Are Wind Turbine Step-Up Transformers the Weak Link in the Wind power supply Chain?

!: Are Wind Turbine Step-Up Transformers the Weak Link in the Wind power supply Chain?

In the rush to cash-in on wind energy, developers are often trading low first costs for higher total costs of possession to be shouldered later by the wind farm owners and operators.

Converting wind power to electrical power is the fastest growing segment of the Us power sector. Today, wind power represents less than 5% of the Us electrical generation and is targeted to reach 20% in the foreseeable future. For this to happen, new sites need to be industrialized in spite of a down turning economy.

Bolstered by ready federal stimulus dollars, we are seeing a virtual modern day 'land-rush'. In the words of one commerce leader, 'if there is a site that has a viable wind profile, entrance to network connections, and entrance for delivery of materials, and we don't manufacture it, some one else will.'

This head long rush to setup more and more wind turbines has outstripped the usual developmental studying curve, where new technologies mature by a process of trial and error, resulting in defining equipment remarkable for the job at hand.

The added economic pressure of today's market has made an already competing market even more demanding. This has, in the view of many commerce insiders, resulted in purchasing decisions for equipment based largely on the lowest introductory cost solutions and not solutions that will contribute the best choice in terms of total cost of ownership, network stability, less down time and lost revenue from high maintenance issues. This is nowhere as apparent as in the Case of Wind Turbine Generator (Wtg) transformers.

Historically this Wtg transformer function has been handled by conventional, 'off the shelf' distribution transformers, but the relatively large numbers of new failures would strongly advise that Wtg transformer designs need to be made substantially more robust. The institution of using approved 'off the shelf' distribution transformers as a low cost solution is folly. In some Cases site operators are maintaining a quantity of spare transformers to combat the frequent outages caused by proper distribution transformers being used where they are not suitable.

The role of the Wind Turbine Generator (Wtg) transformer in this process is primary and, as such, its manufacture needs to be carefully and thoughtfully analyzed and reevaluated.

Transformer Loading:

Wind turbine yield voltages range from 480 volts to 690 volts. The turbine yield is transformed, by the Wtg transformer, to a collector voltage of 13,800 to 46,000 volts. The turbines are highly dependant upon local climatic conditions; and this can succeed in yearly midpoint load factors as low as 35%. The relatively light loading of Wtg transformer has a favorable succeed on insulation life but introduces two unique and functionally primary problems.

The first problem is when lightly loaded or idle, the core losses come to be a more primary economic factor while the coil or winding losses come to be less significant. Typically used price estimation method do not apply to this scenario. Nema Tp1 and Doe efficiencies are not modeled for the operational scenario where midpoint loading is near 30-35% and, consequently, should be cautiously applied when calculating the total cost of possession for Wtg transformers.

The second problem is that the Wtg transformer is subjected to frequent thermal cycling as a function of varying turbine 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.

Harmonics and Non-Sinusoidal loads:

Wtg transformers are switched with solid state controls to limit the inrush currents. While potentially aiding in the introductory energization, these same electronic controls contribute damaging harmonic voltages that, when coupled with the non-sinusoidal wave forms from the turbines, cannot be ignored from a heating point of view. When a rectifier/chopper law is used, the Wtg transformer must be designed for harmonics similar to rectifier transformers, taking the supplementary loading into consideration as well as providing electrostatic shields to preclude the replacement of harmonic frequencies between the former and secondary windings.

Transformer sizing and voltage variation:

Wtg transformers are designed such that the voltage is matched to the wind turbine's yield voltage exactly. There is no 'designed in' over-voltage capacity to overcome voltage fluctuations which are a frequent problem with wind turbines. At the same time, the generator yield current is Monitored at millisecond intervals and the operational limits allow up to 5% over-current for 10 seconds before it is taken off the system. Therefore, the Wtg transformer is designed to match the generator yield with no overload sizing, and the Wtg transformer manufacture must be uniquely robust to function without it

Requirement to withstand Fault Currents:

Typically, approved distribution transformers, power transformers, and other types of step-up transformers will 'drop out' when subjected to a fault. Once the fault has cleared, the distribution transformer is brought back on-line. Wind turbine generators, on the other hand, in order to enounce network stability are not allowed to disconnect from the law due to network disturbances except within certain guidelines industrialized for generating plants. 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 Wtg transformer. Therefore, During faults, the transformer 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. The Wtg transformer must be uniquely designed with enough 'ruggedness' to withstand full short circuit current During the introductory few cycles when the maximum mechanical forces are exerted upon the Wtg transformer windings.

Conclusions:

The role of Wtg transformers in today's wind generation task is unique; it's manufacture must be equally unique and robust. Don't trade long term reliability and lower total cost of possession for low introductory cost.