Nicholas Christofides

A simulation tool to assess the lightning induced over-voltages on dc cables of photovoltaic installations

The development of large scale photovoltaic (PV) plants in rural areas is constantly increasing. This paper describes appropriately scaled laboratory tests and geometrically accurate (real scale) simulation models in an attempt to assess the induced over-voltages on long dc cabling loops.

Lightning protection practice for large-extended photovoltaic installations

This paper aims to analyse the lightning protection system (LPS) of an isolated large and extended Photovoltaic (PV) installation park. The area where the PV plant operates is characterised by the high ground flash density (25 thunderstorm days per year) and the extremely high soil resistivity value (i.e. pure rock with a resistivity of more than 2000m). The paper includes the LPS system design after experimental testing results, which were performed in the laboratory. It also includes solutions to some difficult overcoming problems that were faced during the application of the lightning protection design.

Performance enhancement of MAF based PLL with phase error compensation in the pre-filtering stage

The large scale integration of Renewable Energy Sources (RES) requires sophisticated control techniques for efficient power transfer under faults and/or off-nominal grid conditions. A RES is efficiently integrated to the grid via proper control of the Grid Side Converter (GSC) by accurately estimating the grid voltage phase angle. Moving Average Filter (MAF) based Phase Lock Loop (PLL) techniques provide reduced complexity, however, they present disadvantages under specific grid fault conditions. The most recent MAF based technique is the EPMAFPLL, which provides improved dynamic response and reduces the phase error under off-nominal grid frequencies. However, the EPMAFPLL presents high phase and frequency overshoot at the time of fault. Furthermore, inaccurate harmonic mitigation under off-nominal grid frequencies was not investigated in EPMAFPLL. A modified EPMAFPLL (EPMAFPLL Type 2) is proposed in this paper. The modified EPMAFPLL accurately compensates the offset errors under off-nominal grid frequencies, offers lower frequency overshoot and faster dynamics under faults. In addition, it provides accurate compensation of grid voltage harmonics under off-nominal grid frequencies.

Generalized method for harmonic elimination in two and three level voltage sourced converters

Two and three level VSC inverters are the building blocks of most of inverter applications necessitating the employment of appropriate switching techniques that result in harmonic free sinusoidal output. The paper presents the generalized method for harmonic elimination in two and three level voltage sourced converter. The technique used for suppression of selective low-ordered harmonics form the inverters output voltage waveform is Selective harmonic elimination (SHE). The inverters output voltage waveform is mathematically modeled using Fourier series that results in the system of nonlinear equations. The non-linear equations are solved for eliminating the specific low order harmonics. The main objective in calculating appropriate switching angle is to minimize the value of Total Harmonic Distortion (THD). To solve the system of non-linear equations numerical method Newton Raphson is applied and appropriate switching angles are calculated. The remaining uneliminated higher order harmonics are eliminated by connecting appropriate value of filter at the output stage of VSC inverter. The model of two level VSC is simulated in SimPowerSystem Matlab tool bar together with the algorithm of Newton Raphson technique. The time domain plot and frequency spectrum of inverters output waveform is presented in the results section. The results of two and three level VSC inverters are analyzed and debated together with the comparative analysis on the basis of THD. Moreover, a comparison is presented on the basis of IEEE standard Std. 519 and EN 50160.

Diversifying the role of distributed generation grid side converters for improving the power quality of distribution networks using advanced control techniques

Diversifying the role of grid side converters (GSC) associated with distributed generation (DG) may have a technical and economic impact. The increasing number of residential photovoltaic rooftop systems can be exploited by distribution network (DN) operators in order to support and improve the power quality of the grid without considerable further investment. GSC can support the grid during abnormal conditions but may also be used to improve the power quality by balancing out asymmetries and alleviating the grid from unwanted harmonics. The proposed advanced control technique and associated sophisticated controllers demonstrate through simulation and experimental results that the role of GSC can be upgraded to improve the power quality of the grid. Furthermore, the proposed controller and advanced control scheme is validated on a realistic low-voltage DN using data from Cyprus Electricity Authority. Consequently, conventional practices by DN operators can be reassessed as the necessary electronic hardware for mitigating grid problems are already present within the GSCs of DG systems.

