Page 64 - Energize November 2021

TECHNICAL

Input stage developments
The purpose of the input stage is to
convert incoming AC from the grid to
DC for the DC bridge. This involves AC/
DC conversion through rectification. The
simplest input stage consists of a passive
bridge rectifier circuit, using high power
diodes feeding into a DC bus. The diode Figure 4: A six-pulse (three-phase bridge) rectifier (WEG)
bridge will only conduct when the input
voltage rises above the DC bus voltage,
resulting in zero current flow for part of
the input cycle.
The starting point will be the basic
three-phase rectifier bridge configuration,
which leads to what is known as the six-
pulse system. Figure 4 shows the rectified
voltage and current waveforms. This
results in a distorted current waveform
that has a nonlinear periodic form (more
commonly known as a pulsed waveform).
The shape of the current waveform
will depend on the point at which the
diode conducts, which varies as the load
varies, and the harmonic content will
vary with load. The waveform shows a Figure 5: Broadband passive filter (Schneider)
strong ripple. This wave form is rich in
harmonics, mainly the 5th, 7th, 11th bulky, show a considerably large voltage drop across the filter chokes at nominal load, and
and 13th harmonic components, and often result in increased DC-link voltage in no-load or light load conditions.
the typical total harmonic distortion of When the drive is off, the capacitor can cause power factor and voltage rise problems,
input currents (THDi) is slightly below and in the event of capacitor failure, standard units offer no indication of this failure. In
48%. These rectifier systems do not fulfil addition, they show a low power factor at partial load. The tuned filters could resonate with
the requirements outlined in industry the grid supply frequency, or with existing harmonics on the grid, and have to be damped,
standards, where a total THDi of only 5% which reduces their efficiency. These filters have a relatively low cost compared to multi-
is demanded. pulse converters. Overall, they are not considered the best choice for implementing a low
harmonic MV input stage.
Harmonic mitigation
Methods adopted to reduce harmonics Active harmonic correction filters
include: Active harmonic correction units are high-performance inverters which measure system
• Passive filters harmonics through current transformers and inject harmonics of equal amplitude and
• Active filters opposite phase into the system. These systems are very expensive and are rarely applied
• Multipulse rectifier systems for low-diversity, high-power applications. They can be well-suited to systems where low
• Active front end systems power loads are numerous and diverse, as a single active filter can be applied to mitigate
the harmonic distortion effects of many drives.
Passive filters Active harmonic correction filters also can make an excellent retrofit for existing
Several possibilities are available to systems because they are a shunt-connected device. When lightly loaded, most devices can
comply with stringent input current correct power factor using the capacitance built into them. Drawbacks to this technology
harmonic requirements. One method include the high cost per unit of power and lower energy efficiency.
used to reduce low-frequency harmonics
is the application of passive filters Multi pulse systems
tuned to the low harmonics, mainly to Multipulse solutions, such as 12-pulse, 18-pulse, or 24-pulse rectifier systems, use a
the 5th/7th and 11th/13th harmonic transformer with two or more phase-shifted windings and diode bridges. Due to the
components. A primary inductor with phase shift in the different windings, some low-frequency harmonics are compensated on
relatively high impedance blocks higher the primary side of the transformer. These systems show a high efficiency as well as the
order harmonics. Due to their low tuned advantage of already including the transformer to the MV side in the drive. However, the
frequency, these filters are very heavy and high complexity and cost of the transformer can outweigh these advantages. 4

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