CN116154860A - Improved method of phase-locked loop of photovoltaic inverter for reactive power support under low voltage ride through - Google Patents
Improved method of phase-locked loop of photovoltaic inverter for reactive power support under low voltage ride through Download PDFInfo
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- H—ELECTRICITY
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Abstract
The invention discloses a method for improving a phase-locked loop of a photovoltaic inverter for reactive power support under low voltage ride through, which comprises the following steps: firstly, in a fault occurrence stage, acquiring a transient phase error through an arctangent value of a voltage component under a rotation coordinate system obtained by phase tracking of a phase-locked loop, adding an exponential function link related to a low-voltage fault phase error in the phase-locked loop in real time, and recording the difference between the voltage phases before and after the occurrence of the low-voltage fault; and in the fault recovery stage, an exponential function link related to the low-voltage fault phase jump angle is added into the phase-locked loop, so that the phase angle following speed of the phase-locked loop and the reactive power withdrawing speed of the inverter during fault recovery are accelerated. The invention can effectively improve the accuracy of reactive power control of low voltage faults, inhibit the overvoltage after recovery of the low voltage faults caused by inaccurate reactive power control and the like, and improve the low voltage ride through capability of the system.
Description
Technical Field
The invention relates to the technical field of photovoltaic inverters and phase-locked loops, in particular to an improvement method of a photovoltaic inverter phase-locked loop for reactive power support under low voltage ride through.
Background
With the continuous promotion of the 'double carbon' target in China, new energy sources such as photovoltaic stations and the like are rapidly developed, the new energy sources are accessed into an electric power system in a large scale, the new energy sources and the duty ratio are larger and larger, and a 'double high' electric power system is being formed. Meanwhile, due to the increase of the scale of the photovoltaic field station and uneven geographical distribution of solar energy resources, the grid connection of the light Fu Changzhan shows the characteristic of accessing a weak power grid. In weak grids, photovoltaic sites have a greater impact on the stable operation of the power system. The control strategy of the traditional photovoltaic inverter relies on the phase-locked loop to synchronize along with the power grid voltage, and takes the phase-locked loop (SRF-PLL) with the most widely applied synchronous coordinate system as an example at present, the phase-locked loop directly realizes coordinate synchronization through coordinate transformation, and the information such as amplitude, frequency and phase of the power grid voltage can be accurately obtained under a three-phase ideal power grid. In the low-voltage fault, the voltage drop and the phase jump of the power grid voltage can occur at the same time, which causes the problem of phase-locked loop phase-locked misalignment in the transient process of the occurrence and recovery of the low-voltage fault, the phase-locked misalignment can cause reactive power control misalignment of the photovoltaic inverter, and the high-voltage fault after the low-voltage fault can occur in serious cases.
Many phase-locked loop parameter design or improvement strategies and phase-locked loop models are currently proposed to enhance the control strategy. In order to make the inverter connected to the weak grid have a larger stability margin, researchers have proposed a parameter design method that considers the coupling of the current loop and the phase-locked loop. And a low-pass filtering link is added to the phase-locked loop to enhance the performance of the inverter in a weak power grid. In order to solve the transient stability problem during low voltage faults, researchers shield the integrator of the phase-locked loop PI link to obtain a first-order integrated loop so as to reduce adverse effects caused by angle overshoot. Researchers have proposed using a second order oscillation link instead of the original PI control link to improve phase-locked loop characteristics. And a learner introduces the characteristic of a reference synchronous machine, uses a synchronous machine model to replace a traditional phase-locked loop, and proposes various phase angle control and improvement strategies to ensure transient stability in the low-voltage fault process.
In weak current networks, the reactive power has a remarkable effect on the voltage stability of the power system, but the improvement research of the phase-locked loop rarely considers the effect of the phase-locked loop characteristics on reactive power support during fault transient, the improvement cannot restrain the transient reactive power effect caused by voltage phase jump under low voltage fault, and the realization of the improvement of the phase-locked loop often needs huge calculation cost, so that the improvement is difficult to apply and realize in engineering practice in a photovoltaic inverter.
