Static Equivalent Method And Its Applications In Power Generation,transmission,and Distribution Systems

In order to promote renewable energy accommodation and improve energy efficiency,harmonious operation of interconnected power systems and coupled electricity-gas systems is one of the effective ways.With the fast development of interconnected energy systems,power grids or energy systems are tightly coupled.The interactions of the interconnected systems are significant and cannot be ignored.Therefore,it is necessary to consider the influence of interconnected neighboring systems in the planning,operation,and control of power systems.The integrated analysis of the interconnected systems can accurately account for the interactions.However,there are still two major difficulties for the integrated analysis.On one hand,the calculation of integrated system problems is inefficient and even difficult to solve,because of numerous of components,large-scale systems,and nonlinear models of the interconnected systems.On the other hand,the data exchange is not always possible for interconnected systems.As a result of the interconnected systems are owned by different companies,the data involve the safety information and commercially sensitive information.To overcome the mentioned deficiencies of the integrated analysis,the static equivalent method is a promising solution.By using the static equivalent method,the external complex networks can be replaced by an equivalent network,which is small,simple,easy to maintain,and can protect data privacy.Moreover,the equivalent network can retain the influence of the external network.Many studies have investigated the static equivalent method and made valuable contributions.However,there are still some important problems that have not been solved well.In this paper,static equivalent method and its applications in power generation,transmission,and distribution systems is studied.The detailed contents are shown in the following:For transmission networks,the integrated analysis of interconnected systems is difficult due to data privacy.This paper proposes a static equivalent method for interconnected external networks based on component characteristics using PMU measurements to retain the influence of external networks.This method constructed an equivalent network considering component characteristics.Based on border PMU measurements and the typical operation modes of the external network,the constrained least square model is established without the real-time synchronous information of the entire external system.Base on the established equivalent model,a set of equivalent criteria are developed considering the adequacy of reactive power and voltage.The equivalent criteria are used to determine whether the simplest and the most widely used PV/PQ equivalence is still appropriate.If the PV/PQ equivalence is inappropriate,the proposed equivalent method should be used.The proposed equivalent method and criteria are applied in static security analysis and optimal reactive power flow,which can achieve high calculation accuracy for interconnected power systems without the real-time synchronous information of the entire external system.For distribution networks,the integrated calculation of transmission and distribution networks is inefficient and even difficult to solve,due to large number of components and large-scale systems.In order to simplify active distribution networks,this paper proposes a two-tier static equivalent method of active distribution networks considering sensitivity,power loss and static voltage characteristics.This method proposes a new physical equivalent network.Based on the equivalent network,the two-tier static equivalent model is further constructed.In the upper tier equivalent model,the equivalent model is constructed based on the consistency of sensitivity,power loss,and power states.In the lower tier equivalent model,the least square model based on border measurement equations is established to further considering the static voltage characteristic of loads.The proposed two-tier static equivalent model is applied in static security analysis.The method improves the effectiveness of the analysis of transmission and distribution networks.For generators,the electricity outputs of the natural gas-fired units(NGUs)are imposed by the operational constraints of natural gas networks.The natural gas loss is affected by the electricity outputs of NGUs.The interactions between the electricity network and natural gas network cannot be ignored.However,Electricity and natural gas networks are owned by different utilities,the data exchange is not always possible.This paper proposes a static equivalent method considering operational constraints and loss of natural gas networks.The constraint equivalent model maximizes the outputs of every combined NGUs meeting the operational constraints of the natural gas network.This model can retain the impacts of the natural gas network.The loss equivalent model calculates the natural gas consumption of compressors contributed by NGUs based on “shortest path principle” using the gas loss as weight.The proposed static equivalent model is applied in the optimal power flow(OPF)of electricity-gas systems.The operational constraints and loss of natural gas networks are considered in the OPF model and the data privacy is protected.For applications,in this paper,the static equivalent method is applied in the capacity evaluation of renewable energy accommodation.Facing with the serious problem of wind,solar,and water power curtailment,the flexibility space of power systems can be used to promote renewable energy accommodation.This paper proposes a capacity evaluation of renewable energy accommodation using static equivalent method considering the flexibility space of power systems.This method accounts for the flexibility space of deep peak regulation(the minimum outputs of generators can be further regulated)and capacity increasing of lines under the contingencies(the transmission capacity can be exceeded in short-time scale).By maximizing the outputs of every combined renewable generators and the transmission power of every combined lines,meeting the operational constraints of power systems,the effect of flexibility space on promoting renewable energy accommodation is evaluated.In conclusion,this paper proposes a static equivalent method for power generation,transmission,and distribution systems.The proposed equivalent method can not only simplifie transmission networks,active distribution networks,and natural gas networks,but also represent the operational constraints of external systems.Moreover,the influence of interconnected neighboring networks can be represented and the data privacy can be protected.Besides,the proposed equivalent method is applied on static security analysis,optimal reactive power flow,optimal active power flow,and the capacity evaluation of renewable energy accommodation.Facing with difficulties in the integrated analysis,the proposed method ensures the effectiveness of the analysis and decision of interconnected systems.