An uninterruptible power supply method based on forced commutation of solid-state switching switch

Abstract

The present invention provides an uninterruptible power supply system based on forced commutation of a solid-state switching switch, comprising a solid-state switching switch, a diesel generator, an automatic switching switch, a rectifier unit, an energy storage unit, an inverter unit, a filter unit and a control system. The present invention provides an uninterruptible power supply system and method based on forced commutation of a solid-state switching switch, which can accelerate the forced commutation of an uninterruptible power supply system based on an automatic switching switch in an economic mode when a power grid fails. Compared with the forced commutation of thyristors using LC resonance, the forced commutation method reduces the control complexity and cost of the entire system, taking into account not only power grid failures and loads, but also key power quality issues such as voltage sags and non-unit power factors. At the same time, the system only needs one power supply, which reduces the requirements for power supply security on site and improves the applicability of the entire system.

Description

Technical Field

[0001] The invention relates to the technical field of power equipment control, and in particular to an uninterruptible power supply method based on forced commutation of a solid-state switching switch. Background Art

[0002] Uninterruptible power supply systems have been widely used in IT companies, commercial telephone companies, data centers, cloud computing centers and other industrial sites. At the same time, there is a strong demand for using uninterruptible power supply systems to provide more reliable and safe power supply for key links. Traditional uninterruptible power supply systems are divided into two types: offline and online. From the basic application principle, the uninterruptible power supply system is a power protection device that contains an energy storage device, with an inverter as the main component, and a stable voltage and frequency output. It is mainly composed of a rectifier, a battery, an inverter and a solid-state switching switch. Among them: (1) The rectifier is a rectifying device that converts AC into DC, which mainly provides energy to the inverter and charging voltage to the battery; (2) The battery is a device used by the uninterruptible power supply system to store electrical energy. It is composed of several batteries connected in series. Its capacity determines its discharge time. When the mains power is normal, the electrical energy is converted into chemical energy and stored in the battery. When the mains power fails, the chemical energy is converted into electrical energy and provided to the inverter or load; 3) The inverter is a device that converts DC into AC. It is composed of an inverter bridge, control logic and filter circuit; 4) The solid-state switching switch is a contactless AC switch composed of two thyristors connected in reverse parallel. Its closing and opening are controlled by a logic controller.

[0003] Such uninterruptible power supply systems provide customers with a complex choice of online mode and economic mode. In online mode, the load is powered by an inverter, so it provides excellent immunity to grid frequency fluctuations, but this also leads to an increase in its power electronic switch losses; the economic mode is a very cost-effective and convenient way to improve the efficiency of the uninterruptible power supply system because the load is powered by a solid-state switch. Then, the thyristor commutation problem of the solid-state switch during a grid fault must be fully solved to fully utilize the mechanism. Compared with gate-off or integrated gate-commutated thyristor systems, thyristor-based solid-state switches have many advantages such as low cost and low conduction losses, but the natural commutation mechanism of thyristors takes a long time to turn off. In addition, it is greatly affected by load and grid fault conditions, another power issue, which even prolongs the switching process from economic mode to online mode. However, after a mains disturbance occurs, a typical offline uninterruptible power supply system should take over the load within 1-5ms to resist any voltage interruption, so natural commutation cannot meet these requirements, thereby reducing the effectiveness of the economic mode. To solve the above problems, some scholars have proposed a pulse commutation solid-state switching switch method, which uses LC resonance to achieve forced commutation of thyristors. However, the design of additional system circuits increases the control complexity and cost of the system. Summary of the invention

[0004] In order to solve the above problems, the present invention provides an uninterruptible power supply method based on forced commutation of a solid-state switching switch, and establishes a control mechanism to accelerate the commutation of a solid-state switching switch system based on a thyristor. The method not only considers grid faults and load conditions, but also considers key power quality issues such as voltage sag and non-unity power factor. The specific technical solution is as follows:

[0005] An uninterruptible power supply system based on forced commutation of a solid-state switching switch, comprising a solid-state switching switch, a diesel generator, an automatic switching switch, a rectifier unit, an energy storage unit, an inverter unit, a filter unit and a control system;

[0006] The solid-state switch is connected in series between the AC power grid and the sensitive load, inputs the AC power grid, and outputs to the sensitive load;

[0007] The diesel generator has its output connected to an automatic switching switch;

