A brutal storm ripped through southwestern Minnesota in April and snapped 2,000 power poles. Worthington Public Utilities kept the lights on with a seat-of-the-pants microgrid.
Demanding More from DR
Customer-specific demand-response strategies become more sophisticated.
deliver under-frequency response were tested in the early 1980s, but were shown to be too slow. Instead, DR was used to provide load reductions where real-time operating requirements were not so rigorous, such as supplemental or operating reserves, which typically require that the DR system respond in less than 10 minutes. Now things have changed.
In recent years, there has been a lot of press associated with development of smart appliances, where sensors are installed within the appliance in order to enable it to respond to certain grid events—like under-frequency. This application of DR continues to hold great promise, especially as home area networks become more prevalent in residences so that these appliances can become active participants in a utility’s under-frequency strategy. In the meantime, the utility can it retrofi its existing customer appliances with DR equipment that has under-frequency response built in.
The first example of using direct load-control equipment with built-in under-frequency sensing was in Indiana where similar DR systems were deployed by two large Midwest electric utilities. Those DR systems are still being deployed today, and the control devices contain the ability to implement autonomous under-frequency response. The key issue in the development of these systems was to verify that the under-frequency sensing and response ( i.e., shedding of the connected appliance, in this case residential HVAC systems) was fast enough to win the race to shed that would exist between the DR equipment and the existing under-frequency relays already installed and operational in substations.
To ensure that the DR equipment shed fast enough to win the race, the DR shed logic included setting the appliance under-frequency trip point to operate at a significantly higher frequency than traditional under-frequency feeder control. A typical set point was 59.8 Hz. The other component was to verify that the under-frequency sensing and shedding within the DR equipment was fast enough to compete with traditional under-frequency shedding schemes. Testing with an industry standard event recorder verified that the DR equipment could sense the frequency shift and shed its connected load—including the A/C compressor contactor drop out time—within approximately 11 cycles or 183.3 milliseconds, thereby qualifying it as a true under-frequency resource. The other key specifications designed into this solution were that the under-frequency sensing would be able to be armed and disarmed remotely by the system dispatcher, as well as remotely setting the under-frequency trip point. This capability was tested and verified, and those devices now are being deployed as an accessory application of the DR system. The system also contains the ability to restore load automatically after the frequency has returned to normal, but that restoration time intentionally was delayed by a utility’s configurable time period, so that those affected loads wouldn’t reconnect to the grid at an inappropriate time. The normal operating procedure would be for the dispatcher to enable the load restoration when the system was stable enough to accept the load.
The reason for deploying this type of under-frequency capability was to clearly demonstrate the ability to use DR systems for the provision of true frequency responsive type