The most common methods of measuring the resistance of an earth electrode

When an earth electrode system has been designed and installed, it is usually necessary to measure and confirm the earth resistance between the electrode and “true Earth”. The most commonly… Read more
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Three-phase Y and Delta Configurations | Polyphase AC Circuits

Initially we explored the idea of three-phase power systems by connecting three voltage sources together in what is commonly known as the “Y” (or “star”) configuration. This configuration of voltage sources is characterized by a common connection point joining one side of each source. (Figure below)



Three-phase “Y” connection has three voltage sources connected to a common point.


If we draw a circuit showing each voltage source to be a coil of wire (alternator or transformer winding) and do some slight rearranging, the “Y” configuration becomes more obvious in Figure below.



Three-phase, four-wire “Y” connection uses a “common” fourth wire.


The three conductors leading away from the voltage sources (windings) toward a load are typically called lines, while the windings themselves are typically called phases. In a Y-connected system, there may or may not (Figure below) be a neutral wire attached at the junction point in the middle, although it certainly helps alleviate potential problems should one element of a three-phase load fail open, as discussed earlier.


Three-phase, three-wire “Y” connection does not use the neutral wire.


When we measure voltage and current in three-phase systems, we need to be specific as to where we’re measuring. Line voltage refers to the amount of voltage measured between any two line conductors in a balanced three-phase system. With the above circuit, the line voltage is roughly 208 volts. Phase voltage refers to the voltage measured across any one component (source winding or load impedance) in a balanced three-phase source or load. For the circuit shown above, the phase voltage is 120 volts. The terms line current and phase current follow the same logic: the former referring to current through any one line conductor, and the latter to current through any one component.

Y-connected sources and loads always have line voltages greater than phase voltages, and line currents equal to phase currents. If the Y-connected source or load is balanced, the line voltage will be equal to the phase voltage times the square root of 3:



However, the “Y” configuration is not the only valid one for connecting three-phase voltage source or load elements together. Another configuration is known as the “Delta,” for its geometric resemblance to the Greek letter of the same name (Δ). Take close notice of the polarity for each winding in Figure below.



Three-phase, three-wire Δ connection has no common.


At first glance it seems as though three voltage sources like this would create a short-circuit, electrons flowing around the triangle with nothing but the internal impedance of the windings to hold them back. Due to the phase angles of these three voltage sources, however, this is not the case.

One quick check of this is to use Kirchhoff’s Voltage Law to see if the three voltages around the loop add up to zero. If they do, then there will be no voltage available to push current around and around that loop, and consequently there will be no circulating current. Starting with the top winding and progressing counter-clockwise, our KVL expression looks something like this:



Indeed, if we add these three vector quantities together, they do add up to zero. Another way to verify the fact that these three voltage sources can be connected together in a loop without resulting in circulating currents is to open up the loop at one junction point and calculate voltage across the break: (Figure below)



Voltage across open Δ should be zero.


Starting with the right winding (120 V ∠ 120o) and progressing counter-clockwise, our KVL equation looks like this:



Sure enough, there will be zero voltage across the break, telling us that no current will circulate within the triangular loop of windings when that connection is made complete.

Having established that a Δ-connected three-phase voltage source will not burn itself to a crisp due to circulating currents, we turn to its practical use as a source of power in three-phase circuits. Because each pair of line conductors is connected directly across a single winding in a Δ circuit, the line voltage will be equal to the phase voltage. Conversely, because each line conductor attaches at a node between two windings, the line current will be the vector sum of the two joining phase currents. Not surprisingly, the resulting equations for a Δ configuration are as follows:



Let’s see how this works in an example circuit: (Figure below)



The load on the Δ source is wired in a Δ.


With each load resistance receiving 120 volts from its respective phase winding at the source, the current in each phase of this circuit will be 83.33 amps:



So each line current in this three-phase power system is equal to 144.34 amps, which is substantially more than the line currents in the Y-connected system we looked at earlier. One might wonder if we’ve lost all the advantages of three-phase power here, given the fact that we have such greater conductor currents, necessitating thicker, more costly wire. The answer is no. Although this circuit would require three number 1 gage copper conductors (at 1000 feet of distance between source and load this equates to a little over 750 pounds of copper for the whole system), it is still less than the 1000+ pounds of copper required for a single-phase system delivering the same power (30 kW) at the same voltage (120 volts conductor-to-conductor).

