Abstract: In response to the low power distribution efficiency and poor economic performance of existing data center power stations, and leveraging the advantages of energy storage stations and substation, a multi-station integrated DC power supply system scheme based on energy storage inverters (PCS) is proposed. This scheme establishes a unified, rational, and efficient information management and integrated DC power supply system among the three stations. Through the research of circuit topology and control technology of a multi-port low-voltage DC power supply system, it achieves unified information management and energy control among the three stations. Compared with traditional high-voltage DC solutions, this scheme boasts high operational reliability, low construction costs, and high energy conversion efficiency, providing a theoretical reference for multi-station integration construction.
Keywords: PCS; Data Center Station; DC Power Supply System; Multi-Station Integration
1. Introduction
In traditional power grids, substation responsibilities are limited to unidirectional energy transmission. The entire grid's electricity is supplied by power plants, with relatively simple dispatch control from the dispatch center. As distributed grids are constructed and developed, substation functions will also include energy storage and power conversion, transforming energy transmission from unidirectional to bidirectional. This makes power dispatch extremely complex, with a dramatic increase in the data required to be collected. Therefore, in addition to collecting and transmitting data from substation and energy storage facilities, a data center station is built on the foundation of energy storage stations to enable the function of charging energy storage devices during power surplus and supplying power to the grid when power is insufficient. This article proposes a multi-station integration solution for data centers, energy storage, and substation based on a high-voltage direct current power supply scheme for data centers, offering significant advantages in energy consumption, safety, reliability, post-maintenance, work efficiency, and environmental protection.
2 System Solutions
2.1 System Solution Design
Tongli Comprehensive Energy Service Center has adopted PCS and power electronic transformers to construct a low-voltage DC grid, but the large capacity PCS and power electronic transformers require a significant amount of land, are expensive to invest in, difficult to install, and have a high debugging difficulty, making them unsuitable for energy stations. After the co-location of energy storage, data centers, and substation construction, the individual advantages of the three can be integrated, aligning with the development direction of green data center construction and providing a strong guarantee for ubiquitous Internet of Things. The integration of the three offers the following advantages:
Energy storage can provide backup power for data centers, reducing the configuration capacity of UPS systems and lowering the land use and construction costs for data centers.
(2) The energy storage PCS operates at low power for extended periods. If its structure can be reused for a DC distribution network, it can significantly enhance the utilization of station equipment, further conserving resources.
Based on the aforementioned considerations, a power supply strategy for DC data centers utilizing the redundant capacity of energy storage power station PCS has been proposed. The specific plan is as follows:
Figure 1: Multi-site Integrated Direct Current Power Supply System Diagram
As shown in the figure, the power supply solution consists of an independent low-voltage DC microgrid composed of multiple PCS and DC/DC units, featuring the following characteristics:
(1) Each PCS unit supplies a 120kW DC load, accounting for 20% of the PCS total capacity. The DC sides of two PCS units within each energy storage container are connected to two separate 750V DC busbars, forming a dual power supply.
(2) The DC power supplied by PCS is collected at the nearby energy storage container via a 750V busbar and transmitted to the data center building through a DC cable. The number of sets, N, in the scheme is determined based on the current-carrying capacity of the 750V cable, with N = Current-Carrying Capacity / 120kW.
To prevent power from being sent back through the PCS DC transmission lines, diodes are installed on each transmission line to ensure one-way conduction.
(4) Each group of two 120kW DC/DC units powers a supply source, connected to the 750V DC bus on the data center side, to supply the 120kW data center cabinets. In the event of a power failure on one line, the cabinets can automatically switch to the other power source, establishing a dual power supply mode for the data center.
(5) For A-class data center loads, a third power supply must be configured. This third power supply can be formed by introducing a 10kV cable from a nearby station through a 10MVA 10kV/400V transformer to create a 400V AC bus, from which multiple outgoing lines are connected to the AC side of the PCS.
(6) In this scheme, each DC/DC power supply load forms a small 220V local DC microgrid, while multiple energy storage PCS supply N groups of DC/DC DC loads, creating an independent 750V DC microgrid. Based on the total capacity of the DC loads, multiple independent low-voltage DC microgrids can be constructed. This topology of local networks at various DC low-voltage levels offers high power supply reliability and is a trend in the development of low-voltage DC distribution networks, holding significant engineering demonstration value.
2.2 System Operation Method
The PCS DC power supply solution features three operating modes:
(1) The battery discharge method involves part of the battery's power supplying the DC/DC direct current load, while the other part flows into the grid through the PCS. This operation mode is adopted during peak electricity usage times.
(2) The battery charging method employs power supply from the PCS to both the battery and the DC/DC DC load. This operation mode is used during off-peak electricity consumption.
(Battery) During normal operation, the battery operates without charging or discharging, with power supplied by the PCS to the DC/DC converter for直流 load.
Under the three operating modes, the DC/DC converter is a stable DC load, requiring only a constant power on the high-voltage side and a constant voltage output on the low-voltage side, without needing to adjust strategies based on the operation status of other equipment; the PCS needs to adjust the DC side power reference value based on the DC/DC power, thus achieving a constant power output; the battery can adopt a constant-voltage control strategy, charging and discharging based on the PCS power conditions. With the aforementioned control strategies, the three components can achieve autonomous power balance without the need for coordinated control, aligning with the development trend of DC distribution networks.
