Grid-connected and off-grid switching method and power converter

Abstract

The invention provides a grid-connected and off-grid switching method and a power converter, and relates to the field of power conversion. And in response to the abnormal state of the power grid, obtaining the inversion power of the alternating current side of the inversion circuit and the load power of the load port, and adjusting the output power of the direct current source under the condition that the inversion power is not matched with the load power so as to reduce the power difference value between the inversion power and the load power, thereby reducing the power difference value between the inversion power and the load power. At the moment, the load power is gradually provided by the power converter, and the current between the alternating-current power grid and the power converter is reduced, so that when the switch between the alternating-current power grid and the power converter is switched off, the degree of arc discharge of the switch can be reduced, damage to switch contacts is reduced, and the service life of the switch is prolonged.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power conversion, and more particularly, to a parallel-off-grid switching method and a power converter. BACKGROUND

[0002] The power converter has the ability to take power from the power grid or generate power to the power grid.

[0003] In actual scenarios, the AC side of the power converter is connected to the power grid through a switch. In the case of power grid anomalies, the power converter needs to switch to an off-grid operation state, at which time the switch needs to be disconnected. When the switch breaks the current, an arc phenomenon occurs, which damages the switch contacts and severely reduces the service life of the switch. SUMMARY

[0004] Therefore, the present application provides a parallel-off-grid switching method and a power converter to solve the problem of reduced service life of the switch caused by breaking the current of the switch.

[0005] To solve the above technical problems, the present application adopts the following technical solutions:

[0006] The first aspect of the present application provides a parallel-off-grid switching method, comprising:

[0007] Monitoring the grid state of the AC power grid connected to the parallel grid port of the parallel grid system; wherein the parallel grid system further comprises an inverter circuit, a parallel-off-grid switching switch and a load port, the DC side of the inverter circuit is used to connect a DC source, one end of the parallel-off-grid switching switch is connected to the AC side of the inverter circuit and the load port, and the other end of the parallel-off-grid switching switch is connected to the parallel grid port; the load port is used to connect a load;

[0008] In response to the abnormal grid state, obtaining the inverter power of the AC side of the inverter circuit and the load power of the load port;

[0009] In the case that the inverter power and the load power do not match, adjusting the output power of the DC source to reduce the power difference between the inverter power and the load power;

[0010] When the power difference meets a preset off-grid power condition, controlling the parallel-off-grid switching switch to be disconnected and switching the inverter circuit to operate in an off-grid mode.

[0011] In a possible implementation, the inverter power and the load power do not match, comprising:

[0012] The power difference between the inverter power and the load power is greater than a difference threshold value;

[0013] Or, a power variation ratio of the inverter power relative to the load power is greater than a ratio threshold.

[0014] In a possible implementation, the adjusting the output power of the DC power source to reduce the power difference between the inverter power and the load power comprises:

[0015] The output power of the DC power source is adjusted to control the inverter circuit to perform power output according to a specified power, the specified power being a power value between the inverter power and the load power.

[0016] In a possible implementation, the specified power is determined by:

[0017] Any power value between the inverter power and the load power is taken as the specified power.

[0018] Or, a safety power threshold when the on-grid / off-grid switch is off is obtained, a reference inverter power of the inverter circuit when the on-grid / off-grid switch flows a power equal to the safety power threshold is calculated, and any power value between the reference inverter power and the load power is taken as the specified power.

[0019] In a possible implementation, the adjusting the output power of the DC power source to control the inverter circuit to perform power output according to a specified power comprises:

[0020] The inverter power of the inverter circuit is adjusted to the specified power by using an inverter power control loop; and in the process of adjusting the inverter power of the inverter circuit to the specified power, the output power of the DC power source changes along with the change of the inverter power.

[0021] Or, the output power of the DC power source is adjusted by using a DC power source power loop to adjust the inverter power of the inverter circuit to the specified power.

[0022] In a possible implementation, the DC power source only includes a photovoltaic string.

[0023] The output power of the DC power source is adjusted by using a DC power source power loop to adjust the inverter power of the inverter circuit to the specified power, comprising:

[0024] In a case where the specified power is greater than the inverter power and the photovoltaic string performs power output according to a limited power, if the specified power is less than or equal to a maximum output power of the photovoltaic string, the photovoltaic string is controlled to perform power output according to the specified power, or if the specified power is greater than the maximum output power of the photovoltaic string, the photovoltaic string is controlled to perform power output according to the maximum output power.

[0025] in a case that the specified power is greater than the inverter power and the photovoltaic string outputs power according to the maximum output power, controlling the photovoltaic string to continue outputting power according to the maximum output power;

[0026] in a case that the specified power is less than the inverter power, controlling the photovoltaic string to output power according to a power limiting operation of the specified power.

[0027] In a possible implementation, the direct current source includes a photovoltaic string and an energy storage battery;

[0028] the output power adjustment of the direct current source by using the power loop of the direct current source to adjust the inverter power of the inverter circuit to the specified power, includes:

[0029] in a case that the specified power is greater than the inverter power, calculating a difference value between the specified power and the inverter power;

[0030] in a case that the energy storage battery is charging, controlling the charging power of the energy storage battery to decrease by the difference value, or in a case that the energy storage battery is discharging, controlling the charging power of the energy storage battery to increase by the difference value;

[0031] in a case that the specified power is less than the inverter power, limiting the output power of the photovoltaic string and / or adjusting the power of the energy storage battery to adjust the inverter power of the inverter circuit to the specified power.

[0032] In a possible implementation, limiting the output power of the photovoltaic string and / or adjusting the power of the energy storage battery, includes:

[0033] controlling the photovoltaic string to output power according to the specified power;

[0034] or, in a case that the energy storage battery is charging, acquiring a maximum charging power of the energy storage battery, in a case that the maximum charging power is greater than or equal to the specified power, controlling the charging power of the energy storage battery to increase by a difference value between the specified power and the inverter power, in a case that the maximum charging power is less than the specified power, controlling the charging power of the energy storage battery to increase by a difference value between the maximum charging power and the inverter power, and controlling the output power of the photovoltaic string to decrease by a difference value between the specified power and the maximum charging power;

[0035] Or, in the case of discharging the energy storage battery, acquiring a maximum discharging power of the energy storage battery, in the case that the maximum discharging power is greater than or equal to the specified power, controlling the discharging power of the energy storage battery to decrease by a difference value between the specified power and the inverter power; in the case that the maximum discharging power is less than the specified power, controlling the discharging power of the energy storage battery to decrease by a difference value between the maximum discharging power and the inverter power, and controlling the output power of the photovoltaic string to decrease by a difference value between the specified power and the maximum discharging power.

[0036] In a possible implementation, the controlling the on-off grid switching switch to be disconnected when the power difference value meets a preset off-grid power condition comprises:

[0037] In the case that the power difference value is within a set difference value range and reaches a disconnection time of the on-off grid switching switch, or only reaches the disconnection time of the on-off grid switching switch, outputting a preset level signal to the on-off grid switching switch, the preset level signal being used to control the on-off grid switching switch to perform a disconnection operation.

[0038] The second aspect of the present application provides a power converter, the power converter comprising an inverter circuit, an on-off grid switching switch, a grid-connected port and a load port, a direct current side of the inverter circuit being used to connect a direct current source, one end of the on-off grid switching switch being connected to an alternating current side of the inverter circuit and the load port, and the other end of the on-off grid switching switch being connected to the grid-connected port; the load port being used to connect a load;

[0039] The power converter is used for:

[0040] Monitoring a grid state of an alternating current grid connected to the grid-connected port;

[0041] In response to the grid state being abnormal, acquiring an inverter power of the alternating current side of the inverter circuit and a load power of the load port;

[0042] In the case that the inverter power and the load power are not matched in power, scheduling an output power of the direct current source to decrease a power difference value between the inverter power and the load power;

[0043] In the case that the power difference value meets a preset off-grid power condition, controlling the on-off grid switching switch to be disconnected, and switching the power converter to run in an off-grid mode.

