Cable

Electrical Submersible Cable :

Power is supplied to the electric motor via the electric cable. It is banded to the production tubing.

Cable Selection :

Cable selection is governed by :

• Motor amperage and voltage rating
• Voltage drop
• Temperature
• Fluid type
• Gas composition
• Available space
• Cost

A range of conductor sizes are available to meet application requirements.

Cables are available in either flat or round configurations.

Flat Cable : small overall profile, allowing the cable to be used in wells where space constraints exist.

Most flat cables do not have an overall jacket, making these cables more susceptible to damage.

Round Cable : Round cable offers more resistance to physical damage and is used where space is not an issue, Round cable is also used in applications with highly deviated wells .

Also round cables are electrically better because of the alignment of conductors.

Cables can be installed in wells where bottom hole  temperature is excess of 400 degrees Fahrenheit.

Cable Components :

• CONDUCTOR
• INSULATION
• BARRIER 
• INJECTION TUBE (optional)
• JACKET
• ARMOR

1- Conductor Selection :

• Conductor size
• Conductor configuration
• Ampacity (cable conductor temperature)

The primary consideration in selecting a conductor for a particular application is selecting its appropriate size. In general, selecting a conductor size is a

balance between reliability and cost.

The secondary consideration in selecting a conductor is selecting the conductor configuration (solid/stranded/compact-strand)

Conductor Size :

• The main purpose of the conductor is to carry current from the surface to the motor. 
• The size of a conductor refers to the cross-sectional area
• Standard conductors used in ESP applications are #2/0, #1/0, #1, #2, #4, and #6 gauge
• Increasing gauge numbers give decreasing wire diameters and hence decreasing cross-sectional areas.

Why Conductor Size is Important :

• Conductor size has a direct influence on the cable temperature rating , the smaller the conductor, the higher the resistance, which results in a higher temperature increase in the conductor.
• A higher resistance results in more voltage loss in the conductor. If the voltage loss is too high, this can result in motor starting problems. In addition, of course, voltage loss in the cable is less efficient from an electrical operation standpoint.
• The resistance in the cable plays a role in defining the resonant frequency of the electrical system, which is important for harmonic analysis , especially with PWM-style variable speed drives.

To select the most appropriate conductor size for an application, we first need to determine the voltage drop in the conductor.

Next, we need to evaluate motor starting issues. The length and size of the conductor are the biggest determining factors on starting characteristics for a submersible motor. 

Proper selection of the conductor and starting method can insure that the motor will start reliably.

Conductor Configuration :

• Solid : this conductor has a circular cross-section.
• Stranded (Round) : seven wire stranded configuration, comprised of a center strand and six outer strands that are twisted around
• Compacted : seven wire stranded configuration that has been pulled through several sets of compacting rollers, effectively reducing the diameter of the conductor.

Ampacity :

Ampacity is the RMS electric current which a device can continuously carry while remaining within its temperature rating. It is the maximum amount of electrical current a conductor or device can carry before sustaining immediate or progressive deterioration. It is also described as current rating or current-carrying capacity.

The ampacity of a cable is a function of : 

• size of the conductor
• operating amperage placed on the conductor
• ambient temperature the cable is exposed to in the well
• thermal properties of the various cable components 
• cable configuration 

Therefore, each cable at each size has an ampacity value. 

Ampacity calculations are used to determine the temperature rise in the conductor and is defined by:

Using the equation, ampacity charts have been created for each cable type. An example is below:

2- Insulation :

A true insulator is a material that does not respond to an electric field and completely resists the flow of electric charge.

This does not exist in practice, so materials with a high dielectric constant is considered to be insulators. 

The function of an insulator to support or separate electrical conductors without allowing electrical current through themselves.

There are three different types of dielectric insulation : 

• Polypropylene (PPE)
• Ethylene propylene diene rubber (EPDM)
• Poly-ether-ether-ketone (PEEK)

The insulation and conductor is bonded together with a high temperature adhesive

It is critical to form a good bond between the two components for several reasons:

• Eliminate gas transmission in conductor when gas travelling on the interface reaches an area of lower external pressure the gas will cause the insulation to expand resulting in damage
• Eliminate damage from corrosive gas prevents corrosive from accumulating at the conductor surface
• Eliminate corona discharge current flow can ionize accumulated gas at the conductor surface. The ionization can result in a corona being formed, damaged insulation layer.

There are currently three voltage ratings available: 4, 5 and 8 kV.

This rating is determined by the thickness of the electric insulation layer over the electric conductor

3- Barrier :

• Use of barriers helps prevent chemical attack and gas decompression failures.
• Protects the insulation

Barrier Types :

4- Jacket :

Main function of jackets provide damage resistance for the underline Insulated conductors.

The jacket selection depends upon chemical resistance  properties and temperature considerations.

Jacket Types : 

• 185°F (85°C), HIGH DENSITY POLYETHYLENE (HDPE)
• 250°F (121°C),  NITRILE 1 (Oil resistant nitrile rubber compound)
• 275°F (135°C),  NITRILE 2
• 350°F (176°C),  NITRILE 3
• >350°F (176°C), EPDM  (EPDM rubber compound)

5- Armor :

The Armor is a metallic layer wrapped around the cable core.

• Protects the cable from mechanical damage during handling and installation
• Provides reinforcing hoop strength to protect against jacket swelling

Materials :

1. Galvanized steel: for most downhole applications
2. Stainless steel (316 L): For more corrosive wells
3. MONEL (copper-nickel alloy): For the most severe corrosive well environments

For more corrosive wells, stainless steel can be used; however, this material has temperature limits, especially in the presence of chloride ions, which could result in stress chloride cracking. For the most severe corrosive well environments, Monel armor is the best choice due to its excellent corrosion resistance. Monel is also the standard armor for MLEs because of the potential for high temperature, corrosive environments, and galvanic corrosion as the cable passes by various materials on the ESP equipment.

Nomenclature :

Armor Profile Types :

Motor Connection Options :

1- MOTORLEAD : Typically smaller conductor than power cable, thus runs hotter : 

• KEOTB - 250°F (121°C)
• KELB - 450°F (232°C)
• KELTB - 450°F (232°C)

2- POTHEADS :

• Tape-In Pothead - Tape wrapped around individual connector leads inside motor. 
• Plug-In Pothead - mating block mounted in motor.
• Direct Connect Pothead - Power cable attached directly to the Pothead.  (Plug-In Type)