Low-voltage Microgrid Research Articles

D-STATCOM With Positive-Sequence Admittance and Negative-Sequence Conductance to Mitigate Voltage Fluctuations in High-Level Penetration of Distributed-Generation Systems

Voltage fluctuations resulting from variable output power of renewable energy sources are strictly challenging power quality in distributed-generation systems. This paper presents a control method for distributed static synchronous compensator (D-STATCOM) to alleviate variation of both positive- and negative-sequence voltages. The D-STATCOM simultaneously operates as fundamental positive-sequence admittance and fundamental negative-sequence conductance to restore the positive-sequence voltage to the nominal value as well as reduce the negative-sequence voltage to an allowable level. Both admittance and conductance are dynamically tuned to improve voltage-regulation performances in response to load changes and power variation of renewable sources. A proportional-resonant current regulator with selectively harmonic compensation is realized to control the fundamental current of the D-STATCOM as well as reduce the harmonic current, which could be an advantage in practical applications due to high voltage distortion in low-voltage microgrids. Voltage-regulation performances are discussed for different D-STATCOM locations as well as different D-STATCOM currents. Computer simulations and laboratory tests validate effectiveness.

An Open Platform for Rapid-Prototyping Protection and Control Schemes With IEC 61850

Communications is becoming increasingly important to the operation of protection and control schemes. Although offering many benefits, using standards-based communications, particularly IEC 61850, in the course of the research and development of novel schemes can be complex. This paper describes an open-source platform which enables the rapid prototyping of communications-enhanced schemes. The platform automatically generates the data model and communications code required for an intelligent electronic device to implement a publisher-subscriber generic object-oriented substation event and sampled-value messaging. The generated code is tailored to a particular system configuration description (SCD) file, and is therefore extremely efficient at runtime. It is shown here how a model-centric tool, such as the open-source Eclipse Modeling Framework, can be used to manage the complexity of the IEC 61850 standard, by providing a framework for validating SCD files and by automating parts of the code generation process. The flexibility and convenience of the platform are demonstrated through a prototype of a real-time, fast-acting load-shedding scheme for a low-voltage microgrid network. The platform is the first open-source implementation of IEC 61850 which is suitable for real-time applications, such as protection, and is therefore readily available for research and education.

Arctan Power Frequency Droop for Power Electronics Dominated Microgrids

>The microgrid is an environment constructed with the aim of controlling local distributed resources. The most common power flow control technique utilised in a power electronics dominated or standalone microgrid is a technique known as power frequency droop. This paper develops a novel droop controller based on an arctan power profile that can provide natural limiting of frequency deviations and accurate frequency output. The new control system has been simulated in SABER and results are presented that show the advantages of the control. A low voltage microgrid consisting of two inverters and a passive load has been constructed. Experimental results from the hardware system are presented to confirm the operation of the new droop controller.

Arctan power frequency droop for power electronics dominated microgrids

The microgrid is an environment constructed with the aim of controlling local distributed resources. The most common power flow control technique utilised inside a power electronics dominated or standalone microgrid is- a technique known as power frequency droop. This paper develops a novel droop controller based on an arctan power profile that can provide natural limiting of frequency deviations and accurate frequency output. The new control system has been simulated in SABER and results are presented that show the advantages of the control. A low voltage microgrid consisting of two inverters and a passive load has been constructed. Experimental results from the hardware system are presented to confirm the operation of the new droop controller.

An Integrated Control Strategy Adopting Droop Control with Virtual Inductance in Microgrid

As there exists sorts of distributed generators in microgrid, an integrated control strategy containing different control methods against corresponding generators should be applied. The strategy in this paper involves PQ control and droop control methods. The former aims at letting generators like PV output maximum power. The latter stems from inverter parallel technique and applies to controlling generators which can keep the network voltage steady to make the parallel system reach the minimum circulation point. Due to the unworthiness of droop control applied in low-voltage microgrid of which the impedance ratio is rather high, the paper adopts the droop control introducing virtual generator and virtual impedance. Based on theoretical analysis, simulation in Matlab is also implemented to verify the feasibility of the strategy.

A DC Arc Model for Series Faults in Low Voltage Microgrids

This paper presents a dc arc model to simplify the study of a critical issue in dc microgrids: series faults. The model is derived from a hyperbolic approximation of observed arc voltage and current patterns, which permit analyzing the arc in terms of its resistance, power, energy, and quenching condition. Recent faults staged by the authors on a dc microgrid yielded enough data to develop an arc model for three fault types: constant-gap speed, fixed-gap distance, and accelerated gap. The results in this paper compare experimental and simulation results for the three fault types. It is concluded that because the instantaneous voltage, current, power, and energy waveforms produced by the model agree well with experimental results, the model is suitable for transient simulations.

Protection Scheme for DC Microgrid Distribution System

The low voltage DC (LVDC) distribution system is a new concept and a promising technology to be used in the future smart distribution system having high level cost-efficiency and reliability. In this paper, a low-voltage (LV) DC microgrid protection system design is proposed. Usually, an LVDC microgrid must be connected to an ac grid through converters with bidirectional power flow and, therefore, a different protection scheme is needed. This paper describes practical protection solutions for the LVDC network and an LVDC system laboratory prototype is being experimentally tested by MATLAB/SIMULINK. The results show that it is possible to use available devices to protect such a system. But other problems may arise which needs further study.

