A system and method for treating three-phase imbalance of an agricultural distribution network

Abstract

The application discloses a kind of agricultural distribution network three-phase unbalance treatment system and method, it is related to power system distribution network automation control technical field.For the problem of high cost and low utilization of existing agricultural distribution network installation fixed energy storage equipment, the application proposes to use interphase converter for power fast mutual aid as main adjustment means, and uses the AC V2G charging and discharging pile cluster deployed in the transformer area to assist in adjusting intermediate phase voltage, instead of traditional fixed energy storage unit.System calculates three-phase voltage deviation in real time through state monitoring unit, and triggers control when deviation is out of limit by hierarchical decision unit: first, use the converter to transfer the excess power of the highest voltage phase to the lowest voltage phase to complete the main adjustment;Then, if there is still deviation in the middle phase of voltage, the power battery of online V2G vehicle is aggregated and dispatched for auxiliary charging and discharging fine adjustment;And use the repetitive controller to suppress the characteristic subharmonic in the whole adjustment process.The application improves the adjustment accuracy of interphase mutual aid with low-cost V2G auxiliary means, realizes the socialization of distribution network treatment equipment, greatly reduces the investment of transformer area reconstruction, and at the same time ensures the response speed and power quality of power grid.

Description

Technical Field

[0001] This invention relates to the field of power system distribution network automation control technology, specifically to a method and system for managing three-phase imbalance in rural distribution networks, which uses phase-to-phase power mutual assistance as the main regulation means and AC V2G (vehicle-to-grid) charging and discharging pile clusters as an auxiliary fine-tuning means. Background Technology

[0002] In the daily operation of rural power distribution networks, the problems of uneven three-phase load distribution and an excessively high proportion of single-phase nonlinear loads have long existed. Due to the random connection of single-phase loads such as household lighting, agricultural machinery, and distributed photovoltaic systems in rural areas, the three-phase voltage deviation rate of the power grid often exceeds the limits stipulated by national standards. This imbalance leads to an increase in the zero-sequence current of distribution transformers, a significant increase in line losses, and even causes motor overheating and malfunctions of relay protection devices, seriously affecting the reliability of power supply and the lifespan of equipment.

[0003] In recent years, with the advancement of the electric vehicle (EV) rural development strategy, the large number of scattered single-phase AC charging piles has further exacerbated the three-phase imbalance problem in rural power distribution networks. To address this issue, existing technical solutions typically involve installing centralized fixed energy storage devices or SVG phase-to-phase power regulators on the transformer side of the distribution area. However, existing solutions have significant limitations:

[0004] First, they are not economically viable and have low resource utilization rates: traditional fixed energy storage systems such as large lithium iron phosphate battery cabinets have huge initial investment costs, while load fluctuations in rural areas are often short-term and intermittent, resulting in expensive energy storage equipment being idle for a long time and having extremely low returns on investment.

[0005] Second, the control strategy lacks coordination: existing phase-to-phase power transfer technology and energy storage charging and discharging technology often operate independently, lacking multi-objective coordinated optimization of "vehicle interaction - phase-to-phase mutual assistance - harmonic suppression", resulting in system response lag.

[0006] Third, there is a lack of ability to cope with complex vehicle access scenarios: the existing substation control system does not take into account the heterogeneity of electric vehicle power batteries connected to the grid, and cannot transform charging behavior into an effective resource for grid regulation, which is prone to regulation overshoot.

[0007] Therefore, there is an urgent need for a multi-objective collaborative governance scheme that is centered on low-cost mutual assistance and can flexibly introduce auxiliary pressure regulation measures. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides a method and system for managing three-phase imbalance in rural power distribution networks. Its core concept lies in establishing an architecture where phase-to-phase converter power balance is the primary adjustment mechanism, and V2G vehicle battery charging and discharging serves as auxiliary fine-tuning. Utilizing AC V2G charging and discharging pile technology, the power batteries of electric vehicles connected to the rural power distribution network are used as auxiliary mobile energy storage units, replacing expensive fixed centralized energy storage cabinets. After coarse adjustment is completed through phase-to-phase power balance, low-cost, high-precision auxiliary compensation is performed on the intermediate phase voltage.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A three-phase imbalance control system for agricultural power distribution networks includes:

[0011] Status monitoring unit: Real-time acquisition of three-phase voltage signals and load current in rural power distribution network areas, and calculation of three-phase voltage deviation rate;

[0012] Hierarchical decision unit: When the calculated three-phase voltage deviation rate exceeds the preset unbalance trigger threshold, it identifies the phase with the highest voltage, the phase with the lowest voltage, and the intermediate voltage phase, and generates adjustment commands in combination with the available auxiliary capacity of online V2G vehicles in the transformer area.

