Power grid power failure protection method and device for NPC three-level grid-connected inverter

Abstract

The invention relates to the technical field of three-phase photovoltaic grid connection and energy storage, and discloses an NPC three-level grid-connected inverter power grid power failure protection method and device. Obtaining a voltage average value under the alpha-beta coordinate system, a previous voltage average value under the alpha-beta coordinate system, a voltage average value under the dq coordinate system and a voltage slow average value under the dq coordinate system; judging whether the power grid voltage drops or not according to the difference value between the voltage average value of the previous beat under the alpha-beta coordinate system and the voltage average value under the alpha-beta coordinate system; judging whether the power grid is powered off according to the voltage average value under the alpha-beta coordinate system, the voltage average value of the previous beat under the alpha-beta coordinate system and the voltage average value under the dq coordinate system; when the power grid is powered off, the topology drive of the NPC three-level grid-connected inverter is closed. By judging the drop and amplitude of the voltage parameters of the power grid, the power failure of the power grid and the condition of large load on the grid side are identified, and the NPC three-level drive can be quickly closed.

Description

Technical Field

[0001] The present invention relates to the technical field of three-phase photovoltaic grid-connected and energy storage technology, and in particular to a power outage protection method and device for an NPC three-level grid-connected inverter. Background Art

[0002] At present, in three-phase grid-connected systems such as photovoltaics and energy storage, three-phase grid-connected inverters mostly use the neutral point clamped (NPC) three-level topology. Compared with two-level inverters, three-level inverters have lower output harmonics and higher efficiency, and have gradually become the mainstream circuit topology. If a power outage occurs during the inverter grid-connected operation, due to the existence of grid-side loads, an island operation state will be formed before the grid-connected inverter island or grid voltage and frequency protection, and the grid-connected machine will continue to operate for a period of time.

[0003] When the power grid goes out, if the load power connected to the transformer on the grid side of the machine is much greater than the grid-connected inverter power, the voltage at the machine grid-connected point will tend to 0V. The inverter output energy cannot support the grid, which will cause the load-side voltage to drop sharply, which is similar to a "short circuit". The current control detection has a long detection time for grid-side voltage loss in this working condition, resulting in the machine failing to sample the output voltage for a period of time and continuing to output, resulting in the grid-connected control out of control and the inverter switch tube overvoltage failure. Summary of the invention

[0004] In view of this, the present invention provides a method and device for protecting a power grid from power failure of an NPC three-level grid-connected inverter, so as to solve the problem that the current control detection has a long detection time for grid-side voltage loss under the above working conditions.

[0005] In a first aspect, the present invention provides a power outage protection method for an NPC three-level grid-connected inverter, the method comprising:

[0006] The three-phase grid voltage is processed to obtain the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, the voltage average value in the dq coordinate system, and the voltage slow average value in the dq coordinate system;

[0007] Determine whether the grid voltage has dropped based on the difference between the previous beat voltage average value in the αβ coordinate system and the voltage average value in the αβ coordinate system;

[0008] Determine whether the power grid is out of power based on the average voltage in the αβ coordinate system, the average voltage of the previous beat in the αβ coordinate system, and the average voltage in the dq coordinate system;

[0009] When the grid fails, the NPC three-level grid-connected inverter topology drive is closed.

[0010] The present invention provides a NPC three-level grid-connected inverter power outage protection method, including: processing the three-phase grid voltage to obtain the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, the voltage average value in the dq coordinate system, and the voltage slow average value in the dq coordinate system; judging whether the grid voltage drops according to the difference between the previous beat voltage average value in the αβ coordinate system and the voltage average value in the αβ coordinate system; judging whether the grid is out of power according to the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, and the voltage average value in the dq coordinate system; when the grid is out of power, closing the NPC three-level grid-connected inverter topology drive. According to the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, the voltage average value in the dq coordinate system, and the voltage slow average value in the dq coordinate system, the grid voltage drop state and whether the grid is out of power are detected, which is much faster than the normal island detection and the grid voltage frequency detection speed, and the NPC three-level drive can be quickly closed to avoid the risk of stress exceeding the specification failure of the inverter switch tube.

[0011] In an optional embodiment, the method comprises:

[0012] Determine whether the power grid has recovered based on the three-phase power grid voltage, the average voltage in the αβ coordinate system, the average voltage of the previous beat in the αβ coordinate system, the average voltage in the dq coordinate system, and the slow average voltage in the dq coordinate system;

[0013] When the power grid is restored, the NPC three-level grid-connected inverter topology drive is turned on.

