Inverter, off-grid power supply system, and magnetic bias current control method

Abstract

The application provides an inverter, an off-grid power supply system and a magnetic bias current control method. The inverter comprises a conversion circuit and a controller. The input end of the conversion circuit is coupled to a DC power supply, and the output end of the conversion circuit is coupled to a load through a transformer. The conversion circuit comprises a first DC capacitor and a second DC capacitor, which are connected in series and then connected in parallel across the DC power supply. The controller is configured to obtain the voltage values of the first DC capacitor and the second DC capacitor in the conversion circuit, and adjust the output voltage of the conversion circuit according to the voltage values of the first DC capacitor and the second DC capacitor, so as to increase the output voltage to the rated output voltage and keep the core magnetic flux in the transformer less than the saturation magnetic flux. By using the application, the output voltage of the conversion circuit can be increased to complete the black start when the magnetic bias current is in a controllable range, the reliability of the magnetic bias current control is improved, and the accuracy of the magnetic flux estimation and control is higher.

Description

Technical Field

[0001] This application relates to the field of electronic power, and more particularly to an inverter, an off-grid power supply system, and a method for controlling bias current. Background Technology

[0002] Black start refers to the process where, after a system outage due to a fault, or during the initial startup, the system recovers without relying on other networks or the power grid. This is achieved by starting units with self-starting capabilities, which in turn drive units without self-starting capabilities, gradually expanding the recovery scope and ultimately restoring the entire system. Black start is widely used in various power supply systems (such as microgrids and off-grid systems). During black start, the transformer core of magnetic equipment in the power supply system easily enters a saturation state, meaning the total magnetic flux of the transformer far exceeds the saturation flux of the iron core. This oversaturation generates a very large excitation current, which contains a significant even-order current (i.e., bias current). This bias current can cause the neutral point potential of multi-level inverters to diverge and shift, and excessively strong excitation current may damage the transformer. Currently, controlling the bias current generated during black start mainly involves calculating the theoretical value of the transformer core flux throughout the entire black start process and the maximum value of the core flux not exceeding the saturation threshold to obtain the constraints on the black start time and determine the control strategy. However, this method ignores the residual magnetism in the transformer and the high impedance or certain magnetic loads that may exist in the power supply system. The accuracy of magnetic flux estimation and control is low, and it is not applicable to the scenario of multi-stage cascaded transformers. The reliability of bias current control is low, and its applicability is not high. Summary of the Invention

[0003] This application provides an inverter, an off-grid power supply system, and a bias current control method, which can increase the output voltage of the conversion circuit to complete black start when the bias current is within a controllable range, thereby improving the reliability of bias current control and increasing the accuracy of magnetic flux estimation and control.

[0004] In a first aspect, this application provides an inverter suitable for off-grid power supply systems. The inverter includes a conversion circuit and a controller. The input terminal of the conversion circuit is coupled to a DC power supply, and the output terminal is coupled to a load via a transformer. The conversion circuit includes a first DC capacitor and a second DC capacitor, which are connected in series and then in parallel across the DC power supply. The controller acquires the voltage values ​​of the first and second DC capacitors in the conversion circuit and adjusts the output voltage of the conversion circuit based on these voltage values. This increases the output voltage of the conversion circuit to its rated output voltage while maintaining the magnetic flux of the transformer core below the saturation flux to reduce bias current.

[0005] In this application, the controller in the inverter detects whether the transformer has a bias current due to magnetic flux oversaturation by acquiring the voltage values ​​of the first DC capacitor and the second DC capacitor. When the bias current is within a controllable range, the controller increases the output voltage of the conversion circuit to complete the black start, thereby improving the reliability of the bias current control, increasing the accuracy of magnetic flux estimation and control, and enhancing applicability.

[0006] In conjunction with the first aspect, in a first possible implementation, the controller is used to increase the output voltage of the conversion circuit when the difference between the voltage values ​​of the first DC capacitor and the second DC capacitor is not greater than a balance threshold. Here, the controller in the inverter detects whether the transformer experiences a bias current due to magnetic flux oversaturation by acquiring the voltage values ​​of the first and second DC capacitors. When the bias current is within a controllable range, the controller increases the output voltage of the conversion circuit to complete a black start, improving the reliability of bias current control, increasing the accuracy of magnetic flux estimation and control, and enhancing applicability.

[0007] In conjunction with the first possible implementation of the first aspect, in the second possible implementation, the magnetic flux of the core in the transformer includes both AC and DC magnetic flux. The DC magnetic flux decays over time. The controller is further configured to control the output voltage of the conversion circuit to remain constant and maintain this position for a preset duration when the voltage difference between the first and second DC capacitors exceeds a balance threshold. This ensures that the AC magnetic flux of the transformer core remains constant while the DC magnetic flux decays until the voltage difference between the first and second DC capacitors is less than a set threshold, resulting in the sum of the AC and DC magnetic flux being less than the saturation magnetic flux. The inverter, through the controller, controls the output voltage of the conversion circuit to remain constant and maintain this position for a preset duration when a bias current occurs, thus preventing the transformer core magnetic flux from saturating and generating a bias current, and preventing the neutral point of the DC bus connected to the inverter from shifting and the transformer from being damaged.

