Photovoltaic grid-connected micro inverter

Abstract

The utility model discloses a photovoltaic grid-connected micro inverter, which comprises an active power unit and an active and reactive power unit, the active power unit can only output active power, and the active and reactive power unit can simultaneously output active power and reactive power; when the voltage of the power grid is normal, the active power unit works in an MPPT mode; the active and reactive power unit dynamically adjusts active power and reactive power in the active and reactive power unit to meet the requirement of a power grid for a power factor.

Description

Technical Field

[0001] The utility model relates to the technical field of photovoltaic grid-connected inverters, in particular to a photovoltaic grid-connected micro inverter. Background Art

[0002] In order to control the bus voltage at the grid connection point of the photovoltaic power station, most of the photovoltaic power stations currently in operation are equipped with a dynamic reactive power compensation device (SVG) of 20% to 30% of the installed capacity of the photovoltaic power station in accordance with the requirements of the power grid company. The reactive voltage control system in the photovoltaic power station achieves the control target required by the dispatching agency by controlling the SVG.

[0003] If the reactive power regulation capability of the PV inverter itself can be utilized, there is no need for additional SVG, thus saving a lot of construction costs and subsequent operation and maintenance costs.

[0004] Micro inverters generally refer to inverters with a power of less than or equal to 1 kilowatt and component-level MPPT in photovoltaic power generation systems.

[0005] Usually, the micro-inverter topology circuit includes a DC / DC isolation boost circuit and a secondary DC / AC inverter circuit, wherein the primary side of the DC / DC isolation boost circuit is connected to the photovoltaic module, and a DC high voltage is obtained on the DC bus capacitor through the secondary rectifier circuit, and the secondary DC / AC inverter circuit converts the DC high voltage into AC power and sends it to the grid; the secondary DC / AC inverter circuit adopts an H-bridge structure composed of four high-frequency switching tubes, and by controlling the phase of a given current, a phase difference can be generated between the output AC current and the grid voltage, thereby achieving the purpose of controlling the output reactive power; the advantage of this micro-inverter topology is that it can adjust reactive power and has strong functions, but the disadvantage is high cost and complex control.

[0006] Not all microinverters have reactive power regulation capability.

[0007] Chinese patent application number 202311198626.3, a flyback micro-inverter system and its working method, mentions a commonly used micro-inverter circuit, which consists of a front-stage flyback circuit and a rear-stage industrial frequency commutation circuit. The advantage of this circuit topology is low cost and high efficiency, but the problem in use is that the front-stage flyback is a unidirectional circuit, so it is very difficult for the system to generate reactive power. Conventional control can only send active power but not reactive power. This patent improves this micro-inverter so that it can send reactive power. The improved micro-inverter adds a reactive power compensation circuit, which increases the cost and control difficulty.

[0008] The Chinese patent application number 202311178324.X, a reactive power control method for a flyback micro-inverter, adopts another method to solve the above-mentioned problem, but it will cause the output current waveform to be distorted when sending reactive power.

[0009] In order to ensure that the micro-inverter has the ability to regulate reactive power while maintaining a low cost, the existing micro-inverter needs to be improved. Utility Model Content

[0010] The utility model aims to provide a photovoltaic grid-connected micro-inverter, which has the ability to regulate reactive power while maintaining a low cost.

[0011] The photovoltaic grid-connected micro-inverter of the utility model comprises: an active power unit and an active reactive power unit; the outputs of the active power unit and the active reactive power unit are connected in parallel to provide the output power of the photovoltaic grid-connected micro-inverter; the active power unit can only output active power, and the active reactive power unit can output active power and reactive power at the same time.

[0012] The active power unit of the photovoltaic grid-connected micro-inverter of the utility model outputs active power P1, the active power and reactive power output by the active and reactive power units are P2 and Q2 respectively, and the apparent power S2=sqrt(P2*P2+Q2*Q2); the active power P of the output power of the photovoltaic grid-connected micro-inverter is P1+P2, the reactive power Q is Q2, the apparent power S=sqrt(P*P+Q*Q), and the power factor is P / S; the maximum value of the apparent power of the active and reactive power units is the maximum apparent power S2MAX, and the power factor required by the power grid is cosФ.

