AEMC PEL102, PEL103 User Manual

Understanding Power &

AC

Power Quality Measurements

is the actual transformer voltage. This
system is frequently used for power
loads in commercial and industrial
buildings. In such cases, service to the
premises is made at 208V, three-phase.
Feeders carry the power to panel
boards supplying branch circuits for
motor loads. Lighting loads are usually
handled by a separate single-phase
service. The 480V distribution
is often used in industrial buildings
with substantial motor loads.

ACB

A

The threatened limitations of
conventional electrical power sources
have focused a great deal of attention
on power, its application, monitoring
and correction. Power economics
now play a critical role in industry as
never before. With the high cost of
power generation, transmission, and
distribution, it is of paramount concern to
effectively monitor and control the
use of energy.

The electric utility’s primary goal is
to meet the power demand of its
customers at all times and under
all conditions. But as the electrical
demand grows in size and complexity,
modifications and additions to existing
electric power networks have become
increasingly expensive. The measuring
and monitoring of electric power have
become even more critical because of
down time associated with equipment
breakdown and material failures.

For economic reasons, electric power
is generated by utility companies at
relatively high voltages (4160, 6900,
13,800 volts are typical). These
high voltages are then reduced at
the consumption site by step-down
transformers to lower values which may
be safely and more easily used
in commercial, industrial and residential
applications.

Personnel and property safety are
the most important factors in the
operation of electrical system operation.
Reliability is the first consideration
in providing safety. The reliability of

any electrical system depends upon
knowledge, preventive maintenance
and subsequently the test equipment
used to monitor that system.

Typical Voltage Configurations

Single-Phase Systems

Single-phase residential loads are
almost universally supplied through
120/240V, 3-wire, single-phase services.
Large appliances such as ranges,
water heaters, and clothes dryers
are supplied at 240V. Lighting, small
appliances, and outlet receptacles
are supplied at 120V. In this system the
two “hot” or current carrying conductors
are 180° out-of-phase
with respect to the neutral.

A

0

B

LINE (HOT)

NEUTRAL

LINE (HOT)

Figure 1. 1Ø System

Three-Phase, 3-Wire Systems

In this type of system, commonly known
as the “DELTA” configuration, the
voltage between each pair of line wires

t

B
C

Figure 2. 3Ø, 3-wire system

Three-Phase, 4-Wire Systems

Known as the “WYE” type connection,
this is the system most commonly used
in commercial and industrial buildings.
In office or other commercial buildings,
the 480V three-phase,
4-wire feeders are carried to each floor,
where 480V three-phase is tapped to
a power panel or motors. General area
fluorescent lighting that uses 277V
ballasts is connected between each leg
and neutral; 208Y/ 120 three-phase,
4-wire circuits are derived from step­down transformers for local lighting and
receptacle outlets.

Typical voltage:
phase-to-phase = 208/480V
phase-to-neutral = 120/277V

B

A
Neutral

B

C

Figure 3. 3Ø, 4-Wire System

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Balanced vs. Unbalanced Loads

A balanced load is an AC power
system using more than two wires,
where the current flow is equal in each
of the current carrying conductors.
Many systems today represent an
unbalanced condition due to uneven
loading on a particular phase. This
often occurs when electrical expansion
is affected with little regard to even
distribution of loads between phases or
several nonlinear loads on the
same system.

RMS vs. Average Sensing

The term RMS (root-mean-square)
is used in relation to alternating
current waveforms and simply means
“equivalent” or “effective,” referring
to the amount of work done by the
equivalent value of direct current (DC).
The term RMS is necessary to describe
the value of alternating current, which
is constantly changing in amplitude
and polarity at regular intervals. RMS
measurements provide a more accurate
representation of actual current or
voltage values. This is very important
for nonlinear (distorted) waveforms.

Until recently, most loads were “linear”;
that is, the load impedance remained
essentially constant regardless of
the applied voltage. With expanding
markets of computers, uninterruptible
power supplies, and variable speed
motor drives, resulting nonlinear
waveforms are drastically different.

