A method for controlling on-grid and off-grid switching of AC / DC microgrid converters

Abstract

This invention discloses a control method for switching between grid-connected and off-grid operation of an energy storage converter. It achieves smooth switching from off-grid to grid-connected operation by employing phase, frequency, and amplitude pre-synchronization, as well as grid-connected power value setting. Furthermore, it achieves smooth switching by suppressing control loop state fluctuations, extracting the positive and negative sequence of grid-side current, and boosting grid-side power. The method uses coordinate transformation for pre-synchronization control during the off-grid to grid-connected transition, resulting in low computational complexity and smooth switching. The grid current value is used as the setpoint for the current loop during the grid-connected to off-grid transition. Simultaneously, this setpoint is applied to the output of the off-grid voltage loop controller through compensation, reducing distortion of the control loop state variables during the grid-connected to off-grid transition and ensuring the smoothness of the output voltage.

Description

Technical Field

[0001] This invention relates to the field of power electronics and their control technology, and in particular to a method for switching AC / DC microgrid converters between the grid and off-grid. Background Technology

[0002] Microgrids, as an important component of smart grids, generally consist of energy storage units, distributed power sources, grid-connected / off-grid switching devices, and local loads. They are a crucial means to meet power quality and security requirements and promote the development of new energy sources. The AC / DC microgrid converter (PCS) possesses both grid-connected and off-grid operation modes, making it one of the key devices for realizing grid-connected and off-grid operation of microgrids. When a grid fault or planned power outage is detected, the PCS needs to switch from grid-connected mode to off-grid mode; when the grid returns to normal, the PCS needs to switch back from off-grid mode to grid-connected mode.

[0003] The impact of the transition from off-grid to grid-connected operation is closely related to the logic of hardware switching actions and algorithm switching. Both the transient processes of hardware switching actions and algorithm switching affect the AC-side voltage. During the transition from off-grid to grid-connected operation, the phase, amplitude, and frequency of the off-grid output voltage of the AC / DC microgrid converter must be consistent with the grid voltage; any mismatch in these variables will lead to voltage distortion. During the transition from grid-connected to off-grid operation, the PCS loses grid support the instant the grid-side switch is opened. At this moment, algorithm switching and changes in control loop parameters will cause AC-side voltage distortion. The grid-side switching actions and algorithm switching logic must coordinate to reduce output instability caused by sudden changes in state variables during the switching process, thereby ensuring a smooth transition. If the parameter fluctuations of the control loop state variables are not suppressed during the switching process, significant distortion in the AC-side output voltage will occur, potentially leading to equipment failure and shutdown. Summary of the Invention

[0004] This invention addresses the shortcomings and defects of existing technologies by providing a method for switching AC / DC microgrid converters between the grid and the grid. It achieves a smooth switching from off-grid to grid by using phase, frequency, amplitude pre-synchronization and grid-connected power value setting. It also achieves a smooth switching from grid-connected to off-grid by using control loop state variable fluctuation suppression, grid-side current positive and negative sequence extraction, and grid-side power boosting.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] A method for switching AC / DC microgrid converter between grid and off-grid operation includes the following steps:

[0007] Step 1: Off-grid to on-grid switching, obtaining the phase θ of the grid voltage through SOGI phase-locked loop. g The three-phase voltage V of the power grid is obtained using Clark transform and Park transform. GAV GB With V GC V in the corresponding two-phase rotating coordinate system gd With V gq ;

[0008] Step 2: Obtain the V of the PCS AC side output voltage through coordinate transformation. A V B With V C Corresponding phase θ g V in a two-phase rotating coordinate system d With V q Using V d V q With V gd V gq The phase, frequency, and amplitude differences are calculated to obtain the compensation amount for the PCS voltage setpoint. The PCS then adjusts its AC side output voltage V based on the compensation amount. A V B With V C The phase, frequency, and amplitude are synchronized with the grid voltage.

[0009] Step 3: When the PCS output voltage meets the grid connection conditions, the main controller issues a grid-side switch closing command to close the grid-side switch. Through the feedback signal of the grid-side switch closing, the PCS will switch from off-grid operation mode to grid-connected operation mode.

