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LAB NO 02
Manipulation of the Performance of an overhead Transmission Line in
Matched-Load Operation
OBJECTIVES
Upon successful completion of this experiment, the students will be able to: Measurement of current and voltage relationships of an overhead line in matched-load operation. Interpretation of the terms characteristic wave impedance, lagging and leading operation, efficiency and transmission losses.
THEORY:
This operating case is present when the transmission line is terminated (i.e. matched) by an ohmic consumer resistance equivalent to the characteristic impedance. The power transmitted in this case is called natural load. The line current is just large enough for the reactive power consumption of the line inductor and capacitance to cancel; the transmission line thus does not require any external reactive power for operation. As, in this case, the active power losses in transmission are minimal in real transmission lines (i.e. low-loss), this is to be viewed as the optimum case. However, the load on a system changes constantly according to the performance of the consumers. Operation with natural load thus seldom occurs. When the current in the transmission line changes, the reactive power balance is disturbed. If the current is lower, the line acts capacitive. If the current increases, the line has an inductive performance. In both cases, the active power losses increase in real transmission lines. If the voltage at the beginning of the line is kept constant, an increase in the voltage may be noted at the line end lagging operation (cf. no- load as limiting case).The voltage at the line end drops in leading operation (cf. short- circuit as limiting case). In order to guarantee the consumer a constant voltage, the voltage must be regulated at the supplying transformer in the case of changing system loads. The load capability of overhead transmission lines (i.e. the thermal limit rating) is significantly higher than the natural load. In practical operation, the overhead transmission lines are most often loaded in Leading mode.
In matched-load operation, the transmission line is terminated with an ohmic resistance having the value of the characteristic impedance. The consumer current I2 is in-phase with the voltage U2.
EQUIPMENTS:
1 IT 6017 Three-phase power supply unit
1 IT 6019 Power circuit breaker
1 IT 6003 Three-phase transformer
1 IT 6002 Overhead line model
1 IT 6004 Resistive load
2 IT 6035 Moving-iron ammeter (2, 5 A)
2 IT 6037 Moving-iron voltmeter (600 V)
1 IT 6048 Power meter
CONNECTIONS:
Fig. 2.1 Block Diagram of Transmission Line
EXPERIMENT PROCEDURE:
Assemble the circuit in accordance with the foregoing topographic diagram. Set the primary-side of the three-phase transformer in delta connection 380 V and using bridging plugs set the secondary-side to star UN - 5%. Insert all bridging plugs connecting the capacitance to overhead line model. Connect a three-phase balanced resistive load to end terminals of the line; set the load resistance value to R 1. Set the supply voltage to UN = 380 V. Beginning from the R1 value reduces the resistive load in steps till the R 7 value. for each step measure the following quantities: Voltage U 1 , current I 1 , active power Pi and reactive power Q 1 at the start of the line as well as the voltage U 2 and the current bat the end of the line.
OBSERVATION TABLE: 2.
Determine the particular value of resistive load at which the line no longer consumes any reactive power (i.e. at which matching is achieved) and compare this with the theoretical value specified for the characteristic wave impedance Zw = 240 ohm. As typical result when the resistive load is above R4 there is a capacitive behavior: inductive behavior results at values below that. Between R4 and R6 load values the line consumes no detectable reactive power.
Leaving the resistive load unchanged to the approx value of the characteristic wave impedance (R5 = 213 ohm) ) measure voltage and current at both ends of the line for all possible supply voltage, which can be set on the secondary side of the three-phase transformer.
OBSERVATION TABLE: 2.
In case of matched load only the active power is transmitted so, in accordance with the equation: Calculate the total active power P1 at the start end and the total active power P2 at the end of the line. Calculate the line transmission losses: And the line transmission efficiency:
Since the line and the load form a series circuit the ratio of the transmission power converted in the two elements and thus the efficiency are independent on the magnitude of the supply voltage. However, if a constant power is supposed to be transmitted, then a higher supply voltage would be more favorable, because the line losses drop as current decreases. Note: In real overhead lines corona losses also arise, which have a slight negative effect on the efficiency. Furthermore, the value determined above only applies for the exceptional case of matched load.
Review Questions:-
Q:1 What do you mean by the constants of an overhead transmission lines?
Q:2 what are the requirements of a conductor for a transmission line?
Q:3 Types of conductors
Q4: Is there any difference b/w Vs and V R and why?
Q5: What is the variation in load current when load is is increased?
