Source-grid-load-storage integrated microgrid system and control method thereof for incremental load

Abstract

The application relates to the technical field of power system operation and control, and particularly discloses a source-network-load-storage integrated micro-grid system for incremental load and a control method thereof, which comprises the following steps: in response to a grid-connected point voltage drop, energy storage current, virtual power angle, wind power phase-locked loop phase difference and bus voltage sag depth are collected; a transient stability boundary constraint containing phase synchronization margin and current peak value is constructed and a fault is located; an output voltage phase is discretely pre-adjusted to select an optimal adjustment phase; an energy storage converter is driven and a cooperative reactive power instruction is sent to a wind power converter; a switching time is determined according to the phase synchronization margin and a voltage recovery slope, and an off-grid switching is executed based on the optimal adjustment phase; and the application maintains energy storage network construction capacity and meets low-voltage ride-through grid connection requirements.

Description

Technical Field

[0001] This invention relates to the field of power system operation and control technology, specifically to an integrated microgrid system for incremental loads and its control method. Background Technology

[0002] With the large-scale integration of distributed power sources such as wind power into the distribution network, integrated microgrids combining generation, grid, load, and storage have become an important way to absorb new energy sources and meet the electricity demand of incremental loads such as industrial parks and data centers. These microgrids are typically connected to the main grid via a single circuit and are equipped with static switching switches at the point of common coupling to enable rapid off-grid switching in the event of grid-side faults, ensuring the continuity of power supply to incremental loads.

[0003] The existing technology has the following shortcomings: the overcurrent problem caused by the increase of excitation current to maintain the voltage reference during grid faults in existing grid-connected energy storage converters, and the voltage reference loss and wind power phase-locked loop out-of-synchronization and grid disconnection problems caused by current limiting measures to meet low voltage ride-through compliance. Summary of the Invention

[0004] The purpose of this invention is to provide an integrated microgrid system for source, grid, load and storage oriented to incremental loads and its control method, so as to solve the problems mentioned above.

[0005] The objective of this invention can be achieved through the following technical solutions: Integrated microgrid systems for source-grid-load-storage to meet incremental loads include: The data acquisition module, in response to the grid connection point voltage dropping below the threshold, synchronously acquires the output current of the energy storage converter, the virtual power angle, the phase difference of the wind power converter phase-locked loop, and the voltage sag depth of the load-side bus. The boundary construction module establishes transient stability boundary constraints, including phase synchronization margin and current peak value, based on the virtual power angle and the phase difference of the phase-locked loop, and locates the fault status of the grid connection point based on the output current and the bus voltage sag depth. The phase optimization module, based on fault state and transient stability boundary constraints, performs a finite number of discrete phase angle pre-adjustments on the output voltage phase of the next control cycle, and selects the optimal adjustment phase that satisfies the phase synchronization margin and does not exceed the current peak limit. The collaborative execution module drives the energy storage converter output with the voltage vector under the optimal adjustment phase, and at the same time sends a collaborative reactive power command based on the voltage phase to the wind power converter to maintain the continuous locking of the wind power phase-locked loop. The switching decision module continuously monitors the phase synchronization margin during the low voltage ride-through window and determines the switching timing based on the voltage recovery slope at the grid connection point. When the voltage at the grid connection point remains below the threshold, off-grid switching is performed based on the optimal phase adjustment.

[0006] As a further aspect of the present invention: the process for establishing the transient stability boundary constraints is as follows: The virtual power angle is incremented step by step, and the difference between the incremented virtual power angle and the phase difference of the phase-locked loop is iterated at each increment step. The maximum difference in the difference sequence that satisfies the continuous locking condition of the phase-locked loop is obtained as the phase synchronization margin. The output current is recursively compared with the rated current at each sampling time, and the maximum ratio in the ratio sequence is taken as the current peak value. The phase synchronization margin and peak current are correlated and mapped in the same time dimension to form a two-dimensional transient stability boundary constraint.

[0007] As a further aspect of the present invention: the fault status of the location and grid connection point specifically includes: Under the two-dimensional transient stability boundary constraint, the bus voltage sag depth is decomposed into multiple continuous segments according to the phase period. The phase angle offset of the output current is accumulated and summed in each segment to obtain the cumulative offset value of each segment. The cumulative offset value of each segment is sequentially compared with the preset fault type threshold. When the cumulative offset value of any segment exceeds the corresponding threshold, the phase interval corresponding to the segment is determined as a fault phase, and the fault status including the fault phase and the duration of the temporary drop is output.

[0008] As a further aspect of the present invention: the process for selecting the optimal adjustment phase is as follows: Using the output voltage phase of the current control cycle as the reference phase, the reference phase is shifted step by step in the leading and lagging directions respectively, generating a discrete phase set composed of each shifted phase. Each level of offset phase in the discrete phase set is substituted into the two-dimensional transient stability boundary constraints for verification, and offset phases that do not meet the phase synchronization margin or current peak are eliminated. Among the remaining offset phases that have been verified and retained, the offset phase that is consistent with the voltage phase compensation direction corresponding to the fault phase of the fault state is selected, and the one with the smallest offset step is taken as the optimal adjustment phase.

