Advantages of 6-Pulse VFD with Lineator AUHF vs Active Front End (AFE) Drives

mirus_logo

White Paper (August 20th, 2015)

Published by Anthony (Tony) Hoevenaars, P. Eng , President and CEO Mirus International Inc.

INTRODUCTION

Active Front End (AFE) Drive manufacturers will claim that their technology provides the best solution for treatment of harmonics associated with variable frequency drives (VFDs). They are quick to point out benefits over standard 6-Pulse VFDs equipped with diode bridge rectifiers such as, reduced line current harmonics, improved power factor and inherent regenerative capabilities. But they will hide the fact that current harmonics are much higher when measured above the 50th and that very serious problems can result from the introduction of these higher frequency harmonics. Also, they will downplay a substantial loss in efficiency due to the increased losses in the input IGBTs.

The reality is:

  1. AFE’s are not the best solution for a low harmonic VFD.
  2. A properly designed Wide Spectrum Passive Filter, such as the Lineator AUHF, can outperform AFE especially when harmonics up to the 100th are taken into consideration.
  3. AFE’s generate high frequency harmonics which can have more serious consequences than low frequency harmonics. As a passive device, Lineator AUHF cannot introduce high frequency harmonics and will, in fact, help reduce them when they are present.
  4. 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 conditions.
  5. Although an active solution, AFE’s still require input passive filters (LCL and EMI/RFI filters) to control switching frequency harmonics and to attenuate ripple in the mains side voltage and current.
  6. LCL and EMI/RFI filters are more likely to resonate with the power system at rectifier harmonic frequencies (ie. 5th, 7th, 11th, etc.) than the Lineator AUHF. Also under lightly loaded conditions, the reactive power of the LCL capacitors can cause over-excitation of generators.
  7. AFE’s generate significant levels of ground leakage current which can cause inadvertent ground fault trips and failure of sensitive equipment.
  8. AFE losses are significantly higher and efficiencies much lower than a 6-Pulse VFD with Lineator AUHF.

AFE TOPOLOGY VS LINEATOR/6-PULSE VFD

Fig. 1a shows a typical AFE drive topology. The problems associated with the operation of AFE rectifiers are related to the converter design characteristics, switching frequency and interaction with the power system. You will note that ahead of the input bridge is a passive LCL filter. The function of this filter is to reduce the switching frequency harmonics introduced by the IGBTs. All AFE manufacturers, however, include LCL filters that are only minimally effective because a more effective filter would be much more expensive and physically larger. In fact, this passive filter would very likely be larger and more expensive than the Lineator AUHF that provides equivalent current harmonic mitigation on a simple 6-Pulse VFD (Fig 1b). Both topologies have an input passive filter but the Lineator/6-Pulse VFD topology is much simpler, more reliable and less expensive.

Advantages of 6-Pulse VFD with Lineator AUHF vs Active Front End (AFE) Drives_figure1a

Fig. 1a: AFE Drive topology with LCL filter

 

Advantages of 6-Pulse VFD with Lineator AUHF vs Active Front End (AFE) Drives_figure1b

Fig. 1b: 6-Pulse Drive topology with Lineator AUHF

 

AFE INTRODUCES HIGH FREQUENCY HARMONICS

In order to reduce input current harmonics, AFE Drives use IGBTs instead of a diode bridge rectifier. Current harmonics can be controlled through the switching action of the IGBTs but in so doing, switching frequency harmonics are introduced. Fig. 2 shows various measurements taken at a Paper Mill equipped with AFE Drives, by the authors of a paper on ‘Practical Problems Associated with the Operation of ASDs Based on Active Front End Converters in Power Distribution Systems’ [1]. They compare Ph-to-Gnd voltages and input currents while operating the AFE Drives as simple 6-Pulse Rectifiers and in full AFE operation.

Both operations show Ph-to-Gnd voltage with high frequency components but during AFE operation these distortions are substantially worse. Input current measurements show much lower levels of low frequency harmonics than in 6-Pulse operation but the high frequency ripple is very obvious in the waveform and the spectrum reflects this ripple with higher bars around the 50th.

With a band of harmonics near the 50th, the IGBTs on these Drives would be switching at around 2 – 3 kHz. With higher switching frequencies, the harmonic band would move out to higher harmonic orders. In many cases, these are well above the 50th where almost all power quality analyzers do not measure. Despite AFE Drive manufacturers’ efforts to ignore them, these higher frequency harmonics do certainly exist and most definitely can wreak havoc with connected equipment. These failures can be very difficult to diagnose even for trained power quality professionals.

