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The Unique Role Of Wind Turbine WTSU

The Unique Role Of Wind Turbine WTSU

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 magnetizing inrush 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 non- sinusoidal 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 over- current 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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AUTHOR: Pacific Crest Transformers
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Transformer Ratings

Transformer Ratings

Transformer size or capacity is most often expressed in kVA. “We require 30 kVA of power for this system” is one example, or “The facility has a 480 VAC feed rated for 112.5 kVA”.

However, reliance upon only kVA rating can result insafety and performance problems when sizing transformers to feed modern electronic equipment.

Use of off-the-shelf, general purpose transformers for electronics loads can lead to power quality and siting problems:

  • Single phase electronic loads can cause excessive transformer heating.
  • Electronic loads draw non-linear currents, resulting in low voltage and output voltage distortion.
  • Oversizing for impedance and thermal performance can result in a transformer with a significantly larger footprint.

It is vital for the systems designer to understand all of the factors that affect transformer effectiveness and performance.
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Thermal Performance

Historically, transformers have been developed to supply 60 Hz, linear loads such as lights, motors, and heaters. Electronic loads were a small part of the total connected load. A system designer could be assured that if transformer voltage and current ratings were not exceeded, the transformer would not overheat, and would perform as expected. A standard transformer is designed and specified with three main parameters: kVA Rating, Impedance, and Temperature Rise.
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KVA Rating

The transformer voltage and current specification. KVA is simply the load voltage times the load current. A single phase transformer rated for 120 VAC and 20 Amperes would be rated for 120 x 20 = 2400 VA, or 2.4 KVA (thousand VA).
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Impedance

Transformer Impedance and Voltage Regulation are closely related: a measure of the transformer voltage drop when supplying full load current. A transformer with a nominal output voltage of 120 VAC and a Voltage Regulation of 5% has an output voltage of 120 VAC at no-load and (120 VAC – 5%) at full load – the transformer output voltage will be 114 VAC at full load. Impedance is related to the transformer thermal performance because any voltage drop in the transformer is converted to heat in the windings.
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Temperature Rise

Steel selection, winding capacity, impedance, leakage current, overall steel and winding design contribute to total transformer heat loss. The transformer heat loss causes the transformer temperature to rise. Manufacturers design the transformer cooling, and select materials, to accommodate this temperature rise.

Transformer Heat Loss

Transformer Heat Loss

Use of less expensive material with a lower temperature rating will require the manufacturer to design the transformer for higher airflow and cooling, often resulting in a larger transformer. Use of higher quality materials with a higher temperature rating permits a more compact transformer design.

Transformer Insulation Systems

Transformer Insulation Systems

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“K” Factor Transformer Rating

In the 1980′s, power quality engineers began encountering a new phenomenon: non-linear loads, such as computers and peripherals, began to exceed linear loads on some distribution panels. This resulted in large harmonic currents being drawn, causing excessive transformer heating due to eddy-current losses, skin effect, and core flux density increases.

Standard transformers, not designed for nonlinear harmonic currents were overheating and failing even though RMS currents were well within transformer ratings.

In response to this problem, IEEE C57.110-1986 developed a method of quantifying harmonic currents. A “k” factor was the result, calculated from the individual harmonic components and the effective heating such a harmonic would cause in a transformer. Transformer manufacturers began designing transformers that could supply harmonic currents, rated with a “k” factor. Typical “K” factor applications include:

  • K-4: Electric discharge lighting, UPS with input filtering, Programmable logic controllers and solid state controls
  • K-13: Telecommunications equipment, UPS systems, multi-wire receptacle circuits in schools, health-care, and production areas
  • K-20: Main-frame computer loads, solid state motor drives, critical care areas of hospitals

“K” factor is a good way to assure that transformers will not overheat and fail. However, “K” factor is primarily concerned with thermal issues. Selection of a “K” factor transformer may result in power quality improvement, but this depends upon manufacturer and design.
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Transformer Impedance

Transformer impedance is the best measure of the transformer’s ability to supply an electronic load with optimum power quality. Many power problems do not come from the utility but are internally generated from the current requirements of other loads.

While a “K” factor transformer can feed these loads and not overheat, a low impedance transformer will provide the best quality power. As an example, consider a 5% impedance transformer. When an electronic load with a 200% inrush current is turned on, a voltage sag of 10% will result. A low impedance transformer (1%) would provide only a 2% voltage sag – a substantial improvement. Transformer impedance may be specified as a percentage, or alternately, in Ohms (Ω) from Phase- Phase or Phase-Neutral.
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High Frequency Transformer Impedance

Most transformer impedance discussions involve the 60 Hz transformer impedance. This is the power frequency, and is the main concern for voltage drops, fault calculations, and power delivery. However, nonlinear loads draw current at higher harmonics. Voltage drops occur at both 60 Hz and higher frequencies. It is common to model transformer impedance as a resistor, often expressed in ohms. In fact, a transformer behaves more like a series resistor and inductor.

The voltage drop of the resistive portion is independent of frequency, the voltage drop of the inductor is frequency dependent.

Standard Transformer impedances rise rapidly with frequency. However, devices designed specifically for use with nonlinear loads use special winding and steel lamination designs to minimize impedance at both 60 Hz and higher frequencies. As a result, the output voltage of such designs is far better quality than for standard transformers.
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Recommendations for Transformer Sizing

System design engineers who must specify and apply transformers have several options when selecting transformers.
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Do It Yourself Approach

With this approach, a larger than required standard transformer is specified in order to supply harmonic currents and minimize voltage drop. Transformer oversizing was considered prudent design in the days before transformer manufacturers understood harmonic loads, and remains an attractive option from a pure cost standpoint. However, such a practice today has several problems:

  • A larger footprint and volume than low impedance devices specifically designed for non-linear loads
  • Poor high frequency impedance
  • Future loads may lead to thermal and power quality problems
Standard Isolation Transformer

Standard Isolation Transformer

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“K”-factor Rated Transformers

Selecting and using “K”-factor rated transformers is a prudent way to ensure that transformer overheating will not occur. Unfortunately, lack of standardization makes the “K” factor rating a measure only of thermal performance, not impedance or power quality.

Percent Impedance

Percent Impedance

Some manufacturers achieve a good “K” factor using design techniques that lower impedance and enhance power quality, others simply derate components and temperature ratings. Only experience with a particular transformer manufacturer can determine if a “K” factor transformer addresses both thermal and power quality concerns.
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Transformers Designed for Non-Linear Loads

Transformers designed specifically for non-linear loads incorporate substantial design improvements that address both thermal and power quality concerns. Such devices are low impedance, compact, and have better high frequency performance than standard or “K” factor designs. As a result, this type of transformer is the optimum design solution.

This type of transformer may be more expensive than standard transformers, due to higher amounts of iron and copper, higher quality materials, and more expensive winding and stacking techniques. However, the benefits of such a design in power quality and smaller size justify the extra cost, and make the low impedance transformer the most cost effective design overall.

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