Analysis Method and System for Frequency Characteristics of Wind-Fire-Storage Transmission System under Short-Circuit Fault

Abstract

The present invention discloses a method and system for analyzing the frequency characteristics of a wind-fire-storage power transmission and distribution system under a short-circuit fault, which relates to the technical field of power engineering. The method includes: obtaining the power change information of the wind-fire-storage power transmission and distribution system during a short-circuit fault; based on the above data, respectively determining the active power change amount without energy storage after the short-circuit fault, the active power change amount with energy storage after the short-circuit fault, the equivalent inertia time constant, the equivalent damping parameter, the virtual inertia control parameter, and the virtual damping control parameter, so as to determine the target time-domain expression, and determining the initial frequency change rate and the maximum system frequency of the wind-fire-storage power transmission and distribution system during the short-circuit fault according to the target time-domain expression; the initial frequency change rate and the maximum system frequency are used to characterize the frequency stability of the wind-fire-storage power transmission and distribution system. The present invention can quantitatively analyze the frequency dynamic characteristics of the wind-fire-storage power transmission and distribution system under a short-circuit fault.

Description

Technical Field

[0001] The present invention relates to the technical field of power engineering, and in particular, to a method and system for analyzing the frequency characteristics of a wind-fire-energy storage transmission system under a short-circuit fault. Background Art

[0002] The proportion of new energy represented by wind energy and solar energy in the power system has been continuously increasing, which has caused many problems in the system. Since wind turbines are connected to the grid through power electronic devices and cannot provide inertia and damping support for the power system like traditional thermal power plants, the inertia and damping of the power system are weakened, which brings many new difficulties and challenges to the stable operation of today's power system. For this reason, scholars have proposed different types of virtual inertia control strategies. Among them, using energy storage to provide virtual inertia control for wind turbines has become a key technology for improving the frequency stability of wind turbine grid-connected systems, which can improve the frequency stability of the system while maintaining the maximum power tracking operation of the wind turbines.

[0003] In order to analyze the supporting effect of the energy storage system on frequency, it is very important to analyze the dynamic characteristics of the system frequency after a fault occurs. However, there are few studies on quantitatively analyzing the supporting ability of the energy storage system on the system frequency under a short-circuit fault. Most of the literature only analyzes the influence of the fault ride-through control links of wind turbines and photovoltaic on the system frequency under a three-terminal short-circuit fault through simulation, and does not conduct a theoretical analysis on the dynamic characteristics of the system frequency. There is a literature that analyzes the inertia support ability of energy storage on the system frequency through contribution factors, but does not conduct a quantitative analysis. Another scholar analyzes the transient frequency characteristics based on the power system frequency response model, obtains the analytical expression of the system frequency by simplifying the governor model, and quantitatively analyzes the influence of wind power generation and HVDC transmission on the frequency dynamic characteristics, but does not discuss energy storage. Therefore, how to propose a method for analyzing the frequency characteristics of a wind-fire-energy storage transmission system under a short-circuit fault needs further research. Summary of the Invention

[0004] The purpose of the present invention is to provide a method and system for analyzing the frequency characteristics of a wind-fire-energy storage transmission system under a short-circuit fault, so as to quantitatively analyze the frequency dynamic characteristics of the wind-fire-energy storage transmission system under a short-circuit fault.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] A method for analyzing the frequency characteristics of a wind - fire - storage power transmission and distribution system under short - circuit faults, characterized in that the frequency - characteristic analysis method is applied to a wind - fire - storage power transmission and distribution system, and the wind - fire - storage power transmission and distribution system includes: a wind farm, an energy storage system, a thermal power plant, and a load; the wind farm is respectively connected to the load and the energy storage system; the thermal power plant is connected to the load; the wind farm includes a number of wind turbines; the thermal power plant includes a number of synchronous machines; the energy storage system is used to provide inertia support for the wind turbines by adopting a constant active - reactive power control strategy and an energy - storage - type virtual inertia damping control strategy;

[0007] The frequency - characteristic analysis method includes:

[0008] Obtain the power change information of the wind - fire - storage power transmission and distribution system during the short - circuit fault; the power change information includes: the increment of load active power, the increment of synchronous machine active power, the total power of synchronous machines, the inertia time constant of each synchronous machine, the apparent power of each synchronous machine, the total active power of all wind turbines in the steady state, the active power of each wind turbine, the total power of wind turbines, the total system power, and the active power output of the energy storage system;

[0009] According to the increment of load active power, the increment of synchronous machine active power, the total active power of all wind turbines in the steady state, the active power of each wind turbine, and the active power output of the energy storage system, determine the change amount of active power without energy storage after the short - circuit fault and the change amount of active power with energy storage after the short - circuit fault;

[0010] According to the total power of synchronous machines, the total power of wind turbines, the inertia time constant of each synchronous machine, and the apparent power of each synchronous machine, determine the equivalent inertia time constant without energy storage after the short - circuit fault;

[0011] According to the equivalent inertia time constant without energy storage after the short - circuit fault and the change amount of active power with energy storage after the short - circuit fault, determine the equivalent damping parameter without energy storage after the short - circuit fault;

[0012] Determine the virtual inertia control parameter and the virtual damping control parameter of the wind - fire - storage power transmission and distribution system;

[0013] According to the change amount of active power without energy storage after the short - circuit fault, the equivalent inertia time constant without energy storage after the short - circuit fault, the equivalent damping parameter without energy storage after the short - circuit fault, the virtual inertia control parameter, and the virtual damping control parameter, determine the target time - domain expression; the target time - domain expression is the time - domain expression of the frequency change curve of the wind - fire - storage power transmission and distribution system during the short - circuit fault;

[0014] Determine the initial frequency change rate and the maximum system frequency of the wind-fire-storage power transmission system during the short-circuit fault according to the target time-domain expression; the initial frequency change rate and the maximum system frequency are used to characterize the frequency stability of the wind-fire-storage power transmission system.

