A Microgrid Security Optimization Method

Abstract

The present invention provides a microgrid safety optimization method, comprising the following steps: determining whether a specific scenario change occurs in the current microgrid; after determining whether a specific scenario change occurs in the current microgrid, optimizing the microgrid reactive power; after completing the reactive power optimization of the microgrid, optimizing the energy storage unit in the microgrid; after completing the optimization of the energy storage unit in the microgrid, improving the overall power quality control of the energy storage system. The present invention can adjust the microgrid flow distribution through the microgrid reactive power optimization, ensure the voltage stability of each node in the system, and meet the power quality standards, so that the system can operate safely and stably, and the load can work reliably.

Description

Technical Field

[0001] The present invention relates to the field of microgrid security, and in particular to a microgrid security optimization method. Background Art

[0002] With the development of industry and agriculture and the improvement of electrification level, the whole society has an increasing demand for electricity. However, due to the increasing depletion of fossil energy and the demand for a low-carbon economy, countries have to vigorously develop renewable energy such as wind energy and solar energy. However, the output of renewable energy has the characteristics of uncertainty, which makes it impossible to determine the amount of reactive local compensation when traditional microgrids absorb renewable energy, and network losses and voltage deviations increase. Therefore, it is of great significance to study the power supply security optimization of microgrids. Summary of the invention

[0003] The present invention provides a microgrid safety optimization method, which can adjust the microgrid power flow distribution through microgrid reactive optimization, ensure the voltage stability of each node in the system, and ensure that the power quality meets the standard, so that the system can operate safely and stably, and the load can work reliably.

[0004] A microgrid security optimization method comprises the following steps:

[0005] Determine whether a specific scenario changes in the current microgrid;

[0006] After determining that a specific scenario change occurs in the current microgrid, the microgrid is optimized for reactive power;

[0007] After completing the reactive power optimization of the microgrid, the energy storage unit in the microgrid is optimized;

[0008] After optimizing the energy storage units in the microgrid, the overall power quality control of the energy storage system is improved.

[0009] Furthermore, the determining whether a change in a specific scenario occurs in the current microgrid specifically includes:

[0010] Determine whether there is a high proportion of renewable energy access in the microgrid;

[0011] If a high proportion of renewable energy is connected to the microgrid, determine whether a power quality event occurs in the microgrid;

[0012] After determining that a high proportion of renewable energy is connected to the microgrid and a power quality event occurs in the microgrid, determine whether the microgrid needs to implement demand side management;

[0013] After determining that the microgrid needs to implement demand-side management, determine the changes in specific scenarios that occur in the current microgrid.

[0014] Furthermore, the determining whether a high proportion of renewable energy is connected to the microgrid specifically includes:

[0015] Collect the power generation data of various energy sources in the microgrid in real time, compare the real-time power generation data with the historical data, and analyze the changing trend of renewable energy power generation. If the proportion of renewable energy power generation in the total power generation continues to increase and reaches the preset power generation threshold, it is judged that a high proportion of renewable energy is connected;

[0016] Or if the proportion of renewable energy installed capacity to total installed capacity exceeds a preset proportion, it is judged that a high proportion of renewable energy is connected.

[0017] Furthermore, the determining whether a power quality event occurs in the microgrid specifically includes:

[0018] Determine whether voltage fluctuations occur at key nodes of the microgrid;

[0019] Summarize the number of voltage fluctuations and harmonic interferences occurring simultaneously at each key node of the microgrid;

[0020] Determine whether the number of voltage fluctuations and harmonic interferences occurring simultaneously at each key node of the microgrid exceeds a set ratio and exceeds a set duration. If so, determine that a power quality event has occurred in the current microgrid.

[0021] Furthermore, after determining that a change in a specific scenario occurs in the current microgrid, the microgrid is optimized for reactive power, specifically including:

[0022] Set the comprehensive optimization target F for reactive power optimization of the microgrid:

[0023] ;

[0024] Where: ω1, ω2 are the weight coefficients for reducing voltage deviation and network loss in the microgrid respectively; F1 is the minimum node voltage deviation in the microgrid, and F2 is the lowest network loss in the microgrid. F1 and F2 are respectively expressed by the following formulas:

[0025] ;

[0026] Among them, U i , U N , U imax , U imin are microgrid node voltage, node reference voltage, node maximum allowable voltage and node minimum allowable voltage respectively; P loss is the network loss of the microgrid system; N is the number of nodes in the microgrid; G ij is the conductance between nodes i and j in the microgrid; U j is the voltage at node j; θij is the voltage phase angle difference between nodes i and j in the microgrid;

[0027] According to the comprehensive optimization target F of reactive power optimization of microgrid, reactive power optimization is performed on distributed power sources in microgrid respectively to achieve reactive power optimization of the whole microgrid.

[0028] Furthermore, after completing the reactive power optimization of the microgrid, optimizing the energy storage unit in the microgrid specifically includes:

[0029] In the voltage and current double closed-loop control of the inverter of each distributed power source in the microgrid, a dynamic virtual impedance balancing strategy is introduced to achieve accurate and equal distribution of the active power output by the inverter of each distributed power source;

[0030] Balance the current state of charge of each distributed energy storage unit to achieve a balanced current state of charge between the distributed energy storage units;

[0031] Furthermore, the dynamic virtual impedance balancing strategy is introduced into the voltage and current double closed-loop control of the inverter of each distributed power source in the microgrid to achieve accurate equal distribution of the active power output by the inverter of each distributed power source, specifically including:

[0032] Determine the virtual negative inductance value to be introduced into each inverter parallel control module in the microgrid;

[0033] Determine the virtual resistance value to be added to each inverter parallel control module in the microgrid;

[0034] Determine the dynamic virtual complex impedance added to each inverter parallel control module in the microgrid according to the virtual negative inductance value and the virtual resistance value;

[0035] The dynamic virtual complex impedance is added to each inverter parallel control module in the microgrid to introduce a dynamic virtual impedance balancing strategy to achieve accurate equal distribution of the active power output by the inverters of each distributed power source.

