Partial Discharges in Hydroelectric Generators

The hydroelectric generator is an asset of central importance in hydroelectric plants. Permanent damage, operation accidents or just non-planned halts caused by failure in that equipment have high costs to the power generation company. Regular assessment of the hydroelectric generator condition, which is strongly related to the quality of its isolation system, allows minimization of such losses. Therefore, following the machine condition provides the means to organize scheduled actions by maintenance teams.

During operation, intense electric and magnetic fields inside the generator cause discharges (called partial discharges, PD) to happen. Moreover, aging and mechanical vibration deteriorate the isolation system, increasing even more the intensity of PD. Thus, the integrity of the isolation system can be inferred by monitoring and analysing PD. Such an approach reduces the frequency of direct inspection of the machine isolation system, which is time and money consuming.

The process of monitoring PD comprises mainly two phases: measurement and interpretation. The first step includes procedures and apparatus to effectively measure partial discharge signals. To succeed in this task, in-depth knowledge on partial discharge phenomenon, principles of electrical machines and signal processing techniques is critical. In turn, interpretation aims at classification of fault patterns and pinpointing the location of PD. Feature extraction and machine learning techniques are of particular interest here.

In this book we discuss a rather broad field of PD problems in high voltage generators. The book addresses the measurement, pinpointing and interpretation of PD in hydrogenerators, covering key topics such as physics of the partial discharge phenomenon, types of defects and corresponding PD patterns, sensors and acquisition procedures, applied signal processing techniques, automatic classification of discharge types, and correlation between partial discharge occurrence and ozone generation.

In the introduction, we consider the main types of failures in the high voltage rotating machines, and, principally, in the electrical insulation of the stator windings providing description of terms and concepts of PD. Further, a short description of electromagnetic sensors for PD detection is given. Then, signal processing techniques and an overview of PD online monitoring systems are presented. Thus, we give below a brief review of state of the art of the PD problems in high voltage generators.

Partial discharges occur in electrical insulation systems when the electric field strength exceeds the breakdown value of the material, and the current through the gas or vapor appears produced by the process of ionization. The ionization of the material is a process of separating the electrons of the molecules of the insulation material, producing ions and electrons. This process can be easily seen in gases and liquids, where the ionization process occurs more quickly. In solid materials, the ionization is more difficult to observe, because the electrons are more strongly bound to the atoms.

The breakdown of the material is a stochastic process, i.e. it cannot be accurately predicted. It is possible to estimate the time of occurrence of the breakdown by knowing the electric field strength and the characteristics of the material. This chapter presents the main terms and concepts related to PDs. We focus on the physics of the discharges, specifically when equipment is subjected to high voltage. Special attention will be given to PDs that occur in stator insulation, providing an overview of their sources as well as a description of typical PD patterns.

Partial discharges are small electrical discharges that can occur in the interior or on surfaces of electrical insulation systems (International Electrotechnical Commission, High-voltage test techniques-partial discharge measurements. IEC, Publication-60270, 2000). PDs can also originate from high-voltage metallic surfaces, producing air ioniozation around electrodes. These discharges can arise from many causes, such as impurities, air humidity, temperature and mechanical stress (Rotating Electrical Machines - Part 27-2, On-Line Partial Discharge Measurements on the Stator Winding Insulation of Rotating Electrical Machines. IEC/TS 60034-27-2. International Electrotechnical Commission. Technical Report, 2012). PDs have been known since the nineteenth century and are commonly found in high voltage electrical systems of different applications. It is important to detect them in order to avoid potential damages to high voltage systems, such as hydrogenerators.

