Instrument transformers to increase accuracy to extremely low-current levels
Instrument transformer designs have improved recently and increased the accuracy and stability of the current transformers' performance. This means that accuracy down to extremely low-current levels can now be achieved to record generated power regardless of wind-turbine availability or wind speed. This enables wind farms to gain more revenue on power produced at low levels.
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Instrument transformer designs have improved recently and increased the accuracy and stability of the current transformers’ performance. This means that accuracy down to extremely low-current levels can now be achieved to record generated power regardless of wind-turbine availability or wind speed. This enables wind farms to gain more revenue on power produced at low levels.
Changes in power flow caused by independent power producers (IPPs), and an increasing need for accurate measurement at low currents require new instrument-transformer designs to keep pace with user demands. The recent transformer designs have exceeded the best performance defined in the traditional IEEE C57.13/CSA C60044 industry standards that govern instrument-transformer design, manufacturing, and testing. In addition, up-to-date instrument transformers conveniently interface with the existing equipment at site.
Within the last 10 years, metering needs have evolved faster than the industry IEEE C57.13 standard. Extremely wide-current swings are present in generation metering points for IPP facilities which have low in-flow auxiliary power requirements from the transmission connection. It represents a small fraction of the facility’s generation capacity. Metering inaccuracy at these low-current levels cannot be tolerated in this type of metering application as the transmission grid operator supplies the power, and designates the metering equipment to be used in most installations. So the use of highly accurate bi-directional meters is a must.
The industry standard IEEE C57.13 properly specifies instrument-transformer accuracy and burden requirements to help minimise errors involved in taking readings from the instrument transformer applications from 600V up to 765 kV levels. Typical requirements for most revenue metering installations throughout the US, in accordance with the main IEEE C57.13 standard to measure power sold, has been accuracy Class 0.3.
Class 0.3 transformers are required to meet the following criteria with the instrumentation burden connected to the transformer:
Current transformers (CT): at 100% of current ratio up to the continuous current rating factor of the CT, Transformer Correction Factors (TCF) must be between 0.997 and 1.003; at current levels below 100% and down to 10% of current ratio, TCF must be between 0.994 and 1.006.
Voltage transformers (VT): for operating voltages from 90% to 110% of nominal voltage, the TCF must be between 0.997 and 1.003 for revenue metering.
For high voltage-rated metering for direct connection to the grid, CT designs up to 500 kV with extra wide-range performance maintain 0.15% accuracy from 0.5% to 400% nominal current value. With this 800:1 swing in current without loss of accuracy, one design can meter the generated power into thousands of amperes, as well as reverse power needs at low-current levels for auxiliary power requirements back into the facility.
In an effort to address increasing measurement needs during wide load swings, IEEE C57.13.6 standard published in 2005, established the accuracy classes 0.15 and 0.15s. Specifically for wide load swings found in Wind Farm applications, Class 0.15s transformers are required to meet the following criteria with the instrumentation burden connected to the transformer:
0.15s Current Transformers: at 100% of current ratio up to the continuous current rating factor of the CT, TCF must be between 0.9985 and 1.0015; at current levels below 100% and down to 5% of current ratio, TCF must be between 0.9985 and 1.0015.
Wide range, high-accuracy metering instrument transformers provide the higher accuracy class of 0.15% to align with solid state metering technology better, and are produced with continuous current-rating factors up to 4.0 (400% of nominal current rating).
Since high accuracy, revenue-metering technology is available in outdoor slipover designs, such as the ACCUSlip CT from ABB, support structures and foundations are not needed for their installations. The slipover CTs install over the high-voltage bushings of power transformers, circuit breakers, or cable terminations.
New transformer designs give users access to more metering points for data collection, and better accuracy for optimal performance. Metering decision makers also have access to better products, easier installations, and lower cost options for metering critical loads.
Source: Windpower Engineering & Development
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