[0044] The third aspect of the application provides a power converter, the power converter comprising an inverter circuit, a DC side of the inverter circuit being configured to be connected to a DC source, and an AC side of the inverter circuit being configured to be connected to an AC port of an on-off grid switching device; the power converter is communicatively connected to the on-off grid switching device; the on-off grid switching device further comprises an on-off grid switching switch, a grid-connected port and a load port, one end of the on-off grid switching switch being connected to the AC port of the inverter circuit and the load port, and the other end of the on-off grid switching switch being connected to the grid-connected port; the grid-connected port is configured to be connected to an AC power grid, and the load port is configured to be connected to a load.

[0045] The power converter is configured to:

[0046] monitor a grid state of the connected AC power grid;

[0047] in response to the grid state being abnormal, acquire an inverter power of the AC side of the inverter circuit and a load power of the load port;

[0048] in a case where the inverter power and the load power are not matched, schedule an output power of the DC source to reduce a power difference between the inverter power and the load power;

[0049] in a case where the power difference meets a preset off-grid power condition, switch the power converter to operate in an off-grid mode, and send a control instruction to the on-off grid switching device to make the on-off grid switching device control the on-off grid switching switch to be turned off.

[0050] The application provides an on-off grid switching method and a power converter. In the application, in response to the grid state being abnormal, an inverter power of the AC side of the inverter circuit and a load power of the load port are acquired, and in a case where the inverter power and the load power are not matched, the output power of the DC source is adjusted to reduce a power difference between the inverter power and the load power. At this time, the load power is gradually provided by the power converter, and the current between the AC power grid and the power converter is reduced, so that when the switch between the AC power grid and the power converter is turned off, the degree of arc generation of the switch can be reduced, the damage to the switch contact can be reduced, and the service life of the switch can be prolonged. BRIEF DESCRIPTION OF DRAWINGS

[0051] In order to more clearly illustrate the technical solutions in the embodiments of the application or the related art, the following will briefly introduce the drawings needed to be used in the embodiments or the related art description. Obviously, the drawings in the following description are only embodiments of the application, and for those skilled in the art, other drawings can be obtained without creative labor on the basis of the provided drawings.

[0052] Figure 1An electrical topology diagram of a photovoltaic system provided by an embodiment of the present application;

[0053] Figure 2 Another electrical topology diagram of a photovoltaic system provided by an embodiment of the present application;

[0054] Figure 3 A switch connection schematic diagram provided by an embodiment of the present application;

[0055] Figure 4 A structural schematic diagram of an inverter provided by an embodiment of the present application;

[0056] Figure 5 A structural schematic diagram of an inverter and grid-connected / off-grid switching device provided by an embodiment of the present application;

[0057] Figure 6 A flowchart of a grid-connected / off-grid switching method provided by an embodiment of the present application;

[0058] Figure 7 A structural schematic diagram of an inverter power control loop provided by an embodiment of the present application;

[0059] Figure 8 A structural schematic diagram of a photovoltaic string power loop provided by an embodiment of the present application;

[0060] Figure 9 A structural schematic diagram of a battery power loop provided by an embodiment of the present application. DETAILED DESCRIPTION

[0061] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of protection of the present application.

[0062] A power converter is a device capable of power conversion. Based on different functions of the power converter, the power converter can be divided into a rectifier for converting alternating current into direct current, an inverter for converting direct current into alternating current, etc., or can have a bidirectional converter for direct current-alternating current conversion.

[0063] Taking the power converter as an inverter for example, the inverter can be a photovoltaic inverter, an energy storage inverter, a photovoltaic and energy storage integrated inverter, etc.

[0064] Figure 1An electrical topology of a photovoltaic system constructed by a photovoltaic inverter. The photovoltaic system is usually composed of photovoltaic strings, inverters and other devices. The photovoltaic strings are usually composed of a plurality of photovoltaic components in series. The number of components in each photovoltaic string can be different or the same according to the difference in open-circuit voltage of a single photovoltaic component and the difference in input voltage of the inverter. The boost circuit Boost 1-Boost m in the inverter receives the power input by the corresponding photovoltaic string. After the boost processing in the boost circuit, the power is input to the inverter circuit through the positive electrode (BUS+) and the negative electrode (BUS-) of the DC bus for DC-to-AC operation. The power output by the grid-connected port of the inverter circuit is output to the power grid. In one implementation, the power output by the grid-connected port of the inverter circuit is subjected to voltage transformation by a transformer before being output to the power grid. In another implementation, the inverter circuit is directly connected to the power grid through a switch. A power meter or other device can also be arranged in the line from the inverter circuit to the power grid.

[0065] In the above formula, BUS+ and BUS- can be configured with a bus capacitor therebetween. Figure 1 In the above formula, m and x are positive integers, and m≥x.

[0066] In a system constructed by an energy storage inverter, the DC side of the inverter can no longer be connected to a photovoltaic string, but to an energy storage battery. In a photovoltaic system constructed by a photovoltaic and energy storage integrated inverter, the DC side of the inverter can be connected to both an energy storage battery (through a DC-DC converter) and a photovoltaic string. The structure diagram is shown in Figure 2

[0067] In actual scenarios, the photovoltaic system not only has the ability to generate power in parallel with the power grid, but also has the ability to independently operate with a load in the event of abnormal operation of the power grid, as shown in Figure 2 When the power grid is operating normally, the inverter operates in parallel with the power grid, and the load is powered by at least one of the inverter and the power grid.

[0068] Figure 2 In the above formula, the photovoltaic string can only output power to the inverter, the energy storage battery can output power to the inverter and can also obtain power from the inverter, and the inverter can obtain power from the power grid and can also output power to the power grid.

[0069] As shown in Figure 3 The inverter is provided with a relay K1. The line after the relay K1 can be connected to the power grid through a relay K2. In addition, in one implementation, the inverter can also be connected to a load through a relay K3. In one implementation, the line after the relay K1 can be directly connected to the load without passing through the relay K3. In one implementation, the line after the relay K1 can be connected to the power grid through the relay K2 without being connected to the load. ​

[0070] Among them, relays K2 and K3 can be installed inside the inverter or outside the inverter.

[0071] When the power grid is normal, the inverter operates in grid-connected mode, and the load can draw power from the inverter and / or the grid. During grid-connected operation, if the power matching between the inverter's DC side and the load side is inconsistent, the inverter may draw power from the grid or generate power for the grid. In this case, current flows through relay K2 on the grid side, such as... Figure 3 As shown.

[0072] When the power grid is abnormal, the inverter needs to switch to off-grid operation. At this time, relay K2 needs to be disconnected. When relay K2 is disconnected under current (also known as load disconnection), arcing will occur, which will damage the relay contacts. In severe cases, it may even cause the contacts to stick together and be damaged, reducing the life of the relay.

[0073] Therefore, in this embodiment of the application, before disconnecting relay K2, the current flowing through relay K2 can be reduced, thereby reducing the degree of relay arcing, reducing damage to switch contacts, and improving switch life.

[0074] Based on the above, this application provides a method for switching between grid connection and off-grid operation, wherein the executing entity can be a power converter.

[0075] In one implementation, such as Figure 4 As shown, a power converter, such as an inverter, includes an inverter circuit, a grid-connected / off-grid switching switch, a grid-connected port, and a load port. The grid-connected / off-grid switching switch can be a relay K2, or it can be a circuit breaker, etc.

[0076] The DC side of the inverter circuit is used to connect to a DC source, which can be at least one of the aforementioned photovoltaic strings and energy storage batteries. One end of the grid-connected / off-grid transfer switch is connected to the AC side of the inverter circuit and the load port. When connecting the load port, it can be directly connected or connected via relay K3. The load port is used to connect the load, and this application does not limit the specific type of load. The other end of the grid-connected / off-grid transfer switch is connected to the grid-connected port, which is used to connect to the AC power grid.