According to Institute of Electrical and Electronics Engineers (IEEE Guide for the Measurement of Partial Discharges in AC Electric Machinery. IEEE Std 1434-2014 (Revision of IEEE Std 1434-2000), pp. 1–89, 2014), PDs occur along with various physical manifestations, such as electrical pulses, radio-frequency (RF) signals, acoustic emissions, visible light and chemical reactions involving generator’s cooling gases (mainly air and hydrogen). In this Chapter we discuss electromagnetic methods for PD signal measurements. Electromagnetic measurements may be performed using PD sensors that detect the electric current components of PDs that propagate through the stator windings or by antennae or field sensors which detect the associated radiated electromagnetic waves (Rotating Electrical Machines - Part 27-2, On-Line Partial Discharge Measurements on the Stator Winding Insulation of Rotating Electrical Machines. IEC/TS 60034-27-2. International Electrotechnical Commission. Technical Report, 2012). Methods for performing measurements of displacement currents in capacitors connected to stator windings are standardized to evaluate the windings insulation condition in rotating machines. Although procedures regarding detection of radiated signals using antennae are still not standardized, it is known that the spectra of radiated electromagnetic disturbances produced by a single PD may range from approximately 100 kHz up to several hundreds of MHz (Sena et al., Energies 14(21), 2021). Therefore, with appropriately designed antennas, it is possible to detect PD occurrences from the radiated electromagnetic waves.

This chapter addresses the subject of PD measurements in synchronous generators. We begin by presenting the PD types generally found in that kind of generators as well as the common stator isolation faults. Then, an overview on methods and systems for PD measurement is given. Next, measurements in the field, performed in hydroelectric power plants, are presented and discussed. We also detail a complex study on PD monitoring, carried out during commissioning tests of a new hydrogenerator, i.e., our PD measurements ran parallel the first machine excitation, synchronism procedures, and operating range and heating tests. The study is concluded with PD measurements performed after 2 months of commercial operation of the machine.

In this chapter, we describe a spectral approach based on resonance that may be used to identify concurrent partial discharges in hydrogenerator coils. It is designed to offer a method of diagnosing the coil insulating material by accurately determining the positioning of various discharges. Numerical models of coil structures were created using the full-wave technique finite-difference time-domain (FDTD) to solve Maxwell’s equations. To quantitatively acquire the frequency spectrum for diagnosis, the transient voltages connected to partial discharges occurring at various points in the coil are calculated. Peaks in the spectrum are compared to analytically determined resonance frequencies. It is demonstrated that in almost 90% of the numerical experiments, precise predictions of the simultaneous discharge sites were made. The physical processes that enabled the method’s development were evaluated numerically and in laboratory.

The environment in which PD signals are recorded may exhibit varied levels of noise due to radio frequency interference, thermal activity, and electronic switching devices. To ensure reliable PD monitoring, meticulous use of signal processing techniques to isolate the signal of interest from unwanted disturbances. Section 6.1 briefly discusses the general features of PD measuring systems and their relation to the characteristics of PD signals. Then, in Sect. 6.2, we present basic techniques for signal separation and denoising based on linear time-invariant filters and wavelets. Examples illustrate both approaches applied to synthetic and real signals. The final section of the chapter focuses on classification schemes based on artificial neural networks that take PRPD diagrams as inputs. The aim is to automatically estimate different types of insulation impairments by analysing PRPD data. Classification results by using PD data from power plants are shown. A list of relevant references concludes the chapter.

The main method to monitor the condition of the stator insulation in hydrogenerators is the measurement of partial discharge pulses through capacitive couplers. However, partial discharges exposed to air also produce ozone, whose concentration can be measured through different types of sensors. This parameter can be valuable when assessing the condition of a machine, especially if the measurements are continuous. Since ozone concentration is affected by many variables, such as temperature, humidity, discharge voltage, airflow, etc, its value alone is not very meaningful, but it can be a good complement to typical partial discharge measurements, since it may reveal localized faults that would otherwise go unnoticed. It is also important to understand that very high ozone concentrations can chemically damage a generator, since it combines with atmospheric nitrogen and humidity creating nitric acid, which can corrode metals and degrade insulating materials.