A Hierarchical Coordinated Control Method for Low Voltage Micro-Grid in Intelligent Community

The operation control of the micro-grid system is the foundation to protect micro-grid equipment, to realize associate control and optimize economic operation. Looking on Chenjiazhen micro-grid in Shanghai as model, the paper set three hierarchical controls: protection control, micro-grid coordinated controller control which can achieve millisecond time precision and intelligent community energy management system control. Different level control function has been divided. Control functions in all levels complement each other and collaborate together to achieve the coordination and control of micro-grid running.

Voltage and Frequency Droop Control in an Autonomous Microgrid

In this paper, a method for the parallel operation of inverters in an autonomous microgrid system is adopted. This paper presents the resistive output impedance control scheme that allows multiple voltage source converters (VSCs) to operate in parallel in a VSC fed microgrid. The control loops are taking into account the special nature of a low-voltage microgrid, in which the line impedance is mainly resistive. In contrast with the conventional droop-control method, the proposed controller uses a virtual resistance without communication signals to achieve good power sharing, which is insensitive to line-impedance unbalances.

Matrix perturbation based approach for sensitivity analysis of eigen-solutions in a microgrid

Sensitivities of eigen-solutions to control variables play an important role in microgrid studies, such as coordinated optimal design of controllers and parameters, robust stability analysis on control variables, oscillation modes analysis on a system, etc. Considering the importance of sensitivities and the complexity of state matrix in a microgrid, parameter perturbations are utilized in this paper to analyze the construction characteristics of state matrix. Then, the sensitivities of eigenvalues and eigenvectors to control variables are obtained based on the first-order matrix perturbation theory, which makes the complex derivations of sensitivity formulas and repeated solutions of eigenvalue problem unnecessary. Finally, the effectiveness of the matrix perturbation based approach for sensitivity calculation in a microgrid is verified by a numerical example on a low-voltage microgrid prototype.

Automatic Power-Sharing Modification of $P$/$V$ Droop Controllers in Low-Voltage Resistive Microgrids

Microgrids are receiving an increasing interest to integrate the growing share of distributed-generation (DG) units in the electrical network. For the islanded operation of the microgrid, several control strategies for the primary control have been developed to ensure stable microgrid operation. In low-voltage (LV) microgrids, active power/voltage (P/V) droop controllers are gaining attention as they take the resistive nature of the network lines and the lack of directly coupled rotating inertia into account. However, a problem often cited with these droop controllers is that the grid voltage is not a global parameter. This can influence the power sharing between different units. In this paper, it is investigated whether this is actually a disadvantage of the control strategy. It is shown that with P/V droop control, the DG units that are located electrically far from the load centers automatically deliver a lower share of the power. This automatic power-sharing modification can lead to decreased line losses; therefore, there is overall better efficiency compared to the methods that focus on perfect power sharing. In this paper, the P/V and P/f droop control strategies are compared with respect to this power-sharing modification and the line losses.

An Enhanced Microgrid Load Demand Sharing Strategy

For the operation of autonomous microgrids, an important task is to share the load demand using multiple distributed generation (DG) units. In order to realize satisfied power sharing without the communication between DG units, the voltage droop control and its different variations have been reported in the literature. However, in a low-voltage microgrid, due to the effects of nontrivial feeder impedance, the conventional droop control is subject to the real and reactive power coupling and steady-state reactive power sharing errors. Furthermore, complex microgrid configurations (looped or mesh networks) often make the reactive power sharing more challenging. To improve the reactive power sharing accuracy, this paper proposes an enhanced control strategy that estimates the reactive power control error through injecting small real power disturbances, which is activated by the low-bandwidth synchronization signals from the central controller. At the same time, a slow integration term for reactive power sharing error elimination is added to the conventional reactive power droop control. The proposed compensation method achieves accurate reactive power sharing at the steady state, just like the performance of real power sharing through frequency droop control. Simulation and experimental results validate the feasibility of the proposed method.

Convergence analysis of a distributed voltage support strategy for optimal reactive power compensation

We consider the problem of commanding the electronic power interfaces of the microgenerators in a low voltage microgrid for the task of optimal reactive power compensation. In this work, we analyze the convergence of the strategy proposed by Tenti et al. (2012). The proof of convergence gives some additional insight on the behavior of the algorithm and allows the characterization of its rate of convergence as a function of the microgrid parameters.

A Microcontroller-Based Power Management System for Standalone Microgrids With Hybrid Power Supply

The paper presents a microcontroller-based power management system (PMS) designed for the online operation of an experimental low voltage microgrid equipped with a battery storage system and two power supplies: a kilowatt (kW)-class proton exchange membrane (PEM) fuel cell (FC) and a photo-voltaic (PV) module emulator, both connected to a low voltage ac node. The connections of the energy sources to the common ac bus make use of power inverters with specific functionalities. The ac node feeds electric active and reactive load emulators able to reproduce programmable profiles. The automatic PMS provides the microgrid monitoring and the FC power scheduling in both grid-connected and islanded operating conditions. The paper describes the structure and functionalities of the PMS as well as a specific experimental investigation aimed at assessing the dynamic performance of the microgrid in islanded conditions.

