Abstract: This paper presents a dedicated submersible EV protector designed to meet the characteristics of submersible EVs. This protector provides protection against overload, phase loss, imbalance, overvoltage, undervoltage, and voltage imbalance. The design principles of this product are given, and its differences from conventional motor protectors, along with its hardware and software design principles, are explained.
Keywords: Submersible electric pump, protector, controller
0 Introduction
Submersible electric pumps (SAPs) are medium-pressure mechanical oil extraction devices used in oilfields. With the increasing automation of oilfield development, the original SSP fixed-frequency control cabinets have revealed problems such as incomplete functions, inconvenient data analysis, lack of waveform recording, and lack of communication functions. The main reason for these deficiencies is that protectors not specifically designed for SSPs do not support these functions.
Currently, there are few domestic manufacturers researching and developing submersible electric pumps (ESPs), and those that exist are small-scale, with products lacking serialization and large-scale production. To enhance the competitiveness of domestic ESP protectors and compensate for the shortcomings of older protectors, it is necessary to develop a targeted ESP protector.
1. The difference between submersible pump protector and motor protector
2. Features of Submersible Pump Protector
3 Submersible Pump Protector Product Composition
The submersible electric pump protector consists of a protector body, a human-machine interface module, and a switching power supply, as shown in Figure 1. The protector body performs parameter acquisition, protection, and communication functions; the human-machine interface module enables real-time parameter display, parameter setting, real-time curve display, historical curve display, historical data query, and fault query functions; the switching power supply converts AC/DC 220V (or other voltage levels) to a low voltage for use by the protector body and the human-machine interface module.
- Submersible pump protectors are a sub-category of motor protectors. While they share basic measurement, protection, and communication functions with motor protectors, many protections for submersible pumps, such as underload, current imbalance, overvoltage, and low voltage, utilize inverse-time protection, whereas general-purpose motor protectors typically employ definite-time protection.
- Submersible electric pump protectors need to have historical data recording and export functions, while conventional motor protectors only need to have fault records.
- To facilitate the maintenance and assessment of submersible electric pumps, the submersible electric pump protector needs to have a check-in function for assessment purposes, while conventional motor protectors do not need to have this function.
- The submersible electric pump protector can measure three-phase current, three-phase voltage, and frequency in real time, and can simultaneously display electrical parameters such as active power and electrical energy; it can also transmit the detected and collected switch-type process parameters and 4-20mA-type process parameters to the system via communication.
- The submersible electric pump protector has protection functions such as overload, phase loss, current imbalance, stall, blockage, undervoltage, overvoltage, and voltage imbalance, and can achieve underload self-start.
- The submersible electric pump protector can display real-time and historical curves of parameters such as three-phase current and three-phase voltage.
- The submersible pump protector can export stored historical data to a USB flash drive, and the exported file supports Excel operations. After inserting the USB flash drive into a computer, data analysis can be performed directly without installing additional analysis software. This function can replace a current recorder.
- The submersible electric pump protector supports MODBUS and PROFIBUS communication protocols, and can directly transmit real-time data and send start and stop commands through the communication network.
Figure 1. Schematic diagram of submersible electric pump product composition
4. Design of Submersible Pump Protector
The design of the submersible electric pump protector is divided into two aspects: hardware and software. The hardware design includes power supply, signal acquisition, switch input/output, communication circuits, etc.; the software design mainly includes software architecture, measurement algorithm, protection algorithm, control algorithm, etc.
4.1 Hardware Design
Since the operating voltage of commonly used submersible pumps ranges from AC 200V to AC 2000V, it is not advisable to directly use the submersible pump's operating power supply as the operating power supply for its protection. The conventional practice is to use an isolation transformer to convert the submersible pump's operating voltage to AC 110V control voltage. In this case, a switching power supply is used for the submersible pump protector because it has a wide operating range and high efficiency, making it suitable for this situation.
Signal acquisition includes the acquisition of electrical parameters such as voltage and current. To ensure the reliability of the acquisition, voltage converters (TVs) and current transformers (TAs) are typically used as isolation converters for voltage and current signals. When selecting a TA, attention should be paid to its voltage rating. The standard TA voltage rating is 0.66kV, but when the operating voltage of the submersible pump reaches AC 1000V or even AC 2000V, a 3.3kV TA should be selected to ensure isolation safety.
The design of the digital input/output and communication circuits is conventional and does not have many special features.
Reliability design includes electromagnetic compatibility (EMC) and safety design. In the initial stages of EMC design, all possible problems should be fully considered, and contingency plans should be in place. Common anti-interference methods include: adding EMC filters to the power supply section; adding filtering circuits to the signal acquisition section; adding port protection circuits at the input ports of each signal processing chip; and adding decoupling capacitors at the chip power input.
4.2 Software Design
Commonly used sampling algorithms include DC sampling and AC sampling. AC sampling can measure non-sinusoidal signals and is therefore widely used in electrical parameter measurement. Common measurement algorithms include those based on sinusoidal signals and those based on non-sinusoidal signals. Algorithms based on sinusoidal signals include the maximum absolute value algorithm within half a cycle, the half-cycle absolute integration algorithm, the first derivative algorithm, the second derivative algorithm, the sampled value product algorithm, and the three-sample value algorithm, etc. Algorithms based on non-sinusoidal signals include the Fourier algorithm, the first-order difference post-half-wave Fourier algorithm, and the true RMS algorithm, etc. Each algorithm has its advantages and disadvantages. Although sinusoidal signal-based algorithms are simple and consume fewer resources, they are inaccurate when harmonics are present or the waveform is transformed; while non-sinusoidal signal-based algorithms are more complex, they can guarantee the accuracy of the measured values.
Because designing an inverse time curve algorithm in protection algorithms is quite difficult, some products are labeled as inverse time, but in reality, they are time-limited segmented, such as: (1.2-1.5) times, 60s protection; (1.5-2) times, 40s protection; and above 2 times, 10s protection. This violates the basic principle of inverse time curves. If the signal fluctuates around the critical point, the action time will be inaccurate. A true inverse time curve has a continuous inverse time curve, truly achieving fast action for large multiples and slow action for small multiples.
In software design, in addition to selecting appropriate algorithms, attention should also be paid to the standardization of software design. The basic principles of software design are information hiding and module independence. Good software requires high cohesion (cohesion is a measure of module strength) and low coupling.
Software testing is an essential and indispensable part of software design, and its purpose is to verify whether the software meets the specifications. During development, the principle of early testing and early correction should be followed, with integration testing, acceptance testing, and verification testing only proceeding after unit testing has passed.
5. Conclusion
Submersible electric pumps are a new type of motor protector derived from conventional motor protector products. They are more suitable for submersible pumps and feature inverse time protection against underload, overvoltage, low voltage, current imbalance, and voltage imbalance. They also have measurement functions for current, voltage, power, frequency, energy, and residual current, making them more suitable for customer requirements.
This article is from the December 2013 issue of "Electrical Engineering Technology".
References
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