Demand Reduction of Electric Water Heaters for Load-Shedding Recovery using Stochastic Control

Utility-implemented load-shedding aims to balance the supply and demand of a constrained power grid for a scheduled period of time by switching off supply to selected regions. The unintended consequence of this strategy occurs at the return of supply, where an instantaneous peak demand is experienced as an impulse at localised points of the power system. This results in frequent trips on medium voltage breakers that can damage infrastructure and result in delayed return of supply to end-users. The aim of this paper is to examine demand-side management interventions using intelligent control of electric water heaters (EWHs) to reduce the instantaneous peak demand at return of supply. Domestic water heating load was chosen as it is a deferrable, dispatchable, passive load with a high power requirement. The approach taken simulates the loading effect of 3 600 EWHs using three measured tank states and the combination of the Monte Carlo algorithm for probabilistic population behaviour. A typical 150-litre, 3 kW EWH in horizontal orientation is used, with a maximum draw volume of 50-litre per tank per day. It is shown that system changes to load-shedding schedules from a period of 4.5-hours to 2.5-hours beneficially reduces the instantaneous peak demand with the shorter period. For a 3 600 EWH-population, simulated results comparing uniformly distributed stochastic delays for activation time windows within 15, 30, 45, and 60 minutes show that a 15-minute activation time window could reduce the expected peak demand by a factor of two. This is important because intelligent controller retrofits onto EWHs is becoming more ubiquitous and if this simple sub-routine is built-in to produce a short delay after a period of no supply, it could result in significant passive impact on the power grid.