DC Capacitor Research Articles

Analysis of Frequency Regulation Capability of Doubly Fed Induction Generator and Supercapacitor Energy Storage Based on Dynamic Power Flow

The grid-integrated doubly fed induction generator (DFIG) is required to participate in the frequency regulation of the power system. The supercapacitor energy storage (SES) is capable of enhancing the frequency regulation capability of the DFIG in a coupled manner. The SES is connected to the DC capacitor of the DFIG and provides active power response through the droop control. The dynamic power flow (DPF) model is established to quantify the frequency response of the power system when the DFIG-SES system participates in the frequency regulation. The integration of the SES affects the internal power flow distribution of the DFIG; thus, the detailed model of the DFIG is incorporated into the DPF analysis. Considering the different response speeds of the synchronous generator (SG), the SES, and the DFIG to the frequency regulation, the first-order inertia delay in the governor control of the SG is included in the DPF model. The impact of the delay time constant on the continued operation time of the SES is analyzed. With the same deloading percentage, the output power of the DFIG is adjusted based on a variable droop coefficient scheme to fully utilize its active power reserve. The feasibility and effectiveness of the DFIG-SES scheme to participate in the frequency regulation are analyzed based on the DPF and verified through numerical analysis.

Influence of switching sequences on the neutral point voltage balance in a three-level voltage source invertor

BACKGROUND:Ensuring the voltage balance of the neutral point of the DC link within acceptable limits is one of the mandatory requirements during the operation of a three-level autonomous neutral point clamped voltage source inverter. It is known that neutral point voltage imbalance can adversely affect the operation of the inverter and the load at all, lead to the failure of both power switches and capacitors in the DC link. Neutral point voltage imbalance mostly often occurs due to the asymmetry of the nominal values of DC capacitors, inconsistent properties of switching devices, asymmetric three-phase load, and also due to the imperfection of the converter control algorithm.
 AIMS:Selection of the optimal switching sequence that ensures the smallest deviation of the neutral point voltage and the number of switching of power switches in a three-level voltage source inverter.
 METHODS:For an objective comparison of the considered switching sequences and their influence on the neutral point voltage balance, a simulation model of a three-level neutral point clamped voltage source inverter was developed in theMATLAB/Simulinksoftware. To control this inverter, a space vector modulation method was used with three different switching sequences: five-segment, seven-segment and standard.
 RESULTS:The topology of a three-level neutral point clamped voltage source inverter is presented. The experimental dependencies of the neutral point voltage deviation error was taken with a change in the modulation coefficient, the frequency of the fundamental harmonic at the inverter output, and load characteristics for three different switching sequences.
 CONCLUSIONS:In this paper, the influence of the basic vectors and the influence of switching sequences on the neutral point voltage balance are studied. Combinations of states of large and zero basic vectors do not affect the neutral point voltage. For the medium basic vectors, the neutral point voltage can increase or decrease depending on the operating conditions of the inverter. The small basic vectors significantly affect the neutral point voltage. Considering the abovementioned, the seven-segment sequence that ensures both the optimal balance of the neutral point voltage and the level of switching losses should be considered as the best switching sequence for a three-level inverter.

A Novel Hybrid Control Strategy and Dynamic Performance Enhancement of a 3.3 kW GaN–HEMT-Based iL2C Resonant Full-Bridge DC–DC Power Converter Methodology for Electric Vehicle Charging Systems

The conventional resonant inductor–inductor–capacitor (L2C) DC–DC converters have the major drawbacks of poor regulation, improper current sharing, load current ripples, conduction losses, and limiting the power levels to operate at higher loads for electric vehicle (EV) charging systems. To address the issues of the L2C converter, this paper proposes an interleaved inductor–inductor–capacitor (iL2C) full-bridge (FB) DC–DC converter as an EV charger with wide input voltage conditions. It comprises two L2C converters operating in parallel on the primary side with 8-GaN switches and maintains the single rectifier circuit on the secondary side as common. Further, it introduces the hybrid control strategy called variable frequency + phase shift modulation (VFPSM) technique for iL2C with a constant voltage charging mode operation. The design requirements, modeling, dynamic responses, and operation of an iL2C converter with a controller are discussed. The analysis of the proposed concept designed and simulated with an input voltage of 400 Vin at a load voltage of 48 V0 presented at different load conditions, i.e., full load (3.3 kW), half load (1.65 kW), and light load (330 W). The dynamic performances of the converter during line and load regulations are presented at assorted input voltages. In addition, to analyze the controller and converter performance, the concept was validated experimentally for wide input voltage applications of 300–500 Vin with a desired output of 48 V0 at full load condition, i.e., 3.3 kW and the practical efficiency of the iL2C converter was 98.2% at full load.

