Pre-synchronization starting control strategy of photovoltaic network-forming converter

Abstract

The invention belongs to the technical field of photovoltaic network construction, and discloses a pre-synchronization start control strategy of a photovoltaic network construction type converter. According to the method, an energy balance relation between a photovoltaic array and output power of an inverter of a virtual synchronous machine (VSG) is used for replacing charging and discharging of an energy storage battery on a system, and a photovoltaic virtual synchronous machine (PV-VSG) pre-synchronization slow start control strategy is designed. The strategy can enable the photovoltaic inverter to have the capability of independently supporting the bus voltage and frequency, and can avoid the grid connection failure caused by the destructive influence of the system power shortage caused by external disturbance on the DC bus voltage at the starting moment of the energy-storage-free photovoltaic grid connection system. According to the strategy, PV-VSG network construction control in a power reserve mode is considered, and smooth grid connection of the photovoltaic on-load system is realized. In the step of stabilizing the voltage of the direct current bus, sliding mode control is adopted to replace traditional PI control, and the robustness of a network construction system is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of photovoltaic grid construction and discloses a pre-synchronous starting control strategy for a pure photovoltaic grid construction type converter. Background Art

[0002] Active support for grid voltage and frequency based on photovoltaic virtual synchronous mechanism network technology can avoid the defects of photovoltaic inverters controlled by traditional power electronics in the case of lack of damping and inertia support when connected to the grid. Existing photovoltaic grid inverters usually connect energy storage batteries in parallel on the DC bus side to consume the load power to the rated value, stabilize the DC bus voltage through the charging and discharging of the energy storage battery, and provide a stable operating environment for the entire grid system. The existence of energy storage batteries not only increases the operating cost, but also due to the limited storage capacity, it cannot provide high-quality voltage synchronization signals for the inverter when the battery is overcharged or over-discharged, which will bring more control problems and safety hazards.

[0003] To ensure maximum output power, photovoltaic arrays usually work in maximum power tracking mode, which automatically detects the DC voltage and output current of the main circuit, calculates the output power of the solar array, and tracks the maximum power point. When there is a shortage caused by power imbalance between the source side and the load side, the photovoltaic cells outputting at maximum power cannot effectively adjust the grid voltage and frequency, thus affecting the stability of the grid. This situation is particularly obvious at the moment of grid connection startup.

[0004] Since the output power of the photovoltaic array fluctuates greatly, there is a lag between it and the output power of the virtual synchronous generator. The power difference between the two will cause the DC bus voltage to fluctuate violently. Through extensive reading of literature, it is found that the existing solution is usually to directly connect the photovoltaic grid to the grid at the moment of grid-connected startup, which is equivalent to transferring the power to be regulated to the external grid. Ignoring the ability of the photovoltaic system to regulate power itself, a pre-synchronous startup algorithm combined with the active power-voltage output curve of the photovoltaic array is designed to make the working point of the photovoltaic array lower than the maximum power point, realizing power reserve while considering the influence of the load before grid connection. In addition, in order to further stabilize the bus voltage on the DC side of the inverter, sliding mode control is used instead of the traditional PI controller, so that the DC bus voltage fluctuates only on the sliding mode surface, improving the robustness and accuracy of the system. Summary of the invention

[0005] In order to overcome the problems existing in the related art, the present invention discloses and implements a pre-synchronous starting control strategy for a photovoltaic grid-connected converter.

[0006] The technical solution adopted by the present invention is:

[0007] A pre-synchronous start-up control strategy for a photovoltaic grid-connected converter, characterized by comprising the following steps:

[0008] Step 1: Establish a mathematical model of photovoltaic virtual synchronous generator (PV-VSG);

[0009] Step 2: Design the PV-VSG pre-synchronization slow start control strategy;

[0010] Step 3: Stabilize the DC bus voltage based on sliding mode control.

[0011] In step 1, a control relationship between the photovoltaic array and the virtual synchronous machine is established. The specific method is as follows:

[0012] The bipolar photovoltaic array adopts frequency droop control. When the VSG output frequency is not equal to the reference frequency, the reference power input to the photovoltaic array is:

[0013] P ref =P set +K f (f ref -f)(1);

[0014] In formula (1), P ref is the reference power output by the photovoltaic array, P set is the set PV array output power, which should be less than the maximum output power of the PV cell, K f is the frequency droop coefficient (positive value), f ref is the rated reference frequency of the power grid, and f is the actual frequency of the power grid.

