By Ralph Abbott

For many years a distinction was made between direct load control and indirect load control. Direct load control was the remote control by the utility of, usually, non-essential loads at times of system peak. These were generally voluntary programmes in which the customer received a financial incentive for participating. In other cases, load control was required as a condition of service. These remote load control techniques evolved rapidly in continental Europe in the wake of World War II, when they were used as a rationing technique.

The technology employed was a one-way ‘ripple’ communications, communicating over the power line. By the late 1960s, these techniques were being adopted in the US. Ripple technology enjoyed modest success, and is still in service in a few utilities, but was supplanted in the early 1970s by one-way VHF radio. VHF technology rapidly transitioned from analogue single tone to analogue dual-tone sequential and finally to various digital addressing schemes that are still in widespread use. Direct load control was simple, inexpensive to implement, and produced a known load reduction ‘yield’ when the utility ‘pressed the button’.

Water heaters, HVAC, thermal storage, irrigation pumping and swimming pool filters have been popular loads because the perceived impact on the consumer can be minimised with thoughtful control strategies. An important attribute is the ability to respond to an emergency with a ‘SCRAM’ load shedding command, removing all controlled load for the duration of the emergency. Some utilities have established equivalency of load control with spinning reserve.

The development of Programmable Communicating Thermostats (PCTs) in the past five years is an interesting development. These devices are a form of direct load control that also offer some participation by the customer in setting space temperature preferences, and can also retain the emergency load shedding capability. Communications via paging services, VHF-RF, UHF-RF Home Area Network (HAN) and Broadcast ‘Radio Data Service’ are all readily available options.

With the oil embargo of 1973 and, later, the US Public Utility Regulatory Practices Act of 1978 utilities were strongly encouraged to offer time-differentiated rates, or peak load pricing, as it was more popularly known. In this case the expectation was that the customer would respond to price signals implicit in the electric rates charged, and the customer would alter his patterns of consumption by shifting some load to lower priced off-peak periods. This was often referred to as indirect load control.

Obviously, reduction in peak period demand was a matter of customer choice, and was therefore preferred by some utilities. But it was also true that rate response was less deterministic and less assured over time than direct load control. It also lacked the ability to command an emergency load reduction with the same core equipment.

The acronyms have changed many times in the last 30 years, but the principles are the same. The direct and indirect approaches to load reduction existed under the term demand side management (DSM) for many years. Currently the more popular term is demand response (DR). Today, utilities are selecting AMI systems that can support any sort of complex rate design, even rates for low use residential customers. Today utilities are also asking their internal staff and consultants to address both advanced metering and load control in exploring a portfolio of peak load management techniques. A prominent question is, “Should our AMI and our load management requirements be served by the same system using the same technology?”

Definitions of AMI have shifted a bit, but today it is generally agreed that an AMI system will not depend on mobile field agents, but will use fixed inplace communications infrastructure. Additionally, AMI involves two-way communications that is capable of both acquiring data from any single meter end point and capable of communicating data (for command, control or reprogramming) to end points with an addressing structure that permits individual, group and ‘global’ addressing.

Accordingly, most modern AMI systems are already set up to support direct load control. Indeed, many AMI suppliers offer highly capable load control devices as part of their suite of compatible products. These load control products can be either one-way (receiving control commands only) or two-way (able to confirm command receipt and equipment status). Obviously, equipment does fail, and determining the location of failed or tampered one-way load control equipment has always been a costly challenge. Two-way is preferred, and obviously further argues for making load control capabilities part of the AMI system specifications.

It is sometimes argued that offering TOU rates and direct load control to the same customer can be counterproductive. It is theoretically true that controlled load could be manipulated to be OFF prior to a peak period then enabled ON during the peak period. The controlled load would be recovering at the worst possible time, thus exacerbating the load on peak and the cost to the customer.

Is this a practical concern? Probably not. The good news is that direct load control could be used in conjunction with TOU or dynamic rates, when the customer and the utility agree that the utility will exercise control during periods of premium pricing. Of course the customer could install timers to keep certain loads off at TOU rate peak, and many do. The timers are useless for dynamic rates, such as the Critical Peak Pricing (CPP) rates proposed in California and elsewhere.

So, should AMI and load control reside on the same AMI system? Or is it reasonable to have separate communications for AMI and for load control? There is no single right answer.

For many utilities having a single multi-purpose AMI system will be the right way to go. Beginning with system installation all the way through full operation, there are many simplified virtues to having all combined in a single system. Often the timing is a determining factor in which way to go, or consideration of the system requirements of the related offerings to commercial and industrial customers. But other utilities will find that a separate one-way operation of load control and PCTs is more attractive, especially when public networks are used. The wide area public networks afford some advantages for related applications such as notification of a pending CPP event, regardless of the location of the recipient of this information.

Modern AMI systems have created attractive new technology options for direct load control. Load control isn’t for every utility, but every utility would be remiss in not taking a fresh look at it. The sharing of communications infrastructure between metering and load control can be quite compelling!