Control method of network-constructing photovoltaic system supporting active support of power grid frequency

Abstract

The invention provides a network construction type photovoltaic system control method supporting active support of power grid frequency, and the method comprises the steps: (1) designing a PV system power reduction operation control strategy which is used for enabling a PV system to reserve power to participate in frequency adjustment of a power grid; (2) designing a PI-based DC bus voltage control strategy for adjusting the DC bus voltage to a reference value; (3) designing a load side step-down operation control strategy based on PI, wherein the strategy is used for stabilizing the DC bus voltage in an off-grid mode; (4) designing a VSG control strategy, and enabling the network-forming inverter to simulate a synchronous generator to participate in active frequency modulation; and (5) in a grid-connected mode, designing a pre-synchronization control strategy, and enabling the grid-forming inverter and the power grid to operate in a grid-connected manner. According to the invention, the frequency stability of the new energy high-permeability power system can be improved. In addition, the method only depends on local information, does not need a maximum power point estimator, a central controller or a solar radiation sensor, and has the advantage of low implementation cost.

Description

Technical Field

[0001] The present invention belongs to the technical field of new power systems, and in particular relates to a control method for a grid-connected photovoltaic system that supports active support of grid frequency. Background Art

[0002] With the increasing penetration of power electronics and new energy in new power systems, the system inertia and frequency regulation capabilities are increasingly insufficient, and there is an urgent need to develop new frequency regulation resources to provide additional frequency response for future new power systems. Traditional photovoltaic systems operating in maximum power point tracking (MPPT) mode cannot provide frequency support, which is not conducive to the stability of power system frequency. In recent years, some photovoltaic active frequency regulation control strategies have also been proposed:

[0003] (1) Energy storage can provide certain frequency support. For example, the Chinese invention patent application with publication number CN114977218A provides a method and device for frequency regulation control of energy storage-assisted photovoltaic units under multiple working conditions. However, energy storage will undoubtedly increase the investment and maintenance costs of the system.

[0004] (2) Some photovoltaic active frequency regulation control strategies based on load reduction operation usually have the following limitations:

[0005] 1) A complex maximum power point estimation algorithm is required; [R.Owais and SJIqbal, "A master-slave-based power reserve control approach for solar PV participation infrequency control," Arabian Journal for Science and Engineering, 2023JUL272023.];

[0006] 2) Irradiance and temperature sensors are required; [G. Yan, S. Liang, Q. Jia, and Y. Cai, “Noveladapted de-loading control strategy for PV generation participating in grid frequency regulation,” Journal of Engineering-JOE, no. 16, pp. 3383–3387, MAR 2019.]

[0007] 3) A central controller and centralized communication are required; [X. Zhang, Y. Hu, W. Mao, T. Zhao, M. Wang, F. Liu, and R. Cao, “A grid-supporting strategy for cascaded h-bridge PVconverter using VSG algorithm with modular active power reserve,” IEEE Transactions on Industrial Electronics, vol. 68, no. 1, pp. 186–197, JAN 2021.]

[0008] In order to solve the above problems, people have been seeking an ideal technical solution. Summary of the invention

[0009] The purpose of the present invention is to address the deficiencies of the prior art and thus provide a grid-connected photovoltaic system control method that supports active grid frequency support, thereby improving the frequency stability of the new power system and the new energy access capability.

[0010] In order to achieve the above object, the technical solution adopted by the present invention is:

[0011] In a first aspect, the present invention provides a control method for a grid-connected photovoltaic system supporting active grid frequency support, comprising:

[0012] (1) Design a power reduction control strategy for the PV system to allow the PV system to have power reserve to participate in the frequency regulation of the power grid;

[0013] (1-1) When the PV array operates at MPP, the output power and output voltage satisfy the following relationship:

[0014]

[0015] Among them, P PV is the output power of the PV array, V PV is the output voltage of the PV array, I PV is the output current of the PV array, dI PV / dV PV is the conductance increment of the PV array;

[0016] (1-2) The slope of the photovoltaic power-voltage characteristic curve S0 is defined as:

[0017]

[0018] (1-3) According to the photovoltaic power-voltage characteristic curve, in the grid-connected mode, in order to make the PV system operate below the MPP to participate in frequency regulation, a new photovoltaic power-voltage characteristic curve slope S is defined PV for:

[0019] S PV =S0-ΔS

[0020] Among them, ΔS is the frequency adjustment term

[0021] By adjusting ΔS, the PV array is put into a reduced power operation state;

[0022] (2) Design a PI-based DC bus voltage control strategy to adjust the DC bus voltage to its reference value;

[0023] (2-1) The DC bus voltage reference value V DC,ref And the actual value V DC Input the first PI controller to obtain the PV array output voltage reference value V PV0 :

[0024]

[0025] k p0 and k i0 is the first PI controller parameter, s is a complex variable;

[0026] (2-2) When S0 is greater than 0, that is, the PV system operates in the right half plane of the MPP, the PV array output voltage reference value V PV0 Increase V state Get the new PV array output voltage reference value V PV,ref , so that the PV system works in the left half plane;

[0027] That is, the new PV array output voltage reference value V PV,ref for:

[0028] V PV,ref =V PV0 +V state

[0029] Among them, V state It is the photovoltaic array reference voltage adjustment item;

[0030] (2-3) Photovoltaic array reference voltage adjustment item V state The value rules are:

[0031] 1) When S0 gradually increases from -∞:

[0032]

[0033] 2) When S0 gradually decreases from +∞:

[0034]

[0035] Among them, V step Indicates the unit regulation voltage, S 1P and S UP Indicates the frequency adjustment threshold, S 1P <0, S UP >0;

