CN111525605A - Photovoltaic system low voltage ride through method and system based on variable power output control - Google Patents

Photovoltaic system low voltage ride through method and system based on variable power output control Download PDF

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CN111525605A
CN111525605A CN202010265670.1A CN202010265670A CN111525605A CN 111525605 A CN111525605 A CN 111525605A CN 202010265670 A CN202010265670 A CN 202010265670A CN 111525605 A CN111525605 A CN 111525605A
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power
voltage
current
value
output
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CN111525605B (en
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李建林
王含
杨林
谭宇良
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North China University of Technology
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/381—Dispersed generators
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • H02J2300/22—The renewable source being solar energy
    • H02J2300/24—The renewable source being solar energy of photovoltaic origin
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/56—Power conversion systems, e.g. maximum power point trackers

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Abstract

The invention discloses a photovoltaic system low-voltage ride through method and system based on variable power output control. According to the method, when the voltage drop depth is small, the photovoltaic system can still keep the maximum power output; when the grid voltage drop depth is large, the output power of the photovoltaic system is actively adjusted based on the power balance problem at two sides of the inverter, and unbalanced power generated by grid faults is quickly eliminated; and during the fault period, according to the difference of voltage drop depths, the distribution of active and reactive current values is carried out again, so that the output of the inverter is ensured not to be over-current, and the low-voltage ride-through process is completed. The method and the system can give consideration to the safe and stable operation of the photovoltaic grid-connected power generation system during the grid fault period and the full utilization of solar energy resources, and have more superiority compared with the prior art.

Description

Photovoltaic system low voltage ride through method and system based on variable power output control
Technical Field
The invention relates to the technical field of protection and control of a new energy grid-connected power generation system, in particular to a photovoltaic system low-voltage ride through method and system based on variable power output control.
Background
The access of a high-capacity new energy (photovoltaic, wind energy) grid-connected power generation system brings huge challenges to the stability of a power grid. During a power grid fault (voltage drop), the instantaneous power cut caused by the off-grid operation of a high-capacity grid-connected system can cause huge impact on a power grid.
Under normal operating conditions, the photovoltaic system is usually controlled to work at the maximum power point to fully utilize solar energy resources. During a grid fault (grid voltage drop), because the power balance on the two sides of the inverter is broken, if the photovoltaic system continues to keep the output of the maximum power, the unbalanced power accumulated on the parallel capacitor on the direct current side is gradually increased, the direct current bus voltage is rapidly increased, and after the threshold voltage is reached, the grid-connected system and the grid are disconnected by the electric protection device, so that the grid-connected photovoltaic power generation system is disconnected from running. The grid-connected guide drafted by the national grid company of China stipulates that when grid faults cause voltage drop of a grid-connected point, a photovoltaic system must keep running without grid disconnection, and provides certain reactive power for a grid to support grid voltage recovery, namely, the low-voltage ride through function is realized. Therefore, the output of the photovoltaic system needs to be adjusted during the fault period, the unbalanced power caused by the grid fault is rapidly eliminated, and the stability of the direct-current bus voltage is maintained.
In the prior art, power self-adjustment is usually performed by directly utilizing the P-U (power-voltage) characteristic of a photovoltaic system, or unbalanced power on a direct current bus is eliminated by utilizing the coordinated control of an external energy storage device, but when the voltage drop depth is large, the requirement of rapid power dissipation cannot be met due to the large numerical value of the unbalanced power in a short time, and the voltage of the direct current bus still rises to a threshold voltage, so that the action of a protection device cuts off a grid-connected system, and the low-voltage ride-through process cannot be smoothly completed.
Disclosure of Invention
The invention aims to provide a photovoltaic system low-voltage ride-through method and system based on variable power output control, and aims to solve the problem that the low-voltage ride-through process cannot be smoothly completed by adopting the prior art when the voltage drop depth is large.
In order to achieve the purpose, the invention provides the following scheme:
a photovoltaic system low voltage ride through method based on variable power output control, the photovoltaic system low voltage ride through method comprising:
acquiring inverter output power and direct-current bus voltage in a photovoltaic grid-connected power generation system; the photovoltaic grid-connected power generation system comprises a photovoltaic system, a Boost circuit, an inverter and a power grid;
judging whether to execute variable power output control or not according to the output power of the inverter and the voltage of the direct current bus to obtain a first judgment result;
if the first judgment result is that the variable power output control is not executed, the photovoltaic system in the photovoltaic grid-connected power generation system keeps a maximum power output state;
if the first judgment result is that variable power output control is executed, determining the output duty ratio of a Boost circuit according to the output power of the inverter and the voltage of the direct-current bus;
adjusting the output power of the photovoltaic system according to the output duty ratio of the Boost circuit;
acquiring a grid voltage value after the voltage of the photovoltaic grid-connected power generation system drops;
determining a reactive current set value and an active current set value according to the power grid voltage value after the voltage drop;
and adjusting the reactive power emitted by the inverter in the photovoltaic grid-connected power generation system according to the reactive current given value and the active current given value.