[0014] In an optional implementation, the three-phase grid voltage is processed to obtain a voltage average value in the αβ coordinate system, a previous beat voltage average value in the αβ coordinate system, a voltage average value in the dq coordinate system, and a voltage slow average value in the dq coordinate system, including:

[0015] Acquire a three-phase grid voltage, and perform coordinate transformation on the three-phase grid voltage to obtain a voltage in an αβ coordinate system and a voltage in a dq coordinate system;

[0016] Perform multiple moving average calculations on the voltage in the αβ coordinate system and the voltage in the dq coordinate system to obtain the average voltage in the αβ coordinate system, the average voltage of the previous beat in the αβ coordinate system, and the average voltage in the dq coordinate system;

[0017] The voltage average value in the dq coordinate system is calculated by moving average multiple times to obtain a slow average value of the voltage in the dq coordinate system.

[0018] In an optional implementation, judging whether the grid voltage drops according to the difference between the previous beat voltage average value in the αβ coordinate system and the voltage average value in the αβ coordinate system includes:

[0019] Determine whether the difference between the previous beat voltage average of the α-axis and the α-axis voltage average is greater than a first threshold, and simultaneously determine whether the difference between the previous beat voltage average of the β-axis and the β-axis voltage average is greater than the first threshold;

[0020] When the difference between the previous beat voltage average of the α-axis and the α-axis voltage average is greater than a first threshold and the difference between the previous beat voltage average of the β-axis and the β-axis voltage average is greater than a first threshold, it is determined that a grid voltage drop occurs.

[0021] In an optional implementation, judging whether the power grid is out of power according to the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, and the voltage average value in the dq coordinate system includes:

[0022] Determine whether the average voltage of the α-axis is less than a second threshold value, and determine whether the average voltage of the β-axis is less than a second threshold value;

[0023] When the average voltage of the α-axis is less than the second threshold value and the average voltage of the β-axis is less than the second threshold value, it is determined that the voltage drop amplitude exceeds the limit;

[0024] Determining whether the d-axis voltage average value is less than a third threshold;

[0025] The number of times the accumulated d-axis voltage average value is less than a third threshold value;

[0026] When the number is greater than a fourth threshold, it is determined that a power outage occurs in the power grid.

[0027] In an optional implementation, judging whether the power grid is out of power according to the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, and the voltage average value in the dq coordinate system further includes:

[0028] Determine whether the average voltage of the α-axis is less than a second threshold value, and determine whether the average voltage of the β-axis is less than a second threshold value;

[0029] When the average voltage of the α-axis is less than the second threshold value and the average voltage of the β-axis is less than the second threshold value, it is determined that the voltage drop amplitude exceeds the limit;

[0030] Determine whether the d-axis voltage average value is less than a fifth threshold value, and determine whether the voltage drops;

[0031] The number of times the accumulated d-axis voltage average value is less than the fifth threshold and the voltage drops;

[0032] When the number is greater than a fourth threshold, it is determined that a power outage occurs in the power grid.

[0033] In an optional implementation, judging whether the power grid is restored according to the three-phase power grid voltage, the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, the voltage average value in the dq coordinate system, and the voltage slow average value in the dq coordinate system includes:

[0034] Determine whether the slow average value of the d-axis voltage is greater than a sixth threshold, and determine whether the three-phase grid voltage is greater than the sixth threshold;

[0035] The number of times that the accumulated d-axis voltage slow average value is greater than the sixth threshold and the three-phase grid voltage is greater than the sixth threshold;

[0036] When the number is greater than a seventh threshold, it is determined that the power grid is restored.

[0037] In a second aspect, the present invention provides a power grid power failure protection device for an NPC three-level grid-connected inverter, the device comprising:

[0038] A coordinate conversion module is used to process the three-phase grid voltage to obtain the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, the voltage average value in the dq coordinate system, and the voltage slow average value in the dq coordinate system;

[0039] A first judgment module is used to judge whether the grid voltage drops according to the difference between the previous beat voltage average value in the αβ coordinate system and the voltage average value in the αβ coordinate system;

[0040] The second judgment module is used to judge whether the power grid is out of power according to the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, and the voltage average value in the dq coordinate system;

[0041] The power failure drive module is used to close the NPC three-level grid-connected inverter topology drive when the power grid fails.