[0015] Optionally, the change in active power when no energy storage is equipped after the short-circuit fault is calculated as:

[0016]

[0017] where ΔP s is the change in active power when no energy storage is equipped after the short-circuit fault, ΔP L is the increment of load active power, ΔP G is the increment of synchronous machine active power, P sw is the total active power of all wind turbines at steady state, P wj is the active power of the j-th wind turbine, and m is the number of wind turbines;

[0018] The change in active power when energy storage is equipped after the short-circuit fault is calculated as:

[0019] ΔP s1 = ΔP s - P ESS ;

[0020] where ΔP s1 is the change in active power when energy storage is equipped after the short-circuit fault, and P ESS is the active power output of the energy storage system.

[0021] Optionally, the equivalent inertia time constant when no energy storage is equipped after the short-circuit fault is calculated as:

[0022]

[0023] where H s is the equivalent inertia time constant when no energy storage is equipped after the short-circuit fault, H i is the inertia time constant of the i-th synchronous machine, S i represents the apparent power of the i-th synchronous machine, S s is the total power of the synchronous machines, S w is the total power of the wind turbines, and n is the number of synchronous machines;

[0024] The equivalent damping parameter when no energy storage is equipped after the short-circuit fault is calculated as:

[0025]

[0026] where D sis the equivalent damping parameter without energy storage after a short - circuit fault, ΔP s1 is the change in active power with energy storage after a short - circuit fault, f is the grid system frequency, f0 is the system frequency reference value, and t is time.

[0027] Optionally, the target time - domain expression has the specific formula:

[0028]

[0029] where f(t) is the system frequency at time t, f0 is the system frequency reference value, t0 is the fault start time, t is time, H s is the equivalent inertia time constant without energy storage after a short - circuit fault, D s is the equivalent damping parameter without energy storage after a short - circuit fault, ΔP s is the change in active power without energy storage after a short - circuit fault, H vir is the virtual inertia control parameter, D vir / is the virtual damping control parameter, S N is the total system power.

[0030] Optionally, the initial rate of change of frequency has the specific formula:

[0031]

[0032] where ROCOF1 is the initial rate of change of frequency, f0 is the system frequency reference value, ΔP s is the change in active power without energy storage after a short - circuit fault, H s is the equivalent inertia time constant without energy storage after a short - circuit fault, H vir is the virtual inertia control parameter, S N is the total system power.

[0033] Optionally, the maximum value of the system frequency has the specific formula:

[0034]

[0035] where f max1 is the maximum value of the system frequency, f0 is the system frequency reference value, ΔP s is the change in active power without energy storage after a short - circuit fault, H s is the equivalent inertia time constant without energy storage after a short - circuit fault, D s is the equivalent damping parameter without energy storage after a short - circuit fault, H vir is the virtual inertia control parameter, D vir / is the virtual damping control parameter, S Nis the total system power, t0 is the start time of the fault, and t p is the fault clearing time.

[0036] Optionally, the frequency characteristic analysis method further includes:

[0037] Determine the inertia contribution of the energy storage system according to the virtual inertia control parameter and the total system power;

[0038] Determine the damping contribution of the energy storage system according to the virtual damping control parameter and the total system power;

[0039] The inertia contribution and the damping contribution are used to characterize the support ability of the energy storage system for the frequency of the wind-fire-energy storage power transmission system.

[0040] A frequency characteristic analysis system for a wind-fire-energy storage power transmission system under a short-circuit fault. The frequency characteristic analysis system is applied to a wind-fire-energy storage power transmission system, and the wind-fire-energy storage power transmission system includes: a wind farm, an energy storage system, a thermal power plant, and a load; the wind farm is respectively connected to the load and the energy storage system; the thermal power plant is connected to the load; the wind farm includes a plurality of wind turbines; the thermal power plant includes a plurality of synchronous machines; the energy storage system is used to provide inertia support for the wind turbines by adopting a constant active and reactive power control strategy and an energy storage type virtual inertia damping control strategy;

[0041] The frequency characteristic analysis system includes:

[0042] A data acquisition module for acquiring the power change information of the wind-fire-energy storage power transmission system during a short-circuit fault; the power change information includes: the increment of the load active power, the increment of the synchronous machine active power, the total power of the synchronous machines, the inertia time constant of each synchronous machine, the apparent power of each synchronous machine, the total active power of all wind turbines in the steady state, the active power of each wind turbine, the total power of the wind turbines, the total system power, and the active power output of the energy storage system;

[0043] An active power change amount determination module for determining the active power change amount without an energy storage system after a short-circuit fault and the active power change amount with an energy storage system after a short-circuit fault according to the increment of the load active power, the increment of the synchronous machine active power, the total active power of all wind turbines in the steady state, the active power of each wind turbine, and the active power output of the energy storage system;

