TSG-00536-2012.R1 1 Abstract— This paper describes the droop control method for parallel operation of distributed electric springs for stabilizing ac power grid. It provides a methodology that has the potential of allowing reactive power controllers to work in different locations of the distribution lines of an ac power supply and for these reactive power controllers to support and stabilize the ac mains voltage levels at their respective locations on the distribution lines. The control scheme allows these reactive power controllers to have automatically adjustable voltage references according to the mains voltage levels at the locations of the distribution network. The control method can be applied to reactive power controllers embedded in smart electric loads distributed across the power grid for stabilizing and supporting the ac power supply along the distribution network. The proposed distributed deployment of electric springs is envisaged to become an emerging technology potentially useful for stabilizing power grids with substantial penetration of distributed and intermittent renewable power sources or weakly regulated ac power grid. Index Terms—Smart Gird, Droop Control, Electric Springs, Voltage regulation. I. INTRODUCTION N traditional power systems, the power flow is controlled by the power plants and with the power generation following the load demand on an instantaneous basis. The power flow is centrally controlled by the utility companies and is essentially carried out in a unidirectional manner. In future power grids, increasing proportion of renewable energy sources such as wind and solar energy systems will be installed in a distributed fashion across the load centers (and thus all over the power grids). These distributed renewable energy sources may be known or unknown to the utility companies and a significant portion of them are to be connected to the distribution network close to the loads. Since Manuscript received August 28, 2012. This work was supported in part by the EPSRC (grant no. EP/F029128/1) , the Hong Kong Research Grant Council under Grant HKU10/CRG/10 and the University of Hong Kong Seed Funds (Seed Projects: 201111159239 & 201203159010). C.K. Lee is with the Department of Electrical & Electronic Engineering, The University of Hong Kong (e-mail: cklee@eee.hku.hk). N. Chaudhuri was with the Department of Electrical & Electronic Engineering, Imperial College London and is now with the General Electric, USA (e-mail: nilanjgec@gmail.com). B. Chaudhuri is with the Department of Electrical & Electronic Engineering, Imperial College London (e-mail: b.chaudhuri@imperial.ac.uk). S.Y.R. Hui is with the Department of Electrical & Electronic Engineering, The University of Hong Kong (e-mail: ronhui@eee.hku.hk) and also with Imperial College London (e-mail: r.hui@imperial.ac.uk). it is not possible to determine the total instantaneous power generation from distributed renewable sources since they are non-dispatchable, the increasing use of intermittent renewable energy sources is expected to introduce dynamic instability to the ac power supply, potentially resulting in highly fluctuating or even unstable ac mains voltage [1]. In future power grids, a new control paradigm is needed to ensure that the load demand should follow the power generation [2],[3], instead of the power generation following the load demand as in the case of traditional power systems. To achieve this new control paradigm, demand-side management approaches have been investigated. They can be broadly summarized as: (i) Scheduling of delay–tolerant power demand tasks [4-6] (ii) Real-time pricing [7-9], (iii) Use of energy storage to alleviate peak demands [10] and (iv) Direct load control or on-off control of smart loads [11-13]. Approaches (i) and (ii) can be used to shape the load profile in a pre-determined manner. But they cannot cope with instantaneous imbalance of power generation and load demand. Approach (iii) can help achieve instantaneous power balance, but energy storage has limited capacity and is an expensive option. Approach (iv) can achieve instantaneous power balance, but is intrusive to the consumers. Moreover, approaches (iii) and (iv) often require remote communication to coordinate the action of multiple smart loads and/or energy storage which could be unreliable. A novel concept of “electric springs” has recently been proposed as a new technology [14], [15] to satisfy the new control paradigm of load demand following power generation for future smart grid with substantial penetration of distributed renewable energy sources. Based on the 3-century old Hooke’s law, the mechanical spring concept has been extended to the electrical regime. Electric spring has been practically realized using a reactive power controller with an input-voltage control. Since electric springs are designed to support the ac mains voltage at various points of their installations across the power grid, it is imperative to derive a common control scheme so that they can support the voltage at their locations with consideration for the voltage variations along the distribution lines. In this paper, the droop control concept is applied to the electric spring control so that they can derive their own reference voltages according to their points of installation in the distribution network. Three practical 1 kVA electric springs have been tested individually and in a group. The proposed droop control has been found to be effective in automatically coordinating the electric springs to act in a complimentary fashion without requiring central control and remote communication. Droop Control of Distributed Electric Springs for Stabilizing Future Power Grid C.K. Lee, Member, IEEE, N.R. Chaudhuri, Member, IEEE, B. Chaudhuri, Senior Member, IEEE, and S.Y.(Ron) Hui, Fellow, IEEE I