New Wave of Direct Load Control Update on DLC Systems, Technology

By G. Heber Weller, Science Applications International Corp. (SAIC)

Much has changed with utility-sponsored load control systems that use direct load control (DLC). DLC is a utility-sponsored demand-side management program where customer participation in the program is voluntary, but participation in load control events is not–even though most programs offer ways to opt out of control for specific needs. The control imposed on participating appliances is chosen by the customer when he or she volunteers for the program, and it cannot be changed during a control event.

Most DLC programs offer multiple control tiers, allowing customers to move from one level of control to another and determine which tier works best for them.

This is in contrast to demand response (DR) demand-side management programs that primarily operate by sending to customers energy price signals, which are intended to cause a customer response. Customers might respond by turning off certain appliances or adjusting their heating, ventilating and air conditioning (HVAC) systems’ temperature settings. DR systems that automatically respond to price signals based on a customer’s personal configuration are being implemented. DR allows customers to modify their responses at any time.

Direct Load Control

DLC is the oldest form of dispatchable demand-side management. Many DLC systems that were launched as far back as the late 1970s remain in use throughout the United States. Some of the early domestic DLC programs began full-scale operation in the early 1970s using a basic, often one-way radio communicating technology. Those systems were based on connecting load control receivers to equipment at customer premises, allowing utilities to directly manage the running characteristics of those participating appliances–hence the term direct load control.

As mentioned, customers are not involved in the load control process in a DLC system. They do not interact with the system or provide real-time permission for control actions. These systems usually either shed (turn off) the affected appliance or force some kind of on-off run cycle (duty cycle) that is intended to be shorter than the current natural duty cycle. Most DLC systems provide financial incentives to customers for the right to control their appliances. These programs have restricted levels of control, such as duty cycle, and operational availability, such as hours per day of control, days of control, and so on.

Residential DLC

Residential DLC normally involves HVAC system, water heater and pool pump control. The appliances impacted by a DLC program are determined by how significantly they can contribute to reducing a utility’s peak demand. Load research, therefore, is the first step in DLC implementation. A utility must determine the load’s coincidence with system peak to determine which appliances are appropriate for its DLC system. The load control strategy depends on which loads are chosen for the DLC program. In residential loads, electric water heaters and pool pumps often are shed. These loads are shed rather than duty-cycled because water heaters are storage devices and, therefore, a total shed does not cause customer discomfort as long as the shed period ends before the customer runs out of hot water. In addition, most customers do not mind an occasional shortening of pool pump run time. HVAC systems normally are not shed, but some utilities offer that option. Typical HVAC control consists of either duty cycle restriction or temperature setback. The duty cycle approach to HVAC control has been successful, however, because all HVAC systems are controlled the same (50 percent off and 50 percent on), some customers contribute more than average amounts of load reduction while others contribute no load reduction. In aggregate, the utility still achieves the load reduction needed to cost justify the program. Significant variations exist, however, in load reduction contribution and comfort impacts across the program participant population.

To address this wide variation, utilities in partnership with load control vendors have added local intelligence to the load control receiver (LCR), generically referred to as adaptive control. Adaptive control is intended to determine the optimal control strategy for each customer’s HVAC system. Successful adaptive control will deliver more level load reduction among participants. With adaptive control, the LCR monitors the HVAC system’s running characteristics, time of day, weather conditions and so on and uses that information to predict the next hour’s natural duty cycle. From that prediction, the system can implement a duty cycle reduction percent, or in more sophisticated systems, control to requested load reduction. In the latter scenario, the LCR must be loaded with HVAC nameplate data to perform that on-site calculation.

This advanced distributed adaptive control is the ultimate sophistication in DLC. When the DLC is fully operational, the utility’s generation dispatcher simply commands the DLC system to deliver a specific amount of load reduction–50 MW, for example. The DLC system master station headend then determines the load reduction values that must be delivered by program participants and directs each LCR to calculate the customer-specific load control strategy that will deliver the required load reduction. This kind of adaptive control scheme addresses variable load reduction but does not address variations in customer comfort impacts among program participants.

To address variations in customer comfort during a control event, the focus of the control strategy shifts from level load reduction to equal temperature impacts. Programmable communicating thermostats (PCTs) that offer temperature setback as the form of load control are available. Temperature setback directly adjusts the temperature set point of the HVAC thermostat. This type of control addresses customer comfort impacts and re-establishes variable load reduction among program participants. Some utilities view the temperature setback as an improvement because all customers are equally physically affected during a control event–for example, a 4-degree temperature increase or decrease, depending on whether the utility is running a winter or summer control event. The utility will see adequate aggregate load reduction, but the variation in load reduction among participants remains. At the least, the HVAC systems that continue running will deliver some load reduction. Most PCTs can implement duty cycle control (with or without adaptive control) and temperature setback. The utility determines which program will be most popular with its customers, while still delivering adequate load reduction to meet its cost-effectiveness test.

