A static reactive power generator-based active anti-islanding method for distribution area

Abstract

This invention provides an active anti-islanding method for distribution substations based on a static var generator (SVG), comprising the following steps: S1: The SVG measures the grid voltage in real time and obtains the grid frequency through phase-locked loop (PLL) technology, and calculates the reactive disturbance current based on the frequency and the current reactive current of the equipment; S2: The SVG periodically injects the reactive disturbance current into the grid. This invention solves the problem of blind spots in islanding detection, can effectively and promptly break the islanding operation state of distributed generator substations, and does not require the installation of additional anti-islanding devices or the detection of islanding, thus avoiding voltage fluctuations and other impacts on the grid. It greatly improves the safety and reliability of the distribution substation grid, ensures the power safety of users, and has strong economic practicality.

Description

Technical Field

[0001] This invention relates to the field of distributed photovoltaic power generation anti-islanding technology, and in particular to an active anti-islanding method for distribution stations based on a static var generator. Background Technology

[0002] With the increasing prevalence of distributed photovoltaic (PV) systems, islanding may occur in low-voltage distribution areas, posing challenges to the safety of maintenance personnel and the operational safety of equipment within the island. While grid-connected PV inverters possess anti-islanding protection, they may fail to detect islanding in certain scenarios, creating a blind spot. Therefore, anti-islanding protection devices are needed to provide secondary protection against islanding, ensuring greater safety and reliability. Commonly used low-voltage anti-islanding devices are resistive loads, applied manually before maintenance to disrupt the power balance of the island. However, manual switching lacks initiative and timeliness. Furthermore, since anti-islanding devices are only manually activated before maintenance, grid operators lack the means to disrupt islanding when it occurs unnecessarily. Summary of the Invention

[0003] In view of this, the purpose of this invention is to provide an active anti-islanding method for distribution substations based on a static var generator (SVG), which solves the problem of blind spots in islanding state detection in anti-islanding protection. It eliminates the need to detect whether islanding has occurred. When islanding occurs in the substation during operation, the SVG can automatically adjust its working state to interfere with the islanding state of the substation, break the islanding state, and thus improve the safety and reliability of the distribution substation.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: an active anti-islanding method for distribution radio areas based on a static var generator, comprising the following steps:

[0005] Step S1: The SVG device measures the grid voltage in real time and obtains the grid frequency through phase-locked loop technology. Based on the frequency and the device's current reactive power output current, the reactive power disturbance current is calculated.

[0006] Step S2: The SVG device periodically injects reactive disturbance current into the power grid.

[0007] In a preferred embodiment, the SVG device and the distributed photovoltaic power generation system are in the same distribution substation. The SVG device is used to compensate for reactive power in the substation and improve power quality. The underfrequency / undervoltage protection threshold of the SVG device should be less than or equal to the underfrequency / undervoltage threshold of the photovoltaic device in the substation, and the overfrequency / overvoltage protection threshold should be greater than or equal to the overfrequency / overvoltage threshold of the photovoltaic device.

[0008] In a preferred embodiment, when setting the reactive power disturbance current that the SVG device should emit, it should be able to actively emit disturbance components under normal conditions to detect the existence of islanding. After islanding occurs, it should be able to continuously emit disturbances so that the photovoltaic inverter in the distribution area can sense the existence of islanding and thus trigger the anti-islanding protection of the photovoltaic inverter.

[0009] In a preferred embodiment, the reactive disturbance current in step S1 consists of two parts: a periodic fixed disturbance amount I. q1 and positive feedback disturbance I q2 The calculation method is as follows:

[0010]

[0011] I q2 =k2*|f grid -f n |

[0012] ΔI q =I q1 +I q2

[0013] Among them, I q The current reactive current reference amplitude of the SVG device; T is the applied disturbance period; t p f is the time for applying the disturbance. n f is the reference frequency of the power grid. grid The voltage frequency at the grid connection point is measured in real time; k1 and k2 are disturbance coefficients; ΔI q This represents the amplitude of the reactive disturbance current.

[0014] In a preferred embodiment, the periodic fixed disturbance is set to be small to reduce the impact on the output current of the SVG during normal operation. When there is no islanding phenomenon in the transformer area, the measured frequency is clamped by the grid frequency. If there is no measurement noise interference, the frequency is the normal value. At this time, the positive feedback disturbance is zero, and the disturbance only includes the periodic disturbance, which has little impact on the power quality of the grid.

[0015] In a preferred embodiment, if islanding occurs—that is, an external fault causes the distribution area to become an isolated small power grid, but the anti-islanding protection of the distribution area's photovoltaic inverter fails to identify and activate to break the islanding—the islanding frequency of the distribution area will shift. The disturbance includes not only periodic fixed disturbances but also positive feedback disturbances. These disturbances cause changes in the grid frequency, forming a positive feedback loop that increases the disturbance and frequency deviation, thereby triggering the anti-islanding protection of the distribution area's photovoltaic inverter.

