أسئلة حول المحولات الكهربائية

Chapter 16: Transformers and Equipment

• This chapter reviews aspects of transformers, including losses, loading characteristics, transformer cooling, interpretation of tests, and capacitors.
• Author: Stephen L. Cress is a Department Manager at Kinectrics, Inc. He has over 33 years of experience in power distribution. He holds a US patent on high-voltage current limiting fuses.
• Author: Ali Naderian has degrees from Sharif University of Technology and the University of Tehran. He has experience in high-voltage testing and diagnostics.
• Reviewers: This chapter has reviewers from Consolidated Edison, Northeast Utilities, and other power companies.

Abstract

• This chapter reviews transformer losses, loading characteristics, selection criteria for pad-mounted transformers, transformer cooling, and interpretation of tests on transformers and oil and capacitors.

Transformer Losses

• Losses are categorized as load and no-load losses.
• Load losses vary with the square of the load.
• No-load losses are continuous and constant regardless of load.
• No-load losses are associated with core design (core loss, dielectric loss, exciting current).
• For distribution transformers under 27.6 kV, dielectric loss is negligible.
• The no-load loss in the transformer core is dependent on voltage magnitude, frequency, and waveform.
• Core loss is composed of hysteresis and eddy current losses. Hysteresis loss depends on frequency and peak flux density. Eddy current loss is controlled by laminated cores.
• Load losses (copper loss) are primarily from current in the primary and secondary windings (I2R) and stray losses.
• I2R losses are a major component in distribution transformers
• Stray losses are affected by temperature.

Transformer Efficiency

• Transformer efficiency is the ratio of useful output power to total input power.
• Efficiency is typically between 98% and 99.5% efficient.
• Maximum efficiency generally occurs at 50% loading.
• Efficiency is often mandated by governing energy agencies.

Load Characteristics for Transformers

• Load characteristics are crucial for transformer selection.
• Loads vary by type (domestic, commercial, industrial, etc.).
• Load type affects the required size and characteristics of the transformer.
• Load profiles vary daily (e.g., single hump, double hump).
• Peak load periods are important for design considerations.
• Designing transformers to handle peak loads is crucial to avoid overheating.
• Diversity factor, load factor, and demand factor are important metrics related to load.
• Load diversity is the difference between the sum of individual maximum demands of loads on a system and the actual maximum demand on the system.
• Diversity, load factor, and demand factor are expressed mathematically.

Pad-Mounted Transformer Selection

• Rating considerations involve internal temperatures (hottest spot, top oil, average winding), loss of life, and total lifetime cost.
• Hot spot temperature is crucial in determining transformer life.
• Design criteria and standards are important to ensure appropriate ratings and suitable safety features.
• Overloading of transformers may affect transformer life and reliability.
• Overloading can be considered under emergency conditions.

Interpretation of Test Results

• Methods for testing transformers include oil tests, insulation resistance tests, and polarization index.
• Oil quality tests, such as dielectric strength, dissipation factor, and neutralization number are crucial in determining oil quality.
• Insulation tests such as insulation resistance and polarization index (PI) testing assess insulation condition.
• Values are used to determine the condition and possible problems in transformers.

Capacitors

• Capacitors are used to improve the power factor in distribution systems.
• They decrease energy losses in the system.
• Capacitors come in various types and configurations.
• Determining the appropriate number and type of capacitor is essential for efficiency and economics.
• Placement and sizing are also critical components in distribution system design.

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