Island detection method of grid-forming type virtual synchronous generator grid-connected system

Abstract

The invention discloses an island detection method for a grid-forming type virtual synchronous motor grid-connected system, and belongs to the technical field of new energy power generation. The method comprises the steps that current and voltage signals are sampled, and fundamental wave signals and harmonic wave signals are extracted and separated through a bisecond-order generalized integrator; synchronously controlling the fundamental wave signal and the harmonic wave signal; setting capacitor voltage outer loop control and current inner loop control; and an island detection function is realized through harmonic impedance. According to the invention, the local detection method is adopted to realize island detection of the network-forming inverter, the equipment investment cost is effectively reduced, and the independent control of the fundamental component and the harmonic component is realized, so that the impedance measurement precision is obviously improved, and the island detection precision is further effectively improved. The method is particularly suitable for a grid-forming type virtual synchronous motor grid-connected system, and has good applicability and engineering practice application value.

Description

Technical Field

[0001] The present invention belongs to the field of new energy power generation, and particularly relates to an islanding detection method for a grid-forming virtual synchronous generator grid-connected system. Background Art

[0002] Existing islanding detection methods mainly fall into three categories: communication-based islanding detection technology, active islanding detection methods, and passive islanding detection methods.

[0003] Li Wenlong et al. studied the change in the output frequency of the inverter before and after islanding in the literature "Li Wenlong, Zhang Xinhui, Wang Lei, et al. Passive Islanding Detection Method Based on Rate of Change of Frequency [J]. Smart Power, 2024, 52(03): 47-54.", and used the change in the slope of the frequency-weighted fitting curve as the detection criterion to propose an islanding detection method based on the rate of change of frequency; however, most current domestic and foreign studies are basically aimed at grid-following grid-connected systems, and many islanding detection methods and technologies have been formed. However, all existing detection technologies are not very suitable for the grid-forming mode. Therefore, there is an urgent need to conduct new research on islanding detection technology in the grid-forming mode.

[0004] Wei Youjin et al. proposed an active islanding detection method based on an improved P-f droop equation in the grid-forming mode in the literature "Wei Youjin, Jiang Wang, Huang Shunquan, et al. Active Islanding Detection Method for Grid-Forming Inverters Based on Improved P-f Droop Equation [J]. Electric Engineering, 2024(02): 9-12. DOI: 10.19768 / j.cnki.dgjs.2024.02.003." by combining the characteristics of droop control itself.

[0005] In summary, the existing methods still have the following disadvantages:

[0006] 1) The mainstream islanding detection methods generally only apply to grid-following inverters, and their detection principles are contrary to the operating characteristics of grid-forming inverters;

[0007] 2) Most of the existing grid-forming islanding detection methods are developed for droop control, and there is less research on islanding detection methods for grid-forming inverters with VSG control, which are widely used today;

[0008] 3) Most of the islanding detection methods based on impedance measurement use signal processing technology to directly analyze the output power, and the fundamental wave component and harmonic component are not independent of each other, which easily leads to the harmonic impedance being affected by the fundamental wave fluctuation and reduces the accuracy of islanding detection. Summary of the Invention

[0009] The technical problem to be solved by the present invention is that the existing island detection methods are mainly applicable to grid-connected inverters, and their detection principles are contrary to the operating characteristics of grid-forming inverters. At the same time, most of the island detection methods based on impedance measurement directly analyze the output power by using signal processing technology, and fail to effectively separate the fundamental component and the harmonic component, resulting in the harmonic impedance being affected by the fundamental wave fluctuation, thereby reducing the accuracy of island detection. For this reason, the present invention provides an island detection method for a grid-forming virtual synchronous generator grid-connected system, aiming to overcome the problem that the existing island detection method does not match the characteristics of the grid-forming microgrid. This method separates the fundamental component and the harmonic component by using a double second-order generalized integrator and controls them independently, so that the harmonic component is not interfered by the fundamental component, and the accuracy of impedance measurement is improved. In addition, current limiting control is adopted in the harmonic reactive power control part, reducing the impact on the power quality due to harmonic injection.

[0010] The technical solution of the present invention is as follows.

