A capacity-weighted based multi-inverter aggregation method

Abstract

The application provides a capacity-weighted multi-converter aggregation method, which realizes the aggregation and order reduction of multi-converter grid connection under network configuration type control through a power angle swing curve, determines the same group after the same disturbance through the power angle curve, and replaces the same group with a virtual synchronous machine through equivalent circuit and control parameters, verifies the correctness and effectiveness of the aggregation according to the principle that the active power is unchanged before and after the aggregation, and makes the aggregation effect more accurate compared with the parameter aggregation without considering the capacity of the virtual synchronous machine through capacity-weighted equivalence of the parameter value after the aggregation.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of converter control technology in power grid technology, and particularly relates to a multi-converter aggregation method based on capacity weighting. BACKGROUND

[0002] With the rapid development of new energy power generation technology, a large number of power electronic devices are connected to the grid side. Due to the randomness and volatility of photovoltaic and wind power generation technologies, the application of traditional control strategies to the grid will result in poor system stability. At the same time, due to the fast response speed of power electronic devices and the low overvoltage and overcurrent threshold, large-scale disconnection of devices is easily caused by load shedding or fault conditions, affecting the stable operation of the system. The traditional converter control in the grid mostly adopts centralized control or distributed droop control. However, due to the lack of inertia and damping in the system, the stability and robustness of the system are poor. In view of the problem of lack of inertia of power electronic devices, relevant researchers propose a control strategy of grid-forming virtual synchronous generator (VSG), which simulates the mechanical equation of a synchronous generator to make the output characteristic of the converter close to that of a synchronous generator, thereby providing inertia support for the grid system.

[0003] At present, in the operation of large-scale new energy power systems with multi-converter grid-forming control, in order to facilitate the study of system stability and transient simulation, the model order reduction is often realized by using converter aggregation equivalence (harmonic equivalence) method to reduce the complexity of modeling and simulation. In the aggregation method: specific aggregation methods include data mining and clustering algorithm for classifying the behavior of converters, but such methods are only used for power sources with the same control method, and the models with different control methods need to be further studied; there are methods of using Hamiltonian action to equivalent the state equation of the converter to a physically meaningful equivalent model and using impedance voltage drop between converters as the standard for grouping converters, but the control of the converters is still the traditional method, and whether it is applicable to the converters under the control of grid-forming VSG still needs to be studied. In parameter equivalence: the existing methods include direct superposition of parameters, which does not conform to the physical meaning of parameter equivalence of multi-converter, and may result in poor aggregation equivalence effect; there are methods of using inertia center concept to equivalent the parameters of aggregated converters, which increases the calculation amount. SUMMARY

[0004] The purpose of the present application is to reduce the complexity of modeling and simulation in order to study the external dynamic characteristics of multi-converter grid systems.

[0005] In order to achieve the above-mentioned application purpose, the technical scheme of the present application provides a multi-converter aggregation method based on capacity weighting, comprising the following steps:

[0006] According to the control principle of the virtual synchronous generator, a plurality of converter grid-connected control loops are built, and the virtual synchronous generator control formula analyzed by the control principle is used.

[0007] Obtaining a plurality of virtual synchronous machine output voltages and output currents measured from a large-scale new energy power system in a converter grid-connected control loop, combining the virtual synchronous machine control formula to obtain a plurality of active power and reactive power under the operation of the converter grid-connected control loop, and respectively bringing into the active control loop and the reactive control loop in the corresponding converter grid-connected control loop to obtain a plurality of electromotive forces, and realizing control closed loop according to the plurality of electromotive forces;

[0008] According to the plurality of converter grid-connected control loops after the closed loop, a multi-converter grid-connected circuit diagram is drawn, and when the large-scale new energy power system is stable, a multi-power angle swing curve diagram is drawn by increasing a fault disturbance;

[0009] According to the same tone determination condition, the plurality of converters corresponding to the plurality of power angles in the multi-power angle swing curve diagram are grouped to obtain a plurality of same tone groups;

[0010] The plurality of converters in the plurality of same tone groups are respectively equivalent to corresponding equivalent converters by using a multi-converter equivalent parameter aggregation method, and circuit parameters are weighted and aggregated by using a capacity weighting method to obtain aggregated equivalent parameters. The power angle change data in the equivalent parameters before and after the aggregation is combined with the multi-power angle swing curve diagram to obtain a power angle swing curve diagram before and after the aggregation, and a converter grid-connected circuit is generated to realize multi-converter aggregation.

[0011] Preferably, the same tone determination condition formula is as follows:

[0012]

[0013] In the formula, , is the change amount of the power angle of the converter during the disturbance of the two power generation devices, is the start disturbance time, is the disturbance time, is the set maximum allowed power angle difference.

