Thursday, June 8, 2017

Case Study: Investigation into Combustible Gases in Selector of Tap Changer

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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