CN118040792A - A method for controlling secondary frequency sag in power system based on speed recovery using exponential function - Google Patents

Abstract

The invention discloses a power system frequency secondary drop control method for recovering rotation speed based on an exponential function, which comprises the following steps: when a disturbance occurs in the power system, obtaining frequency deviation delta f and frequency change rate df/dt, and input power P ₀ and rotating speed omega _r of a fan; judging whether the system frequency is stable or not, and if the system frequency is not met, enabling the fan to participate in system frequency modulation by adopting virtual inertia control; judging whether the rotating speed of the fan reaches the lower limit or not, and entering a rotating speed recovery stage; adopting a target power recovery curve based on an exponential function to reduce the secondary influence on the frequency of the power grid until the rotating speed is recovered; and after the rotating speed is recovered, the MPPT mode is adopted to control the output power of the unit. On one hand, the invention can quickly reduce the system power so as to avoid overdrawing of the kinetic energy of the rotor; on the other hand, the stable recovery of the speed can be ensured, the torque influence is reduced, the secondary drop of the system frequency is restrained, the technical support is provided for the research of the frequency stability of a large-scale complex network, and the power-assisted power grid is safe and stable to operate.

Description

Secondary frequency drop control method of power system based on speed recovery based on exponential function

Technical Field

The present invention belongs to the technical field of power system frequency stability analysis, relates to a frequency secondary sag control technology, and specifically relates to a power system frequency secondary sag control method for speed recovery based on an exponential function.

Background technique

As renewable energy sources such as wind power and photovoltaic power generation gradually replace conventional synchronous generators, the inertia level of regional power grids continues to decrease. Doubly fed wind turbines change the output power of wind turbines after disturbances occur through virtual inertia control. When the system frequency decreases, the wind turbine releases the rotor kinetic energy for short-term power support. As the kinetic energy of the rotor is gradually released, when the generator speed drops to the lower limit of the speed, the wind turbine exits the primary frequency regulation and resumes MPPT control for speed recovery. At this time, the sharp drop in electromagnetic torque causes a sudden drop in active power output, causing a secondary drop in system frequency. Therefore, it is necessary to carry out research on wind power inertia secondary drop control technology. Frequency is an important indicator reflecting system stability. For the problem of secondary frequency drop, there is still a lack of efficient and reliable control strategies in the current research process.

Therefore, a new technical solution is needed to solve this problem.

Summary of the invention

Purpose of the invention: In order to overcome the deficiencies in the prior art, a method for controlling secondary frequency drop of an electric power system based on speed recovery using an exponential function is provided.

Technical solution: To achieve the above-mentioned purpose, the present invention provides a method for controlling the secondary frequency drop of a power system based on an exponential function for speed recovery, comprising the following steps:

S1: When a disturbance occurs in the power system, obtain the frequency deviation Δf and the frequency change rate df/dt as well as the input power P ₀ and the speed ω _r of the wind turbine;

S2: Determine whether the system frequency is stable. If the system meets the requirements for safe and stable operation of the power grid, the wind turbine operates in the maximum power point tracking (MPPT) mode. If not, the wind turbine uses virtual inertia control to participate in system frequency regulation.

S3: Determine whether the fan speed has reached the lower limit. If the speed has reached the minimum value, enter the speed recovery phase of step S4;

S4: Use a target power recovery curve based on an exponential function to mitigate the secondary impact on the grid frequency until the speed is restored;

S5: After the speed is restored, the MPPT mode is used to control the output power of the unit.

Furthermore, in step S2, the method for the fan to participate in system frequency modulation by using virtual inertia control is:

The output power of the doubly-fed wind turbine when using virtual inertia control is:

Where _Kp and _Kd are the corresponding frequency modulation parameters, _ΔPW is the output power of the fan;

At this time, the reference value _Pref of active power satisfies:

_Pref = _PMPPT - _ΔPW .

Furthermore, the frequency modulation parameters _Kp and _Kd are determined according to the frequency deviation Δf and the frequency change rate df/dt .

