Autotransformer Connection

 Impeadance Of Auto Transformer

The autotransformer is both the most simple and the most fascinating of the connections involving two windings. It is used quite extensively in bulk power transmission systems because of its ability to multiply the effective KVA capacity of a transformer. Autotransformers are also used on radial distribution feeder circuits as voltage regulators. The connection is shown in Figure 1

FIGURE 1 The boosting autotransformer connection. The output terminals operate

at a higher voltage than the input terminals

The autotransformer shown in Figure 1 is connected as a boosting autotransformer because the series winding boosts the output voltage. Care must be exercised when discussing ‘‘primary’’ and ‘‘secondary’’ voltages in relationship to windings in an autotransformer. In two-winding transformers, the primary voltage is associated with the primary winding, the secondary voltage is associated with the secondary winding, and the primary voltage is normally considered to be greater than the secondary voltage. In the case of  a boosting autotransformer, however, the primary (or high) voltage is associated with the series winding, and the secondary (or low) voltage is associated with the common winding; but the voltage across the common winding is higher than across the series winding.

The other possible connection for an autotransformer is shown in Figure 2. The autotransformer shown in Figure 4.2 is connected as a bucking autotransformer because the series winding bucks, or opposes, the output voltage.

FIGURE 2 The bucking autotransformer connection. The output terminals operate

at a lower voltage than the input terminals

The key feature of an autotransformer is that the KVA throughput of the transformer, i.e., its capacity, is different than the KVA transformed by the common and series windings. The common and series windings are wound on the same core leg, so the transformer laws  apply:

1. The volts per turn in the common winding equal the volts per turn in the series winding. The common winding voltage divided by the series winding voltage is equal to the number of turns in the common winding divided by the number of turns in the series winding.
2. The sum of the ampere-turns of the common winding plus the ampere- turns of the series winding equal the magnetizing ampereturns. The magnetizing ampere-turn are practically zero, so the magnitude of the ampere-turns in the common winding is approximately equal to magnitude of the ampere-turns in the series winding. The series winding current divided by the common winding current is equal to the number of turns in the common winding divided by the number of turns in the series winding.
3. The KVA transformed in the series winding equals the KVA transformed in the common winding.

The capacity multiplication effect stems from the fact that the metallic connection between the input and output circuits allows part of the KVA to flow though the connection and bypass the transformation. This is illustrated in the following example.

Example :

A boosting autotransformer has a common winding voltage of 7200 V and a series winding voltage of 1400 V. The current low-voltage input current is 100 A. Determine the KVA throughput and the KVA transformed. Refer to Figure 3.

KVAthroughput = 7.2 kV x 100 A x720 KVA

KVAcommon = KVAseries x KVAtransformed

7200 V x (100 A  -I0) =1400 V x I0

I0 = 720 KVA / 8600 V= 83.72 A

KVAcommon = 7.2 kV x (100 A - 83.72 A) =  117.2 KVA

KVAseries= 1.4 kV x 83.72 A = 117.2 KVA

In this Example  the ratio of KVA throughput to KVA transformed is 720/117.2 = 6.1, meaning that this autotransformer has 6.1 times the capacity of a two-winding transformer of a similar size and weight. This is a considerable multiplication of KVA capacity.

FIGURE  3 A boosting autotransformer used in Example

read more :

http://en.wikipedia.org/wiki/Autotransformer

Parallel Operation of Transformers

 Parallel Operation of Transformers

For supplying a load in excess of the rating of an existing transformer, two or more transformers may be connected in parallel with the existing transformer. It is usually economical to install another transformer in parallel instead of replacing the existing transformer by a single larger unit. The cost of a spare unit in the case of two parallel transformers (of equal rating) is also lower than that of a single large transformer. In addition, it is preferable to have a parallel transformer for the reason of reliability. With this, at least half the load can be supplied with one transformer out of service. For parallel connection of transformers, primary windings of the

transformers are connected to source bus-bars and secondary windings are connected to the load bus-bars. There are various conditions that must be fulfilled for the successful parallel operation of transformers. These are as follows:

