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Energy storage systems are widely used for compensation of intermittent renewable energy sources and restoration of system frequency and voltage. In a conventional operation, all distributed energy storage systems are clustered into one fixed virtual power plant and their state of charges are maintained at a common value. In this article, it is proposed to dynamically cluster the energy storage systems into several virtual power plants based on the energy storage systems’ power demands and capacities. This results in reduced network power losses. The proposed dynamic clustering algorithm enables to cluster agents (energy storage systems) based on their preselected feature states (local power demands and energy storage capacities). To determine the clusters, the distance of the agents’ current feature states from the average estimates of the states is determined in all clusters. The algorithm also provides average estimates of designated auxiliary states that can be used for control purposes. Presented RTDS-based real-time implementation results verify that clustering energy storage systems (batteries) into dynamic virtual power plants can reduce the network power losses.

In this article, the effect of large double frequency capacitor voltage ripple (CVR) on harmonic performance, efficiency, and capacitor lifetime in the cascaded H-bridge low capacitance static reactive power compensator (StatCom) (LC-StatCom) is analytically determined. With insight provided by the analysis, an LC-StatCom with modular switched capacitors is proposed that provides a degree of freedom to control the CVR magnitude. The premise of the operation is to keep redundant capacitor modules in standby mode when operating with lower currents. In addition, it is shown that by distributing losses between additional dc-side switches and the main H-bridge switches, the proposed concept provides opportunities for choosing an optimal hybrid combination of semiconductor technologies. The proposed concept is evaluated using a simulated 8.5 MVA StatCom and feasibility of the proposed system is confirmed by experiments on a seven-level 0.8 kVA single-phase LC-StatCom prototype.

This paper presents a new framework, utilizing information-gap decision theory (IGDT), for multiobjective robust security-constrained unit commitment of generating units in the presence of wind farms and gridable vehicles. Both the wind power and load demand uncertainties are considered, and modeled using a biobjective model. As the main advantage, the framework enables the system operator to take an appropriate operational decision with respect to the extremity of each uncertainty. The proposed problem is solved using a normal boundary intersection technique. Subsequently, VIsekriterijumska optimizacija i KOmpromisno Resenje (VIKOR), a decision-making tool, is utilized to choose the best Pareto optimal solution. Finally, the IGDT-based framework presented in this paper is validated using a six-bus test system, the IEEE Reliability Test System with 24 buses, and the IEEE 118-bus system.

This paper expands a previous study regarding application of modular variable filter inductor in a cascaded H-bridge low-capacitance StatCom (LC-StatCom). In this paper, the benefit of having such a filter to expand the operating capability of the LC-StatCom into the inductive region is demonstrated. A comparison between the modular filter inductor and thyristor bypassed series inductor solutions is performed through derivation of analytic expressions. The added capacity to operate in the inductive region is also validated via experimental results.

This paper proposes a distributed coordination control strategy for load sharing and energy balancing between heterogeneous energy storages. These control objectives are satisfied through a two-level control structure. At the primary level, the decentralized virtual impedance control, without the requirement of communication links, allocates the low frequency component of the loads to batteries, while the high frequency component is allocated to ultracapacitors. Distributed control strategy, introduced at the secondary level over a sparse communication network, achieves battery state of charge balancing and regulation of the local bus voltages. Two sets of data are exchanged via the communication links, the local bus voltages, and state of charges of batteries. The distributed controller for the restoration of local bus voltages implements an average consensus protocol, while the controller for energy balancing uses a cooperative protocol. In addition, the ultracapacitor voltages are locally restored at a slower time-scale. The proposed control strategy is resilient to communication link failures and features plug-and-play capability. Presented results demonstrate performance of the proposed control strategy for an islanded 380 VDC datacenter with variable loads. Different operating conditions are verified through the RTDS Technologies real-time digital power system simulator using switching converter models and nonlinear battery models.

In this paper, a cascaded H-bridge low-capacitance StatCom (LC-StatCom) with a modular inductive filter is introduced. Replacing the fixed-value filter inductors with thyristor-switched reactors, a well-developed member of the static VAR compensators family, reduces the overall energy storage capacity and voltage rating of the LC-StatCom without violating standard limits on current quality. The proposed modular inductive filter takes advantage of LC-StatCom's ability to synthesize high-quality ac voltage to reduce the overall energy storage requirement of the filter inductance. The proposed modular filter inductor concept adds an additional degree of freedom to control the maximum voltage drop on the filter inductor by changing the inductance value. This additional capability decreases the overall voltage rating of the converter. The effects of adding the modular inductor unit on the I-V characteristic of the LC-StatCom are evaluated. The effectiveness of the proposed solution is demonstrated by experimental results on a seven-level 0.7-kVA LC-StatCom. MOE (Min. of Education, S’pore)

Hybrid energy systems (HESs) bring together different generation, storage, and consumption technologies in a single system, improving the overall benefits compared to a system that depends on a single source. Originally designed as a combination of conventional, nonrenewable generation (e.g., diesel generators) with battery energy storage systems (BESSs), their definition has now expanded to include systems that are 100% based on renewable energy (e.g., solar photovoltaic and wind), or combine different energy storage systems (e.g., BESSs, fuel cells, and supercapacitors). HESs have also grown in capacity from small, off-grid systems of a few kilowatt, typically designed for low voltage DC and AC to larger megawatt systems expanding to medium voltage grid-connected systems.