[0036] Furthermore, the current state of charge of each distributed energy storage unit is balanced and controlled to achieve the current state of charge balance among the distributed energy storage units, specifically including:

[0037]

[0038] Where: sgn is the symbol function used to determine the operation mode of DESU, S oc,i is the current SOC state value of the i-th DESU, S oc,ave is the mean SOC of each DESU, P i , Q iare the active power and reactive power output by the inverter corresponding to the i-th DESU, m and n are the active-voltage droop coefficient and reactive-frequency droop coefficient, respectively; U N is the rated output voltage of the inverter; U i is the voltage amplitude output by the inverter corresponding to the i-th DESU when it is in operation;

[0039] The working process of the method for balancing the SOC of each DESU is as follows:

[0040] First, the operating status of each DESU is determined by the sgn function, and then the SOC state value S of the i-th DESU is obtained by the average consistency algorithm. oc,i The average SOC value of each DESU oc,ave , so as to make adaptive adjustments.

[0041] Furthermore, after the energy storage unit in the microgrid is optimized, the overall power quality control of the energy storage system is improved, specifically including:

[0042] Control the charging and discharging of the energy storage system according to the set strategy to improve the power quality of the microgrid;

[0043] Determine the current command size that can suppress the power fluctuation of the energy storage unit, and add the current command that suppresses the power fluctuation to the energy storage unit for control, specifically including:

[0044] Assume that the active power consumed by the user side is , the active power generated by distributed energy is , the active power at the head end of the microgrid is , then the active power that the energy storage system needs to provide to smooth out power fluctuations is P = - - , if the rotating vector synthesized by the three-phase grid voltage coincides with the d-axis of the synchronous coordinate system, then P is expressed as:

[0045]

[0046] In the formula is the voltage on the d-axis, is the current component on the q-axis in the synchronously rotating coordinate system, Output active current for the energy storage unit and control the output active current of the energy storage unit The active power can be controlled. In the harmonic detection link, Set to 0, add the active power current used to smooth power fluctuations to the obtained compensation current, and finally form the required command current.

[0047] Furthermore, the set strategy includes:

[0048] When the SOC of the energy storage unit is less than 5%, the IGBT corresponding to the energy storage unit is controlled to be turned off, and the energy storage unit cannot perform active power regulation and power quality management of the microgrid;

[0049] When the SOC is between 5% and 95%, the energy storage unit is charged and discharged according to the difference between the distributed energy grid-connected power and the active power reference value of the distribution station area. If the difference is positive, the energy storage unit is charged to store grid-connected energy that exceeds the reference value, and performs reactive power compensation and harmonic control at the same time. If the difference is close to 0, the capacity of the energy storage unit tends to be stable, and only the power quality control of the distribution station area is performed. If the difference is negative, the energy storage unit is discharged to compensate for the grid-connected energy that is less than the reference value, and participates in the power quality control of the distribution station area at the same time.

[0050] When the SOC is greater than or equal to 95%, the corresponding IGBT of the energy storage unit is controlled to be turned off so that the bus voltage is maintained at the reference value. A chopper resistor is connected in parallel on the output side of the bus voltage and controlled through the IGBT so that the energy storage unit does not affect the bus voltage when it exceeds the upper limit.

[0051] The present invention has the following beneficial effects:

[0052] (1) In the present invention, the microgrid reactive power optimization can adjust the microgrid power flow distribution, ensure the voltage stability of each node in the system, and ensure that the power quality meets the standard, so that the system can operate safely and stably, and the load can work reliably. In addition, reactive power optimization can also reduce network losses and improve the economy of system operation.

[0053] (2) The present invention realizes accurate equal distribution of the system output active power by introducing a virtual impedance module; adopts resistive control based on SOC balance, adaptively adjusts the droop coefficient according to the output SOC value of each DESU, and sets the voltage boundary to ensure the system stability during the SOC adjustment process.

[0054] (3) Through the combination of microgrid reactive power optimization and resistive control based on SOC balance, the voltage stability and power quality of each node in the system are guaranteed to meet the standards. At the same time, the system stability is further improved on the basis of achieving system SOC balance.

[0055] (4) It can realize the control of power and energy input and output of microgrid on different time scales, effectively utilize the grid-connected power of distributed energy, stabilize the active power fluctuation of microgrid, improve the stability and operation characteristics of microgrid, and combine energy storage with power quality control devices. It can not only compensate for harmonics and reactive current in microgrid and control the power quality problems of microgrid, but also give full play to the advantage of energy storage unit in stabilizing the active power of microgrid when the grid-connected power of distributed energy fluctuates. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 It is the topological diagram of the microgrid being connected to the grid;

[0057] Figure 2 It is the structural diagram of the island AC microgrid;

[0058] Figure 3 It is a double closed-loop control block diagram of the embodiment of the present invention introducing dynamic virtual impedance balancing;

[0059] Figure 4 is a working principle diagram of an embodiment of the present invention based on improved droop control SOC balancing;

[0060] Figure 5 It is a topological diagram of connecting the energy storage system in parallel at the head end of the microgrid. DETAILED DESCRIPTION

[0061] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0062] like Figure 1 As shown in Figure 1, this is the topological diagram of the microgrid being connected to the grid. Figure 1 In the figure, Vw is the wind speed at the site where the wind turbine is installed, I is the light intensity at the site where the photovoltaic power generation device is installed, Pw is the active output of the wind turbine, Pv is the active output of the photovoltaic device, and P+jQ is the output of the grid-connected inverter.

[0063] The embodiment of the present invention provides a microgrid security optimization method, comprising the following steps:

[0064] Step 1: Determine whether there are any specific scenario changes in the current microgrid. Under these specific scenario changes, it is necessary to start the subsequent reactive power optimization, power quality control and overall energy storage unit improvement steps in the present invention.

