Power Systems for SSC JE Electrical: Transmission Lines, Insulators, Corona, Sag, Cables, Distribution and Load Factors

Power systems cover the generation, transmission and distribution of electrical energy, and SSC JE Electrical asks about the voltage levels, the factors, the transmission line performance, the insulators and the cables. This guide on power systems for SSC JE covers all of these in tables, with solved examples, practice questions and FAQs.

Structure of the Power System

1. Generation (thermal, hydro, nuclear and renewable) at 11 kV to 33 kV.
2. Step-up transformers raise the voltage for transmission.
3. Transmission at extra high voltage (EHV).
4. Sub-transmission and step-down at grid sub-stations.
5. Distribution at 11 kV, then 415 V (three-phase) and 230 V (single-phase) to consumers.

Voltage Levels in India

• The frequency in India is 50 Hz; the Indian grid is organised into regional grids that are now synchronously connected as a single national grid (the Southern Grid joined in December 2013).
• Power Grid Corporation of India (POWERGRID) is the central transmission utility.
• High-voltage transmission reduces the current for a given power and hence the I²R loss and the conductor cost.

Load Curve and Factors

Transmission Lines

• Line parameters: resistance (R), inductance (L), capacitance (C) and leakage conductance (G).
• Classification by length: short line (up to 80 km; only R and L are considered), medium line (80 to 250 km; nominal T or nominal π model) and long line (above 250 km; distributed parameters).
• ABCD constants: for a two-port network, V_s = A V_r + B I_r; AD − BC = 1.
• Voltage regulation = (V_NL − V_FL)/V_FL × 100.
• Ferranti effect: the receiving-end voltage is higher than the sending-end voltage on a long, lightly loaded or open-circuited line because of the line capacitance.
• Skin effect: the AC current flows mainly near the surface of a conductor; it increases the effective resistance; it is more at a high frequency and for large conductors.
• Proximity effect: the current distribution in a conductor is changed by the nearby conductor.
• Transmission efficiency = receiving-end power / sending-end power.
• A long transmission line is compensated by shunt reactors (to absorb the reactive power) or series capacitors.

Corona

• Corona is the ionisation of the air around a conductor when the electric field strength exceeds the breakdown strength of the air (about 30 kV/cm peak or 21.1 kV/cm rms).
• It produces a hissing sound, a violet glow, ozone, radio interference and a power loss.
• Factors affecting corona: the supply voltage, conductor size and spacing, surface condition, atmosphere (humidity increases the loss).
• Corona is reduced by using larger conductors, bundled conductors and a greater spacing between conductors.

Insulators

• Porcelain is the most common material for insulators; glass and polymer (composite) insulators are also used.
• String efficiency = voltage across the whole string / (n × voltage across the unit nearest to the conductor); it is always less than 100% because the voltage distribution is not equal due to the shunt capacitance; the unit nearest to the conductor is subjected to the maximum voltage.
• String efficiency improves with a longer cross-arm, grading of the insulators (different capacitances) and a guard ring (static shielding).
• The string efficiency is less for a longer string.

Sag in Overhead Lines

Sag is the vertical distance between the highest support and the lowest point of the conductor. For the level supports, S = w l² / (8T), where w is the weight per unit length, l is the span and T is the tension. A greater tension gives a lower sag; the sag is more in summer (higher temperature) and with the wind and ice load.

Underground Cables

• Parts of a cable: conductor (copper or aluminium), insulation, metallic sheath, bedding, armouring and serving.
• Insulating materials: paper (PILC – paper-insulated lead-covered), PVC, XLPE (cross-linked polyethylene, used up to 220 kV) and rubber.
• Types of cables by voltage: low tension (up to 1 kV), high tension (up to 11 kV), super tension (up to 33 kV), extra high tension (up to 66 kV), extra super voltage (above 132 kV).
• Underground cables are costlier than overhead lines but safer and more aesthetic; they are used in cities.
• Faults in a cable are located by the Murray loop and Varley loop tests.