[0008] In conjunction with the second possible implementation of the first aspect, in the third possible implementation, the controller is further configured to increase the output impedance of the conversion circuit to increase the transmission line impedance between the inverter and the transformer when the output voltage of the conversion circuit does not exceed the rated output voltage. By increasing the impedance on the transmission line, the decay rate of the DC magnetic flux is accelerated, further preventing the transformer core magnetic flux saturation and the generation of bias current, thus preventing the inverter midpoint potential divergence and transformer damage.

[0009] Secondly, this application provides an off-grid power supply system, which includes a DC power supply and an inverter provided in the first aspect and any possible implementation thereof. The DC power supply is used to provide DC power input to the inverter.

[0010] In conjunction with the second aspect, in a first possible implementation, the aforementioned grid power supply system further includes a DC / DC converter, through which the DC power supply is coupled to the input terminal of the inverter. The DC / DC converter is used to perform DC power conversion based on the DC input provided by the DC power supply and output the converted DC power to the inverter.

[0011] In this application, the inverter provided based on the first aspect above can obtain the voltage values ​​of the first DC capacitor and the second DC capacitor to detect whether the transformer has a bias current due to magnetic flux oversaturation. When the bias current is within a controllable range, the output voltage of the conversion circuit is increased to complete the black start, which improves the reliability of bias current control, and the accuracy of magnetic flux estimation and control is higher, and the applicability is strong.

[0012] Thirdly, this application provides a method for controlling the bias current in an off-grid power supply system. This method is applicable to inverters in off-grid power supply systems, which include a DC power supply, a transformer, and the inverter. The inverter includes a conversion circuit and a controller. The input terminal of the conversion circuit is coupled to the DC power supply, and the output terminal of the conversion circuit is coupled to the load through the transformer. The conversion circuit includes a first DC capacitor and a second DC capacitor, which are connected in series and then in parallel across the DC power supply. In this method, the controller acquires the voltage values ​​of the first and second DC capacitors in the conversion circuit. Based on these voltage values, the controller adjusts the output voltage of the conversion circuit to increase its output voltage to the rated output voltage while maintaining the magnetic flux of the transformer core below the saturation flux to reduce the bias current.

[0013] In this application, the inverter can obtain the voltage values ​​of the first and second DC capacitors in the conversion circuit through a controller, and detect the bias current based on the difference between the voltage values ​​of the first and second DC capacitors. The inverter can adjust the output voltage of the conversion circuit through the controller when the difference between the voltage values ​​of the first and second DC capacitors is not greater than a balance threshold, thereby increasing the output voltage of the conversion circuit to the rated output voltage and keeping the magnetic flux in the transformer core less than the saturation magnetic flux. The controller can also control the output voltage of the conversion circuit to remain constant for a preset time when a bias current occurs, so that the total magnetic flux in the transformer core can exit the saturation magnetic flux as the DC magnetic flux decreases, avoiding the generation of bias current due to transformer core magnetic flux saturation, and preventing neutral point shift of the DC bus connected to the inverter and transformer damage.

[0014] In conjunction with the third aspect, in the first possible implementation, the aforementioned adjustment of the output voltage of the converter circuit based on the voltage values ​​of the first DC capacitor and the second DC capacitor by the controller includes increasing the output voltage of the converter circuit when the difference between the voltage values ​​of the first DC capacitor and the second DC capacitor is not greater than a balance threshold. Here, the controller in the inverter detects whether the transformer exhibits a bias current due to magnetic flux oversaturation by acquiring the voltage values ​​of the first DC capacitor and the second DC capacitor. When the bias current is within a controllable range, the controller increases the output voltage of the converter circuit to complete the black start, thereby improving the reliability of bias current control, increasing the accuracy of magnetic flux estimation and control, and enhancing applicability.

[0015] In conjunction with the first possible implementation of the third aspect, in the second possible implementation, the magnetic flux of the transformer core includes both AC and DC magnetic flux. The DC magnetic flux decays over time. The controller adjusts the output voltage of the conversion circuit based on the voltage values ​​of the first and second DC capacitors. This involves the controller maintaining the output voltage of the conversion circuit unchanged for a preset duration when the voltage difference between the first and second DC capacitors exceeds a balance threshold. This ensures that the AC magnetic flux of the transformer core remains constant while the DC magnetic flux decays until the voltage difference between the first and second DC capacitors is less than a set threshold, resulting in the sum of the AC and DC magnetic flux being less than the saturation magnetic flux. The inverter, through the controller, maintains the output voltage of the conversion circuit unchanged for a preset duration when a bias current occurs to exit the saturation magnetic flux, preventing the transformer core magnetic flux from saturating and generating a bias current, thus preventing neutral point shift of the DC bus connected to the inverter and transformer damage.

