A grid-connected system and synchronization method

Abstract

The present application discloses a grid-connected system and a synchronization method, wherein the grid-connected system comprises: a controller, a main switch, a transformer and a current transformer; the output end of the current transformer is connected to the low-voltage side of the transformer, and the high-voltage side of the transformer is used to connect to the power grid through the main switch; before the current transformer is connected to the grid, the main switch is disconnected; the controller is used to control the current transformer to start off-grid, and according to the phase comparison result of the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer, the voltage frequency of the current transformer started off-grid is closed-loop controlled to make the voltage phase of the high-voltage side of the transformer consistent with the voltage phase of the power grid. The phase with a faster change is converted into a frequency with a slower change speed, and the controller controls the frequency of the output voltage of the current transformer to gradually approach the frequency of the power grid, thereby realizing the control of the phase, thereby realizing the rapid synchronization of the phases on both sides of the main switch.

Description

Technical Field

[0001] The present application relates to the field of grid connection technology, and in particular to a grid connection system and a synchronization method. Background Art

[0002] The grid-connected system includes a converter and a step-up transformer. For example, the grid-connected system can be applied to large-scale photovoltaic power plants or distributed grid-connected power generation systems. The grid-connected system inverts the energy on the DC side and then feeds the energy into the medium and high voltage grid through a step-up transformer.

[0003] When the grid-connected system is a photovoltaic power generation grid-connected system, the grid-connected system will be in standby mode at night or on rainy days when the lighting conditions are very poor, and the step-up transformer will always be connected to the medium and high voltage power grid. In this way, when the grid-connected system is in standby mode, the step-up transformer will generate no-load losses. To solve the above problem, a main switch is set between the high-voltage side of the transformer and the power grid, and the main switch is disconnected at night, thereby reducing the no-load losses of the step-up transformer.

[0004] The difference between the voltage synchronization during the conventional inverter grid-connected process is that since the main switch is disconnected, the inverter cannot sample the grid voltage in real time, and the controller that obtains the grid voltage cannot directly control the output voltage of the inverter. Therefore, the conventional control method cannot achieve the synchronization of the inverter and the grid. Summary of the invention

[0005] In view of this, the present application provides a grid-connected system and a synchronization method, which can quickly synchronize the voltages on both sides of a main switch.

[0006] The present application provides a grid-connected system, comprising: a controller, a main switch, a transformer and a converter;

[0007] The output end of the converter is connected to the low-voltage side of the transformer, and the high-voltage side of the transformer is used to connect to the grid through the main switch; before the converter is connected to the grid, the main switch is disconnected;

[0008] The controller is used to control the converter to start off the grid. According to the phase comparison result of the voltage phase of the grid and the voltage phase of the high-voltage side of the transformer, the voltage frequency of the converter for off-grid starting is controlled in a closed loop so that the voltage phase of the high-voltage side of the transformer is consistent with the voltage phase of the grid.

[0009] In one possible implementation, the controller is also used to close-loop control the voltage amplitude of the off-grid started converter based on an amplitude comparison result of the grid voltage amplitude and the voltage amplitude on the high-voltage side of the transformer, so that the voltage amplitude on the high-voltage side of the transformer is consistent with the voltage amplitude of the grid.

[0010] In a possible implementation, the grid-connected system includes N transformers and M converters, where M and N are both integers, and M is greater than or equal to N; and one transformer is connected to one or more converters.

[0011] In a possible implementation, the controller controls the voltage amplitude of the off-grid started converter in a closed loop according to the comparison result of the voltage amplitude of the power grid and the voltage amplitude of the high voltage side of the transformer, specifically for:

[0012] The difference between the voltage amplitude of the power grid and the voltage amplitude of the high-voltage side of the transformer is passed through a regulator to obtain an amplitude comparison result, the amplitude comparison result is limited to obtain a limited voltage amplitude command, and the voltage amplitude command is sent to the off-grid started converter to control the voltage amplitude of the off-grid started converter.

[0013] In a possible implementation, the controller, based on a phase comparison result of the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer, controls the voltage frequency of the off-grid started converter in a closed loop, specifically for:

[0014] The difference between the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer is passed through a regulator to obtain a frequency adjustment amount, the frequency adjustment amount is limited to obtain a limited frequency adjustment amount, the limited frequency adjustment amount is superimposed on a preset frequency to obtain a frequency command, and the frequency command is sent to an off-grid started converter to control the phase of the off-grid started converter.

[0015] In a possible implementation manner, the preset frequency is the rated frequency of the power grid or the voltage frequency of the power grid detected in real time.

[0016] According to the phase comparison result of the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer; the voltage frequency of the off-grid started converter is closed-loop controlled, and the controller is specifically used for:

[0017] The difference between the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer is passed through a regulator to obtain a frequency adjustment amount, the frequency adjustment amount is limited to obtain a limited frequency adjustment amount, the limited frequency adjustment amount is used as a frequency command, and the frequency command is sent to the off-grid started converter to control the phase of the off-grid started converter.

