Of the 169 substations scattered throughout its bi-state service territory in Kansas and Missouri, Kansas City Power & Light (KCP&L; Kansas City, Missouri, U.S.) has 247 transformers that require dissolved gas analysis (DGA). Although the utility outsourced DGA testing to various laboratories in the past, KCP&L recently decided to consider bringing this function in-house in an effort to reduce maintenance costs and ensure quicker turnaround times.
A key factor in the decision-making process for KCP&L was selecting the right equipment. After researching several different instruments, it found what it was looking for in the Kelman Transport X: efficiency and cost-effectiveness. This technology convinced KCP&L to bring its DGA analysis in-house.
How does the technology work? The test equipment measures gas levels by using a photo-acoustic spectrometer, as opposed to gas-chromatograph techniques. The gas's ability to absorb electromagnetic radiation, such as infrared light, causes the photo-acoustic effect. The absorbed electromagnetic radiation increases the temperature of the gas, proportionally raising the pressure of the gas in a sealed container. By pulsing the light source, the pressure of the gas in the sealed container fluctuates in synchrony, which allows the amplitude of the resultant pressure waves to be detected using sensitive microphones.
There are two key factors that permit the photo-acoustic effect to be used for analytical measurements. First, every gas has a unique absorption spectrum that allows the frequency of the infrared source to be tuned to excite a given substance. Second, the absorption level is directly proportional to the concentration of a given gas. Selecting the appropriate wavelength and measuring the level of the resultant signal makes it possible to detect the presence and concentration of any given gas. This forms the principle of photo-acoustic spectroscopy (PAS).
Through the conceptual design of a practical PAS measurement module, you see the process starts with a hot-wire source, creating broadband radiation across the infrared range. The radiation bounces off a parabolic mirror to a chopper wheel rotating at a constant speed, giving a stroboscopic effect to the light source. Then, the radiation proceeds through an optical filter, one of several filters designed to transmit a specific wavelength and chosen specifically to excite the compound under investigation before it reaches the measurement cell. The compound then reaches the measurement cell of the PAS module for analysis.
Level of Accuracy
The Transport X uses a photo-acoustic spectrometer module, along with a custom-designed system, for the gas extraction of target gases from the sample to give a completely portable, self-contained analysis system.
The equipment uses the conventional manner for drawing an oil sample from a transformer. This involves using the drawn oil sample directly from the syringe and injecting it into the measurement container. Stirring the oil with a Teflon-coated magnet extracts the dissolved gases into the headspace of the sample bottle and circulates it through the sampling loop of the PAS module. When the oil has reached a stable equilibrium, the PAS module analyzes the gases in the headspace. The measurement results show on the LCD display after the device completes the testing cycle. The Transport X also has the ability to test gas samples taken directly from a transformer headspace or from a Buchholz relay.
To test the accuracy and repeatability of the equipment, KCP&L analyzed samples from transformers and compared them with existing historical data along with direct lab comparison. During this time, the utility took multiple oil samples out of various transformers, sent them to an external laboratory for analysis and kept others to test in-house.
During this process, KCP&L noticed that the repeatability of its testing was excellent; however, the results of ethylene and ethane did not always match up with the originating laboratory. To determine the correct result for the ethylene and ethane, KCP&L took nine oil samples out of a single transformer. It sent three of the oil samples to the original laboratory, three to a second independent laboratory and used the Transport X to analyze the gases on the remaining three.
When reviewing the results, it again was determined that each testing source had good internal repeatability, but the measurements of ethane and ethylene were inconsistent. When comparing these two gases, the first laboratory was below the Transport X measurements as before; however, the results from the second laboratory were higher than the Transport X. The inconsistency experienced on measuring the ethane and ethylene from different laboratories made this part of the testing ineffective, so KCP&L decided to focus on repeating the accuracy of its testing.
According to KCP&L, the Transport X is easy to use because it includes embedded PC guides that walk users through the process and does not require any calibration or settling procedures. It also has an instruction sheet inside the top cover for quick reference. The Transport X has the ability to detect the following seven key gases: hydrogen, carbon dioxide, carbon monoxide, ethylene, ethane, methane and acetylene, as well as moisture. It displays gas concentration as “normal,” “caution” or “warning level” when the analysis is complete. The Transport X also rates the overall transformer condition at one of these three levels.
Gas concentration default levels are initially programmed into the equipment using the Transformer Oil Analysis 3 (TOA) from Delta X Research. The user can set these threshold levels to a desired level. A value of any gas in the “caution” or “warning level” shows as yellow or red, respectively. This application is only for the analysis of transformers, although the development of LTC analysis functions is underway.
The operator of the test equipment also has the ability to use the diagnosis options, which include Rogers' Ratios, Key Gas Method and Duval's Triangle analysis tools. Once the DGA results are stored into memory, the operator has the ability to retrieve any previous result for future analysis. An embedded thermal printer also allows for an immediate hard copy of results and diagnosis comments if required.
Kelman also has developed some user-friendly software that uses Microsoft ActiveSync to send information to and from the test equipment. This software creates its own database from the information retrieved from the Transport X and can be searched by results, equipment, locations, sampling points and manufacturer. This software has the ability to pull the equipment database from an existing TOA program, so that it can be loaded into the Transport X. This also allows test results to continually update the TOA program. Kelman is continuing to develop and update this software with additional functionality, such as graphical and numerical trending, report generation and application of established diagnostic tools.
The procedure for testing oil samples at KCP&L did not change after it purchased the Transport X. Substation operation and maintenance personnel test oil samples during low-demand periods of the week. It takes approximately 20 minutes to perform each test. During this time, the person performing the test also performs a dielectric test. The Transport X's primary use at KCP&L has been for yearly DGA testing; however, it also has helped confirm a good transformer at its Grand Avenue Substation within one hour of taking an oil sample. By extending the use of the test equipment in this manner, KCP&L expects a payback on its investment in about one and a half years.
Elijah F. Forney is a substation staff engineer for the substation construction and maintenance department at Kansas City Power & Light. Before joining KCP&L, he worked as a field engineer for Kiewit Industrial. He earned the BSEE degree from Iowa State University. Elijah.Forney@kcpl.com