Abstract: This paper mainly introduces the principle of installing the harmonic elimination device for the busbar voltage transformer of the substation, especially for the power frequency displacement overvoltage and ferromagnetic resonance overvoltage caused by the saturation of the voltage transformer core in the ungrounded system. In the practical application, the advantages and limitations of using the primary and secondary harmonic elimination devices for harmonic elimination are presented, and the advantages of using arc suppression, harmonic elimination, line selection and overvoltage protection are proposed.
Keywords: neutral point ungrounded system arc suppression coil ferromagnetic resonance overvoltage intermittent arc grounding overvoltage
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introduction
Most of the systems below 35 kV in China use the neutral point of the power supply. When this grounding method occurs in single-phase grounding , if the phase C is single-phase grounded, then the relative ground voltages of both A and B are raised from the original phase voltage to the line voltage, that is, the voltage of the original ground voltage is increased to √ 3 The capacitance current of the C- phase grounding is 3 times of the relative capacitive current per normal operation. When a phase is grounded, the line voltage of the line is allowed to operate in a short time regardless of the phase and magnitude . However, with the expansion of urban and rural power grids and the increase of cable outlets, the single-phase grounding capacitor current will further increase. When the grid-to-ground capacitor current reaches a certain value, the arc at the fault point after single-phase grounding cannot self-extinguish, resulting in The gap arc is grounded to overvoltage, damaging the line equipment.
In the neutral point ungrounded system of the grid, the electromagnetic primary winding of the busbar becomes the neutral point ungrounded grid.
For the only metal channel to the ground, the charging and discharging paths of the grid relative to the ground must pass through the primary windings. When the system is single-phase grounded, the fault current will flow through the capacitor current. The voltage of the ungrounded phase (A , B) rises to the line voltage, and its capacitance to the ground is charged with the charge corresponding to the line voltage. During a ground fault, the capacitive current generated by this charge, in the path of the ground point, circulates between the power supply - wire - ground. Since the excitation impedance of the pressure mutual is large, the current flowing therein is small. Once the ground fault disappears, the current path is cut off and the non-ground phase must be instantaneously restored to the normal phase voltage level by the line voltage. However, since the ground fault has been disconnected, the non-grounded phase has been charged to the charge under the line voltage during the grounding period, and only through the pressure of the high-voltage winding, the neutral point of the original grounding enters the earth, and the number of voltages in the primary winding will appear. The inrush current of the amperage amplitude fuses the high voltage fuses. During this transient process, a high-frequency low-frequency saturation current will flow through the high-voltage windings, which will severely saturate the pressure-crossing core, and the magnetizing inductance of the saturated voltage transformer will become smaller. It tends to be inductive. At this time, if the grounding inductance of the system network matches the capacitance to ground, a three-phase or single-phase resonant circuit (see Figure 1 ) is formed , which can excite various ferromagnetic resonance overvoltages. In addition , single-phase arc grounding in the power grid, due to lightning strikes or other reasons, the line is instantaneously grounded, so that the sound phase voltage suddenly rises, resulting in a large inrush current, which will also cause the pressure to burn each other.
 Figure 1 Ferromagnetic resonance equivalent circuit
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Under actual operating parameter, the resonant frequency of the column system, mostly 1/2 and the fundamental resonance frequency division. An area that is inevitable or may occur and is unlikely to resonate, as shown in Figure 2 .
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Figure 2 resonance probability distribution map
It can be seen from the figure:
(a) When the normal operating voltage and R0 are not applied, ( Â Â Â Â Â Â Â Â ) = 0.025 to 0.280, the sub-frequency resonance; ( Â Â Â Â Â Â Â Â Â The fundamental resonance occurs when =0.180 to 0.680 .
(b) As R0 increases, the resonance range decreases. When R0 is greater than a certain threshold, the resonance range disappears, that is, resonance does not occur. When R0≥0.056ωL , all fundamental and frequency-divided harmonics can be eliminated.
According to the above analysis, appropriate measures can be taken to eliminate the resonance, and the overvoltage is limited. The measures that can be taken are various. It is more common to use the harmonic elimination device at both ends of the open-ended triangular winding of the pressure mutual secondary side. The method of pressing the neutral side of the primary side to the ground to eliminate the harmonic resistance is as follows. The following is a comparison of these harmonic elimination methods, so as to be suitable for local conditions and reasonable selection.
