VFD Industry Requirements: Oil and Gas

Industry requirements: Oil and Gas

Our series on industry specific VFD requirements starts with Oil and Gas industry. What are the most important requirements for VFDs utilized in the Oil & Gas market segment? What is the background of those requirements? And how to specify a VFD that will fit the purpose? [1] Keep reading this article to find some answers or at least some inspiration.

VFDs are widely used in the oil and gas industry already for several decades. They are making the operations more flexible and efficient. By utilizing VFDs the users can reduce their energy bills and minimize the impact on the environment.

VFD applications in Oil & Gas

Major applications in oil and gas industry are compressors and pumps. We can find them in the entire process from upstream plants, through mainstream to downstream processing.

Requirements on VFD in oil and gas industry

Now let’s look at some of the most important requirements on the VFDs serving the oil and gas industry. Note that despite many commonalities there can always be some specific project based requirements that do not entirely fit into our general description.

1. Safety

Safety shall be top priority for any industry. In oil and gas, safety is even more pronounced as the risks are comparably higher. Many oil and gas applications are located in hazardous area, i.e. there is a certain probability that explosive gases may be present. Therefore, it is of utmost importance to prevent any risk of explosion.

The VFD itself is located in an electric room or a dedicated e-house and is not exposed to the explosive gases. However, the driven motors are often located in hazardous area and shall have a design suitable for such environment (e.g. IECEx/ATEX certification). As the VFD impacts the motor performance, e.g. by harmonics causing additional losses, it must be ensured that no hot spots appear in the motor that could be a potential source of ignition.

Personal safety also impacts the VFD directly. One of the topics is an arc fault. This applies to any VFD. However, since the power rating in oil and gas applications can be very high (large VFDs in terms of their capacity) the energy levels in case of a fault can be exceptionally high. There are several approaches how to protect personnel against the danger of an electric arc. Some of them are based on reinforced cabinet structures combined with over-pressure flaps. Others intend to duct the arc outside of the room. Another approach separates the control unit from the power hardware that is located in a different room and not accessible when the VFD is energized. An attractive approach with high reliability and superior safety is fast arc detection and elimination. By extinguishing the arc within just few milliseconds it is ensured that the energy level of the fault remains below the critical level.

2. Reliability

Another key aspect is reliability. Unplanned downtimes are very expensive with severe impact on the business. Therefore, effort shall be made to maximize reliability of the VFD and entire drive system.

Unfortunately, it is not so easy to quantify reliability. We have written a lot on this topic in our reliability and availability series [2]. General approach to increase reliability is to include design margins in the product and its sub-components. Oil and gas as a traditionally conservative industry usually includes sufficient margins already in project specifications so above condition is inherently fulfilled. While ‘overdimensioning’ certainly helps, it is not a magic formula guaranteeing superior reliability. Moreover, reliability considerations shall apply to the system level of the entire drive train.

Manufacturer’s experience also plays an important role. Oil and gas customers often require previous references of similar equipment installed in the field to get a confidence about the manufacturer and his technical solution.

3. Availability

Every customer wishes high availability of the equipment. This is especially true in oil and gas. Example: The decision to go for a full electric compressor drive train instead of using gas turbine as a driver is strongly motivated by higher availability of the electric drive system (gas turbine is more maintenance-intensive). Every unplanned downtime is one too much.

Users may specify minimum availability of the VFD or drive system with subsequent penalties when the contractual availability is not reached. This is mainly linked to outages due to equipment faults and quality issues. However, VFD may also trip as a consequence of external disturbance. Therefore, it is important to aim a robust system design. Classical example of such disturbance is a grid fault. The VFD shall be capable to bridge certain grid disturbances of a short duration. Such functionality is called undervoltage ride through or fault ride through.