Disclosure of Invention
The present invention aims to solve at least one of the technical problems in the related art to some extent.
Therefore, the invention provides the photovoltaic inverter phase-locked loop improvement method for reactive power support under low voltage ride through, which is based on the improvement of the traditional synchronous coordinate system phase-locked loop, does not occupy huge calculation cost in engineering practice, and can well inhibit the influence caused by voltage phase jump during the three-phase symmetrical low voltage fault. By performing phase compensation on a phase-locked loop (PLL) of the photovoltaic inverter, the reactive transient characteristic of the photovoltaic inverter during a three-phase symmetrical low-voltage fault transient period is improved, the problem of inaccurate phase locking during the transient period can be restrained, reactive impact of the low-voltage transient period is reduced, reactive withdrawal speed is accelerated, and continuous fault risks of low-voltage ride through and high-voltage ride through are reduced.
Another object of the invention is to propose an improved system for a photovoltaic inverter phase locked loop for reactive support at low voltage ride through.
To achieve the above object, in one aspect, the present invention provides a method for improving a phase-locked loop of a photovoltaic inverter for reactive power support under low voltage ride through, comprising:
grid-connected control of the photovoltaic inverter is performed by utilizing a voltage directional vector control strategy so as to control the voltage of a power grid through a phase-locked loop and monitor whether the power grid fails or not;
based on the monitored three-phase symmetrical low-voltage faults of the power grid, the initial phase error of the phase-locked loop when the low-voltage faults occur is obtained by solving the arctangent value of the voltage component output by the phase-locked loop;
providing phase compensation of the phase-locked loop for a transient phase at which a low voltage fault occurs according to the initial phase error and an exponential function added in the phase-locked loop, and calculating a final phase difference of the grid voltage during low voltage ride through;
and providing phase compensation based on the phase angle following speed of the phase-locked loop for the transient phase of the low-voltage fault recovery based on the final phase difference and an exponential function added in the phase-locked loop, so that the photovoltaic inverter quickly recovers to normal working conditions.
In addition, the improved method of the phase-locked loop of the photovoltaic inverter for reactive power support under low voltage ride through according to the embodiment of the invention can also have the following additional technical characteristics:
further, in an embodiment of the present invention, the low voltage fault based on the detected three-phase symmetry of the power grid provides corresponding reactive support according to the detected voltage drop degree of the power grid, wherein the reactive current requirement is as follows:
I Tq ＝k×(0.9-U T )×I N (U T <0.9)
wherein I is Tq For reactive current target values required to be provided during low voltage faults, U T Is the per-unit value of the voltage during low voltage faults, I N For the photovoltaic inverter rated current, k is an empirical factor with voltage sag.
Further, in one embodiment of the present invention, when the photovoltaic inverter executes a low voltage ride through control strategy, the voltage amplitude U is determined T Calculate I Tq And will I Tq As the value of I in the phase-locked loop coordinate system q The reference value error is:
further, in one embodiment of the present invention, the active current is provided by using the maximum capacity of the switching device of the photovoltaic inverter while the reactive current is provided during the low voltage ride through control of the photovoltaic inverter:
wherein I is Td To consider the maximum active current that the photovoltaic inverter can output during low voltage ride through, I d_max The maximum active current which can be provided under the current working condition is provided.
Further, in one embodiment of the present invention, the active current I is provided when the low voltage ride through control strategy of the photovoltaic inverter is considered d The actual reactive current reference value is:
I' Tq ＝I Tq ×cosΔθ+I Td ×sinΔθ
the error between the target reactive current reference value and the actual reactive current reference value is as follows:
ΔI q ＝I Tq -I' Tq ＝I Tq ×(1-cosΔθ)-I Td ×sinΔθ
further, in one embodiment of the present invention, the phase compensation of the phase locked loop is provided for a transient phase in which a low voltage fault occurs according to the initial phase error and an exponential function added in the phase locked loop:
τ 1 ＝K 1 ×T PLL
wherein U is d ，U q For two voltage components in a rotating coordinate system obtained by performing park transformation on the power grid voltage based on the phase-locked loop output phase, delta theta 1 Is the approximate value of the phase difference of the grid voltage before and after the occurrence of the low-voltage fault, theta 0 For the original output phase of the phase-locked loop, K 1 To take into account the empirical coefficients under actual compensation effects, T PLL Phase-locked loop PI link time constant, θ ω For the phase-locked loop corrected output phase τ 1 Time constant as an exponential function of the failure occurrence phase.
