Frequency Control Method Utilising the Potential of Recovery Process of the ON/OFF Flexible Loads

Abstract

ON/OFF flexible loads (FLs), which account for a large proportion of demand side resources, have been proved to be effective in the frequency control of the power systems. The recovery process, which is essential for ON/OFF FLs, requires FLs to recover to their original ON/OFF status after activated. Other than thermostatically controlled loads (TCLs), non-TCLs cannot realise recovery through natural switching. The recovery control of non-TCLs may limit the capacity of bi-directional frequency control. Focusing on this problem, this paper proposes a bi-directional frequency control method which fully utilises the ON/OFF FLs in the recovery-process. By defining a combined fast-and-slow recovery-process (CFSRP), the activated ON/OFF FLs can still provide active power regulation service during their fast recovery-process (FRP). By designing a hierarchical strategy that considers bi-directional frequency control, the available regulation capacity of FLs and the frequency control capacity are increased. The case study results show that, compared to traditional methods that do not consider the CFSRP strategy, the proposed method reduces the maximum frequency deviation by over 4%.