Islanded Mode Of Operation Research Articles

Direct Phase Angle and Voltage Amplitude Model Predictive Control of a Power Converter for Microgrid Applications

Several control strategies of the finite control set model predictive controls (FCS-MPC) have been proposed for power converters, such as predictive current control (PCC), direct predictive power control (DPPC), and predictive voltage control (PVC). However, for microgrid (MG) applications, the control strategy of the FCS-MPC for a power converter might be changed according to the operation mode of the MG system, which results in a transient response in the system voltage or current during the mode transition. This study proposes a new control strategy of FCS-MPC for use in both islanded and grid-connected operation modes of an MG system. Considering the characteristic of a synchronous generator, a direct phase angle and voltage amplitude model predictive control (PAC) of a power converter is proposed in this study for MG applications. In the islanded mode, the system frequency is directly controlled through the phase angle of the output voltage. In the grid-connected mode, a proportional-integral (PI) regulator is used to compensate for the phase angle and voltage amplitude of the power converter for constant power control. The phase angle of the system voltage can be easily adjusted for the synchronization process of an MG system. A comparison study on the proposed PAC method and existing predictive methods is carried out to show the effectiveness of the proposed method. The feasibility of the proposed PAC strategy is evaluated in a simulation-based system by using the MATLAB/Simulink environment.

Utilization of Supercapacitors in Protection Schemes for Resiliency Against Communication Outages: A Case Study on Size and Cost Optimization

Adaptive protection techniques used for a microgrid rely on a stable communication link to and from protective devices at the point of common coupling to adjust the settings within these corresponding devices for either a grid-connected or islanded mode of operation. However, during communication outages or in the event of a cyberattack, relays’ settings are not changed. Thus, adaptive protection schemes are rendered unsuccessful. Due to their fast response, supercapacitors, which are present in the microgrid to feed pulsed loads, could also be utilized to enhance the resiliency of adaptive protection schemes against communication outages. Proper sizing of the supercapacitors is therefore important in order to maintain a stable system operation and the cost of the protection scheme. This paper presents a two-level optimization scheme for minimizing the supercapacitor size along with optimizing its controllers’ parameters. The latter will lead to a reduction of the supercapacitor fault current contribution and an increase in that of other ac resources in the microgrid in the extreme case of having a fault occurring simultaneously with a pulsed load. It was also shown that the size of the supercapacitor can be reduced if the pulsed load is temporarily disconnected during the transient fault period. Simulation results showed that the resulting supercapacitor size and the optimized controller parameters were feeding enough fault currents for several types of faults in different locations and minimizing the cost of the protection scheme. Experimental results on a hardware microgrid setup validated the simulation results.

Microgrid Economic Dispatch With Energy Storage Systems

This paper presents a formulation to determine the appropriate power dispatch of an energy storage system, whose available energy is dependent on the charging/discharging pattern from previous time periods. The implementation structure is consistent with current dispatch algorithms used in microgrids, and the algorithm can be used in either grid-connected or islanded modes of operation. The proposed approach employs a backcasting algorithm to estimate the net stored energy value, against which the current cost of energy is compared to determine how the storage system should be used to perform arbitrage. The contribution of this paper is a means to include the time-dependent resource in traditional economic dispatch algorithms to reduce the cost of energy in a microgrid while enabling the arbitrage algorithm to continuously adapt to changing market conditions. Results show that the backcasting algorithm is able to reduce the average cost of energy by 8.14% and can reduce the average cost of energy by up to 72.3% of the ideal reduction, as determined by a perfect forecasting dispatch.

Privacy-preserving optimal scheduling of integrated microgrids

The increasing penetration of microgrids in distribution networks, as a viable option for end-use customers to increase load-point reliability and power quality, will result in formation of many interconnected microgrids in a not so far future. This paper considers a case in which multiple microgrids are geographically close and electrically connected, and studies anticipated interactions among these microgrids and also between the microgrids and the utility grid, during grid-connected and islanded operation modes. A model for the optimal scheduling of integrated microgrids is further proposed. The model is first developed with the objective of minimizing the aggregated operation cost and is accordingly decomposed into individual optimal scheduling problems using the Lagrangian relaxation method to take prevailing privacy issues into account. The microgrids capability in operating in the islanded mode for multiple hours is scrutinized by a T–τ islanding criterion. Numerical simulations exhibit the merits and the effectiveness of the proposed model via simulations on a system of integrated microgrids.