[0008] The automatic switching switch is connected to the diesel generator, inputs the AC power grid, and outputs the AC to the rectifier unit;

[0009] The rectifier unit inputs the alternating current of the automatic switching switch and outputs direct current to the energy storage unit;

[0010] The energy storage unit variable is between the rectifier unit and the inverter unit and is used for energy storage;

[0011] The inverter unit inputs direct current from the energy storage unit and inverts and outputs alternating current;

[0012] The filter unit inputs the inverter unit, and the output AC power is filtered and output, and connected to the corresponding phase of the solid-state switch output to supply power to the three-phase sensitive load;

[0013] The control system is the control core of the uninterruptible power supply system, which inputs voltage and current signals from the grid side, load side, and inverter output side, and outputs control signals for solid-state switching switches, automatic switching switches, rectifier units, and inverter units.

[0014] Furthermore, the solid-state switching switch controls the A / B / C three-phase voltage, and each phase uses two thyristors connected in reverse parallel to form a contactless AC switch.

[0015] Furthermore, the rectification unit adopts a three-phase PWM rectifier.

[0016] Furthermore, the energy storage unit adopts a combination of a super capacitor and a battery.

[0017] Furthermore, the inverter unit adopts a three-phase PWM rectifier.

[0018] Furthermore, the filtering unit mainly filters out harmonics generated when the inverter unit is working, and the filtering unit adopts an LC low-pass filter.

[0019] Furthermore, the control system divides the control phase of the uninterruptible power supply system into a grid-connected operation phase, a fault detection phase, a forced commutation phase and an island operation phase;

[0020] The grid-connected operation stage is a stage in which the grid side operates normally without any faults, and the load current is provided by the grid side; the fault detection stage is a stage in which a fault has just occurred on the grid side and the load current is provided by the grid side; the forced commutation stage is a stage in which a fault has just occurred on the grid side and the load current is provided by the grid side and the AC side of the inverter unit; the island operation stage is a stage in which the load current is completely provided by the AC side of the inverter after a fault has occurred on the isolated grid side.

[0021] An uninterruptible power supply method based on forced commutation of a solid-state switching switch, using any of the above-mentioned uninterruptible power supply systems based on forced commutation of a solid-state switching switch, characterized in that the steps include:

[0022] Detect whether a fault occurs on the grid side: detect the three-phase voltages uga, ugb, and ugc on the grid side, transform them to obtain the synchronous reference rotating coordinate components ugd and ugq, and solve them through a digital filter to obtain the voltage measurement values ​​uFd and uFq. The voltage measurement values ​​uFd and uFq are compared with the voltage threshold uth to detect whether a fault occurs on the grid side;

[0023] Start the commutation mechanism: When the control system detects a fault, the forced commutation mechanism is immediately triggered, the drive signal of the solid-state switch changes from 1 to 0, and the diesel generator starts automatically. The automatic switch switches from the grid side to the diesel generator for power supply.

[0024] Furthermore, in the forced commutation stage, the current of the inverter unit tracks the load current through predictive current control, and the voltage of the inverter unit is used as a feedback item. When the current error is reduced to below the current threshold ith, the island operation stage will be transferred. When the grid fault is cleared, the island operation stage can be switched to the grid-connected operation stage.

[0025] Compared with the prior art, the beneficial effects are:

[0026] The present invention provides an uninterruptible power supply system and method based on forced commutation of a solid-state switching switch, which can accelerate the forced commutation of an uninterruptible power supply system based on an automatic switching switch in an economic mode when a power grid fails. Compared with the forced commutation of thyristors using LC resonance, the forced commutation method reduces the control complexity and cost of the entire system, taking into account not only power grid failures and loads, but also key power quality issues such as voltage sags and non-unit power factors. At the same time, the system only needs one power supply, which reduces the requirements for power supply security sites and improves the applicability of the entire system. In addition to providing power when the system fails, the uninterruptible power supply system can also provide reactive power compensation to improve the power factor of the load during normal operation. By cooperating with a diesel generator, uninterruptible power supply to the load can be achieved, and uninterruptible power supply can be provided for sensitive loads, which helps to improve the power supply quality of power supply users, large companies and residential communities. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0028] Figure 1 The block diagram of the uninterruptible power supply system based on forced commutation of solid-state switching switches;

[0029] Figure 2 It is a block diagram of the detection and control phase conversion of an uninterruptible power supply system based on forced commutation of a solid-state switching switch;

[0030] Figure 3 The waveforms of the forced commutation stage of the uninterruptible power supply system based on the forced commutation of the solid-state switching switch;

[0031] Figure 4 The figure is a control block diagram of the forced commutation mechanism of an uninterruptible power supply system based on forced commutation of a solid-state switching switch. DETAILED DESCRIPTION

[0032] The embodiments of the present disclosure are described in detail below with reference to the accompanying drawings.