1. The line voltage ratios of the transformers must be equal (on each tap): If the transformers connected in parallel have slightly different voltage ratios, then due to the inequality of induced emfs in the secondary windings, a circulating current will flow in the loop formed by the secondary windings under the no-load condition, which may be much greater than the normal no-load current. The current will be quite high as the leakage impedance is low. When the secondary windings are loaded, this circulating current will tend to produce unequal loading on the two transformers, and it may not be possible to take the full load from this group of two parallel transformers (one of the transformers may get overloaded).
2. The transformers should have equal per-unit leakage impedances and the same ratio of equivalent leakage reactance to the equivalent resistance (X/R): If the ratings of both the transformers are equal, their per-unit leakage impedances should be equal in order to have equal loading of both the transformers. If the ratings are unequal, their per-unit leakage impedances based on their own ratings should be equal so that the currents carried by them will be proportional to their ratings. In other words, for unequal ratings, the numerical (ohmic) values of their impedances should be in inverse proportion to their ratings to have current in them in line with their ratings. A difference in the ratio of the reactance value to resistance value of the perunit impedance results in a different phase angle of the currents carried by the two paralleled transformers; one transformer will be working with a higher power factor and the other with a lower power factor than that of the combined output. Hence, the real power will not be proportionally shared by the transformers. 

3. The transformers should have the same polarity: The transformers should be properly connected with regard to their polarity. If they are connected with incorrect polarities then the two emfs, induced in the secondary windings which are in parallel, will act together in the local secondary circuit and produce a short
circuit.

The previous three conditions are applicable to both single-phase as well as threephase transformers. In addition to these three conditions, two more conditions are essential for the parallel operation of three-phase transformers:

4. The transformers should have the same phase sequence: The phase sequence of line voltages of both the transformers must be identical for parallel operation of three-phase transformers. If the phase sequence is an incorrect, in every cycle each pair of phases will get short-circuited.
5. The transformers should have the zero relative phase displacement between the secondary line voltages: The transformer windings can be connected in a variety of ways which produce different magnitudes and phase displacements of the secondary voltage. All the transformer connections can be classified into distinct vector groups. Each vector group notation consists of first an uppercase letter denoting HV connection, a second lowercase letter denoting LV connection, followed by a clock number representing LV winding’s phase displacement with respect to HV winding (at 12 o’clock). There are four groups into which all possible three-phase connections can be classified:
Group 1: Zero phase displacement (Yy0, Dd0, Dz0)
Group 2:180° phase displacement (Yy6, Dd6, Dz6)
Group 3: -30° phase displacement (Yd1, Dy1, Yz1)
Group 4: +30° phase displacement (Yd11, Dy11, Yz11)

In above notations, letters y (or Y), d (or D) and z represent star, delta and zigzag connections respectively. In order to have zero relative phase displacement of secondary side line voltages, the transformers belonging to the same group can be paralleled. For example, two transformers with Yd1 and Dy1 connections can be paralleled. The transformers of groups 1 and 2 can only be paralleled with transformers of their own group. However, the transformers of groups 3 and 4 can be paralleled by reversing the phase sequence of one of them. For example, a transformer with Yd1 1 connection (group 4) can be paralleled with that having Dy1 connection (group 3) by reversing the phase sequence of both primary and secondary terminals of the Dy1 transformer.

References

1. Say, M.G. The performance and design of alternating current machines, 2nd edition, Sir Isaac Pitman and Sons, London, 1955.
2. Toro, V.D. Principles of electrical engineering, 2nd edition, Prentice Hall, New Delhi, 1977.
3. Stevenson, W.D. Elements of power system analysis, 4th edition, McGraw- Hill, Tokyo, 1982, pp. 138–162.
4. MIT Press, Magnetic circuits and transformers, 14th edition, John Wiley and Sons, New York, 1962, pp. 259–406.

Stop Corrosive Sulfur

 Stop Corrosive Sulfur

Corrosive Sulfur in transformer oil has been the cause of high profile transformer failures in recent years. Oil treatments consisting of copper passivators have been only partially effective. DSI has discovered a multi-step method that has been proven to change oils with corrosive sulfur into non-corrosive status, as well as significantly slowing oxidation and ageing of insulating oil and paper. This paper outlines the history of the problem as well as the research that led to a successful commercial launch of a product known as DSI Sulfur Inhibitor.

Background:

Corrosive sulfur has recently received a great deal of attention by owners of power transformers. Before the mid-1990s, however, it was considered a phenomenon that occurred only with transformer oils of questionable quality. The consensus was that if one used one of the major brands of oil that this problem would rarely be encountered.

