Virtual synchronous machine control method based on feedforward extraction using sine amplitude integration method

Abstract

The present invention relates to a virtual synchronous machine control method based on the extraction of feedforward using the sine amplitude integral method, which belongs to the technical field of distributed energy converters, and particularly relates to a virtual synchronous generator control method based on the extraction of feedforward using the sine amplitude integral method. The method comprises a VSG link, an SAI link and a feedforward link; the VSG link provides a control method for a grid-connected inverter to simulate a synchronous generator, and achieves an operating performance equivalent to that of a conventional synchronous generator through a control strategy, thereby improving the control characteristics under the access of new energy to the grid; the SAI link extracts the negative sequence and harmonic components of the grid in real time; the feedforward link suppresses the grid harmonics by adding feedforward to the controller based on the negative sequence and harmonic components of the grid voltage provided by the SAI link, so that the VSG can achieve complete cancellation of the negative sequence and harmonic components of the grid voltage under non-ideal grid voltage, thereby achieving the control method's suppression effect on current harmonics under different grid operating conditions.

Description

Technical Field

[0001] The present invention relates to a virtual synchronous machine control method based on the sine amplitude integral method feedforward quantity extraction, which belongs to the technical field of distributed energy converters, and particularly relates to a virtual synchronous generator control method based on the sine amplitude integral method feedforward quantity extraction. Background Art

[0002] Currently, renewable energy sources such as photovoltaic and wind power are increasingly being used, and their share of power generation in the power grid is increasing year by year. Because renewable energy power generation systems are affected by the external environment, their output exhibits a degree of randomness and volatility. The integration of large numbers of renewable energy sources into the grid poses challenges to the stable operation of the grid. Furthermore, compared to conventional large-capacity generators, distributed generation systems based on power electronic converters have lower inertia and damping, making the grid system more sensitive to power fluctuations and exacerbating system stability issues under high renewable energy penetration. Finding solutions to these root causes is urgently needed.

[0003] Traditional synchronous generators provide excellent support for stable grid operation. Virtual synchronous generators (VSGs) simulate the operating characteristics of synchronous generators and, by regulating grid-connected inverters, enable them to have similar external characteristics to synchronous generators, thereby improving grid stability. Conventional VSG control schemes are based on symmetrical grid voltages. However, in real-world grid conditions, such as asymmetric three-phase grid voltages and the presence of harmonics, these control schemes can lead to current distortion, power fluctuations, and other issues, further deteriorating grid quality. Summary of the Invention

[0004] The purpose of the present invention is to provide a virtual synchronous control method based on the extraction of feedforward quantity of the sinusoidal amplitude integration method to address the above-mentioned shortcomings. Combined with the sinusoidal amplitude integration method, a method for extracting the negative sequence and harmonic components of the power grid in real time is proposed, and a VSG grid-connected adaptive control method is developed. The VSG mechanism and its working characteristics are improved. By constructing a feedforward control scheme for the negative sequence and low-order harmonic components of the power grid voltage, the burden of the grid-connected current controller is reduced and grid adaptive control is achieved.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] A virtual synchronous machine control method based on the extraction of feedforward quantities using the sinusoidal amplitude integral method includes a VSG link, an SAI link, and a feedforward link. The VSG link provides a control mode for a grid-connected inverter to simulate a synchronous generator, and achieves operating performance equivalent to that of a conventional synchronous generator through a control strategy, thereby improving the control characteristics under the connection of new energy to the grid. The SAI link extracts the negative sequence and harmonic components of the grid in real time. The feedforward link suppresses the grid harmonics by adding feedforward to the controller based on the negative sequence and harmonic components of the grid voltage provided by the SAI link, so that the VSG can achieve complete cancellation of the negative sequence and harmonic components of the grid voltage under non-ideal grid voltage.

