Transformer Oil Maintenance

Energy transformers are critical components of the energy distribution grid and it is therefore important to have a monitoring and maintenance plan in place to preempt their failure. One of the critical components of an energy transformer is the transformer oil.
A transformer operates in a moisture free environment and even the slightest moisture can seriously reduce its life. Most companies have an oil maintenance schedule to monitor the condition of oil and detect a problem before it causes extensive damage. Oil testing during maintenance also helps detect problems like contact arcing, aging insulating paper and other latent faults.
Steps for Collecting Oil Sample for Transformer Oil Testing:
Oil testing is a critical process it can be done before the transformer start-up, during a routine transformer inspection or in any circumstances indicating possibility of damage to the transformer, particularly when a protective device is triggered.
To collect an oil sample a sampling valve located near the bottom of the tank is used. Transformer oil is a hygroscopic substance and must be protected from contact with moisture. It is therefore important to place the collected oil sample in a clean dry container.

Oil Treatment Guidelines
Following are the oil treatment guidelines which can prolong the life of transformer and save a company thousands of dollars:

• Purify when the acid level is still low, i.e. <0.1 mg
• Regenerate preferably from 0.1 mg KOH/g oil to avoid precipitation of sludge
• Desludge when the acid level is >0.20 mg KOH/g oil
• Dry-out when the solid insulation is wet >3.5 % MDW

Purification, a method of transformer oil maintenance
Purification is the process by which moisture and gasses are removed from the insulating oil. This process readily dries up the oil but not the insulation system, this is because the drying depends on the rate of diffusion of water through the paper into the oil, which is slow. Frequent processing is necessary to attain the degree of dryness desired in the cellulose insulation.
Even though the purification method is not the best, it is an effective moisture management tool. It is used widely in the industry to effectively reduce the moisture content and elevate the dielectric strength of the oil in wet core situations.

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THE UNIQUE ROLE OF WIND TURBINE STEP-UP (WTSU) TRANSFORMERS

Introduction:
Harnessing wind energy to perform work is not a new concept.
Since the earliest of times, wind power has been captured with sails to allow traders,merchants and explorers to ply their trades and discover the world around them.
On land, windmills have been used for irrigation, grinding grains, and performing crude manufacturing for centuries. Even the generation of electricity from wind power is not a new idea. What is new, however, is the scale at which this renewable energy source is being used today.
Early wind generation served a local need, often supplying power for isolated
equipment. Today, wind energy represents nearly 5% of the US electrical generation and is targeted to reach 20% in the foreseeable future.
For this to happen, wind turbine outputs need to be gathered, stepped-up to
transmission levels and passed across the nation’s interconnected power grid to the end users. The role of the Wind Turbine Step-Up (WTSU) transformer in this process is critical and, as such, its design needs to be carefully and thoughtfully analyzed and reevaluated in our view.
Historically this WTSU transformer function has been handled by conventional, “off the shelf” distribution transformers, but the relatively large numbers of recent failures would strongly suggest that WTSU transformer designs need to be made substantially more robust. WTSU transformers are neither conventional “off the shelf” distribution transformers nor are they conventional “off the shelf” power generator step-up transformers. WTSU transformers fall somewhere in between and as such, we believe, require a unique design standard.
Although off-shore wind farms using dry-type transformers are beginning to grow in popularity, for this discussion we will look only at liquid-filled transformers that are normally associated with inland wind farm sites.
Transformer Loading:

Wind turbine output voltages typically range from 480 volts to 690 volts. This turbine output is then delivered to the WTSU transformers and transformed to a collector voltage of 13,800 to 46,000 volts. The turbines are highly dependant upon local climatic conditions; and this dependency can result in yearly average load factor as low as 35%. Both conventional distribution transformers and power generator step-up transformers are typically subjected to more constant loading at, or slightly above, their theoretical maximum rating. This high level of loading stresses insulation thermally and leads to reduced insulation life. On the other hand, the relatively light loading of WTSU transformer has a favorable effect on insulation life but introduces two unique and functionally significant problems with which other types of conventional transformers do not have to deal.

