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Due to the increasing penetration of large-scale renewable energy generation at transmission level, hybrid AC/DC transmission expansion is becoming more relevant by the day, especially if the considered renewable generation is far away from existing AC grid infrastructure. This paper presents a multi-period hybrid AC/DC network expansion planning model that incorporates node voltages, reactive power and network losses. Since the operation of a high voltage DC link is influenced considerably by the converter stations, converter transformer, filter, and phase reactor are modeled in each converter station. Since the AC power flow equations render a non-convex problem whose solution algorithms do not always guarantee convergence or optimality, a second-order conic relaxation is utilized. The resulting model is a mixed-integer convex problem with a convergence guarantee that embodies a better approximation of system operation than its DC linear counterpart. Three case studies are considered to illustrate the proposed model. [ABSTRACT FROM AUTHOR]

This paper presents an optimization approach for the placement of geomagnetically induced current (GIC) blocking devices (BDs) on power system transformers to mitigate the adverse effects of a geomagnetic disturbance (GMD). Solar storms lead to GMDs which, in turn, drive GICs along transmission lines and through transformer windings. GICs cause half-cycle saturation in power transformers, increase their shunt reactive power loss, lead to a significant lack of power system reactive power support, and, as a result, create voltage instability and, potentially, large-scale voltage collapse. To mitigate these detrimental effects, a long-term remedy is to install GIC BDs at the neutral point of transformers. Since these devices are fairly costly, their placement should be performed in an optimal manner. The optimization problem proposed in this paper minimizes the cost of BD placement while satisfying power system voltage and generator maximum reactive power limits. The numerical results demonstrate the effectiveness of the proposed method in maintaining an acceptable voltage profile for the power system in case of GMDs with any degree of severity. [ABSTRACT FROM AUTHOR]

Optimal power flow (OPF) programs must be able to provide sound operating points for power systems under different conditions while meeting a large number of constraints. These constraints, however, either due to the operating conditions at hand or by construction, can have linearly dependent gradients, thus defining ill-behaved feasible sets. This paper analyzes the use of an alternative set of optimality conditions, derived from the Fritz-John (FJ) conditions, to represent OPF solutions in such critical cases. Results are presented for realistic test systems. [ABSTRACT FROM PUBLISHER]

Isolated microgrid power systems are prone to large frequency and voltage deviations, since they have limited generation and finite inertia and may include pulse power loads and large proportion of dynamic loads. Thus, they require effective power management methods to operate optimally, while satisfying operating and security constraints. In this paper, a novel integrated security-constrained model-based power management approach is proposed for isolated microgrids in all-electric ships during the normal/alert operating state. The new power management method was formulated as a multi-objective optimal control problem. A method based on the Nondominated Sorting Genetic Algorithm II was developed to solve the problem. The developed integrated security-constrained power management approach was applied to a notional all-electric ship computer model. The simulation results, conducted in PSCAD, showed the effectiveness of the new power management method. [ABSTRACT FROM PUBLISHER]

The expected impact of distributed generation (DG) into Smart Grid represents a great challenge of the future for power systems. In particular, the integration of DG based on renewable energy sources (RESs) in distribution networks, without compromising the integrity of the grid, requires the development of proper control techniques to allow power delivery to customers in compliance with power quality and reliability standards. This paper proposes a coordinated local control approach that allows distribution system operator (DSO) and independent power producers (IPPs) to obtain benefits offering the voltage regulation ancillary service to DSO and maximizing allowable active power production for each RES unit belonging to the same IPP. The control is based on a cooperation of data transfer between DSO and IPPs. In order to realize such cooperation, a nonlinear constrained optimization problem is formulated and solved by sequential quadratic programming (SQP) method. The validation of the proposed control technique has been conducted through several time series simulations on a real MV Italian distribution system. [ABSTRACT FROM PUBLISHER]

