Using transmission protection technology in distribution relays

As the development of ‘micro-grids’ indicates a need for improved distribution systemtechnologies, Alstom Grid provides a solution.

MiCOM P40 Agile, Alstom Grid’s recentoffering in its substation automation solutionrange, lives up to its name as thewell-known MiCOM P40’s smaller, lighter ‘sibling.’Alstom Grid recognises the mission critical status ofsubstation automation technologies and continuesto adapt leading edge transmission technologies fordistribution systems and industry.

There has been increasing pressure on energyoperators from governments and environmentalgroups to improve system efficiency while also ensuringreliable supply. As more and more operatingcompanies worldwide embrace the concept of theSmart Grid, and offer consumers access to flexibleenergy supply agreements, the composition of distributionsystems will change. One expected changeis the development of ‘micro-grids’ defined as interconnectedloads and distributed energy resourcesacting as integrated systems and operating in parallelwith the main grid or in islanded mode.

Certain generic features, such as programmablescheme logic and supervision functions that haveuntil now primarily been used in transmission systems,are already being sought at medium voltages.It may seem fairly simple to use existing transmissionprotection relays in distribution systems, butit is necessary to consider the possible limitations -alongside the benefits - of transmission protectiontechnologies when applied to distribution systems.

Programmable scheme logic
Programmable scheme logic is a transmission protectiontechnology that enables relay users to configureindividual protection schemes to suit theirunique application. This extremely flexible system,based on programmable logic gates and delay timers,allows users to create their own scheme logicdesign no matter how complex. The inherent flexibilityof programmable scheme logic makes it atechnology with obvious applications for distributionsystems and - when built into distribution relays- improves power reliability and quality.

Supervisory functions
As distribution systems take on a more critical status,it is vital that a greater degree of supervision isbuilt into secondary devices to enable earlier triggersfor predictive maintenance. Some supervisory functionsalready in use in transmission systems such assupply supervision, condition monitoring and tripcircuit supervision can be set in a distribution relay.

Programmable scheme logic can also be used effectivelyin this context. A voltage transformer supervision(VTS) feature is used to detect failure ofalternating current voltage inputs to the relay. VTSlogic in the relay is designed to detect voltage failureand automatically adjust the configuration of protectionelements whose stability would be at risk.

Communication infrastructure
Distribution systems are becoming smarter and,accordingly, they need a communications infrastructurethat is strong and reliable. Serial-basedcommunications protocols such as Modbus can beaugmented by more modern communication protocolssuch as the Ethernet-based IEC61850 to ensureprotection relays can feed as much informationas possible through the communications infrastructure.It is essential to provide options to suit projectrequirements, while maintaining the same interfaceto staff.

Circuit breaker fail protection
Programmable scheme logic can be used oncemore, in this instance, to develop simple circuitbreaker. Incorporating transmission protection technologiessuch as circuit breaker fail protection indistribution relays has several benefits including theenabling of predictive maintenance and ‘self healing’responses to system disturbances.

Increased availability of highly integrated relays
It is possible to increase the availability and effectivenessof a single protection. The increased availabilitycan be achieved by developing the self-diagnosticfeatures within the relay.

These self-diagnostic features can include therelay detecting software-related fatal errors andsubsequently performing a restart of the relevantprocess without having to reboot the entire device.This results in full product operational status beingregained within a few power system cycles; ashorter time period than the seconds it would taketo reboot.

Conclusion
The incorporation of existing transmission protectiontechnologies into distribution relays can reap amyriad of benefits. There may be constraints, however,such as cost, size and withdraw ability. AlstomGrid’s MiCOM P40 Agile has, as its name implies,the inbuilt flexibility and technological sophisticationto overcome these constraints and more, andto successfully bridge the gap between transmissionand distribution relay technologies.

For more information, contact Soon-Wi Lee at +659231 2770 or [email protected]