Many things have changed in the last two decades, however, to make the problem of corrosive sulfur a very real one:

1. Refining and oil purification methods have changed.
2. The transformer oil industry has seen many oil suppliers leave the market, shifting market share to new suppliers.
3. Sources of crude oil have changed; new sources contain different profiles of naturally occurring sulfurous compounds.
4. Transformer operating conditions and designs have changed. Today’s transformers are designed with less cooling oil with respect to the mass of metal available, which raises the relative level of metal ions in the oil.
5. Modern transformers, too, are being operated at higher temperatures, which has several effects.
a. Higher temperatures seem to have the effect of changing a type of sulfur from non- or poorly reactive one into a type that is more highly reactive.
b. Higher temperatures drive chemical reactions to occur at a faster rate which means that the dissolution, reaction and plating effects of sulfur-metal compounds occurs much more quickly than it would have in the past.

Types of Sulfur Compounds : 

Not all sulfur compounds in oil are harmful. Some are not only stable, but actually have antioxidant effects. Others, such as mercaptans, simple sulfides and elemental sulfur are highly reactive. Dibenzodisulfide (DBDS) is thought to be one of the more reactive sulfur species that is found in transformer oils. The types and quantities of sulfur in an oil depends on the source of the crude oil and the refining methods used. Different crude oils have different amounts of each of these sulfur compounds. Different companies’ refining processes can remove or change sulfur compounds from one type to another.

What Happens With Corrosive Sulfur?

Inside a transformer, metals – copper, iron and aluminum, slowly dissolve into the transformer oil. Because of its molecular structure and properties, copper is the most easily dissolve and the most reactive of the metals normally found in transformers. Metal ions in solution combine with sulfur compounds to produce a range of copper-sulfur salts. The exact profile of the metal-sulfur salts that are created depends on the conditions inside the transformer, the types of copper that are present, and the types of sulfur that is present in the transformer oil.  

But copper isn’t the only metal that can take part in chemical reactions. The different alloys of aluminum and iron that are found inside a transformer also dissolve into transformer oil, and their ions can and do interact with sulfur. If copper isn’t present, aggressive sulfur will combine with these other metals to form a variety of sulfur-metal salts.

These sulfur-metal salts, whether they’re from copper, iron, or aluminum, saturate the transformer oil. When the concentration of salts reaches a certain point (which varies, according to the chemistry of the oil and the conditions in the transformer), the salts will grow in a crystalline structure on other surfaces inside
the transformer. These surfaces may be paper, wood, or any surface that can act as a substrate to the growth of copper-sulfur, ferro-sulfur, or alumino-sulfur salts.

This process repeats itself 1 until the source of metal or sulfur ions is used up. Before this happens, however, the transformer will often experience problems because the metal-sulfur salts that are being deposited (“plated out”, in common terms) are conductive. The buildup of these conductive salts leads to transformer failure.

What can be done about this? Studies have shown that sulfur is difficult to remove from transformer oil. Filtration with fuller’s earth or other ion exchange media has very little effect on the concentration of sulfur compounds, although it can remove some of the metal-sulfur salts that are already in solution.

The Traditional Approach : 

Historically, corrosive sulfur has been dealt with not by treating the sulfur, but by hindering copper ions from entering solution. This can be done by using a variety of “Yellow Metal Passivators”. These Yellow Metal Passivators, such as benzotriazole, or tolyl-triazole, form a very thin, non-reactive coating on copper and thereby slow its dissolution into transformer oil.

There are three problems with attempting to stop corrosive sulfur with the simple addition of a yellow-metal passivator, however.

1. There are several different alloys or varieties of copper and brass materials in every transformer. Each different type reacts differently with different copper passivators. Some are very well protected by one chemical passivator, but not another. Some don't bind well to either type.
2. Different iron, steel and aluminum metals are present in transformers, and they are also are reactive to aggressive sulfur. While they're not as reactive as copper, they do combine with sulfur and have a hand in the
plating reactions that occur. Copper passivating chemicals – benzotriazole and tolyltriazole - don't protect transformers from reactions involving iron, aluminum or steel.
3. Copper passivators, by themselves, don't do anything to reduce the corrosive sulfur compounds in transformer oils. They simply attempt to intervene in the dissolution of certain metals. The underlying corrosion
problem is still present.