Optionally, the determining whether to execute variable power output control according to the inverter output power and the dc bus voltage to obtain a first determination result specifically includes:
judging whether the output power of the inverter is smaller than or equal to an active power reference value during the fault period, and obtaining a second judgment result;
if the second judgment result is that the output power of the inverter is greater than the active power reference value during the fault period, determining that the first judgment result is that the variable power output control is not executed;
if the second judgment result is that the output power of the inverter is smaller than or equal to the active power reference value in the fault period, judging whether the voltage of the direct current bus is smaller than or equal to a set voltage value lower limit, and obtaining a third judgment result;
if the third judgment result is that the direct current bus voltage is less than or equal to the lower limit of the set voltage value, determining that the first judgment result is that the variable power output control is not executed;
and if the third judgment result is that the direct current bus voltage is greater than the lower limit of the set voltage value and less than the upper limit of the set voltage value, determining that the first judgment result is to execute variable power output control.
Optionally, the determining the output duty cycle of the Boost circuit according to the output power of the inverter and the dc bus voltage specifically includes:
according to the output power P of the inverterinjAnd the DC bus voltage UdcUsing formula Idc＝Pinj/UdcDetermining DC bus current Idc；
Obtaining a current value I corresponding to the maximum power point of the photovoltaic systemMSum power value PM；
According to the current value IMSum power value PMBy the formula
Figure BDA0002441168890000031
Determining a current reference value at a maximum power point of the photovoltaic system
Figure BDA0002441168890000032
Wherein
Figure BDA0002441168890000033
The output power reference value at the maximum power point of the photovoltaic system is obtained;
according to the DC bus current IdcAnd the current reference value
Figure BDA0002441168890000034
Using a formula
Figure BDA0002441168890000035
And determining the output duty ratio D of the Boost circuit.
Optionally, the determining a reactive current given value and an active current given value according to the grid voltage value after the voltage drop specifically includes:
according to the power grid voltage value U after the voltage drop, adopting an equation set
Figure BDA0002441168890000036
Determining a reactive current setpoint value
Figure BDA0002441168890000037
Wherein U isNRepresenting a grid voltage rating; i.e. iNRepresenting an inverter output current rating;
according to the given value of the reactive current
Figure BDA0002441168890000038
Using a formula
Figure BDA0002441168890000039
Determining an active current setpoint
Figure BDA00024411688900000310
A photovoltaic system low voltage ride through system based on variable power output control, the photovoltaic system low voltage ride through system comprising:
the system parameter acquisition module is used for acquiring the output power of an inverter and the voltage of a direct-current bus in the photovoltaic grid-connected power generation system; the photovoltaic grid-connected power generation system comprises a photovoltaic system, a Boost circuit, an inverter and a power grid;
the variable power output control judging module is used for judging whether to execute variable power output control according to the output power of the inverter and the voltage of the direct current bus to obtain a first judging result;
the maximum power output maintaining module is used for maintaining the maximum power output state of the photovoltaic system in the photovoltaic grid-connected power generation system if the first judgment result indicates that the variable power output control is not executed;
the duty ratio calculation module is used for determining the output duty ratio of a Boost circuit according to the output power of the inverter and the voltage of the direct-current bus if the first judgment result is that variable power output control is executed;
the output power adjusting module is used for adjusting the output power of the photovoltaic system according to the output duty ratio of the Boost circuit;
the grid voltage value acquisition module is used for acquiring a grid voltage value after the voltage of the photovoltaic grid-connected power generation system drops;
the active reactive current given value calculation module is used for determining a reactive current given value and an active current given value according to the power grid voltage value after the voltage drop;
and the reactive power adjusting module is used for adjusting the reactive power emitted by the inverter in the photovoltaic grid-connected power generation system according to the reactive current given value and the active current given value.
Optionally, the variable power output control and judgment module specifically includes:
the output power judgment unit is used for judging whether the output power of the inverter is smaller than or equal to an active power reference value during the fault period, and obtaining a second judgment result;
a first determination result unit, configured to determine that the first determination result is not to perform variable power output control if the second determination result indicates that the inverter output power is greater than an active power reference value during a fault;
a dc bus voltage determining unit, configured to determine whether the dc bus voltage is less than or equal to a lower limit of a set voltage value if the second determination result is that the output power of the inverter is less than or equal to an active power reference value during a fault period, and obtain a third determination result;
a second determination result unit, configured to determine that the first determination result is not to perform variable power output control if the third determination result indicates that the dc bus voltage is less than or equal to a set voltage lower limit;
and a third determination result unit, configured to determine that the first determination result is to perform variable power output control if the third determination result is that the dc bus voltage is greater than a lower limit of a set voltage value and less than an upper limit of the set voltage value.
Optionally, the duty ratio calculating module specifically includes:
a DC bus current calculating unit for calculating the output power P of the inverterinjAnd the DC bus voltage UdcUsing formula Idc＝Pinj/UdcDetermining DC bus current Idc；
A current power parameter obtaining unit for obtaining a current value I corresponding to the maximum power point of the photovoltaic systemMSum power value PM；
A current reference value calculating unit for calculating a current reference value according to the current value IMSum power value PMBy the formula
Figure BDA0002441168890000051
Determining a current reference value at a maximum power point of the photovoltaic system
Figure BDA0002441168890000052
Wherein
Figure BDA0002441168890000053
The output power reference value at the maximum power point of the photovoltaic system is obtained;
a duty ratio calculation unit for calculating the duty ratio according to the DC bus current IdcAnd the current reference value
Figure BDA0002441168890000054
Using a formula
Figure BDA0002441168890000055
And determining the output duty ratio D of the Boost circuit.
