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5 TRANSMISSION LOSS 
 
Generally, in a power system, several plants are situated at different 
places. They are interconnected by long transmission lines. The entire 
system load along with transmission loss shall be met by the power plants 
in the system. Transmission loss depends on i) line parameters ii) bus 
voltages and iii) power flow. Determination of transmission loss requires 
complex computations. However, with reasonable approximations, for a 
power system with N number of power plants, transmission loss can be 
represented as 
 
[ ]
N 2 1 L
P P P P … = 
?
?
?
?
?
?
?
?
?
?
?
?
NN N2 N1
2N 22 21
1N 12 11
B B B
B B B
B B B

   


?
?
?
?
?
?
?
?
?
?
?
?
N
2
1
P
P
P

                                            (15) 
 
where 
N 2 1
P ,......., P , P  are the powers supplied by the plants 1,2,…….,N 
respectively.  
Page 2


5 TRANSMISSION LOSS 
 
Generally, in a power system, several plants are situated at different 
places. They are interconnected by long transmission lines. The entire 
system load along with transmission loss shall be met by the power plants 
in the system. Transmission loss depends on i) line parameters ii) bus 
voltages and iii) power flow. Determination of transmission loss requires 
complex computations. However, with reasonable approximations, for a 
power system with N number of power plants, transmission loss can be 
represented as 
 
[ ]
N 2 1 L
P P P P … = 
?
?
?
?
?
?
?
?
?
?
?
?
NN N2 N1
2N 22 21
1N 12 11
B B B
B B B
B B B

   


?
?
?
?
?
?
?
?
?
?
?
?
N
2
1
P
P
P

                                            (15) 
 
where 
N 2 1
P ,......., P , P  are the powers supplied by the plants 1,2,…….,N 
respectively.  
The coefficient matrix  [ B ]  is called  as LOSS COEFFICIENT MATRIX or  
B
mn
  COEFFICIENT MATRIX or B – COEFFICIENT MATRIX. This matrix is 
symmetric. 
    From eq.(15)     P
L 
= [ ]
N 2 1
P P P … … 
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
=
=
=
N
1 n
n n N
N
1 n
n n 2
N
1 n
n n 1
P B
P B
P B

 
                                  =  P 
N
P B
?
 +  P 
N
P B
?
 +  ……. +  P 
N
P B
?
                                   =  P
1
 
n
1 n
n 1
P B
?
=
 +  P
2
 
n
1 n
n 2
P B
?
=
 +  ……. +  P
N
 
n
1 n
n N
P B
?
=
 
                                  =  
n
N
1 n
n 1 1
P B P
?
=
 +  
n
N
1 n
n 2 2
P B P
?
=
 +  …… + 
n
N
1 n
n N N
P B P
?
=
 
                                  =  
n
P B P
n m
N
1 m
N
1 n
m ? ?
= =
 
Thus  P
L
 can be written as   
P
L
  =  
n
P B P
n m
N
1 m
N
1 n
m ? ?
= =
                                                                               (16) 
Page 3


5 TRANSMISSION LOSS 
 
Generally, in a power system, several plants are situated at different 
places. They are interconnected by long transmission lines. The entire 
system load along with transmission loss shall be met by the power plants 
in the system. Transmission loss depends on i) line parameters ii) bus 
voltages and iii) power flow. Determination of transmission loss requires 
complex computations. However, with reasonable approximations, for a 
power system with N number of power plants, transmission loss can be 
represented as 
 
[ ]
N 2 1 L
P P P P … = 
?
?
?
?
?
?
?
?
?
?
?
?
NN N2 N1
2N 22 21
1N 12 11
B B B
B B B
B B B

   


?
?
?
?
?
?
?
?
?
?
?
?
N
2
1
P
P
P

                                            (15) 
 
where 
N 2 1
P ,......., P , P  are the powers supplied by the plants 1,2,…….,N 
respectively.  
The coefficient matrix  [ B ]  is called  as LOSS COEFFICIENT MATRIX or  
B
mn
  COEFFICIENT MATRIX or B – COEFFICIENT MATRIX. This matrix is 
symmetric. 
    From eq.(15)     P
L 
= [ ]
N 2 1
P P P … … 
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
=
=
=
N
1 n
n n N
N
1 n
n n 2
N
1 n
n n 1
P B
P B
P B

 
                                  =  P 
N
P B
?
 +  P 
N
P B
?
 +  ……. +  P 
N
P B
?
                                   =  P
1
 
n
1 n
n 1
P B
?
=
 +  P
2
 
n
1 n
n 2
P B
?
=
 +  ……. +  P
N
 
n
1 n
n N
P B
?
=
 
                                  =  
n
N
1 n
n 1 1
P B P
?
=
 +  
n
N
1 n
n 2 2
P B P
?
=
 +  …… + 
n
N
1 n
n N N
P B P
?
=
 
                                  =  
n
P B P
n m
N
1 m
N
1 n
m ? ?
= =
 
Thus  P
L
 can be written as   
P
L
  =  
n
P B P
n m
N
1 m
N
1 n
m ? ?
= =
                                                                               (16) 
P
L
  =  
n
P B P
n m
N
1 m
N
1 n
m ? ?
= =
                                                                               (16) 
When the powers are in MW, the B
mn
  coefficients are of dimension  1/ MW. 
If powers are in per-unit, then B
mn
 coefficients are also in per-unit. Loss 
coefficient matrix of a power system shall be determined before hand and 
made available for economic dispatch. 
 