[0036] (3) Design a load-side step-down operation control strategy based on PI to stabilize the DC bus voltage in off-grid mode;

[0037] (3-1) Define the load side voltage regulation term ΔV as:

[0038]

[0039] Among them, k pΔv and k iΔv is the control parameter of the second PI controller, s is a complex variable;

[0040] (3-2) In off-grid mode, the DC bus voltage reference value V DC,ref And the actual value V DC Input the second PI controller to obtain the load side voltage regulation term ΔV;

[0041] (4) Design VSG control strategy to enable grid-connected inverters to simulate synchronous generators and participate in active frequency regulation;

[0042] (4-1) The swing equation of the designed VSG controller is:

[0043] P in -PD·(w-w0-Δw)=J·w0·s·(w-w0)

[0044] Among them, P in is the reference active power provided by the speed regulator, P is the active power output of the grid-type inverter, D is the damping factor, w and w0 are the actual angular frequency and nominal angular frequency of the output voltage of the grid-type inverter respectively, and J is the moment of inertia;

[0045] In the grid-connected mode, Δw is the frequency synchronization compensation term generated by the pre-synchronization controller;

[0046] In off-grid mode, Δw=0;

[0047] (4-2) In order to simulate the droop characteristics of the SG speed regulator, the reference active power P is designed. in for:

[0048] P in =P ref-m w ·(w-w0-Δw)

[0049] Among them, m w is the active power droop coefficient of the frequency converter, P ref is the reference active power;

[0050] In grid-connected mode, P ref The third PI controller is used to obtain the frequency regulation term ΔS, which is used to reduce the power of the PV system so as to release the reserved power of the PV system or increase the power reserve to provide frequency support when the power grid faces frequency disturbances;

[0051] In off-grid mode, P ref =0;

[0052] (4-3) The dynamic characteristics between the active power P and the angular frequency w of the grid-connected inverter are obtained from equations (4-1) and (4-2):

[0053] J·w0·s·(w-w0)=P ref -m w ·(w-w0-Δw)-P

[0054] -D·(w-w0-Δw)

[0055] (4-4) The relationship between the power angle θ and the frequency w is:

[0056]

[0057] Wherein, in the grid-connected mode, Δθ represents the phase angle synchronization compensation term generated by the pre-synchronization controller; in the off-grid mode, Δθ=0;

[0058] (4-5) The dynamic characteristics between the reactive power Q output by the grid-connected inverter and the voltage amplitude reference value V are designed as follows:

[0059] V=V0-m q ·(QQ ref )+ΔV

[0060] Where V0 is the rated voltage amplitude, Q ref and Q are the reference value and actual value of reactive power output by the grid-connected inverter, respectively. q is the reactive power droop coefficient;

[0061] In the grid-connected mode, ΔV represents the voltage synchronization compensation term generated by the presynchronization controller;

[0062] In off-grid mode, ΔV represents the voltage regulation term generated by the second PI controller;

[0063] In off-grid mode, when the system is subject to a large disturbance, the voltage regulation term ΔV is used to reduce the load-side voltage, and the DC voltage is kept stable by stepping down the load-side voltage.

[0064] (5) In the grid-connected mode, a pre-synchronization control strategy is designed to enable the grid-connected inverter to operate in parallel with the grid;

[0065] (5-1) Calculate the voltage amplitude V of the grid-connected inverter GFI GFI , the voltage amplitude of the common coupling point PCC V PCC , the voltage phase angle θ of the grid-connected inverter GFI GFI and the voltage phase angle θ at the common coupling point PCC PCC :

[0066]

[0067] θ PCC =arctan(v d,PCC / v q,PCC )

[0068] θ GFI =arctan(v d,GFI / v q,GFI )

[0069] Among them, v d,GFI 、v q,GFI They are the grid-connected inverter GFI output voltage v abc,GFI The d-axis voltage and q-axis voltage after abc→dq transformation, v d,PCC 、v q,PCC The grid voltage v abc,PCC The d-axis voltage and q-axis voltage after abc→dq transformation;

[0070] (5-2) In order to make the voltage amplitude of the grid-connected inverter equal to the voltage amplitude of the PCC, the voltage amplitude synchronization compensation term ΔV is generated by the fourth PI controller:

[0071]

[0072] Among them, define e V =V PCC -V GFI , k pv and k iv is the control parameter of the fourth PI controller;

[0073] (5-3) The phase angle synchronization compensation term Δθ is generated by the fifth PI controller:

[0074]

[0075] Among them, define eθ =θ PCC -θ GFI , k pθ and k iθ is the control parameter of the fifth PI controller;

[0076] (5-4) The frequency synchronization strategy is designed as follows:

[0077] ① Calculate the voltage phase angle of PCC:

[0078]

[0079] Among them, w PCC is the grid frequency, θ 0,PCC is the initial voltage phase angle of PCC;

[0080] ② Frequency deviation between the grid and the grid-connected inverter e w Defined as:

[0081] e w =w PCC -w GFI

[0082] Among them, w GFI is the grid-type inverter frequency;

[0083] ③Calculate the phase angle deviation e θ :

[0084]

[0085] ④ Calculate the phase angle deviation e θ Converges to the value at steady state:

[0086]

[0087] ⑤In steady state, the frequency deviation e w Phase angle deviation e θ The relationship is as follows:

[0088] e w =k iθ ·e θ (∞)

[0089] If the phase angle deviation e θ Stable, no PLL is needed, and the frequency synchronization compensation term Δw can be generated by the sixth PI controller, as shown in the following formula:

[0090]

[0091] (5-5) According to the above analysis process, the grid connection conditions are designed as follows:

[0092]

[0093] Among them, ε V , ε θ and ε w They are the allowable deviations of voltage, phase angle and frequency deviation respectively.

