VFD vs DOL Selection

Introduction

This article presents, defines, and evaluates the criteria for the selection of the method of starting and operation of medium voltage motors.

The various criteria were presented and discussed for assessing the basis of selection of the recommended method of starting and operation of the motors.

Induction Motor Starting and Operation Method

The following two methods (options) for starting and operation of the motors were studied and evaluated:

1. Direct Online Starting
2. Variable Frequency Drive (VFD)

Direct-on-line (DOL) starter provides a simple and cost-effective way to start and stop motors. Theses are easy to install, operate and maintain. On the other hand, a variable frequency drive (VFD) offers a precise control over motor speed, allowing for energy savings and enhanced motor performance. VFDs are ideal for applications that require speed modulation or torque control.

To determine the best choice for the motor control needs, it is important to consider several factors. Understanding the advantages and limitations of both DOL starters and VFDs is crucial in making the right decision.

So, whether the requirement is a basic and economical motor control solution like DOL starters or require advanced features such as speed control and energy efficiency offered by VFDs, this article will weigh the pros and cons, ultimately serves as a guideline towards the best choice for the motor control requirements.

Selection Criteria

Direct-on-line (DOL) or Variable Frequency Drive (VFD) methods will be analyzed on given criteria provided herein. The analysis shall be performed on each criteria listing their respective strengths and weaknesses which will then be collectively weighted. The method of starting which will emerge as more beneficial to the project requirement shall be duly selected.

Some criteria to be considered during selection are:

• Operational Flexibility
• Production Efficiency
• Physical Dimension
• Life Cycle Cost and Economics
• Troubleshooting, Maintenance and Reliability
• Design and ESP Performance Impact
• Impact on Power Supply Network
• Energy Consumption and Sustainability

Operation Flexibility

DOL starter is a high risk to the motor due to its high starting current and has a very tight operating range to adapt to operating condition changes.

VFDs have a higher reliability and safety during motor start-ups. VFDs are more flexible to adapt to the changes in operating conditions.

Production Efficiency

DOL starter have a relatively lower production efficiency as it uses mechanical throttle device to control process flow, requires shutdown for transformer tap change, and has a narrow operating range to force motor to operate outside its operating range. Variable frequency drive is highly efficient in production as it used frequency adjustment to control process flow, no transformer tap change, no shutdown required for all well condition changes, and large operating range allowing ESP to operate inside its permissible range hence extending motor life.

Physical Dimension

FD / DOL starter comprises of two major components, 1) Indoor Equipment, 2) Outdoor Equipment. The indoor equipment of FD / DOL occupies less space than VFDs, however, DOL requires an additional space for an outdoor transformer while for VFDs the transformer is integral part of the indoor VFD equipment.

12.4 LIFE CYCLE COST AND ECONOMICS 12.4.1 Initial Cost of Equipment DOL starters have a lower initial equipment cost compared to VFDs. The initial investment cost for DOL starters is lower because they do not require complex electronics for motor control. VFDs are generally more expensive upfront compared to the DOL starters due to the additional electronics and control mechanisms required for speed regulation. However, the potential energy savings and improved efficiency provided by VFDs can often justify the higher initial investment over the lifespan of the equipment. 12.4.2 Cost of Installation DOL starter requires a larger area for the equipment footprint due to the outdoor equipment while VFD only requires a smaller area for its indoor equipment. Due to the larger footprint and requirement for a platform extension to locate the outdoor equipment, the total cost of installation is higher for DOL.

