Variable DC Bus Voltage Research Articles

Implementation of Control Strategies for Optimum Utilization of Solar Photovoltaic System with Energy Storage Systems

Solar Photovoltaic (SPV) system primarily based on autonomous system have advanced as a promising solution to the problem of electrification in areas where the network is now not available. The main challenges in designing such systems are as follows: 1) extraction of maximum power from PV system under fast varying irradiance; 2) extraction of high voltage gain with DC-DC converter; 3) development of efficient power management scheme between SPV and Energy Storage System (ESS). As multiple objectives must be satisfied, existing schemes for autonomous systems require a minimum of three conversion steps, which leads to significant reduction in the reliability and efficiency of the system. The above issues are addressed with different control strategies. For extraction of maximum power a modified non iterative Incremental Conductance MPPT method is developed to generate a fine tuned duty cycle for sudden change in irradiance and regulates that always the intersection point of load line and I-V curve represents Maximum power point (MPP). High voltage gain DC-DC converter generally required for industrial applications and can be achieved by high duty ratio which cause reverse recovery problem and voltage stress on power switches. To overcome this problem a soft switch interleaved boost converter (SSIBC) is proposed which is operate on zero current switching to turn ON active switches and it can achieve high voltage gain for lower duty ratios. Lead acid battery is an energy storage device which is used in SPV systems. Finally to achieve power management, variable DC bus voltage based algorithm is developed with super capacitor (SC) and battery (BAT) as ESS. The capability of the proposed control strategies is tested and validated under various scenarios

Resistive–Capacitive Output Impedance Shaping for Droop-Controlled Converters in DC Microgrids With Reduced Output Capacitance

In dc microgrids, droop control is widely employed in power converters interfacing distributed energy resources and common dc bus for automatic power sharing. In order to limit the dc-bus voltage variations even during load transient conditions, it is necessary to shape the output impedance of droop-controlled converters to be always lower than the dc resistive value. A commonly used way is to install bulky output capacitance, which not only increases the system cost, size, and weight, but also generates higher short-circuit fault currents. In order to avoid large output capacitance, this article proposes a design approach for droop-controlled converters, including the selection criterion of the output capacitance and the design of the droop coefficient. Herein, the required output capacitance is calculated based on the dc droop coefficient and the voltage control bandwidth. The droop coefficient is designed as a frequency-dependent term instead of a constant value. As a result, resistive-capacitive output impedance can be obtained with a much smaller output capacitance. The proposed design strategy is applied to buck and boost converters and validated by experimental results performed on a buck-based and a boost-based dc microgrid prototype, respectively.

Incipient Inter-Turn Short Circuit Fault Estimation Based on a Faulty Model Observer and ANN-Method for Induction Motor Drives

Background: According to statistics, short circuit faults are the second most frequent faults in induction motors. Thus, in this paper, we investigated inter turn short circuit faults in their early stage. Methods: A new equivalent model of the induction motor with turn to turn fault on one phase has been developed. This model has been used to establish two schemes to estimate the severity of the short circuit fault. In the first scheme, the faulty model is considered as an observer, where a correction of an error between the measured and the estimated currents is the kernel of the fault severity estimator. However, to develop the second method, the model was required only in the training process of an artificial neural network (ANN). Since stator faults have a signature on symmetrical components of phase currents, the magnitudes and angles of these components were used with the mean speed value as inputs of the ANN. A simulation on MATLAB of both techniques has been performed with various stator frequencies. Results: The suggested schemes prove a unique efficiency in the estimation of incipient turn to turn fault. Besides, the ANN based scheme is less complex which reduces its implementation cost. Conclusion: To monitor the stator of an induction motor, the choice of the appropriate algorithm should be done according to the system in which the motor will be installed. If the motor is directing connected to the grid or fed via an inverter with a variable DC bus voltage, the observer would be better, otherwise, the ANN algorithm is recommended.

