Power converter
By inputting a differential mode voltage signal to the inverter bridge arm and controlling the relay closure, the overheating problem caused by the increase in impedance of the oxidation relay is solved, and effective oxide layer cleaning is achieved, ensuring the safety and reliability of the inverter.
Patent Information
- Application Number
- CN202510296594.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-03-12
- Publication Date
- 2025-07-18
Smart Images
Figure CN120342207A_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of power electronics, and in particular, to a power converter. Background Art
[0002] Most of the grid-connected relay contacts in inverters adopt the process of plating inert metals on the surface. During the operation of the inverter, there may be corrosive gases outside the inverter and enter the interior of the inverter. Or, there may be some strongly chemically corrosive elements inside the inverter. The above corrosive gases and strongly chemically corrosive elements may cause the inert metal plating on the grid-connected relay contacts to oxidize, resulting in an increase in the impedance of the grid-connected relay contacts. Therefore, it is easier to generate heat when the power current passes through, making the inverter at risk of overheating. Summary of the Invention
[0003] An embodiment of this application provides a power converter, which can reduce the contact impedance of the oxidation relay in the power converter.
[0004] In a first aspect, this application provides a power converter. The power converter includes a controller, at least two inverter bridge arms, and at least two relays. The output end of each inverter bridge arm is connected to the power grid through at least one relay. The controller is configured to control at least two inverter bridge arms to convert the received direct current into alternating current and output it. The controller is configured to, during the process of controlling at least two inverter bridge arms to convert the received direct current into alternating current and output it, input a differential-mode voltage signal to a first inverter bridge arm and a second inverter bridge arm among at least two inverter bridge arms, so that the difference in the instantaneous voltage values of the alternating current output by the first inverter bridge arm and the alternating current output by the second inverter bridge arm changes. The controller is configured to first control the relay connected to the first inverter bridge arm to close, and then control the relay connected to the second inverter bridge arm to close within a set time period during the process of inputting the differential-mode voltage signal to the first inverter bridge arm and the second inverter bridge arm, so as to reduce the contact impedance of the relay connected to the second inverter bridge arm, where the absolute value of the instantaneous value of the differential-mode voltage signal is greater than or equal to a set threshold within the set time period.
[0005] In this application, during the cleaning process of the oxidized relay, the cleaning effect of the relay oxide layer is directly related to the voltage difference across the relay when it is closed. When other conditions remain unchanged, when a current loop is formed between the first inverter leg and the second inverter leg, the voltage difference across the relay connected to the second inverter leg is proportional to the absolute value of the instantaneous value of the differential-mode voltage signal input by the controller. Therefore, during the differential-pressure cleaning process of the relay connected to the second inverter leg in the power converter by the controller, the controller controls the oxidized relay to close during the set period when inputting the differential-mode voltage signal to the first inverter leg and the second inverter leg. Since the absolute value of the instantaneous value of the differential-mode voltage signal input during the set period is greater than or equal to the set threshold, the differential-mode voltage signal does not cross zero during the set period, so the input differential-mode voltage signal can adjust the voltage difference across the relay connected to the second inverter leg, making the voltage difference across the relay meet the differential-pressure cleaning requirements during the set period, avoiding the voltage difference across the relay being too low when the relay actually closes due to the conduction delay of the relay, improving the reliability of the differential-pressure cleaning of the relay, reducing the contact impedance of the relay, and preventing the power current from passing through the oxidized relay and causing the power converter to overheat.
[0006] In a possible implementation, the controller is configured to first control the relay connected to the first inverter leg to close, and then control the relay connected to the second inverter leg to close when the absolute value of the instantaneous value of the differential-mode voltage signal increases from less than the set threshold to equal to the set threshold. Here, since the absolute value of the instantaneous value of the differential-mode voltage signal is equal to the set threshold during the set period, when the oxidized relay has a conduction delay, the above set period can cover the time interval from when the controller controls the relay to close to when the relay actually closes, so as to ensure that when the relay actually closes, the absolute value of the instantaneous value of the differential-mode voltage signal is equal to the set threshold, and the differential-mode voltage signal can adjust the voltage difference across the oxidized relay to meet the differential-pressure cleaning requirements, further improving the reliability of the differential-pressure cleaning of the relay.
[0007] In a possible implementation, the controller is configured to first control the relay connected to the first inverter leg to close, and then, after a set waiting duration after the absolute value of the instantaneous value of the differential-mode voltage signal increases from less than a set threshold to greater than or equal to the set threshold, control the relay connected to the second inverter leg to close, where the set waiting duration is less than or equal to the duration of the set period. Here, if the conduction delay of the relay is relatively long, for example, the time interval from when the controller controls the relay to close to when the relay actually closes is equal to the above-mentioned set period, then by controlling the relay to close after the absolute value of the instantaneous value of the differential-mode voltage signal increases from less than the set threshold to greater than or equal to the set threshold and after an interval of the set waiting duration, such that the actual closing time of the relay is between another set period, it is ensured that the absolute value of the instantaneous value of the differential-mode voltage signal is equal to the set threshold when the relay actually closes, and the differential-mode voltage signal can adjust the voltage difference across the relay to meet the requirements of differential-pressure cleaning, further improving the reliability of differential-pressure cleaning of the relay.
