The measurement of power quality is voltage and frequency. Voltage imbalance seriously affects power quality. The increase, decrease or phase loss of phase voltage will affect the safe operation of power grid equipment and user voltage quality to varying degrees. There are many reasons for voltage imbalance in the compensation system. This article introduces The six causes of voltage unbalance are analyzed in detail, and different phenomena are analyzed and dealt with.
Key words: compensation system voltage; unbalance; analysis and processing
1 The generation of voltage unbalance
1.1 The ground capacitance of the phase voltage unbalanced network caused by the inappropriate compensation degree and all arc suppression coils in the compensation system form a series resonant circuit with asymmetric voltage UHC as the power supply, and the neutral point displacement voltage is:
UN=[uo/(P+jd)]·Ux
In the formula: uo is the asymmetry degree of the network, a system compensation degree: d is the damping rate of the network, which is approximately equal to 5%; U is the system power supply phase voltage. It can be seen from the above formula that the smaller the compensation degree, the higher the neutral point voltage. In order to keep the neutral point voltage from being too high during normal operation, resonance compensation and near-resonance compensation must be avoided during operation, but in practical situations However, it often occurs: ① The compensation degree is too small, due to the capacitor current and the inductance current of the arc suppression coil IL=Uφ/2πfL due to the change of the operating voltage and cycle, both IC and IL can change, thus changing the old compensation degree. The system approaches or forms resonance compensation. ②The power supply of the line is stopped. When the operator adjusts the arc suppression coil, he accidentally puts the tap changer in an inappropriate position, causing obvious neutral point displacement, and then the phenomenon of phase voltage imbalance. ③In the under-compensated power grid, sometimes due to line tripping, or power outage due to power limitation and maintenance, or due to the line being put into the over-compensated power grid, there will be close to or form resonance compensation, resulting in serious neutrality. The point is displaced, and the phase voltage unbalance occurs.
1.2 The voltage imbalance caused by PT disconnection at the voltage monitoring point The characteristics of voltage imbalance caused by PT secondary fuse blown and primary knife switch poor contact or non-full-phase operation are; the grounding signal may appear (PT primary disconnection), causing The voltage indication of the disconnected phase is very low or no indication, but there is no voltage rising phase, and this phenomenon only occurs in a certain transformer.
1.3 Voltage unbalance compensation caused by single-phase grounding of the system When the system is normal, the asymmetry is small, the voltage is not large, and the potential of the neutral point is close to the potential of the earth. When a metal grounding occurs at a certain point on a line, busbar or live equipment, it is at the same potential as the ground, and the voltage value of the two normal phases to the ground rises to the phase-to-phase voltage, resulting in serious neutral point displacement. Different resistances, the two normal phase voltages are close to or equal to the line voltage, and the amplitudes are basically the same. The direction of the neutral point displacement voltage is on the same straight line as the ground phase voltage, and the direction is opposite to it. The phasor relationship is shown in Figure 2. shown.
1.4 The voltage imbalance caused by the single-phase disconnection of the line causes the asymmetric change of the parameters in the network after the single-phase disconnection, which makes the asymmetry increase significantly, resulting in a large displacement voltage at the neutral point of the power grid, resulting in the three-phase phase of the system. Unbalanced ground voltage. After the single-phase disconnection of the system, the past experience is that the voltage of the disconnected phase increases and the voltage of the two normal phases decreases. However, due to the difference in the position of single-phase disconnection, operating conditions and influencing factors, the direction and magnitude of the neutral point displacement voltage and the indication of each phase-to-ground voltage are not the same; Equal or equal, the voltage of the power supply to the ground of the disconnected phase decreases; or the voltage of a normal phase to ground decreases, and the voltage of the disconnected phase and the other normal phase to ground increases but the amplitudes are not equal.
1.5 Voltage unbalance caused by inductive coupling of other compensation systems. The two lines of the two compensation systems for power transmission are relatively close and the parallel sections are long, or when the cross opening is erected on the same pole for backup, the two lines are connected in series by the capacitance between the parallel lines. resonant circuit. Phase-to-ground voltage imbalance occurs.
1.6 Phase voltage unbalanced by resonance overvoltage Many nonlinear inductive elements in the power grid, such as transformers, electromagnetic voltage transformers, etc., and the capacitive elements of the system form many complex oscillating circuits. When the empty bus is charged, each phase of the electromagnetic voltage transformer and the ground capacitance of the network form an independent oscillation circuit, which may cause two-phase voltage increase, one-phase voltage decrease or opposite phase voltage imbalance. This ferromagnetic resonance, It only appears on the only one power bus when charging the empty bus through the transformer with a power source of another voltage level. In a system with a voltage level, this problem does not exist when the secondary substation bus is charged by the power transmission main line. To avoid the empty charging bus, a long line must be charged together.
