Grid-connected system and synchronization method

Abstract

A grid-connected system and a grid-connected method. The grid-connected system comprises a controller (100), a main switch (K), transformers (T1, T2, ..., Tn), and converters (10, 20, ..., N0). An output end of each converter is connected to the low-voltage side of the corresponding transformer, and the high-voltage side of the transformer is configured to be connected to a power grid by means of the main switch. The controller is configured to, when the voltage phase sequence of the high-voltage side of the transformer is inconsistent with the grid-side phase sequence of the power grid, take the grid-side phase sequence as a reference to send a phase sequence adjustment command to a converter started in an off-grid mode. The converter started in an off-grid mode is configured to, on the basis of the phase sequence adjustment command, adjust the phase sequence of the output voltage to be consistent with the grid-side phase sequence, thereby enabling grid connection in a correct phase sequence, and realizing consistency between the phase sequence of the output voltage of the converter and the grid-side phase sequence.

Description

A grid-connected system and synchronization method

[0001] This application claims priority to the Chinese patent application with application number 202410088976.2 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 by reference into this application. Technical Field

[0002] The present application relates to the field of off-grid startup technology, and in particular to a grid-connected system and a grid-connected 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 power grid via a step-up transformer. The medium- and high-voltage power grid typically consists of three phases: A, B, and C.

[0004] Due to construction wiring reasons, the phase sequence of the converter port wiring may be inconsistent with the phase sequence of the medium and high voltage power grid wiring at the grid-connected system site, resulting in the converter output voltage phase sequence being inconsistent with the grid-side phase sequence, making it impossible to achieve synchronization between the converter output voltage and the grid voltage.

[0005] Summary of the Invention

[0006] In view of this, the present application provides a grid-connected system and a grid-connected method, which can be connected to the grid with a correct phase sequence, so that the phase sequence of the output voltage of the converter is consistent with the phase sequence on the grid side.

[0007] The present application provides a grid-connected system, comprising: a controller, 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; the controller is used to send a phase sequence adjustment command to the off-grid started converter based on the grid-side phase sequence when the voltage phase sequence on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence of the power grid; the off-grid started converter is used to adjust the phase sequence of the output voltage to be consistent with the grid-side phase sequence according to the phase sequence adjustment command.

[0008] In one possible implementation, the controller is also used to control the off-grid starting converter to perform off-grid starting according to a preset voltage amplitude, preset frequency and preset phase sequence, and after off-grid starting, detect the inverter voltage on the high-voltage side of the transformer to obtain the voltage phase sequence on the high-voltage side of the transformer.

[0009] In one possible implementation, the controller is also used to control the off-grid starting converter to perform off-grid starting according to the amplitude, frequency and grid-side phase sequence of the grid voltage, and after off-grid starting, detect the inverter voltage on the high-voltage side of the transformer to obtain the voltage phase sequence on the high-voltage side of the transformer.

[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 method further includes: a first voltage transformer and a second voltage transformer connected to the controller;

[0012] A first voltage transformer, used for detecting the voltage of the power grid;

[0013] A second voltage transformer, used for detecting the inverter voltage on the high-voltage side of the transformer;

[0014] The controller is specifically used to obtain the grid-side phase sequence according to the voltage of the power grid, and obtain the voltage phase sequence on the high-voltage side of the transformer according to the inverter voltage on the high-voltage side of the transformer.

[0015] In one possible implementation, the off-grid started converter is configured to adjust the phase sequence of the output voltage to be consistent with the grid-side phase sequence according to the phase sequence adjustment command, specifically including:

[0016] The off-grid started converter stops sending drive signals to the switch tube, generates a new drive signal according to the phase sequence adjustment command, and uses the new drive signal to drive the switch tube to make the phase sequence of the output voltage consistent with the phase sequence on the grid side.

