Off-grid parallel system and starting method thereof

Abstract

The present invention provides an off-grid parallel system and a startup method thereof, the method comprising: taking any one PCS of the off-grid parallel system as a host and other PCSs as slaves, sending a first off-grid startup command to the host, and the host starts off-grid with a threshold voltage; sending a second off-grid startup command to the slave, and the slave starts off-grid and is incorporated into the PCC; after all PCSs are incorporated into the PCC, sending a first off-grid voltage raising command to the host, the host executes a first off-grid voltage raising strategy, and the slave executes a second off-grid voltage raising strategy in real time according to the voltage of the PCC, so as to raise the AC side voltage of all PCSs to the rated voltage, and complete the off-grid synchronous startup of all PCSs. The present invention can solve the problem of off-grid startup failure when the load power exceeds the maximum output power of a single PCS, improves the reliability of system startup, and does not rely on communication between PCSs, will not be affected by communication delays, and improves the reliability of off-grid synchronous startup of the system.

Description

Technical Field

[0001] The present invention relates to the field of new energy technology, and in particular to a starting method for an off-grid parallel system and an off-grid parallel system. Background Art

[0002] The off-grid parallel operation of PCS (Power Conversion System, energy storage inverter) generally adopts a non-communication droop control scheme. Based on this scheme, the off-grid startup of the system requires that one of the PCSs be started first to establish the voltage to the rated voltage, and then the other PCSs 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 startup scheme will inevitably lead to failure of the off-grid startup of the system. Therefore, how to achieve reliable and stable startup of high-power loads exceeding the maximum output power of a single PCS under the existing off-grid parallel scheme framework has become a technical problem that needs to be solved urgently by those skilled in the art. Summary of the invention

[0004] The present invention is to solve the above technical problems, and the first purpose is to provide a starting method for an off-grid parallel system.

[0005] The second object of the present invention is to provide an off-grid parallel system.

[0006] The technical solution adopted by the present invention is as follows:

[0007] The embodiment of the first aspect of the present invention provides a method for starting an off-grid parallel system, wherein the off-grid parallel system comprises: a plurality of PCSs connected in parallel to the same AC bus and a load mounted on the AC bus, wherein a common coupling point PCC exists between the AC sides of the plurality of PCSs and the loads, and the starting method comprises the following steps: any one of the PCSs in the off-grid parallel system is used as a host, and all other PCSs are used as slaves, and a first off-grid starting instruction is sent to the host, and after receiving the first off-grid starting instruction, the host starts off-grid with a threshold voltage to raise the voltage of the common coupling point PCC to a certain value. The first off-grid voltage raising strategy is used to raise the voltage of the slave to a threshold voltage, which is lower than the rated voltage of the off-grid parallel system; a second off-grid start-up instruction is sent to the slave, and the slave is started off-grid and connected to the common coupling point PCC after receiving the second off-grid start-up instruction; when all PCSs are connected to the common coupling point PCC, a first off-grid voltage raising instruction is sent to the host, so that the host executes the first off-grid voltage raising strategy, and the slave executes the second off-grid voltage raising strategy in real time according to the voltage of the common coupling point PCC, so as to raise the AC side voltage of all PCSs to the rated voltage, and complete the off-grid synchronous start-up of all PCSs.

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

[0009] According to one embodiment of the present invention, the host specifically adopts the following steps to execute the first off-grid voltage raising strategy: injecting a harmonic signal of a preset amplitude and a 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 delay of a first preset time T, controlling its own output voltage to linearly increase 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 machine specifically adopts the following steps to execute the second off-grid voltage raising strategy: obtaining 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 delaying for a second preset time D, controlling its own output voltage to linearly increase 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 an 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 a harmonic signal of a preset amplitude and a 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 signal injected by the host, and realizes information transmission with the host according to the injection method.

