Electromagnetic Compatibility Consideration and Recommendation

with Variables Speed Drives

Eduardo Gie

Pacific Engineering Manager at Danfoss

Electromagnetic Compatibility (EMC)

• It describes the ability of a device to resist electrical interference and not to emit electrical interference to other devices

• Natural electrical interference– Lightning – Magnetic field that surround the

globe

• Unnatural electrical interference – Interferences occur when

electrical energy is used. This interference can disperse through air or electrical wiring

– i.e light switch may cause flicker on old TVs or radios

Interference diffusion paths

Transmitted through mains and earth cables

Emitted from the VSD housing and motor cables

Conducted Emission

Radiated Emission

50Hz -2.5Hz

Harmonic

2.5KHz –150KHz

150Khz –30Mhz

Cond. RFI

30MHz -400GHzRad. RFI

Harmonic cause and effect

• Power Electronics equipment with input rectifiers have the drawback of increasing the non-sinousoidal currents into the power network

• Pulse input current which does not follow voltage waveform causes Current Distortion THiD

• THiD causes:– Transformer and/or cable overheating– Nuisance switchgear trips– Stress on Power Factor correction banks, etc.

• When multiplying THiD by the impedance of the system, it causes Voltage Distortion THvD

• THvD causes:– Malfunction and/or breakdown of electronic

equipment– Increased losses with motor running in DOL, etc.

• NB! Harmonic distortions are repetitive and continuously deform the voltage or current waveforms.

Harmonics Limits

System Level (THvD limits)

AS 61000.3.6.2001 (TR IEC 61000.3.6.2012)

System Level (THvD Limits)

AS 61000.2.2.2003 (Domestic)

AS61000.2.4.2009 (Industry)

Product Level (THiD Limits)

AS61000.3.2.2007 <=16A (VSD <1kw)

AS61000.3.12.2006 16 to 75A

AS61000.3.4.2007 > 75A

IEEE 519.2014 Utility limit THvD @ PCC

IEEE 519.2014 customer limit TDD @ PCC

Mitigation Techniques

Passive:

DC-Inductors

18-pulse

Advanced Harmonic Filters

(AHF)

Active:

///

Active Front End with PWM-rectifier (AFE)

/// ///

Active filter (AAF)

Low Harmonic Drive (LHD)

D

d

y

D

-20%

+20%

0%

12-pulse

AC-Inductors

Harmonic Considerations

• Identify the applicable THiD/THvD harmonic limit to meet the relevant standards, electricity code and installation rules of the electrical distributor.

• VSD supplier shall provide details of all harmonic currents up to and including the 49th when operating at full load and provide an estimation of the total harmonic voltage distortion resulting at the point(s) of common coupling due to the installation of all the VSDs on the project.

• Details such as – Single Line Diagram (SLD), – Supply transformer size, type, voltages and impedance – short-circuit power or short-circuit current at the primary side of transformer ( SCR has a decisive impact on

the filter performance)– Linear and non-linear loads schedule with their diversity factor – Standby, duty and assist loads– Power factor correction banks– and other relevant supply network data will be supplied to enable this calculation to be made

• If this simulation concludes the total harmonic voltage and/or current distortion is expected to be above the value prescribed by relevant standards or electricity distributor, the VSD manufacturer shall propose additional passive or active advanced harmonic filters to reduce the harmonic distortion to within limits.

High frequency conducted noise

• Use of Active Power Techniques such as Active Front Ends, Low Harmonic Drive, and Active Harmonic Filters make it possible to reduce the harmonic currents, however it may cause High Frequency conducted disturbances & Harmonic Resonances >2Khz

• 2-150kHz frequency range is currently not covered by international standards. Mainly due to the lack of reported cases and complexity to simulate them

• Factors causing significant impact on Harmonic Resonances are:– Switching frequency– EMI filter design– Supply Network Impedance– Supply Network Short circuit Ratio

• Possible solution adding HF Passive Filters to reduce both the discrete frequency and the band of frequencies levels.

How can HF-disturbances be minimised?

• Whenever possible, limit the number of devices using power electronics with switching technologies in the same part of the grid.

