A seamless switching control method for a direct-current micro-grid photovoltaic controller

Abstract

The application discloses a seamless switching control method of a direct-current micro-grid photovoltaic controller, and is based on a direct-current micro-grid composed of a photovoltaic power generation unit with a photovoltaic controller and an MPPT controller, an energy storage unit and a direct-current bus; a direct-current micro-grid simulation model including the photovoltaic controller, the MPPT controller, the energy storage unit and a local load is established first, and then different converters are coordinately controlled according to a direct-current bus voltage fluctuation law; the application combines the MPPT control with the bus voltage adjustment strategy, does not need communication, does not need to change the hardware structure and control parameters, and compared with the prior art, can realize the maximum power output and the current-limiting voltage regulation function of the photovoltaic under the condition of no communication, and can not generate large voltage fluctuation when mode switching, and can effectively improve the stability of the direct-current micro-grid.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of photovoltaic control of power systems, and particularly relates to a seamless switching control method for a direct-current micro-grid photovoltaic controller. BACKGROUND

[0002] In recent years, with the rapid development of distributed direct-current power sources such as photovoltaic cells, storage batteries, fuel cells, etc., direct-current micro-grids have attracted more and more attention from countries around the world. Compared with alternating-current micro-grids, direct-current micro-grids have the following advantages: reducing the intermediate link of energy conversion, which can reduce energy loss and improve energy utilization efficiency; direct-current micro-grids only have active power flow, without reactive power loss and skin effect, which reduces voltage loss and eliminates the need to consider problems such as phase, frequency synchronization and harmonics, effectively improving power quality.

[0003] A direct-current micro-grid is usually composed of a distributed power generation unit (such as a photovoltaic array), an energy storage unit, and local direct-current loads, all of which are connected in parallel to a direct-current bus through respective DC-DC interface converters. At present, in some independent power systems, such as islands, data centers, aerospace systems, and ship power systems, the advantages of direct-current micro-grids have been demonstrated.

[0004] Traditional photovoltaic control strategies can be divided into MPPT control and droop control. MPPT control is a constant power control method that seeks and maintains maximum power output. In a direct-current micro-grid, a photovoltaic power source is connected to a direct-current bus through a photovoltaic controller. Since MPPT control is a constant power control, a photovoltaic power source operating in MPPT mode may cause power surplus in the micro-grid in an island state, leading to direct-current bus voltage imbalance, such as when the energy storage system is partially damaged or the SOC reaches the upper limit and cannot continue to absorb excess power.

[0005] Therefore, in addition to the maximum power output mode, photovoltaic control also needs to adjust the output power according to the change in the direct-current bus voltage. When the photovoltaic power is greater than the sum of the absorption power of the load and the battery, the photovoltaic unit should operate in CVC mode to stabilize the direct-current bus voltage and ensure the safe operation of the direct-current system.

[0006] In the traditional photovoltaic control switching strategy, the photovoltaic unit should switch the control loop to the CVC control mode or the MPPT control mode when needed. The control target of the MPPT controller is the maximum output power of the photovoltaic module PV, while the control target of the CVC controller is the output voltage of the converter. When the operating mode changes, due to the large difference in control targets in the two modes, some unexpected transient problems may occur. For example, switching may cause large voltage fluctuations, and in severe cases, it may even reach the protection value to cause protection misoperation, threatening the stable operation of the micro-grid. Therefore, the prior art has defects and needs to be improved. SUMMARY

[0007] The present application aims to overcome the above deficiencies, and provide a seamless switching control method for a DC micro-grid photovoltaic controller to solve the above problems.

[0008] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application to solve its technical problems is: a seamless switching control method for a DC micro-grid photovoltaic controller, based on a DC micro-grid composed of a photovoltaic power generation unit with a photovoltaic controller and an MPPT controller, an energy storage unit and a DC bus, wherein the photovoltaic power generation unit and the energy storage unit are connected to the DC bus through a boost DC / DC converter and a bidirectional DC / DC converter respectively, the DC bus is connected to a local load and a bidirectional DC / AC converter, and the other end of the bidirectional DC / AC converter is connected to a large power grid through an isolation transformer; comprising the following steps:

[0009] Step one, establish a DC micro-grid simulation model including a photovoltaic controller, an MPPT controller, an energy storage unit and a local load, and track the maximum power point of the DC micro-grid according to the change of solar radiation under the condition of local load change;

