Digital fault recorders (DFR) in the power system are devices that sample the analogue values of voltage and current and convert them to a digital format after being triggered by events or signals from protection relays. This digital data is then used for various analyses of electrical system performance. The analogue measurements are delivered by conventional voltage and current transformers or by modern sensors such as optical current transformers.

DFRs are applied to:

• Detect disturbances and store measurements before and during events.
• Assess power quality, voltage levels and harmonic content.
• Monitor system protection performance.
• Transfer and display measurements to the operator, enabling interpretation and understanding of power system conditions
• Transfer measurements to storage for further analysis.

Storage of measurements may be continuous or triggered by a predetermined disturbance. A disturbance may trigger data storage, which typically captures data from a period before the triggering event as well to help determine the sequence of events leading up to the disturbance.

Sampling frequencies for measurements are adapted to the scope and application of the measurements. The nominal power system frequency is 50 Hz. To observe over-harmonic frequencies, for example, the sampling frequency must be over 1000 Hz, depending on the relevant order of over-harmonic frequencies.

Accurate time stamps for measurements are crucial, as this enables comparison with measurements at other locations and interpretation of sequences and propagation of events. This will become even more important as the share of energy coming from power electronic converters increases, given that they can react in milliseconds to external influence [1].

The application of DFRs offers several benefits, including:

• The ability to detect disturbances and store measurements, which enable the analysis of faults and the origin of events. DFRs make the analysis of previous events easier and more accurate.
• Measurements may reveal unwanted behaviour and interaction between electronic power converters. This is relevant for interoperability, as a large amount of High Voltage Direct Current (HVDC) converters and over- and under-harmonic frequencies are generated by such equipment.
• DFRs may function as phasor measurement units (PMUs) and the measurements may be helpful in real-time operation to monitor and mitigate things like unwanted oscillations.
• Measurements with accurate time stamps make it possible to analyse the location and sequence of events (SOE) leading up to a disturbance.
• A better understanding of the origin and events leading up to a disturbance can significantly reduce restoration time.
• The introduction of power electronic converters introduces new challenges related to the control of the converters, and DFRs aid in monitoring.

The enablers of DFR are listed below:

• The technology for secure, digital communication networks with low latency is available. This enables the exchange of measurements and improves the understanding of events and disturbances in real time.
• Solid-state drives can allow for higher-resolution data to be stored for longer time periods.

To further enhance DFR technology, some suggested R&D activities are listed below:

• Develop triggers and sensor capabilities adapted to phenomena and disturbances in power electronics-dominated systems.
• Integrate DFRs into wide area monitoring protection and control solutions (WAMPAC) systems.
• Further develop secure communication and storage systems to enable close to real-time event analysis and operator awareness.
• Improve and standardise retrieval of global time and time stamps for measurements.
• Standardise sensors, estimation algorithms and interpretation to enable the exchange of consistent measurements. Standardise the exchange of measurements from DFRs between different European locations as part of WAMPAC.

The technology is in line with milestones “Adoption of real-time data sensors and IoT devices for a more efficient, cost-effective and safer remote monitoring” and “Advanced reconfiguration and control of network and assets” under Mission 1 and milestone “Integration of PMU (WAMS) in dynamic security assessment process” under Mission 4 of the ENTSO-E RDI Roadmap 2024-2034.

Examples

TRL 9 for DFR, as the sensor and storage technology are available.
TRL 2 to 3 for DFR data along with appropriate software for automated protection and control of the power system.
TRL 3 to 4 for WAMPAC.

1. J. Perez, “A guide to digital fault recording event analysis.”