2.3 Reliability Analysis
Due to the stringent reliability requirements for power supply in data centers, this site must establish a third power supply as a backup to ensure timely switching in the event of a power outage in either of the two main power supply lines within the substation. Under normal operation, the two operational power supplies are redundant and hot-standby. A fault or maintenance of any component will not disrupt the data center's power supply, as normal power switching alone can ensure uninterrupted operation. When both main power supply lines are lost, servers must instantly switch to energy storage batteries for power. During this period, the third power supply must be switched on to replace the battery within a limited time to prevent over-discharge of the energy storage battery. This plan considers using a 10kV dedicated line as the third power supply for this data center.
Comparison with HVDC Power Supply Solutions
The data center currently employs a HVDC (High-Voltage Direct Current) power supply method, as illustrated in Figure 2. Initially, the 10kV lines are stepped down to AC380V by transformers, forming an AC bus, which is then rectified and converted by the HVDC system. The HVDC system actually involves a cascade of AC/DC and DC/DC topologies, first converting AC380V to DC700V, followed by a DC/DC conversion to 240V, as shown in Figure 3. In addition, the HVDC's DC240V is paralleled with a battery bank for backup. This paper proposes a DC power supply method based on energy storage PCS, which supplies IT loads through DC/DC conversion from the energy storage system.
Figure 2 HVDC Power Supply Solution
Figure 3(a) AC/DC Conversion
Figure 3(b) DC/DC Converter
The topology of the energy storage PCS and DC/DC power electronic transformers in the DC power supply scheme, as shown in Figure 4. From the topology diagram, it can be observed that the energy storage PCS in the DC power supply scheme is equivalent to the AC/DC conversion in the HVDC power supply topology, while the DC/DC power electronic transformer is equivalent to the DC/DC conversion in the HVDC power supply topology.
Figure 4(a) PCS Transformation Topology
Figure 4(b) DC/DC Power Electronic Transformer Topology
Here is a comparison of the two proposals:
(1) In terms of power supply efficiency, the current research results show that transformer efficiency is approximately 98%, PCS is around 98%, isolated DC/DC is about 97%, and HVDC efficiency is roughly 96%. The power supply efficiency of the two schemes is essentially consistent.
(2) In terms of equipment quantity, the DC solution saves a primary transformer, primary AC/DC conversion, and the same energy storage can serve as a UPS, thereby saving a portion of lead-acid batteries. Compared to the HVDC solution, it offers a significant advantage in terms of space and equipment count.
(3) In terms of equipment utilization, the DC solution reuses the energy storage PCS for AC/DC conversion and employs energy storage as a certain UPS, enhancing the utilization rate of lead-acid batteries, thereby giving the DC solution an advantage in equipment utilization.
(4) In terms of reliability, the DC/DC power supply for direct current voltage offers superior quality, including higher stability, reduced voltage ripple, and improved ability to withstand temporary drops in AC voltage.
(5) Economically speaking, the battery in the energy storage PCS power supply solution can supply DC/DC direct current loads during peak electricity usage hours and recharge during off-peak hours, offering good cost-effectiveness.
Ankoray Battery Monitoring System Introduction: Equipment Selection
4.1 Overview
Ankorui's ABAT series lead-acid battery online monitoring system is an online battery monitoring product that can provide early warnings for failing lead-acid batteries and battery balancing, in compliance with ANSI/TIA-942 standards requirements.
The system features the ability to monitor battery voltage, internal resistance, and internal temperature, with easy installation, maintenance, and connectivity. It is primarily composed of the ABAT-S module, ABAT-C module, and ABAT-M collector. Users can access alarm and real-time data, set parameters, and opt for a monitoring platform for networked centralized management.
4.2 System Networking
4.3 Hardware Selection
5 Conclusion
This article investigates the low-voltage DC busbar network topology structure composed of parallel energy storage battery systems. By mastering the reliability design of a multi-port low-voltage DC power supply system through dual-power systems, core device multi-redundant configurations, and energy flow transmission protection strategies, it constructs the overall control system for a multi-port low-voltage DC power supply system. It designs control strategies for power measurement ports, load measurement ports, and bidirectional busbar energy, and combines these strategies with a coordinated control method for multi-port low-voltage DC power supply. This achieves maximized energy utilization. The feasibility and effectiveness of the scheme are verified through topology structure comparisons.
[Reference]
Lingbo, A Brief Introduction to the Working Principle of UPS Uninterruptible Power Supply and Its Application in Power Systems[J]. Science Wind, 2013(23): 92-92
Wang Bichao, Chen Yankui, Liu Yuzhen, Wang Hao, Xu Lika, Li Guozhu, Zhan Jinguo. Research on Direct Current Power Supply Systems Based on Multi-Station Integration [J]. Specialized Technology, 2020(02): 75-76.
Ankorri Enterprise Microgrid Design and Application Manual, 2022.5 Edition