[0013] The embodiment of the second aspect of the present invention provides an off-grid parallel system, comprising: a plurality of PCSs connected in parallel to the same AC bus, a load mounted on the AC bus, and an EMS (Energy Management System). System, energy management system), there is a common coupling point PCC between the AC sides of multiple PCSs and the loads, the EMS communicates with the PCS through a communication line, and any PCS of the off-grid parallel system is used as a host, and all other PCSs are used as slaves, wherein the host starts off-grid with a threshold voltage after receiving a first off-grid startup instruction sent by the EMS, so as to increase the voltage of the common coupling point PCC to the threshold voltage, and the threshold voltage is lower than the rated voltage of the off-grid parallel system, after the host increases the voltage of the common coupling point PCC to the threshold voltage, the EMS sends a second off-grid startup instruction to the slave, and the slave starts off-grid and is incorporated into the common coupling point PCC after receiving the second off-grid startup instruction, and when all PCSs are incorporated into the common coupling point PCC, the EMS sends a first off-grid voltage raising instruction to the host, so that the host executes a first off-grid voltage raising strategy, and the slave executes a second off-grid voltage raising strategy according to the voltage of the common coupling point PCC in real time, so as to raise the voltage of the AC side of all PCSs to the rated voltage, and complete the off-grid synchronous startup of all PCSs.

[0014] The above-mentioned off-grid parallel system of the present invention also has the following additional technical features:

[0015] According to one embodiment of the present invention, the host specifically adopts the following steps to execute the first off-grid voltage raising strategy: injecting a harmonic signal of a preset amplitude and a 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 delay of a first preset time T, controlling its own output voltage to linearly increase 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 machine specifically adopts the following steps to execute the second off-grid voltage raising strategy: obtaining 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 delaying for a second preset time D, controlling its own output voltage to linearly increase 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 an 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 a harmonic signal of a preset amplitude and a 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 signal injected by the host, and realizes information transmission with the host according to the injection method.

[0019] Beneficial effects of the present invention:

[0020] The present invention is applicable to an off-grid parallel system based on a non-communication droop control scheme. First, one of the PCSs is controlled to start at a threshold voltage far lower than the rated voltage to establish the PCC voltage. After the other PCSs are incorporated into the PCC, all PCSs then perform voltage synchronous raising to complete the off-grid synchronous start-up. The problem of off-grid start-up failure when the load power exceeds the maximum output power of a single PCS can be solved, and the reliability of system startup is improved. The system does not rely on communication between PCSs and will not be affected by communication delays, thereby improving the reliability of the off-grid synchronous start-up of the system.

[0021] The present invention uses a harmonic injection combination method to meet the transmission of various status and command signal information between PCSs when there is no communication. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0023] Figure 2 is a flow chart of a method for starting an off-grid parallel system according to an embodiment of the present invention. DETAILED DESCRIPTION

[0024] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0025] Figure 1It is a structural diagram of an off-grid parallel system according to an embodiment of the present invention, including: multiple PCSs connected in parallel on the same AC bus AC BUS and a load Load mounted on the AC bus AC BUS. There is a common coupling point PCC between the AC sides of multiple PCSs and the load Load. There is no communication between the PCSs. The PCSs can communicate with the energy management system EMS through the communication bus (Communication Bus). The communication bus can be RS485, CAN (Controller Area Network), Ethernet, etc. The energy management system EMS is responsible for controlling the PCS startup, shutdown, charging and discharging, and on-grid and off-grid switching. The energy management system EMS can draw power from the AC bus AC BUS, or from the battery of 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 flow chart of a method for starting an off-grid parallel system according to an embodiment of the present invention. Figure 2 As shown, the startup method includes the following steps:

[0028] S1, any PCS of the off-grid parallel system is used as the host, and all other PCSs are used as slaves, and the first off-grid startup command is sent to the host. After receiving the first off-grid startup command, the host starts off-grid with a threshold voltage to increase 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 at this voltage. The threshold voltage may be (5%~30%) Vn, for example, the threshold voltage may be (25% Vn).

[0030] S2, sending a second off-grid startup instruction to the slave machine, and the slave machine starts off-grid and merges into the common coupling point PCC after receiving the second off-grid startup instruction.

[0031] Specifically, any PCS can serve as a host. The host is responsible for establishing the common coupling point voltage (PCC voltage) as the first off-grid startup. All PCSs except the host serve as slaves. After the host establishes the PCC voltage, the slaves are started off-grid and merged into the PCC point.