12.4.3 Operation and Maintenance Cost (Total OPEX) As the speed of the motor is fixed for a DOL, the energy consumption during the operation of the ESP will remain constant. For a variable flow process requirement, a mechanical throttling device (i.e. choke valve) will be employed to limit the flow. Although this is an effective means of control, mechanical and electrical energy is wasted. As VFD alters the power frequency to the motor, speed, flow, and energy consumption are reduced in the system. The comparative maintenance cost for DOL and VFD systems can vary based on several factors, including the complexity of the equipment, the operating environment, and the application requirements. The operating environment and application requirements for both systems will be the same hence comparative maintenance cost will be based on the complexity of the equipment. DOL starters are relatively simple in design and fewer components that can wear out or require regular servicing. However, DOL starters can experience high mechanical stress during motor startup due to the abrupt application of full voltage, which may lead to more frequent maintenance or replacement of ESP over time. The frequent maintenance or replacement of the ESP makes the OPEX for DOL higher than VFD. In addition, the moving parts of the conventional switching element need frequent inspection and maintenance. Failures in the moving parts of conventional circuit breakers due to unusually high number of switchings is a phenomenon which increases maintenance costs of DOL and may cause downtimes in production. VFDs are complex electronic devices consisting of power electronics, control circuitry, and cooling systems. Maintenance requirements for VFDs may include periodic inspections, firmware updates, and replacement of components such as cooling fans, capacitors, and power modules. In addition, the requirement for of skilled personnel to perform VFD maintenance tasks is essential. VFDs are more susceptible to environmental factors and temperature fluctuations, which may require additional maintenance measures to ensure reliable operation. Proper regular preventive maintenance is crucial for maximizing the lifespan and reliability of VFDs. The daily rate of Field Service personnel from OEM is approximately $1,600. The frequency of visit of Field Service personnel is not available at the time of this study, hence the quantitative comparison of VFD and DOL will not be presented. In view of the above, operation and maintenance cost of DOL is higher than VFD. 12.4.4 Loss of Revenue As there is wide uncertainty to evaluate the ESP performances which depend on reservoir conditions / well performance, considered loss of revenue is based on the following two cases: Case 1: Production Optimization Gain assuming well with water cut of 60 to 80 %, VFD can deliver additional production up to 500 to 1000 BPD production where as DOL can deliver up to 200 to 400 BPD. Case 2: Productivity loss by 50%, assuming water cut 60 to 80 %, VFD can maintain production of 500 to 1000 BPD where as DOL could be zero production due to potential ESP failure as it will be operating in downthrust conditions. In the above production scenarios, VFD can deliver higher production volume compared to DOL. 12.5 TROUBLESHOOTING, MAINTENANCE AND RELIABILITY DOL has a limited flexibility for troubleshooting related to well conditions compared to VFD which a better flexibility to changing well conditions. However, DOL is easier to troubleshoot due to lesser component in contrast to the complexity of a VFD. Despite being easier to troubleshoot, DOL requires more maintenance, in particular the outdoor transformer. Based on actual site experience, VFDs are more reliable due to less breakdowns compare to DOL. 12.6 DESIGN AND ESP PERFORMANCE IMPACT The few interfaces of DOL starters make the design simpler compared to VFD. The configuration and parameter setting of DOL will be minimal. Interface with existing systems such as the DCS will be straight forward. The complex requirement of VFDs interfaces with existing system make the design of VFD more elaborate. Special cable requirements, larger footprints, and HVAC requirements are few factors to consider during the design process for VFD systems. 12.6.1 Impact on Electric Submersible Pumps 12.6.1.1 Torsional Stresses The large starting current of an motor startup directly from a DOL introduces severe mechanical stress in the ESP. This mechanical stress exposes the ESP shaft and interconnections to overload conditions and results in ESP failure. Extended exposure to a fluctuating mechanical stress can also introduce fatigue into the ESP system. Sustained fatigue with a high level of mechanical stress may break the shaft and connectors at a premature stage. The unique ESP shaft geometry (long length and small diameter) is primarily accountable for the failure due to torsional stresses. Severe torsional vibration may occur if the vibrational frequency of the shaft reaches its resonant value. Variable frequency drives (VFDs) are deployed for motors to reduce the stress on its mechanical transmission system. VFDs can regulate both the frequency and voltage during startup and normal operation of an ESP system. Using a VFD, the motor starting torque and the magnitude of torsional oscillations are significantly lower than its value during the direct-on-line starting. This allows the rotor to start slowly from a standstill condition, avoids the severe sub-synchronous oscillations and gets stabilized to a steady state. During the ramp up, the rotor accelerates to the full load speed when the voltage is ramped up to its rated value. The cost to replace or repair an ESP unit is very high and failures in ESPs create long downtime and business loss. 12.6.2 Transient Torques When an induction motor is started across the line by the sudden closing of a switch, transient pulsating torques, which typically may reach values from 3 to 6 times the nominal starting torque, are set up in the shafts, gearing, and couplings of the connected system. The actual magnitude of these torques depends not only upon the electrical characteristics of the motor but also on the characteristics of the mechanical transmission system, such as coupling flexibility, motor, and load inertias. Considerations of transient pulsating torques is particularly important where motors are started and stopped frequently or continuously since, under such conditions, fatigue failure may occur on the mechanical transmission system if the mechanical stresses developed exceed the endurance limit of the material. With a variable frequency drive, the low pulsating torque frequency will result in large speed jitter and can induce mechanical resonance which is unacceptable. The solution is VFD inverter technology using pulse width modulation (PWM) technology used in VFD inverter section which modulates the output waveform leading to a lower order harmonic content thus reducing the transient pulsating torque. 