Condition monitoring of metal-oxide surge arresters using leakage current signature analysis

Abstract: Online condition assessment of surge arresters is necessary for early detection of problems and subsequent replacement of the device to avoid unplanned downtime and/or damage to the equipment being protected. The most common technique of monitoring and assessing the condition of the arrester is extraction of the resistive component of the leakage current. There are many drawbacks to this method arising from inaccuracies in the model-based approach and deficiencies in the understanding of the leakage current signal. This paper presents leakage current signature analysis (LCSA) as a method of assessing the condition of the surge arrester where the harmonics components of the current signal, extracted using Bayesian spectrum estimation, are used as indicators of degradation. An experimental methodology comprising measurement of the leakage current of arresters undergoing constant-stress accelerated degradation testing is employed. Results indicate that a consistent increase in the DC component of the leakage current signature in the case of degrading samples is the most likely indicator of condition

On-line monitoring of metal-oxide surge arresters using improved equivalent model with evolutionary optimisation algorithm

Abstract: A resistive-current extraction algorithm to assist with on-line monitoring of surge arresters is proposed in this paper. The algorithm is based on the improved equivalent model for surge arresters and combines evolutionary optimisation with the base-frequency approximating method. A genetic algorithm is used to obtain the optimal capacitance such that the phase shift between the base-components of the branch voltage and current is minimised thereby yielding a good approximation of the resistive current. In this paper, the algorithm is implemented and tested using Matlab/Simulink. Results indicate that the proposed algorithm is able to efficiently and accurately obtain the resistive component of the leakage current, using the improved equivalent model, under both ideal and distorted supply conditions

Investigation of lightning surge effects on a grid-connected PV plant

Abstract: This paper investigates the indirect effects of lightning on a medium-scale photovoltaic plant. The purpose of the investigation is to analyse the surge effects on the photovoltaic plant and interconnected systems in order to develop an understanding of the associated risk of damage to the plant and ascertain the requirements for lightning protection. The investigated plant consists of four photovoltaic arrays, each comprising 2500 modules connected in series-parallel configurations, with inverters and connection to the high voltage transmission grid via a system of parallel buses, insulated power cables and power transformers. The presented study makes use of the electro-magnetic transient program-restructured version (EMTP-RV) software platform to investigate induced effects of lightning on grid-connected PV systems and provide system designers with a means of analysing and mitigating lightning risk. Results obtained show significant risk associated to voltage rise of 7.4% on the DC side and 1.2 p.u on the AC side

Thermographical analysis of turbo-generator rotor

Abstract: Refurbished or newly constructed utility-scale turbo-generator rotors requires stringent acceptance testing before commissioning and subsequent operation thereof. Conventional methods of testing are inadequate in detecting and locating thermally induced problems. This paper presents a thermographic method for carrying out thermal instability testing of generator rotors. An experimental setup is used to map the thermal distribution of the generator rotor. Implementation and testing of the method is carried out in a laboratory setting using a down-scaled turbo-generator rotor

Evaluation of rotor design parameters for minimising torque ripple on a synchronous reluctance machine using multifactor ANOVA

Abstract: This paper presents a method for evaluating, both qualitatively and quantitatively, the effects of specific rotor design parameters on the performance of a synchronous reluctance machine. The method uses multi-factor experimental design, with Analysis of Variance (ANOVA), and Finite Element Analysis (FEA) to determine the optimal rotor design parameter according to a specific objective. Using this method, two factors - rotor flux barrier pitch angle and barrier width - are selected at simultaneously varied levels for assessment with the aim of minimising the response variable, which is, the torque ripple. Results from the investigation show that the influence of the rotor flux barrier pitch angle on the torque ripple is more statistically significant than the influence of the barrier width. However, the effect of the barrier width on the actual torque is more significant

Thermal instability analysis of a synchronous generator rotor using direct mapping

Abstract: Abstract: This paper presents a direct and practical method for mapping the thermal behaviour of a synchronous generator. Since temperature variations can lead to rotor thermal instability which adversely affects the operation of the generating unit, a better understanding of this phenomenon is required. The two main methods of performing thermal instability testing - direct current injection and friction/windage - are found to be practiced internationally without preference. Infrared thermography is used here as a means of determining the thermal performance of the rotor under different testing scenarios. The experimental testing is conducted using a scaled setup of a balancing facility and a 600 MW generator rotor. The results obtained are presented in the form of surface temperature maps. The thermal distribution of the two different methods were found to differ substantially with the friction method exhibiting a uniform surface distribution while the current method exhibited areas of higher temperature concentration around the rotor pole faces

Investigation of thermal instability testing on synchronous generator rotors using an experimental direct mapping method

Abstract: Utilities employ Thermal instability testing (TIT) for final acceptance testing after the construction and refurbishment of turbogenerator rotors. This type of testing is performed through two methodologies namely current injection and friction/windage in order to assess the thermal sensitivity of the machine’s rotor. Although there are distinct differences between the two methods, no apparent preference is shown by service providers/OEMS globally. There is also no definitive evidence or standards that offer a comparison of the two methods and suitability assessment thereof. The presented research investigates these two methods of TIT for a synchronous generator rotor. An experimental setup with infrared thermography is employed to investigate the thermal behaviour of the machine’s rotor for each of the test methods. Experimental results show that the thermal behavior of the generator rotor is significantly different for each methodology. It is also shown in this paper that contemporary TIT practice requires an augmented test methodology

Analysis of lightning surge effects on small-scale rooftop photovoltaic systems

Abstract: Small-scale rooftop PV systems have become an attractive investment for small businesses and home owners. PV systems are inherently exposed to lightning phenomena and hence protection of the electrical system is required. In this paper, a simulation approach using MATLAB and Simulink is adopted to analyse the impacts of lightning induced effects on small-scale, rooftop, grid-connected PV systems. It is found that based on different coupling points, lightning surges could cause damage to the PV array, inverter as well as the connected load. The installation of surge protective devices could mitigate the potential damaging effects. In analysing the surge effects on the PV system, an understanding of the associated risk of damage to the PV system can be developed and hence the requirements for lightning protection of small-scale rooftop grid-connected PV systems can be comprehended

Analysis of thermal instability test methodologies for synchronous generator rotors

Abstract: Thermal instability testing (TIT) is utilised by service providers as a final proving test during the construction, repair and overhaul of large turbo-generator rotors. This test is typically performed using two methodologies – i.e. current injection and friction/windage methods – to evaluate the thermal sensitivity of the generator rotor. Although these methods are distinctly different – service providers/OEMS worldwide show no preference towards a methodology and there is no substantiating evidence or international standards which provide insight into which method is most suitable. This paper investigates these two methods of TIT for synchronous generator rotors. A specialised experimental configuration utilising infrared thermography is used analyse the thermal behaviour a synchronous generator rotor under different test conditions. Experimental results indicate that there are substantial differences in the behaviour of the rotor under the two different methodologies and that an augmented test methodology is required to improve TIT

Investigation of very fast-front transient overvoltages for selection and placement of surge arresters

Abstract: An analysis of very fast front transient overvoltages, generated as a result of induced effects of lightning, across a photovoltaic plant is investigated. The EMTP-RV version 3.3 computer software platform is used to simulate surge voltage magnification. Selection and placement of surge arresters for effective protection of power transformers are conducted. Results show that selected arresters placed on bus 4 and coordinated with those placed on bus 5 and 3 ensure surge amplitude reduction of 33% on bus 4