[0065] Determine whether a specific scenario changes in the current microgrid, as follows:

[0066] Step 1.1: Determine whether there is a high proportion of renewable energy access in the microgrid.

[0067] When a high proportion of renewable energy such as wind and solar energy is connected to the microgrid, due to the volatility and uncertainty of these energy sources, it may be necessary to improve the regulation ability and stability of the system through reactive power optimization and energy storage unit optimization. At the same time, power quality control also becomes particularly important to ensure the stability of power supply.

[0068] Determine whether a high proportion of wind, solar and other renewable energy sources are connected to the microgrid, including:

[0069] The power generation data of various energy sources in the microgrid are collected in real time, including wind power, solar power, energy storage devices and traditional energy. These data are collected and transmitted to the central control system through sensors, smart meters and other equipment.

[0070] Compare real-time power generation data with historical data to analyze the changing trend of renewable energy power generation. If the proportion of wind power, solar power and other renewable energy in total power generation continues to rise and reaches the preset power generation threshold, it can be judged that a higher proportion of renewable energy has been connected.

[0071] By calculating the proportion of wind, solar and other renewable energy generation in the total power generation of the microgrid, we can intuitively understand the access to renewable energy. For example, if the proportion of wind and solar power generation in the total power generation exceeds 50% (this power generation threshold can be set according to actual conditions), it can be considered that a high proportion of renewable energy is connected.

[0072] In addition to the proportion of power generation, the proportion of installed capacity of renewable energy can also be considered, which reflects the potential power generation capacity of renewable energy in the microgrid. If the proportion of installed capacity of renewable energy in the total installed capacity is also high, then in actual operation, the power generation of renewable energy is also likely to account for a high proportion.

[0073] Step 1.2: If a high proportion of renewable energy is connected to the microgrid, determine whether a power quality event occurs in the microgrid.

[0074] If a microgrid has power quality problems, such as voltage fluctuations and harmonic interference, it may be necessary to urgently perform reactive power optimization, energy storage unit configuration, power quality control optimization, and other operations to improve the overall performance of the system.

[0075] First, it is necessary to install power quality monitoring devices at key nodes of the microgrid (such as grid connection points and important load points), such as power quality analyzers and harmonic monitors, to monitor key parameters such as voltage, current, frequency, and harmonics in real time. The specific implementation steps are as follows:

[0076] Step 1.2.1: Determine whether voltage fluctuation occurs at each key node of the microgrid

[0077] Voltage fluctuation refers to the continuous change of the effective value of voltage in a series of instantaneous values. The definition and allowable range of voltage fluctuation are clearly defined and usually measured by the voltage fluctuation rate (the ratio of the change in the effective value of voltage to the rated voltage), which is usually determined according to national standards or industry standards.

[0078] Calculating whether voltage fluctuation occurs at key nodes of the microgrid includes: real-time monitoring of the current voltage value of each key node, and obtaining the voltage value at the last monitoring time, calculating the change in the effective voltage value of each key node according to the current voltage value and the voltage value at the last monitoring time; calculating the voltage fluctuation rate of each key node according to the ratio of the change in the effective voltage value to the rated voltage; comparing the voltage fluctuation rate with the pre-set standard voltage fluctuation rate limit to determine whether there is a voltage fluctuation problem at each key node.

[0079] Assuming that the national standard stipulates that the voltage fluctuation rate should not exceed ±5%, at a certain moment, the voltage fluctuation rate of the grid connection point is +6%, the voltage fluctuation rate of the important load point A is -4%, and the voltage fluctuation rate of the important load point B is +3%. At this time, it can be judged that there are voltage fluctuation problems at the grid connection point and the important load point A, while the voltage fluctuation of the important load point B is within the allowable range.

[0080] Step 1.2.2: Determine whether harmonic interference occurs at each key node of the microgrid

[0081] Harmonics refer to the sinusoidal components of electrical quantities whose frequencies are integer multiples of the fundamental frequency, which can have an adverse effect on the normal operation of the power system and the safety of equipment.

[0082] Calculation of whether harmonic interference occurs at each key node of the microgrid includes:

[0083] Obtain the predetermined allowable content limit of each harmonic (such as 3rd, 5th, 7th, etc.), that is, the standard limit;

[0084] Analyze the harmonic spectrum of each key node and calculate the content rate of each harmonic;

[0085] Compare the harmonic content rate with the standard limit to determine whether there is harmonic interference problem at each key node.

[0086] Assuming that the national standard stipulates that the 5th harmonic voltage content rate shall not exceed 4%, at a certain moment, the 5th harmonic voltage content rate of the grid connection point is monitored to be 5.5%, the 5th harmonic voltage content rate of the important load point A is 3.8%, and the 5th harmonic voltage content rate of the important load point B is 2.5%. At this time, it can be judged that there is a harmonic interference problem at the grid connection point, while the harmonic content of the important load points A and B is within the allowable range.

[0087] Step 1.2.3: Summarize the number of voltage fluctuations and harmonic disturbances occurring simultaneously at each key node of the microgrid

[0088] The voltage fluctuation and harmonic interference analysis results of each key node are summarized to determine the number of voltage fluctuations and harmonic interferences occurring simultaneously at each key node of the microgrid.

[0089] Step 1.2.4: Determine whether the number of voltage fluctuations and harmonic interferences occurring simultaneously at each key node of the microgrid exceeds a set ratio and exceeds a set duration. If so, determine that a power quality event occurs in the current microgrid.

[0090] If voltage fluctuations or harmonic interference problems occur at multiple key nodes at the same time, and exceed the set ratio, and last for a long time (exceeding the set duration), it can be determined that there is a problem with the power quality of the microgrid and a power quality event has occurred in the current microgrid.

