Schweitzer Engineering Laboratories
How protective relay data can help cut costs
Relays and monitoring devices in your power system can help predict and prevent problems, such as motor failure, to increase efficiency and ease workload.
By Mark Zeller, Sales & Customer Service Director, SEL
This article appeared in the August 2021 edition of Rockwell’s The Journal and has been reposted here with permission.
Modern process control systems for industrial plants use both analog and digital sources of information to perform routine functions such as protection and automation. Data collected and intended for one purpose often can provide substantial benefits when applied in other ways. For example, consider modern digital protective relays. They gather data for electrical protection, sampled at frequency rates as high as 1 MHz, that is used to quickly detect atypical electrical conditions and deactivate a problematic section of circuit. Most of the information not being used for protection is discarded. By unifying the process control and power distribution systems, however, that data instead can be used in operational control and troubleshooting to improve reliability and reduce downtime.
Data Easy to Access
Reading operational data from a digital protective relay can provide invaluable process evaluation information at no or a low cost. Most modern relays include some form of communication interface, so collecting data from them often is as easy as configuring the port and defining the information packets to the preferred communication protocol, including common choices such as Modbus, IEC 61850 and EtherNet/IP as used by Rockwell Automation products. Collecting and communicating this data has no impact on the relay’s protective functions. Modern relay architectures include enough processing power to separate protection and communication functions. They’re designed deliberately to operate protection on a fixed scan rate entirely independent of communication processing. Sharing relay information with colleagues in process control or operations is encouraged wherever and whenever possible. As an example, industrial facilities typically separate the electrical protection system from the operational distributed control system (DCS) responsible for controlling manufacturing processes. The energy used to manufacture the end product is tied directly to the electrical energy delivered. As a result of this connection, examining the power delivery system can reveal many process issues. The intelligent electronic devices (IEDs) tasked with protecting the power system are the ideal place to gather such information. While DCS systems scan process inputs several times a second, IEDs scan the power system thousands or even millions of times per second. This improved resolution can identify abnormalities and warn the DCS of impending problems quickly — with earlier detection providing more options for resolving issues before they turn into unscheduled downtime.
The information available in modern protective relays can help:
- Detect pending failures on aging or damaged cables with incipient fault detection.
- Monitor the power system for stray harmonics indicating misfiring adjustable frequency drives.
- Alarm overloaded circuits before they trip, allowing time for operations or automatic load shedding.
- Capture important data before, during and after electrical issues to find the downtime’s root cause.
- Detect downed conductors even without short-circuit currents.
- Detect and alarm for motor damage from excessive starting or loading, including broken rotor bars.
- Alert operations when motor loads drop below normal, signaling process issues such as pump cavitation.
- Predict motor trip times by monitoring motor load and alerting operations before the motor shuts down.
- Indicate process changes by measuring motor starting times and allow the scheduling of repairs before failure occurs
How It All Works Together
Collecting and analyzing motor information helps illustrate how operational data can impact plant processes. For instance, the duration of time a motor requires to reach operating speed can be an excellent indicator of process health. Extended start periods — durations longer than the average start time — offer a glimpse into the process. By monitoring the motor parameters under normal operating conditions, later comparisons have foundational data from which to evaluate the system for process changes. Process instrumentation such as flow rate, temperature, consistency, and valve and damper position can be recorded during operation and then checked later for validity using a simple matrix. This validation and testing system relies heavily on certain motor parameters gathered from the relay, such as current and horsepower, with basic process analysis capable of predicting most variations. This type of evaluation system provides significant benefits in that it can alert operators to issues before they cause equipment failure — meaning operators can recalibrate feedback systems, maintain positioners before failure and conduct maintenance that requires a process shutdown during the next available outage. Current feedback is some of the most commonly used motor information. Pump mechanical issues, process consistency variations, valve position problems or process pipe blockage often are indicated by higher-than-average motor start current. Current that’s lower than normal, on the other hand, can indicate issues with couplings, pump shaft or impeller damage or a lack of pumping material. By evaluating the current signature, operators can predict problems reliably, including pump cavitation, loss of flow, variable-frequency drive issues, bearing problems and even belt or coupling misalignment. Many predictive maintenance programs use this type of signal analysis to monitor the signal emitted by the bearings and evaluate rotating equipment condition. Performing this analysis from within the protective relay offers a host of advantages, including greater available processing power, existing access to the current signal, continuous availability and operator alarming capability. Predictive maintenance technology is yet to be fully explored as a key component in the protection and monitoring of an AC motor.
Use Existing Operational Data to Achieve Savings
Important operational data can be gathered from protective relays to reduce the lost time, money and production associated with unexpected equipment failures. As relays are a vital component already installed within most given systems, extracting information from them is an easy, low-cost operation. To take full advantage of the available data, it’s important to know what information can be extracted in the first place and how to apply it best to model and evaluate process control changes. These process-improvement and cost-saving capabilities will only increase for process control and operation engineers as more reliability-based information becomes available on future iterations of protective relays. Even now, electrical protective relays can offer insights into operating characteristics, key equipment statuses, and maintenance indicators for use in operations and maintenance systems. Those systems can:
- Read key equipment data from the relay and display statuses or generate work instructions.
- Reduce unscheduled downtime from equipment failures by recognizing failure warning signs.
- Improve maintenance effectiveness by scheduling maintenance on a condition basis instead of a time basis.
This additional information shifts operations and maintenance activities to be proactive, rather than reactive, to ease overall workload, lower costs and improve efficiency by focusing efforts where they can provide the most benefit. Ultimately, using existing relays to establish communication channels between DCSs — or maintenance work systems — provides vital information at little or no cost.
Listen to the Podcast
How to Use Protective Relay Data to Cut Costs
In The Journal magazine’s latest Automation Chat podcast, “How to Use Protective Relay Data to Cut Costs,” Executive Editor Theresa Houck chats with Mark Zeller from Rockwell Automation Technology Partner Schweitzer Engineering Laboratories (SEL).
They discuss how your existing digital protective relays gather and use data for electrical protection in manufacturing plants, and how to download the data. Also see examples of how the information can help you monitor health of devices connected to your power system, and thus predict problems before they occur, such as a circuit or motor failure.
Listen on your favorite podcast app or on the web, or watch their chat on YouTube.
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