20190924Avista to Staff rpt-Chas Raymond CS2 10-07b.pdf

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120 Prospect Street
Ballston Spa, NY 12020
Phone [phone]
FAX [phone]
E-mail [e-mail]

Charles T. Raymond, P.E.
Transformer Inspection
Coyote Springs Areva GSU
213/284/355 MVA
Areva S/N 316322
In-Service Gassing
Teardown Investigation

Charles T. Raymond, P.E.
October 29, 2007

2
Summary
This transformer developed combustible gas in excess of industry standards while in
service. The levels were initially detected by the Severon gas monitor. Because of the
high levels of acetylene and ethylene, imminent failure of critical insulation was
highly probable.
A spare transformer was installed and internal inspection and testing of the subject
transformer failed to disclose any evidence of a source of the gassing that could be
addressed and corrected at the site.
No facilities have been identified in North America that can effectively and
economically rebuild this transformer for less than the cost of a new transformer.
Based on this and the prior experience with this transformer, it was decided to scrap it
and purchase a new transformer as replacement.
The salvage dealer agreed to disassemble the transformer in a manner that would
allow reasonable forensic examination of the transformer insulation.
Examination of the winding insulation revealed extensive damage of the critical
winding insulation between the H1 line lead and the regulating winding which is at
the neutral end of the winding. This damage was due to partial discharge that was
particularly intense beneath the lower regulating winding.
This partial discharge was most probably due to manufacturing variations in the
spacing of the cooling ducts within the insulation system. The spacing of the ducts is
critical and fill shims were found in the area of partial discharge that would indicate
excessive variation in oil duct width. These dimensional variations would cause
increased local dielectric stresses and as well as reduction in dielectric withstand
capabilities from design values.
The transformer would not have been serviceable without a complete rebuild to
replace the damaged insulation and correct the underlying cause in all legs. It is
further noted that this transformer was very near to experiencing a complete
insulation breakdown from the incoming line to the grounded neutral in the regulating
winding. This breakdown would have resulted in an arc within the transformer that
would have a current magnitude of that experienced in the 2002 failure of the
duplicate transformer. Based on that experience, tank rupture and fire could be
expected to result.

Charles T. Raymond, P.E.
October 29, 2007

3
Background
This transformer was manufactured in Gebze, Turkey in 2002 as a spare for a
duplicate transformer that failed when first energized and was scrapped following an
intensive failure investigation.
The history of this transformer includes
1. A failure during factory impulse tests,
2. Shipping damage that required factory disassembly and repair at the ESI
repair facility in Westminster, CA.
3. An insulation failure in service that required return to Gebze, Turkey for
repair (this failure was evidenced by the development of combustible gas).
After more than a year in service, it again developed evidence of gassing in service as
detected by the Severon gas monitor. Of particular concern were the levels of
ethylene and acetylene. These gases indicated a high probability of insulation
degradation.
On site testing and inspections followed to assess the condition of the transformer
prior to final disposition.

Site Inspections
Scott Wilson and Sara Koeff supervised on-site tests and made internal inspections
after the oil was removed. I was provided results of the tests and a video of these
inspections as well Mr. Wilson’s descriptions as conveyed through phone
conversations.

The following is a summary of my opinions leading up to this teardown:
1. The makeup of the combustible gases indicated a high probability of
insulation failure.
2. Based on the history of this transformer and its prior duplicate,
continued operation of this transformer with these high gas contents
was not advisable.
3. The in-tank inspections did not indicate any evidence of a problem
that could be identified and corrected without going through
extensive disassembly in a transformer facility.
Charles T. Raymond, P.E.
October 29, 2007

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4. The transformer, if properly constructed, should have been capable
of performing in the service conditions encountered including
operations in back-feed from the 500kV system. This was
investigated in detail previously by Dr. Degeneff both theoretically
and in conjunction with transient measurements with the Areva
provided transient measuring equipment.
5. The transformer could not be considered operational without a
thorough teardown investigation. This would undoubtedly have
necessitated a complete rewind to restore a reasonable level of
serviceability.
6. Because of the cost of a teardown, investigation, and rewind,
replacement was considered a more economical and reliable
alternative especially with the problems encountered with this design
and the absence of any qualified repair facilities willing to undertake
a redesign and rebuild.

Charles T. Raymond, P.E.
October 29, 2007

5

On-Site Teardown Investigation
The Teardown of the transformer was begun on the Coyote Springs site beginning on
October 23, 2007 with the forensic investigation continuing until October 26, 2007.
The tank was removed by burning off the bottom seam weld and lifting the tank. This
operation was completed on October 25, 2007.
On October 26, 2007 the high voltage (HV) lead entrance tubes were removed from all
three (3) phases.

Fig. 1 - Evidence of Partial Discharge at H1 HV Lead Entrance

With the lead entrance shield tubes removed, evidence of tracking could be seen
within the insulation assembly. This was seen on H1 (see Fig. 1) but not on H2 or H3.
The shield tubes themselves did not have any evidence of tracking on the ends or
surfaces.
This tracking was not visible with the shield tubes in place.

