Energy storage hierarchical control strategy for governing bidirectional heavy overload of power distribution network equipment

Abstract

An energy storage hierarchical control strategy for governing bidirectional heavy overload of power distribution network equipment comprises the steps that a power distribution network is divided into four layers of areas according to voltage classes and different power distribution network equipment, namely a 0.4 kV feeder line, a 0.4 kV distribution transformer, a 10kV feeder line and a 10kV transformer substation; aiming at the bidirectional heavy overload of the power distribution network equipment caused by the net power of a common connection point under different voltage grades, constructing a power distribution network equipment bidirectional heavy overload step-by-step governance framework of 0.4 kV feeder, 0.4 kV distribution transformer, 10kV feeder and 10kV transformer substation; based on a bidirectional heavy overload step-by-step governance framework, from a low voltage grade to a high voltage grade, dynamic site selection is carried out on the energy storage device based on a load rate threshold criterion; meanwhile, the capacity and the output power of the energy storage device are determined based on the power operation limit constraint. According to the strategy, equivalently, short-time capacity increasing processing is carried out on equipment with insufficient capacity, so that the equipment has a dynamic capacity increasing function to solve the problem of heavy and overload operation in a short time period, and the measure does not have the negative effects of low utilization rate, long construction period and the like caused by physical capacity increasing of the equipment.

Description

Technical Field

[0001] The present invention relates to the technical field of safe power supply for distribution networks, and particularly to a hierarchical energy storage control strategy for governing two-way heavy overloads of distribution network equipment. Background Art

[0002] In urban distribution networks, the access amounts of distributed photovoltaics and electric vehicles are increasing. The output of distributed photovoltaics is affected by complex and random meteorological factors. When the output is intense, the active power output that is not absorbed is fed back to the grid side, which is extremely likely to cause short-term reverse heavy overloads in the distribution network; while in the disordered charging mode of electric vehicles, their aggregated charging is extremely likely to cause overloading problems in some periods of the distribution network, resulting in the occurrence of short-term forward heavy overload phenomena in transformers and lines. Both of these situations will seriously affect the power supply safety of the distribution network. It is worth noting that at present, with the rapid growth of multi-pole loads and the grid connection of a high proportion of new energy, the distribution network equipment mainly composed of transformers and lines is in an embarrassing transition period from sufficient equipment capacity to completely insufficient openable equipment capacity. Simple or blind construction or expansion and renovation of distribution network equipment will lead to new problems such as low utilization rate of distribution network equipment, high investment cost, and poor economy. In view of this, a hierarchical energy storage control strategy for governing two-way heavy overloads of distribution network transformers - lines has important application value. Summary of the Invention

[0003] To solve the above technical problems, the present invention provides a hierarchical energy storage control strategy for governing two-way heavy overloads of distribution network equipment. This strategy first analyzes the reasons for the two-way heavy overload operation of distribution network equipment and the mechanism of the energy storage device participating in power regulation, and then constructs a hierarchical governance framework for two-way heavy overloads of distribution network equipment of "0.4kV feeder - 0.4kV distribution transformer - 10kV feeder - 10kV substation"; based on this framework, dynamic siting of energy storage devices is carried out from low voltage levels to high voltage levels according to the load rate threshold criterion; at the same time, the capacity and output power of the energy storage device are determined based on the constraints of power operation limits; by targeted configuration of energy storage devices, the problem of short-term two-way heavy overloads of distribution network equipment is solved.

[0004] The technical solution adopted by the present invention is as follows:

[0005] A hierarchical energy storage control strategy for governing two-way heavy overloads of distribution network equipment, comprising the following steps:

[0006] Step 1: Divide the distribution network into 4-layer areas according to voltage levels and different distribution network equipment: 0.4kV feeder, 0.4kV distribution transformer, 10kV feeder, and 10kV substation;

[0007] Step 2: For the net power at the common connection point under different voltage levels that causes two-way heavy overload of distribution network equipment, construct a hierarchical governance framework for two-way heavy overload of distribution network equipment from the "0.4 kV feeder - 0.4 kV distribution transformer - 10 kV feeder - 10 kV substation".

[0008] Step 3: Based on the hierarchical governance framework for two-way heavy overload, from the low voltage level to the high voltage level, dynamically locate the energy storage device based on the load rate threshold criterion; at the same time, determine the capacity and output power of the energy storage device based on the power operation limit constraint. In the above Step 1, the distribution network is divided into 4-layer areas according to different voltage levels and distribution network equipment; specifically as follows:

[0009] First, the voltage levels at which distributed photovoltaic and electric vehicles access the distribution network are not unique. Distributed photovoltaic may access the distribution network at any voltage level according to the single-point grid connection capacity; electric vehicles mainly access from the 220V and 380V voltage levels. That is, distributed photovoltaic and electric vehicles in the distribution network will be spread across all voltage levels, and their access impacts will inevitably affect every voltage level. The standard voltage levels of medium and low voltage distribution networks generally include 10 kV, 380V, and 220V.

