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ESP surface motor controllers
In artificial lift with electrical submersible pumps, the surface controller provides power to the ESP motors and protects the downhole ESP components. This page discusses the features of these controllers.
There are three types of motor controllers used on ESP applications and all are generally specifically designed for application with ESPs. They include the switchboard, soft starter, and the variable speed controller. All units vary in design, physical size, and power ratings. They are offered in two versions:
- Indoor, NEMA 1
- Outdoor, NEMA 3
Normally, all utilize solid state circuitry to provide protection, as well as a means of control for the ESP system.
Motor controller designs vary in complexity from the very simple and basic to the very sophisticated, which offer numerous options to enhance the methods of control, protection, and monitoring of the ESP operation. The selection of the type of controller and optional features depends on the application, supporting economics, and the preferred method of control.
The switchboard, fixed-speed controller, or across-the-line starter consists of a manual fused disconnect switch or circuit breaker, a motor starter, and a control power transformer. Because this controller is only a switch and does not modify the input voltage or current, it provides full-rated, instantaneous voltage to the downhole ESP system. The low inertia characteristics of the ESP allow for it to be at full rated speed within 0.200 seconds. During this starting process, the ESP motor can draw between 4 to 8 times its nameplate, or rated current, allowing it to produce several times its rated torque. This can cause excessive electrical and mechanical stresses on the ESP equipment in some situations. Normally, on deep-set systems with long lengths of power cable, the voltage drop, because of the cable, allows for a reduction in these stresses.
Disconnect switch. The manual disconnect switch allows for the primary power to be shut off from the outside of the unit. It is also fused to provide circuit protection in case of power surges.
Control power transformer (CPT). The CPT generally has multiple taps for selection of a range of output voltages. This allows a switchboard to be used within its rated range for different voltage- and amperage-rated motors.
Recording ammeter. The recording ammeter historically has been a pen-type chart recorder that plots one leg of the three-phase current. Currently, there are digital monitoring systems available that monitor all three-phase currents. They also have capability to store the monitored data in memory and display these data in graphical format.
Control module. These are solid-state devices that offer basic functions necessary to monitor and operate the ESP in a reliable manner. The unit examines the inputs from the CPT and other input signals and compares them with preprogrammed parameters entered by the operator. Some of the functions include:
- Overload and time-delayed underload protection
- Restart time delay
- Protection for voltage or current imbalance
Additional external devices can be connected, which provide for:
- Downhole pump intake pressure protection
- Downhole motor temperature protection
- Surface tank high/low level controls
- Line pressure switches
The soft starter is designed to reduce the high electrical and mechanical stresses that are associated with starting ESP systems. Typically, these are systems that are either on very short cables or are very high HP relative to their mechanical rating. The soft starter is similar to a standard switchboard, except that it is designed to drop the voltage to the motor during the initial startup phase. The drop-in voltage reduces the inrush current, thus "softening" the starting characteristic. These devices use either primary reactors or solid-state devices to control the amount of power delivered to the motor as it is coming up to speed. A soft starter typically extends the time for the motor to reach full speed from 0.200 seconds of the across the line switchboard to 0.500 seconds. After this startup period, the soft-start system switches off and the controller becomes a normal switchboard.
Most ESP design and application software programs evaluate the downhole system for these electrical and mechanical stresses and will advise as to whether a soft start is recommended. If a system is too soft, a motor could be damaged because of cogging or failure to reach starting speed.
Variable speed controllers
A variable speed controller (VSC), also referred to as a variable speed drive, designed for use with ESP systems, was first used in the late 1970s. Since that time, the industry has seen a significant increase in their use. This increase has been a result of the benefits that variable speed ESP operations can bring to the artificial-lift application. With the benefits comes an increase in the complexity and cost of the total system. Therefore, to properly apply and receive the maximum benefit, the end user should understand both the potential benefits and cautions in using VSCs. Benefits include:
- Broadened application range of the ESP pump
- Optimal efficiency of the downhole system
- Maximum well production
- Electrical isolation of the downhole equipment from surface power disturbances
- Reduced starting stresses
- Production matched with surface processes
- Maintenance improvements for operations in high free-gas applications
- Higher initial-capital cost
- Increased design complexity
- Interface with the electrical utility
- Additional motor heating
- Potential increase in voltage stresses
- Possibly higher electrical cost.
One should appreciate and understand the potential for problems or damage to the downhole equipment if certain types of VSCs are not applied and operated correctly. Since the introduction of the first VSC, the design has been simplified and reliability increased. Also, the user understanding of the system and user friendliness of the VSC have been greatly increased.
