Method and device for improving frequency stability of power system, equipment and storage medium

Abstract

The invention provides a method, device and equipment for improving the frequency stability of a power system and a storage medium, and the method comprises the steps: determining the frequency response characteristics during disturbance through obtaining the static frequency characteristics of a load and a generator set, and constructing a generator set frequency response model. And a new energy unit active-frequency response expansion model is formed by combining the mathematical relationship between the new energy generating capacity and the energy storage power supply capacity. And further, determining an association relationship between the maximum disturbance quantity and the load static frequency characteristic, and optimizing the load static frequency characteristic. According to the method, a frequency stability mechanism is deeply analyzed, quantitative evaluation indexes are formulated, and powerful support is provided for safe and stable operation of a power grid.

Description

Technical Field

[0001] The present application relates to the field of power grid frequency control, and in particular to a method, device, equipment and storage medium for improving the frequency stability of a power system. Background Art

[0002] As the global demand for clean and sustainable energy continues to grow, new energy sources such as wind power and photovoltaics have become an important force in alleviating energy supply pressure and promoting social and economic development.

[0003] As the proportion of new energy installed capacity continues to increase, the role of conventional power sources is changing, becoming a regulating and guaranteeing power source. At the same time, the continuous development of system energy storage, demand response and other technologies has gradually decoupled the output of generator sets and power load, providing more possibilities for grid regulation. However, the replacement of synchronous machines with new energy also leads to a decrease in system inertia and a deterioration in frequency response characteristics, which weakens the system's ability to resist power differences. Summary of the invention

[0004] The purpose of the present application is to overcome the above-mentioned prior art and provide a method, device, equipment and storage medium for improving the frequency stability of the power system.

[0005] The present application provides a method for improving the frequency stability of a power system, comprising:

[0006] Obtain the static frequency characteristics of loads in the power system;

[0007] Determining the static frequency characteristics of the generator set according to the static frequency characteristics of the load;

[0008] Determining frequency response characteristics of the power system when a disturbance occurs according to the static frequency characteristics of the load and the generator set;

[0009] Constructing a frequency response model of a generator set according to the frequency response characteristics;

[0010] The frequency response model of the generator set is constrained by the mathematical relationship between the power generation of the new energy in the generator set and the power supply of the energy storage station, so as to generate an extended model of the active power-frequency response of the new energy generator set;

[0011] Determining the correlation between the maximum disturbance of the power system and the static frequency characteristics of the load according to the active power-frequency response expansion model of the new energy unit;

[0012] The optimal load static frequency characteristic in the power system is determined according to the association relationship.

[0013] Optionally, the static frequency characteristic of the load is expressed as follows:

[0014]

[0015] Among them, P f is the total active load of the power system when the frequency is f; P fe is the total active load of the system when the frequency is rated; a k (k=0, 1, 2, ..., n) is the load ratio proportional to the i-th power of the frequency.

[0016] Optionally, the static frequency characteristic of the generator set is expressed as follows:

[0017]

[0018] Its per unit value is:

[0019]

[0020] δ=[(n o -n) / n e ]×100%

[0021] Among them, n0 is the static speed of the generator set when the system is unloaded; n is the static speed of the generator set when the system is fully loaded; n e It is the speed of the generator set when the system is in rated state.

[0022] Optionally, the frequency response characteristic of the power system when a disturbance occurs is expressed as follows:

[0023]

[0024] Where β=(K G +K L ) is the system frequency response characteristic coefficient, which is the sum of the load frequency characteristic coefficient and the generator set frequency characteristic coefficient in the power system.

[0025] Optionally, the motor group frequency response model includes:

[0026]

[0027] Where:

[0028]

[0029]

[0030] Among them, P SP is the ratio of the load disturbance power in the system to the total load of the system; F H K is the high pressure bar power coefficient of the steam turbine; m is the mechanical power gain coefficient; Δω is the system frequency offset; P m P is the mechanical power output of the steam turbine;e is the electromagnetic power of the generator; P a =P m -P e is the acceleration power of the generator; H is the total inertia time constant of the generator; D is the equivalent damping coefficient of the generator; R is the frequency adjustment coefficient of the speed regulator; T R is the reheat time constant of the prime mover.

[0031] Optionally, the association relationship includes:

[0032] The quantitative evaluation index ηS of system frequency static stability is expressed as follows:

[0033]

[0034] Among them, η S is the evaluation index, 0≤η≤0.05, f SEP and f UEP They respectively represent the frequency values ​​corresponding to frequency SEP and frequency UEP, and fN is the rated frequency value.

[0035] Optionally, the new energy power generation includes: wind power and / or photovoltaic power generation.

