CN116154860A - Improvement method of photovoltaic inverter phase-locked loop for reactive power support under low voltage ride through - Google Patents

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

Improved method of photovoltaic inverter phase-locked loop for reactive support under low voltage ride-through

Technical Field

The invention relates to the technical field of photovoltaic inverters and phase-locked loops, and in particular to an improved method for a photovoltaic inverter phase-locked loop used for reactive support under low voltage ride-through.

Background Art

With the continuous advancement of my country's "dual carbon" goals, new energy sources such as photovoltaic stations have developed rapidly, and they have been connected to the power system on a large scale. New energy sources and their proportion are increasing, and a "double high" power system is taking shape. At the same time, due to the increase in the scale of photovoltaic stations and the uneven geographical distribution of solar energy resources, photovoltaic stations are connected to the grid and show the characteristics of being connected to a weak power grid. In a weak power grid, photovoltaic stations have a greater impact on the stable operation of the power system. The control strategy of traditional photovoltaic inverters relies on the phase-locked loop to synchronize with the grid voltage. Taking the most widely used synchronous coordinate system phase-locked loop (SRF-PLL) as an example, this phase-locked loop directly realizes coordinate synchronization through coordinate transformation, and can accurately obtain information such as the amplitude, frequency and phase of the grid voltage under a three-phase ideal power grid. In the case of a low voltage fault, the grid voltage will simultaneously experience a voltage drop and a phase jump, which leads to the problem of phase-locked loop phase misalignment during the transient process of low voltage fault occurrence and recovery. Phase misalignment will cause the reactive power control of the photovoltaic inverter to be misaligned, and in severe cases, a high voltage fault will occur after a low voltage fault.

At present, people have proposed many phase-locked loop parameter design or improvement strategies and phase-locked loop models to enhance control strategies. In order to make the inverter connected to the weak power grid have a larger stability margin, some researchers have proposed a parameter design method that considers the coupling between the current loop and the phase-locked loop. There is also a low-pass filter link added to the phase-locked loop to enhance the performance of the inverter in the weak power grid. In order to solve the problem of transient stability during low-voltage faults, some researchers shielded the integrator of the phase-locked loop PI link to obtain a first-order integral loop to reduce the adverse effects caused by angle overshoot. Some researchers proposed to use a second-order oscillation link to replace the original PI control link to improve the phase-locked loop characteristics. Some scholars have introduced the characteristics of the reference synchronous machine, used the synchronous machine model to replace the traditional phase-locked loop, and proposed various phase angle control and improvement strategies to ensure transient stability during low-voltage faults.

In weak power grids, the impact of reactive power on the voltage stability of the power system is more obvious. However, the above-mentioned studies on the improvement of the phase-locked loop rarely consider the impact of the phase-locked loop characteristics on reactive support during fault transients. These improvements cannot suppress the transient reactive impact caused by voltage phase jumps under low voltage faults. At the same time, the implementation of these phase-locked loop improvements often requires huge computing overhead, which makes it difficult to apply and implement in photovoltaic inverters in engineering practice.

Summary of the invention

The present invention aims to solve one of the technical problems in the related art at least to a certain extent.

To this end, the present invention proposes a method for improving the phase-locked loop of a photovoltaic inverter for reactive support under low voltage ride-through. Based on the improvement of the traditional synchronous coordinate system phase-locked loop, it does not require a huge computational overhead in engineering practice, and can better suppress the impact of voltage phase jump during a three-phase symmetrical low voltage fault. By performing phase compensation on the phase-locked loop (PLL) of the photovoltaic inverter, the reactive transient characteristics during the transient state of the three-phase symmetrical low voltage fault of the photovoltaic inverter are improved, the problem of inaccurate phase locking during the transient state can be suppressed, the reactive impact in the low voltage transient stage can be reduced, and the reactive withdrawal speed can be accelerated, thereby reducing the risk of continuous failures of low voltage ride-through and high voltage ride-through.

Another object of the present invention is to provide an improved photovoltaic inverter phase-locked loop system for reactive power support under low voltage ride-through.