[0007] The VSG link is based on the second-order equivalent model of the synchronous generator. Combining the rotor motion equation and the stator electrical equation, the models of the mechanical part and the electromagnetic part of the virtual synchronous machine are as follows:

[0008]

[0009] Among them, v represents the terminal voltage, e represents the induced electromotive force, i represents the stator armature current, r represents the stator winding armature resistance, L represents the motor stator winding self-inductance, M f Indicates the maximum value of the mutual inductance between the rotors, i f represents the rotor excitation current, θ represents the angle between the rotor winding magnetic field axis and the a-phase stator winding axis, J represents the moment of inertia of the synchronous motor's rotating part, ω represents the angular velocity of the synchronous motor, and T m Represents the virtual mechanical torque, T e Denotes electromagnetic torque, D p represents the friction coefficient of the synchronous motor, ω r It represents the reference rotation angular velocity, P represents the grid-connected active power, and Q represents the grid-connected reactive power.

[0010] The SAI link is the sine amplitude integrator link, and its negative feedback part includes the following two closed-loop complex coefficient transfer functions:

[0011]

[0012]

[0013] Where k is the impact factor, and the k value is selected To ensure that the corresponding harmonic components can be extracted accurately and quickly, n is the harmonic order and w0 is the angular frequency of the fundamental wave.

[0014] The positive-sequence and negative-sequence structural complex coefficients of the sinusoidal amplitude integral are:

[0015]

[0016]

[0017] Where n is the harmonic order and w0 is the angular frequency of the fundamental wave.

[0018] s is the sign of the complex variable of the Laplace transform, and j is the sign of the imaginary part of the complex variable.

[0019] The feedforward is to directly add the voltage harmonics and asymmetric components extracted in real time based on the sinusoidal amplitude integration method to the voltage vector in front of the PWM generation module, so that the VSG can completely offset the negative sequence components and harmonic components of the grid voltage under non-ideal grid voltage.

[0020] The beneficial effects achieved by the present invention are:

[0021] 1. The present invention can overcome the influence of low-order negative sequence components and harmonic components on control and improve grid-connected characteristics;

[0022] 2. The present invention does not require changing the VSG controller design and control parameters. It can maintain the unique voltage source properties of the VSG and effectively control the output three-phase balanced current when the grid voltage is unbalanced or contains harmonics.

[0023] 3. The feedforward control of the present invention does not require additional delay generated by the VSG controller and can complete compensation in real time. Therefore, it can not only effectively improve the adaptability of virtual synchronous motor control to the power grid, but also help reduce active power and reactive power oscillations.

[0024] 4. The control method of the present invention does not depend on the imbalance type of the grid voltage and the line parameters, and does not require fault detection and control mode switching. The control structure is simple and easy to implement in engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention will be further described below with reference to the accompanying drawings:

[0026] Figure 1 This is the control block diagram of the virtual synchronous machine;

[0027] Figure 2 It is the positive sequence sine amplitude integral structure diagram;

[0028] Figure 3 It is the negative sequence sine amplitude integral structure diagram;

[0029] Figure 4 Construct a block diagram for sinusoidal amplitude-integrated negative feedback;

[0030] Figure 5 This is the output current control block diagram of the synchronous inverter;

[0031] Figure 6 This is the principle diagram of the virtual synchronous machine control method based on the sine amplitude integration method to extract the feedforward quantity. DETAILED DESCRIPTION

[0032] The following is combined with Figures 1 to 6 The present invention is further described in the following examples, which are only used to more clearly illustrate the technical solution of the present invention and are not intended to limit the scope of protection of the present invention.

[0033] like Figure 1 As shown in the figure, the VSG link is based on the second-order equivalent model of the synchronous generator. Combining the rotor motion equation and the stator electrical equation, the models of the mechanical part and the electromagnetic part of the virtual synchronous machine are as follows:

[0034]

[0035] Among them, v represents the terminal voltage, e represents the induced electromotive force, i represents the stator armature current, r represents the stator winding armature resistance, L represents the motor stator winding self-inductance, M f Indicates the maximum value of the mutual inductance between the rotors, i f represents the rotor excitation current, θ represents the angle between the rotor winding magnetic field axis and the a-phase stator winding axis, J represents the moment of inertia of the synchronous motor's rotating part, ω represents the angular velocity of the synchronous motor, and T m Represents the virtual mechanical torque, T e Denotes electromagnetic torque, D p represents the friction coefficient of the synchronous motor, ω r It represents the reference rotation angular velocity, P represents the grid-connected active power, and Q represents the grid-connected reactive power.

