The replacement of synchronous generation by inverter-interfaced distributed energy resources (DERs) lowers system inertia and

leaves inter-area oscillations weakly damped, while aggregate frequency response is increasingly set by a deliberate allocation

of virtual inertia and damping. Such allocation can be designed around the center-of-inertia (COI) response. Any supplementary

wide-area damping action must not disturb it. This paper introduces a controller handling inter-area oscillations in reduced low-

inertia systems. Through an internet communication channel the controller feeds back the selectively measured nodal frequencies

and injects aggregate-neutral proportional damping. It adds and subtracts power distributed over the participating buses. To

preserve the COI behaviour, the control action is chosen such that its net injected power sums to zero at every instant. By the

initial- and final-value theorems, the initial COI rate of change of frequency (RoCoF) and the steady-state COI frequency endpoint

are proven invariant. Numerically, the COI frequency nadir is left essentially unchanged. We test on an augmented twelve-bus

Kundur benchmark, evaluated with both linear and nonlinear models. The controller tightens the inter-area frequency spread

around the COI while preserving the designed COI frequency response. The same behaviour holds on the WECC, SC500 and

Texas2000 systems for multiple amounts of participating damping nodes.