[0016] In conjunction with the second possible implementation of the third aspect, in the third possible implementation, the method further includes controlling the output impedance of the conversion circuit to increase when the output voltage of the conversion circuit does not exceed the rated output voltage, thereby increasing the transmission line impedance between the inverter and the transformer. By increasing the impedance on the transmission line, the decay rate of the DC magnetic flux is accelerated, further preventing the transformer core magnetic flux saturation and the generation of bias current, thus preventing the inverter midpoint potential divergence and transformer damage. Attached Figure Description

[0017] Figure 1 This is a schematic diagram illustrating the application scenario of the off-grid power supply system provided in this application;

[0018] Figure 2 This is a schematic diagram of the transformer excitation current waveform;

[0019] Figure 3 This is a structural schematic diagram of the off-grid power supply system provided in this application;

[0020] Figure 4 This is another structural schematic diagram of the off-grid power supply system provided in this application;

[0021] Figure 5 This is another structural schematic diagram of the off-grid power supply system provided in this application;

[0022] Figure 6 This is a schematic diagram showing the relationship between the transformer magnetic flux and the output voltage provided in this application;

[0023] Figure 7 This is a schematic diagram of the transformer magnetic flux provided in this application;

[0024] Figure 8 This is a flowchart illustrating the bias current control method for the off-grid power supply system provided in this application. Detailed Implementation

[0025] Off-grid power supply systems (or microgrid systems) are widely used in remote mountainous areas, areas without electricity, islands, communication base stations, and streetlights. Off-grid power supply systems use photovoltaic arrays to convert solar energy into electrical energy to power loads when there is sunlight, while simultaneously charging battery banks and energy storage batteries. Alternatively, when there is no sunlight, battery banks and energy storage batteries power DC loads, and the batteries also directly supply DC power to the inverter, which then converts the converted AC power to power AC loads. See also Figure 1 , Figure 1This is a schematic diagram illustrating an application scenario of the off-grid power supply system provided in this application. The off-grid power supply system provided in this application may include a DC power supply, an inverter, and a transformer. The DC power supply may consist of a photovoltaic array, and the output terminal of the photovoltaic array may be connected to the input terminal of the inverter (or the DC terminal of the inverter). The output terminal of the inverter (or the AC terminal of the inverter) is coupled to the load through a transformer. Figure 1 In the off-grid power supply system shown, the photovoltaic array can be composed of one or more photovoltaic strings connected in parallel, and a photovoltaic string can be obtained by connecting one or more photovoltaic modules in series. The inverter converts the DC power provided by the photovoltaic array and outputs the converted AC power to the transformer. The transformer performs voltage transformation based on the AC power output by the inverter and outputs stepped-up or stepped-down AC power to the load to supply power to the load (which can be a battery, communication base station, or household appliances, etc.).

[0026] For some feasible implementations, please refer again. Figure 1 The DC power supply may also include an energy storage battery. The output of the energy storage battery can be connected to the input of the inverter, and the output of the inverter is connected to the AC power grid through a transformer. The inverter can convert the DC power provided by the energy storage battery and output the converted AC power to the transformer. The transformer performs voltage transformation based on the AC power output by the inverter and outputs boosted or bucked AC power to the load (which may be a battery, communication base station, or household appliances).

[0027] For some feasible implementations, please refer again. Figure 1 The DC power supply may also include a wind turbine and a wind power converter. The output of the wind turbine is connected to the input of the inverter via the wind power converter, and the output of the inverter is connected to the AC power grid via a transformer. The wind power converter rectifies the AC power provided by the wind turbine to obtain DC power. The inverter converts the DC power provided by the wind power converter and outputs the converted AC power to the transformer. The transformer then performs voltage transformation on the AC power output from the inverter and outputs boosted or stepped-down AC power to the load (which may be a battery, communication base station, or household appliance).

[0028] exist Figure 1In the application scenario shown, the off-grid power supply system can perform a black start to supply power to the loads in the system. During the initial startup or restart after a fault, the off-grid power supply system can operate without the assistance of other networks or the power grid. By starting units with self-starting capabilities, it can drive units without self-starting capabilities, gradually expanding the system's recovery range and ultimately achieving the startup and recovery of the entire system. In the aforementioned off-grid power supply system, during the black start process, the transformer core (or iron core) of magnetic equipment such as transformers is highly susceptible to entering a saturation state (during the black start process, the inverter output voltage gradually increases, leading to an increase in the magnetic flux in the transformer core). This means the total magnetic flux of the transformer far exceeds the saturation magnetic flux of the iron core, and this oversaturation generates a very large excitation current. This excitation current contains a large even-order current (which can be called a bias current). This bias current causes a potential shift at the neutral point of the DC bus connected to the inverter (the degree of shift is proportional to the even-order current content), causing the inverter to stop working during the black start process. Please refer to [further details omitted]. Figure 2 , Figure 2 This is a schematic diagram of the transformer excitation current waveform, such as... Figure 2 As shown, when the total magnetic flux of the transformer far exceeds the saturation magnetic flux of the iron core, and the excitation current contains a large even-order current, the transformer excitation current changes from a symmetrical current waveform (transformer excitation current 1) to an asymmetrical waveform (transformer excitation current 2). Currently, controlling the bias current generated during black start mainly involves calculating the theoretical value of the transformer core magnetic flux throughout the entire black start process and the maximum value of the core magnetic flux not exceeding the saturation threshold to obtain the constraints on the black start time and determine the control strategy. However, this method ignores the residual magnetism of the transformer core and the high reactance or certain magnetic loads that may exist in the power supply system. The accuracy of magnetic flux estimation and control is low, and it is not applicable to scenarios with multi-stage cascaded transformers. The reliability of bias current control is low, and its applicability is limited.

