Calculation and Simulation Study on Transient Stability of Power System Based on Matlab / Simulink

The stability of the power system is destroyed, will cause a large number of users power outage, even cause the collapse of the whole system, extremely serious consequences. Based on the analysis in single machine infinite system as an example, when at the f point two phase ground fault occurs, the fault lines on either side of the circuit breaker tripping resection at the same time,respectively by two kinds of calculation and simulation methods of system transient stability analysis , the conclusion are consistent. and the simulation analysis is superior to calculation analysis .


Introduction
The failure in stability of the power system would cause power outage to many users or even collapse of the whole system, thus bringing very serious consequences.
Therefore, the study on the stability of the power system is of great significance in ensuring safe and reliable operation of the power system.After the power system is subject to a large disturbance, the imbalance between mechanical torque and electromagnetic torque on the generator rotor leads to a change in the relative position between synchronous generator rotors, i.e. the current, voltage and electromagnetic power of the system are changed as the relative angle between generator potentials is changed.
The study on the transient stability of the power system refers to researching whether parallel operated synchronous generators can still operate synchronously and the load is normal after the power system is subject to a large disturbance in an operation mode.The reasons for the large disturbance mainly include: short circuit fault; turning on or off main components of the system, such as generator, transformer and line; sudden change in load, such as addition of large users.The short-circuit fault is usually used to check the transient stability of the system as it is the most severe among them.

Calculation of transient stability of power system
Taking the diagram of single-machine infinite bus system shown in Fig. 1 as an example, the transient stability of the power system is calculated after the faulty line is cut off through simultaneous tripping of circuit breakers on both sides of the line in the case of two-phase ground fault at the point f.The parameters of components in the single-machine infinite bus system shown in Fig. 1 are as follows [1] : Parameters of the generator G: Operation: U 0 =115KV, P 0 =250MW, cosφ 0 =0.98.The equivalent circuit during the normal operation of the system is shown in Fig. 2a), and integrated impedances of the system are as follows:

Calculation of transferring impedance and power characteristic of the system
The equivalent circuit during the normal operation of the system, negative sequence and zero sequence equivalent The additional reactance at the short circuit point in the case of two-phase grounding short circuit is:     The simulation curve in Fig. 6 shows that the speed of the generator gradually decreases with the increase of the time (variation range: 0.99~1.01)and tends to become stable constants in the case of two-phase grounding short circuit fault at the point f after cutting off the faulty line after 0.1s (the clearing time is less than the limit clearing time), therefore the system is stable; the simulation curve in Fig. 7 shows that the speed of the generator gradually increases with the increase of the time in the case of two-phase grounding short circuit fault at the point f after cutting off the faulty line after 0.5s (the clearing time is greater than the limit clearing time), therefore the system is unstable

Conclusion
To sum up, the limit clearing angle obtained by calculating the transient stability of the power system is 64.3 0 , and the limit clearing time obtained with Ronge-Kuta method in Matlab is 0.2s based on the limit clearing angle.If the clearing time is 0.1s (less than the limit clearing time), the maximum angle δ obtained with Ronge-Kuta method in Matlab is 88.44 0 , which is less than 180 0 , and the vibration amplitude value is becoming smaller and smaller, therefore the system is stable transiently.andfor power system transient stability simulation analysis obtained the system is also stable transiently as the speed of the generator gradually decreases with the increase of the time after cutting off the faulty line at 0.1s after the fault.Therefore, the conclusions are consistent In comparison, the simulation analysis is superior to calculation, the reason is the complicated calculation can be avoided.

2
Calculation results of per unit values of operating parameters are as follows: networks in the case of two-phase short circuit at the point f, equivalent network in the case of short circuit and equivalent network after clearing of short circuit are shown in a), b), c), d) and e) in Fig. 2 respectively.

Fig. 2 a
Fig. 2 a) Equivalent circuit during the normal operation b) Negative sequence network c) Zero sequence network d) Equivalent network in the case of short circuit e) Equivalent network after clearing of short circuit As seen in b) negative sequence network and c) zero sequence network in Fig. 2, the negative sequence and zero sequence equivalent reactance at the short circuit

3
Fig.2, the transferring impedance and power characteristic of the system are: