Motor Protection Application Guide - Your Power …

1 Motor ProtectionApplication GuideAbout the AuthorsPaul Lerley has 28 years of utility and electronics experience, including 15 years at Central MainePower Company. He is a graduate of the University of New Hampshire and was Director of Substations Electrical Systems at Central Maine Power prior to joining Basler Electric Company. Mr. Lerley is a Senior Member of the IEEE and participates in four working groups of the Power System Relaying Committee. He has authored articles on testing for the Doble Engineering Conference and Transmission and Distribution magazine. He was previously very active in the Electric Council of New England.

2 Mr. Lerley was Principal Application Engineer at Basler Electric from 1996 to 2000 and was a Planning Engineer at Central Maine Power from 2000 to 2010. He currently works for RLC Engineering as a Senior Power System Young received his MBA from Rollins College in 1983 and BSET from Purdue University in 1971. He worked for Wisconsin Electric Power Company as a Relay Engineer for two years, and for Florida Power Corporation as a Field Relay Supervisor for 21 years. He authored the text "Protective Relaying for Technicians" and co-authored papers for the Georgia Tech Protective Relaying Conference.

3 Mr. Young is a Regional Application Engineer for Basler Electric and is a member of the IEEE. Mr. Young retired from Basler Electric in 2002 and is President of North Idaho Relay (Dan) Ransom, PE has 40 years of industrial and utility electronics experience; including many years in Motor Protection development and Application support for a major US relay manufacturer. He has extensive experience in consulting engineering for Power and communications systems. Dan is an engineering graduate (BSEE) of Gonzaga University, Spokane, Washington; he also holds a liberal arts degree from Washington State University.

4 He is a member of the IEEE IAS (Industry applications ), PES ( Power Engineering), Communications, and Standards societies. To date he has one US patent. He is a Professional Electrical Engineer in 11 states, commonwealths, and territories across the United States. Dan joined Basler Electric in 2010 and is Principal Application Engineer for the West Coast document contains a summary of information for the Protection of various types of electrical equipment. Neither Basler Electric Company nor anyone acting on its behalf makes any warranty or representation, express or implied, as to the accuracy or completeness of the information contained herein, nor assumes any responsibility or liability for the use or consequences of use of any of this printing April 1998 Revision July 20131 Motor Protection Application Guide1.

5 IntroductionWhen applying protective relays to motors or any other equipment, the decision of how much Protection is enough must be made. The answer depends on Motor rewind cost, loss of production, effect of downtime, new versus old installation, metering, control, and the consequences of a Motor failure on the electrical system and Guide presents an overview of Motor hazards and a discussion of detection and Protection options. Typical setting value ranges for the Basler Electric BE1-11m Protection system are given along with considerations to help designers and users when selecting Basler's Motor Protection solution.

6 Most of the Protection functions apply to squirrel cage, wound, induction motors. Additional Protection elements are needed for synchronous motors and are mentioned in this Overview of Motor Hazards Motor Protection is a challenge because many different incidents can affect a Motor and the associated load. Motor hazards include the following: Motor induced High internal temperature Insulation failure (faults within the Motor ) Bearing failure Mechanical failure Failure to start Synchronous motors - loss of fieldLoad induced Overload and underload Jamming High inertia.

7 Especially at startingEnvironment induced High ambient temperature High contaminant level or blocked ventilation Extreme cold or wet conditionsSource induced Loss of phase or phases Voltage unbalance Overvoltage Undervoltage Phase reversal Out-of-step condition resulting from system disturbanceOperation induced Closing out of phase High duty cycle Jogging Rapid reversing3. Protection Stator Phase Fault Overcurrent Protection : Choose the phase CT so the Motor running mode full load amps (FLA) is no less than 50% of the rated phase CT primary.

8 Ideally, the phase CT primary should be chosen so the FLA is 100% of the phase CT primary or slightly less, but never more. Phase-to-phase and three-phase stator faults usually are detected with nondirectional, instantaneous elements. If the available three-phase fault current is a low multiple of the relay setting (weak system), quick pickup is not assured. In this situation, differential relaying should be considered (see Section ). Otherwise, instantaneous phase overcurrent elements typically are applicable when the Motor rating is less than one-half of the supply transformer kVA rating.

9 2 When an instantaneous element is used for stator faults the setting should be as low as possible, yet never operate during the starting period. The locked rotor amps (LRA), ILR is usually six times the full load amps (FLA), IFL. Fig. 1 illustrates the element settings in relation to the starting current and the FLA. The instantaneous phase element (50P) should be set at no less than times the LRA (using the value of LRA at maximum starting voltage), plus a safety margin ( pu plus relay accuracy ( ). This setting assumes the element is sensitive to the transient overreach (DC offset) of an asymmetrical fault.)

10 Lower settings are possible if the element disregards the transient component or if a time delay longer than the transient time (6-15 cycles) is added (50TP). Verify that the minimum available three-phase fault current at the Motor terminals is at least 3 times the element setting. When applying a time delay ensure that all of the equipment can withstand the maximum available fault current for the total clearing time (element operation time, plus auxiliary time delay, plus circuit breaker operating time). FIGURE 1. STATOR SHORT-CIRCUIT Protection WITH 50 OR 50P Differential Protection : Differential Protection is used on motors where the available short-circuit current is close to the value of LRA.