Grid-connected system and synchronization method

Abstract

The present application discloses a grid-connected system and a synchronization method. The grid-connected system comprises a controller, a master switch, a transformer and a converter; an output end of the converter is connected to a low-voltage side of the transformer, and a high-voltage side of the transformer is configured to be connected to a power grid by means of the master switch; before the converter is connected to the grid, the master switch is turned off; the controller is used for controlling the converter to undergo off-grid starting, and on the basis of a phase comparison result between a voltage phase of the power grid and a voltage phase of the high-voltage side of the transformer, performing closed-loop control on a voltage frequency of the converter having undergone off-grid starting, so that the voltage phase of the high-voltage side of the transformer is consistent with the voltage phase of the power grid. A phase changed rapidly is converted into a frequency changed slowly, and the controller controls the frequency of an output voltage of the converter to gradually approach the frequency of the power grid, thereby realizing phase control and rapid synchronization between the phases on both sides of the master switch.

Description

A grid-connected system and synchronization method

[0001] This application claims priority to the Chinese patent application with application number 202410089089.7 and application name “A Grid-Connected System and Synchronization Method” filed with the State Intellectual Property Office of China on January 22, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0002] 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

[0003] A grid-connected system consists of a converter and a step-up transformer. For example, it can be used in large-scale photovoltaic power plants or distributed grid-connected power generation systems. After inverting the DC power, the grid-connected system feeds the energy into the medium- and high-voltage grid via a step-up transformer.

[0004] When a photovoltaic power generation system is connected to the grid, it will be in standby mode at night or on rainy days when lighting conditions are poor, while the step-up transformer will remain connected to the medium- and high-voltage grid. This will cause no-load losses in the step-up transformer during standby. To address this issue, a main switch is installed between the high-voltage side of the transformer and the grid. This switch is disconnected at night, reducing the no-load losses in the step-up transformer.

[0005] The difference from voltage synchronization during conventional inverter grid connection 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, conventional control methods cannot achieve synchronization between the inverter and the grid.

[0006] Summary of the Invention

[0007] 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 the main switch.

[0008] The present application provides a grid-connected system, comprising: a controller, a main switch, a transformer and a converter;

[0009] The output 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;

[0010] 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 startup is closed-loop controlled to make the voltage phase of the high-voltage side of the transformer consistent with the voltage phase of the grid.

[0011] In one possible implementation, 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 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 grid.

[0012] 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.

[0013] In one possible implementation, the controller performs closed-loop control of the voltage amplitude of the off-grid started converter based on a comparison result between the voltage amplitude of the grid and the voltage amplitude of the high-voltage side of the transformer, specifically for:

[0014] The difference between the voltage amplitude of the grid and the voltage amplitude of the high-voltage side of the transformer is passed through the 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.

[0015] In one possible implementation, the controller, based on a phase comparison result between the voltage phase of the grid and the voltage phase of the high-voltage side of the transformer, controls the voltage and frequency of the off-grid started converter in a closed loop, specifically for:

[0016] 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 the preset frequency to obtain 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.

[0017] In a possible implementation, the preset frequency is the rated frequency of the power grid or the voltage frequency of the power grid detected in real time.

[0018] 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 startup converter is closed-loop controlled, and the controller is specifically used to:

[0019] 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.

[0020] 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, thereby changing the phase of the voltage on the high-voltage side of the transformer.

[0021] In a possible implementation, the method further includes: a first voltage transformer and a second voltage transformer connected to the controller;

[0022] A first voltage transformer, used for detecting the voltage of the power grid;

[0023] a second voltage transformer, used to detect the voltage on the high-voltage side of the transformer;

[0024] 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 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.

[0025] In one possible implementation, the controller is also used to ensure that the voltage amplitude on the high-voltage side of the transformer is consistent with the voltage amplitude of the grid, and the voltage phase on the high-voltage side of the transformer is consistent with the voltage phase of the grid, and to control the main switch to close.

[0026] In a possible implementation, the DC side of the converter is used to connect to at least one of a photovoltaic module or an energy storage battery.

[0027] The present application also provides a synchronization method for 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;

[0028] The method includes:

[0029] Control the converter to start up off-grid;

[0030] Based on 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 off-grid started converter is closed-loop controlled to make the voltage phase of the high-voltage side of the transformer consistent with the voltage phase of the grid.

[0031] In one possible implementation, the method further includes: based on a comparison result of the voltage amplitude of the 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 grid;

[0032] 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.

