The Reduction of Harmonics caused by Variable Speed Drives

Harmonics – what are they?

Harmonic distortion is a type of electrical pollution that can cause problems if the sum of the harmonic currents exceeds certain levels. Harmonic currents have a frequency that is a multiple of the fundamental frequency, for example, 250 Hz current in a 50 Hz network is the 5th harmonic. On the network, the 250 Hz current represents energy that cannot be used by devices. Therefore the energy will be converted to heat.

Harmonics: what are their effects?

Harmonics can cause cables to overheat, damaging their insulation. It is also possible for motors to overheat or become noisy, and torque oscillations within the rotor can cause mechanical resonance and vibration. In the most severe cases, capacitors overheat and can explode as their dielectric breaks down. Flickering displays and lighting can occur, circuit breakers can trip, computers can malfunction, and meters can give false readings.

What causes harmonic currents? 

The power distribution system creates harmonic currents and voltages by connecting non-linear loads. By injecting harmonic currents directly into the grid, all power electronic converters used in different types of electronic systems can increase harmonic disturbances. Other non-linear loads include motor starters, variable speed drives, computers and other electronic devices, electronic lighting, welding supplies, and uninterruptible power supplies.

What can be done to reduce the effects of variable speed drives?

Harmonics can be reduced by either modifying the drive system or using external filtering. Currently, harmonics depend on drive construction.

There are a number of factors that increase current harmonics, including:

  • Motor is much larger than the supply transformer
  • Increased motor load
  • Longer supply cable

Harmonics can be decreased by a variety of factors including:

  • DC or AC inductance that is greater
  • Increased number of pulses in the rectifier
  • Transformer of greater size
  • Transformers with lower impedance
  • Short circuit capacity of the supply is higher

Utilizing a 6-pulse, 12-pulse, or 24-pulse rectifier

A six-pulse diode bridge is the most common rectifier circuit in three-phase PWM drives. The rectifier is rugged, robust, and inexpensive, but it contains a large amount of low order harmonics in the input current.

Two six-pulse rectifiers are connected in parallel to feed the same dc bus, forming the twelve-pulse diode bridge. Compared to a single six-pulse bridge, this gives a smoother current waveform. The downside of this arrangement is that a special transformer is needed, which raises the cost of the drive. Similarly, a 24-pulse rectifier is formed by connecting four six-pulse rectifiers.

Using an IGBT bridge

An active IGBT (Integrated Gate Bipolar Thyristor) inverter rectifies incoming AC power instead of a diode rectifier. As a result, the power factor is maintained close to unity because the IGBT is actively modulated to reduce harmonic overtones.

The main benefits include:

  • Safe function in case of power outages
  • Accurate control over the entire range
  • Ability to generate reactive power and even compensate harmonic currents of parallel loads
  • Supply current with low harmonic content. IGBTs have very low harmonics at low frequencies, but higher harmonics at higher frequencies
  • Voltage boost capability.  In the event that the supply voltage falls, the DC voltage can be boosted to keep the motor voltage higher than the supply voltage.

The main disadvantage is the high cost of the IGBT bridge and the additional filtering required.

Using larger DC or AC inductor

In general drives with large DC inductors will have lower harmonic current distortion. Drives with no inductor installed have the highest distortion.

External active filter

Active filters compensate for harmonics generated by nonlinear loads by generating the same harmonic components in the opposite phase. External active filters are most suitable for multiple small drives. Filtering is relatively expensive compared to other methods of attenuating harmonics.

 

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