Reactive power control method based on photovoltaic grid-connected inverter

Abstract

The invention discloses a reactive power control method based on a photovoltaic grid-connected inverter, relates to the field of reactive power control methods, focuses on a new theory of reactive power control of the photovoltaic grid-connected inverter, and provides a reactive power control method based on the photovoltaic grid-connected inverter by deeply analyzing active and reactive power output characteristics of a grid-connected photovoltaic system and influences of the active and reactive power output characteristics on power grid voltage. A single-inverter control strategy based on a two-phase static coordinate system is provided, independent control of active power and reactive power is achieved, and constraint conditions about reactive power output such as inverter capacity, power transmission line limitation and direct current side voltage limitation are considered. The photovoltaic grid-connected inverter reactive power control method aims at improving the permeability of a power grid to photovoltaic energy grid connection by designing a new control method, meanwhile, voltage fluctuation of a point of common coupling (PCC) can be more efficiently restrained, and then the stability of the voltage of the power grid and the reliability of a power system are improved.

Description

Technical Field

[0001] The invention relates to a reactive power control method, in particular to a reactive power control method based on a photovoltaic grid-connected inverter. Background Art

[0002] In recent years, with the rapid development of renewable energy such as solar energy, photovoltaic grid-connected power generation technology has attracted more and more attention. The grid-connected inverter is the core of the photovoltaic system grid-connected. When the inverter is running in the grid, the current control mode is essentially the voltage source input and current source output of the inverter. At present, a lot of work has been carried out at home and abroad in filter selection, grid-connected control strategy and controller design, with the aim of improving the power quality of grid-connected current. Grid-connected photovoltaic systems are usually composed of multiple inverters, so it is necessary to analyze the control strategy and reactive power output constraints of a single inverter. Summary of the invention

[0003] The purpose of the present invention is to provide a reactive power control method based on a photovoltaic grid-connected inverter to solve the problems raised in the above background technology.

[0004] To achieve the above object, the present invention provides the following technical solutions:

[0005] A reactive power control method based on a photovoltaic grid-connected inverter, comprising a Thevenin equivalent circuit of a photovoltaic system connected to a power grid and a power grid C, Z grid is the equivalent impedance on the grid side, and the PV system injects active power and reactive power into the grid;

[0006] When the photovoltaic power generation system is controlled by unity power factor, the grid voltage U pv Change ΔU pv , the corresponding current change ΔI pv :

[0007]

[0008] In formula (1), ΔP pv is the change in active power output of the photovoltaic system, U pv is the rated voltage value at the point of common coupling (PCC), S k is the short-circuit capacity, is the equivalent impedance angle, ΔI p is the active current change of the photovoltaic system. From formula (1), it can be seen that when the output power of the photovoltaic system is greater than a certain value, the PCC voltage will exceed the normal value. When the photovoltaic system is connected to the reactive voltage control mode, the grid voltage changes to:

[0009]

[0010] In formula (2), ΔSpv is the power change of the photovoltaic system, θ is the power factor angle of the photovoltaic grid-connected system, ΔI q is the change in reactive current in the photovoltaic system;

[0011] When the active output of the photovoltaic system increases, ΔU pv Located at the top of the r axis, the reactive power of the grid-connected inverter is adjusted to stabilize the PCC voltage, i.e. |U pv |=|U'pv|;

[0012] Based on a two-phase static coordinate system, a single photovoltaic grid-connected inverter including an active control loop, a reactive power independent control loop and a current control loop is established;

[0013] The control of the grid-connected photovoltaic power generation system includes maximum power point tracking control and inverter control; the front stage of the photovoltaic system includes a solar photovoltaic array with a DC / DC boost converter and maximum power point tracking control, and the back stage three-phase inverter adopts dual-loop control. The current loop controls the output current and tracks the command current of the input power grid. The voltage outer loop maintains the stability of the DC bus voltage and provides active current reference for the current loop.