Furthermore, in step S4, a speed recovery method based on asymptotic approximation of an exponential function is used to recover the speed, which is specifically expressed as follows:

The power reference values during recovery are as follows:

_Pref = _P0- ( _P0 - _PMPPT )(1-e ^-kt )

In the formula, _P0 is the active power at time t1 before the end of the inertia response, _Pref is the power target value in the speed recovery stage, k is the adjustment coefficient, t is time, and the recovery start time t＝0. Wind speed is volatile, and the power or speed of the wind turbine can not be restored to the initial position. At the same time, the recovery time is difficult to predict, which can be represented by "infinite" time. From the above formula, it can be seen that when t tends to infinity, _Pref ＝P _MPPT , that is, the target power is tracked. k and t have the same position characteristics. The size of the k value affects the curvature of the _Pref curve. The larger the k value, the faster _Pref approaches P _MPPT , that is, it is closer to conventional control; the smaller the k value, the slower the speed of _Pref approaching P _MPPT , the smaller the impact of the mechanical shaft system, the lower the corresponding recovery point speed, and the greater the risk of wind speed disturbance. Therefore, the k value should not be too small, and the factors to be considered in its value range are as follows.

Furthermore, the determination of the value range of the adjustment coefficient k in the power reference value formula during recovery includes energy constraints and target power constraints.

Furthermore, the energy constraint is expressed as follows:

It can be seen from the above formula that it is difficult to obtain the range of k value due to the influence of the dual variables of C _p and wind speed v. In order to obtain universality, we can consider the extreme scenario where the wind speed becomes below the cut-in wind speed (the mechanical power input is considered to be 0), and the above formula can be simplified to:

Furthermore, the target power constraint is expressed as follows:

|P _ref -P _MPPT |≤ε|P ₀ -P _MPPT |

In the formula, ε is the error rate, which can be taken as 2% in the specific research process.

Combining the expressions of energy constraint and target power constraint and making approximate simplification, the range of k value can be obtained:

Furthermore, in step S5, the target recovery power _Pref satisfies:

In the formula,

Beneficial effects: Compared with the prior art, the present invention provides a control technology for suppressing the secondary frequency drop by performing speed recovery based on an exponential function. The speed recovery method using the asymptotic approach of an exponential function can, on the one hand, rapidly reduce the system power to avoid overdraft of the rotor kinetic energy; on the other hand, it can ensure smooth speed recovery and reduce the influence of torque, suppress the secondary drop of the system frequency, provide technical support for the study of frequency stability of large and complex networks, and help the safe and stable operation of the power grid.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a flow chart of the method of the present invention;

Figure 2 is a comparison diagram of active power curves of wind turbines;

Figure 3 is a comparison diagram of the speed curves of the fans;

Figure 4 is a comparison chart of the grid frequency change curves.

Detailed ways

The present invention is further explained below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention. After reading the present invention, various equivalent forms of modifications to the present invention by those skilled in the art all fall within the scope defined by the claims attached to this application.

As shown in FIG1 , the present invention provides a method for controlling a secondary frequency drop of a power system based on an exponential function for speed recovery, comprising the following steps:

S1: When a disturbance occurs in the power system, obtain the frequency deviation Δf and the frequency change rate df/dt as well as the input power P ₀ and the speed ω _r of the wind turbine;

S2: Determine whether the system frequency is stable. If the system meets the requirements for safe and stable operation of the power grid, the wind turbine operates in the maximum power tracking MPPT mode. If not, the wind turbine uses virtual inertia control to participate in system frequency regulation:

The method for fans to participate in system frequency modulation using virtual inertia control is:

The frequency modulation parameters _Kp and _Kd are determined according to the frequency deviation Δf and the frequency change rate df/dt. In this embodiment, in combination with a typical case of the SFR model, _Kp = 20 and _Kd = 20 are taken, and a relatively satisfactory simulation result can be obtained.

The output power of the doubly-fed wind turbine when using virtual inertia control is:

Where _Kp and _Kd are the corresponding frequency modulation parameters, _ΔPW is the output power of the fan;

At this time, the reference value _Pref of active power satisfies:

P _ref = P _MPPT - ΔP _W

S3: Determine whether the fan speed has reached the lower limit. If the speed has reached the minimum value, enter the speed recovery phase of step S4;

S4: Use a target power recovery curve based on an exponential function to mitigate the secondary impact on the grid frequency until the speed is restored:

A speed recovery method based on the asymptotic approach of an exponential function is used to recover the speed, which is specifically expressed as follows:

The power reference values during recovery are as follows:

_Pref = _P0- ( _P0 - _PMPPT )(1-e ^-kt )