[0036] The present application also provides a device for improving the frequency stability of a power system, comprising:

[0037] A frequency characteristic module is used to obtain the static frequency characteristics of the load in the power system; determine the static frequency characteristics of the generator set according to the static frequency characteristics of the load; determine the frequency response characteristics of the power system when a disturbance occurs according to the static frequency characteristics of the load and the generator set;

[0038] A model building module, used to build a frequency response model of the generator set according to the frequency response characteristics;

[0039] A model extension module, used to constrain the frequency response model of the generator set by using the mathematical relationship between the power generation of new energy in the generator set and the power supply of the energy storage station to generate an extended model of the active power-frequency response of the new energy generator set;

[0040] A relationship acquisition module, used to determine the correlation relationship between the maximum disturbance of the power system and the static frequency characteristics of the load according to the active power-frequency response expansion model of the new energy unit;

[0041] The characteristic selection module is used to determine the optimal load static frequency characteristic in the power system according to the association relationship.

[0042] The present application also provides a device for improving the frequency stability of a power system, comprising:

[0043] A memory, used to store a computer executable program for the above method for improving the frequency stability of a power system;

[0044] The processor is used to call the computer executable program in the memory to execute: obtaining the static frequency characteristics of the load in the power system; determining the static frequency characteristics of the generator set according to the static frequency characteristics of the load; determining the frequency response characteristics of the power system when a disturbance occurs according to the static frequency characteristics of the load and the generator set; constructing a frequency response model of the generator set according to the frequency response characteristics; constraining the frequency response model of the generator set with the mathematical relationship between the power generation of new energy in the generator set and the power supply of the energy storage station to generate an active-frequency response expansion model of the new energy generator set; determining the correlation between the maximum disturbance of the power system and the static frequency characteristics of the load according to the active-frequency response expansion model of the new energy generator set; and determining the optimal load static frequency characteristics in the power system according to the correlation.

[0045] The present application also provides a storage medium, comprising a computer executable program stored therein, wherein the computer executable program is used to be called by a processor to execute the steps of the above-mentioned method for improving the frequency stability of a power system.

[0046] The beneficial effects of this application are:

[0047] The present application provides a method for improving the frequency stability of an electric power system, including: obtaining the static frequency characteristics of the load in the electric power system; determining the static frequency characteristics of the generator set according to the static frequency characteristics of the load; determining the frequency response characteristics of the electric power system when a disturbance occurs according to the static frequency characteristics of the load and the generator set; constructing a frequency response model of the generator set according to the frequency response characteristics; constraining the frequency response model of the generator set with the mathematical relationship between the power generation of new energy in the generator set and the power supply of the energy storage station, and generating an active-frequency response expansion model of the new energy unit; determining the correlation between the maximum disturbance of the electric power system and the static frequency characteristics of the load according to the active-frequency response expansion model of the new energy unit; and determining the optimal load static frequency characteristics in the electric power system according to the correlation. The frequency stability mechanism is deeply analyzed, and a quantitative evaluation index for the static stability of the system frequency is formulated. The present application proposes a quantitative evaluation index for static frequency stability based on the frequency fluctuation characteristics of the new electric power system, providing strong support for the safe and stable operation of the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 A schematic diagram of a process for improving the frequency stability of a power system in this application;

[0049] Figure 2 It is a schematic diagram of the overall implementation route of the invention in this application;

[0050] Figure 3 It is a schematic diagram of the static frequency characteristics of the load in this application;

[0051] Figure 4 It is a schematic diagram of the static frequency characteristics of the generator set in this application;

[0052] Figure 5 A schematic diagram of primary regulation of the power system frequency in this application;

[0053] Figure 6 This is a schematic diagram of the dynamic frequency response process under high power loss in this application;

[0054] Figure 7 This is a schematic diagram of the ASF model structure in this application;

[0055] Figure 8 This is a schematic diagram of the SFR model structure in this application;

[0056] Fig. 9 This is a simplified schematic diagram of the frequency response of an equivalent single-machine system in this application;

[0057] Fig.10 This is a schematic diagram of the full-power wind power generation system structure in this application

[0058] Fig.11 This is a schematic diagram of the photovoltaic inverter control system in this application;

[0059] Fig.12 This is a schematic diagram of the energy storage inverter control system in this application;

[0060] Fig.13 This is a schematic diagram of the structure of the doubly-fed wind power generation system in this application;

[0061] Fig.14 Schematic diagram of the synchronous generator speed control system model with feedback control in this application

[0062] Fig.15 Schematic diagram of load distribution and system frequency response of synchronous units in parallel operation system in this application

[0063] Fig.16 This is a schematic diagram of the static power-frequency characteristics of the system within a large frequency offset range in this application. DETAILED DESCRIPTION

[0064] Exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0065] Please refer to Figure 1 as well as Figure 2 As shown, the present application provides a method for improving the frequency stability of a power system, and the specific steps of the method include:

[0066] S101. Obtain static frequency characteristics of loads in a power system.

[0067] When the power system is in a stable state, the active load is proportional to the frequency to the kth power (k ≥ 0).

[0068] The static frequency characteristic of the load, also known as the active power-frequency static characteristic of the system load (abbreviated as the power-frequency static characteristic of the load), refers to the characteristic between the total active load and frequency in the power system. Its mathematical expression is as follows:

[0069]

[0070] Where: Pf is the total active load of the power system when the frequency is f; Pfe is the total active load of the system when the frequency is rated; a k (k=0, 1, 2, ..., n) is the load ratio proportional to the i-th power of the frequency.

[0071] like Figure 3 It means that when the frequency in the system does not deviate greatly, an approximate straight line can be used to represent the characteristics between the load active power and the frequency, where the slope of the straight line is:

[0072]

[0073] Where: KL is the load frequency adjustment coefficient; is the frequency increased (or decreased) by the system; is the load increased (or decreased) by the system.

[0074] In the power system, the load has the characteristic of automatic adjustment, and KL can be used to represent this effect of the load. The value of KL changes with the change of the proportion of different loads in the power system, that is, KL is not a constant value. When time and frequency change, it will also change. However, the value of KL is always a positive number, because in the power system, the direction of change of load and frequency is always the same.

[0075] S102: Determine the static frequency characteristics of the generator set according to the static frequency characteristics of the load.

[0076] When the system load changes, the system frequency will also change, which will cause the speed of the generator set to change. The generator sets running in parallel in the power system are equipped with speed governors. At this time, the speed governor of the unit will not act or respond immediately, but when the speed of the generator exceeds the dead zone, the speed governor will adjust the opening size of the prime mover valve to change the output power of the prime mover.

[0077] This action characteristic of the speed governor is called the differential characteristic of the generator set, which is expressed by the differential rate δ:

[0078] δ=[(n o -n) / n e ]×100%

[0079] Among them: n0 is the static speed of the generator set when the system is unloaded; n is the static speed of the generator set when the system is fully loaded; ne is the speed of the generator set when the system is in rated state.

[0080] Due to the characteristic relationship between generator set power and frequency, the output power of the generator set increases when the load of the power system increases, and the system frequency is lower than the initial frequency. Conversely, the output power of the generator set decreases when the load of the power system decreases, and the frequency is higher than the initial frequency.

[0081] like Figure 4 As shown, this characteristic can be represented by a curve and is called the active power-frequency static characteristic of the unit (abbreviated as the power-frequency static characteristic of the unit).

[0082] KG is commonly used to represent the static frequency characteristic slope of the generator set:

[0083]

[0084] Its per unit value is:

[0085]

[0086] S103: Determine frequency response characteristics of the power system when a disturbance occurs according to the static frequency characteristics of the load and the generator set.

[0087] Please refer to Figure 5 As shown in the above analysis, the primary regulation of the power system frequency is completed by the superposition of the different effects of the load and the generator set.

[0088] Set the initial test point of system operation to O (i.e. the intersection of the power-frequency characteristics of the generator set and the power-frequency characteristics of the load). If the active power of the load suddenly increases by ΔP LO , the generator set will slow down, the power system frequency will drop, corresponding to Figure 5 The medium load power-frequency characteristic curve moves upward to PL' .

[0089] Since the active power output of the generator cannot change in time with the sudden increase of load, the speed governor increases the generator power by increasing the prime mover valve, so that the system reaches a new balance point O'. This regulation process is completed by the generator and the load, which is called the primary regulation of the power system frequency. The primary regulation of the power system frequency is differential regulation.

[0090] pass Figure 5 It can also be seen that when the new equilibrium point is reached, the static power-frequency characteristics of the generator set do not change, but the operating frequency of the system is lower than the initial operating frequency.

[0091] After analysis, we can get:

[0092]

[0093] Where β=(K G +K L ) is called the system frequency response characteristic coefficient, which is the sum of the load frequency characteristic coefficient and the generator frequency characteristic coefficient in the power system. It reflects the relationship between the actual load power change and the actual frequency change.

[0094] S104: constructing a frequency response model of the generator set according to the frequency response characteristics.

[0095] The inertial response and primary frequency control after a power disturbance in the system, also known as frequency response, is the first stage of overall frequency control. It is a response to prevent frequency changes after resources and loads locally perceive frequency changes.

[0096] The frequency response is automatic and not driven by any centralized system.

[0097] When analyzing the dynamic process of system frequency response, it can be analyzed from two aspects.

[0098] The first aspect, namely power, is to analyze the power changes made by various components in the system at different times as the frequency changes;

[0099] The second is the frequency aspect, which involves analyzing the frequency changes of some components in the system through their power responses.

[0100] For example:

[0101] Please refer to Figure 6 As shown, a disturbance event of sudden loss of a 1000MW generating unit is assumed.

[0102] Although this is a large disturbance event, smaller events that occur frequently result in frequency response dynamics that are similar.

[0103] The power loss scale of the event only affects the shape of the curve on the graph (i.e., the depth of response of each link in the disturbance), while the contribution of each link in the frequency response process is consistent. Figure 6 The black line in the middle represents the system power shortage, and the red line represents the system frequency (assuming that the initial frequency when the disturbance occurs is 50 Hz).

