Composite island detection method and system considering energy storage charge state

Abstract

The invention discloses a composite island detection method and system considering an energy storage charge state. The method comprises the following steps: respectively calculating a frequency change rate and a voltage phase deviation angle; when the SOC of the distributed energy storage battery is higher than a set threshold value, the calculated frequency change rate is compared with a set first threshold value, the duration time when the frequency change rate exceeds the first threshold value is recorded, and if the time exceeds the set first threshold value, it is judged that the distributed photovoltaic energy storage system is in an island state; when the SOC of the distributed energy storage battery is lower than a set threshold value, the calculated frequency change rate is compared with a set second threshold value, if the frequency change rate reaches or exceeds the second threshold value, voltage phase deviation detection is started, and if the voltage phase deviation angle is larger than the set second threshold value, voltage phase deviation detection is stopped. And if yes, determining that the distributed photovoltaic energy storage system is in an island state. According to the invention, the island detection method can be kept stable and efficient under different working conditions.

Description

Technical Field

[0001] The present invention relates to the technical field of electric power system islanding of distributed photovoltaic energy storage systems, and in particular to a composite islanding detection method and system taking into account the energy storage charge state. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] With the widespread application of distributed photovoltaic energy storage systems, the safe operation of distributed photovoltaic energy storage systems, especially the detection and prevention of islanding effects, has become a key issue in system design. Traditional islanding detection technology is mainly based on the changes in parameters such as frequency and voltage. After adding energy storage, traditional islanding detection methods may not be sufficient to cope with these new challenges.

[0004] Passive islanding detection determines islanding by monitoring the anomalies of electrical quantities such as voltage and frequency at the common coupling point. The traditional frequency offset islanding detection method has the advantages of simple principle, easy implementation in actual systems, and low implementation and maintenance costs. However, frequency offset detection usually requires a long detection time and is not suitable for detecting rapidly changing islanding states. Therefore, although the traditional frequency offset detection method has certain application advantages in islanding detection, its adaptability in distributed photovoltaic systems with energy storage systems is poor. Due to the frequency regulation capability and complex dynamic characteristics of the energy storage system, the traditional method is prone to missed detection and false detection.

[0005] When the system is disconnected from the main grid due to some reason (such as a fault or operating error) and an island phenomenon occurs, the SOC value of the distributed energy storage system has a significant impact on the behavior of the system, especially in terms of frequency changes. When the energy storage SOC value is high (for example, greater than 30%), the energy storage battery has more available power, which enables it to effectively participate in the frequency regulation of the system. During the islanding period, if a sudden increase or decrease in load occurs, the energy storage system can quickly release or absorb energy to help maintain the stability of the grid frequency. Therefore, even if an island occurs, the frequency fluctuation of the system may be small, and the energy storage battery can resist sudden load changes and maintain a more stable output.

[0006] When the SOC value of the distributed photovoltaic energy storage system is low (for example, less than 30%), the available energy of the energy storage is limited, which limits the responsiveness of the energy storage system when facing an island. The frequency change rate detection mainly has the following problems:

[0007] (1) When the energy storage SOC value is low, its regulation and response capabilities are limited in an isolated state. If the system suddenly faces a load change, the energy storage system cannot provide enough energy to stabilize the frequency. The instability of the energy storage output power increases, and the system is more sensitive to frequency disturbances, which affects the sensitivity and accuracy of the rate of change of frequency (ROCOF) method.

[0008] (2) The single frequency change rate method relies on rapid changes in frequency to identify islanding conditions. When the energy storage system SOC is low, the system may not be able to respond quickly enough to frequency changes, which may lead to false triggering of islanding detection or delayed triggering in real islanding events, thereby increasing the risk of grid operation.

[0009] (3) The energy storage system may be forced to over-discharge, which will not only damage the health and life of the battery, but also cause unstable power supply or even complete power outage.

[0010] As energy storage systems are added to increasingly complex power grids, the need for island detection becomes more diverse. Traditional island detection methods mainly rely on a single detection technology, but a single detection method has certain limitations when dealing with complex working conditions and variable load conditions. In particular, when the SOC of the energy storage system changes, the accuracy and sensitivity of island detection will be affected. Summary of the invention

[0011] In order to solve the above problems, the present invention proposes a composite island detection method and system taking into account the energy storage charge state, which adopts a composite detection method combining frequency change rate and voltage phase shift, can effectively cope with complex working conditions and changeable load conditions, and has higher reliability and stronger adaptability.