[0010] Define the charge and discharge power Pc of the DC bus as: P C = P PV + P B - P load , wherein P PV is the output power of the photovoltaic power generation unit, P B is the output power of the energy storage unit, and P load is the power consumed by the local load;

[0011] The DC bus voltage U dc and the equivalent capacitance C of the DC bus in the DC micro-grid are represented as follows:

[0012]

[0013] Step two, coordinate control different boost DC / DC converters, bidirectional DC / DC converters and bidirectional DC / AC converters according to the DC bus voltage fluctuation law;

[0014] Step three, mode judgment based on the size of photovoltaic output power, MPPT control or / and CVC control is used for the photovoltaic controller;

[0015] Step four, based on the "negative region" working state of the photovoltaic P-U characteristic curve, MPPT control and offset control are carried out at the same time to realize seamless switching of the working mode of the photovoltaic controller and maintain the stability of the DC micro-grid; wherein the offset control is completed by moving the MPP voltage during the process of switching the photovoltaic controller from the MPPT control mode to the CVC control mode.

[0016] Furthermore, in the MPPT control mode, the MPPT controller obtains the output voltage U of the photovoltaic power generation unit. PV and current value I PV In the MPPT control mode, the MPPT control algorithm generates a reference voltage, and then calculates the photovoltaic maximum power point voltage U through the MPPT control algorithm. mp and with the photovoltaic output voltage U PV The difference is compared and sent to the PI controller to generate the corresponding PWM signal, which then drives the step-up DC / DC converter.

[0017] Furthermore, in the CVC control mode, the set value of the bus voltage U ref The actual value of the bus voltage U dc The difference is sent to the PIⅠ controller to get the voltage offset signal, and then the PIⅡ controller gets the corresponding PWM signal to drive the DC / DC converter to control the output power of the photovoltaic. dc >U ref When the PIⅠ controller generates a bias signal dP / dU ref , corresponding to the generation of an offset voltage U shift , which will act on U pv The PV output voltage deviates from the maximum power point. In CVC control mode, the offset signal dP / dU of the boost DC / DC converter 3 is compared with the measured dP / dU value, and the corresponding PWM signal is generated by the PIⅡ controller, thereby increasing the PV voltage and reducing the output power.

[0018] Furthermore, the maximum value of the DC / DC converter output voltage U0 is subtracted from the output current I d The difference between the product of the voltage and the droop gain R passes through a lower limiter. The lower limit setting value of the lower limiter is the mode switching voltage value considering the droop. The value of is the bus voltage reference value under this strategy. With the actual value of voltage U dc Compare, if After passing through the PIⅠ controller, its value is greater than zero, and is clamped to zero after passing through the negative limiter. The DC / DC converter operates in MPPT mode. Then the offset signal dP / dU ref When it is negative, the photovoltaic voltage increases, thereby achieving the purpose of limiting the photovoltaic power output.

[0019] The beneficial effects of the present application are: the control method of the present application takes the output voltage of the photovoltaic power generation unit as the control variable of the inner loop, and switches the working mode by moving the maximum power point voltage, and when the control mode switching occurs, only the reference value of dP / dU, that is, the ratio of the change value of photovoltaic power to the change value of photovoltaic voltage is slightly adjusted near the maximum photovoltaic power point, thereby ensuring the seamless switching of the photovoltaic controller between the two control modes, and the given change of dP / dU is very small during the mode switching process, which ensures the transient performance of the control mode switching to a certain extent. Compared with the traditional photovoltaic switching method, the control method of the present application has smaller oscillation amplitude and shorter adjustment time during the control mode switching process. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 It is a structure schematic diagram of the photovoltaic storage direct-current micro-grid of the present application;

[0021] Figure 2 It is an equivalent model of the direct-current bus of the present application;

[0022] Figure 3 It is the principle of the PI controller;

[0023] Figure 4 It is the MPPT control algorithm;

[0024] Figure 5 It is a traditional photovoltaic switching strategy schematic diagram;

[0025] Figure 6 It is the photovoltaic P-U characteristic curve "negative area" diagram of the present application;

[0026] Figure 7 It is an improved photovoltaic control strategy schematic diagram of the present application;

[0027] Figure 8 It is an improved photovoltaic control strategy combined with droop control schematic diagram of the present application;

[0028] Figure 9 Mode switching flow chart.

