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VIRGINIA AND GEORGIA TRANSFORMER CORP – Solar Inverter step-up transformers

With the advent of renewable energy, the number of wind and solar farms has been on the increase, and economical benefits were originally found in the use of pad-mount transformers to step-up the voltage as produced by the wind turbines and solar inverters. The uniqueness of solar inverter step up (ISU) transformers does not permit for standard off the shelf pad-mount transformers to be used in this application. This article looks at factors that we have adopted to meet the requirements of this application and to improve robustness of solar ISU transformers.

Solar ISUs are gaining popularity in the field of green energy, and where economic benefits can be derived in the use of distribution pad-mount transformers and there are sufficient differences that would call for some level of customization. Some of these differences are based on the nature of solar farms and the inverter technology, and others would be based on how they are operated or failure modes. These include:

  • Location of solar farms – As a harvester of solar energy, solar farms are usually located in open areas sometimes with a maximum ambient temperature higher than 40°C, and maximum average daily temperatures over 30°C as per ANSI/IEEE standards.

This location is usually remote and the need for transmission at high voltages to grid connection points is also a requirement. To minimize the number of step-up operations, the voltage ratio on the solar ISU transformer is usually high, and adds to design complexity.

  • Inverter technology – The inverter is the only source of power for the ISU transformer, the sizing of the transformer is based on the sizing of the inverter. Due to limitations in inverter technology on increasing the inverter sizes and ratings, dual low voltage windings are being provided on ISU transformers for optimization of resources and maximizing efficiency. The coupling between low voltage windings, and the coupling between each of the low voltage windings and the high voltage winding needs to be considered.

To ensure the reliability of our designed and manufactured solar ISU transformers, we employ certain considerations for solar ISU transformers applications, including but not limited to:

  1. Impedance and coupling – We work with the customer to understand their system requirements for impedance matching between each of the LV windings and the HV winding. Also of importance is the level of coupling between the LV windings, and we can meet our customers’ requirement for the level of coupling between these LV windings. Where no requirement is stated, we would employ concentric windings but with matching impedances.

                          

  1. Core – As with all designs, stacked cores are used in all solar ISU transformers. Also, the flux density selected is such as to prevent core saturation due to potential dc components in the LV supply. This reduced flux density also caters for operational issues that could lead to voltage unbalance that would otherwise potentially result in core saturation, increased core noise or core heating.
  2. Sizing – We work with the end-user to ensure that the ISU transformer is not sized based on inverter nameplate rating but on the maximum inverter rating, as overloading could be an issue where the maximum inverter rating exceeds the nameplate rating.
  3. Cooling capacity – When solar ISU transformers do not have a high harmonic content, there is still some heating concern as the period of maximum loading usually coincides with the period of maximum ambient temperatures. We allow for this by also considering the maximum losses at the maximum ambient temperature that the transformer will experience, and allow for sufficient cooling based on the load cycle.
  4. Use of Electrostatic Shield – We provide low eddy loss electrostatic shields (ESS) between each LV winding and the HV winding due to the usually high voltage ratio to:
    • Prevent the capacitive coupling between the LV and the HV voltage windings, as the LV windings in this application are usually ungrounded wye connected windings.
    • Prevent the transfer of transients over voltages from the HV winding to the LV winding that may be due to switching operation of circuit breakers or multi-stage capacitor bank.
    • Prevent the transfers of high frequency (harmonics) voltage disturbances that are created during the voltage inversion by the inverter.

This shows that the solar ISU transformer may not be as robust as the wind turbine generator step-up  (WTGSU) transformer, but are more robust than the standard pad-mount distribution transformers.

CONCLUSION: We have used our experience in over 14,000 custom designs to tailor the design and construction of solar ISU transformers to meet the needs of renewable power.

There are other economic considerations not covered in this article, so contact us (540-354-9892) to assist in developing a robust specification.

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