# Flat start for power systems design

1. Sep 16, 2014

### honyeehong8692

Hello all, would like to ask what seems to be a general rule when designing power systems.

In the power system design program which I just started to use. It is advised that:

For every simulation to be run, it is best to have a "flat start" before running any faults.

I'm not too sure of the purpose of having a flat start to the system. Would like an explanation.. Please and thanks.

2. Sep 30, 2014

### OldEngr63

What do you mean by a "flat start"? I have never heard this term before.

3. Jan 18, 2015

### Svein

I expect that they mean that all internal voltages should be set to 0V (including the voltages across any capacitors).

4. Jan 18, 2015

### jim hardy

Let it run a while to get everything settled out at equilibrium .

Like OldEngr, i never heard that term.
But i do know from simulating that you must start from equilibrium, not some assumed set of initial conditions.

That way you know that when you applied your fault, whatever changed was a result of your fault.

5. Jan 19, 2015

### Staff: Mentor

In load flow calculations, "flat start" refers to the initial guess of bus voltages. I.e. All voltages 1.0 per unit at zero degrees angle.

After the initial guess, the program iterates to an actual solution. The initial guess has no influence on the final answer (unless it fails to find a solution at all.)

You don't have to use a flat start, it is merely the initial guess least likely to fail if you have no idea what the solution will be.

6. Jan 19, 2015

### jim hardy

If he's applying faults the program needs to be at equilibrium , else how to know what part of the transient comes from the fault vs from a bad initial guess?

Might be just terminology here , my simulation experience was on a plant simulator not a power system analyzer.

7. Jan 19, 2015

### Staff: Mentor

The solution to which I refer is the initial steady state. Short circuits ( AKA faults)are then studied with static analysis or transient analysis, depending on the purpose of your study.

In the jargon, "load flow" is a static calculation. In transient simulations, we do a load flow of the grid each time step.

Power plant simulators huh? What fun. I spent 17 years building those. Had a great time.

8. Jan 19, 2015

### jim hardy

Ahhhh so you know both worlds

Thanks for the clarification !

old jim