Anti-islanding control method of Sandia frequency shift method based on frequency hopping

Abstract

The invention relates to an anti-islanding control method of a Sandia frequency shift method based on frequency hopping, and belongs to the technical field of inverter control. According to the method, the frequency deviation between the real-time frequency of the voltage at the output end of the current micro inverter and the average frequency of the power grid voltage is calculated, then the chopping factor in the period is calculated, the real-time frequency is calculated based on the chopping factor, positive feedback is achieved, and whether the island fault occurs or not is determined by judging whether the frequency deviation is larger than a threshold value or not. And the response time can be within 1 second. Due to the action of positive feedback, the micro inverter can have larger frequency deviation when a power grid is disconnected, so that compared with the traditional active frequency shift (AFD), the method has the advantages that an undetectable area (NDZ) is smaller, and the effectiveness of detection and the quality of output electric energy can be considered by controlling the value of a chopping factor.

Description

Technical Field

[0001] The present invention relates to the technical field of inverter control, and in particular to an anti-islanding control method of a Sandia frequency shift method based on frequency hopping. Background Art

[0002] In order to prevent the islanding effect from causing electric shock to relevant personnel, the micro inverter needs to have an anti-islanding strategy.

[0003] Anti-islanding strategies are mainly divided into two categories, namely, communication-based anti-islanding strategies and local anti-islanding strategies. The first type of communication-based anti-islanding strategy mainly uses radio communication to detect the islanding effect, and the second type of local anti-islanding strategy detects the islanding effect by monitoring the terminal voltage and current signals of the grid-connected power generation device.

[0004] Local anti-islanding strategies are further divided into passive and active. Passive solutions include: over / under voltage, over / under frequency, voltage harmonic detection, etc. Active solutions include: impedance measurement, automatic phase shift, active frequency shift (AFD), sliding membrane frequency shift, Sandia frequency shift, etc.

[0005] However, conventional anti-islanding passive detection has great limitations and has a large non-detectable zone (NDZ).

[0006] Therefore, how to provide an anti-islanding method that can detect the islanding state faster and more accurately and with a smaller non-detectable zone (NDZ) becomes an urgent problem to be solved in the field. Summary of the invention

[0007] The purpose of the present invention is to overcome the shortcomings of the above-mentioned prior art and provide an active frequency shift-Sandia frequency shift scheme based on positive feedback, add an enhanced system frequency deviation, and introduce the concept of positive feedback to make it a faster and more accurate anti-islanding control method than the AFD algorithm.

[0008] In order to achieve the above-mentioned purpose, the anti-islanding control method of the Sandia frequency shift method based on frequency hopping of the present invention comprises:

[0009] Detect the grid voltage frequency and calculate the average grid voltage frequency ω over multiple cycles grid ;

[0010] Calculate the frequency deviation |Δω| between the real-time frequency of the current micro-inverter output voltage and the average frequency of the grid voltage, Δω=ω-ω grid , where ω is the real-time frequency of the micro-inverter output voltage;

[0011] Calculate the chopping factor cf for the kth cycle k =cf k-1 +GΔω;cf k-1is the chopping factor of the k-1th cycle, G is the directional chopping acceleration gain;

[0012] Based on the chopping factor cf k Calculating the real-time frequency ω of the voltage at the output terminal of the micro-inverter;

[0013] When the kth cycle time is reached, calculate the current real-time frequency ω and the average grid voltage frequency ω grid Frequency deviation |△f|;

[0014] It is determined whether |△f| is greater than the frequency change threshold fth. If so, it is determined to be an island fault and the micro inverter is controlled to stop generating power. If not, it enters the k+1th cycle.

[0015] In the anti-islanding control method based on the frequency hopping Sandia frequency shift method, based on the chopping factor cf k Calculate the real-time frequency ω of the voltage at the output terminal of the micro-inverter; specifically:

[0016] The frequency deviation |Δω| increases with the increase of the voltage frequency at the output of the micro-inverter. The chopping factor cf k The real-time frequency ω of the voltage at the output of the micro-inverter also increases accordingly, forming a positive frequency feedback.

[0017] The anti-islanding control method of the Sandia frequency shift method based on frequency hopping is characterized in that the frequency change threshold value fth is in the range of 0.5 Hz to 0.8 Hz.

[0018] The anti-islanding control method of the Sandia frequency shift method based on frequency hopping also includes:

[0019] By adjusting the initial chopping factor cf0 and the directional chopping acceleration gain G, the total harmonic distortion THD of the grid current and the response time of the anti-islanding effect can meet the design requirements.

[0020] In the anti-islanding control method based on the Sandia frequency shift method of frequency hopping, the average frequency ω of the grid voltage in multiple cycles is calculated. grid Calculated as the average grid voltage frequency ω within 256 cycles grid .