[0012] In some embodiments, the following technical solutions are adopted:

[0013] A composite island detection method considering the energy storage charge state, comprising:

[0014] Real-time measurement of the grid frequency at the common coupling point between the grid and the distributed photovoltaic energy storage system, and calculation of the frequency change rate and voltage phase deviation angle respectively;

[0015] When the SOC of the distributed energy storage battery is greater than the set threshold, the calculated frequency change rate is compared with the set first threshold, and the duration of the frequency change rate exceeding the first threshold is recorded. If the time exceeds the set first threshold value, it is determined that the distributed photovoltaic energy storage system is in an island state;

[0016] When the SOC of the distributed energy storage battery is less than or equal to the set threshold, the calculated frequency change rate is compared with the set second threshold. If the frequency change rate reaches or exceeds the second threshold, the voltage phase shift detection is started. If the voltage phase shift angle is greater than the set second threshold value, it is determined that the distributed photovoltaic energy storage system is in an island state.

[0017] As a further solution, the first threshold value is greater than the second threshold value.

[0018] As a further solution, the average value of the frequency change rate of each data point in the current measurement time window relative to the corresponding data point in the previous measurement time window is calculated to obtain the frequency change rate of the current measurement time window.

[0019] As a further solution, the voltage phase shift angle is calculated as follows:

[0020] Set a fixed integration time period T and initialize the voltage phase deviation angle Δθ int is zero;

[0021] In each sampling period Δt, the frequency signal f at the common coupling point is obtained and the frequency variation Δf in Δt is calculated; the frequency variation Δf is accumulated and calculated to obtain the phase shift angle Δθ;

[0022] After each sampling period, the initial phase offset angle of the current sampling period is added to Δθ to obtain the current phase offset angle, which is used as the initial phase offset angle of the next sampling period.

[0023] When the integration time period T is reached, the voltage phase offset angle is calculated based on the current phase offset angle and the integration time period T; and the current phase offset angle is cleared at the same time;

[0024] The above process is repeated to calculate the voltage phase deviation angle for the next integration time period T.

[0025] As a further solution, after measuring the grid frequency and voltage signals at the common coupling point between the grid and the distributed photovoltaic energy storage system, the signals are filtered.

[0026] As a further solution, after determining that the distributed photovoltaic energy storage system is in an island state, an island alarm is triggered, and corresponding protection actions are performed, disconnecting the circuit breaker at the common coupling point or switching to island mode operation.

[0027] In other embodiments, the following technical solutions are adopted:

[0028] A composite island detection system considering the energy storage charge state, comprising:

[0029] A data acquisition module is used to measure the grid frequency at the common coupling point of the grid and the distributed photovoltaic energy storage system in real time, and calculate the frequency change rate and voltage phase deviation angle respectively;

[0030] The island state judgment module is used to compare the calculated frequency change rate with the set first threshold when the SOC of the distributed energy storage battery is greater than the set threshold, record the duration of the frequency change rate exceeding the first threshold, and if the time exceeds the set first threshold value, determine that the distributed photovoltaic energy storage system is in an island state; when the SOC of the distributed energy storage battery is less than or equal to the set threshold, compare the calculated frequency change rate with the set second threshold, if the frequency change rate reaches or exceeds the second threshold, start voltage phase shift detection, and if the voltage phase shift angle is greater than the set second threshold value, determine that the distributed photovoltaic energy storage system is in an island state.

[0031] In other embodiments, the following technical solutions are adopted:

[0032] A terminal device comprises a memory, a processor and a computer program stored in the memory, wherein the processor executes the computer program to implement the steps of the above method.

[0033] In other embodiments, the following technical solutions are adopted:

[0034] A computer-readable storage medium stores a computer program / instruction, which implements the steps of the above method when executed by a processor.