[0014] Preferably, a simulation platform is used to build the plurality of converter grid-connected control loops in a PSCAD / EMTDC environment.

[0015] Preferably, the fault disturbance is added on the common bus.

[0016] Preferably, the equivalent parameters are corrected by comparing the equivalent parameters before and after using the capacity weighting method.

[0017] Preferably, the multi-converter aggregation method further comprises:

[0018] In the drawing of the multi-power angle swing curve, the aggregate pre-change data of active power and reactive power with time after the increase of the disturbance is synchronously acquired;

[0019] After the equivalent parameters are obtained, the aggregate change data of active power and reactive power in the equivalent parameters with time is acquired, and the swing curve of active power and reactive power before and after aggregation is generated in combination with the aggregate pre-change data;

[0020] According to the swing curve of active power and reactive power before and after aggregation, the accuracy of the multiple coherent groups is verified.

[0021] The technical scheme of the present application provides a multi-converter aggregation method based on capacity weighting, which realizes the aggregation and order reduction of multi-converter grid connection under network type control through power angle swing curve, determines the coherent group through the power angle curve under the same disturbance, replaces the coherent group with a virtual synchronous machine, verifies the correctness and effectiveness of aggregation according to the principle of invariable active power before and after aggregation, and makes the aggregation effect more accurate compared with the parameter aggregation without considering the capacity of the virtual synchronous machine through capacity weighting equivalent of the improved parameter value after aggregation. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 A flowchart of a multi-converter aggregation method based on capacity weighting is provided for the embodiments of the present application;

[0023] Figure 2 A virtual synchronous machine topology diagram is provided;

[0024] Figure 3 A VSG power angle change diagram after disturbance is provided;

[0025] Figure 4 A comparison diagram of power angle change after aggregation is provided;

[0026] Figure 5 A comparison diagram of reactive power and active power after all aggregation is provided;

[0027] Figure 6 A converter equivalent swing curve before and after disturbance is provided. DETAILED DESCRIPTION

[0028] The present application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present application and not to limit the scope of the present application. In addition, it should be understood that those skilled in the art can make various modifications or changes to the present application after reading the content taught by the present application, and these equivalent forms also fall within the scope defined by the appended claims.

[0029] As Figure 1As shown, the embodiment of the present application provides a capacity-weighted multi-converter aggregation method, comprising the following steps:

[0030] Determine the control principle of the virtual synchronous machine;

[0031] According to the control principle, a simulation platform is used to build a five-unit complete virtual synchronous machine grid-connected control loop in the PSCAD / EMTDC environment, and through the power, power angle and other formulas given in the control principle, the actual calculation results of these formulas are established in PSCAD / EMTDC.

[0032] According to the multi-converter grid-connected circuit diagram corresponding to the virtual synchronous machine grid-connected control loop, the power angle swing curve diagram of the five converters and the pre-aggregation change data of active power and reactive power over time after the disturbance is added on the common bus are drawn, and the converters are grouped according to the multi-machine aggregation criterion method.

[0033] According to the criterion condition, the grouping of the converters is obtained through the power angle swing curve diagram, and the same-tuned converters are replaced by an equivalent converter through the multi-converter equivalent parameter aggregation method. The circuit parameters of the converters are weighted and aggregated by considering the capacity weighting method to obtain the equivalent parameters after aggregation. The control parameters are superimposed to obtain a new converter grid-connected circuit.

[0034] According to the equivalent parameters after aggregation, the grid-connected circuit after aggregation is built through PSCAD, and the post-aggregation power angle change data is obtained through the calculation method of the power angle in the following formula (1). The pre- and post-aggregation power angle swing curve diagrams are generated in combination with the power angle swing curve diagram. Then, the post-aggregation active power and reactive power change data over time is obtained through the calculation method of the active power and reactive power in the following formula (3), and the pre- and post-aggregation active power and reactive power swing curve diagrams are obtained in combination with the pre-aggregation change data. The effectiveness of the aggregation method is obtained by comparing the pre-aggregation power and power angle curves in the power angle and power swing curve diagrams. And by comparing with the equivalent parameters without considering the capacity weighting, the accuracy of the improved equivalent parameters is obtained.

[0035] Specifically as follows:

[0036] According to the control principle of the virtual synchronous machine as shown in Figure 2 , the virtual synchronous machine control formula is determined, such as formula (1):

[0037] (1)

[0038] In the formula, is the rotational inertia coefficient of the rotor, , are active power given value and active power output value of new energy large power system respectively, is active-frequency droop coefficient (damping coefficient), is virtual excitation adjustment coefficient, is reactive droop coefficient, is angular frequency of new energy large power system, is stator-rotor mutual inductance coefficient, is excitation current, is reactive power given value, is actual reactive power, is VSG output voltage value, is VSG output rated voltage value, is virtual torque, is low-pass filter time constant.

