A rapid detection method for isolated islands in energy storage systems

Abstract

The present invention discloses a method for rapid detection of isolated islands in an energy storage system, and the steps are as follows: S1: In a frequency calculation detection unit, a PCS collects the grid voltage at a PCC, and after filtering by an all-pass filter, extracts the positive sequence component and the negative sequence component of the grid voltage according to a formula; S2: Perform frequency-locked loop processing on the separated positive and negative sequence components of the grid, respectively, and extracts the positive sequence voltage component f p , the positive sequence component f p Sent to the disturbance control unit; S3: In the active or reactive current calculation unit, the calculated active current i′ d and reactive current i′ q Sent to the disturbance control unit; S4: the disturbance control unit, the main software completes the complete judgment of island detection; S5: in the logic judgment unit, it mainly completes the injection of reactive disturbance current and the accurate judgment of fault occurrence. The present invention can solve the problem of time lag in hardware circuit detection of grid frequency, the problem of PCS misjudgment of island detection in island state and the problem of long detection time in island state.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and in particular to a method for rapid detection of isolated islands in an energy storage system. Background Art

[0002] There are two methods for island detection: active and passive. The passive method determines whether an island has occurred by detecting changes in the PCC power. The basic principle of the active detection method is to detect the occurrence of an island by adding very small disturbance signals such as the frequency and phase of the voltage or current to the control signal of the inverter and detecting the system's response to the disturbance. When the PCS is connected to the grid, it is clamped by the grid and the effect of the disturbance signal is very small; when an island is formed, the system loses the clamping of the grid, so the effect of the disturbance signal is more obvious. At this time, by detecting changes in the corresponding grid parameters at the PCC (Public coupling point), it can be determined whether an island has occurred.

[0003] The existing technology has the following deficiencies:

[0004] 1. The hardware circuit detects the grid frequency based on zero-crossing comparison, which increases the hardware cost;

[0005] 2. Inaccurate island detection causes PCS to misjudge during operation, resulting in equipment downtime;

[0006] 3. The detection time is long, and it is not possible to detect whether the PCS is in an island state within the specified time (2S), which poses a safety hazard;

[0007] 4. When the PCS is operating in an island state, when the power grid is restored, there will be a huge instantaneous surge current, causing damage to the PCS Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide a method for rapid detection of islanding in an energy storage system, which can solve the problem of time lag in hardware circuit detection of grid frequency, the problem of misjudgment of PCS detection of islanding in an islanding state, and the problem of long detection time in an islanding state.

[0009] In order to solve the above technical problems, the present invention provides a method for rapid detection of isolated islands in an energy storage system, comprising a frequency calculation detection unit, an active or reactive current calculation unit, a disturbance control unit, and a logic judgment unit, and the use steps are as follows:

[0010] S1: In the frequency calculation and detection unit, the PCS collects the grid voltage at the PCC, and after filtering it through an all-pass filter, extracts the positive sequence component and negative sequence component of the grid voltage according to the formula;

[0011] S2: Perform frequency-locked loop processing on the separated positive and negative sequence components of the power grid respectively to extract the positive sequence voltage component f p , the positive sequence component fp sent to the disturbance control unit;

[0012] S3: In the active or reactive current calculation unit, the calculated active current i′ d and reactive current i′ q sent to the disturbance control unit;

[0013] S4: Disturbance control unit, the main software completes the complete judgment of island detection;

[0014] S 5: In the logic judgment unit, the injection of reactive disturbance current and accurate judgment of fault occurrence are mainly completed. When a fault is detected, the PCS pulse is immediately blocked and the corresponding relay action is triggered to ensure that the PCS is disconnected from the power grid and the RLC load.

