Frequency Response Analysis Technique for Detection of Winding Displacement and Deformation - Assignment Example

Name Course Name Introduction Maintenance and servicing of power transformer increase pressure on the economy. On the other hand, many transformers are aging causing an increase in loads. Therefore, apparatus used for diagnosing these problems are very necessary especially risky ones (McGrail, 2005). Over the years, there has been more development on various ways of measurements, recording and analysis data. SFRA is not only powerful, but also sensitive and nondestructive method applied in the evaluation of mechanical condition of the transformer windings, the core and its clampings. SFRA measures the electrical transfer functions in a considerable range of frequencies. A low voltage signal which has varied frequencies is connected into one terminal of a transformer and measuring the output signal on the other terminal. This is done on all other windings and the resulting signals are compared to the standard frequency response data. The frequency deviations are caused by changes between or in the network elements. The deviations between the FRA output and the actual measurements show the differences in electrical or position of the components. Failures can be seen over various parts in the frequency range. This diagnostic method has been shown by scientific investigators to be the best as it deliver reliable data about the mechanical state of a power transformer. Literature review FRA is a popular technique for testing deformations such as in turns, coils, layers and others, in transformer windings. Such faults occur due to impact during transportation, aging and short-circuit faults. In 1987, Erven and Dick became the first people to apply FRA technique in detecting deformation in transformer windings (Ryder, 2002). The method was later improved by other researchers. Islam (2000) came up with three frequency ranges for FRA spectrum which include: high, medium and low frequencies. A ladder model was used to represent high voltage winding. For low frequency range, series capacitance was eliminated but at high frequencies range, inductance was discarded. Serious study for transformer parameters for FRA was later undertaken. Therefore, in 2002 Hettiwatte et al. did a research using a 6.6 kV transformer with continuous disk type winding. The model had multiple conductor lines with a single turn. The losses, inductance and inductance were calculated as distributed parameters. Accurate analysis was done by use of impedance measurement, which shows the effects of increasing series capacitance, impedance and inductance. In 2006, a better model which uses FRA to measure frequency response for three phase transformers was developed by Gubanski et al. The model could measure the permittivity of the insulation material like air, paper and the wooden board. Frequency response was developed using lumped parameters. Sofian and Wang, 2009 urged that winding structure affects the values obtained in FRA response. They urged that high series capacitance shows rising trend in the magnitude of FRA, while lower series capacitance display a steady trend with some resonance. The paper gave value to electrical parameters which was going to be obtained later. Distributed parameter model was conducted by Shintemirov et al. 2010 to detect deformation in small winding. They suggested that faults in minor windings can be detected at a frequency of 1 MHz and above, which gave more understanding for sensitive ground capacitance. In 2011, Abu-Siada and Small game up with a new technique for analyzing FRA data with polar plots. The method is automated and shows the relationship between the capacitance, inductance, and capacitance and fault types. Basically, there are two techniques used in FRA for measuring winding deformation in transformer. They include: Low Voltage Impulse FRA and SFRA. The former is also called the impulse technique while the later is swept –frequency method. In both methods transformer impedance is measured in a range of frequencies and the impedance varies due to the conditions in the transformer (Ryder, 2002; McGrail, 2005). SFRA TESTING This type of testing is based on comparative measurement. It means that the results from a test, as a set of curves that represent all windings in a transformer are the baseline data. Three ways of analyzing the FRA data are: a) Comparing currents results with the previous data of the same unit. If measurements have been performed on the same transformer previously, the results are uploaded into FRA instrument software and studied before performing the test. The new data output is then analyzed by comparing with the reference data. b) Comparing FRA results of one transformer with the results of another of the same type. If the old data is not available, the results from the same type of transformer are used for analysis. This is because the transformers are more like to have the same mechanical failures. However, the winding design may be changed when manufacturing, and therefore, different output may be produced. c) Comparing FRA results of one phase with those of another the phases in the same transformer. If there is no reference data, the test can be done according to the international standards guidelines taking onto account the objective of the test. For example: investigating failure, assessing the damage due to transportation or phase characteristics and the amplitude which forms a ‘fingerprint’. The common test is measurement of from end to end while leaving other terminals to float. In many cases the reference data and the input voltage signal are connected to the neutral terminal, and the output signal is obtained from one of the star connected phases (Tang and Wu, 2011). According to IEEE, SFRA is used to: Testing factory short-circuit Used during relocation and Installation Testing after a through-fault event As routine diagnostic testing protocol Used after a transformer alarm like pressure Used in electrical test conditions like a change in winding capacitance Used in system Modeling The SFRA testing equipment is connected as follows: (IEEE Power and Energy Society, 2012) Modeling a transformer The two modeling techniques for the transformer are the terminal (black box) and physical (detailed) models. The former model utilizes insulation coordination in EHV and HV application systems (Guardado et al., 2009). It parameters are measured in frequency or time domain from a physical transformer. Its lack of topological structures results in complicated functions which are difficult to estimate. Node to node impedance functions is obtained. (Donald et al., 1993). The terminal nodes of the network are represented by the resulting admittance matrix. Guardado et al. urged that the black box model represents the character of the terminals of the transformer not its winding displacement (Guardado et al., 2009). The physical model consists of a network of inductance and capacitance from the isolation of self and mutual winding capacitance and inductance. Solution to of the parameters is difficult and requires the measurements of the physical layout with transformer construction details. This model enhances more knowledge on the observed information and is very useful in diagnosing faults (Hosokawa, 1997). This model can be classified into: Leakage inductance model – it is used in modeling multi-section power transfers. It models the leakage inductance of the transformer. Duality model- it is based on leakage flux and it models the iron core accurately, but not leakage inductance and ignores the thickness of the winding. Electromagnetic field model – it is used in the calculation of the design parameters of the transformer. It is commonly used for computing field problems, but it does not compute transients since it is expensive to simulate. Measuring using SFRA During the test, four terminals are divided two pairs: input and output pairs. The two pairs can be modeled as shown below. Figure 1: Two port network The transfer function is represented by Fourier variable, H(jw) and w=2πf. Therefore, The transfer function generates a polynomial ratio, and the half power and resonance occurs at polynomials roots. The numerator and denominator roots are called zeros and poles respectively (Islam et al, 2011). FRA measures the impedance model of the transformer and do not detach true impedance, Z(jw) which has RLC network. The figure below shows RLC circuit with shunt resistor (Islam et al, 2011. Figure 2: LRC and shunt resistor circuit Resonance frequency, . At resonance the transfer function is: What is really measured over R1 is the current. Therefore the impedance is (Islam et al, 2011) When the transfer function of a transformer is analyzed the changes in the capacitance and inductance are shown either as formation of a new resonance or a shift from the old frequency. In other words, the capacitive and inductive components results in a unique frequency response from each unit. The trans-admittance in the winding, which is the transfer function, is measured within the range of 100Hz to 2MHz. The low and high winding voltage transformation ratio is measured within this frequency band. The phase characteristics and the amplitude obtained forms a ‘fingerprint’ which identify the geometry of the windings. The displacement of the coils changes some of the resonance frequencies and can be seen if they are analyzed using the factory data (Tang and Wu, 2011). Analysis The bode diagram uses asymptotic symmetry and logarithmic scale for frequency, and produce better view with large frequency range. The figure below is an example of high response in high voltage star connection. Figure 3: Frequency Analysis Bands (Hoque and Islam, 2011) The sub-bands are mainly the characteristics of the transformer internal components and can exhibit various failures types as shown below. Region Sub-band frequency Component Failure sensitivity 1. Read More

