A method for suppressing low-frequency oscillation of output power in a virtual synchronous generator grid-connected system

Abstract

The present invention discloses a method for suppressing low-frequency oscillations in the output power of a virtual synchronous generator grid-connected system. Based on the control of traditional virtual synchronous generators, the present invention adds a feedforward compensation link from frequency deviation to voltage amplitude deviation, thereby increasing the damping ratio of the system, thereby suppressing the low-frequency oscillations in the output power of the traditional virtual synchronous generator grid-connected system. At the same time, the feedforward compensator ensures that the steady-state operating point of the system is not affected by the added control path. The method of the present invention has a simple structure, does not introduce nonlinear links, and does not rely on an accurate model of the system. It has strong robustness. While suppressing the low-frequency oscillations in the output power of the virtual synchronous generator, it does not change the inherent control parameters of the virtual synchronous generator. The controller design has a high degree of freedom, providing a good reference value for engineering applications.

Description

Technical Field

[0001] The present invention belongs to the field of distributed power grid-connected control, and in particular relates to a method for suppressing low-frequency oscillation of output power of a virtual synchronous generator grid-connected system. Background Art

[0002] With the increasing severity of energy crises and environmental issues, distributed power sources (DGs) using renewable energy sources such as solar and wind power as primary energy sources have garnered increasing attention and application. Typically, DGs rely on power electronic converters, such as inverters, to achieve power conversion, energy management, and off-grid control. However, compared to traditional synchronous generators, DGs based on power electronic converters lack inertia, which can lead to large frequency deviations and frequency variability, deteriorating power supply quality, causing malfunctions of power system automatic control devices, and compromising power system frequency stability.

[0003] Fortunately, the application of virtual synchronous generator technology can effectively solve this problem. By simulating the mechanical equations of motion of traditional synchronous generators, distributed power sources controlled by virtual synchronous generators have similar steady-state and dynamic characteristics to traditional synchronous generators. By simulating inertia at the control level, the equivalent inertia of the distributed power sources is increased, thereby providing support for grid frequency.

[0004] However, the application of virtual synchronous generator technology has resulted in distributed power sources inheriting some of the inherent issues of traditional synchronous generators, such as low-frequency oscillations in output power stemming from the mechanical equations of motion. When the power grid changes or experiences disturbances, both traditional synchronous generators and distributed power sources using virtual synchronous generator technology will experience large, low-frequency oscillations in output power. Because distributed power sources have weak overload capacity, these low-frequency oscillations can easily cause overloads.

[0005] Some literature proposes methods for suppressing power oscillations by carefully designing inherent control parameters such as the virtual inertia, virtual damping coefficient, and virtual output impedance of virtual synchronous generators. While this approach can suppress low-frequency oscillations in the output power of grid-connected systems with virtual synchronous generators to a certain extent, the design freedom is very limited: the adjustment of the virtual inertia is constrained by the requirement for providing virtual inertia; the adjustment of the virtual damping coefficient is constrained by the system's steady-state operating point; and the adjustment of the virtual output impedance is constrained by power quality and stability requirements. Other literature proposes methods for suppressing power oscillations in grid-connected systems with virtual synchronous generators by adaptively adjusting the virtual inertia and virtual damping coefficient. While this approach can also suppress low-frequency oscillations in the output power of grid-connected systems with virtual synchronous generators, the nonlinearities introduced by the adaptive controller hinder the overall system parameter design. Recent literature has also proposed using state feedback and pole placement to design a feedback compensator based on a state-space model of the system to suppress output power oscillations in grid-connected systems with virtual synchronous generators. This approach offers the advantage of greater design freedom compared to the aforementioned methods. However, the pole placement is highly dependent on the accuracy of the system model and parameters, resulting in limited robustness. Some literature has also proposed to connect a proportional-derivative compensator in series in virtual synchronous generator control to improve the system response. However, this method greatly reduces the system's ability to suppress noise. Summary of the Invention

[0006] The purpose of the present invention is to overcome the above-mentioned shortcomings and provide a method for suppressing low-frequency oscillations of the output power of a virtual synchronous generator grid-connected system, which can improve the design freedom and robustness of the controller, simplify the design process of the controller, and effectively suppress the low-frequency oscillation phenomenon of the output power of the virtual synchronous generator grid-connected system.