[0011] An island detection method for a grid-forming virtual synchronous generator grid-connected system, the topological structure involved in the island detection method includes a DC power supply, an inverter, a three-phase LC filter, a line impedance, a local load, a circuit breaker, a grid impedance, and a three-phase grid connected in series in sequence, and the other end of the three-phase grid is grounded;

[0012] Assume that the inverter and the three-phase grid are in the state of supplying power to the local load at the same time, and perform island state detection, including the following steps:

[0013] Step 1, sample the current on the inverter side and record it as the three-phase output current I a , I b , I c , sample the capacitor voltage in the three-phase LC filter and record it as the three-phase capacitor voltage V a , V b , V c , and obtain the αβ-axis components I α , I β of the output current and the αβ-axis components V α , V β of the capacitor voltage through Clarke transformation;

[0014] The αβ-axis components I α , I β of the output current and the αβ-axis components V α , V β of the capacitor voltage are extracted through a double second-order generalized integrator to obtain 4 positive-sequence αβ-axis components and 4 negative-sequence αβ-axis components. Among them, the 4 positive-sequence αβ-axis components include: the fundamental voltage positive-sequence αβ-axis components V fα1, V fβ1 , the harmonic voltage positive-sequence αβ-axis components V sα1,V sβ1 , the positive-sequence αβ-axis components I of the fundamental wave current fα1, I fβ1 , the positive-sequence αβ-axis components I of the harmonic current sα1, I sβ1 ;

[0015] Step 2, calculate the fundamental active power P f , fundamental reactive power Q f , harmonic active power P s and harmonic reactive power Q s ;

[0016] Apply virtual synchronous machine control to the fundamental active power P f and fundamental reactive power Q f to obtain the fundamental output voltage angular frequency ω and fundamental output voltage amplitude V m , and synthesize the two into the fundamental reference voltage V fref ; Apply virtual synchronous machine control with current limiting to the harmonic active power P s and harmonic reactive power Q s to obtain the harmonic output voltage angular frequency ω s and harmonic output voltage amplitude V ms , and synthesize the two into the harmonic reference voltage V sref ;

[0017] Step 3, superimpose the fundamental reference voltage V fref obtained in Step 2 and the harmonic reference voltage V sref to generate the reference voltage V ref , and form a double closed-loop control with the reference voltage V ref , the three-phase capacitor voltage V a ,V b ,V c and the three-phase output current I a ,I b ,I c to generate the PWM wave for modulation, control the turn-off of the switching tubes in the inverter, and make the inverter output three-phase electricity;

[0018] Step 4, after double closed-loop control, re-sample, perform Clarke transformation and extract using a double second-order generalized integrator according to the method in Step 1 to obtain the controlled three-phase output current I a * ,I b * ,I c * and the controlled three-phase capacitor voltage V a * ,V b* , V c * , the αβ-axis components I of the output current after control α * , I β * , the αβ-axis components V of the capacitor voltage after control α * , V β * and the 4 positive-sequence αβ-axis components after control, including the positive-sequence αβ-axis components V of the fundamental voltage after control fα1 * , V fβ1 * , the positive-sequence αβ-axis components V of the harmonic voltage after control sα1 * , V sβ1 * , the positive-sequence αβ-axis components I of the fundamental current after control fα1 * , I fβ1 * , the positive-sequence αβ-axis components I of the harmonic current after control sα1 * , I sβ1 * ;

[0019] The positive-sequence αβ-axis components I of the harmonic current after control sα1 * , I sβ1 * and the positive-sequence αβ-axis components V of the harmonic voltage after control sα1 * , V sβ1 * , after being filtered by a low-pass filter and then subjected to Park transformation, the dq-axis components I of the harmonic current after control are obtained sd * , I sq * and the dq-axis components V of the harmonic voltage after control sd * , V sq * ;

[0020] Step 5, from the dq-axis components I of the harmonic current after control sd * , I sq * and the dq-axis components V of the harmonic voltage after control sd * , V sq * calculate the absolute value of the harmonic current amplitude |I s|Absolute value of harmonic voltage amplitude|V s |, and their calculation formulas are respectively:

[0021]

[0022] Introduce the absolute value of harmonic impedance|Z s |, and its calculation formula is:

[0023]

[0024] Compare the obtained absolute value of harmonic impedance|Z s | with the pre-set threshold|Z s0 | through a delay period T. When|Z s | continuously exceeds|Z s0 | within the delay period T, it indicates that the system is in the island state at this time, and an island detection signal IDC is generated to stop the power supply on the inverter side.