[0035] In other embodiments, the following technical solutions are adopted:

[0036] A computer program product comprises a computer program / instruction, which implements the steps of the above method when executed by a processor.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] (1) When the SOC value is high, the present invention selects a larger detection threshold to avoid false operation and reduce interference with the normal operation of the system; when the SOC value is low, a smaller detection threshold is selected to increase the sensitivity of island detection, quickly disconnect from the grid, and protect the energy storage battery from over-discharge. According to the change of the energy storage SOC value, the threshold of the frequency change rate is dynamically adjusted, so that the island detection method can remain stable and efficient under different working conditions.

[0039] (2) The present invention adapts different islanding detection methods according to different SOC values. When the SOC value is high, islanding detection is performed only based on frequency change rate detection. When the SOC value is low, frequency change rate detection and voltage phase shift detection are combined. The two different detection methods of frequency change rate and voltage phase shift are comprehensively considered, which can more comprehensively capture the changes in the power grid when an islanding occurs. The two detection methods are used to complement each other and provide double protection, which can improve the accuracy and sensitivity of islanding detection and reduce the false alarm rate and missed alarm rate.

[0040] (3) When calculating the voltage phase shift angle, the present invention takes into account that the traditional integration method will accumulate errors over time, so a fixed integration time length T is set. After integrating the integration time length T, the phase shift angle is reset to zero and integrated again. In this way, the accumulation caused by normal disturbances is eliminated, misjudgment is avoided, and the accumulation of errors can be effectively avoided, thereby improving accuracy.

[0041] (4) The present invention uses conventional power system monitoring equipment without adding additional hardware costs. It has the advantages of low cost and good real-time performance. It is suitable for island detection of distributed photovoltaic energy storage systems and is easy to implement in engineering and promote application.

[0042] Other features and advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a flow chart of a composite island detection method considering the energy storage charge state in an embodiment of the present invention;

[0044] Figure 2 This is a schematic diagram of a distributed photovoltaic storage system connected to a target system distribution network in an embodiment of the present invention;

[0045] Figure 3 Schematic diagram of island power flow of distributed photovoltaic storage system in an embodiment of the present invention;

[0046] Figure 4 A schematic diagram of a simulation model of a distributed photovoltaic energy storage system in an embodiment of the present invention;

[0047] Figure 5 1 is the frequency change rate, voltage phase shift and island detection signal waveform of scenario 1 in an embodiment of the present invention;

[0048] Figure 6 1 is the frequency change rate, voltage phase shift and island detection signal waveform of scenario 2 in an embodiment of the present invention;

[0049] Figure 71 and 2 are waveforms of the frequency change rate, voltage phase shift and island detection signal of scenario 3 in an embodiment of the present invention. DETAILED DESCRIPTION

[0050] It should be noted that the following detailed descriptions are illustrative and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0051] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0052] Embodiment 1

[0053] In the island detection of distributed photovoltaic energy storage systems, the change of SOC becomes an important factor. When the SOC value is low, the system stability is poor and islanding is prone to occur, but the frequency change may be small at this time and cannot accurately reflect the change of system status. The change of voltage phase responds more quickly and sensitively to the power balance state, and can more sensitively capture the slight changes in the system, making up for the shortcomings of frequency change rate detection.

[0054] Therefore, in one or more embodiments, a composite island detection method that takes into account the energy storage charge state is disclosed, and voltage phase offset detection is introduced when the distributed photovoltaic energy storage system has a low SOC to improve the detection sensitivity and accuracy, ensure the timely detection and processing of the island effect, and improve the safety and stability of the system operation. At the same time, the composite detection that combines the frequency change rate and the voltage phase offset can effectively cope with complex working conditions and variable load conditions, and has higher reliability and stronger adaptability. The composite island detection method combines a variety of detection technologies through multi-dimensional monitoring, which can more accurately distinguish between normal operating status and island status, significantly reduce false triggering, and can dynamically adjust the detection strategy and threshold according to the SOC value of the energy storage system and the grid conditions.

[0055] Combination Figure 1 The method of this embodiment specifically includes the following process:

[0056] Step (1): Real-time measurement of the grid frequency at the common coupling point (PCC) between the grid and the distributed photovoltaic energy storage system.

[0057] Specifically, the system is initialized first, and the initial state of the distributed energy storage system is measured and recorded, including the SOC, output power and system load of each energy storage unit. The SOC threshold is set (for example, 30%), and the SOC threshold can be set according to actual requirements. When the energy storage SOC is lower than this threshold, the frequency and sensitivity of island detection are increased.