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

[0033] Specifically, after EMS finds out that all PCSs are incorporated into the PCC point, EMS sends the first off-grid voltage raising instruction to the host. After receiving the first off-grid voltage raising instruction, the host executes the first off-grid voltage raising strategy to raise the host AC side voltage to the rated voltage. At the same time, the slave obtains the voltage of the common coupling point PCC in real time, and executes the second off-grid voltage raising strategy in real time according to the voltage of the common coupling point PCC, raising the slave AC side voltage to the rated voltage, raising the AC side voltage of all PCSs to the rated voltage, and completing the off-grid synchronous start-up of all PCSs. After EMS finds out that all PCSs are incorporated into the PCC point, it can also send an off-grid voltage raising instruction to all PCSs. After receiving the off-grid voltage raising instruction, the PCS executes the off-grid voltage raising strategy to raise the AC side voltage of all PCSs to the rated voltage, and completes the off-grid synchronous start-up of all PCSs.

[0034] Therefore, the present invention first controls one of the PCSs to start at a threshold voltage far lower than the rated voltage to establish the PCC voltage. After the other PCSs are incorporated into the PCC, all PCSs then raise the voltage synchronously to complete the off-grid synchronous startup. All PCSs can supply power to the load together, which can solve the problem of off-grid startup failure when the load power exceeds the maximum output power of a single PCS, improves the reliability of system startup, and does not rely on communication between PCSs, will not be affected by communication delays, and improves the reliability of the system off-grid synchronous startup.

[0035] In a specific embodiment of the present invention, the host specifically adopts the following steps to execute the first off-grid voltage raising strategy: injecting a harmonic signal of a preset amplitude and a 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 delay of a first preset time T, controlling its own output voltage to linearly increase from the current threshold voltage to the rated voltage according to a preset slope, where N is a positive integer.

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

[0037] In a specific embodiment of the present invention, the slave machine specifically adopts the following steps to execute the second off-grid voltage raising strategy: obtaining 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 delaying for a second preset time D, controlling its own output voltage to linearly increase 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 obtains the voltage of the common coupling point PCC in real time to extract the harmonic signal injected by the host, and then counts the zero crossing points of the harmonic signal. Due to the noise interference of the environment and the equipment itself, there is a fixed phase difference between the harmonic signal extracted by the slave and the harmonic signal emitted by the host. Therefore, at the Nth zero crossing point, the slave controls its own output voltage to rise linearly from the current threshold voltage to the rated voltage according to a preset slope after a delay of the second preset time D. The second preset time D is greater than the first preset time T. The second preset time D is determined according to the fixed phase difference between the harmonic signal extracted by the slave and the harmonic signal emitted by the host. The fixed phase difference can be obtained in advance according to relevant tests. In this way, the synchronous rise of the master and slave can be achieved, and then the synchronous start of the off-grid parallel system can be achieved.

[0039] In one embodiment of the present invention, the host can inject a harmonic signal of a preset amplitude and a 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 signal 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, different injection methods can be adopted. The injection methods can be: injection of only phase A, injection of only phase B, injection of only phase C, injection of AB at the same time, injection of AC at the same time, injection of BC at the same time, and injection of ABC at the same time. Different injection methods represent different instructions or signals. For details, please refer to Table 1. In order to achieve the transmission of more information, the injection method can be distinguished in combination with the harmonic type. The harmonic type is distinguished by defining different amplitudes and frequencies. The slave extracts the injection method of the harmonic signal injected by the host, realizes the reception of multiple states or command signals according to the injection method, and performs corresponding actions. For example, if the information transmitted by the host is judged to be voltage raising according to the injection method, the second off-grid voltage raising strategy is executed. If the information transmitted by the host is judged to be fault signal 1 according to the injection method, the self-stop is controlled, etc. The specific action can be set in advance. Thus. It can meet the information transmission of multiple states and command signals between PCS when there is no communication.