12.6.3 Motor Bearing Current Motor bearing currents can be a concern DOL starters due to the sudden application of voltage to the motor windings. When a motor is started directly online, the sudden inrush of current which can induce stray currents in the motor rotor, potentially causing electrical discharge through the bearings. However, this only happens during the starting of the motor. VFD systems have issues on motor bearing currents caused by the fast-switching IGBT-inverters. The common mode voltage of the feeding inverter causes high frequency common mode ground currents in the induction motor, mainly between the winding and the stator frame. High frequency currents flow in the bearing of the induction motors due to parasitic couplings in the motor drive system. At higher frequencies, parasitic couplings between stator and rotor windings and the rotor and motor frame are prominent which can cause circulating currents in the bearing when shaft voltage exceeds the breakdown voltage level of the bearing lubricant. This circulating current damages the motor bearing intensely and shortens the life of the motor drastically. An motor using a self-lubricating polymer rotor bearing will not have this issue as the polymer in the bearing acts as an insulation preventing the motor bearing current flow. 12.7 IMPACT ON POWER SUPPLY NETWORK Adding electric submersible pumps (ESPs) into the WHP-1, WHP-2 and WHP11 will impact the power supply network which underscores the importance of careful planning, monitoring, and management to ensure the reliable and efficient operation of the Umm Al Dalkh (UAD) electrical network. The impacts of the addition and operation of ESPs on UAD electrical network are presented and explored hereinafter. 12.7.1 Starting Current An motor starting directly from the switchboard supply creates a large starting current, which can be 6 to 9 times of the motor rated current. The large starting current generates a significant electrical stress on the power supply system. For a weak electrical network with a low X/R ratio, the high starting motor current may create significant voltage dip hence preventing the motor to reach its rated operating speed. In a worst case scenario, the voltage dip may exceed the limit for maximum allowable voltage drop which can cause the circuit breaker to trip. The high starting current allows the motor to deliver several times its rated torque. This can cause excessive electrical and mechanical stress on the ESP equipment, particularly in shallow well applications. An ESP is placed into operation at a depth that requires several hundred meters of power cable. During start-up operations, the cable causes a voltage drop to the motor, hence, the reduced voltage start decreases the initial starting current and torque. Variable frequency drives (VFDs) offer smooth and soft motor starting characteristics. The motor starting current using a variable frequency drive can be reduced to ≤100% of the motor rated current. A lower starting current reduces the electrical stress on the supply and prevents it from significant voltage dips and unnecessary trips in the circuit breaker. 12.7.2 Harmonics Harmonics refer to the frequencies that are integer multiples of the fundamental frequency. When harmonic frequencies are generated by non-linear loads within a power system, the voltage and current waveform will become distorted instead of a pure sinusoidal waveform. The distortion on the voltage and current waveform is called as harmonic distortion. Harmonic distortion is a measure of the amount of deviation from a pure sinusoidal waveform. Fixed Speed Drive (FD) / direct-on-line (DOL) electrical motor starters do not inherently produce harmonics. However, the operation of the motor connected to a DOL starter can introduce minimal harmonics into the power system due to voltage imbalance and mechanical resonance in the motor structure. Switching transients in FD/ DOL system can be minimized by use of surge arrestors and SF6 circuit breakers. A VFD is a non-linear load hence will introduce harmonics into the electrical network creating harmonic distortion on the voltage and current waveform. Harmonic distortions can affect the operation of other devices connected to the same power supply network. Various international standards have developed methods on how to assess the severity of harmonic distortion. One of these is standard IEEE 519 by the Institute of Electrical and Electronic Engineers (IEEE) which recognises that the sensitivity of the equipment to harmonics determines the acceptable level of voltage distortion. IEEE 519 states different limits for different types of installations, hence, variable frequency drives do not always include harmonic filtering. As a rule of thumb, unless the variable frequency drives constitute more than 30% of the load on the transformer from which they receive power, there is little need to be concerned about harmonics and standard 6-pulse drives should be sufficient. If harmonics are higher than desired after performing a harmonic analysis, there are a number of ways to mitigate the harmonic distortion like the use of filters or having a multipulse drives. These VFD configurations generate lower level of harmonic currents. A study of industrial users identified harmonics as a contributor to economic losses. When compared to voltage sags, very rarely does harmonics distortion lead to a process interruption. Most of the costs are attributed to process slow down such as nuisance tripping. According to the report, 25% of the harmonic costs were related to equipment, either in the form of damage or additional maintenance. The list below highlights the effects of excessive harmonics on equipment:  Excessive temperature rise in motors and transformers.  