CN-115275923-B - Practical calculation method for short-circuit current of power system in consideration of influence of photovoltaic power station

CN115275923BCN 115275923 BCN115275923 BCN 115275923BCN-115275923-B

Abstract

The invention discloses a practical calculation method of short-circuit current of an electric power system considering influence of a photovoltaic power station, which comprises the steps of carrying out short-circuit fault analysis on the photovoltaic power station, calculating voltage of a photovoltaic grid-connected node according to short-circuit current under different scenes output by the photovoltaic power station, calculating an average value of the voltage of the photovoltaic grid-connected node as a grid-connected point voltage estimated value after fault, calculating equivalent impedance value in a Davidan equivalent model according to the voltage estimated value of the grid-connected point after fault, and connecting a photovoltaic fault equivalent branch at grid-connected point of the photovoltaic power station of a system positive sequence network, and calculating the whole network short-circuit current distribution by adopting the short-circuit fault. According to the practical calculation method for the short-circuit current of the power system, provided by the invention, the influence of the grid connection of the photovoltaic power station on the short-circuit current of the system is represented by the constant potential source series impedance branch circuit without the need of detailed model parameters of the photovoltaic power station, so that the iterative calculation process in the step of solving the short-circuit current theoretically is avoided, the calculation speed is high, the realizability is strong, and the problem of convergence does not exist.

Inventors

SHI HENGCHU
ZHU QINGCHENG
LI YINYIN
CHEN JING
KONG DEZHI
XU SHOUDONG
YOU HAO
YANG YUANHANG
LI BENYU
ZHAO MING
XIE LIANG
YANG QIAOWEI

Assignees

云南电网有限责任公司

Dates

Publication Date: 20260505
Application Date: 20220919

Claims (5)

1. A practical calculation method for short-circuit current of an electric power system considering influence of a photovoltaic power station is characterized by comprising the following steps: The method comprises the steps of carrying out short-circuit fault analysis on the photovoltaic power station, and calculating the voltage of a photovoltaic grid-connected node according to short-circuit currents under different scenes output by the photovoltaic power station, wherein the short-circuit fault analysis on the photovoltaic power station comprises the following steps of considering a low-voltage ride-through control strategy adopted when a grid-connected inverter of the photovoltaic power station fails, wherein the low-voltage ride-through control strategy is expressed as follows: ; ; Wherein, I f-dref is the active current reference value output during fault, I f-qref is the reactive current reference value output during fault, I N is the rated current of the photovoltaic power station, and I max =K I N is the maximum current value allowed to flow through the grid-connected inverter, and U pcc is the positive sequence voltage of the grid-connected point; the approximate relation between the output short-circuit current of the photovoltaic power station and the grid-connected point positive sequence voltage comprises the following steps: ; Wherein, I pv is the output short-circuit current of the photovoltaic power station, I f-dref 、I f-qref is the output active current and reactive current reference value when in fault, U pcc is the positive sequence voltage of the grid-connected point, and the calculation of the voltage of the photovoltaic grid-connected node comprises the following steps: When the influence of the output short-circuit current of the grid-connected photovoltaic power station is ignored, the voltage of the photovoltaic grid-connected node is expressed as follows: ; Wherein, the For the voltage of any node i in the power network, The normal voltage of the fault point f before short circuit in the power network; for the transimpedance between node i and the failed node f in the power network, As the self-impedance of the fault node f in the power network, when the node i is a grid-connected point, U pcc-pvmin = ; If all photovoltaic power stations in the system output the maximum short-circuit current and the short-circuit current is reactive current, the reactive current does not contain an active component, and the voltage of the photovoltaic grid-connected node is expressed as: ; Wherein, the The voltage of the grid-connected point p of the photovoltaic power station after the fault, The voltage of the grid-connected point of the photovoltaic power station before failure, As the self-impedance of the node p, As the self-impedance of the node f, For the transimpedance between nodes p and f in the node impedance matrix, z f is the transition resistance of the fault point f, and when node p is the parallel point, U pcc-pvmin = ; Calculating the mean value of the photovoltaic grid-connected node voltage as the grid-connected node voltage estimated value after the fault specifically comprises the following steps: The grid-connected point voltage estimated value after the fault is expressed as: ; Wherein U pcc-pvmin is the voltage of the photovoltaic grid-connected node when the output current is zero, and U pcc-pvmax is the voltage of the photovoltaic grid-connected node when the output current is maximum; calculating an equivalent impedance value in the Thevenin equivalent model according to the estimated value of the voltage of the grid-connected point after the fault, and calculating the equivalent impedance value according to the estimated value of the voltage of the grid-connected point after the fault, wherein the method comprises the following steps: Equivalent impedance value, expressed as: ; Wherein z pv is equivalent impedance of the photovoltaic model, E is constant potential source value, U pcc-est is voltage estimation value of grid-connected point of the photovoltaic power station, I N is rated current of the photovoltaic power station, and I max is maximum current allowed to flow through the grid-connected inverter; and (3) connecting a photovoltaic fault equivalent branch at a grid connection point of the system positive sequence network photovoltaic power station, and calculating by adopting a short circuit fault to obtain the whole network short circuit current distribution.
2. The practical calculation method of the short-circuit current of the power system considering the influence of the photovoltaic power station as claimed in claim 1, wherein the Thevenin equivalent model comprises an impedance and a voltage source connected in series, and the circuit is uniformly calculated by equalizing the photovoltaic branch circuit to be a serial branch circuit of a constant potential source and the impedance.
3. The practical calculation method of short-circuit current of the power system considering the influence of the photovoltaic power station according to claim 2, wherein the calculation method of short-circuit fault comprises the steps of calculating short-circuit single current by utilizing node impedance matrix according to three-phase symmetrical fault and asymmetrical fault; A node impedance matrix, expressed as: ; Wherein, the For nodes in a node impedance matrix And The mutual impedance between the two electrodes is equal to the mutual impedance, Is a node Is used to determine the self-impedance of the (c) a, Is a node Is used to determine the amount of voltage deviation of (c), Is the injection current of the photovoltaic grid-connected node, Current is injected for the fault point.
4. A practical calculation method of short-circuit current of an electric power system taking into account the influence of a photovoltaic power plant as claimed in claim 3, wherein the calculation of the short-circuit current by the node impedance matrix for three-phase symmetrical faults comprises: The short circuit current of either branch is expressed as: ; Wherein, the For nodes in an electrical power network Sum node The impedance of the branch between them, For any node in the power network Is used for the voltage of the (c) transformer, For any node in the power network Is used for the voltage of the (c) transformer, For nodes in an electrical power network Sum node The current in the branch between them, For the transformation ratio of the transformer branch, for the non-transformer branch, let 。
5. The method for practical calculation of short-circuit current in an electrical power system taking into account the effects of a photovoltaic power plant as recited in claim 4, wherein calculating short-circuit current by using said node impedance matrix for an asymmetrical fault comprises: The method comprises the steps of dividing a network into a positive sequence network, a negative sequence network and a zero sequence network by adopting a symmetrical component method, carrying out voltage estimation on grid connection points in the positive sequence network only for a power system accessed by a photovoltaic power station, calculating to obtain Thevenin equivalent model parameters, modifying the positive sequence network, adding the calculated photovoltaic equivalent synchronous machine model in the positive sequence network without considering the influence of photovoltaic access, calculating by adopting a traditional asymmetric fault short circuit calculation method, and correcting the short circuit current distribution of the system accessed by the photovoltaic power station.

