Dual power supply intermediate frequency heating power supply and control method thereof

Abstract

The application discloses a dual-power-supply intermediate frequency heating power supply and a control method thereof, and belongs to the technical field of intermediate frequency induction heating power supplies, and comprises: a commercial power input unit, a photovoltaic input unit, and an inverter module; the commercial power input unit comprises a multi-pulse rectifier circuit with an integer multiple of 12 pulses; the photovoltaic input unit comprises a plurality of MPPT modules; the DC input end of the inverter module is connected with the DC output end of the multi-pulse rectifier circuit and the DC output end of the MPPT module respectively, and the number of the inverter module is associated with the pulse number of the multi-pulse rectifier circuit; a control unit is connected with the commercial power input unit, the photovoltaic input unit and the inverter module, and is used for controlling the output voltage and frequency of the inverter module, and controlling the start-stop and maximum power tracking of each MPPT module. The application reduces the network side harmonic through the multi-pulse rectification, realizes the direct coupling of the commercial power and the photovoltaic dual power supply DC bus, reduces the energy conversion link, and realizes the flexible expansion of the system capacity based on the modular configuration associated with the pulse number.

Description

Technical Field

[0001] This invention belongs to the field of medium-frequency induction heating power supply technology, and particularly relates to a medium-frequency heating power supply with dual power supply and its control method. Background Technology

[0002] Medium-frequency induction heating power supplies are widely used in industrial heating fields such as metal smelting, heavy oil extraction, heating, and welding. Traditional medium-frequency heating power supplies typically use three-phase uncontrolled rectification or thyristor phase-controlled rectification to convert AC to DC, and then an inverter circuit converts it back to medium-frequency AC to power the load. With the development of power electronics technology, multi-pulse rectification technology has been introduced into the field of medium-frequency power supplies. Multi-pulse rectification circuits use phase-shifting transformers to shift the three-phase voltage before superimposing it through multiple rectifier bridges, which can significantly reduce the harmonic content of grid-side current and improve the power factor. Multi-pulse rectification technology has significant advantages in improving grid compatibility. Meanwhile, with the decline in the cost of photovoltaic power generation, introducing new energy sources such as photovoltaics into industrial heating applications has become an important way to save energy and reduce emissions. Some existing technologies attempt to connect photovoltaic power to medium-frequency heating systems, for example, by coupling photovoltaic power to the DC bus after DC / DC conversion, or by feeding photovoltaic power into the grid through an independent grid-connected inverter before it is utilized by the heating power supply.

[0003] However, existing technologies still have the following shortcomings. First, regarding multi-pulse rectification technology, although the rectification scheme can effectively reduce harmonics, existing intermediate frequency power supplies typically only use multi-pulse rectification as an independent mains input stage, without systematic integration with the photovoltaic input unit. Second, regarding photovoltaic grid connection, traditional solutions usually require photovoltaic power to be connected to the AC grid through multiple DC / DC converters or independent grid-connected inverters before being utilized by the heating power supply. This results in multiple energy conversion stages, system complexity, and low efficiency. While some solutions achieve dual power supply from photovoltaic and mains, energy storage devices are required, increasing system cost and maintenance difficulty. Third, regarding control strategies, most existing intermediate frequency power supplies only support a single grid-connected operation mode, lacking a dedicated control scheme for off-grid (no mains power) conditions where only photovoltaic power is supplied independently. Especially during off-grid operation, the photovoltaic output power fluctuates drastically with the intensity of sunlight, and existing open-loop or closed-loop control strategies struggle to achieve automatic power balancing and effective limitation without the need for energy storage. Fourth, in terms of system expansion, the inverter modules of existing medium-frequency power supplies are mostly independently configured, lacking a modular parallel expansion scheme based on the number of multi-pulse rectifier pulses.

[0004] In summary, there is an urgent need for a medium-frequency heating power supply that can be compatible with both mains power and photovoltaic power supply, ensuring the power quality of the grid side while making full use of photovoltaic energy, and achieving safe, efficient and flexible control under different operating modes. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention proposes a dual-power supply medium-frequency heating power supply and its control method, thereby resolving the issues present in the prior art.

