Understanding Transformer Vector Groups in Transformer Systems

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Transformer vector groups play a crucial role in determining the phase relationships between high and low voltage sides in transformer windings. Proper understanding of vector groups is essential for parallel connection of transformers to prevent phase differences and potential short circuits. The article delves into the significance of vector groups, their identification on transformer nameplates, and the implications of different winding connections.

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Transformer Cooling of Transformer Transformer Transformer Oil Main Parts Transformer Vector Groups Vector Groups PRESENTED BY PROF. VG PATEL VG PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS Vector Group of Transformers The three phase transformer windings can be connected several ways. Based on the windings connection, the vector group of the transformer is determined. The transformer vector group is indicated on the Name Plate of transformer by the manufacturer. The vector group indicates the phase difference between the HV and LV sides, introduced due to that particular configuration of transformer windings connection. The determination of vector group of transformers is very important before connecting two or more transformers in parallel. When two transformers are connected in parallel, their phase shifts must be identical; if not, then phase difference exist between the secondary of the transformers a short circuit will occur and large circulating current flows between the two transformers when the transformers are energized. 12/8/2024 2 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS 12/8/2024 3 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS Six Ways to wire Delta Winding 12/8/2024 4 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS When Pair of Coil of Transformer have same direction, then voltage induced in both coil are in same direction from one end to other end. When two coil have opposite winding direction, then Voltage induced in both coil are in opposite direction. Winding connection designations First Symbol: for High Voltage: Always capital letters. D=Delta, S=Star, Z=Interconnected star, N=Neutral Second Symbol: for Low voltage: Always Small letters. d=Delta, s=Star, z=Interconnected star, n=Neutral. Third Symbol: Phase displacement expressed as the clock hour number (1,6,11) 12/8/2024 5 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS The phase-bushings on a three phase transformer are marked either ABC, UVW or 123 (HV-side capital, LV-side small letters). Two winding, three phase transformers can be divided into four main categories: Group Group I Group II Group III Group IV Minus indicates LV lagging HV, plus indicates LV leading HV TC O clock 0 o clock, 00 6 o clock, 1800 1 o clock, -300 11 o clock, +300 delta/delta, star/star delta/delta, star/star star/delta, delta/star star/delta, delta/star 12/8/2024 6 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS We know that, the primary and secondary coils on any one limb have induced emfs that are in time-phase. Let's consider two transformers of same number primary turns and the primary windings are connected in star. The secondary number of turns per phase in both transformers are also same. But the first transformer has star connected secondary and other transformer has delta connected secondary. If same voltages are applied in primary of both transformers, the secondary induced emf in each phase will be in same time-phase with that of respective primary phase, as because the primary and secondary coils of same phase are wound on the same limb in the core of transformer. In first transformer, as the secondary is star connected, the secondary line voltage is 3 times of induced voltage per secondary phase coil. But in case of second transformer, where secondary is delta connected, the line voltage is equal to induced voltage per secondary phase coil. 12/8/2024 7 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS If we go through the vector diagram of secondary line voltages of both transformers, we will easily find that there will be a clear 30o angular difference between the line voltages of these transformers. Now, if we try to run these transformers in parallel then there will be circulating current flowing between the transformers as because there is a phase angle difference between their secondary line voltages. This phase difference cannot be compensated. Thus two sets of connections giving secondary voltages with a phase displacement cannot be intended for parallel operation of transformers. The following table gives the connections for which from the view point of phase sequence and angular divergences, transformer can be operated parallel. According to their vector relation, all three phase transformers are divided into different vector group of transformers. All electrical power transformers of a particular vector group can easily be operated in parallel if they fulfill other condition for parallel operation of transformers. 