[0044] An equivalent inertia time constant determination module for determining the equivalent inertia time constant without an energy storage system after a short-circuit fault according to the total power of the synchronous machines, the total power of the wind turbines, the inertia time constant of each synchronous machine, and the apparent power of each synchronous machine;

[0045] An equivalent damping parameter determination module, configured to determine an equivalent damping parameter without energy storage after a short-circuit fault according to the equivalent inertia time constant without energy storage after the short-circuit fault and the change in active power with energy storage after the short-circuit fault;

[0046] A virtual inertia and virtual damping determination module, configured to determine virtual inertia control parameters and virtual damping control parameters of the wind-fire-energy storage power transmission system;

[0047] A target time-domain expression determination module, configured to determine a target time-domain expression according to the change in active power without energy storage after the short-circuit fault, the equivalent inertia time constant without energy storage after the short-circuit fault, the equivalent damping parameter without energy storage after the short-circuit fault, the virtual inertia control parameters, and the virtual damping control parameters; the target time-domain expression is the time-domain expression of the frequency change curve of the wind-fire-energy storage power transmission system during the short-circuit fault;

[0048] A frequency characteristic parameter determination module, configured to determine an initial frequency change rate and a maximum system frequency of the wind-fire-energy storage power transmission system during the short-circuit fault according to the target time-domain expression; the initial frequency change rate and the maximum system frequency are used to characterize the frequency stability of the wind-fire-energy storage power transmission system.

[0049] According to the specific embodiments provided by the present invention, the following technical effects are disclosed by the present invention:

[0050] The frequency characteristic analysis method under short-circuit faults provided by the present invention simultaneously considers the low-voltage ride-through control of wind turbines and energy storage systems during short-circuit faults, can more accurately obtain the frequency dynamic characteristics of the wind-fire-energy storage power transmission system after a short-circuit fault occurs, and can thus more comprehensively analyze the frequency support effect of the energy storage system on the wind-fire-energy storage power transmission system, providing guidance for energy storage capacity configuration in engineering. Description of the Drawings

[0051] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention, and those of ordinary skill in the art can also obtain other drawings based on these drawings without creative efforts.

[0052] Figure 1 It is a flowchart of the frequency characteristic analysis method provided by the present invention;

[0053] Figure 2 It is a topological structure diagram of the wind-fire-energy storage power transmission system provided by the present invention;

[0054] Figure 3The curve graph showing the relationship between active power and grid-connected voltage provided by the present invention;

[0055] Figure 4 The graph showing the variation of the grid-connected voltage of the fan provided by the present invention;

[0056] Figure 5 The graph showing the variation of the grid-connected power of the fan provided by the present invention;

[0057] Figure 6 The comparison graph between the system frequency simulation curve and the theoretical curve provided by the present invention;

[0058] Figure 7 The comparison graph of the system frequency curves before and after equipping with energy storage provided by the present invention;

[0059] Figure 8 The comparison graph between the frequency measured by the phase-locked loop and the system frequency provided by the present invention;

[0060] Figure 9 The simulation structure diagram of a certain actual power grid provided by the present invention;

[0061] Figure 10 The graph showing the variation of the active power of the energy storage system provided by the present invention;

[0062] Figure 11 The curve graph showing the variation of the system frequency provided by the present invention;

[0063] Figure 12 The module diagram of the frequency characteristic analysis system provided by the present invention.

[0064] Symbol description:

[0065] Data acquisition module - 1, active power change determination module - 2, equivalent inertia time constant determination module - 3, equivalent damping parameter determination module - 4, virtual inertia and virtual damping determination module - 5, target time domain expression determination module - 6, frequency characteristic parameter determination module - 7. Specific implementation mode

[0066] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts shall fall within the protection scope of the present invention.

[0067] The purpose of the present invention is to provide a method and system for analyzing the frequency characteristics of a thermal power, wind power, and energy storage transmission system under a short-circuit fault to quantitatively analyze the frequency dynamic characteristics of the thermal power, wind power, and energy storage transmission system under a short-circuit fault.

[0068] At present, the analysis of frequency dynamic characteristics is usually carried out based on load disturbances. There are few literatures on the analysis of frequency dynamic characteristics based on short-circuit faults, and no literature has considered the low-voltage ride-through control of wind turbines and energy storage systems at the same time to analyze the frequency dynamic characteristics of the wind-fire-storage power transmission system. The present invention can realize the low-voltage ride-through control of wind turbines and energy storage systems being considered at the same time, and conduct a more accurate and comprehensive analysis of the frequency dynamic characteristics of the wind-fire-storage power transmission system.

[0069] In order to make the above objects, features and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0070] Embodiment 1

[0071] The present invention provides a method for analyzing the frequency characteristics of a wind-fire-storage power transmission system under short-circuit faults. The frequency characteristic analysis method is applied to a wind-fire-storage power transmission system, and the wind-fire-storage power transmission system includes: a wind farm, an energy storage system, a thermal power plant and a load; the wind farm is respectively connected to the load and the energy storage system; the thermal power plant is connected to the load; the wind farm includes a number of wind turbines; the thermal power plant includes a number of synchronous machines; the energy storage system is used to provide inertia support for the wind turbines by adopting a constant active and reactive power control strategy and an energy storage type virtual inertia damping control strategy.