In a first pressure cross winding varistor installed between neutral and ground (resonance eliminator)
Connect a large enough grounding resistance at the neutral point of the high voltage winding (see Figure 3 ) to play the role of damping and current limiting. When the single phase fault disappears, the low frequency saturated current passes through the resistor Ro and enters the earth. Part of the voltage drop is added to the resistor, which greatly suppresses the low-frequency saturation current, making the high-voltage fuse not easy to be blown. At the same time, due to the resistor Ro connected in series with the zero-sequence voltage loop, most of the voltage of the voltage-saturated overvoltage falls. The resistor Ro is used to avoid core saturation and limit the occurrence of voltage mutual saturation overvoltage.
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Figure 3
The limitation is due to the complexity of the power grid, the capacitance current of each distribution network, the nature of the line fault, the voltage mutual volt-ampere characteristics and the operating environment of the harmonic elimination device. The heat capacity of the primary harmonic elimination device is limited, which is difficult to guarantee. After the harmonic elimination device is installed at the neutral point of the pressure, the equipment is safe, especially when the intermittent arc grounding has a long duration, the individual harmonic elimination resistance will be damaged by overheating, thereby causing the high-voltage fuse to be blown, and even the pressure is burned. A relatively large primary harmonic damper can still damage the device under the intermittent arc grounding overvoltage excitation with long duration; if the value of Ro is too small, it is equivalent to not increasing the zero sequence resistance, limiting the voltage mutual saturation overvoltage. The effect is not large. From the perspective of damping, the larger the resistance value, the better. If Ro → ∞, the neutral point of the high voltage side winding becomes insulated, and the inductance of the pressure does not participate in the zero sequence loop. (such as microcomputer grounding line selection device). Moreover resonance eliminator can limit the pressure of mutual resonance does not occur, other cross-voltage grid is invalid, when the single-phase ground fault occurs, and the high-pressure side of the system in more neutral grounding pressure cross run , it is necessary to install a harmonic elimination resistor at each neutral point for each pressure.
2 Install a secondary harmonic elimination device (damping resistor) on the secondary side of the transformer of the pressure cabinet
The damping resistor Ro is connected at both ends of the triangular winding of the secondary side of the pressure, which is equivalent to a resistor connected in parallel with the Y- junction winding of the high-voltage side of the pressure, and this resistance only appears when the power grid has zero-sequence voltage, and the normal operation At the time, the Ro connected to the zero-sequence voltage winding does not consume energy. The smaller the value of Ro is, the smaller the parallel resistance is on the pressure mutual excitation inductance L. When Ro is less than a certain value, the relative parameters of the network three are basically determined by the equivalent resistance, and the inductance is reduced by the mutual saturation of the pressure. Does not significantly cause the power neutral point displacement voltage. When Ro → 0 , the open triangular winding is short-circuited, the three-phase inductance value of the pressure becomes a leakage inductance, and the three phases are equal, and the voltage mutual saturation overvoltage does not exist.
The limitation is that when single-phase grounding occurs in the power grid, 100V power frequency zero-sequence voltage will appear at both ends of the triangular open winding of the pressure mutual opening, so the capacity of the damping resistor needs to be large enough. When the damping resistance is too small, the resistance itself is on the one hand. It may burn out due to overheating. On the other hand, the pressure may be burnt due to excessive current. When the inrush current occurs, it will short-circuit the secondary opening triangle, which in turn will increase the inrush current amplitude.
3 Adding a microcomputer harmonic elimination device
A microcomputer harmonic elimination device is installed in the double-opening triangular winding of the pressure mutual. When it is determined that there is a power frequency displacement overvoltage or a ferromagnetic resonance overvoltage, the single chip performs a harmonic elimination procedure to emit a high frequency pulse group, so that the reverse is in the opening. The two thyristors at both ends of the triangular winding are alternately zero-crossed to trigger conduction, and the open triangular windings are short-circuited ( if the system is single-phase grounded, the harmonic elimination device is not activated ) , so that the voltage mutual saturation overvoltage is quickly eliminated. Since the shorting time is extremely short, it does not burden the pressure.
