Vector Group of Transformers
• A vector group is the International Electrotechnical Commission (IEC) method of categorizing the high voltage (HV) windings and low voltage (LV) winding configurations of three-phase transformers.
• Vector group is a connection of primary winding , secondary winding and an electrical angle difference .
• The vector group designation indicates the windings configurations and the difference in phase angle between them.
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.
Group 1: Zero phase displacement between the primary and the secondary line voltages .(0 o'clock , 0 degree.) -delta/delta , star/star)
Group 2: 180 deg phase displacement (6 o'clock , 180 deg) -delta/delta , star/star)
Group 3: 30 deg lag phase displacement of the secondary with respect to the primary . (1 o'clock , -30 deg ) -star/delta , delta/star
Group 4: 30 deg lead phase displacement of the secondary with respect to the primary . (11 o'clock , +30 deg) -star/delta , delta/star
[ - indicates LV lagging HV , + indicates LV leading HV]
[ Clock positions show the angular displacement between secondary to primary]
Standard Code for vector groups of transformers
• IEC introduced the system in which the vector group is indicated by a code consisting of two or three letters, followed by one or two numeric digits. The letters indicate the winding configuration as follows:
• 1: D or d: Delta winding , also called a mesh winding. • ( 1st capital letter shows high voltage side and 2nd small letter shows low voltage side . )
• 2: Y or y: Wye winding , (also called a star)
3: Z or z: Zigzag winding , or interconnected star winding.
• 4: N (uppercase): indicates that a system neutral is connected to the high-voltage side.
• 5: n (lowercase): indicates that a system neutral is connected to the low -voltage side.
• 6: And the digit that we use represents the phase difference between between high voltage and low voltage sides .
To understand the phase difference in standard code that we use digit in it.
• A clock contains 12 hours and it is round 360 degree thus in standard codes we use as :
• 1 = 30 degree , 2 = 60 degree , 3 = 90 degree , 6 = 90 degree and 12 = 0 degree or 360 degree .
• Phase rotation is always taken as anti-clockwise . • (Internationaly adopted). We use the hour indicator to • Indicate phase displacement angle .
Standard Code of Vector groups
• To understand the above given standard codes in table we are going to discuss the one example of it .
• Example : Yd1 vector group • Here , Y shows HV star connected winding •
d shows LV delta connected winding • 1 indicates phase shift (LV lags HV by 30 degree)
In the IEC vector group code, each letter stands for one set of windings. The high-voltage (HV) winding is designated with an uppercase letter, followed by medium or low-voltage (LV) windings designated with a lowercase letter. The digits following the letter codes indicate the difference in phase angle between the windings, with HV winding is taken as a reference. The number is in units of 30 degrees. For example, a transformer with a vector group of Dy1 has a delta-connected HV winding and a wye-connected LV winding. The phase angle of the LV winding lags the HV by 30 degrees.
• Note that the high-voltage (HV) side always comes before the low-voltage (LV) side, regardless of which is the primary winding.
What is importance of vector groups when transformers to be connect in parallel
• As different combinations of winding connections will result in different phase angles between the voltages on the windings, to connect transformers into parallel must have the same vector group because the mismatching of phase angles will result in circulating current and other system disturbances.
• If transformers would be belonge to the same vector groups and connected in parallel there would be no difficulty in operating them.
Vector grouping is an indication of the nature of the primary & secondary connections between the three phases of a transformer, along with the phase shift (if any) of the secondary with respect to the primary. For example, Dyn11 indicates that the primary is Delta connected while the secondary is Y or star connected with a neutral available. Further, the secondary line-to neutral phasor is at a clock position of 11 o'clock with respect to that of the primary (12 o'clock as reference), indicating a 30degree phase advance. For transformers to be connected in parallel, it is preferred that they have the same vector grouping such that zero phase shift between the respective secondaries is ensured.