VFD design for oil and gas applications

So how do all above requirements translate into the VFD design specifications? Some common points are listed in this section.

i. Power rating and overload

It is quite common to specify the VFD capacity to be 105% or 110% of the continuous motor rating. In Addition, a short time overload may be specified as well. Then VFD may be rated as 110% continuous / 120% for 60 s every 600 s referring to the motor nominal power.

ii. Speed (frequency) range

Typical speed range of a variable-speed compressor drive train is 70% to 105%. In some (rare) cases the speed range may be as wide as 30% to 120%.

iii. Load torque profile

Most oil and gas applications have square torque behavior [3], i.e. torque proportional to the square of the machine speed. However, there are also services with constant torque profile, e.g. slurry pumps.

iv. Withstand of temporary undervoltage

The VFD shall withstand specified line undervoltage (e.g. -20%) for restricted period of time. A reduction of power is allowed in case the increased current would be outside of VFD capability. In such case, motor speed may decline.

v. Ride through capability

Grid fault is disturbance with consequent voltage deviation of a larger magnitude. The VFD shall mostly be able to ride through a grid fault for specified period of time. It is often defined as number of cycles of the grid (e.g. 10 cycles, i.e. 166.7 ms in a 60 Hz grid or 200 ms in a 50 Hz grid).

vi. Long periods of uninterrupted operation

Sometimes longer periods of uninterrupted operation are requested. Either the product is designed so that it does not need any maintenance stops during such period or the maintenance tasks can be performed while the VFD is in operation (e.g. maintenance works on the cooling unit).

vii. Cooling type

VFD above 10’000 hp shall be liquid-cooled (preference). Air-cooled VFDs are used for lower power ratings or for starting purpose with short time duty. See [4] for more information on VFD cooling.

viii. Speed control loop

Speed control is usually a closed loop control. Speed sensorless algorithms may be preferred over a speed encoder. Optionally also an open loop speed control is used (for asynchronous machines). However, it may compromise the ride through performance described above.

ix. Torque ripple and vibration

Turbo-compressor trains are sensitive to vibration. Therefore, the VFD shall not generate large torque oscillations. More importantly, the inverter fed motor shall not excite any torsional natural frequency of concern [5].

x. Protection of drive system

Protection of the entire drive train is very important. Besides standard protection features there might be additional requirements of transformer or motor protection. An example is a transformer differential protection that may not be a very straight forward as already explained in [6]. Online partial discharge monitoring is another feature that may eventually (not very often) be requested – for transformer and/or for the motor. Lot of attention is paid to arc fault protection [7].

xi. Long motor cables

In oil and gas industry the motor cables might be longer than usual due to physical distances between motor and VFD and site arrangement. It may affect the VFD design. For more details refer to [8].

xii. Offshore installation

Offshore VFD installation is a chapter on its own. There are very specific challenges. The reliability and availability is even more important here as the access to the offshore platform is limited and site work very costly. VFDs installed offshore shall have reinforced mechanical design tolerating certain level of vibration and roll and pitch. There is also an aim to minimize the dimensions and weight of the equipment.

Summary

Oil and gas is an important market for drive applications. VFDs have been serving the industry for several decades. They help to save energy and to better control the process. Safety, reliability and high availability are important criteria. A robust design is required with respect to the drive design as well as high immunity against external disturbances. Lot of attention is paid to smooth torsional behavior. Protection is crucial as well. It is quite common to utilize some more expensive options not used in other industries. Oil and gas is traditionally rather conservative segment with focus on references and field experience. However, there are also some exceptions with companies open for latest technology and innovation. A good example is the offshore portion of oil and gas (topside and especially subsea development).

References

[1] How to choose a medium voltage VFD (series), https://mb-drive-services.com/category/choose-mv-vfd/

[2] Reliability and availability (series), https://mb-drive-services.com/category/reliability-and-availability/

[3] Load types and characteristics, https://mb-drive-services.com/load-types-and-characteristics/

[4] Advantages of liquid cooled drives, https://mb-drive-services.com/advantages-of-liquid-cooled-drives/

[5] Torsional behavior (series), https://mb-drive-services.com/category/torsional/

[6] VFD transformers: Differential protection, https://mb-drive-services.com/differential-protection-of-vfd-transformers/

[7] How to choose medium voltage VFD: Protection concept, https://mb-drive-services.com/long-motor-cables/

[8] Long motor cables, https://mb-drive-services.com/long-motor-cables/

[9] Drives and PLCs for Oil & Gas, https://new.abb.com/drives/segments/oil-and-gas