Decentralized Demand-Side Contribution to Primary Frequency Control

Frequency in large power systems is usually controlled by adjusting the production of generating units in response to changes in the load. As the amount of intermittent renewable generation increases and the proportion of flexible conventional generating units decreases, a contribution from the demand side to primary frequency control becomes technically and economically desirable. One of the reasons why this has not been done was the perceived difficulties in dealing with many small loads rather than a limited number of generating units. In particular, the cost and complexity associated with two-way communications between many loads and the control center appeared to be insurmountable obstacles. This paper argues that this two-way communication is not essential and that the demand can respond to the frequency error in a manner similar to the generators. Simulation results show that, using this approach, the demand side can make a significant and reliable contribution to primary frequency response while preserving the benefits that consumers derive from their supply of electric energy.

I. Introduction

Imbalances between load and generation must be corrected within seconds to avoid frequency deviations that might threaten the stability and security of the power system. Routine deviations from this balance are usually corrected by adjustments in the output of conventional generating units driven by their governor in what is called primary frequency response [1]. The load is used explicitly to restore this balance only when the imbalance is severe and cannot be remedied quickly enough using fast acting generation. In such cases, blocks of loads are interrupted following the action of underfrequency relays. This control philosophy may need to be revised in the coming years as the demand side may take a more active role in the control of the system. As their relative size increases, intermittent and variable output renewable energy sources such as wind farms will contribute larger random fluctuations to the load/generation balance [2]. At the same time, the number of conventional generating plants that have the flexibility required to take part in primary frequency control is likely to decrease as coal-fired plants are decommissioned. One possible scenario would see the bulk of the electrical energy being produced by a combination of renewable sources and nuclear power plants [3]. Under such conditions, performing primary frequency control using only supply-side resources may become not only prohibitively expensive but also technically difficult; see, for example, [4]. It is therefore important to explore how a sufficient proportion of the loads could assume a routine role in primary frequency control to maintain the stability of the system at an acceptable cost, considering this load participation as an example of the contribution that consumers could make to ancillary services [5], [6].