To achieve the above object, the present invention proposes, on one hand, a method for improving a photovoltaic inverter phase-locked loop for reactive support under low voltage ride-through, comprising:

Use voltage-oriented vector control strategy to control the grid connection of photovoltaic inverters to control the grid voltage through a phase-locked loop and monitor whether the grid has faults;

Based on the monitored three-phase symmetrical low voltage fault of the power grid, the initial phase error of the phase-locked loop when the low voltage fault occurs is obtained by calculating the arc tangent value of the voltage component output by the phase-locked loop;

According to the initial phase error and the exponential function added to the phase-locked loop, a phase compensation of the phase-locked loop is provided for the transient stage of the low voltage fault, and a final phase difference of the grid voltage during the low voltage ride-through period is calculated;

Based on the final phase difference and the exponential function added in the phase-locked loop, phase compensation based on the speed of phase angle following of the phase-locked loop is provided for the transient stage of low voltage fault recovery, so that the photovoltaic inverter can quickly recover to normal working condition.

In addition, the photovoltaic inverter phase-locked loop improvement method for reactive support under low voltage ride-through according to the above embodiment of the present invention may also have the following additional technical features:

Further, in one embodiment of the present invention, based on the monitored three-phase symmetrical low voltage fault of the power grid, corresponding reactive support is provided according to the detected degree of voltage drop of the power grid, wherein the requirement of reactive current is:

I _Tq =k×(0.9-U _T )×I _N ( _UT <0.9)

Among them, I _Tq is the reactive current target value that needs to be provided during the low voltage fault, U _T is the voltage per unit value during the low voltage fault, I _N is the rated current of the photovoltaic inverter, and k is the empirical coefficient with the degree of voltage drop.

Further, in one embodiment of the present invention, when the photovoltaic inverter executes the low voltage ride-through control strategy, the value of I _Tq is calculated according to the voltage amplitude _UT , and the value of I _Tq is used as the reference value of I _q in the phase-locked loop coordinate system, and the reference value error is:

Furthermore, in one embodiment of the present invention, during the low voltage ride through control of the photovoltaic inverter, while providing reactive current, the maximum capacity of the switching device of the photovoltaic inverter is used to provide active current:

Among them, I _Td is the maximum active current that the photovoltaic inverter can output during low voltage ride-through, and I _{d_max} is the maximum active current that can be provided under the current working conditions.

Further, in one embodiment of the present invention, when the low voltage ride through control strategy of the photovoltaic inverter considers providing active current I _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:

ΔI _q =I _Tq -I' _Tq =I _Tq ×(1-cosΔθ)-I _Td ×sinΔθ

Further, in one embodiment of the present invention, according to the initial phase error and the exponential function added to the phase-locked loop, a phase compensation of the phase-locked loop is provided for the transient stage of the low voltage fault:

τ ₁ =K ₁ ×T _PLL

Among them, U _d , U _q are two voltage components in the rotating coordinate system obtained after Park transformation of the grid voltage based on the output phase of the phase-locked loop, Δθ ₁ is the approximate value of the phase difference of the grid voltage before and after the low voltage fault occurs, θ ₀ is the original output phase of the phase-locked loop, K ₁ is the empirical coefficient considering the actual compensation effect, T _PLL is the time constant of the PI link of the phase-locked loop, θ _ω is the output phase after correction of the phase-locked loop, and τ ₁ is the time constant of the exponential function in the fault occurrence stage.

Further, in one embodiment of the present invention, the calculating the final phase difference of the grid voltage during the low voltage ride-through period includes:

The overvoltage zero-crossing point T ₁ and the overvoltage zero-crossing point T ₂ before the fault are calculated by zero-crossing detection, and then the final phase difference is calculated by the zero-crossing detection time difference and the grid voltage frequency:

Wherein, Δθ ₂ is the phase difference of the grid voltage before and after the specific known low voltage fault occurs.

Further, in one embodiment of the present invention, providing phase compensation based on the speed of phase angle following of the phase locked loop for the transient stage of low voltage fault recovery based on the final phase difference and the exponential function added in the phase locked loop comprises:

τ ₂ =K ₂ ×T _PLL

Among them, K ₂ is the empirical value coefficient considering the actual effect, and τ ₂ is the time constant of the exponential function in the fault recovery stage.

