Energy measurement errors, requirements for measuring transformers

Choice accuracy class meters depends on the purpose, the method of inclusion and the type of energy measured (active or reactive).

By purpose, measuring devices can be divided into the following categories: calculated and intended for technical (control) accounting, and by the method of inclusion — to meters with direct connection and connected through current and voltage measuring transformers.

The accuracy class of direct connection meters must be at least 2.5 when measuring active energy and at least 3.0 when measuring reactive energy. For measuring instruments connected through measuring transformers, the accuracy class for measuring active and reactive energy must be at least 2.0, respectively, for technical measuring instruments — at least 2.0 and 2.5

When measuring high power, it is recommended to use calculated active power meters of class at least 1.0, reactive — at least 1.5.When working with meters, current and voltage measuring transformers must have a class of at least 0.5 (current transformers of class 1.0 are allowed to be used, provided that their actual load error in the secondary circuit is not more than 0 ,4 Ohm does not exceed the permissible error for current transformers of class 0.5); for work with meters for technical accounting, it is necessary to use transformers of a class not lower than 1.0

The load on the secondary circuits of the measuring transformers should not exceed the nominal load for a given accuracy class. As a result, it is assumed that the resistance of the connecting wires supplied to the secondary circuit of the transformer is no more than 0.2 Ohm. The smallest permissible cross-sections of connecting wires, calculated from these considerations, are given in tableitze.

Length of the wire at one end, m

to 10

10-15

15-25

25-35

35-50

The smallest section of copper wire, mm2

2,5

4

6

8

10

Direct meters read directly in kilowatt-hours or kilovolt-ampere-react-hours.

For meters connected by means of current and voltage measuring transformers and for universal transformer meters intended for inclusion by means of measuring transformers of any transformation factor, the readings are multiplied by the coefficient k = kt NS kn, where knt and kn are the transformation coefficients of current and voltage transformers.

The readings of transformer meters intended for inclusion through meter transformers with a given transformation ratio are not multiplied by the factor.If such a meter is switched on through measuring transformers with transformation ratios other than those specified, then its readings are multiplied by

When switching on the measuring devices through current transformers, it is strictly forbidden to install fuses in the secondary circuit of the transformers. It is recommended to ground the housings as well as the secondary (of the same name) terminals of current and voltage transformers.