Search

Series resistances of overhead transmission lines (OTLs) are largely dependent on the corresponding conductor temperature. The electric current heats the conductor, and the environmental conditions affect the thermal exchange. Therefore, multiple factors related to the thermal balance codetermine the conductor temperature and resistance. However, the traditional power flow analysis neglects the thermal balance effects by treating those related factors as invariables, which may introduce some errors because of the change of the factors. Therefore, in this study, new power flow formulations, which allow for the relationship between the transmission power flow and the thermal balance, are derived. This study demonstrates the errors by applying the new formulations to calculate the power transfer limit (PTL). Moreover, the PTL of the OTL is defined either by encountering the thermal rating (TR) or the steady‐state stability limit. A case study based on an equivalent 2‐bus test system was used to illustrate the efficacy of the thermal balance that affects the PTLs of both types. In this analysis, this study also reveals the relationship between the TR and steady‐state stability limit with various physical and environmental conditions. [ABSTRACT FROM AUTHOR]

In highly polluted regions, it might be necessary to operate power plants enforcing appropriate emission limits to help ensuring air quality. If the air quality is low and pollutants are difficult to diffuse, few additional emissions should be allowed to avoid further deteriorating the air quality. Wind power helps reducing pollution levels as it substitutes polluting thermal production. However, wind and air pollution are generally anti‐correlated. Storage can be used to shift wind power production from hours when higher pollution levels are allowed to hours when lower pollution levels are allowed to help meeting emission constraints at critical hours. Storage availability also achieves a comparatively lower production cost for the system as a whole. To represent the stochastic nature of wind power production and weather‐dependent emission limits, the authors propose a two‐stage stochastic programming model to analyse the trade‐off emission versus cost in an electric energy system with storage facilities. The first stage represents the day‐ahead scheduling, while the second one represents the real‐time operation under different scenarios. The authors analyse the impact of emission limits and/or storage on generation scheduling by comparing different models using an illustrative example and a case study based on the IEEE 118‐node system. [ABSTRACT FROM AUTHOR]

Vanadium redox flow batteries (VRBs) are a promising solution to facilitate the expansion of intermittent renewables. The efficiency of VRBs is dynamically dependent on the instantaneous operating states, including the output power level and state of charge (SOC). In addition, VRB capacity fades over long‐term cycling and thus might limit the market revenue. Modelling these two factors would assist deciding better operating strategy, enhance and more precisely evaluate the economics of VRBs. This study establishes an operation optimisation model that integrates the characteristics of the dynamic efficiency performance and the capacity loss of VRBs. First, the dynamic efficiency functions of the operating states are transformed into VRB power functions using a fitting technique to be compatible with the operation optimisation model and reduce the computational complexity. The VRB power functions are then embedded into the operation optimisation model, which decides the optimal charge–discharge schedule of VRBs to maximise the revenue. The capacity loss model is also implemented to estimate the revenue decrease caused by capacity loss, which could help decide the preventive measures and capacity restoration from an economic perspective. The validity of the proposed model is demonstrated by case study results based on the market prices of ERCOT in 2014. [ABSTRACT FROM AUTHOR]

Due to the unique advantages in computational robustness and convergence, the linear approximation approach is and will remain to be an important method to solve the optimal power flow (OPF) problem, especially for industrial applications. The DC power flow method, which is currently used in the majority of power industries, is the representative. Many studies extend the DC power flow method by including voltage magnitude, reactive power, and losses. This study provides a detailed analysis and breakdown investigation of existing linear approximations of the OPF problem. The formulation and accuracy of existing linear approximations are compared. Taking advantage of the decoupled formulation of linear approximations, the property of power flow equations is illustrated from a new perspective. Why reactive power flow equations are hard to linearise is explained theoretically. The numerical performance of existing linear approximations is demonstrated in IEEE and Polish test systems. Evidence from the theoretical analysis and numerical studies shows that the accuracy of linear approximations could be substantially improved using a mathematical transformation of the non‐linear voltage magnitude term. This finding provides a new research direction for solving the OPF problem using linear approximations. [ABSTRACT FROM AUTHOR]

