A. Arulampalam

Dynamic voltage restorer based on voltage-space-vector PWM control

A dynamic voltage restorer (DVR) based on the voltage-space-vector pulsewidth-modulation algorithm is presented. Phase-jump compensation is achieved using a software phase-locked loop and a lead-acid battery energy store. A battery-charging control technique using the DVR itself is also described. To validate the control of the DVR, a three-phase prototype with a power rating of 10 kVA has been successfully developed. Simulation and experimental results are shown to validate the control methods.

Four-wire dynamic voltage restorer based on a three-dimensional voltage space vector PWM algorithm

A modified voltage space vector pulse-width modulated (PWM) algorithm for a four-wire dynamic voltage restorer (DVR) is described. The switching strategy based on a three-dimensional (3-D) /spl alpha//spl beta/O voltage space is applicable to the control of three-phase four-wire inverter systems such as the split-capacitor PWM inverter and the four-leg PWM inverter. In contrast to the conventional voltage space vector PWM method, it controls positive, negative and zero sequence components of the terminal voltages instantaneously. Three 3-D modulation schemes are analyzed with respect to total harmonic distortion (THD), weighted total harmonic distortion (WTHD), neutral line ripple and switching loss over the whole range of the modulation index when the DVR experiences both balanced and unbalanced sags with phase angle jumps. Experimental results from a 9 kW DVR system using a split-capacitor PWM inverter are presented to validate the simulation results.

Dynamic voltage restorer based on voltage space vector PWM control

A dynamic voltage restorer (DVR) based on the voltage space vector PWM (VSVPWM) algorithm is presented. Phase jump compensation is achieved using a software phase-locked loop (SPLL). A battery charging control technique using the DVR itself is also described. To validate the control of DVR, a three-phase prototype with a potential power rating of 10 kVA has been successfully developed. Simulation and experimental results are shown to validate the control methods.

Mitigation of saturation in dynamic voltage restorer connection transformers

During the transient period at the start of a voltage sag, a DVR injection transformer can experience a flux-linkage that is up to twice its nominal steady-state value. In order to prevent the transformers from saturating it is normal to choose a rating flux that is double that of the steady-state limit. An alternative method is to limit the flux-linkage during the transient switch-on period, thus preventing saturation. It is shown through both simulation and experimental results that an adaptive form factor can be applied to the DVR injected voltage, which minimizes the disturbance seen by a sensitive load, while at the same time preventing saturation. The proposed method removes the need for rating the series injection transformers for the DVR transient switch-on period, and therefore removes the redundancy normally associated with their steady state operation. In economic terms, this may reduce the total cost of a DVR system, thus making it a more attractive solution for voltage sag mitigation.

Software phase-locked loop applied to dynamic voltage restorer (DVR)

In this paper, the analytical and practical design issues of a software phase-locked loop (SPLL) for DVR are presented. A SPLL model that uses a lag/lead loop controller, is derived in order to analyse the system performance and filtering characteristic by the use of bode diagrams and root-locus methods. In DVR applications, parameters of the design of the SPLL controller are not only dependent on the steady state and dynamic state, but also on practical conditions such as utility unbalance, voltage sag/swell magnitude, voltage harmonics, phase jumps and frequency variations. Therefore, the practical aspect of the SPLL implementation has also been discussed. Experimental results demonstrate its phase tracking capability.

Control of power electronic interfaces in distributed generation microgrids

Technological advances and environmental pressures are driving the interconnection of renewable energy sources to the distribution network. The interconnection of large amounts of non-traditional generation however causes problems in a network designed for ‘conventional’ operation. The use of power electronics interfaces and the ‘bundling’ of micro-generation and loads into so-called Microgrids, offers a potential solution. Each Microgrid is designed to operate as a ‘good citizen’ or near ideal conventional load. This paper discusses the various elements of the new Microgrid concept and presents suggestions for some typical control strategies for the various system elements.

Application of a Static Reactive Power Compensator (STATCOM) and a Dynamic Braking Resistor (DBR) for the Stability Enhancement of a Large Wind Farm

A control strategy to improve the stability of a large wind farm using a Static Reactive Power Compensator (STATCOM) and Dynamic Braking Resistor (DBR) is proposed and investigated. The STATCOM supplies the reactive power demand of the wind farm dynamically in order to maintain the network voltage. The DBR is controlled by Liapunovs stability criterion to absorb the active power of the wind farm during the network fault. The performance of the STATCOM and DBR, applied to a large wind farm (60MW), is studied in PSCAD/EMTDC. The simulation results show that effective control of the STATCOM and DBR together can enhance the stability of large wind farms.

Trends in wind power technology and grid code requirements

Developments in wind turbine technology are facilitating the increase of power generation capacity from renewable energy sources. However an electrical utility grid is generally unable to accept a large amount of wind power without imposing strict conditions. Voltage fluctuation, reactive power compensation and fault ride through are the main areas of concern. Variable speed generators, STATCOMs and crowbar circuits are used to comply with grid code requirements. This paper discusses a number of wind turbine topologies and grid code requirements for large scale integration of wind power.

Simulated Onshore-Fault Ride through of Offshore Wind Farms Connected through VSC HVDC:

Effective Onshore-Fault Ride Through was demonstrated by simulation for a Fixed Speed Induction Generator (FSIG) offshore wind farm connected through a Voltage Source Converter HVDC link. When a terrestrial grid fault occurs, power through the onshore converter reduces and the DC link voltage increases. A control system was then used to block the offshore converter. The offshore AC network voltage was reduced to achieve rapid power rejection. Reactive power at the onshore converter was controlled to support the AC network voltage according to the GB Grid Code requirements. Two cases, a 200 ms terrestrial fault and a 50% retained voltage fault of duration 710 ms, at the grid connection point were studied. The simulation results show that power blocking at the offshore converter was effective and the DC link voltage was controlled.

Micro-grid control of PV-Wind-Diesel hybrid system with islanded and grid connected operations

This paper discusses modeling of a micro-grid with PV-Wind-Diesel generator hybrid system and its operations. The PV system is modeled with a DC/AC inverter with a pre-defined Maximum Power Point Tracking (MPPT) control. Fixed speed induction generator based wind turbines are used to model the wind farm. The diesel generator controls are modeled with droop governor and automatic voltage regulator (AVR). The PV inverter controls, including MPPT and voltage droop, are modeled in the DC/AC inverter itself. The proposed MPPT control is more stable with additional input of PV cell temperature. The complete system is modeled in PSCAD/EMTDC software. Performance of this system is studied on (i) fault tripping of the main grid and (ii) islanded operation of micro-grid with intermittent power from the wind and PV plants. The simulation results confirm the smooth operations of the proposed concept for disturbances from main grid as well as due to the intermittent wind and PV renewable power plants.