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What to Consider When Collecting and Analyzing AC Measurements

Abstract

This paper outlines the questions you should consider when collecting and analyzing measurements of alternating current for the purposes of designing or maintaining an AC mitigation system. It addresses three main areas: planning, data collection, and reporting.

What Kind of Coupon Test Station Are You Using?

Triple-coupon test stations are a good choice for calculating AC current density since they have the added benefit of allowing you to collect up to six other measurements. While AC current density is the most important measurement when it comes to AC mitigation, the others provide valuable context that’s useful for monitoring your CP system. These additional measurements include:

• DC current density• AC and DC structure-to-soil• DC structure-to-soil instant off• Native structure-to-soil• AC current drain

Planning Since AC measurements are the topic of this article, we’ll focus on how these measurements apply during the AC mitigation planning process. Our assumption is that you’ve already collected enough data about locations where HVAC transmission systems exist near buried structures to know where AC interference is likely to occur on your system.

Are You Measuring AC Current Density? Once you’ve identified the areas where you’ll need to investigate the effects of AC, you’ll want to install coupon test stations to measure AC current density: one of the most reliable measurable parameters related to AC corrosion. AC current density helps quantify the level of urgency associated with any proposed mitigation plan. Since it can only be derived from a current measurement on a fixed-area coupon, coupon test stations are the best way to measure it.1

Planning Tip

If you’re not yet sure where AC interference is likely to occur on your system, an excellent resource is Mitigating Corrosion on Cathodically Protected Pipelines by Clay Brelsford of Bass Engineering.

What to Consider When Collecting and Analyzing AC Measurements

What’s Your Measurement Duration? NACE recommends that AC potential and current flow measurements be taken for a duration of at least 24 hours in order to characterize the variation in power line load levels and identify the associated peak AC current density.1

Some HVAC transmission systems may be relatively dormant until needed by the utility, so measuring for as long as possible is recommended. Research shows that the duration of monitoring has a major role in deciding likelihood of AC corrosion.2

How Frequently Are You Taking Measurements?

Measurement frequency depends on the phase of the AC mitigation project. Prior to the design and installation of a mitigation system, measurements should be taken quite frequently (at least once per hour) to have a better chance of capturing the peak AC current density. This is because AC current densities fluctuate quite a bit, so measuring them infrequently, for example, once per day, makes it very unlikely that the measurement will coincide with the peak. Measuring once an hour or even once per minute increases the likelihood of successful characterization of the peak AC current densities.

Once the mitigation system is installed, continue with hourly measurements until you’re satisfied that it’s working as designed. Once satisfied, measurement frequency can be reduced to save on communication costs.

Data Collection

Which Measurement Method Should You Choose?

There are three common ways to collect measurement data from coupon test stations: manually with a multimeter, with a remote monitor, and using a data logger. Table 1 includes considerations to take into account when choosing between these methods.

Table 1: Considerations to Take Into Account When Choosing a Measurement Method (Continued on next page)

Measurement Method Considerations

Manually with a Multimeter • Because this method involves sending a technician out to the coupon test station (CTS) to take measurements, it’s expensive and time consuming.

• This method does not accurately capture peaks and valleys as there is little chance the technician just happens to take the measurements at the exact right time.

• Advantages of the manual method are that it doesn’t require any capital outlay and it may be useful as a way to characterize steady-state AC influence in order to prioritize more permanent or accurate data capture methods such as remote monitoring.

Using a Remote Monitor • This method requires installing a remote monitoring unit (RMU) specifically designed to capture coupon test station measurements. The RMU will periodically take measurements and wirelessly transmit them to a data repository where they can be accessed when needed.

• Capital outlay is required to purchase and install the RMU, and ongoing communication costs are typically involved.

• One advantage of remote monitoring is that the automatic and frequent data capture increases the likelihood of identifying the peak current densities.

• Another advantage is that, unlike data loggers, there is no limit on the number of measurements the remote monitor can capture since the captured data is not stored on the unit.

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What to Consider When Collecting and Analyzing AC Measurements

Reporting Frequent data capture over a long duration results in a great deal of data to store and interpret. For example, if you use a triple-coupon test station to capture measurements in addition to AC current density, you’ll be working with up to seven measurements. Seven measurements taken every hour for 90 days results in over 15,000 data points from a single test station. That’s too much data to deal with manually, so we recommended that you track it with compliance management software, if available. This will allow you to:

• Manipulate, refine, analyze and report on the data. • Put the AC data in context with CP inspections, which should be watched carefully as AC can damage CP

systems such as rectifiers.1

Are You Reporting on the Right Data? The first step in generating useful reports from the collected data is knowing which data will be most helpful in understanding the existence and/or degree of possible AC corrosion. Current density expressed as AC amps per square meter is the most useful data for this purpose. Identifying possible safety issues as well as the interaction of the CP system with the induced AC are important so AC voltage, DC voltage and current density can also be useful.