One distinct advantage of a Δ-connected system is its lack of a neutral wire. With a Y-connected system, a neutral wire was needed in case one of the phase loads were to fail open (or be turned off), in order to keep the phase voltages at the load from changing. This is not necessary (or even possible!) in a Δ-connected circuit. With each load phase element directly connected across a respective source phase winding, the phase voltage will be constant regardless of open failures in the load elements.

Perhaps the greatest advantage of the Δ-connected source is its fault tolerance. It is possible for one of the windings in a Δ-connected three-phase source to fail open (Figure below) without affecting load voltage or current!



Even with a source winding failure, the line voltage is still 120 V, and load phase voltage is still 120 V. The only difference is extra current in the remaining functional source windings.


The only consequence of a source winding failing open for a Δ-connected source is increased phase current in the remaining windings. Compare this fault tolerance with a Y-connected system suffering an open source winding in Figure below.



Open “Y” source winding halves the voltage on two loads of a Δ connected load.


With a Δ-connected load, two of the resistances suffer reduced voltage while one remains at the original line voltage, 208. A Y-connected load suffers an even worse fate (Figure below) with the same winding failure in a Y-connected source



Open source winding of a “Y-Y” system halves the voltage on two loads, and looses one load entirely.


In this case, two load resistances suffer reduced voltage while the third loses supply voltage completely! For this reason, Δ-connected sources are preferred for reliability. However, if dual voltages are needed (e.g. 120/208) or preferred for lower line currents, Y-connected systems are the configuration of choice.

  • REVIEW:
  • The conductors connected to the three points of a three-phase source or load are called lines.
  • The three components comprising a three-phase source or load are called phases.
  • Line voltage is the voltage measured between any two lines in a three-phase circuit.
  • Phase voltage is the voltage measured across a single component in a three-phase source or load.
  • Line current is the current through any one line between a three-phase source and load.
  • Phase current is the current through any one component comprising a three-phase source or load.
  • In balanced “Y” circuits, line voltage is equal to phase voltage times the square root of 3, while line current is equal to phase current.
  • In balanced Δ circuits, line voltage is equal to phase voltage, while line current is equal to phase current times the square root of 3.
  • Δ-connected three-phase voltage sources give greater reliability in the event of winding failure than Y-connected sources. However, Y-connected sources can deliver the same amount of power with less line current than Δ-connected sources.

Vector Group of Transformer

Vector Group of Transformer

Introduction:

Three phase transformer consists of three sets of primary windings, one for each phase, and three sets of secondary windings wound on the same iron core. Separate single-phase transformers can be used and externally interconnected to yield the same results as a 3-phase unit.

The primary windings are connected in one of several ways. The two most common configurations are the delta, in which the polarity end of one winding is connected to the non-polarity end of the next, and the star, in which all three non-polarities (or polarity) ends are connected together. The secondary windings are connected similarly. This means that a 3-phase transformer can have its primary and secondary windings connected the same (delta-delta or star-star), or differently (delta-star or star-delta).

It’s important to remember that the secondary voltage waveforms are in phase with the primary waveforms when the primary and secondary windings are connected the same way. This condition is called “no phase shift.” But when the primary and secondary windings are connected differently, the secondary voltage waveforms will differ from the corresponding primary voltage waveforms by 30 electrical degrees. This is called a 30 degree phase shift. When two transformers are connected in parallel, their phase shifts must be identical; if not, a short circuit will occur when the transformers are energized.”

 Basic Idea of Winding:

  • An ac voltage applied to a coil will induce a voltage in a second coil where the two are linked by a magnetic path. The phase relationship of the two voltages depends upon which ways round the coils are connected. The voltages will either be in-phase or displaced by 180 deg
  • When 3 coils are used in a 3 phase transformer winding a number of options exist. The coil voltages can be in phase or displaced as above with the coils connected in star or delta and, in the case of a star winding, have the star point (neutral) brought out to an external terminal or not.

