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Technical Data – General Section Technical Data TD 61
Survey
Survey This general section pertains to the following technical data.
VACUTAP® V T ........................................... TD 124 VACUTAP® V V ........................................... TD 203 VACUTAP® V R ........................................... TD 237 OILTAP® V
........................................... TD 82
OILTAP® MS
........................................... TD 60
OILTAP® M
........................................... TD 50
OILTAP® RM
........................................... TD 130
OILTAP® R
........................................... TD 115
OILTAP® G
........................................... TD 48
DEETAP® U
........................................... TD 51
NOTE These technical data are intended for the calculator and designer of the transformer. This release replaces all previous releases which then become invalid. Dimensional drawings and connection diagrams are subject to change without prior notice. Drawings submitted during bidding and ordering are always binding. Since the on-load tap-changer is delivered to the specifications of the transformer manufacturer, the manufacturer is responsible for selecting the correct properties of the on-load tap-changer so that the requirements of the transformer are met. 2
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Table of Contents
Table of Contents 1
General .................................................................................................................................................................................................................. 1.1 How it functions .................................................................................................................................................................................... 1.2 Basic connections of the tap winding ............................................................................................................................................
5 5 6
2
Characteristic properties of the on-load tap-changer ..................................................................................................................... 2.1 Designations of the on-load tap-changer .................................................................................................................................... 2.2 Through-current, step voltage and step capacity ....................................................................................................................... 2.3 Insulation ................................................................................................................................................................................................. 2.4 Overload .................................................................................................................................................................................................. 2.4.1 Through-currents greater than rated through-current .............................................................................................. 2.4.2 Operation at increased transformer power with improved cooling ....................................................................... 2.4.3 Required specifications for modes which are not defined by IEC 60354 or ANSI C57.91 ............................... 2.5 Short-circuit stress on on-load tap-changers and off-circuit tap-changers .................................................................... 2.6 Forced current division ........................................................................................................................................................................ 2.7 Permissible overexcitation .................................................................................................................................................................. 2.8 Multiple-column on-load tap-changers ........................................................................................................................................ 2.9 On-load tap-changer applications with variable step voltage ............................................................................................... 2.10 Coarse tap winding/tapped winding, leakage inductance ....................................................................................................... 2.11 Potential connection of the tap winding ...................................................................................................................................... 2.12 On-load tap-changer in transformers for arcing furnaces (cf. also ordering sheets) ..................................................... 2.13 Installing the on-load tap-changer and off-circuit tap-changer ......................................................................................... 2.14 Activating the on-load tap-changer during the transformer test ........................................................................................ 2.15 Special applications .............................................................................................................................................................................. 2.16 Insulation oils ......................................................................................................................................................................................... 2.17 Service in arctic areas .......................................................................................................................................................................... 2.18 Hermetically sealed transformer with gas cushion .................................................................................................................... 2.19 Parallel jumpers for parallel connection of tap selector planes ............................................................................................
7 7 10 11 11 11 11 12 12 12 13 13 13 14 14 19 19 19 19 19 19 19 19
3
Important information on design and installation ............................................................................................................................ 3.1 Designation of the terminal contacts of the tap selector and operating positions ........................................................ 3.2 Oil suction pipe ...................................................................................................................................................................................... 3.3 Oil filter unit ...........................................................................................................................................................................................
20 20 21 21
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Table of Contents
Table of Contents 4
Protective relay RS 2001 ............................................................................................................................................................................... 21
5
Motor-drive units ED-S, ED-L ..................................................................................................................................................................... 22 5.1 5.2 5.3
Function .................................................................................................................................................................................................. 22 Type designations .................................................................................................................................................................................. 22 Technical data of the motor-drive unit ......................................................................................................................................... 23
6
Drive shaft ............................................................................................................................................................................................................ 24
7
Selecting the on-load tap-changer ............................................................................................................................................................ 24 7.1 Selection principle ................................................................................................................................................................................ 24 7.2 Examples of selecting the on-load tap-changer ......................................................................................................................... 25
8
Appendix ............................................................................................................................................................................................................... 28
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1 General
1
General
1.1
Function principle
The on-load tap-changer provides uninterrupted voltage regulation of transformers under load. The voltage is regulated by changing the voltage ratio. This is done in steps. The transformer is equipped with a tap winding whose tappings are connected with the tap selector of the on-load tap-changer. This is the reason the on-load tap-changer is designed for immersed installation in the transformer tank (fig. 1) to keep the distances from the tap winding terminals to the tap selectors short. The on-load tap-changer is activated by a motor-drive. Drive shafts and bevel gear units mechanically connect the motor-drive to the on-load tap-changer head.
