An equivalent method suitable for new energy equipment under single bus net framework structure

Abstract

The application discloses an equivalent method suitable for new energy equipment under a single bus net rack structure, and comprises the following steps: (1) according to the design requirements of a system, equivalent is conducted on the power grid on both sides of the bus of a selected voltage grade; (2) a two-level inverter with equal power output is used to replace the new energy equipment; and (3) the connection impedance of the two-level inverter is determined according to three-phase short-circuit current. The application improves the dynamic equivalent method of the existing single bus net rack structure, and makes the three-phase short-circuit current provided by the new energy equipment at the main bus remain unchanged and also be included in the steps of dynamic equivalent, so that the application has better effect in electromagnetic transient simulation and can provide certain reference for future engineering design.

Description

Technical Field

[0001] This invention belongs to the field of power system simulation technology, specifically relating to an equivalent method for new energy equipment under a single busbar grid structure. Background Technology

[0002] With the large-scale integration of new energy sources such as wind and solar power, the proportion of traditional power sources, represented by thermal power, is rapidly declining. Power systems will become increasingly affected by the stability of new energy sources. Furthermore, the surge in power electronic equipment accompanying the increase in new energy sources will significantly impact the stability of power system control on the power supply side, while also introducing power quality issues such as harmonics. There is currently no systematic solution to the problem of establishing electromagnetic transient models for AC / DC power systems. Due to the enormous computational demands of electromagnetic transient simulation, commonly used electromagnetic transient simulation software is significantly limited in terms of simulation efficiency and hardware cost, making large-scale simulations impractical. To improve simulation efficiency and save hardware costs, dynamic equivalent methods suitable for electromagnetic transient simulation are becoming increasingly important.

[0003] The traditional power system equivalent method proposed in the literature [Xu Zheng, Yang Jingping, Duan Hui. An equivalent simplification method suitable for electromagnetic transient simulation[J]. Southern Power Grid Technology, 2007(1):4] will form a negative resistance branch during the equivalence process, so it cannot be applied to electromagnetic transient simulation analysis and requires human intervention to modify a large number of parameters.

[0004] The literature [Weng Hua, Xu Zheng, Wang Xinggang, et al. Simplification method of AC / DC system of Southern Power Grid [J]. Power System Technology, 2012, 36(3):5] proposes a dynamic equivalent method for electromagnetic transient simulation based on the physical equivalence concept. The system is layered according to voltage level, and it is believed that the dynamic characteristics of the large AC / DC system are mainly determined by the system strength of the large-capacity DC line access layer, that is, the support strength of the generator to the backbone network. The equivalent system not only ensures that it has the same stable power flow characteristics as the prototype system, but also ensures that the three-phase short-circuit current of the backbone network is the same. Thus, it simplifies and reduces the order of the original network without affecting the short-circuit current analysis. At present, this method considers that the three-phase short-circuit current of 500kV bus under a single bus network remains unchanged before and after equivalence and the zero-sequence impedance of the equivalent transformer, but does not consider the different operating characteristics of new energy equipment and synchronous generators during faults. Summary of the Invention

[0005] In view of the above, the present invention provides an equivalent method for new energy equipment under a single busbar grid structure. This method is simple to implement, highly applicable, and has great value in engineering design.

[0006] An equivalent method for new energy equipment under a single busbar grid structure includes the following steps:

[0007] (1) Based on the system design requirements and the data from the power system electromechanical transient simulation software, the grids on both sides of the selected voltage level bus are equivalently evaluated for the single bus grid structure with new energy equipment access to be analyzed.

[0008] (2) After the grid is simplified to equivalent value, the portion of the total equivalent load occupied by new energy equipment is replaced by two-level inverters with equal power output, and the power output of the equivalent load is changed accordingly.

[0009] (3) Connect the two-level inverter to the corresponding bus through impedance, and connect it to the reserved main bus through a step-up transformer, and then determine the impedance value by calculating the three-phase short-circuit current.

[0010] Further, in step (1), the selected voltage level U S Then, the voltage level in the power grid is ≥U S The main busbar is the main busbar, and the rest of the lines are low-voltage grids.

