Transformer Protection Relaying Scheme

Prepared by: Angeles, Cris Julius C.2008106430

IntroductionThe two major items of equipment in a power system are the generators and transformers. They have very high chance of fault occurrence and usually takes much time and money to repair the damage.

Faults occurring in TransformersOpen-Circuit faultsEarth faultsPhase-to-Phase faultsInter-Turn faultsOverheating

Factors in choosing Protective Gear for a TransformerType of TransformerSize of the TransformerType of CoolingSystem where usedImportance of service for which it is required

Transformer Relaying Scheme

Buchholz ProtectionAlso known as gas accumulator relay, commonly used on all oil-immersed transformer provided with conservator.

Working Principle:Whenever a fault occur inside the

transformer, the oil of the tank gets overheated and gases are generated. The heat generated by the high local current causes the transformer oil to decompose and produce gas which can be used to detect the winding faults

Buchholz Protection

Core-Balance Leakage ProtectionThis system is used to provide protection against earth faults on high voltage winding. When earth fault occurs, the sum of the three currents is no longer zero and a current is induced in the secondary of the CT causing the trip relay to operate and isolate the transformer from the bus-bars.

Combined Leakage and Overload ProtectionThe core-balance protection cannot provide protection against overload. It is usual practice to provide combined leakage and overload protection for transformer. The earth relay has low current setting and operates under earth faults only. The overload relays have high current setting and are arrange to operate against faults between the phases

In this system, two overload relay and one earth relay are connected. The two overload relays are sufficient to protect against phase to phase faults. The trip contacts of overload relays and earth fault relay are connected in parallel. Therefore the energizing of either one of them, the circuit breaker will tripped.

Description SIEMENS ABB

Multifunction Generator, Motor and

Transformer

SIPROTEC 4 7UM62 REM 543 

 Two- and three-winding transformers

protection

7SG14  RET650 

Voltage regulator for

power transformers with on-load tap-changers

MJ-4A SPAU 341 C

High Impedance Protection

Relay

7VH60 SPAE 011

Sample ComputationA 50MVA, 132/66 kV, / Y three-phase power transformer is protected by percentage differential relays. If the current transformers (CTs) located on delta and wye sides of the power transformer are 300/5 A and 1,200/5 A respectively, determine (a) the output current at full load (b) the relay current at full load (c) the minimum relay current setting to permit 25% overload.

Given:50MVA, 132---66V delta - wyeCT1 = 300---5CT2 = 1200---5

FIND: a) Output current at FLb) Relay current at FLc) Minimum relay current to allow 25% overload

a) Rated current for primary ( - connected side)

= = 218.7 A Ans.

Current in hv side CTs = 218.7 Current in lv side CTs = 437.4 x

b) Relay current at full load = 3.645 = 0.489 A Ans.

c) With 25% overloadLine current in secondaries of CTs

connected on hv side is and line current in secondaries of CTs connected in delta on lv side =

Relay Current = 4.56 – 3.94 = 0.62 ASo relay setting = 62% Ans.

ConclucionOpen-circuit faults, earth faults, phase-to-

phase faults, inter-turn faults and overheating are the fault that are likely occur in a transformer

Relays control output circuits of a much higher power.

Safety is increasedProtective relays are essential for keeping

faults in the system isolated and keep equipment from being damaged.