Performance enhanced RES current controller with reduced computational complexity

The increased penetration of RES requires the development of advanced control techniques that can work accurately with improved performance and less computational resources under unbalanced and harmonically distorted grid conditions. The paper sets out with a benchmarking study of two existing current control techniques developed for compensating the harmonics and gird voltage unbalance. The techniques are analyzed in terms of their performance and computational complexity. Both techniques can work satisfactory under abnormal grid scenarios, but these techniques are computationally complex and the performance of one under unbalanced conditions is not accurate. The proposed controller (Type 1) presented, eliminates oscillations under unbalanced voltage conditions. Furthermore, it enhances the quality of the injected current and works efficiently with improved performance (low overshoot and less oscillation) at the time of fault. However, it slightly increases the computational complexity something that motivated the development of a further controller (Type 2) that can work with less computational resources and improved performance under such conditions.

Harmonic analysis of a 10.8 MW installed capacity wind farm in cyprus

This work presents current and voltage waveform data recorded from Ayia Anna wind farm in Cyprus and the subsequent analysis in Matlab using Fast Fourier Transform (FFT) algorithms. The analysis aims to investigate whether the harmonic content, is within acceptable IEEE standard limits. Recorded data were arranged in descending order with respect to output power generation. Total harmonic distortion (THD) for voltage and current were analyzed by classifying the measured data in three categories depending on the output power so that the harmonic distortion is correlated with the output power generated.

Performance analysis of Fuzzy Logic Control and six-pulse line-commutated converter in DC motor drive application

This paper presents the performance analysis of six-pulse line commutated converter (LCC) and advanced control strategy Fuzzy Logic Control (FLC) for speed control of a dc motor. The speed of motor is maintained at the desired reference value by adjusting the value of armature current. The armature current is varied by controlling the firing angle of LCC. The desired value of reference current is obtained from the corresponding speed value and is fed to the controller. The controller takes the measured and reference value of armature current as an input. The output of the controller is firing angle. To maintain the motor speed at the desired value two controllers the FLC and the conventional Proportional Integral Derivative (PID) controllers are employed and their performance is compared. The proposed controllers and power system is implemented in Control System and SimPowerSystem toolbox of MATLAB. Results of Computer simulation are described and debated together with a comparative analysis of the different control schemes. The simulation results show that the proposed FLC has tracking ability with better steady state error and transient response than conventional PID controller. Moreover, the deleterious effects of converter on input current and power factor are also discussed.

Estimation and control of non-linear plant in ARX form using MDPP controlled Fuzzy Identifier

The paper presents the estimation of an unknown non-linear system in ARX form. The estimated model is used in Adaptive self-tuning regulator to regulate the output of plant at desired reference input using Minimum Degree Pole Placement Controller (MDPP). Most of the literature includes the conventional RLS algorithm to estimate the system parameters. The estimation technique addressed here is Fuzzy Logic Identifier (FLI). The proposed estimator approximates the system using a linear model at each operating point. The linear identification of model necessitates the use of linear control strategy. Therefore, the design of MDPP controller based on linear identified system is also presented. The consequents of FLI are updated using Levenberg-Marqardt (LM) algorithm. MATLAB/Simulink is used for the implementation of proposed estimation techniques. The effectiveness of the proposed estimators and control strategy is tested on a non-linear plant. The results are presented and debated together with a comparative analysis of RLS and FLI. Superiority in performance is found for FLI in estimating and controlling the non-linear using MDPP. The performance indices Mean Square Error (MSE), Mean Absolute Error (MAE) and Mean Time-Weighted Error validate the effectiveness of proposed FLI.