[0010] Second, there are corresponding transformers, lines and other equipment for each voltage level. Generally speaking, whether the power flow is in the forward direction (from the high voltage level to the low voltage level) or the reverse countercurrent (from the low voltage level to the high voltage level), when flowing through the equipment of different voltage levels, the power or current magnitudes are different. At the same time, even at the same voltage level, the power / current operation limits of transformers and lines are also different. Therefore, according to different voltage levels and distribution network equipment, as Figure 1 shown, the medium and low voltage distribution network is divided into 4-layer areas: the "10 kV substation" area composed of 10 kV transformers; the "10 kV feeder" area composed of 10 kV transmission lines; the "0.4 kV distribution transformer" area composed of 380V and 220V transformers; the "0.4 kV feeder" area composed of 380V and 220V transmission lines. Among them, considering that there are many distribution network equipment at the 380V and 220V voltage levels, they are uniformly classified into the corresponding 0.4 kV level without further subdivision.

[0011] In the above Step 2, for any common connection point i, calculate its net power at time t as:

[0012]

[0013] In Equation (1), are the active power of distributed photovoltaic, the charging load power of electric vehicles, and the active power of conventional loads connected to node i at time t, respectively.

[0014] When the net power is 0, it means the line power is 0 and the source-load balance in the distribution substation area is achieved; when the net power is greater than 0, it means reverse power flows through the line. When the reverse power reaches a certain level, it will cause reverse overload of the line or the transformer in the distribution substation area;

[0015] Similarly, when the net power is less than 0, it may cause forward overload of the distribution network equipment. Therefore, keeping the net power between the limit values can eliminate the overload problem.

[0016] If the net power exceeds the limit value, it is adjusted by installing energy storage devices. The nodes where the net power may exceed the limit value are the nodes where distributed photovoltaics or electric vehicles are connected in step 1. Combining the analysis in step 1, the distribution network is divided into 4-layer areas. From the "0.4kV feeder" area to the "10kV substation" area, the overload conditions of the distribution network equipment are judged step by step. If the net power exceeds the limit value, the capacity and output power of the corresponding energy storage device are solved to manage the two-way overload problem. Specifically, as Figure 2 shown, first, obtain data information such as the access power of distributed photovoltaics, electric vehicles, and conventional loads in the distribution network. According to the initial data information, calculate the line load rate and over-limit time information of the "0.4kV feeder" area to judge whether the corresponding line has an overload problem. If so, further calculate the capacity, power, etc. of the energy storage device configured in this area; then put into the energy storage device of the corresponding specification for power regulation; subsequently, on this basis, update the power flow information of the distribution network, calculate the transformer load rate and over-limit time information of the "0.4kV distribution transformer" area to judge whether the corresponding transformer has an overload problem. If so, further calculate the capacity, power, etc. of the energy storage device configured in this area, and then put into the energy storage device of the corresponding specification for power regulation; the subsequent operations are the same, and the screening and management of equipment overload problems in the "10kV feeder" and "10kV substation" areas are carried out in turn.

[0017] In step 3 described above,

[0018] (1). The load rate of line ij at time t is defined as:

[0019]

[0020] In formula (2), is the actual active operation limit of line ij; is recorded as the reverse load rate when it is positive, and is the forward load rate when it is negative;

[0021] The load rate of line ij satisfies:

[0022]

[0023] Traverse all the common connection points in the distribution network, solve the load rates of the adjacent lines. If the condition in Equation (3) is satisfied, the line operates normally; otherwise, immediately enter the warning state and designate the adjacent nodes as the preselected installation nodes for energy storage devices.

[0024] In practice, there is a certain allowable time margin for line overload. Here, if the line load rate does not satisfy Equation (3) and continues to operate for 2 hours or more, it is regarded as the judgment principle for installing energy storage devices for power regulation.

[0025] (2). The load rate of transformer j at time t is defined as:

[0026]

[0027] In Equation (4), are the net power and the actual active operation limit of transformer j respectively.

[0028] The load rate of transformer j satisfies:

[0029]

[0030] Traverse all the distribution transformers in the distribution network to solve their load rates. If the condition in Equation (5) is satisfied, there is no need to install energy storage devices; otherwise, further judge whether energy storage devices should be configured according to the allowable time margin for the transformer to withstand its own overload operation.