[0077] In another implementation, the connection structure of the power converter can also be as follows: Figure 5 As shown, the power converter includes an inverter circuit, and the DC side of the inverter circuit is used to connect to a DC source, which can be at least one of the photovoltaic string and the energy storage battery mentioned above.

[0078] The AC side of the inverter circuit is used to connect the AC port of the on-off grid switching device, and the power converter is in communication connection with the on-off grid switching device. The on-off grid switching device further comprises an on-off grid switching switch, a grid-connected port and a load port. One end of the on-off grid switching switch is connected to the AC port of the inverter circuit and the load port, and the other end of the on-off grid switching switch is connected to the grid-connected port. The grid-connected port is used to connect the AC power grid, and the load port is used to connect the load.

[0079] Regardless of the connection mode of the power converter, the on-off grid switching method in the embodiments of the present application can be applied.

[0080] In one implementation mode, as shown in Figure 6 , an on-off grid switching method can comprise:

[0081] S11, monitoring the grid state of the AC power grid connected to the grid-connected port of the grid-connected system.

[0082] The grid-connected system further comprises an inverter circuit, an on-off grid switching switch and a load port. The DC side of the inverter circuit is used to connect a DC source. One end of the on-off grid switching switch is connected to the AC side of the inverter circuit and the load port, and the other end of the on-off grid switching switch is connected to the grid-connected port. The load port is used to connect a load. The specific connection structure can refer to Figure 4 or Figure 5 .

[0083] In the embodiments, the inverter circuit can take power from the AC power grid or output power to the AC power grid. When the inverter circuit is connected to the AC power grid through the on-off grid switching switch, it can be connected through single-phase, split-phase or three-phase lines. The three-phase line can be balanced or unbalanced. The inverter circuit can be connected to the grid in the form of a current source or a voltage source.

[0084] When the AC power grid is abnormal, the on-off grid switching switch needs to be disconnected to avoid affecting the normal operation of the power converter. Therefore, the grid state of the AC power grid needs to be monitored in the embodiments of the present application. The grid state can be determined by collecting voltage, current and power information of the AC power grid. The grid state can be normal operation or abnormal operation. Abnormal operation can be islanding, over-frequency, under-frequency, over-voltage, under-voltage, imbalance, etc.

[0085] The abnormal operation has corresponding judgment conditions. Taking over-frequency as an example, when the grid frequency exceeds the rated value, it is considered that there is an over-frequency fault. At this time, the grid frequency can be detected to determine whether it is greater than the rated value to determine whether the grid is over-frequency. Other abnormal operation conditions also have corresponding detection conditions. When the corresponding detection conditions are met, the abnormal operation is determined.

[0086] S12, in response to the grid state being abnormal, acquiring the inverter power of the AC side of the inverter circuit and the load power of the load port.

[0087] When the grid state is in abnormal operation, in order to avoid the power converter from being affected, the grid-connected off-grid switch needs to be turned off at this time.

[0088] In actual scenarios, taking the inverter as an example, when the grid state is abnormal and the power of the DC side of the inverter and the load side does not match, the inverter may take power from the grid or generate power to the grid, at this time, as shown in Figure 3 , the grid-connected off-grid switch, such as the relay K2, has current flowing through it.

[0089] When the grid-connected off-grid switch is turned off, the switch is current breaking, at this time, arc phenomenon may occur, in order to reduce the degree of arc phenomenon, by Figure 3 , it can be seen that the AC side of the inverter is connected with the AC grid, at this time, the inverter power of the AC side of the inverter can be adjusted, so that the current flowing through the grid-connected off-grid switch gradually decreases, and after the current flowing through the switch decreases, the arc degree when the switch is turned off will be reduced.

[0090] When adjusting the inverter power of the inverter, the inverter power of the AC side of the inverter circuit and the load power of the load port need to be acquired.

[0091] Among them, the inverter power of the AC side of the inverter circuit can be calculated by collecting the voltage and current of the AC side of the inverter circuit.

[0092] For the load power of the load port, in one implementation manner, as shown in Figure 4 , if the load port is located inside the inverter, at this time, a voltage and current acquisition module can be arranged at the load port, the voltage and current of the load port are acquired through the module, and thus the load power of the load port is obtained based on the voltage and current of the port.

[0093] As shown in Figure 5 , if the load port is located inside the grid-connected off-grid switching device, at this time, a voltage and current acquisition module can be arranged at the load port, the voltage and current of the load port are acquired through the module, and thus the load power of the load port is obtained based on the voltage and current of the port, and the grid-connected off-grid switching device transmits the load power of the load port to the inverter through the communication line.

[0094] In another implementation manner, according to Figure 4 and Figure 5As shown, at node A, according to KCL (Kirchhoff's Current Law) law, the current output by the AC side of the inverter circuit is equal to the current of the AC power grid and the current of the load, so the inverter power output by the AC side of the inverter circuit is equal to the power of the AC power grid and the load power of the load port, and thus the load power of the load port can be indirectly calculated by using the inverter power of the AC side of the inverter circuit and the power of the AC power grid. At this time, the load power of the load port = the inverter power of the AC side of the inverter circuit - the power of the AC power grid.

[0095] In an implementation manner, the inverter power of the AC side of the inverter circuit and the power of the AC power grid is a vector value. In the case that the electric energy flows from the inverter circuit to the AC power grid, the inverter power of the AC side of the inverter circuit and the power of the AC power grid are positive, and in the case that the electric energy flows from the AC power grid to the inverter circuit, the inverter power of the AC side of the inverter circuit and the power of the AC power grid are negative. Alternatively, in the case that the electric energy flows from the inverter circuit to the AC power grid, the inverter power of the AC side of the inverter circuit and the power of the AC power grid are negative, and in the case that the electric energy flows from the AC power grid to the inverter circuit, the inverter power of the AC side of the inverter circuit and the power of the AC power grid are positive. The specific configuration can be performed according to the actual scene.

[0096] At this time, the AC power grid can send the power of the AC power grid to the power converter, and the power converter calculates the load power of the load port based on the inverter power of the AC side of the inverter circuit and the power of the AC power grid.

[0097] S13, in the case that the inverter power and the load power are not matched in power, adjusting the output power of the direct current source to reduce the power difference between the inverter power and the load power.

[0098] In the embodiment, in the case that the inverter power and the load power are not matched in power, it means that the inverter power is not equal to the load power, and at this time, the possible scene is that the inverter circuit outputs electric energy to the load and the AC power grid, or the AC power grid outputs electric energy to the load and the inverter circuit. Regardless of which case, there will be a current at the parallel-off grid switching switch, and when the parallel-off grid switching switch is disconnected, the switching switch belongs to current breaking, and at this time, arc phenomenon may occur. In order to reduce the degree of arc phenomenon, in the embodiment, the output power of the direct current source of the inverter circuit is adjusted, and at this time, the inverter power of the AC side of the inverter circuit is adjusted accordingly. The embodiment adjusts the output power of the direct current source for the purpose of reducing the power difference between the inverter power and the load power.

[0099] After the power difference between the inverter power and the load power decreases, it indicates that the inverter power gradually approaches the load power, and it also indicates that most of the load power is provided by the inverter circuit at this time, and the load power provided by the AC power grid gradually decreases, so that the current at the parallel-off-grid switching switch gradually decreases. Subsequently, when the parallel-off-grid switching switch is controlled to be turned off, compared with the mode of not adjusting the inverter power on the AC side, the current flowing through the parallel-off-grid switching switch is small, which reduces the damage of the arc degree to the contact.

