Resonance suppression method of centralized new energy grid-connected system

Abstract

The invention discloses a resonance suppression method of a centralized new energy grid-connected system, solves the problem of resonance instability of the centralized new energy grid-connected system, and belongs to the technical field of new energy power generation and power grids. According to the method, the power grid impedance and the total output impedance of the new energy grid-connected converter are identified on line, the system resonant frequency and the grid-connected point voltage resonant component are extracted through the impedance identification result, the virtual impedance needing to be compensated is dynamically adjusted based on the adaptive virtual impedance algorithm, and the calculation result is sent to a control loop of the new energy grid-connected converter. A resonance suppression method based on virtual impedance compensation is utilized to remodel output impedance of a grid-connected converter at a resonance frequency band so as to suppress a resonance phenomenon, normal power output of a new energy grid-connected system is ensured, meanwhile, a wide-frequency-domain resonance suppression function is achieved, and an economical and feasible control scheme is provided for safe and stable grid connection of centralized new energy.

Description

Technical Field

[0001] The invention relates to a resonance suppression method for a centralized new energy grid-connected system, belonging to the technical field of new energy power generation and power grid. Background Art

[0002] With the increasing penetration rate of new energy power generation systems connected to the grid, the power grid is developing towards a new power system characterized by a high proportion of new energy and a high degree of power electronics. However, most of the new energy power generation systems are located in remote terminal grid areas. The multi-stage transformer links and long-distance transmission lines cause the system to present a weak grid characteristic of "low short-circuit ratio" and high impedance. As the main interface between new energy power generation devices and the grid, the grid-connected converter plays an important role in the application of new energy grid connection. New energy such as photovoltaics and wind power are mainly connected to the grid in the form of current-controlled converters. Unlike traditional industrial frequency power generation equipment, the output impedance of the grid-connected converter in different frequency bands varies greatly. Since large-scale new energy power generation systems mainly adopt centralized grid connection, and the grid impedance under weak grid cannot be ignored, under this grid connection condition, the coupling interaction between the output impedance of multiple grid-connected converters and the grid impedance can easily cause wide-band resonance problems, which seriously affects the safe and stable operation of the new energy grid-connected system.

[0003] In order to suppress the resonance phenomenon caused by the coupling between the grid impedance and the output impedance of the grid-connected converter and improve the stability of the new power system, most of the existing studies use the grid-connected converter to perform virtual impedance compensation to achieve impedance reshaping to weaken the system impedance coupling. This method is an advanced control method that can change the frequency characteristics of the output impedance of the grid-connected converter without adding actual impedance by changing the current injected into the grid by the grid-connected converter. The existing virtual impedance compensation methods are mainly divided into two categories: fixed impedance compensation and adaptive impedance compensation. The stable operation ability of the new power system is often affected by the system impedance coupling. Under the conditions of centralized new energy grid connection, the volatility of new energy output and the presence of a large number of power electronic conversion devices lead to real-time changes in system impedance. Therefore, how to obtain system impedance information online in real time, comprehensively consider the impact of system impedance coupling on system stability, and achieve adaptive impedance reshaping is one of the key issues in the current application field of new energy power generation grid connection. Summary of the invention

[0004] In view of the problem of resonance instability in a centralized new energy grid-connected system, the present invention provides a resonance suppression method for a centralized new energy grid-connected system.

[0005] A resonance suppression method for a centralized new energy grid-connected system of the present invention, the centralized new energy grid-connected system includes N new energy grid-connected converters, N is a positive integer, and the method includes:

[0006] S1. Online identification of the total output impedance Z of the new energy grid-connected converter o (s) and grid impedance Z g (s);

[0007] S2. Calculate the virtual impedance Z required for compensation v (s), including:

[0008] S21, according to the total output impedance Z of the new energy grid-connected converter o (s) and grid impedance Z g (s), extract the system resonant frequency ω z ;

[0009] S22. Using the virtual impedance phase angle planning model, obtain the reasonable value range of the virtual impedance phase angle [θ vmin ,θ vmax ];

[0010] S23, in the reasonable value range [θ vmin ,θ vmax ], according to the system resonant frequency ω z Determine the optimal virtual impedance phase angle θ in the frequency band v ;

[0011] S24. Using the virtual impedance amplitude planning model, obtain a reasonable value range of the virtual impedance amplitude [Z vmin ,Z vmax ];

[0012] S25, in the reasonable value range [Z vmin ,Z vmax ], the optimal virtual impedance amplitude Z is determined according to the demand of the current grid-connected point voltage resonance component on the resonance suppression speed. v ;

[0013] S26, according to the optimal virtual impedance phase angle θ v and the optimal virtual impedance magnitude Z v , determine the virtual impedance required for compensation s is the Laplace operator;

[0014] S3, the virtual impedance Z required for compensation v (s) is sent into the control loop of the renewable energy grid-connected converter to control the renewable energy grid-connected converter.

[0015] As a preferred embodiment, the virtual impedance phase angle planning model is:

[0016] θ vmin =min∠Z v (s), θ vmax =max∠Z v (s)

[0017]

[0018] Z v (s) is the virtual impedance required for compensation, ∠Z v (s) represents Z v (s) impedance phase angle, Z' o (s c ) represents the impedance of the new energy grid-connected converter after the reshaping of the impedance c The total output impedance at the intermediate variable s c =jω c ,ω c Z g (s) / Z' o (s) shear frequency, PM is Z g (s) / Z' o (s) phase margin, R g is the virtual resistance, L g is the virtual inductance, θ lim is the minimum virtual impedance phase angle that can be compensated by N new energy grid-connected converters, Z lim is the minimum virtual impedance amplitude that can be compensated by N new energy grid-connected converters; ∠Z g (s c ) is Z g (s c ) impedance phase angle, ∠Z' o (s c ) is Z' o (s c )’s impedance phase angle.

