Distribution Automation and the Self-Healing Network
- Written by Robert W. Uluski
Disruptive power outages of the past few years have resulted in customer outcries for a power delivery system that is able to withstand major storms more effectively, and “self healing” is one possible solution. Self-healing is not a new concept, but previous system designs must be re-evaluated to ensure proper operation with a high penetration of distributed energy resources, which pose new challenges and offer new opportunities for improved performance during emergencies.
Many electric distribution utilities are planning to implement automatic sectionalizing facilities on distribution feeders as part of their grid modernization strategy to achieve the vision of a “self-healing” distribution grid. Self-healing is not a new concept. Electric utilities have been implementing automatic reclosers and sectionalizers for decades to restore power without human intervention to customers whose power delivery infrastructure remains intact following a short circuit. Automatic failover schemes have also been used for years to transfer critical facilities to a backup supply when the normal power source is lost. In addition, more advanced automatic service restoration systems that detect and isolate faulted feeder sections, and then restore service to healthy (unfaulted) feeder sections, have been used on distribution feeder loops for over twenty years. These early approaches have generally worked quite well and have played a major role in reducing outage duration for many consumers by 50 percent or more.
Today, electric distribution utilities are facing new challenges that have a significant impact on the self-healing system functionality. Electric distribution facilities are routinely loaded to more than 50 percent of rated capacity, and sufficient capacity may not be available on backup sources when needed. Hence, the recent generation of self-healing systems commonly includes logic for dealing with these issues. Many systems of the past ten years will automatically block any load transfer that would overload adjacent feeders. The more advanced systems will even divide the load into smaller pieces that can be supplied by multiple backup sources without producing unacceptable electrical conditions.
Some electric utilities have experienced problems with self-healing systems that have operated successfully for years. Often these problems are the result of increased DG on the feeder. Distributed generators are a possible source of fault current that, while small compared to the level of fault current from the electric utility’s generators, may be enough to trigger faulted circuit indicators that are downstream of the actual fault. As a result, the incorrect feeder segment will be identified as faulted, and restoration switching activities may re-energize the fault and possibly lock out multiple feeders. This problem can be addressed by using “directional” fault current indicators that identify the direction of current flow or by using centralized modeling approaches that account for short circuit contributions from distributed generators.
Another challenge resulting from the presence of distributed generators is what is referred to herein as the “net load” problem. Todays’ fault location and system restoration (FLISR) systems continuously monitor the power flowing into and out of each feeder section to determine the load that may need to be transferred if a short circuit subsequently occurs. The problem arises when the distributed unit trips off line when a fault occurs. Since the unit is not allowed to reconnect until approximately five minutes following restoration of normal primary voltage on the electric utility lines, the amount of load to transfer may exceed the amount measured just prior to the fault, potentially overloading the backup feeder. A possible solution is to monitor distributed contributions at all times and account for the generation dropping off line when an outage occurs. But this solution may only be practical for larger (utility scale) DG units.
Despite the challenges facing us today, the future is bright for self-healing systems. The presence of distributed generation and other distributed energy resources like storage can also provide opportunities to enhance the performance of future self-healing systems. In cases where FLISR load transfers are frequently blocked due to high load, future FLISR systems will be able to exploit distributed resources like generation and storage to reduce the net load being transferred or to free up available capacity on backup sources.
Future systems may also be used to operate feeder extremities that are isolated following a fault event as microgrids that are powered by distributed generators and other distributed resources. This approach may also enable electric utilities to derive value from self-healing systems during major storms. FLISR systems are often disabled during major storms due to the lack of available backup sources and due to extensive damage to the power delivery infrastructure. The ability to automatically transfer to microgrid “island” mode in such circumstances will enable utilities to continue supplying critical loads during widespread power outages.
Some utilities are taking self-healing to the next level by deploying distribution feeders that are fully networked at the primary voltage level. In such cases, when a fault occurs on the networked feeder, the faulted section can be quickly isolated from both ends with no interruption at all for customers that are served by healthy sections. Short of developing power delivery components that automatically repair themselves, networking capability truly represents the ultimate in self-healing technology.
In all cases, developing a business case that shows that the benefits accrued by self-healing outweigh the costs is a daunting task. Savings derived by the customers who experience fewer and shorter duration outages can be very significant. However, the use of customer outage cost savings is not accepted as cost justification in many jurisdictions. Utilities can help solve future investment challenges through forward-looking investments that prepare the utility for future FLISR deployment. For example, the utility should invest in electrically operable switches whenever an existing manually operated switch needs to be replaced. Also, electric utilities should ensure that AMI communication infrastructure can handle the communication requirements of self-healing systems.
Despite the technical and financial challenges of implementing self-healing networks, it is almost certain that electric distribution utilities will continue to deploy self-healing mechanisms to improve service reliability for the benefit of its customers.