[0015] Furthermore, in the frequency calculation and detection unit, the positive sequence component f p Perform a difference operation with the rated frequency of the grid voltage to obtain the error frequency Calc_ferr, and take the absolute value to obtain |Calc_ferr|;

[0016] In the disturbance control unit, whether an island occurs is determined based on the absolute value of the error frequency |Calc_ferr|. The determination steps are as follows:

[0017] A1: First, determine whether the absolute value of the error frequency |Calc_ferr| is greater than the preset error frequency threshold 1;

[0018] A2: If |Calc_ferr| is greater than the error frequency threshold 1, then determine whether Calc_ferr is greater than 0. If Calc_ferr is greater than 0, the reactive disturbance injected is Calc_iq1 = Kp1*i′ d If Calc_ferr is less than 0, the reactive disturbance injected is Calc_iq2 = -Kp1*i′ d , where Kp1 is the disturbance proportional coefficient 1;

[0019] A3: If |Calc_ferr| is greater than the error frequency threshold 1, then determine whether |Calc_ferr| is greater than the error frequency threshold 2. If so, it is determined that an island has occurred. If not, execute step A1 again to proceed to the next cycle.

[0020] A4: If it is less than the error frequency threshold 1, then determine whether Calc_ferr is greater than 0. If Calc_ferr is greater than 0, the reactive disturbance injected is Calc_iq3 = Kp2*i′ d If Calc_ferr is less than 0, the reactive disturbance injected is Calc_iq4 = -Kp2*i′ d, where Kp2 is the disturbance proportional coefficient 2;

[0021] A5: Superimpose the injected reactive disturbance on the PCS reactive setting to perform reactive closed-loop control;

[0022] In order to reduce the impact of injected reactive power on control, the disturbance proportional coefficient 1 should be much larger than the disturbance proportional coefficient 2; in order to prevent misjudgment of islands, the preset error frequency threshold 2 should be larger than the preset error frequency threshold 1.

[0023] Furthermore, in the active or reactive current calculation unit, the active current i′ d and reactive current i′ q The sources are as follows:

[0024] The PCS performs clark and park transformations on the collected current to obtain the active current i d and reactive current i q , according to the needs of the control system, d and i q Perform a second-order low-pass filter to obtain the current i′ d and i′ q .

[0025] Furthermore, in the logic judgment unit, the logic control step refers to the judgment step in the disturbance control unit.

[0026] The beneficial effects of the present invention are as follows: 1. No hardware frequency detection circuit is required, and the grid frequency can be quickly tracked within ms level through the software frequency calculation unit;

[0027] 2. During normal operation, the injected reactive current is linearly related to the load. When the load is small, the injected current is small, and when the load is large, the injected current is large. When the load is full, the injected current is maintained within a very small range, so the entire control will not be affected.

[0028] 3. In the case of an island, once a system abnormality is detected, the disturbance will be aggravated until the system is quickly determined to be in an island state. The time from the occurrence of the island to the completion of the island judgment is about 200ms, which is much lower than the standard requirement. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a schematic diagram of the island detection connection of the present invention.

[0030] Figure 2 It is a structural schematic diagram of an island detection control unit of the present invention.

[0031] Figure 3 It is the logic control block diagram of the island software of the present invention.

[0032] Figure 4This is the calculation formula for separating the positive and negative sequence of the power grid of the present invention.

[0033] Figure 5 It is the reactive interference current injected when the PCS of the present invention is fully loaded.

[0034] Figure 6 This is the island detection waveform of the 100Kw PCS of the present invention when it is fully loaded. DETAILED DESCRIPTION

[0035] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

[0036] Reference Figures 1 to 5 As shown, an embodiment of a method for rapid detection of isolated islands in an energy storage system of the present invention includes a frequency calculation detection unit, an active or reactive current calculation unit, a disturbance control unit, and a logic judgment unit, and the use steps are as follows:

[0037] In the frequency calculation and detection unit, the PCS collects the grid voltage at the PCC, and after filtering it through an all-pass filter (APF), according to the following Figure 4 The formula extracts the positive sequence component and negative sequence component of the grid voltage;

[0038] 2. Perform frequency-locked loop processing on the separated positive and negative sequence components of the power grid respectively to extract the positive sequence voltage component f p , the positive sequence component f p sent to the disturbance control unit;

[0039] 3. In the active or reactive current calculation unit, the calculated active current i′ d and reactive current i′ q sent to the disturbance control unit;

[0040] 4. Disturbance control unit, the main software completes the complete judgment of island detection, please refer to Figure 3 Flowchart of

[0041] 5. In the logic judgment unit, the logic control steps refer to the execution steps in the disturbance control unit, mainly completing the injection of reactive disturbance current and accurate judgment of fault occurrence. When a fault is detected, the PCS pulse is immediately blocked and the corresponding relay action is triggered to ensure that the PCS is disconnected from the power grid and the RLC load.