2 Judgment and treatment of various voltage imbalances in system operation
When the phase voltage unbalance occurs in the system operation, most of them are accompanied by grounding signals, but the voltage unbalance is not all grounded, so the line should not be selected blindly, and should be analyzed and judged from the following aspects:
2.1 Find the cause from the unbalanced range of phase voltage
2.1.1 If the voltage unbalance is limited to one monitoring point and there is no voltage rising phase, causing the user to have no phase loss response, the unit PT circuit is disconnected. At this time, only consider whether the protection of the voltage component can malfunction and affect the measurement. Whether the cause of the unbalance is due to the unbalanced load connection of the main circuit, which leads to the unbalanced display, and whether it is caused by the failure of the display screen.
2.1.1 If the voltage unbalance occurs at each voltage monitoring point in the system at the same time, the voltage indication of each monitoring point should be checked. The unbalanced voltage is obvious, and there are decreasing phases and increasing phases, and the indications of each voltage monitoring point are basically the same. The situation that causes the abnormal voltage may also be very special such as the poor contact of the busbar voltage transformer. It is also possible that several reasons are mixed together. If the cause of the abnormality cannot be found out, the abnormal part should be withdrawn from operation and handed over to the maintenance personnel for processing. As a dispatcher and operator, it is sufficient to determine that the cause of the abnormality lies in the busbar voltage change and the following circuits, and restore the system voltage to normal. Reasons may be:
①The compensation degree is not suitable, or the adjustment and operation of the arc suppression coil is wrong.
②Under-compensated system, there are line accident trips with equivalent parameters.
③When the load is low, the frequency and voltage change greatly.
4. After the unbalance accident such as grounding occurs in other compensation systems, the neutral point displacement of the system is caused, and the voltage unbalance caused by the compensation problem should be adjusted. The compensation degree should be adjusted.
For the voltage imbalance caused by the tripping of the power grid line in under-compensated operation, it is necessary to try to change the compensation degree and adjust the arc suppression coil. When the load in the network is at a trough, the voltage unbalance occurs when the cycle and voltage rise, and the arc suppression coil can be adjusted after the unbalance disappears naturally. As a dispatcher, you should master these characteristics to accurately judge and quickly deal with various abnormalities that may occur during operation. The judgment of a single feature is relatively easy, and the judgment and processing of the voltage abnormality caused by the compound fault of two or more situations are more complicated. For example, single-phase grounding or resonance is often accompanied by high-voltage fuse blowing and low-voltage fuse blowing. When the high-voltage fuse is not completely blown, whether the grounding signal is sent or not depends on the secondary voltage setting value of the grounding signal and the degree of blown fuse. Judging from the actual operation, when the voltage is abnormal, the secondary circuit is often abnormal. At this time, whether the voltage level and grounding signals are sent out, the reference value is not large. It is especially important to find out the rule of investigation and deal with abnormal voltage.
2.2 Judging the cause according to the magnitude of the phase voltage unbalance. For example, serious phase voltage unbalance occurs in each substation during the operation of the system, indicating that there is a single-phase grounding or single-phase disconnection in the main line in the network, and each voltage monitoring point should be quickly investigated. According to the voltage indication of each phase, make a comprehensive judgment. If it is a simple one-phase grounding, you can select the line to search according to the specified line selection sequence. Select first from the outlet of the power substation, that is, after selecting the grounding trunk according to the principle of “root first, then tip”, and then select the grounding section in sections.
2.3 Judging the reasons based on the operation changes of the system equipment ① An abnormality occurs in a certain phase of the three-phase winding of the transformer, and the asymmetric power supply voltage is delivered. ② The transmission line is long, the cross-section of the conductor is uneven, and the impedance and voltage drop are different, resulting in an unbalanced voltage of each phase. ③ The power and lighting are mixed and shared, and there are many single-phase loads, such as household appliances, electric furnaces, welding machines, etc. are too concentrated on one or two phases, resulting in uneven distribution of power load on each phase, making the power supply voltage and current inconsistent. balance.
To sum up, in the operation of the small current grounding system (compensation system) grounded by the arc suppression coil, the phase voltage unbalance phenomenon occurs from time to time, and due to different reasons, the degree and characteristics of the unbalance are also different. But the general situation is that the power grid is running in an abnormal state, and the increase, decrease or phase loss of the phase voltage will affect the safe operation of power grid equipment and user production to varying degrees.
Post time: Aug-29-2022