Effect of Distribution Automation on Protective Relaying

By Fredric A. Friend, AEP, USA
IEEE Working Group Report

Modern protection technology allows some possibilities that can be used to optimize the operation of the network in coordination with DA applications and, on the other hand, DA application deployment may have some impact on relaying.  Protective relays and controls are often a key part of DA; they provide information and, also, may be used to execute controls. This document expresses some thoughts on this matter by providing a brief history, describing how various schemes can effect relaying, describing the effect on relay applications and settings, and concluding with the impact on system maintenance. The purpose of this document is to explore the effect on protective relaying when DA is applied on a distribution system.  For this purpose, DA is defined as the sectionalization and reconfiguration of distribution circuits that include the use of automatic or remote-controlled transfer switches, line reclosers, fault interrupters, sectionalizers, and / or automated capacitor controls.
The first sections of the document include common definitions, key abbreviations and acronyms that the reader needs to know. This is followed by a short history of distribution automation intended to help protection specialists that do not have experience with DA to better see the relationships with protective relaying. This section covers two aspects and stages in the history: station distribution automation and line distribution automation.

Today's distribution automation applications are described in the next section of the report. DA has evolved to include a wide gamut of applications that include monitoring, control, reconfiguration, reporting, and evaluation:

 Remote monitoring

  • Typically use SCADA protocols such as DNP3 or metering protocols - ANSI C12
  • Fault detection at feeder devices (e.g., faulted circuit indicators - FCI)
  • Circuit measurements (e.g. voltage, steady-state/fault current, and/or real/reactive power from discrete CTs, VTs and/or FCIs for both overhead and underground circuits)
  • Load measurements (e.g. energy, voltage, current and/or real/reactive power from AMI billing or distributed generation meters)

Remote monitoring with control

  • Typically use SCADA protocols such as DNP3
  • Voltage and VAR control (e.g. power measurements and voltage or VAR regulation with line capacitor banks or line voltage regulators)
  • Generation control (e.g. power measurements and generation mode of distributed generation)

Remote monitoring with circuit reconfiguration

  • Typically use SCADA protocols such as DNP3
  • Equipment status (e.g. open or close of station or circuit switches)
  • Fault detection, isolation and restoration (e.g. fault detection, power measurements, and open or close with line reclosers or switchgear with fault interrupters)


  • Typically use file transfer protocols
  • Power quality measurements (e.g. harmonic content from high-end meters or monitoring/control devices)
  • Disturbance recordings (e.g. fault signatures or oscillographics from high-end meters or monitoring/control devices)


  • Remote or local
  • Accurate fault location (e.g. based on analysis of fault currents, voltages and/or disturbance recordings)
  • Spare capacity for circuit reconfiguration (e.g. based on assumed equipment capabilities and historical power measurements)

Many of these applications can share functions, sensors, and especially communications with other DA applications.  This integration, when properly planned, can reduce costs and improve benefits to the global DA system.
DA schemes come in many varieties and complexities that range from simple applications that use local, independent equipment to system-wide, centrally-controlled automation.  Localized schemes that use some form of intelligence can be connected to large control centers which can also be connected to even larger central control centers. 
DA schemes, such as Fault Location, Isolation, and Service Restoration (FLISR) or Optimal Network Reconfiguration, that reconfigure or change the distribution circuit can affect protective relaying. These schemes can also impact the fault level contribution and direction, particularly for systems operating in loop mode or have distributed resources.
Local intelligence: Localized DA schemes are applications where automatic functions occur with minimal communication between devices.  Functionality is contained within the device and occurs based on external stimuli, such as voltage and current.  For example, Figure 1 shows a simple transfer scheme of Breaker T in which Loads X and Y are maintained in the event of the loss of either Line A or B.  Assume a Line A fault that causes Breaker 1 to open.  The Breaker 1 auxiliary contact is an input to the Breaker T control scheme and initiates a closure of Breaker T.  After Breaker 1 is restored, Breaker T automatically opens.  The Breaker T controls may contain transfer or restoration time delays.  In some schemes, loss of voltage initiates breaker operation instead of auxiliary contact logic.

Distributed intelligence -  also referred to as decentralized intelligence applies communication and software to localized sectionalizing and fault-interrupting devices to provide automated control within a defined area that can vary from a simple circuit segment to a region where multiple circuits interconnect several substations. While these devices are primarily controlled at the installed location, rather than from a central location, the shared software and communication distribute the data pertaining to the event or condition among the devices to effect the required circuit reconfigurations.
Central intelligence: DA schemes utilizing central intelligence take the concepts associated with localized or distributed intelligence schemes and apply them across larger control areas.  A centralized intelligence scheme can determine the optimal switching sequences, and if desired, issue the switching commands to optimize FLISR and other advanced functionality, including Optimal Network Configuration, Volt-VAR Optimization (VVO), and Dynamic Equipment Rating (DER).
Requirements for telecommunication: DA depends on good telecommunication systems, which no matter the media and protocols used, must be reliable and robust.  They also must be designed and implemented in an understandable way to ensure any required trouble-shooting is straightforward. They must be designed in such a way that a failure of communications does not cause a failure in basic protection.  Since a good telecommunications system is the cornerstone of distribution automation, potential workforce jurisdictional problems, internal or external, need to be considered, during planning, implementation, and on-going operation.

Effects on Relay Applications and Settings:
Circuit reconfiguration has effect on relay applications and consists in the modification of the topology of a distribution network by operating NC and NO switches. Sectionalizing Switches located along the feeders are NC and allow isolating sections of the feeder when required, while Tie Switches connected between two feeders are NO and allow transferring loads between the feeders when they are closed. The selection of the location of tie and section nodes is based on flexibility criteria. Load serving issues, looping and Fault Location, Isolation and Service Restoration (FLISR) are discussed from the perspective of relay applications later in the section. Figures 2 and 3 show the result of FLISR following a fault in Z2.
The report also analyzes the impact on various protection schemes, zones of protection, protection coordination and adaptive relaying. Protection functional requirements after reactive circuit reconfiguration, such as for the cases of fuse saving/ sacrificing, fault locating, remote or   alternate settings, and especially DR are also described.

Impact of system maintenance on distribution automation is discussed at the end of the report, which will be published on the IEEE PES PSRC web site.  


Background on D11
In 2005 DA was becoming a hot topic and I had a desire to learn more, so I requested the Line Protection Subcommitte to form a task force to study the issue to determine if there was indeed an effect on relaying, which ultimately lead to a working group.  After many presentations from various utility practices, the working group began developing a paper with many lively meetings as DA began evolving in the industry
The working group had many discussions regarding whether we were looking at the effect of DA on relaying, or the effect of protective relaying on DA.  The document ultimately shaped up with sections on the history of DA, the effects on relay applications and settings, and the impact of system maintenance.  Some supplemental information is provided in two annexes, including a history from the Duquesne Light Company.


Let?s start with organization in protection testing