  Deep decarbonization of the electricity sector can be provided by a high
penetration of renewable sources such as wind, solar PV and hydro power.
Flexibility from hydro and storage complements the high temporal variability of
wind and solar, and transmission infrastructure helps the power balancing by
moving electricity in the spatial dimension. We study cost-optimal
highly-renewable Chinese power systems under ambitious CO$ _2 $ emission
reduction targets, by deploying a 31-node hourly-resolved techno-economic
optimization model supported by a validated weather-converted 38-year-long
renewable power generation and electricity demand dataset. With a new realistic
reservoir hydro model, we find that if CO$_2$ emission reduction goes beyond
70\%, storage facilities such as hydro, battery and hydrogen become necessary
for a moderate system cost. Numerical results show that these flexibility
components can lower renewable curtailment by two thirds, allow higher solar PV
share by a factor of two and contribute to covering summer cooling demand. We
show that expanding unidirectional high-voltage DC lines on top of the regional
inter-connections is technically sufficient and more economical than
ultra-high-voltage-AC-connected "One-Net" grid. Finally, constraining
transmission volume from the optimum by up to 25\% does not push total costs
much higher, while the significant need for battery storage remains even with
abundant interconnectivity.
