Indian electricity rules:
Wooden poles:
Note:
Span: Span means the horizontal distance between two adjacent supporting points of an overhead conductor.
Normal span in meters \(= C\sqrt {\frac{{P - L}}{D}}\)
Where,
P = height of conductor support for which the normal span is to be calculated (meter)
L = conductor clearance above level ground (meter)
C = ruling span (meter)
D = conductor sag for ruling span C (meter)
As per Indian Electricity rule in overhead systems, the recommended span of various support poles is given below
Line-support | Recommended span |
Wooden pole | 40-50 meters |
RCC pole | 80 - 100 meters |
Steel pole | 50 to 80 meters |
Steel tower | 300 meters or above |
Three-phase system has the following advantages as compared to the single-phase system:
The supply frequency of a single-phase AC system in India is 50 Hz.
To develop a polyphase system, the armature winding in a generator is divided into the required number of phases.Fig. below shows a 2 wire DC distributor cable AC of 4 km long supplying loads of 100 A and 200 A at distances of 2 km and 4 km from A. The feeder is fed at point A with a voltage of 500 V. The voltage available at the farthest point in the system is ______.
(Assume conductor resistance per km as 0.02 Ω).
Concept:
On the basis of how DC distributors are fed by the feeders, they are classified as:
→ Distributor fed at one end
→ Distributor fed at both ends
→ Distributor fed at the centre.
→ Ring distributor.
Now the type of distribution given in the question is of type “Distributor fed at one end”.
Points to remember in this type of distribution:
1) The current in the various sections of the distributor away from the feeding point goes on decreasing. Thus the current in the section PQ is more than current in the section QR and the current in the section QR is more than current in section RS.
2) The voltage across the loads away from the feeding point goes on decreasing. Therefore minimum voltage occurs at point S.
3) In case a fault occurs at/on any section of the distributor, the whole distributor will have to be disconnected from the supply mains.
Calculations:
Given- conductor resistance per km = 0.02 Ω
But in 2 wire DC distributor system 2 conductors are present
∴ Resistance per km for 2 wire DC distributor = 0.02 × 2 = 0.04 Ω
∴ Resistance of section AB = 0.04 × 2 = 0.08 Ω (R_{AB})
∴ Resistance of section BC = 0.04 × 2 = 0.08 Ω (R_{BC})
Also, I_{2} = 200 A, I_{1} = 100 A
∴ Current in section AB = I_{1} + I_{2} = 100 + 200 = 300 A
∴ current in section BC = I_{2} = 200 A
i.e. I_{AB} = 300 A, I_{BC} = 200 A
Now, Voltage available at load point B
V_{B} = Voltage at A – Voltage drop in AB
V_{B} = 500 V – I_{AB} × R_{AB}
V_{B} = 500 V – (300 × 0.08) V
V_{B} = (500 - 24) V
V_{B} = 476 V
Now, voltage available at point C
V_{C} = voltage at B – voltage drop in BC
V_{C} = 476 V – I_{BC} × R_{BC}
V_{C} = 476 V – (200 × 0.08) V
V_{C }= 476 V – 16 V
V_{C} = 460 V
Therefore the voltage available at the farthest point (C) in the system is 460 V.
Note:
There are a few advantages of other types of the distribution system.
Fig. Distributor fed at the center
Explanation:
Direct Current 2-wire System with Mid-point Earthed:
The DC 2-wire system with mid-point earthed is called DC 2-wire balance system.
Such a system is shown below.
In which, line voltage = 2V
Line current, \({I_\;} = \frac{P}{{2V}} \)
If R be the resistance per conductor than the total loss for both conductor is:
\({P_L} = 2{I^2}R = 2{\left( {\frac{P}{{2V}}} \right)^2}R = \frac{{{P^2}}}{{2{V^2}}}R\)
Important Points
Power Loss in various Supply System:
Let P amount of power is to be transmitted at voltage level V and the power factor of the system is cosϕ.
Types of System |
Loss in watt |
DC 2-wire system with one wire earthed |
\(2{\left( {\frac{P}{V}} \right)^2}R \) |
DC 2-wire system with mid-point earthed |
\(\frac{{{P^2}}}{{2{V^2}}}R \) |
DC 3-wire system |
\(\frac{{{P^2}}}{{2{V^2}}}R \) |
Single-phase AC 2-wire system with one earthed |
\(4{\left( {\frac{P}{{Vcos\phi }}} \right)^2}R \) |
Single-phase AC 2-wire system with mid-point earthed |
\({\left( {\frac{P}{{cos\phi }}} \right)^2}R \) |
Three-phase AC 3-wire star connected system |
\(\frac{2}{3}{\left( {\frac{P}{{Vcos\phi }}} \right)^2}R \) |
______ voltages can be available from a 3 wire DC distribution system.
Three Wire DC Distribution Systems:
This system leaves the following connection choices to a consumer:
Important Point:
Below is given the table which shows the ratio of conductor-material in any system compared with that in the corresponding 2-wire DC system. Cos φ is the power factor in an AC system.
