Independent Power Control Research Articles

A Hybrid Voltage/Current Control Scheme With Low-Communication Burden for Grid-Connected Series-Type Inverters in Decentralized Manner

In this article, a novel hybrid voltage/current control scheme with low-communication burden is proposed for series-type inverters in a decentralized manner. All the inverter units are controlled by their individual local controllers without a real-time central global-communication. Among them, a few inverters serve as the current controlled inverters (CCIs) to regulate the grid current, and the rest inverters work as the voltage controlled inverters (VCIs). Under this control architecture, five main control targets are achieved: 1) frequency self-synchronization of each VCI inverter based on local current phase-angle signals; 2) fault-tolerant capability of all inverters without one-to-all-failure; 3) high disturbance-rejection-capability to grid variations; 4) adjustable grid power factor; and 5) independent power control for inverters with unbalanced power sources. As the central high-bandwidth communication is avoided, it has the features of decreased cost, increased scalability, and reduced geographic restrictions of inverters. Compared with conventional decentralized approaches, only a few CCI units acquire the grid phase-angle signal to achieve grid synchronization by communication, and the most VCI units are local self-synchronization via the local line-current-phase signal rather than real-time grid-voltage-phase communication signal. Thus, the proposed control can be realized with a very low-communication burden, which is more cost-effective and higher reliability in terms of communication faults. The feasibility of the proposed method is verified by experimental results of eight different cases, such as source power change, large power gap, grid voltage/frequency deviation, grid harmonics, and one-fault redundancy.

Analysis of Wind Driven Doubly Fed Induction Generator under Transient Conditions

The Doubly-fed induction generator is one of the most effective and frequently used wind power generation systems in the wind power industry. It offers several enticing benefits, including consistent output voltage, low cost, independent control of active and reactive power, and improved power quality. Despite any of the aforementioned advantages of DFIGs, it is precisely impacted by grid interruption. Any sudden appearance of a fault, such as symmetrical or asymmetrical faults, can cause a reduction in stator voltage, oscillations in stator and rotor currents, DC-link overvoltage, and electromagnetic torque pulsations. Such a phenomenon can considerably affect the performance of the DFIG under a transient state if an appropriate protection system is not considered. Thus, in this thesis work, the crowbar protection system is employed for the enhancement of the FRT capability of the wind turbine. The performance of the machine with the existing capacitor bank and crowbar protection is analyzed and compared. Moreover, the MATLAB/Simulink 2015a software is used for modeling and simulation and all system parameters are obtained from ADAMA-II Wind Farm.

Dual Active Bridge as a DC Link Current Pulsation Compensator in Energy Storage Applications

This paper presents a system for compensating DC link current pulsation in four-wire inverters with energy storage operating under unbalanced load conditions. This phenomenon occurs when an inverter with an independent power control in each of the phases attempts to locally balance the voltage imbalance in the grid. Such a condition creates a DC link current pulsation, which is destructive for energy storage connected to the DC link. The conditions when this situation appears are presented in detail in the paper. A solution to this problem is proposed in the form of a dual active bridge converter and a capacitor bank to actively compensate this pulsation. The control algorithm is proposed based on a proportional-resonant controller. This paper presents the technical background and method by which the controller parameters were calculated, implemented and tested in a real-time system. The test results are presented and discussed, concluding that the proposed solution is an attractive option for protecting the energy storage from DC link current pulsation. The dual active bridge converter combined with resonant controller can compensate the DC link current pulsation almost entirely.