[0100] In the embodiment of the application, in order to avoid the problem of unstable load power supply caused by too fast adjustment of the output power of the DC source, in the specific adjustment, in one implementation manner, a fixed amount can be adjusted each time, for example, 5KW of power is adjusted each time. In another implementation manner, the adjustment can be performed according to a fixed adjustment rate, and how to adjust can be configured according to actual conditions. During the adjustment process, the current flowing through the parallel-off-grid switching switch changes in the decreasing direction, gradually approaches the safety current threshold, and then is less than the safety current threshold, and finally is adjusted to 0A, thereby reducing the degree of current breaking of the switch and protecting the switch. In one implementation manner, when the current flowing through the parallel-off-grid switching switch cannot be minimized to 0A, since the current flowing through the switch changes in the decreasing direction, the switch is broken at a small current, and the degree of arc caused by current breaking is also reduced, thereby protecting the switch.

[0101] The embodiment of the application is simple to implement, and the change of the current flowing through the switch can be monitored by the current sensor.

[0102] On the basis of the embodiment, in one implementation manner, the power mismatch between the inverter power and the load power can be that:

[0103] The power difference between the inverter power and the load power is greater than a difference threshold.

[0104] The difference threshold can be configured according to actual conditions, and in one implementation manner, the difference threshold can be determined based on the disconnection safety threshold of the parallel-off-grid switching switch. When the current or power at the parallel-off-grid switching switch is less than the corresponding disconnection safety threshold, the damage to the contact caused by direct disconnection is not great, and the parallel-off-grid switching switch can be directly disconnected. When the current or power at the parallel-off-grid switching switch is greater than or equal to the corresponding disconnection safety threshold, the damage to the contact caused by direct disconnection is great, and the parallel-off-grid switching switch cannot be directly disconnected at this time.

[0105] After the disconnection safety threshold of the on-off switch is known, if the disconnection safety threshold is a safety current threshold, a safety power threshold at the on-off switch can be derived based on the safety current threshold, and the safety power threshold is the difference threshold in the embodiments of the present application. If the disconnection safety threshold is a safety power threshold, the safety power threshold is the difference threshold in the embodiments of the present application. In addition, the difference threshold can also be determined in other manners, such as being artificially specified or being calculated by other algorithms.

[0106] If the power difference between the inverter power and the load power is greater than the difference threshold, it is considered that the two do not match, and the output power adjustment operation of the DC source needs to be performed. If the power difference between the inverter power and the load power is not greater than the difference threshold, it is considered that the two match, at this time, most of the load power is provided by the inverter circuit, and the load power provided by the AC power grid is very small, at this time, the current at the on-off switch is small, and the damage to the contact caused by direct disconnection is not very large, at this time, the output power adjustment operation of the DC source does not need to be performed.

[0107] In another implementation manner, the power mismatch between the inverter power and the load power can also be that:

[0108] The power change ratio of the inverter power relative to the load power is greater than a ratio threshold.

[0109] The calculation formula of the power change ratio of the inverter power relative to the load power is:

[0110] (inverter power-load power) / load power.

[0111] The ratio threshold can also be configured according to the actual scene, as described above. When the power change ratio of the inverter power relative to the load power is greater than the ratio threshold, it is indicated that the inverter power and the load power differ greatly, at this time, it is considered that the two do not match, and the output power adjustment operation of the DC source needs to be performed. If the power change ratio of the inverter power relative to the load power is less than or equal to the ratio threshold, it is considered that the two match, at this time, most of the load power is provided by the inverter circuit, and the load power provided by the AC power grid is very small, at this time, the current at the on-off switch is small, and the damage to the contact caused by direct disconnection is not very large, at this time, the output power adjustment operation of the DC source does not need to be performed.

[0112] In the actual scene, the difference or the power change ratio can be selected according to the demand to perform the judgment operation of whether the inverter power and the load power match.

[0113] S14, when the power difference meets the preset off-grid power condition, the on-off switch is controlled to be disconnected, and the inverter circuit is switched to operate in the off-grid mode.

[0114] In this embodiment, the preset off-grid power condition is set for the protection of the on-off switch, and can be specifically configured according to actual conditions.

[0115] In an implementation, the power difference satisfies the preset off-grid power condition, including:

[0116] The power difference is within the set difference range and reaches the disconnection time of the on-off switch.

[0117] The set difference range is a range with a small value. In an example, the preset difference range is a range close to zero. In another example, the preset difference range is a range smaller than the disconnection safety threshold (e.g., a safety power threshold) of the on-off switch, so as to ensure that the current flowing through the on-off switch is within the safety current threshold range when the on-off switch is disconnected.

[0118] In an actual scenario, after determining that the grid state is abnormal, the on-off switch needs to be disconnected after a specified time, which is the disconnection time of the on-off switch. For example, the on-off switch is disconnected 1 minute after determining that the grid state is abnormal, and the disconnection time is 1 minute.

[0119] When the power difference is within the set difference range and reaches the disconnection time of the on-off switch, the on-off switch is disconnected at this time. The current at the on-off switch is small, the contact damage is small, and the safety of the on-off switch can be ensured.

[0120] In another implementation, the power difference satisfies the preset off-grid power condition, including:

[0121] Only the disconnection time of the on-off switch is reached.

[0122] In an actual scenario, if the adjustment capability of the DC source of the inverter circuit is large, the power difference between the inverter power and the load power can be quickly adjusted to be within the set difference range within the disconnection time of the on-off switch. At this time, the judgment rule of “the power difference is within the set difference range and reaches the disconnection time of the on-off switch” can be used.

[0123] However, another situation may exist in an actual scenario, that is, the adjustment capability of the DC source of the inverter circuit is small. In an example, the DC source is an energy storage battery. When the energy storage battery needs to increase the output power, if the energy of the energy storage battery is low and cannot provide a large power, the output power of the energy storage battery cannot be adjusted to quickly adjust the power difference between the inverter power and the load power to be within the set difference range within the disconnection time of the on-off switch. At this time, the judgment rule of “reaching the disconnection time of the on-off switch” can be set.

[0124] In the specific implementation, the output power of the DC power source is adjusted to reduce the power difference between the inverter power and the load power. Even if the adjustment capability of the output power of the DC power source is limited, the output power of the DC power source is still adjusted in the direction of reducing the power difference between the inverter power and the load power. When the disconnection time of the on-off switch is reached, compared with the case where the output power of the DC power source is not adjusted, the current flowing through the on-off switch when the on-off switch is disconnected is reduced, the degree of arc phenomenon generated by the on-off switch is reduced, the damage to the contact of the on-off switch is reduced, and the service life of the on-off switch is prolonged.

[0125] In the case where the power difference is within the set difference range and the disconnection time of the on-off switch is reached, or only the disconnection time of the on-off switch is reached, a preset level signal is output to the on-off switch, and the preset level signal is used to control the on-off switch to perform the disconnection operation.

[0126] The preset level signal can be a low-level signal. If the on-off switch is arranged in the inverter, the low-level signal can be output to the on-off switch through an I / O (Input / Output) port of a chip where the inverter circuit is arranged. After the on-off switch receives the low-level signal, the on-off switch performs the disconnection operation. If the on-off switch is arranged in the on-off switching device, a control instruction such as a low-level signal can be output to the on-off switching device through an I / O port of a chip where the inverter circuit is arranged. The on-off switching device controls the on-off switch to be disconnected.

[0127] After the on-off switch is disconnected, the inverter circuit is switched from the grid-connected state to the off-grid state. In the off-grid state, the inverter circuit is disconnected from the AC power grid, and the output voltage of the inverter only needs to maintain the voltage required by the load.

[0128] In the embodiment, in response to the abnormal grid state, the inverter power on the AC side of the inverter circuit and the load power of the load port are obtained. In the case where the inverter power and the load power are not matched, the output power of the DC power source is adjusted to reduce the power difference between the inverter power and the load power. At this time, the load power is gradually provided by the power converter, and the current between the AC power grid and the power converter is reduced. Therefore, when the switch between the AC power grid and the power converter is disconnected, the degree of arc phenomenon generated by the switch is reduced, the damage to the contact of the switch is reduced, and the service life of the switch is prolonged.