[0014] (1) Current control loop

[0015] In the αβ two-phase stationary coordinate system, the current control quantity is sinusoidal, and the quasi-proportional resonant (PR) controller realizes zero steady-state error tracking. The active and reactive currents are uncoupled and independently controlled. The output of the active power and reactive power control loops obtains the current from the active power and reactive power components of the power grid. The dq / αβ transformation provides the command value for the current loop in the stationary coordinate system. The inverter and the power grid are generally synchronized by a phase-locked loop. However, the grid voltage is distorted and unbalanced. When the voltage is greater than zero, the phase-locked loop is locked, affecting the normal operation of the inverter. In the two-phase stationary coordinate system, the positive sequence fundamental voltage is v+1sα and v+1sε. The dq / αβ transformation is shown in formula (3):

[0016]

[0017] in:

[0018]

[0019] Where ω is the system angular frequency, γ +1 is the positive sequence voltage angle;

[0020] When the grid voltage is distorted and unbalanced, the negative sequence and harmonic components of the grid voltage will cause negative sequence and harmonic components in the grid current. The voltage negative sequence and harmonic components are expressed as:

[0021]

[0022] Feed forward v'sα and v'sβ to suppress the influence of interference on current control and eliminate negative sequence and harmonic components in the power grid;

[0023] In the feedforward control strategy, K pwm is the inverter gain, K pwm =v dc / 2;

[0024] When the grid voltage is unbalanced, the DC side voltage fluctuates twice, causing the active power and reactive power to fluctuate twice, making the grid-connected current contain a large third harmonic. In the active and reactive control loops, a double notch filter is introduced to suppress the harmonics in the active and reactive control outputs and the third harmonics of the grid current. The transfer function of the double notch filter is:

[0025]

[0026] Among them, ω0 = 2 × 100π = 200π, and the frequency of the signal needs to be filtered, Q is the quality factor;

[0027] (2) Reactive power control circuit

[0028] The purpose of reactive loop modeling and control is to achieve independent control of reactive power. The output of the system can quickly and accurately track reactive power and reactive power regulation changes. Reactive power controls the inverter to output reactive current i*sq. The reactive power reference value Q* is determined by the output active power and the grid voltage amplitude. When a grid failure occurs, emergency protection is required for the photovoltaic system. Its Q* is under the constraint of reactive output value. The reactive loop is used in conjunction with the PI controller of the low-pass filter. The open-loop transfer function of the control loop is:

[0029]

[0030] The parameters of the controller are designed according to the dynamic indicators of the type I system, where τ Q3 The value of is set by the cutoff frequency of the low-pass filter in the controller;

[0031] Establish a mathematical model of the three-phase grid-connected inverter. When the upper bridge arm switch is closed, s k =1, when the lower bridge arm switch is closed, s k =0, (k=a,b,c);

[0032] Assuming the three-phase voltages are balanced and positive:

[0033]

[0034]

[0035] Consider a three-phase symmetrical system:

[0036]

[0037] All of the above:

[0038]

[0039] The DC side voltage amplitude of the inverter is v dc The maximum phase voltage of AC output is v r Need to meet:

[0040]

[0041] I sa is the inverter A phase output current, V ra is the inverter output A phase voltage, V sa is the A phase voltage at PCC, is the power factor angle;

[0042] Ignoring the filter inductance loss:

[0043]

[0044] According to the DC voltage constraint:

[0045]

[0046] Consider the inverter capacity constraint:

[0047]

[0048] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention accurately adjusts the reactive power output of the inverter, and the proposed control method helps to maintain the stability of the grid voltage and reduce the voltage fluctuation caused by the access of photovoltaic power sources to the grid. The reactive power control theory mentioned in the present invention helps to efficiently utilize renewable energy such as solar energy and promote the widespread application of clean energy. The present invention also analyzes how the photovoltaic system can provide necessary emergency support to the grid by adjusting the reactive power output of the inverter when the grid fails. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is the Thevenin equivalent circuit diagram of the grid-connected photovoltaic system;

[0050] Figure 2 It is the voltage change vector analysis diagram of the photovoltaic system;

[0051] Figure 3 It is the active and reactive power control strategy diagram in the stationary coordinate system;

[0052] Figure 4 It is the reactive power loop control diagram;

[0053] Figure 5 It is the topological structure diagram of three-phase grid-connected inverter;

[0054] Figure 6 is the inverter phase voltage vector diagram. DETAILED DESCRIPTION

[0055] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0056] In the embodiment of the present invention, Figure 1 As shown, the Thevenin equivalent circuit of the photovoltaic system connected to the grid is connected to the grid c. grid is the equivalent impedance on the grid side. The photovoltaic system can inject active power and reactive power into the grid.

