CN107732956B - Variable-power tracking track two-stage type photovoltaic grid-connected system low-voltage ride-through method - Google Patents
Variable-power tracking track two-stage type photovoltaic grid-connected system low-voltage ride-through method Download PDFInfo
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Abstract
The invention discloses a variable power tracking trajectory two-stage photovoltaic grid-connected system low-voltage ride through method, and belongs to the technical field of new energy power systems and micro-grids. On the premise of not changing the original configuration of a two-stage three-phase photovoltaic grid-connected system and not adding additional equipment, the power tracking track is adjusted through the feedforward control of the voltage drop amplitude, the voltage of the photovoltaic cell terminal is changed in real time, the output of the power of the photovoltaic cell is further quickly and effectively controlled, and the balance of the power on two sides of the inverter is realized; meanwhile, through cooperative control with the grid-connected inverter, on the premise that the grid-connected output current of the inverter is not out of limit, active power and reactive power are provided for a power grid as much as possible, the recovery of the voltage of the power grid is supported to the maximum extent, and zero voltage ride through of a photovoltaic grid-connected system is realized. Compared with the traditional control method, the method does not need to add extra equipment, not only realizes the safe and stable operation of the system, but also reduces the cost of the photovoltaic power generation system, and has higher practicability.
Description
Technical Field
The invention belongs to the technical field of new energy power systems and micro-grids, and particularly relates to a variable power tracking trajectory two-stage type photovoltaic grid-connected system low-voltage ride-through method.
Background
With the continuous expansion of the photovoltaic grid-connected power generation scale, the influence of the safe and stable operation of a photovoltaic power generation system on a power grid cannot be ignored. When a short-term fault occurs in a power grid, large-scale off-grid of the unit can seriously affect the stability of the voltage and frequency of the power grid, threaten the safety and stable operation of the power grid, even possibly cause the breakdown of a local power grid and cause the interruption of power supply in a larger area, and therefore the power grid requires that the photovoltaic generator set has certain Low Voltage Ride Through (LVRT) capability like the conventional generator set. The national standard "technical specification of photovoltaic power station access to electric power system" indicates that the photovoltaic power station should meet the low voltage ride through requirement: a) when the voltage of the grid connection point drops to 0, the photovoltaic power station can continuously operate for 0.15 second without disconnecting from the grid; b) when the grid-connected point voltage drops below the solid line in fig. 1, the photovoltaic power plant can be cut out of the grid. In addition, the regulation also requires that the photovoltaic power station should fully utilize the reactive capacity of the grid-connected inverter and the regulation capacity thereof, and exert the dynamic reactive power support capacity thereof.
When the voltage of a power grid drops temporarily, grid-connected current increases suddenly, if the current is limited to the upper limit of the current under normal conditions, the active power output is limited, the energy emitted by the photovoltaic cell is still kept at the maximum power point, redundant energy is accumulated on a direct-current bus capacitor, the voltage of the direct-current bus increases suddenly, and how to process the redundant energy becomes a key for realizing low-voltage ride through. In addition, how to adjust the inverter reactive power control to help the rapid recovery of the grid voltage is also a non-negligible part of it.
In the existing low voltage ride through strategy, parallel connection of the unloading resistor at the dc bus is the most common method. During a fault period, the unloading resistor consumes redundant energy emitted by the photovoltaic cell to keep balance of power on two sides of the inverter, the increase of voltage on a direct current side and the overcurrent of grid-connected current on an alternating current side are limited, and the requirement of low-voltage ride-through is met. Later, some scholars put forward a low-voltage ride-through strategy based on super-capacitor energy storage, control of active power in a low-voltage fault process is achieved by utilizing the inherent rapid charging and discharging characteristic of the super-capacitor, redundant energy is absorbed when the voltage of a direct-current bus exceeds a given value, energy on two sides of an inverter is balanced, and voltage rise on a direct-current side is restrained. However, the addition of the super capacitor greatly increases the hardware cost of the device, makes the system control complicated, increases uncertainty, and brings inconvenience to the installation of the super capacitor due to the huge volume of the super capacitor equipment. Some scholars propose low-voltage ride-through methods for single-stage photovoltaic systems, and experiments prove the effectiveness of the methods, but the methods are only suitable for the single-stage photovoltaic systems and cannot be applied to the two-stage three-phase photovoltaic systems which are common at present, because the strategies do not consider the problem of the overvoltage of the direct-current bus. Since the operating characteristics of a photovoltaic power generation system are closely related to environmental changes and the magnitude of the grid voltage sag, none of the above methods has proven the feasibility of a strategy under different environmental conditions and different magnitude of sag, and the operation of the system in the face of the most severe case of zero voltage ride-through. The traditional low-voltage ride-through strategy usually keeps the photovoltaic cell always working at the maximum power point, and only improves the topological structure of the photovoltaic system or adds auxiliary equipment to consume redundant active power during the fault, so that the cost is huge, and the stability of the system is also influenced to a certain extent.