[0013] The power execution unit includes a main phase-to-phase power mutual assistance module and an auxiliary V2G aggregation regulation module: the main phase-to-phase power mutual assistance module is used to transfer the excess power of the phase with the highest voltage to the phase with the lowest voltage through the three-phase converter; the auxiliary V2G aggregation regulation module is communicatively connected to the V2G charging and discharging piles in the transformer area, and after the main adjustment is activated, it schedules the V2G piles to perform auxiliary charging and discharging control according to the residual deviation of the intermediate voltage phase.

[0014] The harmonic suppression unit is used to add a repetitive controller to the converter control loop to suppress characteristic subharmonics.

[0015] A method for managing three-phase imbalance in agricultural power distribution networks includes the following steps:

[0016] Step 1: Monitor and collect the three-phase voltage and load current of the rural power distribution network area in real time, and calculate the three-phase imbalance.

[0017] Step 2: When the imbalance exceeds the preset threshold, the main phase power mutual assistance is activated first, and the three-phase converter is controlled to transfer the active excess power of the phase with the highest voltage to the phase with the lowest voltage.

[0018] Step 3: Detect the real-time voltage deviation of the intermediate phase. If the absolute value of the deviation exceeds the target threshold, start auxiliary fine adjustment and send a dispatch command to the online V2G charging and discharging piles in the transformer area to utilize the electric vehicle battery to absorb or release power.

[0019] Step 4: Synchronously start the repetitive controller in the converter control loop to suppress the characteristic subharmonics of the power grid.

[0020] The beneficial effects of this invention are as follows:

[0021] 1. Clear distinction between primary and secondary functions, significant economic benefits: Phase-to-phase power mutual assistance is established as the primary voltage regulation method, requiring only V2G charging and discharging piles as auxiliary fine-tuning tools to replace traditional fixed energy storage. By scheduling idle electric vehicle batteries for auxiliary compensation, the high cost of battery cabinet hardware investment in rural power grid management is completely eliminated, achieving a win-win situation for both social vehicle resources and grid auxiliary services.

[0022] 2. Combining coarse and fine adjustments for precise control: Employing a primary and secondary hierarchical decision-making mechanism, the system first achieves rapid primary adjustment of inter-phase power through the converter, and then uses V2G terminals to perform auxiliary fine adjustments of the intermediate phase voltage. This "coarse adjustment first, fine adjustment later" logic, combined with variable parameter PID control, enables the system response time to reach the millisecond level, effectively solving the problem of the intermediate phase adjustment blind spot when relying solely on the converter.

[0023] 3. Improve power quality: The integration of a repetitive controller into the control loop of the V2G bidirectional converter and phase-to-phase converter not only solves the problem of active power imbalance, but also effectively suppresses the 5th / 7th / 11th characteristic harmonics, reducing the total harmonic distortion (THD) to less than 5%, meeting the national standard grid connection requirements. Attached Figure Description

[0024] Figure 1 A diagram illustrating the architecture of a three-phase imbalance management system for agricultural power distribution networks, provided in an embodiment of the present invention.

[0025] Figure 2 A flowchart illustrating a method for managing three-phase imbalance in agricultural power distribution networks, provided in an embodiment of the present invention. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0027] Example 1: System Architecture

[0028] like Figure 1 As shown, this embodiment of the invention provides a three-phase imbalance control system for rural power distribution networks, primarily based on inter-phase power mutual assistance and supplemented by V2G voltage regulation. This system is deployed on the low-voltage side of the transformer in the rural power distribution network area and specifically includes the following modules:

[0029] Status monitoring unit: It adopts a high-precision intelligent sensor terminal to collect the three-phase voltage and current signals of the transformer area in real time at a set high sampling frequency, and calculates the three-phase voltage deviation rate ΔU and the active / reactive power of each phase through the data processing module.

