A photovoltaic linear adaptive frequency control method based on dP / dV

Abstract

The application discloses a photovoltaic linear adaptive frequency control method based on dP / dV, which is used for realizing fast frequency modulation, relieving frequency fluctuation of an alternating current side of a power grid and reducing construction and maintenance costs of an energy storage system. The control method first collects voltage and current parameters of the alternating current side of the power grid, calculates an actual frequency through a phase-locked loop, and then adjusts photovoltaic output power through an adaptive frequency control algorithm according to an error between the actual frequency and a reference frequency, so as to slow down the frequency fluctuation of the alternating current side of the power grid. The control method provided by the application does not need communication between photovoltaic systems, does not need an MPP estimator, has a simple photovoltaic model, and reduces energy storage equipment in the operation process, thereby reducing investment and maintenance costs.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of new energy power generation grid-connected control technology, and particularly relates to a photovoltaic linear adaptive frequency control method based on dP / dV. BACKGROUND

[0002] Smart grid is characterized by distributed renewable energy, focusing on low carbon and environmental sustainability, and is increasingly popular in contemporary society. The widespread use of solar energy and the reduction of photovoltaic module cost have made photovoltaic power generation rapidly become a key component of energy infrastructure in various regions.

[0003] Most photovoltaic controllers extract maximum power from solar energy through a method called MPPT, thereby maximizing economic benefits. However, MPPT strategies usually rely on search methods such as perturb and observe (P&Q) and the like, which do not allow fast frequency regulation. With the increasing capacity of photovoltaic power generation, MPPT strategies can cause problems such as frequency fluctuations, overcharging of energy storage, or exceeding the state of charge (SOC) limit. Expanding the energy storage capacity can solve this problem, but the addition of energy storage devices requires regular maintenance and replacement, which will increase the overall operation and maintenance costs of the device. In addition, in the case of multiple photovoltaic systems operating in parallel, grid-connected control of photovoltaic systems requires communication between photovoltaics to achieve fair distribution of power among photovoltaics, which further increases the communication burden. Therefore, how to balance the impact of the above problems is a research hotspot in the field of new energy power generation grid-connected control. SUMMARY

[0004] To solve the above problems, the purpose of the present application is to propose a photovoltaic linear adaptive frequency control method based on dP / dV, which can adaptively switch the photovoltaic system to the MPPT mode or the power reserve mode according to the high or low of the grid frequency, reduce the frequency change in the primary frequency control stage, and the model is simple and reliable. Multiple photovoltaic systems in parallel adaptively distribute power directly through the change of the grid AC side frequency, so there is no need for communication between photovoltaic systems.

[0005] The purpose of the present application is achieved by the following technical solution: a photovoltaic linear adaptive frequency control method based on dP / dV, comprising the following steps:

[0006] 1: Collect the three-phase voltage and three-phase current parameters of the grid AC side, and obtain the dq axis components of the voltage and current through the rotating coordinate axis abc / dq transformation, and then obtain the actual grid frequency through the phase-locked loop;

[0007] 2: Collect the output voltage and output current of the photovoltaic array on the DC side, the photovoltaic output voltage and output current are used to calculate the value of dP / dV of the photovoltaic array, and the photovoltaic output power is obtained in this way;

[0008] 3: The actual grid frequency obtained from the phase-locked loop in step 1 is input into the adaptive frequency control algorithm to obtain the dP / dV value that the photovoltaic array needs to change;

[0009] 4: The actual dP / dV value of the photovoltaic array is calculated through the photovoltaic output voltage and output current, and the error is obtained by subtracting the dP / dV value obtained in step 3 from the adaptive frequency control algorithm;

[0010] 5: The inductor current of the dc / dc circuit is collected as the reference quantity of PI control in the dc / dc circuit. The error obtained in step 4 is input through the PI link to finally generate a PWM wave to control the opening and closing of the DC / DC circuit IGBT to adjust the output voltage of the photovoltaic, thereby forming a closed-loop control;

[0011] 6: During the operation of the device, the light intensity is reduced, and the adaptive frequency control strategy will release the reserved power to return to the MPPT mode, achieving the purpose of considering the economic efficiency of the device.

