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Transformers are the largest investment in a substation. Combined, they make up one of the largest (if not the largest) investment by an electric utility.

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The instantaneous rate of aging is the tangent to the overall aging curve. The rate of aging is affected by such factors as temperature and moisture content. Temperature is related to loading and cooling efficiency.

The stress here is not just electrical surge stress, although that is one type of stress. Recent data show that through-fault mechanical forces are the number one cause of transformer failure.

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There are many tests that can be performed to determine the state of the insulation inside a transformer. While the tests are less expensive than taking the transformer out of service, they are still costly and time-consuming. There is also the chance that the test itself will be in error, possibly leading to internal inspections that take more time and money.

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A transformer that cost $2 million to replace can cost that much or more in replacement power.

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Having four single-phase transformers is one way of being ready for a failure. In this way, the spare unit can be switched to very quickly when one of the other transformers fails, preventing long outages or costly rerouting of power. Of course, the cost is a 33 percent increase in equipment.

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Protective relays are constantly monitoring power system voltage and current waveforms. This is a good start in understanding power system apparatus operations. Voltage and current magnitudes, phase angles, and the resulting quantities derived from these core values (kW, kVA, kVAR, PF) are often used in various reports generated by the relays. In addition to these quantities, protective relays may also be used to monitor temperatures, station dc voltages, and low-level dc inputs (4 to 20 mA, 0 to 10 Vdc) with high resolution and synchronized time via IRIG-B time-synchronization signals.

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Transformer differential protective relays typically provide sensitive and secure protection for transformers with up to four windings. In addition, some manufacturer’s relays provide through-fault monitoring. Through-fault monitoring records each through-fault event experienced by the transformer and provides a time and date stamp, total number of through faults and their duration in seconds, and accumulated I2 • t through-fault data.

Transformer protection may also provide transformer thermal monitoring based upon the IEEE C57.91-1995 Guide for Loading Mineral-Oil-Immersed Transformers. By measuring transformer load parameters and ambient temperature, some relays can provide calculated top-oil and hot-spot temperatures, as well as insulation aging factors calculated from the IEEE recommended algorithms.

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The timing of the sampling and calculation used for various monitoring points is important. In some cases, like the monitoring of interrupted current during breaker opening operations, the measurement and processing times used for the voltage and current waveforms are the same as those used for the device protection. In other instances, like transformer temperature monitoring, the measurement times can be much longer, on the order of tens of seconds between samples due to the long period of the thermal time constant of most transformers. Essentially, the sampling period should be at least one-half as long as the response time of the variable being monitored.

Unlike some generic remote terminal unit (RTU) devices, protective relays are designed to sample a power system apparatus at intervals that will provide the best results for the protection and monitoring that the relays provide.

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Some data can be calculated while some must be measured. Any estimation of hot-spot temperature must start with a measurement of ambient temperature. If the ambient temperature is known, it is possible to calculate top-oil temperature based on transformer tests and load information. If the top-oil temperature is available, then the calculation for the hot spot is improved.

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Because aging is an accumulated quantity, it must be recorded over time by a device always there to record—such as a protective relay!

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Alarms can be programmed based on either the total loss of life (TLOL) or the rate of aging.

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The relay uses hot-spot temperature as a basis for calculating the insulation aging acceleration factor, FAA, and loss-of-life (LOL) quantities. Use the thermal element to indicate alarm conditions and/or activate control actions when one or more of the quantities shown on the slide exceed settable limits. When appropriate, request a thermal monitor report that indicates the present thermal status of the transformer. Historical thermal event reports and profile data are stored in the relay in hourly format for the previous 24 hours and daily format for the previous 31 days.

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The SEL-387-6 Current Differential and Overcurrent Relay stores the following data on an hourly basis for the last 24 hours (the data are stored at the beginning of each hour):

• One-hour average ambient temperature

• One-hour average calculated top-oil temperature

• One-hour average measured top-oil temperature

• One-hour average winding hot-spot temperature

• One-hour average per-unit load current

• One-hour average insulation aging acceleration factor

Note: When the thermal model is applied on one three-phase transformer (XTYPE = 3), the SEL-387-6 displays only the values for Transformer 1; but when the thermal model is applied on a set of three single-phase transformers (XTYPE = 1), the SEL-387-6 displays the values for Transformer 1, Transformer 2, and Transformer 3.

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The figure on the slide shows a thermal report from a transformer differential relay. This report is updated once per minute and provides information from the transformer thermal model. The thermal model can be configured for three single-phase or one three-phase transformer, and it provides the following:

• Thermal element condition. Compares transformer temperatures, insulation aging factors, and cooling system efficiency to relay settings for these values, and issues warnings if values are exceeded.

• Load (per unit). Unit load as measured by the current transformer (CT) ratio, MVA rating, and secondary current inputs.

• In-service cooling stage. Driven by contact inputs to the relay or logic equations, this displays the cooling stage in use. Cooling Stages 1, 2, and 3 are provided, with separate MVA ratings for each.

