Cooperative Control Of Power System Frequency Based On Differential Games Theory

With the enhancement of the penetration of wind power and other intermittent power tothe power grid, their characteristics such as large variation in output and being difficult topredict, will exacerbate the imbalance of active power and the frequency fluctuations.Moreover, wind turbines are sensitive to the power grid disturbance and easy quit running, tofurther deteriorate the coordination problems among frequency regulation devices.The first is the coordination problem between primary frequency control and secondaryfrequency control in the same area. Primary frequency control responses to a local frequencydeviation from its set value, and requires change in generating unit power output via the speedgovernor, which is a local automatic control. Secondary frequency control responses to thearea control error, and is implemented by the coordinated control system of the frequencyadjustment units, which is a region-wide control. Primary frequency control aims at the rapidrelief of the power shortfall, while secondary frequency control aims to achieve the areapower balance. Their control modes and control objectives are different, but they decide theunit output simultaneously, sometimes leading the anti-regulation phenomenon to take placebetween them. The frequent power fluctuations brought by wind turbines make thisphenomenon even more serious.The second is the coordination problem between the secondary frequency controls ofdifferent areas in interconnected system. In the electricity market environment, each area gridgradually evolves into a separate stakeholder. They pursuit their local frequency stability, andmaking some power support to other areas during the regulation, but they desire their localcontrol cost to be minimum meanwhile, which may impact the control effect of other areas.The large-scale integration of wind power will make this confliction more obvious because ofthe uneven geographic distribution of wind.This paper applies the differential games theory to solve these two problems. Byconsidering the interaction between agents, differential games theory becomes the effectivemeans to solve multi-agent multi-objective decision-making problems. In this paper, twocooperative control models are built, of which the first one is about the primary and secondaryfrequency control in the same area, and the second one is about the secondary frequencycontrols in different areas. Two cooperative controllers are also proposed, which are thecontroller based on open-loop Nash equilibrium and the controller based on feedback Nashequilibrium. Taking a two area interconnected system as the simulation example, we comparethe control effect of the two cooperative controllers with that of the conventional PI controller and the multi-objective optimal controller. Analysis shows that the cooperative controllers canmore effectively alleviate the conflicts between devices, reducing the number of unnecessaryregulation and adjustment, and the controller based on feedback Nash equilibrium has a moresuperior performance in aspects of the dynamic consistency property and the fairness of thepolicy.At last, we add some complex constraints, such as the dead zone of governor, the limit ofcontrolling movement amplitude and the constraint of units climbing speed, to the originalsimulation model, and explore how to obtain the control strategy of the controller based onfeedback Nash equilibrium in this case. Experiment result shows the proposed coevolutionalgorithm can obtain the equilibrium strategy which meets the complex constraints, achievingthe approaching to the practical application of power system.