[0058] The battery management system (BMS) is used to monitor and record the SOC of the energy storage system in real time. At the same time, the grid frequency at the common coupling point between the grid and the distributed photovoltaic energy storage system is continuously monitored.

[0059] In this embodiment, a high-precision frequency sensor is used to monitor the grid frequency at the common coupling point. To ensure the accuracy of the measurement, the acquisition device uses a GPS clock for precise time synchronization to ensure the consistency of time tags between different devices. At the same time, to ensure that the time resolution of the data is sufficient to capture rapid frequency changes, the real-time data sampling frequency should be at least 1kHz.

[0060] In this embodiment, in order to improve the accuracy and reliability of the frequency change rate and voltage phase offset calculation, it is necessary to perform appropriate filtering on the signal to eliminate noise and occasional spikes. A digital low-pass filter or a moving average filter can be used to smooth the frequency data, thereby reducing errors and reducing the impact of transient interference.

[0061] Step (2): Calculate the frequency change rate and voltage phase shift angle respectively.

[0062] In this embodiment, the frequency change rate is calculated as follows:

[0063]

[0064] Wherein, Δt is the measurement time window length, which is generally 2-40 cycles; n is the number of samples sampled within the measurement time window length Δt, and k is the sample sequence number 1, 2, 3, ..., n contained in the measurement time window length Δt. f(t k ) and f(t k -Δt) are the sampling frequencies of the k-th sample and the sample one measurement time window before the k-th sample, respectively.

[0065] During each time window, the system continuously collects frequency-related data points. When the time window slides forward, the oldest data points are replaced by new data points. The ROCOF value is then calculated as the average of the frequency change rate of each data point relative to the data point one measurement time window ago.

[0066] In this embodiment, the calculation process of the voltage phase shift angle is specifically as follows:

[0067] When an island occurs, the inverter is disconnected from the main grid, and the voltage at the inverter output loses the clamping of the main grid, which will cause a voltage phase jump at the common coupling point (PCC). The island can be determined by detecting the degree of voltage phase jump through the setting of the threshold. This type of island detection method is the voltage phase offset island detection method.

[0068] The voltage phase shift angle Δθ is the cumulative value of the frequency change Δf at the PCC before and after the islanding occurs. The phase shift angle is determined by two factors: the magnitude of the frequency change and the duration of the change.

[0069] The traditional integration method will accumulate errors over time. When the unbalanced power between the inverter output power and the load power is very small, the frequency change Δf is also very small. If the integration time is long enough, even a very small Δf will cause a large phase angle shift. Therefore, when calculating the voltage phase shift angle, this embodiment sets a fixed integration time length T. After integrating for a time length T, Δθ is set to zero and integrated again. This eliminates the accumulation caused by normal disturbances, avoids misjudgment, and can effectively avoid the accumulation of errors, thereby improving accuracy. The specific calculation steps are as follows:

[0070] ① Initial setting: Set a fixed integration time period T. Initialize the voltage phase deviation angle Δθ int is zero.

[0071] ②Signal sampling: In each sampling period Δt, the frequency signal f at the common coupling point is obtained and the frequency change Δf within Δt is calculated.

[0072] ③ Phase shift angle integration: In each sampling period, the frequency change Δf is accumulated and calculated to obtain the phase shift angle Δθ = Δf × Δt.

[0073] ④ Update the voltage phase offset angle value: After each sampling period, add the initial phase offset angle of the current sampling period to Δθ, that is, Δθint'=Δθint+Δθ; Δθint' is the current phase offset angle, and also serves as the initial phase offset angle of the next sampling period.

[0074] ⑤ Voltage phase offset value reset: When the fixed integration time T is reached, calculate the voltage phase offset angle θ shift =Δθint' / T. Δθint' is cleared to prepare for the next integration cycle.

[0075] ⑥ Repeat process: Start a new integration cycle and repeat the above steps.

[0076] Step (3): When the SOC of the distributed energy storage battery is greater than a set threshold, the calculated frequency change rate is compared with the set first threshold, and the duration of the frequency change rate exceeding the first threshold is recorded. If the time exceeds the set first threshold value, it is determined that the distributed photovoltaic energy storage system is in an island state.

[0077] The threshold of the SOC of the energy storage battery is set according to actual needs. For example, in this embodiment, it is set to 30%. When the SOC of the distributed energy storage battery is greater than 30%, it is considered that the SOC value of the distributed energy storage battery is high; when the SOC of the distributed energy storage battery is less than 30%, it is considered that the SOC value of the distributed energy storage battery is low.

[0078] When the SOC value of the distributed energy storage battery is high, the energy storage system has better stability and can withstand larger frequency fluctuations. At the same time, the system has a strong dynamic response capability, and the short-term frequency change rate may exceed the threshold, which may lead to misjudgment.

[0079] Therefore, this embodiment adds a duration criterion, which can effectively avoid false alarms caused by short-term disturbances and enhance the accuracy of detection. At this time, a higher frequency change rate first threshold R1 is set. In order to avoid false operation, the first threshold R1 should be selected to be relatively large to reduce interference with the normal operation of the system.

[0080] The calculated real-time frequency change rate ROCOF is compared with the set first threshold R1. When the frequency change rate exceeds R1, the timing starts and the duration of the frequency change rate exceeding R1 is recorded; then it is determined whether the duration Δt1 of the frequency change rate ROCOF greater than R1 exceeds the time threshold Δt 1s .

[0081] If Δt1>Δt 1s , then the island alarm is triggered and the corresponding protection action is performed, disconnecting the circuit breaker at the common coupling point or switching to the island mode. 1s Denoted as island criterion A; when Δt1≤Δt 1s , then return to step (1).

[0082] Step (4): When the SOC of the distributed energy storage battery is less than or equal to the set threshold, the calculated frequency change rate is compared with the set second threshold. If the frequency change rate reaches or exceeds the second threshold, the voltage phase shift detection is started. If the voltage phase shift angle is greater than the set second threshold value, it is determined that the distributed photovoltaic energy storage system is in an island state.

[0083] When the SOC value of the energy storage battery is low, the output power capacity of the energy storage system decreases, and the battery may not be able to maintain high-power output. The battery cannot effectively balance load changes in the low-SOC state, and the frequency fluctuations of the power grid will be more obvious. In this case, even a small load change will cause a large rate of frequency change. In the low-SOC state, reducing the ROCOF threshold can improve the detection sensitivity, quickly capture the rapid change of frequency, detect the islanding state earlier, and avoid system instability caused by excessive frequency change. In this embodiment, a lower second threshold R2 (R2 < R1) of the rate of frequency change is set to increase the sensitivity of islanding detection and quickly disconnect from the power grid to protect the energy storage battery from being over-discharged.

[0084] Although reducing the ROCOF threshold will improve the detection sensitivity, it may also increase the risk of false operation. To avoid this situation, voltage phase offset detection can be combined to provide double protection. In the low-SOC state of the energy storage battery, due to the decrease in the output power capacity of the energy storage system, load changes or other disturbances are more likely to cause voltage and frequency fluctuations. Therefore, the voltage phase offset of the energy storage system may be more obvious. This is because the battery cannot provide enough instantaneous power to balance load changes, resulting in a decrease in the stability of the power grid voltage and phase.

[0085] In this embodiment, when the distributed energy storage system has a low SOC value, the rate of frequency change is used as the starting condition for preliminary screening. When the rate of frequency change reaches or exceeds the preset threshold R2, voltage phase offset detection is started for a more refined islanding state analysis.

[0086] The specific process of voltage phase offset detection is as follows:

[0087] Judge whether the voltage phase offset angle Δθ is greater than the set voltage phase offset angle threshold value Δθ th , if Δθ > Δθ th , it is determined as the islanding state; in this embodiment, R2 < ROCOF < R1 and Δθ > Δθ th is recorded as the islanding criterion B. If Δθ ≤ Δθ th , return to step (1).

[0088] In this embodiment, the islanding detection method combining the rate of frequency change and voltage phase offset is applied to the distributed photovoltaic energy storage system, and reasonable coordination is carried out for the change of the energy storage SOC value; so that the islanding detection method can remain stable and efficient under different working conditions, can more comprehensively capture the power grid changes during islanding, improve the accuracy and sensitivity of islanding detection, and reduce the false alarm rate and missed alarm rate.