[0041] Table 1

[0042]

[0043] In summary, the startup method of the off-grid parallel system according to the embodiment of the present invention is applicable to the off-grid parallel system based on the non-communication droop control scheme. First, one of the PCSs is controlled to start with a threshold voltage far lower than the rated voltage to establish the PCC voltage. After the other PCSs are incorporated into the PCC, all PCSs then perform voltage synchronous raising to complete the off-grid synchronous startup. This can solve the problem of off-grid startup failure when the load power exceeds the maximum output power of a single PCS, improve the reliability of system startup, and does not rely on communication between PCSs, and will not be affected by communication delays, thereby improving the reliability of the off-grid synchronous startup of the system. The harmonic injection combination method is used to meet the transmission of multiple status and command signal information between PCSs when there is no communication.

[0044] Corresponding to the above-mentioned off-grid parallel system startup method, the present invention also proposes an off-grid parallel system. Since the system embodiment of the present invention corresponds to the above-mentioned method embodiment, the same or similar steps are implemented to achieve the purpose of the present invention. For details not disclosed in the system embodiment, refer to the above-mentioned method embodiment, and no further description is given in the present invention.

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

[0046] Multiple PCSs connected in parallel on the same AC bus AC BUS, loads mounted on the AC bus AC BUS, and energy management system EMS. There is a common coupling point PCC between the AC side of multiple PCSs and the loads Load. EMS communicates with PCS through communication lines. Any PCS in the off-grid parallel system is used as the host, and all other PCSs are used as slaves.

[0047] After receiving the first off-grid start-up instruction sent by the EMS, the host starts off-grid with a threshold voltage to increase the voltage of the common coupling point PCC to the threshold voltage, and the threshold voltage is lower than the rated voltage of the off-grid parallel system. After the host increases the voltage of the common coupling point PCC to the threshold voltage, the EMS sends a second off-grid start-up instruction to the slave. After receiving the second off-grid start-up instruction, the slave starts off-grid and is incorporated into the common coupling point PCC. When all PCSs are incorporated into the common coupling point PCC, the EMS sends a first off-grid voltage raising instruction to the host, so that the host executes the first off-grid voltage raising strategy. The slave executes the second off-grid voltage raising strategy in real time according to the voltage of the common coupling point PCC, so as to raise the AC side voltage of all PCSs to the rated voltage, thereby completing the off-grid synchronous start-up of all PCSs.

[0048] According to one embodiment of the present invention, the host specifically adopts the following steps to execute the first off-grid voltage raising strategy: injecting a harmonic signal of a preset amplitude and a 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 delay of a first preset time T, controlling its own output voltage to linearly increase from the current threshold voltage to the rated voltage according to a preset slope, where N is a positive integer.

[0049] According to one embodiment of the present invention, the slave machine specifically adopts the following steps to execute the second off-grid voltage raising strategy: obtaining 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 delaying for a second preset time D, controlling its own output voltage to linearly increase 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 an 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 a harmonic signal of a preset amplitude and a preset frequency into the fundamental voltage of the common coupling point PCC in different ways, the slave extracts the injection method of the harmonic signal 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 the embodiment of the present invention is suitable for an off-grid parallel system based on a non-communication droop control scheme. First, one of the PCSs is controlled to start at a threshold voltage far lower than the rated voltage to establish the PCC voltage. After the other PCSs are incorporated into the PCC, all PCSs then perform voltage synchronous raising to complete the off-grid synchronous start-up. This 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 startup, and does not rely on communication between PCSs, and will not be affected by communication delays, thereby improving the reliability of the off-grid synchronous start-up of the system. A harmonic injection combination method is used to meet the transmission of multiple status and command signal information between PCSs when there is no communication.

[0053] In the description of the present invention, the description with reference to the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms are not necessarily for the same embodiment or example. Any process or method description in a flow chart or otherwise described herein may be understood to represent a module, fragment or portion of a code including one or more executable instructions for implementing the steps of a custom logical function or process, and the scope of the preferred embodiments of the present invention includes additional implementations, in which functions may be performed in a non-sequential order as shown or discussed, including in a substantially simultaneous manner or in reverse order according to the functions involved, which should be understood by a technician in the technical field to which the embodiments of the present invention belong.

[0054] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0055] A person skilled in the art may understand that all or part of the steps in the method for implementing the above-mentioned embodiment may be completed by instructing related hardware through a program, and the program may be stored in a computer-readable storage medium, which, when executed, includes one or a combination of the steps of the method embodiment.

