Page 314 - Handbook of Electrical Engineering

300 HANDBOOK OF ELECTRICAL ENGINEERING

11.10 SOME COMMENTS ON THE APPLICATION OF IEC60363
AND IEC 60909

IEC60363 was first available in 1972 and IEC60909 in 1988. IEC60363 was issued for evaluating
the short circuits in power systems that are used onboard ships. It covers both the transient and
sub-transient fault situations. AC power systems on modern large ships have certain similarities to
those in oil industry, marine and onshore installations, e.g.

• Independent from other sources of power, i.e. ‘island’ operation.
• Generators connected directly to the main busbars.
• The main busbars supply induction motors that have relatively high ratings.
• Short cable routes and therefore minimal attenuation of fault currents.
• Significant contribution of sub-transient fault current from induction motor consumers.

IEC60636 is presented in two parts, the first for AC systems and the second for DC systems.
The first part gives formulae and tables for calculating the steady state and dynamic fault currents at
generators, near to generators and remote from generators. It takes account of the external impedance,
beyond the generator terminals, that alters the values of the various time constants that are frequently
used in short-circuit calculations. (This aspect is sometimes overlooked when dynamic calculations
are being carried out.) The publication also uses only parameters and data that are readily available
from manufacturers or databases, which is very convenient. The decrements in the fault currents
are also described and illustrated by worked examples. Motor contribution to fault currents is also
described and illustrated. The publication briefly addresses the effect of the generators being fully or
highly loaded before the fault occurs. In recent years this subject has become more significant in the
selection of equipment, relatively small variations due to loading should be considered.
IEC60909 is also presented in two parts but does not cater for DC power systems. It addresses
in detail balanced and unbalanced faults near to and far away from a generator. The aspect of a loaded
generator is catered for by using a factor ‘c’ to multiply the rated voltage U n of the generator, see
clauses 6 and 11.4, Table I therein. A second factor K G is also introduced to modify the sub-transient
impedance of the generator, as a function of the load power factor. Appendix A of the publication
gives numerical examples.

11.11 STABILITY STUDIES
So far the power system has been designed to meet the steady state load distribution requirements
and the steady state and transient fault currents that could occur under the worst conditions. Most
power systems in the oil industry have their own generators. Consequently, the transient performance
of the system and its generators is of great concern when relatively large disturbances are applied,
e.g. starting large motors, switching out loaded feeders, recovery from fault clearance.
The analysis and study of the dynamic behaviour of the power system is part of what is
generally called ‘Stability Studies’.
The stability of a power system can be studied in several ways but, generally speaking, only
two ways are important, i.e. steady state and transient stability. The results of these studies usually
cause only minor changes to the system that was originally proposed provided that the system had
been well thought out initially.