[ Abstract ] This paper analyzes the communication characteristics of digital substation automation systems, focuses on the construction of a three-layer digital substation network solution based on MOXA Ethernet switches, introduces the key technologies and features of MOXA switches, and provides three case studies of digital substation communication systems.
Keywords: substation, digitalization, industrial Ethernet, MOXA switch, MOXA embedded management machine
0 Introduction
Digital substations are an essential stage in the journey towards intelligent substations that form a smart grid. Understanding the construction and system characteristics of digital substations is crucial for laying a solid foundation for building future intelligent substations. So, what exactly is a digital substation? Simply put, it's a substation that uses primary and secondary equipment as digitized objects, relies on a high-speed network communication platform, standardizes digitized information to achieve information sharing and interoperability, and uses network data to automate functions such as data measurement and monitoring, control and protection, and information management. This article focuses not on the digital substation itself, but on the high-speed network communication equipment responsible for building it. Specifically, this refers to dedicated power Ethernet switches. These devices must meet the highest levels of electromagnetic interference immunity, industrial reliability, and environmental adaptability requirements.
1. Ethernet Architecture for Digital Substations
1.1 General Ethernet Architecture
Digital substations typically have a three-layer architecture: the station control layer, the bay layer, and the process layer. However, in common practice, Ethernet switches are often placed in the station control layer and the process layer, while Ethernet switches that originally belonged to the bay layer are incorporated into the station control layer. This de facto commonality is often due to the actual needs of the project. But IED devices still strictly adhere to the three-layer architecture.
The switch network at the station control layer is typically deployed in a dual-star topology, a structure inherited from the consistent requirements of power automation systems. This approach offers advantages such as easy cabling and mutual redundancy between the two networks. Currently, there are two switch network solutions for the process layer: one still uses a dual-star topology, and the other uses a ring network. The former is the mainstream solution in China, while the latter is the mainstream solution for digital substations abroad. Each of these network solutions has its advantages: With a dual-star topology, when one communication line fails, the process-layer IEDs can seamlessly switch between the two backup lines, ensuring real-time data exchange. However, the dual-star topology requires two copies of the switches, increasing costs. With a ring network, when any segment of the link fails, the ring network can complete self-healing switching within milliseconds, ensuring almost uninterrupted communication, but its real-time performance is slightly inferior. Since a single-ring network architecture requires only one set of switches, some hardware costs can be saved. Therefore, the process-layer single-ring network solution can be considered a compromise, sacrificing some real-time performance for lower costs.
1.2 System Communication Requirements The communication requirements for the digital substation automation system are as follows:
(1) Protection against harsh electromagnetic interference (EMI) environments: The network equipment complies with the IEC 61850-3 substation environmental certification or the IEEE 1613 environmental and testing requirements for communication network equipment in power distribution stations, providing the best EMI shielding and fault-free communication capabilities.
(2) High availability of the network, with a redundant Ethernet structure to achieve secure data communication;
(3) Robust fiber optic connection: Fiber optic cable connection has the characteristics of noiseless and long-distance transmission, and the bandwidth scalability meets the requirements of future upgrades.
(4) The network switch has a modular design with multiple optical ports, gigabit and 100 Mbps ports, and should meet zero packet loss under full-rate forwarding conditions;
(5) It can withstand extreme environmental changes. The temperature boundary conditions of the substation environment vary greatly, and the substation local area network must be able to operate reliably under wide temperature conditions.
2. MOXA Digital Substation Solution The MOXA 3-layer Ethernet architecture-based digital substation solution is shown in Figure 1.
Figure 1 MOXA Digital Substation Solution
2.1 Overall Network Architecture
The entire substation monitoring system adopts a dual-star network connection. The station control layer is built using MOXA PT7728 power switches, while the process layer uses PT7324 power switches. When a system network fault occurs, alarm information can be output through the relays of the field switches, or the fault point and cause can be reported to the station control layer administrator via email or SNMP trap, facilitating timely judgment and maintenance. The process layer uses traditional instrument transformers, converting their synchronously acquired signals into digital frames through MU merging units and transmitting this information to the bay layer through switches, thus achieving fully digital operation in the monitoring of primary and secondary equipment.
2.2 Advantages of the Solution
The advantages of MOXA's digital substation solutions include:
(1) The dual-star connection effectively avoids the risk of system failure when a single network fails, enabling digital communication to be backed up bidirectionally between networks;
(2) When the backbone network of the monitoring substation fails, it will not affect the communication of the main network;
(3) When the system is expanded, the risk of ring network de-ringing is avoided. With the help of MU equipment and switch equipment, the digital expansion of the system can be quickly realized, which improves the monitoring efficiency of the substation.
2.3 Core Technologies
The core technologies of the MOXA Ethernet architecture include:
(1) The PT7728 high-performance power switch used in the station control layer meets the industrial-grade performance requirements of substation automation systems (IEC 61850-3, IEEE 1613). The PT-7728 can build high-performance gigabit Ethernet backbone networks and redundant ring networks, and has the function of 24/48 VDC or 110/220 VDC/VAC redundant power input. While improving the stability of network communication, it can also save wiring. The modular design provides users with a more relaxed and flexible networking method.
(2) The PT7728 top-level switch supports Turbo Ring and RSTP/STP (IEEE 802.1W/D); IGMP Snooping and GMRP filtering of multicast packets; supports port-based VLANs, IEEE 802.1Q VLANs and GVRP protocols, which can easily realize network management; improves the decision-making mechanism through QOS-IEEE 802.1p/1Q and TOS/DiffServ; adopts 802.3ad and LACP to optimize network bandwidth; supports IEEE 802.1X and HTTPS/SSL to enhance network security; SNMP V1/V2/V3 for different levels of network management; adopts RMON to improve network monitoring capabilities; ABC-01 automatic backup configurator; bandwidth management can prevent unpredictable network conditions, port locking only allows authorized MAC addresses to access; port mirroring is used for online debugging; abnormal events are automatically output with alarm information via E-mail or relays; automatically restores the IP address of connected devices; and Line-swap can be quickly restored.
(3) MOXA's new generation PT series switches have hardware and software functions that support IEEE 1588V1/V2, and when used as a transparent clock, they support both E (End) to E and P (Peer) to P dual modes, achieving time synchronization accuracy at the nanosecond level. Their GMRP function can forward multicast packets to specific groups, thus enabling accurate time synchronization for multi-layered cascading modes in large substations.
3 MOXA Digital Substation Application Cases
3.1 Digital Substation Network Implementation Plan (Fuzhou)
The network scheme for digital substations in Fuzhou is shown in Figure 2.
Figure 2 MOXA Digital Substation Solution
The Fuzhou digital substation network solution is introduced as follows:
(1) Architecture Design. An IEC61850-based substation automation system is composed of IEC61850 backend software, measurement and control and protection devices, remote control workstations, and protocol conversion devices for connecting to other intelligent devices. IEC61850 is implemented at the bay and station control levels, using MOXA PT7728 series switches for digital communication and GOOSE message transmission. The process layer equipment still uses traditional TA, TV, and switchgear. Bay layer equipment is connected to process layer equipment via MU devices. Measurement and control devices are configured according to bays to monitor and control primary equipment. Measurement and control and protection devices communicate with the monitoring backend and remote control workstations using the IEC61850 protocol. Simultaneously, to complete the interlocking function of the bay layer, measurement and control devices need to communicate with each other using the IEC61850 protocol.
(2) The GPS timing unit sends 1PPS and time stamp signals to the GPS module of the merging unit via optical fiber or RS-485 for clock synchronization. Typically, a substation or power plant only needs one GPS timing unit to provide a synchronization clock for all MUs.
The system upgrade adopted a layered and distributed approach, which not only achieved the goal of a digital substation but also facilitated seamless technical upgrades of primary equipment in the future.
3.2 Redundant Ethernet for Building Modern Substations Compliant with IEC 61850 (Africa)
The following is an introduction to the engineering application of redundant Ethernet in a substation of a power company in Africa:
(1) Project Background. A power company provides three 132kV power transmission lines to an aluminum smelting plant, and supplies power to the smelting process workshops after power adjustment through the plant's own substation. The project objective is to upgrade the entire automation network, including upgrading the network of the old substation built in 2000. The original network system is a hub-based Ethernet structure and is a non-redundant network design.
(2) Network requirements. ① Establish a redundant Ethernet structure to ensure data communication security; ② Network equipment must comply with IEC-61850-3 and IEEE 1613 standards to ensure reliable network operation in harsh power environments; ③ Ethernet switches must have module connection capabilities for configuration upgrades.
(3) Features of MOXA Redundant Ethernet Applications. ① MOXA Turbo Ring redundancy technology has multiple fast recovery capabilities (for a network of 250 switches, the recovery time is less than 20 ms); ② The PT-7828 Layer 3 Ethernet switch supports IP routing protocols; ③ The PowerTrans series Ethernet switches have passed KEMA certification and meet the performance tests of IEC-61850-3 and IEEE 1613 standards; ④ The PowerTrans series Ethernet switches adopt a modular design, which facilitates upgrades.
3.3 MOXA Substation Ethernet Switches Improve Power Transmission Reliability (Taiwan, China)
The following is an introduction to the engineering applications of Ethernet switches in improving the reliability of power transmission:
(1) Project Background. CPC is a leading enterprise in Taiwan's petrochemical industry. CPC already has its own power plants and substations. In order to realize remote monitoring of each substation, CPC uses MOXA PowerTrans series Ethernet switches to connect relay protectors and complete data acquisition. In case of emergency, the PowerTrans switch will also issue an alarm relay output signal to realize immediate shutdown.
(2) Network requirements. ① The network equipment should meet the EMI electromagnetic interference shielding requirements in the high-voltage substation environment; ② The fast redundant ring network and RSTP technology should be adopted to ensure a fault-free and reliable network connection; ③ The network equipment should be able to operate in a wide temperature range and operate reliably in an environment without temperature control devices.
(3) Application characteristics of MOXA Ethernet switches. ① MOXA PowerTrans series Ethernet switches comply with IEC-61850-3 and IEEE 1613 standards and have excellent RFI/EMI shielding capabilities; ② MOXA Ethernet switches can establish media redundancy networks, and the network fault recovery time is less than 20ms; ③ PowerTrans series Ethernet switches can meet the operating requirements of extreme temperature environments (-40～85℃).
4. Conclusion
Network systems are the lifeblood of digital substation automation systems. Industrial Ethernet provides traditional industrial bus interconnection systems with new options, directions, and technologies for heterogeneous system interconnection and for driving automation through informatization. In substation interconnection applications, MOXA can significantly promote and improve the efficiency and safe operation of power energy management.
