effect of tap changer location on transformer

7/22/2019 Effect of Tap Changer Location on Transformer

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International Colloquium Transformer Research and Asset Management

Cavtat, Croatia, November 12 – 14, 2009

Farzad Zhalefar Mehrdad KalantariIran-Transfo Co., Zanjan, Iran Iran-Transfo Co., Zanjan, [email protected] [email protected]

Jawad FaizSchool of ECE, University of Tehran, Iran [email protected]

STUDYING THE EFFECT OF LOCATION OF TAP-CHANGER SWITCH ON

MAXIMUM FLUX DENSITY OF MAGNETIC CORE

SUMMARY

This paper discusses about two major strategies of tapping power transformers, namely CFVVand VFVV. In CFVV strategy which is more common, turns number of tapped winding changes so thatsmoothes the voltage across the un-tapped winding. In such strategy, peak value of flux density ofmagnetic core should not change considerably. However, in VFVV this variable would change.

For the case of power transformers which connected to EHV lines with huge short circuit level,CFVV will lead to placing tap-changer at LV side. However, this choice has special difficulties duringdesign and manufacturing processes. In this paper it will be shown that it is possible for such powertransformers to place tap-changer on HV side without facing major problem during operation oftransformers.

Key words: Flux density, OLTC location, power transformer

1. INTRODUCTION

One of the main requirements of any electrical system is that it should provide a voltage to the userwhich remains within closely defined limits regardless of the loading on the system, despite the regulationoccurring within the many supply transformers and cables, which will vary greatly from conditions of lightload to full load. Although in many industrial systems, in particular, the supply voltage must be highenough to ensure satisfactory starting of large motor drives, it must not be so high when the system isunloaded as to give rise to damaging overvoltages on, for example, sensitive electronic equipment [1].

For many decades power transformers equipped with on-load tap-changers (OLTC) have been the

main components of electrical networks and industry. The OLTC allows voltage regulation and/or phaseshifting by varying the transformer ratio under load without interruption [2]. Some industrial processes willnot operate correctly if the supply voltage is not high enough and some of these may even be protectedby undervoltage relays which will shut down the process should the voltage become too low [1], [3].

Most domestic consumers are equally desirous of receiving a supply voltage at all times of day andnight which is high enough to ensure satisfactory operation of television sets, personal computerswashing machines and the like, but not so high as to shorten the life of filament lighting, which is often thefirst equipment to fail if the supply voltage is excessive. In this situation, therefore, and despite thereservations concerning the use of tap-changers, many of the transformers within the public supplynetwork must be provided with on-load tap-changers without which the economic design of the networkwould be near to impossible [2], [4]. In industry, transformers having on-load tap-changers are used in the



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provision of supplies to arc furnaces, electrolytic plants, chemical manufacturing processes and the like[1], [5].

One important aspect on using tap-changers is the location they are used. In fact, such equipmentscould be located on high voltage, middle or low voltage windings. Each of these choices has its benefitsand defects. In this paper, the effect of tap-changer location in a power transformer would be studied.

2. TAP-CHANGING EQUIPMENT

The voltage of busses in generating stations, switching substations and receiving sub-stationsshould be held within permissible limits. The voltages of distribution lines and supply points to consumersshould be held at constant rated values (with permitted deviation) under fluctuating load conditions Thetask of voltage control is closely associated with fluctuating load conditions and correspondingrequirements of reactive power compensation and tap-changing [6].

Almost all transformers incorporate some means of adjusting their voltage ratio by means of theaddition or removal of tapping turns. This adjustment may be made on-load, as is the case for many largetransformers, by means of an off-circuit switch, or by the selection of bolted link positions with thetransformer totally isolated. The degree of sophistication of the system of tap selection depends on thefrequency with which it is required to change taps and the size and importance of the transformer [1], [3].

Transformer users require tappings for a number of reasons [1]:

•

To compensate for changes in the applied voltage on bulk supply and other system transformers.• To compensate for regulation within the transformer and maintain the output voltage constant on

the above types.

• On generator and interbus transformers to assist in the control of system VAr flows.

• To allow for compensation for factors not accurately known at the time of planning an electricalsystem.

• To allow for future changes in system conditions.

All the above represent sound reasons for the provision of tappings and, indeed, the use of tappingsis so commonplace that most users are unlikely to consider whether or not they could dispense withthem, or perhaps limit the extent of the tapping range specified. However, transformers without taps aresimpler, cheaper and more reliable. The presence of tappings increases the cost and complexity of thetransformer and also reduces the reliability. Whenever possible, therefore, the use of tappings should beavoided and, where this is not possible, the extent of the tapping range and the number of taps should be

restricted to the minimum. The following represent some of the disadvantages of the use of tappings ontransformers [1], [5]:

• Their use almost invariably leads to some variation of flux density in operation so that the designflux density must be lower than the optimum, to allow for the condition when it might beincreased.

• The transformer impedance will vary with tap position so that system design must allow for this.

• Losses will vary with tap position, hence the cooler provided must be large enough to cater formaximum possible loss.

• There will inevitably be some conditions when parts of windings are not in use, leading to lessthan ideal electromagnetic balance within the transformer which in turn results in increasedunbalanced forces in the event of close-up faults.

• The increased number of leads within the transformer increases complexity and possibility ofinternal faults.

• The tap-changer itself, particularly if of the on-load type, represents a significant source ofunreliability.

3. VARIOUS TYPES OF TAP-CHANGERS

3.1. Off-Circuit Tap-Changers



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The simplest tap-changing device operates on a ‘‘break before make’’ principle and changes tapson the primary winding. Obviously, such a device cannot be operated when the transformer is carryingload or even when the transformer is energized because it would break load current and/or magnetizingcurrent otherwise. This device is called a tap-changer for deenergized operation. Years ago, it was calleda no-load tap-changer, but this description has fallen out of favor because the name implies that it can beoperated when the transformer is energized but not carrying load, which is not the case. Nowadays, thistype of tap-changer is acalled OCTC or Off-Circuit Tap-Changer [7].

Most tap changers for deenergized operation have a total of five tap positions. There are usuallytwo tap positions above the nominal voltage rating and two tap positions below the nominal voltage plus atap at the nominal voltage. The voltage increments between taps are generally 2.5% of the nominalvoltage, so the full tap range is 10% [1], [7].

Tap changers for deenergized operation are designed to be moved infrequently. The tap settingsare generally specified for the particular location on the electrical system and the settings do not changeunless system conditions permanently change. Because a good electrical contact often depends oncontact ‘‘wiping,’’ it is generally a good idea to operate the tap changer periodically (when the transformeris out of service) to wipe the contacts clean. The contacts themselves are generally silver- or tin-coatedsince bare copper has a tendency to develop a copper sulfate film under oil which increases the contactresistance. This can lead to a thermal runaway effect from oil coking. As the temperature around thecontacts increases, the oil around the contacts can coke or turn into carbon. This forms a carbon film thatcan actually force the contacts apart so the load current must pass through a layer of highresistancecarbon. This increases the temperature still further, leading to more coking and so forth until the contacts

overheat and are destroyed [3], [7].

3.2. On-Load Tap-Changers

For many decades power transformers equipped with on-load tap-changers (OLTC) have beenthe main components of electrical networks and industry. The OLTC allows voltage regulation and/orphase shifting by varying the transformer ratio under load without interruption [2].

When load levels and/or system voltages change frequently, it is sometimes necessary to adjustthe transformer tap ratio to follow the changes in system conditions. It is obviously impractical to do thiswhile the transformer is deenergized or unloaded, so a special type of tap changer has been developed tochange taps under full-load conditions [7].

Whereas the tap changer for deenergized operation is a break-before-make switching device, anon-load tap-changer (OLTC) must be a make-before-break switching device, requiring bridging over two

adjacent taps before moving on to the next tap. If an electrical short circuit were placed between two taps,then the short-circuit current would be extremely large based on the large number of ampere turns withfew turns. Therefore, an impedance must be inserted between the taps in order to limit the short-circuitcurrent that flowing in the bridging position [7], [5]. Structure of a practical OLTC using [2] is shown infigure 1.

Figure 1 - Structure of a practical OLTC



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From the beginning of OLTC development, two switching principles have been used for the load-transfer operation, the high-speed-resistance type and the reactance type. Over the decades, bothprinciples have been developed into reliable transformer components available in a broad range ofcurrent and voltage applications to cover the needs of today’s network and industrial-processtransformers as well as ensuring optimum system and process control [2].

For transformers with high amounts of rated voltage and power, it is preferred to use OLTCsrather than OCTCs. This is due to higher range of voltage variation of OLTCs and also their ability to

tapping in loaded condition.

4. IEC STATEMENTS ON TAP-CHANGING STRATEGIES

The short notation of tapping ranges and tapping steps indicates the variation range of the ratio ofthe transformer. But the assigned values of tapping quantities are not fully defined by this alone andadditional information is necessary. This can be given either in tabular form with tapping power, tappingvoltage and tapping current for each tapping, or as text, indicating “category of voltage variation” andpossible limitations of the range within which the tappings are “full power tapping”[8], [9].