[0019] As a preferred option, the optimal virtual impedance phase angle θ v for:

[0020]

[0021] Among them, ω1 is the mid-low frequency resonance boundary angle frequency, and ω2 is the mid-high frequency resonance boundary angle frequency.

[0022] As a preferred embodiment, the virtual impedance amplitude planning model is:

[0023] Z vmin =min|Z v (s)|,Z vmax =max|Z v (s)|

[0024]

[0025] Among them, Z v (s) is the virtual impedance required for compensation, ∠Z v(s) represents Z v (s) impedance phase angle, Z' o (s c ) represents the impedance of the new energy grid-connected converter after the reshaping of the impedance c The total output impedance at the intermediate variable s c =jω c ,ω c Z g (s) / Z' o (s) shear frequency, PM is Z g (s) / Z' o (s) phase margin, R g is the virtual resistance, L g is the virtual inductance, θ lim is the minimum virtual impedance phase angle that can be compensated by N new energy grid-connected converters, Z lim is the minimum virtual impedance amplitude that can be compensated by N new energy grid-connected converters; ∠Z g (s c ) is Z g (s c ) impedance phase angle, ∠Z' o (s c ) is Z' o (s c )’s impedance phase angle.

[0026] As a preferred option, the optimal virtual impedance amplitude Z v for:

[0027]

[0028] U pcc is the effective value of the grid connection point voltage, U pcch is the effective value of the resonant component of the grid-connected point voltage.

[0029] As a preferred embodiment, according to the total output impedance Z of the new energy grid-connected converter o (s) and grid impedance Z g (s), using the resonant frequency extraction model, extract the system resonant frequency ω z : The resonant frequency extraction model is:

[0030] ω z =minω

[0031]

[0032] Where s = jω, ω represents the angular frequency, PM1 is Z g (s) / Z o (s) phase margin;

[0033] Solve the resonant frequency extraction model to obtain the system resonant frequency ω z .

[0034] As a preferred method, online identification of the total output impedance Z of the new energy grid-connected converter o (s) and grid impedance Z g (s) methods, including:

[0035] Let a single renewable energy grid-connected converter work in the grid-connected mode, and identify the positive sequence admittance G of the current grid online through pulse response analysis. gp (s) and negative sequence admittance G gn (s);

[0036] When N grid-connected converters are connected to the grid and operating normally at the same time, the N grid-connected converters are sequentially identified online through impulse response analysis for the positive sequence admittance and negative sequence admittance at the current grid connection point:

[0037]

[0038] Among them, G pi (s) and G ni (s) are the online identification results of the positive-sequence admittance and negative-sequence admittance at the grid-connected point of the i-th grid-connected converter, G opk (s) and G onk (s) are the positive-sequence output admittance and negative-sequence output admittance of the kth grid-connected converter respectively;

[0039] For N grid-connected converters, G pi (s) and G ni (s) is calculated to obtain the total positive sequence output admittance G of the new energy grid-connected converter op (s) and negative sequence output total admittance G on (s):

[0040]

[0041] Positive sequence admittance G gp (s) and the total positive sequence output admittance G of the new energy grid-connected converter op (s) is calculated to obtain the grid impedance Z g (s) and the total output impedance Z of the new energy grid-connected converter o (s):

[0042]

[0043] As a preference, the virtual impedance Z to be compensated is v (s) is sent into the control loop of the grid-connected converter. The method for controlling the grid-connected converter includes:

[0044] According to the system resonant frequency ωz Adjusting the resonant frequency ω of the proportional-resonant controller n and the center frequency of the bandpass filter ω r ;

[0045] Use a bandpass filter to filter the grid voltage signal u pcc Filter and obtain the resonant component u of the grid-connected point voltage pcch ;

[0046] According to the resonant component u of the grid-connected voltage pcch and the virtual impedance Z required for compensation v (s) Get the given value of the virtual impedance compensation current loop of the grid-connected converter

[0047] Set the grid-connected converter virtual impedance compensation current loop given value i href The α and β axis components i hαref 、i hβref The two differences are respectively subtracted from their actual values, and the two differences are sent to the proportional-resonant controller for operation to obtain the α and β axis modulation signals. The α and β axis modulation signals are subjected to Clark inverse transformation to obtain the virtual impedance compensation modulation signal v h , the virtual impedance compensation modulation signal v h With the power modulation signal v b After superposition and PWM modulation, a switch control signal of the grid-connected converter is obtained, and the grid-connected converter is controlled according to the switch control signal.

[0048] As a preferred embodiment, according to the given value and actual value of the grid-side filter inductor current and the converter-side filter inductor current of the grid-connected converter, the power modulation signal v is obtained by the current double closed-loop full decoupling control method in the dq coordinate system. b ,include:

[0049] The current control outer loop sets the grid-side filter inductor current of the grid-connected converter to a given value i L2derf 、i L2qerf and their actual values ​​i L2d 、i L2q The two differences are sent to the proportional-integral controller for operation to obtain the given value i of the filter inductor current on the converter side. L1derf 、i L1qerf ;

[0050] The current control inner loop sets the converter side filter inductor current to a given value i L1derf 、i L1qerf and their actual values ​​i L1d 、i L1qThe two differences are respectively sent to the proportional-integral controller for operation. The operation results are superimposed on the d-axis and q-axis decoupling feedforward components to obtain the d-axis and q-axis modulation signals. The d-axis and q-axis modulation signals are divided by the DC side voltage V dc The result of the normalization is subjected to Park inverse transformation to obtain the power modulation signal v b .

[0051] The present invention also provides a control system based on the resonance suppression method of the centralized new energy grid-connected system, comprising:

[0052] System impedance online identification unit, online identification of the total output impedance Z of the new energy grid-connected converter o (s) and grid impedance Z g (s);

[0053] Adaptive virtual impedance calculation unit, calculates the virtual impedance Z required for compensation v (s);

[0054] The virtual impedance compensation unit converts the virtual impedance Z v (s) is sent into the control loop of the grid-connected converter to control the grid-connected converter.