[0031] According to the transformer load-related guidelines, when the load rate of the distribution transformer is higher than 100%, it indicates that the distribution transformer has an overload problem. When the overload is 10%, the transformer can continue to operate for 180 minutes; when the overload is 20%, the transformer can continue to operate for 150 minutes; when the overload is 30%, the transformer can continue to operate for 120 minutes.

[0032] In Step 3, it includes dynamically configuring energy storage devices, specifically as follows:

[0033] First, calculate the line load rate for the 0.4 kV feeder to obtain the installation nodes of the energy storage devices at this level. After the energy storage devices at this level are configured, then conduct targeted energy storage configuration for the 0.4 kV distribution transformers; in the hierarchical order of "feeder - distribution transformer - feeder - substation", screen and manage the equipment overload problems level by level.

[0034] In Step 3, the capacity of the energy storage device should be at least equal to the area enclosed by the net power curve crossing the power operation limit, and the power operation limit is obtained by inversely solving from the load rate constraints of the distribution network equipment. For line ij, there is:

[0035]

[0036] In Equation (6), The capacities of the energy storage devices for preventing reverse and forward overloads of line ij, respectively; △t′ is the power data update time interval; M and N are the total numbers of effective time periods including △t′ for reverse and forward overload operations of the line within the research time scale, respectively; m and n are the ordinals of each △t′ in M and N, respectively. is the net power of line ij at time t m ; is the net power of line ij at time t n ;

[0037] The final capacity of energy storage device i is denoted as:

[0038]

[0039] The rated power of the energy storage device is the upper limit of the electrical energy that can be provided or received per unit time. Then, the rated power of energy storage device i is set as:

[0040]

[0041] In formula (8), d is the power margin of the energy storage device; is the net power of line ij at any time t within the research time scale.

[0042] In step 3, for distribution transformer j, according to the relevant regulations of the load rate of the distribution transformer, the capacity of the energy storage device that should be installed in the distribution substation area is deduced inversely;

[0043]

[0044] In formula (9), are the capacities of the energy storage devices for preventing reverse and forward overloads of transformer j, respectively; S ESS,j is the final capacity of the energy storage device for preventing double - sided heavy overload of transformer j; △t′ is the power data update time interval; M′ and N′ are the total numbers of effective time periods including △t′ for reverse and forward overload operations of the transformer within the research time scale, respectively; m and n are the ordinals of each △t′ in M′ and N′, respectively; is the net power of transformer j at time t m ; is the net power of transformer j at time t n ;

[0045] The rated power of energy storage device j is set as:

[0046]

[0047] In formula (10), d is the power margin of the energy storage device; is the net power of transformer j at any time t within the research time scale.

[0048] When the output of the energy storage device tracks the fluctuation of the net power at the grid connection point in the reverse direction, the local balance between the power source and the load can be maximally achieved. Therefore, when the net power exceeds the power operation limit value, in order to most effectively manage the problem of heavy overload of distribution network equipment, the real-time output of the energy storage device should satisfy:

[0049]

[0050] The energy storage hierarchical control strategy for managing the bidirectional heavy overload of distribution network equipment in the present invention has the following technical effects:

[0051] 1). The present invention is applicable to the distribution network in the construction transition period, that is, when the bidirectional heavy overload condition of the equipment has occurred, but it is still within the scope of short-term emergency heavy overload and is not yet sufficient for large-scale retirement transformation of conventional distribution network equipment, to solve the problem of insufficient capacity of short-term and seasonal distribution network equipment and delay the construction or transformation and upgrading of distribution network equipment.

[0052] 2). While solving the problem of short-term heavy overload of distribution network equipment, this strategy will not cause new problems such as low equipment utilization rate and long construction period due to simply and blindly building or expanding and reforming distribution network equipment, and can provide a technical reference for power companies to solve such problems.

[0053] 3). In step 2 of the present invention, the reasons for the bidirectional heavy overload operation of distribution network equipment and the mechanism of energy storage participating in power regulation are analyzed. Subsequently, considering the integrity of the system power flow and more intuitively reflecting the scope of influence of the bidirectional heavy overload problem of distribution network equipment, a comprehensive screening is carried out and problems are solved targeted, and a hierarchical governance framework for bidirectional heavy overload of distribution network equipment of "0.4kV feeder - 0.4kV distribution transformer - 10kV feeder - 10kV substation" is constructed, which improves the accuracy and pertinence of the energy storage device configuration.

