Power management system for string-type solar inverters

Abstract

This invention provides a power management system and control method for a string-type solar inverter that provides a stable power supply to an AC grid. [Solution] The system includes a first DC / DC converter 114 coupled between a first solar panel string 112 and a DC voltage bus, a large power point (MPP) tracker 101 that performs a maximum power point tracking (MPPT) control scheme for the first DC / DC converter, a power reservoir 102 coupled between a second solar panel string 122 and a DC voltage bus and including a second DC / DC converter 124, and an inverter 154 coupled between the DC voltage bus and a power grid that operates in multiple modes including grid forming (GFM) mode and grid following (GFL) mode, in GFM mode, the second DC / DC converter adjusts the voltage on the DC voltage bus.

Description

[Technical Field]

[0001] Embodiments of this disclosure relate to photovoltaic (PV) energy conversion systems, and more particularly to power management and control methods for string-type photovoltaic inverters capable of providing grid forming (GFM) functionality. [Background technology]

[0002] As renewable energy deployment continues worldwide, power systems face new stability challenges. Traditional power grids rely heavily on synchronous generators, where the inertia and damping inherently provided by the rotational mass of these generators contribute to maintaining grid frequency stability. However, inverter-based renewable energy sources, such as photovoltaic (PV) systems, typically operate using grid-following (GFL) control. In grid-following control, the inverter behaves as a current source synchronized to the grid via a phase-locked loop (PLL). GFL inverters have extremely low internal energy storage and their control mechanisms are not designed to mimic the behavior of synchronous generators, so they do not establish grid voltage or frequency and do not provide significant inertia.

[0003] Recent efforts have focused on grid forming (GFM) inverter technology, which allows inverters to act as a voltage source capable of setting grid voltage and frequency, supporting autonomous operation, and providing combined inertia and damping. GFM inverters can provide important stabilization services, including frequency response and black start functionality. These functions are effectively applied in battery energy storage systems where sufficient energy is available on the DC side of the inverter. However, conventional solar inverters typically operate each solar string at its maximum power point (MPP), and there is no power margin to provide inertia, making it difficult to easily implement GFM control.

[0004] Two approaches have been proposed to enable solar inverters to provide grid forming capabilities: (1) adding a dedicated energy storage device to the DC side of the inverter, or (2) operating the PV strings away from the MPP so that headroom is available when grid support is needed. The former solution increases the cost and complexity of the system, while the latter solution requires careful coordination of the DC / DC conversion stage to ensure that sufficient reserve power is available without compromising the overall system performance.

[0005] In conventional inverter architectures, the DC bus is controlled by the inverter, while each DC / DC converter independently executes a maximum power point tracking (MPPT) algorithm. However, under GFM operation, the inflow and outflow power to the DC bus are determined by unrelated control targets, which can lead to DC bus instability in such a configuration. Without direct coordination, undesirable voltage fluctuations can occur in the DC bus, limiting the feasibility of GFM operation in PV energy conversion systems. Therefore, there is a need for an improved power management architecture for string-type solar inverters that enables stable grid forming control without the need for additional external energy storage devices. This disclosure addresses this need. [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] These and other problems are generally resolved or avoided by preferred embodiments of the present disclosure that provide a method for power management and control of string-type solar inverters, and technical advantages are generally achieved. [Means for solving the problem]

[0007] According to one embodiment, the system includes a maximum power point (MPP) tracker coupled between a first string of solar panels and a DC voltage bus, comprising a first DC / DC converter and configured to perform a maximum power point tracking (MPPT) control scheme on the first DC / DC converter; a power reservoir coupled between a second string of solar panels and the DC voltage bus, comprising a second DC / DC converter; and an inverter coupled between the DC voltage bus and a power grid, configured to operate in a plurality of modes, including grid forming (GFM) mode and grid following (GFL) mode, wherein in GFM mode, the second DC / DC converter is configured to regulate the voltage on the DC voltage bus.

[0008] According to another embodiment, the method includes the steps of: configuring a maximum power point (MPP) tracker to perform a maximum power point following (MPPT) control scheme for a first DC / DC converter coupled between a first solar panel string and a DC voltage bus in a string inverter power system; configuring an inverter coupled between the DC voltage bus and a power system to operate in a plurality of modes, including grid forming (GFM) mode and grid following (GFL) mode; configuring a second DC / DC converter in a power reservoir to regulate the voltage on the DC voltage bus when the inverter is configured to operate in the GFM mode, and configuring the inverter to regulate the voltage on the DC voltage bus when the inverter is configured to operate in the GFL mode, wherein the second DC / DC converter is coupled between a second solar panel string and the DC voltage bus.

[0009] According to yet another embodiment, the system includes N DC / DC converters respectively coupled between N strings of solar panels and a DC voltage bus, an inverter coupled between the DC voltage bus and a power grid and configured to operate in a plurality of modes including a grid-forming (GFM) mode and a grid-following (GFL) mode, and a control circuit configured to designate one of the N DC / DC converters as a maximum power point (MPP) tracker, operate the designated DC / DC converter according to a maximum power point tracking (MPPT) control scheme, and operate the remaining DC / DC converters of the N DC / DC converters as power reserve converters. In the GFM mode, the power reserve converters are configured to adjust the voltage on the DC voltage bus.

