Variable frequency drives

In the case of variable frequency drives (VFDs), either applied as stand-alone drives or within a common DC bus system, the two basic PQ problems the same.

  • Harmonic voltage distortion
  • Common mode voltage

AC VFDs – Harmonic voltage distortion

The negative effects of harmonics are acknowledged in many technical papers (see Resources section for more information) and do not require any explanation here other than to state that they generally fall into two basic categories:

  • Excessive heating caused by additional I2R losses, iron losses, skin effect, etc. in cables and equipment (e.g. generators, transformers and motors).
  • Voltage distortion resulting from harmonic currents, at the various frequencies, passing through the system impedances and leakage inductances of the power system, disrupting or destroying susceptible equipment.

Harmonic voltage distortion is essentially ‘pollution’ of the supply voltage and is ‘seen’ by all equipment connected to the power system.

Most conventional variable AC and DC speed drives offshore are ‘6 pulse’ (i.e. one three phase rectifier). All rectifiers, when fed with sinusoidal voltages, draw non-sinusoidal or ‘non-linear current’ from the supply and are hence termed ‘non-linear loads’. When the supply voltage is distorted and/or imbalanced, uncharacteristic harmonic currents and voltage are also drawn from the supply.

Fig 8 shows the relationship between the Uthd, line voltage and current waveforms and Ithd on a 560kW VFD. Note the high frequency components in the line voltage waveforms.

Fig 9 : Relationship between Uthd, line voltage and current waveforms and Ithd on a 560kW VFD

Note :

In VFDs with no AC line or DC bus reactance, the Ithd (total harmonic current distortion) is typically 85-130%. With 3% AC line reactors installed in the VFD, the Ithd is around 28-32% based on generator subtransient reactance (X”d) of around 16-18%. Since the harmonic voltage distortion (Uthd) is a function of the harmonic current being drawn by the non-linear load(s) it will be obvious that Uthd will be reduced slightly of AC line reactors are installed. 3% reactance is a comprise between cost, size and performance and will permit active filters to be used for harmonic mitigation, if required. At least 3% AC line reactors are essential if active filters are to be used for harmonic mitigation on VFDs, and other rectifier loads. DC SCR drives usually require 4% reactance.

Common Mode Voltage

Over the last 5-10 years the use VFDs have increased tremendously on in the marine industry, from main propulsion to pumps and fans. This popularity however as resulted in a dramatic increase in a phenomenon of “common mode voltage” also known as “common mode shift”, which can have serious consequences on the operational integrity and safety of electrical other equipment.

Common mode voltage originates at output of VFDs due to the non-sinusoidal and high du/dt (i.e. rate of rise of voltage) characteristic of the rapidly switched output voltages as illustrated in Fig 10. The PWM (pulse width modulated) outputs from VFDs results in most EMC problems.

The excessive du/dt can also be a serious problem when VFDs are retrofitted.  Depending on the class and age of the winding insulation, motors may not be above to withstand excessive du/dt and burn out.

Fig 10 : High du/dt PWM output voltages results in many EMC problems including common mode voltage

Excessive common mode voltage (CMV) can disrupt sensitive electronic equipment resulting operational serious operational problems.  CMV is considered by many as the ‘IED’ of the marine electrical world.   Highly disruptive to susceptible equipment, it is rarely measured and can occur on any installation which has VFDs. The cause is often the incorrect installation of VFD equipment from an EMC (i.e. electromagnetic compatibility) perspective.

Common mode voltage (CMV) is an EMC (electromagnetic compatibility) issue. It can be measured between each phase and ground using an oscilloscope and spectrum analyser or a suitable PQ recorder. An explanation of common mode voltage is complex and outwith the remit of this section but a detailed explanation can be found in the Resource section of this website via the drilling paper, “AADE-11-NTCE-7 The Price of Poor Power Quality”. Click here to download a copy of  AADE-11-NTCE_7_The Price of Poor Power Quality_Evans IC & Richards MJR_Rev 5_20th March 2011    (The common mode voltage section of the paper is still valid despite being a drilling industry paper).

‘EMC’ covers electromagnetic phenomena over a very wide range of frequencies; the European EU Directive limits the frequency range from 0Hz (DC) to 400GHz. North America has its own standards via the FCC Regulations.

VFDs are powerful emitters of electro-magnetic noise due to the rapid switching of output voltage and current. SCRs, as used in DC drives are benign by comparison, switch relatively slowly, limiting their emission spectrum to around 1MHz. VFDs which use IGBTs emit frequencies up to around 50MHz with most problematic emissions in the range 1-150kHz (for VFDs).

In the European Union there is a legal requirement to use specially designed EMC filters designed for 150kHz- 30MHz. Variable speed drives must also be installed in strict compliance with the drive manufacturer’s EMC instructions (e.g. specific type of cable, cable routing, enclosure design and layout, grounding and bonding, etc.) to minimise emissions of EMI. Common mode EMI problems due to VFDs usually occur below 150kHz (i.e. usually 1-150kHz) and may require special filters and techniques.

The majority of offshore power installations are IT networks (i.e. isolated neutrals). These networks cannot use standard EMC filters since the filter capacitors have to be connected to ground and are destroyed should a ground fault appear on the system.

Fig 11 : Line voltage and common mode voltage on a 630kW VFD.