[0010] Step 4: Grid-connected to off-grid switching, via the grid-side current I GA , I GB , I GC The positive sequence, negative sequence and zero sequence components are extracted, and the current values ​​of the three phases on the grid side are calculated as the given values ​​of the PCS grid-connected output current, thereby increasing the output power of the PCS and reducing the output power on the grid side.

[0011] Step 5: Convert the PCS AC side output voltage V A V B With V C As the setpoint for the voltage loop, the setpoint I of the grid-connected current loop is used. d * , I q * The off-grid voltage loop output value is compared with the PI controller to obtain the compensation amount of the off-grid voltage loop, so as to suppress the sudden change of the control loop state quantity at the moment of grid connection to off-grid.

[0012] Step 6: When the PCS output power meets the conditions, the main controller issues a grid-side switch disconnection command to disconnect the grid-side switch. Based on the feedback signal of the grid-side switch disconnection, the PCS switches from grid-connected operation mode to off-grid operation mode.

[0013] Furthermore, in step 1, the grid voltage V is obtained through Clark transformation and PARK transformation. GA V GB With V GC V in the corresponding two-phase rotating coordinate system gd V gq The calculation expression is:

[0014] [ V g d V g q ] = 2 3 [ c o s ( i g ) c o s ( i g − 2 π 3 ) c o s ( i g + 2 π 3 ) − s i n ( i g ) − s i n ( i g − 2 π 3 ) − s i n ( i g + 2 π 3 ) ] [ V G A V G B V G C ]

[0015] Furthermore, in step 2, the V of the PCS AC side output voltage is obtained through coordinate transformation. A V B With V C Corresponding phase θ g V d V q The calculation expression is:

[0016] [ V d V q ] = 2 3 [ c o s ( i g ) c o s ( i g − 2 π 3 ) c o s ( i g + 2 π 3 ) − s i n ( i g ) − s i n ( i g − 2 π 3 ) − s i n ( i g + 2 π 3 ) ] [ V A V B V C ]

[0017] Furthermore, in step 2, V q With V gq The comparison value is used as the compensation value given by the off-grid operating voltage frequency of the PCS, V q With V gq After reaching a consensus, proceed with V. d With V gd The comparison, V d With V gd The comparison value is used as the compensation value for the given voltage amplitude of the PCS off-grid operation, thereby realizing the pre-synchronization of the PCS off-grid output voltage with the grid voltage.

[0018] Furthermore, in step 4, the current I GA , I GB with I GC The positive sequence current component is expressed as:

[0019]

[0020] Among them, I a+ , I b+ , I c+ For the positive sequence fundamental components of each three-phase current, I m+ φ is the amplitude of the fundamental positive sequence current. 1+ ω1 is the initial phase of the fundamental positive sequence current, and ω1 is the fundamental angular frequency.

[0021] Furthermore, in step 4, the current I GA , I GB with I GC The negative sequence current component is expressed as:

[0022]

[0023] Among them, I a- , I b- , I c- For the negative sequence fundamental components of each three-phase current, I m- φ is the amplitude of the fundamental negative sequence current. 1- This represents the initial phase of the fundamental negative sequence current.

[0024] Furthermore, the zero-sequence current component I0 in step 4 is expressed as:

[0025]

[0026] Beneficial technical effects of the present invention:

[0027] 1. Shared phase θ g The information obtained is used with V d V q of and V gd V gq By performing phase, frequency, and amplitude difference calculations, the number of controllers required for off-grid to grid-connected pre-synchronization is reduced, thus decreasing the computational load.

[0028] 2. By using positive-sequence, negative-sequence, and zero-sequence current extraction methods, the unbalanced power output of each phase on the grid side is calculated and compensated, thereby reducing the output current on the grid side and avoiding excessive current during grid-side switch shutdown, which could affect the switch's service life.

[0029] 3. Using the given value I of the grid-connected current loop d , I q The voltage loop output is compared with the off-grid voltage loop output. The comparison value is then processed by the PI controller to obtain the compensation amount of the voltage loop, so as to suppress the sudden change of the control loop state quantity at the moment of switching from grid connection to off-grid. Attached Figure Description

[0030] Figure 1 This is a system topology diagram of the present invention for the on-grid / off-grid switching control method of AC / DC microgrid converter.