[0008] In a possible implementation, the differential-mode voltage signal is a periodically varying AC signal, and the controller is configured to control the relay connected to the second inverter leg to close once after a set waiting duration after the absolute value of the instantaneous value of the differential-mode voltage signal increases from less than a set threshold to greater than or equal to the set threshold each time. Among them, the set waiting duration increases after each time the controller controls the relay connected to the second inverter leg to close, and the set waiting duration is less than or equal to the duration of the set period. By setting the set waiting duration to increase after each time the controller controls the relay to close, such that the time when the controller controls the relay to close each time is at a different position in each set period, it is possible to avoid the problem that the differential-mode voltage signal is at zero at the moment when the relay actually closes due to the uncertain factor of the conduction delay of the relay, ensuring that the absolute value of the instantaneous value of the differential-mode voltage signal is equal to the set threshold when the relay actually closes and ensuring that the voltage difference across the relay meets the requirements of differential-pressure cleaning.
[0009] In a possible implementation, after the controller controls the relay connected to the second inverter leg to close, the controller is configured to, if the contact impedance of the relay connected to the second inverter leg is greater than the set impedance threshold, control the relay connected to the second inverter leg to open, and then control the relay connected to the second inverter leg to close again within a set period during the process of inputting the differential-mode voltage signal to the first inverter leg and the second inverter leg. Here, the voltage across the relay is detected by a sampling circuit, and the contact impedance of the relay is obtained by combining the voltage across the relay and the current flowing through the relay at present. If the contact impedance of the relay is still too large, the relay is controlled to open for the next differential-pressure cleaning, thereby further improving the cleaning effect on the oxidized relay.
[0010] In a possible implementation, the controller is configured to control the amplitude of the differential-mode voltage signal to increase as the number of times the relay connected to the second inverter leg is closed increases. During the cleaning process of the oxidized relay, the cleaning effect of the relay oxide layer is directly related to the voltage difference across the relay when it is closed, and the voltage difference across the relay is proportional to the absolute value of the instantaneous value of the differential-mode voltage signal input to the controller. Since the absolute value of the instantaneous value of the differential-mode voltage signal is proportional to the amplitude of the differential-mode voltage signal, by increasing the amplitude of the differential-mode voltage signal, the absolute value of the instantaneous value of the differential-mode voltage signal can be increased to increase the voltage difference across the relay when it is closed, thereby improving the cleaning effect on the oxidized relay.
[0011] In a possible implementation, the controller is configured to control the amplitude of the differential-mode voltage signal to increase as the operating temperature of the power converter during its last grid connection increases. Since, when other conditions remain unchanged, the heat generated by the relay is proportional to its contact impedance. Therefore, as the operating temperature of the power converter during its last grid connection is higher, the controller increases the amplitude of the differential-mode voltage signal, which can increase the absolute value of the instantaneous value of the differential-mode voltage signal to increase the voltage difference across the relay when it is closed, thereby improving the cleaning effect on the oxidized relay.
[0012] In a possible implementation, the differential-mode voltage signal is a square wave. Compared with a triangular wave of the same frequency, when the square wave is used as the differential-mode voltage signal and input to the inverter leg, the change in the instantaneous value of the square wave in one period is smaller, ensuring that when cleaning the voltage difference across the oxidized relay, the voltage difference across the relay meets the cleaning requirements.
[0013] In a possible implementation, the differential-mode voltage signal is a trapezoidal wave. Compared with a triangular wave of the same frequency, when the trapezoidal wave is used as the differential-mode voltage signal and input to the inverter leg, the change in the instantaneous value of the trapezoidal wave in one period is smaller, ensuring that when cleaning the voltage difference across the oxidized relay, the voltage difference across the relay meets the cleaning requirements. Description of the Drawings
[0014] Figure 1 is a schematic diagram of an application scenario of the power supply system provided by the present application;
[0015] Figure 2 is another schematic diagram of an application scenario of the power supply system provided by the present application;
[0016] Figure 3 is a schematic diagram of a structure of the power converter provided by the present application;
[0017] Figure 4 is another schematic diagram of a structure of the power converter provided by the present application;
[0018] Figure 5It is a control schematic diagram for cleaning the power converter relay provided by this application;
[0019] Figure 6 It is another control schematic diagram for cleaning the power converter relay provided by this application;
[0020] Figure 7 It is another control schematic diagram for cleaning the power converter relay provided by this application. Detailed implementation manners
[0021] Refer to Figure 1 , Figure 1 It is a schematic diagram of an application scenario of the power supply system provided by this application. The power supply system provided by this application may include a DC power supply and at least one power converter. Among them, the DC power supply is a photovoltaic module. Taking the power supply system including multiple power converters as an example, the DC terminals of the power converters are used to connect the photovoltaic module, and the AC terminals of the power converters are connected in parallel and then used to connect the power grid and the load. The power converter can perform an inversion conversion on the direct current provided by the photovoltaic module and output the alternating current obtained after the inversion conversion to the power grid and the load for power supply.