The first problem is that, when lightly loaded or idle, the core losses become a more significant economic factor while the coil or winding losses become less significant and de-emphasized. Typically used price evaluation formulae do not apply to this scenario. NEMA TP1 and DOE efficiencies are not modeled for the operational scenario where average loading is near 30-35% and, consequently, should be cautiously applied when calculating the total cost of ownership for WTSU transformers
The second problem is that the WTSU transformer goes into thermal cycling as a
function of these varying loads. This causes repeated thermal stress on the winding,clamping structure, seals and gaskets. Repeated thermal cycling causes nitrogen gas to be absorbed into the hot oil and then released as the oil cools, forming bubbles within the oil which can migrate into the insulation and windings to create hot spots and partial discharges which can damage insulation. The thermal cycling can also cause accelerated aging of internal and external electrical connections.
These cumulative effects put the WTSU transformer at a higher risk of insulation and dielectric failure than either the typical “off the shelf” distribution transformer or the power generator step-up transformer experiences.
Harmonics and Non-Sinusoidal loads:
Another unique aspect of WTSU transformers is the fact that they are switched in the line with solid state controls to limit the inrush currents. This differs widely from the typical step-up transformer which must be designed to withstand high magnetizinginrush currents which cause core saturation, and in the extreme Ferroresonance.
While potentially aiding in the initial energization, these same electronic controls
contribute damaging harmonic voltage frequencies that, when coupled with the nonsinusoidal wave forms from the wind turbines, cannot be ignored from a heating point of view. Conventional distribution transformers do not typically see non-linear loads that require preventative steps due to harmonic loading. When a rectifier/chopper system is used, the WTSU transformer must be designed for harmonics similar to rectifier transformers, taking the additional loading into consideration as well as providing electrostatic shields to prevent the transfer of harmonic frequencies between the primary and secondary windings, quite dissimilar to conventional distribution transformers.
Transformer sizing and voltage variation:
WTSU transformers are designed such that the voltage is matched to the generator (e.g. wind turbine) output voltage exactly. There is no “designed in” over-voltage capacity to overcome voltage fluctuations, as is typically done on distribution and power transformer designs which allow for up to 10% over-voltage. Further, it should be noted that the generator output current is monitored at millisecond intervals and the generator limited to allow up to 5% over-current for 10 seconds before it is taken off the system. Therefore, the WTSU transformer size ( kVA or MVA) is designed to match the generator output with no overload sizing. Since overload sizing is a common protective practice with “off the shelf” distribution or power step-up generator transformers, the WTSU transformer design must be uniquely robust to function without it.
Requirement to withstand Fault Currents:
Typically, conventional distribution transformers, power transformers, and other types of step-up transformers will “drop out” when subjected to an under-voltage or overcurrent situation caused by a fault. Once the fault has cleared, the distribution transformer is brought back on-line either individually or with it’s local feeder in conjunction with automatic reclosures. Wind turbine generators, on the other hand, in order to maintain network stability are only allowed to disconnect from the system due to network disturbances within certain, carefully controlled network guidelines developed for generating plants. Depending upon the specific network regulations, the length of time the generator is required to stay on line can vary. During this time the generator will continue to deliver an abnormally low voltage to the WTSU transformer.Therefore, during near-to generator faults, the generator may be required to carry as low as 15% rated voltage for a few cycles and then ramp back up to full volts a few seconds after fault clearing. This means that the WTSU transformer must be uniquely designed with enough “ruggedness” to withstand full short circuit current during the initial few cycles when the maximum mechanical forces are exerted upon the WTSU transformer windings.
Since wind turbines must stay connected during disturbances in the network, the WTSU transformers must be designed to withstand the full mechanical effects of short circuits.
Conclusions:
The role of WTSU transformers in today’s wind generation scheme is unique; it’s design must be equally unique and robust. The combination of wide variations in loading; harmonic loads from associated control electronics and generators; sizing without protection for over-voltages, under-voltages or over-loading; and the requirement to “ride through” transient events and faults sets the WTSU apart from it’s more conventional, “off the shelf” counterparts. It is neither a conventional distribution transformer nor is it a conventional generator step-up transformer.
“Off the shelf” . . . doesn’t belong . . . “down on the farm”!

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The Differences between Grounding Transformers and Distribution Transformers

Grounding transformers (GT) differ from "standard distribution transformers" (DT) because they are used to establish a return path for ground fault currents on a system which is otherwise isolated or effectively un-grounded. This differentiates the construction in a couple of ways.
Grounding transformers must be designed to meet two basic criteria:

1. They must be able to carry the continuous phase and neutral currents without exceeding their temperature ratings.
2. They must be able to carry the fault current without excessive heating that deteriorates the conductors or adjacent insulation.

It is in the second parameter which most widely separates grounding transformers from distribution transformers. DTs are designed to carry a fault current, which is limited by their impedance, for a maximum duration of 2 seconds per standards.  Whereas the GT must carry a fault current that is not limited by its impedance, for durations exceeding the 2 second limitation. Often this time is 10 seconds or more. The GT design must be such that at the end of this extended time period, the conductor temperature is below the critical thermal limit as identified in the standards.

