The ongoing decommissioning of conventional power plants decreases the
installed reactive power reserves for voltage control in transmission
grids. Hence, an efficient planning of compensation devices substituting
this lack of reactive power is required. Grid operators must allocate
these devices for steady-state voltage control and for dynamic voltage
control ensuring voltage stability. A separate determination of this
static and dynamic VAR demand, however, fails to exploit synergies and
disregards that VAR compensation in steady-state reduces the reserves
for dynamic compensation. This paper proposes a coupled determination of
the system static and dynamic VAR demand. An optimisation method
applying mixed-integer programming identifies an efficient allocation
and portfolio consisting of different compensation technologies. It
includes constraints for voltage limits during steady-state and
contingencies as well as for long-term voltage stability. Results
emphasise that the method identifies an efficient portfolio for various
operation and fault scenarios, while providing the required voltage
stability margin.