Physical Removal of Sulfur:

Several attempts have been made to find a way to physically remove sulfur from transformer oil. Sulfur is very difficult to remove, however, from existing transformer oil. Filtration with fuller’s earth or other ion exchange media has very little effect on levels of sulfur found in oil.

Chemical Removal of Sulfur:

Research at DSI has found that aggressive sulfur compounds in transformer oil can be changed by introducing them to one of several reactive “sulfur scavengers”. These compounds “tie up” sulfur in oil, preventing its reaction with any metal ions. The sulfur-metal salts are effectively prevented from forming, so they can't “plate out” onto cellulose insulation.

These sulfur scavengers are large, complex molecules that are especially reactive to corrosive sulfur compounds, but not to other chemicals found in transformer oil. They effectively combine with reactive sulfur and hold it in suspension, preventing it from combining with metal ions in oil. Analysis has shown that some of the more aggressive types of sulfur, such as dibenzo disulfide (DBDS) can actually be changed to a less aggressive compound of sulfur.

Relationship Between Corrosive Sulfur and Oxidation:Field and anecdotal evidence describes a correlation between low oxidation resistance of an oil and its propensity to develop problems with corrosive sulfur, given the same application conditions. While the basis for this correlation is not well understood, it has been noticed and discussed at CIGRE and IEEE, and was considered significant enough to take into account during this investigation.

Commercial Development and Application:In 2006 and 2007, continuation of this research program resulted in the development of a commercially available product to protect transformers from corrosive sulfur in transformer oil.

This new sulfur protection and reduction scheme protects transformers in three different ways, which work in synergistic manner:

1. First, DSI Sulfur Inhibitor uses a blend of several different metal passivators. Our research has found that a mixture of metal passivators are much more effective at preventing dissolution of copper into oil than a single compound. A years' worth of study and laboratory testing yielded precise ratios for the use of different metal passivators that would prevent the different chemicals from interfering with one another, and to work together to protect the maximum number of types of copper metals found in transformers. 

2. Second, DSI exploited the causal link between oxidation stability of paper and oil and the ability of the sulfur-metal interaction to proceed. It is our understanding and belief that oils that have lower stability against oxidation are more likely to promote the dissolution of metal and its reaction with aggressive sulfur compounds. For this reason, the product that we developed contains a powerful blend of different antioxidant chemistries that protect oil and paper from accelerated ageing and inhibits their ability to enter into chemical reactions.

Third, DSI developed a mixture of sulfur scavenging and passivating compounds. These chemicals seek out corrosive sulfur in oil and bind with it to prevent its interaction with metals, paper, or oil. The bound sulfur is effectively rendered harmless. The concentration of the most reactive types, such as DBDS, is actually lower after treatment with DSI Sulfur Inhibitor. This goes far beyond the protection provided by simple yellow metal passivators.

Taken together, these three mechanisms have been proven to be very effective in protecting metals and reducing the amounts and types of corrosive sulfur compounds found in transformer oil.

Laboratory Testing:

ASTM D1275b Corrosive Sulfur in Oil Test:

DSI Sulfur Inhibitor was first tested with four different oil samples that tested positive for corrosive sulfur. ASTM D1275B, which is the standard ASTM test for Corrosive Sulfur. This test measures the effect of subjecting clean copper strips to the oil being tested. We tested oil samples before and after treatment with DSI Sulfur Inhibitor. The oil samples were provided by different independent laboratories and the analyses were performed at a major independent laboratory.

Sample 1 Test result:

Untreated sample:                4b Corrosive
Treated Sample                   3a Tarnished

Sample 2 Test result:

Untreated sample:                3b Tarnished
Treated Sample                    3b Tarnished

Sample 3 Test result:

Untreated sample:                   4c Corrosive
Treated Sample                       3a Tarnished

Sample 4 Test result:

Untreated sample:                      4b Corrosive
Treated Sample                         No Change (no corrosion or tarnish)

In each of the samples that had “Corrosive” status, DSI Sulfur Inhibitor changed the sample to “non-corrosive” status.

Dibenzo Disulfide (DBDS) Testing:

Dibenzo Disulfide is a chemical found in transformer oils that has been linked to corrosion of copper and other metals. Reduction in Dibenzo Disulfide (DDS) content of the oil is considered to be very closely linked to reduction in corrosive behavior.