[0009] As a further aspect of the present invention: the removal of offset phases that do not meet the phase synchronization margin or current peak value specifically includes: Based on the fault phase in the fault state, the offset phases at each level in the discrete phase set are grouped according to their respective phase intervals to obtain multiple phase groups; Within each phase group, each offset phase is checked to see if it meets the phase synchronization margin with the phase-locked loop phase difference. After removing the offset phases that do not meet the margin, a retained phase is obtained. The predicted current under each retained phase is compared with the current peak value one by one. After removing the offset phases where the predicted current exceeds the current peak value, the second retained phase is used as the valid offset phase that passes the verification.

[0010] As a further aspect of the present invention: the process of sending the coordinated reactive power command is as follows: The optimal adjustment phase is used as the output voltage phase reference of the energy storage converter. The pulse width duty cycle of each phase is allocated according to the output voltage phase reference to drive the output of the energy storage converter. The optimal adjustment phase is compared with the real-time phase of the phase-locked loop of the wind power converter to obtain the phase lag value of the real-time phase of the phase-locked loop relative to the optimal adjustment phase. The reactive current compensation amount is determined based on the phase lag value, and the reactive current compensation amount is sent to the wind power converter as a coordinated reactive power command so that the output current phase of the wind power converter follows the optimal adjustment phase.

[0011] As a further aspect of the present invention: the process of obtaining the phase lag value is as follows: Using the optimal adjustment phase as the reference phase benchmark, the real-time phase of the phase-locked loop is continuously sampled according to the control cycle to obtain the real-time phase value at each sampling time. The real-time phase value at each sampling time is compared with the reference phase benchmark one by one to obtain the initial difference value at each sampling time. The initial differences obtained in multiple consecutive control cycles are arranged in chronological order, and multiple continuous subsequences are extracted from the arranged sequence by a sliding window. The initial differences in each subsequence are accumulated and averaged to obtain the lag cumulative value corresponding to each subsequence. The largest value among all accumulated hysteresis values ​​is selected as the phase hysteresis value of the real-time phase of the phase-locked loop relative to the optimal adjusted phase.

[0012] As a further aspect of the present invention: the determination of the switching timing specifically includes: During the low voltage ride-through window, the phase synchronization margin is sampled at fixed time intervals and the margin value at each sampling time is recorded. At the same time, the amplitude of the grid connection point voltage at each sampling time is sampled and the amplitude difference between adjacent sampling times is divided by the sampling interval to obtain the voltage recovery slope. The margin value at each sampling time is compared with the preset minimum locking margin, and the voltage recovery slope at each sampling time is compared with the preset lower limit of the recovery slope. When the margin value is lower than the minimum locking margin and the voltage recovery slope is lower than the lower limit of the recovery slope for two consecutive sampling times, it is determined that it is time to perform off-grid handover. Using the optimal adjustment phase as the off-grid phase reference, an off-grid command is sent to the static switching switch so that the load-side bus voltage phase follows the off-grid phase reference to complete the off-grid switching.

[0013] A control method for integrated source-grid-load-storage microgrids oriented towards incremental loads includes the following steps: S1, in response to the grid connection point voltage dropping below the threshold, synchronously collects the energy storage converter output current, virtual power angle, wind power converter phase-locked loop phase difference and load-side bus voltage sag depth. S2, based on the virtual power angle and the phase difference of the phase-locked loop, establish transient stability boundary constraints including phase synchronization margin and current peak value, and locate the fault status of the grid connection point based on the output current and bus voltage sag depth. S3, based on the fault state and transient stability boundary constraints, performs a finite number of discrete phase angle pre-adjustments on the output voltage phase of the next control cycle, and selects the optimal adjustment phase that satisfies the phase synchronization margin and does not exceed the current peak limit; S4 drives the energy storage converter output with the voltage vector under the optimal adjustment phase, and sends a coordinated reactive power command based on the voltage phase to the wind power converter to maintain continuous locking of the wind power phase-locked loop. S5 continuously monitors the phase synchronization margin during the low voltage ride-through window and determines the switching timing based on the voltage recovery slope at the grid connection point. When the voltage at the grid connection point remains below the threshold, off-grid switching is performed based on the optimal phase adjustment.

[0014] The beneficial effects of this invention are: (1) By establishing a two-dimensional transient stability boundary constraint that includes phase synchronization margin and current peak value, and by performing discretization pre-adjustment and step-by-step verification of the output voltage phase of the next control cycle based on the constraint, the energy storage converter can maintain the output characteristics of the grid-type voltage source without the need for current limiting measures during low voltage ride-through. At the same time, it ensures that the phase difference of the phase-locked loop of the wind power converter is always within the range that can be continuously locked, avoiding the problem of wind power disconnection caused by phase loss, and reducing the risk that the static switching switch cannot switch quickly due to the large phase difference between the voltages at both ends.