             Six Ways to wire Star Winding:

 

            Six Ways to wire Delta Winding:

 

 Polarity:

  • An ac voltage applied to a coil will induce a voltage in a second coil where the two are linked by a magnetic path.  The phase relationship of the two voltages depends upon which way round the coils are connected.  The voltages will either be in-phase or displaced by 180 deg.
  • When 3 coils are used in a 3 phase transformer winding a number of options exist.  The coil voltages can be in phase or displaced as above with the coils connected in star or delta and, in the case of a star winding, have the star point (neutral) brought out to an external terminal or not.

 

  • When Pair of Coil of Transformer have same direction than voltage induced in both coil are in same direction from one end to other end.
  • When two coil have opposite winding direction than Voltage induced in both coil are in opposite direction.

Winding connection designations:

  • First Symbol: for High Voltage: Always capital letters.
  •  D=Delta, Y=Star, Z=Interconnected star, N=Neutral
  • Second Symbol: for Low voltage: Always Small letters.
  •  d=Delta, y=Star, z=Interconnected star, n=Neutral.
  • Third Symbol: Phase displacement expressed as the clock hour number (1,6,11)
  • Example – Dyn11
    Transformer has a delta connected primary winding (D) a star connected secondary (y) with the star point brought out (n) and a phase shift of 30 deg leading (11).
  • The point of confusion is occurring in notation in a step-up transformer. As the IEC60076-1 standard has stated, the notation is HV-LV in sequence. For example, a step-up transformer with a delta-connected primary, and star-connected secondary, is not written as ‘dY11’, but ‘Yd11’. The 11 indicates the LV winding leads the HV by 30 degrees.
  • Transformers built to ANSI standards usually do not have the vector group shown on their nameplate and instead a vector diagram is given to show the relationship between the primary and other windings.

Vector Group of Transformer:

  • 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 primary and secondary 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. If two transformers of different vector groups are connected in parallel then phase difference exist between the secondary of the transformers and large circulating current flows between the two transformers which is very detrimental.
  • Phase Displacement between HV and LV Windings:

    •  The vector for the high voltage winding is taken as the reference vector. Displacement of the vectors of other windings from the reference vector, with anticlockwise rotation, is represented by the use of clock hour figure.
    • IS: 2026 (Part 1V)-1977 gives 26 sets of connections star-star, star-delta, and star zigzag, delta-delta, delta star, delta-zigzag, zigzag star, zigzag-delta. Displacement of the low voltage winding vector varies from zero to -330° in steps of -30°, depending on the method of connections.
    • Hardly any power system adopts such a large variety of connections. Some of the commonly used connections with phase displacement of 0, -300, -180″ and -330° (clock-hour setting 0, 1, 6 and 11).
    • Symbol for the high voltage winding comes first, followed by the symbols of windings in diminishing sequence of voltage. For example a 220/66/11 kV Transformer connected star, star and delta and vectors of 66 and 11 kV windings having phase displacement of 0° and -330° with the reference (220 kV) vector will be represented As Yy0 – Yd11.
    • The digits (0, 1, 11 etc) relate to the phase displacement between the HV and LV windings using a clock face notation. The phasor representing the HV winding is taken as reference and set at 12 o’clock. Phase rotation is always anti-clockwise. (International adopted).
    • Use the hour indicator as the indicating phase displacement angle. Because there are 12 hours on a clock, and a circle consists out of 360°, each hour represents 30°.Thus 1 = 30°, 2 = 60°, 3 = 90°, 6 = 180° and 12 = 0° or 360°.
    • The minute hand is set on 12 o’clock and replaces the line to neutral voltage (sometimes imaginary) of the HV winding. This position is always the reference point.
    • Example:
    • Digit 0 =0° that the LV phasor is in phase with the HV phasor

    Digit 1 =30° lagging (LV lags HV with 30°) because rotation is anti-clockwise.

