Evaluation of Lightning Impulse Test by Frequency Response Analysis

In this work are presented the basis for improving the interpretation of transformer lightning impulse test and the development of a graphical user interface system, which allows comparisons of time domain data and frequency response. The frequency response is obtained from deconvolution of voltage and neutral current records. A quantitative comparison of frequency response is performed using the techniques applied to displacement detection through Frequency Response Analysis, such as correlation and spectral deviation. The system is implemented using 8 bit digitizers to acquire the voltage and neutral current records. The quantization error and reliability of the frequency response obtained is handled through the use of the coherence function and tolerance bands. The system is thoroughly tested applying a lightning impulse test to a dry type distribution transformer, simulating an interdisc fault with a spark gap. Failure detection is confirmed

Demonstration of sustained and useful converter responses during balanced and unbalanced faults in microgrids

In large power grids where converter penetration is presently low and the network impedance is predominantly reactive, the required response from converters during faults is presently specified by phrases such as “maximum reactive output”. However, in marine and aero power systems most faults are unbalanced, the network impedance is resistive, and converter penetration may be high. Therefore a balanced reactive fault current response to an unbalanced fault may lead to over-voltages or over/under frequency events. Instead, this paper presents a method of controlling the converter as a balanced voltage source behind a reactance, thereby emulating the fault response of a synchronous generator (SG) as closely as possible. In this mode there is a risk of converter destruction due to overcurrent. A new way of preventing destruction but still providing fault performance as close to a SG as possible is presented. Demonstrations are presented of simulations and laboratory testing at the 10kVA 400V scale, with balanced and unbalanced faults. Currents can be limited to about 1.5pu while still providing appropriate unbalanced fault response within a resistive network

Electrical interferences in SFRA measurements

Power transformers are still among the most costly and critical components in an electrical power network. Thus, the importance of a reliable condition assessment of these assets increases due to the aging of transformer fleets. The Sweep Frequency Response Analysis (SFRA) has become an important standard test and provides comprehensive information about the mechanical and electrical integrity of the active part of power transformers. Electrical as well as geometrical changes in the magnetic core, the winding assembly and the clamping structure can be detected by a comparison of an actual measurement with a reference measurement. In contrast to traditional diagnostic methods, the SFRA is sensitive to external electrical interferences which may limit the comparability and can consequently lead to misinterpretation of the measurement results. For an optimum suppression of narrowband and broadband noise, software-based as well as hardware-based techniques can be utilized. This article discusses the theory of different noise sources and noise suppression techniques. Different case studies show the efficacy of measurements even in harsh conditions

Cable Health Monitoring System Built Into Power Converter Using Time Domain Reflectometry

Aging cables are a critical reliability concern for many electrical systems. Cables run over long distances, are often exposed to harsh environments and are intended to work for several decades. Cable faults can be a serious problem because cables are usually hard to repair or replace. Condition based maintenance is a popular approach that provides high reliability and low maintenance cost at the same time. This is a non-destructive method that collects information about the health condition of the cable under test without causing any potential damage. Nowadays, in several applications power cables are connected to power converters and it would be desirable to be able to implement condition based maintenance by using the existing converter for cable health monitoring during normal system operation. In this thesis, we investigate the feasibility of using existing power converters to perform online cable health monitoring and propose an add-on circuit which can be added to an existing power converter so that it can perform the health monitoring function. We use the time domain reflectometry method to characterize and locate faults in the cable connecting the output of a power converter to its load

A review of transformer FRA measurement and diagnosis techniques

Frequency Response Analysis (FRA) is a successful technique to detect mechanical damage in power transformers with greater sensitivity than other measurements. The SFRA method is commonly used today and requires a benchmark FRA measurement for comparative diagnostic methods. The use of numerical indices to quantify differences in two FRA signatures is described. It is important to understand the importance of understanding the physical meaning behind the frequency responses of windings through modelling and simulation

Online condition monitoring of MV cable feeders using Rogowski coil sensors for PD measurements

Condition monitoring is a highly effective prognostic tool for incipient insulation degradation to avoid sudden failures of electrical components and to keep the power network in operation. Improved operational performance of the sensors and effective measurement techniques could enable the development of a robust monitoring system. This paper addresses two main aspects of condition monitoring: an enhanced design of an induction sensor that has the capability of measuring partial discharge (PD) signals emerging simultaneously from medium voltage cables and transformers, and an integrated monitoring system that enables the monitoring of a wider part of the cable feeder. Having described the conventional practices along with the authors' own experiences and research on non-intrusive solutions, this paper proposes an optimum design of a Rogowski coil that can measure the PD signals from medium voltage cables, its accessories, and the distribution transformers. The proposed PD monitoring scheme is implemented using the directional sensitivity capability of Rogowski coils and a suitable sensor installation scheme that leads to the development of an integrated monitoring model for the components of a MV cable feeder. Furthermore, the paper presents forethought regarding huge amount of PD data from various sensors using a simplified and practical approach. In the perspective of today's changing grid, the presented idea of integrated monitoring practices provide a concept towards automated condition monitoring.This work is done under the project Smart Condition Monitoring of Power Grid that is funded by the Academy of Finland (Grant No. 309412)

Online condition monitoring of MV cable feeders using Rogowski coil sensors for PD measurements

Condition monitoring is a highly effective prognostic tool for incipient insulation degradation to avoid sudden failures of electrical components and to keep the power network in operation. Improved operational performance of the sensors and effective measurement techniques could enable the development of a robust monitoring system. This paper addresses two main aspects of condition monitoring: an enhanced design of an induction sensor that has the capability of measuring partial discharge (PD) signals emerging simultaneously from medium voltage cables and transformers, and an integrated monitoring system that enables the monitoring of a wider part of the cable feeder. Having described the conventional practices along with the authors’ own experiences and research on non-intrusive solutions, this paper proposes an optimum design of a Rogowski coil that can measure the PD signals from medium voltage cables, its accessories, and the distribution transformers. The proposed PD monitoring scheme is implemented using the directional sensitivity capability of Rogowski coils and a suitable sensor installation scheme that leads to the development of an integrated monitoring model for the components of a MV cable feeder. Furthermore, the paper presents forethought regarding huge amount of PD data from various sensors using a simplified and practical approach. In the perspective of today’s changing grid, the presented idea of integrated monitoring practices provide a concept towards automated condition monitoring.fi=vertaisarvioitu|en=peerReviewed