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The static VAr compensator (SVC) recently commissioned by the UK National Grid Company plc incorporates a three-phase chain-cell STATCOM. The installation demonstrates the advantages of the chain-cell converter in providing dynamically variable exchange of reactive power with a transmission system to facilitate voltage control. However, the chain-cell converter in this form is not able to exchange significant real power with the transmission system. Such ability would be of great benefit in operations ranging from damping transients through to load levelling.. This paper demonstrates that the chain-cell converter, when modified for the task of both real and reactive power exchange, has the potential to compensate for many transient and dynamic effects in a transmission system. The required cell ratings are discussed in terms of the usable stored energy of the converter and compared to other converters used for interfacing storage elements. A state-space model is also presented for incorporation into system studies.