[0017] In a possible implementation, the controller obtains the grid-side phase sequence according to the voltage of the grid, specifically including: the controller obtains the grid-side phase sequence according to the voltage of the grid through a software phase-locked loop or a hardware zero-crossing comparison circuit;

[0018] The controller obtains the voltage phase sequence on the high-voltage side of the transformer according to the inverter voltage on the high-voltage side of the transformer, specifically including: the controller obtains the voltage phase sequence on the high-voltage side of the transformer through a software phase-locked loop or a hardware zero-crossing comparison circuit according to the inverter voltage on the high-voltage side of the transformer.

[0019] In one possible implementation, the controller is specifically used to phase-lock the voltage on the high-voltage side of the transformer based on the grid-side phase sequence, and when the phase locking is unsuccessful, it is determined that the voltage phase sequence on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence of the power grid; or, when the lead angle between the three-phase voltages of the voltage on the high-voltage side of the transformer is inconsistent with the lead angle of the grid voltage based on the grid-side phase sequence, it is determined that the voltage phase sequence on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence of the power grid.

[0020] In one possible implementation, the controller is also used to control the closing of the main switch when the voltage phase sequence on the high-voltage side of the transformer is consistent with the phase sequence on the grid side, and the amplitude, phase and frequency of the inverter voltage on the high-voltage side of the transformer are synchronized with the amplitude, phase and frequency of the voltage of the grid respectively.

[0021] 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.

[0022] The present application also provides a grid-connected method for a grid-connected system, the grid-connected system comprising: a controller, 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;

[0023] The method includes: when the voltage phase sequence on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence of the power grid, sending a phase sequence adjustment command to the off-grid started converter based on the grid-side phase sequence; controlling the off-grid started converter to adjust the output voltage phase sequence to be consistent with the grid-side phase sequence according to the phase sequence adjustment command.

[0024] In a possible implementation, the method further includes: controlling the off-grid starting converter to perform off-grid starting according to a preset voltage amplitude, a preset frequency, and a preset phase sequence, and detecting the inverter voltage on the high-voltage side of the transformer after the off-grid starting to obtain the voltage phase sequence on the high-voltage side of the transformer.

[0025] In a possible implementation, the method further includes: controlling the off-grid starting converter to perform off-grid starting according to the amplitude, frequency and grid-side phase sequence of the grid voltage, and detecting the inverter voltage on the high-voltage side of the transformer to obtain the voltage phase sequence on the high-voltage side of the transformer after off-grid starting.

[0026] In one possible implementation, the phase sequence of the output voltage is adjusted to be consistent with the phase sequence on the grid side according to the phase sequence adjustment command, specifically including: suspending sending the drive signal to the switch tube, generating a new drive signal according to the phase sequence adjustment command, and driving the switch tube with the new drive signal to make the phase sequence of the output voltage consistent with the phase sequence on the grid side.

[0027] In a possible implementation, obtaining the grid-side phase sequence according to the voltage of the grid specifically includes: obtaining the grid-side phase sequence according to the voltage of the grid through a software phase-locked loop or a hardware zero-crossing comparison circuit;

[0028] The voltage phase sequence on the high-voltage side of the transformer is obtained according to the inverter voltage on the high-voltage side of the transformer, specifically including:

[0029] The voltage phase sequence on the high-voltage side of the transformer is obtained through a software phase-locked loop or a hardware zero-crossing comparison circuit according to the inverter voltage on the high-voltage side of the transformer.

[0030] In one possible implementation, it is determined that the voltage phase sequence on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence of the power grid by:

[0031] The voltage on the high-voltage side of the transformer is phase-locked based on the grid-side phase sequence. If the phase-lock fails, it is determined that the voltage phase sequence on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence of the power grid.

[0032] or,

[0033] When the leading angle between the three-phase voltages on the high-voltage side of the transformer is inconsistent with the leading angle of the grid voltage based on the grid-side phase sequence, it is determined that the voltage phase sequence on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence of the grid.