[0011] Furthermore, in step (1) for a single busbar grid structure power grid, the low-voltage grid is connected to the main busbar. The equivalent circuit structure of the low-voltage grid consists of an equivalent generator, an equivalent load, and an equivalent reactive power compensation device. The equivalent generator is connected to the main busbar through a step-up transformer, and the equivalent load and the equivalent reactive power compensation device are connected to the main busbar through a step-down transformer.

[0012] Furthermore, the new energy equipment connected to the power grid in step (1) is equivalent to a constant power load. The power absorbed by the load is the opposite of the power output of the new energy equipment under steady state, that is, the power absorbed by the load is negative.

[0013] Furthermore, in step (2), the equivalent load of the new energy equipment is replaced by a two-level inverter. The capacity of the two-level inverter is equal to the total capacity of the new energy equipment, and its power output is equal to the power output of the new energy equipment under steady state.

[0014] Furthermore, the total equivalent load consists of the original grid equivalent load and the new energy equipment equivalent load. When the new energy equipment equivalent load is replaced by a two-level inverter, it is removed from the total equivalent load.

[0015] Furthermore, in step (3), the two-level inverter is connected to the low-voltage side bus of the step-up transformer through impedance, and then connected to the main bus through the step-up transformer.

[0016] Furthermore, the specific method for calculating the three-phase short-circuit current in step (3) is as follows: apply a three-phase short-circuit fault to the main bus, and then use electromechanical transient simulation software to calculate the three-phase short-circuit current injected into the corresponding connecting bus by each low-voltage network; in actual engineering applications, the current loop limiting control strategy adopted by new energy equipment will generally clamp the upper limit of the short-circuit current to 1.2 times the rated current.

[0017] Furthermore, the principle for determining the connection impedance value of the two-level inverter in step (3) is to ensure that the magnitude of the three-phase short-circuit current provided by the new energy equipment to the main bus remains unchanged before and after the equivalent value.

[0018] Current methods only consider the equivalent values ​​before and after, and the three-phase short-circuit current provided by the synchronous generator at the main bus remains unchanged. This invention improves the existing dynamic equivalent method for single busbar grid structures, and also incorporates the three-phase short-circuit current provided by the new energy equipment at the main bus into the dynamic equivalent step, making it more effective in electromagnetic transient simulation and providing a certain reference for future engineering design. Attached Figure Description

[0019] Figure 1 This is a structural diagram of a single busbar grid before its equivalent.

[0020] Figure 2 This is a schematic diagram of the equivalent structure of a single busbar grid.

[0021] Figure 3 A schematic diagram of injecting short-circuit current into a two-level inverter during a three-phase short-circuit fault. Detailed Implementation

[0022] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] This invention applies to the equivalent method for new energy equipment under a single busbar grid structure, and includes the following steps:

[0024] (1) Based on the system design requirements and the data from the power system electromechanical transient simulation software, for the single busbar grid structure power grid with new energy equipment access to be analyzed, the power grid on both sides of the selected voltage level busbar is equivalent, and the new energy equipment participates in the equivalent process as a constant power load.

[0025] Selected voltage level U S Afterwards, voltage level ≥ U SAll busbars are main busbars, and the remaining lines are low-voltage grids. For a single busbar grid structure, its low-voltage grid is connected to the main busbar. The equivalent circuit structure of this low-voltage grid consists of an equivalent generator, an equivalent load, and an equivalent reactive power compensation device. The equivalent generator is connected to the main busbar through a step-up transformer, and the equivalent load and the equivalent reactive power compensation device are connected to the main busbar through a step-down transformer.

[0026] The new energy equipment is equivalent to a load, and the power absorbed by this load is the power output P of the new energy equipment under steady state. V / Q V The opposite of -P V / -Q V That is, the value of the load is negative.

[0027] (2) After the original power grid network is simplified, the portion of the equivalent load occupied by new energy equipment is replaced by a two-level inverter with equal power output, and the power output of the equivalent load is changed accordingly.