A Multivariable Fuzzy Rule-Based Relay for Short Circuits in AC Micro-grids

Objectives: This work seeks to present a new protective relay for short circuits in AC micro-grids, under different control strategies as well as in both grid-connected and islanded modes of operation. Methods/Statistical Analysis: An index test-bed which consists of two wind turbines as micro-sources and other requisite network elements is developed. From the dynamic short circuit analysis of the test-bed, the rules which govern interaction of four parameters are framed in fuzzy form. Using the framed rules, fuzzy logic controllers are designed for micro-source and feeder sub-relays. The hardware of each sub-relay is realized in SimPowerSystems using combinational logic components. Findings: Offline and online response tests of the proposed relay show that it generates logic 1 during short circuits and logic 0 during normal operating conditions in both grid-connected and islanded modes of operation of the micro-grid. The proposed relay also provides equivalent response under both voltage and reactive power control strategies. This is consistent with response of a reliable protective relay as reported in related literature. The proposed relay also supports plug-and-play and peer-to-peer requirements of micro-grids. Similar to digital relays reported in literature, the proposed relay is a departure from classical relays wherein protection is based on threshold of short circuit current. In the proposed relay, protection is based on parameters of micro-sources and feeders. Application/Improvements: The proposed relay finds use in providing protection for personnel and system against short circuit currents. It reliably protects against single line-to-ground, line-to-line and three-phase bolted short circuits. Keywords: Fuzzy, Micro-grids, Multivariable, Relay, Short Circuits

Compensation in Complex Variables for Microgrid Power Flow

The solution of the distribution network power flow in practical applications is based either on the forward–backward sweep method for radial networks, or the current injection method for meshed networks. While the power flow in microgrids that operate in grid-connected mode could be resolved using the above-mentioned approaches, their operation in island mode would require simulating local generation droop controllers for sharing the complex power load and network loss among the generators. This letter proposes a complex power compensation approach, which is based on Wirtinger calculus, for extending the applicability of practical distribution power flow methods to microgrids operating in island mode. Supporting numerical results are reported on microgrids with up to 3139 nodes.

Phase Fault Protection Scheme for Reliable Operation of Microgrids

In a microgrid, the overcurrent relays see significant variation in the short-circuit currents during transition between grid-connected and islanded modes of operation. Further, with the disconnection of one or more distributed generators (DGs), the changes in the short-circuit currents are observed by the relays. As a consequence, the existing protection scheme may either delay the relay operation or fail to operate. This paper proposes an optimal and single protection scheme suitable for all operating modes of microgrid along with every type of phase faults in the system, i.e., three-phase symmetrical ( $3\text{-}\phi$ ), line-to-line, and double line-to-ground faults. The optimal setting of the directional overcurrent relays is obtained using a metaheuristic-based optimization algorithm. Moreover, the proposed scheme is validated using GE Multilin, model-750/760, overcurrent relay in ETAP (industrial simulation software) on the modified IEEE 9-bus 4-DG microgrid.

A Communication-Supported Comprehensive Protection Strategy for Converter-Interfaced Islanded Microgrids