[0033] In one possible implementation, closed-loop controlling the voltage amplitude of the off-grid started converter based on a comparison result between the voltage amplitude of the grid and the voltage amplitude on the high-voltage side of the transformer includes:

[0034] The difference between the voltage amplitude of the 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;

[0035] In one possible implementation, based on a 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 startup converter is controlled in a closed loop, specifically for:

[0036] 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 the preset frequency to obtain 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.

[0037] In a possible implementation, the preset frequency is the rated frequency of the power grid or the voltage frequency of the power grid detected in real time.

[0038] 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 converter for off-grid startup is controlled in a closed loop, specifically including:

[0039] 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.

[0040] In one possible implementation, controlling the phase of a converter for off-grid startup includes:

[0041] The frequency of the modulation wave of the off-grid starting converter is changed according to the frequency command to change the phase of the voltage on the high voltage side of the transformer.

[0042] In a possible implementation, the method further includes: when the voltage amplitude on the high-voltage side of the transformer is consistent with the voltage amplitude of the grid, and the voltage phase on the high-voltage side of the transformer is consistent with the voltage phase of the grid, controlling the main switch to close.

[0043] It can be seen that this application has the following beneficial effects:

[0044] The grid-connected system provided by 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 closed-loop cooperative control of the controller and the converter. Regarding the conversion of phase control to frequency control, since frequency changes can cause the phase to change rapidly, because the slower-changing frequency is used to influence the faster-changing phase, the converter voltage phase can achieve real-time tracking effects such as slow-catch-up of the grid voltage phase by the converter voltage phase. For example, if the converter voltage phase lags behind the grid voltage phase, the converter voltage frequency is increased to catch up with the grid voltage phase. In any case, when the converter voltage phase leads the grid voltage phase, the converter output voltage frequency is reduced to wait for the grid voltage phase. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] FIG1 is a schematic diagram of a grid-connected system provided in an embodiment of the present application;

[0046] FIG2 is a schematic diagram of another grid-connected system provided in an embodiment of the present application;

[0047] FIG3 is a schematic diagram of a converter voltage lagging behind a grid voltage according to an embodiment of the present application;

[0048] FIG4 is a schematic diagram of a converter voltage leading a grid voltage according to an embodiment of the present application;

[0049] FIG5 is a schematic diagram of a closed-loop control principle of voltage amplitude provided in an embodiment of the present application;

[0050] FIG6 is a schematic diagram of a closed-loop control principle of a voltage phase according to an embodiment of the present application;

[0051] FIG7 is a schematic diagram of another closed-loop control principle of voltage phase provided in an embodiment of the present application;

[0052] FIG8 is a flowchart of a synchronization method for a grid-connected system provided in an embodiment of the present application. DETAILED DESCRIPTION

[0053] 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 will be first described below with reference to the accompanying drawings.

[0054] Refer to Figure 1, which is a schematic diagram of a grid-connected system provided in an embodiment of the present application.

[0055] A grid-connected system includes M converters and N transformers, where M is an integer greater than or equal to 1 and greater than N. This means there can be multiple converters or a single converter, but generally there are multiple converters. N is an integer greater than or equal to 1. A transformer can be connected to one converter, or multiple converters. For example, a transformer can be connected to one or more centralized inverters or multiple string inverters.

[0056] The converter provided in the embodiment of the present application can be a bidirectional converter, which can realize both DC to AC conversion and AC to DC conversion. The embodiment of the present application does not specifically limit the type of DC power supply connected to the DC side of the converter. For example, it can be a photovoltaic module, or it can be an energy storage battery, etc., or it can include both photovoltaic modules and energy storage batteries. The converter can be at least one of a photovoltaic inverter or an energy storage converter. For example, the DC side of the photovoltaic inverter is connected to the photovoltaic module, and the DC side of the energy storage converter is connected to the energy storage battery. It should be understood that when there are multiple converters, some converters can be photovoltaic inverters connected to photovoltaic modules, and some converters can 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 can also be a photovoltaic system, or the grid-connected system can also be an energy storage system. None of these are limited in the embodiment of the present application, and all of the above systems can use the technical solutions provided in the embodiment of the present application.

[0057] In the following embodiments, for the convenience of description and understanding, the number of converters and the number of transformers are equal, and N=M is ​​used as an example for illustration.

[0058] Figure 1 uses a DC / AC converter as an example. The N converters are DC / AC1, DC / AC2, and DC / ACN. The N transformers are T1, T2, and finally 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.