[0091] Assume that a microgrid system in a certain area consists of photovoltaic power generation, wind power generation, energy storage system and multiple key load nodes. The system is designed to provide a stable and reliable power supply to meet the daily life and industrial production needs of local residents. Recently, the microgrid monitoring system showed that voltage fluctuations and harmonic interference occurred simultaneously at multiple key nodes (such as grid connection points, important load points, etc.).

[0092] The voltage fluctuation range at multiple key nodes exceeds ±5%, far exceeding the ±2% voltage fluctuation limit recommended by the International Electrotechnical Commission (IEC).

[0093] Among the harmonic contents detected at multiple key nodes, the 3rd, 5th and 7th harmonic contents increased significantly, reaching 5%, 3% and 2% respectively, far exceeding the preset harmonic limits of general microgrid systems (usually the content of each harmonic does not exceed 1%).

[0094] The analysis results show that the number of key nodes with voltage fluctuations and harmonic interference at the same time is 3 (assuming that the total number of key detection nodes is 10), and the proportion of key nodes with voltage fluctuations and harmonic interference at the same time is 3 / 10=30%, which exceeds the set ratio of 20%, and the duration is about 1 minute. Therefore, it can be judged that there is a problem with the power quality of the current microgrid, and a power quality event has occurred in the current microgrid.

[0095] Step 1.3: After determining that a high proportion of renewable energy is connected to the microgrid and a power quality event occurs in the microgrid, determine whether the microgrid needs to implement demand side management.

[0096] If it is determined that the microgrid needs to implement demand-side management strategies, such as demand response or load control, it is determined that the microgrid needs reactive power optimization, optimized energy storage units, and power quality control to respond more flexibly to load changes.

[0097] Among them, the method of determining whether the microgrid needs to implement demand-side management can be based on the manager's request confirmation, or the manager can set the management strategy in advance. When it is determined that a high proportion of renewable energy is connected to the microgrid and a power quality event occurs in the microgrid, it is automatically determined whether the microgrid needs to implement demand-side management. At this time, it is determined whether a specific scenario change occurs in the current microgrid, and it is necessary to start the subsequent reactive power optimization, power quality control, and overall improvement of the energy storage unit in the present invention.

[0098] Step 2: After determining in step 1 that a specific scenario change occurs in the current microgrid, the microgrid is optimized for reactive power.

[0099] Reactive power optimization mainly focuses on improving the voltage stability of the microgrid and reducing transmission losses. Reactive power optimization directly affects the basic operating conditions and efficiency of the power grid, and is the first step in this solution.

[0100] When the traditional microgrid does not absorb the output of the microgrid, static VAR compensators, electrostatic capacitors, transformer tap voltage regulators and other devices are generally selected to compensate the microgrid for reactive power. The main principles to be followed are: combining local balance with full network balance and combining decentralized compensation with centralized compensation. However, when the microgrid absorbs the output of the microgrid, the uncertainty of its output will affect the power flow distribution of the microgrid, resulting in the inability of the traditional discrete reactive compensation method to respond quickly. For this reason, it is urgent to propose a new reactive optimization compensation strategy for the access of microgrids to traditional microgrids.

[0101] Therefore, in order to more accurately describe the fluctuation of microgrid output, the microgrid reactive power optimization in the present invention can adjust the microgrid power flow distribution, ensure the voltage stability of each node in the system, and meet the power quality standards, so that the system can operate safely and stably, and the load can work reliably. In addition, reactive power optimization can also reduce network losses and improve the economy of system operation.

[0102] Step 2 specifically includes:

[0103] Step 2.1: First, set the comprehensive optimization target F for reactive power optimization of the microgrid

[0104] The comprehensive optimization objective F for reactive power optimization of microgrid is as follows:

[0105]

[0106] Where: ω1, ω2 are the weight coefficients for reducing the voltage deviation and network loss in the microgrid respectively; different control objectives of each microgrid can be achieved by adjusting the values ​​of ω1 and ω2.

[0107] F1 is the minimum node voltage deviation in the microgrid, and F2 is the lowest network loss in the microgrid. F1 and F2 are respectively expressed by the following formulas:

[0108]

[0109] Among them, U i , U N , U imax , U imin are microgrid node voltage, node reference voltage, node maximum allowable voltage and node minimum allowable voltage respectively; P loss is the network loss of the microgrid system; N is the number of nodes in the microgrid; G ij is the conductance between nodes i and j in the microgrid; U j is the voltage at node j; θ ij is the voltage phase angle difference between nodes i and j in the microgrid.

[0110] In order to distinguish the different voltage requirements between the head end node and the end node of the line in the microgrid, the present invention can set different lower limits for the voltage of different nodes in the microgrid. Considering the voltage drop on the line, the lower limit of the voltage of the head end node of the line is set larger. For example, U 1min , U 2min and U 5min Set it to 1.05 and set U 18min , U 22min and U 33min The voltage lower limit of the remaining nodes is set to 0.9.

[0111] During the reactive power optimization process of each distributed power source in the microgrid, the reactive output of the photovoltaic power source and the wind turbine power source must also meet the following constraints:

[0112]

[0113] Where: Q PV、 Q w are the reactive power outputs of the photovoltaic power source and the wind turbine power source in the microgrid respectively; Q PV_ma x, Q PV_min , Q w_max , Q w_min They are the maximum and minimum reactive outputs of the photovoltaic power source and the wind turbine power source in the microgrid, respectively.

[0114] Step 2.2: According to the comprehensive optimization target F of the reactive power optimization of the microgrid, the distributed power sources in the microgrid are optimized respectively to achieve the reactive power optimization of the microgrid as a whole.

[0115] For example, the above formula can be used to complete the reactive power optimization of photovoltaic power source and wind turbine power source in the microgrid respectively.

[0116] like Figure 1As shown in Figure 1, the photovoltaic power source and the wind turbine power source are connected to the microgrid through the grid-connected inverter. Assuming the rated power of the grid-connected inverter is S, the active power P and reactive power Q output by the wind turbine and photovoltaic grid-connected inverter must meet P 2 + Q 2 ≤S 2 Therefore, in windless or dark weather, the grid-connected inverter can still output reactive power and perform reactive power optimization.

