The harmonics injecting into power systems are increasing from day to day with the increasing use of power electronics and nonlinear loads. These harmonics are to be monitored and analyzed to measure their impact on the quality of the power flowing through the lines during normal and abnormal conditions. It is easy to measure the fundamental frequency and it's off nominals of power signal using DFT and its extended methods when the power system frequency is stable. But, with increase in micro-grids and renewable energy sources like wind into the power network, the frequency of the power system in not stable. The DFT leads to errors in frequency estimation during this unstable state .This work uses the demodulation method for estimating the amplitudes and phase angle of fundamental frequency and its off-nominals of a power system having dynamics in frequency. This demodulation principle is part of the orthogonal frequency division multiplexing (OFDM) used for digital multi-carrier modulation.