A brief discussion on how to improve the stability of power system

The loss of stability of the power system during operation is the most serious accident of the power system. For this reason, in the design and operation of the power system, when it is found that the stability of the system is not high enough after calculation. Technical measures should be taken to ensure the safe and stable operation of the power system. In addition, once the system loses stability, corresponding measures should be taken to limit the scope of the accident, reduce the losses caused by it, and restore the normal operation of the system as soon as possible.
The stability analysis of the power system should be carried out from two aspects: static stability and transient stability. Generally speaking, the system with high static stability should also have higher transient stability. Static stability refers to the ability of the system to maintain its own stability under normal operation. If a system cannot completely maintain its stability in normal operation, it is even more difficult to guarantee the stability after a large disturbance, that is, transient stability. Therefore, in order to improve the static stability of the system, fundamental measures must be taken, that is, to increase the system stability reserve and reduce the electrical distance. For transient stability, because it considers the stability of the system after a large disturbance, it is more difficult to maintain the transient stability of the system than to maintain the static stability of the system, and there are more measures accordingly. We discuss the measures to improve the stability of the power system from these two aspects.
1. Measures to improve static stability
The static stability of the power system refers to the ability of the power system to automatically recover to the original operating state after a certain small disturbance disappears without self-excited oscillation or asynchronous loss of step.
From the simple system power-angle characteristic equation below, it can be seen that under the condition of constant transmission power, the greater the possible limit power of the generator, the higher the static stability limit, and the better the corresponding static stability performance. To improve the static stability limit, the power supply potential and the receiving terminal voltage can be increased. Reduce the reactance. To increase the power supply potential and system voltage, the system and generator must first have sufficient reactive power; to reduce the reactance, the power supply capacity must be increased. At the same time, shorten the "electrical distance" between the generator and the system.
1. The generator uses an automatic adjustment excitation device
When the generator does not use an automatic adjustment excitation device, the no-load potential Eq is a constant, and the reactance of the generator is the synchronous reactance Xd. After the automatic adjustment excitation device is adopted, the generator can achieve Eq' or Vg as a constant. When Eq' is a constant, Xd is reduced to Xd', while when Vg is a constant, Xd will have no effect on system stability. Therefore, installing an advanced automatic excitation device on the generator is equivalent to shortening the "electrical distance" between the generator and the system. Since the installation of an automatic excitation device is inexpensive and effective, it is the preferred measure to improve static stability.
2. Reduce line reactance
Reducing line reactance and strengthening the connection between systems can increase the static stability limit and the degree of stability. The following methods can be used to directly reduce line reactance: 1) Replace overhead lines with cables; 2) Use expanded diameter conductors; 3) Use split conductors. The first two methods are difficult to implement universally due to high investment or other technical problems. Therefore, the method of directly reducing line reactance is mainly to use split conductors. For example, for 500kV overhead lines, when a single conductor is used, the reactance is about 0.43Ω/km; when three split conductors are used, it is about 0.3Ω/km; the reactance value is reduced by one third. Therefore, split conductors are mostly used in 220kV and above systems.
3. Increase the rated voltage level of the line
From the power-angle characteristic equation, it can be seen that increasing the rated voltage level of the line can increase the static stability limit and the level of static stability. However, increasing the voltage level requires additional investment, especially the system needs to have sufficient reactive power.
4. Use series capacitor compensation
Series capacitor compensation can improve the static stability of the power system by voltage regulation or by reducing line reactance. In the latter case, its compensation degree should be determined by calculation. Generally speaking, the greater the compensation degree, the smaller the equivalent reactance of the line, which is beneficial to improving stability. However, when the compensation degree is too large, a series of problems will arise: causing the damping power coefficient D to be negative, causing spontaneous low-frequency oscillation of the system, easily causing the generator to generate self-excitation, causing difficulties in the operation of relay protection, and increasing short-circuit current. Considering the above factors, the compensation degree of series capacitor compensation used to improve stability should generally be less than 0.5.
Series capacitor compensation generally adopts centralized compensation. For dual power supply lines, it is installed at the midpoint, and for single power supply lines, it is installed at the end.
5. Improve system structure
Improving system structure and strengthening system connection can improve the stability of the power system. The methods include: 1) increasing the transmission line loop and reducing the line reactance; 2) strengthening the internal connection of the systems at both ends of the line and reducing the system equivalent internal reactance; 3) connecting to the intermediate power system, so that the voltage in the middle of the long-distance transmission line can be maintained constant, which is equivalent to segmenting the transmission line, thereby reducing the reactance; 4) installing a synchronous phase regulator on the step-down transformer in the middle of the transmission line, and the synchronous phase regulator is equipped with an advanced automatic excitation device, which can maintain its terminal voltage and even the high-voltage bus voltage of the substation constant. This is also equivalent to segmenting the long-distance transmission line and reducing the line reactance.