S.G. Abeyratne

Current-clamped, modified series resonant DC-link power converter for a general purpose induction motor drive

A new AC/AC power converter topology, in which all the switches operate in a resonant fashion to reduce switching losses, is proposed. The topology enables conduction-period control of individual current pulses, whereby pulse-width modulation (PWM) could be achieved to a fair degree of accuracy with the associated controller. The scheme implements current peak (resonant) limiting by a simple diode clamp. Improved switch utilization (voltage /spl times/ current) and reduced part-count could be cited as the merits of the circuit over the previous soft-switched current-sourced AC/AC configurations. It is experimentally verified that the output PWM controller could be used to implement constant V/F operation, and the results are presented. In-depth design criteria for the topology that gives optimized voltage stresses are presented. A charge-based, line current feed-forward, mode-controller is introduced at the input and digitally verified. Feasibility of the simultaneous control over both input power-factor and smooth input-output line currents are studied and the digital verification is presented.

Fault Ride Through operation of a DFIG wind farm connected through VSC HVDC

The electromechanical transients during a deloading of a DFIG turbine and the Fault Ride Through (FRT) capability of a DFIG wind farm connected through HVDC transmission lines are discussed. The electromechanical oscillations during a deloading operation of a DFIG wind turbine generator are simulated using BLADED software. Then power reduction control during a fault was achieved by reducing the power from the wind farm as a whole and by deloading the individual wind generator. A new power blocking technique applied at the offshore converter station was used to reduce the wind farm power output. Simultaneous control of the wind farm and wind turbine power outputs enabled a smooth power reduction during the fault.

Grid side converter controller of DFIG for wind power generation

With the advancement of technology, electricity generation using wind has drawn an increased attention in the world. As the amount of wind power integrated into the system is increased, it causes power quality and stability problems. Therefore power utilities are developing stringent grid codes which are to be satisfied by the wind power producers. Voltage control capability, reactive range capability, frequency control ability and fault ride through capability are among the major requirements stipulated in the grid codes. Due to the variable speed operation capability of Doubly Fed Induction Generators (DFIGs), they are becoming popular in wind power generation. The voltage control of the DFIG wind farm has been identified as the latest challenge with the present grid code requirements. This paper presents the design methodology of a stator side controller of the DFIG, which produces the terminal voltage control in addition to the DC link voltage regulation. The developed controller is simulated using PSCAD/EMTDC software to verify the performance of the controller. Further, a 60 MW typical wind farm is modeled in both PSCAD/EMTDC and IPSA+ and applicability of the above controller is justified.

Extraction of parameters for simulating photovoltaic panels

Presented in this paper is a method of simulating photovoltaic modules by taking the manufacturer specified data at standard test conditions as inputs. A method for estimating unknown parameters of a simplified version of the single diode solar cell model corresponding to a given photovoltaic panel is described. In the proposed approach, the equivalent circuit represents an entire solar panel, composed of several solar cells. A new method, which is simple, is also proposed to identify the ideality factor of the equivalent diode along with the series resistance. Estimated unknowns are used in generating module output curves at different environmental conditions. The generated curves are compared with manufacturers data. The curves are also compared with curves generated with an alternative method available in the literature. The superiority of the proposed technique is demonstrated.

Comparative analysis of speed of convergence of MPPT techniques

In order to extract maximum power available from a solar cell in a photovoltaic system, maximum power point tracking should be activated. Several maximum power point tracking (MPPT) techniques have been developed over the past years. In an environment where, varying irradiance levels prevail, time taken to track the maximum power point is important. In this paper, three widely used MPPT tracking techniques are compared with regard to their convergence speed at irradiance disturbances. The results of the simulation study are validated on a laboratory prototype and findings are presented. It has been shown that the incremental conductance method is superior at irradiance transients.

