Analysis of Two Types of Surge Faults in Fuel Generator Sets and Summary of Well Site Interference Handling
When the fuel generator set at the oilfield well site experiences speed fluctuations and runaway surging, the faults can be classified into two types, and the handling approaches are completely different. The first category: The unit experiences runaway and surge when it is no-load. This fault belongs to the matching problem of the speed regulation closed-loop system. By adjusting parameters such as the stability, gain, and speed regulation of the electronic speed control board, the response characteristics of the throttle actuator can be optimized to eliminate the speed oscillation. The second category: The unit operates smoothly under no-load conditions, but surge occurs only after the variable frequency pumping unit load is put into operation. At this point, simply adjusting the parameters of the speed control board often fails to eliminate the fault. When the frequency conversion equipment is in operation, common-mode interference is continuously generated. The PE protective ground wire inside the power cable at the well site should be reliably grounded at both ends in accordance with safety regulations. However, the overall grounding pipeline network construction conditions at the well site are limited, resulting in potential differences among multiple grounding points. Eventually, this is manifested as the equipment casing and the common ground wire being electrified. The original design of the matching magnetic-electric tachometer sensor is that one end is connected to the equipment housing. The potential fluctuation of the ground wire is directly superimposed on the tachometer feedback signal, causing the speed control board to collect distorted signals, continuously misadjusting the throttle, and causing the unit to surge. Due to the constraints of the site and construction conditions, it is difficult to implement the large-scale rectification of the grounding grid at the well site. On-site practical solution: Install the electronic speed control board with floating ground insulation to isolate the ground potential interference caused by the shell, ensuring that the speed signal collection is not disturbed and completely solving the surge problem. In addition, pay attention to the distinction of concepts: The yellow-green PE protective ground wire inside the cable must be grounded at both ends for personal safety reasons. For the outer metal shielding layer of the cable, the grounding method should be flexibly selected in combination with the on-site grounding quality. In the well site environment with unbalanced ground potential, it is recommended that the shielding layer be grounded at one end to reduce the electromagnetic interference caused by ground current.
On-site stratified and segmented node measurement and fault source tracing methods
When troubleshooting such hidden electrical interference faults, it is strictly prohibited to draw conclusions based on single-point measurement. Instead, the method of comparing the entire line's segmented nodes must be adopted. At the three key nodes of the generator output end, the incoming line end of the distribution cabinet, and the incoming line end of the frequency conversion cabinet, the three-phase line voltages, the voltages of each relative neutral line, and the voltages of each relative shell to ground are uniformly measured in sequence. At the same time, independent temporary ground wires are laid. The potential difference between the original grounding grid and the temporary ground wire is compared at multiple points. Through the data difference of the entire line, the fault zone is precisely located.
Precise determination logic
All nodes along the entire line experienced shell drift voltage. After the shutdown and power-off, there was no voltage, only when the switch was closed and the load was on. Moreover, there was no abnormality when resistive and ordinary inductive loads were connected. Only when the variable-frequency load was put into operation did the entire line experience potential offset. It can be directly determined that the root cause of the fault is: the common-mode interference generated by the high-frequency operation of the frequency converter, combined with the unequal potential defect of the grounding grid at the well site, forming ground loop interference. 2. If an abnormal voltage to ground occurs alone in a certain section of the line, with an abnormal voltage to ground at a local node and a low insulation value, while the rest of the sections are normal, it can be determined that the insulation of the power cable in this section is damaged and there is hidden leakage, rather than an interference problem with the frequency converter. 3. The entire line voltage is normal, but there is a potential difference throughout the grounding grid. The power supply voltage at each node is balanced and there is no leakage feature. However, there is always a voltage difference between the original grounding grid and the temporary ground wire. It can be diagnosed that the grounding system at the well site is imperfect, the soil resistivity is too high, and the grounding at multiple points is unbalanced, causing a potential gradient throughout the entire area.
Key judgment criteria: Determine faults differently based on working conditions and loads
Identify the core and key points, strictly distinguish the operational differences between no-load and loaded conditions, and at the same time cross-verify by switching different load types to prevent misjudgment
1. The unit operates smoothly under no-load conditions.
2. When resistive loads such as electric tracing and conventional inductive loads like motors are put into operation, the entire process is stable without surging.
3. Only when connected to a nonlinear load of the frequency converter does the speed fluctuation and runaway surge occur immediately.
By combining the measurement data of all sections along the entire line, the essence of the fault can be thoroughly distinguished: surging under full load conditions → it belongs to the parameter matching problem of the speed regulation system; Only the variable-frequency load triggered surge, and the ground potential interference was detected throughout the entire line → it belongs to the signal distortion fault caused by electrical common-mode interference + ground defect. Before the comparison of all section nodes along the entire line and the cross-testing of working conditions and loads are completed, it is strictly prohibited to blindly adjust the parameters of the speed control board. Otherwise, it will mask the true electrical faults and increase the difficulty of fault diagnosis. Personal opinion, for reference only. Welcome to leave comments and exchange views.