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

Power system stability analysis should be carried out from two aspects: static stability and transient stability. Generally speaking, the system with high static stability has higher transient stability. Static stability refers to the ability of a system to maintain its own stability under normal operation mode. If a system cannot fully maintain its stability under normal operation mode, it is more difficult to ensure the stability after large interference, 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 the stability of the system after large interference is considered, it is more difficult to maintain the transient stability of the system than the static stability of the system, and there are more measures accordingly. We discuss the measures to improve the stability of power system from these two aspects.

1、 Measures to improve static stability

The static stability of power system refers to that after the power system is subject to some small disturbance and the disturbance disappears, there is no self-excited oscillation or asynchronous out of step. The ability to automatically return to the original running state.

As can be seen from the simple system power angle characteristic equation below, under the condition of certain 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 receiver voltage can be increased. Reduce reactance. To improve the power potential and system voltage, first of all, the system and generator are required to have sufficient reactive power supply; To reduce the reactance, it is necessary to increase the power capacity. At the same time, shorten the "electrical distance" between the generator and the system.

1. The generator adopts automatic regulating excitation device

When the generator does not adopt the automatic regulating excitation device, the no-load potential EQ is constant and the reactance of the generator is synchronous reactance XD. When the automatic regulating excitation device is adopted, the generator can make EQ 'or VG constant. It means that VD 'is a constant, while XD' does not play a role in the stability of the system. Therefore, the installation of advanced automatic regulating excitation device in the generator is equivalent to shortening the "electrical distance" between the generator and the system. Because of the low price and obvious effect of installing the automatic regulating excitation device, it is a measure to improve the static stability.

2. Reduce line reactance

Reducing the line reactance and strengthening the connection between systems can improve the static stability limit and the stability degree. The following methods can be adopted to directly reduce the line reactance: 1) replace the overhead line with cable; 2) . adopt expanded diameter conductor; 3) . split conductor is adopted. The first two methods are difficult to be generally realized due to high investment or other technical problems. Therefore, the method of directly reducing the line reactance is mainly to use split conductor. For example, for 500kV overhead lines, when a single conductor is used, the reactance is about 0.43 Ω / km; When three bundle conductor is adopted, it is about 0.3 Ω / km; The reactance value decreased 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 improve the static stability limit and the level of static stability. However, increasing the voltage level requires increasing investment, especially the system has sufficient reactive power supply.

4. Series capacitor compensation

Series capacitor compensation can improve the static stability of power system by adjusting voltage and reducing line reactance. In the latter case, the compensation degree shall be determined by calculation. Generally speaking, the greater the compensation degree, the smaller the equivalent reactance of the line, which is beneficial to improve the stability. However, when the compensation is too large, there will be a series of problems: causing the damping power coefficient D to be negative, causing the spontaneous low-frequency oscillation of the system, it is easy for the generator to generate self excitation, making it difficult for the relay protection operation, increasing the short-circuit current, etc. 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 lines, it is installed at the midpoint and for single power 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 power system. The methods are as follows: 1) increase the transmission line circuit and reduce the line reactance; 2) Strengthen the internal connection of each system at both ends of the line to reduce the equivalent internal reactance of the system; 3) . connect 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, so as to reduce the reactance; 4) The step-down transformer in the middle of the transmission line is equipped with a synchronous condenser, which is equipped with advanced automatic regulating excitation device, which can maintain its terminal voltage and even the voltage of the high-voltage bus of the substation. In this way, it is also equivalent to the segmentation of long-distance transmission line, which reduces the line reactance.

2、Measures to improve transient stability

Improving the transient stability of power system refers to whether the system can reach a new stable operation state or return to the original state through the transient process after a sudden large interference under a certain operation condition. However, due to the sharp interference of the operating system, there will be a large difference between the electrical excitation rate of the generator and the mechanical power, which is the main reason for the destruction of the transient stability of the system. Therefore, the measures to improve transient stability should first consider the temporary measures to shorten the action time of unbalanced power and reduce the power difference.