[0015] (2) When determining the timing of off-grid switching, phase synchronization margin and grid connection point voltage recovery slope are introduced as dual judgment conditions. The switching is only performed when the conditions are met at two consecutive sampling times. This reduces the probability of false switching due to single sampling noise or instantaneous disturbance, improves the reliability of off-grid switching decision, and sends off-grid command to static switching switch with the optimal adjustment phase as the off-grid phase reference. This keeps the load side bus voltage phase continuous before and after switching, reducing the disturbance to incremental load power supply. Attached Figure Description

[0016] The invention will now be further described with reference to the accompanying drawings.

[0017] Figure 1 This is a system block diagram of the present invention; Figure 2 This is a flowchart of the process for establishing transient stable boundary constraints in this invention; Figure 3 This is a flowchart of the optimal phase adjustment screening process in this invention; Figure 4 This is a flowchart of the method of the present invention. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Please see Figure 1 As shown, this invention is an integrated microgrid system for incremental loads, comprising: The data acquisition module, in response to the grid connection point voltage dropping below the threshold, synchronously acquires the output current of the energy storage converter, the virtual power angle, the phase difference of the wind power converter phase-locked loop, and the voltage sag depth of the load-side bus. The boundary construction module establishes transient stability boundary constraints, including phase synchronization margin and current peak value, based on the virtual power angle and the phase difference of the phase-locked loop, and locates the fault status of the grid connection point based on the output current and the bus voltage sag depth. The phase optimization module, based on fault state and transient stability boundary constraints, performs a finite number of discrete phase angle pre-adjustments on the output voltage phase of the next control cycle, and selects the optimal adjustment phase that satisfies the phase synchronization margin and does not exceed the current peak limit. The collaborative execution module drives the energy storage converter output with the voltage vector under the optimal adjustment phase, and at the same time sends a collaborative reactive power command based on the voltage phase to the wind power converter to maintain the continuous locking of the wind power phase-locked loop. The switching decision module continuously monitors the phase synchronization margin during the low voltage ride-through window and determines the switching timing based on the voltage recovery slope at the grid connection point. When the voltage at the grid connection point remains below the threshold, off-grid switching is performed based on the optimal phase adjustment.

[0020] In the data acquisition module, in response to the grid connection point voltage dropping below a threshold, the output current of the energy storage converter, the virtual power angle, the phase difference of the wind power converter's phase-locked loop, and the voltage sag depth of the load-side bus are synchronously acquired, specifically including: The three-phase voltage signal is obtained by the secondary side of the voltage transformer installed at the grid connection point. After filtering and amplification, the signal is sent to the analog-to-digital converter for continuous sampling at a sampling rate of 128 points per cycle. The effective value of the three-phase voltage is then calculated by the digital signal processor.

[0021] The voltage drop threshold is preset to 0.2 times the rated voltage. When any phase of the calculated effective value of the three-phase voltage is lower than this threshold, it is determined that a voltage drop event has occurred, triggering subsequent data acquisition actions.

[0022] For the acquisition of the output current of the energy storage converter, a Hall effect current sensor is installed on the AC output side of the energy storage converter. The sensor converts the primary current into a voltage signal of 0 to 5 volts proportionally. This signal is then connected to another channel of the analog-to-digital converter after being filtered by anti-aliasing.

[0023] The three-phase current is sampled synchronously with the same sampling clock as the voltage sampling to obtain the instantaneous digital quantity, which is then converted into the output current value in amperes by the digital signal processor.

[0024] To obtain the virtual power angle, the phase angle of the virtual internal potential is calculated in real time in the control program of the energy storage converter. This phase angle is obtained by the virtual synchronous machine control algorithm based on the active power setpoint and the grid frequency deviation.

[0025] The phase angle of the grid-connected voltage is obtained by passing the secondary side signal of the voltage transformer at the grid connection point through a phase-locked loop. The virtual internal potential phase angle is then subtracted from the grid-connected voltage phase angle to obtain the value of the virtual power angle.

[0026] To obtain the phase difference of the phase-locked loop of the wind power converter, the real-time phase angle output by the phase-locked loop inside the wind power converter is read through the communication bus. The phase-locked loop uses the grid connection point voltage as the reference input.

[0027] The phase angle of the wind power converter phase-locked loop is read and compared with the phase angle of the grid connection point voltage at the same time. The difference between the two is calculated as the phase difference of the wind power converter phase-locked loop.

[0028] For the acquisition of the voltage sag depth of the load-side bus, a voltage transformer is installed at the load-side bus. Its secondary side signal is processed by the same filtering, amplification and analog-to-digital conversion to calculate the effective value of the three-phase voltage.

[0029] Based on the rated voltage of the load-side bus, the collected real-time effective value is converted into a per-unit value, and the sag depth is obtained by subtracting the per-unit value from 1, expressed as a percentage.

