According To Length

According to length the Short Transmission line can be categorized as

• Short Transmission Line
• Medium Transmission Line
• Long Transmission Line

Short Transmission Line

Short transmission lines are the transmission line that are having a length less than 80Km. Short transmission line have the voltage level ranging between 30Kv to 50Kv. As there length is short so there is no capacitance effect taking place or it is negligible. So while making the equivalent circuit we only consider resistance and inductance.

Some Applications of short transmission line are

• Distribution of power from substations to local areas.
• Power transmission with industries.

Advantages of Short Transmission Line

• It is very cost effective as its price is not high.
• Due to short distances the amount of power loss is very less which is helpful.
• It is very simple to understand and its functions are easy to use.

Disadvantages of Short Transmission Line

• It can transfer power upto a certain distance, after that the losses increases.
• If the distance is more then there will be voltage drop.

Medium Transmission Line

Medium transmission lines are the transmission line that are having a length ranging from 80Km to 150Km. Medium transmission line have the voltage level ranging between 20Kv to 100Kv. As there length and voltage is increased compared to short transmission line so there is capacitance effect taking place and have to be considered. So while making the equivalent circuit we consider resistance, inductance and capacitance.

Some Applications of medium transmission line are

• Interconnecting the substations with a region.
• Connecting wind farms and solar farms to the grid line.

Advantages of Medium Transmission Line

• There cost is balanced, that means it is not more nor less.
• Power Losses are more as compared to short transmission line, but they are manageable.
• They are not much complex as compared to Long transmission line.

Disadvantages of Medium Transmission Line

• The voltage drop is more as compared to Short Transmission line.
• As the capacitance is not negligible then then we have to maintain and monitor the reactive power.

Methods in Medium Transmission Line

There are Three main methods in Medium Transmission Line, they are as follows

• Nominal Pi Method
• Nominal T Method
• End Condenser Method

Nominal Pi Method

It is the method in the medium transmission line which is used to analyze the medium transmission lines on the basis of its parameters like resistance, inductance and capacitance in the form of pi [Tex]\pi [/Tex] configuration. This model represents the parameters in pi form by the help of

• Series Impedance (Z) : This represents combined resistance (R) and inductance (L) which are placed in the middle of the model.
• Shunt Admittance (Y) : This represents line capacitance (C) that is divided in two equal parts and is placed at both sending end and receiving end of the model.

Phasor Diagram of Nominal Pi method can be given as

Advantages of Nominal Pi Model

• It is the most used model in medium transmission line making it easy to understand.
• It is very accurate model.
• It is very simple and manageable.

Disadvantages of Nominal Pi Model

• If the line length is more then the approximate value will decrease and not be much accurate.
• Due to this method the exact behavior of the line is not taken which is not proper.

Calculation in Nominal Pi Method

[Tex]A=D=1+\frac{ZY}{2} [/Tex]

[Tex]B=Z[/Tex]

[Tex]C=Y(1+\frac{ZY}{4})[/Tex]

Calculation of Sending end voltage and current

[Tex]I_R[/Tex] = load current per phase

R = resistance per phase

C = capacitance per phase

[Tex]X_L[/Tex] = inductive reactance per phase

[Tex]cos\phi_R[/Tex] = receiving end power factor (lagging)

[Tex]V_S[/Tex] = sending end voltage per phase

[Tex]\vec{V_R}=V_R+j0[/Tex]

[Tex]\vec{I_R} = I_R(cos\phi_R – jsin\phi_R)[/Tex]

line current is [Tex]\vec{I_L}=\vec{I_R}+\vec{I_{c1}}[/Tex]

sending end voltage [Tex]\vec{V_S}=\vec{V_R}+\vec{I_L}\vec{Z}=\vec{V_R}+\vec{I_L}(R+jX_L)[/Tex]

Charging the current at sending end –

[Tex]\vec{I_{c2}}=j\omega (\frac{C}{2}\vec{V_S})[/Tex]

[Tex]j\pi fC\vec{V_S}[/Tex]

Sending end current,

[Tex]\vec{I_S}=\vec{I_L}+\vec{I_{c2}}[/Tex]

Nominal T Method

It is the method in the medium transmission line which is used to analyze the medium transmission lines on the basis of its parameters like resistance, inductance and capacitance in the form of T shape. All the parameters are stored in the middle point of the model.