Optionally, the active reactive current given value calculation module specifically includes:
a reactive current given value calculation unit for adopting an equation set according to the power grid voltage value U after the voltage drop
Figure BDA0002441168890000056
Determining a reactive current setpoint value
Figure BDA0002441168890000057
Wherein U isNRepresenting a grid voltage rating; i.e. iNRepresenting an inverter output current rating;
an active current given value calculation unit for calculating the reactive current given value according to the reactive current given value
Figure BDA0002441168890000058
Using a formula
Figure BDA0002441168890000059
Determining an active current setpoint
Figure BDA00024411688900000510
According to the specific embodiment provided by the invention, the invention discloses the following technical effects:
the invention provides a photovoltaic system low-voltage ride through method and a system based on variable power output control, wherein the method comprises the steps of firstly, judging whether to execute variable power output control according to inverter output power and direct-current bus voltage in a photovoltaic grid-connected power generation system, and if the variable power output control is not executed, keeping the photovoltaic system in a maximum power output state; if variable power output control is executed, determining the output duty ratio of a Boost circuit according to the output power of the inverter and the voltage of the direct-current bus, and adjusting the output power of the photovoltaic system according to the output duty ratio of the Boost circuit; meanwhile, obtaining a grid voltage value after the voltage of the photovoltaic grid-connected power generation system drops, and calculating a reactive current given value and an active current given value; and adjusting the reactive power emitted by the inverter in the photovoltaic grid-connected power generation system according to the reactive current given value and the active current given value. According to the method, when the voltage drop depth is small, the photovoltaic system can still keep the maximum power output; when the grid voltage drop depth is large, the output power of the photovoltaic system is actively adjusted based on the power balance problem at two sides of the inverter, and unbalanced power generated by grid faults is quickly eliminated; and during the fault period, according to the difference of voltage drop depths, the distribution of active and reactive current values is carried out again, so that the output of the inverter is ensured not to be over-current, and the low-voltage ride-through process is completed. The method and the system can give consideration to the safe and stable operation of the photovoltaic grid-connected power generation system during the grid fault period and the full utilization of solar energy resources, and have more superiority compared with the prior art.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without inventive exercise.
FIG. 1 is a flow chart of a photovoltaic system low voltage ride through method based on variable power output control according to the present invention;
FIG. 2 is a schematic diagram of a photovoltaic system low voltage crossing method based on variable power output control according to the present invention;
fig. 3 is a structural diagram of a two-stage photovoltaic grid-connected power generation system provided by the present invention;
FIG. 4 is a schematic diagram of a voltage hierarchy based on hysteresis control according to the present invention;
FIG. 5 is a control schematic diagram of mode switching provided by the present invention;
FIG. 6 is a block diagram of the inverter control provided by the present invention;
FIG. 7 is a schematic diagram of a simulation result of the photovoltaic system low voltage ride through method based on variable power output control according to the present invention; FIG. 7(a) is a schematic of photovoltaic system output power; FIG. 7(b) is a schematic diagram of DC bus voltage; fig. 7(c) is a schematic diagram of the inverter output active power; fig. 7(d) is a schematic diagram of the inverter output reactive power.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The invention aims to provide a photovoltaic system low-voltage ride-through method and system based on variable power output control, and aims to solve the problem that the low-voltage ride-through process cannot be smoothly completed by adopting the prior art when the voltage drop depth is large.
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.
FIG. 1 is a flow chart of a photovoltaic system low voltage ride through method based on variable power output control according to the present invention; fig. 2 is a schematic diagram of a photovoltaic system low-voltage crossing method based on variable power output control according to the present invention. Referring to fig. 1 and fig. 2, the photovoltaic system low voltage ride through method based on variable power output control provided by the present invention specifically includes:
step 101: and acquiring the output power of an inverter and the voltage of a direct current bus in the photovoltaic grid-connected power generation system.
The method is applied to the existing photovoltaic grid-connected power generation system, and the photovoltaic grid-connected power generation system comprises a photovoltaic system, a Boost circuit, an inverter and a power grid. Fig. 3 is a structural diagram of a two-stage photovoltaic grid-connected power generation system provided by the invention. As shown in fig. 3, a complete two-stage three-phase photovoltaic grid-connected power generation system includes a photovoltaic system, a Boost circuit, an inverter, a power grid, and the like. The photovoltaic system is connected with a Boost circuit on the right side, the Boost circuit is connected with an inverter on the right side, and the output of the inverter is connected with a power grid. The Boost circuit controls the output power of the photovoltaic cell, raises the voltage to a proper value, and is inverted by the inverter and then is incorporated into a power grid.
In fig. 3, the photovoltaic system is connected to the Boost circuit, and the photovoltaic system cannot meet the requirement of the grid-connected power generation system due to a small output voltage value of the photovoltaic system, so that the photovoltaic system boosts the voltage through the Boost circuit and then through the Boost circuitThe circuit completes Maximum Power Point Tracking (MPPT). The Boost circuit is connected with the inverter through a direct current bus, and is connected with the grid through the inversion function of the inverter. C in FIG. 3pvA photovoltaic side is connected with a capacitor in parallel; d is the output duty ratio of the Boost circuit; cdcIs a DC side filter capacitor, UdcIs the dc bus voltage. T is1～T6The inverter comprises 6 switching tube elements. i.e. ia、ib、icFor the inverter to output three-phase currents ua、ub、ucFor outputting three-phase voltages to the inverter, ea、eb、ecIs the three-phase voltage of the power grid. And L is a filter inductor.