Further, in one embodiment of the present invention, the calculating the final phase difference of the grid voltage during the low voltage ride through includes:
calculating the zero crossing point T of the overvoltage before fault by zero crossing detection 1 Zero crossing point T of overvoltage before fault 2 And calculating the final phase difference through the zero-crossing detection time difference and the power grid voltage frequency:
wherein, delta theta 2 A phase difference of the grid voltages before and after occurrence of a fault of low voltage, which is specifically known.
Further, in one embodiment of the present invention, the providing phase compensation for the transient phase of the low voltage fault recovery based on the final phase difference and an exponential function added in the phase locked loop based on a speed of phase angle following of the phase locked loop includes:
τ 2 ＝K 2 ×T PLL
wherein K is 2 To take into account the empirical value coefficients under practical effects τ 2 Time constant as an exponential function of the fault recovery phase.
To achieve the above object, another aspect of the present invention provides a phase-locked loop improvement system for a photovoltaic inverter for reactive support under low voltage ride through, comprising:
the power grid fault monitoring module is used for carrying out grid-connected control on the photovoltaic inverter by utilizing a voltage directional vector control strategy so as to control the power grid voltage through the phase-locked loop and monitoring whether the power grid has faults or not;
the phase error calculation module is used for obtaining an initial phase error of the phase-locked loop when the low-voltage fault occurs by solving the arctangent value of the voltage component output by the phase-locked loop based on the monitored low-voltage fault of the three-phase symmetry of the power grid;
the initial phase compensation module is used for providing phase compensation of the phase-locked loop for the transient phase of the low voltage fault according to the initial phase error and an exponential function added in the phase-locked loop, and calculating the final phase difference of the grid voltage during the low voltage ride through;
and the recovery phase compensation module is used for providing phase compensation based on the speed followed by the phase angle of the phase-locked loop for the transient phase of low-voltage fault recovery based on the final phase difference and an exponential function added in the phase-locked loop so as to enable the photovoltaic inverter to quickly recover to a normal working condition.
According to the improved method and the system for the phase-locked loop of the photovoltaic inverter for reactive power support under the low voltage ride through, disclosed by the embodiment of the invention, the phase compensation is carried out on the phase-locked loop (PLL) of the photovoltaic inverter, so that the reactive transient characteristic of the photovoltaic inverter during the three-phase symmetrical low voltage fault transient period is improved, the problem of inaccurate phase locking during the transient period can be restrained, the reactive power impact of the low voltage transient period is reduced, the reactive power withdrawal speed is increased, and the continuous fault risk of the low voltage ride through and the high voltage ride through is reduced.
Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
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The foregoing and/or additional aspects and advantages of the invention will become apparent and readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings, in which:
fig. 1 is a flow chart of a photovoltaic inverter phase-locked loop improvement method for reactive support at low voltage ride through according to an embodiment of the present invention;
fig. 2 is another flow chart of a photovoltaic inverter phase-locked loop improvement method for reactive support at low voltage ride through according to an embodiment of the present invention;
FIG. 3 is a block diagram of an SRF-PLL phase locked loop according to an embodiment of the present invention;
FIG. 4 is a vector diagram of voltage phase jump without consideration of active current according to an embodiment of the present invention;
FIG. 5 is a graph of Q-axis reference error versus phase jump for various voltage sag levels regardless of active current, according to an embodiment of the present invention;
FIG. 6 is a vector diagram illustrating voltage phase jump taking into account active current according to an embodiment of the present invention;
FIG. 7 is a graph of Q-axis reference error versus phase jump for various voltage sag levels of the active current under consideration in accordance with an embodiment of the present invention;
FIG. 8 is a schematic diagram of phase-locked loop phase compensation during low voltage occurrence based on a block diagram of the SRF-PLL phase-locked loop structure according to an embodiment of the present invention;
fig. 9 is a block diagram of a photovoltaic inverter phase-locked loop improvement system for reactive support at low voltage ride through according to an embodiment of the present invention.