[0006] To achieve the above objectives, in a first aspect, the present invention provides a medium-frequency heating power supply with dual power supply, comprising: The mains power input unit includes a multi-pulse rectifier circuit, wherein the pulse number of the multi-pulse rectifier circuit is an integer multiple of 12; A photovoltaic input unit includes multiple MPPT modules and one or more inverter units. The DC input terminal of the inverter unit is connected to the output terminal of the MPPT module, and the AC output terminal of the inverter unit is used to connect to a load. An inverter module, wherein the DC input terminal of the inverter module is connected to the DC output terminal of the multi-pulse rectifier circuit and the DC output terminal of the MPPT module, respectively. The number of inverter modules is related to the number of pulses in the multi-pulse rectifier circuit; The control unit is connected to the mains input unit, the photovoltaic input unit and the inverter module, and is used to control the output voltage and frequency of the inverter module, as well as control the start-up, shutdown and maximum power point tracking of each MPPT module.

[0007] Preferably, the multi-pulse rectifier circuit includes a phase-shifting transformer, an uncontrolled rectifier bridge connected to the secondary side of the phase-shifting transformer, a medium-voltage switch disposed on the primary side of the phase-shifting transformer, and a current sensor for detecting the mains input current.

[0008] Preferably, the phase-shifting transformer has multiple sets of secondary windings, the output voltage of each set of secondary windings has a predetermined phase difference, each set of secondary windings is connected to one of the uncontrolled rectifier bridges, and the output terminals of two uncontrolled rectifier bridges are connected in parallel to form a set of 12-pulse DC output.

[0009] Preferably, the AC output terminals of multiple inverter modules are connected in parallel, the AC output terminals of the inverter modules are connected to the primary winding of the intermediate frequency isolation transformer, and the secondary winding of the intermediate frequency isolation transformer is used to connect the load.

[0010] Preferably, the photovoltaic input unit further includes one or more inverter units, the DC input terminal of the inverter unit is connected to the output terminal of the MPPT module, and the AC output terminal of the inverter unit is used to connect to the load.

[0011] In a second aspect, the present invention provides a control method for a dual-power supply medium-frequency heating power supply, applied to the medium-frequency heating power supply described in the first aspect, the control method comprising: Power grid connection operation control steps: The control unit controls the start of the mains input unit to establish the DC bus voltage; The control unit controls the inverter module to operate in a constant frequency mode and adjusts the modulation voltage amplitude of the inverter module according to the power required by the load to regulate the output power; The control unit controls the operating state of the MPPT module and determines whether the MPPT module performs maximum power point tracking based on the photovoltaic power. Off-grid operation control steps: The control unit controls the mains power input unit to stop working; The control unit activates each MPPT module to perform maximum power point tracking in order to adjust the output voltage of the MPPT module; The control unit controls the inverter module to operate at a fixed frequency and a fixed modulation ratio, so that the output voltage of the inverter module changes in accordance with the change of its DC input voltage; When the total output power of the MPPT module reaches a preset limit, the control unit reduces the modulation ratio of the inverter module to limit the output power.

[0012] Preferably, in the grid connection operation control step, adjusting the modulation voltage amplitude of the inverter module according to the power required by the load specifically involves: collecting the output power as a feedback quantity, comparing it with the given power command, obtaining the voltage adjustment quantity after calculation by the power regulator, and adjusting the modulation voltage amplitude of the inverter module according to the voltage adjustment quantity.

[0013] Preferably, in the off-grid operation control step, the fixed modulation ratio is the maximum modulation ratio allowed in this mode; the control unit reduces the fundamental amplitude of the inverter module's output voltage by lowering the modulation ratio, thereby limiting the output power.

[0014] Preferably, in the grid connection operation control step, when the photovoltaic power exceeds the set power, the MPPT module exits the maximum power tracking mode and the power is automatically limited; in the off-grid operation control step, the control unit also controls the inverter unit in the photovoltaic input unit to start, and drives the load after inverting the photovoltaic power.

[0015] Thirdly, the present invention also discloses a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the method described in the second aspect.