[0010] The above is a fairly general overview of the features and technical advantages of the present disclosure so that the detailed description of the present disclosure described below can be better understood. Additional features and advantages that form the subject matter of the claims of the present disclosure are described below. It should be understood by those skilled in the art that the disclosed concepts and specific embodiments can be readily utilized as a basis for modifying or designing other structures or processes for accomplishing the same purposes of the present disclosure. It should also be understood by those skilled in the art that such equivalent configurations do not depart from the spirit and scope of the present disclosure as set forth in the appended claims.

Brief Description of the Drawings

[0011] To more fully understand the present disclosure and its advantages, reference is made to the following description taken in conjunction with the accompanying drawings.

[0012] [Figure 1] FIG. 1 shows a block diagram of a string inverter power system according to various embodiments of the present disclosure.

[0013] [Figure 2] FIG. 2 shows a schematic diagram of a first implementation of the string inverter power system shown in FIG. 1 according to various embodiments of the present disclosure.

[0014] [Figure 3] A control block diagram of the string inverter power system shown in FIG. 2 operating in GFM mode according to various embodiments of the present disclosure is shown.

[0015] [Figure 4] A control block diagram of the string inverter power system shown in FIG. 2 operating in GFL mode according to various embodiments of the present disclosure is shown.

[0016] [Figure 5] A schematic diagram of a second implementation of the string inverter power system shown in FIG. 1 according to various embodiments of the present disclosure is shown.

[0017] [Figure 6] A control block diagram of the string inverter power system shown in FIG. 5 according to various embodiments of the present disclosure is shown.

[0018] [Figure 7] A flowchart of a control method for starting the string inverter power system shown in FIG. 1 according to various embodiments of the present disclosure is shown.

[0019] [Figure 8] A flowchart of a control method for operating the string inverter power system shown in FIG. 1 according to various embodiments of the present disclosure is shown.

[0020] Corresponding numbers and symbols in different drawings generally refer to corresponding parts unless otherwise specified. The drawings are drawn to clearly show relevant aspects of various embodiments and are not necessarily drawn to scale.

MODE FOR CARRYING OUT THE INVENTION

[0021] The manufacture and use of currently preferred embodiments are described in detail below. However, it should be understood that this disclosure provides many applicable inventive concepts that can be embodied in various specific circumstances. The specific embodiments described are merely illustrative of specific methods for manufacturing and using this disclosure and do not limit the scope of this disclosure.

[0022] This disclosure describes preferred embodiments in a specific context, namely, a power management and control method for a string-type solar inverter capable of providing grid forming (GFM) functionality. However, this disclosure is also applicable to various power conversion systems. Various embodiments will be described in detail below with reference to the accompanying drawings.

[0023] Figure 1 shows a block diagram of a string inverter power system according to various embodiments of the present disclosure. This string inverter power system includes a plurality of solar panel strings divided into two functional groups, namely, a first solar panel string group 11 and a second solar panel string group 21. The string inverter power system also further includes N DC / DC converters divided into two functional groups. The first DC / DC converter group 10 includes M DC / DC converters, each converter in the first DC / DC converter group 10 connected to the corresponding solar panel string in the first solar panel string group 11. The second DC / DC converter group 20 includes the remaining (NM) DC / DC converters, each converter connected to the corresponding solar panel string in the second solar panel string group 21.

[0024] In the illustrated embodiment, a first DC / DC converter group 10 is coupled between a first solar panel string group 11 and a DC voltage bus Vdc, and a second DC / DC converter group 20 is coupled between a second solar panel string group 21 and a DC voltage bus Vdc. Furthermore, an inverter 30 is coupled between the DC voltage bus Vdc and an external power system. The inverter 30 converts the voltage on the DC voltage bus to an AC output and operates in multiple modes, including grid forming (GFM) mode and grid following (GFL) mode, as will be described in detail later. The DC / DC converters of the first and second DC / DC converter groups 10 and 20 jointly manage the extraction of power from the solar panel strings and the regulation of the voltage on the DC voltage bus.

[0025] During the startup process of the string inverter power system in Figure 1, all DC / DC converters in the first DC / DC converter group 10 and the second DC / DC converter group 20 initially operate in DC bus adjustment mode to establish the voltage on the DC voltage bus to a predetermined rated value. First, a pre-charge operation is performed to charge the DC bus capacitor to a safe level, followed by a soft-start process that gradually increases the voltage on the DC voltage bus to the rated operating voltage. After the voltage on the DC voltage bus stabilizes, inverter 30 enters pre-synchronization mode to match the voltage of its output filter capacitor to the grid voltage. Next, inverter 30 closes its AC side breaker and enters grid forming mode. Once inverter 30 is operating stably, one of the N DC / DC converters (typically one DC / DC converter in the first DC / DC converter group 10) is designated as the Maximum Power Point (MPP) tracker and begins MPPT ramp-up, while the remaining DC / DC converters continue to adjust the voltage on the DC voltage bus as power reservoirs. During the MPPT ramp-up period, power reserves are unavailable.