[0129] On the basis of any of the above embodiments, before the inverter power on the AC side of the inverter circuit and the load power of the load port are obtained, the current flowing through the on-off switch can be detected. In the case where the current is greater than a safety current threshold, the inverter power on the AC side of the inverter circuit and the load power of the load port are obtained.

[0130] In the embodiments of the present application, in the case where the AC side of the inverter circuit is connected to both the AC power grid and the load, the current of the AC power grid can be detected by the current sensor arranged on the AC power grid side, and the current flowing through the switch can also be detected by the current sensor arranged near the switch.

[0131] In actual scenarios, Figure 3 The relay K2 in the above formula can be provided with a corresponding safety current threshold, and the value of the safety current threshold can be determined according to the type and specifications of the relay. If the current flowing through the relay K2 when the relay K2 is disconnected is less than the safety current threshold, the influence of the current breaking on the relay is small, and the degree of arc is low. Therefore, in the embodiments of the present application, the output power of the DC power source can be adjusted only when the current flowing through the switch is greater than the safety current threshold. When the current flowing through the switch is less than or equal to the safety current threshold, the output power of the DC power source can not be adjusted, and the switch can be directly disconnected. In addition, when the current flowing through the switch is less than or equal to the safety current threshold, the output power of the DC power source can also be adjusted to better protect the switch.

[0132] On the basis of any of the above embodiments, when adjusting the output power of the DC power source, there needs to be an adjustment target to adjust the output power of the DC power source according to the adjustment target. In one implementation, the adjustment target can be a specified power, which refers to a power expected to be output by the inverter circuit. When the inverter circuit outputs power according to the specified power, the current when the off-grid switch performs the disconnection operation can be reduced.

[0133] At this time, adjusting the output power of the DC power source to reduce the power difference between the inverter power and the load power can be adjusting the output power of the DC power source to control the inverter circuit to output power according to the specified power.

[0134] The specified power has a numerical requirement, and specifically, the specified power is a power value between the inverter power and the load power.

[0135] When the state of the power grid is abnormal, the inverter power output by the inverter circuit is a specific value, and the load power is a specific value. If the specified power is set to be between the two, it means that the inverter power is adjusted in the direction of the load power, so that the proportion of the load power provided by the inverter circuit increases, and the proportion of the load power provided by the AC power grid decreases, which can reduce the current at the off-grid switch.

[0136] For example, the power converter outputs power to the load and the AC grid, the inverter power on the AC side of the inverter circuit in the power converter is 20KW, the power received by the load is 10KW, the power received by the AC grid is 10KW, when the inverter power on the AC side of the inverter circuit changes from 20KW to 10KW, the 10KW is only used to power the load, and the power flowing to the AC grid is zero, so the current flowing to the AC grid is also zero, that is, the current flowing through the relay K2 is zero, at this time, the relay is disconnected, and the arc degree is extremely low.

[0137] In the actual scene, if the 10KW required by the load is fixed, during the process that the inverter power on the AC side of the inverter circuit changes from 20KW to 10KW, the power delivered by the inverter circuit to the AC grid gradually decreases, so the current flowing through the relay K2 also gradually decreases, and the arc degree when the relay K2 is disconnected can be reduced to a certain extent. Therefore, in the embodiment of the application, the load power is calculated by voltage and current, or the load power is calculated by inverter power and AC grid power, and then a power value between the inverter power on the AC side of the inverter circuit and the load power is determined as the specified power, for example, a power value of 15KW in the process that the above-mentioned 20KW changes to 10KW, which can be used as the specified power.

[0138] In an implementation manner, any power value between the inverter power and the load power can be used as the specified power, for example, a power value between the inverter power and the load power can be randomly selected as the specified power.

[0139] In another implementation manner, the safe power threshold when the off-grid switch is disconnected is obtained, the reference inverter power of the inverter circuit when the power flowing through the off-grid switch is the safe power threshold is calculated, and any power value between the reference inverter power and the load power is used as the specified power.

[0140] The safety power threshold can be configured according to an actual scene. When the power flowing through the on-off switch is the safety power threshold, the damage of the on-off switch to the contact is small, and at this time, the reference inverter power of the inverter circuit can be calculated based on the safety power threshold. Specifically, the load power is known, and the safety power threshold at the off-grid switch is known. According to the principle of power conservation, the reference inverter power of the inverter circuit = the safety power threshold at the off-grid switch + the load power. Among them, the three powers are vectors and have positive and negative signs. For example, the inverter power of the inverter circuit on the AC side of the power converter is 20KW, the power received by the load is 10KW, and the safety power threshold at the off-grid switch is 5KW. In the case of power flowing from the power converter to the AC grid, the inverter power is positive; in the case of power flowing from the AC grid to the power converter, the inverter power is negative. As a reference, the reference inverter power of the inverter circuit = 5+10=15KW. At this time, the specified power is any power value between 15KW and 10KW. When the specified power is 15KW, the power at the off-grid switch is the safety power threshold, and at this time, the damage of the off-grid switch to the contact is small. When the specified power is less than 15KW, the power at the off-grid switch is less than the safety power threshold, and at this time, the damage of the off-grid switch to the contact is smaller.

[0141] After determining the specified power, the current or power of the DC source can be adjusted to change the inverter power of the inverter circuit in the direction of the specified power until the inverter circuit outputs power according to the specified power.

[0142] In this embodiment, two ways of determining the specified power are provided. The specified power can be selected according to the needs of the actual scene to control the inverter circuit to output power according to the specified power, reduce the current when the off-grid switch is turned off, and protect the off-grid switch.

[0143] On the basis of any of the above embodiments, in an implementation, adjusting the output power of the DC source can be divided into a direct adjustment mode and an indirect adjustment mode. The direct adjustment mode refers to directly adjusting the output power of the DC source. After the output power of the DC source is adjusted, the inverter power output by the AC side of the inverter circuit also changes accordingly because the power provided by the DC source to the inverter circuit changes. When the output power of the DC source is directly adjusted, the power loop of the DC source can be used to adjust the output power of the DC source to adjust the inverter power of the inverter circuit to the specified power.

[0144] The power loop can be a PID (Proportion Integration Differentiation) control loop. In an implementation, the DC source can be classified into two cases: only including photovoltaic strings, and including both photovoltaic strings and energy storage batteries.

[0145] For photovoltaic strings, a corresponding photovoltaic string power loop is provided, and for energy storage batteries, a corresponding battery power loop is provided. Therefore, the output power of photovoltaic strings and energy storage batteries can be adjusted through the photovoltaic string power loop and the battery power loop at the same time, and the inverter power on the AC side of the inverter circuit is passively accepted to change, so as to adjust the inverter power of the inverter circuit to the specified power. In addition, the output power of the photovoltaic string can also be adjusted through the photovoltaic string power loop only, so as to adjust the inverter power of the inverter circuit to the specified power.

[0146] The indirect adjustment mode refers to adjusting the inverter power on the AC side of the inverter circuit, so that the output power of the DC source changes with the change of the inverter power.

[0147] Specifically, the inverter power of the inverter circuit can be adjusted to the specified power by using the inverter power control loop. During the process of adjusting the inverter power of the inverter circuit to the specified power, the output power of the DC source (i.e. photovoltaic strings and energy storage batteries) is passively accepted to change, and will change with the change of the inverter power.

