Transformers are electrical equipment used by the power grid to transform voltage and transmit electrical energy. It is also the carrier of the power grid to supply power to users. The safe and reliable operation of transformers is very important. However, in the operation of the power grid, overvoltage will be generated due to many reasons, and the insulation level of the transformer is relatively weak. Among the causes of transformer damage, the probability of damage caused by overvoltage is the largest. During the operation of the power grid, overvoltage is generated due to some reasons, which will inevitably lead to the damage of the transformer. Its insulation level is mainly determined by the lightning withstand voltage and the power frequency withstand voltage.
Overvoltage refers to a sudden voltage increase that is dangerous to the insulation. This abnormal voltage increase can reach several times the rated voltage of the equipment and seriously threaten the safety of the transformer insulation. If the overvoltage lasts for a long time, It will inevitably cause damage to the transformer. In order to ensure the safety of transformers in grid operation, in addition to selecting high-quality transformers, reasonable and effective overvoltage protection measures must be set up for the transformers.
Ⅰ. The mechanism of power grid overvoltage
The overvoltage of the power system can generally be divided into temporary overvoltage, including power frequency overvoltage, resonance overvoltage, arc grounding overvoltage, operating overvoltage, lightning overvoltage and so on. Temporary overvoltage is mainly caused by single-phase ground fault, resonance, etc. Resonant overvoltage refers to the failure of electrical equipment in the power grid, or frequent operation of the equipment causes the inductance and capacitance in the power grid to match to form a resonant circuit. Under certain conditions, electrical energy is generated and magnetic energy is converted. The ferromagnetic resonance generated by the ground capacitance will cause a higher overvoltage. Arc grounding overvoltage is an overvoltage caused by arc discharge at the grounding point due to a single-phase grounding fault in the system.
The operating overvoltage is the overvoltage generated when the energy of the electromagnetic field is rapidly changed due to the sudden change of the grid state, or the switching of large-capacity equipment, or the operation error of the equipment, which causes the rapid release of energy. Mainly manifested in the no-load circuit, the breaking and reclosing of the transformer, etc.
Lightning overvoltage is an overvoltage caused by thundercloud discharge when a large number of positively charged thunderclouds meet with negatively charged thunderclouds in the atmosphere. Lightning overvoltage can be divided into direct lightning overvoltage and induced overvoltage. Direct lightning overvoltage is generated by the direct discharge of thunderclouds to conductors such as equipment and components, while induced overvoltage is generated by sharp changes in the electromagnetic field.
Ⅱ. The harm of power grid overvoltage to transformer
Several overvoltages generated in the power grid really affect the insulation and protection devices of the transformer provided by distribution transformer factory, mainly depending on the waveform of the overvoltage. Amplitude and duration. There are three types of voltage waveforms for assessing the insulation level of equipment: short-wave front lightning waves, long-wave front operating waves and low-frequency voltage waves. The resistance of the equipment insulation to lightning, operation or power frequency voltage shall be tested by the corresponding waveform voltage.
In the accident that overvoltage causes damage to the transformer, the probability of insulation breakdown damage caused by lightning overvoltage is the greatest. When the power grid is struck by lightning, a non-periodic pulse voltage wave with large amplitude and short action time will be generated on the line wires. It is transmitted along the line at the speed of light. When a lightning arrester is protected by a transformer, it will produce a voltage drop (residual voltage) and act on the transformer. If there is a certain electrical distance between the transformer and the arrester, the residual voltage will oscillate in the wire of this distance before entering and cause the voltage to rise, causing the voltage applied to the transformer to be higher than the residual voltage, thus posing a threat to the insulation safety of the transformer. Therefore, when installing the lightning arrester of the transformer, it is necessary to keep zero distance between the arrester and the transformer as much as possible.
The resonance overvoltage or operating overvoltage that appears in the power grid has a high overvoltage amplitude and a longer duration. It will also threaten the safety of transformer operation, and even cause insulation breakdown and destroy the transformer.