[0059] Each converter is connected to the grid via a corresponding transformer. Because the converter's output voltage is relatively low, to match the grid voltage, the transformer is typically a step-up transformer. The present embodiment does not specifically limit the grid voltage level; for example, it can range from several kilovolts to tens of kilovolts.

[0060] For example, when the grid-connected system is a photovoltaic power generation system, at night or on rainy days, when lighting conditions are poor, the grid-connected system will be in standby mode, while the step-up transformer will remain connected to the medium- and high-voltage grid. This will cause no-load losses in the step-up transformer during standby. To address this issue, a main switch K can be installed between the high-voltage side of the transformer and the grid. Main switch K can be disconnected at night, thereby reducing the no-load losses of the step-up transformer. When the photovoltaic panels are outputting energy, the converter slowly starts up to establish voltage, synchronizing the voltages on both sides of main switch K and achieving zero-impact closing.

[0061] During the conventional grid-connected converter process, the grid synchronization method is that the converter samples the grid voltage in real time, calculates information such as the grid phase and amplitude, and outputs an inverter voltage synchronized with the grid based on the real-time information of the grid voltage.

[0062] Unlike grid synchronization during conventional converter grid connection, in this grid-connected system, the distance between the converter and the controller is generally relatively far, and the communication delay is relatively long, for example, in the microsecond level, which can reach hundreds of milliseconds. The communication delay process may last for several sine wave cycles. Therefore, the communication response is too slow, and the converter cannot quickly synchronize according to the grid voltage detected by the controller. This is equivalent to the converter being unable to know the amplitude and phase of the grid voltage, that is, the converter cannot sample the grid voltage in real time (at the us level), and the controller that can sample the grid voltage in real time cannot directly control the output voltage of the converter. Therefore, conventional control methods cannot achieve medium and high voltage grid synchronization.

[0063] Some traditional synchronous technologies require additional devices for soft start or flexible switching, but both increase system costs and complicate the hardware architecture.

[0064] 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.

[0065] See FIG2 , which is a schematic diagram of another grid-connected system provided in an embodiment of the present application.

[0066] The grid-connected system provided in this embodiment 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 this embodiment, M = N is used as an example. The N transformers are T1, T2, through TN, and the N converters are 10, 20, through N0.

[0067] The output of each converter is connected to the low-voltage side of the corresponding transformer. The high-voltage side of each transformer is used to connect to the grid via a 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 even the high-voltage side of TN are connected in parallel and connected to the first end of a main switch K. The first end of main switch K is connected to the grid. It should be understood that when the grid-connected system is a three-phase system, main switch K also includes a three-phase switch, such as the main switch K shown in Figure 2.

[0068] 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 .

[0069] 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.

[0070] The second voltage transformer, PT2, detects the inverter voltage on the high-voltage side of the transformer. Since K is disconnected, PT2 can only detect the voltage on the high-voltage side of the transformer. Because the high-voltage sides of all transformers are connected in parallel, the voltages on the high-voltage sides of each transformer are equal.

[0071] The grid-connected system provided in an 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 are in one-to-one correspondence;

[0072] The output 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;

[0073] The controller 100 is configured to control at least one converter for off-grid startup. Based on a phase comparison result between the grid voltage phase and the voltage phase on the high-voltage side of the transformer, closed-loop control is performed on the voltage frequency of the converter during off-grid startup to align the voltage phase on the high-voltage side of the transformer with the grid voltage phase. Because the phase has 360 degrees in a grid voltage cycle and changes rapidly, real-time tracking is difficult. Therefore, the grid-connected system provided in an embodiment of the present application converts phase control into frequency control. The frequency changes relatively slowly, for example, from 50 Hz to 55 Hz, and the change in value does not span a large span. Phase synchronization of the converter's output voltage and the grid voltage is achieved by controlling the slowly changing frequency.

[0074] Main switch K is always in the off state. During this time, the off-grid startup converter can be one or more. The main function of the off-grid startup converter is to establish the voltage required for off-grid startup. Main switch K is closed only when the voltages on both sides of main switch K are consistent. This consistency includes amplitude, frequency, and phase. Specifically, main switch K is closed only when the voltage on the high-voltage side of the transformer is synchronized with the grid voltage. This prevents voltage discrepancies across main switch K, which could cause shock to the switch when closed.

[0075] The controller 100 can communicate with each converter.

[0076] 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 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.

