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Combined CT/PT Sets for Metering Application

It is a common practice of many utilities to meter the power supplied to a customer on the H.T. side so that the transformer (provided by the consumer) losses are also metered. This practice is followed for consumers who receive supply at 11 kV or 22 kV. Certain utilities also use metering sets in the cases of consumers who receive supply at 36 kV. Use of 22 kV and 33 kV combined metering sets is not recommended; as such sets are likely to be of large bulk. Besides this, there is an added disadvantage that in the vent of failure of any of the component units of the metering  set, the entire metering set may have to be out of commission. The required repairs/replacement leads to a much longer period of inconvenience to the concerned utility an dits customer, It is, therefore suggested that CT/PT sets for use on 22 kV and 33 kV system may be formed from two independent C.Ts and 3 Nos. P.Ts connected in Star/Star.

CBIP had referred to IEEMA for analysis of the causes for frequent failures of 11 kV metering sets. The matter was then fully discussed with Mr. P.K. Lal and the opinion of instrument Transformers Committee was conveyed to Mr. Lal on 2.3.94. Some of the observations made by the committee are listed below. This is likely to be of interest to the utilities as far as procurement of these sets is concerned

a)   The specifications of C.Ts and P.Ts required to be housed in the metering set are unrealistic and need to be reviewed.

b)   We recommend a burden of 7.5 VA with accuracy class 0.5/1 for C.Ts. and 25 VA with accuracy class 0.5/1 for P.Ts. (Also 1 Amp. Secondary for C.Ts if the lead length is long.)

c)   We recommend O.C.F. as 100 for 1 second instead of fault current corresponding to the system fault level. This is particularly so because the C.T. ratios are rather low and the installation of the set is as per enclosed Figure 2. It is apparent from the figure that there is almost no chance that the system fault current is likely to flow through the C.T. It has also been explained to C.B.I.P. that, by incorporating above changes, the cost of the set will reduce markedly and perhaps, the prices quoted by reputed and knowledgeable manufacturers may be competitive, which will ensure longer life for CT/PT metering sets.