[0029] The reference signs are: 1-photovoltaic power generation unit, 2-energy storage unit, 3-boost DC / DC converter, 4-bidirectional DC / DC converter, 5-bidirectional DC / AC converter, 6-isolation transformer. DETAILED DESCRIPTION

[0030] The present application will be further described below in combination with the drawings and specific embodiments, but the following embodiments are by no means any limitation on the present application.

[0031] With the increase of photovoltaic penetration in DC microgrid, photovoltaic power supply with traditional MPPT control strategy is difficult to meet the demand of collaborative operation, therefore, the application discloses a seamless switching strategy of DC microgrid photovoltaic controller mode, so that the photovoltaic controller can smoothly operate between MPPT control mode and CVC control mode.

[0032] Referring to Figure 1 The DC microgrid of the application is composed of a photovoltaic power generation unit 1 with a photovoltaic controller and an MPPT controller, an energy storage unit 2 and a DC bus, wherein the photovoltaic power generation unit 1 and the energy storage unit 2 are connected to the DC bus through a boost DC / DC converter 3 and a bidirectional DC / DC converter 4 respectively, the DC bus is connected to a local load and a bidirectional DC / AC converter 5, and the other end of the bidirectional DC / AC converter 5 is connected to a large power grid through an isolation transformer 6. The energy storage unit 2 in the embodiment is preferably a battery.

[0033] The application discloses a seamless switching control method of a DC microgrid photovoltaic controller, comprising the following steps.

[0034] Step one, build a DC microgrid simulation model including a photovoltaic controller, an MPPT controller, an energy storage unit 2 and several different types of local loads; then formulate a photovoltaic control seamless switching strategy, track the maximum power point of the DC microgrid according to the change of solar radiation under the condition of local load change, and realize the maximum power point tracking under the condition of solar radiation change.

[0035] DC bus voltage stability is the most important control target of the DC microgrid, referring to Figure 2 The charge and discharge power Pc of the DC bus is defined as P C = P PV + P B - P load , wherein P PV is the output power of the photovoltaic power generation unit 1, P B is the output power of the energy storage unit 2, P load is the power consumed by the local load, and P C represents the charge and discharge power of the DC bus.

[0036] The DC bus voltage U dc and the equivalent capacitance C of the DC bus in the DC microgrid are represented as follows:

[0037]

[0038] When the power of PV unit 1, energy storage unit 2, and the local load is unbalanced, the DC bus voltage changes. The DC bus voltage represents the power balance. A decrease in the DC bus voltage indicates that the power provided by the source cannot meet the local load's needs. An increase in the DC bus voltage indicates that the power provided by the source exceeds the local load's requirements. Therefore, by monitoring changes in the DC bus voltage, the output of PV unit 1 and energy storage unit 2 can be appropriately dispatched.

[0039] Step 2: Coordinate and control the different boost DC / DC converters 3, bidirectional DC / DC converters 4 and bidirectional DC / AC converters 5 according to the DC bus voltage fluctuation law.

[0040] Step 3: Based on the photovoltaic output power, the mode is judged and the photovoltaic controller is controlled by MPPT control and CVC control. The PI control principle involved is as follows: Figure 3 As shown; MPPT control principle see Figure 4 As shown; the traditional switching control strategy is as follows Figure 5 In traditional control strategies, the MPPT controller controls the maximum output power of the photovoltaic module PV, while the CVC controller controls the output voltage of the converter. In contrast, in the seamless switching control strategy proposed in this invention, the PV controller always acts as a voltage source in both modes, and its controlled variable is the output voltage of PV power generation unit 1.

[0041] Step 4: Based on the negative region of the PV PU characteristic curve, MPPT control and offset control are performed simultaneously to achieve seamless switching of the PV controller's operating mode and maintain the stability of the DC microgrid. Offset control refers to the process of switching the PV controller from MPPT control mode to CVC control mode by shifting the MPP voltage.

[0042] observe Figure 6 It can be found that in the photovoltaic PU characteristic curve, there is an interval that makes dP / dU≤0 (this interval is defined as the "negative zone"), and as the output voltage U pv In order to make the photovoltaic output current and photovoltaic output power change in the same direction, the output voltage U pv The operating area is limited to [U mp , U oc ], when the value of dP / dU decreases from 0, the photovoltaic output power gradually decreases from the maximum power, and the dP / dU in the "negative zone" shows a good control characteristic for the output power. mp is the maximum power point (MPP) voltage of the photovoltaic array, that is, the photovoltaic power generation unit 1, U OC is the open circuit voltage of the photovoltaic power generation unit 1.

