Use Of On Load Tap Changers

On Load Tap Changers (OLTC) are just what the name implies. They are used to vary the turns ratio while the transformer is in service to maintain a relatively constant output voltage. In most cases they are located in the lower voltage side of the transformer. They are severe-duty switching systems, requiring arcing contacts and bypass contacts to ensure continuity while switching from one tap to another. The voltage change from one tap to the next is typically less than 1%, with many (30+) taps to provide a reasonable range of operation. OLTC's usually operate automatically to compensate for changes in supply voltage and line sag on the secondary side as load increases.

Off Load (or No Load) Tap Changers (NLTC) are not designed to be operated unless both sides of the transformer are deenergized. The operating handle is padlocked to prevent damage due to inadvertent operation. NLTC's are much lighter construction, since they don't need to make and break any voltage, and are usually installed in the higher voltage side. NLTC's consist of a few, widely spaced taps. Most have 5 taps, at +5%, +2.5%, 0, -2.5% & -5% of nominal voltage.

For transformers with both an OLTC and a NLTC, the NLTC acts as a "coarse adjustment", providing a centerpoint around which the "fine adjustment" OLTC operates.

Dear pwr2thepeople,

Your post is great. And the sentence "line sag on the secondary side as load increases" is very good. Such a sag always comes up when the receiving point (load center) is far away from the transformer.

But,

you said:

"In most cases they are located in the lower voltage side of the transformer"

Our 21MVA, 66/6.6kv power transformer (Bosch) has an on load tap changer (MR) installed on the high voltage side, and I thought that tap changers are always installed on the high voltage side.

Am I right? and if not, would you pls. guide me to a web site to illustrate this point?

Thanks

Samak

You are correct on the tap changers location. the are on the hi side of a transformer.

Samak,

I have (15) 25MVA, 67/13.8kv and (2) 25MVA 132/13.8kv power transformers in service, all with OLTC's on the low side. Here is a brochure for one type of OLTC I use. Note that the maximum voltage rating is 15kv. All of my OLTC's are separate tanks mounted on the side of the transformer main tank. Although I understand that there are OLTC's available which are inside the main tank, I've never seen one.

The original question was for a 500/220kv transmission autotransformer. The separation distances to prevent internal arcing between taps on 220kv would be rather big. On the 500kv side, the OLTC tank might need to be larger than the transformer itself (just a guess).

I suspect your 66kv tap changer gets a lot of wear & tear. The voltage difference between adjacent taps on my 33-step OLTC's is 86V (5/8% of 13,800). For a similar device installed on the 66KV side, the voltage difference would be over 400V. That's a lot of contact arc during tap movement. If your transformer supplies a single facility with a relatively constant load, the tap changer probably doesn't move a lot so contact wear would not be an issue. Mine supply a wide variety of residential, commercial and industrial loads, so the load may vary 50% or more over a 24 hour period. The tap changer might mover from the #8 lower tap to the #6 raise tap twice in that period. Even so, I only inspect the OLTC's once every 5 years, and seldom have to do more than clean up the contacts with emery cloth.

Regards,

Tony

Tony,

Thank you for your reply.

My 66/6.6 kv transformer has the following OLTC:

mr maschinenfabrik reinhausen.
fab.no. 137 348 1989.
type v III d 200 - 060 - 10193 g 8.5 ohm
typ ma 9 - 137 348
pot 16x10 ohm
VDE 0532, IEC 214

The technical data from site of MR http://www.reinhausen.com/mr/en/file/download/t/download/id/2057/name/22802010en.pdf

says, for (V III 200 D) oil-tap, the max. rated step voltage U_im is 1500V (i.e. greater than 400) and the highest voltage for equipment U_m is 123/76KV.

Also,I believe that, the decision of which side the OLTC should be installed is a mater of current not voltage (precisely the power). That is, OLTC should be installed in the side of lower current not lower voltage.

What do you think?

Regards...

Samak

The Tap change switch is for getting secondary voltage in required limit by varying input voltage of primary voltage on + or - with variation of 1.25 % or 2.5 % ., usually selected with the details of variation in voltage of grid power

This in case OLTC can be local and / OR remote as per requirement

nascon

I would expect that the OLTC would always be on the side of the incoming voltage regardless of the current. The amount of current required to be switched would dictate the capacity of the tap changer.

For an OLTC to function properly, it should be on the incoming voltage side of the transformer and the voltage sensing being done on the load side of the transformer.

Normally, the load side (down stream) is the low voltage side.

How it will be (OLTC in which side) if the transformer is a step up one.

My experience is (OLTC or no-load TC is always connected to the high voltage side).

What do you think?

Samak

Samak, I agree with you.

My experience is (OLTC or no-load TC is always connected to the high voltage side).

There may be some in existence but I haven't seen any applications of OLTCs used on step up transformers.

Generally, step up transformers are used for power transmission and the high side voltage is not too critical. Any voltage adjustment that is required would be done at the receiving side where it is stepped down. The high side of the stepdown transformer would likely be where the OLTC is located to adjust the voltage to meet the low side distribution requirements.