[0055] The beneficial effects of the present invention are as follows: the present invention provides a method for online identification of system impedance, which is applicable to a centralized new energy grid-connected system containing multiple grid-connected devices. The method can obtain the real-time grid impedance and the total output impedance of the new energy grid-connected devices online, and provide an accurate impedance information model for system stability analysis and improvement, so as to realize accurate monitoring and effective management of the system's stable operation capability.

[0056] The present invention realizes direct control and elimination of the system resonant components through a resonance suppression method based on virtual impedance compensation. While ensuring the normal power output of the new energy grid-connected device, it fully utilizes the device's spare capacity and control resources to reshape the output impedance of the new energy grid-connected device at the resonant frequency band, so as to solve the resonance problem existing in the new energy grid-connected device and improve the stable operation capability of the power system.

[0057] The present invention adopts an adaptive virtual impedance algorithm, fully considers factors such as system impedance, resonant frequency and resonant component, dynamically adjusts the virtual impedance to be compensated, and minimizes the impact of virtual impedance compensation on the power output control of the new energy grid-connected device while ensuring system stability. At the same time, it improves the system resonance smoothing speed and reduces power loss, so as to improve the efficiency, safety and economic benefits of system operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 This is a schematic diagram of the structure of a centralized new energy grid-connected system;

[0059] Figure 2 This is a schematic diagram of the circuit structure of a new energy grid-connected converter;

[0060] Figure 3 This is a flow chart of the impedance online identification and adaptive resonance suppression method for centralized renewable energy grid-connected systems;

[0061] Figure 4 This is a flow chart of the impedance online identification method for a centralized new energy grid-connected system;

[0062] Figure 5 The equivalent circuit model of the system before and after virtual impedance compensation, where (a) is before virtual impedance compensation, and (b) is after virtual impedance compensation;

[0063] Figure 6 is the flow chart of adaptive virtual impedance algorithm;

[0064] Figure 7 This is the control block diagram of the impedance online identification and adaptive resonance suppression method. DETAILED DESCRIPTION

[0065] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0066] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0067] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but they are not intended to limit the present invention.

[0068] The schematic diagram of the centralized renewable energy grid-connected system is as follows: Figure 1 As shown in the figure, N sets of renewable energy power generation devices are connected in parallel to the renewable energy grid connection point through N sets of renewable energy grid-connected converters, forming a centralized renewable energy power generation system, which is connected to the power grid and transmission system through transformers. pcc is the grid connection point voltage, i pcc is the grid-connected point current, i.e. the output current of the centralized renewable energy power generation system, R g and L g is the equivalent impedance of the transmission line, u g with i g are the grid side voltage and current respectively.

[0069] In the centralized renewable energy power generation system, N renewable energy grid-connected converters adopt LCL type grid-connected converter topology. The circuit structure diagram is as follows: Figure 2 As shown in the figure, the main structure of the grid-connected converter is a three-phase bridge converter, with a capacitor C on the DC side. dc Combined with new energy power generation device in parallel, V dc and I dc They are the DC side voltage and DC side output current respectively. Each grid-connected converter is connected to the new energy grid connection point in parallel through an LCL filter. L1 is the converter side filter inductor, L2 is the grid side filter inductor, and C d is the filter capacitor, i L1 is the current flowing through the inductor L1, i.e. the AC output current of the grid-connected converter, i L2 is the current flowing through the inductor L2, i.e. the grid-connected output current of the grid-connected converter, u C is the filter capacitor voltage.

[0070] The resonance suppression method of the centralized new energy grid-connected system of this embodiment online identifies the grid impedance and the total output impedance of the new energy grid-connected converter, extracts the system resonant frequency and the grid-connected point voltage resonant component through the impedance identification result, dynamically adjusts the virtual impedance required for compensation based on the adaptive virtual impedance algorithm, and sends the calculated result to the control loop of the new energy grid-connected converter. The resonance suppression method based on virtual impedance compensation is used to reshape the output impedance of the grid-connected converter at the resonant frequency band to suppress the resonance phenomenon, while ensuring the normal power output of the new energy grid-connected system, realizing the wide-band resonance suppression function, and providing an economically feasible control solution for the safe and stable grid connection of centralized new energy. Specifically including:

[0071] Step 1: In a centralized renewable energy grid-connected system with N grid-connected converters, online identification of the total output impedance Z of the renewable energy grid-connected converters o (s) and grid impedance Z g (s):

[0072] First, a single renewable energy grid-connected converter is operated in the grid-connected mode, and the positive sequence admittance G of the current grid is identified online through pulse response analysis. gp (s) and negative sequence admittance G gn (s).

[0073] In the implementation case, each grid-connected converter in the centralized renewable energy grid-connected system adopts an impedance online identification method based on pulse response analysis. This method injects a disturbance pulse into the grid-connected point through a renewable energy grid-connected converter to obtain the system impedance or admittance information other than the grid-connected converter. Since the grid-connected converter generally adopts current type control, the current pulse is used as the disturbance pulse in this case. The specific process includes:

[0074] Step 1: Detect the current output current i of the new energy power generation system pcc Whether the peak value is reached, if so, proceed to step 2, otherwise, continue to step 1;

[0075] Step 2: applying a disturbance signal to the current injected into the grid through the new energy grid-connected converter;

[0076] Step 3: Measure the grid connection voltage u pcc and the output current i of the new energy power generation system pcc Sampling is performed to obtain current disturbance and voltage response signals, and the current and voltage signals when not disturbed are subtracted therefrom to remove the influence of inherent harmonics of the power grid;

[0077] Step 4: Perform discrete Fourier transform (DFT) analysis on the sampled voltage response and current disturbance signal to obtain their respective frequency domain components;

[0078] Step 5: Separate the positive and negative sequence frequency domain components of the voltage response and current disturbance. The ratio of the positive sequence frequency domain component in the current disturbance to the positive sequence frequency domain component in the voltage response is the positive sequence frequency domain admittance at the grid connection point. Similarly, the negative sequence frequency domain admittance at the grid connection point can be obtained. The positive and negative sequence frequency domain impedance can be obtained by taking the inverse of the positive and negative sequence frequency domain admittance.