[0013] When the grid voltage is normal, the active power unit works in MPPT mode; the active and reactive power units dynamically adjust the active power P2 and reactive power Q2 in the active and reactive power units with the maximum apparent power S2MAX as the constraint condition to meet the grid's demand for power factor COSФ.

[0014] The dynamic adjustment includes the following specific steps:

[0015] Steps11: Determine whether S2 is less than S2MAX, if yes, go to Steps12, otherwise go to Steps13;

[0016] Step 12: Determine whether P / S is less than COSФ. If so, the active and reactive power units work in MPPT mode and Q2 decreases. Otherwise, Q2 increases.

[0017] Steps13: Determine whether P / S is less than COSФ. If so, Q2 decreases, otherwise the input current of the active and reactive power units decreases.

[0018] When the grid voltage is abnormal, the active and reactive power units of the photovoltaic grid-connected micro-inverter of the present application provide the reactive power demand of the grid.

[0019] The beneficial effects of the utility model are as follows: the active power unit can only output active power, and the active reactive power unit can output active power and reactive power at the same time; by dynamically adjusting the active power P2 and reactive power Q2 in the active reactive power unit, the power grid's demand for power factor COSФ is met; at the same time, due to the use of low-cost active power units, the photovoltaic grid-connected micro-inverter maintains a low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The present invention provides specific steps for dynamically adjusting the active power and reactive power in the active and reactive power units of the photovoltaic grid-connected micro-inverter in the embodiment of the present application. DETAILED DESCRIPTION

[0021] The present application will be described in detail below with reference to the accompanying drawings.

[0022] The photovoltaic grid-connected micro-inverter of the utility model comprises: an active power unit and an active reactive power unit; the outputs of the active power unit and the active reactive power unit are connected in parallel to provide the output power of the photovoltaic grid-connected micro-inverter; the active power unit can only output active power, and the active reactive power unit can output active power and reactive power at the same time.

[0023] The active power unit of the photovoltaic grid-connected micro-inverter of the utility model outputs active power P1, the active power and reactive power output by the active and reactive power units are P2 and Q2 respectively, and the apparent power S2=sqrt(P2*P2+Q2*Q2); the active power P of the output power of the photovoltaic grid-connected micro-inverter is P1+P2, the reactive power Q is Q2, the apparent power S=sqrt(P*P+Q*Q), and the power factor is P / S; the maximum value of the apparent power of the active and reactive power units is the maximum apparent power S2MAX, and the power factor required by the power grid is cosФ.

[0024] When the grid voltage is normal, the active power unit works in MPPT mode; the active and reactive power units dynamically adjust the active power P2 and reactive power Q2 in the active and reactive power units with the maximum apparent power S2MAX as the constraint condition to meet the grid's demand for power factor COSФ.

[0025] As attached Figure 1 As shown, the dynamic adjustment includes the following specific steps:

[0026] Step 11: Determine whether S2 is less than S2MAX. If so, it means that the current S2 can be increased, and the process goes to Step 12. Otherwise, it means that the current S2 needs to be reduced, and the process goes to Step 13.

[0027] Step 12: Determine whether P / S is less than COSФ. If so, the active and reactive power units work in MPPT mode and Q2 decreases. Otherwise, Q2 increases.

[0028] Steps13: Determine whether P / S is less than COSФ. If so, Q2 decreases, otherwise the input current of the active and reactive power units decreases.

[0029] From the perspective of specific implementation, increasing P2 means increasing the amplitude of the given current that is in phase with the output voltage, and increasing Q2 means increasing the amplitude of the given current that is 90 degrees ahead or 90 degrees behind the output voltage, and whether it is ahead or behind depends on the needs of the power grid; reducing P2 and Q2 means reducing the amplitude of the given current accordingly; and the total output current given by the active and reactive power units is the vector superposition of the above two current givens with a phase difference of 90 degrees.

