Zhengshuo Li

Emission-Concerned Wind-EV Coordination on the Transmission Grid Side With Network Constraints: Concept and Case Study

This paper presents a study of emission-concerned wind-electric vehicle (EV) coordination on the transmission grid side. An aggregator model representing a cluster of controllable EVs is proposed, and is coordinated with large-scale wind power on the transmission side. Using these EV aggregators, a conceptual framework is developed for wind-EV coordination, based on a three-level hierarchy. At the top level, the control center determines the optimal plant outputs and EV charging strategies from the proposed wind-EV dispatch model, where emission costs, thermal plant generation, controllable EVs, and CO2 capture power plants are taken into account and set as multi-objectives. The time-varying number of EV connections, plant ramp rate limits, and network constraints, including interface and branch transmission capacities, are also considered in the model. The model is used to verify the benefits of wind-EV coordination in an IEEE 14-bus system. Compared with uncoordinated charging, wind curtailment, emissions, total optimized cost, and EV charging fees are all greatly reduced by coordination. Moreover, interface transmission capacity and discharging price are found to be important factors in coordination, and the impacts are investigated.

Master–Slave-Splitting Based Distributed Global Power Flow Method for Integrated Transmission and Distribution Analysis

With the recent rapid development of smart grid technology, the distribution grids become more active, and the interaction between transmission and distribution grids becomes more significant. However, in traditional power flow calculations, transmission and distribution grids are separated, which is not suitable for such future smart grids. To achieve a global unified power flow solution to support an integrated analysis for both transmission and distribution grids, we propose a global power flow (GPF) method that considers transmission and distribution grids as a whole in this paper. We construct GPF equations, and develop a master-slave-splitting (MSS) iterative method with convergence guarantee to alleviate boundary mismatches between the transmission and distribution grids. In our method, the GPF problem is split into a transmission power flow and a number of distribution power flow sub-problems, which supports on-line geographically distributed computation. Each sub-problem can be solved using a different power flow algorithm to capture the different features of transmission and distribution grids. An equivalent method is proposed to improve the convergence of the MSS-based GPF calculation for distribution grids that include loops. Numerical simulations validate the effectiveness of the proposed method, in particular when the distribution grid has loops or distributed generators.

Sufficient Conditions for Exact Relaxation of Complementarity Constraints for Storage-Concerned Economic Dispatch

Storage-concerned economic dispatch (ED) problems with complementarity constraints are strongly non-convex and hard to solve. In this letter, an exact relaxation method is proposed to relax the non-convex ED to a convex form under two sufficient conditions, which can be further generalized for other problems of similar structures. A mathematical proof is provided, and the numerical tests verify the effect of this method.

Transmission Contingency Analysis Based on Integrated Transmission and Distribution Power Flow in Smart Grid

In future smart grids, with distribution networks having loops more frequently, current transmission contingency analysis (TCA) which usually neglects the distribution power flow variations after a contingency may leave out severe outages. With more distribution management systems deployed on the distribution side, a new transmission CA method based on global power flow (GPF) analysis which integrates both the transmission and distribution power flow is proposed in this paper (named as GTCA) to address the problem. The definition and new features of GTCA are first introduced. Then, the necessity of GTCA is physically illustrated. Difference in the results of GTCA and TCA is mathematically analyzed. A GPF-embedded algorithm of performing GTCA is then provided. The data exchange process and the performance with communication interruptions are discussed. As multiple contingencies are considered in GTCA, several approaches are proposed and discussed to reduce communication burdens and improve the computational efficiency. Plenty of numerical tests are performed in several systems to verify the theoretical analysis. With theoretical analysis and numerical verification, it is suggested that GTCA should be performed instead of TCA to avoid potential false alarms, especially in the condition that DNs are more frequently looped in the future smart grids.

A New Real-Time Smart-Charging Method Considering Expected Electric Vehicle Fleet Connections

This letter presents a real-time electric vehicle (EV) smart-charging method (N-RT), that not only considers currently connected EVs, but also uses a prediction of the EVs that are expected to plug in in the future. Numerical tests show that the N-RT method improves valley-filling under various levels of prediction accuracy.

Research on architecture of ITS based Smart Charging Guide System

Nowadays, environmental-friendly electric vehicles (EVs) attract great attention around the world. However, the adoption of EVs will impact the power system greatly. Different from previous work which concentrated on “when to charge”, research on “where to charge” to minimize the impacts on power grid is presented based on the interaction with Intelligent Transportation System (ITS). With the transportation information from ITS, a Smart Charging Guide System (SCGS) is designed in this paper. Architecture based on interaction between TCC (Traffic Control Center) and PSCC (Power System Control Center) is presented with the following three functions. Firstly, initial routes navigating to destination are selected with necessary charging stations. Secondly, load forecasting is carried out to determine the future power demands on different charging stations. Finally, based on the forecasted load, security evaluation on power system is adopted to produce a security index (SI). The SI will be used to revise the traffic distance to an electrical distance, which involves the contribution to power system operation. Two typical simulation cases are presented to explain the ideas of SCGS and further research prospects are discussed.

PMU Uncertainty Quantification in Voltage Stability Analysis

This letter presents an uncertainty quantification method for phasor measurement units (PMUs) in voltage stability assessment. The effect of local phasor measurement uncertainty on the Thevenin equivalent impedance used for voltage stability analysis is quantified analytically. The results can be used to specify the requirements for PMU uncertainty in voltage stability assessment.

Transmission Contingency Screening Considering Impacts of Distribution Grids

As traditional transmission contingency screening (TCS) methods neglect impacts of distribution grids, the contingency selection result may not be always satisfactory, especially when distribution networks are more frequently looped in a smart grid. Two new contingency screening methods considering impacts of distribution grids are proposed in this letter. The first one is based on global power flow, and the second only utilizes distribution network equivalencing. Numerical tests show both new methods can give more reliable results than TCS.

Coordinated Transmission and Distribution AC Optimal Power Flow

The current separate transmission and distribution energy management system faces multiple challenges associated with integrating distributed generators (DGs) into future grids. These challenges, such as a voltage rise issue resulting in curtailment of DGs, are difficult to solve via the current separate energy management. Thus, coordination between transmission and distribution is suggested, and a coordinated transmission and distribution AC optimal power flow (TDOPF) is proposed in this paper. A mathematical TDOPF model is established and analyzed in a master-slave structure. A heterogeneous decomposition algorithm (HGD), which is inspired by heterogeneous transmission and distribution characteristics, is proposed to solve the TDOPF in a distributed manner. The HGD is compared to other typical multi-area OPF decomposition algorithms and the differences are discussed. Numerical tests verify the benefit of the TDOPF to both transmission and distribution systems. The TDOPF improves economic operations, mitigates voltage rises and decreases boundary bus mismatches. Hence, more DGs can be accommodated by the grid. In addition, a series of numerical tests indicate that the HGD competitively solves the TDOPF.

Current energy management technologies research in China considering EVs integration

Some latest research works on EVs integration in China are presented in this paper. A future energy management framework supporting large-scale EVs fleets is proposed, involving five related topics: 1) modeling of EV swarms charging load based on naturalistic travel patterns; 2) global power flow based assessment and early warning technologies considering real-time traffic information; 3) emergency control by regulating EVs charging infrastructures; 4) optimized coordination with EVs and renewable energies; 5) interaction between power system and transportation system. Finally, some common features of the above researches are surveyed.