Voltage level in Power lines

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Voltage Levels in Power Transmission and Distribution

As an Electrical Engineer, one of the most important concepts to understand in power systems is how voltage levels are varied at different stages of generation, transmission, and distribution. Let’s walk through the process step by step.



1. Voltage Levels in Power Generation

At the generating station, electrical power is produced at a medium voltage level, typically in the range of 11 kV to 25 kV.

  • This voltage is available at the stator terminals of the alternators.

  • However, this level is not suitable for long-distance transmission, as losses would be excessively high.

Therefore, the generated power is immediately passed through a step-up transformer, raising the voltage to 33 kV or higher.

2. Primary Transmission

From the generating substation, the voltage is stepped up further by transmission transformers:

  • 33 kV → 66 kV → 132 kV → 220 kV → 400 kV → 765 kV → even up to 1000 kV (UHV level).

This extra high voltage (EHV) or ultra-high voltage (UHV) is necessary for long-distance transmission, as higher voltage minimizes transmission losses.

The main purpose of this stage is bulk power transfer over hundreds of kilometers with minimum losses.

3. Secondary Transmission

At the receiving end of the primary transmission, substations step down the voltage:

  • From 400 kV / 220 kV → 132 kV.

This marks the beginning of secondary transmission, which typically supplies regional substations.

4. Primary Distribution

At the end of secondary transmission, power transformers step the voltage further down:

  • 132 kV → 33 kV or 11 kV.

This begins the primary distribution stage, which feeds various local distribution substations.

5. Secondary Distribution

Finally, local distribution transformers reduce the voltage:

  • 11 kV / 33 kV → 415 V (line voltage, i.e., 230 V phase-to-neutral).

This 415 V / 230 V supply is what reaches residential, commercial, and small industrial consumers.

Why Higher Voltage is Used in Long Transmission Lines?

Transmission line losses are proportional to the square of the current:

PLoss=3I2RP_{Loss} = 3 I^2 R

Where:

  • II = line current

  • RR = resistance per phase

Since transmitted power P=3VIcos⁡θP = \sqrt{3} VI \cos \theta,

I=P3Vcos⁡θI = \frac{P}{\sqrt{3} V \cos \theta}

Thus,

PLoss∝1V2P_{Loss} \propto \frac{1}{V^2}

👉 This means that higher transmission voltage results in much lower power losses.
👉 It also reduces conductor size, saving material cost.

AC vs DC in Long-Distance Transmission

Why HVAC (High Voltage AC) is Widely Used?

  • Voltage transformation is simple with transformers (step-up or step-down as required).

  • AC substations are cheaper and easier to maintain.

  • The entire power system (generation, transmission, distribution) is designed for AC.

  • No need for additional converters as in HVDC systems.

Why HVDC is Sometimes Preferred?

In certain very long-distance or special cases, High Voltage DC (HVDC) transmission is more economical. Its advantages include:

  • Requires only two conductors (positive and negative).

  • No skin effect → lower effective resistance.

  • No reactive power loss due to capacitance or inductance.

  • No phase angle issues → better voltage regulation.

  • No surge problems.

  • Lower insulation requirements.

  • Reduced corona effect.

  • Very stable and easier to synchronize.

Limitations of Each System

Disadvantages of HVAC Transmission:

  • Requires more conductor material compared to DC.

  • Line charging current due to capacitance increases continuously.

  • Skin effect increases effective resistance.

  • Reactive power management is necessary.

Disadvantages of HVDC Transmission:

  • Power cannot be generated directly in DC form; it must be converted from AC.

  • Conversion equipment (rectifiers & inverters) is expensive and complex.

  • Voltage cannot be stepped up or down using simple transformers.

  • DC switchgear and circuit breakers are costly and have operational limitations.

Why Distribution Uses Lower Voltages?

At the distribution level:

  • Power is handled at 11 kV or 33 kV, and then stepped down to 415 V for end users.

  • Since distances are short, conductor resistance is not very large.

  • Although the current is higher at lower voltages, the distribution losses are minimal due to short distances.

Hence, low and medium voltages are more practical and economical for final delivery to consumers.

Conclusion

The electrical power system is carefully structured with different voltage levels at each stage:

  • Generation (11–25 kV)

  • Primary Transmission (220–765 kV or above)

  • Secondary Transmission (132 kV)

  • Primary Distribution (11–33 kV)

  • Secondary Distribution (415 V / 230 V)

The use of higher voltages in transmission ensures lower losses and efficiency in bulk transfer, while lower voltages in distribution ensure safety and practicality for end-users.

⚡ Disclaimer:
This explanation is for educational and professional reference purposes only. Actual voltage levels, equipment ratings, and grid configurations may vary based on country standards, regulatory codes, and utility practices. Always consult local grid codes, IS/IEC/IEEE standards, and safety regulations when working on power systems.


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