[0018] In a possible implementation, the controller is specifically configured to change the frequency of a modulation wave of an off-grid started converter according to a frequency command to change the phase of the voltage on the high voltage side of the transformer.

[0019] In a possible implementation, the method further includes: a first voltage transformer and a second voltage transformer connected to the controller;

[0020] A first voltage transformer, used for detecting the voltage of the power grid;

[0021] A second voltage transformer, used for detecting the voltage on the high voltage side of the transformer;

[0022] The controller is specifically used to obtain the voltage amplitude and voltage phase of the power grid according to the voltage of the power grid, and to obtain the voltage amplitude and voltage phase of the high-voltage side of the transformer according to the voltage of the high-voltage side of the transformer.

[0023] In a possible implementation, the controller is also used to make the voltage amplitude on the high-voltage side of the transformer consistent with the voltage amplitude of the power grid, and the voltage phase on the high-voltage side of the transformer consistent with the voltage phase of the power grid, and control the main switch to close.

[0024] In a possible implementation, the DC side of the converter is used to connect at least one of a photovoltaic module or an energy storage battery.

[0025] The present application also provides a synchronization method of a grid-connected system, the grid-connected system comprising: a main switch, a transformer and a converter; the output end of the converter is connected to the low-voltage side of the transformer, and the high-voltage side of the transformer is used to connect to the grid through the main switch; before the converter is connected to the grid, the main switch is disconnected;

[0026] The method includes:

[0027] Control the converter to start off-grid;

[0028] According to the phase comparison result between the voltage phase of the grid and the voltage phase of the high-voltage side of the transformer, the voltage frequency of the off-grid started converter is controlled in a closed loop to make the voltage phase of the high-voltage side of the transformer consistent with the voltage phase of the grid.

[0029] In a possible implementation, the method further includes: according to a comparison result of the voltage amplitude of the power grid and the voltage amplitude of the high-voltage side of the transformer, closed-loop controlling the voltage amplitude of the off-grid started converter so that the voltage amplitude of the high-voltage side of the transformer is consistent with the voltage amplitude of the power grid;

[0030] In a possible implementation, the grid-connected system includes N transformers and M converters, where M and N are both integers, and M is greater than or equal to N; and one transformer is connected to one or more converters.

[0031] In a possible implementation, based on a comparison result of the voltage amplitude of the power grid and the voltage amplitude of the high-voltage side of the transformer, the voltage amplitude of the off-grid starting converter is controlled in a closed loop, including:

[0032] The difference between the voltage amplitude of the power grid and the voltage amplitude of the high-voltage side of the transformer is passed through a regulator to obtain an amplitude comparison result, the amplitude comparison result is limited to obtain a voltage amplitude command after limiting, and the voltage amplitude command is sent to the off-grid started converter to control the voltage amplitude of the off-grid started converter;

[0033] In a possible implementation, based on a phase comparison result of the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer, the voltage frequency of the off-grid started converter is controlled in a closed loop, specifically for:

[0034] The difference between the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer is passed through a regulator to obtain a frequency adjustment amount, the frequency adjustment amount is limited to obtain a limited frequency adjustment amount, the limited frequency adjustment amount is superimposed on a preset frequency to obtain a frequency command, and the frequency command is sent to an off-grid started converter to control the phase of the off-grid started converter.

[0035] In a possible implementation manner, the preset frequency is the rated frequency of the power grid or the voltage frequency of the power grid detected in real time.

[0036] According to the phase comparison result of the voltage phase of the power grid and the voltage phase of the high voltage side of the transformer; the voltage frequency of the off-grid starting converter is controlled in a closed loop, specifically including:

[0037] The difference between the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer is passed through a regulator to obtain a frequency adjustment amount, the frequency adjustment amount is limited to obtain a limited frequency adjustment amount, the limited frequency adjustment amount is used as a frequency command, and the frequency command is sent to the off-grid started converter to control the phase of the off-grid started converter.

[0038] In a possible implementation, controlling the phase of an off-grid starting converter includes:

[0039] The frequency of the modulation wave of the off-grid started converter is changed according to the frequency command to change the phase of the voltage on the high voltage side of the transformer.

[0040] In a possible implementation, it also includes: the voltage amplitude on the high-voltage side of the transformer is consistent with the voltage amplitude of the power grid, and the voltage phase on the high-voltage side of the transformer is consistent with the voltage phase of the power grid, and the main switch is controlled to close.