The invention outputs power P to the inverterinjSampling and comparing the active power reference value P during the fault period*And PinjIf P is the size ofinj>P*The photovoltaic system keeps the maximum power output state; if Pinj≤P*Then the principle of hysteresis control is used to set two threshold voltages U1、U2Sampling the DC bus voltage UdcValue when Udc≤U1When the photovoltaic system is in the maximum power output state, the photovoltaic system is executed to maintain the maximum power output state, otherwise, the variable power output control is executed, namely step 104 is executed.
Step 102: and judging whether to execute variable power output control or not according to the output power of the inverter and the voltage of the direct current bus to obtain a first judgment result.
Fig. 4 is a schematic diagram of a voltage layering mode based on hysteresis control provided by the present invention, where the abscissa of fig. 4 represents a dc bus voltage value and the ordinate represents a control mode. U in FIG. 41To set the lower limit of the voltage value, U2To set the upper voltage value limit. As shown in fig. 4, when unbalanced power is generated due to a grid fault, the dc bus voltage U is applieddcWhen U risesdc≤U1In the process, the photovoltaic system keeps the maximum power output and finishes MPPT through a Boost circuit; when U is turned1<Udc<U2In time, the control is switched to variable power output control (variable power control for short), and the direct current bus voltage U in the fault period is preventeddcRises to a threshold voltage U0。
Therefore, the step 102 specifically includes:
judging the output power P of the inverterinjWhether or not it is less than or equal to the active power reference value P during a fault*Obtaining a second judgment result;
if the second judgment result is the output power P of the inverterinjGreater than the active power reference value P during a fault*Determining that the first judgment result is that the variable power output control is not executed; step 103 is executed, and the photovoltaic system in the photovoltaic grid-connected power generation system keeps a maximum power output state;
if the second judgment result is the output power P of the inverterinjIs less than or equal to the active power reference value P during the fault*Then, the DC bus voltage U is judgeddcWhether the lower limit of the set voltage value U is less than or equal to1Obtaining a third judgment result;
if the third judgment result is the DC bus voltage UdcLess than or equal to the lower limit U of the set voltage value1Determining that the first judgment result is that the variable power output control is not executed; step 103 is executed, and the photovoltaic system in the photovoltaic grid-connected power generation system keeps a maximum power output state;
if the third judgment result is the DC bus voltage UdcGreater than the lower limit U of the set voltage value1And is less than the upper limit U of the set voltage value2Determining that the first judgment result is to execute variable power output control; then step 104 is executed, according to the inverter output power PinjAnd the DC bus voltage UdcAnd determining the output duty ratio D of the Boost circuit.
Step 103: and if the first judgment result is that the variable power output control is not executed, the photovoltaic system in the photovoltaic grid-connected power generation system keeps a maximum power output state.
Step 104: and if the first judgment result is that variable power output control is executed, determining the output duty ratio of a Boost circuit according to the output power of the inverter and the voltage of the direct-current bus.
Fig. 5 is a control schematic diagram of mode switching provided by the present invention. As shown in FIG. 5, the present invention implements different control strategies based on the voltage stratification results, such as when Udc≤U1When the MPPT is controlled, the maximum power output of the photovoltaic system is kept, and the MPPT control of the photovoltaic system is completed through a Boost circuit; when U is turned1<Udc<U2And meanwhile, switching to variable power control, and adjusting the output power of the photovoltaic system by changing the output duty ratio D of the Boost circuit. The expression of the duty ratio D of the output circuit obtained by the working principle of the Boost circuit and the power conservation equation is as follows:
Figure BDA0002441168890000091
d in formula (1) is the output duty ratio of the Boost circuit, Idc、UdcThe dc bus current and voltage values are respectively.
Figure BDA0002441168890000092
The standard values of the current and the power of the maximum power point of the photovoltaic system are respectively, namely the current and the output power reference value of the maximum power point of the photovoltaic system. I isM、PMThe current and the power value corresponding to the maximum power point of the photovoltaic system are respectively, namely the actual sampling values of the current and the power when the photovoltaic system outputs the maximum power. PinjAnd outputting power for the inverter.
Therefore, the step 104 of determining the output duty cycle of the Boost circuit according to the output power of the inverter and the dc bus voltage specifically includes:
according to the output power P of the inverterinjAnd the DC bus voltage UdcUsing formula Idc＝Pinj/UdcDetermining DC bus current Idc；
Obtaining a current value I corresponding to the maximum power point of the photovoltaic systemMSum power value PM；
According to the current value IMSum power value PMBy the formula
Figure BDA0002441168890000093
Determining a current reference value at a maximum power point of the photovoltaic system
Figure BDA0002441168890000094
Wherein
Figure BDA0002441168890000095
The output power reference value at the maximum power point of the photovoltaic system is obtained;
according to the DC bus current IdcAnd the current reference value
Figure BDA0002441168890000096
Using a formula
Figure BDA0002441168890000097
And determining the output duty ratio D of the Boost circuit.