A 13.56-MHz Full-Bridge Class-D ZVS Inverter With Dynamic Dead-Time Control for Wireless Power Transfer Systems

This paper presents the development of a Class-D full-bridge zero-voltage switching (ZVS) inverter, applicable to wireless power transfer (WPT) systems, operating at 13.56 MHz switching frequency with dynamic dead-time control (DDTC). Resonant-coupled WPT systems are being designed at ultrahigh switching frequencies to reduce the size of the wireless link and the passive components. Maintaining ZVS while controlling the output power delivered to a fixed or variable load is one of the major challenges of designing inverters at multi-MHz switching frequencies. DDTC is the approach deployed in this paper to sustain soft switching of a Class-D full-bridge inverter over the full range of output power while regulating the input dc bus voltage. Simulation results are presented to show that dynamically controlling the dead-time during input dc bus voltage variations reduces switch-node voltage overshoot, prevents large current spikes in the switching devices, and reduces associated high switching loss. Practical results obtained show that DDTC reduces switch-node voltage overshoot, increases the inverter efficiency, and reduces the steady-state temperature of the inverter during output power regulation.

Cost-Based Droop Scheme for Converters in Interconnected Hybrid Microgrids

Hybrid microgrids are the future of the power system as they bring the advantages of ac and dc networks under a common roof. As the power system continues to expand, hybrid microgrids will be connected together for increased reliability. One of the main concerns of such an arrangement is economical power sharing among multiple grids. This paper aims to address this issue by proposing a cost-based droop scheme for interlinking converters (ILCs). At the basic level, a novel voltage-frequency droop is presented which manages bidirectional power flow among the grids. Any change in ac grid frequency is translated into a dc bus voltage variation which allows active power flow from an underloaded to the overloaded grid. The conventional secondary control within the individual ac and dc grids is used to eliminate the inherent frequency and dc voltage deviations, restoring them to the nominal values. Economical power sharing among multiple grids is realized by embedding the incremental costs into the voltage-frequency droop and minimizing the total active power generation cost based on equal incremental cost principle. Different operational modes are discussed for interconnected grids, and simulations performed in power system computer-aided design (PSCAD) are used to confirm the validity of the scheme for each mode.

Maximum Efficiency Tracking of an Integrated Two-Staged AC–DC Converter Using Variable DC-Link Voltage

This paper presents an approach to achieve maximum conversion efficiency of a two-staged-cascaded ac–dc converter. In this study, the integrated ac–dc converter consists of a three-phase boost-type power factor correction rectifier and a phase-shifted full-bridge dc–dc converter. The detailed loss modeling of the integrated stage implies that the total power loss varies in a nonmonotonic fashion with intermediate dc-link voltage. In order to ensure maximum conversion efficiency, an optimum dc-link voltage level can be set depending on the load power level. A comparison between the efficiency variation under two different cases is carried out with extensive mathematical analyses: 1) fixed dc bus and 2) variable dc-bus voltage control. In addition, the experimental results obtained from a 6-kW-integrated ac–dc hardware prototype show a unity power factor along with a peak efficiency of 97.2%, which is 2.5% more than the fixed dc-link control approach.

Improved Dynamic Voltage Regulation in a Droop Controlled DC Nanogrid Employing Independently Controlled Battery and Supercapacitor Units

DC bus voltage signaling (DBS) and droop control are frequently employed in DC nano and microgrids with distributed energy resources (DERs) operating in a decentralized way. This approach is effective in enforcing the desired contributions of power sources and energy storage systems (ESSs) in steady-state conditions. The use of supercapacitors (SCs) along with batteries in a hybrid energy storage system (HESS) can mitigate the impact of high and fast current variations on the losses and lifetime of the battery units. However, by controlling the HESS as a single unit, one forfeits the potential contribution of the SC and its high power capabilities to dynamically improve voltage regulation in a DC nanogrid. This paper discusses an approach where the SC interface is controlled independently from the battery interface, with a small droop factor and a high pass filter (HPF), to produce high and short current pulses and smooth DC bus voltage variations due to sudden power imbalances in the DC nanogrid. Experimental results are presented to show that, unlike in a conventional HESS, the SC unit can be used to improve the dynamic voltage regulation of the DC nanogrid and, indirectly, mitigate the high and fast current variations in the battery.