[0041] It can be seen that this application has the following beneficial effects:

[0042] The grid-connected system provided in the embodiment of the present application does not require an additional synchronization device, and the output voltage of the converter can be synchronized with the grid voltage through the closed-loop cooperative control of the controller and the converter. For the conversion of phase control to frequency control, since the frequency change can make the phase change quickly, because the slower changing frequency is used to affect the faster changing phase, the voltage phase of the converter can achieve the real-time tracking effect of the phase of the grid voltage, such as catching up with the phase of the grid voltage. For example, if the phase of the converter voltage lags behind the phase of the grid voltage, the frequency of the converter voltage is increased to catch up with the phase of the grid voltage. Anyway, when the phase of the converter voltage leads the phase of the grid voltage, the frequency of the converter output voltage is reduced to wait for the phase of the grid voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 A schematic diagram of a grid-connected system provided in an embodiment of the present application;

[0044] Figure 2 A schematic diagram of another grid-connected system provided in an embodiment of the present application;

[0045] Figure 3 A schematic diagram of a converter voltage lagging behind a grid voltage provided in an embodiment of the present application;

[0046] Figure 4 A schematic diagram of a converter voltage leading a grid voltage provided in an embodiment of the present application;

[0047] Figure 5 A closed-loop control principle diagram of a voltage amplitude provided in an embodiment of the present application;

[0048] Figure 6 A closed-loop control principle diagram of a voltage phase provided in an embodiment of the present application;

[0049] Figure 7 Another closed-loop control principle diagram of voltage phase provided in an embodiment of the present application;

[0050] Figure 8 A flowchart of a synchronization method for a grid-connected system provided in an embodiment of the present application. DETAILED DESCRIPTION

[0051] In order for those skilled in the art to better understand and implement the technical solutions provided in the embodiments of the present application, the architecture of the grid-connected system is first introduced below with reference to the accompanying drawings.

[0052] See also Figure 1 , which is a schematic diagram of a grid-connected system provided in an embodiment of the present application.

[0053] The grid-connected system includes M converters and N transformers, where M is an integer greater than or equal to 1 and greater than N, that is, there can be multiple converters or one converter, and generally there are multiple converters. N is an integer greater than or equal to 1. One transformer can be connected to one converter, and one transformer can also be connected to multiple converters. For example, one transformer can be connected to one or more centralized inverters, and one transformer can also be connected to multiple string inverters.

[0054] The converter provided in the embodiment of the present application may be a bidirectional converter, which can realize both direct current to alternating current conversion and alternating current to direct current conversion. The embodiment of the present application does not specifically limit the type of direct current power source connected to the direct current side of the converter, for example, it may be a photovoltaic module, it may also be an energy storage battery, etc., or it may include both a photovoltaic module and an energy storage battery. The converter may be at least one of a photovoltaic inverter or an energy storage converter, for example, the direct current side of the photovoltaic inverter is connected to a photovoltaic module, and the direct current side of the energy storage converter is connected to an energy storage battery. It should be understood that when there are multiple converters, some converters may be photovoltaic inverters connected to photovoltaic modules, and some converters may be energy storage converters connected to energy storage batteries, that is, the grid-connected system is a photovoltaic storage grid-connected system. In addition, the grid-connected system may also be a photovoltaic system, or the grid-connected system may be an energy storage system, which is not limited in the embodiment of the present application, and the above various systems may use the technical solutions provided in the embodiment of the present application.

[0055] In the following embodiments, for the convenience of description and understanding, the number of converters and the number of transformers are equal to each other for illustration. In the embodiments of the present application, N=M is ​​used as an example for illustration.

[0056] Figure 1 Taking the converter as DC / AC as an example, the N converters are DC / AC1, DC / AC2, and DC / ACN. The N transformers are T1, T2, and TN. It can be seen that the AC side of DC / AC1 is connected to the low-voltage side of T1, the AC side of DC / AC2 is connected to the low-voltage side of T2, and the AC side of DC / ACN is connected to the low-voltage side of TN.

[0057] Each converter is connected to the grid through a corresponding transformer. Since the output voltage of the converter is relatively low, in order to match the grid voltage, the transformer is generally a step-up transformer. The embodiment of the present application does not specifically limit the voltage level of the grid, for example, it can range from several kilovolts to tens of kilovolts.

[0058] For example, when the grid-connected system is a photovoltaic power generation grid-connected system, the grid-connected system will be in standby mode at night or on rainy days when the lighting conditions are very poor, and the step-up transformer will always be connected to the medium and high voltage power grid. In this way, when the grid-connected system is in standby mode, the step-up transformer will generate no-load losses. To solve the above problem, a main switch K can be set between the high-voltage side of the transformer and the power grid, and the main switch K can be disconnected at night to reduce the no-load losses of the step-up transformer. When the photovoltaic panel has energy output, the voltage is established through the converter slow start, the voltage on both sides of the main switch K is synchronized, and the zero-impact closing function is realized.