  • Digit 11 = 330° lagging or 30° leading (LV leads HV with 30°)
  • Digit 5 = 150° lagging (LV lags HV with 150°)
  • Digit 6 = 180° lagging (LV lags HV with 180°)
  • When transformers are operated in parallel it is important that any phase shift is the same through each. Paralleling typically occurs when transformers are located at one site and connected to a common bus bar (banked) or located at different sites with the secondary terminals connected via distribution or transmission circuits consisting of cables and overhead lines.
  • Phase Shift (Deg)

    Connection

    0

    Yy0

    Dd0

    Dz0

    30 lag

    Yd1

    Dy1

    Yz1

    60 lag

     

    Dd2

    Dz2

    120 lag

     

    Dd4

    Dz4

    150 lag

    Yd5

    Dy5

    Yz5

    180 lag

    Yy6

    Dd6

    Dz6

    150 lead

    Yd7

    Dy7

    Yz7

    120 lead

     

    Dd8

    Dz8

    60 lead

     

    Dd10

    Dz10

    30 lead

    Yd11

    Dy11

    Yz11

     

     

     

     

    •  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
    GroupO’clockTC
    Group I0 o’clock, 0°delta/delta, star/star
    Group II6 o’clock, 180°delta/delta, star/star
    Group III1 o’clock, -30°star/delta, delta/star
    Group IV11 o’clock, +30°star/delta, delta/star
    Minus indicates LV lagging HV, plus indicates LV leading HV

     Clock Notation: 0

    Clock Notation : 1

     Clock Notation: 2

    Clock Notation: 4

    Clock Notation: 5

    Clock Notation: 6

    Clock Notation: 7

    Clock Notation: 11

     Points to be consider while Selecting of Vector Group:

    • Vector Groups are the IEC method of categorizing the primary and secondary winding configurations of 3-phase transformers. Windings can be connected as delta, star, or interconnected-star (zigzag). Winding polarity is also important, since reversing the connections across a set of windings affects the phase-shift between primary and secondary. Vector groups identify the winding connections and polarities of the primary and secondary. From a vector group one can determine the phase-shift between primary and secondary.
    • 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.
      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.
      6. 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. 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 30deg 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 (ie, 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.

    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.

    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 configurations 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.

     For selecting Distribution Transformer:

    •  The first 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.
    • The second criterion to consider is what phase-shift you want between primary and secondary. For example, Dy11 and Dy5 transformers are both delta-star. If we don’t care about the phase-shift, then either transformer will do the job. Phase-shift is important when we are paralleling sources. We want the phase-shifts of the sources to be identical.
    • If we are paralleling transformers, then you want them to have the same the same vector group. If you are replacing a transformer, use the same vector group for the new transformer, otherwise the existing VTs and CTs used for protection and metering will not work properly.
    • There is no technical difference between the one vector groups (i.e. Yd1) or another vector group (i.e. Yd11) in terms of performance. The only factor affecting the choice between one or the other is system phasing, ie whether parts of the network fed from the transformer need to operate in parallel with another source. It also matters if you have an auxiliary transformer connected to generator terminals. Vector matching at the auxiliary bus bar

     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.

    (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.
  • (3)  Delta – Delta (Dd 0 or Dd 6)

    • 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.
    • 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 zigzag connection (interconnection between phases), third harmonic voltages are reduced. It also allows unbalanced 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.

    (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.

     (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)  Yz-5

    • For Distribution Transformer up to 250MVA for local distribution system.
    • The Neutral point can be loaded with rated Current.

     Application of Transformer according  according to Uses:

    •  Step up Transformer: It should be Yd1 or Yd11.
    • Step down Transformer: It should be Dy1 or Dy11.
    • Grounding purpose Transformer: It should be Yz1 or Dz11.
    • Distribution Transformer: We can consider vector group of Dzn0 which reduce the 75% of harmonics in secondary side.
    • Power Transformer: Vector group is deepen on application for Example : Generating Transformer : Dyn1 , Furnace Transformer: Ynyn0.

