Jeevan Adhikari

Overview of high altitude wind energy harvesting system

High Altitude Wind Power (HAWP) can supply clean energy at low cost and high capacity factor than Conventional Wind Power (CWP) system. The concept of harvesting high altitude wind power using air-borne wind turbine-cum electric generator supported by light gas filled blimp/aerostat has been proposed in the paper. An air-borne wind turbine at high altitude extracts kinetic energy from high speed streamlined wind using buoyancy provided by the blimp. Using a suitable power electronic converter (PEC), harvested electrical power is transmitted to the ground by using a tether. Blimp is tethered to the ground and provides mechanical strength to hold the blimp and the same tether consisting of electrical conductors is used for transmitting the generated electrical power. The paper outlines major components used for harvesting high altitude wind power. Transmission of the electrical power at medium voltage DC reduces the transmission loss and volume of the conducting cable. The optimal transmission voltage level that gives minimum weight of the tether has been calculated for a given power level. The proposed HAWP system requires high power density PEC, which converts low voltage AC to medium voltage DC in an airborne unit and a ground based PEC that transforms medium voltage DC to distribution level grid voltage. An air-borne PEC converter consists of a rectifier and an isolated DC-DC converter that supports the unidirectional power flow. Further, the paper also proposes the ground based grid-side PEC for distributed grid interface. In addition, comparative study between conventional wind energy harvesting system and high altitude wind energy harvesting system shows that high altitude wind power is better in terms of capacity factor, Cost of Electricity (COE), ease of construction, and power density than conventional wind power generating system.

Harnessing high altitude wind power using light gas filled blimp

This paper presents a simple concept of harvesting high altitude wind power using air-borne wind turbine-cum-electric generators supported by light gas filled blimp/aerostat. Air-borne wind turbine at high altitude extracts kinetic energy from high speed streamlined wind using buoyancy provided by the blimp. Using a suitable power electronic converter (PEC), extracted electrical power is sent to the ground using a tether. Blimp is tethered to the ground and the same tether is used for electricity transmission as well. This paper outlines major components used for harvesting high altitude wind power. Transmission of power at medium voltage DC reduces the transmission loss and volume of the conducting cable. The optimal transmission voltage level is determined that gives minimum weight of the tether for a given power level and proposes a simple and high power density power electronic converter, which converts low voltage AC to medium voltage DC in an air-borne unit. Simulation results obtained prove efficient power conversion using the proposed power electronic converter. In addition, comparative study between conventional wind energy harvesting system and high altitude wind energy harvesting system shows high altitude wind power is better in terms of capacity factor, cost of electricity (COE), ease of construction and power density than conventional wind power generating system.

Generation and Transmission of Electrical Energy in High-Altitude Wind Power Generating System

This paper presents a high-altitude wind power generating system supported by a light gas filled blimp/aerostat that extracts electrical energy from high-altitude streamlined wind. The optimal generation and transmission mechanisms that give suitable power-to-weight (P/W) ratio and efficiency of the overall system are investigated. The variations in weight and total losses of the permanent magnet synchronous generator with changes in generation voltage and pole-pair number (frequency) have been analyzed. The design of the tether that transmits electrical power to the ground-based station is also presented. AC and dc transmission mechanisms using aluminum/copper-based conductors are studied and compared to find optimal weight of the tether. It is found that aluminum conductor gives better P/W ratio than using copper conductor. From the detailed analysis of generation and transmission mechanisms, it is concluded that the optimal electrical power architecture is medium voltage (MV) ac generation as well as transmission. It exhibits better P/W ratio and efficiency in comparison with low-voltage ac generation and MV dc transmission. The selected optimal electrical architecture simplifies electric system by transferring the power electronic converter from the airborne unit to the ground-based station and thereby improves the overall P/W ratio by a factor of 7% approximately.