The losses, inductance and inductance were calculated as distributed parameters. Accurate analysis was done by use of impedance measurement, which shows the effects of increasing series capacitance, impedance and inductance. In 2006, a better model which uses FRA to measure frequency response for three phase transformers was developed by Gubanski et al. The model could measure the permittivity of the insulation material like air, paper and the wooden board. Frequency response was developed using lumped parameters.

Sofian and Wang, 2009 urged that winding structure affects the values obtained in FRA response. They urged that high series capacitance shows rising trend in the magnitude of FRA, while lower series capacitance display a steady trend with some resonance. The paper gave value to electrical parameters which was going to be obtained later. Distributed parameter model was conducted by Shintemirov et al. 2010 to detect deformation in small winding. They suggested that faults in minor windings can be detected at a frequency of 1 MHz and above, which gave more understanding for sensitive ground capacitance.

In 2011, Abu-Siada and Small game up with a new technique for analyzing FRA data with polar plots. The method is automated and shows the relationship between the capacitance, inductance, and capacitance and fault types. Basically, there are two techniques used in FRA for measuring winding deformation in transformer. They include: Low Voltage Impulse FRA and SFRA. The former is also called the impulse technique while the later is swept –frequency method. In both methods transformer impedance is measured in a range of frequencies and the impedance varies due to the conditions in the transformer (Ryder, 2002; McGrail, 2005).

SFRA TESTING This type of testing is based on comparative measurement. It means that the results from a test, as a set of curves that represent all windings in a transformer are the baseline data. Three ways of analyzing the FRA data are: a) Comparing currents results with the previous data of the same unit. If measurements have been performed on the same transformer previously, the results are uploaded into FRA instrument software and studied before performing the test. The new data output is then analyzed by comparing with the reference data. b) Comparing FRA results of one transformer with the results of another of the same type.

If the old data is not available, the results from the same type of transformer are used for analysis. This is because the transformers are more like to have the same mechanical failures. However, the winding design may be changed when manufacturing, and therefore, different output may be produced. c) Comparing FRA results of one phase with those of another the phases in the same transformer. If there is no reference data, the test can be done according to the international standards guidelines taking onto account the objective of the test.

For example: investigating failure, assessing the damage due to transportation or phase characteristics and the amplitude which forms a ‘fingerprint’. The common test is measurement of from end to end while leaving other terminals to float. In many cases the reference data and the input voltage signal are connected to the neutral terminal, and the output signal is obtained from one of the star connected phases (Tang and Wu, 2011). According to IEEE, SFRA is used to: Testing factory short-circuit Used during relocation and Installation Testing after a through-fault event As routine diagnostic testing protocol Used after a transformer alarm like pressure Used in electrical test conditions like a change in winding capacitance Used in system Modeling The SFRA testing equipment is connected as follows: (IEEE Power and Energy Society, 2012) Modeling a transformer The two modeling techniques for the transformer are the terminal (black box) and physical (detailed) models.

The former model utilizes insulation coordination in EHV and HV application systems (Guardado et al., 2009).

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