[0079] Furthermore, when N renewable energy grid-connected converters are connected to the grid normally, the N grid-connected converters are sequentially identified online through pulse response analysis for positive sequence admittance and negative sequence admittance at the current grid connection point, as shown in the following formula:

[0080]

[0081] Among them, G pi (s) and G ni (s) are the online identification results of the positive and negative sequence admittance at the grid connection point of the i-th grid-connected converter, G opk (s) and G onk (s) are the positive and negative sequence output admittances of the kth grid-connected converter, respectively. It can be seen that the online identification result of the admittance at the grid connection point of the i-th renewable energy grid-connected converter is the sum of the admittances of all components of the system except the grid-connected converter.

[0082] Furthermore, the online identification results of the admittance of N new energy grid-connected converters are calculated, and the following formula can be obtained:

[0083]

[0084] Among them, G op (s) and G on (s) are respectively the total positive-sequence output admittance and the total negative-sequence output admittance of the renewable energy grid-connected converter, that is, the sum of the positive-sequence output admittance of N renewable energy grid-connected converters.

[0085] According to the above formula, the online identification results of the admittance of N new energy grid-connected converters are calculated to obtain the total positive sequence output admittance G of the new energy grid-connected system. op (s) and negative sequence output total admittance G on (s), as shown in the following formula:

[0086]

[0087] Furthermore, the positive and negative sequence admittance of the power grid and the total positive and negative sequence output admittance of the new energy grid-connected converter are calculated to obtain the power grid impedance Z g (s) and the total output impedance Z of the centralized renewable energy grid-connected system o (s), as shown in the following formula:

[0088]

[0089] Under three-phase balanced conditions, the renewable energy power generation grid-connected system often works in a positive-sequence environment. The main factor affecting the stability of the system is the positive-sequence impedance characteristic. Therefore, in this embodiment, only the influence of the system positive-sequence admittance or impedance on the system is analyzed.

[0090] The flow chart of the online impedance identification method for centralized renewable energy grid-connected systems is as follows: Figure 4 As shown, the specific steps are:

[0091] Step 1: Make a single renewable energy grid-connected converter work in grid-connected mode, and identify the current grid positive sequence admittance G online through pulse response analysis. gp (s) and negative sequence admittance G gn (s);

[0092] Step 12: When N new energy grid-connected converters are connected to the grid and run normally, determine whether i≤N (the initial value of i is 1). If yes, go to step 13; if not, go to step 14.

[0093] Step 13: Let the i-th renewable energy grid-connected converter identify the positive sequence admittance G at the current grid-connected point online pi (s) and negative sequence admittance G ni (s), update i=i+1 and go to step 1-2;

[0094] Step 14: Calculate the online identification results of the admittance of N new energy grid-connected converters to obtain the total positive sequence output admittance G of the new energy grid-connected converter op (s) and negative sequence output total admittance G on (s);

[0095] Step 2: Use the adaptive virtual impedance algorithm to calculate the virtual impedance Z required for compensation v (s), including:

[0096] The adaptive virtual impedance algorithm combines the online identification results of the grid impedance and the output impedance of the new energy grid-connected converter to obtain the system resonant frequency and the grid-connected point voltage resonant component, and dynamically adjusts the virtual impedance required for compensation to achieve resonance suppression in a wide frequency domain and improve the system resonance suppression speed. The specific idea of ​​the algorithm is to obtain the system resonant frequency through the impedance online identification results, and adjust the virtual impedance phase angle θ according to the frequency band of the current system resonant frequency. v ; Send the system resonant frequency to the bandpass filter to obtain the grid-connected point voltage resonant component, and adjust the virtual impedance amplitude Z according to the current resonant component's demand for the resonance suppression speed v ;Finally, according to the adjusted virtual impedance phase angle θ v With amplitude Z v , calculate the virtual impedance Z required for compensation v (s).

[0097] Step 21: Based on the total output impedance Z of the centralized new energy grid-connected converter o (s) and grid impedance Z g (s), using the resonant frequency extraction model, extract the system resonant frequency ω z , the resonant frequency extraction model is:

[0098] ω z =minω

[0099]

[0100] Where s = jω, ω represents the angular frequency, PM1 is Z g (s) / Z o (s) phase margin;

[0101] Solve the resonant frequency extraction model to obtain the system resonant frequency ω z .

[0102] The system resonant frequency ω of this step z There are two functions, one is in step 23, and the other is step 25 to obtain the grid connection point voltage resonance component.

[0103] Step 2: Use the virtual impedance phase angle planning model to obtain the reasonable value range of the virtual impedance phase angle [θ vmin ,θ vmax ];

[0104] Furthermore, according to the influence of impedance coupling relationship on system stability, the reasonable value range of virtual impedance is obtained. Figure 5 The equivalent circuit model of the system before and after virtual impedance compensation is given, where I s(s) is the output current of the centralized renewable energy power generation system, I g (s) is the grid current, U g (s) is the grid voltage, Z o (s) is the total output impedance of the new energy grid-connected converter, Z g (s) is the grid impedance, Z v (s) is the virtual impedance to be compensated and satisfies Z v (s) = R v +sL v .according to Figure 5 The equivalent model of (a) can be obtained as follows:

[0105]

[0106] Since the coupling effect between the grid impedance and the total output impedance of the renewable energy grid-connected converter is the main reason for the resonance of the system, and under the condition that the grid voltage is stable, the output power, circuit parameters and control loop of the centralized renewable energy power generation system remain unchanged, it can be considered that I in the above formula is s (s), U g (s), Z o (s) is a stabilizing term, and the stability of the grid current depends on the second half of the above equation, H(s), which is expressed as follows:

