Benefits of transformer online dissolved gas monitoring Introducing the ABB CoreSense intelligent Dissolved Gas Analyzer

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1 White Paper Benefits of transformer online dissolved Introducing the ABB CoreSense intelligent Dissolved Gas Analyzer Luiz Cheim, Principal R&D Engineer, ABB Transformer Technology Center Thomas Buijs, Product Manager, ABB Measurement and Analytics ABSTRACT Following ABB s strategic view on the ever growing demand for sensors and techniques for the online condition assessment of power transformers the company has recently launched a hydrogen and moisture sensor (CoreSense TM ) as an initial step in a much broader and consistent product development program to provide utilities and users alike with the best available technology to support their efforts in keeping transformers operating longer and reliably. The simplicity and at the same time robustness of the CoreSense is a proof that ABB is employing the best available know-how and technologies to help its customers to have a low-cost high-quality device that can operate as a first level diagnostic tool to flag issues and trigger more comprehensive transformer condition assessment techniques with the sole aim of detecting incipient faults and act as an early warning system. This white paper provides an insight into the most common faults that involve the paper-oil insulation and the importance of hydrogen and moisture detection as a powerful means of avoiding failure and deferring investments. It describes the chemical structure of the solid and liquid insulation as well as the result of the breakdown of that insulation under thermal, mechanical and electrical stresses. The paper also sheds light onto the complex issues of gas detection and measurement such as laboratory accuracy as well as sampling errors against the more consistent and less subject to variability sensor s readings. The author s intent is to provide the reader with the scientific and technical justification for a cost effective solution capable of detecting a large number of problems that sooner or later can take the transformer to a major failure.

2 Introduction Today s electrical grids are comprised of millions of transformers that interconnect power generation, transmission and distribution. These transformers are critical assets that require proper maintenance in order to provide long uninterrupted electrical service. In this white paper, we look at the typical structure of a large transformer and explain why it is important to monitor dissolved gases. We then look at the characteristics of sensors that can do this online and explain the ABB CoreSense online dissolved gas analyzer. Transformer Insulation Transformer insulation is fundamentally composed of oilimpregnated cellulose paper surrounding conductors and layers of multiple conductors that form the transformer windings. A large amount of cellulose is also present in pressboards, wood and mechanical supports utilized during the manufacturing process as illustrated below [1-2]. the oil or through forced air and oil circulation in the presence of fans and/or pumps. In normal operation of power transformers, there occurs natural aging of the insulating system due to temperature and load cycles. This aging produces some combustible and non-combustible gases that only become a matter of concern at higher rates of gas formation when compared to normal conditions. Formation of Gases in Oil Both mineral oil and cellulose have carbon based molecular structures rich in hydrogen as illustrated below. The decomposition of oil and cellulose forms a large number of byproducts, including combustible and non-combustible gases. Hydrogen is naturally present in most of those compounds. Figure 2. Naphthenic aromatic molecule of insulating mineral oil (there are several other aromatic rings present in mineral oil, all rich in Hydrogen) Figure 1. Transformer solid insulation windings, barriers and supports Figure 3. Molecular structure of cellulose The entire structure illustrated in Figure 1 is thoroughly impregnated by insulating oil that also serves as a cooling agent to extract heat from the windings during the operation of the transformer. Heat may be extracted through natural convection of Cellulose is highly sensitive to heat, oxygen and moisture. The higher the temperature the faster the aging process of the paper (thermal aging). In the presence of higher amounts of oxygen and moisture, the paper breaks down through oxidation and hydrolytic processes, which generate additional moisture, acids and other components that accelerate the aging process even further. The 2 Benefits of transformer online dissolved ABB white paper

