CEMIG l ND 2.2 l Aula 01 l Instalações Básicas de Redes de Distribuição Aéreas Rurais

Introduction to CEMIG Distribution Standards

Overview of the Video Series

• The video introduces a playlist focused on the CEMIG (Companhia Energética de Minas Gerais) distribution standards, specifically aimed at preparing viewers for a competitive exam.

• The first lesson covers basic norms related to aerial distribution networks, referencing document AMD 2.2 as the foundational material.

Key Concepts in Distribution Systems

• The instructor discusses the multi-grounded neutral system defined in standard 2.2, emphasizing its connection to substation grounding.

• An illustrative image is presented to explain how the neutral point is grounded at various points along the distribution network.

Understanding Conductors Used in Rural Distribution Networks

Types of Conductors

• The second question addresses which conductors are utilized in rural distribution networks (RDr), with answers sourced from standard ND 2.2.

• It is noted that aluminum conductors with steel reinforcement (C&A type) are primarily used, highlighting their structural benefits.

Specifications and Measurements

• Various conductor sizes are discussed, including specifications like AWG (American Wire Gauge), with examples provided for clarity.

• Specific sizes such as 4mm² and corresponding AWG measurements are detailed, illustrating how these relate to practical applications in rural settings.

Types of Cables and Their Applications

Cable Varieties

• Five types of cables used in rural networks are identified: 21mm², 34mm², 54mm², 107mm², and 170mm².

• Emphasis is placed on using C&A type cables for spans up to 80 meters within construction guidelines set by the norm.

Neutral Conductor Requirements

• For sections utilizing C&A cables, it’s specified that the neutral conductor must be made of aluminized steel or covered aluminum wire.

Understanding Installation Challenges in Rural Areas

Installation Difficulties with Sky Cable

• The speaker discusses challenges faced when installing Sky cable in areas where installation is difficult or impossible, particularly in agricultural zones.

• Concerns are raised about potential accidents due to machinery like tractors operating nearby, which complicates the installation process.

Environmental Considerations

• Emphasis is placed on the importance of preserving local vegetation and ecosystems, especially in areas with valuable tree species.

• The speaker notes that structures cannot be installed if they threaten these trees, leading to alternative solutions being necessary.

Technical Specifications for Cable Use

• Discussion includes using smaller cables (cabo de Vitória) to reduce spans between poles and lessen weight on structures.

• The maximum span length for installations using specific cables is outlined as 65 meters, necessitating detailed studies before implementation.

Justification for Engineering Decisions

• A detailed study must justify the use of certain materials and methods based on costs, visual aspects, terrain conditions, and other factors.

• Clarification is provided regarding the first and last spans of new lines being constructed with a maximum length of 80 meters using specific cables.

Depth Calculation for Simple Engastamento

• The method for calculating depth and width for simple engastamento (embedding posts) is explained; it involves dividing the post height by ten plus a fixed amount.

• An example calculation shows that a 10-meter post requires an engastamento depth of at least 1.60 meters.

Soil Compaction Techniques

• Importance of compacting soil every 20 cm during installation to ensure stability is highlighted; this technique aids in achieving proper embedding depth.

• Reference to norms from Cemig indicates that many procedures are standardized across different utility companies due to their similarities.

Special Cases for High Resistance Posts

Engastamento e Normas de Segurança em Postes

Engastamento de Postes

• O engastamento de postes não é um processo simples, especialmente quando a resistência do poste é elevada (300 danças). A profundidade e o método de engastamento são cruciais para garantir a estabilidade.

• Para um poste de 15 metros, será necessário cavar até 2,10 metros para uma base concretada. Isso está alinhado com as normas da Cemig, especificamente na seção ND 2.2.

• A profundidade aumentada do engastamento deve ser considerada ao trabalhar com postes maiores. Por exemplo, um poste de 15 metros requer uma análise cuidadosa da profundidade para garantir segurança.