Detailed Description
It should be noted that, without conflict, the embodiments of the present invention and features of the embodiments may be combined with each other. The invention will be described in detail below with reference to the drawings in connection with embodiments.
In order that those skilled in the art will better understand the present invention, a technical solution in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in which it is apparent that the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the present invention without making any inventive effort, shall fall within the scope of the present invention.
The following describes a photovoltaic inverter phase-locked loop improvement method and system for reactive support at low voltage ride through according to embodiments of the present invention with reference to the accompanying drawings.
Fig. 1 is a flow chart of a photovoltaic inverter phase-locked loop improvement method for reactive support at low voltage ride through according to an embodiment of the present invention.
As shown in fig. 1, the method includes, but is not limited to, the steps of:
s1, grid-connected control of a photovoltaic inverter is performed by utilizing a voltage directional vector control strategy so as to control the voltage of a power grid through a phase-locked loop and monitor whether the power grid fails;
s2, based on the monitored three-phase symmetrical low-voltage faults of the power grid, obtaining an initial phase error of the phase-locked loop when the low-voltage faults occur by solving the arctangent value of the voltage component output by the phase-locked loop;
s3, providing phase compensation of the phase-locked loop for a transient phase of low voltage fault occurrence according to the initial phase error and an exponential function added in the phase-locked loop, and calculating a final phase difference of the power grid voltage during low voltage ride through;
and S4, providing phase compensation based on the phase angle following speed of the phase-locked loop for the transient phase of low-voltage fault recovery based on the final phase difference and an exponential function added in the phase-locked loop, so that the photovoltaic inverter quickly recovers to normal working conditions.
It can be understood that the phase compensation is added in the transient phase of the low voltage fault based on the synchronous coordinate system phase-locked loop, so as to inhibit phase locking misalignment in the transient process, reduce the error of reactive reference values in the transient process, and reduce the problems of low voltage fault occurrence, high voltage fault connection, continuous high-low voltage ride through fault and the like caused by reactive control error or untimely reactive withdrawal in the recovery process.
The method of the invention firstly passes U in the fault occurrence stage q And U d The phase-locked loop and the phase-locked loop are subjected to inverse tangent to obtain transient phase errors, an exponential function link related to the low-voltage fault phase errors is added in real time in the phase-locked loop, and the difference between the voltage phases before and after the occurrence of the low-voltage fault is recordedThe method comprises the steps of carrying out a first treatment on the surface of the And in the fault recovery stage, an exponential function link related to the low-voltage fault phase jump angle is added into the phase-locked loop, so that the phase angle following speed of the phase-locked loop and the reactive power withdrawing speed of the inverter during fault recovery are accelerated.
Furthermore, the method of the invention considers the problem of insufficient calculation overhead cost in the actual photovoltaic inverter in engineering application, and is realized by adding a correction link to the traditional common phase-locked loop, thereby greatly improving the usability. Simulation results prove that the method is effective, can effectively inhibit the problems of overvoltage after low-voltage faults and the like caused by inaccurate reactive power control, and improves the low-voltage ride-through capability of the system.
As an embodiment of the present invention, fig. 2 is a specific flowchart of the improved method of the phase-locked loop of the photovoltaic inverter for reactive support under low voltage ride through of the present invention. The method of the invention will be described in detail with reference to the accompanying drawings.
Specifically, grid-tie control of photovoltaic inverters typically uses a voltage-directed vector control strategy to achieve decoupling by a phase-locked loop following the grid voltage, with the most widely used phase-locked loop architecture being the synchronous coordinate system phase-locked loop (SRF-PLL), as shown in fig. 3.
When a system has three-phase symmetrical low-voltage faults, the national standard requires that the photovoltaic field station is kept not to be disconnected for a period of time, and corresponding reactive support is provided according to the detected voltage drop degree, wherein the reactive current requirements are as follows:
I Tq ＝k×(0.9-U T )×I N (U T <0.9)
wherein I is Tq A reactive current target value to be provided during a low voltage fault, which provides capacitive reactive for positive and inductive reactive for negative; u (U) T Is the voltage per unit value during low voltage faults; i N And k is an empirical coefficient along with the voltage drop degree, and the value range is 1.5-2.5.