[0030] Please see Figure 2 As shown, in the boundary construction module, transient stability boundary constraints including phase synchronization margin and current peak are established based on the virtual power angle and the phase-locked loop phase difference. The fault state of the grid connection point is located based on the output current and bus voltage sag depth, specifically including: To establish transient stability boundary constraints, the virtual power angle value obtained at the current moment is first used as the initial value, and then the value is increased sequentially in increments of 0.5 degrees of electrical angle, with an upper limit of 90 degrees of electrical angle.

[0031] At each increment step, the incremented virtual power angle value corresponding to that step step is subtracted from the phase difference value of the wind power converter phase-locked loop collected at the same time to obtain the difference value at that step step.

[0032] The difference is compared with a pre-stored phase-locked loop (PLL) continuous locking condition, which states that the rate of change of the PLL phase difference over three consecutive control cycles does not exceed 0.1 degrees per millisecond.

[0033] The differences obtained at each incrementing step size are compared in sequence, and the largest difference that satisfies the locking condition is selected as the phase synchronization margin, which is expressed in degrees of electricity.

[0034] The output current values ​​at each sampling time are extracted point by point in chronological order. The output current value at each sampling time is divided by the rated current value of the energy storage converter to obtain the current ratio at that time.

[0035] The current ratios at each sampling time are compared one by one, and the ratio with the largest value is selected as the current peak value, which is expressed as a per-unit value.

[0036] The phase synchronization margin and current peak value are paired on the same time axis according to their respective time labels to form a two-dimensional numerical pair consisting of the phase synchronization margin value and the current peak value. This two-dimensional numerical pair is the transient stability boundary constraint.

[0037] For locating the fault status at the grid connection point, based on the two-dimensional numerical pair determined by the transient stability boundary constraints mentioned above, the bus voltage sag depth value is expressed as a percentage. This value is divided by 0.1 electrical degrees to obtain a decomposed quantity value, which is then rounded to obtain the total number of segments. The sum of these segments covers a complete voltage phase cycle, i.e., 360 electrical degrees.

[0038] Within each segment, the phase angle of the output current is subtracted from the phase angle of the grid connection point voltage to obtain the phase angle offset at that sampling moment. The phase angle offsets at all sampling moments within the segment are summed to obtain the cumulative offset value of the segment.

[0039] Three sets of fault type thresholds are preset. The first set of thresholds corresponds to single-phase ground faults, and the threshold is 1.5 times the average of the cumulative offset values ​​of each section. The second set of thresholds corresponds to two-phase short-circuit faults, and the threshold is 2.5 times the average value; The third set of thresholds corresponds to three-phase short-circuit faults, and the threshold is 4 times the average value.

[0040] The cumulative offset value of each segment is compared with the three sets of thresholds one by one in the order of phase interval from zero degrees to 360 degrees. That is, it is first compared with the first set of thresholds. If it does not exceed the threshold, it is compared with the second set of thresholds. If it still does not exceed the threshold, it is compared with the third set of thresholds.

[0041] When the cumulative offset value of a segment exceeds the corresponding threshold in any set of comparisons, the phase interval covered by the segment is determined as the fault phase corresponding to the fault type of the set of faults, and the determination result is output. The result includes the identification information of the fault phase and the duration of the bus voltage sag depth. The duration is in milliseconds and is counted from the time of trigger acquisition.

[0042] Please see Figure 3 As shown, in the phase optimization module, based on the fault state and transient stability boundary constraints, the output voltage phase of the next control cycle is pre-adjusted by a finite number of discrete phase angles to select the optimal adjustment phase that satisfies the phase synchronization margin and does not exceed the current peak limit. Specifically, this includes: The output voltage phase of the current control cycle is used as the reference phase, and the value of the reference phase is used as the zero point. The offset step size is set to 0.5 electrical degrees.

[0043] The phase angle is shifted step by step in the leading direction (the direction of increasing phase angle) and the lagging direction (the direction of decreasing phase angle), with each shift being one step size. The maximum number of shift levels is 10, which means the maximum shift is ±5 electrical degrees.

[0044] All offset phase values ​​are numbered according to the offset direction (leading or lagging) and offset level (from 1 to 10) to form a discrete phase set consisting of offset phases of each level. Each element in the set contains an offset direction identifier, an offset level, and a corresponding offset phase value.

[0045] Each level of offset phase in the discrete phase set is substituted into the two-dimensional transient stability boundary constraints for verification.

[0046] During the calibration process, the predicted peak current at this offset phase is calculated.

[0047] Let the peak value of the currently collected output current be The unit is amperes; the depth of voltage sag is... The value is expressed in per-unit values, ranging from 0 to 1; the offset phase angle is... Expressed in radians, its value is equal to the offset level multiplied by 0.5 electrical degrees and then converted back to radians (multiplied by...). ).