Therefore, a novel low-voltage ride-through method for a two-stage three-phase photovoltaic power generation system is expected, the control of the photovoltaic system in the prior art is improved, the system can realize low-voltage ride-through, the problems in the prior art are avoided, and the stable operation of the future photovoltaic power generation system is guaranteed, which is the key point of the research in the prior art.
Disclosure of Invention
The invention aims to provide a variable power tracking track two-stage type photovoltaic grid-connected system low-voltage ride-through method which is characterized by comprising the following steps of:
1) the power tracking track of the photovoltaic cell is adjusted during the fault period, namely the power tracking track is adjusted through the feedforward control of the voltage drop amplitude, the terminal voltage of the photovoltaic cell is changed in real time, the generation of redundant energy is reduced from the source, the power output of the photovoltaic cell is controlled rapidly and effectively, and the balance of the power at the two sides of the inverter is realized; the direct-current bus overvoltage and grid-connected current overcurrent are avoided, the low-voltage ride-through of the two-stage photovoltaic grid-connected system with the variable power tracking track is realized on the premise of not changing a topological structure and not increasing auxiliary equipment, and the cost of the system is greatly reduced;
2) by the control of the grid-connected inverter, on the premise of ensuring that the grid-connected output current is not out of limit, active power and reactive power are provided for the power grid as much as possible, and the recovery of the voltage of the power grid is supported to the maximum extent;
3) when a low-voltage fault occurs at the power grid side, in order to realize energy balance, the terminal voltage of the photovoltaic cell cannot be kept unchanged at the original value, and a power grid voltage feedforward regulating quantity U related to the voltage drop depth is added in the maximum power point tracking processPV＿LVRTThe maximum power point tracking process is changed, so that the terminal voltage of the photovoltaic cell is changed, the energy emitted by the photovoltaic cell is actively reduced, and the energy balance is realized; thereby changing the target of power point tracking from the maximum power point to the optimum power point more conducive to achieving low voltage ride through.
In the step 1), the power tracking track is adjusted through feedforward control of voltage drop amplitude, the voltage of the photovoltaic cell terminal is changed in real time, the power tracking track is adjusted and controlled by a Boost circuit, and first, a given value of the voltage of the photovoltaic cell terminal is defined
Is composed of
Wherein U isPV-MPPTTerminal voltage of photovoltaic cell, U, corresponding to maximum power pointPV-LVRTFeeding forward the regulated quantity for the grid voltage;
when the power grid normally operates or the voltage fluctuation value of the power grid does not exceed 10% of rated voltage, the system is in a normal working state at the moment, the influence of the power grid voltage is not considered, the photovoltaic cell operates at the maximum power point, UPV_LVRT＝0，
When the voltage of the power grid is lower than 90% of rated voltage and exceeds the normal operation range, the voltage drop fault is considered to occur, and the feedforward adjustment quantity U of the voltage of the power grid isPV＿LVRTIs positive and has a direct proportion relation with the voltage drop amplitude, and the voltage set value of the photovoltaic cell terminal at the moment
The terminal voltage of the photovoltaic cell is reduced, and the generated energy is reduced, so that the balance between the input power of the photovoltaic cell and the power flowing into a power grid is quickly realized;
after the low voltage ride through process is finished, because the terminal voltage of the photovoltaic cell is at a lower value, the maximum power point tracking needs to be performed again within a certain time, so that the cell operates at the maximum power point again,
when the voltage sag is over, even if the voltage sag lasts for only a few cycles; the terminal voltage of the photovoltaic cell is the terminal voltage at the maximum power point maintained before, and is not required to be smaller from the temporary reduction process
Maximum power point tracking is carried out again at the value position, so that the conversion time is saved, and the stability of the system is improved.
