Theoretical Study Of Power System Synergistic Economic Dispatch

With the rapid development of China’s social economy, energy demand is rapidly increasing, which leads to the shortage of energy and environmental pollution problem. Thus, optimizing and adjusting the structure of energy utilization, reducing the dependence on non-renewable fossil energy, and the development of clean renewable energy have become an inevitable choice. Electric power industry is an important part of energy utilization, and in this situation the development of conventional fossil energy power generation has been slowed down or curbed, and renewable energy generation including wind power and solar photovoltaic has got rapid development. With the development of renewable energy generation, electric vehicles, energy storage, controllable and interruptible load and other technologies continually emerge.Under this background, the composition of generation and load in power system presents a diversified development trend, and the mode of active power balance in power system is increasingly complex. At the same time, due to the uncertain characteristics of the renewable energy generation with strong randomness and intermittent, the uncertainty in power system dispatch and control is greatly enhanced. To achieve power balance in the allowed frequency range, it is necessary to make full use of various active resources in power system operation, so that the limited amount of active variables can cope with the uncertainty of passive variable.In this paper, to mine the synergy ability in power system operation, the frequency regulation effect in the allowed frequency range is used as a resource to cope with uncertainty, which can expand the solution space of economic dispatch. On the basis of the coordination of active power sources, the cooperation between active load and passive variables is introduced in order to cope with uncertainty. The operation status of the transmission elements are brought into economic dispatch, which shows the nature of the cooperation between source and network. In the research process, this paper gives the definition of the synergy ability, the synergy state, and the conflict degree of between source and network. On this basis, to establish the relevant model and analyze the mechanism, two level optimization, reinforcement learning and other mathematical optimization theories are introduced. The whole research refers to many specific aspects, such as the establishment and solution method of dispatch model in different time scales, the strategy and process of the cooperation, and the implementation mechanism of synergistic economic dispatch. Main contributions and innovations of this paper are described as follows:(1)To make the full use of the frequency regulation effect in economic dispatch, and cope with the fluctuations of load and renewable energy generation in normal states, the idea for synergistic economic dispatch is given from a new perspective in this paper. Based on automatic generation control, the synergy ability is divided into three types without considering the influence of voltage change, namely automatic synergy ability, controllable synergy ability and re-controllable synergy ability. Accordingly, on the premise of closely integrated with dispatch, three synergy states are introduced. On the basis of above, the relevant economic dispatch models are established with the unit output power and frequency as the decision variables, and the solution and mechanism analysis are given. A 10-unit system is used to verify the validity of the proposed approach that it can enhance the ability of power system to keep source power balance under uncertainty.(2) On the basis of above, this paper further considers the role of active load to cope with the uncertainty, and tries to mine the synergy ability in the dispatch of different time scales. For this purpose, firstly, a two level optimization model for dynamic economic dispatch in wind and energy storage integrated system is established. The upper level problem takes the allowable wind power output interval as decision variable, to minimize the difference between the allowable wind power output interval and the wind power prediction interval, i.e. to maximize wind power penetration ability; the lower level problem is to minimize the operating cost. The energy storage system can be used to cope with the uncertainty of wind power by adjusting its charging or discharging power, which can help achieve the expected power balance and the real time energy balance, which can also reduce the burden of the AGC unit. At the same time, the frequency regulation effect is considered, which is equivalent to consider the process of automatic control in dispatch. The proposed model is a linear two level optimization model, which can be transformed into a single level model and be solved by the mixed integer linear programming algorithm. Finally, the validity of the model and the solving algorithm is verified by the 6-bus system.(3)For the problem of how to use active load effectively in unit commitment, this paper proposes a synergistic unit commitment model based on two level optimization. The upper level problem is to minimize the operation cost of active power source, and to meet the cooperation results between active load and passive variables in the lower level problem. Automatic synergy ability is considered in the upper level problem and unit operating status and reserve configuration are optimized. The lower level problem is guided by the price information given by the upper level problem, to optimize active load charging and discharging power and the control range. According to the predetermined control strategy, the prediction error scene is compensated, and the uncertainty of the prediction is reduced. The model is solved by the alternating iteration of the upper and lower problem, and the integration of the coordination and cooperation is realized. The simulation and analysis of 10-unit system show that active load not only helps to meet the expected power balance constraints, can also directly work as reserve, which can avoid unit’s frequent start and stop, and improve the ability of renewable energy generation penetration.(4)In order to consider the cooperation between source and network in economic dispatch, this paper takes economic dispatch as the clue, and performs the argumentation on the relation between source optimal operation and power grid structure by a simple 3-bus system. Argumentation results show that to cope with the changeful power system in the future, it is necessary to consider the operation status of transmission elements as decision variables in economic dispatch. On the basis of above, the concept of conflict degree between source and network and a two level optimization formulation of this problem are proposed. The conflict degree between source and network can be used to evaluate the restriction of network to the optimal operation of source. Taking the conflict degree as the core, the upper level problem is to minimize the change of the power grid structure, so that conflict degree between source and network is within the acceptable range. The lower level problem optimizes the operation of generation units. Finally, the significance of this study is illustrated with 5-bus and 30-bus system.(5) The idea of synergistic economic dispatch is expected to realize in the way of distributed autonomous integrated with overall planning, the local system composed of wind power and energy storage system is studied. The core is that the wind farms and energy storage form a hybrid system in the way of cooperation to participate in the power trade, so that the hybrid system has a certain autonomy ability. Under the premise that energy storage capacity and price information are given, the optimal decision-making method in the cooperation of wind power and energy storage based on reinforcement learning algorithm is proposed. The state of charge and wind power forecast are taken as the state input of the established controller.The controller has the ability of on-line learning, and the gain of hybrid system is taken as feedback information in the process of learning, and the controller gradually has the ability to make decisions on the energy storage charge/discharge power and the purchased reserve capacity. Simulation using real data shows that, after the pre learning statge of the exploration of trial and error, the controller gradually accumulates experience, and gradually has the ability to give the best action strategy. In online learning state, the control effect is getting better gradually, which can reduce the burden of power system regulation and realize the distributed autonomous of wind and energy storage.