Case Study - Are you sure your Generators/Motors are safe? | System Protection

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Case Study - Are you sure your Generators/Motors are safe?

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Background

It was just another day and anoth-

We immediately dispatched 2 of

er morning of January 2019, on w

our engineers along with the test

hich we received a call from a new

sets required, by the first flight

Customer in Metal Sector, from

available and were soon on the

Chhattisgarh. And urgently, they

job. In less than a day, we were

wanted us to test and commission

up and working.

an old 10 MW, 11 kV Generator along with its relaying, schemes

Findings‐ at Site

and complete testing. ‐ The initial forensics were found On enquiring about such urgency, we learnt that their original Generator rated 12 MW, 11kV got burnt (stator) and was sent for repairing, this 10 MW was taken on rental basis and a temporary solution. As we all know that in our industry, Generator failure is nothing less than an air‐crash and this criticality warranted extensive

from the fault records of the Relay. We learnt that the Relay (and GCB eventually) was tripped on NPS protection, which is normally a time delayed protection. It was a surprise, because normally when such fault occurs (stator failure), other protections take care, NPS is something like last line of defense.

and methodical analysis like an air‐crash investigation.

‐ So, such a surprising observation just at the start of the

Hearing this, the day went into a

job prompted us to be extra

tizzy and we immediately started

vigilant. This protection system.

preparing a checklist (we’ll see it later in a diagram) to find out the Root Cause. And unless we find the Root Cause and solve it, this rented Generator was in danger and the upcoming repaired Generator too!

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‐ And then came the moment of

‐ Following actual images clearly

reckoning, and you know from

show the fault. So, 90 % Stator

where? NCT wiring was the

Earth Fault Protection (51S) did

culprit for the Generator burn‐out!

not operate due to non‐wiring of

Least expected fault for a

NCT secondary S2 to Relay.

Protection Engineer so to speak (It

Generator kept running on this

was found while carrying out

serious a fault which finally led

Stator Earth Fault scheme

to heating and eventually, NPS

checking).

operation operated on time delay.

But by the time NPS operated, the damage was already done.

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‐ NCT, which is used for Stator

‐ Reverse Power and Forward

E/F Protection, was wired up to

Power Protections were set but

TB in CRP correctly. But from TB

with wrong logic. Logic was so

to Relay, wire S2 was missing/not

wrongly set that neither Alarm

connected on Relay. So even in

could be generated nor Tripping!

case of Stator E/F, Relay wouldn’t

With the set logic, these

have sensed nor operated nor

protections were rendered

tripped the relevant breakers. This

useless. Corrected by changing

fault was simply corrected by

the logic.

connecting a 2.5 sq. mm wire. ‐ Similarly, this wrong logic error ‐ Differential Protection setting

was observed in Under Voltage,

was found high, besides, the

Under Frequency and Over

reference for the settings was not

Frequency Protections. All these

available, we recommended to

were set alright, but the set logic

furnish us the OEM data,

didn’t let these protections

compiled during commissioning

generate any Alarm or Tripping

and to carry out the fresh Studies

signals, imagine! Corrected by

on Relay Co‐ ordination, Power

changing the logic.

System and Load Flow. ‐ LBB Protection tripping was ‐ Suppose both the above

assigned to Output No. 3, but as

protections, Stator E/F and

per the OEM Manual of the

Differential, are alright and

Generator Protection Relay, this

operate and generate alarm and

Output No. 3 must be used for

trip signals faithfully. And as we

other designated functions and

all know, both these are Class A

not this. Corrected by changing

protection and must cut the very

the logic and wiring.

source of the fault. Here in this case, it’s AVR. And you know what we found? AVR could not be tripped because inter‐panel wiring was not done. Corrected and now stands rectified.

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� Following Diagram will help us understand this critical event better.

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Conclusion – How our insightful actions helped the Industry

‐ Not only our team verified that

‐ And at last, we w're happy to

the newly installed Generator of

"wow" the customer, who learnt

10 MW has all the protections

at last, the difference between

available, they verified that it

symptom‐solving‐agents and

operates, and operates correctly.

roper Doctors.

And generate a correct tripping sequence too. No burning of Generator fro now onwards,

‐ The above are in fact the major

the way a safe and reliable

problems found and corrected.

protection system should be.

Though, during our very busy 3‐ days (and nights too) at site, we also observed many minor

‐ By doing so, we also ensured

abnormalities and corrected as

complete protection of 12 MW

many as possible. Well, to know

Generator which will be put in

more, call/write to us, we’ll be

place after due repairs. Settings

happy to save your assets too!

need to be verified/corrected with rewinding data but more or less, the machine will be protected.

- And if you’re looking for a dedicated agency who understands the direct link

‐ Generator is an expensive asset

between your assets and your

itself, and the loss of production

revenue, we’ll be happy to assist

in a continuously running plant

you, anytime, anywhere.

due to failure of a Generator can run in to crores. All our efforts are viewing the problems from customers’ point and resolving them in totality. Testing, as such, is an isolated job, but when it is

Regards System Protection

done in a way which affects the end‐results (production/revenue), th And that’s what we delivered.

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CONTACT INFORMATION 301/2/306, Silverline Complex,Opposite BBC Tower,Sayajigunj,Vadodara - 390 005, Gujarat, India. bdm@systemprotection.in

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