Selection and integration of adjustable load in multiple scenarios

In the context of the “carbon peaking and carbon neutrality goals”, the new electricity systems are evolving towards source-load friendly interactions. At the same time, the increasing necessity for adjustable loads to participate in the power balance of the new electricity systems is highlighted. Consequently, a large-scale source-grid-load design scheme is proposed. It employs three independent communication channels to facilitate communication between users and intelligent grid-load terminals. Based on analysis of the individual loads and equipment characteristics of each user, principles for user shunt load access principle and adjustable loads are formulated by combining the maximum duration, frequency constraints, and compliance costs. Furthermore, it proposes construction plans for both the customer side in a single distribution room and the customer side distributed across different locations. According to the experimental analysis of 245 users, the response time of the experimental system is less than 600 ms. These advancements enhance the emergency response speed and responsiveness, contributing to the advancement of precise, real-time, and lean management of the demand side of the power grid.


Introduction
With the adjustment of industrial structure and the continuous improvement of people's living standards, the seasonal variations in electricity demand have gradually become more apparent, especially with the increasing use of air conditioning, which has led to a more pronounced peak in electricity demand.In recent years, the comprehensive commencement of the Ultra-High-Voltage Direct Current (UHVDC) projects implies that faults in high-power UHVDC transmission lines may have a significant impact on the power grid.Therefore, it is necessary to comprehensively manage the allocation of electrical resources and loads.This can be achieved through the classification, categorization, and regional management of load resources.It enables real-time and precise control of loads for different categories of users directly by the grid operators, avoiding the complete disconnection of entire substations or power lines.This approach fosters mutual support between power sources, the power grid, and user loads, ultimately minimizing the societal impact of power grid failures [1][2][3] .
The source-grid-load system refers to an operational mode where interactions between power sources, loads (electricity consumers), and the electrical grid take place in various forms.The goal is to enhance the dynamic power balance capabilities of the electrical system in a more economical, efficient, and secure manner while maximizing the utilization of energy resources.The system primarily consists of the grid-side system control center, power-side outgoing switches, and customerside system terminals.Communication between the control center and system terminals is established through optical fiber connections.In the event of a power grid emergency, the source-grid-load system can automatically disconnect customer-side branch switches at the millisecond level via system terminals to ensure grid safety [4][5] .
Grid-load interaction encompasses various interactions between electricity loads (i.e., electricity consumers) and the electrical grid.These interactions include adjustable loads, which can dynamically adapt their electricity consumption based on real-time grid demands to maintain supply-demand balance, as well as the integration of distributed energy resources.Most importantly, grid-load interaction plays a pivotal role during emergency situations by swiftly adjusting or disconnecting loads to safeguard the stability of the electrical system [6] .Currently, numerous researchers have conducted extensive studies on grid-load interaction systems.For instance, starting from grid-load interaction, optimal load-shedding strategies have been proposed by considering a comprehensive assessment of grid frequency and voltage.This has effectively enhanced the efficiency of emergency load control within the grid [7] .Additionally, in terms of rapid user load control, grid-load interaction terminals have been designed to meet the requirements of fine-grained load data collection, real-time communication, and secure rapid control across multiple control stations [8] .These efforts hold significant importance for improving the stability of the power system and achieving efficient operations.However, there remains room for improvement, particularly in enhancing the intelligence and automation levels of grid-load interaction systems, especially in the context of fine-grained coordination of the power supply network and end-users.
Adjustable loads refer to the instantaneous load monitored and recorded by the source-grid-load system terminal just before the branch switches connected to the customer source-grid-load systems trip.These loads are interruptible without causing substantial impacts on the production or daily life of electricity consumers.They possess both load characteristics and power source characteristics, as they can be started or stopped in real time based on the dynamic changes in power sources and the electrical grid [9][10][11] .
Based on the above problems, this paper proposes a large-scale source-grid-load interaction system to realize the friendly interaction between the grid-load interactive terminal and the user's production load in large enterprises.The system effectively manages and controls the adjustable load through the research on the selection and access method of adjustable load in multi-dimensional scenarios to meet the stability, reliability, and efficiency requirements of power systems in different environments, aiming to improve power system flexibility, resilience, and management efficiency.