3 molecular structure of the insulating fluid also breaks down in the presence of higher temperatures and electrical faults such as partial discharges and arcing of several degrees of intensity. Both cellulose and insulating oil may also degrade in presence of contaminants, such as, for example, the ingress of moisture through leaks, or maintenance activities, etc. The formation of gases is common to almost all types of insulation degradation. These gases then dissolve in the oil, allowing detection and analysis of the defect through an adequate monitoring of the amounts and evolution of these gases, the so-called Dissolved Gas Analysis (DGA) procedure. Dissolved Gas Analysis (DGA) Dissolved Gas Analysis (DGA) is among the most powerful tools for detecting faults in power transformers. DGA analytical techniques and interpretation methods are continuously being investigated and improved. Online monitors are particularly useful for applications where gas formation in remote electrical equipment must be followed at frequent time intervals (eg, in strategic or expensive equipment, or where significant faults have already been detected) which is typically not practical through off-line laboratory analysis. The results of years of experience by the power industry is well summarized in IEEE C and IEC DGA Theory as in the IEEE C57.104/2008 [3] 4: The two principal causes of gas formation within an operating transformer are thermal and electrical disturbances. 4.1 Cellulose Decomposition The thermal decomposition of oil-impregnated cellulose insulation produces carbon oxides (CO, CO2) and some hydrogen and methane (H2, CH4) due to the oil 4.2 Oil Decomposition Mineral transformer oils are mixtures of many different hydrocarbon molecules, and the decomposition processes for these hydrocarbons in thermal or electrical faults are complex. The fundamental steps are the breaking of carbon hydrogen and carbon carbon bonds. Active hydrogen atoms and hydrocarbon fragments are formed. These free radicals can combine with each other to form gases, molecular hydrogen, methane, ethane, etc., or they can recombine to form new, condensable molecules. Further decomposition and rearrangement processes lead to the formation of products such as ethylene and acetylene and, in the extreme, to modestly hydrogenated carbon in particulate form. 4.3 Application to Equipment Internal faults in oil produce the gaseous byproducts hydrogen (H2), methane (CH4), acetylene (C2H2), ethylene (C2H4), and ethane (C2H6). When cellulose is involved, the faults produce methane (CH4), hydrogen (H2), carbon monoxide (CO), and carbon dioxide (CO2). Each of these types of faults produces certain gases that are generally combustible. DGA Theory as in the IEC60599:2007 [4] 4.1 Decomposition of oil Scission of some of the C-H and C-C bonds may occur as a result of electrical and thermal faults, with the formation of small unstable fragments, in radical or ionic form, such as H, CH3, CH2, CH or C (among many other more complex forms), which recombine rapidly, through complex reactions, into gas molecules such as hydrogen (H-H), methane (CH3-H), ethane (CH3-CH3), ethylene (CH2 = CH2) or acetylene (CH CH) Low-energy faults, such as partial discharges of the cold plasma type (corona discharges), favor the scission of the weakest C-H bonds (338 kj/ mole) through ionization reactions and the accumulation of hydrogen as the main recombination gas. Note: underlined text introduced by ABB authors in both texts From the texts above it is clear that hydrogen is a fundamental gas that occurs in the presence of thermal and electrical issues leading to oil and cellulose degradation and thus it plays a key role in the early detection of any abnormal conditions or failure modes inside the transformer. DGA Laboratory Issues The main issues associated with laboratory DGA are: a) The cost, resources and time to manually collect samples particularly in remote locations b) Sample contamination (during and/or after sampling on-site) c) Sample degradation between the time of sampling and the time of analysis d) Laboratory precision (or repeatability) and accuracy e) Laboratory reproducibility It is well known in the industry that many laboratories provide reasonably accurate DGA results to their customers but that many others provide quite inaccurate results. Even the best laboratories produce results with some inaccuracy, which, therefore, needs to be known to determine the reliability and accuracy of the diagnoses. It is strongly recommended that each laboratory evaluate and provide its own accuracy figures. When this is not possible, default accuracy values based on international surveys can be used. M. Duval, 2005 [5] ABB white paper Benefits of transformer online dissolved 3