[0022] In some feasible implementation manners, the DC power supply is a storage battery, and the DC terminals of each power converter can be used to connect the storage battery. Please refer to Figure 2 , Figure 2 It is another schematic diagram of an application scenario of the power supply system provided by this application. Among the multiple power converters of the power supply system, the DC terminals of some power converters are used to connect the storage battery, and the AC terminals of the multiple power converters are connected in parallel and then used to connect the power grid and the load. Each power converter can perform an inversion conversion on the direct current provided by the photovoltaic module or the storage battery and output the alternating current obtained after the inversion conversion to the power grid and the load for power supply. Here, the power converter connected to the photovoltaic module in the power supply system can be a photovoltaic inverter, and the power converter connected to the storage battery in the power supply system can be a current converter.
[0023] In Figure 1 or Figure 2In the application scenario of the power supply system shown, the power converter includes a controller, a power conversion circuit, and a relay. The input end of the power conversion circuit is connected to a photovoltaic module or an energy storage battery, and the output end of the power conversion circuit is connected to the power grid and a load through the relay. The above-mentioned controller is used to control the power conversion circuit to convert the received direct current into alternating current after the power converter is connected to the photovoltaic module or the energy storage battery, and control the relay to conduct to connect the power conversion circuit with the power grid and the load, so as to supply power to the power grid and the load. It should be understood that when other conditions remain unchanged, the heat generated by the relay is proportional to its contact impedance. Here, since the surface of the relay contacts in the power conversion circuit mostly adopts the process of plating inert metal, during the operation of the power converter, the following situations may occur: there may be corrosive gases outside the power converter and enter the interior of the power converter; or there may be some strongly chemically corrosive elements inside the power converter. The above-mentioned corrosive factors such as corrosive gases and strongly chemically corrosive elements may cause the relay to oxidize, resulting in an increase in the contact impedance of the relay. When the power current passes through, the relay is prone to generate more heat and pose an overheating risk, thus affecting the normal operation and safety of the power converter. By applying a specific voltage across the oxidized relay, especially ensuring a certain voltage difference across the relay when it is closed, an electrochemical reaction occurs on the oxide layer of the relay, removing the oxide layer on the surface of the relay contacts and restoring its conductivity and switching performance. This cleaning method is also called voltage difference cleaning.
[0024] Currently, during the voltage difference cleaning of the oxidized relay, a common-mode voltage signal such as a triangular wave is injected into the power converter while controlling the relay to conduct, so as to form a voltage difference across the relay, and the relay is closed during the period when there is a voltage difference across the relay, thereby performing voltage difference cleaning on the oxidized relay. However, during the injection of the common-mode voltage signal such as a triangular wave, the voltage difference between the output voltage of the power converter and the grid voltage may be zero. Due to the different conduction delays of the relay, the relay conduction action time is uncertain. Therefore, the voltage difference at the moment of relay closing may be in a large voltage difference range or near zero crossing, and the voltage difference cleaning effect cannot be guaranteed.
[0025] In order to improve the voltage difference cleaning effect on the oxidized relay, in the power converter provided by the present application, the power converter includes a controller, at least two inverter bridge arms, and at least two relays, and each inverter bridge arm is connected to the power grid through at least one relay. For example, taking the power converter including three inverter bridge arms as an example, please refer to Figure 3 , Figure 3is a schematic structural diagram of a power converter provided by this application. The power converter includes an inverter leg A, an inverter leg B, and an inverter leg C. The output terminals of the inverter leg A, the inverter leg B, and the inverter leg C serve as the A-phase output terminal, the B-phase output terminal, and the C-phase output terminal of the power converter respectively. The inverter leg A, the inverter leg B, and the inverter leg C in the power converter are respectively connected to the power grid through relays Ka, Kb, and Kc. In addition, the power converter includes a series-connected bus capacitor C1 and a bus capacitor C2, and the inverter leg A is connected in parallel across the series-connected bus capacitor C1 and bus capacitor C2. Taking the power converter connected to a photovoltaic module as an example, the bus capacitor C1 is connected to the positive electrode of the photovoltaic module through the DC bus BUS+, and the bus capacitor C2 is connected to the negative electrode of the photovoltaic module through the DC bus BUS-. For example, before the power converter is connected to the power grid for operation, differential pressure cleaning is performed on the relays connected to the power converter. The controller in the power converter ( Figure 3(not shown) is used to control inverter leg A, inverter leg B, and inverter leg C to convert the direct current from two bus capacitors into alternating current and output it. Further, among the multiple inverter legs included in the power converter, there are a first inverter leg and a second inverter leg. When the relay connected to the second inverter leg is oxidized and needs to be cleaned, the controller inputs a differential-mode voltage signal to the first inverter leg and the second inverter leg, causing the difference in the instantaneous values of the voltages of the alternating current output by the first inverter leg and the alternating current