Single Source Multi-Frequency AC-AC Converter for Induction Cooking Applications

In recent years, induction heating (IH) applications aided by electronic power control have gained significance. Particularly, for cooking applications, an appropriate control technique is required to feed power from a single source to multiple loads with minimum switching losses. Additionally, when multiple loads are used, it requires independent control and operation for each of the loads. The main idea of this work is to develop a single-stage AC-AC converter topology to feed power to multiple loads independently with a single source, with a reduced number of switching devices and with minimum switching losses. The proposed topology uses a frequency bifurcation concept to feed power to multiple loads by placing the transmitting coil and work coil at a distance of 3 cm. The source is resonated at a 25 kHz switching frequency, with the designed bifurcated frequencies of 20 kHz and 33 kHz. The resonant capacitors are appropriately chosen to operate at those frequencies. For real-time applications, simultaneous and independent power control are inevitable in multi load-fed IH applications. This is achieved through a pulse density modulation scheme with minimum switching losses. The simulation of the proposed system is performed in MATLAB/Simulink, and also the 1 kW system is validated using a PIC16F877A microcontroller. The real-time thermal variation in the load is also recorded using a FLIR thermal imager. The experimental and simulation results are observed, and the obtained efficiency of the system is plotted for various duty cycles of pulse density modulation control.

Research on Factors Affecting Power Flow of Direct Current Grid Considering Different Control Modes

Direct Current grid technology based on flexible Direct Current transmission is one of the key means to solve large-scale grid integration and consumption of new energy. The flexible Direct Current transmission technology developed in recent years has superior control performance, strong flexibility, and independent control of active power. And reactive power, can be connected to the weak Alternating Current system. This article derives the power flow equation of the Direct Current transmission network by establishing a mathematical model of the Direct Current transmission network and combining different system-level control strategies in the Direct Current transmission network. The Newton-Raphson method is used to solve the power flow distribution of the Direct Current network when the master-slave control and droop control are adopted. The influence of the two control methods on the power flow distribution of the Direct Current network is compared through a calculation example.

Power Decoupling Control for Wind Turbine Converter Based on Series-Connected Modular Multilevel Converter

A power decoupling control for wind turbine converter based on series-connected modular multilevel converter (SCMMC) and open-winding permanent magnetic synchronous generator (OW-PMSG) is proposed in this paper. The phase dc links of the SCMMC are series connected and the windings of the OW-PMSG are independent, which is different from conventional MMC. To improve the robustness of the system under imbalanced power disturbance, an ac power decoupling control is proposed with perfect power independent control of the three phase power and phase dc-link voltages. Besides, multi-frequency-bandwidth proportional-resonant (MFBPR) controller in the abc stationary frame without coordinates transformation is presented, which simplifies the control of the SCMMC. The accurate current control is based on the MFBPR controller and it is easy to improve the quality of the current with this controller. The proposed power decoupling control is verified through simulation and a 3 kW laboratory prototype.

Distributed Power Sharing Control for Islanded Single-/Three-Phase Microgrids With Admissible Voltage and Energy Storage Constraints

This article investigates power sharing and power quality improvement issues of islanded single-/three-phase microgrids (S/T-MGs) where both sources and loads are unbalanced. A hierarchical distributed control approach is proposed, which consists of 1) a phase-independent virtual synchronous generator (P-VSG) control used for primary control of distributed generators (DGs), 2) a distributed secondary power flow regulator used for power sharing control among DGs and among phases, and 3) a distributed secondary voltage regulator used for voltage restoration and power quality improvement. Compared with conventional methods, the proposed control has several salient features: 1) the P-VSG control allows for independent and flexible power control and voltage regulation for each phase and accurate phase shifts; 2) distributed containment control proposed in the secondary power control and voltage regulation layer guarantees admissible output phase powers, voltage profiles and power quality; 3) the constraint operator developed for the secondary controllers makes charging/discharging power of the energy storage system (ESS) within permitted values; 4) communication delays are also considered in the proposed distributed approach. Simulation results are presented to demonstrate the proposed control method.

Attenuation of DC-Link Pulsation of a Four-Wire Inverter during Phase Unbalanced Current Operation

This paper proposes a control algorithm for a hybrid power electronic AC/DC converter for prosumer applications operating under deep phase current asymmetry. The proposed system allows independent control of active and reactive power for each phase of the power converter without current pulsation on the DC link connected to an energy store. The system and its algorithm are based on a three-phase converter in four-wire topology (AC/DC 3p-4w) with two dual-active bridge (DC/DC) converters, interfaced with a supercapacitor and an energy storage. The control algorithm tests were carried out in a Hardware in the Loop environment. Obtained results indicate that operation with deep unbalances and powers of opposite signs in individual phases leads to current oscillations in the DC link. This phenomenon significantly limits energy storage utilization due to safety and durability reasons. The proposed algorithm significantly reduces the level of pulsation in the DC link which increases safety and reduces strain on lithium-ion storage technology, enabling their application in four-wire converter applications.