In the step 2), through the control of the grid-connected inverter, the grid-connected photovoltaic inverter needs to provide reactive current for the power grid when the voltage of the power grid drops, so that the recovery of the voltage of the power grid is supported, and the generation of the reactive current becomes another key for meeting the low-voltage ride-through requirement. According to the national standard regulation, the ratio coefficient Q of the required reactive current to the rated current of the inverter is adjusted for different degrees of voltage dropratioThe following formula is used to calculate the formula,
wherein U isgrmsAnd UnorThe effective values of the network voltage at the moment and under the normal condition are respectively, and the required reactive current reference value is
In the formula iratedIs the rated value of the inverter grid-connected current. When the voltage of the power grid is normal, the inverter operates under a unit power factor, QratioZero, the output reactive power is zero; voltage sagDuring which, in order to supply the reactive energy required for voltage recovery, QratioThe value of (3) is related to the voltage drop depth, and the reactive current is output according to the formula (3).
In order to improve the energy capture of the photovoltaic array, it is desirable to generate as much active power as possible during voltage sag; under a synchronous rotation dq coordinate system, the system operates at a unit power factor and an active current reference value i when working normallydrefTo rated current value iratedRemain substantially the same; during low voltage ride through, because the power on two sides of the inverter is balanced, the grid-connected current is still kept at a rated value, the direct-current bus voltage is similar to that in normal, the direct-current bus voltage is basically kept near a given value, and the direct-current bus voltage cannot fluctuate too much, wherein i at the momentdrefBasically the same as the normal condition, the given value of the active current of the d-axis of the grid-connected inverter during the fault is considered to be the same as the given value of the active current of the d-axis of the grid-connected inverter during the fault, considering the reactive current support of the inverter to the grid voltage and the relation between the active current and the reactive current
Wherein the ratio coefficient of the active current to the rated current of the inverter
Can be calculated from equation (2).
The method has the advantages that the voltage at the end of the photovoltaic cell is changed by adjusting the power tracking track through the feedforward of the voltage drop amplitude, so that the power output of the photovoltaic cell is quickly and effectively controlled, the power balance of two sides of the inverter is ensured, the voltage rise of the direct current bus is accurately and effectively inhibited, the voltage of the direct current bus is stabilized, the output power of the photovoltaic cell is changed by changing the power tracking track on the premise of not changing the original configuration of a two-stage three-phase photovoltaic grid-connected system and not adding auxiliary equipment such as a parallel unloading resistor or an energy storage device, the low voltage ride through of the photovoltaic grid-connected system is finally realized, the cost of the photovoltaic power generation system is reduced while the safety and the stable operation of the system are ensured, and the economy of the photovoltaic power.
Drawings
FIG. 1 is a diagram of a two-stage three-phase photovoltaic grid-connected system;
FIG. 2 is a schematic diagram of a Boost circuit control for variable power tracking trajectory;
FIG. 3 is a schematic diagram of an inverter control strategy;
FIG. 4 is a diagram illustrating simulation results under standard environmental conditions; wherein (a) - (c) are simulation results of the photovoltaic system facing different voltage drops;
FIG. 5 is a diagram illustrating simulation results under non-standard environmental conditions. Wherein (a) - (b) are simulation results of photovoltaic system facing different levels of voltage drop.
Detailed Description
The invention provides a variable power tracking trajectory two-stage type photovoltaic grid-connected system low-voltage ride-through method, which is described in more detail below with reference to the embodiments and the accompanying drawings.
The voltage of the photovoltaic cell terminal is changed by adjusting the power tracking track through the feedforward of the voltage drop amplitude, so that the output of the power of the photovoltaic cell is controlled quickly and effectively, the balance of the power of two sides of the inverter is ensured, the rise of the voltage of the direct current bus is restrained accurately and effectively, the voltage of the direct current bus is stabilized, the output power of the photovoltaic cell is changed by changing the power tracking track on the premise of not changing the original configuration of a two-stage three-phase photovoltaic grid-connected system and not adding auxiliary equipment such as a parallel unloading resistor or an energy storage device, and finally the low voltage ride through of the photovoltaic grid-connected system is realized.