[0059] The conventional way of grid synchronization during the inverter grid connection process is that the inverter samples the grid voltage in real time, calculates information such as the grid phase and amplitude, and the inverter outputs an inverter voltage synchronized with the grid based on the real-time information of the grid voltage.

[0060] Unlike the grid synchronization during the conventional inverter grid-connected process, in this grid-connected system, the distance between the inverter and the controller is generally relatively far, and the communication delay is relatively long, for example, in the microsecond level, it can reach hundreds of milliseconds, and the communication delay process may have passed several sine wave cycles. Therefore, the communication response is too slow, and the inverter cannot quickly synchronize according to the grid voltage detected by the controller, which is equivalent to the inverter being unable to know the amplitude and phase of the grid voltage, that is, the inverter cannot sample the grid voltage in real time (us level), and the controller that can sample the grid voltage in real time cannot directly control the output voltage of the inverter. Therefore, conventional control methods cannot achieve medium and high voltage grid synchronization.

[0061] Some traditional synchronous technologies require additional devices to perform soft start or flexible switching, but both increase system costs and complicate the hardware architecture.

[0062] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the embodiments of the present application are further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0063] See also Figure 2 , which is a schematic diagram of another grid-connected system provided in an embodiment of the present application.

[0064] The grid-connected system provided in the embodiment of the present application includes: a controller 100, a main switch K, N transformers and M converters, where M and N are both integers, and M is greater than or equal to N; one transformer is connected to one or more converters; in the embodiment of the present application, M=N is used as an example for introduction. The N transformers are T1, T2 to TN, and the N converters are 10, 20 to N0.

[0065] The output end of each converter is connected to the low-voltage side of the corresponding transformer, and the high-voltage side of each transformer is used to connect to the grid through the main switch; that is, 10 is connected to the low-voltage side of T1, 20 is connected to the low-voltage side of T2, and N0 is connected to the low-voltage side of TN. T1, T2 and the high-voltage side of TN are connected in parallel to the first end of the main switch K, and the first end of the main switch K is connected to the grid. It should be understood that when the grid-connected system is a three-phase system, the main switch K also includes a three-phase switch, for example Figure 2 Main switch K shown.

[0066] For example, the grid-connected system provided in the embodiment of the present application further includes: a first voltage transformer PT1 and a second voltage transformer PT2 connected to the controller 100 .

[0067] The first voltage transformer PT1 is used to detect the voltage of the power grid. At this time, since K is disconnected, PT1 can only detect the voltage of the power grid.

[0068] The second voltage transformer PT2 is used to detect the inverter voltage on the high-voltage side of the transformer; at this time, since K is disconnected, PT2 can only detect the voltage on the high-voltage side of the transformer. Since the high-voltage sides of all transformers are connected in parallel, the voltages on the high-voltage sides of each transformer are equal.

[0069] The grid-connected system provided in the embodiment of the present application includes: a controller, a main switch, N transformers and N converters, where N is an integer greater than or equal to 1; the N transformers and the N converters correspond one to one;

[0070] The output end of each converter is connected to the low-voltage side of the corresponding transformer, and the high-voltage side of each transformer is used to connect to the grid through a main switch; before the converter is connected to the grid, the main switch is disconnected;

[0071] The controller 100 is used to control at least one converter to start off the grid, and according to the phase comparison result of the voltage phase of the grid and the voltage phase of the high-voltage side of the transformer, the voltage frequency of the converter started off the grid is controlled in a closed loop, so that the voltage phase of the high-voltage side of the transformer is consistent with the voltage phase of the grid. Since the phase has 360 degrees in a cycle of a grid voltage, the phase changes very quickly and it is difficult to track in real time. Therefore, the grid-connected system provided in the embodiment of the present application converts the control of the phase into the control of the frequency. The frequency changes relatively slowly, for example, from 50Hz to 55Hz, and the change value will not span too large. The output voltage of the converter is synchronized with the phase of the grid voltage by controlling the frequency that changes slowly.

[0072] The main switch K is always in the disconnected state. At this time, there can be one or more converters for off-grid startup. The converter for off-grid startup is mainly used to establish the voltage for off-grid startup. The main switch K is closed only when the voltages on both sides of the main switch K are consistent. The voltage consistency includes amplitude, frequency and phase consistency, that is, the voltage on the high-voltage side of the transformer is synchronized with the grid voltage before the main switch K is closed to avoid the voltages on both ends of the main switch K being different, which may cause impact on the main switch K when it is closed.

[0073] The controller 100 can communicate with each converter.

[0074] The controller provided in the embodiment of the present application is also used to close-loop control the voltage amplitude of the off-grid started converter based on the amplitude comparison result of the voltage amplitude of the power grid and the voltage amplitude on the high-voltage side of the transformer, so that the voltage amplitude on the high-voltage side of the transformer is consistent with the voltage amplitude of the power grid.

