Asier Mugarra

For power grid operators, knowing the transient response of the synchronous generators (SGs) included in their grids is important in order to simulate and monitor faults and other contingencies. However, the time constant of the automatic voltage regulator (AVR) and speed governors of SGs are not fast enough to show their transient dynamics in the case of a fault in the grid. This paper presents a fieldwork carried out in more than 60 gas power plants, where the response of their controllers was studied. These power plants are running and supplying electricity to the Spanish grid. The study consists of recording some SG responses in different situations, varying the AVR or the speed governor setpoints while the generator is running at no-load conditions, and also performing load rejection tests, achieving a real fault emulation. Once all the data are gathered, a fitting of the SG parameters is performed by computer simulations using GENSAL, GAST and SEXS models replicating the performed field tests. This work allows us to build an accurate network model for the whole power system and check which plants are having trouble in the case of contingencies in the grid.

Sweep Frequency Response Analysis (SFRA) is an extended and non-invasive technique used for transformer condition assessment. In the last years, its proven sensitivity and ease of testing have generated interest for rotating machine applications. Since SFRA measurements are curves plotted on Bode diagrams, the failures can be detected by comparison between a reference curve (performed at healthy conditions), and a curve obtained after an incident. However, the interpretation of results is a hard task because the usual way of interpreting the results still being by visual inspection, then, extensive knowledge and appropriate experience are required. To solve it, in previous publications, the authors developed a method called “Fault Diagram” to analyze SFRA results based on fitting the measurement with an equivalent three-variable circuit. This paper presents several improvements in the method based on changing the equivalent circuit, the optimization algorithm and the frequency range analyzed. Furthermore, the paper provides experimental results of the method's application in a 40 MVA synchronous machine pole. SFRA results presentation into a 3D plot provide information about the fault type (ground or inter-turn fault), location, number of faulty turns and fault resistance with more defined tendencies than in previous research.

Inter-turn faults are among the most common failures in electrical machines. While Frequency Response Analysis, (FRA) has been extensively used for power transformers diagnosis for the last 40 years. In this conventional, accurate and non-destructive technique for power transformer diagnosis, it is a usual practice to connect the chassis of the FRA test device to the transformer housing, normally grounded. The same practice has been applied for pole diagnosis, although no procedure has yet been established as standard since pole diagnosis using FRA is still an experimental technique. Nevertheless, this paper shows a different approach to diagnose inter-turns faults in poles by isolating the FRA chassis from the iron core. In this way, the capacity to ground has less influence, increasing FRA test sensitivity for inter-turn fault diagnosis. The method has been verified by experimental results in several spare poles belonging to 106 MVA and 40 MVA synchronous hydro-generators and also by two simulation models with both types of connections, grounded and FRA-isolated, achieving positive results.

Frequency Response Analysis (FRA) is an extended technique for transformer condition assessment. In recent times, its proven sensitivity for detecting different type of faults have generated interest for rotating machine applications. However, at present the interpretation of FRA results is mostly made by visual inspection of curves and is a hard task for non-expert maintenance personnel. This paper presents a mathematical model to simplify diagnostic tasks from FRA results. The developed model is called Fault Diagram and it is conceptualized to be applied to any coil, so it could be extensive to any type of machine. The fault diagram is based on a paradigm which states that a healthy machine is a machine with infinite impedance fault. The model uses a first order frequency response for the purpose of fitting a linear function R3. The input of the model is the FRA curve of the machine, and the output is a point in a 3D space. This approach can give an accurate diagnosis without ambiguous interpretations about if the machine is healthy or not and can provide an estimation of the fault location in case of damage. For validating the method, it was applied in a pole of a 40 MVA synchronous generator.

Frequency Response Analysis (FRA) is a technique under research for its application in rotary electrical machines. Even though FRA has been used for around 40 years in power transformers diagnosis, its application is still brand new in synchronous machines. The FRA technique is based on the comparison to a previous reference test that is done in healthy conditions. This paper presents numerous experimental factory tests in a pole of a 40 MVA synchronous machine and on-site tests upon a 106 MVA synchronous generator in a hydro power plant where the FRA test was performed on a pole inside and outside the machine. Furthermore, simulations were executed in order to calculate the impact of the stator core on the FRA reference test. The main contribution of this work is demonstrating that the stator core has a significant influence on poles FRA results. Therefore, FRA tests done inside the machine cannot be compared to FRA tests performed outside the machine or vice versa, in order to avoid fault positives. FRA can be considered a suitable technique for the ground and interturn faults detection in poles of large synchronous machines.