[0036] When the input is an orthogonal signal, the complex coefficient transfer function expression can be obtained:

[0037]

[0038]

[0039] The specific structure diagram of the above formula (2) can be found in Figure 2 、 Figure 3 The Bode diagrams of equations (2) and (3) show that the amplitude gain of the sine amplitude integral is infinite at the corresponding frequency, and the output has no phase delay. Using the above characteristics, we can construct Figure 4 The negative feedback system shown in FIG2 can obtain the corresponding closed-loop complex coefficient transfer function, as shown in FIG2 . The following equation (4) corresponds to positive sequence harmonic detection, and the following equation (5) corresponds to negative sequence harmonic detection.

[0040]

[0041]

[0042] In the formula, k is the impact factor, and the k value is selected as To ensure that the corresponding harmonic components can be extracted accurately and quickly.

[0043] Figure 5 This is a method for controlling the inductor current of the VSG. The induced electromotive force is directly generated by the power outer loop control and acts on both ends of the inductor with the grid voltage. The difference between the two is used to control the change trend of the inductor current. When the filtering method of the grid-connected inverter is single inductor, the grid current i g and the inductor current i L The control is completely equivalent. At this time, the derived grid-connected current transfer function is:

[0044]

[0045] As can be seen from the above formula, the grid-connected current is affected by two factors: 1. The output induced electromotive force e obtained by the power outer loop. The harmonics in the actual voltage and current will cause the output power to be unstable, and the power loop e1 will contain negative grid sequence and low-order harmonic components. 2. The disturbance of the grid voltage u g The grid voltage u including background harmonics g Therefore, when the grid is not ideal, the three-phase voltage imbalance and grid voltage harmonics will cause grid current imbalance and harmonics, reducing the quality of the grid current.

[0046] Since the sine amplitude integration method can extract both negative sequence components and harmonic components in the power grid, the sine amplitude integration method can be used to extract specific subvoltage components. Figure 6 This is a feedforward control scheme for specific subgrid voltage components based on the sine amplitude integral method (SAI). In order to obtain the negative sequence and harmonic components of the grid voltage, it is necessary to use a sine amplitude integrator to extract the subvoltage components from the grid voltage, and then multiply each component by a certain proportional coefficient and feed it forward to the PWM module. From the perspective of the equivalent mathematical model, since the inverter switching frequency is much higher than the power frequency, the inverter transfer function can be equivalent to a proportional link. Therefore, as long as the negative sequence and harmonic components of the grid voltage can be accurately detected, their influence can be fully compensated in the control. Figure 5 、 Figure 6 As shown, k SVPWM is the inverter PWM transfer function, and setting its reciprocal in the feedback loop can achieve complete cancellation of specific low-order harmonics. m (m=1,2,3…) is the order of the voltage harmonic component. When there is three-phase voltage imbalance in the power grid, the above harmonic extraction method can be used to realize the feedforward of the negative sequence component of the power grid voltage. In this case, n1=-1. When the power grid contains harmonics, for example, there are 5th and 7th harmonic components in the power grid. Since the 5th is a negative sequence component, n5=-5. The 7th is a positive sequence component, so n7=+7 can be taken.

[0047] Conventional virtual synchronous machine control strategies only consider the three-phase balanced condition of the power grid. The three-phase inverter only outputs the positive-sequence voltage component, which cannot offset the negative-sequence component in the power grid. When the power grid is unbalanced, the grid-connected current suffers from severe distortion. However, extracting the negative-sequence component of the power grid through the sinusoidal amplitude integration method and feeding it forward can simply and effectively offset the negative-sequence component in the power grid without affecting the virtual synchronous machine control implementation, thus meeting the grid-connected control requirements when the power grid is unbalanced. The inductor current is determined by the voltage difference across the inductor. After adding voltage component feedforward control, the negative-sequence and harmonic components in the power grid are output through the inverter terminal voltage, thus canceling each other out across the inductor, ensuring that the grid-connected current only contains the positive-sequence component of the power grid, thus achieving grid-adaptive control of the VSG under non-ideal power grids. Due to the introduction of feedforward, negative sequence and harmonic components on the grid side can be offset, and there is no need to suppress them through the control of the virtual synchronous machine. On the one hand, the burden on the controller is reduced, and the dynamic performance is also improved, which is also conducive to improving the grid-connected control performance under the condition of fluctuating output of renewable energy. On the other hand, the control scheme of the virtual synchronous machine has not been changed, but the introduction of feedforward control is conducive to reducing active and reactive oscillations.

[0048] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A virtual synchronous machine control method based on the sine amplitude integration method for extracting feedforward quantity, characterized in that: It includes VSG link, SAI link and feedforward link; the VSG link provides a control mode for a grid-connected inverter to simulate a synchronous generator. Through the control strategy, it achieves operating performance equivalent to that of a conventional synchronous generator, improves the control characteristics under the connection of new energy to the grid, and the control implementation method of the VSG inductor current is to directly generate an induced electromotive force by the power outer loop control, and act on both ends of the inductor with the grid voltage, and use the difference between the two to control the change trend of the inductor current; the SAI link extracts the negative sequence and harmonic components of the grid in real time, and needs to use a sinusoidal amplitude integrator to extract each voltage component from the grid voltage, and then multiply each component by a certain proportional coefficient and feed forward to the PWM module; the feedforward link suppresses the grid-connected harmonics by adding feedforward in the controller based on the negative sequence and harmonic components of the grid voltage provided by the SAI link, so that the VSG can achieve complete cancellation of the negative sequence and harmonic components of the grid voltage under non-ideal grid voltage; The grid-connected current transfer function of the VSG inductor current control implementation method is: Among them, K pwm is the inverter PWM transfer function.

2. The virtual synchronous machine control method based on the sine amplitude integration method feedforward quantity extraction according to claim 1 is characterized in that: The VSG link is based on the second-order equivalent model of the synchronous generator. Combining the rotor motion equation and the stator electrical equation, the models of the mechanical part and the electromagnetic part of the virtual synchronous machine are as follows: Among them, v represents the terminal voltage, e represents the induced electromotive force, i represents the stator armature current, r represents the stator winding armature resistance, L represents the motor stator winding self-inductance, M f Indicates the maximum value of the mutual inductance between the rotors, i f represents the rotor excitation current, θ represents the angle between the rotor winding magnetic field axis and the a-phase stator winding axis, J represents the moment of inertia of the synchronous motor's rotating part, ω represents the angular velocity of the synchronous motor, and T m Represents the virtual mechanical torque, T e Denotes electromagnetic torque, D p represents the friction coefficient of the synchronous motor, ω r It represents the reference rotation angular velocity, P represents the grid-connected active power, and Q represents the grid-connected reactive power.

3. The virtual synchronous machine control method based on the sine amplitude integration method feedforward quantity extraction according to claim 1 is characterized in that: The SAI link is the sine amplitude integrator link, and its negative feedback part includes the following two closed-loop complex coefficient transfer functions: Where k is the impact factor, and the k value is selected To ensure that the corresponding harmonic components can be extracted accurately and quickly, n is the harmonic order, and ω0 is the angular frequency of the fundamental wave; s is the sign of the complex variable of the Laplace transform, and j is the sign of the imaginary part of the complex variable.

4. The virtual synchronous machine control method based on the sine amplitude integration method feedforward quantity extraction according to claim 1 is characterized in that: The positive-sequence and negative-sequence structural complex coefficients of the sinusoidal amplitude integral are: Where n is the harmonic order, ω0 is the angular frequency of the fundamental wave; s is the sign of the complex variable of the Laplace transform, and j is the sign of the imaginary part of the complex variable.

5. The virtual synchronous machine control method based on the sine amplitude integration method feedforward quantity extraction according to claim 1 is characterized in that: The feedforward is to directly add the voltage harmonics and asymmetric components extracted in real time based on the sinusoidal amplitude integration method to the voltage vector in front of the PWM generation module, so that the VSG can completely offset the negative sequence components and harmonic components of the grid voltage under non-ideal grid voltage.

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