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Introduction

1.1 Why Perform Partial Discharge Measurements?

This book is focused on the practical aspects of the measurement of partial discharge (PD) and corona in 50/60 Hz power system equipment such as generators, motors, power cables, air‐ and gas‐insulated switchgear (GIS), and transformers, all usually rated 3 kV and above. Such electrical equipment uses solid electrical insulation, for example polyethylene, epoxy, and polyester, or insulation composites such as oil–paper, fiberglass‐reinforced polymers, or epoxy‐mica, to separate high‐voltage conductors from ground or to separate one AC phase from another. If this insulation fails, the equipment experiences a phase‐to‐ground fault or a phase‐to‐phase fault, which will activate protective relays to isolate the equipment from the power system. Such a failure may manifest itself as a power outage in a residential area or hospital, a loss of electrical power production capacity, or a reduction in power system reliability. In industries such as petrochemical, cement, steel, aluminum, paper, or semiconductor fabrication, these failures can be extremely expensive because modern production processes are continuous; an electrical power failure of even a few minutes may necessitate taking the entire factory out of production for days or weeks. In addition, such insulation failures can cause collateral damage to adjacent components that can greatly increase the cost of repair. For example, a large utility generator or power transformer failure can cost millions to repair, and result in a plant shutdown that can last for months, causing tens of millions of dollars in lost production.

Partial discharges are small electrical “micro‐sparks” that can occur in insulation systems operating with high electric fields. The physics of PD and how it is manifested are discussed in Chapters 2, 3, and 5. PD activity can directly lead to insulation degradation and equipment failure. PD is also sometimes a symptom of poor manufacturing and/or aging of the insulation due to high temperature, mechanical forces, contamination, etc. In this case, PD might not directly lead to failure but may indicate that insulation aging due to other mechanisms is occurring and maintenance may be needed. Thus, by measuring PD activity, equipment manufacturers can often determine that the insulation system on the equipment was properly made, and equipment owners can determine if aging is occurring that could lead to failure.

Each partial discharge is accompanied by a current pulse. As presented later in this book, these current pulses can be detected by various types of sensors and measurement instruments. In addition to measuring the PD current, PD can be detected from radio frequency (RF) radiation, light emissions, acoustic noise, and by chemical changes in the local environment. PD testing involves the measurement of the PD current pulses and other signals that are produced by PD.

PD testing using 50/60 Hz AC is widely employed as a factory quality assurance (QA) test for all types of high‐voltage equipment. Many IEEE and IEC technical standards have been published to indicate how the PD should be measured for each type of equipment, often providing guidance on interpretation, and sometimes providing information on pass/fail criteria. The premise is that if newly manufactured equipment successfully passes the PD test, then premature insulation failure due to electrical stress is unlikely.

In recent decades, with the development of digital hardware, often with powerful disturbance suppression methods and signal processing, PD testing has increasingly been applied to high‐voltage equipment that has been installed in the power system or industrial plants with a view to assess if the high‐voltage insulation system is degrading and may have a high risk of failure. Thus, the purpose of PD testing, once equipment has entered service, is to help with insulation condition assessment and determining the need for maintenance. There are relatively few IEEE and IEC standards for such PD testing applications. Hence, an important function of this book is to provide information for both onsite (offline) and online PD testing/monitoring of the different types of high‐voltage equipment.

In this book, for simplicity, we will use the term “high‐voltage insulation system,” rather than the more cumbersome “medium‐ and high‐voltage insulation system.” What voltage ratings are associated with medium voltage (MV) and high voltage (HV) depends on the type of equipment. A medium‐voltage motor is usually rated between 3 and 7 kV, whereas a high‐voltage motor is 11 kV or higher. In electrical power transmission systems, there is a wide variation of what is meant by medium and high voltage.

1.2 Partial Discharge and Corona

There are many definitions for partial discharge. Perhaps the most widely used definition of PD comes from IEC 60270, where it is described as “a localized electrical discharge that only partially bridges the insulation between the conductors and which can or cannot occur adjacent to a conductor.” That is, PD is a localized electrical breakdown of the insulation that does not immediately progress to a complete breakdown across the insulator (e.g. between the high‐voltage conductor and ground). In contrast, a “complete discharge” essentially means a phase‐to‐phase or phase‐to‐ground fault has occurred, which would typically trigger protective relaying to open‐circuit breakers. As is discussed in Chapters 2 and 3, since gases (and air in particular) have a dielectric strength that is a small fraction of the dielectric strength of a solid or liquid insulation, PD tends to occur where there is a gas under high electrical stress. Thus, PD almost always occurs when there is a gas‐filled void within the solid or liquid insulation, or there is gas adjacent to the solid/liquid insulation along a surface. PD can also occur in a gas adjacent to metal conductors where the electric field is rapidly decreasing the greater the distance from the metal conductor. Thus, PD can occur in all types of high‐voltage apparatus, regardless of the insulation system, and may even occur at relatively low voltages if distances are small (Chapter 18).

A corona discharge is a particular type of PD. In IEC 60270, corona is described as “a form of partial discharge that occurs in a gaseous media that is around conductors that are remote from solid or liquid insulation.” The most common type of corona occurs in overhead electric transmission lines, from which its distinctive crackling sound can often be heard, especially during rainy/snowy/foggy weather. Such corona is caused by localized breakdown of the air due to the high electric field adjacent to the bare aluminum conductors. The corona is very localized, since the electric field more than a few centimeters away from the high‐voltage conductors is too low for electrical breakdown to occur. Thus, there is no “complete breakdown” between the transmission line conductors and ground. The term “corona” has been reserved for this type of PD since, on dark nights, the glow of the “corona” surrounding the lines can often be observed visually. To clarify, corona is often visible and caused by nonuniform electric fields in the air or gas. Corona itself does not directly damage the “electrical insulation” since, for the most part, electron and ion bombardment of gas molecules have no lasting effect, and although metals may experience some discoloration and pitting, and corona can produce by‐products such as ozone, this usually does not impair the function of the HV apparatus. Also, the glass and ceramic insulators that hold up the overhead transmission lines are inorganic and extremely resistant to corona. In fact, the only real negative impact of corona is the radio and television interference they cause, as well as the energy losses due to corona on the transmission line.

A hundred years ago, the terms “ionization” and “corona” were used for what is now called PD. In the 1920s, the term corona became more popular than ionization. After the 1940s, more and more papers referred to both corona and (partial) discharges interchangeably. Once the definition of corona and partial discharge were clarified by many standard‐making organizations in the 1960s, corona and PD should no longer be used as synonyms. In reviewing the literature, Europeans adapted more quickly and tended not to use the corona and PD as synonyms after the 1960s. North Americans tended to use corona and PD interchangeably well into the 1980s (and a few older persons still get mixed up). In this book, PD will refer to all types of incomplete discharges. Corona will be used to refer to a particular type of PD that is associated with highly divergent electric fields around metal conductors in air.

1.3 Categories of PD Tests

PD testing has two main purposes:

• as a factory test on new equipment; and
• as a test to determine if insulation aging is taking place in installed high‐voltage equipment.

The first is an offline test (that is an external AC supply is needed to energize the equipment to the test voltage). There are subcategories of factory tests: PD tests during the development stage of new equipment; type...