This paper examines the practical design issues of sliding-mode (SM) controllers as applied to the control of dc-dc converters. A comprehensive review of the relevant literature is first provided. ...Major problems that prevent the use of SM control in dc-dc converters for industrial and commercial applications are investigated. Possible solutions are derived, and practical design procedures are outlined. The performance of SM control is compared with that of conventional linear control in terms of transient characteristics. It has been shown that the use of SM control can lead to an improved robustness in providing consistent transient responses over a wide range of operating conditions.
This paper presents a discrete sliding mode control (DSMC) scheme for a series-series compensated wireless power transfer (WPT) system to achieve fast maximum energy efficiency (MEE) tracking and ...output voltage regulation. The power transmitter of the adopted WPT system comprises a dc/ac converter, which incorporates the hill-climbing-search-based phase angle control in achieving minimum input current injection from its dc source, thereby attaining minimum input power operation. The power receiver comprises a buck-boost converter that emulates an optimal load value, following the MEE point determined by the DSMC scheme. With this WPT system, no direct communication means is required between the transmitter and the receiver. Therefore, the implementation cost of this system is potentially lower and annoying communication delays, which deteriorate control performance, are absent. Both the simulation and experiment results show that this WPT system displays better dynamic regulation of the output voltage during MEE tracking when it is controlled by DSMC, as compared to that controlled by the conventional discrete proportional-integral (PI) control. Such an improvement prevents the load from sustaining undesirable overshoot/undershoot during transient states.
A family of bidirectional switched-capacitor (SC) converters with high-gain ratio of any positive integer is proposed in this paper for distributed energy resources applications. As compared with ...other existing SC converters achieving a same conversion gain, the main advantages of the proposed converters are that they require a relatively lower number of switches and capacitors, have a relatively lower switch's and capacitor's stress, and that their associated driver circuits are simpler to realize. Importantly, with the achievable conversion ratio being flexible and that the input and output of the proposed converters are of common ground, the proposed converters are widely suitable for many applications. Moreover, as the proposed converters do not possess magnetic component or any component that can severely degrade the converters' performance at high temperature, they are especially useful for high-temperature applications. Besides, the proposed converters are capable of delivering bidirectional power, which is a key requirement for emerging applications with battery storages. Different aspects of the proposed converters, including a simple auxiliary power supply circuit for the MOSFETs' drivers, will be discussed in this paper. A nine-time SC converter prototype that operates with 20-V input voltage, 100-W output, and at 75 kHz, is constructed and tested. Experiment results show that the maximum efficiency achievable with this prototype is over 98% (without driver's loss) and the efficiency over the entire load range between 25 and 100 W is over 95.5% including the driver's loss. The output voltage ripple of the SC converter is less than 1%. When the SC converter is open-loop controlled, the load voltage regulation is relatively well kept at less than 5% between full load and no load conditions.
Existing control schemes for single-phase ac-to-dc converters with active power-decoupling function typically involve a dedicated power-decoupling controller. Due to the highly coupled and nonlinear ...nature of the single-phase system, the design of the power-decoupling controller (typically based on the small-signal linear control techniques) is cumbersome, and the control structure is complicated. Additionally, with the existing power-decoupling control, it is hard to achieve satisfied dynamic responses and robust circuit operation. Following a recently proposed automatic-power-decoupling control scheme, this paper proposes a nonlinear control method that can achieve enhanced large-signal dynamic responses with strong disturbance rejection capability without the need for a dedicated power-decoupling controller. The proposed controller has a simple structure, of which the design is straightforward. The control method can be easily extended to other single-phase ac-to-dc systems with active power-decoupling function. Simulation and experimental results validate the feasibility of the proposed control method on a two-switch buck-boost PFC rectifier prototype.
Voltage and frequency fluctuation associated with renewable integration have been well identified by power system operators and planners. At the microgrid level, a novel device for the implementation ...of dynamic load response, which is known as the electric springs (ES), has been developed for mitigating both active and reactive power imbalances. In this paper, a comprehensive control strategy is proposed for ES to participate in both voltage and frequency response control. It adopts the phase angle and amplitude control which respectively adjust the active power and the reactive power of the system. The proposed control strategy is validated using a model established with power system computer aided design/electro-magnetic transient in dc system. Results from the case studies show that with appropriate setting and operating strategy, ES can mitigate the voltage and frequency fluctuation caused by wind speed fluctuation, load fluctuation, and generator tripping wherever it is installed in the microgrid.