[0012] Further, the method for calculating the actual grid frequency in step 1 through abc / dq transformation is specifically:

[0013]

[0014] Where a, b, and c represent the voltages of the a-phase, b-phase, and c-phase of the alternating current side, d represents the d-axis component in the two-phase rotating coordinate system, q represents the q-axis component in the two-phase rotating coordinate system, and k represents the transformation coefficient. If , it represents constant amplitude transformation, and if , it represents constant power transformation, and the power remains unchanged before and after transformation. After obtaining the dq-axis parameters through the formula conversion, the rotational speed of the dq coordinate axis is adjusted through the PI link until the q-axis parameter is 0, at which point the rotational angular velocity ω of the coordinate axis can be obtained. Whereby the actual grid frequency f is obtained.

[0015] Further, the specific process of calculating the dP / dV value of the photovoltaic array using the photovoltaic output voltage and output current in step 2 is as follows:

[0016]

[0017] Where V pv is the photovoltaic output voltage, I pv is the photovoltaic output current, and R pv is the photovoltaic output power. The variable ΔR pv is set to

[0018] Further, the ΔR pvThe reference value is determined by the following method:

[0019]

[0020]

[0021] b1=ΔR pv，min -k1·f max

[0022] In the initial stage of the adaptive frequency control algorithm, the parameter f is set. max f nom ΔR pv，max and ΔR pv，min And it remains fixed throughout the entire operation; f nom and f max The algorithm's operating range in terms of frequency is limited; where f nom This represents the standard frequency under normal system operation. If the grid frequency is lower than f... nom Then the photovoltaic system operates in MPPT mode; f max This indicates the maximum regulation frequency specified by the algorithm. If the grid frequency exceeds f... max Then the photovoltaic system will always be in maximum power reserve mode, that is, operating at ΔR. pv，min The corresponding power point; ΔR pv，min This indicates that when the frequency is at or above f max At that time, photovoltaic power is operating in maximum power reserve mode. Value; ΔR pv，max This indicates that when the frequency is at or below f nom At that time, the photovoltaic system operates in MPPT mode. The value is 0; k1 and b1 are natural numbers, and their values ​​are determined by the values ​​set above.

[0023] Further, step 5 specifically involves: adjusting the error ΔR obtained in step 4... pv，dev Make a judgment and generate a photovoltaic output voltage reference quantity V. pv，ref :

[0024]

[0025] In the formula V step It is a fixed value, representing the reference voltage V for each calculation. pv，ref Both increase or decrease the same voltage value.

[0026] Next, with V pv，ref For reference, adjust the input voltage V of the DC / DC circuit. pv The size of the photovoltaic output power P is thus changed. pv The collected inductor current is I LThis process uses PI-based double-loop control:

[0027] The voltage outer loop:

[0028] I L，ref = K P，v · ΔV + K I，v · (T · ΔV + V sum )

[0029] V sum = T · ΔV + V sum

[0030] The current inner loop:

[0031] d = K P，i · ΔI + K I，i · (T · ΔI + I sum )

[0032] I sum = T · ΔI + I sum

[0033] Where ΔV = V pv - V pv，ref , ΔV represents the error between the photovoltaic output voltage V pv and the voltage reference V pv，ref , V sum represents the integrator of the PI element representing the voltage outer loop; K P，v and K I，v represent the proportional and integral parameters of the voltage outer loop, respectively; ΔI = I L - I L，ref , I L，ref represents the inductor current reference, I sum represents the integrator of the PI element representing the current inner loop, and ΔI represents the error between the inductor current I L and the inductor current reference I L，ref . T represents the sampling time, and d represents the duty cycle of the control IGBT on-off output to the DC / DC circuit.

[0034] The beneficial effects of the present application are:

[0035] (1) The present application can estimate the MPP point by voltage and current sampling, and control the power output of the photovoltaic by dp / dv. This process does not need to add other sensors, and the control is simple and the cost is low.