This article presents a predictive home energy management system (HEMS) for a residential building with integration of a plug-in electric vehicle (PEV), a photovoltaic array, and a heat pump. A stochastic model predictive control (MPC) strategy is applied in the HEMS in order to minimize the home's electricity cost and reduce the PEV battery degradation cost. Moreover, the MPC ensures that home load demand, PEV battery charging requirements, and household thermal comfort conditions are met. The MPC operates in real-time and thus minimizes the effects of gap between the forecasted and real data on the HEMS performance by updating its control decisions and the forecast data as the stochastic parameters are realized in each time step. The obtained simulation results demonstrate that the proposed control strategy reaches 96% to 97% of ideal performance achieved by off-line optimization counterpart with perfect data.

Microgrid is a power system incorporating renewable energy sources and energy storage systems in a small geographic area. Since the microgrid covers small area its voltage rating is low. Therefore, the low voltage microgrid has highly resistive line impedances unlike a conventional power system with highly inductive line impedances. Thus, the power decoupling assumption based on the highly inductive power lines is no longer valid and the primary level droop control and higher level controls have to be redesigned. This paper proposes a bi-level control structure, combining the secondary level control with the primary level droop control, for the highly resistive low voltage microgrids with distributed photovoltaic generation sources and energy storage systems. The primary level droop control implements P −V and Q − f strategy. Then, the primary level droop control frequency and voltage references are set by the secondary level control for the frequency and average voltage restorations, state of charge balancing and reactive power sharing between renewable energy sources and energy storage systems. Presented simulation results carried out on the RTDS real-time digital simulator verify the proposed control structure on a microgrid with switching level converter models.

The cascaded H-bridge (CHB) low-capacitance StatCom (LC-StatCom) has a limited operating area in the inductive region compared to a conventional StatCom's V–I characteristic. This limitation for operation in the inductive region is considered to be the biggest disadvantage of CHB LC-StatCom. In this paper, the effect of adding parallel and series reactors on the LC-StatCom system's V–I characteristic is analyzed. Then, a new configuration, which fully compensates for the lost operating area of the LC-StatCom, is introduced. A scaled down single-phase seven-level laboratory prototype is used to confirm practicability of the proposed system.

This paper proposes a novel centralized switching controller for the state of charge balancing of battery energy storage systems distributed in a DC microgrid. The main advantage of the proposed controller is that it can tolerate non-uniform and time-varying delays in the communication links between a central processor and each battery energy storage system. The central processor generates predictive estimates of the renewable energy generation/load and state of charge of each battery energy storage. The switching controller uses the predictive estimates to generate participation rates of battery energy storage systems. The proposed control strategy is integrated with the primary droop control and the secondary voltage control for the power sharing and microgrid voltage restoration. In addition, the proposed control method guarantees that the active battery energy storage systems are either charging or discharging thus eliminating circulating currents. RTDS real-time simulator is used to verify the proposed large time delay tolerant control scheme on a DC microgrid with four distributed battery energy storage systems, loads and a PV generation. Moreover, mathematically rigorous analysis of the proposed control scheme is given.

In the integrated reconfigurable converter (IRC) topology for high-voltage series-connected battery module systems, the converter can be reconfigured into a range of operating modes: “feeding a load” mode, “battery modules balancing” mode, “regenerative” mode, “charging” mode, and “backup” mode. In most battery applications, a battery-balancing system operates while another converter is used as a controllable power interface between the battery system and a load. However, during the conventional IRC operation, the “battery modules balancing” and “feeding a load” modes cannot be operated simultaneously. In this paper, a new balancing scheme is proposed that allows balancing the battery modules while they are powering the load. The proposed balancing strategy is able to distribute different amount of load between the battery modules based on their state of charge (SOC). When the load is disconnected, the battery modules can be still balanced using the conventional “battery modules balancing” mode. A scaled down prototype of the IRC is implemented with three series-connected battery modules. Experimental results are provided to verify the viability of the proposed balancing method.

The boost-inverter-based battery-supercapacitor hybrid energy storage systems (HESSs) are a popular choice for the battery lifetime extension and system power enhancement. Various sliding mode (SM) controllers have been used to control the boost inverter topology in the literature. However, the traditional SM controllers for the boost inverter topology operate with a high and variable switching frequency which increases the power losses and system components design complexity. This can be alleviated by a pulse width modulation (PWM)-based fixed-frequency SM controller proposed in this paper. The SM controller is implemented using variable amplitude PWM carrier signals generated using the output capacitor voltage and inductor current measurements, thus eliminating the requirement of the output capacitor currents measurement. The battery-connected inductor reference currents for the SM controller are generated by a supercapacitor energy controller which is responsible for the HESS power allocation. First, the theoretical aspects of the SM controller, the operation and parameter selection of the supercapacitor energy controller, and the supercapacitor sizing for the HESS are discussed in the paper. Then, the proposed control system is experimentally verified, and it is shown that the HESS is able to satisfy the HESS output power requirements, while allocating the ripple current and the fast power fluctuations to the supercapacitor while maintaining operation of the supercapacitor within predefined voltage limits. The main advantage of the proposed SM controller, as compared with the traditional double-loop control method, is in eliminating possible DC current injection into the grid when the equivalent series resistance values of the boost inductors become unequal due to the tolerances and temperature variations.

