VSG frequency modulation control method and system based on segmented integral coefficient

Abstract

The present application belongs to the technical field of secondary frequency modulation control of virtual synchronous generator, and discloses a VSG secondary frequency modulation control method and system based on segmented integral coefficients, which proposes the determination of the value of the frequency integral coefficient based on the segmentation of the frequency offset, and determines two fixed integral coefficients respectively. When the frequency deviation is lower than the threshold value, the integral coefficient value is small, and when the frequency deviation is higher than the threshold value, the integral coefficient value is large. The control method proposed in the present application reduces the frequency integral coefficient adjustment frequency while reducing the maximum frequency deviation, improves the frequency dynamic adjustment performance of the virtual synchronous generator and the stability of the system, and through the segmented setting of the integral coefficient, not only the frequency deviation is reduced, but also the frequency dynamic adjustment time is shortened. The control method proposed in the present application changes the integral coefficient only twice in the dynamic adjustment process, reduces the change frequency of the integral coefficient, and improves the stability of the frequency change in the dynamic process.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of secondary frequency modulation control of virtual synchronous generator, and particularly relates to a VSG secondary frequency modulation control method and system based on segmented integral coefficients. BACKGROUND

[0002] With the increasing of renewable energy installed capacity and proportion, the power electronic degree of power system is continuously improved. For the new power electronic power system, a series of new control strategies are constantly proposed. In view of the characteristics of low inertia and small damping of power electronic equipment, the virtual synchronous generator (VSG) control technology emerges as the times require.

[0003] According to the adjustment characteristics of the output frequency of VSG, the VSG parameter adaptive control can be divided into two types of primary frequency modulation control and secondary frequency modulation control. The primary frequency modulation is a differential adjustment, and the frequency cannot return to the initial value in the final stable state. The secondary frequency modulation is to compensate for the frequency deviation, and the frequency deviation is zero in the steady state. When the VSG is in the off-grid condition, if the system suddenly switches to heavy load, the system frequency is most likely to drop out of the safe range of frequency. Therefore, under the off-grid condition, the secondary frequency modulation control of VSG can improve the safety and reliability of the power system.

[0004] The traditional fixed parameter value control method cannot solve the contradiction between system regulation time and frequency oscillation. Although the improved integral coefficient adaptive control can improve the frequency regulation performance, the integral coefficient is adjusted too frequently, which will cause the frequency fluctuation to be larger in the frequency regulation process.

[0005] Li Bin [1] in "Microgrid inverter secondary frequency modulation scheme based on improved virtual synchronous generator algorithm", adopts a fixed integral coefficient, and determines the value of the integral coefficient according to the power proportion of VSG. The integral coefficient of each VSG is a constant value. Zhu Zuo-bin [2] in "Microgrid inverter VSG control strategy based on integral compensation", adopts a fixed integral coefficient to realize the secondary regulation of VSG frequency.

[0006] The existing technology has the defect that the integral coefficient is a constant value, which cannot solve the contradiction between frequency regulation time and overshoot. Only the function of frequency secondary regulation is realized, and the regulation performance is not improved.

[0007] Through the above analysis, the problems and defects of the existing technology are:

[0008] (1) The traditional fixed parameter value control method cannot solve the contradiction between system regulation time and frequency oscillation. Although the improved integral coefficient adaptive control can improve the frequency regulation performance, the integral coefficient is adjusted too frequently, which will cause the frequency fluctuation to be larger in the frequency regulation process.

[0009] (2) integral coefficient is a fixed value, which cannot solve the contradiction between frequency adjustment time and overshoot. Only the function of frequency quadratic regulation is realized, and the regulation performance is not improved. SUMMARY

[0010] In view of the problems existing in the prior art, the application provides a VSG quadratic frequency control method based on segmented integral coefficient.

[0011] The application is implemented as follows: a VSG quadratic frequency control method based on segmented integral coefficient comprises the following steps:

[0012] Step 1: design a VSG quadratic frequency structure;

[0013] Step 2: design a segmented quadratic frequency control strategy.

[0014] Further, the method for designing the VSG quadratic frequency structure comprises the following steps:

[0015] (1) design a VSG primary frequency control block diagram;

[0016] According to the VSG active frequency control equation:

[0017]

[0018] The active frequency control block diagram can be obtained;

[0019] (2) design a VSG quadratic frequency control block diagram;

[0020] An integral element is added to the active frequency control equation:

[0021]

[0022] The frequency quadratic regulation control block diagram can be obtained.