[0036] (2) The present application can control the photovoltaic to run below the MPP, reserve power to respond to the change of the grid frequency, and reduce the fluctuation range of the grid frequency. This method does not need additional energy storage devices, dump devices, and other power storage and power consumption devices, reducing the operation cost and maintenance cost.

[0037] (3) The application initially defines the upper and lower limits of ΔR pv , but the adjustment margin of power can be changed by adjusting the value of ΔR pv，min , which can be more reasonably adapted to different load change conditions.

[0038] (4) The application adjusts photovoltaic by collecting power grid frequency, and the power grid frequency is the only independent variable. Therefore, when multiple photovoltaics are connected in parallel, only the initial parameters need to be set, and the photovoltaics can realize automatic power distribution without communication. Reduce communication costs and complexity caused by communication process.

[0039] (5) The application can automatically release standby power or switch to MPPT mode when external light weakens, achieving economic benefits of photovoltaic power generation. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 is the overall architecture diagram of the photovoltaic system;

[0041] Figure 2 is the photovoltaic panel dP / dV and photovoltaic output power curve diagram;

[0042] Figure 3 is the photovoltaic system P pv -ΔR pv droop curve diagram;

[0043] Figure 4 is the droop curve diagram of the adaptive frequency adjustment strategy ΔR pv -f;

[0044] Figure 5 is the DC / DC converter control block diagram;

[0045] Figure 6 is the DC / AC converter control block diagram;

[0046] Figure 7 is the experimental configuration architecture diagram;

[0047] Figure 8 is the frequency change comparison diagram of using MPPT mode and adaptive frequency control strategy mode after increasing or decreasing the load;

[0048] Figure 9 is the output power diagram of two photovoltaic systems after increasing or decreasing the load;

[0049] Figure 10 is the power change diagram of the energy storage battery after increasing or decreasing the load;

[0050] Figure 11is a frequency variation comparison diagram of the adaptive frequency control strategy mode after changing the illumination;

[0051] Figure 12 is an output power diagram of two photovoltaic systems after changing the illumination. DETAILED DESCRIPTION

[0052] The specific embodiments of the present application are further described in detail below with reference to the accompanying drawings.

[0053] As Figure 1 shown, the present application provides a photovoltaic linear adaptive frequency control method based on dP / dV, and the specific implementation scheme is as follows:

[0054] Step 1: as Figure 1 shown, the three-phase voltage V abc and three-phase current parameters I abc of the AC side power grid of the photovoltaic inverter are collected, and the dq axis components I d and I q of the voltage and current are obtained through the rotating coordinate axis abc / dq transformation, and then the actual frequency f of the power grid is obtained through the phase-locked loop:

[0055]

[0056] wherein a, b, and c represent the voltages of the a-phase, b-phase, and c-phase of the AC side, d represents the d-axis component in the two-phase rotating coordinate system, q represents the q-axis component in the two-phase rotating coordinate system, k represents the transformation coefficient, if then it represents constant amplitude transformation, if then it represents constant power transformation, and the power remains unchanged before and after the transformation; after obtaining the dq axis parameters through the formula conversion, the rotation speed of the dq coordinate axis is adjusted through the PI link until the q-axis parameter is 0; at this time, the rotation angular velocity ω of the coordinate axis can be obtained; wherein the actual frequency f of the power grid is obtained through .

[0057] Step 2: the output voltage V pv and output current I pv of the DC side photovoltaic array are collected, and V pv and I pv are used to calculate the value of dP / dV of the photovoltaic array. As Figure 2 shown, the size of the photovoltaic output power and the value of dP / dV present a curve relationship of first rising and then falling, and the vertex of the curve is the maximum power point. By comparing the dP / dV and the photovoltaic output power curve, the photovoltaic output power is obtained:

[0058]

[0059] wherein V pv is the photovoltaic output voltage, Ipv For photovoltaic output current, P pv This represents the photovoltaic output power. For simplicity, the variable ΔR will be used in the following text. pv Represented as

[0060] Step 3: Based on the actual power grid frequency f obtained in Step 1, substitute it into the adaptive frequency control algorithm. The control scheme is as follows: Figure 3 As shown, ΔR is set. pv，min These are the settings for maximum power reserve mode, corresponding to power point P. pv，min Set ΔR pv，max The value is 0, which corresponds to the maximum power point P. pv，mpp To adapt to the droop curve of the power grid, the adaptive frequency control algorithm requires the photovoltaic system to enter power storage mode if the grid frequency is above 50Hz, and MPPT mode if the grid frequency is below 50Hz.