On-load tap-changer (diverter switch-tap selector principle) The tap selector selects the desired tap which is then connected to the no-load side of the diverter switch. This tap then accepts the service current with the next diverter switch operation. The functions of the diverter switch and tap selector are time-correlated during the tap change, see fig. 2a for connection principle.
Exception: The on-load tap-changer VACUTAP® VT is fixed to the active part of the dry-type transformer.
Tap selector
Diverter switch
KHW 370 - 4
Fig. 2a Connection principle of the on-load tap-changer consisting of diverter switch and tap selector 8997590D
Fig. 1
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Transformer with on-load tap-changer, drawing 1 - On-load tap-changer 2 – Motor-drive 3 – Protective relay 4 – Oil conservator
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1 General
On-load tap-changer (selector switch principle)
1.2
Basic connection of the tap winding (see fig. 3)
It combines the characteristics of a diverter switch and a tap selector. The change-over from one tap to the next takes place in only one switching process, see fig. 2b for connection principle.
For possible basic connections, see the technical data of the pertinent tap-changer type.
a
b
c KHW 114-4
EV1004
Fig. 3
Fig. 2b Connection principle of the on-load tap-changer based on the selector switch principle
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Basic connections a – Without change-over selector b – With reversing switch c – With coarse tap selector
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2 Characteristic properties of the on-load tap-changer
2
Characteristic properties of the on-load tap-changer
2.1
Designations of the on-load tap-changer
M
Each type of on-load tap-changer is available in many designs, varying by number of poles, maximum rated through-current, highest voltage for equipment Um, tap selector size, and basic connection diagram. For this reason, the designation of a certain on-load tap-changer model must also indicate these features (see fig. 4). This gives the on-load tap-changer an unique identification. Number of steps and basic connection diagram The tap selector can be extensively adjusted to the required number of steps and the connection of the tap winding. The applicable basic connection diagrams are differentiated by tap selector contacts (10 to 18), number of operating positions, number of mid-positions, and the change-over selector design. Fig. 5 shows the designation of the basic connection diagram. The adjustment position is the position in which the on-load tap-changer is delivered. The on-load tap-changer must be in „adjustment position“ mode during maintenance work (removal or installation of the on-load tap-changer unit). For further details, see the pertinent operating/maintenance instructions. Each design connection diagram of the on-load tap-changer explicitly specifies the adjustment position. The mid-position is the position in which the „K“ contact is connected in the reversing switch or coarse tap design. The mid-position is usually also the adjustment position (see design connection diagram of the on-load tap-changer).
I
601 – 123 /
B
–
10 19 1 W
Type Basic connection diagram Number of poles
Tap selector size
Max. rated through-current in A and additional identification of the design
Highest voltage for equipment Um in kV
Fig. 4 Designation of the on-load tap-changer Example: On-load tap-changer, type M, 1-pole, max. rated through-current 600 A, highest voltage for equipment Um = 123 kV, tap selector size B, tap selector in acc. w. basic connection diagram 10 19 1 W
10
19
1
Tap selector pitch
1 mid-position: With 1 mid-position, there is no position with the same voltage before or after the „K“ contact.
10 12 14 16 18
3 mid-positions: With 3 mid-positions, there is no change in voltage before and after the „K“ contact. Jumpered contacts are not considered as mid-positions.