12/8/2024 8 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS CONSIDERING ANTICLOCKWISE ROTATION GROUP CONNECTION CONNECTION Yy0 Dd0 12/8/2024 9 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS CONSIDERING ANTICLOCKWISE ROTATION GROUP CONNECTION CONNECTION Yy6 Dd6 12/8/2024 10 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS CONSIDERING ANTICLOCKWISE ROTATION GROUP CONNECTION CONNECTION Yd1 Dy1 12/8/2024 11 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS CONSIDERING ANTICLOCKWISE ROTATION GROUP CONNECTION CONNECTION Yd11 Dy11 12/8/2024 12 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS Fig.: Designation of symbols for three phase transformers 12/8/2024 13 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS Transformer vector group depends upon: 1. Removing harmonics: Dy connection y winding nullifies 3rd harmonics, preventing it to be reflected on delta side. 2. Parallel operations: All the transformers should have same vector group & polarity of the winding. 3. Earth fault Relay: A Dd transformer does not have neutral. to restrict the earth faults in such systems, we may use zig zag wound transformer to create a neutral along with the earth fault relay. 4. Type of Non Liner Load: systems having different types of harmonics & non linear Types of loads e.g. furnace heaters, VFDS etc for that we may use Dyn11, Dyn21, Dyn31 configuration, wherein, 30 deg. shifts of voltages nullifies the 3rd harmonics to zero in the supply system. 12/8/2024 14 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS 5. Type of Transformer Application: Generally for Power export transformer i.e. generator side is connected in delta and load side is connected in star. For Power export import transformers i.e. in Transmission Purpose Transformer star star connection may be preferred by some since this avoids a grounding transformer on generator side and perhaps save on neutral insulation. Most of systems are running in this configuration. May be less harmful than operating delta system incorrectly. Yd or Dy connection is standard for all unit connected generators. There are a number of factors associated with transformer connections and may be useful in designing a system, and the application of the factors therefore determines the best selection of transformers. 12/8/2024 15 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS For example: For selecting Star Connection: A star connection presents a neutral. If the transformer also includes a delta winding, that neutral will be stable and can be grounded to become a reference for the system. A transformer with a star winding that does NOT include a delta does not present a stable neutral. Star-star transformers are used if there is a requirement to avoid a 30 deg phase shift, if there is a desire to construct the three-phase transformer bank from single-phase transformers, or if the transformer is going to be switched on a single-pole basis (i.e. one phase at a time), perhaps using manual switches. Star-star transformers are typically found in distribution applications, or in large sizes interconnecting high-voltage transmission systems. Some star-star transformers are equipped with a third winding connected in delta to stabilize the neutral. 12/8/2024 16 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS For selecting Delta Connection: A delta connection introduces a 30 electrical degree phase shift. A delta connection traps the flow of zero sequence currents. For selecting Delta-Star Connection: Delta-star transformers are the most common and most generally useful transformers. Delta-delta transformers may be chosen if there is no need for a stable neutral, or if there is a requirement to avoid a 30 electrical degree phase shift. The most common application of a delta-delta transformer is as tan isolation transformer for a power converter. 12/8/2024 17 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS For selecting Zig zag Connection: The Zig Zag winding reduces voltage unbalance in systems where the load is not equally distributed between phases, and permits neutral current loading with inherently low zero-sequence impedance. It is therefore often used for earthing transformers. Provision of a neutral earth point or points, where the neutral is referred to earth either directly or through impedance. Transformers are used to give the neutral point in the majority of systems. The star or interconnected star (Z) winding configura- tions give a neutral location. If for various reasons, only delta windings are used at a particular voltage level on a particular system, a neutral point can still be provided by a purpose-made transformer called a neutral earthing . 12/8/2024 18 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS For selecting Distribution Transformer: The criterion to consider in choosing a vector group for a distribution transformer for a facility is to know whether we want a delta-star or star-star. Utilities often prefer star-star transformers, but these require 4-wire input feeders and 4-wire output feeders (i.e. incoming and outgoing neutral conductors). For distribution transformers within a facility, often delta-star are chosen because these transformers do not require 4-wire input; a 3-wire primary feeder circuit suffices to supply a 4-wire secondary circuit. That is because any zero sequence current required by the secondary to supply earth faults or unbalanced loads is supplied by the delta primary winding, and is not required from the upstream power source. The method of earthing on the secondary is independent of the primary for delta-star transformers. There is no technical difference between the one vector groups (i.e. Dy1) or another vector group (i.e. Dy11) in terms of performance. The only factor affecting the choice between one or the other is system phasing, i.e whether parts of the network fed from the transformer need to operate in parallel with another source. 