Lab5 Transmission Lines PDF

Preview

Summary

Lab 5 ...

Description

EE 335L Electric Power Generation, Transmission and Distribution

Lab 05

Lab 05 Short and Medium Transmission Lines Name:

5.1

Student ID:

Objective

To investigate the performance of short and medium transmission line models at different loading conditions using Simulink

5.2

Introduction to Transmission Line Modelling

Transmission lines are the conductors which transfer electric energy from generating units at various locations to the distribution system which supplies the load. They also interconnect neighboring utilities. Since, transmission lines are conductors, they exhibit electrical properties of resistance (R), inductance (L), capacitance (C) and conductance (G). These 4 are called transmission line constants.  The series resistance relies basically on the physical composition of the conductor at a given temperature.  The series inductance and shunt capacitance are produced by the presence of magnetic and electric fields around the conductors, and depend on their geometrical arrangement.  The shunt conductance is due to leakage currents flowing across insulators and air. As leakage current is considerably small compared to nominal current, it is usually neglected, and therefore, shunt conductance is normally not considered for the transmission line modeling. These parameters are obtained on per phase basis and are quite useful in obtaining correct models for transmission lines. Transmission lines are represented by equivalent models with appropriate circuit parameters on per-phase basis. Phase voltages and phase currents are used and therefore, the three phase system is reduced to equivalent single phase system. The obtained models are useful in calculation of voltages, currents and power flows from sending end to receiving end. The dominant properties of a transmission line vary with its length. Therefore, there are three different models, namely: 1. Short Line Model 2. Medium Line Model 3. Long Line Model It is convenient to represent a transmission line as a two-port network as shown in Figure 1, where, Vs and Is are per-phase sending end voltage and current, and VR and IR are per-phase receiving end voltage and current, respectively.

Figure 1: Two-port network representation

EE 335L Electric Power Generation, Transmission and Distribution

Lab 05

The relation between the sending and receiving end quantities is given as:

V s  AV R  BI R I s  CV R  DI R Where, the parameters A, B, C and D depend on transmission line constants (R, L, C and G) and are determined as per the model configuration.

4.4

Voltage Regulation and Efficiency of Transmission Line

The percentage change in voltage at receiving end of the line in going from no load to full load expressed as percentage of full load voltage is called voltage regulation of line.

PercentVR 

| VR NL |  | VR FL | 100 | VRFL |

Where, VRNL  The transmission line efficiency is given by:  

4.5

VS A

PR (3 ) PS (3 )

100

Short Line Model

When the length of transmission lines is lesser than 80 km (50 miles) then they are represented by short line model. In this model, only the series resistance and reactance are included and shunt admittance is neglected or, in other words, the effect of capacitance is ignored.

Figure 2: Short-line model

If per-phase, per unit length resistance ‘r’ and inductance ‘L’ are known, the total series impedance can be found by multiplying it with the length ‘l’ of transmission line. Then, from figure 2, the per-phase sending end voltage and current can be expressed as: V s  V R  ZI R Is IR Where,

Z  ( r  jL)l Z  R  jX

Figure 3: Phasor diagram of short transmission line at unity power factor

EE 335L Electric Power Generation, Transmission and Distribution

Lab 05

Task 1: To calculate ABCD parameters of a short line model Comparing the given equations for sending end voltage and current of a short transmission line with two-port network, express the ABCD parameters of a short line model: B   ____ ____  D   ____ ____ A three-phase transmission line is 40 km long. The resistance per phase 0.15Ω per km and the inductance per phase is 1.3263mH per km. The shunt capacitance is negligible. Using short line model for the transmission line, find the following: Z= ______________________ A C 

A  C

B  D

 ____ ____  ____ ____  

Task 2: To verify short line model using MATLAB Simulink 1. Let the transmission line mentioned above has to supply a load of 381 MVA at lagging power factor of 0.8 at 220 kV (line-line), calculate the parameters given in Table 1. System frequency is 60 Hz. Table 1: Calculated sending and receiving end parameters of the given short-transmission line

Sending end current: Is

Receiving end current: IR

Sending end voltage: Vs Sending end voltage: |Vs L-L|

Receiving end voltage: VR Receiving end voltage: |VRL-L|

2. Implement the above system in Similink as shown in Figure 4. Note that short line model is implemented using Series RLC branch in Simulink as model has resistance and inductance in series.

Figure 4: Short-line modelled in Simulink

3. Select base voltage equal to Vs (line-line). Obtain its load flow solution by making source as swing and load as PQ bus. Verify your results and fill in Table 2. To display the RMS values of sending and receiving end voltages, connect display blocks with three-phase voltage and current measurement blocks through RMS blocks. Apply the results of load flow to your model and then run the simulation to get RMS values of voltage and currents in volts and amperes.

