Load in modelling
Load in modelling
(OP)
Hi guys,
how do you model load on the distribution feeder for steady condition Power Load Flow?
program I work with have options:
1. Constant Power load
2. Constant Current load
3. Constant Impedance load.
problem I have is at my company, we are so used to presenting loads (Power Flow Studies) as Constant Power.
Well, when changing the voltage on the TS bus, say increasing by 5%, my current in the model will go down ( due to S=const). Well I humbly don't believe that is the case in the "real life". I would expect current to follow voltage, 5% voltage increase on line will bring increase in current and vice versa. (in steady condition)
Am I right?
If so, what determines type of load for my study?
Thank you for any comment
how do you model load on the distribution feeder for steady condition Power Load Flow?
program I work with have options:
1. Constant Power load
2. Constant Current load
3. Constant Impedance load.
problem I have is at my company, we are so used to presenting loads (Power Flow Studies) as Constant Power.
Well, when changing the voltage on the TS bus, say increasing by 5%, my current in the model will go down ( due to S=const). Well I humbly don't believe that is the case in the "real life". I would expect current to follow voltage, 5% voltage increase on line will bring increase in current and vice versa. (in steady condition)
Am I right?
If so, what determines type of load for my study?
Thank you for any comment






RE: Load in modelling
Constant kVA
Constant Impdance
Constant Current
I have attempted to vary the % of each load type based on the loads served for each individual distribution feeder. For example, feeders serving mostly residentail type loads have a high % of Constant Impedance load type. Feeders serving mostly industrial loads have a high % of Constant kVA load type. As a check, I have installed voltage recorders at various locations in the distribution system to check the model's results. You can use these field measurements to help fine tune your load type allocations (assuming of course that your model is 100% accurate).
RE: Load in modelling
Constant current or constant power are more conservative if you are worried about voltage regulation.
RE: Load in modelling
I would expect industrial loads to have more of a constant power component because of motor loads, so you might use a mix of 50% constant power and 50% constant current if the software allows it. You might also use a mix of constant impedance with constant current for residential loads.
The reason that a constant power load model is appropriate for transmission studies is that most distribution substations have voltage regulators. Changes in the transmission voltage will not affect the customer voltage, so the power will be constant.
RE: Load in modelling
"Constant current or constant power are more conservative if you are worried about voltage regulation."?
Can you please put some more details
RE: Load in modelling
Loads such as motors, which usually are responsible for a high percentage of most commercial/industrial loads, will decrease current when the voltage is increased (within certain parameters).
Your motor load should be modeled as 'Constant Power', and probably most of the rest, (like lighting, etc.) should be modeled as 'Constant Impedance'.
RE: Load in modelling
With a constant current model, the current stays the same as voltage drops, and with constant kVA it increases as voltage drops. So the most conservative approach is to model as constant power or constant kVA.
RE: Load in modelling
RE: Load in modelling
RE: Load in modelling
RE: Load in modelling
RE: Load in modelling
RE: Load in modelling
A constant current model would give 1%, a constant impedance model would give 2% for small changes, and a constant power model would give 0%.