[0072] As Figure 1 shown, the frequency characteristic analysis method includes:

[0073] Step S1: Obtain the power change information of the wind-fire-storage power transmission system during short-circuit faults; the power change information includes: load active power increment, synchronous machine active power increment, total power of synchronous machines, inertia time constant of each synchronous machine, apparent power of each synchronous machine, total active power of all wind turbines at steady state, active power of each wind turbine, total power of wind turbines, total system power and energy storage system output active power.

[0074] Step S2: Determine the active power change amount without energy storage after short-circuit faults and the active power change amount with energy storage after short-circuit faults according to the load active power increment, the synchronous machine active power increment, the total active power of all wind turbines at steady state, the active power of each wind turbine and the energy storage system output active power.

[0075] Step S3: Determine the equivalent inertia time constant without energy storage after short-circuit faults according to the total power of synchronous machines, the total power of wind turbines, the inertia time constant of each synchronous machine and the apparent power of each synchronous machine.

[0076] Step S4: Determine the equivalent damping parameter without energy storage after the short - circuit fault according to the equivalent inertia time constant without energy storage after the short - circuit fault and the active power change amount with energy storage after the short - circuit fault.

[0077] Step S5: Determine the virtual inertia control parameter and the virtual damping control parameter of the wind - fire - storage power transmission system.

[0078] Step S6: Determine the target time - domain expression according to the active power change amount without energy storage after the short - circuit fault, the equivalent inertia time constant without energy storage after the short - circuit fault, the equivalent damping parameter without energy storage after the short - circuit fault, the virtual inertia control parameter and the virtual damping control parameter; the target time - domain expression is the time - domain expression of the frequency change curve of the wind - fire - storage power transmission system during the short - circuit fault.

[0079] Step S7: Determine the initial frequency change rate and the maximum system frequency of the wind - fire - storage power transmission system during the short - circuit fault according to the target time - domain expression; the initial frequency change rate and the maximum system frequency are used to characterize the frequency stability of the wind - fire - storage power transmission system.

[0080] Further, the frequency characteristic analysis method further includes:

[0081] Step S8: Determine the inertia contribution of the energy storage system according to the virtual inertia control parameter and the total system power; determine the damping contribution of the energy storage system according to the virtual damping control parameter and the total system power; the inertia contribution and the damping contribution are used to characterize the frequency support ability of the energy storage system for the wind - fire - storage power transmission system.

[0082] The above steps are discussed in detail below.

[0083] First, the topological structure diagram of the wind - fire - storage power transmission system is given, as Figure 2 shown, including a direct - drive wind farm, an energy storage system, a thermal power plant and a load. The direct - drive wind turbines in the wind farm adopt traditional machine - side control (MSC) and grid - side control (GSC) strategies. The wind farm is equipped with a substation - level energy storage system to provide inertia support for the wind turbines, and the energy storage system converter adds an energy - storage - type virtual inertia damping control strategy on the basis of adopting typical P / Q control.

[0084] When the grid - connected voltage of the wind turbine drops to the range of 0.2 p.u. - 0.9 p.u., the low - voltage ride - through control of the wind turbine comes into play. At this time, the wind turbine improves the grid - connected voltage stability by outputting reactive power. At the same time, the grid - connected active power of the wind turbine drops, which will cause dynamic changes in the system frequency. The injected reactive current and the injected active current of the wind turbine satisfy the following formula:

[0085]

[0086] wherein, i gqref is the reference value of the reactive current injected into the wind turbine, k iq is the reactive current regulation coefficient of the wind turbine, usually taken as 1.5, u gpu is the per-unit value of the grid-connected voltage of the wind turbine, I N is the rated current value, i gmax is the limit value of the current allowed to be injected by the wind turbine, k i is a constant representing the maximum current ratio coefficient of the wind turbine, usually taken as 1.05, i gd is the active current injected, and the * indicates the per-unit value.

[0087] Assume that the grid-connected voltage of the wind turbine is a constant value during the fault. Then, the per-unit value of the active power output by the wind turbine during the short-circuit fault is P w :

[0088] P w = u gpu i gd * (2)

[0089] By combining Equation (1) and Equation (2), the following Equation (3) can be obtained.

[0090]

[0091] From Equation (3), it can be analyzed that as the grid-connected voltage u gpu of the wind turbine decreases, the active power output by the wind turbine during the short circuit gradually decreases, and the active power difference of the system gradually increases, thus deteriorating the frequency stability of the system.

[0092] According to Figure 2 it can be known that the active power output (per-unit value) of the energy storage system satisfies the following equation.

[0093]

[0094] wherein, P ESS is the active power output of the energy storage system, S N is the total system power, f0 is the system frequency reference value, H vir is the virtual inertia control parameter, D vir / is the virtual damping control parameter, f pll (t) is the frequency measured by the phase-locked loop.

[0095] Similar to the low-voltage ride-through control of the wind turbine, the energy storage system also has corresponding low-voltage control to ensure the stability of the energy storage grid-connected voltage. Different from the voltage protection range of the wind turbine, the voltage protection range of the energy storage system is 0 - 0.85U EN wherein, U ENRepresents the rated value of the grid-connected voltage of the energy storage system. The dynamic reactive current I injected by the energy storage converter into the power system E is as follows:

[0096]

[0097] where U Epu is the per-unit value of the grid-connected voltage of the energy storage, k ie is the reactive current regulation coefficient of the energy storage, and k is the current proportion coefficient of the energy storage system. According to the energy storage grid-connection guide, k ie is taken as 1.6, and k is taken as 1.04.

