DC Systems - Systems for Hydro and Thermal Power Plants

"Low-voltage direct current system" is applied to hydraulic and thermal power plants, various substations and other users who use direct current equipment to provide direct current for signal equipment, protection, automatic devices, emergency lighting, emergency power supply and circuit breaker opening and closing operations. The power supply for the power supply. The DC system is an independent power supply, which is not affected by the generator, plant power consumption and system operation mode, and in the event of an external AC power interruption, it is an important device that ensures that the backup power supply-battery continues to provide DC power.


The high-voltage direct current system is mainly used for power transmission. The current high-voltage direct current transmission voltage can reach ±1000KV, the transmission current is 5000A, and the transmission capacity is 8000MW (8 million kilowatts). It is the main component of the West-East Power Transmission.


DC system


1. Brief description of DC system


The DC system is a very important part of the substation. Its main task is to provide power for relay protection devices, circuit breaker operations, and various signal circuits. The normal operation of the DC system is related to the correct operation of relay protection and circuit breakers, which will affect the safe operation of substations and even the entire power grid.


The DC system is composed of charging equipment, battery, DC screen, DC split screen, insulation monitoring device, load, etc.


2. System composition of DC system


2.1 Rectifier module system

The power rectification module is a stand-alone module that rectifies AC power into DC power. It is usually named by the size of the passing current (such as 2A module, 5A module, 10A module, 20A module, etc.). According to different design concepts, it can also be divided into: Wind Cold modules, independent air duct modules, self-cooling modules, self-energy air-cooling modules and self-energy self-cooling modules. It can be used in parallel to achieve N+1 redundancy. The module output is 110V, 220V stable and adjustable DC voltage. The module itself has relatively complete various protection functions such as: input overvoltage protection, output overvoltage protection, output current limiting protection and output short circuit protection, etc.


2.2 Monitoring system

The monitoring system is the control and management core of the entire DC system. Its main tasks are: to conduct long-term automatic monitoring of each functional unit and battery in the system, to obtain various operating parameters and states in the system, and to process them in time according to the measurement data and operating states. , and based on this, the system is controlled to realize the fully automatic management of the power system and ensure the continuity, reliability and safety of its work. The monitoring system is divided into two types: one is the button type and the other is the touch screen type. The monitoring system provides man-machine interface operation to realize system operation parameter display, system control operation and system parameter setting.


2.3 Insulation monitoring unit

The DC system insulation monitoring unit is a device for monitoring the insulation of the DC system. It can monitor the line-to-ground leakage resistance in real time. This value can be set according to specific conditions. When the line-to-ground insulation is reduced to the set value, an alarm signal will be issued. The DC system insulation monitoring unit includes bus insulation monitoring and branch insulation monitoring.


2.4 Battery inspection unit

The battery inspection unit is a device for inspection of the online voltage of the battery. It can detect the voltage of each battery in real time. When the voltage of any battery is higher or lower than the setting, an alarm signal will be sent, and the monitoring system can display which battery is faulty. The battery inspection unit can generally detect 2V ~ 12V batteries and patrol inspection 1 ~ 108 batteries.


2.5 Switch value detection unit

The switching value detection unit is a device for on-line detection of switching value and output of alarm stem node. For example, in the whole system, which circuit breaker trips or which fuse is blown, the switch value detection unit will send out an alarm signal, and can display which circuit breaker has a fault trip or which fuse is blown through the monitoring system. device fuse. The switching value detection unit can collect 1 to 108 switching values and multiple passive dry node alarm outputs.


2.6 Buck unit

The step-down unit is a step-down and voltage-stabilizing device. It is a combined mother voltage input step-down unit, and the step-down unit then outputs to the control mother to adjust the control mother voltage within the set range (110V or 220V). When the combined bus voltage changes, the step-down unit automatically adjusts to ensure the output voltage is stable. The step-down unit is also rated by the size of the output current. There are two types of step-down units, one is a step-down silicon chain, and the other is a stepless step-down chopper. The step-down silicon chain has 5-stage step-down and 7-stage step-down, and the voltage adjustment point is 3.5V, that is to say, when the combined mother voltage rises or falls by 3.5V, the step-down silicon chain automatically adjusts and stabilizes the control mother voltage. The stepless step-down chopper is a step-down module. It is smaller than the step-down silicon chain. It has no voltage adjustment point, so the output voltage is more stable than the step-down silicon chain. There are also overvoltage, overcurrent, and battery overdischarge. and other functions. However, the step-down step-down chopper technology is not very mature and often fails, so the step-down silicon chain is still widely used.