Mid-position(s)
Max. operating positions
without changeover selector
10 12 14 16 18
0 1 3
W
Change-over selector
G Coarse tap selector
W Reversing switch
with change-over selector
09 11 13 15 17
19 23 27 31 35
Fig. 5 Designation of the basic connection diagram Example: Tap selector pitch 10, max. of 19 operating positions, 1 mid-position, change-over selector designed as reversing switch 061/02/01/0
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2 Characteristic properties of the on-load tap-changer
On-load tap-changer designations x Type
No. of poles
I Um
No. of configured sectors (only 1-pole) Application
Um
Tap selector size
Contacts 9 10 12 14 16 18 22 Number of max. operating positions
Mid-positions
Reversing switch / coarse tap 1)
x
xxxx
®
9 10 12 14 16 18 22 0 1 3 W G
x -
xxx / x -
xx
xx
xx
Basic connection diagram
OILTAP V ® OILTAP M / MS ® OILTAP R / RM ® OILTAP G ® VACUTAP VT ® VACUTAP VV I 1 pole II 2 pole III 3 pole A 200 V (not V I) 250 V (special design, not V I), VV 300 MS 350 M, V 400 V (special design), VV 500 M, VT I 600 M, RM, VV 800 MI 1200 MI, RMI; R 1500 MI, RMI 1600 G 2000 RI 2400 RI (forced current division) 3000 RI, GI 1 1 sector 2 2 sectors 3 3 sectors Y Application with neutral point D Application not with neutral point Not for single-pole tap-changers kV 36 VT 40 V, VV 72,5 MS, M, RM, R, G, 76 V, VV 123/76 V III D 123 MS, M, RM, R, G, VIII Y 145 VV 170 MS, M, RM, R, G 245 M, MS, RM, R, G 300 MI, RMI, RI, GI 362 On request B M, MS C M, RM, R D M, RM, R, G DE M, RM E R, G Not with selector switches and VACUTAP® VV without change-over with change-over selector VT M, MS, R, RM, G, V, VV M, MS, R, RM, G, V, VV M, MS, R, RM, G, V, VV M, MS, R, RM, G, V, VV V, M, MS, R, RM, G V, M, MS, R, RM, G M, R, RM, G M, R, RM, G M, R, RM, (G) M, R, RM, (G) M without change-over with change-over selector 9 10 19 12 23 14 27 16 31 18 35 22 0 mid-positions (without change-over selector) 1 mid-position 3 mid-positions Reversing switch Coarse tap
1) Up to max. 107 operating positions (only type M)
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Off-circuit tap-changer designations x
x
xxxx
-
xxx -
xx
xx
xx
Basic connection diagram
Off-circuit tap-changer Number of poles Max. rated throughcurrent
U III 300 600 800 1000 >1000
Highest voltage for equipment Um [kV]
Contact circle pitch
Max. number of operating positions Type of connection
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06
3 poles 300 A 600 A 800 A 1000 A on request 17.5 36 72.5 123 170 >170 on request Contact circle Æ 350 mm, max. of 5 operating positions
12
Contact circle Æ 550 mm, max. of 11 operating positions
18
Contact circle Æ 750 mm, max. of 17 operating positions
05 11 17 Y D ME MD SP YD S
5 operating positions 11 operating positions 17 operating positions Linear off-circuit tap-changer for neutral application Linear off-circuit tap-changer for delta application Single bridging off-circuit tap-changer Double bridging off-circuit tap-changer Series-parallel off-circuit tap-changer Star-delta off-circuit tap-changer Special connection
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2 Characteristic properties of the on-load tap-changer
Through-current, step voltage and step capacity
The through-current is the current flowing through the onload tap-changer and off-circuit tap-changer while in service. The through-current of an on-load tap-changer usually varies along the voltage regulating range (e. g., while the rated power of the transformer remains the same). The maximum through-current which a transformer can handle continuously must be used for the rating of the onload tap-changer and off-circuit tap-changer. This maximum continuously permissible through-current of the transformer is the rated through-current Iu of the on-load tap-changer or of the off-circuit tap-changer.
The rated step capacity PStN is the product of rated through-current Iu and related rated step voltage Ui: P = Iu x Ui StN
Ui = Uim
Ui
2.2
The step voltage is the operating voltage between adjacent taps. The step voltage can remain the same or vary over the entire setting range. If the step voltage varies, the maximum step voltage Ust of the transformer is used to rate the onload tap-changer and the off-circuit tap-changer.
Iu = Ium
The maximum rated through-current Ium varies with the design and is the maximum through-current of an on-load tap-changer and off-circuit tap-changer to which the current-related type tests refer. The rated step voltage Ui of an on-load tap-changer is the highest permissible step voltage for a certain value of the rated through-current Iu. Together with a rated throughcurrent, it is known as the related rated step voltage. The max. rated step voltage Uim varies with the design and is the max. permissible step voltage of an on-load tapchanger and off-circuit tap-changer. The transition resistors of the on-load tap-changer are designed for the existing values of the maximum step voltage Ust and the rated through-current Iu of the transformer for which the on-load tap-changer is to be used. Since the permissible rated through-current Iu and the permissible step voltage Ust vary with the value of the transition resistors, these rated values refer to the particular application. If an on-load tap-changer is to be used with values for step voltage and through-current other than those declared in the order (e. g., transformer power increased due to improved cooling or use of the on-load tap-changer in another transformer), MR must determine whether this is possible or whether the transition resistors must be changed. This also applies when the desired new rated values Iu and Ust are below the original values since the design of the transition resistors not only affects the switching capacity stress of the contacts but uniform contact wear is also desired.