The need for continuous temperature monitoring and thermal management is unquestionable as operating temperature is a critical factor affecting many electrical/electronic components and devices, including energy storage such as batteries and supercapacitors (SCs). On the other hand, many applications require the battery or SC cells to be stacked up to meet the required voltage or power level which introduces uneven temperature distribution among the cells. Hence, thermal-model-based temperature monitoring and management has been widely utilized, using state estimation to allow the use of minimum number of temperature sensors. This paper explores the performance of multiple-model-based abnormal overheating detection in a SCs string with minimum number of sensors based on simulation and experimental results. The performance of the estimator/detector system is evaluated for the case of an abnormally overheating cell as well as for different values of the estimator parameters. Preliminary indication from the experimental results shows satisfying performance of the estimator/detector system.

In conventional parallel hybrid energy storage systems (HESSs), there are usually two cascaded converters active when transferring energy between its energy storage systems (ESSs) that result in the overall efficiency drop. Moreover, the dc-link capacitor has to be oversized to limit the voltage ripple during simultaneous energy exchange between the ESSs and load feeding. The integrated reconfigurable configuration for HESSs proposed in this paper allows direct energy transfer between the ESSs by using only a single converter and bypassing the dc link. This reduces the stress on the dc link, reduces the dc-link capacitor size, and improves the efficiency. The proposed configuration can be reconfigured to operate in different operating modes: “feeding a load” mode, “regenerative” mode, “energy exchange” mode, “energy exchange and feeding a load” mode, “balancing” mode, “charging” mode, and “backup” mode. To control the proposed HESS, a battery peak power limiting method is used to allocate the power components between the ESSs. The operating principle of the proposed configuration and its different modes are explained and experimentally verified.

This paper proposes a multi-agent control strategy to coordinate power sharing between heterogeneous energy storage devices distributed throughout a DC microgrid. Without requiring a central controller, the proposed control strategy extends the benefits offered by hybrid energy storage systems to DC microgrids with batteries and ultracapacitors spatially distributed at different levels of the power distribution hierarchy. The proposed control strategy has the following advantages. 1) The high frequency microgrid load is provided by the ultracapacitors. 2) The low frequency load is provided by batteries used for bulk energy storage during islanded mode, and the main grid during grid connected operation. 3) The ultracapacitor voltages are regulated at a desired reference. 4) State of charge balancing is provided between the batteries. 5) The energy storage systems cooperate based on neighbor-to-neighbor output feedback over a sparse communication network. The only communication requirement is a spanning tree from the ultracapacitor leaders and battery leaders to their respective followers. Simulations are presented demonstrating the performance of the proposed control strategy for a 380 VDC datacenter during grid connected and islanded operation.

This paper presents a distributed control strategy for state-of-charge balancing between the battery modules of a reconfigurable battery energy storage system. The autonomous modules share state-of-charge information with their neighbors over a sparse communication network and cooperate to reach a balanced state of charge. The proposed control strategy provides advantages in terms of reduced communication requirements and increased modularity, over a centralized battery management system. Steady-state analysis provides bounds on the mean error and mean-squared error of the distributed average state-of-charge estimates used for autonomous balancing control. The proposed control strategy enforces module topology constraints specific to the particular reconfigurable battery implementation. The state-of-charge balancing mode current controller prevents current spikes during module topology transitions. Experimental results demonstrate the performance of the proposed control strategy for a reconfigurable energy storage system made up of four 6-V, 7-Ah lead-acid battery modules.

Battery energy storage systems (BESS) are becoming common with the increasing penetration of renewable energy sources. Real-time simulations (RTS) can provide an effective approach in the studies of BESS by reducing initial experimental cost and increasing model and test flexibility. Therefore, the need for developing accurate models of BESS for RTS is rising. This paper develops three RTS models for BESS with detailed power electronics, including ripple-port inverter, boost inverter and H-bridge with series-connected flyback converter. Moreover, the second harmonic current (SHC) affects the battery lifetime. To prolong the lifetime of BESS, the topologies and corresponding control schemes are designed to mitigate the SHC in battery. The operation of the real-time models and the effectiveness of the three topologies is demonstrated through RTS results that also show how the topologies and implementated control schemes can reduce the SHC significantly.