[0075] The grid-connected system provided in the embodiment of the present application does not require an additional synchronization device, and the output voltage of the converter can be synchronized with the grid voltage through the closed-loop cooperative control of the controller and the converter. The voltage is directly closed-loop controlled, and the phase control is converted into frequency control to achieve a real-time tracking effect of slow catching up, that is, if the phase of the converter voltage lags behind the phase of the grid voltage, the frequency of the converter voltage is increased to catch up with the phase of the grid voltage. Anyway, when the phase of the converter voltage leads the phase of the grid voltage, the frequency of the converter output voltage is reduced to wait for the phase of the grid voltage.

[0076] The grid-connected system provided by the embodiment of the present application is not limited by the influence caused by the communication delay between the controller and each converter, although the converter cannot sample the phase of the power grid in real time at the us level because the phase changes very quickly. However, the embodiment of the present application converts the faster changing phase into a slower changing frequency, and the controller controls the frequency of the output voltage of the converter to gradually approach the frequency of the power grid, thereby achieving phase control.

[0077] In order to more intuitively understand the technical solution provided by the embodiments of the present application, please refer to Figure 3 The schematic diagram of the converter voltage lagging behind the grid voltage is shown, and Figure 4 The schematic diagram shown is that the voltage of the converter leads the grid voltage.

[0078] from Figure 3It can be seen that the output voltage Vout of the converter and the grid voltage Vg are both sinusoidal waves. The phase of the output voltage Vout of the converter lags behind the phase of the grid voltage Vg. Therefore, it is necessary to increase the frequency of the output voltage of the converter, that is, the frequency of the output voltage of the converter is higher and the phase changes faster, so that the phase of the output voltage of the converter can catch up with the phase of the grid voltage Vg, so that the phase of the output voltage of the converter is consistent with the phase of the grid voltage Vg.

[0079] from Figure 4 It can be seen that the phase of the output voltage Vout of the converter leads the phase of the grid voltage Vg. Therefore, it is necessary to reduce the frequency of the output voltage of the converter, that is, the frequency of the output voltage of the converter is lower and the phase change speed is reduced, so that the phase of the output voltage of the converter can wait for the phase of the grid voltage Vg, thereby making the phase of the output voltage of the converter consistent with the phase of the grid voltage Vg.

[0080] The following describes the synchronous control of voltage amplitude in an embodiment of the present application in conjunction with the closed-loop control principle diagram.

[0081] See also Figure 5 , which is a closed-loop control principle diagram of a voltage amplitude provided in an embodiment of the present application.

[0082] The grid-connected system provided in the embodiment of the present application uses the voltage amplitude Uref of the grid as a reference value for closed-loop control of the voltage amplitude. The controller, based on the comparison result of the voltage amplitude Uref of the grid and the voltage amplitude Uinv on the high-voltage side of the transformer, closes the loop to control the voltage amplitude of the off-grid started converter, specifically for:

[0083] After the difference between the voltage amplitude Uref of the power grid and the voltage amplitude Uinv of the high-voltage side of the transformer passes through a closed-loop regulator, the voltage amplitude adjustment amount is output. For example, the regulator can output the voltage amplitude adjustment amount through a proportional-integral algorithm or a proportional algorithm, and the amplitude comparison result Ure is limited to obtain the voltage amplitude command Ucmd after limiting, and the voltage amplitude command Ucmd is sent to the off-grid-started converter DC / AC to control the voltage amplitude of the off-grid-started converter DC / AC.

[0084] It can be seen from the closed-loop control introduced above that the closed-loop control of the voltage amplitude provided in the embodiment of the present application can make the voltage amplitude of the power grid gradually approach the amplitude of Uref and the voltage amplitude Uinv on the high-voltage side of the transformer, and finally reach consistency, that is, when the difference between the two is within the allowable range, the two are considered to be consistent.

[0085] See also Figure 6 , which is a closed-loop control principle diagram of a voltage phase provided in an embodiment of the present application.

[0086] The grid-connected system provided in the embodiment of the present application uses the voltage phase θref of the grid as a reference value for closed-loop control of the voltage phase. The controller, based on the phase comparison result of the voltage phase θref of the grid and the voltage phase θinv of the high-voltage side of the transformer, controls the voltage frequency of the off-grid started converter in a closed loop, specifically for:

[0087] The difference between the voltage phase θref of the power grid and the voltage phase θinv on the high-voltage side of the transformer is passed through a closed-loop regulator to obtain the frequency amplitude adjustment amount θc. For example, the regulator can output the frequency amplitude adjustment amount Δω through a proportional-integral algorithm or a proportional algorithm, and the frequency amplitude adjustment amount Δω is limited to obtain the limited frequency Δω1. The limited frequency Δω1 is superimposed with the preset frequency ω0 to obtain the frequency command ωref, and the frequency command ωref is sent to the off-grid started converter DC / AC to control the phase of the off-grid started converter DC / AC.

