In the rush to cash-in on wind energy, developers are often trading low first costs for higher total costs of ownership to be shouldered later by the wind farm owners and operators.
Converting wind energy to electrical power is the fastest growing segment of the US energy sector. Today, wind energy 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 developed in spite of a down turning economy.
Bolstered by available federal stimulus dollars, we are seeing a virtual modern day 'land-rush'. In the words of one industry leader, 'if there is a site that has a viable wind profile, access to network connections, and access for delivery of materials, and we do not develop it, some one else will.'
This head long rush to install more and more wind turbines has outstripped the usual developmental learning curve, where new technologies mature by a process of trial and error, resulting in dismantling equipment suited for the job at hand.
The added economic pressure of today's market has made an already competitive market even more demanding. This has, in the view of many industry insiders, resolved in purchasing decisions for equipment based largely on the lowest initial cost solutions and not solutions that will provide the best choice in terms of total cost of ownership, network stability, less down time and lost revenue from high maintenance issues. This is now 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 reliably large numbers of recent failures would strongly suggest that WTG transformer designs need to be made substantively more robust. The practice of using conventional '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 outouts caused by standard distribution transformers being used where they are not suitable.
The role of the Wind Turbine Generator (WTG) transformer in this process is critical and, as such, its design needs to be carefully and thoughtfully analyzed and reevaluated.
Wind turbine output voltages range from 480 volts to 690 volts. The turbine output is converted, by the WTG transformer, to a collector voltage of 13,800 to 46,000 volts. The turbines are highly dependent upon local climatic conditions; and this can result in annual average load factors as low as 35%. The relatively light loading of WTG transformer has a favorable effect on insulation life but introduces two unique and functionally significant problems.
The first problem is when lightly loaded or idle, the core losses become a more significant economic factor while the coil or winding losses becomes less significant. Typically used price evaluation formula does 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 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 converters are switched with solid state controls to limit the inrush currents. While potentially assisting in the initial energization, these same electronic controls contribute harmful harmonic voltages that, when coupled with the non-sinusoidal wave forms from the turbines, can not be ignored from a heating point of view. When a rectifier / chopper system is used, the WTG 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.
Transformer sizing and voltage variation:
WTG transformers are designed such that the voltage is matched to the wind turbine's output 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 output 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 output with no overload sizing, and the WTG transformer design must be completely robust to function without it
Requirement to withstand Fault Currants:
Typically, conventional 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 maintain network stability are not allowed to disconnect from the system due to network distortances except within certain guidelines developed 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 specifically designed with enough 'ruggedness' to withstand full short circuit current during the initial few cycles when the maximum mechanical forces are executed upon the WTG transformer windings.
The role of WTG transformers in today's wind generation scheme is unique; it's design must be equally unique and robust. Do not trade long term reliability and lower total cost of ownership for low initial cost.