[0042] The present invention provides an NPC three-level grid-connected inverter power grid power failure protection device, comprising: a coordinate conversion module, used for processing a three-phase grid voltage to obtain a voltage average value in an αβ coordinate system, a previous beat voltage average value in the αβ coordinate system, a voltage average value in a dq coordinate system, and a voltage slow average value in the dq coordinate system; a first judgment module, used for judging whether the grid voltage drops according to a difference between a previous beat voltage average value in the αβ coordinate system and a voltage average value in the αβ coordinate system; a second judgment module, used for judging whether the grid is powered off according to the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, and the voltage average value in the dq coordinate system; and a power failure drive module, used for closing a NPC three-level grid-connected inverter topology drive when the grid is powered off. The voltage drop state of the grid and whether the grid is out of power are detected according to the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, the voltage average value in the dq coordinate system and the slow voltage average value in the dq coordinate system. Compared with normal island detection, the grid voltage frequency detection speed is greatly improved, and the NPC three-level drive can be quickly turned off to avoid the risk of failure of the inverter switch tube due to stress exceeding the specification.

[0043] In a third aspect, the present invention provides a computer device, comprising: a memory and a processor, the memory and the processor are communicatively connected to each other, the memory stores computer instructions, and the processor executes the NPC three-level grid-connected inverter power grid power outage protection method of the first aspect or any corresponding embodiment thereof by executing the computer instructions.

[0044] In a fourth aspect, the present invention provides a computer-readable storage medium having computer instructions stored thereon, the computer instructions being used to enable a computer to execute the NPC three-level grid-connected inverter power grid power outage protection method of the above-mentioned first aspect or any corresponding embodiment thereof. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0046] Figure 1 It is a schematic diagram of the high voltage blackout strategy of the three-level grid-connected system;

[0047] Figure 2 It is the equivalent diagram of high voltage power outage in three-level grid-connected system and the schematic diagram of voltage change at ABC points;

[0048] Figure 31 is a flow chart of a method for protecting a power grid from power outage of an NPC three-level grid-connected inverter according to an embodiment of the present invention;

[0049] Figure 4 A power outage protection strategy for a three-level grid-connected system according to an embodiment of the present invention;

[0050] Figure 5 is a structural block diagram of a power grid power failure protection device of an NPC three-level grid-connected inverter according to an embodiment of the present invention;

[0051] Figure 6 It is a schematic diagram of the hardware structure of a computer device according to an embodiment of the present invention. DETAILED DESCRIPTION

[0052] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0053] like Figure 1 As shown, when the grid-connected power of the three-phase grid-connected inverter is relatively small relative to the load, the output current Ia / Ib / Ic in the system is small and cannot support the load power of the load Ra / Rb / Rc. The grid-end current Ir / Is / It will provide a very large current to stabilize the load power and the voltage at points A / B / C, that is, the corresponding current of the three-phase load is: Ira=Ia+Ir, Irb=Ib+Is, Irc=Ic+It. Since the grid-connected inverter output does not control the voltage, it can be equivalent to a current source, and the grid voltage support capability is very strong, so it can be equivalent to a voltage source. When S1 is disconnected, it is equivalent to cutting off the circuit of the voltage source. At this time, the inverter output power is very small and is not enough to drive the load. See Figure 2 The load-end voltage is Ua=Ia*Ra, Ub=Ib*Rb, and Uc=Ic*Rb. At the moment of power failure, the voltage at points A / B / C will drop rapidly to 0V. This phenomenon will cause the inverter switch tube to have the risk of failure due to stress exceeding the specification.

[0054] To this end, an embodiment of the present invention provides a power outage protection method for an NPC three-level grid-connected inverter. Figure 3 is a flow chart of a power outage protection method for an NPC three-level grid-connected inverter according to an embodiment of the present invention. Figure 3 As shown, the process includes the following steps:

[0055] Step S1, processing the three-phase grid voltage to obtain the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, the voltage average value in the dq coordinate system, and the voltage slow average value in the dq coordinate system.

[0056] Specifically, step S1 includes:

[0057] Step S11, acquiring the three-phase grid voltage, and performing coordinate transformation on the three-phase grid voltage to obtain the voltage in the αβ coordinate system and the voltage in the dq coordinate system.

[0058] Step S12, performing multiple moving average calculations on the voltage in the αβ coordinate system and the voltage in the dq coordinate system to obtain the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, and the voltage average value in the dq coordinate system.

[0059] Step S13, performing multiple moving average calculations on the voltage average value in the dq coordinate system to obtain a slow average value of the voltage in the dq coordinate system.