‘Floating’ EMC filters may be used with caution but this often raises causes safety concerns and successful implementation requires considerable EMC expertise and experience. Isolation transformers with electrostatic shielding can be very effective but also large and expensive. In some cases, capacitors to earth can be utilised to provide some, if inelegant, attenuation.

Fig 12 : Common mode voltage/current paths for VFDs on IIT networks (Cherry Clough)

The higher the frequency, the lower the impedance of the stray capacitances, which means that the VFD switching ‘noise’, consisting of very brief transient ‘spikes’ (see Fig 10) at the switching instants easily pass through the stray capacitances.

Common mode currents flow through cable insulation, through the air and through the metal structure of the hull and any item of electrical equipment connected to it. Installing EMC filters (i.e. ungrounded types) or isolating transformers at the VFD input essentially provides a shorter path for common mode currents so they flow through less items of equipment, thus sparing their control systems the EMI exposure and subsequent problems which may result.

Some examples of the problems caused by common mode voltage/current follow.  (Examples 1 and 2 as from the offshore drilling industry but are still valid as they can occur also in the marine industry):

Example One. This involved a jack-up rig with a hybrid drilling package (i.e. SCR DC drives for mud pumps and VFDs for draw-works and top drive). The red trace in Fig 13 represents the Phase One to ground voltage when no drilling package VFDs are assigned or running. All equipment on the MODU (mobile offshore drilling unit) operated without any problems during this period.

Fig 13
Fig 13 : Common mode voltage (red) with VFDs running or assigned

In Fig 14 the red trace represents the Phase V1 to ground voltage when either of the drilling package VFDs were assigned or running. The consequences of the voltage rendered all three deck cranes ‘dangerous’ as the common mode voltage interfered with the crane electronic control systems and other equipment. The rig was taken off contract until the problems could be resolved.

Fig 14 : Red trace is common mode voltage when the draw-works and top drive VFDs running

The common mode voltage spectrum (Fig 15) can be seen at 153V at 1.98kHz (i.e. switching frequency of the VFDs); the highest recorded voltage was 203.54V between phase and ground at 1.98kHz.

Fig 15 : Harmonic voltage spectrum of common mode voltage when the VFDs assigned or running

Note that the 3rd harmonic voltage in the Fig 15 voltage spectrum is due to the dissimilar pitch phenomena which occurs when generators are paralleled.

Example Two. This example illustrates the effects of common mode voltage on the operation of a fire and gas detection system on a jack-up drilling rig. The left trace (Fig 16) illustrates the control pulses when the system operated normally. The right trace illustrates the result when a new 1200HP/900kW pump VFD was connected to the system.

Fig 16 : Fire and Gas Detection System. Left trace is normal control pulse train. Right trace is with a 630kW VFD running.

When the new pump VFD was operating (right trace, Fig 16) the fire and gas detection system faulted continuously, resulting in spurious gas alarms and was disabled until the cause of the failures were established and the system made fully operational again. This left the rig and the personnel at risk for some days.

 

Example Three. The path for common mode voltage (Fig 17) and current is from the VFD IGBT output bridge, along the cables to the motor, across the air gap to the rotor and also via the bearings to ground (i.e. the hull of the ship).

Fig 17 : Typical common mode voltage waveform (1.26kHz VFD switching frequency)

Fig 18 illustrates the effect of excessive common mode voltage through electrostatic discharges (ESD) on a bulk carrier 2500kW marine shaft generator with VFD voltage controller. The cause of the damage to the flexible coupling and bearings was incorrect EMC installation procedures and practices.

The effect of common mode current and AC motors

Above we have only considered the disruption to equipment due to common mode voltage.  However, the associated common mode current can be a serious problem for the AC motors connected to VFDs AND to fixed speed motors (where VFDs are present) which are connected to the same ground (e.g. the hull) as the common mode current transits from the VFD IGBT inverter stage along the connecting cable to the motor.  The common mode current is constantly looking for a ground and finds it via the motor bearings where the bearing(s) is damaged/destroyed by EDM (electrical discharge machining).

 

The path for common mode current is show in Fig 20.  In Fig 21 an example of damage (fluting) to a VFD fed motor is illustrated.

Fig 20 : Path for common mode current in VFD fed induction motors
Fig 21 : Example of ‘fluting’ on VFD fed induction motor bearing outer race due to common mode current

Note : Excessive common mode voltage/current and electrostatic discharges (including EDM) are serious issues for all VFD fed explosion-proof motors due to potential damage to bearings and flame paths.    In addition, high frequency components in VFD waveforms can result in “hot rotors”, which on rotor critical machines cam result in dangerous high rotor surface temperatures, well outside the temperature class.

Common mode current can also damage and destroy the bearings on fixed speed AC motors.  The illustration below (Fig 22) shows an example of the effects of EMD due to micro-arcing (at VFD switching frequencies) on >1000kW MV EExd motors due to the common mode voltage/current produced by multiple, large PWM VFDs.   In this case motor(s) bearing life did not exceed 1400-1600hrs.

Fig 22 : Example of ‘pitting’ due to electrical discharge machining (EDM) on fixed speed EExd motor

The common mode current micro-arcing also a serious potential danger if gas, vapour or condensate is present in the hazardous area.  Common mode current can be doubly dangerous in EExd (flameproof) motors as any bearing damage may affect the flameproof paths with potentially catastrophic results.