• Ambient temperature. Ambient temperature is provided by resistance temperature detector (RTD) inputs or via a communications processor that is receiving thermal data from a transformer monitor. A default ambient temperature can be used in place of a measured temperature.

• Calculated top oil. Transformer top-oil temperature is calculated from transformer loading levels, ambient temperatures, and transformer thermal time constants based on IEEE C57.91-1995.

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• Measured top oil. Measured top-oil temperature is provided by an RTD or via a communications processor that is receiving thermal data from a transformer monitoring device.

• Winding hot spot. The transformer winding hot spot is calculated from the transformer loading, measured top oil (or calculated top oil if measured temperature is not available), and transformer hot-spot thermal time constants based on IEEE C57.91-1995.

• Aging acceleration factor. Compares the calculated winding hot-spot temperature to a reference temperature from IEEE C57.91-1995. If a hot spot is above the reference, transformer insulation is aging faster than normal, and this value is greater than 1.0.

• Rate of LOL. Measures insulation aging based upon the aging acceleration factor over a 24-hour period and is scaled in percent.

• Total accumulated LOL. The accumulated LOL data since last reset.

• Time-assert TLOL. The estimated time before assertion of the TLOL alarm. The TLOL alarm is triggered when accumulated LOL levels rise above a TLOL limit setting in the relay.

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The hourly thermal report from the relay provides time- and data-stamped information for ambient, top oil, hot spot, load current, and insulation aging acceleration factor.

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The daily thermal report provides thermal and acceleration aging factors every 24 hours, updated at midnight each day. This report shows remaining LOL and TLOL values for the transformer insulation.

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Two leading causes of transformer failure are close-in faults and through faults. High-magnitude faults lead to transformer aging.

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A close-in fault on the outgoing feed from a transformer produces huge mechanical forces on the windings and insulation structure.

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According to William Bartley of the Hartford Steam Boiler Inspection and Insurance Company (an insurance company for equipment), through-fault stresses are the number one cause of transformer failure in recent years*.

*W. H. Bartley, “An Analysis of Transformer Failures—1988 through 1997,” The Locomotive, Hartford Steam Boiler Inspection and Insurance Company.

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Insulation that has aged becomes brittle and less able to withstand the mechanical forces of through faults.

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Note that the same stress a new or even less severely used transformer could survive will cause a high-mileage transformer to fail.

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The figure on the slide shows a through-fault report from an SEL-387-5 Relay. All SEL-387 Relays provide the same through-fault report. This report shows the number of through faults, accumulated I2 • t per phase, and individual through-fault time, date, duration, and primary amperes per phase.

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Use transformer history as a predictor of the present state.

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The figure on the slide shows how a customer is using temperature signals in an existing installation.

Recent advances allow the connection of the RTD outputs to an RTD module and then directly to the relay, as shown in the next slide.

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Fiber-optic connections avoid the possibility of noise in the radio or wire as it goes through the station.

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Temperature data are obtained via one of the relay’s serial ports. These data come from an SEL-2030 Communications Processor, which receives the temperature data from either an SEL-2600 RTD Module or a programmable logic controller (PLC), as shown on the slide. The SEL-2030 must receive the temperature data in Modbus®, SEL Fast Message, or ASCII protocol.

The SEL-2030 passes these data on to the SEL-387-6 in the form of an SEL Fast Message. While the SEL-387-6 can receive temperature data at any rate, the thermal element uses these data at a rate of once per minute.

The SEL-2600 can also be connected directly to an SEL-387-6.

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Another method of collecting data is to access the reporting data from various relays within a substation with the communications processor. Use the communications processor equipped with an Ethernet card as a Telnet switch that is accessed by remote computers via a wide-area network (WAN). In this type of system, web server computers are used within the firewalled control room to gather and store the relevant asset management information. Remote computers connected to the WAN access data from the web server with the correct user ID and password.

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The figure on the slide shows the system one-line display. The webpage HTML contains an embedded Java™ applet for gathering the data that will be displayed on this page. The system one line is the default startup page for the thermal monitor viewer.

The View Details pushbutton contains a link to the thermal monitor webpage. This page provides a graphical display of the thermal monitor data for the respective transformer.

The Show Relays pushbutton on the page is a link to another webpage that shows the various relays and communications processors used to implement the thermal monitoring system.

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The thermal viewer webpage provides all of the information contained in the relay’s thermal report in a graphical form. Relay information is displayed in the upper left-hand corner. The current conditions section allows selection of any of the listed parameters for display in the conditions profile area of the graphical user interface (GUI).

Current alarms are shown in red for active and green for clear. They include thermal limit, insulation aging, cooling system efficiency, and communications error alarm indications.

A 24-hour window with zoom capabilities is provided for the thermal report and alarm data.

A scrolling window at the top of the webpage provides a means for selecting thermal monitoring data from any recorded day available in the MySQL database history.

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