New energy grid-connected control circuit and SST control method based on virtual synchronous machine

Abstract

The present invention discloses a new energy grid-connected control circuit and an SST control method based on a virtual synchronous machine. The present invention aims at connecting a new energy grid to a three-stage SST composed of an input stage, an isolation stage and an output stage, and arranges a new energy grid-connected control circuit based on a VSG at the output stage. Based on the circuit, current balance, active power and reactive power constant control under unbalanced voltage are performed through an SST control method, and a control strategy for grid stability is calculated through an SST control method based on a virtual synchronous machine under unbalanced voltage, thereby improving the power supply quality of the grid and improving the grid-connected current, voltage and power quality output by the SST output stage.

Description

Technical Field

[0001] The present invention belongs to power system control technology, and specifically relates to a new energy grid-connected control circuit and an SST control method based on a virtual synchronous machine. Background Art

[0002] In low-voltage distribution networks, the diversity and complexity of power loads have caused increasingly prominent power quality issues in low-voltage distribution networks, especially the access of distributed new energy sources, which has aggravated the voltage quality of low-voltage distribution networks and caused imbalance between high and low voltages. The existing control and management technologies have not been effective. Most of the power loads in low-voltage distribution networks are single-phase loads. Due to factors such as differences in power consumption time, three-phase imbalance problems exist objectively and in real time. The three-phase imbalance problem seriously restricts transformer output and voltage quality problems, and at the same time causes increased losses in low-voltage distribution lines. Moreover, with the development of the economy, there are more and more nonlinear loads, and harmonic and reactive power problems are becoming more and more prominent, with application limitations.

[0003] On the other hand, under unbalanced grid voltage, the inverter controlled by the traditional VSG control strategy will face problems such as output current imbalance, active and reactive power fluctuations, and most of the traditional strategies for dealing with three-phase voltage imbalance are not suitable for virtual synchronous generator control. Summary of the invention

[0004] Purpose of the invention: In order to solve the problems of current imbalance, power oscillation and current quality caused by the SST output stage in the traditional VSG control under the condition of three-phase unbalanced voltage in the low-voltage distribution network, the first purpose of the present invention is to provide a new energy grid-connected control circuit based on VSG. Based on this control circuit, the second purpose of the present invention is to provide an SST control method based on a virtual synchronous machine under unbalanced voltage.

[0005] Technical solution: A VSG-based new energy grid-connected control circuit, in which the new energy grid is connected to the isolation stage of the three-stage SST topology structure main circuit consisting of an input stage, an isolation stage and an output stage, including an SST control circuit based on a virtual synchronous machine under unbalanced voltage, and the SST control circuit includes a VSG controller, a current reference value generator, a positive and negative sequence current regulator based on PI, and a midpoint potential balance controller;

[0006] In the SST control circuit, the VSG controller provides a voltage reference value for the current reference generator, and the modulation wave generator controls the current injected into the grid by the output stage under the coordinated output of the PI-based positive and negative sequence current regulator and the midpoint potential balance controller, so that only the positive sequence component is allowed to be included.

[0007] In the above circuit, the input stage includes a half-bridge MMC submodule topology structure, the intermediate isolation stage includes a dual active bridge series resonant converter, and the output stage includes a T-type three-level inverter structure.

[0008] Furthermore, the input stage is connected to the medium and high voltage AC grid with a half-bridge MMC converter, and can be adapted to different voltage levels by increasing or decreasing the number of MMC submodules;

[0009] The isolation stage includes a dual active bridge series resonant converter, which modulates the output DC voltage on the input side into a high-frequency AC voltage through a single-phase full-bridge inverter circuit, couples to the secondary side through a high-frequency transformer, and converts it into DC by a single-phase full-bridge rectifier circuit, thereby realizing DC voltage conversion and electrical isolation of the input and output stages, allowing power to flow in both directions. The isolation stage adopts an input series / output parallel structure to realize input voltage division and output current division, and its low-voltage DC bus provides a DC port for new energy grid connection or load.

[0010] The output stage includes a T-type three-level inverter, which implements mid-point clamping of the bridge arm output voltage through reverse series IGBTs, and converts the low-voltage DC output of the isolation stage into a stable three-phase industrial frequency voltage, which is input into the low-voltage distribution network to supply power to users.