Joe,

Thanks

________________________________________________________Marginal___

If you are interested/have a time, you may take a look:

http://cr4.globalspec.com/thread/13023/Reality-of-Magnetic-Actuator-CB

Samak

Samak,

Every OLTC I have ever seen or worked on (50 or so) has been on the low-voltage side of the transformer. That may be a uniquely American design. I am aware that there are many possible transformer designs, as this entire thread indicates. Although I have significant experience in operation & maintenance of tap-changing transformers, I had not paid much attention to the design criteria.

Rather than go with my initial impression, I looked for recommendations from transformer designers. Here's what I found:

The IEEE (Institute of Electrical and Electronic Engineers) standard for power transformers, C57.12.10-1997, specifies that the NLTC shall adjust the high voltage windings, and the OLTC shall adjust the low voltage windings. This standard has been adopted by ANSI (the American National Standards Institute) as the recommended standard for USA transformers.

I believe the comparable IEC standard is 60076-8 (1997), but I have not been able to obtain a copy to review. It appears that many or most of the nations in Europe, Asia and Africa adopt IEC standards as their own national standards, so my US experience with ANSI and IEEE may be a minority of the total transformer population in existence.

I have not been able to find the design philosophy as to why the tap changers are to be placed as stated. I have emails out to a few colleagues at transformer manufacturers. If I find anything more, I'll certainly let you know.

Tony

Thank you Tony, I appreciate your efforts and I hope you get some comments from your colleagues to cover this point for us.

"C57.12.10-1997, specifies that the NLTC shall adjust the high voltage windings, and the OLTC shall adjust the low voltage windings."

My comment is that the above sentence doesn't mean that the NLTC/OLTC should be installed in high/low voltage side respectively. Why!, because the adjustment of a voltage of a side is a matter of "where your voltage sensor is".

If, for example, the voltage sensor is connected to the high voltage side then your controller is installed to maintain the voltage of that side, that is off-course in case of OLTC.

However, I don't have C57.12.10 , If I may ask, can you send me an e-copy.

samak@alexandria.cc

Thank you very much, and have a nice day.

Samak

Full load (on-line) tap changers allow for automatic voltage adjustment and function as a voltage regulator. They generally have secondary voltage sensing circuits and a motor driven tap changer which will change the primary tap setting to maintain the secondary voltage witin a certain range.

Off-line tap changers require that the transformer be taken out of service or at least the load be removed before making any changes to the tap settings.

on such a high rating both in terms of KV and KVA on load tap changers have to be used as interruptions cannot be tolerated on such supplies.

Off-circuit tap changer

It is a device fitted in the transformer, which is used to vary the voltage transformation ratio. Here the voltage levels can be varied only after isolating the primary voltage of the transformer.

On load tap changer (OLTC)

The voltage levels can be varied without isolating the connected load to the transformer. To minimise the magnetisation losses and to reduce the nuisance tripping of the plant, the main transformer (the transformer that receives supply from the grid) should be provided with On Load Tap Changing facility at design stage. The down stream distribution transformers can be provided with off-circuit tap changer.

The On-load gear can be put in auto mode or manually depending on the requirement.

OLTCs provide fine (much closer than 1%), automatic control. They are needed at plants where voltage variation affects production, such as plants producing high speed, solid state devices, or where wide voltage variations are expected. Otherwise use NLTC. OLTC will not compensate rapidly enough to overcome the effect of motor starting or other inrush.

Since the above is Auto Transformer, OLTC on Secondary Side is preferred.

Thank You

Ramana.

My experience is that NLTC or OLTCs are installed at the secondary side of Auto transformers of 500KV/220 KV and 220 KV/132 KV rating whereas the OLTCs on primary side of 132KV/11.5 KV rating power transformers.

Obviously, the OLTC is connected to the secondary of the Auto transformer. Such a case may be considered as a logical replacement to a shunt reactor on an OHTL.

However, the max. voltage for operational equipment U_m of which an OLTC can be installed is stated in the following link (MR: one of the biggest OLTC manufacturers).

U_m=362KV

____________________________________________________________________________

http://www.reinhausen.com/mr/en/products/oltc/oiltap/

OILTAP® R The on-load tap-changer for the highest voltages and transformer output rates

Max. rated through-current: 1,000A (3-phase)
3,000A (single-phase)
Max. rated step voltage: 4,000V
Max. rated switching capacity: 6,000kVA (single-phase)
Max. voltage for operational equipment um: 362kV
Application up to 1,200 A (3-phase): Star point (neutral point) (Um=245kV)
Application up to 3.600 A (single-phase): At any randomly selectable winding position (Um =362kV)
Operating positions: Without change-over selector: 18 max.
With change-over selector: 35 max.

____________________________________________________________________________

Samak

The tap changer can be connected several different ways depending on whether it is the high voltage or the low voltage side has the variation, and how the manufacturer actually designs the windings.

Many people refer to DETC (deenergized tap changers) rather than NLTC(no load tap changer). Not only does abrieviating on and off lead to an obvious problem, but furthermore the transformer must be denergized rather than just not loaded.

Dear All,

Tap changers are provided at the HV side only because at HV side load(current) is very small as compared to LV side. Secondly at HV side no. of turns are more for better voltage regulation hence Taps can be changed at HV side easily.

Can anyone tell me what is the requirement of On load tap changer, in what cases we should prefer on load tap changer instead of off circuit tap changer.

Plz reply