[0088] The embodiments of the present application do not specifically limit the specific value of the preset frequency. For example, the preset frequency may be the rated frequency of the power grid, such as 50 Hz or 60 Hz. In addition, the preset frequency may also be the voltage frequency of the power grid detected in real time, such as the actually detected voltage frequency of 52 Hz or 48 Hz. The above values ​​are only examples and can be set according to actual conditions.

[0089] In addition, the embodiment of the present application also provides a closed-loop control principle of voltage phase, see Figure 7 .

[0090] Figure 7 and Figure 6 The difference is Figure 7 The frequency feedforward control is not included in the method. Instead, the frequency amplitude adjustment amount Δω output by the regulator is directly limited to obtain the limited frequency, and the limited frequency is used as the obtained frequency command ωref. The frequency command ωref is sent to the off-grid started converter DC / AC to control the phase of the off-grid started converter DC / AC. This control will be simpler and does not require frequency feedforward control.

[0091] The grid-connected system provided in the embodiment of the present application does not specifically limit the manner in which the converter adjusts the frequency of its output voltage. It should be understood that the converter adjusts the output voltage mainly by adjusting the pulse width modulation (PWM) signal that drives the switch tube. The PWM signal is generally generated by a carrier and a modulation wave. A possible implementation method is a controller that is specifically used to change the frequency of the modulation wave of the off-grid started converter according to a frequency command, thereby changing the PWM signal. Different PWM signals result in different actions of the switch tube, thereby changing the frequency of the converter's output voltage. The phase of the output voltage is changed by the frequency of the output voltage, thereby changing the phase of the voltage on the high-voltage side of the transformer, so that the phase of the voltage on the high-voltage side of the transformer is consistent with the phase of the voltage of the power grid.

[0092] In the embodiments of the present application, closed-loop control of voltage amplitude and closed-loop control of voltage phase both involve limiting. The purpose of limiting is to prevent the controlled object from deviating and to ensure that the voltage amplitude and phase are within a safe and controllable range.

[0093] The above embodiments introduce the control of voltage amplitude and voltage phase. The controller provided in the embodiments of the present application is also used to make the voltage amplitude on the high-voltage side of the transformer consistent with the voltage amplitude of the power grid, and the voltage phase on the high-voltage side of the transformer consistent with the voltage phase of the power grid, to control the closing of the main switch, that is, the main switch can only be closed when the voltage amplitude and phase at both ends of the main switch are consistent, so as to avoid different voltages at both ends, which may cause a large impact on the main switch.

[0094] Based on a grid-connected system provided in the above embodiment, an embodiment of the present application further provides a synchronization method for a grid-connected system, which is described in detail below with reference to the accompanying drawings.

[0095] See also Figure 7 , which is a flow chart of a synchronization method for a grid-connected system provided in an embodiment of the present application.

[0096] The embodiment of the present application provides a synchronization method for a grid-connected system, wherein the grid-connected system comprises: a main switch, a transformer and a converter; the output end of the converter is connected to the low-voltage side of the transformer, and the high-voltage side of the transformer is used to connect to the grid through the main switch; before the converter is connected to the grid, the main switch is disconnected;

[0097] The method includes:

[0098] S701: Control the converter to start up off the grid; it should be understood that at least one converter among the multiple converters may be controlled to start up off the grid.

[0099] S702: Based on the phase comparison result between the voltage phase of the grid and the voltage phase on the high voltage side of the transformer, close-loop control the voltage frequency of the off-grid started converter to make the voltage phase on the high voltage side of the transformer consistent with the voltage phase of the grid.

[0100] The synchronization method of the grid-connected system provided in the embodiment of the present application does not require additional synchronization devices, and the output voltage of the converter can be synchronized with the grid voltage through the closed-loop cooperative control of the controller and the converter. For the conversion of phase control to frequency control, since the frequency change can make the phase change quickly, because the slower changing frequency is used to affect the faster changing phase, the voltage phase of the converter can achieve the real-time tracking effect of the phase of the grid voltage, such as slowing down and speeding up. For example, if the phase of the converter voltage lags behind the phase of the grid voltage, the frequency of the converter voltage is increased to catch up with the phase of the grid voltage. Anyway, when the phase of the converter voltage leads the phase of the grid voltage, the frequency of the converter output voltage is reduced to wait for the phase of the grid voltage.

[0101] In another implementation method provided in the embodiment of the present application, the embodiment of the present application may also include S703: based on the amplitude comparison result of the voltage amplitude of the power grid and the voltage amplitude of the high-voltage side of the transformer, closed-loop control of the voltage amplitude of the off-grid started converter makes the voltage amplitude of the high-voltage side of the transformer consistent with the voltage amplitude of the power grid.

