by Alex Apostolov, USA
Directional comparison protection based on superimposed components, also known as incremental quantities, represents one of the most important developments in high-speed transmission line protection. It has been my favourite communications-based protection for half a century and remains like this today as well.
Unlike conventional phasor-based protection, which relies on steady-state sinusoidal quantities, incremental-quantity protection uses the sudden changes in currents and voltages that occur immediately after fault inception. These transient quantities contain information about the fault direction and location and can be processed extremely quickly, often within only a few milliseconds.
The principle emerged from research into transient phenomena and traveling waves during the 1960s and 1970s. One of the earliest practical implementations was the RALDA protection developed by ASEA in Sweden. RALDA used analog techniques to derive directional information from incremental currents and voltages associated with faults. Even with the limitations of analog hardware at the time, the scheme demonstrated the potential for very high-speed fault detection and directional comparison.
A further important evolution came with the LFDC relay developed by GEC Alsthom. LFDC became one of the best-known commercial implementations of directional comparison protection using incremental quantities and traveling-wave-related techniques. The relay demonstrated excellent performance on long and heavily loaded transmission lines where conventional distance relays could experience limitations. Its fast-operating time contributed significantly to improved system stability and reduced fault clearing times on EHV networks.
In France, the EPAC protection scheme represented another major step in the development of transient-based line protection. EPAC explored the use of superimposed quantities and directional comparison principles to achieve secure and rapid fault clearing on EHV transmission systems. These developments showed that incremental quantities were less affected by load flow, power swings, and weak source conditions than traditional distance protection methods.
The widespread adoption of digital signal processing in the 1990s and 2000s enabled a new generation of numerical relays based on the same principles. Modern devices such as Schweitzer Engineering Laboratories’ SEL-411L combine incremental-quantity directional and differential elements with conventional phasor-based protection functions. By analyzing the changes in currents and voltages immediately after a fault, the relay can make very fast and secure tripping decisions, even under challenging system conditions such as series compensation, CT saturation, or inverter-based resource penetration.
Today, incremental-quantity directional comparison protection is receiving renewed attention because modern power systems are becoming increasingly dynamic and complex. Its immunity to changing operating conditions, ability to operate before severe CT saturation develops, and compatibility with process bus architectures make it especially attractive for future digital substations. What began decades ago as experimental transient-based protection research has evolved into one of the most advanced approaches to transmission line protection.