output by the second inverter leg to change. For example, when the controller is used to control inverter leg A, inverter leg B, and inverter leg C to convert the received direct current into alternating current and output it, when the relay Kb connected to inverter leg B is oxidized and needs to be cleaned, the controller inputs a differential-mode voltage signal to inverter leg A and inverter leg B, or inputs a differential-mode voltage signal to inverter leg A and inverter leg C. Taking the example of the controller inputting a differential-mode voltage signal to inverter leg A and inverter leg B (i.e., inverter leg A is the above-mentioned first inverter leg, and inverter leg B is the above-mentioned second inverter leg), after inputting the differential-mode voltage signal to inverter leg A and inverter leg B, the difference in the instantaneous values of the voltages of the alternating current output by inverter leg A and the alternating current output by inverter leg B (i.e., the line voltage Uab between the A-phase output terminal and the B-phase output terminal) changes. Then, the controller is further used to control the relay Ka connected to inverter leg A to close, and then, within a set period during the process of inputting the differential-mode voltage signal to inverter leg A and inverter leg B, control the relay Kb connected to inverter leg B to close, so that a current loop is formed between inverter leg A and inverter leg B, thereby forming a voltage difference across its two ends when the relay Kb closes, so as to reduce the contact impedance of the relay Kb. Among them, the absolute value of the instantaneous value of the differential-mode voltage signal within the above-mentioned set period is greater than or equal to a set threshold. Here, since during the cleaning process of the oxidized relay, the cleaning effect of the relay oxide layer is directly related to the voltage difference across the two ends of the relay when it is closed, and when other conditions remain unchanged, when a current loop is formed between inverter leg A and inverter leg B, the voltage difference across the two ends of the relay Kb connected to inverter leg B is proportional to the absolute value of the instantaneous value of the differential-mode voltage signal input by the controller. Therefore, the controller controls the relay Kb connected to inverter leg B to close within the set period of inputting the differential-mode voltage signal. Since the absolute value of the instantaneous value of the differential-mode voltage signal within the set period is greater than or equal to the set threshold and there is no zero-crossing point within the set period, the voltage difference across the two ends of the relay Kb meets the differential-pressure cleaning requirements within the set period, avoiding the voltage difference across the two ends of the relay Kb being too low when the relay Kb actually closes due to the conduction delay of the relay Kb, improving the reliability of the differential-pressure cleaning of the relay Kb, reducing the contact impedance of the relay Kb, and avoiding overheating of the power converter. It can be understood that the cleaning process for other relays in the power converter is similar to the cleaning process of the above-mentioned relay Kb, and will not be elaborated here.
[0026] Optionally, the output terminal of each inverter leg can also be connected to two serially connected relays. Please refer to Figure 4 , Figure 4 which is another schematic structural diagram of the power converter provided by this application. As Figure 4 shown, the output terminals of each inverter leg are all connected to two relays. The output terminal of inverter leg A is connected to relay Ka1 and relay Ka2, the output terminal of inverter leg B is connected to relay Kb1 and relay Kb2, and the output terminal of inverter leg C is connected to relay Kc1 and relay Kc2. By increasing the number of relays connected to each inverter leg, the situation where some relays among the relays connected to any one inverter leg fail and cannot be disconnected from the power grid can be avoided. In the Figure 4 power converter shown, the controller ( Figure 4 not shown) in the power converter is used to control inverter leg A, inverter leg B, and inverter leg C to convert the direct current from two bus capacitors into alternating current and output it. Before the power converter is connected to the power grid for operation, when Figure 4 any relay in the power converter shown needs to be cleaned due to oxidation, for example, when relay Kb1 connected to inverter leg B needs to be cleaned, the controller is used to input a differential mode voltage signal to inverter leg A and inverter leg B, so that the instantaneous value difference of the voltages of the alternating current output by inverter leg A and the alternating current output by inverter leg B changes. Then, the controller is further used to control relay Ka1, relay Ka2 connected to inverter leg A, and relay Kb2 connected to inverter leg B to close, and the closing sequence of the above relays is not limited. Then, the controller controls relay Kb1 connected to inverter leg B to close again within the set time period when inputting the differential mode voltage signal to inverter leg A and inverter leg B. Among them, the absolute value of the instantaneous value of the differential mode voltage signal within the set time period is greater than or equal to the set threshold. Here, the controller controls relay Kb1 connected to inverter leg B to close within the set time period when inputting the differential mode voltage signal. Since the absolute value of the instantaneous value of the differential mode voltage signal within the set time period is greater than or equal to the set threshold, there will be no zero crossing within the set time period, so the input differential mode voltage signal can make the voltage difference across relay Kb1 meet the differential pressure cleaning requirements within the set time period, avoiding the situation that the voltage difference across relay Kb1 is too low when relay Kb1 actually closes due to the conduction delay of relay Kb1, improving the reliability of differential pressure cleaning of relay Kb, reducing the contact impedance of relay Kb1, and avoiding overheating of the power converter. It can be understood that the cleaning process for other relays in the power converter is similar to the cleaning process of the above relay Kb1, and will not be elaborated here.
