What you can’t see can still hurt you

Underground cable fault detection can reduce unplanned outages and save costs

Preventing is better than care

Unplanned power outages are a significant expense and negatively impact resource planning, efficiency, and profitability for utilities. Many unplanned outages are caused by simple feeder cable failures.

The key to avoiding such outages and the costs associated with them is to identify potential problems before they occur. This can be challenging, as utilities struggle with budgets and personnel, especially when it comes to maintenance. In addition, not all cables are easily inspected – as is the case with the millions of miles of underground cables installed over the last several decades.

But advances in sensor technologies, innovative algorithms, and communications capabilities between the distribution system and control center are helping utilities predict failures before they occur, so they can do something about them.

When it comes to cable failure, both overhead and underground cables are a concern. Naturally, cables above ground or overhead lines are easier to address because work crews can easily see them.

Underground cables are a bigger challenge from an accessibility and visibility view point. With advances in insulation technology and social pressure on overhead lines, many utilities began installing underground cables in the 1970s and there are now millions of miles of underground cables installed.

While protected from elements such as wind or ice, underground cables are susceptible to insulation degradation and damage caused by water and animals. Additionally, splices from previously repaired cables are prone to failures. Also, many of these cables are reaching the end of their expected life, based on average life span.

As the cable insulation degrades and water seeps in, a short duration, high current fault may occur. Often, the heat evaporates the water and the problem resolves itself. These are termed self-clearing or incipient faults. They rarely cause an outage, but warn of cable failure that could result in an unplanned outage.  Sensing these incipient faults is therefore critical in predicting cable failures.

Predicting cable failure – A smarter solution

A smarter solution would be to detect feeder cable failures before they result in fault causing outages and proactively focus maintenance activities on those cables most likely to fail. Advancements in monitoring and control applications, as well as the development of algorithms which utilize new digital signal processing techniques, have made this solution possible for utilities.

Three key components are needed to enable underground cable fault detection:

• Intelligent electronic devices (IED) – This may be a sensor or a microprocessor-based relay with highly accurate data acquisition circuits and high-speed digital signal processing capabilities.

• SCADA system – The SCADA system enables the real-time cable fault detection information to be transmitted to the control room dispatcher for analysis and action.

• Network capability – This provides the communications link to every IED that feeds the SCADA system.

These same components can also detect incipient faults in overhead lines caused by factors such as animals or tree growth and allow the utility to proactively prioritize maintenance on these lines.

When choosing an IED, it’s necessary to select one that provides a dedicated real-time cable fault detection feature, such as ABB’s Relion® family relay REF615 which can be installed alongside an existing protection system. Alternatively, since the ABB REF615 also provides a configuration for both feeder protection and cable fault detection, utilities can invest in one comprehensive unit that performs both functions for the most cost effective solution.

Just analyzing a SCADA point may prove difficult. Sending an e-mail with an attached picture of the cable fault provides significantly more information from which intelligent decisions and confirmations may be made. In addition to the dispatcher, the e-mail can be sent to managers who are also responsible for the outage restoration process. Utilizing a substation solution such as ABB’s COM600 or MicroSCADA system can address the network capability and SCADA requirements.

The bottom line

Many utilities are still trying to predict cable failure using traditional methods. At the same time, public utility commissions and government agencies are increasing their scrutiny of outages and other related grid problems. Traditional approaches to cable fault detection do little to improve reliability. Nor do they do much to alleviate the unplanned outages that lead to millions of dollars in fines and misdirected maintenance budgets every year.

As demand for power puts additional burden on the system, the scrutiny on unplanned outages is expected to continue to increase. The smart utility needs to leverage technology to cut operating expenses and improve reliability of service in readying itself for the dynamics of the future.