Regarding the different capabilities, linear motors have been employed in many applications in industry. Among different linear motors, single-sided linear induction motors (SLIMs) have been widely used due to their simplicity and low construction cost. However, these types of motors suffer from low efficiency and power factor and also, existence of so called end effect. The end effect produces braking force that deteriorates the performance of the motor, especially at high speeds. In this paper, analytical equations are derived for end effect braking force, efficiency, power factor, and output thrust. Employing the derived equations and considering all phenomena involved in the single-sided SLIM, a simple design procedure is presented and the effect of different design variables on the performance of the motor is analyzed. A multiobjective optimization method based on genetic algorithm is introduced to maximize efficiency and power factor, as well as to reduce primary weight and end effect braking force, simultaneously. All effective design variables are considered in optimization. The results show significant improvements in the objective function. Finally, the 2-D and 3-D finite element method is employed to validate the results obtained by the analytical method. [ABSTRACT FROM PUBLISHER]

This paper proposes an active-reactive power dispatch procedure that is expressed via a bilevel optimization problem whose upper and lower level criteria are, respectively, the minimum opportunity cost and minimum offered price of active power. It analyzes the impact of minimizing such criteria on system operation, and it investigates the behavior of marginal prices and opportunity costs and their use in mechanisms to compensate generators for power provision. The bilevel problem is solved using a version of interior point methods derived for mathematical programs with complementarity constraints. Results obtained with three test systems are analyzed. [ABSTRACT FROM PUBLISHER]

In this paper, a receding-horizon multi-step optimization is proposed to correct nonviable transmission voltages and prevent long-term voltage instability. The proposed control scheme is based on real-time control, inspired by model predictive control, and steady state power-flow-based equations. In order to anticipate load behavior and avoid using dynamic equations in the control scheme, explicit formulations are used to model evolution of load with time. The simulation results of the proposed technique are presented on the Nordic32 test system. [ABSTRACT FROM PUBLISHER]

Connection of offshore wind farms to shore requires the use of submarine cables. In the case of long HVAC connections, the capacitive charging currents limit the transfer capability and lead to high losses. This paper shows that the losses can be substantially reduced by continuously adjusting the cable operating voltage according to the instantaneous wind farm power production. Calculations for a 320 MW wind farm connected to shore via a 200 km cable at 220 kV nominal voltage show that an annual loss reduction of 9% is achievable by simply using a \pm 15\% tap changer voltage regulation on the two transformers. Allowing a larger voltage regulation range leads to further loss reduction (13% for 0.4–1.0 p.u. voltage range). If the wind farm has a low utilization factor, the loss reduction potential is demonstrated to be as high as 21%. The methodology can be applied without introducing new technology that needs to be developed or qualified. [ABSTRACT FROM PUBLISHER]

This paper presents a risk-based advanced distribution network management system (NMS) aimed at maximizing wind energy harvesting while simultaneously managing congestion and voltages. The NMS allows the adoption of multi-minute control cycles so the volume of actions from on-load tap changers (OLTCs) and distributed generation (DG) plants can be reduced while effectively catering for the effects of wind power uncertainties. This work presents the quantification of benefits and impacts from adopting different control cycles as well a comparison with a deterministic approach. The risk-based approach is implemented by adapting and expanding an AC optimal power flow. A risk level is used to determine the extent to which congestion and voltage rise could exist during a control cycle. The proposed NMS is applied to a real-life U.K. MV network from the North West of England to assess its effectiveness in managing high penetration of wind power considering minute-by-minute simulations for one week. The results show that the risk-based approach can effectively manage network constraints better than the deterministic approach, particularly for multi-minute control cycles, reducing also the number of control actions but at the expense of higher levels of curtailment. [ABSTRACT FROM PUBLISHER]

This paper presents an effective optimization scheme for the measurement-based load modeling based on the sensitivity analysis of composite load model parameters. Each parameter of load model has different effects on its dynamic response. Moreover, some parameters are insensitive to the change of others. To estimate the dynamic interactions between parameters, their sensitivity is analyzed by using the eigenvalues of Hessian matrix used in the optimization algorithm. Also, the linear dependence between two load model parameters is then identified by examining the condition number of Jacobian matrix. With this parameter analysis, the performance of optimization process for measurement-based composite load modeling is improved by reducing the number of necessary parameters to consider. The performance of proposed method is verified with the practical data measured at a feeder in a real substation. [ABSTRACT FROM PUBLISHER]