[0016] Compared with the prior art, the present invention has the following advantages and technical effects: This invention employs a multi-pulse rectifier circuit with a pulse number that is an integer multiple of 12 in the mains input unit. Through a phase-shifting transformer, the output voltage waveforms of multiple rectifier bridges are superimposed to cancel low-order harmonics, significantly reducing the harmonic content of the grid-side current. Compared to conventional 6-pulse rectification, no additional filtering device is required to meet the power quality requirements of the grid, reducing system components and costs. Furthermore, a higher pulse number results in stronger suppression of high-order harmonics.

[0017] In this invention, the output terminal of the MPPT module of the photovoltaic input unit and the DC output terminal of the multi-pulse rectifier circuit are respectively connected to the DC input terminal of the inverter module. That is, the DC power output by the photovoltaic can be directly coupled to the DC bus without going through an independent grid-connected inverter to be inverted to AC before being used by the heating power supply. This reduces the energy conversion links, reduces conversion losses, and improves the overall utilization efficiency of photovoltaic power.

[0018] The DC input terminal of the inverter module of this invention is simultaneously connected to the DC output terminal of the multi-pulse rectifier circuit and the DC output terminal of the MPPT module, forming a structure in which the dual power sources naturally converge on the DC bus side, allowing the mains power and photovoltaic power to supply power to the load simultaneously or individually. The control unit can coordinate the working states of the mains power input and photovoltaic input according to the photovoltaic output, thereby achieving flexible power complementarity, prioritizing the absorption of photovoltaic energy, and automatically supplementing the insufficient portion by the mains power.

[0019] The number of inverter modules in this invention is related to the pulse count of the multi-pulse rectifier circuit, meaning that each additional 12-pulse rectifier unit corresponds to one additional inverter module. The AC outputs of the inverter modules are connected in parallel and then connected to the load via an intermediate frequency isolation transformer. This configuration allows for flexible expansion of system capacity according to actual needs, overcoming the power bottleneck of a single inverter module. Furthermore, the parallel operation of multiple modules ensures that if one module fails, the others can continue to supply power, improving the system's safety and redundancy.

[0020] The control unit of this invention is connected to the mains input unit, the photovoltaic input unit, and the inverter module respectively. It collects the grid-side voltage and current and the inverter-side output voltage and current in a unified manner, and centrally controls the on / off of the medium-voltage switch, the output voltage and frequency of the inverter module, and the start / stop and maximum power point tracking of the MPPT module. This enables full-domain coordinated management and control of the mains and photovoltaic dual power sources, ensuring that the system can operate safely and stably under different operating conditions, while improving the system's ability to cope with grid fluctuations and photovoltaic fluctuations. Attached Figure Description

[0021] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings: Figure 1This is a schematic diagram of the structure of a medium-frequency heating power supply with dual power supply according to an embodiment of the present invention; Figure 2 This is a flowchart of a control method for a dual-powered medium-frequency heating power supply according to an embodiment of the present invention; Figure 3 This is a power closed-loop control block diagram of the grid access operation control mode according to an embodiment of the present invention; Figure 4 This is a block diagram of automatic output power balancing and power limiting control in the off-grid operation control mode according to an embodiment of the present invention. Detailed Implementation

[0022] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0023] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0024] Example 1 like Figure 1 As shown, this embodiment provides a medium-frequency heating power supply with dual power supply, including: The mains power input unit includes a multi-pulse rectifier circuit, wherein the pulse number of the multi-pulse rectifier circuit is an integer multiple of 12; Furthermore, the multi-pulse rectifier circuit includes a phase-shifting transformer, an uncontrolled rectifier bridge connected to the secondary side of the phase-shifting transformer, a medium-voltage switch disposed on the primary side of the phase-shifting transformer, and a current sensor for detecting the mains input current.

[0025] Furthermore, the phase-shifting transformer has multiple sets of secondary windings, the output voltage of each set of secondary windings has a predetermined phase difference, each set of secondary windings is connected to one of the uncontrolled rectifier bridges, and the output terminals of two uncontrolled rectifier bridges are connected in parallel to form a set of 12-pulse DC output.

[0026] In a preferred embodiment, the multi-pulse rectifier circuit employs a 12-pulse structure, and the phase-shifting transformer has secondary windings connected in both star and delta configurations to achieve a 30° phase shift. The outputs of two sets of three-phase uncontrolled rectifier bridges are connected in series to form a DC bus. A current sensor is connected in series after the medium-voltage switch, and its output is connected to the control unit to perform fast protection when the current exceeds a threshold.