[0031] Figure 2 This is a control structure diagram of the on-grid / off-grid switching control method for AC / DC microgrid converters according to the present invention.

[0032] Figure 3 This is a switching principle diagram of the on-grid / off-grid switching control method for AC / DC microgrid converters according to the present invention.

[0033] Figure 4 This is a flowchart illustrating the implementation process of the present invention for the on-grid / off-grid switching control method of AC / DC microgrid converters.

[0034] Figure 5 This is the simulation result of the off-grid to on-grid switching control method for AC / DC microgrid converters in this invention.

[0035] Figure 6 This is the simulation result of the grid-to-off-grid switching control method for AC / DC microgrid converters in this invention. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and do not limit the scope of the invention.

[0037] The system topology diagram of the AC / DC microgrid converter grid-to-off-grid switching control method applied in this invention is as follows: Figure 1 As shown, the AC / DC microgrid converter grid-connected / off-grid switching control system mainly consists of an energy storage device, a DC / AC converter based on a T-type three-level topology, and a grid-connected / off-grid switching switch. The grid-connected / off-grid switching switch function is implemented by the grid-side switch KM2. When switching from off-grid to grid-connected, KM2 will be closed, and when switching from grid-connected to off-grid, KM2 will be opened. Figure 2 This demonstrates the off-grid and grid-connected operation modes of the AC / DC microgrid converter PCS. In off-grid operation, the PCS employs dual-loop voltage and current control, acting as a voltage source to control the voltage output. In grid-connected operation, the PCS uses current loop control, obtaining the output current by setting the power setpoint; in this mode, the PCS acts as a current source to control the current output. Figure 3 The diagram illustrates the switching principle of the AC / DC microgrid converter on-grid and off-grid switching control method. The switching process is mainly divided into two parts: hardware switching and software switching. The hardware switching and software switching work together to avoid large current distortion.

[0038] Figure 4 The implementation process of this invention is illustrated. During the off-grid to grid-connected transition, a pre-synchronization function synchronizes the output voltage of the AC / DC microgrid converter with the grid voltage. Then, a hardware switch is closed, and based on the feedback signal from the hardware switch, the operating mode of the AC / DC microgrid converter is switched to grid-connected mode. During the grid-connected to off-grid transition, the grid-side power is reduced, and then the hardware switch is opened. Similarly, based on the feedback signal from the hardware switch, the operating mode of the AC / DC microgrid converter is switched to off-grid mode.

[0039] The present invention provides a method for controlling the switching between grid connection and off-grid operation of an AC / DC microgrid converter, comprising the following steps:

[0040] Step 1: Off-grid to on-grid switching, obtaining the phase θ of the grid voltage through SOGI phase-locked loop. g The grid voltage V is obtained using Clark transform and Park transform. GA VGB With V GC V in the corresponding two-phase rotating coordinate system gd With V gq The expression is:

[0041] [ V g d V g q ] = 2 3 [ c o s ( i g ) c o s ( i g − 2 π 3 ) c o s ( i g + 2 π 3 ) − s i n ( i g ) − s i n ( i g − 2 π 3 ) − s i n ( i g + 2 π 3 ) ] [ V G A V G B V G C ]

[0042] Step 2: Obtain the V of the PCS AC side output voltage through coordinate transformation. A V B With V C Corresponding phase θ g V in a two-phase rotating coordinate system d With V q Using V d V q With V gd V gq The phase, frequency, and amplitude differences are calculated to obtain the compensation amount for the PCS voltage setpoint. V d V q The expression is:

[0043] [ V d V q ] = 2 3 [ c o s ( i g ) c o s ( i g − 2 π 3 ) c o s ( i g + 2 π 3 ) − s i n ( i g ) − s i n ( i g − 2 π 3 ) − s i n ( i g + 2 π 3 ) ] [ V A V B V C ]

[0044] Using the same θ g After coordinate transformation, the difference in the q-axis components of the two synchronous rotating coordinate systems represents the difference in frequency and phase. This difference is then calculated using proportional-integral calculations, and the result is used as the compensation value for the off-grid voltage frequency setpoint of the PCS. The resulting frequency setpoint compensation value Δf for the AC / DC microgrid converter during pre-synchronization is:

[0045]

[0046] Where K p K i These are the proportional and integral coefficients of the frequency and phase pre-synchronization PI controllers, respectively.