    Convert One Group of Transformer to Other Group by Channing External Connection:

    (1)  Group I: Example: Dd0 (no phase displacement between HV and 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(-120°) or Dd8(+120°) connection. The same is true for internal connected Dd4 or Dd8 transformers.

    (2)  Group II: Example: Dd6 (180° displacement between HV and 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(-60°) or Dd10(+60°) connection.

    (3)  Group III: Example: Dyn1 (-30° displacement between HV and 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(-150°) or Dyn9(+90°) connection.

    (4)  Group IV: Example: Dyn11 (+30° displacement between HV and 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(+150°) or Dyn3(-90°) connection.

    Point 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.

    Why 30°phase shift occur in star-delta transformer between primary and secondary?

    • The phase shift is a natural consequence of the delta connection. The currents entering or leaving the star winding of the transformer are in phase with the currents in the star windings. Therefore, the currents in the delta windings are also in phase with the currents in the star windings and obviously, the three currents are 120 electrical degrees apart.
    • But the currents entering or leaving the transformer on the delta side are formed at the point where two of the windings comprising the delta come together – each of those currents is the phasor sum of the currents in the adjacent windings.
    • When you add together two currents that are 120 electrical degrees apart, the sum is inevitably shifted by 30 degrees.
    •  The Main reason for this phenomenon   is that the phase voltage lags line current by 30degrees.consider a delta/star transformer. The phase voltages in three phases of both primary and secondary. you will find that in primary the phase voltage and line voltages are same, let it be VRY(take one phase).but, the corresponding secondary will have the phase voltage only in its phase winding as it is star connected. the line voltage of star connected secondary and delta connected primary won’t have any phase differences between them. so this can be summarized that “the phase shift is associated with the wave forms of the three phase windings.

     Why  when Generating Transformer is Yd1 than Distribution Transformer is Dy11:

    • This is the HV Side or the Switchyard side of the Generator Transformer is connected in Delta and the LV Side or the generator side of the GT is connected in Star, with the Star side neutral brought out.
    • The LV side voltage will “lag” the HV side voltage by 30 degrees.
    • Thus, in a generating station we create a 30 degrees lagging voltage for transmission, with respect to the generator voltage.
    • As we have created a 30 degrees lagging connection in the generating station, it is advisable to create a 30 degrees leading connection in distribution so that the user voltage is “in phase” with the generated voltage. And, as the transmission side is Delta and the user might need three phase, four-wire in the LV side for his single phase loads, the distribution transformer is chosen as Dyn11.
    • There is magnetic coupling between HT and LT. When the load side (LT) suffers some dip the LT current try to go out of phase with HT current, so 30 degree phase shift in Dyn-11 keeps the two currents in phase when there is dip.
    • So the vector group at the generating station is important while selecting distribution Transformer.

    Vector Group in Generating-Transmission-Distribution System:

    • Generating TC is Yd1 transmitted power at 400KV, for 400KV to 220KV Yy is used and by using Yd between e.g. 220 and 66 kV, then Dy from 66 to 11 kV so that their phase shifts can be cancelled out. And for LV (400/230V) supplies at 50 Hz are usually 3 phase, earthed neutral, so a “Dyn” LV winding is needed. Here GT side -30lag (Yd1) can be nullify +30 by using distribution Transformer of Dy11.
    • A reason for using Yd between e.g. 220 and 66 kV, then Dy from 66 to 11 kV is that their phase shifts can cancel out and It is then also possible to parallel a 220/11 kV YY transformer, at 11 kV, with the 66/11 kV (a YY transformer often has a third, delta, winding to reduce harmonics). If one went Dy11 – Dy11 from 220 to 11 kV, there would be a 60 degree shift, which is not possible in one transformer. The “standard” transformer groups in distribution avoid that kind of limitation, as a result of thought and experience leading to lowest cost over many years.

    Generator TC is Yd1, Can we use Distribution TC Dy5 instead of Dy11.