In the conventional integrated reconfigurable converter (IRC) topology, the battery modules charging mode was implemented using a unidirectional DC/DC buck converter supplied by an external DC source. This paper proposes an enhanced IRC configuration which allows bidirectional energy transfer between the battery modules and the AC grid. The bidirectional energy transfer is facilitated by the IRC DC/DC converter and an H-bridge. During the ‘battery to grid’ mode the active/reactive power control is realized by cooperation of the DC/DC converter operating in the boost mode and the H-bridge in the inverting mode. Conversely, during the ‘grid to battery’ mode, the DC/DC converter operates in the buck mode and the H-bridge as a rectifier. The proposed configuration is compact as the same inductor can be shared between all IRC operating modes. A scaled down prototype of the IRC is implemented with three series-connected battery modules. Presented experimental results verify viability of the proposed configuration.

© 1969-2012 IEEE. This paper proposes multi-agent energy storage system aggregation as a means of scaling energy management to low voltage microgrids with distributed energy storage systems. Based on this concept, a hierarchical control strategy is developed for an AC microgrid with distributed battery and ultracapacitor energy storage systems. On the tertiary control level, the energy management problem is made scalable by considering each type of energy storage system in aggregate. This addresses the 'curse of dimensionality,' since additional energy storage systems do not increase the optimization problem dimension, and allows nonlinear energy storage models to be used for optimization, accounting for variable efficiency, self-discharge, and lifetime degradation. On the secondary control level, multi-agent state of charge balancing, reactive power sharing, frequency restoration, and voltage restoration are combined, to aggregate energy storage systems for the tertiary control. This includes the novel use of multi-agent sliding mode control for state of charge balancing between AC microgrid energy storage systems. Unlike a linear state of charge balancing strategy, circulating currents are prevented, increasing efficiency and reducing lifetime degradation. An RTDS Technologies real-time digital simulator was used to verify the performance of the proposed control strategy.

In this paper, a cascaded H-Bridge (CHB)-multilevel converter (MC)-based photovoltaic (PV) system with no voltage or current sensors at the dc-side is proposed. Eliminating the dc-side sensors simplifies the hardware, leading to lower cost and higher reliability of the PV system. A novel scheme estimating the capacitors’ voltages from the ac output voltage of the inverter is developed. The scheme allows replacing all dc-side voltage sensors by a single-voltage sensor at the ac-side of the converter. Furthermore, the dc current sensors, conventionally required for the maximum power point tracking (MPPT), are also eliminated. Instead, the outputs from the capacitors’ voltage control systems are utilized for the MPPT. The effectiveness of the proposed dc-side sensorless system is experimentally demonstrated on a 2-kW single-phase seven-level CHB converter-based PV system.

A boost DC/AC converter is popular in AC line-integrated energy storage systems (ESSs) based on low-voltage DC sources such as battery, fuel cell, or supercapacitor. The direct DC/AC power conversion in the boost inverter introduces a second-order harmonic ripple current at the DC side of the boost converter, which leads to internal heating of the energy storage devices and degradation of their lifetime. In this paper, a novel current feedback method is proposed to mitigate the second-order harmonic current component at the DC side. The proposed method is able to significantly reduce the second-order harmonic component in the DC-side current of the boost inverter without increasing other harmonic components. Performance of the proposed method is validated on a single-phase grid-connected DC/AC boost inverter-based battery ESS experimental prototype.

In series-connected battery systems, typically, separate converters for intramodule balancing bidirectional connection to a load and charging from an external power source are employed. In this paper, an integrated reconfigurable converter for high-voltage series-connected battery storage systems is proposed. The main advantage of the proposed converter is that it can be reconfigured to operate in different operating modes: feeding the load from the battery system, feeding the load from a backup power source, regenerative mode, intramodule balancing mode, and charging mode. Unlike conventional systems, the proposed topology shares the semiconductor devices and an inductor among the different operating modes which makes it compact. The proposed converter exhibits redundant modes which together with a backup mode increase its reliability. Additionally, the proposed topology minimizes stress on the batteries during the charging/discharging cycles. A scaled down experimental prototype of the proposed converter is implemented for a five-module battery system to verify its viability and demonstrate all operating modes.

Multilevel-cascaded H-bridge converters are promising candidates for next generation photovoltaic power converters. They feature reduced switching losses and higher conversion efficiency with modular structure; characteristics vital for large-scale photovoltaic power plants. However, the stochastically-variable nature of irradiance levels and ambient temperatures affects the normal operation of this topology, because power levels in the three phases can be unequal. The existing zero sequence injection method can deal with the power imbalance problem, but it is limited in its application. The paper proposes a zero sequence injection method to optimize the converter power balance, extending the converter operation with severe power imbalance. Based on the proposed optimal method, a simplified optimal zero sequence injection method requiring less calculation effort is derived and compared with the optimal method. Simulation and experimental results validate the effectiveness and feasibility of the proposed methods.

Single conversion stage DC/AC boost inverters are an attractive solution when integrating energy storage devices, such as a battery, fuel cell, or supercapacitor to a single-phase AC grid. However, a second-order harmonic current ripple appears at the DC side of the inverter increasing the internal heat and losses in the energy storage device and degrading its lifetime. In this paper, a rule-based controller is proposed to reduce such harmonic current ripple component. A key feature and advantage of the proposed controller is its ability to reduce the ripple current amplitude in all four inverter output power operating quadrants without being affected by the capacitor tolerances and the internal resistance of the inductors. Presented experimental results validate the performance of the proposed controller on a single-phase grid-connected DC/AC boost inverter-based battery energy storage system.