Modular interleaved ZVS current fed isolated DC-DC converter for harvesting high altitude wind power

Three phase low voltage power is generated using an air-borne wind turbine and electric generator in a high altitude wind power (HAWP) generation system supported by light gas filled blimp/aerostat. Generated power is transmitted at an optimal medium voltage DC to reduce the weight of an electro-mechanical tether and to increase the transmission efficiency. A 100 kW isolated DC to DC converter is proposed which converts DC link voltage to an optimal transmission voltage. Modified interleaved current fed converter is designed which gives zero voltage switching in the primary side for a wide range of load. Converter is interleaved to reduce the device rating, conduction loss and the size of passive components used in the converter. Two modules of insulated gate bipolar transistor (IGBT) H-bridge are connected in parallel in primary side and four modules of full bridge diode rectifier are connected in series in secondary side. Two extra IGBT switches for zero voltage switching (ZVS) are used to design clamp circuits for the proposed converter. This modular concept simplifies the design procedures of converter at high power level and improves the stray losses associated with in the converter. The steady state analysis of the converter is carried out and verified with the simulation results and presented in this paper. Device ratings and passive components size are determined using the steady state analysis for 100kW HAWP generation system.

Reduction of Input Current Harmonic Distortions and Balancing of Output Voltages of the Vienna Rectifier Under Supply Voltage Disturbances

This paper proposes a modified proportional resonant (PR) based control scheme for a three-level Vienna rectifier that reduces the input current harmonic distortion [total harmonic distortion (THD)] for harmonic contaminated and unbalanced supply voltage conditions. Low-order current harmonics that are present in the source side currents can be selectively eliminated using this proposed control strategy. In addition, the complete control technique includes the proportional integral (PI) based voltage balancing technique to balance the output voltages of the Vienna rectifier. This method includes the advantages of both hysteresis-based current control (fast transient response) and PI-based vector control (fixed frequency operation) methods. Various computational blocks such as space vector modulation (SVM) block, current/voltage transformation blocks, etc., are not necessary to implement this control method. Therefore, this scheme is relatively easy to implement using inexpensive digital controllers. A 1-kW prototype of the rectifier is built and tested in the laboratory environment using the proposed control method. The converter provides a unity power factor operation with low-current THD at the input side, balances the capacitor voltages, and tracks the voltage reference at the output side.

Maximum power-point tracking of high altitude wind power generating system using optimal vector control technique

In a high altitude wind power (HAWP) generating system, the generation of wind power is carried out at high altitude above the earth surface and control of power generation is accomplished at the ground based station. Three phase medium-voltage permanent magnet synchronous generator (PMSG) is used as the source of power generation in the air-borne unit. The ground based power electronic converters (PECs) are used for maximum power-point tracking (MPPT) of the air-borne wind turbine (AWT). Optimal torque/vector control of the PMSG is best suited for MPPT of HAWP application. The proposed method does not use mechanical sensor in the air-borne system to measure the rotor position. The phase-lock-loop (PLL) is employed to estimate the rotor position, which is used for calculation of MPPT torque/current reference. The AWT is controlled using ground based power conversion system comprising of three-level neutral point clamped (NPC) rectifier for generation side MPPT control. The proposed MPPT control algorithm of the PMSG is validated using MATLAB/Simulink. A 1 kW PMSG generation system prototype is designed in the laboratory and used for experimental validation of the proposed MPPT method.

Ground-based step-down AC-AC power electronic converter for high altitude wind energy harvesting system

High altitude wind power (HAWP) generating system poses several benefits over conventional wind power (CWP) generating system. An air-borne electric generator is held at high altitude above the ground surface with buoyancy provided by light gas filled blimp/aerostat. In order to minimize the size of the blimp and to reduce the number of electric components in the air-borne system, generation of electrical power is carried out at three phase medium voltage AC (MV-AC) and transmitted to the ground station (without any power conversion) using electromechanical tethers. Thus, transmitted power is interfaced into the distribution level grid at 415 V and 50 Hz. This paper evaluates possible power electronic converter (PEC) topologies that can be used to convert variable voltage and variable frequency three phase medium voltage AC power into constant frequency distribution level grid voltage. Three different PEC topologies are proposed that allow generation and grid side current control and generation side maximum power point tracking (MPPT) control. In addition, the proposed PECs provide step down of voltage and electrical isolation with the distribution grid. Suitable active and passive components are selected for 100 kW HAWP system and overall semiconductor losses are evaluated. The converters are simulated using computer software programs PSIM-9 and MATLAB. The converter that exhibits good efficiency, easy control of generation and grid side current as well as facilitates MPPT control of HAWP generation side is selected for grid interface of isolated HAWP system.