[0023] Further, the method for designing the segmented quadratic frequency control strategy comprises the following steps:

[0024] 1) calculate the value range of the frequency integral coefficient:

[0025] The integral coefficient k satisfies the condition that the frequency integral coefficient corresponds to the damping ratio in the 0.1-0.3 interval of the second-order system.

[0026]

[0027] According to the values of Kp, D and J, the value range of the integral coefficient k is determined as:

[0028]

[0029] 2) determine the method for segmented quadratic frequency:

[0030] The piecewise control strategy is designed as follows by taking k1 and k2 in the value range of the integral coefficient k respectively.

[0031]

[0032] The value of epsilon is determined according to the change range of the frequency offset.

[0033] The control block diagram of the piecewise secondary frequency modulation.

[0034] Another object of the present application is to provide a piecewise integral coefficient-based VSG secondary frequency modulation control system for implementing the piecewise integral coefficient-based VSG secondary frequency modulation control method as described above, comprising:

[0035] A VSG primary frequency modulation control module is configured to design a VSG primary frequency modulation control block diagram and obtain an active frequency control block diagram according to a VSG active frequency control equation.

[0036] A VSG secondary frequency modulation control module is configured to add an integral element to the active frequency control equation to obtain a frequency secondary regulation control block diagram.

[0037] A piecewise integral coefficient calculation module is configured to calculate the value range of the frequency integral coefficient, set the damping ratio of the frequency integral coefficient corresponding to the 0.1-0.3 interval of the second-order system, and determine the value range of the integral coefficient k.

[0038] A piecewise control strategy design module is configured to take k1 and k2 in the value range of the integral coefficient k respectively, determine the values according to the change range of the frequency offset, and design the control strategy of the piecewise secondary frequency modulation.

[0039] Further, the VSG primary frequency modulation control module comprises:

[0040] According to the active frequency control equation:

[0041]

[0042] The designed active frequency control block diagram;

[0043] The VSG secondary frequency modulation control module adds an integral element to the active frequency control equation:

[0044]

[0045] The designed frequency secondary regulation control block diagram.

[0046] Further, the piecewise control strategy design module comprises:

[0047] The value range of the frequency integral coefficient k is calculated, and the value range of the integral coefficient k is determined according to the values of Kp, D and J.

[0048]

[0049] The designed segmented control strategy is as follows when the integral coefficient k takes values k1 and k2 in the value range of the integral coefficient k:

[0050]

[0051] The value of ε is determined according to the change range of the frequency offset.

[0052] In combination with the technical scheme and the solved technical problem, the technical scheme to be protected by the application has the following advantages and positive effects:

[0053] Firstly, the application proposes to determine the value of the integral coefficient based on the frequency offset, and two fixed integral coefficients are determined, when the frequency deviation is lower than the threshold value, the integral coefficient value is small, and when the frequency deviation is higher than the threshold value, the integral coefficient value is large. The control method proposed in the application reduces the frequency maximum offset while reducing the adjustment frequency of the integral coefficient, and improves the performance of the virtual synchronous generator frequency dynamic adjustment and the stability of the system.

[0054] By segmenting the integral coefficient, the frequency offset is reduced, and the dynamic adjustment time of the frequency is shortened. Compared with other parameter adaptive control methods, the integral coefficient of the proposed control method changes only twice in the dynamic adjustment process, greatly reducing the number of integral coefficient changes and improving the stability of the frequency change in the dynamic process.

[0055] 1) The frequency offset and the dynamic adjustment time of the frequency are reduced.

[0056] 2) The adjustment frequency of the integral coefficient is reduced, and the stability of the system is improved.

[0057] The application proposes a segmented control strategy for determining the value of the integral coefficient based on the frequency offset. The dynamic adjustment of the frequency is divided into two parts in the longitudinal direction, which are the part of the frequency offset lower than the threshold value and the part of the frequency offset higher than the threshold value. For the part of the frequency offset higher than the threshold value, a larger integral coefficient is used to improve the dynamic performance of the frequency response. For the part of the frequency offset lower than the threshold value, a smaller integral coefficient is used to reduce the overshoot in the steady state.