[0039] According to the control working principle, the virtual synchronous machine grid-connected control loop is built, the virtual synchronous machine output voltage and output current measured from the new energy large power system in the virtual synchronous machine grid-connected control loop are obtained, the active power and the reactive power under the operation of the virtual synchronous machine grid-connected control loop are obtained in combination with the virtual synchronous machine control formula, and the two are substituted into the active control loop and the reactive control loop in the virtual synchronous machine grid-connected control loop respectively to obtain the control output , the virtual excitation flux , and finally the switching device is realized through the electromotive force as the control PWM signal, and the on-off of the switching device is realized, and the closed loop control is finally achieved.

[0040] According to the above virtual synchronous machine grid-connected control loop, the corresponding multi-converter grid-connected circuit diagram is drawn, when the new energy large power system is stable, the method of increasing fault disturbance on the common bus is used to draw the swing curve diagram of the power angle of five converters and the aggregation change data of the active power and the reactive power with time after the disturbance is increased, and then the converters are divided into groups to obtain the coherent group according to the multi-machine aggregation criterion method, i.e. the coherent equivalent determination condition. The coherent determination condition is shown in formula (2):

[0041] (2)

[0042] In the formula, , are the change amounts of the power angle of two power generation equipments during the disturbance, is the start disturbance time, is the disturbance time, is the set maximum allowed power angle difference.

[0043] Table 1 Parameters based on virtual synchronous machine control

[0044]

[0045] The time-domain simulation results of the swing curve equations for converters 1-5 are as follows: Figure 3 As shown. From Figure 3 It can be seen that the power angle swing curves of converters 1-3 and 4-5 are similar, and they can be regarded as two sets of synchronous converters. Furthermore, it can be visually observed that the power angle swing curves of converter 4-5 are significantly different from those of converter 1-3. Based on formula (2), the maximum power angle difference for each converter is calculated in Table 2. The table shows that the power angle difference for converters 1-3 is within 5°, while the difference with converter 4-5 is greater than 15°. Therefore, VSG1, VSG2, and VSG3 are considered to be synchronously equivalent, and VSG4 and VSG5 are classified into the same synchronous group. Figure 4 As shown, after parameter equivalence aggregation, the five VSGs are equivalent to two VSGAs and VSGBs, and the power angle swing curves before and after aggregation are generated by combining the power angle swing curves. Among them, VSG1, VSG2, and VSG3 are in group A, and VSG4 and VSG5 are in group B.

[0046] Table 2 Maximum power angle difference of VSGs

[0047]

[0048] Based on the aggregated homology group, the multiple converters are aggregated through parameter equivalence, and the circuit parameters are equivalent using the capacity weighting method, as shown in formula (3) below:

[0049] (3)

[0050] In the formula, , These represent the equivalent line impedance; , These represent the equivalent active and reactive power, respectively. , , , These represent the equivalent control parameters; Indicates converter The ratio of the capacity of the aggregated VSG to the total capacity of the equivalent VSG. For converter Capacity. By combining active and reactive power changes over time after the addition of disturbances, fluctuation curves of active and reactive power before and after aggregation are generated. The reactive power of the two units after full aggregation is compared with that before aggregation. Figure 5 As shown in (a), the active power of the two machines after full aggregation is compared with that before aggregation. Figure 5The power fitting degree of (b) in the figure is high before and after the aggregation, and is in the error range.

[0051] According to the aggregation parameter equivalent method, the effectiveness of the aggregation equivalent parameter is proved, and the simulation comparison of the method and other equivalent methods (without considering the capacity of the converter to directly parallel equivalent circuit parameters) is carried out, and the accuracy of the equivalent method of the embodiment of the application is further highlighted through the comparison results.

[0052] Figure 6 The (a) in the figure is the equivalent swing curve of the converter without applying fault disturbance, and the (b) in the figure is the equivalent swing curve of the converter with applying fault disturbance. Figure 6 As shown in (a) in the figure, in the swing curve of the active power and the reactive power graph without applying the fault (0~3s), the fitting curve of the improved parameter and the actual power is better, and the swing curve of the unimproved parameter ignores the converter capacity, and the equivalent parameter calculated cannot truly reflect the real-time value in the equivalent process of the converter parameter.

[0053] Figure 6 The (b) in the figure is the equivalent swing curve of the converter with applying fault disturbance, and the (b) in the figure is the equivalent swing curve of the converter with applying fault disturbance. Figure 6 As shown in (b) in the figure, in the system dynamic swing process, the swing curve of the improved parameter is also better fitted before the aggregation, and it is observed that the change trend of the improved parameter and the unimproved parameter on the swing curve is mainly determined by the control parameter after the aggregation, and it can be seen that the change trend of the improved circuit parameter and the unimproved circuit parameter after the aggregation is basically consistent.