[0029] In the off-grid power supply system provided in this application, the inverter may include a conversion circuit and a controller. The input terminal of the conversion circuit is coupled to a DC power supply, and the output terminal of the conversion circuit is coupled to the load through a transformer. The conversion circuit may include a first DC capacitor and a second DC capacitor, which are connected in series and then in parallel across the DC power supply. Here, the first and second DC capacitors can be connected to the DC power supply via a DC bus. The connection point of the first and second DC capacitors can serve as the neutral point of the DC bus. That is, the neutral point offset state of the DC bus in the off-grid power supply system (or whether the inverter's neutral point potential diverges) can be determined by the voltage difference between the first and second DC capacitors. In other words, the magnitude of the voltage difference between the first and second DC capacitors can be used to detect whether the transformer experiences a bias current due to magnetic flux oversaturation (the even-order current in the bias current causes the DC bus neutral point offset). The controller in the aforementioned inverter can acquire the voltage values ​​of the first and second DC capacitors in the conversion circuit. When the voltage difference between the first and second DC capacitors is not greater than a balance threshold, the controller adjusts the output voltage of the conversion circuit to reach the rated output voltage and maintain the magnetic flux of the transformer core below the saturation flux. Here, the controller detects whether the transformer experiences a bias current due to magnetic flux oversaturation by acquiring the voltage values ​​of the first and second DC capacitors. When the bias current is within a controllable range, the controller increases the output voltage of the conversion circuit to complete a black start, improving the reliability of bias current control, increasing the accuracy of magnetic flux estimation and control, and enhancing applicability.

[0030] See Figure 3 , Figure 3 This is a structural schematic diagram of the off-grid power supply system provided in this application. Figure 3 The off-grid power supply system shown includes a DC power source, an inverter, and a transformer. The DC power source can be a solar panel or an energy storage battery, etc. The input terminal of the inverter is connected to the DC power source in this off-grid power supply system, and the output terminal of the inverter is coupled to the load through the transformer. Figure 3 In the off-grid power supply system shown, the inverter converts the DC power output from the DC power supply and outputs the AC power (which can be the first AC power) obtained after the conversion to the transformer. The transformer performs voltage transformation based on the AC power output from the inverter and outputs stepped-up or stepped-down AC power (which can be the second AC power) to the load to supply power to the load (which can be a battery, communication base station, or household appliances, etc.).

[0031] See Figure 4 , Figure 4This is another structural schematic diagram of the off-grid power supply system provided in this application. Figure 4 The off-grid power supply system shown includes a DC power source, an inverter, a DC / DC converter, and a transformer. The DC power source can be a solar panel or an energy storage battery, etc. The input terminal of the inverter is coupled to the DC power source in the off-grid power supply system through the DC / DC converter, and the output terminal of the inverter is coupled to the load through the transformer. Figure 4 In the off-grid power supply system shown, the DC / DC converter performs voltage conversion on the DC power output from the DC power source, the inverter performs inversion conversion on the DC power output from the DC / DC converter, and outputs the AC power (which can be the first AC power) obtained after inversion to the transformer. The transformer performs voltage conversion based on the first AC power and outputs stepped-up or stepped-down AC power (which can be the second AC power) to the load to supply power to the load (which can be a battery, communication base station, or household appliances, etc.).