[0148] In an implementation, the inverter power control loop can refer to Figure 7 , wherein PinvRef is the specified power, PinvFbk is the real-time inverter power, IinvFbk is the real-time inverter current, and the controller can be a PI (Proportional-Integral Regulator) closed-loop regulator, a PR (Proportional-Resonant) closed-loop regulator, etc. Execution refers to driving semiconductor devices by using PWM (Pulse Width Modulation), and the semiconductor devices can include IGBT (Insulated Gate Bipolar Transistor), MOS (Metal Oxide Semiconductor), GAN (Gallium Nitride), etc. conventional devices or wide bandgap devices.

[0149] The operation principle of the inverter power control loop is as follows:

[0150] The controller calculates a specified current using the difference between PinvRef and PinvFbk, calculates a PWM control signal of the semiconductor device using the difference between the specified current and IinvFbk, and controls the on-off of the corresponding semiconductor device using the PWM control signal.

[0151] By directly scheduling the inverter power in this way, the inverter power is directly controlled to gradually approach the specified power and is finally adjusted to the specified power.

[0152] In this embodiment, two power adjustment modes are provided, so that a suitable power adjustment mode can be selected according to actual conditions to perform the adjustment operation of the inverter power.

[0153] On the basis of the above embodiment, the direct adjustment mode is introduced in the embodiments of the present application. In one implementation mode, the direct current source only includes a photovoltaic string. At this time, the output power of the direct current source is adjusted by using the power loop of the direct current source to adjust the inverter power of the inverter circuit to the specified power, which can be as follows:

[0154] In the case where the specified power is greater than the inverter power and the photovoltaic string outputs power according to the limited power, if the specified power is less than or equal to the maximum output power of the photovoltaic string, the photovoltaic string is controlled to output power according to the specified power, or if the specified power is greater than the maximum output power of the photovoltaic string, the photovoltaic string is controlled to output power according to the maximum output power.

[0155] Here, the specified power is greater than the inverter power, which means that the inverter power needs to be increased. The photovoltaic string outputs power according to the limited power, which means that the photovoltaic string does not output power according to the MPPT (Maximum Power Point Tracking), which may be due to the fact that the power required on the alternating current side of the inverter circuit is not very large, and the photovoltaic string does not need to output power according to the maximum power.

[0156] In the above case, if the inverter power needs to be increased, the power limitation of the photovoltaic string can be removed to increase the output power of the photovoltaic string.

[0157] If the specified power is less than or equal to the maximum output power of the photovoltaic string, it indicates that the photovoltaic string can provide the specified power, at this time, only part of the power limit can be removed, and finally the photovoltaic string is controlled to output power according to the specified power. If the specified power is greater than the maximum output power of the photovoltaic string, it indicates that the photovoltaic string cannot provide the specified power, at this time, the photovoltaic string can only be controlled to output power according to the maximum output power, and the inverter power on the AC side of the inverter circuit will be adjusted to the specified power direction. Since the power adjustment capability of the photovoltaic string is insufficient, the final inverter power will be less than the specified power.

[0158] In an implementation, in the case that the specified power is greater than the inverter power and the photovoltaic string outputs power according to the maximum output power, it indicates that there is no power limit for the photovoltaic string at this time, and the photovoltaic string outputs power according to the maximum output power. At this time, it is impossible to achieve the purpose of improving the output power of the photovoltaic string by removing the power limit, and the photovoltaic string can only be controlled to continue to output power according to the maximum output power. Since the specified power is greater than the maximum output power, the power adjustment capability of the photovoltaic string is insufficient, and the final inverter power will be less than the specified power.

[0159] In an implementation, in the case that the specified power is less than the inverter power, it indicates that the inverter power needs to be reduced, at this time, the photovoltaic string can be controlled to output power according to the specified power by the power limiting operation of the photovoltaic string.

[0160] In the specific power adjustment of the photovoltaic string, the structure of the photovoltaic string power loop used is referred to Figure 8 The photovoltaic string power loop has two structures, corresponding to two adjustment modes. Regardless of which adjustment mode, the voltage adjustment mode is adopted.

[0161] In the first structure, two controllers are provided. Among them, UpvRef is the target voltage, in the case that the specified power is greater than the inverter power, the photovoltaic string outputs power according to the limited power, and the specified power is less than or equal to the maximum output power of the photovoltaic string, UpvRef takes the value of the voltage corresponding to the specified power. In the case that the specified power is greater than the inverter power, the photovoltaic string outputs power according to the limited power, and the specified power is greater than the maximum output power of the photovoltaic string, UpvRef takes the value of the voltage corresponding to the maximum output power. In the case that the specified power is greater than the inverter power and the photovoltaic string outputs power according to the maximum output power, UpvRef takes the value of the voltage corresponding to the maximum output power. In the case that the specified power is less than the inverter power, UpvRef takes the value of the voltage corresponding to the specified power. UpvFbk is the real-time voltage of the photovoltaic string, and the difference between the two is calculated by the controller to obtain the target current. The difference between the target current and the actual current IpvFbk is calculated by the controller to obtain the PWM control signal of the semiconductor device in the boost circuit in the power converter, and the on-off of the corresponding semiconductor device is controlled by using the PWM control signal, so as to realize the power adjustment operation of the photovoltaic string.

[0162] In the second structure, one controller is provided. The difference between the target voltage and the real-time voltage is calculated by the controller to obtain the PWM control signal of the semiconductor device in the boost circuit in the power converter, and the on-off of the corresponding semiconductor device is controlled by using the PWM control signal, so as to realize the power adjustment operation of the photovoltaic string.

[0163] In actual scenarios, the structure and control mode of the photovoltaic string power loop can be selected according to the needs.

[0164] In this embodiment, the output power of the photovoltaic string can be adjusted by unlimiting and limiting the output power of the photovoltaic string, so as to adjust the inverter power on the AC side of the inverter circuit, and gradually adjust to the specified power.

[0165] In an implementation manner, the direct current source includes a photovoltaic string and an energy storage battery. The battery power loop of the energy storage battery can refer to the battery power loop shown in Figure 9 .

[0166] PbatRef is the target power of the energy storage battery, PbatFbk is the real-time power of the energy storage battery, and IbatFbk is the real-time current of the energy storage battery. The specific content executed by the controller can refer to other embodiments.

[0167] At this time, the output power adjustment of the direct current source by using the power loop of the direct current source to adjust the inverter power of the inverter circuit to the specified power can include:

[0168] 1) in the case that the specified power is greater than the inverter power, the difference between the specified power and the inverter power is calculated, in the case that the energy storage battery is charging, the charging power of the energy storage battery is controlled to decrease by the difference, or in the case that the energy storage battery is discharging, the charging power of the energy storage battery is controlled to increase by the difference.

[0169] Wherein, the specified power is greater than the inverter power, which means that the inverter power needs to be increased, and the difference between the specified power and the inverter power is the value of the inverter power that needs to be increased. When the direct current source includes both photovoltaic strings and energy storage batteries, the power adjustment capability of the energy storage battery is stronger and can quickly adjust the power, while the photovoltaic string is easily affected by light and thus has poor power adjustment capability and cannot quickly adjust the power. For this reason, in the case that both photovoltaic strings and energy storage batteries are provided, the energy storage battery is preferentially used for power adjustment operation. When adjusting the power of the energy storage battery, the charging and discharging conditions of the energy storage battery also need to be considered. In the case that the energy storage battery is charging, the inverter power needs to be increased, that is, the output power of the direct current side of the inverter circuit needs to be increased, and thus the charging power of the energy storage battery should be controlled to decrease by the difference. In the specific adjustment, Figure 9 PbatRef=PbatFbk-difference, after the power absorbed by the energy storage battery is reduced, the output power of the direct current side is increased by the difference, and thus the inverter power is increased by the difference. In the case that the energy storage battery is discharging, the discharging power of the energy storage battery is controlled to increase by the difference, and thus Figure 9 PbatRef=PbatFbk+difference, so that the output power of the direct current side is increased by the difference, and thus the inverter power is increased by the difference.

