An exemplary method and arrangement for estimating the frequency and the positive sequence of a fundamental component of a measured three-phase voltage are disclosed (ναβ). The method includes measuring the three-phase voltage (ναβ), calculating estimates of a fundamental frequency voltage component ({circumflex over (ν)}αβ,1) and a difference ({circumflex over (φ)}αβ,1) between positive and negative sequence components of the fundamental frequency voltage from an error ({tilde over (ν)}αβ) between the measured three-phase voltage (ναβ) and at least one of the calculated estimates of the fundamental frequency voltage component ({circumflex over (ν)}αβ,1) and from an estimate of the angular frequency ({circumflex over (ω)}0) of the fundamental frequency voltage component, wherein the estimate of the angular frequency ({circumflex over (ω)}0) of the fundamental frequency voltage component is calculated from at lest one of the estimated differences ({circumflex over (φ)}αβ,1) and from the error ({tilde over (ν)}αβ) between the measured three-phase voltage and at least one of the calculated estimates of the fundamental frequency voltage component.