Water heater control also has been improved significantly. Water heaters can be shed for significantly longer periods than in the past. A new controller that monitors the temperature at the bottom and top of the water heater is available. By monitoring temperature on-site, the controller can allow individual water heaters to override control events if needed to keep specific customers from running out of hot water. This frees the utility to control these loads without fear of causing customer discomfort. Water heater control durations are limited to the amount of time it takes program participants to begin to run out of hot water.

A 4-hour shed period is typically the longest shed cycle implemented because of that risk. Customer impact studies show that only a small percentage (some 4 to 5 percent) of program participants run out of hot water at that time. The utility, however, must restore service to all water heaters because it doesn’t know which ones are running out of hot water. An LCR that can monitor the water temperature and, if necessary, override control allows the utility to extend significantly a water heater load control event if necessary to maximize the load reduction benefits without negatively impacting customers. A direct benefit from this kind of water heater control is that the water heater becomes an energy storage device, almost like a battery. This functional capability allows a utility with high electric water heater appliance saturation to alter its daily load factor significantly by shifting the water heater load off peak. This is possible only because the risk of customer discomfort has been almost eliminated. Any positive change in a utility’s load factor delivers huge benefits to the utility and its customers. From a generation equivalency perspective, the value of this load reduction could be compared to an intermediate generator rather than a simple-cycle gas turbine peaker, the generation most often compared to DLC. If the DLC system lets the utility avoid construction of an intermediate generator vs. a peaker, the DLC system’s value increases significantly.

Wind Balancing

A creative application for DLC that will allow loads to be controlled solely to balance the fluctuations associated with wind generation is being developed. This is an attractive alternative to using standby generation, which simply reduces wind’s value. Loads with storage characteristics are natural targets, but traditional loads also are being reviewed for compatibility with this application. The control strategies are significantly more complex and must operate much closer to real time than traditional DLC applications.

Additional available features include:

Cold load pickup: Cold load pickup means that the controlled load remains shed for at least one time period (configurable) after electric service is restored to the grid. This starting delay allows the grid to begin to re-establish some natural diversity among appliances before these large loads are returned to service. Cold load pickup reduces chances of blowing fuses, tripping circuits, etc., as a result of excessive current on the circuit, which can occur when load is restored after a sustained outage.

Distributed intelligence: DLC can destroy appliances’ natural diversity, causing wide load swings on the grid as these loads become coincident during a control event. This always occurs when the load control strategy is operated from the DLC headend with no local intelligence to restore load diversity. A centralized control strategy that attempts to maintain appliance diversity becomes too complex to be practically implemented. To solve this problem, a design feature sometimes referred to as distributed intelligence is included in LCRs so that while under duty cycle control, each one can determine its own start and stop time. By implementing these start times linearly but randomly, the DLC system can control hundreds of thousands of appliances without creating an unbalanced load shape.

Under frequency shed: Under frequency shed means that the load control receiver autonomously can detect under frequency events on the grid and react to those events by shedding the controlled appliance. LCRs with this feature react fast enough to shed these loads before other under frequency relays connected to substation feeder breakers and other equipment can react to the grid disturbance. The goal is to shed enough load fast enough to stop the frequency decline before more drastic load shedding is implemented. The trip frequency also is configurable when these loads return to service.

Under voltage shed: Under voltage shed means that if the service voltage drops below a configurable level, the controlled load will be shed. This feature is used when the electric grid is under stress and there is risk that the voltage may dip below acceptable operational levels for HVAC compressors. If the voltage dips too low, the compressor can stall and go into a locked-rotor condition. Compressors in a locked-rotor condition will hit the already over-stressed circuit with orders of magnitude higher load than when running normally, likely causing the circuit to trip or, at a minimum, exacerbate an already overloaded grid.

Financial DLC Auctions

DLC auctions impact payment structures to end-use customers who participate in dispatchable DLC aggregation. Curtailment service providers (CSP) typically work with electric customers to curtail energy usage during peak demand, helping grid operators prevent blackouts and avoid new power plant construction. In return, operators pay a set price per megawatt to CSPs, who then pass on a portion of this payment to customers. Since launching an auction process, visibility into these transactions has provided participating customers market access with transparency, price discovery and liquidity. These auctions allow customers to participate in the DLC market.

Auctions are no longer the exclusive tool of independent system operators (ISOs) used to allot load-relief capacity to DLC CSPs. Competitive DLC auctions are moving these transactions closer to customers and dramatically reshaping how they participate in DLC aggregation programs. The result is this new auction process that maximizes customers’ share of DLC revenue paid by ISOs. These auctions also enhance DLC’s growth as a tradable resource, benefiting utilities and customers.

Maximizing the potential of auction-based DLC requires utilities to use technology solutions and program structures and, more specifically, base their load control strategies on customer behavior and comfort rather than yesterday’s outdated, generic, one-size-fits-all type of DLC.

Heber Weller, director of smart grid solutions for SAIC, is a professional engineer specializing in the design and implementation of advanced technologies for the electric utility industry.

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