Selection of Tap Changer on Transformer

[jbond]

Hi,

I've had to select two 20MVA transformers for some transmission lines for a project that I'm involved in and I selected off-load tap changers because an ex-colleague told me to do it before he left. These transmission lines will be feeding an old substation which in turn supplies the power to a small town.

The thing is, I don't know much about off-load tap changers. I presumed that if the transformer were to change tap, then it would have to do it while the load is on, because how can it disconnect to change the tap in this scenario? Won't the town lose all its power for a split second? (or however long it takes for an off-load tap changer to change it's taps - how long is that by the way?)

Your help is much appreciated,
Matt.

 

[RalphChristie]

On-load tapchanger - Change transformer taps while load is still connected. (Automatic or manual)
Off-load tapchanger - Change transformer taps when power is off.

I rather call an off-load tapchanger a off-circuit tapchanger, and NOTE: never change taps while a transformer with an off-circuit tapchanger is still energized.

In my opinion an off-circuit tapchanger is many times a waste of time, an extra cost and one extra piece that may fail. The voltage level is set when you put the transformer into operation, once set, the transformer may remain at that setting for the reminder of its operating life. (if you want to change the voltage, you have to put the transformer off, change taps, then put it back into service.)
If the voltage fluctuates a lot due to the load, I'd rather add an automatic on-load tapchanger, then the tapchanging can be done automatically, while the transformer is into service.

 

[ScottyUK]

mhumble,

Please take Ralph's warning about never operating off-circuit tapchangers while the transformer is live with the utmost seriousness. A gross breakdown of procedure and some contributing mistakes led to this exact event occuring at our site. The generator auxiliary transformer blew up after the tapchanger disintegrated under the enormous stress of shorting two taps together. Three colleagues died as a result; the fourth survived with massive injuries.

"I selected off-load tap changers because an ex-colleague told me to do it before he left" - you need to understand what you are specifying before you spec it, or get someone else to spec it and let you learn. The application sounds like you may need the on-load tapchanger if you can't interrupt the supply to the load when a tap change is necessary.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[tinfoil]

Off load tapchangers have their uses, but I too wish to stress the point: Off-load means off-potential as well. DO NOT OPERATE AN OFFLOAD TAPCHANGER while a device is energized if you place any value on your equipment or your personnel.

 

[tinfoil]

The reason for spec'ing off-load taps is to correct for gross long-term steady-state voltage issues on a system.

For example, some of a utility's substations may be at the end of long supply lines, where the voltage will generally be several percent less than the voiltage at the supply end of the line, for most of the typical loads seen on the line.

Having a gross ability to change the secondary voltage by 2.5 or 5 percent (or whatever the off-load taps are) moves the secondary voltage back into the middle of the desired operating range. Then downstream voltage regulating devices can make 'fine' or short-term adjustments as required.

The same transformer can be relocated elsewhere in the system, simply by adjusting its fixed turns-ratio via the off-load tapchager, and other transformers can be swapped in, assuming they have similar tap arrangements.

In addition, it is not unusual for utilities to regulate 'sending' or initial voltages to be 5% higher than nomimal, to give them more to drop as the feeders/lines extend away from the source, allowing them to reach further or load their lines more heavily before requiring additional capital investment, like auto-tapchanging devices downstream.

Customers very near these sources would normally see this 5% increase, unless off-load taps were used to bring the secondary voltage back closer to nominal.

Off-load taps to not replace the need for automatic voltage regulation, but they can make them more effective, or reduce the number of them.

 

[jghrist]

Off-load tap changers need to be set so that voltages on the secondary are within the range of the OLTC or voltage regulator, considering the possible range of high-side voltages and transformer loads. Maximum voltage on low side surge arresters also need to be considered.

 

[jbond]

Thanks tinfoil! And to everyone else also, your time is very appreciated! You helped me to understand what my ex-colleague said.

My ex-colleague gave me the reason that we are at the head-end of the supply system, so that essentially we only want to be able to tap to account for any voltage losses down our transmission lines (or something to that effect) which are about 5km (3.5miles) im length. And it makes sense to me that the on-load tap changers occur closer to the consumers.

Thanks for your help everyone.
Matt.

 

[bofu2000]

You never operate an off load tachanger with the transformer energized. The tap changer contacts are not made for thatAn off load tapchanger has its own advantages like low cost and little or no maintenance.

 

[Sargardani]

Just to add to the list of precautions ... personally I would feel comfortable in energizing the transformer, after each tap changer operation, only if the transformer turn ratio is checked. Perhaps checking the contunity of winding won't hurt either.


Sarg

 

[vtpower]

Whenver we go into a sub to do routine transformer maintenance, usually every 3 to 5 years at a sub, we pull a mobile substation in, and de-energize the transformer, to do all necessary maintenance. At this time we also see what the regulators have been doing. If they are continously bucking, the bus voltage could stand to be lowered, and also if they are continously boosting the bus voltage could be lowered, which is why we will change the no load tap changer. The more your regulators remain near neutral the less wear and tear on the contacts. The voltage will fluctuate depending on transmission load, generation, capacitance among other issues. Over the course of 50 years, a great deal can change with a system, which can be the life of the transformers. We do not purchase a transformer without a No load tap change.

 

[bacon4life]

A few years ago we operated some of our substations with a lower than nominal transmission voltage without changing the No Load Tap Changer setting, causing the On Load Tap Changers to be continuously boosting. When the routine maintenance was done on these transformers, carbon buildup was discovered on some parts of the On Load Tap Changer. The proposed cause was that the contacts never swept through the Neutral point.

 

[mc5w]

One of the other purposes of a deenergized tap changer is if you have some say 22,000 volt subtransmission and say 14,400Y42,940 volt distribution. Several decades ago Duquesne Light Company in Pittsburgh, PA upgraded their subtransmission from 22,000 delta to 14,400Y24,940 volts. They did this raise rather gradually so that the 2400Y4160 volt substations could cope with increased voltage until their primary taps were changed. This happened when they added a

Their tariffs list the newer voltage as 13,200Y23,000 volts but the distribution transformers have the primary taps at 14,400 volts in case the line voltage is a little high due to line drop compensation. The secondary voltages are listed in their tariffs as 115, 230, 460 and so forth. This seems to be a safety factor because I have seen a power quality study for an Ohio Edison customer where their 277Y480 volts was experiencing voltage swells on the order of more like 290Y502 or 295Y510 volts which was blowing out electronic ballasts. The electronic ballasts were supposed to take continuous exposure to 305 volts but a design flaw made that rating bogus.