Off-grid parallel system and startup method therefor

Abstract

Provided in the present invention are an off-grid parallel system and a startup method therefor. The method comprises: using any PCS of an off-grid parallel system as a master unit and other PCSs as slave units, and sending a first off-grid startup instruction to the master unit, such that the master unit performs off-grid startup at a threshold voltage; sending a second off-grid startup instruction to the slave units, such that the slave units perform off-grid startup and connect to a PCC; and after all the PCSs are connected to the PCC, sending a first off-grid voltage rise instruction to the master unit, such that the master unit executes a first off-grid voltage rise strategy, and the slave units execute a second off-grid voltage rise strategy in real time on the basis of the voltage of the PCC, so as to raise alternating-current-side voltages of all the PCSs to a rated voltage, thereby completing off-grid synchronous startup of all the PCSs. The present invention can solve the problem of an off-grid startup failure when a load power exceeds the maximum output power of a single PCS, thereby improving the reliability of system startup, and the present invention does not rely on communication between PCSs and is not affected by communication latency, thereby improving the reliability of off-grid synchronous startup of the system.

Description

Off-grid parallel system and its startup method Technical Field

[0001] This invention relates to the field of new energy technology, specifically to a startup method for an off-grid parallel system and an off-grid parallel system. Background Technology

[0002] PCS (Power Conversion System, Energy Storage Inverter) off-grid parallel operation generally adopts a communication-free droop control scheme. Based on this scheme, the off-grid startup of the system requires one of the PCS to start first to establish the voltage to the rated voltage, and then the other PCS are connected to the PCC (Point of Common Coupling).

[0003] However, when the load power exceeds the maximum output power of a single PCS, the above-mentioned startup scheme will inevitably lead to the failure of off-grid startup. Therefore, how to achieve reliable and stable startup of high-power loads exceeding the maximum output power of a single PCS within the existing off-grid parallel connection scheme framework has become a technical problem that urgently needs to be solved by those skilled in the art. Technical issues

[0004] To solve the above-mentioned technical problems, the first objective of this invention is to propose a startup method for an off-grid parallel system.

[0005] The second objective of this invention is to propose an off-grid parallel system. Technical solutions

[0006] The technical solution adopted in this invention is as follows:

[0007] An embodiment of the first aspect of the present invention provides a startup method for an off-grid parallel system. The off-grid parallel system includes: multiple PCSs connected in parallel to the same AC bus and loads connected to the AC bus. A common coupling point PCC exists between the AC side of the multiple PCSs and the loads. The startup method includes the following steps: designating any one PCS of the off-grid parallel system as a master and all other PCSs as slaves, sending a first off-grid startup command to the master; and upon receiving the first off-grid startup command, the master starts off-grid with a threshold voltage to raise the voltage of the common coupling point PCC. The voltage is raised to a threshold voltage, which is lower than the rated voltage of the off-grid parallel system. A second off-grid start-up command is sent to the slave device, which starts up off-grid and connects to the common coupling point (PCC) upon receiving the second off-grid start-up command. After all PCS are connected to the common coupling point (PCC), a first off-grid voltage rise command is sent to the master device, so that the master device executes the first off-grid voltage rise strategy. The slave device executes the second off-grid voltage rise strategy in real time according to the voltage of the common coupling point (PCC) to raise the AC side voltage of all PCS to the rated voltage, thus completing the off-grid synchronous start-up of all PCS.

[0008] The startup method for the off-grid parallel system proposed in this invention may also have the following additional technical features:

[0009] According to one embodiment of the present invention, the host specifically employs the following steps to execute the first off-grid voltage boosting strategy: injecting a harmonic signal with a preset amplitude and preset frequency into the fundamental voltage of the common coupling point PCC, and then counting the zero-crossing points of the harmonic signal; at the Nth zero-crossing point, after a first preset time T, controlling its own output voltage to linearly rise from the current threshold voltage to the rated voltage according to a preset slope, where N is a positive integer.

[0010] According to one embodiment of the present invention, the slave device specifically employs the following steps to execute the second off-grid voltage boosting strategy: acquiring the voltage of the common coupling point PCC to extract the harmonic signal injected by the host, and then counting the zero-crossing points of the harmonic signal; at the Nth zero-crossing point, after a second preset time D, controlling its own output voltage to linearly rise from the current threshold voltage to the rated voltage according to a preset slope, wherein the second preset time D is greater than the first preset time T.

