Power frequency overvoltage refers to the overvoltage in the system whose frequency is equal to or close to the power frequency (50Hz) and is higher than the maximum operating voltage of the system caused by line no-load, asymmetric grounding fault and load shedding.
The power frequency overvoltage of UHV power grid mainly considers two types of power frequency overvoltage: ungrounded three-phase load rejection and single-phase grounded three-phase load rejection.The system structure, power capacity, transmission power flow, line parameters and high impedance compensation of the line are the key factors affecting the power frequency overvoltage of the system.
According to the planning of China's ultra-high voltage power grid, the length of single section lines in the ultra-high voltage AC transmission channel varies greatly, from tens of kilometers to hundreds of kilometers, and due to the requirements of altitude and line corridor, some ultra-high voltage lines will be erected in a mixed way of double circuit and single circuit on the same tower;Due to the different nature of the power supply at the transmission end of the line, the UHV system can be divided into a "plant to network" structure (the transmission end is the power plant) and a "network to network" structure (the transmission end is the power grid).The above conditions may lead to large differences in power frequency overvoltage levels of different UHV systems[1]。
Power frequency overvoltage system
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Power frequency overvoltage The overvoltage that occurs during operation or ground fault in a system with a frequency equal to or close to the power frequency (50 Hz) and higher than the maximum operating voltage of the system.Within hundreds of milliseconds after system operation and grounding tripalternatorMedium flux linkage can not change abruptly, generatorAutomatic voltage regulatorThe inertia effect of makes the electromotive force of the generator remain unchanged. The power frequency overvoltage during this period is called temporary power frequency overvoltage.With the increase of time, the automatic voltage regulator of the generator acts to make the electromotive force of the generator decrease and become stable. At this time, the power frequency overvoltage is called steady state power frequency overvoltage.The main causes of power frequency overvoltage are: the capacitive effect of long no-load lines, the positive sequence, negative sequence and zero sequence voltage components caused by asymmetric grounding, and the sudden load shedding of the system to accelerate the rotation of the generator.Special measures shall be taken to limit power frequency overvoltage according to specific conditions. Common methods are:Shunt reactorCompensate the capacitance effect of no-load long line and select a reasonable systemNeutral point operation mode, fast voltage adjustment and control of the generator, etc.
Power frequency overvoltage difference of different types of lines
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Firstly, under the same system conditions, three types of power frequency overvoltages, namely single circuit erection, double circuit erection on the same tower and single/double circuit hybrid erection (respectively represented by S, D and S&D), are calculated and analyzed for 400 km long UHV transmission lines. The results are listed in Table 3.The single/double circuit mixing ratio is 1:1.In addition, when the line is erected by D and S&D, two modes of double circuit operation and single circuit operation (represented by LD and LS respectively) are studied. Under LS mode, two situations are considered, namely, the grounding switches on both sides of the shutdown line are suspended and grounded (represented by LS-H and LS-G respectively).
1) The ratio of zero sequence reactance to positive sequence reactance of lines between three types of lines: single circuit erection, double circuit erection on the same tower, and double circuit and single circuit hybrid erectionX0/X1. There are differences, which makes the line side power frequency overvoltage values of three lines of the same line length are different when single-phase grounding load rejection occurs, while the overvoltage differences between the fault free load rejection bus side and the line side and the single-phase grounding load rejection bus side are small.
2) For double circuit lines on the same tower, the power frequency overvoltage caused by load shedding is more serious in single circuit operation than in double circuit operation;Under the single circuit operation mode, the grounding of grounding switches on both sides of the shutdown line can significantly restrain the power frequency overvoltage.For double circuit and single circuit lines on the same tower, the power frequency overvoltage suppression effect of grounding switches on both sides of the shutdown line should be related to the mixed ratio of single circuit and double circuit lines. Under 1:1 condition, there is almost no suppression effect.
The research results of line side power frequency overvoltage under load can be seen as follows:
1) Under the same system conditions, the power frequency overvoltage level of double circuit lines on the same tower is the lowest among the three lines of different lengths, and the overvoltage level of single circuit lines is about 0.1 pu higher than it.
