Transformer Tertiary Winding Basic Application
Tertiary winding in transformers have been widely used in the transmission and distribution of electricity – from power transformers up to distribution transformers. In most cases, the role of the tertiary is for voltage stability and suppression of third harmonic voltages. The discussion will begin with a brief introduction to wye-wye transformers, their advantages and potential issues associated with their operation.
Wye-Wye Transformer
Wye-wye connected transformers are very common and have been used throughout the electric power industry for several reasons. The following are the most notable reasons for using wye-wye transformers.
- 0° shift between primary and secondary voltages
- Availability of a secondary neutral point for grounding
- One end of the winding can be placed at low potential to ground (except for ungrounded neutral)
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Voltage Stability
The conveniences offered by a wye-wye transformer come with a stability issue to the line-to-neutral voltage which may become unstable when
- Unbalanced line-to-neutral loads with the primary not effectively grounded
- Third harmonic currents cannot flow through the primary or secondary windings
Unbalanced Loads and Isolated Primary Neutral
Image taken from IEEE Std C57.158™-2017
Unbalanced loads on a wye-wye transformer with isolated primary neutral presents a voltage stability issue as illustrated in the above figure. The load current which flows through the neutral of the secondary side produces a zero-sequence voltage drop which shifts the secondary terminal’s voltage to neutral. However, the increase in the line-to-neutral voltage is limited to 1.4pu if secondary is effectively grounded.
- Isolated primary neutral exhibits high zero-sequence impedance
- Generally difficult to connect primary neutral through a low impedance path
The above figure shows a simplified network with generator, power line, and a wye-wye connected transformer with isolated primary neutral, and the equivalent zero-sequence network. By isolating the transformer primary neutral, zero-sequence currents are prevented to flow.
Harmonic Distortion
Image taken from IEEE Std C57.158™-2017
The absence of zero-sequence path for the third harmonic currents to flow results to the distortion voltages as shown in the above figure.
Application of Stabilizing Winding: The Tertiary Winding
The absence of path for zero-sequence currents to flow presents potential problems in the operation of wye-wye connected transformers with isolated primary neutral.
This is also true for wye-wye transformers with both neutral connected to ground but is located very far away from the source, i.e. high zero-sequence impedance of transmission lines.
The introduction of a delta-connected tertiary winding as a stabilizing winding addresses these by providing a path for zero sequence currents to flow. As a result,
- provides stability to neutral point voltage,
- suppresses third harmonic voltages, and
- minimizes telephone interference
The figure below shows a zero-sequence network with a source, transmission line , and a three-winding transformer equivalent circuit.
Stability of Neutral Point Voltage
The neutral point voltage can be expressed as a function of load current and zero-sequence impedance according to the following equation.
With a stabilizing winding, the equivalent zero-sequence impedance is significantly reduced and thus limiting the neutral point voltage.
Suppression of Third Harmonic Voltage
Harmonic voltage distortion results from a lack of zero-sequence path or high zero-sequence impedance. Generally, harmonic currents flow through the neutral line, delta winding, and line-to-earth capacitance. In wye-wye connected transformers with both sides connected to neutral, the neutral line presents a path for zero-sequence currents to flow. As neutral lines run along telephone lines, communication circuits are affected by third harmonics.
The presence of a stabilizing winding presents an alternate path with significantly lower zero-sequence impedance for the harmonic currents to flow.
Effect of Upstream Ground Faults
From among the different three-winding transformer configurations, there is a configuration susceptible to upstream ground faults – wye-wye connected transformers with both sides effectively grounded and with a delta-connected tertiary winding.
Three-Winding Transformer Equivalent Circuit
This figure shows a line-to-ground fault on a simplified network with a source, a transmission line, and a three-winding transformer. The figure also shows the equivalent sequence network of the line-to-ground fault. Notice how the stabilizing winding offers a parallel path for zero-sequence currents to flow.
For power transformers, this poses no problems since ground fault protection is available. However, for distribution transformers which only have fuses for protection, the transformer may be left unprotected on upstream ground faults.
Fault currents passing through fuse can have lower magnitudes especially for high-impedance faults, and may overload the stabilizing winding for longer periods. This is especially true when using ANSI/NEMA type T and K links, as these types of links are 150% rated. On the other hand, high-impedance ground faults may not be detected by upstream overcurrent protection devices or may take longer time to operate due to low magnitude fault currents.
The following gallery shows pictures of a burned three-winding transformer due to upstream ground faults.
References
“IEEE Guide for the Application of Tertiary and Stabilizing Windings in Power Transformers,” in IEEE Std C57.158-2017 , vol., no., pp.1-80, 27 April 2018, doi: 10.1109/IEEESTD.2018.8352755
“Application of Primary Fuses“, Hubbell Power Systems, Inc., 2000, Centralia, MO.