[0032] In some feasible implementations, Figure 3 or Figure 4 In the off-grid power supply system shown, the inverter includes a conversion circuit and a controller. The inverter can convert the DC power output from the DC power supply through the conversion circuit. The conversion circuit (which may be a midpoint clamping three-level inverter circuit, etc.) may also include a first DC capacitor and a second DC capacitor. Figure 3 Taking the off-grid power supply system shown as an example, the first DC capacitor and the second DC capacitor mentioned above are connected in series and then in parallel across the DC power supply. Please refer to [the relevant documentation / reference]. Figure 5 , Figure 5 This is another structural diagram of the off-grid power supply system provided in this application, such as... Figure 5 As shown, the first and second DC capacitors can be connected to the DC power supply via the DC bus. The connection point of the first and second DC capacitors can serve as the neutral point of the DC bus. That is, the neutral point offset state of the DC bus in the off-grid power supply system can be determined by the voltage difference between the first and second DC capacitors. The inverter can obtain the voltage values ​​of the first and second DC capacitors in the conversion circuit through the controller. When the voltage difference between the first and second DC capacitors is not greater than the balance threshold, the inverter adjusts the output voltage of the conversion circuit to increase the output voltage to the rated output voltage and maintain the magnetic flux of the transformer core below the saturation flux. Here, the controller in the inverter obtains the voltage values ​​of the first and second DC capacitors to detect whether the transformer experiences a bias current due to magnetic flux oversaturation. When the bias current is within a controllable range, the inverter increases the output voltage of the conversion circuit to complete the black start, improving the reliability of bias current control, increasing the accuracy of magnetic flux estimation and control, and enhancing applicability.

[0033] The following will combine Figures 6 to 8 The inverter provided in this application is illustrated by example. In some feasible implementations, the inverter can acquire the voltage values ​​of the first and second DC capacitors in the conversion circuit through a controller during the black start process of the off-grid power supply system, and detect the bias current based on the difference between the voltage values ​​of the first and second DC capacitors. The inverter can adjust the output voltage of the conversion circuit by the controller when the difference between the voltage values ​​of the first and second DC capacitors is not greater than the balance threshold, so as to increase the output voltage of the conversion circuit to the rated output voltage and keep the magnetic flux of the iron core in the transformer less than the saturation magnetic flux. Here, the controller in the inverter acquires the voltage values ​​of the first and second DC capacitors to detect whether the transformer has a bias current due to magnetic flux oversaturation. When the bias current is within a controllable range, the controller increases the output voltage of the conversion circuit to complete the black start, which improves the reliability of bias current control, the accuracy of magnetic flux estimation and control is higher, and the applicability is strong.

[0034] In some feasible implementations, the magnetic flux of the core in the transformer can include AC magnetic flux (or steady-state magnetic flux) and DC magnetic flux (or transient magnetic flux). Here, the AC magnetic flux can be the peak value of the AC magnetic flux, and the DC magnetic flux of the core in the transformer decays over time. The controller can control the output voltage of the conversion circuit to remain constant and maintain it for a preset time when the voltage difference between the first DC capacitor and the second DC capacitor in the conversion circuit is greater than a balance threshold, until the voltage difference between the first DC capacitor and the second DC capacitor is less than a set threshold (here, the set threshold can be less than the balance threshold). This ensures that the AC magnetic flux of the core in the transformer remains constant and the DC magnetic flux decays, so that the sum of the AC and DC magnetic flux is less than the saturation magnetic flux. Specifically, the magnetic flux of the core in the transformer can be expressed as:

[0035]

[0036] Where ω is the angular frequency of the output voltage of the converter circuit in the inverter, a is the initial phase angle of the output voltage of the converter circuit, and R and L are the resistance and inductance of the primary winding of the transformer, respectively. This refers to the residual magnetism of the transformer core. Here, in the above expression...

[0037] or Representing alternating magnetic flux (or, steady-state magnetic flux), it can be represented by... This means that in the above expression Representing DC magnetic flux (or transient magnetic flux), it can be represented by... express. It can be represented as:

[0038]

[0039] Among them, U m The controller can detect when the voltage difference between the first and second DC capacitors exceeds the balance threshold, i.e., when the total magnetic flux of the transformer far exceeds the saturation magnetic flux of the iron core, resulting in a bias current, and can maintain the output voltage of the conversion circuit constant (U). m The value of remains unchanged, making Since the value of the inverter remains unchanged, the AC magnetic flux of the inverter also remains unchanged. And according to the above expression for the DC magnetic flux of the inverter... The DC magnetic flux decays exponentially over time, and its decay rate is proportional to the R / L ratio of the transmission line between the inverter and the transformer. Therefore, the controller can maintain the output voltage of the conversion circuit constant for a preset duration. The total magnetic flux in the transformer core can exit the saturation region (i.e., fall below the saturation flux) as the DC magnetic flux decreases. In other words, the sum of the AC and DC magnetic flux in the transformer core is less than the saturation flux. The inverter, through the controller, maintains the output voltage of the conversion circuit constant for a preset duration when a bias current occurs, thus preventing the transformer core from saturating and generating a bias current, and preventing the neutral point of the DC bus connected to the inverter from shifting and the transformer from damage.

[0040] In some feasible implementations, the controller is further configured to increase the output impedance of the inverter to increase the transmission line impedance between the inverter and the transformer when the output voltage of the conversion circuit does not exceed the rated output voltage, i.e., before the black start process of the off-grid power supply system ends. Specifically, from the expression for the DC flux of the inverter... In this process, the DC magnetic flux decays exponentially over time, and its decay rate is directly proportional to the R / L ratio of the transmission line between the inverter and the transformer. This means that the decay rate of the DC magnetic flux can be accelerated by reshaping the impedance of the transmission line. For example, the output resistance R of the inverter can be increased by setting a virtual impedance, or the output inductance L can be decreased, thus increasing the R / L ratio. By increasing the impedance of the transmission line to accelerate the decay rate of the DC magnetic flux, it is possible to further prevent the transformer core from saturating and generating a bias current, thus preventing the inverter midpoint potential from diverging and the transformer from being damaged.

