A new modal technique to analyse the effects of a signal to be fed back by stabilisers in a plant with identical units is proposed. The contribution of a mode on an output signal is used as a measure of the coupling input-mode-output to verify the effectiveness of the signal. The effects of stabilisers on oscillation modes are then analysed. Attention is then paid to selective control of modes. It was concluded that not only a sum of speeds, but many other signals may be used for cancellation of the contribution of intraplant modes on system response. With this cancellation, stabilisers may be applied on a plant without deteriorating the damping of these modes.