This paper proposes a unified distributed control strategy for DC microgrid operating modes, without bus voltage signaling or mode detection mechanisms that are normally required for decentralized control strategies. The proposed control strategy is based on the novel integration of distributed controllers for energy balancing between DC microgrid energy storage systems with distributed controllers used to regulate the average DC microgrid bus voltage, and a new method for controlling the grid connected rectifier that maintains the distributed control structure. Under the proposed control strategy, seamless mode transitions are achieved between qualitatively different operating modes, namely, 1) grid connected operation with the rectifier providing load balancing, 2) grid connected operation with the rectifier charging the energy storage systems, and 3) islanded operation. In all operating modes, the average DC microgrid bus voltage is regulated to the microgrid voltage reference, and the energy storage systems are controlled independently of the operating mode to achieve and maintain a balanced energy level. Simulations are presented demonstrating the performance of the proposed control strategy for a 380 VDC datacenter with intermittent photovoltaic generation and communication delays expected from a WiFi control network implementation.

Multilevel cascaded H-bridge converters are promising candidates for large-scale photovoltaic power plants. They allow direct connection to medium-voltage distribution networks without the presence of bulky line frequency power transformers. Owing to the stochastically variable nature of irradiance level, ambient temperature, and other factors, power levels in the three phases are expected to be unequal. The power imbalance condition creates unexpected problems with this topology, which was initially designed to operate under balanced power conditions. To deal with this issue, the paper proposes three novel zero-sequence injection methods as an expansion to the conventional zero-sequence injection method. Results obtained from simulations and a 430-V 8-kW three-phase seven-level cascaded H-bridge prototype are presented to verify the effectiveness and feasibility of the proposed methods.

In conventional control systems for a static synchronous compensator (STATCOM) based on a cascaded H-bridge multilevel converter, the action of individual voltage controllers that balance capacitor voltages can alter the output of a cluster voltage controller and a current controller due to a coupling effect. In this paper, first, the conditions that eliminate this coupling effect are derived, and then, a control system that enforces the derived decoupling conditions is proposed. It is also shown that an additional advantage of the proposed decoupled control system is that it enables the linearization of a cluster voltage control loop. Experimental studies on a single-phase seven-level STATCOM demonstrate the effectiveness of the proposed decoupled control system in improving the transient performance of the STATCOM.

A cascaded H-bridge (CHB) converter is one of the viable options for next-generation large-scale photovoltaic (PV) power conversion. Owing to the increased number of components involved, converter reliability and fault-tolerant control are important issues. A CHB converter must also manage unequal power generation among bridges, which is inherent in PV applications because of unequal solar irradiance and/or module temperatures. This paper proposes a fault-tolerant strategy, which maintains balanced three-phase grid currents during faults with unequal power generation in healthy H-bridges. The effectiveness of the presented fault-tolerant control is verified using the results obtained from a 430-V 10-kW laboratory prototype.

Multi-energy integrated sources are increasingly being used as the bundling-sale for electric systems. Estimating the power supply reliability and regulation performance of the fixed-speed Pumped-Storage Generation Systems (PSGSs) in suppressing power fluctuations of intermittent energy is essential to the operational safety and reliability of a system. However, the regulation process relates to the coupling fluctuations of hydraulic-mechanical-electrical factors, leading to a multiple time-scale effect to the whole hybrid power system. This makes the PSGS’s power response lagging behind the wind power fluctuation and further impacts the power supply reliability of the hybrid power system. To enable the analysis, a numerical model which describes operating states of the presented integrated system, including wind energy and PSGS, is developed in MATLAB/Simulink. Then, ten wind scenarios are selected to study complementation and regulation ability of the PSGS based on a framework of evaluation indicators. The results show that longer power response delay and larger guide vane distance opening result in weaker complementary response of PSGS to wind disturbances. It is also shown that the PSGS provides good complementary capability at lower random wind standard deviation (0.5 vs 1.5), larger wind speed mean value (15 m/s vs 13 m/s) and lower wind speed deviation (5 m/s vs 12 m/s). Furthermore, an unexpected regulation behavior is demonstrated: fast settling and peak times coincide with large variation in overshoot, undershoot and peak value of the power response. This presents a challenge in assessing performance of the integrated generation system. The proposed evaluation method and presented results are important steps to increasing the national power grid capability to accept high integration of multiple energy sources.

This paper proposes a multi-agent dynamic optimal power flow (DOPF) strategy for microgrids with distributed energy storage systems. The proposed control strategy uses a convex formulation of the ac DOPF problem developed from a d-q reference frame voltage–current model and linear power flow approximations. The convex DOPF problem is divided between autonomous agents and solved based on local information and neighbour-to-neighbour communication over a sparse communication network, using a distributed primal subgradient algorithm. Each agent is only required to solve convex quadratic sub-problems, for which robust and efficient solvers exist, making the control strategy suitable for receding horizon model predictive control. Also, the agent sub-problems require limited power network information and include only a subset of the centralised optimisation problem decision variables and constraints, providing scalability and data privacy. Unlike existing distributed optimal power flow methods, such as alternating direction method of multipliers, under the proposed control strategy the information required by each agent is independent of the communication network topology, providing increased flexibility and robustness. The performance of the proposed control strategy was verified for an ac microgrid with distributed lead-acid batteries and intermittent photovoltaic generation, using an RTDS Technologies real-time digital simulator.

