capacitor bank effect on the transformer HV side 10

[prismpower]

hello,

we have HV/ MV power transformers. we've installed capacitors on the MV side of the transformer. is there any chance that the capacitor affect the HV side by increasing the voltage or its only affecting the MV side of the transformer?

regards,

 

[davidbeach]

The presence of the low-side caps will change the flow of vars through the transformer. That will have some impact on the high-side. Whether or not that impact is noticeable will depend of a whole variety of things.

 

[HamburgerHelper]

I don't know the arrangement of your system but your transformer taps can be used to direct the vars.

 

[prismpower]

i was thinking that power; including Vars; will not flow back into the HV grid.the effect will be major on the MV side only by increasing the voltage of the MV side. am i correct?

 

[davidbeach]

If the transformer is feeding radial load, say that load consumes (making up numbers) 4.5Mvar that presently comes from the high-side through the transformer to supply the need. Now say you install two 3Mvar banks. If one bank is switched in, that flow through the transformer will be reduced from 4.5Mvar to 1.5Mvar.; still from high-side to low-side. Now turn on the other bank; the magnitude will still be 1.5Mvar, but it will be the opposite direction; low-side to high-side because there's no place else for it to go.

But if the transformer is networked and not radial, there are many other considerations that will impact how the vars flow. With an LTC you could have some steps where they go from low-side to high-side and others where they don't; the LTCs on other transformers will also matter.

 

[waross]

The capacitors will reduce the reactive current in the primary feed conductors. Thus the reactive voltage drop on the primary will be reduced. If the MVARs of correction connected exceeds the load MVARs, there will be a reactive current in the opposite direction to the real current in the supply conductors that will cause a reactive voltage rise.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

Response to original query- When you install capacitive load on MV, only the MV terminal voltage will be affected and not much of HV. Theoretically, as waross mentioned, it can change due to the reactive drop in the HV system impedance. But system impedance is so low compared to transformer impedance( 2-3 % of transformer impedance for 20-10 MVA at 132 kV), this is negligible compared to voltage drop or rise in transformer impedance.With 4.5 MVAR reactive load, MV terminal voltage will dip compared to secondary open circuit voltage. When net load on transformer becomes 1.5 capacitive MVAR, the load current lead the voltage and cause a voltage rise compared to open circuit voltage which remains the same. There will not be any reverse current flow to system as per my understanding.Open circuit voltage will change when you change the transformer tap and it is operated so as to get constant terminal voltage on load.

 

[prismpower]

in theory, the capacitor will supply the MV side loads with reactive power (MVAR). which will reduce the load on the HV side. but the excess MVARs at MV side will cause voltage rise on the MV side but will not flow to the HV side. the only effect is reducing the load on the HV grid. i might be wrong!

 

[prc]

prismpower, The net current passing through the transformer is the vectorial sum of load current plus capacitor current. If the net current is leading the voltage (ie capacitive),then voltage regulation is positive ie terminal voltage will be more than the open circuit voltage. If the net current is lagging the voltage (ie reactive) voltage regulation is negative ie terminal voltage will drop from open circuit voltage by the voltage regulation value(vectorially)

 

[waross]

Warning {Anecdote on}
A real world example:
A city in Central America originally ran on diesel power.
About 30 or more years ago a large hydro-electric project was completed on the other side of the country.
The diesel plant was mothballed.
As the city grew, and the load increased, the capacity of the transmission and substation system was exceeded.
The capacity of the line was limited by the voltage drop and the ability of the OLTCs to compensate for the voltage drop.
A large part of the voltage drop was due to line reactance.
The diesel plant was taken out of mothballs and put back online. Due to the age of the equipment and the cost of fuel, the plants were only loaded enough to reduce slobbering.
The plants were used to supply MVARs to the system and reduce the reactive voltage drop on the transmission line.
This solution allowed the system to supply greater energy than the original design while maintaining the voltage at an acceptable level.{anecdote off}

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[VTer]

Assuming your load is downstream of the MV side and we are talking about PFCC on a simple radial system, the capacitors on the MV side will reduce your MVAR demand to the point of the PFCC. The reactive energy will alternate and transfer between the inductive load downstream and these capacitors. This will result in lower current magnitude upstream of PFCC and therefore lower voltage drop across the line and the transformer. If your capacitor rating increases above the load VAR demand, then your PF will be leading upstream of the caps. In that case, depending on the system and line parameters, voltage rise may also become an issue.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[stevenal]

but the excess MVARs at MV side will cause voltage rise on the MV side but will not flow to the HV side. the only effect is reducing the load on the HV grid. i might be wrong!