[0047] When V q With V gq When they tend to be consistent, it indicates that the frequency and phase of the AC / DC microgrid converter and the grid voltage are synchronized. At this time, the difference between the d-axis components of the two-phase synchronous rotating coordinate system represents the difference in their amplitudes. By performing proportional-integral calculation on this difference, the compensation value of the PCS off-grid operating voltage amplitude is obtained, and the compensation value ΔV of the AC / DC microgrid converter amplitude during pre-synchronization is obtained as follows:

[0048]

[0049] Among them, K pu K iuThese are the proportional coefficient and integral coefficient of the amplitude pre-synchronization PI controller, respectively.

[0050] Step 3: When the PCS output voltage meets the grid connection conditions, the main controller issues a grid-side switch closing command to close the grid-side switch. Through the feedback signal of the grid-side switch closing, the PCS will switch from off-grid operation mode to grid-connected operation mode.

[0051] Step 4: Grid-connected to off-grid switching, via the grid-side current I GA I GB with I GC The positive sequence, negative sequence and zero sequence components are extracted, the current values ​​of the three phases on the grid side are calculated, and the detected current value is used as the given value of the PCS grid-connected output current, thereby increasing the output power of the PCS and reducing the output power on the grid side.

[0052] Grid-side current I GA I GB with I GC The positive order components are:

[0053]

[0054] Among them, I a+ I b+ I c+ For the positive sequence fundamental components of each three-phase current, I m+ φ is the amplitude of the fundamental positive sequence current. 1+ ω1 is the initial phase of the fundamental positive sequence current, and ω1 is the fundamental angular frequency.

[0055] Grid-side current I GA I GB with I GC The negative order components are:

[0056]

[0057] Among them, I a- I b- I c- For the negative sequence fundamental components of each three-phase current, I m- φ is the amplitude of the fundamental negative sequence current. 1- This represents the initial phase of the fundamental negative sequence current.

[0058] Grid-side current I GA I GB with I GC zero-order components for:

[0059]

[0060] AC / DC microgrid converters are used in grid-connected operation. Figure 2 The power setting method shown controls the output power. When switching from grid connection to off-grid, the values ​​of the positive and negative components, negative sequence component, and zero sequence component of the grid-side current are used as the setpoint values ​​of the grid-connected control current loop of the AC / DC microgrid converter. By increasing the output value of each phase current of the AC / DC microgrid converter, the output value of the grid-side current is reduced, thereby reducing the impact of the grid-side output power on the output voltage of the AC / DC microgrid converter during the grid-connected to off-grid transition.

[0061] Step 5: Measure the voltage V at the grid connection port. A V B With V C As the setpoint for the voltage loop, the setpoint I of the grid-connected current loop is used. d * , I q * The off-grid voltage loop output value is compared with the PI controller to obtain the compensation amount of the off-grid voltage loop, so as to suppress the sudden change of the control loop state quantity at the moment of grid connection to off-grid.

[0062] voltage loop compensation ΔV d , ΔV q The expression is:

[0063]

[0064] Among them, I ud , I uq K is the output value of the voltage control loop. p1 , K i1 These are the proportional and integral coefficients of the PI controller, representing the compensation values ​​for the grid-connected current loop setpoint and the off-grid voltage loop output value, respectively.

[0065] Step 6: When the PCS output power meets the conditions, the main controller issues a grid-side switch disconnection command to disconnect the grid-side switch. Based on the feedback signal of the grid-side switch disconnection, the PCS switches from grid-connected operation mode to off-grid operation mode.

[0066] The method of this invention for switching AC / DC microgrid converters between on-grid and off-grid is shown in the simulation results of off-grid to on-grid conversion. Figure 5 The simulation results of grid-connected to off-grid switching are shown in the figure. Figure 6 As demonstrated, the method of the present invention exhibits good smoothness in the switching process between AC / DC microgrid converters and grid connection, indicating the effectiveness of the present invention.

[0067] The above embodiments are descriptions of specific implementations of the present invention, and not limitations thereof. Those skilled in the art can make various modifications and changes without departing from the spirit and scope of the present invention to obtain corresponding equivalent technical solutions. Therefore, all equivalent technical solutions should be included in the patent protection scope of the present invention.