This paper considers achieving the minimum total harmonic distortion (THD) or frequency-weighted THD (WTHD) of the staircase-modulated output voltage of single-phase multilevel inverters, with or without elimination of the lowest order harmonics. The minimal THD values, together with the corresponding step angles and dc voltage source ratios, have been obtained for the 5-, 7-, 9-, 11-, and 13-level cases; accounting for the device voltage drops when the load is resistive or is moderately inductive is described. Similarly, the minimal WTHD values, together with the corresponding step angles and dc source voltage ratios, have been obtained for the five-, seven-, and nine-level waveform cases. The results show that requiring harmonic elimination leads to larger W/THD than the minimum W/THD that can be achieved without this requirement. Furthermore, a 13-level waveform is needed to attain a voltage THD less than 5%, and a nine-level waveform is needed to attain a WTHD less than 0.5%. Experimental measurements are presented as verification of the analytical results. [ABSTRACT FROM PUBLISHER]

System restoration involves status assessment, optimization of generation capability, and load pickup. The optimization problem needs to take complex constraints into consideration, and therefore, it is not practical to formulate the problem as one single optimization problem. The other critical consideration for the development of decision support tools is its generality, i.e., the tools should be portable from a system to another with minimal customization. This paper reports a practical methodology for construction of system restoration strategies. The strategy adopted by each power system differs, depending on system characteristics and policies. A new method based on the concept of “generic restoration milestones (GRMs)” is proposed. A specific restoration strategy can be synthesized by a combination of GRMs based on the actual system conditions. The decision support tool is expected to reduce the restoration time, thereby improving system reliability. The proposed decision support tool has been validated with cases based on a simplified Western Electricity Coordinating Council (WECC) 200-Bus system and Hawaiian Electric Company system. [ABSTRACT FROM PUBLISHER]

This article develops a two-stage volt-VAr control strategy coordinating the discrete controls of legacy grid devices and the operation of smart inverters (SIs) in power distribution grids with high penetration of photovoltaic generation. The first stage dispatch problem optimally coordinates the tap settings of on-load tap changers, on/off status of shunt capacitor banks, and SIs’ active power output. The second stage incorporates the SI volt-VAr mode, i.e., $\mathbf{Q(V)}$ as per IEEE-1547, and an adaptive volt-VAr droop function, $\mathbf{Q(\Delta V)}$ , to dispatch the SI reactive power generation. Both dispatch problems are formulated as mixed-integer linear programming problems to maintain tractable formulations and reduce the complexity and computational burden that usually arise when solving large-scale optimal power flow problems. The proposed two-stage strategy is tested on a modified IEEE 123-bus system considering different droop settings. The dispatch strategy augmented with $\mathbf{Q(\Delta V)}$ droop outperforms the model with only $\mathbf{Q(V)}$ droop in stabilizing the PCC voltage while maintaining less active power curtailment. [ABSTRACT FROM AUTHOR]

We propose an approach to solving the AC network-constrained unit commitment problem (AC-NCUC) of large-scale power systems. This approach relies on solving convex (continuous and mixed-integer) optimization problems. First, we formulate a second-order conic relaxation of the original optimization problem, which renders a mixed-integer second-order conic optimization problem. To solve this problem, we use Benders’ decomposition, which decomposes it into a master problem, which is mixed-integer linear, and a subproblem, which is continuous and second-order conic. To improve the convergence of the Benders’ algorithm, we add several linear feasibility constraints to the master problem. Since the second-order relaxation of the original problem cannot guarantee AC feasibility of the solution found, we ensure such feasibility by solving a sequence of continuous convex optimization problems. We numerically validate the proposed approach using a Texas 2000-bus system. [ABSTRACT FROM AUTHOR]