Q6: What is the effects on active power and power factor when load is varied without and with capacitive
LAB NO 3
To Mend and Measure the Performance of an overhead Transmission
Line in ohmic-inductive load
OBJECTIVES
Upon successful completion of this experiment, the students will be able to: Measuring and interpreting the current and voltage ratios of a transmission line with mixed ohmic-inductive and pure inductive loads. Interpreting the current and voltage ratios of a transmission line with mixed ohmic-inductive and pure inductive loads.
THEORY:
Transmission line possesses resistance R, inductance L, leakage conductance G and capacitance. All low voltages overhead lines having length up to 80 km are categorize as short line. In a short line, the shunt capacitance C and shunt conductance G are neglected. The series resistance R and series inductance for the total length of the line is considered. A single phase supply line is short in length and operates at low voltage. It has two conductors. Each conductor has resistance R1 and inductance L1. The inductance is affected equivalent to the inductive reactance X1=2pifL1. A balance three phase circuit consisting of three separate identical single phase circuit therefore the calculation for balance three phase line are carried out in similar manner as singe phase line, the difference being that per phase base is adopted. In this line all the given voltages are line to line values that all the current are line current. However, the load on a system changes constantly according to the performance of the consumers. When the current in the transmission line changes, the reactive power balance is disturbed. If the current is lower, the line acts capacitive. If the current increases, the line has an inductive performance. In both cases, the active power losses increase in real transmission lines. If the voltage at the beginning of the line is kept constant, an increase in the voltage may be noted at the line end lagging operation (cf. no- load as limiting case).The voltage at the line end drops in leading operation (cf. short- circuit as limiting case). In order to guarantee the consumer a constant voltage, the voltage must be regulated at the supplying transformer in the case of changing system loads.
Loads that power electrical motors are inductive loads. These are found in a variety of household items and devices with moving parts, including fans, vacuum cleaners, dishwashers, washing machines and the compressors in refrigerators and air conditioners. In contrast to resistive loads, in a purely inductive load, current follows a sinusoidal pattern that peaks after the voltage sine wave peaks, so the maximum, minimum and zero points are out of phase.
EQUIPMENTS:
1 IT 6017 Three-phase power supply unit 1 IT 6019 Power circuit breaker 1 IT 6003 Three-phase transformer 1 IT 6002 Overhead line model 1 IT 6004 Resistive load 1 IT 6005 Inductive load 1 IT 6048 Power meter 1 IT 6049 Power factor meter 2 IT 6035 Moving-iron ammeter (2,5 A) 2 IT 6037 Moving-iron voltmeter (600 V)
EXPERIMENT PROCEDURE:
Step 1: Assemble the circuit in accordance with the foregoing topographic diagram, Set primary-side of the three-phase transformer in delta connection 380 V and using bridging plugs set the secondary-side to star UN + 5%. Step 2: Insert all bridging plugs connecting the capacitance to overhead line model. Step 3: To end terminals of line connect a three-phase balanced ohmic-inductive load: Step 4: Set the load resistance value to R1 and begin with the value L4 = 1.27 H of the inductive load. Step 5: Starting at R1 value reduce the resistance value in steps to R3, R4 and R5 in that order. Step: 6 For each step measure the following quantities: voltage U1, current I1, active power P1 and reactive power Q1 at the beginning of the line, and voltage U2, current I and cos_2 at the line end.
1) At Inductive load: L4 = 1.27 H.
Table: 3 .1 Observations
2) At Inductive load: L5 = 0.9 H.
Table: 3.2 observations
3) At Inductive load: L6 = 0.64 H.
Table: 3.3 observations
In all measurements the voltage at the line end is considerably lower than the voltage at the line beginning and decreases as the load current increases. Now remove the connection to the resistive load and repeat the measurement for At L 4 = 1.27 H.
Table: 3.4 observations
Task: A current-voltage vector diagram for the case of a mixed ohmic inductive
load with power factor of 0.8. Note: (The operating capacitance of the line is
disregarded here).)
Review Questions:-
Q1: What is the Effect of increase in capacitive and inductive load on Apparent Power.
Q2: What is the effect of change in inductive and capacitive load on VR
Q3: What is the change in Voltage drop when inductive and capacitive loads are varied
Q4: Write down the change in Power factor and angle in all cases?
Q5 : How overhead transmission line are classified?
Q6 : What is the effect of load power factor on regulation and efficiency of a transmission line?
