Power quality optimization control system and control method of grid-connected inverter

Abstract

The application discloses a grid-connected inverter output power quality optimization control system and a control method. The steps are as follows: in the grid-connected inverter, a model predictive control is used to replace a PWM modulation module, a sine function orthogonal characteristic is used to extract a harmonic component of a point of common coupling (PCC) voltage and a grid current, a harmonic voltage at the grid is calculated by prediction, the calculated harmonic voltage is multiplied by a coefficient to be added to a reference voltage, and the model predictive control is used for modulation. The harmonic voltage amplitude on the grid impedance is controlled by controlling the coefficient of the harmonic voltage, and flexible suppression of the grid current harmonic and the PCC voltage harmonic is realized. The method has a simple structure and is easy to realize. The grid-connected inverter based on the method can realize higher quality power output, and is suitable for various grid-connected inverters.

Description

Technical Field

[0001] This invention relates to grid-connected inverter systems, and more particularly to a power quality optimization control system and method for grid-connected inverter outputs. Background Technology

[0002] New energy distributed grid-connected generation systems include local loads, energy storage devices, and distributed generation equipment (grid-mounted inverters). They supply power to local loads and also need to be connected to the grid. Therefore, improving the output voltage quality and grid-connected current quality of grid-mounted inverters in new energy distributed grid-connected generation systems has received widespread attention and discussion.

[0003] First, distributed grid-connected power generation systems for new energy sources need to supply power to local loads. These local loads include some nonlinear loads, which will generate harmonic currents. These harmonic currents increase the total harmonic distortion (THD) of the PCC voltage and the THD of the grid-connected current. Second, distributed grid-connected power generation systems for new energy sources are often located in remote areas, where background harmonics may exist in the grid, also leading to increased THD of the PCC voltage and the THD of the grid-connected current. How to suppress harmonics in the output voltage and grid-connected current of grid-connected inverters is a key problem that is currently being solved or will be solved in the future.

[0004] Existing literature has studied methods for optimizing the output power quality control of grid-connected inverters. Common methods include using a harmonic suppression scheme with grid current feedforward to reshape the converter's output impedance, thereby improving the converter's output current quality; using a voltage feedforward scheme to reshape the converter's output impedance and suppress PCC voltage harmonics or grid current harmonics; and some researchers have used a combination of voltage and current feedforward to achieve flexible suppression of PCC voltage harmonics or grid current harmonics. Although existing power quality optimization schemes for grid-connected inverters can achieve a certain degree of harmonic suppression, significant digital delays cause phase errors in harmonic suppression methods using current and voltage feedforward, and traditional schemes have slow dynamic responses and poor harmonic suppression effects.

[0005] Therefore, for grid-connected inverters, it is necessary to study a harmonic suppression scheme with faster control speed and more accurate harmonic extraction and compensation, so as to improve the output power quality of the inverter and achieve high-quality grid connection of the inverter. Summary of the Invention

[0006] The purpose of this invention is to provide a method for optimizing the output power quality of a grid-connected inverter. This method can achieve accurate harmonic extraction and suppression through model predictive control, reduce harmonic phase errors, and thus improve the output power quality of the inverter.

[0007] To achieve the above objectives, the solution of the present invention is:

[0008] A grid-connected inverter output power quality optimization control system is disclosed. The control system includes a grid-connected inverter, which comprises a bridge inverter topology and an inverter-side inductor L. f Filter capacitor C f After being connected to the local load, it is connected to the power grid.

[0009] Based on the above control system, this application also provides a control method thereof, the control method comprising the following steps:

[0010] 1) Sample the inverter output voltage sampling signal u oabc , Current sampling signal i gabc The model prediction calculation yields a harmonic signal u that approximates the grid voltage. rehabc It outputs the optimal switching vector obtained from the previous calculation and predicts the inverter inductor current, output voltage, and output current at the next moment based on the sampled signal and switching state.

[0011] 2) The harmonic signal u calculated in step 1) rehabc Multiply the result by the coefficient x, add the result to the reference voltage calculated by the grid-type inverter control to obtain a new reference voltage, and use model predictive control to track the new reference voltage; use the inverter inductor current, output voltage and output current at the next moment predicted to perform model predictive control calculation, select the switching vector that makes the output voltage closest to the reference voltage, and save it to the next sampling calculation for output;

[0012] By adjusting the voltage difference across the grid impedance, flexible suppression of grid current harmonics and PCC voltage harmonics can be achieved.