Step 105: and adjusting the output power of the photovoltaic system according to the output duty ratio of the Boost circuit.
The calculated duty ratio D is applied to a switching tube of the Boost circuit in fig. 3, and the output power of the photovoltaic system is changed by adjusting the duty ratio D.
According to the invention, the power output of the photovoltaic system is adjusted during the fault period, and the output power of the photovoltaic system is changed in real time, so that the target tracked by the power point is changed from the maximum power point to the optimal power point which is more beneficial to realizing low voltage ride through, the energy emitted by the photovoltaic cell is actively reduced, the generation of redundant energy is reduced from the source, the output of the power of the photovoltaic cell is rapidly and effectively controlled, the balance of the power at two sides of the inverter is realized, and the overvoltage of a direct current bus and the overcurrent of grid-connected current are avoided.
Step 106: and acquiring the voltage value of the power grid after the voltage of the photovoltaic grid-connected power generation system drops.
Fig. 6 is a diagram illustrating a control structure of the inverter according to the present invention. Fig. 6 shows a classical dual closed-loop control manner of an inverter in the photovoltaic grid-connected power generation system. Wherein iqFor inverter transmissionSampling values of q-axis components of the outgoing current are also expressed as sampling values of reactive current; i.e. idSampled values for the d-axis component of the inverter output current.
Figure BDA0002441168890000101
Respectively representing a reactive current given value and an active current given value. e.g. of the typed、eqThe components d and q of the power grid voltage are respectively, omega is the power grid angular frequency, and L is the filter inductance. U shaped、UqD, q components of the obtained control reference voltage, respectively, like Uα、UβThe control reference voltage obtained is α and β components, SVPWM (space vector pulse width modulation) is a space vector modulation mode of the inverter.
In order to realize space vector modulation of the inverter during the fault period, the method firstly obtains a power grid voltage value U after the voltage of the photovoltaic grid-connected power generation system drops.
Step 107: and determining a reactive current set value and an active current set value according to the power grid voltage value after the voltage drop.
During a fault, the inverter is to deliver a certain amount of reactive power to support the restoration of the grid voltage, at which time the given value i of the reactive current* qComprises the following steps:
Figure BDA0002441168890000102
i in formula (2)* qFor the q-axis component of the inverter output current, i is obtained from the instantaneous power theory* qCan be expressed as a reactive current setpoint. i.e. iNThe rated value of the output current of the inverter is a technical nameplate parameter. And U is an actual measured value after the voltage of the power grid falls, namely the voltage value of the power grid after the voltage falls. U shapeNRepresenting the grid voltage rating, typically 380V. U/UNI.e. representing the grid voltage sag depth. i.e. i* qThe method is obtained by calculation according to the actual grid-connected power requirement and capacity.
In order to prevent the output of the inverter from overflowing, namely not exceeding 1.1 times of the rated current value, the method is obtainedDesired active current set value during fault
Figure BDA0002441168890000111
Comprises the following steps:
Figure BDA0002441168890000112
therefore, the step 107 of determining the reactive current set value and the active current set value according to the grid voltage value after the voltage drop specifically includes:
according to the power grid voltage value U after the voltage drop, adopting an equation set
Figure BDA0002441168890000113
Determining a reactive current setpoint value
Figure BDA0002441168890000114
Wherein U isNRepresenting a grid voltage rating; i.e. iNRepresenting an inverter output current rating;
according to the given value of the reactive current
Figure BDA0002441168890000115
Using a formula
Figure BDA0002441168890000116
Determining an active current setpoint
Figure BDA0002441168890000117
Active current set value
Figure BDA0002441168890000118
And reactive current set point
Figure BDA0002441168890000119
Both values are used as the outer loop current control set values in the inverter control strategy. The invention completes the low voltage ride through process by realizing the self-adaptive adjustment of the output power of the photovoltaic system, and the low voltage ride through processIs the control of the inverter.
Step 108: and adjusting the reactive power emitted by the inverter in the photovoltaic grid-connected power generation system according to the reactive current given value and the active current given value.
As can be seen in fig. 6, the inverter adopts a single-loop control mode during a fault, and the active and reactive current reference values i during the fault* dAnd i* qAccording to the method described in step 107, the calculation is carried out, so that the inverter is controlled according to the active and reactive current reference values i* dAnd i* qA certain amount of reactive power is emitted to support the restoration of the grid voltage.
According to the invention, through 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.
By adopting the method, when the power grid fails, in order to realize the maximization of power balance and solar energy resource utilization, when the voltage drop depth is smaller (less than 30%), the photovoltaic system can still keep the maximum power output. When the voltage drop depth is large (more than 50%), because the unbalanced power accumulated by the direct-current side capacitor is large, the unbalanced power can not be eliminated in a short time, two threshold voltages are set according to the hysteresis control principle to realize the switching of two control modes, the unbalanced power is quickly eliminated, and the low-voltage ride-through process is completed. Wherein the variable power control of the photovoltaic system is realized by changing the duty ratio D of the Boost circuit, and step 104 provides a method for calculating the duty ratio D. Step 104 is carried out for the purpose of adjusting the output power of the photovoltaic system, step 107 is carried out for calculating a given value of the control current command, and the control objects of step 105 and step 108 are the photovoltaic system and the inverter respectively. The photovoltaic system has the advantages that the problem of power imbalance under the working condition of power grid faults when the voltage drop depth is large is solved, the stability of direct-current bus voltage and the full utilization of solar energy resources are considered, and the photovoltaic system can still keep the maximum power output when the voltage drop depth is small; when the voltage drop depth is large, the unbalanced power under the fault working condition is quickly eliminated by changing the output of the photovoltaic system, and the stability of the photovoltaic grid-connected power generation system during the low-voltage ride through period is improved.
