Page 349 - Handbook of Electrical Engineering

336 HANDBOOK OF ELECTRICAL ENGINEERING

12.7 HIGH VOLTAGE INDUCTION MOTOR PROTECTION

Most oil industry plants use high voltage induction motors to drive pumps and compressors. Unlike
industries that take power from a utility the oil industry usually generates its own power. Most utility
companies restrict the size of induction motors that are to be started direct-on-line. This restriction
seldom applies in the oil industry. There are some applications where direct-on-line starting is avoided
e.g. large compressors in LNG plants, but these may be regarded as special cases. The starting time
for high voltage induction motors varies typically from one second to as long as 30 seconds. Pumps
and low speed machines tend to have the shorter times. A high-speed compressor driven through a
gearbox will usually have a long starting time.
High starting currents and long starting times can give rise to difficulties in choosing suitable
protective relays for the motor. Not all motor relays have a wide enough range of settings to adequately
protect the motor during the starting time.
High voltage induction motors are normally provided with the following protective devices,
some or all of which may be incorporated in the one device (occasionally called a motor managing
relay). Modern motor relays are based on microcomputer technology and these relays not only provide
most of the protection functions but also provide a full range of measurements, indications and alarms.
They also communicate by media such as fibre optics through networks to management and SCADA
computers, see also sub-section 7.6.2.

a) Main functions:
• Overloading or thermal image (49).
• Instantaneous or high-set overcurrent (50).
• Negative phase sequence (46).
• Core balance earth fault (51N).
• Differential stator current (87).
b) Additional functions:
• Stalling current.
• Limitation to the number of successive starts.
• Undercurrent (37).
• High winding temperature.
• High bearing temperature.
• Excessive vibration.

Figure 12.15 shows the application of the above functions to small and large high volt-
age motors.
Most modern electronic motor relays are designed to meet the requirements of IEC60255
Part 8. This standard defines the thermal image or overloading curves of the relay. Some modern
motor relays are very sophisticated and their literature needs to be studied carefully in order to ensure
that the relay chosen fully satisfies the characteristics of the motor. Not all manufacturers use the
same terminology to describe the functions of their relays. This makes the process of comparing
different makes and models of relays somewhat difficult.