[0033] The following describes the embodiments of the present disclosure through specific examples, and those skilled in the art can easily understand other advantages and effects of the present disclosure from the contents disclosed in this specification. Obviously, the described embodiments are only a part of the embodiments of the present disclosure, rather than all of the embodiments. The present disclosure can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present disclosure. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict. Based on the embodiments in the present disclosure, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present disclosure.

[0034] like Figure 1 As shown, an uninterruptible power supply system based on forced commutation of a solid-state switching switch is characterized in that it includes a solid-state switching switch, a diesel generator, an automatic switching switch, a rectifier unit, an energy storage unit, an inverter unit, a filter unit and a control system.

[0035] The solid-state switching switch is connected in series between the AC power grid and the sensitive load, inputs the AC power grid and outputs to the sensitive load; the diesel generator, whose output is connected to the automatic switching switch; the automatic switching switch is connected to the diesel generator, inputs the AC power grid, and outputs AC to the rectifier unit; the rectifier unit inputs the AC power of the automatic switching switch, and outputs DC to the energy storage unit; the energy storage unit variable is between the rectifier unit and the inverter unit, and is used for energy storage; the inverter unit inputs the DC power of the energy storage unit, and inverts and outputs AC power; the filter unit inputs the inverter unit, and the output AC power is filtered and output, and is connected to the corresponding phase of the solid-state switching switch output to supply power to the three-phase sensitive load; the control system is the control core of the uninterruptible power supply system, which inputs the voltage and current signals of the grid side, the load side, and the inverter output side, and outputs the control signals of the solid-state switching switch, the automatic switching switch, the rectifier unit and the inverter unit.

[0036] In this embodiment, the solid-state switch controls the A / B / C three-phase voltage, and two thyristors are connected in reverse parallel for each phase to form a contactless AC switch. According to the rated capacity of the uninterruptible power supply system of 250kVA, the calculation adopts the 400A movement of the integrated chassis, which fully meets the design requirements.

[0037] In this embodiment, the diesel generator mainly provides backup power for the uninterruptible power supply system. According to the rated capacity of the uninterruptible power supply system, a diesel generator set is designed with a rated capacity of 250kVA, a rated current of 360A, a steady-state voltage regulation rate of no more than ±1%, a voltage fluctuation rate of no more than ±0.5%, a voltage stabilization time of no more than 1s, a frequency stabilization time of no more than 3s, and a frequency fluctuation rate of no more than ±0.5. When the diesel generator is working, it can be incorporated into the rectifier unit without synchronization with the uninterruptible power supply system.

[0038] In this embodiment, the automatic transfer switch is used to switch between the AC power grid and the diesel generator. The automatic transfer switch is used, with a rated current of 400A and a switching time of 800ms, and is equipped with NSX plastic case, NS large plastic case or MT frame switch. Considering that it takes a certain amount of time for the diesel generator to start and output stably, the time coordination with the automatic transfer switch can fully meet the requirements of the uninterruptible power supply system.

[0039] In this embodiment, the rectifier unit adopts a three-phase PWM rectifier, which is convenient for connection with other electrical equipment such as the energy storage unit, and realizes the sinusoidalization of the grid-side current and the adjustable power factor. When it is detected that the voltage of the energy storage unit does not meet the requirements, the rectifier unit can be started to control the output DC voltage to charge the energy storage unit. In order to reduce the switching loss of the uninterruptible power supply system, the switching frequency of this uninterruptible power supply system adopts 5kHz.

[0040] In this embodiment, the energy storage unit uses a combination of supercapacitors and batteries, which is mainly considered based on the cost-effectiveness of the uninterruptible power supply system. When a fault occurs on the grid side, the supercapacitor and the battery are started at the same time to provide power to the load through the inverter unit, and forced to commutate at the same time, and enter island operation, and the diesel generator is started synchronously, and the automatic switching switch is switched to the diesel generator for power supply. The diesel generates electricity and charges the energy storage unit through the rectifier unit. After the diesel generator is stable, the battery power supply is exited, and the battery is operated offline to increase the life of the battery. When the fault of the uninterruptible power supply system disappears, the output of the inverter unit is turned off, and the solid-state switching switch is triggered to make the uninterruptible power supply system re-enter the grid-connected operation.

