2. Demand response
2. ไ. Service description
In the United Kingdom, National Grid operates the electricity grid, maintaining it as tightly as possible around the desirable frequency of 50 Hz. In case of a significant drop in frequency, as illustrated by Fig. 1, National Grid recognizes two mechanisms relevant for this work, frequency response and reserve energy. Within two seconds of an incident that causes the frequency to drop, the frequency response services are brought on-line to stabilize the grid. Reserve energy providers are then brought on-line to enable the fast responding frequency services to be switched off so they could be used again at future events. The services considered here that can provide frequency response (FR) through demand response are the Firm Frequency Response (FFR) and Frequency Control by Demand Management (FCDM). The reserve energy provision service considered here is the Short Term Operational Reserve (STOR), which is brought on-line within 20-30 min [14]. The faster reacting fast reserve service is not considered since the 50 MW minimum power delivery is prohibitively large for a typical WDS.
The first demand response service considered for the WDS case studies is STOR since the technical requirements suggest that it can be implemented in a WDS more readily. A STOR provider offers a steady demand reduction and must deliver the reduction within 4 h after being called and may be required to reduce the demand for up to 2 h. However, the tender records show that the mean call duration in 2013 was 82 min and that National Grid prefers services that can respond within 10-20 min [9], Since the minimum offered power requirement for STOR participation in the UK is 3 MW, only large WDSs would be able to participate in a STOR scheme directly. However, through an aggregation, a company that aggregates several consumers and bids their capacities to National Grid, a smaller WDSs could participate in these mechanisms by sharing the profits generated with the aggregation. To offer STOR National Grid recognizes a range of pathways to suit the wide range of suppliers. The pathways modeled here are based on offering STOR services during both availability windows or just in one, this can be achieved through tendering either a committed or flexible service. The STOR windows and tariff structure is described in further detail in Section 2.2 and in Fig. 4.
The second method for demand response energy provision considered here is the provision of frequency response services through FFR or FCDM. National Grid requires that an FFR provider is able to deliver a minimum of 10 MW response power; smaller users can offer FFR through an aggregation. For the secondary response service, which is considered here, the response must occur within seconds and be maintained for a few minutes. The service may be tendered for any time period, with National Grid preferring tenders that can offer and deliver the service most times. Furthermore, there are requirements detailing the metering and communication systems in place as well as qualification assessments that need to be performed [15,16]. FCDM is a bespoke service arranged through bilateral agreements with National Grid. In general an FCDM provider must provide the demand reduction within 2 ร of instruction and deliver for a minimum of 30 min. The minimum demand reduction to be delivered is 3 MW, which may be achieved by aggregating a number of smaller loads at same location. FCDM calls occur only ten to thirty times per annum [17].
For our analysis, the frequency response services FFR and FCDM are approximated by removing the minimum power delivery constraint and requiring the WDS to be able to deliver demand response throughout the day. The event duration for which water must be supplied to customers with reduced pump power is set to 30 min. For the analysis of the financial viability of DR the