[0027] Specifically, the mains input unit is connected to the mains power grid (e.g., 10kV). A medium-voltage switch (which can be a vacuum contactor or circuit breaker) is located on the primary side to connect and disconnect the mains power. A current sensor (such as a Hall effect current sensor) is connected in series between the medium-voltage switch and the primary winding of the phase-shifting transformer to detect the current in real time and feed it back to the control unit. The phase-shifting transformer is a three-phase dry-type transformer with two sets of windings on the secondary side: one set is star-connected, and the other is delta-connected, with a 30° phase shift between them. The secondary star winding is connected to the first uncontrolled rectifier bridge, and the delta winding is connected to the second uncontrolled rectifier bridge. The outputs of the two sets of uncontrolled rectifier bridges are connected in series to form upper and lower buses, with a total DC voltage of approximately 1000V, constituting the output terminal of the multi-pulse rectifier circuit. The series connection helps to improve the DC bus voltage level. This 12-pulse rectifier structure significantly reduces the 5th and 7th harmonics in the grid-side current.

[0028] In this embodiment, the mains input unit employs a multi-pulse rectifier circuit. This multi-pulse rectifier circuit includes a phase-shifting transformer, an uncontrolled rectifier bridge connected to the secondary side of the phase-shifting transformer, a medium-voltage switch located on the primary side of the phase-shifting transformer for connecting or disconnecting the mains power, and a current sensor for detecting the mains input current. The phase-shifting transformer shifts the three-phase AC mains power, generating multiple sets of secondary voltages with predetermined phase differences. Each set of secondary voltages is rectified by the uncontrolled rectifier bridge to form a low-ripple DC voltage. By selecting the phase-shifting angle and rectifier bridge combination, 12-pulse, 18-pulse, or even 24-pulse rectification can be achieved, significantly reducing the harmonic content of the input current and meeting the power quality standards of the power grid. The medium-voltage switch is installed on the high-voltage end of the transformer's primary side, serving both as an actuator for power start-up and shutdown and as a means to quickly disconnect the mains power in case of a fault. The current sensor is used to monitor the grid-side current in real time, providing a basis for overcurrent protection.

[0029] Photovoltaic input unit, the photovoltaic input unit including multiple MPPT modules; Furthermore, the photovoltaic input unit also includes one or more inverter units, the DC input terminal of the inverter unit is connected to the output terminal of the MPPT module, and the AC output terminal of the inverter unit is used to connect to the load.

[0030] In this embodiment, the photovoltaic input unit includes multiple MPPT modules and one or more inverter units. The input of each MPPT module is connected to the photovoltaic array, and it typically contains a boost converter circuit and an MPPT controller. By adjusting the duty cycle, the photovoltaic array is kept operating at its maximum power point, and the output provides DC power. The outputs of multiple MPPT modules can be connected in parallel to the DC bus to adapt to different lighting conditions and component configurations.

[0031] In this embodiment, the Boost circuit of the MPPT module adopts an inductor and switching transistor structure. The switching transistor is controlled by the MPPT controller according to the perturbation observation method or the conductance increment method, which can quickly track the maximum power point of the photovoltaic array.

[0032] Specifically, the photovoltaic input unit is connected to the photovoltaic array. In this embodiment, two MPPT modules are provided, each connected to a different photovoltaic string. Each MPPT module consists of a boost circuit and an MPPT controller. The boost circuit raises the low and fluctuating voltage of the photovoltaic array to a stable DC bus voltage, achieving maximum power point tracking. The output of the MPPT module is connected in parallel to the DC bus. In addition, the photovoltaic input unit also includes an inverter unit, whose DC side is connected to the output of the MPPT module, and whose AC side can be connected to the grid via a switch to perform grid-connected inverter function, feeding excess photovoltaic power into the grid. This inverter unit can operate in current source mode, synchronized with the grid.