Confirmed, you are wrong. Reactive power like real power flows from source to load whichever way is needed through any transformers in the path. See Davidbeach's example above.

 

[bacon4life]

The voltage change due to a cap bank switching can be estimated using the short circuit level as explained in

http://electrical-engineering-portal.com/capacitor-banks-in-power-system-part-three

On a weak 69 kV system, 6 MVAR could move the HV side more than 1%. On a strong 115 kV system, 6 MVAR could move the HV side less than 0.1%.

 

[prc]

VTer, "The reactive energy will alternate and transfer between the inductive load downstream and these capacitors." I like that, the best way to describe the phenomenon.

 

[crshears]

Allow me to post from a transmission operator's perspective...

Within the more robust portions of my utility, our LT/MV capacitors [ 14, 28 & 44 kV ], being nominally rated at 20, 25 and 30 MX and located at our substations, do indeed have profound effects on the HV transmission system; our Independent Electrical System Operator does in fact in large part dispatch our LT capacitors to control the 115, 230 and 500 kV system voltages. Incidentally we do have 115 and 230 kV cap banks as well, typically with a nominal rating [ meaning at rated voltage ] of 100, 200, 300 and 400 MX, generally near or within major load centres, or at major transmission nexi.

There are some more remote and weaker portions of our system that have lengthy 115 kV lines and therefore significant impedance; at substations connected to these circuits even an 11 MX cap bank can cause major voltage perturbations, necessitating considerable prudence in dispatching same.

Certainly the HV caps do deliver great gouting amounts of reactive power, and have their place; but it is the LT caps that can often be tweaked to a relative nicety, maximizing the real power capability of the HV circuits and the substation transformers in question, something of great importance during both winter and summer peaks as well as during maintenance outages in the shoulder seasons.

It's just gone midnight here, and for us it is now Christmas Eve; I'm expecting the IESO to be calling fairly soon to request a bunch of said LT caps removed from service...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[thefredq]

Hello, I'm new to this forum and this look's like great information! I'm an Electrical Project Engineer in charge of a Power quality study and would like to pass something by the community.

The scenario is this: We have CAP banks installed at site #1 on the 13.8KV secondary's of 2 site transformers (138KV:13.8KV) and generally sit at PF of 99 to unity most days. We have a second site fed from the same transmission line at 138KV with poor PF dipping down to 87 some days which is about 10KM away from Site #1 as the crow flies.

Given that there is poor quality at the site #2, one of the Maintenance Engineers has suggested that we switch in the CAP banks at site #1 (overcompensating) in order to dump MVAR's through the secondary of the Transformers up to the Primary 138KV side in order to mitigate the PF at the second site.

To make matters more convoluted, the low PF at the second site is due to inductive load which is up to another 6KM away (on top of the 10KM) at 25KV (Underground Mining) for a total of about 16KM.

The total impedance of the 138KV line between the 2 sites is unknown at this point, but at a high level I'm concerned about the effect of voltage rise at 13.8KV system at site #1 . Will an independent PF correction CAP bank be a better solution at Site #2 vs. trying to utilize Site #1's CAP Banks? By switching in additional CAP banks at site #1, my thought is that we would be overcompensating for MVAR demand at site #1 and potentially cause an overvoltage condition at Site #1 due to the distance between the 2 sites. Given that Site #2's MVAR demand will counter the Additional MVAR's dumped onto the transmission line it could be argued that the net effect will achieve the desired outcome, but I'm not fully convinced that this would be the case.

Does anyone have expertise on this that could weigh in from a high-level standpoint?

I thank you in advance for any input on this!

Best Regards!

 

[waross]

The first consideration is the wishes of the operator of the 138 kV system.
Adding caps at one site will not affect the metering of KVARs or PF at a second site.
If the system operator will allow you to combine the kWHr and KVARHr consumption of both sites and bill the PF penalty on the combined total this will allow you to correct the PF on the 138 kV line by adding caps at one site.
One site would have a leading PF and the second site would have a lagging PF, but the effect on the incoming 138 kV line could be a unity PF.
However it is doubtful that the sys-op's tariffs will allow combining of the two power bills.
The voltage rise will be greatest in most cases at the point where the caps are connected.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[thefredq]

Hi Bill,

As far as the system operator goes, that would be a whole other set of negotiations for the company to carry out regarding combining the 2 power bills which is out of my realm of influence and highly unlikely at this point.

You have confirmed what I was thinking earlier regarding the voltage rise and that makes me wary of pursuing this scenario.

Thanks for your input!