[0030] However, the increase or decrease of P2 is related to the primary input power of the active and reactive power unit; increasing the primary input power will increase the input power obtained on the secondary DC bus capacitor, which will also lead to an increase in the output active power, and vice versa, reducing the primary input power will reduce the input power obtained on the secondary DC bus capacitor, which will also lead to a decrease in the output active power; the voltage feedback of the secondary DC bus can be compared with the voltage reference of the secondary DC bus to adjust the output active power through the 3P3Z regulator.

[0031] Therefore, in order to increase P2, it is only necessary to increase the primary input power. Conversely, in order to reduce P2, it is only necessary to reduce the primary input power. In order to change the reactive power Q2, it is necessary to change the current setting of the reactive current.

[0032] In Steps12, determine whether P / S is less than COSФ. If it is, it means that P2 needs to be increased or Q2 needs to be reduced. However, as mentioned above, increasing P2 requires increasing the primary input power. The input power of the photovoltaic panel changes with the sunlight. The primary input power can only be increased as much as possible, but it cannot be guaranteed that the input power will definitely increase. Therefore, the active and reactive power units work in MPPT mode and Q2 is reduced, so that P / S approaches COSФ.

[0033] In Steps12, it is determined whether P / S is less than COSФ. If not, Q2 increases. Since it has been determined in Steps11 that S2 can increase, P2 can remain unchanged while Q2 increases.

[0034] In Steps13, it is determined whether P / S is less than COSФ. If so, Q2 is reduced. Since it has been determined in Steps11 that S2 needs to be reduced, P2 can remain unchanged while Q2 is reduced.

[0035] In Steps13, it is determined whether P / S is less than COSФ. If not, the input current of the active and reactive power units is reduced. Because it has been determined in Steps11 that S2 needs to be reduced, Q2 can remain unchanged while P2 is reduced. The photovoltaic panel is connected to the input end of the micro inverter. As long as the input current is continuously reduced, the input power can be reduced, thereby reducing P2, so that P / S approaches COSФ.

[0036] Grid voltage anomalies include low voltage crossing and other situations. Low voltage crossing stipulates that when the grid voltage is disturbed or fails for a period of time, power generation and grid-connected equipment such as photovoltaic inverters can remain connected to the grid and have a certain amount of dynamic reactive power to support the grid voltage to achieve low voltage crossing. When the grid voltage is abnormal, the active and reactive power units of the photovoltaic grid-connected micro-inverter of the present application provide the reactive power demand of the grid. During this process, the active power unit of the photovoltaic grid-connected micro-inverter can operate in a waiting state and output active power of zero. After the low voltage crossing is completed, the output active power is gradually restored.

Claims

1. A photovoltaic grid-connected micro-inverter, characterized in that: include: Active power unit, active reactive power unit; the outputs of the active power unit and the active reactive power unit are connected in parallel to provide the output power of the photovoltaic grid-connected micro-inverter; the active power unit can only output active power, and the active reactive power unit can output active power and reactive power at the same time.

2. The photovoltaic grid-connected micro-inverter according to claim 1, characterized in that: The active power unit outputs active power P1; the active power and reactive power output by the active and reactive power units are P2 and Q2 respectively, and the apparent power S2 = sqrt(P2*P2+Q2*Q2); the output power of the photovoltaic grid-connected micro-inverter is active power P = P1+P2, reactive power Q = Q2, apparent power S = sqrt(P*P+Q*Q), and the power factor is P / S; the maximum value of the apparent power of the active and reactive power units is the maximum apparent power S2MAX, and the power factor required by the power grid is cosФ; When the grid voltage is normal, the active power unit works in MPPT mode; the active and reactive power units dynamically adjust the active power P2 and reactive power Q2 in the active and reactive power units with the maximum apparent power S2MAX as the constraint condition to meet the grid's demand for power factor COSФ.

3. The photovoltaic grid-connected micro-inverter according to claim 1, characterized in that: When the grid voltage is abnormal, the active and reactive power units provide the reactive power demand of the grid.

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