Four samples of oil were treated with DSI Sulfur Inhibitor. Of these four, three had significant reductions in the concentration of Dibenzo DiSulfide. Some samples showed a greater reduction than others, but taken as an average, DSI Sulfur Inhibitor lowered the concentration of Dibenzo DiSulfide (DDS) in the oil samples by an average of 14250 ppb, or 26% of the DDS concentration of the pooled samples.

Field Application:

Since its commercial introduction, DSI Sulfur Inhibitor has been used in transformers (GSUs and substation units of several types) of varying ages and sizes. Results of oil tests on treated transformers have shown significantly lower dibenzo disulfide levels, lower oxidation rates, and enhanced protection for oil and paper. Transformer oil samples that have been tested have indicated that they are “non-corrosive” status.

Application Example:

A Chinese utility had two power transformers (Panyu 150 kV, 80 MVA) with oil that was extremely corrosive when tested with DIN and ASTM tests. A small sample of the oil was treated in the laboratory with DSI Sulfur Inhibitor, which rendered the oil “non-corrosive”. The utility then decided to treat both transformers with Sulfur Inhibitor. After treatment, the oil in both transformers tested “non-corrosive”.

DSI performed a full analysis on the oil from these transformers, before and after treatment. Analysis of the oil showed that the concentration of several corrosive sulfur compounds had been reduced or eliminated. For example, the concentration of 1-methyl dibenzothiophene had been reduced by 82% in one transformer and by 92% in the other.

How is DSI Sulfur Inhibitor used?

The completed DSI Sulfur Inhibitor is a pre-mixed, liquid concentrate blend of advanced passivators, oxidation inhibitors and sulfur scavengers and stabilizers.

It is added directly to transformers where there is corrosive sulfur or low oxidation inhibitor content.

Because DSI Sulfur Inhibitor is a liquid, it is simply added to a de-energized transformer. No further mixing or blending is required. As delivered, DSI Sulfur Inhibitor is dried, degassed, and highly processed. It is compatible with all brands of standard mineral oil and fire resistant petroleum oils.

DSI Sulfur Inhibitor is made for field application in transformers that are filled with standard mineral oil. DSI Sulfur Inhibitor is added to transformer oil to inhibit and prevent corrosion, plating, or other problems caused by corrosive sulfur. Although it has a blend of advanced antioxidants, sulfur inhibitor is not made to slow or reverse problems of “gumming”, polymerization, or premature oxidation of soybean oil dielectric fluids.

Production Of Corrosive Sulfur

 Production Of Corrosive Sulfur

Corrosive sulphur in transformer oils has been a growing concern for the electrical transmission industry for the past few years. There have been a number of documented cases of transformer failures which are believed to be due to corrosive sulphur compounds contained in the transformer oils. This has led to an industry-wide investigation into the causes and possible remedies for these transformer failures [1,2,3,4].

From this investigative work, new or updated test methods have been, or are being developed to help detect corrosive sulphur species in transformer oil. The updated ASTM 1275B and proposed CIGRE Covered Conductor Deposition (CCD) test methods are a direct result of this work. Some naphthenic transformer oil suppliers have chosen to address this corrosive sulphur problem by using additives called passivators to help try and slow down the effects of corrosive sulphur in their oils. However, even with the field addition of passivators to transformer oils known to contain corrosive sulphur, failures related to corrosive sulphur have still been reported [5].

The electrical industry has been asking transformer oil manufacturers to come up with better technologies to resolve this issue. Passivators may only be a “quick fix” for the short term and may have long term implications that are not yet fully understood [6]. Transformer oil manufacturers are taking this issue seriously as the liability of supplying corrosive transformer oil is a significant consideration for all. Most manufacturers understand that eliminating corrosive sulphur through more thorough refining and purification of the base oils used to manufacture the transformer fluids is a key to preventing corrosive sulphur related failures [7].

Corrosive sulphur free base oils that do not require the use of passivators to manufacture a transformer fluid are not a new development, and in fact have been commercially available for over ten years. These  iso-paraffin base oils are produced using state-of-the-art refining instead of the conventional methods used to process some naphthenic based transformer oils.

