7.4 Transformer applications in power systems

Transformers are the unsung heroes of power systems, quietly working behind the scenes to keep electricity flowing smoothly. They're like voltage shape-shifters, morphing high voltages into lower ones and vice versa, making long-distance power transmission possible and safe.

But transformers aren't just one-trick ponies. They come in all shapes and sizes, from tiny isolation transformers keeping your gadgets safe to massive three-phase beasts powering entire cities. Some even have special talents, like measuring crazy-high currents or voltages without breaking a sweat.

Voltage Conversion and Isolation

Power Transmission and Distribution Systems

• Transformers play a crucial role in efficient electricity transmission over long distances
• Step-up transformers increase voltage at power plants for long-distance transmission
• Step-down transformers reduce voltage for local distribution networks
• High-voltage transmission minimizes power losses due to reduced current flow
• Substations use multiple transformers to gradually step down voltage for end-users

Voltage Level Conversion Techniques

• Transformers adjust voltage levels between different parts of the power system
• Primary to secondary winding turn ratio determines voltage transformation
• Step-up transformers increase voltage (fewer primary turns than secondary)
• Step-down transformers decrease voltage (more primary turns than secondary)
• Voltage conversion formula: $V_s = V_p \times (N_s / N_p)$
• Power remains constant during ideal voltage conversion: $P_p = P_s$

Isolation and Safety Applications

• Isolation transformers provide galvanic separation between primary and secondary circuits
• Prevent direct electrical connection between input and output
• Enhance safety by blocking DC voltages and reducing noise
• Commonly used in medical equipment and sensitive electronic devices
• Typically have 1:1 turn ratio for maintaining the same voltage level
• Autotransformers offer compact design for small voltage adjustments
• Share a common winding between primary and secondary sides
• More efficient than standard transformers for small voltage changes
• Used in variable voltage power supplies and motor starting circuits

Specialized Transformers

Instrument Transformers for Measurement

• Current transformers (CTs) measure high currents safely
  • Connect in series with the load
  • Provide proportionally reduced current for measurement devices
  • Typical ratios include 100:5 A or 1000:1 A
• Voltage transformers (VTs) measure high voltages safely
  • Connect in parallel with the circuit
  • Step down voltage for measurement equipment
  • Common ratios include 11000:110 V or 33000:110 V
• Both types improve safety and enable use of standardized measuring instruments
• Accuracy classes define maximum allowable errors (0.1%, 0.2%, 0.5%, etc.)

Three-Phase Transformer Configurations

• Combine three single-phase transformers or use a single three-phase core
• Common connections include delta-delta, wye-wye, and delta-wye
• Delta connection forms a triangle with windings
  • Provides good harmonic suppression
  • Allows operation with one phase damaged in emergencies
• Wye (star) connection has a common neutral point
  • Provides access to both line-to-line and line-to-neutral voltages
  • Commonly used for distribution transformers
• Phase shift occurs in delta-wye connections (typically 30° lag)
• Vector group notation describes winding connections and phase relationships

Tap Changers for Voltage Regulation

• Allow adjustment of transformer turn ratio during operation
• On-load tap changers (OLTC) can change taps while energized
  • Use complex switching mechanisms to maintain power flow
  • Common in large power transformers for grid voltage control
• Off-circuit tap changers require de-energizing the transformer
  • Simpler design with fixed tap positions
  • Used in distribution transformers for occasional voltage adjustments
• Typical tap range is ±5% to ±10% of nominal voltage in 2.5% steps
• Electronic tap changers use solid-state switches for faster, more precise control
• Automatic voltage regulators use tap changers to maintain stable output voltage