To achieve the above object, the present invention provides, on the other hand, a photovoltaic inverter phase-locked loop improvement system for reactive support under low voltage ride-through, comprising:

A grid fault monitoring module, which is used to control the photovoltaic inverter grid connection by using a voltage-oriented vector control strategy to control the grid voltage through a phase-locked loop and monitor whether a grid fault occurs;

A phase error calculation module is used to obtain the initial phase error of the phase-locked loop when the low voltage fault occurs by calculating the arc tangent value of the voltage component output by the phase-locked loop based on the monitored three-phase symmetrical low voltage fault of the power grid;

An initial phase compensation module is used to provide phase compensation of the phase-locked loop for the transient stage of the low voltage fault according to the initial phase error and the exponential function added to the phase-locked loop, and calculate the final phase difference of the grid voltage during the low voltage ride-through period;

The recovery phase compensation module is used to provide phase compensation based on the speed of the phase angle following of the phase locked loop for the transient stage of low voltage fault recovery based on the final phase difference and the exponential function added in the phase locked loop, so that the photovoltaic inverter can quickly recover to normal working conditions.

The improved method and system of the photovoltaic inverter phase-locked loop for reactive support under low voltage ride-through in the embodiment of the present invention improves the reactive transient characteristics of the photovoltaic inverter during the three-phase symmetrical low voltage fault transient state by performing phase compensation on the phase-locked loop (PLL) of the photovoltaic inverter, thereby suppressing the problem of inaccurate phase locking during the transient state, reducing the reactive impact in the low voltage transient stage, accelerating the reactive withdrawal speed, and reducing the risk of continuous failure of low voltage ride-through followed by high voltage ride-through.

Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and/or additional aspects and advantages of the present invention will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

1 is a flow chart of a method for improving a photovoltaic inverter phase-locked loop for reactive support under low voltage ride-through according to an embodiment of the present invention;

2 is another flow chart of a method for improving a photovoltaic inverter phase-locked loop for reactive support under low voltage ride-through according to an embodiment of the present invention;

FIG3 is a block diagram of a SRF-PLL phase-locked loop according to an embodiment of the present invention;

4 is a vector diagram of a voltage phase mutation without considering active current according to an embodiment of the present invention;

5 is a diagram showing a change in Q-axis reference error with phase mutation under different voltage drop levels without considering active current according to an embodiment of the present invention;

6 is a vector diagram of a voltage phase mutation considering active current according to an embodiment of the present invention;

7 is a diagram showing a change in Q-axis reference error with phase mutation under different voltage drop levels of active current according to an embodiment of the present invention;

8 is a schematic diagram of phase-locked loop phase compensation during low voltage occurrence based on the SRF-PLL phase-locked loop structure block diagram according to an embodiment of the present invention;

9 is a structural diagram of an improved photovoltaic inverter phase-locked loop system for reactive support under low voltage ride-through according to an embodiment of the present invention.

DETAILED DESCRIPTION

It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only embodiments of a part of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

The following describes a photovoltaic inverter phase-locked loop improvement method and system for reactive power support under low voltage ride-through according to an embodiment of the present invention with reference to the accompanying drawings.

FIG1 is a flow chart of a method for improving a photovoltaic inverter phase-locked loop for reactive support under low voltage ride-through according to an embodiment of the present invention.

As shown in FIG1 , the method includes but is not limited to the following steps:

S1, using voltage-oriented vector control strategy to control the photovoltaic inverter grid connection to control the grid voltage through a phase-locked loop and monitor whether the grid fails;

S2, based on the monitored three-phase symmetrical low voltage fault of the power grid, the initial phase error of the phase-locked loop when the low voltage fault occurs is obtained by calculating the arc tangent value of the voltage component output by the phase-locked loop;

S3, providing phase compensation of the phase-locked loop for the transient stage of low voltage fault occurrence according to the initial phase error and the exponential function added to the phase-locked loop, and calculating the final phase difference of the grid voltage during the low voltage ride-through period;

S4, based on the final phase difference and the exponential function added to the phase-locked loop, a phase compensation based on the speed of the phase angle following of the phase-locked loop is provided for the transient stage of low voltage fault recovery, so that the photovoltaic inverter can quickly return to normal working conditions.

