ht means high tension voltage, ht voltage starts from 6.6kv
onwards to 11kv, 33kv, 66kv, 110kv, 220kv and so on
generally ht are used in transmission and distribtuion in
order to reduce the power loss i.e. when you send power at
high voltages current will be of small proportion and hence
decreasing the size of conductors and also the supporting
structures required to hold the conductors need not too
strong so over all a great amount of cost reduction can be
done by sending power at very high voltages and other than
transmission and distributing section ht voltages are used
for motors of greater capacity around 220kw, 315kw, 900kw.
And same priciple applies to motor as well and more over if
you go for lt voltages for the above range of motors the
insulation required for the stator and armature will be
more because at starting the motor takes 6 to 7 times the
full load current of motor.