A Novel and Generalized Three-Phase Power Flow Algorithm for Islanded Microgrids Using a Newton Trust Region Method
TL;DR: In this paper, a generic three-phase power flow algorithm is formulated for islanded microgrids, where the features of distribution systems, i.e., threephase feeder models, unbalanced loads and load models have been taken in consideration.
Abstract: A new formulation is required to provide a proper power flow analysis in islanded microgrids taking into consideration their special philosophy of operation. In this paper, a novel and generic three-phase power flow algorithm is formulated for islanded microgrids. The algorithm is novel since it adapts the real characteristics of the islanded microgrid operation; i.e., 1) some of the distributed generation (DG) units are controlled using the droop control methods and their generated active and reactive power are dependent on the power flow variables; 2) the steady-state system frequency is considered as one of the power flow variables. The proposed algorithm is generic, where the features of distribution systems, i.e., three-phase feeder models, unbalanced loads and load models have been taken in consideration. Further, all possible operation modes of DG units (droop, PV, or PQ) have been considered. The problem has been formulated as a set of nonlinear equations. A globally convergent Newton-trust region method has been proposed to solve this set of nonlinear equations. The proposed algorithm is a helpful tool to perform accurate steady state studies of the islanded microgrid. Different case studies have been carried out to test the effectiveness and the robustness of the proposed algorithm.
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01 Jan 2009
TL;DR: In this article, the authors proposed a power flow control between utility and microgrid through back-to-back converters, which facilitates desired real and reactive power flow between the utility and the microgrid.
Abstract: 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 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 ar-rangement has been validated through simulation for various operating conditions. The model of the microgrid power system is simulated in PSCAD.
258 citations
TL;DR: In this article, the authors proposed a simple and effective modifications to the conventional method (Newton Raphson) to compute the power flow for micro-grids, which can be easily integrated in current commercially available power system software and can be applied for power system studies.
Abstract: The study of power flow analysis for microgrids has gained importance where several methods have been proposed to solve these problems. However, these schemes are complicated and not easy to implement due to the absence of a slack bus as well as the dependence of the power on frequency as a result of the droop characteristics. This paper proposes simple and effective modifications to the conventional method (Newton Raphson) to compute the power flow for microgrids. The presented method provides a simple, easy to implement, and accurate approach to solve the power flow equations for microgrids. The proposed method is applied to two test systems: a 6-bus system and a 38-bus system. The results are compared against simulation results from PSCAD/EMTDC which validate the effectiveness of the developed method. The proposed technique can be easily integrated in current commercially available power system software and can be applied for power system studies.
202 citations
Cites background or methods or result from "A Novel and Generalized Three-Phase..."
...A static load model can be represented as [5], [17]...
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...In a multi-source islanded microgrid, the assumption of any DG to act as a slack bus is inoperative as there is no single DG capable of maintaining the system frequency and its local bus voltage constant [5]....
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...Thus, new methods have been proposed to solve the power flow analysis for islanded microgrids [5]–[7]....
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...A new power flow formulation that incorporates the droop bus has been presented as a set of nonlinear equations and solved using a globally convergent Newton-trust region method in [5]....
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...The simulation results, presented in this paper, closely match the results presented using the Newton Trust region method proposed in [5] and the results obtained using PSCAD....
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TL;DR: The mathematical models for both types of MGs considering the concept of virtual impedance are used to be in conformity with the practical control of the DGs and calculation accuracy is improved, comparing with previous methods without considering virtual impedance.
Abstract: In the low-voltage (LV) ac microgrids (MGs), with a relatively high R/X ratio, virtual impedance is usually adopted to improve the performance of droop control applied to distributed generators (DGs). At the same time, LV dc MG using virtual impedance as droop control is emerging without adequate power flow studies. In this paper, power flow analyses for both ac and dc MGs are formulated and implemented. The mathematical models for both types of MGs considering the concept of virtual impedance are used to be in conformity with the practical control of the DGs. As a result, calculation accuracy is improved for both ac and dc MG power flow analyses, comparing with previous methods without considering virtual impedance. Case studies are conducted to verify the proposed power flow analyses in terms of convergence and accuracy. Investigation of the impact to the system of internal control parameters adopted by DGs is also conducted by using proposed method.
155 citations
Cites background or methods from "A Novel and Generalized Three-Phase..."
...The problem of tailoring conventional power flow programs for microgrid applications has been recently addressed in [10]–[12]....
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...To extend the proposed model into three-phase, without considering any unbalance compensation (the proper value of virtual impedance for this purpose is out the scope of this work), equal virtual impedance to each phase and its corresponding virtual droop bus can be added based on the model used in [12]....
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...The method proposed in [12] represents correctly the actual distributed slack buses by modelling the DG units as droop buses....
