Studies On Effect Of Different Load Models On Voltage Stability

Currently, electric power networks dispersed geographically in China are being interconnected with the emphasis on transferring large amount of power from the west region to the east and keeping a generation/load balance between the south region and the north. The most traditional system analysis method are carried on under the static environment, or the simpler systematic dynamic link, in this case, the unsafe conclusions are always available. Compared with the models of generator and transmission network that have been developed a lot, load model develops too slowly. Taylor C.W. said that the problem of voltage stability is the problem of load stability. So load modelling plays an important role in the research field of voltage stability. We will draw different conclusions even errors when do some power system simulation or calculation using different load models. Therefore, it is very important to analyze the effect on voltage stability using different load model.In this thesis, studies are made mainly on the saddle-node bifurcation of voltage stability in power system based on bifurcation theory of the non-linear dynamics analysis method. In addition, preliminary research is made on Hopf bifurcation and singularity induced bifurcation. The main body of this thesis consists of the items as follows:1. Firstly, taking a single machine-single load system as an example, the emphasis are using contiuation power flow method and bifurcation theory to research the effects of static and dynamic load model on the voltage stability. Speaking on the voltage stability, constant power model is easier to lose stability than constant current model, and constant impedance model won't have the voltage stability problem. When mechanical transient model and mechanical electric transient model of induction motor were used to describing electricity load, the saddle node bifurcation points were very close.2. Taking New England 10 machines 39 nodes system as an example, the saddle node bifurcation will be found when constant impedance load model is used. The limit of load power is larger than constant power and constant current load model, and the threshold voltage is much lower. The margin of system voltage stability is larger as the rate of constant impedance load in total load is increasing. And the system stability is poorer with the rate of constant power load being larger.3. Taking WSCC 3 machines 9 nodes system as an example, the emphasis are using contiuation power flow method and bifurcation theory to research the effects of synthetic load model on the voltage stability. The effect of different parts of synthetic load model on voltage stability is different. Var compensation at load bus can improve stability limit and voltage level of stable equilibrium points. The rate of induction motor in synthetic load model have greater effect on system stability limit. The effect of static model components becomes greater with its rate in synthetic load model increasing, and the voltage level of stable equilibrium points increases, too.4. Based on above work, considering dynamic characteristic of generator, in addition to saddle node bifurcation we found Hopf bifurcation points when using bifurcation theory to research. At the Hopf bifurcation points system begin to occur oscillatory voltage instability. Appropriate var compensation can shorten or even vanish the instability area of system or make it down to the curve of PV so that the stability area broaden. When the rate of induction motor in synthetic load model increasing, the load margin decreases, Hopf bifurcation points and instability area go up along the PV curve, and then the system becomes easily to lose stability.5. In this thesis, changing parameter and program expanding is conveniencely in the whole program. It has comparative practicability.