Research On Power System LFC Considering Cross-level Hierarchical Structure

Load Frequency Control(LFC)is a widely used method to control system frequency and tie-line of power grid.There is a special grid structure called Cross-level Hierarchical Structure in the operation of power grid in our country,in which some generators(direct-dispatching generators)are directly controlled by upper control center(not provincial control center but regional' control center).Large quantities of experience of power grid operation show that direct-dispatching generator plays an important roles on the recovery of frequency deviation after active disturbance and improvement of stability of system frequency and tie-line transmission power of power grid.With the power grid with Cross-level Hierarchical Structure mentioned being taken as research subject,some problems about the effectiveness and the role direct-dispatching generator may play will be studied deeply in this paper.And the main results of this paper are as follows:1.The necessity and effectiveness of the closed area method have been proved in theory.In order to solve the problem that regulating of direct-dispatching generators would have impacts on the calculation of ACE of the province in power grid,'Closed area method' is usually adopted.The necessity and effectiveness of this method will be proved in detail with area control model.Meanwhile,digital simulation on how closed method is applied in actual power grids will be carried out with load frequency control model.The result shows that closed area method significantly reduces the impacts on ACE of provincial control area from direct-dispatching generations,and the requirement of ACE of the provincial control area in which direct-dispatching generations locate can be met as planned thus.2.The role direct-dispatching generators can play in LFC of grid with simple grid interconnected structure has been discussed.Firstly,the problem on how direct-dispatching generators influence LFC of two regional interconnected power grid will be studied.Then,the question on the effectiveness of power support from direct-dispatching generators to make up for nonadjacent tie-lines in multi-area line-shape girds will be researched.Finally,digital simulations will be carried out accordingly.The result shows that direct-dispatching generators do make a contribution to the stability of system frequency and tie-line transmission power of two regional interconnected power grid.Nevertheless,whether main responsibility or auxiliary responsibility direct-dispatching generators take depends on the priority of regulating speed and regulating cost,and power support from direct-dispatching generators to make up for nonadjacent tie-lines in multi-area line-shape girds could not be beneficial to recovery of tie-line transmission power.3.The role direct-dispatching generators can play in LFC of loop-shape grids has been discussed,evolved from the closed area of direct-dispatching generators.Based on the previous work from our research group,the idea that AGC subarea control scheme is implemented by direct-dispatching generators is proposed in this paper.Specially,'virtual zone' created by closed area method will replace the 'physical zone' used before.Then corresponding digital simulations will be carried out.The result shows that the idea that AGC subarea control scheme is implemented by direct-dispatching generators can transfer tie-line transmission power deviations in multi-area loop-shape girds to the inner of area in which direct-dispatching generators locate,which can eliminate tie-line power deviations in multi-area loop-shape girds thus.