[0011] According to one embodiment of the present invention, the threshold voltage is (5%~30%)Vn, where Vn is the rated voltage.

[0012] According to one embodiment of the present invention, the host injects harmonic signals of preset amplitude and preset frequency into the fundamental voltage of the common coupling point PCC in different ways, and the slave extracts the injection method of the harmonic signals injected by the host and realizes information transmission with the host according to the injection method.

[0013] A second aspect of the present invention provides an off-grid parallel system, comprising: multiple PCS connected in parallel to the same AC bus, loads connected to the AC bus, and an EMS (Energy Management System). The Energy Management System (EMS) consists of multiple Process Control Systems (PCS) with a common coupling point (PCC) between their AC sides and the loads. The EMS communicates with the PCS via communication lines, designating any one PCS as the master and all other PCS as slaves. Upon receiving a first off-grid start command from the EMS, the master starts off-grid with a threshold voltage to raise the PCC voltage to the threshold voltage, which is lower than the rated voltage of the off-grid parallel system. After the master raises the PCC voltage to the threshold voltage, the EMS sends a second off-grid start command to the slaves. Upon receiving the second off-grid start command, the slaves start off-grid and connect to the PCC. Once all PCS are connected to the PCC, the EMS sends a first off-grid voltage boost command to the master, causing the master to execute a first off-grid voltage boost strategy. The slaves then execute a second off-grid voltage boost strategy in real time based on the PCC voltage to raise the AC side voltage of all PCS to the rated voltage, completing the synchronous off-grid start of all PCS.

[0014] The off-grid parallel system described above in this invention also has the following additional technical features:

[0015] According to one embodiment of the present invention, the host specifically employs the following steps to execute the first off-grid voltage boosting strategy: injecting a harmonic signal with a preset amplitude and preset frequency into the fundamental voltage of the common coupling point PCC, and then counting the zero-crossing points of the harmonic signal; at the Nth zero-crossing point, after a first preset time T, controlling its own output voltage to linearly rise from the current threshold voltage to the rated voltage according to a preset slope, where N is a positive integer.

[0016] According to one embodiment of the present invention, the slave device specifically employs the following steps to execute the second off-grid voltage boosting strategy: acquiring the voltage of the common coupling point PCC to extract the harmonic signal injected by the host, and then counting the zero-crossing points of the harmonic signal; at the Nth zero-crossing point, after a second preset time D, controlling its own output voltage to linearly rise from the current threshold voltage to the rated voltage according to a preset slope, wherein the second preset time D is greater than the first preset time T.

[0017] According to one embodiment of the present invention, the threshold voltage is (5%~30%)Vn, where Vn is the rated voltage.

[0018] According to one embodiment of the present invention, the host injects harmonic signals of preset amplitude and preset frequency into the fundamental voltage of the common coupling point PCC in different ways, and the slave extracts the injection method of the harmonic signals injected by the host and realizes information transmission with the host according to the injection method. Beneficial effects

[0019] The beneficial effects of this invention are:

[0020] This invention is applicable to off-grid parallel systems based on a communication-free droop control scheme. It first controls one PCS to start at a threshold voltage far below its rated voltage to establish the PCC voltage. After the other PCS are connected to the PCC, all PCS then synchronously raise their voltages to complete the off-grid synchronous start-up. This solves the problem of off-grid start-up failure when the load power exceeds the maximum output power of a single PCS, improving the reliability of system startup. Furthermore, it does not rely on communication between PCS and is not affected by communication delays, further enhancing the reliability of off-grid synchronous start-up.

[0021] This invention uses a harmonic injection combination method to satisfy the transmission of various state and command signal information between PCS when there is no communication. Attached Figure Description

[0022] Figure 1 is a schematic diagram of an off-grid parallel system according to an embodiment of the present invention;

[0023] Figure 2 is a flowchart of a startup method for an off-grid parallel system according to an embodiment of the present invention. The best embodiment of the present invention

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Figure 1 is a schematic diagram of an off-grid parallel system according to an embodiment of the present invention, including: multiple PCS connected in parallel to the same AC bus and loads mounted on the AC bus. A common coupling point PCC exists between the AC side of the multiple PCS and the loads. There is no communication between the PCS. The PCS can communicate with the energy management system (EMS) via a communication bus, which can be RS485, CAN (Controller Area Network), Ethernet, etc. The energy management system (EMS) is responsible for controlling the PCS's startup, shutdown, charging and discharging, and on-grid / off-grid switching. The energy management system (EMS) can draw power from the AC bus or from the batteries in the energy storage cabinet of the off-grid parallel system.