[0025] Preferably, the active power P f , reactive power Q f , harmonic active power P s , harmonic reactive power Q s in step 2 are calculated by the following formulas respectively:

[0026]

[0027] Among them, ω cp is the cut-off frequency of the low-pass filter, and s is the Laplace operator;

[0028] The virtual synchronous motor control is adopted for the fundamental active power P f and fundamental reactive power Q f through the fundamental active power control loop and the fundamental reactive power control loop;

[0029] The fundamental active power control loop satisfies the following equation:

[0030]

[0031] The fundamental reactive power control loop satisfies the following equation:

[0032] V m -V 0 =D q (Q ref -Q f )

[0033] Among them, J is the virtual inertia of the virtual synchronous generator, D p is the active power droop coefficient, D q is the reactive power droop coefficient, Pref is the fundamental rated active power, Q ref is the fundamental rated reactive power, V 0 is the fundamental rated voltage amplitude, ω 0 is the fundamental rated voltage angular frequency;

[0034] For the harmonic active power P s and the harmonic reactive power Q s Adopt virtual synchronous motor control plus current limiting, and carry out through the harmonic active power control loop and the harmonic reactive power control loop;

[0035] The harmonic active power control loop satisfies the following equation:

[0036]

[0037] The harmonic reactive power control loop satisfies the following equation:

[0038]

[0039] Among them, Q sref is the harmonic rated reactive power, V 0s is the harmonic rated voltage amplitude, ω s0 is the fundamental rated voltage angular frequency, K p is the proportional coefficient of the PI controller of the reactive power loop, K i is the integral coefficient of the PI controller of the reactive power loop, I s is the harmonic current amplitude, I sref is the reference value of the harmonic current amplitude.

[0040] Preferably, the dq-axis components I sd * , I sq * of the controlled harmonic current and the dq-axis components V sd * , V sq * of the controlled harmonic voltage have the following transformation formulas respectively:

[0041]

[0042] Among them, θ s is the electrical angle at the harmonic frequency.

[0043] Preferably, the double second-order generalized integrator in step 1 includes a second-order generalized integrator for extracting the fundamental wave signal and a second-order generalized integrator for extracting the harmonic signal. Among them, the positive-sequence αβ-axis components I sα1, I sβ1Positive sequence αβ-axis component V of harmonic voltage sα1, V sβ1 ;

[0044] The linear model of the second-order generalized integrator for extracting harmonic signals is established as follows:

[0045]

[0046] Among them, G d (s) is the first transfer function in the second-order generalized integrator for extracting harmonic signals, and G q (s) is the second transfer function in the second-order generalized integrator for extracting harmonic signals, and their expressions are respectively:

[0047]

[0048] In the formula, k SOGIf is the first gain of the second-order generalized integrator for extracting harmonic signals, k SOGIs is the second gain of the second-order generalized integrator for extracting harmonic signals, ω f0 is the rated fundamental angular frequency, ω s0 is the rated harmonic angular frequency, and s is the Laplace operator.

[0049] Compared with the prior art, the beneficial effects of the present invention are:

[0050] 1. The local method is adopted for islanding detection of the grid-forming inverter, without the need for remote communication, saving costs;

[0051] 2. The fundamental component and the harmonic component are made independent of each other, improving the accuracy of impedance measurement, that is, improving the accuracy of islanding detection;

[0052] 3. It is applicable to the virtual synchronous generator grid-forming grid-connected system, having practical application significance. Brief description of the drawings

[0053] Figure 1 is the main circuit topology structure in the embodiment of the present invention.

[0054] Figure 2 is the control block diagram of the double second-order generalized integration part in the present invention.

[0055] Figure 3 is the control block diagram of the fundamental wave control part in the present invention.

[0056] Figure 4 is the control block diagram of the harmonic control part in the present invention.

[0057] Figure 5 is the flow diagram of the present invention.

[0058] Figure 6It is the simulation waveform diagram of the harmonic impedance before and after the occurrence of the island. Specific implementation mode

[0059] The present invention will be described in detail below in conjunction with the accompanying drawings and embodiments.

[0060] Taking a single-grid-connected inverter system in a simulation software Matlab / Simulink as an example, this embodiment clarifies an island detection method for a grid-forming virtual synchronous generator grid-connected system.