Claims

1. A method for switching AC / DC microgrid converter between grid and off-grid, characterized in that, Includes the following steps: Step 1: Off-grid to on-grid switching, obtaining the phase of the grid voltage through SOGI phase-locked loop. θ g The three-phase voltages of the power grid are obtained using the Clark transform and the Park transform. V GA , V GB and V GC The corresponding two-phase rotating coordinate system V gd and V gq ; Step 2: Obtain the output voltage of the PCS AC side through coordinate transformation. V A , V B and V C Corresponding phase θ g Two-phase rotating coordinate system V d and V q ,use V d , V q and V gd , V gq The phase, frequency, and amplitude difference are calculated to obtain the compensation amount for the PCS voltage setpoint, and the PCS AC side output voltage. V A , V B and V C Based on whether the compensation amount increases or decreases; Step 3: When the PCS output voltage meets the grid connection conditions, the main controller issues a grid-side switch closing command to close the grid-side switch. Through the feedback signal of the grid-side switch closing, the PCS will switch from off-grid operation mode to grid-connected operation mode. Step 4: Grid-connected to off-grid switching, by controlling the grid-side current. I GA , I GB , I GC The positive sequence, negative sequence and zero sequence components are extracted, and the current values ​​of the three phases on the grid side are calculated as the given values ​​of the PCS grid-connected output current, thereby increasing the output power of the PCS and reducing the output power on the grid side. Step 5: Convert the output voltage of the PCS AC side V A , V B and V C As the setpoint for the voltage loop, the setpoint for the grid-connected current loop is... I d * , I q * The off-grid voltage loop output value is compared with the PI controller to obtain the compensation amount of the off-grid voltage loop, so as to suppress the sudden change of the control loop state quantity at the moment of grid connection to off-grid. Step 6: When the PCS output power meets the conditions, the main controller issues a grid-side switch disconnection command to disconnect the grid-side switch. Based on the feedback signal of the grid-side switch disconnection, the PCS switches from grid-connected operation mode to off-grid operation mode.

2. The AC / DC microgrid converter grid-to-off-grid switching control method according to claim 1, characterized in that, Step 1 obtains the grid voltage through Clark and Park transformations. V GA , V GB and V GC In the corresponding two-phase rotating coordinate system V gd , V gq The calculation expression is: 。 3. The AC / DC microgrid converter grid-to-off-grid switching control method according to claim 1, characterized in that, In step 2, the output voltage of the PCS AC side is obtained through coordinate transformation. V A , V B and V C Corresponding phase θ g of V d , V q The calculation expression is: 。 4. The AC / DC microgrid converter grid-to-off-grid switching control method according to claim 1, characterized in that, In step 2 V q and V gq The comparison value is used as the compensation value given by the off-grid operating voltage frequency of the PCS. V q and V gq After reaching a consensus, proceed V d and V gd The comparison, V d and V gd The comparison value is used as the compensation value for the given voltage amplitude of the PCS off-grid operation, thereby realizing the pre-synchronization of the PCS off-grid output voltage with the grid voltage.

5. The AC / DC microgrid converter grid-to-off-grid switching control method according to claim 1, characterized in that, The current in step 4 I GA , I GB and I GC The positive sequence current component is expressed as: ， in, I a+ , I b+ , I c+ These are the positive-sequence fundamental components of each three-phase current. I m+ This represents the amplitude of the fundamental positive-sequence current. φ 1+ This represents the initial phase of the fundamental positive-sequence current. ω 1 represents the fundamental angular frequency.

6. The AC / DC microgrid converter grid-to-off-grid switching control method according to claim 1, characterized in that, The current in step 4 I GA , I GB and I GC The negative sequence current component is expressed as: ， in, I a- , I b- , I c- These are the negative sequence fundamental components of each three-phase current. I m- This represents the amplitude of the fundamental negative sequence current. φ 1- This represents the initial phase of the fundamental negative sequence current.

7. The AC / DC microgrid converter grid-to-off-grid switching control method according to claim 1, characterized in that, The zero-sequence current component in step 4 I 0 is represented as: 。

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