[0026] After the startup process is complete, the inverter 30 operates in either GFM mode or GFL mode, depending on the relationship between the grid power demand and the maximum power that can be supplied from the DC / DC converter. In particular, if the output power demand of the string inverter power system is less than or equal to the available power output that the string inverter power system can provide under the MPPT control scheme, the inverter 30 is configured to operate in GFM mode. On the other hand, if the output power demand of the string inverter power system is greater than the power output that the string inverter power system can provide under the MPPT control scheme, the inverter 30 is configured to operate in GFL mode.

[0027] In GFM mode, the inverter 30 functions as a voltage source that establishes and maintains the grid voltage and grid frequency, and the DC / DC converters in the second DC / DC converter group 20 adjust the voltage on the DC voltage bus while operating the corresponding solar panel strings at a voltage higher than the maximum power point voltage in order to maintain power reserve. Power reserve (ΔP) represents the additional active power that the string inverter power system can provide beyond the current operating point. In particular, power reserve (ΔP) represents the additional active power that can be provided by reducing the operating voltage of the solar panel strings of the second solar panel string group 21 toward their respective maximum power points.

[0028] During operation, if a frequency drop occurs in the grid or additional active power is required, the inverter 30 releases the necessary additional power by releasing its power reserve. During this time, the DC / DC converters in the second DC / DC converter group 20 draw additional power by reducing the operating voltage of the relevant solar panel strings toward their respective MPP voltages. If the grid power demand exceeds the maximum available power from the DC / DC converters of DC / DC converter groups 10 and 20 as defined by the MPPT power limit, the inverter 30 transitions from GFM mode to GFL mode. In GFL mode, the inverter 30 behaves as a current source synchronized with the grid voltage, adjusting the voltage on the DC voltage bus while supplying maximum available power from all solar panel strings. Thus, through coordinated GFM / GFL operation and controlled utilization of power reserve, the inverter 30 can provide inertial response, frequency support, and stable power supply under changing grid conditions.

[0029] Figure 2 shows a schematic diagram of a first implementation of the string inverter power system shown in Figure 1, according to various embodiments of the present disclosure. As shown in Figure 2, the string inverter power system includes an MPP tracker 101, a power reservoir 102, a capacitor C1, an inverter 154, a first filter formed by an inductor Lf and a capacitor Cf1, and a breaker S1, and a second filter formed by a capacitor Cf2 and an inductor Lg.

[0030] The MPP tracker 101 includes a first DC / DC converter 114 and associated control circuits (not shown). The first DC / DC converter 114 is coupled between a first solar panel string 112 and a DC voltage bus Vdc. The first solar panel string 112 powers the first DC / DC converter 114, which is configured to operate under an MPPT control scheme. The power reservoir 102 includes a second DC / DC converter 124 and associated control circuits (not shown). The second DC / DC converter 124 is coupled between a second solar panel string 122 and a DC voltage bus Vdc. The second solar panel string 122 powers the second DC / DC converter 124, which is configured to regulate the voltage on the DC voltage bus.

[0031] In practice, a string inverter power system can include multiple solar panel strings and multiple DC / DC converters. Depending on application requirements and design needs, the string inverter power system may also include additional MPP trackers, each containing a DC / DC converter, and additional power reservoirs configured similarly. To clearly illustrate the innovative aspects of this disclosure, Figure 2 shows an embodiment having only a single MPP tracker and a single power reservoir. Those skilled in the art will understand that the operating principles described herein are equally applicable to systems having any appropriate number of MPP trackers and power reservoirs.

[0032] As shown in Figure 2, capacitor C1 is connected between the inputs of inverter 154. The first filter, formed by inductor Lf and capacitor Cf1, is connected to the output of inverter 154. The second filter, formed by capacitor Cf2 and inductor Lg, is connected to the output of the first filter via breaker S1. As shown in Figure 2, breaker S1 is connected between the common node of inductor Lf and capacitor Cf1 and the common node of capacitor Cf2 and inductor Lg. The output of the second filter is connected to the power grid.

[0033] In some embodiments, the first and second DC / DC converters 114 and 124 shown in Figure 2 are implemented as buck-boost converters. However, depending on system requirements, other suitable converter topologies such as boost converters, interleaved boost converters, or stacked dual boost converters (but not limited to these) may also be used. Similarly, inverter 154 can be implemented as a single-phase inverter, a three-phase inverter, or any other inverter configuration suitable for the intended grid connection and power rating.

[0034] During operation, the first DC / DC converter 114 is controlled to perform maximum power point tracking, while the second DC / DC converter 124 acts as a power reservoir, regulating the voltage on the DC voltage bus and supplying backup power as needed. When the inverter 154 is configured to operate in GFM mode, the second DC / DC converter 124 is configured to control the output voltage of the second solar panel string 122 to be higher than the maximum power point voltage of the first solar panel string 112. In the inertia response operation of the string inverter power system in GFM mode, the second DC / DC converter 124 configures the second solar panel string 122 to supply additional power by reducing the output voltage of the second solar panel string 122 until the output voltage of the second solar panel string 122 reaches its maximum power point voltage.

