Work Plan for Live Inspection of Distribution Transformers and Ring Main Units

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I. Overview: Live inspection of electrical equipment is an effective method for identifying potential operational hazards in such equipment, and it serves as an important guarantee for the safe and stable operation of electrical equipment. To standardize and carry out live inspection of power equipment effectively, and with reference to relevant domestic and international standards, this work plan is formulated on the basis of the **Power Grid Company’s Document No. Primary power transformation (2011) 11, \"Notice on Issuing the (Provisional) Technical Specifications for Live Inspection of Power Equipment,\" taking into account the actual conditions of our company. This solution is primarily suitable for the live inspection of field equipment such as pole-mounted transformers, American-style box transformers, European-style box transformers, ring main units, cable distribution boxes, cable terminal boxes, and cable intermediate boxes in 10KV distribution networks. This plan specifies the principles of live detection, detection items, judgment criteria, detection procedures, detection frequency, etc. II. Referenced Standards 1) DL/T596-1996 \"Code for Preventive Tests of Electrical Equipment\" 2) DL/T664-2008 \"Guidelines for the Application of Infrared Diagnosis Technology to Live Equipment\" 3) QGDW 11400-2015 \"Guidelines for the Field Application of High-Frequency Partial Discharge Detection Technology for Electrical Equipment\" III. Terms and Definitions The following terms and definitions apply to this plan. 3.1 Partial discharge refers to an electrical discharge that occurs in a partially broken down part of the insulation system of electrical equipment; such discharge can take place near conductors (electrodes) or at other locations. 3.2 Live detection specifically refers to various real-time inspections of equipment carried out without interrupting its normal operation. 3.3 Infrared thermal imaging inspection: The use of infrared technology to inspect and diagnose live electrical equipment in power systems that experiences heating effects due to current, voltage, or other factors. 3.4 High-frequency partial discharge detection: A detection method that involves the acquisition, analysis, and evaluation of partial discharge signals with frequencies ranging from 3 MHz to 30 MHz. 3.5 Ultrasonic signal detection: A detection method that involves the acquisition, analysis, and evaluation of acoustic signals with frequencies ranging from 20 kHz to 200 kHz. 3.6 Ultra-high frequency partial discharge detection: A detection method that involves the acquisition, analysis, and evaluation of partial discharge signals with frequencies ranging from 300 MHz to 3000 MHz. 3.7 Transient voltage detection: When local discharge occurs, an instantaneous voltage is generated on the grounded metal surface; this voltage propagates in various directions along the surface of the metal. Local discharge in electrical equipment is identified and located by detecting the ground voltage. 3.8 50Hz correlation of partial discharges refers to the probability that a partial discharge occurs only once within one power cycle; the higher this probability, the stronger the 50Hz correlation. 3.9 Partial discharge 100Hz correlation refers to the probability that partial discharges occur 2 times within one power cycle; the higher this probability, the stronger the 100Hz correlation. 3.10 Symbol dB: A form of expressing the intensity of partial discharge signals, represented by the logarithm of the ratio of the signal amplitude to a reference value, that is, 20log(signal amplitude/reference value), with the unit being dB. mV: Converts the collected partial discharge signals into electrical signals, using voltage values to represent the signal strength. dBmV: A notation used to represent the dB value of partial discharge relative to a reference value of 1 mV; for example, if the actual amplitude of a signal is 1 mV, its dB value is 20log(1 mV/1 mV) = 0 dBmV. IV. Detection Principle 4.1 Principle of Infrared Thermal Imagers Infrared thermal imagers utilize infrared detectors, optical imaging lenses, and a mechanical scanning system (advanced focal plane technologies eliminate the need for such a mechanical scanning system) to capture the distribution pattern of infrared radiation energy from the object being measured, which is then reflected onto the photosensitive elements of the infrared detector. Between the optical system and the infrared detector, there is a mechanical scanning mechanism (which is not present in focal plane thermal imagers) that scans the infrared image of the object and focuses it on the individual detectors. These detectors convert the infrared radiation energy into electrical signals, which are then amplified and transformed into standard video signals that are displayed as infrared thermal images on a television screen or monitor. This thermal image corresponds to the distribution field on the object’s surface ； In fact, due to the extremely weak signals, the thermal