[0061] The specific algorithm is as follows: Figure 4 As shown, ΔR is calculated using an adaptive frequency control strategy. pv The reference value is determined by the following method:

[0062]

[0063]

[0064] b1=ΔR pv，min -k1·f max

[0065] In the initial stage of the adaptive frequency control algorithm, the parameter f is set. max f nom ΔR pv，max and ΔR pv，min And it remains fixed throughout the entire operation; f nom and f max The algorithm's operating range in terms of frequency is limited; where f nom This represents the standard frequency under normal system operation. If the grid frequency is lower than f... nom Then the photovoltaic system operates in MPPT mode; f max This indicates the maximum regulation frequency specified by the algorithm. If the grid frequency exceeds f... max Then the photovoltaic system will always be in maximum power reserve mode, that is, operating at ΔR. pv，min The corresponding power point; ΔR pv，min This indicates that when the frequency is at or above f max At that time, photovoltaic power is operating in maximum power reserve mode. Value; ΔR pv，max This indicates that when the frequency is at or below f nomAt that time, the photovoltaic system operates in MPPT mode. The value is 0; k1 and b1 are natural numbers, and their values ​​are determined by the values ​​set above.

[0066] Step 4: Calculate the actual output voltage and current of the photovoltaic array. The numerical value, and the ΔR obtained from the adaptive frequency control algorithm. pv By comparing the two values ​​and subtracting them, the error ΔR is obtained. pv，dev .

[0067] Step 5: As Figure 5 As shown, the inductor current of the DC / DC circuit is collected and used as a reference quantity for PI control in the DC / DC circuit. The obtained error is passed through the PI stage to generate a PWM wave, which is used to control the IGBT switching of the DC / DC circuit to adjust the output voltage of the photovoltaic system, thus forming a closed-loop control. Specifically, ΔR... pv，dev Make a judgment and generate a photovoltaic output voltage reference quantity V. pv，ref :

[0068]

[0069] In the formula V step It is a fixed value, representing the reference voltage V for each calculation. pv，ref Both increase or decrease the same voltage value.

[0070] With V pv，ref For reference, adjust the input voltage V of the DC / DC circuit. pv The size of the photovoltaic output power P is thus changed. pv The collected inductor current is I L This process uses PI-based dual-loop control:

[0071] Voltage outer loop:

[0072] I L，ref =K P，v ·ΔV+K I，v ·(T·ΔV+V sum )

[0073] V sum =T·ΔV+V sum

[0074] Inner current loop:

[0075] d = K P，i ·ΔI+K I，i ·(T·ΔI+I sum )

[0076] I sum =T·ΔI+Isum

[0077] Where, ΔV=V pv -V pv，ref ΔV represents the photovoltaic output voltage V. pv With voltage reference quantity V pv，ref The error between them, V sum The PI circuit represents the integrator of the outer voltage loop; K P，v and K I，v These represent the proportional and integral parameters of the outer voltage loop, respectively. ΔI = I L -I L，ref I L，ref I represents the reference value for inductor current. sum The PI circuit represents the integrator of the inner current loop, and ΔI represents the inductor current I. L With inductor current reference quantity I L，ref The error between them; T represents the sampling time, and d represents the duty cycle of the control IGBT to be turned on and off in the DC / DC circuit.

[0078] like Figure 6 As shown, the output voltage of the DC / DC converter circuit is the same as the output voltage of the DC / AC inverter circuit. Its voltage value should be kept constant, and this voltage can be stably controlled through the outer voltage loop and the inner current loop.