For a three-phase symmetric power grid, after three-phase voltage and current are subjected to Park conversion, the reactive power output of the system can be described as:
when a low-voltage fault occurs, the power grid voltage suddenly drops, the phase is hopped, and the phase-locked loop cannot instantaneously follow the phase hopping of the power grid voltage, so that the phase-locked loop needs to follow the phase again in the transient process. During this time, sudden changes in phase angle can misalign the reference value of the reactive current, and can cause the Q-axis current to follow inaccurately. Such an error caused by the phase jump will be analyzed in detail, taking the case when a fault occurs as an example.
In particular, the low voltage control strategy does not take into account the effects of phase locked loop misalignment in the active time. As shown in fig. 4, when the grid voltage is from U s Abrupt delta theta is U s When (only phase difference is considered), the D-Q coordinate system will also become the D '-Q' coordinate system, and the output θ of the phase-locked loop ω Will not change suddenly, at this time the grid-connected current I s Can be decomposed according to a D-Q coordinate system obtained by phase-locked loop q 、I d But the real components thereof should be I d '、I q '。
When switching to the control strategy of low voltage ride through, the control system controls the voltage amplitude U T Calculate I Tq And takes this as I in the phase-locked loop coordinate system q Is the reference value of (1) in the real coordinate system Tq ' obviously different from the target value, the two have certain errors, and the specific reference value errors are as follows:
as can be seen from the above, when the voltage sag level is determined, that is, the magnitude of the reactive current reference value is determined, the reactive current reference value error increases with an increase in Δθ, and is independent of the positive and negative thereof. At the same time, when the fault is more serious, the reactive current target value I Tq Also, the larger the reactive current reference value error is, as shown in FIG. 5, wherein the horizontal axis is voltage phase jumpThe vertical axis of the variation is the error of the reactive current reference value.
Since there is an error in the reactive current reference value, and a certain time is required for the phase-locked loop to follow the phase, the error in the reactive current reference value during this period may cause an error in the reactive current process as well, which may cause reactive support misalignment and even cause continuous high/low voltage faults after the low voltage is ended.
Further, providing active power while providing sufficient reactive power in a low voltage fault of the power system contributes to a stable operation of the power system. Thus during low voltage ride through of current photovoltaic inverters, the maximum capacity of the inverter switching devices is fully utilized to provide active current while reactive current is provided according to a standard:
wherein I is Td To consider the maximum active current that the photovoltaic inverter can output during low voltage ride through, I d max The maximum active current which can be provided under the current working condition is provided.
Specifically, as shown in fig. 6, the active current I is considered to be provided when the low voltage ride through control strategy of the photovoltaic inverter d At this time, due to phase mutation, I d And I q Will be against the actual reactive current I q ' all have an effect, the actual reactive current reference value is:
I' Tq ＝I Tq ×cosΔθ+I Td ×sinΔθ
the error between the target reactive current reference value and the actual reactive current reference value is as follows:
ΔI q ＝I Tq -I' Tq ＝I Tq ×(1-cosΔθ)-I Td ×sinΔθ
in determining the reactive current reference value, unlike the control strategy that does not consider the active current, as shown in fig. 7, the positive and negative of Δθ have different effects on the target reactive reference value. When delta theta is positive, namely the phase of the power grid voltage suddenly advances, the error of the reactive reference value has a maximum value along with the increase of the phase angle, and then the error is reduced; when Δθ is negative, i.e., the grid voltage phase suddenly lags (such faults are more common in LVRT), the error of the target reference value increases monotonically with increasing phase angle, and the error value is larger.
In summary, the phase locked loop may cause the problem of reactive power control misalignment due to phase abrupt change when the low voltage fault occurs and is cut off. The scale of the photovoltaic field station is gradually increased, the strength of the accessed power grid is gradually reduced, and the influence of the problem on the power system in the recovery process of the low-voltage fault is more obvious.
