Time-Domain Based Protection For Transmission Lines: Secure and Reliable Solution For Complex Networks
Faults on transmission lines are one of the most dangerous events in power grids. Secure and dependable fault detection and clearance of transmission line faults is important for reducing electrical equipment stress, enhancing power system stability, and improving power quality.
Distance protection is popular in transmission networks and constitutes a large installed base across grid systems. The most widely implemented distance protection is based on voltage and current phasors, derived from the measured voltages and currents at the terminal where the relay is located, and is centered on the principle that an impedance measured at the relay location is indicative of a fault on the line if it falls within the relay’s zone of operation. Though these methods are reliable and proven in the field over decades, they may be impacted by CT saturation, power swings, CVT performance, series compensation, and the addition of power electronics based renewable energy resources (IBRs). Today the electric grids are in fact becoming increasingly complex and dynamic in nature with the increasing deployment of power electronic converter interfaced systems such as renewable energy resources, series compensated lines, HVDC, FACTS, etc., necessitating a need for fast, secure and reliable protection decisions. In these networks, the phasor-based distance protection is prone to reliability problems and has intrinsic speed limitations.
Ultra-fast protection solutions based on fault induced traveling waves are commercially available. Although promising, such solutions may miss faults occurring near voltage zero-crossings, and falsely detect non-faulty disturbances as faults. Therefore, these solutions may lack wider acceptance in modern transmission networks on lines where protection security is a non-negotiable requirement.
In comparison, time-domain based transmission line protection solutions have appeal in terms of reliability and speed. In addition, an ideal time-domain transmission line protection should be applicable to various networks such as series compensated lines, lines connected to renewable energy resources and long underground/sub-sea cables. This paper proposes a method for time-domain based transmission line protection (i.e. using incremental quantities of the voltages and currents measured at the relay location) to meet these requirements.
The proposed method
- identifies the fault zone by comparing a novel dynamic restraining quantity (which is the calculated rate of change of the incremental quantity of the fault loop current at the relay location for a solid fault at the set reach point) with a current-based operating quantity (which is the rate of change of the incremental quantity of the fault loop current at the relay location)
- inherently overcomes the influence of series compensation, renewable integration and provides a fast and reliable identification of in-zone and out-of-zone faults
The proposed method has been tested for various networks using laboratory experimental data and field records. Further it was implemented in an IED platform and tested with RTDS simulations in a hardware-in-the-loop mode. The method was found to work satisfactorily and consistently over the extensive test scenarios.
The paper will cover details of the proposed method (including the operate and restraint quantities), illustrative cases (including simulation results and analysis, that will cover also performance assessment during special scenarios in the power grid like CT saturation, power swings and faults during power swings, etc. including for power grids with renewables), and validation with field data.