WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Supply Systems, Voltage Drop
Calculations in DC/AC Distributers, & Design Numericals.
AC Supply Systems
- Electric power
produced at
power plant is
transmitted to end
consumer by
Transmission &
Distribution System.
- Transmission system
delas with medium
and high voltages,
while Distribution
system deals with
low voltages.
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 1
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
AC Voltage Levels
M.Sc. Electrical Engineering
Generation
- 3-phase alternators (Y)
- 11 – 13.8 kV
- Step up transformers (Y/∆)
Primary Transmission
- 132 kV, 220 kV, 400 kV, 500 kV, 750 kV
- 3-phase, 3-wire system
- Steel Towers
Secondary Transmission
- Receiving stations
- Received HV.
- Step down transformers
- 3-phase, 3-wire system
- 33 kV or 22 kV
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 2
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
Primary Distribution
- Sub-station involved
- Received 22 kV or 33 kV.
- Step down transformers
- 11 KV, 6.6 KV, 3.3 KV
- 3-phase, 3-wire system
- To feeders
Secondary Distribution (Distributers)
- Step down distribution transformers
- 11 KV to 400V or 230 V.
- 3-phase, 4-wire system
- To service mains
M.Sc. Electrical Engineering
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 3
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
Typical Distribution System
M.Sc. Electrical Engineering
a) Distribution
substation
b) Feeders
c) Distribution
Transformers
d) Distributor
conductors
e) Service mains
conductors
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 4
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
Distribution Substation (Dist. S/S)
- Transforms
voltage
M.Sc. Electrical Engineering
from
one level to other level.
- Located near or inside
city/town/village/industrial
area.
- At Dist. S/S, HV from transmission line is stepped down by a
step-down transformer to primary distribution level voltage
(11 kV).
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 5
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
Distribution Feeders
M.Sc. Electrical Engineering
- Feeders are conductors emanating from a substation to
supply distribution transformer.
- They connect sub-station to area where power is to be
distributed.
- Feeder forms Primary Distribution System.
- Voltage levels of dist. feeder are 11- 33 kV.
- Feeders are 3-phase, 3-wire connection system.
- Feeder is not directly connected to load.
- As no tappings are taken from the feeders so that the
current remains same throughout.
- Feeder is designed for its current carrying capacity.
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 6
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
Distribution transformer
M.Sc. Electrical Engineering
- A distribution transformer, is a stepdown 3-phase transformer.
- Steps down 11 kV to 400 V.
- Line to line voltage is 400 V & Phase
voltage is 230 V.
- Primary winding is usually delta connected, while
secondary side is star connected.
- Equipment & system earthing is provided for human and
device safety.
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 7
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
Distributors
M.Sc. Electrical Engineering
- Distributor is a line or conductor which takes the output of
distribution transformer and supplies power to consumers.
- It is 3-phase, 4-wire connection system.
- Voltage levels of distributor are 400 V and 230 V.
- Distributor is directly connected to load.
- Tappings are taken along the length of a distributor
conductor for power supply to the end consumers.
- The current through a distributor is not constant as
tappings are taken at various places throughout its length.
- Voltage drop along the length is the main consideration
while designing a distributor conductor.
Service Mains
- Small cables which connect distributers to consumer DB.
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 8
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
Primary Distribution System
M.Sc. Electrical Engineering
- It is that part of an AC
distribution
system
which operates at
somewhat
higher
voltages
than
general
residential
consumer utilization.
- Commonly
used
primary
distribution
voltages
in
most
countries are 11 kV,
6.6 kV and 3.3 kV.
- Primary distribution handles large consumers such as
factories and industries.
- Primary distribution is carried out by 3-phase, 3-wire system.
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 9
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
Secondary Distribution System
M.Sc. Electrical Engineering
- This part directly supplies
to the residential end
consumers.
- Domestic consumers are
fed with single phase
supply at 230 volts.
- Three phase supply may
also be provided at 400
volts for big properties,
commercial
buildings,
small factories etc.
- Secondary transmission in most countries is carried out by
3-phase, 4-wire system.
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 10
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
Radial Distribution System
M.Sc. Electrical Engineering
- Radial system has only one power
source (substation) for a group of
consumers.
- Feeders
radiate
from
one
substation
and
feed
the
distributors at one end.
- Power flow is in only one
direction.
- It is the simplest system and has the lowest initial cost.
- Consumers closer to feeder will get maximum voltages,
while at far end of feeder, will get low voltages.
- As consumers are dependent on single feeder and
distributer, any fault on any of these two causes
interruption in supply to all consumers.
- Radial dist. system is used when when substation is located
among the consumers.
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 11
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Parallel Feeders Distribution System
- Limitation of radial system is minimized by parallel feeders.
