A new multilevel inverter (MLI) topology is proposed using a level doubling network (LDN). The LDN takes the form of a half-bridge inverter to almost double the number of output voltage levels. The ...concept (of the proposed LDN) has the capability of self-balancing during positive and negative cycles without any closed-loop control/algorithm, and it does not consume or supply any power. The topology uses a symmetric cascaded H-bridge MLI but offers an equivalent performance of an asymmetric topology in terms of the number of levels. Also, it maintains the merit of uniform loading of the individual cell for a symmetric MLI. The topology is implemented by connecting only a three-arm H-bridge (only two switches per phase) with the entire three-phase inverter to double the number of levels. Thus, it significantly improves the power quality, reduces the switching frequency, and reduces the cost and size of the power filter. Operation of the circuit is verified by simulation result and experiments from a laboratory prototype.
The proposed inverter topology is emerged from the multiple level-doubling-network (LDN) based topology for grid-connected solar photovoltaic (PV) system, where dc buses of three phases could not be ...merged without electrical isolation. Three-phase T-neutral point clamped (TNPC) is used to merge all the three phases without transformer. Due to LDN operation, balancing the capacitors of TNPC (main bridge) is a major challenge. A dc-dc converter is customized in this work, which is normally required for two-stage operation of solar PV inverter. This converter topology reduces the size and losses of filter inductors due to high-resolution inverter output voltage in addition to restricting the leakage current well below the relevant standards. To validate the concept, experiments are carried out using hardware prototype and those matched well with simulation result.
A seven-level packed U-cell inverter is presented in this article. The converter uses a single dc source and two floating capacitors, whose voltages are balanced in open loop, to produce multilevel ...output voltage. Peak magnitude of the output phase voltage is equal to the magnitude of dc source. Voltages across floating capacitors add intermediate voltage-levels by establishing an asymmetric ratio (with respect to the available dc voltage in the circuit). The average energy exchange (when the network is in steady state) of the capacitors with the rest of the inverter circuit will be zero. This helps the capacitors to maintain desired voltages and thus create intermediate levels of steady dc voltages. Performance of the converter is validated in simulation by MATLAB/Simulink and testing of the converter is done for resistive as well as inductive loads. Experimental verification of the converter is carried out on a laboratory prototype and the obtained results match well with the simulation.
A Novel Method of Frequency Regulation in Microgrid Bose, Upama; Chattopadhyay, Sumit K.; Chakraborty, Chandan ...
IEEE transactions on industry applications,
2019-Jan.-Feb., 2019-1-00, 20190101, Letnik:
55, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Owing to generation and demand mismatch, the frequency deviation and the rate of change of frequency (RoCoF) may become drastic at times in an islanded microgrid. This is due to increased penetration ...of noninertial renewable energy sources that brings down the system's resilience to any kind of disturbance. Therefore, efficient control algorithms are required to mimic the inertial behavior of a conventional synchronous machine to arrest/limit any sudden change. This work focuses on frequency regulation of microgrid during transient conditions by means of fast-responding external energy reserve. The characteristics of a weak grid has been studied and simulated through modeling a virtual synchronous machine. An inverter model is also developed to integrate the energy storage system (ESS) to the grid and a double second-order generalized integrator phase locked loop (DSOGI-PLL) is implemented that is capable of synchronizing the system under distorted and unbalanced situation. A new technique to estimate the frequency of the microgrid is reported. A simple proportional controller-based approach for different level of pulsed power injection (using ultracapacitors and batteries) is proposed and simulated using MATLAB. Experimental demonstration is made using batteries only but with two different current-limits. The controller is implemented using dSPACE1103.
An improved version of power flow-controller (PFC) which is based on multi-winding transformer (MWT) is presented in this paper. The objective of this article is to control the power flow in a ...power-line to alleviate overload in a heavily loaded system and increase power flow of an under loaded line. The novelty of multi-winding transformer based power-flow controller is the use of unequal voltage ratio in the tap-changer windings of MWT with bi-directional switches to provide a compensating voltage required to achieve the desired power flow. This new model drastically increases the number of compensating points to make it more compact with higher flexibility. It aims to utilize the full potential of a line (bound by its thermal limits) to contribute in power flow by providing several times higher resolution with limited number of switches and reduced amount of copper and iron volume compared to existing state-of-the-art technology. The proposed MWT model has been included in Newton Raphson Load Flow (NRLF) method for power flow analysis. The feasibility of the model has been realized using a standard 5 bus test system and a modified IEEE 57 bus system. Operation of the PFC has been demonstrated using a hardware laboratory prototype for generating nearest compensating points (voltage vectors) corresponding to the obtained compensating voltages after load flow solution. The results obtained are verified by simulating the controller in MATLAB/Simulink.
