[Part 2] Network Theorems & Transformations Study Notes for GATE & Electrical Eng. Exams

By Yash Bansal|Updated : July 27th, 2021

Network Theorems & Transformation study notes explained in Part 2 cover the topics such as Reciprocity Theorem, Tellegen's Theorem, Millman's Theorem, Substitution Theorem & Star-delta Transformation. These topics are important to be asked in the GATE, ISRO, SSC JE, ESE, and other Electrical Engineering exams.

Table of Content


Millman’s theorem helps to reduce 'N' number of parallel voltage sources to one. It can be observed in the given figure below. This technique permits finding the current through or voltage across RL without applying any method such as mesh analysis, nodal analysis, superposition, and so on. For example, the given three voltage sources can be reduced to one voltage source.

Generally, three steps are included in its application

Step 1: Convert all voltage sources into current sources.

 Step 2: Convert the parallel current source into resulting network as shown below

 IT= I1+I2+I  &    GT = G1+G2+G3

Step 3: Convert the resulting current source to a voltage source by transformation, and the desired single-source network will be shown as



The plus-and-minus signs appears in the last equation to include those cases where the sources may not be supplying energy in the same direction.

In terms of resistances values:



The reciprocity theorem is applicable to the single-source networks only. The theorem states that the current I in any branch of a network due to a single voltage source E present anywhere else in the network will be equal to the current across the branch in which the source was originally placed if the source is placed in the branch in which the current I was originally(initially) measured.

In other words, the location of the voltage source and the resulting current may be interchanged without a change in magnitude of current. The theorem postulates that the polarity of the voltage source have the same adaptation with the direction of the branch current in each position.

  • Example: Verify Reciprocity theorem.



  • Interchanging (or reciprocating) the location of I and E in the last figure to demonstrate the validity of the reciprocity theorem.




  • Tellegen’s theorem is based upon two Kirchhoff’s laws and is also applicable for any lumped network having elements that are linear or non-linear, active or passive, time-varying or time-invariant.
  • For a lumped network whose element assigned by associate reference direction for branch voltage vand branch current jk.The product vkjk is the power delivered at time t by the network to the element k.
  • If all branch voltages and branch currents satisfies KVL and KCL then


Application of Tellegen's Theorem: 

As seen from the last equation, the Tellegen’s Theorem implies the law of energy conservation.“The sum of power delivered by the independent sources to the network elements is equal to the sum of the power absorbed by all the branches of the network.” So, the application of Tellegen's theorem can be classified as 

  • Conservation of energy
  • Conservation of complex power
  • The real(or active) part and the phase of driving point impedance
  • Driving point impedance

Example: Find all branch voltages and currents for both networks N1, N2, and then verify Tellegen’s theorem.



The substitution theorem states that "If the voltage across any branch and the current flowing through that branch of a dc bilateral network are known, then that branch can be substituted by any one of the combinations that can consist of the same voltage and current through that chosen branch.

More simply, the theorem states that for branch equivalence, the voltage across the terminal and current through the terminal must be the same. Consider the circuit in which the voltage across and current through the branch a-b are determined. Through the use of the substitution theorem, few number of equivalent a-a′ branches are shown. Note that for each equivalent circuit, the terminal voltage & the current remains same.

  • By the use of the substitution theorem, the number of equivalent branches are:

Note : for each equivalent, the terminal voltage and current are the same and known potential difference and current in a network can be replaced by an ideal voltage source and current source respectively.

  • Example: The current source equivalence where a known current is replaced by an ideal current source, permitting the isolation of R4 and R5 as shown below

Recall the discussion of bridge networks that V = 0 and I = 0 were replaced by a short-circuit and an open circuit respectively. 


  • A part of a bigger circuit that is configured with the three-terminal network Y (or Δ) has to be converted into an equivalent Δ (or Y) through transformations.
  • Applications of these transformations will be studied by solving the resistive circuits.

Delta (Δ) – Wye (Y) conversion:

Let us consider the network shown below and assumed the resistances ( RAB, RBC, RCA) in Δ network are known. Now the requirement is to measure the resistance values of the branches of the Wye (Y) network that would produce same resistances when measured across similar pairs of terminals of Δ network. For this We have to write the equivalence resistance between any two terminals in the following form.


  • on solving above equations, we get values for star network resistances 


Conversion from Wye or Star (Y) to Delta (Δ):

  • To convert a Wye (Y ) to a Delta (Δ ), the relationships RAB, RBC & R3 must be obtained in terms of the Wye (Y)  resistances RA RB and RC Considering the Y connected network, we can write the current expression through RA resistor as


  • After equating the coefficients of VAB and VAC in both sides, we get the following relationship


  • similarly we can obtain for RBC for equivalent delta configuration


  • Observations: With a view to the symmetry of the transformation equations, the Wye (Y) and Delta (Δ) networks have been superimposed on each other.
  • The equivalent Wye (Y) resistance connected to a given terminal is equal to the product of the two Delta (Δ) resistances connected to that same terminal divided by the sum of all the Delta (Δ) resistances.
  • The equivalent Delta (Δ) resistance between the two-terminals is the product of the two-star (Y) resistances connected to those terminals divided by the third-star resistance plus the sum of the two same star (Wye) resistances.

The network theorems and transformation explained in the 2nd part is an important chapter of the Network theory as it most asked in the GATE EESSC JE EEESE IES EE, ISRO EE, and other electrical branch exams.

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Yash BansalYash BansalMember since Nov 2018
Content Manager (GATE)
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Prasad Kumbhar
In tellegen's theorem
Branch currents satisfy kvl & kcl mean what will be happen please tell about tellegen's theorem

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