[0089] This embodiment takes the distribution network of the distributed photovoltaic and energy storage access system as the research object. Figure 2The figure shows the schematic diagram of the target system distribution network access of distributed photovoltaic storage. The DC power generated by photovoltaic power is converted into 400V AC through local inverter, and then connected to the low-voltage side of the photovoltaic boost transformer after being merged by the AC combiner box, and then boosted to 10 kV by the boost transformer, and then connected to the photovoltaic collector interval of the switch station. The energy storage unit is converted into AC through the energy control device PCS of the battery energy storage system, and then boosted by the matching boost transformer and connected to the energy storage incoming line interval of the switch station. After the photovoltaic energy storage is collected at the switch station, it is finally connected to the grid through a 10 kV line.

[0090] Before the island occurs, when the distributed photovoltaic energy storage system is connected to the grid normally, the inverter in the distributed photovoltaic system converts the DC power generated by the photovoltaic cells into AC power and sends it to the grid. The excess power may be transmitted to the grid. The distributed energy storage system transmits power to the grid or absorbs power from the grid to maintain the stability of the grid according to the battery SOC value and control strategy. Figure 3 shown.

[0091] After the system circuit breaker Sg is disconnected, the distributed photovoltaic energy storage belt and local load form an island, and power imbalance may occur between power generation and load in the system. If the total output power of photovoltaic power generation and energy storage equipment in the system is greater than the load demand, there will be excess power. If the system's load demand is greater than the total output power of photovoltaic power generation and energy storage equipment, there will be insufficient power. Unbalanced power can cause changes in system frequency and fluctuations in voltage. In island mode, the energy storage system needs to respond quickly to balance the power difference. The SOC of the energy storage system directly affects its regulation ability. When the SOC is low, the power output capacity of the energy storage system is weakened, and it is difficult to effectively compensate for the load demand, resulting in greater frequency and voltage fluctuations.

[0092] In PSCAD / EMTDC platform, Figure 4 The distributed photovoltaic energy storage system simulation model shown in the figure sets the simulation time to 0.7 seconds, and the distributed photovoltaic energy storage system changes from grid-connected to islanded state. After detecting any signal of islanding criterion A or B, the boundary switch at the PCC is disconnected. The grid-connected power factor of the DPSS is set to 1, and the rated capacity is set to 0.7MW.

[0093] The specific parameter settings are shown in Table 1. When the simulation time is 0.7s, the switch K is disconnected to form an island. The frequency change rate, voltage phase shift and waveform of the island detection signal at the time of island occurrence in three different scenarios are shown in Figure 1. Figure 5 , Figure 6 and Figure 7 As shown; wherein, if signal A or signal B is zero, it means that no island is detected, and as long as 1 appears, it means that an island is detected.

[0094] Table 1

[0095]

[0096] The following is the specific detection process of the composite island detection method combining the rate of change of frequency (ROCOF) and the voltage phase offset angle in a distributed photovoltaic energy storage system. Set the sampling frequency of the distributed photovoltaic energy storage system to 6.4KHz, the high threshold of the frequency change rate R1 = 1.2Hz / s, and the duration threshold greater than R1 is set to 0.2s. Set R2 = 0.5Hz / s, the voltage phase offset angle threshold is set to 15°, and the integration time period T = 0.1s. Collect the voltage and frequency data at the PCC, and calculate the frequency change rate and the voltage phase offset angle value. Select different island detection processes according to the current SOC value of the energy storage system.

[0097] Assume that the current SOC>30%, enter Figure 1 The left side detection process:

[0098] ① If the current frequency change rate calculated value ROCOF>1.2Hz / s, then proceed to the duration judgment greater than 1.2Hz / s;

[0099] (a) If the duration Δt1 of ROCOF>1.2 Hz / s is greater than 0.2 s, the distributed photovoltaic energy storage system is determined to be in an island state.

[0100] (b) If ROCOF < 1.2 Hz / s, return to frequency sampling and continue detection.

[0101] ②If ROCOF<R1, return to frequency sampling and continue detection.