[0021] The anti-islanding control method of the Sandia frequency shift method based on frequency hopping of the invention is adopted. By calculating the frequency deviation between the real-time frequency of the current micro-inverter output voltage and the average frequency of the grid voltage, the chopping factor within the cycle is calculated, and then the real-time frequency is calculated based on the chopping factor to achieve positive feedback. By judging whether the frequency deviation is greater than the threshold value fth, it is determined whether it is an islanding fault, and the response time can be within 1 second. Due to the effect of positive feedback, the micro-inverter will have a larger frequency deviation when the grid is disconnected. Therefore, compared with the traditional active frequency shift (AFD), this method has a smaller non-detectable zone (NDZ), and then by controlling the value of the chopping factor, the effectiveness of detection and the output power quality can also be taken into account. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic flow chart of the steps of the anti-islanding control method of the Sandia frequency shift method based on frequency hopping of the present invention;

[0023] Figure 2 It is a flow chart of the anti-islanding control method of the Sandia frequency shift method based on frequency hopping in practical application of the present invention;

[0024] Figure 3 A schematic diagram of island state determination using the method of the present invention;

[0025] Figure 4 The waveform diagram is of the grid voltage, the output voltage signal of the micro inverter and the output current signal when the grid is disconnected using the method of the present invention. DETAILED DESCRIPTION

[0026] In order to more clearly understand the technical content of the present invention, the following embodiments are given in detail.

[0027] See also Figure 1 , which is a schematic flow chart of the steps of the anti-islanding control method of the Sandia frequency shift method based on frequency hopping of the present invention.

[0028] In one embodiment, the anti-islanding control method of the Sandia frequency shift method based on frequency hopping includes the following steps:

[0029] Detect the grid voltage frequency and calculate the average grid voltage frequency ω over multiple cycles grid , can be 256 cycles;

[0030] Calculate the frequency deviation |Δω| between the real-time frequency of the current micro-inverter output voltage and the average frequency of the grid voltage, Δω=ω-ω grid , where ω is the real-time frequency of the micro-inverter output voltage;

[0031] Calculate the chopping factor cf for the kth cycle k =cf k-1 +GΔω;cf k-1 is the chopping factor of the k-1th cycle, G is the directional chopping acceleration gain;

[0032] Based on the chopping factor cf k Calculating the real-time frequency ω of the voltage at the output terminal of the micro-inverter;

[0033] When the kth cycle time is reached, calculate the current real-time frequency ω and the average grid voltage frequency ω grid Frequency deviation |△f|;

[0034] It is determined whether |△f| is greater than the frequency change threshold fth. If so, it is determined to be an island fault and the micro inverter is controlled to stop generating power. If not, it enters the k+1th cycle.

[0035] In a preferred embodiment, the chopping factor cf k Calculate the real-time frequency ω of the voltage at the output terminal of the micro-inverter; specifically:

[0036] The frequency deviation |Δω| increases with the increase of the voltage frequency at the output of the micro-inverter. The chopping factor cf k The real-time frequency ω of the voltage at the output of the micro-inverter also increases accordingly, forming a positive frequency feedback.

[0037] In another preferred embodiment, the frequency change threshold value fth ranges from 0.5 Hz to 0.8 Hz.

[0038] In a more preferred embodiment, the method further comprises: adjusting the values ​​of the initial chopping factor cf0 and the directional chopping acceleration gain G so that the total harmonic distortion THD of the grid current and the anti-islanding response time meet the design requirements.

[0039] In practical applications, the application process of the present invention is as follows Figure 2 As shown, it is an active frequency shift scheme that introduces positive feedback to the frequency of the micro-inverter output voltage UG.

[0040] First, the chopping factor cf is defined as a function of the voltage frequency at the output of the micro-inverter and the grid voltage frequency deviation, as follows:

[0041] cf k =cf k-1 +GΔω

[0042] Among them, cf k is the chopping factor of the kth cycle, cf k-1is the chopping factor of the k-1th cycle, G is the directional chopping acceleration gain, Δω is the frequency deviation between Ua and the grid voltage frequency grid, that is, Δω=ω-ω grid .

[0043] When the micro photovoltaic grid-connected inverter is connected to the grid, small frequency fluctuations are detected in real time, and the frequency enhancement strategy is used to try to accelerate the frequency change. However, the stability of the entire grid prevents this frequency change, so the frequency is generally stable at the grid frequency. When the micro inverter is disconnected from the grid, if the frequency shifts upward, the frequency deviation will increase with the micro inverter output voltage U G As the frequency increases, the chopping factor also increases accordingly, causing the micro photovoltaic grid-connected inverter to increase the frequency of the system output current, forming a frequency positive feedback. The entire positive feedback process continues until the frequency change exceeds the limit value and shuts down. The frequency change threshold fth range is 0.5hz to 0.8hz. For the case of frequency under-bias, the principle is similar.