In this paper, a nonlinear controller is presented for regulating an emerging class of high power density, high power-conversion efficiency, and high reliability (H 3 ) single-phase power converters ...featuring small buffering capacitors. The proposed controller combines the best features of input-output feedback linearization and an automatic-power-decoupling (APD) control strategy, and achieves enhanced dynamic performances as compared to existing solutions based on linear control techniques. By feedback linearization, the plant models of H 3 single-phase converters are fully linearized and decoupled, and thus fast dynamics, large-signal asymptotic tracking, and global stability can be achieved with simple linear controllers. Additionally, the APD strategy further strengthens the robustness of the closed-loop system as active pulsating power buffering (of the instantaneous power difference between the ac-port and the dc-port of the converter) of basically any form can be achieved. With the proposed nonlinear controller, we are able to further exploit the potential and unlock several new features intrinsic in H 3 single-phase converters, e.g., active voltage holdup function, allowing extended holdup time (comparable to existing products with large dc-link capacitors) with only a small energy storage. The general theory, design procedures, and practical considerations of the nonlinear controller are detailed. A 100-W hardware prototype is also built to demonstrate the advantages of the proposed solution. New opportunities of H 3 single-phase power converters with the aid of the proposed nonlinear control are also suggested for future research.
Electric spring (ES) was originally proposed as a distributed demand-side management (DSM) technology for stabilizing power distribution network in the presence of intermittent power generation ...without using communication. This paper explores the practical use of consensus control for a cluster of ESs through a WiFi communication layer for new functions not previously realized in practice. This approach can be considered as a form of DSM for smart grid technology. A novel consensus control is introduced to enable distributed ES circuits to provide local voltage and system frequency regulations in a microgrid with shared responsibility of active and reactive power compensation. The practical implementation details of consensus control for a cluster of ESs are addressed. New plug-and-play functions of ESs are practically demonstrated for the first time under consensus control. Practical results indicate that droop control (without communication) and consensus control (with communication) are complementary. Under normal condition when the communication network is available, distributed ESs can perform with shared power compensation efforts based on consensus control. If the communication network fails, ESs can revert to perform under droop control.
DC microgrids fed with substantial intermittent renewable energy sources face the immediate problem of power imbalance and the subsequent dc bus voltage fluctuation problem (that can easily breach ...power system standards). It has recently been demonstrated that dc electric springs (DCES), when connected with series non-critical loads, are capable of stabilizing the voltage of local nodes and improving the power quality of dc microgrids without large energy storage. In this paper, two centralized model predictive control (CMPC) schemes with: 1) non-adaptive weighting factors and 2) adaptive weighting factors are proposed to extend the existing functions of the DCES in the microgrid. The control schemes coordinate the DCES to mitigate the distribution power loss in the dc microgrids, while simultaneously providing their original function of dc bus voltage regulation. Using the DCES model that was previously validated with experiments, simulations based on MATLAB/Simulink platform are conducted to validate the control schemes. The results show that with the proposed CMPC schemes, the DCES are capable of eliminating the bus voltage offsets as well as reducing the distribution power loss of the dc microgrid.
There is a growing interest in using dc power systems and microgrids for our electricity transmission and distribution, particularly with the increasing penetration of photovoltaic power systems. ...This paper presents an electric active suspension technology known as the dc electric springs (DC-ES) for voltage stabilization and power quality improvement. The basic operating modes and characteristic of a DC-ES with different types of serially connected non-critical loads will first be introduced. Then, the various power delivery issues of the dc power systems, namely bus voltage variation, voltage droop, system fault, and harmonics, are briefly described. The operating limits of a DC-ES in a dc power grid is studied. It is demonstrated that the aforementioned issues can be mitigated using the proposed DC-ES technology. Experiment results are provided to verify the feasibility of the proposed technology.
The conventional control methods for the battery systems of photovoltaic (PV) battery systems in standalone dc microgrids are designed to stringently regulate the bus voltages at the maximum power ...points (MPP) of PV modules while the state of charge (SOC) of the battery packs is regulated within the tolerances. In this paper, a local hierarchical control (LHC) is proposed for the battery system to improve the energy efficiency of the entire PV-battery system at the MPP of PV modules while the SOC of the battery pack is still regulated within the tolerance. Specifically, by allowing the dc bus voltage to deviate within a preset allowable tolerance, the secondary control of the LHC is employed to compute real-time optimal references to its primary control, such that the energy conversion of the entire PV-battery system can be optimized. Simulation studies exhibit significant efficiency improvement of a 12-PV-battery system under both uniform and nonuniform insolation conditions on a cloudy day and a 600-kW PV-battery system on a sunny day using the proposed LHC. Experimental results validate that the energy efficiency of a single-PV-module-battery system controlled by the LHC can be enhanced using shortened sunny-day and cloudy-day irradiance profiles for various PV modules. The proposed control scheme can be easily implemented in digital controllers without additional hardware costs.
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