10
KHW 371-3
Iu
Fig. 5
Rated step capacity diagram of a diverter switch 1 - upper limit point 2 - lower limit point
Fig. 5 shows the typical load limits of a diverter switch. This means that the permissible range on the voltage side is limited by the max. rated step voltage Uim and, on the current side, by the max. rated through-current Ium. The points of the curve located between limit points 1 and 2 are determined by the permissible rated switching capacity. The permissible switching capacity between limit points 1 and 2 corresponds to related pairs of values for Iu and Ui and may be constant or varying. The rated step capacity diagram as well as individual values for Iu and Ui in limit points 1 and 2 are specified separately for each type of on-load tap-changer (see TD of the particular type). The limit step capacity is the greatest step capacity which can be safely transferred. Every MR on-load tap-changer can switch at least twice the rated through-current Iu at step voltage Ust for which the on-load tap-changer was designed. This limit switching capacity is documented with the type test as prescribed by IEC 60214. Tap change operations with currents greater than twice the rated through-current Iu must be prevented with suitable measures.
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2.3
Insulation
2.4
The insulation strength of the various insulation distances of the on-load tap-changer and off-circuit tap-changer is determined by the individual technical data of the particular type of tap-changer. The specified rated withstand voltages of the insulation apply to new, thoroughly dried insulating material in treated transformer oil (at an ambient temperature of at least 10 °C). Selection of an on-load tap-changer and off-circuit tapchanger requires the following information. The maximum network-frequency voltages during service
-
The power frequency test voltages occurring during the transformer test
The transformer manufacturer is responsible for the correct selection of the rated withstand voltages as required by onsite insulation coordination. The required rated withstand voltages must be provided for the different insulation distances. Against ground
-
With multiple-phase types, between the phases
-
Between contacts of one phase
The required specifications depend on the type of regulation (e.g., with on-load tap-changers: regulation without changeover selector, reversing switch arrangement, coarse tapping arrangement) and the type of tap-changer. The relevant insulation distances and their relation to the voltages of the transformer windings are described in the technical data of the particular tap-changer type.
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MR on-load tap-changers and off-circuit tap-changers can be used for all transformer loads as described in IEC 60354:1991 („Loading guide for oil-immersed transformers“) and ANSI C57.91-1995 („Guide for loading mineral-oilimmersed transformers“).
-
Normal cyclic loading
-
Long-time emergency cyclic loading
-
Short-time emergency loading
ANSI C57.91 differentiates between four modes:
The impulse voltages (lightning impulse, switching impulse, wave cut off at the back and wave cut off at the front) occurring during the transformer test
-
2.4.1 Through-currents higher than rated throughcurrent
IEC 60354 differentiates between three modes.
-
-
Overload
-
Normal life expectancy loading
-
Planned loading beyond nameplate rating
-
Long-time emergency loading
-
Short-time emergency loading
Suitability of an on-load tap-changer for the above modes is documented with the type test in accordance with IEC 60214-1989. With „normal cyclic loading“ or „normal life expectancy loading“ mode, through-currents greater than the rated through-current may occur during a daily load cycle. If the operating conditions described by IEC 60354 or ANSI C57.91 are adhered to (i.e., duration and power during a daily cycle, transformer oil temperature, and so on), this constitutes normal service and not an extraordinary load. For this reason, through-currents greater than the rated throughcurrent which may occur briefly in the above modes do not need to be given special consideration when selecting the on-load tap-changer.
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2 Characteristic properties of the on-load tap-changer
2.4.2 Operation at increased transformer load with improved cooling Keep the following points in mind when specifying the required rated through-current of an on-load tap-changer. When a transformer is run with different capacities due to different conditions (e.g., type of cooling, ambient temperature), the greatest load must be taken as the basis for the rated power when selecting the on-load tap-changer. See also IEC 60076-1:1993. This is necessary since the oil temperature in the transformer is not reduced despite increased transformer cooling due to the increased load and, in contrast to the transformer, the external conditions of the on-load tap-changer are not improved. Another reason is the design of the transition resistors of on-load tap-changers based on the greatest through-current so that the switching capacity stress on the contacts of the on-load tap-changer is limited to permissible values. 2.4.3 Required specifications for modes which are not defined by IEC 60354 or ANSI C57.91 When asked about overload conditions, MR requests a definition based on the above modes to avoid misunderstandings and to clearly describe the conditions of service. If the requested mode cannot be defined in relation to IEC 60354 or ANSI C57.91, the following specifications become necessary.
