Transformers and Power Distribution

Transformers and Power Distribution

Chapter Introduction

This chapter covers all the knowledge required for the Red Seal exam concerning transformers and electrical power distribution. You will find the fundamental principles, common configurations, essential calculations, the requirements of the Canadian Electrical Code (CE Code), and typical pitfalls to avoid. Mastering this content is essential, as questions on this topic represent a significant portion of the exam.

Fundamental Principles of Transformers

Faraday's Law and Electromagnetic Induction

A transformer operates on the principle of electromagnetic induction discovered by Michael Faraday. When an alternating current flows through the primary winding, it creates a varying magnetic flux in the ferromagnetic core. This varying flux induces an electromotive force (EMF) in the secondary winding.

The fundamental relationship is expressed by Faraday's Law:

E = N × (dΦ/dt)

Where:

Transformation Ratio

The transformation ratio (a) is the ratio between the number of turns in the primary and that of the secondary:

a = N₁/N₂ = V₁/V₂ = I₂/I₁

For an ideal transformer (no losses):

Calculation Example: A transformer has 480 turns on the primary and 120 turns on the secondary. If the primary voltage is 600 V, what is the secondary voltage?

V₂ = V₁ × (N₂/N₁) = 600 × (120/480) = 600 × 0.25 = 150 V

Types of Transformers by Application

Three-Phase Transformers

Connection Configurations

Three-phase transformers can be connected in various configurations. Each one has specific advantages and disadvantages.

Wye-Wye (Y-Y)

Delta-Wye (Δ-Y)

Wye-Delta (Y-Δ)

Delta-Delta (Δ-Δ)

Clock Hour Index

The clock hour index indicates the phase shift between primary and secondary voltages. It is expressed in multiples of 30°. For example, a Δ-Y connection with a 30° phase shift is designated Dyn1 or Dyn11 depending on the direction of the phase shift.

Power and Current Calculations

Apparent, Active, and Reactive Power

For a balanced three-phase system:

Calculating Rated Current

Example: A three-phase transformer rated 750 kVA, 600 V/208 V, Δ-Y connection. Calculate the rated current on the secondary.

I₂ = S / (√3 × V₂) = 750,000 / (1.732 × 208) = 750,000 / 360.3 = 2,082 A

Transformer Efficiency

η = (P_output / P_input) × 100%

Losses are divided into:

Maximum efficiency occurs when copper losses equal iron losses.

Grounding and Bonding

Canadian Electrical Code Requirements

The Canadian Electrical Code, Part I (CE Code) imposes specific rules for the grounding of transformers.

Rule 10-204 — Grounding of Transformers

The metal enclosure and core of every transformer must be grounded in accordance with the rules in Section 10. The secondary neutral must be connected to ground at the service point or at the transformer itself.

Rule 10-206 — Grounding Conductor

The grounding conductor must be:

Rule 10-208 — Grounding Electrode

The electrode must have a resistance of 25 Ω or less. If this value cannot be achieved, an additional electrode must be installed.

Grounding System Schemes (System Grounding Types)

Grounding systems are classified by three letters:

Transformer Protection

Overcurrent Protection

CE Code Rule 26-248

This rule requires that every transformer be protected against overcurrents on the primary side. The protective device must be set:

If the exact setting is not available, you may round up to the next standard size.

Secondary Protection

Secondary protection is required when:

Protection Against Internal Faults

Instrument Transformers

Current Transformers (CTs)

Current transformers reduce high currents to measurable values (typically 5 A or 1 A on the secondary).

Important characteristics:

Potential Transformers (PTs)

Potential transformers reduce high voltages to measurable values (typically 120 V or 69.3 V on the secondary).

Power Distribution in Industrial Buildings

Typical Distribution Systems

Rule 8-200 — Load Calculations

Rule 8-200 of the CE Code specifies minimum demands for load calculations. Demand factors apply according to the type of load:

Conductors and Raceways

Rule 4-004 — Ampacity of Conductors

The ampacity of conductors is determined according to Tables 1 to 4 of the CE Code, based on:

Correction Factors

Harmonics and Power Quality

Sources of Harmonics

Non-linear loads (variable frequency drives, switching power supplies, electronic lighting) generate harmonic currents that:

K-Factor Transformers

K-factor transformers are designed to withstand harmonic currents. The K-factor indicates the transformer's ability to handle harmonics:

Rule 26-250 — Transformers and Harmonics

The CE Code requires that the neutral of 3-phase, 4-wire systems supplying non-linear loads be sized to handle the resulting neutral current. In some cases, the neutral must be full size or even oversized.

Testing and Commissioning

Tests Prior to Energization

Energized Tests

Energization Procedure

Pitfalls to Avoid

Summary

Review Questions

This chapter covers the essential concepts for the Red Seal exam. Make sure you master the calculations, CE Code rules, and safety procedures before moving on to the exam.