VSCs used with ESPs should be designed for the specific requirements of the downhole ESP motor and pump. This is because of the unique design and characteristics of the downhole centrifugal pump and submersible motor as compared to their surface counterparts. Generally, the VSC is designed to provide a constant volts/hertz output through a broad range of frequency variations. The magnetic flux that is generated in the stator of the submersible motor and passes through the rotors is directly proportional to the voltage and inversely proportional to the frequency of the applied power. The result is a constant magnetic flux density in the motor. Because the output torque of the motor is proportional to the magnetic flux density, the motor is a constant-torque variable-speed device. Also, because of its low inertia characteristics and unique rotor design, it does not have the same high-operating-speed restrictions as a typical surface induction motor. Therefore, a VSC is typically applied to frequencies from 30 to 90 Hz, with its minimum and maximum frequencies restricted only by the mechanical limitations of the downhole ESP equipment.
The fundamental building blocks of variable speed technology are an input rectifier or converter, a DC bus, and an output inverter, as shown in Fig. 1. In general, a converter is a piece of electrical equipment that changes electrical energy from one form to another. It may change the voltage and current magnitudes, change AC to DC or DC to AC, and change the frequency. VSCs applied to ESP equipment are AC to AC converters. They convert the input 460 volts, 60-Hz power (380 to 460 volts, 50 Hz) to output 40 to 480 volts, 10- to 120-Hz power.
Input rectifier (converter). This unit converts the input AC voltage and current to DC current and power. Current input rectifiers contain either diode bridges or silicon controlled rectifiers (SCRs). There are several types of input rectifiers, which are discussed next.
First, there is the three-phase full bridge rectifier. This most common rectifier in high-power electronics uses six devices, which are usually diodes or SCRs, to form the bridge. Two of the devices are connected to each of the incoming power phases. One device connects to the positive DC bus and the other to the negative bus. Each of these devices conducts during either the positive or negative half cycle of its respective phase. This means that we get two pulses on each incoming phase; thus, in total, it is a six-pulse converter. These converters are somewhat invariant and can cause input current total harmonic distortion (THD) levels of 25 to 35%.
Multipulse converter rectifiers are also used. They reduce input current harmonics in high-power electronic equipment. Most systems used today are multiple three-phase bridge rectifiers connected in parallel via phase or time-shifted power supplies. In multipulse systems, two pulses per phase are still achieved. Thus, the pulse number is always twice its input phase number. A phase-shifted power supply is accomplished by using a phase-shifting transformer. The transformer is connected to three-phase power and, through a vector combination of these three phases, develops the required number output phases. The most common multiphase system is a twelve-pulse bridge. It uses two six-pulse converters that are phase-shifted by 30 degrees. Normally, this converter can reduce the THD to a level of about 8%. Higher-pulse number converters further reduce the input current distortion levels. For example, an eighteen-pulse converter will produce less than 3% THD.
DC bus. The DC bus of the VSC is composed of passive, noncontrolled devices. Typical elements include inductors, capacitors, and resistors. These devices form a damped low-pass filter to smooth the DC voltage and current that is provided from the input rectifier. Depending on the design of the VSC, the DC bus provides a smooth DC voltage or current source to the output inverter. Typically, in medium-horsepower VSC units, the DC bus is composed of multiple inductors, capacitors, and/or resistors to achieve the design voltage and current ratings. Some designs include only inductors; others have only capacitors and resistors, while some have all three. The selection of the design and size of the components determines the effect of the VSC’s input current distortion and its overall performance.
Output inverter. The output inverter converts the DC power provided by the DC bus to a variable-frequency, AC power. This inverter can be either a voltage or current source inverter. In a voltage source inverter, the output voltage waveform is controlled, and the load applied determines the output current waveform. The current source inverter is just the opposite. In it, the output current waveform is controlled, and the load applied determines the output voltage waveform. Most VSCs for ESP applications use the voltage source inverter.
Current source inverters. In a current source inverter, large inductors are used to supply a current source to the inverter. The current is normally controlled by an SCR. The current source inverter controls only the output frequency of the drive, while the input converter controls the current and voltage. The inverter may operate in six steps or with pulse-width-modulated (PWM) inverters.
Voltage source inverters. In a voltage source inverter, large banks of capacitors act as low impedance DC voltage sources for the inverter. The inverter changes the DC voltage by one of several switching methods. These methods generally fall into two categories:
- Variable voltage inverter (VVI)
- Constant voltage inverter (CVI)
A VVI usually employs a controlled rectifier to control the DC bus voltage and, thereby, the output voltage of the inverter. In a CVI, the output is controlled by the method of switching.
