On land-rigs, offshore drilling rigs/drill-ships, oilfield ESPs, FPSOs and other oilfield installations the nature of power quality problems can usually be characterised by the type of variable speed drives (e.g. DC SCR drives and/or VFDs) being utilised.

The section below serves to provide a basic introduction to both DC SCR drives and VFD related problems, common on drilling packages and other oilfield applications….

For information on PQGuard local/remote power and power quality monitoring for offshore/oilfield applications click here.

DC Drilling Packages

On rigs with DC SCR drives the common PQ problems encountered are:

Note : 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 (due to VFDs), 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 VFD 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 (Fig 3) 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 1 illustrates the voltage distortion a land-rig despite the load current being relatively sinusoidal. The maximum total harmonic voltage distortion (Uthd) show is 19.2%. The maximum Uthd during the site visit was 26.3%.  The high Uthd was due to the line notching and switching of the SCRs.

Note : It is not common older rigs, the majority of them with DC SCR drilling packages, to have harmonic mitigation installed as marine classification rules are not retrospective. However, the operational effects of excessive harmonic voltage distortion and other related PQ problems on rigs without harmonic mitigation can have a high cost in terms of lost production, maintenance and safety. These considerations may be not fully realised by their owners and operators.

DC drives – Line notching

Due to the commutation process within DC SCR drives the line voltages are short circuited as one thyristor (SCR) device transfers current to the next device. The depth of voltage notch is exacerbated if there are no AC line reactors in the DC SCR drives as only the inductance of the generators limit the short circuit current.

The result can be spurious zero crossovers (of the voltages) leading to misfiring on equipment which depends on true zero crossovers for timing (e.g. AVRs and SCR firing control boards). Line notching, by interacting with cable and other stray impedances can excite repetitive voltage spikes which disrupt and damage equipment.

Fig 2 below shows typical line notching due to three SCR drives operating at differing control angles (i.e. output DC voltage). Note the primary line notching does not tend towards zero due the lack of low speed, high torque operation (i.e. low DC voltage, high DC current).

Fig 3 below depicts the harmonic line voltage spectrum (Uthd) during time period of the waveforms shown in Fig 2.

Note that on this occasion there are three discrete harmonic voltage spectrums (however there will be of course be some frequency overlap, over the three spectrums) :

  1. The 3rd to 43rd harmonic due to the non-linear load current drawn by the DC SCR drive acting on the system impedances.
  2. The 52nd to 70th due to the top drive VFD common mode voltage impressed on the switchboard busbars via the ground connection. The CMV travelled 180m through the ground.
  3. The 75th to 186th due mainly to the high frequency components in the line voltage notches.

Please note with conventional power quality analysers only the harmonic voltages to the 50th or 63rd order will only be visible. This restriction does not assist troubleshooting as only portion of the overall PQ picture can be observed. For this reason, Harmonic Solutions Oil and Gas use advanced cycle by cycle power quality analysers which capture every single cycle and harmonic orders to 512st in addition to 200MHz scope-meters with inbuilt spectrum analysers and EMC spectrum analysers).

DC Drives – Voltage Spikes

On drilling rigs and other installations with DC SCR drives, interaction between the line notching and stray cable and other capacitance (e.g. VFD EMC filters) can result in ringing (i.e. voltage oscillations) and resonance within the line notches (Fig 4) and repetitive voltage spikes (Fig 5).

The effect of the voltage spikes can be serious and very disruptive. On one jack-up drilling rig, for example, the following failures, recorded over a three month period, were attributed directly to repetitive voltage spikes :

  • Four VFD EMC filters totally destroyed
  • Multiple failures (>10 off) 24V and 48V switched mode power supplies
  • Multiple failures (>12 off) electronic thermostats
  • Numerous failures of fire & gas detection system input filters
  • Repeated and multiple failures of fluorescent lighting capacitors
  • Continual tripping of laundry washing machines

DC Drives – Low generator displacement power factor

For DC SCR drives, the displacement power factor (DPF – fundamental components) and true power factor (TPF), which includes the harmonic components, are similar in value. Both are dependent on the SCR drive output DC voltage (DC motor speed) and torque demand (DC current). At low speed/high torque operation, the generators have to supply large amounts of reactive power (kVAr) to the SCR drives, reducing their power factor significantly, often to <0.1-0.2 lag and limiting the generators capability to supply the design kW load without overheating.

