Generator Rotor Angle Control And Its Application In Power System Operation

The power system operation and control face more problems with the development of smart grid and clean energy (like wind power and photovoltaic). The problems include how to solve traditional problems (like low frequency oscillations) better and how to implement distributed autonomous control (with central coordination). A new principle is proposed in this paper, namely, the principle of generator rotor angle control. This principle tries to fix generator rotor positions in GPS coordinate system so that their relative position can be fixed. This method can suppress low frequency oscillation, increase transient stability, implement automatic load following at the same time. The researches on these aspects are of great theoretic and possible practical value.Currently PSS provide damping torque by measuring speed and relative position between local generators. This mechanism cannot take effect between generators far apart. Consequently, remote speed (and position) informations are needed to suppress inter-area low frequency oscillations. This problem can be solved if the angle position of every generator rotor is fixed in the spinning coordinate system determined by GPS. This way, the oscillations can be suppressed more easily, and the power system will change from chaos to order.The rotor angle controller can also improve transient stability. For the time being, the absolute rotor angle of every generator is changing from time to time, so it’s less likely to improve transient stability by using the rotor angle measurement. Things are no longer so after absolute rotor angle controllers are deployed across the power system. The frequency of the power system will remain constant, and the absolute rotor angle of every generator will be kept at the aim value given by the dispatching center most of the time. After a fault happens, the angle controller can then give order to turbine valve or power electronic braking devices to accelerate or decelerate the generator rotor so that it comes back to its original position. Consequently, stability and order will be restored. This is the reason why rotor angle control can improve transient stability.Some experiments are carried out in the laboratory of Shandong University to verify the practicability of the new principle and the new load following mechanism. Physical experiment shows that:(1) It is possible to implement absolute rotor angle control based on measurement and control technology nowadays. All generator rotor angles can be fixed on their aim values in the GPS coordination system. There’s no stability/controllability/observability problem. (2) The rotor angle of a generator stops changing after it is connected to a power system that runs on fix angle mode. Besides, if a power system changes to variable angle mode after large disturbance, it can come back to fix angle mode easily when the frequency deviation is small enough. (3) The physical experiments also show that the generators operating on different modes (constant power/primary frequency control/rotor angle control) can run side by side. The primary frequency control and AGC currently used in power system can be fulfilled in a new manner under the new control strategy. The generators can follow loads quickly and precisely without intervention of a dispatching center. The increase of any load will be shared between all generators running on fix angle mode. Besides, if a generator running on fix angle mode reaches its output up-limit, it will lose the ability to maintain angle. But nearby generators will provide assistance if the loads keep on growing, so system frequency can be kept at the rated value.Because the magnitude of the generator internal potential source changes dramatically for different levels of load demands, the DC power flow calculation method should not be used for power systems running in the angle control mode. The Newton-Raphson method or a new power flow calculation method derived from the PQ decoupled method should be used instead. The differences between the new method and the traditional PQ decoupled method are as follows: When calculating B matrix before the iteration, the diagonal elements should be modified by adding the corresponding d-axis admittance. During the iteration, this mtrix should be modified again after voltage magnitude corrections are got. The angle corrections of all buses can be got by using the modified matrix. The new magnitude of generator internal source can then be calculated. The new method also converges quickly. It may be the recommended power flow calculation method since the algorithm and codes currently used can be reused after slight modification.Please note that, since the position (and the speed) variations reflect active power imbalance, changing turbine mechanical power is selected as the means to fulfil rotor angle control in this paper. However, this is not the only choice. The principle of rotor angle control can also be fulfilled by exciter control since exciters act more quickly and precisely when compared with turbines. But the benefits will take other forms at that time. From this point of view, what’s important is not the forms (and possible difficulties) of methods used in this paper, but the idea (and the benefits) behind the appearance.The main achievements of our research are as follows:1) The principle of generator rotor angle control and its implementation by means of PID control. The main aim of the principle is to fix generator rotor position is the coordinate system determined by GPS, so that the form composed by generators remain unchanged, and system frequency be fixed at the rated value.The automatic balancing between generation and loads can also be fulfilled after this principle is implemented across the system.2) The method to suppress inter-area low frequency oscillations more effectively. After the angle controllers are deployed, the rotor angle and speed remain constant under normal conditions, so it is possible to get information of all kinds of oscillations (including inter-area low frequency oscillation) through local measurements. When used together with PSS, the angle controller can suppress inter-area oscillations effectively without help of any remote measurement. Simulation also proves its effectiveness.3) The method to improve transient stability by means of rotor angle control. After fault happens, rotor angle controller can give orders to turbine or braking devices to accelerate or decelerate the generator rotor so that it moves back towards its original position. The decelerating area can be increased by these means. Simulation results in both single machine infinite bus system and multi machine system are given. The critical clearing time can be increased dramatically in both cases.4) The new load following mechanism and its experimental verification. Analysis shows that the primary frequency control and automatic generation control currently used can be fulfilled in a new manner under the new control strategy. The generators can follow loads quickly and precisely. The load balancing can be reached without intervention from the dispatching centre. Moreover, the new mechanism can distribute load variance mainly to nearby generators. This can decrease network losses and tie line power flow fluctuations.5) The new power flow calculation method used under rotor angle control mode. A new method is derived from PQ decoupled method, and a comparison is made between the new method and the traditional power flow calculation method.