The Specificity of GIS Partial Discharge and Its Importance in Detection
Characteristics of partial discharge in GIS:
Sealing: The equipment is completely sealed and cannot be directly in contact with the internal components. Non-invasive detection methods must be employed.
Insulating medium: Inside, it is filled with high-pressure SF6 gas, which has excellent insulation performance. However, once there is a defect, the discharge will develop rapidly and may quickly escalate into a failure.
Typical defect types: Discharge defects within the GIS are usually quite typical, such as:
Free metal particles: Jumping or adhering to the bottom of the shell or to the high-voltage conductor.
Conductor spurs: Corona discharge caused by sharp protrusions on high-voltage conductors or enclosures.
Insulator surface contamination: There are metal particles or moisture on the surface of the insulator.
Insulator internal air gap: There are manufacturing defects inside the bowl-shaped insulator.
The consequences are severe: GIS is the core equipment of the substation. If it fails, it will cause widespread power outages, with long repair periods and significant economic losses.
Therefore, conducting partial discharge detection on GIS is the most effective method for identifying early insulation defects and preventing sudden failures.
Ultrasonic Partial Discharge Detector
The main methods and principles of GIS partial discharge detection instrument
Based on the characteristics of GIS, the mainstream detection methods include the following types:
Ultrahigh Frequency Method (UHF - Ultra-High Frequency) 【The currently most mainstream and sensitive method】
Principle: When local discharge occurs within the GIS, an extremely steep rising current pulse will be generated (at the nanosecond level), which will excite electromagnetic waves with frequencies ranging from 300 MHz to 3 GHz. These electromagnetic waves can propagate very far within the GIS cavity like a waveguide.
Sensor: These electromagnetic wave signals are received through built-in or external UHF sensors (couplers) installed on the GIS bushing insulator.
Advantages:
Extremely sensitive: Capable of detecting weak discharges as small as several picocoulombs.
Strong anti-interference capability: The interference frequencies of conventional power systems, such as corona discharge, are relatively low (<300 MHz), and the UHF method can effectively avoid these.
It can be located (through the time difference of signals from multiple sensors).
It can be monitored online without affecting the operation of the equipment.
Disadvantages:
It is difficult to calibrate the absolute discharge quantity. Usually, relative values such as dBm are used to represent the signal strength.
The installation position of the sensor has a significant impact on the detection effect.
2. Ultrasonic Method (AE - Acoustic Emission)
Principle: When partial discharge occurs, acoustic and ultrasonic signals (with frequencies typically ranging from 20kHz to 300kHz) are generated. These signals propagate through the SF6 gas and the GIS housing.
Sensor: The ultrasonic sensor is attached to the exterior of the GIS metal casing to receive sound signals.
Advantages:
The positioning accuracy is extremely high, making it the best method for physical positioning. By comparing the time it takes for signals to reach different sensors, precise positioning can be achieved down to the meter or even decimeter level.
Completely non-intrusive, with flexible sensor installation.
Not sensitive to electromagnetic interference in an external environment.
Disadvantages:
The signal attenuates significantly in metals and gases, and the detection range is limited.
Prone to being disturbed by environmental noises (such as wind and rain, vibrations).
The sensitivity is usually lower than that of the UHF method.
3. Transient Earth Voltage Method (TEV)
Principle: Part of the electromagnetic waves generated by internal discharge will leak out through the gaps of the bowl-shaped insulator, causing a transient ground voltage pulse on the metal casing of the GIS.
Sensor: The TEV sensor is used to detect at the shell joint.
Advantages: Portable, easy to operate.
Disadvantages: Primarily used in switch cabinets, it has relatively low sensitivity for fully enclosed GIS and is less frequently used.
The function and detection range of the GIS partial discharge detector
Function:
Early diagnosis of insulation defects: During withstand voltage tests and during operation, these various typical defects can be promptly identified.
Fault location: By combining UHF and ultrasonic methods, it is possible to precisely identify the gas chamber or specific location where the discharge defect occurs, significantly reducing the maintenance time and scope.
Quality Control:
Factory test: This is a mandatory inspection item for GIS after manufacturing.
On-site commissioning test: After the installation is completed, it is carried out simultaneously with the AC withstand voltage test. This is an international practice (as required by the IEC 62271-203 standard). It can detect new defects that arise during transportation and installation under high voltage conditions.
State monitoring and early warning: Conduct regular inspections or online monitoring of operating GIS, assess the changing trend of insulation status, and achieve predictive maintenance.
Detection scope (application scenarios):
Manufacturer: 100% factory test.
New station construction/extension: Commissioning and acceptance tests after installation.
Operating substations: Regular inspections, fault investigations, and verification after major overhauls.
