POWER DISTRIBUTION--Distribution Automation

Throughout its existence, the electric power industry has been a leader in the application of electric, electronic, and later computer technology for monitoring, control, and automation. Initially, this technology consisted of simple meters to show voltages and flows, and telephones to call the manned sub stations to do control operations. Yet as early as the 1950s supervisory control and the associated telemetering equipment was in widespread use with a 1955 AIEE (American Institute of Electrical Engineers) report [25] indicating 31% of transmission level stations (switching stations) had such control, and that the U.S. electric industry had more than 30,000 channel miles (48,000 channel km) for communication, and that continuous monitoring of watts, vars, and voltage was widespread. By the late 1960s increasingly sophisticated energy management systems (EMSs) were beginning to be deployed in electric utility control centers with applications that included automatic generation control, alarming, state estimation, on-line power flows, and contingency analysis. However, initially all of this monitoring, control, and automation was confined to the generators and the transmission level substations.

Because of its larger number of devices and more diffuse nature, the costs of monitoring and automating the distribution system could not be justified.

The monitoring, control, and automation of the distribution system is known under the general rubric of distribution automation (DA). While proto type DA systems date back to the 1970s, it has only been in the last decade or so, as communication and computer costs have continued to decrease, that they have started to become widespread.

A primary reason for DA is to reduce the duration of customer outages.

As was mentioned in Section 2, distribution systems are almost always radial, with sectionalizing fuses used to avoid having prolonged outages for most customers upstream from the fault location. Then sectionalizing switches can be used to further isolate the faulted area, and by closing normally open switches the unfaulted downstream sections of the feeder can be fed from adjacent feeders. DA can greatly reduce the time necessary to complete this process by either

1. providing the distribution system operators with the ability to remotely control the various sectionalizing switches; or 2. automating the entire process.

Real-time monitoring of the voltages and power flows is used to ensure there is sufficient capacity to pickup the unfaulted load on the adjacent feeders.

An example of this situation is illustrated in Ill. 22, which can also be seen in PowerWorld Simulator by loading case Ill. 22. This case represents a feeder system modeled using the primary loop approach from Section 2 with a nominal 13.8 kV feeder voltage. A total of 10 loads are represented, with each load classified as either primarily residential ('res'), primarily commercial ('com'), or industrial (' ind'); the bus name suffix indicates the type. The left side of the loop goes from the substation distribution bus 2 to bus 8ind; the right side of the loop goes from the substation distribution bus 3 to bus 13ind. Substation buses 2 and 3 are connected by a normally open bus tie breaker, while buses 8ind and 13ind are joined together by a normally open feeder segment to complete the loop. Each feeder line segment is assumed to use 336,400 26/7 ACSR line conductors and to be 0.6 miles (1 km) in length giving a per unit impedance (on a 100 MVA base) of 0.0964 = 1995. The two 12 MVA 138/13.8 kV transformers have an impedance of 0.1 = 0.8 per unit.

Assume a persistent fault occurs immediately downstream from the bus 3 breaker. This fault would be cleared by the bus 3 breaker, outaging all of the customers of the right branch feeder. Without DA, a line crew would need to be dispatched to locate the fault and then manually change the status of the appropriate sectionalizers to restore service to most customers on this feeder.

In contrast even with a simple DA, which just consisted of having the ability to remotely control the sectionalizers and monitor flow values/voltages, service could be more quickly restored to most customers on the feeder. This could be accomplished by first opening the sectionalizer downstream from bus 9com, then closing the sectionalizer between buses 8ind and 12res, then closing the distribution substation bustie breaker between buses 2 and 3 (after first balancing their taps to prevent circulating reactive power). This new configuration is shown in Ill. 23.

Another important use for DA is the use of switched capacitor banks to minimize distribution losses and to better manage the customer voltage. Since the feeder load is continually changing, in order to maintain the desired feeder voltages and to minimize system losses, the status of the capacitors often needs to be changed. Without DA various techniques using only local information have been employed including temperature, current, voltage and reactive power sensors, or just simple timers. While these approaches are better than nothing, they all have limitations. By providing a more global view of the entire feeder, DA can greatly improve the situation.

To illustrate, again consider the Ill. 22 case, which includes six 1.0 Mvar (nominal voltage) switched capacitors. The one-line display also shows the total system losses, and allows variation in the load multiplier for each of the three customer classes (residential, commercial, industrial) with a typical value for each ranging between 0.5 and 1.25. Initially all of the capacitors are in-service with total losses of 0.161 MW. However by manually opening the capacitors at buses 7res and 12res the losses can be decreased modestly to 0.153 MW. Under lighter loading situations the loss reduction from capacitor optimization can be even more significant.

While the benefits of DA can be substantial, what had been holding back more widespread adoption of DA was the costs, both for the initial up dated equipment installations, and the ongoing costs associated with maintaining the monitoring and control infrastructure such as communication costs. However many of these costs have continued to decrease resulting in more widespread adoption of DA technology.