Varaible Speed Drive VSD

Why use a VSD in  your ESP application?
The VSD adds Flexibility to the ESP system :

• Provides lightning/surge protection for the downhole equipment
• Eliminates the need for chokes in the system
• Provides inherent soft start, so no inrush currents
• Protects the downhole equipment from pump off
• Enables remote operation/automation
• Enables smooth response to torque changes
• Converts incoming AC to DC and back to AC.  This allows the drive to change the speed from 0Hz to 60Hz
• Monitors more parameters than a switchboard
• Protects the motor by shutting down power if anything goes wrong
• Provides constant torque through the entire speed range
• Can be used to manually set the output voltage of the drive
• Protects power source from harmonics generated going upstream
• Protects downhole ESP system from harmonics generated going down stream

VSDs add value to the ESP application :

• Every ESP application can benefit from variable speed :

                         - Extend your equipment life

                         - Improve your system efficiency

                         - Enable optimization as well conditions change

The VSD controls the output Voltage and Frequency to the motor by :

• Changing the input AC signal to DC signal
• Chopping the resulting DC signal, and finally
• Varying the output voltage and frequency to the motor

By varying the voltage and frequency to the motor, we are changing what is called the V/Hertz ratio.

As we have seen, a VSD is simply a device for taking a fixed frequency power supply; and changing it to a variable frequency to drive the downhole equipment.

There is nothing "magic" about a VSD.  As the frequency increases, so does the motor speed as well as its horsepower capability.

The increased speed causes the pump to put out more head and flow; and, in an effort to obey the laws of physics, the brake horsepower required to do this task also increases.

We can predict the change in pump performance with affinity laws.

Sizing a VSD application is then a matter of matching the pump and motor at the speed of interest.

VSD Main Block Diagram :

Converter is the Input end of VSD : its function is to convert AC power to DC power .

DC Link : 1. To help smooth out the ripples

                   2. Stores the electric energy

Inverter : its function is to invert  DC power into a varying AC power.

The output of the VSD is a square wave due to the Pulse Width Modulation (PWM) type of inverter system used. 

A PWM output works on average voltage principle of each waveform. As the DC bus is a constant as per incoming supply voltage, the inverter has to use the DC voltage in pulses to create the output voltage required.

The rate of the pulses of DC voltage  (IGBT firing on/off) is controlled by carrier frequency setting – Default 2.2Khz.

In VSD, Carrier Signal B is kept constant.

If the voltage OR frequency of A change, the pulse width in C will change accordingly

The sine wave A is “translated” into a chain of pulses C with vary pulse width. In another word, the pulse width in C carries all the waveform information of A.

Because the pulse width is resulted from the modulation of a sine wave (reference signal A) , so the Average Voltage is in sinusoidal wave shape.

While the voltage (PWM signal) applied to the motor windings is NOT sinusoidal, the motor current is a function of the Average Voltage applied. So the motor current is very sinusoidal in nature. In another word, the motor will see a sinusoidal current.

The motor will rotate smoothly due to the sinusoidal current.

The square wave output is not ideal for ESP systems as :

• It increases vibration & heating in motor windings
• Creates high voltage harmonics
• Possible Resonant frequency issues & electrical failures

Multiples of the fundamental frequency, that make up distorted waveforms :

Input harmonics are the harmonics produced by the converter of the VSD. For diode converters the issue is high current harmonics. These can be improved by using :

• 12 pulse VSD system (12 pulse VSD & phase shift transformer)
• Mirrus lineator filter
• Using a Sinewave Drive SWD
• Medium Voltage Drive (MVD) – High voltage applications

Affinity Laws :

The pump/motor speed will not exactly vary with the ratio of the change in speed.

In other words, if a unit on a VSD is turning exactly at 3500 RPM at 60 Hz, it will not turn exactly at 2917 RPM at 50 Hz -- it will turn a little faster.

The reason is that the unit will operate where the pump and motor torques are equal.

As can be seen, the motor output torque is essentially constant but the pump torque varies with the square of the speed.

This is usually not worth considering in terms of production calculations and the "steeper" the motor speed-torque curve at the operating point, the less the impact will be.

If the change in production does become significant, this can be compensated for with less than a 1 hertz change in frequency in most cases.