[0041] In some feasible implementation methods, please refer to the following: Figure 6 , Figure 6 This is a schematic diagram illustrating the relationship between the transformer magnetic flux and the output voltage provided in this application. Figure 6 As shown, when u(t) = 0, the transformer is connected to a load, and the transformer core will exhibit... The magnetic flux in the iron core cannot change abruptly; therefore, a constant flux must be generated. to offset non-periodic components Then consider the residual magnetism of the transformer. Due to the influence of this, the maximum magnetic flux appeared after half a cycle. This value is much greater than the transformer's saturation magnetic flux. Before the magnetic flux of the transformer core reaches saturation, the excitation current is very small and its value can be ignored (it can be...). Figure 6 When the magnetic flux of the transformer core exceeds the saturation flux (where I1 is the current magnitude or less), the excitation current will increase sharply, for example... Figure 6 When the inverter reaches its maximum magnetic flux At this time, the corresponding bias current is much larger than I2 of I1, and the excitation current contains a large even-order current. Excessive even-order current can cause the bias current to diverge at the inverter's midpoint potential and damage the transformer. Please refer to [further details]. Figure 7 , Figure 7 This is a schematic diagram of the transformer magnetic flux provided in this application. (For example...) Figure 7 As shown, the magnetic flux in the transformer core is maintained when the inverter output voltage is held. and As time decays The corresponding output voltage is less than The corresponding output voltage and initial remanence are both Saturation magnetic flux is That is, the peak value of the transformer's magnetic flux is proportional to the output voltage. Under the same residual magnetism and closing angle, a higher output voltage will generate a larger magnetic flux in the transformer core, making the core more likely to enter the saturation region.

[0042] In this application, the controller in the inverter can acquire the voltage values ​​of the first DC capacitor and the second DC capacitor in the conversion circuit, and detect the bias current based on the difference between the voltage values ​​of the first DC capacitor and the second DC capacitor. The inverter can adjust the output voltage of the conversion circuit by the controller when the difference between the voltage values ​​of the first DC capacitor and the second DC capacitor is not greater than a balance threshold, thereby increasing the output voltage of the conversion circuit to the rated output voltage and keeping the magnetic flux of the transformer core less than the saturation magnetic flux. The controller can also control the output voltage of the conversion circuit to remain constant for a preset time when a bias current occurs, so that the total magnetic flux in the transformer core can exit the saturation magnetic flux as the DC magnetic flux decreases, avoiding the generation of a bias current due to transformer core magnetic flux saturation, and preventing the neutral point of the DC bus connected to the inverter from shifting and transformer damage. Here, the controller in the inverter detects whether the transformer has a bias current due to magnetic flux oversaturation by acquiring the voltage values ​​of the first DC capacitor and the second DC capacitor. When the bias current is within a controllable range, the output voltage of the conversion circuit is increased to complete the black start, which improves the reliability of bias current control, and the accuracy of magnetic flux estimation and control is higher, making it more applicable.

[0043] See Figure 8 , Figure 8 This is a flowchart illustrating the bias current control method for off-grid power supply systems provided in this application. The bias current control method for off-grid power supply systems provided in this application is applicable to the above-mentioned... Figures 2 to 7 The inverter in any off-grid power supply system shown is an example of an off-grid power supply system comprising a DC power source, a transformer, and the inverter. The inverter includes a conversion circuit and a controller. The input of the conversion circuit is coupled to the DC power source in the off-grid power supply system, and the output of the conversion circuit is coupled to the load through the transformer. The conversion circuit includes a first DC capacitor and a second DC capacitor, which are connected in series and then in parallel across the DC power source. Figure 8 As shown, the bias current control method for off-grid power supply systems provided in this application includes the following steps:

[0044] S801 obtains the voltage values ​​of the first DC capacitor and the second DC capacitor through the controller.

[0045] S802, determine whether the difference between the voltage values ​​of the first DC capacitor and the second DC capacitor is greater than the balance threshold. If the determination result is yes, then proceed to step S803. If the determination result is no, then proceed to step S804.

[0046] S803 controls the output voltage of the conversion circuit to remain constant and for a preset duration via a controller.

[0047] S804, the output voltage of the conversion circuit is increased by the controller.

