Pursuant to Article 116 of the Energy Law

4.2.11.2. Selection of the type of protection devices for 110 kV transmission system 
4.2.11.2.1. In the process of protection selection, it will be required to take into consideration 
the specific features of the previously installed equipment, particularly the requirements of 
the existing technical control system. 
4.2.11.2.2. For 110/x kV transformer protection, gas relay (Buholtz protection of transformer 
tank and OLTC breaker), contact thermometer, overpressure relay and electric protection 
against internal and external faults, as well as overload protection should be installed. 
Required protective functions by electric protection devices should be: 
a) for main protection devices on 110 kV side: 
- transformer differential protection; 

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- restricted earth-fault protection ; 
- "tank" protection, only as necessary alternative solution for restricted earth fault 
protection of 110 kV windings 
- overload protection - thermal image 
- overload / overcurrent protection; 
- multi stage three phase overcurrent protection; 
- multi stage earth fault overcurrent protection. 
- protection against continuous current misbalance; 
- breaker failure protection; 
- pole discordance protection breaker protection (if not realized by an internal 
breaker’s scheme ) ; 
- Trip circuit supervision of the breaker (as an external device for each tripping 
coil); 
b) for backup protection device on 110 kV side: 
- multi stage three-phase overcurrent protection (autonomous or conventional 
design depending on whether one or two accu-batteries are applied in TS 110/x 
kV); 
- multi stage earth fault overcurrent protection. 
c) protection device for the x side kV (x = 35, 20, 10, 6): 
- multi stage three phase overcurrent protection; 
- multi stage earth fault overcurrent protection. 
- simplified bus bar protection for x kV
- multi stage single-phase overcurrent protection for protection of the earthing 
equipment in neutral of the x kV side 
- trip circuit supervision for the breaker (as external device or internal function in 
the protection device).