Availability of more number of levels for the same number of switching devices has made asymmetrical topologies superior over symmetrical structures. While the 16 : 4 : 1 configuration is reported as ...an optimal asymmetry, a detailed performance is not available in literature. This paper presents the performance and control of the 16 : 4 : 1 asymmetrical topology. A new analytical approach is introduced to find the complete information of the available combination of space vectors for any asymmetric ratio. A limit chart is introduced, which represents the number of ways to generate a particular space vector. Asymmetrical hexagonal decomposition technique is used to select the switching devices from the high-, medium-, and low-voltage cells to realize a space vector. The converter is simulated in MATLAB/Simulink. Supporting results from a laboratory prototype have confirmed the usefulness of the 16:4:1 asymmetrical configuration.
This article presents a new device named flexible step-voltage regulator (FSVR) based on a multiwinding transformer for unbalanced distribution networks. While the traditional step-voltage regulator ...(SVR) can be used to control the voltage magnitude only, the proposed device can be used to control both voltage magnitude and phase angle for each phase independently. This allows greater flexibility during voltage control in unbalanced distribution networks. A mathematical model of the FSVR is derived for implementation in unbalanced power flow analysis. Simulation studies for IEEE 13 bus and IEEE 34 bus test networks demonstrate the effectiveness of the FSVR over the traditional SVR in reducing real power losses and voltage imbalance in the distribution network. A scaled-down model of the FSVR is designed in the lab to demonstrate the proof of concept.
The proposed inverter topology is evolved from multiple level-doubling-network (LDN) based state-of-the-art topology, where power frequency isolation-transformer is used at the ac output. The ...proposed work removes the transformer by replacing the main bridges by three-phase TNPC (T-type neutral point clamped) converter. The major issue of this hybrid topology is capacitor balancing of NPC with LDN operation. To solve this issue two different methods are proposed in this article. In these methods, an additional circuitry is added for capacitor balancing. The worst-case power handled by this additional converter is negligible compared to power handling capability of the entire converter. Hence, a full-capacity transformer is replaced by a nonisolated dc-dc converter of fractional rating. Both manufacturing cost and operating cost (losses) are substantially reduced. The proposed topology does not require any isolated auxiliary power supply as well as only one dc source is used for entire topology. Operation of the proposed topology is verified with PV fed grid-connected single stage operation and restricted the leakage current within limit. To validate the concept simulation is carried out in MATLAB/Simulink. A laboratory prototype is developed for experimental verification of the concept. Hardware results matched well with the simulation.
A new technique to interconnect HVAC and high-voltage-direct-current (HVDC) network is reported in this paper. An alternative dc-breaker-less voltage-source-converter topology with level doubling ...network based open-loop control is presented. The objective is to reduce the cell-capacitor size and number of cells (with reduced cumulative volt-ampere rating of all switches) without compromising with the efficiency. A detail comparison with state-of-the-art topology is presented. The proposed system using a full-bridge converter with naturally balanced modular cascaded H-bridge wave-shapers is expected to be useful in offshore HVDC transmission especially for power evacuation from offshore wind farms.
Asymmetrical cascaded topology can produce more number of levels. This can be further boosted by using level doubling networks (LDNs). An LDN is a floating capacitor fed two-switch topology that ...almost doubles the number of levels without necessitating any closed-loop control for the voltage balancing of the floating capacitors. Topological modules (TMs) are formed by using LDN. This paper shows the uniqueness of 1:7 ratio of asymmetry (between the TMs) to maximize the number of levels without having any loss in the intermittent voltage levels for the line-to-line voltage in a three-phase system. A new control technique to select the right space vector without generating any dc component in the output phase voltage is reported. Step-by-step description of the controller is presented in this paper. Extensive simulations using MATLAB/Simulink and supporting experimental results using a laboratory prototype have confirmed the usefulness of the proposed controller.