[0054] The beneficial effects of the embodiment of the application are:

[0055] Based on the working principle of the virtual synchronous machine, a multi-converter grid connection and a same frequency criterion condition are built in the PSCAD / EMTDC environment, a virtual power angle change curve classification is designed as a multi-virtual synchronous machine aggregation condition, under the same disturbance, the same frequency group is determined through the power angle curve, the equivalent circuit and the control parameter, and the same frequency group is replaced by a virtual synchronous machine, and then the correctness and effectiveness of the aggregation are verified according to the principle of the invariable active power before and after the aggregation, and the capacity weighted equivalent of the improved parameter value after the aggregation is used, so that the aggregation effect is more accurate compared with the parameter aggregation without considering the capacity of the virtual synchronous machine.

Claims

1. A capacity-weighted aggregation method for multiple converters, characterized in that, Includes the following steps: Based on the control principle of the virtual synchronous machine, a grid-connected control loop for multiple converters is constructed. The control formula for the virtual synchronous machine, derived from the control principle analysis, is as follows: In the formula, The moment of inertia coefficient of the rotor. , These are the active power setpoint and active power output of a large-scale new energy power system, respectively. The active power-frequency droop factor is... This is the virtual excitation adjustment coefficient. This is the reactive power droop factor. For the angular frequency of large-scale new energy power systems, The mutual inductance coefficient between the stator and rotor is... For excitation current, The reactive power setpoint, This represents the actual reactive power. This refers to the VSG output voltage value. The rated voltage value for VSG output. For virtual torque, This is the time constant for the low-pass filter; Multiple virtual synchronous machine output voltages and currents, measured from a large-scale renewable energy power system, are obtained in the converter grid-connected control loop. These are then combined with the virtual synchronous machine control formula to obtain multiple active power values ​​under the converter grid-connected control loop operation. and reactive power The values ​​are then substituted into the corresponding active and reactive power control loops of the converter grid-connected control circuit to obtain the control output. Virtual excitation flux The control loop is achieved by using the electromotive force as the signal to control the PWM. Based on the grid-connected control loops of multiple converters after the closed loop, draw the grid-connected circuit diagram of multiple converters. After the large-scale power system of new energy is running stably, draw the multi-power angle swing curve by adding fault disturbance. Based on the coherence determination criteria, the multiple converters corresponding to multiple power angles in the multi-power angle swing curve are grouped to obtain multiple coherence groups. The equivalent parameter aggregation method for multiple converters is adopted, which equates multiple converters in multiple homogeneous groups to corresponding equivalent converters. The circuit parameters are then aggregated using a capacity-weighted method to obtain the aggregated equivalent parameters. The capacity-weighted method formula is as follows: In the formula, , These represent the equivalent line impedance, , These represent the equivalent active and reactive power, respectively. , , , These represent the equivalent control parameters. Indicates converter The ratio of the capacity of the aggregated VSG to the total capacity of the equivalent VSG. For converter capacity; By combining the power angle change data after aggregation in the equivalent parameters with the multi-power angle swing curve, the power angle swing curve before and after aggregation is obtained, and the converter grid-connected circuit is generated to realize multi-converter aggregation.

2. The capacity-weighted multi-converter aggregation method as described in claim 1, characterized in that, The formula for determining homology is as follows: In the formula, , This represents the change in power angle during converter disturbances in the two power generation devices. The start time of the disturbance. For the disturbance time, This is the maximum permissible power angle difference.

3. The capacity-weighted multi-converter aggregation method as described in claim 1, characterized in that, The grid-connected control loops of the multiple converters were built using a simulation platform in the PSCAD / EMTDC environment.

4. The capacity-weighted multi-converter aggregation method as described in claim 1, characterized in that, The aforementioned fault disturbance is added to the common bus.

5. The capacity-weighted multi-converter aggregation method as described in claim 1, characterized in that, The equivalent parameters are corrected by comparing the equivalent parameters before and after using the capacity weighting method.

6. The capacity-weighted multi-converter aggregation method as described in claim 1, characterized in that, The multi-converter aggregation method further includes: When plotting the multi-power angle swing curve, the changes in active power and reactive power before aggregation over time after the addition of disturbance are simultaneously acquired. After obtaining the equivalent parameters, the aggregated change data of active power and reactive power in the equivalent parameters over time are obtained, and the swing curves of active power and reactive power before and after aggregation are generated by combining the change data before aggregation. The accuracy of multiple harmonic groups was verified by analyzing the swing curves of active and reactive power before and after aggregation.

Images