2.7 Power distribution unit

The power distribution unit is mainly the devices used in the DC panel to realize functions such as AC input, DC output, voltage display, and current display, such as: power cords, terminal blocks, AC circuit breakers, DC circuit breakers, contactors, lightning protection devices, Shunts, fuses, transfer switches, push button switches, indicator lights, and current and voltmeters, etc.


3. Equipment classification of DC system


3.1 DC screen system

The general name of DC screen is intelligent maintenance-free DC power supply screen, referred to as DC screen, and the general model is GZDW. Simply put, a DC screen is a device that provides a stable DC power supply. (When the input has 380V power supply, it is directly converted to 220V, and when there is no input (mains power and backup power), it is directly converted to battery power supply - DC 220V, in fact, it can also be said to be an industrial special emergency power supply) . The power operating power supplies in power plants and substations are all DC power supplies today, which provide power for control loads, power loads, and DC accident lighting loads, and are the basis for control and protection of contemporary power systems. The DC screen is composed of a power distribution unit, a charging module unit, a step-down silicon chain unit, a DC feed unit, a power distribution monitoring unit, a monitoring module unit and an insulation monitoring unit. It is mainly used in small and medium-sized power plants, hydropower stations, various substations, and other users using DC equipment (such as petrochemical, mining, railway, etc.) in the power system. Electrical protection and fault lighting and other occasions.


The DC screen is a new type of DC system with digital control, protection, management and measurement. The monitoring host part is highly integrated and adopts a single-board structure (All in one), which includes functions such as insulation monitoring, battery inspection, grounding line selection, battery activation, silicon chain voltage stabilization, and microcomputer central signal. The host is equipped with a large LCD touch screen, and various operating states and parameters are displayed in Chinese characters. The overall design is convenient and concise, and the man-machine interface is friendly, which is in line with user habits. The DC screen system provides powerful functions for remote detection and control, and has remote control, remote adjustment, telemetry, remote signaling functions and remote communication interfaces. Through the remote communication interface, the operating parameters of the DC power system can be obtained remotely, and the operating status and fixed values can also be set and modified through this interface to meet the requirements of power automation and power system unattended substations; equipped with standard RS232/485 serial Line interface and Ethernet interface, which can be easily incorporated into the power station automation system.


3.2 DC Power System

DC power supply (DC power) has two electrodes, positive and negative. The potential of the positive electrode is high, and the potential of the negative electrode is low. Form a current from the positive pole to the negative pole. A constant water flow cannot be maintained solely by the difference in water level, but a constant water level difference can be maintained by means of a water pump to continuously send water from a low place to a high place to form a steady water flow. Similar to this, the electrostatic field generated by the charge alone cannot maintain a constant current, but with the help of a DC power supply, the non-electrostatic effect (referred to as "non-electrostatic force") can be used to move the positive charge from the negative electrode with a lower potential. Return to the positive electrode with higher potential through the power supply to maintain the potential difference between the two electrodes, thus forming a stable current. Therefore, a DC power supply is an energy conversion device that converts other forms of energy into electrical energy supply circuits to maintain a steady flow of current. The non-electrostatic force in a DC power supply is directed from the negative pole to the positive pole. When the DC power supply is connected to the external circuit, a current from the positive pole to the negative pole is formed outside the power supply (external circuit) due to the promotion of the electric field force. And inside the power supply (internal circuit), the action of non-electrostatic force makes the current flow from the negative pole to the positive pole, so that the flow of charge forms a closed cycle.


4. Substations for DC systems


The DC power supply part of the power system is composed of battery packs, charging equipment, DC panels and other equipment. Its function is: to provide closing DC power supply for the circuit breaker in the substation under normal conditions; in case of failure, when the power supply of the plant and station is interrupted, it will provide relay protection and automatic devices, circuit breaker tripping and closing, carrier communication, The factory machinery driven by the DC motor of the power plant provides working DC power. Whether it is normal or not directly affects the safe and reliable operation of the power system.


In the past, each substation in the power system was manned, and the operation status of the DC equipment could be checked regularly, so abnormal phenomena could be detected and dealt with in time to ensure the safe and stable operation of the substation. The power system promotes unattended substations. Although the dispatching center can obtain real-time information on the operation of the substation through the telecontrol channel, it can only obtain a small amount of important information for the DC part (including: remote signaling - AC power failure of the charger, charging Machine failure, DC insulation grounding, abnormal DC power supply voltage; remote measurement - control bus voltage). It cannot reflect the detailed information on the operation of the DC system, especially it cannot detect the abnormal operation of the system at the beginning, and the dispatch is not sent until the long-term abnormal operation develops into a fault. At this time, the accident has expanded. If the abnormal phenomenon can be detected and dealt with in time when it first appears, the abnormal situation can be prevented from expanding. Therefore, equipment maintenance personnel are required to conduct regular inspections on it. In addition, the control of DC equipment operation is also performed on-site by maintenance personnel. There are many substations and few maintenance personnel, so it is obviously impossible to guarantee the completion on schedule and quantity. In this case, the DC monitoring system came into being. Its main function is to send the DC equipment information of each substation to the monitoring center for its query, and the monitoring center can also send control commands to each station. In this way, maintenance personnel can not only remotely monitor the DC equipment in the monitoring center, but also discover the abnormal state of the equipment in time and deal with it in time, without waiting for it to develop into an accident. Therefore, the establishment of the DC monitoring system can save manpower and material resources and improve work efficiency.


4.1 Channel selection

All kinds of information sent from the substation to the dispatching center, such as telemetry, remote signaling, remote control, main equipment status and alarm information, etc., are transmitted through the telecontrol channel. These information have high requirements for real-time performance and do not want other information to occupy And make the channel crowded, affect the normal work of scheduling. Therefore, the operation information of the DC equipment must be transmitted remotely through another channel. In addition to the remote control channel in the substation, there is also a telephone channel, which is generally used by the staff when working on site, and other auxiliary systems such as security alarm systems are used when necessary. Usually this channel is idle, but it must be set, so it can be used as the information channel of the DC monitoring system.


The amount of data information in the DC monitoring system is small, and the channel time is short when sending. In this way, you can dial up during work, occupy the channel, hang up after the end, and use the channel time-sharingly with other systems, thus ensuring the normal operation of each system.


4.2 System configuration

The monitoring center computer is connected to the telephone network through a modem. The monitor is also connected to the telephone network through a modem. Both modems can call each other, establish a communication link through both modems and the telephone network, and exchange information. In this way, the computer of the monitoring center can take the information of the monitors of each station and send control commands through this communication link, and the monitors of each station can also report the daily timing operation data and abnormal situation information to the center. The system consists of 3 parts: the monitor is the front-end system, which is responsible for the collection of equipment data, the control of the operating status and the transmission of information; the monitoring center is the background system, which runs on the PC and is responsible for the monitoring of all substations The controller sends commands, receives its operation data, and processes and analyzes the data; the data communication between the two depends on the establishment of modem and telephone network. Monitoring Center and Monitor are one-to-many systems.


4.3 Monitor design

4.3.1 Monitor principle

The monitor is a system installed in each substation. It collects the operating status information of each DC equipment, controls it, and sends each data information to the monitoring center and other monitoring units.


The monitor is designed with an industrial computer, and its I/O ports are used for input and output. It can directly acquire information such as measurement quantities, state quantities, and battery insulation status from DC equipment, and can also control and adjust DC equipment, such as charging The switching action of the machine, the change of the equalization and floating charge, the change of the equalization and floating charge voltage, and the opening and closing of the feeder, etc. On the other hand, the microcomputer controller sends the four-remote signal to the RTU or dispatching center through RS232 or modem, and sends the operation information of all DC equipment through the modem to the monitoring center where the equipment maintenance personnel are located through the telephone network.


4.3.2 Monitor software design

The monitor software is composed of 4 modules: communication module, data transceiver module, I/O module and data processing module.


The role of the communication module is to prepare for communication for data transmission, including the functions of opening and closing the modem and automatic dialing. The software can be set to turn on and off the modem at regular intervals, which enables this system to use the telephone channel in time-sharing with other alarm systems. Automatic dialing is to automatically dial the modem of the monitoring center and send corresponding alarm information to it when an abnormal event occurs in the DC equipment.


The data sending and receiving module is mainly responsible for data sending and receiving after the communication link is established. The data sent includes: the operation information of each DC equipment at that time; the fixed time historical data stored in the monitor within 24 hours; all the alarm information within 24 hours. The received data includes control and regulation commands from the monitoring center, etc.


The functions of the I/O module include: providing the man-machine input interface of the monitor, collecting and controlling the quantity of each DC equipment by the monitor.


The data processing module is the core module of the monitor. On the one hand, it processes the data obtained by the I/O module, and stores the daily data in the database according to the set time interval, and waits for the data transceiver module to send it to the monitoring center. These data are updated daily. On the other hand, it analyzes the control commands received by the transceiver module and provides control information to the I/O module.


4.3.3 Monitoring Center Design

The monitoring center is a microcomputer on which the background software of the monitoring center runs.


The software of the monitoring center mainly includes 4 parts. A system for remote monitoring of DC equipment in an unattended substation is proposed. It uses the telephone channel in the station to realize remote monitoring of DC equipment, query of DC equipment operation history, and equipment operation. Abnormal situation reporting and other functions. When the factory and station are part of the communication module, the database formation module, the main control module and the report printing module.


The function of the communication module is the same as that of the monitor. It is used to dial the monitor modem of the substation, establish a communication link, and send down control command information. In addition, it can also be called by the other party to receive the uploaded information. This module is developed with Visual Basic 5.0. It only needs to dial the modem and establish a communication link according to the communication requirements. The specific data that can be sent has nothing to do with it, and the main control software part is responsible for processing.


The data processing module has two functions: one is to form the information database of the DC equipment of each station; the other is to organize the daily data of each station collected every day into the database to form a historical database of all substation DC equipment. According to the actual situation, the user can flexibly establish the database of each substation and all DC equipment in the station, and flexibly maintain all the information in the station. After maintenance, the system automatically stores the database, which is flexible and easy to operate. In addition, each substation monitor sends the daily data to the monitoring center regularly every day, and the latter stores it in its own database according to the name of each station after receiving it, forming a historical database for the query of the report printing system.


The main control software provides a man-machine interface, which is based on the Windows 95 operating system, and uses the mouse and keyboard to operate the graphics on the screen. As long as the mouse is used to select the graphic elements corresponding to the equipment components, the equipment can be queried and controlled, and the parameters of the equipment can also be input with the keyboard to achieve the purpose of remotely adjusting the operating parameters of the equipment. The maintenance personnel can query and control the equipment in the running station through the mouse and keyboard. After the communication module establishes the communication link, it can open the data information sent from the remote place, display it on the screen, and hand over the data to the data processing module for processing. In addition, it also sends the operator's control commands to the remote place in the form of data packets. It is the core of this system.


According to the needs of the work, the report printing module queries the historical database saved by the data processing module, makes daily reports, monthly reports and other reports and makes corresponding data curves for analysis and query, and finally checks for errors.


5. Monitoring and maintenance of DC system


Battery normal operation inspection items


5.1 The indoor temperature of the battery is between 5°C and 35°C, which is the best working environment temperature for the battery. However, it should not exceed 30°C for a long time, so as to prevent the battery capacity from being seriously reduced and the operating life shortened due to excessive ambient temperature.


5.2 The battery voltage and electrolyte level are normal, the temperature of the battery casing and pole is normal, and the battery casing cover has no deformation or seepage.


5.3 The battery room should be dry, well-ventilated, and well-lit. The guardrail or net of the explosion-proof lamp should be firm and reliable to prevent the battery from short-circuiting due to falling.


5.4 Smoking is strictly prohibited in the battery room, and it is strictly forbidden to carry and store flammable items.


5.5 The floating charge voltage difference between the batteries is <±50mV.


5.6 All joints should be fastened without loosening.


5.7 The charger should work in an environment where the room temperature is not lower than -10°C and not higher than 40°C, the average relative humidity of the air does not exceed 90% (daily average ≤95%), there should be no conductive and explosive dust around, and no corrosion Metals and gases and vapors that destroy insulation. The inspectors mainly monitor whether the AC input voltage value, the output voltage and current value of the charging device, the battery pack voltage value, the DC bus voltage value, the floating charge current value and the insulation voltage value are normal for the DC power supply device in operation. The inspection personnel should check the working status of the DC power supply device and the microcomputer monitor, various signal lights, and sound alarms every day. If it is abnormal, it should exit the operation in time and notify the maintenance personnel to repair it.


6. System usage of DC system


Application of DC system: Widely used in hydraulic and thermal power plants, various substations and other users using DC equipment (such as power plants, substations, distribution stations, petrochemical, steel, electrified railways, real estate, etc.), for signal equipment, Protection, automatic devices, accident lighting, and circuit breaker opening and closing operations provide DC power. It is also widely used in communication departments, computer rooms, hospitals, mines, hotels, and high-rise buildings. The reliable emergency power supply has a wide range of uses. In addition, the heart of the DC system is the battery, and the scientific maintenance of the battery is the core work of the DC system.

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