Page 229 - Offshore Electrical Engineering Manual

216 CHAPTER 3 Motor Faults and Protection

High winds blowing directly into ventilation fans, causing them to run backwards at
high speed (‘windmilling’), is a similar problem. In this case, nonreturn dampers or
brakes which release when the motor is started can cure the problem.

ABNORMALITIES IN THE SUPPLY SYSTEM

The supply system depends on a limited number of platform generators which are, in
turn, dependent for their fuel, cooling water, etc. on the correct operation of other equip-
ment on the installation. Therefore, there is a significant probability that frequency and
voltage fluctuations of sufficient magnitude will occur to affect the running of motors.
With low-frequency problems, stalling protection will adequately deal with the situa-
tion. High-frequency problems should be catered for by overload protection devices.
However, with large reciprocating compressors, it may be advisable to fit timed over-
and/or underfrequency trips to avoid running the machine for long periods at a fre-
quency close to tolerance limits. Generator under-/overfrequency trip settings should
be a compromise between taking the most sensitive plant into account and introducing
the likelihood of generator nuisance tripping. Running at abnormally low system volt-
age will cause motors to draw larger currents and increase heating effects in motor
windings and supply cables. A lower system voltage will also increase the likelihood of
stalling, as the motor torque is proportional to the square of the voltage at a given speed.
An abnormally high voltage will reduce heating effects in windings and supply
cables and give faster starting times because of the increased torque. Because of
this, it is the practice on some installations to run the system at a few percent above
the nominal voltage. Unfortunately, prospective fault currents increase in proportion
with the square of the voltage. Therefore, if the system is to be normally run at a
voltage several percent above nominal, the prospective fault currents should be recal-
culated to reflect this and switchgear fault ratings checked accordingly.

OVERLOAD PROTECTION
The heating effects due to motor overloads can be simulated to some degree by a
bimetal element or an electronic thermal replica (Fig. 4.3.2). Both the stator wind-
ings and the rotor will begin to heat up at the onset of an overload. At ratings below
about 20 kW, depending on the design and type of machine, the stator windings will
tend to suffer thermal damage in a shorter time than the rotor, in which case the
motor is termed ‘stator critical’. Stator critical motors are easier to protect than rotor
critical types because the stator windings are more accessible than the rotor and can
be fitted with temperature sensors if the thermal overload device is felt to be inad-
equate. If the motor is driven by a variable-speed drive (VSD), where it is likely to
run for some time at low speed, the thermal effects of reduced ventilation must be
taken into account in the choice of motor and the setting of the overload. The VSD
unit is usually fitted with sophisticated overload protection as a standard feature.
Electronic replica type protection relays are normally reserved for the protection of
larger machines. However, a drive of only a few tens of kilowatts which has a par-
ticularly critical function, or is located at some part of the platform where no form of