[0030] The hierarchical decision-making unit, acting as the "brain" of the system, is comprised of the edge computing gateway. When ΔU exceeds the 5% trigger threshold, the decision-making unit identifies the phase with the highest voltage (overloaded phase), the phase with the lowest voltage (lightly loaded phase), and the intermediate voltage phase by comparing the three-phase voltage amplitudes. Simultaneously, the decision-making unit obtains the status information of all vehicles plugged into V2G charging / discharging piles within the current distribution area via the CAN / RS485 local communication network or the 4G / 5G IoT cloud platform, including SOC and maximum allowable charging / discharging power D, and comprehensively generates a primary / secondary scheduling strategy.

[0031] Power Execution Unit: Contains two core hardware modules, with a clear distinction between primary and secondary components:

[0032] The main phase-to-phase power balance module is a three-phase two-level converter. Its AC side is connected to the three-phase lines of the rural power distribution network via filter inductors to achieve direct and rapid energy transfer between phases; the DC side has a 750V~800V DC bus. This module is responsible for transferring the vast majority of unbalanced power.

[0033] The auxiliary V2G aggregation and regulation module consists of several intelligent bidirectional AC V2G charging piles deployed within the transformer substation. The DC terminals of these charging piles are connected to the aforementioned DC bus. After the main regulation operation is completed, if voltage fluctuations still occur in the intermediate phase, this module receives auxiliary regulation commands from the decision-making unit, aggregates the power batteries of multiple electric vehicles, and uniformly executes charging or discharging operations, thereby replacing the traditional fixed battery pack.

[0034] DC bus voltage regulation control: The DC bus adopts a hysteresis control strategy, with a preset safe voltage range of 720V~820V. When the power transfer between the converter and the V2G pile causes the bus voltage to momentarily exceed the upper limit (>820V), the decision unit immediately dispatches the online V2G vehicle battery to absorb the excess power (i.e., issues an additional charging command); when the bus voltage momentarily exceeds the lower limit (<720V), the V2G vehicle battery is dispatched to release the deficient power to the bus (i.e., issues a discharge command), thereby maintaining the stability of the DC bus voltage and avoiding converter lock-up or V2G pile shutdown caused by the bus voltage exceeding the limit.

[0035] Harmonic suppression unit: Based on software algorithms, it is embedded in the DSP control board of the converter. By adding a repetitive controller based on the internal mode principle to the inner current control loop, it performs high-gain suppression of the 5th, 7th, and 11th harmonics generated by common nonlinear loads in rural power grids, ensuring that the total harmonic distortion (THD) of the output current is controlled within 5%.

[0036] Example 2: Control Strategy and Method Flow

[0037] like Figure 2 As shown, this embodiment of the invention provides a method for coordinating the above-mentioned system, and the specific steps are as follows:

[0038] Step 1: Real-time data acquisition and status assessment.

[0039] The monitoring unit collects the three-phase voltage (Ua, Ub, Uc) at the main incoming line of the transformer area in real time and calculates the three-phase voltage imbalance and deviation rate.

[0040] Step 2: Phase-to-phase power master adjustment and mutual assistance.

[0041] When ΔU > 5%, the first level of master control is triggered. The decision unit controls the three-phase converter to start, transferring the excess active power of the phase with the highest voltage to the phase with the lowest voltage steplessly through the DC bus. During this stage, if the system detects that the voltage drop in the lowest phase is mainly caused by the reactive power deficit of the line, the converter synchronously injects reactive power Q into that phase to support voltage recovery. This is the main means of mitigation.

[0042] Step 3: V2G vehicle auxiliary fine-tuning control.

[0043] After the first-level master control, the voltage of the highest and lowest phases is effectively improved. At this point, the system detects the "voltage intermediate phase" in a transitional state. If the absolute value of the intermediate phase voltage deviation is still greater than 5%, the second-level auxiliary fine-tuning control is triggered. The decision unit issues instructions based on the current vehicle status connected to the V2G charging / discharging station:

[0044] Scenario A: If the intermediate phase voltage is too high, the decision unit selects vehicles with a battery SOC that is not full (preferably SOC < 80%), issues a dynamic charging command, and uses a variable parameter PID algorithm to accelerate the charging of the vehicle and help absorb excess power from the grid.