[0016] In a preferred embodiment, in the early stages of islanding, the frequency remains unchanged or changes very little, and periodic fixed disturbances play a dominant role. As the frequency shifts, positive feedback disturbances become dominant.

[0017] In a preferred embodiment, the periodic disturbance coefficient k1 is proportional to the total capacity of the distributed photovoltaic (PV) system in the distribution area. The larger the total capacity of the distributed PV system, the greater the reactive power required to break the islanding balance; therefore, the coefficient k1 should be appropriately increased. The positive feedback disturbance coefficient k2 is determined based on the required anti-islanding protection action time of the PV inverter in the distribution area. The shorter the required anti-islanding protection action time, the larger k2 should be, and the faster the anti-islanding protection will be triggered.

[0018] In a preferred embodiment, I q2 The measurement frequency can vary and may have a dead zone, i.e., |f grid -f n When | is less than a certain level, k2 = 0, I q2 =0; |f grid -f n When it exceeds a certain level, k2 becomes a non-zero value, I q2 The value is not zero. This setting avoids the influence of frequency measurement noise during normal operation on the reactive power output current of the SVG. When k2 is set in this way, k1 can be increased appropriately; when k2 is not set in this way, k1 can be decreased appropriately.

[0019] In a preferred embodiment, the reactive current output by the SVG can be bidirectional. To ensure that the SVG can cause the maximum power disturbance to the islanded operation of the transformer area, the direction of the reactive disturbance current in step S2 is opposite to the direction of the current output current of the current device.

[0020] Compared with the prior art, the present invention has the following beneficial effects: it solves the problem of blind zone detection of islanding status, eliminates the need to install additional anti-islanding devices, eliminates the need to detect whether islanding has occurred, and can automatically and timely break the islanding operation status of the distribution transformer area, greatly improving the safety and reliability of the power grid of the distribution transformer area, ensuring the power safety of users, and has strong practicality. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the electrical connection of the SVG in the photovoltaic distribution area in this invention.

[0022] The reference numerals in the attached diagram are as follows: 1 is the photovoltaic power generation device, 2 is the user load, 3 is the SVG device, 4 is the grid connection point of the SVG device, 5 is the main power supply switch of the distribution substation, and 6 is the transformer of the distribution substation. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0024] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0025] It should be noted that the terminology used herein is for the purpose of describing particular implementations only and is not intended to limit the exemplary implementations according to this application; as used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise; furthermore, it should be understood that when the terms “comprising” and / or “including” are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0026] An active anti-islanding method for distribution radio areas based on static var generators includes the following steps:

[0027] Step S1: The SVG device measures the grid connection point voltage U. pcc The grid frequency f is obtained based on phase-locked loop. grid , power grid reference frequency f n and the current reactive current output reference value i of the equipment qref Calculate the amplitude of reactive disturbance current ΔI q ;

[0028] Step S2: During SVG operation, fundamental positive sequence reactive disturbance current is periodically injected into the grid. When islanding occurs and the anti-islanding protection of the photovoltaic inverter fails to identify it, the distribution substation enters islanding. The reactive current sent by the SVG will disrupt the islanding. At this time, the distributed photovoltaic power generation system shuts down due to over-frequency / under-frequency. After the photovoltaic shutdown, the substation loses power, thus causing the SVG to shut down.

[0029] Furthermore, the SVG device is located in the same distribution substation as the distributed photovoltaic power generation system. The SVG device is used to compensate for reactive power in the substation and improve power quality, but it also acts as an active anti-islanding device.

[0030] Furthermore, the underfrequency / undervoltage protection threshold of the SVG device should be less than or equal to the underfrequency / undervoltage threshold of the photovoltaic device in the distribution area, and the overfrequency / overvoltage protection threshold should be greater than or equal to the overfrequency / overvoltage threshold of the photovoltaic device.

[0031] Furthermore, in step one, the reactive disturbance current ΔI q It consists of two parts: periodic disturbance I q1 and positive feedback disturbance I q2 The amplitude is calculated as follows:

[0032]

[0033] I q2 =k2*|f grid -f n |

[0034] ΔI q =I q1 +I q2

[0035] Among them, I q Reference i for the current reactive current of the SVG device qref Amplitude; T is the period of the applied disturbance, T is recommended to be 1s; t p For the time to apply the disturbance, t is recommended. p It is 0.24s; f n f is the power grid reference frequency (50Hz). grid The voltage frequency at the grid connection point is measured in real time; k1 and k2 are disturbance coefficients, with k1 recommended to be 0.0625 and k2 to be 0.5.

[0036] When a transformer substation is connected to the grid, the frequency at the connection point is clamped by the grid, and the value of the positive frequency feedback is zero. Each time, the reactive power generated only has a fixed periodic value component. When the transformer substation is disconnected from the grid, if an islanding effect occurs, the frequency at the connection point changes due to the reactive power disturbance. This change forms a positive feedback, causing the reactive power generated to become larger and larger, and the frequency deviation to become larger and larger.