[0117] In the present invention, reactive power optimization of the microgrid can ensure that the voltage of each node in the system is stable and the power quality meets the standard, so that the system can operate safely and stably and the load can work reliably. In addition, reactive power optimization can also reduce network losses and improve the economy of system operation.

[0118] Step 3: After completing the reactive power optimization of the microgrid, optimize the energy storage unit in the microgrid.

[0119] The effective use of energy storage units needs to be based on the stability of the power grid. When the microgrid needs to improve its adaptability to special scenarios (such as the specific scenario in step 1, the volatility of renewable energy, etc.), the optimization of energy storage units becomes particularly important. In the present invention, after the reactive power optimization of the microgrid, the energy storage units in the microgrid are optimized to balance supply and demand, while further reducing dependence on the external power grid and reducing costs.

[0120] Due to the intermittent, uncertain and random nature of renewable energy, it is difficult to maintain stable output power, so a distributed microgrid needs to be equipped with an energy storage system, which can include multiple distributed energy storage units (DESU) to improve power supply reliability.

[0121] Currently, AC microgrids are usually connected to low-voltage power distribution systems. In isolated low-voltage AC microgrids, the DC power output by each microgrid unit needs to be converted into AC power through an inverter to power the load, and multiple DESUs are configured to provide support for continuous power supply to the microgrid.

[0122] However, due to the difference in output line impedance of each inverter, the output power distribution is unreasonable, and the initial state of charge (SOC) of each DESU is inconsistent. It is difficult to achieve SOC balance between each microgrid unit using traditional resistive droop control, which leads to overcharging or over-discharging of DESU, reducing the service life and capacity utilization of DESU. If multiple DESUs exit the system due to SOC imbalance, the reliability of microgrid power supply will be significantly reduced, and even cause system collapse in severe cases. Therefore, it is of great strategic significance to study the SOC balancing strategy of each DESU in the low-voltage microgrid.

[0123] Aiming at the SOC imbalance problem of DESU in an isolated low-voltage microgrid, the present invention first ensures the decoupling of droop control power and the accurate equalization of output active power, providing necessary conditions for achieving SOC balance; then, a SOC adjustment item is added to the droop control so that the droop coefficient is adaptively adjusted according to the SOC of each DESU to ensure SOC balance among DESUs, and voltage and frequency compensation control is introduced to further improve the system operation stability; finally, the effectiveness of the proposed strategy is verified through a simulation model.

[0124] Depend on Figure 2 It can be seen that the isolated AC microgrid generates DC power from the photovoltaic and wind turbine distributed units, and then supplies power to the load through the inverter via the output line. In addition, each DESU is connected in parallel with each other through the interface inverter and connected to the public AC bus to provide power support for the isolated microgrid load.

[0125] Due to the difference in line impedance, the active power cannot be evenly distributed, resulting in the SOC imbalance problem of DEUS. The present invention proposes an improved SOC balancing strategy to optimize the energy storage unit in the microgrid.

[0126] In the present invention, the strategy for optimizing the energy storage unit in the microgrid is divided into three parts: the first part is to introduce dynamic virtual complex impedance in the voltage and current dual closed-loop control to achieve accurate equalization of the inverter output active power; the second part is to improve the traditional resistive droop control to achieve SOC balance among the DESUs.

[0127] Step 3 specifically includes:

[0128] Step 3.1: Introduce a dynamic virtual impedance balancing strategy into the voltage and current dual closed-loop control of the inverter of each distributed power source in the microgrid to achieve accurate equal distribution of the active power output by the inverter of each distributed power source.

[0129] In order to achieve SOC balance among DESUs, the problem of uneven distribution of active power output by the inverters of distributed power sources in the microgrid should be solved first. To address this problem, a dynamic virtual impedance balancing strategy is proposed. Dynamic virtual complex impedance is introduced into the dynamic virtual impedance balancing strategy. The dynamic virtual complex impedance includes virtual negative inductance and virtual resistance. Among them, the virtual negative inductance is used to eliminate the inductive components in the inverter output impedance and line impedance, thereby solving the power coupling problem in resistive droop control: the virtual resistance is used to balance the difference in output line impedance of each inverter, so that the output active power is reasonably evenly distributed. The dynamic virtual impedance balancing control block diagram is introduced in the inverter parallel control module as shown in the figure. Figure 3 shown.

[0130] Figure 3 In, K pu With K piare the voltage outer loop and current inner loop control coefficients in the inverter parallel control module, K pwm is the double closed-loop control gain coefficient, u ref with u ref * are the reference voltages before and after the dynamic virtual impedance balancing strategy is introduced into the inverter parallel control module, u0 and i0 are the output voltage and output current values, R f , L f and C f are filter resistor, inductor and capacitor respectively, Z v is the introduced dynamic virtual complex impedance (including virtual negative inductance and virtual resistance).

[0131] Step 3.1 specifically includes:

[0132] Step 3.1.1: Determine the virtual negative inductance value to be introduced into each inverter parallel control module in the microgrid; specifically, step 3.1.1 includes:

[0133] Step 3.1.1.1: Calculate the output impedance Z of each inverter parallel control module in the microgrid after the dynamic virtual complex impedance is introduced eq (s)

[0134] Z eq (s) is expressed as:

[0135]

[0136] in:

[0137]

[0138] Step 3.1.1.2: Based on the output impedance Z eq (s) Determine the inductive component of the inverter output impedance.

[0139] The output impedance will be expressed in the form of a complex number, with the real part representing the resistive component and the imaginary part representing the inductive component.

[0140] Step 3.1.1.3: Let the line inductance after the virtual negative inductance is introduced into each inverter parallel control module in the microgrid be equal to the inductive component of the inverter output impedance, and determine the value of the virtual negative inductance to be introduced.