[0041] In this embodiment, the inverter unit adopts a three-phase PWM rectifier, and adopts a device with the same parameters as the rectifier unit. In addition to providing power for sensitive loads and forced commutation of solid-state switching switches when the power grid fails, the inverter unit can also provide reactive power compensation for sensitive loads when the uninterruptible power supply system is operating normally, and is used to provide power factors for sensitive equipment. In order to reduce the switching loss of the uninterruptible power supply system, the switching frequency of this uninterruptible power supply system adopts 5kHz.

[0042] In this embodiment, the filter unit mainly filters out the harmonics generated when the inverter unit is working. In a high-power uninterruptible power supply system, it is usually necessary to design a low switching frequency to meet the switching loss requirements, so the passive filter unit designs an LC low-pass filter. Among them, the filter inductor L is designed according to the 5% standard unit in the traditional voltage source inverter to achieve the maximum balance between the modulation utilization rate and the current output quality; the filter capacitor is generally large, connected in parallel with the load, and generates a compensation current. To solve this problem, the calculation of the additional capacitor current command is as follows:

[0043] ;

[0044] In the formula, is the load voltage change; is the instantaneous value change of voltage and current of each phase; k It is a constant, which mainly depends on the capacitor of the filter. If its parameters are unknown, it can be taken as 0.9. This uninterruptible power supply system uses 600uF filter capacitor and 0.162mH filter inductor.

[0045] Therefore, the overall command of the inverter can be obtained as:

[0046] ;

[0047] In the formula, is the load current.

[0048] In this embodiment, the control system divides the control phase of the uninterruptible power supply system into a grid-connected operation phase, a fault detection phase, a forced commutation phase and an island operation phase.

[0049] In economic mode, the inverter operates independently during the grid connection phase until a grid fault is detected. The controller of the control system can confirm the result of the fault detection within 5ms and accelerate the entire transfer process through a forced commutation mechanism.

[0050] The grid-connected operation stage is the stage where the grid side operates normally without any faults and the load current is provided by the grid side. .

[0051] The fault detection stage is the stage when a fault just occurs on the grid side and the load current is still provided by the grid side. .

[0052] The forced commutation stage is the stage when a fault just occurs on the grid side and the load current is provided by the grid side and the AC side of the inverter unit. .

[0053] The island operation stage is the stage where the load current is completely provided by the inverter AC side after a fault occurs on the isolated grid side. .

[0054] An uninterruptible power supply method based on forced commutation of a solid-state switching switch, the detailed steps are as follows:

[0055] The grid-connected operation stage is the stage where the grid side operates normally without any faults and the load current is provided by the grid side. .

[0056] The fault detection stage is the stage when a fault just occurs on the grid side and the load current is still provided by the grid side. .like Figure 2 It can be seen that by detecting the three-phase voltage on the grid side u ga 、u gb 、u gc , transform to obtain the synchronous reference rotation coordinate component u gd 、u gq , the voltage measurement value is obtained through digital filter solution u Fd 、u Fq , then the voltage measurement value u Fd 、u Fq With voltage threshold u th Compare and detect whether there is a fault on the grid side.

[0057] The formula for transforming the reference rotation coordinate components is as follows:

[0058]

[0059]

[0060] In the formula, x for g / i / l , g Represents the grid side, i Represents the inverter output side, l represents the load side, represents the grid frequency, represent x The three-phase voltage on the side, represent x The three-phase current on the side, Represents the voltage Quantity, Represents the current Quantity, Represents the voltage d、 q Quantity, Represents the current d, q Quantity.

[0061] The digital filter is mainly used to attenuate transient voltage and 120Hz voltage ripple caused by unbalanced sequence components. u Fd 、u Fq Convert to , with the voltage threshold u th Voltage threshold value when the grid side is in normal operation u th Is the upper limit and lower limit Voltage threshold u th The calculation formula is:

[0062] ;

[0063] In the formula, , are the voltages before the fault on the grid side d, q Components and composite values.

