Power Quality and Protective Device Coordination:

Problems & Solutions

by Robert E. Fuhr, P.E.

 

Part 1: “Undersizing of Utility Main Service Transformers”

 

Part 2: “High Inrush Currents for Dry Type Transformers”

 

The use of electronic equipment has dramatically increased during the last 20 years. Some of these electronic devices include variable speed drives, programmable controllers, computers, and even solid state protective relays. Electronic equipment is much more sensitive to power quality. A lot of attention has been given to power quality in the last several years. Most of this focus has been on harmonics, power sags, and surges.

 

One important aspect of power quality that is extremely important is Protective Device Coordination. Protective Device Coordination Studies determine settings for circuit breakers and relays. These studies also verify the correct fuse size and determine the protection required for conductors, transformers, and other equipment. The goal of this study is to determine settings which would reduce damage to equipment and to isolate only the circuit that has a short circuit or fault. This causes the least amount of disturbance to the remainder of the distribution system.

 

Part 1 & Part 2 will focus on two unique problems that are affecting power system protection, owners, and design engineers. Click on the links below to read the papers outlining these 2 problems:

 

• Part 1 - Under-sizing of utility main service transformers for buildings, plants, and facilities.

 

• Part 2 - High inrush currents on low temperature or K-Rated transformers.