[0102] At this time, there is scenario 1: set the distributed energy storage SOC to 80%, the local load active power 1.0MW, and the reactive power 0.3Mvar; combined Figure 5 When the island occurs, the distributed energy storage battery is fully charged and the inverter output power P DG With local load power P L When the unbalanced power is large, the instantaneous frequency changes significantly, the ROCOF value is large and lasts for a long time within a large value range, and the islanding criterion A sends an islanding signal and disconnects the switch at the PCC.

[0103] Assuming the current SOC ≤ 30%, enter Figure 1 Right side detection process:

[0104] ① If the current frequency change rate calculation value ROCOF>0.5Hz / s, the voltage phase deviation angle is determined.

[0105] (a) If Δθ>15°, the distributed photovoltaic energy storage system is judged to be in an island state.

[0106] (b) If Δθ<15°, return to frequency sampling and continue detection.

[0107] ②If ROCOF < 0.5Hz / s, return to frequency sampling and continue detection.

[0108] At this time, there is scenario 2: the distributed energy storage SOC value is set to 30%, the local load active power is 1.2MW, and the reactive power is 0.1Mvar. Figure 6 When the island occurs, the distributed energy storage battery has less power and the inverter output power P DG With local load power P L When the unbalanced power is small, the instantaneous frequency change value does not reach R1 but is between R1 and R2. At this time, the voltage phase offset formed by the frequency accumulation during the integration time period reaches the threshold value, and the islanding criterion B sends an islanding signal and disconnects the switch at PCC. Compared with the single frequency change rate detection method, this composite detection method can effectively cope with complex working conditions when the energy storage system SOC is low, improves the accuracy and sensitivity of islanding detection, and ensures the safe operation of the system.

[0109] Scenario 3: Set the distributed energy storage SOC to 80%. Considering the impact of disturbances during the normal grid-connected operation of the distributed photovoltaic energy storage system, 1.0MW of active load and 0.5Mvar of reactive load are put into operation in 0.3 seconds, and the load is removed in 0.4 seconds. Figure 7 ,The impact of larger load fluctuation on the frequency change rate does not reach the low threshold, the voltage phase shift detection will not be entered, and the island detection will not be misjudged.

[0110] Embodiment 2

[0111] In one or more embodiments, a composite island detection system considering the state of charge of energy storage is disclosed, comprising:

[0112] A data acquisition module is used to measure the grid frequency at the common coupling point of the grid and the distributed photovoltaic energy storage system in real time, and calculate the frequency change rate and voltage phase deviation angle respectively;

[0113] The island state judgment module is used to compare the calculated frequency change rate with the set first threshold when the SOC of the distributed energy storage battery is greater than the set threshold, record the duration of the frequency change rate exceeding the first threshold, and if the time exceeds the set first threshold value, determine that the distributed photovoltaic energy storage system is in an island state; when the SOC of the distributed energy storage battery is less than or equal to the set threshold, compare the calculated frequency change rate with the set second threshold, if the frequency change rate reaches or exceeds the second threshold, start voltage phase shift detection, and if the voltage phase shift angle is greater than the set second threshold value, determine that the distributed photovoltaic energy storage system is in an island state.

[0114] The specific implementation of the above modules is the same as that in Example 1 and will not be described in detail.

[0115] Embodiment 3

[0116] In one or more embodiments, a terminal device is disclosed, including a server, the server including a memory, a processor, and a computer program stored in the memory and executable on the processor, and the processor implements the composite island detection method considering the energy storage charge state in embodiment 1 when executing the program. For the sake of brevity, the specific process is not repeated.

[0117] It should be understood that in this embodiment, the processor may be a central processing unit CPU, and the processor may also be other general-purpose processors, digital signal processors DSP, application-specific integrated circuits ASIC, off-the-shelf programmable gate arrays FPGA or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor may be a microprocessor or the processor may also be any conventional processor, etc.

[0118] The memory may include a read-only memory and a random access memory, and provide instructions and data to the processor. A portion of the memory may also include a non-volatile random access memory. For example, the memory may also store information about the device type.

[0119] In the implementation process, each step of the above method can be completed by an integrated logic circuit of hardware in a processor or an instruction in the form of software.

[0120] Embodiment 4

[0121] In one or more embodiments, a computer-readable storage medium is disclosed, on which a computer program / instruction is stored. When the computer program / instruction is executed by a processor, the steps of the method described in the first embodiment are implemented.