A Unified Capacitor Stress Emulation Method for High-Power Converter Applications

This paper proposes a unified capacitors stress emulation method, which has two unique test capabilities: 1) concurrent electrical stress emulation to AC capacitors and DC capacitors; 2) different electrical stress levels (including different ripple current, ripple voltage, and DC voltage) can be individually controlled for the testing samples. It preserves the advantages of a recently reported method with a minimum required power supply and is robust to testing sample degradation. This method is suitable for application-oriented stress emulation testing with different types of DC / AC capacitors and multiple electrical stresses in high power converter systems. The circuit architecture and testing ability of this method are presented. Moreover, analytical models are derived to size the key components to realize various testing requirements. Proof-of-concept experimental results are presented to verify the feasibility of the proposed test method.

Performance Analysis of Solar PV system integrated with SEPIC and ZETA converter

The renewable energy source that is now expanding the fastest is solar energy. It is expanding quickly because of its many applications and affordable upkeep. In order to maintain a consistent output voltage, the DC-DC power electronic converter is integrated with the solar PV system. This study examines SEPIC and ZETA converters in conjunction with a solar PV array. In this study, many characteristics like DC voltage, inductance, capacitance, and voltage are discussed. This research also compares and contrasts the SEPIC and ZETA converters. The MATLAB/Simulink environment is used for all work. When all the parameters are set to certain standard values, waveforms are produced. This demonstrates how changing a few parameters could produce distinct waveforms.

SRF theory-based shunt active power filter for power quality improvement in grid-interfaced solar photovoltaic system

This paper uses Shunt Active Power Filter (SAPF) in a polluted and unbalanced mains voltage for the compensation of current harmonics along with the reactive power. During compensation the reference current is calculated by the theory of Synchronous Reference Frame (SRF). This study utilizes the Photovoltaic (PV) energy, a renewable energy source, for which a maximum power point tracking (MPPT) method, an incremental conductance (InCnd), is employed. To modify the energy storage of dc-voltage, an artificial neural network controller (ANN) is used. The power system toolbox of MATLAB/SIMULINK is used to simulate the system being proposed. The SAPF is compared and assessed on the basis of PI control, FUZZY controller, and Artificial Neural Network (ANN). The results demonstrate the ANN control’s efficiency in optimizing the DC capacitor energy storage, sinusoidal form of the current, and reactive power compensation. A low Total Harmonic Distortion (THD) will be achieved with the proposed system which shows the efficacy of the method presented.

A Digital Twin System of Capacitive DC Bank Using Rogowski Coil to Monitor Individual Capacitors

The reliability of dc link capacitors is closely related to the operational safety of the whole power electronic system. To meet the application requirements, it is usually necessary to use a series or parallel structure of multiple capacitors to increase the capacity and withstand voltage, which also makes it difficult to monitor the individual capacitors in the capacitor bank. This article proposed a digital twin system for dc-link capacitor banks with PCB integrated Rogowski coils as sensors. The system uses highly integrated Rogowski coils to obtain the operating currents of individual capacitors in order to realize parameter correction of the digital twin system and ensure that the digital twin system can accurately reflect the electrothermal behavior of the dc-link capacitor bank. In addition, the effect of operating temperature on capacitor parameters is also considered, and the capacitor parameters and the operating temperature monitored by the digital twin system simultaneously are used to standardize the parameters. The standardized parameters are used as indicators to realize the health status assessment of each capacitor in the capacitor bank, which further provide an important basis for operation and maintenance.

Backstepping Control of NPC Multilevel Converter Interfacing AC and DC Microgrids

This work introduces modified backstepping methods to design controllers for neutral point clamped (NPC) converters interfacing a DC/AC microgrid. The modified backstepping controllers are derived from a proper converter model, represented in dq coordinates, and are designed to regulate the DC voltage and to balance the two NPC converter DC capacitor voltages through a DC offset in the sinusoidal pulse width modulation (SPWM) carriers. The averaged and separated dynamics backstepping controllers also enforce nearly sinusoidal AC currents at a given power factor. The two proposed NPC converter controllers are evaluated through MATLAB/Simulink simulations and experimental implementation using a laboratory prototype. Simulations and experimental results show that the two modified backstepping controllers regulate the microgrid DC voltage in steady state and in transient operation, even with load disturbances or DC voltage reference changes, while enforcing nearly AC sinusoidal currents at a given power factor or injected reactive power. The modified backstepping-controlled NPC converter is bidirectional, converting energy from DC renewable energy sources or storage systems to AC or charging storage systems from AC. The results also highlight the effective balancing of the NPC DC capacitor voltages.

