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Wednesday, May 4, 2011

low energy reading on EIG statistical meter (part 1)

We have been monitoring the statistical meter of our customer for the past three months. This is because the energy reading is very low compared to the downloaded billing meter at the customer's end. We have been proposing that the current transformer used at our side be replaced with a current transformer of lower CT ratio. The CT ratios available in our unit are 1000:5 and 500:5. The customer has a 4 MVA power transformer at their substation with seven 350-kW motor pumps. Considering the rating of their power transformer at full load with unity power factor, their full load current will be

Power=(3)VIcosφ; at unity power factor cos φ = 1

Therefore,

4MW = (3)(69kV)I

I= 33.47 A


Since the lowest CT ratio available is 500:5, this is used for the statistical meter. Secondary current I2 will be

I1/I2=500:5

I2=(33.47)5/500

I2=0.3347A


Since the value of this current is very small, they placed an interposing CT going to the statistical meter so that the secondary current will be stepped up. The ratio of the interposing CT is 20:5.

I2/I2’=5:20

I2’ = (0.3347)20/5

    = 1.33A


The problem is the connected customer only operates at two pumps (350kW) at most. Therefore this secondary current will still be reduced.
EIG meter front

EIG Meter back

 

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