Based on the photovoltaic system low voltage ride through method provided by the invention, the invention also provides a photovoltaic system low voltage ride through system based on variable power output control, and the photovoltaic system low voltage ride through system comprises:
the system parameter acquisition module is used for acquiring the output power of an inverter and the voltage of a direct-current bus in the photovoltaic grid-connected power generation system; the photovoltaic grid-connected power generation system comprises a photovoltaic system, a Boost circuit, an inverter and a power grid;
the variable power output control judging module is used for judging whether to execute variable power output control according to the output power of the inverter and the voltage of the direct current bus to obtain a first judging result;
the maximum power output maintaining module is used for maintaining the maximum power output state of the photovoltaic system in the photovoltaic grid-connected power generation system if the first judgment result indicates that the variable power output control is not executed;
the duty ratio calculation module is used for determining the output duty ratio of a Boost circuit according to the output power of the inverter and the voltage of the direct-current bus if the first judgment result is that variable power output control is executed;
the output power adjusting module is used for adjusting the output power of the photovoltaic system according to the output duty ratio of the Boost circuit;
the grid voltage value acquisition module is used for acquiring a grid voltage value after the voltage of the photovoltaic grid-connected power generation system drops;
the active reactive current given value calculation module is used for determining a reactive current given value and an active current given value according to the power grid voltage value after the voltage drop;
and the reactive power adjusting module is used for adjusting the reactive power emitted by the inverter in the photovoltaic grid-connected power generation system according to the reactive current given value and the active current given value.
Wherein, the power-variable output control judging module specifically comprises:
the output power judgment unit is used for judging whether the output power of the inverter is smaller than or equal to an active power reference value during the fault period, and obtaining a second judgment result;
a first determination result unit, configured to determine that the first determination result is not to perform variable power output control if the second determination result indicates that the inverter output power is greater than an active power reference value during a fault;
a dc bus voltage determining unit, configured to determine whether the dc bus voltage is less than or equal to a lower limit of a set voltage value if the second determination result is that the output power of the inverter is less than or equal to an active power reference value during a fault period, and obtain a third determination result;
a second determination result unit, configured to determine that the first determination result is not to perform variable power output control if the third determination result indicates that the dc bus voltage is less than or equal to a set voltage lower limit;
and a third determination result unit, configured to determine that the first determination result is to perform variable power output control if the third determination result is that the dc bus voltage is greater than a lower limit of a set voltage value and less than an upper limit of the set voltage value.
The duty ratio calculation module specifically includes:
a DC bus current calculating unit for calculating the output power P of the inverterinjAnd the DC bus voltage UdcUsing formula Idc＝Pinj/UdcDetermining DC bus current Idc；
A current power parameter obtaining unit for obtaining a current value I corresponding to the maximum power point of the photovoltaic systemMSum power value PM；
A current reference value calculating unit for calculating a current reference value according to the current value IMSum power value PMBy the formula
Figure BDA0002441168890000131
Determining the photovoltaic systemCurrent reference value at maximum power point
Figure BDA0002441168890000132
Wherein
Figure BDA0002441168890000133
The output power reference value at the maximum power point of the photovoltaic system is obtained;
a duty ratio calculation unit for calculating the duty ratio according to the DC bus current IdcAnd the current reference value
Figure BDA0002441168890000134
Using a formula
Figure BDA0002441168890000135
And determining the output duty ratio D of the Boost circuit.
The active reactive current given value calculation module specifically comprises:
a reactive current given value calculation unit for adopting an equation set according to the power grid voltage value U after the voltage drop
Figure BDA0002441168890000141
Determining a reactive current setpoint value
Figure BDA0002441168890000142
Wherein U isNRepresenting a grid voltage rating; i.e. iNRepresenting an inverter output current rating;
an active current given value calculation unit for calculating the reactive current given value according to the reactive current given value
Figure BDA0002441168890000143
Using a formula
Figure BDA0002441168890000144
Determining an active current setpoint
Figure BDA0002441168890000145
The following utilizes simulation models toFurther illustrating the technical effects of the method and system of the present invention. A simulation model of a two-stage three-phase photovoltaic grid-connected power generation system with the capacity of 21kW is established in Matlab/Simulink, and the feasibility of the photovoltaic system low-voltage ride through method based on variable power output control provided by the invention is verified. The experimental simulation simulates the standard environment condition (the illumination S is 1000W/m) of the photovoltaic test2And the simulation result of the grid voltage drop at the temperature T of 25 ℃ is shown in FIG. 7, and shows the dynamic change process of the operation state of the grid-connected system in the low-voltage ride-through process.