[0035] Figure 3 shows a control block diagram of the string inverter power system shown in Figure 2 operating in GFM mode, according to various embodiments of the present disclosure. The control circuit includes an MPPT unit 311, a power management unit 350, an MPPT PWM generator 313 including a voltage controller 312 and a current controller 314, a power reservoir PWM generator 323 including a voltage controller 322 and a current controller 324, and an inverter PWM generator 353 including a GFM unit 351, a voltage controller 352 and a current controller 354.

[0036] Returning to Figure 2, the MPPT PWM generator 313 generates a PWM signal to control the operation of the first DC / DC converter 114. TRACKER The power reservoir PWM generator 323 is configured to generate a PWM signal for controlling the operation of the second DC / DC converter 124. RESERVOIR The inverter PWM generator 353 is configured to generate a PWM signal for controlling the operation of the inverter 154. INVERTER It is configured to generate.

[0037] As shown in Figure 3, the MPPT unit 311 measures the current I of the first solar panel string 112. PVTRACKER And the measured voltage V at the output of the first solar panel string 112 PVTRACKER The MPPT unit 311 is configured to receive both measurements. The MPPT unit 311 uses both measurements to evaluate the instantaneous operating point on the IV and PV curves of the first solar panel string 112. From these measurements, the reference panel voltage V at the maximum power point is determined by a well-known MPPT algorithm (e.g., Perturb and Observe and Incremental Conductance). PVREF Calculate.

[0038] When the inverter 154 operates in the GFM mode and the first DC / DC converter 114 and the associated first solar panel string 112 are configured as an MPP tracker, the two-pole single-throw switch 315 is switched from the lower position to the upper position. The reference panel voltage V PVREF is input to the MPPT PWM generator 313.

[0039] The MPPT PWM generator 313 has two loops: an outer voltage loop and an inner current loop. The voltage controller 312 represents the outer voltage loop of the MPPT PWM generator 313. The current controller 314 represents the inner current loop of the MPPT PWM generator 313. Since both the outer voltage loop and the inner current loop are well-known in the art, they will not be described here.

[0040] During operation, the reference panel voltage V PVREF is compared with the measured voltage V PVTRACKER in the voltage controller 312 to determine the current reference value input to the current controller 314. The current reference value is compared with the measured current I PVTRACKER in the current controller 314 to determine the PWM signal PWM TRACKER for controlling the operation of the first DC / DC converter 114.

[0041] The power reservoir PWM generator 323 has two loops: an outer voltage loop and an inner current loop. The voltage controller 322 represents the outer voltage loop of the power reservoir PWM generator 323. The current controller 324 represents the inner current loop of the power reservoir PWM generator 323. Since both the outer voltage loop and the inner current loop are well-known in the art, they will not be described here.

[0042] As shown in FIG. 3, a predetermined reference voltage V DCREF is input to the power reservoir PWM generator 323. In some embodiments, the predetermined reference voltage V DCREF is a reference value of the voltage on the DC voltage bus and is generated by a DC voltage bus controller (not shown). The predetermined reference voltage VDCREF This is determined by the desired AC output voltage of the inverter 154. A predetermined reference voltage V DCREF teeth, It is larger than JPEG2026116751000002.jpg414 and can be selected with additional design margins to accommodate the modulation index limitations. Vo is the RMS value of the AC output voltage.

[0043] During operation, a predetermined reference voltage V DCREF This is compared with Vdc in the voltage controller 322 to determine the current reference value that is input to the current controller 324. The current reference value is the measured current I of the second solar panel string 122 in the current controller 324. PVPR This is compared with a PWM signal for controlling the operation of the second DC / DC converter 124. RESERVOIR To decide.

[0044] As shown in Figure 3, the measured voltage V PVTRACKER and the measured current I PVTRACKER This is input to the multiplication unit 310, where the maximum power P from the MPP tracker 101 is input. MPP The following is generated. The power management unit 350 controls the maximum power P MPP The power management unit 350 is configured to receive the following. The power management unit 350 generates different power setpoints for the inverter 154 based on different operating modes. In particular, when the inverter 154 is configured to operate in GFM mode, the power setpoint P INVREF is N×P MPP It is equal to the value obtained by subtracting a predetermined power reserve ΔP from it. On the other hand, if the inverter 154 is configured to operate in GFL mode, the power setpoint P INVREF This is the power required by the power grid, or the total maximum available power N × P MPP It is equal to the power limited by [the specified factor].

[0045] The inverter PWM generator 353 includes a GFM unit 351, a voltage controller 352, and a current controller 354. The inverter PWM generator 353 controls the power setpoint P INVREFIt receives a PWM signal to control the operation of inverter 154. INVERTER It is configured to generate.