[0045] Scenario B: If the intermediate phase voltage is too low, the decision unit selects vehicles with a battery SOC higher than the safety lower limit (e.g., SOC>40%) and issues a V2G discharge command. The vehicle battery sends power to that phase in reverse through the V2G pile to help fill the power deficit until the absolute value of the intermediate phase voltage deviation is ≤2%.

[0046] Vehicle available capacity assessment: The decision unit performs the vehicle available capacity assessment step before issuing instructions — when the SOC of the connected vehicle is lower than the preset lower limit (preferably 20%), only a charging instruction is generated to avoid battery damage caused by over-discharge; when the SOC of the connected vehicle is higher than the preset upper limit (preferably 80%), a discharging instruction is generated preferentially to avoid battery aging caused by over-charge. Vehicles in the intermediate range of 20% < SOC < 80% are regarded as fully available resources and can be dispatched bidirectionally.

[0047] Variable-parameter PID adaptive adjustment: The variable-parameter PID control algorithm is adopted for the dynamic adjustment of auxiliary charging and discharging power. The specific relationship is as follows:

[0048] Kp = Kp0·(1+α·|ΔU| / ΔUn)

[0049] Ki = Ki0·(1+α·|ΔU| / ΔUn)

[0050] Kd = Kd0·(1+α·|ΔU| / ΔUn)

[0051] where Kp0, Ki0, and Kd0 are the reference proportional, integral, and differential gains (recommended values: Kp0=1.0, Ki0=0.2, Kd0=0.05), α is the adaptive coefficient (recommended value: 0.5~1.5), ΔUn is the reference value of the rated voltage deviation (set to 5%), and |ΔU| is the absolute value of the intermediate phase voltage deviation. When |ΔU| increases, all three gains adaptively increase according to the above formula, so as to realize the dynamic response of "strong adjustment for large deviations and precise adjustment for small deviations", and avoid the response lag of the traditional fixed-parameter PID when the deviation is large or the overshoot oscillation when the deviation is small.

[0052] Step 4: Dynamic harmonic coordinated suppression.

[0053] During the entire high-frequency switching operation of the converter in Step 2 and Step 3, the system continuously activates the repetitive controller in the harmonic suppression unit to monitor and cancel the 5th / 7th / 11th harmonic components in real time, so as to ensure the grid-connected power quality.

[0054] The above descriptions are merely preferred embodiments of the present invention, and are not intended to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall all fall within the protection scope of the present invention.

Claims

1. A three-phase imbalance control system for agricultural power distribution networks, characterized in that, include: Status monitoring unit: Real-time acquisition of three-phase voltage signals and load current in rural power distribution network areas, and calculation of three-phase voltage deviation rate; Hierarchical decision unit: When the calculated three-phase voltage deviation rate exceeds the preset unbalance trigger threshold, it identifies the phase with the highest voltage, the phase with the lowest voltage, and the intermediate voltage phase, and generates adjustment commands in combination with the available auxiliary capacity of online V2G vehicles in the transformer area. The power execution unit includes a primary phase-to-phase power mutual assistance module and an auxiliary V2G aggregation regulation module. The main control phase-to-phase power mutual assistance module is used to directly transfer the excess power of the phase with the highest voltage to the phase with the lowest voltage through the three-phase converter, as the main control means for three-phase imbalance control; the AC side of the three-phase converter is connected to the three-phase lines of the rural power distribution network through filter inductors, and a DC bus is led out from the DC side. The auxiliary V2G aggregation and regulation module is connected to at least one AC V2G charging and discharging pile in the transformer area. The DC end of the V2G charging and discharging pile is coupled to the DC bus and shares the bus voltage. After the main regulation action is executed, the V2G charging and discharging pile is scheduled to perform charging or discharging auxiliary control according to the residual voltage deviation value of the intermediate phase and the received regulation command. The harmonic suppression unit is used to add a repetitive controller to the converter control loop of the three-phase converter and V2G charging and discharging pile to suppress characteristic harmonics.

