Managing wind-farm assets can be inefficient without using the proper metering instrument transformer to monitor produced and transmitted power. Recent improvements to instrument transformer designs have increased the accuracy and stability of the current transformer’s performance range. This provides accuracy down to extremely low-current levels to capture generated power regardless of wind-turbine availability or wind speed. So with this capability, it allows Wind Farms to capture additional revenues on the power they produce at levels never seen before in the industry.

Nick S. Powers, High Voltage Instrument Transformer Marketing Manager, ABB Kuhlman, www.ABB.com

Instrument transformers convert high voltages and high currents to standardized values for measurement and control equipment, such as meters and relays. The changing electric utility market requires metering instrument transformers with improved performance to meet the functional specifications of a deregulated industry. Wide, bi-directional swings 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 good news is that 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.

Designers of such equipment are reacting to real market needs to get better performance. They are also working to place the equipment into service quicker and expending fewer funds to accurately meter diverse locations, such as wind farms, base load-generation facilities, or other IPPs such as solar farms. Applications commonly found in many new metering locations are identified in this article.

More metering points
When electric utilities own the complete power supply system – generation, transmission, distribution, and delivery to users as shown in the Single entity power supply system illustration – metering at intermediate points is not needed because the power is controlled by one entity. In this arrangement the actual power is not metered in most instances until the end-user takes delivery.

Metering within the power grid of North America has changed significantly over the past years due to the impact of deregulation and independent power generation. In the move to open the utility power grid to competition, the activities requiring metering have increased in scope and complexity. Many states require metering all generation sites, as well as power-use points. This has occurred in Texas with metering oversight provided by the Electric Reliability Council of Texas. Also, if generation assets are being sold by local utilities to outside interests, these points are retrofitted with metering installations.

IPPs build generation facilities (generally either combined-cycle gas, solar, or wind energy) in locations that require the power capacity, and are sited convenient to the prime mover source and transmission grid. IPPs normally prefer to meter power output on the high-voltage side of the facility at a point closest to the actual connection to the grid. Even in today’s cautious power environment, some electric utilities are evaluating ways to meter their distribution and retail substations in preparation for the eventual separation of the electric utility grid. This is in addition to normal metering that occurs at system-tie points, and customer-metering points connected and currently buying power at high-voltage levels.

Over many decades, IEEE C57.13 has set the pace for metering technology to meet site needs pertaining to industrial and utility applications – metering from small individual loads to large blocks of power. Within the last 10 years, however, metering needs have evolved faster than this industry standard. Extremely wide-current swings are present in generation metering points for IPP facilities. These facilities 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 due to the fact that the transmission grid operator supplies this power, and also designates the metering equipment to be used in most installations. So the use of highly accurate bi-directional meters is a must.

Instrument transformer metering standards
In the stringent world of revenue metering (and for all concerned parties), it is important to accurately measure the available power. When the voltages and loads become high enough to need instrument transformer-rated metering, the industry standard IEEE C57.13 properly specifies instrument-transformer accuracy and burden requirements. This defines standardized acceptance criteria, and within practical limits, helps minimize 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 U.S., 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.

CT operation is sensitive to the actual load-current levels measured. Generally, higher current-level measurements provide better accuracy results, and low-current levels tend to suffer from more inaccurate readings due to excitation error of the CT core. VT operation is normally unaffected by operating conditions because the voltage tends to be relatively stable, within ±10% around nominal rated values.

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.

This class is an improvement over previous 0.3 and also 0.15 class units because it maintains the 0.15% accuracy over a wider current swing. It still does not, however, cover the extremely low-current swing needed for auxiliary power measurement on power flow from the grid back into the facility for auxiliary power requirements. Many manufacturers have developed better than Class 0.15s wide range-metering current transformers for IPP applications. In fact with advanced free-standing designs, we have been able to certify accuracy performance to even 0.25% of nominal current. The result – IPPs receive a better return on investment using a 0.15% accuracy design, while the connection utility can accurately meter the reverse power supplied back into the facility. This is a system fair for all parties involved.

Advancements in asset metering
This accuracy class 0.15% improvement has pushed accurate measurement of current ratings down to 10%, 5%, and 1% of nominal current and below.

Wide range, high-accuracy metering instrument transformers provide the higher accuracy class of 0.15% to better align with solid state metering technology, and are produced with continuous current-rating factors up to 4.0 (400% of nominal current rating). This provides better metering range by increasing the top-end measurement using Class 20 meters for full access to the CT performance range. The transformers also have traceable accuracy going into the low-current levels much below that of the traditional Class 0.3 designs.