DC-Bus Voltage Range Extension in 1500 V Photovoltaic Inverters

Solar plants based on single-stage conversion photovoltaic (PV) inverters (no dc–dc boost stage) have gained popularity due to their simplicity, high efficiency, and cost effectiveness. Existing PV plants mostly operate under 1000 V and are subject to wide dc-bus voltage variations due to the effect of PV cell temperature and the voltage of the maximum power point (well below the open-circuit voltage). In particular, attractive locations, such as U.S.–Canada mountain and central regions, are subject to extreme PV surface temperatures, making the dc-bus voltage variation a challenge for single-stage solar inverters. This paper investigates 1500 V solar inverters with a focus on dc-bus voltage range extension capabilities through novel modulation and power devices utilization. A comparative analysis with the existing 1000 V solar inverters is presented to illustrate the significant advantages of the wide dc-bus range in 1500 V systems. A lower voltage limit in the dc-bus is achieved by employing a novel voltage reactive power dc-bus control strategy and modified modulation. A higher dc-bus peak voltage is obtained by maximizing the use of power switches. As a result, the 1500 V inverter dc-bus voltage is significantly extended to capture energy under extreme PV surface temperatures, greatly improving the limited range of traditional 1000 V inverters. Simulations and experimental results using a three-phase three-level neutral point-clamped inverter level are presented to validate the proposed dc-bus extension range, control strategy, and modulation scheme.

Frequency-Adaptive Filtering of Low-Frequency Harmonic Current in Fuel Cell Power Conditioning Systems

Based on one of the most researched multiinput dc-dc converter topologies for renewable energy systems, the multi-input dual-active bridge (DAB) dc-dc converter, the effectiveness of harmonic current absorption by the energy storage branch in fuel cell power conditioning systems is critically evaluated. The closed-loop output impedances of the converter under single-voltage-loop and dual-loop controls are derived and compared. It is shown that both control strategies can effectively reduce the converter's closed-loop output impedance, thus favoring the flow of harmonic current and prevent it from being drawn from either the fuel cell branch or the dc-link capacitor. However, as shown by experimental results, the use of conventional PI control alone still produces noticeable voltage ripple on the dc voltage bus due to harmonic current being drawn from the dc-link capacitor. Proportional-resonant control is proposed to effectively compensate for the dc-bus voltage variation by generating an extremely low impedance path for harmonic current flow at specific frequency. An analog-based frequency tracking circuit is further proposed to adjust the resonant frequency for compensating the effect of harmonic frequency variation.

A comparison study of different semi-active hybrid energy storage system topologies for electric vehicles

In this paper, four different semi-active hybrid energy storage systems (HESSs), which use both supercapacitors (SCs) and batteries, are compared based on an electric city bus running the China Bus Driving Cycle (CBDC). The SC sizes of the different HESS topologies are optimized by using the dynamic programming (DP) approach, based on a dynamic degradation model of the LiFePO4 battery. The operation costs of different HESSs, including the electricity and the battery degradation costs over a whole CBDC, are minimized in the optimization process. Based on the DP results, near-optimal control strategies of different HESSs for on-line uses are proposed. Finally, the four HESS topologies are comprehensively compared from different aspects, including operation cost, initial cost, and DC bus voltage variation. Simulation results show that all HESS topologies have their merits and drawbacks, and can be used in different applications with different requirements. In addition, about 50% of the operation cost of the energy storage system is reduced by the semi-active HESSs when compared to the battery-only topology. Thus the effectiveness of adopting the SC in the HESS is verified.

Hierarchical Control System for PV Microgrid Applications

DC microgrid can effectively play to the value and benefits of the distributed power supply, communication than micro grid-connected have the ability to stronger more flexible refactoring and therefore become a new trend micro grid-connected technology research. Based on photovoltaic DC microgrid as example, this paper aimed at the DC microgrid voltage stability problem, proposes a layered coordination control DC bus voltage, the method is based on the detection and control of DC bus voltage variation to coordinate photovoltaic battery energy storage interface, net side interface and the interface converter works, ensure that under different conditions can keep the grid-connected in active power balance. Using Matlab/Simulink simulation and experimental verification, the results verified the effectiveness and feasibility of the method.

DC-Bus Voltage Control With a Three-Phase Bidirectional Inverter for DC Distribution Systems

This paper presents dc-bus voltage control with a three-phase bidirectional inverter for dc distribution systems. The bidirectional inverter can fulfill both grid connection and rectification modes with power factor correction. The proposed control includes two approaches, one line-cycle regulation approach (OLCRA) and one-sixth line-cycle regulation approach (OSLCRA), which take into account dc-bus capacitance and control dc-bus voltage to track a linear relationship between the dc-bus voltage and inverter inductor current. Since both of the approaches require the parameter of dc-bus capacitance, this paper first presents determination of dc-bus capacitor size and an online capacitance estimation method. With the OLCRA, the inverter tunes the dc-bus voltage every line cycle, which can reduce the frequency of operation-mode change and current distortion. The OSLCRA adjusts current command every one-sixth line cycle to adapt to abrupt dc-bus voltage variation. The two approaches together can prevent dc-bus voltage from wide variation and improve the availability of the dc distribution systems without increasing dc-bus capacitance. Experimental results measured from a three-phase bidirectional inverter have verified the feasibility of the discussed control approaches.