DT: Main Concerns
The DT main concern is for heating caused by loading. Radiators are added to the transformer to help the insulating fluid control the steady state temperature rise, but these do not help during fault conditions. Heat generated during a fault happens in such a short period of time (usually seconds) that the calculation assumes "all heat is stored" in the conductor because heat dissipation does not occur fast enough to combat the rapidly heating conductors. The GT takes this into account and is designed such that the conductor can handle the fault heating without relying on insulating oil for heat transfer during the fault.

Many GT specifications recognize this and allow the steady state cooling to be calculated using the magnetizing current and HV I2R loss resulting from energizing the core only. This leads to some misconception that the DT is better cooled, but the opposite is during faults.

Another subtle difference is the way the two devices "see" faults. The DT typically sees a line to ground fault or maybe, a line to line fault, but since the GT is providing a return path to the network, it typically sees a zero sequence fault which impresses the fault current equally on all three legs simultaneously. To combat the forces generated, GT conductors are always copper for maximum strength to cross section ratio, and because copper has a higher thermal withstand capability. GT coils are always circular on cruciform cores to gain the maximum form stability. Distribution transformers often utilize rectangular coil construction which does not have the same form stability offered by the circular coil technology.

How to Choose a Transformer

When choosing a transformer, there are two primary concerns: the load and the application. Several factors must be evaluated carefully while making the choice, to ensure that the needs of both primary concerns are met.
To use a cliché, it is typically a ‘no-brainer’ to choose smaller transformers. A unit with a kVA rating that is larger from the anticipated load can quickly be picked up. But if you are selecting a large unit for an electrical utility system, to be part of a large distribution network, you are typically making a much larger investment; thus the evaluation process is much more detailed and elaborate. With over 90 years of experience in this industry, Pacific Crest Transformers has put together a quick checklist to help you make your choice judiciously.
Top Questions
There are three major questions that influence your choice:

• Does the chosen unit have enough capacity to handle the expected load, as well as a certain amount of overload?

• Can the capacity of the unit be augmented to keep up with possible increase in load?

• What is the life expectancy of the unit? What are the initial, installation, operational, and maintenance costs?

Evaluation Factors
The cost and capacity of the transformer typically relate to a set of evaluation factors:
1. Application of the Unit
Transformer requirements clearly change based on the application.
For example: in the steel industry, a large amount of uninterrupted power is required for the functioning of metallurgical and other processes. Thus, load losses should be minimized – which means a particular type of transformer construction that minimizes copper losses is better suited. In wind energy applications, output power varies a great extent at different instances; transformers used here should be able to withstand surges without failure. In smelting, power transformers that can supply constant, correct energy are vital; in the automotive industry, good short-term overload capacity is a necessary attribute. Textile industries, using motors of various voltage specifications, will need intermittent or tap-changing transformers; the horticulture industry requires high-performance units that suit variable loading applications with accurate voltage.
These examples serve to underline that type of load (amplitude, duration, and the extent of non-linear and linear loads) and placement are key considerations. If standard parameters do not serve your specific application, then working with a manufacturer that can customize the operating characteristics, size and other attributes to your needs will be necessary. Pacific Crest regularly builds custom transformers for unique applications.

2. Insulation Type (Liquid-Filled or Dry Type)
While there is still debate on the relative advantages of the available types of transformers, there are some performance characteristics that have been accepted:

• Liquid-filled transformers are more efficient, have greater overload capability and longer life expectancy.
• Liquid-filled units are better at reducing hot-spot coil temperatures, but have higher risk of flammability than dry types.
• Unlike dry type units, liquid-filled transformers sometimes require containment troughs to guard against fluid leaks.

Dry type units are usually used for lower ratings (the changeover point being 500kVA to 2.5MVA). Placement is also a crucial consideration here; will the unit be indoors serving an office building/apartment, or outdoors serving an industrial load? Higher-capacity transformers, used outdoors, are almost always liquid-filled; lower capacity, indoor units are typically dry types. Dry types typically come in enclosures with louvers, or sealed; varnish, vacuum pressure impregnated (VPI) varnish, epoxy resin or cast resin are the different types of insulation used.
3. Choice of Winding Material
Transformers use copper or aluminum for windings, with aluminum-wound units typically being more cost-effective. Copper-wound transformers, however, are smaller – copper is a better conductor - and copper contributes to greater mechanical strength of the coil. It is important to work with a manufacturer that has the capability and experience to work with either material to suit your specific requirement.