All MR on-load tap-changers and off-circuit tap-changers meet at least the requirements of IEC 60214:1989 pertaining to short-circuit strength. Calculation of permissible shortcircuit duration with stress of short-time currents lower than the rated short-time withstand current, or calculation of the permissible short-time current for short-circuit durations longer than the rated short-circuit duration is possible with the help of the following equation. Ix² * tx = IK² * tK with IK : Rated short-time withstand current tK : Rated short-circuit duration Ix : Permissible short-time current for short-circuit duration tx (with tx always greater than tk) tx : Permissible short-circuit duration for stress with Ix (with Ix always smaller than Ik) Due to the dynamic stress alone from the impulse current, an impulse current greater than the rated peak withstand current is not permitted. This is the reason that recalculation of the rated values for higher impulse currents and short-time currents for shorter short-circuit durations is not permitted!
-
Through-currents and related load duration during one day’s cycle
-
Oil temperature of the transformer during one day’s cycle
Short-circuit stresses usually occur only rarely on transformers in service. With applications with very frequent shortcircuit stresses (e.g., special test transformers), this must be allowed for by selecting an on-load tap-changer with greater short-circuit resistance. Information on amount and frequency of the expected short-circuit stresses is necessary for this.
-
Expected number of tap changes during the load phases of one day’s cycle (only for on-load tap-changers)
2.6
-
Duration of overload service in days/weeks/months
-
Frequency of this overload service (e. g., „once a year“ or „rarely, only when other transformers fail“)
2.5
Stress on on-load tap-changers and off-circuit tap-changers due to short circuit
Permissible stress due to short circuits is listed below. - Rated short-time withstand current as r.m.s. value of permissible short-circuit current - Rated peak withstand current as highest permissible peak value of the short-circuit current - Rated short-circuit duration as permissible short-circuit duration during stress with rated short-time withstand current 12
Forced current division
With single-pole on-load tap-changers and off-circuit tapchangers for large rated through-currents, current paths are connected in parallel. A distinction is made between applications with and without „forced current division.“ Applications with and without „forced current division“ with the same rated through-current require different on-load tap-changer and off-circuit tap-changer designs. The meaning of „forced current division“ differs for on-load tap-changers and off-circuit tap-changers. On-load tap-changer During the change-over operation of the diverter switch, uniform division of the current on the parallel contacts must be ensured. This always requires a divided tap winding and a divided main winding. Leakage impedance between the parallel main windings must be at least three times the value of the transition resistor of the on-load tap-changer. TD 61/02
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2 Characteristic properties of the on-load tap-changer
It is imperative that MR be consulted about these applications. You will need a drawing of the complete winding arrangement with all parallel winding parts. Off-circuit tap-changer: The tap winding must be completely divided. In addition, some windings on the tap winding next to the main winding must also be divided. In arrangements with forced current division, parallel contacts may not be jumpered. The voltage between the parallel tap windings when stressed with impulse voltage must be considered. The transformer manufacturer must specify the required impulse voltage strength between the parallel tap windings. 2.7
Permissible overexcitation
MR on-load tap-changers meet the requirements of IEC 60076-1:1993 (5% overexcitation) and ANSI IEEE C57.12.002000 (10% overexcitation) . 2.8
Multiple-column on-load tap-changers
Regardless of whether activated by one or more motordrives, multiple-column on-load tap-changers (e.g., 3 x M I) do not switch synchronously. When delta connections with a very large regulation range and very low voltage in an end position are regulated where the voltage can then only be generated from a few taps, this can cause excessively high circulating currents in the delta winding (varies with connection group and short-circuit impedance of the transformer). In such cases, the transformer manufacturer must specify the circulating current for different positions of the on-load tap-changer in the three phases so that MR can consider the required increased switching capacity when selecting the on-load tap-changer and designing the transition resistors. 2.9
On-load tap-changer applications with variable step voltage
When different value pairs of step voltage and related through-current are required for an on-load tap-changer, the combination must consist of maximum step voltage and maximum through-current within the permissible switching capacity range of the pertinent on-load tap-changer type, even when the step voltage and this through-current do not occur at the same time. Example: A transformer is being run at constant power within a large range of fluctuating line voltage. Then the highest step voltage occurs with highest system voltage together with a low through-current in relation to the transformer load, and the greatest through-current occurs together with the lowest step voltage at the lowest system voltage. The on-load tap-changer must then be designed as if the highest step voltage occurs together with the highest through-current. The reason for this is the necessary adjustment of the transition resistance to both the step voltage and the throughcurrent. In general, the following applies to this adjustment: High step voltages require high values for the transition resistance. In contrast, high through-currents require low values for the transition resistance. Therefore, a solution to transition resistance adjustment is only possible if there is a resistance value which is suitable both for the highest step voltage and the highest throughcurrent. Otherwise, the value of the transition resistance in the above example would have to be continuously adjusted to the different system voltages. There is always a suitable resistance value if the value pair of highest step voltage and highest through-current is within the permissible switching capacity. If this pair of values is located just outside the permissible switching capacity range, MR must check individual cases to determine whether a solution for the adjustment of the transition resistance is still possible. When the permissible switching capacity range is significantly exceeded, an on-load tap-changer type with greater switching capacity must be used.