[0044] Among them, as the acceleration gain G increases, the detection blind zone (NDZ) is significantly reduced, but too large an acceleration gain will directly increase the circuit's THD (total harmonic distortion), affecting the stability of the system. The initial chopping cf0 has no significant effect on the size of the detection blind zone, so changing the value of cf0 has little effect on improving the island detection performance. However, the size of cf0 directly affects the harmonic content of the output current. Therefore, the value range of cf0 and G can be flexibly adjusted to make the total harmonic distortion (THD) of the grid current and the response time of anti-islanding meet the requirements of different standards.

[0045] like Figure 3 As shown, the change of cf is determined by the frequency offset. When cf exceeds a certain value, the frequency offset is calculated. If the offset is greater than the threshold, grid_sta1 represents the island state, 1 means normal operation, and 0 means the island state is detected.

[0046] This method adds the judgment of the frequency offset threshold, directly detecting whether the frequency offset in the unit cycle reaches the threshold fth. If it reaches it, it will shut down immediately and report the island fault. It no longer relies on the threshold shutdown of over-frequency or under-frequency. This can distinguish whether the system shutdown is caused by an island fault or a real power grid over-frequency or under-frequency fault.

[0047] like Figure 4 As shown in FIG. 1 , when the grid is disconnected, the micro inverter stops generating power after about 900 ms.

[0048] Therefore, after adopting the anti-islanding control method of the present invention, the micro inverter can meet the requirements of various standards, and when the quality factor Qf is under the condition of 1±0.05, the response time can be within 1 s.

[0049] Compared with the active frequency shift (AFD) in the active detection scheme, due to the positive feedback in the present invention, the micro inverter will have a larger frequency offset when the grid is disconnected, and a smaller non-visible detection zone (NDZ) can be obtained.

[0050] The anti-islanding control method of the Sandia frequency shift method based on frequency hopping of the invention is adopted. By calculating the frequency deviation between the real-time frequency of the current micro-inverter output voltage and the average frequency of the grid voltage, the chopping factor within the cycle is calculated, and then the real-time frequency is calculated based on the chopping factor to achieve positive feedback. By judging whether the frequency deviation is greater than the threshold value fth, it is determined whether it is an islanding fault, and the response time can be within 1 second. Due to the effect of positive feedback, the micro-inverter will have a larger frequency deviation when the grid is disconnected. Therefore, compared with the traditional active frequency shift (AFD), this method has a smaller non-detectable zone (NDZ), and then by controlling the value of the chopping factor, the effectiveness of detection and the output power quality can also be taken into account.

[0051] In this specification, the present invention has been described with reference to specific embodiments thereof. However, it is apparent that various modifications and variations may be made without departing from the spirit and scope of the present invention. Therefore, the specification and drawings should be regarded as illustrative rather than restrictive.

Claims

1. An anti-islanding control method based on Sandia frequency shift method with frequency hopping, characterized in that: The method includes: Detect the grid voltage frequency and calculate the average grid voltage frequency ω over multiple cycles grid ; Calculate the frequency deviation |Δω| between the real-time frequency of the current micro-inverter output voltage and the average frequency of the grid voltage, Δω=ω-ω grid , where ω is the real-time frequency of the micro-inverter output voltage; Calculate the chopping factor cf for the kth cycle k =cf k-1 +GΔω;cf k-1 is the chopping factor of the k-1th cycle, G is the directional chopping acceleration gain; Based on the chopping factor cf k Calculating the real-time frequency ω of the voltage at the output terminal of the micro-inverter; When the kth cycle time is reached, calculate the current real-time frequency ω and the average grid voltage frequency ω grid Frequency deviation |△f|; It is determined whether the frequency deviation |△f| is greater than the frequency change threshold fth. If so, it is determined to be an island fault and the micro inverter is controlled to stop generating power. If not, it enters the k+1th cycle.

2. The anti-islanding control method of Sandia frequency shift method based on frequency hopping according to claim 1, characterized in that: Based on the chopping factor cf k Calculate the real-time frequency ω of the voltage at the output terminal of the micro-inverter; specifically: The frequency deviation |Δω| increases with the increase of the voltage frequency at the output of the micro-inverter. The chopping factor cf k The real-time frequency ω of the voltage at the output of the micro-inverter also increases accordingly, forming a positive frequency feedback.

3. The anti-islanding control method of Sandia frequency shift method based on frequency hopping according to claim 2, characterized in that: The frequency change threshold value fth ranges from 0.5 Hz to 0.8 Hz.

4. The anti-islanding control method of Sandia frequency shift method based on frequency hopping according to claim 1, characterized in that: The method further includes: By adjusting the initial chopping factor cf0 and the directional chopping acceleration gain G, the total harmonic distortion THD of the grid current and the response time of the anti-islanding effect can meet the design requirements.

5. The anti-islanding control method of Sandia frequency shift method based on frequency hopping according to claim 1, characterized in that: The calculation of the average frequency ω of the grid voltage within a plurality of cycles grid Calculated as the average grid voltage frequency ω within 256 cycles grid .

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