Urban rail transit stations contain numerous low-voltage electrical systems, such as communication, signaling, integrated monitoring, environmental monitoring, office automation, access control, automatic fare collection, automatic fire alarm, platform screen doors, emergency lighting, and substation integrated automation. These systems are responsible for monitoring the passenger transport environment, transmitting information, and guiding passengers in subway stations. They are important or particularly important first-level loads and require highly reliable power supplies to ensure power quality and continuity.
In previous rail transit projects, each station's low-voltage electrical system was equipped with its own power supply system, resulting in drawbacks such as redundant equipment configuration, low utilization rate, large footprint, and economic inefficiency. During operation and maintenance, there was a lack of dedicated power supply maintenance personnel for each system, leading to inadequate or improper maintenance, which resulted in reduced battery capacity and failure to meet backup time requirements. There have been instances in domestic subway systems where train service was disrupted due to battery issues.
With the development of manufacturing processes and application technologies for power electronic equipment, large-capacity power supply systems and advanced control technologies are being used maturely in communication and power systems, providing favorable conditions for the integration of various low-voltage power supply systems in rail transit engineering. Preliminary attempts at integrating station low-voltage power supply systems have already been made in existing subway projects in China, such as the Beijing Subway Airport Line.
Purpose and Principles of Station Power System Integration Purpose of Station Power System Integration
The batteries and devices of various power systems are integrated and centrally located to facilitate unified maintenance and management. Centralized layout reduces equipment room space and lowers station construction costs. Hardware integration reduces redundant equipment configuration, enabling resource sharing and saving equipment investment. Unified selection of power supply brands facilitates equipment bidding.
Key Principles of Station UPS Integration Station integration should primarily follow these principles.
(1) The integrated power system should meet the technical requirements of all integrated systems for power supply, and ensure the reliable and safe operation of each system.
(2) The integrated power supply room should be located close to the center of the weak current load to facilitate the layout of the feeder cable, reduce line voltage drop and line loss, and improve the safety and reliability of the power supply line.
(3) The integration of power systems should be based on the technical requirements such as the load characteristics and power demand of each system being integrated, so as to make the necessary hardware configuration.
(4) Power integration should minimize the mutual influence between power supplies of different systems.
Power demand analysis for various low-voltage systems; Load type and demand analysis for various low-voltage systems.
(1) Communication system. Load type: Computer network communication equipment of subsystems such as transmission, wireless, official telephone, private telephone, closed-circuit television, broadcasting, and clock are required to be able to continuously supply AC220V power.
(2) Signaling system. Load type: signaling system, passenger information system, and other computer and network equipment and control equipment; power requirements: able to continuously supply AC380/220V power.
(3) Integrated monitoring system (including environmental monitoring and access control). Load type: computer, network equipment, environmental monitoring and access control equipment; power requirements: continuous AC220V power supply.
(4) Automatic Fare Collection (AFC) system. Load type: computer, network equipment, AFC terminal equipment. Power requirements: able to continuously supply AC220V power.
(5) Office automation system. Load type: computer, network equipment: power supply requirement: continuous AC220V power supply.
(6) Platform screen door system. Load type, power requirements for platform screen door control and drive equipment: control equipment requires a continuous AC220V control power supply, drive equipment requires a DC110V drive power supply.
(7) Automatic fire alarm system. Load type: Computer network equipment, alarm controllers, etc. require a continuous AC220V power supply.
(8) DC auxiliary power supply for substation. Load type: device power supply, control power supply, motor power supply. The power supply is required to continuously supply DC220V power.
(9) Station emergency lighting system. Load type: Includes evacuation lighting and backup lighting. Power supply requirement: Able to continuously supply AC220V power.
UPS power integration scope analysis
Based on the power requirements and characteristics of each low-voltage system, the following analysis is performed:
(1) Communication, signal, integrated monitoring (including environmental and equipment monitoring, access control) office automation system automatic ticketing and fire alarm are mainly capacitive loads such as computers and network equipment, requiring AC380/220V power supply, and are suitable for power supply.
(2) The load of the platform screen door system is mainly distributed on the platform, and the drive motor of the platform screen door is an inductive load with a relatively low power factor and a large inrush current. If a power supply integration system is used, it will significantly affect the voltage stability and reduce the power quality. Therefore, the platform screen door system is not suitable for power supply integration system. It is advisable to set up an independent backup power supply and not include it in the scope of power supply integration system.