[0058] Secondly, the present application solves the problem of slow response speed and low regulation accuracy in the prior art VSG secondary frequency modulation control method. In the frequency regulation process of the traditional VSG control method, it is often difficult to quickly respond to load changes, resulting in large frequency deviation and reduced system stability. Especially in the case of large load fluctuations, the existing control strategy is difficult to ensure that the dynamic performance and steady-state performance of the system are met at the same time. Therefore, there is an urgent need for a VSG secondary frequency modulation control method that can improve the response speed and regulation accuracy.

[0059] The present application significantly improves the response speed and regulation accuracy of the system by designing a segmented integral coefficient secondary frequency modulation control strategy. The segmented integral coefficient control strategy selects appropriate integral coefficients according to the different frequency deviation, which can quickly respond to large load changes and reduce frequency deviation. At the same time, by accurately calculating the value range of the frequency integral coefficient, the system can maintain high regulation accuracy under various operating conditions, thereby effectively improving the stability of the system.

[0060] In addition, the segmented control strategy adopted by the present application can adaptively adjust the integral coefficient in real time according to the load change, overcoming the problem of fixed integral coefficient in the traditional method that cannot balance the dynamic performance and steady-state performance. The segmented control strategy not only improves the flexibility of frequency regulation, but also provides the best control effect under different load conditions, significantly improving the overall performance of the system.

[0061] In summary, the present application not only solves the problems of the prior art, but also achieves significant technical progress. By optimizing the VSG secondary frequency modulation control structure and strategy, the present application not only improves the response speed and regulation accuracy of the system, but also enhances the stability and adaptability of the system, providing strong support for the popularization of VSG technology in practical applications.

[0062] Thirdly, the technical solution of the present application solves the technical problem that the fixed integral coefficient in the traditional method cannot balance the dynamic performance and steady-state performance. BRIEF DESCRIPTION OF DRAWINGS

[0063] Figure 1 is the VSG secondary frequency modulation control method flowchart based on segmented integral coefficient provided by the embodiment of the present application.

[0064] Figure 2 is the VSG secondary frequency modulation control system structure block diagram based on segmented integral coefficient provided by the embodiment of the present application.

[0065] Figure 3 is the VSG secondary frequency modulation control system detailed structure block diagram based on segmented integral coefficient provided by the embodiment of the present application.

[0066] Figure 4 is a block diagram of active frequency control provided by an embodiment of the present application.

[0067] Figure 5 is a block diagram of secondary frequency regulation control provided by an embodiment of the present application.

[0068] Figure 6 is a block diagram of segmented secondary frequency regulation control provided by an embodiment of the present application.

[0069] Figure 7 is a schematic diagram of segmented secondary frequency regulation provided by an embodiment of the present application.

[0070] Figure 8 is a simulation effect diagram provided by an embodiment of the present application. DETAILED DESCRIPTION

[0071] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application is further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0072] As shown in Figure 1 , a secondary frequency regulation control method for VSG based on segmented integral coefficients provided by an embodiment of the present application includes the following steps:

[0073] S101, designing a secondary frequency regulation structure for VSG;

[0074] S102, designing a segmented secondary frequency regulation control strategy.

[0075] The detailed working principle of an embodiment of the present application.

[0076] 1. Primary frequency control design:

[0077] The primary frequency control mainly stabilizes the frequency by adjusting the output power. The control block diagram is based on the active frequency control equation, and the preliminary stabilization of the frequency is realized by adjusting the damping coefficient D.

[0078] In actual application, the output power P of the VSG will be adjusted according to the difference between the current frequency ω and the reference frequency ω * , to ensure the preliminary stabilization of the frequency.

[0079] 2. Secondary frequency control design:

[0080] The secondary frequency control increases the integral control link on the basis of the primary frequency control, to eliminate the frequency deviation and realize the accurate control of the frequency.

[0081] The integral term K i is added to the active frequency control equation. *)dt, the frequency deviation can be accumulated and compensated, so as to realize more accurate frequency control.

[0082] Segmented integral coefficient design: in order to improve the response speed and stability of frequency control, the integral coefficient K i Designed as a segmented function, different integral coefficients are used according to different frequency deviation ranges, and the control performance is optimized.