[0170] 2) in the case that the specified power is less than the inverter power, the output power of the photovoltaic string is limited, and / or the power of the energy storage battery is adjusted to adjust the inverter power of the inverter circuit to the specified power.

[0171] Specifically, the specified power is less than the inverter power, which means that the inverter power needs to be reduced. Although the power of the photovoltaic string cannot be quickly increased in the absence of power limitation, the output power of the photovoltaic string can be quickly reduced through power limitation operation, and thus the output power of the direct current side can be reduced by limiting the output power of the photovoltaic string, thereby indirectly reducing the inverter power. In a specific implementation, the photovoltaic string can be controlled to perform power limitation output operation according to the specified power, that is, the output power of the photovoltaic string is limited to the specified power, and thus Figure 8 UpvRef in the formula takes the value of the voltage corresponding to the specified power.

[0172] In addition, when the power is limited, the output power of the DC side can also be reduced by adjusting the power of the energy storage battery, thereby indirectly reducing the inverter power. In addition, when the adjustment capability of the photovoltaic string and the energy storage battery is insufficient, the output power of the photovoltaic string can also be limited, and the power of the energy storage battery is adjusted, thereby indirectly reducing the output power of the DC side and reducing the inverter power.

[0173] In an implementation, the energy storage battery has a maximum charging and discharging capability, the energy storage battery has a maximum charging power in a charging scenario, and the energy storage battery has a maximum discharging power in a discharging scenario.

[0174] In the case of charging the energy storage battery, the maximum charging power of the energy storage battery is obtained. When the maximum charging power is greater than or equal to a specified power, it indicates that the energy storage battery can provide the specified power. At this time, the charging power of the energy storage battery is controlled to increase by a difference between the specified power and the inverter power. The difference between the specified power and the inverter power is the power value required to be reduced by the inverter. After the charging power of the energy storage battery is increased, the output power of the DC side is reduced, thereby reducing the inverter power. When the maximum charging power is less than the specified power, it indicates that the energy storage battery cannot provide the specified power. At this time, the power of the photovoltaic string and the energy storage battery needs to be adjusted simultaneously. Since the power adjustment speed of the energy storage battery is high, the charging power of the energy storage battery can be first controlled to increase by a difference between the maximum charging power and the inverter power, that is, the energy storage battery increases the charging power according to the maximum charging capability, so as to reduce the output power of the DC side. Then, the output power of the photovoltaic string is controlled to decrease by a difference between the specified power and the maximum charging power. For example, the maximum charging power of the energy storage battery is A, the specified power is B, and the inverter power is C. The charging power of the energy storage battery should be controlled to increase |A-C|, and the output power of the photovoltaic string should be controlled to decrease |B-A|. At this time, the sum of the power reduced by the two is |B-C|.

[0175] In an implementation, in the case of discharging the energy storage battery, the maximum discharging power of the energy storage battery is obtained. When the maximum discharging power is greater than or equal to a specified power, it indicates that the energy storage battery can provide the specified power. At this time, the discharging power of the energy storage battery is controlled to decrease by a difference between the specified power and the inverter power. After the discharging power of the energy storage battery is reduced, the output power of the DC side is reduced, thereby reducing the inverter power. When the maximum discharging power is less than the specified power, it indicates that the energy storage battery cannot provide the specified power. At this time, the power of the photovoltaic string and the energy storage battery needs to be adjusted simultaneously, and the power of the energy storage battery is preferentially adjusted. The discharging power of the energy storage battery can be controlled to decrease by a difference between the maximum discharging power and the inverter power, and the output power of the photovoltaic string can be controlled to decrease by a difference between the specified power and the maximum discharging power. The specific implementation is opposite to the charging process described above.

[0176] It should be noted that, no matter how to adjust the power of the photovoltaic string and the energy storage device, the inverter power cannot be adjusted to the specified power, such as in bad weather, when the energy storage device is almost out of power, the power adjustment capability of the direct current side is poor, and the required power cannot be provided. At this time, the power provided to the alternating current side as close to the specified power as possible is only possible to reduce the current flowing through the switch.

[0177] In this embodiment, the adjustment of the inverter power can be realized based on the power adjustment operation of the photovoltaic string only, the adjustment of the inverter power can be realized based on the power adjustment operation of the energy storage battery only, and the adjustment of the inverter power can be realized by simultaneously performing the power adjustment operation of the photovoltaic string and the energy storage battery. The specific way can be configured according to the actual configuration.

[0178] On the basis of any of the above embodiments, in an actual scene, the power converter can be connected to the alternating current grid through a single-phase, split-phase or three-phase line. Among them, the three-phase line can be three-phase balanced or three-phase unbalanced. The split-phase can also be two-phase balanced or two-phase unbalanced. In order to be able to perform the power adjustment operation in the embodiments of the present application when each phase sequence is balanced or unbalanced, in the embodiments of the present application, the power is adjusted by adjusting each phase sequence respectively. At this time, the output power of the direct current side is the sum of the specified powers of each phase sequence of the alternating current side of the inverter circuit.

[0179] On the basis of the above embodiment of the grid-connected and off-grid switching method, the power converter in the embodiment of the present application further comprises an inverter circuit, a grid-connected and off-grid switching switch, a grid-connected port and a load port. The direct current side of the inverter circuit is used to connect a direct current source. One end of the grid-connected and off-grid switching switch is connected to the alternating current side of the inverter circuit and the load port. The other end of the grid-connected and off-grid switching switch is connected to the grid-connected port. The load port is used to connect a load. For specific structure, refer to Figure 4 .

[0180] The power converter is used for:

[0181] monitoring the grid state of the alternating current grid connected to the grid-connected port;

[0182] in response to the abnormal grid state, acquiring the inverter power of the alternating current side of the inverter circuit and the load power of the load port;

[0183] in the case that the inverter power and the load power do not match, scheduling the output power of the direct current source to reduce the power difference between the inverter power and the load power;

[0184] when the power difference meets a preset off-grid power condition, controlling the grid-connected and off-grid switching switch to be disconnected, and switching the power converter to run in an off-grid mode.

[0185] For specific implementation process, refer to the above corresponding description.

[0186] On the basis of the above-mentioned embodiments of the off-grid switching method, the power converter in the embodiments of the present application further comprises an inverter circuit, a direct current side of the inverter circuit is used to connect a direct current source, and an alternating current side of the inverter circuit is used to connect an alternating current port of the on-off grid switching device; the power converter is in communication connection with the on-off grid switching device; the on-off grid switching device further comprises an on-off grid switching switch, a grid-connected port and a load port, one end of the on-off grid switching switch is connected to the alternating current port of the inverter circuit and the load port, and the other end of the on-off grid switching switch is connected to the grid-connected port; the grid-connected port is used to connect an alternating current power grid, and the load port is used to connect a load, and the specific structure is referred to Figure 5 .

[0187] The power converter is used for:

[0188] monitoring a grid state of the connected alternating current power grid;

[0189] in response to an abnormal grid state, acquiring an inverter power of the alternating current side of the inverter circuit and a load power of the load port;

[0190] in the case that the inverter power and the load power do not match in power, scheduling an output power of the direct current source to reduce a power difference between the inverter power and the load power;

[0191] in the case that the power difference meets a preset off-grid power condition, switching the power converter to run in an off-grid mode and sending a control instruction to the on-off grid switching device to make the on-off grid switching device control the on-off grid switching switch to be disconnected.

[0192] The specific implementation process is referred to the above-mentioned corresponding description.

[0193] Another embodiment of the present application further provides a power conversion system comprising the above-mentioned power converter.

[0194] The embodiments of the present application further provide a computer program product comprising computer readable instructions, which, when executed on an electronic device, cause the electronic device to implement any one of the on-off grid switching methods provided by the embodiments of the present application.

