Defining & Specifying Transformers for Wind and Solar Applications

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Wind and solar power transformers have some very unique characteristics that require careful consideration. Their primary purpose is to transform voltage but in order to do so properly, quite a few other characteristics need consideration. From harmonics to voltage disruption to even simply how often a transformer is loaded off and on in a single day, any number of these considerations, if not properly managed, could have a big impact on the efficiency and safety of wind or solar power transformer.

To help our members keep up with the latest and best thinking in wind and solar power transformers, IEEE Power & Energy Society has created a number of resources on this subject. Here, we will highlight one of those resources, as well as additional content focused on this subject:


Defining & Specifying Transformers for Wind and Solar Applications

By Philip J Hopkinson

Fundamental Connection

In virtually every installation, the fundamental connection is from a 600-volt class low voltage circuit to a 34.5 kV medium voltage circuit.  Early wind power generators were ac but more sophisticated new wind and solar generators are 600 volt class dc, which requires an inverter to connect to the ac inverter transformer.  In fact, due to the electronic inverters with electronic IGBT (Insulated Gate Bipolar Transistors), the low voltage side may be delta or Y connected but not grounded.  The high voltages may also be delta or Y.

Power Levels

Most modern wind and solar transformers have power levels from 1000 kVA to 3000 kVA.  However, some of the offshore wind transformers have power levels of 10,000 to 20,000 kVA. For this discussion, power levels of 1000 to 3000 kVA will be examined.

Ambient

Ambient temperatures can vary significantly from -30C  in higher altitudes to + 50 C in hot desert-like locations.  Humidity can vary from 0 to nearly 100%.  Transformers in coastal regions can also have very high salt levels, which is especially important for dry type transformers that are often used in the nacelles.

Inverters

Solar transformers and dc wind generators require inverters to convert the dc voltages to ac.  Most such inverters use IGBT power electronic switches to produce the ac output.  Internal grounding of the electronic switches mandates that the LV input windings of the inverter-transformer remain ungrounded.   The examples below are common circuits, taken from IEEE C57.159:

Sept TT 01

Sept TT 02

Overvoltage

Per IEEE C57.12.00, normal transformers are designed to handle 110% of rated voltage on the North American System.  Interestingly, for wind power transformers, the overvoltage requirement is 115% of rated voltage. However, the frequency is allowed to be a minimum of 95% of rated frequency, resulting in a net volts/Hz allowance of 121% of rated.

Switching Transients

Virtually all of the medium voltage transformers are installed with vacuum circuit breakers.  For wind and solar transformers, there is a strong likelihood that switching operations will occur in service and the circuits will have very low power factor at the point of switching.  When such conditions occur, high voltage switching transients often result in transformer winding failures.  Resistor-Capacitor Snubbers have proven to be very effective for damping such transients when they are installed.   IEEE C57.142 IEEE Guide to Describe the Occurrence and Mitigation of Switching Transients Induced by Transformers, Switching Device, and System Interaction

Harmonics

Normal harmonics are assumed to be 5%.  Any harmonics in excess of 5% are to be indicated by the manufacturer.  Generally speaking, early concerns that harmonics were a leading failure cause have largely been discarded.

Core Construction and Grounding

Magnetic cores may be 5-leg wound or 3, 4, o4 5-leg stacked.  A large percentage of the wound core transformers have electrical windings with the LV on the inside and the HV on the outside.  When this occurs, the two outer most phases (phase A and phase C) have capacitive coupling to the core elements that can produce very high voltage drops within the cores, resulting in partial discharges and large amounts of hydrogen gas.  This phenomenon is well documented in the latest drafts of IEEE C57.12.00 and IEEE C57.12.90.

Load swings

Large load swings on a relatively short period of time are inevitable with solar and wind turbine transformers.  At night, there is virtually no load.  As the sun rises in early day, solar cells begin to conduct and the wind begins to blow.  This results in very large load swings as well as pressure changes within the tanks of liquid filled transformers.

 

PES Resources and Content

Read About this Topic in Electrification Magazine

vol8 issue1 electrification resizedIn addition to these articles, you can also find more info on this subject in volume 8, issue 1 of IEEE Electrification Magazine

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IEEE PES Standards

Two very important standards have been written by the IEEE Transformers Committee that addresses both wind and solar transformers that complement and add significant concepts to the normal documents:

  1. IEEE C57.159 Guide on Transformers for Application in Distributed Photovoltaic (DPV) Power Generation Systems
  2. IEC/IEEE C57.60076-16 for Wind Transformers for Wind Turbine Application - This document was jointly developed by both IEEE and IEC TC 14 (Power Transformers).

Additional Groups

This topic is one being worked on by a variety of different groups both inside and outside of IEEE PES.

If you are really interested in machine learning applications to energy forecasting and analytics check out this committee for even more info: