The Impact of Current-Transformer Saturation on Transformer Differential Protection

Current-transformer (CT) saturation has a major impact on the operation of differential protection schemes. Transformer differential protection must maintain security during CT saturation for external faults while preserving high sensitivity and speed of operation for lower magnitude internal faults. Relay manufacturers provide very little information or direction for protection engineers to set relays adequately to achieve these goals. If an engineer lacks a deep understanding of relay behavior during CT saturation, he or she could make setting implementation mistakes that may lead to spurious trips or failures to trip with catastrophic consequences. This work investigated the impact of CT saturation on transformer differential relays in order to provide guidance on scheme behavior. It was discovered that harmonic blocking is triggered by CT saturation-a significant effect which is not described in the literature or relay manuals. This effect was modeled and the model's accuracy was verified with CT simulations, relay secondary injections, and high current primary injections. It is demonstrated that harmonic blocking provides effective security against nuisance trips caused by CT saturation. It is also shown that the traditional use of a high slope 2 is mostly redundant wherever harmonic blocking is enabled.

I. Introduction

Since the invention of differential schemes, current-transformer (CT) saturation has been one of the main problems for differential transformer protection. CT saturation leads to the inaccurate measurement of primary currents that may cause relay malfunction. Saturation also increases current measurement error that creates a false differential current. It is impossible to predict or even estimate this error due to the unpredictable nature of CT saturation (many factors, such as fault level, fault inception angle, system configuration, CT remanence, etc. affect the level of saturation and measurement error). This false spill current may cause spurious relay operation under through-faults.