Advanced control strategies are vital components for realization of microgrids. This paper reviews the status of hierarchical control strategies applied to microgrids and discusses the future trends. This hierarchical control structure consists of primary, secondary, and tertiary levels, and is a versatile tool in managing stationary and dynamic performance of microgrids while incorporating economical aspects. Various control approaches are compared and their respective advantages are highlighted. In addition, the coordination among different control hierarchies is discussed.

Hierarchical Structure of Microgrids Control System

DC–DC converters with voltage boost capability are widely used in a large number of power conversion applications, from fraction-of-volt to tens of thousands of volts at power levels from milliwatts to megawatts. The literature has reported on various voltage-boosting techniques, in which fundamental energy storing elements (inductors and capacitors) and/or transformers in conjunction with switch(es) and diode(s) are utilized in the circuit. These techniques include switched capacitor (charge pump), voltage multiplier, switched inductor/voltage lift, magnetic coupling, and multistage/-level, and each has its own merits and demerits depending on application, in terms of cost, complexity, power density, reliability, and efficiency. To meet the growing demand for such applications, new power converter topologies that use the above voltage-boosting techniques, as well as some active and passive components, are continuously being proposed. The permutations and combinations of the various voltage-boosting techniques with additional components in a circuit allow for numerous new topologies and configurations, which are often confusing and difficult to follow. Therefore, to present a clear picture on the general law and framework of the development of next-generation step-up dc–dc converters, this paper aims to comprehensively review and classify various step-up dc–dc converters based on their characteristics and voltage-boosting techniques. In addition, the advantages and disadvantages of these voltage-boosting techniques and associated converters are discussed in detail. Finally, broad applications of dc–dc converters are presented and summarized with comparative study of different voltage-boosting techniques.

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Step-Up DC–DC Converters: A Comprehensive Review of Voltage-Boosting Techniques, Topologies, and Applications

DC microgrids (MGs) have been gaining a continually increasing interest over the past couple of years both in academia and industry. The advantages of dc distribution when compared to its ac counterpart are well known. The most important ones include higher reliability and efficiency, simpler control and natural interface with renewable energy sources, and electronic loads and energy storage systems. With rapid emergence of these components in modern power systems, the importance of dc in today's society is gradually being brought to a whole new level. A broad class of traditional dc distribution applications, such as traction, telecom, vehicular, and distributed power systems can be classified under dc MG framework and ongoing development, and expansion of the field is largely influenced by concepts used over there. This paper aims first to shed light on the practical design aspects of dc MG technology concerning typical power hardware topologies and their suitability for different emerging smart grid applications. Then, an overview of the state of the art in dc MG protection and grounding is provided. Owing to the fact that there is no zero-current crossing, an arc that appears upon breaking dc current cannot be extinguished naturally, making the protection of dc MGs a challenging problem. In relation with this, a comprehensive overview of protection schemes, which discusses both design of practical protective devices and their integration into overall protection systems, is provided. Closely coupled with protection, conflicting grounding objectives, e.g., minimization of stray current and common-mode voltage, are explained and several practical solutions are presented. Also, standardization efforts for dc systems are addressed. Finally, concluding remarks and important future research directions are pointed out.

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DC Microgrids—Part II: A Review of Power Architectures, Applications, and Standardization Issues

Multimegawatt wind-turbine systems, often organized in a wind park, are the backbone of the power generation based on renewable-energy systems. This paper reviews the most-adopted wind-turbine systems, the adopted generators, the topologies of the converters, the generator control and grid connection issues, as well as their arrangement in wind parks.

Overview of Multi-MW Wind Turbines and Wind Parks

In this paper, a novel control method of grid-connected voltage-source converters (VSCs) is proposed. The method can be generally applied for all grid-connected VSCs but may be of most importance in high-voltage dc (HVDC) applications. Different from the previous control methods, the proposed method utilizes the internal synchronization mechanism in ac systems, in principle, similar to the operation of a synchronous machine. By using this type of power-synchronization control, the VSC avoids the instability caused by a standard phase-locked loop in a weak AC-system connection. Moreover, a VSC terminal can give the weak ac system strong voltage support, just like a normal synchronous machine does. The control method is verified by both analytical models and time simulations.

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Power-Synchronization Control of Grid-Connected Voltage-Source Converters

This comprehensive reference guides the reader through all HVDC technologies, including LCC (Line Commutated Converter), 2-level VSC and VSC HVDC based on modular multilevel converters (MMC) for an in-depth understanding of converters, system level design, operating principles and modeling. Written in a tutorial style, the book also describes the key principles of design, control, protection and operation of DC transmission grids, which will be substantially different from the practice with AC transmission grids. The first dedicated reference to the latest HVDC technologies and DC grid developments; this is an essential resource for graduate students and researchers as well as engineers and professionals working on the design, modeling and operation of DC grids and HVDC.