[0083] 3. Specific implementation steps:

[0084] Step 1: design VSG primary frequency control block diagram:

[0085] Based on the VSG active frequency control equation, the primary frequency control block diagram is designed to ensure that it can respond and adjust quickly when the frequency deviation is small.

[0086] Step 2: design VSG secondary frequency control block diagram:

[0087] On the basis of the primary frequency control block diagram, the integral control link is added, and the frequency deviation is eliminated through integral control to ensure accurate control of the frequency.

[0088] Segmented integral coefficient is used, different integral coefficients K i Are designed according to different frequency deviation ranges to optimize control response and stability.

[0089] Through the above steps and methods, the VSG secondary frequency control based on segmented integral coefficient can be realized, and the frequency control performance and stability of the VSG are effectively improved.

[0090] The method for designing the VSG secondary frequency control structure provided by the embodiment of the application comprises:

[0091] (1) design VSG primary frequency control block diagram;

[0092] According to the VSG active frequency control equation:

[0093]

[0094] The active frequency control block diagram can be obtained;

[0095] (2) design VSG secondary frequency control block diagram;

[0096] An integral link is added in the active frequency control equation:

[0097]

[0098] The frequency secondary regulation control block diagram can be obtained.

[0099] The method for designing the segmented secondary frequency control strategy provided by the embodiment of the application comprises:

[0100] 1) Calculate the value range of the frequency integration coefficient:

[0101] Set the damping ratio of the frequency integration coefficient corresponding to the second-order system in the interval of 0.1-0.3, then the condition that the integration coefficient k satisfies is:

[0102]

[0103] According to the values of Kp, D and J, the value range of the integration coefficient k is determined as:

[0104]

[0105] 2) Method for determining segmented quadratic frequency modulation:

[0106] Take k1 and k2 in the value range of the integration coefficient k respectively, and design the segmented control strategy as follows:

[0107]

[0108] The value of ε is determined according to the change range of the frequency offset.

[0109] Control block diagram of segmented quadratic frequency modulation.

[0110] As Figure 2 shown, the VSG quadratic frequency modulation control system based on segmented integration coefficient provided by the embodiment of the application comprises:

[0111] A frequency modulation structure design module for designing a VSG quadratic frequency modulation structure;

[0112] A control strategy design module for designing a segmented quadratic frequency modulation control strategy.

[0113] The application is specifically implemented as follows:

[0114] 1. Overall idea of technical scheme

[0115] For the part with a frequency offset higher than the threshold value, a larger integration coefficient is used to improve the dynamic performance of the frequency response. For the part with a frequency offset lower than the threshold value, a smaller integration coefficient is used to reduce the overshoot in the steady state.

[0116] 2. Specific implementation of technical scheme Figure 3 ;

[0117] Among them, C dc is the DC bus capacitor, I abc is the inverter output current, r and L are the filter inductance and resistance respectively, C is the filter capacitor, R is the load, U s , I sV and I are the load end voltage and current, respectively. P and Q are the active power and reactive power, P m is the mechanical power, J is the moment of inertia, ω0 is the reference rotational angular frequency, D is the damping coefficient, k is the integral coefficient, Δf is the frequency offset, ε is the frequency offset threshold, K q is the voltage droop coefficient, Q ref is the reference reactive power, K u is the excitation regulation coefficient, U s is the synchronous generator output voltage, U ref is the synchronous generator reference output voltage, E0 is the reference excitation electromotive force.

[0118] Step 1: Design the secondary frequency modulation structure of VSG

[0119] Step 11: Design the primary frequency modulation control block diagram of VSG

[0120] According to the active frequency control equation of VSG:

[0121]

[0122] The active frequency control block can be obtained Figure 4 :

[0123] Step 12: Design the secondary frequency modulation control block diagram of VSG

[0124] Add an integral element to the active frequency control equation:

[0125]

[0126] The frequency secondary regulation control block can be obtained Figure 5 :

[0127] Step 2: Design the segmented secondary frequency modulation control strategy

[0128] Step 21: Calculate the value range of the frequency integral coefficient:

[0129] Set the damping ratio of the frequency integral coefficient corresponding to the 0.1-0.3 interval of the second-order system, then the integral coefficient k satisfies the condition:

[0130]

[0131] According to the values of Kp, D, and J, the value range of the integral coefficient k is determined as:

[0132]

[0133] Step 22: Determine the segmented secondary frequency modulation method, as shown in Figure 7 :

[0134] The integral coefficient k takes values k1 and k2 in the range of the integral coefficient k, and the segmented control strategy is designed as follows:

[0135]

[0136] The value of epsilon is determined according to the range of frequency offset.

