Page 252 - Offshore Electrical Engineering Manual

IEC 60909 239

Method of Calculation
Standard
Type of Circuit Specification Breaking Current Making Current

Systems of ships IEC 61363 Ac decrement based Dc and ac decrements
and mobile & fixed on generator transient calculated
offshore units impedance
Primarily for land- IEC 60909 Use of c and k factors.
based systems Refer to IEC TR 60909-1
All systems IEEE 141/ANSI Use of impedance correction factors for rotating
C37 machines can significantly affect results.

IEC 61363
Calculation methods that include generator and motor short circuit decrement will
tend to produce the lowest acceptable values of short circuit current. As ship and off-
shore systems normally consist of ‘Island’ generation where machine decrement has
significant effect, IEC Standard 61363-1, usually, produces the most accurate result,
i.e., it produces study results that represent conditions that may affect typical marine
or offshore installations more significantly than land-based systems, including more
emphasis on generator and motor decay.
Where feeder cable impedance tends to reduce decrement effects, the study
can take advantage of the Equivalent Generator approach outlined in IEC 61363-1
section 7. For more simple marine electrical systems, the equivalent generator
method will involve extensive calculations and produce results little different
from more simple methods. In this case, IEC 60909 may be a better choice for
your study.

IEC 60909

The calculation method, used in the IEC Standard 60909 determines the short circuit
currents at the location F using equivalent voltage source:

cUn
√
3
This source is defined as the voltage of an ideal source applied at the short
circuit location in the positive sequence system, whereas all other sources are
ignored. All network components are replaced by their internal impedances (see
Fig. 4.6.1).