In applications with variable step voltage, the greatest possible step voltage must always be specified for the selection of the on-load tap-changer. Examples of such applications include: -
Variable magnetic flow Tap windings with different numbers of turns Load and position-dependent step voltage for phaseshifter transformers Service under unusually great fluctuation of the system voltage
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2 Characteristic properties of the on-load tap-changer
2.10 Coarse tap winding/tapped winding, leakage inductance
2.11 Potential connection of the tap winding The tap winding is briefly galvanically isolated from the main winding during the change-over operation of the reversing switch or coarse tap selector. It assumes a potential resulting from the voltages of the adjacent windings and the coupling capacities to these windings or to grounded parts. This potential shift of the tap winding generates corresponding voltages between the breaking change-over selector contacts since one contact is always connected with the tap winding and the other contact is always connected with the main winding. This voltage is called the recovery voltage Uw. When the change-over selector contacts open, a capacitive current caused by the above coupling capacities of the tap winding must be interrupted. This current is called the breaking current Is. The recovery voltage Uw and the breaking current Is may cause excessive discharges on the change-over selector. The permissible range of recovery voltage Uw and breaking current Is is shown in fig. 7 for the various on-load tapchanger types.
During the change-over operation from the end of the tapped winding to the end of the coarse tap winding (midposition, see fig. 6) and the reverse switching direction, all turns of the coarse tap winding and the entire tapped winding are located between the selected and pre-selected tap. With these switching operations, this results in a much higher leakage inductance for the circuit of the on-load tapchanger as the internal resistance of the step voltage than for all other switching operations during which only the leakage inductance of a step takes effect and this inductance can be ignored for the function of the on-load tapchanger. The leakage inductance described above for coarse tap winding/tapped winding generates a phase shift between breaking current and recovery voltage on the resistor contacts of the diverter switch which may cause longer arcing times. This leakage inductance must be specified for adjustment of the on-load tap-changer to these operating conditions.
Without tie-in resistors
Recovery voltage Uw (kV)
In extreme cases, this leakage inductance can be the determining factor for selecting the on-load tap-changer type. 50.00 45.00 40.00 35.00
M, MS, RM
30.00 25.00 20.00 15.00 10.00 5.00 0.00
VV
R, G
V 0
20
40
60
80 100 120 140 160 180 200 220 240 260 280 300 320 Breaking current Is [mA]
Fig. 7
Standard values without tie-in resistors
KHW 447-4
Fig. 6
14
Leakage inductance in the mid-position
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Due to the potential connection of the tap winding with a tie-in resistor, the recovery voltage Uw is decreased on the change-over selector contacts while the breaking current Is is increased by the additional current via the tie-in resistor. Fig. 9 shows for the different on-load tap-changer types the range of recovery voltage Uw and breaking current Is which can be used without consulting with MR when tie-in resistors are used. This figure applies to all cases where the breaking current Is is primarily determined by the tie-in resistor.
If appropriate calculations result in values outside the permissible range shown in fig. 7, the tap winding must be connected to a fixed potential during the switching operation. This is accomplished with the following measures (see fig. 8).
