Voltage fluctuations and motor tripping

VOLTAGE FLUCTUATIONS
&
MOTOR TRIPPINGMOTOR TRIPPING
By:
D K Singhal
deveshksinghal@gmail.com
Digitally signed
by D K Singhal
Date:
2018.07.27
'12:32:13 +05'30

Voltage Fluctuations
Voltage fluctuations create unnecessary and
unwanted problems. Sudden change inunwanted problems. Sudden change in
voltage for a moment and hence tripping of
electrical appliances is a major cause for
production loss in several industries. In
addition, electrical motor and other items
failure rate is increased due to fluctuations.failure rate is increased due to fluctuations.
2Voltage Fluctuation & Tripping, D K Singhal, July 2018

Source
Usually, the source is upstream of the supply
system.The power supply station cateringsystem.The power supply station catering
several feeders may generate fluctuation
when any one feeder is switched on or off;
particularly if that switchgear is too old.
Improper wiring, cabling, loose connections
particularly in OLTC or bushing in servo mayparticularly in OLTC or bushing in servo may
also create voltage fluctuations.
Weak neutral can also result in voltage
fluctuations.

Critical Areas
As obvious, the problem of voltage
fluctuation is more if the supply is from afluctuation is more if the supply is from a
substation having old switchgears, in an old
unit, or with staggered growth of unit where
new equipment are added with time, thus
leaving possibility for some error and making
the whole system sensitive.the whole system sensitive.
Many of the grid supplied units observe
voltage fluctuation several times a year.

Seriousness of the Issue
A fluctuation may create sudden change in
process parameters, through momentarily. Inprocess parameters, through momentarily. In
several instances, this may lead to quality
problems.
But, in case any of the equipments trips, this
may cause a production loss, and becomesmay cause a production loss, and becomes
more serious issue.
Obviously, nobody wants a production loss.

Objectives
Anybody concerned with the problem would
like to-like to-
Identify the source of the problem
Reduce the magnitude of fluctuations
Try to survive with fluctuation without production
and quality loss.

Problem
Unfortunately, many process-electrical
engineers consider fluctuation beyond theirengineers consider fluctuation beyond their
control, and just shrug off.
Sufficient information is not available even
on internet to quantify the magnitude and
impact of fluctuations.impact of fluctuations.
No low cost test equipment or device is
available to monitor fluctuations.

Initial Approach
In a typical mill having more than 300
electrical motors, it was noted that duringelectrical motors, it was noted that during
rainy season, voltage fluctuation was the
major electrical problem.
Well, with direct feeder through open supply
lines (33kV) from a 132kV sub-station, with
several trees near the line, fluctuationsseveral trees near the line, fluctuations
cannot be avoided.
The approach was to study and gather
preliminary information about the same.

The Cabling
Generally, several systems are used for
neutral-earth to supply power to variousneutral-earth to supply power to various
equipments.An overview of the same is given
on the next slides.
Please note that all images have been taken
with thanks from…with thanks from…
http://miorzaini.blogspot.com/2014/06/types-
of-earthing-system.html

TN-C System

TN-S System

TN-C-S System

TT System

IT System

Installation of VFDs
With most of the systems, the diagrams show
supply and the consumer as the main pointssupply and the consumer as the main points
to connect the neutral.
WithVFDs, some suppliers prefer that the
neutral should NOT be connected toVFD,
while some other demand so.
In severalVFD panels, the neutral and earthIn severalVFD panels, the neutral and earth
junctions are not clearly mentioned, and
electrical engineers join these two, resulting
in poor signal quality issues.

Other Loads
Furthermore, single phase loads also play
their role in the whole system.their role in the whole system.
To achieve higher PF, many mills have
installed capacitor directly connected to
feeder cables, instead of installing these near
motors.motors.
Thus, the overall system becomes too
complicated.

So, What should be done?
The approach includes the following points-
1. Identify if there is a weak neutral
2.Try to quantify the magnitude of fluctuation
3. Identify the possible solutions to minimize tripping
4. Discuss with experts4. Discuss with experts

Weak Neutral
Weak neutral reduces the possibility of
current impulse to go to supply neutral andcurrent impulse to go to supply neutral and
hence the tripping of motor may be high.
However, a weak neutral is generally difficult
to identify in most cases.
The proposed system indicates how the weakThe proposed system indicates how the weak
neutral can be identified.

Supervisory Neutral Bus
As the name implies, this is an insulated
cable, connected to transformer neutral atcable, connected to transformer neutral at
one end, and having separate branches, each
connected to the neutral of every motor
through an MCB of say 1-2 Amps.
The cable size should not be too heavy toThe cable size should not be too heavy to
suppress the main earth. For connecting
individual motors (branches), just a single
core cable size of 0.75 or 1.0mm2 is sufficient.

Supervisory Natural Bus
So, what’s next. Ideally, the neutral being in
place and working well, all of the MCBs shallplace and working well, all of the MCBs shall
be ON as the current passing through these
would always be less than 1 ampere or so.
But, in case there is any problem of voltage
fluctuation, or sudden problem with motor,
and that particular neutral is not able to do itsand that particular neutral is not able to do its
duty, the current will pass through this
network, and in case this fault current is more
than MCB rating, the MCB will trip.

Supervisory Neutral Bus
On tripping of the MCB, you know that there
had been some problem with that particularhad been some problem with that particular
motor, and the conventional neutral might be
weak or broken!
You can, then, take corrective actions as
required.required.

