We conducted a 32-channel EEG and used FFT to calculate each frequency's Power-Value. Some of them are negative and I don't know how to interprete those im comparison to positive Power-values.
Power values are absolute values of complex numbers derived from FFT. They can not be negative unless they are contrasted to baseline or other conditions. If so then simply it means you have more power in the baseline or the other condition for that specific time/frequency bin.