[0102] The embodiment of the present application does not specifically limit the order of S702 and S703.

[0103] According to the amplitude comparison result of the voltage amplitude of the power grid and the voltage amplitude of the high-voltage side of the transformer, the voltage amplitude of the off-grid started converter is controlled in a closed loop, including: obtaining an amplitude comparison result by subjecting the difference between the voltage amplitude of the power grid and the voltage amplitude of the high-voltage side of the transformer to a proportional integral algorithm or a proportional algorithm, limiting the amplitude comparison result to obtain a voltage amplitude command after limiting, sending the voltage amplitude command to the off-grid started converter, and controlling the voltage amplitude of the off-grid started converter;

[0104] According to the phase comparison result of the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer; the voltage frequency of the off-grid started converter is controlled in a closed loop, including: obtaining a phase comparison result by subjecting the difference between the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer to a proportional-integral algorithm or a proportional algorithm, limiting the phase comparison result to obtain a phase after limiting, superimposing the phase after limiting with a preset frequency to obtain a frequency command, sending the frequency command to the off-grid started converter, and controlling the phase of the off-grid started converter.

[0105] The preset frequency is the rated frequency of the power grid or the voltage frequency of the power grid detected in real time.

[0106] Controlling the phase of the off-grid started converter includes: changing the frequency of the modulation wave of the off-grid started converter according to a frequency command to change the phase of the voltage on the high voltage side of the transformer.

[0107] The synchronization method of the grid-connected system provided in the embodiment of the present application also includes: the voltage amplitude on the high-voltage side of the transformer is consistent with the voltage amplitude of the power grid, and the voltage phase on the high-voltage side of the transformer is consistent with the voltage phase of the power grid, and the main switch is controlled to close.

[0108] The grid-connected system provided by the embodiment of the present application is not limited by the influence caused by the communication delay between the controller and each converter, although the converter cannot sample the phase of the power grid in real time at the us level because the phase changes very quickly. However, the embodiment of the present application converts the faster changing phase into a slower changing frequency, and the controller controls the frequency of the output voltage of the converter to gradually approach the frequency of the power grid, thereby achieving phase control.

[0109] It should be noted that the various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments, and the same or similar parts between the various embodiments can be referred to each other. For the system or device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part description.

[0110] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A grid-connected system, characterized in that: include: Controllers, main switches, transformers and converters; The output end of the converter is connected to the low voltage side of the transformer, and the high voltage side of the transformer is used to connect to the power grid through the main switch; Before the converter is connected to the grid, the main switch is disconnected; The controller is used to control the converter to start off-grid, obtain a frequency command according to a phase comparison result of the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer, and close-loop control the voltage frequency of the converter for off-grid start-up based on the frequency command, so that the voltage phase of the high-voltage side of the transformer is consistent with the voltage phase of the power grid; The controller is also used to close-loop control the voltage amplitude of the off-grid started converter based on the amplitude comparison result of the voltage amplitude of the power grid and the voltage amplitude of the high-voltage side of the transformer, so that the voltage amplitude of the high-voltage side of the transformer is consistent with the voltage amplitude of the power grid.

2. The grid-connected system according to claim 1, characterized in that: The grid-connected system includes N transformers and M converters, wherein both M and N are integers, and M is greater than or equal to N; one transformer is connected to one or more converters.

3. The grid-connected system according to claim 1, characterized in that: The controller, according to the comparison result of the voltage amplitude of the power grid and the voltage amplitude of the high-voltage side of the transformer, controls the voltage amplitude of the off-grid started converter in a closed loop, specifically for: The difference between the voltage amplitude of the power grid and the voltage amplitude of the high-voltage side of the transformer is passed through a regulator to obtain an amplitude comparison result, the amplitude comparison result is limited to obtain a limited voltage amplitude command, and the voltage amplitude command is sent to the off-grid started converter to control the voltage amplitude of the off-grid started converter.

4. The grid-connected system according to claim 1, characterized in that: The controller obtains a frequency command according to a phase comparison result of the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer, and controls the voltage frequency of the off-grid started converter in a closed loop based on the frequency command, specifically for: The difference between the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer is passed through a regulator to obtain a frequency adjustment amount, the frequency adjustment amount is limited to obtain a limited frequency adjustment amount, the limited frequency adjustment amount is superimposed on a preset frequency to obtain a frequency command, and the frequency command is sent to an off-grid started converter to control the phase of the off-grid started converter.

5. The grid-connected system according to claim 4, characterized in that: The preset frequency is the rated frequency of the power grid or the voltage frequency of the power grid detected in real time.