[0034] In a possible implementation, it also includes: when the voltage phase sequence on the high-voltage side of the transformer is consistent with the phase sequence on the grid side, and the amplitude, phase and frequency of the inverter voltage on the high-voltage side of the transformer are synchronized with the amplitude, phase and frequency of the voltage of the grid respectively, controlling the main switch to close.

[0035] It can be seen that this application has the following beneficial effects:

[0036] The grid-connected system provided in the embodiment of the present application, before being connected to the grid, first detects whether the voltage phase sequence on the high-voltage side of the transformer is consistent with the grid-side phase sequence of the power grid during off-grid startup. When they are inconsistent, the controller generates a phase sequence adjustment command based on the grid-side phase sequence, and sends the phase sequence adjustment command to the off-grid started converter, so that the converter adjusts the phase sequence of the output voltage, thereby making the voltage phase sequence on the high-voltage side of the transformer consistent with the grid-side phase sequence of the power grid, thereby achieving synchronization between the output voltage of the converter and the grid voltage, and enabling the converter to successfully achieve grid-connected power generation. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG1 is a schematic diagram of a grid-connected system provided in an embodiment of the present application;

[0038] FIG2 is a schematic diagram of the correct phase sequence of a grid-connected system provided in an embodiment of the present application;

[0039] FIG3 is a schematic diagram of an erroneous phase sequence of a grid-connected system provided in an embodiment of the present application;

[0040] FIG4 is a schematic diagram of another grid-connected system provided in an embodiment of the present application;

[0041] FIG5 is a schematic diagram of a phase sequence after adjustment of a grid-connected system provided in an embodiment of the present application;

[0042] FIG6 is a flow chart of a grid-connected method for a grid-connected system provided in an embodiment of the present application;

[0043] FIG7 is a flow chart of another grid-connected method of a grid-connected system provided in an embodiment of the present application. DETAILED DESCRIPTION

[0044] 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.

[0045] Refer to Figure 1, which is a schematic diagram of a grid-connected system provided in an embodiment of the present application.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] See Figure 2, which is a schematic diagram of the correct phase sequence of a grid-connected system provided in an embodiment of the present application.

[0053] For example, a three-phase grid-connected system includes phases A, B, and C. Under normal circumstances, the three phases at the converter port are INVa, INVb, and INVc. The high-voltage side of the transformer is connected to the grid via a main switch K.

[0054] The three-phase sequence on the low-voltage side of the transformer is a, b and c, and the three-phase sequence on the low-voltage side of the transformer is a1, b1 and c1. The grid-side phase sequence is A, B and C. When the ports of the converter are connected correctly, the phase sequences among the converter, transformer and grid are consistent.

[0055] Refer to FIG3 , which is a schematic diagram of an erroneous phase sequence of a grid-connected system provided in an embodiment of the present application.

[0056] Figure 3 shows an incorrect connection to the converter ports. For example, converter port INVa is connected to phase b on the transformer's low-voltage side. INVa outputs voltage phase a, but output voltage phase Va is connected to phase b on the transformer's low-voltage side. Similarly, converter port INVb is connected to phase a on the transformer's low-voltage side. INVb outputs voltage phase Vb, but output voltage phase Vb is connected to phase b on the transformer's low-voltage side.

[0057] The grid-connected system includes multiple converters, and the on-site wiring is complex. Moreover, since the phase sequence of the transformer is the same as that of the power grid, if the wiring between the converter and the transformer is incorrect, the phase sequence of the converter will be inconsistent with the phase sequence of the power grid, and the converter cannot be normally connected to the grid.

[0058] However, the converter cannot identify the grid-side phase sequence of the power grid and the wiring phase sequence of the converter port. When there is energy on the DC side, the controller controls the converter to start slowly. When the wiring phase sequence of the converter port is inconsistent with the grid-side phase sequence, voltage synchronization cannot be achieved, and the circuit breaker cannot be closed or a large impact current will occur when the circuit breaker is forced to close, resulting in equipment failure and power generation loss.