Fig. 7 is a schematic diagram of a simulation result of the photovoltaic system low voltage ride through method based on variable power output control provided by the present invention, and simulation parameters are set as follows: the voltage of a power grid is 380V, the frequency is 150Hz, a symmetrical drop fault occurs in 0.5s, the drop depth is 70%, and the recovery is carried out in 0.8 s; in FIG. 6, two PI regulator parameters are set to K respectivelyp1＝10，Kp2＝17，Ki1120, Ki2, 1200; the hysteresis voltages are 730V and 675V respectively; maximum allowable voltage U of direct current bus0Is 740V.
Wherein fig. 7(a) is a schematic diagram of the output power of the photovoltaic system, the abscissa is time, and the ordinate is the output power of the photovoltaic system; fig. 7(b) is a schematic diagram of dc bus voltage, with time on the abscissa and dc bus voltage on the ordinate; fig. 7(c) is a schematic diagram of the active power output by the inverter, where the abscissa is time and the ordinate is the active power output by the inverter; fig. 7(d) is a schematic diagram of the reactive power output by the inverter, the abscissa is time, and the ordinate is the reactive power output by the inverter. As can be seen from fig. 7(a), when the voltage drop depth is 70%, the photovoltaic system initially operates at the maximum power point, and as the dc bus voltage rises, the photovoltaic system starts to adjust its own output power value to complete the low voltage ride through process. As can be seen from fig. 7(a) and (b), when the dc bus voltage exceeds 730V, the photovoltaic system reduces the output power thereof to stabilize the dc bus voltage below the threshold voltage 740V. As shown in fig. 7(c) and (d), the inverter performs a fully reactive operation, and meets the power output requirement during the low voltage ride through.
The invention discloses a photovoltaic system low-voltage ride through method and a photovoltaic system low-voltage ride through system based on variable power output control, belonging to the technical field of new energy grid-connected power generation system protection and control, wherein when the grid voltage drop depth is larger, the output of a photovoltaic system is actively adjusted based on the power balance problem at two sides of an inverter, and unbalanced power generated due to grid faults is quickly eliminated; and during the fault period, according to the difference of voltage drop depths, the distribution of active and reactive current values is carried out again, so that the output of the inverter is ensured not to be over-current, and the low-voltage ride-through process is completed. By adopting the scheme of the invention, the safe and stable operation of the photovoltaic grid-connected power generation system during the grid fault period and the full utilization of solar energy resources can be considered, and compared with the prior art, the scheme of the invention has more superiority.
The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other. For the system disclosed by the embodiment, the description is relatively simple because the system corresponds to the method disclosed by the embodiment, and the relevant points can be referred to the method part for description.
The principles and embodiments of the present invention have been described herein using specific examples, which are provided only to help understand the method and the core concept of the present invention; meanwhile, for a person skilled in the art, according to the idea of the present invention, the specific embodiments and the application range may be changed. In view of the above, the present disclosure should not be construed as limiting the invention.

Claims (8)

1. A photovoltaic system low voltage ride through method based on variable power output control is characterized by comprising the following steps:
acquiring inverter output power and direct-current bus voltage in a photovoltaic grid-connected power generation system; the photovoltaic grid-connected power generation system comprises a photovoltaic system, a Boost circuit, an inverter and a power grid;
judging whether to execute variable power output control or not according to the output power of the inverter and the voltage of the direct current bus to obtain a first judgment result;
if the first judgment result is that the variable power output control is not executed, the photovoltaic system in the photovoltaic grid-connected power generation system keeps a maximum power output state;
if the first judgment result is that variable power output control is executed, determining the output duty ratio of a Boost circuit according to the output power of the inverter and the voltage of the direct-current bus;
adjusting the output power of the photovoltaic system according to the output duty ratio of the Boost circuit;
acquiring a grid voltage value after the voltage of the photovoltaic grid-connected power generation system drops;
determining a reactive current set value and an active current set value according to the power grid voltage value after the voltage drop;
and adjusting the reactive power emitted by the inverter in the photovoltaic grid-connected power generation system according to the reactive current given value and the active current given value.
2. The method for photovoltaic system low voltage ride through according to claim 1, wherein the determining whether to perform variable power output control according to the inverter output power and the dc bus voltage to obtain a first determination result specifically includes:
judging whether the output power of the inverter is smaller than or equal to an active power reference value during the fault period, and obtaining a second judgment result;
if the second judgment result is that the output power of the inverter is greater than the active power reference value during the fault period, determining that the first judgment result is that the variable power output control is not executed;
if the second judgment result is that the output power of the inverter is smaller than or equal to the active power reference value in the fault period, judging whether the voltage of the direct current bus is smaller than or equal to a set voltage value lower limit, and obtaining a third judgment result;
if the third judgment result is that the direct current bus voltage is less than or equal to the lower limit of the set voltage value, determining that the first judgment result is that the variable power output control is not executed;
and if the third judgment result is that the direct current bus voltage is greater than the lower limit of the set voltage value and less than the upper limit of the set voltage value, determining that the first judgment result is to execute variable power output control.