REFINING BASICS : 

Crude oil as it comes out of the earth contains molecules made up of mostly carbon and hydrogen. Relatively smaller, but significant, amounts of other undesirable substances such as sulphur, oxygen, nitrogen, inorganic salts, metals and aromatic compounds can also be found in the crude [8,9]. The purpose of refining is to remove these undesirable constituents, as required, to develop the desired range of products with the necessary properties. The rest of this section will focus on the manufacture of lubricant base oil stocks (from which transformer oil is made) and the treatment methods involved.

In the early days of refining, chemical or clay treatment was the best technology for removing unwanted compounds from base oil stocks. In the late 1920’s to early 1930’s, an alternative method was developed known as Solvent Refining or Solvent Extraction. By contacting the base oil with an extraction solvent, such as furfural or phenol, increased removal of aromatics and other undesirable species could be achieved that led to a more thermally stable (higher viscosity index) and moderately higher quality of base oil.

Around the same timeframe (mid to late 1930’s) another process called Hydrogen Refining (commonly called Hydrocracking or Hydrotreating) was being considered. The main benefit over solvent extraction was that the undesirable constituents in the feed were converted into desirable lubricant base oil components or other substances that could be readily removed. It never really caught hold in the industry since hydrogen at that time was expensive, and the overall cost of solvent refining was more economical. However, in the early 1970’s there was a resurgence of this technology due to the poorer quality of crude slates available and the difficulty of processing them with conventional solvent refining techniques. The issue of expensive hydrogen was also alleviated by use of a cheaper source of hydrogen from catalytic reformers.

Finally in the early 1990’s, the latest version of Hydrogen Refining was introduced, called Severe Hydrocracking/Hydroisomerization. Instead of using a “chill dewaxing” step to remove wax from the base oil, this process converts the wax to oil by passing it over a catalyst at high temperatures and very high pressures which selectively cracks and recombines the molecules resulting in a water-white, corrosive sulphur free base fluid [10]. It is this technology that the iso-paraffin transformer oils are based upon.

BENEFITS OF ISO-PARAFFIN TRANSFORMER OIL : 

As mentioned above the state-of-the-art technology for making iso-paraffin base oils is known as Severe Hydrocracking/Hydroisomerization. Some of the main benefits of this technology for producing transformer fluid base oils are:

1- Water-white colour
2- Virtually no sulphur, nitrogen, aromatics or other impurities
3- No corrosive sulphur
4- Excellent response to oxidation inhibitors
5- Fully saturated iso-paraffin and cyclo-paraffin content
6- Increased thermal stability
7- Excellent cold temperature properties

What does this mean to a transformer oil user?

A significant benefit of using iso-paraffin base oils to manufacture transformer oils is the fact that it has no corrosive sulphur. The description “virtually no sulphur” above indicates that if sulphur content was analyzed using sophisticated equipment capable of measuring sulphur with extremely low detection limits, there may be trace amounts (< 1 ppm) of sulphur in the oil. However, due to the severe nature of the refining process, this remaining sulphur would be noncorrosive in nature. While the typical high pressure hydrotreating and hydrocracking processes operating at >1,200 psig may be the minimum level necessary to remove many of the corrosive sulphur compounds [7], the Severe Hydrocracking / Hydroisomerization process operates around 2,500 psig to ensure that iso-paraffin base oils used for transformer fluids can consistently provide corrosive sulphur free performance.

Many corrosive sulphur studies and tests have been performed on the iso-paraffin transformer oils in the past few years. Some of these include:

1- ASTM D1275 and ASTM D1275B
2- A major industry investigative study on Corrosive Sulphur (including copper strip in oil tests and covered conductor deposition studies).
3- ABB and Siemens evaluations (varying versions of a Covered Conductor Deposition or CCD test which uses copper and paper in oil). The Siemens version is currently under review by CIGRE committee A2.32 for adoption as a standard.

In all cases to date, test results have reported that the iso-paraffin base transformer fluids successfully provided non-corrosive performance. What is important to note is that all of these were achieved without the use of a copper passivator additive.

Iso-paraffin base oils used for manufacturing transformer oil are also water-white. Some conventional naphthenic transformer oils have a pale yellowish colour to them, which is a result of the aromatic compounds left in the oil after refining. Sometimes, various aromatics can impart specific properties that a refiner may want for blending products, such as motor oil or hydraulic fluid. In a transformer oil, small amounts of certain types of aromatics can aid in providing negative gassing tendencies. However, too many aromatic compounds or the wrong types of aromatic compounds can have a detrimental effect on the biodegradability and toxicity of the oil. Although this is not a typical requirement for a transformer oil today, it is a concern that is continuously being raised in the industry.