With tie-in resistors
Recovery voltage Uw (kV)
40.00 35.00 30.00
M, MS, RM
25.00
VV
20.00
R, G
15.00 10.00
V
5.00 0.00 0
20
40
60
80 100 120 140 160 180 200 220 240 260 280 300 320 Breaking current Is [mA]
Fig. 9
a
Fig. 8
Potential connections (Reversing switch is in mid-position) a – Tie-in resistor Rp b – With potential switch Sp and tie-in resistor Rp
b KHW 164-2
a – Connection to potential of the tap winding by a permanently installed ohmic resistor (tie-in resistor) b – Potential switch Connection to potential of the tap winding by a ohmic resistor which is only inserted (by a potential switch) during the change-over selector operation. The constructive solutions for a and b vary depending on the type of on-load tap-changer. For additional details pertaining to our delivery program, see TD 48, TD 50, TD 60, TD 82, TD 115, TD 130, TD 203 and TD 237.
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Standard values for tie-in resistor, breaking current Is primarily determined by tie-in resistor.
Since the recovery voltage Uw and breaking current Is are not the only important criteria for evaluating the permissible switching capacity of a change-over selector, an evaluation by MR is required when the ranges in fig. 7 and fig. 9 are exceeded. Since a decrease in the recovery voltage Uw due to the tie-in resistor is always accompanied by an increase in the breaking current Is, a solution with permissible change-over selector capacity cannot always be found for winding arrangements with poor capacitive coupling. If this is true, a change-over selector with a higher permissible breaking current must then be used or the winding arrangement must be changed. This is why prompt examination of the change-over selector capacity is particularly important for high-power transformers (i.e., large coupling capacities) and high operating voltages (i. e., great potential shift of the tap winding during changeover selector operation).
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2 Characteristic properties of the on-load tap-changer
Calculation of the recovery voltage Uw and the breaking current Is as well as the design of the possibly necessary tiein resistor can be handled by MR. The following information is required for this. -
Winding arrangement (i.e., location of the tap winding with respect to the adjacent windings)
-
Capacitance of the tap winding to the adjacent windings or capacitance of the tap winding against ground or adjacent grounded windings
Example of the estimated calculation of the recovery voltage on the change-over selector Transformer Rated power: 13 MVA High voltage winding: 132 kV + 10% Delta connection: 50 Hz Tap winding with reversing switch (fig. 10)
In addition, the following information is needed to dimension the tie-in measure.
Double concentric arrangement of the high voltage winding with inside main winding (disk-type coils) and outside tap winding (see page 17, fig. 11). Winding capacities: C1 = 1810 pF (between main and tap winding) C2 = 950 pF (between tap winding and ground)
-
Expected stress due to lightning impulse voltage across half the tap winding
On-load tap-changer: MS I 301/MS II 302 - 170/B-10 19 3 W
-
A.c. voltage across half the tap winding under operating and test conditions can usually be deduced from the normal ordering specifications for the on-load tapchangers.
-
A.c. operating voltage across windings or the positions of the windings which are adjacent to the tap winding
Fig. 10
16
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Connection of the high voltage winding
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2 Characteristic properties of the on-load tap-changer
Assuming that winding capacities C1 and C2 are effective in the middle of the winding, the following equation applies to recovery voltages Uw+ and Uw- (see fig. 12 below).
Core
Tank
as well as for voltage exceeding C1
US
OS
Fig. 11
Winding arrangement with the related winding capacities
Fig. 12
Vectorial diagram for calculation of the recovery voltages on change-over selector contacts (+) and (-)
and consequently, the vector variable and the corresponding absolute value can be calculated as follows:
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17
2 Characteristic properties of the on-load tap-changer
For C1 = 1810 pF
–
C2 = 950 pF
–
U1 = 132 kV
–
UF = 13,2 kV
the following computational values result for the amount of recovery voltages UW+ and UW-:
The breaking currents Is+ und Is– are:
U1 Is+ = 2 Ö3
. .
. .
U1 Is-- = 2 Ö3
U1 + UF wC2 + j 2
U1 – UF wC2 + j 2
. w (C1 + C2) . w (C1 + C2)
Using the above number values, the following results:
Is+ = 63.97 mA Is– = 52.75 mA Due to the high values for Uw a tie-in resistor is required. After a tie-in resistor Rp = 280 kW is installed, the following values are obtained:
Uw+ = 19.6 kV
Uw– = 14.4 kV
Is+ = 72.0 mA
Is– = 53.0 mA
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2 Characteristic properties of the on-load tap-changer
2.12 On-load tap-changer in transformers for arcing furnaces (cf. also ordering sheets)
2.17 Service in artic areas
When in service, overloads of up to 2.5 times the rated transformer load occur in on-load tap-changers which are used in transformers for arcing furnaces. The on-load tapchangers must be adjusted to these operating conditions. Types V V, MS, M, RM, R and G: The permissible step voltage is reduced for the required rated through-current to 80 % of the relevant rated step voltages specified in the applicable TD. Type V: V 200 (250) is not designed for this type of service. With V 350 (400) the rated through-current is limited to 200 A.
