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 :