[0042] Figure 1 This is a schematic diagram of the PCS island test connection built for the experiment. The main control units of this system are as follows: Figure 2As shown, the frequency calculation detection unit, active or reactive current calculation unit, disturbance control unit and logic judgment unit. The frequency calculation detection unit completes the accurate calculation of the grid voltage frequency; the active or reactive unit obtains the active or reactive current by performing relevant vector coordinate transformation on the current sampled by the system, and at the same time, in order to eliminate the influence of interference, a second-order low-pass filter is performed before sending it to the disturbance control unit; the disturbance control unit mainly injects reactive current and superimposes it into the control loop; the logic judgment unit mainly completes the logic control of the injected reactive current and fault judgment, which is specifically performed in the following steps:

[0043] 1. In the frequency calculation and detection unit, the PCS collects the grid voltage at the PCC, and after filtering it through an all-pass filter (APF), the voltage is calculated according to the following formula: Figure 4 The formula extracts the positive and negative sequence components of the grid voltage;

[0044] 2. Perform frequency-locked loop processing on the separated positive and negative sequence components of the power grid respectively to extract the positive sequence voltage component f p , the positive sequence component f p Perform a difference operation with the rated frequency of the grid voltage to obtain the error frequency Calc_ferr, and take the absolute value to obtain |Calc_ferr|;

[0045] 3. In the active / reactive current calculation unit, PCS performs clark and park transformations based on the collected current to obtain the active current i d and reactive current i q ;

[0046] 4. According to the needs of the control system, d and i q Perform a second-order low-pass filter to obtain the current i′ d and i′ q , change i′ d sent to the disturbance control unit;

[0047] 5. The disturbance control unit determines whether an island occurs based on the absolute value of the error frequency |Calc_ferr|. The software flow chart is as follows: Figure 3 As shown, the specific steps are as follows:

[0048] A1: First, determine whether the absolute value of the error frequency |Calc_ferr| is greater than the preset error frequency threshold 1;

[0049] A2: If |Calc_ferr| is greater than the error frequency threshold 1, then determine whether Calc_ferr is greater than 0. If Calc_ferr is greater than 0, the reactive disturbance injected is Calc_iq1 = Kp1*i′ dIf Calc_ferr is less than 0, the reactive disturbance injected is Calc_iq2 = -Kp1*i′ d , where Kp1 is the disturbance proportional coefficient 1;

[0050] A3: If |Calc_ferr| is greater than the error frequency threshold 1, then determine whether |Calc_ferr| is greater than the error frequency threshold 2. If so, it is determined that an island has occurred. If not, execute step A1 again to proceed to the next cycle.

[0051] A4: If it is less than the error frequency threshold 1, then determine whether Calc_ferr is greater than 0. If Calc_ferr is greater than 0, the reactive disturbance injected is Calc_iq3 = Kp2*i′ d If Calc_ferr is less than 0, the reactive disturbance injected is Calc_iq4 = -Kp2*i′ d , where Kp2 is the disturbance proportional coefficient 2;

[0052] A5: Superimpose the injected reactive disturbance on the PCS reactive setting to perform reactive closed-loop control;

[0053] In order to reduce the impact of injected reactive power on control, the disturbance proportional coefficient 1 should be much larger than the disturbance proportional coefficient 2; in order to prevent misjudgment of islands, the preset error frequency threshold 2 should be larger than the preset error frequency threshold 1;

[0054] 6. In the logic judgment unit, the logic control steps refer to the execution steps in the disturbance control unit, mainly completing the injection of reactive disturbance current and accurate judgment of fault occurrence. When a fault is detected, the PCS pulse is immediately blocked and the corresponding relay action is triggered to ensure that the PCS is disconnected from the power grid and the RLC load;

[0055] 7. Figure 5 It is the A-phase current and reactive disturbance current of the inverter side when the 100Kw PCS is working normally at full load. Figure 6 The output current and disturbance current of a 100Kw PCS under full-load islanding state. It can be seen from the figure that it takes less than 200ms from the occurrence of islanding to the detection of islanding state, which verifies the feasibility of the control method.

