STRICTLY TECHNICAL: Hearing from Our Tech Teams
The selection of transformer connections and the type of neutral grounding has a significant impact on system performance. This impact spreads over various aspects such as protective relaying, sizing of arresters and quality of transformer output, restraining the harmonic levels.
Transformers are connected in various configurations, some of which are delta/wye, wye/wye and wye/delta. Transformer connections have an impact on a number of criteria of system performance, including the following:
-
Phase shifts impact relay protection and system operability
-
Neutral grounding of wye connected winding impacts ground relaying and overall system performance
-
Flow of ground fault current impacts relay protection and system performance
-
Passage of third harmonic current is needed to produce sinusoidal transformer output voltage
-
Connections enabling twelve pulse rectification
Impact on differential protection
One of the impacts is related to transformer differential protection. To understand this impact, we must review the phase angle shifts that result from various connections.
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 1. Wye-wye connection Figure 2. Delta-wye connection
Figure 2 shows the delta-wye connection (which can also be wye-delta). In this connection the primary and secondary currents in all phases are not in phase. There is a 30-degree phase shift and hence, a compensation is needed. This compensation is performed through current transformer connections, or, internally, in a microprocessor differential relay.
Figure 3 shows common transformer connections that are in use. The phase shifts are designated in terms of hour positions on a clock. For example, Dd0 means that the primary and the secondary current are in phase with A phase of windings at 12 o’clock or zero hours.
Similarly, DY1 connection means that the primary winding is connected in delta with A phase pointing at 12 o’clock and the secondary winding is connected in wye with A phase pointing towards 1 o’clock, resulting in a phase shift with wye winding lagging delta winding by 30°.
If we look at a three-winding transformer, a connection designated as YNy0d1 means that the primary is wye grounded at 12 o’clock and the secondary winding is wye ungrounded also set at 12 o’clock or zero hours. The delta tertiary is shifted towards 1 o’clock resulting in a phase shift of thirty degree from the wye windings. The letter N or n stands for grounded neutral.
Figure 3. Common transformer connections
Figure 4. A typical powerplant layout
Phasing issues
Transformer connections can lead to phasing issues which can be solved by using appropriate winding connections.
Figure 4 shows a layout at a typical powerplant. Transformer T5, which has been circled in the diagram, feeds the auxiliary load. When the generator is started and brought online, the power to the auxiliary bus is fed by transformer T6. Breakers B and T are in close position and breaker A is open. The auxiliary bus feeds motors and other loads that are necessary to support the generator. Once the generator is synchronized, breaker A is closed followed by opening of breaker B. Hence, the two sources fed by breakers A and B are paralleled for a short duration. Hence, the output of transformer T5 and T6 should be in phase. To achieve this, the transformer T5 has to be of certain connection, as shown in Figure 4. If the generator step up is connected as YNd1, T5 will have to be connected as DYn1.
Shifting of transformer damage curve
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
Managing flow of third harmonic
Excitation current required by transformers is between 1-5% of the rated current of the transformers. The excitation current contains 60 Hz along with few harmonics. Most prominent harmonics are the 3rd, 5th, 7th, 9th, and 11th. The 3rd harmonic, which is most prominent, requires special attention. To understand the issue, we must understand the characteristics of this harmonic.
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.