[0011] A SST control method based on a virtual synchronous machine under unbalanced voltage, the SST control method is used for current balance control, active power constant control and reactive power constant control after new energy is connected to the grid, the method includes extracting the positive and negative sequence components of the output current based on the positive and negative sequence separation of a double second-order generalized integrator, and includes the following calculation and modulation process:

[0012] (1) Based on the VSG controller output voltage reference value u * k , the negative sequence current reference value is calculated by instantaneous active power and reactive power, so that the current injected into the grid by the SST control output stage contains only positive sequence components;

[0013] (2) Based on current balance control, calculate and solve the VSG controller voltage reference value u * k With the grid positive sequence voltage u + gd 、Current i + i The relationship between the component reference values ​​can be expressed as follows:

[0014]

[0015] Where: u* kd 、*u kq They are respectively the voltage reference value u of the VSG control module * kThe components under the d and q axes after Park transformation, i + id 、i + iq They are the grid positive sequence current i + i Components under the d and q axes after Park transformation;

[0016] (3) Considering the correlation between the negative sequence current reference value of the active power constant control and the positive sequence current reference value of the current balance control in the active power constant control, the negative sequence current reference value is calculated, and the following expression exists:

[0017]

[0018] Where: u g + 、u g - They are low voltage distribution network voltage U g + and U g - The instantaneous value of i + 、i i - They are the output current I i + and I i - The instantaneous value of

[0019] In the reactive power constant control, the positive sequence current reference value in the current balance control is correlated with the negative sequence current reference value in the active power and reactive power constant control, and is the inverse of the negative sequence current reference value in the active power constant control;

[0020] (4) A current reference generator is set to control the negative sequence current reference value output under active power constant control and reactive power constant control, and a flexible coefficient H is introduced. Under the condition of unbalanced grid voltage, the control coefficient H is used to realize grid stability control, including the following control calculations:

[0021] When H=0, current balance control is achieved; when H=1, active constant control is achieved; when H=-1, reactive constant control is achieved, and its expression is as follows:

[0022]

[0023] (5) By setting a PI-based positive and negative sequence current regulator to adjust the output current, the positive and negative sequence current reference values ​​can be quickly tracked. The following expression is used to calculate the adjustment value:

[0024]

[0025] Where: e * is the modulation voltage; Kp and Ki are the proportional integral coefficients;

[0026] (6) The voltage balance between the two capacitors of the T-type three-level output stage is performed by a midpoint potential balance controller, wherein the zero-sequence component is generated as follows:

[0027]

[0028] Where: v0 is the zero-sequence component, C is the capacitive reactance of the voltage divider capacitor, i npav is the midpoint current, i np0 is the midpoint current after superimposing the zero-sequence component, f s is the carrier frequency.

[0029] Furthermore, in the above method, in the positive and negative sequence separation based on the double second-order generalized integrator, the second-order generalized integrator transfer function is as follows:

[0030]

[0031] Where: D(s) and E(s) are bandpass and low-pass filters respectively; ω r is the resonant frequency; k is the damping coefficient.

[0032] In the method, the current injected into the grid by the SST output stage is only allowed to contain the positive sequence component, the negative sequence current reference value is set to zero, the positive sequence current reference value is calculated by the voltage reference value generated by the VSG control module and the output stage positive sequence circuit equation, and the SST output stage output voltage stability value u ik The VSG module output voltage reference value u k * Approximately equal, the relationship between the voltage reference value of the VSG control module and the reference value of the positive sequence voltage and current components of the power grid can be obtained as follows:

[0033]

[0034] Where: u id 、u iq and i id 、i iq are the d-axis and q-axis components of the output voltage and current of the output stage; u gd 、u gq is the dq-axis component of the grid voltage;

[0035]

[0036] Where: u kd 、u kq They are the components of the VSG controller voltage reference value on the d and q axes respectively.

[0037] Furthermore, according to the calculation relationship of the components of the VSG controller voltage reference value under the d and q axes, it is converted into a positive sequence current reference value expression and transformed into the complex frequency domain through the Laplace final value theorem, and the following relationship is obtained:

[0038]

[0039] Where: s is the differential operator; R is the total resistance from the inverter to the grid;

[0040] According to Laplace's final value theorem, the simplified and converted expression to the time domain is as follows:

[0041]

[0042] When the negative sequence current reference value is set to zero, the double frequency power fluctuation component is as follows:

[0043]

[0044] From the above, it can be seen that the double frequency power fluctuation component is not completely eliminated; weakening power oscillation and achieving the current balance control goal is at the expense of active and reactive power oscillation.

[0045] Furthermore, in the active power constant control, when the SST is used as the grid-connected interface of the photovoltaic or energy storage system, the active power needs to be constantly controlled so that the double frequency active power fluctuation component is zero and only the average active power component is generated. The expression is as follows:

[0046]

[0047] It can be seen from the above formula that the negative sequence current reference value of the active power constant control and the positive sequence current reference value of the current balance control are related to each other.

[0048] In the reactive power constant control, when the large-capacity power generation equipment is connected to the grid through SST and provides stable reactive power support, the double frequency reactive power fluctuation component is made zero. The expression for the output average reactive power component is as follows:

[0049]

[0050] It can be seen from the above formula that the negative sequence current reference value of reactive power constant control and the positive sequence current reference value of current balance control are related to each other.