12/8/2024 19 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS Application of Transformer according to Vector Group 1) Dyn11, Dyn1, YNd1, YNd11 Common for distribution transformers. Normally Dyn11 vector group using at distribution system. Because Generating Transformer are YNd1 for neutralizing the load angle between 11 and 1. We can use Dyn1 at distribution system, when we are using Generator Transformer are YNd11. In some industries 6 pulse electric drives are using due to this 5thharmonics will generate if we use Dyn1 it will be suppress the 5th harmonics. Star point facilitates mixed loading of three phase and single phase consumer connections. The delta winding carry third harmonics and stabilizes star point potential. A delta-Star connection is used for step-up generating stations. If HV winding is star connected there will be saving in cost of insulation. But delta connected HV winding is common in distribution network, for feeding motors and lighting loads from LV side. 12/8/2024 20 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS 2) Star-Star (Yy0 or Yy6) Mainly used for large system tie-up transformer. Most economical connection in HV power system to interconnect between two delta systems and to provide neutral for grounding both of them. Tertiary winding stabilizes the neutral conditions. In star connected transformers, load can be connected between line and neutral, only if (a) the source side transformers is delta connected or (b) the source side is star connected with neutral connected back to the source neutral. In this transformers. Insulation cost is highly reduced. Neutral wire can permit mixed loading. Triple harmonics are absent in the lines. These triple harmonic currents cannot flow, unless there is a neutral wire. This connection produces oscillating neutral. Three phase shell type units have large triple harmonic phase voltage. However three phase core type transformers work satisfactorily. A tertiary mesh connected winding may be required to stabilize the oscillating neutral due to third harmonics in three phase banks. 12/8/2024 21 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS 3) Delta Delta (Dd0 or Dd6) This is an economical connection for large low voltage transformers. Large unbalance of load can be met without difficulty. Delta permits a circulating path for triple harmonics thus attenuates the same. It is possible to operate with one transformer removed in open delta or V connection meeting 58 percent of the balanced load. Three phase units cannot have this facility. Mixed single phase loading is not possible due to the absence of neutral. 4) Star-Zig-zag or Delta-Zig-zag (Yz or Dz) These connections are employed where delta connections are weak. Interconnection of phases in zigzag winding effects a reduction of third harmonic voltages and at the same time permits unbalanced loading. 12/8/2024 22 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS This connection may be used with either delta connected or star connected winding either for step-up or step-down transformers. In either case, the zigzag winding produces the same angular displacement as a delta winding, and at the same time provides a neutral for earthing purposes. The amount of copper required from a zigzag winding in 15% more than a corresponding star or delta winding. This is extensively used for earthing transformer. Due to zig-zag connection (interconnection between phases), third harmonic voltages are reduced. It also allows unbalan-ced loading. The zigzag connection is employed for LV winding. For a given total voltage per phase, the zigzag side requires 15% more turns as compared to normal phase connection. In cases where delta connections are weak due to large number of turns and small cross sections, then zigzag star connection is preferred. It is also used in rectifiers. 12/8/2024 23 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS 5) Zig-zag/ star (ZY1 or Zy11) Zigzag connection is obtained by inter connection of phases.4- wire system is possible on both sides. Unbalanced loading is also possible. Oscillating neutral problem is absent in this connection. This connection requires 15% more turns for the same voltage on the zigzag side and hence costs more. Hence a bank of three single phase transformers cost about 15% more than their 3- phase counterpart. Also, they occupy more space. But the spare capacity cost will be less and single phase units are easier to transport. Unbalanced operation of the transformer with large zero sequence fundamental mmf content also does not affect its performance. Even with Yy type of poly phase connection without neutral connection the oscillating neutral does not occur with these cores. Finally, three phase cores themselves cost less than three single phase units due to compactness. 