[0013] Furthermore, the specific implementation process of step 1) is as follows:

[0014] 1.1) Utilizing the orthogonalization property of the sine function to sample the inverter output voltage signal u within a cycle. oabc The inverter output voltage harmonic signal u is obtained through processing and calculation. ohabc ;

[0015] 1.2) Utilizing the orthogonalization property of the sine function to sample the grid current i within a period. gabc The process is performed to calculate the harmonic signal i of the grid-connected current. ghabc ;

[0016] 1.3) The result obtained in step 1.2) i ghabc Multiply by the virtual impedance L v Obtain the virtual inductor voltage harmonic signal u vhabc ;

[0017] 1.4) The inverter output voltage harmonic signal u mentioned in step 1.1)ohabc Subtract the result u obtained from step 1.3) vhabc The predicted voltage harmonic signal u on the right side of the virtual inductor is obtained. rehabc .

[0018] Furthermore, steps 1.1) to 1.2) specifically refer to:

[0019] S11 calculates the inverter's A-phase output voltage u based on the orthogonality of trigonometric functions. oa The amplitudes of the sine and cosine components of each harmonic component are calculated by integrating the voltage signals at times k+1 and k+2, yielding the following results:

[0020]

[0021] S12 further obtains W in the discrete calculation of model predictive control. s1 The expression is:

[0022]

[0023] Where u oa (j) is the instantaneous value of the inverter's A-phase output voltage at time j, and sin(θ(j)) is the instantaneous phase angle of the output voltage at time j;

[0024] S13 Calculate the amplitude W of the sinusoidal component of the nth harmonic component of the inverter's A-phase output voltage. sn And the amplitude W of the cosine component of the nth harmonic component of the A-phase output voltage. cn :

[0025]

[0026] S14 inverter output voltage sampling signal u oabc , Current sampling signal i gabc Substituting into equations (8) and (9), the inverter output voltage harmonic signal u can be calculated. ohabc and the grid-connected current harmonic signal i ghabc .

[0027] Furthermore, step 1.3) is as follows:

[0028] The harmonic voltage across the virtual inductor is obtained using the relationship between inductor current and voltage.

[0029]

[0030] Furthermore, step 1.4) is as follows:

[0031] The harmonic components of the voltage at the right node of the virtual inductor are obtained according to the KVL equation:

[0032] urehabc =u ohabc -u vhabc (11)

[0033] The grid-connected converter system using the present invention has the following characteristics:

[0034] 1) By using model predictive control instead of PWM modulation module, the control system has a faster response speed and greater anti-interference ability;

[0035] 2) The harmonic extraction scheme used has a smaller phase error;

[0036] 3) The compensation control structure is simple and easy to implement;

[0037] 4) By adjusting the coefficient x, the PCC voltage harmonics and grid current harmonics can be flexibly suppressed. Attached Figure Description

[0038] Figure 1 This is a diagram of the main circuit structure of the grid-connected inverter used in the power quality optimization control method for grid-connected inverters in this application, wherein: u gabc The mains voltage; u PCCabc The voltage of the grid at the point of common coupling; i gabc For grid connection current; u oabc i is the inverter output voltage; oabc i is the inverter output current; Labc This refers to the inverter filter inductor current. An LC-filtered grid-connected inverter consists of a bridge inverter topology and an inverter-side inductor L. f Filter capacitor C f and passive damping R d composition.

[0039] Figure 2 This is a control block diagram of the delay compensation scheme adopted in this application, which includes the first step of sampling and updating the drive signal, using the sampled and switching signals to predict the voltage and current of the k+1 switching cycle, and finally selecting the optimal switching state so that the output voltage at time k+2 is consistent with the reference voltage.

[0040] Figure 3 For a grid-connected inverter with a reference output power of 10kW, a grid impedance of 5mH, a grid with 3% fifth-order negative harmonic voltage and 3% seventh-order positive harmonic voltage, and a nonlinear load power of 1.2kW, the PCC voltage u of the grid-connected inverter without harmonic suppression control is... PCC and grid current i g Steady-state test waveform;

[0041] Figure 4For a grid-connected inverter with a reference output power of 10kW, a grid impedance of 5mH, a grid with 3% fifth-order negative harmonic voltage and 3% seventh-order positive harmonic voltage, and a nonlinear load power of 1.2kW, what is the PCC voltage u of the grid-connected inverter when harmonic suppression control is applied and x is 0.3? PCC and grid current i g Steady-state test waveform;