[0043] Therefore, the size of dP / dU can be changed in the "negative zone" according to the change of the DC bus voltage U dc , so as to realize full-range regulation of the photovoltaic output power. By reducing the photovoltaic output power, the power balance can be maintained. Conversely, the process of switching the photovoltaic controller to the MPPT control mode is to adjust the photovoltaic power generation unit operating point back to its MPP. Therefore, whether in MPPT control or CVC control mode, the photovoltaic output voltage U PV is taken as the control object, and the photovoltaic output power is regulated in combination with the MPPT algorithm and the bias controller. When the mode is switched, there is no need to switch the control object and the control algorithm, and the offset of the corresponding photovoltaic voltage is adjusted by changing the value of dU / dU, and only a small given change near the MPP can reduce the output power of the photovoltaic.

[0044] According to the change of the DC bus voltage U dc , the size of dP / dU is changed in the range of dP / dU≤0, at this time, the value of dP / dU starts from 0 and continuously decreases, and the photovoltaic output power gradually decreases from the maximum power. The dP / dU in the "negative zone" presents good control characteristics for the output power, and full-range regulation of the PV output power is realized.

[0045] The switching control process is shown in Figure 7 . As shown in the figure, the photovoltaic voltage U pv and the photovoltaic current I pv are collected, the MPPT algorithm adopts the incremental conductance method, and the voltage reference value U mp is calculated by the MPPT controller.

[0046] In the MPPT control mode, the DC bus voltage U dc is less than its set value U ref , and the output of the bias control is forced to be zero under the action of the negative limiter, so the dP / dU reference value of the PIⅠ controller is zero, that is, the photovoltaic voltage is U mp . The value of dP / dU is obtained by the incremental conductance method. The principle of the PI controller is as follows Figure 3The PI controller is composed of a proportional element and an integral element. The proportional element is used to reduce the static error of the current automatic control system by adding the difference between the current value and the set value to the system in a certain proportion, so as to reduce the system error err and make the output value closer to the set value. However, the proportional element has a steady-state error, so the integral element is needed to eliminate it. The so-called integral is that when there is a deviation input err, the integral controller will accumulate the deviation over time, that is, the speed of integral accumulation is proportional to the size of the deviation err and the integral speed. As long as there is a deviation err, the output of the integral controller will change, that is, the integral is always in effect, and only when the deviation does not exist, the integral will stop. Of course, there are some problems in using the conductance increment method to calculate the value of dP / dU, that is, when the denominator is small or zero, it is easy to cause the calculation result to be out of range, thereby causing the measurement to fail. In order to avoid the out-of-range situation, when it is detected that the value of dP / dU is out of range, the calculation result is blocked, and the voltage of the 5V photovoltaic is increased, and the upper limit of the value of dP / dU is limited. In the MPPT control mode, the MPPT controller obtains the output voltage and current values of the photovoltaic power generation unit 1, and then calculates the maximum power point voltage U mp of the photovoltaic power generation unit 1. When dP / dU=0, it is compared with the actual value of dP / dU, and then sent to the PI II controller to generate a corresponding PWM signal, and then drive the boost DC / DC converter 3.

[0047] When the U dc is greater than the set value U ref , the PI I controller will generate a bias signal dP / dU ref , and a corresponding offset voltage U shift will be generated, which will act on U pv to make the photovoltaic output voltage deviate from the maximum power point. In the CVC control mode, the offset signal dP / dU of the boost DC / DC converter 3 is compared with the measured value of dP / dU, and the corresponding PWM signal is obtained through the PI II controller, so as to increase the photovoltaic voltage and reduce the output power.

[0048] The power distribution combined mode switching is as shown in Figure 8 . The droop gain R of the droop control determines the power distribution, and the offset control determines the mode switching. The highest value U0 of the output voltage of the DC / DC converter 3 minus the product of the output current I d and the droop gain R, through a lower limit limiter, the lower limit set value of the lower limit limiter is the mode switching voltage value considering the droop. The value of output from the lower limit limiter is the bus voltage reference value under this strategy, and is compared with the actual voltage U dc , and if is greater than U dc, the value of which is greater than zero, is clamped to zero by the negative limiter, and the DC / DC converter 3 operates in the MPPT mode; if U dc , the offset signal dP / dU ref is negative, the photovoltaic voltage increases, thereby limiting the photovoltaic power output.