[0026] EMS can control the startup of off-grid parallel systems in the following ways:

[0027] Figure 2 is a flowchart of a startup method for an off-grid parallel system according to an embodiment of the present invention. As shown in Figure 2, the startup method includes the following steps:

[0028] S1, take any one PCS of the off-grid parallel system as the master and all other PCS as slaves, and send the first off-grid start command to the master. After receiving the first off-grid start command, the master starts off-grid with the threshold voltage to raise the voltage of the common coupling point PCC to the threshold voltage, which is lower than the rated voltage of the off-grid parallel system.

[0029] Specifically, the threshold voltage is much lower than the rated voltage Vn (Vn=220V) to ensure that general electrical loads cannot work under this voltage. The threshold voltage can be (5%~30%)Vn, for example, the threshold voltage can be (25%Vn).

[0030] S2, send the second off-grid start command to the slave device. After receiving the second off-grid start command, the slave device starts up off-grid and connects to the common coupling point PCC.

[0031] Specifically, any PCS can act as the master, and the master is responsible for establishing the common coupling point voltage (PCC voltage) as the first off-grid start-up. All other PCS except the master act as slaves. After the master establishes the PCC voltage, the slaves start off-grid and connect to the PCC point.

[0032] S3. After all PCS are connected to the common coupling point PCC, the first off-grid voltage rise command is sent to the host so that the host executes the first off-grid voltage rise strategy. The slave executes the second off-grid voltage rise strategy in real time according to the voltage of the common coupling point PCC so as to raise the AC side voltage of all PCS to the rated voltage and complete the off-grid synchronous start-up of all PCS.

[0033] Specifically, after the EMS identifies all PCS connected to the PCC point, it sends a first off-grid voltage rise command to the host. Upon receiving this command, the host executes a first off-grid voltage rise strategy, raising its AC side voltage to the rated voltage. Simultaneously, the slave devices acquire the voltage of the common coupling point (PCC) in real time and execute a second off-grid voltage rise strategy based on this voltage, raising their AC side voltage to the rated voltage. This process raises the AC side voltage of all PCS to the rated voltage, completing the off-grid synchronous startup of all PCS. Alternatively, after identifying all PCS connected to the PCC point, the EMS can also send an off-grid voltage rise command to all PCS. Upon receiving this command, each PCS executes its off-grid voltage rise strategy, raising its AC side voltage to the rated voltage, thus completing the off-grid synchronous startup of all PCS.

[0034] Therefore, this invention first controls one of the PCS to start at a threshold voltage far below the rated voltage to establish the PCC voltage. After the other PCS are connected to the PCC, all PCS then perform voltage synchronous boosting to complete the off-grid synchronous start-up. All PCS can supply power to the load together, which can solve the problem of off-grid start-up failure when the load power exceeds the maximum output power of a single PCS, improve the reliability of system start-up, and is not dependent on communication between PCS, so it is not affected by communication delay, thus improving the reliability of off-grid synchronous start-up.

[0035] In a specific embodiment of the present invention, the host specifically employs the following steps to execute the first off-grid voltage boosting strategy: injecting a harmonic signal with a preset amplitude and preset frequency into the fundamental voltage of the common coupling point PCC, and then counting the zero-crossing points of the harmonic signal; at the Nth zero-crossing point, after a first preset time T, controlling its own output voltage to linearly rise from the current threshold voltage to the rated voltage according to a preset slope, where N is a positive integer.

[0036] Specifically, a low-order harmonic with a pre-set amplitude and frequency is injected into the fundamental voltage. For example, a harmonic with an amplitude of 10%Vn and a 9th fundamental frequency (9×50Hz) is injected. Then, the zero-crossing points of the harmonic signal are counted. The counting methods include, but are not limited to, the zero-crossing point judgment method and the peak value judgment method. At the Nth zero-crossing point, a pre-set time delay of T milliseconds is performed. After T milliseconds, the output voltage is linearly raised from the current threshold voltage to the rated voltage according to a pre-set slope.