Phasor Diagram of Nominal T Method can be given as

Advantages of Nominal T Method

• It is less complex as compared to the pi model which is helpful.
• It is also accurate.
• It is more effective way.

Disadvantages of Nominal T Method

• For the more high voltage medium transmission line the Nominal T Method is not used more as it becomes less accurate.
• It does not have distributed capacitance in the model.

Calculation of Nominal T Method

[Tex]A=D=1+\frac{ZY}{2}[/Tex]

[Tex]B=Z(1+\frac{ZY}{4})[/Tex]

[Tex]C=Y[/Tex]

Calculation of Sending end voltage and current –

[Tex]I_R [/Tex]= load current per phase

[Tex]X_L[/Tex] = inductive reactance per phase

[Tex]cos\phi _R[/Tex] = receiving end power (lagging)

R = resistance per phase

C = capacitance per phase

[Tex]V_S[/Tex] = sending end voltage per phase

[Tex]V_1[/Tex] = voltage across capacitor

Now,

receiving end voltage,

[Tex]\vec{V_R} =V_R+j0[/Tex]

load current is [Tex]\vec{I_R} =I_Rcos\phi_R-jsin\phi_R[/Tex]

Voltage across C is,

[Tex]\vec{V_1} =V_R+I_R(cos\phi_R-jsin\phi_R)(\frac{R}{2}+j\frac{X_L}{2})[/Tex]

Therefore we get ,

capacitive current as – [Tex]\vec{I_C}=j\omega C\vec{V_1}=j2\pi fC\vec{V_1} [/Tex]

Sending end current as – [Tex]\vec{I_S} = \vec{I_R}+\vec{I_C}[/Tex]

Sending end voltage as – [Tex]\vec{V_S}=\vec{V_1}+\vec{I_S}\frac{\vec{Z}}{2}=\vec{V_1}+\vec{I_S}(\frac{R}{2}+j\frac{X_L}{2})[/Tex]

End Condenser Method

It is also a method to analyze the performance of medium transmission line. It is the most easier and quicker way to analyze the performance. It is very simple method as compared to both nominal pi and nominal t method. In this method the whole capacitance is focused at a single point, most of the time at receiving end.

Phasor Diagram for End Condenser can be given as

Advantages of End Condenser Method

• It is the most simple method among all three.
• It is suitable for quick estimation of the voltage regulation and power transfer.

Disadvantages of End Condenser Method

• As it mostly focuses on receiving end the capacitance effect is overestimated.
• It is not much accurate.

Calculation of End Condenser Method

[Tex]V_R=V_R+j0[/Tex]

Load current [Tex]I_R=I_R\phi_R[/Tex] (lagging)

[Tex]=I_R(cos\phi_R-jsin\phi_R)[/Tex]

Now shunt capacitor is [Tex]I_C=jl_C[/Tex]

But we know [Tex]I_C=V_R/(1/\omega C)=\omega CV_R[/Tex]

[Tex]I_C=j\omega CV_R=j2\pi fCV_R[/Tex]

Now,

[Tex]I_S=I_R+I_C[/Tex]

[Tex]I_R(cos\phi_R-jsin\phi_R)+j2\pi fCV_R[/Tex]

[Tex]i_Rcos\phi_R+j(2\pi fCV_R-I_Rsin\phi_R)[/Tex]

Now we know,

[Tex]V_S=V_R+I_SR+jI_SX[/Tex]

[Tex]=V_R+I_S[R+jX][/Tex]

So we calculated sending end voltage and sending end current. Now the sending end power can also be calcluated.

The Voltage Regulation of medium line by end condenser method is –

[Tex]\%Voltage Regulation = (V_S-V_R)\times 100/V_R \%[/Tex]

In this article, we will be going through a Transmission line, first, we will start our article with the introduction of the Transmission line, then we will go through then we will go through different types of transmission lines with their methods, After that we will go through Voltage regulations and efficiency of the Transmission, At last, we will conclude our Article with some applications, properties, protection of transmission line with some FAQs.