[0042] Figure 5 For a grid-connected inverter with a reference output power of 10kW, a grid impedance of 5mH, a grid with 3% fifth-order negative harmonic voltage and 3% seventh-order positive harmonic voltage, and a nonlinear load power of 1.2kW, what is the PCC voltage u of the grid-connected inverter when harmonic suppression control is applied and x is set to 0.5? PCC and grid current i g Steady-state test waveform;

[0043] Figure 6 For a grid-connected inverter with a reference output power of 10kW, a grid impedance of 5mH, a grid with 3% fifth-order negative harmonic voltage and 3% seventh-order positive harmonic voltage, and a nonlinear load power of 1.2kW, what is the PCC voltage u of the grid-connected inverter when harmonic suppression control is applied and x is 0.7? PCC and grid current i g Steady-state test waveform;

[0044] Figure 7 For a grid-connected inverter with a reference output power of 10kW, a grid impedance of 5mH, a grid with 3% fifth-order negative harmonic voltage and 3% seventh-order positive harmonic voltage, and a nonlinear load power of 1.2kW, what is the PCC voltage u of the grid-connected inverter when harmonic suppression control is applied and x is set to 0.95? PCC and grid current i g Steady-state test waveform. Detailed Implementation

[0045] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings.

[0046] Figure 1 This diagram shows the main circuit structure of the grid-connected inverter used in the power quality optimization control method for grid-connected inverters in this application. This application uses model predictive control instead of the PWM modulation module in the grid-connected inverter to achieve a higher response speed. The inverter output voltage sampling signal u is obtained through sampling. oabc , Current sampling signal i gabc The voltage harmonic signal u is obtained by model prediction calculation. refhabcThe predicted voltage harmonic signal is multiplied by a coefficient x and added to the reference voltage of the grid-connected inverter to obtain a new reference voltage. Model predictive control calculations are performed on the reference voltage to generate the bridge arm drive signal, update the inverter output voltage, reduce the voltage difference across the grid impedance, and control the grid impedance harmonic voltage by changing the value of the coefficient x, thereby achieving flexible suppression of PCC voltage quality and grid current quality.

[0047] Example 1:

[0048] This embodiment is a power quality optimization and control system for a grid-connected inverter. The control system includes a grid-connected inverter, which comprises a bridge inverter topology and an inverter-side inductor L. f Filter capacitor C f After being connected to the local load, it is connected to the power grid. For example... Figure 1 As shown, u gabc The mains voltage; u PCCabc The voltage of the grid at the point of common coupling; i gabc For grid connection current; u oabc i is the inverter output voltage; oabc i is the inverter output current; Labc This refers to the inverter filter inductor current. An LC-filtered grid-connected inverter consists of a bridge inverter topology and an inverter-side inductor L. f Filter capacitor C f and passive damping R d Composition. The inverter power loop control is a grid-type virtual synchronous machine control, which combines model predictive control to calculate the reference harmonic voltage and obtain the drive signals for the inverter switching transistors.

[0049] Example 2:

[0050] The power quality optimization and control method for grid-connected inverters in this application includes the following steps:

[0051] 1) Model predictive control is used instead of the traditional PWM modulation module. The specific implementation method is as follows: Equation (1) is the state equation expression of the inverter. A zero-order hold is used to transform the state equation of the circuit into a discrete domain equation (2).

[0052]

[0053] Where T s For sampling time i Labc (k+1) represents the predicted inductor current, u oabc (k+1) represents the predicted inverter output voltage, i Labc (k) represents the sampling inductor current, u oabc (k) represents the inverter output voltage, i oabc (k) represents the output current sampling, uiabc (k) is the output voltage vector of the DC-AC converter.

[0054] Two-level inverters have eight switching vector states. Substituting the expression for each switching vector state into (2) yields the output voltage for that switching vector state. The goal of model predictive control in this application is to achieve stable control of the output voltage of the two-level inverter. Therefore, it is necessary to select the switching vector that makes the output voltage closest to the reference voltage. The square of the difference between the reference voltage and the candidate output voltage represents the straight-line distance between the reference voltage and the candidate output voltage. Let the value function be the square of the difference between the reference voltage and the candidate output voltage. The value functions corresponding to the eight switching vectors are calculated. The candidate voltage that has the smallest value function is closest to the reference voltage. The switching vector corresponding to the candidate voltage should be output. The value function is expressed as:

[0055] J = (u aref -u a (k+1)) 2 +(u bref -u b (k+1)) 2 +(u cref -u c (k+1)) 2 (3)

[0056] Where u aref ,u bref ,u cref u is the capacitor reference voltage. a (k+1),u b (k+1),u c (k+1) represents the predicted inverter output voltage.

