The electromagnetic voltage transformer (hereinafter referred to as PT) in the 6~35kV neutral point ungrounded power grid, when the bus is no-load or there are few outgoing wires, is excited due to closing charging or elimination of ground fault during operation, etc., the voltage transformer will be oversaturated, and ferromagnetic resonance overvoltage may occur. Abnormalities such as unstable phase-to-ground voltage, malfunctioning of the grounding indicator, and blown PT high-voltage fuse may occur. In serious cases, the PT may be burned out, leading to other accidents.
For example, if the neutral point of the 6~35kV power grid is not grounded, Y on the bus. The connected PT primary winding will become the only metallic channel of the power grid to the ground. When a single phase is grounded or disappears, the grid-to-ground capacitance undergoes a charge and discharge transition process through the PT primary winding. At this time, there is often a power frequency half-wave inrush current with the highest amplitude of several amperes passing through the PT. This current may blow the PT high-voltage fuse (0.5A). The electronic harmonic elimination device installed on the secondary side of the PT generally cannot eliminate this inrush current.
XPT type, XPT (D) type harmonic elimination resistor is installed in the primary winding Y of 6~35kVPT. The high-capacity nonlinear resistor between the neutral point of the wiring and the ground serves as a damping and current limiting function, eliminating voltage ferromagnetic resonance and preventing single-phase high-voltage fuse from blowing. The accessory "third harmonic limiter" installed at both ends of the open triangle of the transformer residual voltage winding can limit the voltage drop of the transformer's third harmonic current and fundamental unbalanced current on the harmonic eliminator within a certain range. At the same time, the voltage across the harmonic elimination resistor in the primary winding is also reduced. The voltage across the harmonic elimination resistor can be reduced to less than a quarter of its original value. This is more beneficial for voltage transformers with low insulation at the neutral point and long-term operation at low voltage (less than 100V).
The harmonic eliminator resistor body is made of SiC as the base material, and the connector is made of die-cast aluminum or copper. It is small in size, light in weight, high in strength, easy to dissipate heat, and easy to install.
The product uses a single large-flow non-linear resistor, which eliminates the uneven flow in the original multi-chip parallel structure harmonic eliminator and makes the performance more stable.
If current flows through the harmonic eliminator when it is in use, the high voltage it generates is directly applied to the neutral point of the PT, which may burn out the neutral point insulation of the weakly insulated PT. The harmonic eliminator provides harmonic eliminators with D parameter components (with D type) for weakly insulated PTs. This component can effectively limit the voltage at both ends of the harmonic eliminator and protect the neutral point insulation.
This harmonic eliminator is smaller in size and is especially suitable for PT cabinets and handcart cabinets. The entire harmonic eliminator has no porcelain sleeve, so there is no risk of explosion.
After the transformer cabinet has been put into operation, a harmonic eliminator is installed. It is inconvenient to open holes on the transformer cabinet panel to install accessories, and it is also necessary to reduce the voltage rise at both ends of the open triangle. You can configure a simple accessory and connect it directly to the open triangle da and dn ends of the terminal block to reduce the increase in the open triangle voltage caused by the harmonic eliminator.
The XPT type harmonic eliminator adopts most voltage transformers. The excitation current under the rated line voltage is greater than 10mA (peak/√2）Characteristics, the resistance value is calibrated based on the AC 10mA (peak value/√2) current on the resistor which is greater than 6% of the rated line voltage of the transformer. Therefore, the relationship between the resistance value of the harmonic elimination resistor and the required harmonic elimination resistance value is very clear. Calibrate 1mA (peak/√2), which establishes the electrical parameters of the harmonic eliminator, so it is more reasonable.
The electrical wiring diagram of the XPT type harmonic eliminator and its accessories is shown in Figure 1. The resistor is connected between the neutral point of the transformer primary winding and the ground, and the accessories are connected to both ends of the transformer open triangle. The AC100V power supply is provided by the line voltage of the transformer secondary winding, and the required power is no more than 5VA. If you use a simple accessory, you only need to tighten the two support wires of the simple accessory directly to the da and dn ends of the terminal strip.
1. R=10~20Ω during normal operation, R≥22kΩ at 80V, R is the internal resistance of the limiter (when K is closed).
2. When the input voltage is greater than 80V, the resonance counter will be activated.
3. Display opening triangle voltage range: 0~200V.
1. XPT type harmonic eliminator is generally installed indoors. Years of operation experience have proven that it can also be installed outdoors. When installing in a pressure transformer cabinet, it is generally installed vertically. If the vertical installation in the cabinet is difficult to arrange, it can also be installed horizontally.
2. The distance between the resistor installed in the transformer cabinet and the live body should meet the requirements of the national standard "3~110kV High Voltage Distribution Equipment Design Code" for the safe distance between the live body and the grounded body. The distance between the upper end of the resistor and the grounding body should be no less than 20mm.
3. The upper end of the XPT type harmonic eliminator resistor has an insulated wire about 1m long. The insulated wire can be laid in the air or along the wiring board.
4. XPT type harmonic eliminator accessories need to be installed on the transformer cabinet panel or control cabinet panel to facilitate observation by operators. Regarding the operation of accessories during operation, there are text instructions on the accessory panel. Follow the instructions.
5. Installation and operation of the simple accessory of the XPT type harmonic eliminator: The simple accessory can be directly fastened to the terminal block with a pressure-transforming open triangular contact, and the two Φ1.2 copper wires of the simple accessory can be fastened to the da and dn contacts of the terminal block. If the transformer is in operation, a screwdriver with an insulated handle can be used to install it live. Before installation, use a multimeter to measure the voltage between da and dn. If the voltage between da and dn of the terminal block is between 3 and 15V, it means that the primary winding of the voltage transformer is connected to a harmonic elimination resistor. After installing simple accessories, the voltage dropped to about one-tenth of its original value. If simple accessories are not installed and the voltage is between 0.3V and 3V, the harmonic elimination resistor connected to the primary winding may be short-circuited, and the insulation of the transformer neutral point needs to be carefully checked. If the transformer is out of service when a simple accessory is installed, after installing the simple accessory, when the transformer is running, measure the voltage at both ends da and dn in time to check whether there is a short circuit at both ends of the open triangle of the transformer and the harmonic eliminator. Because the simple attachment does not have an alarm function, manual inspection is required to ensure the safety of the pressure change.
Our company is equipped with a "Special Measurement Instrument for AC Peak Parameters of Harmonic Elimination Resistors" that can easily measure the AC current through the resistor at 1mA (peak value/√2), 10 mA (peak value/√2) when the AC voltage across the resistor (peak/√2). Please note that because the resistor is a non-linear resistor, the current flowing through the non-linear resistor and the voltage on the resistor are not sine waves. The values measured with ordinary milliamp meters and voltmeters are different from the values measured by the peak meter. Considering that users have difficulty in measuring the peak value, here are two measurement methods and mutual comparison coefficients.