2) The overvoltage level of single/double circuit hybrid lines decreases with the increase of the proportion of double circuits on the same tower. When the proportion of double circuits on the same tower is 50%, the overvoltage level is equivalent to that of single circuit lines on the whole line;When the proportion of double circuit lines on the same tower is more than 50%, the overvoltage level is lower than that of single circuit lines, but still higher than that of double circuit lines on the same tower. If the proportion is 83.3%, the overvoltage level is about 0.05 pu higher than that of single circuit lines.Considering that more types of double circuit lines on the same tower are used in subsequent UHV projects, this paper will focus on the power frequency overvoltage characteristics of UHV double circuit transmission system on the same tower[2]。
Analysis of the Influence of Line Series Compensation on Power Frequency Overvoltage
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1) The influence of series compensation on power source potential and line transmission power flow.
After the UHV line is installed with series compensation, partial reactance of the line is compensated, which is equivalent to shortening the line length. On the one hand, it will increase the transmission power flow of the line, which may increase the power frequency overvoltage;On the other hand, under the same transmission power flow, the power supply potential at the power transmission end is lower than that without series compensation, which is beneficial to reducing power frequency overvoltage.
2) Influence of series compensation on grounding coefficient.
After the single-phase grounding fault occurs in the line, if the series compensation does not bypass, the positive sequence reactance of the line will decrease and the line will increase, which will lead to the increase of power frequency overvoltage caused by the single-phase grounding load rejection of the line.But in fact, when it is judged as a single-phase permanent grounding fault and the circuit breakers on both sides of the three-phase line trip, it will command the series compensation bypass switch to close, and the series compensation will only shed load on single-phase grounding within a short time (generally less than 50ms) when the bypass switch is closedSwitching overvoltageIt has a certain impact, but has no effect on the power frequency overvoltage after the switching overvoltage.
3) Influence of series compensation on voltage distribution along the line.
For lines equipped with series compensation, the arrangement of series compensation at the side of high impedance line is generally adopted to improve the voltage distribution characteristics along the line.When load shedding occurs on the line, the line will have no load. At this time, capacitive reactive power flows through the series compensation, so the series compensation has a reduction effect on the line side voltage, but the capacitive current is small, and the effect is not obvious.In conclusion, the series compensation mainly affects the transmission power flow of the line and the power supply potential before load shedding, thus affecting the power frequency overvoltage level caused by load shedding.
Analysis of the Influence of Setting Switchyard on Power Frequency Overvoltage
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Generally, when the transmission distance exceeds 600 km, it is considered to set up a switching station in the middle of the line, which is expressed as 2300 km, 2350 km.For such lines, the following line operation modes shall be considered.
Mode 1: full line double circuit operation, represented by "LD-LD";
Mode 2: single circuit operation of the whole line, represented by "LS-LS";
Mode 3: the front line s1 operates in double circuit and the rear line s2 operates in single circuit, which is represented by "LD-LS";
Mode 4: the front line s1 operates in a single circuit, and the rear line s2 operates in a double circuit, which is represented by "LS-LD".
Taking the 2300 km and 2400 km long UHV double circuit lines on the same tower as an example, the impact of switching stations in the middle of the line on the power frequency overvoltage of the system and the power frequency overvoltage of the 600 km long single section line are studied.The compensation degree of high impedance is considered as 87%.
It can be seen that:
1) As the switchyard has no load drop point, this is actually the middle point of the line. Under various operation modes, the power frequency overvoltage level on the bus side is higher than that of the single section line, which occurs on the switchyard bus.
2) After the switchyard is set in the middle of the line, under the symmetrical operation mode of double circuit and single circuit of the whole line, the power frequency overvoltage level on the line side of the substation or switchyard is basically equivalent to that of the single section line.
3) The asymmetric operation mode of the front and rear sections of the UHV double circuit line on the same tower with switchyard in the middle increases the ratio of zero sequence reactance to positive sequence reactance of the line, and the power frequency overvoltage of single-phase grounding load rejection is significantly higher than that of the single section line.If the line is long and the transmission power flow is heavy, the power frequency overvoltage problem under the above conditions may be more prominent.In fact, from the operation point of view, for such lines, when one circuit of a line exits from operation, it is not necessary that both circuits of the other line are in operation. Therefore, the single circuit operation mode of the whole line can be adopted to avoid such asymmetric operation mode[3]。
