Customer Satisfaction Will Remain High if we minimize the number and duration of outages a customer experiences. To continue already high customer-satisfaction ratings, PPL Electric Utilities (Allentown, Pennsylvania, U.S.) is commissioning its first two fully automated distribution circuits with a system it calls Supervised Intelligent Switching to Reduce SAIDI (SISRS). PPL believes this is the first distribution automation (DA) of its kind installed in North America.

PPL has approximately 1.4 million customers in its 10,000-sq mile (25,900-sq km) service territory located in central eastern Pennsylvania. The utility has about 1100 12-kV circuits, the vast majority of which are overhead construction. PPL has several metropolitan areas, with the biggest cities being Harrisburg, Allentown, Lancaster and Scranton. Additionally, the territory is mostly rural with rolling hills and mountains.

DISTRIBUTION AUTOMATION JUSTIFICATION

PPL tried unsuccessfully for many years to justify DA on an economic basis. When it chose to find ways to increase reliability for its customers, DA showed its benefits.

A team was commissioned to examine ways to lower the system average interruption duration index (SAIDI) and to set up a plan to go forward. The SAIDI team investigated many alternatives to reduce the duration of outages. The SAIDI savings of each alternative was calculated, as well as an estimated cost to achieve those savings. The cost was divided by the SAIDI savings to come up with a dollar/SAIDI-minute value.

Alternatives with lower values were implemented immediately. Some of the alternatives were: 24/7 trouble men, restoring as many customers as possible before repairing the trouble, increased sectionalizing (fusing), increased line maintenance and inspection, equipment replacement, increased tree trimming, increased load-transfer capabilities and DA. This is not an all-inclusive list of the ways PPL addressed lowering its SAIDI, but enough to give an idea of what the utility tried. The dollar/SAIDI-minute method is not meant to indicate how much it costs to reduce SAIDI. It is meant to put each alternative on a relative basis to see which projects can provide more relative investment benefit and which ones should be done first.

SAIDI is the product of the number of customers affected by a permanent outage and the duration of that outage divided by the total number of customers in the company. From this equation, to reduce SAIDI, the number of customers who see a permanent outage must be lowered or the duration of that outage must be reduced, or both. The total number of customers is fixed. PPL saw SISRS as a way to drastically reduce the number of customers who see a permanent outage. However, the reduction in those who see a permanent outage comes at a cost, those customers who are restored will still see a momentary interruption. So a reduction in SAIDI with SISRS may lead to an increase in the momentary average interruption frequency index.

DESIGNING WITH AUTOMATIC DEVICES

Preventing an outage in the tree-rich Pocono Mountains area is a daunting effort. PPL accepted that outages are going to happen, but that it would try to limit the number of customers who experience a permanent outage. The utility designed its system with enough automatic devices to limit any given permanent outage to 500 customers.

Once PPL decided to embark on an automation effort, it needed to find the circuits that would show the most benefit. The search began with the top 20% of the worst-performing circuits. These are circuits where history indicates that many outages have occurred. After all, why install the equipment on a circuit that has never had any outages?

Next, PPL discounted single-phase outages because the utility doesn't use single-phase ties. PPL then looked for outages that occurred at the circuit breaker. Outages at the breaker were in the first section of the circuit and all the customers were out. This condition allows for the largest percentage of customers to be restored with automation.

Lastly, if all the aforementioned conditions were met, we checked to see if there were any existing ties to the circuit, and the tie had to have the capacity to handle most of the adjacent circuit being transferred to it during restoration.

This analysis left about 20 circuits. PPL averaged the historical outages over the last five years to get an estimated annual SAIDI for each circuit. The utility assumed 50% of the permanent outages could be eliminated. If PPL used four automated devices per circuit, and assumed a normal distribution of outages, some outages would restore 75% of the customers, some would restore 50% of the customers, some would restore 25% and some would restore 0%. So PPL assumed the average of 50% even though it targeted the circuits where the circuit breaker operated most of the time. Two circuits were selected for the pilot project: the Blooming Glen 06-2 circuit and the Lanark 23-1 circuit.

LOCAL VERSUS GLOBAL

When PPL did due diligence on the automation systems that were available, the utility noticed there were two kinds of systems: local and global. These are not industry terms but PPL observations.

Local systems tend to be individually programmed devices that perform required tasks based on input from each other. The switching devices are intelligent and work quite well if every scenario is determined beforehand on each circuit. If the circuit is rearranged, they must be reprogrammed. At that time, the systems did not provide notification to anyone about what they had done. The customers are back in service, but the operator or dispatcher does not know it.

Global systems are more complex and require robust communication networks. The switching devices are intelligent and all of them communicate back to a single large computer with their status and alarms. This computer is usually centrally located back at the main office or operations center. The restoration solution is determined at the central computer and the instructions are sent to the individual switches, which communicate back after they are done. This system has the advantage of being able to show the operator or dispatcher what is going on.

PPL liked the global solution better, but did not have the funding for large computers and big communication networks. The utility came up with a hybrid design that can grow into the global system in the future. This will allow for a gradual build out and spread the cost out over 20 years. An approach such as this requires a standard protocol (DNP3) so that many different devices can be used.

THE SYSTEM

PPL selected Advanced Control Systems (ACS; Norcross, Georgia, U.S.) to develop the system, which consists of a host computer located at a substation. This computer has an operational model of the circuit and adjacent tie circuits. The host performs a load flow on the model before composing a restoration solution. Since the host knows the condition of the circuit at any time, it can even attempt to restore customers if a second fault occurs based on abnormal conditions.

All of the switching devices on the trial circuits are ABB electronic vacuum reclosers. Some of the reclosers interrupt fault current and the others are used as switches to isolate the fault and restore load. PPL decided to use reclosers for all switching devices to keep the development to a minimum, even though the cost was somewhat higher. Once PPL figured out how to program one, the others were easy. The utility has already begun testing motorized air-break switches at single locations with operator control. These will be phased into the SISRS program as PPL becomes more familiar with them.