What is a Phase-Shifting Transformer?
VFD multi-winding transformers typically use phase-shifting feature in order to eliminate (cancel) certain harmonic orders in the spectrum of the current drawn from the grid. There are, however, other transformers that also use phase-shifting, but it a completely different power range and application. This blog post presents two different applications of phase-shifting transformers.
What is phase shifting?
Phase-shifting is a means to achieve the desired phase displacement between several three-phase systems. Phase shifting can refer to a relative phase displacement between primary and secondary winding or among several secondary windings (in case of multi-winding transformer).
Phase-shifting transformer in drive applications
In VFD applications the phase-shifting transformers usually supply a multi-pulse rectifier. By using higher rectifier pulse number and suitable relative phase displacement between the supply windings, certain harmonic orders can be cancelled inside the transformers and thus not appearing on the grid side. Simplest phase-shifting transformer is a 3-winding transformer: one primary (HV) winding and two secondary (LV) windings with relative displacement of 30 degree. Such transformer is suitable to supply a 12-pulse rectifier. In this case, the displacement can be achieved by using one secondary winding connected as a star (y) and the other secondary winding connected as a delta (d): e.g. Yy0d11 or Dd0y1. Relative phase displacement less than 30 degree requires use of ‘alternative’ winding connections, such as extended delta, polygon delta or zig-zag (“broken star”). The designation of the winding system incl. information about relative phase displacement is called (winding) connection or vector group.
Phase-shifting multi-winding transformers are frequently used to supply medium voltage VFDs and other power electronic converters (SFCs, STATCOM etc). Main benefit is a reduced harmonic distortion due to elimination of certain harmonic orders. Thus, multi-winding phase-shifting transformers contribute to a cleaner power drawn by the power electronics. Systems combining multi-winding phase-shifting transformers and multi-pulse rectifiers mostly comply with international standards regarding harmonics without the need of installing harmonic filters.
Advantages of a multi-winding phase-shifting transformer:
- Elimination or minimization of certain harmonic orders in the frequency spectrum
- Reduction of total harmonic distortion (THD) seen from the grid
- Compliance with harmonic standards without the need for harmonic filters
Phase-shifting transformer in power grids
Phase-shifting transformers (PST) in power transmission systems are used to control the power flow by means of phase angle regulation. When the source feeds power into the load, there is a voltage drop and phase angle shift between those two systems. In complex power transmission systems there are typically two or more parallel paths connecting the subsystems. By regulating the phase angle of the phase-shifting transformer, the loading of that specific path can be adjusted (increasing or reducing the load). Such phase-shifting transformers gain importance in modern power grid with increasing portion of renewable power sources and less predictable power generation. They are being installed in the most important nodes of the power grid (in the grid intertie links).
These transformers can also be called phase angle regulating transformers.
Advantages of a phase-shifting power transformer:
- Increasing grid stability by actively controlling the power flow
- Removal of grid bottlenecks
- Protection of transmission line against thermal overload
PST in a power transmission system are two-winding transformers with adjustable angle between primary and secondary side (e.g. +/- 70°). Single or dual core design can be used, depending on the power rating. Dual core design can be placed in one common tank or be split into two separate tanks.
Summary
In the field of variable frequency drives (VFD), phase-shifting transformers are used to supply multi-pulse rectifiers. The phase shifting refers to a creation of a relative phase displacement between the multiple secondary windings. However, when typing “phase-shifting transformer” in the search engine, you get search results mainly for the other application described in this article: PST as large power transformers that can adjust the phase angle between source system and load system and thus control the active power flow in the transmission system.
Take away
Phase-shifting transformer in VFD applications
= Input isolation transformer supplying a multi-pulse rectifier of a VFD (or STATCOM, high-power rectifier, static frequency converter etc) and eliminating certain harmonic frequencies from the spectrum
- Medium power multi-winding transformers
- Relative phase displacement among individual secondary windings
- Purpose to eliminate certain harmonics
- Phase displacement is fix
Phase-shifting transformer in power transmission system
= Power transformer able to actively adjust power flow inside critical lines of the power transmission system contributing to higher reliability and better utilization of a power grid (reduced risk of grid overloading and blackout)
- Large special transformers in the power grid
- Phase displacement between primary and secondary winding
- Purpose to control the power flow
- Phase displacement is adjustable
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References
[1] VFD transformers: Multi-winding design, MB Drive Services, November 2019, https://mb-drive-services.com/vfd_transformer_design/
[2] Phase-shifting transformers, ENTSO-E Technopedia, https://www.entsoe.eu/Technopedia/techsheets/phase-shifting-transformers
[3] Quadrature booster, Wikipedia – The Free Encyclopedia, https://en.wikipedia.org/wiki/Quadrature_booster
[4] Phase-shifting transformers (PST), Hitachi Energy, https://www.hitachienergy.com/us/en/products-and-solutions/transformers/power-transformers/system-intertie-transformers/phase-shifting-transformers
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