[0054] 3). When performing dynamic siting of the energy storage device in step 3 of the present invention, while considering the load rate constraint of the distribution network equipment, the tolerable time margin of the heavy overload of the distribution network equipment is also taken into account, which can more accurately reflect the risk degree of the heavy overload of the distribution network equipment and avoid misjudgment problems caused by a single criterion.

[0055] 4). In step 3 of the present invention, the capacity of the energy storage device is related to the area enclosed by the net power curve exceeding the power operation limit value, and the output of the energy storage device tracks the fluctuation of the net power at the grid connection point in the reverse direction, so that the local balance between the power source and the load can be maximally achieved and problems such as heavy overload of distribution network equipment can be most effectively managed.

[0056] 5). The present invention conducts numerical example verification and theoretical analysis of the governance strategy, verifying the effectiveness of the strategy. The installation of the energy storage device can successfully avoid the occurrence of short-term bidirectional heavy overload operation of the equipment in the previous distribution network system. Description of the Drawings

[0057] Figure 1 It is a schematic diagram of the hierarchical area division of the distribution network.

[0058] Figure 2 It is a framework diagram of the implementation process of the present invention.

[0059] Figure 3 It is a simplified topology of the distribution transformer area containing distributed photovoltaics and electric vehicles.

[0060] Figure 4 It is a modified IEEE 33-node distribution network system.

[0061] Figure 5(a) is the output power curve of the distributed photovoltaic connected to the simulation system;

[0062] Figure 5(b) is the conventional load power curve connected to the simulation system;

[0063] Figure 5(c) is the charging power curve of the electric vehicle connected to the simulation system.

[0064] Figure 6(a) is the daily output power curve of the energy storage (ESS-T3 daily output power curve);

[0065] Figure 6(b) is the daily output power curve of the energy storage (ESS-T4 daily output power curve);

[0066] Figure 6(c) is the daily output power curve of the energy storage (ESS-T1 daily output power curve);

[0067] Figure 7(a) is the net power image of the node before and after the energy storage governance (T3 low-voltage side node);

[0068] Figure 7(b) is the net power image of the node before and after the energy storage governance (T4 low-voltage side node);

[0069] Figure 7(c) is the net power image of the node before and after the energy storage governance (T1 low-voltage side node). Detailed implementation manners

[0070] A hierarchical energy storage control strategy for governing two-way heavy overload of distribution network transformers and lines. First, data such as the output power of distributed photovoltaics connected to the distribution network, conventional loads, and the charging load power of electric vehicles are obtained. Then, under the framework of the step-by-step governance of two-way heavy overload of distribution network equipment in the "0.4 kV feeder - 0.4 kV distribution transformer - 10 kV feeder - 10 kV substation" constructed, the load rates and cumulative over-limit durations of distribution network equipment within each level are calculated step by step from the low voltage level to the high voltage level, the heavy-overloaded transformers and feeders are identified, and further calculations of energy storage device information and installation of energy storage devices are carried out for the abnormal distribution network equipment. Finally, by absorbing or releasing corresponding energy through the energy storage device, the distribution network equipment operates within the power operation limit. In the simulation, the modified IEEE 33-node distribution network system is used as an example to verify the effectiveness of the proposed hierarchical governance strategy for heavy overload of distribution network equipment. The flow chart is as Figure 2 shown, including the following steps:

[0071] Step 1: Based on the different voltage levels and equipment, construct a step-by-step governance framework for two-way heavy overload of distribution network equipment in the "0.4 kV feeder - 0.4 kV distribution transformer - 10 kV feeder - 10 kV substation":

[0072] The access of distributed photovoltaics makes the local area of the distribution network system have a "source", and the output power of photovoltaics has the characteristics of strong volatility and intermittency; the unordered charging of electric vehicles (EVs) will make the original load curve "peak on peak", and the power consumption load in the substation area will increase significantly in some periods. The two lead to a complex situation of diversified and randomized power flow distribution in the distribution network, making the system operate in multiple states with coexistence of heavy and light loads. The typical structure of a distribution substation area containing distributed photovoltaics and EVs is as Figure 3 shown.