Specifically, for the power system, the time and the fault scene of occurrence of the low-voltage fault are completely random, and cannot be predicted and predicted, so that when the fault occurs, the system cannot directly acquire the phase difference of the grid voltage before and after the fault. Therefore, we need to consider the approximation of the phase angle error of the PLL, and obtain the phase error approximately by the arctangent values of Uq and Ud, and for the PLL (SRF-PLL) of the conventional synchronous coordinate system, a phase compensation module is added to the control structure to reduce the error caused by the phase jump to the control system, and the specific control structure block diagram is shown in fig. 8, and the specific amounts can be described as follows:
τ 1 ＝K 1 ×T PLL
wherein U is d ，U q For two voltage components in a rotating coordinate system obtained by performing park transformation on the power grid voltage based on the phase-locked loop output phase, delta theta 1 Is the approximate value of the phase difference of the grid voltage before and after the occurrence of the low-voltage fault, theta 0 For the original output phase of the phase-locked loop, K 1 To take into account the empirical coefficients under actual compensation effects, T PLL Phase locked loop PI link time constant.
Further, for a three-phase symmetrical low-voltage fault, the phase difference of the grid voltage before and after the occurrence of the low-voltage fault can be obtained easily, so that the re-jump value of the grid voltage can be predicted in advance when the fault is recovered. After the transient phase of the low voltage fault occurs, the voltage phase tends to be stable, and the phase detection of the voltage is carried out after the voltage phase is stable. First, calculating the zero crossing point T of the overvoltage before fault through zero crossing detection 1 Zero crossing point T of overvoltage before fault 2 Secondly, calculating a phase difference between the zero crossing detection time difference and the power grid voltage frequency:
in the transient state of low-voltage fault recovery, a phase compensation module is added into a control structure of a traditional synchronous coordinate system phase-locked loop (SRF-PLL) to reduce errors caused by phase abrupt change to a control system. Unlike the failure occurrence stage, Δθ at this time 2 For a specifically known phase difference of the grid voltages before and after occurrence of the low voltage fault, a control structure block diagram thereof may be described as shown in fig. 7, and a specific amount thereof may be described as:
τ 2 ＝K 2 ×T PLL
wherein Δθ 2 A phase difference of the grid voltages before and after occurrence of a particularly known low voltage fault; t (T) PLL K is the phase-locked loop PI link time constant 2 To consider the empirical value coefficient under actual effect, θ 0 Adding the output phase, θ, before phase correction to the phase locked loop ω For the phase-locked loop corrected output phase τ 2 Time constant as an exponential function of the fault recovery phase.
According to the improved method for the phase-locked loop of the photovoltaic inverter for reactive power support under low voltage ride through, disclosed by the embodiment of the invention, the phase compensation strategy is added to the phase-locked loop in the fault occurrence stage and the recovery stage, so that the error of reactive power caused by phase misalignment of the phase-locked loop can be reduced, the reactive power support or withdrawal speed is accelerated, continuous high-low voltage ride through faults possibly occurring in the low voltage fault recovery stage are reduced, and the system performance is improved.
In order to implement the above embodiment, as shown in fig. 9, a photovoltaic inverter phase-locked loop improvement system 10 for reactive support under low voltage ride through is also provided in this embodiment, and the system 10 includes a grid fault monitoring module 100, a phase error calculation module 200, an initial phase compensation module 300, and a recovery phase compensation module 400.
The grid fault monitoring module 100 is configured to perform grid-connected control of the photovoltaic inverter by using a voltage directional vector control strategy to control a voltage of a grid through a phase-locked loop, and monitor whether the grid has a fault;
the phase error calculation module 200 is configured to obtain an initial phase error of the phase-locked loop when the low-voltage fault occurs by solving an arctangent value of a voltage component output by the phase-locked loop based on the monitored low-voltage fault of the three-phase symmetry of the power grid;
an initial phase compensation module 300, configured to provide phase compensation of the phase-locked loop for a transient phase in which a low voltage fault occurs according to an initial phase error and an exponential function added to the phase-locked loop, and calculate a final phase difference of the grid voltage during a low voltage ride through;
the recovery phase compensation module 400 is configured to provide phase compensation based on a speed followed by a phase angle of the phase-locked loop for a transient phase of low-voltage fault recovery based on the final phase difference and an exponential function added in the phase-locked loop, so that the photovoltaic inverter quickly recovers to a normal working condition.