Fig 5 below shows the relationship between L12 voltage and L1 current at low displacement power factor.

Fig 7 below illustrates the rms current, power factor and reactive power demand (kVAr) for the SCR draw-works on a land-rig. As can be seen, the power factor reduces (and kVAr demand increases) during periods of increased current demand. The power factor and reactive power demand are dynamic and also dependent on the number of generators operating in parallel.

Note: that VFDs have a high ‘displacement power factor’ (DPF), around 0.96-0.97 lag, irrespective of load. With VFDs the ‘true power factor’ (TPF), which includes the ‘harmonic distortion factor’ always is lower and is dependent on the harmonic current magnitude, which varies with load.

This high VFD (or common DC bus diode rectifiers) DPF can lead to problems with generator leading power factor if some types of passive harmonic mitigation are used incorrectly as they can inject excessive reactive power into the power system at light or no load.

Variable frequency drives

In the case of variable frequency drives (VFDs) either applied as stand-alone drives on applications including ESPs, compressors or as components within a common DC bus system based drilling package the two basic PQ problems the same. Namely :

  • 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 (Fig 7) 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 voltages.

Fig 8 : Relationship between Uthd, line voltage and current waveforms and Ithd on a 560kW VFD

Note : VFDs, without AC line or DC bus reactance have 85-135% Uthd. In this instance 3% AC line reactors were installed in the VFD. The Ithd was around 28-24% 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 if 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, 4% if SCR drives and other rectifier loads.

Rigs built in recent years tend to have VFD based drilling packages and usually have some type harmonic mitigation installed in an attempt to comply with marine classification bodies rules. However, this may not always the case unfortunately. Proper compliance or verification testing is rarely carried out during trails or work up.

Common mode voltage

Over the last 5-10 years the use VFDs have increased tremendously on offshore and onshore installations and drilling rigs. 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 dv/dt (i.e. rate of rise of voltage) characteristic of the rapidly switched output voltages as illustrated in Fig 9. The PWM (pulse width modulated) outputs from VFDs results in most EMC problems.

Fig 9 : High du/dt PWM output voltages results in many EMC problems including common mode voltage

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

Some examples of common mode interactions on drilling rigs and oilfields are provided later in this section.

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 in the 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

‘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 1-30kHz (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 1-15kHz) 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 10 : Line voltage and common mode voltage on a 630kW top drive 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.

In addition to the conductive paths in a VFD (Fig 10), the common mode voltages and currents flow between the phases and ground through any stray capacitances [e.g. cables, motor/generator windings] to the grounded metalwork (i.e. the hull of the rig or ground of a land-rig).

Fig 11 : 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 actual site common mode voltage problems on drilling rigs are :

Example One. This involved jack-up rig with a hybrid drilling package (i.e. SCRs for mud pumps and VFDs for draw-works and top drive). The red trace in Fig 11 represents the Phase One to ground voltage when no drilling package VFDs are assigned or running. All equipment on the MODU operated without any problems during this period.

Fig 11 : Common mode voltage (red) with no drilling package VFDs running or assigned

In Fig 11 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. The rig was taken off contract until the crane problems could be resolved.

Fig 12 : Red trace is common mode voltage when the draw-works and top drive VFDs running

The common mode voltage spectrum (Fig 12) 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 12 : Harmonic voltage spectrum of common mode voltage when the VFDs assigned or running

Note that the 3rd harmonic voltage in the Fig 12 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 jack-up fire and gas detection system. The left trace (Fig 13) 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 13 : Fire and Gas Detection System. Left trace is normal control pulse train. Right trace is with a 1200HP/900kW VFD running.

When the new pump VFD was operating (right trace, Fig 13) 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 14) 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 or the ground if a land-rig).

