Power Factor Correction CT 2

[ukgraduate]

Hello,

I am having a bit of trouble determining what CT burden I will need for a PFC unit which will be connected to a 415V MCC.

I have read a bit of information on the requirements of standard PFC units and from what I can gather they require a Class 1 metering CT and 15 VA burden with 5A secondary.

My question is - can I connect a CT with a greater burden than 15VA? I can only source a 60VA CT due to the size of the busbar it needs to connect to. My thoughts are that it will be ok and that the PFC unit will only draw what current it needs and a burden greater than 15VA will not damage the PFC unit.

Also 1 more question - I have been told to put the CT on the red phase by several sources (manufactures). Why would they recommend this? Surley it doesnt matter what phase the CT is connected to.

Thanks

 

[LSpark]

I'm not familiar with PFC CTs but the main reason I don't order metering CTs with a higher burden rating than I need is that they cost more. They also tend to be less accurate at low values of burden (have a look at the applicable standard for your CTs it will give you the error tolerances).

The burden on the CT is determined by the attached cabling and instruments so I can't see that you'd damage the PFC unit

 

[scottf]

ozgraduate-

In short the answer is yes, you can use a 60VA burden CT.

Lspark-

Not all CTs tend to be less accuracy at low burdens. In general, the accuracy improves at low burdens. However, in the IEC world, CT accuracy only has to be maintained from 25% to 100% of rated burden. Some manufacturers use that to "shift" the load line by using parallel turns correction to reduce the size of the core, thus saving on material cost. In my opinion, the 25% burden range is something that needs to be changed in the IEC standard, given modern device burdens being so low.

 

[waross]

As you know, power factor is determined by comparing the current phase angle with the voltage phase angle. If you use other than the red phase you will have to change the voltage sensing accordingly. If space limitations make it easier to instal on other than the red phase, you will have to "roll" the potential connections to the PFC unit to maintain the correct voltage to current relationship.

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

 

[LionelHutz]

All relays have current limitations on the current transformer input. You can damage the relay if you over-drive the inputs. Protection relays typically are rated for very currents on the CT inputs so they can meter faults. Relays such as a PFC unit are often rated for much less current and may be damaged by the high current that relay class CT's provide.

However, in the end, it often ends up being a compromise because you can't get a CT with a low enough VA so you can expect it to saturate before providing enough current to damage the controller.

The power factor controller should have a connection diagram in the manual. It won't be capable of properly calculating the power factor if you don't follow the connection diagram.