[0015] In step 2, a PV-VSG pre-synchronization control algorithm is designed, and the specific method is as follows:

[0016] 1) The essence of active frequency regulation and reactive voltage regulation of virtual synchronous machine is droop control. The rotor mechanical equation of primary frequency regulation is:

[0017]

[0018] In formula (2), J is the virtual moment of inertia, D is the virtual damping coefficient, K p is the active frequency droop coefficient, P m is the virtual mechanical power, P N is the rated input active power, P e is the actual output power, w is the actual output angular frequency, w0 is the rated input angular frequency, θ is the actual output power angle of VSG, and t is the time;

[0019] 2) The control equation of reactive power and voltage droop of virtual synchronous machine is:

[0020]

[0021] In formula (3), K q is the reactive voltage integral coefficient, Ku is the reactive voltage droop coefficient, Q N is the rated input reactive power, Q is the actual output reactive power, U N is the rated input phase voltage, U is the actual output phase voltage, and E is the VSG output electromotive force amplitude.

[0022] In step 2, a PV-VSG pre-synchronization control algorithm is designed, and the specific method is as follows:

[0023] 1) The essence of pre-synchronization control is the secondary frequency and voltage regulation of the power grid system. However, the traditional pre-synchronization control relies on directly adjusting the amplitude and phase of the grid connection point voltage to reduce the deviation on both sides, which causes the VSG to be in a drooping working state all the time, causing interference to the stability of the power grid. Before the pre-synchronization control is started, the voltage difference Δu between the grid connection point voltage and the grid voltage to be connected to the grid side (taking phase a as an example) is:

[0024]

[0025] In formula (4), u ga and u a are the instantaneous values ​​of phase a voltage of the power grid and the grid connection point, U g and U a are the effective values ​​of the voltage of the power grid and the grid connection point, respectively, ga and w a are the a-phase angular frequency values ​​of the power grid and the grid connection point, respectively. and are the phase values ​​of phase a of the power grid and the grid connection point, U is the effective value of the actual output voltage of the grid connection point, and w N is the rated angular frequency value;

[0026] 2) According to formula (4), the power fluctuation generated during grid connection is mainly caused by the phase and frequency of the voltage on both sides of the grid connection point. Therefore, the VSG pre-synchronization control is divided into three parts: amplitude pre-synchronization, phase pre-synchronization and frequency pre-synchronization. The corresponding control equations are:

[0027]

[0028] In formula (5), K p is the proportional coefficient of the PI controller, K i is the integral coefficient of the PI controller, U, U N and U g are the actual output voltage of the grid connection point, the rated voltage of the grid connection point and the effective value of the grid voltage, and are the actual output phase angle of the grid-connected point, the rated phase angle of the grid-connected point and the grid phase angle, respectively. N and f gThey are the actual output frequency of the grid-connected point, the rated frequency of the grid-connected point and the grid frequency.

[0029] In step 2, a PV-VSG slow start control algorithm is designed, and the specific method is as follows:

[0030] 1) To achieve the purpose of slow start and grid connection, the power emitted by the photovoltaic array is always greater than the output power of the VSG, avoiding excessive power shortage caused by hysteresis, and also suppressing the impact of excessive power changes on the system during grid connection startup, reducing component damage, and designing a slow start algorithm flow chart; the algorithm sets three different operating states according to the system operation time, state 0 (state = 0) is the primary frequency and voltage regulation stage; state 1 (state = 1) is the secondary frequency and voltage regulation stage, that is, pre-synchronization control is turned on; state 2 (state = 2) is the grid connection stage, and pre-synchronization control is turned off. When state 2 is completed, the operating state of state 0 is restored;

[0031] 2) At the beginning of state 2, the VSG output power gradually increases from the power required by the load to the set photovoltaic cell output power, t is the rising slope, and the changes in active and reactive power during the slow start grid connection process are:

[0032]

[0033] In formula (6), P pv is the active power output of the photovoltaic array, Q ref is a given reactive power reference value (assuming that the PV array does not generate reactive power), δP is the active power integral in the VSG pre-synchronization stage, δQ is the reactive power integral in the VSG pre-synchronization stage, and P N is the VSG rated active power, Q N is the rated reactive power of VSG.