[0057] When the photovoltaic power generation system is controlled by unity power factor, the grid voltage U pv Change ΔU pv , the corresponding current change ΔI pv .

[0058]

[0059] In formula (1), ΔP pv is the change in active power output of the photovoltaic system, U pv is the rated voltage value at the point of common coupling (PCC), S k is the short-circuit capacity, is the equivalent impedance angle, ΔI p is the active current change of the photovoltaic system. From formula (1), it can be seen that when the output power of the photovoltaic system is greater than a certain value, the PCC voltage will exceed the normal value. When the photovoltaic system is connected to the reactive voltage control mode, the grid voltage changes as follows:

[0060]

[0061] In formula (2), ΔS pv is the power change of the photovoltaic system, and θ is the power factor angle of the photovoltaic grid-connected system. ΔI q is the change in reactive current in the photovoltaic system.

[0062] like Figure 2 As shown in Figure 2, when the active output of the photovoltaic system increases, ΔU pvLocated at the top of the r axis. At this time, the reactive power of the grid-connected inverter can be adjusted to stabilize the PCC voltage, that is, |U pv |=|U'pv|. It makes ΔU pv lie in Figure 2 Therefore, the grid-connected photovoltaic system should make full use of the reactive power control of the inverter to regulate reactive power and voltage.

[0063] Single inverter control strategy

[0064] The grid-connected photovoltaic power generation system is composed of multiple inverters. The current photovoltaic automatic voltage control (AVC) system construction requires the control of the inverter, so we should first model and control a single inverter before proceeding to other steps.

[0065] The present invention firstly establishes the overall control block diagram of a single photovoltaic grid-connected inverter based on a two-phase static coordinate system, which mainly includes an active control loop, a reactive independent control loop and a current control loop. Figure 3 shown.

[0066] The control of the grid-connected photovoltaic power generation system includes maximum power point tracking control and inverter control. Figure 3 In the PV system, the front stage includes a solar PV array with a DC / DC boost converter and maximum power point tracking control. The rear stage three-phase inverter adopts dual-loop control. The current loop controls the output current and tracks the command current of the input grid. The voltage outer loop maintains the DC bus voltage stability and provides active current reference for the current loop.

[0067] (1) Current control loop

[0068] In the αβ two-phase stationary coordinate system, the current control quantity is sinusoidal. The quasi-proportional resonant (PR) controller can achieve zero steady-state error tracking, and the active and reactive currents are uncoupled and can be controlled independently. The output of the active power and reactive power control loops can obtain current from the active power and reactive power components of the power grid. The dq / αβ transformation provides a command value for the current loop in the stationary coordinate system. The synchronization of the inverter and the grid is generally achieved by a phase-locked loop, and the grid voltage is distorted and unbalanced, especially when the voltage is greater than zero, the phase-locked loop is locked, affecting the normal operation of the inverter. In the two-phase stationary coordinate system, the positive sequence fundamental voltages are v+1sα and v+1sε. Figure 3 The dq / αβ transformation in is shown in formula (3).

[0069]

[0070] in:

[0071]

[0072] Where ω is the system angular frequency, γ +1 is the positive sequence voltage angle.

[0073] When the grid voltage is distorted and unbalanced, the negative sequence and harmonic components of the grid voltage will cause negative sequence and harmonic components in the grid current. The voltage negative sequence and harmonic components can be expressed as:

[0074]

[0075] Feeding forward v'sα and v'sβ can suppress the influence of interference on current control and eliminate negative sequence and harmonic components in the power grid.

[0076] Feedforward control strategy such as Figure 3 As shown, K pwm is the inverter gain, K pwm =v dc / 2.