[0051] Beneficial effects: Compared with the prior art, firstly, the traditional VSG control strategy does not consider the negative sequence component, resulting in active and reactive power oscillations, which are transmitted to the output voltage reference amplitude and phase angle through the active loop and reactive loop in the VSG control, causing the output current of the SST output stage to be unbalanced, resulting in poor output power quality of the SST. An output stage control strategy based on an improved VSG is proposed. By adding the control link i*=f(uk*), the output current balance and the reduction of active or reactive power fluctuations are achieved without changing the voltage source characteristics of the VSG. The significant effects of the control circuit and control method described in the present invention are as follows:

[0052] (1) The improved VSG control strategy proposed in the present invention retains the basic voltage source characteristics of VSG. The positive-sequence current reference value is deduced through the positive-sequence circuit of the SST output stage in the VSG control, and the negative-sequence current reference value of different control targets is deduced in combination with the instantaneous power theory. The positive-sequence and negative-sequence current regulation modules are then cascaded to achieve grid-connected current balance, reduce active / reactive power oscillation and grid-connected current harmonic content, and by introducing a flexible coefficient, reliable switching of the three control targets is achieved.

[0053] (2) The present invention includes the proposed midpoint potential balancing method for zero-sequence component injection, combined with an improved VSG control strategy, which is suitable for unbalanced voltage conditions, and can well balance the voltage of the T-type three-level output stage voltage divider capacitor, effectively reduce the injection of low-order harmonics, and improve the output stage power quality.

[0054] (3) When implementing current balance VSG control or active power constant VSG control, the present invention effectively suppresses the double frequency fluctuation of the SST low-voltage DC bus voltage, thereby improving the adaptability of DC microgrid access and the reliability of DC load power supply. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 It is the main circuit topology diagram of the solid-state transformer (SST);

[0056] Figure 2 It is a circuit diagram of the new energy grid connection and controller connection described in the present invention;

[0057] Figure 3 It is a control circuit structure and control strategy framework diagram of the present invention;

[0058] Figure 4 is a control block diagram of the VSG in the embodiment;

[0059] Figure 5 It is a control structure block diagram of the current reference value generator in the present invention;

[0060] Figure 6 It is a block diagram of the positive and negative sequence current control and modulation strategy in the present invention;

[0061] Figure 7 It is the simulation result of the existing VSG controlling the SST output stage;

[0062] Figure 8 This is the simulation result of the output stage of the current balance and reactive power constant VSG control in the embodiment;

[0063] Fig. 9 This is the simulation result of the current balance and active power constant VSG control output stage in the embodiment;

[0064] Fig.10 It is the simulation result of the target switching of the output stage of the VSG control implemented according to the present invention. DETAILED DESCRIPTION

[0065] In order to explain the technical solution of the present invention in detail, further elaboration is given below in conjunction with the accompanying drawings and specific embodiments of the specification.

[0066] The present invention provides a new energy grid-connected control circuit and an SST control method based on a virtual synchronous machine, including providing a new energy grid-connected control circuit based on VSG, and based on the regulation strategy and stability parameter calculation performed by the circuit, providing an SST control method based on a virtual synchronous machine under unbalanced voltage.

[0067] In the development of the power system, in order to adapt to the changes in the characteristics of the power system after a high proportion of new energy is connected and to ensure the power quality requirements, a three-level SST topology structure is used to form the main circuit, which consists of an input stage, an isolation stage, and an output stage. Figure 1 As shown. Each stage can use a different structure. In the present invention, the input stage adopts a half-bridge MMC submodule topology structure, the intermediate isolation stage uses a dual active bridge series resonant converter, and the output stage adopts a T-type three-level inverter structure. The present invention meets most of the grid control requirements through this structural combination. Compared with the existing output stage, which mostly adopts a three-phase two-level structure or a three-phase four-wire structure, the T-type three-level inverter structure is less used for the output stage, and it better meets the control requirements of high-quality new energy grid connection.

[0068] Combination Figure 1 and Figure 2 As shown in the figure, in the main circuit topology of the solid-state transformer, the input stage is connected to the medium and high voltage AC power grid with a half-bridge MMC converter. By increasing or decreasing the number of MMC sub-modules, it can adapt to different voltage levels, reduce the withstand voltage requirements of power switch components, improve the power quality control capability and power density, and achieve unity power factor or constant power factor operation.

[0069] The isolation stage uses a dual active bridge series resonant converter, so that the output DC voltage on the input side is modulated into a high-frequency AC voltage through a single-phase full-bridge inverter circuit, coupled to the secondary side through a high-frequency transformer, and converted into DC by a single-phase full-bridge rectifier circuit, realizing DC voltage conversion and electrical isolation of the input and output stages, allowing power to flow in both directions. Among them, the isolation stage adopts an input series / output parallel structure to achieve input voltage division and output current division, and its low-voltage DC bus provides a DC port for new energy grid connection or load.

[0070] The output stage adopts a T-type three-level inverter, which is an improvement of the diode NPC three-level topology. Its main function is to achieve mid-point clamping of the bridge arm output voltage through the reverse series IGBT. The output stage converts the low-voltage DC output of the isolation stage into a stable three-phase power frequency voltage, which is input into the low-voltage distribution network to supply power to users.

[0071] After the new energy is connected at the isolation level, a control circuit is set at the output level, that is, the new energy grid-connected control circuit based on VSG proposed by the present invention. The installation of the control circuit is as follows: Figure 2 The control logic of the control circuit is as shown. Figure 3 As shown. Based on the control circuit, a control strategy based on a virtual synchronous machine is implemented, and a VSG-SST control strategy taking into account double-sequence current is implemented. By measuring the parameter value of the low-voltage distribution network, the controller controls and adjusts the positive and negative sequence components of the output current of the SST (fixed transformer) output stage, thereby adjusting the SVPWM modulation signal in real time and adding it to the 12 IGBTs of the output stage, thereby changing the various parameters of the low-voltage distribution network to meet the power quality requirements. In view of the current imbalance, power oscillation and current quality problems caused by the SST output stage caused by the traditional VSG (synchronous virtual machine) control under the condition of three-phase unbalanced voltage in the low-voltage distribution network, an improved T-type three-level output stage joint control strategy based on VSG is proposed, namely "SST control strategy based on virtual synchronous machine taking into account double-sequence current", which improves the output grid-connected current and power quality of the SST output stage. The present invention obtains the positive sequence current reference value by using the output voltage reference value of the VSG master control strategy, and calculates the negative sequence current reference value by the instantaneous power theory, so as to realize current balance control and active / reactive constant control under unbalanced voltage.

[0072] Combination Figure 3 The control circuit is mainly composed of a traditional VSG controller, a current reference value generator, a PI-based positive and negative sequence current regulator, and a midpoint potential balance controller. However, the T-type three-level inverter structure in the present invention is used for the output stage. Compared with the three-phase two-level structure or three-phase four-wire structure that is mostly used in the existing structure, it has a stronger ability to suppress harmonics, but it will introduce a new problem, that is, it brings about midpoint imbalance. The present invention sets a midpoint potential balance link to solve this problem.

[0073] Based on the above control circuit, the present invention provides a SST control method based on a virtual synchronous machine under unbalanced voltage, the method includes adopting a positive and negative sequence separation method based on a double second-order generalized integrator, with good filtering function, to achieve good extraction effect of positive and negative sequence components of grid voltage and output current. The transfer function of the second-order generalized integrator is as shown in formula (1).

[0074]

[0075] Where: D(s) and E(s) are bandpass and low-pass filters respectively; ω r is the resonant frequency; k is the damping coefficient.

[0076] For the VSG controller, the circuit structure of the VSG controller is as follows Figure 4 As shown, for Figure 4 , there is the following parameter correspondence:

[0077] U dc * U represents the reference value of the low-voltage DC bus voltage; the reference value is the ideal expected value, which is set manually in the simulation of the embodiment. dcL Indicates the actual value of the DC bus voltage, which is obtained through on-site measurement. ref Indicates the reference value of active power; Q ref Indicates the reference value of reactive power; P e is the instantaneous active power output; the actual value is obtained by measurement and calculation: P e =U ik_a I ik_a +U ik_b I ik_b +U ik_c I ik_c .Q e Indicates the instantaneous reactive power output, the actual value, obtained by measurement and calculation:

[0078]

[0079] K q , D, and J are the integral gain coefficient, virtual damping coefficient, and inertia coefficient, respectively; these three parameters are set to the following values ​​in this embodiment based on debugging and experience: gain coefficient = 0.14, virtual damping coefficient = 7, and inertia coefficient = 0.1. * Indicates the rated angular velocity, that is, the angular velocity of the power grid; (50Hz industrial frequency AC, the angular velocity is ω * =2πf=314rads -1 , i.e. 314 rad / s); E and θ are the reference amplitude and phase angle of the VSG output voltage respectively; the voltage reference u * kIt can be written in the form of amplitude and phase angle of E∠θ. Uo represents the no-load electromotive force of VSG, and the given setting value is 311V; * k is the output voltage reference value of the VSG controller.

[0080] The present invention improves the structure of the VSG control circuit. Since the SST output stage outputs a stable value of the voltage u ik The VSG module output voltage reference value u k * Approximately equal, the relationship between the voltage reference value of the VSG control module and the reference value of the positive sequence voltage and current components of the power grid can be obtained from formula (2), as shown in formula (3):

[0081]

[0082] Where: u id 、u iq and i id 、i iq are the d-axis and q-axis components of the output voltage and current of the output stage; u gd 、u gq is the dq axis component of the grid voltage

[0083]

[0084] Where: u kd 、u kq are the components of the voltage reference value of the VSG control module under the d and q axes respectively. Convert equation (3) into the positive sequence current reference value expression and transform it to the complex frequency domain through the Laplace final value theorem, as shown in equation (4):

[0085]

[0086] Where: s is the differential operator; R is the total resistance from the inverter to the grid.