[0073] Figure 3 In the figure, bus1 is the common connection point of distributed photovoltaics, EVs, and conventional loads, and the arrow direction in the figure is recorded as the positive direction. Compared with when the photovoltaic output power is 0, the increase in photovoltaic output power will cause the line current I0 to decrease in the positive direction; when the photovoltaic output power is equal to the total load of the substation area, theoretically, the substation area reaches source-load balance and I0 decreases to 0; if the photovoltaic output power continues to increase, that is, is greater than the total load of the substation area, I0 begins to increase in the reverse direction, the power begins to reverse, and subsequently, it may lead to reverse heavy overload operation of the line and the distribution transformer in the substation area. The analysis of the situation of forward heavy overload of distribution network equipment is similar. That is, subtracting the total load of the substation area (conventional load + EV charging load) from the photovoltaic output power will obtain a power input, and the magnitude of this power directly affects the current flow direction and the magnitude of the flowing power of the adjacent line. Now, this power is denoted as the net power. For any common connection point i, the net power at time t can be calculated as:

[0074]

[0075] In formula (1), They are respectively the active power of distributed PV connected at node i at time t, the EV charging load power, and the active power of the conventional load.

[0076] When the net power is 0, it means the line power is 0 and the source-load balance in the substation area is achieved; when the net power is greater than 0, reverse power flows through the line, and when its value reaches a certain level, it will cause reverse overloading of the line or the distribution transformer in the substation area; similarly, when the net power is less than 0, it may cause forward overloading of the distribution network equipment. Therefore, by making the net power fall between the limit values, the overloading problem can be eliminated. Analyzing in combination with formula (1), it can be known that by optimizing the EV charging load, the net power can be minimized as much as possible. If the net power still exceeds the limit value, an energy storage device can be installed for further adjustment.

[0077] In addition, whether it is the reverse overloading of the distribution network equipment corresponding to the power backfeeding or the forward overloading of the distribution network equipment corresponding to the forward power flow, both are caused by the net power at the point of common coupling, that is, the net power at the point of common coupling is the source of the two-way overloading of the distribution network equipment. The voltage levels at which distributed PV and EV are connected to the distribution network are not unique. Distributed PV may be connected to any voltage level of the distribution network according to the single-point grid-connected capacity; EVs are mainly connected from the 220V and 380V voltage levels. That is, the points of common coupling in the distribution network will be spread across various voltage levels.

[0078] To intuitively reflect the scope of influence of the two-way overloading problem of the distribution network equipment and comprehensively screen and solve the problem targeted. As Figure 1 shown, the entire distribution network equipment is divided into 4-layer areas according to voltage levels and different equipment: 0.4kV feeder, 0.4kV distribution transformer, 10kV feeder, and 10kV substation. A hierarchical governance framework for the two-way overloading of the distribution network equipment of "0.4kV feeder - 0.4kV distribution transformer - 10kV feeder - 10kV substation" is constructed for the two-way overloading of the distribution network equipment caused by the net power at the points of common coupling at different voltage levels. Based on this framework, electrochemical energy storage devices are gradually configured from the low voltage level to the high voltage level to govern the overloading problems of the line and the transformer. This is also considering the integrity of the system power flow and determining the energy storage information step by step in different regions to improve the accuracy of the energy storage device. Among them, the distribution network equipment at the 220V voltage level is classified into the corresponding 0.4kV level without further subdivision.

[0079] Step 2: Based on the above framework, conduct dynamic siting of energy storage based on the load rate threshold criterion from the low voltage level to the high voltage level:

[0080] The load rate of line ij at time t is defined as:

[0081]

[0082] In formula (2), is the actual active power operation limit of line ij. When it is positive, it is recorded as the reverse load rate; When it is negative, it is the forward load rate.

[0083] According to relevant national standards, the load rate of line ij should satisfy:

[0084]

[0085] Traverse all common connection points in the distribution network, solve the load rates of their adjacent lines. If the formula (3) is satisfied, the line can operate normally; otherwise, it should immediately enter the warning state, and the adjacent nodes are designated as the preselected installation nodes for energy storage. In practice, there is a certain tolerable time margin for line heavy overload. Here, if the line load rate does not satisfy formula (3) and continues to operate for 2 hours or more, it is regarded as the judgment principle for installing energy storage devices for power regulation.

[0086] For a distribution transformer, the load rate of distribution transformer j at time t is defined as:

[0087]

[0088] In formula (4), are the net power and the actual active power operation limit of transformer j respectively.

[0089] According to relevant national standards, the load rate of distribution transformer j should satisfy:

[0090]

[0091] Based on this, traverse all distribution transformers in the distribution network to solve their load rates. If the formula (5) is satisfied, there is no need to install energy storage devices; otherwise, further judge whether a distribution transformer should be configured according to the tolerable time margin of the distribution transformer for its own heavy overload operation. It can be known from the relevant guidelines for power transformer loads that when the load rate of a distribution transformer is higher than 100%, it means that the distribution transformer has an overload problem. When it is overloaded by 10%, the distribution transformer can continue to operate for 180 min; when it is overloaded by 20%, the distribution transformer can continue to operate for 150 min; when it is overloaded by 30%, the distribution transformer can continue to operate for 120 min.