Measuring nonsinusoidal voltage and
current waveforms requires a True
RMS meter. Conventional meters
usually measure the average value of
amplitudes of a waveform. Some meters
are calibrated to read the equivalent
RMS value (.707 x peak); this type
calibration is a true representation only
when the waveform is a pure sine wave
(i.e., no distortion). When distortion
occurs, the relationship between
average readings and True RMS values
changes drastically. Only a meter which
measures True RMS values gives
accurate readings for a nonsinusoidal
waveform. RMS measuring circuits
sample the input signal at a high
rate of speed. The meter’s internal
circuitry digitizes and squares each
sample, adds it to the previous samples
squared, and takes the square root of
the total. This is the True RMS value.

V

÷

Ø

I

Figure 4. Nonlinear current waveform

Demand

The amount of electrical energy
consumed over time is known as
demand. Demand is the average load
placed on the utility to provide power
(kilowatts) to a customer over a utility­specified time interval (typically 15 or
30 minutes). If demand requirements
are irregular, the utility must have
more capability available than would
be required if the customer load
requirements remained constant. To
provide for this time-varying demand,
the utility must invest in the proper size
equipment to provide for these power
peaks. Brief high peaks such
as those present when large equipment
initially comes on line are not critical
in the overall equation because the
duration is short with respect to the
demand averaging interval.

Consumption

Watts and vars are instantaneous
measurements representing what is
happening in a circuit at any given
moment. Since these parameters
vary so greatly within any period, it is
necessary to integrate (sum) electrical
usage over time.

The fundamental unit for measuring
usage is the watt hour (Wh), or more
typically the kilowatt hour (kWh). This
value represents usage of 1000W for
one hour. Typical costs in the United
States for one kilowatt hour range
from 8 to 15 cents.

Power Factor

Power factor is the ratio of ACTUAL
POWER used in a circuit to the
APPARENT POWER delivered by a
utility. Actual power is expressed in
watts (W) or kilowatts (kW); apparent
power in voltamperes (VA) or
kilovoltamperes (kVA). Apparent power
is calculated simply by multiplying the
current by the voltage.

Power Factor = Actual Power = kW

Apparent Power kVA

Certain loads (e.g., inductive type
motors) create a phase shift or delay
between the current and voltage
waveforms. An inductive type load
causes the current to lag the voltage by
some angle, known as the phase angle.

On purely resistive loads, there is
no phase difference between the
two waveforms; therefore the power
factor on such a load will be 0 degrees,
or unity.

The following examples of a soldering
iron and a single-phase motor illustrate
how power factor is consumed in
different types of loads. In a soldering
iron, the apparent power supplied by
the utility is directly converted into heat,
or actual power. In this case, the actual
power is equal to the apparent power,
so that the power factor
is equal to “1” or 100% (unity).

KVA

ø

Kw

V

√

Figure 5. Power factor on nondistorted
sine wave.

Kvar (inductive)

Ø

I

In the case of a single-phase motor,
the actual power is the sum of several
components:

a. the work performed by the system;

that is, lifting with a crane, moving air
with a fan, or moving material, as with
a conveyer.

b. heat developed by the power lost in

the motor winding resistance

c. heat developed in the iron through

eddy currents and hysteresis losses

d. frictional losses in the moor bearings

e. air friction losses in turning the motor

rotor, more commonly known as
windage losses.

We now observe that with a single­phase motor, the apparent power
obtained is greater than the actual
power. This difference is the power
factor.

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Power factor reflects the difference
which exists between loads. The
soldering iron is a purely resistive load
which absorbs the current, which is
then absorbed directly into heat.
The current is called actual current
because it directly contributes to the
production of actual power.

On the other hand, the single-phase
electric motor represents a partially
inductive load consisting of actual
current which will be converted into
actual power, and magnetizing current
which generates the magnetic field
required to operate the electric motor.
This magnetizing current, called the
reactive current, corresponds to an
exchange of energy between the
generator and the motor, but it is not
converted into actual power.