[0037] In a specific embodiment of the present invention, the slave device specifically employs the following steps to execute the second off-grid voltage boosting strategy: acquiring the voltage of the common coupling point PCC to extract the harmonic signal injected by the host, and then counting the zero-crossing points of the harmonic signal; at the Nth zero-crossing point, after a second preset time D, controlling its own output voltage to linearly rise from the current threshold voltage to the rated voltage according to a preset slope, wherein the second preset time D is greater than the first preset time T.

[0038] Specifically, the slave device acquires the voltage of the common coupling point (PCC) in real time to extract the harmonic signal injected by the master device. It then counts the zero-crossing points of this harmonic signal. Due to environmental and equipment noise interference, the harmonic signal extracted by the slave device and the harmonic signal emitted by the master device have a fixed phase difference. Therefore, at the Nth zero-crossing point, the slave device, after a second preset time D, controls its output voltage to linearly rise from the current threshold voltage to the rated voltage according to a preset slope. The second preset time D is greater than the first preset time T. The second preset time D is determined based on the fixed phase difference between the harmonic signal extracted by the slave device and the harmonic signal emitted by the master device. This fixed phase difference can be obtained in advance through relevant experiments. This allows for synchronous rise of the master and slave devices, thereby achieving synchronous startup of the off-grid parallel system.

[0039] In one embodiment of the present invention, the host can inject harmonic signals of preset amplitude and preset frequency into the fundamental voltage of the common coupling point PCC in different ways, and the slave can extract the injection method of the harmonic signals injected by the host and realize information transmission with the host according to the injection method.

[0040] Specifically, as shown in Table 1 below, when the host injects harmonic signals into the fundamental voltage of the common coupling point (PCC), it can adopt different injection methods. These methods include: injection of phase A only, injection of phase B only, injection of phase C only, simultaneous injection of phases A and B, simultaneous injection of phases AC, simultaneous injection of phases BC, and simultaneous injection of phases ABC. Different injection methods represent different commands or signals, as detailed in Table 1. To achieve further information transmission, the injection methods can be further differentiated based on harmonic types, which are defined by different amplitudes and frequencies. The slave device extracts the injection method of the harmonic signal injected by the host and receives various status or command signals according to the injection method, executing corresponding actions. For example, if the information transmitted by the host is determined to be voltage rise based on the injection method, the second off-grid voltage rise strategy is executed; if the information transmitted by the host is determined to be fault signal 1, the slave device is controlled to shut down, etc. Specific actions can be pre-set. Therefore, the transmission of various status and command signal information between PCSs can be satisfied when there is no communication.

[0041] Table 1

[0042]

[0043] In summary, the off-grid parallel system startup method according to embodiments of the present invention is applicable to off-grid parallel systems based on a communication-free droop control scheme. It first controls one PCS to start at a threshold voltage far below its rated voltage to establish the PCC voltage. After the other PCS are connected to the PCC, all PCS then undergo voltage synchronous boosting to complete the off-grid synchronous startup. This method can solve the problem of off-grid startup failure when the load power exceeds the maximum output power of a single PCS, improving the reliability of system startup. Furthermore, it does not rely on communication between PCS and is not affected by communication delays, further enhancing the reliability of off-grid synchronous startup. The use of harmonic injection combination satisfies the transmission of various state and command signal information between PCS when communication is unavailable.

[0044] Corresponding to the above-described off-grid parallel system startup method, this invention also proposes an off-grid parallel system. Since the system embodiments of this invention correspond to the above-described method embodiments, and they implement the same or similar steps to achieve the objectives of this invention, details not disclosed in the system embodiments can be found in the above-described method embodiments, and will not be repeated here.

[0045] Figure 1 is a schematic diagram of an off-grid parallel system according to an embodiment of the present invention. As shown in Figure 1, the off-grid parallel system includes:

[0046] The system comprises multiple PCS (Power Control System) units connected in parallel to the same AC bus, loads mounted on the AC bus, and an energy management system (EMS). A common coupling point (PCC) exists between the AC side of the multiple PCS and the loads. The EMS communicates with the PCS via communication lines, designating any one PCS in the off-grid parallel system as the master and all other PCS as slaves.

