The input porcelain is a very important part of the structure of transformers, breakers and other high voltage electrical equipment. According to statistics more than 10 failure of the pressure transformers due to damage to the input porcelain. A failure at the input terminals can result in the failure of an entire transformer.
PHAM VAN PHUONG - Central Electrical Testing Company
I. Overview of input porcelain
The insulators are located on the face of the main transformer tank, the input insulators are all hollow with a circular guide through. The MBA porcelain is specially made, reinforced with many ceramic canopy. Porcelain inputs include 2 types: ceramic capacitor type and non-capacitor type. The condenser-type porcelain used very popularly in 110kV transformers is the insulating part of the oil impregnated paper, which is also divided into two types: pressure and non-pressure porcelain.
Pressure ceramic is insulating oil that is pressurized to the inside of the porcelain, on top is fitted with an oil expander for the purpose of reducing the oil pressure in the porcelain when the temperature rises. Each porcelain has a watch that only monitors oil pressure. Non-pressure ceramic type is the oil loaded inside the non-pressure porcelain, the upper also has an oil expansion chamber. Non-capacitor ceramic is only ceramic and liquid insulation
Figure 1 Structure of the input porcelain capacitor type
II. The importance and failure mechanism of the input porcelain
The input porcelain is a very important part of the structure of transformers, breakers and other high voltage electrical equipment. According to statistics more than 10 failure of the pressure transformers due to failure of the inlet ceramic (Figure 2). A failure at the input terminals can result in the failure of an entire transformer.
Figure 2 Rate of damage to components in a pressure transformer
Below is a typical failure pattern of inlet porcelain:
Figure 3. Failure mechanism of transformer input ceramic.
III. Evaluate the quality of incoming porcelain according to traditional tests
Transformer input insulators are classified into two types: capacitor and non-capacitor types. The non-capacitor type is usually ceramic for low voltage, the capacitor for voltage over 69kV is made of capacitive layers centered on the conductor and flange grounded. Concentric capacitors generate voltage and divide voltage evenly. Figure 4 shows the input ceramic with 10 equal capacitance layers, the capacitance of each layer is C, the total capacitance is 0.1C if one layer is broken, the total capacitance will be 0.11C, which means the capacitance increases. ten‰.
Figure 4: Equivalence diagram of capacitor input ceramic
Evaluation of the insulation quality of the capacitor input insulators through a dielectric loss angle test includes:
- Main insulation C1 is the conductor insulation with the electrode tip (Navel measurement tgd-test tap).
- Insulate the terminal C2 between the electrode tip (test tap) and the flange.
Angular test of the total dielectric loss of the input inserts of the single-wire capacitance measurement navel allows the test of the input inserts while it is in the apparatus without the need to separate any connections from the inserts. . The dielectric loss angle shall be measured through a non-earthed test object test to eliminate the effect of the transformer winding insulation, which is unaffected by external operating conditions.
Figure 5. Principle diagram of conventional loss measurement method
IV Introduction to MBA input porcelain diagnostic methods
In-depth testing and evaluation to be able to make accurate and timely conclusions is essential in the testing, management and safe operation of electrical equipment. In this article, I would like to introduce 2 methods of diagnosing the fault of capacitor input capacitors: frequency domain dielectric response (FDS), local discharge analysis (PD). .
1. Frequency domain dielectric response (FDS) analysis.
a. Measuring method for new enameled types RIP, RPB and OIP.
This method is made to measure with frequencies in a wide range from 15 to 400Hz. The loss coefficient curves of the new high voltage porcelain are shown in figure 6. The test voltage value is measured at a voltage of 2kV and the frequency range from 15Hz to 400Hz.
b. Effect of moisture on dielectric loss in RIP porcelain.
The picture shows us clearly that the influence of the humidity of the input ceramic storage environment will greatly affect the insulation of the porcelain. Without the protective oil layer, moisture from the environment will be absorbed into the insulation layers over time and will change the coefficient of loss most dramatically. When the humidity is high, the loss coefficient will be of great value at low frequencies and gradually decrease to the minimum of the curve when at high frequencies. c. Diagnostic on ceramic input RBP. We analyze the dielectric response of input porcelain type RBP 123kV. As shown in Figure 8: the red line shows the dielectric response of phase C and the blue path the dielectric response of phase A).
Figure 8. Through the figure we can see a strong increase in the dielectric loss coefficient according to the increasing frequency of the frequency.
2. Analysis of partial discharge (PD).
Will traditional experimental methods tell us if the discharge has occurred inside? But not aware of the level and evolution of the processes causing local discharges inside as well as the porcelain surface of the input porcelain; the time of discharge occurred, not monitoring in real time, data was not continuous, results were inconsistent; location of discharge occurred and need to be cut off, time consuming to analyze. From there we can see that in order to be able to detect potential problems in time, we must use advanced diagnostic technology that is a real-time local discharge monitoring method.
Begin with a PD measurement based on a representation on a phase sequence diagram. All the PD signals we can see in phase PD in Figure 11. However, it is not possible to identify this PD source. Therefore, the PD sources will be separated by the center frequency relationship graph (3CFRD). The 3CFRD distinction between the generation of PD pulse sources from a variety of sources is based on their frequency spectrum.
The spectrum is simulated to simulate three different spectrums: 500kHz, 2.8MHz and 8MHz. Amplitude at each frequency is added and the result is bundled with the separate PD signals of each PD source. The individual signal aggregation can be displayed in the phase sequence diagram to facilitate analysis of the PD types. This technology can be used to determine the magnitude of the source causing PD as well as measure multiple platforms affecting the PD measurement on the device.
V. Operation and maintenance of inlet porcelain
To increase service life and minimize potential damage to the load cell transformer inputs. The operator must have good control from the selection of the ceramic type, the manufacturer; preservation before installation; monitor the installation process and monitor the parameters and working conditions of the inlet porcelain. During the operation, the following issues should be noted:
Figure 14. Using infrared camera to measure the temperature of the ceramic junction
1. Clean the insulating surface. The transformer operates in a polluted environment or near the sea, dust will stick on the insulating surface of the porcelain and make the insulation resistance lower, increasing surface leakage current. Therefore, there must be a plan for cleaning and cleaning the porcelain surfaces. You can use high pressure water or you can use a damp cloth.
2. Use infrared camera to check the porcelain junction. When the MBA is operating at full load, the temperature in the ceramic companies is about 35-450C. If you use an infrared camera, you can check the heat in the porcelain, when the high temperature is measured, the junction has bad contact.
3. The operator must regularly check for oil leakage on the porcelain surface.
4. Check the oil pressure and oil level on the porcelain indicator.