Dissolved Gas Analysis Plays Key Role in Transformer Operations

Purity of gases is critical to preventing atmospheric contamination and resultant power failures

So much attention is paid to what comes out of the stack and for good reason, pollutants such as SO₂ and NO are detrimental to the environment. However, as the electricity is delivered to the power grid, transformers play an important role in converting high voltage to domestic use. It is in these mineral oil-filled transformers that constant monitoring and maintenance is critical in preventing ill-timed failures. Blackouts give power companies black eyes and reduced profits.

The dissolved gas analysis (DGA) technique is an important tool for monitoring and troubleshooting a transformer’s operational condition. There are four basic transformer fault types categorized by severity.¹ Arching, the most severe transformer fault, produces significant amounts of hydrogen and acetylene as the mineral oil breaks down. If cellulose insulating paper is exposed to the arching, then carbon dioxide (CO₂) and carbon monoxide (CO) will be present.

Next in severity is localized heating or sparking due to intermittent high voltage flash without current. The symptomatic gases produced are increased levels of methane and ethane. Third in severity is localized overheating. Overheating as an example may be caused by electrical contact failure, which produces ethylene and methane gases. If severe overheating occurs, then trace amounts of acetylene may be present.

Lowest is severity is a low-energy electrical discharge that is sometimes referred to as a corona event. This low-order fault will produce hydrogen and methane with traces of ethane and ethylene. If the low-energy discharge occurs within the cellulose insulation paper, then CO₂ and CO will be present.²

Periodically, transformer oil samples are collected as a key part of a preventative maintenance program. The sample is then subject to high vacuum, degassed and analyzed by gas chromatography. The following discussion of support gas systems highlights the gas chromatograph (GC) use of the thermal conductivity detector (TCD) and the flame ionization detector (FID) in DGA. Figure 1³ illustrates the basic components of a GC system. The system includes a carrier gas source, flow controller, sample injection port, column, column oven, detector and data reporting system. The collected sample is introduced into the system and swept by the carrier gas into the column for separation. The column at elevated temperatures elutes or exhausts compounds at distinctive intervals for analysis by the detector.

It is not uncommon for the GC to have a TCD preceding a FID for detection of molecules without the carbon-hydrogen bond. For instance, when a fault occurs near the cellulose insulation, nitrogen (N₂), oxygen (O₂), CO₂ and CO may be produced. The TCD compares the thermal conductivity of the carrier gas to the sample-carrier gas mixture. The design utilizes separate reference and sample cells, which use metal filaments or thermistors to sense temperature change. An electrical resistance is established, producing a signal proportional to the concentration of the sample in question.⁵ Helium is the desired carrier gas because hydrogen may be the fault gas of interest.

Figure 1. Basic gas chromatograph components

The FID is an extremely sensitive method for detecting hydrocarbon fault gases like methane, ethane, ethylene and acetylene. These gases are key indicators of arcing, corona or sparking type transformer faults. Differing slightly from the TCD, the sample elutes from the column and is mixed with hydrogen and a make-up gas before entering the FID flame nozzle head as illustrated in Figure 2.³ The mixture is combined with air and burned while being exposed to a voltage bias between the nozzle and collector plate or electrode. The sample gets converted to ions. A current is establishes and converted to a voltage signal proportional to the samples mass as a function of time.4 The exhaust gases are then vented out the top of the detector.

Figure 2. FID nozzle schematic

In either detector gas purity plays an important role in column performance and stability. Chromatogram peak trailing, ghost peaks or drifting base lines are symptoms that column contamination or damage has occurred. It is imperative to minimize atmospheric contamination. Polymer tubing should not be used for the carrier gas because it permeates oxygen and moisture over time. The partial pressure differential provides the driving force for the impurity to permeate the polymer tubing wall. Ideally, stainless tubing should be used in all gas connections because copper will permeate moisture. A dual-stage cylinder regulator as illustrated in Figure 3 should be constructed from barstock raw material to minimize the whetted surface area. The internal geometry can be machined to eliminate dead ends and other entrapment sites to enhance purging. It is critical that the diaphragm material be 316L stainless versus neoprene to eliminate atmospheric permeation and plasticizer off gassing.

Figure 3. (GC) Dual-Stage regulator

Another key feature of a gas chromatograph (GC) regulator is the integral gas bottle gland check valve. The location of the check valve prevents atmosphere from being exposed to the gas system during a cylinder exchange. As an additional precaution the (GC) regulator should have an interstage purge. This feature will minimize the exposure of the .0225 cm3 of trapped atmosphere between the cylinder and the internal check valve. The GC regulator utilizes the high load marginal spring of the second-stage seat for isolation. The interstage diaphragm valve can then be used to flush or displace the contaminants. The first stage preset pressure of 225 psi enables the technician to perform a pressure-time cycle purge. This will reduce the contaminant level below the 1 ppm level.

The DGA is a critical element of any oil transformer maintenance program. With proper care of the GC gas delivery system, flame ionization and thermal conductivity detectors can provide the resolution to monitor the type and concentration of fault gases. This will provide insight of impending transformer problems so the power company can schedule preventative maintenance. Ultimately, the black eye can be avoided while meeting the public’s insatiable demand for electricity.

References:

1. Northern Technology & Testing, Fault Types, http://www.nttworldwide.com/faults.htm
2. USA Industrial Group, Transformer Oil Testing, http://www.usaindustrialgroup.com/transoiltest6.htm
3. Sheffield Hallam University, Biosciences http://teaching.shu.ac.uk/hwb/chemistry/tutorials/chrom/gaschrm.htm
4. Hinshaw, J, “The Flame Ionization Detector” LCGC North America, Dec. 2005. http://chromatographyonline.findpharma.com/lcgc/GC/The-Flame-Ionization-Detector/
5. Hinshaw, J, “The Thermal Conductivity Detector” LCGC North America, Jan. 2006. http://chromatographyonline.findpharma.com/lcgc/GC/

NOTE: This article appeared in the June 2008 issue of Pollution Engineering)

Richard Green is Manager of Business Development, CONCOA, manufacturers of gas flow control systems and equipment, headquartered in Virginia Beach, VA, 800-225-0473, richard.green@concoa.com, www.concoa.com.