protection of HV motors
Motor protection" groups all the
protection devices used to prevent
serious damage due to abnormal
operating conditions at supply, motor or
process level.
The protection devices to be installed
are chosen according to the following
criteria:
c operating conditions;
c importance of the operation
performed by the motor;
c the degree of dependability required;
c the relative cost of the protection
device with respect to the motor;
c the likelihood of the faults considered
occurring.
As well as:
c the type of load driven;
c disturbances which could occur on
the network;
c the type of motor protected.
The faults listed below may therefore
require use of a protection device.
main fault types
Asynchronous motors
c overloads;
c short-circuits;
c phase breaking, reversal and
unbalance;
c insulation fault between turns;
c stator frame;
c under and overvoltage;
c incomplete starting.
Synchronous motors
All the above faults, plus:
c loss of synchronism;
c loss of excitation;
c rotor frame;
c prolonged operation in asynchronous
mode on starting;
c overloads and short-circuits in the
exciting winding;
c reverse power (operation on
AC generator).
Other faults linked to the process or
load
c over frequent starts;
c locked rotor;
c underpower or undercurrent.
The detection/protection processes for
the main fault types are studied in the
paragraph below.
protection principles
Overloads
Overloads can be detected by reverse
time overcurrent relays, thermal image
relays or heat sensors.
The relays process the information
"current absorbed by the motor" which
is generally detected by current
transformers.
The heat sensors are inserted in the
live parts of the motor.
c reverse time overcurrent relays
Their use requires:
v either an operating curve I(t) allowing
starting, or a device blocking the relay
during starting,
v an operating threshold I0 close to the
rated current In of the motor
I0 » 1.10 In .
These relays do not memorise the
overloads.
c thermal image relays
These relays are certainly the most
suitable since they ensure the greatest
possible advantage is derived from
motor overload possibilities without
damage.
The operating curve I(t) of the relay
must enable the starting current to flow
without tripping and be approved by the
motor manufacturer.
c heat sensors
These are resistors, the ohmic value of
which varies with temperature.
In theory these devices are not used by
themselves, but rather back up the
relays using the current absorbed as a
measuring means.
Overloads due to temperature rise of a
bearing are, in theory, insufficient for
detection by the overload relays.
Bearings must therefore be protected
by thermostats or heat sensors.
Short-circuits
Short-circuits on circuit-breaker
equipment are detected by
instantaneous operation overcurrent
relays, set above the starting current.
On fuse-contactors, short-circuits are
cleared by the fuses.
However, an useful solution is to add
slightly time delayed overcurrent relays
to the fuses. This means that the
contactor can be used right up to its
breaking capacity.
Phase breaking, reversal and
unbalance
These faults are detected by a filter
which highlights the negative phase
sequence components.
It is vital to monitor phase breaking and
unbalance since these faults give rise
to:
c increased current, in the stator,;
c additional temperature rise by Joule
effect, due to the fact that all out-ofbalance
states result in the appearance
of reverse currents flowing through the
rotor at twice supply frequency in the
rotor.
Phase reversal is detected either by
currents or voltages:
c by currents: this reversal is detected
after contactor closing: the driven
machine receives the fault;
c by voltages: this means contactor
closing can be prohibited, if necessary,
if phase order is not the normal network
one.
Insulation fault in winding
Stator windings may have faults
between turns on the same phase or
between windings of different phases.
As a result of its electrical position, the
fault may not be detected quickly
enough by the overload protection
device, thus causing serious damage.
These faults are normally detected by
current comparison.
c longitudinal earth leakage protection
Provides protection against faults
between windings of different phases.
For this, the ends of the motor windings
must be accessible on the neutral side.
Locked rotor
Jamming of a motor for mechanical
reasons causes an overcurrent roughly
equivalent to starting current. The
resulting temperature rise is
considerably greater since rotor losses
are maintained at their highest value
throughout jamming and ventilation is
no longer present if connected to rotor
rotation. As a result, when there is a
risk of jamming, the "locked rotor"
protection is necessary since overload
relays sometimes have an excessively
long response time.
This fault is detected by an overcurrent
relay set at a value less than the
starting current. This value is validated
control monitoring and protection of HV motors