The deployment of distributed generators (DGs) gives rise to several challenges for a microgrid or conventional distribution feeder, regarding control and protection issues. The major ones are: bi-directional flow of power, changes in fault current magnitude, and continuous changes in operational configuration due to both the plug-and-play of DGs and loads, and the intermittency of the renewable DGs. This issue is exacerbated when the microgrid contains several converter-interfaced DGs and operates in the islanded mode of operation. Hence, conventional protection strategies and relaying techniques will no longer be sufficient to protect islanded microgrids against network faults and disturbance conditions. This paper proposes a fast and reliable communication-supported protection strategy for ensuring the safe operation of converter-interfaced islanded microgrids. The strategy is implementable using commercially accessible microprocessor based digital relays, and is applicable for the protection of low voltage islanded microgrids. It provides backup protection to handle communication failures and malfunctions of protective devices. The paper also presents the detailed structural layout of the digital relay, which executes the proposed protection strategy. A number of improvements are proposed to find an alternative method for conventional overcurrent relays to reliably detect small-magnitude fault currents and high impedance faults, commonly encountered in converter-interfaced islanded microgrids. A simple and economical bus protection method is also proposed. Several simulations are conducted on a comprehensive model of a realistic operational industrial microgrid (Goldwind Smart Microgrid System) using PSCAD/EMTDC software environment—for different case studies and fault scenarios—to verify the effectiveness of the present strategy and its digital relay.

QZS Inverter as Synchronverter in Small-Scale Micro-Grid

Focus is on the case study of a synchronveter model use in the qZS inverter working in an islanding operation mode. Main tasks and control principles of a sychronverter in the microgrid are explained. Theoretical assumptions are tested and verified with Psim simulation model with different types and powers of microinverter loads. General waveforms are presented and discussed. Also, the effectiveness of synchronverter implementation is verified by means of grid regulation capability.Additionally, we address the most challenging tasks for further developments and improvements of the qZS inverter based synchronverter in microgrids.DOI: http://dx.doi.org/10.5755/j01.eie.24.2.20636

Day-ahead optimal charging/discharging scheduling for electric vehicles in microgrids

Microgrid as an important part of smart grid comprises distributed generators (DGs), adjustable loads, energy storage systems (ESSs) and control units. It can be operated either connected with the external system or islanded with the support of ESSs. While the daily output of DGs strongly depends on the temporal distribution of natural resources such as wind and solar, unregulated electric vehicle (EV) charging demand will deteriorate the unbalance between the daily load curve and generation curve. In this paper, a statistic model is presented to describe daily EV charging/discharging behaviors considering the randomness of the initial state of charge (SOC) of EV batteries. The optimization problem is proposed to obtain the economic operation for the microgrid based on this model. In day-ahead scheduling, with the estimated power generation and load demand, the optimal charging/discharging scheduling of EVs during 24 h is achieved by serial quadratic programming. With the optimal charging/discharging scheduling of EVs, the daily load curve can better track the generation curve. The network loss in grid-connected operation mode and required ESS capacity in islanded operation mode are both decreased.

A Review of Communication Failure Impacts on Adaptive Microgrid Protection Schemes and the Use of Energy Storage as a Contingency

A main challenge in the practical implementation of a microgrid is the design of an adequate protection scheme in both grid-connected and islanded modes of operation. Microgrid protection requires a fast, reliable, and robust communication system to adjust relay settings to the appropriate current level according to the operation mode. Various approaches have been implemented to deal with this problem, yet the most promising ones are the use of adaptive protection techniques abiding by the International Electrotechnical Commission (IEC) 61850 communication standard. This paper presents a review of the technical challenges for the use of classical protection techniques and the need for an enhanced, adaptive, smart protection system. However, the risk of communication link failures and cyber security threats still remain a challenge in implementing a reliable adaptive protection scheme. A contingency is essential in the event that a communication failure prevents a relay from adjusting between grid-connected and islanded mode. An adaptive protection scheme is proposed, which utilizes energy storage devices to enhance resiliency against communication outages. The proposed solution does not incur additional costs to the network, as the energy storage devices are already utilized for other purposes. A case study experimentally verifies this contingency under a worst-case scenario, where a fault occurs simultaneously with a pulsed load while operating in islanded mode with a communication failure.

Passivity-Based Control for DC-Microgrids with Constant Power Terminals in Island Mode Operation

This paper presents a Passivity Based Control (PBC) for a dc microgrid. PBC is commonly used in electrical systems such as power electronics converters and electric machines, but there are few applications in microgrids operating in island mode. PBC is based on properties of passive systems and energy exchange between subsystems. The proposed strategy performs primary and secondary control of the hierarchical architecture. Local communications and measurements are needed at the nodes where dc/dc converters are placed. Simulation studies are performed in MATLAB to validate the control in a realistic test system composed of renewable energies sources, loads and energy storage units. Results show that the proposed control ensures stability and fast response of the dc-bus voltage under different operating conditions.