[0034] In step 3, stabilizing the DC bus voltage based on a sliding mode controller specifically includes the following steps:

[0035] Step 3.1: Establish the mathematical model of Boost circuit;

[0036] The current i on the output side of the Boost converter out for:

[0037] i out =P pv / u dc (7);

[0038] In formula (7), u dc is the DC bus voltage output by the Boost converter;

[0039] DC-AC inverter side input current i in for:

[0040] i in =P dc / u dc =P e / u dc (8);

[0041] In formula (8), P dc is the output power of the Boost converter;

[0042] Under ideal conditions, the actual value of the DC bus voltage is equal to the reference value, that is, u dc =u ref , flows through the DC bus side capacitor C dc The current is 0, so:

[0043]

[0044] Substituting formula (7) and (8) into formula (9), we can get the state variable u to be controlled: dc The derivative of is:

[0045]

[0046] Step 3.2: Calculate the control law of the DC bus voltage;

[0047] DC bus voltage error e dc for:

[0048] e dc =u dc -u dcref (11);

[0049] Define the proportional-integral sliding surface S dc for:

[0050] S dc =(k s1 +k s2 / s)e dc (12);

[0051] In formula (12), k s1 is a positive proportionality coefficient; k s2 is a positive integral coefficient;

[0052] The exponential adaptive reaching law is helpful to reduce the system chattering, and the derivative of the sliding surface It can be expressed as:

[0053]

[0054] In formula (13), η, ξ, and γ are all positive coefficients of the control rate;

[0055] Substituting formula (12) into formula (13), the control law of the DC bus voltage can be obtained as follows:

[0056]

[0057] In formula (14), is the input power command of VSG;

[0058] Step 3.3: Proof of stability;

[0059] Choose to build the Lyapunov function V(S dc ) proves the stability of the control strategy as follows:

[0060]

[0061] Derivative of formula (15) yields:

[0062]

[0063] In formula (16), since η, ξ, and γ are all positive values, That is, the designed control strategy is stable.

[0064] The present invention provides a pre-synchronous start-up control strategy for a photovoltaic grid-connected converter, and the technical effects are as follows:

[0065] 1) In view of the unpredictability of external environmental changes, a flexible control algorithm for the photovoltaic array is designed to make its operating point lower than the maximum power point, realize power reserve and take into account the impact of the load before grid connection, avoiding the DC bus voltage collapse caused by power imbalance.

[0066] 2) A pre-synchronization grid-connected slow start algorithm combined with the photovoltaic array was designed to solve the problem that the photovoltaic array output power lags behind the VSG output power at the moment of grid connection, and to ensure the stability of the grid-connected voltage under external interference.

[0067] 3) The traditional DC bus voltage PI controller is replaced by a sliding mode controller. The existence of the sliding mode surface improves the system robustness, reduces the DC bus voltage regulation rate, and effectively suppresses the output voltage ripple. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] The accompanying drawings herein are incorporated in and constitute a part of the specification, illustrate embodiments consistent with the present disclosure, and together with the description, serve to explain the principles of the present disclosure;

[0069] Figure 1 An overall control diagram provided for an embodiment of the present invention;

[0070] Figure 2(a) is a photovoltaic array control diagram provided by an embodiment of the present invention;

[0071] Figure 2 (b) is a flow chart of a photovoltaic array control algorithm provided by an embodiment of the present invention;

[0072] Figure 3 (a) is a VSG pre-synchronization control diagram provided by an embodiment of the present invention;

[0073] Figure 3 (b) is a flow chart of a VSG pre-synchronization slow start control algorithm provided by an embodiment of the present invention;

[0074] Figure 4 (a) is a DC bus voltage sliding mode control strategy diagram provided by an embodiment of the present invention;

[0075] Figure 4 (b) is a comparison diagram of DC bus voltage values ​​under three conditions provided by an embodiment of the present invention;

[0076] Figure 5 (a) is the active power output of the photovoltaic array provided in the embodiment of the present invention (P pv / W), VSG output active power (P e / W) and VSG output reactive power (Q e / Var) map;

[0077] Figure 5 (b) is the DC bus voltage (U dc ) and the effective value of the grid voltage (U a )picture;

[0078] Figure 5 (c) is a frequency variation curve diagram provided by an embodiment of the present invention; Figure 5 (d) is the grid voltage (u) provided in the embodiment of the present invention g ) and VSG output voltage (u a ) Single-phase instantaneous value diagram; Figure 6 An operating parameter diagram provided for an embodiment of the present invention. DETAILED DESCRIPTION

[0079] The present invention is described in detail below in conjunction with the accompanying drawings so that those skilled in the art can better understand the present invention and implement it, but this is not intended to limit the scope of protection of the present application.