Charles T. Raymond, P.E.
October 29, 2007

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Fig. #2 – Extensive Partial Discharge Under H1 Weidmann Separation Rings

The upper regulating windings and Weidmann separating rings were removed.
Extensive partial discharge tracking was evident on the surface of the insulation
barriers that are assembled concentrically under the Weidmann rings.
There was no evidence of tracking on the Weidmann rings themselves.
It was also noted that the Weidmann rings were made up of four (4) radial sections of
equal thickness. The original design was made of a single thickness of Weidmann
transformerboard.

Charles T. Raymond, P.E.
October 29, 2007

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Fig. #3 Intense Partial Discharge Under H1 Regulating Winding

The regulating windings were removed.
An area of intense partial discharge was located on the insulating cylinder under the
lower regulating winding. The cylinder was burned through in an area approximately
.75” across.
The intensity of the heat damage in this location indicated that the partial discharge
activity most likely started at this location.

Charles T. Raymond, P.E.
October 29, 2007

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Fig. #4 – Close-up of Intense Partial Discharge Under H1 Regulating Winding

Close examination of this partial discharge damage revealed a fairly uniform area
where the insulating transformer board was eroded by heating. The carbon traces on
the cylinder surface indicated an upward plume of gases, carbon products and ionized
oil.
These contaminants most probably caused the partial discharge activity noted on the
insulation around the lead entrance.

Charles T. Raymond, P.E.
October 29, 2007

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Fig. #5 – Pattern of Partial Discharge Tracking Under H1 Lead Entrance

The pattern of partial discharge tracking was found around the lead entrance at
multiple locations on the insulating cylinders above the lower regulating winding.

Charles T. Raymond, P.E.
October 29, 2007

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Fig. #6 – Partial Discharge Tracking on H1 Insulating Cylinder

There were some areas of partial discharge tracking on the insulating cylinders that
extended around to the low voltage side of the winding.
This activity was most likely progressing at the time the transformer was removed
from service and was caused by disruptions in the electrostatic field by the partial
discharge on the HV front previously noted.

Charles T. Raymond, P.E.
October 29, 2007

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Fig. #7 – Additional Fill Shims Added to Oil Duct Spacers in Area of Tracking

Fill shims were found under the cylinder on the regulating winding where the intense
partial discharge erosion was noted.
These fill shims were added to fill gaps between the insulating cylinders and the
support spacers. This results in variations in the radial dimensions of oil ducts in the
insulating structure.
It is a well established principle in the design of composite oil-cellulose insulation
systems that the dielectric strength of an oil volume (volts/mm) decreases as the
dimension of the oil path in the direction of the dielectric stress increases. The goal in
a composite structure such as employed here is to keep the duct radial dimensions
uniform and limit or control the duct thickness to some particular value depending on
the design. This limit is typically 7 mm (.28 inch).
The fill added here was not uniform and served to decrease the dielectric strength of
the duct at critical locations. This would also have the effect of disrupting the
electrostatic field and increasing the stress in the solid insulation that is in series with
the oil duct.

Charles T. Raymond, P.E.
October 29, 2007

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Fig. #8 – Tracking on H1 Lead Entrance Insulation

Tracking was found on the lead entrance snouts around the H1 lead.
This was most probably a consequence of the insulation degradation products and
ionized oil from the partial discharge activity below it.

Charles T. Raymond, P.E.
October 29, 2007

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Fig #9 – Typical Tracking Found on H1 Pressboard Insulating Cylinders

Evidence of partial discharge tracking was found throughout the solid insulation
indicating the progression of breakdown throughout the insulation system between
the H1 lead entrance and the regulating winding.

Charles T. Raymond, P.E.
October 29, 2007

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Fig. #10 – H1 HV Winding on Left H2 Regulating Windings on Right

The insulation under the regulating windings was removed down to expose the high
voltage winding.
No evidence of partial discharge or other stress defect was found on the high voltage
winding itself.

Charles T. Raymond, P.E.
October 29, 2007

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Discussion / Conclusions

This transformer was definitely not serviceable at the time it was removed from
service. A very significant risk of sudden catastrophic failure clearly existed based on
the insulation degradation and experience with prior failures at this station.
The construction of this winding arrangement requires assembly of the regulating
windings individually above and below the lead entrance after the high voltage and
low voltage windings are completed. Sliding these windings and the Weidmann rings
over the underlying insulation structure is difficult without providing clearance which
must then be taken up with fill shims. Extreme care must be exercised to keep the
shims to a minimum and make sure they are uniformly placed. This care has been
insufficient on this transformer.
There is no evidence that operating this transformer in the “back-feed mode” when
not generating contributed to the failure. Concern for this operating mode centers
around transfer of transient over-voltages from the high voltage winding to the low
voltage. Failures that might be experienced in this mode would be in the low voltage
windings and possibly from low-voltage to ground. There has been no evidence of
this in any of the problems experienced in the transformers at CS2. The problems to
date have all involved insulation breakdown between the high voltage lead entrances
and ground.

C.T. Raymond
10/29/07