[0061] First, in the simulation software Matlab / Simulink, according to Figure 1 Build the modeling topological structure involved in the present invention. This topological structure includes a DC power supply, an inverter, a three-phase LC filter, a line impedance, a local load, a circuit breaker, a grid impedance, and a three-phase grid connected in series in sequence. The other end of the three-phase grid is grounded. In this topology, the inverter and the three-phase grid can supply power to the local load simultaneously or separately.

[0062] In Figure 1 , V dc is the DC power supply voltage, Z line is the line impedance, Z load is the local load, Z g is the grid impedance, L and C are the filtering inductor and filtering capacitor in the three-phase LC filter respectively, and QF is the circuit breaker.

[0063] In this embodiment, V dc = 1300V. The switching frequency of the grid-connected inverter is 5430π (rad / s), the fundamental frequency is 100π (rad / s), the fourth harmonic is adopted for the harmonic, that is, the harmonic frequency is 400π. Three AC voltage sources are used to form the grid, and the effective value of the grid phase voltage is 364 (V).

[0064] Figure 2 is the control block diagram of the double second-order generalized integral part in step 1 of the present invention, Figure 3 is the control block diagram of the fundamental wave control part in step 2 of the present invention, Figure 4 is the control block diagram of the harmonic control part in step 2 of the present invention, Figure 5 is the step diagram of the present invention. As Figures 2 - 5 can be seen, the present invention provides an island detection method for a grid-forming virtual synchronous generator grid-connected system. Assuming that the inverter and the three-phase grid are in the state of supplying power to the local load simultaneously, the island state detection is carried out, including the following steps:

[0065] Step 1, sample the current on the inverter side and record it as the three-phase output current I a , I b , I c, sample the capacitor voltages in the three-phase LC filter and denote them as the three-phase capacitor voltages V a , V b , V c , and obtain the αβ-axis components I α , I β of the output current and the αβ-axis components V α , V β .

[0066] Extract the αβ-axis components I α , I β of the output current and the αβ-axis components V α , V β of the capacitor voltage through a double second-order generalized integrator to obtain 4 positive-sequence αβ-axis components and 4 negative-sequence αβ-axis components. Among them, the 4 positive-sequence αβ-axis components include: the fundamental voltage positive-sequence αβ-axis component V fα1, V fβ1 , the harmonic voltage positive-sequence αβ-axis component V sα1, V sβ1 , the fundamental current positive-sequence αβ-axis component I fα1, I fβ1 , the harmonic current positive-sequence αβ-axis component I sα1, I sβ1 .

[0067] In Figure 2 , the positive and negative sequence separator is used to separate the positive-sequence αβ-axis components and the negative-sequence αβ-axis components.

[0068] Step 2, calculate the fundamental active power P f , the fundamental reactive power Q f , the harmonic active power P s and the harmonic reactive power Q s based on the separated 4 positive-sequence αβ-axis components.

[0069] Apply virtual synchronous motor control to the fundamental active power P f and the fundamental reactive power Q f to obtain the fundamental output voltage angular frequency ω and the fundamental output voltage amplitude V m , and synthesize the two into the fundamental reference voltage V fref ; apply virtual synchronous motor control with current limiting to the harmonic active power P s and the harmonic reactive power Q s to obtain the harmonic output voltage angular frequency ω s and the harmonic output voltage amplitude V ms , and synthesize the two into the harmonic reference voltage V sref .

[0070] In this embodiment, the active power Pf , Reactive power Q f , Harmonic active power P s , Harmonic reactive power Q s The calculation formulas are as follows:

[0071]

[0072] Among them, ω cp is the cut-off frequency of the low-pass filter, and s is the Laplace operator.

[0073] The control of the fundamental active power P f , fundamental reactive power Q f is carried out through the fundamental active power control loop and the fundamental reactive power control loop by adopting virtual synchronous motor control.

[0074] The fundamental active power control loop satisfies the following equation:

[0075]

[0076] The fundamental reactive power control loop satisfies the following equation:

[0077] V m -V 0 = D q (Q ref -Q f )

[0078] Among them, J is the virtual inertia of the virtual synchronous generator, D p is the active droop coefficient, D q is the reactive droop coefficient, P ref is the fundamental rated active power, Q ref is the fundamental rated reactive power, V 0 is the fundamental rated voltage amplitude, ω 0 is the fundamental rated voltage angular frequency.