If we know the pump performance at 60Hz, we can correct it to another frequency by the affinity laws : 

If we know the motor 60Hz nameplate rating, we can calculate the output horsepower rating at any other frequency with :

If we prefer to work with 50Hz as a base, we can substitute in 50 in place of 60 everywhere it appears in the equations.

If we know the pump BHP at 60Hz and we know what is the maximum frequency we desire to run at, we can determine the minimum permissible 60Hz motor HP rating as :

If we know the pump BHP at 60Hz and we know what our motor size is at 60Hz, we can calculate the maximum allowable frequency before overloading the motor as :

If we know the voltage at 60Hz, we can calculate it at another frequency as :

If we know the pump BHP at 60Hz and the motor rated Hp at 60Hz, we can determine the motor load at any frequency as :

At any frequency, if we know the volts and amps, we can calculate the KVA as :

If we know the drive KVA rating at one input voltage, we can convert it to another input voltage as :

If we know the pump shaft HP rating at 60Hz, we can convert it to another frequency as :

If we know the pump shaft HP rating at 60Hz and the pump BHP requirement at 60Hz, we can determine the maximum frequency allowable before we exceed the shaft capability as :

Wellhead

Wellhead :

The Wellhead is the equipment that is installed at the surface of the well bore.

It’s purpose is: to suspend the tubing string in the well, and to monitor and control high pressures conditions often present within the well.

The fowlloing figures shows the wellhead connector assembly and the wellhead pack off assembly :

Junction Box

Junction box / Vent Box 

FUNCTION :

• Provides a connection point for the surface cable from the motor control panel to the power cable in the wellbore coming from the wellhead .
• Allows for any gas to vent that may have migrated through to the power cable.
• Provides easy accessible test point for electrically checking downhole equipment.
• Minimum required distance from wellhead : 35 ft .
• Connection terminals, easier to reverse motor rotation and to take initial readings.

Switchboard

Switchboard : 

• Monitors incoming voltage and current along with a few other things
• Protects the motor by shutting down power if voltage or current wander outside acceptable parameters
• Direct on Line (DoL) device, this means that the line power coming in is the same as the power going out.  The switchboard does not modify the power

Components and Function :

1- Basic Circuit :

The basic circuit can be divided into two parts:

1. The control circuit which operates at 120 Volts, and up to 5 Amps.
2. The power circuit which operates at the voltage and current of the ESP (allowing for cable loss).

2- Power Circuit :

1. Disconnect  Switch: Incoming power to the controller is applied to the manually operated disconnect switch.
2. Vacuum Contactor: The controller controls the opening and closing operation of the contactor. Auxiliary contacts provide electrical indication of the position of the contactor.
3. Power/Control Fuses: The current limiting power fuses selected can withstand repeated starting of the motor while maintaining short circuit protection. These particular fuses were selected for motor instantaneous overcurrent protection.
4. Lightning Arrestor: The lightning arrestors provide protection of control and motor circuits from lightning strikes. 

3- Control Circuit :

1. Control Power Transformer: The transformer primary accepts voltages of 220 to 5050 AC and converts down to approximately 120 volts AC used to run all the control circuitry in the Switchboard.
2. Current Module It is used to convert the high primary current to a low secondary current that is suitable for input to the controller.
3. Instrument Potential Transformers The primary taps provide different voltage ranges “selected based on the input”  to provide approximately 120 volts AC at the secondary terminals. The secondary voltage is input to the controller for monitoring, controlling and display purposes.

Potential Transformers :

1. Used to step-down the high voltage (from between 330 to 4900 volts) to a control voltage of 120 volts.
2. The 120 volt control voltage is used to run all the control circuitry in the Switchboard.
3. Motor controllers will monitor the control voltage and using the P.T. ratio calculate the line voltage in the switchboard.
4. Accuracy is tested to a tolerance of +/- 2%.
5. Two types: Control PT & Instrument PT.

Current Transformers :

1. To monitor the motor current it has to be reduced to a level which is easier to measure.
2. If this was not reduced then measuring devices would be large and cumbersome in order to cope with the high currents.
3. To reduce the level to a more manageable one current transformers are used
4. Since most control circuits operate in a 0 – 5 amp range, the CT is used to convert actual amperage to a value useful to the controller.

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)