Measuring Riddle No.7 - Effect of burden of CT/VT on measuring accuracy
Will it affect the accuracy of CT or VT if we connect a lower burden than rated to a CT or VT for tariif meter.  What is the minimum percentage of burden to be connected to maintain stated accuracy?. For example if I have 30VA CT and if I connect 2VA to it whether the accuracy will be lower than stated.  Similary for VT also, If I connect lower burden will it affect measruing accuracy?

For a 66kV Tariff meter we had connected 2VA burden to a 30VA, 0.2S Class CT, but our client asked to change the burden to 10VA telling minimum connected VA shall be 20%.  Is it really required to change

In riddle No.1 it is given as the %age of rated burden, but in the %age of current is mentioned in the table.  I think the current is the matter not the burden. Please clairify?

#1
Mon, November 28th, 2011 - 10:14
Generally the accuracy limit factor of measuring transformers is related to amount of transformer magnetizing current , therefore working point of core magnetizing curve influence the error. Also working point of magnetizing curve is related to secondary load (Is.Zs) or voltage.

The primary current contains two components:

An exciting current, which magnetizes the core and supplies the eddy current and hysteresis losses, etc.
A remaining primary current component, which is available for transformation to secondary current in the inverse ratio of turns.

The exciting current is not being transformed and is therefore the cause of transformer errors. The amount of exciting current drawn by a CT depends upon the core material and the amount of flux that must be developed in the core to satisfy the output requirements of the CT. that is, to develop sufficient driving voltage required, pushing the secondary current through its connected load or burden. This can be explained vectorally in Figure below.

According to relevant standards, measuring accuracy defined for specified region of secondary current (burden). For tariff measuring class this region is very wide and is acceptable between 1%-125% rated burden.

Definition and standardised values as per IEC 60185 and DIN VDE 0414-1:
Measuring core rated output: 2.5 – 5.0 – 10 – 15 – 30 VA; burden output factor cosφ= 0.8

Classes: 0.1 – 0.2 – 0.5 – 1: valid in the range of 25 % and 100 % of the rated burden.

Classes: 0.2 s and 0.5 s: For special applications (electrical meters that measure correctly between 50 mA and 6 A, i.e. between 1% and 120% of the rated current of 5A)
3 – 5: valid in the range 50% to 100% of the rated burden

#2
Sat, September 8th, 2012 - 12:36
sir,

CT ratio is 50/ 5 A, having only metering core, class 0.5 rated CT burden 10 VA.

burden of Energy meter 2 VA, wire used 2.5 mm2, LENGTH = 3 FEET ONE WAY.

If connected burden is 3 VA, will this effect CT accuracy IF YES THEN BY HOW MUCH.

#3
Sat, September 8th, 2012 - 18:13
2 VA/10VA=%20
3 VA/10 VA=%30
According to a.m. table for accuracy class 0.5, as you can see:
1- The error current for %20 to %30 of load consumption which could considered in %20 to %100 region  is %0.75
2- The phase displacement angle error for %20 to %30 of load consumption which could considered in %20 to %100 region is 45 to 30 minute.