What is a short circuit load and faulty synch load? 1

[rte4563]

I have been given a static load(N), short circuit load(N) and faulty synch load(N) which I need to apply at certain points on a structure. I am quite familiar with static loads but not with the last two, they're supposed to be coming from some machinery on top of the structure. Would these most likely be dynamic loads? It says the short circuit load and faulty synch load are intended as both traction and compression loads.

This makes me believe the behavior of the type of loads, for example short circuit load of +/- 1000 N, might start at +1000 N, then go down to -1000 N, then back up to +1000 N and so on. The load might even be rotatin 360 degrees all while this is happening. Does this make sense to anyone?

 

[waross]

Is this an electrical question or a structural question? You are in an electrical forum.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[FacEngrPE]

I concur this is mostly a structural question, specifying a generator support structure able to withstand the reaction loads from infrequent events, short circuits and out of sync paralleling.

Please be aware - I do not have experience with utility generator foundations so the following are suggestions about how I would frame the issues in your problem statement. Once I think understand the problem statement I would look for a structural engineer (a second opinion) to validate my approach.

A short circuit,that is promptly cleared, an out of sync paralleling event, and a poll slip will generate forces at the generator feet that are impact like.

static load(N),
[ul][li]I would treat this as dead load. It is probably the self weight of the generator. (Please verify)[/li][/ul]
short circuit load(N)
[ul]
[li]As operating reaction loads are not provided, I would treat this number as part of the live load. There may be other components of live load. Hopefully short circuits are infrequent enough this is conservative.[/li]
[/ul]
faulty synch load(N)
[ul][li]Treat as a (hopefully) infrequent impact load.[/li][/ul]

Be aware that out of sync events and pole slips can be sufficiently violent that generators and their attached prime movers often incur internal damage (up to and including rapid disassembly, parts out through the roof). I suggest you make an effort to understand the degree angle of out of sync your customer is attempting to address, their risk tolerance, and then review other threads here on out of sync events.

 

[waross]

Mechanical reaction forces in a generator are dependant on peak current, not RMS current.
Worst case short circuit the current may approach 2x(2^2) times the RMS current based on the inductive reactance of the generator.
Worst case out of sync may approach 2 times the worst case short circuit.
An inadvertent interchange of two wires to the sync gear during maintenance may result in a 180 electrical degree out of sync.
That is the worst case, and possibly one of the most likely causes of a worst case out of sync.
I know of a coupling key sheared on a 600 kW diesel generator due to a wrongly connected synchro-scope, and a 60 degree out of sync closure.
I made a lot of money on the repair of a 400 HP wound rotor motor when the contactor flashed phase to phase.
The induction motor became an induction generator feeding a dead short.
The motor shaft was bent, the fan shaft was bent and pieces of the coupling where found all over the machine room.
I have seen a short in a welding motor-generator result in the machine rolling up on one skid, almost to the tipping point. The breaker cleared and the machine fell back down.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

The direction of the force; The generator will attempt to rotate around the main axis, that is around the shaft. On one side of the machine the force will be such as to compress the vibration isolaters.
On the other side of the machine the force will be upwards, trying to break the hold down bolts or separate the vibration isolaters.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

Clarification:
The worst case electrical is at 180 electrical degrees out of sync.
The worst case mechanical is at about 90 electrical degrees out of sync.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Gr8blu]

rte4563: Static force actually has two components because it is the force that is acting "all the time" during normal operation. The general equation is:

Static force = XXX (units of force) +/- 0.5*(weight of machine)

The first piece is the force exerted by the torque of the machine as it tries to rotate the stationary portion in opposition the shaft rotation. In this case, the force is the torque divided by the horizontal distance from the shaft centerline to the hold down bolt location. With that said, it acts to "push" the machine into the foundation on one side and "pull" it away on the other side. The second piece is a force related to the weight of the machine itself, acting downward because of gravity. If the machine is mounted above a beam, the force acts to push the machine into the beam. If it is hung under a beam, the force acts to pull the machine away from the beam. The total combined force is (usually) divided between two or four points of contact - essentially, where the rotating machine (or its sub-base) is bolted to the foundation.

Short Circuit force is a result of an electrical incident. There are multiple cases, but effectively the description is that one (or more) phases become shorted to another phase or to ground. When this occurs, the instantaneous torque increases dramatically (up to as much as 8X the normal rated torque). It usually doesn't last long when a machine is motoring (i.e., driving a process), but can last a while when generating (i.e., driven by a prime mover or process). For your purposes, it can be viewed as a "torque" event, which is the first part of the "static" equation above. Instead of XXX units of force, you'd use (8)*(XXX) units of force and thus get a new total. You'd still have to account for the weight of the machine for foundation reactions, since sometimes it acts with the torque load and sometimes against.

Faulty synch is just what it implies - the machine attempts to synchronize out of phase with the electrical supply. In this case, the peak torque is going to be roughly 2X the rated torque, which occurs when the breaker or contactor is closed at 180 degrees phase opposition. As per the previous cases, you still have to account for the machine weight for foundation reactions as the "fault" force is related to torque, and the weight sometimes acts to aid and sometimes to oppose it.

Converting energy to motion for more than half a century

 

[waross]

In a self contained machine, torque loading is not continuous. External torque is developed when the load is changing or when the speed is changing.
Anyone who has seen an electric motor energized on the bench when not properly restrained will have seen the torque transient as the motor tries to roll while accelerating.
When the motor is up to speed with no load, there is no external torque developed.
In a diesel generator, the bed plate or sub frame is designed to accept the torque between the prime mover and the generator. Any relative movement may affect the alignment and may lead to eventual drive coupling damage.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

rte4563 (Mechanical)(OP)26 Feb 23 18:27
"....I have been given a static load(N), short circuit load(N) and faulty synch load(N) which I need to apply at certain points on a structure. I am quite familiar with static loads but not with the last two, they're supposed to be coming from some machinery on top of the structure. Would these most likely be dynamic loads? It says the short circuit load and faulty synch load are intended as both traction and compression loads...."
1. .... "...Mechanical reaction forces in a generator are dependant on peak current, not RMS current. Worst case short circuit the current may approach 2x(2^2) times the RMS current based on the inductive reactance of the generator. Worst case out of sync may approach 2 times the worst case short circuit" , are irrelevant to the question asked. These forces are within the motor/generator winding in the stator or the busbar in a switchboard.
2. The question pertaining to "...short circuit load(N) and faulty synch load(N) "...are for the anchor bolt and the block foundation design, in the civil/structure engineering.
3. As an example:
(a) if the anchor bolts are too small in diameter or of wrong material, the bolts may shear/break.
(b) if the anchor bolts embedment is too shallow, the bolts may be up-rooted.
(c) the number of bolts and the distance apart and depth to ensure that the concrete does not breakout. etc....
Che Kuan Yau (Singapore)

 

[waross]

These forces are within the motor/generator winding in the stator

Not always.

The worst case mechanical is at about 90 electrical degrees out of sync.

At 180 electrical degrees, the forces are contained within the machine. At any other angle other than zero, the inertia of the rotating mass is partially transmitted to the base in a magnitude limited by the peak current and the angle at of the sync error.
I know of several sheared keyway keys from out of sync closures.
There was a wiring error in the synchroscope and a 30 degree phase angle error when the 'scope indicated "Safe to close".

--------------------
Ohm's law
Not just a good idea;
It's the LAW!