Voltages in Power Transmission Lines or Transmission Voltages

In power stations, electrical power is generated at medium voltage levels, typically between 11 kV and 25 kV.

This generated power is sent to a step-up transformer to increase the voltage. From here to the user end, the voltage varies at different stages. Let’s explore these steps.

• A voltage level between 11 kV and 25 kV is maintained at the alternator stator terminals to generate electrical power in the station.
• This generated power is fed to the generating step up transformer to make this medium voltage level to higher level, i.e. up to 33 kV.
• Power at 33 kV is sent to the generating substation. There the transformer increases the voltage level to 66 kV or 132 kV.
• From the generating substation, power is sent to a nearby substation to increase the voltage further, possibly to 400 kV, 765 kV, or even 1000 kV. These high voltage levels are used for long-distance transmission, known as primary transmission.
• At the end point of primary transmission of power, in the substation, the step down transformers are used to step down the voltage level to 132 kV. Secondary transmission of power starts from this substation.
• Power transformer at the end of the secondary transmission, just makes 132 kV voltage level steps down to 33 kv or 11 kV as per requirement. From this point, the primary distribution of power starts to distribute power to different distribution stations.
• At the end of the primary distribution, the distribution stations receive this power and step down this voltage level of 11 kV or 33 kV to 415 V (Line Voltage). From these distribution stations to consumer ends, 415 V is kept to sustain for utilization purpose.

Type of Power Lines

From the very beginning of power generation to the user end transmission lines are broadly classified based on different voltage levels.

Why High Voltage is used for Long Transmission Line?

Generally long distant transmission lines are designed to operate at high voltage, extra high or ultra high voltage level. It is because of line power loss reduction purpose.

Practically long distant transmission line resistance is comparatively more than medium and short transmission line. Due to this higher valued transmission line resistance considerable amount of power is lost. So we need to decrease the amount of current through each conductor by making the operating voltage very high for same amount of power transmission.
We know that the power in AC system to transmit is

Total power loss P_Loss = 3I_L²R considering three phases altogether.
R is the resistance in ohm per phase of the transmission line.
Now, rearranging Equation (1) we get,

So,

Again in DC system, there is no phase difference between voltage and current, i.e. cosƟ = 1, and only two conductors (positive and negative) are used. So, in DC system transmitted power P = VI, and power loss

From equations (2) and (3), we see that power loss in a transmission line is inversely proportional to the square of the line voltage. Higher line voltage means less power loss. Therefore, thinner conductors can be used, saving material costs.

Why HVAC is Used for Long Transmission Line?

Now-a-days electrical energy is generated, transmitted and distributed in AC form. Especially for long distant transmission line high voltage AC is transmitted for several reasons, they are:

1. AC voltage can be stepped up or down as per requirement easily by transformer.
2. Maintenance of AC substation is easy and cheaper.
3. Throughout electrical power system AC voltage is handled. So no extra hazard of rectification or inversion like DC voltage transmission.

Why HVDC is Used for Long Transmission Line?

High Voltage DC is used at extra or ultra high voltage level. HVDC transmission is used at fixed level of voltage in primary transmission only as it cannot be stepped up or down by transformer. Only in long distant transmission line it is used only, because:

1. Only two conductors (positive and negative) are required as compared to three of AC transmission.
2. The absence of inductance, capacitance and phase displacement power loss is very less. Hence better voltage regulation.
3. Surge problem never occurs.
4. No skin effect.
5. Less insulation requires due to less potential stress.
6. Less corona discharge (i.e. the corona effect), and hence less power loss.
7. Highly stabilized and synchronized.

Why Low and Medium Voltage is Used in Distribution Line?

In primary distribution, power is handled at 11 kV or 33 kV. As voltage level gets stepped down from 132 kV to 11 kV or 33 kV, current level gets higher valued. But this high valued current distributed among various local distribution stations (distribution transformers) nearby. These distribution transformers again steps down the voltage to 415 V. It is because; Power at 415 V is used at the user end. Distance between these distribution transformers and the primary distribution stations is very short, hence conductor resistance is not large. Very small amount of power is lost in this section.

Disadvantages of AC or HVAC Transmission

The main disadvantages of AC transmission are

1. AC lines require more conductor material than DC.
2. AC transmission line construction is more complicated than DC.
3. Effective resistance is increased due to skin effect, hence power loss.
4. Continuous power loss due to charging current because of line capacitance.

Disadvantages of DC or HVDC Transmission

The main disadvantages of DC transmission are

1. Electric power is not generated in HVDC form due to commutation problem. Only HVDC is achieved for transmission from HVAC by rectification. So special arrangement is required for this conversion.
2. DC voltage cannot be stepped up or down for transmission.
3. DC switches and circuit breakers are expensive and with certain limitations.