[0046] The GFM unit 351 implements a gridforming control scheme incorporating virtual inertia and damping parameters, denoted as J and D, respectively. In this configuration, J represents a virtual inertia component that mimics the inertial behavior of a synchronous generator, enabling the inverter 154 to limit the rate of change of frequency and provide stabilization support during grid disturbances. Parameter D represents a damping coefficient that produces a damping torque effect, which suppresses oscillation after failures or abrupt changes in operating conditions and improves system stability. In typical implementations such as droop control and virtual synchronous generator (VSG) control, the GFM unit 351 uses the J and D parameters within its power loop to mimic the dynamic response of a synchronous generator and generate corresponding reference voltage and frequency signals for the inverter 154. By establishing these reference signals, the GFM unit 351 enables the inverter 154 to synthesize grid support characteristics, maintain stable voltage and frequency, and contribute to overall grid resilience in renewable energy-dominated power systems.

[0047] The inverter PWM generator 353 has two loops: an outer voltage loop and an inner current loop. The voltage controller 352 represents the outer voltage loop of the inverter PWM generator 353. The current controller 354 represents the inner current loop of the inverter PWM generator 353. Since both the outer voltage loop and the inner current loop are well known in the art, they will not be described here.

[0048] During operation, the reference voltage signal generated by the GFM unit 351 is used in the voltage controller 352. ABCINV This is compared to determine the current reference value input to the current controller 354. In some embodiments, V ABCINVThis is the output voltage of inverter 154. The current reference value generated by voltage controller 352 is the measured current I of inverter 154 in current controller 354. ABCINV This is compared with a PWM signal for controlling the operation of inverter 154. INVERTER Determine. In some embodiments, I ABCINV This is the current flowing through the output of inverter 154.

[0049] Figure 4 shows a control block diagram of the string inverter power system shown in Figure 2 operating in GFL mode, according to various embodiments of the present disclosure. Control block diagrams of conventional GFL control schemes are well known in the art. However, in this disclosure, this control block diagram is used as one of the operating modes of the string inverter power system. In this GFL mode, inverter 154 functions as a current source that synchronizes with the grid voltage via a phase-locked loop (PLL) and injects active and reactive currents according to commanded setpoints. During operation, if the available power reserve ΔP is insufficient to support grid forming operation, inverter 154 transitions from GFM mode to the well-known GFL operating mode, thereby allowing inverter 154 to adjust the voltage on the DC voltage bus and track the grid voltage waveform. In this configuration, inverter 154 no longer establishes its own voltage or frequency. Instead, inverter 154 tracks the grid voltage and supplies the maximum power that can be supplied collectively by all the solar panel strings.

[0050] As shown in Figure 4, the GFL control scheme adjusts the voltage on the DC voltage bus Vdc by adjusting the current injection of the inverter, ensuring that the inverter 154 maintains synchronization with the grid while safely supplying the maximum available power from solar power, even under conditions where grid forming support cannot be maintained.

[0051] Figure 5 shows a schematic diagram of a second implementation of the string inverter power system shown in Figure 1, according to various embodiments of the present disclosure. The string inverter power system in Figure 5 is similar to that shown in Figure 2, except that there are M MPP trackers and (NM) power reservoirs, where M and N are both positive integers, and N is greater than M. To clearly illustrate the innovative aspects of the present disclosure, Figure 5 shows an embodiment having three of the M MPP trackers and one of the (NM) power reservoirs. Those skilled in the art will understand that the operating principles described herein are similarly applicable to systems having any appropriate number of MPP trackers and power reservoirs.

[0052] As shown in Figure 5, the MPP tracker 101 includes a first DC / DC converter 114 and associated control circuits (not shown). The first DC / DC converter 114 is coupled between a first solar panel string 112 and a DC voltage bus Vdc. The first solar panel string 112 supplies power to the first DC / DC converter 114, which is configured to operate under an MPPT control scheme.

[0053] The MPP tracker 102 includes a second DC / DC converter 124 and associated control circuits (not shown). The second DC / DC converter 124 is coupled between a second solar panel string 122 and a DC voltage bus Vdc. The second solar panel string 122 powers the second DC / DC converter 124, which is configured to operate under an MPPT control scheme.

[0054] The MPP tracker 103 includes a third DC / DC converter 134 and associated control circuits (not shown). The third DC / DC converter 134 is coupled between a third solar panel string 132 and a DC voltage bus Vdc. The third solar panel string 132 powers the third DC / DC converter 134, which is configured to operate under an MPPT control scheme.

[0055] The power reservoir 104 includes a fourth DC / DC converter 144 and associated control circuits (not shown). The fourth DC / DC converter 144 is coupled between the fourth solar panel string 142 and the DC voltage bus Vdc. The fourth solar panel string 142 supplies power to the fourth DC / DC converter 144, which is configured to regulate the voltage of the DC voltage bus Vdc.