- If any feeder gets faulty, other one can provide power.
- More reliable than radial feeder.
- Load management (load sharing & load shedding) can
be done easily.
- High initial cost as number of feeders is doubled.
- Distribution transformer is fed with common bus-bar.
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 12
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
Ring Main Distribution System
M.Sc. Electrical Engineering
- Each distribution transformer is fed with two feeders but in
different paths.
- Feeders form a loop which starts from substation bus-bars,
runs through load area feeding distribution transformers
and returns to substation bus-bars.
- Reliable system similar to parallel feeders’ system.
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 13
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
- Feeder covers the whole
area of supply in the ring
fashion and terminates at
the substation from where it
was started.
- Primaries of distribution
transformers form a loop,
hence the name given.
- Substation supplies to closed Feeder LMNOPQRS.
- Distributors are tapped from different points M, O and Q.
- If fault occurs at any point “F” of section SLM of Feeder,
then section SLM of Feeder can be isolated for repair and
supply can be maintained to all consumers via Feeder
SRQPONM.
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 14
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Interconnected Distribution System
- Feeder
ring
is
energized by two
or more than two
substations.
- Feeder ring ABCD is
supplied by S1 and
S2 at points D & C
respectively.
- Distributors are fed
at points O, P, Q
and R via distribution transformers.
- More efficient & reliable system.
- Feeders can be fed according to load demand.
- Area fed from one substation during peak load hours can
be fed from the other generating station or substation for
meeting power requirements from increased load.
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 15
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
DC Distribution Calculations
M.Sc. Electrical Engineering
- Concentrated Loads: Loads that act at a particular point
of distributor. For example, domestic loads which are
tapped off from a particular point of distributor.
- Distributed Loads: Loads which act uniformly at all points
of distributor. Ideally, there are no distributed loads.
- In dc distribution calculations, point of minimum potential
is determined on the distributor. It should not be less than
6% of the rated voltage at consumer’s terminals.
- DC distribution calculations for concentrating loads:
1.
2.
3.
4.
Distributor fed at one end
Distributor fed at both ends
Ring Distributor
Interconnected distributor
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 16
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
DC Distributor Fed at One End
M.Sc. Electrical Engineering
AB → dc distributor fed at point “A” having loads I1 tapped at point C, I2 at D, I3 at E and I4 at F.
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 17
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 18
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 19
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
DC Distributor Fed at Both Ends with Equal Voltages
- Consider a distributor AB fed at both ends with equal
voltages V volts and having concentrated loads I1 , I2 , I3 ,
I4 and I5 at points C, D, E, F and G respectively.
- As we move away from one of the feeding points, say A,
p.d. goes on decreasing till it reaches the minimum value
at some load point, say E, and then again starts rising and
becomes V volts as we reach the other feeding point B.
- All currents tapped off between points A and E (minimum
p.d. point) will be supplied from feeding point A while
those tapped off between B and E will be supplied from
feeding point B.
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 20
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
- Current tapped off at point E will be partly supplied from A and partly
from B. If these currents are x and y respectively, then, I3= x + y
- The point at which current comes from both ends of the distributor is
called point of minimum potential.
- If current supplied by feeding end A is Ia, current distribution in various
sections of distributor is given as:
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 21
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 22
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Actual distribution of currents in various sections of distributor AB is given as:
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 23
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Currents are coming to point E from both sides. Hence, E is point of min. potential.
Distributor fed at both ends with unequal voltages
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 24
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 25
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 26
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 27
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 28
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 29
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
Ring Distributor with Interconnector
M.Sc. Electrical Engineering
- If distributor serving area is large, voltage drop will be high.
- To reduce voltage drop, different points of distributor are joined
by interconnector conductor.
- Interconnector is disconnected to find Thevenin voltage (VTH,
or Vo) across it.
- Thevenin equivalent resistance (RTH, or Ro) of network lines is
calculated viewed from interconnector.
- Finally, current of interconnector is calculated by using VTH, RTH
and resistance of interconnector.
𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪 𝒊𝒊𝒊𝒊 𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊 𝑩𝑩𝑩𝑩 =
𝑬𝑬𝒐𝒐
𝑹𝑹𝒐𝒐 + 𝑹𝑹𝑩𝑩𝑩𝑩
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 30
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 31
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 32
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 33
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
AC Distribution Calculations
- Voltage drops in various sections of line are due to R, X and
C of line.
- Power factor of sending and receiving end of TL is taken
into account.
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 34
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 35
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
Page 36
WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
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WEEK-11, LEC-21 & 22
Electrical Power System Engineering
M.Sc. Electrical Engineering
Dr. Riaz Ahmad Rana _ Electrical Engineering Department, FOE, UCP Lahore
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