Experiment — 2
Power Factor Correction
Objectives
To understand the power diagram, active, reactive, apparent power, power
factor correction and the effect on transmission line losses.
PreLab:
Download and Install PowerWorld on your computer. Create a oneline Diagram with
a transmission line and a series R-L Load connected to a generator. Place a variable capacitor
across the load. Run the program and debug any issues. Values you’ll need to enter for the
generator, transmission line and loads are given in the appendix on Power world. Submit your
one-line diagram file via Moodle.
Theory
The relevant theory related to real, reactive, and apparent power should be
reviewed in your ECE 231 book before doing the experiment. The following ―in quote,‖
is taken from Wikipedia.org http://en.wikipedia.org/wiki/Apparent_power under the
Creative Commons Attribution-ShareAlike License
―In a simple alternating current (AC) circuit consisting of a source and a linear
load, both the current and voltage are sinusoidal. If the load is purely resistive, the two
quantities reverse their polarity at the same time. At every instant the product of voltage
and current is positive, indicating that the direction of energy flow does not reverse. In
this case, only real power is transferred.
If the load is purely reactive, then the voltage and current are 90 degrees out of phase. For
half of each cycle, the product of voltage and current is positive, but on the other half of
the cycle, the product is negative, indicating that on average, exactly as much energy
flows toward the load as flows back. There is no net energy flow over one cycle. In this
case, only reactive energy flows—there is no net transfer of energy to the load.
Practical loads have resistance, inductance, and capacitance, so both real and reactive
power will flow to real loads. Power engineers measure apparent power as the vector sum
of real and reactive power. Apparent power is the product of the root-mean-square
voltage and current.
Engineers care about apparent power, because even though the current associated with
reactive power does no work at the load, it heats the wires, wasting energy. Conductors,
transformers and generators must be sized to carry the total current, not just the current
that does useful work.
Another consequence is that adding the apparent power for two loads will not accurately
give the total apparent power unless they have the same displacement between current
and voltage (the same power factor).
If a capacitor and an inductor are placed in parallel, then the currents flowing through the
inductor and the capacitor tend to cancel out rather than adding. Conventionally,
capacitors are considered to generate reactive power and inductors to consume it. This is
the fundamental mechanism for controlling the power factor in electric power
transmission; capacitors (or inductors) are inserted in a circuit to partially cancel reactive
power 'consumed' by the load.
The apparent power is the vector sum of real and reactive power.
Real power (P)
Reactive power (Q)
Complex power (S)
Apparent Power (|S|)
Phase of Current (φ)
Engineers use the following terms to describe energy flow in a system (and assign each
of them a different unit to differentiate between them):





Real power (P) or active power [1]: watt [W]
Reactive power (Q): volt-amperes reactive [Var]
Complex power (S): volt-ampere [VA]
Apparent Power (|S|), that is, the absolute value of complex power S: voltampere [VA]
Phase of Current (φ), the angle of difference (in degrees) between voltage and
current; Current lagging Voltage (Quadrant I Vector), Current leading voltage
(Quadrant IV Vector)
In the diagram, P is the real power, Q is the reactive power (in this case positive), S is the
complex power and the length of S is the apparent power.
Reactive power does not transfer energy, so it is represented as the imaginary axis of the
vector diagram. Real power moves energy, so it is the real axis.
The unit for all forms of power is the watt (symbol: W), but this unit is generally reserved
for real power. Apparent power is conventionally expressed in volt-amperes (VA) since it
is the product of rms voltage and rms current. The unit for reactive power is expressed as
var, which stands for volt-amperes reactive. Since reactive power transfers no net energy
to the load, it is sometimes called "wattless" power. It does, however, serve an important
function in electrical grids and its lack has been cited as a significant factor in the
Northeast Blackout of 2003.[2]
Understanding the relationship between these three quantities lies at the heart of
understanding power engineering. The mathematical relationship among them can be
represented by vectors or expressed using complex numbers,
(where j is the
imaginary unit).
Power factor
The ratio between real power and apparent power in a circuit is called the power factor.
It's a practical measure of the efficiency of a power distribution system. For two systems
transmitting the same amount of real power, the system with the lower power factor will
have higher circulating currents due to energy that returns to the source fro m energy
storage in the load. These higher currents produce higher losses and reduce overall
transmission efficiency. A lower power factor circuit will have a higher apparent power
and higher losses for the same amount of real power.