Frequency Support Control of Multi-Terminal Direct Current System Integrated Offshore Wind Farms Considering Direct Current Side Stability

The frequency stability of modern power systems is challenged due to widespread application of large-scale renewable energy systems, of which the offshore wind farm (OWF) is one of the dominant resources. The OWFs are usually integrated into the grid by multi-terminal direct current (MTDC) transmission systems, which makes the energy flow complicated and the frequency control design challenging. A frequency support control method of MTDC system integrated OWFs (referred to as the OWF-MTDC system) is proposed in this paper. First, the wind turbine generation system (WTGS) is controlled to reserve a certain amount of available power according to the real-time wind speed for more comprehensive frequency regulation. Then, the frequency support control of OWFs is designed, and they can release the rotor kinetic energy and reserved power to support the onshore grid frequency. In addition, the virtual inertia control of a modular multi-level converter (MMC) is designed, which can also provide frequency support in an emergency by use of the DC capacitor. To ensure that the frequency control of the OWF-MTDC system does not degrade the stability of the system, a detailed DC impedance model of the MMC-based MTDC systems is developed, considering the constant power control and DC voltage control. Based on the impedance model, the impact of the frequency control coefficients on the DC side stability of the MTDC system is analyzed. Simulation results validate the stability analysis and verify the proposed frequency control method, which can effectively provide frequency support to the onshore power grid.

Reduction of DC Capacitor Size in Three-Phase Input/Single-Phase Output Power Cells of Multi-Cell Converters through Resonant and Predictive Control: A Characterization of Its Impact on the Operating Region

Cascaded H-bridge drives require using a significant-size capacitor on each cell to deal with the oscillatory power generated by the H-bridge inverter in the DC-link. This results in a bulky cell with reduced reliability due to the circulating second harmonic current through the DC-link capacitors. In this article, a control strategy based on a finite control set model predictive control and a proportional-resonant controller is proposed to compensate for the oscillatory power required by the H-bridge inverter through the cell’s input rectifier. With the proposed strategy, a DC-link second harmonic free operation is achieved, allowing for the possibility of reducing the capacitor size and, in consequence, the cell dimensions. The feasibility of the proposed control scheme is verified by experimental results in one cell of a cascade H-bridge inverter achieving an operation with a capacitance 141 times smaller than required by conventional control approaches for the same voltage ripple.

Power line Voltage Sag Mitigation by Dynamic Voltage Restorer using ANN optimization Technique Approach

Voltage sag is one of the most severe power quality disturbances to be dealt with by the industrial sector, as it can cause severe process disruptions and results in substantial economic loss. A short-term decreasein voltage lasting anywhere from milliseconds up to a few seconds is called voltage sag. At present, a wide range of very flexible controllers, which capitalize on newly available power electronics components, are emerging for custom power applications. Among these, the distribution static compensator (D- STATCOM) and the dynamic voltage restorer (DVR) are most effective devices for mitigation of voltage sag, both of them based on the voltage source converter (VSC) principle. A DVR injects a voltage in series with the system voltage and a D-STATCOM injects a current into the system to correct the voltage sag. Control unit is the heart of DVR and D- STATCOM where the main function of it is to detect the presence of voltage sag in the system and calculating the required compensating voltage for the above compensating devices. Due to approximation in the process of the control design the existing control schemes used for DVR and D-STATCOM have some tracking error. This work evaluates the performance of the above compensating devices with a novel algorithm called two neuron control algorithm for simultaneous sag compensation and replenishing dc bus energy. This algorithm uses the simplest pre-sag supply voltage boosting technique. Besides, a self-charging technique is used which maintains the dc capacitor voltage at the desired level. The salient advantages of this method are compensating long duration deeper voltage sags, reduction in size of dc-link capacitor and simplicity of algorithm for digital implementation. Comprehensive results are presented to assess the performance of the two neuron control algorithm for each device as a potential custom power solution. Keywords: Voltage sag, DVR, D-STATCOM, FACTS Devices, Power quality

DC Capacitor-Less Two-Terminal Unified Active Capacitor and Inductor

Flexible characteristics, tunable parameters, and improved power density are the desired attributes that have led to widespread research on active capacitors and inductors (ACI) in power conversion systems. However, the two-terminal realisation of ACI is challenging due to different trade-offs between power density, device stress, and reliability of the added components. Moreover, the requirement of handling second-harmonic ripple makes bulk electrolytic capacitors nearly indispensable in two-terminal ACIs. Thus, the scope of power density improvement becomes constrained. Hence, active capacitors requiring minimum or no bulk dc capacitance are seen as an attractive technology in recent literature. However, such solutions can mimic either inductive or capacitive behaviour with a single converter unit, restricting their adaptability across different applications. This limitation is addressed in this work with the proposed unified active capacitor and inductor (UACI), realised without any bulk dc capacitor. It emulates variable capacitance and inductance while offering a smooth transition from one characteristic to another. High effective impedance due to a small susceptance at the transition duty prevents undesirable current overshoot during the transition. A laboratory-scale prototype of the proposed ACI is fabricated, and the hardware is verified up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 450 VAR.

