K-rated or harmonic mitigating transformers

Just as current harmonics can cause additional heating and losses in the magnetic core of a motor, they can produce much the same effect in the iron core of a transformer. However, greater losses from harmonics can occur in the windings of the transformer.

09/17/2012


Eddy currents are circulating current in the conductors induced by the sweeping action of leakage magnetic field on the conductors. Eddy current concentrations are higher at the ends of transformer windings due to the crowding effect of the leakage magnetic fields at the coil extremities. Eddy current losses increase with the square of the frequency of the harmonic content of the current. Transformers that supply power to nonlinear loads generate more internal heat than if the same load was strictly linear in type, meaning that the transformers aren’t capable of meeting their nameplate capacities without running at damaging high temperatures. 

By the early 1980s, commercial buildings had a significantly high percentage of nonlinear loads in proportion to total load; as a result, overheating was regularly seen on heavily loaded transformers. Electrical manufacturers responded with transformers specially designed to handle harmonic-rich loads, called k-rated transformers. 

For this reason, where transformers will be serving nonlinear loads comprising more than 35% to 50% of their nameplate rating, k-rated transformers ensure the transformer does not overheat and possibly fail. 

The k-rating of a transformer is defined as highest k-factor load that the transformer can serve at its nameplate rating. In effect, the higher the k-factor, the more resulting heat the transformer is able to handle without exceeding its load carrying rating. In practical terms, the most common k-rated transformers typically have a k rating of 4, 7, 13, or 20, which corresponds to harmonic producing equipment totaling up to 35%, 50%, 75%, and 100% of downstream load, respectively. 

A key weakness of k-rated transformers is that while they reduce losses within the transformer itself, they do not actually reduce harmonics within the power system. Thus all loads connected to the secondary side of the transformer see the same voltage harmonics, and the upstream building electrical system is exposed to the current harmonics drawn by the transformer. 

Harmonic mitigating transformers (HMTs) are designed to greatly reduce certain harmonics based on their design, and thus reduce exposure to the rest of the electrical system from current harmonics drawn by downstream loads such as VFDs.



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
2016 Product of the Year; Diagnose bearing failures; Asset performance management; Testing dust collector performance measures
Safety for 18 years, warehouse maintenance tips, Ethernet and the IIoT, GAMS 2016 recap
2016 Engineering Leaders Under 40; Future vision: Where is manufacturing headed?; Electrical distribution, redefined
SCADA at the junction, Managing risk through maintenance, Moving at the speed of data
Safety at every angle, Big Data's impact on operations, bridging the skills gap
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Applying network redundancy; Overcoming loop tuning challenges; PID control and networks
Driving motor efficiency; Preventing arc flash in mission critical facilities; Integrating alternative power and existing electrical systems
Package boilers; Natural gas infrared heating; Thermal treasure; Standby generation; Natural gas supports green efforts

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role of plant safety and offers advice on best practices.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me