[0087] According to Laplace's final value theorem, equation (4) can be simplified and converted to the time domain as follows:

[0088]

[0089] The negative sequence current reference value is set to zero, and the double frequency power fluctuation component can be obtained according to equation (5), as shown in equation (6).

[0090]

[0091] From equation (6), we can see that the double frequency power fluctuation component is not completely eliminated, but the power oscillation is greatly weakened. Therefore, the current balance control goal is achieved at the expense of active and reactive power oscillation.

[0092] For the current reference value generator, the current reference value calculation module is the core of the control strategy, and the VSG master control strategy outputs the voltage reference value u * k The positive sequence current reference value is obtained, and the negative sequence current reference value is calculated by instantaneous power theory to achieve current balance control and active / reactive power constant control under unbalanced voltage.

[0093] The specific control strategy includes three aspects: current balance control, active power constant control and reactive power constant control, including the following derivation and calculation process:

[0094] (1) Current balance control

[0095] In order to obtain high-quality grid-connected current under the condition of unbalanced grid voltage, the current injected into the grid by the SST output stage is only allowed to contain positive sequence components. Therefore, the negative sequence current reference value needs to be set to zero, and the positive sequence current reference value is calculated by the voltage reference value generated by the VSG control module and the output stage positive sequence circuit equation, such as Figure 5 shown.

[0096] Derivation and calculation process: VSG control module voltage reference value u * k With the grid positive sequence voltage u + gd 、Current i + i The relationship between the component reference values ​​is as follows:

[0097]

[0098] Where: u* kd 、*u kq They are respectively the voltage reference value u of the VSG control module * k Components under the d and q axes after Park transformation; i + id 、i + iq They are the grid positive sequence current i + i Components under the d and q axes after Park transformation.

[0099] Convert equation (6) into the positive sequence current reference value expression and transform it into the complex frequency domain through Laplace transformation, as shown in equation (7).

[0100]

[0101] Where: s is the differential operator; R represents the total resistance from the inverter to the low-voltage distribution network.

[0102] According to Laplace's final value theorem, equation (7) can be simplified and converted to the time domain, as shown in equation (8).

[0103]

[0104] Conclusion: The double frequency power fluctuation component is not completely eliminated, but the power oscillation is greatly weakened. The current balance control goal is achieved at the expense of active and reactive power oscillation.

[0105] (2) Active power constant control

[0106] When SST is used as the grid-connected interface of photovoltaic or energy storage system, the power is controlled constantly, so that the double frequency active power fluctuation component P in equation (9) is s2 and P c2 is zero, only the average active power component is generated, and equation (10) is obtained.

[0107]

[0108] Where: u g + 、u g - They are low voltage distribution network voltage U g + and U g - The instantaneous value of i + 、i i - They are the output current I i + and I i - The instantaneous value of .

[0109]

[0110] It can be seen from formula (10) that the negative sequence current reference value of active power constant control and the positive sequence current reference value of current balance control are related to each other. The negative sequence current reference value is calculated using formula (8), as shown in formula (12).

[0111]

[0112] Conclusion: The negative sequence current component is not eliminated, which leads to larger reactive power oscillation in active power constant control than in current balance control, and the grid-connected current of the SST output stage is unbalanced.

[0113] (3) Reactive power constant control

[0114] When large-capacity power generation equipment is connected to the grid through SST and provides stable reactive power support, the main thing is to make the double frequency reactive power fluctuation component Q in equation (9)s2 and Q c2 is zero, and only the average reactive power component is output, as shown in formula (13).

[0115]

[0116] It can be seen from formula (13) that the negative sequence current reference value of reactive power constant control and the positive sequence current reference value of current balance control are related to each other. The negative sequence current reference value is calculated using formula (8), as shown in formula (14).

[0117]

[0118] Conclusion: The negative sequence current component is not eliminated, which leads to greater active power oscillation in reactive power constant control than in current balance control, and unbalanced grid-connected current in the SST output stage.

[0119] (4) PI-based positive and negative sequence current regulator, modulation wave generator, and midpoint potential balance controller

[0120] It can be seen from equations (8), (10) and (13) that the positive sequence current reference value in current balance control is related to the negative sequence current reference value in active and reactive power constant control, and the negative sequence current reference values ​​in equations (12) and (14) are opposite to each other. Therefore, a current reference generator is designed, and a flexible coefficient H is introduced. Under the condition of unbalanced grid voltage, the control coefficient H is used to select the control target to meet different requirements, such as Figure 6 shown.

[0121] When H=0, current balance control is achieved; when H=1, active power constant control is achieved; when H=-1, reactive power constant control is achieved.

[0122] Figure 6 In the block diagram, formula (15) and formula (16) are:

[0123]

[0124] Where: v0 is the zero-sequence component; C is the capacitive reactance of the voltage divider capacitor. The two voltage divider capacitors are the same, that is, C = C1 = C2; i npav is the midpoint current; i np0 is the midpoint current after superimposing the zero-sequence component; f s is the carrier frequency, which is 20000Hz.