[0007] In order to achieve the above object, the present invention comprises the following steps:

[0008] Connect the distributed power source to the grid bus through the inverter;

[0009] The inverter adopts a virtual synchronous generator control method to collect capacitor voltage amplitude, output instantaneous active power and reactive power information, and calculate angular frequency deviation and voltage amplitude deviation;

[0010] The angular frequency deviation is used as the input signal of the frequency feedforward compensator;

[0011] The output signal of the frequency feedforward compensator is used as the compensation signal and is summed with the voltage amplitude deviation to obtain the total voltage amplitude deviation;

[0012] Sum the angular frequency deviation and the rated frequency to obtain the angular frequency reference value;

[0013] The total voltage amplitude deviation is summed with the rated voltage amplitude to obtain the voltage amplitude reference value;

[0014] A voltage reference value is generated according to an angular frequency reference value and a voltage amplitude reference value for inner loop control.

[0015] Control the inverter as a voltage source.

[0016] The capacitor voltage amplitude of the inverter V C , the instantaneous active power P and reactive power Q output are calculated by the following formula:

[0017]

[0018] P=v Ca i oa +v Cb i ob +V Cc i oc

[0019]

[0020] Among them, v Ca 、v Cb and v Cc are the measured values ​​of the three-phase capacitor voltages, i oa 、i ob and i oc are the measured values ​​of the three-phase output current, v Cα and v Cβ are the α-axis component and β-axis component of the capacitor voltage in the two-phase stationary coordinate system.

[0021] The calculation method of angular frequency deviation Δω and voltage amplitude deviation ΔE1 is as follows:

[0022]

[0023]

[0024] Among them, P0 and Q0 are the given values ​​of active power and reactive power respectively, ω0 and E0 are the rated angular frequency and rated voltage amplitude respectively, J is the virtual inertia, K is the integral coefficient, D p is the steady-state droop coefficient between active power and angular frequency, D q is the steady-state droop coefficient between reactive power and voltage amplitude, Q is reactive power, V C is the capacitor voltage amplitude.

[0025] The frequency feedforward compensator is composed of a first-order high-pass filter and a linear controller connected in series.

[0026] The transfer function of the linear controller G in the feedforward compensator is:

[0027]

[0028] Among them, K damp is the gain of the controller, and parameters T1, T2 and β are used to adjust the phase compensation amount.

[0029] The output compensation signal ΔE2 of the frequency feedforward compensator and the total voltage amplitude deviation ΔE are calculated by the following formula:

[0030] ΔE2=G HPF (s)G(s)Δω

[0031] ΔE=ΔE1+ΔE2

[0032] Among them, G HPF (s) is the transfer function of the first-order high-pass filter, ΔE1 is the voltage amplitude deviation, and ΔE2 is the output compensation signal of the frequency feedforward compensator.

[0033] The calculation method of the angular frequency reference value ω is as follows:

[0034] ω=ω0+Δω

[0035] Where ω0 is the rated angular frequency and Δω is the angular frequency deviation.

[0036] The calculation method of the voltage amplitude reference value E is as follows:

[0037] E=E0+ΔE

[0038] Where E0 is the rated voltage amplitude and ΔE is the total voltage amplitude deviation.

[0039] Compared with the existing technology, the present invention adds a feedforward compensation link from frequency deviation to voltage amplitude deviation to the traditional virtual synchronous generator control, increasing the system's damping ratio and thus suppressing low-frequency oscillations in the output power of the traditional virtual synchronous generator grid-connected system. At the same time, the feedforward compensator ensures that the system's steady-state operating point is not affected by the added control path. The method of the present invention has a simple structure, does not introduce nonlinear links, and does not rely on a precise system model. It is highly robust and suppresses low-frequency oscillations in the virtual synchronous generator's output power without changing the inherent control parameters of the virtual synchronous generator. This provides a high degree of freedom in controller design and provides excellent reference value for engineering applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 is a control block diagram of the present invention;

[0041] Figure 2Figure 2 shows the simulated waveforms of active power given value changes under traditional virtual synchronous generator control and the control method proposed in this invention; (a) frequency reference value waveform; (b) instantaneous active power waveform; (c) capacitor line voltage RMS waveform; (d) instantaneous reactive power waveform.

[0042] Figure 3 Experimental waveforms of the frequency reference value, output active power, and reactive power when the active power set value changes under two control methods; (a) the waveform under traditional virtual synchronous generator control; (b) the waveform under the control method proposed in this invention. DETAILED DESCRIPTION

[0043] The present invention will be further described below with reference to the accompanying drawings.