According to IEC 60076-1&4 standards, the extreme categories of tapping voltage variation are[8], [9]:

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Constant flux voltage variation (CFVV), and,• Variable flux voltage variation (VFVV)

They are defined as follows:

• CFVV: The tapping voltage in any untapped winding is constant from tapping to tapping. Thetapping voltages in the tapped winding are proportional to the tapping factors.

• VFVV: The tapping voltage in the tapped winding is constant from tapping to tapping. Thetapping voltages in any untapped winding are inversely proportional to the tapping factors.

There is also another strategy considerable for tap-changing which is a combination of the introducedmethods, namely CbVV (Combined Voltage Variation). In many applications and particularly withtransformers having a large tapping range, a combination is specified using both principles applied todifferent parts of the range; combined voltage variation (CbVV). The change-over point is called

“Maximum Voltage Tapping”.

For the CbVV system the following applies [8], [9]:

• CFVV: Applies for tappings with tapping factor below the maximum voltage tapping factor.

• VFVV: Applies for tappings with tapping factor above the maximum voltage tapping factor.

Graphic presentation of tapping voltage variation categories using [8], [9] are shown in figure 2. In thisfigure:

U A, I A is Tapping voltage and tapping current in the tapped winding,UB, IB is Tapping voltage and tapping current in the untapped winding,S AB is Tapping power



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Figure 2 - Graphic Presentation of Tapping Voltage Variation Categories,a) CFVV, b) VFVV and c) CbVV

5. PROBLEM DEFINITION

As indicated in section 4, major tapping strategies are CFVV and VFVV. In the first strategy, themain assumption is that voltages of un-tapped windings are constant in rms and a variable voltage isapplied to the tapped winding. In this case, due to presence of constant voltage value on un-tappedwindings, rms value of flux density of magnetic core would be constant. Therefore, this strategy could benamed as Constant Flux Voltage Variation. However, in VFVV, voltages applied to un-tapped windingshave variable values as the voltage associated to tapped winding is constant. In the later case, the rmsvalue of magnetic flux would vary as voltage varies.

For these purposes, most of electric power companies as well as power transformermanufaturers prefer to use CFVV strategy to be able to use all capacity of their magnetic core while theyhave appropriate voltages on their loads. However, for the case of transformers which are connected toEHV lines, due to high amount of short circuit level of EHV systems, the rms voltage value of these linescould be assumed to be constant. In this case, presence of OLTC (On-Load Tap-Changer) switch on HVside of transformers (connected to EHV line) might lead to VFVV structure. Therefore, it is usuallyemphasized by purchasers that they need EHV transformers with OLTC on their LV side.

However, this request could lead to special difficulties. One of these difficulties is need to OLTCswith high amounts of rated current, as load current of LV side has a greater rms value in comparision withHV side. Another problem for locating OLTC on LV side is that turn numbers assosiated with each voltagestep might have as little as it would be difficult to manufacture such tapping winding.

In following sections, it would be shown that for the case of transformers connected to such EHVlines, location of OLTC (HV or LV side) does not have considerable effect of rms value of flux densityinside magnetic core. Therefore, it is simply possible to move OLTC from HV side to LV.

6. SYSTEM STUDIED

For the purpose of studying the effect various locations of OLTC on maximum flux density ofmagnetic core, two large power transformers have been chosen for simulation. Both power transformersare designed and manufactured by Iran-Transfo Co., Zanjan, Iran. Rated values associated with thesetransformers are indicated in table I.



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Table I - Rated values associated simulated transformers

Transformer

StudiedRated Voltage Rated Power Vector Group Core Structure

A (400±15%)/63 kV 120/160/200 MVA YNd11 Three limb

B 400/(63±15%) kV 120/160/200 MVA YNd11 Three limb

As shown in table I, two selected transformers for simulation are very similar to each other,except for the location of OLTC at which they use such equipment in their HV and LV sides, respectively.

7. SIMULATION RESULTS

Using detail parameters associated with mentioned transformers, appropriate simulations havebeen performed at with magnitude of flux density on different taps obtained. For this purpose, self andmutual inductances method has been chosen [10]. These simulations are performed using MATLAB andEMTP softwares which verified each-other.

In the performed simulations, both transformers are fed from HV side assuming that short circuitlevel of this side in infinite, as shown in figure 3. Therefore, in all full-load and no-load cases, rms voltage

value associated with HV side would be 400 kV. Also, load current of each tap is chosen so that rmsvalue of full-load voltage of LV side is 60 kV. Obtained results could be acceded in tables II and III.

