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This study proposes a decentralised droop control method for guaranteeing precise load sharing among distributed resources in an islanded microgrid. Distributed resources are usually fed through power electronic converters and the switching harmonics produced by them are eliminated by third-order output LCL filters. Output impedance is considered as a major factor for finding exact power-angle droop coefficient in droop design - neglecting this factor can affect the load sharing accuracy. Even though an acceptable active power sharing can be achieved with higher droop gains, increased droop gain may adversely affect the microgrid stability. Here, a modified angle droop control is proposed such that the dependence on the output inductance on the real power sharing is removed. Thus, the lower droop coefficients are sufficient for droop sharing and the system stability is not endangered. It has been assumed that the microgrid is converter-dominated, where a proportional-resonant controller has been utilised for converter switching control. This controller has an outer voltage loop and an inner current loop. A harmonic term has been added to the voltage loop to facilitate more accurate reactive power sharing. Simulation studies are conducted using PSCAD/EMTDC to validate the efficacy of the proposed controller. [ABSTRACT FROM AUTHOR]

This paper proposes a distributed control approach to coordinate multiple energy storage units (ESUs) to avoid violation of voltage and thermal constraints, which are some of the main power quality challenges for future distribution networks. ESUs usually are connected to a network through voltage source converters. In this paper, both ESU converters active and reactive power are used to deal with the above mentioned power quality issues. ESUs' reactive power is proposed to be used for voltage support, while the active power is to be utilized in managing network loading. Two typical distribution networks are used to apply the proposed method, and the simulated results are illustrated in this paper to show the effectiveness of this approach. [ABSTRACT FROM PUBLISHER]

In this paper, the performance of voltage-source converter-based shunt and series compensators used for load voltage control in electrical power distribution systems has been analyzed and compared, when a nonlinear load is connected across the load bus. The comparison has been made based on the closed-loop frequency response characteristics of the compensated distribution system. A distribution static compensator (DSTATCOM) as a shunt device and a dynamic voltage restorer (DVR) as a series device are considered in the voltage-control mode for the comparison. The power-quality problems which these compensator address include voltage sags/swells, load voltage harmonic distortions, and unbalancing. The effect of various system parameters on the control performance of the compensator can be studied using the proposed analysis. In particular, the performance of the two compensators are compared with the strong ac supply (stiff source) and weak ac-supply (non-stiff source) distribution system. The experimental verification of the analytical results derived has been obtained using a laboratory model of the single-phase DSTATCOM and DVR. A generalized converter topology using a cascaded multilevel inverter has been proposed for the medium-voltage distribution system. Simulation studies have been performed in the PSCAD/EMTDC software to verify the results in the three-phase system. [ABSTRACT FROM AUTHOR]

This paper proposes a method for power flow control between utility and microgrid through back-to-back converters, which facilitates desired real and reactive power flow between utility and microgrid. In the proposed control strategy, the system can run in two different modes depending on the power requirement in the microgrid. In mode-1, specified amount of real and reactive power are shared between the utility and the microgrid through the back-to-back converters. Mode-2 is invoked when the power that can be supplied by the distributed generators (DGs) in the microgrid reaches its maximum limit. In such a case, the rest of the power demand of the microgrid has to be supplied by the utility. An arrangement between DGs in the microgrid is proposed to achieve load sharing in both grid connected and islanded modes. The back-to-back converters also provide total frequency isolation between the utility and the microgrid. It is shown that the voltage or frequency fluctuation in the utility side has no impact on voltage or power in microgrid side. Proper relay-breaker operation coordination is proposed during fault along with the blocking of the back-to-back converters for seamless resynchronization. Both impedance and motor type loads are considered to verify the system stability. The impact of dc side voltage fluctuation of the DGs and DG tripping on power sharing is also investigated. The efficacy of the proposed control arrangement has been validated through simulation for various operating conditions. The model of the microgrid power system is simulated in PSCAD. [ABSTRACT FROM AUTHOR]

