Abel González Gómez
Induction motors are the workhorse of the modern age and they constitute up to 90% of the installed motor base. Motor protection is very important and when done properly can save companies hundreds of thousands of dollars a year in down time.
Modern motor protection devices can monitor all relevant operational variables like line voltages and currents, frequency, speed, temperature, etc. To configure and implement different sets of protection functions using these variables, adequate knowledge of the operational theory of the machine is required. However, proper design and configuration of the motor protection devices and systems is only the first step when it comes to their correct operation. A systematic approach to testing is equally important to detect design flaws, improper configurations, system reliability, and settings changes as well as device malfunctions or failures The ability to create tests that simulate normal as well as faulted motor operating conditions is key to properly evaluating the behavior of the relay.
In this paper a systematic approach for testing the most common induction motor protection functions, is presented. The protection functions presented in the paper are, current and voltage unbalance, under and over voltage, thermal overload, mechanical jam protection, sensitive ground protection, and motor starting elements. A proper approach to testing each of these different motor protection functions will be shared.
Since proper testing of protection functions starts with the right understanding of their operation, the reasons behind the existence and the theory of operation of each of the motor protection functions is presented as well as how their settings can be related to the motor’s nameplate information.
Testing motor protection is a very involved and work-intensive process that can become very complicated if it is not well prepared with basic information to get started as in motor nameplate information, settings files in the relays’ proprietary format, and drawings of the protection scheme for the protective relay under test. As previously mentioned, a review of a typical process will be covered to help outline what a technician might need to do in the field.