[0104] Even though generation has been interrupted and the power injected by the generators has been removed from the grid, the load side still has the same demand for active power. The “law of conservation of energy” dictates that 1000MW must be supplied to the grid if energy balance is to be maintained, and this extra 1000MW is generated by extracting the kinetic energy stored in the rotating mass of all the synchronous generators and motors, essentially using the device as a giant flywheel.

[0105] The energy extracted from the flywheel energy storage is used to maintain the energy balance of the power grid. This energy is called balancing inertia.

[0106] The slowing down of rotating equipment in the grid provides both stored kinetic energy and a reduction in the grid frequency. Figure 6 This is illustrated by the change trend of the middle yellow line, which just covers and offsets the power shortage. During the period from the beginning of the large disturbance in the system to the beginning of the governor's response, the change pattern of the balancing inertia and the active power shortage is consistent, which shows that even if there is an active power shortage in the system, the instantaneous active power balance can still be maintained through the action of balancing inertia.

[0107] When the frequency is reduced, the synchronous motors slow down and the power they provide decreases, resulting in a reduction in load, which is called load damping. This load damping is the reason for the initial drop in power loss. The synchronously running motors will contribute to the load damping, while the speed regulator, which is decoupled from the grid frequency, does not contribute to load damping.

[0108] Generally, any load that does not vary with grid frequency (including resistive loads) will not contribute to load damping or frequency response. The reduced system load is:

[0109] ΔP=D×Δf

[0110] Among them, D is called the damping coefficient. Figure 5 It can be seen that under the effect of load damping, the load demand begins to decrease and the system power deficit decreases accordingly.

[0111] After a short delay, the speed regulator responds and the system frequency starts to rise rapidly, such as Figure 6 The blue line shows the changing trend.

[0112] The governor response exactly offsets the power loss at the point in time when the frequency drop is arrested. At this point, the balancing inertia provides its contribution to reliability because it is replaced by the governor response and its power contribution is reduced to zero.

[0113] If the time delay associated with the transmission of the speed regulator response is reduced, the interference events required to limit the frequency change required to balance the inertia can also be reduced. The mechanical equipment action time and energy conversion time are two important factors that determine the size of the speed regulator delay time.

[0114] From the above analysis, we can see that in the early stage of a large disturbance event in the system, the speed regulator does not act immediately, but there is a certain delay in action. During this period, the synchronous motor and the motor rotor provide balanced inertia, and its response capability depends on the inertia time constant of each unit and the comprehensive inertia time constant of the system.

[0115] Among them, the inertia time constant (s) of the unit is calculated as follows:

[0116]

[0117] System comprehensive inertia time constant (s):

[0118]

[0119] The expression of the system dynamic frequency response under disturbance derived from the rotor motion equation is:

[0120]

[0121] The speed regulation rate of the i-th unit governor:

[0122]

[0123] In the above formula: J is the rotor moment of inertia; ω is the rotor angular velocity; S is the rated installed capacity of a single unit; H i is the inertia time constant of the i-th unit; S i is the installed capacity of the i-th machine; P g P is the active output of the generator; l is the active power demand of the load; f0 is the rated frequency of the system; Δf is the change of system frequency.

[0124] The power system is highly complex, and its detailed mathematical model has high order and strong nonlinearity, making it very difficult to solve such a complete model.

[0125] In the early days, the system frequency dynamics was analyzed by simplifying the system, and a frequency response model based on mathematical analytical methods was proposed. This method ignores the frequency oscillation between generator sets, and equates the complex power system to a simple system with a single machine and concentrated load, which reduces the difficulty of solving the system frequency dynamic response.

[0126] Currently, there are two commonly used system stand-alone equivalent models: Average System Frequency Model (ASF Model) and System Frequency Response Model (SFR Model).

[0127] Figure 7 As shown in the figure, the ASF model is a simplified method for analyzing the frequency response of power systems. This method ignores the network impact, assumes that the frequency of the entire network is unified, and aggregates the motion equations of all generator rotors in the entire network into a single machine model. However, the dynamic behavior of each generator prime mover-speed governor in the power system is retained, and the total mechanical power output by the prime mover-speed governor acts on the equivalent rotor of the system, which is formed by the equivalent aggregation of all generator rotors in the system. At the same time, a concentrated load is used to replace all loads in the system, and its model structure is as follows.

[0128] This method reduces the scale of the system, reduces the amount of calculation, can better reflect the dynamic frequency changes of the system, and has a good application in the primary frequency regulation analysis of the power grid. However, this model can only provide the average frequency information of the whole network, but cannot reflect the system active dynamics and the frequency dynamic differences in different regions, which has certain limitations.

[0129] like Figure 8 As shown in the figure, based on the ASF model, the SFR model is further simplified by using a single prime mover-speed governor to approximate the dynamics of all prime movers-speed governors in the system through aggregation equivalence. The rest of the simplified processing is the same as the ASF model.