Variable voltage inverters. VVI drives are most generally six step inverters. The unit consists of six switches, each turning on and off one time during every output cycle. The name comes from the fact that each cycle is divided into six 60° periods. During each period, there is a unique combination of power devices activated. This results in a phase-to-phase voltage waveform that has six identifiable "steps" to approximate a sine wave (Fig. 2). This is also referred to as a "quasisine wave" inverter. The inverter controls only the output frequency, and the electrical stresses on the power devices are significantly reduced over other output topologies.
Pulse-width-modulated inverters. PWM inverters also consist of six switches, but they switch many times per output cycle to control both the output voltage and frequency. The voltage waveform is divided into many small time periods that range from several hundred to several thousand (Fig. 3). During each period, the instantaneous output voltage is approximated by a square wave at some duty cycle. A 100% duty cycle would represent full voltage, while 0% would represent zero voltage.
To generate a sine wave, these pulses start at zero width and build, sinusoidally, to 100% duty cycle at the 90° point on the waveform. Then, they would decrease in width sinusoidal to zero at the 180° point of the waveform. The output voltage level is the integral of these pulse widths of DC bus voltage height over any given cycle. This integration is performed by the inductance of the motor, and the resultant current waveform becomes more and more sinusoidal as more pulses are used. To vary the average voltage, each pulse width is multiplied by a scale factor (to get half the output voltage, each pulse must be one-half its original width).
The electrical stresses on the power devices of a PWM inverter are significantly higher than a six-step inverter. Each switching transition causes high losses in the power devices, occurring hundreds or thousands of times per cycle. Therefore, extra care must be taken to ensure that these electrical stresses are managed properly.
Control module. This unit functions the same as explained in the switchboard section. When used with a VSC, the controller can be programmed to provide some speed adjustment depending on certain input conditions. If the input indicates the unit is approaching or is in a shut-down parameter range, the controller could send a signal to the VSC to change its frequency, within its preprogrammed, allowable frequency range. If this specific parameter moved back into its preprogrammed operating range, the controller would maintain this VSC frequency until the next input parameter occurrence. In most cases, there is a preset time function for this adjustment to take effect; otherwise the unit would shut down.
Application considerations for VSCs
Because of the relationship of the performance of a centrifugal pump to its rotational speed (see ESP centrifugal pump), the variable speed controller (VSC) allows for wider flexibility of the downhole ESP system. The effect on pump operation is shown in Fig. 4. This is the same pump that is represented in the 60-Hz fixed-speed performance curve of Fig. 5. This allows the designer to select the flow rate and speed of the system on the initial design. For this pump stage, it can be operated between 1,800 B/D at 30 Hz (minimum recommended operating point) and 10,200 B/D at 90 Hz (maximum recommended operating point).
Benefits of VSC use
Broadened application range. On fixed-speed operation, a pump stage has a recommended minimum and maximum flow rate. Beyond these points, the pump can operate in a detrimental run-life or reliability area. By operating at reduced frequency, the minimum recommended operating point is reduced, and, at higher frequencies, the maximum operating point is increased. This allows the application of ESPs in low-productivity-index (PI) wells and higher flow rates to be obtained from small bore casings. It also allows a limited inventory of pumps to be applied over a broader flow range.
ESP efficiency optimization. Either when an ESP system is initially designed or after it is deployed, adjusting the frequency of the unit can maximize the total system efficiency. In light of wellbore PI uncertainties, this allows the operator some flexibility between the requirements of the initial design and the actual operating conditions of the equipment in the wellbore.
Maximize well production. If the well PI is greater than that for the original design, either through data error or changing wellbore parameters, the ESP operating point can be increased with a VSC. The HP rating of the motor limits the frequency increase. Remember, the horsepower (HP) load from the pump increases with the cube of the frequency ratio, and the HP capability of the motor increases directly to the speed ratio. Therefore, the designer must consider using an oversized motor if there is a potential need of higher flow rates.
Minimum well production. If the well PI is lower than that for the original design, the ESP operating point can be decreased with the VSC. The total developed head (TDH) of the pump is the limiting factor on the minimum VSC frequency. The produced head of the pump decreases with the square of the frequency ratio. Therefore, the designer must consider initially oversizing the pump lift, if there is a potential for reduced-frequency operation.
There may also be cases where the ESP is operated at reduced frequency to reduce stresses on the reservoir. This could prevent reservoir damage or control the influx of unconsolidated sand or frac materials because of sudden pressure differentials across the wellbore face.
ESP electrical isolation. In a fixed-speed ESP application, the downhole motor is connected directly to the power source via the switchboard contactor, with isolation only from the transformer and cable impedances. When a VSC is connected, automatic isolation occurs. The input converter and output inverter are decoupled or isolated by the DC bus. Also, high-energy transients open fuses or destroy solid-state semiconductor devices in the VSC instead of potentially damaging the electrical components (motor, cable, electrical penetrators downhole).
