ABB KOFA MEDIUM VOLTAGE PRODUCT Indoor current transformers

ABB KOFA  MEDIUM VOLTAGE PRODUCT  Indoor current transformers

Technical parameters Value

Rated frequency 50; 60 Hz

Rated secondary current 5 (1.2) A

Rated burdens 2.5; 5; 7.5; 10; 15 ; 20; 25; 30 VA

Accuracy class 0.2; 0.5; 1; 5P10; 5P20

Mechanical strength

of primary terminals

5 kN

Standards other data

will be offered on

request

IEC, BS, ANSI, ...

Description

The primary winding, core and secondary winding

of the transformer are encapsulated in cast-resin.

Secondary conductors

The fixing screws in the light alloy base are positioned

within easy reach also from above. The

conduit used for the secondary conductors can

be fixed into a U-shape slot entry in the secondary

terminal box. This design obviates the drawing

of the secondary conductor through an entrance

hole into the terminal box.When specified

on order the box can be provided with a threaded

entry for cable termination accessories. One

2.5...10 sq. mm or two 2.5...6 sq. mm (in case of

three core transformer one 2.5...6 sq. mm ) conductors

can be directly connected to the secondary

terminals. The terminal cover is sealable. Degree

of protection by enclosure is IP 30.

Primary terminals

M12 size screws are used in primary terminals.

Primary connection can be changed without removing

the already fitted primary conductors.

The transformer specification can in such a case

be for example:

Type KOFA 24 D2

Service voltage 17.5 kV

Test voltages 38 kV 1 min./95 kV impulse

The normal transformer type KOFA 24 D2 would

have been suitable if used below 1 000 m.

Permissable torques for screw connections

M5 max 3.5 Nm min 2.8 Nm

M8 max 20 Nm min 16 Nm

M12 max 70 Nm min 56 Nm

Environment and altitudes

The KOFA transformers are made for indoor

mounting. IEC recommandation gives requirement

for ambient temperature between -5°C and

+40°C. The transformers, however, may be used

at a temperature as low as -40°C. The transformers

must be protected against unusually

heavy deposits of dust or similar pollution, as well

as against direct sunshine.

In mountaineous areas the height above sea level

may be higher than the height specified in the IEC

standards as design criteria (1 000 m). The thin

air at high altitudes affects the cooling of the

transformers as well as the required creepage &

free air distance to earth and between phases.

The manufacturer should be consulted in such

cases.

The problem with creepage and free air distance

can be solved by choosing a transformer type designed

for higher voltages. The requirement for

impulse test voltage (BIL-level) should preferably

NOT be higher than normal for the service voltage

used as this may have a negative effect on the

available output from the secondaries.

Isulation levels for KOFA models

12/28/75 kV

17.5/38/95 kV

24(25)/50/125 kV

Description for preferred transformers

A limited selection of the KOFA-type current

transformers are available as preferred types,

which gives two advantages:

1. Having factory documentation and material

ready for the most frequent ratings thus offering

short delivery times for urgent needs.

2. Simplifying the choice and ordering by giving

each standard c.t. a specific order code. On ordering

transformers according to the table below

please give only the order code, observing the

voltage code.

Available preferred current transformers:

Type: KOFA 12 D 2 for 12 kV

KOFA 24 D 2 for 24 kV

Secondary current: 5 A

Number of cores: 2

Weight: 17 and 19 kg

Standards: International standard

IEC 60044-1

Normally not labelled: Rct (winding resistance at

75°C), Uk (knee-point voltage)

and Io (exciting current).