One of the most challenging threats to electrical grids is the occurrence of temporary overvoltages, which can cause substantial damage to electrical systems and require sophisticated protection solutions. In this article, we will explore in detail what temporary overvoltages are, their causes and effects, as well as the importance of surge arrester withstand capability in mitigating these threats.
Temporary Overvoltages: Understanding the Threat
Temporary overvoltages, as the name suggests, are momentary peaks in the electrical voltage of a electrical grids. These disturbances can be triggered by a variety of events, including:
1. Load Rejections: When an electrical load is disconnected abruptly, a temporary overvoltage may occur. This phenomenon is common in electrical systems due to planned or unplanned load interruptions.
2. Single-Phase Short-Circuits: Failures in one of the conductors of an electrical grids can result in momentary overvoltages, which propagate through the grid in search of grounding.
3. Ferroresonance: This complex phenomenon occurs when the energy stored in a transformer is released in an oscillatory manner, generating significant voltage spikes.
4. Atmospheric Discharges: The occurrence of lightning strikes hitting electrical structures or transmission lines can induce dangerous overvoltages.
Oscillatory and Weakly Damped Overvoltages: In-Depth Analysis
A distinctive feature of temporary overvoltages is their oscillatory nature and lack of adequate damping.
This means that after the initial event that caused the overvoltage, oscillations in electrical voltage continue to occur for an extended period.
These oscillatory and weakly damped characteristics can create conditions of extreme stress for electrical components, especially those sensitive to voltage variations.
Oscillatory overvoltages are particularly challenging due to their persistence, and the lack of adequate damping makes them more difficult to control. As a result, it is essential to implement effective protection and mitigation measures to prevent damage to electrical systems and maintain electrical grid reliability.
Surge Arresters: Guardians Against Temporary Overvoltages
One of the primary lines of defense against temporary overvoltages is surge arresters. These devices are designed to safely divert energy from overvoltages, stemming from atmospheric discharges, to the ground, thus protecting sensitive electrical equipment and systems. The effectiveness of surge arresters is a critical factor in the protection of electrical grids.
Surge Arrester Withstand Capability
The surge arrester withstand capability is a crucial aspect to consider. This feature refers to the ability of surge arresters to absorb and dissipate the energy of an overvoltage without failing.
A notable category of surge arresters is those that use zinc oxide (ZnO) as a key component. ZnO-based surge arresters are widely recognized for their effectiveness in protection against overvoltages, thanks to their ability to dissipate the heat generated during a prolonged overvoltage. This is essential for maintaining the functionality of the surge arrester and ensuring continuous protection of electrical grids.
Dielectric Stress and Thermal Capacity
The importance of dielectric stress phase-to-ground or phase-to-phase concerning the thermal capacity of ZnO surge arresters cannot be underestimated. Dielectric stress represents the study of voltage oscillations that occur after events such as load rejections, single-phase short circuits, atmospheric discharges, and ferroresonance. These oscillations have a direct impact on the thermal capacity of ZnO surge arresters.
The thermal capacity of ZnO surge arresters is crucial for their effectiveness. It determines the amount of heat the surge arrester can safely dissipate during a prolonged overvoltage. When dielectric stress results in oscillatory and weakly damped overvoltages, the thermal capacity of ZnO surge arresters plays a critical role in ensuring the stability of the electrical system.
Determining the Thermal Capacity of ZnO Surge Arresters
The assessment of the thermal capacity of ZnO surge arresters is a complex and crucial process. It involves a detailed analysis of the device's design, physical properties, materials, and its heat management capability. Understanding how these surge arresters respond to voltage disturbances resulting from dielectric stress is essential to ensure their ability to effectively protect electrical grids.
SPDA Design
The design of Lightning Protection Systems (SPDA, Sistema de Proteção contra Descargas Atmosféricas) concerns the study, analysis, calculation, development, and installation of devices that are intended to capture and drain the energy from lightning discharges to the ground in a safe and reliable manner.