High Voltage Direct Current Transmission: Converters, Systems and DC Grids

This research addresses the special requirements of phase locked loops (PLLs) for a typical application with FACTS elements. A new PLL system that uses adaptation algorithms is developed with the aim of improving speed of responses, robustness to AC voltage depressions, and harmonic rejection. The adaptive PLL consists of the three control units that individually control frequency, phase angle, and voltage magnitude. The voltage controller output is used to compensate for reduced gain caused by the AC voltage magnitude depressions. The output phase angle and its derivative, the frequency signal, are controlled in two independent control systems in order to enable elimination of frequency and phase error without compromising transient responses. The simulation results are compared with a PLL available with the PSB MATLAB block-set and noticeable improvements are demonstrated. In particular, settling time and overshooting are significantly lower with conditions of reduced AC voltage magnitude.

Phase locked loop system for FACTS

This paper outlines the design of an offshore wind farm using a dc offshore grid based on resonant dc-dc converters. Multiphase resonant dc-dc converters are studied to step up the dc voltage from individual wind generators to a Medium Voltage (MV) dc bus, and from the MV bus to an HVDC line that will connect the wind farm to shore. Compared to an equivalent ac grid-based wind farm, a dc grid-based wind farm has slightly higher losses, but the weight of the magnetic components and cables is substantially lower. The analysis of operating permanent-magnet synchronous generators at variable and constant dc voltages shows that a fixed dc voltage has marginally higher efficiency than a variable dc voltage. However, using a variable dc voltage gives lower harmonics at the generator facing the voltage-source converter and the dc-dc step-up converter. An aggregated model of multiple parallel connected wind generators is developed and shown to accurately approximate a detailed PSCAD model during varying wind conditions and transients.

Analysis and Design of an Offshore Wind Farm Using a MV DC Grid

The construction of the North Sea Super Grid is the major step towards meetingthe future demand for electric power transmission in northern Europe This gridwill likely also extend onshore towards the load centres, and eventually form theEuropean Super GridLarge-scale electric power generation at remote locations will lead tosignificant long distance power flows with a preferred flow direction A methodto identify these unidirectional flows has been developed and applied on a casestudy, indicating the importance to consider unidirectional flows when designinga super gridVoltage source converter based HVDC appears to be the best technicalsolution for the implementation of long distance transmission in such an offshoresuper grid AC technology appears to be the most convenient choice for offshorenodes, but DC might also gain importance in this field, if reliable and affordableDC protection systems become available A meshed DC grid offers significantadvantages towards a solution with many independent point-to-point HVDClinks, but also here protection is an unsolved issue that has to be overcome firstReliability assessment of HVDC-based super grids is still very difficult,because operational experience with new technologies like the modularmultilevel converter is limited This leads to a lack of data to calculate thefailure probabilitiesA test system with a DC grid and the connected AC grids has been developedto serve as a common reference for a variety of DC grid studiesUnlike classical AC grids, DC grids will be dominated by power electronicsand the system behaviour will be determined to a large extent by the controllersof those power electronic systems Large-scale implementation of powerelectronics with inappropriate control design has led to problems in AC systemsbefore Photovoltaic generation systems in Germany are a good example forthisA simplified AC frequency model has been developed to assess how powerelectronic systems influence the grid frequency This model has been used to simulate how photovoltaic generation systems in Germany can endanger systemstability A ‘grid-friendly’ charging controller for plug-in electric vehicles withbattery storage has been developed, and simulations have indicated that thiscontrol can contribute significantly to system stabilityEven though AC and DC grids have some significant differences, some ofthe general concepts and lessons regarding balancing are true for both, andtomorrow’s DC grids can learn from today’s AC challengesThe balance in a DC grid should be defined as a current balance rather thanan active power balance (as it is used in AC grids), and the voltage can serve asa balance indicator, similar to AC frequency in AC grids The control base forcontrolling the voltage should also be current instead of active power, leadingto linear system behaviour and a linear control taskHVDC converter control methods can be regarded as cases of droop controlwith one or more linear segments in the characteristic control curve Withinone linear segment of the control curve, a HVDC converter can be representedby the Thevenin or Norton equivalent circuitTo unify a variety of proposed control concepts, Undead-band droop controlhas been proposed as a general piece-wise linear voltage control, which includesall other proposed methods as special implementations of undead-band droopcontrol This concept could also be applied for other tasks than DC voltagecontrol like AC frequency control

The CIGRE B4 DC Grid Test System

This paper studies a bidirectional dc-dc converter concept which is capable of achieving very high stepping ratios with MW level power transfers. The converter can find potential application in connecting high-power dc sources, interfacing to high-voltage dc transmission or to flexible ac transmission system elements. The converter is based on two resonant circuits which share a common ac capacitor. The topology is simple and utilizes thyristors with potentially all soft switchings. Complete analytical modelling is presented which enables systematic design procedure for the converter. The detailed digital simulation on PSCAD platform confirms satisfactory operation on a 5-MW test system, which connects 4-kV dc source to an 80-kV high voltage dc grid. The converter shows good responses to rapid changes in power magnitude/direction and it is concluded that robustness to terminal voltage disturbances is excellent. Since the highest-power phase-control thyristors are employed, the converter can potentially be used at much higher power levels. The passive components are of reasonable size.

Dragan Jovcic

Papers

High Voltage Direct Current Transmission: Converters, Systems and DC Grids

Phase locked loop system for FACTS

Analysis and Design of an Offshore Wind Farm Using a MV DC Grid

The CIGRE B4 DC Grid Test System

Bidirectional, High-Power DC Transformer