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Overcurrent Coordination Setting Guidelines Generators

 
     
  The information presented in this application guide is for review, approval, interpretation and application by a registered professional engineer only. SKM disclaims any responsibility and liability resulting from the use and interpretation of this information.

Reproduction of this material is permitted provided proper acknowledgement is given to SKM Systems Analysis Inc.

Introduction

The proper selection and coordination of protective devices is mandated in article 110.10 of the National Electrical Code. To fulfill this requirement an overcurrent coordination study is required. The electrical engineer is always responsible for this analysis. It is an unfortunate fact of life that many times the engineer who specified and purchased the equipment will not set the devices. Therefore, compromises are inevitable.
There are three fundamental objectives to overcurrent coordination that engineers should keep in mind while selecting and setting protective devices.

The first objective is life safety. Life safety requirements are met if protective devices are rated to carry and interrupt maximum available load currents, as well as, withstand and interrupt maximum available fault currents. Life safety requirements are never compromised.
 
  •   The second objective is equipment protection. Protection requirements are met if overcurrent devices are set above load operating levels and below equipment damage curves. Feeder and transformer damage curves are defined in applicable equipment standards. Motor and generator damage curves (points) are machine specific, and are normally provided in the vendor data submittal package. Based on system operating and equipment sizing practices equipment protection is not always possible.
•   The last objective is selectivity. Selectivity requirements are met if in response to a system fault or overload, the minimum area of the distribution system is removed from service. Again, based on system operating and equipment selection practices selectivity is not always possible.
 
  Purpose  
  The purpose of this guide is to provide overcurrent protective device setting guidelines for generators to meet the objectives listed above.  
  MV Generator Switchgear Feeder Unit with Voltage Controlled 51V  
  Industry standard backup overcurrent protection schemes for MV generators fed from switchgear circuit breakers include either a voltage controlled or voltage restrained overcurrent relay (device 51V). The voltage controlled overcurrent relay will be covered in this section. The 51V relay characteristics are plotted on a phase TCC along with the generator decrement and overload curves, and the feeder damage curve.

The purpose of the relay is to allow the generator to operate, and to provide backup fault protection for the generator and cable. To accomplish this, the relay pickup must be to the left of the generator armature steady state current. Also, the relay time delay characteristics must be above and to the right of the generator decrement curve with constant excitation, and to the left and below the generator overload and feeder damage curves, and the amp rating of the cable. The time delay must also be set to be selective with downstream feeder relays.

Suggested margins are listed below that have historically allowed for safe operation of the generator and cable while reducing instances of nuisance trips.
 
     
 
Device Function Recommendations Comments
CT Size 125-150% of FLA  
51V Pickup 80-90% of Id = 1/Xd Assume Xd = 1.5 ê p.u. if Xd unknown.
51 Time Dial Above knee of generator decrement curve with constant excitation Set above downstream feeder relays.
      Set below cable damage curve.
 
     
   
  Fig. 1 MV generator switchgear feeder unit w/VC 51V - one line  
   
  Fig. 2 MV generator switchgear feeder unit w/VC 51V - phase TCC  
  MV Generator Switchgear Feeder Unit with Voltage Restrained 51V

Industry standard backup overcurrent protection schemes for MV generators fed from switchgear circuit breakers include either a voltage controlled or voltage restrained overcurrent relay (device 51V). The voltage restrained overcurrent relay will be covered in this section. The 51V relay characteristics are plotted on a phase TCC along with the generator decrement and overload curves, and the feeder damage curve.

The purpose of the relay is to allow the generator to operate, and to provide backup fault protection for the generator and cable. To accomplish this, the relay pickup at 0%V restraint must be to the left of the generator armature steady state current, and at 100%V restraint must be to the right of the generator full load amps. Also, the relay time delay characteristics must be above and to the right of the generator decrement curve with constant excitation, and to the left and below the generator overload and cable damage curves, and the cable amp rating. The time delay must also be set to be selective with downstream feeder relays.

Suggested margins are listed below that have historically allowed for safe operation of the generator and cable while reducing instances of nuisance trips.
 
     
 
Device Function Recommendations Comments
CT Size 125-150% of FLA  
51V Pickup 125-150% of FLA @ 100% Voltage Restraint
Set below Overload Curve
51 Time Dial Above knee of generator decrement curve with constant excitation Set above downstream feeder relays.
      Set below cable damage curve.
 
   
  Fig. 3 MV generator switchgear feeder unit w/VR 51V - one line  
   
  Fig. 4 MV generator switchgear feeder unit w/VR 51V - phase TCC  
     
  LV Generator Molded Case Circuit Breaker or Power Circuit Breaker Feeder Unit

Industry standard phase overcurrent functions purchased with molded case or power circuit breakers serving LV generators include long time, short time and instantaneous functions. The circuit breaker (CB) characteristics are plotted on a phase TCC along with the generator characteristics, and the feeder damage curve.

The purpose of the CB is to allow the generator to operate, and to protect the generator and cable from overloads and faults. To accomplish this, the CB curve should be above the generator FLA, intersect the generator decrement curve in the short time region, fall to the left and below the cable damage curve and amp rating, and be above the generator decrement curve in the instantaneous region.

Suggested margins are listed below that have historically allowed for safe operation of the generator and cable while reducing instances of nuisance trips.
 
 
Device Function Recommendations Comments
CB LTPU 115-125% of FLA Set at or below cable ampacity.
CB LTD, STPU &
STD
Minimum required Set to intersect with generator decrement curve.
CB I2T Out If I2T in the breaker may never trip.
CB INST 200% of AC decrement curve Set below cable damage curve.
      Cable damage curve must be above the point defined by the maximum fault current and the CB instantaneous clear curve.
 
   
  Fig. 5 LV generator circuit breaker feeder unit - one line  
   
  Fig. 6 LV generator circuit breaker feeder unit - phase TCC.  
     
  References  
  •   Other Application Guides offered by SKM Systems Analysis at www.skm.com
•   Electrical Transmission and Distribution Reference Book, ABB Power T&D Company, Raleigh, North Carolina, 1997
•   Protective Relaying Theory and Applications, 2nd Edition, Marcel Dekker, New York, 2004
 
  The latest revision of:  
  •   IEEE Std 242, IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (IEEE Buff Book)
•   IEEE Std C37.102, IEEE Guide for AC Generator Protection
•   EEE Std C37.101, IEEE Guide for Generator Ground Protection
•   ANSI C50.13, Cylindrical-Rotor Synchronous Generators
•   NEMA Std MG-1, Motors and Generators