It can be understood that the present invention is based on a synchronous coordinate system phase-locked loop, and adds phase compensation in the transient stage of a low-voltage fault to suppress phase-locking misalignment during the transient process, while reducing the error of the reactive reference value during the transient period, thereby reducing the occurrence of low-voltage faults, and the low-voltage faults followed by high-voltage faults and continuous high-low voltage crossing faults caused by reactive control errors or untimely reactive withdrawal during the recovery process.

The method of the present invention first obtains the transient phase error by calculating the inverse tangent of _Uq and _Ud during the fault occurrence stage, adds an exponential function link related to the low voltage fault phase error in real time to the phase-locked loop, and records the voltage phase difference before and after the low voltage fault occurs; secondly, during the fault recovery stage, adds an exponential function link related to the low voltage fault phase jump angle to the phase-locked loop, and accelerates the phase angle following speed of the phase-locked loop and the reactive power withdrawal speed of the inverter during the fault recovery period.

Furthermore, the method of the present invention takes into account the problem of insufficient computational overhead cost in actual photovoltaic inverters in engineering applications, and achieves this by adding a correction link to the traditional commonly used phase-locked loop, which greatly improves the availability. The simulation results prove the effectiveness of this method, which can effectively suppress problems such as overvoltage after low voltage faults caused by inaccurate reactive power control, and improve the system's low voltage ride-through capability.

As an embodiment of the present invention, Fig. 2 is a specific flow chart of the improved method of photovoltaic inverter phase-locked loop for reactive support under low voltage ride-through of the present invention. The method of the invention is described in detail below with reference to the accompanying drawings.

Specifically, the grid-connected control of photovoltaic inverters usually uses a voltage-oriented vector control strategy, and decoupling is achieved by following the grid voltage through a phase-locked loop. The most widely used phase-locked loop structure is the synchronous reference frame phase-locked loop (SRF-PLL), as shown in Figure 3.

When a three-phase symmetrical low voltage fault occurs in the system, the national standard requires that the photovoltaic station remain connected to the grid for a period of time and provide corresponding reactive power support according to the detected voltage drop. The reactive current requirement is:

I _Tq =k×(0.9-U _T )×I _N ( _UT <0.9)

Among them, I _Tq is the reactive current target value that needs to be provided during the low voltage fault. When it is positive, capacitive reactive power is provided, and when it is negative, inductive reactive power is provided; U _T is the voltage per unit value during the low voltage fault; I _N is the rated current of the photovoltaic inverter, and k is the empirical coefficient with the degree of voltage drop, ranging from 1.5 to 2.5.

For a three-phase symmetrical power grid, after the three-phase voltage and current are transformed by Park, the reactive power output of the system can be described as:

When a low voltage fault occurs, the grid voltage will suddenly drop and the phase will also jump. The phase-locked loop cannot instantly follow the phase jump of the grid voltage, so the phase-locked loop needs to follow the phase again during the transient process. During this period, the sudden change of the phase angle will cause the reference value of the reactive current to be inaccurate, and will also cause the Q-axis current to follow inaccurately. The following will take the situation when a fault occurs as an example to analyze in detail the error caused by the sudden change of phase.

Specifically, the low voltage control strategy does not consider the impact of the phase-locked loop inaccuracy during active operation. As shown in Figure 4, when the grid voltage suddenly changes from _Us to _Us ' by Δθ (considering only the phase difference), the DQ coordinate system will also become the D'-Q' coordinate system, and the output _θω of the phase-locked loop will not change suddenly. At this time, the grid-connected current _Is will still be decomposed into _Iq and _Id according to the DQ coordinate system obtained by the phase-locked loop, but its real components should be _Id ' and _Iq ' respectively.