EE 335L Electric Power Generation, Transmission and Distribution

Lab 05

4. For no-load receiving end voltage, disconnect the load and re-compute the load flow. Again apply the results to your model and run the simulation. Table 2: Sending and receiving end parameters of short-line model (from Simulink)

Sending end current: Is

Receiving end current: IR

Sending end voltage: Vs L-L Sending end power: SS Total line losses: SL Line efficiency: η

Receiving end voltage: VR L-L Receiving end power: SR No load receiving end voltage: VRNL % Voltage regulation: % VR

Task 3: To observe the performance of short transmission line with leading load 1. Modify the above model such that now the transmission line has to supply a load of 381 MVA at 0.8 leading power factor. Obtain load flow solution and complete Table 3. Note: Supply voltage Vs remains unchanged and receiving end voltage VR is to be found. In Table 3, you may write magnitudes only for Voltage (kV) and Current (A). Table 3: Short-line performance with leading load

Sending end current: Is

Receiving end current: IR

Sending end voltage: Vs L-L Sending end power: SS Total line losses: SL Line efficiency: η

Receiving end voltage: VR L-L Receiving end power: SR No load receiving end voltage: VRNL % Voltage regulation: % VR

2. Analyze the obtained results.

Task 4: To draw phasor diagram for short-line model supplying loads 1. Compare the results in Table 2 and 3. Interpret and justify your results with the help of phasor diagrams at leading and lagging power factors loads (refer Figure 3).

EE 335L Electric Power Generation, Transmission and Distribution

4.6

Lab 05

Medium Line Model

The transmission lines whose length is greater than 80 km (50 miles) and lesser than 250 km (150 miles) are termed as medium length lines. For such lines, line charging current and the shunt capacitance are considered. Therefore, the line model includes the total shunt admittance Y along with the total series impedance Z. Shunt admittance of the line is given by: Y  ( g  jC )l Here, ‘C’ is the line to neutral capacitance per unit length and ‘g’ is the shunt conductance per unit length which represents leakage current. It is negligible and assumed to be zero. Medium length lines can be modelled using π model and T model. In nominal π model, half of the shunt capacitance is lumped at each end of the line as shown in Figure 5. While in T model, the shunt admittance is lumped at the center and half of the series impedance is lumped at each end of line.

Figure 5 : Medium line models (T and pi)

Applying KCL and KVL on the π model,

Y I L  I R  VR 2 V S  V R  ZI L Y I S  I L  VS 2

Task 5: To calculate the ABCD parameters of π model for medium length lines 1. Using the equations given above, express Vs and Is in terms of VR, IR and line parameters to find the ABCD parameters π model.

 A B   _______ ________  C D    _______ _________    

EE 335L Electric Power Generation, Transmission and Distribution

Lab 05

2. For a transmission line which is 130 km long which is operating at 60 Hz, the line parameters per unit length are given as: r= 0.01273 Ω/km L=0.9337mH/km C=0.01274µF/km Find the corresponding ABCD (using MATLAB commands will be easier than using calculator): Y=_____________________________ Z=_____________________________

 A B   _______ ________ C D    _______ _________    

Task 6: To calculate sending-end and receiving-end voltage and current using pi model parameters If the transmission line is supplying a load of 270 MVA with 0.8 power factor lagging at 325kV, calculate the following: Table 4: Calculate sending and receiving end voltage and currents of a medium length transmission line

Sending end current: Is

Receiving end current: IR

Sending end voltage: Vs

Receiving end voltage: VR

Sending end voltage: |Vs L-L|

Receiving end voltage: |VRL-L|

Task 7: To verify medium line pi model using MATLAB Simulink Using Simulink, simulate the system given in Task 7 as done in Task 3 but for modelling medium length transmission line, use three-phase pi section line block and verify the results of Task 7. Again, use sending end voltage as system base voltage. Connect a small resistance (0.5 Ω) in series with pi-model after source as MATLAB doesn’t allow to connect source directly with capacitance. Table 5: Simulated results for medium length transmission line

Sending end current: Is

Receiving end current: IR

Sending end voltage: Vs L-L

Receiving end voltage: VR L-L

Sending end power: SS

Receiving end power: SR

Total line losses: SL

No load receiving end voltage: VRNL

Line efficiency: η

% Voltage regulation: % VR

EE 335L Electric Power Generation, Transmission and Distribution

Lab 05

1. Is the receiving end voltage greater than the sending end voltage at load and at no-load? Compare voltage regulation at inductive load with that of the short-line model and justify your interpretation.

2. How the voltage regulation would change if the medium transmission line supplies a capacitive load?

3. What would be the receiving end voltage in comparison to the sending end voltage for a short-line and a medium length line if the lines supply a small purely resistive load?

Post Lab Task Task 8: To study about the importance of transmission line capacity in power system 1. “Transmission line congestion is said to be a ‘bottleneck’ in power system”. Discuss the statement. 2. Study about Matiari-Lahore HVDC transmission line project and list down its prominent features and advantages. How is it going to improve Pakistan power system? 3. Study about TP-1000 project by K-Electric. How much additional power (Mega Watts) transmission will be made possible through it?

Reference: 

Power System Analysis, Second Edition by Hadi Saadat

EE 335L Electric Power Generation, Transmission and Distribution

Lab 05

Assessment Rubric Lab 05

Short and Medium Transmission Line Name:

Student ID:

Points Distribution LR2 Simulation

Task No.

LR 4 Data Collection

LR 5 Figures

Task 1

LR6 Calculations

LR 10 Analysis

CLO Mapped

/4

Task 2

/8

/8

Task 3

/4

/4

/8 /4

Task 4

/4

/4 CLO 3

Task 5

/4

Task 6

/8

Task 7

/8

/8

/8

Task 8

/16

CLO

Total Points

3

100

Total

100

Points Obtained

For description of different levels of the mapped rubrics, please refer the provided Lab Evaluation Assessment Rubrics and Affective Domain Assessment Rubric...