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

摘要

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 world wide. The solution described in this paper makes the energization of transformers without any nuisance trips. It is now being used at customer''s facility and h--ad already shown satisfactory performance during the first three years after its implementation. The solution implemented can be extended to any other similar industrial installation without additional hardware extensions or modifications as it is uniquely based on software and logic configuration. The use of power system simulation package has allowed that the analysis, development of new solution for differential application on schemes composed by cables + transformers and tests were possible to be performed in a lab. This approach eliminated the difficulties and problems typically encountered when doing testing in the field‥