Online Electrical Engineering
Sweep Frequency Response Analysis Test | SFRA Test
Electrical Power Transformer | Definition and Types of Transformer
What is transformer? Definition & Working Principle of Transformer
Theory of Transformer on load and no load operation
EMF Equation of Transformer | Turns Voltage Transformation Ratio of Transformer
Resistance & Leakage Reactance or Impedance of Transformer
Equivalent Circuit of Transformer referred to Primary and Secondary
Voltage Regulation of Transformer
Hysteresis Eddy Current Iron or Core Losses and Copper Loss in Transformer
Open and Short Circuit Test on Transformer
What is Auto Transformer ?
Tertiary Winding of Transformer | Three Winding Transformer
Parallel operation of Transformers
Transformer Cooling System and Methods
Core of Transformer and Design of Transformer Core
Transformer Insulating Oil and Types of Transformer Oil
Transformer Accessories | Breather and Conservator Tank | Radiator
DGA or Dissolved Gas Analysis of Transformer Oil | Furfural or Furfuraldehyde Analysis
Over Fluxing in Transformer
Single Three Phase Transformer vs bank of three Single Phase Transformers
Current Transformer CT class Ratio Error Phase Angle Error in Current Transformer
Voltage Transformer or Potential Transformer Theory
Accuracy Limit Factor and Instrument Security Factor of Current Transformer
Knee Point Voltage of Current Transformer PS Class
What is Earthing Transformer or Grounding Transformer
Differential Protection of Transformer | Differential Relays
Restricted Earth Fault Protection of Transformer | REF Protection
Buchholz Relay in transformer | Buchholz Relay operation and principle
Transformer Testing | Type Test and Routine Test of Transformer
Transformer Winding Resistance Measurement
Voltage and Turn Ratio Test of Transformer
Vector Group Test of Power Transformer
Insulation Dielectric Test of Transformer
Transformer Oil and Winding Temperature Rise Test
Impulse Test of Transformer
Magnetizing Inrush Current in Power Transformer
Air Core Transformer
High Voltage Transformer
Silica Gel Breather of Transformer
Conservator Tank of Transformer
Maintenance of Transformer
Radiator of Transformer | Function of Radiator
Magnetic Oil Gauge or MOG | Magnetic Oil Level Indicator of Transformer
Oil Winding and Remote Temperature Indicator of Transformer
On Load and No Load Tap Changer of Transformer | OLTC and NLTC
Distribution Transformer | All Day Efficiency of Distribution Transformer
Installation of Power Transformer
Commissioning of Power Transformer
Design of High Frequency Pulse Transformer
Vector Group Test of Power Transformer
Under Electrical Transformer
Vector Group Test of TransformerThe vector group of transformer is an essential property for successful parallel operation of transformers. Hence every electrical power transformer must undergo through vector group test of transformer at factory site for ensuring the customer specified vector group of transformer.
The phase sequence or the order in which the phases reach their maximum positive voltages, must be identical for two paralleled transformers. Otherwise, during the cycle, each pair of phases will be short circuited.
The several secondary connections are available in respect of various primary three phase connection in a the three phase transformer. So for same primary applied three phase voltage there may be different three phase secondary voltages with various magnitudes and phases for different internal connection of the transformer.
Let's have a discussion in detail by example for better understanding.
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 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. If we go through the vector diagram of secondary line voltages of both transformer, 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 a circulating current flows between the transformers as because there is a phase angle difference between their secondary line voltages. This phase difference can not be compensated. Thus two sets of connections giving secondary voltages with a phase displacement can not 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 transformer. 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.
( - 30o)
Procedure of Vector Group Test of TransformerLet’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 test of transformer for other group can also be done in similar way.