[0094] In a second aspect, the present invention provides a grid-connected photovoltaic system control system that supports active grid frequency support, including:

[0095] PV system power reduction operation controller is used to allow the PV system to have power reserve to participate in the frequency regulation of the power grid. The input is the output power P of the PV array. PV 、PV array output voltage V PV 、PV array output current I PV , the output is the slope S of the new photovoltaic power-voltage characteristic curve PV ;

[0096] The PI-based DC bus voltage controller is used to adjust the DC bus voltage to its reference value. The input is the DC bus voltage reference value V DC,ref And the actual value V DC , the output is the PV array output voltage reference value V PV,ref ;

[0097] The load-side step-down operation controller based on PI is used to stabilize the DC bus voltage in off-grid mode. The input is the DC bus voltage reference value V DC,ref And the actual value V DC , the output is the load side voltage regulation term ΔV;

[0098] The VSG controller is used to make the grid-type inverter simulate the synchronous generator to participate in active frequency regulation, and the input is the reference active power P in And actual active power P, reactive power reference value Q ref And the actual value Q, the voltage adjustment term ΔV and the phase angle adjustment term Δθ, the output is the output voltage amplitude reference value V and the power angle θ of the grid-forming inverter;

[0099] Pre-synchronization controller, used to make the grid-connected inverter run in parallel with the grid, the input is the output voltage v of the grid-connected inverter GFI abc,GFI and grid voltage v abc,PCC The output is the voltage amplitude synchronous compensation term ΔV, the phase angle synchronous compensation term Δθ and the grid-connected signal s w ;

[0100] The PV system power reduction operation controller is connected to the PI-based DC bus voltage controller, and the VSG controller is respectively connected to the PI-based load side voltage reduction operation controller and the pre-synchronization controller, so as to implement the grid-forming photovoltaic system control method supporting active grid frequency support.

[0101] In a third aspect, the present invention provides a grid-connected photovoltaic system that supports active grid frequency support, comprising:

[0102] PV arrays;

[0103] DC / DC converter;

[0104] DC / AC grid-connected inverter;

[0105] The PV array is connected to the grid through DC / DC converters and DC / AC grid-forming inverters in turn;

[0106] It also includes the grid-forming photovoltaic system control system that supports active grid frequency support, which is used to actively control the frequency regulation of the DC / DC converter and the DC / AC grid-forming inverter.

[0107] The present invention has the following beneficial effects and advantages:

[0108] 1. By adjusting the output voltage of the photovoltaic array, the DC bus voltage can be stabilized, and the output power of the photovoltaic array can be reduced, so that the photovoltaic array is in a reduced power operation state.

[0109] 2. In off-grid mode, when the system is subject to a large disturbance, the DC bus voltage can be maintained stable by stepping down the load side.

[0110] 3. By simulating the characteristics of synchronous generators, grid-connected inverters can actively participate in the frequency response of the power system, which reduces the requirements for the energy storage system.

[0111] 4. The control method of the present invention only requires local information of the photovoltaic system, does not require a complex maximum power point estimator, a central controller or expensive solar radiation and temperature sensors, and the system is easy to implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0112] Figure 1 It is a structural diagram of the control method of a grid-connected photovoltaic system supporting active grid frequency support according to Example 1.

[0113] Figure 2 It is the structural diagram of the pre-synchronization controller and the VSG controller.

[0114] Figure 3 Microgrid simulation system.

[0115] Figure 4System simulation results under off-grid mode with small load disturbance.

[0116] Figure 5 System simulation results under large load disturbance scenario in off-grid mode.

[0117] Figure 6 System simulation results under off-grid mode irradiance disturbance scenario.

[0118] Figure 7 Grid-connected signal deviation.

[0119] Figure 8 Parameter changes before and after grid connection.

[0120] Fig. 9 System simulation results under load disturbance scenario in grid-connected mode.

[0121] Fig.10 System simulation results under grid-connected mode irradiance disturbance scenario.

[0122] Fig.11 It is a structural diagram of a grid-connected photovoltaic system that supports active grid frequency support according to Example 3.

[0123] Fig.12 is the photovoltaic power-voltage characteristic curve.

[0124] Fig.13 is the P of the PV array PV With dP PV / dV PV Graph. DETAILED DESCRIPTION

[0125] The present invention is further described in detail below in conjunction with the accompanying drawings.

[0126] Example 1

[0127] like Figure 1 As shown, this embodiment provides a control method for a grid-connected photovoltaic system supporting active grid frequency support, including:

[0128] (1) Design a power reduction control strategy for the PV system to allow the PV system to have power reserve to participate in the frequency regulation of the power grid;

[0129] (1-1) When the PV array operates at MPP, the output power and output voltage satisfy the following relationship:

[0130]

[0131] Among them, P PV is the output power of the PV array, V PV is the output voltage of the PV array, I PVis the output current of the PV array, dI PV / dV PV is the conductance increment of the PV array;

[0132] (1-2) The slope of the photovoltaic power-voltage characteristic curve S0 is defined as:

[0133]

[0134] (1-3) According to Fig.12 The photovoltaic power-voltage characteristic curve shown in the figure is as follows. In order to make the PV system participate in frequency regulation when the system is operated below the MPP in the grid-connected mode, a new photovoltaic power-voltage characteristic curve slope S is defined. PV for:

[0135] S PV = S0-ΔS (3)

[0136] Among them, ΔS is the frequency adjustment term;

[0137] By adjusting ΔS, the PV array is put into a reduced power operation state.

