Battery storage facility import/export 2

[kaspor]

Hello

I am trying to wrap my head around how battery storage facility import/export power.

For these scenarios, assume grid voltage is 66kV, inverter output voltage is 660V, battery facility rated capacity is 20MVA, consisting of multiple inverters (for simplicity assume all inverters are equal in size and are lumped into one inverter).

For the import scenario, my understanding is as follows:

Inverters will decrease generated voltage angle amplitude to charge batteries or absorb reactive power up to the limits on the inverters PQ curve. Current will lead inverter voltage implying a leading power factor. On the limits of the PQ curve, a current will flow throughout the facility into the inverters and its imperative that these voltages are held within the inverters operating tolerance (e.g. +/- 10% 660V).

For the export scenario, my understanding is as follows:

Inverters will increase voltage angle/voltage amplitude to export real/reactive power into the grid up to the limits of the inverters PQ curve. Current will lag inverter voltage implying a lagging power factor. On the limits of the PQ curve, a current will flow throughout the facility into the grid. For this scenario, can you assume that the grid voltage will be held constant and if impedances within the facility (i.e. cables etc.) are too small, then the inverter will not be able to export at rated capacity? What happens in this situation? Is the inverter limited as to how much it can increase voltage angle/voltage amplitude?

p.s. apologies if I'm using the wrong terminology, by voltage angle I mean the phase angle difference between inverter generated voltage and grid voltage (I think some people call this load angle on a synchronous generator).

 

[bacon4life]

I would expect that the inverters can control real power independently of control for reactive power.

The installation should be designed to keep the inverters within the acceptable voltage range while meeting the applicable requirements from the transmission operator for providing/absorbing reactive power.

 

[itsmoked]

Wouldn't you call the import hardware "chargers" rather than inverters?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[xnuke]

I currently work in microgrid design, control, and implementation, so I see these types of questions frequently regarding battery energy storage systems (BESS).

Knowledgeable people use the term power conversion system (PCS) when discussing a BESS instead of inverter because they allow bidirectional power flow, i.e., rectifying for charging and inverting for discharging. Many provide sourcing/sinking both active and reactive power (full four-quadrant operation). However, recent experience has exposed me to a lot of people who call them inverters, and don't know what a PCS is and get confused when I use the term. As I'm something of a pedant, I usually explain the difference, trying to get them to stop saying inverter and start saying PCS, but it seems to be a losing battle. I even slip up once in a while and say "inverter" now due to all the influence from others - then I get mad at myself and vow never to do it again. Until it happens again. Then I repeat that vicious cycle.

p.s. apologies if I'm using the wrong terminology, by voltage angle I mean the phase angle difference between inverter generated voltage and grid voltage (I think some people call this load angle on a synchronous generator).

Most people say power angle instead of voltage angle, but it is defined exactly as you wrote it. The angular difference occurs across a series output inductive reactance such as that provided by an inductive filter element or a transformer. It is in fact the same angle as the power angle and the torque angle of a synchronous machine.

For the import scenario, my understanding is as follows:
Inverters will decrease generated voltage angle[/]amplitude to charge batteries or absorb reactive power up to the limits on the inverters PQ curve. On the limits of the PQ curve, a current will flow throughout the facility into the inverters and its imperative that these voltages are held within the inverters operating tolerance (e.g. +/- 10% 660V).

A PCS will produce a negative power angle to absorb active power and can decrease generated voltage amplitude to absorb reactive power up to the equipment rated limits. Equipment limits may be set at a maximum reactive power value based on the apparent power rating and the power factor rating when maximum real power is being produced, but this can be vary by model. If allowed to produce zero real power output, it may be possible to produce 100% reactive power out in either direction in some models, operating similar to a STATCOM.

The system voltage and frequency are controlled by the grid when connected (PCS is in grid-following mode) and by the PCS when islanded (PCS is in grid-forming mode, unless in parallel with a generator, in which case it will typically be in grid-following mode and let the generator regulate island voltage and frequency).

For the export scenario, my understanding is as follows:
Inverters will increase voltage angle/voltage amplitude to export real/reactive power into the grid up to the limits of the inverters PQ curve.

A PCS will produce a positive power angle to source active power and can increase generated voltage amplitude to source reactive power up to the equipment rated limits. Equipment limits are as discussed previously.

Current will lead inverter voltage [when charging] implying a leading power factor....Current will lag inverter voltage [when discharging] implying a lagging power factor.

Power factor is a separate issue somewhat unrelated to charging and discharging, and can be leading, lagging, or unity. It is often controlled to near unity at the point of common coupling between the power system and the utility grid, therefore providing power factor correction for the power system loads. However, since most systems require reactive power to operate, absorption of reactive power by BESS typically doesn't happen, but production of reactive power does.

On the limits of the PQ curve, a current will flow throughout the facility into the grid. For this scenario, can you assume that the grid voltage will be held constant and if impedances within the facility (i.e. cables etc.) are too small, then the inverter will not be able to export at rated capacity? What happens in this situation? Is the inverter limited as to how much it can increase voltage angle/voltage amplitude?

Export to the grid cannot occur at rated capacity if facility impedances are too large or the PCS serves parasitic load on the facility side of the grid connection, as both losses and load eat up capacity. Export also cannot occur at rated capacity if restricted by the utility, and they often place export limits on the microgrid systems I work with, but not typically on a stand-alone BESS that is supposed to export near capacity. A PCS is limited by its power ratings, which limit how much it can increase power angle/voltage amplitude, not the other way around.

I would expect that the inverters can control real power independently of control for reactive power.

A PCS can do this. For more information on how, look for information on space vector modulation. I don't want to attempt to discuss it here - it's complicated. However, there are PCS functions that link reactive power production to active power production or to system voltage, so it may not always be independently adjusted.

For those of you who want more information on operating functionality, I suggest EPRI's Common Functions for Smart Inverters, 4th Edition found HERE, which discusses both PV and BESS PCS inverter functions.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[itsmoked]

then I get mad at myself and vow never to do it again. Until it happens again. Then I repeat that vicious cycle

As the guys in lab coats dragged him away mumbling something about PCS PCP what's the difference.

Thanks xnuke, that makes perfect sense. "BESS" I will join you in your quest. May we be successful in our struggle! I recall a time when most people called VFDs "inverters".

Keith Cress
kcress -

http://www.flaminsystems.com