When switching to the low voltage ride-through control strategy, the control system calculates the value of I _Tq according to the voltage amplitude _UT and uses it as the reference value of I _q in the phase-locked loop coordinate system. Its I _Tq ' in the real coordinate system is obviously different from the target value, and there is a certain error between the two. The specific reference value error is:

It can be seen from the above formula that when the voltage drop degree is determined, that is, when the reactive current reference value is determined, the reactive current reference value error will increase with the increase of Δθ, and has nothing to do with its positive or negative. At the same time, when the fault is more serious, the reactive current target value I _Tq is also larger, which will also make the reactive current reference value error larger, as shown in Figure 5, where the horizontal axis is the change in voltage phase jump, and the vertical axis is the error of the reactive current reference value.

Since there is an error in the reactive current reference value, and the phase follow-up of the phase-locked loop requires a certain amount of time, the error in the reactive current reference value during this period will also cause an error in the reactive current process, which will cause the reactive support to be inaccurate and even cause continuous high/low voltage faults after the low voltage ends.

Furthermore, providing sufficient reactive power and active power during low voltage faults in the power system is conducive to the stable operation of the power system. Therefore, during the low voltage ride-through period of the current photovoltaic inverter, while providing reactive current according to the standard, the maximum capacity of the inverter switching device is fully utilized to provide active current:

Where I _Td is the maximum active current that the photovoltaic inverter can output during low voltage ride-through, and I _{d max} is the maximum active current that can be provided under the current working conditions.

Specifically, as shown in FIG6 , when the low voltage ride-through control strategy of the photovoltaic inverter considers providing active current I _d , due to the phase mutation, both I _d and I _q will have an impact on the actual reactive current I _q ', and 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:

ΔI _q =I _Tq -I' _Tq =I _Tq ×(1-cosΔθ)-I _Td ×sinΔθ

When determining the reactive current reference value, unlike the control strategy that does not consider the active current, as shown in Figure 7, the positive and negative values of Δθ have different effects on the target reactive reference value. When Δθ is positive, that is, the grid voltage phase suddenly leads, as the phase angle increases, the error of the reactive reference value has a maximum value and then decreases; when Δθ is negative, that is, the grid voltage phase suddenly lags (this type of fault is more common in LVRT), the error of the target reference value increases monotonically with the increase of the phase angle, and the error value is large.

In summary, the phase-locked loop will cause reactive power control inaccuracy due to phase mutation when a low voltage fault occurs and is removed. As the scale of photovoltaic stations gradually increases and the strength of grid access gradually decreases, this problem has a more obvious impact on the power system during the recovery process of low voltage faults.

Specifically, for the power system, the time and scenario of low voltage faults are completely random and cannot be predicted or anticipated. Therefore, when a fault occurs, the system cannot directly obtain the phase difference of the grid voltage before and after the fault. Therefore, we need to consider the approximate value of the phase angle error of the phase-locked loop, and approximate the phase error by the inverse tangent value of Uq and Ud. For the traditional synchronous reference frame phase-locked loop (SRF-PLL), a phase compensation module is added to its control structure to reduce the error caused by the phase mutation to the control system. The specific control structure block diagram is shown in Figure 8, and its specific quantity can be described as:

τ ₁ =K ₁ ×T _PLL

Wherein, U _d , U _q are two voltage components in the rotating coordinate system obtained after Park transformation of the grid voltage based on the phase-locked loop output phase, Δθ ₁ is the approximate value of the phase difference of the grid voltage before and after the low voltage fault occurs, θ ₀ is the original output phase of the phase-locked loop, K ₁ is the empirical coefficient considering the actual compensation effect, and T _PLL is the time constant of the PI link of the phase-locked loop.

Furthermore, for a three-phase symmetrical low voltage fault, the phase difference of the grid voltage before and after the low voltage fault can be easily obtained, so the next jump value of the grid voltage can be predicted in advance when the fault is restored. After the transient phase of the low voltage fault ends, the voltage phase tends to be stable, and the voltage phase detection is performed after stabilization. First, the overvoltage zero crossing point T ₁ before the fault and the overvoltage zero crossing point T ₂ before the fault are calculated through zero crossing detection, and then the phase difference is calculated through the zero crossing detection time difference and the grid voltage frequency:

In the transient stage of low voltage fault recovery, a phase compensation module is also added to the control structure of the traditional synchronous frame phase-locked loop (SRF-PLL) to reduce the error caused by the phase mutation to the control system. Different from the fault occurrence stage, Δθ ₂ is the phase difference of the grid voltage before and after the low voltage fault occurs. The control structure block diagram can be described as shown in Figure 7, and the specific quantity can be described as:

τ ₂ =K ₂ ×T _PLL

Wherein Δθ ₂ is the phase difference of the grid voltage before and after the specific known low voltage fault occurs; T _PLL is the time constant of the phase-locked loop PI link, K ₂ is the empirical value coefficient considering the actual effect, θ ₀ is the output phase of the phase-locked loop before adding phase correction, θ _ω is the output phase of the phase-locked loop after correction, and τ ₂ is the time constant of the exponential function in the fault recovery stage.

