Advancements In Line Differential Protection: Model-Based Line Differential Protection
It’s getting more and more common in modern power systems that long AC overhead lines and long AC cables can have several sections with different construction and varying parameters, including mixed lines comprising both overhead line and cable sections. They have large series impedance but also quite large and distributed capacitance to ground which cannot be ignored anymore by the line protection. Commonly, they are used together with fixed and/or variable shunt reactors, and/or series capacitors as well to form the overall transmission path.
Such complex installations introduce both magnitude change and phase angle shift in the phase currents along the line length. Disturbance recordings from existing installations show the difference in current magnitudes can be several hundred Amperes primary and phase-shifts can deviate by up to 30° from the ideal 180° (e.g. local-end in current angle 0°, remote-end out current angle (180-30)°). In addition, these changes are also variable and dependent on through-load conditions and actual voltage magnitude, i.e. they are not constant but are dependent on the actual operating conditions. Therefore, when traditional line differential protection is used it will measure quite large false differential currents during normal operating conditions. Different operating and contingency scenarios only make things worse for the line protection which shall be able to cover all of them. Hence, the minimum pickup of the classical (traditional) line differential protection 87L needs to be severely desensitized to cope with all operating conditions.
The model-based differential protection takes into account the exact information about the protected object (overall transmission path, including different line sections, shunt reactors (fixed or variable), series compensation, etc.) which is modelled within the relay by using sequence matrices. Based on the model and local measurements the expected remote-end phase currents can be accurately estimated by the local relay. This effectively provides exact charging current compensation when these estimated currents are compared with the actual currents from the remote-end which are measured at the local relay via a communication link. As a result, the model-based differential protection pickup can be set quite sensitive irrespective of the protected object properties (overall transmission path).
Note that the model-based line differential protection does not operate standalone. It operates in parallel with the classic (traditional) differential protection, integrated together as one overall line differential protection function able to handle the most complex transmission paths. The role of the model-based line differential protection is to provide the required sensitivity in those complex applications where the classical line differential protection needs to be severely desensitized to cope with all operating conditions.
The model-based differential protection can adapt to any such installation in order to provide optimal protection for complex transmission paths.
The paper will present the theoretical basis and the practical implementation of the model-based line differential protection.