The limitation is that in the neutral point ungrounded grid, the electromagnetic pressure mutual high voltage fuse is not necessarily caused by the voltage mutual saturation overvoltage. When the grid-to-ground capacitance is large, and the low-frequency saturation current formed when the grid is intermittently grounded or grounded disappears, it appears in the 1/4 to 1/2 power-frequency period after the single-phase ground disappears. The current amplitude can be much larger than the frequency-divided resonant current. (The frequency-resolved resonant current is more than 100 times the rated excitation current ) . Since the low-frequency saturation current has the characteristics of high amplitude and short action time, half of the cycle after the single-phase grounding disappears can fuse the fuse, and the addition of the microcomputer harmonic elimination device cannot suppress the low-frequency saturation current, which is suitable for the grid to be small and Where the ground capacitance is not large; the microcomputer harmonic elimination device is also difficult to correctly distinguish between fundamental resonance and single-phase grounding. At present, the main difference between the fundamental resonance and the single-phase ground fault criterion is the level of the zero-sequence voltage U0 . Typically, when U0≥150V as the fundamental resonance; when 30 V≤U0 <145V as single-phase ground fault. In order to prevent the situation from being burned by the device being mis-operated during the single-phase grounding due to the device being mis-operated for a long time, the criterion voltage of the fundamental frequency resonance of the microcomputer harmonic elimination device is generally set relatively high. Thus, in the case where the power frequency shift voltage is not very high (such as an empty bus switch), the device will not operate, and some fuses with poor excitation characteristics and iron cores may be blown. In addition, under the intermittent arc overvoltage excitation with a long duration, the current flowing through the high voltage windings of the voltage will increase significantly, and the pressure mutuals may still be burned out.
4 in the pressure mutual neutral point in series with a single voltage of the rated voltage as the line voltage
A single-phase voltage mutual current with a rated voltage of line voltage is connected in series at the neutral point of the pressure (see Fig. 4 ), that is, a zero-sequence voltage transformer, and the primary winding of each voltage transformer is connected in a star shape, and the main voltage transformer is One of the secondary windings is used for measurement protection, and the other is connected to the closed triangle. Without any load, it only acts as a harmonic elimination. The secondary winding of the zero-sequence voltage transformer acts as an alarm when single-phase grounding occurs. use. By using a neutral point string resistor (ie, the equivalent resistance R0 of the zero-sequence voltage transformer, the resonance range decreases as R0 increases, and after R0 is greater than a certain threshold, the resonance range disappears, that is, resonance does not occur) And the closed triangle winding wiring method (to minimize the damping resistance value of the open delta connection), so that they cooperate and interact with each other to enhance the harmonic elimination effect.
Figure 4
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The limitation is that the PT handcart is relatively small, and it is difficult to install four pressures; the resonance is an LC oscillation, and the neutral point series connection of single phase pressure can be simplified into an LC circuit, and its main function comes from the oscillation point. The offset does not change the nature of the oscillation. In theory, there is still the possibility of resonance; the secondary wiring is cumbersome and easy to be connected; due to the influence of the zero-sequence inductance, the measurement may cause errors; the resistance and capacity of R0 are determined. It is related to the parameters of the system, and the calculation is complicated. In order to exert a strong damping effect, a high-resistance dedicated zero-sequence transformer must be selected.
5 comprehensive application of arc suppression, harmonic elimination line selection and overvoltage protection
In summary, the harmonic elimination effects of various harmonic elimination devices have their own advantages and disadvantages. When the harmonic elimination measures are adopted for the power distribution system, in addition to the selection of electromagnetic voltage transformers with good excitation characteristics and difficult core saturation, it is necessary to Depending on the situation, the primary harmonic elimination device and the secondary harmonic elimination device can be used together to complement each other to ensure that the pressure mutual self does not participate in the resonance. In the same distribution network, the harmonic elimination and the second are used as much as possible. At the same time as the secondary harmonic elimination measures, in order to ensure the safety of the equipment, measures to limit the intermittent arc grounding overvoltage should also be taken. The arc suppression and harmonic elimination line selection and overvoltage comprehensive protection are used to limit various overvoltages (arc grounding overvoltage, resonant overvoltage, and operating overvoltage) in the power grid and accurately select the grounding line of the system to enhance the operation of the power system. Maintenance management, ensuring the safe, stable and reliable operation of the power grid will have a very positive effect.
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