[0027] In some feasible embodiments, the controller is configured to first control the relay connected to the first inverter leg to close, and then control the relay connected to the second inverter leg to close when the absolute value of the instantaneous value of the differential-mode voltage signal increases from less than the set threshold to equal the set threshold. Please refer to Figure 5 , Figure 5 which is a control schematic diagram for cleaning the relay of the power converter provided by the present application. Taking the differential-mode voltage signal input by the controller to the inverter leg in the power converter as a trapezoidal wave as an example, and taking Figure 3 the power converter shown as an example, when the relay Kb connected to the inverter leg B is oxidized and needs to be cleaned, as Figure 5 shown, the controller inputs a differential-mode voltage signal Udm to the inverter leg A and the inverter leg B. The differential-mode voltage signal Udm has the same phase and frequency as the line voltage Uab between the output terminals of the inverter leg A and the output terminals of the inverter leg B. After inputting the differential-mode voltage signal Udm, the alternating current output by the inverter leg A changes from Ua1 to Ua2, and the alternating current output by the inverter leg B changes from Ub1 to Ub2. Among them, the phase of the differential-mode voltage signal injected into the inverter leg A is opposite to the phase of the differential-mode voltage signal injected into the inverter leg B, so that the line voltage between the output terminals of the inverter leg A and the output terminals of the inverter leg B changes from Uab1 to Uab2. Taking the maximum value of the differential-mode voltage signal Udm as the above set threshold (alternatively, the above set threshold can also be set lower than the maximum value of the differential-mode voltage signal Udm), then the set period of the differential-mode voltage signal Udm is between t1 and t2, or can also be between t3 and t4. Among them, between t1 and t2, the absolute value of the instantaneous value of the differential-mode voltage signal Udm is equal to the set threshold; between t3 and t4, the absolute value of the instantaneous value of the differential-mode voltage signal Udm is equal to the set threshold, and the direction is opposite to the direction of the differential-mode voltage signal Udm between t1 and t2. In other words, within the set period, the absolute value of the instantaneous value of the differential-mode voltage signal Udm is greater than or equal to the set threshold. Taking the set period between t1 and t2 as an example, the controller is configured to control the relay Ka connected to the inverter leg A to close, and then when the absolute value of the instantaneous value of the above differential-mode voltage signal Udm increases from less than the set threshold to just equal the set threshold, that is, at the moment t1, control the relay Kb connected to the inverter leg B to close. Here, since the absolute value of the instantaneous value of the differential-mode voltage signal Udm between t1 and t2 is equal to the set threshold, when the relay has a conduction delay, the set period between t1 and t2 can cover the time interval from when the controller controls the relay Kb to close to when the relay Kb actually closes, so as to ensure that when the relay Kb actually closes, the absolute value of the instantaneous value of the differential-mode voltage signal Udm is equal to the set threshold. The differential-mode voltage signal Udm can adjust the voltage difference between the two ends of the relay Kb connected to the inverter leg B, so that the voltage difference between the two ends of the relay Kb meets the differential-pressure cleaning requirements, further improving the reliability of the differential-pressure cleaning of the relay.
[0028] In some feasible embodiments, if the conduction delay of the relay is relatively long, the controller is configured to first control the relay connected to the first inverter leg to close, and then, after a set waiting duration after the absolute value of the instantaneous value of the differential-mode voltage signal increases from less than the set threshold to greater than or equal to the set threshold, control the relay connected to the second inverter leg to close, where the set waiting duration is less than or equal to the duration of the set period. Also, taking the Figure 3 power converter shown as an example, when the relay Kb connected to the inverter leg B is oxidized and needs to be cleaned, please refer to Figure 5 again. If the conduction delay of the relay Kb is relatively long, for example, the time interval from when the controller controls the relay Kb to close to when the relay Kb actually closes is equal to the above-mentioned set period, the controller can be configured to control the relay Ka connected to the inverter leg A to close, and then, after the absolute value of the instantaneous value of the differential-mode voltage signal Udm increases from less than the set threshold to just equal to the set threshold and after an interval of the set waiting duration Ts, that is, at the t5 moment after the t1 moment, control the relay Kb connected to the inverter leg B to close. The actual closing moment of the relay Kb is between another set period, that is, between t3 and t4. Thus, it is ensured that when the relay Kb actually closes, the absolute value of the instantaneous value of the differential-mode voltage signal Udm is equal to the set threshold, and the differential-mode voltage signal Udm can adjust the voltage difference across the relay Kb connected to the inverter leg B, so that the voltage difference across the relay Kb meets the differential-pressure cleaning requirements, further improving the reliability of the differential-pressure cleaning of the relay.