This paper investigates the problem of appropriate load sharing in an autonomous microgrid. High gain angle droop control ensures proper load sharing, especially under weak system conditions. However, it has a negative impact on overall stability. Frequency-domain modeling, eigenvalue analysis, and time-domain simulations are used to demonstrate this conflict. A supplementary loop is proposed around a conventional droop control of each DG converter to stabilize the system while using high angle droop gains. Control loops are based on local power measurement and modulation of the d-axis voltage reference of each converter. Coordinated design of supplementary control loops for each DG is formulated as a parameter optimization problem and solved using an evolutionary technique. The supplementary droop control loop is shown to stabilize the system for a range of operating conditions while ensuring satisfactory load sharing. [ABSTRACT FROM AUTHOR]

In rural low-voltage networks, distribution lines are usually highly resistive. When many distributed generators are connected to such lines, power sharing among them is difficult when using conventional droop control, as the real and reactive power have strong coupling with each other. A high droop gain can alleviate this problem but may lead the system to instability. To overcome this, two droop control methods are proposed for accurate load sharing with frequency droop controller. The first method considers no communication among the distributed generators and regulates the output voltage and frequency, ensuring acceptable load sharing. The droop equations are modified with a transformation matrix based on the line R/X ratio for this purpose. The second proposed method, with minimal low bandwidth communication, modifies the reference frequency of the distributed generators based on the active and reactive power flow in the lines connected to the points of common coupling. The performance of these two proposed controllers is compared with that of a controller, which includes an expensive high bandwidth communication system through time-domain simulation of a test system. The magnitude of errors in power sharing between these three droop control schemes are evaluated and tabulated. [ABSTRACT FROM AUTHOR]

Abstract-In this paper, an optimization-based control algorithm for the compensation of steady-state load under distorted supply voltages is presented. For balanced and sinusoidal supply voltages, load compensation using shunt active filter produces perfect harmonic cancellation (PHC) and unity power factor (UPF) source currents. However, when the supply voltages are distorted, compensation for PHC will not provide UPF, as the harmonics in the supply voltages are not utilized for delivering the average power. In the same way, to achieve UPF source currents, the compensated source currents should have the same harmonics, unbalance, shape, and be in phase with the respective supply voltages, thereby violating the perfect harmonic cancellation objective. Hence, there should be an optimal operation between these two important compensation characteristics. The optimization-based control algorithm presented in this paper gives the best power factor while satisfying the constraints such as total harmonic distortion limits and average power balance of source currents. It is also flexible to adopt different compensation characteristics based upon the supply voltage conditions. MATLAB and its optimization toolbox are used for simulation studies and solving the nonlinear optimization problem, respectively. The algorithm is validated by using a prototype of digital signal processor (TMS320F2812PGFA)-based shunt active power filter. Detailed experimental results are presented. [ABSTRACT FROM AUTHOR]

Abstract: The paper discusses the voltage control of a critical load bus using a dynamic voltage restorer (DVR) in a distribution system. The critical load requires a balanced sinusoidal waveform across its terminals preferably at system nominal frequency of 50Hz. It is assumed that the frequency of the supply voltage can vary and is different from the system nominal frequency. The DVR is operated such that it holds the voltage across the critical load bus terminals constant at system nominal frequency irrespective of the frequency of the source voltage. In case of a frequency mismatch, the total real power requirement of the critical load has to be supplied by the DVR. To compensate for frequency variation, the dc link of the DVR is supplied through an uncontrolled rectifier that provides a path for the real power required by the critical load to flow. A simple frequency estimation technique is discussed which uses a moving average process along with zero-crossing detector. Through detailed analysis and simulation studies using PSCAD/EMTDC it is shown that the voltage is completely controlled across the critical load. [Copyright &y& Elsevier]

