A hierarchical voltage regulation method and system for power distribution network-line-transformer-customer

Abstract

The present invention discloses a distribution network station-line-transformer-user layered voltage regulation method and system. The method targets the voltage over-limit problem of the distribution network and carries out linkage management in a top-down order of "station-line-transformer-user". The problem of voltage over-limit in a large area is solved through substations and lines, and the problem of voltage over-limit in a dispersed area and user is solved through substations and lines. The method of the present invention optimizes the AVC control strategy within the substation level, regulates the voltage through medium-voltage line voltage regulators and reactive compensation devices at the line level, and solves the problem of voltage over-limit at the distribution transformer level by adjusting the gear and switching on and off reactive compensation devices according to the voltage at the outlet of the low-voltage side of the distribution transformer and the voltage of the user at the distribution transformer level. On-site investigation is carried out on the reasons for the voltage over-limit of individual users, and targeted measures such as adding household reactive compensation devices and modifying the down-line are adopted. The present invention solves the problem of prominent high voltage problems of current users but insufficient voltage regulation capabilities of single-level through multi-level voltage linkage, thereby improving the safety, reliability and economy of power grid operation.

Description

Technical Field

[0001] This invention patent relates to the field of voltage regulation strategies for power distribution networks. In response to the problem of voltage exceeding limits in distribution networks, it promotes coordinated governance in a top-down order of "station-line-transformer-customer". It addresses large-scale voltage exceeding-limit problems through substations and lines, and addresses decentralized voltage exceeding-limit problems through distribution areas and users. Background Technology

[0002] my country's regional power distribution network is relatively dispersed, and long-distance power supply is a common phenomenon. For example, the length of 10kV lines is more than 15km and the length of 400V lines is more than 500m. Due to the long transmission distance, the resistance is large and the voltage difference between the beginning and end of the line is large. The voltage distribution is relatively dispersed, and voltage management cannot be solved by a single-level voltage regulation method.

[0003] With the large-scale integration of distributed power sources and random loads such as electric vehicles, the operating environment of power distribution networks is becoming increasingly complex, with strong randomness in power amplitude fluctuations and direction changes, placing higher demands on voltage stability and the speed of regulation. In passive power distribution network operation, the voltage drop direction is the same as the power flow direction, with the lowest voltage point at the end. However, when distributed photovoltaic power is integrated into the distribution network, the power flow reverses, resulting in highly uncertain and volatile voltage distribution among users. Traditional single voltage regulation strategies are insufficient to meet the development needs of current new power systems.

[0004] Existing technical document 1 (CN115603368 A) discloses a multi-level reactive power coordination optimization method considering photovoltaic access at the substation, line, and transformer levels. This document takes photovoltaic absorption level, power supply reliability, and grid economic operation level as comprehensive objectives, and outputs reactive power, tap changer, and other resource actions. However, these resources exist at multiple levels such as substations, lines, and distribution transformers, and the actions and their order at each level are not specified. At the equipment level, the main control equipment is reactive power compensation devices, photovoltaics, and energy storage, without considering line voltage regulators, user-level voltage regulation resources, etc., and without fully considering all voltage regulation resources, thus failing to achieve optimal voltage regulation. At the algorithm level, the adjustment strategy is generated by iterative optimization. Voltage is a complex mathematical problem of a system, and may face unsolvable situations, limiting its applicability to specific scenarios.

[0005] Existing technical document 2 (CN115459283A) discloses an AVC active adaptive cooperative control method for preventing voltage over-limit in the main grid. This document only optimizes the AVC control logic within the substation, without considering voltage regulation resources at the line, distribution transformer, and user levels. Summary of the Invention

[0006] This invention proposes a top-down voltage over-limit management strategy of "station-line-transformer-customer". It solves the problem of large-scale voltage over-limit through substations and lines, and solves the problem of scattered voltage over-limit through distribution areas and customers.

[0007] To achieve the above objectives, the first aspect of this invention discloses a hierarchical voltage regulation method for power distribution network-line-transformer-customer; the steps are as follows:

[0008] At the substation level, time-sharing AVC control is implemented on the 10kV busbar of the substation;

[0009] At the line level, for lines located with a power supply radius longer than the set distance or with daily voltage fluctuations greater than the set range, voltage regulation is achieved through medium-voltage line regulators and line reactive power compensation devices.