In the description of the present invention, it is to be understood that the terms "center", "longitudinal", "lateral", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in fig. 1 and 2,
when a low-voltage fault occurs at the power grid side, in order to realize energy balance, the terminal voltage of the photovoltaic cell cannot be kept unchanged at the original value, and a voltage drop depth related to the maximum power point tracking process is addedOf the network voltage feed-forward regulation UPV＿LVRTThe tracking process of the maximum power point is changed, so that the terminal voltage of the photovoltaic cell is changed, the energy emitted by the photovoltaic cell is actively reduced, and the energy balance is realized. The goal of power point tracking is changed from the maximum power point to the optimum power point that is more conducive to achieving low voltage ride through,
a complete two-stage three-phase photovoltaic grid-connected system is shown in fig. 1 and includes a photovoltaic cell, a Boost circuit, an inverter, a control system, and the like. The control system consists of a Boost circuit control part and an inverter control part, wherein the Boost part controls the output power of the photovoltaic cell and raises the voltage to a proper value so that the inverter can work normally; the inverter part controls the direct current bus voltage through a voltage and current double closed loop and transmits required current to the power grid side. Aiming at the low voltage ride through process of a two-stage three-phase photovoltaic grid-connected system when three-phase symmetrical drop occurs to the grid voltage, the invention adjusts the power tracking track through the feedforward of the voltage drop amplitude to change the voltage of the photovoltaic cell terminal on the premise of not changing the original configuration of the two-stage three-phase photovoltaic grid-connected system and not adding additional equipment, thereby rapidly and effectively controlling the output of the power of the photovoltaic cell, ensuring the balance of the power at two sides of an inverter, accurately and effectively inhibiting the voltage rise of a direct current bus and stabilizing the voltage of the direct current bus. And through cooperative control with the grid-connected inverter, on the premise of ensuring that the grid-connected output current is not out of limit, a certain reactive power can be generated while providing as much active power support as possible for the power grid, the recovery of the voltage of the power grid is supported to the maximum extent, and finally the low-voltage ride-through process, even the zero-voltage ride-through process of the photovoltaic grid-connected system is realized.
Generally, after the low voltage ride through process is finished, since the terminal voltage of the photovoltaic cell is at a lower value, it takes a certain time to perform maximum power point tracking again, so that the cell operates at the maximum power point again, even if the voltage sag lasts for a few cycles. The strategy designs a decoupling module and an improved disturbance observation MPPT method, decouples a maximum power point tracking process controller and a low voltage ride through controller, and defines the disturbance observation methodJudgment Condition Pin(k)＝Vpv(k)Iin(k)Spv(k) Increase the decoupling coefficient Spv. When the output of the low voltage ride through controller is positive, the voltage sag is shown to occur at the moment, and the output S of the decoupling coefficient modulepvAt 0, the output of the MPPT controller will remain constant at the maximum power point; when the output of the low-voltage ride-through controller is negative, the power grid operates in a normal range at the moment, and the output S of the decoupling coefficient modulepvAt 1, the decoupling coefficient has no effect on the power tracking process. When the voltage sag is over, the terminal voltage of the photovoltaic cell is the terminal voltage at the maximum power point maintained before, and the smaller terminal voltage in the sag process is not needed
And maximum power point tracking is carried out again at the value position, so that the conversion time is saved, and the stability and the economy of the photovoltaic power generation system are improved.
Fig. 2 is a schematic diagram of a control flow of a Boost circuit for tracking a track with variable power;
defining a given value of terminal voltage of a photovoltaic cell
Is composed of
Wherein U isPV-MPPTTerminal voltage of photovoltaic cell, U, corresponding to maximum power pointPV-LVRTAnd feeding forward the regulated quantity for the power grid voltage.
When the power grid normally operates or the voltage fluctuation value of the power grid does not exceed 10% of rated voltage, the power grid system is in a normal working state at the moment, the influence of the power grid voltage is not considered, the photovoltaic cell operates at the maximum power point, UPV_LVRT＝0，
When the voltage of the power grid is lower than 90% of rated voltage and exceeds the normal operation range, the voltage drop fault is considered to occurFeed-forward regulation U of the network voltagePV＿LVRTIs positive and has a direct proportion relation with the voltage drop amplitude, and the voltage set value of the photovoltaic cell terminal at the moment
The terminal voltage of the photovoltaic cell is reduced, the generated energy is reduced, and the balance between the input power of the photovoltaic cell and the power flowing into a power grid can be quickly realized.