For a two plant system, the expression for the transmission loss is For a two plant system, the expression for the transmission loss is 
 
[ ]
?
?
?
?
?
?
?
?
?
?
?
?
=
2
1
22 21
12 11
2 1 L
P
P
B B
B B
P P P  
 
     [ ]
2 1
P P =
?
?
?
?
?
?
+
+
2 22 1 21
2 12 1 11
P B P B
P B P B
 =  
22
2
2 21 2 1 12 2 1 11
2
1
B P B P P B P P B P + + + 
Page 4


5 TRANSMISSION LOSS 
 
Generally, in a power system, several plants are situated at different 
places. They are interconnected by long transmission lines. The entire 
system load along with transmission loss shall be met by the power plants 
in the system. Transmission loss depends on i) line parameters ii) bus 
voltages and iii) power flow. Determination of transmission loss requires 
complex computations. However, with reasonable approximations, for a 
power system with N number of power plants, transmission loss can be 
represented as 
 
[ ]
N 2 1 L
P P P P … = 
?
?
?
?
?
?
?
?
?
?
?
?
NN N2 N1
2N 22 21
1N 12 11
B B B
B B B
B B B

   


?
?
?
?
?
?
?
?
?
?
?
?
N
2
1
P
P
P

                                            (15) 
 
where 
N 2 1
P ,......., P , P  are the powers supplied by the plants 1,2,…….,N 
respectively.  
The coefficient matrix  [ B ]  is called  as LOSS COEFFICIENT MATRIX or  
B
mn
  COEFFICIENT MATRIX or B – COEFFICIENT MATRIX. This matrix is 
symmetric. 
    From eq.(15)     P
L 
= [ ]
N 2 1
P P P … … 
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
=
=
=
N
1 n
n n N
N
1 n
n n 2
N
1 n
n n 1
P B
P B
P B

 
                                  =  P 
N
P B
?
 +  P 
N
P B
?
 +  ……. +  P 
N
P B
?
                                   =  P
1
 
n
1 n
n 1
P B
?
=
 +  P
2
 
n
1 n
n 2
P B
?
=
 +  ……. +  P
N
 
n
1 n
n N
P B
?
=
 
                                  =  
n
N
1 n
n 1 1
P B P
?
=
 +  
n
N
1 n
n 2 2
P B P
?
=
 +  …… + 
n
N
1 n
n N N
P B P
?
=
 
                                  =  
n
P B P
n m
N
1 m
N
1 n
m ? ?
= =
 
Thus  P
L
 can be written as   
P
L
  =  
n
P B P
n m
N
1 m
N
1 n
m ? ?
= =
                                                                               (16) 
P
L
  =  
n
P B P
n m
N
1 m
N
1 n
m ? ?
= =
                                                                               (16) 
When the powers are in MW, the B
mn
  coefficients are of dimension  1/ MW. 
If powers are in per-unit, then B
mn
 coefficients are also in per-unit. Loss 
coefficient matrix of a power system shall be determined before hand and 
made available for economic dispatch. 
 
For a two plant system, the expression for the transmission loss is For a two plant system, the expression for the transmission loss is 
 
[ ]
?
?
?
?
?
?
?
?
?
?
?
?
=
2
1
22 21
12 11
2 1 L
P
P
B B
B B
P P P  
 
     [ ]
2 1
P P =
?
?
?
?
?
?
+
+
2 22 1 21
2 12 1 11
P B P B
P B P B
 =  
22
2
2 21 2 1 12 2 1 11
2
1
B P B P P B P P B P + + + 
Since B
mn
 coefficient matrix is symmetric, for two plant system 
22
2
2 12 2 1 11
2
1 L
B P B P P 2 B P P + + =                                                                   (17) 
In later calculations we need the Incremental Transmission Loss ( ITL ),  
i
L
P
P
?
?
. 
 For two plant system 
 
2 12 1 11
L
P B 2 P B 2
P
P
+ =
?
?
                                                                                    (18) 
2 12 1 11
1
P B 2 P B 2
P
+ =
?
                                                                                    (18) 
2 22 1 12
2
L
P B 2 P B 2
P
P
+ =
?
?
                                                                                   (19) 
 