[0092] It should be noted that configuring an energy storage device at any node in the distribution network will inevitably affect the power flow of the distribution network, especially the power magnitude in the adjacent area of the installation node, thus affecting the calculation results of the load rates of lines or transformers. Therefore, the present invention proposes to dynamically configure energy storage devices.

[0093] Combined with the aforementioned governance framework, first calculate the line load rate of the 0.4 kV feeder to obtain the installation nodes of the energy storage devices at this level. After the configuration of the energy storage devices at this level is completed, then conduct targeted configuration of the energy storage devices for the 0.4 kV distribution transformer, and screen and manage the problems of overloading and heavy loading of distribution network equipment step by step in the hierarchical order of "feeder - distribution transformer - feeder - substation".

[0094] Step 3: While dynamically locating the energy storage devices, determine the capacity and output power of the energy storage devices based on the constraints of power operation limits;

[0095] The energy storage devices should absorb or release corresponding energy to keep the load rate of the distribution network equipment within a reasonable range. Therefore, the capacity of the energy storage devices should be at least equal to the area enclosed by the net power curve crossing the power operation limits, and the power operation limits are obtained by inversely solving the load rate constraints of the distribution network equipment. For line ij, there is:

[0096]

[0097] In Equation (6), are the capacities of the energy storage devices to prevent reverse and forward heavy loading of line ij respectively; △t′ is the time interval for power data update; M and N are the total numbers of effective time periods including △t′ for reverse and forward heavy loading operation of the line within the research time scale respectively. The final capacity of energy storage device i is denoted as:

[0098]

[0099] The rated power of the energy storage is the upper limit of the electric energy that can be provided or received per unit time, so the rated power of energy storage device i is set as:

[0100]

[0101] In Equation (8), d is the energy storage power margin.

[0102] Similarly, for distribution transformer j, according to the relevant regulations of the load rate of the distribution transformer, the capacity of the energy storage devices that should be installed in the distribution substation area is inversely deduced.

[0103]

[0104] In Equation (9), are the capacities of the energy storage devices to prevent reverse and forward heavy loading of distribution transformer j respectively; M′ and N′ are the total numbers of effective time periods including △t′ for reverse and forward heavy loading operation of the distribution transformer within the research time scale respectively.

[0105] The rated power of energy storage device j is set as:

[0106]

[0107] When the output of the energy storage device tracks the net power fluctuation at the grid connection point in the reverse direction, the in-situ balance of the source and load can be maximally achieved. Therefore, when the net power exceeds the power operation limit, in order to most effectively address issues such as heavy overload of distribution network equipment, the real-time output of the energy storage device should satisfy:

[0108]

[0109] Step 4: Taking the reformed IEEE 33-node distribution network system as an example, verify the effectiveness of the proposed hierarchical governance strategy for heavy overload of distribution network equipment:

[0110] Based on the historical data of a certain urban distribution network, obtain data such as the output of distributed photovoltaics, conventional load, and EV charging load power that can reflect the volatility of the source and load. First, take the "0.4 kV feeder" as the object, calculate the line load rate and cumulative over-limit time within this level, identify the overloaded feeders, and then further calculate and install energy storage information for the abnormal feeder; after the energy storage device configuration at the "0.4 kV feeder" level is completed, conduct a risk investigation and governance of heavy overload for the distribution transformers at the next level, namely the "0.4 kV distribution transformer"; the risks of heavy overload of distribution network equipment at the subsequent two levels are also screened and excluded step by step in turn.

[0111] The reformed IEEE 33-node distribution network system is as Figure 4 shown. Select a typical day as the research time scale. The power data of distributed photovoltaics, conventional load, and EV charging and power consumption all come from the historical data of a certain urban distribution network, as Figures 5(a) to 5(c) shown. Among them, EV charging is not a single charging mode, but a mixed charging mode that takes into account both unordered and ordered charging modes.

[0112] Using the strategy of the present invention to perform corresponding processing on the distribution network system, information such as the net power and load rate of each level is obtained in turn. After comparative analysis, the net power of three nodes exceeds the power operation limit. Specifically, the three nodes are the low-voltage side nodes of T3 and T4 and the T1 substation node. Transformer T3 has a positive heavy overload phenomenon for about 180 minutes during the evening period, with a maximum load rate reaching 114%; transformer T4 has a reverse heavy load phenomenon for about 200 minutes during the noon period, with a maximum load rate of 94%, and at the same time, it also has a positive heavy load phenomenon for about 190 minutes during the evening period, with a maximum load rate of 98%. The situation of T1 substation is the worst. It has a reverse heavy overload phenomenon for about 230 minutes during the noon period, with a maximum load rate reaching 128%. In addition, it also has a positive heavy overload phenomenon lasting nearly 14 hours, with a maximum load rate as high as 180%.