Design of intelligent grid-load interactive terminal
The overall design of the large-scale source-grid-load friendly interactive system is shown in Figure 1.The intelligent grid-load interactive terminal installed in the user's power distribution room responds to instructions issued by different master stations in three communication directions.A communication loop is three independent communication channels that are physically isolated.
The first channel is the emergency load control and fast-cutting command from the load center station.A power grid stability emergency control center station is set up in the UHV converter station to monitor the operation status of the UHV power grid.When a fault signal is detected, it immediately sends a stability control emergency load-shedding command to the user's smart grid-load interactive terminal in the predetermined strategy.The terminal cuts off the user's corresponding internal shunt load according to the settings of the current emergency control load-shedding outlet trip matrix.This channel uses a dedicated line connection and sends the current responsive load value to the central station during normal operation for the central station to adjust the strategy.Overall scheme of large-scale source-grid-load friendly interaction system.The second channel is the partition control command from the main station of the grid-load interactive system.The main station of the grid-load interactive system is the main management station of the intelligent grid-load interactive terminal installed on the user's site.It is responsible for the terminal's file management, daily operation monitoring, operation strategy, and parameter adjustment.The receiving dispatch D5000 system gives the "sub-emergency control" according to the emergency response of the power grid System" and "zonal load control" command according to the established strategy of the main station.A secondary emergency load-cutting instruction is issued to the designated terminal, and the terminal is set according to the current emergency control and load-cutting matrix to cut off the corresponding shunt load of the user.The terminal can also cut off the user's diversion load one by one according to the requirements of the marketing orderly power consumption management system.
The third channel is the meter reading command from the management information area to complete the data collection task of the user's meter.
According to the above business requirements, the intelligent grid-load interactive terminal adopts optical fiber information intranet communication.The data transmission has the security protection capability, and the terminal has the encryption function.In terms of functional design, the intelligent grid-load interactive terminal is mainly designed for real-time measurement and control.It has functions such as AC sampling, remote control, remote signaling, event sequence recording, etc.It can realize the collection and control of electrical quantities on multiple lines.It is a kind of high-speed network based on A real-time measurement and control device with rapid response as its main purpose [12] .

User adjustable load selection
In the event of a large power grid accident, it is necessary to ensure the safety of the power grid and reduce the social impact of the power outage as much as possible.In order to increase the speed of UHV emergency response and reduce the impact area, it is necessary to carefully distinguish the shunt loads within the user and connect non-main production loads, temporary interruptions, and production loads that cause small losses as adjustable loads to the grid-load interactive terminal inside.

User load and device classification
Different industries, different categories, and different natures of power users have large differences in electrical equipment, load characteristics, and electricity consumption habits.During the research on the construction plan of this project, users are divided into the following 6 categories.Each user's load and equipment Features are as follows.
(1) Characteristics of high-energy-consuming enterprises such as steel and cement: Such industries have large electricity loads, and the nature of production is generally continuous operation in three shifts and is not affected by time, season, or climate.Adjustable loads include those of electric furnaces, scrap steel processing lines, cement mills, raw meal mills, and other high-power-consuming electrical equipment that can be quickly deactivated.Adjustable loads have strong rapid control capabilities.
(2) Continuous production enterprises such as the chemical industry: This type of industry has a large electricity load, and the nature of production is mostly continuous operation in three shifts and is not affected by time period, season, and climate.Under the premise of not jeopardizing safe production, the adjustable equipment only includes auxiliary production equipment such as office electrical equipment and material pumps.In contrast, the main production equipment is continuous and important (high-risk), so it is not suitable to participate in rapid regulation.
(3) Non-continuous production enterprises such as machinery: Most of the users in this industry are ordinary industrial users with level three loads.Most enterprises adopt a day shift system and non-continuous production, which is not affected by time, season, and climate.Adjustable loads include non-productive loads such as office lighting, computers, and air conditioners, as well as production equipment such as electric saws, cutting machines, and machine tools.The corresponding equipment can be quickly cut off.
Street lighting, lighting, and other power users: Such industries generally have a small power load and are mainly concentrated at night.The load can be removed quickly, but due to the small power capacity, the effect is not obvious enough.
Electric power users such as hotels, shopping malls, and office buildings: The load operation of users in these industries is mainly concentrated in the morning peak, waist peak, and evening peak phases.The operating time is relatively fixed, involving many electrical equipment and dense personnel, so they are more dependent on a reliable power supply.User loads in this industry are basically important (high-risk) electricity loads, and adjustable loads are basically air-conditioning loads [13][14][15] .
Electricity users in important units such as schools and hospitals: The electricity load in this type of industry is mainly concentrated from Monday to Friday, mostly for office electricity.The electricity consumption period is fixed, and the electricity elasticity coefficient is relatively stable.Users such as schools and hospitals have electricity loads during summer peak hours, which are basically guaranteed and do not have adjustable capabilities.The adjustable loads are only electrical equipment, such as exterior wall lighting and lawn lighting.

Adjustable load model
Adjustable load is an important resource for demand-side management.Coordinating with rotating standby to maintain the balance of system power and load can effectively relieve the pressure of the power system during the peak period of power supply, improve system reliability, and reduce the economic loss of insufficient power generation.In a large-scale source-grid-load friendly interaction system, the model of the adjustable load is: (1) Maximum adjustable duration constraint: where To.t is the adjustable time; To, max is the maximum adjustable duration for user o; T is the total duration.
(2) Adjustable frequency constraints The interruptible load frequency constraints are as follows: , ,max 1 where Ncut, t is the actual number of adjustments that can be; Ncut,max is the maximum number of adjustions.
(3) Adjustable load cost During scheduling, the call cost of adjustable loads is as follows: where fo represents the total compensation price of the user o; fo,1 are the capacity compensation costs; fo,2 are the power compensation costs of the user in the t period; Cq,o is the capacity compensation price of the user o; po,max is the maximum adjustable capacity of user o; Cp,o is the power compensation price of the user o; po,t is the actual adjusted power of the user o; uo,t indicates whether the adjustable load is called during the t period, if uo,t =1, uo,t was called, uo,t =0 means it is not called.

User shunt load access principle
(1) User load circuit connection selection order: It is selected step by step according to the main incoming line, the auxiliary load and non-production load that will have a lower impact after the interruption, and the production load that may cause a certain loss after the interruption.The importance of road controllable load is gradually increasing.
(2) After the terminal wiring is completed, the corresponding relationship between tripping rounds and load circuits is set through parameter configuration.Each round can be arbitrarily corresponding to multiple load switches.It is recommended that load switches with different degrees of influence cannot be configured in the same round.In principle, 1-2 rounds correspond to auxiliary loads and non-production loads that are less affected after the interruption; 3-6 rounds correspond to production loads that may cause certain losses after interruption; 7-8 rounds correspond to the user's main and backup master switches.
(3) The monitorable load connected to the user's 1-6 wheel switch shall not be less than 50% of the user's normal power load and shall not be higher than 70% of the user's normal production load.

User load access and debugging
For users with a single power distribution room design, the high and low voltage switches are arranged in the same power distribution room, generally for 10 kV users, with small capacity and concentrated load.This type of user has the advantages of small construction difficulty, less workload, convenient maintenance, and low cost.
The user side of the single power distribution room design adopts different load access methods according to the user's main wiring diagram structure, power supply quantity, production process, etc., but mainly includes three aspects of load: total load, important production load, and auxiliary nonproduction load.The user-side construction plan of a single power distribution room is shown in Figure 2. What is marked in blue is the incoming line main switch or main transformer low-voltage main switch connected to the user, which is used to monitor the total load of the user in real-time; what is marked in red is the important production load of the user, which is used for energy efficiency monitoring; what is marked in green is the incoming line main switch or main transformer lowvoltage main switch that is connected to the user.The auxiliary non-production loads of incoming users, such as air conditioners, canteens, dormitories, and other non-production loads, are used to participate in the source network load response.The specific access switch loads are shown in Table 1.When the source network load responds in real-time, only the switch marked in green is disconnected, and the normal production of the user will not be interrupted.This type of user borrows the user's internal cable channel to lay optical fiber or communication cables from the terminal of the main power distribution room to the distribution room of the user.It uses smart instruments in the distribution room to realize the collection and monitoring of the user's switching equipment on the spot.The specific construction scheme is shown in Figure 3.The technical solution has the advantages of small construction difficulty, little impact on the user's original equipment, high real-time data collection, remote control function, and strong anti-interference ability.
By organizing the accurate load-cutting time verification work of the source-gird-load, the number of households participating in the actual cut verification this time is 245.After receiving the emergency load-cutting instruction from the central station, 250,000 kW, the adjustable load is quickly cut off, and the response time is less than 600 ms, which verifies the overall design goal of the system.

Conclusion
The overall scheme of the large-scale source-grid-load interactive system proposed in this paper analyzes the power consumption characteristics of various loads.It adopts three isolated independent communication channels to respond to the instructions issued by different main stations.It takes realtime measurement and control as the main purpose.It has many functions, such as AC sampling, remote control, remote signaling, etc., so as to achieve rapid response and realize communication between users and intelligent grid-load terminals.
By promoting precise and real-time demand-side lean management, the large-scale source-gridload friendly interactive system can better cope with the changes in power forms, such as the transformation of large-scale power grids, large-scale clean energy, and electric vehicle access in recent years.The emergency handling and fault recovery requirements of various types of UHV faults can be met through multi-level, batch-by-batch coordination and precise control so as to ensure the stable and reliable operation of the power grid.

Figure 1 .
Figure 1.Overall scheme of large-scale source-grid-load friendly interaction system.The second channel is the partition control command from the main station of the grid-load interactive system.The main station of the grid-load interactive system is the main management station of the intelligent grid-load interactive terminal installed on the user's site.It is responsible for the terminal's file management, daily operation monitoring, operation strategy, and parameter adjustment.The receiving dispatch D5000 system gives the "sub-emergency control" according to the emergency response of the power grid System" and "zonal load control" command according to the established strategy of the main station.A secondary emergency load-cutting instruction is issued to the designated terminal, and the terminal is set according to the current emergency control and load-cutting matrix to cut off the corresponding shunt load of the user.The terminal can also cut off the user's diversion load one by one according to the requirements of the marketing orderly power consumption management system.The third channel is the meter reading command from the management information area to complete the data collection task of the user's meter.According to the above business requirements, the intelligent grid-load interactive terminal adopts optical fiber information intranet communication.The data transmission has the security protection capability, and the terminal has the encryption function.In terms of functional design, the intelligent grid-load interactive terminal is mainly designed for real-time measurement and control.It has functions such as AC sampling, remote control, remote signaling, event sequence recording, etc.It can realize the collection and control of electrical quantities on multiple lines.It is a kind of high-speed network based on A real-time measurement and control device with rapid response as its main purpose[12] .

Figure 2 .
Figure 2. User-side construction plan of a single power distribution room.

Figure 3 .
Figure 3.Total score of off-site user side construction plan.

Table 1 .
Access load of single power distribution room user.different places in the main distribution room are composed of the main transformer and several distribution rooms.High-voltage switchgear and low-voltage switchgear are arranged in different power distribution rooms.They are generally users of 20 kV and above.The capacity of this type of user is relatively large, and the load is relatively dispersed.