The power factor is one when the voltage and current are in phase. It is zero when the
current leads or lags the voltage by 90 degrees. Power factors are usually stated as
"leading" or "lagging" to show the sign of the phase angle, where leading indicates a
negative sign.
Purely capacitive circuits cause reactive power with the current waveform leading the
voltage wave by 90 degrees, while purely inductive circuits cause reactive power with the
current waveform lagging the voltage waveform by 90 degrees. The result of this is that
capacitive and inductive circuit elements tend to cancel each other out.
Where the waveforms are purely sinusoidal, the power factor is the cosine of the phase
angle (φ) between the current and voltage sinusoid waveforms. Equipment data sheets
and nameplates often will abbreviate power factor as "cosφ" for this reason.
Example: The real power is 700 W and the phase angle between voltage and current is
45.6°. The power factor is cos(45.6°) = 0.700. The apparent power is then: 700 W /
cos(45.6°) = 1000 VA.
Reactive power
Reactive power flow on the alternating current transmission system is needed to support
the transfer of real power over the network. In alternating current circuits energy is stored
temporarily in inductive and capacitive elements, which can result in the periodic reversal
of the direction of energy flow. The portion of power flow remaining after being
averaged over a complete AC waveform is the real power, which is energy that can be
used to do work (for example overcome friction in a motor, or heat an element). On the
other hand the portion of power flow that is temporarily stored in the form of electric or
magnetic fields, due to inductive and capacitive network elements, and returned to source is
known as the reactive power.
AC connected devices that store energy in the form of a magnetic field include inductive
devices called reactors, which consist of a large coil of wire. When a voltage is initially placed
across the coil a magnetic field builds up, and it takes a period of time for the current to reach
full value. This causes the current to lag the voltage in phase, and hence these devices are said
to absorb reactive power.
A capacitor is an AC device that stores energy in the form of an electric field. When current is
driven through the capacitor, it takes a period of time for charge to build up to produce the full
voltage difference. On an AC network the voltage across a capacitor is always changing – the
capacitor will oppose this change causing the voltage to lag behind the current. In other words
the current leads the voltage in phase, and hence these devices are said to generate reactive
power.
Energy stored in capacitive or inductive elements of the network give rise to reactive power
flow. Reactive power flow strongly influences the voltage levels across the network. Voltage
levels and reactive power flow must be carefully controlled to allow a power system to be
operated within acceptable limits.‖
Equipment
1. An inductor with a separate magnetic core in cabinet.
2. A capacitor bank, in cabinet.
3. Resistive Load Cart
4. Two multimetrers and two wattmeters.
5. Power Transformer
6. Variable Resistance Box (OR Variable resistor?)
Procedure
1.
2.
3.
5.
Without the iron core inserted, measure the resistance of the inductor
Apply a variable voltage making sure that the inductor current does not
exceed the rated current.
Apply a few volts to the inductor and
measure
the current and power factor. Repeat for 120 Volts.
Insert the iron core in the inductor and perform an experimental study to measure the
effect of the iron core on the inductance.
6. Add a resistance in series with the inductor so that the power factor angle is
approximately 45 degrees.
7.
Apply 120 Volts to the inductor in series with the resistance, and measure the current,
the voltage, and power.
7.
Connect a variable capacitor across the RL combination and measure the current,
voltage and power factor for a variety of capacitance values.1
10. Use a transformer and a resistance box to create a model of a transmission line. You
should connect as many of the four coils of the transformer in series (in the
appropriate order) to create an inductance of about 500 mH.to 200 mH with a
parallel resistance of about 5 to 10 Ohms. You may not need to add any resistance
as there will be losses in the transformer. You should measure the power factor that
you observe exciting the inductor. Try and make sure you operate the inductor in
it’s linear region.
11. Connect your AC source through your transmission line to your parallel combination of
Capacitance and R-L combination. Arrange to measure the Power into and out of
your transmission line. Measure the real and reactive power lost in your tranmssion
line as a function of your balancing capacitance. Determine the capacitance value
that produces the lowest transmission loss. Plot MW and MVAR loss vs capacitance
for in your report. NOTE: you may wish to first attach both power meters to the
same side of your tranmsion line to ensure they are both calibrated the same before
swithching the configuration to measure the Power flow in and out of the tranmssion
line.