[0077] The grid-connected system provided by the embodiment of the present application does not require an additional synchronization device. The output voltage of the converter can be synchronized with the grid voltage through closed-loop cooperative control of the controller and the converter. The voltage is directly closed-loop controlled, and the phase control is converted to frequency control to achieve real-time tracking effects such as slow and fast. 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. In other words, 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.

[0078] The grid-connected system provided by the embodiments of the present application is not limited by the impact of communication delays between the controller and each converter. Although the converter cannot sample the grid phase in real time at the microsecond level due to the rapid phase changes, the embodiments of the present application convert the rapidly changing phase into a slower-changing frequency. The controller controls the frequency of the converter's output voltage to gradually approach the grid frequency, thereby achieving phase control.

[0079] In order to more intuitively understand the technical solution provided by the embodiments of the present application, please refer to the schematic diagram of the converter voltage lagging behind the grid voltage shown in Figure 3, and the schematic diagram of the converter voltage leading the grid voltage shown in Figure 4.

[0080] As shown in Figure 3, the converter's output voltage Vout and the grid voltage Vg are both sinusoidal. The phase of the converter's output voltage Vout lags behind that of the grid voltage Vg. Therefore, the converter's output voltage frequency needs to be increased. That is, the converter's output voltage frequency should be higher and the phase change should be faster. This will allow the converter's output voltage phase to catch up with the grid voltage Vg, aligning the phase of the converter's output voltage with that of the grid voltage Vg.

[0081] As can be seen from Figure 4, the phase of the converter's output voltage Vout leads the phase of the grid voltage Vg. Therefore, it is necessary to reduce the frequency of the converter's output voltage, that is, the frequency of the converter's output voltage is lower and the phase change speed is reduced. In this way, the phase of the converter's output voltage can wait for the phase of the grid voltage Vg, thereby making the phase of the converter's output voltage consistent with the phase of the grid voltage Vg.

[0082] 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.

[0083] See FIG5 , which is a schematic diagram of a closed-loop control principle of voltage amplitude provided in an embodiment of the present application.

[0084] The grid-connected system provided in the embodiment of the present application uses the grid voltage amplitude Uref as a reference value for closed-loop voltage amplitude control. The controller, based on the comparison result of the grid voltage amplitude Uref and the voltage amplitude Uinv on the high-voltage side of the transformer, performs closed-loop control of the voltage amplitude of the off-grid startup converter, specifically for:

[0085] After passing the difference between the grid voltage amplitude Uref and the transformer high-voltage side voltage amplitude Uinv through a closed-loop regulator, a voltage amplitude adjustment variable is output. For example, the regulator can output the voltage amplitude adjustment variable using a proportional-integral algorithm or a proportional algorithm. The amplitude comparison result Ure is then clipped to obtain a clipped voltage amplitude command Ucmd. This voltage amplitude command Ucmd is then sent to the off-grid DC / AC converter to control the voltage amplitude of the off-grid DC / AC converter.

[0086] 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.

[0087] See FIG6 , which is a schematic diagram of a closed-loop control principle of a voltage phase provided in an embodiment of the present application.

[0088] The grid-connected system provided by the embodiment of the present application uses the grid voltage phase θref as a reference value for voltage phase closed-loop control. The controller, based on the phase comparison result of the grid voltage phase θref and the voltage phase θinv on the high-voltage side of the transformer, controls the voltage and frequency of the off-grid startup converter in a closed-loop manner, specifically for:

[0089] 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 a frequency amplitude adjustment value θc. For example, the regulator can output a frequency amplitude adjustment value Δω through a proportional-integral algorithm or a proportional algorithm. The frequency amplitude adjustment value Δω is limited to obtain a limited frequency Δω1. The limited frequency Δω1 is superimposed on the preset frequency ω0 to obtain a 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.

[0090] 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.

[0091] In addition, the embodiment of the present application also provides a closed-loop control principle of voltage phase, see Figure 7.

[0092] The difference between Figure 7 and Figure 6 is that Figure 7 does not include frequency feedforward control. Instead, the frequency amplitude adjustment value Δω output by the regulator is directly limited to obtain a limited frequency. This limited frequency is used as the obtained frequency command ωref, which is sent to the off-grid start-up DC / AC converter to control the phase of the off-grid start-up DC / AC converter. This control method is simpler and does not require frequency feedforward control.

[0093] 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 voltage phase of the grid.

[0094] In the embodiment of the present application, the closed-loop control of the voltage amplitude and the closed-loop control of the 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.

[0095] 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 ensure that 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 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 are consistent, avoiding different voltages at both ends, which may cause a large impact on the main switch.