[0080] Figure 1 The photovoltaic virtual synchronous machine pre-synchronization control strategy diagram provided for the embodiment of the present invention includes the following technical requirements and is the overall topology structure of the present invention.

[0081] Figure 2 (a) is a control diagram of a photovoltaic array provided in an embodiment of the present invention, which adopts dual-mode control, with the photovoltaic array output power always being equal to the VSG output power as the core, wherein mode 1 (S pv1 =0, S pv2 =P e ) is the off-grid mode before grid connection, which only considers the constant power and constant frequency control of local loads, and the output power of photovoltaic cells is the active power required by the load; Mode 2 (S pv1 =dP,S pv2 =P set ) is the frequency modulation control after grid connection, and the output power of the photovoltaic cell is the preset output power value of the photovoltaic array;

[0082] Step 1: Establish a mathematical model of photovoltaic virtual synchronous generator (PV-VSG);

[0083] In step 1, the bipolar PV array adopts frequency droop control. When the VSG output frequency is not equal to the reference frequency, the reference power input to the PV array is:

[0084] P ref =P set +K f (f ref -f)(1);

[0085] In formula (1), P ref is the reference power output by the photovoltaic array, P set is the set PV array output power, which should be less than the maximum output power of the PV cell, K f is the frequency droop coefficient (positive value), f ref is the rated reference frequency of the power grid, and f is the actual frequency of the power grid;

[0086] Figure 2 (b) is a flow chart of a photovoltaic array control algorithm provided in an embodiment of the present invention. A voltage control algorithm is designed based on the disturbance observation method in combination with the photovoltaic array PU curve. When the output power of the photovoltaic cell is not equal to the set reference power, the direction of change of the power and voltage at the current moment is determined by sampling to determine whether the working point is on the left or right side of the PU curve. Then, the voltage change at the next moment is determined by determining the magnitude relationship between the current power and the reference power. This process is repeated to obtain stable output power and output voltage.

[0087] In step 1, a mathematical model of a virtual synchronous machine is established. The specific method is as follows:

[0088] 1) The essence of active frequency regulation and reactive voltage regulation of virtual synchronous machine is droop control. The rotor mechanical equation of primary frequency regulation is:

[0089]

[0090] In formula (2), J is the virtual moment of inertia, D is the virtual damping coefficient, K p is the active frequency droop coefficient, P m is the virtual mechanical power, P N is the rated input active power, P e is the actual output power, w is the actual output angular frequency, w0 is the rated input angular frequency, θ is the actual output power angle of VSG, and t is the time;

[0091] 2) The control equation of reactive power and voltage droop of virtual synchronous machine is:

[0092]

[0093] In formula (3), K q is the reactive voltage integral coefficient, K u is the reactive voltage droop coefficient, Q N is the rated input reactive power, Q is the actual output reactive power, U N is the rated input phase voltage, U is the actual output phase voltage, and E is the VSG output electromotive force amplitude.

[0094] Step 2: Design the PV-VSG pre-synchronization slow start control strategy;

[0095] Figure 3 (a) is a VSG pre-synchronization control diagram provided by an embodiment of the present invention. In step 2, a PV-VSG pre-synchronization control algorithm is designed. The specific method is as follows:

[0096] 1) The essence of pre-synchronization control is the secondary frequency and voltage regulation of the power grid system. However, the traditional pre-synchronization control relies on directly adjusting the amplitude and phase of the grid connection point voltage to reduce the deviation on both sides, which causes the VSG to be in a drooping working state all the time, causing interference to the stability of the power grid. Before the pre-synchronization control is started, the voltage difference Δu between the grid connection point voltage and the grid voltage to be connected to the grid side (taking phase a as an example) is:

[0097]

[0098] In formula (4), u ga and u a are the instantaneous values ​​of phase a voltage of the power grid and the grid connection point, U g and U a are the effective values ​​of the voltage of the power grid and the grid connection point, respectively, ga and w a are the a-phase angular frequency values ​​of the power grid and the grid connection point, respectively. and are the phase values ​​of phase a of the power grid and the grid connection point, U is the effective value of the actual output voltage of the grid connection point, and wN is the rated angular frequency value;