[0138] (2) Design a PI-based DC bus voltage control strategy to adjust the DC bus voltage to its reference value;

[0139] (2-1) The DC bus voltage reference value V DC,ref And the actual value V DC Input the first PI controller to obtain the PV array output voltage reference value V PV0 :

[0140]

[0141] k p0 and k i0 is the first PI controller parameter, s is a complex variable;

[0142] (2-2) By Fig.13 The PV array shown in P PV With dP PV / dV PV The curve graph shows that the slope sensitivity of the left side curve of MPP is small, while the slope sensitivity of the right side curve is large. The PV system is unstable when it works in the right half plane, so the PV system is not allowed to work in the right half plane. When it works in the right half plane, it is controlled to return to the left half plane. When S0 is greater than 0, that is, when the PV system works in the right half plane of MPP, the PV array output voltage reference value V PV0 Increase V state Get the new PV array output voltage reference value V PV,ref , so that the PV system works in the left half plane;

[0143] That is, the new PV array output voltage reference value V PV,ref for:

[0144] V PV,ref =V PV0 +V state

[0145] Among them, V state It is the photovoltaic array reference voltage adjustment item;

[0146] (2-3) In order to reduce the jump of the system at S0=0, the PV array reference voltage adjustment item V state The value rules are:

[0147] 1) When S0 gradually increases from -∞:

[0148]

[0149] 2) When S0 gradually decreases from +∞:

[0150]

[0151] Among them, V step Indicates the unit regulation voltage, S 1P and S UP Indicates the frequency adjustment threshold, S 1P <0, S UP >0.

[0152] (3) Design a load-side step-down operation control strategy based on PI to stabilize the DC bus voltage in off-grid mode;

[0153] (3-1) Define the load side voltage regulation term ΔV as:

[0154]

[0155] Among them, k pΔv and k iΔv is the control parameter of the second PI controller, and s is a complex variable.

[0156] (3-2) In off-grid mode, the DC bus voltage reference value V DC,ref And the actual value V DC Input the second PI controller to obtain the load side voltage regulation term ΔV.

[0157] (4) Design VSG control strategy, such as Figure 2 As shown, the grid-type inverter simulates the synchronous generator to participate in active frequency regulation;

[0158] (4-1) The swing equation of the designed VSG controller is:

[0159] P in -PD·(w-w0-Δw)=J·w0·s·(w-w0) (6)

[0160] Among them, P in is the reference active power provided by the speed regulator, P is the active power output of the grid-type inverter, D is the damping factor, w and w0 are the actual angular frequency and nominal angular frequency of the output voltage of the grid-type inverter respectively, and J is the moment of inertia;

[0161] In the grid-connected mode, Δw is the frequency synchronization compensation term generated by the pre-synchronization controller;

[0162] In off-grid mode, Δw=0;

[0163] (4-2) In order to simulate the droop characteristics of the SG speed regulator, the reference active power P is designed. in for:

[0164] P in =P ref -m w ·(w-w0-Δw) (7)

[0165] Among them, m w is the active power droop coefficient of the frequency converter, P ref is the reference active power. Figure 1 As shown, in the grid-connected mode, P ref It can be obtained through the third PI controller, and the PV system is operated at reduced power through the frequency adjustment term ΔS. When the power grid faces frequency disturbances, the reserved power of the PV system can be released or the power reserve can be increased to provide frequency support; in off-grid mode, P ref =0.

[0166] (4-3) The dynamic characteristics between the active power P and the angular frequency w of the grid-connected inverter are obtained from equations (4-1) and (4-2):

[0167]

[0168] (4-4) The relationship between the power angle θ and the frequency w is:

[0169]

[0170] Wherein, in the grid-connected mode, Δθ represents the phase angle synchronization compensation term generated by the pre-synchronization controller; in the off-grid mode, Δθ=0;

[0171] (4-5) The dynamic characteristics between the reactive power Q output by the grid-connected inverter and the voltage amplitude reference value V are designed as follows:

[0172] V=V0-m q·(QQ ref )+ΔV (10)

[0173] Where V0 is the rated voltage amplitude, Q ref and Q are the reference value and actual value of reactive power output by the grid-connected inverter, respectively. q is the reactive power droop coefficient;

[0174] In the grid-connected mode, ΔV represents the voltage synchronization compensation term generated by the presynchronization controller;

[0175] In off-grid mode, ΔV represents the voltage regulation term generated by the second PI controller;

[0176] In off-grid mode, when the system is subject to a large disturbance, the voltage regulation term ΔV is used to reduce the load side voltage, and the DC voltage is kept stable by stepping down the load side.

[0177] (5) In grid-connected mode, if Figure 2 As shown, a pre-synchronization control strategy is designed to enable the grid-connected inverter to operate in parallel with the power grid;

[0178] (5-1) Calculate the voltage amplitude V of the grid-connected inverter GFI GFI , the voltage amplitude of the common coupling point PCC V PCC , the voltage phase angle θ of the grid-connected inverter GFI GFI and the voltage phase angle θ at the common coupling point PCC PCC :

[0179]

[0180] Among them, v d,GFI 、v q,GFI They are the grid-connected inverter GFI output voltage v abc,GFI The d-axis voltage and q-axis voltage after abc→dq transformation, v d,PCC 、v q,PCC The grid voltage v abc,PCC The d-axis voltage and q-axis voltage after abc→dq transformation.

[0181] (5-2) In order to make the voltage amplitude of the grid-connected inverter equal to the voltage amplitude of the PCC, the voltage amplitude synchronization compensation term ΔV is generated by the fourth PI controller:

[0182]

[0183] Among them, define e V =V PCC -V GFI , k pv and k ivis the control parameter of the fourth PI controller;

[0184] When GFI is connected to PCC, it will connect to the grid through the signal s w Change the input signal of the fourth PI controller from e V Switch to 0-ΔV, at this time, ΔV=0, that is, the input of the fourth PI controller is 0, which will not affect the reactive power distribution of formula (10).

