Electricity + Control January 2017

DRIVES, MOTORS + SWITCHGEAR

AFE – Active Front End ANSI – American National Standards Institute AUHF – Advanced Universal Harmonic Filter EMI – Electromagnetic Interference IEEE IGBT – Insulated Gate Bipolar Transistor Isc – Short Circuit Current IL – Load Current LCL – Inductor/Capacitor/Inducor RFI – Radio Frequency Interference TDD – Total Demand Distortion THD – Total Harmonic Distortion THVD – Total Harmonic Voltage Distortion VFD – Variable Frequency Drive VSD – Variable Speed Drive

– Institute of Electrical and Electronics Engineers

Abbreviations/Acronyms

that, in turn, relates to a comparable harmonic output for any given size. Some manufacturers offer VSD products with built-in harmonic mitigation components in the form of inductors/reactors that will, in turn, further reduce output harmonics. For those that don’t, external input reactance can be added to achieve similar results. Inmany applications, a 6 pulse VSD rectifier with an external input reactor or one with internal reactors on the dc bus, will meet the IEEE 519 [1] recommendations perfectly. When this is not possible, some of the typical available solutions for the reduction in harmonic currents are to increase the number of rectifier pulses, using 12, 18 or 24 pulses, the use of Active Input Rectifiers (AFE Drives) or using standard 6 pulse VSDs in conjunction with specialised passive harmonic filters. A variety of information is required to determine whether or not any formof harmonic mitigation is necessary to comply with a recom- mended level of distortion as defined by IEEE 519 [1]. This includes: • The utility source short circuit power • The total number of drives and their cumulative power must be defined (few small drives on a very large network will have very little effect) • The total user demand load current (non-linear plus linear) Using this information, an estimate of the current and voltage distor- tion can bemade. More accurate calculations can bemade if additional information is available such as existing harmonic distortion levels and cable lengths/sizes between the drives and the PCC. As a rule of thumb, unless the drives constitute more than 30% of the load on the main distribution transformer from which they receive power, there is little need to be concerned about harmonics and standard 6 pulse drives should be sufficient. If any doubt remains, however, a harmonic analysis of a given installation should be performed. Given all the required details, the majority of reputable suppliers will be able to assist with a theoreti- cal study. AFE drives versus alternative harmonic mitigation techniques For new or existing VSD installations where harmonic requirements are stringent, engineers often opt for AFE drives, as these designs are commonly known to generate less harmonic content, without knowing that there are comparable alternative solutions available. If an AFE supply is under consideration for the aim of purely re- ducing harmonic content, then the following should be considered.

VSD manufacturers often push AFE technology as the best solution for treatment of harmonics associated with VSDs. Benefits over standard 6 pulse VSDs equipped with diode bridge rectifiers such as reduced line current harmonics, improved power factor and inherent regenerative capabilities are often pointed out. But the fact that current harmonics are much higher when measured above the 50 th harmonic and that very serious problems can result from the introduction of these higher frequency harmonics are often overlooked. Over and above this, there is often a substantial loss in efficiency due to the increased losses in the input IGBTs.

Figure 3: AFE drive topology with LCL filter.

Figure 4: 6 Pulse drive topology with lineator AUHF.

The reality is: • AFEs are not the best solution for a low harmonic VSD • A properly designed Wide Spectrum Passive Filter, such as the Lineator AUHF, can outperformAFEs, especially when harmonics up to the 100 th order are taken into consideration • AFEs generate high frequency harmonics which can have more serious consequences than low frequency harmonics. As a pas- sive device, a lineator AUHF cannot introduce high frequency

January ‘17 Electricity+Control

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