Enhancement of the Voltage Control Response in Three-Phase Photovoltaic Inverters With Small DC Capacitors

In the case of photovoltaic (PV) inverters, an adequate dc voltage regulation is fundamental to maximize or limit the power injected into the grid. However, the traditional control requires a large dc capacitance to ensure stability in the whole operating range while the existing alternatives, despite achieving a stable control with a small capacitance, become too slow in the open-circuit area. This paper proposes two control methods to improve this performance. Firstly, a new voltage control with virtual impedance emulation is presented, showing that the response becomes faster in all operating points. Secondly, the control with impedance emulation is combined with a feed-forward compensation, further improving the dynamic response. Both methods are very simple to implement and their superior performance when using a small dc capacitance is verified by means of simulation results.

Three Parts Modulation and Hybrid DC Capacitor Voltage Balancing for a Single-Phase Two-Leg Five-Level NPC Inverter

Capacitor voltage unbalance in NPC multilevel inverters is an inherent challenge and has a severe effect on five and higher-level inverters. Current modulation-based voltage balancing schemes for five-level NPC inverters require a high number of switching, especially at high modulation index operations. Moreover, hardware-based voltage balancing techniques come with a high component count. This paper proposes a unique three-part modulation technique combined with a hybrid voltage balancing approach to reduce switching losses and maintain voltage balancing. The modulation is composed of different carrier-based modulations, while the hybrid balancing scheme combines modulation-based and hardware-based balancing techniques. Comparative switching loss analysis demonstrates a reduction in inverter switching associated with the modulation. While the voltage unbalance characteristics of the modulation are determined considering linear loading conditions. Furthermore, balancing capacity analysis and switching loss reevaluation are used to justify the hybrid balancing performance. The concept verification use Matlab-Simulink simulation and hardware experiments on a five-level single-phase NPC inverter setup operated at different modulation index and power factor conditions.

Instant EOFF measurement error in cathodically protected pipelines: A parametric assessment study

Buried pipeline systems benefit from mitigation wires (earthing systems) in order to be protected against electric shock hazards. Moreover, cathodic protection (CP) systems are incorporated into pipeline systems to provide protection against corrosion by injecting an impressed DC current. DC decoupling devices are normally installed between the pipeline's metallic wall and the earthing wire, functioning as a filter that blocks the DC component (of the CP system) while allowing the hazardous AC interference current to be dissipated into the earth. However, the internal capacitance of the DC decoupling devices introduces an error in the routine survey measurement of the CP effectiveness, as frequently reported by pipelines’ system operators. In this paper, the factors influencing this measurement error are investigated by modeling the electrical behavior of the CP-pipeline system. This investigation concluded that the capacitive discharge time constant and by extension, the measurement error highly depends on the pipeline resistance to remote earth (coating resistance), as well as on the number and the capacitance C of the capacitive DC decoupling devices. To this extent, methods for minimizing Eoff measurement error are proposed.

Symmetrical Cross-Connected T-Type Multilevel Inverter With Reduce Device Count

This paper presents a symmetrical, cross-connected T-type (CT-type) multilevel inverter (MLI) configuration with reduced device count (RDC). It comprises eight power switches, two DC sources, and four capacitors with self-voltage balancing. The generalized topology is formed by two CT-type basic modules connected by two power switches. These topologies operate using DC sources and capacitors having the same or different voltage levels, which can be extended by cascading the modules to attain a higher voltage level suitable for high-power applications. Therefore, the proposed CT-Type MLI is a good choice for renewable energy applications that require a lower input voltage source magnitude to attain high voltage levels. This paper uses the Matlab/Simulink platform to simulate the proposed CT-type topology using the level-shifted pulse width modulation (LS-PWM) technique. Further, a laboratory prototype is developed to test and validate the feasibility and effectiveness of the proposed MLI. Finally, a comparative analysis of the number of DC sources, capacitors, power switches, drivers, and total standing voltage (TSV) is presented against similar recent topologies.

