Sparse network topology (e.g. radial systems) and small number of interconnections between
individual areas of the system can also cause rather poor convergence due to insufficient reactive
power distribution in the network. Too narrowed operation limits, such as var constraints for PV buses
and ranges of tap-changing transformers, along with incorrectly chosen slack bus, negative line
reactances (i.e. series capacitors) and long lines are the most common reasons of the numerical
instability. Especially, lightly loaded long lines may consume significant volume of transmitted
reactive power.