[0195] The embodiments of the present application further provide a computer readable storage medium, which carries one or more computer programs, which, when executed by an electronic device, can cause the electronic device to implement any one of the on-off grid switching methods provided by the embodiments of the present application.

[0196] The foregoing description of the disclosed embodiments enables a person skilled in the art to make or use the application. Modifications of these embodiments will occur to persons of skill in the art, and that the generic principles defined herein can be applied to other embodiments without departing from the spirit or scope of the application. Therefore, the present application is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for switching between parallel and offline networks, characterized in that, include: The system monitors the grid status of the AC grid connected to the grid-connected port of the grid-connected system. The grid-connected system further includes an inverter circuit, a grid-connected / off-grid switching switch, and a load port. The DC side of the inverter circuit is used to connect to a DC source. One end of the grid-connected / off-grid switching switch is connected to the AC side of the inverter circuit and the load port, while the other end is connected to the grid-connected port. The load port is used to connect a load. In response to the abnormal grid condition, the inverter power on the AC side of the inverter circuit and the load power at the load port are obtained. If the inverter power and the load power are mismatched, adjust the output power of the DC source to reduce the power difference between the inverter power and the load power. When the power difference meets the preset off-grid power condition, the grid-connected / off-grid switching switch is opened, and the inverter circuit is switched to off-grid mode.

2. The on-grid / off-grid handover method according to claim 1, characterized in that, The power mismatch between the inverter power and the load power includes: The power difference between the inverter power and the load power is greater than the difference threshold. Alternatively, the ratio of the inverter power to the load power is greater than a ratio threshold.

3. The on-grid / off-grid handover method according to claim 1 or 2, characterized in that, Adjusting the output power of the DC source to reduce the power difference between the inverter power and the load power includes: Adjust the output power of the DC source to control the inverter circuit to output power at a specified power, which is a power value between the inverter power and the load power.

4. The on-grid / off-grid handover method according to claim 3, characterized in that, The process of determining the specified power includes: The specified power is any power value located between the inverter power and the load power. Alternatively, obtain the safe power threshold when the grid-connected / off-grid switching switch is open, calculate the reference inverter power of the inverter circuit when the power flowing through the grid-connected / off-grid switching switch is the safe power threshold, and take any power value between the reference inverter power and the load power as the specified power.

5. The on-grid / off-grid handover method according to claim 3, characterized in that, Adjusting the output power of the DC source to control the inverter circuit to output power according to a specified power includes: The inverter power of the inverter circuit is adjusted to the specified power using an inverter power control loop; wherein, during the process of adjusting the inverter power of the inverter circuit to the specified power, the output power of the DC source changes in accordance with the change in the inverter power; Alternatively, the output power of the DC source can be adjusted using the power loop of the DC source to adjust the inverter power of the inverter circuit toward the specified power.

6. The on-grid / off-grid handover method according to claim 5, characterized in that, The DC source consists only of photovoltaic strings; The method of adjusting the output power of the DC source using the power loop of the DC source to adjust the inverter power of the inverter circuit to the specified power includes: If the specified power is greater than the inverter power and the photovoltaic string outputs power according to the limited power, then if the specified power is less than or equal to the maximum output power of the photovoltaic string, the photovoltaic string is controlled to output power according to the specified power; or, if the specified power is greater than the maximum output power of the photovoltaic string, the photovoltaic string is controlled to output power according to the maximum output power. When the specified power is greater than the inverter power and the photovoltaic string outputs power at the maximum output power, the photovoltaic string is controlled to continue to output power at the maximum output power. When the specified power is less than the inverter power, the photovoltaic string is controlled to perform a power limiting output operation according to the specified power.

7. The on-grid / off-grid handover method according to claim 5, characterized in that, The DC source includes a photovoltaic string and an energy storage battery; The method of adjusting the output power of the DC source using the power loop of the DC source to adjust the inverter power of the inverter circuit to the specified power includes: If the specified power is greater than the inverter power, calculate the difference between the specified power and the inverter power; When the energy storage battery is charging, the charging power of the energy storage battery is controlled to decrease the difference, or when the energy storage battery is discharging, the charging power of the energy storage battery is controlled to increase the difference. If the specified power is less than the inverter power, limit the output power of the photovoltaic string, and / or adjust the power of the energy storage battery to adjust the inverter power of the inverter circuit toward the specified power.

8. The on-grid / off-grid handover method according to claim 7, characterized in that, Limiting the output power of the photovoltaic string and / or adjusting the power of the energy storage battery includes: Control the photovoltaic string to perform power limiting output operation according to the specified power; Alternatively, while the energy storage battery is charging, the maximum charging power of the energy storage battery is obtained. If the maximum charging power is greater than or equal to the specified power, the charging power of the energy storage battery is controlled to increase the difference between the specified power and the inverter power. If the maximum charging power is less than the specified power, the charging power of the energy storage battery is controlled to increase the difference between the maximum charging power and the inverter power, and the output power of the photovoltaic string is controlled to decrease the difference between the specified power and the maximum charging power. Alternatively, when the energy storage battery is discharging, the maximum discharge power of the energy storage battery is obtained; if the maximum discharge power is greater than or equal to the specified power, the discharge power of the energy storage battery is controlled to decrease the difference between the specified power and the inverter power; if the maximum discharge power is less than the specified power, the discharge power of the energy storage battery is controlled to decrease the difference between the maximum discharge power and the inverter power, and the output power of the photovoltaic string is controlled to decrease the difference between the specified power and the maximum discharge power.

9. The on-grid / off-grid handover method according to claim 1, characterized in that, The step of controlling the grid connection / disconnection switch to open when the power difference meets the preset off-grid power condition includes: When the power difference is within a set difference range and reaches the disconnection time of the grid-connected / off-grid switch, or only reaches the disconnection time of the grid-connected / off-grid switch, a preset level signal is output to the grid-connected / off-grid switch. The preset level signal is used to control the grid-connected / off-grid switch to perform a disconnection operation.

10. A power converter, characterized in that, The power converter includes an inverter circuit, a grid-connected / off-grid switching switch, a grid-connected port, and a load port. The DC side of the inverter circuit is used to connect to a DC source. One end of the grid-connected / off-grid switching switch is connected to the AC side of the inverter circuit and the load port, and the other end of the grid-connected / off-grid switching switch is connected to the grid-connected port. The load port is used to connect to a load. The power converter is used for: Monitor the grid status of the AC power grid connected to the grid connection port; In response to the abnormal grid condition, the inverter power on the AC side of the inverter circuit and the load power at the load port are obtained. When the inverter power and the load power are mismatched, the output power of the DC source is adjusted to reduce the power difference between the inverter power and the load power. When the power difference meets the preset off-grid power condition, the grid-connected / off-grid switching switch is opened, and the power converter is switched to off-grid mode.

11. A power converter, characterized in that, The power converter includes an inverter circuit. The DC side of the inverter circuit is used to connect to a DC source, and the AC side of the inverter circuit is used to connect to the AC port of the grid-connected / off-grid switching device. The power converter is communicatively connected to the grid-connected / off-grid switching device. The grid-connected / off-grid switching device further includes a grid-connected / off-grid switching switch, a grid-connected port, and a load port. One end of the grid-connected / off-grid switching switch is connected to the AC port of the inverter circuit and the load port, and the other end of the grid-connected / off-grid switching switch is connected to the grid-connected port. The grid-connected port is used to connect to the AC power grid, and the load port is used to connect to a load. The power converter is used for: Monitor the status of the connected AC power grid; In response to the abnormal grid condition, the inverter power on the AC side of the inverter circuit and the load power at the load port are obtained. When the inverter power and the load power are mismatched, the output power of the DC source is adjusted to reduce the power difference between the inverter power and the load power. When the power difference meets the preset off-grid power condition, the power converter is switched to off-grid mode and a control command is sent to the on-grid switching device so that the on-grid switching device controls the on-grid switching switch to open.

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