[0056] Figure 6 shows a control block diagram of the string inverter power system shown in Figure 5, according to various embodiments of the present disclosure. The control implementation shown in Figure 6 is similar to the configuration shown in Figure 3, except that it includes two additional MPPT units. In the embodiment of Figure 6, MPPT unit 311 controls the reference panel voltage V for the first solar panel string 112. PVREF1 The PWM signal for the first DC / DC converter 114 is the reference panel voltage V. PVREF1 It is generated based on the following. The MPPT unit 321 uses a reference panel voltage V for the second solar panel string 122. PVREF2 The PWM signal for the second DC / DC converter 124 is the reference panel voltage V. PVREF2 It is generated based on the following. The MPPT unit 331 uses a reference panel voltage V for the third solar panel string 132. PVREF3 The PWM signal for the third DC / DC converter 134 is the reference panel voltage V. PVREF3 It is generated based on this. Thus, the string inverter power system is equipped with three MPP trackers 101, 102, and 103, and three MPPT units 311, 321, and 331. Each MPPT PWM generator has a corresponding voltage control loop and current control loop that operate similarly to the PWM control loop described above for Figure 3.

[0057] In some embodiments, when multiple MPPT units are available as shown in Figure 6, the string inverter power system can dynamically rotate the role of MPP tracker among the DC / DC converters. For example, the controller can select M DC / DC converters from multiple converters coupled to a solar panel string and operate those converters as active MPP trackers by executing an MPPT control scheme on one or more of the selected converters. The designation of the active MPP tracker can be rotated periodically among the M converters based on solar irradiance, variations in shading patterns, or other operating conditions of the corresponding solar panel string. This rotational assignment allows the string inverter power system to more accurately identify the maximum available power from different strings over time, improving overall power estimation and reserve power allocation under non-uniform environmental conditions.

[0058] As shown in Figure 6, the power outputs from the three MPPT units are supplied to an average or maximum value unit 345, which generates either the average or maximum value of the three MPPT outputs, depending on the design needs. The resulting value P MPP This is then provided to the power management unit 350, which then sets the inverter power setpoint P INVREF Determine the parameters and operate the inverter PWM generator in the same manner as shown in Figure 3. This multi-tracker configuration, shown in Figure 6, can improve the accuracy of estimating available solar power when using multiple solar panel strings under fluctuating environmental conditions.

[0059] Figure 7 shows a flowchart of a control method for starting the string inverter power system shown in Figure 1, according to various embodiments of the present disclosure. The flowchart shown in Figure 7 is merely an example and does not unduly limit the scope of the claims. Those skilled in the art will understand that many variations, substitutions, and modifications are possible. For example, the various steps shown in Figure 7 can be added, deleted, replaced, rearranged, and repeated.

[0060] In step 702, the startup process of the string inverter power system is initiated by pre-charging a capacitor coupled between the DC voltage bus and ground.

[0061] In step 704, the capacitor is further charged during the soft-start process so that the voltage on the DC voltage bus reaches a predetermined voltage.

[0062] In step 706, the DC / DC converter in the string inverter power system is configured to adjust the voltage on the DC voltage bus. The inverter then starts the startup sequence by entering pre-synchronization mode, in which the voltage across the inverter's output filter capacitor matches the grid voltage. After synchronization is achieved, the inverter transitions to GFM mode by closing the circuit breaker.

[0063] In step 708, the output power of the string inverter power system is ramped up by executing an MPPT control scheme on the first DC / DC converter, while the remaining DC / DC converters continue to regulate the voltage on the DC voltage bus.

[0064] In step 710, the initial power output setpoint of the inverter is provided based on the power output of the first DC / DC converter.

[0065] In step 712, it is determined whether the output power demand is less than or equal to the available power estimated under the MPPT control scheme. If the condition is met, the method proceeds to step 714; otherwise, the method proceeds to step 716.

[0066] In step 714, the inverter is configured to operate in GFM mode.

[0067] In step 716, the inverter is configured to operate in GFL mode.

[0068] Under normal operation, the inverter is configured to operate in GFM mode. If a sudden power loss or significant load increase occurs in the grid, the grid frequency may drop rapidly. In response to such load transients, the combined inertia function of GFM control causes the inverter to release additional active power by driving the operating voltage of the solar panels, which act as power reservoirs, from a higher voltage (e.g., open-circuit level) to their respective MPP voltages. Once the system frequency begins to stabilize, the inverter's droop control function adjusts the active and reactive power setpoints to continue assisting in grid recovery.

[0069] The droop control method is a distributed strategy that mimics the natural behavior of a synchronous generator by introducing an inverse linear relationship between frequency and active power (as the inverter outputs more active power, the frequency drops slightly) and an inverse linear relationship between voltage and reactive power (as the inverter outputs more reactive power, the voltage drops slightly). Under this scheme, as the inverter supplies more active power, its frequency drops slightly, and as it supplies more reactive power, its terminal voltage drops slightly, so the inverter can self-regulate and stably share power with other distributed resources without communication. When the grid reaches a new steady-state frequency, both the inverter's output power and the operating voltage of the solar panels for the power reservoir settle to the corresponding steady-state values ​​determined by the droop control settings.

[0070] While GFM mode can maintain stable operation under typical disturbances, the available power reserve may become insufficient to maintain gridforming control. In such cases, if the grid frequency exceeds acceptable limits, or if changes in weather conditions reduce available power and the power reserve becomes insufficient to maintain gridforming functionality, the inverter transitions from GFM mode to GFL mode. In GFL mode, the inverter adjusts the DC voltage to track the grid voltage waveform and delivers the maximum available power from the solar panel string according to the conventional GFL control scheme.