[0185] (5-3) In order to eliminate the phase angle difference between GFI and PCC, the phase angle synchronization compensation term Δθ is generated by the fifth PI controller:

[0186]

[0187] Among them, define e θ =θ PCC -θ GFI , k pθ and k iθ is the control parameter of the fifth PI controller;

[0188] During the short transient process after the GFI is connected to the PCC, the frequency of the GFI will be consistent with the frequency of the PCC. From formula (7), it can be seen that the active load sharing is not affected by θ. Therefore, after the GFI is connected to the PCC, the value of θ remains unchanged.

[0189] (5-4) The frequency synchronization strategy is designed as follows:

[0190] ① Calculate the voltage phase angle of PCC:

[0191]

[0192] Among them, w PCC is the grid frequency, θ 0,PCC is the initial voltage phase angle of PCC;

[0193] ② Frequency deviation e between the grid PCC and the grid-connected inverter GFI w Defined as e w =w PCC -w GFI , where w GFI is the grid-type inverter frequency;

[0194] ③ From equations (9), (13) and (14), we can get the phase angle deviation e θ :

[0195]

[0196] ④ From formula (13), we can see that the phase angle deviation e θ In steady state, it converges to the following value:

[0197]

[0198] ⑤In steady state, the frequency deviation e w Phase angle deviation e θ The relationship is as follows:

[0199] e w =k iθ ·e θ (∞) (17)

[0200] If the phase angle deviation e θ If stable, the frequency deviation e can be estimated by formula (15) w , without using PLL, the frequency synchronization compensation term Δw can be generated by the sixth PI controller, as shown in the following formula:

[0201]

[0202] In the formula, k pw and k iw is the control parameter of the sixth PI controller;

[0203] In order to accurately estimate the frequency deviation e by formula (17) w , phase angle deviation e θ The convergence speed should be faster than the frequency deviation e w In addition, according to formula (8), when the GFI is connected to the PCC, in order to properly distribute the active power, the input signal e of the sixth PI controller for frequency synchronization should be w Switching from 0-Δw, at this time, the input of the sixth PI controller is 0, such as Figure 2 shown.

[0204] (5-5) According to the above analysis process, the grid connection conditions are designed as follows:

[0205]

[0206] Among them, ε V , ε θ and ε w They are the allowable deviations of voltage, phase angle, and frequency deviation respectively. According to IEEE Standard 1547-2018, the synchronization parameters of the grid-connected inverter should meet the following restrictions before it is connected to the grid: Frequency deviation ε w <0.1Hz, voltage deviation ε V <3%V nom , phase angle deviation ε θ <20°.

[0207] Simulation Analysis

[0208] In order to verify the effectiveness of the control method proposed in this embodiment, a control system is built based on the MATLAB / Simulink platform. Figure 3 The simulation system shown in the figure is simulated and analyzed. The simulation system includes a distributed generator (DG), a PV power generation unit, a transformer, and a system rated frequency of 60Hz. The PV power generation unit is connected to the grid through a distribution transformer, which reduces the 4.16kV voltage to 480V. Other main system parameters are shown in Table 1.

[0209] Table 1 Main system parameters

[0210]

[0211] 1. Performance verification in off-grid mode

[0212] 1.1. Performance verification under small load disturbance scenario

[0213] In off-grid mode, when the load disturbance is small, that is, does not exceed the maximum output power of PV, the control strategy proposed in this embodiment is tested for performance, and the relevant parameters are set as follows: irradiance is 600W / m 2 The temperature is constant at 25°C. The load increases suddenly at t=2.5s and decreases suddenly at t=3.5s. The simulation results are as follows: Figure 4 shown.

[0214] Depend on Figure 4 It can be seen that when the load suddenly increases at t=2.5s, the DC bus voltage decreases and the system frequency decreases. In order to maintain the DC bus voltage stable and suppress the frequency decrease, the PV output power increases. When the load suddenly decreases at t=3.5s, the DC bus voltage increases and the system frequency increases. In order to maintain the DC bus voltage stable and suppress the frequency increase, the PV output power decreases. During the whole process, the load voltage amplitude remains unchanged, about 640V.

[0215] 1.2 Performance verification under large load disturbance scenario

[0216] In off-grid mode, when the load disturbance is large, that is, exceeds the maximum output power of PV, the control strategy proposed in this embodiment is tested for performance, and the relevant parameters are set as follows: irradiance is 600W / m 2 Constant, the temperature is constant at 25℃, the load suddenly increases at t=2.5s, and suddenly decreases at t=3.5s. The simulation results are as follows Figure 5 shown.

[0217] Depend on Figure 5It can be seen that when the load suddenly increases at t=2.5s, the DC bus voltage decreases and the system frequency decreases. In order to maintain the DC bus voltage stability and suppress the frequency decrease, the PV output power increases. Since the PV output exceeds its maximum output power, the DC bus voltage is maintained stable by load voltage reduction. When the DC bus voltage is stable, the load voltage amplitude is about 550V; when the load suddenly decreases at t=3.5s, the DC bus voltage increases and the system frequency increases. In order to maintain the DC bus voltage stability and suppress the frequency increase, the PV output power decreases. When the DC bus voltage is stable, the load voltage amplitude returns to 640V.