Advantages of 6-Pulse VFD with Lineator AUHF vs Active Front End (AFE) Drives_figure2

Fig. 2: Variable Frequency Drive Voltage and Current Waveforms and Spectrums for an AFE Drive in a Paper Mill [1]

 

When AFE Drives are used on marine vessels with weak generator supplies, this problem can become even worse. The American Bureau of Shipping (ABS) has acknowledged this in several locations in Section 13 of its ‘Guidance Notes on Control of Harmonics in Electrical Power Systems’, such as:

iii) Total harmonic current distortion (Ithd), harmonic current spectrum up to 50th harmonic (or up to 100th for equipment with “active front ends”) and total magnitude of total harmonic current per unit, per circuit and per installation at rated load, as applicable. [2]

Fig. 3 shows frequency spectrums of the voltage at the Bridge Distribution Panel of a catamaran equipped with Main and Propulsion AFE Drives [3]. Measurements were taken over three frequency bands – up to 50th harmonic, 50th to 10 kHz and 10 kHz to 50 kHz. Although the voltage harmonics were very low in the lower frequency range (VTHD = 1.68%), they were very high in the frequency range above the 50th (VTHD = 8.14%) with a band around 3450 Hz (69th harmonic) produced by the AFE Drives operating at a 3.6 kHz switching frequency [3]. Most power quality analyzers that only measure up to the 50th harmonic would not have highlighted these high distortion levels.

These higher frequency harmonics will undoubtedly cause problems with connected equipment such as standard AC 6-Pulse VFDs, including those manufactured by the same supplier as the AFE Drives. The following statement is from the ‘Practical Problems’ paper sited earlier.

“From the power distribution point of view, the AFE rectifier operates as a current source, and as such injects high frequency current harmonics into the grid. If ASDs that use diode-based rectifiers (standard ASD) are connected to the same ac grid, the high frequency current components are pushed into their dc bus. This is due to the fact that they offer a low impedance path to these high frequency current components (due to the dc link capacitor presence), overloading the respective converter. Moreover, if the standard ASD is operating at light load, its dc bus voltage will tend to increase until the converter shuts down, hopefully by means of the dc link over voltage protection.” [1]

Advantages of 6-Pulse VFD with Lineator AUHF vs Active Front End (AFE) Drives_figure3

Fig 3: Voltage harmonic spectrum of a Marine Vessel with Main and Auxiliary Propulsion AFE Drives [3]

 

The authors of [1] also noted that the high speed IGBT switching action of AFE Drives introduces ground leakage currents (common-mode) that can cause inadvertent operation of ground fault protection equipment. Fig. 4 shows the neutral-to-ground voltage and currents of an AFE Drive running in both 6-Pulse operation and AFE operation. High frequency common-mode noise increases substantially while in AFE operation.

Advantages of 6-Pulse VFD with Lineator AUHF vs Active Front End (AFE) Drives_figure4

Fig. 4: Variable Frequency Drive Neutral-to-Ground Voltage and Current for an AFE Drive in a Paper Mill [1]

 

LINEATOR AUHF MATCHES AFE IN REDUCTION OF LOW FREQUENCY HARMONICS (UP TO 50th) WITHOUT INTRODUCING HIGH FREQUENCY HARMONICS

Lineator AUHF is a series connected, wide spectrum, passive harmonic filter designed to eliminate harmonics generated by 3-phase, 6-Pulse variable frequency drives. It performs as well as an Active Front End Drive in reducing harmonics in the low frequency range (up to 50th harmonics) while substantially outperforming AFE in the high frequency range. Lineator will provide some reduction in high frequency harmonics while the AFE Drive actually introduces these more damaging harmonics into the power system as described earlier.

The Lineator AUHF consists of a reactor with multiple windings on a common core and a relatively small capacitor bank (Fig. 5). This design exploits the mutual coupling between the windings to improve harmonic mitigation performance, making it far superior to conventional passive filter solutions. To prevent importation of upstream harmonics, the resonant frequency, as seen from the input terminals, is near the 4th harmonic, comfortably below the predominant harmonics of 3-phase rectifiers.

Advantages of 6-Pulse VFD with Lineator AUHF vs Active Front End (AFE) Drives_figure5 

Fig. 5: Lineator AUHF Wide Spectrum Passive Filter Schematic

 

One key advantage of the unique reactor design is that it allows for the use of a significantly smaller capacitor bank (< 15% reactive power as a percent of the full load rating). This reduces voltage boost and reactive power at no load to ensure compatibility with generators. All other passive harmonic filter solutions introduce higher capacitive reactive power at light loads (typically 30% to 40%). Even the LCL filters on AFE Drives have higher capacitive reactance than the Lineator AUHF.