12/8/2024 24 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS 6) Yd5 Mainly used for machine and main Transformer in large Power Station and Transmission Substation. The Neutral point can be loaded with rated Current. 7) Yz5 For Distribution Transformer up to 250 KVA for local distribu- tion system. The Neutral point can be loaded with rated Current. 12/8/2024 25 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS Application of Transformer according to Uses Step up Transformer Step down Transformer It should be Dy1 or Dy11. Grounding purpose Transformer Distribution Transformer Dzn0 which reduce the 75% of harmonics in secondary side. Power Transformer Vector group depend on application, eg: Generating Transformer: Dyn1, Furnace Transformer: Ynyn0. It should be Yd1 or Yd11. It should be Yz1 or Dz11. We can consider vector group of 12/8/2024 26 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS Convert One Group of Transformer to Other Group by Channing External Connection Group I: Example: Dd0 (no phase displacement between HV & LV) The conventional method is to connect the red phase on A/a, Yellow phase on B/b, and the Blue phase on C/c. Other phase displacements are possible with unconventional connections (for instance red on b, yellow on c and blue on a) By doing some unconventional connections externally on one side of the Transformer, an internal connected Dd0 transformer can be changed either to a Dd4(-1200) or Dd8(+1200) conne- ction. The same is true for internal connected Dd4 or Dd8 transformers. 12/8/2024 27 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS Group II: Example: Dd6 (1800 displacement between HV & LV) By doing some unconventional connections externally on one side of the Transformer, an internal connected Dd6 transformer can be changed either to a Dd2(-600) or Dd10(+600) connection. Group III: Example: Dyn1 (-300 displacement between HV & LV) By doing some unconventional connections externally on one side of the Transformer, an internal connected Dyn1 transformer can be changed either to a Dyn5(-1500) or Dyn9(+900) connection. Group IV: Example: Dyn11 (+300 displacement between HV & LV) By doing some unconventional connections externally on one side of the Transformer, an internal connected Dyn11 transformer can be changed either to a Dyn7(+1500) or Dyn3(-900) connection. 12/8/2024 28 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS Points to be remembered For Group-III & Group-IV: By doing some unconventional connections externally on both sides of the Transformer, an internal connected Group-III or Group-IV transformer can be changed to any of these two groups. Thus by doing external changes on both sides of the Transformer an internal connected Dyn1 transformer can be changed to either a: Dyn3, Dyn5, Dyn7, Dyn9 or Dyn11 transformer, This is just true for star/delta or delta/star connections. For Group-I & Group-II: Changes for delta/delta or star/star transformers between Group-I and Group-III can just be done internally. 12/8/2024 29 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS VECTOR GROUP CHECK TEST Connect terminals 1U, 2U together. Apply three phase 415 V, 50 HZ, AC to HV terminals 1U,1V,1W. Measure voltages between terminals 1V-2V, 1W-2V, 1W-2W & 1V-2n, 2n- 1U, 1V-1U. 1V-2W, 1V-2V & 1V-2W will be equal, while 1W-2V will be greater than 1W-2W and 1V-2n plus 2n-1U will be equal to 1V-1U for Dyn11 vector group. 12/8/2024 30 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS VECTOR GROUP CHECK TEST Let s have a YNd11 transformer. Connect neutral point of star connected winding with earth. Join 1U of HV and 2W of LV together. Apply 415 V, three phase supply to HV terminals. Measure voltages between terminals 2U-1N, 2V-1N, 2W-1N, that means voltages between each LV terminal and HV neutral. Also measure voltages between terminals 2V-1V, 2W-1W and 2V-1W. For YNd11 transformer, we will find, 2U-1N > 2V-1N > 2W-1N ; 2V-1W > 2V-1V or 2W-1W. The vector group check test of transformer for other groups can also be done in similar way. 12/8/2024 31 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS For parallel operation of transformers, vector group of both the transformers should be the same. Q - Can Dy1 and Yd11 vector group transformers be paralleled? If answer is NO, why? If answer is YES, how? 12/8/2024 32 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS Please answer the Question. Unless you speak answer before computer screen, you can not move to next slide. 12/8/2024 33 PROF. V. G. PATEL

TRANSFORMER ENCYCLOPAEDIA VECTOR GROUP OF TRANSFORMERS Answer: In order to have zero relative phase displacement of LV side line voltages, the transformers belonging to the same group can be paralleled. For example, two transformers with Yd1 and Dy1 connections can be paralleled. The transformers with different groups (+300 & -300 ---- 11 & 1) can be paralleled by reversing the phase sequence of one of them. For example, A transformer with Yd11 connection can be paralleled with that having Dy1 connection by reversing the phase sequence of both primary and secondary terminals of 12/8/2024 the Dy1 transformer. 34 PROF. V. G. PATEL