    • With regards to theory, there are no special advantages of Dyn11 over Dyn5.
    • In Isolation Application: In isolated applications there is no advantage or disadvantage by using Dy5 or Dy11. If however we wish to interconnect the secondary sides of different Dny transformers, we must have compatible transformers, and that can be achieved if you have a Dyn11 among a group of Dyn5’s and vice versa.
    • In Parallel Connection: Practically, the relative places of the phases remain same in Dyn11 compared to Dyn5.
    • If we use Yd1 Transformer on Generating Side and Distribution side Dy11 transformer than -30 lag of generating side (Yd1) is nullify by +30 Lead at Receiving side Dy11) so no phase difference respect to generating Side and if we are on the HV side of the Transformer, and if we denote the phases as R- Y-B from left to right, the same phases on the LV side will be R- Y -B, but from left to Right.
    • This will make the Transmission lines have same color (for identification) whether it is input to or output from the Transformer.
    • If we use Yd1 Transformer on Generating Side and Distribution side Dy5 transformer than -30 lag of generating side (Yd1) is more lag by  -150 Lag at Receiving side (Dy5) so Total phase difference respect to generating Side is 180 deg (-30+-150=-180) and if we are on the HV side of the Transformer, and if we denote the phases as R- Y-B from left to right, the same phases on the LV side will be R- Y -B, but from Right to Left.
    • This will make the Transmission lines have No same color (for identification) whether it is input to or output from the Transformer.
    • The difference in output between the Dyn11 and Dny5 and is therefore 180 degrees.

    Control of Synchronous Generators with Droop and Cross-Current Compensation.

    Control of Synchronous Generators with Droop and Cross-Current Compensation.

    The excitation of a synchronous generator is usually done by an AVR (Automatic Voltage Regulator) that uses generator voltage and/or current as inputs in order to control its output to a pre-set value.

    AVRs include different control modes to optimise performance depending on whether the generator is connected to the grid, or in island mode. Therefore, they can be set to maintain the voltage, the PF or the reactive power.
    In this report, we will analyse the principle of operation of the voltage control mode of the AVR, known as droop compensation, when one or more generators operate in island mode or are connected to the grid. Based on droop control limitations, we will study techniques to improve its performance, and compare it with the cross-current compensation method.

    1.  Voltage control mode – Droop Compensation

    In the voltage control or droop mode, the AVR is regulated by a droop characteristic, which is shown in the following drawing. 

     

    Figure 1. AVR Set-point V vs reactive power Q

    The droop characteristic represents a graph of the AVR voltage set-point V as a function of the generator reactive power produced. This set-point regulates the generator terminal voltage when in island mode. 
    The interpretation of the above graph is that as the reactive power demand from the generator increases, the generator terminal voltage decreases. The set-point in the AVR is chosen so that when the generator reactive power Q supplied is zero, the generator VN is equal to the nominal voltage. If the initial AVR set-point is not changed, VL will be the voltage due to droop that the generator terminal voltage will reach operating in island mode against reactive load QL.
    The reactive power generated is calculated from the generator voltage and current signals, fed back to the AVR. Droop compensation is set as percentage drop of the nominal voltage VN for maximum reactive power QL generated. Depending on the AVR, maximum reactive power is usually defined either as the reactive power exported at rated power factor, or as the MVA rating of the generator. 
    Droop setting can be given values from 0%, which effectively disables the droop, to a maximum of usually 20%, which could cause VL to drop to 0.8 p.u. Typically, a setting of 4-6% is chosen.

    Droop compensation is a control technique designed when the generator is connected to the grid, so it is not required when one generator is in island mode.
    On the other hand, when connected to the grid, droop compensation is required and the droop characteristic is used below to explain the control of the AVR.

     

    Figure 2. Representation of AVR control when connected to the grid.

    When a generator is directly connected to the grid, the grid voltage VG is fixed and cannot be controlled by the AVR. Any requirement for reactive power from the generator will result in the AVR internal voltage set-point V to change to meet the new demand. So, in the diagram of figure 4, the increased reactive power demand QL causes the AVR set-point to increase from VG to VL because of the droop compensation control.

