Current Step Generation and Measurement with Rise-time in the Range of Nanoseconds

A current step generator based on a charged coaxial cable is designed and tested for characterizing impulse current shunts. This thesis has developed a traceable calibration infrastructure for fast shunts and other current sensors, defined measurement techniques for a current step and improved the test procedure and measurement capabilities. For calibration of shunts, current coil sensors are used in the measurement circuits. Since no calibration services are currently available for impulse current measuring systems, a best circuit combination is proposed for current step generation with a rise time of less than 5 ns, along with a proposed reference shunt that aims to provide the best and most stable measurement results with negligible noise, oscillations, and droop in the measured current step. Based on techniques found in the literature, current steps are generated, and different sensors were used to measure the generated steep front current steps. The generation system consists of a 110-m long, 50-Ω coaxial cable and a spark gap. Various spark gap switches, including the SF6 spark gap, are used for generating current steps. With the coaxial cable charged from one end, a current step is generated after reflecting back from the open end with a step length of twice the cable transmission delay. The cable is than discharged to the shunt (or coil) through the spark gap. The measurement system consists of shunts and coil current sensors, 5:1 and 6.6:1 attenuators based on the requirement of the sensors. The recording instrument is a 1-GHz, 8-bit, 1-GS/s digitizer. The proposed step generator can produce current steps with a stable current of up to 100 A. The rise time of the step varies from 1.6 ns to 15 ns, depending on the spark gap used for switching. The produced current is constant within 0.5% for a step length of 960 ns generated with a coaxial cable 110 m in length. To improve the test procedure and measurement capabilities, the thesis also analyzed factors affecting current step measurement, such as the type of coaxial cable, type of connection, extra shielding, clearances, interference sources, media of the spark gap, and the spark gap electrode distance (arc length). It is found that the measurement system and the rise time of current step is affected by many factors, including the coaxiality of the connection, impedance mismatch, interference, clearances, stray capacitances, and stray inductances. These results will enable future standardization of impulse current sensors

A test technique for measuring lightning-induced voltages on aircraft electrical circuits

The development of a test technique used for the measurement of lightning-induced voltages in the electrical circuits of a complete aircraft is described. The resultant technique utilizes a portable device known as a transient analyzer capable of generating unidirectional current impulses similar to lightning current surges, but at a lower current level. A linear relationship between the magnitude of lightning current and the magnitude of induced voltage permitted the scaling up of measured induced values to full threat levels. The test technique was found to be practical when used on a complete aircraft

SOLVING EMC PROBLEMS IN THE DESIGN OF NEW HV TEST LABORATORY

The paper deals with solving electromagnetic compatibility (EMC) problems in the design of a new, case study, industrial high voltage test laboratory, intended to be used for testing of transformers and other apparatus up to 550 kV rated voltage. Modern high voltage test facilities are equipped, apart from primary test devices like AC, DC and impulse voltage generators etc., also with sophisticated numerical measuring instruments and informatics technology. Since such devices are sensitive to transient overvoltages, the highest degree of EMC is to be secured. This can be achieved by proper earthing and screening of test laboratory, what shall be designed in a way to satisfy all requirements conditioned by building lightning protection, personal protection and system earthing, avoiding electromagnetic compatibility disturbances at the same time. One of the main tasks is solving electromagnetic compatibility problems caused by outdoor electromagnetic disturbances originating from various unknown sources. Those disturbances and interferences may seriously influence measuring accuracy and readings of test devices, what consequently leads to false results. The stated is especially relating to partial discharge measurements. As to avoid such disturbances, the laboratory shall be completely screened with a net forming optimally designed Faraday cage. On the other hand, at high voltage tests with impulse voltages, especially with chopped tail waves, steep transient overvoltages may be generated. As a consequence, high transient potential differences between particular points along the earth electrode may occur, what can even lead to flashovers between parts of it. Therefore is of utmost importance to provide proper earthing and low inductance current return path for impulse high voltage test equipment where high frequency transients are to be anticipated. Improper earthing and bonding may result, apart from mentioned flashovers, in severe induced voltages in secondary cables with consequential influence on test results, possible destruction of measuring instruments and hazardous touch voltages for personnel. For analyzing transient potential differences, it is important to model, with maximum accuracy, impulse test circuit (impulse generator, chopping spark gap, voltage divider, Faraday cage, fundament earth electrode, earthing strips, earthing rods etc.). Magnitude of transient potential difference between particular points is proportional to earth electrode inductance, i.e. low inductance of earth electrode will result in decrease of transient potential difference

Grounding methodology in a 550 kv ac power transmission line in the Amazon - a case study

Atmospheric discharges are indispensable for the electrical sector due to their responsibility on the shutdown of the power lines, what may trigger a sequence of events that led the interconnected electrical system to collapse. The aim of this case study is to demonstrate the development of a viable methodology in order to reduce the resistance values of the grounding system of the power lines, reducing shutdown occurrences due to returning disruptive discharges, or backflashover, mitigating the damages caused to the electrical system and to society, in order to improve the quality of the electrical power distribution. Considering the factors of implementation costs, technical environmental viability, the encompassment of this solution for other structures lines, and the reduction of the implementation percentage, it was concluded that the most adequate solution to implement a more robust grounding system in the transmission power lines is the solution 03 tested on the tower 585, with a reduction of 66,98%

Review of recent research towards power cable life cycle management

Power cables are integral to modern urban power transmission and distribution systems. For power cable asset managers worldwide, a major challenge is how to manage effectively the expensive and vast network of cables, many of which are approaching, or have past, their design life. This study provides an in-depth review of recent research and development in cable failure analysis, condition monitoring and diagnosis, life assessment methods, fault location, and optimisation of maintenance and replacement strategies. These topics are essential to cable life cycle management (LCM), which aims to maximise the operational value of cable assets and is now being implemented in many power utility companies. The review expands on material presented at the 2015 JiCable conference and incorporates other recent publications. The review concludes that the full potential of cable condition monitoring, condition and life assessment has not fully realised. It is proposed that a combination of physics-based life modelling and statistical approaches, giving consideration to practical condition monitoring results and insulation response to in-service stress factors and short term stresses, such as water ingress, mechanical damage and imperfections left from manufacturing and installation processes, will be key to success in improved LCM of the vast amount of cable assets around the world