At 12:40 P.M. on Tuesday, May 30, 2006, Western Power was faced with an unplanned outage at its Clarence Street (CL) Substation located in an inner suburb of the capital city, Perth, Western Australia. The outage occurred due to a flashover on a circuit breaker. It left approximately 10,000 customers without power in the high-rise residential area, which includes a large shopping and entertainment district. This outage also included two different types of sensitive customers: two hospitals and the city zoo.

The suburb's geographic location makes it somewhat isolated as it is bounded by the Swan River from the west and the north, which limited the utility's options for interconnection. There was only one nearby substation, the Collier (COL) Substation, which connected to CL Substation's old 11-kV network.

There are three lines that can backfeed power from the COL Substation to the CL Substation. These sections have weaknesses that were identified and managed by the controllers and engineers. Limited interconnections made it imperative that all these sections run at full capacity to feed as many customers as possible, including those normally fed by the CL Substation.

Managing distribution network operations has changed considerably since the days of wallboard displays. Western Power, situated in the southwest of Western Australia, operates one of the world's largest isolated interconnected network. GE Energy's Electrical Network Management and Control (ENMAC) distribution management system (DMS) assists Western Power in almost every aspect of planned and unplanned network operations. These systems were particularly effective during the CL Substation outage. Both ENMAC and the distribution power analysis (DPA) programs were used to efficiently restore the maximum number of customers and to prevent failure of critical conductors.

The ENMAC DMS is a multifunctional tool used for the operational management of the Western Power distribution network. Its major function is to provide a real-time single-line diagrammatic view of the distribution network from zone substations down to distribution transformers. It is used in production mode by the controllers and other key personnel in the control center to monitor and create planned and unplanned switching programs for operating the network.

Once the fault occurred, the single-line view highlighted the CL feeders to show the areas lost. A variety of methods were used to assist the control center in determining the best way to restore the network:

• Interconnecting feeders between CL and COL were easily seen through the single-line schematic by using the trace function. Selected normal open points between the two feeders were closed to successfully backfeed to CL.

• Symbols such as blue stars and red crosses were used to represent sensitive and life-support customers on distribution transformers to allow controllers to prioritize restoration.

• Real-time telemetered values of the loads of the interconnecting COL feeders were viewed to determine how much spare capacity was available on each individual feeder by comparison to the alarm limits.

• Trending of CL and COL feeders was performed with graphs showing the past load trends of selected feeders, reclosers or power transformers for a specified range. This function was particularly useful because it enabled the controllers to view the expected trend for the feeder. By viewing the developing evening peak, controllers were able to determine when it would be necessary to curtail load.

• The controller created a job showing all the proposed switching steps and could confirm each step with the fault crews as they happened. This increases safety for the switching operators as the operations are planned and documented.

• Other controllers and the coordinator also created smaller jobs to plan for the advent of an offload due to morning and evening peaks. These switching documents are also stored to allow for investigation after the event.

• Western Power enlisted a number of emergency-response generators to aid in restoration. ENMAC assisted in the strategic placement of these assets and generator documents were placed on the point of connection on the single-line diagram to view their placement. This allowed the generators to be safely managed while connected to the network.

• To enable the controllers to clearly see assets that required repair and that may not operate correctly, “request for repair” documents were placed on the affected asset. This allowed controllers to determine the best switching point available.

• For a closer study of the system in key areas, such as checking the conductor types and lengths for ratings details, the planning engineers used a map to GIS function. Key nodes can be mapped from ENMAC to a geographical network diagram for more details of the network in a particular area. This function also enables a geographical study of how far the fault crews needed to travel for each step of the switching program. This was especially useful in situations where switching operations included a large load transfer that could be minimized if the steps were done soon after one another.

Distribution Power Analysis (DPA) is an integrated load-flow and fault-level tool that provides indicative load values and fault levels at selected points in the network. DPA has several modes of operation:

• Load allocation is a real-time load-flow program using available real-time telemetry, such as power transformer, feeder, circuit breaker, voltage regulator, load break switch and recloser currents at any part of the switching program. Load allocation takes real-time telemetered measurements, and scales loads to force a solution that matches the telemetered measurements. This study is suitable for real-time operations such as faults and outages.

• Load profile is a load-flow study that takes into account the distribution transformer load profiles for the date, time and temperature over the course of the year. This is suitable for forecast studies.

• Periodic manager automatically and continually calculates the network to see if any limits are violated. Recalculation of the entire network is done every 12 minutes. Currently, it is used to indicate potential loading issues at known trouble spots using colored current values.

The DPA tool was used throughout the restoration to assist in judging the exact extra load that the three interconnecting COL feeders could support.

Western Power controllers used DPA to gain a better understanding of how much load would be transferred in the switching operations. DPA was also used in the job schedules to see how much load would be transferred at particular switching steps and produced indicative load values on interconnecting feeders.

Additionally, the operational engineers used quick load-flow, load-profile and load-allocation studies to gauge the effect of the extra load at the morning and evening peaks. This eased the decision of where and how much load should be shed if the peak load was greater than the available capacity on each of the interconnecting feeders. The control center could then identify which customers would be affected in an offload, so Western Power could proactively attempt to advise sensitive customers.

Using the advanced network management facilities, as well as DPA, enabled approximately 99.66% of CL to be backfed from COL Substation four hours after the occurrence of the initial fault.

One of the major feeders used to backfeed CL customers was COL 302. Through the use of DPA, the weak section of the conductor never failed due to overload. The alarm limits of COL 302 were consulted to provide an indication of the expected spare capacity. The section of the three-phase line with 7#12 American wire gauge copper (each strand is 2.7 mm [0.106 inches] in diameter) conductor on the backbone of COL 302 (rated for 266 A) meant there was not enough capacity to support the extra evening load. It was decided that some load would be shed during the peak period.

DPA was used to simulate the COL 302 network with the interconnection under peak conditions to view the estimated load at each switching point from the ends of the feeders. The amount of offload depended on the load of the particular night. Western Power requested larger customers to conserve power and the emergency generators were connected, reducing the load so that it would not peak as it had on previous evenings. These load-shed points were also chosen to keep a constant supply to the hospital.

The limiting component of the three-phase line with 7#12 copper conductor can be overloaded for short periods of time before failing by roughly 20% to about 319 A. However, the maximum overload before failure depends on other factors including weather conditions. As there was no direct telemetry on this part of the network, DPA was crucial in determining when to start offloading to save the conductor and keep as many customers supplied as possible.

Quick load-flow studies were undertaken as the evening peak approached to see the DPA estimate of current through the conductor. The load through the weak section showed roughly a 35% overload.