A phase-shifting transformer is a type of rectifier transformer. The rectifier transformer is the power transformer of the rectifier equipment. The single-phase conductive effect of the rectifier device causes the distortion of the alternating magnetic field waveform of the rectifier transformer; The magnitude of the distortion is determined by the proportion of DC capacity to the grid capacity and the frequency and harmonic order of the harmonic current flowing into the grid. One effective way to suppress harmonics is to shift the phase on the high-voltage side of the rectifier transformer, which can basically eliminate the large amplitude of low-order harmonics.

A phase-shifting transformer is a special type of transformer whose primary function is to change the phase of the output voltage to meet the requirements of a particular electrical system or equipment. The design principle of this transformer is based on the law of electromagnetic induction. By adjusting the connection and combination of the transformer windings, smooth or stepwise phase changes can be achieved.

The working principle of a phase-shifting transformer can be understood through a simple model of a double-coil transformer. When the primary side of two double-coil transformers of the same specification is connected to the same phase power supply, the voltages on the secondary side are added together to form a new voltage with a phase difference of 180°. If the two coils are connected to different phases of the power supply, the sum of the secondary side voltages will result in a new voltage with a phase difference of 120°. The phase of the output voltage can be flexibly controlled by changing the wiring of the primary side and the secondary side.

Phase shifting principle

→ For every doubling of the pulse count, the phase shift Angle is halved ($\theta = \frac{30^\circ}{N/6}$, where N is the pulse count)

→ Construct phase-complementary voltage vectors through multi-winding spatial phase shifting, causing the converter trigger pulses to be offset and superimposed

Harmonic elimination mechanism

→ Characteristic harmonic order $h = k \times N \pm 1$(k is an integer)

→ 12 pulses raise the lowest harmonic to the 11th ($h_{min}=12-1=11$), and 24 pulses raise to the 23rd

Economic balance

For every doubling of the pulse count, transformer costs increase by 35% to 50%, but filter costs decrease by 60% to 70%

→ 12 pulses is the cost-performance inflection point (the lowest overall cost)

Pulse type
Pulse count	Phase difference	Implementation method	Harmonic suppression effect	Typical application scenarios
6 Pulses	60 °	Single-group three-phase bridge rectification	Basic harmonic suppression	General industrial equipment
12 pulses	30 °	1 transformer (Y/Δ winding) + 2 sets of rectifier Bridges	Eliminate 5th and 7th harmonics, THD≈6 to 10 percent	Industrial frequency converters, medium power electrolysis equipment
18 pulses	20 °	1 autotransformer + 3 sets of rectifier Bridges	Eliminate the 5th, 7th, 11th and 13th harmonics, with THD≈5 to 8%	Electroplating power supply, data center power supply
24 pulses	15 °	2 transformers (±7.5° phase shift) + 4 sets of rectifier Bridges	Eliminate harmonics below the 23rd order, THD≤4.4%	Rail transit, high voltage direct current transmission
36 pulses	10 °	3 groups of phase difference power supply (such as extended triangle wiring)	Eliminate harmonics below the 35th	High voltage frequency converters, Marine power systems
48 pulses	7.5 °	4 transformers or special winding designs	Better harmonic suppression	Shore power systems, precision industrial power supplies
60/72 Pulse	6 ° / 5 °	Multi-winding complex structures (such as axial split type)	Near sinusoidal output	West-east Gas Pipeline booster station, super-large capacity rectifier system
96 pulses	3.75 °	Modular superimposed design (such as a linear transformer)	Ultra-low harmonic pollution	Experiment-grade high power quality scenarios