The filter is connected in series between the main supply and the drive. Current Total Harmonic Distortion (ITHD) is typically reduced to < 6% (a < 5% ITDD version is available) when applied to a 6-pulse AC PWM drive regardless of whether the drive is equipped with an AC or DC reactor or no reactor at all.

Lineators can be applied to AC drives with diode or SCR pre-charge input rectifiers ranging in size from 5HP/4kW to 3500HP/2600kW. They can be applied to single or multiple drives but only drive loads should be connected as the filter is designed specifically for rectifier operation. The filter can usually be retrofitted to existing drives without the requirement for drive modifications, whether for single drive or for multiple drive applications. A model is also available for operation on fully controlled SCR bridges, as used in DC Drives.

Figures 6 and 7 provide typical performance results measured in the Mirus Harmonics & Energy Lab on a Lineator AUHF. They show voltage and current waveforms and spectrums at the input to the Lineator measured up to the 500th harmonic using an ION 7650 Power Quality Analyzer.

Advantages of 6-Pulse VFD with Lineator AUHF vs Active Front End (AFE) Drives_figure6

Fig. 6: Input Current Waveform and Spectrum for 200HP, 480V Lineator AUHF

 

Advantages of 6-Pulse VFD with Lineator AUHF vs Active Front End (AFE) Drives_figure7 

Fig. 7: Input Voltage Waveform and Spectrum for 200HP, 480V Lineator AUHF

 

Current Total Harmonic Distortion (ITHD) is only 5.12% even when all harmonics up to the 500th are included. Clearly noticeable is that, unlike the AFE Drive, there are extremely low levels of harmonic currents past the 50th.

Voltage Total Harmonic Distortion (VTHD), including all harmonics up to the 500th, is only 2.54%. This is well below the 5% maximum recommended by IEEE Std 519. Again, the harmonics above the 50th are virtually non existent, while for the AFE Drive, these were the highest harmonics present.

LINEATOR AUHF’s DESIGN PROTECTS AGAINST POWER SYSTEM RESONANCE

As a series connected passive filter, the Lineator’s combined inductance and capacitance presents a resonant frequency to the upstream power system. To prevent inadvertent resonance with the power system at a common characteristic harmonic frequency, the input resonant frequency is designed near the 4th harmonic to be comfortably below the 5th and other 6-Pulse rectifier harmonics.

Fig. 8a and 8b show a simple power system 1-Line and its equivalent diagram. Fig. 8c shows the reactance curves of the Lineator AUHF and the resonance point which occurs where these curves intersect. Since power systems are inherently inductive (unless installed Power Factor Correction capacitors are overcompensating which should always be avoided), the inductance curve will shift upwards moving the resonant frequency lower and further away from characteristic harmonics (Fig. 8d).

Advantages of 6-Pulse VFD with Lineator AUHF vs Active Front End (AFE) Drives_figure8

Fig 8: Lineator AUHF and Power System Resonance

 

The passive LCL and EMI/RFI filters required by AFE Drives, on the otherhand, are always tuned at a frequency above the 5th harmonic. The added inductive reactance of the power system will then lower the overall resonant frequency. When the resultant frequency matches a predominant harmonic on the power system, resonance will occur with its serious consequences. Therefore, the AFE Drive is much more susceptible to power system resonance than the Lineator AUHF.

AFE HAS HIGHER LOSSES RESULTING IN LOWER EFFICIENCY

Although the introduction of high frequency harmonics should in itself be enough justification to avoid the use of AFE Drives that do not have sufficient input passive filtering, there are many other reasons why the Lineator AUHF combined with a simple 6-Pulse VFD is a better solution. One significant reason is the higher losses and lower efficiency resulting from the operation of the input IGBT rectifier of the AFE.

Tables 1 and 2 show a major electrical manufacturer’s technical data for their AFE and 6-Pulse Drives, respectively. Table 3 provides a comparison of electrical losses and efficiency using the power loss statistics of a 75 kW (100 HP) and 400 kW (500 HP) Drive from these tables. With the losses of a Lineator AUHF added to the 6-Pulse VFD, this combination is still 1.7% more efficient than the AFE Drive. It is important to note that the stated AFE losses are for operation at the lowest IGBT switching frequencies. Losses increase with higher switching rates, further widening the efficiency gap.