[0057] Ideally, the predictive control calculation at time k can be completed in a very short time, i.e., sampling and switching device triggering occur simultaneously. However, in reality, there are influencing factors such as measurement delay, calculation delay, and switching action delay. Therefore, the control process has a certain delay, affecting the actual control effect. This paper employs a two-step prediction method to compensate for the delay. The system outputs the optimal switching vector calculated at the previous time k, then samples and predicts the voltage and current at time k+1. Finally, it predicts the optimal switching vector solution calculated at time k+2 and retains it for output during the next sampling. The control block diagram is shown below. Figure 2 As shown.

[0058] 2) The inverter output voltage sampling signal u is recorded within one fundamental frequency cycle using a sliding window recording method. oabc and the sampling signal of the grid current i gabc And the corresponding harmonic components are calculated using the orthogonality of the sine function.

[0059] According to the Fourier series theorem, both the inverter output voltage and current can be Fourier expanded into a superposition of countless sine waves. Taking the inverter A-phase output voltage u... oa For example, by performing Fourier decomposition on the sampled signal, we can obtain:

[0060]

[0061] Among them, W s1 W represents the amplitude of the sinusoidal component of the fundamental component of the A-phase output voltage. c1 W represents the amplitude of the cosine component of the fundamental component of the A-phase output voltage. sn W represents the amplitude of the sinusoidal component of the nth harmonic component of the A-phase output voltage. cn The amplitude of the cosine component of the nth harmonic component of the output voltage of phase A is given.

[0062] Therefore, the output voltage u of phase A of the inverter can be calculated based on the orthogonality of trigonometric functions. oa The amplitudes of the sine and cosine components of each harmonic component. (U...) oa The amplitude W of the sinusoidal component of the fundamental wave s1 For example

[0063]

[0064] In digital control, the sampling and calculation of the inverter are both discrete, so equation (5) needs to be transformed into a discrete integral form to be applied in digital control. The model predictive control delay compensation scheme adopted in this paper is a two-step predictive control, that is, sampling and outputting the switching vector at time k, using the current sampled signal and the switching vector to predict the voltage and current signals of the inverter at time k+1, and then bagging the eight possible switching vectors into the value function, rolling optimization to select the best switching vector at time k+2 that is closest to the reference voltage, and outputting it at the beginning of the next sampling period. In order to reduce the influence of phase error, the voltage signals at times k+1 and k+2 are included in the integral for calculation, and combined with the above equation, we get:

[0065]

[0066] Further, we can obtain W in the discrete computation of model predictive control. s1 The expression is,

[0067]

[0068] Where u oa (j) is the instantaneous value of the output voltage of phase A at time j, and sin(θ(j)) is the instantaneous phase angle of the output voltage at time j.

[0069] Similarly, the amplitude W of the sinusoidal component of the nth harmonic component of the A-phase output voltage in the discrete calculation of model predictive control is given below. sn And the amplitude W of the cosine component of the nth harmonic component of the A-phase output voltage. cn .

[0070]

[0071] The sampled signal u oabc and i gabc Substituting into equations (8) and (9), the inverter output voltage harmonic signal u can be calculated. ohabc and the grid-connected current harmonic signal i ghabc .

[0072] 3) The harmonic voltage across the virtual inductor can be obtained using the relationship between inductor current and voltage.

[0073]

[0074] Furthermore, the harmonic components of the voltage at the node on the right side of the virtual inductor are obtained based on the KVL equations.

[0075] u rehabc =u ohabc -u vhabc (11)

[0076] The voltage harmonic u on the right side of the virtual inductor rehabc The harmonic components are added as feedforward compensation to the reference voltage of the grid-connected inverter. This is achieved by controlling the PCC voltage harmonic components and u... rehabc Equal values ​​can reduce the harmonic voltage difference of the power grid line impedance, achieve cancellation compensation, and thus reduce the harmonic content of the grid current.