[0048] In some feasible implementations, the controller in the inverter can acquire the voltage values ​​of the first and second DC capacitors in the conversion circuit, and detect the bias current based on the difference between their voltage values. The inverter can adjust the output voltage of the conversion circuit by the controller when the difference between the voltage values ​​of the first and second DC capacitors is not greater than a balance threshold, thereby increasing the output voltage of the conversion circuit to the rated output voltage and maintaining the magnetic flux of the transformer core below the saturation flux. Here, the controller in the inverter acquires the voltage values ​​of the first and second DC capacitors to detect whether the transformer experiences a bias current due to magnetic flux oversaturation. When the bias current is within a controllable range, it increases the output voltage of the conversion circuit to complete black start, improving the reliability of bias current control, increasing the accuracy of magnetic flux estimation and control, and enhancing applicability.

[0049] In some feasible implementations, the magnetic flux of the core in the transformer can include AC magnetic flux (or steady-state magnetic flux) and DC magnetic flux (or transient magnetic flux). Here, the AC magnetic flux can be the peak value of the AC magnetic flux, and the DC magnetic flux of the core in the transformer decays over time. The controller can control the output voltage of the conversion circuit to remain constant and maintain it for a preset time when the voltage difference between the first DC capacitor and the second DC capacitor in the conversion circuit is greater than a balance threshold, until the voltage difference between the first DC capacitor and the second DC capacitor is less than a set threshold (here, the set threshold can be less than the balance threshold). This ensures that the AC magnetic flux of the core in the transformer remains constant and the DC magnetic flux decays, so that the sum of the AC and DC magnetic flux is less than the saturation magnetic flux. Specifically, the magnetic flux of the core in the transformer can be expressed as:

[0050]

[0051] Where ω is the angular frequency of the output voltage of the converter circuit in the inverter, a is the initial phase angle of the output voltage of the converter circuit, and R and L are the resistance and inductance of the primary winding of the transformer, respectively. This refers to the residual magnetism of the transformer core. Here, in the above expression... or Representing alternating magnetic flux (or, steady-state magnetic flux), it can be represented by... This means that in the above expression Representing DC magnetic flux (or transient magnetic flux), it can be represented by... express. It can be represented as:

[0052]

[0053] Among them, U m The controller can detect when the voltage difference between the first and second DC capacitors exceeds the balance threshold, i.e., when the total magnetic flux of the transformer far exceeds the saturation magnetic flux of the iron core, resulting in a bias current, and can maintain the output voltage of the conversion circuit constant (U). m The value of remains unchanged, making Since the value of the inverter remains unchanged, the AC magnetic flux of the inverter also remains unchanged. And according to the above expression for the DC magnetic flux of the inverter... The DC magnetic flux decays exponentially over time, and its decay rate is proportional to the R / L ratio of the transmission line between the inverter and the transformer. Therefore, the controller can maintain the output voltage of the conversion circuit constant for a preset duration. The total magnetic flux in the transformer core can exit the saturation region (i.e., fall below the saturation flux) as the DC magnetic flux decreases. In other words, the sum of the AC and DC magnetic flux in the transformer core is less than the saturation flux. The inverter, through the controller, maintains the output voltage of the conversion circuit constant for a preset duration when a bias current occurs, thus preventing the transformer core from saturating and generating a bias current, and preventing the neutral point of the DC bus connected to the inverter from shifting and the transformer from damage.

[0054] S805 controls the output impedance of the conversion circuit to increase when the output voltage does not exceed the rated output voltage.

[0055] In some feasible implementations, the controller is further configured to increase the output impedance of the inverter to increase the transmission line impedance between the inverter and the transformer when the output voltage of the conversion circuit does not exceed the rated output voltage, i.e., before the black start process of the off-grid power supply system ends. Specifically, from the expression for the DC flux of the inverter... In this process, the DC magnetic flux decays exponentially over time, and its decay rate is directly proportional to the R / L ratio of the transmission line between the inverter and the transformer. This means that the decay rate of the DC magnetic flux can be accelerated by reshaping the impedance of the transmission line. For example, the output resistance R of the inverter can be increased by setting a virtual impedance, or the output inductance L can be decreased, thus increasing the R / L ratio. By increasing the impedance of the transmission line to accelerate the decay rate of the DC magnetic flux, it is possible to further prevent the transformer core from saturating and generating a bias current, thus preventing the inverter midpoint potential from diverging and the transformer from being damaged.

[0056] In this application, the inverter can obtain the voltage values ​​of the first and second DC capacitors in the conversion circuit through a controller, and detect the bias current based on the difference between the voltage values ​​of the first and second DC capacitors. The inverter can adjust the output voltage of the conversion circuit through the controller when the difference between the voltage values ​​of the first and second DC capacitors is not greater than a balance threshold, thereby increasing the output voltage of the conversion circuit to the rated output voltage and keeping the magnetic flux in the transformer core less than the saturation magnetic flux. The controller can also control the output voltage of the conversion circuit to remain constant for a preset time when a bias current occurs, so that the total magnetic flux in the transformer core can exit the saturation magnetic flux as the DC magnetic flux decreases, avoiding the generation of bias current due to transformer core magnetic flux saturation, and preventing neutral point shift of the DC bus connected to the inverter and transformer damage. Here, the controller in the inverter detects whether the transformer has a bias current due to magnetic flux oversaturation by acquiring the voltage values ​​of the first DC capacitor and the second DC capacitor. When the bias current is within a controllable range, the output voltage of the conversion circuit is increased to complete the black start, which improves the reliability of bias current control, and the accuracy of magnetic flux estimation and control is higher, making it more applicable.

