Autotransformers in EHV Power Systems: Theory, Maths & Practice

Autotransformers are among the most misunderstood yet critically important assets in modern power systems. While they appear deceptively simple at low voltages, at EHV levels (380–500 kV) they become system-defining components that directly influence fault levels, insulation coordination, grounding, and grid stability.

This article provides a complete, utility-grade explanation of autotransformers, covering theory, mathematics, IEC standards, and real-world design practice.

1. What Is an Autotransformer?

An autotransformer is a transformer in which part of the winding is common to both the primary (source) and secondary (load).

Unlike a conventional two-winding transformer:

There is no galvanic isolation
Power is transferred by:
- Magnetic induction
- Direct electrical conduction

This fundamental difference is the reason autotransformers are:

Smaller
More efficient
Cheaper
—but also more demanding from a system-engineering perspective.

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2. Autotransformer vs Two-Winding Transformer

Aspect	Two-Winding	Autotransformer
Electrical isolation	Yes	No
Power transfer	Induction only	Induction + conduction
Size & weight	Larger	Smaller
Losses	Higher	Lower
Fault level control	Better	Poorer
Typical use	Distribution	HV / EHV interties

3. Step-Up and Step-Down Autotransformers

Step-Up Autotransformer

Load voltage > Source voltage
Series winding in series with load
Common winding shared

Step-Down Autotransformer

Load voltage < Source voltage
Series winding in series with source
Common winding shared

At EHV level, both operate on the same principle — only scale and risk change.

4. Key Parts of an Autotransformer

Common Winding

Electrically shared
Transfers power inductively
Carries difference of currents

Series Winding

Carries full line current
Transfers power conductively

This division directly impacts:

Thermal design
Short-circuit withstand
Protection philosophy

5. Power Transfer Mathematics (Core Concept)

Total apparent power is divided into:

Transformed (Inductive) Power

$S_{trans} = S \times \frac{V_{HV} – V_{LV}}{V_{HV}}$ Strans=S×VHVVHV−VLV

Conducted Power

$S_{cond} = S – S_{trans}$ Scond=S−Strans

Only the transformed portion requires full magnetic coupling, which explains the material and efficiency advantage.

6. Example: 380 / 132 kV – 502.5 MVA Autotransformer

Line Currents

380 kV side $I_{380} = \frac{502.5 \times 10^6}{\sqrt{3} \times 380 \times 10^3} = 763 \text{ A}$ I380=3×380×103502.5×106=763 A

132 kV side $I_{132} = \frac{502.5 \times 10^6}{\sqrt{3} \times 132 \times 10^3} = 2198 \text{ A}$ I132=3×132×103502.5×106=2198 A

Power Split

$S_{trans} = 502.5 \times \frac{380 – 132}{380} = 328 \text{ MVA}$ Strans=502.5×380380−132=328 MVA $S_{cond} = 174.5 \text{ MVA}$ Scond=174.5 MVA

➡ Only ~65% of power is magnetically transformed

7. Autotransformers & IEC 60076

IEC 60076 fully governs autotransformers:

IEC 60076-1 – Ratings & general requirements
IEC 60076-2 – Temperature rise
IEC 60076-3 – Insulation & dielectric tests
IEC 60076-5 – Short-circuit withstand
IEC 60076-8 – Application guide

IEC clearly states:

The rated power of an autotransformer is the total apparent power, provided winding current limits are not exceeded.

8. Why Utilities Use Autotransformers at 380–500 kV

Autotransformers dominate EHV grids because:

Voltage ratio is relatively small (e.g. 500/380, 400/220)
Two-winding transformers become impractical
Material and transport constraints dominate
Efficiency > 99.7%

A 500/380 kV two-winding transformer would be:

Much larger
~30–40% more expensive
Harder to transport

9. Vector Group Explained – YNa0d1

For a 380/132/13.8 kV autotransformer:

Y – Star HV winding
N – Neutral brought out
a – Autotransformer (common winding)
0 – 0° phase shift between 380 & 132 kV
d1 – Delta tertiary, 30° phase displacement

This configuration ensures:

Grid synchronism
Harmonic suppression
Zero-sequence control

10. Delta Tertiary (13.8 kV) – Why It Exists

The tertiary winding is not optional.

Functions:

Absorbs 3rd harmonics
Provides zero-sequence path
Stabilizes voltage during unbalanced faults
Supplies:
- Shunt reactors
- Station service
- FACTS devices

Typical rating: 30–40% of main MVA

11. Grounding Philosophy (Critical at EHV)

Because there is no isolation:

Ground faults propagate between voltage levels
Neutral grounding defines fault current magnitude

Common practice:

Neutral grounded via reactor
Sometimes resistance grounded
Rarely solid at EHV

12. Protection Challenges of Autotransformers

Autotransformer protection is significantly more complex than two-winding transformers.

Mandatory protections:

Biased differential (87T)
Restricted earth fault (REF)
Overfluxing (V/Hz)
Neutral displacement
Buchholz
Sudden pressure
Temperature supervision

CT placement and zero-sequence compensation are design-critical.

13. Insulation Coordination & Surge Transfer

Key reality:

Lightning and switching surges on HV side partially appear on LV side.

Design implications:

LV insulation levels are increased
Surge arresters required on all windings
Coordination per IEC 60071

14. Fault Levels & System Impact

Autotransformers:

Have low impedance (typically 12–18%)
Do not block fault current
Increase short-circuit levels at lower voltage

This affects:

GIS ratings
Circuit breaker duty
Busbar design

15. Advantages of Autotransformers

✔ Higher efficiency
✔ Lower cost
✔ Smaller size
✔ Higher MVA capability
✔ Ideal for EHV interties

16. Disadvantages & Risks

❌ No electrical isolation
❌ Higher fault currents
❌ Surge transfer risk
❌ Complex protection
❌ Common winding failure impact

17. Where Autotransformers Should NOT Be Used

Large voltage ratios (e.g. 380/13.8 kV)
Distribution systems requiring isolation
Systems with strict fault-level limits

In such cases, two-winding transformers are mandatory.

18. Final Expert Takeaway

An autotransformer is not just a transformer — it is a grid-coupling element.

At EHV level, success depends on:

Mathematics
Insulation coordination
Protection engineering
Grounding design
IEC compliance

When designed correctly, an autotransformer delivers:
✔ Efficiency
✔ Reliability
✔ Long service life (>40 years)

When misunderstood, it becomes a system risk.