[0047] After receiving the first off-grid start command from the EMS, the master unit starts off-grid with a threshold voltage to raise the voltage of the common coupling point (PCC) to the threshold voltage, which is lower than the rated voltage of the off-grid parallel system. After the master unit raises the PCC voltage to the threshold voltage, the EMS sends a second off-grid start command to the slave unit. Upon receiving the second off-grid start command, the slave unit starts off-grid and connects to the PCC. Once all PCS are connected to the PCC, the EMS sends a first off-grid voltage rise command to the master unit, causing the master unit to execute the first off-grid voltage rise strategy. The slave unit executes the second off-grid voltage rise strategy in real time according to the voltage of the PCC to raise the AC side voltage of all PCS to the rated voltage, thus completing the off-grid synchronous start of all PCS.

[0048] According to one embodiment of the present invention, the host specifically employs the following steps to execute the first off-grid voltage boosting strategy: injecting a harmonic signal with a preset amplitude and preset frequency into the fundamental voltage of the common coupling point PCC, and then counting the zero-crossing points of the harmonic signal; at the Nth zero-crossing point, after a first preset time T, controlling its own output voltage to linearly rise from the current threshold voltage to the rated voltage according to a preset slope, where N is a positive integer.

[0049] According to an embodiment of the present invention, the slave device specifically employs the following steps to execute the second off-grid voltage boosting strategy: acquiring the voltage of the common coupling point PCC to extract the harmonic signal injected by the host, and then counting the zero-crossing points of the harmonic signal; at the Nth zero-crossing point, after a second preset time D, controlling its own output voltage to linearly rise from the current threshold voltage to the rated voltage according to a preset slope, wherein the second preset time D is greater than the first preset time T.

[0050] According to one embodiment of the present invention, the threshold voltage is (25%Vn), where Vn is the rated voltage.

[0051] According to one embodiment of the present invention, the host injects harmonic signals of preset amplitude and preset frequency into the fundamental voltage of the common coupling point PCC in different ways, and the slave extracts the injection method of the harmonic signals injected by the host, and realizes information transmission with the host according to the injection method.

[0052] In summary, the off-grid parallel system according to embodiments of the present invention is applicable to off-grid parallel systems based on a communication-free droop control scheme. It first controls one PCS to start at a threshold voltage far below its rated voltage to establish the PCC voltage. After the other PCS are connected to the PCC, all PCS then synchronously raise their voltages to complete the off-grid synchronous start-up. This solves the problem of off-grid start-up failure when the load power exceeds the maximum output power of a single PCS, improving the reliability of system startup. Furthermore, it does not rely on communication between PCS and is not affected by communication delays, further enhancing the reliability of off-grid synchronous start-up. The use of harmonic injection combination satisfies the transmission of various state and command signal information between PCS when communication is unavailable.

[0053] In the description of this invention, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Any process or method description in the flowcharts or otherwise herein can be understood as representing a module, segment, or portion of code comprising one or more executable instructions for implementing custom logical functions or processes, and the scope of preferred embodiments of the invention includes additional implementations in which functions may be performed not in the order shown or discussed, including substantially simultaneously or in reverse order according to the functions involved, as should be understood by those skilled in the art to which embodiments of the invention pertain.

[0054] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0055] Those skilled in the art will understand that all or part of the steps of the methods in the above embodiments can be implemented by a program instructing related hardware. The program can be stored in a computer-readable storage medium, and when executed, the program includes one or a combination of the steps of the method embodiments.

[0056] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A startup method for an off-grid parallel system, characterized in that, The off-grid parallel system includes: multiple PCS connected in parallel to the same AC bus and a load connected to the AC bus, wherein there is a common coupling point PCC between the AC side of the multiple PCS and the load, and the startup method includes the following steps: Using any one PCS of the off-grid parallel system as the master and all other PCS as slaves, a first off-grid start command is sent to the master. After receiving the first off-grid start command, the master starts off-grid with a threshold voltage to raise the voltage of the common coupling point PCC to the threshold voltage, which is lower than the rated voltage of the off-grid parallel system. A second off-network start command is sent to the slave device. Upon receiving the second off-network start command, the slave device starts up off-network and connects to the common coupling point (PCC). Once all PCS are connected to the common coupling point PCC, a first off-grid voltage boosting command is sent to the host, causing the host to execute the first off-grid voltage boosting strategy. The slave devices execute the second off-grid voltage boosting strategy in real time according to the voltage of the common coupling point PCC, so as to boost the AC side voltage of all PCS to the rated voltage and complete the off-grid synchronous startup of all PCS.