[0033] In this embodiment, the positive and negative terminals of the DC bus are connected to the DC input terminals of the inverter module, respectively. The inverter module adopts a full-bridge topology, consisting of four IGBTs with anti-parallel freewheeling diodes. Its output terminal is connected to the primary side of the intermediate frequency isolation transformer. The turns ratio of the intermediate frequency isolation transformer is set according to the load requirements, for example, 10:1, converting the high-voltage intermediate frequency output from the inverter into a low-voltage, high-current suitable for the induction heating coil. The secondary side is connected to the heating load, namely the induction heating coil and its load.

[0034] An inverter module, wherein the DC input terminal of the inverter module is connected to the DC output terminal of the multi-pulse rectifier circuit and the DC output terminal of the MPPT module, respectively. Furthermore, the AC output terminals of multiple inverter modules are connected in parallel, and the AC output terminals of the inverter modules are connected to the primary winding of the intermediate frequency isolation transformer, while the secondary winding of the intermediate frequency isolation transformer is used to connect the load.

[0035] The number of inverter modules is related to the number of pulses in the multi-pulse rectifier circuit; In this embodiment, the DC input terminal of the inverter module is connected to the output terminal of the multi-pulse rectifier circuit and the output terminal of the MPPT module, respectively. That is, the DC bus obtained by the mains rectification is connected in parallel with the DC bus output by the photovoltaic MPPT. This parallel structure allows energy to naturally merge when both power sources are available simultaneously, or the power source can be selected according to the control strategy. The inverter module converts DC power into the intermediate frequency AC power required by the load. It is usually composed of fully controlled power devices such as IGBTs and MOSFETs to form a full-bridge or half-bridge topology and adopts a pulse width modulation strategy.

[0036] In this embodiment, the primary winding of the intermediate frequency isolation transformer is connected to the AC output terminal of the inverter module, while the secondary winding is used to connect the induction heating load. This transformer operates at an intermediate frequency (e.g., hundreds of Hz to tens of kHz) and features small size and high efficiency. It not only provides voltage matching but also electrical isolation, ensuring the safety of the load and personnel.

[0037] The control unit is connected to the mains input unit, the photovoltaic input unit and the inverter module, and is used to control the output voltage and frequency of the inverter module, as well as control the start-up, shutdown and maximum power point tracking of each MPPT module.

[0038] Specifically, the control unit is based on a digital signal processor and is equipped with analog sampling and conditioning circuits, drive circuits, and communication interfaces. The control unit receives feedback signals such as the status of the medium-voltage switch, signals from the current sensor, DC bus voltage, inverter output current and voltage, and outputs switch control commands, inverter module PWM signals, and MPPT enable and control signals.

[0039] In this embodiment, the control unit is the core of the entire system, connected to the medium-voltage switch, inverter module, and each MPPT module. The control unit collects voltage, current, and power signals, executes logical judgments and control algorithms, and achieves coordinated operation of the components under different operating modes. Specifically, this includes: controlling the on / off state of the medium-voltage switch to start or stop the multi-pulse rectifier circuit; controlling the output voltage, frequency, and modulation strategy of the inverter module; controlling the start, stop, and maximum power point tracking (MPPT) algorithms of each MPPT module; in addition, the control unit may also have a communication interface to receive power commands or status indications from the host computer.

[0040] The beneficial effects of this embodiment: (1) This embodiment effectively suppresses grid-side current harmonics and improves grid compatibility through multi-pulse rectification technology, without the need for additional filtering devices; (2) This embodiment integrates a photovoltaic input unit, which supports direct coupling of photovoltaic power to the DC bus, reducing energy conversion links and improving efficiency; (3) In grid-connected mode, constant voltage-frequency ratio closed-loop regulation is achieved, resulting in high power control accuracy; (4) In off-grid mode, the inverter module operates in open loop with a fixed modulation ratio, automatically adapts to photovoltaic power fluctuations, and actively limits power when the limit is reached. The control is simple and robust. (5) The medium frequency isolation transformer achieves voltage matching and electrical isolation, adapts to various loads, and is safe and reliable.