1. Quick cut out fault

After failure, the power difference on the rotor shaft, i.e. unbalanced power, will accelerate the rotor. According to the equal area rule, in order to obtain transient stability of the system, the acceleration area must be reduced and the deceleration area must be increased as much as possible. Only in this way can the accelerated rotor return to the synchronous speed and restore the normal synchronous operation of the system. The most direct way to reduce the acceleration area is to quickly remove the fault. Another positive effect of fast fault removal is that it can make the motor terminal voltage rise rapidly, reduce the risk of motor stall and stop, and improve the stability of load operation. In order to realize fast fault removal, fast acting relay protection device and fast acting circuit breaker must be selected.

2. Reclosing device is adopted

Most of the faults of power system, especially high-voltage transmission line, are transient faults rather than permanent faults. The automatic reclosing device is adopted, that is, after the fault occurs and the circuit breaker disconnects the fault line, the automatic reclosing device will put the line into operation again after a certain time. If the fault 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 also is beneficial to the transient stability of the system. The faster the reclosing action is, the better the stability is. However, the action time of reclosing is limited by the de dissociation time at the short circuit. In general, arc often appears at short-circuit points. If the reclosing is too fast, the short-circuit points that produce arc may rekindle the arc due to insufficient de dissociation, making the reclosing unsuccessful and even expanding the fault. Especially for single-phase reclosing, the latent current generated by the phase capacitance and mutual inductance between the fault phase and the two normal phases maintains the combustion of the arc and prolongs the de dissociation time. The failure of reclosing is very unfavorable to transient stability, which is equivalent to a big impact on the system in a very short time. At the same time, it increases the burden of the circuit breaker, which should be paid attention to in practical use. If the unsuccessful reclosing increases the acceleration area and makes the system lose transient stability, measures should be taken to avoid this result.

3. Forced excitation

When the voltage at the generator end is reduced due to external short circuit, so as to reduce its output electromagnetic power, strong excitation device can be used to increase its electromagnetic power output and reduce the unbalanced power of rotor. General generator automatic regulation excitation system has forced excitation device. When the generator terminal voltage VG is lower than 85% of the rated voltage, the low voltage relay acts, and the regulating resistance of the excitation device is forcibly short circuited through the intermediate relay, which greatly increases the excitation current of the exciter. Thus, the excitation current and excitation voltage of the generator increase rapidly, so as to improve the generator potential and increase the electromagnetic power output. So as to reduce the unbalanced power of the rotor and improve the transient stability.

4. The neutral point of transformer is grounded through small resistance

When asymmetric grounding short circuit occurs in neutral grounded power system, zero sequence current component will be generated. If the neutral point of the star connected transformer in the system is grounded through a small resistance, power loss will occur in this resistance when the zero sequence current flows. This power loss can reduce the unbalanced power of the rotor and is conducive to the transient stability of the system. At the same time, small grounding resistance is connected, which is reflected in the positive sequence augmented network, which is equivalent to increasing the additional impedance, reducing the system connection impedance and increasing the electromagnetic power. The size and installation location of grounding resistance shall be determined through calculation. Generally, the value of grounding resistance is close to the short-circuit reactance value of transformer.

5. Reduce the mechanical power output of prime mover

When the fault reduces the electromagnetic power, if the mechanical power output by the prime mover can be reduced, the residual power acting on the rotor can be reduced and its transient stability can be improved. The measures to reduce the mechanical power output of prime mover are as follows: 1) for steam turbine, fast automatic speed regulation system or fast closing of inlet valve can be adopted; 2) . interlocking machine cutting, that is, while cutting off the fault, interlocking cutting off one or several generator units in the sending end power plant; 3) The method of mechanical braking, i.e. direct braking of rotor, is adopted to reduce the power difference, so as to improve the transient stability of power system operation.