Claims

1. An inverter suitable for off-grid power supply systems, characterized in that, The inverter includes a conversion circuit and a controller. The input terminal of the conversion circuit is coupled to a DC power supply, and the output terminal of the conversion circuit is coupled to the load through a transformer. The conversion circuit includes a first DC capacitor and a second DC capacitor, which are connected in series and then in parallel across the DC power supply. The controller is used to acquire the voltage values ​​of the first DC capacitor and the second DC capacitor in the conversion circuit, and adjust the output voltage of the conversion circuit according to the voltage values ​​of the first DC capacitor and the second DC capacitor, so as to increase the output voltage of the conversion circuit to reach the rated output voltage and keep the magnetic flux of the iron core in the transformer less than the saturation magnetic flux to reduce the bias current. The magnetic flux of the core in the transformer includes AC magnetic flux and DC magnetic flux. The DC magnetic flux decays over time. The controller is also used to control the output voltage of the conversion circuit to remain unchanged until the voltage difference between the first DC capacitor and the second DC capacitor is less than a set threshold when the difference between the voltage values ​​of the first DC capacitor and the second DC capacitor is greater than a balance threshold, so that the AC magnetic flux of the core in the transformer remains unchanged and the DC magnetic flux decays.

2. The inverter according to claim 1, characterized in that, The controller is used to increase the output voltage of the conversion circuit when the difference between the voltage values ​​of the first DC capacitor and the second DC capacitor is not greater than the balance threshold.

3. The inverter according to claim 2, characterized in that, The set threshold is less than the balance threshold, and the sum of the AC magnetic flux and the attenuated DC magnetic flux is less than the saturation magnetic flux.

4. The inverter according to claim 3, characterized in that, The controller is also configured to increase the output impedance of the conversion circuit to increase the transmission line impedance between the inverter and the transformer when the output voltage of the conversion circuit does not exceed the rated output voltage.

5. An off-grid power supply system, characterized in that, The off-grid power supply system includes a DC power supply and an inverter as described in any one of claims 1-4; The DC power supply is used to provide DC power input to the inverter.

6. The off-grid power supply system according to claim 5, characterized in that, The off-grid power supply system also includes a DC / DC converter, and the DC power supply is coupled to the DC side of the inverter through the DC / DC converter; The DC / DC converter is used to perform DC power conversion based on the DC input provided by the DC power supply, and outputs the DC power converted DC power to the inverter.

7. A method for controlling the bias current of an off-grid power supply system, characterized in that, The method is applicable to inverters in off-grid power supply systems. The off-grid power supply system includes a DC power supply, a transformer, and the inverter. The inverter includes a conversion circuit and a controller. The input terminal of the conversion circuit is coupled to the DC power supply, and the output terminal of the conversion circuit is coupled to the load through the transformer. The conversion circuit includes a first DC capacitor and a second DC capacitor, which are connected in series and then in parallel across the DC power supply. The method includes: The controller acquires the voltage values ​​of the first DC capacitor and the second DC capacitor in the conversion circuit, and adjusts the output voltage of the conversion circuit based on the voltage values ​​of the first DC capacitor and the second DC capacitor to increase the output voltage of the conversion circuit to the rated output voltage and keep the magnetic flux of the iron core in the transformer less than the saturation magnetic flux to reduce the bias current. The magnetic flux of the core in the transformer includes AC magnetic flux and DC magnetic flux. The DC magnetic flux decays over time. When the difference between the voltage values ​​of the first DC capacitor and the second DC capacitor is greater than a balance threshold, the controller controls the output voltage of the conversion circuit to remain unchanged until the difference between the voltage values ​​of the first DC capacitor and the second DC capacitor is less than a set threshold, so that the AC magnetic flux of the core in the transformer remains unchanged and the DC magnetic flux decays.

8. The method according to claim 7, characterized in that, The step of adjusting the output voltage of the conversion circuit based on the voltage values ​​of the first DC capacitor and the second DC capacitor by the controller includes: The controller increases the output voltage of the conversion circuit when the difference between the voltage values ​​of the first DC capacitor and the second DC capacitor is not greater than the balance threshold.

9. The method according to claim 8, characterized in that, The set threshold is less than the balance threshold, and the sum of the AC magnetic flux and the attenuated DC magnetic flux is less than the saturation magnetic flux.

10. The method according to claim 9, characterized in that, The method further includes: When the output voltage of the conversion circuit does not exceed the rated output voltage, the controller controls the output impedance of the conversion circuit to increase, thereby increasing the transmission line impedance between the inverter and the transformer.

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