[0141] In order to ensure that the resistive droop control output active power is reasonably evenly distributed, it is necessary to solve the power coupling problem caused by the inductive component in the inverter output impedance. A virtual negative inductance is introduced to offset the inductive component in the output impedance, thereby making the output impedance resistive.

[0142] The expression of virtual negative inductance introduced is:

[0143]

[0144] in:

[0145]

[0146] Where: X li is the inverter line inductive reactance.

[0147] The value of the virtual negative inductance to be introduced is adaptively determined according to the inductive component of the line impedance, which can eliminate the inductive component of the inverter output impedance, solve the power coupling problem in the resistive droop control, and realize that the inverter output impedance is approximately pure resistive.

[0148] Step 3.1.2: Determine the virtual resistance value to be added to each inverter parallel control module in the microgrid.

[0149] By introducing virtual resistance into each inverter parallel control module in the microgrid, the active power output by each inverter is reasonably evenly distributed. The resistance expression of the virtual resistance introduced into each inverter parallel control module in the microgrid is:

[0150]

[0151] Where: R v0 is the initial value of the virtual resistance, which is generally set to 0.8~3.5 times the output line impedance; K R is the set virtual resistance adjustment coefficient.

[0152] The dynamic virtual resistance expression introduced is a droop equation with virtual resistance and active power as control variables. By making P1, P2...Pi equal, the virtual resistance of different distributed power sources in the microgrid can be determined, and then the virtual resistance can be dynamically adjusted to achieve accurate equal distribution of output active power.

[0153] Step 3.1.3: Determine the dynamic virtual complex impedance added to each inverter parallel control module in the microgrid according to the virtual negative inductance value and the virtual resistance value, that is, the combination of the virtual negative inductance and the virtual resistance;

[0154] Step 3.1.4: Add dynamic virtual complex impedance to each inverter parallel control module in the microgrid to introduce a dynamic virtual impedance balancing strategy to achieve accurate equal distribution of the inverter output active power of each distributed power source.

[0155] Step 3.2: Balance the current state of charge (SOC) of each distributed energy storage unit (DESU) to achieve SOC balance among the DESUs.

[0156] On the basis of introducing dynamic virtual complex impedance to realize basic equal distribution of active power, it is difficult to balance the system SOC because the initial SOC states of each DESU are different. Therefore, a method for balancing the SOC of each DESU is proposed, as shown in the following formula:

[0157]

[0158] Where: sgn is the symbol function used to determine the operation mode of DESU, S oc,i is the current SOC state value of the i-th DESU, S oc,ave is the mean SOC of each DESU, P i , Q i are the active power and reactive power output by the inverter corresponding to the i-th DESU, m and n are the active-voltage droop coefficient and reactive-frequency droop coefficient, respectively; U N is the rated output voltage of the inverter; U i is the voltage amplitude output by the inverter corresponding to the i-th DESU when it is running.

[0159] The working process of the method for balancing the SOC of each DESU is as follows:

[0160] First, the operating status of each DESU is determined by the sgn function, and then the SOC state value S of the i-th DESU is obtained by the average consistency algorithm. oc,i The average SOC value of each DESU oc,ave The working principle of the method for balancing the SOC of each DESU is as follows: Figure 4 shown.

[0161] The SOC state value S of the i-th DESU is obtained by the average consistency algorithm oc,i The average SOC value of each DESU oc,ave The difference between , and thus adaptive adjustment is performed. The specific implementation method is as follows:

[0162] Depend on Figure 4 It can be seen that in the discharge mode, if the S of the DESU oc,i Greater than S oc,ave , then its active power-voltage curve moves upward, increasing the output active power of the DESU; if the S oc,i Less than S oc,ave , then its active power-voltage curve moves down, reducing the output active power rate of the DESU. In charging mode, if the S oc,i Greater than S oc,ave, then its active power-voltage curve moves up, reducing the output active power rate of the DESU, and vice versa. In addition, the voltage boundary limit is set during the adjustment process to ensure that the system voltage changes within ±5%U N When the S of each DESU oc,i Equal to S oc,ave ,To achieve balanced system SOC, the traditional resistive droop control is adopted again.

[0163] The average SOC value of each DESU oc,ave , obtained through the dynamic consistency algorithm. The energy storage system in the microgrid can be represented by n nodes G and n edges E, and each DESU can be regarded as a node G i , Edge E i is the communication status between nodes, that is, (G i ,E i ). First, each DESU samples the local SOC information and exchanges SOC status information with the adjacent DESU using low-bandwidth communication. Then, solve S once according to equation (1): oc,ave , and for the desired S oc,ave According to formula (2), the convergence judgment is performed. If formula (2) is not satisfied, an update iteration is performed to recalculate S oc,ave , until the convergence requirements are met. Finally, if the convergence conditions are met, the solution process ends and the final S is output. oc,ave result.

[0164] (1)

[0165] (2)

[0166] Where: S oc,ave , (k+1) is the average SOC of the i-th DESU after iteration k+1 times; S oc,ave (k) is the SOC value of the i-th DESU after k iterations; δ ij (k) is the average SOC difference between adjacent DESUs after k iterations, and δ ij (0)=0;X ij is the connection status between the i-th and j-th DESU, and ε is the DESU edge weight.

[0167] Step 4: After optimizing the energy storage units in the microgrid, improve the overall power quality control of the energy storage system.

[0168] The energy storage technology in the present invention can realize the control of the power and energy input and output of the microgrid on different time scales, effectively utilize the distributed energy grid-connected power, stabilize the active power fluctuation of the microgrid, improve the stability and operating characteristics of the microgrid, and combine the energy storage with the power quality management device. It can not only compensate for the harmonics and reactive current in the microgrid and manage the power quality problems of the microgrid, but also give full play to the advantage of the energy storage system in stabilizing the active power of the microgrid when the distributed energy grid-connected power fluctuates.