[0130] It is worth noting that the SFR model does not need to use the step-by-step integration method to solve the system frequency dynamics, maintains the analytical characteristics of the frequency, and can extract many key information from the analytical expression, such as the initial drop rate of the dynamic frequency, the maximum frequency difference, the steady-state frequency difference, etc., and is widely used in low-frequency load reduction design.

[0131] The system frequency deviation expression of the SFR model is:

[0132]

[0133] Where:

[0134]

[0135]

[0136] Where: P SP is the ratio of the load disturbance power in the system to the total load of the system; F H K is the high pressure bar power coefficient of the steam turbine; m is the mechanical power gain coefficient; Δω is the system frequency offset; P m P is the mechanical power output of the steam turbine; e is the electromagnetic power of the generator; P a =P m -P e is the acceleration power of the generator; H is the total inertia time constant of the generator; D is the equivalent damping coefficient of the generator; R is the frequency adjustment coefficient of the speed regulator; T R is the reheat time constant of the prime mover.

[0137] like Fig. 9 As shown, since only one frequency response is considered, let P SP =0, only the frequency change when Pe changes is considered. In this way, the system frequency deviation expression is simplified to:

[0138]

[0139] When the disturbance is a step function as follows:

[0140] P d (t) = P step u(t)

[0141] The corresponding frequency domain expression is:

[0142] P d (s) = P step / s

[0143] Here, the disturbance power P is defined d is the input variable of the system, and stipulates: P d > 0, it simulates a sudden increase in active power of generator (or a sudden decrease in active power of load), P d <0 simulates a sudden increase in load active power (or a sudden decrease in generator active power).

[0144] When ξ<1, the time domain expression of the system frequency deviation is:

[0145]

[0146] Where:

[0147]

[0148]

[0149]

[0150] By taking the derivative of the time domain expression of the frequency deviation of the system, the initial frequency drop rate of the system can be obtained at time t = 0:

[0151]

[0152] make The lowest point time of the system frequency can be obtained as:

[0153]

[0154] Substituting the lowest frequency point time of the system into the time domain expression of the frequency deviation of the system, the maximum frequency difference of the system can be obtained as:

[0155]

[0156] According to the final value theorem, the steady-state frequency difference of the system under disturbance input is:

[0157]

[0158] S105, constraining the frequency response model of the generator set based on the mathematical relationship between the power generation of the new energy in the generator set and the power supply of the energy storage station, and generating an extended model of the active power-frequency response of the new energy generator set.

[0159] Based on the theoretical foundation of frequency response modeling technology of new energy and energy storage stations, an extended model of active-frequency response of new energy units considering the power-frequency characteristics of energy storage stations is established. The operating data of actual new energy units is analyzed to derive and establish a mathematical relationship between new energy power generation and energy storage station energy supply, which is used as a constraint on the input supply-side power generation.

[0160] Taking into account the impact of large-scale frequency deviation on the output of energy storage stations, the frequency dynamic response process of 0.1 second is simulated more accurately, and a more detailed frequency response calculation model is established and programmed for the new power system. The system frequency dynamic simulation function under large frequency deviation is expanded, the calculation accuracy is improved, and the adaptability of conventional frequency response models is improved.

[0161] The effectiveness of the extended model was verified through case analysis, providing support for subsequent research such as frequency safety and stability analysis, evaluation, and control optimization decision-making.

[0162] This application is based on three typical new energy power sources: full-power wind power generation system, energy storage and photovoltaic grid-connected system, and doubly-fed wind power generation system, to establish a corresponding power generation system model.

[0163] like Fig.10As shown in the figure, the electrical part of a full-power wind turbine is mainly composed of a generator, a converter and a power grid. "Subsystem #1" corresponds to a permanent magnet direct-drive generator; "Subsystem #2" corresponds to a machine-side converter and its control system; "Subsystem #3" corresponds to a grid-side converter and its control system; and "Subsystem #4" corresponds to a Thevenin equivalent external power grid.

[0164] like Fig.11 , Fig.12 As shown in the figure, the photovoltaic power generation and energy storage inverter grid-connected system are operated in parallel, which essentially analyzes the interaction and grid-connected stability of the two inverters with different control methods. Among them, the energy storage inverter is PQ control, corresponding to "subsystem #1"; and the photovoltaic grid-connected system is DC voltage control, corresponding to "subsystem 2#".

[0165] like Fig.13 As shown, the doubly-fed generator set is the main equipment for wind power generation. Its structure and grid-connected mode are different from those of the direct-drive permanent magnet wind turbine set and the photovoltaic power generation component. The stator of the doubly-fed wind turbine set is directly connected to the grid. Among them, "Subsystem #1" corresponds to the doubly-fed generator and the machine-side converter and its control system; "Subsystem #2" corresponds to the grid-side converter and its control system; "Subsystem #3" corresponds to the Thevenin equivalent external grid.