Supervisory Natural Bus
In case you are not willing to install MCB, as
an alternate you may consider using LEDan alternate you may consider using LED
indicators of suitable rating.A glowing
indicator indicates that the current is passing
through that circuit, and neutral cabling is
needed to be evaluated properly.

Magnitude of Fluctuation
Whenever we face a voltage fluctuation, we
may be interested in knowing the magnitudemay be interested in knowing the magnitude
of the same.
A fluctuation may be quantified by the
amplitude of the change in voltage and as
time duration of persistence of fluctuation.time duration of persistence of fluctuation.
For most practical cases, time is practically
small, so change in voltage is of main concern
over here.

Magnitude Measurement
The proposed scheme of measurement of
voltage fluctuation measurement involvesvoltage fluctuation measurement involves
having a set of three purely resistive loads,
connected in star to each of three phases.
The star junction voltage is measured with
reference to transformer (or supply) ground.reference to transformer (or supply) ground.
Theoretically, under normal operating
condition, it should be zero.

Voltage Fluctuation & Tripping, D K Singhal, July 2018 25
Image Courtesy: https://electronics.stackexchange.com/questions/286419/controlling-three-phase-heating-element

If all three resistances R1, R2 & R3 are equal,
measurement of voltage at point N with
1 2 3
measurement of voltage at point N with
reference to PE, should be near zero in an
ideal case.

However, in practical cases where, there
exists a little imbalance amongst threeexists a little imbalance amongst three
phases, this may be of some small value.
It is now possible to install a voltage sensing
relay connected with a counter to display
number of counts when the fluctuationnumber of counts when the fluctuation
exceeded a predefined value.

In many cases, shop floor people know about
a voltage fluctuation whenever the glow ofa voltage fluctuation whenever the glow of
conventional incandescent bulb alters for a
fraction of second.
By the proposed arrangement, having
different units at supply side as well as in thedifferent units at supply side as well as in the
plant, one can know if the fluctuation was
from the supply side or it was just appeared
due to floating neutral.

Correcting the Problem
In case the fluctuation appears from the
supply side, practically little can be done.supply side, practically little can be done.
However, if the fluctuation is being caused by
the floating neutral, one need to look into the
issue within the installation.

Floating Neutral
Checking voltage difference between neutral
and ground points at different 3-pin socketsand ground points at different 3-pin sockets
can easily give a very good indication of
floating neutral.
A voltage difference beyond 5V suggests that
the neutral is floating and a thoroughthe neutral is floating and a thorough
inspection of the wiring/cabling must be done
at priority.

Stabilizer Imbalance
Almost all mills install servo voltage
stabilizers to save costly electrical machinesstabilizers to save costly electrical machines
from over and under voltages.
In some cases, when each phase is separately
governed, there might be occasional issues
when an imbalance appears momentarily,when an imbalance appears momentarily,
resulting in tripping of machines.

Stabilizer Monitoring
As indicated earlier, star junction voltage
measurement immediately after each servomeasurement immediately after each servo
voltage stabilizer may be a good approach.
In case you observe that there were no
fluctuation from supply side, but fluctuation
is recorded after servo, that means there isis recorded after servo, that means there is
some issue with servo.

Tripping Management
Electrical machines are not only costly, but
their failure can also cause heavy productiontheir failure can also cause heavy production
and other losses.
That is why protective switchgear are must to
protect these by tripping in case of any
problem.problem.
However, in some cases, when tripping
system gets activated at not so significant
issues, one may need to look into the same.

Tripping Management
In some cases, where just a few motors were
getting tripped during voltage fluctuations,getting tripped during voltage fluctuations,
the following approaches were tried and
found useful.

Control Supply Change
In a particular case, the control supply was
being given from Red phase.The motor wasbeing given from Red phase.The motor was
110kW, operating with 3∅ 415V supply.
After changing control supply from Red to
Yellow, the problem got reduced by more
than 90%.than 90%.
Probably the fluctuation was appearing in
Red phase (voltage reduction), and hence
contactors were getting released.

Replacement of Contactor
Frequent tripping in one case, without any
noticeable fluctuation was creating problem.noticeable fluctuation was creating problem.
Just to clear doubts, the contactor of this
motor was replaced with a new one.
The problem was solved completely; maybe
the contactor coil was weak. Maybe this
contactor was not genuine.contactor was not genuine.
After the same, that mill made a policy to
procure electrical switchgears from
authorized dealers only.

Neutral Restoration
In some cases, the motors were tripping, and on
thorough inspection, it was observed that thethorough inspection, it was observed that the
cabling was replaced due to some problem long
back.While replacing the cables, the neutral was
kept loose at one end (electrician negligence).
When everything is OK, the neutral does no
significant useful work, so it remained unnoticedsignificant useful work, so it remained unnoticed
for long. Later, while the same was connected
again, the problem reduced significantly.

A Request
The author is NOT an electrical engineer, but
has tried to understand and reduce problemshas tried to understand and reduce problems
after discussing several electrical engineer
friends, suppliers, referring to internet etc.
I humbly request you to please correct me
wherever needed, either by updating thiswherever needed, either by updating this
presentation, or by sharing some useful
information.

ThankYouThankYou
D K Singhal
deveshksinghal@gmail.com

Voltage fluctuations and motor tripping

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