[0044] See also Figure 1 , the present invention comprises the following steps:

[0045] The first step is to connect the distributed power generation to the grid bus through the inverter;

[0046] In the second step, the inverter adopts the virtual synchronous generator control method to collect the capacitor voltage amplitude, the output instantaneous active power and reactive power information, calculate the angular frequency deviation and voltage amplitude deviation; and control the inverter into a voltage source. C , the instantaneous active power P and reactive power Q output are calculated by the following formula:

[0047]

[0048] P=v Ca i oa +v Cb i ob +V Cc i oc

[0049]

[0050] Among them, v Ca 、v Cb and v Cc are the measured values ​​of the three-phase capacitor voltages, i oa 、i ob and i oc are the measured values ​​of the three-phase output current, v Cα and v Cβ are the α-axis component and β-axis component of the capacitor voltage in the two-phase stationary coordinate system.

[0051] The calculation method of angular frequency deviation Δω and voltage amplitude deviation ΔE1 is as follows:

[0052]

[0053]

[0054] Among them, P0 and Q0 are the given values ​​of active power and reactive power respectively, ω0 and E0 are the rated angular frequency and rated voltage amplitude respectively, J is the virtual inertia, K is the integral coefficient, D p is the steady-state droop coefficient between active power and angular frequency, D q is the steady-state droop coefficient between reactive power and voltage amplitude, Q is reactive power, V C is the capacitor voltage amplitude.

[0055] In the third step, the angular frequency deviation is used as the input signal of the frequency feedforward compensator; the frequency feedforward compensator is composed of a first-order high-pass filter and a linear controller in series. The transfer function of the linear controller G in the feedforward compensator is:

[0056]

[0057] Among them, K damp is the gain of the controller, and parameters T1, T2 and β are used to adjust the phase compensation amount.

[0058] In the fourth step, the output signal of the frequency feedforward compensator is used as the compensation signal and summed with the voltage amplitude deviation to obtain the total voltage amplitude deviation. The output compensation signal ΔE2 of the frequency feedforward compensator and the total voltage amplitude deviation ΔE are calculated by the following formula:

[0059] ΔE2=G HPF (s)G(s)Δω

[0060] ΔE=ΔE1+ΔE2

[0061] Among them, G HPF (s) is the transfer function of the first-order high-pass filter, ΔE1 is the voltage amplitude deviation, and ΔE2 is the output compensation signal of the frequency feedforward compensator.

[0062] Step 5: Sum the angular frequency deviation and the rated frequency to obtain the angular frequency reference value. The calculation method of the angular frequency reference value ω is as follows:

[0063] ω=ω0+Δω

[0064] Where ω0 is the rated angular frequency and Δω is the angular frequency deviation.

[0065] Step 6: Sum the total voltage amplitude deviation and the rated voltage amplitude to obtain the voltage amplitude reference value. The calculation method of the voltage amplitude reference value E is as follows:

[0066] E=E0+ΔE

[0067] Where E0 is the rated voltage amplitude and ΔE is the total voltage amplitude deviation.

[0068] In the seventh step, a voltage reference value is generated according to the angular frequency reference value and the voltage amplitude reference value to perform inner loop control.

[0069] The simulation model of the present invention consists of a system consisting of a three-phase inverter connected to the grid bus through line impedance. In order to clearly compare with the traditional virtual synchronous generator control, a simulation model of the traditional virtual synchronous generator control is also established for simulation. The results are shown in Figure 3 A set of simulation waveforms is presented, comparing the power oscillation phenomenon under conventional virtual synchronous generator control and the control method proposed in this invention when the power setpoint changes. (a)-(d) are waveforms of the inverter frequency reference value, instantaneous active power, capacitor line voltage RMS value, and instantaneous reactive power under the two control methods, respectively. At 0.1 seconds, the active power reference value increases from 1 kW to 4 kW. As can be seen from the simulation waveforms, the control method proposed in this invention can effectively suppress low-frequency oscillations in the active and reactive power output of the virtual synchronous generator grid-connected system.

[0070] The experimental platform consists of a three-phase inverter (MWINV-9R144) connected to a three-phase grid simulator (Chroma 61860) via line impedance and solid-state relays. Figure 3 The experimental waveforms of two control methods when the active power set value changes are given, among which (a) is the inverter frequency reference value, output active power and reactive power waveforms under the control of the traditional virtual synchronous generator; (b) is the frequency reference value, inverter output active power and reactive power waveforms under the control method proposed by the present invention. It can be seen from the experimental waveforms that when the traditional virtual synchronous generator is controlled, when the active power set value increases by 2 kilowatts, the inverter frequency reference value, active power and reactive power all show large-scale low-frequency oscillations; while when the control method proposed by the present invention is used, when the active power set value increases by 2 kilowatts, the inverter frequency reference value, active power and reactive power respond more quickly, and the low-frequency oscillation phenomenon is effectively suppressed. The experiment proves that the method of the present invention can effectively suppress the low-frequency oscillation phenomenon of the output power of the virtual synchronous generator grid-connected system.