[0113] All of the above situations have seriously affected the power supply safety of the corresponding transformer equipment. Further, the configuration information of the energy storage device is calculated as shown in Table 1, and its output curve is as Figures 6(a) to 6(c) shown.

[0114] After installing the energy storage device in the distribution network system, the net power of the above three nodes is controlled within a reasonable range. The comparison of the treatment effects is as Figures 7(a) to 7(c) shown.

[0115] Table 1 Energy storage configuration information

[0116]

[0117] Figures 7(a) to 7(c) It shows that after configuring the energy storage device, the net power of the nodes is basically limited within the power red line, and the distribution network equipment can basically operate within the power red line, that is, there will be no situation of double-sided heavy overload of the distribution network equipment power. The original situation of reverse heavy overload caused by the reverse flow of the power flow during the peak photovoltaic output at noon and the serious overload operation caused by the peak charging and discharging of the load at night have been greatly improved. The installation of the energy storage device has successfully avoided the occurrence of heavy load or overload operation of the distribution network system equipment in the past, and the strategy regulation effect has reached the expected goal. The above process verifies the effectiveness of the present invention.

[0118] This strategy is equivalent to performing short-term capacity increase treatment on the distribution network equipment with insufficient capacity, enabling it to have the function of dynamic capacity increase to solve the situation of short-term heavy overload operation of itself. This measure will not have negative effects such as low utilization rate and long construction period brought by physical capacity increase of the distribution network equipment.

Claims

1. A storage layered control strategy for managing bidirectional heavy overload of distribution network equipment, characterized in that The following steps are involved: Step 1: Divide the distribution network into four areas according to the voltage level and distribution network equipment: 0.4kV feeder, 0.4kV distribution transformer, 10kV feeder and 10kV substation; Step 2: Aiming at the bidirectional heavy overload of distribution network equipment caused by the net power of the public connection point at different voltage levels, a step-by-step management framework for bidirectional heavy overload of distribution network equipment of "0.4kV feeder-0.4kV distribution transformer-10kV feeder-10kV substation" is established; Step 3: Based on the bidirectional heavy overload step-by-step management framework, dynamically select the location of the energy storage device from low voltage level to high voltage level based on the load rate threshold criterion; at the same time, determine the capacity and output power of the energy storage device based on the power operation limit constraint.

2. According to claim 1, a storage layered control strategy for managing bidirectional heavy overload of distribution network equipment is characterized by: In the step 1, the medium and low voltage distribution network is divided into four areas according to the voltage level and the distribution network equipment: a "10kV substation" area composed of 10kV transformers; a "10kV feeder" area composed of 10kV transmission lines; a "0.4kV distribution transformer" area composed of 380V and 220V transformers; and a "0.4kV feeder" area composed of 380V and 220V transmission lines. Among them, there are many distribution network equipment at 380V and 220V voltage levels, so they are uniformly classified into the corresponding 0.4kV level.

3. According to claim 1, a storage layered control strategy for managing bidirectional heavy overload of distribution network equipment is characterized by: In step 2, for any common connection point i, the net power at time t is calculated as: In formula (1), They are the distributed photovoltaic active power, electric vehicle charging load power, and conventional load active power connected to node i at time t; When the net power is 0, it means that the line power is 0 and the source and load in the distribution station area are balanced; when the net power is greater than 0, it means that the line flows through reverse power. When the reverse power is large to a certain extent, it will cause the transformer of the line or distribution station area to reverse overload. When the net power is less than 0, the distribution network equipment will be heavily overloaded; therefore, making the net power between the limits can eliminate the heavy overload problem.

4. According to claim 3, a storage layered control strategy for managing bidirectional heavy overload of distribution network equipment is characterized by: If the net power exceeds the limit, it is adjusted by installing an energy storage device, as follows: First, the data information of distributed photovoltaic, electric vehicles, and conventional load access power in the distribution network is obtained. Based on the initial data information, the line load rate and over-limit time information of the "0.4kV feeder" area are calculated to determine whether the corresponding line has a heavy overload problem. If so, the capacity and power of the energy storage device in the area are further calculated. Then, energy storage devices of corresponding specifications and sizes are put into use for power regulation; Subsequently, the power flow information of the distribution network is updated, and the transformer load rate and over-limit time information in the "0.4kV distribution transformer" area are calculated to determine whether the corresponding transformer has a heavy overload problem; if so, the capacity and power of the energy storage device configured in the area are further calculated; Then, energy storage devices of corresponding specifications and sizes are put into use for power regulation; subsequent operations are similar, and the screening and management of equipment overload problems in the "10kV feeder" and "10kV substation" areas are carried out in turn.