[0048] The predicted peak current is calculated using the following formula: ; in, Indicates the predicted peak current. This is a preset, extremely small positive number, with a value of 0.01, used to prevent the denominator from being zero. In this formula, This represents the per-unit value of the remaining voltage at the grid connection point, and its squared term reflects the effect of voltage drop on the current amplitude. It reflects the modulation effect of the current phase change caused by the offset phase on the amplitude; the square root of the sum of the squares of the two and the reciprocal is used as the current amplification factor, which is multiplied by the current peak value to obtain the predicted value.

[0049] Calculate the predicted phase synchronization margin at this offset phase. Let the current phase synchronization margin be... The unit is electrical degrees; the preset maximum allowable offset angle is... The value is taken as 30 electrical degrees, which is converted to radians. The predicted phase synchronization margin is calculated using the following formula: ; in, Indicates the predicted phase synchronization margin. This represents the normalized ratio of the offset phase to the maximum allowable offset angle. Its squared term reflects the attenuation effect of the offset on the margin. Adding 1, taking the square root, and then taking the reciprocal is used as the margin preservation coefficient. Multiplying it by the current margin gives the predicted margin.

[0050] The calculated Compare with the preset peak current limit (1.15 times the rated current) and... Compare with the preset minimum locking margin (valued at 30 electrical degrees).

[0051] like Less than or equal to the peak current limit and If the offset phase is greater than or equal to the minimum locking margin, it meets the verification condition and is retained; otherwise, the offset phase is discarded.

[0052] Based on the fault phase in the fault state, the offset phases of each level retained after preliminary verification are grouped according to their respective phase intervals.

[0053] Specifically, the complete phase period of 360 electrical degrees is divided into three intervals, each covering 120 electrical degrees, corresponding to the voltage phase ranges of phase A, phase B, and phase C, respectively.

[0054] Based on the fault phase (e.g., A-phase ground fault), determine the corresponding phase interval, and then assign all the offset phases in the discrete phase set to the corresponding phase groups according to the intervals their values ​​fall into, thus obtaining multiple phase groups.

[0055] Within each phase group, each offset phase is checked to see if its phase difference with the phase-locked loop of the wind power converter meets the phase synchronization margin requirement, that is, whether the absolute value of the difference is greater than or equal to the minimum locking margin of 30 electrical degrees. Offset phases that do not meet the condition are eliminated, and the remaining ones are used as primary reserved phases.

[0056] For each primary retained phase, the predicted current peak value is recalculated according to the aforementioned predicted current peak value calculation formula. The predicted value is compared with the current peak limit value (1.15 times the rated current) one by one. The offset phases whose predicted values ​​exceed the limit value are eliminated, and the remaining ones are used as secondary retained phases. These secondary retained phases are the valid offset phases that have passed the verification.

[0057] Among the valid offset phases that have been verified and retained, the offset phase that corresponds to the voltage phase compensation direction of the fault phase in the fault state is selected.

[0058] Specifically, if the faulty phase is phase A, then the offset direction is selected as the leading direction (because the voltage phase of phase A needs to be compensated in advance). If it is phase B or phase C, then select the corresponding compensation direction (which can be preset).

[0059] Among the valid offset phases that meet the directional conditions, the offset phase corresponding to the one with the smallest offset order is taken as the optimal adjustment phase, because the smallest offset order means the smallest adjustment to the current output voltage phase, which is beneficial to maintaining system stability.

[0060] The optimal adjustment phase is the output voltage phase adjustment value for subsequent control cycles.

[0061] In the collaborative execution module, the energy storage converter output is driven by the voltage vector under optimal phase adjustment, while a collaborative reactive power command based on the voltage phase is sent to the wind power converter to maintain continuous locking of the wind power phase-locked loop. Specifically, this includes: The optimal adjustment phase is used as the output voltage phase reference for the energy storage converter.

[0062] Based on this phase reference, the control program inside the energy storage converter calculates the pulse width duty cycle of each of phases A, B, and C according to the three-phase sinusoidal modulation method.

[0063] Specifically, with this phase reference as the reference zero point, the three phases lag by 120 electrical degrees and 240 electrical degrees respectively. The corresponding sinusoidal modulation wave amplitude is obtained by looking up the phase value at each moment. Then, the amplitude is compared with the preset carrier triangular wave to generate the conduction time ratio of each phase power switching device, i.e., the pulse width duty cycle.

[0064] The pulse width duty cycle signals of each phase are output to the power switching transistors of the energy storage converter through the drive circuit, so that the output of the energy storage converter is consistent with the optimal adjustment phase of the three-phase AC voltage.

[0065] The optimal adjustment phase is compared with the real-time phase of the phase-locked loop inside the wind power converter to obtain the phase lag value of the real-time phase of the phase-locked loop relative to the optimal adjustment phase.

[0066] The acquisition process is implemented as follows: the optimal adjustment phase is used as the reference phase benchmark, which is locked to a fixed value at the beginning of the control cycle.

[0067] Starting from this initial moment, the real-time phase of the phase-locked loop of the wind power converter is sampled once for each control cycle (the control cycle length is 50 microseconds) to obtain the real-time phase value at each sampling moment.