In modern power systems, renewable energy, such as solar and wind, has been increasingly exploited to replace traditional fossil fuels in hope of resolving the issue of energy crisis. Normally, the renewable generators are integrated into the power grid through power converters. A common issue with the photovoltaic (PV) powered single-phase inverter systems is that the performance and lifespan of the dc source might be undermined by the second-order current ripple (SCR) due to the double line frequency instantaneous power oscillating at the ac side. This issue tends to exacerbate when the inverter interfaces more complicated nonlinear loads, where the input current is adulterated with multiple low-frequency ripple components besides SCR. To solve this issue, this paper presents a repetitive controller based dual-mode control method for PV powered single-phase buck-boost inverter under nonlinear load scenario. Without introducing any additional circuit or passive electrical components, the proposed method manages to significantly mitigate ripple components in the input current with the reduced dc-link capacitance. Finally, simulated results are demonstrated to verify its effectiveness in current ripple reduction.

This paper presents an overview of the state of the art control strategies specifically designed to coordinate distributed energy storage (ES) systems in microgrids. Power networks are undergoing a transition from the traditional model of centralised generation towards a smart decentralised network of renewable sources and ES systems, organised into autonomous microgrids. ES systems can provide a range of services, particularly when distributed throughout the power network. The introduction of distributed ES represents a fundamental change for power networks, increasing the network control problem dimensionality and adding long time-scale dynamics associated with the storage systems’ state of charge levels. Managing microgrids with many small distributed ES systems requires new scalable control strategies that are robust to power network and communication network disturbances. This paper reviews the range of services distributed ES systems can provide, and the control challenges they introduce. The focus of this paper is a presentation of the latest decentralised, centralised and distributed multi-agent control strategies designed to coordinate distributed microgrid ES systems. Finally, multi-agent control with agents satisfying Wooldridge’s definition of intelligence is proposed as a promising direction for future research.

Battery-supercapacitor hybrid energy storage systems (HESSs) are popular as a way of extending the battery lifetime by reducing the battery current fluctuations. In this paper, a current sensor reduction method for a recently proposed interleaved boost inverter based battery-supercapacitor HESS is proposed. A pulse width modulation (PWM) and inductor current reconstruction scheme is developed to achieve the current sensor reduction for the HESS. The method is able to measure four inductor current waveforms using two current sensors while ensuring the interleaved operation of the boost converter legs.

© 1986-2012 IEEE. Vehicle to grid (V2G) reactive power compensation using electric vehicle (EV) onboard chargers helps to ensure grid power quality by achieving unity power factor operation. However, the use of EVs for V2G reactive power compensation increases the second-order harmonic ripple current component at the DC-side of the charger. For single-phase, single-stage EV chargers, the ripple current component has to be supplied by the EV battery, unless a ripple compensation method is employed. Additionally, continuous usage of EV chargers for reactive power compensation, when the EV battery is not charging from the grid, exposes the EV battery to these undesirable ripple current components for a longer period and discharges the battery due to power conversion losses. This paper presents a way to provide V2G reactive power compensation through a boost inverter-based single stage EV charger and a DC-side capacitor without adversely affecting the EV battery. The operation of the boost inverter-based EV charger with second-order harmonic and switching frequency ripple current reduction, the dynamic behavior of the system, the transition between different operating modes, the DC-side capacitor voltage control above a minimum allowed voltage, and the DC-side capacitor sizing are extensively analyzed. The performance of the proposed system is verified using an experimental prototype, and presented results demonstrate the ability of the system to provide V2G reactive power compensation both with and without the EV battery.

This paper proposes the novel use of multi-agent sliding mode control for state of charge balancing between distributed dc microgrid battery energy storage systems. Unlike existing control strategies based on linear multi-agent consensus protocols, the proposed nonlinear state of charge balancing strategy: 1) ensures the battery energy storage systems are either all charging or all discharging, thus eliminating circulating currents, increasing efficiency, and reducing battery lifetime degradation; 2) achieves faster state of charge balancing; 3) avoids overloading the battery energy storage systems during periods of high load; and 4) provides plug and play capability. The proposed control strategy can be readily integrated with existing multi-agent controllers for secondary voltage regulation and accurate current sharing. The performance of the proposed control strategy was verified with an RTDS Technologies real-time digital simulator, using switching converter models and nonlinear lead-acid battery models.

