Unlocking New Sources of Flexibility T THE SIGNIFICANT GROWTH OF WIND AND photovoltaic generation experienced in many countries around the world will soon challenge the ability of transmission system operators (TSOs) to guarantee the security of supply. To cope with such future low-carbon electricity systems, it is imperative to increase the portfolio of flexibility sources in a cost-effective and sustainable manner. This is likely to require exploring solutions beyond the use of traditional players connected at higher voltages, such as fast-acting generation plants and large customers. Managing loads at lower voltages can play a key role in providing short-term flexibility by reducing demand for short periods (≤60 min) when needed by the TSO for balancing and other purposes; by the distribution network operator (DNO) to, for example, reduce asset congestion; or even by the customers themselves to react to time-of-use tariffs and other conditions. Although By Andrea Ballanti, Luis (Nando) Ochoa, Kieran Bailey, and Steve Cox Digital Object Identifier 10.1109/MPE.2017.2660799 Date of publication: 19 April 2017 52 ieee power & energy magazine 1540-7977/17©2017IEEE may/june 2017