[0125] In order to achieve the regulation of output current and quickly track the positive and negative sequence current reference values, a PI-based positive and negative sequence current regulation module is designed, as shown in formula (17).

[0126]

[0127] Where: e * is the modulation voltage; Kp and Ki are the proportional-integral coefficients.

[0128] The three-phase horn modulation wave is generated by the three-level SVPWM modulation strategy, which can achieve a certain balance effect on the midpoint potential. However, since the current flows through the midpoint of the two voltage-dividing capacitors on the DC side of the T-type three-level output stage, the capacitor voltage will fluctuate or offset, affecting the output power quality. Therefore, the midpoint potential balance method of zero-sequence component injection is adopted, and the zero-sequence component is injected to participate in the modulation, which effectively realizes the voltage balance of the two capacitors of the T-type three-level output stage, and generates the zero-sequence component as shown in the following formula (18).

[0129]

[0130] Where: v0 is the zero-sequence component; C is the capacitive reactance of the voltage divider capacitor; i npav is the midpoint current; i np0 is the midpoint current after superimposing the zero-sequence component; f s is the carrier frequency.

[0131] Example 1

[0132] To verify the effectiveness of the control strategy, Figure 2 The topological structure is constructed by building an SST system simulation model on the Matlab / simulink software platform for simulation analysis.

[0133] The effective value of the input stage voltage is 10 kV, the high-voltage DC bus voltage is 16 kV, the low-voltage DC bus voltage is 750 V, the output stage AC port grid-connected voltage is 380 V, and the SST output stage simulation parameter settings are shown in Table 1.

[0134] Table 1 Output stage parameter settings

[0135]

[0136] Operation condition setting: During the period from 0s to 0.3s, the three-phase grid voltage is balanced; during the period from 0.3s to 0.9s, the three-phase grid voltage is unbalanced - the voltage of phase A increases by 20%, the voltage of phase B remains unchanged, and the voltage of phase C decreases by 20%; at 0.6s, the midpoint potential balance control is added. The operation results are as follows Figure 7 shown.

[0137] Depend on Figure 7 It can be seen that: in the period from 0s to 0.3s, when the grid voltage is balanced, the SST output stage outputs a three-phase balanced grid-connected current with an amplitude of 34.8A, and the low-voltage DC bus, output active power and reactive power remain stable;

[0138] During the period of 0.3s to 0.9s, when the grid voltage is unbalanced, the three-phase unbalance of the grid-connected current output by the SST output stage occurs, with a maximum peak value of 38.5A and a harmonic content of 6.92%. The low-voltage DC bus, output active power and reactive power experience double frequency fluctuations, and the low-voltage DC bus voltage fluctuation amplitude is 2.5V, the active power fluctuation amplitude is 5.2kW, and the reactive power fluctuation amplitude is 6.6kvar.

[0139] 0.6s before adding the midpoint potential balance control, the output stage DC side voltage divider capacitor voltage U C1 and U C2 The fluctuation peaks are 385V and 365V respectively; after adding the midpoint potential balance control at 0.6s, the voltages of the two voltage-dividing capacitors are kept balanced at the target potential of 375V.

[0140] According to the control described in the present invention, the operating condition switching includes the following three aspects:

[0141] (1) Current balance control to reactive power constant control

[0142] The operating condition settings are as follows: from 0s to 0.3s, the grid voltage is balanced; from 0.3s to 0.9s, the three-phase grid voltage is unbalanced; before 0.6s, the current balance is the goal; after 0.6s, the reactive power is constant. Figure 8 shown.

[0143] Depend on Figure 8 It can be seen that: when the three-phase grid voltage is unbalanced, the SST output stage current under the current balance VSG control maintains three-phase balance, with an amplitude of 35A and a harmonic content of 1.49%. The output active and reactive power fluctuations are reduced, but the low-voltage DC bus voltage and output active / reactive power still have double frequency fluctuations, and the fluctuation amplitudes of the three are 1.5V, 3.9kW, and 3.7kvar respectively.

[0144] Under the reactive constant VSG control, the output reactive power fluctuation amplitude of the SST output stage is reduced to 0.49kvar, but the three-phase imbalance of the grid-connected current, the low-voltage DC bus voltage and the output active power fluctuations become larger. The peak value of the grid-connected current fluctuation is 38.5A, the harmonic content is 0.58%, and the low-voltage DC bus and active power fluctuation amplitudes are 3.1V and 7.6kW respectively.

[0145] The above results show that VSG control with current balance as the goal can ensure the balance of output grid-connected current, reduce harmonic content and output active / reactive power; VSG control with constant reactive power as the goal can achieve constant output reactive power, but will lead to greater current imbalance and active power fluctuation.