Power conversion system for low power high altitude wind power generating system

In high altitude wind power (HAWP) generating system, medium voltage AC (MV-AC) permanent magnet synchronous generator (PMSG) is used. The generated electrical power is transmitted to ground without any power conditioning in the air-borne unit. The ground based power conversion system (PCS) interfaces variable voltage and variable frequency medium voltage power into distribution level grid voltage. The proposed PCS consists of a three-level vienna rectifier for generation side control, a half bridge DC-DC converter for isolation and step-down purpose and a grid/load connected inverter for load side active power control. Three-level operation in the generation side converter reduces the switch/diode voltage stress to half and therefore allows to use low voltage rating power semiconductor devices. The vienna rectifier is controlled for sensorless maximum power-point tracking (MPPT) of the air-borne wind turbine. The phase lock loop (PLL) is used for speed and rotor position detection of the PMSG for optimal torque control of the airborne wind turbine (AWT). Simulation studies have been carried out using computer programs like PSIM and MATLAB. For the validation of the proposed methodology, scaled down laboratory based prototype is built and tested. The obtained experimental results confirm the performance of the PCS for interfacing HAWP generation system to the grid.

Modelling, design and control of grid connected converter for high altitude wind power application

High altitude wind based renewable energy generating system can be connected to a distribution level grid. The generated power at high altitude above the ground is transmitted at medium voltage DC to the ground based station. Thus, transmitted power is interfaced with the distribution grid at the ground station. This paper presents the power electronic converter (PEC) rated at 100 kW HAWP application that converts medium voltage DC to three phase distribution level grid voltage. The proposed converter topology consists of a neutral point clamped (NPC) three level DC-DC converter followed by three phase grid connected two level inverter. The designed power electronic converter uses four high voltage (HV) rating power semiconductor switches for buck converter before inversion to three phase AC distribution voltages. The active and passive components selection for two stage conversion is presented in the paper. The grid side current is controlled using quadrature axes current control method and inverter switches are switched using space vector modulation (SVM) technique. Simulations of the proposed PEC and control of the inverter are carried out using software programs PSIM-9 and MATLAB. The designed converter converts the 8 kV DC transmission voltage to 415 V grid side voltage with current total harmonic distortion (THD) of about 1.2%.

Angle compensation based rotor position estimation for sensorless vector control of the permanent magnet synchronous motor

This paper proposes a new sensorless method for estimating the mechanical speed and rotor position of the Permanent Magnet Synchronous Motor (PMSM). A voltage sensor is employed to measure the terminal voltage of the inverter. The phasor of the measured terminal voltage (phase-A) of the inverter has the same angular frequency as that of the back electro-motive force (emf) of the PMSM. Therefore, the angular frequency of the measured terminal voltage is used for computing the rotational speed of the PMSM. A simplified dynamic angle compensation term is derived that calculates the phase/angle shift between the terminal voltage phasor and the back emf phasor. The calculated phase/angle shift (angle compensation) in terms of time is then used to time-shift the terminal voltage phasor to obtain the exact rotor position of the PMSM. This proposed method does not require any complex estimation/observer based algorithm. The estimated rotor position and mechanical speed are employed for the vector control of the PMSM. A 1 kW laboratory prototype is developed and tested to assess the effectiveness of the proposed method. The proposed rotor position estimation approach is capable of estimating the rotor position with less than 1% error and consequently, tracks the reference speed with less than 0.1% steady-state error.