[0096] Based on the grid-connected system provided in the above embodiment, the embodiment of the present application further provides a synchronization method for the grid-connected system, which is described in detail below with reference to the accompanying drawings.

[0097] See FIG7 , which is a flow chart of a synchronization method for a grid-connected system provided in an embodiment of the present application.

[0098] The embodiment of the present application provides a synchronization method for a grid-connected system, wherein the grid-connected system includes: 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 power grid through the main switch; before the converter is connected to the grid, the main switch is disconnected;

[0099] The method includes:

[0100] S701: Control the converter to perform off-grid startup; it should be understood that at least one converter among the multiple converters may be controlled to perform off-grid startup.

[0101] S702: Based on a phase comparison result between the voltage phase of the grid and the voltage phase of the high-voltage side of the transformer, close-loop control the voltage frequency of the off-grid started converter to make the voltage phase of the high-voltage side of the transformer consistent with the voltage phase of the grid.

[0102] The synchronization method of the grid-connected system provided in the embodiment of the present application does not require additional synchronization devices. 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. Regarding the conversion of phase control to frequency control, since the frequency change can cause the phase to change rapidly, the slower-changing frequency is used to influence the faster-changing phase, thereby achieving the real-time tracking effect of the converter voltage phase on the grid voltage phase, 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. In any case, 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.

[0103] In another implementation method provided in an embodiment of the present application, the embodiment of the present application may further 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 the voltage amplitude of the off-grid started converter to make the voltage amplitude of the high-voltage side of the transformer consistent with the voltage amplitude of the power grid.

[0104] The embodiment of the present application does not specifically limit the order of S702 and S703.

[0105] Closed-loop control of the voltage amplitude of the off-grid started converter based on an amplitude comparison result between the voltage amplitude of the grid and the voltage amplitude of the high-voltage side of the transformer, including: applying a proportional-integral algorithm or a proportional algorithm to the difference between the voltage amplitude of the grid and the voltage amplitude of the high-voltage side of the transformer to obtain an amplitude comparison result, limiting the amplitude comparison result to obtain a limited voltage amplitude command, and sending the voltage amplitude command to the off-grid started converter to control the voltage amplitude of the off-grid started converter;

[0106] 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, including: the difference between the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer is subjected to a proportional-integral algorithm or a proportional algorithm to obtain a phase comparison result, the phase comparison result is limited to obtain a phase after limiting, the phase after limiting is superimposed on the preset frequency to obtain a frequency command, the frequency command is sent to the off-grid started converter, and the phase of the off-grid started converter is controlled.

[0107] The preset frequency is the rated frequency of the power grid or the voltage frequency of the power grid detected in real time.

[0108] 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.

[0109] 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.

[0110] The grid-connected system provided by the embodiments of the present application is not limited by the impact of communication delays between the controller and each converter. Although the converter cannot sample the grid phase in real time at the microsecond level due to the rapid phase changes, the embodiments of the present application convert the rapidly changing phase into a slower-changing frequency. The controller controls the frequency of the converter's output voltage to gradually approach the grid frequency, thereby achieving phase control.

[0111] It should be noted that the various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. Reference can be made to the common and similar parts between the various embodiments. For the systems or devices disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple, and the relevant parts can be referred to the method description.

[0112] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present application. Various modifications to these embodiments will be readily apparent to one 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 is not limited to the embodiments shown herein, but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A grid-connected system, characterized in that, Including: A controller, a main switch, a transformer, and a converter; The output terminal 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. According to the phase comparison result between the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer, it closed-loop controls the voltage frequency of the off-grid starting converter to make the voltage phase of the high-voltage side of the transformer consistent with the voltage phase of the power grid.

2. The grid-connected system according to claim 1, wherein The controller is also used to closed-loop control the voltage amplitude of the off-grid starting converter according to the amplitude comparison result between 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.

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

4. The grid-connected system according to claim 2, characterized in that, The controller, according to the amplitude comparison result between the voltage amplitude of the power grid and the voltage amplitude of the high-voltage side of the transformer, closed-loop controls the voltage amplitude of the off-grid starting converter, specifically: Obtain the amplitude comparison result by passing the difference between the voltage amplitude of the power grid and the voltage amplitude of the high-voltage side of the transformer through a regulator, perform amplitude limiting on the amplitude comparison result to obtain the amplitude-limited voltage amplitude command, and send the voltage amplitude command to the off-grid starting converter to control the voltage amplitude of the off-grid starting converter.