In the formula, _P0 is the active power at time _t1 before the end of the inertia response, _Pref is the power target value in the speed recovery stage, k is the adjustment coefficient, t is time, and the recovery start time t = 0. Wind speed is volatile, and the power or speed of the wind turbine can not be restored to the initial position. At the same time, the recovery time is difficult to predict, which can be represented by "infinite" time. From the above formula, it can be seen that when t tends to infinity, _Pref = P _MPPT , that is, the target power is tracked. k and t have the same position characteristics. The size of the k value affects the curvature of the _Pref curve. The larger the k value, the faster _Pref approaches P _MPPT , that is, it is closer to conventional control; the smaller the k value, the slower the speed of _Pref approaching P _MPPT , the smaller the impact of the mechanical shaft system, the lower the corresponding recovery point speed, and the greater the risk of wind speed disturbance. Therefore, the k value should not be too small. The factors to be considered in its value range are as follows;

The determination of the value range of the adjustment coefficient k in the power reference value formula during the recovery period includes energy constraints and target power constraints;

The energy constraint is expressed as follows:

It can be seen from the above formula that it is difficult to obtain the range of k value due to the influence of the dual variables of C _p and wind speed v. In order to obtain universality, we can consider the extreme scenario where the wind speed becomes below the cut-in wind speed (the mechanical power input is considered to be 0), and the above formula can be simplified to:

The target power constraint is expressed as follows:

|P _ref -P _MPPT |≤ε|P ₀ -P _MPPT |

Wherein, ε is the error rate, which is 2% in this embodiment.

Combining the expressions of energy constraint and target power constraint and making approximate simplification, the range of k value can be obtained:

S5: After the speed is restored, the MPPT mode is used to control the output power of the unit, and the target recovery power _Pref satisfies:

In the formula,

The present invention provides a control technology for suppressing the secondary frequency drop by performing speed recovery based on an exponential function. The speed recovery method using the asymptotic approach of an exponential function is adopted. On the one hand, the system power can be rapidly reduced to avoid overdraft of the rotor kinetic energy; on the other hand, it can ensure smooth recovery of the speed and reduce the influence of torque, suppressing the secondary frequency drop of the system, providing technical support for the research on frequency stability of large and complex networks, and facilitating the safe and stable operation of the power grid.

In order to verify the practical effect of the present invention, this embodiment has carried out simulation experiments, and the simulation results are as follows:

As shown in Figure 2, it can be seen that after the system adopts the improved strategy, the output power of the fan drops rapidly and stabilizes after a certain period of time, thus avoiding the overdraft of the rotor kinetic energy;

As shown in Figure 3, it can be seen that after the system adopts the improved strategy, the recovery of the fan speed is more stable, the torque influence is reduced, and the secondary drop of the system frequency is better suppressed;

As shown in Figure 4, it can be seen that after adopting the improved strategy, the grid frequency will basically not experience obvious secondary drops after the first drop, which suppresses the secondary drop of the system frequency and helps the grid to operate safely and stably.

This embodiment also provides an electric power system frequency secondary drop control system for speed recovery based on an exponential function, the system comprising a network interface, a memory and a processor; wherein the network interface is used to receive and send signals during the process of sending and receiving information between other external network elements; the memory is used to store computer program instructions that can be run on the processor; the processor is used to execute the steps of the above-mentioned consensus method when running the computer program instructions.

The present embodiment also provides a computer storage medium, which stores a computer program, and the method described above can be implemented when the processor executes the computer program. The computer readable medium can be considered to be tangible and non-temporary. Non-limiting examples of non-temporary tangible computer readable media include non-volatile memory circuits (such as flash memory circuits, erasable programmable read-only memory circuits or mask read-only memory circuits), volatile memory circuits (such as static random access memory circuits or dynamic random access memory circuits), magnetic storage media (such as analog or digital tapes or hard drives) and optical storage media (such as CDs, DVDs or Blu-ray discs), etc. The computer program includes processor executable instructions stored on at least one non-temporary tangible computer readable medium. The computer program may also include or rely on stored data. The computer program may include a basic input/output system (BIOS) that interacts with the hardware of a special-purpose computer, a device driver that interacts with a specific device of a special-purpose computer, one or more operating systems, user applications, background services, background applications, etc.

Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application may adopt the form of a computer program product implemented in one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code.