image showing the distribution of infrared radiation from various parts of the object under inspection lacks depth and three-dimensionality compared to visible light. Therefore, in order to more effectively assess the infrared thermal field of the target during actual operations, various auxiliary measures are often employed to enhance the functionality of the instrument, such as controlling image brightness and contrast, performing practical corrections, using pseudo-colors for representation, as well as calculating and printing histograms. In short, an infrared thermal imager is a detection device that measures infrared heat through non-contact means, converts it into thermal images and temperature values for display on a monitor, and is capable of calculating these temperature values. Infrared thermal cameras can accurately quantify the heat detected, enabling precise identification and thorough analysis of the faulty areas that are generating heat. Infrared thermal imaging inspection is divided into general inspection and precise inspection. For more details, refer to DL/T664-2008 \"Guidelines for the Application of Infrared Diagnosis Technology to Live Equipment\", section 4.2 Principle of High-Frequency Partial Discharge Detection. High-frequency current transformers are primarily used for detecting partial discharges in high-voltage electrical equipment, and they operate on the principle of pulse currents. Since most high-voltage electrical equipment has distributed capacitance on its high-voltage and low-voltage sides or in its grounding parts, when discharge occurs in areas with high electric field strengths, it is coupled to the grounding parts and enters the ground through the grounding wires. The HFCT card is placed on the cable itself or the ground wire, and the pulse current signals generated by the partial discharges are detected, thereby obtaining information regarding the partial discharges of the device under inspection. In high-voltage cables, the conductor and the metal shielding are separated by insulating material, resulting in distributed capacitance; this capacitance is around a few hundred pF and provides a path for high-frequency signals. Therefore, the high-frequency partial discharge signals are transmitted as a circuit formed by the distributed capacitance and the ground lead. At the moment internal discharge occurs, a high-frequency pulse current is generated. This high-frequency pulse current flows from the core, which is at a higher potential, to the metal sheath (armoring), which is at a lower potential, through the distributed capacitance between them; it then enters the ground via the grounding wires at the cable joints or terminals. Therefore, by connecting a high-frequency current transformer (HFCT) to the ground wire at the intermediate joint or terminal, the high-frequency pulse partial discharge current can be coupled into the HFCT, and then transmitted to the inspection device via the test cable between the HFCT and the inspection device for signal acquisition and analysis. 4.3 Principles of Ultrasonic Partial Discharge Detection 4.3.1 Principle of Ultrasonic (US) Measurement When partial discharge signals are generated inside electrical equipment, shock vibrations and sounds are produced. The ultrasonic method measures partial discharge signals by installing ultrasonic sensors on the outer wall of the device chamber or by detecting ultrasonic signals in the air. The advantage of this method is that the sensor has no connection to the electrical circuit of the geographic device, so it is not affected by electrical interference; however, when used in the field, it is prone to being influenced by ambient noise or mechanical vibrations from the equipment. Due to the significant attenuation of ultrasound signals in the insulating materials commonly used in electrical equipment, the detection range of ultrasonic testing is limited, but it has the advantage of high positioning accuracy. 4.3.2 Ultrasonic testing of transformers: Over time, transformers may develop issues such as loose internal components, contamination on the insulating surfaces, or air gaps within the insulation, all of which can lead to non-penetrating discharge phenomena inside the equipment. During discharge, shock vibrations and sound waves are generated. The spectrum of the sound waves produced by partial discharges is wide, ranging from several dozen Hz to several MHz. Signals with frequencies below 20 KHz can be heard by the human ear, while ultrasonic signals with frequencies above this value must be detected using ultrasonic sensors. The intensity of the discharge is inferred by installing ultrasonic sensors on the outer wall of the device chamber to measure the sound pressure of the ultrasonic signals. 4.3.3 Ultrasonic testing of ring main units: By detecting ultrasonic signals in the air, the sound pressure of partial discharge signals can be measured, thereby allowing an estimation of the intensity of the discharge. During ultrasonic testing, the ultrasonic sensor should be scanned along the gaps on the switchgear for inspection. 