Technical Articles

## Transformer Connections: Types and Their Impact on System Performance STRICTLY TECHNICAL: Hearing from Our Tech Teams

###### Figure 1 shows the wye-wye connection. In this connection the primary and secondary currents, in all phases, are in phase and hence there is no need for compensation. ###### Figure 5 shows a delta-wye transformer with neutral grounded. In the event of a line to ground fault on the wye side of the transformer, the ground relays on the delta side will not see this ground fault. This is due to infinite zero sequence impedance between the two windings. However, the phase relays will see this as a phase-to-phase fault with magnitude diminished to 58% of what it would see if there were a three-phase fault on the wye side. The transformer damage curve, which represents the capability of the transformer to withstand a through fault is plotted for a three-phase fault. In effect, the phase relays on the delta side are seeing only 58% of the current and hence will not trip in time to protect the transformer. Hence, the solution is to shift both the damage curve and the characteristics of the phase relay by 58% towards the left. In effect, the setting for the phase overcurrent relay needs modification. Figure 5. A delta-wye transformer with neutral grounded

###### Figure 6 shows the sequence of the 1st through 3rd harmonic. The first, second and third harmonics exhibit positive, negative and zero sequence characteristics, respectively. Using the first harmonic phase angles as reference, we obtain the sequence angles for 2nd and 3rd harmonics by multiplying the 60 Hz angles by two and three, respectively. As shown in Figures 6 and 7, the 2nd and 3rd harmonics exhibit characteristics of negative and zero sequence, respectively. ###### Figure 6. The sequence of the 1st through 3rd harmonic ###### Figure 8. The 3rd harmonic in the wye-delta connection ###### Figure 9 shows a delta-delta connection. This connection is not a preferred option for various reasons. However, this connection does provide a closed path for the third harmonic to flow. ###### Figure 9. A delta-delta connection ###### The wye-wye connections with neutrals ungrounded as connection shown in Figure 10 has other issues such as neutral instability, overvoltage, etc. and hence is not preferred unless the transformer has a core type winding. ###### There is no need for corresponding 3rd harmonic to flow in the secondary winding since balancing ampere turns is not required as far as 3rd, 6th, 9th harmonics are concerned. ###### Figure 15 shows the GSU connected grounded-wye/grounded-wye. This connection is not desirable because the magnitude of ground fault flowing in each winding is indeterminant during a ground fault on either side of the GSU. This connection makes it difficult to determine proper settings for ground protection relays. ###### One solution is to install a grounding bank on the high side of GSU as shown in Figure 17. This will provide a path for the ground fault current to flow through the grounding transformer. This will enable tripping of breaker B via relay applied in the neutral of the grounding bank. ###### Commonly used schemes are 6 pulse and 12 pulse. Figure 14 shows a 12-pulse rectification using twelve diodes/thyristors. The disadvantage of cheaper option 6 pulse scheme is two-fold. The DC output is not smooth, and the rectification creates significant harmonics that are injected into the incoming AC power supply. The two main harmonics that create voltage and current distortions are the 5th and the 7th harmonics. ###### The issue of DC current quality is solved by using a more expensive option of 12-pulse rectifier, which uses 12 diodes/thyristors scheme. The issue of harmonic distortion is solved by introducing a 30-degree phase shift between the two secondary windings as shown in Figure 19. Due to proper selection of transformer connection, a 30-degree phase shift is achieved resulting in significant reduction in harmonics injected into the incoming AC power supply. 