[0037] Furthermore, in step two, the reactive disturbance current ΔI q The direction of the current reactive reference current i of the device qref If the direction is opposite, then the reference value Δi of the reactive power disturbance current of the SVG device is... * As shown below, where ph i is i qref The real-time phase.

[0038] Δi * =i qref -ΔI q *ph i

[0039] The above embodiments are descriptions of specific implementations of the present invention, and not limitations thereof. Those skilled in the art can make various modifications and changes without departing from the spirit and scope of the present invention to obtain corresponding equivalent technical solutions. Therefore, all equivalent technical solutions should be included in the patent protection scope of the present invention.

Claims

1. A method for active anti-islanding of distribution radio areas based on static var generators, characterized in that, Includes the following steps: Step S1: The SVG device measures the grid voltage in real time and obtains the grid frequency through phase-locked loop technology. Based on the frequency and the device's current reactive power output current, the reactive power disturbance current is calculated. Step S2: The SVG device periodically injects reactive disturbance current into the power grid; The reactive disturbance current in step S1 consists of two parts: a periodic fixed disturbance amount. and positive feedback disturbance The calculation method is as follows: in, This is the current reactive current reference amplitude for the SVG device; To apply the disturbance period; The duration of the applied disturbance; The reference frequency for the power grid. The frequency of the grid connection point voltage is measured in real time; and The disturbance coefficient; This represents the amplitude of the reactive disturbance current. The periodic fixed disturbance is set small to reduce the impact on the output current of the SVG during normal operation. When there is no islanding phenomenon in the transformer area, the measured frequency is clamped by the grid frequency. If there is no measurement noise interference, the frequency is the normal value. At this time, the positive feedback disturbance is zero, and the disturbance only includes the periodic disturbance, which has little impact on the power quality of the grid. If islanding occurs, meaning an external fault causes the distribution area to become an isolated small power grid, but the anti-islanding protection of the distribution area's photovoltaic inverter fails to identify and activate to break the islanding, the islanding frequency of the distribution area will shift. The disturbance includes not only periodic fixed disturbances but also positive feedback disturbances. The disturbance causes a change in the grid frequency, and this change forms a positive feedback loop, making the disturbance larger and larger, and the frequency deviation larger and larger, thereby triggering the anti-islanding protection of the distribution area's photovoltaic inverter.

2. The active anti-islanding method for distribution radio areas based on a static var generator according to claim 1, characterized in that, The SVG device is located in the same distribution substation as the distributed photovoltaic power generation system. The SVG device is used to compensate for reactive power in the substation and improve power quality. The underfrequency / undervoltage protection threshold of the SVG device is less than or equal to the underfrequency / undervoltage threshold of the photovoltaic device in the substation, and the overfrequency / overvoltage protection threshold is greater than or equal to the overfrequency / overvoltage threshold of the photovoltaic device.

3. The active anti-islanding method for distribution radio areas based on a static var generator according to claim 1, characterized in that, When setting the reactive power disturbance current emitted by the SVG device, it is possible to actively emit disturbance components under normal circumstances to detect the existence of islanding. After islanding occurs, it can continuously disturb the system so that the photovoltaic inverter in the distribution area can sense the existence of islanding and thus trigger the anti-islanding protection of the photovoltaic inverter.

4. The active anti-islanding method for distribution radio areas based on static var generators according to claim 1, characterized in that, In the early stages of islanding, the frequency remains unchanged or changes very little, and periodic fixed disturbances play a dominant role. As the frequency shifts, positive feedback disturbances become dominant.

5. The active anti-islanding method for distribution radio areas based on a static var generator according to claim 1, characterized in that, Periodic disturbance coefficient The reactive power required to break the islanding balance is directly proportional to the total capacity of the distributed photovoltaic system in the area. The larger the total capacity of the distributed photovoltaic system, the greater the reactive power required to break the islanding balance. Increase appropriately; positive feedback disturbance coefficient The time required for the anti-islanding protection to operate is determined based on the requirements for the anti-islanding protection operation time of the photovoltaic inverter in the distribution area. The shorter the required anti-islanding protection operation time, the better. The higher the setting, the faster the anti-islanding protection will be triggered.

6. The active anti-islanding method for distribution radio areas based on a static var generator according to claim 1, characterized in that, It has a dead zone as the measurement frequency changes, i.e. When it is below a certain level, =0, =0; When it exceeds a certain level, Non-zero value Non-zero, this setting avoids the influence of frequency measurement noise during normal operation on the reactive power output current of the SVG; when When this setting is adopted, Increase appropriately; when When this setting method is not adopted, Reduce appropriately.

7. The active anti-islanding method for distribution radio areas based on a static var generator according to claim 1, characterized in that, The reactive current output by the SVG is bidirectional. In order to ensure that the SVG can cause the maximum power disturbance to the islanded operation of the transformer area, the direction of the reactive disturbance current in step S2 is opposite to the direction of the current output current of the current device.

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