[0071] Figure 8 shows flowcharts of control methods for operating the string inverter power system shown in Figure 1, according to various embodiments of the present disclosure. The flowcharts shown in Figure 8 are merely examples and do not unduly limit the scope of the claims. Those skilled in the art will understand that many variations, substitutions, and modifications are possible. For example, the various steps shown in Figure 8 can be added, deleted, replaced, rearranged, and repeated.

[0072] In step 802, the Maximum Power Point (MPP) tracker is configured to perform a Maximum Power Point Tracking (MPPT) control scheme for the first DC / DC converter. The first DC / DC converter is coupled between the first solar panel string and the DC voltage bus in a string inverter power system.

[0073] In step 804, the inverter is configured to operate in multiple modes, including grid forming (GFM) mode and grid following (GFL) mode. The inverter is coupled between the DC voltage bus and the power grid.

[0074] In step 806, a second DC / DC converter in the power reservoir is configured to regulate the voltage on the DC voltage bus when the inverter is configured to operate in GFM mode, and to regulate the voltage on the DC voltage bus when the inverter is configured to operate in GFL mode. The second DC / DC converter is coupled between the second solar panel string and the DC voltage bus.

[0075] The method further includes the steps of: pre-charging a capacitor coupled between a DC voltage bus and ground during the startup process of a string inverter power system; charging the capacitor during a soft-start process so that the voltage on the DC voltage bus reaches a predetermined voltage; configuring the DC / DC converters in the string inverter power system to adjust the voltage on the DC voltage bus; ramping up the output power of the string inverter power system by executing an MPPT control scheme on the first DC / DC converter while the remaining DC / DC converters continue to adjust the voltage on the DC voltage bus; and providing an initial power output setpoint for the inverters based on the power output of the first DC / DC converter.

[0076] The method further includes the steps of configuring the inverter to enter GFM mode after the startup process is complete, and providing an operating power output setpoint for the inverter based on the power output and power reserve of the first DC / DC converter.

[0077] The method further includes the steps of configuring the inverter to operate in GFM mode when the output power demand of the string inverter power system is less than or equal to the available power output that the string inverter power system can provide under an MPPT control scheme, and configuring the inverter to operate in GFL mode when the output power demand of the string inverter power system is greater than the available power output that the string inverter power system can provide under an MPPT control scheme.

[0078] The method further includes the steps of operating the inverter as a voltage source to maintain the voltage and frequency of the AC system by releasing additional power to the AC system in GFM mode, and operating the inverter as a current source to inject power into the AC system in GFL mode.

[0079] The method further includes the steps of: selecting M DC / DC converters from a plurality of DC / DC converters coupled between a corresponding solar panel string and a DC voltage bus; operating at least one of the M DC / DC converters as an MPP tracker by executing an MPPT control scheme on that at least one DC / DC converter; and periodically rotating the role of MPP tracker among the M DC / DC converters based on fluctuations in solar irradiance or operating conditions of the solar panel string.

[0080] The method further includes the step of operating the inverter in GFM mode to provide inertia function, damping function, and primary frequency response function.

[0081] The method further includes the steps of: controlling a second DC / DC converter in GFM mode to set the output voltage of a second solar panel string higher than the maximum power point voltage of a first solar panel string; and configuring a second solar panel string in the inertial response operation of a string inverter power system in GFM mode to supply additional power by controlling a second DC / DC converter to reduce the output voltage of the second solar panel string until the output voltage of the second solar panel string reaches the maximum power point voltage of the second solar panel string.

Claims

1. A maximum power point (MPP) tracker coupled between a first solar panel string and a DC voltage bus, comprising a first DC / DC converter and configured to perform a maximum power point tracking (MPPT) control scheme with respect to the first DC / DC converter, A power reservoir including a second DC / DC converter is coupled between the second solar panel string and the DC voltage bus, An inverter coupled between the DC voltage bus and the power system and configured to operate in multiple modes, including grid forming (GFM) mode and grid following (GFL) mode, Includes, In the GFM mode, the second DC / DC converter is configured to adjust the voltage on the DC voltage bus. system.

2. In the GFM mode, the inverter is configured to provide inertia function, damping function, and primary frequency response function. When the inverter is configured to operate in the GFL mode, the inverter is configured to adjust the voltage on the DC voltage bus. The system according to claim 1.

3. It further includes multiple MPP DC / DC converters and multiple power reservoir DC / DC converters, Each of the aforementioned MPP DC / DC converters is located within a corresponding MPP tracker coupled between at least one solar panel string and the DC voltage bus. Each of the aforementioned power reservoir DC / DC converters is coupled between at least one solar panel string and the DC voltage bus. The system according to claim 1.

4. The inverter is configured to operate in GFM mode when the output power demand of the system is less than or equal to the available power output that the system can provide under the MPPT control scheme. The inverter is configured to operate in GFL mode when the output power demand of the system is greater than the available power output that the system can provide under the MPPT control scheme. In the GFM mode, the inverter is configured as a voltage source that maintains the voltage and frequency of the power system by releasing additional power to the power system, and in the GFL mode, the inverter is configured as a current source that injects power into the power system. The system according to claim 1.