Distribution

• Feeder carries the power from the sub-station to the distributor with no tapping; distributor supplies the consumers with tappings; service mains connect the distributor to the consumer.
• Kelvin’s law: the most economical conductor size is that for which the annual cost of the energy loss is equal to the annual interest and depreciation charges on the capital cost.
• Tariffs: flat rate, block rate, two-part (fixed charge + running charge), maximum demand and three-part tariffs.
• Power factor improvement is done with static capacitors, synchronous condensers and phase advancers.
• Earthing protects the people and equipment; the earth resistance should be low.

Solved Examples

Example 1. The maximum demand is 50 MW and the average load is 30 MW. Find the load factor.
30/50 = 0.6.

Example 2. A conductor of weight 2 kg/m is on a span of 200 m with a tension of 2,000 kg. Find the sag.
S = 2 × 200² / (8 × 2000) = 5 m.

Example 3. Why does the receiving-end voltage rise on a lightly loaded long line?
Because of the Ferranti effect (the line capacitance).

Example 4. What is the use of a bundled conductor?
To reduce the corona loss and the line reactance.

Quick Tips

• Memorise the definitions of the load factor, demand factor and diversity factor.
• Remember that the diversity factor is always greater than 1.
• Remember the formula for sag.
• Remember that the unit near the conductor has the maximum voltage in a string.

Quick Revision Points

• The load factor is always less than 1; the diversity factor is greater than 1.
• The Ferranti effect occurs on a long, lightly loaded line.
• Corona is reduced by bundled conductors.
• Suspension insulators are used above 33 kV.
• Sag = wl²/8T.

Practice Questions With Answers

1. What is the load factor?
   Answer: Average load / maximum demand
2. What is the diversity factor always?
   Answer: Greater than 1
3. Which effect causes a rise of the receiving-end voltage on long lines?
   Answer: Ferranti effect
4. Which insulators are used above 33 kV?
   Answer: Suspension (disc) insulators
5. How is corona loss reduced?
   Answer: By bundled conductors and a larger conductor diameter
6. What is the formula for sag?
   Answer: S = wl²/8T
7. Which unit of a string has the maximum voltage?
   Answer: The unit nearest to the conductor
8. What is the length of a short transmission line?
   Answer: Up to 80 km
9. Which cable insulation is used for up to 220 kV?
   Answer: XLPE
10. What is the nominal voltage of primary distribution?
    Answer: 11 kV

How to Practise This Chapter

Practise 10 questions every day and write the steps on paper. Then attempt timed sets from our practice papers and the previous year papers page. See all topics on the Electrical Engineering notes page, read the previous topic Synchronous Machines and the next topic Switchgear, Protection and Circuit Breakers.

Frequently Asked Questions

What is the load factor?

The ratio of the average load to the maximum demand.

What is the diversity factor?

The ratio of the sum of the individual maximum demands to the maximum demand of the system.

What is the Ferranti effect?

The rise in the voltage at the receiving end of a lightly loaded long line.

What is corona?

The ionisation of the air around a conductor at a high voltage.

What is the skin effect?

The tendency of the AC to flow near the surface of the conductor.

What are the types of insulators?

Pin, suspension, strain and shackle insulators.

What is string efficiency?

The ratio of the voltage across the string to n times the voltage across the unit nearest to the conductor.

What is sag?

The vertical distance between the highest support point and the lowest point of the conductor.

What is Kelvin’s law?

The economical conductor size is where the cost of the energy loss equals the interest and depreciation.

Is this topic asked in SSC JE?

Yes. Power systems are asked in every SSC JE Electrical paper.

What is a feeder?

A conductor that carries the power from the sub-station to the distribution area without tapping.

What is a ring main system?

A distribution system in which the feeder forms a closed loop and is fed from two sides.