Figure 3 – Schematic Operation Diagram of Studied Transformers

Table II – Numerical results of simulations performed on transformer A of table I

Tap

HV Side

Full-Load

Voltage

LV Side Full-

Load Voltage

LV Side

No-Load

Voltage

Full-Load

Flux Density

(T)

No-Load

Flux Density

(T)

Variation of

Flux

Density

(%)Maximal

(+15%)400 kV 60 kV 54.8 kV 1.52 1.49 -10.59 %

Nominal

(0%)400 kV 60 kV 63 kV 1.67 1.7 -1.77 %

Minimal

(-15%)400 kV 60 kV 74.1 kV 1.8 1.99 5.88 %



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Table III – Numerical results of simulations performed on transformer B of table I

Tap

HV Side

Full-Load

Voltage

LV Side Full-

Load Voltage

LV Side

No-Load

Voltage

Full-Load

Flux Density

(T)

No-Load

Flux Density

(T)

Variation of

Flux

Density

(%)

Maximal

(+15%)400 kV 60 kV 72.45 kV 1.614 1.719 -6.13 %

Nominal(0%)

400 kV 60 kV 63 kV 1.687 1.719 -1.86 %

Minimal

(-15%)400 kV 60 kV 53.55 kV 1.786 1.719 3.87 %

Comparing obtained numerical simulation results, some considerable points could be fetched. Intable II which associates with transformer A with OLTC on HV side, peak value of no-load flux density isdifferent for various taps. This is due to the fact that HV side is fed by a 400 kV source, directly.Therefore, changing tap position would lead to change in effective number of turns in HV winding andtherefore, various values for flux density would be obtained.

However, the important and useful results in these cases is obtained values for flux density in full-load condition. This is due to the fact that tap position in large power transformers is usually controlled by AVR which compares load side voltage with a reference number. Therefore, for the case of transformer A, AVR would never lets the transformer to operate in no-load condition at tap position in minimal.Therefore, in table II, for calculation of variation of flux density from no-load condition the value related tonominal tap is chosen as reference. In this case, obtained value of 1.99 T for flux density of no-loadcondition would not be a major problem anymore.

According to results of table III, no-load flux density does not depends on tap position. This is dueto the fact that number of turns of the fed winding (HV) is constant as OLTC is located in LV side.

A comparison between simulation results of transformers A and B shows that movement of OLTClocation from LV side in transformer B to HV side in transformer A leads to increment of flux densityvariation at full-load condition. However, this increment is less than 5% for this case (10.59-6.13=4.46%),while, no-load voltage variation of untapped winding (LV) is ±15% for transformer A. This is due to thefact that voltage variation of untapped winding does not reflect to magnetic core, directly. In fact, there isa considerable voltage drop on transformer impedance that smoothes the effects of voltage variation onflux density of magnetic core. Therefore, it would be reasonable to use OLTC on HV side of transformersand chose smaller nominal value for flux density rather than design transformers with OLTC on LV sides.

8. CONCLUSION

This paper was focused on effect of OLTC location on magnetic flux density in powertransformers. In this order, IEC 60076-1 says there are three strategies for tapping transformers of whichCFVV (Constant-Flux Voltage Variation) and VFVV (Variable-Flux Voltage Variation) strategies are thebasic strategies. Among these strategies, CFVV is more common and preferred at which peak value offlux density is less variable. Such strategy for the case of large power transformers which connects toEHV lines with huge amounts of short circuit level leads to use OLTC on L side. However, this choice hasits difficulties that are pointed some of them in paper.

In this paper it was shown that using OLTC on HV side for EHV transformers has less variationrange of flux density in comparison to voltage of untapped voltage. Therefore, it is completely economicaland reasonable to move OLTC from LV side to HV side and prevent from placing tap-changer at LV side.

9. REFERENCES

[1] M.J. Heatcote, “the J&P transformer book,“ 12th edition, Planta Tree, 1998[2] J.H. Harlow, et al, “electric power transformer engineering”, CRC Press LCC, 2004[3] K. Imhof, F. Oesch and I. Nordanlycke, “modeling of tap-changer in an energy managment

system“, IEEE Trans. Power Systems, vol. 11, no. 1, Feb. 1996, pp. 428-434[4] L. Van der Sluis, “transients in power systems“, John Wiley and Sons, 2001



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[5] S.V. Kulkarni and S.A. Khaparde, “transformer engineering: design and practice”, Marcel DekkerInc., 2004

[6] S. Rao, “power transformers and special transformers,“ 3rd edition, Khanna Publishers, 2004[7] J.J. Winders, “power transformers-principles and applications,“ Marcel Dekker Inc., 2002[8] IEC Standars 60076-1[9] IEC Standars 60076-4[10] F. Zhalefar and M. Sanaye-Pasand, "transformer winding detailed modeling for protective

studies", IEEE Power India Conference, New Delhi, India, March 2006

effect of tap changer location on transformer

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