4 A survey carried out by Cigre [5] SCD1 TF involving 25 experienced laboratories from 15 countries using gas-in-oil standards found an average repeatability of 7% at medium gas concentrations ( ppm) and 27% at low gas concentrations (1 10 ppm). The average accuracy found was 15% at medium gas concentrations and 30% at low gas concentrations. Figure 4. Illustration of the interplay between precision and accuracy a) Low precision, low accuracy b) High precision, low accuracy c) Low precision, high accuracy d) High precision, high accuracy Repeatability or precision [6]: related to observed differences when multiple samples of the same oil are analyzed by the same laboratory over a short period of time typically of less than one day. Accuracy is related to differences observed between values measured by a given laboratory and the nominal value contained in the gas-in-oil sample prepared according to standard procedures. In addition to the repeatability and accuracy limitations of laboratories, the insulating oil sample will degrade over time due to the mobility of the dissolved gases in general and the high mobility of hydrogen gas in particular which will slowly escape into the atmosphere even from a sealed container. Online Gas Monitors Online gas monitors are installed on a transformer at the factory or in service and provide frequent readings (typically several readings per day) of gas concentrations dissolved in the oil of the transformer without the need for a manual sample. A major advantage of online monitors, as compared to laboratory analysis, is the capability to detect abnormal gas formation and faults occurring in near real time between manual oil samplings. For regular maintenance with laboratory analysis, manual samplings are typically performed every year or every 6 months. With online gas monitors, gas analysis is performed much more frequently thus providing a powerful early detection system that avoids excessive site visits and manual samplings with the advantage of performing an almost continuous observation of gas levels and trends. A number of challenges must be addressed when installing, commissioning and operating systems to avoid missing fault conditions (false negatives) and also to avoid false alarms when no fault exists (false positives). 1 The oil sampled by the monitor must be representative. If the monitor is installed such that it is always sampling the same pocket of stagnant oil there is a high probability that developing fault conditions will go unnoticed giving a false sense of security. 2 The monitoring system oil sampling mechanism must be designed in a fail-safe manner to reduce the likelihood of any oil leaks. Even a small leak can lead over time to a drop in the oil level of the main transformer tank. If the oil drops below a critical threshold a safety shut down of the transformer will be initiated automatically or the transformer will fail catastrophically if there is no safety system. 3 The gas-sensing elements in the monitor must exhibit longterm stability under real world transformer conditions. Indeed the same conditions that lead to the aging of the transformer itself can age and/or deteriorate the electronics and sensors present in the monitoring system. Some of the conditions that must be accounted for are: a. Temperature cycling b. Presence of moisture and oxygen dissolved in the transformer oil c. Presence of reactive chemicals in the transformer oil generated by the aging and breakdown of the cellulose insulation and the oil itself; eg, carbon monoxide, organic acids, alcohols, furans 4 The gas-sensing elements should not exhibit cross interference; eg, react to other gases that may be present and provide an incorrect reading. 5 The monitoring system should not consume the gas in order to measure it as this can lead to a depletion in the oil sample the sensor is reading and lead to false trends in the gas levels. 6 The monitoring system needs to exhibit long term reliability and should self-diagnose itself to avoid having a dead sensor be interpreted as condition normal, no gas detected. There are three types of online gas monitors commonly used in the industry: one gas, gas combination and multi-gas monitors, each with its pros and cons. 4 Benefits of transformer online dissolved ABB white paper

5 One Gas Monitors These inexpensive sensors are widely deployed thanks to their low cost and high reliability. Most one gas monitors measure hydrogen gas in oil due to its central role in the detection of abnormal operation of transformers. Not only is hydrogen almost always present when a fault occurs but it typically is also the first gas formed because C-H bonds are relatively easier to break than C-C or C-O bonds present in the insulating oil and cellulose. Gas Combination Monitors Gas combination sensors, sometimes called Total Combustible (oxidizable) Gas sensors are another common type of online gas monitor commonly deployed to transformers for early fault warning purposes. These devices use sensors that give a single readout in response to a combination of all the oxidizable gases. Their typical response may look something like Readout = A x H 2 + B x CO + C x CH 4 + D x C 2 H 2 + E x C 2 H 4 + F x C 2 H 6 Figure 5. Approximate gas formation temperature in C The readout is dependent on the concentrations of all the gases with different relative sensitivities for each gas. The idea is that the readout will provide an early warning of gas formation regardless of what gas is actually being formed. The main issue with this type of sensor is that the interpretation of the readout can be very difficult because a perfectly normal transformer always has some background gas present in its oil. Table 1. Ranges of 90% typical gas concentration values observed in power transformers in μl/l (Source IEC60599:2007) C 2 H 2 H 2 CH 4 C 2 H 4 C 2 H 6 CO CO 2 All transformers , ,000 No OLTC Communicating OLTC These simple monitoring systems can be widely deployed even in remote locations where access for routine maintenance is slow, expensive and complicated. This leads to some important considerations when choosing one systems Long-term reliability is critical to keep maintenance requirements low. In practice, this means avoiding monitoring systems that make use of moving parts or complicated oil handling such as mechanical circulation pumps, cooling fans, valves, membranes, etc. The monitoring system should not require any regular calibration effort to stay within specifications. The monitoring system should not have any consumables that need replacing on a regular basis. The problem is that it is not possible to distinguish a high hydrogen condition from a high carbon monoxide condition for example. It can be seen from Table 1, that carbon monoxide may be responsible for a somewhat high reading of a gas combination monitor under normal conditions. This may mask the beginning of a rise in hydrogen which would otherwise indicate the development of a fault. This leads to an apparent decrease in sensitivity which reduces the overall usefulness and confidence in the monitoring system as shown in the images bellow. The one-gas hydrogen specific sensor does not suffer this apparent decrease in sensitivity (Figure 6). In addition, gas combination sensors typically use an oxidizing fuel cell as their sensor mechanism and this type of sensor functions by consuming oxidizable gases to produce an electrical current. If the rate of depletion by the sensor is greater than the rate of local replenishment, the gases will be depleted leading to incorrect readouts. ABB white paper Benefits of transformer online dissolved 5