[0028] This invention replaces the equivalent load of new energy equipment with a two-level inverter, the capacity of which is S. I Total capacity S of new energy equipment V The values ​​are equal, and the power output P I / Q I With steady-state power output P of new energy equipment V / Q V equal.

[0029] The total equivalent load consists of the original grid equivalent load and the new energy equipment equivalent load. After replacing it with a two-level inverter, the new energy equipment equivalent load is removed from the total equivalent load.

[0030] (3) Connect the two-level inverter to the corresponding bus through impedance, and connect it to the reserved main bus through a step-up transformer. The impedance value is determined according to the three-phase short-circuit current.

[0031] The two-level inverter is connected to the low-voltage side of the step-up transformer via impedance, and is then connected to the main busbar through the step-up transformer.

[0032] The calculation method for three-phase short-circuit current is as follows: A three-phase short-circuit fault is applied to the main bus, and then the three-phase short-circuit current injected into the corresponding connected bus by each low-voltage network is calculated using electromechanical transient simulation software. In practical engineering applications, the current loop limiting control strategy adopted by new energy systems generally clamps the upper limit of the short-circuit current to 1.2 times the rated current.

[0033] The principle for determining the connection impedance of a two-level inverter is to ensure that the magnitude of the three-phase short-circuit current supplied by the new energy equipment to the main bus remains unchanged before and after the equivalent value.

[0034] Currently, the main backbone of my country's power grid is mainly composed of 500kV AC lines. Therefore, in the following embodiments, we choose to connect the 500kV busbars only through backbone lines of 500kV and above. The new energy equipment is equivalently represented in the single busbar network structure according to the following method.

[0035] (1) Based on the system design requirements and the data from the power system electromechanical transient simulation software, for the single busbar grid structure power grid with new energy equipment access to be analyzed, the power grid on both sides of the selected voltage level busbar is equivalent, and the new energy equipment participates in the equivalent process as a constant power load.

[0036] After selecting a voltage level of 500kV, the power grids on both sides of the main grid are equipotentially balanced. The selected voltage level U... S Afterwards, voltage level ≥ U S All busbars are main busbars, and the remaining lines are low-voltage grids. The output of the new energy equipment is 39MW, 0MVar, and the capacity is 50MVA. The new energy equipment is equivalent to a constant power load of -39MW. Based on the basic principle of physical equivalence, the system is equivalently evaluated by keeping the short-circuit current of the main grid constant and the power flow of the system constant. According to the requirement that the three-phase short-circuit current of the main grid remain constant, the relevant parameters of the equivalent generator and transformer are determined, and the size of the equivalent load is adjusted so that the power flow of the equivalent system is basically consistent with that of the prototype system.

[0037] Equivalent space frame structure before and after, such as Figure 1 and Figure 2 As shown, for a single busbar grid structure, its low-voltage grid is connected to the main busbar. The equivalent circuit structure of the low-voltage grid consists of an equivalent generator, an equivalent load, and an equivalent reactive power compensation device. The equivalent generator is connected to the main busbar through a step-up transformer, and the equivalent load and the equivalent reactive power compensation device are connected to the main busbar through a step-down transformer. The main parameters used are shown in Table 1.

[0038] Table 1

[0039]

[0040] (2) After the original power grid network is simplified, the portion of the equivalent load occupied by new energy equipment is replaced by a two-level inverter with equal power output, and the power output of the equivalent load is changed accordingly.

[0041] The active and reactive power outputs of the equivalent load are P L +jQ LThe portion of the equivalent load occupied by renewable energy equipment will be replaced by a two-level inverter with a power output of 39MW, 0MVar, and a capacity of 50MVA. The total equivalent load consists of the original grid equivalent load and the equivalent load of renewable energy equipment. After replacing it with a two-level inverter, the equivalent load of renewable energy equipment will be removed from the total equivalent load, and the output of the equivalent load will become (P L +39)+jQ L .

[0042] (3) Connect the two-level inverter to the corresponding bus through impedance, and connect it to the reserved main bus through a step-up transformer. The impedance value is determined according to the three-phase short-circuit current.

