Functionality of DMS

The functionality of DMS can be divided into steady-state performance improve-ment and dynamic performance improveimprove-ment.

2.4.1 Steady-state performance improvement

In this section the main functions associated with the analysis of the steady-state performance of the system are considered. These functions are characterized by fulfilling targets previously defined in planning studies.

2.4.1.1 Volt/VAR control

Volt/VAR control deals mainly with the detection and prediction of voltage violations which require close control of the relationship between the voltage and the reactive power equipment. The equipment involved includes mainly

Figure 2.5 Screen shots of a typical EMS

capacitor, static VAR control (SVC) devices, load tap changers and voltage regulator controls.

2.4.1.2 Feeder reconfiguration

Feeder reconfiguration refers to the operation of switches, breakers, or reclosers to reconfigure the topology of feeders and improve the operating condition of the system. It is done under normal conditions in order to reduce losses and increase reliability. It also helps to minimize voltage drop on the feeders. Feeder reconfi-guration represents great benefits. However, it is not always possible to achieve all the objectives simultaneously.

2.4.1.3 Demand side management (DSM)

DSM refers to a method for controlling the load at the user’s premises, based on an agreement previously established with the utility.

2.4.1.4 Advanced metering infrastructure (AMI)/automatic meter reading (AMR)

AMI/AMR plays an important role as it aids utilities in establishing competitive strategies that conduct load profile gathering in order to characterize the value of individual customers to the utility. It identifies the major customers (aggregated load) and offers load information to customers as a special service.

One of the technologies used by AMI is power line communication – PLC – that uses the wires of the distribution system including the customer’s incoming lines. The signal rides on the fundamental power line frequency and modulation occurs at the zero-cross. It does not require repeaters or line conditioning equipment, resulting in lower installation costs. This technology does not affect power quality. Figure 2.7 illustrates a typical PLC system that uses the technol-ogy known as TWACS (two-way automatic communication system). This system uses modulation for the outbound signals and modulation for inbound signals as shown in Figure 2.8.

Figure 2.6 Screen shots of a typical DMS

2.4.2 Dynamic performance improvement

In this section the main functions associated with the analysis of the dynamic performance of the system are considered. These functions are characterized by actions to be taken during faults, unpredicted events, and emergency conditions.

2.4.2.1 Fault location, isolation, and service restoration (FLISR) The use of automated feeder switching aids in detecting feeder faults, determining the fault location (between two or more switches), isolating the faulted section of the feeder, and providing the possibility of restoring service to ‘‘healthy’’ portions of the feeder.

Service restoration allows to find alternate options to quickly restore power to healthy parts of the system making sure that voltage levels are within accepted ranges and that any overload is avoided. Cold load pickup has to be considered for long outages in order to avoid tripping when re-energizing the feeders. Customer prioritization is carefully considered.

Meter retrofitted with module CSS

Client workstations SCADA, meter data (PC)

management, outage

Figure 2.7 Illustration of power line communication (PLC)

V

Figure 2.8 (a) Modulation for the outbound signal; (b) modulation for the inbound signal

Automating this process renders immense benefits as the outage time is greatly reduced, as shown in Figure 2.9.

2.4.2.2 Trouble call system

When a fault occurs, protective relays and fault locators should raise the corre-sponding alarms that outages have occurred. However, the report of users affected is very important not only to reaffirm the fault occurrence but also to help locating it more quickly.

Utilities now have modern facilities where the calls from users are readily received, classified, and matched with the database of the billing system to identify other users who may be associated with the problem. Thus, they can achieve solution schemes that identify and involve all the customers affected by the event.

These facilities are trouble call systems that are manned but assisted with state-of-the-art communication and software capabilities. A typical process of a TCS is illustrated in Figure 2.10, which requires a close relationship with the customer information system (CIS).

2.4.2.3 Alarm triggering

Alarms are triggered by the operation of the protection relays. They send coded information to distribution control centers so that operators are aware of what is happening on the network. Other alarms indicate the state of the power system, for example, voltages at various locations and load flows on the more important cir-cuits. These alarms provide one of the main sources of information flowing in real time into a distribution control center and are normally channeled to one printer in the control room where a hard copy can be produced. The alarm streams are also channeled to the operator’s or control engineer’s console where they can be dis-played on computer screens.

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Restoration time considering DA

Restoration time with manual procedure Outage %

Figure 2.9 Comparison of restoration time with and without DA

A third avenue for alarm streams is storage in a data logger where the principal function is to retain a history of the alarm streams. This can be used for post-system fault analysis if it is needed later.

It is worth nothing that, with this arrangement, the alarms are not processed.

Several events may occur simultaneously, or at a close time proximity with respect to each other, and each incident may trigger many alarms resulting in a large number of alarms flowing into the control center in close succession. The operator would then have to use his judgment to determine what has happened to the system.

Subsequent telephone calls from customers may also help to determine the exact location of these incidents such as a blackout in a certain district. The aim of an alarm processor is to help the operator to arrive at a sensible conclusion speedily and to discard redundant information in the alarm streams.

2.4.2.4 Work orders

In order to expedite the coordination of crews responsible for the maintenance or feeder fixing, a proper work order system is essential. After a fault is reported, the system should be capable of automatically locating the most convenient crew and traveling means, and to determine the essential elements required to reestablish service as quickly as possible. This requires a proper coordination with the DMS, the resource scheduler, and the operation manager of the utility, as depicted in Figure 2.11.