[0060] In an embodiment of the present invention, the three-phase grid voltages Uan, Ubn, and Ucn are collected by an NPC three-level grid-connected inverter to obtain the three-phase grid voltages Uan, Ubn, and Ucn. Clark transformation and Park transformation are performed on the three-phase grid voltages Uan, Ubn, and Ucn to obtain the voltages Ualpha and Ubeta in the αβ coordinate system and the voltage Ud in the dq coordinate system. The voltages Ualpha and Ubeta in the αβ coordinate system and the voltage Ud in the dq coordinate system are calculated by N1 moving averages to obtain the voltage average values ​​UalphaAvg and UbetaAvg in the αβ coordinate system, the previous beat voltage average values ​​UalphaAvgPre and UbetaAvgPre in the αβ coordinate system, and the voltage average value UdAvg in the dq coordinate system. The voltage average value UdAvg in the dq coordinate system is calculated by N2 moving averages to obtain the slow voltage average value UdAvgSlow in the dq coordinate system. Among them, N1 and N2 are determined according to the actual sampling and control design scheme of the NPC three-level grid-connected inverter. In the embodiment of the present invention, the switching frequency of the NPC three-level grid-connected inverter is 16 kHz, N1 is taken as 4 times, and N2 is taken as 320 times. This is only taken as an example and is not limited thereto.

[0061] Step S2, judging whether the grid voltage drops according to the difference between the previous beat voltage average value in the αβ coordinate system and the voltage average value in the αβ coordinate system.

[0062] Specifically, step S2 includes:

[0063] Step S21, determining whether the difference between the α-axis previous beat voltage average value UalphaAvgPre and the α-axis voltage average value UalphaAvg is greater than a first threshold value Uslope, and determining whether the difference between the β-axis previous beat voltage average value UbetaAvgPre and the β-axis voltage average value UbetaAvg is greater than a first threshold value Uslope.

[0064] Step S22, when the difference between the α-axis previous beat voltage average value UalphaAvgPre and the α-axis voltage average value UalphaAvg is greater than the first threshold Uslope and the difference between the β-axis previous beat voltage average value UbetaAvgPre and the β-axis voltage average value UbetaAvg is greater than the first threshold Uslope, it is determined that the grid voltage has dropped.

[0065] In the embodiment of the present invention, the power outage protection strategy of the three-level grid-connected system is as follows: Figure 4 As shown, first determine whether (UalphaAvgPre-UalphaAvg)>Uslope is satisfied. When (UalphaAvgPre-UalphaAvg)>Uslope is satisfied, set UalphaFallFlag=1; then determine whether (UbetaAvgPre-UbetaAvg)>Uslope is satisfied. When (UbetaAvgPre-UbetaAvg)>Uslope is satisfied, set UbetaFallFlag=1. In addition, when (UalphaAvgPre-UalphaAvg)>Uslope is not satisfied, determine whether the α-axis voltage average value UalphaAvg is greater than the second threshold value Ufall and whether the d-axis voltage average value UdAvg is greater than the threshold value U1, that is, determine whether (UalphaAvg>Ufall)&&(UdAvg>U1) is satisfied. When (UalphaAvg>Ufall)&&(UdAvg>U1) is satisfied, it is determined that the power grid is normal, and UalphaFallFlag=0 is set at this time. When (UbetaAvgPre-UbetaAvg)>Uslope is not satisfied, it is determined whether the β-axis voltage average value UbetaAvg is greater than the second threshold value Ufall and whether the d-axis voltage average value UdAvg is greater than the threshold value U1, that is, it is determined whether (UbetaAvg>Ufall)&&(UdAvg>U1) is satisfied. When (UbetaAvg>Ufall)&&(UdAvg>U1) is satisfied, it is determined that the power grid is normal, and UbetaFallFlag=0 is set at this time.

[0066] When (UalphaAvgPre - UalphaAvg) > Uslope and (UbetaAvgPre - UbetaAvg) > Uslope, it is determined that the grid voltage has a rapid drop. At this time, set UalphaFallFlag = 1 and UbetaFallFlag = 1. Here, UalphaFallFlag and UbetaFallFlag are user-defined software recognition flags, with a default value of 0. Uslope takes 0.5Un, and Un is the rated grid voltage.

[0067] Step S3, determine whether the grid is powered off based on the average voltage in the αβ coordinate system, the average voltage of the previous beat in the αβ coordinate system, and the average voltage in the dq coordinate system.

[0068] Specifically, the following judgment process is required to determine whether the grid is powered off: 1) First, determine whether the voltage drop amplitude exceeds the limit; 2) On the basis of determining that it exceeds the limit, determine whether the grid is powered off.

[0069] When determining whether the voltage drop amplitude exceeds the limit, the specific operation steps are as follows:

[0070] Step S31, determine whether the average voltage UalphaAvg of the α-axis is less than the second threshold Ufall, and at the same time determine whether the average voltage UbetaAvg of the β-axis is less than the second threshold Ufall.

[0071] Step S32, when the average voltage UalphaAvg of the α-axis is less than the second threshold Ufall and the average voltage UbetaAvg of the β-axis is less than the second threshold Ufall, it is determined that the voltage drop amplitude exceeds the limit.

[0072] In the embodiment of the present invention, if (UalphaAvg < Ufall) && (UbetaAvg < Ufall), it indicates that the voltage drop amplitude exceeds the limit. At this time, set UalphaFallFlag = 1 and UbetaFallFlag = 1. Here, the Ufall voltage takes 0.05Un.

[0073] After determining that the voltage drop amplitude exceeds the limit, further determine whether the grid is powered off. Two methods can be used to determine whether the grid is powered off. One of the judgment processes is as follows:

[0074] Step S33, determine whether the average voltage of the d-axis is less than the third threshold Ublackout.

[0075] Step S34, accumulate the number of times that the average voltage of the d-axis is less than the third threshold Ublackout.

[0076] Step S35, when the number is greater than the fourth threshold N3, it is determined that the grid is powered off.

[0077] In an embodiment of the present invention, when it is judged that (UdAvg < Ublackout) is satisfied, GridBlackoutCnt is incremented. When the increment exceeds N3 times, it is considered that a grid power outage has occurred, and GridBlackOutFlag = 1 is set. Among them, N3 is taken as 5.

[0078] Another judgment process is as follows:

[0079] Step S33, judge whether the average value of the d-axis voltage UdAvg is less than the fifth threshold value 0.7Un, and at the same time judge whether the voltage drops.

[0080] Step S34, accumulate the number of times when the average value of the d-axis voltage UdAvg is less than the fifth threshold value 0.7Un and the voltage drops.

[0081] Step S35, when the number of times is greater than the fourth threshold value N3, it is determined that a grid power outage has occurred.

[0082] In an embodiment of the present invention, when it is judged that ((UdAvg < 0.7Un) && UalphaFallFlag && UbetaFallFlag) is satisfied, GridBlackoutCnt is incremented. When the increment exceeds N3 times, it is considered that a grid power outage has occurred, and GridBlackOutFlag = 1 is set. Among them, N3 is taken as 5.

[0083] Combining the above two judgment processes, it can be summarized as the following judgment formula: (UdAvg < Ublackout) || ((UdAvg < 0.7Un) && UalphaFallFlag && UbetaFallFlag). When it is judged that (UdAvg < Ublackout) || ((UdAvg < 0.7Un) && UalphaFallFlag && UbetaFallFlag) is satisfied, GridBlackoutCnt is incremented. When the increment exceeds N3 times, it is considered that a grid power outage has occurred, and GridBlackOutFlag = 1 is set. When it is judged that (UdAvg < Ublackout) || ((UdAvg < 0.7Un) && UalphaFallFlag && UbetaFallFlag) is not satisfied, then GridBlackOutFlag = 0 is set. Among them, GridBlackoutCnt is a custom counter. In order to avoid misjudgment, it is necessary to accumulate the number of times when the average value of the d-axis voltage UdAvg is less than the fifth threshold value 0.7Un and the voltage drops, and judge the accumulated value.

[0084] Step S4, when a grid power outage occurs, block the drive of the NPC three-level grid-connected inverter topology.

[0085] Specifically, if a power outage occurs, the NPC three-level topology drive is closed to avoid grid-connected outages that may cause abnormal stress failure of the device. By identifying a high-voltage power outage in the power grid, the switch tube is actuated in advance to avoid excessive voltage stress failure of the switch tube. This embodiment quickly identifies the power grid while taking into account the low-voltage ride-through requirements of safety regulations. The solution does not require additional circuits and costs, greatly improving the reliability of three-phase grid-connected machines during high-voltage power outages in the power grid, and has high practical value.

[0086] This implementation uses the commonly used three-phase control coordinate transformation and judges the drop and amplitude of the grid Ud, Ualpha, and Ubeta to identify the situation where the grid is out of power and there is a large load on the grid side. Compared with normal island detection, the grid voltage and frequency detection speed is greatly improved. The detection time of this solution can be within a few milliseconds, and the NPC three-level drive can be quickly turned off to avoid the risk of the inverter switch tube failing due to stress exceeding the specification.

[0087] In an optional embodiment, the method comprises:

[0088] Step S5, judging whether the grid has recovered according to the three-phase grid voltage, the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, the voltage average value in the dq coordinate system, and the voltage slow average value in the dq coordinate system.

[0089] Specifically, step S5 includes:

[0090] Step S51, determining whether the slow average value of the d-axis voltage UdAvgSlow is greater than a sixth threshold value ZvrtVolt, and determining whether the three-phase grid voltage is greater than the sixth threshold value ZvrtVolt.