[0122] Embodiment 5

[0123] In one or more embodiments, a computer program product is disclosed, including a computer program / instruction, which implements the steps of the method described in Example 1 when executed by a processor.

[0124] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. A composite island detection method considering the energy storage charge state, characterized in that: include: Real-time measurement of the grid frequency at the common coupling point between the grid and the distributed photovoltaic energy storage system, and calculation of the frequency change rate and voltage phase deviation angle respectively; When the SOC of the distributed energy storage battery is greater than the set threshold, the calculated frequency change rate is compared with the set first threshold, and the duration of the frequency change rate exceeding the first threshold is recorded. If the time exceeds the set first threshold value, it is determined that the distributed photovoltaic energy storage system is in an island state; When the SOC of the distributed energy storage battery is less than or equal to the set threshold, the calculated frequency change rate is compared with the set second threshold. If the frequency change rate reaches or exceeds the second threshold, the voltage phase shift detection is started. If the voltage phase shift angle is greater than the set second threshold value, it is determined that the distributed photovoltaic energy storage system is in an island state.

2. A composite island detection method considering the energy storage charge state as claimed in claim 1, characterized in that: The first threshold value is greater than the second threshold value.

3. A composite island detection method considering the energy storage charge state as claimed in claim 1, characterized in that: The average value of the frequency change rate of each data point in the current measurement time window relative to the corresponding data point in the previous measurement time window is calculated to obtain the frequency change rate of the current measurement time window.

4. A composite island detection method considering the energy storage charge state as claimed in claim 1, characterized in that: The calculation of the voltage phase shift angle is specifically as follows: Set a fixed integration time period T and initialize the voltage phase deviation angle Δθ int is zero; In each sampling period Δt, the frequency signal f at the common coupling point is obtained and the frequency variation Δf in Δt is calculated; the frequency variation Δf is accumulated and calculated to obtain the phase shift angle Δθ; After each sampling period, the initial phase offset angle of the current sampling period is added to Δθ to obtain the current phase offset angle, which is used as the initial phase offset angle of the next sampling period. When the integration time period T is reached, the voltage phase offset angle is calculated based on the current phase offset angle and the integration time period T; and the current phase offset angle is cleared at the same time; The above process is repeated to calculate the voltage phase deviation angle for the next integration time period T.

5. A composite island detection method considering the energy storage charge state as claimed in claim 1, characterized in that: After measuring the grid frequency and voltage signals at the common coupling point between the grid and the distributed photovoltaic energy storage system, the signals are filtered.

6. A composite island detection method considering the energy storage charge state as claimed in claim 1, characterized in that: After determining that the distributed photovoltaic energy storage system is in an island state, an island alarm is triggered and corresponding protection actions are taken to disconnect the circuit breaker at the common coupling point or switch to island mode operation.

7. A composite island detection system considering the energy storage charge state, characterized in that: include: A data acquisition module is used to measure the grid frequency at the common coupling point of the grid and the distributed photovoltaic energy storage system in real time, and calculate the frequency change rate and voltage phase deviation angle respectively; The island state judgment module is used to compare the calculated frequency change rate with the set first threshold when the SOC of the distributed energy storage battery is greater than the set threshold, record the duration of the frequency change rate exceeding the first threshold, and if the time exceeds the set first threshold value, determine that the distributed photovoltaic energy storage system is in an island state; when the SOC of the distributed energy storage battery is less than or equal to the set threshold, compare the calculated frequency change rate with the set second threshold, if the frequency change rate reaches or exceeds the second threshold, start voltage phase shift detection, and if the voltage phase shift angle is greater than the set second threshold value, determine that the distributed photovoltaic energy storage system is in an island state.

8. A terminal device, comprising a memory, a processor and a computer program stored in the memory, characterized in that: The processor executes the computer program to implement the steps of the method according to any one of claims 1 to 6.

9. A computer-readable storage medium having a computer program / instruction stored thereon, characterized in that: When the computer program / instructions are executed by a processor, the steps of the method according to any one of claims 1 to 6 are implemented.

10. A computer program product comprising a computer program / instructions, characterized in that When the computer program / instructions are executed by a processor, the steps of the method according to any one of claims 1 to 6 are implemented.

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