The loss of stability of power system in operation is the most serious accident of power system. Therefore, in the design and operation of the power system, 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. In general, a system with high static stability will have higher transient stability. Static stability refers to the ability of a system to maintain its own stability under normal operation mode. Even if a system cannot maintain its stability completely in normal operation mode, it will be more difficult to guarantee the stability after large interference, namely transient stability. Therefore, in order to improve the static stability of the system, fundamental measures must be taken, that is, to increase the stable reserve of the system and reduce the electrical distance. As for transient stability, it is more difficult to maintain transient stability than static stability of the system because the stability of the system is considered after a large disturbance, so there are more measures. We discuss measures to improve the stability of the power system from these two aspects respectively.
Measures to improve static stability
Static stability of power system refers to that the power system is subjected to a small disturbance and the disturbance disappears without self-excited oscillation or non-synchronous out-of-step. The ability to automatically revert to the original operating state.
As can be seen from the following simple system power-angle characteristic equation, under the condition of a certain transmission power, the higher the possible limit power of the generator is, the higher the static stability limit is, and the corresponding static stability performance is better. The static stability limit can be increased by increasing the power potential and terminal voltage. Reduce reactance. To improve the power potential and system voltage, the system and generator should have enough reactive power. To reduce the reactance, it is necessary to increase the power supply capacity. It also shortens the "electrical distance" between the generator and the system.
1, the generator adopts automatic adjustment excitation device
When the generator does not use the automatic adjusting excitation device, the no-load potential Eq is constant, and the reactance of the generator is synchronous reactance Xd. With automatic excitation, the generator can be set to Eq 'or Vg as a constant. Eq 'being constant means that Xd decreases to Xd', while Vg being constant means that Xd will have no effect on system stability. Therefore, the installation of advanced self-regulating excitation devices in the generator is equivalent to shortening the "electrical distance" between the generator and the system. Because of its low cost and obvious effect, the automatic excitation device is the first choice to improve the static stability.
2, reduce the line reactance
The static stability limit and stability degree can be improved by reducing the line reactance and strengthening the connection between systems. The following methods can be used to directly reduce the line reactance: 1) Replace overhead lines with cables; 2) Using expanding wire; 3) Use split wire. The first two methods are difficult to achieve because of high investment or other technical problems. Therefore, the method to directly reduce the reactance of the line is mainly to split the wire. For example, for 500kV overhead line, when using a single wire reactance is about 0.43 ω /km; It is about 0.3 ω /km when using tri-split conductor; Reactance dropped by a third. Therefore, 220kV and above systems mostly use split wire.
3. Improve the rated voltage level of the line
It can be seen from the power-angle characteristic equation that the static stability limit and the static stability level can be improved by increasing the rated voltage grade of the line. But raising the voltage level requires additional investment, especially if the system has enough reactive power.
4, using series capacitor compensation
Series capacitor compensation can improve the static stability of power system by regulating voltage and reducing line reactance. In the latter case, the degree of compensation shall be determined by calculation. Generally speaking, the larger the compensation degree is, the smaller the equivalent reactance of the line is, which is beneficial to improve the stability. However, when the compensation is too large, a series of problems will occur: damping power coefficient D is negative, causing the system spontaneous low-frequency oscillation, easy to generate generator self-excitation, causing difficulties to relay protection operation, increase 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.
The series capacitor compensation generally adopts centralized compensation. For dual power lines installed at the midpoint, for single power lines installed at the end.
5. Improve the system structure
The stability of power system can be improved by improving system structure and strengthening system connection. The methods are as follows: 1) Increase transmission line loop and reduce line reactance; 2) Strengthen the internal connection of the systems at both ends of the line to reduce the equivalent internal reactance of the system; 3) Access to the intermediate power system, so that the voltage in the middle of the long-distance transmission line can be maintained constant, equivalent to dividing the transmission line, so as to reduce the reactance; 4) The step-down transformer in the middle of the transmission line is equipped with synchronous adjusting camera, and the synchronous adjusting camera is equipped with advanced automatic adjustment excitation device, which can maintain the terminal voltage and even the high voltage bus voltage of the substation to be constant. In this way, it is equivalent to the segmentation of long distance transmission lines and reduces the line reactance.