[0107]

[0108] H(s) can be regarded as a vector with a forward gain of 1 and a feedback gain of Z. g (s) / Z o The closed-loop transfer function of the negative feedback control system (s) is, according to control theory, only when Z g (s) / Z o (s) When the improved Nyquist stability criterion is met: at the cutoff frequency ω c Z g (s) and Z o (s) The phase difference is less than 180°, that is, the phase margin PM>0, which means that the system is stable. The expression of the phase margin PM is:

[0109] PM=180°-(∠Z g (jω c )-∠Z o (jω c ))

[0110] It can be seen from the above formula that the change of impedance coupling relationship will have a certain impact on the stability of the system, thereby causing resonance phenomenon. Virtual impedance compensation can be performed through the new energy grid-connected converter to achieve impedance reshaping and suppress the occurrence of system resonance phenomenon. Figure 5(b) The equivalent model shows that virtual impedance compensation is essentially to adjust the total output impedance of the new energy grid-connected converter to ensure that the system has sufficient phase margin. In the present invention, each new energy grid-connected converter needs to perform power output control and virtual impedance compensation at the same time. In order to minimize the impact of virtual impedance compensation on the power output control of the new energy grid-connected converter, resistive-inductive virtual impedance compensation is adopted to achieve safe and stable operation of the new energy grid-connected converter. When selecting the parameters of the resistive-inductive virtual impedance, the phase angle and amplitude can be considered. The virtual impedance phase angle can reflect the proportion of the resistive component and the inductive component in the virtual impedance. The smaller the phase angle, the greater the proportion of the resistive component, and the smaller the proportion of the inductive component. At the same time, the resistive and inductive virtual impedances correspond to the consumption of active and reactive power, respectively. When the system resonant frequency is at a low frequency, the resistive-inductive virtual impedance has a good resonance suppression characteristic, but the proportion of the resistive component should be reduced as much as possible to reduce its consumption of active power to avoid affecting the power output control; as the system resonant frequency gradually increases, the impedance value corresponding to the inductive virtual impedance continues to increase. At this time, the proportion of the inductive component should be reduced to ensure a good resonance suppression characteristic. In addition, when the virtual impedance amplitude is smaller, the suppression speed of the grid resonance is faster, and the resonance suppression effect is more obvious. However, too small a virtual impedance amplitude will cause more current to be absorbed, resulting in a decrease in the overall efficiency of the system. Therefore, it is necessary to select a suitable virtual impedance amplitude to suppress the resonance of the distribution network while minimizing power loss as much as possible. In summary, it is necessary to reasonably select the phase angle and amplitude of the virtual impedance, while ensuring the normal power output of the new energy grid-connected converter, and minimize the power consumption caused by the virtual impedance compensation, so as to maximize the system resonance suppression performance. First, the virtual impedance phase angle planning model shown below can be used to calculate the reasonable value range of the virtual impedance phase angle;

[0111] The virtual impedance phase angle planning model is:

[0112] θ vmin =min∠Z v (s), θ vmax =max∠Z v (s)

[0113]

[0114] Z v (s) is the virtual impedance required for compensation, ∠Z v (s) represents Z v (s) impedance phase angle, Z' o (s c ) represents the impedance of the new energy grid-connected converter after the reshaping of the impedance c The total output impedance at the intermediate variable s c =jω c,ω c Z g (s) / Z' o (s) shear frequency, PM is Z g (s) / Z' o (s) phase margin, R g is the virtual resistance, L g is the virtual inductance, ∠Z g (s c ) is Z g (s c ) impedance phase angle, ∠Z' o (s c ) is Z' o (s c )’s impedance phase angle, θ lim is the minimum virtual impedance phase angle that can be compensated by N new energy grid-connected converters, Z lim is the minimum virtual impedance amplitude that can be compensated by N new energy grid-connected converters. The value can be determined by comprehensively considering the remaining capacity resources, efficiency indicators and loss requirements of the new energy grid-connected converter in addition to power output. From the above formula, we can get the range of virtual impedance phase angles [θ vmin ,θ vmax ].

[0115] Step 2: In the reasonable value range [θ vmin ,θ vmax ], according to the system resonant frequency ω z Determine the optimal virtual impedance phase angle θ in the frequency band v :

[0116]

[0117] Wherein, ω1 is the mid-low frequency resonance boundary angular frequency, and ω2 is the mid-high frequency resonance boundary angular frequency. In this implementation case, ω1 takes the angular frequency corresponding to 100 Hz, and ω2 takes the angular frequency corresponding to 1 kHz. Step 24: Use the virtual impedance amplitude planning model to obtain a reasonable value range of the virtual impedance amplitude [Z vmin ,Z vmax ]:

[0118] Z vmin =min|Z v (s)|,Z vmax =max|Z v (s)|

[0119]

[0120] Step 25: Within the reasonable value range [Z vmin ,Zvmax ], obtain the grid point voltage effective value U pcc And the effective value of the resonant component of the grid-connected point voltage U pcch , according to the demand of the current grid-connected point voltage resonance component on the resonance suppression speed, determine the optimal virtual impedance amplitude Z v :

[0121]

[0122] Step 26: According to the optimal virtual impedance phase angle θ v and the optimal virtual impedance magnitude Z v , determine the virtual impedance Z required for compensation v (s):

[0123]

[0124] s is the Laplace operator;

[0125] Step 3: Set the virtual impedance Z to be compensated v (s) is sent to the control loop of the grid-connected converter, and the grid-connected converter is controlled based on the resonance suppression method of virtual impedance compensation;

[0126] The resonance suppression method based on virtual impedance compensation controls N sets of new energy grid-connected converters to absorb the resonant current in the system, thereby making the N sets of grid-connected converters equivalent to additional resistive-inductive virtual impedances in the resonant frequency band, so as to achieve the purpose of reshaping the output impedance of the grid-connected converter and suppressing the resonance of the system. Specifically, it includes:

[0127] First, a bandpass filter is used to filter the collected grid-connected point voltage signal u pcc Filter and obtain the resonant component u of the grid-connected point voltage pcch , calculate the effective value of the grid connection point voltage U pcc And the effective value of the resonant component of the grid-connected point voltage U pcch , the transfer function of the bandpass filter G u The expression of (s) is as follows:

[0128]

[0129] Among them, ω r is the center frequency, ω u is the bandpass width. The function of the bandpass filter is to extract the frequency ω r signal component, so ω r The value of should be consistent with the system resonant frequency, and the passband width ω u It needs to be consistent with the actual impedance identification frequency step accuracy.