[0071] This paper presents a frequency feedforward-based method for suppressing low-frequency oscillations in the output power of a virtual synchronous generator grid-connected system. To verify the feasibility of the control method, the authors constructed a simulation model of a three-phase inverter connected to the grid via a line impedance in the simulation software Matlab / Simulink. They then constructed an experimental platform using an MWINV-9R144 inverter, line impedance, and a Chroma 61860 power grid simulator for hardware experiments. Both simulation and experimental results demonstrate that the method can effectively suppress low-frequency oscillations in the output power of a virtual synchronous generator grid-connected system. The method is accurate and reliable, providing valuable insights for engineering applications.

Claims

1. A method for suppressing low-frequency oscillation of output power of a virtual synchronous generator grid-connected system, characterized in that: The following steps are involved: Connect the distributed power source to the grid bus through the inverter; The inverter adopts the virtual synchronous generator control method to collect the capacitor voltage amplitude, output instantaneous active power and reactive power information, calculate the angular frequency deviation and voltage amplitude deviation; angular frequency deviation Δ ω and voltage amplitude deviation Δ E 1 is calculated as follows: in, P 0 and Q 0 are the given values ​​of active power and reactive power respectively, and E 0 are the rated angular frequency and rated voltage amplitude, J is the virtual inertia, K is the integration coefficient, D p is the steady-state droop coefficient between active power and angular frequency, D q is the steady-state droop coefficient between reactive power and voltage amplitude, is the reactive power, is the capacitor voltage amplitude; The angular frequency deviation is used as the input signal of the frequency feedforward compensator; The output signal of the frequency feedforward compensator is used as the compensation signal and is summed with the voltage amplitude deviation to obtain the total voltage amplitude deviation; Sum the angular frequency deviation and the rated frequency to obtain the angular frequency reference value; The total voltage amplitude deviation is summed with the rated voltage amplitude to obtain the voltage amplitude reference value; A voltage reference value is generated according to an angular frequency reference value and a voltage amplitude reference value for inner loop control.

2. A method for suppressing low-frequency oscillation of output power of a virtual synchronous generator grid-connected system according to claim 1, characterized in that: Control the inverter as a voltage source.

3. The method for suppressing low-frequency oscillation of output power of a virtual synchronous generator grid-connected system according to claim 1, characterized in that: Capacitor voltage amplitude of the inverter V C , output instantaneous active power P and reactive power Q , calculated by the following formula: in, v Ca 、 v Cb and v Cc are the measured values ​​of the three-phase capacitor voltages, i oa 、 i ob and i oc are the measured values ​​of the three-phase output current, v Cα and v Cβ They are the capacitor voltages in the two-phase stationary coordinate system. α Axis components and β Axis component.

4. The method for suppressing low-frequency oscillation of output power of a virtual synchronous generator grid-connected system according to claim 1, characterized in that: The frequency feedforward compensator is composed of a first-order high-pass filter and a linear controller connected in series.

5. The method for suppressing low-frequency oscillation of output power of a virtual synchronous generator grid-connected system according to claim 1, characterized in that: Linear Controller in Feedforward Compensator G The transfer function is: in, K damp is the controller gain, parameter T 1. T 2 and β Both are used to adjust the phase compensation amount.

6. The method for suppressing low-frequency oscillation of output power of a virtual synchronous generator grid-connected system according to claim 1, characterized in that: The output compensation signal Δ of the frequency feedforward compensator E 2 and the total voltage amplitude deviation Δ E , calculated by the following formula: in, is the transfer function of the first-order high-pass filter, is the voltage amplitude deviation, is the output compensation signal of the frequency feedforward compensator.

7. The method for suppressing low-frequency oscillation of output power of a virtual synchronous generator grid-connected system according to claim 1, characterized in that: Angular frequency reference value ω The calculation method is as follows: in, is the rated angular frequency, is the angular frequency deviation.

8. The method for suppressing low-frequency oscillation of output power of a virtual synchronous generator grid-connected system according to claim 1, characterized in that: Voltage amplitude reference value E The calculation method is as follows: in, is the rated voltage amplitude, is the total voltage amplitude deviation.

Images