According to the improved method of the photovoltaic inverter phase-locked loop for reactive support under low voltage ride-through according to the embodiment of the present invention, by adding a phase compensation strategy to the phase-locked loop in the fault occurrence stage and the recovery stage, the error of reactive power caused by the phase misalignment of the phase-locked loop can be reduced, the reactive support or withdrawal speed is accelerated, the continuous high and low voltage ride-through faults that may occur 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 Figure 9, this embodiment also provides a photovoltaic inverter phase-locked loop improvement system 10 for reactive support under low voltage ride-through, 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.

A power grid fault monitoring module 100, for controlling the photovoltaic inverter grid connection by using a voltage-oriented vector control strategy to control the grid voltage through a phase-locked loop, and monitoring whether a fault occurs in the grid;

The phase error calculation module 200 is used to obtain the initial phase error of the phase-locked loop when the low voltage fault occurs by calculating the arc tangent value of the voltage component output by the phase-locked loop based on the monitored three-phase symmetrical low voltage fault of the power grid;

The initial phase compensation module 300 is used to provide phase compensation of the phase-locked loop for the transient stage of the low voltage fault according to the initial phase error and the exponential function added to the phase-locked loop, and calculate the final phase difference of the grid voltage during the low voltage ride-through period;

The recovery phase compensation module 400 is used to provide phase compensation based on the speed of the phase angle following of the phase-locked loop for the transient stage of low voltage fault recovery based on the final phase difference and the exponential function added in the phase-locked loop, so that the photovoltaic inverter can quickly recover to normal working conditions.

According to the improved photovoltaic inverter phase-locked loop system for reactive support under low voltage ride-through according to the embodiment of the present invention, by adding a phase compensation strategy to the phase-locked loop during the fault occurrence stage and the recovery stage, the error of reactive power caused by phase misalignment of the phase-locked loop can be reduced, the reactive support or withdrawal speed is accelerated, the continuous high and low voltage ride-through faults that may occur during the low voltage fault recovery stage are reduced, and the system performance is improved.

It should be noted that the above explanation of the embodiment of the photovoltaic inverter phase-locked loop improvement method for reactive support under low voltage ride-through is also applicable to the photovoltaic inverter phase-locked loop improvement device for reactive support under low voltage ride-through of this embodiment, and will not be repeated here.

In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

Claims (9)

1. A method for improving a photovoltaic inverter phase-locked loop for reactive power support under low-voltage ride-through, characterized in that it comprises the following steps: Use the voltage-oriented vector control strategy to control the grid-connected photovoltaic inverter to control the grid voltage through the phase-locked loop, and monitor whether the grid is faulty; Based on the detected three-phase symmetrical low-voltage fault of the power grid, the initial phase error of the phase-locked loop is obtained when the low-voltage fault occurs by calculating the arctangent value of the voltage component output by the phase-locked loop; According to the initial phase error and the exponential function added in the phase-locked loop, phase compensation of the phase-locked loop is provided for the transient phase of the low-voltage fault, and the final phase difference of the grid voltage during the low-voltage ride-through period is calculated; Based on the final phase difference and the exponential function added in the phase-locked loop, the phase compensation based on the phase angle following speed of the phase-locked loop is provided for the transient phase of the low-voltage fault recovery, so that the photovoltaic inverter can quickly return to normal operation condition. 2. The method according to claim 1, characterized in that, based on the detected three-phase symmetrical low-voltage fault of the power grid, corresponding reactive power support is provided according to the detected degree of power grid voltage drop, wherein the reactive current The requirements are: I _Tq ＝k×(0.9-U _T )×I _N (U _T <0.9) Among them, I _Tq is the reactive current target value that needs to be provided during the low voltage fault period, U _T is the per unit value of the voltage during the low voltage fault period, I _N is the rated current of the photovoltaic inverter, k is the experience with the voltage drop coefficient. 3. The method according to claim 2, wherein when the photovoltaic inverter executes the low voltage ride through control strategy, the value of _ITq is calculated according to the voltage amplitude U _T , and the value of _ITq is used as a lock The reference value of I _q in the phase ring coordinate system, the error of the reference value is:

