maintenance is -5.78842, and the system stability is poor. Repeat the above operations in various working conditions to complete the frequency domain evaluation of grid oscillation risk under different operation modes.

[0023] Step 6: Time domain typical working condition voltage recording analysis; From step 5, it can be concluded that the operation mode with the lowest stability margin is line 1 switching to maintenance. Further time domain simulation and waveform analysis are carried out to evaluate the impact range and oscillation propagation law of the most serious broadband oscillation event in the regional power grid. Based on the equivalent simulation model after the above parameter optimization, the circuit breaker 1 is disconnected and the voltage oscillation waveforms of each node in the regional power grid are recorded as follows: Figure 7 As shown in the waveform, it can be seen that the oscillation diverges, which is consistent with the frequency domain analysis results in step 5, and the oscillation amplitude is 57kV.

[0024] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A method for assessing the risk of broadband oscillation in a regional power grid based on electromagnetic transient equivalent simulation, characterized in that: The following steps are involved: Step 1: On-site wave recording and oscillation feature extraction; Step 2: Grid-side equivalent modeling; Step 3: Equivalent modeling of wind power side; Step 4: Optimize and adjust wind turbine control parameters; Step 5: Frequency domain stability assessment; Step 6: Time domain voltage recording analysis of typical operating conditions.

2. A method for assessing the risk of broadband oscillation in a regional power grid based on electromagnetic transient equivalent simulation according to claim 1, characterized in that Step 1 uses fault recording equipment to record data on the grid connection point of the new energy station where the accident occurred. By performing spectrum analysis on the recorded data and extracting oscillation characteristics, a real and reliable data basis is provided for subsequent simulation modeling, so that the modeling work of this method is based on actual accident scenarios.

3. A method for assessing the risk of broadband oscillation in a regional power grid based on electromagnetic transient equivalent simulation according to claim 1, characterized in that Step 2 determines the regional scope of the power grid and builds the grid side model in RTDS based on the collection results.

4. A method for assessing the risk of broadband oscillation in a regional power grid based on electromagnetic transient equivalent simulation according to claim 1, characterized in that Step 3 determines the total capacity of wind turbines according to the actual installed capacity of the site, and constructs an electromagnetic simulation model of the wind farm using the power multiplication form of typical units.

5. A method for assessing the risk of broadband oscillation in a regional power grid based on electromagnetic transient equivalent simulation according to claim 1, characterized in that Step 4 extracts 5 types of control parameters, sets the same wind power grid-connected operating power as the on-site fault recording in step 1, adjusts the 5 groups of wind power control parameters, and completes the simulation reproduction of the voltage oscillation frequency, amplitude and other characteristics of the wind power grid-connected point at the project site.

6. A method for assessing the risk of broadband oscillation in a regional power grid based on electromagnetic transient equivalent simulation according to claim 1, characterized in that Step 5: For a power grid under a certain operating mode, a certain section in the network is selected, a broadband disturbance voltage is injected in series into the interconnection line between two nodes, and a ratio of two voltage recording data obtained by a set of broadband disturbance injection is calculated under a broadband band, and whether the node is stable can be analyzed according to the Nyquist stability criterion.

7. A method for assessing the risk of broadband oscillation in a regional power grid based on electromagnetic transient equivalent simulation according to claim 1, characterized in that Step 6: Based on different situations, typical operating modes are set, and the stability margin of each typical operating mode is evaluated from the frequency domain perspective. The operating mode with the lowest stability margin is selected for further time domain simulation and recording analysis to evaluate the impact range and oscillation propagation law when the most serious broadband oscillation event occurs in the regional power grid.

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