[0059] 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.

[0060] See FIG4 , which is a schematic diagram of another grid-connected system provided in an embodiment of the present application.

[0061] The grid-connected system provided in the embodiments 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 embodiments of the present application, M=N is used as an example. There is a one-to-one correspondence between the N transformers and the N converters; the N transformers are T1, T2, through TN, and the N converters are 10, 20, through N0.

[0062] 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.

[0063] The controller 100 is configured to send a phase sequence adjustment command to at least one off-grid started converter based on the grid-side phase sequence when the voltage phase sequence on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence of the power grid.

[0064] It should be understood that until the phase sequence on the high-voltage side of the transformer aligns with the grid-side phase sequence, the main switch K remains disconnected. During this time, the off-grid startup converter can be one or more. The main purpose of the off-grid startup converter is to establish the off-grid startup voltage. Main switch K is closed only when the voltages on both sides of main switch K are consistent. This voltage consistency includes amplitude, frequency, and phase. In other words, 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 a shock to main switch K when closed.

[0065] The controller 100 can communicate with each converter and send a phase sequence adjustment command to the converter to be started off-grid.

[0066] When there are multiple converters for off-grid startup, the controller 100 may send phase sequence adjustment commands to the multiple converters for off-grid startup.

[0067] It should be understood that when only part of the converters are started off-grid, and not all converters are started off-grid, the remaining converters can automatically track the converters started off-grid to establish output voltage.

[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 controller 100 is specifically configured to obtain a grid-side phase sequence according to the voltage of the grid, and obtain a voltage phase sequence on the high-voltage side according to the inverter voltage on the high-voltage side.

[0072] The grid-connected system provided in the embodiment of the present application does not specifically limit the manner in which the controller obtains the grid-side phase sequence and the voltage phase sequence on the high-voltage side of the transformer, and can be obtained by software or hardware.

[0073] The controller obtains the grid-side phase sequence according to the voltage of the grid, specifically including: the controller obtains the grid-side phase sequence according to the voltage of the grid through a software phase-locked loop or a hardware zero-crossing comparison circuit.

[0074] The controller obtains the voltage phase sequence on the high-voltage side according to the inverter voltage on the high-voltage side, specifically including: the controller obtains the voltage phase sequence on the high-voltage side through a software phase-locked loop or a hardware zero-crossing comparison circuit according to the inverter voltage on the high-voltage side.

[0075] The off-grid started converter is used to adjust the phase sequence of the output voltage to be consistent with the grid-side phase sequence according to the phase sequence adjustment command.

[0076] Generally, the grid-side phase sequence of the power grid is correct. Therefore, the grid-side phase sequence of the power grid is used as a reference to adjust the phase sequence of the output voltage of the converter to ensure that the voltage phase sequence on the high-voltage side of the transformer is consistent with the grid-side phase sequence.

[0077] The grid-connected system provided in the embodiment of the present application, before being connected to the grid, first detects whether the voltage phase sequence on the high-voltage side of the transformer is consistent with the grid-side phase sequence of the power grid during off-grid startup. When they are inconsistent, the controller generates a phase sequence adjustment command based on the grid-side phase sequence, and sends the phase sequence adjustment command to the off-grid started converter, so that the converter adjusts the phase sequence of the output voltage, thereby making the voltage phase sequence on the high-voltage side of the transformer consistent with the grid-side phase sequence of the power grid, thereby achieving synchronization between the output voltage of the converter and the grid voltage, and enabling the converter to successfully achieve grid-connected power generation.