[0099] 2) According to formula (4), the power fluctuation generated during grid connection is mainly caused by the phase and frequency of the voltage on both sides of the grid connection point. Therefore, the VSG pre-synchronization control is divided into three parts: amplitude pre-synchronization, phase pre-synchronization and frequency pre-synchronization. The corresponding control equations are:

[0100]

[0101] In formula (5), K p is the proportional coefficient of the PI controller, K i is the integral coefficient of the PI controller, U, U N and U g are the actual output voltage of the grid connection point, the rated voltage of the grid connection point and the effective value of the grid voltage, and are the actual output phase angle of the grid-connected point, the rated phase angle of the grid-connected point and the grid phase angle, respectively. N and f g They are the actual output frequency of the grid connection point, the rated frequency of the grid connection point and the grid frequency;

[0102] Figure 3 (b) is a flow chart of a VSG pre-synchronization slow start control algorithm provided by an embodiment of the present invention. In step 2, a PV-VSG slow start control algorithm is designed. The specific method is as follows:

[0103] 1) To achieve the purpose of slow start and grid connection, the power emitted by the photovoltaic array is always greater than the output power of the VSG, avoiding excessive power shortage caused by hysteresis, and also suppressing the impact of excessive power changes on the system during grid connection startup, reducing component damage, and designing a slow start algorithm flow chart; the algorithm sets three different operating states according to the system operation time, state 0 (state = 0) is the primary frequency and voltage regulation stage; state 1 (state = 1) is the secondary frequency and voltage regulation stage, that is, pre-synchronization control is turned on; state 2 (state = 2) is the grid connection stage, and pre-synchronization control is turned off. When state 2 is completed, the operating state of state 0 is restored;

[0104] 2) At the beginning of state 2, the VSG output power gradually increases from the power required by the load to the set photovoltaic cell output power, t is the rising slope, and the changes in active and reactive power during the slow start grid connection process are:

[0105]

[0106] In formula (6), P pv is the active power output of the photovoltaic array, Q refis a given reactive power reference value (assuming that the PV array does not generate reactive power), δP is the active power integral in the VSG pre-synchronization stage, δQ is the reactive power integral in the VSG pre-synchronization stage, and P N is the rated active power of VSG, Q N is the rated reactive power of VSG.

[0107] Step 3: Stabilize the DC bus voltage based on sliding mode control.

[0108] Figure 4 (a) is a DC bus voltage sliding mode control strategy diagram provided by an embodiment of the present invention. In step 3, the DC bus voltage is stabilized based on sliding mode control, which specifically includes the following steps:

[0109] Step 3.1; Establish the mathematical model of Boost circuit;

[0110] The current i on the output side of the Boost converter out for:

[0111] i out =P pv / u dc (7);

[0112] In formula (7), u dc It is the DC bus voltage output by the Boost converter;

[0113] DC-AC inverter side input current i in for:

[0114] i in =P dc / u dc =P e / u dc (8);

[0115] In formula (8), P dc is the output power of the Boost converter;

[0116] Under ideal conditions, the actual value of the DC bus voltage is equal to the reference value, that is, u dc =u ref , flows through the DC bus side capacitor C dc The current is 0, so:

[0117]

[0118] Substituting formula (7) and (8) into formula (9), we can get the state variable u to be controlled: dc The derivative of is:

[0119]

[0120] Step 3.2: Calculate the control law of the DC bus voltage;

[0121] DC bus voltage error e dc for:

[0122] e dc =u dc -u dcref (11);

[0123] Define the proportional-integral sliding surface S dc for:

[0124] S dc =(k s1 +k s2 / s)e dc (12);

[0125] In formula (12), k s1 is a positive proportionality coefficient; k s2 is a positive integral coefficient;

[0126] The use of exponential adaptive reaching law is beneficial to reduce system chattering, and the derivative of the sliding surface It can be expressed as:

[0127]

[0128] In formula (13), η, ξ, and γ are all positive coefficients of the control rate;

[0129] Substituting formula (12) into formula (13), the control law of the DC bus voltage can be obtained as follows:

[0130]

[0131] In formula (14), is the input power command of VSG;

[0132] Step 3.3: Proof of stability;

[0133] Choose to build the Lyapunov function V(S dc ) proves the stability of the control strategy as follows:

[0134]

[0135] Derivative of formula (15) yields:

[0136]

[0137] In formula (16), since η, ξ, and γ are all positive values, That is, the designed control strategy is stable.