    2.  Generator operation modes

    According to the network topology, the following operating scenarios can be identified:

    • Operating in island mode as a stand-alone generator.
    • Synchronised to the grid.
    • Operating in island mode, but in parallel with other generators.

    These three scenarios are analysed separately below.

    2.1.   Island mode operation with a single generator

    This is the simplest case in terms of AVR control, as there is only one active component in the circuit that can affect the busbar voltage and react to any reactive load changes.

    A single synchronous machine operating in island mode is only responsible for two actions:

    • Control the busbar voltage to the required nominal level.
    • Supply the load with the required reactive power and respond fast to any load changes to meet the demand at any time.

    The diagram below presents the simple case described.

     

    Figure 3. One generator in island mode with droop enable contact.

    The AVR in this case does not require droop compensation to control its output. In order to eliminate the droop effect, which would otherwise drop the circuit voltage with any increase in the reactive load, there are two possibilities:

    • Set the droop setting within the AVR to zero %.
    • Close the droop enable contact shown in the diagram above, so that the compounding CT current would not flow into the AVR.

     

    2.2.   Synchronised to the grid

    In the case where there is connection to the grid, the AVR needs droop compensation in order to control its output. The circuit configuration and the droop characteristic for this case are presented in figures 3 and 4 respectively.

     

    Figure 4. One generator synchronised to the grid.

    The configuration above shows that by the operation of a simple contact the droop can be enabled or disabled, allowing the flexibility to disable it when in island operation and to enable it before connecting it to the grid. This eliminates the undesired effect of lower than nominal voltages when in island operation.

    2.3.   Island operation with paralleled generators

    For the case of island mode operation with at least two generators connected in parallel to supply the load, the control of the voltage and the reactive power requirements have to be shared between the generators in parallel. 

    There are two control methods for the generator AVRs to achieve this:

    • Control with droop compensation.
    • Control with cross-current compensation.

     

    2.3.1.   Control with droop compensation

    In this case the following assumptions have to be satisfied:

    • The generators must be of equal size.
    • The AVRs must have the same droop characteristic and the same setting applied.

    In the simplest case, the AVRs can operate in droop compensation mode to obtain equal sharing of the reactive load. The relevant diagram is shown below.

     

    Figure 5. Island mode with two generators in parallel in droop mode.

    The two generators in figure 5 share equally the reactive load connected according to the droop characteristic of the AVRs and the setting applied.

    Although this control mode is ideal when there is grid connection, in island mode it results in the voltage output being dependent on the reactive power demand. So, as the requirement for reactive power increases, the output voltage from the generators decreases due to the droop compensation. 

    2.3.2.   Control with cross-current compensation

    Cross-current compensation or reactive differential is a method that allows two or more paralleled generators to share equally a reactive load, given that the following assumptions are satisfied:

    • There is no grid connection, i.e. the generators operate in island mode.
    • The generators are of equal size.
    • The AVRs  have the same droop characteristic, which is set to its maximum setting.

    The secondary wiring of the compounding CTs of all the generators to be paralleled have to be interconnected. Below, the wiring configuration for two generators set up for cross-current compensation is included.

     

    Figure 6. Island mode with two generators in parallel with cross-current compensation

    According to this method, the same current develops through the compounding CT’s of the generators in parallel, since they are identical, and when the CCC contact closes, it stops flowing through the AVRs, but only flows through the CTs. 
    The configuration above shows that by the operation of a simple contact the CCC can be enabled or disabled, allowing the flexibility to enable it when in island operation and to disable it before connecting it to the grid. This eliminates the droop effect and allows the paralleled generators to operate in island mode at nominal voltage when the reactive load increases.

    The figure below shows the complete configuration with all the techniques explained previously incorporated for maximum functionality. This includes both the droop-compensation and the CCC enable-disable contacts.
    In this case, when the generators are connected to the grid, all contacts must be open.
    For paralleled generators in island mode, the droop contacts must be open and the CCC contact closed. 

     

    Figure 7. Island mode with two generators in parallel with cross-current compensation and droop disable contacts.