Case Study - Importance of Relay Coordination in Process Industries

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ISO 9001:2015
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Certified Company
Importance of Relay Coordination in
Process Industries
Case Study

2. There are certainly quite a few cases of inadvertent tripping in cement plants, steel plants,
fertilizer plants, FMCG and other industries that many Electrical Engineers witness at a certain
point of time. Such scenarios occur in the majority of industries, not for the reason that the
protection plan of these industries is not properly coordinated but because changes in the
electrical system take place on a day-to-day basis. Pinned below is the SLD of a Cement Plant
which failed due to poor Relay Coordination, we will discuss the same in this case study.
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During one instance, the Clinker Hammer
Crusher Motor tripped in overload due to
jamming. It was after 30 seconds that the
control room gave the command to start
the crusher again as it was observed in the
past that jamming could be cleared by
heavy starting torque but this time
unexpectedly when the command was
given to crusher motor, the entire plant
tripped. It was unexpected because the
Clinker crusher jamming happens at least 3
to 4 times in a year and the plant was
running for the last 4 years and such co-
ordination problems never occurred. This
issue had arisen for the second time in the
last 3 months and our team was called to
address the issue.
The very first thing that we did was to check
whether the complete electrical system was
properly coordinated or not and it was
found that the system was well coordinated
since the commissioning stage and they
had the records for the same.

3. Thereafter, we inquired about any
modification done in the distribution team
like replacing the existing motor with less
KW or adding any extra load on that MCC
because of any process requirement. They
told us that a 37 kW old compressor was
removed as it was no longer being used and
one 18- kW compressor was shifted from
another MCC to the present MCC as the
load on that MCC was around 100 %. They
also told us that one more change was
made. A 75 kW high-pressure pump that
was used for breaking jamming in Kiln was
installed as per the requirement of
process/production. Therefore, a total of
around 217 kW was added and the settings
were manually adjusted according to the
MCC incomer and PCC outgoing panel.
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It was a 5000 TPD plant and this
breakdown cost the plant around 410 tons
of clinker which is approximately 500 tons
of cement (10000 Bags of cement). It
turned around to be a loss of INR 2.5 to 2.8
millions in just 2 hours (Total of INR 5 – 5.5
million for 2 breakdowns). Besides, the time,
all the efforts put in for the modification to
improve stability and efficiency went waste.
Ideally, the MCC-6 Incomer should have
tripped and not the motor because the
motor started normally with just the extra
load on it.
Knowing all these details, we concluded
that the reason for such a problem was that
there was a clinker lump and the clinker
crusher motor was tripped. Based on the
experience, they took action and restarted it
again after 30 seconds but as the whole
plant was running except clinker crusher,
MCC was already at 80% load and when the
315 kW motor started, the starting current
was around 4 to 5 times of motor FLC. The
total current crossed the threshold of that
relays and they forgot to make changes in
the setting of 6.6 kV Side as mentioned in
SLD. This made the complete PCC bus dead
and the total plant was completely tripped
and it took around 2 hours to start back.
Thus, it was concluded that to avoid such
an issue which incurs heavy losses, each
time any kind of big modification is made
to the electrical distribution system viz.
adding load or adding any source, the
complete relaying and protection should be
coordinated again.

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