(3) Substations and station low-voltage systems are generally located at both ends of the station, with the power supply close to the load center of each low-voltage system. Therefore, the distance between the substation load and the power supply room spans the entire station, the power supply line is circuitous, and the line voltage drop is large. Therefore, the substation operating power supply should be set up independently.
(4) Emergency lighting is mainly an inductive load, and its load is distributed throughout the entire station area with many points and a wide coverage. The power supply lines are circuitous, so the emergency lighting system should not be included in the power supply integration scope. In summary, communication, signaling, integrated monitoring (including environmental monitoring and access control), office automation, automatic fare collection, and automatic fire alarm systems are included in the power supply integration scope, that is, each system shares a power supply. Platform screen door systems, substation auxiliary power supplies, and emergency lighting are each set up with independent backup power supplies and are not included in the power supply integration scope.
Power integration system composition
The low-voltage electrical systems in the subway have very high requirements for power reliability. Power outages can cause subway shutdowns and even personal injury, resulting in incalculable losses. Therefore, each station's integrated power system is equipped with two sets of power supply devices (including rectifiers, inverters, and batteries), load synchronization controllers, and intelligent control units. The composition of the integrated power system is shown in Figure 1.
UPS power integration system hardware composition
(1) Power Supply and Battery Setup. Two power supply units will be installed. During the project implementation, the power demand data for each piece of equipment will be collected to calculate a more accurate capacity. To ensure long-term safe and reliable operation, it is recommended that the maximum load capacity of the UPS be 60% to 80% of the UPS's rated output power. Based on relevant engineering experience, the total load of the low-voltage systems in a typical subway station is generally about 130kVA, so the capacity of each UPS can be selected as 160kVA.
The two battery banks offer flexible operation. One battery bank can be deactivated for maintenance without affecting operation. It boasts high reliability and maintainability.
(2) Load synchronization controller. When the two UPS power supplies are not synchronized, the load voltage may be disturbed during the switching process, and the current may be too large when the switch is turned on, causing the machine to stop. If the switching is to be reliable when the power supplies are not synchronized, the interruption time must be increased (usually 13ms). This will be very detrimental to the load (the interruption time of the computer load power supply should not be greater than 6ms).
Setting up a load synchronization controller can ensure that the voltage amplitude, waveform, and frequency output by the two UPS power supply units are the same, achieving a smooth and reliable switching.
(3) Intelligent control unit. The intelligent control unit is responsible for controlling and managing the UPS power supply unit's batteries and uploading monitoring information of the UPS power supply integration system.
UPS power integration system operation mode
The UPS power integration system draws one AC380V power supply from the two 0.4V busbars of the step-down substation as the input power for the two UPS units.
During normal operation, the two UPS units operate redundantly in parallel, sharing the load current to provide AC380/220V uninterruptible power to various power systems. The capacity of each UPS unit can meet the total system capacity requirements. An intelligent control unit is also provided, responsible for sequentially disconnecting various loads according to a set schedule.
The system configuration diagram is shown in Figure 1.
During operation, if a UPS fails, it can be automatically disconnected from the UPS parallel system through selective operation. The system can still provide high-quality power with high availability to its load, thereby improving the reliability of the UPS power supply system.
The system allows for scheduled maintenance/fault repair operations on another UPS unit in a parallel UPS system without power supply, while the UPS is powered by its inverter, thus improving the maintainability of the UPS power supply system.
UPS power supply integration considerations
Reasonable and regular maintenance and upkeep can reduce equipment failure rates, extend equipment lifespan, and ensure the availability of the power system. After a period of use, UPS systems require regular inspection, including checking for any abnormalities in their appearance and the presence of any unpleasant odors. Since UPS systems are constantly powered, the batteries remain in a charging state for extended periods, leading to decreased battery activity. Therefore, UPS systems need to be periodically (generally once a month) discharged to maintain battery activity.
After power system integration, the entire power system is highly independent and clearly demarcated from other disciplines, allowing for the establishment of a power maintenance department staffed with professional technicians to perform maintenance and management. This prevents serious equipment damage caused by erroneous operations, thus reducing equipment reliability and lifespan. Furthermore, the provision of professional maintenance and repair tools, coupled with a comprehensive maintenance and repair management system, ensures that the power system equipment is in good operating condition, enabling the advanced power system to play a crucial role in critical moments.
Application of UPS power supply integration in urban rail transit