Optimal power allocation strategy for black start in VSC-MTDC systems considering dynamic impacts

The ability of voltage source converters (VSCs) to restore blackout grid has been widely accepted based on their abilities in flexible and independent power control. However, the traditional literature only studied the case of a two-terminal hybrid system in which all required black start (BS) power is provided by a single healthy AC grid. Meanwhile, the dynamic changes at the sending end, including AC grid and converter, are not considered. In this paper, an optimal power allocation strategy is proposed to perform BS in the VSC-based multi-terminal direct current (VSC-MTDC) systems. The new strategy ensures that all the healthy AC grids can cooperate to transfer BS power together to the blackout grid. Thus, each grid only needs to provide a proportion of the required power. Furthermore, since the grids and converters in practical VSC-MTDC projects are generally limited to small sizes, the capacities together with main dynamic characteristics are also considered in the new strategy. By reasonably setting the optimization objective and constraints, the overall dynamic impacts can be significantly reduced, and the stability of AC grids and converters can be guaranteed. Finally, the comparison tests on a typical four-terminal system verify the feasibility and effectiveness of the proposed method.

Multi-Input High Step-Up DC–DC Converter With Independent Control of Voltage and Power for Hybrid Renewable Energy Systems

In this article, a new nonisolated multi-input step-up converter is presented. By implementing a new switching algorithm in the proposed topology, the converter output voltage, as well as the power absorbed from each source, can be controlled independently, resulting in a simple controller, which is a distinctive virtue for this converter in comparison to similar multi-input converters. Current sharing between various sources is achieved independently. There is no need to control the different parameters of switches' gate-source voltage, such as the delay between switches, and thus, independent maximum power point tracking algorithm can be used for each input source. Providing soft-switching condition and transferring power from all inputs to the load independently are the other features of this converter. Operating modes of the converter as well as experimental results obtained for a 300-W prototype for supplying stand-alone load are presented.

Control of a Four-wire Hybrid Prosumer Converter for Balancing Utility Grids

Abstract The paper presents the project of the power electronic AC/DC converter operating under a multiresonant control algorithm for prosumer applications. This design allows independent control of active and reactive power for any or each phase. Both the converter and its algorithm are based on a three-phase converter of four-wire topology (AC/DC 3p-4w) with a DC bus, which couple the converter to a renewable energy source and energy storage. Further, model and simulation tests were carried out in the Matlab-SIMULINK programming environment. The results obtained indicate that operation with deep unbalances and powers of opposite signs in individual phases results in current variations (oscillations) in the DC line, which is a significant limitation, and it can be concluded that the level of asymmetry should be limited to the level acceptable to the energy storage device.

Cascaded full‐bridge resonant inverter configuration for different material vessel induction cooking

This paper presents a cascaded full-bridge resonant inverter configuration for different material vessel induction cooking. The proposed inverter configuration features simultaneous heating of three different material vessels, and independent power control. In this proposed work, three different induction heating loads are simultaneously operated at their respective resonant frequencies. Vessels of iron, steel, and aluminium materials are used. The output powers are independently controlled by an asymmetric duty cycle control. The proposed inverter is designed and hardware prototype has been implemented. Experimental results are validated with simulation results.