5. According to claim 1, a storage layered control strategy for managing bidirectional heavy overload of distribution network equipment is characterized by: In step 3, the load rate of line ij at time t is defined as: In formula (2), is the actual active power operating limit of line ij; When it is a positive value, it is recorded as the reverse load rate. When it is a negative value, it is a positive load rate; The load rate of line ij satisfies: Traverse all public connection points in the distribution network and solve the load rate of the adjacent lines. If equation (3) is satisfied, the line is operating normally; otherwise, it immediately enters the warning state and the adjacent nodes are set as the pre-selected installation nodes of the energy storage device.

6. According to claim 5, a storage layered control strategy for managing bidirectional heavy overload of distribution network equipment is characterized by: In practice, there is a certain time margin for heavy overload of the line. Here, when the line load rate does not meet the formula (3) and continues to operate for 2 hours or more, it is considered as the evaluation principle for installing energy storage devices for power regulation.

7. According to claim 6, a storage layered control strategy for managing bidirectional heavy overload of distribution network equipment is characterized by: The load factor of transformer j at time t is defined as: In formula (4), are the net power and actual active operating limit of transformer j respectively; The load rate of transformer j satisfies: Traverse all distribution transformers in the distribution network to solve their load rates. If they satisfy equation (5), there is no need to install an energy storage device. Otherwise, the time margin that the transformer can withstand its own heavy overload operation is used to further determine whether an energy storage device should be configured.

8. According to claim 7, a storage layered control strategy for managing bidirectional heavy overload of distribution network equipment is characterized by: Step 3 includes dynamically configuring the energy storage device as follows: First, the line load rate of the 0.4kV feeder is calculated to obtain the installation node of the energy storage device at this level. After the energy storage device at this level is configured, targeted storage is then configured for the 0.4kV distribution transformer. In accordance with the hierarchical order of "feeder-distribution transformer-feeder-substation", the heavy overload problem of equipment is screened and managed step by step.

9. The energy storage hierarchical control strategy for managing bidirectional heavy overload of distribution network equipment according to claim 1 is characterized by: In step 3, the capacity of the energy storage device should be at least equal to the area enclosed by the net power curve crossing the power operation limit, and the power operation limit is obtained by inversely solving the load rate constraint of the distribution network equipment; for line ij: In formula (6), are the capacities of the energy storage device to prevent reverse and forward overloading of the line ij power respectively; △t′ is the time interval for power data update; M and N are the total number of effective time periods containing △t′ in reverse and forward heavy-load operation of the line within the research time scale; m and n are the ordinal numbers of each △t′ in M ​​and N respectively; For line ij at t m Net power at the moment; For line ij at t n Net power at the moment; The final capacity of energy storage device i is recorded as: The rated power of the energy storage device is the upper limit of the electric energy it can provide or receive per unit time. The rated power of energy storage device i is set to: In formula (8), d is the power margin of the energy storage device; It is the net power of line ij at any time t within the study time scale.

10. The energy storage hierarchical control strategy for managing bidirectional heavy overload of distribution network equipment according to claim 9 is characterized by: In step 3, for distribution transformer j, according to the relevant regulations on the load rate of distribution transformer, the capacity of the energy storage device that should be installed in the distribution area is inferred; In formula (9), are the capacity of the energy storage device to prevent the distribution transformer j from overloading in the reverse and forward directions respectively; S ESS,j The final capacity of the energy storage device to prevent the distribution transformer j from bidirectional heavy overload; △t′ is the time interval for power data update; M′ and N′ are the total number of effective time periods containing △t′ in the reverse and forward heavy load operation of the distribution transformer within the research time scale; m and n are the ordinal numbers of each △t′ in M′ and N′ respectively; For the distribution transformer j in t m Net power at the moment; For the distribution transformer j in t n Net power at the moment; The rated power of energy storage device j is set as: In formula (10), d is the power margin of the energy storage device; The net power of distribution transformer j at any time t within the study time scale; The output of the energy storage device reversely tracks the net power fluctuation of the grid connection point, which can maximize the local balance of source and load; therefore, when the net power exceeds the power operation limit, in order to most effectively control the heavy overload problem of distribution network equipment, the real-time output of the energy storage device should meet the following requirements:

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