This study consists of, through one of the validated methods (protection angle, spheres, or meshes), determining the shape of the lightning rod (rod or mesh), the material, height, and distribution of these devices, such as quantity and distance; so that a certain coverage area remains safe from the interactions of lightning strikes and their harmful effects.
Guard Cable
Specifically in Transmission Lines, there is the presence of "guard cable" used for protection against lightning strikes, located above the phase conductors, being the most likely point for the lightning strike to hit; its connections with transmission towers ensure a correct and safe discharge of the energy from the lightning strike to the ground.
OPGW Cable
The OPGW cable is a type of guard cable that takes advantage of the need for the installation of a guard cable, and the space that this guard cable occupies and displaces, and integrates into this cable a layer of fiber optic communication cable, that is, we have a single cable that protects transmission lines and their surroundings against lightning strikes and transmits communication data via fiber optics.
Ensuring the Protection of Electrical Grids
In summary, temporary overvoltages represent a constant threat to electrical grids, and the oscillatory and weakly damped nature of these disturbances makes them particularly challenging to deal with. The surge withstand capability of surge arresters, especially those based on ZnO, plays a critical role in protecting electrical grids against these threats.
Understanding the dielectric stress phase-to-ground or phase-to-phase is essential for assessing the thermal capacity of ZnO surge arresters and ensuring that they can dissipate the heat generated during prolonged overvoltages.
Protecting electrical grids is a complex but essential task to maintain the reliability and safety of the power supply. The continuous study of these phenomena and the implementation of appropriate protection measures are imperative to address the challenges of the constantly evolving electrical sector.
HVEX: Leadership and Pioneering!
We are the pioneer in the creation of the Impulse Generator, a unique solution that places us as the only national manufacturer of this equipment. This device is designed to emulate voltage and current surges, as well as common lightning strikes, providing ideal conditions for impulse tests. These tests assess the dielectric withstand capability of materials and equipment, allowing them to be classified for reliability and safe application in the electrical system.
The Impulse Generator works by amplifying the potential and delivering it impulsively, either in terms of voltage or current, to the test specimen. A key feature is the exclusive supervisory software, the HVEX LIAS, which intuitively manages the operation of these devices. Additionally, the software is capable of automatically generating technical reports in .pdf and .doc formats.
HVEX Impulse Generators are divided into two categories:
Voltage Impulse Generator: This device is equipped with amplification circuits, resistors, and dividers designed to inject voltage following an impulse function into the terminals where the test specimen is connected. HVEX Impulse Generators are custom-designed to meet the needs of each customer and can be applied according to standards such as IEC 60060, NBR 61083, NBR 6939, NBR 5356, and NBR 5440. They are ideal for testing transformers (transmission, distribution, protection, and measurement), insulators, cables, bushings, terminations, circuit breakers, capacitors, reactors, surge arresters, SPDs (Surge Protective Devices), and other electrical components.
Current Impulse Generator: This equipment injects an impulse function in the form of current. The main difference from the Voltage Impulse Generator lies in the organization of the divider resistors. This allows prioritizing the voltage or current to which the object is subjected. The HVEX impulse current test system can generate up to 200kA, with residual voltage of up to 100kV, following standards such as IEC 60060, IEC 60099-4, IEC 62305-1, IEC 61643-11, and NBR 6939. Its compact structure, with reactor configuration that allows generating long-duration and exponential current pulses, can be customized to meet the specific testing requirements of each customer. Applications include testing surge arresters, varistors, EMP (Electromagnetic Pulse) and NEMP (Nuclear Electromagnetic Pulse) devices, circuit breakers, SPDs, vehicles (cars, buses, trucks, trains, aircraft), wind generators, photovoltaic generators, and research.
Portable Impulse Generator: This equipment operates on the same principle as the Voltage Impulse Generator, but on a reduced scale. This implies a limitation in the power of the device, but makes it highly versatile and easy to transport. It can be carried in a pickup truck, for example, to meet specific testing demands in the field.
HVEX continues to lead the way in innovation and technological advancement in the electrical sector, providing high-quality solutions to customers across the spectrum of power transmission, generation, and trading.