[0146] (2) Current balance control to active power constant control

[0147] The operating condition settings are as follows: from 0s to 0.3s, the grid voltage is balanced; from 0.3s to 0.9s, the three-phase grid voltage is unbalanced; before 0.6s, the current balance is the goal; after 0.6s, the active power is constant. The operating results are as follows Fig. 9 shown.

[0148] Depend on Fig. 9 It can be seen that: when the three-phase voltage unbalanced condition occurs, under the active constant VSG control, the SST output stage output active power fluctuation amplitude is reduced to 0.5kW, and the low-voltage DC bus voltage fluctuation amplitude is reduced to 0.4V, but the output grid-connected current is unbalanced in three phases, with a fluctuation peak of 37.5A and a harmonic content of 0.65%. The output reactive power fluctuation becomes larger, with a fluctuation amplitude of 7.54kvar.

[0149] The above results show that VSG control with the goal of constant active power can achieve constant output active power, but it leads to greater current imbalance and reactive power fluctuation.

[0150] (3) Flexible coefficient switching control target

[0151] Operation condition settings: 0s to 0.3s, the grid voltage is balanced; 0.3s to 0.9s, the grid voltage is unbalanced; 0s to 0.5s, the current balance is the goal; 0.5s to 0.7s, the reactive power is constant as the goal; 0.7s to 0.9s, the active power is constant as the goal. The operation results are as follows: Fig.10 shown.

[0152] Depend on Fig.10 It can be seen that:

[0153] Under the condition of three-phase unbalanced grid voltage, the midpoint potential balance algorithm effectively controls the voltage U of the voltage divider capacitor. C1 and U C2 balance;

[0154] When H = 0, VSG control with current balance as the goal is realized, and the SST output stage outputs a three-phase balanced grid-connected current with an amplitude of 35A;

[0155] When H=1, VSG control with constant reactive power as the goal is realized, and the output reactive power fluctuation amplitude of the SST output stage is 0.49kvar;

[0156] When H=-1, VSG control with the goal of constant active power is achieved, and the active power fluctuation of the SST output stage is 0.5kW.

[0157] The above results show that the SST output stage based on the improved VSG can achieve stable operation with different control objectives, and can smoothly switch between three different control objectives by changing the flexible parameters.

Claims

1. A VSG-based new energy grid-connected control circuit, characterized in that: In the circuit, the input stage includes a half-bridge MMC submodule topology structure, the intermediate isolation stage includes a dual active bridge series resonant converter, and the output stage includes a T-type three-level inverter structure; the new energy grid is connected to the isolation stage of the three-level SST main circuit topology structure composed of the input stage, the isolation stage and the output stage, including an SST control circuit based on a virtual synchronous machine under an unbalanced voltage for stable grid regulation, and the SST control circuit includes a VSG controller, a current reference value generator, a positive and negative sequence current regulator based on PI, and a midpoint potential balance controller; In the SST control circuit, the VSG controller provides a voltage reference value for the current reference generator, and the modulation wave generator controls the current injected into the grid by the output stage to only contain the positive sequence component under the coordinated output of the PI-based positive and negative sequence current regulator and the midpoint potential balance controller; The SST control circuit based on the virtual synchronous machine adopts the following control method, which is used for current balance control, active power constant control and reactive power constant control after the new energy is connected to the grid. The control method includes extracting the positive and negative sequence components of the output current based on the positive and negative sequence separation of the double second-order generalized integrator, and includes the following calculation and modulation process: (1) Based on the VSG controller output voltage reference value u * k , the negative sequence current reference value is calculated by instantaneous active power and reactive power, so that the current injected into the grid by the SST control output stage contains only positive sequence components; (2) Based on current balance control, calculate and solve the output voltage reference value u of the VSG controller * k With the grid positive sequence voltage u + g , positive sequence current i + i The relationship between the component reference values ​​can be expressed as follows: Where: u* kd 、*u kq They are the VSG controller output voltage reference value u * k The components under the d and q axes after Park transformation, They are the grid positive sequence current i + i Component i under the d and q axes after Park transformation + id 、i + iq The reference value, R1 represents the output stage filter resistor, L s It is the filter inductor of the VSG control module; (3) Considering the correlation between the negative sequence current reference value of the active power constant control and the positive sequence current reference value of the current balance control in the active power constant control, the negative sequence current reference value is calculated, and the following expression exists: Where: u g + 、u g - They are low voltage distribution network voltage U g + and U g - The instantaneous value of the Park change on the d-axis and q-axis is u + gd 、u + gq and u - gd 、u - gq； ;i i + 、i i - They are the output current I i + and I i - The instantaneous value of They are the grid negative sequence current i - i Component i under the d and q axes after Park transformation - id 、i - iq The baseline value of In the reactive power constant control, the positive sequence current reference value in the current balance control is correlated with the negative sequence current reference value in the active power and reactive power constant control, and is the inverse of the negative sequence current reference value in the active power constant control; (4) A current reference generator is set to control the negative sequence current reference value output under active power constant control and reactive power constant control, and a flexible coefficient H is introduced. Under the condition of unbalanced grid voltage, the control coefficient H is used to realize grid stability control, including the following control calculations: When H=0, current balance control is achieved; when H=1, active constant control is achieved; when H=-1, reactive constant control is achieved, and its expression is as follows: Where R represents the total resistance from the inverter to the grid; (5) By setting a PI-based positive and negative sequence current regulator to adjust the output current, the positive and negative sequence current reference values ​​can be quickly tracked. The following expression is used to calculate the adjustment value: Where: e * is the modulation voltage; Kp, K i is the proportional-integral coefficient; (6) The voltage balance between the two capacitors of the T-type three-level output stage is performed by a midpoint potential balance controller, wherein the zero-sequence component is generated as follows: Where: v0 is the zero-sequence component, C is the capacitive reactance of the voltage divider capacitor, i npav is the midpoint current, i np0 is the midpoint current after superimposing the zero-sequence component, f s is the carrier frequency, U C1 and U C2 is the voltage of the voltage-dividing capacitor on the DC side of the output stage; 2. The VSG-based new energy grid-connected control circuit according to claim 1, characterized in that: The input stage is connected to the medium and high voltage AC grid with a half-bridge MMC converter, and can adapt to different voltage levels by increasing or decreasing the number of MMC submodules; The isolation stage includes a dual active bridge series resonant converter, which modulates the output DC voltage on the input side into a high-frequency AC voltage through a single-phase full-bridge inverter circuit, couples to the secondary side through a high-frequency transformer, and converts it into DC by a single-phase full-bridge rectifier circuit, thereby realizing DC voltage conversion and electrical isolation of the input and output stages, allowing power to flow in both directions. The isolation stage adopts an input series / output parallel structure to realize input voltage division and output current division, and its low-voltage DC bus provides a DC port for new energy grid connection or load. The output stage includes a T-type three-level inverter, which implements mid-point clamping of the bridge arm output voltage through reverse-series IGBTs, and converts the low-voltage DC output of the isolation stage into a stable three-phase industrial frequency voltage, which is input into the low-voltage distribution network to supply power to users.