Application of cascaded H-bridge distribution-static synchronous series compensator in electrical distribution system power flow control

In this study, the power flow control of the electrical distribution network using multilevel converter-based distribution-static synchronous series compensator is presented. The conventional radial distribution network can be changed into a meshed grid because of this device. Using multilevel converter topology, the device can be installed directly in series with the line, connecting two separate feeders from different substations together. Two control methods are proposed to control the power flow in the line. The first control strategy uses a fixed DC bus voltage. Enhancing the performance of the multilevel converter, a novel control strategy using variable DC bus voltage is introduced while maintaining the appropriate dynamic response. The new strategy can reduce the total harmonic distortion (THD) of the injected voltage and losses of the power converter compared with the fixed DC bus method. A new soft start method is also proposed to connect the separate feeders together safely. The feasibility of the control strategies is investigated through simulation and experimental results.

A Low-Power AC–DC Single-Stage Converter With Reduced DC Bus Voltage Variation

A new low-power single-stage ac-dc converter is proposed in the paper. The outstanding feature of the converter is that it can operate with a sinusoidal input current and a low primary-side dc bus voltage that is much less variable than that found in other single-stage converters. The operation of the converter is discussed in the paper and its various modes of operation are explained in detail. An analysis of the converter's steady-state characteristics is performed and the results are used in the design of the converter. Experimental results obtained from a prototype converter are also presented.

Unity power factor rectifier–inverter structure operating under unbalanced supply and variable DC bus voltage

A recently introduced method of improving converter input current waveforms as well as power factor are re-evaluated under non-ideal operating conditions, that is, variable DC bus and unbalanced supply. The proposed method makes use of a novel controlled front-end diode rectifier of a rectifier–inverter structure. The technique involves the use of bi-directional bi-pass switches across the front-end rectifier, with a dSPACE-based intelligent control algorithm. The operation of the converter is fully analysed as possible DC/AC drives and a complete design example is provided. The main feature of the topology is low cost, small size, high power factor and simplicity. It is found to be a universal retrofit for DC drives, and in the front-end rectifier of existing three-phase AC drives, UPS etc. for power factor correction without any passive or active filtering.

Three-Dimensional Flux Vector Modulation of Four-Leg Sine-Wave Output Inverters

The time integral of the output voltage vector of a three-phase inverter is often termed the inverter flux vector. This paper addresses the control of a three-phase four-leg sine-wave output inverter having an LC filter at its output, by controlling the flux vector in three dimensions. Flux vector control has the property that an output filter resonance is actively damped by an output voltage control loop alone. Furthermore, an inverter switching action inherently regulates the output voltage rapidly against dc-bus voltage variations. The flux vector control of sine-wave output inverters finds several applications in three-phase four-wire systems. This paper presents a flux modulation method for three-phase four-leg inverters feeding unbalanced and nonlinear loads. All the necessary steps for the digital implementation of the flux modulator are presented. The switching behavior of the modulator has been evaluated, which is useful for the variable fundamental frequency applications of the inverters. To provide experimental validation, the modulator is implemented as a part of the control system for a stand-alone three-phase four-leg inverter with an LC filter at its output. Control system details are also provided. Experimental results indicate the effectiveness of the modulator and control system in providing balanced voltages at the output of the LC filter even under highly unbalanced conditions with nonlinear loads. The resonance damping and voltage regulation properties of the modulator are also apparent from the experimental results.

High-dynamic direct average torque control for switched reluctance drives

A technique was developed to estimate online average torque of switched reluctance machines. This novel online average torque and energy ratio estimation technique can be used for closed-loop torque control, i.e., direct average torque control. This closed-loop torque control algorithm continuously adjusts the reference torque by changing switching angles and reference current to maintain constant and accurate average shaft torque at a commanded torque level. Using the estimated average torque for regulation, the control structure can be simplified and decoupled from DC-bus voltage variations and other secondary control input parameters, such as temperature. The switched reluctance drive controller used in this study was developed and implemented for a 55 W electric-vehicle traction drive.