This can be generalized as 
n
P B 2
P
P
N
1 n
mn
m
L
?
=
=
?
?
      m = 1,2,………., N                                                       (20) 
Page 5


5 TRANSMISSION LOSS 
 
Generally, in a power system, several plants are situated at different 
places. They are interconnected by long transmission lines. The entire 
system load along with transmission loss shall be met by the power plants 
in the system. Transmission loss depends on i) line parameters ii) bus 
voltages and iii) power flow. Determination of transmission loss requires 
complex computations. However, with reasonable approximations, for a 
power system with N number of power plants, transmission loss can be 
represented as 
 
[ ]
N 2 1 L
P P P P … = 
?
?
?
?
?
?
?
?
?
?
?
?
NN N2 N1
2N 22 21
1N 12 11
B B B
B B B
B B B

   


?
?
?
?
?
?
?
?
?
?
?
?
N
2
1
P
P
P

                                            (15) 
 
where 
N 2 1
P ,......., P , P  are the powers supplied by the plants 1,2,…….,N 
respectively.  
The coefficient matrix  [ B ]  is called  as LOSS COEFFICIENT MATRIX or  
B
mn
  COEFFICIENT MATRIX or B – COEFFICIENT MATRIX. This matrix is 
symmetric. 
    From eq.(15)     P
L 
= [ ]
N 2 1
P P P … … 
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
=
=
=
N
1 n
n n N
N
1 n
n n 2
N
1 n
n n 1
P B
P B
P B

 
                                  =  P 
N
P B
?
 +  P 
N
P B
?
 +  ……. +  P 
N
P B
?
                                   =  P
1
 
n
1 n
n 1
P B
?
=
 +  P
2
 
n
1 n
n 2
P B
?
=
 +  ……. +  P
N
 
n
1 n
n N
P B
?
=
 
                                  =  
n
N
1 n
n 1 1
P B P
?
=
 +  
n
N
1 n
n 2 2
P B P
?
=
 +  …… + 
n
N
1 n
n N N
P B P
?
=
 
                                  =  
n
P B P
n m
N
1 m
N
1 n
m ? ?
= =
 
Thus  P
L
 can be written as   
P
L
  =  
n
P B P
n m
N
1 m
N
1 n
m ? ?
= =
                                                                               (16) 
P
L
  =  
n
P B P
n m
N
1 m
N
1 n
m ? ?
= =
                                                                               (16) 
When the powers are in MW, the B
mn
  coefficients are of dimension  1/ MW. 
If powers are in per-unit, then B
mn
 coefficients are also in per-unit. Loss 
coefficient matrix of a power system shall be determined before hand and 
made available for economic dispatch. 
 
For a two plant system, the expression for the transmission loss is For a two plant system, the expression for the transmission loss is 
 
[ ]
?
?
?
?
?
?
?
?
?
?
?
?
=
2
1
22 21
12 11
2 1 L
P
P
B B
B B
P P P  
 
     [ ]
2 1
P P =
?
?
?
?
?
?
+
+
2 22 1 21
2 12 1 11
P B P B
P B P B
 =  
22
2
2 21 2 1 12 2 1 11
2
1
B P B P P B P P B P + + + 
Since B
mn
 coefficient matrix is symmetric, for two plant system 
22
2
2 12 2 1 11
2
1 L
B P B P P 2 B P P + + =                                                                   (17) 
In later calculations we need the Incremental Transmission Loss ( ITL ),  
i
L
P
P
?
?
. 
 For two plant system 
 
2 12 1 11
L
P B 2 P B 2
P
P
+ =
?
?
                                                                                    (18) 
2 12 1 11
1
P B 2 P B 2
P
+ =
?
                                                                                    (18) 
2 22 1 12
2
L
P B 2 P B 2
P
P
+ =
?
?
                                                                                   (19) 
 
This can be generalized as 
n
P B 2
P
P
N
1 n
mn
m
L
?
=
=
?
?
      m = 1,2,………., N                                                       (20) 
6 ECONOMIC DIVISION OF SYSTEM LOAD BETWEEN VARIOUS PLANTS  
   IN THE POWER SYSTEM 
 
It is to be noted that different plants in a power system will have different 
cost characteristics. 
 
Consider a power system having N number of plants. Input-output 
characteristics of the plants are denoted as  ) P ( C ...., ),........ P ( C ), P ( C
N N 2 2 1 1
.  
 
Our problem is for a given system load demand P
D
,  find the set of  plant 
generation  
N 2 1
P ., ,......... P , P  which minimizes the cost function 
C
T
 = ) P ( C .. .......... ) P ( C ) P ( C
N N 2 2 1 1
+ + +                                                       (21) 
subject to the constraints 
0 ) P .. .......... P P ( P P
N 2 1 L D
= + + + - +                                                             (22) 
and   
max i i min i
P P P = =  I = 1,2,……….,N                                                         (23) 
 
Inequality constraints are omitted for the time being. 
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