The 33rd China International Exhibition on Electric Power Equipment and Technology
Shanghai International Energy Storage Technology Application Expo / Hydrogen Energy Expo
Power transmission and distribution secondary equipment encompasses all apparatus that operates at low voltage (typically 110 V DC or 220 V AC) to perform monitoring, protection, control, metering, and communication functions within a substation or along a distribution network. Key secondary equipment categories include protection relays (overcurrent, distance, differential, and earth fault), remote terminal units (RTUs), intelligent electronic devices (IEDs), substation automation systems (SAS), energy meters, power quality analysers, fault recorders, and communication equipment. Secondary equipment derives its input signals from current transformers (CTs) and voltage transformers (VTs) connected to the primary circuit, and outputs control commands to circuit breakers, disconnectors, and other primary apparatus. The IEC 61850 standard has transformed secondary equipment design, enabling interoperability between devices from different manufacturers and supporting the transition to fully digital substations.
5 Key Questions About Power T&D Secondary Equipment
Protection relays continuously monitor electrical quantities (current, voltage, frequency, impedance) measured by instrument transformers and issue trip commands to circuit breakers when abnormal conditions are detected. Modern numerical protection relays combine multiple protection functions in a single IED, provide event recording and fault analysis capabilities, and communicate via IEC 61850 or other protocols with the substation automation system. Fast, selective protection is essential for limiting fault damage and maintaining supply to healthy parts of the network.
IEC 61850 is the international standard for communication networks and systems in substations, defining a common data model and communication protocols (GOOSE, Sampled Values, MMS) that enable interoperability between IEDs from different manufacturers. IEC 61850 has enabled the replacement of hardwired control circuits with process bus communication, reducing copper wiring by up to 80% in digital substations. It also enables centralised protection and control architectures where protection functions run on servers rather than dedicated relay hardware, reducing equipment footprint and lifecycle costs.
A substation automation system integrates all secondary equipment functions — protection, control, monitoring, and metering — into a unified platform communicating via a station bus (typically IEC 61850 MMS over Ethernet). The SAS provides operators with a real-time single-line diagram of the substation, enables remote control of primary equipment, logs all events and alarms, and interfaces with the utility's SCADA system. Modern SAS platforms support advanced functions including automatic voltage control, load shedding, and integration with wide-area monitoring systems using phasor measurement units (PMUs).
AI and machine learning are being applied to secondary equipment in several areas: adaptive protection settings that automatically adjust relay parameters based on network topology changes; predictive maintenance analytics that identify equipment approaching failure from event log patterns; automated fault location using travelling wave analysis; and intelligent alarm management that filters and prioritises alarms during disturbances to help operators focus on critical actions. Chinese utilities and equipment manufacturers are actively developing AI-enhanced secondary systems as part of the smart grid programme.
Secondary equipment connected to utility communication networks is subject to cybersecurity requirements under China's Cybersecurity Law and the Critical Information Infrastructure Protection regulations. IEC 62351 provides the technical standard for securing IEC 61850 and other power system communication protocols. Requirements include device authentication, encrypted communications, role-based access control, security audit logging, and vulnerability management. Utilities increasingly require cybersecurity certification for secondary equipment as part of their procurement specifications.
Key Takeaways
Secondary equipment is the intelligence layer of the power grid, enabling real-time monitoring, automated protection, and remote control of high-voltage primary apparatus. The transition to digital substations based on IEC 61850 is transforming secondary equipment design and creating new opportunities for software-defined protection and control. EP Shanghai showcases the latest advances in protection relays, substation automation, and digital secondary systems.
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