In addition, the harm of reverse conversion overvoltage to the transformer cannot be ignored. When the transformer adopts a lightning arrester for lightning protection, the grounding wire of the arrester, the neutral wire of the transformer and the transformer shell are grounded in a three-in-one manner. If the high-voltage side of the transformer is struck by lightning during operation, it will cause the arrester to discharge, and the resulting residual voltage will act on the high-voltage winding. Because the impedance of the high-voltage winding is large and the capacitive reactance is small, the lightning current only flows through the high-voltage winding and the capacitance to the ground. When the circuit is discharged through the grounding point, it will produce a large impulse voltage drop on the grounding resistance. This voltage passes The neutral wire will also be applied to the low-voltage winding, and the lightning current flowing through the low-voltage winding will also generate magnetic flux. According to the principle of electromagnetic induction, this magnetic flux will generate a very high induced voltage on the high-voltage winding according to the change of the transformer. This voltage is called inverse conversion overvoltage. The voltage amplitude is several to several tens of times larger than the residual voltage, and it will also cause breakdown and damage to the transformer insulation.
Ⅲ. Protective measures against overvoltage in the power grid
1. Install lightning rod protection
The lightning rod can effectively guide the lightning current to itself and safely into the ground, which is an effective measure to protect the direct lightning strike. Substations and electrical equipment are generally protected by lightning rods, and the protection range depends on the height and number of lightning rods. If multiple lightning rods are used for protection, the larger the range and the better the effect. At the same time, in order to prevent counterattack accidents, the grounding grid setting and grounding resistance value of the lightning rod must meet the technical specifications, and the distance between it and the frame, transformer, circuit breaker and other equipment must meet the technical requirements to achieve a good lightning protection effect.
2. Erection of lightning protection wires for protection
Setting up lightning protection wires above the power inlets in the near area of the substation can greatly reduce the probability of the power inlets suffering from lightning intrusion waves. If lightning strikes on a line other than the lightning protection line, the lightning current flowing through the arrester will be limited due to the impedance of the incoming line wire itself, and the impact of the corona on the wire will reduce the steepness and amplitude of the lightning intrusion wave, So that the residual voltage of the lightning arrester that protects the transformer is reduced, which is conducive to the coordination with the transformer insulation, and thus plays a good role in the lightning protection of the transformer.
3. Install a lightning arrester for protection
The surge arrester for power grid protection over voltage, whether it is a zinc oxide arrester with no gap or an ordinary valve arrester with a gap, a common principle for the selection and use of the arrester is: the rated voltage of the arrester should not be lower than the temporary overvoltage at the installation location of the arrester; The rated voltage of the neutral point arrester of the transformer should not be lower than the highest phase voltage of the transformer. If the rated voltage of the arrester is selected low, for the valve arrester, if a single-phase grounding fault occurs in the line, the arc cannot be extinguished due to the overvoltage at the neutral point of the transformer, causing an explosion; for the zinc oxide arrester without gaps, the same will Make it absorb too much energy under one-time overvoltage and degrade and damage. Conversely, if the rated voltage of the arrester is selected high, the corresponding impulse discharge voltage and residual voltage will increase, and the voltage limiting effect of protecting electrical equipment will become poor.
For ordinary valve arresters with gaps, the resistance of the valve plate changes with the passing current. When a large lightning current passes through the valve plate, its nonlinear resistance presents a large conductivity, so that the residual voltage of the arrester is not high. Under normal voltage, the conductivity of the non-linear resistor will drop, limiting the power frequency freewheeling to a small value, creating good conditions for the spark gap to cut off the freewheeling, and reducing the impulse discharge voltage of the arrester for a short time, thereby protecting To ensure the safe operation of the transformer.
The three-in-one grounding method is used in the lightning protection of the transformer. Whether the grounding device and grounding resistance value can meet the technical requirements, whether the electrical distance between the arrester and the transformer can be achieved zero distance, this is whether the transformer can be effective The key to protection. When the transformer is struck by lightning, the lightning current will also produce a voltage drop through the grounding resistance. This voltage and the residual voltage will be superimposed on the transformer windings, which will also threaten the safety of the transformer.
In short, the lightning protection effect of the arrester depends on the residual voltage of the arrester, the steepness of the intruding wave and the electrical distance between the arrester and the transformer. In the selection of lightning arrester, the upper limit of the volt-second characteristic must be lower than the lower limit of the transformer volt-second characteristic. The residual voltage of the arrester should also be less than the allowable degree of insulation withstand voltage of the transformer, and its value should also be less than the amplitude of the shock wave. In this way, the arrester will have the effect of protecting the overvoltage.