[0079] The control of the harmonic active power P s , harmonic reactive power Q s is carried out through the harmonic active power control loop and the harmonic reactive power control loop by adopting virtual synchronous motor control plus current limiting.

[0080] The harmonic active power control loop satisfies the following equation:

[0081]

[0082] The harmonic reactive power control loop satisfies the following equation:

[0083]

[0084] Among them, Q sref is the harmonic rated reactive power, V 0s is the harmonic rated voltage amplitude, ω s0 is the fundamental rated voltage angular frequency, K p is the proportional coefficient of the reactive power loop PI controller, K i is the integral coefficient of the reactive power loop PI controller, I s is the harmonic current amplitude, I sref is the reference value of the harmonic current amplitude.

[0085] In this embodiment, P ref = 1575 kW, J = 0.08, D p = 478.7426, D q = 1.3064×10 -5 , K p = 0.8, K i = 50.

[0086] Step 3: Superimpose the fundamental reference voltage V fref obtained in Step 2 and the harmonic reference voltage V sref to generate the reference voltage V ref . Compare the reference voltage V ref with the three-phase capacitor voltages V a , V b , V c and the three-phase output currents I a , I b , I c to form a double closed-loop control to generate the PWM wave used for modulation, control the turn-off of the switching tubes in the inverter, and make the inverter output three-phase electricity.

[0087] Step 4: After double closed-loop control, re-sample, perform Clarke transformation and extract using a double second-order generalized integrator according to the method in Step 1 to obtain the controlled three-phase output currents I a * , I b * , I c * , the controlled three-phase capacitor voltages V a * , V b * , V c * , the αβ-axis components I α * , I β * of the controlled output current, and the αβ-axis components V α * , Vβ * and the four positive-sequence components of the αβ axes after control, including the positive-sequence αβ-axis components of the fundamental voltage after control, V fα1 * , V fβ1 * , the positive-sequence αβ-axis components of the harmonic voltage after control, V sα1 * , V sβ1 * , the positive-sequence αβ-axis components of the fundamental current after control, I fα1 * , I fβ1 * , the positive-sequence αβ-axis components of the harmonic current after control, I sα1 * , I sβ1 * .

[0088] The positive-sequence αβ-axis components of the harmonic current after control, I sα1 * , I sβ1 * and the positive-sequence αβ-axis components of the harmonic voltage after control, V sα1 * , V sβ1 * , after being filtered by a low-pass filter and then subjected to Park transformation, the dq-axis components of the harmonic current after control, I sd * , I sq * and the dq-axis components of the harmonic voltage after control, V sd * , V sq * .

[0089] In this embodiment, the transformation formulas of the dq-axis components of the harmonic current after control, I sd * , I sq * and the dq-axis components of the harmonic voltage after control, V sd * , V sq * are respectively:

[0090]

[0091] where θ s is the electrical angle at the harmonic frequency.

[0092] Step 5, from the dq-axis components of the harmonic current after control, I sd * , Isq * and the dq-axis components V of the controlled harmonic voltage sd * , V sq * The absolute value of the calculated harmonic current amplitude |I s | and the absolute value of the harmonic voltage amplitude |V s |, and their calculation formulas are respectively:

[0093]

[0094] The absolute value of the introduced harmonic impedance |Z s |, and its calculation formula is:

[0095]

[0096] Compare the obtained absolute value of the harmonic impedance |Z s | with the pre-set threshold |Z s0 | through a delay period T. When |Z s | is continuously greater than |Z s0 | within the delay period T, it indicates that the system is in the islanding state at this time, and an islanding detection signal IDC is generated to stop the power supply on the inverter side.

[0097] In this embodiment, the bi-second-order generalized integrator described in step 1 includes a second-order generalized integrator for extracting the fundamental wave signal and a second-order generalized integrator for extracting the harmonic signal. Among them, the positive-sequence αβ-axis components I of the harmonic current are obtained through the second-order generalized integrator for extracting the harmonic signal sα1, I sβ1 and the positive-sequence αβ-axis components V of the harmonic voltage sα1, V sβ1 .