Figure 1 below shows the results of the Modified Sturm Test (OECD 301B) that measures the amount of oil degraded by microbes after a set period of time. For the “Readily Biodegradable” zone, a major threshold that the oil must achieve is 60% degradation in 28 days. Although an oil based on natural esters or vegetable oil may be able to achieve this rapid rate of biodegradation it is also subject to hydrolysis leading to poor in-use fluid stability. Iso-paraffin based transformer oils can approach this higher rate of biodegradation without loss of fluid stability. Compared to the conventional naphthenic transformer oil in this chart, the iso-paraffin based transformer oil showed very good results and achieved a level known as “Inherently Biodegradable”.

OECD 301B Testing

Figure 1 – Biodegradability Data – Modified Sturm Test

Another benefit of the Severe Hydrocracking/Hydroisomerization technology is that of stability and the response to oxidation inhibitors. When a transformer oil oxidizes and breaks down, it will form harmful oxidation byproducts, such as acids, varnish, and sludge. The industry combats oxidation by adding oxidation inhibitors to the oil formulation at specified concentration limits.
Since there are various reaction mechanisms by which oxidation can occur, this is not a foolproof solution and some oxidation of the oil can still happen. This effect has been shown in lab tests (see Figure 2 below) as well as tests run by Canadian Utilities [11]. Relative to conventional naphthenic products, iso-paraffin based transformer oils have demonstrated excellent response to oxidation inhibitors to provide outstanding oxidation stability.

Figure 2 – Oxidation Stability – Modified ASTM D2440

                                                   Iso-paraffin                           Conventional Oils
                                                                                                  Naphthenic

-  Severe oxidation stability testing

-  ASTM D2440, modified

-  Temperature raised to 120 °C

-  Duration extended to 500 hours

The final benefit of the iso-paraffin transformer oil that will be covered in this paper is its heat transfer property. Heat transfer is essential in a transformer to take the heat generated at the core and windings and move it to the outer shell where it can be dissipated. Qualities such as improved heat capacity and thermal conductivity have lead to direct comparisons of the isoparaffin oil with typical naphthenic oil. Figure 3 below shows this comparison during a temperature rise test:

Temperature Rise Test 

Copper Conductor, 800 Amperes

Figure 3 – Temperature Rise Test
(Measuring the temperature of the copper conductor in various transformer oils)

The lower temperature of the copper conductor is a good indication that the iso-paraffin oil could keep a transformer cooler than the conventional naphthenic oils tested. Based on these and other lab tests of a similar nature, a study was conducted by a U.S. electric utility company that showed improved heat transfer with the iso-paraffin [12].

STANDARDS :

The industry has a number of key standards and specifications (including ASTM D3487-00, CAN/CSA C50-97, Doble TOPS 2006, and IEC 60296:2003) that buyers of transformer oil can use as a basis for their purchases [13,14,15,16]. The iso-paraffin transformer oils have been tested and were found to meet or exceed the inhibited and/or uninhibited (trace) oil performance requirements for each of these standards.

SUMMARY :

The transformer industry has been trying to resolve the issue of corrosive sulphur in transformer oils for a number of years. The “quick fix” that some transformer oil manufacturers are promoting is the use of copper passivators to try and slow down the effects of corrosive sulphur in their oil. From industry investigations, it is becoming more apparent that copper passivators may not be the long term answer. Better refining technology is required to lower or eliminate the corrosive sulphur content of transformer oils.

REFERENCES : 

1. Claiborne, C. Clair, “Recent Increases in Transformer Failures Due to Corrosive Sulfur,” Proceedings of TechCon 2006, TJ|H2b Analytical Services, Lake Buena Vista, FL, 2006.
2. Hajek, Jan et al, “Quality of Oil Makes the Difference, ABB Discovers the Solution to Transformer Breakdowns”, ABB Review, Zurich, Switzerland, 2004.
3. Lewand, Lance R., “The Role of Corrosive Sulfur in Transformers and Transformer Oil,” Proceedings of the 69th Annual International Doble Client Conference, Boston, MA, 2002.