Table 1: Considerations to Take Into Account When Choosing a Measurement Method (Continued from previous page)

Measurement Method Considerations

Using a Data Logger • Data loggers are portable devices that can be temporarily installed on a coupon test station and can take very frequent measurements, sometimes as often as once per second. This gives them the best chance to identify the AC current density peak.

• Collecting the data is expensive as it requires a tech to retrieve the data logger device and ex-tract the data. Eventually, the data logger will reach its storage capacity. For these reasons, data loggers are best used during the pre-design phase of an AC mitigation project in circumstances where AC current is likely to be quite variable.

Current Density Threshold Likelihood of Induced AC Corrosion

0-20 A/m2 No or low likelihood

20-100 A/m2 Unpredictable

100+ A/m2 Very high likelihood

Table 2: Thresholds That Specify When AC Corrosion Is Likely

Which Thresholds Indicate a Potential AC Corrosion Issue?

When reporting on AC data, it’s important to define the thresholds where a particular measurement may warrant concern or further investigation. Table 2 shows the thresholds that the industry has defined as indicating how likely it is that AC corrosion will occur.2 Similarly, some research shows that CP may impact AC corrosion, so defining thresholds for the ratio of AC to DC current density may also be helpful. A ratio of AC to DC current density greater than 10 indicates a high risk of corrosion.2

What to Consider When Collecting and Analyzing AC Measurements

Do Your Reports Show How Often Measurements Exceed Thresholds? You should have at least one report that shows how often measurements at a particular test station exceed the thresholds you defined. This report will be used to identify areas that need action or further investigation. For example, the report in Figure 1 shows the percentage of readings during a month-long period that exceeded the 20 A/m2 threshold for “action” and the percentage that exceeded the 100 A/m2 “urgent” threshold.

Figure 1: A Sample AC Monitoring Report That Shows How Coupon Current Density Compares to the Established Thresholds

Test Station % Urgent >100 A/m2

% Action 20-100 A/m2

% Normal<20 a/m2

CTS1 MP32.031 99.3% 0.0% 0.7%

CTS5 MP121.934 95.5% 4.5% 0.0%

CTS3 MP94.064 21.0% 24.6% 54.4%

CTS4 MP120.345 0.0% 100.0% 0.0%

CTS2 MP93.134 0.0% 97.6% 2.4%

CTS6 MP167.831 0.0% 97.2% 2.8%

CTS9 MP163.845 0.0% 0.0% 100.0%

CTS8 MP250.598 0.0% 0.0% 100.0%

CTS6 MP250.251 0.0% 0.0% 100.0%

If your AC monitoring report reveals areas that need further investigation, a graph showing AC current density readings over time can reveal:

• When concerning current density levels first began• If exceeding the thresholds is a regular occurrence• If the peak current density levels occur at a regular time each day

The above information can help with mitigation system design, testing, and monitoring. For example, Figure 2 is a graph that shows AC current density over time for test station CTS3 MP94.064 from Figure 1.

Figure 2: A Sample AC Monitoring Report That Shows Current Density Over Time for CTS MP94.064

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What to Consider When Collecting and Analyzing AC Measurements

Summary Using AC mitigation systems to keep personnel safe and protect structures from AC induced corrosion is a sophisticated process, so it’s important to make sure that your existing or future system is functioning properly. To help with this, we recommend that you consider the questions outlined in this document, which we’ve summarized in Table 3.

Table 3: Questions to Consider When Designing or Maintaining an AC Mitigation System

Phase of the AC Mitigation Process

Questions to Consider

Planning • Are you measuring AC current density?• What kind of coupon test station are you using?

Data Collection • What’s your measurement duration?• How frequently are you taking measurements?• Which measurement method should you use?

Reporting • Are you reporting on the right data?• Which thresholds indicate a potential AC corrosion issue?• Do your reports show how often your measurements exceed thresholds?

References

1. NACE SP0177-2014, Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems

2. NACE, AC Corrosion State-of-the-Art: Corrosion Rate, Mechanism, and Mitigation Requirements

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What to Consider When Collecting and Analyzing AC Measurements

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