How does negative (or export) kVAR and kW affect maximum demand charges?

Question

I am trying to calculate enegy costs using data from an energy meter that provides me with halfhourly data points for the following 4 channels:

• Import kW
• Export kW
• Import kVAr
• Export kVAr

and in particular I want to determine the Maximum Demand (kVA) used in a period to verify municipal bills received. I am using the power triangle to calculate the maximum demand for each halfhourly point and then finding the maximum of those point namely: kVA = sqrt (kW^2 + kVAR^2)

However I am not sure how to treat the import/export channels. I can think of two approaches that makes sense to me:

1. Calculate the kVA using only the import kW and import kVAR since this is what is "imported" from the energy utility and all that is paid for.
2. Calculate the kVA using the net kW and net kVAR (i.e. kW imp - kW exp as well as kVAR imp - kVAR exp) since this is the capacity of the power flow that the utility must be able to handle.

(There are scenarios in which the data point has a reading for both import and export kW - I assume this is becasue in one halfhour there can be a time period during which the site is importing energy followed by a time it is exporting energy)

For most sites it is only the export kVAR channel that influences this, but for sites with solar installations where energy exports are allowed the kVA can be dramatically influenced if the negtaive, export kW is included. I have even seen the scenario where the Maximum kVA would be as a result of the kW exported to the grid in the middle of the day, rather than as a result of energy imported from the grid at any other time.

Just to emphasise, my main interest is finding the "accepted" way to calculate this i.e. how I can expect the utility bill to be calculated by various billing authorities (different municipalities). However, any explantions of the underlying theory of why this is would also be appreciated.

Answer 1

It is entirely possible that a supplier would want to charge a penalty for short term increases in kVAR regardless of direction. In some areas of the world there may be one or a few "accepted" ways of calculating electric utility charges. It is likely that individual utilities have several classes of customers with different calculation methods used for each class. In large complex markets with many suppliers, like USA, there are likely more than a few ways for calculating charges.

It could probably be considered to be abnormal for an individual utility customer to supply kVAR unless the customer produces electric power with a wound-field synchronous generator (WFSG). Even in that case, that seems unlikely. The equipment that can supply kVAR is limited to WFSGs and capacitors. Individual facility operators ordinarily try to avoid operating at a power factor higher than about 0.95 lagging. Operating at 1.0 or leading pf can cause the facility voltage to be too high or to exhibit some kind of instability. That is a statement that deserves further research as to accuracy and underlying theory.

Re: Export kW (e.g. from solar installations) with regards to calculating the kVA for maximum demand charges?

The electrical energy wholesale market, the evolving nature of the customer base and of the technology, infrastructure maintenance costs, harmonic distortion, regulatory restrictions and other factors influence what utility companies would like to do and are permitted to do. Any utility billing strategy that is in place is likely to change.