1. Introduction
Electric Submersible Pumps (ESPs) are widely used in onshore deep-well oil extraction, offshore oil & gas production, and chemical liquid medium transfer. The motor and pump are integrated and directly submerged in harsh downhole environments with high temperature, high pressure, and corrosive fluids. In actual engineering, the power cable between the drive and the downhole motor typically reaches hundreds or even thousands of meters.
The distributed capacitance effect and voltage reflection caused by long-distance cables can lead to overvoltage at the motor terminals, accelerating insulation aging and damage. At the same time, the periodic positive and negative DC signals injected by the downhole sensor choke system can superimpose on the drive's sampling circuits, causing control disturbances and frequent fault trips.
Addressing the core challenges of ESP drive applications, Nancal Electric has introduced a solution for ESP medium voltage drive systems based on cell series multi-level topology, leveraging core technologies such as long-cable adaptive control and choke signal compatibility processing. This solution has been successfully applied in long-cable projects such as a propane submerged pump, effectively resolving issues including long-cable overvoltage, choke signal interference, and control accuracy degradation.
2. System Core Challenges
The ESP variable frequency drive system mainly consists of four parts: the medium voltage drive, long-distance power cable, downhole ESP unit, and choke monitoring system. In actual operation, the system faces three core challenges.
Figure 1: Simplified Schematic of ESP Variable Frequency Drive System
First, long-distance transmission is the primary factor affecting system stability. The distributed capacitance effect of the long cable causes voltage reflection, leading to overvoltage at the motor terminals. At the same time, the capacitive charging current causes output current distortion, threatening the motor insulation and the safe operation of drive power devices.
Second, signal interference from the choke system is a technical challenge that cannot be overlooked. The choke system, used for downhole condition monitoring, injects periodic positive and negative DC signals into the main circuit. These signals superimpose on the drive's output voltage, interfering with voltage sampling and vector control algorithms, and can easily cause false tripping of overcurrent and ground fault protection.
Third, the harsh operating environment places higher demands on the reliability of the variable frequency drive system. In applications such as offshore platforms, wellheads, or chemical plants, medium voltage drives often face harsh conditions including high temperature, high humidity, salt spray corrosion, vibration, and power fluctuations. Offshore platforms, in particular, must also contend with extreme conditions such as typhoons and storm surges. Such operating conditions impose requirements on environmental adaptability, control accuracy, protection reliability, and system redundancy that are far higher than those in conventional industrial scenarios.
3. Long Cable Issues and Solutions
Under long-distance cable transmission, the electrical coupling characteristics between the drive and the motor change, primarily manifesting as four core issues: voltage reflection, distributed capacitance effect, steady-state voltage drop, and system resonance risk.
When PWM pulses are transmitted along the cable, reflection occurs at the motor end due to impedance mismatch. The superposition of incident and reflected waves can significantly increase the voltage amplitude at the motor terminals, in severe cases approaching twice the DC bus voltage, posing a direct threat to the motor winding inter-turn insulation.
At the same time, the long cable acts as a large distributed capacitance. The periodic charging and discharging process caused by PWM pulses generates additional high-frequency capacitive currents. These currents, superimposed on the load current, may cause distortion of the drive output current waveform and increase the switching stress and temperature rise of power devices.
In addition, the resistance and inductance of the cable itself produce a steady-state voltage drop under load current, causing the actual voltage at the motor terminals to be lower than the drive output voltage, which may lead to reduced motor output torque.
The distributed inductance and capacitance of the cable form an LC resonant circuit, which may excite harmonic oscillations at specific frequencies, causing abnormal amplification of voltage and current. In severe cases, this can lead to system instability or protective tripping.
To address the above issues, Nancal Electric has proposed a systematic solution for long cable applications. At the topology level, the cell series multi-level topology is adopted, producing a stepped output waveform close to a sine wave with extremely low du/dt, suppressing voltage reflection at the source and adapting to long-distance cable transmission. At the same time, a sine wave filter is configured on the drive output side to further filter high-frequency PWM harmonics, making the output voltage waveform closer to a sine wave and significantly reducing overvoltage and resonance risks.
At the control algorithm level, the drive features built-in automatic voltage drop compensation for long cables, which corrects the output voltage and current sampling in real time, ensuring that the motor receives voltage close to the rated value while maintaining control accuracy. In addition, by optimizing the thresholds and action times of ground fault protection and overcurrent protection, false tripping caused by the charging current of the long cable is effectively avoided, ensuring continuous and stable system operation.
4. Choke Signal Interference and Suppression
The ESP choke system is an integrated unit for downhole sensor power supply and signal transmission. The core functions of the system include: injecting periodic positive and negative DC signals into the motor main circuit to provide power for downhole sensors measuring temperature, pressure, motor insulation status, and leakage current; and receiving return signals from downhole sensors and transmitting them to the surface control panel, enabling real-time online monitoring of downhole conditions.