Optimal Performance Design Guideline of Hybrid Reference Frame Based Dual-Loop Control Strategy for Stand-Alone Single-Phase Inverters

The dual-loop control strategy in a hybrid reference frame (HRF) for single-phase voltage-source inverters in islanded operation mode is studied, which applies a capacitor voltage shaping loop in the synchronous reference frame (SRF) and a capacitor current shaping loop in the stationary reference frame (HRF-based $v+ i_{c}$ control strategy). This strategy is able to achieve the purpose of active damping, fast dynamic response, and zero reference tracking error. However, due to the inherent characteristics of SRF-based voltage loop and the digital control delay, the performance of the system is degraded and the control parameter design of the HRF-based $v+ i_{c}$ control strategy shows great difficulties. To overcome these shortcomings, in this paper, a systematic parameter design guideline for the HRF-based $v+ i_{c}$ control strategy is proposed to ensure the system stability and optimize the performance of the system under control delay condition. The mathematic model of the HRF-based $v+ i_{c}$ control strategy is established with the consideration of control delay in this paper. Based on this model, a satisfactory region of the system stability indexes can be obtained by stability specifications of the system, and the optimal control parameters can be calculated according to the stability indexes selected from the satisfactory region. By using this method, the system stability and robustness can be guaranteed. Finally, the experimental results are presented to validate the effectiveness of the presented optimal control parameter design methodologies.

Power Management of Islanded Self-Excited Induction Generator Reinforced by Energy Storage Systems

Self-Excited Induction Generators (SEIGs), e.g., Small-Scale Embedded wind generation, are increasingly used in electricity distribution networks. The operational stability of stand-alone SEIG is constrained by the local load conditions: stability can be achieved by maintaining the load’s active and reactive power at optimal values. Changes in power demand are dependent on customers’ requirements, and any deviation from the pre-calculated optimum setting will affect a machine’s operating voltage and frequency. This paper presents an investigation of the operation of the SEIG in islanding mode of operation under different load conditions, with the aid of batteries as an energy storage source. In this research a current-controlled voltage-source converter is proposed to regulate the power exchange between a direct current (DC) energy storage source and an alternating current (AC) grid, the converter’s controller is driven by any variation between machine capability and load demand. In order to prolong the system stability when the battery reaches its operation constraints, it is recommended that an ancillary generator and a dummy local load be embedded in the system. The results show the robustness and operability of the proposed system in the islanding mode of the SEIG under different load conditions.

Reactive power sharing improvement of droop‐controlled DFIG wind turbines in a microgrid

This study presents an innovative control scheme to improve the power sharing among doubly-fed induction generator (DFIG) wind units in a medium-voltage (MV) microgrid. The control objectives of DFIGs in an islanded mode of microgrid operation are to achieve: (i) stabilisation of the microgrid voltage amplitude and frequency, (ii) proper active/reactive power sharing among wind units. To satisfy these requirements, the DFIG control loop based on the traditional droop control is designed. This method, however, cannot satisfactorily operate in a MV microgrid with dominantly resistive line impedances from power sharing point of view. To overcome this problem, a modified control strategy is proposed in this study. The mathematical modelling is developed, and time-domain simulations are presented to verify the novel control scheme in a typical microgrid case study.

A flexible active distribution system expansion planning model: A risk-based approach

This paper presents an active distribution network expansion planning framework, which concurrently uses the renewable distributed generations and energy storage systems as capacity expansion options. In order to enhance the network reliability, the model takes into account the island mode operation of the renewable resources and energy storage systems. The proposed planning framework, which is modeled as a probabilistic bi-level optimization problem quantifies and controls the economic risk level associated with the stochastic nature of these resources. The master level is devoted to the here-and-now decisions in the planning phase, whereas the slave level, which is formulated as a two-stage model, is related to the wait-and-see decisions in the operational phase. At the first stage of the slave level, the network operational behaviour is determined by performing an optimal power flow modeled as a mixed-integer linear programming problem. At the second stage of the slave problem, the network reliability is optimized considering island mode operation and taking into account energy-limited nature of storage systems. The effectiveness of the proposed active distribution network planning model is demonstrated through several case studies. Simulation results demonstrate that the proposed approach can result in a flexible low-risk plan for the expansion of the network.