• A resistência do engastamento aumenta conforme a profundidade se torna maior. É importante seguir as diretrizes normativas para determinar a profundidade correta.

Distâncias Mínimas entre Condutores e Solo

• A norma estabelece distâncias mínimas entre condutores de média tensão e o solo em áreas urbanas. Essas informações podem ser encontradas na página 3-1 do normativo.

• As distâncias variam dependendo da tensão da rede; por exemplo, em áreas exclusivas para pedestres, a distância mínima é de 600 mm entre os cabos aterrados e o solo.

• Para redes de baixa tensão (até 1000 Volts), a distância mínima também é estabelecida em seis metros do solo, tanto para BT quanto MT.

Faixa de Segurança e Aceiro

• A largura da faixa de segurança (ou faixa de servidão) ao redor dos postes deve ser cuidadosamente mantida. Essa área deve ter pelo menos 15 metros no total: 7,5 metros em cada lado do poste.

Vegetation Management and Safety Regulations

Importance of Clear Zones Around Structures

• The "aceiro" is a critical area of 2 meters around structures that must be kept clear of obstacles and vegetation to ensure safety.

• It is permissible to plant low-growing crops (maximum height of 1 meter) within the designated area, but existing taller vegetation must be removed to maintain safety standards.

Restrictions on Planting and Construction

• Only low-growing vegetation (up to 1 meter high) is allowed; construction of buildings, pens, or pools in this zone is prohibited.

• The minimum diameter for clearing around poles should be at least 2 meters for all types of posts.

Vegetation Removal Guidelines

• Cut vegetation must be removed from the central axis towards the edges within the safety zone limits, ensuring no debris remains in the cleared area.

Understanding Basic Insulation Levels

Definition and Application of MBI

• MBI refers to "Nível Básico de Isolamento," which translates to Basic Insulation Level, crucial for electrical safety.

• This standard applies to both three-phase and single-phase structures with insulation levels at 95kV, 170kV, and 300kV.

Voltage Ratings and Insulator Functionality

• An insulator rated for 15 kV can handle voltages up to that level while supporting lower voltage lines (e.g., 7.9 kV).

• The MBI ensures protection during short circuits lasting milliseconds; it can withstand brief voltage spikes below specified thresholds without failure.

Impact of Electromagnetic Fields

• High-voltage spikes caused by nearby lightning can affect conductors; however, if these spikes are transient (lasting only milliseconds), they may not cause damage as long as they remain below certain voltage levels.

Insulation Levels Based on Structure Type

Variations in MBI Depending on Material

• Different materials influence insulation levels; concrete poles have different ratings compared to wooden ones due to their internal metal structure.

• For example, an insulator rated at 15 kV provides a basic insulation level (MBI) of 95 kV when considering its distance from grounded points like crossarms or poles.

Grounding Considerations in Rural Networks

• In rural setups with wooden poles, grounding points significantly impact insulation ratings; proper installation ensures adequate protection against electrical faults.

Understanding Insulators in Electrical Structures

Overview of Insulator Types and Applications

• The speaker discusses the use of a specific insulator type, mentioning a 15kV isolator with an MBI (Maximum Breaking Index) of 95kV, relevant for both monophase and triphase structures.

• Clarification is provided that "encabeçamento" refers to anchoring structures; these can utilize polymeric insulators instead of traditional glass insulators.

• The transition from glass to polymeric insulators is highlighted, noting that glass insulators are increasingly being replaced due to their fragility.

Structural Considerations for Insulator Installation

• A description of a setup involving two isolators yielding an MBI of 170kV is given, emphasizing the importance of proper installation and configuration.

• The impact of using concrete versus wooden poles on MBI values is discussed; concrete poles can support higher MBIs compared to wooden ones.

Enhancements in Insulator Design

• It’s noted that increasing the number of isolators in series can enhance insulation capacity, particularly when using polymeric materials.

• The distance between energized points and grounded points significantly affects performance; this must be considered during installation.