[0029] In some feasible embodiments, the differential-mode voltage signal input by the controller to the inverter leg in the power converter can be a square wave. Please refer to Figure 6 Figure 6 which is another control schematic diagram of the relay cleaning of the power converter provided by this application. Also, taking the Figure 3 power converter shown as an example, when the relay Kb connected to the inverter leg B is oxidized and needs to be cleaned, as Figure 6 As shown, the controller inputs a differential-mode voltage signal Udm to inverter leg A and inverter leg B. The differential-mode voltage signal Udm has the same phase and frequency as the line voltage Uab between the output terminals of inverter leg A and inverter leg B. After inputting the differential-mode voltage signal Udm, the alternating current output by inverter leg A changes from Ua1 to Ua2, and the alternating current output by inverter leg B changes from Ub1 to Ub2. Among them, the phase of the differential-mode voltage signal injected into inverter leg A is opposite to that of the differential-mode voltage signal injected into inverter leg B, so that the line voltage between the output terminals of inverter leg A and inverter leg B changes from Uab1 to Uab2. Taking the maximum value of the differential-mode voltage signal Udm as the above-mentioned set threshold (or, the above-mentioned set threshold can also be set lower than the maximum value of the differential-mode voltage signal Udm), the set period of the differential-mode voltage signal Udm is between t1 and t2, or can also be between t2 and t3. Among them, between t1 and t2, the absolute value of the instantaneous value of the differential-mode voltage signal Udm is equal to the set threshold; between t2 and t3, the absolute value of the instantaneous value of the differential-mode voltage signal Udm is equal to the set threshold, and the direction is opposite to that of the differential-mode voltage signal Udm between t1 and t2 above.
[0030] Similarly, the controller is used to control the relay Ka connected to inverter leg A to close, and then when the absolute value of the instantaneous value of the above-mentioned differential-mode voltage signal Udm rises from less than the set threshold to just equal to the set threshold, that is, at time t1, control the relay Kb connected to inverter leg B to close. Here, since the absolute value of the instantaneous value of the differential-mode voltage signal Udm is equal to the set threshold between t1 and t2, when the relay has a conduction delay, the set period between t1 and t2 can cover the time interval from when the controller controls the relay Kb to close to when the relay Kb actually closes, so as to ensure that when the relay Kb actually closes, the absolute value of the instantaneous value of the differential-mode voltage signal Udm is equal to the set threshold, and the differential-mode voltage signal Udm can adjust the voltage difference across the relay Kb connected to inverter leg B.
[0031] In some feasible embodiments, the differential-mode voltage signal is an alternating current signal with periodic changes, and the controller controls the relay to close multiple times to clean the voltage difference of the relay. Specifically, the controller is used to control the relay connected to the second inverter leg to close once every time the absolute value of the instantaneous value of the differential-mode voltage signal rises from less than the set threshold to greater than or equal to the set threshold and after a set waiting duration. Among them, the set waiting duration increases after each time the controller controls the relay connected to the second inverter leg to close, and the set waiting duration is less than or equal to the duration of the set period.
[0032] Please refer to Figure 7 , Figure 7This is another control schematic diagram for the cleaning of the power converter relay provided by this application. Taking the differential-mode voltage signal input by the controller to the inverter bridge arm in the power converter as a square wave with a periodic change as an example, and taking the relay Kb connected to the inverter bridge arm B in the power converter shown in Figure 3 as an example for differential pressure cleaning, as shown in Figure 7 , taking the maximum value of the differential-mode voltage signal Udm as the set threshold, the set time period of the differential-mode voltage signal Udm includes t1 to t2, t2 to t3, t3 to t4, t4 to t5, t5 to t6, and t6 to t7, etc. The controller is used to control the relay Kb connected to the inverter bridge arm B to close once every time the absolute value of the instantaneous value of the differential-mode voltage signal Udm is equal to the set threshold and after a set waiting duration. Taking the initial value of the above set waiting duration as 0 and the duration incremented each time the set waiting duration as T0 / 6 as an example, T0 is the duration when the differential-mode voltage signal reaches the maximum value in each cycle. First, the absolute value of the instantaneous value of the differential-mode voltage signal Udm is equal to the set threshold at the moment t1, and the controller controls the relay Kb connected to the inverter bridge arm B to close once at the moment t1, and then controls the relay Kb to open. The absolute value of the instantaneous value of the differential-mode voltage signal Udm is equal to the set threshold at the moment t2, the set waiting duration is incremented by T0 / 6, and at the moment t8, which is T0 / 6 after the moment t2, the controller controls the relay Kb connected to the inverter bridge arm B to close, and then controls the relay Kb to open. The absolute value of the instantaneous value of the differential-mode voltage signal Udm is equal to the set threshold at the moment t3, the set waiting duration is incremented by T0 / 6, and at the moment t9, which is 2T0 / 6 after the moment t3, the controller controls the relay Kb connected to the inverter bridge arm B to close, and then controls the relay Kb to open. Similarly, the absolute value of the instantaneous value of the differential-mode voltage signal Udm is equal to the set threshold at the moments t4, t5, and t6, and the controller controls the relay Kb connected to the inverter bridge arm B to close at the moments t10, which is 3T0 / 6 after the moment t4, t11, which is 4T0 / 6 after the moment t5, and t12, which is 5T0 / 6 after the moment t6, respectively. Here, by controlling the relay Kb connected to the inverter bridge arm B to close once every time the absolute value of the instantaneous value of the differential-mode voltage signal Udm is equal to the set threshold and after a set waiting duration, and the above set waiting duration is incremented each time the controller controls the relay to close, it is ensured that in multiple times of the controller controlling the relay to close, the moment of each control of the relay to close is at a different position in each set time period. For example, as shown in Figure 7As shown, the moment when the controller first controls the relay to close is at the starting moment (i.e., t1) within the set time period t1 to t2. And as the number of closures increases, the moment when the controller controls the relay to close gradually moves backward. For example, it is at the middle moment (i.e., t10) within the set time period t4 to t5, and at the end moment (i.e., t12) within the set time period t6 to t7. The moment when the controller controls the relay to close each time is at different positions within each set time period, which can avoid the problem that the differential mode voltage signal Udm is at zero at the actual closing moment of the relay due to the uncertain conduction delay of the relay, ensuring that when the relay actually closes, the absolute value of the instantaneous value of the differential mode voltage signal Udm is equal to the set threshold. The differential mode voltage signal Udm can adjust the voltage difference across the relay Kb connected to the inverter bridge arm B, ensuring that the voltage difference across the relay meets the voltage difference cleaning requirements.