[0091] Step S52, accumulating the number of times that the slow average value of the d-axis voltage is greater than the sixth threshold value ZvrtVolt and the three-phase grid voltage is greater than the sixth threshold value ZvrtVolt.

[0092] Step S53: when the number of times is greater than the seventh threshold value N4, it is determined that the power grid is restored.

[0093] In an embodiment of the present invention, if UdAvgSlow>ZvrtVolt&&Uan>ZvrtVolt&&Ubn>ZvrtVolt&&Ucn>ZvrtVolt, GridBlackoutBackCnt is accumulated. When the accumulation exceeds N4 times, it is considered that the power grid has recovered, and GridBlackoutBackCnt=0 is set. Among them, N4 is 80. Among them. GridBlackoutBackCnt is a custom counter. In order to avoid misjudgment, it is necessary to accumulate the number of times that the slow average value of the d-axis voltage is greater than the sixth threshold ZvrtVolt and the three-phase power grid voltage is greater than the sixth threshold ZvrtVolt, and judge the accumulated value. ZvrtVolt is a custom grid voltage judgment threshold, and its value is designed according to the corresponding safety regulations, for example, it is designed to be 0.8Un. This solution takes into account the safety regulations for low voltage crossing requirements. Through the threshold design, it can be compatible with normal low voltage crossing and power grid power outage protection, which can effectively reduce the risk of machine failure and has high practical value.

[0094] Step S6: when the power grid is restored, start the NPC three-level grid-connected inverter topology drive.

[0095] Specifically, when it is determined that the power grid has recovered, the NPC three-level grid-connected inverter topology drive is turned on normally.

[0096] In this embodiment, a NPC three-level grid-connected inverter power grid power failure protection device is also provided, which is used to implement the above-mentioned embodiments and preferred implementation modes, and the descriptions that have been made will not be repeated. As used below, the term "module" can implement a combination of software and / or hardware of a predetermined function. Although the devices described in the following embodiments are preferably implemented in software, the implementation of hardware, or a combination of software and hardware, is also possible and conceivable.

[0097] This embodiment provides a NPC three-level grid-connected inverter power grid power failure protection device, such as Figure 5 As shown, including:

[0098] The coordinate conversion module 51 is used to process the three-phase grid voltage to obtain the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, the voltage average value in the dq coordinate system, and the voltage slow average value in the dq coordinate system.

[0099] The first judgment module 52 is used to judge whether the grid voltage drops according to the difference between the previous beat voltage average value in the αβ coordinate system and the voltage average value in the αβ coordinate system.

[0100] The second judgment module 53 is used to judge whether the power grid is out of power according to the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system and the voltage average value in the dq coordinate system.

[0101] The power failure driving module 54 is used to close the NPC three-level grid-connected inverter topology driving when the power grid fails.

[0102] The further functional description of each of the above modules and units is the same as that of the above corresponding embodiments and will not be repeated here.

[0103] The NPC three-level grid-connected inverter power grid power outage protection device in this embodiment is presented in the form of a functional unit, where the unit refers to an ASIC (Application Specific Integrated Circuit) circuit, a processor and memory that executes one or more software or fixed programs, and / or other devices that can provide the above functions.

[0104] The present invention provides an NPC three-level grid-connected inverter power grid power failure protection device, comprising: a coordinate conversion module, used for processing a three-phase grid voltage to obtain a voltage average value in an αβ coordinate system, a previous beat voltage average value in the αβ coordinate system, a voltage average value in a dq coordinate system, and a voltage slow average value in the dq coordinate system; a first judgment module, used for judging whether the grid voltage drops according to a difference between a previous beat voltage average value in the αβ coordinate system and a voltage average value in the αβ coordinate system; a second judgment module, used for judging whether the grid is powered off according to the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, and the voltage average value in the dq coordinate system; and a power failure drive module, used for closing a NPC three-level grid-connected inverter topology drive when the grid is powered off. The voltage drop state of the grid and whether the grid is out of power are detected according to the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, the voltage average value in the dq coordinate system and the slow voltage average value in the dq coordinate system. Compared with normal island detection, the grid voltage frequency detection speed is greatly improved, and the NPC three-level drive can be quickly turned off to avoid the risk of failure of the inverter switch tube due to stress exceeding the specification.

[0105] The embodiment of the present invention also provides a computer device having the above Figure 5 The NPC three-level grid-connected inverter power grid power failure protection device shown.