Challenges with Glass Insulators

• Issues related to vandalism affecting glass insulators are addressed. Polymer alternatives provide better durability against such disturbances.

• Polymer insulators are described as more robust and less prone to damage from external factors like vandalism compared to glass counterparts.

Installation Guidelines for Poles in Distribution Networks

• Instructions on installing double T poles indicate that they should face the direction with lesser resistance when no deflection angle exists, optimizing structural integrity under load conditions.

Understanding Network Structures and Load Resistance

Key Concepts in Network Structure Design

• The angulation of the network structure is crucial; a slight angle can be beneficial depending on the situation, particularly when positioning the drawer side to favor the network.

• In double T-post structures without longitudinal stays, the side with greater resistance should face the direction of the network to manage load effectively.

• For structures that bear weight from networks, such as double T-posts, it's essential that the top side faces upward to support significant loads.

• In three-phase networks with double T-posts and longitudinal stays, similar principles apply: position the stronger side towards the network for enhanced safety and load management.

• The design must ensure that one side of a post supports more weight; thus, it’s critical to orient it correctly based on structural demands.

Structural Considerations for Monophase Networks

• For single-phase networks using double T-post structures with longitudinal stays, place the weaker side facing towards the network due to reduced cable tension compared to three-phase setups.

• The question arises regarding which structures are suitable for header configurations in R Dr systems with specific cable sizes (1/0 AWG and 4/0 AWG).

• M3 and M4 IPE type structures have limitations for conductors up to 10 AWG; N3 and N4 types are recommended for larger conductors like 1/0 AWG.

Types of Structures Used in Header Configurations

• Only HT and Ágape types should be used for encabeçamento due to their ability to handle heavier loads effectively.

• HT structures are discussed extensively in previous materials; they provide robust support necessary for heavy cables like 4/0 AWG or 336.4 MCM.

• Observing different header configurations reveals how multiple posts can distribute weight better than simpler designs, allowing for increased load capacity.

Limitations and Guidelines on Maximum Spans

• Understanding various header configurations helps clarify why certain designs can accommodate more weight than others—specifically those designed with multiple supporting posts.

• It is emphasized that only HP-type structures should be utilized when dealing with heavy cables due to their superior strength characteristics under load conditions.

• A representation of a four-header structure illustrates its capability compared to simpler designs like Finger structures which have reduced spans and capacities.

Conclusion on Structural Integrity

• The discussion concludes by highlighting how different structural designs impact overall performance under varying loads, emphasizing careful selection based on intended use cases.

Understanding Cable Anchoring and Distances in Electrical Networks

Importance of Anchoring Cables

• The speaker emphasizes the necessity of anchoring cables to prevent continuous movement without support, indicating that they must rest at intervals.

• It is stated that cables should be anchored every 1,500 meters maximum to avoid excessive weight on the network, particularly for non-reinforced conductors.

Specifications for Different Circuit Types

• For monophase and triphase circuits using reinforced conductors (C A), a reduction in distance is required every 1,000 meters.

• Non-reinforced conductors have a stricter limit of 600 meters due to their inability to withstand heavy loads.

Distance Requirements Between Conductors

• The minimum distance between phase and neutral conductors in networks exceeding 300 meters is specified as 1.45 meters according to IP15 guidelines.

• This distance applies universally except for specific structures like BHT and HTTP types.

Structural Considerations

• The central phase's fixation point serves as a reference for measuring distances from the neutral conductor, which must maintain a minimum separation of 1.45 meters.

• The speaker illustrates how this measurement helps prevent accidents by ensuring safe distances from grounded components.

Safety Measures and Exceptions

• Emphasis is placed on grounding neutral conductors properly to mitigate risks associated with electrical faults or disturbances.

• An exception exists for certain structures (PHT and HTTP), where different spacing rules apply; however, the general guideline remains crucial for safety.

Conclusion and Call to Action

• The speaker expresses hope that viewers find the material helpful in their studies related to electrical engineering standards.