Matching surface processes. If the well has any surface processing constraints, the wellhead flow and/or pressure can be controlled by the operating frequency. This would include items such as tank level control and flowline pressure. Also, where multiple wells are manifolded together and a constant flow rate is desired, any drop off in the rate of one or more wells can be made up by an increase in speed to one or more wells.
Reduced starting stresses. With a VSC, maximum current starting levels can be controlled. At startup, the frequency to produce minimum starting torque can be used with a controlled ramp up to operational speeds and power settings. This produces the optimum soft start. With any added benefits and capabilities, there are also some cautions.
Factors to consider
Increased design complexity. The ESP application design becomes much more complex with the use of a VSC and, in all practicality, requires the use of a design software program to do it properly.
Utility interface. When using a VSC, it is desirable not to feedback problems to the utility-power system that could interact with other users on the system. The problems could include a poor input power factor or high-input current distortion (harmonics). A poor power factor leads to unnecessarily high-input current levels, thereby reducing the overall capacity to serve other loads. Input current distortion, which is injected into the power system, can reduce the life of other equipment connected to the system or cause electronic devices to malfunction. Leuthen gives a detailed description of the impact of VSCs on these two utility concerns—how each VSC topology measures up and the method to control or diminish the impact. The guidelines for the harmonic control of electrical power systems is provided in the Inst. of Electrical and Electronic Engineers (IEEE) Stand. 519-1992. 
Cautions with a VSC
To fully achieve the benefits that a VSC can bring to an ESP application, care must be taken to understand its impact on the downhole system and minimize any potential damaging influences. Several concerns include:
- Excessive motor heating
- Increased voltage stress
- Maximization of motor torque performance
Motor heating. Excessive motor heat can impact the motors performance and, in the long term, its overall life. Operation of an ESP motor with a VSC causes additional heating from two main sources: increased winding losses because of higher current values and increased core losses because of high-frequency components. Because all drives provide a modified sine wave to the motor, it is distorted and contains other frequency components. Therefore, the total current values of VSC operation over across-the-line values are higher. This increased current level produces higher resistive losses in the motor windings, causing increased heat. Increased core losses, because of the changes in applied terminal voltage, also result in higher motor temperatures. When the motor core experiences changes in the applied terminal voltage, the magnetic dipoles must realign to the new magnetic field present. Every time this happens, the friction of the motion of the dipoles releases heat. Therefore, it is important to minimize the subcycle voltage fluctuations at the motor terminals, although testing has shown heat rise to be very small.
Increased voltage stress. Even though the VSC is a buffer to input power surges or spikes, their power inverters have the potential to generate higher peak voltages than those from a true sinusoidal voltage source. This is because inverters are inherently digital in nature rather than analog. Basically, the output can only change in discrete voltage steps and the transition from step to step happens very rapidly. When this power is applied to the complex impedance of a downhole ESP system, the natural response is a damped sine wave and the resultant ringing is a normal response. On a VVI drive, the ringing has time to decay to zero between each vertical edge of the waveform. On a PWM waveform, the vertical edge of the VSC output waveform can occur on top of the ringing of a previous vertical edge. Under the worst conditions, this effect can produce peak voltages in multiples of the original applied voltage, and they can occur many times per cycle. This impact can be reduced by the application of filters on the output side of the VSC and step-up transformer. Each PWM application should be reviewed for this potential condition, but, generally, the user should be concerned about high-voltage and -horsepower equipment and long lengths of cable.
Motor torque. The ESP motor has the capability to deliver a large percentage of its full torque capability over a wide speed range. It is important to examine the VSC’s capability to deliver the necessary current to achieve required torque levels. Any of the VSC types can be matched to the ESP motor when properly set up.
- Leuthen, M. 1997. Variable Speed Drives: Definitions, Applications, and Comparisons. Paper presented at the 1997 SPE Gulf Coast ESP Workshop, Houston, 30 April–May 2.
- Standard 519 Recommended Practice and Requirements for Harmonic Control in Electrical Power Systems. 1992. New York City: Inst. of Electrical and Electronic Engineers, New York City.
Noteworthy papers in OnePetro
Akerson, J. (2003, January 1). Overcoming The Previous Limitations Of Variable Speed Drives On Submersible Pump Applications. Society of Petroleum Engineers. doi:10.2118/81131-MS
Patterson, M. M. (1996, February 1). On the Efficiency of Electrical Submersible Pumps Equipped With Variable Frequency Drives: A Field Study. Society of Petroleum Engineers. doi:10.2118/25445-PA
Wilson, B. L., & Liu, J. C. (1985, January 1). Electrical Submersible Pump Performance Using Variable Speed Drives. Society of Petroleum Engineers. doi:10.2118/13805-MS
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Jose Caridad, BSME & MSc ME