Question about LTC Transformers

[user1011]

I'm performing Reactive power study for a solar plant to determine compliance with FERC 827 and quantify if there is a need for additional reactive power compensation device. Should I include in the study the effect of LTC of the main transformer on the reactive power flow at the POI?

 

[waross]

It depends.
LTC adjusts the voltage.
Voltage adjustments tend to vary the reactive current.
Due diligence suggests considering the effect of LTC operation.
The effect may be significant or it may be negligible.
It depends on information not given.

 

[JJ_Roy]

Yes. Compliance with FERC 827 requires the reactive power capabilities to met at the high side of the station transformer. Some interconnections requirements say it's at the point of interconnection (POI).

As warross noted, the LTC could have a significant impact....

I'd check with the interconnecting utility (or system operator) as there is probably some sort of planning procedure that dictates how you determine the reactive capability.

 

[crshears]

The Technical Interface Requirements or equivalent of the Authority Having Jurisdiction [AHJ] will largely determine the answer to your question.

 

[wcaseyharman]

Biggest impact of LTCs I think of on IBRs is the loss of VAR capability at high voltage. If the taps aren’t set right the VAR capability can be severely limited. Right now for that reason I plan to set the LTC in our new wind farm to manual - I just don’t see much value in the LTC anymore with the turbines and plant controllers capable of controlling VARs, and it has the potential to mess with the VAR capability of the plant.

 

[user1011]

Yes. Compliance with FERC 827 requires the reactive power capabilities to met at the high side of the station transformer. Some interconnections requirements say it's at the point of interconnection (POI).

As warross noted, the LTC could have a significant impact....

I'd check with the interconnecting utility (or system operator) as there is probably some sort of planning procedure that dictates how you determine the reactive capability.

Thanks for the reply. I'm just wondering why it needs to be considered since LTC operation is slow and not dynamic compared to SVC/STATCOMs.

 

[waross]

The tap position may affect the power factor and may affect the ASCC. (Available Short Circuit Current)

 

[JJ_Roy]

Thanks for the reply. I'm just wondering why it needs to be considered since LTC operation is slow and not dynamic compared to SVC/STATCOMs.

You need to consider the "steady state" reactive power capabilities as well.

Below is a clip from the ISO-NE Planning procedure 5-6 regarding reactive power capability testing. (FYI: it says wind park but it actually applies to all inverter-based generation.) I've (poorly) highlighted the section where it requires "various transformer tap settings".



Your interconnecting authority probably has something similar.

Just to add more color to wcasey's comment, below is a graph showing the reactive capability for a 240 MW plant with +/- 80 MVAR at the xfmr low side at different tap settings. This is for a xfmr with fixed taps. It give you a good idea of the transformer reactive losses at full power depending on tap setting.



Hope this helps....

 

[cyriousn]

Typically we run a pf test before submitting to the ISO using PSSE to determine if we need adjust quantity of inverters, adjust taps of GSU transformers, or increase Main Power Transformer base rating. We typically try to avoid Cap Banks and OLTCs altogether. On large PV projects with very long overhead lines, the losses there can also be an issue.