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Influence of Circuit Breaker Maintenance Tests on Busbar Differential Protection Scheme

Influence of Circuit Breaker Maintenance Tests on Busbar Differential Protection Scheme

Relay Protection and Automation for Electric Power Systems 2017

In this paper, different methods of inhibiting the unwarranted operation of busbar protection relay during the CB testing is discussed with appropriate logics. This paper also focuses on providing proper security for bus faults by implementing relay logics to ensure minimal human intervention (prevention of bus bar blocking, throwing of knife switches for CT secondary shorting etc…) and acceptable modifications to the current protection scheme layout. Various methods are recommended that can be incorporated based on the suitability of the respective system condition. The discussed methods are developed in Intelligent Electronic Devices (IED)s using the advanced logics and algorithms. 

Author(s): Dinesh Chhajer, Daniel Carreno, Dr K N Dinesh Babu, Vijay Shanmugasundaram, Jorge Cardenas
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Low resistance micro ohm measurement is a routine diagnostic test performed on Circuit Breakers (CB) and bus bars in medium and high voltage substations. This test is important to check the integrity of CB contacts, detection of high resistance joints and terminations for installed bus work. Although the test is very simple and easy to perform, it can be problematic for the differential protection scheme associated with the circuit under test. The DC current used for measuring the contact resistance may result in a flow of the CT secondary current causing an imbalance in the differential circuit leading to unwarranted tripping of the busbar which is against the power system protection principle. Factors such as quality of the applied DC current, relay settings, CT ratio, accuracy class and its magnetization can impact the selectivity and sensitivity of the protection functions. The paper will thoroughly explain the root cause of misoperation using real life case study and lab simulations. 

In this paper, different methods of inhibiting the unwarranted operation of busbar protection relay during the CB testing is discussed with appropriate logics. This paper also focuses on providing proper security for bus faults by implementing relay logics to ensure minimal human intervention (prevention of bus bar blocking, throwing of knife switches for CT secondary shorting etc…) and acceptable modifications to the current protection scheme layout. Various methods are recommended that can be incorporated based on the suitability of the respective system condition. The discussed methods are developed in Intelligent Electronic Devices (IED)s using the advanced logics and algorithms.