3. The VSG-based new energy grid-connected control circuit according to claim 1 is characterized in that: In the positive and negative sequence separation based on the double second-order generalized integrator, the second-order generalized integrator transfer function is as follows: Where: D(s) and E(s) are bandpass and low-pass filters respectively; ω r is the resonant frequency; k is the damping coefficient.

4. The VSG-based new energy grid-connected control circuit according to claim 1, characterized in that: In the method, the current injected into the grid by the SST output stage is only allowed to contain positive sequence components, the negative sequence current reference value is set to zero, the positive sequence current reference value is calculated by the voltage reference value generated by the VSG control module and the output stage positive sequence circuit equation, and the SST output stage output voltage stability value u ik The VSG module output voltage reference value u k * Approximately equal, the relationship between the voltage reference value of the VSG control module and the reference value of the positive sequence voltage and current components of the power grid can be obtained as follows: Where: u id 、u iq and i id 、i iq are the d-axis and q-axis components of the output voltage and current of the output stage; u gd 、u gq is the dq-axis component of the grid voltage; Where: u kd 、u kq They are the components of the VSG controller voltage reference value on the d and q axes respectively.

5. The VSG-based new energy grid-connected control circuit according to claim 1 is characterized in that: According to the calculation relationship of the components of the VSG controller voltage reference value under the d and q axes, it is converted into the positive sequence current reference value expression and transformed into the complex frequency domain through the Laplace final value theorem to obtain the following relationship: Where: s is the differential operator; R is the total resistance from the inverter to the grid; According to Laplace's final value theorem, the simplified and converted expression to the time domain is as follows: When the negative sequence current reference value is set to zero, the double frequency power fluctuation component is as follows: From the above, it can be seen that the double frequency power fluctuation component is not completely eliminated; weakening power oscillation and achieving the current balance control goal is at the expense of active and reactive power oscillation.

6. The VSG-based new energy grid-connected control circuit according to claim 1, characterized in that: In the active power constant control, when SST is used as the grid-connected interface of photovoltaic or energy storage system, the active power needs to be constantly controlled so that the double frequency active power fluctuation component is zero and only the average active power component is generated. The expression is as follows: It can be seen from the above formula that the negative sequence current reference value of the active power constant control and the positive sequence current reference value of the current balance control are related to each other.

7. The VSG-based new energy grid-connected control circuit according to claim 1, characterized in that: In the control method for constant reactive power, when large-capacity power generation equipment is connected to the grid through SST and provides stable reactive power support, the double frequency reactive power fluctuation component is made zero. The expression for the output average reactive power component is as follows: It can be seen from the above formula that the negative sequence current reference value of reactive power constant control and the positive sequence current reference value of current balance control are related to each other.

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