The system typically completes one round of data acquisition per hour. For example, the positive DC signal is maintained for approximately one hour, after which the polarity reverses to negative DC and is maintained for several seconds, and the cycle repeats. However, the choke signal has a significant negative impact on the normal operation of the drive. The positive and negative DC offset superimposed on the AC output voltage interferes with vector control calculations, causing inaccuracies in flux observation and torque calculation. During loaded operation, the polarity reversal of the DC signal can easily trigger overcurrent or ground fault protection, preventing continuous system operation. In addition, the DC bias disrupts the control algorithm, potentially causing motor speed fluctuations.
To effectively suppress the above interference, Nancal Electric has carried out targeted optimization of the drive hardware, software, and protection strategies. On the hardware side, the output voltage sampling circuit of the main control board was redesigned, and a DC bias isolation circuit was added to physically block the DC signal from entering the sampling channel. On the software side, a DC bias software compensation module was added to remove the DC component from the sampling signal in real time, and the vector control algorithm was optimized to shield interference during choke signal transitions. On the protection strategy side, the motor ground fault protection parameters were recalibrated, and the detection time setting of the output ground fault protection was adjusted to avoid false protection triggered by signal transitions.
5. Nancal Electric Medium Voltage Drive Technical Advantages and Reliability Assurance
For ESP long-cable applications, the medium voltage drives offered by Nancal Electric have significant advantages in hardware configuration, control technology, and engineering application. The drive hardware architecture is shown in the figure below.
Figure 2: Cell Series Multi-Level Medium Voltage Drive Topology
In terms of hardware configuration, this series of drives adopts a cell series multi-level topology, delivering high-quality sinusoidal output waveforms with low du/dt. Depending on the cable length between the drive and the motor, a sine wave filter can be configured, making it professionally suited for long-cable applications. The system offers optional features such as main control system redundancy and power cell automatic bypass, further enhancing reliability and stability.
In terms of control technology, this series of drives employs open-loop vector control, precisely adapted to downhole motor drive requirements, with accurate flux observation and fast torque response. For choke system interference, it features dual protection through hardware isolation and software compensation, minimizing the impact of DC bias signals. The long-cable adaptive compensation function automatically identifies cable impedance and compensates for voltage drop in real time. The motor parameter online identification technology automatically identifies stator resistance and rotor time constant, adapting to parameter drift caused by motor heating and saturation.
From an engineering application perspective, this series of drives offers high reliability. All boards are coated with UV conformal coating, providing moisture, mold, and salt spray protection. The power cell boards and power devices feature independent sealed structures with high protection ratings, making them suitable for harsh environments and meeting long-term continuous operation requirements. The products have been successfully commissioned in multiple projects, and all solutions have been verified under severe operating conditions.
6. Application Case
In a chemical liquid propane transfer submerged pump project, the pump body is submerged in a liquid environment. The system configuration includes a medium voltage drive, long cable (approximately 1,000 m), submerged pump, and choke monitoring system. The medium voltage drive features hardware and functions including main control system redundancy, power cell automatic bypass, cabinet-top fan redundancy, and sine wave filter, significantly enhancing reliability.
Figure 3: System Singl Line Diagram
Table 1: Motor and Drive Basic Configuration
Figure 4: Whole Drive and Choke Box On-Site Installation
Figure 5: Choke System DC Signal
During project implementation, Nancal Electric effectively resolved a series of key technical issues by optimizing control circuit routing, redesigning the hardware sampling circuit with added DC bias isolation, introducing software compensation algorithms, and recalibrating protection parameters. These included on-site signal interference and false tripping of overcurrent and ground fault protection caused by the 120 V periodic DC bias signal from the choke system (positive polarity maintained for approximately one hour, followed by polarity reversal to negative for approximately 1.5 seconds).
The project was officially commissioned in December 2024. Actual operation has demonstrated that the drive operates stably, with no overvoltage or resonance in long-cable transmission, good compatibility between the choke system and the drive, and precise motor control. The product has received high recognition from the customer.
7. Conclusion
As a critical downhole conveying equipment, the ESP variable frequency drive system faces three major challenges: long-cable transmission, choke signal interference, and harsh on-site operating environments. Nancal Electric's medium voltage drives, based on cell series multi-level topology, with long-cable adaptive control as the core and choke system compatibility as a distinctive feature, effectively address industry pain points through deep integration of hardware optimization and software algorithms.
Nancal Electric will continue to deepen its expertise in ESP variable frequency drive applications, continuously iterating and upgrading long-cable adaptation, complex signal compatibility, and high-reliability redundancy technologies. The company is committed to providing safer, more efficient, and more stable variable frequency drive solutions for the petroleum, chemical, and deep-sea extraction sectors, contributing to the localization of high-end equipment and the high-quality development of the industry.