• Consider adding HF-harmonics filters.

• Avoid switching frequencies of the power electronic equipment in the frequency band allocated for communications, or if these switching frequencies are close to possible network resonance points.

• Always consider harmonic resonance, even when applying small power rated equipment with power electronics. High harmonic order resonances can be especially troublesome, because of significant heat and interference which could created.

• EN61000-2-4 recommendations for compatibility levels in the frequency range 2-9kHz are not considered in any EMC susceptibility tests for electrical equipment. As a result, applying these recommendations does not guarantee safe operation for all consumers at the power network.

The nature of the RFI

• VSDs typically uses PWM for the generation of the 3 phase voltages by switching the transistors ON and OFF at a very fast rate

• The high frequency switching of relatively high currents by a VSDs output transistors and the very short rise times of each pulse can result in high levels of RFI noise radiated and/or conducted onto the mains supply, control wiring and devices

Why it’s important to follow manufacturer's instructions

Power Drive System

• AS 61800.3-2005 (IEC61800-3:2004) standard specify the EMC requirements and the recommended engineering practice for power drive systems, NOT only VSDs.

• The Power Drive System consists of a Complete Drive Module (VSD), the motor and its interconnection such as motor cables, control cables, supply cables, junction box, contactors, etc. except for the driven load

RFI Mitigation Techniques

Conducted Emission

Radiated Emission

150Khz –30Mhz

RFI

30MHz -400GHz

RFI

• Install RFI Filter, a low pass filter to reduce the HF common-mode current to acceptable levels.

• The RFI Filters designed is based on the input current to the VSD and maximum. allowable motor cable length.

• Install Screened Motor Cable bonded at both ends to ensure a low impedance path for HF common-mode current to go back to the VSD

• Install Screened Motor Cable to avoid capacitive coupling, inductive coupling and Electromagnetic coupling.

• Follow Cable Segregation and Routing as per manufacturer.

• It is essential that motor screened cables are “properly” glanded and terminated at both ends. This means ensuring that the screen makes contact through 360 of the gland.

• Do not break the screen at any point between the VSD and the motor.

• No Pigtail at cable screen!!

Bearing Motor Currents - EDM

• Fast switching pulse width modulation (PWM) causes high frequency voltage pulses with respect to motor ground. High Frequency capacitivelycoupled currents between the motor stator windings & rotor; and between the motor stator windings & motor frame, can flow causing premature bearing failures.

Bearing and Shaft current categorised

• Low Frequency nature,– These currents which can be localised in

the bearing or are driven through the bearing are due to asymmetries in the motor material properties or construction; and even occurrin DOL motors.

• High Frequency nature,– Fast switching pulse width modulation

(PWM) causes high frequency voltage pulses with respect to motor ground. High Frequency capacitive coupled currents between the motor stator windings & rotor; and between the motor stator windings & motor framecan flow.

EDM Mitigation Techniques

Low Frequency Currents

• Motor design optimised for symmetry

• Insulate one bearing

High Frequency Currents• Follow strict EMC installation practice state by VSD’

manufacturer documentation

• It is essential that screened cables are “properly” glandedand terminated.

• Protective Conductor Earth (PE) shall be inside the cable screen and terminated by very short connection at the same place as screen at both ends

• Do not break the screen at any point between the VSD and the motor. If there is a local isolator between the VSD and motor for safety purposes, connect the screen continuously with the help of a metal back plate. Connect the screen to the back plate using proper metal cable clamps.

• Proper HF grounding of the motor

• Proper HF ground connection between motor chassis & driven load

• Shaft grounding brush & isolating couplings can be used as additional mitigation

• HF Common Mode Filter cores can also be used as a preventative measure

• Reduce Switching frequency

Care must be taken when specifying, selecting and installing VSD and options

• “Prevention is better than cure”

• The best time to consider all aspect of EMC is during the preliminary design.

• Specify relevant limits• Select the correct product and

options• Plan EMC Plan for PDS• Validate the performance with

instruments Avoid the need to call the Electromagnetic Compatibility Crime Scene Investigator (EMC-CSI)

Thank You