This paper proposes a novel solution for transient frequency compensation in weak 3-phase Microgrids (MGs) based on a Luenberger observer and a transient frequency detector. Unlike in conventional grids, the low inertia of the generators coupled to a MG could make their rotor speeds to be affected by load changes, varying the grid frequency and compromising the grid quality and stability. This problem has been approached in the literature by the Virtual Inertia (VI) concept. However, the existing solutions are affected by the decoupling of the grid frequency reference and the frequency estimation bandwidth. The proposed paper addresses these problems by the use of a transient frequency drift estimator based on a transient detector and a Luenberger type observer that provides a nearly-zero lag frequency estimation. The proposed alternative is analytically compared with the existing techniques and validated through simulation and exhaustive experimental results in an islanded MG. The developed method enables a 1Hz reduction in the transient frequency deviation when compared with the existing alternatives and improves the system stability. PCTI-FICYT under grants ID BP13-138, BP14- 135 and BP16-133 Research, Technological Development and Innovation Program Oriented to the Society Challenges of the Spanish Ministry of Economy and Competitiveness under grants ENE2013-44245-R and ENE2016-77919-R. European Union through ERFD Structural Funds (FEDER).