Selection of Tap Changer on Transformer
Selection of Tap Changer on Transformer
(OP)
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.
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.






RE: Selection of Tap Changer on Transformer
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.
RE: Selection of Tap Changer on Transformer
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.
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If we learn from our mistakes,
I'm getting a great education!
RE: Selection of Tap Changer on Transformer
RE: Selection of Tap Changer on Transformer
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.
RE: Selection of Tap Changer on Transformer
RE: Selection of Tap Changer on Transformer
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.
RE: Selection of Tap Changer on Transformer
RE: Selection of Tap Changer on Transformer
Sarg
RE: Selection of Tap Changer on Transformer
RE: Selection of Tap Changer on Transformer
RE: Selection of Tap Changer on Transformer
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.