[0068] The real-time phase value obtained at each sampling moment is subtracted from the reference phase benchmark to obtain the initial difference at that sampling moment. This difference is expressed in degrees of electricity and can be positive or negative. A positive value indicates leading and a negative value indicates lagging.

[0069] The 10 initial differences obtained in 10 consecutive control cycles are arranged into a sequence according to the sampling time.

[0070] In this sequence, starting from the first initial difference, five consecutive initial differences are taken as a subsequence, that is, the first to the fifth are the first subsequence, the second to the sixth are the second subsequence, and so on, resulting in a total of six subsequences.

[0071] The arithmetic mean of the five initial differences within each subsequence is obtained by adding the five initial differences together and dividing by five.

[0072] The cumulative average lag values ​​of the six subsequences are compared, and the average value with the largest value is selected. This maximum value is the phase lag value of the real-time phase of the phase-locked loop relative to the optimal adjustment phase, and the unit is degree-of-electricity angle.

[0073] This hysteresis value reflects the degree to which the output current phase of the wind power converter lags behind the output voltage phase of the energy storage converter.

[0074] The reactive current compensation amount is determined based on the phase lag value.

[0075] A proportional coefficient is preset, which is the reactive current value that needs to be compensated for each 1 degree electrical angle of phase lag, for example, it is set to 0.5 amperes per degree.

[0076] Multiply the absolute value of the phase lag (since the lag value is negative, its absolute value is taken) by the proportional coefficient to obtain the value of the reactive current compensation, in amperes.

[0077] The reactive current compensation amount is used as a coordinated reactive power command and sent to the controller of the wind power converter via the field communication bus.

[0078] After receiving the instruction, the wind power converter adjusts the reactive current setpoint of its grid-side converter, causing the phase of its output current to be adjusted in the leading direction, thereby reducing the gap with the optimal adjustment phase.

[0079] Since the instruction is based on the voltage phase, the phase-locked loop of the wind power converter continuously tracks the voltage phase at the grid connection point during the adjustment process, and its locked state is maintained, so it will not lose lock due to phase change.

[0080] Throughout the process, the energy storage converter always outputs in the optimal adjustment phase, and the output current phase of the wind power converter gradually follows this reference, achieving coordinated operation between the two.

[0081] In the handover determination module, the phase synchronization margin is continuously monitored during the low-voltage ride-through window, and the handover timing is determined based on the grid connection point voltage recovery slope. When the grid connection point voltage remains below the threshold, off-grid handover is performed based on the optimal phase adjustment, specifically including: During the low-voltage ride-through window, the window is timed from the trigger acquisition time and lasts for 0.2 seconds. The phase synchronization margin is sampled at fixed time intervals and the margin value at each sampling time is recorded.

[0082] The sampling interval is set to 0.5 milliseconds, meaning a total of 400 samples are taken within a 0.2-second window.

[0083] During each sampling, the virtual power angle at the current moment is iterated with the phase difference of the phase-locked loop in accordance with the method of establishing transient stability boundary constraints, and the phase synchronization margin value at the current moment is obtained and recorded in degrees of electricity.

[0084] The amplitude of the grid connection point voltage is sampled at the same sampling time. The sampling method is the same as the voltage acquisition method. The effective value of the three-phase voltage is obtained by analog-to-digital conversion through the secondary side signal of the voltage transformer. The average value of the three-phase effective value is taken as the voltage amplitude of the grid connection point at that time and recorded in volts.

[0085] The voltage amplitudes at the grid connection point recorded at each sampling time are arranged in chronological order.

[0086] Starting from the second sampling moment, the voltage amplitude at the current sampling moment is subtracted from the voltage amplitude at the previous sampling moment to obtain the change in voltage amplitude; this change is divided by the sampling interval of 0.5 milliseconds to obtain the voltage recovery slope corresponding to that moment, in volts per millisecond.

[0087] The phase synchronization margin value at each sampling time is compared with the preset minimum locking margin, which is 30 electrical degrees.

[0088] The voltage recovery slope calculated at each sampling time is compared with the preset lower limit of the recovery slope. The lower limit of the recovery slope is set to 0.5 volts per millisecond. This means that when the voltage recovery rate is lower than 0.5 volts per millisecond, the grid connection point voltage is considered to be unable to automatically recover to the rated value.

[0089] When two consecutive sampling times simultaneously meet the following two conditions, it is determined that the off-grid handover is to be performed: the first condition is that the phase synchronization margin is lower than the minimum locking margin of 30 electrical degrees; the second condition is that the voltage recovery slope is lower than the lower limit of the recovery slope of 0.5 volts per millisecond.

[0090] The determination of two consecutive sampling times is used to avoid misjudgment caused by noise in a single sampling and to improve the reliability of the switching determination.

[0091] Once the timing for off-grid handover is determined, the optimal adjustment phase obtained will be used as the off-grid phase reference.