[0130] Furthermore, the virtual impedance Z required for compensation is obtained by an adaptive virtual impedance algorithm. v (s), and use the following relationship to get the current given value:

[0131]

[0132] Among them, i href is the given value of the virtual impedance compensation current loop of each renewable energy grid-connected converter, that is, the resonant current that each renewable energy grid-connected converter needs to absorb; Z v (s) is the virtual impedance that needs to be compensated at present. At this time, N grid-connected converters are equivalent to additional resistive and inductive impedance in the resonant frequency band.

[0133] Furthermore, in order to accurately track the given AC resonant current at a specific frequency, current control is implemented in the αβ coordinate system. The current controller adopts a proportional-resonant (PR) controller, and its transfer function G i (s) are as follows:

[0134]

[0135] In the formula, K pa and K ra are the proportional coefficient and resonant coefficient of the PR controller, ω n is the resonant frequency of the PR controller, ω d is the resonant bandwidth. The function of the PR controller is to n The signal component is tracked and controlled, so ω n The value of should be consistent with the system resonant frequency, the resonant bandwidth ω d It needs to be consistent with the actual impedance identification frequency step accuracy.

[0136] In order to improve the extraction and tracking accuracy of the resonant component, the system resonant frequency ω z The frequency control parameters are adjusted as follows:

[0137] ω r =ω n =ω z

[0138] Based on the above, the virtual impedance Z v (s) is sent to the control loop of the new energy grid-connected converter. The process of controlling the new energy grid-connected converter includes:

[0139] Step 31: According to the system resonant frequency ω z Adjusting the resonant frequency ω of the proportional-resonant controller n and the center frequency ω of the bandpass filter r;

[0140] Step 3.2: Use a bandpass filter to filter the grid voltage signal u pcc Filter and obtain the resonant component u of the grid-connected point voltage pcch ;

[0141] Step 3: According to the resonant component u of the grid-connected point voltage pcch and the virtual impedance Z required for compensation v (s) Get the given value of the virtual impedance compensation current loop of the new energy grid-connected converter

[0142] Step 34: Set the virtual impedance compensation current loop of the new energy grid-connected converter to the given value i href The α and β axis components i hαref 、i hβref The two differences are respectively subtracted from their actual values, and the two differences are sent to the proportional-resonant controller for operation to obtain the α and β axis modulation signals. The α and β axis modulation signals are subjected to Clark inverse transformation to obtain the virtual impedance compensation modulation signal v h , the virtual impedance compensation modulation signal v h With the power modulation signal v b After superimposed PWM modulation, a switch control signal of the new energy grid-connected converter is obtained, and the new energy grid-connected converter is controlled according to the switch control signal.

[0143] This embodiment provides a control system based on the above method, and its overall control block diagram is as follows: Figure 7 As shown, it includes a signal acquisition and processing unit, a coordinate transformation unit, a system impedance online identification unit, an adaptive virtual impedance calculation unit, a power and current given calculation unit, a power output control unit, and a virtual impedance compensation unit.

[0144] The centralized renewable energy power generation system contains N LCL-type renewable energy grid-connected converters. Each grid-connected converter is independently controlled to realize functions such as online identification of system impedance, virtual impedance compensation and power output. Figure 7 The control flow diagram of each grid-connected converter is given.

[0145] Signal acquisition and processing unit, used to collect the output current i of the centralized new energy power generation system at the current moment pcc and grid voltage u pcc At the same time, it is sent to the system impedance online identification unit and the virtual impedance compensation unit to collect the grid-side filter inductance current i of the new energy grid-connected converter at the current moment. L2 , converter side filter inductor current i L1 And the filter capacitor voltage u c The coordinate transformation unit is sent to collect the DC side voltage V of the new energy grid-connected converter at the current moment.dc The current grid-connected point voltage u is obtained through the phase-locked loop (PLL). pcc The phase angle θ is sent to the coordinate transformation unit and the power output control unit to calculate the effective value of the grid connection point voltage U pcc The effective value of the resonant component of the grid-connected point voltage U pcch Send to the adaptive virtual impedance calculation unit.

[0146] Coordinate conversion unit, used to compensate the current loop given value i of the virtual impedance of the new energy grid-connected converter href and the grid-side filter inductor current i of the new energy grid-connected converter L2 Perform Clark transformation to obtain its α and β axis components i hαref 、i hβref and i L2α 、i L2β , sent to the virtual impedance compensation unit, using the grid point voltage u pcc The phase angle θ of the LCL type new energy grid-connected converter grid-side filter inductor current i L2 , converter side filter inductor current i L1 、Filter capacitor voltage u c and grid voltage u pcc Perform Park transformation to obtain the d and q axis current and voltage components: i L2d 、i L2q 、i L1d 、i L1q 、u cd 、u cq 、u pccd 、u pccq , sent to the power current given calculation unit and the power output control unit.

[0147] System impedance online identification unit is used to utilize Figure 4 The process shown in the figure obtains the grid impedance Z g (s) and the total output impedance Z of the new energy grid-connected converter o (s) and sent to the adaptive virtual impedance calculation unit.

[0148] The power current given calculation unit is used to use the instantaneous power theory to calculate the power current given value according to the following formula and send it to the power output control unit.

