Nowadays, fluctuations 7. Frequency variations 2.5 Waveform distortion Wave

Nowadays, electrical device, equipment, facility and
etc. has become an essential element in our daily life, so to ensure its
operation and functionality power quality become a very important factor. According
to IEEE the definition of power quality is the concept of powering and
grounding electronic equipment in a manner that is suitable to the operation of
that equipment and compatible with the premise wiring system and other
connected equipment. 1

Power is the rate of the energy being deliver and is
proportional to the product of the voltage and current as the formula

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P = VI

The power supply system can only control by the
voltage quality and the current that might draw by a particular load is cannot
be control. Therefore, power quality is control by maintaining the supply
voltage within certain limits.

So why we concerned about the power factor? Economic
value is the main reason. Power quality have a direct impact on both the utility
and industry consumer due to the use of modern technology today in their
management and production area so, if any disturbance or problem occur it will
cause a huge economy and financial lost. Furthermore, it also causes impact to
the commercial customer indirectly for example they are not able to use the
internet due to the serve is interrupted.

2.0 Type of power quality problems

 1. Transients

2. Interruptions

3. Sag / Undervoltage

4. Swell / Overvoltage

5. Waveform distortion

6. Voltage fluctuations

7. Frequency variations

 

 

2.5 Waveform distortion

Wave distortion is defined as a steady-state deviation
from idea sine wave of power frequency principally characterized by the spectral
content of the deviation. 3 The primary types of waveform distortion are

1. DC offset

2. Harmonics

3. Interharmonics

4. Notching

5. Noise

2.5.1 DC offset

DC offset occur when there is a DC current or voltage
exist in an AC power system. The existence of DC current in an AC distribution
system is normally due to the failure of rectifiers within several AC to DC
conversion technologies that have proliferated modern equipment. DC can pass
through the AC power system and add unnecessary current to the device that had
operating at its rated level. This will causes additional heating and decrease
in transformer life. In addition, these additional heat not only make the
transformer hot but also causes it to be unable to deliver full power to the
load and subsequently create more instability in the system due to the
distortion of waveform.  

Figure
1: DC offset

 

 

 

 

2.5.2 Harmonics

When, the sinusoidal voltage or current have multiples
time higher frequency than the frequency of the supply system that is designed
then harmonic distortion occur. The fundamental frequency of the supply system is
normally 50 Hz or 60 Hz. Waveform that had distorted periodically can be break
down into a sum of fundamental frequency and the harmonics. Harmonic distortion
originates in the nonlinear characteristics of device and load of power system.
3 Nonlinear loads are the one that producing harmonic current. These harmonic
current produce will interact with the power system equipment and can cause
interference with telecommunication lines and result error in the power
metering.

Furthermore, it will also the reason of overheated
transformers, neutral conductor and other electrical powered equipment, tripping
of circuit breaker and loss of synchronization of a timing circuit. 2

Figure
2: typical harmonic waveform
distortion

 

2.5.3 Interharmonics

When, the sinusoidal voltage or current have frequency
that are not multiples time higher than the frequency of the supply system that
is designed then interharmonic distortion occur. 3 The main source of this
type of waveform distortion are static frequency converter, cycloconverters,
induction furnace, and arcing device. Signal that carried by the power line can
also be considerate as interharmonics. The most common effect of the harmonics
distortion is the flickering of the visual display and the light bulb.

Figure
3: Interharmonics waveform distortion

 

2.5.4 Notching

Notching is a periodic voltage disturbance caused by the power
electronic device under normal operation when current is commutated from one
phase to another. 3 Notching can be characterized through the harmonic
spectrum of the affected voltage since it occur continuously. The problem will
face when notching occur in a system usually are system halt, data loss, data
transmission problem and etc. 2

Figure
4: Notching

 

2.5.5 Noise

Noise is the unwanted electrical signal with broadband
spectrum content lesser than 200 kHz superimposed upon the power system voltage
or current in phase conductor or found on neutral conductors or signal lies. There
are three type of noise in the electrical power system, which is the line to
neutral, neutral to ground, line to ground.

