Electrical Power Systems

278 Electrical Power Systems
The range of inertia constants for certain machines are given in Table - 11.1
Table 11.1: Typical Inertia Constants of Synchronous Machines
Type of machine Inertia constant
H (MJ/MVA.)
Turbine generator:
Condensing, 1800 rpm 9–6
3600 rpm 7–4
Noncondensing, 3600 rpm 4–3
Waterwheel generator:
Slow speed, < 200 rpm 2–3
Hign speed, >200 rpm 2–4
Synchronous Condenser:
Large 1.25
Small 1.0
Synchronous motor with 2.0
varies from 1.0 to 5.0
and higher for heavy
flywheels.
* Where range is given, the first valve applies to machines of smaller MVA rating. [rom
Weshting house Electrical Transmission and Distribution Reference Book, 1964, p. 486].
ig. 11.1: low of powers in a synchronous generator.
Consider a synchronous generator developing an electromagnetic torque Te (and a
corresponding electromagnetic power Pe ) while operating at the synchronous speed ws . If the
input torque provided by the prime mover, at the generater shaft is Ti , then under steady-state
conditions (i.e., without any disturbance)
Te = Ti ...(11.10)
Here we have neglected any retarding torque due to rotatianal losses. Therefore we have
Te ws = Ti ws ...(11.11)
and Ti ws – Te ws = Pi – Pe = 0 ...(11.12)
If there is a departure from steady-state occurs, for example, a change in load or a fault,
then input power Pi is not equal to Pe , if the armature resistance is neglected. Therefore
left-side of eqn. (11.12) is not zero and an accelerating torque comes into play. If Pa is the
corresponding accelerating (or decelerating) power, then