Transformer Phase Displacement

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Vector Groups

Transformer nameplates carry a vector group reference such at Yy0, Yd1, Dyn11 etc. Thisrelatively simple nomenclature provides important information about the way in which three

phase windings are connected and any phase displacement that occurs.

Winding ConnectionsHV windings are designated: Y, D or Z (upper case)LV windings are designated: y, d or z (lower case)

Where:Y or y indicates a star connectionD or d indicates a delta connectionZ or z indicates a zigzag connectionN or n indicates that the neutral point is brought out

Phase DisplacementThe digits ( 0, 1, 11 etc) relate to the phase displacement between the HV and LV windingsusing a clock face notation. The phasor representing the HV winding is taken as referenceand set at 12 o'clock. It then follows that:

Digit 0 means that the LV phasor is in phase with the HV phasorDigit 1 that it lags by 30 degreesDigit 11 that it leads by 30 degreesetc

All references are taken from phase-to-neutral and assume a counter-clockwise phaserotation. The neutral point may be real (as in a star connection) or imaginary (as in a deltaconnection)

When transformers are operated in parallel it is important that any phase shift is the samethrough each. Paralleling typically occurs when transformers are located at one site andconnected to a common busbar (banked) or located at different sites with the secondaryterminals connected via distribution or transmission circuits consisting of cables and overheadlines

Basic Theory An ac voltage applied to a coil will induce a voltage in a second coil where the two are linkedby a magnetic path. The phase relationship of the two voltages depends upon which wayround the coils are connected. The voltages will either be in-phase or displaced by 180 deg asbelow:

In phase 180deg displacement

When 3 coils are used in a 3 phase transformer winding a number of options exist. The coilvoltages can be in phase or displaced as above with the coils connected in star or delta and, inthe case of a star winding, have the star point (neutral) brought out to an external terminal ornot.


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Example - Dyn11We now know that this transformer has a delta connected primary winding (D) a starconnected secondary (y) with the star point brought out (n) and a phase shift of 30 deg leading(11). Connections and vector diagrams are as follows::

HV

LV

Other ConfigurationsBy connecting the ends of the windings in other ways a wide range of options becomesavailable as set out below.

Phase shift (deg) Connections

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


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Differential Protection

Differential protection is a unit scheme that compares the current on the primary side of atransformer with that on the secondary side. Where a difference exists (other than that due to

the voltage ratio) it is assumed that the transformer has developed a fault and the plant isautomatically disconnected by tripping the relevant circuit breakers. The principle of operationis made possible by virtue of the fact that large transformers are very efficient and hence undernormal operation power-in equals power-out. Differential protection detects faults on all of theplant and equipment within the protected zone, including inter-turn short circuits.

Principle of Operation The operating principle employed by transformer differential protection is the Merz-Pricecirculating current system as shown below. Under normal conditions I1and I2 are equal andopposite such that the resultant current through the relay is zero. An internal fault produces anunbalance or 'spill' current that is detected by the relay, leading to operation.

Design Objectives An ideal scheme is required to be:

Extremely stable under through fault conditions Very fast to operate for an internal fault

Design Considerations A number of factors have to be taken into account in designing a scheme to meet theseobjectives. These include:

The matching of CT ratios Current imbalance produced by tap changing Dealing with zero sequence currents Phase shift through the transformer

Magnetising inrush current Each of these is considered further below.

The Matching of CT Ratios The CTs used for the Protection Scheme will normally be selected from a range of currenttransformers with standard ratios such as 1600/1, 1000/5, 200/1 etc. This could mean that thecurrents fed into the relay from the two sides of the power transformer may not balanceperfectly. Any imbalance must be compensated for and methods used include the applicationof biased relays (see below) and/or the use of the interposing CTs (see below).

Current Imbalance Produced by Tap Changing A transformer equipped with an on-load tap changer (OLTC) will by definition experience achange in voltage ratio as it moves over its tapping range. This in turn changes the ratio of

primary to secondary current and produces out-of-balance (or spill) current in the relay. As the


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transformer taps further from the balance position, so the magnitude of the spill currentincreases. To make the situation worse, as the load on the transformer increases themagnitude of the spill current increases yet again. And finally through faults could producespill currents that exceed the setting of the relay. However, none of these conditions is 'inzone' and therefore the protection must remain stable ie. it must not operate. Biased relaysprovide the solution (see below).

Dealing with Zero Sequence Currents Earth faults down stream of the transformer may give rise to zero sequence current,depending upon winding connections and earthing arrangements. Since zero sequencecurrent does not pass through a transformer, it will be seen on one side only producing spillcurrent and possible relay operation for an out-of-zone fault. To prevent such occurrence,zero sequence current must be eliminated from the differential scheme. This is achieved byusing delta connections on the secondary side of any CTs that are associated with maintransformer windings connected in star.

Where CT secondaries are connected in star on one side of a transformer and delta on theother, allowance must be made for the fact that the secondary currents outside the delta willonly be 1/√3 of the star equivalent.

Phase Shift Through the Transformer Having eliminated the problem of zero sequence currents (see above) through faults will stillproduce positive and negative sequence currents that will be seen by the protection CTs.These currents may experience a phase shift as they pass through the transformer dependingupon the transformer vector group. CT secondary connections must compensate to avoidimbalance and a possible mal-operation.

Magnetising Inrush Current When a transformer is first energised, magnetising inrush has the effect of producing a highmagnitude current for a short period of time. This will be seen by the supply side CTs only andcould be interpreted as an internal fault. Precautions must therefore be taken to prevent aprotection operation. Solutions include building a time delay feature into the relay and the use

of harmonic restraint driven, typically, by the high level of second harmonic associated withinrush current.

Other Issues

Biased Relays The use of a bias feature within a differential relay permits low settings and fast operatingtimes even when a transformer is fitted with an on-load tapchanger (see above). The effect ofthe bias is to progressively increase the amount of spill current required for operation as themagnitude of through current increases. Biased relays are given a specific characteristic bythe manufacturer.

Interposing CTs The main function of an interposing CT is to balance the currents supplied to the relay wherethere would otherwise be an imbalance due to the ratios of the main CTs. Interposing CTs areequipped with a wide range of taps that can be selected by the user to achieve the balancerequired.

As the name suggests, an interposing CT is installed between the secondary winding of themain CT and the relay. They can be used on the primary side or secondary side of the powertransformer being protected, or both. Interposing CTs also provide a convenient method ofestablishing a delta connection for the elimination of zero sequence currents where this isnecessary.


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Modern Relays It should be noted that some of the newer digital relays eliminate the need for interposing CTsby enabling essentials such as phase shift, CT ratios and zero sequence current elimination tobe programed directly into the relay.

As an independent focal point for the industry, Transformer World provides an opportunity tocollect information about transformer faults and problems occurring around the world with aview to building a dynamic database that can be accessed on line by members. This facility iscurrently under development.Preliminary proposals are set out below.If you are interested in taking part or have any comments please contact us [email protected]

Objective To build a global database of transformer failures to help owners, operators, contractors and

manufacturers benefit from experiences in the field.

Preliminary Proposal Participation will be by membership. Members will be granted facilities to enter data on-linewith minimal input effort. Members will also be able to search the database for information of atechnical nature.

Database Records Input - all fields Output - bold entries only

Name of person reporting Name of company Contact details Substation name Transformer name Manufacturer Voltage ratio Rating Year of Manufacture Design Type Type of Cooling Type of Tapchanger Date of fault Nature of failure Component that failed Cause of failure
mailto:[email protected]:[email protected]:[email protected]

Transformer Phase Displacement

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