[0098] The maximum value (per-unit value) of the active power output by the energy storage during the short-circuit fault, P Emax satisfies:

[0099]

[0100] where P SEmax is the maximum value of the P ESS variation curve.

[0101] The frequency characteristic analysis method under short-circuit faults provided by the present invention is specifically implemented as follows:

[0102] Step 1: Establish an energy storage virtual inertia damping control model to provide inertia damping support for the thermal-wind power sending-end system.

[0103] Among them, the energy storage virtual inertia damping control strategy respectively introduces the differential link and the proportional link of the system frequency to provide inertia damping support, including using the inertia response support power provided by the energy storage system PESS1 and the damping support power P ESS2 . Its expression is as follows:

[0104]

[0105] where H vir and D vir are respectively the virtual inertia damping control parameters, Δf pll is the frequency change amount obtained by using the phase-locked loop, P ESSref is the rated value of the active power output by the energy storage, and s represents the Laplace operator.

[0106] Step 2: Consider the low-voltage ride-through control of the wind turbine and analyze the change of the active power of the wind turbine during the short-circuit fault.

[0107] After the short-circuit fault occurs, the relationship between the active power of the wind turbine and the energy storage system and the grid-connected voltage is as Figure 3As shown, the low-voltage control voltage protection ranges of the two can be clearly seen. When the grid-connected voltage is less than 0.2 p.u., the active power output by the wind turbine is 0, and the maximum value of the active power output by the energy storage is less than 0.03 times the rated value. Moreover, as the grid-connected voltages of the wind turbine and the energy storage decrease, the active power output during the short-circuit period gradually decreases, and the difference in system active power gradually increases, thus deteriorating the frequency stability of the system.

[0108] Step 3: Considering the low-voltage ride-through control of the energy storage system, analyze the variation of system active power during the short-circuit fault.

[0109] According to the analysis of Equations (1) to (6), considering the low-voltage ride-through control of the wind turbine and the energy storage system, the expression for the change in system active power during the short-circuit fault is:

[0110]

[0111] where ΔP s is the change in active power without energy storage after the short-circuit fault, ΔP L is the increment of load active power, P sw is the total active power of all wind turbines in the steady state, P wj is the active power of the jth wind turbine, and m is the number of wind turbines. It should be noted that during the short-circuit fault, the active powers of the synchronous machine and the wind turbine are basically unchanged, so it is assumed that ΔP s is a constant value.

[0112] Step 4: Considering the low-voltage ride-through control of the wind turbine and the energy storage system, analyze the relationship between active power and system frequency during the short-circuit fault.

[0113] The inertia response stage of the frequency of the wind-fire-energy storage power transmission system satisfies the following equation:

[0114]

[0115] where H s is the equivalent inertia time constant of the system, f is the grid system frequency, and f0 is the system frequency reference value, which is 50 Hz.

[0116] For a wind turbine grid-connected system with multiple synchronous motors and wind turbines, ignoring the inertia of the wind turbines, the inherent equivalent inertia time constant H s satisfies the following relationship:

[0117]

[0118] where n represents the number of synchronous machines, H i represents the inertia time constant of the ith synchronous machine, S i represents the apparent power of the ith synchronous machine, Ss is the total power of the synchronous machine, S w is the total power of the fan, and n is the number of synchronous machines.

[0119] When the damping parameter of the synchronous machine is taken into account, the following relationship is satisfied between the active power difference of the system and the system frequency:

[0120]

[0121] where D s is the damping parameter of the system.

[0122] Assume that the phase-locked loop can accurately measure the real-time system frequency change, that is, the frequency f pll measured by the phase-locked loop is equal to the system frequency f. The time-domain expression of the system frequency change curve during the short-circuit fault can be obtained.

[0123]

[0124] where f(t) is the system frequency at time t, and t0 is the start time of the system fault.

[0125] And when the system is not equipped with energy storage, that is, P ESS = 0, the time-domain expression of the system frequency f2 without energy storage can be obtained:

[0126]

[0127] Step 5: Derive the initial change rate of the system frequency, the maximum value of the system frequency during the short-circuit fault, and obtain the inertia contribution and damping contribution of the energy storage system to the wind-fire-energy storage power transmission system.

[0128] It can be seen from equations (12) and (13) that after equipping with energy storage, the equivalent inertia time constant H s1 and the equivalent damping parameter D S1 of the wind-fire-energy storage power transmission system increase, and the contributed inertia and damping are H vir / S N and D vir / S N , respectively, that is:

[0129]

[0130] Taking the derivative of equation (12) above, the initial frequency change rate can be obtained:

[0131]

[0132] It can be seen from equation (15) above that adding an energy storage system can enable the fan to participate in the inertia response regulation of the system frequency. And as the virtual inertia H virThe larger the , the smaller the absolute value of the initial frequency change rate of the system, and the stronger the frequency stability.

[0133] If the time for the active power recovery of the fan is not considered, the moment when the maximum system frequency appears is the fault clearing moment. Let the fault clearing time be t p , the maximum system frequency f can be obtained max1 is:

[0134]

[0135] Through the above process, the frequency dynamic characteristics of the thermal-wind-storage sending-end system under short-circuit faults can be quantitatively analyzed, and the inertia and damping contributions of the energy storage to the system can be obtained.

[0136] The following provides a specific embodiment to verify the correctness of the method provided by the present invention.