[0041] Example 2 This embodiment provides a control method for a dual-power supply medium-frequency heating power supply, such as... Figure 2As shown, in the medium-frequency heating power supply described in Embodiment 1, after the system is powered on, the control unit determines whether the mains power is normal and available. If the mains power is normal and heating is required, it enters the grid-connected operation control mode; if the mains power is unavailable or the off-grid mode is manually selected, it enters the off-grid operation control mode. The control method includes: Grid-connected operation control steps: The control unit controls the start of the mains input unit to establish the DC bus voltage; The control unit controls the inverter module to operate in a constant frequency mode and adjusts the modulation voltage amplitude of the inverter module according to the power required by the load to regulate the output power; Furthermore, the modulation voltage amplitude of the inverter module is adjusted according to the power required by the load. Specifically, the output power is collected as a feedback quantity, compared with a given power command, and the voltage adjustment quantity is obtained after calculation by the power regulator. The modulation voltage amplitude of the inverter module is then adjusted according to the voltage adjustment quantity.

[0042] The control unit controls the operating state of the MPPT module and determines whether the MPPT module performs maximum power point tracking based on the photovoltaic power. Furthermore, in the grid-connected operation control step, when the photovoltaic power exceeds the set power, the MPPT module exits the maximum power tracking mode and the power is automatically limited; in the off-grid operation control step, the control unit also controls the inverter unit in the photovoltaic input unit to start, and drives the load after inverting the photovoltaic power.

[0043] In this embodiment, as Figure 2 As shown, the grid-connected operation control mode is as follows: Step S1: The control unit closes the medium-voltage switch, the phase-shifting transformer is energized, the multi-pulse rectifier circuit works, and the DC bus establishes a voltage of about 1000V.

[0044] Step S2: The control unit checks the status of the photovoltaic input unit. If photovoltaic power is available, the MPPT module output can be set to constant voltage mode to inject power into the DC bus. In this embodiment, the MPPT module output voltage is set higher than the multi-pulse rectifier output voltage, and photovoltaic power is used to supply the inverter module first, with the insufficient photovoltaic power supplemented by the mains power.

[0045] Step S3: The control unit starts the inverter module, using constant voltage-frequency ratio control. A given intermediate frequency f_set (e.g., 1kHz) is established, and an initial voltage amplitude value is preset according to the heating process. The output AC voltage and current are acquired through the voltage and current detection module, and the actual output power P_out is calculated.

[0046] Step S4: Compare P_out with the power demand command P_ref issued by the host computer. The error is calculated by the PI regulator and used as the voltage amplitude correction amount. The control unit adjusts the amplitude of the SPWM modulation wave, thereby changing the output voltage of the inverter module so that the output power tracks the command value. Figure 3 The power closed-loop block diagram is shown, showing the power error → PI regulator → limiting → generation of modulation amplitude value.

[0047] Step S5: Monitor the current sensor signal in real time. If an overcurrent occurs, immediately disconnect the medium-voltage switch. Simultaneously monitor protection signals such as DC bus voltage and inverter temperature.

[0048] In this embodiment, the grid-connected operation control mode is suitable for situations where the mains power is normal and a large heating power is required. The control unit closes the medium-voltage switch, the multi-pulse rectifier circuit starts, and a stable DC bus voltage is established. At this time, the photovoltaic input unit can be powered by DC power output from the MPPT module, prioritizing the consumption of photovoltaic energy. The inverter module operates in constant voltage-frequency ratio control mode, that is, it outputs medium-frequency AC power with a set frequency and voltage, and the frequency and voltage meet the load matching requirements. The control unit also performs negative feedback adjustment according to the power required by the load: it detects the output power in real time, compares it with the given power command, and outputs an adjustment amount through a power regulator (such as a PI regulator) to change the amplitude of the inverter module's modulation voltage, thereby adjusting the output voltage and realizing power closed-loop control. This mode can provide precise and controllable heating power to meet the requirements of industrial heating processes.

[0049] Off-grid operation control steps: The control unit controls the mains power input unit to stop working; The control unit activates each MPPT module to perform maximum power point tracking in order to adjust the output voltage of the MPPT module; The control unit controls the inverter module to operate at a fixed frequency and a fixed modulation ratio, so that the output voltage of the inverter module changes in accordance with the change of its DC input voltage; When the total output power of the MPPT module reaches a preset limit, the control unit reduces the modulation ratio of the inverter module to limit the output power.