6 Figure 6. Example of masking effect on hydrogen trend by carbon monoxide in gas combination monitors 6 Benefits of transformer online dissolved ABB white paper

7 Multi-gas Monitors Figure 7. Computer simulation showing stagnant oil when no thermal pump is used Multi-gas sensors provide individual readouts for up to 9 gases simultaneously. They significantly increase the usefulness of the monitoring system by providing complete information on dissolved gases in a very similar form to what is obtained from a laboratory DGA analysis, but in real time. This capability empowers the asset owner to do real time diagnostics on critical transformers semi-continuously. This advantage is somewhat mitigated by the significant cost and complexity of some of the multi-gas sensors on the market today. The result being that this type of monitor is often deployed for the most critical of assets or for assets known to be developing faults. Today many utilities use the approach of installing simple low cost one-gas monitors to provide a warning and then perform a laboratory DGA to get a further diagnostic. As multi-gas sensor technology matures, it is expected that adoption rates will increase. The ABB CoreSense Hydrogen and Moisture Sensor Figure 8. Computer simulation of CoreSense thermal pump showing oil inflow at bottom and outflow at top The ABB CoreSense hydrogen and moisture sensor is a new gas monitoring system that represents ABB s answer to the challenges of designing a good online gas in oil analyzer. The CoreSense utilizes two solid-state sensors to measure hydrogen and moisture directly in transformer insulating oil without the need for any complex sample handling to condition the oil or separate the dissolved gas from the oil. It addresses the question of stagnant oil and representative sampling with an innovative thermal pump that ensures fresh oil is always in contact with the sensors by forced convection. This approach is very simple and ultra-reliable since it has no membrane and no moving parts. The CoreSense can be mounted safely on any type of valve since this does not create any intrusion to the valve nor to the transformer. The CoreSense can easily be installed by customers and requires no special protective actions prior to oil treatment activities, a great plus over membrane based systems that can easily be damaged by pressure fluctuations. The solid-state hydrogen sensor used in the CoreSense has been extensively tested and characterized for long-term stability and reliability. It makes use of a patented protective coating to avoid aging and degradation effects from reactive gases, acids and other species that can be present in transformer oil. It is tempera- ABB white paper Benefits of transformer online dissolved 7

8 ture stabilized to avoid any thermal influence on hydrogen readings and makes use of a reversible hydrogen specific phase change in a palladium metal alloy to detect hydrogen levels. In addition, this sensor does not consume hydrogen, which eliminates the possibility that the hydrogen reading could be incorrect due to local depletion. Figure 10. The ABB CoreSense with solid metal enclosure The CoreSense has a built in embedded computer that continuously monitors all of its functions. It provides status at a glance capability with three super bright LEDs that report on the condition of the sensor and on hydrogen and moisture levels in the transformer oil. The CoreSense also has a built in web server that publishes a simple man machine interface to enable local and remote network access to the gas and moisture levels as well as the current status of the monitoring system. The interface is very intuitive, easy to read and efficient. Web pages are accessible through any internet browsers and do not require installation of specific software. Figure 9. Screenshot of the web HMI The CoreSense accurately measures hydrogen in oil down to a detection limit of 25 ppm with an error of ±10 ppm and a fast T90 time of under one minute. It is designed to provide accurate readings for 10 to 15 years with no calibration or routine maintenance requirements and no consumables. These characteristics make the CoreSense ideal for large-scale deployments even in flood areas or remote locations with potential weather extremes and access difficulties. The CoreSense is designed for durability and long life with no moving parts, solid state microelectronic gas in oil sensors that function directly in situ in transformer oil and a robust all metal IP67 rated submersible waterproof enclosure. This philosophy was applied to all aspects of the design, including the use of a super-capacitor to keep time in the event of a power interruption instead of a battery that would have a finite lifetime and need to be replaced at regular intervals. Bibliography [1] ABB Transformer Handbook [2] ABB Transformer Service Handbook [3] ANSI IEEEC57.104/2008 Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers [4] IEC60599:2007 Mineral oil-impregnated electrical equipment in service Guide to the interpretation of dissolved and free gases analysis [5] M. Duval, J. Dukarm, Improving the Reliability of Transformers Gas-in-Oil Diagnosis, IEEE Elect. Insul. Mag., Jul-Aug 2005, [6] IEC60567:2011, Oil-filled electrical equipment Sampling of gases and analysis of free and dissolved gases Guidance 8 Benefits of transformer online dissolved ABB white paper

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