Fig. 3 is a schematic diagram of an inverter control strategy, and the grid-connected photovoltaic inverter needs to provide reactive current for the grid when the grid voltage drops, so as to support the recovery of the grid voltage, and the generation of the reactive current becomes another key for meeting the low voltage ride through requirement. According to the national standard regulation, the ratio coefficient Q of the required reactive current to the rated current of the inverter is adjusted for different degrees of voltage dropratioThe calculation is given by the following formula.
Wherein U isgrmsAnd UnorThe effective values of the network voltage at the moment and under the normal condition are respectively, and the required reactive current reference value is
In the formula iratedIs the rated value of the inverter grid-connected current. When the voltage of the power grid is normal, the inverter operates under a unit power factor, QratioZero, the output reactive power is zero; during a voltage sag, Q, in order to supply the reactive energy required for voltage recoveryratioThe value of (3) is related to the voltage drop depth, and the reactive current is output according to the formula (3).
In addition to this, in order to improve the energy capture of the photovoltaic array, it is desirable to generate as much active power as possible during the voltage sag. Under a synchronous rotation dq coordinate system, the system operates at a unit power factor and an active current reference value i when working normallydrefTo rated current value iratedRemain substantially the same. Because the power on two sides of the inverter is balanced, the grid-connected current is still kept at a rated value, the direct-current bus voltage is similar to that in the normal state and is basically kept near the set value, and the direct-current bus voltage cannot fluctuate too much, i at the momentdrefBasically the same as the normal condition, the given value of the active current of the d-axis of the grid-connected inverter during the fault is considered to be the same as the given value of the active current of the d-axis of the grid-connected inverter during the fault, considering the reactive current support of the inverter to the grid voltage and the relation between the active current and the reactive current
Wherein the ratio coefficient of the active current to the rated current of the inverter
Can be calculated from equation (2).
A simulation model of a two-pole three-phase photovoltaic grid-connected system with the capacity of 5kW is established in Matlab/Simulink, and the feasibility of the designed control strategy is verified. Simulating the standard environment condition of photovoltaic test (the illuminance S is 1000W/m)2Temperature T of 25 ℃ and non-standard environmental conditions (illuminance S of 700W/m)2And the temperature T is 20 ℃) under two different environmental conditions, when the grid voltage drops in different amplitudes, the working condition of the system is adopted by the variable power tracking track low-voltage ride through strategy provided by the invention.
In order to fully verify the feasibility of the low-voltage ride through method of the variable power tracking track, the simulation result shows the dynamic change process of the operation state of the grid-connected system in the low-voltage ride through process from 6 key observation quantities of grid-connected voltage, grid-connected current, output active power, output reactive power, direct-current bus voltage and photovoltaic cell terminal voltage.
Fig. 4 reflects changes in the operating conditions of the photovoltaic power generation system under the standard photovoltaic measurement conditions when the grid voltage drops by different amplitudes. Fig. 4(a) shows a simulation result when the voltage drops to 70% of the rated voltage in 0.6s-0.8s, and it can be seen that the grid-connected output current only jitters at the beginning or at the end of the drop, but the amplitude of the grid-connected output current does not exceed 1.1 times of the limited current and is within the standard range, so that the device is not damaged or the breaker is not turned off due to overcurrent of the grid-connected current or due to touch protection. The dc bus voltage fluctuates slightly during state transitions where the sag begins or ends, but remains substantially around the reference voltage of 800V throughout the fault. The active power generated by the system during the voltage sag is reduced from the rated power of 5000W to about 2800W, the reactive power is increased from 0Var to 2050Var, and the reactive current value required to be provided by the utilization standard is basically the same as the value calculated by the power grid voltage at the moment. In the fault stage, the terminal voltage of the photovoltaic cell is reduced to about 230V, the power value emitted by the photovoltaic cell is reduced, and when the falling state is finished, the terminal voltage of the photovoltaic cell is rapidly restored to the value before falling.
