7/27/2019 The Application of 600 Volt Class Current Transformers.pdf

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Fall 2002 1

ANSI medium-voltage switchgear commonly uses 600-volt class

window type current transformers in medium-voltage circuits.For many years manufacturers have installed 600-volt class cur-

rent transformers in medium-voltage switchgear. The preferred location

for the current transformers is the circuit breaker spouts.

The Application of 600 Volt Class

Current Transformersin Medium-Voltage Switchgear

Tech Brief 

 Jim Bowen,Technical Director 

Powell Electrical Mfg. Co.

Table of BIL Tests on 6.5 Inch Bus MountedWindow Type Current Transformer 

Insulation

Material

Number of

Bus Bars

Per Phase

and Size

(inches)

(1) 1/2 x 3

(1) 1/4 x 4

(1) 3/4 x 4

(2) 1/2 x 3

(2) 1/2 x 4

(2) 3/4 x 4

(1) 1/4 x 4(2) 3/4 x 4

(3) 3/4 x 4

(sandwiched)

Standard

Current

Transformer

Increased

Dielectric

Current

Transformer

Hipoxy 2000®

GE-Noryl

Scotch BBI-4A

Passed

Failed @ 82 kV

Failed @ 78 kV

Failed @ 94 kV

Failed @ 74 kV

Failed @72 kVNo Test

No Test

No Test

Passed

Passed

Passed

Passed

Passed

Passed

PassedFailed @ 94 kV

Passed

Note: Standard current transformers would be suitable for allswitchgear designs requiring 60 kV.

In order to certify the dielectric capability of theequipment, the switchgear type tests are performedwith the maximum number of current transformers

mounted on the circuit breaker’s feed through bush-ings. A typical configuration of the switchgear withthe smallest air gaps is then subjected to the series of  basic impulse level (BIL) tests required by ANSI. Thesuccess of the BIL testing in the area around the cir-cuit breaker feed through bushings relies on the fol-lowing combination of insulation:

• The solid dielectric of the circuit breaker’s feedthrough bushings.

• The air gap between the conductor and spout.

• The air gap between the spout and the case of thecurrent transformer.

• The 600 volt class insulation of the current trans-former.

This series combination of various insulating me-diums provides an insulating system that goesthrough the dielectric testing along with the entireswitchgear system. During the power frequency test(high potential) and the impulse test required in ANSIC37.20.2, it is proven that the system dielectric strengthexceeds the nominal BIL rating for the voltage class.

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2 NETA WORLD

Bus-Mounted Current TransformersOccasionally the client’s current transformer re-

quirements exceed the physical mounting capacity of the circuit breaker spouts. In these cases it becomesnecessary to add additional window type currenttransformers. These additional current transformersare mounted in the cable compartment around therun-back bus.

Since these are not standard components, they do

not get included in the switchgear BIL testing. Tomaintain the BIL of the equipment, an industry ruleof thumb for both factory and field installations re-quiring bus mounted window type current transform-ers is to allow a one inch air gap between the insu-lated bus and the current transformer housing.

Powell went through the process of testing the oneinch air gap rule of thumb with various bus insulation systems that we use in the manufacture of thswitchgear. We have established an internal matrithat applies to the various insulating systems, the associated standard current transformer, and a specia“increased dielectric” current transformer. The tabl below indicates what configurations were acceptablwhen tested for 95 kV BIL.

 Jim Bowen graduated from Texas A&M University in 1976 wita BSEE. He has worked for SIP Engineering as a power engineeand for Exxon in all facets of electrical engineering in the petrochemical process. He held the position of regional engineer foExxon Chemicals Europe for three years. In January of 1997, Jim

 joined Powell Electrical Manufacturing Company as TechnicaDirector, providing leadership, training, and mentoring to botinternal and external electrical communities.