The majority of all power cuts that affects individuals and communities are caused by earth faults on the power transmission lines at 10kV and 20kV level [1]. If these power cuts could be eliminated, less disturbances and interrupts would lead to large amounts of money savings.

Swedish Neutral has developed such a protection system. When an earth fault occurs power is injected into the neutral point of the transformer. The RCC (Residual Current Compensation) protection calculates a compensation current exactly 180 degrees out of phase to the fault current. Doing this, the voltage at the fault location becomes very close to zero, without affecting the power transmission.

The protection system can only compensate automatically for the fundamental frequency (50Hz), and manually for the 3^rd, 5^th, 7^th and 9^th harmonics. In most cases, when the harmonics are very small it is not necessary to compensate for them. There are though cases when compensation for the harmonics is necessary.

This thesis focuses on finding the best method to extract the content of a sampled signal regarding both simplicity and speed. Both amplitude and phase of each harmonic must be calculated. Is the proposed method suitable for the current computer system and how can it easily be implemented.

Because the fundamental frequency is known and the harmonics are all multiples of the fundamental frequency it makes the task less complex. It is not necessary to use the FFT algorithm. The DFT can be calculated using correlation. Both phase and amplitude can be calculated very precisely with few samples and not so many computer operations.