[0092] An off-grid command is sent via a communication bus to the static switching switch connecting the microgrid and the main grid. This command contains the value of the off-grid phase reference.

[0093] After receiving the instruction, the static switching switch sets the target phase value of the load-side bus voltage as the off-grid phase reference in its internal control logic. By adjusting the conduction angle of the thyristors in the static switching switch, the actual phase of the load-side bus voltage gradually approaches and eventually equals the off-grid phase reference. This allows the load-side bus voltage phase to follow the off-grid phase reference to complete the switch from power supply from the main grid to autonomous power supply from the microgrid. During the switching process, the load-side voltage remains continuous and no power outage occurs.

[0094] After the switch is completed, the energy storage converter continues to output voltage according to the off-grid phase reference voltage to provide a continuous power supply for incremental loads.

[0095] Please see Figure 4 As shown, the integrated source-grid-load-storage microgrid control method for incremental loads is characterized by the following steps: S1, in response to the grid connection point voltage dropping below the threshold, synchronously collects the energy storage converter output current, virtual power angle, wind power converter phase-locked loop phase difference and load-side bus voltage sag depth. S2, based on the virtual power angle and the phase difference of the phase-locked loop, establish transient stability boundary constraints including phase synchronization margin and current peak value, and locate the fault status of the grid connection point based on the output current and bus voltage sag depth. S3, based on the fault state and transient stability boundary constraints, performs a finite number of discrete phase angle pre-adjustments on the output voltage phase of the next control cycle, and selects the optimal adjustment phase that satisfies the phase synchronization margin and does not exceed the current peak limit; S4 drives the energy storage converter output with the voltage vector under the optimal adjustment phase, and sends a coordinated reactive power command based on the voltage phase to the wind power converter to maintain continuous locking of the wind power phase-locked loop. S5 continuously monitors the phase synchronization margin during the low voltage ride-through window and determines the switching timing based on the voltage recovery slope at the grid connection point. When the voltage at the grid connection point remains below the threshold, off-grid switching is performed based on the optimal phase adjustment.

[0096] The foregoing has provided a detailed description of one embodiment of the present invention, but this description is merely a preferred embodiment and should not be construed as limiting the scope of the invention. All equivalent variations and modifications made within the scope of the claims of this invention should still fall within the patent coverage of this invention.

Claims

1. A microgrid system integrating source, grid, load, and storage for incremental loads, characterized in that, include: The data acquisition module, in response to the grid connection point voltage dropping below the threshold, synchronously acquires the output current of the energy storage converter, the virtual power angle, the phase difference of the wind power converter phase-locked loop, and the voltage sag depth of the load-side bus. The boundary construction module establishes transient stability boundary constraints, including phase synchronization margin and current peak value, based on the virtual power angle and the phase difference of the phase-locked loop, and locates the fault status of the grid connection point based on the output current and the bus voltage sag depth. The phase optimization module, based on fault state and transient stability boundary constraints, performs a finite number of discrete phase angle pre-adjustments on the output voltage phase of the next control cycle, and selects the optimal adjustment phase that satisfies the phase synchronization margin and does not exceed the current peak limit. The collaborative execution module drives the energy storage converter output with the voltage vector under the optimal adjustment phase, and at the same time sends a collaborative reactive power command based on the voltage phase to the wind power converter to maintain the continuous locking of the wind power phase-locked loop. The switching decision module continuously monitors the phase synchronization margin during the low voltage ride-through window and determines the switching timing based on the voltage recovery slope at the grid connection point. When the voltage at the grid connection point remains below the threshold, off-grid switching is performed based on the optimal phase adjustment.

2. The integrated microgrid system for incremental load as described in claim 1, characterized in that, The process of establishing the transient stability boundary constraints is as follows: The virtual power angle is incremented step by step, and the difference between the incremented virtual power angle and the phase difference of the phase-locked loop is iterated at each increment step. The maximum difference in the difference sequence that satisfies the continuous locking condition of the phase-locked loop is obtained as the phase synchronization margin. The output current is recursively compared with the rated current at each sampling time, and the maximum ratio in the ratio sequence is taken as the current peak value. The phase synchronization margin and peak current are correlated and mapped in the same time dimension to form a two-dimensional transient stability boundary constraint.

3. The integrated microgrid system for incremental load as described in claim 1, characterized in that, The fault status of the location and grid connection point specifically includes: Under the two-dimensional transient stability boundary constraint, the bus voltage sag depth is decomposed into multiple continuous segments according to the phase period. The phase angle offset of the output current is accumulated and summed in each segment to obtain the cumulative offset value of each segment. The cumulative offset value of each segment is sequentially compared with the preset fault type threshold. When the cumulative offset value of any segment exceeds the corresponding threshold, the phase interval corresponding to the segment is determined as a fault phase, and the fault status including the fault phase and the duration of the temporary drop is output.

