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On the Functions of DC Systems in Substations
Release time:
2020-10-16
The DC power supply section of a power system consists of battery banks, charging equipment, DC switchboards, and other related equipment.
The DC power supply section of a power system comprises battery banks, charging equipment, DC switchboards, and other related devices. Its primary functions are: under normal operating conditions, to provide the DC control power required for circuit breaker closing; and in the event of a fault or loss of plant and substation auxiliary power, to supply operational DC power to relay protection and automatic devices, circuit breaker tripping and closing mechanisms, carrier communication systems, and plant-driven machinery powered by DC motors at power plants. The proper functioning of this system directly impacts the safe and reliable operation of the entire power system.
In the past, all substations in the power system were staffed, enabling regular inspections of the operating status of DC equipment. This allowed for the timely detection and resolution of any abnormalities, thereby ensuring the safe and stable operation of the substations. Currently, unattended substations are being widely implemented across the power system. Although the control center can obtain real-time operational data from substations via telecontrol channels, for the DC portion only a limited amount of critical information is available—namely, remote signals such as AC power supply failure of the rectifier, rectifier malfunction, DC insulation grounding, and abnormal DC bus voltage, as well as remote measurements like control bus voltage. This information fails to provide a detailed picture of the DC system’s operation; in particular, it cannot detect the very early stages of abnormal operation, and only when prolonged abnormal conditions evolve into actual faults is the issue reported to the control center. By that time, the incident has already escalated.
If abnormalities are detected and addressed promptly as soon as they first emerge, their escalation can be prevented. Therefore, equipment maintenance personnel are required to conduct regular inspections. In addition, the control of DC equipment operation is also carried out on-site by maintenance staff. However, with a large number of substations and a limited number of maintenance personnel, it is clearly impossible to ensure that inspections are completed on schedule and to the required extent. Under these circumstances, DC monitoring systems have been developed.
Its primary function is to transmit DC equipment information from each substation to the monitoring center for query, while the monitoring center can also send control commands to the substations. In this way, maintenance personnel can not only remotely monitor DC equipment from the monitoring center but also promptly detect abnormal operating conditions and take timely corrective actions, preventing such conditions from escalating into accidents. Therefore, the implementation of a DC monitoring system can save manpower and resources and enhance operational efficiency.
(1) Channel Selection
Currently, all types of information transmitted from substations to the dispatch center—such as telemetry, teleindication, remote control signals, status of major equipment, and alarm messages—are conveyed via the SCADA communication channel. These data have stringent real-time requirements, and it is undesirable for other traffic to occupy this channel, leading to congestion and disrupting normal dispatch operations. Therefore, operational information from DC equipment must be transmitted remotely over a separate channel. In addition to the SCADA channel, substations typically also have a dedicated telephone channel, which is generally used by on-site personnel and, when necessary, by other auxiliary systems such as the security alarm system. Although this channel usually remains idle, it is nonetheless a mandatory requirement; thus, it can be utilized as the information transmission channel for the DC monitoring system.
The DC monitoring system generates a small volume of data and requires only a brief channel occupancy during transmission. Consequently, it can establish a connection during operation, occupy the channel for the duration of the transmission, and then disconnect once the task is complete, thereby sharing the channel with other systems on a time-division basis and ensuring the normal operation of each system.
(2) System Composition
The monitoring center computer is connected to the public telephone network via a modem, and each monitoring unit is also connected to the telephone network through its own modem. The modems on both ends can call each other, establishing a communication link over the telephone network to exchange information. In this way, the monitoring center computer can use this communication link to acquire data from the monitoring units at various substations and transmit control commands, while the monitoring units can report daily scheduled operational data and any abnormal conditions back to the center. The system comprises three main components: the monitoring units serve as front-end systems, responsible for collecting equipment data, controlling operating states, and transmitting information; the monitoring center functions as a back-end system running on a PC, tasked with sending commands to all substation monitoring units, receiving their operational data, and performing data processing and analysis; and data communication between the two is facilitated by modems and the telephone network. The monitoring center and the monitoring units operate in a one-to-many configuration.
(3) Monitor Design
3.1 Principle of the Monitor
The monitoring system is installed at each substation; it collects operational status information from all DC equipment, performs control functions, and transmits all data to the monitoring center and other monitoring units.
The monitor is designed using an industrial PC, with its I/O ports serving both input and output functions. It can directly acquire measurement data, status information, and battery insulation condition from DC equipment, as well as perform control and adjustment operations on such equipment, including switching the charger on and off, toggling between equalization and float charging modes, adjusting the equalization and float charging voltages, and switching feeder circuits on and off. On the other hand, the microcomputer controller transmits the “four remote” signals to the RTU or dispatch center via RS-232 or modem, and sends all operational information of the DC equipment to the monitoring center where the equipment maintenance personnel are located through a modem over the telephone network.
3.2 Monitor Software Design
The monitor software consists of four modules: the communication module, the data transmission and reception module, the I/O module, and the data processing module.
The communication module is responsible for preparing the system for data transmission, including functions such as turning the modem on and off and initiating automatic dialing. The software allows users to schedule the modem to turn on and off at specific times, enabling the system to share telephone lines with other systems, such as alarm systems, on a time-division basis. Automatic dialing is triggered when an abnormal event occurs in the DC power supply system; the system automatically dials the monitoring center’s modem and transmits the corresponding alarm information.
The data transmission and reception module is primarily responsible for data exchange once the communication link has been established. The data transmitted includes: real-time operating information from each DC device; historical data stored in this monitor at fixed intervals over the past 24 hours; and all alarm messages generated within the past 24 hours. The data received includes control and adjustment commands from the monitoring center, among other items.
The functions of the I/O module include providing a human-machine interface for the monitor, as well as the monitor’s acquisition and control of various DC equipment parameters.
The data processing module is the core component of the monitor. On one hand, it processes the data acquired by the I/O module and, according to the configured time interval, stores the daily data in the database for subsequent transmission to the monitoring center via the data transceiver module. This data is updated on a daily basis. On the other hand, it analyzes the control commands received from the transceiver module and then provides the corresponding control information to the I/O module for execution.
(4) Monitoring Center Design
The monitoring center is a microcomputer that runs the monitoring center’s backend software.
The monitoring center software primarily comprises four key components: a system designed for remote monitoring of DC equipment in unattended substations, which leverages the substation’s existing telephone communication channels to enable functions such as remote DC equipment monitoring, historical operation data retrieval, and reporting of operational anomalies. In power systems, the DC power supply section consists of battery banks, charging equipment, DC switchboards, and other related devices. Its primary functions include providing closing DC power to circuit breakers within the substation under normal operating conditions; and, in the event of a fault, supplying power to plant and station auxiliary systems. The system further includes a communication module, a database formation module, a master control module, and a report printing module.
The communication module performs the same function as the monitoring device’s communication module: it dials the monitoring device’s modem at the substation to establish a communication link, sends control command messages downstream, and can also be called by the remote end to receive uploaded information. This module is developed using Visual Basic 5.0; its sole task is to dial the modem and establish the communication link in accordance with the communication requirements. It does not handle the transmission of specific data—this responsibility lies with the main control software.
The data processing module serves two primary functions: first, it establishes an information database for the DC equipment at each substation; second, it organizes and stores the daily data collected from each substation, thereby creating a historical database of DC equipment across all substations. Users can flexibly configure databases for individual substations and for all DC equipment within each substation based on actual operational needs, and can also conveniently maintain all in-station information. Once maintenance is completed, the system automatically saves the updated data to the database, offering high flexibility and simple, user-friendly operation. In addition, the monitoring systems at each substation periodically transmit daily data to the monitoring center, which, upon receipt, stores the data in the corresponding substation-specific database, thus building a historical database that can be accessed by the report-printing system for query purposes.
The master control software provides a human–machine interface based on the Windows 95 operating system, allowing users to interact with on-screen graphics using a mouse and keyboard. By simply clicking with the mouse on the graphical elements corresponding to specific equipment components, users can query and control those devices; alternatively, device parameters can be entered via the keyboard to enable remote adjustment of operational settings. Maintenance personnel can perform queries and controls on in-station equipment through the mouse and keyboard. Once the communication module establishes a communication link, it can retrieve data transmitted from remote locations, display it on the screen, and forward this data to the data processing module for further handling. In addition, it packages operator control commands into data packets and transmits them to remote sites. This module serves as the core of the entire system.
The report printing module queries the historical database maintained by the data processing module based on operational requirements, generates daily reports, monthly reports, and other types of reports, and produces corresponding data curves for analysis and retrieval.
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