REPORT: In addition to the usual structure of a report make certain to include the following
items.
1. Calculate the inductance and the resistance of the Inductor at low and high voltages without
the iron core inserted and with the core inserted. Compare these resistances to the measured
resistance using a DMM. Explain any differences observed.
2. Your calculation of the needed resistance to create a power factor angle of 45. What
Resistance did you use and what pf resulted?
3. Compare the values of pf measured for the capacitance in paralell with your R-L load
with the results of computation.
4. Use your power world model with an additional capacitive load inserted. Run your
model while changing value of the balancing capacitor. Plot MW and MVAR losses in
your transmission line vs. Capacitance. Your results won’t match your experiment
exactly but comment on the characteristic differences and similarities.
PowerWorld Appendix
Download the version designed for the ECE 442 textbook by Glover Sarma and Overbye, here.
On-line training is provided by Powerworld here
Power World is a graphical editor which allows easy creation of power systems combined with a
powerful analytical tool for doing a variety of power flow analysis on power systems, both
steady state and transient. It was created for teaching power systems analysis at universities.
Because power systems typically work with significantly larger quantities of voltage, current and
power than we do in this lab, our use of Power world will require us to boost the level of our
signals in the model. The default units are kV and MW or MVAR. So rather than modelling
208V, or 120V we’ll use 208 kV or 120 kV.
For this Lab you will primarily be using the Draw and Tools toolbars. The Draw toolbar to
create your one-line diagram and the Tools toolbar to run the power flow calculation.
Open power world
File  New Case.
Opens new oneline diagram sheet
You add items to your oneline diagram from the Draw toolbar. Your generator, busses (nodes),
transmission line and loads will be entered by clicking on the network drop-down menu on the
Draw toolbar, selecting the item that you wish to add, moving the curser to the location on your
diagram where you wish to place the item and clicking again. An exception is the tranmssion
line item. Each time you click the line is extended to the location of your click. The line is
terminated with a double-click. You shculd first click on one of the busses to be connected and
then a few more times to route the T-line and then double clicking on the final bus.
Whenever you add an element a dialog box opens up to allow the entry of parameters for the
element you are adding. It isn’t necessary at this stage for you to understand all of the
parameters, but there are some important ones to enter. The table below indicates the important
information for each element
Element
Bus
Generator
Load
Information you should enter
1. Bus Name (optional)
2. Nominal Voltage (a variable that calculations will change
except for the slack bus)
3. One bus must have the slack checkbar marked – for this lab
it should be the one attached to the generator to ensure it is
a constant voltage source.
1. Set initial values for MW and MvarsOutput
2. Ensure the Enforce MW output box is checked.
3. Enter a value of 100 for the Generator MVA Base
1. You need to have numbers in all of the Base Load Model
boxes. Normally the load is given in Constant Power
terms. For this lab you will want to use Constant
Impedance. The values you enter are not however the
impedance values but the values of MW and Mvar in p.u.
that would be dissipated in your impedances for 1 pu
voltage applied.
Transmission Line
1. In the per unit Impedance Parameters area enter 0.012 for
resistance, .111 for reactance, .28 for Shunt charging and 0
for shunt conductance.
Take some time to look at the other tabs in the dialog boxes and play with thenm if you like.
Some just effect the looks of the diagram and others effect how the element behaves.
When you have any questions, you can always hover your curser over an item, entry area, etc
and press F1 which brings up the on-line help for the item you’re hovering over.
When you have entered your elements you must switch from the edit to the run mode on the far
left of the ribbon bar. Then select the Tool toolbar. In the power Flow tools section look for the
arrow – if it is green pushing it will continuously solve your case. If it is not green, click on
solve and select the Full Newton method. After this the arrow should now be green.
Fields and variable fields. To display fields (solution values) for elements one needs to go to the
draw toolbar, Select the Fields drop down menu from the Individual Insert section, choose the
type of element whose field you’d like to see then click on the specific instance of that element.
Choose which field to show from the dialog box that opens. You can make the field adjustable
during the run by entering a value in the “Delta per click” box.