[0106] See also Figure 6 , Figure 6 is a schematic diagram of the structure of a computer device provided by an optional embodiment of the present invention, such as Figure 6As shown, the computer device includes: one or more processors 10, a memory 20, and interfaces for connecting various components, including high-speed interfaces and low-speed interfaces. Various components are connected to each other using different buses for communication, and can be installed on a common mainboard or installed in other ways as needed. The processor can process the instructions executed in the computer device, including instructions stored in or on the memory to display the graphical information of the GUI on an external input / output device (such as, a display device coupled to the interface). In some optional embodiments, if necessary, multiple processors and / or multiple buses can be used together with multiple memories and multiple memories. Similarly, multiple computer devices can be connected, and each device provides some necessary operations (for example, as a server array, a group of blade servers, or a multi-processor system). Figure 6 A processor 10 is taken as an example.

[0107] The processor 10 may be a central processing unit, a network processor or a combination thereof. The processor 10 may further include a hardware chip. The hardware chip may be a dedicated integrated circuit, a programmable logic device or a combination thereof. The programmable logic device may be a complex programmable logic device, a field programmable gate array, a general purpose array logic or any combination thereof.

[0108] The memory 20 stores instructions executable by at least one processor 10, so that the at least one processor 10 executes the method shown in the above embodiment.

[0109] The memory 20 may include a program storage area and a data storage area, wherein the program storage area may store an operating system, an application required for at least one function; the data storage area may store data created according to the use of the computer device, etc. In addition, the memory 20 may include a high-speed random access memory, and may also include a non-transient memory, such as at least one disk storage device, a flash memory device, or other non-transient solid-state storage device. In some optional embodiments, the memory 20 may optionally include a memory remotely arranged relative to the processor 10, and these remote memories may be connected to the computer device via a network. Examples of the above-mentioned network include, but are not limited to, the Internet, an intranet, a local area network, a mobile communication network, and combinations thereof.

[0110] The memory 20 may include a volatile memory, such as a random access memory; the memory may also include a non-volatile memory, such as a flash memory, a hard disk or a solid state drive; the memory 20 may also include a combination of the above types of memory.

[0111] The computer device further comprises a communication interface 30 for the computer device to communicate with other devices or a communication network.

[0112] The embodiment of the present invention also provides a computer-readable storage medium. The method according to the embodiment of the present invention can be implemented in hardware, firmware, or can be implemented as a computer code that can be recorded in a storage medium, or can be implemented as a computer code that is originally stored in a remote storage medium or a non-temporary machine-readable storage medium and will be stored in a local storage medium through a network download, so that the method described herein can be stored in such software processing on a storage medium using a general-purpose computer, a dedicated processor, or programmable or dedicated hardware. Among them, the storage medium can be a magnetic disk, an optical disk, a read-only storage memory, a random access memory, a flash memory, a hard disk or a solid-state hard disk, etc.; further, the storage medium can also include a combination of the above types of memories. It can be understood that a computer, a processor, a microprocessor controller, or programmable hardware includes a storage component that can store or receive software or computer code. When the software or computer code is accessed and executed by a computer, a processor, or hardware, the method shown in the above embodiment is implemented.

[0113] Although the embodiments of the present invention have been described in conjunction with the accompanying drawings, those skilled in the art may make various modifications and variations without departing from the spirit and scope of the present invention, and such modifications and variations are all within the scope defined by the appended claims.

Claims

1. A method for protecting a power grid from power outage of an NPC three-level grid-connected inverter, characterized in that: The method comprises: The three-phase grid voltage is processed to obtain the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, the voltage average value in the dq coordinate system, and the voltage slow average value in the dq coordinate system; Determine whether the grid voltage has dropped based on the difference between the previous beat voltage average value in the αβ coordinate system and the voltage average value in the αβ coordinate system; Determine whether the power grid is out of power based on the average voltage in the αβ coordinate system, the average voltage of the previous beat in the αβ coordinate system, and the average voltage in the dq coordinate system; When the grid fails, the NPC three-level grid-connected inverter topology drive is closed.

2. The NPC three-level grid-connected inverter power grid power failure protection method according to claim 1, characterized in that: The method comprises: Determine whether the power grid has recovered based on the three-phase power grid voltage, the average voltage in the αβ coordinate system, the average voltage of the previous beat in the αβ coordinate system, the average voltage in the dq coordinate system, and the slow average voltage in the dq coordinate system; When the power grid is restored, the NPC three-level grid-connected inverter topology drive is turned on.