[0078] It should be understood that when the controller controls the off-grid-start converter to perform off-grid startup, it can send an off-grid startup command to the converter, where the off-grid startup command includes the voltage amplitude, frequency, and phase sequence. For example, the controller is further configured to control the off-grid-start converter to perform off-grid startup according to a preset voltage amplitude, preset frequency, and preset phase sequence, or to control the off-grid-start converter to perform off-grid startup according to the grid voltage amplitude, frequency, and grid-side phase sequence, and after off-grid startup, detect the inverter voltage on the high-voltage side of the transformer to obtain the voltage phase sequence on the high-voltage side. However, even if the off-grid-start converter is started according to the grid-side phase sequence of the grid, wiring errors between the converter and the transformer may cause the voltage phase sequence on the high-voltage side of the transformer to be inconsistent with the grid-side phase sequence.

[0079] Since the controller can obtain the grid voltage through PT1, the amplitude, frequency, phase and phase sequence of the grid voltage can be obtained.

[0080] Because the off-grid-start converter has already generated ripples to the switches during off-grid startup—that is, sent drive signals to cause the switches to switch—when the controller determines that the phase sequence is inconsistent, the off-grid-start converter suspends ripple generation. Specifically, the off-grid-start converter adjusts the output voltage phase sequence to align with the grid-side phase sequence based on the phase sequence adjustment command. This specifically involves suspending the drive signals to the switches, generating new drive signals based on the phase sequence adjustment command, and using the new drive signals to drive the switches to align the output voltage phase sequence with the grid-side phase sequence.

[0081] The new driving signal will cause the converter to output voltage again according to the grid-side phase sequence. For details, please refer to Figure 5, which is a schematic diagram of the phase sequence after adjustment of a grid-connected system provided in an embodiment of the present application.

[0082] As can be seen from Figure 5, the phase sequence is inconsistent due to the wiring error between the converter and the transformer. However, the converter adjusts the phase sequence of the output voltage to make the phase sequence consistent. For example, the converter port INVa no longer outputs the a-phase voltage, but outputs the b-phase voltage. As a result, the converter and the transformer can achieve phase consistency even in the case of wiring errors.

[0083] The grid-connected system provided in the embodiment of the present application does not need to change the incorrect wiring on site. Instead, it controls the phase sequence of the voltage output by the converter to ensure that the phase sequence of the high-voltage side of the transformer is consistent with the phase sequence of the grid in the case of incorrect wiring, and only ensures that the phase sequence of the output voltage of the converter is consistent with the voltage phase sequence of the grid.

[0084] The grid-connected system provided in the embodiment of the present application does not specifically limit the manner in which the controller determines whether the phase sequence is inconsistent. For example, the grid-connected system provided in the embodiment of the present application, the controller is specifically used to phase-lock the voltage on the high-voltage side based on the grid-side phase sequence. When the phase locking is unsuccessful, it is determined that the voltage phase sequence on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence of the power grid; that is, when the transformer is phase-locked according to the grid-side phase sequence, if the phase locking cannot be successful, it indicates that the phase sequence is inconsistent.

[0085] In another implementation, the controller is specifically configured to determine, based on the grid-side phase sequence, that when the lead angle between the three-phase voltages on the high-voltage side is inconsistent with the lead angle of the grid voltage, the voltage phase sequence on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence of the grid. For example, the grid-side phase sequence is such that phase A leads phase B by 120 degrees, and phase B leads phase C by 120 degrees. If the voltage phase sequence on the high-voltage side of the transformer obtained by the controller is such that phase b leads phase a by 120 degrees, then it indicates that the voltage phase sequence on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence of the grid.

[0086] The grid-connected system provided in the embodiments of the present application closes the main switch K only when the voltage phase sequence on the high-voltage side of the transformer is consistent with the grid-side phase sequence, and the voltage on the high-voltage side of the transformer is synchronized with the grid voltage. That is, the controller is further configured to control the main switch to close when the voltage phase sequence on the high-voltage side is consistent with the grid-side phase sequence, and the amplitude, phase, and frequency of the inverter voltage on the high-voltage side are synchronized with the amplitude, phase, and frequency of the grid voltage, respectively.

