Under frequency load shedding (UFLS) scheme has been widely implemented as a safety net to prevent system collapse following major disturbances which results in large mismatch between load and generation. However, voltage related issues post activation of UFLS stages are also vital concerns in preserving system stability and should be properly analysed. Fixed and switched shunt capacitors that are in service during normal operation for maintaining system voltage and dynamic MVAR reserve can generate surplus reactive power post operation of UFLS relays which may result in over voltage issues, generator under excitation and some undesirable conditions such as transformer saturation, ferro-resonance etc. This study addresses the need for coordinated under frequency load and capacitor shedding and its implementation approach to effectively preserve system stability following small and large disturbances. To confirm the feasibility of the approach, the proposed method has been used to design coordinated UFLS and under frequency capacitor shedding (UFCS) schemes for a real and actual power network. In addition, the proposed coordinated UFLS and UFCS scheme has been combined with automatic switching of shunt reactors to optimise the performance of the scheme.

Distributed generations (DGs) are recently in growing attention as a solution to environmental and economical challenges caused by conventional power plants. In this study, a multi-objective framework as a nonlinear programming (NLP) is proposed for optimal placement and sizing of DG units. Objective functions include minimising the number of DGs and power losses as well as maximising voltage stability margin formulated as a function of decision variables. The objective functions are combined into one objective function. To avoid problems with choosing appropriate weighting factors, fuzzification is applied to objective functions to bring them into the same scale. DG units are placed at more efficient buses rather than end buses of radial links as usually determined by previous methods for improving voltage stability. Also, power system constraints including branch and voltage limits are observed in the problem. The proposed method not only is able to model all types of DG technologies but also it employs adaptive reactive limits for DGs rather than fixed limits. In addition, a three-stage procedure is proposed to gradually solve the multi-objective problem in order to prevent infeasible solutions. Also, a new technique is proposed to formulate the number of DGs without converting the NLP problem into mixed-integer NLP. Results of testing the proposed method show its efficiency.

This study proposes a secondary voltage and frequency control scheme based on the distributed cooperative control of multi-agent systems. The proposed secondary control is implemented through a communication network with one-way communication links. The required communication network is modelled by a directed graph (digraph). The proposed secondary control is fully distributed such that each distributed generator only requires its own information and the information of its neighbours on the communication digraph. Thus, the requirements for a central controller and complex communication network are obviated, and the system reliability is improved. The simulation results verify the effectiveness of the proposed secondary control for a microgrid test system.

In this study, a novel dynamic paradigm aimed at solving the power flow equations in both solvable and unsolvable region is proposed. The challenging idea is to reformulate the power flow equations into a set of ordinary differential equations, whose equilibrium points represent the power flow problem solutions. Starting from the Lyapunov theory, the authors demonstrate that this system of dynamic equations is characterised by an exponential asymptotic convergence to equilibrium points. This feature allows us to overcome the inherent limitations of the traditional iterative minimisation algorithms that can fail to converge because of the highly non-linearities of the first-order condition. Extensive simulation studies aimed at demonstrating the effectiveness of the proposed methodology are presented and discussed.

Batteries play an important role in sustainable energy systems because the energy stored in batteries can be dispatched at any time. Series-connected batteries can furnish a load with higher voltage and consequently reduce the I ² R loss during power conversion and transmission. However, if imbalanced voltage occurs, a battery with high state of charge (SOC) would react more drastically than that with lower SOC under both charging and discharging conditions. Imbalanced charge or discharge would give rise to potential damage to the battery and also shorten the battery life cycle. This study presents a modified charge equaliser (CE) along with an optimal charge equalisation algorithm (CEA). The proposed circuit scheme can distribute an imbalanced charge in a more efficient way without increasing additional circuit cost. To further expedite charge equalisation among a battery string, the activated CE duty ratio is fixed at the upper limits throughout the charge equalisation phase. An optimal CEA with a view towards maximising the final battery string voltage is developed to resolve the CE operating sequence and length. The proposed equalisation strategy is realised using a peripheral interface controller that uses a buck–boost converter as the CE. The experimental results confirm the performance of the proposed strategy.

The overload phenomenon in distribution networks is a prevalent event, which usually causes the unwanted outages happen in distribution networks. The malfunction of the protection system as a result of high load currents during the overload situations yields network outages which are often unpredictable. The reliability indices in distribution systems are currently calculated neglecting the effect of overload condition. Having a correct estimation of the number of the overload-related outages in distribution networks enables the network planners and operators to better choice a proper solutions to avoid or decrease the side-effects of this event. In this study, in order to estimate the outage rates in distribution networks because of the overload phenomenon, a stochastic model for the electrical loads is presented. The normal cumulative distribution function is used to model the magnitude of the load demand at any time of the day. Monte Carlo approach is applied for random selection of the load magnitude. Load model results are compared with real measured data and the validity of the results is investigated. Electrical loads of the IEEE 34-bus system are modified by incorporating the real load variation curves to provide more realistic results. The results are presented and analysed in detail.