[0098] Considering the influence of the second-order generalized integrator on the extraction effect of the harmonic signal on the impedance measurement accuracy, the second-order generalized integrator for extracting the harmonic signal is designed.

[0099] The linear model of the second-order generalized integrator for extracting the harmonic signal is established as follows:

[0100]

[0101] Among them, G d (s) is the first transfer function in the second-order generalized integrator for extracting the harmonic signal, and G q (s) is the second transfer function in the second-order generalized integrator for extracting the harmonic signal, and their expressions are respectively:

[0102]

[0103] In the formula, kSOGIf The gain one, k, of the second-order generalized integrator for extracting harmonic signals SOGIs The gain two, ω, of the second-order generalized integrator for extracting harmonic signals f0 The rated fundamental angular frequency, ω s0 The rated harmonic angular frequency, and s is the Laplace operator.

[0104] In this embodiment,

[0105] To prove the beneficial effects of the present invention, simulations were carried out. Figure 6 is the waveform diagram of the harmonic impedance before and after the occurrence of islanding. From Figure 6 it can be seen that the system switches from the grid-connected state to the islanding state at 10 seconds. After the state change, the output impedance |Z es | has an obvious change and |Z s | remains greater than the pre-set threshold |Z s0 | within the delay period T, thus successfully detecting the islanding state.

Claims

1. An island detection method for a grid-connected virtual synchronous generator system, the topology structure involved in the island detection method includes a DC power supply, an inverter, a three-phase LC filter, a line impedance, a local load, a circuit breaker, a grid impedance and a three-phase grid connected in series in sequence, and the other end of the three-phase grid is grounded; It is characterized in that Assuming that the inverter and the three-phase power grid are in a state of simultaneously supplying power to the local load, performing island state detection includes the following steps: Step 1: Sample the inverter side current and record it as the three-phase output current I a ,I b ,I c , sample the capacitor voltage in the three-phase LC filter and record it as the three-phase capacitor voltage V a ,V b ,V c And the output current αβ axis component I is obtained by Clarke transformation α ,I β and the capacitor voltage αβ axis component V α ,V β ; The output current αβ axis component I α ,I β and the capacitor voltage αβ axis component V α ,V β Four positive-sequence αβ-axis components and four negative-sequence αβ-axis components are extracted by a double second-order generalized integrator, wherein the four positive-sequence αβ-axis components include: fundamental voltage positive-sequence αβ-axis component V fα1, V fβ1 , harmonic voltage positive sequence αβ axis component V sα1, V sβ1 , fundamental current positive sequence αβ axis component I fα1, I fβ1 , harmonic current positive sequence αβ axis component I sα1, I sβ1 ; Step 2: Calculate the fundamental wave active power P according to the separated four positive sequence αβ axis components f , fundamental wave reactive power Q f , Harmonic active power P s and harmonic reactive power Q s ; Fundamental active power P f , fundamental wave reactive power Q f Take virtual synchronous motor control to obtain the fundamental output voltage angular frequency ω and fundamental output voltage amplitude V m and synthesize the two into the fundamental reference voltage V fref ; For harmonic active power P s , harmonic reactive power Q s Take virtual synchronous motor control plus current limiting to obtain the harmonic output voltage angular frequency ω s and harmonic output voltage amplitude V ms and synthesize the two into a harmonic reference voltage V sref ; Step 3: convert the fundamental reference voltage V obtained in step 2 fref With the harmonic reference voltage V sref The reference voltage V is generated by superposition ref , the reference voltage V ref The three-phase capacitor voltage V obtained in step 1 a ,V b ,V c , three-phase output current I a ,I b ,I c A double closed-loop control is formed to generate the PWM wave used for modulation, control the switch-off of the switch tube in the inverter, and make the inverter output three-phase electricity; Step 4: After double closed-loop control, re-sample, Clarke transform and double second-order generalized integrator extraction are performed according to the method in step 1 to obtain the controlled three-phase output current I a * ,I b * ,I c * , the three-phase capacitor voltage after control V a * ,V b * ,V c * 、The output current αβ axis component I after control α * ,I β * 、The αβ-axis component of the capacitor voltage after control V α * ,V β * and the four αβ axis positive sequence components after control, including the fundamental voltage positive sequence αβ axis component V fα1 * ,V fβ1 * , the harmonic voltage positive sequence αβ axis component V after control sα1 * ,V sβ1 * , the positive sequence αβ axis component of the fundamental current after control I fα1 * ,I fβ1 * , the positive sequence αβ axis component of the harmonic current after control I sα1 * ,I sβ1 * ; The positive sequence αβ axis component of the harmonic current after control I sα1 * ,I sβ1 * And the positive sequence αβ axis component V of the harmonic voltage after control sα1 * ,V sβ1 * , filtered by a low-pass filter and then subjected to Park transformation, the controlled harmonic current dq axis component I sd * ,I sq * And the controlled harmonic voltage dq axis component V sd * ,V sq * ; Step 5: The dq-axis component I of the harmonic current after control sd * ,I sq * And the controlled harmonic voltage dq axis component V sd * ,V sq * The absolute value of the harmonic current amplitude is calculated as |I s |Absolute value of harmonic voltage amplitude|V s |, and their calculation formulas are: Introducing the absolute value of harmonic impedance |Z s |, the calculation formula is: The absolute value of the harmonic impedance |Z s |With the threshold set in advance|Z s0 |After a delay period T, the comparison is performed. When |Z s |Continuously greater than |Z s0 |, it indicates that the system is in an island state, and an island detection signal IDC is generated to stop power supply to the inverter side.