[0087] Based on the grid-connected system provided in the above embodiment, the embodiment of the present application further provides a grid-connected method for the grid-connected system, which is described in detail below with reference to the accompanying drawings.

[0088] Refer to FIG6 , which is a flow chart of a grid-connected method for a grid-connected system provided in an embodiment of the present application.

[0089] The grid-connected system includes: a controller, a main switch, a transformer, and a converter; 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;

[0090] The method includes:

[0091] S601: When the voltage phase sequence on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence of the power grid, a phase sequence adjustment command is sent to the off-grid started converter based on the grid-side phase sequence; this application does not specifically limit the number of off-grid started converters, which can be one or more.

[0092] It should be understood that the main switch remains open until the phase sequence on the high-voltage side of the transformer matches the grid-side phase sequence. During this time, the off-grid startup converter can be one or more. The main switch is closed only when the voltages on both sides of the main switch are aligned. This includes voltage consistency in amplitude, frequency, and phase. Specifically, the main switch is closed only when the voltage on the high-voltage side of the transformer is synchronized with the grid voltage. This prevents voltage differences across the main switch, which could cause a shock to the switch upon closing.

[0093] S602: Control the off-grid startup converter to adjust the phase sequence of the output voltage to be consistent with the grid-side phase sequence according to the phase sequence adjustment command.

[0094] The grid-connected system provided in the embodiment of the present application does not specifically limit the manner in which the controller obtains the grid-side phase sequence and the voltage phase sequence on the high-voltage side of the transformer, and can be obtained by software or hardware.

[0095] The controller obtains the grid-side phase sequence according to the voltage of the grid, specifically including: the controller obtains the grid-side phase sequence according to the voltage of the grid through a software phase-locked loop or a hardware zero-crossing comparison circuit.

[0096] The controller obtains the voltage phase sequence on the high-voltage side according to the inverter voltage on the high-voltage side, specifically including: the controller obtains the voltage phase sequence on the high-voltage side through a software phase-locked loop or a hardware zero-crossing comparison circuit according to the inverter voltage on the high-voltage side.

[0097] The off-grid started converter is used to adjust the phase sequence of the output voltage to be consistent with the grid-side phase sequence according to the phase sequence adjustment command.

[0098] Generally, the grid-side phase sequence of the power grid is correct. Therefore, the grid-side phase sequence of the power grid is used as a reference to adjust the phase sequence of the output voltage of the converter to ensure that the voltage phase sequence on the high-voltage side of the transformer is consistent with the grid-side phase sequence.

[0099] The grid-connected method of the grid-connected system provided in the embodiment of the present application, before grid connection, at off-grid startup, first detects whether the voltage phase sequence on the high-voltage side of the transformer is consistent with the grid-side phase sequence of the power grid. When inconsistent, the controller generates a phase sequence adjustment command based on the grid-side phase sequence, and sends the phase sequence adjustment command to the off-grid started converter, so that the converter adjusts the phase sequence of the output voltage, and then makes the voltage phase sequence on the high-voltage side of the transformer consistent with the grid-side phase sequence of the power grid, thereby achieving synchronization between the output voltage of the converter and the grid voltage, so that the converter can smoothly achieve grid-connected power generation.

[0100] The following is a detailed description with reference to the accompanying drawings.

[0101] Refer to FIG7 , which is a flow chart of a grid-connected method for another grid-connected system provided in an embodiment of the present application.

[0102] S701: Detect the grid-side phase sequence.

[0103] Specifically, the grid voltage can be obtained through a voltage transformer, and the grid-side phase sequence can be obtained based on the grid voltage.

[0104] S702: Control at least one converter to start off-grid.

[0105] S703: Detect the voltage phase sequence on the high-voltage side of the transformer.

