writeup for trnaformer protection

7/23/2019 Writeup for Trnaformer Protection

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Objective of Electrical Protection System:

The objectives of electrical protection relaying system are-

A. To identify and isolate the faulty zone/ section

B. To maintain electric power availablity on heathy zones/ sections

To achieve the above two objectives, electrical networs are divided into various zones of

protection and different types of relays are provided to identify and clear the fault with in theirzones of protection.

Transformer Protection:

!rotection "ystem for Transformers consist of following components#

$. !rotection achieved through protection relays#

$.$ %vercurrent and &arth 'ault (elay )"hown as ($ relay in 'igure-$*

$.+ ifferential !rotection (elay )"hown as (+ relay in 'igure-$*

+. !rotection achieved through alarming relays mounted on transformer body#

+.$ Bucholtz (elay generate Alarm and Trip "ignals

+.+ inding Temperature ndicator generate Alarm and Trip "ignals

+.0 %il Temperature ndicator generate Alarm and Trip "ignals+.1 !ressure (elease evice generate Alarm and Trip "ignals

+.2 %il 3evel ndicator generate Alarm and Trip "ignals

The above relays at point no. $ nad + are meant for operation with in very well definded zones of

protection. These zones are here by described in figure-$ below#

FIGURE-1Zone-1:4one-$ is the zone of operation for ($ relay which is an overcurrnet and earth fault relay.

Any fault occuring with in 4one-$ should ideally be piced up by ($ relay. The ideal scheme is that


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($ relay sends a trip signal to immediate upstream circuit breaer )5B$ in 'igure-$* which will

lead to undervoltage at downstream circuit breaer )5B0 in 'igure-$* incoming terminals and 5B0

will also trip due to undervoltage. Therefore the faulty section will be isolated from both sides. Also

the locout )67* feature of zone watchdog relay )here it is ($* should also become active and the

relay contact should remain latched until it is physically reset by the electrical operator. 3ocout

relay feature is a safety feature for &8uipment with in the faulty zone and prevent reoccurance of

fault with in the faulty zone. (esetting of latch locout relay )67* will re8uire detailed checig offaulty section e8uipment and may prove to be a time consuming affair as all the electrical apparatus

with in faulty zone will re8uire physical testing/inspection before going for re-energization.

The locout feature of only upstream 4one watchdog relay )5B$ in 'igure-$* is made active but not

the downstream breaer )5B0 in 'igure-$* as it will simply confuse the operator about the location

of fault. The zone distrimination will have no meaning then and it will lead to distruption of

electrical power in 9ealthy &lectrical "ystems. Also, diagnostic and fault finding will tae more

time/efforts and will lead to increased downtime resulting in production losses to industries. e can

visualize the importance of zone selectivity and discrimination feature of protection relaying system

by simply thining that if therefore is no zone discrimination/selectivity, each time any fault

occuring any where on the electrical networ, it may lead to shutting down/tripping of complete

electrical networ.

Zone-:Any fault occuring with in zone-+ shall be piced up by (+ relay. Also, the transformer

mounted alarming systems )Bucholtz, T, %T, !(, %3* senses the fault in this zone. All fault

sensing relays/devices should send trip signal to immediate upstream circcuit breaer )5B$ in

'igure-$*.

Zone-!: Any fault occuring with in 4one-0 shall be piced up bt (0 relay and cleared by tripping of

circuit breaer 5B0.

"ac#na $it% Omifco 11&'()*)&' Transformer Protection Sc%eme:

The alarm/trip signals from transformer mounted protection devices )Bucholtz, T, %T, !(,

%3* are wired to zone-0 protection relay. deally, actuation of these devices should send trip signal

to 5B$ as they are detecting fault in 4one-+ but in actual they are sending trip signal to downstream

circuit breaer )5B0 in 'igure-$* and 3ocout relay of a healthy zone, 4one-0, is actuating.

This wrong scheme may result in two problems#

a* The electrical operator may consider healthy 4one as faulty and faulty 4one as healthy. The

operator may therefore go for re-energizing the actually faulty zone as no locout relay is latched

for this zone. "o the probability of errors are increased due to this scheme.

b* The power supply of healthy system will remain affected with out any fault.c* t is tae more time to diagnose the fault as the fault trip alarm are misleadign the electrical

operator to heathy zone instead of faulty zone.

The transformer fault inputs from Bucholtz, T, %T, !(, %3 might have been wired to relay

(0 instead of ($ relay so as to save cost of cable and enhnaced cost of ($ relay due to greater

number of fault inputs. e may note that (elay ($ is mounnted at $$v ::"-$ switchgear where

as relay (0 is mounted at :"-2A switchgear which is very near to transformer T::2A.

Also, other problem with this wiring of transformer fault inputs to relay (0 is that there is no alarm

signal 3& is programmed on (elay front to identify the type of fault. The electrical operator will

not be able to now from the relay front which alarm either Bucholtz or T or %T or !( or %3have actuated. 9e will have to do a detailed checing of the electrical circuit, then only he will be

able to now the cause of fault alarm/trip.


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writeup for trnaformer protection

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