According to the photovoltaic inverter phase-locked loop improvement system for reactive power support under low voltage ride through, disclosed by the embodiment of the invention, the phase compensation strategy is added to the phase-locked loop in the fault occurrence stage and the recovery stage, so that the error of reactive power caused by phase misalignment of the phase-locked loop can be reduced, the reactive power support or withdrawal speed is accelerated, continuous high-low voltage ride through faults possibly occurring in the low voltage fault recovery stage are reduced, and the system performance is improved.
It should be noted that the foregoing explanation of the embodiment of the method for improving a phase-locked loop of a photovoltaic inverter for reactive power support under low voltage ride through is also applicable to the device for improving a phase-locked loop of a photovoltaic inverter for reactive power support under low voltage ride through of this embodiment, and will not be repeated here.
In the description of the present specification, a description referring to terms "one embodiment," "some embodiments," "examples," "specific examples," or "some examples," etc., means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present invention. In this specification, schematic representations of the above terms are not necessarily directed to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, the different embodiments or examples described in this specification and the features of the different embodiments or examples may be combined and combined by those skilled in the art without contradiction.
Furthermore, the terms "first," "second," and the like, are used for descriptive purposes only and are not to be construed as indicating or implying a relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defining "a first" or "a second" may explicitly or implicitly include at least one such feature. In the description of the present invention, the meaning of "plurality" means at least two, for example, two, three, etc., unless specifically defined otherwise.
Claims (9)
1. A method for improving a phase-locked loop of a photovoltaic inverter for reactive support under low voltage ride through, comprising the steps of:
grid-connected control of the photovoltaic inverter is performed by utilizing a voltage directional vector control strategy so as to control the voltage of a power grid through a phase-locked loop and monitor whether the power grid fails or not;
based on the monitored three-phase symmetrical low-voltage faults of the power grid, the initial phase error of the phase-locked loop when the low-voltage faults occur is obtained by solving the arctangent value of the voltage component output by the phase-locked loop;
providing phase compensation of the phase-locked loop for a transient phase at which a low voltage fault occurs according to the initial phase error and an exponential function added in the phase-locked loop, and calculating a final phase difference of the grid voltage during low voltage ride through;
and providing phase compensation based on the phase angle following speed of the phase-locked loop for the transient phase of the low-voltage fault recovery based on the final phase difference and an exponential function added in the phase-locked loop, so that the photovoltaic inverter quickly recovers to normal working conditions.
2. The method according to claim 1, wherein the low voltage fault based on the detected three-phase symmetry of the grid provides a corresponding reactive support according to the detected voltage sag level of the grid, wherein the reactive current requirement is:
I Tq ＝k×(0.9-U T )×I N (U T <0.9)
wherein I is Tq For reactive current target values required to be provided during low voltage faults, U T Is the per-unit value of the voltage during low voltage faults, I N For the photovoltaic inverter rated current, k is an empirical factor with voltage sag.
3. The method of claim 2, wherein when the photovoltaic inverter implements a low voltage ride through control strategy, according to a voltage magnitude U T Calculate I Tq And will I Tq As the value of I in the phase-locked loop coordinate system q The reference value error is:
4. a method according to claim 3, characterized in that during low voltage ride through control of the photovoltaic inverter, reactive current is provided while active current is provided by means of the maximum capacity of the switching devices of the photovoltaic inverter:
wherein I is Td To consider the maximum active current that the photovoltaic inverter can output during low voltage ride through, I d_max The maximum active current which can be provided under the current working condition is provided.