4. The method according to claim 3, characterized in that, during the low voltage ride through control period of the photovoltaic inverter, while providing reactive current, the maximum capacity of the switching device of the photovoltaic inverter is used to provide active current :

Among them, I _Td is the maximum active current that the photovoltaic inverter can output during the low voltage ride-through period, and I _{d_max} is the maximum active current that can be provided under the current working condition. 5. method according to claim 4, is characterized in that, when the low voltage ride through control strategy of photovoltaic inverter considers to provide active current _Id , actual reactive current reference value is: I' _Tq ＝I _Tq ×cosΔθ+I _Td ×sinΔθ The error between the target reactive current reference and the actual reactive current reference is: ΔI _q ＝I _Tq -I' _Tq ＝ _ITq ×(1-cosΔθ) _-ITd ×sinΔθ 6. The method according to claim 1, characterized in that, according to the initial phase error and the exponential function added in the phase-locked loop, the phase compensation of the phase-locked loop is provided to the transient phase of the low-voltage fault occurrence :

τ ₁ =K ₁ ×T _PLL Among them, U _d and U _q are the two voltage components in the rotating coordinate system obtained by Parker transformation of the grid voltage based on the output phase of the phase-locked loop, Δθ ₁ is the approximate value of the phase difference of the grid voltage before and after the low voltage fault occurs, θ ₀ is the original output phase of the phase-locked loop, K ₁ is the empirical coefficient considering the actual compensation effect, T _PLL phase-locked loop PI link time constant, θ _ω is the output phase of the phase-locked loop after correction, τ ₁ is the fault phase The time constant of the exponential function. 7. The method according to claim 6, wherein the calculating the final phase difference of the grid voltage during the low voltage ride-through period comprises: Calculate the overvoltage zero-crossing point T ₁ before the fault through zero-crossing detection, and the over-voltage zero-crossing point T ₂ before the fault, and then calculate the final phase difference through the zero-crossing detection time difference and the grid voltage frequency:

Among them, Δθ ₂ is the phase difference of the grid voltage before and after the specific known low-voltage fault occurs. 8. The method according to claim 7, wherein the phase angle based on the phase lock loop is provided to the transient phase of low voltage fault recovery based on the final phase difference and the exponential function added in the phase lock loop Phase compensation for following velocity, including:

τ ₂ =K ₂ ×T _PLL Among them, K ₂ is the empirical value coefficient considering the actual effect, and τ ₂ is the time constant of the exponential function in the fault recovery stage. 9. An improved photovoltaic inverter phase-locked loop system for reactive power support under low-voltage ride-through, characterized in that it includes: The power grid fault monitoring module is used to use the voltage oriented vector control strategy to control the grid connection of photovoltaic inverters to control the grid voltage through the phase-locked loop, and to monitor whether the grid is faulty; The phase error calculation module is used to obtain the initial phase error of the phase-locked loop when the low-voltage fault occurs by obtaining the arctangent value of the voltage component output by the phase-locked loop based on the monitored three-phase symmetrical low-voltage fault of the power grid; The initial phase compensation module is used to provide phase compensation of the phase-locked loop for the transient phase of the low-voltage fault according to the initial phase error and the exponential function added in the phase-locked loop, and calculate the grid voltage during the low-voltage ride-through period The final phase difference of The recovery phase compensation module is used to provide phase compensation based on the phase angle follow-up speed of the phase-locked loop for the transient phase of the low-voltage fault recovery based on the final phase difference and the exponential function added in the phase-locked loop, so that the photovoltaic The inverter quickly returns to normal working conditions.

Images