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TL;DR: A decision making framework is proposed based on IGDT model to solve the information gap decision theory problem and the effectiveness of the proposed tool is assessed and demonstrated by applying it on a large distribution network.
Abstract: This paper presents the application of information gap decision theory (IGDT) to help the distribution network operators (DNOs) in choosing the supplying resources for meeting the demand of their customers. The three main energy resources are pool market, distributed generations (DGs), and the bilateral contracts. In deregulated environment, the DNO is faced with many uncertainties associated to the mentioned resources which may not have enough information about their nature and behaviors. In such cases, the classical methods like probabilistic methods or fuzzy methods are not applicable for uncertainty modeling because they need some information about the uncertainty behaviors like probability distribution function (PDF) or their membership functions. In this paper, a decision making framework is proposed based on IGDT model to solve this problem. The uncertain parameters considered here, are as follows: price of electricity in pool market and demand of each bus. The robust strategy of DNO is determined to hedge him against the risk of increasing the total cost beyond what it is willing to pay. The effectiveness of the proposed tool is assessed and demonstrated by applying it on a large distribution network.
127 citations
TL;DR: In this paper, a multistage centralized control scheme for droop-controlled islanded microgrids with high penetration of plug-in electric vehicles (PEVs) is proposed, which optimally coordinates the distributed generation units' droop characteristics, the shedding of microgrid power demand (during inadequate generation periods), and the PEVs charging/discharging decisions to support the IMG operation for a large time frame.
Abstract: This paper proposes a multistage centralized control scheme for droop-controlled islanded microgrids (IMGs) with high penetration of plug-in electric vehicles (PEVs). The proposed control scheme optimally coordinates the distributed generation (DG) units’ droop characteristics, the shedding of microgrid power demand (during inadequate generation periods), and the PEVs charging/discharging decisions to support the IMG operation for a large time frame. This coordination allows the PEVs to play a pivotal role in the successful and optimized operation of the IMG systems. To this end, a novel multistage droop-based optimal power flow algorithm is proposed in order to: 1) minimize the load shedding; 2) satisfy the PEVs customers’ requirements; and 3) minimize the microgrid cost of operation. The proposed algorithm takes into consideration: 1) the special features and operational philosophy of droop-controlled IMG systems; 2) the variability associated with the output power of renewable DG units; and 3) the random behavior of PEV charging. Several case studies have been carried out to show the effectiveness and robustness of the proposed control scheme. The simulation studies show that the proposed control scheme can enhance the operation of IMG systems and facilitate a successful implementation of the IMG concept in the presence of high PEV penetration.
112 citations
Cites background from "A Novel and Generalized Three-Phase..."
...Hence, it did not account for the special features and operational philosophy of droop-controlled IMG systems [8]....
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...Nonetheless, other system operational requirements might not be satisfied, where 1) the reactive power sharing between the DG units is not exact and depends on the system parameters [5]–[7]; 2) even though the voltage regulation requirements are met at the DG units’ PCC, still a voltage violation might occur at some load points due to voltage drops along the feeders [7], [8]; 3) the conventional droop characteristic does not consider the optimized economical operation of IMGs [9], [10]....
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References
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TL;DR: An overview of the structures for the DPGS based on fuel cell, photovoltaic, and wind turbines is given and the possibility of compensation for low-order harmonics is discussed.
Abstract: Renewable energy sources like wind, sun, and hydro are seen as a reliable alternative to the traditional energy sources such as oil, natural gas, or coal. Distributed power generation systems (DPGSs) based on renewable energy sources experience a large development worldwide, with Germany, Denmark, Japan, and USA as leaders in the development in this field. Due to the increasing number of DPGSs connected to the utility network, new and stricter standards in respect to power quality, safe running, and islanding protection are issued. As a consequence, the control of distributed generation systems should be improved to meet the requirements for grid interconnection. This paper gives an overview of the structures for the DPGS based on fuel cell, photovoltaic, and wind turbines. In addition, control structures of the grid-side converter are presented, and the possibility of compensation for low-order harmonics is also discussed. Moreover, control strategies when running on grid faults are treated. This paper ends up with an overview of synchronization methods and a discussion about their importance in the control
4,655 citations
"A Novel and Generalized Three-Phase..." refers background in this paper
...The majority of DG units are interfaced via a power electronic converter and an output filter [1], [21]....
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TL;DR: In this paper, the authors developed a model for autonomous operation of inverter-based micro-grids, where each sub-module is modeled in state-space form and all are combined together on a common reference frame.