Fig 14 : Typical common mode voltage waveform (1.26kHz VFD switching frequency)

Fig 15 illustrates the effect of excessive common mode voltage through electrostatic discharges (ESD) on a 2500kW marine shaft generator with VFD voltage controller. The cause of the damage to the flexible coupling and bearings was incorrect EMC installation procedures (e.g. no special VFD rated cables from the VFD to the switchboard, incorrect glanding of cable shields and excessively long ground pigtails).

Fig 15 : Pitting on a flexible coupling of a 2500kW shaft generator due to common mode voltage/current
Fig 16 illustrates the voltage measured across the shaft of the generator

Note : that excessive common mode voltage/current and electrostatic discharges are serious issues for all VFD fed explosion-proof motors, due to potential damage to bearings and flame paths unless special precautions are taken.

Note : Whilst harmonics, line notching, voltage spikes and low generator power factor can be readily resolved by varies means, common mode voltage problems due to high powered VFDs can be a lot more challenging. Often it is not possible, for operational reasons, to carry out the required remedial work and maintain production (e.g. significant re-cabling may be required). In these circumstances it may be necessary to isolate susceptible equipment from common mode voltage in order to maintain operational integrity in expectation that the remedial work with be carried out at the earliest opportunity. Experience however suggests that this may not always occur and that the ‘sticking plaster’ approach is often the solution (until more susceptible equipment is hopefully installed).

Fixed speed explosion-proof motors with supplies containing harmonics

It is appreciated that harmonic voltages and currents affect generators, transformers and induction motors which require thermal derating in their presence

Typical explosion proof motor

There is however a further ‘complication’ regarding fixed speed explosion-proof motors based on IEC and UL/NEMA standards.

These explosion-proof motors are only certified for use on sinusoidal supplies (0% Uthd). For example, the IEC standard relating to electrical machines, IEC60034-1, specifies the requirements regarding 2-3% ‘harmonic voltage factor’ (HVF) due to the effects of harmonics on winding temperature. IEC 60079-1 (hazardous area equipment) however does not currently have any requirement for HVF regarding compliance testing or certification for any explosion-proof motor protection concepts (i.e. different type of explosion-proof motors). It is a similar situation for NEMA/UL motors under standard MG-1.

Therefore, if these motors were subject to voltage supplies with >0% Uthd then are uncertified as they are “operating outwith the conditions envisaged when they were certified”. This does not mean the motors are unsafe (although under certain conditions they could be) but that the operator has lost any third party (e.g. NEMA/UL/ PTB, CSA, BASEEFA et al) verification as to their safety under all operating conditions.

There is a second serious and practical consideration regarding flameproof induction motors and harmonic voltage distortion. The flameproof motor (i.e. EExd in IEC codification) relies on the principle that no matter what happens inside the flameproof enclosure (e.g. an internal explosion) it cannot transmit to the surrounding hazardous area. While that statement may be perfectly valid for sinusoidal voltage supplies it is not valid for voltages supplies polluted with harmonics.

A flameproof motor relies solely on the flameproof enclosure and flame paths in the end housings to contain any internal explosion in the event of gas or vapour entering the machine. However, in the presence of harmonics, most notably on motors with deep bar or double cage rotors, the rotor temperature rise can be excessive and possibly exceed the motor temperature class (e.g. 200 deg C for an EExd IIB T3 motor).  High rotor temperatures can affect the bearings as the lubrication degrades, exposing them to excessive wear. Hot rotors can also degrade the flame paths and if there is an internal explosion, perhaps more likely due to high rotor temperatures, then it may not be contained and transmission to external hazardous areas may result with potentially disastrous consequences.

In order to overcome this ‘deficiency’, the standards authorities (IEC, NEMA et al) place the sole responsibility on operators to maintain their harmonic voltage distortion at a safe and acceptable level (i.e. 5-8% Uthd) such that explosion-proof motor safety is not compromised. Harmonic Solutions Oil & Gas is unsure how the same authorities would react to the reality of explosion-proof motors being regularly subjected to 20%-35% harmonic voltage distortion levels as witnessed during a number of offshore (and onshore) oilfield harmonic measurements.