ABB RET670 - Page 510

Table 120: ANSI requirements for unbalanced faults on synchronous machines
Types of Synchronous Machine
Permissible
[ ]
2
2
I t K s=
Salient pole generator 40
Synchronous condenser 30
Cylindrical rotor generators: Indirectly cooled 30
Directly cooled (0 – 800 MVA) 10
Directly cooled (801 – 1600 MVA) See Figure 213
Fig 213 shows a graphical representation of the relationship between generator
2
2
I t
capability and generator MVA rating for directly cooled (conductor cooled) generators.
For example, a 500 MVA generator would have K = 10 seconds and a 1600 MVA
generator would have K = 5 seconds. Unbalanced short-time negative sequence current
I
2
is expressed in per unit of rated generator current and time t in seconds.
en08000358.vsd
IEC08000358 V1 EN
Figure 213: Short-time unbalanced current capability of direct cooled generators
Continuous I
2
- capability of generators is also covered by the standard. Table 121
below (from ANSI standard C50.13) contains the suggested capability:
Section 3 1MRK504116-UUS C
IED application
504
Application manual