4. Protection of transformer neutral point overvoltage
The special design of transformer neutral earthing has a good effect on protecting the neutral point of the main transformer from overvoltage. The substation is located in the mine-prone area and has a single power supply line. The three-phase lightning has a high probability of intruding waves. Therefore, the neutral point of the main transformer needs to be protected by a lightning arrester. The overvoltage protection of the neutral point of the transformer can be set separately to protect the neutral point of the transformer, and the gapless zinc oxide arrester (referred to as the neutral point MOA) to protect the neutral point of the transformer can be used separately. The neutral point MOA is used to protect against lightning overvoltage and operating overvoltage. Its advantages are: sensitive action, low residual voltage, large current capacity, and has a good effect on protecting the neutral point of the main transformer from overvoltage.
In addition, the current neutral point MOA has a higher rated voltage, and when the neutral point potential does not deviate much, the MOA still has a better protective effect. If a single-phase grounding fault occurs in the effective grounding system, the neutral point of the main transformer will generate twice the power frequency phase voltage, and this voltage will not cause damage to the MOA.
If the neutral point potential deviates seriously, the safety of the arrester will be threatened during operation. When the neutral point overvoltage reaches more than twice the power frequency phase voltage, the arrester will be damaged due to insufficient flow and may endanger the main transformer. Security. Although the probability of this occurrence is very small, for the safety of the main transformer, in addition to using the neutral point MOA to protect the neutral point overvoltage of the main transformer, a horizontal bar gap can also be connected in parallel on the side of the arrester to limit the power frequency overvoltage. The protective function of the rod gap parallel lightning arrester is: lightning and operating overvoltage are protected by the arrester, and the gap is used to limit the high-amplitude power frequency overvoltage appearing on the arrester. This kind of protection setting not only protects the neutral point of the transformer from overvoltage, but also achieves the purpose of mutual protection.
5. Protection of three-winding transformer
The three-winding transformer has three windings of high, medium, and low voltage. If lightning strikes the high voltage side during operation, when there is lightning wave intrusion, it will transmit overvoltage to the low voltage winding through electrostatic coupling and electromagnetic induction. When the high and medium voltage windings are running and the low voltage winding is open, the capacitance of the low voltage winding to ground is small, and the electrostatic induction voltage component on the winding can reach a higher value, which will endanger the insulation safety of the low voltage winding and limit the damage of overvoltage, The low-voltage winding needs to be protected by a lightning arrester installed outside the outlet.
During the operation of the transformer, if an asymmetric ground fault occurs in the system, or a zero-sequence voltage occurs due to the non-full-phase operation of the circuit breaker, this voltage will be transferred to the low-voltage winding through capacitive coupling. Since this voltage has power frequency overvoltage characteristics, it will also endanger the safety of the low-voltage winding insulation. For this reason, in addition to selecting circuit breakers with good performance in the same period, 3 Ys are usually installed on the bus bridge on the low-voltage side of the transformer. The wired capacitor is used to increase the capacitance to the ground on the low-voltage side, which can effectively absorb and reduce the amplitude and steepness of the overvoltage, thereby protecting the effect of overvoltage.
6. Protection of transformer low-voltage system
The low-voltage side of the transformer has a long power supply line and is prone to lightning waves. When the low-voltage line is struck by lightning, the voltage is on the low-voltage winding and the grounding resistance respectively. The intruding lightning current will be generated on the high-voltage winding according to the transformation ratio of the transformer due to electromagnetic induction. Induced overvoltage. In order to prevent the over-voltage of the reverse conversion from causing harm to the high-voltage winding, a set of metal oxide arresters must be installed at the low-voltage side outlet to suppress the impulse magnetic flux generated by the low-voltage winding, thereby protecting the high-voltage winding.
In addition, due to the characteristics of the winding structure of the transformer of the Y and Zn11 connection group, if this type of transformer is used, it will also have a good effect on inhibiting the inverse conversion over-voltage, and can effectively protect the transformer from damage caused by the inverse conversion over-voltage.
Ⅳ. Conclusion
The transformer is an important equipment of the power system, and its insulation level is relatively weak, and the probability of transformer damage caused by overvoltage is the greatest. In order to ensure the safe and reliable operation of the transformer, in addition to the selection of high-quality transformers, it is still necessary to set up effective overvoltage protection for the operating transformers. According to the operating characteristics of the power grid and the grounding method, there are more chances of overvoltage at the neutral point of the transformer. In addition to over-voltage protection in substations and power grids, reasonable over-voltage protection should be set up at the neutral point of the transformer to ensure the safe and stable operation of the transformer.