Reactive Compensation Power

Reactive compensation power refers
to the capacitive values required to
correct low power factor to as close
to unity (1.0) as possible. Most
industrial loads are inductive, so the
load current lags the line voltage by
some degree. In order to bring the
value closer to unity, something must
be added to the load to draw a leading
current. This is done by connecting a
capacitor in parallel with the load. Since
a capacitor will not dissipate any real
power, the charge for real power will be
the same.

AEMC recommends consulting a power
factor correction capacitor manufacturer
prior to any installation to reduce
the possible effects of harmonics,
resonance, etc.

Electrical Harmonics

Until fairly recently, power quality
referred to the ability of the electric
utilities to supply electric power
without interruption. Today, the phrase
encompasses any deviation from a
perfect sinusoidal waveform. Power
quality now relates to short-term
transients as well as continuous state
distortions. Power system harmonics
are a continuous state problem with
dangerous results. harmonics can be
present in current, voltage, or both. It is
estimated that as many as 60% of all
electrical devices operate with non­linear current draw.

Utility companies invest millions of
dollars each year to ensure that voltage

supplied to their customers is as close
as possible to a sinusoidal waveform.
If the power user connects loads to the
system which are resistive, such as
incandescent light bulb, the resulting
current waveform will also be sinusoidal.
However, if the loads are nonlinear,
which is typically the case, the current
is drawn in short pulses and the current
waveform will be distorted. Total current
that is then drawn by the nonlinear load
would be the fundamental as well as all
the harmonics.

Fundamental

3rd Harmonic

Resultant Distorted Waveform

Figure 6. Composite waveform

Harmonic distortion can cause serious
problems for the users of electric
power, from inadvertent tripping
of circuit breakers to dangerous
overheating of transformers and neutral
conductors, as well as heating in motors
and capacitor failure. Harmonics
can cause problems that are easy to
recognize but tough to diagnose.

It is becoming increasingly important
to understand the fundamentals of
harmonics, and to be able to recognize
and monitor the presence of damaging
harmonics. Harmonics within an
electrical system vary greatly within
different parts of the same distribution
system and are not limited simply to
the supply of the harmonic producing
device. Harmonics can interact within
the system through direct system
connections or even through capacitive
or inductive coupling.

A harmonic may be defined as an
integer multiple of a fundamental
frequency. Harmonics are designated

by the harmonic number. For our
discussion, we will focus on the 60Hz
power frequency. The second harmonic
would be two times the fundamental or
120Hz. The third would be three times
the fundamental or 180Hz, and so on.

Nonlinear equipment generates
harmonic frequencies. The nonlinear
nature of a device draws current
waveforms that do not follow the
voltage waveform. Electronic
equipment is a good example. While
this broad category encompasses many
different types of equipment, most of
these devices have one characteristic
in common. They rely on an internal DC
power source for their operation.

Loads which produce harmonic
currents include:

•Electroniclightingballasts

•Adjustablespeeddrives

•Electricarcfurnaces

•Personalcomputers

•Electricweldingequipment

•Solidstaterectifiers

•Industrialprocesscontrols

•UPSsystems

•Saturatedtransformers

•Solidstateelevatorcontrols

•Medicalequipment

This is by no means an exhaustive
list of equipment which generates
harmonics. Any electronic-based
equipment should be suspected of
producing harmonics.

Due to the ever increasing use
of electronics, the percentage of
equipment which generates harmonic
current has increased significantly.
The harmonic problem manifests itself
with proliferation of equipment using
diode capacitor input power supplies.
This type of equipment draws current
in a short pulse only during the peak
of the sine wave. The result of this
action, aside from improved efficiency,
is that high frequency harmonics are
superimposed onto the fundamental
60Hz frequency.

The harmonics are produced by the
diode-capacitor input section which
rectifies the AC signal into DC. The
circuit draws current from the line
only during the peaks of the voltage
waveform, thereby charging a capacitor
to the peak of line voltage.

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