[0050] Furthermore, the fixed modulation ratio is the maximum modulation ratio allowed in this mode; the control unit reduces the fundamental amplitude of the inverter module's output voltage by lowering the modulation ratio, thereby limiting the output power.

[0051] In this embodiment, as Figure 2 As shown, the off-grid operation control mode: Step L1: The control unit confirms that the mains power is unavailable or actively disconnects the medium voltage switch, and the multi-pulse rectifier circuit stops operating.

[0052] Step L2: Activate all MPPT modules of the photovoltaic input unit. Each MPPT controller executes the maximum power point tracking algorithm, continuously adjusting the Boost duty cycle to maximize the output power of the photovoltaic array. The output terminals of the MPPT modules are connected in parallel to maintain the DC bus voltage, which fluctuates with changes in sunlight and load.

[0053] Step L3: The control unit controls the inverter module to operate in open loop at a fixed frequency f_set (e.g., 1kHz) and a fixed modulation ratio m_fix. The fixed modulation ratio can be set to the maximum allowed modulation ratio m_max in this mode (e.g., 0.95). At this time, the frequency of the SPWM modulation wave is fixed, and the ratio of the modulation wave amplitude to the triangular carrier amplitude is fixed. Therefore, the fundamental amplitude of the inverter output voltage V_out = m_fix × V_dc (ignoring dead time and other nonlinearities) strictly follows the DC bus voltage V_dc.

[0054] Step L4: Since the heating load can be equivalent to an inductive impedance, at a certain frequency, the power is approximately proportional to the square of V_out. When the illumination increases, the MPPT module increases the output voltage, V_dc rises, V_out automatically increases, and the load power increases accordingly. No additional feedback control is required, achieving natural power balance.

[0055] Step L5: The control unit calculates the total output power in real time. When the power reaches the preset maximum power limit P_max, power limiting control is initiated. The control unit gradually reduces the modulation ratio m of the inverter module from its current value, causing V_out to decrease, thereby limiting the power to P_max. If the power drops due to reduced illumination, the power can be restored to a fixed modulation ratio m_fix. Figure 4 The process logic is shown.

[0056] Step L6: Perform over-temperature and over-current protection as well.

[0057] In this embodiment, the off-grid operation control mode is suitable for scenarios where the mains power is disconnected or the photovoltaic system supplies power independently. The control unit disconnects the medium-voltage switch, and the multi-pulse rectifier circuit stops working. Since only the photovoltaic input unit provides energy, the control unit activates each MPPT module, enabling it to independently perform maximum power point tracking, adjusting the output voltage to capture as much photovoltaic power as possible. At this time, the inverter module adopts a special open-loop control method: it operates at a fixed frequency and a fixed modulation ratio, that is, it keeps the frequency and amplitude of the modulation wave relative to the DC voltage constant. In this way, the amplitude of the AC fundamental voltage output by the inverter module is proportional to the DC input voltage. Therefore, when the output voltage of the MPPT module changes with the light intensity, the output voltage of the inverter module will automatically follow the change, maintaining power balance without active adjustment. This characteristic is perfectly adapted to the fluctuation of photovoltaic output power: as the light intensity increases, the MPPT output voltage increases, the inverter output voltage automatically increases, and the output power increases accordingly; conversely, it decreases. When the light intensity is extremely strong and the total output power of the MPPT module reaches the preset maximum power limit, in order to avoid overload or equipment damage, the control unit controls the inverter module to reduce the modulation ratio, thereby reducing the output voltage and limiting the power within a safe range. This control strategy, which combines open-loop fixed modulation ratio with power-limited modulation ratio reduction, is simple and reliable, making full use of photovoltaic energy while ensuring system safety.

[0058] In this embodiment, the dual-power supply medium-frequency heating power supply can provide high-quality electrical energy for heating when the mains power is normal, with low harmonic pollution and flexible power adjustment. Under off-grid conditions, it relies on independent photovoltaic power supply, and can automatically adjust the output power according to the sunlight without the need for energy storage batteries, and effectively limit the maximum power to ensure system safety.

[0059] Example 3 This embodiment also discloses a computer-readable storage medium storing a computer program thereon, which, when executed by a processor, implements the steps of the method described in Embodiment 2.