Abstract: The analysis of the small-signal stability of conventional power systems is well established, but for inverter based microgrids there is a need to establish how circuit and control features give rise to particular oscillatory modes and which of these have poor damping. This paper develops the modeling and analysis of autonomous operation of inverter-based microgrids. Each sub-module is modeled in state-space form and all are combined together on a common reference frame. The model captures the detail of the control loops of the inverter but not the switching action. Some inverter modes are found at relatively high frequency and so a full dynamic model of the network (rather than an algebraic impedance model) is used. The complete model is linearized around an operating point and the resulting system matrix is used to derive the eigenvalues. The eigenvalues (termed "modes") indicate the frequency and damping of oscillatory components in the transient response. A sensitivity analysis is also presented which helps identifying the origin of each of the modes and identify possible feedback signals for design of controllers to improve the system stability. With experience it is possible to simplify the model (reduce the order) if particular modes are not of interest as is the case with synchronous machine models. Experimental results from a microgrid of three 10-kW inverters are used to verify the results obtained from the model
2,482 citations
"A Novel and Generalized Three-Phase..." refers background or methods in this paper
...The choice of the minimum and maximum allowable values of the frequency and voltage magnitude depend on the required voltage and frequency regulation [3]....
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...These characteristics are usually justified by assuming that the output impedance of the converter ismainly inductive due to the coupling inductor used at the converter output [1], [3], the large inductor of the output filter or by the use of virtual inductive output impedance [25]....
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...To validate the applicability of the novel power flow algorithm and demonstrate its accuracy, the results of the proposed algorithm are compared with the steady-state results obtained from a detailed time-domainmodel [1], [3] of an islandedmicrogrid....
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TL;DR: In this article, real and reactive power management strategies of EI-DG units in the context of a multiple DG microgrid system were investigated. And the results were used to discuss applications under various microgrid operating conditions.
Abstract: This paper addresses real and reactive power management strategies of electronically interfaced distributed generation (DG) units in the context of a multiple-DG microgrid system. The emphasis is primarily on electronically interfaced DG (EI-DG) units. DG controls and power management strategies are based on locally measured signals without communications. Based on the reactive power controls adopted, three power management strategies are identified and investigated. These strategies are based on 1) voltage-droop characteristic, 2) voltage regulation, and 3) load reactive power compensation. The real power of each DG unit is controlled based on a frequency-droop characteristic and a complimentary frequency restoration strategy. A systematic approach to develop a small-signal dynamic model of a multiple-DG microgrid, including real and reactive power management strategies, is also presented. The microgrid eigen structure, based on the developed model, is used to 1) investigate the microgrid dynamic behavior, 2) select control parameters of DG units, and 3) incorporate power management strategies in the DG controllers. The model is also used to investigate sensitivity of the design to changes of parameters and operating point and to optimize performance of the microgrid system. The results are used to discuss applications of the proposed power management strategies under various microgrid operating conditions
1,531 citations
"A Novel and Generalized Three-Phase..." refers background in this paper
...Further, the droop control can be used to optimize the islanded microgrid operation via different power management strategies [4]–[8]....
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TL;DR: In this paper, a new control method for the parallel operation of inverters operating in an island grid or connected to an infinite bus is described, where each inverter supplies a current that is the result of the voltage difference between a reference ac voltage source and the grid voltage across a virtual complex impedance.
Abstract: In this paper, a new control method for the parallel operation of inverters operating in an island grid or connected to an infinite bus is described. Frequency and voltage control, including mitigation of voltage harmonics, are achieved without the need for any common control circuitry or communication between inverters. Each inverter supplies a current that is the result of the voltage difference between a reference ac voltage source and the grid voltage across a virtual complex impedance. The reference ac voltage source is synchronized with the grid, with a phase shift, depending on the difference between rated and actual grid frequency. A detailed analysis shows that this approach has a superior behavior compared to existing methods, regarding the mitigation of voltage harmonics, short-circuit behavior and the effectiveness of the frequency and voltage control, as it takes the R to X line impedance ratio into account. Experiments show the behavior of the method for an inverter feeding a highly nonlinear load and during the connection of two parallel inverters in operation.
1,528 citations
TL;DR: An algorithm for the problem of minimizing a quadratic function subject to an ellipsoidal constraint is proposed and it is shown that this algorithm is guaranteed to produce a nearly optimal solution in a finite number of iterations.
Abstract: We propose an algorithm for the problem of minimizing a quadratic function subject to an ellipsoidal constraint and show that this algorithm is guaranteed to produce a nearly optimal solution in a finite number of iterations. We also consider the use of this algorithm in a trust region Newton's method. In particular, we prove that under reasonable assumptions the sequence generated by Newton's method has a limit point which satisfies the first and second order necessary conditions for a minimizer of the objective function. Numerical results for GQTPAR, which is a Fortran implementaton of our algorithm, show that GQTPAR is quite successful in a trust region method. In our tests a call to GQTPAR only required 1.6 iterations on the average.
1,434 citations
"A Novel and Generalized Three-Phase..." refers background in this paper
...It can be proved [32] that the solution of this constrained minimization satisfies...
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