Properly planning predictive and reliability-centered maintenance

Wind-power generation facilities must maintain reliable and dependable high-voltage systems and substations to maintain profitability. Grid reliability agencies, both national and regional, require maintenance testing. With the testing, it is good practice to ensure collector systems, transformers, and substations are all operating properly to avoid downtime and maintain consistent output.

Some wind plants have chosen to stay online for extended periods and just wait until an outage occurs. However, availability and production are critical for generation facilities and a failure in the high-voltage system can take down the entire wind plant, not just one turbine.

So how can wind-farm owners plan the maintenance necessary to their collector and high-voltage systems and reduce downtime? Predictive and reliability-centered maintenance are key concepts used to reduce downtime while providing the level of maintenance needed to keep generators online and producing power. Developing a maintenance program based on these two concepts is not an easy task. Here are a few guidelines to include in a maintenance strategy for high-voltage equipment and why.

Maintenance planning
The testing frequency for this type of equipment varies based on many criteria. Every site has specific needs depending on the equipment design and manufacturer. Some of these factors include:

• The critical nature of the equipment,
• Historical reliability,
• Age of the equipment, and
• Requirements of the power purchase agreement with local utility.

Basing a maintenance program on national consensus standards is said to bring credibility. We’d recommend referencing sources such as:

• NFPA 70B, Recommended Practice for Electrical Equipment Maintenance,
• InterNational Electrical Testing Association (NETA) MTS-2011,
• Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems, and
• Several IEEE standards.

The NETA testing standard also offers guidelines for the frequency of maintenance tests within Annex B of the document. A maintenance checklist should include:

• Generator step-up transformers
• SF6 circuit breaker
• Vacuum-feeder breakers
• Arresters
• Current transformers
• Air switch
• Disconnect switch
• Station-service transformer
• Capacitor banks
• Battery system
• Battery charger

Other critical items include a medium-voltage wiring in the collector system as well as the grid interconnection relays and controls as required by regulations. Monthly and quarterly checks on the collector system can be performed by in-house personnel when they are qualified or by a third-party contractor when the work is beyond the abilities and training of the in-house personnel. Online testing can also present additional risks and hazards with which in-house personnel may not be familiar. Normally, the overall impact to operations of these regular checks is minimal, and typically outages are not required. Trending such factors as generator heat, vibration, and transformer-oil analysis data often minimizes downtime and disruption to operations with time.

Qualifying electrical workers
Technicians performing electrical tests and inspections should be trained and qualified to understand the hazards associated with operating, switching, and maintaining electrical-power equipment. An independent, third-party contractor often provides the necessary expertise required to perform online testing, interpret and trend the data, and create a reliability-centered maintenance program. Selecting a qualified contractor with an industry-recognized electrical testing accreditation, such as NETA accredited companies, will help ensure that a qualified and competent testing organization performs the necessary testing.

Whether staff or outside technicians are chosen, due diligence to their capabilities and qualifications is essential. Wind-farm owners and electrical workers must be cautious of arc-flash hazards, particularly on pad-mounted transformers. Wearing the proper personal protective equipment and employing a qualified safety backup can lessen the risk.

NFPA 70B explains how proper maintenance of electrical equipment is critical to maintaining workplace safety. Regardless of what PPE is used, as determined by arc flash studies and good safety practice, if the equipment fails in service or does not operate as designed, serious injury can result. Carefully following the OSHA, NEC, and industry guidelines ensures the reliability of the equipment and safety of the workers. WPE