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

Claims

1. A method for starting an off-grid parallel system, characterized in that: The off-grid parallel system comprises: a plurality of PCSs connected in parallel on the same AC bus and a load mounted on the AC bus, a common coupling point PCC exists between the AC sides of the plurality of PCSs and the load, and the startup method comprises the following steps: Any one PCS of the off-grid parallel system is used as a host, and all other PCSs are used as slaves, and a first off-grid startup instruction is sent to the host. After receiving the first off-grid startup instruction, the host starts off-grid with a threshold voltage to increase the voltage of the common coupling point PCC to a threshold voltage, and the threshold voltage is lower than the rated voltage of the off-grid parallel system; Sending a second off-grid startup instruction to the slave machine, and the slave machine starts off-grid and merges into the common coupling point PCC after receiving the second off-grid startup instruction; After all PCSs are incorporated into the common coupling point PCC, a first off-grid voltage raising instruction is sent to the host, so that the host executes the first off-grid voltage raising strategy, and the slave executes the second off-grid voltage raising strategy in real time according to the voltage of the common coupling point PCC, so as to raise the AC side voltage of all PCSs to the rated voltage, and complete the off-grid synchronous start-up of all PCSs; The host injects a harmonic signal of a preset amplitude and a 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 signal injected by the host, and realizes information transmission with the host according to the injection method. The injection methods include: injection of only phase A, injection of only phase B, injection of only phase C, injection of AB phases at the same time, injection of AC phases at the same time, injection of BC phases at the same time, and injection of ABC phases at the same time. Different injection methods represent different instructions or signals.

2. The startup method of the off-grid parallel system according to claim 1, characterized in that: The host specifically implements the first off-grid voltage raising strategy by the following steps: Injecting a harmonic signal of a preset amplitude and a 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 delay of the first preset time T, the output voltage is controlled to linearly increase 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 implements the second off-grid voltage raising strategy by the following steps: Obtaining 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 delay of a second preset time D, the output voltage is controlled to linearly increase 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. An off-grid parallel system, characterized in that: include: Multiple PCSs connected in parallel on the same AC bus, loads mounted on the AC bus, and EMSs, a common coupling point PCC exists between the AC sides of multiple PCSs and the loads, the EMS communicates with the PCSs through a communication line, any one PCS of the off-grid parallel system is used as the host, and all other PCSs are used as slaves, wherein: After receiving the first off-grid start-up instruction sent by the EMS, the host starts off-grid with a threshold voltage to increase the voltage of the common coupling point PCC to the threshold voltage, and the threshold voltage is lower than the rated voltage of the off-grid parallel system. After the host increases the voltage of the common coupling point PCC to the threshold voltage, the EMS sends a second off-grid start-up instruction to the slave, and the slave starts off-grid and is incorporated into the common coupling point PCC after receiving the second off-grid start-up instruction. After all PCSs are incorporated into the common coupling point PCC, the EMS sends a first off-grid voltage raising instruction to the host, so that the host executes the first off-grid voltage raising strategy, and the slave executes the second off-grid voltage raising strategy in real time according to the voltage of the common coupling point PCC, so as to raise the AC side voltage of all PCSs to the rated voltage, and complete the off-grid synchronous start-up of all PCSs; The host injects a harmonic signal of a preset amplitude and a 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 signal injected by the host, and realizes information transmission with the host according to the injection method. The injection methods include: injection of only phase A, injection of only phase B, injection of only phase C, injection of AB phases at the same time, injection of AC phases at the same time, injection of BC phases at the same time, and injection of ABC phases at the same time. Different injection methods represent different instructions or signals.

6. The off-grid parallel system according to claim 5, characterized in that: The host specifically implements the first off-grid voltage raising strategy by the following steps: Injecting a harmonic signal of a preset amplitude and a 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 delay of the first preset time T, the output voltage is controlled to linearly increase from the current threshold voltage to the rated voltage according to a preset slope, where N is a positive integer.

7. The off-grid parallel system according to claim 6, characterized in that: The slave device specifically implements the second off-grid voltage raising strategy by the following steps: Obtaining 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 delay of a second preset time D, the output voltage is controlled to linearly increase 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.

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

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