System |
Same maximum voltage to earth |
Same maximum voltage between conductors |
DC system: Two-wire |
1 |
1 |
DC: Two-wire mid-point earthed |
0.25 |
1 |
DC: 3 wire |
0.3125 |
1.25 |
Single-phase: 2 wire |
2/cos2ϕ |
2/cos2ϕ |
Single-phase: 2 wire mid-point earthed |
0.5/cos2ϕ |
2/cos2ϕ |
Single-phase: 3 wire |
0.625/cos2ϕ |
2.5/cos2ϕ |
2-phase: 4 wire |
0.5/cos2ϕ |
2/cos2ϕ |
2-phase: 3 wire |
1.457/cos2ϕ |
2.914/cos2ϕ |
3 phase, 3 wire |
0.5/cos2ϕ |
1.5/cos2ϕ |
3 phase, 4 wire |
0.667/cos2ϕ |
2/cos2ϕ |
The major advantage of a radial distribution system is:
Radial Distribution System:
This system is used only when the substation is located at the center of the consumers. In this system, different feeders radiate from a substation and feed the distributors at one end.
Thus, the main characteristic of a radial distribution system is that the power flow is in only one direction.
The major advantage of a radial distribution system is that it is simplest system and has the lowest initial cost.
Major drawbacks of a radial distribution system are,
Ring main system:
In this system, each distribution transformer is fed with two feeders but in different paths. The feeders in this system form a loop which starts from the substation bus-bars run through the load area feeding distribution transformers and returns to the substation bus-bars.
Ring main distribution system is the most preferred due to its following advantages:
Interconnected distribution system:
When a ring main feeder is energized by two or more substations, it is called as an interconnected distribution system. This system ensures reliability in an event of transmission failure. Also, any area fed from one generating station during peak load hours can be fed from the other generating station or substation for meeting power requirements from the increased load.
Total current supplied by the distributor, I = 0.5 × 1000 = 500 A
Total resistance of the distributor R = 2 × 0.05 × 1 = 0.1 Ω
For a uniformly loaded DC - distributed wire fed from both sides with equal voltages, the V_{min} occurs at the mid-point (x = l/2).
So, the maximum voltage drop is at the mid-point
V_{drop }= \({i.r \times{(l-x)^2} \over 2}={i.r \times{(l-\frac{l}{2})^2} \over 2}={{i.l \times r.l} \over 2}\)
Maximum voltage drop
\(= \frac{{IR}}{8} = \frac{{500 \times 0.1}}{8} = 6.25\;V\)
Economic Distance For HVDC distribution lines:
Advantages of HVDC distribution:
Disadvantages of HVDC distribution:
Conclusion:
Difference between HVDC and HVAC distribution system
HVDC distribution System |
HVAC distribution System |
Low losses. |
Losses are high due to the skin effect and corona discharge |
Better Voltage Regulation and Control ability. |
Voltage regulation and Control ability are less. |
Transmit more power over a longer distance. |
Transmit less power compared to an HVDC system. |
Less insulation is needed. |
More insulation is required. |
Reliability is high. |
Low Reliability. |
Asynchronous interconnection is possible. |
Asynchronous interconnection is not possible. |
Reduced line cost due to fewer conductors. |
The line cost is high. |
Towers are cheaper, simple, and narrow. |
Towers are bigger compared to HVDC. |
Radial Distribution System:
This system is used only when the substation is located at the center of the consumers. In this system, different feeders radiate from a substation and feed the distributors at one end. Thus, the main characteristic of a radial distribution system is that the power flow is in only one direction. It is the simplest system and has the lowest initial cost but it is not highly reliable.
Major drawbacks of a radial distribution system are,
Ring main system:
In this system, each distribution transformer is fed with two feeders but in different paths. The feeders in this system form a loop which starts from the substation bus-bars run through the load area feeding distribution transformers and returns to the substation bus-bars.
Ring main distribution system is the most preferred due to its following advantages:
Interconnected distribution system:
When a ring main feeder is energized by two or more substations, it is called as an interconnected distribution system. This system ensures reliability in an event of transmission failure. Also, any area fed from one generating station during peak load hours can be fed from the other generating station or substation for meeting power requirements from the increased load.
Concept:
In a uniformly loaded distributor fed at one end, the maximum total voltage drop = IR/2
In a uniformly loaded distributor fed at both ends, the maximum total voltage drop = IR/8
The maximum voltage drop in the case of uniformly loaded distributor fed at both ends is one-fourth of the maximum voltage drop in the case of uniformly loaded distributor fed at one end.