5. The grid-connected system according to claim 1, characterized in that The controller, according to the phase comparison result between the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer; closed-loop controls the voltage frequency of the off-grid starting converter, specifically: Obtain the frequency adjustment amount by passing the difference between the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer through a regulator, perform amplitude limiting on the frequency adjustment amount to obtain the amplitude-limited frequency adjustment amount, superimpose the amplitude-limited frequency adjustment amount with a preset frequency to obtain the frequency command, and send the frequency command to the off-grid starting converter to control the phase of the off-grid starting converter.

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

7. The grid-connected system according to claim 1, wherein The controller, according to the phase comparison result between the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer; closed-loop controls the voltage frequency of the off-grid starting converter, specifically: Obtain the frequency adjustment amount by passing the difference between the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer through a regulator, perform amplitude limiting on the frequency adjustment amount to obtain the amplitude-limited frequency adjustment amount, use the amplitude-limited frequency adjustment amount as the frequency command, and send the frequency command to the off-grid starting converter to control the phase of the off-grid starting converter.

8. The grid-connected system according to any one of claims 5-7, characterized in that, The controller is specifically used to change the frequency of the modulation wave of the off-grid starting converter according to the frequency command to change the phase of the voltage of the high-voltage side of the transformer.

9. The grid-connected system according to any one of claims 1-7, characterized in that, Also including: 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 configured to obtain the voltage amplitude and voltage phase of the power grid according to the voltage of the power grid, and obtain the voltage amplitude and voltage phase of the high-voltage side of the transformer according to the voltage on the high-voltage side of the transformer.

10. The grid-connected system according to any one of claims 1-7, characterized in that, The controller is further configured to control the main switch to close when the voltage amplitude of the high-voltage side of the transformer is consistent with the voltage amplitude of the power grid and the voltage phase of the high-voltage side of the transformer is consistent with the voltage phase of the power grid.

11. The grid-connected system according to claim 9, 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.

12. A synchronization method for a grid-connected system, characterized in that, The grid-connected system includes: 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 power grid through the main switch; before the converter is grid-connected, the main switch is open; The method includes: Controlling the converter to start off-grid; 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 to make the voltage phase of the high-voltage side of the transformer consistent with the voltage phase of the power grid.

13. The method according to claim 12, wherein It further includes: 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 closed-loop controlled to make the voltage amplitude of the high-voltage side of the transformer consistent with the voltage amplitude of the power grid; 14. The method according to claim 12 or 13, characterized in that, The grid-connected system includes N transformers and M converters, where both M and N are integers and M is greater than or equal to N; one transformer is connected to one or more of the converters.

15. The method according to claim 13, characterized in that, The step of, 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, closed-loop controlling the voltage amplitude of the off-grid started converter includes: Obtaining an amplitude comparison result by passing the difference between the voltage amplitude of the power grid and the voltage amplitude of the high-voltage side of the transformer through a regulator, obtaining a limited voltage amplitude command by limiting the amplitude comparison result, and sending the voltage amplitude command to the off-grid started converter to control the voltage amplitude of the off-grid started converter; 16. The method according to claim 12 or 13, 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; the step of closed-loop controlling the voltage frequency of the off-grid started converter specifically includes: Obtaining a frequency adjustment amount by passing the difference between the voltage phase of the power grid and the voltage phase of the high-voltage side of the transformer through a regulator, obtaining a limited frequency adjustment amount by limiting the frequency adjustment amount, obtaining a frequency command by superimposing the limited frequency adjustment amount with a preset frequency, and sending the frequency command to the off-grid started converter to control the phase of the off-grid started converter.

17. The method according to claim 16, wherein The preset frequency is the rated frequency of the power grid or the voltage frequency of the power grid detected in real time.

18. The method according to claim 12 or 13, 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; the step of closed-loop controlling the voltage frequency of the off-grid started converter 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 starting converter to control the phase of the off-grid starting converter.

19. The method according to any one of claims 16 - 18, characterized in that, The controlling the phase of the off-grid starting converter includes: Changing the frequency of the modulation wave of the off-grid starting converter according to the frequency command to change the phase of the voltage of the high-voltage side of the transformer.

20. The method according to any one of claims 12-18, characterized in that, It further includes: When the voltage amplitude of the high-voltage side of the transformer is consistent with the voltage amplitude of the power grid and the voltage phase of the high-voltage side of the transformer is consistent with the voltage phase of the power grid, control the main switch to close.

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