[0137] In this embodiment, based on the single-machine system with wind energy storage and the actual power grid of a certain province respectively, PSASP is used to simulate and verify the correctness of the theoretical analysis. When Figure 2 both the thermal power unit and the wind power unit in are 1 unit, taking Figure 2 as an example, a time-domain simulation is carried out in the PSASP software.

[0138] At 0.5 s, a non-metallic three-phase short circuit occurs near the fan node, and the fault is cleared at 0.6 s. The grid-connected voltage and grid-connected power of the fan are respectively as Figure 4 and Figure 5 shown.

[0139] After a three-phase short circuit occurs, according to the active power change of the load during the short-circuit fault, the active power change of the synchronous machine, the active power change of the fan, etc., the active power difference of the system can be obtained, so that the system inertia and system damping value can be calculated. Finally, the theoretical value of the system frequency can be obtained. The comparison diagram of the simulation curve and the theoretical curve of the system frequency is as Figure 6 shown. From Figure 6 it can be analyzed that the correctness of the frequency dynamic characteristic analysis method provided by the present invention.

[0140] Figure 7 is the comparison diagram of the system frequency change before and after energy storage is equipped. According to the method provided by the present invention, it can be calculated that the initial frequency change rate of the system is 13.21 Hz / s, and the maximum frequency is 51.32 Hz. Compared with the Figure 7 simulation value in , the correctness of the frequency dynamic characteristic analysis of the thermal-wind-storage sending system is verified.

[0141] Since in the actual power grid system, the system frequency on the grid-connected side of the fan is usually measured based on the phase-locked loop, the change of the frequency measured by the phase-locked loop and the actual system frequency is analyzed by time-domain simulation as Figure 8As shown. From Figure 8 it can be obtained that the dynamic characteristics of the phase-locked loop during the fault are basically the same as the overall change of the measured frequency, verifying the rationality of the hypothesis of the present invention.

[0142] Next, based on the actual power grid model of a certain province built by PSASP, in the sending-end power grid, a 100MW direct-drive wind farm model with energy storage type substation-level virtual inertia damping control is built and connected to the actual power grid of a certain province for simulation. The system mainly contains synchronous machines and wind turbines. Figure 9 This is the simulation structure diagram of the actual power grid. The active power capacity of the energy storage is set to 20MW, that is, 20% of the wind farm capacity, and the charge and discharge efficiency of the energy storage is set to 1p.u.

[0143] When a three-phase short circuit occurs at a certain node near the wind farm, considering the low-voltage ride-through control of the energy storage, at this time, the grid-connected voltage of the energy storage system drops to 0.5p.u., and the active power change curve of the energy storage output is as Figure 10 shown. Through theoretical calculation, the active power output by the energy storage during the fault is approximately 0.4441p.u., verifying the correctness of the theoretical analysis.

[0144] To clearly analyze the frequency dynamic characteristics of the system after considering the low-voltage ride-through control of the energy storage, it is necessary to import the data in the PSASP curve reading room into Matlab for curve plotting. Figure 11 This is the simulation result diagram of a certain actual power grid. From Figure 11 it can be seen that when using an energy storage with a power capacity of 20MW, due to the limitation of the low-voltage ride-through control, the maximum output of the energy storage is 8.8MW. At this time, the maximum frequency of the wind turbine grid connection point is 51.65Hz, which will cause the wind turbine to trip off the grid. It verifies the correctness of the qualitative frequency analysis of the method provided by the present invention. As the virtual inertia and virtual damping increase, the system stability gradually improves.

[0145] Therefore, the frequency characteristic analysis method provided by the present invention is applicable to the analysis of the frequency dynamic characteristics of the system after a short-circuit fault in the system, and can more correctly analyze the frequency dynamic characteristics of the wind-fire-energy storage sending system after a short-circuit fault.

[0146] Embodiment 2

[0147] To implement the method corresponding to the first embodiment above to achieve the corresponding functions and technical effects, the following provides a frequency characteristic analysis system of a wind-fire-energy storage power transmission system under a short-circuit fault. The frequency characteristic analysis system is applied to a wind-fire-energy storage power transmission system, and the wind-fire-energy storage power transmission system includes: a wind farm, an energy storage system, a thermal power plant, and a load; the wind farm is respectively connected to the load and the energy storage system; the thermal power plant is connected to the load; the wind farm includes a number of wind turbines; the thermal power plant includes a number of synchronous machines; the energy storage system is used to provide inertia support for the wind turbines by adopting a constant active and reactive power control strategy and an energy storage type virtual inertia damping control strategy.

[0148] As Figure 12 shown, the frequency characteristic analysis system includes:

[0149] A data acquisition module 1, configured to acquire the power change information of the wind-fire-energy storage power transmission system during a short-circuit fault; the power change information includes: the increment of load active power, the increment of synchronous machine active power, the total power of synchronous machines, the inertia time constant of each synchronous machine, the apparent power of each synchronous machine, the total active power of all wind turbines in the steady state, the active power of each wind turbine, the total power of wind turbines, the total system power, and the active power output of the energy storage system.

[0150] An active power change amount determination module 2, configured to determine the active power change amount without energy storage after a short-circuit fault and the active power change amount with energy storage after a short-circuit fault according to the increment of load active power, the increment of synchronous machine active power, the total active power of all wind turbines in the steady state, the active power of each wind turbine, and the active power output of the energy storage system.

[0151] An equivalent inertia time constant determination module 3, configured to determine the equivalent inertia time constant without energy storage after a short-circuit fault according to the total power of synchronous machines, the total power of wind turbines, the inertia time constant of each synchronous machine, and the apparent power of each synchronous machine.

[0152] An equivalent damping parameter determination module 4, configured to determine the equivalent damping parameter without energy storage after a short-circuit fault according to the equivalent inertia time constant without energy storage after a short-circuit fault and the active power change amount with energy storage after a short-circuit fault.

[0153] A virtual inertia and virtual damping determination module 5, configured to determine the virtual inertia control parameter and the virtual damping control parameter of the wind-fire-energy storage power transmission system.

[0154] The target time-domain expression determination module 6 is configured to determine a target time-domain expression according to the active power change amount without energy storage after the short-circuit fault, the equivalent inertia time constant without energy storage after the short-circuit fault, the equivalent damping parameter without energy storage after the short-circuit fault, the virtual inertia control parameter, and the virtual damping control parameter; the target time-domain expression is the time-domain expression of the frequency change curve of the wind-fire-storage power transmission system during the short-circuit fault.

[0155] The frequency characteristic parameter determination module 7 is configured to determine the initial frequency change rate and the maximum system frequency of the wind-fire-storage power transmission system during the short-circuit fault according to the target time-domain expression; the initial frequency change rate and the maximum system frequency are used to characterize the frequency stability of the wind-fire-storage power transmission system.

[0156] The various embodiments in this specification are described in a progressive manner. The key point of each embodiment is the difference from other embodiments. The same or similar parts among the embodiments can be referred to each other. For the system disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the description of the method part.

[0157] Specific examples are used in this article to elaborate on the principles and implementation manners of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manners and application scopes. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A method for analyzing the frequency characteristics of a wind-fire energy storage and transmission system under a short-circuit fault, characterized in that, The frequency characteristic analysis method is applied to a wind-fire-energy storage power transmission system, which includes a wind farm, an energy storage system, a thermal power plant, and a load. The wind farm is connected to the load and the energy storage system respectively. The thermal power plant is connected to the load. The wind farm includes a number of wind turbines. The thermal power plant includes a number of synchronous machines. The energy storage system is used to provide inertia support for the wind turbines by adopting a constant active and reactive power control strategy and an energy storage type virtual inertia damping control strategy. The frequency characteristic analysis method includes: Obtaining the power change information of the wind-fire-energy storage power transmission system during a short-circuit fault. The power change information includes: the increment of load active power, the increment of synchronous machine active power, the total power of synchronous machines, the inertia time constant of each synchronous machine, the apparent power of each synchronous machine, the total active power of all wind turbines in the steady state, the active power of each wind turbine, the total power of wind turbines, the total system power, and the output active power of the energy storage system. Determining the active power change amount without energy storage after the short-circuit fault and the active power change amount with energy storage after the short-circuit fault according to the increment of load active power, the increment of synchronous machine active power, the total active power of all wind turbines in the steady state, the active power of each wind turbine, and the output active power of the energy storage system. Determining the equivalent inertia time constant without energy storage after the short-circuit fault according to the total power of synchronous machines, the total power of wind turbines, the inertia time constant of each synchronous machine, and the apparent power of each synchronous machine. Determining the equivalent damping parameter without energy storage after the short-circuit fault according to the equivalent inertia time constant without energy storage after the short-circuit fault and the active power change amount with energy storage after the short-circuit fault. Determining the virtual inertia control parameter and the virtual damping control parameter of the wind-fire-energy storage power transmission system. Determining the target time-domain expression according to the active power change amount without energy storage after the short-circuit fault, the equivalent inertia time constant without energy storage after the short-circuit fault, the equivalent damping parameter without energy storage after the short-circuit fault, the virtual inertia control parameter, and the virtual damping control parameter. The target time-domain expression is the time-domain expression of the frequency change curve of the wind-fire-energy storage power transmission system during the short-circuit fault. Determining the initial frequency change rate and the maximum system frequency of the wind-fire-energy storage power transmission system during the short-circuit fault according to the target time-domain expression. The initial frequency change rate and the maximum system frequency are used to characterize the frequency stability of the wind-fire-energy storage power transmission system.

2. The frequency characteristic analysis method of the wind-fire-energy-storage power transmission and distribution system under short-circuit faults according to claim 1, characterized in that The formula for the active power change amount without energy storage after the short-circuit fault is: Among them, ΔP s is the change in active power without energy storage after a short-circuit fault, ΔP L is the increment of load active power, ΔP G is the increment of synchronous machine active power, P sw is the total active power of all wind turbines at steady state, P wj is the active power of the j-th wind turbine, and m is the number of wind turbines; The formula for the active power change amount with energy storage after the short-circuit fault is: ΔP s1 = ΔP s - P ESS ; Among them, ΔP s1 is the change in active power when energy storage is equipped after a short-circuit fault, and P ESS is the active power output by the energy storage system.