[0049] Figure 9 is a flow chart of the mode switching.

[0050] Here, by collecting the photovoltaic output voltage U PV and the photovoltaic output current I PV , the value of dP / dU is zero in the MPPT control mode, i.e. the reference value of the photovoltaic voltage is U mp , a bias signal is generated in the CVC control mode, then the dP / dU reference value is compared with the actual value of the feedback signal dP / dU, and the duty cycle driving photovoltaic controller is obtained by the PIⅡ controller to reduce the output power. As shown in Figure 6 , at this time, the working point of the photovoltaic moves along the "negative region" curve to the right, so that even if the output power of the photovoltaic power generation unit 1 changes greatly, the value of U shift is usually relatively small. On this basis, seamless switching control of the MPPT control and the CVC control can be realized.

[0051] The above embodiments only exemplarily illustrate the principles and effects of the present application, and part of the applied embodiments, for those skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which all belong to the protection scope of the present application.

Claims

1. A seamless switching control method for a DC microgrid photovoltaic controller, based on a DC microgrid consisting of a photovoltaic power generation unit (1) with a photovoltaic controller and an MPPT controller, an energy storage unit (2), and a DC bus, wherein the photovoltaic power generation unit (1) and the energy storage unit (2) are connected to the DC bus via a boost DC / DC converter (3) and a bidirectional DC / DC converter (4), respectively, the DC bus is connected to a local load and a bidirectional DC / AC converter (5), and the bidirectional DC / AC converter (5) is connected to a large power grid via an isolation transformer (6); characterized in that: The following steps are involved: Step 1: Establish a DC microgrid simulation model including a photovoltaic controller, an MPPT controller, an energy storage unit (2) and a local load, and track the maximum power point of the DC microgrid according to the change of solar irradiation under the condition of local load change; Define the charging and discharging power of the DC bus ,in P PV is the output power of the photovoltaic power generation unit (1), P B is the output power of the energy storage unit (2), P load Power consumed by local loads; DC bus voltage U dc and the equivalent capacitance on the DC bus C satisfy ; Step 2: Coordinate and control the boost DC / DC converter (3), the bidirectional DC / DC converter (4) and the bidirectional DC / AC converter (5) according to the DC bus voltage fluctuation law; Step 3: Based on the photovoltaic output power, the photovoltaic controller performs MPPT control and / or CVC control; Step 4: Based on the PV PU characteristic curve, MPPT control and offset control are simultaneously performed to achieve seamless switching of the PV controller and maintain the stability of the microgrid. The offset control is to shift the MPP voltage when the PV controller switches from MPPT control to CVC control. When MPPT control is selected, the MPPT controller obtains the output voltage of the photovoltaic power generation unit (1) U PV and current value I PV In the MPPT control mode, the MPPT control algorithm generates a reference voltage, and then calculates the photovoltaic maximum power point voltage through the MPPT control algorithm. U mp and with the photovoltaic output voltage U PV Compare the difference and send it to the PI controller to generate the corresponding PWM signal, which then drives the step-up DC / DC converter (3); When CVC control is selected, the bus voltage setting value U ref The actual value of the bus voltage U dc The difference is sent to the PIⅠ controller to get the voltage offset signal, and then the PIⅡ controller gets the corresponding PWM signal to drive the DC / DC converter (3) to control the output power of the photovoltaic. U dc > U ref When , the PIⅠ controller will generate a bias signal dP / dU ref , which generates an offset voltage U shift , acting on the output voltage of the photovoltaic power generation unit (1) U PV Make the photovoltaic output voltage deviate from the maximum power point; The maximum output voltage of the DC / DC converter (3) U 0 Subtract output current I d With droop gain R The difference of the product of the two passes through a lower limiter, and the lower limit setting value of the lower limiter is the mode switching voltage value considering the droop; the output from the lower limiter The value of is the bus voltage reference value under this strategy. and actual voltage value U dc Compare, if > U dc , after passing through the PIⅠ controller, its value is greater than zero, and is clamped to zero after passing through the negative limiter. The DC / DC converter (3) operates in the MPPT mode; if < U dc , the offset signal is negative and the photovoltaic voltage increases, thereby achieving the purpose of limiting the photovoltaic power output.

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