Failure of Transformer Cable box A case study

Transcription

1 Failure of Transformer Cable box A case study Presented by Janardan Choudhary, Executive Director (O&M) 1

2 INTRODUCTION The 280 MW Dhauliganga Power Station, NHPC Ltd, situated at remote place of Pithoragarh in Uttarakhand, India, was commissioned in 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). 2

3 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. 3

4 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 4

5 Damaged and detached XLPE Cable XLPE Cable 5

6 XLPE cable detached from Oil to Air Bushing installed at HVCB 6

7 Damaged Flexible link inside the HVCB 7

8 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 DGA of Transformer Oil was due to mixing of HVCB oil. Gases After Before o So faulty transformer along with bushing fault 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. (ppm) fault 1. H₂ H₂O CO₂ CO C₂H₄ C₂H₆ CH₄ C₂H₂ Total down time was 21 days (including two weeks taken for arranging modified flexible link). 8

9 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 9

10 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.. 10

11 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 11

12 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. 12

13 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. 13

14 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. 14

15 THANK YOU 15