Impact of Current Transformer Saturation on Fault Protection and Power System Stability

Abstract

In transmission networks experiencing rapid development, short circuit levels can increase gradually approaching existing Current Transformers (CT) capability. If the CT rating is not adequately set to reflect highest possible primary current flow, CT saturation may occur which can affect protection relay operation. This presentation covers the impacts of CT saturation on distance and over current protection. Protection malfunction due to CT saturation can be a concern when the existing CTs are exposed to higher short-circuit currents due to energization of new generators in their vicinity. In Alberta, this can be a critical issue in areas with rapid generation growth. This presentation will provide a brief overview on CT type and ratio selection criteria and how it may impact fault detection. The presentation will cover a case study where short circuit levels increase to a level where CT saturation was possible and how the situation can be addressed in the interim until the needed CT upgrades are implemented. The primary concern in this case study was the potential delay in fault clearing if CT saturation occurred during fault conditions without CT upgrade implementation. To analyze the risks associated with delay in clearing faults, transient stability analysis was conducted to identify the needed critical clearing time. The study concluded that temporary CT saturation conditions are manageable if the duration through which the CT saturation occurs could be accommodated, i.e. Below the needed critical clearing time. It was concluded that three phase faults do not result in saturation of CTs in all three phases ensuring adequate protective relay operation under three phase faults. For single phase faults, it was found that the needed critical clearing time is longer than the duration of saturation that a CT may experience under fault condition and hence, the protective relaying will be able to detect the fault.