[0064] The forced commutation stage is the stage when a fault just occurs on the grid side and the load current is provided by the grid side and the AC side of the inverter unit. .like Figure 3 It can be seen that in phase 1 and phase 2, the load current is mainly provided by the grid side. Once a fault is detected, the forced commutation mechanism is immediately triggered, the drive signal of the solid-state switch changes from 1 to 0, and the diesel generator starts automatically, and the automatic switch switches from the grid side to the diesel generator for power supply. Due to the semi-controlled characteristics of the thyristor, the solid-state switch can only be turned off when the current flowing through it is reduced to zero. The current generated by the inverter unit can accelerate the turn-off of the internal thyristor.

[0065] like Figure 4 As shown, the load voltage, load current and inverter current are converted into corresponding synchronous rotating components respectively. sw It is the control signal for switching from the forced commutation phase to the island operation phase. sw = 0, i.e., the forced commutation stage, the current of the inverter unit tracks the load current through predictive current control, and the voltage of the inverter unit is used as a feedback item. When the current error is reduced to the current threshold i th The uninterruptible power supply system will transfer to sw =1, that is, the island operation stage. The specific control process is as follows: Figure 2 shown.

[0066] The performance of forced commutation depends entirely on the instantaneous current output capacity of the inverter unit. Based on Kirchhoff's voltage law KVL, the instantaneous current error of the inverter unit can be expressed as:

[0067] ;

[0068] In the formula, is the inverter output voltage reference value, is the sampling interval, L is the inductance of the filter unit.

[0069] Assuming that sampling delay and switching error are negligible, the output voltage of the inverter is for:

[0070] ;

[0071] In the formula, K is the compensation coefficient, generally taken as 0.9.

[0072] The dynamic compensation performance of the inverter unit can be expressed as:

[0073] ;

[0074] In the formula, the load current Expressed as load voltage Functional relationship .

[0075] Slave load current Expressed as load voltage From the functional relationship of , it can be seen that the forced commutation performance is greatly affected by the load voltage and current. In a three-phase system, the instantaneous values ​​of the voltage and current of each phase are not the same, so there is inconsistency in the closing drive signal of the solid-state switch. Based on this relationship, forced commutation can be discussed in two stages:

[0076] Phase 1: The solid-state switch changes from three-phase conduction to one-phase disconnection and the remaining two phases conduction. The superior performance of current compensation is required in the early stage of forced commutation to reduce the current passing through the solid-state switch. At this time, the inverter unit must bear the heavy burden brought by the adjustment, and it may even trigger the amplitude edge of the linear modulation. Taking the B-phase disconnection as an example, based on Kirchhoff's current law KCL, it can be obtained:

[0077] ;

[0078] At this time, the time required for this stage is expressed as .

[0079] Phase 2: The solid-state switching switch changes from one phase disconnected and the remaining two phases on to two phases disconnected and the remaining one phase on. Taking the disconnection of phases B and C as an example, based on Kirchhoff's current law KCL, we can get:

[0080] ;

[0081] At this time, the time required for this stage is expressed as .

[0082] Then the total time of forced commutation can be expressed as: ;

[0083] The island operation stage is the stage where the load current is completely provided by the inverter AC side after a fault occurs on the isolated grid side. The sensitive load is isolated from the faulty AC grid, so the inverter unit acts as a backup power source and adjusts the load voltage to operate at the rated amplitude and frequency. Figure 4 The control block diagram showing the island operation phase ( sw=1 ), all variables are converted to synchronous reference coordinates, and the traditional PI controller is used to adjust the voltage set value and the feedback load voltage signal The error between the two is calculated, and the current command generated is then input into the predictive current controller to obtain the modulation signal, and the corresponding gate drive signal is output. The island operation stage must be operated until the grid fault is cleared before switching to the normal grid-connected operation stage.

[0084] The present invention provides an uninterruptible power supply system and method based on forced commutation of a solid-state switching switch, which can accelerate the forced commutation of an uninterruptible power supply system based on an automatic switching switch in an economic mode when a power grid fails. Compared with the forced commutation of thyristors using LC resonance, the forced commutation method reduces the control complexity and cost of the entire system, taking into account not only power grid failures and loads, but also key power quality issues such as voltage sags and non-unit power factors. At the same time, the system only needs one power supply, which reduces the requirements for power supply security sites and improves the applicability of the entire system. In addition to providing power when the system fails, the uninterruptible power supply system can also provide reactive power compensation to improve the power factor of the load during normal operation. By cooperating with a diesel generator, uninterruptible power supply to the load can be achieved, and uninterruptible power supply can be provided for sensitive loads, which helps to improve the power supply quality of power supply users, large companies and residential communities.