[0138] Figure 4 (b) is a comparison chart of DC bus voltage values ​​under three conditions provided by the embodiment of the present invention, which respectively compares and verifies the DC bus voltage output effects of traditional PI control, sliding mode control, and no controller, and the rated reference value is 800V. When the DC bus voltage does not use a controller, it will produce a large impact and is very sensitive to any small fluctuations in the system. When the PI controller is used, although it can maintain stability, a part of the loss is generated, and this part of the loss is usually compensated by energy storage batteries. In the absence of energy storage batteries, the sliding mode controller designed by the present invention is selected. Although the response speed is slightly slower than the previous two, it has the best stability and the voltage regulation rate is only 1.25%.

[0139] Example: With a light intensity of 1000W / m 2 The temperature condition of 25°C is used as the standard operating condition, and changes are made according to a fixed step size based on the standard operating condition (the light intensity decreases by 200W / m every 0.5 seconds). 2 , the temperature drops by 1°C every 0.5 seconds).

[0140] according to Figure 5 (a) It can be seen that after the grid connection, the VSG output active power rises in a ramp shape to the set rated active power of 12kW, and the PV array output active power rises to near the rated reference value at the moment of grid connection, which solves the problem of excessive system power shortage caused by the lag in the power output of the PV array, which in turn causes the DC bus voltage to collapse; Figure 5 (b) It can be seen that after the successful grid connection, the effective values ​​of the DC bus voltage and the grid voltage quickly stabilized at around 790V and 220V respectively, thus ensuring the stability of the overall operating environment; Figure 5 (c) It can be seen that the overall stability is extremely high, which shows that the system has strong frequency support capability; Figure 5 (d) It can be seen that the pre-synchronization process gradually reduces the deviation by tracking the grid voltage, suppressing the power fluctuation and voltage and current impact generated during grid connection. At t = 0.25s, the pre-synchronization is turned on, and the VSG output voltage gradually coincides with the grid voltage, effectively suppressing the impact caused by the difference in amplitude and phase, and improving the power quality during grid connection.

[0141] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principles of the present invention should be covered within the protection scope of the present invention.

[0142] Any matters not described in the present invention are applicable to the prior art.

Claims

1. According to claim 1, a pre-synchronous start-up control strategy for a photovoltaic grid-connected converter is characterized in that: In step 1, a control relationship between the photovoltaic array and the virtual synchronous machine is established, and the specific method is as follows: The bipolar photovoltaic array adopts frequency droop control. When the VSG output frequency is not equal to the reference frequency, the reference power input to the photovoltaic array is: P ref =P set +K f (f ref -f) (1); In formula (1), P ref is the reference power output by the photovoltaic array, P set is the set PV array output power, which should be less than the maximum output power of the PV cell, K f is the frequency droop coefficient (positive value), f ref is the rated reference frequency of the power grid, and f is the actual frequency of the power grid.

2. According to claim 2, a pre-synchronous start-up control strategy for a photovoltaic grid-connected converter is characterized in that: The mathematical model of the virtual synchronous machine is established as follows: 1) The essence of active frequency regulation and reactive voltage regulation of virtual synchronous machine is droop control. The rotor mechanical equation of primary frequency regulation is: In formula (2), J is the virtual moment of inertia, D is the virtual damping coefficient, K P is the active frequency droop coefficient, P m is the virtual mechanical power, P N is the rated input active power, P e is the actual output power, w is the actual output angular frequency, w0 is the rated input angular frequency, θ is the actual output power angle of VSG, and t is the time; 2) The control equation of reactive power and voltage droop of virtual synchronous machine is: In formula (3), K q is the reactive voltage integral coefficient, K u is the reactive voltage droop coefficient, Q N is the rated input reactive power, Q is the actual output reactive power, U N is the rated input phase voltage, U is the actual output phase voltage, and E is the VSG output electromotive force amplitude.