With the rapid growth of distributed energy resources (DERs) and microgrids (MGs), the volatility of renewable generations will impact the economical and secure operation of the power system. To exert the grid‐friendly manner of MGs, an optimal joint‐dispatch scheme of energy and reserve is proposed in this study for combined cooling, heating and power (CCHP)‐based MGs. In the scheme, the total operation cost of the MG is minimized under pre‐defined reserve requirements. The energy demands are satisfied via dispatching various DERs. Distinguished from the electric demand balanced at any given instant, the unbalance between thermal supply and demand is acceptable because the temperatures can be maintained due to the thermal inertia. When the electric supply is in shortage, the energy provided for the thermal systems can be temporarily reduced so that more energy can be utilised to support the electrical systems. The dynamics of the MG is modelled by discretising thermal balance differential equations. With the coordination of CCHP systems, the reserve capability of the MG is fully utilised, thereby ensuring the security of the bulk power system. Case studies based on an industrial park in Chicago, USA validate that the proposed scheme can provide more reserve capability. [ABSTRACT FROM AUTHOR]

In this study, the authors introduce a new solution to the alternating current optimal power flow problem based on successive linear approximation of power flow equations. The polar coordination of the power flow equations is used to take advantage of the quasi‐linear P–θ relationship. A mathematical transformation for the crossing term of voltage magnitude is used, which guarantees the accuracy of the linear approximation when the quality of initial points regarding voltage magnitude is relatively low in the first few iterations. As a result, the accuracy of the proposed approximation becomes very high in very few iterations. Linearisation method for the quadratic apparent branch flow limits is provided. Methods to recover the AC feasibility from the obtained optimal power flow solution and correct possible constraint violations are introduced. The proposed method is tested in several IEEE and Polish benchmark systems. The difference in objective functions relative to MATPOWER benchmark results is generally <0.1% when the algorithm terminates. [ABSTRACT FROM AUTHOR]

This study presents a closed‐loop coordination mode and model between generation maintenance scheduling (GMS) and long‐term security‐constrained unit commitment (SCUC) considering energy constraints and N − 1 contingencies that aims at improving the security and economy of power system operations. Given the calculation complexity, this study employs constraint transformation techniques and an efficient approach termed the relaxation induced method, which is based on the solution of the relaxed mixed‐integer programming (MIP) model. The proposed approach can quickly obtain a near‐optimal solution. If the near‐optimal solution is not acceptable for system operators, it can be used to warm‐start the solution of the original MIP problem. The modified IEEE 30‐bus test system and a provincial power system in China are employed to demonstrate the effectiveness of the proposed model and algorithm. [ABSTRACT FROM AUTHOR]

The traditional N − 1 security assessment approaches give a limited perspective on the degree of power system security. This study proposes a novel security assessment approach using the index of post‐contingency steady‐state security distance (PCSSD). Two PCSSD models are established for contingencies causing or not causing system overloads, respectively. If the system is secure following a contingency, the PCSSD provides the shortest distance of the operating point (OP) to the post‐contingency steady‐state security region (PCSSR) boundaries, to indicate the OP's remaining security margin. If an overload condition arises after a contingency, the PCSSD provides the shortest distance of the OP to the PCSSR, to show the difficulty in eliminating power grid violations. With the index of the PCSSD, one can continuously quantify post‐contingency OPs whether they are secure or insecure and rank all possible contingencies according to their severity. The PCSSD model is a large‐scale non‐linear optimisation model which has to be calculated numerous times. A novel algorithm is proposed to improve the computational efficiency. It identifies the active boundaries of the PCSSR, and then approximates most of the PCSSDs by the corresponding ante‐contingency ones, reducing the number of optimisation problems to be solved. Case studies are performed to demonstrate the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

Steady‐state security region (SSR) provides a region‐wise approach to assess the steady‐state security of power systems. Based on the SSR model, we introduce the concept of 'steady‐state security distance' (SSD) to provide exact 'quantitative analysis' on the scale of security margins for system operators. Then, mathematical models of SSD are formulated. On this basis, implementation of SSD is discussed towards various uncertainties within the power system operation. As the calculation of SSD is in essence a large‐scale non‐linear optimisation process, it requires very long computation time, and thus could not be utilised in practical application. In order to enhance the efficiency of computation, a novel algorithm is proposed, which decomposes the complex optimisation process into two steps: active SSR boundary identification and partial constrained solution. The proposed algorithm remarkably cuts the scale of the optimisation model as well as the number of calculations, thus significantly reduces the computation time to meet requirements of assessment towards day ahead and hours ahead generation schedules. In the end, an IEEE‐30 bus case and a practical case on a provincial power system are both studied to testify the effectiveness of the proposed model and algorithm. [ABSTRACT FROM AUTHOR]