MEDIUM VOLTAGE DIRECT CURRENT (MVDC), A NEW SYSTEM FOR THE MANAGEMENT AND OPERABILITY OF ELECTRICITY DISTRIBUTION GRIDS

The challenges in the operation of the electricity grids of the future, especially due to an increase in demand and connected distributed generation, are forcing utilities to propose solutions that improve the controllability and capacity of the grids. The connection of direct current links in medium voltage (MVDC) may be one of these solutions whose features could improve the efficiency and flexibility of these networks. The aim of this article is to describe this technology, its basic operation and different control strategies, and to indicate the applications that these links currently have, with special emphasis on those that will improve operation and control in medium-voltage distribution networks, some of which are already in operation. KEYWORDS: MVDC, VSC, distribution grids, direct current links, independent power control

Independent control of active and reactive power for grid connected DFIG using reference power based improved field-oriented control scheme

The increase in the wind speed or rotor speed will decrease the doubly-fed induction generators (DFIG) terminal voltage which will increase the undesired reactive power flow from it. Hence, in this paper extraction of optimal power and control of reactive power is analysed. For this, independent control of real and reactive power is done using the proposed reference power-based improved field-oriented control scheme. The rotor side controller is designed to run the rotor at specified optimal speed and also control of reactive power from the rotor side based on the grid requirement. In this work, only a wind speed sensor is required, and there is no need for rotor speed sensor, real power sensor, and torque sensor and hence, is cost-effective and lesser parameter dependency. Rotor speed and torque are predicted using measured stator voltage and stator and rotor current and also using the derived rotor flux. The performance of the DFIG system is examined in a simulation environment for rapid changes in rotor speed, wind speed, grid reactive power. There is smoother rotor speed changing, and rapid reactive power support from the DFIG.

Power conditioning unit with independent extreme power control for two sections of small spacecraft solar arrays

the article analyzes the main approaches applied in the development process of power conditioning unit (PCU) model to ensure the efficient power use of non-orientable solar arrays for small satellites operated in low Earth orbits with altitudes ranging from 300 to 1500 km. Particular attention is paid to the description of PCU structure choice, as well as the way to achieve the technical specification goals, such as efficiency factor and specific power. A conclusion is made that the optimal structure to ensure PCU efficiency and reliability is the one with a series regulator and section-by-section solar array power extreme control. It is noted that this structure will make it possible to receive power from solar array sections regardless of the difference in temperature and illumination between them. Based on the quantity and duration analysis of the solar battery charge and discharge cycles, the charge and discharge modes of the batteries are defined as basic ones, so the efficiency factor in these modes is maximized. The results of the study described in the article can be used by enterprises specialized in developing small satellites and power conditioning units as a stepping stone for developing a PCU flight model to ensure the efficient power use of non-orientable solar arrays for small satellites.

Long‐distance wireless power transfer system powering multiple loads with constant voltage outputs using S‐SP compensation

This paper proposes a long-distance multi-load wireless power transfer (WPT) system with a constant voltage (CV) output for each load. A repeater unit consisting of two bipolar repeater coils is designed to power the load. Multiple repeater units are placed in a line and the power can be transferred from one unit to the next. Each repeater unit corresponds to one load and multiple loads can be powered simultaneously. The two repeater coils in the same repeater unit are placed perpendicularly to eliminate the magnetic coupling between them. In each repeater unit, a compensation capacitor is placed in series with the transmitting coil while two extra compensation capacitors form a series-parallel (SP) structure for the receiving coil. When neglecting the coil's parasitic resistance, the CV characteristics can be obtained, which enables the independent and flexible power control of each load. Moreover, the influence of the parasitic resistances on the system performance is analysed. Such a system is especially suitable for powering the gate drivers in a high-voltage converter where multiple insulated gate bipolar transistors (IGBTs) are connected in series. An experimental setup with six loads is constructed to validate the proposed system. The maximum system efficiency can reach 88.1%.

