Q6Power System 2
Question
Q.6. Define per unit inertia constant known as H constant.
Answer
The per-unit inertia constant H is defined as the ratio of the kinetic energy stored in the rotating mass at synchronous speed to the machine's rated MVA, expressed in units of MJ/MVA or seconds, quantifying how quickly a machine accelerates for a given power imbalance.
The per-unit inertia constant H is defined as the kinetic energy stored in the rotor (turbine plus generator) at synchronous speed, divided by the machine's rated MVA (apparent power) rating: H = (kinetic energy at synchronous speed in MJ) / (rated MVA), giving H units of MJ/MVA, which numerically simplifies to seconds. Physically, H represents the time (in seconds) for which the machine could supply its own rated MVA purely from its stored rotational kinetic energy if all mechanical input were suddenly removed; H is a key parameter in the swing equation, since it directly determines how rapidly a machine's rotor angle accelerates or decelerates in response to any imbalance between mechanical input and electrical output power, with typical H values for large steam turbo-generators ranging roughly from about 2 to 9 seconds depending on the specific machine type and size.