Sensitive electronic equipment malfunctions  SCADA issues  Accelerated aging of equipment  Tripping of circuit breakers  Cable insulation breakdown 12.7.3 Electromagnetic Interference Electromagnetic interference (EMI) refers to the high frequency disturbance generated by electromagnetic radiation from an external source that disrupts the operation of an electronic device or system. This interference can degrade the performance of sensitive equipment, cause malfunctions, or even lead to complete system failure if not properly managed. EMI can be categorized into two types: conducted interference and radiated interference. Conducted interference occurs when electromagnetic energy is conducted along power or signal cables and enters the device through its electrical connections. Common sources of conducted interference include power lines, motors, and switching power supplies. Radiated interference is caused by electromagnetic waves propagating through the air and inducing unwanted signals in nearby electronic devices. Radiated interference sources can include radio transmitters, mobile phones, and high-voltage power lines. Fixed Speed Drive (FD) / Direct on-line (DOL) can produce electromagnetic interference (EMI) during motor starting, although the extent and severity may vary depending on factors such as the size of the motor, the power supply system, and the surrounding environment. The high inrush currents and voltage transients during motor starting are abrupt changes in current and voltage and can generate electromagnetic interference across a range of frequencies. However, this scenario only happens during the motor starting duration and not during normal motor operation. Variable frequency drives (VFDs) produce electromagnetic interference (EMI) due to the switching action and the rapid changes in voltage and current they generate. The switching of power semiconductors within VFDs, such as insulated gate bipolar transistors (IGBTs) or power MOSFETs, can create high-frequency harmonics and noise that can interfere with nearby electronic equipment. The layout and length of cables connected to the VFD can as well affect the amount of electromagnetic interference produced. Longer cables act as antennas, radiating electromagnetic energy, while improper cable routing can increase coupling with nearby sensitive equipment. 12.7.4 Switching Transients An electrical transient occurs on a power system each time an abrupt circuit change occurs. This circuit change is usually the result of a normal switching operation, such as breaker opening or closing or simply turning a light switch on or off. Switching transients are oscillatory and are characterized by their transient period, very short when compared with the power frequency. The duration of the transients is mostly in the range of microseconds to several milliseconds and depends on circuit parameters. Transients are extremely important because at such times, the circuit components and electrical equipment are subjected to the greatest stresses resulting from abnormal transient voltages and currents. Transients cause over-voltage and overcurrent. Over-voltages caused may result in flashovers or insulation breakdown, while overcurrent may damage equipment due to electromagnetic forces and excessive heat generation. Flashovers usually cause temporary power outages due to tripping of the protective devices, but insulation breakdown usually leads to permanent equipment damage. Switching occurs during the start – stop operation of the DOL. The sudden change in the system can initiate damped oscillations with high frequencies that depend on resonant frequencies of the network. Magnitude and duration of the switching overvoltages depends on many parameters such as type of the load and the switching device. Switching transients in FD/ DOL system can be minimized by use of surge arrestors and SF6 circuit breakers. Overvoltages generated by the switching in power electronic converters are like the switching overvoltages in nature, with the difference that they occur continuously and with high frequencies. The frequency of the switching transient is determined by values of inductance, capacitance, and resistance. The advancements in electronics have increased switching frequency while reducing the losses. VFDs have benefited from these fast switches in the form of higher flexibility and performance. On the downside, the major problems associated with these drives come from their three inherent features: the fast rise time of the pulses (or high dv/dt), the pulse repetition frequency, and the overvoltage. In a VFD, the output voltage of the inverter is near square wave shaped. However, the voltage wave reflection phenomenon in a cable can create a high frequency overvoltages at the end of the cable that can exceed twice the DC link voltage. These high frequency voltage spikes, created by the interaction of the inverter and the cable, have some unfavorable effects on the components of the system. Impact of high frequency voltage spikes in VFD output on motors can be minimized by use of a combination of appropriate earthing conductors, sine wave filters and dv/dt filters. 12.8 ENERGY CONSUMPTION AND SUSTAINABILITY Variable flow is achieved with DOL starters by using a mechanical throttling device to limit the flow. Applying a throttling device to the system reduces the flow but changes the pump curve is not altered and continues to operate at full speed. This creates mechanical stresses, excessive pressure and temperature on the pump system, which can cause premature seal or bearing failures. More importantly, this also consumes a tremendous amount of energy. A VFD allows a great degree of control in the acceleration of the load that is not as readily available with the other types of reduced voltage starting methods. Applying a VFD to the pump allows control of the pump speed electrically while using only the energy needed to produce a given flow. This is similar to applying a new pump with a smaller impeller. Also, the pressure is reduced, which helps reduce the mechanical stresses generated by throttling devices.