Electricity + Control January 2017

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.

DRIVES, MOTORS + SWITCHGEAR

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

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

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

Abbreviations/Acronyms

AFE – Active Front End

ANSI

– American National Standards Institute

AUHF – Advanced Universal Harmonic Filter

EMI

– Electromagnetic Interference

IEEE

– Institute of Electrical and Electronics Engineers

IGBT – Insulated Gate Bipolar Transistor

Isc

– Short Circuit Current

– 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

January ‘17

Electricity+Control