How do transformers and harmonic filters suppress harmonics?
Harmonic Mitigation Techniques for Three-Phase Loads
Three-phase loads do not produce triplen harmonics. Therefore, in environments dominated by three-phase loads, harmonic issues primarily arise from currents at the 5th, 7th, 17th, 19th, and higher harmonics. A harmonic mitigating transformer (HMT) can help reduce these harmonics by utilizing dual secondary windings or multiple transformers with a 30° phase shift between them. This phase shift ensures that the harmonic components of one secondary are out of phase with those of another, aiding in their cancellation.
Mixed Single-Phase and Three-Phase Load Scenarios
When a transformer supplies both single-phase and three-phase loads, a combined approach is necessary. Typically, pairs of delta-zigzag transformers with a 30° phase shift are used in a separate transformer bank. This phase shift helps reduce the 5th, 7th, 17th, and 19th harmonics, while the secondary zigzag windings significantly reduce triplen harmonics.
Voltage sags during startup can be monitored with a power quality meter.
For optimal performance, single-phase, line-to-neutral, nonlinear loads should be balanced across two panels powered by two separate HMTs. One HMT should be a delta-zigzag with a 0° phase shift, and the second HMT should be either a delta-wye or a wye-zigzag with a 30° phase shift. Using these two transformers will help eliminate the 5th, 7th, 17th, and 19th harmonics, and harmonic attenuation is more effective when loads are balanced.
For instance, if a main power panel feeds single-phase nonlinear loads requiring 200 A, it is better to use two separate panels of 100 A each. Two transformers, one in a delta-zigzag configuration and the other in a delta-wye or wye-zigzag configuration, can be used to feed these panels with a 30° phase shift between them. The harmonic currents from computer loads will then cancel each other, resulting in a system that draws current with very low total harmonic distortion (THD).
Transformer banks with a phase shift between them are employed to cancel out harmonics.
HMT Impedance Considerations
The two HMTs should have the same impedance values, be located close to the source bus, and have similar load harmonic profiles. With a zigzag secondary, the impedance is less than the transformer's nameplate impedance rating. In a delta-wye or delta-delta transformer, the single-phase impedance matches the positive and negative sequence impedance as indicated on the nameplate.
For delta-zigzag or wye-zigzag transformers, the phase-to-neutral impedance is approximately 75% to 85% of the positive and negative sequence impedance. This can result in a higher fault current during a single-phase fault to neutral or ground, possibly requiring an overcurrent protection device with a higher rating. The nameplate impedance represents the positive/negative sequence impedance, so any fault current should be assumed to be about 133% of the calculated fault current, which is critical for arc flash protection coordination.
The single-phase impedance of a zigzag transformer is approximately 75% to 85% of its nameplate impedance.
Harmonic Filters
Harmonic filters reduce harmonic components and THD. Single-phase harmonic filters minimize third and other triplen harmonics from nonlinear single-phase loads. Three-phase harmonic filters, also known as trap filters, reduce harmonics from single-phase nonlinear loads on a three-phase system or three-phase loads such as AC variable-speed motor drives. These filters are typically tuned to just below the fifth harmonic, providing a low-impedance path that traps the fifth and most of the seventh harmonics. Harmonic filters should be installed as close to the nonlinear load as possible, and for three-phase drives, they are usually installed at the service equipment.
Types of Harmonic Filters
Passive Harmonic Filters
Passive harmonic filters use capacitors and inductors tuned to remove specific harmonic frequencies, functioning like band-pass or low-pass filters. These filters allow low frequencies (60 Hz) to pass while removing higher frequencies (180 Hz and above). However, they can cause issues like ringing, unwanted resonances, and overcompensation. Single-phase harmonic sources like SMPSs do not generate significant phase shifts between current and voltage, so passive filters can inadvertently cause circuits to switch from lagging to leading. Additionally, passive filters are often large and relatively expensive.
A passive harmonic filter uses a set of resistors, capacitors, and inductors tuned to remove harmonic frequencies.
Active Harmonic Filters
Active harmonic filters use electronics to provide variable impedance to remove harmonics or generate an adaptive current waveform that is 180° out of phase with the harmonics. Although traditionally expensive and less available, advancements in electronics are making active harmonic filters more accessible and cost-effective.
Subscribe to us to enjoy event prices and get some of the best prices.