[0166] The dynamic process of synchronous units is relatively complex. In order to facilitate the study and analysis of the frequency regulation of synchronous units, the following assumptions must be made for synchronous units:

[0167] 1. It is assumed that the generator magnetic circuit is not saturated, that is, the magnetomotive force will not produce flat-top waves, and the effects of hysteresis loss, eddy current, skin effect, etc. on the operation of the unit are ignored;

[0168] 2. It is assumed that the motor rotor is symmetrical in structure on the direct axis and the quadrature axis respectively;

[0169] 3. The stator abc three-phase windings are symmetrical in spatial structure and are 120° apart from each other, generating a uniformly sinusoidally distributed magnetomotive force in the air gap;

[0170] 4. The stator of the motor has a smooth surface, that is, the grooves on the stator and rotor do not affect the self-inductance and mutual inductance of the stator and rotor;

[0171] 5. When the motor rotates at synchronous speed without load, the no-load electromotive force generated by the magnetomotive force induction of the rotor winding is a sinusoidal function.

[0172] In the circuit, the positive direction of the stator flux is selected as the demagnetization effect, that is, the generator direction, and the positive direction of the rotor flux is selected as the magnetization effect, that is, the motor direction.

[0173] For a power system dominated by synchronous machines, it is very necessary to ensure the stability of the system frequency. The stability of the frequency in the power system can ensure that the rotor speed of the asynchronous motor units on the load side and the synchronous units on the generating side of the system is constant, and ensure that the mechanical structure of the units is not damaged due to speed fluctuations. The synchronous units dominate the system frequency.

[0174] The frequency stability of the system depends on the stability of the rotor speed of the synchronous units in the system, and the stability of the rotor speed depends on the balance of active power in the system. Under the condition of ignoring the temporal and spatial distribution characteristics of frequency, frequency is a common factor of the system. The supply and demand relationship of active power at a certain point in the system will lead to changes in the frequency of the entire system. Therefore, the speed governor of each synchronous unit can assist in regulating the system frequency by controlling the opening and closing of the turbine valve to adjust the rotor speed.

[0175] like Fig.14 As shown, the active power-frequency control system of the synchronous unit is composed of a gas valve / water gate, a turbine, a speed control system, a rotor mass block, etc.

[0176] Driven by steam or water, the turbine transfers its internal energy to the rotor and generates mechanical torque to make the rotor rotate. The mechanical torque is converted into mechanical power (prime mover power) through the stator and rotor magnetic circuits. The system side load power is characterized by the electromagnetic power of each unit.

[0177] It is necessary to ensure that the mechanical power of the synchronous unit in the system is equal to the electromagnetic power of each unit to keep the rotor speed as constant as possible. When unbalanced power is generated in the system, the electromagnetic power at the end of the synchronous unit is not equal to the mechanical power, so the unbalanced torque is felt on the rotor. Under the action of the unbalanced torque, the rotor speed changes, and the change in speed is amplified, and a reset signal ΔY is formed through the integral circuit (speed governor) to adjust the opening of the steam turbine valve or the opening of the turbine gate, so that the turbine adjusts the mechanical torque applied to the rotor, so the mechanical power of the unit changes. After this adjustment process, the rotor speed returns to the reference speed, and the mechanical power and electromagnetic power of the synchronous unit are kept equal again, realizing the closed-loop control of the rotor speed, and the system frequency returns to stability.

[0178] When a power disturbance occurs in a system with multiple synchronous machines running in parallel, the speed of the units in the system changes synchronously to ensure that the speed of each unit is equal after adjustment to ensure the stability of the system frequency.

[0179] like Fig.15 As shown, taking the increase in load of a system with two synchronous units in parallel as an example, the two units have different regulation characteristics. In the initial state, the output powers of the two units are P10 and P20 respectively. After the system is disturbed, the speed changes under the action of rotor inertia. The regulation characteristics of each unit are different. In steady state, the additional power of the two units is ΔP1 and ΔP2 respectively. Under the joint action of the two units, the change in system frequency is Δf.

[0180] S106. Determine the correlation between the maximum disturbance of the power system and the static frequency characteristics of the load according to the active power-frequency response expansion model of the new energy generator set.

[0181] like Fig.16 As shown in the figure, when the auxiliary machine power-frequency characteristics are taken into account, the unit output no longer increases monotonically as the frequency decreases. Instead, the unit output first increases and then decreases as the frequency decreases. When the frequency drops to a certain level, an inflection point G appears on the unit power-frequency characteristic curve.

[0182] When studying the static power-frequency characteristics of the unit and load within a larger frequency variation range, a new balance point will be generated on the left side of the inflection point on the static power-frequency characteristic curve of the unit. Fig.16 The medium load characteristic PL1 intersects with PG at points A1 and B1, and PL2 intersects with PG at points A2 and B2.

[0183] Relevant standards have been formulated for system frequency deviation at home and abroad. When the frequency deviation exceeds the allowable range, the system is in an abnormal operating state. If the acceptability of steady-state frequency deviation is not considered, these two intersection points can both meet the supply and demand balance between power generation and load, but the stability characteristics of these two balance points after disturbances are different.

