What are the two main aspects to improve the stability of the power system?

The loss of stability in the operation of the power system is the most serious accident of the power system. Therefore, 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 resulting losses, and restore the normal operation of the system as soon as possible.

The stability analysis of the power system should be conducted from two aspects: static stability and transient stability. Generally speaking, a system with a high degree of static stability also has a 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 under normal operation, it is even more difficult to ensure 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, namely, increasing the system stability reserve and reducing 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 measures to improve the stability of the power system from these two aspects.

1. Measures to improve static stability

The static stability of a power system refers to the ability of the power system to automatically recover to its original operating state after a slight disturbance disappears without self-excited oscillation or asynchronous loss of step.

From the simple system power-angle characteristic equation, we can see 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 increase the static stability limit, the power supply potential and the receiving end 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, the "electrical distance" between the generator and the system can be shortened.

1. The generator adopts automatic excitation device

When the generator does not use an automatic excitation device, the no-load potential Eq is a constant, and the reactance of the generator is the synchronous reactance Xd. When an automatic excitation device is used, the generator can make Eq' or Vg a constant. Eq' being a constant means that Xd is reduced to Xd', and Vg being a constant means that Xd will not have any 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 improve the 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 main method to directly reduce line reactance is 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, 220kV and above systems mostly use split conductors.

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 increased investment, especially requiring the system to have sufficient reactive power supply.

4. Use series capacitor compensation

Series capacitor compensation can be used to adjust the voltage or to improve the static stability of the power system by reducing the line reactance. In the latter case, the compensation degree should be determined by calculation. Generally speaking, the greater the compensation degree, the smaller the line equivalent reactance, 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 self-excite, 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 the system structure and strengthening the system connection can improve the stability of the power system. The methods are: 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) accessing 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. In this way, it is also equivalent to segmenting the long-distance transmission line and reducing the line reactance.

2. Measures to improve transient stability

Improving transient stability of power system refers to whether the system can reach a new stable operating state or return to the original state after a sudden large disturbance under certain operating conditions. However, after the running system is subjected to a sharp disturbance, a large difference will appear between the generator's excitation rate and mechanical power, which is the main reason for the destruction of transient stability of the system. Therefore, measures to improve transient stability should first consider temporary measures to shorten the time of unbalanced power action and reduce the power difference.

1. Quickly cut out the fault

After a fault occurs, the power difference on the rotor shaft, i.e., unbalanced power, will cause the rotor to accelerate. According to the equal area rule, in order to obtain transient stability of the system, the acceleration area must be reduced as much as possible and the deceleration area must be increased. Only in this way can the accelerated rotor return to synchronous speed and the system resume normal synchronous operation. To reduce the acceleration area, the most direct way is to quickly remove the fault. Another positive effect of quickly removing the fault is that the motor terminal voltage can be quickly restored, reducing the risk of the motor stalling and stopping, and improving the stability of load operation. In order to achieve rapid fault removal, fast-acting relay protection devices and fast-acting circuit breakers must be selected.

2. Use reclosing device

Most of the faults in the power system, especially the high-voltage transmission lines, are transient faults rather than permanent faults. The automatic reclosing device is used, that is, when a fault occurs and the circuit breaker disconnects the faulty line, the automatic reclosing device will put the line back into operation after a certain period of time. If the faulty line is transient, the system may resume normal operation after the circuit breaker is reclosed. This not only improves the reliability of power supply, but is also beneficial to the transient stability of the system. The faster the reclosing action is, the more beneficial it is to stability. However, the action time of the reclosing is limited by the deionization time of the short-circuit. Generally, arcs often appear at short-circuit points. If the reclosing is too fast, the short-circuit point where the arc is generated may reignite the arc due to insufficient deionization, making the reclosing unsuccessful and even expanding the fault. Especially for single-phase reclosing, the latent current generated by the phase-to-phase capacitance and mutual inductance between the faulty phase and the two normal phases maintains the burning of the arc, which prolongs the deionization time. Failure of reclosing is very unfavorable to transient stability, which is equivalent to giving the system a big shock in a very short time. At the same time, it increases the burden on the circuit breaker, which should be paid attention to in actual use. If the reclosing is unsuccessful, the acceleration area will increase and the system will lose transient stability. Generally, measures should be taken to avoid this result.