[0060] The above are merely preferred embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A medium-frequency heating power supply with dual power supply, characterized in that, Includes the following steps: The mains power input unit includes a multi-pulse rectifier circuit, wherein the pulse number of the multi-pulse rectifier circuit is an integer multiple of 12; A photovoltaic input unit includes multiple MPPT modules and one or more inverter units. The DC input terminal of the inverter unit is connected to the output terminal of the MPPT module, and the AC output terminal of the inverter unit is used to connect to a load. An inverter module, wherein the DC input terminal of the inverter module is connected to the DC output terminal of the multi-pulse rectifier circuit and the DC output terminal of the MPPT module, respectively. The number of inverter modules is related to the number of pulses in the multi-pulse rectifier circuit; The control unit is connected to the mains input unit, the photovoltaic input unit and the inverter module, and is used to control the output voltage and frequency of the inverter module, as well as control the start-up, shutdown and maximum power point tracking of each MPPT module.

2. The medium-frequency heating power supply with dual power supply according to claim 1, characterized in that, The multi-pulse rectifier circuit includes a phase-shifting transformer, an uncontrolled rectifier bridge connected to the secondary side of the phase-shifting transformer, a medium-voltage switch disposed on the primary side of the phase-shifting transformer, and a current sensor for detecting the mains input current.

3. The medium-frequency heating power supply with dual power supply according to claim 2, characterized in that, The phase-shifting transformer has multiple sets of secondary windings, and the output voltage of each set of secondary windings has a predetermined phase difference. Each set of secondary windings is connected to one of the uncontrolled rectifier bridges. The output terminals of two uncontrolled rectifier bridges are connected in parallel to form a 12-pulse DC output.

4. The medium-frequency heating power supply with dual power supply according to claim 1, characterized in that, The AC output terminals of multiple inverter modules are connected in parallel. The AC output terminals of the inverter modules are connected to the primary winding of the intermediate frequency isolation transformer, and the secondary winding of the intermediate frequency isolation transformer is used to connect the load.

5. A control method for a dual-power supply medium-frequency heating power supply, applied to the medium-frequency heating power supply as described in any one of claims 1 to 4, characterized in that, The control method includes: Power grid connection operation control steps: The control unit controls the start of the mains input unit to establish the DC bus voltage; The control unit controls the inverter module to operate in a constant frequency mode and adjusts the modulation voltage amplitude of the inverter module according to the power required by the load to regulate the output power; The control unit controls the operating state of the MPPT module and determines whether the MPPT module performs maximum power point tracking based on the photovoltaic power. Off-grid operation control steps: The control unit controls the mains power input unit to stop working; The control unit activates each MPPT module to perform maximum power point tracking in order to adjust the output voltage of the MPPT module; The control unit controls the inverter module to operate at a fixed frequency and a fixed modulation ratio, so that the output voltage of the inverter module changes in accordance with the change of its DC input voltage; When the total output power of the MPPT module reaches a preset limit, the control unit reduces the modulation ratio of the inverter module to limit the output power.

6. The control method according to claim 5, characterized in that, In the grid connection operation control steps, adjusting the modulation voltage amplitude of the inverter module according to the power required by the load specifically involves: collecting the output power as a feedback quantity, comparing it with the given power command, obtaining the voltage adjustment quantity after calculation by the power regulator, and adjusting the modulation voltage amplitude of the inverter module according to the voltage adjustment quantity.

7. The control method according to claim 5, characterized in that, In the off-grid operation control step, the fixed modulation ratio is the maximum modulation ratio allowed in this mode; the control unit reduces the fundamental amplitude of the inverter module's output voltage by lowering the modulation ratio, thereby limiting the output power.

8. The control method according to claim 5, characterized in that, In the grid connection operation control step, when the photovoltaic power exceeds the set power, the MPPT module exits the maximum power tracking mode and the power is automatically limited; in the off-grid operation control step, the control unit also controls the inverter unit in the photovoltaic input unit to start, and drives the load after inverting the photovoltaic power.

9. A computer-readable storage medium having a computer program stored thereon, the computer program, when executed by a processor, implementing the steps of the method of claim 5.

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