[0149]

[0150] Among them, P ref and Q ref are the active power set value and reactive power set value of the new energy grid-connected converter, i L2drefis the given value of the grid-side filter inductor d-axis current of the new energy grid-connected converter, that is, the given value of the active power current, i L2qref It is the given value of the q-axis current of the grid-side filter inductor of the new energy grid-connected converter, that is, the given value of the reactive power current.

[0151] The power output control unit is used to realize the independent control and output functions of active and reactive power of the new energy grid-connected converter by adopting the current double closed-loop full decoupling control method in the dq coordinate system. The current control outer loop sets the grid-side filter inductor current of the new energy grid-connected converter to the given value i L2derf 、i L2qerf and their actual values ​​i L2d 、i L2q The difference is sent to the proportional-integral (PI) controller for calculation to obtain the given value i of the converter side filter inductor current L1derf 、i L1qerf The current control inner loop sets the filter inductor current on the converter side to a given value i L1derf 、i L1qerf and their actual values ​​i L1d 、i L1q The difference is sent to the proportional-integral (PI) controller for calculation, and the calculation results are superimposed on the d-axis and q-axis decoupling feedforward components G d (s), G q (s) to obtain the d and q axis modulation signals, which are divided by the DC side voltage V of the new energy grid-connected converter dc After normalization, the power modulation signal v is obtained after Park inverse transformation. b And sent to the virtual impedance compensation unit. d, q axis decoupling feedforward component G d (s), G q (s) is shown in the following formula:

[0152]

[0153] Among them, L2 is the grid-side filter inductor of the LCL type new energy converter, L1 is the converter-side filter inductor, C d is the filter capacitor, ω0 is the fundamental angular frequency.

[0154] Adaptive virtual impedance calculation unit for utilizing Figure 6 The process shown in the figure calculates the system resonant frequency ω z and the virtual impedance Z required for compensation v (s) is sent to the virtual impedance compensation unit.

[0155] The virtual impedance compensation unit first uses a bandpass filter to collect the grid connection point voltage signal u pcc Filter and obtain the resonant component u of the grid-connected point voltage pcchThe signal is sent to the signal acquisition and processing unit to convert the resonant component u of the grid connection point voltage pcch Divide by the virtual impedance Z required for compensation v (s) Get the given value i of the virtual impedance compensation current loop of the new energy grid-connected converter href The coordinate transformation unit is used to set the α and β axis current loop given values ​​i hαref 、i hβref and their actual values ​​i L2α 、i L2β The difference is sent to the proportional-resonant controller for operation to obtain the α and β axis modulation signals, and the modulation signal is subjected to Clark inverse transformation to obtain the virtual impedance compensation modulation signal v h , the power modulation signal v b With virtual impedance compensation modulation signal v h The switching control signal is obtained after being modulated by the PWM modulation module, which controls the new energy grid-connected converter to perform virtual impedance compensation and power output. While providing power support to the power grid, it absorbs the resonant current and realizes impedance reshaping to improve system stability.

[0156] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. It should therefore be understood that many modifications may be made to the exemplary embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in a manner different from that described in the original claims. It should also be understood that features described in conjunction with individual embodiments may be used in other described embodiments.

Claims

1. A resonance suppression method for a centralized new energy grid-connected system, wherein the centralized new energy grid-connected system comprises N new energy grid-connected converters, where N is a positive integer, and is characterized in that: The method comprises: S1. Online identification of the total output impedance Z of the new energy grid-connected converter o (s) and grid impedance Z g (s); S2. Calculate the virtual impedance Z required for compensation v (s), including: S21, according to the total output impedance Z of the new energy grid-connected converter o (s) and grid impedance Z g (s), extract the system resonant frequency ω z ; S22. Using the virtual impedance phase angle planning model, obtain the reasonable value range of the virtual impedance phase angle [θ vmin ,θ vmax ]; S23, in the reasonable value range [θ vmin ,θ vmax ], according to the system resonant frequency ω z Determine the optimal virtual impedance phase angle θ in the frequency band v ; S24. Using the virtual impedance amplitude planning model, obtain a reasonable value range of the virtual impedance amplitude [Z vmin ,Z vmax ]; S25, in the reasonable value range [Z vmin ,Z vmax ], the optimal virtual impedance amplitude Z is determined according to the demand of the current grid-connected point voltage resonance component on the resonance suppression speed. v ; S26, according to the optimal virtual impedance phase angle θ v and the optimal virtual impedance magnitude Z v , determine the virtual impedance required for compensation s is the Laplace operator; S3, the virtual impedance Z required for compensation v (s) is sent into the control loop of the renewable energy grid-connected converter to control the renewable energy grid-connected converter.

2. The resonance suppression method of the centralized new energy grid-connected system according to claim 1 is characterized in that: The virtual impedance phase angle planning model is: i vmin =min∠Z v (s),θ vmax =max∠Z v (s) Z v (s) is the virtual impedance required for compensation, ∠Z v (s) represents Z v (s) impedance phase angle, Z' o (s c ) represents the impedance of the new energy grid-connected converter after the impedance remodeling. c The total output impedance at the intermediate variable s c =jω c ,ω c Z g (s) / Z' o (s) shear frequency, PM is Z g (s) / Z' o (s) phase margin, R g is the virtual resistance, L g is the virtual inductance, θ lim is the minimum virtual impedance phase angle that can be compensated by N new energy grid-connected converters, Z lim is the minimum virtual impedance amplitude that can be compensated by N new energy grid-connected converters; ∠Z g (s c ) is Z g (s c ) impedance phase angle, ∠Z' o (s c ) is Z' o (s c )’s impedance phase angle.

3. The resonance suppression method of the centralized new energy grid-connected system according to claim 2 is characterized in that: Optimal virtual impedance phase angle θ v for: Among them, ω1 is the mid-low frequency resonance boundary angle frequency, and ω2 is the mid-high frequency resonance boundary angle frequency.

