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Virginia Transformer has a Solution for Gassing in Wind Farm Pad Mount Transformers
Wind farms have been rapidly added to the power grid in the last decade. Wind now contributes to
more than 60,000 MW of generation capacity in USA and has been achieving double digit growth in the
last couple of years. The wind turbines have become more matured over the years. But one of the
crucial pieces of equipment, namely the Pad Mount Transformer that links the turbine to the grid has
shown a major deficiency. Reports presented at the 2013 Doble conference have shown that pad
mount transformers have experienced a gassing rate for conditions 3 & 4 of the IEEE standard at close
to 11% level and though the turbines are designed for 25-30 year life cycle the pad mount transformers
may be failing in as little as 3-5 years. This is alarming and will cause high O&M costs to the owners of
these Wind Farms in the future.
Virginia Transformer has analyzed this ever increasing problem and brought its expertise of 40 years in
the transformer business to resolve the problems and achieve 25 years of life matching that of the
turbine. A brief description of the problems related to wave distortion, harmonic generation and gas
production of the step-up transformers is provided. Standard distribution transformers have been used
at these wind farms in the past though there is now a need for a much more stringent duty cycle with
the newer turbines using electronic conversion of the power output. As such more care must be taken
to design and manufacture these units.
The Wind Turbine Generators (WTGs), like other generators, produce generator related distortions
which in turn produce harmonic waveforms. In addition to these harmonics, the solid state equipments
used to control the output of the step-up transformers also produce damaging harmonics. The
generator systems using rectifier/chopper circuits present a very special problem for the transformers.
This requires a reduction of the eddy losses to compensate for the harmonic currents.
The specification used for the transformer which is working with rectifier/chopper type controllers
should be similar to the transformer designed for arc furnace, drive duty, or rectifier transformer. If
these concerns are not addressed the combined non-sinusoidal wave from the turbines and the wave
forms from the switching induced harmonics will create excessive heating and hot spots within the
transformer. If the conventional distribution transformers are used the extra burden imposed on them
by such harmonic currents will shorten their life span considerably.
The excessive harmonic voltages when imposed on the cores of the distribution transformers which
were designed for maximum allowable flux density level can saturate the cores and create excessive
voltages. The use of electrostatic shields was not common in the earlier generation of the step-up
transformers but if provided they can filter such harmonics, thus helping the system and the
transformers.
It has been recommended that additional over voltage capability be allowed to the WTGs. The standards
allow for 10 % overvoltage for the distribution transformers but additional 5 % over voltage should be
allowed for step up transformers.
The sizing of the transformer using the tables of standard distribution transformers creates another
problem. The ratings of the generators do not match up with those of the transformers. Typically a
lower rating transformer is selected considering the fact that rarely the turbines will be running at full
capacity. However, this poses a problem since the transformer will be overloaded at times which in turn
will lower the useful life of the transformer. VTC recommends that either a higher rated transformer is
selected from the standard or an exact rating be specified so that all the components may be sized to
match with the WTG.
Low Voltage Ride Through (LVRT) requirement for WTG has been made more stringent. The new
standard requires that the WTGs stay on line at 0 p.u. voltage for 4 to 9 cycles rather than 15 % of the
rated voltage per the earlier requirements. This fault ride-through capability places larger electrical,
thermal and mechanical demands on the WTGs and the transformers connected to them.
There are several other concerns which need to be addressed. Switching surges and transient voltages
generated by the vacuum breaker on HV side and other breakers on LV side produce a transient
recovery voltage (TRV) wave which enters the step up transformer. This combined with the capacitance
of the cables produces standing waves and ringing that are many times the original voltage level. TRV
surges coincide with the internal resonant frequencies of the winding thus elevating the electric stresses
on the winding and are much higher than the withstand capabilities of the winding.
It may be better to specify the 200 kV BIL rating for the coil and leave the bushings and other
components at 150 kV BIL rating. This will protect the coils which are most difficult to replace.
The cyclical nature of the load causes the gases to be absorbed in the fluid and then released when the
fluid heats up and cools down. The gas bubble generated will travel though the winding and may cause
the ionization of the gas trapped in the bubble under right conditions. This will increase the bubble size
which may further escalate the problem and arcing may start - thereby short-circuiting the windings and
leading to the failure of the unit. As mentioned earlier the harmonics may further exacerbate the
situation.
VTC has taken into consideration all these factors in designing true GSU type pad mounts for WTGs. .
Additionally the connections have been made permanent to eliminate the failure due to loosening of
connections. Initial tests have shown very positive results. VTC has been manufacturing specialized and
customized transformers for the last 40 years. The knowledge gained over the years has been put to use
in designing these transformers.