Busbar Protection


7SS52 System features

  • 7SS52 System features (1)

– 4-system busbar or 3 systems with transfer bus
– 48 feeders
– 12 independant protection zones
– 16 couplers (1 CT) / 8 couplers (2 CTs)

– proven measurement principle which is used by
SIEMENS in analog and numerical systems
– trip time < 15 ms at the command contact
– CT saturation tolerated after 3ms

  •  7SS52 System features (2)

– phase selective measurement for each zone
– additional isolator independant ‘checkzone’
– dead zone (coupler) /end fault (feeder) protection for faults between CB and CT
– additional tripping characteristic for earth faults in resistive earthed networks
– integrated versatile two step breaker failure protection

  • 7SS60 Comparison versus 7SS52

             High complexity for multiple busbars

       – increased costs for engineering and documentation
       – increased wiring, testing, commissioning

            Extension of existing schemes is more difficult

            Isolator replica and preference switching by mechanical contacts

      – increased maintenance
      – no supervision of isolator status (auxiliary contacts)


     – requires less space
     – lower price

  • 7SS52 Benefits

– Reduced expenditure of engineering, installation and commissioning
– Maintenance reduced to testing of input quantities and trip circuits due to high degree of internal supervision
– Few different system components (reduced stock of spare parts)
– Easy and fast extension of protection system

Further Readings
The Art & Science of Protective Relaying 
By : C Russel Mason

The 7UT6 Family

7UT612: protection objects up to 2 ends / 7 feeders
7UT613: protection objects up to 3 ends / 9 feeders
7UT633: protection objects up to 3 ends / 9 feeders (extended IOs)
7UT635: protection objects up to 5 ends / 12 feeders (extended IOs)

  • 7SS60 Hardware Components

  • 7SS6 System configuration
Components are for example only !
  • The 7SS52 Distributed Busbar Protection Concept

Busbar Protection & Pickup Characteristics

  • Principle of Busbar Differential Protection (4)
  • Busbar Protection for n Feeders, Stabilized
  • Current Comparison in Stabilized Busbar Protection
  • Pickup Characteristics
  • Isolator Replica
Principle of assigning feeder current and TRIP command via bistable relays,depending on the isolator auxiliary contacts

Busbar Protection with Reverse Interlocking

For radial networks only

1 External short-circuit
2 Busbar short-circuit

  • Principle of Busbar Differential Protection (1)

  • Principle of Busbar Differential Protection (2)
Busbar protection for n feeders, not stabilized 
Prerequisite: Same ratio of all transformers

  • Principle of Busbar Differential Protection (3)
Busbar protection for n feeders, not stabilized, with trip circuit representation
Prerequisite: Same ratio of all transformers

Busbar Protection

Principles of system protection technology 

Busbar Protection
  • Need for busbar protection

A busbar is a point of convergence of many circuits

  1.  generation 
  2.  transmission 
  3.  load
  • Principles of busbar protection
  1. Reverse interlocking
      – Limited application range (no power reversal, single bus) 
  2. Distance protection
      – Slow fault clearance time (> 300ms) 
  3. Differential protection
      – fast, selective, wide application range
  • Typical scope of applications

– Single busbar, up to 12 feeders *, w/o sectionalizer 7UT6x
– Single busbar, any feeders, with sectionalizers 7SS60
– Simple double busbar, with couplers 7SS60
– Complex multi busbar (e.g. transfer bus) 7SS52
– 1½ breaker schemes 7UT6x, 7SS60, 7SS52 

 Depending on the version of relay (7UT612, 7UT613/633, 7UT635) and type of measurement (1/3-phase)