Managing power delivery today requires utilities to maximize grid capacity and extend equipment life in a safe manner. Better system designs and monitoring have garnered much of the focus to date, but significant opportunities are available simply by improving transformer cooling and insulating capability.

First, consider that many transformers installed in North America are approaching or have passed their recommended operational life. According to Hartford Steam Boiler, substation transformer failures are expected to increase 500% within the next 15 years. Many of these units were installed in the 1950s and 1960s, as the United States quickly expanded its electrical infrastructure.

Eventually, old transformers will fail. Despite safeguards in place to prevent such occurrences, some transformers will fail in a violent, eventful manner, resulting in tank ruptures and fires. The associated cost of the failures will include downtime, damage to buildings and property, and the potential loss of life.

Paradoxically, substations and distribution transformers, well past their prime and potentially dangerous, are being pushed harder than ever. Unfortunately, it is difficult for substation managers to obtain the capital required to replace older equipment. It is increasingly common to drive units well beyond their recommended IEEE loading guide. So the question becomes, is it possible to increase the safety of an aged, installed transformer while pushing more power through the unit? The answer is yes.

Reedy Creek Improvement District

In 1997, a new office building was added to the Disney-MGM Studios Theme Park at Walt Disney World near Orlando, Florida, U.S. The facility, which housed Disney cast members, was designed to be expanded. The original construction placed the electrical room, complete with a 1500-KVA distribution transformer, on an exterior wall. However, future building expansion would place the electrical room in the center of the building.

Reedy Creek Improvement District (RCID), the local utility district, performed a risk assessment. The assessment concluded that only a transformer with an FM-listed, high fire point insulating coolant could be installed in the current electrical room.

FM initially adopted transformer installation guidelines to emphasize built-in transformer safety in 1994. This gave transformer manufacturers the opportunity to offer an FM-approved and labeled transformer with an FM-approved less flammable liquid.

Reedy Creek Energy Services, the design contractor for RCID, specified the transformer to accommodate the existing deluge system. The design engineers also decided to beta test Cooper Power Systems' Envirotemp FR3 seed-oil-based transformer fluid. Because the transformer uses a less flammable fluid, the building owner or the utility would not need to make any modifications to the transformer area when the expansion occurred.

In the seven years since the transformer was installed, RCID has not experienced any problems with the transformer. None of the regularly scheduled transformer fluid sample tests have indicated any problems, leading RCID to expect a long life from this transformer.

Alliant Energy

In 1997, Alliant Energy (AE; Madison, Wisconsin, U.S.) experienced a low-current fault in an underground cable beneath a row of 13.8-kV metal-clad switchgear. Because the breaker did not trip, the fault continued to burn, eventually knocking out power to a large section of downtown Dubuque, Iowa, U.S. Fortunately, only the switchgear and cable were damaged. However, because there had been magnesium on the cable, the fire that night could be seen for miles, catching the attention of the media, which gave the event extensive coverage.

Recognizing the fire could have occurred on the substation transformer end of the circuit, AE chose to participate in a beta test of Envirotemp FR3 transformer coolant at several substations, including the retrofilling of a 1957 vintage step-up transformer. The decision was based on the fire and environmental safety benefits of the fluid. It also allowed life extension testing in a real-world application.

After initial testing, AE realized that the same properties of the seed-oil-based fluid that lead to life extension (specifically, removal of water from the paper insulation) could help it provide more power to customers without the need to upgrade transformers.

Recent tests demonstrated that AE could load the transformer to 109.4% of its rated capacity by increasing the top oil temperature from 65∞C to 75∞C (149∞F to 167∞F) without any loss of insulation life. Some power transformers may be loaded up to 115% by increasing their top oil temperature to 86∞C (187∞F), depending on whether certain components of the transformer can resist softening at this higher temperature. Following the results of these tests, AE chartered a Lean Six Sigma team to recommend transformers for mineral oil to seed-oil retrofills throughout its entire four-state service territory. By extending the capacity of a 20-MW transformer by 10%, AE believes it can delay replacing the transformer for three to five years. Initial forecasts indicate that delaying the purchase of a $250,000 transformer could save AE $35,000 annually in carrying charges, or $175,000 over five years.

