Powell Technical Brief #85 The Application of 600 Volt Class ...

Powell Technical Brief #85The Application of 600 Volt Class Current Transformers inMedium Voltage SwitchgearAugust 10, 1999ANSI Medium Voltage Switchgear commonly uses 600-Volt Class window type current transformers inmedium voltage circuits. For many years manufacturers have installed 600-volt class currenttransformers in medium voltage switchgear. The preferred location for the current transformers is thecircuit breaker spouts.To certify the dielectric capability of the equipment the switchgear type tests are performed with themaximum number of current transformers mounted on the circuit breakers cell’s feed through bushings.A typical configuration of the switchgear with the smallest air gaps is then subjected to the series of BasicImpulse Levels Tests required by ANSI. The success of the BIL testing in the area around the circuitbreaker feed through bushings relies on the following combination of insulation:• The solid dielectric of the circuit breaker’s feed through bushings• The air gap between the conductor and spout• The air gap between the spout and the case of the current transformer• The 600 volt class insulation of the current transformerThis series combination of various insulating mediums provides an insulating system that goes throughthe dielectric testing along with the entire switchgear system. During the power frequency test (highpotential) and the impulse test required in ANSI C37.20.2, it is proven that the system dielectric strengthexceeds the nominal BIL rating for the voltage class.Bus Mounted Current TransformersOccasionally the client’s current transformer requirements exceed the physical mounting capacity of thecircuit breaker spouts. In these cases it becomes necessary to add additional window type currenttransformers. These additional current transformers are mounted in the cable compartment around therun-back bus.Since these are non-standard components, they do not get included in the switchgear BIL testing. Tomaintain the BIL of the equipment an industry rule of thumb for both factory and field installationsrequiring bus mounted window type current transformers is to allow a 1 inch air gap between theinsulated bus and the current transformer housing.Powell went through the process of testing the 1-inch air gap rule of thumb with various bus insulationsystems that we use in the manufacture of the switchgear. We have established an internal matrix thatapplies to the various insulating systems, the associated standard current transformer and a special"increased dielectric" current transformer. The table below indicates what configurations were acceptablewhen tested for 95kV BIL.Powell Electrical GroupPowell Electrical Systems, Inc.PO Box 12818 • Houston, TX • 77217©2005 Powell Industries, Inc. • All rights reserved.Tel: 713.944.6900 • Fax: 713.947.4453www.powellind.cominfo@powellind.com

Powell Technical Brief #85The Application of 600 Volt Class Current Transformers inMedium Voltage Switchgearpage 2Table of BIL Tests on 6.5 Inch Bus Mounted Window Type Current TransformerInsulationMaterialNumber of BusBars per Phaseand Size (inches)Standard CurrentTransformerIncreased DielectricCurrent TransformerHipoxy-2000 ® (1) 1/2 x 3 Passed Passed(1) 1/4 x 4 Failed @ 82kV Passed(1) 3/4 x 4 Failed @ 78kV Passed(2) 1/2 x 3 Failed @ 94kV Passed(2) 1/2 x 4 Failed @ 74kV Passed(2) 3/4 x 4 Failed @ 72kV PassedGE-Noryl (1) 1/4 x 4 No Test Passed(2) 3/4 x 4 No Test Failed @ 94kVScotch BBI-4A(3) 3/4 x 4(sandwiched)No TestPassedNote: Standard current transformers would be suitable for all switchgear designs requiring 60kV.Jim BowenTechnical DirectorPowell Electrical GroupPowell Electrical Systems, Inc.PO Box 12818 • Houston, TX • 77217©2005 Powell Industries, Inc. • All rights reserved.Tel: 713.944.6900 • Fax: 713.947.4453www.powellind.cominfo@powellind.com