[0079] Step 6: When the light intensity decreases, the system frequency will also decrease. At this time, based on the adaptive frequency control strategy, the photovoltaic system will automatically operate at a higher power operating point or enter MPPT mode to achieve economical system operation.

[0080] To demonstrate the control effect of this invention, a comprehensive dynamic model of an AC microgrid consisting of two photovoltaic power sources, one frequency-drooping energy storage battery, and one AC load was established using Matlab / Simulink. Simulation experiments were conducted, and the experimental configuration architecture is as follows: Figure 7 As shown, the energy storage battery is used to maintain the voltage and frequency of the microgrid. The converter for the energy storage battery adopts traditional Pf droop control with a droop factor of 200 kW / Hz. The specific parameters of the two photovoltaic systems are shown in Table 1. The installed capacity of photovoltaic system 2 is twice that of photovoltaic system 1.

[0081] Table 1: Photovoltaic Array Parameters

[0082]

[0083]

[0084] In this test, the irradiance and temperature were both set to 1000 W / m. 2and 25°C, with an initial load of 405 kW. At t = 1 s, the load was reduced by 100 kW, and at t = 2 s and t = 3 s, the load was increased by 50 kW and 150 kW, respectively. During this test, the conventional MPPT control mode and the adaptive frequency control strategy mode were tested under the above environmental conditions and load changes, respectively. The frequency changes under different control strategies, the output power changes of the two photovoltaic systems, and the output power changes of the energy storage battery are shown in Figure 8 , Figure 9 and Figure 10 It can be seen that when t is less than 1 s, both photovoltaics are working in MPPT mode, the frequency is maintained at 50 Hz, and the energy storage battery is neither charging nor discharging. When the load is reduced by 100 kW, the system frequency under the MPPT mode rises to 50.5 Hz, while the adaptive frequency control mode enters the power reserve state, alleviating the frequency fluctuation, and the system frequency is lower than 50.5 Hz. After t = 2 s, the load is increased, but since the photovoltaics are still operating in the power reserve mode, the adaptive frequency control strategy mode also shows better frequency improvement effect compared with the MPPT control mode. After t = 3 s, the load is increased to about 505 kW, at which time the system frequency drops below 50 Hz, so both control modes enter the MPPT mode to operate and output maximum power. In addition, photovoltaic 1 and photovoltaic 2 always maintain proportional power sharing during the test, achieving the function of power distribution without communication.

[0085] In the variable light test scenario, the load is set to 300 kW. The photovoltaics are controlled by the adaptive frequency control strategy and tested under three different irradiance conditions of 1000 W / m 2 , 800 W / m 2 , and 600 W / m 2 . From 0 s to 1 s, the irradiance is set to 1000 W / m 2 . From 1 s to 1.5 s, the irradiance is set to 800 W / m 2 . After 1.5 s, the irradiance is set to 600 W / m 2 . The system frequency changes and the output power changes of the two photovoltaic power sources are shown in Figure 11 and Figure 12 From 0 s to 1.5 s, the system frequency is higher than 50 Hz, indicating that the photovoltaics are operating in the power reserve mode under the conditions of 1000 W / m 2 and 800 W / m 2 . After 1.5 s, the irradiance is changed to 600 W / m 2At this time, the system frequency is lower than 50Hz, which indicates that the irradiance is not enough to support the normal operation of the whole system, so the photovoltaic automatically cuts in the MPPT mode to improve the economy of the system operation. The simulation results verify that the adaptive frequency control strategy has the automatic adjustment ability to the light change, and also show that the photovoltaic has higher frequency support ability under high irradiance conditions.

[0086] The above examples are used to explain and illustrate the present application, but not to limit the present application, any modification and change made to the present application within the spirit and protection scope of the claims of the present application, fall into the protection scope of the present application.