[0218] 1.3. Performance verification under large irradiance disturbance scenario

[0219] In off-grid mode, when the irradiance disturbance is large, that is, the maximum PV output power cannot meet the load, the control strategy of this embodiment is tested for performance, and the relevant parameters are set as follows: the load is constant, the temperature is constant at 25°C, and the irradiance increases from 600W / m at t=2.5s. 2 Sudden drop to 300W / m 2 , at t = 3.5s from 300W / m 2 Sudden increase to 800W / m 2 The simulation results are as follows: Figure 6 shown.

[0220] Depend on Figure 6 It can be seen that when the irradiance suddenly decreases at t=2.5s, the PV output decreases, the system frequency increases, and the DC bus voltage decreases. Since the maximum PV output power cannot meet the load at this time, in order to maintain the stability of the DC bus voltage, the load voltage reduction operation is used to reduce the power required by the load. When the DC bus voltage is stable, the load voltage amplitude is about 570V; when the irradiance suddenly increases at t=3.5s, the PV output increases, the system frequency decreases, and the DC bus voltage increases. When the DC bus voltage is stable, the load voltage amplitude returns to 640V.

[0221] 2. Grid connection process performance verification

[0222] Figure 7 and Figure 8 FIG. 4 shows the synchronization process using the pre-synchronization control strategy of this embodiment. Figure 7 As shown in Figure 1, during the period of t = 1 to 1.8 s, the voltage amplitude, frequency and phase angle of the grid-connected inverter are adjusted to be synchronized with the PCC. At around t = 1.8 s, when the synchronization conditions are met, the pre-synchronization controller issues a grid-connection command to connect the GFI to the PCC.

[0223] Figure 8The changes of system parameters before and after grid connection. After grid connection, DG output power decreases and PV output power increases. As the total active power output of the system increases, the system frequency increases.

[0224] 3. Performance verification in grid-connected mode

[0225] 3.1 Performance Verification under Load Disturbance Scenario

[0226] In grid-connected mode, the designed control strategy is tested for performance when the load is disturbed. The relevant parameters are set as follows: irradiance is 600W / m 2 Constant, temperature 25℃ constant, load suddenly increases at t=3.3s, and suddenly decreases at t=4.3s. The simulation results are as follows Fig. 9 shown.

[0227] Depend on Fig. 9 It can be seen that when the load suddenly increases at t=3.3s, the DC bus voltage decreases and the system frequency decreases. Since the PV has a certain power reserve, in order to maintain the DC bus voltage stability and suppress the frequency drop, the PV output power increases. When the load suddenly decreases at t=4.3s, the DC bus voltage increases and the system frequency increases. In order to maintain the DC bus voltage stability and suppress the frequency increase, the PV output power decreases.

[0228] 3.2 Performance verification under irradiance disturbance scenario

[0229] In the grid-connected mode, the control strategy of this embodiment is tested for performance when the irradiance is disturbed. The relevant parameters are set as follows: the load is constant, the temperature is constant at 25°C, the irradiance increases suddenly at t=3.5s, and decreases suddenly at t=5s. The simulation results are as follows: Fig.10 shown.

[0230] Depend on Fig.10 It can be seen that when the irradiance suddenly increases at t=3.5s, the PV output increases, the system frequency increases, and the DC bus voltage increases. In order to maintain the stability of the DC bus voltage and suppress the increase of the system frequency, the DG output power decreases. When the irradiance suddenly decreases at t=5s, the PV output decreases, the system frequency decreases, and the DC bus voltage decreases. In order to maintain the stability of the DC bus voltage and suppress the decrease of the system frequency, the DG output power increases.

[0231] The control method of a grid-connected photovoltaic system that supports active support of the grid frequency proposed in this embodiment, through the designed control strategy, the PV system can smoothly switch between off-grid and grid-connected operation. In the grid-connected mode, the PV system can provide power reserves and adjust the frequency by releasing reserved power or increasing the power reserve of the PV system. In addition, inertial support is provided by simulating the virtual inertial characteristics of traditional synchronous generators to improve the system frequency stability. Compared with the traditional grid-connected PV system, the control strategy of this embodiment can actively build the frequency and automatically reach the synchronous state without the use of PLL. In addition, the control strategy of this embodiment does not require a maximum power curve, a detailed PV model, an irradiance sensor, an MPP estimator, and any communication with the central system, so it is easy to implement.

[0232] Example 2

[0233] This embodiment provides a grid-connected photovoltaic system control system that supports active grid frequency support, including:

[0234] PV system power reduction operation controller is used to allow the PV system to have power reserve to participate in the frequency regulation of the power grid. The input is the output power P of the PV array. PV 、PV array output voltage V PV 、PV array output current I PV , the output is the slope S of the new photovoltaic power-voltage characteristic curve PV ;

[0235] The PI-based DC bus voltage controller is used to adjust the DC bus voltage to its reference value. The input is the DC bus voltage reference value V DC,ref And the actual value V DC , the output is the PV array output voltage reference value V PV,ref ;

[0236] The load-side step-down operation controller based on PI is used to stabilize the DC bus voltage in off-grid mode. The input is the DC bus voltage reference value V DC,ref And the actual value V DC , the output is the load side voltage regulation term ΔV;

[0237] The VSG controller is used to make the grid-type inverter simulate the synchronous generator to participate in active frequency regulation, and the input is the reference active power P in And actual active power P, reactive power reference value Q ref And the actual value Q, the voltage adjustment term ΔV and the phase angle adjustment term Δθ, the output is the output voltage amplitude reference value V and the power angle θ of the grid-forming inverter;

[0238] Pre-synchronization controller, used to make the grid-connected inverter run in parallel with the grid, the input is the output voltage v of the grid-connected inverter GFIabc,GFI and grid voltage v abc,PCC The output is the voltage amplitude synchronous compensation term ΔV, the phase angle synchronous compensation term Δθ and the grid-connected signal s w ;

[0239] The PV system power reduction operation controller is connected to the PI-based DC bus voltage controller, and the VSG controller is respectively connected to the PI-based load side voltage reduction operation controller and the pre-synchronization controller, so as to implement the grid-connected photovoltaic system control method supporting active grid frequency support described in Example 1.

