Transformer Hot spot CTs - How and when to use? 1

[Uquresh1]

I have a Virginia Transformer 42MVA 69/13KV. There are two CTs- one on the B phase of High side winding and one on the B-phase Low side winding. These CTs are wired to the Transformer Monitor relay and I am guessing that these are meant for Hot Spot calculations. So, any confirmation on what these CT are for (other than Hot Spot)? How to use them for that purpose? Should both be used or just one is fine?

Thanks.

 

[DTR2011]

Traditionally, a single CT was used on the H2 (B Phase) of the HV side of the transformer. This was fed to hot spot temperature gauge, which also took oil temperature from a well indicating top oil temperature.

You mention a transformer monitoring relay, something intelligent. Not knowing what you have, the temperature monitor device may take 2 CT inputs. If you choose not to use the second (X2 - LV B Phase CT), be sure to short it out!

If this is a new transformer, pay close attention when commissioning the CT's. The Hot Spot CT's will typically have a similar ratio to any protection CT's, but will have a much lower kneepoint voltage. You do not want to have a Hot spot CT feeding a protection circuit!

The transformer monitor relay can control fans, send alarms, trips, metering info. Please indicate what device you have if you have further questions.

 

[Uquresh1]

Thanks smallgreek. The relay used is SEL-2414. ALso, is HS CT preferred over the LS CT? why?

 

[DTR2011]

Assuming the HV side is the source, use that CT. This will account for transformer temp under no load conditions.

 

[Uquresh1]

Smallgreek, after much brainstorming with couple of other young engineers, we still couldn't see how what you said in the last post would matter, as far as using the HS or LS Hot spot CT is concerned. BTW, high side is the source and is 69KV.

 

[DTR2011]

The transformer will have a certain amount of no load losses. If you check the factory test report or the design specifications, you will see that they are not insignificant, although transformer OEM's try to reduce them. In reality, the time for this to make a significant difference would be considerable.

The SEL device may also have provisions for a 2nd CT for the LV winding temp - depends on what cards were ordered with it.

 

[bacon4life]

Our transformers use a CT on LV A phase for temperature monitoring and CT on LV B phase for LTC control. If one of these CTs feeds a heated well to simulate winding temperature, the CT will have quite a bit of burden on it. If the transformer may be used in paralleling applications with a Beckwith controller, having a dedicated CT for the circulating current paralleling method is desirable.

Per SEL application note AN2011-04 the 2414 can be used as an LTC controller in addition to a temperature monitor/fan controller.

http://www.selinc.com/WorkArea/DownloadAsset.aspx?id=98307

Our three winding transformers are the only ones with high side hot spot CT's.

Smallgeek, in this transformer would the no load losses be accounted for by the top oil temperature measurement?

Depending on the transformer and LTC design, the theoretical hot spot may be in either winding or may shift from one winding to the other as the LTC operates. Our temperature monitors are programmed with the parameters for the higher rise for the worst case combination of DETC and LTC positions. Perhaps the original specifier cared about a few theoretical degrees?

 

[prc]

There are several models of these winding temperature indicators. Basically these are slaves which will show what is programmed for. The CTs will be with current ratio, full load current /2 or 5 A. CTs may be provided in only one winding or on both HV and Lv winding. CT secondary will show current proportional to load current.This CT current will be heating a coil inside the temperature sensing bulb on top of transformer. Hence the coil heating will add temperature over and above the top oil temperature.There will be a graph heater current Vs the gradient over the top oil tempearture.From the temperature rise test, we can find out the hot spot gradient( Hot spot temperature -Top oil temperature) of each winding.The current throuh heater will be adjusted by a rheostat so that the gradient will correspond to hot spot temperature gradient from test at full load current.

In some other models the CT current will heat a bellow in the instrument so that the gradient will add up to the bourdon tube movement to show hot spot over top oil which will be picked up by a top oil sensor.

When only one WTI is used the maximum hot spot from tests will be used for calibration.