3. According to claim 1, a pre-synchronous start-up control strategy for a photovoltaic grid-connected converter is characterized in that: In step 2, a PV-VSG pre-synchronization control algorithm is designed, and the specific method is as follows: 1) The essence of pre-synchronization control is the secondary frequency and voltage regulation of the power grid system. However, the traditional pre-synchronization control relies on directly adjusting the amplitude and phase of the grid connection point voltage to reduce the deviation on both sides, which causes the VSG to be in a drooping working state all the time, causing interference to the stability of the power grid. Before the pre-synchronization control is started, the voltage difference Δu between the grid connection point voltage and the grid voltage to be connected to the grid side (taking phase a as an example) is: In formula (4), u ga and u a are the instantaneous values ​​of phase a voltage of the power grid and the grid connection point, U g and U a are the effective values ​​of the voltage of the power grid and the grid connection point, respectively, ga and w a are the a-phase angular frequency values ​​of the power grid and the grid connection point, respectively. and are the phase values ​​of phase a of the power grid and the grid connection point, U is the effective value of the actual output voltage of the grid connection point, and w N is the rated angular frequency value; 2) According to formula (4), the power fluctuation generated during grid connection is mainly caused by the phase and frequency of the voltage on both sides of the grid connection point. Therefore, the VSG pre-synchronization control is divided into three parts: amplitude pre-synchronization, phase pre-synchronization and frequency pre-synchronization. The corresponding control equations are: In formula (5), K p is the proportional coefficient of the PI controller, K i is the integral coefficient of the PI controller, U, U N and U g are the actual output voltage of the grid connection point, the rated voltage of the grid connection point and the effective value of the grid voltage, and are the actual output phase angle of the grid-connected point, the rated phase angle of the grid-connected point and the grid phase angle, respectively. N and f g They are the actual output frequency of the grid-connected point, the rated frequency of the grid-connected point and the grid frequency.

4. According to claim 4, a pre-synchronous start-up control strategy for a photovoltaic grid-connected converter is characterized in that: Design the PV-VSG slow start control algorithm. The specific method is as follows: 1) To achieve the purpose of slow start and grid connection, the power emitted by the photovoltaic array is always greater than the output power of the VSG, avoiding excessive power shortage caused by hysteresis, and also suppressing the impact of excessive power changes on the system during grid connection startup, reducing component damage, and designing a slow start algorithm flow chart; the algorithm sets three different operating states according to the system operation time, state 0 (state = 0) is the primary frequency and voltage regulation stage; state 1 (state = 1) is the secondary frequency and voltage regulation stage, that is, pre-synchronization control is turned on; state 2 (state = 2) is the grid connection stage, and pre-synchronization control is turned off. When state 2 is completed, the operating state of state 0 is restored; 2) At the beginning of state 2, the VSG output power gradually increases from the power required by the load to the set photovoltaic cell output power, t is the rising slope, and the changes in active and reactive power during the slow start grid connection process are: In formula (6), P pv is the active power output of the photovoltaic array, Q ref is a given reactive power reference value (assuming that the PV array does not generate reactive power), δP is the active power integral in the VSG pre-synchronization stage, δQ is the reactive power integral in the VSG pre-synchronization stage, and P N is the VSG rated active power, Q N is the rated reactive power of VSG.

5. According to claim 1, a pre-synchronous start-up control strategy for a photovoltaic grid-connected converter is characterized in that: In step 3, stabilizing the DC bus voltage based on the sliding mode controller specifically includes the following steps: Step 3.1: Establish the mathematical model of Boost circuit; The current i on the output side of the Boost converter out for: and out =P pv / in dc (7); In formula (7), u dc is the DC bus voltage output by the Boost converter; DC-AC inverter side input current i in for: and in =P dc / in dc =P e / in dc (8); In formula (8), P dc is the output power of the Boost converter; Under ideal conditions, the actual value of the DC bus voltage is equal to the reference value, that is, u dc =u dcref , flows through the DC bus side capacitor C dc The current is 0, so: Substituting formula (7) and (8) into formula (9), we can get the state variable u to be controlled: dc The derivative of is: Step 3.2: Calculate the control law of the DC bus voltage; DC bus voltage error e dc for: yes dc =in dc -in dcref (11); Define the proportional-integral sliding surface S dc for: S dc =(k s1 +k s2 / s)e dc (12); In formula (12), k s1 is a positive proportionality coefficient; k s2 is a positive integral coefficient; The use of exponential adaptive reaching law is beneficial to reduce system chattering, and the derivative of the sliding surface It can be expressed as: In formula (13), η, ξ, and λ are all positive coefficients of the control rate; Substituting formula (12) into formula (13), the control law of the DC bus voltage can be obtained as follows: In formula (14), is the input power command of VSG; Step 3.3: Proof of stability; Choose to build the Lyapunov function V(S dc ) proves the stability of the control strategy as follows: Derivative of formula (15) yields: In formula (16), since η, ξ, and λ are all positive values, That is, the designed control strategy is stable.

Images