What is the difference between an Extended Period Simulation (EPS) and a transient simulation in HAMMER?
A transient simulation typically spans a few seconds or minutes where you analyze the transient wave that is caused by a valve closing/opening, a pump turning off/on, a turbine starting/stopping, etc. An EPS simulation typically spans a matter of hours or days.
The key is that an EPS is just a series of steady state simulations; it does not account for what happens due to any change in momentum between those timesteps. It essentially assumes that any transient effects have settled down to a steady state. A transient simulation on the other hand looks at what happens between two steady states - the effects from momentum changes. So, HAMMER needs to start with a steady state simulation and along with the configured transient event (the cause of a sudden momentum change such as an emergency pump shutdown, valve closure, etc) it simulates the transition from that starting steady state to a new steady state (if given enough time to settle on it). If the momentum change is severe enough, it can result in damaging high or low pressures and introduction of vapor or air.
Establishing that starting steady state is the Initial Conditions, which is typically a simple steady state run. However since an EPS is just a series of steady states, HAMMER has an option whereby you can select a specific time step (steady state) from an EPS. See below article for more on that: How to set initial start time for a transient run in HAMMER? For more, I recommend reading the Help topic called "Hydraulic Transient Theory".