[0184] When the system runs at point A1, the system frequency is f1 and the load is P1.

[0185] When the load is disturbed and the load characteristic shifts from PL1 to PL2, the system reduces the unbalanced power of the system at the cost of frequency shift. At this time, the unit output increases, and the system re-establishes the balance between power and load at point A2.

[0186] Therefore, for small disturbances near this equilibrium point, the system will return to a stable state after the disturbance. Point A1 is called the system's static frequency stable equilibrium point (Stable Equilibrium Point, SEP).

[0187] On the other hand, when the system operates at point B1, the frequency is fb and the load is Pb. At this time, even if the frequency drops a little, it will lead to an avalanche-like drop in frequency.

[0188] Since the load is greater than the power generation, the system frequency decreases and cannot be restored to the power balance state, which eventually leads to a continuous decrease in system frequency and frequency collapse. Therefore, the static equilibrium point B1 is called the system static frequency unstable equilibrium point (Unstable Equilibrium Point, UEP).

[0189] In order to compare the differences in frequency stability of different load characteristics, a quantitative evaluation index ηS of system frequency static stability is proposed based on the frequency SEP and UEP, as shown in the following expression:

[0190]

[0191] Wherein: ηS is an evaluation index, 0≤η≤0.05, fSEP and fUEP represent the frequency values ​​(Hz) corresponding to the frequencies SEP and UEP respectively, the minimum value of fUEP is 95% of the rated frequency value, and fN is the rated frequency value (Hz).

[0192] The frequency static stability quantitative evaluation index ηS can be used to compare the static frequency stability of different load frequency characteristics, reflecting the difference in frequency stability of the system under different load frequency characteristics or corresponding operating modes. The larger the index, the better the static frequency stability of the system.

[0193] According to the analysis of different load characteristics, the corresponding static frequency stability quantitative evaluation index is obtained to obtain the maximum disturbance that the system can withstand under different load characteristics. By continuously increasing the load disturbance until the system frequency becomes unstable, the maximum disturbance ΔPmax of the system can be obtained.

[0194] S107. Determine the optimal load static frequency characteristic in the power system according to the association relationship.

[0195] When the system load frequency characteristics change from a certain load characteristic to another load characteristic, the frequency stability quantitative index changes accordingly. According to the change of the maximum disturbance ΔPmax, it can be seen that the larger the margin index is, the greater the maximum disturbance that the system can withstand. Then the system frequency stability characteristics under different initial load conditions are further analyzed. When the load frequency characteristics remain unchanged and only the initial load size is changed, the smaller the initial load is, the greater the disturbance that the system can withstand, the larger the value of the frequency stability quantitative index is, and the better the frequency stability of the system is.

[0196] The present application also provides a device for improving the frequency stability of a power system, comprising:

[0197] A frequency characteristic module is used to obtain the static frequency characteristics of the load in the power system; determine the static frequency characteristics of the generator set according to the static frequency characteristics of the load; determine the frequency response characteristics of the power system when a disturbance occurs according to the static frequency characteristics of the load and the generator set;

[0198] A model building module, used to build a frequency response model of the generator set according to the frequency response characteristics;

[0199] A model extension module, used to constrain the frequency response model of the generator set by using the mathematical relationship between the power generation of new energy in the generator set and the power supply of the energy storage station to generate an extended model of the active power-frequency response of the new energy generator set;

[0200] A relationship acquisition module, used to determine the correlation relationship between the maximum disturbance of the power system and the static frequency characteristics of the load according to the active power-frequency response expansion model of the new energy unit;

[0201] The characteristic selection module is used to determine the optimal load static frequency characteristic in the power system according to the association relationship.

[0202] The present application also provides a device for improving the frequency stability of a power system, comprising:

[0203] A memory, used to store a computer executable program for the above method for improving the frequency stability of a power system;

[0204] The processor is used to call the computer executable program in the memory to execute: obtaining the static frequency characteristics of the load in the power system; determining the static frequency characteristics of the generator set according to the static frequency characteristics of the load; determining the frequency response characteristics of the power system when a disturbance occurs according to the static frequency characteristics of the load and the generator set; constructing a frequency response model of the generator set according to the frequency response characteristics; constraining the frequency response model of the generator set with the mathematical relationship between the power generation of new energy in the generator set and the power supply of the energy storage station to generate an active-frequency response expansion model of the new energy generator set; determining the correlation between the maximum disturbance of the power system and the static frequency characteristics of the load according to the active-frequency response expansion model of the new energy generator set; and determining the optimal load static frequency characteristics in the power system according to the correlation.

[0205] The present application also provides a storage medium, comprising a computer executable program stored therein, wherein the computer executable program is used to be called by a processor to execute the steps of the above-mentioned method for improving the frequency stability of a power system.