An analytical electromagnetic model of “Sen” transformer with multi-winding coupling

In flexible AC transmission systems, the “Sen” transformer (ST) is an economical and effective method for voltage regulation and power flow control, which can achieve independent control of active and reactive power. The ST is a three-phase multi-winding transformer based on the on-load tap-changing technology. In order to reveal the internal characteristics of the ST, this paper proposes an analytical electromagnetic model considering the multi-winding coupling in the ST with the three-phase three-limb structure. Firstly, according to the electromagnetic coupling relationship in the ST, a magnetic equivalent circuit model consisting of self-inductance coefficients and mutual inductance is derived via the unified magnetic equivalent circuit (UMEC). Secondly, an electric circuit model for the internal voltage and current of the ST is established based on the electrical connection between the ST and the external system. Finally, an analytical electromagnetic model considering multi-winding coupling for the ST is obtained by combining the magnetic equivalent circuit model and the electric circuit model. Two case studies have been carried out on a three-phase three-limb model of the ST with the help of MATLAB. The effectiveness of the proposed model is verified by comparing the obtained analytic results with the existing results from time-domain simulation about the series compensating voltage of the ST. In addition, the voltages and currents of windings in the ST in the conditions of different transformer core structures and ferromagnetic materials are investigated for evaluating the influence of magnetic coupling on the results.

Steady State and Transient Analysis of FSIG and DFIG Integration to Grid for Different Penetration Levels using VSC-HVDC

Among all the Renewable Energy Sources Wind energy is the fastest growing energy source over the last decade mainly due to crucial developments of technology in wind energy. Nowadays, the penetration of wind energy is increasing in many countries in the world including India. The power system stability with large penetration of wind power is a concern for many electrical utilities. The common technical issues with increased penetration of wind energy are voltage and reactive power control, frequency control and Low Voltage Ride Through (LVRT) capability. The VSC-HVDC system with its benefit of independent control of active and reactive power promises to enhance the system stability at high penetration levels. The maximum wind penetration levels in to the grid is analysed for Fixed Speed Induction Generator (FSIG) & Doubly Fed Induction Generator (DFIG). The penetration levels are further enhanced by considering the evacuation of wind power with VSC-HVDC system without losing system stability. Different controllers for VSC-HVDC system are used to improve the stability and LVRT capability. Standard Benchmark System is considered, and the simulations are performed by using power system simulation software SIMPOW. Results shows that wind power evacuated through VSC-HVDC system has better stability and LVRT Capability compared to AC system at high wind penetration levels.

Research on the Asymmetrical Multilevel Hybrid Energy Storage System Based on Hybrid Carrier Modulation

In this article, when the cascaded H-bridge topology is used in the asymmetrical multilevel hybrid energy storage system (AM-HESS), the asymmetry of the energy storage components' types and voltages makes it difficult for traditional modulation methods to realize an independent power control of each energy storage unit. Thus, these methods cannot be directly applied to the realization of the complex working modes of AM-HESS. To this end, an AM-HESS hybrid level shifted pulsewidth modulation (PWM) (HLS-PWM) technique is proposed. According to the directions of supercapacities' (SCs') output power and the total output power of the energy storage system, the HLS-PWM is divided into the two fundamental modes-the same (MP) and opposite (MN) modes. Subsequently, the SC charge and discharge control under both the unit power factor and the nonunit power factor is analyzed for the two modes. Furthermore, five operation modes and the power control method of AM-HESS are implemented through the charge and discharge control of the SC. Finally, simulation and experimental results verify that the proposed HLS-PWM can realize an efficient power control of the AM-HESS.

A Load-Independent Wireless Power Transfer System With Multiple Constant Voltage Outputs

A novel wireless power transfer (WPT) system is proposed to power the gate drivers in a multilevel converter to achieve a high insulation level. There are two requirements for the proposed WPT system. First, the WPT system should contain multiple outputs, each corresponding to one gate driver. Second, a constant voltage source is needed for each gate driver. For a multiload WPT system, it is challenging but necessary to realize independent power control of each load. To achieve this goal, the repeater unit consisting of two repeater coils is designed to power a load. Many loads can be powered simultaneously by using multiple repeater units together. The LCC-S compensation topology is adopted for the proposed WPT system. Constant voltage outputs are obtained for all the loads when neglecting the coil's parasitic resistance. An experimental platform with six loads was developed to validate the proposed WPT system.