Constraint on steady state output imposed by zeros at s=0 and servo synthesis using unstable weight with application to multi-area frequency control of power plants

When an MIMO plant has zeros at s=0, it can be shown that this imposes a constraint on the steady state output of the plant. Therefore, reference tracking/disturbance rejection is possible only if certain structural condition is satisfied. In the optimal servo synthesis using unstable weight, such as the one using unstably weighted H_∞ control, the structure of unstable weight can not be selected freely in this case. The required structure is exposed in this paper. Further, the proposed method is applied to the multi-area frequency control of power plants which has a zero at s=0