In this paper, an analytic capacitor voltage filtering scheme for a cascaded H-bridge (CHB) multilevel converter-based static synchronous compensator (STATCOM) is proposed. The scheme is able to filter out a low-frequency ripple of any magnitude and introduces a negligible delay to the control system. Therefore, it facilitates using high-reliability film capacitors with low capacitance instead of the conventional electrolytic capacitors. The proposed analytic filtering scheme is implemented and verified on a single-phase seven-level CHB multilevel converter-based STATCOM. The parameters' sensitivity evaluation is provided to demonstrate practicability of the proposed scheme. Simulation and experimental results are provided to compare the performance of the proposed filtering scheme with the performance of the most commonly used second-order filter-based approach to demonstrate its effectiveness.

This paper proposes dynamic energy level balancing between distributed storage devices as a strategy to improve frequency regulation and reliability in droop controlled microgrids. This has been achieved with a distributed multi-agent cooperative control system which modifies the output power of droop controlled storage devices so that they reach a balanced energy state. As the storage devices approach a common energy level they are able to contribute their full power capacity to deal with generation and demand fluctuations in the microgrid. The cooperative control system also provides secondary frequency control, restoring the microgrid to the reference frequency. Simulations have been completed showing that the cooperative control system improves frequency regulation compared to traditional droop control strategies when the storage devices begin at different energy levels and the microgrid experiences generation or demand variability. A control input saturation constraint has been developed which ensures that the cooperative control system will not overload the storage devices.

An explicit model predictive control (EMPC) system for a hybrid battery-ultracapacitor power source is proposed and experimentally verified in this paper. The main advantage of using the EMPC system is that the control law computation is reduced to evaluation of an explicitly defined piecewise linear function of the states. Separate EMPC systems for the total output current loop, the battery loop and the ultracapacitor loop are designed. This modular design approach allows evaluation of the performance of each individual EMPC system separately and also improves the convergence of the EMPC system design algorithm as the models used to design each loop are smaller. In order to protect the hybrid power source, the designed EMPC systems maintain operation of the hybrid power source within specified constraints, namely, battery and ultracapacitor current constraints, battery state of charge constraints and ultracapacitor voltage constraints. At the same time, the total output current EMPC system allocates high frequency current changes to the ultracapacitor and the low frequency current changes to the battery thus extending the battery lifetime. Presented experimental results verify that the hybrid power source operates within the specified constraints while allocating high and low frequency current changes to the ultracapacitor and battery respectively.

This paper proposes a new method for measuring capacitor voltages in multilevel flying capacitor (FC) converters that requires only one voltage sensor per phase leg. Multiple dc voltage sensors traditionally used to measure the capacitor voltages are replaced with a single voltage sensor at the ac side of the phase leg. The proposed method is subsequently used to balance the capacitor voltages using only the measured ac voltage. The operation of the proposed measurement and balancing method is independent of the number of the converter levels. Experimental results presented for a five-level FC converter verify effective operation of the proposed method.

A dc hybrid power source based on the combination of ultracapacitor and lead-acid battery is considered in this paper. The various control systems for such hybrid power source reported in the technical literature thus far are rather complex. A low complexity control system for such hybrid power source is proposed in this paper. The key feature of the proposed control system is its capability to maintain operation of the hybrid power source within all important operational limits. The proposed control system allows one to allocate the high-frequency current demands to the ultracapacitor and specify the current limits for both the battery and the ultracapacitor. It also maintains operation of the battery within its state of charge limits and the ultracapacitor voltage at a predefined value while charging the ultracapacitor from the battery rather than from the common dc bus. Presented experimental results verify the satisfactory operation of the power source utilizing the proposed control system.

A model predictive control system for a hybrid battery-ultracapacitor power source is proposed and experimentally verified in this paper. The main advantage of the proposed system is that the battery current, the battery state of charge, and the ultracapacitor current and voltage are maintained within predefined limits during the operation. In addition, the controller allocates fast current changes to the ultracapacitor while the battery responds mainly to slow current changes which helps to increase the battery lifetime. The presented experimental results verify operation of the proposed system.

The three-phase boost inverter is a differential inverter topology which can provide both boosting and inversion functions in a single power processing stage. If the switching frequency ripple current component is ignored, the DC side current of the three-phase boost inverter contains only a DC component when all three converter legs are identical. However, the capacitance values of the output capacitors can differ from their nominal values due to tolerance, temperature variation, and ageing. In this paper, it is shown that the mismatched capacitances of the output capacitors lead to a significant ripple current component at the DC side of the three-phase boost inverter. An input current AC component feedback method is proposed in this paper to mitigate such DC side ripple current components. The effectiveness of the proposed method is verified using simulation results.

This paper presents a hierarchical control strategy for a multi-module battery energy storage system, with distributed sliding mode controllers for state of charge balancing between modules. Under the proposed control strategy, a central controller generates a nominal module output power reference to regulate the total battery energy storage system output power. The nominal output power reference is sent to distributed battery module controllers, which use local state of charge information and neighbour to neighbour communication over a sparse network to coordinate their output powers so that they move towards a balanced state of charge. Unlike a linear control approach, the proposed control strategy ensures that the battery modules are either all charged or all discharged, depending on whether the total power demand is negative or positive. This eliminates circulating currents, increasing efficiency and reducing battery lifetime degradation. As the state of charge of a battery module approaches a boundary layers around the average of its neighbours' state of charge, the sliding mode control swaps to a linear interpolation that prevents chattering. State of charge synchronisation under the proposed control strategy is achieved for very general assumptions on the nonlinear battery dynamics. The performance of the proposed control strategy was verified with an RTDS Technologies real-time digital simulator, using switching converter models and nonlinear lead-acid battery models.

