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How to Reduce Harmonics

by making structural modifications in the AC drive system

Factors in the AC drive which affect harmonics 

Harmonics reduction can be achieved either by structural modifications in the drive system or by using external filtering. The structural modifications may be to strengthen the supply, or to use 12 or more pulse drives, to use a controlled rectifier or to improve the internal filtering in the drive. The current harmonics depend on the drive construction and the voltage harmonics are the current harmonics multiplied by the supply impedances

Drive system features affecting harmonics schematic
Fig 1 Drive system features affecting harmonics

Using a larger DC or AC choke The harmonics of a voltage source AC drive can be significantly reduced by connecting a large enough choke to its AC input or DC bus. The trend has been to reduce the size of converter while the choke size has been also reduced, or in several cases it has been omitted totally. The effect of this can be seen from the curve forms in figure 2. The chart in figure 3 shows the effect of the size of the DC choke on the harmonics. For the first 25 harmonic components the theoretical THD minimum is 29%. That value is practically reached when the inductance is 100 mH for a 1 kW motor or 1 mH for a 100 kW motor (415 V, 50 Hz). In practice optimum dimensioning can be reached when the product of the motor power in kW and the inductance in mH is close to 25.

Fig 2 The effect of a choke on the line current.

Fig 3 Harmonic current as a function of DC inductance

The voltage distortion with certain current distortion depends on the short circuit ratio RSC of the supply. The higher the ratio, the lower the voltage distortion. This can be seen in Figure 4

Fig 4 THD voltage vs type of AC drive and short circuit ratio.

Figure 5 introduces a simple nomogram for the estimation of harmonic voltages. On the graph below select first the motor kilowatt, then the transformer kVA and then move horizontally to the diagonal line where you move upwards and stop at the curve valid for your application. Then turn left to the y-axis and read the total harmonic voltage distortion. 

Input data for calculations: 

• Rated motor for the drive 
• Constant torque load 
• Voltage 415 V 
• Drive efficiency = 97% 
• Supply Impedance = 10% of transformer impedance

Fig 5 Total harmonic distortion nomogram.

Results from laboratory tests with drive units from different manufacturers are shown in figure 4.6. Drive A with a large DC choke has the lowest harmonic current distortion, whereas drives with no choke installed have the highest distortion.

Fig 6 Harmonic current with different
Dc-inductances. A = Large DC-inductance, B, C = Small DC-inductance, D, E = Without DC-inductance

Using 12-pulse or 24-pulse rectifiers instead of 6-pulse rectifiers The connections for different rectifier solutions are shown in figure 7. The most common rectifier circuit in 3-phase AC drives is a 6-pulse diode bridge. It consists of six diodes and a choke, which together with a DC-capacitor form a low-pass filter for smoothing the DC-current. The choke can be on the DC- or AC-side or it can be left totally out. 

The 6-pulse rectifier is simple and cost effective but it generates a high amount of low order harmonics 5th, 7th and 11th especially with a small smoothing inductance. The current waveform is shown in figure 7. If the major part of the load consists of converters with a 6-pulse rectifier, the supply transformer needs to be oversized and meeting the requirements in the standards may be difficult. 

Often some harmonics filtering is needed. A 12-pulse rectifier is formed by connecting two 6-pulse rectifiers in parallel to feed a common DC-bus. The input to the rectifiers is provided by a three-winding transformer. The transformer secondaries are at a 30° phase shift. The benefit with this arrangement in the supply side is that some of the harmonics are in opposite phase and thus eliminated. In theory the harmonic component with the lowest frequency seen at the primary of the transformer is the 11th. The major drawbacks are the need for special transformers and a higher cost than with the 6-pulse rectifier.

Fig 7 Line current waveforms with different rectifier constructions.

The principle of the 24-pulse rectifier is also shown in figure 7. It has two 12-pulse rectifiers in parallel with two three-winding transformers having a 15° phase shift. The benefit is that practically all low frequency harmonics are eliminated but the drawback is the high cost. In the case of a high power single drive or large multidrive installation a 24-pulse system may be the most economical solution with the lowest harmonic distortion. 

A phase controlled rectifier is accomplished by replacing the diodes in a 6-pulse rectifier with thyristors. Since a thyristor needs a triggering pulse for the transition from a nonconducting to a conducting state, the phase angle at which the thyristor starts to conduct can be delayed. By delaying the firing angle over 90o, the DC-bus voltage turns negative. This allows a regenerative flow of power from the DC-bus back to the power supply. 

Voltage source inverter configurations do not allow a polarity change of the DC-voltage and it is more common to connect another thyristor bridge antiparallel with the first one in order to allow the current polarity reversal. In this configuration the first bridge conducts in rectifying mode and the other in regenerating mode. The current waveforms of phase controlled rectifiers are similar to those of the corresponding 6, 12 and 24-pulse diode rectifiers, but the displacement power factor is lower when the firing angle is greater than zero. Thus the power factor in braking is lower than in normal operation. In addition to these problems, a converter utilizing phase control causes larger commutation notches in the utility voltage waveform. The angular position of the notches varies along with the firing angle.

Fig 8 Harmonic components with different rectifiers

Fig 9 Distortion of different supply unit types. Values may vary case by case. Distortion is in % of RMS values

Using an IGBT supply unit (ISU), i.e. low harmonic drive Introducing a rectifier bridge, made of self commutated components, brings several benefits and opportunities compared to phase commutated ones. Like a phase controlled rectifier, an active supply unit allows both rectification and regeneration, but it makes it possible to control the DC-voltage level and displacement power factor separately regardless of the power flow direction. 

The main benefits are: 

• Improved ride-through in case of mains supply disappearance. 
• High dynamics of the drive control even in the field weakening range. 
• Ability to generate reactive power. 
• Nearly sinusoidal supply current with low harmonic content. 
 • Voltage boost capability

Measured results for one drive are shown in figure 10. When comparing with figures from figure 7 to 9 we can see a clear difference. The active supply unit has very low harmonics at lower frequencies, but somewhat higher at higher frequencies.  In case of low supply voltage the DC voltage can be boosted to keep the motor voltage higher than the supply voltage. One drawback is the high frequency common mode distortion of the phase to neutral and phase to ground voltages. Dedicated filtering to suppress high frequency content is needed to prevent interference.

Regenerative rectifier unit (RRU) 

An alternative form of IGBT bridge is the Regenerative Rectifier Unit (RRU) where the IGBTs are controlled to conduct at the same intervals as diodes in a 6-pulse bridge. As the current can flow in either direction in the IGBT bridge it is possible to feed energy back to the AC grid during braking. The current harmonics are naturally similar to the 6-pulse diode bridge ones.

Fig 10 Harmonics in line current active supply unit

This article was extracted from the Technical Guide No 6- Guide to harmonics with AC drives by ABB

The entire free guide can be downloaded from the link below

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