Control of Voltage Collapse

The effect of the tap changer ratio of the power system is that the system Hopf range can be decreased or even eliminated by suitable changing tap changer ratio.

Further, with the existence of power system equilibria, it is found that the power system stable region can be enlarged by changing tap changer ratio. Moreover, the control of Hopf bifurcation is also a good design of prevention of voltage collapse.

The details are discussed in [39].

In Chapter 3, we succeed to develop a means to detect the occurrence of voltage collapse in a power system and to generate a warning signal to admonish us. We also find that we can regulate the voltage level to raise the electric quality of the electric power system by utilizing tap changer in Chapter 4. Here, we will utilize the tap changer to prevent the occurrence of voltage collapse at the time of the detected equipment send out a warning signal. A detail scheme is shown in Figure 5.1.

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Figure 5.1: A scheme of prevention of voltage collapse

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5.2 Simulation Results

Figures 5.2-5.3 show two examples of proposed scheme. In these simulations, the system initial state is x0 = [0, 0, 0, 1.1], the reactive power demand of the load is Q1 = 11.3. It makes the power system operation exceeding its stability limit. It is observed from Figure 5.2(b) and 5.3(b) that the system undergoes voltage collapse for such heavy load. Figure 5.2(b) shows the load voltage collapse around t=1.13.

However, by properly adjusting the threshold, we can detect it before it occurs. In these simulations, we select threshold to be 0.06 p.u. As displayed in Figure 5.2(d) and 5.3(d), the voltage collapse can be successfully detected and the alarm signal is fired before t=1. It makes us having enough time to take a proper control action to avoid such instability. In Figure 5.2(e), we increase tap changer ratio to enlarge the power system stable region. It is found in Figure 5.2(f) that the voltage collapse behavior disappears and the load voltage reaches a new equilibrium point after a transient of oscillation. Furthermore, we shall employ the VSC control law designing in section 4.1 to achieve regulating the load voltage. This can be seen from Figure 5.3(e) and Figure 5.3(f). Simulation results have demonstrated the effectiveness of our proposed scheme of prevention of the voltage collapse.

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Figure 5.2: (a) load variation Q( 1) (b) time response of load voltage without control (c) residual signal (d) alarm signal by FIDF (e) tap changer ratio n (f) time response of load voltage with control given by (e)

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Figure 5.3: (a) load variation Q( 1) (b) time response of load voltage without control (c) residual signal (d) alarm signal by FIDF

(e) variation of tap changer ratio n

(f) time response of load voltage with control given by (e)

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CHAPTER 6

Conclusions and Suggestions for Further Research

In this thesis, we have studied the detection of voltage collapse using a Dobson and Chiang's power system model. By treating the difference between the output of the power system model and that of its linearized model as a fault vector and employing a FIDF design technique, the occurrence of voltage collapse is shown to be successfully detected by inspecting the residual signal generated from the FIDF. The performance of detecting voltage collapse depends on the setting of the threshold. Simulations in Chapter 3 are given to demonstrate the effectiveness of this approach.

To raise the voltage quality of power supply for satisfactory operation of a power system, we add an extra tap changer parallel to the nonlinear load to Dobson and Chiang's power system model. We have applied Variable Structure Control design scheme to adjust the tap changer ratio to achieve voltage regulation. According to the

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simulations in Section 4.1, it is shown that the designed controller can achieve the desired performance. However, in practical power system, the system dynamics may have well known dynamics at the beginning, but will experience unpredictable load variation as the control operation goes on. For this reason, we have proposed a parameter estimator as the load monitor to provide the load variation of the power system in Section 4.2. It provides the accurate load variation to the VSC voltage controller to have better regulating capacity.

To prevent the voltage collapse, we have proposed a scheme of prevention of voltage collapse based on prior designs. We utilize the tap changer to prevent the occurrence of voltage collapse. At the time of the detected equipment send out a warning signal, we tune the tap changer ratio to prevent the occurrence of voltage collapse. Further, we have employed the VSC controller to regulate the load voltage.

Simulations in Chapter 5 demonstrate the effectiveness of this scheme.

In the following, we indicate some directions for further research. Firstly, for detection of voltage collapse, we provide a means for quick detection of voltage collapse but not the only one. Recently, the issue of detecting voltage collapse has attracted more and more attention [12,14,30]. It is a way for further research.

Secondly, for voltage control, in addition to the use of Tap changer, another feasible means can also be added, such as shunt capacitors or series capacitors. To consider voltage control with capacitors is also a direction of study. Finally, in this thesis, we focus on voltage control. However, an efficient and reliable operation of power systems should have the property that the voltage and frequency should remain nearly constant. Thus, the final direction is frequency control.

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