Why do we need precision timing in DSS?

Digital Substations (DSS) rely on devices distributed across different locations and must have the same time. This time synchronization is essential to calculate the accurate value of currents and support differential protection schemes and other critical functions that ensure the grid is highly reliable and expensive equipment is protected during faults. Protection functions, particularly differential protection, require that current measurements from different points be compared in real-time and thus be accurately synchronized. The measurements must be sampled and time stamped accurately to avoid discrepancies in time, which could lead to false tripping or failure to trip.

Time synchronization is also essential for accurately recording power measurements and logging events to support the sequence of events (SOE) analysis by humans or intelligent machines, which determines the cause of the disturbance and suggests remedial actions.  Other advanced applications such as fault location, broad area protection, and phasor measurement rely on accurate time synchronization to remain accurate. Finally, detecting potential cybersecurity attacks and rapidly acting upon them also require precise timing information at every step of the digital grid.

What’s the IEEE 1588 Precision Time Protocol?

The IEEE 1588 Precision Time Protocol is the standard for clock synchronization recommended for digital substations. Clocks are monitored within protection relays, fault recorders, merging units, and other intelligent electronic devices (IEDs) using standard messages that continuously inform the time differences in a hierarchy of clocks defined as one grandmaster clock, several master clocks, and their dependent slave clocks. A slave clock exchanges messages with the master clock to calculate the time delay due to network latency. Then, it can adjust its time to correct potential drift. The slave clocks can also exchange messages with other slave clocks (peer-to-peer) to perform similar delay calculations and time correction. The master clock keeps its time synchronized with the grandmaster clock, often a GPS service connected to the substation. The grandmaster clock can be selected dynamically from other master clocks based on accuracy, availability, and other parameters.

The IEC 61850-9-3 standard includes a profile that defines how best to apply the IEEE 1588 PTP protocol to digital substations. It calls for sub-microsecond time accuracy for GOOSE messages and Sample Values in the process bus. It defines redundant clock schemes and ethernet network requirements to ensure the interoperability and performance of devices integrated into the substation.

What are the best practices for time synchronization in DSS?

Best practices for time synchronization in digital substations are essential to ensure the precise operation of protection, automation, and control systems. Time synchronization issues in digital substations, particularly those using IEC 61850-based systems, can lead to incorrect protection function operation, inaccurate data logging, and unreliable device communication.

Best practices include:

1. Adopt a High-Accuracy Time Protocol: The IEEE 1588 Precision Time Protocol (PTP), specified in IEC 61850-9-3, is widely recommended due to its sub-microsecond accuracy. PTP ensures that sampled values and event-triggered data from IEDs are precisely aligned across the substation​​. Misconfiguration of PTP settings, such as clock priorities, synchronization intervals, and delay mechanisms, can cause synchronization drifts or inaccuracies. Ensuring all devices are correctly configured to use the appropriate time sources and intervals is essential. Periodic PTP synchronization monitoring should be in place to verify that all devices are synchronized within acceptable tolerances. Tools such as network analyzers or built-in synchronization monitoring features can be used to track the time synchronization status across the network. These tools can monitor real-time latency, jitter, and synchronization offsets, helping detect and resolve issues before they impact substation operations.
2. Implement Redundant Time Sources: Ensuring the resilience of time synchronization is critical. Redundant GPS or GNSS receivers, combined with holdover capabilities (e.g., using high-quality clocks like atomic clocks or quartz oscillators), provide continuous synchronization even if the primary source fails​​.   – Implementing a redundant network topology can mitigate synchronization issues caused by network failures. The Parallel Redundancy Protocol (PRP) or High-availability Seamless Redundancy (HSR) protocols provide failover capabilities, ensuring uninterrupted time synchronization even if one part of the network fails. Devices can be equipped with dual network interfaces to receive time synchronization signals from two independent network paths. – Simple Network Time Protocol (SNTP) or Network Time Protocol (NTP) can be used as backup time synchronization mechanisms, particularly for less critical applications, or as a fallback solution in case PTP is temporarily unavailable. While SNTP/NTP provide lower accuracy than PTP (typically in the millisecond range), they can still ensure basic timekeeping functionality during outages of the primary time source.
3. Network Architecture to Minimize Time Delay: Time signal delays caused by cabling, network switches, and other devices should be compensated accurately. Modern PTP systems include mechanisms for measuring and correcting these delays, helping to maintain precision​​. Time synchronization performance can be significantly affected by network architecture. Networks should be designed with minimal jitter and latency using dedicated paths for timing signals when possible. Software-defined networking (SDN) is increasingly utilized to manage and optimize time-sensitive traffic, ensuring that critical signals, such as time-stamped data, are prioritized​​. Use VLANs (Virtual Local Area Networks) or Quality of Service (QoS) configurations to prioritize time-sensitive traffic, such as PTP messages, over other types of data traffic. This ensures that synchronization messages are not delayed or lost due to network congestion.
4. Monitor and Detect Time Synchronization Issues: Continuous monitoring of time synchronization is necessary to detect and address any issues proactively. Tools that provide alerts for timing drift, loss of synchronization, or errors in GPS signals are vital to maintaining system reliability​. Some devices may experience clock drift over time, leading to synchronization errors. Implement alarms and event notifications that trigger when devices become unsynchronized or experience time-related errors. This allows operators to proactively address time synchronization issues before they lead to failures or incorrect operations in the substation. Conduct thorough testing during the commissioning phase to ensure all devices are appropriately synchronized. Simulate network failures or time source outages to verify that the substation system can maintain synchronization under abnormal conditions. Factory Acceptance Tests (FAT) and Site Acceptance Tests (SAT) should include time synchronization tests to verify the robustness of the PTP setup under varying network conditions.
5. Use GPS and Alternative Timing Solutions: While GPS is the most common synchronization method, it’s vulnerable to jamming and spoofing. Using alternative satellite systems or terrestrial-based solutions (e.g., atomic clocks or fiber-based systems) can enhance the robustness of the time distribution infrastructure​​.

By adhering to these best practices, digital substations can achieve precise, resilient, and reliable time synchronization, which is crucial for ensuring the correct operation of protection and control systems.