Traditional voltage control schemes in the power distribution system are typically centralized or local. Local control often gives nonoptimal solutions utilizing local measurements and lacks global coordination. Centralized control gives optimal solutions but lack scalability and robustness, especially with the increasing number of distributed energy resources (DERs). Recent advancement in power electronics with improved automation and digitalization in power grid control provides an opportunity to enable distributed optimization approaches for voltage control. This article proposes an optimal distributed voltage control algorithm based on augmented Lagrangian multiplier theory and primal-dual gradients for three-phase unbalanced distribution systems. In the developed algorithm, each node uses its own local measurements and neighboring nodes measurements to compute optimal reactive power setpoints, while meeting reactive power injection constraints for DERs. The proposed algorithm also considers practical constraints, such as communication delays, measurement noises, modeling errors, limited control devices, and infrequent switching required for field implementation. The proposed algorithm is validated for voltage control in an unbalanced, three-phase IEEE 123-bus feeder modeled in OpenDSS to demonstrate superior performance. [ABSTRACT FROM AUTHOR]

Distribution systems commonly operate with a radial topology, so all models of optimization problems in these distribution systems should consider radiality in their formulation. This work presents a literature review, a critical analysis, and a proposal for incorporating the radiality constraints in mathematical models of optimization problems for radial distribution systems. The objective is to show that the radiality constraints on such optimization problems can be considered in a simple and efficient way. The reconfiguration and expansion planning problems of distribution systems are used to test and verify the proposed radiality constraints. A generalization of radiality constraints is also examined. [ABSTRACT FROM PUBLISHER]

Capacitor planning in a distribution system (DS) must account for forecasted load and planned wind generators (WGs). In some DSs with large penetration, WGs support a significant part of the total load on an average and the power flow reverses to the substation during high-wind-low-load conditions. With annual forecasts of peak loads and generations being probabilistic in nature with differing probability distribution functions, their proper representation in a capacitor planning exercise is imperative. [ABSTRACT FROM PUBLISHER]

For the nonlinear power flow problem specified with standard PQ, PV, and slack bus equality constraints, we present a sufficient condition under which the specified set of nonlinear algebraic equations has no solution. This sufficient condition is constructed in a framework of an associated feasible, convex optimization problem. The objective employed in this optimization problem yields a measure of distance (in a parameter set) to the power flow solution boundary. In practical terms, this distance is closely related to quantities that previous authors have proposed as voltage stability margins. A typical margin is expressed in terms of the parameters of system loading (injected powers); here we additionally introduce a new margin in terms of the parameters of regulated bus voltages. [ABSTRACT FROM PUBLISHER]

The objective of this work is to design a suitable rule for the energy reference selection in the AC optimal power flow (ACOPF)-based locational marginal price (LMP) calculation. In the case of the ACOPF model, the congestion component of the LMP vector exhibits strong dependence on the energy reference. This in turn makes the hourly values of financial transmission rights (FTRs) to be reference dependent, and, thus, is a source of dispute. In order to resolve this dispute, a direct optimization framework is developed in this work exploring the interrelationship between the congestion components of locational marginal prices at individual buses. The objective function and the constraints of this optimization model are carefully designed looking at the true motive of the LMP decomposition. At the same time, necessary attention is paid so as to ensure revenue adequacy in the FTR settlement. The optimization problem that is ultimately to be solved is a simple quadratic programming problem with only one variable and two constraints. The effectiveness of the proposed methodology is verified through a case study on a modified IEEE 30-bus system. The framework developed also unveils several interesting features of the LMP components that were under cover until now. [ABSTRACT FROM PUBLISHER]

This letter proposes a methodology for finding the equilibrium point of a microgrid operating in a wide range of loading levels, which includes the possible exhaustion of the power reserve of some of the generating units. The search of the equilibrium would require the use of two different models when using Newton-Raphson-like methods. It is shown herein that the problem can be reformulated instead within the nonlinear constrained minimization framework by employing a complementarity condition. [ABSTRACT FROM PUBLISHER]