Overcurrent Coordination Setting Guidelines Transformers

  
     
  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.

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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 transformers to meet the objectives listed above.  
  MV Transformer Switchgear Feeder Unit  
  Industry standard overcurrent protection schemes for MV transformers fed from switchgear circuit breakers include an instantaneous overcurrent relay (device 50/51). The 50/51 relay characteristics are plotted on a phase TCC along with the transformer and feeder damage curves.

The purpose of the phase overcurrent relay is to allow for full use of the transformer, and to protect the transformer and cable from overloads and faults. To accomplish this, the relay curve should be to the right of the transformer FLA rating and inrush point, and to the left of the transformer and cable damage curves and the cable amp rating.

Suggested margins are listed below that have historically allowed for safe operation of the transformer and cable while reducing instances of nuisance trips.		  
     
 
Device Function Recommendations Comments
CT Size 200% of FLA FLA on base rating.
51 Pickup 110-140% of FLA Set below the transformer damage curve.
      Set at or below cable ampacity.
  Time Dial let-thru current @ 1.0 second Set below the transformer damage curve.
Set at or above low voltage main device.
50 Pickup 200% of let-thru
or                 
200% of inrush		 Set below cable damage curve.
Cable damage curve must be above the maximum fault current at 0.1 seconds.
 
     
   
  Fig. 1 MV transformer switchgear feeder unit - one line  
   
  Fig. 2 MV transformer switchgear feeder unit - phase TCC  
  MV Transformer Fused Switch Feeder Unit

E-rated power fuses are typically used in fused switches serving MV transformers. Fuses rated 100E or less must trip in 300 seconds at currents between 200 and 240% of their E ratings. Fuses above 100E must trip in 600 seconds at currents between 220 and 264% of their E ratings. The fuse characteristics are plotted on a phase TCC along with the transformer and feeder damage curves.

The purpose of the fuse is to allow for full use of the transformer, and to protect the transformer and cable from faults. To accomplish this, the fuse curve should be to the right of the transformer inrush point and to the left of the cable damage curve. Typically the fuse will cross the transformer damage curve. The secondary main device provides overcurrent protection for the circuit.

Suggested margins are listed below that have historically allowed for safe operation of the transformer and cable while reducing instances of nuisance trips.		  
     
 
Device Function Recommendations Comments
50 Fuse Size E-rating > FLA FLA based on the top rating of the unit.
Size at or below cable ampacity.
      Size above transformer inrush point of 12 x nominal FLA at 0.1 seconds.
      Size above transformer inrush point of 25 x nominal FLA at 0.01 seconds.
     

Cable damage curve must be above the maximum fault current at 0.01 seconds.
 
   
  Fig. 3 MV transformer fused switch feeder unit - one line  
   
  Fig. 4 MV transformer fused switch feeder unit - phase TCC  
     
  LV Transformer CB Feeder Unit

Industry standard overcurrent protection schemes for LV transformers fed from circuit breakers equipped with long-time, short-time and instantaneous functions. The circuit breaker characteristics are plotted on a phase TCC along with the transformer and feeder damage curves.

The purpose of the circuit breaker is to allow for full use of the transformer, and to protect the transformer and cable from overloads and faults. To accomplish this, the circuit breaker curve should be to the right of the transformer FLA rating and inrush point, and to the left of the transformer and cable damage curves and the cable amp rating.

Suggested margins are listed below that have historically allowed for safe operation of the transformer and cable while reducing instances of nuisance trips. 		 
 
Device Function Recommendations Comments
51 LTPU 100-125% of FLA

Set below the transformer damage curve.
Set at or below cable ampacity.
  LTD, STPU &
STD		 Set to coordinate with downstream devices

Set below the transformer damage curve.
50 Instantaneous 200% of inrush 

Set below cable damage curve.
Cable damage curve must be above the maximum fault current at the CB total clear curve.
 
   
  Fig. 5 LV transformer CB feeder unit - one line  
   
  Fig. 6 LV transformer CB feeder unit - phase TCC  
     
  LV Transformer Fused Switch Feeder Unit

Class J (<600A) and Class L (>600A) fuses are typically used in fused switches serving LV transformers. The fuse characteristics are plotted on a phase TCC along with the transformer and feeder damage curves.

The purpose of the fuse is to allow for full use of the transformer, and to protect the transformer and cable from faults. To accomplish this, the fuse curve should be to the right of the transformer inrush point and to the left of the cable damage curve. Typically the fuse will cross the transformer damage curve. The secondary main device provides overcurrent protection for the circuit.

Suggested margins are listed below that have historically allowed for safe operation of the transformer and cable while reducing instances of nuisance trips.		  
 
Device Function Recommendations Comments
50 Fuse Size 100-125% of FLA FLA based on the top rating of the unit.
Size at or below cable ampacity.
Size above transformer inrush point of 12 x nominal FLA at 0.1 seconds.
      Size above transformer inrush point of 25 x nominal FLA at 0.01 seconds.
     

Cable damage curve must be above the maximum fault current at 0.01 seconds.
 
   
  Fig. 7 LV transformer fused switch feeder unit - one line  
   
  Fig. 8 LV transformer fused switch 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 C37.91, IEEE Guide for Protective Relay Applications to Power Transformers 		 
     
     
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