3. The frequency characteristic analysis method of the wind-fire-energy-storage power transmission and distribution system under short-circuit faults according to claim 1, characterized in that The formula for the equivalent inertia time constant without energy storage after the short-circuit fault is: Among them, H s is the equivalent inertia time constant without energy storage after a short-circuit fault, and H i is the inertia time constant of the i-th synchronous machine, and S i represents the apparent power of the i-th synchronous machine, and S s is the total power of the synchronous machines, and S w is the total power of the wind turbines, and n is the number of synchronous machines; The formula for the equivalent damping parameter without energy storage after the short-circuit fault is: Among them, D s is the equivalent damping parameter without energy storage after a short-circuit fault, and ΔP s1 is the change in active power with energy storage after a short-circuit fault. f is the grid system frequency, f0 is the system frequency reference value, and t is time.

4. The frequency characteristic analysis method of the wind-fire energy storage and transmission system under short-circuit fault according to claim 1, characterized in that The specific formula for the target time-domain expression is: Among them, f(t) is the system frequency at time t, f0 is the system frequency reference value, t0 is the fault start time, t is time, H s is the equivalent inertia time constant without energy storage after the short-circuit fault, D s is the equivalent damping parameter without energy storage after the short-circuit fault, ΔP s is the active power change without energy storage after the short-circuit fault, H vir is the virtual inertia control parameter, D vir / is the virtual damping control parameter, S N is the total system power.

5. The frequency characteristic analysis method of the wind-fire-storage power transmission and distribution system under short-circuit fault according to claim 1, characterized in that The specific formula for the initial frequency change rate is: Among them, ROCOF1 is the initial rate of change of frequency, f0 is the system frequency reference value, and ΔP s is the change in active power without energy storage after a short-circuit fault, and H s is the equivalent inertia time constant without energy storage after a short-circuit fault, and H vir is the virtual inertia control parameter, and S N is the total system power.

6. The method for analyzing the frequency characteristics of the wind-fire energy storage and transmission system under short-circuit faults according to claim 1, characterized in that The specific formula for the maximum system frequency is: where f max1 is the maximum value of the system frequency, f0 is the reference value of the system frequency, and ΔP s is the change in active power without energy storage after a short - circuit fault, H s is the equivalent inertia time constant without energy storage after a short - circuit fault, D s is the equivalent damping parameter without energy storage after a short - circuit fault, H vir is the virtual inertia control parameter, D vir / is the virtual damping control parameter, S N is the total system power, t0 is the start time of the fault, and t p is the fault clearing time.

7. The method for analyzing the frequency characteristics of the wind-fire energy storage and transmission system under short-circuit faults according to claim 1, characterized in that The frequency characteristic analysis method further includes: Determine the inertia contribution of the energy storage system according to the virtual inertia control parameter and the total system power; Determine the damping contribution of the energy storage system according to the virtual damping control parameter and the total system power; The inertia contribution and the damping contribution are used to characterize the frequency support ability of the energy storage system for the wind-fire-storage power transmission system.

8. A frequency characteristic analysis system of a wind-fire-energy-storage power output system under short-circuit faults, characterized in that, The frequency characteristic analysis system is applied to the wind-fire-storage power transmission system, and the wind-fire-storage power transmission system includes: a wind farm, an energy storage system, a thermal power plant, and a load; the wind farm is respectively connected to the load and the energy storage system; the thermal power plant is connected to the load; the wind farm includes a number of wind turbines; the thermal power plant includes a number of synchronous machines; the energy storage system is used to provide inertia support for the wind turbines by adopting a constant active and reactive power control strategy and an energy storage type virtual inertia damping control strategy; The frequency characteristic analysis system includes: A data acquisition module, configured to acquire the power change information of the wind-fire-storage power transmission system during a short-circuit fault; the power change information includes: the increment of the load active power, the increment of the synchronous machine active power, the total power of the synchronous machines, the inertia time constant of each synchronous machine, the apparent power of each synchronous machine, the total active power of all the wind turbines in the steady state, the active power of each wind turbine, the total power of the wind turbines, the total system power, and the active power output of the energy storage system; An active power change amount determination module, configured to determine the active power change amount without energy storage after the short-circuit fault and the active power change amount with energy storage after the short-circuit fault according to the increment of the load active power, the increment of the synchronous machine active power, the total active power of all the wind turbines in the steady state, the active power of each wind turbine, and the active power output of the energy storage system; An equivalent inertia time constant determination module, configured to determine the equivalent inertia time constant without energy storage after the short-circuit fault according to the total power of the synchronous machines, the total power of the wind turbines, the inertia time constant of each synchronous machine, and the apparent power of each synchronous machine; An equivalent damping parameter determination module, configured to determine the equivalent damping parameter without energy storage after the short-circuit fault according to the equivalent inertia time constant without energy storage after the short-circuit fault and the active power change amount with energy storage after the short-circuit fault; A virtual inertia and virtual damping determination module, configured to determine the virtual inertia control parameter and the virtual damping control parameter of the wind-fire-storage power transmission system; A target time-domain expression determination module, configured to determine a target time-domain expression according to the active power change amount without energy storage after the short-circuit fault, the equivalent inertia time constant without energy storage after the short-circuit fault, the equivalent damping parameter without energy storage after the short-circuit fault, the virtual inertia control parameter, and the virtual damping control parameter; the target time-domain expression is the time-domain expression of the frequency change curve of the wind-fire-storage power transmission system during the short-circuit fault; A frequency characteristic parameter determination module, configured to determine an initial frequency change rate and a maximum system frequency of the wind-fire-storage power transmission system during a short-circuit fault according to the target time-domain expression; the initial frequency change rate and the maximum system frequency are used to characterize the frequency stability of the wind-fire-storage power transmission system.

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