The following report highlights the problem
of the production of combustible gases in the selector of the tap changer of
gen/motor transformer 1 at A Pumped Storage Scheme. This may be caused either by abnormal arcing or
a leak between the diverter and selector compartments.
From
the oil results in 2002 it was noticed that there was an increase in the level
of combustible gases in the selector tank of gen/motor transformer 1. This gas
level was monitored and a gradual increasing trend is noticed. However the
gases fluctuate around a level that is above limits. Last
year (September 2002) it was proposed that an internal inspection be carried
out in the selector to identify a possible cause for the high gassing. Due to
production reasons and an outage not being available it was decided to rather
monitor the transformer with regular oil samples (weekly to three weekly) until
the outage in April 2003 where an internal inspection of the selector and
diverter compartment is carried out.
The
first possible cause highlighted has to do with the operating limits of the
unit linked to the range of tap changing. The tap changer is operated at least
twice a day across the change over tap 9 which would result in a small arc that
could be the source of the combustible gases. Comparing the levels of gases on
all the other units, Unit 1 is at an elevated level suggesting an area of
higher levels of arcing. If this is the case we need to as soon as possible
identify and sort the problem out before it develops into an electrical fault
resulting in the failure of the transformer.
The
second possible cause is that there might be a possible leak between the
diverter tank and selector. The diverter because of its operation will produce
high levels of acetylene and combustible gases. If there is a leak between
these compartments there will be a migration of the gases into the selector.
Inspection of the U2
tap changer at the Power Station was carried out due to suspected faults as a
result of high combustible gases in the selector tank. The following is an
explanation by the OEM of the cause. It must however be stressed that the level
of the U1 gases are far above that experienced on U2.
The dissolved gas
analysis of oil samples taken from the selectors of the generator transformers
indicated hydrogen and acetylene values higher than normal. This initiated an
internal inspection of the selector switch on Unit 2 in February 1999.
This inspection
revealed some evidence of sparking on the changeover contacts of the selector.
The operations of these transformers are somewhat different from the other
generator transformers in the network.
The pumped storage operation requires generating and pumping modes using the
same transformer. This then gives rise to frequent tap changer operations and
also a different tapping range. Unlike other generating stations the tapping
range includes the changeover position on a daily basis.
During the
changeover operation the polarity of the tap winding is reversed. To allow this
to happen the tap winding is disconnected electrically. When the reconnection
takes place even when it is not under load, the capacitive coupling to the
other windings on the same phase induces some voltage in this winding. The
contact to which it then connects is at a different voltage and then a minor
arc occurs when this contact makes. This only happens to the changeover contact
and it happens in both directions – tapping up and down. It is better explained
when referring to the rating plate diagram.
Figure 1:
The changeover occurs on position
9. 9A and 9B are the transitional
positions but have the same output voltage as position 9. This is where the
polarity of the tapping winding is reversed. Consider the operation from tap
position 8 to 10. The taps occur from 8 to 9A. Then to 10 via 9 and 9B without
stopping on 9 or 9B. At the point of 9
the tap winding is completely disconnected and then connected in the reverse
polarity on position number 9B. When going back from position 10 back to 8 the
actual arc takes place between position 9 and 9A.
After it was discovered that there is an increasing trend of
combustible gases especially acetylene in the selector tank, special DGA
sampling at weekly intervals were established. This was then increased to every
three weeks when the level stabilised. While this was monitored the tapping
profile was also recorded for 17/10/2002
to 4/11/2002 .
This information is discussed below. The
next long outage on this unit was only planned in April 2003 and it was agreed
that the tap changer be closely monitored until the outage or if there were
further increases in the gas level to warrant an emergency outage. The oil
results of the selector may be found in appendix 1 and the tapping profile data
may be found in appendix 2. Figure 2 below clearly portrays the tapping profile
of gen/motor transformer 1. As was explained in section 4 above the taps move
across taps 8-10 crossing the transition tap 9 on most occasions thus producing
a small amount of arcing which results in the production of combustible gases.
Figure 2:
Figure 3 illustrates the number of taps that are changed per day for the
period 17/10/02
to 4/11/02 . The
average number of tap changes for one day is estimated at 8 taps per day. This
is considered to be frequent for a generator/motor transformer but not unusual
considering its application.
Figure 3:
Figure 4 below gives an indication of the number of taps per mode. From this profile it is evident that most taps are during generating and SCO modes.
Figure 4:
TEMPERATURE & HYDRAN
Weekly oil and winding
temperature readings taken reveal that gen/motor transformer 1 is also relatively
and consistently warmer that its other three counterparts. This increase has
been noticed from just six months now. The Hydran reading has also increased a
bit to 162 ppm were it was previously sitting at mid 130’s.
OUTAGE – APRIL 2003
During
the outage in April it is proposed that an internal inspection be carried out
to identify the cause of the high gassing. The monitoring of the gases over the
past three months gives no indication of a decreasing trend. The risk of a
possible failure would thus be considered high. If the internal inspection is
not carried out in the April outage Peaking will have to live with the
knowledge of a possible failure.
The
oil samples taken for the past three months was taken to keep track of further
increases so as pick up any problems as quick as possible. The hydran on the
transformer is only monitoring the main tank which is isolated from the
selector by a shut off valve. This further emphasises the need for this
inspection.
CONCLUSION
From
the report above it is clear that there is an arcing problem in the selector of
gen/motor transformer 1. We however need to establish the criticality and
source of this arcing. The possibility of a leak between the selector and
diverter must also be investigated so that plans can be put in place for the
replacement of the diverter barrel if such leak exists.
It
must also be noted that if the recommendations made below are followed it must
be coordinated with the conservator bag project which is scheduled in the April
2003 outage due to interfaces on the handling of oil.
RECOMMENDATIONS
The recommendations below can be implemented in the April 2003 outage.
1. Inspect tap changer (selector &
diverter) tank for source of combustible gases.
2. Test diverter and selector to
identify if there is a leak between selector and diverter tanks.
3. If tap changer is scheduled for 18
month service, carry out the service.
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