Noise usually caused by the power electronic device,
arcing equipment, control circuit, load with solid-state rectifier, switching
power supplies and etc. The existence of noise are usually because of the
improper grounding that fails to conduct noise away. 3

Figure
5: Noise

 

2.6 Voltage Fluctuation

Voltage fluctuation are systematic variation of the
voltage envelop or a series of voltage randomly change. Loads that show
continuous and rapid change in the load current magnitude will cause voltage
fluctuation that called flicker. The term flicker in this case is actually
derived from the voltage fluctuation happen on a lamp and cause the lamp to flicker.
2

Figure
6: Voltage fluctuation

 

Arc flicker is one of the most cause of voltage
fluctuation on distribution system and utility transmission. The flicker signal
is define by its rms magnitude systems expressed as a percent of the
fundamental. Sensitivity of the human eye is used as a standard to measure the
voltage flicker. For a perceptible lamp to flicker the magnitudes is as low as
0.5 percent and if the frequencies are between 6 to 8 Hz. 3

 

2.7 Power frequency variation

Power frequency variations are defined as the change
of the power system fundamental frequency from it specified nominal value. The
rotational speed of the generators supplying system is directly related to the
power system frequency. When the dynamic balance between load and generation
changes there will have a slight variation in frequency. The load
characteristics and the response of the generation control system to load
changes define the size of the frequency shift and its duration. 3

Power frequency variation is very rare and unlikely to
happen in a stable utility power system, especially system that is
interconnected through power grid. This variation of frequency are more likely
to happen in load that are supplied by a generator that is isolated from the
utility system. 3

Figure
7: Power frequency variation

 

Method to solve power quality problem

1.      Troubleshooting
and replacing

To solve this DC offset problem,
first is to troubleshoot the device circuit board and find out the faulty
component that causes DC offset and replace the faulty component with the new
one.

2.      Filters

Adding filters is one of the method
use to remove harmonic and noise. Noise filter remove the unwanted noise of the
system. The type of filters are the low pass filter, high pass filter, band
pass and notch. Different types of filters are used to remove different type of
noise that the system desire. The most common filter use are the high pass
filter and low pass filter to remove the unwanted high or low frequency noise.

 

Harmonics filter reduce the unwanted
harmonics in the system.  There are 2
group of harmonics filter which is the passive filter and ate active filter. To
remove the harmonics of several components amount of passive filter is connect
in parallel to the component. But there’s a problem face by passive filter
which is when the harmonics component is change, the passive filter will become
useless and will even cause resonance to the system. Therefore, active filter
is introduced, it calculated the current consumed by the component and then
generate a current that will cancel the harmonics current produced by the
component.  The reason why in the pass
day people choose to use passive filter but not active filter is that active
filter is very expensive compare to the passive filter.

 

3.      Uninterruptible
Power Supply (UPS)

UPS can solve many power quality
disturbance such as interharmonics, notching, voltage fluctuation and power
frequency variation. UPS help to avoid complete power interruption which is
also called black out. During the event of momentary interruption UPS provide
power continuously to the loads. There are 3 types of UPS:

i.                   
Offline UPS

This
type of UPS is a low cost method for a small, non-critical, independent system.
It provide power to the load by electrical utility for example battery, and
give protection during sag, swells and power interruption. It’s low cost and
high efficiency, but it has a problem which is the capacity of power suppling
is limited by the battery and it has regulation at the output voltage and
noticeable transfer time.

ii.                 
Line interactive UPS

This
type of UPS are used when the rate of voltage fluctuation high. Unlike the
offline UPS which has poor output regulation line interactive can regulate the
output voltage well and without depletion the battery. Line interactive UPS has
better output regulation and high efficiency, but it has noticeable transfer
time.

iii.               
True on-line UPS

This
type of UPS has the highest level of power protection among the others. It is
suitable to use in the critical power system. It is able to convert from AC to
DC for battery charging and inverter powering. To power the critical loads, DC
that converted will then convert again back to AC. It has no noticeable transfer
time and good in eliminate voltage fluctuation, but it has less efficiency
compare to the others and may has more audible noise.

 

 

 

 

4.      Voltage
regulator

It is the most common type method used
to remove or control the voltage fluctuation problem in the input side of the
circuit. There are three type of voltage regulator:

i.                   
Tap charger

It
able to adjust the changing input voltage by transferring taps automatically on
a power transformer. It is high efficiency, large input range, high capability
in overload current and less noise, but noise may produce during the changing
of taps and waveform that gone wrong won’t have any correction.

ii.                 
Buck boost

It
has exactly same function as tap charger just the transformer is not isolated.
It is high efficiency, suitable to operate with high in-rush current, but
facing the problem as tap charges and also have bad noise isolation.

iii.               
Constant voltage transformer (CVT)

CTV
able to maintain nearly constant output voltage during large variation in input
voltage. It has very good noise isolation and for overload protection it is
able to precisely limiting the output voltage and current, but it very large in
size and may have audible noise and low efficiency.