Problems and solutions of line differential application in cable + transformer protection

In industrial installations typically end users want to take advantage of the transformer differential protection or the line current differential protection in order to protect with a single system, relative short portions of cable lines, usually in the range of 10 km, that feed power transformers. The protection requirements for cable line and for transformer are different, thus protection scheme that combines cable line and transformer requires specific features that must fulfil the combined needs without affecting the basic protection rules as speed, accuracy reliability and dependability. Existing product in the plant uses of line current differential protection for combined protection of a transformer and cable line has a limited scope due to difficulties associated with the management of the transformer inrush current. In the present authors propose a solution for transformer protection based on the use of a line current differential protection combined with other functions and tools available in the modern numerical relay such as distance protection, overcurrent protection and internal logic control. The realized studies were performed in scope of the service contract agreement for protection relays between the solution provider and the end customer. After the preliminary system analysis several areas of possible improvement were identified. Following the initial analysis a detailed studies were proposed and based on simulation of several scenarios using the RTDS simulator in order to optimise the total protective relaying scheme performance. The presented tests results were satisfactory and the conclusion from the customer side was the implementation of the recommendations in the actual installation of the customer, which is the RasGas company based in Qatar, one of the major LNG manufacturers worldwide. The