[0240] Example 3

[0241] This embodiment provides a grid-connected photovoltaic system that supports active grid frequency support, such as Fig.11 As shown, including:

[0242] PV arrays;

[0243] DC / DC converter;

[0244] DC / AC grid-connected inverter;

[0245] The PV array is connected to the grid through DC / DC converters and DC / AC grid-forming inverters in turn;

[0246] It also includes the grid-type photovoltaic system control system that supports active grid frequency support as described in Example 2, which is used to actively control the frequency regulation of the DC / DC converter and the DC / AC grid-type inverter.

[0247] It should be noted that, in this embodiment, the control signal obtained by the control system of the grid-forming photovoltaic system supporting active grid frequency support needs to be converted into corresponding instructions in order to drive the DC / DC converter and the DC / AC grid-forming inverter to perform active frequency modulation control, wherein the specific instruction conversion method can adopt conventional technical methods, such as:

[0248] The output voltage of the PV array V PV , output current I PV And the output voltage reference value V PV,ref By inputting the PI-based voltage loop and current loop controller, the DC bus voltage regulation instruction can be obtained, and then transmitted to the PWM module to obtain the driving signal of the DC / DC converter of the PV system, thereby adjusting the DC bus voltage to its reference value;

[0249] The output voltage amplitude reference value V and power angle θ of the grid-forming inverter output by the VSG controller are first transformed from abc to dq, and then the virtual impedance is calculated. Finally, the voltage loop and current loop controller calculation based on PI is performed to obtain the DC / AC grid-forming inverter adjustment instruction, which is then transmitted to the PWM module to obtain the driving signal of the DC / AC grid-forming inverter of the PV system, so that the grid-forming inverter simulates the synchronous generator to participate in active frequency modulation.

[0250] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A control method for a grid-connected photovoltaic system supporting active grid frequency support, characterized in that: include: (1) Design a power reduction control strategy for the PV system to allow the PV system to have power reserve to participate in the frequency regulation of the power grid; (1-1) When the PV array operates at MPP, the output power and output voltage satisfy the following relationship: Among them, P PV is the output power of the PV array, V PV is the output voltage of the PV array, I PV is the output current of the PV array, dI PV / dV PV is the conductance increment of the PV array; (1-2) The slope of the photovoltaic power-voltage characteristic curve S0 is defined as: (1-3) According to the photovoltaic power-voltage characteristic curve, in the grid-connected mode, in order to make the PV system operate below the MPP to participate in frequency regulation, a new photovoltaic power-voltage characteristic curve slope S is defined PV for: S PV =S0-ΔS Among them, ΔS is the frequency adjustment term By adjusting ΔS, the PV array is put into a reduced power operation state; (2) Design a PI-based DC bus voltage control strategy to adjust the DC bus voltage to its reference value; (2-1) The DC bus voltage reference value V DC,ref And the actual value V DC Input the first PI controller to obtain the PV array output voltage reference value V PV0 : k p0 and k i0 is the first PI controller parameter, s is a complex variable; (2-2) When S0 is greater than 0, that is, the PV system operates in the right half plane of the MPP, the PV array output voltage reference value V PV0 Increase V state Get the new PV array output voltage reference value V PV,ref , so that the PV system works in the left half plane; That is, the new PV array output voltage reference value V PV,ref for: V PV,ref =V PV0 +V state Among them, V state It is the photovoltaic array reference voltage adjustment item; (2-3) Photovoltaic array reference voltage adjustment item V state The value rules are: 1) When S0 gradually increases from -∞: 2) When S0 gradually decreases from +∞: Among them, V step Indicates the unit regulation voltage, S 1P and S UP Indicates the frequency adjustment threshold, S 1P <0, S UP >0; (3) Design a load-side step-down operation control strategy based on PI to stabilize the DC bus voltage in off-grid mode; (3-1) Define the load side voltage regulation term ΔV as: Among them, k pΔv and k iΔv is the control parameter of the second PI controller, s is a complex variable; (3-2) In off-grid mode, the DC bus voltage reference value V DC,ref And the actual value V DC Input the second PI controller to obtain the load side voltage regulation term ΔV; (4) Design VSG control strategy to enable grid-connected inverters to simulate synchronous generators and participate in active frequency regulation; (4-1) The swing equation of the designed VSG controller is: P in -P-D·(w-w0-Δw)=J·w0·s·(w-w0) Among them, P in is the reference active power provided by the speed regulator, P is the active power output of the grid-type inverter, D is the damping factor, w and w0 are the actual angular frequency and nominal angular frequency of the output voltage of the grid-type inverter respectively, and J is the moment of inertia; In the grid-connected mode, Δw is the frequency synchronization compensation term generated by the pre-synchronization controller; In off-grid mode, Δw=0; (4-2) In order to simulate the droop characteristics of the SG speed regulator, the reference active power P is designed. in for: P in =P ref -m w ·(w-w0-Δw) Among them, m w is the active power droop coefficient of the frequency converter, P ref is the reference active power; In grid-connected mode, P ref The third PI controller is used to obtain the frequency regulation term ΔS, which is used to reduce the power of the PV system so as to release the reserved power of the PV system or increase the power reserve to provide frequency support when the power grid faces frequency disturbances; In off-grid mode, P ref =0; (4-3) The dynamic characteristics between the active power P and the angular frequency w of the grid-connected inverter are obtained from equations (4-1) and (4-2): J·w0·s·(w-w0)=P ref -m w ·(w-w0-Δw)-P -D·(w-w0-Δw) (4-4) The relationship between the power angle θ and the frequency w is: Wherein, in the grid-connected mode, Δθ represents the phase angle synchronization compensation term generated by the pre-synchronization controller; in the off-grid mode, Δθ=0; (4-5) The dynamic characteristics between the reactive power Q output by the grid-connected inverter and the voltage amplitude reference value V are designed as follows: V=V0-m q ·(Q-Q ref )+ΔV Where V0 is the rated voltage amplitude, Q ref and Q are the reference value and actual value of reactive power output by the grid-connected inverter, respectively. q is the reactive power droop coefficient; In the grid-connected mode, ΔV represents the voltage synchronization compensation term generated by the presynchronization controller; In off-grid mode, ΔV represents the voltage regulation term generated by the second PI controller; In off-grid mode, when the system is subject to a large disturbance, the voltage regulation term ΔV is used to reduce the load-side voltage, and the DC voltage is kept stable by stepping down the load-side voltage. (5) In the grid-connected mode, a pre-synchronization control strategy is designed to enable the grid-connected inverter to operate in parallel with the grid; (5-1) Calculate the voltage amplitude V of the grid-connected inverter GFI GFI , the voltage amplitude of the common coupling point PCC is V PCC , the voltage phase angle θ of the grid-connected inverter GFI GFI and the voltage phase angle θ at the common coupling point PCC PCC : θ PCC =arctan(v d,PCC / v q,PCC ) θ GFI =arctan(v d,GFI / v q,GFI ) Among them, v d,GFI 、v q,GFI They are the grid-connected inverter GFI output voltage v abc,GFI The d-axis voltage and q-axis voltage after abc→dq transformation, v d,PCC 、v q,PCC The grid voltage v abc,PCC The d-axis voltage and q-axis voltage after abc→dq transformation; (5-2) In order to make the voltage amplitude of the grid-connected inverter equal to the voltage amplitude of the PCC, the voltage amplitude synchronization compensation term ΔV is generated by the fourth PI controller: Among them, define e V =V PCC -V GFI , k pv and k iv is the control parameter of the fourth PI controller; (5-3) The phase angle synchronization compensation term Δθ is generated by the fifth PI controller: Among them, define e θ =θ PCC -θ GFI , k pθ and k iθ is the control parameter of the fifth PI controller; (5-4) The frequency synchronization strategy is designed as follows: ① Calculate the voltage phase angle of PCC: Among them, w PCC is the grid frequency, θ 0,PCC is the initial voltage phase angle of PCC; ② Frequency deviation between the grid and the grid-connected inverter e w Defined as: have been w =w PCC -w GFI Among them, w GFI is the grid-type inverter frequency; ③Calculate the phase angle deviation e θ : ④Calculate the phase angle deviation e θ Converges to the value at steady state: ⑤In steady state, the frequency deviation e w Phase angle deviation e θ The relationship is as follows: yes w =k iθ ·e θ (∞) If the phase angle deviation e θ Stable, no PLL is needed, and the frequency synchronization compensation term Δw can be generated by the sixth PI controller, as shown in the following formula: (5-5) According to the above analysis process, the grid connection conditions are designed as follows: Among them, ε V , ε θ and ε w They are the allowable deviations of voltage, phase angle and frequency deviation respectively.

