442 HANDBOOK OF ELECTRICAL ENGINEERING
Equation (16.1) can be rearranged to determine the largest motor rating for a given volt-
drop limit:
Sm^2 =Sg(XgSLsinφLV )
KXg( 1 −V )( 16. 2 )
Table 16.4 shows the largest motor rating allowed for different system conditions and designs,
whereSg=1 pu.
It can be seen that the most restricting factors are a low standing load or a standing load
with a power factor near to unity. In these cases the motor rating is limited to approximately
27% of the total generator kVA capacity. If an allowance is also made for additional volt-drop
in the generator and motor cables then the 27% limit would be reduced to between 20 and 22%.
Thus a ‘rule-of-thumb’ ratio of 5:1 between the kW or kVA ratings of the generators and the
motor to be started is reasonable for quickly assessing a satisfactory system performance, see also
Appendix G.
In view of the above findings it would be reasonable to have the PMS inhibit the starting of
the large motors when there is insufficient kW capacity of running generators. The PMS would raise
an alarm and advise that an extra generator should be brought on-line so that the particular motor can
be started. The PMS could incorporate an equation such as (16.2) since all the constants are known
from the nameplate data and the variables can be found from the system measurements received at
the PMS for other uses.
16.3.5 VDU display of one-line diagrams
A PMS can be programmed to display the status of the power system in the form of one-line diagrams.
The one-line diagrams can be displayed in colour, where different colours can be used for different
functions, such as:-
- Green busbars and feeders are alive.
- Red busbars and feeders are de-energised.
Table 16.4. Relative magnitudes of different parameters that effect the
amount of volt-drop experienced during the starting of a large induction
motor
Case Sm V K Xg SL sinφL
a 0.34 0.1 6 0.25 0.5 0.436
b 0.40 0.15 6 0.25 0.5 0.436
c 0.46 0.2 6 0.25 0.5 0.436
d 0.41 0.1 4 0.25 0.5 0.436
e 0.37 0.1 5 0.25 0.5 0.436
f 0.40 0.1 6 0.15 0.5 0.436
g 0.36 0.1 6 0.2 0.5 0.436
h 0.39 0.1 6 0.25 1.0 0.436
i 0.27 0.1 6 0.25 0.0 0.436
j 0.27 0.1 6 0.25 0.5 0.0
k 0.36 0.1 6 0.25 0.5 0.6