5. In the GFM mode, the second DC / DC converter is configured to control the output voltage of the second solar panel string to be higher than the maximum power point voltage of the first solar panel string. In the inertial response operation of the system in GFM mode, the second DC / DC converter configures the second solar panel string to supply additional power by reducing the output voltage of the second solar panel string until the output voltage of the second solar panel string reaches the maximum power point voltage of the second solar panel string. The first DC / DC converter is a first buck-boost converter, and the second DC / DC converter is a second buck-boost converter. The system according to claim 1.

6. The steps include configuring a maximum power point (MPP) tracker to perform a maximum power point tracking (MPPT) control scheme for a first DC / DC converter coupled between a first solar panel string and a DC voltage bus in a string inverter power system, The steps include configuring an inverter coupled between the DC voltage bus and the power system to operate in multiple modes, including grid forming (GFM) mode and grid following (GFL) mode, If the inverter is configured to operate in the GFM mode, the second DC / DC converter in the power reservoir is configured to adjust the voltage on the DC voltage bus; if the inverter is configured to operate in the GFL mode, the inverter is configured to adjust the voltage on the DC voltage bus. Includes, The second DC / DC converter is coupled between the second solar panel string and the DC voltage bus. method.

7. The startup process of the string inverter power system includes the step of pre-charging a capacitor coupled between the DC voltage bus and ground, In the soft start process, the steps include: charging the capacitor so that the voltage on the DC voltage bus reaches a predetermined voltage; The steps include configuring the DC / DC converter in the string inverter power system to adjust the voltage on the DC voltage bus, The steps include ramping up the output power of the string inverter power system by executing the MPPT control scheme on the first DC / DC converter while the remaining DC / DC converters continue to adjust the voltage on the DC voltage bus, The steps include providing an initial power output setpoint for the inverter based on the power output of the first DC / DC converter, After the completion of the startup process, the inverter is configured to enter the GFM mode. The steps include providing an operating power output setpoint for the inverter based on the power output and power reserve of the first DC / DC converter, Further including, The method according to claim 6.

8. If the output power demand of the string inverter power system is less than or equal to the available power output that the string inverter power system can provide under the MPPT control scheme, the step of configuring the inverter to operate in the GFM mode, If the output power demand of the string inverter power system is greater than the available power output that the string inverter power system can provide under the MPPT control scheme, the step of configuring the inverter to operate in the GFL mode, Further including, The method according to claim 6.

9. In the GFM mode, the inverter is operated as a voltage source to maintain the voltage and frequency of the AC system by releasing additional power to the AC system. In the GFL mode, the steps include operating the inverter as a current source for injecting power into the AC system, Further including, The method according to claim 6.

10. The steps include selecting M DC / DC converters from a plurality of DC / DC converters coupled between the corresponding solar panel string and the DC voltage bus, The steps include: Executing the MPPT control scheme on at least one of the M DC / DC converters to operate at least one of the M DC / DC converters as the MPP tracker; The steps include periodically rotating the role of the MPP tracker among the M DC / DC converters based on fluctuations in solar radiation or the operating conditions of the solar panel string, Further including, The method according to claim 6.

11. In the GFM mode, the steps include operating the inverter to provide an inertia function, a damping function, and a primary frequency response function, In the GFM mode, the steps include controlling the second DC / DC converter to set the output voltage of the second solar panel string higher than the maximum power point voltage of the first solar panel string, The steps of configuring the second solar panel string such that, in the inertial response operation of the string inverter power system in GFM mode, the second DC / DC converter is controlled to supply additional power by reducing the output voltage of the second solar panel string until the output voltage of the second solar panel string reaches the maximum power point voltage of the second solar panel string, Further including, The method according to claim 6.

12. N DC / DC converters are coupled between N solar panel strings and a DC voltage bus, An inverter coupled between the DC voltage bus and the power system and configured to operate in multiple modes, including grid forming (GFM) mode and grid following (GFL) mode, A control circuit is configured to designate one of the N DC / DC converters as a maximum power point (MPP) tracker, operate the designated DC / DC converter according to a maximum power point tracking (MPPT) control scheme, and operate the remaining DC / DC converters as power reservoir converters. Includes, In the GFM mode, the power reservoir converter is configured to adjust the voltage on the DC voltage bus. system.

13. The system according to claim 12, wherein the control circuit is further configured to periodically rotate the designation of the MPP tracker among the N DC / DC converters based on fluctuations in solar radiation or the operating conditions of the solar panel string.

14. The control circuit is further configured to specify M DC / DC converters to operate as MPP trackers by executing their respective MPPT control schemes, and to operate the remaining DC / DC converters as power reservoir converters, where M is greater than 1 and less than N, according to claim 12.

15. During the ramp-up period when the designated MPP tracker increases output power under the MPPT control scheme, the remaining DC / DC converters do not provide power reserves. After the ramp-up period is complete, the power reservoir converter adjusts the DC voltage bus in GFM mode based on the available power reserve from the corresponding solar panel string. The system according to claim 12.

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