2. The three-phase imbalance control system for agricultural power distribution networks according to claim 1, characterized in that, The total available capacity of the auxiliary V2G aggregation and regulation module is obtained by using the battery capacity of all electric vehicles in the aggregation zone that are in standby mode and whose state of charge (SOC) is within a preset working range. This capacity serves as an auxiliary regulation resource for interphase power mutual assistance, replacing traditional stationary chemical energy storage units.

3. The three-phase imbalance control system for agricultural power distribution networks according to claim 1, characterized in that, The AC side of the three-phase converter of the main phase-to-phase power mutual assistance module is connected to the three-phase line of the rural power distribution network through filter inductors, and the DC side is led out by a DC bus; the DC end of the V2G charging and discharging pile is coupled to the DC bus and shares the bus voltage.

4. The three-phase imbalance control system for agricultural power distribution networks according to claim 3, characterized in that, The DC bus adopts a hysteresis control strategy. When the bus voltage exceeds the preset voltage safety range, the auxiliary V2G aggregation and regulation module schedules the vehicle battery to absorb or release DC-side power to achieve voltage stabilization.

5. The three-phase imbalance control system for agricultural power distribution networks according to claim 1, characterized in that, The hierarchical decision-making unit adopts a primary-secondary collaborative control strategy: First-level master control: When the voltage deviation rate is greater than the first threshold, phase-to-phase power transfer is triggered first to reduce the voltage difference between the highest and lowest phases to the target range. Second-level auxiliary control: If the absolute value of the voltage deviation of the intermediate phase is still greater than the second threshold, the auxiliary V2G aggregation adjustment module is triggered to perform charge and discharge compensation fine-tuning.

6. The three-phase imbalance control system for rural power distribution networks according to claim 1, comprising the following steps: Step 1: Monitor and collect the three-phase voltage and load current of the rural power distribution network area in real time, and calculate the three-phase voltage imbalance. Step 2: When the imbalance exceeds the preset threshold, the main phase power mutual assistance is activated first, and the three-phase converter is controlled to transfer the active excess power of the phase with the highest voltage to the phase with the lowest voltage. Step 3: Detect the real-time voltage deviation of the intermediate phase. If the absolute value of the voltage deviation exceeds the target threshold of the intermediate phase, start auxiliary regulation and send a power dispatch command to the online V2G charging and discharging piles in the transformer area to absorb excess power or release insufficient power using the electric vehicle power battery. Step 4: During the power transfer and charge / discharge regulation process described above, the repetitive controller in the converter control loop is started synchronously to suppress the characteristic subharmonics of the power grid.

7. The method for managing three-phase imbalance in agricultural power distribution networks according to claim 6, characterized in that, In step three, before issuing scheduling instructions to V2G charging and discharging stations, a vehicle availability capacity assessment step must also be performed: When the SOC of the connected vehicle is lower than the preset lower limit (preferably 20%), only a charging command is generated; When the SOC of the connected vehicle is higher than the preset upper limit (preferably 80%), a command to discharge to the grid is generated first.

8. The method for managing three-phase imbalance in agricultural power distribution networks according to claim 6, characterized in that, In step three, the auxiliary charging and discharging power of the V2G charging and discharging pile is dynamically adjusted using a variable parameter PID control algorithm. The proportional, integral, and derivative parameters increase adaptively with the increase of the absolute value of the intermediate phase voltage deviation. Specifically, the relationship is Kp = Kp0·(1+α·|ΔU| / ΔUn), where Kp0 is the reference proportional gain, α is the adaptive coefficient, and ΔUn is the reference value of the rated voltage deviation. The integral gain Ki and the derivative gain Kd are adaptively adjusted with the same coefficient.

9. The method for managing three-phase imbalance in agricultural power distribution networks according to claim 6, characterized in that, In step two, when the voltage deviation of the phase with the lowest voltage is mainly caused by reactive power deficit, the converter is controlled to inject reactive power into the phase with the lowest voltage. The reactive power compensation amount is calculated based on the line impedance and voltage deviation.

10. The method for managing three-phase imbalance in agricultural power distribution networks according to claim 6, characterized in that, The repeating controller in step four is specifically used to suppress the 5th, 7th and 11th characteristic harmonics, so that the total harmonic distortion (THD) at the system grid connection point is reduced to within the preset standard value.

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