4. The resonance suppression method of a centralized new energy grid-connected system according to claim 1, characterized in that: The virtual impedance amplitude planning model is: WITH vmin =min|Z v (s)|,Z vmax =max|Z v (s)| Among them, Z v (s) is the virtual impedance required for compensation, ∠Z v (s) represents Z v (s) impedance phase angle, Z' o (s c ) represents the impedance of the new energy grid-connected converter after the impedance remodeling. c The total output impedance at the intermediate variable s c =jω c ,ω c Z g (s) / Z' o (s) shear frequency, PM is Z g (s) / Z' o (s) phase margin, R g is the virtual resistance, L g is the virtual inductance, θ lim is the minimum virtual impedance phase angle that can be compensated by N new energy grid-connected converters, Z lim is the minimum virtual impedance amplitude that can be compensated by N new energy grid-connected converters; ∠Z g (s c ) is Z g (s c ) impedance phase angle, ∠Z' o (s c ) is Z' o (s c )’s impedance phase angle.

5. The resonance suppression method of the centralized new energy grid-connected system according to claim 4 is characterized in that: Optimal virtual impedance amplitude Z v for: U pcc is the effective value of the grid connection point voltage, U pcch is the effective value of the resonant component of the grid-connected point voltage.

6. The resonance suppression method of a centralized new energy grid-connected system according to claim 1, characterized in that: According to the total output impedance Z of the new energy grid-connected converter o (s) and grid impedance Z g (s), using the resonant frequency extraction model, extract the system resonant frequency ω z : The resonant frequency extraction model is: oh z =minω Where s = jω, ω represents the angular frequency, PM1 is Z g (s) / Z o (s) phase margin; Solve the resonant frequency extraction model to obtain the system resonant frequency ω z .

7. The resonance suppression method of a centralized new energy grid-connected system according to claim 1, characterized in that: Online identification of total output impedance Z of new energy grid-connected converter o (s) and grid impedance Z g (s) methods, including: Let a single renewable energy grid-connected converter work in the grid-connected mode, and identify the positive sequence admittance G of the current grid online through pulse response analysis. gp (s) and negative sequence admittance G gn (s); When N grid-connected converters are connected to the grid and operating normally at the same time, the N grid-connected converters are sequentially identified online through impulse response analysis for the positive sequence admittance and negative sequence admittance at the current grid connection point: Among them, G pi (s) and G ni (s) are the online identification results of the positive-sequence admittance and negative-sequence admittance at the grid-connected point of the i-th grid-connected converter, G opk (s) and G onk (s) are the positive-sequence output admittance and negative-sequence output admittance of the kth grid-connected converter respectively; For N grid-connected converters, G pi (s) and G ni (s) is calculated to obtain the total positive sequence output admittance G of the new energy grid-connected converter op (s) and negative sequence output total admittance G on (s): Positive sequence admittance G gp (s) and the total positive sequence output admittance G of the new energy grid-connected converter op (s) is calculated to obtain the grid impedance Z g (s) and the total output impedance Z of the new energy grid-connected converter o (s):

8. The resonance suppression method of a centralized new energy grid-connected system according to claim 1, characterized in that: The virtual impedance Z v (s) is sent into the control loop of the grid-connected converter. The method for controlling the grid-connected converter includes: According to the system resonant frequency ω z Adjusting the resonant frequency ω of the proportional-resonant controller n and the center frequency ω of the bandpass filter r ; Use a bandpass filter to filter the grid voltage signal u pcc Filter and obtain the resonant component u of the grid-connected point voltage pcch ; According to the resonant component u of the grid-connected voltage pcch and the virtual impedance Z required for compensation v (s) Get the given value of the virtual impedance compensation current loop of the grid-connected converter Set the grid-connected converter virtual impedance compensation current loop given value i href The α and β axis components i hαref 、i hβref The two differences are respectively subtracted from their actual values, and the two differences are sent to the proportional-resonant controller for operation to obtain the α and β axis modulation signals. The α and β axis modulation signals are subjected to Clark inverse transformation to obtain the virtual impedance compensation modulation signal v h , the virtual impedance compensation modulation signal v h With the power modulation signal v b After superposition and PWM modulation, a switch control signal of the grid-connected converter is obtained, and the grid-connected converter is controlled according to the switch control signal.

9. The resonance suppression method of the centralized new energy grid-connected system according to claim 8, characterized in that: According to the given value and actual value of the grid-side filter inductor current and the converter-side filter inductor current of the grid-connected converter, the power modulation signal v is obtained by the current double closed-loop full decoupling control method in the dq coordinate system. b ,include: The current control outer loop sets the grid-side filter inductor current of the grid-connected converter to a given value i L2derf 、i L2qerf and their actual values ​​i L2d 、i L2q The two differences are sent to the proportional-integral controller for operation to obtain the given value i of the filter inductor current on the converter side. L1derf 、i L1qerf ; The current control inner loop sets the converter side filter inductor current to a given value i L1derf 、i L1qerf and their actual values ​​i L1d 、i L1q The two differences are respectively sent to the proportional-integral controller for operation. The operation results are superimposed on the d-axis and q-axis decoupling feedforward components to obtain the d-axis and q-axis modulation signals. The d-axis and q-axis modulation signals are divided by the DC side voltage V dc The result of the normalization is subjected to Park inverse transformation to obtain the power modulation signal v b .

10. A control system based on the resonance suppression method of a centralized new energy grid-connected system according to any one of claims 1 to 9, characterized in that: include: System impedance online identification unit, online identification of the total output impedance Z of the new energy grid-connected converter o (s) and grid impedance Z g (s); Adaptive virtual impedance calculation unit, calculates the virtual impedance Z required for compensation v (s); The virtual impedance compensation unit converts the virtual impedance Z v (s) is sent into the control loop of the grid-connected converter to control the grid-connected converter.

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