[0056] The above-described embodiments are only preferred embodiments for fully illustrating the present invention, and the protection scope of the present invention is not limited thereto. Equivalent substitutions or changes made by those skilled in the art based on the present invention are within the protection scope of the present invention. The protection scope of the present invention shall be subject to the claims.

Claims

1. A method for rapid detection of isolated islands in an energy storage system, characterized in that: It includes a frequency calculation detection unit, an active or reactive current calculation unit, a disturbance control unit, and a logic judgment unit, and the use steps are as follows: S1: In the frequency calculation and detection unit, the PCS collects the grid voltage at the PCC, and after filtering it through an all-pass filter, extracts the positive sequence component and negative sequence component of the grid voltage according to the formula; S2: Perform frequency-locked loop processing on the separated positive and negative sequence components of the power grid respectively to extract the positive sequence voltage component f p , the positive sequence component f p sent to the disturbance control unit; S3: In the active or reactive current calculation unit, the calculated active current i′ d and reactive current i′ q sent to the disturbance control unit; S4: Disturbance control unit, the main software completes the complete judgment of island detection; S 5: In the logic judgment unit, the injection of reactive disturbance current and accurate judgment of fault occurrence are mainly completed. When a fault is detected, the PCS pulse is immediately blocked and the corresponding relay action is triggered to ensure that the PCS is disconnected from the power grid and the RLC load.

2. The energy storage system island rapid detection method according to claim 1, characterized in that: In the frequency calculation and detection unit, the positive sequence component f p Perform a difference operation with the rated frequency of the grid voltage to obtain the error frequency Calc_ferr, and take the absolute value to obtain |Calc_ferr|; In the disturbance control unit, whether an island occurs is determined based on the absolute value of the error frequency |Calc_ferr|. The determination steps are as follows: A1: First, determine whether the absolute value of the error frequency |Calc_ferr| is greater than the preset error frequency threshold 1; A2: If |Calc_ferr| is greater than the error frequency threshold 1, then determine whether Calc_ferr is greater than 0. If Calc_ferr is greater than 0, the reactive disturbance injected is Calc_iq1 = Kp1*i′ d If Calc_ferr is less than 0, the reactive disturbance injected is Calc_iq2 = -Kp1*i′ d , where Kp1 is the disturbance proportional coefficient 1; A3: If |Calc_ferr| is greater than the error frequency threshold 1, then determine whether |Calc_ferr| is greater than the error frequency threshold 2. If so, it is determined that an island has occurred. If not, execute step A1 again to proceed to the next cycle. A4: If it is less than the error frequency threshold 1, then determine whether Calc_ferr is greater than 0. If Calc_ferr is greater than 0, the reactive disturbance injected is Calc_iq3 = Kp2*i′ d If Calc_ferr is less than 0, the reactive disturbance injected is Calc_iq4 = -Kp2*i′ d , where Kp2 is the disturbance proportional coefficient 2; A5: Superimpose the injected reactive disturbance on the PCS reactive setting to perform reactive closed-loop control; In order to reduce the impact of injected reactive power on control, the disturbance proportional coefficient 1 should be much larger than the disturbance proportional coefficient 2; in order to prevent misjudgment of islands, the preset error frequency threshold 2 should be larger than the preset error frequency threshold 1.

3. The energy storage system island rapid detection method according to claim 1, characterized in that: In the active or reactive current calculation unit, the active current i′ d and reactive current i′ q The sources are as follows: The PCS performs clark and park transformations on the collected current to obtain the active current i d and reactive current i q , according to the needs of the control system, i d and i q Perform a second-order low-pass filter to obtain the current i′ d and i′ q .

4. The energy storage system island rapid detection method according to claim 2, characterized in that: In the logic judgment unit, the logic control steps refer to the judgment steps in the disturbance control unit.

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