3. The NPC three-level grid-connected inverter power grid power failure protection method according to claim 1, characterized in that: The three-phase grid voltage is processed to obtain the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, the voltage average value in the dq coordinate system, and the voltage slow average value in the dq coordinate system, including: Acquire a three-phase grid voltage, and perform coordinate transformation on the three-phase grid voltage to obtain a voltage in an αβ coordinate system and a voltage in a dq coordinate system; Perform multiple moving average calculations on the voltage in the αβ coordinate system and the voltage in the dq coordinate system to obtain the average voltage in the αβ coordinate system, the average voltage of the previous beat in the αβ coordinate system, and the average voltage in the dq coordinate system; The voltage average value in the dq coordinate system is calculated by moving average multiple times to obtain a slow average value of the voltage in the dq coordinate system.

4. The NPC three-level grid-connected inverter power grid power failure protection method according to claim 1, characterized in that: Whether the grid voltage drops is determined based on the difference between the previous beat voltage average value in the αβ coordinate system and the voltage average value in the αβ coordinate system, including: Determine whether the difference between the previous beat voltage average of the α-axis and the α-axis voltage average is greater than a first threshold, and simultaneously determine whether the difference between the previous beat voltage average of the β-axis and the β-axis voltage average is greater than the first threshold; When the difference between the previous beat voltage average of the α-axis and the α-axis voltage average is greater than a first threshold and the difference between the previous beat voltage average of the β-axis and the β-axis voltage average is greater than a first threshold, it is determined that a grid voltage drop occurs.

5. The NPC three-level grid-connected inverter power grid power failure protection method according to claim 1 or claim 4, characterized in that: Whether the power grid is out of power is determined based on the average voltage value in the αβ coordinate system, the average voltage value of the previous beat in the αβ coordinate system, and the average voltage value in the dq coordinate system, including: Determine whether the average voltage of the α-axis is less than a second threshold value, and determine whether the average voltage of the β-axis is less than a second threshold value; When the average voltage of the α-axis is less than the second threshold value and the average voltage of the β-axis is less than the second threshold value, it is determined that the voltage drop amplitude exceeds the limit; Determining whether the d-axis voltage average value is less than a third threshold; The number of times the accumulated d-axis voltage average value is less than a third threshold value; When the number is greater than a fourth threshold, it is determined that a power outage occurs in the power grid.

6. The NPC three-level grid-connected inverter power grid power failure protection method according to claim 1 or claim 4, characterized in that: Judging whether the power grid is out of power according to the average voltage value in the αβ coordinate system, the average voltage value of the previous beat in the αβ coordinate system, and the average voltage value in the dq coordinate system, further includes: Determine whether the average voltage of the α-axis is less than a second threshold value, and determine whether the average voltage of the β-axis is less than a second threshold value; When the average voltage of the α-axis is less than the second threshold value and the average voltage of the β-axis is less than the second threshold value, it is determined that the voltage drop amplitude exceeds the limit; Determine whether the d-axis voltage average value is less than a fifth threshold value, and determine whether the voltage drops; The number of times the accumulated d-axis voltage average value is less than the fifth threshold and the voltage drops; When the number is greater than a fourth threshold, it is determined that a power outage occurs in the power grid.

7. The NPC three-level grid-connected inverter power grid power failure protection method according to claim 2, characterized in that: Whether the power grid is restored is determined based on the three-phase power grid voltage, the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, the voltage average value in the dq coordinate system, and the voltage slow average value in the dq coordinate system, including: Determine whether the slow average value of the d-axis voltage is greater than a sixth threshold, and determine whether the three-phase grid voltage is greater than the sixth threshold; The number of times that the accumulated d-axis voltage slow average value is greater than the sixth threshold and the three-phase grid voltage is greater than the sixth threshold; When the number is greater than a seventh threshold, it is determined that the power grid is restored.

8. A NPC three-level grid-connected inverter power grid power failure protection device, characterized in that: The device comprises: A coordinate conversion module is used to process the three-phase grid voltage to obtain the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, the voltage average value in the dq coordinate system, and the voltage slow average value in the dq coordinate system; A first judgment module is used to judge whether the grid voltage drops according to the difference between the previous beat voltage average value in the αβ coordinate system and the voltage average value in the αβ coordinate system; The second judgment module is used to judge whether the power grid is out of power according to the voltage average value in the αβ coordinate system, the previous beat voltage average value in the αβ coordinate system, and the voltage average value in the dq coordinate system; The power failure drive module is used to close the NPC three-level grid-connected inverter topology drive when the power grid fails.

9. A computer device, characterized in that: include: A memory and a processor, wherein the memory and the processor are communicatively connected to each other, the memory stores computer instructions, and the processor executes the NPC three-level grid-connected inverter power grid power failure protection method according to any one of claims 1 to 7 by executing the computer instructions.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores computer instructions, and the computer instructions are used to enable a computer to execute the NPC three-level grid-connected inverter power grid power failure protection method according to any one of claims 1 to 7.

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