Automatic Generation Control (AGC)

We saw in the previous example how load sharing in a multi-generator power system can be achieved using droop characteristics of governors. The sharing according to droop is irrespective of load location.

However if non-zero governor droops are used (which is necessary for appropriate sharing), a steady state frequency error will remain which needs to be corrected. Moreover, since all the governors respond to the load change irrespective of load location, there may be undesirable exchange of power between different areas of the grid. This is manifested as a change in the flows of lines interconnecting these areas.

To ensure that frequency steady state error is corrected and generators in a particular area take on the burden of their own load, the load reference (Pm0) of governors is adjusted slowly. This control is also called "secondary control". This correction may be done over several minutes as opposed to 5-10 seconds for initial or "primary" control action of governors.

Thus, while primary control (governor action) ensures that a large and sudden frequency fall or rise is prevented, secondary control or Automatic Generation Control ensures that frequency is brought back to the nominal value and inter-area power flow is regulated.