Load Sheding

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augie47

augie47

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State Electrical Inspector (Retired)
Fairly often during an inspection I am asked about generators. I am aware that most units have load shed capability but I have not seen them in operation so I have a question.
Lets say I have a 20kw generator and I decide to put a A/C with a 10kw heat strip on the load shed contacts to meet Code requirements.. When the other loads are reduced to the point that the 10kw can be operated, will the generator allow it to run and then turn it back off as needed or is the load shed completely off as long as the gen is running ?
 
augie47 said:
Fairly often during an inspection I am asked about generators. I am aware that most units have load shed capability but I have not seen them in operation so I have a question.
Lets say I have a 20kw generator and I decide to put a A/C with a 10kw heat strip on the load shed contacts to meet Code requirements.. When the other loads are reduced to the point that the 10kw can be operated, will the generator allow it to run and then turn it back off as needed or is the load shed completely off as long as the gen is running ?
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AFAIK, commonly the latter (highlighted). Means of dynamic load shedding exist, but guessing rarely implemented in residential and small business systems.
 
Same opinion here. Either way is an option but dynamic load shedding probably has a higher price tag.
 
Load Sheding

I guess each manufacturer is different.

Generac uses frequency to determine if the load is too great. If the frequency drops below a threshold the load shedding module will disable all loads connected to the load shed module

After a preset time it will enable Priority 1 (Load 1)
Then the other Priorities 2 thru 4 will be enabled after preset times.

The first priority is 5 minutes then the others are 30 seconds after load ahead of it on the priority list is enabled.

In the scenario as you described when the generator is started all loads are on. (Heat is on Priority 1)

If the frequency drops due to load, all priorities are disabled and all the loads connected to the module are shed.

After 5 minutes, priority 1 ( in this case the heat) is enabled.

If the frequency is stable 30 seconds later priority 2 is enabled and eventually to priority 4.

Say for example is this case priority 1 (heat) is enabled and then priority 2 is enabled but that load causes to big of a load, the load shedding module will "start over" again disabling all loads.

Priority 1 will enable as per normal (5 minutes) but the priority that caused the overload and all priorities after it will be locked out for 30 minutes.


Hope this helps

Joe Villani
 
O.K.,,, let me see if I have this...
Power fails and the generator starts,,, and the ATS switches over all the connected loads...

(At that initial start will the generator allow the priority (potential shed) loads to connect ? )

In any event, if the generator senses a load problem it disconnects all priority loads and
after a 5 min wait, it attempts to restore priority one, and on 30 second intervals 2,3 & 4 however,once it senses a problem, all priority loads drop out and the priority one waits the 5 min but all others have to wait 30.
Correct ?
 
augie47 said:
O.K.,,, let me see if I have this...
Power fails and the generator starts,,, and the ATS switches over all the connected loads...

(At that initial start will the generator allow the priority (potential shed) loads to connect ? )

In any event, if the generator senses a load problem it disconnects all priority loads and
after a 5 min wait, it attempts to restore priority one, and on 30 second intervals 2,3 & 4 however,once it senses a problem, all priority loads drop out and the priority one waits the 5 min but all others have to wait 30.
Correct ?
Click to expand...

At Generator start all loads (including the load shedding module) are enabled (connected)

Correct.

So if priority 2 caused to generator to "bog down", the load shedding module would disable all the loads connected to it. (Priority 1 thru 4)

After 5 minutes priority 1 will be enabled

Priority 2, will now have to wait 30 minutes to become enabled

Priority 3 30 seconds after priority 2 (30 minutes 30 seconds)

Priority 4 30 seconds after priority 3 (31 minutes)

If priority 3 causes the generator to "bog down" the cycle will repeat as above with Priority 3 having to wait 30 minutes.