4. The integrated microgrid system for incremental load as described in claim 1, characterized in that, The process for selecting the optimal adjustment phase is as follows: Using the output voltage phase of the current control cycle as the reference phase, the reference phase is shifted step by step in the leading and lagging directions respectively, generating a discrete phase set composed of each shifted phase. Each level of offset phase in the discrete phase set is substituted into the two-dimensional transient stability boundary constraints for verification, and offset phases that do not meet the phase synchronization margin or current peak are eliminated. Among the remaining offset phases that have been verified and retained, the offset phase that is consistent with the voltage phase compensation direction corresponding to the fault phase of the fault state is selected, and the one with the smallest offset step is taken as the optimal adjustment phase.

5. The integrated microgrid system for incremental load as described in claim 4, characterized in that, The removal of offset phases that do not meet the phase synchronization margin or current peak value specifically includes: Based on the fault phase in the fault state, the offset phases at each level in the discrete phase set are grouped according to their respective phase intervals to obtain multiple phase groups; Within each phase group, each offset phase is checked to see if it meets the phase synchronization margin with the phase-locked loop phase difference. After removing the offset phases that do not meet the margin, a retained phase is obtained. The predicted current under each retained phase is compared with the current peak value one by one. After removing the offset phases where the predicted current exceeds the current peak value, the second retained phase is used as the valid offset phase that passes the verification.

6. The integrated microgrid system for incremental load as described in claim 1, characterized in that, The process of sending the coordinated reactive power command is as follows: The optimal adjustment phase is used as the output voltage phase reference of the energy storage converter. The pulse width duty cycle of each phase is allocated according to the output voltage phase reference to drive the output of the energy storage converter. The optimal adjustment phase is compared with the real-time phase of the phase-locked loop of the wind power converter to obtain the phase lag value of the real-time phase of the phase-locked loop relative to the optimal adjustment phase. The reactive current compensation amount is determined based on the phase lag value, and the reactive current compensation amount is sent to the wind power converter as a coordinated reactive power command so that the output current phase of the wind power converter follows the optimal adjustment phase.

7. The integrated microgrid system for incremental load as described in claim 6, characterized in that, The process for obtaining the phase hysteresis value is as follows: Using the optimal adjustment phase as the reference phase benchmark, the real-time phase of the phase-locked loop is continuously sampled according to the control cycle to obtain the real-time phase value at each sampling time. The real-time phase value at each sampling time is compared with the reference phase benchmark one by one to obtain the initial difference value at each sampling time. The initial differences obtained in multiple consecutive control cycles are arranged in chronological order, and multiple continuous subsequences are extracted from the arranged sequence by a sliding window. The initial differences in each subsequence are accumulated and averaged to obtain the lag cumulative value corresponding to each subsequence. The largest value among all accumulated hysteresis values ​​is selected as the phase hysteresis value of the real-time phase of the phase-locked loop relative to the optimal adjusted phase.

8. The integrated microgrid system for incremental load as described in claim 1, characterized in that, The determination of the switching timing specifically includes: During the low voltage ride-through window, the phase synchronization margin is sampled at fixed time intervals and the margin value at each sampling time is recorded. At the same time, the amplitude of the grid connection point voltage at each sampling time is sampled and the amplitude difference between adjacent sampling times is divided by the sampling interval to obtain the voltage recovery slope. The margin value at each sampling time is compared with the preset minimum locking margin, and the voltage recovery slope at each sampling time is compared with the preset lower limit of the recovery slope. When the margin value is lower than the minimum locking margin and the voltage recovery slope is lower than the lower limit of the recovery slope for two consecutive sampling times, it is determined that it is time to perform off-grid handover. Using the optimal adjustment phase as the off-grid phase reference, an off-grid command is sent to the static switching switch so that the load-side bus voltage phase follows the off-grid phase reference to complete the off-grid switching.

9. A control method for an integrated source-grid-load-storage microgrid oriented towards incremental load, characterized in that, A method for controlling an integrated microgrid system for incremental load as described in any one of claims 1-8, comprising the following steps: S1, in response to the grid connection point voltage dropping below the threshold, synchronously collects the energy storage converter output current, virtual power angle, wind power converter phase-locked loop phase difference and load-side bus voltage sag depth. S2, based on the virtual power angle and the phase difference of the phase-locked loop, establish transient stability boundary constraints including phase synchronization margin and current peak value, and locate the fault status of the grid connection point based on the output current and bus voltage sag depth. S3, based on the fault state and transient stability boundary constraints, performs a finite number of discrete phase angle pre-adjustments on the output voltage phase of the next control cycle, and selects the optimal adjustment phase that satisfies the phase synchronization margin and does not exceed the current peak limit; S4 drives the energy storage converter output with the voltage vector under the optimal adjustment phase, and sends a coordinated reactive power command based on the voltage phase to the wind power converter to maintain continuous locking of the wind power phase-locked loop. S5 continuously monitors the phase synchronization margin during the low voltage ride-through window and determines the switching timing based on the voltage recovery slope at the grid connection point. When the voltage at the grid connection point remains below the threshold, off-grid switching is performed based on the optimal phase adjustment.

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