Claims

1. A dP / dV based photovoltaic linear adaptive frequency control method, characterized in that, It comprises the following steps: (1): Collecting three-phase voltage and three-phase current parameters of the AC side of the power grid, and obtaining dq axis components of the voltage and current through rotating coordinate axis abc / dq transformation, and then obtaining the actual frequency of the power grid through phase-locked loop; (2): Collecting the output voltage and output current of the photovoltaic array on the DC side, and using the photovoltaic output voltage and output current to calculate the value of dP / dV of the photovoltaic array, so as to obtain the output power of the photovoltaic array; (3): the actual grid frequency obtained from the phase-locked loop in step (1) is brought into the adaptive frequency control algorithm to obtain the reference value of dP / dV that the photovoltaic array needs to change ;​ ； ； ； Setting parameters in initial stage of adaptive frequency control algorithm 、 、 and and fixed throughout the operation; and limit the action range of the algorithm on frequency; wherein, represents the standard frequency under normal operation of the system, if the grid frequency is lower than , the photovoltaic works in MPPT mode; represents the highest regulation frequency specified by the algorithm, if the grid frequency exceeds , the photovoltaic will be in maximum power reserve mode, i.e. operating at corresponding power point; represents the value of when the frequency is at or exceeds , the operation of the photovoltaic in maximum power reserve mode; represents the value of when the frequency is at or lower than , i.e. 0, the operation of the photovoltaic in MPPT mode; 、 is a natural number, the value is determined by the values set above; (4): Calculating the actual dP / dV value of the photovoltaic array through the photovoltaic output voltage and output current, and subtracting the dP / dV value obtained by the adaptive frequency control algorithm in step (3) to obtain the error; (5): Collecting the inductor current of the DC / DC circuit, generating the reference quantity of the photovoltaic output voltage based on the error obtained in step (4) , the reference quantity is sent into the outer loop PI controller of the double closed loop control, and the outer loop outputs the reference value of the inductor current of the DC / DC circuit as the reference of the inner loop, the inner loop is the inductor current PI control loop, and finally generates the PWM wave to control the opening and closing of the IGBT of the DC / DC circuit to adjust the output voltage of the photovoltaic, so as to form the closed loop control; (6): Reducing the light intensity during the operation of the device, and releasing the reserved power by the adaptive frequency control strategy to return to the MPPT mode.

2. A dP / dV-based photovoltaic linear adaptive frequency control method according to claim 1, characterized in that, The method for calculating the actual frequency of the power grid through abc / dq transformation in step (1) is specifically: ； Wherein, a, b, c represent the voltage of a phase, b phase and c phase of alternating side, d indicates the d axis component under two-phase rotating coordinate system, q indicates the q axis component under two-phase rotating coordinate system, k indicates transformation coefficient, if k= , it represents constant amplitude transformation, if k= , it represents constant power transformation, the power is unchanged before and after transformation;After obtaining the dq axis parameters by formula conversion, the rotating speed of dq coordinate axis is adjusted through PI link until the q axis parameter is 0, at this time, the rotating angular velocity of coordinate axis can be obtained ;Wherein, the actual frequency of power grid is obtained by .​ 3. A dP / dV-based photovoltaic linear adaptive frequency control method according to claim 1, characterized in that, The specific process for calculating the value of dP / dV of the photovoltaic array by using the photovoltaic output voltage and output current in step (2) is: ； wherein is the photovoltaic output voltage, is the photovoltaic output current, is the photovoltaic output power; set variable denotes .

4. A dP / dV-based photovoltaic linear adaptive frequency control method according to claim 1, characterized in that, The step (5) is specifically: judging the error obtained in step (4) to generate the photovoltaic output voltage reference quantity : ； wherein, is a fixed value representing the reference voltage all increase or decrease by the same voltage value; Next, the input voltage of the DC / DC circuit is regulated to change the PV output power The inductance current collected is This process uses a PI-based double-loop control:​ Voltage outer loop: ； ； Current inner loop: ； ； wherein, , represents the error between the photovoltaic output voltage and the voltage reference quantity, represents the integrator of the PI element representing the voltage outer loop; and respectively represent the proportional and integral parameters of the voltage outer loop; , represents the inductor current reference quantity, represents the integrator of the PI element representing the current inner loop, represents the inductor current and the error between the inductor current and the inductor current reference quantity; T represents the sampling time, represents the duty cycle output to the DC / DC circuit to control the on-off of the IGBT.​

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