A fast decoupled load flow (FDLF) iteration consists of four basic steps:
Compute real power mismatches.
Update the voltage angles.
Compute reactive power mismatches.
Update the voltage magnitudes.
The tap adjustment algorithm is applied after the voltage magnitude update.
Chan and Brandwajn (1) suggest that instability is introduced into the solution by adjusting TCUL devices before the load flow is moderately converged. Operationally, moderate convergence is defined by starting criterion for tap adjustments. The criterion is either:
A minimum iteration count (no transformer taps are changed before the designated iteration) or
A maximum bus mismatch (no transformer taps are changed unless the maximum reactive bus mismatch is less than the criterion).
Chan and Brandwajn (1) also suggest that an auxiliary solution be performed after the tap update. The tap adjustment creates an incremental reactive mismatch of –bps(anew–aold)tnew at the transformer’s terminals.