Electricity + Control January 2017

DRIVES, MOTORS + SWITCHGEAR

harmonics and will, in fact, help reduce them when they are present

• If there is a mixture of 6 pulse and AFE drives on the same switchboard, the ripple in voltage from the AFE drive can raise the dc bus voltage in the 6 pulse VFDs creating overvoltage condi- tions • Although an active solution, AFEs still require input pas- sive filters (LCL and EMI/RFI filters) to control switching frequency harmonics and to attenuate ripple in the mains side voltage and current • LCL and EMI/RFI filters are more likely to resonate with the power system at rectifier harmonic frequencies (ie. 5 th , 7 th , 11 th , etc.) than the Lineator AUHF. Also under lightly loaded conditions, the reactive power of the LCL capacitors can cause over-excitation of generators • AFEs generate significant levels of ground leakage current which can cause inadvertent ground fault trips and failure of sensitive equipment • AFE losses are significantly higher and efficiencies much lower than a 6 pulse VFD with Lineator AUHF

Type

VSD Rat- ing (KW)

VSD Losses (KW)

AUHF Losses (KW)

Total Losses (KW)

Efficiency

Difference

AFE Drive 6-P with Lineator

75

4,4

4,1

94,80%

1,9

0..8

2,7

96,50%

1,70%

AFE Drive 6-P with Lineator 20 < 50

400

20

20

95,20%

9,1

3,6

12,7

96,90%

1,70%

Figure 5: Efficiency comparison – AFE vs 6-P VSD with lineator.

Conclusion In conclusion, the criteria for each application need to be taken on its own merit, in conjunction with the VSD manufacturer, in order to ensure the correct product supply for your requirements.

• IEEE 519 was published in 1981. • IEEE 519 is not a mandatory standard but rather a reference to specify harmonic distortion limits. • IEEE 519 was revised in 1992 to provide recommendations on maximum allowable levels of harmonic current distortion.

take note

Bibliography [1] Hoevenaars A. Advantages of 6-Pulse VFD with Lineator AUHF vs Active Front End (AFE) Drives. Mirus International Inc.

Glen Ward was born in South Africa in 1972. Glen has more than 20 years’ experience with VSDs and associated products. He completed an N6 diploma and T4 between 1993 and 1995. Thereafter he worked for various contract electronics companies that operated in the industrial mo- tion control sector (Variable speed drives). In 2004 Glen

Reference [1] IEEE 519: 1992. Recommended practice and requirements for harmonic control in electric power systems. (ANSI/IEEE Standard 519 − Guide for harmonic control and reac- tive compensation of static power converters − first published in 1981).

joined Zest Electric Motors as a sales engineer to support the WEG range of automation products. In 2009 he was promoted to Drives and Automation manager in the KZN region where he has remained until present. His main area of interest has been to investigate and highlight areas of efficiency improvement. Enquiries: Kirsten Larkan. Tel. +27 (0) 11 723 6000 or email marketing@zestweg.com

Electricity+Control January ‘17

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