Power Quality And Harmonic Loads

This paper presents the results of a research done on the harmonic distortion of load current and voltage in low voltage distribution networks owing to the extensive use of peak-detecting type capacitor-filter rectifier systems (often the front-end of non linear electronic loads). A classical case to be studied is the compact fluorescent lamps (CFLs) without power factor correction being implemented. It has become evident that the use of CFLs alone is more or less accountable to the non-linear loads generated in rural electrification schemes in Sri Lanka. A case study was also done by the authors on a chosen low voltage distribution network to analyze the impact of CFL loads and the capabilities of the network with regard to IEEE 519 regulations. The methods of analysis and the results are presented in this paper and PSCAD software has been used to simulate the system. The commercial aspects of using CFLs and energy saving have been discussed. Harmonic effects on the distribution transformers are studied as applied to the line segment considered in the study

PWM series resonant DC-link converter with current clamping by the use of saturable core

A series resonant DC link power converter with pulsewidth modulation (PWM) capability and current peak limiting is proposed. When desired, a resonant transition creates notches in the DC link current, allowing the switches of the power converter to synchronize with external PWM strategy. For that, it requires only an additional bidirectional switch. A saturable core (SC) achieves current peak limiting of which the clamping level is automatically adjusted by the current load. Analysis and design procedure are presented for the proposed topology, which does not use any dedicated devices for startup. Simulated and experimental results corroborate the theoretical studies.

Current clamped, modified series resonant DC-link power converter for a general purpose induction motor drive

A modified power converter topology, derived from the series resonant DC-link converter is proposed in which all switches operate in a resonant fashion to reduce switching losses. The scheme enables PWM control over individual current pulses whereby true PWM over a two pulse period can be achieved to a fair degree of accuracy. The scheme also implements current peak limiting by a simple diode clamp. Improved switch utilization factor, reduced cost burden on filter capacitors and better control over current pulses in the discrete pulse domain could be cited as the main advantages of the circuit over previous configurations. A control algorithm has been developed to achieve simultaneous control over both input power factor and smooth input-output line currents. The auxiliary switch performs PWM on each pulse by devising a simple dynamic solution for the switching instants. Digital simulation results for the developed method are presented. Also, open-loop PWM control at the output has been implemented on a three-phase laboratory prototype and experimental results are presented.<<ETX>>

Statechart based modeling and controller implementation of complex reactive systems

Statechart formalism has been a preferred choice for modeling complex reactive systems (CRS) in recent years. It has inbuilt powerful features of orthogonality, hierarchy, intermodular communication and history. Once statechart based system modeling is done the next issues to be addressed are (1) modular verification of the system for failsafe operation under all possible working conditions (2) progressive controller implementation together with the supervisory control while maintaining traceability and re-configurability and (3) facilitation of controller adaptation for progressive incorporation of security features and supervisory specifications. An elevator system was designed and built to reflect exigencies of a typical CRS hardware/software platform. A controller was designed to meet the above requirements and tested on the platform to validate the feasibility of model-based control design/verification methodology for real scale systems. Modularity was achieved by developing the statechart model of the plant into a tree of communicating language generators. Progresively verified modules were then translated into sequential function charts (SFC) which were finally integrated to form a complete flat SFC. The SFC was then implemented on a PLC platform (Telemechanique). The program was first validated in simulation using Telemechanique “Twidosuite” for different operating conditions and finally tested on the elevator system.

Operational restrictions with maximum power extraction of DFIG connected wind farms

Doubly Fed Induction Generator (DFIG) based wind farms are one of the major contributors to wind power generation. The capability of generating active power over an extended range of speed has made it attractive to maximize the power extraction from wind. This has also facilitated reduction of mechanical stress in the turbine. However recently it has been reported that achieving the maximum power extraction at high wind speeds is a major issue for large wind farms while satisfying grid code requirements. This paper discusses the limitations of the DFIG on the maximum power exaction control and a study is presented to improve its performance. The DFIG model is briefly discussed. Difficulty on achieving reactive power control at higher rotor speeds is explained in detail with variation of stability margin along the maximum power extraction curve. A controller is proposed to validate the steady state results. The effects of DFIG machine parameters on the stability and performance also have been studied.