6. The grid-connected system according to claim 1, characterized in that: According to the phase comparison result of the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer, a frequency command is obtained, and the voltage frequency of the off-grid started converter is closed-loop controlled based on the frequency command, which is specifically used for: The difference between the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer is passed through a regulator to obtain a frequency adjustment amount, the frequency adjustment amount is limited to obtain a limited frequency adjustment amount, the limited frequency adjustment amount is used as a frequency command, and the frequency command is sent to the off-grid started converter to control the phase of the off-grid started converter.

7. The grid-connected system according to any one of claims 4 to 6, characterized in that: The controller is specifically used to change the frequency of the modulation wave of the off-grid started converter according to the frequency command to change the phase of the voltage on the high voltage side of the transformer.

8. The grid-connected system according to any one of claims 4 to 6, characterized in that: Also includes: A first voltage transformer and a second voltage transformer connected to the controller; The first voltage transformer is used to detect the voltage of the power grid; The second voltage transformer is used to detect the voltage on the high voltage side of the transformer; The controller is specifically used to obtain the voltage amplitude and voltage phase of the power grid according to the voltage of the power grid, and to obtain the voltage amplitude and voltage phase of the high-voltage side of the transformer according to the voltage of the high-voltage side of the transformer.

9. The grid-connected system according to any one of claims 1 to 6, characterized in that: The controller is also used to make the voltage amplitude on the high-voltage side of the transformer consistent with the voltage amplitude of the power grid, and the voltage phase on the high-voltage side of the transformer consistent with the voltage phase of the power grid, and control the main switch to close.

10. The grid-connected system according to claim 8, characterized in that: The DC side of the converter is used to connect at least one of a photovoltaic module or an energy storage battery.

11. A synchronization method for a grid-connected system, characterized in that: The grid-connected system comprises: a main switch, a transformer and a converter; the output end of the converter is connected to the low-voltage side of the transformer, and the high-voltage side of the transformer is used to connect to the grid through the main switch; before the converter is connected to the grid, the main switch is disconnected; The method includes: Controlling the converter to start off-grid; Obtaining a frequency command according to a phase comparison result of the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer, and controlling the voltage frequency of the off-grid started converter in a closed loop based on the frequency command so that the voltage phase of the high-voltage side of the transformer is consistent with the voltage phase of the power grid; According to the comparison result of the voltage amplitude of the power grid and the voltage amplitude of the high-voltage side of the transformer, the voltage amplitude of the off-grid started converter is controlled in a closed loop so that the voltage amplitude of the high-voltage side of the transformer is consistent with the voltage amplitude of the power grid.

12. The method according to claim 11, characterized in that The grid-connected system includes N transformers and M converters, wherein both M and N are integers, and M is greater than or equal to N; one transformer is connected to one or more converters.

13. The method according to claim 11, characterized in that The closed-loop control of the voltage amplitude of the off-grid started converter according to the comparison result of the voltage amplitude of the power grid and the voltage amplitude of the high-voltage side of the transformer comprises: The difference between the voltage amplitude of the power grid and the voltage amplitude of the high-voltage side of the transformer is passed through a regulator to obtain an amplitude comparison result, the amplitude comparison result is limited to obtain a limited voltage amplitude command, and the voltage amplitude command is sent to the off-grid started converter to control the voltage amplitude of the off-grid started converter.

14. The method according to claim 11, characterized in that The frequency command is obtained according to the phase comparison result of the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer, and the voltage frequency of the off-grid started converter is closed-loop controlled based on the frequency command, which is specifically used for: The difference between the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer is passed through a regulator to obtain a frequency adjustment amount, the frequency adjustment amount is limited to obtain a limited frequency adjustment amount, the limited frequency adjustment amount is superimposed on a preset frequency to obtain a frequency command, and the frequency command is sent to an off-grid started converter to control the phase of the off-grid started converter.

15. The method according to claim 14, characterized in that The preset frequency is the rated frequency of the power grid or the voltage frequency of the power grid detected in real time.

16. The method according to claim 11, characterized in that The method of obtaining a frequency command based on a phase comparison result of the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer, and controlling the voltage frequency of the off-grid started converter in a closed loop based on the frequency command, specifically includes: The difference between the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer is passed through a regulator to obtain a frequency adjustment amount, the frequency adjustment amount is limited to obtain a limited frequency adjustment amount, the limited frequency adjustment amount is used as a frequency command, and the frequency command is sent to the off-grid started converter to control the phase of the off-grid started converter.

17. The method according to any one of claims 14 to 16, characterized in that: The phase of the converter for controlling off-grid startup includes: The frequency of the modulation wave of the off-grid started converter is changed according to the frequency command to change the phase of the voltage on the high voltage side of the transformer.

18. The method according to any one of claims 14 to 16, characterized in that: Also includes: The voltage amplitude on the high-voltage side of the transformer is consistent with the voltage amplitude of the power grid, and the voltage phase on the high-voltage side of the transformer is consistent with the voltage phase of the power grid, and the main switch is controlled to close.

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