3. The photovoltaic system low voltage ride through method according to claim 2, wherein the determining the Boost circuit output duty cycle according to the inverter output power and the dc bus voltage specifically comprises:
according to the output power P of the inverterinjAnd the DC bus voltage UdcUsing formula Idc＝Pinj/UdcDetermining DC bus current Idc；
Obtaining a current value I corresponding to the maximum power point of the photovoltaic systemMSum power value PM；
According to the current value IMSum power value PMBy the formula
Figure FDA0002441168880000021
Determining a current reference value at a maximum power point of the photovoltaic system
Figure FDA0002441168880000022
Wherein
Figure FDA0002441168880000023
The output power reference value at the maximum power point of the photovoltaic system is obtained;
according to the DC bus current IdcAnd the current reference value
Figure FDA0002441168880000024
Using a formula
Figure FDA0002441168880000025
And determining the output duty ratio D of the Boost circuit.
4. The photovoltaic system low-voltage ride-through method according to claim 3, wherein the determining the reactive current given value and the active current given value according to the grid voltage value after the voltage drop specifically comprises:
according to the power grid voltage value U after the voltage drop, adopting an equation set
Figure FDA0002441168880000026
Determining a reactive current setpoint value
Figure FDA0002441168880000027
Wherein U isNRepresenting a grid voltage rating; i.e. iNRepresenting an inverter output current rating;
according to the given value of the reactive current
Figure FDA0002441168880000028
Using a formula
Figure FDA0002441168880000029
Determining an active current setpoint
Figure FDA00024411688800000210
5. A photovoltaic system low voltage ride through system based on variable power output control, the photovoltaic system low voltage ride through system comprising:
the system parameter acquisition module is used for acquiring the output power of an inverter and the voltage of a direct-current bus in the photovoltaic grid-connected power generation system; the photovoltaic grid-connected power generation system comprises a photovoltaic system, a Boost circuit, an inverter and a power grid;
the variable power output control judging module is used for judging whether to execute variable power output control according to the output power of the inverter and the voltage of the direct current bus to obtain a first judging result;
the maximum power output maintaining module is used for maintaining the maximum power output state of the photovoltaic system in the photovoltaic grid-connected power generation system if the first judgment result indicates that the variable power output control is not executed;
the duty ratio calculation module is used for determining the output duty ratio of a Boost circuit according to the output power of the inverter and the voltage of the direct-current bus if the first judgment result is that variable power output control is executed;
the output power adjusting module is used for adjusting the output power of the photovoltaic system according to the output duty ratio of the Boost circuit;
the grid voltage value acquisition module is used for acquiring a grid voltage value after the voltage of the photovoltaic grid-connected power generation system drops;
the active reactive current given value calculation module is used for determining a reactive current given value and an active current given value according to the power grid voltage value after the voltage drop;
and the reactive power adjusting module is used for adjusting the reactive power emitted by the inverter in the photovoltaic grid-connected power generation system according to the reactive current given value and the active current given value.
6. The photovoltaic system low voltage ride through system of claim 5, wherein the variable power output control and determination module specifically comprises:
the output power judgment unit is used for judging whether the output power of the inverter is smaller than or equal to an active power reference value during the fault period, and obtaining a second judgment result;
a first determination result unit, configured to determine that the first determination result is not to perform variable power output control if the second determination result indicates that the inverter output power is greater than an active power reference value during a fault;
a dc bus voltage determining unit, configured to determine whether the dc bus voltage is less than or equal to a lower limit of a set voltage value if the second determination result is that the output power of the inverter is less than or equal to an active power reference value during a fault period, and obtain a third determination result;
a second determination result unit, configured to determine that the first determination result is not to perform variable power output control if the third determination result indicates that the dc bus voltage is less than or equal to a set voltage lower limit;
and a third determination result unit, configured to determine that the first determination result is to perform variable power output control if the third determination result is that the dc bus voltage is greater than a lower limit of a set voltage value and less than an upper limit of the set voltage value.
7. The photovoltaic system low voltage ride through system of claim 6, wherein the duty cycle calculation module specifically comprises:
a DC bus current calculating unit for calculating the output power P of the inverterinjAnd the DC bus voltage UdcUsing formula Idc＝Pinj/UdcDetermining DC bus current Idc；
A current power parameter obtaining unit for obtaining a current value I corresponding to the maximum power point of the photovoltaic systemMSum power value PM；
A current reference value calculating unit for calculating a current reference value according to the current value IMSum power value PMBy the formula
Figure FDA0002441168880000041
Determining a current reference value at a maximum power point of the photovoltaic system
Figure FDA0002441168880000042
Wherein
Figure FDA0002441168880000043
The output power reference value at the maximum power point of the photovoltaic system is obtained;
a duty ratio calculation unit for calculating the duty ratio according to the DC bus current IdcAnd the current reference value
Figure FDA0002441168880000044
Using a formula
Figure FDA0002441168880000045
And determining the output duty ratio D of the Boost circuit.
8. The photovoltaic system low voltage ride through system of claim 7, wherein the active reactive current setpoint calculation module specifically comprises:
a reactive current given value calculation unit for adopting an equation set according to the power grid voltage value U after the voltage drop
Figure FDA0002441168880000046
Determining a reactive current setpoint value
Figure FDA0002441168880000047
Wherein U isNRepresenting a grid voltage rating; i.e. iNRepresenting an inverter output current rating;
an active current given value calculation unit for calculating the reactive current given value according to the reactive current given value
Figure FDA0002441168880000048
Using a formula
Figure FDA0002441168880000049
Determining an active current setpoint
Figure FDA00024411688800000410
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