[0043] The two-level inverter is connected to the low-voltage side of the step-up transformer via impedance, and then connected to the main busbar through the step-up transformer. For example... Figure 3 As shown, a three-phase short-circuit fault is applied to the main busbar, and then electromechanical transient simulation software is used to calculate the three-phase short-circuit current injected into the corresponding connected busbars by each low-voltage network. In practical engineering applications, the current loop limiting control strategy adopted by new energy systems generally clamps the upper limit of the short-circuit current at the outlet of new energy equipment to 1.2 times the rated current. A three-phase short circuit is a relatively serious fault; therefore, the short-circuit current at the outlet is generally clamped to the rated current I. V The connection impedance X of a two-level inverter is 1.2 times that of the two-level inverter. V The principle for determining this is to ensure that the magnitude of the three-phase short-circuit current supplied by the new energy equipment to the main bus remains unchanged before and after the equivalent value is calculated. The three-phase short-circuit current supplied by the new energy equipment to the main bus is the short-circuit current at the output. If the output voltage of the two-level inverter is U... V ,but

[0044] The single-busbar grid structure power grid in this embodiment is equivalently evaluated, and the per-unit values ​​of the three-phase short-circuit current before and after the evaluation are shown in Table 2.

[0045] Table 2

[0046]

[0047] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. An equivalent method for new energy equipment under a single busbar grid structure, comprising the following steps: (1) Based on the system design requirements and the data from the electromechanical transient simulation software of the power system, the grids on both sides of the selected voltage level bus are equivalently evaluated for the single bus grid structure with new energy equipment access to be analyzed. (2) After the power grid is simplified, the portion of the total equivalent load occupied by the new energy equipment is replaced by a two-level inverter with the same power output. The power output of the equivalent load is changed accordingly. Specifically, the equivalent load of the new energy equipment is replaced by a two-level inverter with the same capacity as the total capacity of the new energy equipment and the same power output as the power output of the new energy equipment under steady state. The total equivalent load consists of the original power grid equivalent load and the new energy equipment equivalent load. When the equivalent load of the new energy equipment is replaced by the two-level inverter, it is removed from the total equivalent load. (3) Connect the two-level inverter to the corresponding bus through impedance, and connect it to the main bus through the step-up transformer. Then, determine the impedance value by calculating the three-phase short-circuit current. Specifically, connect the two-level inverter to the low-voltage side bus of the step-up transformer through impedance, and then connect it to the main bus through the step-up transformer. The principle for determining the connection impedance value of the two-level inverter is to ensure that the magnitude of the three-phase short-circuit current provided by the new energy equipment to the main bus remains unchanged before and after the equivalent value.

2. The equivalent method according to claim 1, characterized in that: In step (1), the voltage level U is selected. S Then, the voltage level in the power grid is ≥U S The main busbar is the main busbar, and the rest of the lines are low-voltage grids.

3. The equivalent method according to claim 2, characterized in that: In step (1), for a single busbar grid structure power grid, the low-voltage grid is connected to the main busbar. The equivalent circuit structure of the low-voltage grid consists of an equivalent generator, an equivalent load, and an equivalent reactive power compensation device. The equivalent generator is connected to the main busbar through a step-up transformer, and the equivalent load and the equivalent reactive power compensation device are connected to the main busbar through a step-down transformer.

4. The equivalent method according to claim 1, characterized in that: In step (1), the new energy equipment connected to the power grid is equivalent to a constant power load. The power absorbed by the load is the opposite of the power output of the new energy equipment under steady state, that is, the power absorbed by the load is negative.

5. The equivalent method according to claim 1, characterized in that: The specific method for calculating the three-phase short-circuit current in step (3) is as follows: apply a three-phase short-circuit fault to the main bus, and then use electromechanical transient simulation software to calculate the three-phase short-circuit current injected into the corresponding connecting bus by each low-voltage network; in actual engineering applications, the current loop limiting control strategy adopted by new energy equipment will generally clamp the upper limit of the short-circuit current to 1.2 times the rated current.

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