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Failure of Transformer Cable box

A case study

Presented by –

Janardan Choudhary, Executive Director (O&M)

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INTRODUCTION

The 280 MW Dhauliganga Power Station, NHPC Ltd, situated at remote

place of Pithoragarh in Uttarakhand, India, was commissioned in 2005.

4 x 70 MW power is generated at 11 KV voltage level and stepped up to

220 KV level through twelve single phase bank of generator transformer

and the power evacuation is done through two 220 KV line.

Unit is connected to a bank of 3 nos. Single phase transformer, 29 MVA,

11 KV/220 KV/√3, OFWF. LV side of transformer bank is directly

connected to generator through isolated phase busduct and high

voltage terminal of the transformer is connected to 220 KV GIS by

220KV XLPE cable through High Voltage Cable Box (HVCB).

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DESCRIPTION OF EVENT

On 22/08/2016 all four units were running on full load (280 MW).

At 13:29 hrs Unit#2 got tripped on XLPE cable differential protection of

R-phase.

The oil from the HVCB splashed & spilled all over the transformer

The XLPE cable connected to one side of HVCB got detached from the

bushing and fell apart.

The sprinkler fire protection system operated and fire got extinguished

promptly without much spreading of fire.

From the event list it was observed that PRV and Buchholz relay of

HVCB and Buchholz relay of main transformer were also operated

during the event.

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DESCRIPTION OF EVENTS- Extent of damage

Cable box, insulated copper flexible cable (connecting XLPE cable and

HV bushing of transformer), HV bushing of transformer and XLPE cable

end termination were damaged beyond repair.

Transformer also got severly damaged

Radius of outer winding, i.e. HV winding, increased due to huge

tensile stress caused by electromechanical force.

Looseness of sector and support, deformation of lower & upper

winding end support and coil clamping rings were also observed

Complete Core assembly is decided to be replaced

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Damaged and detached XLPE Cable

XLPE Cable

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XLPE cable detached from Oil to Air

Bushing installed at HVCB

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Damaged Flexible link inside the HVCB

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RESTORATION OF UNIT

The oil of the R-phase transformer of Unit#2 got contaminated with the

HVCB oil which was subjected to high energy arc.

The transformer was subject to huge electromagnetic force.

All oil from the HVCB got drained out. Therefore, after fault DGA of

HVCB oil could not be done.

DGA of main transformer showed higher hydrocarbon gas content which

was due to mixing of HVCB oil.

Total down time was 21 days (including two weeks taken for arranging modified

flexible link).

Gases

(ppm)

After

fault

Before

fault

1. H₂ 53 0

2. H₂O 32 5

3. CO₂ 894 176

4. CO 153 3

5. C₂H₄ 104 5

6. C₂H₆ 21 0

7. CH₄ 40 1

8. C₂H₂ 5.5 0

DGA of Transformer Oil

o So faulty transformer along with bushing

and XLPE cables could not be used

immediately.

o To avoid further generation/capacity loss,

the faulty transformer along with all other

affected equipment were replaced with the

spare one available at site.

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ANALYSIS & ROOT CAUSE

Detailed investigation on damaged HVCB and connection of flexible link was

checked in healthy transformer.

It was observed that the flexible copper link (connecting XLPE cable and HV

bushing of transformer) was not as per drawing.

As per drg., the end connection should form “L” shape to maintain proper

clearance to ground

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ANALYSIS & ROOT CAUSE

Existing connection: The end connection of existing link was provided

with a socket bolted with connector of bushings horizontally.

Horizontal Connection

With time, the bend at both ends near socket joints loses rigidity (firmness)

resulting into increased sag near the midpoint of the flexible link..

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ANALYSIS & ROOT CAUSE

The sag in the existing link led to the reduced clearance with ground

near halfway of the link (approx. 4 mm).

As a result, Partial discharge might have started, which got ultimately

converted into an arcing ground with HVCB bottom and failure.

HVCB bottom

side

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PREVENTIVE ACTION

Following measures were carried out to ensure the overall insulation

level of flexible copper link within HVCB :

Extra insulation (craft paper & paper board) was applied on flexible copper link to

increase its rigidity.

Modification in connection of link with the connector of the bushings.

By above measures, appropriate clearnce of link to ground (HVCB bottom) was

maintained over entire the span.

Vertical

Connection

Modified rigid flexible link

having “L” shaped end connectors

was connected vertically with the

connector of the bushings.

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PREVENTIVE ACTION

With modification in end connection and extra insulation, clearance

between flexible copper link and HVCB surface is made uniform

throughout its span

All flexible links for the balance transformers has been planned to be

replaced in phased manner. However, tightness of the flexible link at

end connection and proper clearance with HVCB bottom has been

ensured in all transformers.

The depletion of oil level over time in HVCB shall also adversely affect

the overall insulation level of flexible copper link. Therefore, regular

monitoring of oil level in HVCB and its top up as required has been

ensured.

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RECOMMENDATIONS

The transformer having oil filled HVCB must be type tested and

installation/commissioning should be done as per approved drawing.

Suitable alternative solution should be explored to minimise the use of oil

filled High Voltage Cable Box for high voltage transformer to minimise

such type of faults.

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THANK YOU