Calculation:
Length of distributor = 200 m = 0.2 km
Current supplied by distributor = 2 amperes/meter
Total current supplied by distributor (I) = 200 × 2 = 400 A
The resistance of single wire = 0.3 Ω/km
Total resistance = 0.3 × 0.2 = 0.06 Ω
Maximum voltage drop \( = \frac{{IR}}{2} = \frac{1}{2} \times 400 \times 0.06 = 12\;V\)
Primary transmission :
The electric supply (132 kV, 220 kV, 500 kV or greater) is transmitted to load center by three-phase three-wire (3 phase - 3 wires) overhead transmission system.
Secondary transmission :
At the receiving station, the level of voltage reduced by step-down transformers up to 132 kV, 66 or 33 kV and electric power is transmitted by three-phase three-wire (3 phase - 3 wires) overhead system to different substations.
Primary distribution :
At a substation, the level of secondary transmission voltage (132KV, 66 or 33KV) is reduced to 11 kV (in a three-phase three-wire overhead system) by step down transformers.
Secondary distribution :
Important Point:
Below is given the table which shows the ratio of conductor-material in any system compared with that in the corresponding 2-wire DC system. Cos φ is the power factor in an AC system.
System |
Same maximum voltage to earth |
Same maximum voltage between conductors |
DC system: Two-wire |
1 |
1 |
DC: Two-wire mid-point earthed |
0.25 |
1 |
DC: 3 wire |
0.3125 |
1.25 |
Single-phase: 2 wire |
2/cos2ϕ |
2/cos2ϕ |
Single-phase: 2 wire mid-point earthed |
0.5/cos2ϕ |
2/cos2ϕ |
Single-phase: 3 wire |
0.625/cos2ϕ |
2.5/cos2ϕ |
2-phase: 4 wire |
0.5/cos2ϕ |
2/cos2ϕ |
2-phase: 3 wire |
1.457/cos2ϕ |
2.914/cos2ϕ |
3 phase, 3 wire |
0.5/cos2ϕ |
1.5/cos2ϕ |
3 phase, 3 wire |
0.583/cos2ϕ |
1.75/cos2ϕ |
Concept:
\(R = \frac{{\rho l}}{A}\)
R = resistance, l = length, A = area of cross-section and ρ = resistivity
SI unit of resistance is the ohm (Ω).
Voltage drop = I R
Active power = V I cos ϕ
Application:
Given V2 = n V1
⇒ V2 / V1 = n
Since there is no change in the connected load, to maintain constant power transfer,
P2 = P1
V2 I2 cos ϕ2 = V1 I1 cos ϕ1
Since the type of connected load is also the same, the power factor is also constant.
⇒ I2 = I1 / n
Given that the voltage drop is same,
I2 R2 = I1 R1
R2 = n R1
ρl / A2 = nρl / A1
A2 = A1 / n
Hence, reduces the x-section of the conductor by n-times
A uniformly loaded DC distributor is fed at both ends with equal voltages. As compared to a similar distributor fed at one end only, the drop at the middle point is:
When distributed is fed at one end only, the voltage drop at the middle point is \( = \frac{{IR}}{2}\)
When distributed is fed at both ends with equal voltages, the voltage drop at the middle point is \( = \frac{{IR}}{8}\)
Where I is the current fed to the distributor
R is the total resistance of the distributor
Therefore, a uniformly loaded DC distributor is fed at both ends with equal voltages the drop at the middle point is one fourth as compared to a similar distributor fed at one end only.The electric supply authority supplies power to the consumers through a low voltage three-phase four-wire distribution system is called ______.
Primary transmission: The electric power at 132 kV is transmitted by 3-phase, 3-wire overhead system to the outskirts of the city. This forms the primary transmission.
Secondary transmission: The primary transmission line terminates at the receiving station which usually lies at the outskirts of the city. At the receiving station, the voltage is reduced to 33kV by step-down transformers. From this station, electric power is transmitted at 33kV by 3-phase, 3-wire overhead system to various sub-stations (SS) located at the strategic points in the city. This forms the secondary transmission.
Primary distribution: The secondary transmission line terminates at the sub-station where voltage is reduced from 33 kV to 11kV, 3-phase, 3-wire. The 11 kV lines run along the important roadsides of the city. This forms the primary distribution.
Secondary distribution: The electric power from the primary distribution line (11 kV) is delivered to distribution sub-stations (DS). These sub-stations are located near the consumers’ localities and step down the voltage to 400 V, 3-phase, 4-wire for secondary distribution.
Primary distribution system:
Concept:
In a uniformly loaded distributor fed at one end, the maximum total voltage drop = IR/2
In a uniformly loaded distributor fed at both ends, the maximum total voltage drop = IR/8
The maximum voltage drop in case of uniformly loaded distributor fed at both ends is one fourth of the maximum voltage drop in case of uniformly loaded distributor fed at one end.
Calculation:
For a uniformly loaded dc distribution fed at one end, the maximum voltage drop = 10 V
If it is fed at both ends at the same voltage, the maximum voltage drop will be one fourth of 10 V.
= 0.25 × 10 = 2.5 V
Distribution Transformers can be mounted outdoors in one of the following ways
Plinth mounting:
H-pole mounting:
Direct mounting: