DC Power Systems - Systems for Controlling DC Power

The DC power system supply has two electrodes, positive and negative. The potential of the positive electrode is high, and the potential of the negative electrode is low. When the two electrodes are connected to the circuit, a constant potential difference can be maintained between the two ends of the circuit, thereby forming an external circuit from the positive electrode to the negative electrode. negative current. 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.


DC Power System


1. System introduction of DC power system


The DC system is used in hydraulic and thermal power plants and various substations. The DC system is mainly composed of battery packs, charging devices, DC feeder screens, DC power distribution cabinets, DC power monitoring devices, and DC branch feeders. A huge DC power supply network all over the place provides safe and reliable working power for various subsystems such as relay protection devices, circuit breaker tripping and closing, signal systems, DC chargers, UPS, communications, etc.


2. Working principle of DC power system


There are two kinds of working principles, one is to use mains power to convert AC to DC; the other is to use DC.


2.1 AC to DC

When the mains power passes through the input switch and connects the transformer to convert the mains voltage into the designed voltage, it enters the pre-stabilization circuit. The pre-stabilization circuit is to initially stabilize the desired output voltage. The tube pressure drop between the input and output of the tube reduces the power consumption of the high-power regulating tube and improves the working efficiency of the DC power supply. The pre-stabilized power supply is generally controlled by a thyristor stepless phase-shift adjustment type and uses a relay to switch the output tap of the transformer. for voltage stabilization. After the pre-stabilized power supply and filter, the voltage obtained is basically stable, and the DC power with relatively small ripples will be accurately and quickly pressed by the high-power regulator controlled by the control circuit, and the voltage stabilization accuracy and performance will meet the standards. After the DC voltage is filtered by the filter, I can get the output DC I need. In order to get the output voltage value or steady current value I need, we also need to sample and detect the output voltage and current values. Send it to the control/protection circuit, the control/protection circuit compares the detected output voltage value and current value with the value set by the voltage/current setting circuit, and then drives the pre-regulator circuit and high-power regulator to use The DC stabilized power supply can output the voltage and current value we set. At the same time, when the control/protection circuit detects abnormal voltage or current value, the protection circuit will be activated to make the DC power supply enter the protection state.


2.2 DC power supply

Two AC incoming lines output one AC (or only one AC incoming line) through the switching device to supply power to each charging module. The charging module converts the input three-phase AC into DC to charge the battery and supply power to the closing bus load at the same time. The closing bus supplies power to the control bus through a step-down device (some designs do not require a step-down device).


Each monitoring unit in the system is managed and controlled by the main monitoring unit, and the information collected by each monitoring unit is sent to the main monitoring unit for unified management through the RS485 communication line. The main monitor can display various information in the system. Users can also query system information and realize the "four remote" function on the main monitor display screen by touching or pressing keys. System information can also be accessed through the host computer communication interface on the main monitor. Remote monitoring system. In addition to the basic unit of comprehensive measurement, the system can also be equipped with functional units such as insulation monitoring, battery inspection, and switch value monitoring, which are used to comprehensively monitor the DC system.


3. The main purpose of the DC power system


3.1 Sophistication of electrolytic capacitors and empowerment of tantalum capacitors

3.2 Resistors, relays, motors and other electronic components are experienced and routinely tested

3.3 Laboratory, electronic equipment, automatic test equipment

3.4 Electronic inspection equipment, production line equipment, communication equipment

3.5 All other occasions that need to use DC power supply


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.


4. Classification of DC power systems


4.1 Distributed

Distributed DC power supply system is a new type of DC power supply equipment, mainly used in small switch stations and user terminals, to provide reliable and uninterrupted work for secondary control lines (such as intelligent terminals such as microcomputer protection and indicator lights, analog indicators, etc.) Power supply, avoiding the failure of microcomputer protection when AC power is lost, and solving the problem of failure of microcomputer protection due to factors such as operating overvoltage and harmonics. At the same time, it can also provide DC operating power for primary switching equipment (spring mechanism vacuum circuit breaker, permanent magnet mechanism vacuum circuit breaker, electric load switch, etc.) that meet the power requirements of the device.


The distributed DC power supply has two modes of mains input and PT input, and the output mode ranges from DC220V to DC24V in various specifications, which can meet various application occasions. The maximum output power of the device can reach 600W, which can meet the needs of different loads.


The distributed DC power supply is small in size and convenient in installation and wiring, and is suitable for scattered installation in various types of switchgear. It makes it more reliable and economical than the general DC screen system (more obvious for small user terminals), saves space, reduces line loss and installation engineering, and is easy to maintain, providing a new option for the reliability of power supply in the power system .


The distributed DC power supply adopts high-frequency power supply technology, and the battery is maintained by an automatic charging management module, which greatly prolongs the service life of the battery and makes the operation of the device safer and more reliable. The device has communication and alarm functions, which can easily realize unattended remote automatic management.


4.2 Wall-mounted

The wall-mounted DC power supply is suitable for small switch stations, small user substations and small 10KV substations. Constituting the system and other characteristics; the monitoring module adopts LCD Chinese character menu display, which has perfect functions for system monitoring and battery automatic management, has four remote interfaces to connect with the automation system, provides two communication interface options of RS232 and RS485, and provides three kinds of RTU, CDT, and MODBUS Communication protocol selection.


4.3 Basic functions

The wall-mounted DC power supply mainly provides DC power for the switch opening and closing of the DC operating mechanism, protection and control circuits, accident tripping, and emergency lighting. It has functions such as battery intelligent management, system parameter setting, DC parameter detection, system fault sound and light alarm, and automatic voltage stabilization of the control mother.


4.4 Product Features

4.4.1 Wall-mounted structure, small size, space-saving, convenient installation and low cost. the

4.4.2 The rectifier module, monitoring module and step-down module are fastened in the cabinet with a compact structure.

4.4.3 The module adopts self-cooling and heat dissipation method without dust accumulation.

4.4.4 The rectifier module adopts advanced soft switching conversion technology and built-in CPU technology, with small voltage distortion, high efficiency, high precision of voltage and current stabilization, small ripple coefficient, stable and reliable. the

4.4.5 The step-down module adopts 7-level voltage regulation mode, automatic voltage regulation, no external controller and switch are needed. the

4.4.6 All components are international or domestic high-quality brand-name products with reliable quality and excellent performance. the

4.4.7 Adopt hardware low-dropout independent current sharing technology, and the unbalance degree of current sharing between modules is ≤±3%. the

4.4.8 The system has advanced battery intelligent management technology, realizes automatic management of battery charge and discharge and maintenance, and online battery temperature monitoring, prolongs battery life and reduces maintenance.

4.4.9 The unique heat dissipation technology of the screen cabinet system, through the combination of temperature monitoring and intelligent heat dissipation system, greatly improves the operating environment temperature of all equipment in the screen cabinet.


5. System Composition of DC Power System


The DC power system is mainly composed of two parts. One part is the battery screen and the other part is the DC charging screen (DC screen). The battery screen is a cabinet (800×600×2260) that can place multiple batteries. The battery in the battery screen is generally composed of 2V-12V batteries connected in series with 9 to 108 cells, and the corresponding voltage output is 110V or 220V. The batteries currently used are mainly valve-regulated sealed maintenance-free lead-acid batteries. The DC screen is mainly composed of a cabinet, a rectifier module system, a monitoring system, an insulation monitoring unit, a battery inspection unit, a switching value detection unit, a step-down unit, and a series of power distribution units such as AC input, DC output, voltage display, and current display. .


5.1 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 N1 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.


5.2 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 currently divided into two types: one is the button type and the other is the touch screen type. The monitoring system provides a man-machine interface to realize system operation parameter display, system control operation and system parameter setting.


5.3 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 currently has busbar insulation monitoring and branch insulation monitoring.


5.4 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 1-108 batteries.


5.5 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. At present, the switching value detection unit can collect 1-108 switching values and multiple passive dry node alarm outputs.


5.6 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 currently two types of step-down units, one is a stepped 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 current stepless step-down chopper technology is not very mature and often fails, so the step-down silicon chain is still widely used.


5.7 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.

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