Table 1: Technical Data for AFE Drives of a Major Drive Manufacturer [7]

Advantages of 6-Pulse VFD with Lineator AUHF vs Active Front End (AFE) Drives_table1

 

Table 2: Technical Data for 6-Pulse VFDs of the same Drive Manufacturer [8][9]

Advantages of 6-Pulse VFD with Lineator AUHF vs Active Front End (AFE) Drives_table2-1

Advantages of 6-Pulse VFD with Lineator AUHF vs Active Front End (AFE) Drives_table2-2

 

Table 3: Efficiency Comparison – AFE vs 6-P VFD with Lineator

Advantages of 6-Pulse VFD with Lineator AUHF vs Active Front End (AFE) Drives_table3

difference in efficiency can result in very substantial savings in energy and operating costs. The following example on a 400kW VFD calculates the annual savings when using the 6-P with Lineator vs an AFE Drive:

Assumptions:

𝐿 = 400 𝑘W, VFD load (motor rating)

𝐿%𝑎vg = 0.7, Average %Load (assumes 70%)

𝑡 = 8760 𝑦r, Operating time (assumes 24/7 operation)

𝐸 = 0.12 $ ⁄ 𝑘Wh, Energy cost (assumes $0.12/kWh)

𝐺 = 0.017, Efficiency %Gain (1.7% from Table 3)

Where:

Cost Savings / year = 𝐿 × 𝐿%𝑎vg × 𝑡 × 𝐸 × 𝐺

= 400 × 0.7× 8760 × 0.12 × 0.017

= 5004 $ ⁄ 𝑦r

Therefore, by using a Lineator and 6-pulse VFD under the conditions above, an additional cost savings of approximately $5,004 per year can be expected, in comparison to using an AFE Drive.

CONCLUSION AND SUMMARY

AFE Drive technology is not the best solution for a low harmonic variable frequency drive despite claims by their manufacturers. It is true that they reduce the low frequency harmonics introduced by a VFD, but they do so with very significant negative consequences. These include (i) introduction of high levels of high frequency harmonics, (ii) an input passive LCL filter that performs poorly and can resonate with the power system, (iii) higher levels of common-mode ground leakage current, (iv) much higher losses, (v) increased complexity which reduces reliability and (vi) significantly higher costs.

A much better solution is the combination of a Lineator AUHF Wide Spectrum Filter with a simple 6-Pulse VFD. This package meets the most severe requirements for harmonic reduction without the negative consequences of AFE technology. Key advantages are no introduction of high frequency harmonics, compatibility with the power system (including resistance to resonance and low capacitive reactance for generators), higher efficiencies, improved reliability and both lower installed and operating costs.

References:

[1] Luis Moran, Jose Espinoza, Mauricio Ortiz, Jose Rodriguez, Juan Dixon, “Practical Problems Associated with the Operation of ASDs Based on Active Front End Converters in Power Distribution Systems”, IEEE Transactions on Industrial Applications, 2004

[2] “Guidance Notes on the Control of Harmonics in Electrical Power Systems”, American Bureau of Shipping, May 2006

[3] Mariusz Szweda, Tomasz Tarasiuk, “An assessment of distortions of supply voltage waveform in All-Electric Ship Power Network – Case Study”, 9th International Conference, Electrical Power Quality and Utilization, Barcelona, Oct. 2007

[4] Janusz Mindykowski, Tomasz Tarasiuk, Mariusz Szweda, Ian C Evans, “Electric Power Quality Measurements on an All-Electric Ship with AC Active Front End Propulsion Drives”, PRS Technical Report No. 68, Feb. 2007

[5] Ian C Evans, “AFEs: Not the Only Answer for Tackling Harmonics”, Letter to the Editor, Drives & Controls magazine, Nov/Dec 2007

[6] Ian C Evans, “Using Active Front End Drives – Substance or Spin”, World Pumps magazine, July 2008

[7] Active Front End SIMOVERT MASTERDRIVES, Siemens Automation & Drives, MK.MC.30MAST.52.2.03 WS11013

[8] SINAMICS G120P, SINAMICS DRIVES Answers for industry, Siemens Automation & Drives, A5E35319202B AA

[9] SINAMICS G130 Drive Converter Chassis Units, SINAMICS DRIVES Answers for industry, Siemens Automation & Drives, E86060-D4001-A510-C9-7600

Published by PQTBlog

Electrical Engineer

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s