Capacitor Bank on Primary vs Secondary? 1

[MichPEng]

I have a question regarding placing a capacitor bank on the primary high side (transmission level) or on the secondary side (distribution) and relating this to the transformer tap regulation.

Is it beneficial to place a cap bank on the high side of the transformer? Since you have better voltage regulation (approx 5%) on the secondary side where the transformer tap would move less and prolong the life of the transformer?

Or does putting the cap bank on the secondary side of the transformer would cause the transformer tap on the high side of the transformer to move a lot which would degrade the life of the transformer. The reason for tap movement is to regulate the voltage level on the secondary side that may have a load factor of 60% ?

Any insights?

Many thanks

 

[davidbeach]

Transmission caps go on the high side, distribution caps go on the low side. If you need VArs on the high side put the caps there, if you need VArs on the low side put the caps there. Stations can have both if needed.

 

[cranky108]

There is a limit to how large of a bank you put on the low side, or tertuary.

So what is the need for the capacitors? Distribution or Transmission?
In general it's better to place capacitors close to the location needing correction.

 

[MikiBg]

What is the purpose of cb installation? Energy savings, voltage profile improvement,or both? Are we talking about automatic regulation? If so, pfc regulator will change the number of connected units just when load demand/nature change, for constant load there will not be tap change initiations just because of capacitors. That's why I wouldn't worry much about tap changer life time, especially if we're talking about fast (thyristor based) compensation. Other aspects (purpose of compensation, amount of kVArs needed, current THD...) can lead to primary/secondary point of installation choice through b/c analysis.

 

[Marmite]

Cost will be a major factor. High voltage capacitor banks are significantly more expensive than low voltage capacitors, as is the controlling switchgear and protection.
Regards
Marmite

 

[MichPEng]

Sorry, I forgot to mention that the capbank would be fixed on on the secondary side on the distribition buswork not on the feeders. However, for the primary high side, the capbank would be point on wave switching.

Marmit is absolutely right. It will definitely be cheaper to install it on the secondary side. That is why I am considering it; however, I am worried about the transformer tap moving to often due to the nature of the load (daily load going from light load conditions say 5MW up to 12MW) and due to the fixed capbank on the low side which may result in more tap movement due to the low side regulation. This may increase exposure to transformer damage which may in turn cost even more in the future. Is this second paragraph I wrote true?

 

[MikiBg]

Of course HV cb and following equipment will be much more expensive, that's why I mentioned b/c analysis.
What about MVAr load oscillations? In general, for wide load oscillations, fixed banks are bad solution. Eventually you can cover lower demands with fixed banks and let higher demands uncompensated. If you go above that, you are risking overcompensation and over voltages during low demands. Again, it seems to me that load oscillations magnitude will have key influence on tap changing frequency, not cap bank itself. However, the question isn't so simple, typical load diagram, transformer and switchgear data will help for more accurate answer.