[0106] Specifically, the voltage on the high-voltage side of the transformer can be obtained through a voltage transformer, and thus the voltage phase sequence on the high-voltage side of the transformer can be obtained according to the voltage on the high-voltage side of the transformer.

[0107] S704: Determine whether the grid-side phase sequence is consistent with the high-voltage side voltage phase sequence. If so, execute S706; otherwise, execute S705.

[0108] S705: A phase sequence adjustment command is issued, and the converter adjusts the output voltage according to the phase sequence adjustment command.

[0109] S706: Continue to adjust the inverter voltage amplitude, phase, and frequency to meet the conditions for closing the main switch.

[0110] The grid-connected method of the grid-connected system provided in the embodiment of the present application does not require changing the erroneous wiring on site. Instead, it controls the phase sequence of the voltage output by the converter to ensure that the phase sequence of the high-voltage side of the transformer is consistent with the phase sequence of the grid in the case of erroneous wiring, and only ensures that the phase sequence of the output voltage of the converter is consistent with the voltage phase sequence of the grid.

[0111] 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. The same and similar parts between the various embodiments can be referenced to each other.

[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; The controller is configured to send a phase sequence adjustment command to the converter starting in off-grid mode with the grid-side phase sequence as a reference when the phase sequence of the voltage on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence; The converter starting in off-grid mode is configured to adjust the phase sequence of the output voltage to be consistent with the grid-side phase sequence according to the phase sequence adjustment command.

2. The grid-connected system according to claim 1, wherein The controller is further configured to control the converter starting in off-grid mode to start in off-grid mode according to a preset voltage amplitude, a preset frequency, and a preset phase sequence, and after starting in off-grid mode, detect the inverted voltage on the high-voltage side of the transformer to obtain the phase sequence of the voltage on the high-voltage side of the transformer.

3. The grid-connected system according to claim 1, characterized in that The controller is further configured to control the converter starting in off-grid mode to start in off-grid mode according to the amplitude, frequency, and grid-side phase sequence of the grid voltage, and after starting in off-grid mode, detect the inverted voltage on the high-voltage side of the transformer to obtain the phase sequence of the voltage on the high-voltage side of the transformer.

4. The grid-connected system according to any one of claims 1-3, 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 converters.

5. The grid-connected system according to claim 4, characterized in that, Further including: A first voltage transformer and a second voltage transformer connected to the controller; The first voltage transformer is configured to detect the voltage of the power grid; The second voltage transformer is configured to detect the inverted voltage on the high-voltage side of the transformer; The controller is specifically configured to obtain the grid-side phase sequence according to the voltage of the power grid, and obtain the phase sequence of the voltage on the high-voltage side of the transformer according to the inverted voltage on the high-voltage side of the transformer.

6. The grid-connected system according to any one of claims 1-5, characterized in that, The converter starting in off-grid mode is configured to adjust the phase sequence of the output voltage to be consistent with the grid-side phase sequence according to the phase sequence adjustment command, specifically including: The converter starting in off-grid mode pauses sending drive signals to the switching tubes, generates new drive signals according to the phase sequence adjustment command, and uses the new drive signals to drive the switching tubes to make the phase sequence of the output voltage consistent with the grid-side phase sequence.

7. The grid-connected system according to claim 5, characterized in that, The controller obtains the grid-side phase sequence according to the voltage of the power grid, specifically including: the controller obtains the grid-side phase sequence through a software phase-locked loop or a hardware zero-crossing comparison circuit according to the voltage of the power grid; The controller obtains the phase sequence of the voltage on the high-voltage side of the transformer according to the inverted voltage on the high-voltage side of the transformer, specifically including: the controller obtains the phase sequence of the voltage on the high-voltage side of the transformer through a software phase-locked loop or a hardware zero-crossing comparison circuit according to the inverted voltage on the high-voltage side of the transformer.