Abstract: The paper discusses the operation and control algorithms of shunt and series active filters. Performance of each of these filters is studied separately when it is connected to protect sensitive loads in a power distribution system. It is assumed that the distribution system is polluted due to the presence of the loads that may be unbalanced and draw interharmonic currents from the feeder. Separate algorithms have been proposed for the shunt and the series active filters. These are operated such that the voltage waveform at the sensitive load bus terminal is a clean balanced sinusoid. The proposed algorithms are verified through the computer simulation studies using PSCAD/EMTDC software package. [Copyright &y& Elsevier]

In this paper, two techniques of game theory are considered for sizing and comparative analysis of a grid-connected networked microgrid, based on a multi-objective imperialistic competition algorithm (ICA) for system optimization. The selected networked microgrid, which consists of two different grid-connected microgrids with common electrical load and main grid, might have different combinations of generation resources including wind turbine, photovoltaic panels, and batteries. The game theory technique of Nash equilibrium is developed to perform the effective sizing of the networked microgrid in which capacities of the generation resources and batteries are considered as players and annual profit as payoff. In order to meet the equilibrium point and the optimum sizes of generation resources, all possible coalitions between the players are considered; ICA, which is frequently used in optimization applications, is implemented using MATLAB software. Both techniques of game theory, Shapley values and Nash equilibrium, are used to find the annual profit of each microgrid, and results are compared based on optimum sizing, and maximum values of annual profit are identified. Finally, in order to validate the results of the networked microgrid, the sensitivity analysis is studied to examine the impact of electricity price and discount rates on maximum values of profit for both game theory techniques. [ABSTRACT FROM AUTHOR]

Presence of large number of single-phase, customer-owned, distributed energy resources that are installed randomly in any phase of Low Voltage (LV) distribution feeders, may lead the generation, in a particular phase, to exceed its load demand. This may result in reverse power flow in that phase towards the upstream network while in the other phases, power flows from the upstream network to the LV feeder. To avoid such problems, this paper proposes the use of Distribution Static Compensators (DSTATCOM) to circulate the reverse flowing power from one phase to the other phases. In this paper, the installation of DSTATCOMs is considered in the medium voltage feeders. The DSTATCOM can be connected in either wye or delta, both of these connections are explored here. Different control strategies are developed for each connection. The main objective of the DSTATCOM is to provide a path for the power, to circulate among the phases, through its DC link. The DSTATCOM controller is further developed to balance its upstream powers and to supply the load reactive power demand. The proposed configurations and the developed control methods for the DSTATCOM are numerically validated through extensive computer simulation studies in PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

• Cooperative game technique used to make a clustered microgrid, and each microgrid has generation resources and batteries. • All possible coalitions are discussed to find the optimum results, and an algorithm is developed to find most feasible solution. • Capacity is allocated to each player, and maximized the profit for clustered system, and sensitivity analysis is performed. • Player's careful selection is important for a game formation to get maximum profit. In this paper a cooperative type of game theoretical technique is proposed at the planning stage to model a grid-connected clustered microgrid. In order to make it more applicable, the selected microgrid consists of different combinations of generation resources like wind turbines, solar cells, and batteries. In the game model, generation resources are considered as players and their profit as the payoff. The technique of the Nash bargaining solution is adopted in this study for capacity allocation of generation resources and batteries, and also to maximize the annual profit of individual microgrids and its cluster. As a cooperative game model, all possible coalitions are discussed between the players to find their optimum sizes, and most the suitable one is selected based upon the game theory technique and maximum payoff value. For this purpose, a particle swarm optimization (PSO) algorithm is developed to find the most feasible Nash bargaining solution using MATLAB software. In the simulations and for the system analysis, the realistic electrical load data and weather forecast is taken for a remote town Mount Magnet in Western Australia. Finally, a sensitivity analysis is performed to validate and show the reasonability of the optimized results concerning the proposed clustered system. [ABSTRACT FROM AUTHOR]