  We present a new implementation for active galactic nucleus (AGN) feedback
through small-scale, ultra-fast winds in the moving-mesh hydrodynamic code
AREPO. The wind is injected by prescribing mass, momentum and energy fluxes
across a spherical boundary centred on a supermassive black hole according to
available constraints for accretion disc winds. After sweeping-up a mass equal
to their own, small-scale winds thermalise, powering energy-driven outflows
with dynamics, structure and cooling properties in excellent agreement with
those of analytic wind solutions. Momentum-driven solutions do not easily
occur, because the Compton cooling radius is usually much smaller than the
free-expansion radius of the small-scale winds. Through various convergence
tests, we demonstrate that our implementation yields wind solutions which are
well converged down to the typical resolution achieved in cosmological
simulations. We test our model in hydrodynamic simulations of isolated Milky
Way - mass galaxies. Above a critical AGN luminosity, initially spherical,
small-scale winds power bipolar, energy-driven super-winds that break out of
the galactic nucleus, flowing at speeds $> 1000 \rm \, km \, s^{-1}$ out to
$\sim 10 \, \rm kpc$. These energy-driven outflows result in moderate, but
long-term, reduction in star formation, which becomes more pronounced for
higher AGN luminosities and faster small-scale winds. Suppression of star
formation proceeds through a rapid mode that involves the removal of the
highest-density, nuclear gas and through a slower mode that effectively halts
halo gas accretion. Our new implementation makes it possible to model
AGN-driven winds in a physically meaningful and validated way in simulations of
galaxy evolution, the interstellar medium and black hole accretion flows.