An accompanying photo (above, right) provides an example of better than class 0.15s designs successfully used in wind farms and other IPP sites in which 345kV Oil-filled free-standing CTs and VTs offer the high accuracy measurement advantage 0.15% accuracy from 400% rated current down to 0.5%% of nominal, and 0.3% accuracy to 0.25% of rated current. In this case, the 200:5 ratio CTs with RF=4.0 provides 0.15% metering accuracy from 1A to 800A, and 0.3% to 0.5A! The accuracy traceability to low-current levels allow capturing generated power regardless of wind-turbine availability or wind speed.

Given the recent improved accuracy of instrument transformers, increased scrutiny is now directed toward proper solid-state meter selection to ensure accurate power measurement at low-load levels. Historically, instrument transformers have been the limiting elements in the metering line-up. Modern, high-accuracy designs rated to supply accurate performance at secondary currents of 12.5mA and lower, now puts pressure on the solid-state meters to keep pace with instrument transformer development.

Easier Installation
Now with high accuracy, revenue-metering technology 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. This saves space and installation cost, and prevents having to place high voltage insulated CTs in the substation. What’s more, slipover designs cost a fraction of the purchase price of oil-filled freestanding designs. But this is generally recommended for base-load facilities that have more consistent current flow, and do not dip down below 5% of nominal ratio levels.

For wider range performance with extremely low end current applications, another method to reduce support structures and installation costs uses transformers called combination metering units. These designs house both current and voltage transformers in a common configuration.

Single phase combination current and voltage-transformer designs start at 5 kV and go to 230 kV. Three phase designs are available from 5kV to 46kV. These designs come with the same accuracy and burden ratings as provided by the individual current and voltage transformers, even in high accuracy, wide-range format. By combining the compact format of the metering units with the 0.15s high accuracy performance of the advanced metering designs, one design can usually meet many ratio range needs. This can greatly reduce the number of inventory items required. With retrofit applications multiplying due to generation addition and deregulation metering, the slipovers and metering units are valuable solutions to the problem of forcing metering into existing installations. The convenience of installation is a major consideration in new and retrofit metering locations, and these newer products help meet that need.

Quicker shipments come on-line faster
Another innovation to meet the needs of wind-farm developers has been the introduction of a multi-tap, wide range, high accuracy, reconnectible (Type KXM-SP) combination metering unit that quickly connects on site for use as either a 200:5, 300:5, 400:5, 600:5, 800:5, 1200:5, or 1600:5 ratio CT with a rating factor as high as 4.0. This design is 0.15%B1.8 capable on each current tap from 0.5% nominal current to rating factor value. So one part number, per voltage class, defines a unit that measures from 1A to 3,200A in the 0.15% accuracy and supply 50 kA RMS short-time rating. In addition, it’s available for quick shipment because this is a stocked standard design, or worse case, a 12-week shipment.

Due to its standardization, core and coils are prebuilt prior to customer orders to shorten the lead-times. Planned stock at the factory also supports emergency needs. This series parallel, primary-bus design uses primary re-connectible windings and tapped secondary windings. It has a special core steel to achieve a high accuracy, wide-range performance, but with multiple ratios to choose from. This greatly increases its range of effective currents, AND shortens the time for energization of your site!

Certification of existing instrument transformers
With infrastructure already in place and connected to the electrical grid, it is sometimes more expedient and economical to consider using existing installed bushing-current transformers and voltage transformers to connect to power meters. However, there are a number of factors that currently limit their use. For instance:

• CTs installed on major electrical apparatuses, such as power transformers, circuit breakers, or generators, are not rated for metering accuracy by design, but are more often relaying type units.
• CTs and VTs do not have accuracy certifications to support their metering application.
• Connected burden on the application have not been measured and are unknown on the installation. Therefore, IT performance at those burden levels is undefined.

Innovative testing techniques for identifying transformer errors on installed current and voltage transformers within generators, power transformers, and switch-gears are being used for revenue metering.

These tests can be conveniently performed on installed CTs and VTs without their removal from the major equipment. ABB, for instance, has tested thousands of CTs and VTs in place using these tests and provided accurate results on many current and voltage transformers. These have been installed within generators, station service-power transformers, and other locations in and around generation facilities and substations. Transformers tested using these methods can be considered usable for metering installations, provided that test results confirm their performance accuracy. WPE

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