2. A grid-connected photovoltaic system control system that supports active grid frequency support, characterized in that: include: PV system power reduction operation controller is used to allow the PV system to have power reserve to participate in the frequency regulation of the power grid. The input is the output power P of the PV array. PV 、PV array output voltage V PV 、PV array output current I PV , the output is the slope S of the new photovoltaic power-voltage characteristic curve PV ; The PI-based DC bus voltage controller is used to adjust the DC bus voltage to its reference value. The input is the DC bus voltage reference value V DC,ref And the actual value V DC , the output is the PV array output voltage reference value V PV,ref ; The load-side step-down operation controller based on PI is used to stabilize the DC bus voltage in off-grid mode. The input is the DC bus voltage reference value V DC,ref And the actual value V DC , the output is the load side voltage regulation term ΔV; The VSG controller is used to make the grid-type inverter simulate the synchronous generator to participate in active frequency regulation, and the input is the reference active power P in And actual active power P, reactive power reference value Q ref And the actual value Q, the voltage adjustment term ΔV and the phase angle adjustment term Δθ, the output is the output voltage amplitude reference value V and the power angle θ of the grid-forming inverter; Pre-synchronization controller, used to make the grid-connected inverter run in parallel with the grid, the input is the output voltage v of the grid-connected inverter GFI abc,GFI and grid voltage v abc,PCC The output is the voltage amplitude synchronous compensation term ΔV, the phase angle synchronous compensation term Δθ and the grid-connected signal s w ; The PV system power reduction operation controller is connected to the PI-based DC bus voltage controller, and the VSG controller is respectively connected to the PI-based load side voltage reduction operation controller and the pre-synchronization controller, so as to implement the grid-connected photovoltaic system control method supporting active grid frequency support as described in claim 1.

3. A grid-building photovoltaic system that supports active grid frequency support, comprising: PV arrays; DC / DC converter; DC / AC grid-connected inverter; The PV array is connected to the grid through DC / DC converters and DC / AC grid-forming inverters in turn; It is characterized in that it also includes a grid-type photovoltaic system control system that supports active grid frequency support as described in claim 2, which is used to actively control the frequency modulation of the DC / DC converter and the DC / AC grid-type inverter.

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