[0137] Control block of segmented secondary frequency modulation Figure 6

[0138] 3、Technical effects:

[0139] By setting the integral coefficient in segments, the frequency offset is reduced, and the dynamic adjustment time of the frequency is shortened. Compared with other parameter adaptive control methods, the integral coefficient of the proposed control method changes only twice in the dynamic adjustment process, greatly reducing the number of integral coefficient changes and improving the stability of frequency changes in the dynamic process.

[0140] The expected control effect is as follows Figure 7

[0141] 4、Technical effects brought by the technical scheme of the present application

[0142] 1) The frequency offset and the dynamic adjustment time of the frequency are reduced.

[0143] 2) The number of integral coefficient adjustments is reduced, and the stability of the system is improved.

[0144] 5、For the technical scheme in 4, are there any other alternative schemes that can also achieve the purpose of the invention

[0145] There are no other schemes that can achieve the same invention purpose.

[0146] 6、Technical key points and pre-protection points of the present application

[0147] 1) A segmented control strategy based on the integral coefficient value determined by the frequency offset is proposed. The dynamic adjustment of the frequency is divided into two parts longitudinally, which are the frequency offset below the threshold and the frequency offset above the threshold. For the part of the frequency offset above the threshold, a larger integral coefficient is used to improve the dynamic performance of the frequency response. For the part of the frequency offset below the threshold, a smaller integral coefficient is used to reduce the overshoot in the steady state.

[0148] The following are two specific industrial application examples of a VSG secondary frequency modulation control method based on segmented integral coefficients:

[0149] Example 1: Frequency regulation control in a wind power generation system

[0150] ​​1) Primary frequency control block diagram design: Based on the VSG active frequency control equation, a primary frequency control block diagram is designed for the wind power generation system to ensure that the system can quickly respond and adjust when the frequency deviation caused by wind speed changes is small, maintaining frequency stability.

[0151] 2) Secondary frequency control block diagram design: On the basis of the primary frequency control block diagram, an integral control link is added to eliminate frequency deviation through segmented integral coefficients. Different integral coefficients k1, k2 are designed according to different frequency deviation ranges (such as ±0.1 Hz, ±0.1 Hz to ±0.5 Hz) to optimize control response and stability.

[0152] 3) Determination of segmented integral coefficients:

[0153] The value range of the frequency integral coefficient is calculated, the damping ratio is set between 0.1 and 0.3, and the value range of the integral coefficient k is determined as k1 between 0.05 and 0.15 and k2 between 0.15 and 0.3.

[0154] According to the frequency deviation range, a segmented control strategy is designed. For example, when the frequency deviation is within ±0.1 Hz, the integral coefficient k1 is used; when the frequency deviation is between ±0.1 Hz and ±0.5 Hz, the integral coefficient k2 is used.

[0155] 4) Application results: Through the secondary frequency control of segmented integral coefficients, the wind power generation system can quickly recover when the frequency deviation is large, ensuring the frequency stability of the power grid and improving the grid performance and system stability of wind power generation.

[0156] Example Two: Frequency regulation control in microgrid

[0157] 1) Primary frequency control block diagram design: In the microgrid system, based on the VSG active frequency control equation, a primary frequency control block diagram is designed to ensure that the system can quickly respond and adjust when the frequency deviation caused by load changes is small, maintaining frequency stability.

[0158] 2) Secondary frequency control block diagram design: On the basis of the primary frequency control block diagram, an integral control link is added to eliminate frequency deviation through segmented integral coefficients. Different integral coefficients k1, k2 are designed according to different frequency deviation ranges (such as ±0.2 Hz, ±0.2 Hz to ±0.6 Hz) to optimize control response and stability.

[0159] 3) Determination of segmented integral coefficients:

[0160] The value range of the frequency integral coefficient is calculated, the damping ratio is set between 0.1 and 0.3, and the value range of the integral coefficient k is determined as k1 between 0.05 and 0.12 and k2 between 0.12 and 0.25.