Economic dispatch (ED) is an optimisation tool that is used to allocate active load demands to the generating units through optimising the fuel generation cost function subject to the different operational constraints. The high non-linearity of the power system imposes mathematical challenges in formulating the generation cost function models, which makes the ED problem hard to solve. This study introduces two ideas to solve issues related to the ED problem. First, a dynamic formulation technique is developed to optimally allocate the change in the total active load demand to the generating units. This technique is shown to be insensitive to the optimality of the initial active load distribution unlike the base point and participation factor method. Moreover, it guarantees an optimal distribution among the generating units due the change in the active load demand. Second, a novel approximation of the non-convex generation cost function is developed to solve non-convex ED problem with the transmission losses. This approximation enables the use of gradient and Newton techniques to solve the non-convex ED problem with valve point loading effect and transmission losses in an analytic approach. This approximation is compared with some heuristic optimisation techniques.

In this study, a hybrid method is proposed to calculate oscillatory parameters and the approximate mode shapes (AMSs) of low frequency oscillations based on the measured signals from wide area measurement system. This method not only overcomes the limitations of the single algorithm, but also integrates the advantages of the used techniques. First of all, two-level decomposition is presented to minish the scale-mixing influence in the empirical mode decomposition. Then, the Gibbs phenomenon of traditional Hilbert transform (HT) is reduced by using the normalised HT. Next, the relative phase calculation algorithm is introduced to realise the generator grouping and mode shape identification. Finally, the AMSs of dominant oscillation at different time ranges and frequency distributions are determined. The EPRI–36 bus simulation model and actual measured signals are used to evaluate the performances and validities of the proposed hybrid method.

The control paradigms of the distributed generation (DG) sources in the smart grid are realised by either utilising virtual power plant (VPP) or by employing MicroGrid structures. Both VPP and MicroGrid are presented with the problem of control of power flow between their comprising DG sources. This study depicts this issue for VPP and proposes a novel and improved universal active and reactive power flow controllers for three-phase pulse width modulated voltage source inverters (PWM-VSI) operating in the VPP environment. The proposed controller takes into account all cases of R–X relationship, thus allowing it to function in systems operating at high, medium (MV) and low-voltage (LV) levels. Also proposed control scheme for the first time in an inverter control takes into account the capacitance of the transmission line which is an important factor to accurately represent medium length transmission lines. This allows the proposed control scheme to be applied in VPP structures, where DG sources can operate at MV LV levels over a short/medium length transmission line. The authors also conducted small signal stability analysis of the proposed controller and compared it against the small signal study of the existing controllers.

This study investigates the effect of voltage profile and steady-state voltage stability margin when a wind-driven squirrel cage induction generator (SCIG)-based distributed generator (DG) is integrated with radial distribution system (RDS). The study has been carried out considering different configurations of SCIG. The node at which a DG has to be installed is identified based on voltage collapse index (VCI), which is a measure of steady-state voltage stability margin. The computational procedure for studying the voltage profile of RDS without and with DG has been developed. The developed algorithm has been tested on the 33-bus RDS with 1 MW SCIG integrated at the identified node and results are furnished. Finally, a configuration of SCIG having star and delta switching arrangements with permanently connected capacitor across each phase winding of the stator is found to be superior in providing improved system performance over a wide range of wind velocity. A switching criterion for this type of DG is proposed based on VCI for the improved performance of RDS.

In deregulated electricity markets, currently, most independent system operators adopt the offer cost minimisation (OCM) auction, and pay-as-market clearing price (MCP) mechanism is used as settlement rule to determine the amount of payments. In such a structure, auction mechanism and settlement rule are not completely compatible because minimised costs differ from payment costs. In this regard, payment cost minimisation (PCM) mechanism is an alternative option as it minimises the total payment cost directly. Although, there exist numerous efficient solution methods for solving the OCM problem, the solution methods for solving PCM problem are very restricted and deficient. As the objective of study, the mixed-integer linear programming (MILP) formulation for solving the PCM problem is presented. Based on several case studies, the effectiveness of the MILP method for solving PCM problem, in comparison with the only acceptable existing method, is illustrated. Additionally, a novel solution methodology is presented for solving MILP formulation of PCM problems. Further, in this study, exponential start up cost curves are utilised instead of single block start up cost functions, as considered in previous studies. In addition, the effects of the shape of load profile on the performance of OCM mechanism, in comparison with PCM, are analysed.

In this study, a deterministic single-stage transmission expansion planning model considering line losses and deployment of energy storage systems (ESSs) is proposed. A piecewise linearisation approach using secant segments is adopted to estimate non-linear line losses, and the optimal partitioning method is studied. ESSs are introduced to reduce the overall cost, and their siting and sizing are determined by nodal power balance and load duration curve. The effectiveness of the proposed model is shown through case studies simulated on two test systems, and the potential of ESSs reducing network investment costs is illustrated in a quantitative manner.