[0077] 4) The calculated harmonic reference voltage u rehabc After multiplying by a coefficient x, the coefficient x is added to the reference voltage of the grid-connected converter. Adjusting the coefficient x can achieve flexible suppression of PCC voltage harmonics and grid-connected current harmonics. Figures 3 to 7 For a grid-connected inverter with a reference output power of 10kW, a grid impedance of 5mH, a grid with 3% fifth-order negative harmonic voltage and 3% seventh-order positive harmonic voltage, and a nonlinear load power of 1.2kW, the PCC voltage u of the grid-connected inverter with harmonic suppression control is calculated. PCC and grid current i g In the steady-state test waveform, as the coefficient x increases, the harmonic content of the grid-connected current decreases, while the harmonic content of the PCC voltage increases. By adjusting the coefficient x, flexible suppression of the PCC voltage harmonics and the grid-connected current harmonics can be achieved.

[0078] The above embodiments are merely illustrative of the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solutions based on the technical concept proposed in this invention shall fall within the scope of protection of this invention.

Claims

1. A control method of a grid-forming inverter output power quality optimization control system, characterized in that, The control system comprises a grid-forming inverter, which includes a bridge inverter topology, an inverter-side inductance L f , a filter capacitance C f , and is connected with the power grid after accessing a local load. The control method comprises the following steps: 1) sampling inverter output voltage sampling signal u oabc , network current current sampling signal i gabc , combined with model prediction calculation close to grid voltage harmonic signal u rehabc ; and output the last calculated optimal switching vector, according to the sampling signal and switching state prediction next time inverter inductance current, output voltage and output current; 2) multiplying the harmonic signal u rehabc The new reference voltage is obtained by multiplying the coefficient x and adding the result to the reference voltage calculated by the grid-connected inverter control. The model predictive control is used to track the new reference voltage. The model predictive control is calculated by using the predicted inductor current, output voltage and output current of the inverter at the next moment, and the switching vector that makes the output voltage closest to the reference voltage is selected and saved for output at the next sampling calculation. The reference voltage of the grid-forming inverter control calculation is calculated by the following process: sampling the inverter output voltage signal u oabc , sampling the inverter output current signal i oabc , calculating the actual active power P e and the actual reactive power Q e of the current system, sending the actual active power P e and the actual reactive power Q e into the grid-forming inverter control VSG module to obtain the reference voltage Uvsg-ref of the grid-forming inverter control calculation; By adjusting the voltage difference on the grid impedance, the flexible suppression of the grid current harmonic and the PCC voltage harmonic is realized; The implementation process of step 1) is specifically as follows: 1.1) Utilizing the orthogonalization property of the sine function to the sampled inverter output voltage signal u oabc within a period is processed to calculate the inverter output voltage harmonic signal u ohabc ; 1.2) Utilizing the orthogonalization property of the sine function to the in-network current current sampling signal i gabc is processed to calculate the in-network current harmonic signal i ghabc ; 1.3) Result of step 1.2) i ghabc Multiply by virtual impedance L v Get virtual inductance voltage harmonic signal u vhabc ; 1.4) The inverter output voltage harmonic signal u ohabc Subtracting the result of step 1.3) u vhabc The predicted virtual inductance right side voltage harmonic signal u rehabc is obtained.

2. The control method according to claim 1, characterized by, The step 1.1) to step 1.2) are specifically as follows: S11 the inverter A phase output voltage u is calculated according to the orthogonal property of trigonometric function oa The sine component amplitude and the cosine component amplitude of each harmonic component of the voltage signal are calculated, the voltage signals at k+1 and k+2 are taken into the integral to calculate, and the following is obtained: S12 further obtains the expression of W in the discrete calculation of model predictive control s1 is expressed as: where u oa (j) is the instantaneous value of the inverter A phase output voltage at time j, and sin(θ(j)) is the instantaneous phase angle of the output voltage at time j. S13 calculates the sine component amplitude W of the n-th harmonic component of the inverter A-phase output voltage sn and the cosine component amplitude W of the n-th harmonic component of the A-phase output voltage cn : S14 inverter output voltage sampling signal u oabc , network current current sampling signal i gabc Substitute the sine component amplitude W sn And the cosine component amplitude W cn The inverter output voltage harmonic signal u ohabc And the network current harmonic signal i ghabc Is calculated.

3. The control method according to claim 1, characterized by, The step 1.3) is: The harmonic voltage across the virtual inductor is obtained by using the relationship between the inductor current and the voltage, 4. The control method according to claim 1, characterized by, The step 1.4) is: According to the KVL equation, the harmonic component of the voltage at the right node of the virtual inductor is obtained: u rehabc = u ohabc - u vhabc (11).

Images