[0169] The present invention utilizes the similarity of the topological structure between the energy storage system and the active power filter (APF), and improves the control strategy and the power regulation of the energy storage system while having the function of regulating the power quality.

[0170] As a common power quality management device, APF can generate command current from detected reactive current and harmonics through operation circuits, and then transmit it to the control circuit, and send it to the drive circuit after modulating the PWM signal, and then send the drive signal to the converter control switch tube, and then filter it through the filter, and finally achieve the effect of power quality management. However, APF can only achieve the effect of power quality management, and cannot stabilize the power fluctuations in the microgrid caused by the randomness and intermittency of distributed energy.

[0171] In order to effectively solve the power quality problem caused by the microgrid, the energy storage system is connected in parallel at the head end of the microgrid. The topology is as follows: Figure 5 As shown in the figure, the energy storage lithium battery first performs DC / DC conversion through a bidirectional half-bridge, and then connects to a four-arm inverter. While providing energy for the inverter, it can also stabilize the bus voltage. Then it is filtered by L and connected to the head end of the 10kV / 380V microgrid. L and R are the filter inductor and parasitic resistance respectively. The energy storage system can effectively improve the power quality of the microgrid and provide guarantee for the operation of the microgrid under the grid connection of distributed energy.

[0172] Step 4 specifically includes:

[0173] Step 4.1: Control the charging and discharging of the energy storage system according to the set strategy to improve the power quality of the microgrid.

[0174] The energy storage system can be charged and discharged based on the difference between the current state of charge (SOC) of each energy storage unit and the active power at the head end of the microgrid.

[0175] When the SOC of the energy storage unit is less than 5%, the control Figure 2The insulated gate bipolar transistor (IGBT) corresponding to the energy storage unit is turned off, and the energy storage unit cannot perform active power regulation and power quality management of the microgrid.

[0176] When the SOC is between 5% and 95%, the energy storage unit can be charged and discharged according to the difference between the distributed energy grid-connected power and the active power reference value of the distribution station area. If the difference is positive, the energy storage unit is charged to store the grid-connected energy in excess of the reference value, and simultaneously performs reactive power compensation and harmonic control. If the difference is close to 0, the capacity of the energy storage unit tends to be stable, and only the power quality control of the station area is performed. If the difference is negative, the energy storage unit is discharged to compensate for the grid-connected energy that is less than the reference value, and simultaneously participates in the power quality control of the station area.

[0177] When the SOC is greater than or equal to 95%, in order to prevent the energy storage unit from charging and discharging beyond the limit, the IGBT is controlled to be turned off so that the bus voltage is maintained at the reference value. In order to prevent a short-term charging process when the SOC is greater than or equal to 95%, a chopper resistor is connected in parallel on the bus voltage output side and controlled by the IGBT so that the energy storage unit does not affect the bus voltage when it exceeds the limit.

[0178] Step 4.2: Determine the current command size that can suppress the power fluctuation of the energy storage unit, and add the current command that suppresses the power fluctuation to the energy storage unit for control.

[0179] In order to enable the energy storage system to stabilize the active power of the microgrid, current instructions that can suppress power fluctuations are added while controlling harmonic currents.

[0180] Assume that the active power consumed by the user is , the active power generated by distributed energy such as photovoltaic, wind power, and hydropower is , the active power at the head end of the microgrid is , then the active power that the energy storage system needs to provide to smooth out power fluctuations is P = - - , if the rotating vector synthesized by the three-phase grid voltage coincides with the d-axis of the synchronous coordinate system, then P can also be expressed as:

[0181]

[0182] In the formula is the voltage on the d-axis, is the current component on the q-axis in the synchronously rotating coordinate system, The energy storage unit outputs active current. From the above formula, we can know that the control energy storage unit outputs active current The active power can be controlled. In the harmonic detection link, Set to 0, add the active power current used to smooth power fluctuations to the obtained compensation current, and finally form the required command current.

[0183] The time scale of 50Hz cycle corresponds to the smoothing of power fluctuations, while the time scale of 50Hz cycle corresponds to the harmonic control, such as the 5th and 7th harmonics, which correspond to the 5×50Hz or 7×50Hz cycle. Therefore, a reasonable control strategy can achieve the unification of the two in terms of time scale.

[0184] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A microgrid security optimization method, characterized in that: The steps include: Determine whether a specific scenario changes in the current microgrid; After determining that a specific scenario change occurs in the current microgrid, the microgrid is optimized for reactive power; After completing the reactive power optimization of the microgrid, the energy storage unit in the microgrid is optimized; After optimizing the energy storage units in the microgrid, the overall power quality control of the energy storage system is improved; The determining whether a change in a specific scenario occurs in the current microgrid specifically includes: Determine whether there is a high proportion of renewable energy access in the microgrid; If a high proportion of renewable energy is connected to the microgrid, determine whether a power quality event occurs in the microgrid; After determining that a high proportion of renewable energy is connected to the microgrid and a power quality event occurs in the microgrid, determine whether the microgrid needs to implement demand side management; After determining that the microgrid needs to implement demand-side management, determine the changes in the specific scenario that occur in the current microgrid; The determining whether a high proportion of renewable energy is connected to the microgrid specifically includes: Collect the power generation data of various energy sources in the microgrid in real time, compare the real-time power generation data with the historical data, and analyze the changing trend of renewable energy power generation. If the proportion of renewable energy power generation in the total power generation continues to increase and reaches the preset power generation threshold, it is judged that a high proportion of renewable energy is connected; Or if the proportion of renewable energy installed capacity to total installed capacity exceeds a preset proportion, it is judged that there is a high proportion of renewable energy access; The determining whether a power quality event occurs in the microgrid specifically includes: Determine whether voltage fluctuations occur at key nodes of the microgrid; Summarize the number of voltage fluctuations and harmonic interferences occurring simultaneously at each key node of the microgrid; Determine whether the number of voltage fluctuations and harmonic interferences occurring simultaneously at each key node of the microgrid exceeds a set ratio and exceeds a set duration. If so, determine that a power quality event has occurred in the current microgrid.