[0033] In some feasible embodiments, after the controller controls the relay connected to the second inverter bridge arm to close, if the contact impedance of the relay connected to the second inverter bridge arm is greater than the set impedance threshold, the controller is used to control the relay connected to the second inverter bridge arm to open, and within a set time period during the process of inputting the differential mode voltage signal to the first inverter bridge arm and the second inverter bridge arm, control the relay connected to the second inverter bridge arm to close again. Taking the cleaning of the relay Kb connected to the inverter bridge arm B in the power converter shown as an example, the controller is used to input a differential mode voltage signal to the inverter bridge arm A and the inverter bridge arm B during the process of controlling the inverter bridge arm A, the inverter bridge arm B, and the inverter bridge arm C to convert the received direct current into alternating current and output, so as to change the difference in the instantaneous voltage values of the alternating current output by the inverter bridge arm A and the alternating current output by the inverter bridge arm B. Then, the controller is further used to control the relay Ka connected to the inverter bridge arm A to close, and within a set time period during the process of inputting the differential mode voltage signal to the inverter bridge arm A and the inverter bridge arm B, control the relay Kb connected to the inverter bridge arm B to close to reduce the contact impedance of the relay Kb. When the relay Kb is closed, the controller obtains the contact impedance of the relay Kb. For example, by sampling the voltage across the relay and combining the voltage across the relay and the current flowing through the relay at present (i.e., the current at the output end of the B phase of the power converter), the contact impedance of the relay Kb is obtained. If the contact impedance of the relay Kb is still too large, such as greater than the set impedance threshold, then control the relay Kb to open for the next differential pressure cleaning, thereby further improving the cleaning effect on the oxidized relay. Figure 3 Taking the cleaning of the relay Kb connected to the inverter bridge arm B in the power converter shown as an example, the controller is used to input a differential mode voltage signal to the inverter bridge arm A and the inverter bridge arm B during the process of controlling the inverter bridge arm A, the inverter bridge arm B, and the inverter bridge arm C to convert the received direct current into alternating current and output, so as to change the difference in the instantaneous voltage values of the alternating current output by the inverter bridge arm A and the alternating current output by the inverter bridge arm B. Then, the controller is further used to control the relay Ka connected to the inverter bridge arm A to close, and within a set time period during the process of inputting the differential mode voltage signal to the inverter bridge arm A and the inverter bridge arm B, control the relay Kb connected to the inverter bridge arm B to close to reduce the contact impedance of the relay Kb. When the relay Kb is closed, the controller obtains the contact impedance of the relay Kb. For example, by sampling the voltage across the relay and combining the voltage across the relay and the current flowing through the relay at present (i.e., the current at the output end of the B phase of the power converter), the contact impedance of the relay Kb is obtained. If the contact impedance of the relay Kb is still too large, such as greater than the set impedance threshold, then control the relay Kb to open for the next differential pressure cleaning, thereby further improving the cleaning effect on the oxidized relay.
