Topics: Tap changing position
on General Discussion
Tap changing position
we are frequently changing tap position of Unit station transformer due to voltage problem. what are the impacts on transformer life and is there any solution to minimize this?
09-17-2013 07:19 PM
Is it auo or manual/.
09-17-2013 09:35 PM
09-18-2013 12:26 AM
Read the manual for oil testing frequency & tap inspection frequency
09-18-2013 02:49 AM
If the transformer has a load tap changer (low side), then are you changing the high side no load tap to try and keep the low side voltage where you need it? I am not clear about the question. Load tap changers (LTC) for substations that feed cities (we are talking now about big transformers with load tap changers) will often change taps due to the varying loads. A good rule of thumb is that they should not tap more than 250 times per week. That will get you around 500,000 operations in about 40 years, which is the life span (the operations) of a typical LTC (the LTC contacts will likely be changed during this period). You may need to adjust the bandwidth and or the timer to keep the operations down to a reasonable level. If that does not work then the LTC controller may need to be replaced.
09-18-2013 04:57 AM
sir it is station transformer of 25MVA .So number of operation you told is too huge i.e:250 times per week. Because of frequent tap changing the oil in OLTC became very degrade.
09-18-2013 07:35 AM
Your original question is about transformer life expectancy, and tap changing will have little shortening effect of the life expectancy. It was after all designed with it that in mind in the factory.
Your question has now drifted onto oil degradation, that should not be related to tap changing, so why do you think so ? Your oil is being contaminated, moisture usually, or getting too hot. Anyway you can trace the cause- oil analysis should give you a clue.
09-18-2013 10:17 AM
Since it appears that you are talking about OLTC, then 250 taps per week is the maximum level that is reasonable in my opinion for a transformer serving varying loads, such as a city. I worked for electric utilities in the US for 23 years and looked at load tap changing counts every week for over 450 MW of transformers (15 MVA to 46 MVA all serving city loads). This count is the top end we would allow. The average count was in the 125-150 range per week (summer loads, with wide varying loads each day, winter loads caused less tapping per week). Oil does not degrade rapidly in the OLTC (that is operating properly) even with a maximum of 250 counts per week, but we would take oil samples every year of the OLTC and the transformer to keep tabs on their overall health. If the oil in the OLTC does degrade rapidly, then their is a good chance that the alignment of the taps is improper and arcing may be occurring during the tap changing.
Tony is correct, OLTC has little or no effect on the life of the transformer. Also, there are two separate oil compartments, one for the OLTC and one for the transformer.
09-18-2013 12:22 PM
Whats the make of OLTC and AVR Relay....
09-18-2013 02:25 PM
I understand your view point however utilities in Mumbai , Delhi are changing it approx. 10 to 30 times in a day and there is no problem observes. If you decide less number of operation then you also need to think the influence on the loads which are connected on the secondary side of transformer. If you need more info about special control scheme then you could inform me and I will organise it for you. It is also possible to use your capacitor bank effectively.
09-18-2013 04:27 PM
Is there an operations counter to record the number of times it has operated?.
09-18-2013 06:55 PM
OLTC (On Load Tap Changer) should be inside its own dedicated partition which is inside its own oil. It is design to have frequent tap changing activities. The effect is: you need to change the oil more frequent if you have a frequent tap changing activity.
Eventhough you manual syas -for instance: change OLTC oil every 2 years. But because you have a frequent tap changing activity, it could probably be shorten to 1 and 1/2 years, or 1 year etc. Therefore, do not believe your manual 100%. Rather, perform Predictive Maintenance on the OLTC instead of just Preventive Maintenance. Wallahu a'lam
09-18-2013 09:07 PM
Having more tap changing per week is not bad, but it wears out the tap changer faster and does require more maintenance. We set our bandwidth at 1.5 volts, 0.75 up and 0.75 down, with a minimum timer of 30 seconds (voltage has to be out of bandwidth for more than 30 seconds for tap changer to move). Voltage for the OLTC controller is based on a 120V base. This normally worked well for our city loads, but perhaps your loads vary even more. I have used a bandwidth of 2 volts maximum with good success to keep the OLTC from tapping more than I liked (250 taps per week, and naturally if your loads swing more than what we had then your taps per week are going to be higher). The 250 count per week maximum is just a goal we set to try and maximize the life of our tap changers and minimize our maintenance. Looking at your timer and bandwidth may help reduce the taps per week. When the tap count per week jumps up suddenly you can suspect the controller might be bad. One more thing, I never use the X setting, just the R. I would draw the voltage "curve" versus the current and figure out my maximum voltage based on the maximum current. This worked well for me for my 23 years of utility work (again, these are city loads, base power factor during the summer was 85%). The power factor would be higher in the winter and lower in the summer (summer at 85% and winter was over 95% because in the winter we had no air conditioning loads). That is why I did not use the X setting (one setting year round).
09-18-2013 11:45 PM
the only solution to minimize the tap changing is increase the bandwidth of the voltage controller anf fix a minimum timer between the changes. If the loads doesn´t vary so fast you can set the bandwidth at 1.3%, althougt you can increase the bandwidth
until 2,5% (depends the type of load and the limits of the Regulation).
If the tap changer has its own oil the costs of the maintenance is low, your problem will be change the oil with the transformer out of service.
09-19-2013 02:25 AM
Lets look at the basics here:
When tap changers, arc will appear. This is normal as if inside Circuit Breakers. They were meant to perform this arching operation. It is due to this arching activity the oil inside the OLTC compartment gets ware out.
Now, for these arching type equipments, what do we need to do?
1) Change the OLTC oil during outage
2) Bring out and service the tap changer during outage.
It's a matter of WHEN shall we perform such maintenance. Therefore, to answer the WHEN question, need to look at how frequent we change tap.
09-19-2013 04:37 AM
Your question still lacks many important information needed to answer your questions. Although you said it is changing taps frequently you did not state how the on- load tap changer (OLTC) is moving. Is it raising the voltage output or is it lowering the voltage output of your 25 MVA transformer. Let us know up to how many steps it raises or lower. There are usually 16 steps "raise" and 16 steps "lower" on the OLTC. How do you observe the maximum "raise" it indicated daily, and how do you observe the "maximum" lower it indicated daily? Are you reading/ logging the indicator position daily and resetting the indicator daily? Are you reading / or are you logging the voltage input to your transformer. Are you keeping the historical profile of your voltage input and load current? These are all vital information that will be needed in order to have a good analysis and solution to your voltage fluctuation problem. is there a heavy cyclic load connected to your transformer? Does your Utility company's power supply to your substation transformer reliably stable?
On the your question about the effect on the degradation of the transformer life, I do not see any serious impact on the transformer life. Your main concern will be on the life of your automatic tap changer contacts which will be eroded due to frequent changing of the contact positions. There will also be some carbonization on the insulating oil in your tap changer compartment that you may need earlier maintenance schedule than the normal period.