Linear Quadratic Regulator-Based Bumpless Transfer in Microgrids

This paper presents a linear quadratic regulator (LQR)-based bumpless transfer controller for achieving seamless transition between the grid connected and the islanded modes of operation in microgrids. During the grid connected mode, the inverter-based distributed generators (DGs) operate in current controlled mode where they supply a constant active and reactive power. During the islanded mode, the inverters operate in voltage controlled mode. During transition between the two control modes severe transients can occur in the system which affect the voltage and frequency responses. The proposed bumpless compensator reduces the error between the outputs of two controllers at the instant of transition by optimally reducing an error measure based on LQR theory. The efficacy of the proposed controller is tested on a microgrid test system. Several operating scenarios on the microgrid have been tested using the proposed approach. It has been found that the proposed controller is effective in reducing the voltage and the frequency transients during transition.

Meandering Vector Control Strategy as D - STATCOM in Renewable Cluster Grid for Power Optimization

The aim of this project is to develop a dynamic model of a converter fed Microgrid operated in Islanded DER Control Mode to regulate its voltage and frequency in the islanded mode of operation. Controllers like PI, FUZZY, FUZZY-PI to be implemented to fine which is best controller for the developed control scheme. The microgrid has two operating modes: the grid-connected mode and the islanded mode. In the grid-connected mode, the MMS is locked and the microgrid can interchange the energy with the host lattice network. In this mode, the DER systems exchange real and reactive powers with the distribution network, according to the corresponding set points; the difference between the aggregate power generated by the DER systems and the power demanded by the local loads is balanced by the upstream network.

A Big Data Scale Algorithm for Optimal Scheduling of Integrated Microgrids

The capability of switching into the islanded operation mode of microgrids has been advocated as a viable solution to achieve high system reliability. This paper proposes a new model for the microgrids optimal scheduling and load curtailment problem. The proposed problem determines the optimal schedule for local generators of microgrids to minimize the generation cost of the associated distribution system in the normal operation. Moreover, when microgrids have to switch into the islanded operation mode due to reliability considerations, the optimal generation solution still guarantees for the minimal amount of load curtailment. Due to the large number of constraints in both normal and islanded operations, the formulated problem becomes a large-scale optimization problem and is very challenging to solve using the centralized computational method. Therefore, we propose a decomposition algorithm using the alternating direction method of multipliers that provides a parallel computational framework. The simulation results demonstrate the efficiency of our proposed model in reducing generation cost, as well as guaranteeing the reliable operation of microgrids in the islanded mode. We finally describe the detailed implementation of parallel computation for our proposed algorithm to run on a computer cluster using the Hadoop MapReduce software framework.

Neutral Earthing Reactance Design in Mitigating Third Harmonic Current at the Generator Neutral

Third harmonic (TH) current from salient-pole synchronous generator that return to its neutral via cable capacitance or other neutral grounding points in the network can cause the neutral earthing resistor (NER) to experience high temperature. Generator neutral earthing reactance (NERX) can be used to mitigate the TH current flowing in the neutral under normal condition. This paper aims to study the suitable design of NERX to be implemented in Universiti Teknologi PETRONAS (UTP) distribution system to reduce the TH current at the generator neutral. The model of UTP distribution system is developed using MATLAB Simulink in order to simulate the flow of TH current. NERX is then designed and implemented to the system to mitigate the magnitude of TH current. Various values of NERX are applied to the system as the neutral current and fault current are recorded and graphed. The application of 0.175 H NERX increased the TH neutral current in island mode and normal operation to 33.18 %. However, the NERX causes the TH fault current to reduce to 44.33 % under fault. For parallel mode, 78.27 % of TH neutral current and also 42.91 % of TH fault current are reduced under normal operation and fault respectively.