[0161] According to the frequency deviation range, a segmented control strategy is designed. For example, when the frequency deviation is within ±0.2 Hz, the integral coefficient k1 is adopted; when the frequency deviation is between ±0.2 Hz and ±0.6 Hz, the integral coefficient k2 is adopted.

[0162] 4) Application results: through the secondary frequency modulation control of the segmented integral coefficient, the micro-grid system can quickly recover when the load changes greatly, ensuring the frequency stability of the power grid, improving the independent operation and stability of the micro-grid, and enhancing the anti-disturbance ability of the system.

[0163] The two embodiments show the actual effect and advantages of the VSG secondary frequency modulation control method based on the segmented integral coefficient in different industrial applications.

[0164] The simulation model is built in Simulink, when the load suddenly increases from 35KW to 70KW, the frequency change curve is as follows Figure 8 As shown, compared with the control of the fixed integral coefficient, the maximum frequency deviation and the frequency regulation time under the control of the segmented integral coefficient are greatly reduced, verifying the actual control effect of the application.

[0165] The above is only a specific embodiment of the application, but the protection scope of the application is not limited thereto, any modification, equivalent replacement and improvement within the technical range disclosed by the application, which is within the spirit and principles of the application, should be covered within the protection scope of the application.

Claims

1. A method for frequency modulation control of a segmented integral coefficient based VSG, characterized in that, Comprising the following steps: Step 1, based on the VSG active frequency control equation, design a primary frequency control block diagram to ensure that when the frequency deviation is small, it can quickly respond and adjust; Designing the VSG primary frequency control block diagram includes; According to the VSG active frequency control equation: The active frequency control block diagram can be obtained; Step 2, design the VSG secondary frequency control block diagram includes: Add an integral element to the active frequency control equation: The frequency secondary adjustment control block diagram can be obtained; On the basis of the frequency modulation control block diagram, the integral control link is added to eliminate the frequency deviation through integral control and ensure the accurate control of frequency; the sectional integral coefficient is adopted to design different integral coefficients K according to different frequency deviation ranges i , so as to optimize the control response and stability. The value range of the frequency integral coefficient includes: Set the damping ratio of the frequency integral coefficient corresponding to the second-order system in the range of 0.1~0.3, then the integral coefficient k satisfies the condition: According to the values of Kp, D and J, the value range of the integral coefficient k is determined as: The method for determining the segmented secondary frequency includes: Respectively take k1 and k2 in the value range of the integral coefficient k, and design the segmented control strategy as follows: The value of epsilon is determined according to the change range of the frequency deviation.

2. A piecewise integration coefficient based VSG frequency chirp control system implementing the VSG frequency chirp control method of claim 1, wherein, Comprise: The VSG primary frequency control module is used for designing the VSG primary frequency control block diagram, and the active frequency control block diagram is obtained according to the VSG active frequency control equation; The VSG secondary frequency control module is used for adding an integral element to the active frequency control equation to obtain the frequency secondary adjustment control block diagram; The segmented integral coefficient calculation module is used for calculating the value range of the frequency integral coefficient, setting the damping ratio of the frequency integral coefficient corresponding to the second-order system in the range of 0.1~0.3, and determining the value range of the integral coefficient k; The segmented control strategy design module is used for respectively taking k1 and k2 in the value range of the integral coefficient k, determining the value according to the change range of the frequency deviation, and designing the control strategy of the segmented secondary frequency.

3. The piecewise integration coefficient based VSG frequency modulation control system of claim 2, wherein, The VSG primary frequency control module includes: According to the active frequency control equation: The designed active frequency control block diagram; The VSG secondary frequency control module adds an integral element to the active frequency control equation: The designed frequency secondary adjustment control block diagram.

4. The piecewise integration coefficient based VSG frequency modulation control system of claim 2, wherein, The segmented control strategy design module includes: The value range of the frequency integral coefficient k is calculated, and the value range of the integral coefficient k is determined according to the values of Kp, D and J: Respectively take k1 and k2 in the value range of the integral coefficient k, and the designed segmented control strategy is as follows: The value of epsilon is determined according to the change range of the frequency deviation.

Images