Description

Practical calculation method for short-circuit current of power system in consideration of influence of photovoltaic power station Technical Field The invention relates to the technical field of short circuit calculation and analysis of power systems, in particular to a practical calculation method of short circuit current of a power system considering influence of a photovoltaic power station. Background When the photovoltaic power station is connected into the system for grid-connected power generation, the photovoltaic power station has certain low voltage ride through capability, and keeps certain time without off-grid operation in the power grid fault period. In particular, when grid-tie voltage U pcc drops below 0.9pu, the photovoltaic power plant should output a certain reactive current to provide support for grid stability. Because the failure characteristic of the photovoltaic power station is influenced by the control strategy of the grid-connected inverter, the output current and the grid-connected point voltage show a strong nonlinear relation, and have a difference with the characteristic of the traditional synchronous machine. Therefore, a photovoltaic power station model suitable for practical calculation of the short-circuit current is established, and the short-circuit current calculation method taking the grid-connected influence of the photovoltaic power station into consideration has important significance. When considering the influence of the fault current output by the photovoltaic power station on the power system, the existing simulation calculation software such as PSCAD/EMTDC, matlab/Simulink and the like needs to build a detailed model of the photovoltaic power station, in engineering calculation, detailed model parameters are difficult to obtain, and the simulation needs a large amount of time expenditure and does not meet the actual engineering requirements. In the existing short-circuit current theoretical analysis of an electric power system connected with a photovoltaic power station, the photovoltaic power station presents nonlinear current source characteristics controlled by grid-connected point voltage under the influence of an inverter control strategy, current calculation of each branch in the system needs to be solved in an iterative mode, the current calculation method is different from a traditional calculation method without iteration, most short-circuit current calculation software is not supported at the present stage, meanwhile, due to the fact that a fault output voltage current characteristic curve of the photovoltaic power station has an unguided point, the situation that convergence cannot occur possibly occurs due to the fact that an iteration algorithm suitable for a continuously-conducted nonlinear equation set is adopted. In summary, the existing equivalent model of the photovoltaic power station cannot be matched with the existing traditional model in the setting software, and cannot be realized in engineering application, so that the current distribution of the whole network cannot be obtained quickly and effectively. Disclosure of Invention This section is intended to outline some aspects of embodiments of the application and to briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section as well as in the description of the application and in the title of the application, which may not be used to limit the scope of the application. The present invention has been made in view of the above-described problems occurring in the prior art. Therefore, the invention solves the technical problems that the existing software simulation calculation model is complex, the convergence exists in the calculation by a theoretical iteration method, and the engineering calculation speed and efficiency requirements are not met. In order to solve the technical problems, the invention provides the following technical scheme that: Carrying out short-circuit fault analysis on the photovoltaic power station, and calculating the voltage of the photovoltaic grid-connected node according to the short-circuit current under different scenes output by the photovoltaic power station; calculating the average value of the photovoltaic grid-connected node voltages as a grid-connected node voltage estimated value after the fault; Calculating an equivalent impedance value in the Thevenin equivalent model according to the grid-connected point voltage estimated value after the fault; and (3) connecting a photovoltaic fault equivalent branch at a grid connection point of the system positive sequence network photovoltaic power station, and calculating by adopting a short circuit fault to obtain the whole network short circuit current distribution. The invention relates to a practical calculation method for short-circuit current of a power system considering influence of a photovoltaic power station, which comprises the following steps: consideri