IEEE C37.230, Guide for Protective Relay Applications to Distribution Lines.

The IEEE PESPSRC Line Protection Subcommittee formed Working Group D5 to develop a guide to describe principles, practices and special topics in protecting distribution lines. The result of the working group's efforts is IEEE Standard C37.230 Guide for Protective Relay Applications to Distribution Lines.

The Guide gives a thorough overview of the fundamentals in protecting distribution systems. Among the topics discussed are the various types of faults that can occur on distribution lines, the impact of fault impedance, characteristics of loads, harmonics and the impact of transients on distribution lines. These topics assure that the user has the needed basics to use the Guide.

Three main methods of grounding for distribution circuits are discussed:

• solidly grounded systems
• ungrounded systems
• impedance grounded systems

The different types of bus configurations used in distribution substations in the industry are described, taking into account load characteristics, continuity of service to customers, operation, maintenance and cost.

Application of protective relays for distribution circuits is discussed. This includes ground fault protection, protection of overhead conductors and cables, application of line reclosers and sectionalizers,  use of expulsion and current limiting fuses, providing backup protection.

The Guide also discusses the fundamentals of CTs and VTs used for protective relaying, including sizing, accuracy and their characteristics and performance under fault conditions. The application and setting criteria for phase, ground, and negative sequence overcurrent relays to protect distribution circuits is provided, including tradeoffs in the selection of settings.

Additional topics include considerations for protective device reach and unbalance current, characteristics of cold load pickup and transformer magnetizing inrush, fuse saving and fuse blowing schemes, and the application of voltage sensing elements to provide protection for distribution equipment. These topics allow the reader to evaluate their protection philosophy.

A section on protective device coordination is provided. The section includes discussions on coordinating time intervals, accounting for transformer magnetizing inrush, cold load pickup, reclosing, and reset characteristics of protective relays and fuses. Several examples of overcurrent device coordination for distribution circuits are provided, including coordination of distribution overcurrent relays, reclosers and fuses, coordination of devices with different time-current characteristics, and coordination of devices with different reset characteristics.

The final section of the Guide covers special topics for distribution protection that may not be common on every distribution system. What follows is a summary of some of the topics:

Often, multiple distribution circuits supplied from the same source substation are built in close proximity to each other. In these situations, a fault can occur on one circuit and eventually involve one or more of the other circuits. While the protective relays for each circuit will only react to the fault current flowing on its corresponding circuit, this will only be a fraction of the total fault current. However, the relays at the common source of the circuits will react to the total fault current.

This can lead to miscoordination between the source relays and the individual circuit relays. To assure proper coordination between source and circuit relays, a number of different options are presented, including providing additional coordinating time between source and circuit devices, and by use of adaptive relaying to disarm certain overcurrent elements when situations are identified in which circuits are faulted simultaneously.

Distribution Automation (DA) is becoming more popular with electric utilities to provide both prompt response to faults and efficient operation of the distribution system. DA can allow a system operator to monitor the distribution system by providing real-time amps, volts, and power flow data. It can provide both status and control of circuit breakers, reclosers, and switches on distribution circuits, provide disturbance direction detection, identify trouble locations on circuits and dispatch personnel to those areas. There is also the ability to monitor loading of circuits to assure that circuit ratings are not exceeded during peak load conditions and provide automatic sectionalizing of circuits during storm conditions.

Underfrequency relays are applied to some distribution circuits to provide load shedding to impede region-wide underfrequency excursions caused by loss of generation or transmission facilities. Undervoltage relays can be applied to distribution circuits to shed load to prevent a total collapse of system voltage due to a lack of reactive power support.

Adaptive relaying can be used on distribution circuits to optimize distribution protection dependability and security for varying system conditions. One historical application of adaptive relaying is permitting fast tripping of circuit interrupting devices for the first one or two trip operations during a fault event and then disabling them for the remainder of the event.

Another application where numerical relays with multiple setting groups are installed is automatic changing of setting groups to provide appropriate protection functionality to adjust for changes in system conditions.

Interconnection of Distributed Resources (DR) on distribution circuits is becoming increasingly more common in the utility industry. DR can impact the protection of the corresponding distribution circuit, including coordination of protective devices on the interconnected circuit with those of adjacent circuits. DRs also affect the coordination with the substation transformer backup protection, the sensitivity of utility relays for faults downstream of the DR interconnection, system grounding, reclosing and islanding of utility load on the distribution circuit with the DR.

The use of communications channels connected to microprocessor-based relays in order to communicate with other IEDs has been increasing on distribution circuits in order to improve the speed and dependability of protective schemes to improve reliability and power quality. For looped-radial and looped distribution systems, the substation relays and IEDs can communicate with each other to determine the location of a fault, and then execute the appropriate tripping and closing of devices to isolate the faulted section and provide continuity of service to the remainder of the distribution system.

Single phase tripping of distribution circuits offers some advantages to three phase tripping for improved reliability and power quality.

For instance, upon occurrence of a single phase-to- ground fault on a distribution circuit, single phase tripping can provide continuity of service to customers supplied from the two unfaulted phases while the faulted phase is removed from service. However, care should be exercised applying single phase tripping to circuits with ungrounded capacitor banks, and with three phase motors. Also, ferroresonant overvoltages are possible for circuits with grounded capacitor banks and ungrounded transformers.

Methods of detection of ground faults on ungrounded systems and on resonant ground systems are also discussed in the Guide.

The Guide is available through the IEEE Standards Department.

At the time this standard was completed, the working group had the following membership:

William P. Waudby, Chair; Randy Crellin, Vice Chair; John Appleyard, Ron Beazer, Ken Behrendt, Martin Best, Kenneth A. Birt, Gustavo Brunello, Jim Burke, Pat Carroll, Albert Darlington, Ratan Das, Ahmed Elneweihi, Fredric Friend, Rafael Garcia, Irwin Hasenwinkle, Roger Hedding, Randy Horton, Richard Hunt, Shoukat Khan, Ljubomir Kojovic, Ed Krizauskas, Raluca Lascu, Larry Lawhead, Walter McCannon, Michael McDonald, Dean Miller, Anthony Napikoski, Robert Pettigrew, Sam Sambasivan, Mark Schroeder, Tony Seegers, Tarlochan Sidhu, Charlie Sufana, Jon Sykes, Betty Tobin, Joe Uchiyama, Ron Westfall, Inma Zamora, Karl Zimmerman, Stan Zocholl