This paper proposes a model predictive control strategy for power flow optimisation between battery energy storage systems distributed in a DC Microgrid. The proposed control strategy uses a new convex formulation of the DC microgrid dynamic optimal power flow problem, based on a static voltage-current model and linear power flow approximations. Unlike optimisation strategies based on a single bus model, line losses and line voltage drops are included in the optimisation. The availability of fast and robust solvers for convex optimisation problems makes this a scalable solution for a real-time model predictive control implementation. To verify the performance of the proposed control strategy, real-time simulations were carried out for a DC microgrid with distributed lead-acid batteries and intermittent photovoltaic generation. The simulations were completed on an RTDS Technologies real-time digital simulator, using switching converter models and non-linear battery models.

Battery-supercapacitor hybrid energy storage systems (HESSs) are popular as a way of extending the battery lifetime by reducing the battery current fluctuations. In conventional single phase HESSs, both the second-order harmonic power component and the fast power fluctuations are allocated to the supercapacitor. However, the ripple current component leads to supercapacitor degradation due to its internal heating. In this paper, a current-feedback ripple current reduction method is applied to a single-phase battery-supercapacitor HESS in order to alleviate the second-order harmonic supercapacitor current component. Effectiveness of the ripple current reduction method is theoretically analyzed. Experimental results show that the current-feedback method is able to significantly decrease the second-order harmonic component in the supercapacitor current without introducing additional harmonics.

In conventional hybrid energy storage systems (HESSs), parallel bi-directional boost converters are used as interface between each storage system and the dc link. One of the disadvantages of the configuration is that during the energy transfer between two energy storage systems two converters are active which in turn lowers the overall efficiency. In this paper, a new multiple converter topology for HESSs is proposed. In the proposed topology one of the boost converters is replaced with a bi-directional buck-boost converter and by adding one additional semiconductor switch the topology can be reconfigured to bypass one of the converters during the energy exchange interval. This improves the overall efficiency. Furthermore, the buck-boost converter allows extending the range of the output voltage. The principle of operation and different operating modes of the proposed topology are explained in detail and verified in simulation.

An application of an advanced anti-windup compensation scheme to a recently developed large-span track-seeking and track-following method for dual-stage actuator systems in hard disk drives (HDDs) is presented. In the original dual-stage servo control approach, micro-actuator saturation was addressed using the Circle Criterion to guide the controller design. However, such an approach does not allow the fine-tuning of seeking and settling speeds and also limits the class of linear controllers for the secondary loop due to the Circle Criterion. Anti-windup compensation removes many of these limitations and improves settling duration, disturbance rejection, recovery from saturation in the secondary actuator loop, and design flexibility. A general anti-windup (AW) compensator is presented in discrete time bringing together a number of AW concepts and results previously given in continuous time. Robustness is taken into account and performance requirements are expressed via frequency weights. The robust performance design is based on an l 2-optimization approach using linear matrix inequalities (LMIs).

A novel approach for the design of a discrete-time sliding mode controller is presented showing that the dynamics of a sliding-mode state-feedback controller can be designed using a singular LQR approach. The weighting of the control signal is set to zero and dead-beat behaviour for the sliding mode reaching dynamics is achieved. The reaching dynamics are modified when the states are a significant distance away from the sliding surface to avoid any high magnitude control action due to the partial dead-beat approach. The control law also takes into account the constraints on the actuator amplitudes and a stability analysis is presented using a discrete-time version of the Popov criterion. The control approach is demonstrated in conjunction with a recently developed large-span track-seeking and track-following method for dual-stage actuator systems in a hard disk drive (HDD). It is shown how the discrete-time sliding mode control scheme can be incorporated into the observer-based control system for the secondary actuator. Superior performance for the track-seeking and track-settling process is demonstrated using an existing practical set-up of a dual-stage HDD servo system. Copyright © 2007 John Wiley & Sons, Ltd.

This paper presents a communication delay robust multi-agent control strategy for state of charge balancing between energy storage systems in droop controlled AC micro-grids. Under the multi-agent control strategy microgrid energy storage systems communicate over a sparse communication network and adjust their output powers so that they approach a balanced state of charge, while regulating the microgrid frequency and voltage. The proposed control strategy provides a robustness condition for balanced communication topologies with uniform communication delays. The robustness condition depends only on the individual energy storage system dynamics and is thus scalable to microgrids with many ES systems. It is shown that the structure of the proposed control strategy preserves the normal synchronous frequency of the droop controlled microgrid, so frequency regulation is not affected. Simulations are presented demonstrating the performance of the proposed control strategy for an AC microgrid with variable PV generation and energy storage systems connected by a sparse communication network with communication delays.