4.3.4 Ultrasonic testing of pole-mounted transformers: When measuring the bushings and leads of pole-mounted transformers, due to the large distance involved, the ultrasonic signals generated by local discharges in high-voltage electrical equipment are very weak. To carry out measurements from a distance, a remote ultrasonic inspection device equipped with a wave concentrator can be used, thereby improving the sensitivity of the measurements and the detection range. Remote ultrasonic inspection devices can detect surface discharge and corona discharge caused by moisture, contamination, cracks in transformer bushings, as well as sharp burrs at the leads. 4.4 Principle of transient geoelectric wave detection: In the field, the instantaneous earth voltage (TEV) detection method is used for the live inspection of ring main units, incoming cabinet of box-type substations, high-voltage metering cabinets, outgoing cabinets, and ring main units. When partial discharge occurs in a high-voltage switchgear, a pulse current with an extremely short duration is generated along the discharge path, which in turn induces transient electromagnetic waves. The discharge process is relatively short, the current pulses have a high steepness, and it has a strong ability to emit high-frequency electromagnetic waves, which can propagate outward through openings in the metal casing; these openings can be gaps around the sealing gaskets of the casing or other insulating components. When these high-frequency electromagnetic waves propagate outside the switchgear, they generate an instantaneous voltage relative to ground on the metal enclosure. The instantaneous voltage ranges from a few millivolts to several volts, with a rise time of only a few nanoseconds. Dedicated TEV sensors are placed outside the switchgear; this non-invasive approach is used to detect partial discharge activity. When applying the transient ground voltage detection method, the switchgear being inspected must have a reliable grounding electrode. The locations detected by the transient to ground voltage method are mainly the busbars (connections, bushings passing through walls, supporting insulators, etc.), circuit breakers, as well as the areas on the cabinet wall corresponding to devices such as CTs, PTs, and cables. Most of these devices are located in the middle and lower parts of the front panel of the switch cabinet, as well as in the upper, middle, and lower parts of the back panel and in the upper, middle, and lower parts of the side panels. 4.5 Principle of UHF detection: The cause of insulation failures in electrical systems is the distortion of the internal electric field, which is often accompanied by partial discharges that generate pulse currents. The rise time and duration of these current pulses are on the order of nanoseconds (nS). Such pulse currents give rise to high-frequency electromagnetic waves, with a main frequency range of 0.3–3 GHz. These electromagnetic waves can leak out through gaps in the equipment or through observation windows. UHF sensors (operating in the 0.3–3 GHz range) are used to measure the electromagnetic waves at the insulation gaps, and the severity of partial discharges can be analyzed based on the strength of the received signals. V. Testing Requirements and Precautions 5.1 Environmental Requirements 1) Ambient temperature: 0°C to 40°C. 2) Relative environmental humidity: not more than 80%. 3) During live testing, there should be no abnormal strong winds or thunderstorms. 5.2 Requirements for testers: (1) Be familiar with the basic principles and diagnostic procedures of live testing for distribution networks; understand the working principles, technical parameters, and performance of live testing instruments, and master the operating procedures and usage methods of these instruments. (2) Understand the structural features, working principle, operating conditions of the equipment under inspection, as well as the basic factors that cause equipment failures. (3) Familiar with the \"Application Specifications for Live Detection and Diagnosis\", and have received training in live detection techniques. (4) Possess certain on-site work experience, and be familiar with as well as able to strictly comply with the relevant safety management regulations for power production and work sites. 5.3 Precautions 1) Make sure to use instruments that have been certified by authoritative agencies as testing tools. 2) During testing, pay attention to the selection of background signals and the saving of the spectra. 3) Both internal and external synchronization methods are employed, along with phase correlation and typical patterns, to eliminate the influence of external interference; special attention is paid to corona interference from the towers outside the ring main unit as well as external discharge interference. 4) When using contact ultrasonic testing on American-style box transformers, be sure to apply ultrasonic coupling agent. 5) If a suspicious discharge is detected, be careful to shorten the tracking period. 6) When making measurements, pay attention to whether the environmental temperature and humidity as well as the load conditions have an impact.