Maintenance Strategies for Digital Substation Automation Systems

The impact of Digital SAS on maintenance strategies needs to be clarified, and there is also an impact of maintenance constraints on the design of Digital SAS. The task of WG B5.06 is the description of these aspects. The scope of work was thus defined as: The analysis and consolidation of strategies for conserving the capability of hardware and software repair, correction and update during the lifetime of the DSAS, taking into account the items and limits listed below:

1. Items within the scope:

  • Conservation of knowledge and abilities on the systems
  • Management of spare-part stocks
  • Maintenance strategies and frequencies for DSAS
  • Appropriate use of local and remote maintenance
  • Cases of systems using IEC 61850 and proprietary protocols.

2. Items excluded from the Scope:

  • Refurbishment (replacement of DSAS elements)
  • Migration strategies
  • Version management
    As usual the deliverable of the working group was a technical brochure describing the strategies for the maintenance of digital SAS.

Types of DSAS maintenance

The types of maintenance considered in this report as common to all SAS can be grouped as follows:

  • Preventive maintenance - planned within defined time-periods or after an advice by a manufacturer, whose goal is to verify that the equipment is working correctly and within the given tolerances. It also can consist in replacing equipment identified at risk of future malfunction. It can be based on different maintenance strategies - Reliability; Condition ;Time
  • Corrective maintenance that is event driven and has to be done after a malfunction of the whole system, a device or a component
  • Updating maintenance, aimed at updating the software or the firmware of an equipment or the information in a database, in order to solve a problem and, usually, without enhancing the functions. It may be done in the general context of a corrective or a preventive action and, like preventive maintenance, it can be planned in advance
    A maintenance action can cover one or more of the above described types of maintenance.

The changes in maintenance due to the introduction of digital Substation Automation Systems: A very common way of thinking is that new systems, having auto diagnosis on board, do not need any more preventive maintenance, but we are still far from its elimination. DSASs operate as digital devices, so their settings don’t have any drift and their faults can be detected by self-monitoring function: for this reason periodical IED test are moving towards I/O and communication testing.

The changes in Preventive Maintenance: The self-monitoring function in modern IEDs can detect over 90 % of internal faults of devices. In addition, measuring circuit and trip circuit monitoring functions help to detect faults outside the relays, increasing the reliability of the total protection system. Testing period for modern IEDs have been doubled (In many utilities the test interval for numerical protection is around six years now) and, in some cases, utilities have totally stopped doing periodical tests. It has to be remembered that these self-monitoring functions are not 100 % coverable, so some kind of periodical inspections are still needed, but we can for sure say that the self-monitoring functions of modern SAS systems have significantly reduced the costs of periodical maintenance and, at the same time, the reliability of protection systems has increased.

How Corrective Maintenance changes: With conventional SAS technique it was possible for big utilities with large own resources to do also in-house repair work for electronic relays and RTU devices. With digital SAS systems this self-made repair is not any more reasonable or possible. Faulty IEDs have to be replaced with spare parts or, at least, faulty boards should be replaced and the faulty units have to be sent to manufacturer for repair. In case of faults involving substation master units and software, utilities are almost completely dependent on vendors or service providers: it’s a question both of competence and of authorization to access to codes.

The evolution of Updating Maintenance: With DSAS based on software, it is well known that it is not possible to check every combination of conditions and events during project development, FAT and SAT, in order to avoid malfunctioning when systems are in service. When a fault occurs to a system in service, a new correct version has to be released by the manufacturer that requires to reinstall the software on the devices that are already in service.

Another problem is the need for refurbishing or renewing or expanding a part of a substation already in service. In other words, after the introduction of DSAS, Updating Maintenance increases significantly.

Insourcing or outsourcing of maintenance: One of the main decisions is the strategy to be adopted by the utility for doing the maintenance of DSAS. It can be based on one of the following two extreme positions:

  • Insourcing
  • Outsourcing

Advantages of insourcing are: high knowledge level and skill of the maintenance staff in the utility, no need of additional supervision of the subcontracted personnel, shorter response times, and faster information flow inside the utility.

Drawbacks of insourcing are: no strong commitment to reduce maintenance costs and the need to train people (with all the related cost) in order to guarantee the necessary competence.

The advantage of outsourcing is the reduction of responsibility and training cost for the utility.

The main drawback of outsourcing is the dependence on the vendor (or third party) and the consequent risk, especially in case of bad performance of DSAS and/or not fulfillment of contract requirements by the maintenance company.

There are also intermediate positions, consisting in a combination of the previous two strategies.

The company know how: New systems require new skill: no more only protection and relay, but also communication, software and configuration. The level of knowledge about the DSAS inside the utility is strongly dependent on the degree of insourcing of maintenance considered in the company strategy.

Testing the system: The kind of tests performed (and the way in which they are performed) during the different phases of the development of a DSAS project, play a significant role on the later DSAS maintenance. As the whole specification, also the tests have to be defined with maintenance in mind, therefore it’s important that the utility’s specification of DSAS includes a detailed test classification. The inclusion of the Qualification Tests, FAT and SAT in the specification can help in minimizing the maintenance to be performed during the life cycle of the DSAS.

The management of spare parts: Another important issue is the strategy concerning spare parts management that is also related to the theme of outsourcing/insourcing of maintenance. Every utility should have a clear and defined spare part policy for all its SAS, depending on the insourcing or outsourcing strategy adopted for maintenance.

The economic aspects: The long-term strategy for maintenance should take into account technical and economic aspects. Utilities, according to their strategy, have to find the best balance between preventive maintenance vs outage/fault.

Conclusions: The introduction of DSAS brings changes for maintenance in terms of subject, content, time periods. The dependence of utilities from vendors surely increases at the beginning, but its level can be mitigated, according to the utility strategy, by means of an adequate training program. The whole life cycle of a DSAS, from the philosophy to the decommissioning, has to be rethought in a more integrated way and human resources always must be considered at the center of the process.

The new technology not only renewal of the old one, but is a real new approach, that, on one side, can solve some problems related to the conventional SAS but also induces new problems such as the reduction of the duration of the system life cycle - a critical issue for utilities and for vendors.

 

Relion advanced protection & control.
BeijingSifang June 2016