8. The grid-connected system according to claim 7, characterized in that, The controller is specifically configured to lock the phase of the voltage on the high-voltage side of the transformer with the grid-side phase sequence as a reference, and when the phase locking is unsuccessful, determine that the phase sequence of the voltage on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence; or, specifically configured to determine that the phase sequence of the voltage on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence when the leading angle between the three-phase voltages of the voltage on the high-voltage side of the transformer is inconsistent with the leading angle of the grid voltage with the grid-side phase sequence as a reference.

9. The grid-connected system according to claim 8, characterized in that, The controller is further configured to control the main switch to close when the voltage phase sequence on the high-voltage side of the transformer is consistent with the grid-side phase sequence, and the amplitude, phase, and frequency of the inverted voltage on the high-voltage side of the transformer are respectively synchronized with the amplitude, phase, and frequency of the grid voltage.

10. 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.

11. A grid connection method for a grid connection system, characterized in that, The grid-connected system includes: a controller, 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; The method includes: When the voltage phase sequence on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence of the grid, a phase sequence adjustment command is sent to the converter starting in the off-grid mode with the grid-side phase sequence as a reference; Controlling the converter starting in the off-grid mode to adjust the phase sequence of the output voltage to be consistent with the grid-side phase sequence according to the phase sequence adjustment command.

12. The method according to claim 11, wherein It further includes: Controlling the converter starting in the off-grid mode to start in the off-grid mode according to a preset voltage amplitude, a preset frequency, and a preset phase sequence. After starting in the off-grid mode, the inverter voltage on the high-voltage side of the transformer is detected to obtain the voltage phase sequence on the high-voltage side of the transformer.

13. The method according to claim 11, wherein It further includes: Controlling the converter starting in the off-grid mode to start in the off-grid mode according to the amplitude, frequency, and grid-side phase sequence of the grid voltage. After starting in the off-grid mode, the inverter voltage on the high-voltage side of the transformer is detected to obtain the voltage phase sequence on the high-voltage side of the transformer.

14. The method according to claim 11, wherein Adjusting the phase sequence of the output voltage to be consistent with the grid-side phase sequence according to the phase sequence adjustment command specifically includes: Pausing to send a driving signal to the switching tube, generating a new driving signal according to the phase sequence adjustment command, and using the new driving signal to drive the switching tube to make the phase sequence of the output voltage consistent with the grid-side phase sequence.

15. The method according to claim 11, wherein Obtaining the grid-side phase sequence according to the voltage of the grid specifically includes: obtaining the grid-side phase sequence through a software phase-locked loop or a hardware zero-crossing comparison circuit according to the voltage of the grid; Obtaining the voltage phase sequence on the high-voltage side of the transformer according to the inverter voltage on the high-voltage side of the transformer specifically includes: Obtaining the voltage phase sequence on the high-voltage side of the transformer through a software phase-locked loop or a hardware zero-crossing comparison circuit according to the inverter voltage on the high-voltage side of the transformer.

16. The method according to any one of claims 11-15, characterized in that, The voltage phase sequence on the high-voltage side of the transformer is determined to be inconsistent with the grid-side phase sequence of the grid in the following manner: Phase-locking the voltage on the high-voltage side of the transformer with the grid-side phase sequence as a reference. When the phase-locking is unsuccessful, it is determined that the voltage phase sequence on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence of the grid; Or, When it is determined that the leading angle between the three-phase voltages of the voltage on the high-voltage side of the transformer is inconsistent with the leading angle of the grid voltage with the grid-side phase sequence as a reference, it is determined that the voltage phase sequence on the high-voltage side of the transformer is inconsistent with the grid-side phase sequence of the grid.

17. The method according to any one of claims 11-15, characterized in that, It further includes: Controlling the main switch to close when the voltage phase sequence on the high-voltage side of the transformer is consistent with the grid-side phase sequence, and the amplitude, phase, and frequency of the inverted voltage on the high-voltage side of the transformer are respectively synchronized with the amplitude, phase, and frequency of the grid voltage.

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