Claims

1. A method for improving the frequency stability of a power system, characterized in that: include: Obtain the static frequency characteristics of loads in the power system; Determining the static frequency characteristics of the generator set according to the static frequency characteristics of the load; Determining frequency response characteristics of the power system when a disturbance occurs according to the static frequency characteristics of the load and the generator set; Constructing a frequency response model of a generator set according to the frequency response characteristics; The frequency response model of the generator set is constrained by the mathematical relationship between the power generation of the new energy in the generator set and the power supply of the energy storage station, so as to generate an extended model of the active power-frequency response of the new energy generator set; Determining the correlation between the maximum disturbance of the power system and the static frequency characteristics of the load according to the active power-frequency response expansion model of the new energy unit; The optimal load static frequency characteristic in the power system is determined according to the association relationship.

2. The method for improving the frequency stability of a power system according to claim 1, characterized in that: The static frequency characteristic of the load is expressed as follows: Among them, P f is the total active load of the power system when the frequency is f; is the total active load of the system when the frequency is rated; a k (k=0, 1, 2, ..., n) is the load ratio proportional to the i-th power of the frequency.

3. The method for improving the frequency stability of a power system according to claim 1, characterized in that: The static frequency characteristic of the generator set is expressed as follows: Its per unit value is: δ=[(n o -n) / n e ]×100% Among them, n0 is the static speed of the generator set when the system is unloaded; n is the static speed of the generator set when the system is fully loaded; n e It is the speed of the generator set when the system is in rated state.

4. The method for improving the frequency stability of a power system according to claim 1, characterized in that: The frequency response characteristics of the power system when a disturbance occurs are expressed as follows: Where β=(K G +K L ) is the system frequency response characteristic coefficient, which is the sum of the load frequency characteristic coefficient and the generator set frequency characteristic coefficient in the power system.

5. The method for improving the frequency stability of a power system according to claim 1, characterized in that: Motor group frequency response model, including: Where: Among them, P SP is the ratio of the load disturbance power in the system to the total load of the system; F H K is the high pressure bar power coefficient of the steam turbine; m is the mechanical power gain coefficient; Δω is the system frequency offset; P m P is the mechanical power output of the steam turbine; e is the electromagnetic power of the generator; P a =P m -P e is the acceleration power of the generator; H is the total inertia time constant of the generator; D is the equivalent damping coefficient of the generator; R is the frequency adjustment coefficient of the speed regulator; T R is the reheat time constant of the prime mover.

6. The method for improving the frequency stability of a power system according to claim 1, characterized in that: The association relationship includes: The quantitative evaluation index ηS of system frequency static stability is expressed as follows: Among them, η S is the evaluation index, 0≤η≤0.05, f SEP and f UEP They respectively represent the frequency values ​​corresponding to frequency SEP and frequency U EP, and fN is the rated frequency value.

7. The method for improving the frequency stability of a power system according to claim 1, characterized in that: The new energy power generation includes: wind power and / or photovoltaic power generation.

8. A device for improving the frequency stability of a power system, characterized in that: include: Frequency characteristic module, used to obtain the static frequency characteristics of the load in the power system; Determine the static frequency characteristics of the generator set according to the static frequency characteristics of the load; determine the frequency response characteristics of the power system when a disturbance occurs according to the static frequency characteristics of the load and the generator set; A model building module, used to build a frequency response model of the generator set according to the frequency response characteristics; A model extension module, used to constrain the frequency response model of the generator set by using the mathematical relationship between the power generation of new energy in the generator set and the power supply of the energy storage station to generate an extended model of the active power-frequency response of the new energy generator set; A relationship acquisition module, used to determine the correlation relationship between the maximum disturbance of the power system and the static frequency characteristics of the load according to the active power-frequency response expansion model of the new energy unit; The characteristic selection module is used to determine the optimal load static frequency characteristic in the power system according to the association relationship.

9. A device for improving the frequency stability of a power system, characterized in that: include: A memory for storing a computer executable program of a method for improving frequency stability of a power system according to any one of claims 1 to 7; The processor is used to call the computer executable program in the memory to execute: obtaining the static frequency characteristics of the load in the power system; determining the static frequency characteristics of the generator set according to the static frequency characteristics of the load; determining the frequency response characteristics of the power system when a disturbance occurs according to the static frequency characteristics of the load and the generator set; A frequency response model of a generator set is constructed according to the frequency response characteristics; the frequency response model of the generator set is constrained by the mathematical relationship between the power generation of new energy in the generator set and the power supply of the energy storage station, and an active-frequency response expansion model of the new energy generator set is generated; according to the active-frequency response expansion model of the new energy generator set, a correlation relationship between the maximum disturbance of the power system and the static frequency characteristics of the load is determined; and according to the correlation relationship, the optimal static frequency characteristics of the load in the power system are determined.

10. A storage medium, characterized in that: The invention comprises storing a computer executable program, wherein the computer executable program is used to be called by a processor to execute the steps of a method for improving the frequency stability of a power system as described in any one of claims 1 to 7.

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