An alternative principle for line protection that is quickly becoming the norm is differential protection. Differential protection is based on Kirchhoff’s laws, stating that all current into a network node shall add up to 0 in an ideal system. What this practically means for a line protection application is that the current that is measured to flow into the line should also be measured to come out.
What sounds as a trivial subtraction of outgoing current from ingoing current is in reality not trivial at all, since the power system is anything from an ideal system. Capacitive charging currents, non-transposed lines or asymmetrical communication causing phase rotation, drifting time synchronization, tapped lines and transformers are some examples of features that leak or transform current flowing on the line or influence the data sent from IED (Intelligent Electronic Device) to IED. It requires advanced stabilization methods to create a reliable line differential protection scheme.
Differential protection is easier to integrate into a protection philosophy as distance protection, as it is unit protection. It delivers more consistent performance, at the cost of increased hardware dependency. Some form of high bandwidth, low latency communication is required in between the line ends in order to exchange analog information for the line differential algorithm to process. Typically this is a direct fiber optic cable.
In order to cope with any situation, a modern protection IED needs to be configurable to fit within its intended application area. Hitachi Power Grids' solution to this requirement is the Relion® series of IEDs. The Relion® series offers a standardized library of the functionality on a high performance, common hardware architecture, and provides the flexibility and configurability to suit any application area.
The Relion® 650 series differential protection IEDs include all required back-up functionality and the full power and flexibility of the Relion® platform.
The Relion® 670 series differential protection IEDs improve upon the 650 series to fulfill more demanding applications, offering improved performance, increased customization options of hardware and software and allowing for increased functional integration and scalability.
PSL 6603U is ideal as an alternative to the Relion® series alternative offering, ultimately suitable as a main 2 solution as it uses a different hardware architecture and its own protection algorithms. Based on a reliable architecture with a redundant CPU and A/D conversion, it applies advanced line differential and distance protection with back-up protection in a configured package, making it very user friendly and intuitive.
Relion® RED670 and RED650 offer either a dedicated line differential protection IED, or a so called multi-function device, whereby distance protection is integrated with the IED as well. PSL 6603U is always configured as a multi-function device.