Operation of a Utility Connected Microgrid Using an IEC 61850-Based Multi-Level Management System

This paper presents a management system for a utility connected low-voltage microgrid composed of three nodes: a wind turbine, a battery and a variable load. These nodes are not commercial equipment but emulators. Each unit is composed of two identical voltage sources in back-to-back configuration. The microgrid is managed by a three-layered hierarchical automation system: the RMU layer, the iNode layer and the iSocket layer. The automation system permits two possible control modes: a centralized mode and a distributed mode. It has been developed an active and reactive power control that can be used for both management modes. Communications among the iNode and the iSockets are according to IEC 61850 standard. To implement IEC 61850 it has been used MMS-EASE Lite solution from SISCO, Inc. A commercial IEC 61850 SCADA has also been used.

Performance of the Grid-Connected PV System in LV Microgrid Operation

Microgrids are a future power system configuration providing clear economic and environmental benefits compared to the legacy power systems, as the Grid-Connected PV penetration increases, its reaction in Low Voltage (LV) microgrid has to be taken into account during relative system studies. This paper presents a mathematical model for the Grid-Connected PV, it’s developed by User Define (UD) module on Power System Analysis Software Package (PSASP), PV behavior under several typical weather and typical 1-day load curve is studied in detail, Flexible Operation Strategy to achieve the reasonable voltage level are both considered, PSASP simulation environment is used to analyze the probable operation scenarios of LV microgrid, useful conclusions are summarized at last.

Stage Fault Test of a Low Voltage Microgrid for Development of Protection Scheme

One of the key elements of microgrid is protection system. To design the protection system for a 380 V microgrid, a stage fault test has been conducted in a microgrid test bed built at the Institute of Nuclear Energy Research (INER), Taiwan. The special feature of ground fault voltage of the 380 V microgrid is investigated in the test, based on which a protection scheme is proposed. This paper presents the INER microgrid test bed and the stage fault test conducted as well as the preliminary evaluation results of the proposed protection scheme.

A Protection Strategy and Microprocessor-Based Relay for Low-Voltage Microgrids

One of the major challenges associated with microgrid protection is to devise an appropriate protection strategy that is effective in the grid-connected as well as islanded mode of operation. This paper proposes a protection strategy based on microprocessor-based relays for low-voltage microgrids. Further, the structure of a new relay enabling the proposed protection strategy is presented. One of the salient feature of the developed protection scheme is that it does not require communications or adaptive protective devices. Moreover, it is to a large extent independent of the fault current magnitude and the mode of operation. Transient time-domain simulation studies are conducted to demonstrate the effectiveness of the proposed protection strategy and its enabling relay, using the PSCAD/EMTDC software package.

A Control Strategy for Islanded Microgrids With DC-Link Voltage Control

New opportunities for optimally integrating the increasing number of distributed-generation (DG) units in the power system rise with the introduction of the microgrid. Most DG units are connected to the microgrid via a power-electronic inverter with dc link. Therefore, new control methods for these inverters need to be developed in order to exploit the DG units as effectively as possible in case of an islanded microgrid. In the literature, most control strategies are based on the conventional transmission grid control or depend on a communication infrastructure. In this paper, on the other hand, an alternative control strategy is proposed based on the specific characteristics of islanded low-voltage microgrids. The microgrid power is balanced by using a control strategy that modifies the set value of the rms microgrid voltage at the inverter ac side as a function of the dc-link voltage. In case a certain voltage, which is determined by a constant-power band, is surpassed, this control strategy is combined with P/V -droop control. This droop controller changes the output power of the DG unit and its possible storage devices as a function of the grid voltage. In this way, voltage-limit violation is avoided. The constant-power band depends on the characteristics of the generator to avoid frequent changes of the power of certain DG units. In this paper, it is concluded that the new control method shows good results in power sharing, transient issues, and stability. This is achieved without interunit communication, which is beneficial concerning reliability issues, and an optimized integration of the renewable energy sources in the microgrid is obtained.

Power Management of Inverter Interfaced Autonomous Microgrid Based on Virtual Frequency-Voltage Frame

This paper presents the power management scheme for a power electronics based low voltage microgrid in islanding operation. The proposed real and reactive power control is based on the virtual frequency and voltage frame, which can effectively decouple the real and reactive power flows and improve the system transient and stability performance. Detailed analysis of the virtual frame operation range is presented, and a control strategy to guarantee that the microgrid can be operated within the predetermined voltage and frequency variation limits is also proposed. Moreover, a reactive power control with adaptive voltage droop method is proposed, which automatically updates the maximum reactive power limit of a DG unit based on its current rating and actual real power output and features enlarged power output range and further improved system stability. Both simulation and experimental results are provided in this paper.