[0085] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

[0086] The above is only for explaining the implementation mode of the present invention and is not intended to limit the present invention. For those skilled in the art, any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention without creative work should be included in the protection scope of the present invention.

Claims

1. An uninterruptible power supply method based on forced commutation of a solid-state switching switch, characterized in that: The steps of the uninterruptible power supply method include: Detect whether there is a fault on the grid side: Detect the three-phase voltage on the grid side u ga 、u gb 、u gc , transform to obtain the synchronous reference rotation coordinate component u gd 、u gq , the voltage measurement value is obtained through digital filter solution u Fd 、u Fq , voltage measurement value u Fd 、u Fq With voltage threshold u th Compare and detect whether a fault occurs on the grid side; Start the commutation mechanism: When the control system detects a fault, the forced commutation mechanism is immediately triggered, the drive signal of the solid-state switch changes from 1 to 0, and the diesel generator starts automatically, and the automatic switch switches from the grid side to the diesel generator for power supply; The uninterruptible power supply method based on forced commutation of a solid-state switching switch is performed by an uninterruptible power supply system, which includes a solid-state switching switch, a diesel generator, an automatic switching switch, a rectifier unit, an energy storage unit, an inverter unit, a filter unit and a control system; The solid-state switch is connected in series between the AC power grid and the sensitive load, inputs the AC power grid, and outputs to the sensitive load; The diesel generator has its output connected to an automatic switching switch; The automatic switching switch is connected to the diesel generator, inputs the AC power grid, and outputs the AC to the rectifier unit; The rectifier unit inputs the alternating current of the automatic switching switch and outputs direct current to the energy storage unit; The energy storage unit variable is between the rectifier unit and the inverter unit and is used for energy storage; The inverter unit inputs direct current from the energy storage unit and inverts and outputs alternating current; The filter unit inputs the inverter unit, and the output AC power is filtered and output, and connected to the corresponding phase of the solid-state switch output to supply power to the three-phase sensitive load; The control system is the control core of the uninterruptible power supply system, inputs voltage and current signals of the grid side, load side, and inverter output side, and outputs control signals of the solid-state switching switch, automatic switching switch, rectifier unit, and inverter unit; The control system divides the control phase of the uninterruptible power supply system into a grid-connected operation phase, a fault detection phase, a forced commutation phase and an island operation phase; The grid-connected operation stage is a stage in which the grid side operates normally without any faults, and the load current is provided by the grid side; the fault detection stage is a stage in which a fault has just occurred on the grid side and the load current is provided by the grid side; the forced commutation stage is a stage in which a fault has just occurred on the grid side and the load current is provided by the grid side and the AC side of the inverter unit; the island operation stage is a stage in which the load current is completely provided by the AC side of the inverter after a fault has occurred on the isolated grid side.

2. The uninterruptible power supply method based on forced commutation of a solid-state switching switch according to claim 1 is characterized in that: In the forced commutation phase, the current of the inverter unit tracks the load current through predictive current control, and the voltage of the inverter unit is used as a feedback term. When the current error is reduced to the current threshold i th When the power grid fault is cleared, the island operation stage can be switched to the grid-connected operation stage.

3. The uninterruptible power supply method based on forced commutation of a solid-state switching switch according to claim 1, characterized in that: The solid-state switch controls the A / B / C three-phase voltage, and each phase uses two thyristors connected in reverse parallel to form a contactless AC switch.

4. The uninterruptible power supply method based on forced commutation of a solid-state switching switch according to claim 1, characterized in that: The rectifying unit adopts a three-phase PWM rectifier.

5. The uninterruptible power supply method based on forced commutation of a solid-state switching switch according to claim 1, characterized in that: The energy storage unit is a combination of a super capacitor and a battery.

6. The uninterruptible power supply method based on forced commutation of a solid-state switching switch according to claim 1, characterized in that: The inverter unit adopts a three-phase PWM rectifier.

7. The uninterruptible power supply method based on forced commutation of a solid-state switch according to claim 1, characterized in that: The filtering unit mainly filters out the harmonics generated when the inverter unit is working, and the filtering unit adopts an LC low-pass filter.

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