[0077] When the grid voltage is unbalanced, the DC side voltage fluctuates twice, causing the active power and reactive power to fluctuate twice, making the grid-connected current contain a large third harmonic. In the active and reactive control loops, the introduction of a double notch filter to suppress the harmonics in the active and reactive control outputs can suppress the third harmonic of the grid current. The transfer function of the double notch filter is:

[0078]

[0079] Among them, ω0 = 2 × 100π = 200π, and the frequency of the signal needs to be filtered. Q is the quality factor.

[0080] (2) Reactive power control circuit

[0081] The purpose of reactive loop modeling and control is to achieve independent control of reactive power. The output of the system can quickly and accurately track reactive power and reactive power regulation changes. Reactive power controls the inverter to output reactive current i*sq. The reactive power reference value Q* is determined by the output active power and the grid voltage amplitude. When a grid failure occurs, emergency protection is required for the photovoltaic system, and its Q* is constrained by the reactive output value. The reactive loop can be used in conjunction with a PI controller of a low-pass filter, and its closed-loop control structure is as follows: Figure 4 shown.

[0082] The open-loop transmission function of the control loop is:

[0083]

[0084] The parameters of the controller can be designed according to the dynamic indicators of the type I system. Q3The value can be set by the cut-off frequency of the low-pass filter in the controller.

[0085] Inverter power constraints

[0086] The above analysis provides emergency protection for the photovoltaic grid-connected system in case of grid emergency. The reactive power output capacity of the inverter is affected by the following three conditions. First, the active power output is limited by the inverter capacity; second, the power transmission is limited; and finally, the inverter capacity needs to be large enough. In this section, the reactive power constraint of the inverter is derived from the DC side voltage limit. Figure 5 Shown is a schematic diagram of the structure of a three-phase photovoltaic grid-connected system.

[0087] In order to establish the mathematical model of the three-phase grid-connected inverter, the following assumptions need to be made: First, the potential of the grid is a pure sine wave with three-phase balance. Second, the filter inductance is linear and saturation is not considered. Finally, the equivalent resistance of the filter inductance on the inverter side is ignored. When the upper bridge arm switch is closed, s k =1. When the lower bridge arm switch is closed, s k =0.

[0088] (k=a,b,c)

[0089] Assuming the three-phase voltages are balanced and positive:

[0090]

[0091] Among them, v NO for Figure 5 The voltage between point N and point O in the three-phase grid-connected inverter topology shown.

[0092] Consider a three-phase symmetrical system:

[0093]

[0094] All of the above:

[0095]

[0096] The present invention adopts SPWM modulation. In order to prevent the output voltage of the inverter from being too low, the DC side voltage amplitude v dc The maximum phase voltage of AC output is v r Need to meet:

[0097]

[0098] Figure 6 The voltage vector diagram of the inverter when the grid is running at a non-unity power factor is shown, and the analysis is performed on phase A. sa is the inverter A phase output current, V rais the inverter output A phase voltage, V sa is the A phase voltage at PCC, is the power factor angle.

[0099] Ignoring the filter inductance loss, Figure 6 (a) It is possible to obtain:

[0100]

[0101] According to the DC voltage constraint:

[0102]

[0103] Consider the inverter capacity constraint:

[0104]

[0105] From equations (15) and (16), it can be seen that when an emergency occurs in the power grid and the photovoltaic system is supported to operate for a short time, the power supply should be reduced.