5. The method of claim 4, wherein the active current I is provided when a low voltage ride through control strategy of the photovoltaic inverter is considered d The actual reactive current reference value is:
I' Tq ＝I Tq ×cosΔθ+I Td ×sinΔθ
the error between the target reactive current reference value and the actual reactive current reference value is as follows:
ΔI q ＝I Tq -I' Tq ＝I Tq ×(1-cosΔθ)-I Td ×sinΔθ
6. the method of claim 1, wherein the providing phase compensation of the phase locked loop for transient phases in which low voltage faults occur is based on the initial phase error and an exponential function added in the phase locked loop:
τ 1 ＝K 1 ×T PLL
wherein U is d ，U q Is a rotary seat obtained by performing park transformation on the power grid voltage based on the phase-locked loop output phaseTwo voltage components under the standard, delta theta 1 Is the approximate value of the phase difference of the grid voltage before and after the occurrence of the low-voltage fault, theta 0 For the original output phase of the phase-locked loop, K 1 To take into account the empirical coefficients under actual compensation effects, T PLL Phase-locked loop PI link time constant, θ ω For the phase-locked loop corrected output phase τ 1 Time constant as an exponential function of the failure occurrence phase.
7. The method of claim 6, wherein calculating the final phase difference of the grid voltage during the low voltage ride through comprises:
calculating the zero crossing point T of the overvoltage before fault by zero crossing detection 1 Zero crossing point T of overvoltage before fault 2 And calculating the final phase difference through the zero-crossing detection time difference and the power grid voltage frequency:
wherein, delta theta 2 A phase difference of the grid voltages before and after occurrence of a fault of low voltage, which is specifically known.
8. The method of claim 7, wherein the providing phase compensation for the transient phase of low voltage fault recovery based on the final phase difference and an exponential function added in the phase locked loop based on a speed of phase angle follow of the phase locked loop comprises:
τ 2 ＝K 2 ×T PLL
wherein K is 2 To take into account the empirical value coefficients under practical effects τ 2 Time constant as an exponential function of the fault recovery phase.
9. A photovoltaic inverter phase-locked loop improvement system for reactive support at low voltage ride through comprising:
the power grid fault monitoring module is used for carrying out grid-connected control on the photovoltaic inverter by utilizing a voltage directional vector control strategy so as to control the power grid voltage through the phase-locked loop and monitoring whether the power grid has faults or not;
the phase error calculation module is used for obtaining an initial phase error of the phase-locked loop when the low-voltage fault occurs by solving the arctangent value of the voltage component output by the phase-locked loop based on the monitored low-voltage fault of the three-phase symmetry of the power grid;
the initial phase compensation module is used for providing phase compensation of the phase-locked loop for the transient phase of the low voltage fault according to the initial phase error and an exponential function added in the phase-locked loop, and calculating the final phase difference of the grid voltage during the low voltage ride through;
and the recovery phase compensation module is used for providing phase compensation based on the speed followed by the phase angle of the phase-locked loop for the transient phase of low-voltage fault recovery based on the final phase difference and an exponential function added in the phase-locked loop so as to enable the photovoltaic inverter to quickly recover to a normal working condition.
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN116388211A (en) * | 2023-06-07 | 2023-07-04 | 国网上海能源互联网研究院有限公司 | Distributed photovoltaic grid-connected voltage out-of-limit treatment method, device, system and medium |
| CN118367629A (en) * | 2024-06-20 | 2024-07-19 | 宁波公牛新能源科技有限公司 | Reactive power control method and device of photovoltaic inverter, controller and storage medium |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN116388211A (en) * | 2023-06-07 | 2023-07-04 | 国网上海能源互联网研究院有限公司 | Distributed photovoltaic grid-connected voltage out-of-limit treatment method, device, system and medium |
| CN116388211B (en) * | 2023-06-07 | 2023-08-11 | 国网上海能源互联网研究院有限公司 | Distributed photovoltaic grid-connected voltage out-of-limit treatment method, device, system and medium |
| CN118367629A (en) * | 2024-06-20 | 2024-07-19 | 宁波公牛新能源科技有限公司 | Reactive power control method and device of photovoltaic inverter, controller and storage medium |
| CN118367629B (en) * | 2024-06-20 | 2024-09-06 | 宁波公牛新能源科技有限公司 | Reactive power control method and device of photovoltaic inverter, controller and storage medium |
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