Measures to improve transient stability
Improved transient stability of power system refers to whether the system can reach a new stable operation state or recover to the original state after being suddenly disturbed by a large transient process in a certain operation condition. However, when the system is disturbed rapidly, there will be a big difference between the excitation rate and the mechanical power, which is the main reason for the transient stability failure of the system. Therefore, the measures to improve the transient stability should first consider the temporary measures to shorten the time of unbalanced power and reduce the power difference.
1. Fast cutting out of failure
After the failure, the power difference on the rotor shaft, namely the unbalanced power, will accelerate the rotor. According to the equal area rule, in order to achieve transient stability of the system, the acceleration area must be reduced as far as possible, and the deceleration area must be increased. In this way, it is possible to speed up the rotor back to the synchronous speed, so that the system to restore normal synchronous operation. The most direct way to reduce the acceleration area is to remove faults quickly. Another positive effect of fast fault removal is to make the motor terminal voltage rise quickly, reduce the danger of motor stall, improve the stability of load operation. In order to realize fast fault cutting, fast relay protection device and fast circuit breaker must be selected.
2. Use reclosing device
Most of the faults of power system, especially the high voltage transmission lines, are transient rather than permanent. Using automatic reclosing device, is when the fault occurs and the circuit breaker will break off the fault line, after a certain period of time by the automatic reclosing device will make the line into operation again. If the faulty line is transient, the system may be restored to normal once the circuit breakers overlap. 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 more beneficial it is to the stability. However, the reclosing action time is limited by the dissociation time of the short circuit. The general short circuit point often appears arc, if the coincidence is too fast, the short circuit point of the arc, may cause the arc to reignite because of insufficient dissociation, so that the reclosing is not successful, and even expand the fault. Especially for single-phase reclosing, the latent power supply flow generated by the capacitance and mutual inductance between the fault phase and the two normal phases maintains the combustion of the arc and lengthens the dissociation time. Unsuccessful reclosing is detrimental to transient stability, which is equivalent to another big shock to the system in a very short period of time. At the same time, it increases the burden of circuit breakers, which should be paid attention to in practical use. In general, measures should be taken to avoid the situation that the system loses transient stability due to the failure of reclosing.
3, forced excitation
When the voltage of the generator is reduced due to the external short circuit, so that the output electromagnetic power is reduced, the strong excitation device can be used to increase the electromagnetic power output and reduce the unbalanced power of the rotor. The general generator automatic excitation system has a forced excitation device. When Vg of the machine is lower than 85% of rated voltage, the low voltage relay will operate and short-circuit the regulating resistance of the excitation device through the intermediate relay, so that the excitation current of the exciter will be greatly increased. So that the generator excitation current, excitation voltage are rapidly increased, to improve the generator potential, increase the electromagnetic power output. Thus, the unbalanced power of the rotor can be reduced to improve the transient stability.
4, transformer neutral point grounding by small resistance
When an asymmetric grounding short-circuit occurs in a neutral grounded power system, a zero-sequence current component is generated. If the neutral point of the transformer with star wiring in the system is grounded through a small resistance, the zero-sequence current flowing through this resistance will produce a power loss. This power loss can reduce the unbalanced power of the rotor and is beneficial to the transient stability of the system. At the same time, access to small grounding resistance, reflected in the positive sequence augmented network, equivalent to increase the additional impedance, reduce the system connection impedance, but also improve the electromagnetic power. The size of the grounding resistance and the installation location should be determined by calculation. Generally, the grounding resistance value is approximately close to the short-circuit reactance of the transformer.
5, reduce the prime mover output mechanical power
When the electromagnetic power is reduced due to the fault, 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 mechanical power measures to reduce the prime mover output are: 1) for the steam turbine can use a fast automatic speed control system or fast closing the intake valve; 2), the chain cutting machine, that is, when the fault is removed, the chain cutting of one or several generator sets in the power plant; 3) Adopt mechanical braking, namely direct braking of the rotor, to reduce the power difference and improve the transient stability of the power system operation.
