Transformer local line monitoring FAQ: from selection to operation and maintenance of one-stop answer

• From selection to operation and maintenanceThis article compiles the most common issues encountered in the online monitoring of partial discharges in transformers—ranging from understanding the principles, sensor selection, and installation and commissioning to spectrum analysis and daily operation and maintenance—and organizes them by topic for easy reference.
• Practical Q&A: All Q&As are based on real, frequently asked questions from engineering practice, and the answers are designed to solve practical problems.

1. Fundamentals

1.1 Q: What is partial discharge? Why is it necessary to monitor it?

Answer: Partial discharge is a non-penetrating discharge phenomenon that occurs in localized areas of a transformer’s insulation system; it serves as an early indicator of insulation defects and degradation. Partial discharge is monitored because it provides an early warning before insulation failures progress to breakdown incidents, with a time window that can range from several months to several years.

1.2 Q: What is the relationship between partial discharge monitoring and oil chromatography monitoring?

Answer: Partial discharge detection directly monitors the discharge events themselves, while oil gas chromatography analyzes the chemical byproducts generated by the discharges. Partial discharge detection identifies discharge-related faults earlier than oil gas chromatography; the two methods are complementary—partial discharge detection provides early warning for discharge-related faults, while oil gas chromatography covers overheating and comprehensive analysis.

1.3 Q: Which of the three testing methods (ultrasonic, high-frequency current, and ultra-high frequency) is the best?

A: There is no single "best" option; each has its own strengths. Ultrasonic sensors excel at positioning and are the easiest to install; high-frequency current sensors offer the highest sensitivity; and ultra-high-frequency sensors provide the strongest resistance to interference. A multi-sensor fusion approach combines the advantages of all three.

2. Product Selection and Configuration

2.1 Q: Should I choose 4 channels or 6 channels?

A: Four channels are sufficient for most main transformers rated at 110 kV or below. Six channels are suitable for large transformers rated at 220 kV or above, or for scenarios requiring more precise positioning. More channels are not necessarily better; the number of channels should be matched to the sensor configuration and actual requirements.

2.2 Q: Do distribution transformers need to be equipped with partial discharge monitoring?

A: Not every transformer requires one. Priority should be given to transformers that supply power to critical users, as well as those that have been in service for a long time or have experienced failures in the past. Selecting a lightweight, single-sensor ultrasonic solution allows for cost-effective deployment.

2.3 Q: What are the differences between partial discharge solutions for retrofitting existing substations and those for building new substations?

Answer: For retrofits of existing substations, the focus is on installing external sensors without power interruption; both ultrasonic and high-frequency current sensors can be installed without power interruption. For new substation construction, built-in ultra-high-frequency sensors can be requested during the transformer ordering process, resulting in a more optimal layout.

2.4 Q: Does the partial discharge monitoring system need to be connected to the network?

A: While the system can perform basic monitoring and generate local alarms when operating independently, connecting it to a network allows data to be transmitted to a remote monitoring center for remote diagnostics. We recommend that the system have at least basic communication capabilities to allow for future expansion.

3. Installation and Debugging

3.1 Q: Does the installation require a power outage? How long will it take?

A: Both ultrasonic and high-frequency current sensors can be installed without powering down the system, and the installation typically takes half a day to a full day. UHF built-in sensors must be installed while the transformer is powered down.

3.2 Q: Where is the best place to install the ultrasonic sensor?

A: Install them on the four sides of the transformer tank, giving priority to the areas of the tank corresponding to the high-voltage windings. Multiple sensors receive signals from different locations and use the time difference in signal arrival to locate the discharge.

3.3 Q: How long does it take to get the system up and running after installation?

A: On-site installation and wiring take half a day, while system integration and communication testing take several hours. We recommend continuously monitoring data stability for the first 1–2 weeks after initial operation to rule out issues such as poor coupling or loose connections that may occur during the initial installation phase.

4. Chart Interpretation

4.1 Q: How do I read a PRPD chart?

A: The horizontal axis represents the power-frequency phase (0–360°), the vertical axis represents the discharge amplitude, and the shade of color indicates the discharge repetition rate. Different types of discharges exhibit distinct distribution patterns on the phase-amplitude plane; the diagnostic software automatically identifies the type and provides a confidence level.

4.2 Q: Does the presence of a signal on the spectrum necessarily indicate partial discharge?

A: Not necessarily. There are various sources of interference in the field environment—cell phone signals, walkie-talkies, transients from switch operations, nearby welding work, and so on—all of which can trigger a response from the sensors. A good system uses cross-validation across multiple sensors and pattern recognition algorithms to distinguish between genuine partial discharges and interference.

4.3 Q: What should I do if I can't understand the diagram?

A: There are two options: use the system’s built-in self-diagnostic function to obtain discharge type labels and diagnostic recommendations; if you still have questions, contact the supplier to request remote diagnostic services. When signing the contract, we recommend confirming whether remote diagnostics are included in the scope of services.

5. Operations and Maintenance

5.1 Q: Does the partial discharge monitoring system require routine maintenance?

A: The sensors are passive devices and require virtually no maintenance. We recommend checking the operational status and communication links of the data acquisition unit once every quarter. The coupling between the ultrasonic sensor and the tank wall should be checked once a year, and coupling agent should be reapplied as needed.

5.2 Q: How often does the system need to be calibrated?

Answer: Unlike oil chromatography, partial discharge monitoring systems generally do not require periodic calibration using a standard signal source. However, it is recommended to perform an annual system functional check, including sensor response verification and channel consistency testing.

5.3 Q: How long is the data stored?

A: Since partial discharge monitoring generates a large volume of raw data (due to high-speed data acquisition), the system typically stores raw waveforms and long-term statistical data for a certain period. The specific retention period depends on the storage configuration; we recommend verifying whether external storage expansion is supported.

Disclaimer: The content of this article is for technical exchanges and reference only, and does not constitute any form of procurement commitment or contract offer. Product technical parameters, configuration programs and prices are subject to the actual signed contracts and technical agreements. The technical data and cases involved in this article are from public information and engineering practice, if updated without notice.

Have more questions about online partial discharge monitoring technology for transformers? Feel free to contact Innotongda—our team of engineers is here to provide personalized support. Service Hotline: 13959168359 (also available on WeChat).