2. The microgrid security optimization method according to claim 1, characterized in that: After determining that a change in a specific scenario occurs in the current microgrid, the microgrid is optimized for reactive power, specifically including: Set the comprehensive optimization target F for reactive power optimization of the microgrid: Where: ω1, ω2 are the weight coefficients for reducing voltage deviation and network loss in the microgrid respectively; F1 is the minimum node voltage deviation in the microgrid, and F2 is the lowest network loss in the microgrid. F1 and F2 are respectively expressed by the following formulas: F2=min(P loss ) Among them, U i , U N , U imax , U imin are microgrid node voltage, node reference voltage, node maximum allowable voltage and node minimum allowable voltage respectively; P loss is the network loss of the microgrid system; N is the number of nodes in the microgrid; G ij is the conductance between nodes i and j in the microgrid; U j is the voltage at node j; θ ij is the voltage phase angle difference between nodes i and j in the microgrid; According to the comprehensive optimization target F of reactive power optimization of microgrid, reactive power optimization is performed on distributed power sources in microgrid respectively to achieve reactive power optimization of the whole microgrid.

3. The microgrid security optimization method according to claim 1, characterized in that: After the reactive power optimization of the microgrid is completed, the energy storage unit in the microgrid is optimized, which specifically includes: In the voltage and current double closed-loop control of the inverter of each distributed power source in the microgrid, a dynamic virtual impedance balancing strategy is introduced to achieve accurate and equal distribution of the active power output by the inverter of each distributed power source; The current state of charge of each distributed energy storage unit is balanced and controlled to achieve a balance between the current state of charge of each distributed energy storage unit.

4. The microgrid security optimization method according to claim 3, characterized in that: The dynamic virtual impedance balancing strategy is introduced into the voltage and current double closed-loop control of the inverter of each distributed power source in the microgrid to achieve accurate equal distribution of the active power output by the inverter of each distributed power source, specifically including: Determine the virtual negative inductance value to be introduced into each inverter parallel control module in the microgrid; Determine the virtual resistance value to be added to each inverter parallel control module in the microgrid; Determine the dynamic virtual complex impedance added to each inverter parallel control module in the microgrid according to the virtual negative inductance value and the virtual resistance value; The dynamic virtual complex impedance is added to each inverter parallel control module in the microgrid to introduce a dynamic virtual impedance balancing strategy to achieve accurate equal distribution of the active power output by the inverters of each distributed power source.

5. The microgrid security optimization method according to claim 3, characterized in that: The said balancing control of the current state of charge of each distributed energy storage unit to achieve the current state of charge balance among the distributed energy storage units specifically includes: Where: sgn is the sign function used to determine the DESU operation mode, S oc,i is the current SOC state value of the i-th DESU, S oc,ave is the mean SOC of each DESU, P i , Q i are the active power and reactive power output by the inverter corresponding to the i-th DESU, m and n are the active-voltage droop coefficient and reactive-frequency droop coefficient, respectively; U N is the rated output voltage of the inverter; U i is the voltage amplitude output by the inverter corresponding to the i-th DESU when it is in operation; The working process of the method for balancing the SOC of each DESU is as follows: First, the operating status of each DESU is determined by the sgn function, and then the SOC state value S of the i-th DESU is obtained by the average consistency algorithm. oc,i The average SOC value of each DESU oc,ave , so as to make adaptive adjustments.

6. The microgrid security optimization method according to claim 1, characterized in that: After the optimization of the energy storage unit in the microgrid is completed, the overall improvement of the power quality control of the energy storage system is carried out, which specifically includes: Control the charging and discharging of the energy storage system according to the set strategy to improve the power quality of the microgrid; Determine the current command size that can suppress the power fluctuation of the energy storage unit, and add the current command that suppresses the power fluctuation to the energy storage unit for control, specifically including: Assume that the active power consumed by the user is P load , the active power generated by distributed energy is P DG , the active power at the head end of the microgrid is P ref , then the active power that the energy storage system needs to provide to smooth out power fluctuations is P = P load -P ref -P DG , if the rotating vector synthesized by the three-phase grid voltage coincides with the d-axis of the synchronous coordinate system, then P is expressed as: Where e d is the voltage on the d-axis, i q is the current component on the q axis in the synchronously rotating coordinate system, i d Output active current for the energy storage unit, control the energy storage unit to output active current i d The active power can be controlled. In the harmonic detection link, i q Set to 0, add the active power current used to smooth power fluctuations to the obtained compensation current, and finally form the required command current.

7. The microgrid security optimization method according to claim 6, characterized in that: The set strategies include: When the SOC of the energy storage unit is less than 5%, the IGBT corresponding to the energy storage unit is controlled to be turned off, and the energy storage unit cannot perform active power regulation and power quality management of the microgrid; When the SOC is between 5% and 95%, the energy storage unit is charged and discharged according to the difference between the distributed energy grid-connected power and the active power reference value of the distribution station area. If the difference is positive, the energy storage unit is charged to store grid-connected energy that exceeds the reference value, and performs reactive power compensation and harmonic control at the same time. If the difference is close to 0, the capacity of the energy storage unit tends to be stable, and only the power quality control of the station area is performed. If the difference is negative, the energy storage unit is discharged to compensate for the grid-connected energy that is less than the reference value, and participates in the power quality control of the station area at the same time. When the SOC is greater than or equal to 95%, the corresponding IGBT of the energy storage unit is controlled to be turned off so that the bus voltage is maintained at the reference value. A chopper resistor is connected in parallel on the output side of the bus voltage and controlled through the IGBT so that the energy storage unit does not affect the bus voltage when it exceeds the upper limit.

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