2. The method for detecting an island in a grid-connected virtual synchronous generator system according to claim 1, characterized in that: Step 2: Active power P f , reactive power Q f , Harmonic active power P s , harmonic reactive power Q s The calculation formulas are: Among them, ω cp is the cutoff frequency of the low-pass filter, s is the Laplace operator; The fundamental wave active power P f , fundamental wave reactive power Q f Virtual synchronous motor control is adopted through fundamental wave active power control loop and fundamental wave reactive power control loop; The fundamental active power control loop satisfies the following equation: The fundamental reactive power control loop satisfies the following equation: V m -V0=D q (Q ref -Q f ) Where J is the virtual inertia of the virtual synchronous generator, D p is the active power droop coefficient, D q is the reactive power droop coefficient, P ref is the fundamental rated active power, Q ref is the fundamental wave rated reactive power, V0 is the fundamental wave rated voltage amplitude, ω0 is the fundamental wave rated voltage angular frequency; The harmonic active power P s , harmonic reactive power Q s Adopt virtual synchronous motor control plus current limiting, through the harmonic active power control loop and harmonic reactive power control loop; The harmonic active power control loop satisfies the following equation: The harmonic reactive power control loop satisfies the following equation: Among them, Q sref is the harmonic rated reactive power, V 0s is the harmonic rated voltage amplitude, ω s0 is the fundamental rated voltage angular frequency, K p is the proportional coefficient of the reactive loop PI controller, K i is the integral coefficient of the reactive loop PI controller, I s is the harmonic current amplitude, I sref It is the reference value of harmonic current amplitude.

3. The method for detecting an island in a grid-connected virtual synchronous generator system according to claim 1, characterized in that: The harmonic current dq axis component I after control in step 4 sd * ,I sq * And the controlled harmonic voltage dq axis component V sd * ,V sq * The transformation formulas are: Among them, θ s is the electrical angle at the harmonic frequency.

4. The method for detecting an island in a grid-connected virtual synchronous generator system according to claim 1, characterized in that: Step 1: The dual second-order generalized integrator includes a second-order generalized integrator for extracting fundamental signals and a second-order generalized integrator for extracting harmonic signals, wherein the harmonic current positive-sequence αβ-axis component I is obtained by extracting the second-order generalized integrator for the harmonic signal. sα1, I sβ1 Positive sequence αβ axis component of harmonic voltage V sα1, V sβ1 ; The second-order generalized integrator linear model for extracting harmonic signals is established as follows: Among them, G d (s) is the transfer function of the second-order generalized integrator for extracting harmonic signals, G q (s) is the transfer function 2 in the second-order generalized integrator for extracting harmonic signals, and its expressions are: In the formula, k SOGIf is the gain of the second-order generalized integrator for extracting harmonic signals, k SOGIs is the gain of the second-order generalized integrator for extracting the harmonic signal, ω f0 is the rated fundamental angular frequency, ω s0 is the rated harmonic angular frequency, and s is the Laplace operator.

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