The present application is described with reference to the flowchart and/or block diagram of the method, device (system) and computer program product according to the embodiment of the present application. It should be understood that each process and/or box in the flowchart and/or block diagram, and the combination of the process and/or box in the flowchart and/or block diagram can be realized by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device produce a device for realizing the function specified in one process or multiple processes in the flowchart and/or one box or multiple boxes in the block diagram.

These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer-readable memory produce a manufactured product including an instruction device that implements the functions specified in one or more processes in the flowchart and/or one or more boxes in the block diagram.

These computer program instructions may also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, whereby the instructions executed on the computer or other programmable device provide steps for implementing the functions specified in one or more processes in the flowchart and/or one or more boxes in the block diagram.

Claims (9)

1. The power system frequency secondary drop control method for recovering the rotating speed based on the exponential function is characterized by comprising the following steps of:

S1: when a disturbance occurs in the power system, obtaining frequency deviation delta f and frequency change rate df/dt, and input power P ₀ and rotating speed omega _r of a fan;

s2: judging whether the system frequency is stable or not, if the system meets the requirement of safe and stable operation of the power grid, the fan operates in a maximum power tracking (MPPT) mode, and if the system does not meet the requirement, the fan adopts virtual inertia control to participate in system frequency modulation;

s3: judging whether the rotating speed of the fan reaches a lower limit, and if the rotating speed reaches a minimum value, entering a rotating speed recovery stage of the step S4;

s4: adopting a target power recovery curve based on an exponential function to reduce the secondary influence on the frequency of the power grid until the rotating speed is recovered;

S5: and after the rotating speed is recovered, the MPPT mode is adopted to control the output power of the unit.

2. The method for controlling the frequency secondary drop of the power system for recovering the rotational speed based on the exponential function according to claim 1, wherein the method for controlling the frequency modulation of the participating system by using the virtual inertia in the step S2 is as follows:

the output power of the doubly-fed fan when virtual inertia control is adopted is as follows:

Wherein, K _p and K _d are corresponding frequency modulation parameters, and DeltaP _W is the output power of the fan;

The reference value P _ref of the active power at this time satisfies:

P_ref＝P_MPPT-ΔP_W。

3. the method for controlling the frequency secondary drop of the electric power system for recovering the rotational speed based on the exponential function according to claim 2, wherein the frequency modulation parameters K _p and K _d are determined according to the frequency deviation Δf and the frequency change rate df/dt.

4. The method for controlling the frequency secondary drop of the electric power system for recovering the rotational speed based on the exponential function according to claim 2, wherein the rotational speed recovery is performed by adopting a rotational speed recovery method based on the gradual approach of the exponential function in the step S4, specifically expressed as follows:

the power reference values during recovery are as follows:

P_ref＝P₀-(P₀-P_MPPT)(1-e^-kt)

in the formula, P ₀ is the active time t1 before the end of inertia response, P _ref is the power target value in the rotational speed recovery stage, k is the adjustment coefficient, t is the time, and the recovery starting time t=0.

5. The method for controlling the frequency secondary drop of the power system for recovering the rotating speed based on the exponential function according to claim 4, wherein the determination of the value range of the adjustment coefficient k in the power reference value formula during the recovery period comprises an energy constraint and a target power constraint.

6. The method for controlling the frequency secondary drop of the power system for recovering the rotational speed based on the exponential function according to claim 5, wherein the energy constraint is expressed as follows:

From the above equation, it can be seen that, under the influence of the bivariate of C _p and the wind speed v, it is actually difficult to obtain the k value range, in order to obtain universality, the wind speed may be considered to be the extreme scene below the cut-in wind speed, and the above equation may be simplified as follows:

7. The method for controlling the frequency secondary drop of the power system for recovering the rotational speed based on the exponential function according to claim 6, wherein the target power constraint is expressed as follows:

|P_ref-P_MPPT|≤ε|P₀-P_MPPT|

where ε is the error rate.

8. The power system frequency secondary drop control method for speed recovery based on exponential function according to claim 7, wherein the k value range can be obtained by combining the expressions of energy constraint and target power constraint and performing approximate simplification:

9. The method for controlling the frequency secondary drop of the electric power system for rotational speed recovery based on the exponential function according to claim 7, wherein the target recovery power P _ref in step S5 satisfies:

In the method, in the process of the invention,