Fig. 4(b) is a case when the grid voltage drops to 20% of the rated voltage in 0.6s-0.8s, and at this time, the grid-connected current still can be guaranteed not to be overcurrent, and the dc bus voltage is not overvoltage. According to the requirement for reactive current specified in the low voltage ride through requirement, when the grid voltage at fault is lower than 50% of the normal value, the grid-connected current should be all reactive current, i.e. 100% reactive current. At this time, due to the relationship between the active current and the reactive current, the active current should be 0, and the system only outputs the reactive power and does not output the active power. The voltage of the power grid drops to 20% of rated voltage, the amplitude of the grid-connected current is basically the same as a normal value, and the reactive power value is about 20% of the normal output active power value. From fig. 4(b), we can see that the active power is reduced to 0W, and the reactive power is about 1000Var, which meets the requirement of reactive current in the specification and is helpful for the recovery of the grid voltage. At the moment, the voltage of the photovoltaic cell is reduced to about 100V, and the phase difference between grid-connected voltage and grid-connected current is 90 degrees.
Fig. 4(c) is a case of 0.6s to 0.75s when the grid voltage is reduced to 0, and it can be seen that with the help of the present strategy, the photovoltaic system can keep 150ms without disconnecting from the grid, at this time, the grid-connected current is still basically kept at the rated value except for being jittered during state conversion, and the dc bus voltage is not out of limit, so that zero voltage ride through is realized, and the requirement of the national grid is satisfied. At this time, the grid-connected current is 100% of reactive current, and the active current is reduced to 0, but since the grid voltage is 0 at this time, the active power and the reactive power are both 0 at this time.
Considering the influence of environmental factors such as illumination and temperature on the running state of the photovoltaic cell, the photovoltaic cell can be operated under non-standard environmental conditions (the illumination S is 700W/m)2The simulation results at a temperature T of 20 ℃ are shown in FIG. 5.
Fig. 5(a) shows the operation of the system when the grid voltage drops to 70% of the rated voltage in 0.6s to 0.8 s. The photovoltaic system can still realize low voltage ride through well under the nonstandard environment, and can provide certain reactive power to help the recovery of the grid voltage according to the requirements. Fig. 5(b) shows the system operation state when the grid voltage drops to 0, at this time, the system can keep continuous operation without grid disconnection for 150ms, zero voltage ride through is realized, and 100% of reactive current can be provided for the grid.
The simulation result verifies the feasibility of the invention under different environmental factors and different dropping amplitudes, and the low-voltage ride through, even zero-voltage ride through of the photovoltaic system can be well realized under the condition of not adding additional equipment.
Claims (2)
1. A variable power tracking track two-stage type photovoltaic grid-connected system low-voltage ride-through method comprises the following steps:
1) the power tracking track of the photovoltaic cell is adjusted during the fault period, namely the power tracking track is adjusted through the feedforward control of the voltage drop amplitude, the terminal voltage of the photovoltaic cell is changed in real time, the generation of redundant energy is reduced from the source, the power output of the photovoltaic cell is controlled rapidly and effectively, and the balance of the power at the two sides of the inverter is realized; the direct-current bus overvoltage and grid-connected current overcurrent are avoided, the low-voltage ride-through of the two-stage photovoltaic grid-connected system with the variable power tracking track is realized on the premise of not changing a topological structure and not increasing auxiliary equipment, and the cost of the system is greatly reduced;
2) by the control of the grid-connected inverter, on the premise of ensuring that the grid-connected output current is not out of limit, active power and reactive power are provided for the power grid as much as possible, and the recovery of the voltage of the power grid is supported to the maximum extent; it is characterized by also comprising:
3) when a low-voltage fault occurs at the power grid side, in order to realize energy balance, the terminal voltage of the photovoltaic cell cannot be maintained unchanged at the original value, the terminal voltage of the photovoltaic cell is adjusted in real time by adjusting a power tracking track through feedforward control of voltage drop amplitude in the maximum power point tracking process, and the power tracking track is adjusted by Boost circuit control, first defining given value of terminal voltage of photovoltaic cell
Is composed of
Wherein U isPV-MPPTTerminal voltage of photovoltaic cell, U, corresponding to maximum power pointPV-LVRTFeeding forward the regulated quantity for the grid voltage;
when the power grid normally operates or the voltage fluctuation value of the power grid, the terminal voltage of the photovoltaic cell is changed, the energy emitted by the photovoltaic cell is actively reduced, and the energy balance is realized; thereby changing the target of power point tracking from the maximum power point to the optimal power point more conducive to low voltage ride through; when the rated voltage exceeds 10%, the system is in a normal working state, the influence of the voltage of a power grid is not considered, the photovoltaic cell operates at the maximum power point, UPV_LVRT＝0，
When the voltage of the power grid is lower than 90% of rated voltage and exceeds the normal operation range, the voltage drop fault is considered to occur, and the feedforward adjustment quantity U of the voltage of the power grid isPV＿LVRTIs positive and has a direct proportion relation with the voltage drop amplitude, and the voltage set value of the photovoltaic cell terminal at the moment
The voltage of the photovoltaic cell terminal is reduced, and the energy generated is reducedThe balance between the input power of the photovoltaic cell and the power flowing into a power grid is quickly realized;
after the low voltage ride through process is finished, because the terminal voltage of the photovoltaic cell is at a lower value, the maximum power point tracking needs to be performed again within a certain time, so that the cell operates at the maximum power point again,
when the voltage sag is over, even if the voltage sag lasts for only a few cycles; the terminal voltage of the photovoltaic cell is the terminal voltage at the maximum power point maintained before, and is not required to be smaller from the temporary reduction process
Maximum power point tracking is carried out again at the value position, so that the conversion time is saved, and the stability of the system is improved.