While AE will be operating the transformers beyond its original recommended capacity, the company is not concerned about eventful failures due to overloading for several reasons. First, all the transformers being considered for the retrofills are isolated from the public. Second, the transformers will not be overloaded beyond their new ratings. Third, even with some residual mineral oil, the fire point remains well above 300∞C (572∞F). Because AE is making a technology improvement to the transformers by retrofilling them, the faceplates can be changed to reflect the higher operating limits.

Safety and increased performance are not the only two reasons to specify seed-oil-based transformer fluids. Unlike traditional transformer mineral oils, seed oils are completely biodegradable, safer for plant life, animals and people, and are not hazardous to handle. (Unlike mineral oil-based coolants, seed-oil-based transformer coolants are not subject to the Federal Regulation of Used Oils [Title 40, No. 270].) Utilities and energy companies close to major water sources or serving primarily residential areas may want to investigate retrofilling transformers or installing transformers with seed-oil coolant to help protect the environment as well as their stakeholders.

Reedy Creek and Alliant Energy are both monitoring the effects of seed-oil transformer coolants on the life extension, or increased capacity, of their transformers. AE has been actively testing FR3 in transformers for more than five years. Because this is a new technology, both companies are moving ahead cautiously, constantly monitoring the transformers for any potential operating problems. So far, initial testing has yielded only positive results.

Rob Weber is a leads standards engineer for Alliant Energy, where he has worked for 13 years. He earned his BSEE degree from the University of Nebraska — Lincoln, has 31 years of experience in distribution engineering and is a 25-year member of IAEI.

Jerry Murphy received his BSEE degree from Texas A&M University. He has more than 20 years of experience designing utility distribution and substation systems and has been working with Reedy Creek Energy Services for 13 years. He is a member of IAEI (25 years) and IAS (3 years).

Safety and Reliability

In the 1990s, electrical distribution manufacturers began experimenting with seed-oil-based natural esters as transformer coolants, primarily to provide improved environmental benefits. Traditional transformer coolants are petroleum-based mineral oils that do not provide utilities with the same level of fire safety or environmental compliance benefits as seed-oil-based transformer coolants. Though environmental compliance was the initial goal of the seed oil transformer coolant programs, research demonstrated seed oils also offered higher fire ratings and, with certain seed-oil coolants, insulation life extension.

Some of these fluids comply with the National Electric Code and National Electric Safety Code requirements, even for most indoor installations. Factory Mutual and Underwriters Laboratories also classify these fluids as fire-resistant fluids. While seed oils are readily biodegradable and more environmentally responsible, researchers also discovered they have high fire points in the range of 340∞C to 360∞C (644∞F to 680∞F) compared to the 155∞C to 165∞C (311∞F to 329∞F) fire-point range of conventional mineral oils. These natural ester dielectric fluids are now recognized as “less flammable” per Section 450.23 of the NEC. This allows transformers under 10 MVA filled with listed vegetable-oil-based transformer coolants to be placed indoors, typically without the requirement of sprinklers, deluge systems or vaults.

Seed-oil-based coolant has been proven to draw the humidity out of the insulation paper, extending the functional life of a transformer. Testing per C57.100 shows that Kraft paper aged in mineral oil is too brittle to be useful after 3002 hours. Kraft paper aged in mineral oil, then retrofilled with vegetable oil, retains 36% tensile strength after 3002 hours. A separate test indicated that after a transformer was retrofilled with the seed oil, the relative humidity dropped from 11.6% to 3.4% after just six weeks.

More Power

Transformer performance relates directly to the condition of its paper insulation. As the insulation paper begins to age and fail, so does the transformer. Water retention is one of the primary causes of paper insulation degradation. Traditional mineral oil transformer fluids allow the insulation paper to gather water in its fiber, causing the paper to age. Seed-oil-based coolants actually draw moisture out of the paper, extending its life and its performance capabilities.

Because of this, transformers filled with seed-oil-based fluid can 1) be run within their standard operating parameters longer, extending the life of the transformer, or 2) be operated beyond the faceplate ratings without loss of insulation life. (For the latter, it is important that other transformer components be assessed to determine if they can support the higher operating temperatures.) In either case, using seed-oil-based transformer coolant offers utilities a lower dollar per kVA capacity. Using seed-oil-based coolant allows utilities to provide up to 15% more power to their consumers with existing equipment as well as avoid transformer upgrade costs.