Harmonic Mitigation Using Phase-shifting Transformers and Harmonic Filters

Three-phase loads do not generate triplen harmonics. Therefore, harmonic problems in situations where 3-phase loads dominate are primarily from currents flowing at the 5th, 7th, 17th, 19th, or higher harmonics. A harmonic mitigating transformer (HMT) can use dual secondary windings or pairs of transformers to reduce these harmonics. A bank of two or more transformers with a 30° phase shift between them can be used to treat these harmonics. This degree of phase shift is chosen to ensure that the harmonic components of one secondary are out of phase with those of another.

In the case where a transformer is supplying both single-phase and 3-phase loads, a combination approach is needed. Pairs of delta zigzag transformers with a 30° phase shift are often used as part of a separate transformer bank. The 30° phase shift between the transformers reduces the 5th, 7th, 17th, and 19th harmonics. The secondary zigzag windings greatly reduce the triplen harmonics.

 

Note:

Voltage sags during startup can be recorded using a power quality meter.

To achieve the best results, the single-phase, line-to-neutral, nonlinear load must be balanced between two panels fed by two separate HMTs. One of the HMTs should be a delta-zigzag with a 0o phase shift. The second HMT could be either a delta-wye or a wye-zigzag with a phase change of 30o. The use of the two transformers would help eliminate the 5th, 7th, 17th, and 19th harmonics. Also, the harmonic attenuation will be more effective when the loads are balanced.

For example, if a main power panel feeding single-phase nonlinear loads requires 200 A, it is better to use two separate panels of 100 A each. See Figure 5. Two transformers are used to feed the separate panels. One transformer is wired in a delta-zigzag configuration, and the other transformer is wired in a delta-wye or a wye-zigzag configuration. The two transformers are 30° out of phase with each other. The computer loads draw current in pulses, and the harmonics move back through the transformers to the main power panel. The harmonics add together so that the overall system draws current in a waveform with very low THD.

 

Figure 5. Banks of transformers with a phase shift between them are used to cancel out harmonics.

 

HMT Impedance

The two HMTs should have the same impedance values, be located close to the source bus, and have the same load harmonic profiles. With a zigzag secondary, the impedance is less than the transformer nameplate impedance rating. In a delta-wye or delta-delta transformer, the single-phase impedance is the same as the positive and negative sequence impedance. This is the impedance on the nameplate.

With a delta-zigzag or a wye-zigzag transformer, the phase to neutral impedance is approximately 75% to 85% of the positive and negative sequence impedance. This results in a higher fault current in the event of a single-phase fault to neutral or ground. See Figure 6. This may require an overcurrent protection device with a higher rating. The impedance value given on the nameplate of the transformer is the positive/negative sequence impedance. Therefore, it is best to assume that any fault current is about 133% of a calculated fault current. This is very important when conducting a coordination study for arc flash protection.

 

Figure 6. The single-phase impedance of a zigzag transformer is about 75% to 85% of the nameplate impedance.

 

Harmonic Filters

A harmonic filter is a device used to reduce harmonic components and THD. A single-phase harmonic filter is used to reduce the harmonics from nonlinear single-phase loads by minimizing the third and other triplen harmonics. Three-phase harmonic filters, also called trap filters, are used to reduce harmonics produced by single-phase nonlinear loads connected to a 3-phase system or 3-phase loads such as AC variable-speed motor drives connected to the system. A 3-phase harmonic filter's primary purpose is to reduce the fifth and seventh harmonic currents produced by six-pulse (six-diode) converters that convert AC to DC. The filter is usually tuned to just below the fifth harmonic and offers a low-impedance path that traps the fifth and most of the seventh harmonic. Harmonic filters should be installed as close as possible to the nonlinear load. With 3-phase drives, they are typically installed at the service equipment.

Harmonic filters may include different types of circuits or components designed to reduce harmonic currents, such as combinations of capacitors, inductors, and other components. Harmonic filters are typically classified as passive harmonic filters and active harmonic filters.

 

Passive Harmonic Filters

A passive harmonic filter uses capacitors and inductors that are tuned to remove particular harmonic frequencies. See Figure 7. The passive harmonic filter works like a band-pass or low-pass filter in an electronic circuit. It allows low frequencies (60 Hz) to pass through unchanged while removing higher frequencies at 180 Hz and above. Passive harmonic filters can be difficult to use because they often cause other problems like ringing, unwanted resonances, and overcompensation. Single-phase harmonics sources like SMPSs generally do not generate very much phase shift between current and voltage. Therefore, a passive filter can easily cause a circuit to switch from lagging to leading. In addition, passive harmonic filters tend to be fairly large and can be somewhat expensive.

 

Figure 7. A passive harmonic filter uses a set of resistors, capacitors, and inductors tuned to remove harmonic frequencies. Image courtesy PSCAD

 

 

Active Harmonic Filters

An active harmonic filter uses electronics to provide a variable impedance to remove harmonics from the circuit or to generate an adaptive current waveform that is 180° out of phase with the harmonics.

Figure 8. Image courtesy of Delta Power Solutions.

 

See Figure 8. Active filters have typically been very expensive and not widely available. However, advances in electronics are making these types of devices more available and cost-effective.