[0034] In some feasible embodiments, when the controller controls the relay to close multiple times for differential pressure cleaning of the relay, the controller controls the amplitude of the differential mode voltage signal to increase as the number of times of controlling the relay connected to the second inverter bridge arm to close increases. Specifically, the expression of the differential mode voltage signal input by the controller to the power converter is:
[0035] g(t) = A m *δ(2πf g *t)
[0036] where A m is the differential-mode voltage amplitude, which can be set to a positive value or a negative value, f g is the fundamental frequency, and δ(t) is a pulse function or a trapezoidal wave function. It should be understood that during the cleaning process of the oxidized relay, the cleaning effect of the relay oxide layer is directly related to the voltage difference across the relay when it is closed, and the voltage difference across the relay is proportional to the absolute value of the instantaneous value of the differential-mode voltage signal input to the controller. After the controller controls the relay to close multiple times to clean the relay by differential pressure, if the contact impedance of the relay is still too large, it means that when the relay is closed, the input differential-mode voltage signal cannot make the voltage difference across the relay reach the cleaning requirement. Since the absolute value of the instantaneous value of the differential-mode voltage signal is proportional to the amplitude of the differential-mode voltage signal, therefore, by increasing the amplitude of the differential-mode voltage signal, the absolute value of the instantaneous value of the differential-mode voltage signal can be increased to increase the voltage difference across the relay when it is closed, thereby improving the cleaning effect on the oxidized relay.
[0037] In some feasible embodiments, the controller is configured to control the amplitude of the differential-mode voltage signal to increase as the operating temperature of the power converter during the previous grid connection increases. Since, under other unchanged conditions, the heat generated by the relay is proportional to its contact impedance. Therefore, before cleaning the oxidized relay, obtain the operating temperature of the power converter during the previous grid connection. If the temperature is higher, the contact impedance of the relay is higher and the degree of oxidation is greater. For example, by setting a temperature detection device on the corresponding relay, the temperature detection device can be set on the wiring terminal of the relay or beside the contact of the relay, and the heating temperature when the relay is closed is obtained through the temperature detection device to obtain the operating temperature of the power converter during grid connection. As the operating temperature of the power converter during the previous grid connection is higher, the controller increases the amplitude of the differential-mode voltage signal, which can increase the absolute value of the instantaneous value of the differential-mode voltage signal to increase the voltage difference across the relay when it is closed, thereby improving the cleaning effect on the oxidized relay.
Claims
1. A power converter, characterized in that, The power converter includes a controller, at least two inverter bridge arms, and at least two relays. The output end of each inverter bridge arm is connected to the power grid through at least one of the relays; The controller is configured to control the at least two inverter bridge arms to convert the received direct current into alternating current and output it; During the process of controlling the at least two inverter bridge arms to convert the received direct current into alternating current and output it, the controller is configured to input a differential-mode voltage signal to a first inverter bridge arm and a second inverter bridge arm among the at least two inverter bridge arms, so that the difference in the instantaneous voltage values of the alternating current output by the first inverter bridge arm and the alternating current output by the second inverter bridge arm changes; The controller is configured to first control the relay connected to the first inverter bridge arm to close, and then control the relay connected to the second inverter bridge arm to close within a set time period during the process of inputting the differential-mode voltage signal to the first inverter bridge arm and the second inverter bridge arm, so as to reduce the contact impedance of the relay connected to the second inverter bridge arm, where the absolute value of the instantaneous value of the differential-mode voltage signal is greater than or equal to a set threshold within the set time period.
2. The power converter according to claim 1, wherein The controller is configured to first control the relay connected to the first inverter bridge arm to close, and then control the relay connected to the second inverter bridge arm to close when the absolute value of the instantaneous value of the differential-mode voltage signal increases from less than the set threshold to equal to the set threshold.
3. The power converter according to claim 1, wherein The controller is configured to first control the relay connected to the first inverter bridge arm to close, and then control the relay connected to the second inverter bridge arm to close after a set waiting duration after the absolute value of the instantaneous value of the differential-mode voltage signal increases from less than the set threshold to greater than or equal to the set threshold, and the set waiting duration is less than or equal to the duration of the set time period.
4. The power converter according to claim 3, characterized in that, The differential-mode voltage signal is an alternating current signal with periodic changes. The controller is configured to control the relay connected to the second inverter bridge arm to close once after a set waiting duration after the absolute value of the instantaneous value of the differential-mode voltage signal increases from less than the set threshold to greater than or equal to the set threshold each time; Wherein, the set waiting duration increases after each time the controller controls the relay connected to the second inverter bridge arm to close, and the set waiting duration is less than or equal to the duration of the set time period.
5. The power converter according to any one of claims 1-4, characterized in that, After the controller controls the relay connected to the second inverter bridge arm to close, if the contact impedance of the relay connected to the second inverter bridge arm is greater than a set impedance threshold, the controller is configured to control the relay connected to the second inverter bridge arm to open, and then control the relay connected to the second inverter bridge arm to close again within a set time period during the process of inputting the differential-mode voltage signal to the first inverter bridge arm and the second inverter bridge arm.
6. The power converter according to claim 5, characterized in that, The controller is configured to control the amplitude of the differential-mode voltage signal to increase as the number of times the relay connected to the second inverter bridge arm closes increases.
7. The power converter according to any one of claims 1-6, characterized in that, The controller is configured to control the amplitude of the differential-mode voltage signal to increase as the operating temperature of the power converter during the previous grid connection increases.
8. The power converter according to claim 1, characterized in that, The differential-mode voltage signal is a square wave.
9. The power converter according to claim 1, wherein The differential-mode voltage signal is a trapezoidal wave.