2. The variable power tracking trajectory two-stage photovoltaic grid-connected system low voltage ride through method of claim 1, wherein in step 2), through the control of the grid-connected inverter, the grid-connected photovoltaic inverter needs to provide reactive current for the grid when the grid voltage drops, so as to support the recovery of the grid voltage, and the generation of the reactive current becomes another key for meeting the low voltage ride through requirement; according to the national standard regulation, the ratio coefficient Q of the required reactive current to the rated current of the inverter is adjusted for different degrees of voltage dropratioThe following formula is used to calculate the formula,
wherein U isgrmsAnd UnorThe effective values of the network voltage at the moment and under the normal condition are respectively, and the required reactive current reference value is
In the formula iratedFor current incorporated into the inverterA nominal value; when the voltage of the power grid is normal, the inverter operates under a unit power factor, QratioZero, the output reactive power is zero; during a voltage sag, Q, in order to supply the reactive energy required for voltage recoveryratioThe value of (3) is related to the voltage drop depth, and reactive current is output according to the formula (3);
in order to improve the energy capture of the photovoltaic array, it is desirable to generate as much active power as possible during voltage sag; in synchronous rotation dqUnder a coordinate system, the system operates at a unit power factor and an active current reference value i when in normal operationdrefTo rated current value iratedRemain substantially the same; during low voltage ride through, because the power on two sides of the inverter is balanced, the grid-connected current is still kept at a rated value, the direct-current bus voltage is similar to that in normal, the direct-current bus voltage is basically kept near a given value, and the direct-current bus voltage cannot fluctuate too much, wherein i at the momentdrefBasically the same as the normal condition, the given value of the active current of the d-axis of the grid-connected inverter during the fault is considered to be the same as the given value of the active current of the d-axis of the grid-connected inverter during the fault, considering the reactive current support of the inverter to the grid voltage and the relation between the active current and the reactive current
Wherein the ratio coefficient of the active current to the rated current of the inverter
Can be calculated from equation (2).
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| CN110581565B (en) * | 2018-06-11 | 2023-03-28 | 香港理工大学 | Control method and device in photovoltaic power generation grid-connected system |
| CN108923458A (en) * | 2018-07-06 | 2018-11-30 | 华北电力大学（保定） | The Variable power for improving conductance increment method tracks photovoltaic virtual synchronous machine grid-connected coordination control method |
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| CN111525605B (en) * | 2020-04-07 | 2021-08-31 | 北方工业大学 | Photovoltaic system low voltage ride through method and system based on variable power output control |
| CN112421651B (en) * | 2020-10-15 | 2022-10-04 | 中国电力科学研究院有限公司 | System power balancing method and device in low-voltage ride through process of photovoltaic inverter |
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| CN114172212B (en) * | 2021-08-20 | 2023-08-18 | 华北电力大学（保定） | Method for improving transient active power output of photovoltaic unit during low voltage ride through |
| CN113794208B (en) * | 2021-08-31 | 2023-10-24 | 南京智睿能源互联网研究院有限公司 | Control method for reducing high-voltage cascading SVG low-voltage ride through off-grid |
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