[0010] At the distribution transformer level, when there are issues with voltage exceeding the upper and lower limits, if it is an off-excitation distribution transformer, the appropriate voltage level should be calculated based on the voltage qualification rate of the user voltage, and the level with the highest user voltage qualification rate should be selected as the target level; if it is an on-load tap-changing distribution transformer, the VQC strategy should be adopted to improve the user voltage qualification rate at each time period through dynamic real-time adjustment.

[0011] At the distribution area level, when the voltage exceeds the upper limit due to the photovoltaic installed capacity reaching the maximum capacity of the distribution area, and exceeds the voltage regulation range of the substation level, line level, and distribution transformer level, it is resolved through energy storage charging and V2G mode. If the problem of exceeding the upper limit still exists, the voltage qualification rate is improved by controlling the photovoltaic output. For the problem of the voltage exceeding the lower limit at the distribution transformer outlet, it is resolved through energy storage discharge, V2G, and the installation of reactive power compensation devices. If the problem of bidirectional voltage exceeding the limit still exists for users in the distribution area after adjustment at the substation level, line level, and distribution transformer level, line voltage regulation equipment can be installed to solve the problem.

[0012] For users who still experience voltage exceedance issues after the above measures have been taken, on-site investigations should be conducted, and targeted measures such as adding household reactive power compensation devices and upgrading the service line should be adopted based on the actual situation.

[0013] Preferably, the AVC control is executed in time slots as follows:

[0014] The daily voltage curve of the 10kV busbar in the substation exhibits a W-shaped characteristic. A refined AVC control strategy is implemented in different time periods to lower the voltage threshold during the midday period (12-2 pm) and raise the voltage threshold during the evening period (18-22 pm). The threshold range should be between 10kV and 10.7kV.

[0015] Preferably, the 10kV line voltage regulator is installed at a point where the line voltage is above the upper limit or below the lower limit.

[0016] Preferably, when the voltage at the voltage regulator installation point fluctuates between 8kV and 10kV, a unidirectional voltage regulator with a voltage regulation range of 0-20% is selected; when the voltage at the voltage regulator installation point fluctuates between 8.5kV and 10.5kV, a unidirectional voltage regulator with a voltage regulation range of -5% to +15% is selected; when the voltage at the voltage regulator installation point fluctuates between 9kV and 11kV, a unidirectional voltage regulator with a voltage regulation range of -10% to +10% is selected; and for lines with new energy access, a bidirectional voltage regulator with a voltage regulation range of -20% to +20% is selected.

[0017] Preferably, the distance is set to 15km, and the range is between -7% and +7%.

[0018] Preferably, the VQC strategy is as follows:

[0019] When the power factor is less than 0.9, if the voltage is lower than 0.9U... N The reactive power compensation devices are grouped and activated up to the upper limit of the power factor. If the voltage still does not meet the requirements, the tap changer of the on-load tap changer is adjusted to boost the voltage. If the voltage is higher than 1.07U... N Adjust the on-load tap changer to reduce the voltage. After the voltage is qualified, disconnect the reactive power compensation devices in groups until the power factor is between 0.9 and 1. U N This is a qualified voltage.

[0020] Preferably, when the power factor is between 0.9 and 1, if the voltage is below 0.9U... N Adjust the tap changer of the on-load tap changer to boost the voltage. If the tap changer is in the highest position, capacitors should be connected in groups. If the voltage is higher than 1.07U... N Adjust the tap changer of the on-load tap changer to reduce the voltage. If the tap changer is in the lowest position, disconnect the capacitors in groups.

[0021] Preferably, for the problem of bidirectional voltage overruns in the distribution area, if the bidirectional overruns occur at the same time, a voltage regulator for the distribution area line should be installed to solve the problem; if the bidirectional overruns occur at different times, an on-load tap changer should be installed to solve the problem.

[0022] Preferably, the on-site investigation and problem-solving specifically involves:

[0023] For cases where the grid-connected photovoltaic capacity is less than the maximum capacity of the distribution area, check for any unauthorized photovoltaic connections. If such connections are found, correct the photovoltaic system records. Also check for any anomalies in the monitoring data. If any anomalies are found, calibrate the monitoring equipment and add a data verification step to the equipment.

[0024] A second aspect of the present invention discloses a hierarchical voltage regulation system for power distribution network-line-transformer-customer, based on the above-mentioned hierarchical voltage regulation method for power distribution network-line-transformer-customer, comprising:

[0025] The substation voltage regulation module is used to perform time-sharing AVC control on the 10kV busbar of the substation.

[0026] The line voltage regulation module is used to regulate the voltage at the line level through the medium-voltage line voltage regulator and the line reactive power compensation device;

[0027] The distribution transformer voltage regulation module is used for tap adjustment of off-load distribution transformers or VQC control of on-load tap-changing distribution transformers to solve the problem of distribution transformer level voltage exceeding the upper limit and voltage exceeding the lower limit.

[0028] The transformer substation voltage regulation module is used to solve the problems of voltage exceeding the upper limit, voltage exceeding the lower limit, and voltage exceeding the limit in both directions in the transformer substation.

[0029] The low-voltage user upgrade module is used to carry out targeted upgrades for individual low-voltage users.

[0030] The present invention has the following advantages:

[0031] 1. The voltage regulation strategy proposed in this invention fully considers that voltage quality problems are systemic and comprehensive issues. Through coordinated governance at all levels, it solves the problem of insufficient voltage regulation resources at the current single level.

[0032] 2. Fully consider the current status of large-scale grid connection of distributed photovoltaic power under the new power system construction model, and meet the current needs for bidirectional voltage management where high and low voltages coexist.

[0033] 3. By optimizing the control logic at each level, the value of voltage regulation resources at each level is fully explored, which increases the power grid regulation capability and improves the safety, reliability and economy of power grid operation.

[0034] 4. Clarify the voltage regulation logic of various types of voltage regulation resources at all levels, provide guidance for grassroots work, achieve optimal control, and realize efficient governance. Attached Figure Description

[0035] Figure 1 This is a schematic diagram showing the distribution of voltage regulation resources for stations, lines, and substations.

[0036] Figure 2 Flowchart of the hierarchical voltage regulation strategy for the "station-line-transformer-customer" distribution network;

[0037] Figure 3 This refers to the voltage regulation principle of the power distribution network. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this application are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, other embodiments obtained by those skilled in the art without creative effort are all within the protection scope of this invention.

[0039] The first embodiment of the present invention provides a hierarchical voltage regulation method for power distribution network-line-transformer-customer. The present invention will be further described in detail below with reference to the accompanying drawings.

[0040] To achieve reactive power balance and stable voltage at all times and in all locations within the regional power grid, this invention divides the distribution network into five domains: station domain, line domain, transformer domain, distribution area, and user domain. The distribution of voltage regulation resources for stations, lines, transformers, and users is shown in the appendix to the specification. Figure 1 As shown in the appendix to the specification, the principle of the voltage regulation strategy of this invention is as follows. Figure 3 As shown.

[0041] As per the instruction manual Figure 2 As shown, the voltage regulation strategy for the distribution network "station-line-transformer-customer" is as follows:

[0042] Step 1: Optimize the AVC control strategy of the substation to solve the problems of widespread voltage exceedance and reactive power flow. Due to the backfeeding of photovoltaic power, the voltage rises layer by layer, resulting in high voltage concentration for users between 12:00-14:00. The daily voltage curve of the substation's 10kV busbar exhibits a W-shaped characteristic, with significant downward adjustment potential during the midday period. Therefore, a refined AVC control strategy is implemented in different time periods to lower the voltage threshold during the midday period. Specifically, this time-period adjustment refers to lowering the voltage threshold between 12:00-14:00 and raising the voltage threshold between 18:00-22:00, thereby achieving the effect of "voltage reduction during peak hours and voltage increase during off-peak hours." The threshold should be determined based on actual site requirements, within the range of 10kV-10.7kV.

[0043] Refinement refers to the AVC control strategy achieving refined voltage regulation within the substation by using multiple finely grouped capacitor banks. For example, the original 8000kvar capacitor bank can be refined into 8 banks of 1000kvar each, and fine voltage control can be achieved through multi-group switching. The threshold range should preferably be between 10.1-10.6kV.

[0044] Step 2: At the line level, for lines with long power supply radii or large daily voltage fluctuations, voltage regulation is achieved by adding line voltage regulators and line reactive power compensation devices to medium voltage lines. Generally, a power supply radius longer than 15km is considered long. If a line has node voltages that are 7% lower than the qualified voltage and 7% higher than the qualified voltage, then the daily voltage fluctuation is considered large.

[0045] Regarding the installation location: The installation point of the 10kV line voltage regulator should be at the upper or lower limit of the line voltage. Generally, the installation point of the unidirectional voltage regulator is at 1 / 2 or 2 / 3 of the distance from the load, and the installation point of the bidirectional voltage regulator is at 1 / 3 or 1 / 2 of the distance from the load.

[0046] Regarding installation capacity: The capacity of a unidirectional voltage regulator is determined based on the electrical load after the installation point; the capacity of a bidirectional voltage regulator is determined based on the electrical load and power supply capacity before and after the installation point; the voltage regulator capacity can be selected as 1000kVA, 2000kVA, or 4000kVA.

[0047] Regarding the selection of voltage regulation range: In radial distribution networks, when the voltage at the regulator installation point fluctuates between 8kV and 10kV, a unidirectional voltage regulator with a regulation range of 0-20% is selected; when the voltage fluctuates between 8.5kV and 10.5kV, a unidirectional voltage regulator with a regulation range of -5% to +15% is selected; and when the voltage fluctuates between 9kV and 11kV, a unidirectional voltage regulator with a regulation range of -10% to +10% is selected. For lines with high-power hydropower and distributed photovoltaic, wind power, and other new energy sources, a bidirectional voltage regulator with a regulation range of -20% to +20% is generally selected.

[0048] Step 3: At the distribution transformer level, if it is an off-excitation distribution transformer, calculate the appropriate tap level based on the voltage qualification rate of user voltage over the past 1-3 months, and select the tap level with the highest user voltage qualification rate as the target tap level; if it is an on-load tap-changing distribution transformer, a VQC strategy (voltage-reactive power joint control strategy) should be adopted to realize reactive power compensation switching and on-load tap-changing tap level adjustment, thereby improving the user voltage qualification rate at various time periods through dynamic real-time adjustment; the switching mechanism of the reactive power compensation device in the distribution area should not be limited to the power factor, but should incorporate voltage into the control logic. The joint control logic of the on-load tap-changing distribution transformer and the reactive power compensation device should be as shown in the table below (U in the table) N For acceptable voltage, the acceptable power factor range is between 0.9 and 1.

[0049]

[0050]

[0051] Step 4: At the distribution area level, for cases where high photovoltaic (PV) capacity in the distribution area leads to severe voltage exceeding the upper limit and exceeds the voltage regulation range of Steps 1-3, prioritize resolving the high voltage issue through energy storage charging and V2G (Vehicle-to-Grid) mode. Secondly, consider controlling PV output, including but not limited to rigid and flexible control, to improve voltage compliance. PV output is considered user asset and should be controlled last. For voltage exceeding the lower limit at the distribution transformer outlet, address this through energy storage discharge, V2G, and reactive power compensation devices. If, after adjustments in Steps 1-3, bidirectional voltage exceeding the limit still exists for users in the distribution area, replace the distribution transformer with an on-load tap-changing transformer or install line voltage regulators in the distribution area.

[0052] Specifically, the voltage regulation at the distribution substation level addresses two main issues: First, the voltage at the substation's output point exceeds the regulation range. For voltage exceeding the upper limit due to a large amount of photovoltaic grid connection, priority is given to resolving the high voltage issue in the substation through energy storage charging and V2G modes. Secondly, photovoltaic output control methods such as flexible or rigid control, or the installation of deeply regulating transformers, are considered. For low voltage at the transformer's output due to factors such as a medium-voltage line power supply radius exceeding 15km or high power, voltage management can be achieved by combining line reactive power compensation devices with deeply regulating transformers. Second, the issue of bidirectional voltage exceeding limits for substation users. For problems where the voltage difference between the beginning and end points exceeds 17% due to an excessively long power supply radius and high power in the substation, if the bidirectional exceedance occurs simultaneously, a substation line voltage regulator should be installed; if the bidirectional exceedance occurs at different times, it can be resolved by installing on-load tap-changing transformers.

[0053] Step 5: For users who still have voltage over-limit problems after taking steps 1-4, on-site investigation should be carried out, and targeted modifications should be made based on the actual situation, such as adding household reactive power compensation devices and modifying the service line.

[0054] The specific process of on-site investigation and problem-solving is as follows: For cases where the grid-connected photovoltaic capacity is less than the maximum capacity of the distribution area, check for unauthorized photovoltaic connections. If unauthorized connections are found, correct the photovoltaic system ledger. Check for anomalies in monitoring data. If anomalies are found, calibrate the monitoring equipment and add a data verification step to the equipment. For cases where voltage exceeds limits, targeted modification measures include adding household reactive power compensation devices and modifying the service line.

[0055] As one of the most prominent substantive features of this invention and one of the significant advancements it brings to the prior art, this invention aims to regulate voltage and achieve the most efficient voltage regulation by aggregating various voltage regulation resources. It clarifies the operational logic of each level and type of voltage regulation resource at the station, line, transformer, and customer levels in a top-down logical order.

[0056] A second embodiment of the present invention provides a hierarchical voltage regulation system for power distribution network-line-transformer-customer, comprising the following modules:

[0057] The substation voltage regulation module is used to reduce the voltage threshold of the substation between 12:00 and 14:00 and to increase the voltage threshold of the substation between 18:00 and 22:00.

[0058] The line voltage regulation module is used to regulate the voltage at the line level through the medium-voltage line voltage regulator and the line reactive power compensation device;

[0059] The distribution transformer voltage regulation module is used to solve the problem of voltage exceeding the upper limit and voltage exceeding the lower limit of the distribution transformer level by adjusting the tap of the off-load distribution transformer or controlling the VQC of the on-load tap-changing distribution transformer.

[0060] The transformer substation voltage regulation module is used to solve the problems of voltage exceeding the upper limit, voltage exceeding the lower limit, and voltage exceeding the limit in both directions in the transformer substation.

[0061] The low-voltage user upgrade module is used to carry out targeted upgrades for individual low-voltage users.

[0062] This disclosure can be a system, method, and / or computer program product. A computer program product may include a computer-readable storage medium having computer-readable program instructions loaded thereon for causing a processor to implement various aspects of this disclosure.

[0063] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A hierarchical voltage regulation method for power distribution network-line-transformer-customer, characterized in that, Includes the following steps: At the substation level, time-sharing AVC control is implemented on the 10kV busbar of the substation; The specific implementation of AVC control in different time periods is as follows: The daily voltage curve of the 10kV busbar in the substation exhibits a W-shaped characteristic. A refined AVC control strategy is implemented in different time periods to lower the voltage threshold during the midday period (12-14 o'clock) and raise the voltage threshold during the evening period (18-22 o'clock). The threshold range should be between 10kV and 10.7kV. At the line level, for lines located with a power supply radius longer than the set distance or with daily voltage fluctuations greater than the set range, voltage regulation is achieved by installing voltage regulators and reactive power compensation devices on medium-voltage lines. When there is a voltage limit violation problem at the distribution transformer level, if it is an off-excitation distribution transformer, the appropriate level is calculated based on the voltage qualification rate of the user voltage, and the level with the highest user voltage qualification rate is selected as the target level; if it is an on-load tap-changing distribution transformer, the VQC strategy is adopted to improve the user voltage qualification rate in each time period through dynamic real-time adjustment. At the distribution area level, when the voltage exceeds the upper limit due to the photovoltaic installed capacity of the distribution area reaching the maximum capacity of the distribution area, and exceeds the voltage regulation range of the substation level, line level, and distribution transformer level, it is solved by energy storage charging and V2G mode. If the problem of exceeding the upper limit still exists, photovoltaic output control is adopted. For the problem of the voltage at the distribution transformer outlet exceeding the lower limit, it is solved by energy storage discharge, V2G, and the installation of reactive power compensation devices. If, after adjustment at the substation, line, and distribution transformer levels, there is a problem of bidirectional voltage exceeding limits for users in the distribution area, line voltage regulating equipment can be installed to solve the problem. The installation point for a 10kV line voltage regulator is located where the line voltage is close to the upper or lower limit. When the voltage at the voltage regulator installation point fluctuates between 8kV and 10kV, a unidirectional voltage regulator with a voltage regulation range of 0-20% should be selected. When the voltage at the voltage regulator installation point fluctuates between 8.5kV and 10.5kV, a unidirectional voltage regulator with a voltage regulation range of -5% to +15% should be selected. When the voltage at the voltage regulator installation point fluctuates between 9kV and 11kV, a unidirectional voltage regulator with a voltage regulation range of -10% to +10% should be selected. For lines with new energy access, a bidirectional voltage regulator with a voltage regulation range of -20% to +20% should be selected. For users who still experience voltage exceedance issues after taking the above measures, on-site investigations should be conducted, and targeted measures such as adding household reactive power compensation devices and upgrading the service line should be adopted based on the actual situation.

2. The hierarchical voltage regulation method for power distribution network-line-transformer-customer as described in claim 1, characterized in that: The set distance is 15km, and the set range is between -7% and +7%.

3. The hierarchical voltage regulation method for power distribution network-line-transformer-customer as described in claim 1, characterized in that: The VQC strategy is as follows: When the power factor is less than 0.9, if the voltage is lower than 0.9U... N The reactive power compensation devices are grouped and activated up to the upper limit of the power factor. If the voltage still does not meet the requirements, the tap changer of the on-load tap changer is adjusted to boost the voltage. If the voltage is higher than 1.07U... N Adjust the on-load tap changer to reduce the voltage. After the voltage is qualified, disconnect the reactive power compensation devices in groups until the power factor is between 0.9 and 1. U N This is a qualified voltage.

4. The hierarchical voltage regulation method for power distribution network-line-transformer-customer as described in claim 3, characterized in that: When the power factor is between 0.9 and 1, if the voltage is below 0.9U... N Adjust the tap changer of the on-load tap changer to boost the voltage. If the tap changer is in the highest position, capacitors should be connected in groups. If the voltage is higher than 1.07U... N Adjust the tap changer of the on-load tap changer to reduce the voltage. If the tap changer is in the lowest position, disconnect the capacitors in groups.

5. The hierarchical voltage regulation method for power distribution network-line-transformer-customer as described in claim 1, characterized in that: For the issue of bidirectional voltage over-limit for users in the distribution area, if the over-limit occurs in both directions at the same time, a voltage regulator for the distribution area line should be installed to resolve the issue. If the over-limit occurs in both directions at different times, an on-load tap changer should be installed to resolve the issue.

6. The hierarchical voltage regulation method for power distribution network-line-transformer-customer as described in claim 1, characterized in that: The on-site investigation and problem-solving specifically included: For cases where the grid-connected photovoltaic capacity is less than the maximum capacity of the distribution area, investigate whether there are any unauthorized photovoltaic connections. If such connections are found, correct the photovoltaic system records. Investigate whether there are any abnormalities in the monitoring data. If such data is found to be abnormal, calibrate the monitoring equipment and add a data verification step to the equipment.

7. A distribution network-line-transformer-customer hierarchical voltage regulation system, operating the distribution network-line-transformer-customer hierarchical voltage regulation method as described in any one of claims 1 to 6, characterized in that, include: The substation voltage regulation module is used to perform time-sharing AVC control on the 10kV busbar of the substation. The line voltage regulation module is used to regulate the voltage at the line level through the medium-voltage line voltage regulator and the line reactive power compensation device; The distribution transformer voltage regulation module is used to solve the problem of voltage exceeding the upper limit and voltage exceeding the lower limit of the distribution transformer level by adjusting the tap of the off-load distribution transformer or controlling the VQC of the on-load tap-changing distribution transformer. The transformer substation voltage regulation module is used to solve the problems of voltage exceeding the upper limit, voltage exceeding the lower limit, and voltage exceeding the limit in both directions in the transformer substation. The low-voltage user upgrade module is used to carry out targeted upgrades for individual low-voltage users.

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