Remember that ambient temperatures below -45 °C may destroy the gaskets of on-load tap-changers and accessories.
2.13 Installing the on-load tap-changer and off-circuit tap-changer The on-load tap-changer and off-circuit tap-changer must be installed vertically. The maximum deviation is 1°.
2.18 Hermetically sealed transformer with gas cushion This is available on request. Requests to MR for information on implementation must contain the maximum gas cushion density below the transformer cover.
2.14 Activating the on-load tap-changer during the transformer test
2.19 Parallel jumpers for parallel connection of tap selector planes
When the transformer is excited, the on-load tap-changer may only be activated under rated frequency. This also applies to no load service.
a) With forced current division: Parallel jumpers are not permitted
2.15 Special applications Examples: High voltage direct current, generator operation, phase shifter, traction transformer, portion of the harmonic waves > 10 % Please see extra ordering sheet. 2.16 Insulation oils To fill the oil compartment and the related oil conservator, use only new mineral oil for transformers in accordance with IEC 60296 (specification for unused mineral insulating oils for transformers and switchgear) and IEC 60422 (supervision and maintenance guide for mineral insulating oils in electrical equipment).
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If on-load tap-changer oil temperatures of less than -25 °C are expected, this must be specified on the ordering sheet so that a temperature control can be provided for reliable service. The temperature control consists of the thermo sensor and the measuring amplifier. The thermo sensor is installed in the on-load tap-changer head cover. It records the temperature of the oil in the on-load tap-changer. The measuring amplifier is installed in the motor-drive. It is connected to the control current circuit so that the motordrive is blocked for electrical service when the temperature control is activated.
b) With unforced current division: Parallel jumpers on the tap selector terminals are still mandatory even when the tap winding was wound in two or more wires and each of these coil taps is connected as a tapping connection to the terminal contacts. This reliably prevents the following: – Introduction of circulating currents into the current paths of tap selector and diverter switch – Arcing on movable tap selector contacts due to commutation – Overvoltages between adjacent tap selector terminals connected in parallel
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The parallel jumpers are also required to ensure the tie-in resistor takes effect for all parallel connected winding parts when tie-in measures are used.
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3 Important information on transformer design and tap-changer installation
3
Important information on transformer design and tap-changer installation
3.1
Designation of the tap selector terminals and operating positions
A connection diagram is individually prepared for every onload tap-changer. This diagram is the sole binding document for connection of the on-load tap-changer to the transformer. In addition to the electrical connections, this diagram contains a drawing of the physical arrangement of the terminals as seen from above. This diagram specifies the designation of the tap selector terminals and the operating positions for the related on-load tap-changer in accordance with customer specifications.
Example: Fig. 13 shows the contact designation of the two tap selector planes as seen from above with 1 to 9, K (clockwise). The on-load tap-changer is in position 2 at the moment and the changeover selector connects contacts 0 and +. Position 1 is reached by activating the other tap selector contact bridges counterclockwise (seen from above). With manual drive, this means: turning the hand crank to the right (clockwise). With motor-drive, this means: activating motor contactor K2 The direction of rotation on the on-load tap-changer remains unchanged regardless of which arrangement of the drive shaft is selected.
Special design
The contact designations used in the dimensional drawings for on-load tap-changers comply with MR’s standard designations.
If necessary, the following parts of the standard design can be changed:
The position designation of the on-load tap-changer is identical to that of the motor-drive.
1. The designation of the operating positions, 2. The specification of the activation directions „higher“/ “lower“ on the motor-drive
MR standard designation In the MR standard designation of the tap selector terminals and operating positions, tap selector terminal 1 is currentcarrying in operating position 1. Operating position 1 is at the same time an end position and is reached by running through the regulating range with the movement of the tap selector contact bridges counterclockwise.
Position
19
18
17
11
10
9
3
2
1
Current-carrying selector switch terminal
9
8
7
1
K
9
3
2
1
Change-over selector connects
0– 0–
>
0– 0–
0– 0+
0+ 0+
0+ 0+
0–
Operation in direction
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