2. The startup method of the off-grid parallel system according to claim 1, characterized in that, The host specifically employs the following steps to execute the first off-grid voltage boosting strategy: A harmonic signal with a preset amplitude and preset frequency is injected into the fundamental voltage of the common coupling point PCC, and then the zero-crossing points of the harmonic signal are counted. At the Nth zero crossing, after a first preset time T, the output voltage is controlled to linearly rise from the current threshold voltage to the rated voltage according to a preset slope, where N is a positive integer.

3. The startup method of the off-grid parallel system according to claim 2, characterized in that, The slave device specifically employs the following steps to execute the second off-grid voltage boosting strategy: The voltage at the common coupling point PCC is obtained to extract the harmonic signal injected by the host, and then the zero-crossing points of the harmonic signal are counted. At the Nth zero crossing, after a second preset time D, the output voltage is controlled to linearly rise from the current threshold voltage to the rated voltage according to a preset slope, wherein the second preset time D is greater than the first preset time T.

4. The startup method of the off-grid parallel system according to claim 1, characterized in that, The threshold voltage is (5%~30%)Vn, where Vn is the rated voltage.

5. The startup method of the off-grid parallel system according to claim 3, characterized in that, The host injects harmonic signals of preset amplitude and preset frequency into the fundamental voltage of the common coupling point PCC using different methods. The slave extracts the injection method of the harmonic signals injected by the host and realizes information transmission with the host according to the injection method.

6. An off-grid parallel system, characterized in that, include: The system comprises multiple PCSs connected in parallel to the same AC bus, a load mounted on the AC bus, and an EMS. A common coupling point PCC exists between the AC side of the PCSs and the load. The EMS communicates with the PCSs via a communication line. Any one PCS in the off-grid parallel system is designated as the master, and all other PCSs are designated as slaves. After receiving the first off-grid start command from the EMS, the host starts off-grid with a threshold voltage to raise the common coupling point (PCC) voltage to the threshold voltage, which is lower than the rated voltage of the off-grid parallel system. After the host raises the PCC voltage to the threshold voltage, the EMS sends a second off-grid start command to the slave. Upon receiving the second off-grid start command, the slave starts off-grid and connects to the PCC. Once all PCs are connected to the PCC, the EMS sends a first off-grid voltage boost command to the host, causing the host to execute the first off-grid voltage boost strategy. The slave executes the second off-grid voltage boost strategy in real time according to the PCC voltage to raise the AC side voltage of all PCs to the rated voltage, thus completing the off-grid synchronous start of all PCs.

7. The off-grid parallel system according to claim 6, characterized in that, The host specifically employs the following steps to execute the first off-grid voltage boosting strategy: A harmonic signal with a preset amplitude and preset frequency is injected into the fundamental voltage of the common coupling point PCC, and then the zero-crossing points of the harmonic signal are counted. At the Nth zero crossing, after a first preset time T, the output voltage is controlled to linearly rise from the current threshold voltage to the rated voltage according to a preset slope, where N is a positive integer.

8. The off-grid parallel system according to claim 7, characterized in that, The slave device specifically employs the following steps to execute the second off-grid voltage boosting strategy: The voltage at the common coupling point PCC is obtained to extract the harmonic signal injected by the host, and then the zero-crossing points of the harmonic signal are counted. At the Nth zero crossing, after a second preset time D, the output voltage is controlled to linearly rise from the current threshold voltage to the rated voltage according to a preset slope, wherein the second preset time D is greater than the first preset time T.

9. The off-grid parallel system according to claim 6, characterized in that, The threshold voltage is (5%~30%)Vn, where Vn is the rated voltage.

10. The off-grid parallel system according to claim 8, characterized in that, The host injects harmonic signals of preset amplitude and preset frequency into the fundamental voltage of the common coupling point PCC using different methods. The slave extracts the injection method of the harmonic signals injected by the host and realizes information transmission with the host according to the injection method.

Images