An Enhanced AC Fault Ride through Scheme for Offshore Wind-Based MMC-HVDC System

This study presents an improved, communication-free Fault Ride-Through (FRT) strategy for type-3 and type-4 wind turbine integrated modular multilevel converter-based high-voltage direct current (MMC-HVDC) systems in offshore wind power plants (OWPPs). The research aims to enhance the reliability and resilience of OWPPs by ensuring their connection with AC grids remains intact during and after faults. Simulation results conducted on a 580 kV, 850 MW MMC-HVDC system using PSCAD/EMTDC software v.4.6.2 demonstrate quick post-fault recovery operation and the ability to effectively manage DC link and capacitor voltages within safe limits. Furthermore, the circulating current (CC) and capacitor voltage ripple (CVR) remain within acceptable limits, ensuring safe and reliable operation. The study’s major conclusion is that the proposed FRT strategy effectively mitigates the adverse effects of short circuit faults, such as a rapid rise in DC-link voltage, on the performance of the MMC-HVDC system. By promptly suppressing DC-link overvoltage, the proposed FRT scheme prevents compromising the safe operation of various power electronics equipment. These findings highlight the significance of FRT capability in OWPPs and emphasize the practical applicability of the proposed strategy in enhancing the reliability of offshore wind power generation.

A comprehensive study of grid impedance and its reliability effects on variable frequency drive

&lt;p class="Text"&gt;Grid impedance is one of the most important parameters for the satisfactory operation of variable frequency drive (VFD) and their interaction with other utilities connected to the same grid. In this research investigation a new approach to analytical evaluation and verification of grid impedance, and short circuit current is discussed. VFD under identical operating and loading conditions is connected to different grid capacities and configurations, being considered in this investigation. Grid impedance variation in terms of measured or estimated values, due to different grid connectivity affects the commutation of uncontrolled front-end rectifiers. This impacts the nonlinearity of input current wave shape, causing a significant change in current harmonics (THDi) and ripple current of DC link. This paper comprehensively investigates the reduction of DC capacitor lifetime and losses of DC inductor, and diode with various grid capacity and configurations under standard load conditions. High-frequency harmonics impact (2-9 kHz) on DC inductor losses is also analyzed. A VFD of 250 kW is considered for entire practical measurement including distortion effects and MATLAB simulation. This research may give a good insight into the sizing of VFD front-end devices. The reliability impact of VFD is also one of the important outcomes of this research.&lt;/p&gt;

Two-Stage Grid-Connected Frequency Regulation Control Strategy Based on Photovoltaic Power Prediction

The large number of photovoltaics connected to the distribution network via power electronic converters squeezes the functional space of traditional synchronous generators in the power system and reduces the inertia of the network itself. However, due to the random and fluctuating nature of PV power generation, different types of meteorological conditions can also affect the inertia support capability of PV output power. Therefore, this paper proposes a frequency regulation control strategy based on the dynamic characteristics of the grid-side DC capacitor. Firstly, the control strategy of the grid-side inverter is improved and the mechanism of the frequency dynamic response model under PV penetration is analysed. Secondly, data from different weather types are correlated and analysed to predict PV power, and wireless sensor technology is used to introduce the data signals into the control link. Finally, a simulation model of a two-stage PV power generation system is developed and the fast-response capability of the system under different control parameters when the load is increased or decreased is analysed. The results show that under the proposed frequency regulation strategy, the larger the regulation coefficient and virtual inertia time constant, the greater the virtual inertia provided by the PV power generation, which improves the stability of the distribution network.

A Novel Unidirectional Three-Phase Multilevel Rectifier Composed of Star-Connected Three Single-Phase Topology Based on Five-Level Flying Capacitor DC–DC Converter

A novel unidirectional three-phase multilevel rectifier composed of star-connected three single-phase topology based on five-level flying capacitor DC-DC converter is proposed, which can be widely applied to many industrial applications, especially for the front-end of multilevel converters for AC adjustable motor drives with open-end winding connection. In addition to not requiring transformers (either line-frequency or high-frequency), compared to other high voltage rectifiers, this proposed topology features fewer active switches with lower voltage stress, a high voltage common DC bus, high input power factor and low input current harmonics. In this paper, a comprehensive analysis to the basic operating principles including the topology configuration, the steady-state modeling and phase-shifted SPWM modulation are presented. A hybrid control strategy is studied, including the input power factor control, the dc-link voltage balancing control and the flying capacitor voltage balancing control with balanced and unbalanced loads. Finally, the experimental results are presented to validate the feasibility and correctness of the proposed solution.