[0106] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

[0107] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A reactive power control method based on a photovoltaic grid-connected inverter, characterized in that: The Thevenin equivalent circuit of the photovoltaic system connected to the grid and the grid c, Z grid is the equivalent impedance on the grid side, and the PV system injects active power and reactive power into the grid; When the photovoltaic power generation system is controlled by unity power factor, the grid voltage U pv Change ΔU pv , the corresponding current change ΔI pv : In formula (1), ΔP pv is the change in active power output of the photovoltaic system, U pv is the rated voltage value at the point of common coupling (PCC), S k is the short-circuit capacity, is the equivalent impedance angle, ΔI p is the active current change of the photovoltaic system. From formula (1), it can be seen that when the output power of the photovoltaic system is greater than a certain value, the PCC voltage will exceed the normal value. When the photovoltaic system is connected to the reactive voltage control mode, the grid voltage changes to: In formula (2), ΔS pv is the power change of the photovoltaic system, θ is the power factor angle of the photovoltaic grid-connected system, ΔI q is the change in reactive current in the photovoltaic system; When the active output of the photovoltaic system increases, ΔU pv Located at the top of the r axis, the reactive power of the grid-connected inverter is adjusted to stabilize the PCC voltage, i.e. |U pv |=|U'pv|; Based on a two-phase static coordinate system, a single photovoltaic grid-connected inverter including an active control loop, a reactive power independent control loop and a current control loop is established; The control of the grid-connected photovoltaic power generation system includes maximum power point tracking control and inverter control; the front stage of the photovoltaic system includes a solar photovoltaic array with a DC / DC boost converter and maximum power point tracking control, and the back stage three-phase inverter adopts dual-loop control. The current loop controls the output current and tracks the command current of the input power grid. The voltage outer loop maintains the stability of the DC bus voltage and provides active current reference for the current loop. (1) Current control loop In the αβ two-phase stationary coordinate system, the current control quantity is sinusoidal, and the quasi-proportional resonant (PR) controller realizes zero steady-state error tracking. The active and reactive currents are uncoupled and independently controlled. The output of the active power and reactive power control loops obtains the current from the active power and reactive power components of the power grid. The dq / αβ transformation provides the command value for the current loop in the stationary coordinate system. The inverter and the power grid are generally synchronized by a phase-locked loop. However, the grid voltage is distorted and unbalanced. When the voltage is greater than zero, the phase-locked loop is locked, affecting the normal operation of the inverter. In the two-phase stationary coordinate system, the positive sequence fundamental voltage is v+1sα and v+1sε. The dq / αβ transformation is shown in formula (3): in: Where ω is the system angular frequency, γ +1 is the positive sequence voltage angle; When the grid voltage is distorted and unbalanced, the negative sequence and harmonic components of the grid voltage will cause negative sequence and harmonic components in the grid current. The voltage negative sequence and harmonic components are expressed as: Feed forward v'sα and v'sβ to suppress the influence of interference on current control and eliminate negative sequence and harmonic components in the power grid; In the feedforward control strategy, K pwm is the inverter gain, K pwm =v dc / 2; When the grid voltage is unbalanced, the DC side voltage fluctuates twice, causing the active power and reactive power to fluctuate twice, making the grid-connected current contain a large third harmonic. In the active and reactive control loops, a double notch filter is introduced to suppress the harmonics in the active and reactive control outputs and the third harmonics of the grid current. The transfer function of the double notch filter is: Among them, ω0 = 2 × 100π = 200π, and the frequency of the signal needs to be filtered, Q is the quality factor; (2) Reactive power control circuit The purpose of reactive loop modeling and control is to achieve independent control of reactive power. The output of the system can quickly and accurately track reactive power and reactive power regulation changes. Reactive power controls the inverter to output reactive current i*sq. The reactive power reference value Q* is determined by the output active power and the grid voltage amplitude. When a grid failure occurs, emergency protection is required for the photovoltaic system. Its Q* is under the constraint of reactive output value. The reactive loop is used in conjunction with the PI controller of the low-pass filter. The open-loop transfer function of the control loop is: The parameters of the controller are designed according to the dynamic indicators of the type I system, where τ Q3 The value of is set by the cutoff frequency of the low-pass filter in the controller; Establish a mathematical model of a three-phase grid-connected inverter. When the upper bridge arm switch is closed, s k =1, when the lower bridge arm switch is closed, s k =0, (k=a,b,c); Assuming the three-phase voltages are balanced and positive: Consider a three-phase symmetrical system: All of the above: The DC side voltage amplitude of the inverter is v dc The maximum phase voltage of AC output is v r Need to meet: I sa is the inverter A phase output current, V ra is the inverter output A phase voltage, V sa is the A phase voltage at PCC, is the power factor angle; Ignoring the filter inductance loss: According to the DC voltage constraint: Consider the inverter capacity constraint:

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