meter retrofits 2

[GOTWW]

We are starting to investigate replacement of indicating KW meters with new digital KWH meters, and adding new KWH meters where only Amp meters were before (by tieing into the PT bus). These are to be installed in substation instrumentation cabinets.

These meters share the CT's used with older induction disk relays 50/51's mainly.

Question 1. Will a recalibration of the relays be required due to the CT burden changes?

Question 2. For AMR, telephone line sharing is the desired approach. Any special requirements on running the phone lines through the cabinents?

 

[busbar]

One approach is to disconnect CT leads and inject 5 amperes in the installed “load”/burden, and see what kind of voltage and phase angle is seen by the CT.

Telecomms guys freak out when they hear of dialtone lines run into switchgear. A compromise is to run pairs through Bentley-Harris 7500V silicone-rubber/glass “motor lead” sleeving. {Their brick-red color kind of stands out.}

 

[GOTWW]

I believe that there are 3-CT's Star connected (judging from the 4-wires going to the ammeter switches). Are there test sets used to inject the 3-5A phase currents at the required phase relationship to perform the test? The meter salesman has indicated that the new meters are such low burden as this is not an issue.

 

[scottf]

I'd agree with the meter salesman. If it's a modern electronic meter, the added burden will be insignificant to the CTs protection performance.

If you're using these CTs for revenue metering, make sure that the CTs also have a metering accuracy and that the total connected burden does not exceed the nameplate metering accuracy burden listed. With electro-mechanical relays connected in the same circuit, the CT's metering burden rating may very well be exceeded, meaning that the CT's may not be that accurate...at least not within the normal 0.3 metering class.

 

[RRaghunath]

If the protection CTs you have are 5P20 class, then these CTs will have an accuracy of class 1 in the metering range and may be OK for your purpose.

Burden also is not an issue.

The problem could be with the meter ability to stand high current that flows in the secondary of the CT during system faults and can be as high as 20 times the rating or even more. The meter should be able to stand such high current, may be, for 0.5seconds (typical fault clearing time). This would not be an issue when the meters are connected to metering class CTs as these are designed to saturate at little over the rated current and thus protect the meters.

Generally speaking, the present day meters are suitable for connecting to the protection class CTs, but I would confirm before actually doing it.

 

[GOTWW]

Thanks rraghunath, That pretty much kills the idea of using the substation relay CT's. Now I am stuck to using pole mounted primary metering for most cases, which looks expensive and requires outages. This is not going to be much fun, but better than getting burnt from 5% accuracy, or blown out meters (testing the relays, or fault conditions).

 

[stevenal]

Regarding the blown out meters, this seems to be related to whether your equipment is built to ANSI or IEC standards.
thread238-88638

 

[GOTWW]

I think the bushing slip-over CT's will solve the problem in some cases, the next is the pesky PT's and the fusing requirements.