Hope this helps,

Joe Villani
 
augie47 said:
O.K.,

(At that initial start will the generator allow the priority (potential shed) loads to connect ? )
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mkgrady said:
No not until a period of time. A few minutes I think.
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I may have worded that incorrectly.... I know there is a transition time before the ATS transfers, but what I meant to ask Joe was if all loads initially connect on the transfer switch closing or if there is a delay for the priority loads.
 
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augie47 said:
I may have worded that incorrectly.... I know there is a transition time before the ATS transfers, but what I meant to ask Joe was if all loads initially connect on the transfer switch closing or if there is a delay for the priority loads.
Click to expand...

There is a delay.
 
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We have also installed aux contacts that lock out non-essential loads from operating at all while on standby operation.

Like a decorative fountain pump, for example.

Or a hot tub.

Things like that which don't really need to be operating while the main power is out.
 
kbsparky said:
We have also installed aux contacts that lock out non-essential loads from operating at all while on standby operation.

Like a decorative fountain pump, for example.

Or a hot tub.

Things like that which don't really need to be operating while the main power is out.
Click to expand...
Lots of things don't need to run when the main power is out, but today's demanding society wants to run as much as possible - just because.

I personally only need heat, water, somewhat minimal light, would be nice to keep the refrigerator going but don't absolutely have to, maybe an extra 1000 VA of extra capacity for whatever, and that whatever can be on a rotating as needed basis and doesn't have to be constantly supplied. If the capacity is there I will run my electric water heater - probably only long enough to heat enough water for a shower and then will likely shut it down again so there is capacity for other items. But I also don't have an ATS but do have a manual TS that transfers the entire service to standby which is not large enough to run the entire service in normal loading conditions, so I do have to monitor what is allowed to run and what is not.

Anyone know how much propane or NG costs to run say a 16 kVA standby at say 50-60% load? I know when I ran my portable gasoline powered generator which is only a 8kVA unit a few years ago when we were out for about 10 days, the rate of gas I was using was going to cost me about $900 to run for a month at that time, the usual electric bill was only maybe $150 - 200, and I was able to run more things as well.
 
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kbsparky said:
We have also installed aux contacts that lock out non-essential loads from operating at all while on standby operation.

Like a decorative fountain pump, for example.

Or a hot tub.
Click to expand...
So you would connect the hot tub through a NO contactor and control the coil with the aux contacts?
 
Or we would interrupt the control wiring with a simple "ice cube" type plug-in relay.

That's how we locked out 20 kW of electric strip heat on a geo-thermal system.
 
kbsparky said:
Or we would interrupt the control wiring with a simple "ice cube" type plug-in relay.

That's how we locked out 20 kW of electric strip heat on a geo-thermal system.
Click to expand...

Not all loads are that simple and/or inexpensive to control, but that is a great way to control those that work that way.
 
Is there a way to connect the load relays remotely? on an existing house you can't always get a control circuit to the distribution panel or equipment, I was thinking of wireless comunication to the load relay.
 
acrwc10 said:
Is there a way to connect the load relays remotely? on an existing house you can't always get a control circuit to the distribution panel or equipment, I was thinking of wireless comunication to the load relay.
Click to expand...

Somebody told me that GE generators load shed relays work wirelessly
 
mkgrady said:
Somebody told me that GE generators load shed relays work wirelessly
Click to expand...

Only the GE Symphony II system.

This text was cut from http://www.gegenerators.com/support/how-to-articles/storm-preparedness/home-standby-generator-accessories-make-maintenance-easy

GE Home Standby Generator Modules

Last but certainly not least are generator modules from GE that are able to coordinate with the Symphony II? Transfer Switch to adjust your generator output during a blackout. All Modules comes with a three year limited warranty and are installed in your home with no additional wiring required.

The GE Low Voltage Generator Module manages thermostatically controlled appliances like an air conditioner or furnace.
The GE 50 Amp Module manages 120-240 volt appliances like an electric stove or electric heat.
The GE Low Voltage Dual Module is able to control any two thermostatically controlled appliances like two air conditioners or furnaces.
Click to expand...

Sam
 
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